Electrical Protection

FOREWORD The subject entitled ‘Electrical Protection’ of Electrical Technology. This and other attend our trai ning programmes.

is one of a series which comes under the heading series have been prepared for our employees who .~ _..

The three ‘Fundamentals of Electricity’ manuals introduce and provide a basis for further study of related’ topics such distribution, motors,~control, protection a6d electrical safety. so that it can be used for self-study;~and, w.ith this in mind, a has been incorporated.

the subject of electrical power as power systems, generation, This manual has been designed series of questions and answers

A list of the electrical power manuals is included inside the front training manuals in the s’eries is given on the inside of the back cover.

Harry

cover,

and a list of all

To/t

Shell Expro

Training,

Aberdeen.

Shell Expro is the major operator exploring for and producing oil and gas in the United Kingdom -~ working alone and in joint ventures with third parties. In the North Sea it is the operator for a SO/50 joint venture with Esso, where the projected output from fields already in operation will meet more than a third of the UK’s estimated oil needs and over 12% of its natural gas requirements.

CONTENTS Page ._

to Protection

--

1

Chapter

1’

Introduction

Chapter

2

Faults

Chapter

3

Overcurrent

Chapter

4.

Discrimination

Chapter

5

Earth-fault

Chapter

6

Differential

Chapter

7

Undervoltage

Chapter

8

Additional

Chapter

9

Generator

Chapter

10

Transformer

Chapter

11

Cable Protection

65

Chapter

12

Motor

68

Chapter

13

Questions

and Fault

4

Levels

11

Protection

30 and Earth-leakage

Protection

34

Protection and System Forms

42 Undervoltage

of Protection

Protection Protection.

Protection and~Answers

Protection

48 50 52 59

80 91

CHAPTER INTRODUCTION

1.1

THE

REASONS

FOR

1

TO PROTECTION

PROTECTION

Electrical plant, machines and distribution systems may occur through abnormal conditions arising. Abnormal

conditions

may be grouped

(a)

Operation outside ing of the system.

(b)

Fault

conditions

into two

the designed

must be protected

t~bverloadifigor

breakdown

Condition (a) is usually ‘chronic’ -that ii, able for a limited period. It may give rise machines and equipment, but, unless these causes sudden or catastrophic failure. It can or a fault condition.

damage which

types: ratingsde

due usuallyvto

against

of some part’of

incorrectfunction-

the system.

it may persist for some time and is often acceptto temperatures outside the design limit of the are very excessive or very prolonged, it seldom usually be corrected before it leads to breakdown

Condition (b) on the other hand is ‘acute’ and arises from electrical or.,mechanical failure which, once established, produces a condition beyond.control. It usually gives rise to very severe excess currents which will quickly cause catastrophic failure of other electrical and mechanical plant in the system unless the fault is rapidly isolated. It may be caused by a breakdown of insulation due to a material failure or overheating or to external conditions such as weathe~r, or it may be due to physical damage to an item of plant or cable. Automatic protection against conditions (a) and (b) is possible~ in electrical installations because it is easy to measure various parameters, to detect abtiormalities, and to set in motion the protective action the instant an abnormality arises. Protection

of an electrical

system

is provided

(a)

To maintain electrical fault has been isolated.

(b)

To protect conditions

(c)

To protect the consumer (e.g. overload).

(d)

To isolate faulted

(e)

To limit

supplies

the generators and faults.

for,one

to as much

and other

equipment

equipment

or more

plant

of the following

of the system

against

damage

principles:

as possible

after

a

due to abnormal

against damage due to abnormal

conditions

to limit the risk of fire locally.

damage to the cable system

resulting,from

a fault.

These principles will determine the type of protective equipment fitted in any installation. It will be noted that principle (a) conflicts with the other requirements to some extent. For example, the best way to protect a generator againstdamageby fault currents is to disconnect it, but it would not then be available to supply other consumers. For this reason a method of disconnection (called ‘discrimination’) has been devised. It is fairly complex and has to be very carefully ,engineered. ,It is dealt with in Chapter 4.

1

Ail the protection (VT) or current voltage systems, busbars through operated devices

devices in this manual are assumed to be fed from voltage transformers transformers (CT). In high-voltage systems this is always the case. In lowsuch as 44OV, voltage-operated devices are sometimes fed direct from the fuses without an intervening VT. Except where currents are small, currentalways use CTs even on low-voltage systems.

The above five principles relate mainly to the protection system. There remains of course the protection of personnel.

of plant

and equipment

in the

An arc may occur at the point of the fault. Apart from possibly burning or blinding anyone in the vicinity, an arc in a high-power system may produce enough heat to melt heavy copper bars or even structural steel in a very short time, and a rapidly spreading fire may result. Arcs are particularly dangerous in areas where flammable gas may be present. It is vital therefore that the source of power which is feeding the arc should be cut off as quickly as possible.

1.2

PRINCIPLES

OF PROTECTION

The fundamental principle of protection is to disconnect and isolate the faulty part of the system so that the fault is not sustained and aggravated by a continuing flow of power into it, and the rest of the system is not damaged and can revert to its normal state. Generally speaking this means automatically detecting the fault condition by means of a suitable device and disconnecting the faulty section by means of a circuit-breaker or other interrupter. For some purposes the two functions are combinrd in one item of switchgear, as in a moulded-case circuit-breaker. In many cases protection is provided by fuses, in which the functions are inseparable. Protection relays as devices xe described in the manual ‘Electrical Control Devices’, and fuses are discussed in Chapter 3 of this present manual. The occurrence of a fault is indicated by various quantities, usually excess currents, ing upon the nature of the fault. The way in which the protection devices respond in terms both of magnitude and of time, is very important for several reasons:

1.3

-

The affected part of the system av&dable damage is done.

-

The protection should not operate unnecessarily. Transient disturbances are liable to occur on most systems for many reasons connected with operation, and most electrical plant is capable of operating safely with moderate overloads for short periods.

-

If the amount of .equipment disconnected is to be kept to the minimum necessary to clear the fault, the sensitivities of the various protection devices which respond to the fault must, as far as possible, be so graded and related that only that device needed to clear the fault actually operates. This is the principle known as ‘discrimination’, which is discussed in Chapter 4.

DISCONNECTION

The means of detecting There are three categories

2

dependto faults,

should

be disconnected

quickly,

before

any

DEVICES a fault and the means oi disconnecting it are equally of devices used to disconnect faulty circuits:

important.

Circuit-breakers. These are generally capable of interrupting the maximum fault currents that can flow in the circuits which they feed. Since under some fault conditions the current may rise to ten or more times the normal full-load current, the design and selection of circuit-breakers is of great importance. Several types of circuit-breaker are in use with different arrangements for arc suppression air-break, oil-break, sulphur hexafluoride (SF,) and vacuum; these are described in detail in the manual ‘Electrical Distribution Equipment, Part A’, Contactors. Contactors are rated to close onto the most severe’faults but have limited breaking capacity; in most cases this is less than the maximum possible fault current in the circuit which they feed. Therefore they have usually to be supplemented by fuses. Contactors may be air-break or vacuum-break; they are described in more detail in the manual ‘Electrical Distribution Equipment, Part A’. Fuses. A fuse constitutes an intentional ‘weak--link’ in an electrical circuit and,, suitably rated, is particularly apt for the quick interruptionofshort-circuit~currents. Fuses, are described in Chapter 3. They are expendable and have to be replaced after they have operated. They are very costly.

1.4

PROTECTION

AND

SYSTEM

DESIGN

No system of protection can be designed without knowing the conditions in the network which it.has to protect. This means that the level of fault currents at various pojnts of the network must be known in advance so that the right type of switchgear may be installed and a proper system of~protection worked,out. The first tasks therefore is to calculate the ‘fault levels’ at all those where switching is to take place. Fortunately this is not a difficult described in Chapter 2.

points in the, network calculation, and it is

3

CHAPTER FAULTS

2.1

AND FAULT

2 LEVELS

GENERAL

An electrical network normally operates within its designed rating. Generators, transformers, cables, transmission lines, switchboards, busbars and connected apparatus are each designed to carry a certain maximum current. Most can carry a moderate overload for a short time without undue overheating. However, if a fault should develop somewhere in the system, that is to say short-circuit or a phase-to-earth breakdown, then all connected generators extremely high currents into that fault, which will be limited only by the complete circuit from generator to fault. Fault currents can be ten or more full-load current.

a phase-to-phase will at first feed impedance of the times the normal

Such currents will quickly cause intense overheating of conductors and windings, leading to almost certain breakdown unless the.y are quickly disconnected. They will also give rise to severe mechanical forces between the current-carrying conductors or windings. All such apparatus must be manufactured to withstand these forces. A fault current of 50 OOOA (rms) flowing in two busbars 3 inches apart will produce between~them a peak mechanical force of nearly half a ton-force per foot run of bars. The pui-pose of automatic protection is to remove the fault from the system and so break the fault current as quickly as possible. Before this can be achieved, however, the fault current will have flowed for a finite, if small, time, and much heat energy will have been released. Also the severe mechanical forces referred to above will already have occurred and will have subjected all conductors to intense mechanical stress.

2.2

FAULT

LEVEL

CALCULATION

In order td design electrical equipment to withstand the expected thermal and mechanical stresses, and to engineer the protective system to operate decisively and quickly, it must be possible to calculate the maximum ~fault current to be expected anywhere in the system under the worst possible conditions. Phase-to-phase and phase-to-earth faults may be nietal-to-metal, but more probably they will be arcing faults where the arc itself has some resistance which will reduce the flow of fault current. However, for calculation purposes the worst condition is considered, and shortcircuits are assumed to be ‘bolted’ - that is, it is assumed that all three conductors are firmly bolted together and that the fault itself has zero impedance. In order to understand the fault conditions what happens in the simpler d.c. case. 2.2.1

in an a.c. network,

it will be helpful

to consider

D.C. Case

Referring to Figure 2.1, suppose the full-load current I of a d.c. generator is produced with an external load resistance R. If E is the emf and r the internal resistance of the generator, then the internal voltage drop is /.r an~d the terminal voltage (that is, rated voltage) V.of the generator at full load is E - /.r. Suppose the internal drop is, say, 20% of the open-circuit voltage E (assuming that there is no automatic voltage regulation).

4

FIGURE D.C. GENEYATOR

2.1 ON LOAD

If now the external load R is decreased so that the current doubles to 2/, then the internal drop increases to 2/x, or 40%. If R is further reduced so that the current is trebled to 3/, then the internal drop increases to 3/x, or 60%. Similarly a current of 4/ will cause an internal drop of 80%, and 5/ would produce 100%. A 100% internal drop means that the whole emf is used in overcoming the internal drop, and there is no voltage left between the terminals - that is, they are effectively at shortcircuit. Put the other way, a dead short-circuit across the generator produces B current five times (1 + 20% or 1 + 0.2) full-load current. The generator is then said to have an internal resistance of 20%. This is an alternative way of expressing it instead of in ohms. 2.2.2

A.C. Case

The same argument applies to the ax. generator shown in Figure 2.2, except that, instead of external and internal resistance, there is now impedance. However, in all ax. generators the internal impedance is almost wholly reactive, and it is therefore customary to talk of a generator’s ‘reactance’ x and to ignore the resistance. It is, like the resistance in the d.c. case; expressed as a percentage. Therefore a generator with a reactance of 20% will deliver 1 + 0.20, or five times, full-load current on short-circuit. This method of using percentages rather than or voitage of the particular generator. The ohms avoids having to con,sider the size (kVA) above applies whatever its size or voltage.

FIGURE A.C. GENERATOR

2.2 ON LOAD 5

An a.c. genes-ator has in fact a varying reactance, which increases as the fault proceeds, due to its complicated magnetic behaviour. For fault calculation purposes however the lowest reactance at the beginning of the fault is always taken; it is the ‘subtransient’ reactance and is typically between 8% and 20% on most generators. This is the most severe condition. A similar argument applies to transformers and 10%. A transformer reactance however

where the reactances are typically between 4% is fixed and does not vary as the fault proceeds. 5000kVA BASE FOR CALCULATION

-iI u 15% ‘5%

15% -15%

=

P

5% 50% 7=

t .5;!+, Q b lo.575

Q

(b)

(a)

TYPICAL

2 7%%)F

WITH

LV SECTION

BREAKER

at0

I

OPEN

N,ETWORK

(c)

SIMPLE

WITH LV SECTION BREAKER CLOSED

FIGURE 2.3 FAULT CALCULATION

Figure 2.3(a) shows a typical, but simple, network comprising two generators, two transformers and an WV and LV distribution system. A fault at a point P on one of the HV feeders would, if all HV breakers were closed, be fed by both generators in parallel. A fault at a point Q on the LV system would be fed by both generators (as before), but they would be in series with one transformer- if the LV section breaker web-e open, or with both transformers in parallel if it were closed. The exact calculation should, strictly, also take into account the resistances of thegeneratol-s and transformers as well as the impedances of the connecting cables, but for a rough calculation with platform-sized lengths of cable these can be disregat-ded.

6

Figures 2.3(b) and 2.3(c) show the reactance equivalent of each of the elements of the network, with the percentage reactance placed against each. Since the size of each generator is 5 OOOkVA the impedance of all other elements, such as that of the transformers, mustbe raised to th~is ‘base’. So, though the transformers are each rated 5% at 5OOkVA, they are entered as 50% at 5 OOOkVA, giving the same short-circuit current. The ‘adjusted’ reactances are shown in Figures 2.3(b) and (c) in red. it should be noted that any figure, such as 100 OOOkVA, may be chosen as a base; it makes no difference to the result. Choosing as a base the kVA of the largest generator is merely a convenience. ,(For onshore grid network calculations 100 OOOkVA base is usually chosen. In those cases generator and transformer resistances and cable impedances cannot be ignored.) The they both base

adjusted reactances are then resolved by ordinary series-parallel network methods until become a single reactance. Thus in Figure 2.3(b) the reactance up to the point P with generators connected is two 15% in parallel, equivalent to one 7%%. To the chosen of 5 OOOkVA the fault level at P is:

5 006 0.075

(Note. For the purpose 7%% = 0.075 p.u.)

of

calculation,

= 67 OOOkVA,

percentages

or 67MVA

are expressed

For point Q with one transformerconnected there is a further add, making 57%% in all. The fauhlevel at Q would be:.

m

For point Q with two transformers tance of 25% to add (derived from level at Q would then be:

‘= 8 7OOkVA,

series

as ‘per reactance

unit’.

Thus

of 50% to

or 8.7MVA

in parallel (Figure 2.3(c)) there is a further series reacthe two 50% in parallel), making 32%% in all. The fault

5 0.325

= 1,5 4OOkVA,

or 15.4MVA

If the generators had been of different sizes - say 5 OOOkVA and 2 5OOkVA, each with reactance 15% - the larger would have been chosen as ‘base’ and the smaller raised to it that is, call it 5 OOOkVA at 30%, and proceed as before: This calculation, though much simplified, illustrates the basic method of making fault calculations. It shows too the advantage of regarding all reactances as percentages; the actual voltage levels have not come into the calculation. It also illustrates the considerable reducing effect of transformers on a system fault level. The fault levels so calculated would apply respectively to the whole HV system and the whole LV system, and they are usually marked in MVA on drawings. The switchgear at each level must be capable of breaking the currents appropriate to those levels, and all conductors, busbars, cables, etc, must be able to withstand the thermal and mechanical stresses induced by those currents. Armed with the result of his fault calculations, the designer will specify exactly what the various items of equipment are required to withstand.

7

Fault levels calculated as shown are expressed in MVA, which is usually purposes, but if act+1 fault currents are needed, the MVA is converted dividing by,/3 times the voltage (kV). Thus:

67MVAat

2.3

-6.g3

= 5.9kA

at 440V

= 0.4;&3

= 11.4kA=ll

15.4MVA

at 440V

= ,,&$,

= 20.2kA

These are all runs symmetrical the above calculations

=

8.7MVA

(Note

From

6.6kV

that MVA

+43kV

gives the current

sufficient to current

for most (kA) by

=5 900A

400A

= 20 200A

in kA.)

currents. two

points

should

be noted:

-

A single transformer between the source of supply and an LV switchboard greatly ‘cushions’ the MVA fault level beyond it and reduces it drastically. In this case an HV fault level of 67MVA is reduced by the transformer to 8.7MVA.

-

Notwithstanding the great reduction of MVA by a transformer, the actual fault current on the LV side is actually increased - in this case from 5 900A to 11 400A. This is because the smaller MVA is obtained fror,l a still smaller voitage.

FAULT

LEVELS

AT LOW VOLTAGE

SWITCHBOARDS

Whereas apple designs of switchgear exist to handle the fault levels to be found on the largest offshore and onshore high-voltage systems, this is not the case on the low-voltage boards. As will be seen from the above calculation, the LV fault currents are in general higher than the HV - in some cases much higher indeed. On many offshore installations the LV fault level with the LV section breaker closed (that is, with both transformers feeding in parallel) exceeds the breaking capacity of the largest LV circuit-breaker available.’ It is therefore ~necessary to ensure that the switchgear fitted is not subjected to such a possible fault. From the above calculation it can also be seen that the LV fault level with one transformer feeding is about half of that with two. Therefore it is arranged in such cases that the two transformers should no? be allowed to feed in parallel. The LV section breaker is normally kept open when both transformers are feeding (the normal condition). The section brea!;er may only be closed if one or other transformer supply breaker is open. This is known as the ‘two-out-of-thl-ee’ method. Any two LV breakers out of the three (two incomers and one section) may be closed at any one time, but not all three. Interlocks .~I. .~p@vetit this. However, this arrangement may be temporarily ‘cheated’ and the vulnerability accepted when changing over from one transformer to the other. FOI- a short period the operation of the interlock is delayed to avoid interrupting supplies to the board. If one of the three breakers is not opened within that short time (typically 30 seconds) the section breaker will trip automatically.

8

2.4

ASYMMETRICAL

FAULT

CURRENTS

The fault currents as derived from the above calculation. the various items of plant, are all ‘rms symmetrical’.

usins the oercentage

reactances

of

rms Symmetrical Current

FIGURE ASYMMETRICAL

2.4 FAULT

CURRENT

The actual currents which occur in the early part of a fault however are generally asymmetrical, giving a greater heating rate. Moreover the highest mechanical forces will occur with the first asymmetrical peak of current, as shown in Figure 2.4. This can, with 100% asymmetry, be up to 2.55 times the rms symmetrical value, so the 67MVA at 6.6kV referred to above, equivalent to 5.9kA rms symmetrical, can rise to 15kA asymmetrical peak. It is this latter figure which determines the mechanical strength of.the busbars and other equipment. When a fault current is quoted in kA it is always wise to add the words ‘rms symmetrical’ if that is what is meant. This avoids confusion with ‘kA peak asymmetrical’ which is also often quoted in addition. Fault levels quoted in MVA are always rms symmetrical.

2.5

PROSPECTIVE

FAULT

CURRENT

In, some cases the actual fault current, even with a bolted short-circuit, may be limited by the operation of the protection device itself - notably in the case of a current-limiting fuse which can interrupt the current before it reaches its first peak (see Chapter 3, para. 3.3). In that case the current never attains its calculated level. However for the purposes of calculation it is assumed that no such limiting effect occurs and that the current will reach its calculated value. This value is called the ‘prospective fault current’, even though, in certain given systems, the fault current will not reach that level. Fuses are given notwithstanding

the credit in their ratings for interrupting the full prospective that they do so by preventing it ever happening.

fault

current

9

2.6

MOTOR

CONTRIBUTION

A factor that may have a significant effect on the fault level of a system which includes an .~ appreciable proportion of motor loads is that known as ‘motor contribution’. This refers to short-circuit current which is generated for a very brief period by a short-circuited induction motor as a result of magnetising currents still circulating in the rotor. (Synchronous motors also generate short-circuit current but are not likely to be encountered in Shell installations.) The magnitudes of such currents are not easy to determine with any accuracy, but they are commonly roughly estimated on the basis of a percentage reactance in the motor assumed to be about 30%. Thus a motor that presents a full load of 1 MVA is calculated to contribute l/O.3 = 3.3MVA to the fault ievel of the circuit which supplies it, and it will contribute to the fault level at any point in the system as if it were a IMVA generator with a subtransient reactance of 30%. This should be added to the calculated system fault level.

2.7

NON

SYMMETRICAL

FAULTS

A short-circuit between two phases results in a lower fault cukent than does a symmetrical short-circuit, because it is driven through the impedance of two phases by the line voltage which is only ,/3 times the phase .voltage. This condition therefore requires no further consideration here.

2.8

EARTH

FAULTS

Earth-fault currents, especially when iimited by earthing resistors,are dealt with in Chapter 5. Havir;g only 1143 of the system voltage behind them, they are in general lower than the short-circuit fault currents and will not therefore influence the fault level calculations.

10

CHAPTER OVERCURRENT

3.1

BASIS

OF OVERCURRENT

3 PROTECTION

PROTECTION

Overcurrent protection is related primarily to heating effects in, and in some circumstances to electromechanical forces on, electrical conductors and may cover both fault and overload protection. Overcurrent protection of device used:.

methods

may be divided

-

Overcurrent relay tripping para. 1.3 for the limitations

-

Direct

-

Fuse.

tripping

into three categories

a circuit-breaker of contactors).

device (‘release’)

A distinction should be made between~ the mechanical overloading of a machine will occur from causes other than overloading motor, short-circuits Or earth faults, none of

or contactor

on a circuit-breaker

according (but

to the type

see Chapter

1,

or Contactor.

terms ‘overcurrent’ and ‘overload’. Whereas the certainly cause overcurrents, OvercUrrents can - for example, stalling or single-phasing of a which is an overload.

The term ‘overload’ should be reserved for mechanical should be used in Its literal sense. All the devicesdescribed

loading, and the word ‘overcurrent’ in thischapterare trueovercurrent

sensors.

3.2

OVERCURRENT

PROTECTION

RELAYS

Overcurrent devices, though all depend on an excess of current to operate them, are of several different forms; these are described below. The official abbreviation (BS 3939) for each type is also given. 3.2.1

instantaneous

Overcurrent

(OC)

An instantaneous overcurrent relay is shown pictorially in Figure 3.1. It consists of a simple iron armature attracted by a coil carrying the current from a line current transformer and restored to its rest position either by gravity or by a control spring. When the current in the coil just exceeds a certain preset value, the pull on’the armature overcomes the spring or gravity and causes it to close. In so doing it operates auxiliary contacts which initiate a tripping circuit or other desired function. Though termed ‘instantaneous’ this type of relay nevertheless requires ‘a small but finite time to operate; this /s usually taken to be a maximum of 0.2 seconds, but it is often much less. The current/time characteristic is thus a ‘square’ one as indicated in Figure 3.1 (c). The value of the current required to operate the relay is set by the screw adjustment at the top. In a single-phas6 system a current transformer in one line,is connected to the relay coil (see Figure 3.1 (a)). In a 3-phase system a current transformer in each phase is connected to one of three relay coils (see Figure 3.1(b)). The three relay elements may be enclosed in one case or in separate cases. However, in a 3-phase, 3-wire, system any overcurrent in one line

Current

Setting Idjustment

Trip Circuit

(a) SINGLE PHASE

OC R&v

-T--Fe (b)

s

Trip Circuit

4

*I

Operating Current Setting

3-PHASE

TYPICAL

I

I

(c)~ CHARACTERISTIC CURVE

FIGURE 3.1 INSTANTANEOUS OVERCURRENT

RELAY

must be accompanied by an overcurrent in one or both of the return lines. Therefore, to achieve complete overcurrent protection in a 3-wire system, it is only necessary to provide overcurrent relay elements in two of the three phases (see also Figure 5.2).

3.2.2

Inverse Time Overcurrent

(OCIT)

’

An inverse-time overcurrent relays is shown pictorially in Figure 3.2. It has an ‘inductiontype’ movement similar to that of a household meter. It consists of a rotating aluminium disc driven by a shaded-pole magnet element which receives the driving current from the CT in the circuit to be monitored. As in a household meter, the disc also revolves between the poles of an eddy-current brake magnet; it is restrained by a light pre-tensioned control hairspring. The relay is used with current transformers in single-phase or 3-phase systems as described for simple overcurrent type and as shown in Figures 3.2(a) and (b). When normal current flows from the CT a driving torque prevented from rotating by the pre-tensioned spring. If the value the disc begins to move and is driven, against the drag contact on the spindle touches a fixed contact. The greater 12

is applied to the disc, but it is current exceeds a certain preset of the brake, right round until a the excess of current above this

Time Multiplier Adjustment

I

OCIT Relay

1:

6-

-.

Trip Circuit

-e

-id

Hieh Set El&&t (as Fig 3.1) be added

(a) SINGLE PHASE .

.

.

III

OCIT Relay

CU~~~~t Setting

Ii

Setting Sockets

Trip Circuit

Minimum Operating Current (b)

(c) CHARACTERISTIC

3-PHASE

TYPICAL

INVERSE

FIGIJRE 3.2 TIME OVERCURRENT

CURVE

RELAY

value, the greater the drive torque and the faster the disc tries to rotate. But the drag of the eddy-current brake also increases with the speed of rotation, and itsslowingeffect isgreatest at the highest currents. The combined effect of this is to produce a time/current characteristic as shown in Figure 3.2(c). For currents less than a certain .min,imum the disc does not move at all. For currents in excess of the ‘just move’ minimum the disc moves, and the operating time becomes shorter with increasing current - that is, it has an ‘inverse-time’ characteristic. This is very important when dealing with ‘discrimination’ (Chapter 4). Two setting adjustments are possible with this relay: current and time. Current adjustments are made by fixed taps on the driving coil. They are +sually set by moving a peg between a number of holes on the front of~the relay face. Typically the range is from 50% to 200% of the normal operating current (1 A or 5A depending on the CT used). The time adjustment is made by moving the ‘fixed’ contact so as to increase or decrease the travel of the disc before the contacts touch. The relay is fitted either with a time-scale marked in seconds, or more usually with a ‘time multiplier’ adjustment, which is used in conjunction with curvessupplied with the relay. 13

Relays could be custom-made to operate with any given CT and any given circuit data, but in practice relays are manufactured t.o certain standard conditions, and adjustments are provided to match this standard relay into a wide variety of circuit arrangements. This results in a fairly complicated setting procedure which is described in detail below.

Movable

Plug

I

I OUT Relay Operating Coil

FIGURE 3.3 OClT RELAY CONNECTION 3.2.3

Setting of an Inverse Time Overcurrent

Relay

To understand the setting of the current and timing adjustments and the interaction between them, consider the particular circuit shown in Figure 3.3. A standard inverse-timeovercurrent relay designed to work on a nominal secondary current of IA is fed from a 400/1A current transformer. Suppose the normal full-load line current is SOOA, then at full load the CT secondary current will be 1.25A - higher than the relay’s designed working current. The relay must. therefore be slightly desensitised. The operating coil has several taps, and a tap which reduces the effective turns by 20% is chosen by inserting the plug in the 125% socket. Thus 1.25A (the actual or ‘effective’ current) through 20% less turns has the same effect (ampere-turns) as the designed IA through the full turns; that is to say, with the 1.25A coming in, the relay will operate as designed for a IA input, and it will have the same designed characteristic time/current curve. Thus the current plug setting compensates for any deviation between the CT rated primary and the actual full-load current. If there is no deviation the plug is set at 100%. For cases where the CT primary current rating is greater than the full-load current, the relay must be made more sensitive, and the tappings are extended below the nominal 1A (100%) so as to increase the effective turns (an ‘auto-transformer’ effect). Hence there are 75% and 50% plug positions. It should always be remembered that settings below 100% make the relay more sensitive, and settings above 100% make it less sensitive. The plug settings 50% to 200% are seven discrete sockets, and no intermediate position is possible. If a calculated setting (e.g. 1 loo/)D comes between two positions, the next higher setting should be used. Some relays, instead of having plugs marked in current percentages as already described, are marked in CT secondary amperes --for example 0.5, 0.75, 1.0, 1.25, 1.5, 1.75 and 2.OA instead of 50%, 75%, lOO%, 125%, 150%, 175% and 200% - but the purpose is the same. Other relays are designed for use with 5A current transformer secondaries. If the plugs are marked in current amperes, the markings would be 2.5, 3.75, 5.0, 6.25, 7.5, 8.75 and 1 O.OA. If the calculated line fault current is, say, 5 OOOA - that is, IO times the normal full-load cur-rent in the case of Figure 3.3 - the CT secondary will then give 12.5A. Note that this is 10 times the plug setting, not 10 times the nominal 1 A (= 100% setting). Consequently the horizontal axis of the characteristic (set Figure 3.4) is scaled in ‘Current (Multiples of Plug Setting 50% - 200%)‘, not simply in multiples of full-load current. 14

The purp~ose of this exercise is to determine what current and timing settings should be put on the relay to achieve any desired time delay in its operation when subjected to a given short-circuit current. (The desired time delay will come out of the discrimination caiculations which~determine the delays required at various points of the network -see Chapter4.) It therefore only remains, having determined the current operating plug setting for the calculated short-circuit current at the point where the relay is installed, to choose the time multiplier which will give the desired time delay.

aperating Time (Secondsl

(= Time Multiplier Settings)

1 0.8

0.6

_

1 Current$lultiples40f Plut S&:50%

RELAY

Figure 3.4 is a set of time/current relay. Both scales are logarithmic.

- 2:%)

Relay

Operating

Line

30

FIGURE 3.4 SETTING CURVES

characteristic

curves as provided

with a typical

OCIJ

For the circuit of Figure 3.3 it has already been determined that a plug setting of 125% is required, and that a short-circuit current of 5 OOOA will mean a current 10 times that of the plug setting. If in Figure 3.4 a vertical line is drawn through the current multiplier point ‘IO’ on the horizontal axis, this is the ‘relay operating line’ for that short-circuit current. Suppose the d~iscrimination calculation requires a time delay of 1.35 seconds at this point in the network. Draw a horizontal line through the point 1.35s on the ‘Operating Time’ (that is vertical) axis. Let it cut the vertical relay operating line at point P. It will be seen from Figure 3.4 that P lies b~etween the Time Multiplier curves 0.4 and 0.5. By interpolation it would be 0.44. Although such a setting would be possible, it is usual to 15

choose the next higher, namely 0.5 (point Q). This setting will actually give a time delay of 1.5 seconds at 5 OOOA - very slightly higher than the desired 1.35, which errs on the safe side. Similarly, if a calculated current plug-setting came between two sockets, the next higher plug-setting should be used. This too errs on the safe side in making the relay marginally less sensitive, needing slightly more time to operate for a given fault current.

It is required to determine the current and timing settings on an OCIT relay to give a 1.35s delay with a short-circuit current of 5 OOOA. Full-load current is 450A and the CT-ratio is 400/1A. (Note that in this example the full-load current is slightly different from before in order to show how an ‘in-between’ figure should be interpolated.) The setting sequence is shown in Figure 3.5 and uses the curves of Figure 3.4. Full load current of circuit Rated primary current of

LI

I

i EFFECTlVE LuRRENT 0, -9..__ --&:-^

CT /Y”

(as a fraction)

I

(If intermediate.

I

L.lse next higherj

FIND RELAY OPERATING LINE ‘SC

Effqctive

DRAW HORIZONTAL LINE THROUGH THE REQUIRED TIME DELAY TO CUT VERTICAL RELAY

OPERATING

Current

LINE,

POINT

Short circuit C”“e”t (i.e. calculated short circuit current at point of fault)

=

-

‘P’

Example Plugsetting=$$x 400/l ,~

,,,=45OA

~

l&.jOOOA

100=112.5%(use125%)

Effective

current

= 400 x 1.25 = 500A

Relay operating

I----

q

Desired

R

Desired

Time Delay 1.35s

Time multiplier

Reading

FIGURE 3.5 TO FIND THE SETTINGS

16

line = E

= lb

time delay = 1.35s

setting at crossover (‘P’) = 0.44 bf interpolation. Use next higher 0.5 (‘Q’)

across, actual time delay achieved = 1.5s

It should be noted that the plug setting in this case comes out at 1 .125, or 112.5%. no such plug position, so the next higher one, 125%, is chosen.

Th,ere is

From here, the plug having been inserted in the 125% position, carry on as before. The 5 OOOA short-circuit current represents 10 times the chosen plug setting. Draw a vertical line through ‘10’ on the Current Multiplier axis of Figure 3.4 and let itcut the horizontal through the desired delay time of 1.35 seconds. The crossing point Pfalls between theTime Multiplier curves 0.4 and 0.5, so the larger is chosen. This will give an actual time delay (horizontal through Q) of 1.5 seconds, slightly longer than the 1.35 desired.

3.2.4

Combined

inverse

Time

Overcurrent

and High Set Instantaneous

Relay

(OCIT/OC)

An inverse-time relay may be equipped with an additional instantaneous element in the same casing but operating at a ‘high-set’ current value. This gives it the feature ofa combined ‘inverse-time and high-set instantaneous’ relay, the instantaneous feature overriding the time delay only on the most severe faults. An eiample of this additional feature is shown dotted in the drawing of Figure 3.2. This arrangement is particularly desirable where overcurrent protection is installed,near the generator end of a network. It is at this end that discrimination requires the longest delays, and a purely inverse-time relay would allow a severe short-circuit to persist until eventually cleared. .An overriding high-set instantaneous overcurrent relay, fed through the same CTs and actuating the same trip circuit, would clear such currents very quickly. It would however operate only on severe faults and would take no part in fault currents below its own high setting. This feature is further investigated in para. 3.2.6, ‘Busbar Protection’. Usually high-set

3.2.5

a 3-element OCIT relay (one per phase) would be combined with elements in two phases only’all in a single case (30CIT/ZOC).

Inverse

and Definite

Minimum

Time

two

instantaneous

(OCIDMT)

An inverse and definite minimum time overcurrent relay is shown pictorially in Figure 3.6. The current transformer arrangements with single-phase or 3-phase systems are similar to those for the simple overcurrent relay and are shown in Figures 3.6(a) and (b). This relay is simply a variation of the inverse-time type shown in Figure 3.2, but here the characteristic, instead of tending towards zero time for the highest fault currents, now tends towards a definite and finite small value, as in Figure 3.6(c). This is built into the relay and cannot be adjusted. The relay is similar

in construction

to the normal

inverse-time

type shown

in Figure

3.2.

The purpose of this variation is to render the relay settings more accurate. All characteristic curves are subject to tolerance, and the separation of the sloping curves of Figure 3.2 at the high-current end for different relays has to be enough to allow for such tolerances. Therefore tripping delays would need to be longer than would be necessary with more accurate curves. The definite minimum time feature at the highest currents, making the curves horizontal at those currents, enables greater accuracy (that is, smaller tolerance) to be achieved, resulting in less separation of the curves and consequently shorter tripping times. An OCIDMT relay may be combined with instantaneous high-set overcurrent elements described for an OCIT relay in para. 3.24. It is shown in dotted outline in Figure 3.6.

as

17

Time Multiplier Adjustment

OCI DMT Relay

Trip Circuit Eldment (as Fig may be added (a) SINGLE PHASE

III

OCIDMT

Sett,ing Plug

Relay

Definite IIlinimum +-‘---Time

SoZkets

:---.. WI

I

* Minimum 4

Operating Current (b)

TYPICAL

3.2.6

Busbar

3.PHASE

INVERSE

ANG

(c)

DEFINITE

FIGURE 3.6 MINIMUM

CHARACTERISTIC

TIME

OVERCURRENT

CURVE

RELAY

Protection

A fault on a high-voltage busbar generator overcurrent relays - the busbars - to operate and clear it. on the discrimination ‘ladder: will heavy fault current; it may well be

will produce severe overcurrents which should cause only protection that the generator has upstream of However the generator inverse-time relays, being the have a comparatively long operating time, despite of the order of 2 to 3 seconds in some networks.

the the last the

During this time the short-circuit current will have passed from its initial large subtransient value, thl-ough the transient value and probably well towards its final synchronous value which, as already shown, could well be even less than normal full-load. In this state there is no overcurrent at all, and the generator protection relays would probably not operate; there would then be no protection against a busbar fault which, by its very nature, could be highly damaging. There are several ways in which this situation can be dealt with, and these are descl-ibed below. (4

Instantaneous

Instantaneous protection is b y the addition of simple high-set instantaneous overcurrent elements to the OCIT relay. With heavy faults these elements would cause an immediate trip before the fault current had started to run down from its subtransient level and before the 18

inverse-time element had worked off its long d&y. but not in the context of a busbar fault. (b)

Voltage

This was briefly

mentioned

in para. 3.2.4

Restrained

This type of protection the generator. Operating T:-IcpC”“rfC\

is by using

‘voltage-restrained’

inverse-time

overcurrent

relays

for

,O 8 6 4 3

1 0.8 0.6 0.4 0.3 0.2

0.1 1

30 Current&ult?ples4af

VOLTAGE

RESTRAINED

PluzSet8tinTSO%

FIGURE INVERSE

3.7 TIME

- 22d%)

OVERCURRENT

RELAY

The operation of this type of relay is explained in the manual ‘Electrical Central Devices’, Chapter 2. It is a normal overcurrent inverse-time relay with an additional voltage-sensitive element which, under normal voltage conditions, produces a counter-torque and so restrains the drive of the overcurrent element. When voltage falls this restraint is lifted, allowing the overcurrent element to operate more rapidly - that is, it becomes more sensitive as voltage falls. The black curve in Figure 3.7 shows the normal (restrained) time/current characteristic similar to Figure 3.4 with time multiplier 0.3, and the red curve shows the change of characteristic (with the same time multiplier) as the voltage falls and the restraint is completely removed. For example, an overcurrent of three times the plug setting will cause the relay to operate in 1.8 seconds with normal voltage (restrained), but it will operate in 0.5 seconds under low voltage with the restraint removed. This allows the generator breaker to clear a busbar fault in a far shorter time than it would with normal OCIT protection (if indeed it would at all). The change of characteristic as the voltage falls is a gradual process, and the black and red curves are the two limits - that is with full restraint and with no restraint. For partial fall of voltage there would be a whole family of curves in between. 19

There is another type of voltage-restrained overcurrent relay where the change of characteristic is not gradual but is abrupt. This type of relay is not strictly restrained, btit its sensitivity is increased by a discrete step when the voltage falls to a specific percentage, either 60% or 30% of the nominal value. This is achieved not by use of a voltage element producing a counter-torque as with the first type but by altering the main driving torque produced by the current element. The sensitivity change is initiated by a voltage-sensitive changeover relay which alters the pole-shading on the current element. Figure 3.7 can be regal-ded as illustrating this type also if the black curve is the normal characteristic and the red cut-ve the more sensitive characteristic when the voltage has fallen and the changeover relay has operated. Its effect on speeding the operation of the relay under short-circuit conditions is just the same as described above for the truly voltagerestrained type of relay.

(c)

Flame

A different protection,

Leakage

way of dealing with a busbar fault which is indicated ip. Figure 3.8.

FRAME

It is assumed that a The frame is lightly current transformer-. both from the deck CT.

FIGURE LEAKAGE

is by the method

known

as ‘Frame

Leakage’

3.8 PROTECTION

busbar arcing fault will rapidly go to earth on the switchboard frame. insulated from the deck and is connected to the true earth through a If the switchboard consists of several sectionsasshown,each is insulated 2nd from its neighbours. Each section is separately earthed through a

If there is a busbar fault involving the frame, the frame becomes live and current flows earth through whichever CT is invqlved. This frame leakage current is used to provide instantaneous trip on all generators and section breakers feeding into that section.

to an

Care must this would

as

(4

be taken not to allow any conducting shunt the CT connection. Zone

material

to lean against theswitchboard,

Protection

This method provides each incomer, each feeder and each section breaker on theswitchboard with a set of current transformers and applies differential protection (see Chapter 6) to the entire group. Under heaithy conditions all the currents entering the switchboard from the incomers and section breakers are exactly balanced by those leaving it through the feeders. Any busbar fault upsets this balance and causes an instantaneous trip.

20

This method is known set up. It is only likely

as ‘Zone Protection’. It is by its nature to be found in major shore netw~orks.

In most normal installations busbar not provided. This is the case with provided at certain onshore plants.

3.2.7

Relays

faults Shell

very

costly

and difficult

are considered so rare that special offshore installations, but busbar

protection protection

to is is

- General

Most protective relays are fitted with flags which indicate when they have operated. They show the operator, for example, which of the protective systems may have caused a turbogenerator to have tripped out. Such relays are themselves narmally self-resetting - that is, ,they revert to their normal state as soon as the fault has been removed. This may occur either because the circuit-breaker has tripped, so disconnecting the fault, or because the fault itself has disappeared. The flag however remains showing until it has been reset by hand. Ian some protective systems, particularly for generators and transformers, all the protective relays trip the breaker through an intervening hand-reset trip, or ‘lock-out’, relay (TH). It too has a flag, but this relay, having once operated, does not reset itself automatically and so prevents the breaker being reclosed until the relay has been deliberately reset by hand. This prevents accidental reclosure onto a fault, and the breaker remains locked out until cleared by the operatorresetting the lock-out relay. Whenever an item of plant has tripped because one of the protective systems has operated, it is most important that the operator should not reset the relay flags until he has carefully noted down which. flags are showing. If this is not done, all evidence of the cause of the malfunction will be lost. The lock-out relay must on no account be reset until it is safe to operate the plant again.,

3.2.8

Electronic

Relays

Those relays which have so far been described are of the ‘electromagnetic’ type, where electromagnet provides the driving force to a mechanical system of moving armature rotating disc and mechanical contacts.

an or

Many of these relays are now being superseded on offshore, and numerous onshore, installations by electronic types which are entirely static except for their final output contacts. Electronic circuits carry out the detection, processing and timing; only the output circuit is passed through normal electromagnetic auxiliary contacts to the external trip circuits. This also isolates the trip circuits proper from the electronics. Though using different methods, electronic relays reproduce similar characteristics td’those of the electromechanical types, and they have similar adjustments such as for current and. time setting. Their use does not affect the principles of protection described in Chapter 1. An electronic counterpart exists for almost every relay described in this and succeeding chapters. To illustrate the principle of operation, a single-phase, electronic inverse-time and instantaneous overcurrent relay (OClTjOC) is described here and shown in Figure 3.9. The input from the line current transformer is fed through a small adapting transformer to a low-pass filter Rl-Cl which suppresses transient voltage surgesA voltage proportional to the input current is developed across the current-setting potentiometer R2. This voltage is applied to the bridge rectifier.

21

Time

(a)- CIRCUIT

input Filter & Transducer

Current setting

--ve

DIAGRAM

M

I

I nstantaneous Trip

(b) CORRESPONDING

FIGURE ELECTRONIC

I

RLA’

BLOCK DIAGRAM

3.9

OVERCURRENT

RELAY

The d.c. output voltage, which is proportional to the line current, is used to charge the capacitor C2 through the potentiometer R5. Tbe setting of this potentiometer determines the rate at which the voltage across C2 increases and hence the timing of the inverse-time operating characteristic of the relay. When the’voltage across C2 reaches a predetermined value, the detector circuit operates to switch the electromechanical relay RLA through the output amplifier and power transistor T2. instantaneous operation is obtained by applying the output voltage of the bridge directly to the input of the amplifier through R4. Thus; for higher values of fault the inverse-time delay circuit is bypassed.

rectifier current,

The power supply for the solid-state circuits is applied through D3 and R6. It is stabilised by zener diode DZl, and spike protection is affor-ded by R7 and C3. The diode D3 protects against reversed polarity of the d.c. power supply. By suitable choice of elements the electronic relay current/time characteristic can be made to reproduce exactly that of the equivalent electromagnetic type. Having virtually no moving parts, they are, in general, more robust, smaller and lighter. Current and time settings in this case are applied through simple variable resistors. 22

3.3

FUSES

3.3.1

The High Rupturing

Capacity

(HRC)

Fuse

A fuse consists essentially of a length of metallic wire or strip carrying the cil-cuit current which, if that current exceeds a certain stated value for a certain minimum time, will melt and break the path of the current in that circuit. It has both a normal current rating corresponding to its service current and a breaking current rating corresponding to the maximum fault current of that part of the system in which it will be used. Originally fuses consisted merely of a length of suitable wire stretched between the terminals of, a holder, the holder being designed to plug into permanent fixed sockets. These had the disadvantages of having much exposed live metal, and the melting open wire tended, under some conditions, to give rise to severe arcing and risk of fire. The wire also tended to corrode and weaken with the passage of time. The open-wire fuse is no .longer used, having been superseded by the cartridge type. That used on offshore and onshore installations consists of an outer ceramic tube in which there is a silver fusible element completely surrounded by quartz powder, as.shown on the left of Figure 3110. Quartz I

Silver Wire Element \

Powder

Filling

C‘eramic Body

Before Fusing

PRINCIPLE

FIGURE 3.10 OFF THE HIGH RUPTURING

After

Fusing

CAPACITY

FUSE

If sufficient current flows through the silver element for sufficient time, the element melts and vaporise:; it reacts chemically with the quartz, under the heat of the arc, to produce a block of highly insulating material in the path of the arc, as shown on the right of Figure 3.10. This rapidly suppresses the arc and, unless the current is much in excess of the fuse rating, it will break the fault current within a matter of milliseconds. Such fuses can break very large prospective fault currents by simply preventing those currents from ever building up. The fuses are consequently known as ‘High Rupturing Capacity’ (HRC) type.

23

(a) OPEN WIRE FUSE

Load Current Befpre Fault

jr\ ;

\d \

Prospective Fault Current

Tqtal

Clearing

Time

(b) HRC FUSE

FAULT

CURRENT

FIGURE 3.11 IN OPEN WIRE

AND

HRC FUSES

Figure 3.11 (a) shows a fault current passing through an open-wire fuse. The current continue for several cycles of arcing before it is eventually broken at a current zero.

may

When a moderate fault current passes through an HRC fuse the melting time will be comparatively slow, and the current may continue for several cycles before it is broken - in fact the HRC fuse will behave just like the open-wire type in Figure 3.11 (a). If however the fault current is very large, the melting time will be less than one-quarter of a cycle. The ensuing arcing time is so short that the current is broken even before it reaches its first peak, as shown in Figure 3.11 (b). Such a fuse is said to exhibit ‘cut-off’. If the current,wave is wholly asymmetrical the first peak is not reached until half a cycle has elapsed, and cut-off may occur if the melting time is somewhat longer - i.e. less than half a cycle. If it had not been for this cut-off, the fault current would have risen to its full-fault peak (called the ‘prospective current’ peak) before reaching its first zero. The fuse, by cutting off, has protected the whole system from the effects of this severe peak. It is therefore given

24

credit for having interrupted the full prospective current, even though in fact the current may never reach it because of cut-off. The fault rating of an HRC fuse is consequently very ’ , it is shown how such fuses are used to back h~igh for its size. In Chapter 4, ‘Discrimination up switchgear of lower fault rating capacity. There is often confusion between the ‘normal’ and ‘breaking’ cxrent ratings of a fuse. The normal rating is matched to the load and is the maximum value of current which the fuse can carry continuously without melting or deteriorating. The breakingratingisthe maximum prospective current which the fuse can safely interrupt at its rated voltage; it is usually quoted in kiloamperes (kA) rms symmetrical and is related to the system fault level. The energy needed~to melt Xfuse~ is the product of the rate of heat generatio+(in watts) due to the fault current.in the resistance of the element and of the total time during which such heat is being generated. It is /‘R x t, where I is the rms current, R the resistance of the element and t the total time. Since R is virtually fixed for.atiy given size of fuse, the energy released is proportional to I* t. A specific fuse element requires a given Izt to melt it. Therefore when (the melting time) will be very small, as indicated in Figure 3.11(b):/‘t to as the ‘let through’ energy.

I is very large, t is often referred

Pre-arcing Time (seconds)

Minihun ~Fusing CUrWlt 125A

Current

(amps)

FIGURE 3.12 HRC FUSE CHARACTERISTIC If the melting (or ‘pre-arcing’) time t is plotted against I (usually on log paper),~the curve of Figure 3.12 is produced. Most of this is the familiar inverse-time curve which many relays also have. There is of course a minimum current which will never melt the fuse however long it is applied, but above this lower limit the fusin, - time varies inversely as the current. The upper limit is set by the ability of a given make and size of fuse to absorbs the /2t energy and to handle the mechanical forces involved. 2s

As the fault current becomes higher, the melting (or pre-arcing) time becomes shorter until the point is reached where it is less than one-quarter of a cycle (0.004 seconds at 60Hz), and cut-off begins. From this point on the characteristic changes and becomes almost linear, as shown on the extreme right of Figure 3.12 (this is because ‘rms’ no longer has any meaning). With a fully asymmetrical current wave, cut-off may occur up to one-half of a cycle (0.008 seconds at 60Hz) after the onset of the fault. 3.3.2

Fusing

Element

Figure 3.10 shows a silver wire as the fusing element. This is normal with small fuses, but for larger ones a silver (or sometimes copper).strip is often used, as shown in Figure 3.13.

Copper~End Caps /

Ceramic Tube uartz Powder Filling Constricted

BASIC

FIGURE 3.13 CONSTRUCTION OF AN HRC

FUSE

Silver

LINK

The strip has a number of constrictions which form hot spots and assist rapid melting under short-circuit conditions. For the heaviest currents a number of such strips may be connected in parallel within the common housing, or many separate fuse-links may be permanently bonded in parallel to form a single multiple link.

3.3.3

Fuse Mountings

Within an equipment, especially high-voltage assemblies, fuses are often mounted without individual enclosure on pillar insulators or directly on busbars. Reliance for safety is placed on the metal enclosure of the HV compartment which houses them. Interlocks prevent the compartment being opened until the circuit has been made safe. On low-voltage distribution boards fuses are housed in a fuse assembly such as the typical one shown in Figure 3.14. The replaceable ceramic cartridge with its metal terminal caps is known as the ‘fuse-link’ and is held in an insulated ‘fuse-carrier’ which completely shrouds all live metal. The carrier is supported on an insulated ‘fuse base’, where it is firmly fixed by various mechanical means, amongst them tongue contacts, butt contacts held by insulated screw pressure, or wedge contacts pressed in by insulated screws. A tongue-contact type is shown in Figure 3.14. 26

1

COMPLETE

3.3.4

Fusing

Low

Fuse Carrier

FIGURE 3.14 VOLTAGE FUSE ASSEMBLY

(TYPICAL)

Factor

which it can carry continuously A fuse has a ‘normal current rating’, which is the current without melting or deteriorating and without altering its characteristic. The current which, under specified ambient temperature conditions, will just cause the fuse to melt after a prolonged time (usually taken to be four hours) is termed the ‘minimum fusing current’. The ratio minimum

fusing

current.

normal rated current

Fuses are manufactured classification accordingly

to different as follows:

Class Q2 Class Ql Class P

IS called the ‘fusing fusing

factors

Fusing Fusing Fusing

factor factor factor

factor’ for various

of that fuse. applications.

They

are given

greater than 2.0 between 1.5 and 2.0 between 1.2 and 1.5

In Figure 3.12 the time/current characteristic of the IOOA (normal rating) fuse is shown to become almost vertical after IO seconds, at which point the minimum fusing current is 125A. After four hours it will still be only 125A, as the curve is vertical. The fusing factor in this case is 125/100 or 1’.25, and its Class is therefore P.

27

Fuses which protect motor circuits often have a dual rating. Against steady overloads a fusing factor of about 1.2 (Class P) is usual, but, to allow starting currents to flow, a shorttime overcurrent rating of the order of 1.6 times normal is given (this is inherent in their time/current character&tics). Such a fuse with a normal 1 OOA rating would be termed ‘1 OOMI 60’. A marking of this type indicates that the fuse is specifically for motor protection. Obviously the surrounding temperature, as well as the manufacturing tolerances, will affect the precise current at which the fuse will melt; the higher the ambient temperature, the less the current (and therefore the heat) needed to melt the metal. Consequently the fusing factor, though easy enough to understand, is not a precise quantity and should therefore be used with care. To overcome this the International Standards, and even the latest British Standard No. 88, no longer refer to fusing factor but use instead two other quantities: the ‘maximum nonfusing current’ and the ‘minimum fusing current’, which represent the lower and upper limits of the grey area in between. The maximum non-fusing current is that current which, under any normal operating conditions, will never melt the fuse no matter how long it is sustained. The minimum fusing current is that current which, under any normal operating conditions, will be guaranteed to melt the fuse if sustained for the specified time (usually stated to be four hours or one hour). These two quantities are usually given, like the fusing factor, as a multiple (or percentage) of the normal rated current: e.g. a maximum non-fusing current of 1.2 times rated current and a minimum fusing current of 1.6 times. Such a dual rating would previously have been referred to as a fusing factor of 1.4, being the average of the two. In practice, ~with varying ambient temperature and with normal manufacturing tolerances, the current which will just fuse the element when-sustained for the specified time will fall somewhere between these two extremes. Thus a 1 OOA normal rated fuse may have a maximum non-fusing current of 120A and a minimum fcsing current of 160A. We know then that it will never fuse below 120A and that it is guaranteed to fuse above 160A if sustained for the specified time. In practice it may fuse anywhere between. Because of these uncertainties, effective in the low overcurrent from shoyt-circuit protection. against overload.

3.3.5

Service

particularly at low overcurrents, fuses are not particularly region and are not generaily suitable for ove~rload as distinct In such cases the contactor, if fitted, provides the protection

of Fuses

Fuses may be used~ in various ways services may be grouped as follows:

to

protect

General Application. The fuse protects of overcurrent, the degree of protection fuse selected.

equipment

downstream

of them.

These

all equipment and cable against the effects depending on the type and rating of the

Close Excess Current Protection. The fuses used have a low fusing factor the ratio of the minimum fusing current to the normal full-load current enables the cables to be used to their fullest rating.

- that is, - which

Motor ProtectionThe fuses have special time/current characteristics which enable them to pass the repeated large, and sometimes prolonged, motor starting currents without melting or deteriorating but will still protect the motor against steady overloading (with small motors) and stalling. Motor fuses generally have to run well below their rated full-load currents to allow for direct-on-line starting currents to flow without melting them. Consequently they do not protect the motor against

28

normal overloads, although they do protect the motor and supply system against short-circuits. Most motor fuses are specially designed to withstand repeated starting surges without fatigue, which would otherwise shorten their life. Fuses intended for application to motor supplies are given restricted continuous current ratings (‘M’ ratings) ascompared with theirfusing characteristics. Thusa ‘TIA32M63’ fuse has a continuous rating of 32A but a rating of 63A for the period of starting. Semiconductor Protection. Fuses are used for two purposes in semiconductor equipment. In a large equipment which contains groups of diodes or thyristors in parallel individual ceils are often fused to enable them to be disconnected from &he circuitiftheyfail tosho~-circuit,soallowingtheequipmenttocontinueoperating. A more demanding duty is the protection of semiconductor devices or assemblies against external faults. Semiconductor devices have very limited overcurrent capability, and it is generally necessary to use special fast-acting fuses carefully matched to the semiconductor ratings and the circuit characteristics. DC. Fuses. In a d.c. circuit there are not the periodic current zeros that occur in a.c. circuits, so the mode of fuse operation described above in relation to moderate overcurrents does not apply. For this reason fuse protection against low or moderate overcurrents in d.c. circuits demands very careful consideration to avoid a dangerous arcing situation within the fuse, particularly if the circuit is inductive. Back-up Fuses. Fuses used to back up contactors or other switchgear used in areas where the fault level exceeds the breaking capacity of the switch are discussed in Chapter 4, ‘Discrimination’.

3.3.6

Specification

In order

to specify

3.3.7

of Fuses a fuse its properties

must

be given in four

-

Application

-

Electrical characteristics (e.g. voltage, phase or 3-phase system).

-

Operating protection,

-

Physical design (e.g. type of contacts, protection, non-interchangeability).

Trigger

classes. These are:

(e.g. industrial,-domestic)

characteristics semiconductor

normal

current,

(e.g. general purpose, protection, time/current repiacement

breaking

current,

close-protection, characteristic). of links,

singlemotor

degree of physical

Fuses

Certain fuses are fitted with a device which releases a trigger when the fuse blows. This may be actuated by a spring which is held in tension until the element melts, or it may be operated by a small explosive charge. The trigger, when released, may be used merely to indicate the blown fuse or else to trip a circuit-breaker mechanically by a trip-bar, or it may close a contact which trips it electrically. Trigger fuses with tripping facilities are a protection against the effects of single-phasing. If any one of the three fuses protectjng a motor blows, its trigger makes (or breaks) a contact which trips the contactor and opens all three phases. Trigger-operated contacts are also used to gjve a ‘Fuse Blown alarm. There are special symbols diagrams to indicate trigger

(BS 3939, fuses.

Nos.

3.10.5

and 3.10.6)

which

should

be used

on

29

CHAPTER

4

DISCRIMINATION

4.1

THE NEED

In Chapter 1, ‘Principles of Protection’, it was stated that one of the main purposes of protection was to remove a faulty equipment or circuit from the electrical system so that as much as possible of the system could continue to function normally. It is therefore desirable that any particular fault should be cleared by that protection device which will perform the service with the least effect on healthy parts, and not by some device further upstream which would disconnect an unnecessarily large section of the system. For example, if a fault occurs on one of a number of circuhs fed from one transformer, it is better to isolate that particular circuit by its own circuit-breaker or fuse than that the transformer should be disconnected from the supply by its primary overcurrent protection or thegenerator tripped. This preferential or selective operation of protection devices is known as ‘discrimination’.

4.2

THE APPLICATION

Almost all switchgear is fitted with overcurrent protection of some sort. If a fault develops low down in the system, fault current will flow right through the network from the supply generator, through every intervening switch, down to the fault point itself. All these over-

Load II

Load IV

SIMPLE 30

FIGURE 4.1 DISCRIMINATION

currents will be detected by the relays of each individual to prevent it, all might trip together, so shutting down have been a purely local fault.

switch, and, if no steps were taken the whole system for what might

The overall protection system is therefore deveioped so that the breaker (or fuse) nearest the fault operates first, thereby isolating only the fault itself. If this does not clear it, the breaker nearest upstream of the fault operates next, thereby isolating only the minimum number of consumers. If this one does not clear, the next upstream breaker operates, and this continues until the generator breaker trips, but only as a last resort. Each Sreaker backs up the one below it. lt.has already been shown that most protective devices, such as overcurrent relays and fuses, have an inverse-time characteristic as shown in the middle column of Figure 4.1. This causes the tripping time to vary inversely as the magnitude of thee fault current. It has also been shown that in relays the characteristic curve can be altered by adjustment of the relay current and time settings. For fuses the characteristic cannot be altered, but a different characteristic can be obtained by choosing a different fuse. In Figure

4.1 it has been assumed

that relay-settings

-

for the generator

-

for the HV feeder

circuit-breaker

-

for the LV feeder

overcurrent

circuit-breaker

have been chosen

(breaker (breaker

and applied:

C), B),

device (breaker

or fuse A),

as shown in the characteristic curves of the middle column. For the purposes of direct comparison the three curves have been drawn to the same scales of time and current referred to a common base voltage. All three curves are superimposed on the right. If ,the settings have been properly chosen, the curves should appear as shown, each clear of the other at ail points. Since these curves are subject to tolerance (a relay accuracy of +7% is usual, and there will be other errors), the curves should all be well clear of each other. If a fault of current value F (adjusted to a common base voltage) appears at point P on the network, the fault current flows through all the breakers A, B and C. Characteristics of A, B and C show that this current would trip (or blow the fuse) A in time Tr , B in time Tz and C in time T3. Provided that A does trip or blow in time. T,, the fault will be removed and B and C will not trip at all and all the other consumers on both boards will remain in service. Should A fail to trip or blow, or if the fault were at point Q higher in the network, the first would breaker to trip. would be B in time Tz, but C would remain closed. More consumers be lost, but the generator would remain on-line feeding all others. Only if both A and B failed to clear would C trip and take the generator itself off-line. It should be noted that the time delay increases as the tripping point moves nearer the supply source (in this cdse the generator). For this reason generators and their HV switchgear have to have a 3.sec.ond through-fault rating under British and European rules, calling in general for heavier copperwork, whereas distribution switchgearnormally hason!ya l-second through-fault rating. (The 3-second rating does not apply in the US.) Restricted earth-fault and differential protection, it should be noted, whichare instantaneous and cover only faults within the protected zone, do not form part of a discriminating protective system. They may however be used together with one. If, for example, a fault occurred within a transformer, the differential protection would deal with it instantly without waiting for the time-delayed transformer HV breaker to trip.

31

4.3

DISCRIMINATION

BETWEEN

FUSES,

MCCBs

AND

MCBs

To achieve adequate discrimination between two fuses of similar type, it is usual to give the major fuse about three times the normal current ratingof the minorfuse. Between a mouldedcase circuit-breaker and a minor fuse the ratio can be reduced to about two. Because Moulded Case Circuit-breakers (MCCBS) and Miniature Circuit-breakers (MCBS) have instantaneous trips in addition to their normal thermal trips, they will not discriminate with each other at the higher currents. For this reason it is bad practice to install two MCCBs or MCBs in series, even though they may have different trip units.

4.4

BACKUP

FUSES

Contactors, MCCBs and MCBs are all described in the manual ‘Electrical Distribution Equipment, Part A’. Though they can all close onto a fault and carry it for a very short time their breaking capacities are strictly limited and are far below those of conventional circu’it-breakers. When used to control equipment in networks, their breaking capacities are tz.ually much lower than the fault levels of the syunearthed systems a line which is faulted to earth can be detected proprietary devices; an example is the ‘Vigilohm’ which continuously monitors earth resistances.

5.4.1

Earth Leakage Circuit

by various the line-to-

Breakers

There are certain types of equipment where an!internal fault which produces even a small earth-leakage current may present a danger to the operator - for example with portable tools, welding equipment, etc. In other types it may prevent the equipment from functioning correctly, as with some trace heating systems. Such a small earth leakage would certainly not be sufficient to blow a fuse or operate a normal overcurrent control device, and in these cases the circuit is often provided with an ‘Earth Leakage Circuit-breaker’ (ELCB). Such miniature circuit-breakers can be arranged to trip with an earth-leakage current as low as 30mA. The principle is shown in Figure 5.3. The ‘go’ and ‘return’ wires both pass through a magnetic ring. As long as there is no leakage of current to eal-th both wires carry equal and opposite currents, and there is no net magnctisation of the ring. However, if a small part of the current in one wire leaks away to earth, the currcntj in the two wires are unbalanced, and the ring becomes magnetised and acts as a ring-type current transformer. A toroidal secondary winding on the ring develops a current which causes the circuit-breaker to release. 36

From Supply N L

Magnetic

To Circuit

TYPICAL

EARTH

FIGURE 5.3 LEAKAGE CIRCUIT

BREAKER

(red TYPE)

This arrangement is self-powered and requires no separate trip supply. A difference in the line currents of as little as 3OrilA is enough to cause an ‘instantaneous’ trip within about 30 milliseconds. After tripping t’le ELCB must be reset by hand. The type of breaker shown in Figure 5.3 and described above is known as a ‘residual current device’ (red), or ‘residual current circuit-breaker’ (rccb). The device works on the imbalance of current in the wires which thread its core and functions independently of voltage. It can be tested by a Fault Test pushbutton. This bypasses part of the outward (but not the return) current thiough a resistance, ~thereby unbalancing the two wire currents inside the ring and causing the ELCB to trip. The pushbutton latches in and must be released by a second mechanical button. A different type of ELCB depends on voltage, not current. The voltage developing on the earthed casing of a faulty piece 05 equipment is sensed and operates a shunt trip on the breaker if it exceeds about 4OV, which is a voltage considered not to endanger life. The ELCB behaves much as a very sensitive overvoltage relay and acts almost instantaneously. This type is suitable where the earth resistance is too high to permit enough earth-leakage current to flow, but elsewhere it is now being superseded by the residual current type. ELCBs are being increasingly fitted in domestic and industrial installations. Indeed it is now required in certain sittiations by the IEE Wiring Regulations (15th Edition), particularly where sockets are used to feed portable apparatus. Their prime purpose is to protect personnel against serious electric shock due to faulty apparatus. If a person in good contact with earth accidentally touches a live conductor or the case of a piece of apparatus made live by an internal fault, an earth-leakage current wil,l flow through him. A current of 30mA, if sustained for even a short time, could prove fatal. An ELCB which operates at 30mA or lessand breaks the supply within about 30ms, while not preventing a shock, will not allow sufficient energy to pass to prove fatal or even dangerous to most people. 37

5.4.2

Earth

Leakage

Protection

in D.C. Systems

Most d.c. battery-supported supplies (other earthed directly or through a low resistance, single earth fault does not give rise to any currents, and the methods of earth-fault and not suitable for d.c.

than communications power supplies) are not as in the case of ax. systems. Consequently a considerable unba!ance between go and return earth-leakage detection used in ax. systems are

R

R Earth Leakage R&y T

$a) NO LEAKAGE

(b) WITH EARTH LEAKAGE PATH

EARTH

LEAKAGE

FIGURE 5.4 DETECTION IN A D.C. SYSTEM

The usual meth~od of earth-leakage detection in d.c. systems is based on a potential divider consisting of a pair of equal highavalue resistors connected in series across the supply, as shown in Figure 5.4. As long as there is no leakage to earth (Figure 5.4(a)) the supply voltage is equally divided between the two resistors - that is, V, =~ V, = V/2. A voltageoperated relay is placed between the mid-point and earth. Since normally no current flows to earth (as there is no leakage), there is no potential difference across the relay, and it is not affected. Both sides are at earth potential. Suppose now a leakage to earth developed in the negative line (Figure 5.4(b)). ?he leakage resistance is R, and the consequent leakage current I,. This leakage current will flow, as indicated in red, from the positive line, through the left-hand resistor R, through the midpoint connection, through the relay to earth and back through the leakage resistance R, to the negative line. This leakage loop current can be considered as superimposed upon the steady potentiometer current, which continues to be V/2R.

38

.~.

The total

current

in the left-hand

resistor

limb is therefore ‘V[2R + iE

It flows

through

the resistor

R, giving a volt drop (V/2R

across

the resistor

of

+ /,)R

= v/2 t- I, .R. This is therefore the new potential of the mid-point, This voltage appears across the relay and, if sufficient,

which was previously causes it to operate.

at earth potential.

A similar situation would exist if the leakage had occurred on the positive line, except that the currents~ would be in the opposite direction. If the relay were of the ‘2-way’ type it could be made to distinguish between a nositive or a negative line leakage.

5.5

RESTRICTED

EARTH

FAULT

(REF)

PROTECTION

Restricted earth-fault protection is similar to that for a normal earth fault already described, but it is arranged to provide protection against earth faults occurring within a limited zone only. It can only be used on earthed systems. There are two variants: one where the neutral is earthed.at the switchboard (in a 4-wire system), and the other where it is locally earthed at the source of supply (at the transformer secondary star-point in the case of an LV system). Roth systems are found in Shell installations.

5.5.1

Switchboard

Earthing

Figure 5.5 shows the connections as applied to the secondary of a star-connected transformer which is earthed through the LV switchboard neutral busbar. Three CTs are arranged as for normal earth-fault protection, and a fourth CT is provided in the neutral line between the star-point and the switchboard earth and is paralleled with the other three. The earth relay ins, as before, in parallel with the CTs and is operated by the resultant spill current from all four. Under normal 4-wire conditions load currents, which may be unbalanced, flow in all four CTs, but no resultant spill current flows in the earth relay. Even under phase-to-phase or phase-to-neutral fault conditions the overall balance of the four CTs is not upset,~ and the earth relay is not affected. A ‘through-earth fault’ - that is, a fault downstream of the CTs- will produce the conditions of Figure 5.5(a). The path of the primary fault current is shown in‘red; the earth path between the fault point and the switchboard neutrai earth is shown dotted. The fault current path passes in opposite directions through one line CT and the neutral CT. The only CT secondaries affected are those shown in green, and~it will be seen that they are equal and opposite. There is therefore no spill current due to the fault, and the earth relay will ignore it. If however the earth fault occurs upstream of the CTs as shown in Figure 5.5(b), the primary fault current (red) passes only through the neutral CT and through none of the others. Its secondary current therefore can only pass through the earth relay (green), which is thus actuated and operates the trip contacts as shown.

39

No Spill

/current

b rcuit

(a) THROUGH

RESTRICTED

EARTH FAULT

EARTH

FAULT

(b) EARTH FAULT IN PROTECTED ZONE

FIGURE RELAY

5.5 WITH

SWITCHBOARD

EARTHING

This system is therefore immune to earth faults downstream of the CTs (‘through faults’) and also to line-to-line faults. It will respond only to earth faults upstream of the CTs: that means to faults only in the transformer secondary windings, cables and cable boxes or switchgear up to the CTs, but not beyond. This is called the ‘protected zone’ - hence the name ‘Restricted Earth Fault’. It serves partly the same purposeas true differential protection as described in Chapter 6 but should not be confused with it.

5.5.2

Local

Earthing

.

With this system there is a fifth CT in the earth connection, in parallel with the other four. Figure 5.6(a) shows the primary fault current flow (red) with a through-earth fault, and the two affected CTs secondaries are in green. Their currents cancel out, and there is no spill current; the earth relay is not affected. Figure 5.6(b) shows an earth fault within the protected zone - that is, upstream Here only one CT, that in the earth connection, is affected, resulting in spill operation of the earth relay. Here again the arrangement ignores through-earth faults and responds within the protected zone, but in this case five CTs are required. 40

only

of the CTs. current and

to earth

faults

n 4 Protected Zone

I I I I. A I I I I I I I I I

‘? Cable

I I I I

(a) THROUGH

EARTH

RESTRICTED

FAULT

EARTH

(b) EARTH.FAULT IN PROTECTED ZONE

FIGURE 5.6 FAULT RELAY WITH

LOCAL

EARTHING

As restricted earth-fault protection is insensitive to through faults and’does not take part in the discrimination pattern (see Chapter 4), an REF relay is given a very low current setting.

5.5.3

Protected Zone

It should be noted that REF protection is the only means of protecting the transformer secondary winding, cable box and cable. With unrestricted protection only faults downstream of the CTs, which are usually in the LV switchboard, will be detected; any within the transformer or between it and the CTs will not be sensed, and the whole of that zone wifi remain unprotected. Only restricted protection will cover it. Because REF protection is confined to the limited protected zone only and is not affected by protection systems outside it, it can be made instantaneous in operation and need not form a step in the discrimination ladder. It is worth remarking that an earth fault occurring near.the earthed star-point of the transformer secondary winding may, due to the low voltage there, not develo,p sufficient fault current to actuate the REF protection. Although a fault at such a point is unlikely, this is a small.loophole in REF protection.

CHAPTER DIFFERENTIAL

6.1

THE

6

PROTECTION

PRINCIPLE

Differential protection depends on a method of fault detection based on the principle that the total current flowing into one part of a system is equal to the total current flowing out of it unless there is some unintended alternative path for it in between. This is just another statement of Kirchoff’s Law. This type of protection ignoring those occurring some respects similar to REF guards only against covers also phase-to-phase faults within one phase at the two ends.

is used to gu~ard against faults arising only within the protected unit, outside it. The unit itself then becomes the ‘protected zone’. It is in restricted earth-fault protection but should not be cotifused with it. earth faults in the protected zone, whereas differential protection faults within the zone. It does not however deal with inter-turn - say in a generator - since that will not c&e differing currents

Differential protection is insensitive to through faults-that is, tofaultsoutside the protected zone - because the same fault current flows through both ends of the zone. It may therefore be used to provide relatively sensitive protection for the equipment inside the protected zone without its being affected by the discrimination scheme of the whole network. The advantage of this is particularly apparent in the case of generators and large bulk power transformers, which may demand rapid and sensitive protection against internal faults but which, because of their position at the high-level end of the power supply system, would be among the last items to be tripped in the went of a through-fault.

(a) CIRCULATING

-

CURRENT

emf

-

emf I

Relay (b) BALANCED VOLTAGE

FIGURE 6.1 DIFFERENTIAL PROTECTION

42

The term ‘differential protection’ (symbol DIP) is used generally~throughout the offshore installations, but elsewhere it may be known by the names ‘Merz Price’ (after the original ‘Circulating Current’ or ‘Balanced Voltage’ protection. All these terms inventors), ‘Unit’, may be met as well as such trade names as ‘Translay’ and ‘Solkor’ which introduce variations into the basic scheme. Differential described

6.1.1

protection in principle

Circulating

is basically below.

Current

of two

kinds,

as shown

in Figure

6.1. The two

kinds

are

(cc) Principle

Figure 6.1(a) shows identical current transformers connected at each end of any piece of electrical equipment - a generator, a motor, or even a‘length of cable - through which a current is flowing. A single-phase circuit has been used for simplicity. The CT secondaries are connected by ‘pilot cables’ in a loop as shown, and a voltage-sensitive relay is connected across the pilots at about their mid-points. L Current flowing through the electrical unit’causes a-secondary currentthrough both CTs to circulate round the pilot circuit without producing any current in the relay. A fault within the zone between the two CTs (the protected zone) will on the other hand cause secondary currents of differing values in the two CTs, and their difference current will flow through the relay. If this difference is sufficient, the relay will operate.

6.1.2

Balanced

Voltage

(bv) Principle

Figure 6.1 (b) shows another arrangement where the two current transformers are connected in opposition and the relay is in series. With the same primary current flowing through both, the secondary emfs oppose each other and no secondary current flows in the pilot circuit the voltages are balanced. In the event of an internal fault causing differing opposing secondary emfs will no longer be equal, circuit, causing the series relay to operate.

primary currents and current will

in the CTs, the two flow round the pilot

It should be noted that in the balanced voitage system no CT secondary current flows normally, and the CTs are effectively on open circuit, giving high voltages on the pilot lines. Moreover thiscondition would cause theoverburdened CTs tosaturateand become inaccurate. Special CTs are used having an air-gap or other non-magnetic gap to avoid saturation.

6.2

CIRCULATING

6.2.1

Voltage

CURRENT

SYSTEiM

Distribution

The simpiified explanation of circulating current protection as given in para. 6.1.1 needs some further attention in order to understand how it works in practice. In particular the distribution of voltages round the secondary loop will be described. If the potentials at all points round the secondary loop are plotted, beginning at 0 the potential is zero, the curve will be as shown red in Figure &2(a). From 0 to potential will rise due to the emf in the CT; from A to B it will steadily fall due resistance of the pilot leg AB; from B to C it will rise again within the CT; and from it will fall once more to zero due to the resistance of the leg CO.

where A the to the C to 0

43

Electrical Unit

CT Secondary

(a) NORMAL

OR THROUGH

FAULT

Electrical Unit

/.

CT Secondary

(b) INTERNAL

CIRCULATING

CURRENT

FAULT

FIGURE 6.2 PROTECTION

VOLTAGE

DISTRIBUTION

At a certain point P midway between the two CTs the potentials of the two secondary (red) will be equal because of symmetry. A voltmeter applied across them there would zero. If a relay were connected across the lines at that point it would be unaffected.

lines read

If now a fault or leakage developed somewhere inside the equipment, part (or all) of the ‘go’ current would be shunted into the return line, so thar the currents I, and I2 on either side of the equipment would be unequal. So, therefore, would be the CT secondary voltages, and the potential curves would be distorted as shown red in Figure 6.2(b), the voltage gradients on the faulty side being greater than on the other. They are no longer symmetrical, and the crossover voltage-balance point has moved from P to some other point Q. At P there is now, a voltage difference between the lines (PI--P,), and the relay (an attracted-armature instactaneous type) inserted at that point would be energised. If the relay setting were sufficient, it would operate to trip whichever circuit-breakers it was necessary to open. The relay setting range is typically 5 to 20% of normal full-load current. It has been shown that the relay must be connected at the point in the pilot lines where, under normal conditions, the voltages are equal. In practice such a point is not easy to find.

44

Electrical

CIRCULATING

CURRENT

FIGURE 6.3 PROTECTION

WITH

RESISTANCE

What is done is to insert resistances intb the pilot circuit so that most of the voltage drop in each lilie is concentrated in the resistors. The crossover point is then bound to besomewhere in the resistors themselves,, so they are provided with tappings, which can be adjusted until the balance point is found. By this means the crossover point, instead of being at some unknown place far from ?he switchboards, is brought as a ‘resistance box’ right into the switchboard where the relay itself is installed. The resistances add to the burden on the CTs, but this is acceptable. For satisfactory operation it is essential that the pairs of CTs be accurate and perfectly matched. Therefore they are usually of the special class of accurdcy (Class X -see manual ‘Electrical Distribution Equipment, Part B’, Chapter 4) and are supplied as matched pairs. Since differential protection operates only over a limited zone, it does not form a step m me discrimination ladder. It is therefore instantaneous in operation and the relay can be given a very low setting.

6.2.2

3-Phase Protection

Figures 6.1 to 6.3 show, for simplicity, a single-phase system, but the principle applied -and usually is-to 3-phase systems.

can be

Three carefully balanced pairs of CTs of high accuracy are inserted, one pair into each of the three phases, and voltage balance is measured between each secondary line and neutral by a 3-element relay. A resistance box containing three tapped resistors is used as described above. This is shown in Figure 6.4. The 3-phase system requires four pilot lines between the sets of CTs, with further lines from the relay contacts to trip the circuit-breaker. For long lines variations of the system such as ‘Translay’ and ‘Solkor’ operate over only two pilot lines and can initiate tripping simultaneously at both ends. It should be noted also that differential protection will operate 1 on both internal phase-to-phase and earth faults, and in this respect it issuperior to restricted earth-fa Ilt protection. 45

Electrical Unit

1 6

1

1

&*-?iP

Circuit

--Differential Relay

DIFFERENTIAL

6.2.3

Differential

FIGURE 6.4 (CIRCULATING CURRENT)

Protection

PROTECTION

(3-PHASE)

of a Transformer

.j

The differential protection so far described, whether circulating current or balanced voltage, depends on identical and matched current transformers at both ends of each phase. For most electrical units the incoming and outgoing currents are, or should be, the same. This applies to generators, motors and cables, but it is not true of transformers. Th,e outgoing current in any phase of a transformer differs, ideally, from the incoming in inverse proportion to the voltage ratio. For example a 2 OOOkVA, 6 600/44OV transformer (ratio 15:l) has a primary current of 175A but asecondary current of 2 62SA (ratio 1:15). Therefore, to achieve balance of the CT secondaries, the CT ratios must be inversely proportional to the main transformer voltage ratio, as shown i,n Figure 6.5(a). Most distribution transformers are delta/star connected, and this affects the line current in the individual phases by a factor ofJ3. If the main transformer is delta/star connected, then the three CT secondaries must be connected in the opposite sense, namely star/delta. This is shown in Figure 6.5(b). ratio

/, 46

Ratio n:l 1 pM

-

n

Relay + (a) VOLTAGE

I

I

1

!

RATIO

1

I

i

Relays

(b) PHASE CONNECTION

DIFFERENTIAL

6.3 As stated type.

FIGURE CURRENT)

(CIRCULATING

BALANCED

VOLTAGE

in para.

6.1 the balanced

6.5 PROTECTION

OF A TRANSFORMER

SYSTEM voltage

system

is less used than

the circulating

current

One consequence of the high voltage on the pilot lines is that it can give rise to appreciable shunt capacitive currents if the pilot cable is long; these can lead to inaccurate operation unless special steps are taken to deal with them. It is for these and other reasons preferred. In the US the balanced

that the circulating current type of protection voltage system is referred to as ‘transactor’.

is generally

47

CHAPTER UNDERVOLTAGE

7.1

UNDERVOLTAGE

Undervoltage

protection

7

AND SYSTEM UNDERVOLTAGE

PROTECTION

PROTECTION may be required

for a variety

of purposes.

The principal

-

to avoid risk or danger parts of a system without

-

to shed load in order to avert a threatened collapse of the system of a severe drop in voltage resulting from a disturbance.

that could arise if power were restored due precautions after an,interruption,

ones are:

to particular in the event

Under the first heading it is usually necessary, if power is lost for any reason, to clear the system of major loads. This is to avoid the danger of motors restarting without warning when power is restored and to avoid overloading of the system by many motors restarting simultaneously. In addition, any interconnected switchboards must be disconnected. Ordinary contactors will drop out on loss of power, thereby automatically disconnecting their loads, but circuit-breakers (and some special contactors) are latched-in and will not trip on loss of voltage unless they are given a positive trip signal. Such a signal is given, where needed, by an undervoltage relay. This is an ordinary voltage-operated relay fed either directly or through a voltage transformer from the supply side of the circuit to be protected. The relay is calibrated to drop out when the voltage fails to a predetermined level (typically 55% of nominal), whereupon its contacts, which are open when the relay is energised, close and initiate the trip signal. An undervoltage relay is usually fitted with a time delay, either within the case or as a separate relay, to prevent operation due to momentary dips in the system voltage. This type of relay is sometimes incorrectly referred to as a ‘No-volt’ not be used. The relay operates not only when no voltage is present a preset level; its function is ‘undervoltage’.

7.2

SYSTEM

UNDERVOLTAGE

relay. This term should but also when it falls to

PROTECTION

When a whole system undergoes a serious disturbance, with a consequent drop in voltage, a complete collapse may result unless large blocks of consuming equipment are temporarily removed~ from it to enable it to recover. To avoid this,, the reduction in system voltage is detected by an undervoltage relay, typically set to release at 55% of nominal voltage (settings vary from system to system), and loads are disconnected in groups. The items most likely to prevent system recovery are the largest motors, and they must be disconnected. This mode of protection, known as ‘System Undervoltage Trippihg’, is concerned specifically with the eventual recovery of the system. It should be distinguished from load-shedding which is for the purpose of keeping the total steady-state load on a system within the available generation capacity. Figure 7.1 shows one form of system undervoltage tripping applied in an offshore installation. The largest motors in this case are 9 240kW and 8 600kW gas re-injection and booster compression motors and six water injection motors, of 2 500kW each, all supplied at 6.6kV. Undervoltage relays connected to the main 6.6kV busbars and the gas compression switchboard trip the reciprocating compressor (the largest) instantaneously and the booster compressor and the injection and other loads after a time delay if the system has not already recovered sufficiently.

48

Trip

L

Trip (3 set) iK

j

-i

/ /

r

6.6kV

Gas Compression Switchboard

I i

INST

3 x 2500kW water Injection Pumps

water

i:

LJ Q

Injection Pumps

M

9240kW Re-iniection

Gas Compressor

Compressor Tl(

T3

(

t

!I83 3i%w Lean Oil Pumps’

T4 9 ”

440V

SYSTEM

UNDERVOLTAGE

FIGURE TRIPPING

7.1 IN AN OFFSHORE

Gas Compression Switchboard

INSTALLATION

49

CHAPTER ADDITIONAL

8.1

8

FORMS OF PROTECTION

GENERAL

The forms of protection described in Chapters 3 to 7 are the most common forms in offshore and onshore installations. A number of other forms are used for specific purposes, based on the particular requirements of individual items of equipment. They are in some cases simple applications of relays and principles already referred to. Others entail special principles, and these are described below. Forms of protection which are special to individual categories of equipments are described in later chapters.

8.2

OVERTEMPERATURE

Overtemperature and transformers Three

protection is often used to safeguard the windings of generators, motors against excessive temperattme rise due to overloading or fault conditions.

main types.of -

The principles

PROTECTION

temperature

sensor

Thermocouple Resistance Temperature Thermistor. of these methods

are used:

Device

are discussed,

(RTD)

in the manual

‘Electrical

Control

Devices’.

Sensors are used for temperature-monitoring instruments in oil-cooled (or Askarel-cooled) transformers. There~they ire typically suspended in the oil in a housing with a heating element which carries a current (from a current transformer) proportional to the transformer current. The housing is designed to reflect the thermal characteristics of the transformer, and the~sensing element thus experiences something yery close to the actual winding temperature; the technique is known as ‘thermal imaging’. The thermistor is connected into a resistance bridge, whose output may operate indicating instruments as well as actuating alarms and trips through an electronic detector circuit. Whereas NTC thermistors can operate over associated measuring circuits, a PTC thermistor temperature, subject to a small tolerance. It at particular locations in equipment - for windings into which they can be embedded passes through its critical temperature, the actuate an alarm or even to give a trip signal. PTC thermistors

come in three

classes,

a ranges of temperatures by adjustment of the is made to change its resistance at a particular is more suitable for detecting overtemperature example, at hot spots in generator or motor during manufacture. As the PTC thermistor sudden change of resistance can be made to

depending

Class B Class F Class H

8.3 Arc

50

ARCING detection

offers

on their critical

temperatures,

as follows:

145°C 165°C 190°C

PROTECTION a rapid

means

of protecting

against

arcing faults

- in switchboards,

for

x_.,

example - which can result in considerable damage if they are cleared by the normal protection. Protection is provided so located and of such a sensitivity that they respond to the to ambient light. A number of photocells can be linked protection over an area.

REVERSE

8.4

POWER

PROTECTION

(RP)

Protection against the reversal of power flow (such as~when by a relay which is sensitive to the direction of power flow. Such protection is used mainly Chapter 9, ‘Generator Protection’.

8.5

OVERPRESSURE

(but

are allowed to persist until they by light-sensitive cells which are ultra-violet light of arcs but not to one control unit to pravide

not

exclusively)

a generator

with

generators

‘motors’)

is provided

and is discussed

in

PROTECTION

iOverpressure protection is used in liquid-filled 10, ‘Transformer Protection’.

PHASE

SEQU;NCE

transformer

8.6

NEGATIVE

(NPS)

Negative discussed

phase sequence protection is used mainly, in Chapter 11, ‘Motor Protection’.

tanks

and is discussed

PROTECTION but not exclusively,

with

It is sometimes also used in large generators to protect against overheating generator suffers~ a seriously unbalanced load, a negative sequence current stator windings, giving rise to a counter-rotating fie(d. This field cuts the rotor at double the system frequency (i.e. at 100 or i20Hz) and can cause ing of the rotor iron due to eddy currents and hysteresis. When NPS protection current.

8.7

OTHER

is applied

FORMS

Other forms of protection, are as follows: -

to a generator,

in Chapter

it is set typically

motors

and is

of the rotor. If a is present in then forward-running serious overheat-

at 12% negative

sequence

OF PROTECTION discussed

in Chapters

9 and 10 for generators

and transformers,

Overfrequency (OF) Underfrequency (UF) Overvoltage (OV) Field Failure (FF) Diode Failure (DF).

All types of protection, for which British Standard abbreviations have been given, may be identified also by an international system of numbers, from 1 to 99, which are given in BS 3939, Appendix II to the Guiding Principles.

51

CHAPTER GENERATOR

9.1

STANDARD

9

PROTECTION

PROTECTION

All offshore installations have main generators driven by gas-turbines, and both offshore and onshore installations have diesel-driven auxiliary or basic services generators. Each electrical generator has its own protection system, which is usually more complex on the main generators than on the smaller diesel sets. Many of the general protection measures described in Chapters 3 to 8 are applied to generators, but there are also some more specific ones. A typical gasturbine generator protection scheme is shown in Figure 9.1.

---- _______ Q. r g----------c

+-----------, ---------_

I

I

I --.

c CT---,------‘------

ii

i iii All relays generator

DIF FF OCiT SRSPE TH

GENERATOR

I I

I I

I I i

~_

MAIN

7’, ’

j

I

FIGURE 9.1 PROTECTION

are located on the panel of main switchboard

DifFerential Field Failure Overcurrent (Inverse Time) Reverse Power Standby Earth Fault Trip, Hand Reset (Lockout)

(TYPICAL)

1b

The scheme

of Figure

9.1 includes

the following

standard

-

Overcurrent protection (30CIT) current settings will be determined

-

Differential

-

Standby

-

Lock-out hand-reset the generator field.

protection earth-fault

(DIF)

relay

fed from protective CTs. by the overall discrimination

fed from

protection

features:

special

(Class

(SBE) fed from (TH).

This

X) protective

neutral

also trips

The time plan.

and

CTs.

CT.

the turbine

and suppresses

When a generator is subject to short-circuit the overcurrent in the initial subtransient stage is at first high. The short-circuit current, if it is not cleared, quickly falls as the generator passes through the transient stage until, by the synchronous stage, it may have fallen to less than the normal full-load current due to armature reaction - that is to say,‘by then there will be no overcurrent at all.

.-@--*---,----@ I I I I 1 L _____

TYPICAL

I I I I I ---------(---------------J

FIGURE INTERCONNECTOR

11.1 CABLE

PROTECTION 65

in the figure

will

be seen the fol,lowing

features

already

described

in Chapters

3 to 8:

-

Overcurrent (2-element) inverse-time (20CIT) protection fed from a set of protective CTs. The relay trips the production-end circuit-breaker through a lock-out relay and thence the main-end breaker by intertripping (see para. 11.2). The time and current settings will be determined by the overall di5crimination plan. It will also trip the main end direct.

-

Earth-fault inverse-time (EIT) protection fed from the same set of protective CTs as above. The relay trips the production-end circuit-breaker through the lock-out relay (TH) and then the main-end breaker direct, and by intertripping.

-

Differential (DIF) protection between ‘both ends of sets of special Class X CTs, those at the main end wires to those at the production end. The relay breakers at both ends together through the lock-out

-

Undervoltage (UV) protection at the product~ion end. Fed by a p~rotective VT from the production busbars, the relay trips the production-end interconnector breaker when production voltage fails; then the main-end breaker by intertripping.

-

Lock-out

-

Three ammeter instruments, and one in the Electrical CT.

hand-reset

the cable. ~Fed from two being connected by pilot instarttaneously trips the relay.

relay (TH). one each at th,e production and main switchboards Control Room. All fed from a single measurement

Differential protectio~n is provided in the case of an interconnector cable to ensure that, if there is a fault in the cable itself, there will be instantaneous isolation at both ends even before the overcurrent protection at eithe: end operates to disconnect other consumers. It will also minimise any fire risk at the fault point, especially in hazardous areas, by removing the power sources in the quickest possible time. Because differential protection is insensitive to through-faults and takes no part in the discrimination pattern, the relay is given a very light setting. If the busbars of the production switchboard aredead,the undervoltage tripand the interlock (see para. 11.2) would prevent the circuit-breakers at both ends of the interconnector from being closed. To allow a dead production switchboard to be energised from the main switchboard, a key-operated switch is provided on the Electrical Control Panel which defeats the undervoltage trip while the interconnectors are being closed. By using this switch the production-end interconnector breaker is first closed by hand; this lifts the interlock and allows the main-end breaker to be closed. Power now flows from main to production switchboard, and the latter becomes energi,sed and so lifts the undervoltage trip. The keyoperated ‘W inhibit’ switch must now be restored to its normal position and the key removed.

11.2

INTERLOCK

AND

INTERTRIP

An intertrip facility is provided so that, if the production-end whether tripped by a fault or, through the undervoltage relay, manually, the main-end breaker (if closed) trips in sympathy. An interlock facility ensures that, main-end breaker cannot, be closed

66

breaker opens for any reason, by failure of power or opened

if the production-end breaker is open for any reason, the until the production-end breaker has first been closed.

It will be seen from Figure 11 .l that a fault on the cable operates through the lock-out relay and trips the breakers at both ends simultaneously. This is to ensure that after such a fault the cable is isolated from both ends and cannot be energised from either switchboard. The intertrip acts as a back-up for this, but it is also needed of the main-end breaker when the production-end breaker from by a fault.

to ensure sympathetic opening is opened by hand, as~distinct

It should be noted that with the interlocking arrangement described, if it is desired to feed power from the main to the production switchboard (i.e. from right to left in Figure 11 .l, which will eventually be the normal arrangement), it is necessary first to close the production-end breaker, using the ‘UV Inhibit’ switch as described, and then the main-end breaker. Final control will thus always be from the main switchboard. There will be minor variations of the system described for different interconnector applications, but the basic requirements remain the same, namely overcurrent, earth-fault and differential protection, with intertripping and interlocking.

11.3

OTHER

CABLES

The above example is based on a typical offshore, high-voltage cable interconnector cable runs of a few hundred yards are involved. Cables in onshore installations, somewhat longer, would, if it were-required to protect them, be dealt with by similar but perhaps with different detail.

where though means,

By shore standards (other than in a limited installation)~platform cable rut&are very short. In Supply Authorities’ distributio.? networks they may be many miles long, and beyond that lies the still greater network of cables and overhead transmission lines. In these cases such methods as differential protection, for example, involve long runs of pilot connections with the special problems that they bring. instead, very sophisticated manual. They are mentioned of protection.

protection methods are used which are beyond the scope of this only so that the reader~may be aware of this vastly greater field

.-

67

CHAPTER

12

MOTOR PROTECTION

12.1

GENERAL

Virtually every offshore motor, whether operating on high or low voltage, is of the squirrelcage induction type, and all except the very largest are direct-on-line started. Except in the living quarters all motors are totally enclosed, fan cooled (TEFC), the largest being supplemented by water cooling of the circulating air. Exceptionally, the variable-speed drives for drilling, drawworks and mud pumping are d.c. motors.

6.6k V Switchboard

8 1 lo\:

440/l 1ov -_

-z-&J StOp

stop

d.c.

i +$---Cl0 c;

e

Bi-meta Elemen’

(b) HV MOTOR WITH PROTECTION RELAY

E NPS TTlOC uv

TYPICAL

68

Earth Fault Negative Phase Sequence Trip, Hand Reset (Lockout) Thermal Overcurrent Undervoltage

FIGURE 12.1 MOTOR PROTECTION

CIRCUITS

AL,-,,

12.2

CONTROL

Most motors are switched on and off by contactors, and in most cases it is necessary to provide back-up fuses lo protect the contactor itself, as well as the circuit, against~ high-level faults such as short-circuits which are beyond the breaking capacity of the contactor. The fuses have special ratings both toensure discrimination in relation tothe trippingcharacteristic of the contactor and to prevent their being blown by the starting current of the motor. They do not protect the motor against overloads; this is a function of the contactor.

12.3

PROTECTION-GENERAL

Like any other piece of electrical plant a motor, whether HV or LV, must be protected against overcurrent and usually against earth fault. In larger motors overtemperature may also need to be monitored in both the cooling air and the windings themselves. Overcurrent

in a motor -

Other

forms -

can be caused by any of four

mechanical overloading short-circuit stalling single-phasing. of abnormal

motor

fbnctioning

include:

earth fault or leakage undervoltage.

All the above will

be considered

in turn

below,

Figure 12.1 shows the control and protection typical of the types actually met in service. HV type.

12.4

conditions:

MECHANICAL

diagrams of motors which On the left is a LV motor

may be regarded as and on the right a

OVERLOADING

A motor may be overloaded mechanically by either overloading the driven end (e.g. pump or compressor) beyond its rating or by some internal mechanical malfunction such as a stiff bearing. Either may cause a rise of active current above the full-load rated current of the motor. Mechanical overloading is probably the commonest cause of overcurrenrin a motor.

L

The motor is protected by an inverse-time overcurrent device which will cause the contactor to trip if the overcurrent is sufficiently high and persists. The device usually takes the form of a thermal element in each phase, either directly or CT-operated. It has an inverse-time characteristic which is more nearly matched to the thermal behaviour of the motor itself than that of the inverse-time electromagnetic overcurrent relay described in Chapter 3. It must allow the large starting current (up to five times full-load current) to flow during the run-up period without operating, but it must trip the motor if even a small overcurrent persists for a longer period, A typical setting of such a device would be 120% full-load continuous current with the appropriate time setting. F.or short starting times the inversetime characteristic must be such that the starting current and run-up time are taken into account. In this respect it must be remembered that high-inertia loads such as a motorgenerator set or a compressor take much longer to run up than, say, a centrifugal fan.

69

For all LV motors and a few HV the inverse-time device is thermal, as described above. For the smaller LV motors it is in series with the motor itself (see Figure 12.1 (a)), but for the others it is a separate relay operated through CTs. For most HV motors on the later platforms however, the device is wholly electronic but with a similar characteristic; it too is a separate relay, CT-operated (see Figure 12.1 (b)); Where these relays are separate the overcurrent device is combined with certain other features into a single ‘Motor Protection Relay’ which is further discussed in para. 12.10. Both types of overcurrent protection will be found in shore installations. A characteristic of inverse-time relays which is particularly noticeable in thermal relays, and which has to be taken into account in allowing for starting current, is ‘overshoot’ (or ‘overrun’). This means that if the relay is energised with something more than its minimum operating current it may close its contacts even after the current has subsequently fallen below the operating level. For example, a motor could be’ tripped after it had safely started and reached~full speed, even though the relay had not operated during the starting period. This can have a considerable effect on the discrimination that can be achieved between starting and overload currents, unless complications are added to the protection scheme. Whereas the contactor with its inverse-time overcurrent device (thermal or electronic) provides overload protection for the motor, such contactors cannot in general clear a fault of short-circuit proportions. For this they must be backed up by series HRC fuses. When used with motors such fuses must have special characteristics. They must have a continuous rating which will allow them to pass the full-load current of the motor continuously, and they must also allow the considerably greater starting current to pass for the period of the run-up time without melting the fuses. These special motor fuses are described in Chapter 3, para. 3.3.5, ‘Motor Protection’.

12.5

SHORT

CIRCUIT

A short-circuit in a motor circuit will cause a severe overcurrenr. One of the more vulnerable places to short-circuit in a motor is the incoming cable box where a too-small radius or imperfect jointing could lead to weakness. As many process motors are located in hazardous areas, it is essential to clear the circuit in the quickest possible time under these conditions. The overcurrent produced by a short-circuit fault will operate the inverse-time device in a relatively short time, but in general not short enough to cause a trip before the fuse blows. Indeed it is important that the contactor should not operate as it is not rated to clear fault currents. Where HV or LV motor feeders are provided with back-up fuses, it is these that will blow first and clear the fault very quickly (see Chapter 4 ‘Discrimination’ and especially Figure 4.2). Where very large motors are fed through circuit-breakers and there are no backup fuses, the inverse-time protection will be backed up by a high-set instantaneous overcurrent relay element (see Chapter 3) to trip the circuit-breaker instantaneously without waiting for the inverse-time elem%it to operate. Its setting will be swell above the ~motor overload and starting current levels.

12.6

EARTH

FAULTS

Earth faults, especially those occurring in solidly earthed systems, will also give rise to severe overcurrents in the affected phases. They may be dealt with by an earth-fault relay (E) which trips the contactor. With small motor starters where the thermal overcurrent devices are direct-connected, the relay is usually energised by a core-balance CT through which the three cables pass (see Chapter 5, para. 5.2). This arrangement is shown in Figure 12.1 (a).

With larger motors where there are CTs, these may be used to provide an earth-fault signal by their spill current. The earth-fault relay may be separate or, as shown in Figure 12.1 (b), may be part of the composite ‘Motor Protection Relay’, which is further described in para. 12.10. The same consideration will apply to earth faults in HV motors. Nearly all HV systems are resistance-earthed, which limits earth-fault currents to a low level that will not actuate the fuses or high-set instantaneous relays. Here also Bn earth-leakage relay operated through CTs is necessary, but the fault current is then well within the breaking capacity.of the contactor.

12.7

STALLING

Stalling can occur if the motor becomes~heavily overloaded -for example by a mechanical seizure of a bearing or of the driven element, or it may be unable to start against an excessive load. In all these cases the stalled motqr draws its ‘locked rotor’current (that is, its maximum starting current) as long as the supply remains connected, and severe overheating results. The condition is aggravated by the lack of ventilation while the rotor is stationary. There is also a temptation to make repeated attempts to start if unsuccessful the first time. More rarely, stalling can occur if the whole power network goes unstable and begins to run down, causing all motors throughout the system to lose speed. If the system recovers while the run-down is proceeding, all the motors in the system will find themselves running at a large slip and all taking nea~rky their full starting currents. The combined effect on the generators of all these simultaneous starting currents will be to depress the system voltage to such a level that some of the larger motors may not develop sufficient torque to recover against their loads. They will then continue to run down and st;.ll. This is sometimes called the ‘Patrickson Effect’. The long period of drawing ‘starting current’, though not actually starting, will appear as a normal overcurrent and should, in principle, be dealt with by the motor thermal overcurrent protection, but difficulty arises when this protection has to discriminate between normal starting current (which is present during the run-up time but then disappears) and the stalled motor current (which persists). This problem is particularly difficult if the run-up time of the motor is of the same order as the motor stall (or locked rotor) withstand time. It is still worse in the case of high-inertia loads, where the run-up time may well be longer than the stall withstand time. For these situations ‘stall relays’ are sometimes used, especially with HV motors. Stall relays are of two types: one using electric sensing of the motor starting current, and the other using detection of actual rotation. The former uses a current-sensitive element and a timer. On a normal start the current-sensitive element energises, but the timer prevents its causing a trip unless the normal run-up time has elapsed. On a genuine stall the timer will trip the motor after run-up time has expired if the overcurrent is still present. This type of relay is often fitted with a ‘thermal memory’ which prevents a restart until sufficie~nt cooling time has elapsed. The stall relay is sometimes included with other elements in a combined ‘Motor Protection Relay’ (see para. 12.10). The other type of relay uses a shaft rotation detector. This form is most suitable when the motor safe stall withstand time is very close to the motor run-up time, or even less as in the case of the very large gas t-e-injection motors on certain offshore installations. The rotation method is a more accurate indication of a stall condition. However it does not incorporate any ‘memory’ to protect against quick restarting, and it must be used in conjunction with some form of lock-out protection.

71

12.8

SINGLE

PHASING

A problem special to 3-phase motors is single-phasing. Any such induction motor needs three phases to produce its rotating field and to provide the necessary starting torque, but once running, the removal of any one of the phases will not necessarily stop it. It will however reduce the driving torque and will also increase the current in the two remaining phases. If the motor were already well loaded it would eventually trip on sustained overcurrent. If, however, the mechanical loading on the motor were not too high, there might still be sufficient torque to drive the load. Also the current, although increased, might still not be enough to actuate the inverse-time overcurrent relay set, typically, to 120% full-load current. The situation could therefore pass unnoticed except for a high-pitched whine from the motor, and no actual harm would result. There is a much greater risk, however, when attempting to~start. If the single-phasing had occurred while the motor was running partially loaded and had not been noticed, the motor would have been stopped ~normally when the operation was complete, but still in its singlephased state. If later an attempt were made to restart it, there would be excessive starting current but no starting torque, and it would remain stationary. As the overcurrent relay is set to allow adequate run-up time, this situation could persist for a dangerously long time with no ventilation in the stationary motor. Still worse, the operator might make several attempts to start, and each time the relay would reset and allow full run-up time afresh. Eventually the motor would probably burn out before the overcurrent relay disconnected it. Therefore-~.if a motor fails to start after two attempts, the operator must make no further attempt to start it until the cause has been found and corrected. Shell have laid down a general instruction successive starts (even if successful) when a cooling period of 30 minutes atstandstill. any one, hour.

that motors over 50kW are only suitab~le for two cold. Another one-start attempt is allowable after No more than three starts may be attempted~in

Damage to a motor by single-phasing is caused by overheating of the windings due to the prolonged excessive currents in two of the phases. it is normally prevented by the protection system. In the case of small motors provided with thermal overcurrent protection, the three thermal elements are mounted in such a way that unequal heating produces a differential ‘. movement which causes the contactor to trip. With some HV and larger LV motors where protection is through current transformers and where thermal overcurrent protection is used, th,e single-phasing protection is’ provided by these same overcurrent elements where unequal heating produces the differential movement. With motors which are protected by electronic relays the device includes a special element which detects the single-phase condition, whether the motor is running or being started. It is referred to as a ‘Single-Phasing’ or ‘Negative-Phase-Sequence’ (NPS) relay. Single-phasing can be caused by the possible welding of contactor contacts due to damage or vibration. It will not with external striker-pin contacts which

12.9 Consider R-Y-B-R.

72

NEGATIVE

PHASE

blowing of one of t,he three back-up fuses, by the or, less probably, by the open-circuiting of one line occur however with HV fuses where they are fitted trip the contactor if any one of them blows.

SEQUENCE

a balanced 3-phase system in which the phases This is indicated by the vector diagram of Figure

rise in the 12.2(a).

normal

sequence

R, L/ BI (a)

I 3

POSITIVE

4

SEQUENCE

(b)

NEGATIVE

R2

SEQUENCE

-B (c)

COMBINATION

FIGURE UNBALANCED

(UNBALANCED)

12.2 SYSTEM

Consider next another balanced 3-phase system, but one of different magnitude and in which the phases rise in the reverse order R-B-Y-R, as shown in Figure 12.2(b). The phase relation of the second to the first system is quite random. If voltages corresponding to both systems are applied simultaneously to a set of busbars, the voltage of each bar_woul$ be the vector sum_of the two voltages applit& to>: that is, red voltage would be RI + R z, yellow voltage Y, + Y, and blue voltage 5, + B2. This vector addition is done in Figure 12.2(c), giving resultant voltages RT yand L7? It will be seen that R, Y and 5 are now of different magnitudes and are no longer spaced 120” apart. This resultant system is still 3-phase, but it is now unbalanced. The system R, Y, 5, in which the phases rise in the normal order is known asa ‘positive phase.sequence’, and the system R2 Bz Y, in which the phases rise in the opposite order is a ‘negative phase sequence’. It should be particularly noted that, although the negativesequence vectors come up in the reverse order, they are still considered to rotate in the conventional anti-clockwise direction, the same as the positive-sequence vectors. (if they did not, it would not be possible to combine them as has been done in the lower diagram.) It is not difficult to see how a pair of different, but balanced, positiveand negativesequences combine to produce an u,nbalaticed system. It is far less easy to see the converse: that any unbalanced system may be resolved into two balanced systems, positiveand negativesequence. This can be proved mathematically, but is not included in this manual. 73

In Figure 12.2 the negative-sequence system was different in magnitude, and its angular position relative to the positive-sequence was quite arbitrary. The negative system might have been assumed to be in any other angular position, and the combined unbalanced set would then be different in each case. A special case is when both positive and negative systems have~the same magnitude but the negative-sequence red phase is diametrically opposite (1 go”, or anti-phase) to the positive, as shown in Figures 12.3(a) and (b).

(a) POSITIVE R-Y-B-R

(b) NEGATIVE R-B-Y-R

R,+RZ=O (c) COMBINATION

SINGLE

(d) SINGLE PHASING

FIGURE PHASED

MOTOR

12.3 CONDITION

If these two are combined in the same way as was used in Figure 12.2, it produces the vector diagram of Figure 12.3(c). It will be seen that the two reds cancel out, and the two blues and the two yellows combine to produce resultants at 9 o’clock and 3 o’clock respectively - that is, equal and opposite to each other. If these are considered as current vectors, this is just the situation which exists in a motor whose red phase supply has become open-circuited (Figure 12.3(d)) but whose other two phases are intact. In that case no current ~flows in red phase, but the current flow in yellow phase returns via the star point through blue phase - that is to say, the yellow and blue currents are equal and opposite, exactly as indicated in the vector diagram. 74

It can be inferred therefore that a single-phased condition, which unbalance, can be represented by two balanced systems, one positive equal in magnitude and where in the negative system the-phase circuited is anti-phase with its positive colour.

is an extreme case of and one negative, both which has been open-

It will be seen that the currents represented by the thick horizontal lines in Figure 12.3(c) are greater than the phase currents of the original positive-sequence system by a factor of d/3:1 - that is to say, when a running motor starts to single-phase, the line currents in the two remaining phases increase by more than 70% over the original currents being taken just before the event. In fact they increase even more than this. A single-phase condition also causes some reduction in driving torque, and the motor will slow down slightly so that its torque will rise again to meet that of the driven load. This increase of slip results also in increased current, and in practice the currents in the sound phases will about double when a running motor starts to single-phase. This doubling of current, if the motor had been fully loaded at the time, than enough to operate the thermal overcurrent protection and trip it. If been only half-loaded or less however, even the doubled current would not operate its overcurrent protection. Therefore the normal protection cannot in all cases to trip a motor if it should lose a phase while running.

would be more the motor had be sufficient to be relied’upon

Although the concept of negative phase sequence is purely mathematical, and one is tempted to say that it does not really exist, its presence can nevertheless be detected by a relay and used to indicate unbalance of currents in a 3-phase system. How this relay operates is explained below. In the extreme ‘case a large negative-sequence component will indicate an open-circuit condition in one phase. The relay is called ‘Negative Phase Sequence’ (NPS) and is particularly used to protect motors against single-phasing. As already explained, this condition is not serious if it occurs while a motor is running, but it could be very serious indeed if an attempt were made to start a motor while in a single-phased state. The negative-phase-sequence connected as shown in Figure

relay uses a simple 12.4(a).

network

and a voltage-sensitive

relay,

Current transformers in R and B phases feed respectively a resistance RI and a resistance R, with a reactance X in series. R, and X are chosen so that the circuit has a ower factor has cos @ = 0.5 (so that 6 = 60” lagging) and their combined impedancez (= d*) RZ the same ohmic value as RI Then the current I, flowing in the red CT secondary causing a voltage I, R, (or V,) across it in phase with II.

passes

through

the resistance

R,,

The current I, flowing in the blue CT secondary passes through the impedance Z, causing a voltage /,Z (or V,) across it. Since the !a circuit is an inductive one, the current /a lags 60” on the voltage across Z, or, put another way, the voltage across Z (= /,Z) leads 60” on the current I,. If we look at the positive-sequence (= I, R,) across R, is in phase with leads 60” on the blue current. Since are equal and opposite. The voltage these two cancel out, so the voltage to positive-sequence currents. Consider now the corresponding diagram of Figure 12.4(c).

current vector diagram of Figure 12.4(b), the voltage I/, the red current, whereas the voltage V, (= I,Z) across Z R, and Z have the same ohmic value, thee two FItages across the relay is the vector sum of V, and Va, and across the relay is zero. Therefore the relay is insensitive

effect

on the

negative-sequence

currents

-

the vector

75

(a) VOLTAGE

RELAY

‘R

‘. Qh3

‘B

(b) POSITIVE

SEQUENCE

NEGATIVE

(c)

FIGURE 12.4 PHASE SEQUENCE

NEGATIVE

SEQUENCE

RELAY

/2. As before VI (= /,R,) is in phase with II, and 1/Z (= /,Z) leads 60” on the blue current The voltage vector Vz ic now at ‘2 o’clock and-no longer cancels out V, Since the voltage as across the relay is the vector sum of V, and V,, this resultant voltage is at ‘1 o’clock’ shown in Figure 12.4(c) and has a definite value. Therefore the relay is sensitive to n~egativesequence currents, and, if the resultant voltage is greater than ~the setting of the relay, the relay will close and trip the cbntactor and lock it out. Thus an NPS relay has been devised ,which will ignore the positive-sequence currents which are normally present but will detect the presence of negative. In particular it will give protection against a single-phase condition and will prevent, starting a motor while i” this state., ,.The foregoing description deals with the concept of balanced positive- and negative-sequence currents as an aid to dealing-with single-phasing. In electrical engineering practice they have a far wider application, which is beyond the scope of this manual. There is a third type of balanced system called ‘zero-phase sequence’. It comes into being when there is current in the neutral conductor of a 4.wire system, or with an earth fault on an earthed 3-wire system (which effectively provides a fourth path). Zero sequence is not dealt with in this manual but comes into the wider application referred to above. The three ccmcepts positive-, as ‘Symmetrical Components’. 76

negative-

and zero-phase

sequences

are collectively

referred

to

-~

12.10

MOTOR

PROTECTION

RELAYS

The thermal overcurrent, earth-fault and single-phase prote6tive~devices for HV and the larger LV motors have been referred to previously as if they were separate elements. In practice they are usually combined into a single ‘Motor Protection Relay’ which contains all ,these elements. Its form differs between manufacturers. One widely used type, made by P.B. Engineering (commonly called ‘P & B Golds’), is shown in Figure 12.5; it uses flags to indicate which element has operated. Another type is wholly electronic and indicates by lamu. In this type the single-phasing condition is detected by an electronic relay.

FIGURE 12.5 MOTOR PROTECTION

P & B GOLDS

In some installations see para. 12.7.

12.11

MOTOR

Overtemperature tection afforded Three

main types -

a motor

WINDING

protection

relay

TEMPERATURE

protection is sometimes by the Motor Protection of temperature

may

RELAY

also incorporate

a stall relay element

-

PROTECTION used, in addition to the thermal overcurrent Relay, to safeguard the windings of a motor.

pro-

sensor are used:

Thermocouple ,Resistance Temperature Thermistor.

Device (RTD)

The principles of these methods are discussed in the manual ‘Electrical The sensing elements arc normally hang.

Control

embedded in the winding insulation,

Devices’.

usually in the over-

77

12.12

UNDERVOLTAGE

In any distribution system involving motors it is important that, if system voltage lost, all motors should be disconnected so that they do not all restart together system voltage returns. They must be carefully restarted in a controlled sequence.

should be when the

All LV and some HV contactors are closed and held-in by their operating solenoids. It follows that, if the operating voltage is lost or falls below a certain value, the contactor will drop off and will not. retlose until given a positive closing signal. Such a contactor is said to have an ‘inherent undervoltage’ facility and needs no other undervoltage protection. Some HV contactors however are, like circuit-breakers, of th,e latching-in type and are operated from a separate d.c. closing and tripping supply. If main voltage is lost they remain latched and will therefore not trip. Such contactors are fitted with separate protection in the form of an undervoltage reiay connected through a VT across the mains. When mains voltage fails the relay trips the contactor through its tripping circuit, and it will not reclose when mains voltage is restored until given a positive closing signal. After motors have tripped on undervoltage certain installations with ‘re-acceleration units’ automatically restart their motors when voltage has been restored. Re-acceleration is discussed System Control’, Chapter 8.2.

12.13

STARTING

which have been provided in a pm-arranged sequence in the manual ‘Electrical

TIME

A major problem in motor design and protection is to ensure that the starting current can flow fur long enough to accelerate the motor without bringing out the motor’s over-current protection, while at the same time not impairing the close protection given to the motor while it is running. For this purpose a time-quantity motor’s windings, while carrying from the maximum temperature perature to the limiting allowable its nameplate.

This is defined as the time taken for the ‘t, ’ is considered. to be further heated the starting current I, continuously, reached in rated service and in a maximum ambient temtemperature. The te time of the motor should be given on

20 min 10 min 8 min 4 min 2 min

‘42

-.

20s 10s 8s 4s 2s

I 1

TYPICAL

MOTOR

2

II 3

II 4

6

I * 8

10

FIGURE 12.6 PROTECTION RELAY THERMAL

la/IN ratio

CHARACTERISTIC

The thermal relay for protecting a squirrel-cage motor should be selected so that the tripping time read from the thermal relay’s time/current characteristic using the /,/I, ratio (ratio of starting to normal rated current) is not larger than the motor’s stated tE time. Example A motor

has a rated

therefore

z

current

of 80A

and a starting

= 6.0. The tE value on the nameplate

current

of 480A.

The /,/I,

isgiven as 16 seconds.

ratio

is

Is thisacceptable?

The characteristic of the particular motor thermal protection relay is shown in Figure 12.6, where the tripping time (t) is plotted against the /,/I, ratio. in this example the ratio is 6.0, so that the tripping time is 8 seconds. This is well within the stated tE time of 16 seconds for the motor and is therefore acceptable. It will not trip when starting or restarting. Had the tripping time need to be chosen.

been greater

than

tE, a relay

with

a different

It must be realised that, if the motor had been at maximum temperature it would be at a still higher temperature immediately after the start. start would then not necessarily be permissible until after a cooling-down the run-up time is to the tE value, the more important this becomes.

characteristic

would

when first restarted, A second or further period. The closer

79

CHAPTER QUESTIONS

13

AND ANSWERS

13.1

QUESTIONS

1.

Name five principal

reasons

2.

Name four

consequences

3.

What are the’particular

4.

What are the three principal disconnection devices and what are the differences in how~they function?

5.

What is meant expressed?

6.

What is the most a particular duty?

7.

What is the effect of large motors they taken into account?

8.

What is the impedance terminals, and why?

~9.

possible

for having

dangers

by the ‘fault important

a protection

of a fault associated

level’

system.

condition. with

at a point

arcs? on which

in a system,

rating of a circuit-breaker

on a system

of an a:c. generator

in assessing

Calculate the rms symmetrical where th,e fault level is 12MVA

11.

The network opposite shows two generators supplying a number of LV motors totailing 800kVA through a transformer. The subtransient readtances of the generators ,and the reactance of the transformer are indicated. Calculate the fault level at the LV board: (a)

ignoring

the motor

(b)

taking account contribution.

its suitability

the fault

many times

How

is it for

are

level at its

rated full-load

system

contribution, of

the

motor

/ Total

80

units

short-circuit current at a point in a 3-phase and the normal voltage is 440V.

\

12.

depends,

short-circuit?

determines

If the subtransient reactance of a generator is 25%, how current will the initial short-circuit current be?

10.

and in what

undergoing that

protection

800kVA

In the network of Q.11 the HV board operates at 6.6kV and the LV board at 440V. Would you expect the short-circuit (rms symmetrical) current at the LV board to be less than, or greater than, the short-circuit current at the HV board? Give the current figures In kA (rms sym,metrical).

13.

What is the significance

14.

What

15.

What are the types

16.

What, approximately, reiav?

17.

A circuit is fitted with inverse-time overcurrent relays fed from current transformers of ratio 500/1A. To protect the circuit against overloads the protection must operate within 1.2 seconds when the current exceeds 3 OOOA. What plug setting would you give to the relays:

is overcurrent

of the term

protection

‘prospective

primarily

of overcurrent

(a)

if the maximum

(b)

if the maximum of CUT?

(Use the relay setting is the purpose

continuous

most likely

to be met?

‘instantaneous’

is increased

3, Figure

overcurrent

to 600A

without

change

3.4.)

18.

What

19.

What is the significance of ‘HRC’ in relation make it particularly effective for short-circuit

20.

How does the operation of fuses with moderate overcurrents differ high prospective current? Why does fuse protection in d.c. ti.zuits consideration?

21.

What do you understands

22.

What is the purpose

23.

What is normally

24.

What is the basis of discrimination between two fuses and how, roughly, would discrimination be ensured?

25.

Why should

26.

Can an MCCB

27.

Why is discrimination

28.

Sketch circuit.

29.

Make a sketch showing how earth-fault protection current protection in a single relay case and fed from

can be combined the same CTs.

with

30.

Describe an Earth Leakage is it principally used?

What

and where

three

two

hand-reset’

current’?

is 300A,

load current

of Chapter

of a ‘trip,

devices

current

rms fault

to protect?

time of a high-set

continuous~load

curves

intended

protective

is the operating

symmetrical

by ‘fusing

of a trigger

the purpose

MCCBs

factor’?

to a fuse, and how protection?

What

of discrimination

system?

does its behaviour

from that with require careful

range of values does it have?

in overcurrent

never be used in series with

between

relay in a protective

fuse?

and a fuse be used in series?

methods

or lock-out

a contactor

of applying

Circuit

Breaker

in series

what

and its back-up earth-fault

(ELCB).

in a short-circuit,

each other?

If so, under

(unrestricted)

protection?

conditions? fuse important? protection

is its purpose,

to a 3-phase

over-

81

31.

How

32.

What is the principle of restricted earth-fault earthed at the switchboard neutral busbar?

33.

What is the principal

34.

What is the advantage earth-fault protection?

35.

Name two

36.

What loads are most drop in voltage?

likely

37.

What is the essential perature sensing?

difference

38.

Why

39.

Against

40.

What

41.

Why is a special generator?

42:

What is the purpose

of generator

43.

What is the purpose

of a neutral

44.

How

would

45.

How

is a differential

46.

Why is a short ential protection

47.

What are the functions (a) (b)

is earth leakage

advantage

possible

d.c. system

of restricted

of full

reasons

is differential what

in an unearthed

differential

protection

earth-fault

to prevent

NTC

particularly

malfunctions

would

you expect

means of detection

of a system

from

a temporary

used for tem-

for generators?

a large generator

to detect

resistor

to be protected?

excitation

diode failure

system

associated

with

in a

a generator?

fed by a transformer

balanced

int,he

be cleared?

case of a transformer? measure)

necessary

in differ-

of: protection,

48.

What interlocking and intertripping is provided LV switchgear? What is their purpose?

49.

Why is differential

50.

What type of motor

51.

Against

52.

What is the most

53.

Why do motor

54.

What is ‘overshoot’

82

restricted

field suppression?

on the LV busbars

Qualitrol transformer a Buchholz relay?

protection is mostly

applied

to high-voltage

would

cause of overcurrent

have special in a thermal

between

a transformer’s

HV and

cables?

used in Shell installations?

of malfunction

likely

fuses

is

protection.

time delay (or some more complicated for transformers?

types

which

protection?

necessary

earthing

protection

with

and PTC thermistors

useful

kind of relay is used for reverse-power

what

in comparison

the recovery

between

in a system

protection?

protection

for using undervoltage

protection

a short-circuit

detected?

ratings? relay?

you expect

a large motor

in a motor?

to be protected?

55.

Why may stalling

56.

What is the purpose and in what respects

57.

How

58.

What results components (a) (b)

can current

protection

be necessary

for a motor?

of negative-phase-sequence is it superior to overcurrent

unbalance

in a motor

from the addition of current:

be detected

of positive-and

if they are unequal, if they are equal in amplitude

protection protection?

for an induction

in a simple

motor,

way?

negative-phase-sequencesymmetrical

and in anti-phase

in one phase?

59.

A P & B Golds motor protection relay includes are they, and explain briefly how they function.

many

60.

Why is separate undervoltage protection sometimes restarting of motors upon restoration of an interrupted

protective

elements.

necessary to prevent supply and sometimes

What the not?

83

13.2

ANSWERS (Figures

1.

2.

3;

4.

in brackets

after

each answer

refer to the~relevant

(4

To maintain electrical fault has been isolated.

supplies

to as much

(b)

To protect generators ditions and fault.

and other

(cl

To protect the consumer ditions (e.g. overload).

(4

To isolate faulted

(4

To limit

(4

An arc.

(b)

Overheating

(4

Mechanical

overstress

(4

Disruption

of operation.

(4

Damage

6’)

Fire.

(4

Explosion

(4

Circuit-breakers: currents.

U-4

Contactors: to break it.

(4

Fuses: can be rated for short-circuit protection to be replaced after~operation. This is costly.

plant

equipment

equipment

damage to the cable system

of the system

against

damage

against

to limit

paragraphs

damage

in the text.)

as possible

after

a

due to abnormal

con-

due to abnormal

con-

the risk of fire locally. resulting

from

a fault.

(1.1)

of conductors. on conductors. (1.1)

to eyes, and burns.

in hazardous

areas.

normally

(1.1)

rated

to make,

rated to make and carry

carry

maximum

and break

fault

current

the maximum

fault

but not normally

but are expendable

and have (1.3)

5.

It is a measure of the energy that can be released at the point in a network where a full ‘bolted’ 3-phase short-circuit may occur. It isJ3 times the product of system line voltage and the calculated prospective short-circuit current at that point. It is measured in volt-amperes, or more usually in MVA. (2.2)

6.

Its fault

7.

For a very short period they act as generators and contribute, together with the main generators, to the total fault current and so to the fault level at the point of the’system in question. They are taken into account by considering them as extra (2.2.2) generators of the same kVA but with a reactance of 30%.

8.

The subtransient reactance, because itgoverns the most severe condition immediately following a short-circuit, which is not greatly affected by the level of excitation. (2.2.2)

9.

1 + 0.25 = 4; four times the normal

a4

level rating or breaking

capacity,

usually

full-load

expressed

current.

ins MVA.

(2.2.2)

(2.2.2)

10.

L.’

I

kVA J3kV

sc =

=

12 000 J3

x 0.44

=

15700A (2.2.2)

= 15.7kA 11 Base 30MVA Without

Fault level

Motor

With Motor

30 = ,.425 =

2lMVA

= 24MVA N.B.

800kVA of m~fors raised to 30MVA base gives a factor of 30/0.8 = 37.5. Motors assumed to have ~reactance 3ll%, which multiplied by 37.5 gives 1125%.

(2.2.2) 12.

From the first part of the answer to Q.ll 30/0.225 The faulr d/3 Z’o.44

= 133MVA.

The short-circuit

level without = 27.6kA,

current is therefore

133

d/3 x 6.6

at the LV board is 21MVA,

and with motord3

Ptlthough the LV fault that of the HV.

‘v

motor

above, the fault level at the HV board is

Fo.44=

level is lower in MVA,

= 11.6kA.

giving a current

of

31SkA.

the LV fault current

13.

It is the rms current that would flow in a bolted short-circuit, had disappeared, if not limited by the action of the protection

14.

Heating effect in conductors, and in some cases electromechanical conductors, due to fault currents.

is higher than ~~_. ~(2.3)

after any asymmetry device itself. (2.5) forces between (3.1) 85

Instantaneous overcurrent relay. Inverse-time overcurrent~relay. Inverse and definite minimum time relay. Electronic relays of all types. Fuse. Approximately

0.2 seconds

(a)

load 300A

Maximum

(200ms),

Plug setting

300 = 500

Effective

= 500 x 0.75

Multiple

current

of plug setting

but often

= 0.6. Choose

(3.2, 3.3) less.

0.75 (75%).

= 375A.

at 3 OOOA = e

(3.2.1)

:~ = 8.

From curves (Figure 3.4), the horizontal time delay line for 1.2 seconds the vertical ‘8’ line between time multiplier setting curves 0.3 and 0.4. Choose (b)

next higher TMS setting

Maximum

of 0.4, giving a trip

in 1.35 seconds.

load 600A

Plug setting

600 = 500

Effective

= 500 x 1.5 = 750A.

:lilultiple

current

of plug setting

= 1.2. Choose

I:5 (150%)

at 3 OOOA = m&L

(1.25A

is too close).

4.

From curves ~(Figure 3.4), the horizontal time delay line for 1.2 seconds the vertical ‘4’ line between time multiplier setting curves 0.2 and 0.3. Choose

cuts

next higher TMS setting

of 0.3, giving

a trip in 1.5 seconds.

cuts

(3.2.3)

18.

The trip signal for almost all protective relays trips the circuit-breaker through a lock-out relay (TH). Unlike most other protective relays, it does not reset itself when the cause of operation has been removed. When the lock-out relay has operated it drops a flag, but the breaker which it has tripped cannot be reclosed, either locally or remotely, until the lock-out relay has deliberately been reset by (3.2.7) hand.

19.

HRC (High Rupturing Capacity) signifiesan ability to interrupt very high prospective fault currents safely. HRC fuses have a current-limiting action whereby a steeply rising current can be interrupted very rapid,ly before it reaches its prospective peak (3.3.1) value.

20.

At a moderate current level a number of half cycles of current are needed to melt the fuse, and the arc is extinguished at a natural current zero. With a high prospective current the fuse melts before the prospective peak is reache~d and interrupts the current at a considerably lower value. In d.c. circuits there are no natural current (3.3.1) zeros, and dangerous arcing can occur within the fuse.,

21.

Fusing

factor

is the ratio

It varies between 86

minimum normal

approximately

fusing current rated current 1.2 and 2.0.

(3.3.4)

22.

Some fuses are fitted with a device which releases a trigger when the fuse blows. It either makes a contact which trips the contactor electrically or strikes a trip bar which releases the contactor mechanically. Its purpose is to ensure the opening of all three phases even if only one fuse blows. (3.3.7)

23.

To ensure system.

24.

It is necessary to ensure that the minor fuse blows before the major. This means that the i*t ‘let-through’ energy of the minor fuse must be less than the melting or pre-arcing /‘t of the major fuse. This is achieved when the normal current ratings of the major and min~or fuses are in the ratio of at least 3:l. (4.3)

25.

Although MCCBs with different trip units would discriminate between each other with their thermal trips, their electromagnetic trips are instantaneous and would not discriminate. Therefore there is no certainty that the major MCCB would not trip before the minor. (4.3)

I&i.

YesJhe normal at least 2:l.

27.

The fuse protects the contactor from the danger of attempting to interrupt a current beyond its breaking capacity (as well as protecting the circuit), and the contactor prevents unnece+ry and possibly dangerous operation of the fuse on low overcurrent. , (4.4)

28.

As Chapter

5, Figures

29.

As Chapter

5, Figure

30.

In an earthed single-phase system the go any, return wires both pass through a ring-type current transformer. Under normal conditions the currents through both wires are equal, and there is no net magnetisation of the CT and hence no secondary current. If either wire develops a leakage to earth the balance is upset, and the CT secondary operates the ELCB trip release in under 30ms. Leakage currents as low as 30mA can be detected.

clearance

of a fault

current

ratings

with

the minimum

of the MCCB

effect

on healthy

and the fuses should

parts

of the (4.1)

have a ratio of (4.3)

5.1 (a), (b) and (c). 5.2.

ELCBs are used principally in domestic and industrial applications to protect personnel from serious shock if they should come into contact with live metal. Particularly effective in circuits to which portable apparatus can be connected. (5.4.1) 31.

By placing a high-resistance potential divider between positive and and connecting a voltage-operated relay between the centre tap and is no earth leakage, no current flows through the relay, but if there is upset, the centre ~tap takes up a voltage above (or below) earth operates.

32.

The spill current from the three line CTs is balanced against the current from a neutral CT. Only an earth fault on the supply side of the line CTs gives rise to a neutral current that is not balanced by the spill current from the line CTs, and so (5.5.1) the earth-fault relay is energised.

33.

It can provide sensitive protection the overall discrimination scheme. (i.e. upstream of the CTs located secondary windings, would have no

34.

It protects

against

phase-to-phase

negative lines earth. If there is, the balance atid the relay (5.4.2)

within a defined zone without having to fit into Without it the equipment in the defined zone in the switchboard), including the transformer (5.5.3) earth-fault protection at all.

as well

as earth faults

within

the protected

zone. (6.1)

87

35.

(a)

To avoid uncontrolled supply failure.

starting

of motors

(b)

To avert following

collapse

of the system

a threatened a disturbance.

restoration

of power

by shedding~large

Large

37.

The resistance of an NTC (Negative Temperature Coefficient) thermistor increasing temperature; that of a PTC (Positive Temperaturecoefficient) rises sharply from a low to a high value~at a certain critical temperature.

39.

-

a

,loads

motors

It provides instantaneous a discrimination scheme delay. Againstymost

motor

after

(7.1)

36.

38.

induction

upon

and sensitive protection against in which the generator protection

falis with thermistor (8.2)

internal faults in spite of is subject tomthe longest (9.1)

of the following:

Overcurrent Earth fault Standby earth fault Reverse power Differential Field failure Diode failure Winding over-temperature Overspeed

and sometimes -

Underfrequency Overvoltage Negative phase sequence.

40.

A 2-w~inding delay.

41.

A diode failure may not noticeably affect the operation of the excitation and the AVR will usually compensate for it. It is unlikely to be noticed special means of detectron is fitted.

42.

By suppressing the generated emf it protects the generator against excessive damage from the internal fault continuing after the generator breaker has tripped on differential protection. (9.2.8)

43.

It limits the earth-fault current in order the severity of any arcing while allowing protective relay.

44.

By the overcurrent

45.

By using primary and secondary current transformers with ratios in inverse proportion to the power transformer ratio and with reversed star/delta secondary connections as necessitated by the power transformer configuration. (10.2)

46.

To allow

88

induction

(9.1) 9.2)

disc power

protection

for magnetising

inrush

(wattmetric)

built-in

or separate

time (9.2.1)

rectifier, unless a (9.2.2)

to minimise damage to the generator and enough fault current to flow to operate a (9.4)

on the HV side.

current.

relay with

(10.1)

(10.2)

47.

(a)

To sense, and if necessary

(b)

To sense evolved conservator.

relieve,

overpressure

in a sealed transformer.

gas or a surge of oil in an unsealed

transformer

fitted

with a (10.4)

48.

An interlock is provided to prevent the LV breaker being closed- unless the HV breaker is already closed. An intertrip is provided which causes the LV breaker to trip if ever the HV breaker trips or is opened manually. This ensures that a transformer can never be energised from the LV side. (10.4.3)

49.

Because of their susceptibility to damage and the need to minimise particularly in hazardous areas.

50.

Squirrel-cage

51.

Mechanical overloading. Short-circuit. Stalling. Single-phasing. Earth fault. Undervoltage. Overtemperature (sometimes).

(12.3)

52.

Mechanical

overloading.

(12.4)

53.

To ensure ~.o starting

discrimination current.

54.

An effect has fallen

55.

With high-inertia loads it may be difficult to differentiate current and a damaging stalled condition purely by protection.

56.

To detect the presence of negative-sequence current and in particular of a singlephased state. In tripping the motor contactor instantly it prevents overheating the windings in this condition and, most important, prevents any attempt to start.

induction

the risk of fire, (11.1)

motors.

with

(12.1)

contactor

characteristics

whereby the contacts may operate below the operating level.

some time

and to avoid

after

blowing

its energising

due (12.4)

current (12.4)

between normal starting inverse-time overcurrent (12.7)

It is superior to. normal thermal overcurrent protection because the line currents in a single-phased state do not accurately reflect their heating effect in the motor, and overcurrent protection alone cannot be relied upon to act quickly enough to save (12.8) the motor. 57.

By means differential relay.

of a thermal overcurrent relay in which tripping can be initiated by movement of the elements, or by use of a negative-phase-sequence (12.8 and 12.9, Figures 12.4 and 12.5)

58.

(a)

Unbalanced

(b)

A single-phase

3.phase current.

currents. (12.9)

89

59.

60.

90

A P & B Golds

motor

protection

relay contains:

(a)

Thermal elements

overcurrent elements in all phases. caused by excessive line current.)

(b)

Earth-fault element. (This operates through a separate trip coil.)

(c)

Single-phase ele.ment. (This is a mechanical linkage unequal heating of the current elements and operates

from

Undervoltage protection is provided automatically not by latching contactors and circuit-breakers. vdltage~?elay to proCide~‘a positive~~shit’ht trip.

(These spill

operate

current

from

by heating

of the

the three

phases

which is unbalay;,$ the trip.)

by non-latching conta&ors but These require a separate under(12rW)

REFERENCES Protective Relays Application Guide - General Electric Co Measurements

(GEC)

Protection of Industrial Power Systems - T. Davies (Pergamon Press) Protection Relaying - S.H. Horowitz

for Power Systems (John Wiley)

IEEE Recommended Practice ,; .Commercial Power Systems - IEEE (John Wiley) Electric Power Systems - B.M. Weedy (John El&tric Fuses - A. Wright

for Protection-and

Co-ordination

of industrial

and

Wiley)

and P.G. Newberry

Electrical Engineer’s Reference - M. G. Say (Butter-worth)

(Peter Book

Peregrinas)

(13th

Edition)

Advanced Electrical Technology - H. Cotton, DSc (Pitman) Alternating - Philip

Current Electrical Engineering (10th Edition) Kemp, MSc Tech, C Eng, MIEE (Macmillan)

91