Training _ Power System Protection _AREVA

TRAINING ON POWER SYSTEM PROTECTION

APPS COMBINED 'COURSE

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INTRODUCTION TO POWER SYSTEM PROTECTON

CONTENTS Overview Of Protection Fundamentals Notes Overcurrent Protection Directional Overcurrnt Transformer Protec:tion Notes Transformer Setting Tutorials Generator and Generator Transf - Protection Generators Setting Criteria Distance Protection Notes Distance Protectiorr Schemes Busbar Protection Motor Protection A C Motor Protection Motor Setting Criteria Notes 1 C T S Notes Additional Analysis Notes Unbalanced Faults Tutorial Balanced Faults Tutorial Grading Examples Tutorials Generator Protection Tutorial C T Selection Tutorial Busbar Protection

Overview Of Protection Fundamentals

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OVERVIEW OF PROTECTION FUNDAMENTALS 1.0

INTRODUCTION

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Relays are compact devices that are connected throughout the power system to detect intolerable or unwanted conditions within an assigned area. They are in effect, a form of active insurance designed to maintain a high degree of service continuity and limit equipment damage. They are "Silent Sentinels". While protective relays will be the main emphasis of this chapter, other types of relays, applied on a more limited basis or used as part df a total protective relays system will also be covered. 2.0

CLASSIFICATION OF RELAYS

Relays can be divided into five functional categories:

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a. P r o t e c t i v e Relays, which detect defective lines, defective apparatus, or other dangerous or intolerable conditions. These relays can either initiate or permit switching or simply provide an alarm.

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b. M o n i t o r i n g R e l a y s , which verify conditions on the power system or in the protection system. These relays include fault detectors, alarm units, channel-monitoring relays, synchronism verification, and network phasing. Power system conditions that do not involve opening circuit breakers during faults can be monitored by these relays.

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c. P r o g r a m m i n g R e l a y s , which establish or detect electrical sequences. Programming relays are used for reclosing and synct-~ronising.

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d. R e g u l a t i n g R e l a y s , which are activated when an operating parameter deviates from predetermined limits. Regulating relays function through supplementary equipment to restore the quantity to the prescribed limits.

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e. Auxiliary Relays, which operate in response to the opening or closing of the operating circuit to supplement another relay or device. These include timers, contact-multiplier relays, sealing units, receiver relays, lock-out relays, closing relays and trip relays. i

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In addition to these functional categories, relays may be classified by input, operating principle or structure and performance characteristic: Input Current voltage Power Pressure Frequency Temperature Flow Vibration (ii)

> > > > >

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Operating Principle of Structure Percentage Multi-restraint Product Solid state Electromechanical Thermal.

and definitions are based on the ANSI Standard The above c~assifi~ation 37.90 (IEEE 313).

3.0

PROTECTIVE RELAYING SYSTEMS AND THEIR DESIGN

Technically, most relays are small systems within themselves. Throughout this chapter, however, the term systems will be used to indicate a combination of relays of the same or different types. Properly speaking, the protective relaying system includes circuit breakers as well as relays. Relays and circuit breakers must function together; there i s little or no value in applying one without the other. Protective relays or systenls are not required to function during normal power system operation, but must be immediately availa,ble to handle intolerable system conditions and avoid serious outages and damage. Thus,. the true operating life of these relays can be on the order of a few seconds, even though they are connected in a system for many years. In practice, the relays operate far more during t.esting and maintenance than in response to'adverse service conditions. -

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In theory, a relay system should be able to respond to the infinity of abnormalities that can possibly occur within the power system. In practice, the relay engineer must arrive at a compromise based on the four factors that influence a n y re!oy rrpp!icatisn: a. Economics - Initial, operating and maintenance. b. Available measure of fault or trouble - Fault magnitudes and location of current transformers and voltage transformers. c. Operating practices - Conformity to standard and accepted practices; ensuring efficient system operation. d. Previous experience - History and anticipation perhaps better expressed of trouble likely to be encountered within-thesystem-. The third and fourth considerations are perhaps better expressed as the "personality of the system and the relay engineer". Since it is simply not feasible to design a protective relaying system capable of handling any potential problem, compromises must be made. In general, only those problems, which according to past experience are likely to occur, receive primary consideration. Naturally, this makes relaying somewhat of an art. Different relay engineers will, using sound logic, design significantly different proteclive systems for essentially the same power system. As a result there is little standardisation in protective relaying. Not only may the type of relaying system vary, but also will the extent of the protective coverage. Too much protection i s almost as bad as little. Nonetheless, protective relaying i s a highly specialised technology requiring an in-depth understanding of the power system as a whole. The relay engineer must know, not only the technology of the abnormal, but have a basic understanding of all the system components and their operation in the system. Relaying, then, i s a "Vertical" specialty requiring a "horizontal" viewpoint. This horizontal, or total system, concept of relaying includes fault protection and the performance of the protection system during abnormal system operation such as severe overloads, generation deficiency, out-of-step conditions, and so forth. Although these areas are vitally important to the relay engineer, his concern has not always been fully appreciated or shared by his colleagues. For this reason, close and continued communication between the planning, relay design, and operation systems should be mandatory, since power systems grow and operating conditions change.

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A complex relaying system may result from poor system design or the economic need to use fewer circuit breakers. Considerable savings can be realized by using fewer circuit breakers and a more complex relay system. Such systems usually involve design compromises requiring careful evaluation, if acceptable protection is to be maintained. -

4.0

DESIGN CRITERIA

The application logic of protective relays divides the power system into several zones, each requiring its own group of relays. In all cases, the five design criteria listed below are common to any well-designed and efficient protective system or system segment: a. Reliability - the ability of the relay p r relay system to perform correctly when needed (dependability) and to avoid unnecessary operalion (security). b. Speed - minimum fault time and equipment damage: c. Selectivity - maximum service continuity with minimum system disconnection. d. Economics - maximum protection at minimum cost. e. Simplicity - minimum equipment and circuitry. Since it is impractical to fully satisfy all these design criteria simultaneously the necessary compromhes must be evaluated on the basis of comparative risks.

4.1 Reliability System reliability consists of two elements - dependability and security. Dependability is the certainty of correct operation in response to system trouble, while security i s the ability of the system to avoid mis-operation between faults. Unfortunately, these aspects of reliability tend to counter one another: increasing security tends to decrease dependability and vice versa. In general, however, modern relaying systems are highly reliable and provide practical compromise between security and dependability. Protective relay system must perform correctly under adverse sysfem and environmental conditions. Regardless of whether other systems are momentarily blinded during this period, the relays must perform accurately and dependably. They must either operate in response to trouble in their assigned area or block correctly i f the trouble is outside their designated area.

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Dependability can be checked relatively easily in the laboratory or during installation by simulated tests or staged faults. Security on the other hand is much more difficult to check. A true test of system security would have to measure response to an almost infinite variety of potential transients and counterfeit trouble indicalions in the power system and its environment. A secure system is usually the result of a good background in design combined with extensive miniature power system testing and can only be confirmed in the power system itself and its environment. 4.2 Speed Relays that could anticipate a fa~lltwo!~ldbe utopian. But, even if 'available, they would doubtlessly raise the question of whether or not the fault gr trouble really required a trip-out. The development of faster relays must always be measured against the increased probability of more unwanted or unexplained operations. Time, no matter how short, is still the best method of distinguishing between real and counterfeit trouble.

Applied to a relay, high speed indicates that the operating time usually does not exceed 50 ms (3 cycles on a 60-hertz base). The term instantaneous indicates that no delay is purposely introduced in the operation. In practice, the terms high speed and instantaneous are frequently used interchangeably. 4.3 Selectivity versus Economics High speed relays provide greater service continuity by reducing fault damage and hazards to personnel. These relays generally have a higher initial cost, which, however, cannot always be justified. Consequently, both low and high-speed relays are used to protect power systems. Both types have high reliability records. Records on protective relay operations consistently show 99.5% and better relay performance.

4.4 Simplicity As in any other engineering discipline, simplicity in a protective relay system is always the hallmark of a good design. The simplest relay system, however, is not always the most economical. As previously indicated, major economies are possible with a complex relay system that uses a minimum number of circuit breakers. Other factors being equal, simplicity of design improves system reliability - if only because there are fewer elements that can malfunction.

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FACTORS INFLUENCING RELAY PERFORMANCE

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Relay performance i s generally classed as:

( 1 ) Correct (2) No conclusion ( 3 ) lncorrect lncorrect operation may be either failure to trip or false tripping. The cause of incorrect operation may be, a) Wrong application, b) lncorrect settings, c ) A personnel error or 4) Equipment mal-function. Equipment that can cause an incorrect operation includes current transformers, voltage transformers, circuit breakers, cable and wiring, relays, channels or station batteries. lncorrect tripping of circuit breakers not associated with the trouble area is often as disastrous as c failure to trip. Hence, special care must be taken in both appiication and installation to ensure against the possibility of incorrect tripping. " No conclusion" is the last resort when no evidence is -available for a correct or incorrect operation. Quite often this is a personnel involvement. 6.0 Zones of Protection The general philosophy of relay application is to divide the power system into protective zones that can be protected adequately with the mininwm amount of the system disconnected. The power system is divided into protective zones for:

i1 ii) iii) iv) v) .

Generators Transformers Buses Transmission and distribution circuits Motors

A typical power system and its zones of protection are shown in Figl. The purpose of the protective system is to provide the first line of protection, within the guide-lines outlined above. Since failures .do occur, however some form of backup protection is provided to trip out the adjace13f breakers or zones surrounding the trouble area. Protection in each zone is overlapped to avoid the possibility of unprotected areas

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The device switching equipment are referred to by numbers, with appropriate suffix letters when necessary, according to the functions they perform. These numbers are based on a system adopted as standard for automatic switchgear by IEEE and incorporated in American Standard C37.2 - 1970. 'This system is used in connection diagrams, in instruction books and in specifications.

8.1 Device Numb'erina Device Number Definition Master Elemenl 1

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Function It is an initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly or through such permissive devices as protective and time to delay relays. place an equipment in or out of operation. Time Delay It i s a device which functions to give Starting or a desired amount of time delay before or after any point of Closing Relay operation in a switching sequence or protective relaying system, except as specifically provided by device function 48, 62 and 79 1 described later. Checking or It is a device which operates in response to the position of a number Interlocking of other devices (or to a number of Relay predetermined conditions), in an equipment, to allow an operating sequence to proceed, to stop, or to provide a check of the position of these devices or of these conditions i for any purpose. -

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Overview Of Protection Fundamentals Page 1 1 of 0

Function It is a device, generally controlled by the device No.1 or equivalent, and the required perrr~issive and protective devices, that serve to make and break the necessary control circuits to place an equipment into operation under the desired conditions and to take it out of operation under other or abnormal conditions. Stopping Device It is a control device used primarily to shut down an equipment and hold it out of operation This device may be manually or Electrically actuated, but excludes the function of electrical lockout (see device 1 function 86) on abnormal conditions. 1 Starting Circuit It is a device whose principal) function is to connect a machine.to Breaker its source of staitina voltaae. I Anode Circuit It is one used in-theanode circuits of a power rectifier for the primary Breaker purpose of interrupting the rectifier circuit if an arc back should occur. Control Power It is a disconnecting device - such as a knife switch, circuit breaker or Disconnecting pullout fuse block, used for the Device purpose of connecting and disconnecting the source of control power to and from the control bus or equipment. Note: Control power is considered to include auxiliary power, which supplies such apparatus as sn~all motors and heaters. It is used for the purpose of reversing Reversing a machine field or for performing 1 Device 1 / any other reversina functions. I ( Unit Sequence ( It is used to change the sequence in 1 which units may be placed in and Switch Definition Master Contactor

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The device switching equipment are referred to by numbers, with appropriate suffix letters when necessary, according to the functions they perform. These numbers are based on a system adopted as standard for automatic switchgear by IEEE and. incorporated in American Standard C37.2 - 1970. This system is used in connection diagrams, in instruction books and in specifications.

8.1 Device Numb'erina Device Number -Definition

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Function Master Element It is an initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly or through such permissive devices as protective and time delay relays. to place an equipment in or out of operation. Time Delay It is a device which functions to give Stariing or a desired amount of time delay before or after any point of Closing Relay operation in a switching sequence or protective relaying system, except as specifically provided b y device function 48, 62 and 79 described later. It is a device which operates in Checking response to the position of a number ( Interlocking of other devices (or to a number of Relay predetermined conditions), in an equipment, to allow an operating sequence to proceed, to stop, or to provide a check of the position of these devices or of these conditions

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Device Nurr~ ber 4

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Definition Master Contactor

Function It is a device, generally controlled b y the device No.1 or equivalent. and the required permissive and I protective devices, that sr=rL/rs to make and break the rlecessarY control circuits to place art, equipment into operation vrde! 'the I desired ~onditiof?s end to toke i i out of operation ~ n d m TJ~I-!F-: Or abnormal conditic:;~. ____---. Stopping Device It is a coritiol d,-\:jCe usee ;,:irr~arily I to shut down c;;equipn.5~1: arid) . hold it out of oge:2-i&n- -,;rli: C;e/ice i -. may be mancc:ii. or I:5c.::i~~lly , actuated, but exc,,zss ;r,5 f;,..ctior~ 1 of elect!-ical I C ~ ~ - , - 1-, ~ :;edice ~ 5 ' .. I function 86)on c ~ - - ~ .~ ~ = : :.~__ ; ,-T ! C!J T~ I ~ . ~i , lt i s a device .....--zs5 5:ir!.cipaI / Breaker function is to c o ~ - - ~~ ~; ~- ~ vto! ! ~ r its source of stariir:.~-.G ; T C ~ ~ . - . ( . 2i:i of Anode Circuit It is one used in i.-e a power- recjifie- :-r,gy,gr'j ; - ,:- 4:;'l t l ~ e r Breaker purpose oi inte---, ---2 1 circuit if ar: arc F7 :T;~,.J~. Control Power It i s a discanne;- -; ,I. , - ;:,C~I Disconnecting as a knife switc: L-.-=_ - L-3r;/er or Device pullout fuse b!.zqzq - ---the . ; : I d purpose of I:-------, disconnecting ii-5 5 z . - - z-l -' -_,5MVA). CTs on the HV side are balanced against'CTs on the LV side. There are a number of different connections but there are some important points that are applicable to all schemes. Transformer Connection Consider a deltalstar transformer. An external earth fault on the star side will result in zero sequence current flowing in the line but due to the effect of the delta winding there will be no zero sequence current in the line associated with the delta winding. In order to ensure stability of the protection this zero sequence current must be eliminated from the secondary connections on the star side of the transformer, ie the CTs on the star side of the transformer should be connected in delta. With the CTs on the delta side of the transformer connected in star, the 30" phase shift across the transformer is also catered for. Since the majorlty of faults are caused by flashovers at the transformer bushings, it is advantageous to locate the CTs in the adjacent switchgear. Interposing CT (ICT) Where it is not possible to correct for zero sequence current and the phase shift across the transformer by using delta connected line CT's on the star side of the transformer, or were CT ratio mismatch exists between primary and secondary CT's, then interposing CT's are used. Tranditional ICT's were external devices, however modern numerical relays are able to account for ratio error, phase shift and zero sequence current within the relay. This eliminates the use of external ICT's and allows the protection to be set up and installed more easily. General Rules for CT Connections CT connections opposite to main-transformer : ie.

star CTs on delta side delta CTs on star side

If similar primary terminals ie PI or P2 are towards the transformer, then delta and star connection for the CTs should be the same as the transformer (or 180" opposite). It is usual to assume that if current flows from P i flow from S2 -+ SI.

--+

P2 then the secondary current will

Note :If the transformer induced voltage is A1 --+ A2 then the secondary induced voltage will be a1 -+ a2. Therefor?, current flo'w will be A1 --+ A2 and a2 -+ a1

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Tap Changers Any differential scheme can only be balanced at one point and it is usual to choose CT ratios that match at the mid point of the tap range. Note that this might not necessarily be the normal rated voltage. For example, if the tapping range is +1O0h, -20% then the CT ratio should be based on a current corresponding to the -5% tap. The theoretical maximum out of balance in the differential circuit is then +_ 15%. Three Winding Transformers Differential protection of three winding transformers is essentially similar to that of two winding transformers. The same rules regarding CT connections still apply but the CT ratios used shpuld be based on the MVA rating of one of the windings (usually the highest rated winding) and not on the ratings of each individual winding. For example, consider a 13213311'I kV transformer with windings rated for 100/60/40 MVA respectively, then the current transformer ratios at all voltage levels should be based on 100 MVA, ie 44011. 176011 and 528011 respectively (these effective ratios are normally obtained by the use of interposing CTs which means that, for example, all the main CTs associated with the 11 kV system can be made equal to 200011 - rated current). If there is a source associated with only one of the transformer windings, then a relay with only two bias coils can be used.- the CTs associated with the other two windings being connected in parallel. If there is more than one source of supply then it is necessary to use a relay with three bias windings in order to ensure that bias is available under all external fault conditions.

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Combined Differential and Restricted Earth Fault Protection

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Although it is preferable to use separate CTs for restricted earth fault protection, it can be combined with differential protection using the same current transformers, together with interposing current transformers. A CT is required in the neutral connection and should be the same ratio as the line current transformers.

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DIFFERENTIAL

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Magnetising Inrush

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When a transformer is first energised, a transient magnetising currqnt flows, which may reach instantaneous peaks of 8 to 30 times those of full load. The factors controlling the dirration and magnitude of the magnetising inrush are :

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Size of the transformer bank Size of the power system Resistance in the power system from the source to the transformer bank Residual flux level Type of iron used for the core and its saturation level.

There are three conditions which can produce a magnetising inrush effect :

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i)

First energisation

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Voltage recovery following external fault clearance

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Sympathetic inrush.due to a parallel transformer being energised.

Under normal steaay state cond'itions the flux in the core changes from maximum negative value to maximum positive value duriqg one half of the voltage cycle, ie a change of 2 0 maximum. Since flux cannot instantly be created or destroyed this transformers are normally designed and run at values of flux approaching the saturation value, an increase of flux to double this value corresponds to relationship must always be true. Thus, if the transformer is energised at a voltage zero when the flux would normally be at its maximum negative value, the flux would rise to twice its normal value over the first half cycle of voltage. This initial rise could be further increased if there was any residual flux in the core at the moment the transformer was energised. Since extreme saturation which requires an extremely high value of magnetising current. As the flux enters the highly saturated portion of the magnetising characteristic, the inductance falls and the current rises rapidly. Magnetising impedance is of the order of 2000% but under heavily saturated conditions this can reduce to around 40% ie an increase in magnetising current of 50 times normal, This figure can represent 5 or 6 times normal full load current. Analysis of a typical magnitude inrush current wave shows (fundamental = 100%) :

Component

-DC

2nd H

3rd H

4th H

5th H

6th H

7th H

55%

63%

26.8%

5.1%

4.1%

3.7%

2.4%

The offset in the wave is only restored to normal by the circuit losses. The time constant of the transient can be quite long, typ~cally0.1 second for a 100 KVA transformer and up to 1 second for larger units. Initial rate of decay is high due to the low value of air core reactance. When below saturation level rate of decay is much slower. The magnitude of the inrush current is limited by the air core inductance of the windings under extreme saturation conditions. A transformer with concentric windings will draw a higher magnetising current when energised from the LV side, since this winding is usually on the inside and has a lower air core inductance. Sandwich windings have approximately equal magnitude currents for both LV and HV. ' Resistance in the source will reduce the magnitude current and increase the'iate of decay.

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Effect on Differential Relays Since magnetising inrush occurs on only one side of the transformer, the effect is similar to a fault condition as far as differential protection is concerned. The following methods are used to stabilise the relay during magnetising inrush period.

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Time delayed - acceptable for small transformers or where high speed operation is not so important. (Note : necessary time delay when associated with parallel transformers could be excessive). -

Harmonic restraint - usual to use 2nd H restraint since magnitude inrush current contains pronounced 2nd harmonics. Note : 3rd H restraint should not be used for two reasons : a)

Due to-delta connections in the main transformer and in the CT circuits (which provide a closed path for third harmonic currents), no third harmonic current would reach the relay.

b)

CT saturation under internal fault conditions'also produces harmonics of which the 3rd is the most predominant. Second harmonics are also produced under these conditions (combination of dc offset and fundamental) so excessive saturation of CTs should be avoided.

The problem of any restraining tendency due to 2nd H currents produced by CTs saturating under heavy internal fault conditions is usually overcome by using high set instantaneous un~ts set at 8-10 x rated current. While the second harmonic produces a useful restraint during external faults, it can produce unwanted restraint for Internal faults, due to dc saturation of CTs. Extremely large CTs are required such that they do not saturate and affect the operating times of the differential relay. Gap Detection - If the various current waveforms that occur during magnetising inrush are analysed, it can be found that the magnetising currents have a significant period in each cycle where the current is substantially zero. Fault current, on the other hand, passes through zero very quickly. Detection of this zero is considered a suitable criteria.

Thus, a transformer differential relay can be made to restrain if zero is detected In a cycle for more than a certain period (typically 114f seconds). With the above principle of detection of magnetising inrush, fast operation of the relay can be achieved for internal faults and economically designed CTs can be used, without affecting the speed of operation.

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VARIATION OF EARTHFAULT CURRENTS ON TRANSFORMER WINDINGS An earthfault is the most common type of fault that occurs in a transformer.

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For an earthfault current to flow, the following conditions must be satisfied :

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a path exists for the current to flow into and out of the windings, ie a zero sequence path

the ampere turns balance is maintained between the windings.

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The magnitude of earthfault current is dependent on the method of earthing solid or resistance and the transformer connection.

Star Winding

- Resistance Earthed

An eadhfault on such a winding will give rise to a current which is dependent on the value of earthing impedance and is also proportional to the distance of the fault from the neutral point, since the fault voltage will be directly proportional to this distance. The ratio of transformation between the primary winding and the short circuited turns also varies with the position of the fault, so that the current which flows through the transformer terminals will be proportional to square of the fraction of the winding which is short circuited.

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If the earthing resistor is rated to pass full load current, then

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Assuming V, = V,. then T, = 43 TI

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For a fault at x p.u. distance from the neutrgl,

Effective turns ratio = T2 I x TI

Primary C.T. ratio is based on lF.L. for differential protection.

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C.T. secondary current (on prin~aryside of transformer)

xL

= --F \I

I f differential setting = 20%

For relay operation

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> 20°/c

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thus x > 59% i.e. 59% of winding is unprotected. Differential relay setting

Oh

of winding protected

If as multiple

of ~ F . L .

Pase 10

3

Star Winding

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Solidly Earthed

In this case, the fault current is limited only by the leakage reactance of the winding, which varies in a complex manner with the position of the fault. For the majority of the winding the fault current is approximately 3 x Iflc, reaching a maximum of 5 x Iflc.

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From a study of the various current distributions shown for earth faults, ~tis evident that overcurrent relays do not provide aaequate earth fault protection. If the system is solidly earthed, some differential relays adequately cover the majority of faults, but in general separate earth fault protection is necessary.

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EARTH FAULT PROTECTION

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Balanced earth fault for a delta (or unearthed star) winding can be provlded by connecting three line CTs in parallel (residual connection). The relay will only operate for internal earth faults since the transformer itself cannot supply zero sequence current to the system. The transformer must obviously be connected to an earth source.

It is usual to provide instantaneous earth faultprotection to transformers since it is relatively easy to restrict the operation of the protection to transformer faults only, ie the protection remains-stable for external faults. This protection is called balanced Gr restricted earth fault and the high impedance principle is utilised. However, modern numerical relays provide do provide both high and low impedance restricted earthfault protection.

Source (Earthed)

w

Balanced Earth Fault

For an earthed star winding, the residual connection of line CTs are further connected in parallel with a CT located in the transformer neutral. Under external earth fault conditions the current in the line CTs is balanced by the current in the neutral CT. Under internal fault conditions. current only flows in the neutral CT and since there is no balancing current from the line CTs, the relay will operate. On four wire systems in order to negate the effect of the neutral return current a further CT placed in the neutral and wired in parallel with the existing CT's. On a four wire system with the transformer earthed at the neutral point 5 CT's are required. However, if the transformer is earthed at the LV switch board only 4 CT's are required. If no neutral CT is used then therelay will have to be set above the maximum expected unbalance current in the neutral return.

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A relay, insensitive to the dc component of fault current is normally used for this type of .. . protection. If a "current operated" relay is used, an external stabilising resistor is placed in series with the relay to ensure protection stability under through fault conditions. The protection setting voltage is calculated by conventional methods. To reduce the setting voltage it is often useful to run three cores from the neutral CT in order that the relay is connected across equ~potentialpoints.

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Typical Settings for REF Protection (From ESI 48-3 1977) 10-60% of winding rated cur-rent

Solidly earthed Resistance earthed

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10-25% minimum earth fault current for fault at transformer terminals

Unrestricted Earthfault Protection Unrestricted earth fault or backup earth fault protection can be provided by utilising a single CT mounted on an available earth connection eg transformer neutral, or (on an earthed systern) by using a residual connection of three line CTs. In this case, the relay should be of the inverse or definite time type in order to ensure correct discrimination.

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On resistance earthed system, unrestricted earth fault protection is referred to as standby earth ,..; fault protection. An inverse time relay is used which matches the thermal characteristic of the .:: earthing resistor. Earthing resistors normally have a 30 second rating and are designed to limit : the earth fault current to transformer full load current. 4

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FAULT CURRENT DISTRIBUTION IN TRANSFORMER WINDINGS Under fault conditions, currents are distributed in different ways according to winding connections. Understanding of the various fault current distribution is essential for the design of differential protection. performance of directional relays and settings of overcurrent relays. Fault current distribution on a delta-star transformer, star-star transformer with unloaded tertiary and star-delta transformer with earthing transformer for phase and earthfaults are shown in the diagrams below :

b2 Source

_

_

P

__t

PH-E Fault

I3 Source

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-

B

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c2

PH-PH Fault

Fault Current Distribut~onon a Star - Star Transformer with Unloaded Tertiary

BUCHHOLZ PROTECTION All types o i fault wlth~na transformer w~llproduce heat which will cause decomposition of the transformer oil The resulting gases that are formed rise to the top of the tank and then to the conservator. A buchholz relay connected between the tank and conservator collects the gas and glves an alarm when a certain volume of gas has been collected. A severe fault causes so much gas to be produced that pressure is built up in the tank and causes a surge of oil. The buchholz relay will also detect these oil surges and under these conditions is arranged to trip the transformer circu~tbreakers. The maln advantage of the buchholz re4ay is that it will detect incipient faults which would not oiherw~sebe detected by conventional protection arrangements. The relay is often the only way of detect~nginterturn faults which cause a large current to flow in the shorted turns but due to the large ratlo between the shorted turns and the rest of the winding, the change in terminal currents IS very small

PARALLEL TRANSFORMERS Parallel transformers are typically protected by directional overcurrent and earthfault protection on the LV side set to look back into the transformers. Where an LV bussection exists the directional relays can be replaced by non-directional relays, with the addition of a non-directional overcurrent and earthfault relay at the bus-section.

If a transformer is connected in parallel with another transformer which is already energised, magnetising inrush will occur in both transformers. The dc component of the inrush associated with the switched transformer creates a voltage drop across the line resistance between the source and the transformer. This voltage causes an inrush in the opposite direction in the transformer that was already connected. After a time the two currents become substantially equal and since they flow in opposite directions in the transmission line they cancel and produe no more voltage drop in the line resistance. The two currents then become a single circulating current flowing around the loop circuit made up of the two transformers in series -the rate of decay being determined by the R/L ratio of the transformer.

Page 14

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OVERLOAD PROTECTION Overloads can be sustained for long periods with the limiting factor being the allowable temperature rise in the windings and the cooling medium. Excessive overloading will result in deterioration of insulation and subsequent failure.

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p s far as protection IS concerned, non-harmonic restraint should not be used due to the long time delay required. A harmonic restrained relay should be used for each transformer since if a common relay were used the 2nd harmonic resGaint could be lost due to cancellation as described above.

Overloads can be split into two categories : Overloads which do not reduce the normal expectation of life of the transformers. Overloads in this category are possible because the thermal time constant of the transformer means that there is a con,siderable time lag before the maximum temperature correspond to a particular load is reached. Quite high overloads can therefore be carried for short period.

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Overloads in which an allowance is made for a rapid use of life than normal. The length of life of insulation is not easily determined but it is generally agreed that the rate of using life is doubled for every 6°C temperature increase over the range 80-140°C (below 80°C the use of life can be considered negltgible).

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A hot spot temperature of 98°C gives what may be considered the normal rate of using life, ie a normal life of some tens of years. This temperature corresponds to a hot spot temperature.rise of 78°C above an ambient temperature of 20°C. The graph below indicates the relative'rate of using life against hot spot temperature.

Relative rate of

using life

80 90 100 110 120 130 140 "C

Hot Spot T e m p

Protection f o r Overloads Since overloads cause heating of the transformer above the normal recommended temperatures, protection against overloads is normally based on winding temperature

Page 15

Transformer Setting Tutorials

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Transformer setting Criteria & Tutorials Page 1 of 33

INTRODUCTION

Power Transformer- is one of the most important links in a power system. W~thTransformers of larger capacity , a single transformer fault can cause large interruption to power supplies. If faulted transformer is not isolated quickly, this can cause serious damage and also power system stability problems. Protective system applied to transformers thus play a vital role in the economics and operation of a power system. In common with other electrical plants, choice of suitable protection is governed by economic considerations brought more into prominence Ly the range of size of transformers which is wider than for most items of eiectrical plant. For transformers of the lower ratings , only the simplest protection such as fuses can be justified and for large rating transformers , comprehensive protection scheme should be applied.

Transformer faults are generally classified into four categories: 1 ) Windlng terminal faults' 2) Core faults 3 ) Abnormal operating conditions , such as overvoltage, Overfluxing and overload 4 ) Sustained or uncleared external faults

TRANSFORMER CONNECTIONS With the development of poly phase systems with more complex transformer connections and also poss~ble phase displacement between primary and secondary windings, standardisation was necessary to ensure universal compatability( BS 171: 1970) There are a number of possible transformer connections but the more common connections are divided into four groups. Group1

Odegree phase displacement -

E.g YyO DdO ZdO

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Group2

180degree phase displacement

Group3

30degree Lagphase displacement

E.g Yy6 Dd6 Dz6 E.g Ydl DY1 Yzl

Group4

30degree Leadphase dispiacement E.g Yd 1 1 Dyl 1 Yzl 1

High voltage windings are indicated by capital letters and low voltage windings by snmll letters (reference to high and low is relative). The numbers refers to positions on a clock face and indicate the phase displacement of the low voltage phase to neutral vecior , e.9,Yd 1 indicates that the low voltage phase vectors lag the high voltags phase vectors by 30 degree (-30 degree phase shift] Individual phases are ind~catedby the letters A,B &C , again capital letters for the low voltage winding. All windings on the same limb of a core are given the same letter. A further numerical subscript serves to differentiate between each end of the winding.

PROTECTION APPLIED TO TRANSFORMERS Over current and earth fault protection(Unrestricted)

Plain overcurrent and earth fault protection utilising IDMTL relays are used primarily to protect the transformer against the effects of exiernal short circuits and excess overloads. The current settings of the protection must be above the permitted sustained over load allowance and below the minimum short circuit current. The ideal characteristic i s the extremely inverse (CDG14)as it is closely approximates to the thermal curve of the transformer. The protection is located on the supply side of the transformer and is arranged to trip both the H V and LV circuit breakers. In many cases the requirements for protecting the transformer and maintaining discrimination with similar relays in the remainder of the power syslern are not corilpatibile. In these circumstances , negative sequence filter protecrion or under voltage blocking may be used to obtain the desired ser?sii~vity.. 1 f

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1 . High set overcurrent cut-Off: On small transformers where the main protection is provided with overcurrent devices and where the transformer i s fed from one end only, a high set instantaneous relay i s utilised to provide protection against terminal and internal winding faults. The ,relay is set to be above the short circuit level on the secondary(load ) -side of the transformer and below: that for a terminal fault on the primary (supply)sideof the transformer. On choosing the type and setting of the high set relay, it i s important to consider the magnetising inrush currents under normal switching , offset fault currents and starting currents of motors.The first two problems can be overcome by using a relay sensitive only to fundamental frequency currents, while the third is overcome by setting the relay above the max. starting current level.

2. Stand-by earth fault protection Where transformers are earthed via an earthing resistance which is short time rated , stundby earth fault protection is applied to protect the resistor from damage when an earth fault persists for a time longer than the rating of the resistors. The relay is energised from a CT in the neutral connection and its time of operation is made to match the thermal rating of the resistor. It is arranged to completely isolate . the transformer. Some times a two stage relay is employed, each stage set to operate at a different time. The first staqe arranged to trip the LV breaker and if still the fault is persisting, Ihe second stage relay trips the HV side breaker thus isolating the transiormer completely.

DIFFERENTIAL PROTECTION

The funciion of differential protection is to provide faster and more discriminative phase fault protection than that obtainable from simple over current relays.CTs on the primary and the secondpry sides

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are connected to form a circulating current system. The following

figure illustrates the principle.

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Basic Considerations for transformer differential protection

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1. Line current transformer primary ratings

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The rated currents of the primary and the secondary sides of a two winding transformer will depend on the MVA rating of thetransformer and will be in inverse ratio to the corresponding voltages. For three winding transformers the rated current will depend on the MVA rating of the relevar-rt winding. Line current transformers should therefore have primary ratings equal to or above the rated currents of the transiormer windings to which they are applied. -

2. Current transformer connections

The CT connections should be arranged , where necessary to compensate for phase difference between line currents on each side of the power transformer. If the transformer is connected in delta/star as shown in figure, balanced three phase through current suffers a phase angle of 30 degree which must be corrected in the CT secondary leads by appropriate connection of the CT secondary wind~ngs. Further more , zero sequence current flowing on the star side of the power transformer .will not produce current outside the delta on the other side. The zerosequence therefore be eliminated from the star side by connecting the CTs in delta, from which i t follows that the CTs on the delta side of the transformers must be connected in star, in order to give 30 degree phase shift. This is a general rule ; if the #

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Figure 1

When Cis are connected I n delta, their secondary rating must be 1 / 43 times the secondary rating of star connected reduced to Cis , inorder that the currents outside the-delta may balance with the secondary currents of the star connected CTs. When line CT ratios proiide adequate matching between currents supplied to the differential relay under through load and through fault conditions , the necessry phase shift can be obtained by suitable connection of the' line CTs . Figure 1 above shows the required connections for various power transformer winding arrangemenis. When delta connected CTs are required it is a common practice to use star conr;ecled line CTs and to obtain fhe delta connection by means of stal-/delta interposing CTs.

3. Bias to cover Tap-Changing facility and CT mismatch

If the transfor-n~er hcs a tapping range enabling its ratio to be varied , this must be allowed for in the differential system. This is because the CTs selecled to balance, for the mean ratio of the power transformer, , a variation in ratio from the mean will create an unbalance proportional to the ratio change. At maximum through fault current , the spill oputput produced by the small percentage unbalance may be substantial. Differential protection should be provided with a proportional bias of at-\ amount which exceeds in effect the maximum ratio deviation. This stabilises ihe protection under through fault conditions while still permitting the system to have good system sensitivity. The bias characteristic for a typical differential protection is shown in figure2, iron-\ which it can be seen that the cursent required to operate the relay increases as the through fault increases.

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When applying a differential relay, care should be taken that its characteristic will prevzr-it operation due to the combination of tap change variation and CT mismatch . To mininiise the effect of tap change variations , current inpuis lo the differeniial relay are usually matched at the mid poini of the tap range..

Figure 3 below shows percentage biased differential protection for a two winding transformer. The two bias windings per phase are conimonly provided on the same electromagnet or auxiliary 1r-a~isfornier core. .

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Advanced Industrial Power System Protection -

The Merz-price principle remains valid for a system having more than two connections, so a transformer with three or more windings can still be protected by the application of above principles. When the power transforn~erhas only one of i t s three windings connected to a source of supply, with the other two windings feeding loads at differ-er~tvoltages, a relay of the same design. as that used for two winding transformer can be employed.

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Figure 5 The pro!eciiori of three winding transfor-nier-sis complicated by the fact that line CTs for each winding ar-e riormally based on different MVA levels and will 1701 ttienlselves achieve balance under ttirough current ronditior~s.To achieve correct balarice , it is necessary to use inlerposir~gCTs wllicti hlill provide the relay with raied currerli when the rating of the highest rated winding is applied to all windings.

An exaniple for a 500KV/138KV/13.45KV, 120MVA/90MVA/30MVA, star/slar/delta transfornler is shown in the figure 6. Load cur rent a i 599

KV =

120 x 10" - ~ x500x 3 10"

= 138.6A

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Load current at 138 KV = 90 x 106 ~ 3 138x x 103 Load current at 13.45 KV = 30 x 106 i 3 x 13.45 x 103

= 376.5 A

= 1288 A

Line CT ralio at 500KV = 20015 A I-ine CT ratio at 138KV = 40015 A . Line CT ratio at 13.45KV = 150015 A Current at 138 KV corresponding to 120 MVA = 120 x 1 O6 3 x 138x lo3

Current at 13.45 KV corresponding to 120 MVA = 120 x 1 O6 \I3 x 13.45 x lo"

= 502 A

~ 5 1 5 1A

Secondary current from 500KV line CTs corresponding to 120 MVA =138.6 x 5 =3.46 A 200

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SOMVA 1 W V

RELAY FATED CURKEICT Figure 6

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-. lnele iore ratio of required starldelta interposing CTs = 3.461 5 A OR 3.461 2.89 A

\'3 Secondary current from 138 KV line CTs corresponding io 120 M V A

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There fore ratio of required starldelta interposing CTs

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Secondary current from 13.45 KV line CTs corresponding to 120 MVA = 5151 x 5 = 17.17 A 1 500 Therefore ratio of required star/star interposing CTs = 17.1715 A

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Under full load conditions of 30MVA, for the 13.45 KV delta winding , the current appearing in the primary of the 17.17/5 A inter posing CT will only be 4.29 A , the corresponding secondary current being 1.25 A . However the ratings of the primary and the secondary windings should ideally be 17.1 7 A and 5 A respectively to minimise winding resistances.

STABILIZATION OF DIFFERENTIAL PROTECTION DURING MAGNETlSlNG INRUSH Tl~en~agnetisinginrush phenomenon produces current input to the

energised winding which has no equivalent on the other sides of the transformer. The whole of the inrush current appears therefore as unbalance and is no1 distingushable from internal fault current. The normal bias is not , iherefore effective and increase of protection setting to a value which would avoid the operation would make the protection of little value. Harmonic Restraint.

The inrush current, clthough y!~.nerallyresembling an inzone fault current, differs greaily when the waveforms are compared. The distinctive difference in the4 woveforms can be used to distinguish

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between the condition;. The inrush contains all orders of harmonics, but these are not all equally suitable for providing bias. The study of this svbject is complex, as the wave form depends on the degree of saturation and on the grade of iron in the core.

a) D.C Offset component (Zero harmonic) A uni-directional component will usually be present in the inrush current of the single phase transformer and in the principal inrush currents of a three phase unit. However if at the instant of switching the residual flux for any phase is equal to the flux which would exist in the steady staie at that point on the voltage wave , then no transient disturbance should take place on that phase.

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Large inrush c ~ r ~ - e nwill i s flow in the other two phases , corresponding to high peak lux values established in these phase cores. The high flux circulates through the yokes , the saturation of which affects the iir-s: phase , L-:iiich. would have had no inrush effect, causing c substantial transient current to flow in this phase as well. This latter current , however will not be off set from the zero axis , althougt-1the current waveform will be distorted.. If the uni-directional current component were used to stabilise a

differential sysiem, some sort of cross phase biasing would be of this effect. required becc~lse

b) Second Harmonic component This connpone!3t is present in all inrush wave forms . It is typical of wave forms in which successive half period portions do not repeat with reversal oi polarity but-in which the mirror image symmetry can be found abo:lt certain ordinates. The portion of second harmonics varies some what with the degrce of saturation of core , but is always present as long as the unidireciior~ai~ormponentof flux exists. It has been shown to have a minimum value of about 20% of the amount by which the inrush current exceecjs the steady state magnetising current.

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Normal fault current do not contain second or other even harmonics, nor do distorted currents flowing in saturated iron cored coils under steady state conditions. -

The output current of a current transformer which is energized into t steady state saturation will also contain odd harmonics but i ~ oeven harmonics. However, should the current transformer be saturated by the transient component of the fault current, the resulting saturation i s not symmetrical and even harmonics are introduced into the output current. This can have the advantage of improving the through fault stability performance of a differential relay, but it also has the adverse effect of increasing the operatioh time for internal faults. The second harmonic is therefore an attractive basis for a stabilizing bias against inrush effects, but care must be taken to ensure that the current transformers are sufficiently large so that the harmonics produced by transient saturation do not delay normal operation of the relay. .

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The differential current is passed through a filter which extracts the second harmonic; this component is then applied to produce a restraining quantity sufficient to overcome theoperating tendency due to the whole of the inrush current which flows in the operating circuit. By this means a sensitive and high speed system can be obtained. With the type DTH relay, a static design, a setting of 15% is obtained with an operating time of 45 milliseconds for all fault currents of twice or more times the current rating. The relay will restrain when the second harmonic component exceeds 20% of the current. Third harmonic The third harmonic is also present in the inrush current in roughly comparable proportion to the second harmonic. The separate phase inrush currents are still related in phase to the primary applied electromotive forces and the harmonics have a similar time spacing, which brings the third harmonic waves in the three windings into phase. If the windings are connected in delta, the line currents are each the difference of two phase currents. As the inrush components vary during the progress of the transient condition it is possible for this qifference to pass,through zero, so that the third

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harmonic component in the line current vanishes; this component, therefore, be regarded as a reliable source of bias. To this must be added the further consideration that a sustained third harmonic component is quite likely to be produced by CT saturation under heavy in-zone fault .conditions. All this means that the third harmonic is not a desirable means of stabilizing a protective system against inrush effects.

d. Higher harmonics , All other harmonics are theoretically present in inrush current but the relative magnitude diminishes rapidly as the order of harmonic increases; there may be 5% of fourth harmonic in a given inrush 'current. This component would be similar in response to the second harmonic but the small magnitude hardly justifies the provision of an extra filter circuit. ,

A still smaller proportion of fifth harmonic will be present. This component is not subject to cancellation as is the third harmonic, and can be present in the output of a CT in an advanced state of saturation, therefore offering no benefit. Still hlgher harmonics are of magnitude too small to be worth consideration. The percentage of fifth harmonic in the transformer magnetizing current increases significantly when the transformer is subjected to a temporary overvoltage condition. Some manufacturers apply a measure of fifth harmonic bias to the relay to restrain operation for this condition. Typically such relays are restrained if the magnetizing current contains 30% fifth harmonic.

RESTRICTED EARTH FAULT PROTECTION

A simple overcurrent and earth fault system will not give good

protection cover for a star-connected primary winding, part~cularlyil the neutral is eaithed through an impedance. The degree of protection is very much improved by the application of a ur-lil differential earth fault system or restricted earth fault protection, as shown in Figure 7. The residual current of three line current

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transformers is balanced against the output of a current transformer in the neutral conductor. The relay is of the high impedance type.

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The system is operalive for faults within the region between current transformers, t h a t Is, fs: fai;lts on the star winding in question. The system will remain stable for all faults outside this zone

HIGH lMPEDANCE RELAY

Restric ted earth fault protection for a star winding. Figure 7 The gain in protection performance comes not only from using an instantaneous relay with a low setting, but also because the whole fault current i s measured, not merely the transformed component in the HV primary winding. Hence, although the prospective current level decreases as fault positions progressively nearer the neutral end of the winding are considered, the square law which controls the primary line current is notapplicable, and with a low effective setting, a good percentage of the winding can be covered. Restricted earth fault protection is often applied even when the neutral is solidly earthed. Since fault current then remains at a high value even to the last turn of the winding , virtually complete cover

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P a g e 1: for earth faults if obtained, which is a gain compared with the performance of systems which do not measure the neutral conductor current.

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Earth fault protection applied to a delta-connected or unearthed star winding in inherently restricted, since no zero sequence component can be transmitted through the transformer to the secondary system. A high impedance relay can therefore be used, giving fast operation and phase fault stability.

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For HV generators, impedance is usually inserted i n stator earthing connection t o limit the magnitude earth fault current. There is a wide variation i n the e: fault current chosen, common values being: 1. rated current

2 . 200A-400A (low impedance earthing] 3. IOA-20A (high impedance earthing)

A modern yencrating u n i t is a complex system comprising the generator stator winding, associated transformer and u n i t transformer (if present), the rotor w i t h its field winding and excitation system, and the prime mover w i t h its associated auxiliaries. Faults of many kinds can occur wi:hin this syslem for which diverse forms of electrical and mechanical protection are

The main methods of impedance-earthing a genet are shown i n Figure 17.3. Low values of earth ' current may limit the damage caused from a fault they simultaneously make detection of a fault tov the stator winding star point more difficult. Excel special applications, such as marine, LV generator normally solidly earthed t o comply w i t h s requirements. Where a step-up transformer is ap

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the generator and the lower voltage winding o f the transformer can be treated as an isolated system that is not influenced by the earthing requirements o f the power system.

sufficient that the transformer be designed to have a primary winding knee-point e m f . equal t o 1.3 times the generator rated line voltage.

Failure of the stator windings or connection insulation can result in severe damage to the windings and stator core. The extent of the damage will depend on the magnitude and duration of the fault current.

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An earthicg transformer or a series impedance can be impedance. If an earthing transformer is ntinuous rating is usually in the range 5250kVA. The secondary winding is loaded with a resistor of a value which, when referred through the rransformrr turns ratio, will pass the chosen short-time earth-fault current. This is typically i n the range of 5-20A. The resistor prevents the production o f high transient overvoltages i n the event of an arcing earth fault, which i t does by discharging the bound charge i n the circuit Capacitance. For this reason, the resistive component of fault current should not be Icss. than the residual Capacitance current. This is the basis of the design, and in practice values of between 3-5 I,,, are used. It is .important that the earthing transformer never

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becomes saturated; otherwise a very undesirable Condition of ferroresonance may occur. The normal rise ofthe generated voltage above the rated value caused by a sudden loss of load or by field forcing must be as well as flux doubling in the transformer point-on-wave o f voltage application. 11 is

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The most probable mode of insulation failure is phase to earth. Use of an earthing impedance limits the earth fault current and hence stator damage. An earth fauit involving the stator core results in burning of the iron at the point of fault and welds laminations together. Replacement of the faulty conductor may not be a very serious matter (dependent on set rating/voltage/construction) but the damage to the core cannot be ignored, since the welding o f laminations may result in local overheating. The damaged area can sometimes be repaired, but i f severe damage has occurred, a partial core rebuild will be necessary. A flashover is more likely to occur in the end-winding region, where electrical stresses are highest. The resultant forces on the conductors would be very large and they may result in extensive damage, requiring the partial or total rewinding o f the generator. Apart from burning the core. the greatest danger arising from failure to quickly deal with a fault is fire. A large portion of the insulating material is inflammable, and in the case of an air-cooled machine, the forced ventilation can quickly cause an arc flame to spread around the winding. Fire will not occur in a hydrogen-cooled machine, provided the stator system remains sealed. In any case, the length of an outage may be considerable, resulting in major financial impact from loss of generation revenue and/or import o f additional energy.

Phase-phase faults clear o f earth are less common; they may occur on the end portion of stator coils or in the slots if the winding involves two coil sides i n the same slot. In the latter case, the fault will involve earth i n a very short time. Phase fault current is not limited by the method of earthing the neutral point.

lnterturn faults are rare, but a significant fault-loop current can arise where such a fault does occur.

Conventional generator protection systems would be blind t o a n interturn fault, b u t the extra cost and complication of providing detection of a purely interturn fault is n o t usually justified. I n this case, an interturn fault must develop into an earth fault before it can be c!eared. An exception may be where a machine has an abnormally complicated or multiple winding arrangement, where the probqbility o f an interturn fault might be increased.

calculation, after measurement of the individual q : secondary currents. I n such relay designs, there is full.: galvanic separation o f the neutral-tail and terminal Q.; secondary circuits, as indicated i n Figure 17.5(a). This is not the case for the application of high-impedance differential protection. This difference can impose some special relay design requirements t o achieve s t a b i l i t y f ~ ~ , biased differential protection i n some applications. .@

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To respond quickly t o a phase fault w i t h damaging heavy current, sensitive, high-speed differential protection is normally applied t o generators rated in excess of 1MVA. For large generating units, fast fault clearance will also maintain stability of the main power system. The zone of differential protection can be extended t o include an associated step-up transformer. For smaller generators, IDMT/instantaneous overcurrent protection is usually the only phase fault protection applied. Sections 17.5-17.8 detail the various methods that are available for stator winding protection.

The relay connections for this form of protection are shown in Figure 17.5(a) and a typical bias characteristic is shown i n Figure 17.5(b). The differential current threshold setting I,, can be set as low as 5% o f rated :. generator current, to provide protection for as much of: the winding as possible. The bias slope break-point$& threshold setting I;, would typically be set t o a value.?$, above generator rated current, say 12O01o, to achieve::?. , external fault stability i'n the event of transient,{2 asymmetric CT saturation. Bias slope I;, setting would:? - ! typically be set at 150%. > .: c:

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The theory o f circulating current differential protection is discussed fully in Section 10.4. Stator

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High-speed phase fault protection is provided, by use of the connections shown i n Figure 17.4. This depicts the derivation of differential current through CT secondary circuit connections. This protection may also offer earth fault protection for some moderate impedance-earthed applications. Either biased differential or high impedance differential techniques can be applied. A subtle difference w i t h modern, biased. numerical generator protection relays is that they usually derive the differential currents and biasi'ng currents by algorithmic i

This d~ffersfrom biased differential protection by manner in which relay stability is achieved for eXt faults and by the fact that the differential current be attained through the electrical connections secondary circuits. I f the impedance of each re Figure 17.4 is high, the event oC one CT bec saturated by the through fault cutrent (leadin!

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17/06/02

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1 0 ~ 4 4 page 285

latively low CT impedance), will allow the current from unsaturated CT t o flow mainly through the saturated rather than through the relay. This provides the uired protection stability where a tuned relay element is employed. I n practice, external resistance is added t o the relay circuit i o prwidc the necessary high impedance. The principle of high-impedance protection application is illustrated i n Figure 17.6, together with a summary of the calculations required t o determine ttie value o f external stabilising resistance.

To calculate the primary operating current, the following expression is used:

I,, = N X(is, + nl,) where:

lop = prima y operating current

N = CT ratio ls = relay l setting

n = number of CT's in parallel with relay element I, = CT magnetising currerft at

i

Hcalthy CT

Saturated CT

*I L', = K V ,

wherc J . O < K r l . S Stabilising resistor, R,, limits spill currcnt to