Protection of Turbogenerator

CHAPTER – 1 INTRODUCTION

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1.1 INTRODUCTION TO VSP Visakhapatnam steel plant, the first coal based steel plant in India is located, 16km South West city of Destiny i.e. Visakhapatnam. Bestowed with modern technologies, VSP has an installed capacity of 3 million tons per annum of liquid steel and 2.656 million tones of saleable steel. At VSP there is emphasis on total automation, seamless integration and efficient up gradation, which result in wide range of long and structural products to meet stringent demands of discerning customers within India and abroad.VSP products meet exactly international quality standards such as JIS, DIN, BIS, BS etc. VSP is certified to all the three international standards of quality. ISO- 9001 for Quality management, ISO-14001 for Environmental Management system and OHSAS-18001 for Occupational health and safety. The certificates covers quality systems of all operational maintenance service units besides purchase systems, training and marketing functions spreading over four regional marketing offices & 22 stock yards located all over country. VSP by successfully installing and operating efficiently RS.460 crores worth of pollution control and Environment control equipments and converting the barren landscape by planting more than 3 million plants has a made the Steel Plant , Steel Township and surrounding areas into heaven of lush greenery. This has made Steel Township a greener, cleaner and cooler place, which can boost of 3 to 4ºc lesser temperature even in the peak summer compared to Visakhapatnam city. VSP exports Quality Pig Iron and Steel products to Sri Lanka, Myanmar, Nepal, Middle East, and USA. RINL-VSP has awarded “STAR TRADING HIUSE” status during 1997-2000. Having established a total manpower of about 16,613 VSP has envisaged a labour productivity of 265 tonnes per man year of Liquid Steel which is the best in the country and comparable with international levels.

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1.2 INTRODUCTION TO THERMAL POWER PLANT Thermal power plant (TPP) is the captive power plant in VSP. The average power demand at all units of VSP when operating in full capacity will be 221MW. The captive power generation capacity of 270 MW is sufficient to meet all plant needs in normal operating time. The objective of TPP is to supply uninterrupted essential power. In case of partial outage of power generation capacity due to break down, shutdown or other reasons, the shortfall of power is available from APSEB grid. The agreement with APSEB provides for exporting of surplus power to APSEB. The TPP can only generate 247.5MW of power; rest of the power is generated by Back Pressure Turbines and Gas Expansion turbines located at other departments in the VSP. TPP has 3 Turbo Generators of each 60mw and 1 Turbo Generator of 67.5MW. The Generators are air cooled and the turbines have Electro Hydraulic Turbine Governing System. Central admission of steam is used to reduce axial thrust.

TPP also meets the air blast

requirements of Blast Furnaces through 3 Turbo Blowers. TPP has 5 Boilers each of CAPACITY -330 Tons/ Hr, PRESSURE- 101 ata and TEMPERATURE - 540o C. The fuel used in boilers is composed of Pulverized coal, Blast Furnace gas, Coke-oven gas, Furnace oil. Oil is generally used to create initial and quick ignition. The calorific values and the composition of fuel used in the boilers are shown in below table. Sl. No

Name of the fuel

Percentage of fuel

Calorific value

1

Blast furnace gas(BFG)

23%

800 kcal/nm3

2

Coke-Oven Gas(COG)

30%

4425 kcal/nm3

3

Furnace Oil

0.07%

10000 kcal/kg

4

Coal

46.8%

3200-4500 kcal/kg

3

Table no: 1 The entire plant is configured as 5 electrical load blocks (LBSS 1-5) and step-down substations are provided in each block with 220kv transformers to step-down to 33/11/6.6Kv for further distribution.

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CHAPTER – 2 INTRODUCTION TO PROTECTION

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2.1 POWER SYSTEM PROTECTION: The word 'protection' is generally used to describe the whole concept of protecting a power system. Power system protection is the process of making the production, transmission, and consumption of electrical energy as safe as possible from the effects of failures and events that place the power system at risk. The function of protective equipment is not the preventive one its name would imply, in that it takes action only after a fault has occurred: it is the ambulance at the foot of the cliff rather than the fence at the top. The special equipment adopted to detect such possible faults is referred to as ‘protective equipment or protective relay’ and the system that uses such equipment is termed as ‘protection system

2.2 NECESSITY OF PROTECTION: It is fair to say that without discriminative protection it would be impossible to operate a modern power system. The protection is needed to remove as speedily as possible any element of the power system in which a fault has developed. So long as the fault remains connected, the whole system may be in jeopardy from three main effects of the fault, namely: •

It is likely to cause the individual generators in a power station, or groups of generators in different stations, to lose synchronism and fall out of step with consequent splitting of the system;

•

A risk of damage to the affected plant; and

•

A risk of damage to healthy plant.

There is another effect, not necessarily dangerous to the system, but important from the consumers' viewpoint, namely, a risk of synchronous motors in large industrial premises falling

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out of step and tripping out; with the serious consequences that entails loss of production and interruption of vital processes. It is the function of the protective equipment, in association with the circuit breakers, to avert these effects. This is wholly true of large H.V. networks, or transmission systems. In the lower-voltage distribution systems, the primary function of protection is to maintain continuity of supply. This, in effect, is achieved incidentally transmission systems if the protection operates correctly to avert the effects mentioned above; indeed it must be so, because the ultimate aim is to provide 100percent continuity of supply. Obviously this aim cannot be achieved by the protection alone. In addition the power system and the distribution networks must be so designed that there are duplicate or multiple outlets from power sources to load centers (adequate generation may be taken for granted), and at least two sources of supply (feeders) to each distributing station. There are certain conventional ways of ensuring alternative supplies, as we shall see, but if full advantage is to be taken of their provision (always a costly matter) the protection must be highly selective in its functioning. For this it must possess the quality known as discrimination, by virtue of which it is able to select and to disconnect only the faulty element in the power system, leaving all others in normal operation so far as that may be possible. With a few exceptions the detection and tripping of a faulty circuit is a very simple matter; the art and skill lie in selecting the faulty one, bearing in mind that many circuits generators, transformers, feeders are usually affected, and in much the same way by a given fault, this accounts for the multiplicity of relay types and systems in use.

2.3 BASIC REQUIRMENTS OF PROTECTION: A protection apparatus has three main functions/duties: •

Safeguard the entire system to maintain continuity of supply

•

Minimize damage and repair costs where it senses fault

•

Ensure safety of personnel.

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These requirements are necessary, firstly for early detection and localization of faults, and Secondly for prompt removal of faulty equipment from service.

In order to carry out the above duties, protection must have the following qualities: •

Speed

•

Selectivity

•

Sensitivity

•

Reliability

•

Simplicity

•

Economy

2.3.1 Speed: Protective relaying should disconnect a faulty element as quickly as possible. This is desirable for many reasons. •

Improves power system stability.

•

Decreases the amount of damage incurred.

•

Reduces annoyance to electric power consumers and decreases total outage time for

•

power consumers.

Decreases the likelihood of development of one type of fault into other more severe type.

•

Permits use of rapid reclosure of circuit breakers to restore service to customers.

To decrease the time taken to disconnect the faulty element of the system, high speed protection should be operated in conjunction with high speed circuit breakers. The time interval within which a faulty system is disconnected from the system is called “clearing time” which is the sum of operating time of the protective relaying and breaker interrupting time.

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Modern high-speed protective relaying has operating time 0.02 to 0.04 sec. and CBS have interrupting time 0.05 to 0.06 sec. Hence clearing time may be about 0.07 to 0.10 sec.

2.3.2 Selectivity: It is ability of the protective systems to determine the point at which the fault occurs and select the nearest of circuit breaker tripping of which will lead to clearing of fault with minimum or no damage to the system. In fact, opening of any other breaker to clear the fault will lead to greater part of the system being isolated. Therefore every time a fault occurs, only those breakers which are nearest to the fault should be opened. This gives us an idea about dividing the power system into protective zones which can be adequately protected with minimum part of the system isolated. Any failure occurring within a given zone will cause the opening of all breakers within that zone. The system can be divided into the following protective zones. •

Generator or generator transformer units.

•

Transformers

•

Bus bars

•

Transmission lines

•

Distribution circuits

2.3.3 Sensivity: It is the capability of the relaying to operate reliably under the actual conditions that produce the least operating tendency. It is desirable to have the protection as sensitive as possible in order that it shall operate for low values of actuating quantity.

2.3.4 Reliability:

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The protection relaying must be ready to function reliable and correct in operation at all times under any kind of fault and abnormal conditions of the power system for which it has been designed.

2.3.5 Simplicity: Simplicity of construction and good quality of the relay, correctness of design and installation qualified maintenance and supervision etc. are the main factors which influence protective reliability. As a rule, the simple the protective scheme and the lesser the no. of relays, circuits and contacts it contains, the greater will be its reliability.

2.3.6 Economy: As with all good engineering design economics play a major role. Too much protection is as bad as to little and the relay engineer must strike a sensible with due regard to practical situation considered

2.4 BASIC COMPONENTS OF PROTECTION: Protection of any distribution system is a function of many elements .Following are the main components of protection. •

Relays

•

Instrument Transformers

•

Circuit breakers

•

Tripping Batteries

2.4.1 Relays:

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A relay is a device which makes a measurement or receives a controlling signal in consequence of which it makes sudden pre-determined changes in one or more electrical circuits. A protective relay is a relay which responds to abnormal conditions in an electrical power system, to control a circuit-breaker so as to isolate the faulty section of the system, with the minimum interruption to service.

Functions of Protective Relaying The function of protective relaying is to cause the prompt removal from service of any element of the power system. When it suffers a short circuit, or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system. The relaying equipment is aided in this task by circuit breakers that are capable of disconnecting the faulty element when they are called up to do so by the relaying equipment. Circuit breaker is generally located so that each generator, transformer, bus, transmission line etc. can be completely disconnected from the rest of the system. Fusing is employed where protective relays and circuit breakers are not economically justifiable. Although the principal function of protective relaying is to mitigate the effects of short circuits other abnormal operating condition arise that also require service of protective relaying. A secondary function of a protective relaying is to provide indication to the location and type of failure. Such data not only assist in expediting repair but also, by comparison with human observation and automatic oscillograph records, they provide means of analyzing the effectiveness of the fault prevention and mitigation features including the protective relaying itself.

CLASSIFICATION OF PROTECTIVE RELAYS: Protective relays can be broadly classified into three types

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•

Electromagnetic relay

•

Static relay

•

Micro processor based relay

Electromagnetic: An Electromagnetic relay has one or more coils, movable elements, contact system etc. The operation of such relay on whether operating torque / force are greater than the restraining torque / force. When the actuating quantity exceeds a certain predetermined value, an operating torque is developed which is applied on the moving part. This causes the moving part to travel and to finally close a contact to energize the trip coil of the circuit breaker. Electromagnetic relays include attracted armature, moving coil, and induction disc and cup type relays.

Static Relays: Static relays contain electronic circuitry, which may include transistor, ICs, Diodes, and Logic Gates etc. There is compactor circuit in the relay, which compares two or more currents or voltages and gives an output, which is applied to either a slave relay or a thyristor circuit. The slave relay is an electromagnetic relay, which finally closes a contact. A static relay containing a slave relay is a semi static relay. The electromagnetic relay used as a slave relay provides a number of output contacts at low cost. A relay using a thyristor circuit is a wholly a static relay. Static relay possess the advantage of having low burden on the CT and PT fast operation, absence of mechanical inertia and contact trouble, long life and less maintenance.

Microprocessor Based Relays:

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Microprocessor based relays are the latest development. With the developments in VLSI technology, sophisticated and fast microprocessors are coming up. The main features, which have encouraged the design and development of microprocessor based, protective relays, are their economy, compactness, reliability, flexibility and improved performance over conventional relays. A number of relaying characteristics such as over current, directional, impedance, reactance, mho, quadrilateral, elliptical etc. can be obtained using the same interface. Using a multiplexer, a microprocessor can get the desired signals to obtain particular relaying characteristics. Different programs are used to obtain different relaying characteristics using the same interfacing circuitry, Microprocessor based protective schemes have attractive compactness in addition to flexibility.

2.4.2 INSTRUMENT TRANSFORMERS: The voltage transformers and current transformers continuously measure the voltage and current of an electrical system and are responsible to give feedback signals to the relays to enable them to detect abnormal conditions. The values of actual currents in modern distribution systems varies from a few amperes in households, small industrial/commercial houses, etc. to thousands of amperes in power-intensive plants, national grids, etc., which also depend on the operating voltages. Similarly, the voltages in electrical systems vary from few hundreds of volts to many kilo volts. However, it is impossible to have monitoring relays designed and manufactured for each and every distribution system and to match the innumerable voltages and currents being present. Hence the voltage transformers and current transformers are used which enable same types of relays to be used in all types of distribution systems ensuring the selection and cost of relays to be within manageable ranges. The main tasks of instrument transformers are: •

To transform currents or voltages from usually a high value to a value easy to handle for relays and instruments.

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•

To insulate the relays, metering and instruments from the primary high-voltage system

•

To provide possibilities of standardizing the relays and instruments, etc. to a few rated currents and voltages.

Instrument transformers are special versions of transformers in respect of measurement of current and voltages. The theories for instrument transformers are the same as those for transformers in general.

Basic theory of operation: The transformer is one of the high efficient devices in electrical distribution systems, which are used to convert the generated voltages to convenient voltages for the purpose of transmission and consumption. A transformer comprises of two windings viz., primary and secondary coupled through a common magnetic core. When the primary winding is connected to a source and the secondary circuit is left open, the transformer acts as an inductor with minimum current being drawn from the source. At the same time, a voltage will be produced in the secondary open-circuit winding due to the magnetic coupling. When a load is connected across the secondary terminals, the current will start flowing in the secondary, which will be decided by the load impedance and the open-circuit secondary voltage. A proportionate current is drawn in the primary winding depending upon the turn’s ratio between primary and secondary. This principle of transformer operation is used in transfer of voltage and current in a circuit to the required values for the purpose of standardization. A voltage transformer is an open-circuited transformer whose primary winding is connected across the main electrical system voltage being monitored. A convenient proportionate voltage is generated in the secondary for monitoring. The most common voltage produced by voltage transformers is 100–120 V (as per local country standards) for primary voltages from 380 V to 800 kV or more. However, the current transformer is having its primary winding directly connected in series with the main circuit carrying the full operating current of the system. An equivalent current is produced in its secondary, which is made to flow through

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the relay coil to get the equivalent measure of the main system current. The standard currents are invariably 1 A and 5 A universally.

Voltage transformers There are basically, two types of voltage transformers used for protection equipment. •

Electromagnetic type (commonly referred to as a VT)

•

Capacitor type (referred to as a CVT).

Electromagnetic type The electromagnetic type is a step down transformer whose primary (HV) and secondary (LV) windings are connected as below.

Fig 1: Electromagnetic Relay

The number of turns in a winding is directly proportional to the open-circuit voltage being measured or produced across it. The above diagram is a single-phase VT. In the three-phase system it is necessary to use three VTs at one per phase and they being connected in star or delta depending on the method of connection of the main power source being monitored. This type of electromagnetic transformers are used in voltage

15

circuits up to 110/132 kV. For still higher voltages, it is common to adopt the second type namely the capacitor voltage transformer (CVT).

Capacitor Voltage Transformer The basic connection adopted in this type is here the primary portion consists of capacitors connected in series to split the primary voltage to convenient values. The magnetic voltage transformer is similar to a power transformer and differs only so far as a different emphasis is placed on cooling, insulating and mechanical aspects. The primary winding has larger number of turns and is connected across the line voltage; either phaseto-phase or phase-to-neutral. The secondary has lesser turns however, the volts per turn on both primary and secondary remains same. .

Fig 2: Capacitor Voltage Transformer The capacitor VT is more commonly used on extra high-voltage (EHV) networks. The capacitors also allow the injection of a high-frequency signals onto the power line conductors to provide

end-to-end

communications

between

substations

for

distance

relays,

telemetry/supervisory and voice communications. Hence, in EHV national grid networks of utilities, the CVTs are commonly used for both protection and communication purposes

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Current Transformers: All current transformers used in protection are basically similar in construction to Standard transformers in that they consist of magnetically coupled primary and secondary windings, wound on a common iron core, the primary winding being connected in series with the network unlike voltage transformers. They must therefore withstand the networks shortcircuit current. There are two types of current transformers: 1

Wound primary type

2

Bar primary type.

The wound primary is used for the smaller currents, but it can only be applied on low fault level installations due to thermal limitations as well as structural requirements due to high magnetic forces for currents greater than 100 A.

Fig 3: Wound primary type CT The bar primary type is used as. If the secondary winding is evenly distributed around the complete iron core, its leakage reactance is eliminated Protection CTs are most frequently of the bar primary, toroidal core with evenly distributed secondary winding type construction.

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Fig 4: Bar Primary Type CT

2.4.3 CIRCUIT BREAKERS: Where fuses are unsuitable or inadequate, protective relays and circuit breakers are used in combination to detect and isolate faults. Circuit breakers are the main making and breaking devices in an electrical circuit to allow or disallow flow of power from source to the load. These carry the load currents continuously and are expected to be switched ON with loads (making capacity). These should also be capable of breaking a live circuit under normal switching OFF conditions as well as under fault conditions carrying the expected fault current until completely isolating the fault side (rupturing/breaking capacity). Under fault conditions, the breakers should be able to open by instructions from monitoring devices like relays. The relay contacts are used in the making and breaking control circuits of a circuit breaker, to prevent breakers getting closed or to trip breaker under fault conditions as well as for some other interlocks.

Protective Relay-Circuit Breaker Combination: The protective relay detects and evaluates the fault and determines when the circuit should be opened. The circuit breaker functions under control of the relay, to open the circuit when required. A closed circuit breaker has sufficient energy to open its contacts stored in one form or another (generally a charged spring). When a protective relay signals to open the circuit, the store energy is released causing the circuit breaker to open. Except in special cases where the protective relays are mounted on the breaker, the connection between the relay and

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circuit breaker is by hard wiring. From the protection point of view, the important parts of the circuit breaker are the trip coil, latching mechanism, main contacts and auxiliary contacts. The roles played by these components in the tripping process and the following step by step procedure take place while isolating a fault (the time intervals between each event will be in the order of a few electrical cycles i.e. milliseconds):

Fig 5: Protective Relay-Circuit Breaker Combination

• •

The relay receives information, which it analyzes, and determines that the circuit breaker is unlatched and opens its main contacts under the control of the tripping spring. The trip coil is deenergized by opening of the circuit breaker auxiliary contacts. Circuit breakers are normally fitted with a number of auxiliary contacts, which are used in a variety of ways in control and protection circuits (e.g. to energize lamps on a remote panel to indicate whether the breaker is open or closed) circuit should be opened.

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•

Relay closes its contacts energizing the trip coil of the circuit breaker.

2.4.4 TRIPPING BATTERIES: The operation of monitoring devices like relays and the tripping mechanisms of breakers require independent power source, which does not vary with the main source being monitored. Batteries provide this power and hence they form an important role in protection circuits. The relay/circuit breaker combination depends entirely on the tripping battery for successful operation. Without this, relays and breakers will not operate, becoming ‘solid’, making their capital investment very useless and the performance of the whole network unacceptable. It is therefore necessary to ensure that batteries and chargers are regularly inspected and maintained at the highest possible level of efficiency at all times to enable correct operation of relays at the correct time.

2.5 TYPES OF PROTECTIONS: Very Frequently, for attaining higher reliability, speeded-up action and improvements in operating flexibility of protection schemes, the separate elements of a power system, in addition to main or primary protection, are provided with a back-up and auxiliary protection.

2.5.1 Main Protection: The main protection is the first line of defence and ensures quick-acting and selective clearing of faults within the boundary of circuit section or element it protects. Main protection as rule is provided for each section as an electrical installation.

2.5.2 Back-up Protection: Back-up protection is the name given to protection which backs up the main protection whenever the later fails in operation, is cut out for repairs etc. It is the second line of defence which functions to isolate a faulty section of the system in case of the main function fails to

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function properly. The failure of the main protection may be due to any of the following reasons: •

The dc supply to tripping circuit fails.

•

The current or voltage supply to relay fails.

•

The circuit breaker fails to operate.

•

The main protective relay fails

Back-up protection may be provided either on same circuit breakers which would be normally opened by main protection or, still better, if the second line of protection makes use of different circuit breakers. Back-up protection usually for economic reasons not as fast as discriminative as the main protection.

2.5.3 Auxiliary Protection: Auxiliary protection is employed in separate cases to accelerate the main protection operation during faults within the protected zone and also protect what is termed the dead zone of the main protection scheme.

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CHAPTER -3 INTRODUCTION TO TURBO GENERATOR PROTECTION

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3.1 NECESSITY OF GENERATOR PROTECTIONS In order to generate power and transmit it to customers million of rupees must be spending on power system equipment. This equipment is designed to work under specified normal conditions however a short circuit may occur due to failure of insulation caused by: •

Over voltage due to switching

•

Over voltage due to the direct and indirect lightning strokes

•

Bridging of conductors by birds

•

Breakdown of insulation due to decrease of its dielectric strength

•

Mechanical damage to the equipment

In modern power system it is necessary to eliminate faults to a large degree by careful system design. Careful insulation coordination, correct operation and maintenance, it is not obviously possible to ensure cent percent reliability and therefore the possibility of faults must be accepted.

3.2 Generator Faults: Different types of faults that can occur within a generator can be classified as follows: •

Stator faults

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•

Rotor faults

•

External faults

3.2.1 Stator Faults: Stator faults involve the main current carrying conductors and must therefore be cleared quickly from the power system by a complete shutdown of the generator. They may be faults to earth, between phases or between turns of a phase, singly or in combination. The great danger from all faults is the possibility of damage to the laminations of the stator core and stator windings due to the heat generated at the point of fault. If the damage so caused is other than superficial, the stator would have to be dismantled, the damaged laminations and windings replaced and the stator rebuilt, all of which is a lengthy and costly process. Limitation of generator stator earth-fault current by means of resistance earthing is normal practice and serves, among other things, to minimize core burning. Phase-to-phase faults and interturn faults are both less common than earth faults. It is relatively easy to provide protection for phase-to-phase faults, but interturn faults are, on the other hand more difficult to detect and protection is not usually provided. Generally speaking, interturn faults quickly involve contact with earth via the stator core and are then tripped by stator earth-fault protection.

3.2.2 Rotor Faults: Rotor faults may be either to earth or between turns and may be caused by the severe mechanical and thermal stresses acting upon the winding insulation; these are aggravated by a variable load cycle. The field system is not normally connected to earth so that a single earth fault does not give rise to any fault current. However, a second fault to earth would short circuit part of the field winding and thereby produce an asymmetrical field system, and unbalanced forces on the rotor. Such forces will cause excess pressure on bearings and shaft distortion, if not quickly removed. Under the general heading of rotor faults can be included loss of excitation. This may be caused by an open circuit in the main field winding or a failure

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elsewhere in the excitation system. Loss of excitation in a generator connected to a large interconnected power system results in a loss of synchronism and slightly increased generator speed, since the power input to the machine is unchanged. The machine behaves as an induction generator drawing its exciting current from the remainder of the system in the form of wattless current whose magnitude approximates to that of the full load rating of the machine. This may cause overheating of the stator winding and increased rotor losses due to the currents induced in the rotor body and damper winding. This condition should not be allowed to persist indefinitely and corrective action either to restore the field, or to off-load and shut down the machine should be taken. With generator outputs above half rated load, pole-slipping caused by weak field condition, would cause severe voltage variations which may, in turn, cause operation of the under voltage protection on the boiler auxiliaries. The resultant operation of 'loss of boiler protection would then shut down the generator unit. Other generators connected to the same bus bar may also be caused to 'swing' and system instability would result. Pole slipping may also result from insufficiently fast clearance of a system fault and require the tripping of the unit.

3.2.3 Mechanical Condition: The mechanical conditions requiring consideration are over speed due to sudden loss of load, loss of drive due to prime mover failure and loss of condenser vacuum. The problem of over speed limitation, particularly in relation to sudden load changes, is considered later. With modem large units it is essential to anticipate over speed and take corrective action. Mechanical over speed devices which operation the steam stop valves are invariably fitted. In the event of failure of the prime mover, a generator will continue to run synchronously drawing power from the system. This can sometimes lead to a dangerous mechanical condition if allowed to persist, although the condition is immediately obvious to the attendant. Sets having an internal combustion prime mover must be protected against engine failure, where, if the alternator continues to motor serious engine damage may result.

3.3 EXTERNAL FAULTS:

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Turbo alternators must be protected against the effects ofsustained external faults, for ex ample faults on lines or bus bars which are notcleared by the appropriate protection. The main condition of interest is that of an unsymmetrical fault producing negative phase sequence currents in the stator winding. The effect of these currents is to produce a field rotating in opposite sense to the d.c field system producing a flux which cuts the rotor at twice the rotational frequency thereby inducing double frequency currents in the field system and the rotor body. These currents produce severe rotor heating and modern machines have a limited negative phase sequence current capability. Automatic tripping is therefore required for the higher negative phase sequence current conditions. This capability limit applies to all modem hydrogen-cooled machines and many air-cooled machines, but some of the older air-cooled machines are designed to withstand full negative sequence currents continuously. In large modern alternators, particularly

those employing direct cooling of the stator and rotor

conductors, the temperature rise caused by abnormally high stator currents is more rapid than in the less highly rated machines and the capability limit is therefore lower. In the event of fault the circulating relay contact is closed and the trip coils TC1, TC2 and TC3 are energized. The trip coil TC1 opens the main C.B while the trip coil TC3 opens the neutral circuit breaker. The trip coil TC2 opens the upper contacts, shorts the lower contacts so as to short circuit the field winding through resistor R. Thus the energy of the generator is dissipated in the resistor R.

3.4 NAME PLATE DETALS OF TURBO GENERATOR USED IN TPP Rating

-

60 MW

Power factor

-

0.8

KVA

-

75000

Voltage

-

11000 Volts

Current

-

3639

Amps

Voltage

-

300

Volts

STATOR ROTOR

26

Current

-

596

Amps

Frequency

-

50 HZ

No. of phases

-

3

Cooling system

-

Air

Type of Insulation

-

Class B

Connection

-

Star

3.5 THE FOLLOWING ARE THE MAIN PROTECTION SCHEMES ADOPTED FOR GENERATOR 1. Generator Differential Protection 2. Stator Inter Turn Protection 3. Reverse Power Protection 4. Automatic Field suppression 5. Local Breaker Backup Protection 6. Stator Earth fault Protection 7. Negative Sequence Protection 8. Rotor earth Fault Protection 9. Over Flux Protection 10. Over Voltage Protection 11. Stator Frame Over Heating Protection 12. Abnormal Frequency Protection 13. Loss of Field Protection 14. Pole Slipping Protection 15. Over Speed Protection

27

16. Protection Against Vibrations 17. Bearing Over Heating Protection 18. Protection Against Motoring

.

CHAPTER 4 PROTECTION SCHEMES I

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4.1 GENERATOR DIFFERENTIAL PROTECTION: This is the most common system employed for protection of stator windings against earth faults and phase to phase makes use of circulating current principle. In this scheme of protection currents at the two ends of protected sections are compared. Under normal operating conditions these currents are equal but may differ in the occurrence of fault in the protected system. The difference of currents under faulty conditions is made to flow through the relay operating coil. The relay then closes its contacts and makes the circuit breaker to trip and thus isolates the protected section from the system. Such a protection system is called a Merz-Price circulating current system. This protective scheme is very effective for earth faults and faults between phases.

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Fig 6: Generator Differential Protection

The schematic arrangement of differential protection scheme is shown in fig: There are two sets of identical CT’s each set is mounted on either side of stator phase windings. The secondaries of these CTs are connected in star, the neutral point being connected to current transformer common neutral and the outer ends of each of the three pilot wires. The fault setting required from the differential protection is determined by the value of the neutral earthing resistor and also by the amount of winding to be protected. Under normal healthy conditions, the currents at both ends of each winding will be equal, emfs induced in secondaries of CTs will be equal and so no current will flow through the operating coils of relays. When an earth fault or phase to phase fault occurs, this condition no longer holds good and the differential current flowing through the relay operating coil makes the circuit breaker to trip.

4.2 STATOR INTERTURN FAULT PROTECTION: The differential current protection described in Section 4.1 cannot detect interturn faults which remain clear of earth, since there is a balance of the currents entering and leaving the winding despite the presence of a large current circulating the shorted turns. Interturn faults are not normally protected against because of the technical difficulty of so doing. If interturn faults

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occur in the stator slots, they quickly develop into faults to earth and are cleared by the stator earth fault protection. There is, however, the possibility that they may occur at the winding ends and so cause extensive damage to the generator before the fault evolves to one detectable by other protection. The primaries of CTs are inserted in these parallel paths and secondaries are cross connected. When there is no fault currents flowing through the two parallel paths of the stator winding will be equal and therefore no current will flow through the relay operating coil. But during inter-turn fault in the phase winding, the currents flowing through the two parallel paths will be different and a current proportional to the difference of two currents will flow through the relay operating coil which will close the trip circuit and isolate the machine from the power system. This kind of protection is extremely sensitive.

Fig 7: Stator Interturn Fault Protection

4.3 REVERSE POWER PROTECTION:

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Motoring of a generator will occur when turbine output is reduced such that it develops less than no-load losses while the generator is still on-line, the generator will operate as a synchronous motor and driving the turbine. The generator will not be harmed by synchronous motoring and a steam turbine can be harmed through overheating during synchronous motoring if continued long enough. The motoring of the turbine output can be detected by reverse power protection relay which is powered by both C.T and P.T’s. To avoid false tripping due to power swings a time delay is incorporated before tripping signal is generated. If the unit trips on reverse power protection, the input power to the turbine is increased as quickly as possible. Even after two to three attempts, if the machine trips on the same protection; probably the governor of turbine is faulty. The condition is informed to maintenance staff for rectification.

4.4 AUTOMATIC FIELD SUPPRESSION: In the event of fault on the turbo generator winding even if the generator C.B is tripped, the fault continues to be fed as long as long as excitation will exist because emf is induced in the generator itself. For the quick removal of fault during emergency, it is necessary to disconnect the field simultaneously with the disconnection of the generator. Thus it is absolutely necessary to disconnect the magnetic field in the shortest possible interval of time. Hence it is to be ensured that all the protection system not only trips the generator circuit breaker but also trips the automatic field discharge switch. The field discharge switch is an automatic control unit designed to remove voltage from the generator after its isolation from the system.

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Fig 8: Automatic Field Suppression

4.5 LOCAL BREAKER BACK UP PROTECTION: For most of the faults, the generator breaker involves tripping the generator from the system. Failure of the breaker to open probably results in loss of protection & other problems such as motoring action or single phasing if one or two poles of the generator breaker fail to open due to mechanical failure in breaker mechanism, the result can be a single phasing &negative phase sequence currents induced on the rotor. The LBB protection is energized when the breaker trip is initiated after a suitable time interval if conformation of the breaker tripping from 3poles is not received .the energized tripping signal from local breaker back up protection will trip all 220kv generator breakers and all 220kv feeder breakers through bus bar protection.

4.6 STATOR EARTH FAULT PROTECTION: Normally the generator stator neutral operates at a potential close to ground. If a faulty phase winding connected to ground, the normal low neutral voltage could rise as high as line-toneutral voltage depending on the fault location. Although a single ground fault will not necessarily cause immediate damage, the presence of one increases the probability of a second. A second fault even if detected by differential relay, may cause serious damage. The usual method of detection fault is by measuring the voltage across the secondary of neutral grounding transformer (NGT). In the voltage-operated type, a standard induction

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disk type overvoltage relay is used. It is also to be noted that the relay is connected across the secondary winding of the transformer and the relay shall be suitably rated for the higher continuous operating voltage. Further, the relay is to be insensitive for third harmonic current. The generator neutral, the overvoltage element will not pick up because the voltage level will be below the voltage element pickup level. In order to cover 100% of the stator windings, two over lapping zones to detect stator ground faults in a high impedance grounded generator system, the two zones are put together cover 100% stator winding for earth faults. A fundamental frequency neutral over voltage relay covers about 0-95% of the stator zonal winding for all faults except those near the neutral.

Fig 9: Stator Earth Fault Protection

Another third harmonic neutral under voltage relay covers remaining 96-100% of the stator zone 2 winding on neutral side. a small amount of third harmonic voltage will be produced by most generators at their neutral and terminals. Normally they would be higher at

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full load. If a fault develops near the neutral, the third harmonic neutral voltage will approach zero and the terminal voltage will increase. Use of a third harmonic under voltage at the neutral it will pick up for a fault at the neutral. `

CHAPTER 5 PROTECTION SCHEMES II

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5.1 NEGATIVE SEQUENCE OR CURRENT UNBALANCE PROTECTION: When the machine delivering the equal currents in three phases, no unbalance are negative phase sequence current is produced as the vector sum of these currents is zero, when the generator is supplying an unbalanced load to a system, a negative phase sequence current is imposed on the generator. The system unbalance is due to opening of lines, breaker failures are system faults. The negative sequence current in the stator winding creates a magnetic flux wave in the air gap which rotates in the opposite direction to that of rotor synchronous speed. This flux induces currents in the rotor body, wedges, retaining rings at twice the line frequency. Heating occurs in these areas and the resulting temperatures depend upon the level and duration of the unbalanced currents. Under these conditions it is possible to reach temperatures at which the rotor material no longer contains the centrifugal forces imposed on them resulting in serious damage to the turbine generator set. Any machine as per design data will permit some level negative sequence currents for continuous period. An alarm will annunciate at annunciation panel it negative sequence currents exceed a normal level. Reduce the MVAR power on the machine if necessary load also and keep the machine for some time till the alarm vanishes at annunciation panel. If the machine trips on the negative sequence protection never take the machine into the service until the temperature on the rotor parts settle down its lower value. Resynchronize the machine to the grid after considerable time under grid and feeder parameters all within limits. If the unit trips again on

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the same protection the machine is stopped after consideration time so as to cool down the rotor parts and inform to the maintenance staff for through examination of the system. A specialized relay to detect the circulating currents called a negative sequence current relay (since the induced currents are called negative sequence currents) is used to detect the phase imbalance with the generator during the unbalancing fault conditions. A negative sequence relay provides protection to generators and rotors against unbalanced loading that may result from the phase to phase faults the equipment consists of network energized from three CTs and a single pole relay having an inverse time characteristic

Fig 10: Negative Sequence or Current Unbalance Protection The equipment consists of network energized from three C.Ts and an single pole relay having an inverse time characteristic connected across the network. The network consists of four impedances Z1, Z2, Z3, and Z4 of equal magnitude connected in a bridge formation. Z1,

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Z2 are non inductive resistances while Z3, Z4 are composed of both resistance and reactance. The values of Z2, Z4 are so adjusted that the currents flowing in these lag behind those in impedances Z1, Z2 by 60degrees. Resolving Ir, Ib there components IZ1, IZ3 and IZ2, IZ4 we find that the actual current flowing in the relay is Iy. Thus the relay is operated under the influence of Iy.

5.2 ROTOR EARTH FAULT PROTECTION: Any rotor field winding of the generator is electrically isolated from the ground. Therefore the existence of one ground fault in the field winding will usually not damage the rotor. However the presence of two or more ground faults in the winding will cause magnetic and thermal imbalance plus localized heating and damage to the rotor metallic parts. The rotor earth fault may be caused due to insulation failure of winding or inter-turn fault followed by localized heat.

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Fig 11: Rotor Earth Fault Protection

The field circuit of our generator comprises the rotor winding, the armature of the exciter, field circuit breaker and connecting cables. This total system is an isolated one and if an earth fault occurs, rotor winding turns will get short circuited which decreases the field circuit resistance and increases the current flowing through it. This current over heats the rotor winding and the exciter supplying it, causes further destruction at the points of faults and may lead to burning of the rotor insulation. The protection scheme consists of a high resistance connected across the rotor circuit and its mid point is grounded through a sensitive relay

5.3 OVER FLUX OR OVER EXCITATION PROTECTION: This problem may occur in turbo generators that are connected to the grid if they experience generating voltage regulation problems. It may also occur for units during startup or resynchronizing following a trip. Over excitation in these instances may be result of equipment problems or operating errors in AVR. Moderate over fluxing (105-110%) increases core loss resulting in increase of core temperatures due to hysteresis and a eddy currents loss long term operation at elevated temperatures can short term the life of the stator insulation severe over fluxing can break down the insulation followed by rapid local core melting. A specialized volts/hertz (Per unit voltage divided by per unit frequency commonly called volts/hertz) relay is used to detect this condition, and will trip the generator if excessive volts/hertz conditions are detected.

5.4 OVER VOLTAGE PROTECTION: Generator voltage is at present value under normal operating conditions as selected by operator in AVR. If it parts from preset value, may be due to AVR mal functioning or due to increase in prime-mover speed due to sudden loss of loss of load. Severe over voltage can cause over fluxing and winding insulation failure.

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The over voltage protection is provided with an over voltage relay which has two units – one instantaneous relay set for pick up at about 130-150% of rated voltage and the another IDMT relay set for pick up at about 110% of rated voltage . These relays are energised from a PT. If an over voltage persists the generator main circuit breaker and the exciter field breaker will be tripped. If the generator trips for over voltage then raise the generator voltage slowly with manual mode in AVR. And keep generator voltage within the limits of normal voltage. If it is unable to control the generator voltage, the field breakers are tripped and informed to the maintenance staff of the AVR.

5.5 STATOR FRAME OVER HEATING PROTECTION: For the protection of the turbo generator against any possible fire accident twelve fire detector relays are provided on either side of the stator winding. These relays have a set of normally open contacts. The set of contacts will close when the temperature surrounding the fire relay exceeds 80O C. The other relay set of contacts close when the temperature exceeds 100O C. These contacts are wired to CO2 fire extinguishing system. CO2 gets discharged due to any of the following causes maintained below: •

Contact of the 100O C fire detector closed.

•

Generator restricted earth fault relay operates.

•

Generator differential relay operates.

•

CO2 discharge system activated from turbo generator control test or manual to the box or push button located near the generator.

All the above cases will actuate CO 2 release mechanism and CO2 gas will be flooded into the generator air ducts.

5.6 ABNORMAL FREQUENCY PROTECTION:

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For a generator connected to a system, abnormal frequency operation is a result of a severe system disturbance. The generator can tolerate moderate over frequency operation provided voltage is within an acceptable limits. The machine operated at higher speeds at which the rotor material no longer contain the centrifugal forces imposed on them resulting in serious damage to the turbine-generator set. The abnormal over frequency on the machine may be due to improper speed control adjustment or disoperation of the speed controller or severe grid disturbance or sudden load thrown off. If the unit trips due to abnormal frequency protection then change the governor speed until machine reaches full speed. Even after 2to3 attempts the machine is running at lower speed, probably the governor of the turbine is faulty. Inform to maintenance staff for rectification of the same.

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CHAPTER 6 PROTECTION SCHEMES III

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6.1 LOSS OF FIELD OR LOSS OF EXCITATION PROTECTION: This fault is caused due to open circuit of field, short circuit of field, accidental tripping of field breaker, poor brush contact. This may also occur due to AVR failure. When the turbo generator is with excitation and connected to the grid, it generates reactive power along with active power to the grid. Loss of field or loss of excitation results in loss of synchronism between rotor flux & stator flux. The synchronous machine operates as an induction machine at higher speed and draws reactive power from the grid. This will result in the flow of slip frequency currents in the rotor body as well as severe torque oscillations in the rotor shaft. As the rotor is not designed to sustain such currents or to withstand the high alternating torques which results in rotor overheating, coupling slippage and even rotor failure. If the generator is not disconnected immediately when it loses excitation wide spread instability may very quickly develop and major system shutdown may occur. When loss of excitation alarm annunciates at annunciation panel, the machine may probably be running with less excitation at leading MVAR power. Increase the excitation on the machine until it reaches on lagging MVAR power. The machine trips on the same protection along with alarm resynchronize the machine and try to stabilize at required MVAR power. If not possible, trip the machine immediately and inform to the maintenance staff for thorough checking of the Automatic Voltage Regulator (AVR) and its associated parts.

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Many protection schemes are employed to protect the turbo generators from loss of field. The protection scheme used in TPP is the method generally adopted for the modern turbo generators. As modern large generators may be required to operate with very low values of excitation, when a generator loses synchronism, the quantity which changes most is its impedance as measured at the stator terminals. Loss of field will cause the terminal voltage of the generator to begin to fall, while the current begins to increase. The apparent impedance of the machine will therefore be seen to decrease and its power factor to change. The mho relay is placed which is designed to detect the change of impedance from the normal load value may therefore be used to provide protection against asynchronous operation resulting from the loss of excitation.

Fig 12: Loss of Field or Loss of Excitation Protection

6.2 POLE SLIP PROTECTION: A generator may lose synchronising with power system without failure of excitation system, because of severe fault disturbance or operation at a high load with leading power factor and hence a relatively weak field. A system power shock may make a generator rotor

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oscillate.

Oscillation will create severe vibrations in machine and subsequent damage of

machine bearings. Incase the angular displacement of rotor exceeds the stable limit; the rotor will slip a pole pitch. During this time machine should be isolated from the system. When a generator loses synchronism, the resulting high current peaks & off frequency operation may cause winding stresses, pulsation torques and mechanical resonances that have the potential danger to turbine generator.

Fig 13: Pole Slip Protection

6.3OVER SPEED PROTECTION: The turbo generator is fitted with over speed limiting gear designed to detect sudden loss of load and to close emergency valves immediately, to limit the magnitude of the temporary

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speed rise. In a typical scheme this is achieved by monitoring the electrical output of the generator using a watt metric relay. This relay will detect a sudden loss of output and operate instantaneously to close its contacts. A second relay monitors the steam input to the turbines at a chosen stage and the contacts are held dosed when the steam pressure is in the full load region. A sudden loss of load will give instantaneous operation of the output relay but the steam input relay does not operate immediately because steam is being admitted to and expanding in the turbine. Under this condition the emergency valve solenoids are energised giving instantaneous control of steam admission. The emergency valves remain closed until falling pressure or restoration of load restores the machines to normal control. The action of this equipment is clearly much faster than that obtainable from the governing system which requires an actual overspeed to produce a response and take corrective action. It is for this reason that overspeed limiting equipment of the type described is often installed where reheat turbines are used, because the long steam pipes of relatively large bore interconnecting each reheater and the reheat sections of the associated boiler plant present special problems due to the large volume of steam entrained. The overspeed limiting equipment then operates additionally into the interceptor emergency stop valves associated with each interceptor steam chest to give instantaneous control of the steam entering the turbine at all stages. In the ultimate, over speeding of the machine beyond the safe limit (10%) will cause operation of the overspeed bolts, and shut the stop valves.

6.4 PROTECTION AGAINST VIBRATIONS: Rotor earth fault protection and negative sequence protection of a generator against unbalanced loads, described in topics 4.4 and 5.2 respectively to prevent or reduce vibration under those circumstances. A vibration detector may be mounted on one of the bearing pedestals in the case of a horizontal shaft generating set, or on the upper guidebearing in case of a vertical shaft generating set. It may be set to trip the machine or initiate an alarm when the radial deflections of certain duration exceed a per-determined value.

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6.5BEARING OVER HEATING PROTECTION: Bearing over heating can be detected by a relay actuated by a thermometer-type bulb inserted in a hole in the bearing, or by a resistance-temperature-detector relay, such as used for stator over heating protection, with the detector embedded in the bearing. In case lubricating oil is circulated through the bearing under pressure, the oil temperature may be monitored if the system has provision for giving an alarm if the oil circulation is stopped. Such protection is provided for all unattended generators where the size or importance of the generator warrants it. Such protection for attended generators is generally limited only to sound an alarm.

6.6 PROTECTION AGAINST MOTORING: In the event of prime-mover failure the generator continues to rotate as a synchronous motor drawing electrical power from the system and driving the prime -mover . The motoring operation the machine is also known as inverted operation. This operation is not desirable, as firstly the machine ceases generation and no purpose is served by its motoring operation. The machine faults such as failure of bearings or lubricating troubles etc, are protected by special indicating instruments/protective devices which may stop the prime-mover automatically but such faults cannot be detected on the electrical side. However the inverted operation of the machine can be protected by employing reverse power relay. During the motoring operation of a generator the power flows from the bus-bars to the machine and the conditions in the three phases are balanced. Hence single element directional powers relay (reverse power relay) sensing the direction of power flow in any one phase is sufficient. Intentional time lag is provided to prevent operation by synchronous surges and power oscillations following system disturbances

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CONCLUSION A generator is the most important and most costly equipmenent in a power system. It is subjected to more number of troubles than any other equipment. The basic function of protection applied to generators is therefore to reduce the outage period to a minimum by rapid discriminative clearance of faults. While selecting the scheme for generator protection, the protection of complete unit and the stability of the system due to disturbance, in a generator should be considered in addition to protection of the generator itself. In our industry oriented mini project we have learned the importance of generator protection and studied various protection schemes employed for a 60MW Turbo Generator installed in thermal power plant (captive power plant in Visakhapatnam steel plant).

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BIBLOGRAPHY [1] “A COURSE IN ELETRICAL POWER” – J.B.Gupta. [2] “ELETRICAL POWER”—D.r. S.l.Uppal. [3] “POWER SYSTEM PROTECTION AND SWITCH GEAR”—Badri Ram and Vishwakarma. [4] “PRINCIPLES OF POWER SYSTEM”—V.K.Mehta. [5]“ELECTRIC POWER SYSTEMAlexandra von Meir, IEEE Press.

A

CONCEPTUAL

INTRODUCTION”-

[6]“PROTECTION OF GENERATORS, TRANSFORMERS, GENERATORTRANSFORMER UNITS AND TRANSFORMER FEEDERS”- J.Rushton, revised by K.G.M.Mewes. [7] “A TEXT BOOK OF POWER SYSTEM ENGINEERING”— M.L.Soni, P.V.Gupta, U.S.Bhatnagar. [8] “POWER SYSTEMS HAND BOOK”- Leonard l.Grisbby. [9] “INDUSTRIAL POWER SYSTEMS”- Shoiab Khan.

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[10] “THE ART AND SCIENCE OF PROTECTIVE RELAYING”-C.Russell Mansion.

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