protection of turbogenerator

7/27/2019 Protection of Turbogenerator

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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 coolerplace, which can boost of 3 to 4c 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.

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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


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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 protectiveequipment 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 turns 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 100120 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

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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 phase-

to-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 short-

circuit 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 aroundthe 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 contactsare 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 oneform 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 remotepanel 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 maintainedat 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 protectionwhenever 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 unbalancedforces 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 - 75000STATOR Voltage - 11000 Volts

Current - 3639 Amps

ROTOR Voltage - 300 Volts

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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

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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 CTs 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.Ts. 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 circuitbreaker 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-to-

neutral 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 onthe 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 CO2 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 guide-

bearing 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.

6.6PROTECTION 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 POWERD.r. S.l.Uppal.

[3] POWER SYSTEM PROTECTION AND SWITCH GEARBadri Ram and

Vishwakarma.

[4] PRINCIPLES OF POWER SYSTEMV.K.Mehta.

[5]ELECTRIC POWER SYSTEM- A CONCEPTUAL INTRODUCTION-Alexandra von Meir, IEEE Press.

[6]PROTECTION OF GENERATORS, TRANSFORMERS, GENERATOR-TRANSFORMER 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.

protection of turbogenerator

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