ksm project

8/6/2019 KSM Project

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ABSTRACT

TITLE: STUDY PROJECT ON PROTECTION OF

TURBOALTERNATORS AND TRANSFORMERS

IN THERMAL POWER STATION WITH A

SPECIFIC AND BRIEF STUDY OF RELAYS ANDTHEIR APPLICATIONS WITH REFERENCE TO

KTPS C STATION

The entire modern Turbo-Alternators is 2 pole machines and are driven by steam

turbines. Since the efficiency of steam turbine is high at large speeds, the Turbo

Alternators are designed for speed up to 3000 rpm. Modern Turbo Alternators havetypical ratings of 60 MW, 120 MW, and 250 MW, 500 MW, 1000 MW. In this project

we are studying the protection of Turbo Alternator.

The Turbo-Alternator protection can be provided with two types of relays, one is

with single function relays and the other is comprehensive multi function relay having

encapsulated in one single module. The latter has a number of protection functions

within the relay case and does have a number of advantages as compared with single

functions generator protection relays. The relay is more compact and requires less

external connections. Using numerical techniques, enhanced features such as

measurement functions, event and disturbances recording, alternative setting group can

also provide remote communications, printing and testing facilities. These features can

simplify the tasks of commissioning, maintenance, and trouble-shooting and post-fault

analysis.


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In addition, a self monitoring feature, together with remote communications,

allows the monitoring of the relay and the generator on demand, will improve availability

of the protection system.

CONTENTS

PART I:

1. INTRODUCTION

2. BRIEF HISTORY OF K.T.P.S.

3. IMPORTANCE OF PROTECTION.

4. CIRCUIT BREAKERS IN POWER PLANT.

5. OVER VIEW OF RELAYS.

PART II:

1. PROTECTION OF GENERATOR.

2. PROTECTION OF TRANSFORMER.3. PROTECTION OF GENERATOR TRANSFORMER UNIT.

4. CONCLUSION.

BIBILOGRAPHY


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PART I


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INTRODUCTION

THIS PROJECT DEALS WITH THE PROTECTION OF TURBO-

ALTERNATOR & TRANSFORMER.

The generators are the most expensive piece of equipment in A.C. power system

and are subjected to most possible troubles than any other equipment. The Aim of the

protection system is to protect against all these abnormal conditions and yet to keep the

protection simple and reliable has resulted in considerable divergence of opinion on the

choice of the protection. Transformers used in the power system are also subjected to so

many troubles. These are used to step-up or step-down the voltage. In order to have

continuous power supply we have to protect transformer also..

The purpose of the power system is to generate & supply electrical energy to

consumers. The system should be managed to deliver this energy to the utilization points

with both reliability and economy.

The power system represents a very large capital investment so it should be

protected so as to give the best service to the consumers.


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BRIEF HISTORY OF THE KOTHAGUDEM

THERMAL POWER STATION

Kothagudem Thermal Power Station is one of the major power generating station

of the Andhra Pradesh. The main raw material is coal is supplied by Singareni Collieries,

Kothagudem and Water sources is from Kinnerasani project, which is about 12 Kms

from the Paloncha.

Kothagudem power station comprises of four powerhouses namely KTPS A,

KTPS B, KTPS C, KTPS 5 TH stage

KTPS A -- 4 X 60 MW

KTPS B -- 2 X 120 MW

KTPS C -- 2 X 120 MW

KTPS 5 TH Stage -- 2 X 250 MW

Total capacity = 1220MW.

A PROFILE OF KOTHAGUDEM THERMAL POWER STATION

Foundation for Indias power sector was laid in the year 1897 with commencement of

2000 KW micro hydel project at Darjeeling, being the load center.

The Andhra Pradesh state electricity board. Here it is often called as APSEB. The

APSEB is responsible for promoting the coordination of generation, transmission and

distribution of electrical energy through out the state of Andhra Pradesh.

In a step to power sector reforms, APSEB has been divided in to two corporations.

i.e. Andhra Pradesh Power Generation Corporation Limited (APGENCO) and Andhra

Pradesh Power Transmission Corporation Limited respectively on 1 st February, 1999.


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IMPORTANCE OF PROTECTION:

A fail free power system is neither economically justifiable nor technically

feasible. Faults can occur in any power system components generator, transformer,

motors, buses and lines though the transmission lines being exposed to environment

are most vulnerable. Faults fall into two general categories - short circuit faults and open

circuit faults. Short-circuit faults are most ever kind, resulting in a very abnormal high

currents. If allowed to persist even for short period of time, short circuit can lead to

extensive damage to equipment. Undesirable effects of short-circuit faults are unmerited

below:

Arcing faults (most common) can vaporize equipment in vicinity leading to,

possibly, fire and explosion, e.g. in transformers and circuit breakers. Power system

components carrying abnormal currents gets over heated, with the consequent reduction

in the life span of their insulation. Operating voltages can go above or below their

acceptable values, leading to development of another fault or damage to utilization

equipment. Consequent unbalanced system operation causes overheating of generator rotors. Power flow is severely restricted, or even completely blocked, while the short

circuit lasts.

As a consequence of blockage of power flow, power system areas can lose

synchronism. The longer the fault last, the more is possibility of loss of synchronism.

Open circuit faults cause abnormal system operating and danger to personnel. Voltage

tends to rise well beyond acceptable values in certain parts of the system with possibility

of insulation failure and development of short circuit fault. While open circuit faults can

be tolerated for a long period of time than short circuit fault, these cannot be allowed to

persist, and must be removed. We shall devote our attention to most severe type of fault,

i.e. the short circuit faults. There are also other abnormal operating conditions, which

require remedying, but do not fall under two categories of faults mentioned. Two

unbalanced conditions are one is heavily unbalanced generator and the other is loss of

excitation. Faults should be instantly detected and faulty section be isolated from thesection in the shortest possible time. It is obviously not possible to do this manually, and

it must, therefore, be accomplished automatically. Faults are detected automatically by


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by means of relays, and faulty section isolated by C.B. connected to the boundaries of

section. The combination of relays & C.B. is known as protective system.

CIRCUIT BREAKERS IN POWER PLANT

INTRODUCTION

Power switching ON and OFF operations in any electrical system needs

isolation equipments. In a situation where power handled is very low, say a fan or a

lamp, this can be achieved by a simple ON/OFF switch. But as load capacity increases,

for example an ordinary water heater where current is of the order of 7 to 10 amps, the

switch used becomes spring loaded so that the make and break actions becomes fast and

need not depend totally on manual operation. For higher loads, special circuit breaking

equipment called circuit breakers are used. A circuit breaker (CB) consists essentially of

current carrying contacts called electrodes. These contacts remain engaged when the

circuit is ON, but under predetermined conditions, gets separated to interrupt the circuit.

When the contacts are separated in order to interrupt the current, an ARC is struck

between them.

ARC PHENOMENA

The arc consists of a column of noised gas with temperature of about

25000K, in which the molecules have lost one or more of their electrons resulting in

positive ions and electrons. The electrons, which have a negative charge and being light,

are attracted towards the positive contact (the anode) very rapidly and the positive ions

are attracted towards the negative contact (the cathode) relatively slowly. For the

initiation of the arc, electrons must be emitted from the cathode as soon as the contacts

begin to separate and this emission is mainly because of (I) Field Emission and (II)

Thermal Emission. The electrons so liberated from the cathode make many collisions

with the atoms and molecules of the gases and vapour existing between the two contacts

during their travel towards the anode. These collisions cause ionization of atoms and the

molecules thus liberating more electrons.


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For the current to be interrupted at zero passage, two processes are to be

considered. Firstly, cooling of the arc must be strong enough to bring the arc temperature

down to the values where the gas is no longer ionized. The current will then be prevented

from re-establishing in the opposite direction. Secondly, after the current has ceased toflow, the dielectric strength between the arcing contacts must be high enough to

withstand the voltage, which will immediately start to build up.

ARC INTERRUPTION

The common methods used for interruption the arc are

(i) High Resistance Interruption:

In this method the arc is so controlled that its effective resistance increases with time so

that the current is reduced to a value insufficient to maintain it.

The resistance can be increased by

Lengthening the arc

Cooing the arc

Splitting the arc Constraining the arc

(ii) Low Resistance Interruption (Current Zero Interruption):

- In this method, the arc resistance is kept low until the current zero where the arc

extinguishes itself naturally and is prevented from re-striking inspite of high re-striking

voltage. A rapid increase of dielectric strength is necessary for successful interruption

and this can be achieved by

Lengthening of the gap

Cooling

Blast effect

Following are the recommended properties for a good arc quenching medium.

1. High Dielectric Strength

2. High Thermal Conductivity

3. Good Physical & Chemical Stability

4. Non-inflammable

5. Good Arc Extinguishing Properties


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TYPES OF CIRCUIT BREAKERS

1. Low Voltage Circuit Breakers:

For voltage level of 1500 volts AC and 3000 volts DC, air is used as the mediumof arc interruption and these circuit breakers are termed as low voltage circuit breakers.

These breakers are fast in operation, free from fire hazards and can be easily maintained.

For arc extinguishion, and arc chute is provided which elongates the arc length thus

increases the resistance in the path, resulting into more cooling effect. These improved

effects help in early arc extinguishion. In manually operated breakers a thermal release is

provided which trips the breaker in case current goes beyond the pre-set value.

Electrically operated breakers are provided with optional separate relay system for added

protection of the circuit.

2. Medium Voltage Circuit Breakers

Generally circuit breakers up to 33 kV are categorized as medium voltage

circuit breakers. Usually these circuit breaker uses Oil, Sulphur Hexaflouride Gas (SF 6)

or Vacuum as medium for arc interruption. These breakers, normally, are of metal clad

design and the form of truck can be raked in or racked out in switchgear. The arc

extinguishing principle differs with different extinguishing medium.

3. High Voltage Circuit Breakers:

Circuit Breaker of 66 kV and above are normally termed as high voltage breakers.

These breakers can be broadly classified into (I) Oil Circuit breakers and (II) Oil lesscircuit breakers. The arc extinguishing medium normally used in these breakers are Oil,

SF 6 and Air Blast etc.

3a. Oil Circuit Breakers

Oil circuit breakers are further sub divided into

(I). Bulk Oil Circuit Breakers and

(II). Minimum Oil Circuit Breakers.

In bulk oil breakers large quantity of oil is used for arc extinguishing as

well as insulating the current carrying parts. In minimum oil breakers oil is used only for


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arc quenching purpose, their current carrying parts are insulated by air and porcelain or

organic insulating materials. During breaker opening operation, the beat generated from

arcing decomposes the oil, which liberates mainly hydrogen, carbon and copper vapour.

More the breaking current, larger the arc, more the hydrogen vapour is generated. Thehydrogen provides the cooling effect and takes away heat of arc as the gas is generated in

a small chamber, gas gets pressurized and its cooling effectiveness increases thus

resulting in arc extinguishion. Further a small vent is provided in arcing chamber which

causes a small blast of hydrogen inside the chamber.

3b. SF6 Circuit Breakers

SF 6 gas is stable up to 500 0C and can be used with insulating materials

such as ceramics, glass, epoxy etc. up to 150 0Cwithout affecting them. At normal

pressure in the interrupters, the gas does not condense up to 40 0C approx. This makes

SF6 gas suitable for use in all temperature ranges. General properties of SF6 gas are

Non toxic

Colourless

OdourlessGenerally two types of SF6 breakers are found.

(a) Flow type generally used in high voltage system

(b) Puffer type used in medium voltage system.

The arc extinguishion in flow type circuit breaker is done by establishing a flow

of gas from breaking contacts. The flow is established at breaking contacts from high-

pressure zone to low pressure zone. For gas vapourization, a compressor and reservoir is

provided on the breaker. In puffer type breaker the gas is enclosed in a sealed chamber.

The chamber is also having breaker contact and piston attached to the moving contact.

When breaker operates the gas on one side of the piston gets pressurized due to

compression effect. This compressed gas passes through circuit breaker contact through a

hole in the piston thus extinguishes the arc.


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3c. Air Blast Circuit Breakers

In these breakers a blast of air is established at breaking contacts for compressed

air system at a pressure of 18-20 Kg/cm2. The blast do the neccessary cooling and thusextinguishes the arc. The advantages of ABCBs over an Oil CB are

Elimination of fire hazard

High speed operation

Short and consistent arc duration

As the arc energy is small, contact burning is less.

Suitable for frequent operation Less maintenance

3d. Vacuum Breakers:

A high vacuum of the order of 10 -8 to 10 -9 torr is established in the arcing

chamber. This increases the mean free path of the particles to several meters there by

avoiding any electron collision and avalanche effect. Thus the arcing is extinguished.

Advantages of Vacuum Breakers are

Vacuum is a superior dielectric medium

Small and compact size of interruption unit

High breakdown for short gaps.


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OVER VIEW OF RELAYS

A protective relay is a device that detects the fault and initiates the operation of

the circuit breaker is isolate the defective element from the rest of the system. The relays

detect the abnormal conditions in the electrical circuits by constantly measuring the

electrical quantities, which are different under normal and fault conditions. As the

technology in the protective system is developed, the relays are also developed in the

complex power system. The different types of relays are

i. Electromagnetic type.

- Attractive type

- Induction type

ii. Static Relays

Microprocessor based relays are

a. RAMDE

b. SPAM 150C

iii. Numerical Relays

ELECTROMAGNETIC RELAY TYPE:i. Attractive Type:

These are the first and fore most types of relays. The relays operate by virtue

of an armature being attracted to the poles of an electromagnet or a plunger

being drawn into a solenoid. Such relays may be actuated by A.C. or D.C.

quantities.

ii. Induction type:

These type of relays operate on the principle of induction motor and are

widely used for protecting relaying purposes involving A.C. quantities. These

are not used for D.C. quantities.

STATIC RELAYS:

Static techniques for power system protection are now well established and have

been developed over a period of more than twenty years use of silicon transistor, MST,

microprocessor and static techniques has made it possible to design high performance


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and more sophisticated characteristics into relays which are necessary to meet the

requirement modern complex power system.

It is now possible for static relays to be designed to replace all functions of

previously performed by electromagnetic relays. The power consumed by static relay islow & are fast in operation there is no moving parts hence it requires less maintenance

use of printed circuit board avoid wiring errors.

DISADVANTAGES:

a. The characteristics vary with the temperature and aging.

b. The reliability of the scheme depends upon a large number of small components

and their electrical connections.

c. The relays have low short time over load capacity compared with electromagnetic

relays.

RAMDE:

RAMDE is an integrated microprocessor controlled RMS measuring motor

protection relay. Ramde can be used for both synchronous & asynchronous motors.

Two types:

RAMDE 1 used for synchronous motors.

RAMDE2 used for asynchronous motors or other applications.

The current measurements are based on RMS values. Therefore taking into

considerations the harmonics common in today industrial environments. The current

unbalance protection is independent of frequency and phase sequence. And therefore also

works for motors fed view frequency converters or for reversible motors.

APPLICATIONS:

For large motors and those which are vital to the process a sensitive differential

protection is recommend for supplementing the short circuit protection in RAMDE.

DISADVANTAGES:

1. No under current protection.

2. No memory locations (registers).


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NUMERICAL RELAYS

These are the latest relay device used in the present world. This numeric relays works on

the numeric values of specified quantity from which it is to be measured. There are based

on the approximation of specified value. There are no mechanical settings in this relays.

THE ADVANTAGES OF NUMERICAL RELAYS

Several setting groups

under range of parameter adjustment

Remote communications built in

Internal Fault diagnosis

Power System measurements available

Distance to fault locator

Disturbance recorder

Auxilia

ry protection functions (broken conductor, negative sequence, etc.)

CB Monitoring (State, condition)

User - definable logic

Backup protection functions in-built

Consistency of operation times - reduced grading margin

It consists of one or more DSP microprocessors, some memory, digital and

analogue input/output (I/O), and a power supply. Where multiple processors are provided,

it is usual for one of them to be dedicated to executing the protection relay algorithms,


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while the remainder implements any associated logic and handles the Human Machine

Interface (HMI). By organizing the I/O on a set of plug-in printed circuit boards (PCB's),

additional I/O up to the limits of the hardware/software can be easily added. The internalcommunications bus links the hardware and therefore is critical component in the design.

It must work at high speed, use low voltage levels and yet be immune to conducted and

radiated interference from the electrically noisy substation environment. Excellent

shielding of the relevant areas is therefore required. Digital inputs are optically isolated to

prevent transients being transmitted to the internal circuits Analogue inputs are isolated

using precision transformers to maintain measurement accuracy while moving harmful

transients. Additionally, the input signals must be amplitude limited to avoid them

exceeding the power supply voltages, as otherwise the waveform will appear distorted.

Analogue signals are converted to digital form using an A/D converter. The cheapest

method is to use a single A/D converter, provided by a multiplexer to connect each of the

input signals in turn to the converter. The signals may be initially input to a number of

simultaneous sample-and-hold circuits prior to multiplexing, or the time relationship

between successive samples must be known if the phase relationship between signals is

important. The alternative is to provide each input with a dedicated A/D converter, and

logic to ensure that all converters perform the measurement simultaneously.

The frequency of sampling must be carefully considered, as the Nyquist criterionapplies :

f s > 2 x f h

Where :

f s

= sampling frequency

f s = highest frequency of interest


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If too low a sampling frequency is chosen, aliasing of the input signal can

occur (Figure 7.12) resulting in high frequencies appearing as part of signal in the frequency

range of interest. Incorrect results will then be obtained. The solution is to apply an anti-aliasing filter, coupled with an appropriate choice of sampling frequency, to the analogue

signal, so those frequency components that could cause aliasing are filtered out. Digital sine

and cosine filters are used, with a frequency response, to extract the real and imaginary

components of the signal. Frequency tracking of the input signals is applied to adjust the

sampling frequency so that the desired number of samples/cycle is always obtained. A

modern numerical relay may sample each analogue input quantity at between 16 and 24

samples per cycle.

All subsequent signal processing is carried out digitally in software, final digital

outputs use relays to provide isolation or arc sent via an external communications bus to

other devices.The relevant software algorithm is when applied. Firstly, the values of the

quantities of interest have to be determined from the available information contained in the

data samples. This is conveniently done by the application of the Discrete Fourier

Transform (DFT), and the result is magnitude and phase information for the selected

quantity. This calculation is repeated for all of the quantities of interest.

The DSP chip in a numerical relay is normally of sufficient processing capacity that

calculation of the relay protection function only occupies part of the processing capacity.The excess capacity is therefore available to perform other functions. Of course, care must

be taken never to load the processor beyond capacity, for if this happens, the protection

algorithm will not complete its calculation in the required time and the protection function

will be compromised.

Typical functions that may be found in a numerical relay besides protection functions

are described in this section. Note that not all functions may be found in a particular relay. In

common with earlier generations of relays, manufacturers, in accordance with their perceived


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market segmentation, will offer different versions offering a different set of functions.

Function parameters will generally be available for display on the front panel of the relay and

also via an external communication port, but some by their nature may only be available atone output interface.

Numerical relays perform their functions by means of software. The process used for

software generation is no different in principle to that for any other device using real-time

software, and includes the difficulties of developing code that is error-free. Manufacturers

must therefore pay particular attention to the methodology used for software generation and

testing to ensure that as far as possible, the code contains no errors. However, it is virtually

impossible to perform internal tests that cover all possible combinations of external effects,

etc., and therefore it must be accepted that errors may exist. In this respect, software used in

relays is no different to any other software, where users accept that field use may uncover

errors that may require changes to the software. Obviously, type testing can be expected to

prove that the protection functions implemented by the relay are carried out properly, but it

has been known for failures or rarely used auxiliary functions to occur under some

conditions.

Where problems are discovered in software subsequent to the release of a numerical

relay for sale, a new version of the software may be considered necessary. This process then

requires some form of software version control to be implemented to keep track of :

a) The different software versions in existence

b) The differences between each version

c) The reasons for the change.

d) Relays fitted with each of the versions.


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With an effective version control system, manufacturers are able to advise

users in the event of reported problems if the problem is a known software related problem

and what remedial action is required. With the aid of suitable software held by a user, it may be possible to download the new software version instead of requiring a visit from a service

engineer.

A numerical relay usually provides many more features than a relay using

static or electromechanical technology. To use these features, the appropriate data must be

entered into the memory of the relay. Users must also keep a record of all of the data, in

case of data loss within the relay, or for use in system studies, etc. The amount of data per

numerical relay may be 10-50 times that of an equivalent electromechanical relay, to which

must be added the possibility of user-defined logic functions. The task of entering the data

correctly into a numerical relay becomes a much more complex task than previously,

which adds to the possibility of a mistake being made. Similarly, the amount of data that

must be recorded is much larger, giving rise potentially to problems of storage.

The problems have been addressed by the provision of software to automate the

preparation and download of relay setting data from a portable computer connected to a

communication port of the relay. As part of the process, the setting data can be read back

from the relay and compared with the desired settings to ensure that the download has

been error-free. A copy of the setting data (including user defined logic schemes whereused) can also be stored on the computer, for later printout and/or upload to the users

database facilities.

Following are the various protections recommended for the generator and generator

transformer protection.


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PART II


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INTER TRIPS

When considering protection of the generator the prime mover must always be

included. For example in the response to a winding failure inside the generator it would

not be sufficient just to trip out the main breaker and disconnect the unit from the

electrical system. We would also need to trip the stop valve and shutdown the prime

mover so as to prevent further damage.

Similarly if a problem occurs within the prime mover which necessitates

tripping the unit, inter trips must be provided to trip out the generator breaker as well.

When the generator is tripped from the system, its excitation system must

also be deenergised by tripping the field breaker.

FIGURE


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CLASSIFICATION OF PROTECTIONS

protection of alternators are classified based on the rating of the m achine and the

purpose for which the machine employed .

Basically protection schemes are categorized into three classes.1. CLASS-A Protection.

2. CLASS-B Protection.

3. CLASS-C Protection.

1.Class-A-Protection:-

This scheme gives the protection against the faults within the unit and auxiliaries

directly connected to the unit where in the entire unit with its auxiliaries and prime

mover to be shutdown instantly.

The unit is tripped in this scheme via reverse power protection.

Class-A-Protection schemes:-

1.Generator differential protection

2.Generator stator 0-to-100% Earth fault protection

3.Buchholz protection (UAT&GT)

4.Generator Transformer & unit auxiliary Transformer overall differential

protection

5.Protection against interturn faults

6.Generator rotor Earth fault protection

7.Generator loss of excitation

8.Backup impedance protection

9.Protection against motoring

a) Low forward power protectionb) Reverse power protection

10.Generator,Generator Transformer and unit auxiliary over fluxing protection

11.Generator Transformer unit protection

12.Generator over voltage protection

13.Breaker fail protection

2.Class-B-Protection:-

This scheme gives the protection against the fault occurring in the prime mover

or Auxillaries where safe shutdown of the unit is possible .


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Class-B-Schemes: .

1.Generator Transformer oil temperature very high.

2.Commands from automatic voltage regulator control

III. Class-C-Protection:-This scheme gives the protection against the faults occurring in the grid affecting

the unit where the unit can be isolated from the grid but the primemover and auxillaries

retained for synchronizing the unit back to grid at the earliest.

Class-C-Schemes:

1. Generator negative phase sequence protection.

2. Generator overload protection.

3. Generator pole slipping.

4. Under frequency protection .

5. Generator Transformer over current & Earth fault protection.

CLASS-A-PROTECTION SCHEMES

1.Generator differential protection:-

Fig .1 shows the schematic diagram of percentage differential protection. It is

used for the protection of generators above 1MW. It protects against winding faults i.e. phase to phase and phase to ground faults. This is also called biased differential

protection. The polarity of the secondary voltage of C.T. s at a particular moment for an

external fault has been shown in the fig. In the operating coil, the current sent by the

upper C.T. is cancelled by the current sent by the lower C.T. and the relay does not

operate. For an internal fault the polarity of the secondary voltage of the upper C.T. is

reversed as shown in the fig 2. Now the operating coil carries the sum of the currents sent

by the upper C.T. and the lower C.T. and it operates and trip the circuit breaker.

The percentage differential protection does not respond to external faults and overloads.

It provides complete protection against phase to phase faults. It provides protection against

ground faults about 80 to 85% of the generator winding it does not provides protection to 100%

of the winding because it is influenced by the magnitude of the earth fault current which

depends upon the method of neutral grounding. When the neutral is grounded through an

impedance, the differential protection supplemented by sensitive earth fault relays.


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Due to the difference in the magnetizing currents of the upper and lower C.T.s the

current through the operating

FIGURE


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The current entering and leaving the protected object are determined current

transformers and compared by relays by means of differential circuit as shown in fig. A

fault inside the protected zone is fed from either one side or both sides depending upon

the current sources present, thus producing a difference current in the differential circuit.If this differential circuit exceeds a set of percentage of the current flowing in the

protected object, the relay picks up.

The relay used is designated 87G & is RADHA & RADSB type. It is to operate

10% (0.5 amp) relay current which corresponds to 1000 Amp fault current.

2. EARTH FAULT PROTECTION:

a. STATOR EARTH FAULT (MAIN):

The generator neutral is earthed through the primary winding of the neutral

grounding transformer of rating 500 KVA, 15.75/.24 kV ratios. The secondary winding

of transformer is shorted through loading resistance of .42 ohms for an earth fault in the

generator the E/F current flow in the primary of the neutral grounding transformer. As a

result a voltage across the resistor is developed which activates stator earth fault sensing

relay. The reason for this kind of protection is due to mechanical damages resulting from

insulation fatigue creep age of conductor bases, vibration of conductor or other fittings of

cooling systems.

The earth fault relay designated is RAGEA type 64G. The relay has an inverse

definite minimum time characteristics. Generally 5% Generator winding starting from

neutral point remains unprotected because a fault in this portion will generate too low

voltage for relay operation.

b. STATOR EARTH FUALT PROTECTION:

The relay is connected across of the generator PT secondary edges. When there is

no E/F the sum of phase voltages of the generator. Hence the voltage across the relay is

zero. The voltage across the point a & b will assume a positive value when one phase

voltage drops because of earth fault on that phase.

The relay designated is 64G , RAGEA static type relay. It has inverse time voltage

characteristics.


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C. ROTOR EARTH FAULT:

First ground leakage in the rotor circuit of the generator does not adversely effect

the operation,Danger arise if a second fault occur causing current to be diverted in part at

least, from the intervening turns which can burn the conductor causing severe damage tothe rotor. If a large portion of winding is shorted the field flux pattern may change the

flux concentration at one pole and wide dispensation at the other the attractive forces

which is proportional to the square of the flux density will be stronger at one pole than

the other which will cause high vibrations and may damage the bearings and may

sufficiently displace the rotor thereby fouling the stator.

Rotor E/F protection is provided by monitoring the I/R value of rotor winding.

< 5.5 K alarm

< 2.2 K Trip

3. Stator Inter Turn Fault:

When leakage occurs between the turns in the same phase of a winding the

induced voltage is reduced and there will be a voltage difference between the center of

the voltage triangle and neutral of the machine.

Therefore, in a generator having one winding per phase, a voltage transformer is

connected between each phase terminals and, the neutral of the winding, the secondary

winding transformer leads being connected in open delta, when inter turn leakage occur

at the ends of the open delta, it is detected by the polarized voltage relay, for generators

having several parallel windings per phase, the neutral ends are connected together to as

many neutrals as parallel windings per phase. These neutrals are then joined through

current transformer to current relay, or though voltage transformer to voltage relay. If an

inter turn fault occur in the machine, the current transformer carries transient current or alternatively voltage transformer produce thereby picking up relay and tripping the

generator.

The relay designated 59 ( U>) and RXEG / RXEDK type static relay is

used for this type of protection.


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GENERATOR INTER-TURN FAULT

4. NEGATIVE PHASE SEQUENCE:

The currents in three-phase machine are normally in balance, but if a fault occurs

on the supplied or supplying system, this balance can be influenced. Single phase andtwo-phase faults, phase rupture or asymmetric loading on the system can give rise to

unbalanced currents, hence negative sequence currents.

Three currents generate a machine stator flux that has the same rotational speed as

the rotor flux but rotates in the opposite direction. Relative to the rotor, the stator flux

therefore rotates at double the power system frequency and generators eddy currents in

the rotor. The high frequency of these eddy currents causes the outer parts of the rotor,

and the winding to become heated. If the negative sequence current is of high

magnitude, or if it persists for long periods of time, these rotor parts can be damaged

due to overheating. It is normally assumed that a generator can sustain negative

sequence currents which exceed a given minimum value for a period of time t, which is

determined from the following equation:

t = K ( I m / I nsc ) 2

where

I m = rated current of the machine

I nsc = negative sequence current

K = a constant in seconds that is characteristic for the generator. This

constant represents the length of time the machine can withstand a negative

phase-sequence current equal to rated current.

The validity of this equation is based on the assumption that all the energygenerated by the negative sequence current is transmitted in the form of heat to the rotor

without any losses, to the surroundings.


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FIGURE


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The relay is used is designated 46G and RARIB static relay is using for this type

of protection.

5. GENERATOR BACKUP IMPEDANCE PROTECTION:

Three-phase zone impedance is provided for the back-up protection of generator

against external three phases and phase to phase fault in 400 KV systems. The zone of

impedance relay should be extended beyond 400 kV switchyard and it should be

connected to trip the generator after a time delay of 1 to 1.5 seconds so that the generator

is tripped only when 400 kV protection has not cleared the fault even in the second zone.

The relay used is designated 21 G and RAKZB type relay is used for this protection..

6. LOSS OF EXCITATION:

Failure of field system leads to losing synchronism and resulting in running above

the synchronous speed. It acts as an induction motor, the main flux being produced by

wattles stator current drawn from the system. Operation as an induction generator

necessitates the flow of slip frequency current in the rotor, damper windings, float

wedges excitation under these conditions requires a large reactive component which

approaches the value of rated out put of the machine. The induced currents in the rotor as

result of this condition, rotor would get over heated due to the slip frequency current.

The magnitude of the reactive power drawn from the system is a function of the machine

reactance and the system source impedance. The resultant current will not be steady

state, but is pulsing, and conventional time delayed over current relays cannot protect thegenerator. Also it could over load the grid, which may not be able to supply the required

MVAR.

When loss of excitation is accompanied by under voltage it will initiate class. A

trip other wise class B trip if the grid is able to sustain the voltage dip.

The relay used is designated 40 G and RAGPC is suitable for all types of

synchronous machines to protect from the loss of excitation.


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7. POLE SLIPPING:

The asynchronous operation of machine while the excitation is still the intact

unlike the loss of excitation, cause sever shock to the both machine and grid due toviolent operation oscillations in the both active power and reactive power. Because of

these machine fall out of step or usually known as pole slipping trip. The oscillations

may disappear in few seconds, in that case it is not desirable to trip the machine. If

however an angular displacement of the rotor exceeds the stability limit of the rotor will

slip a pole pitch. If this disturbances has been sufficiently reduced by the time this has

occurred, the machine should regain synchronism, But if it does not. It must be isolated

from the system.

The swing curves can be detected by the impedance relay. The relay has two

measuring elements set at two values near the impedance as seen from the relay as a

relay impedance seen by the relay changes it comes in the operating zone of the two

relays one after the other. The sequential operation is observed by auxiliary relays. Since

the fault would be with in the 55 ms. However, during pole slipping, two elements would

operate sequentially and a trip command is given when both have operated.

The relay can be set to be in the operation for swing up to + _ 90 deg.

Corresponding to the stability limit of the generator.

The relay used is designated 98 G and is of solid-state design of ZTO type.

In order to discriminate against swing on the grid the tripping is through an

impedance relay (98 GY) set with a reach up to the 400 kV yard. RXZF / RXPE relay is

used for protection against pole slip or out of step.

7. OVER VOLTAGE:The generator winding is rated for 15.75 kV terminal voltage, sustained over

voltage would unduly stress the winding insulation and may lead to failure after some

time. To protect the machine against the over voltage the protection relay senses the

voltage at the secondary of the bus duct PTs. The relay is set to operate at 10% rise in the

terminal voltage. A time delay of 3 seconds is provided to take care of transient over

voltage arising from line charging, switching capacity faults etc. The relay used is

designated 59 G & RXEG type static relay used for this protection.


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G.T. OVER FLUXING:

The iron core of the generator transformer carries the flux to produce required

EMF. If the flux increases UN dually the magnetic circuits of the generator, generator

transformer become over saturated resulting in high magnetizing current. This in turnleads to higher to iron losses, which will increase the winding temp of the transformer.

Since core can be damaged because of this over heating, protection has to be provided

against it. The flux is dependent on ration of voltage & frequency.

The condition of over fluxing could arise in the case the voltage at the machine

terminals rise or its frequency drops or both occurring simultaneously. Particularly this

condition will arise if the machine AVR misbehaves thereby unduly increasing voltage

even if the grid frequency is low.

The relay used is designated 99 GT and is GTT 21 type which senses v/f At the

secondary of the bus duct P.T. And it gives alarm and trip signals at different time delay.

The adopted setting for relay if v/f = 1.2 P.U. i.e. 20% higher than rated v/f ratio.

Alarm is set at 0.5 to 1 sec. & trip at 12 sec.

This v/f relay generates an AVR raise block.

Surge voltage originating from lines because of switching or atmospheric

disturbances are dealt with directly by lightening arrested and surge diverter.

8. LOW FORWARD POWER PROTECTION:

When the generator synchronized with the grid, losses it driving force the

generator remains in synchronism. The generator should be isolated from the grid after

the steam flow ceases and the flow of power to the grid reduces to the minimum i.e. the

point when generator starts drawing power from grid and acts as motor.When load on generator drops to less then 0.5%, generator low forward power

relay gets energized and with turbine tripped or stop closed, trip the generator with the

time delay of 2 seconds. This is protection to trip generator on the other than the

electrical fault. And also this protection used for few electrical faults where the generator

trip can be delayed.

However, provision for time lag unit is there to prevent UN desired operation

from transient power reversal.

The power relay used is designated 32 G1 and RXPDK type relay used for this

type of protection.


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9. REVERSE POWER PROTECTION:

The generator must be disconnected from the grid as soon as turbine stop valve is

closed, completely shutting off the steam. Continued full speed turbine rotation causeslot of turbulence of the trapped steam, which result in increase of temperature. Thus

turbine will be subjected to excessive thermal overstress, vibrations and distortion. So

there is back-up arrangement to trip generator if it does not trip with in 2 seconds i.e. on

low forward power (L.F.P.) protection. This is known as reverse power protection, which

acts in two stages.

1.Reverse power relay operates after 5 seconds time delay and includes stop valve

closing/turbine trip.

2.Reverses power relay acts after 15 seconds time delay which trips the generator

irrespective of the either stop valve closing or turbine trip. This acts as a final back up to

L.P.F. protection.

10. UNDER FREQUENCY PROTECTION (81G):

The under frequency protection

Prevents the steam turbine & Generator from exceeding the permissible operating

time at reduced frequencies.

Ensures that the Generating unit is separated from the network at a present value

of frequency that is less than the final stage of system load shedding. Prevents the AVR from exiting the machine at reduced speeds when protective

relays may not perform at all.

Prevents over fluxing of the generator. The over fluxing relay is used to protect

against small over fluxing for long periods while the over voltage and under

frequency relay also protected against large over fluxing for short times.

The stator under frequency relay measures the frequency of the stator terminal

voltage.


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Though under frequency tripping is recommended by turbine manufacturers, care

should be taken by grid operator personal in ensuring, that machines not run at

lower frequencies and instead resort to means like load shedding in the event of over load.

Requirements:

1. Have one alarm stage two tripping stages.

2. Shall have settings of range 45 Hz to 55 Hz with a least count of 0.1 Hz for each

stage.

3. Shall have under voltage blocking.

11. LOCAL BREAKER BACK-UP PROTECTION:

This is protection against the main generator breaker failure which may occur due to-

(1) Mechanical failure

(2) Trip circuit not healthy

Hence, this protection acts as a back up to the main generator by tripping all breakers connected to that particular bus. The relay designated as 51 and RAICA used

for breaker failure protection.

RELAY SENSING:

(1) D.C. to the relay extended through trip command (either 86 G or 286 G or

B/B Protection trip)

(2) Over current element senses actual fault persisting.

When both the above conditions are satisfied LBB protection acts as with a time

(0.2 sec) to trip all other breaker connected to the bus.

The LBB protection initiates bus bar protection.


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PROTECTION OF TRANSFORMERS

NATURE OF TRANSFORMER FAULTS:

Power transformers, being static, totally enclosed and oil immersed develops

faults only rarely but the consequences of even a rare fault may be serious unless the

transformer is quickly disconnected from the system. For the purpose of discussion,

faults can be divided into three main classes:

1) Faults in the auxiliary equipment, which is part of the transformer.

2) Faults in the transformer windings and connections.

3) Overloads and external short circuits.

1. FAULTS IN AUXILIARY EQUIPMENT:

The detection of faults in auxiliary equipment is necessary to prevent ultimate

failure of the main transformer windings. The following can be considered as auxiliary

equipment.

i) Transformer Oil:

Low oil is a dangerous condition in a transformer because live parts and the leads

to bushings, etc. which have to be under oil, gets exposed if the oil drops belowthe specified level. Oil level indicators with alarm contacts are available to give

indication for immediate attention.

ii) Oil pumps and Forced Air Fans:

The top oil temperature normally gives indication of the load on the transformer.

Increased oil temperature might be an indication of an overload or it might be due to

fault in the cooling system, such as failures of the oil pump or the blocking of a

radiator value, or no operation of fans. A thermometer with alarm contacts will

indicate rise in the oil temperature due to any of these faults. An oil flow indicator is

commonly used to indicate proper operation of oil pumps.


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iii) Core and Windings Insulators:

Incipient faults may occur initially, which may develop into major faults if not

taken care of at the initial stages. Insulation failure may develop because of the

following:a) The insulation of laminations and core bolts may be of Poor quality or

has been damaged accidentally during erection.

b) The insulation between the windings, between winding and the core

and the conductor insulations may be of poor quality; may have been damaged

mechanically; may be brittle because of aging or overloading.

c) Badly made joints or connections:

The incipient faults need to be attended to immediately and as soon as possible. Gas

actuated relays described later in this chapter provide indications alarm for incipient

faults.

2. WINDING FAULTS:

Electrical faults, which cause immediate serious damages and are detected by

unbalance current or voltage, may be divided into the following classes:

i) Faults between adjacent turns or parts of coil such as phase-to-phase faults

on HV and LV external terminals or on the windings itself or short-circuit between

turns of HV and LV windings.

ii) Faults to ground or across complete windings such as phase-to-earth faults

on the HV and LV external terminals or on the windings.

A short circuit between turns can start with a point contact resulting from

mechanical forces or insulation deterioration due to excessive overload or a loose

connection, breakdown of transformer insulation by an impulse voltage. The puncture of the turn insulation by an impulse is supposed to cause a path of destruction, through

which normal frequency voltage can maintain an arc. However, if the turn voltage is

insufficient to maintain the arc it will quench by the oil at the first current zero.

Faults to ground, or across a large part of the winding will result in large values of

fault currents as well as the emitting of large amounts of gas due to the decomposition of

oil This type fault is not difficult to detect, but rapid clearance of fault is essential to

avoid excessive damage and to maintain system stability.


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3. OVERLOADS AND EXTERNAL SHORT CIRCUITS:

Excessive overloading will result in deterioration of insulation and subsequent

failure. It is usual to monitor the winding and oil temperature conditions and an alarm is

initiated when the permitted temperature limits are exceeded. External short circuits mayonly be limited by the transformer reactance and where this is low fault currents may be

excessive.

DIFFERENTIAL PROTECTION OF TRANSFORMERS:

Differential protection is the most important type of protection used for

internal phase-to-phase and phase-to-earth faults and is generally applied to transformers

having ratings of 5 MVA and above. Any deviation from the normal ratio of the current

intensities, at the input and output ends, must of necessity be caused by a fault in the

protected part, so that the unbalance current can be employed directly for tripping and

indicating the fault. For this reason current differential protection combines highest

selectivity with the lowest tripping time.


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The differential protection of transformer is also known as Merz-Price

Protection for the transformer. Figure shown an ordinary differential protection for a

three-phase star-delta power transformer. In a star-delta transformer, the load currents in

the two windings are not in direct phase transformer, the load currents in the twowindings are not in direct phase opposition, but are displaced by 30 degree centigrade

and to allow for this the CT secondaries are connected in delta on the star side and in star

on the delta side.

PROBLEMS ENCOUNTERED IN DIFFERENTIAL

PROTECTION OF TRANSFORMER:

The differential scheme described suffers from the following drawbacks:

i) Unmatched characteristics of current transformer.

ii) Ratio change as a result of tapping.

iii) Magnetizing inrush current.

CT Characteristics:

Unless saturation is avoided, the difference in CT characteristic due to different

ratios being required in the circuits of different voltages may cause appreciabledifference in the respective secondary currents whenever through-faults occur. This

trouble is aggravated in the case of transformers due to unequal ration current

transformers being employed on either side of protected transformer. A source of ratio

error, which results in circulating currents under through-fault condition, is unequal

burden imposed on the current transformers due to unequal lead lengths.

Ratio Change as a Result of Tapping:

Tap changing equipment is a common feature of a power transformer that

effectively alters the turns ratio. Compensating for this effect by varying the tapings on

differential-protection current transformers is impracticable.

Based or percentage differential relays ensure stability with the amount with the

amount of unbalance occurring at the extremities of tap-change range. Based relays are

better suited to the overall protection of variable-ratio transformers.

Magnetizing Inrush Current:


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When the transformer is energized, the transient inrush of magnetizing current

following into the transformer may be as great as ten times full-load current and it decays

relatively slowly. This is bound to operate the differential protection of the transformer

falsely. This magnitude of the magnetizing inrush current is a function of the permanentflux trapped in the transformer core and the instant on the voltage cycle when it is

switched on.

There are a number of ways of ensuring immunity from operation by magnetizing

surges. Firstly, the relay may be given a setting higher than maximum inrush current;

secondly the time setting may be made long enough for the magnetizing current to fall to

a value below the primary operating current before the relays operates. The simple

remedies are incompatible with high speed and low primary operating current. In third

method the harmonic content of the current flowing in the operating circuit is filtered out

and through a restraining coil.

Gas Actuated Relays:

When a fault occurs inside the transformer tank gas is usually generated, slowly

for an incipiently fault and violently for heavy faults most short circuits developed either

by impulse breakdown between adjacent turns at the turns of the winding or as a very

poor initial contact which will immediately heat to arcing temperature the heat

produced by the high local current causes the transformer oil to decompose and produce

a gas which can be made use of to detect the winding faults.Based on this following

relays are available:

1. Gas accumulator relay popularly known as buchholz relay actuated by the gas

formed.

2. Rate of pressure rise relay that acts on the measurement of the rate o9f formationof gas.

3. Pressure relays and pressures relief devices, which have for their actuation a

measurement of the total accumulated pressure.

4. Gas analyzers, which act on the analysis of products of decomposition.

Buchholz relay is the simplest for protection, which is commonly used in all

transformers provided with conservator. It consists of a chamber connected in the

upper side of the pipe run between the oil conservator and the transformer tank, and

containing two cylindrical floats, one near the top of the chamber and other opposite

orifice of the pipe to the transformer. Under the normal conditions the floats are up,


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but on the occurrence of, say a between-turns fault gas bubbles produced by the

breakdown of the oil flow out of the transformer in the direction of the conservator.

On reaching the Buchholz relay they are trapped and there by reduce the oil level in

the chamber and cause the upper float to fall. This is usually a slowly fault and whenthe float has fallen through a pre determined distance a pair of contacts, which are

controlled by the float, close and give an audible or visual warning.

However, if the fault is heavy, the surge of the gas and oil up the pipe towards the

conservator engages the lower float, which is pushed over instantaneously and engages

its associated contacts, which in turn the circuit breaker. Leakage of oil causes the upper

float to operate. Care must be taken during oil changing to avoid spurious signals.

The main advantage of the Buchholz relays are that they indicate incipient faults,

for e.g., between turns faults or core heating and so many enables a transformer to be

taken out of service before serious damage occurs. One important limitation, however, is

that they do not protect the connecting cables which must therefore have a separate

protection.

Hermetically sealed transformers are common in the United States. These

transformers are some times fitted with hydraulic relays. Which respond to the rate of

change of pressure in the gas cushion above the oil.

ON LOAD TAP CHANGER (OLTC):

On load tap changer for use with power transformers could be in their own tank

suitable for housing in the same tank along with transformers.

On load tap changer suitable for mounting in the same tank along with the

transformers are available in three phase unit for neutral end connection and single phase

Unit for connection in the middle of the winding. The design of this on load tap changer is based on modular construction and can be used at neutral end of windings up to 400

KV system or at the line end of windings up to 220 KV system.

The important features of the OLTC are:

i) The tap changer is of resistor type and this ensures minimum arcing at the

diverter switch.

ii) The main current is never interrupted during a tap change.

iii) The ohmic values of the transition resistors are chosen to reduce the voltage

variations during tap a change to a minimum.


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PROTECTION OFGENERATOR TRANSFORMER UNITS

In large high voltage systems generators are connected through step-up power

transformers to the main transmission circuit. For supplying generator auxiliaries a unit

of transformer is connected at the generator terminals.

It is usual to provide common differential protection for both generator and step-

up transformer and the intervening connections. This differential protection which covers

a zone from generator neutral to HV circuit breaker must take into account the phase

shift and current transformation in the step-up transformer. Magentizing inrush surges

due to restoration of voltage on fault clearance may cause unwanted operation of

protection scheme unless precautions are taken. Also the effective setting will differ

depending on position type of fault, i.e.HV phase or earth.

A typical overall differential protective scheme is shown in fig. A biased

differential relay with a setting of 20% and a bias of 20% is generally satisfactorily. This

overall differential protection does not include the until transformer, which has separate

differential protection.

For protection against earth-faults an earth-fault relay can be put in the generator

neutral or in the secondary winding of the step-up transformer. The differential

protection is effective against earth-faults in case the HV star side of the generator-

transformer is resistance earthen and thus restricted earth-fault protection provided. In

case it is solidly earthen the differential protection is quite adequate, but still a separate

restricted earth-fault protection of the instantaneous type is preferred and the differential

protection under such a case acts as a backup protection to the restricted earth-fault

protection. A scheme of restricted E/F protection along with differential protection for a

resistance earthen transformer on the HV side shown in the fig.


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FIGURE


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MOTOR PROTECTION

INDUCTION MOTORS

In order to understand the type of protection required, it is necessary first to

review the main characteristics of both the induction motor and the synchronous motor.

In the induction motor the stator windings are placed around the stator in such a

manner that when a three-phase voltage is applied, a rotating magnetic field is set-up. For

example, with the applied voltage having a frequency of 60 Hz on a four-pole machine,

the speed of the field is 1800 rpm. This is called synchronous speed. This rotating field

cuts the rotor conductors. The rotor consists of a series of solid conductor bars short-circuited at each end to form what looks like a squirrel cage. In order to improve the

magnetic circuit, the conductors are embedded in a laminated iron core.

The flux of the rotating magnetic field induces current in the rotor

conductors. Interaction of this current with the flux produces a torque, which causes the

rotor to rotate. The magnetic field rotates at synchronous speed but the rotor turns at a

slightly lower speed. This is essential in order to maintain some induced current in the

rotor conductors. The actual difference between operating speed and synchronous speed

is known as slip. Typically, the value of slip will vary between 18 and 33 as the load

increases from no load to full load.

When the stator winding is first When the stator winding is first energized, the

rotor is stationary, so a very large current is induced into the rotor conductors as the rate

of cutting by the rotating flux is maximum. Correspondingly, a very large stator current

is drawn, perhaps 7 or 8 times full load current. As soon as the rotor begins to accelerate

the current drops rapidly to about 5 or 6 times full load current. As soon as the rotor

begins to accelerate the current drops rapidly to about 5 or 6 times full load current and

remain fairly Constant at this level until the speed reaches about 80 percent of

synchronous speed. At this point, as the rotor approaches synchronous speed and the slip

rate reduces, the stator current falls of rapidly until at 100 percent synchronous speed it is

zero.


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Torque to the amount of mechanical turning force, which is available to drive the

load, says a large fan. The torque required by the fan is almost a straight line from zero

up to operating speed. When the motor is first switched on, the small amount of excess

torque causes the rotor to accelerate. At about 40% synchronous speed.This point is known as the pullout torque. From this point, as the speed

approaches synchronous speed, the amount of torque available rapidly falls, until it is

zero at synchronous speed. This is because the magnitude of current induced in the rotor

conductors decreases as it approaches synchronous speed. The motor will operate at the

point where the torque developed by the motor, equals the torque required by the fan.

The actual speed at which pullout torque is determined by the design of the motor

and depends upon the ratio of the resistance to the reactance of the rotor conductors. The

characteristic of the motor can be changed to meet the requirements of certain loads by

adding or subtracting resistance in the rotor winding. This is achieved by using a wound

rotor instead of the squirrel cage. The rotor winding is no longer short circuited, instead,

it is brought out to slip rings and these in turn are attached to an external resistance

which can be varied to suit requirements. Another effect of this is to provide variable

speed control.

i. INSULATION FAILURE:

The most common type of motor fault that can occur is insulation failure on the

stator windings. This causes a phase to ground fault, as the insulated conductors are fitted

tightly into the slots of the iron core. In many cases this ground fault leads to a phase to

phase short circuit.

ii. OVER HEATING:

Another common problem with motors is overheating, which may lead todeterioration and eventual failure of the insulation. Overheating can be caused by

overload, where the driven machine, say, a fan or pump, is working above the motors

rated capacity.

iii. STALLING OR LOCKED ROTOR:

If the overload is very great, perhaps due to a mechanical defect, then the motor will

stall and, the stator current will increase to a high value, perhaps 7 times normal rated

current. This condition is called locked rotor and can quickly cause severe damage to

the rotor and stator windings because there is no cooling with a stationary rotor.


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iv. LOSS OF VOLTAGE:

Another cause of overheating is low voltage. Even when the driven load is

normal, with low voltage, excess current will be drawn by the stator windings,

overheating can also be caused by the loss of one phase of the supply voltage. For example, if a single fuse were to fail the remaining two phases would draw excessive

current to continue driving the load. Even a difference between the three phase voltages

of the power supply can cause overheating. The unbalanced system conditions cause

negative sequence currents to circulate in the stator windings and this produces

overheating in the iron core.

v. RAPIC RESTART AT REDUCED SPEEDS:

A hazardous condition can occur with complete loss of voltage, particularly on

large, heavy machines. If the voltage is rapidly restored while the machine is still running

down in speed, an extremely large current will be drawn by the motor as it attempts to

arrest the fall in speed and then accelerate the machine up to normal speed again. In

some cases, due to the inertia of the heavy machine, this acceleration has even caused the

driveshaft to break.

BASIC MOTOR PROTECTION:

Generally speaking, motors up to about 1,000 horsepower operate at 600 volts

or less, while larger motors commonly operate at 4,000 volts or higher.

Small motors are generally energized through a contractor switch, which provides

some built-in protection. The contactor is held closed by the contractor coil. If the supply

voltage falls, the contractor opens and remains open until the operator effects a manualstart. A thermal overcorrect relay is usually built-in to protect the motor against over

currents. For medium size motors, say, 300 horse power and up, the contractor switch is

operated by external protection relays. In order to close the contractor contacts and start

the motor, the contractor coil is energized by closing the push button. This will close the

seal-in circuit allowing the push button to be released. As long as there is voltage on the

contractor coil, the contractor remains closed and the motor will run. To stop the motor

we must press the stop button to open the circuit. Protective relays operate in the same

manner that is, by opening their contacts in the contractor coil circuit.


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The thermal relay (49) provides overcorrect protection. It consists of a heater

element, which operates a bi-metallic switch. Excessive current flow will cause the

switch to open and thus interrupt the power supply to the contactor coil.

The thermal relay must be chosen and set to march the damage (thermal limit)curve of the motor as provided by the manufacturer.

In practice, we try to adjust the thermal relay so that its tripping curve is quite to

the damage curve in order to prevent inadvertent tripping. However, it is impossible to

get the curves to match exactly, and we will probably find that, for high values of

current, the bi-metallic switch is too slow.

One way around this problem is to add a time overcurrent relay. The time

overcurrent relay is set for a higher pick-up. The bi-metallic relay protects for lower

values of overload, while the overcurrent relay protects for higher values.

An improved method of protection against overheating directly measures the

actual temperature of the winding and stator core. When the motor is constructed,

resistance temperature detectors (RTDs) are embedded in the stator slots along with the

stator winding. These RTDs (also known as search coils) are connected detector. The

detector is made of material, which has a linear change in resistance as temperature

changes. Hence, by measuring the value of resistance, the relay can accurately determine

the temperature inside the motor. When the temperature exceeds a specified limit the

relay operates and de-energizes the motor.

Overheating can be caused by phase unbalance. Protection is often provided

against unbalance by the installation of a negative sequence overvoltage relay (47). The

relay is connected to measure negative sequence only. This is normally zero or at least

very low under balanced conditions. The relay is set to operate when negative sequencevoltage rises to, say 4% of normal terminal voltage. When the relay operates it trips all of

the motors on the bus. This relay also protects against phase reversal of the power supply

which could occur, for during a maintenance period.

For a high magnitude fault current, both the time overcurrent relay and the

thermal relay take over 10 seconds to operate. Moreover, in many instances the

contractor does not have sufficient interrupting capacity for high fault current. The motor

is normally protected against this condition by a set of fuses which are located up-stream

of the contractor. Typically, if the rated current of the motor is, say 120 amps. It operates

before the motor damage curve is reached.


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SYNCHRONOUS MOTORS:

The main feature of the synchronous motor is that the rotor winding must be

provided with DC current in order to set-up its own magnetic field. The rotating

magnetic field, produced by the stator, locks together with the rotor field and causes therotor to rotate at precisely synchronous speed. Synchronous speed depends upon

frequency, and on the number of poles in the rotor and stator. For example, at 60 Hz a 2-

pole motor has a synchronous speed of 3600 rpm; a 4-pole motor 1800 rpm, and so on.

As load increases, the rotor will continue to rotate at synchronous speed but the stator

will draw more current from the power supply system. The DC supply for the rotor may

be produced by a DC generator, which is coupled to the motor, or perhaps the supply

may be from an outside DC source. Synchronous motors are used where a precise,

constant speed is require. Another advantage of the synchronous motor is that any

adjustment of the DC excitation current adjusts the power factor of the motor. Reducing

the field current of the synchronous motor increases the demand for VARs from the

system and so causes the lagging power factor to fall. Increasing the field current

actually delivers VAR to the power system, so the motor operate at leading power factor.

This feature is most useful in large industrial installations where the synchronous motor

is commonly employed for power factor correction, thus reducing electricity bills.

Protection for the synchronous motor is similar to that for the induction motor, but

with the addition of a few special relays. One particular condition to protect against is

that of over-excitation of the rotor. Conceivably automatic power factor control or

voltage control could result in excessive DC current flowing through the rotor field

windings leading to overheating of the rotor. Protection is provided by either an

overexcitation relay (76) or a thermal bi-metallic relay (49) connection into the fieldwinding circuit. Another more direct method measures the resistance of the field winding

and from this calculates the temperature. These relays are wired to sound an alarm, or

perhaps in an unattended station, to trip the unit.Overheating of the stator winding is

protected by overcurrent relays, thermal relays, and current balance relays, just as with

the large induction motor. Another relay often found on synchronous motors is the loss

of excitation relay (40). If field current is interrupted for some reason or another, the

motor will operate as an induction motor, at lower speed, drawing a heavy reactive load

from the power system. This will cause serious overheating of both the rotor and the

stator windings. The loss of excitation relay operates by measuring current in the


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FIGURE


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excitation circuit on by measuring the flow of vars into the stator windings.

In power factor relay (5) is often used which is set to operate the power factor

falls below about 0.5 lagging. This also protects against loss of excitation.

Alternatively, an impedance relay may be used, as described in an earlier tape on

generator protection. The loss of excitation relay may be connected to only alarm the

operator, so that he may restore excitation. Alternatively, in an unattended station it will

trip the unit.

In special cases the synchronous motor is protected against loss of synchronism.

The motor could fall out of step if to correct power factor, the excitation was at a

minimum setting and the mechanical load increased. This could also happen if there gas

a sudden reduction in supply voltage. When pole slipping does occur, the state current

increases and the lagging powerfactor falls to a very low value, about.25.

If the motor continues to slip, the stator current and power factor will fluctuate

over a wide range as the rotor tries to pull into step.

Then out-of-step relay (78) detects the initial decrease in power factor and trips

the motor during the first slip cycle.

Synchronous motors are fitted with undervoltage relays (27) so as to prevent rapid

re-start if loss of voltage occurs. This is the same as in induction motors. Additional

protection for this condition in sometimes provided by an underpower relay (37). Thisrelay measures power being fed into the synchronous motor and it is usually set to

operate when power falls below 38 of non load maximum operation, that is, close to

zero.

Alternatively, a reverse power relay (32) could be installed. This relay will only

operate if the motor actually feeds power back into the bus and it can only do this if the

bus is connected to other motors which are drawing power. In all probability this could

be the case as most synchronous motors are used in large industrial installations.


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SUMMARY OF MOTOR PROTECTION:

1) STATOR FAULT

Instantaneous Overcurrent Relays 50Instantaneous Ground Relays 50 G

Inverse Time Ground Relays 51 G

Differential Protection Relay 87

2) OVERLOAD

Thermal Relays 49

Time Overcurrent Relays 51

3) LOCKED ROTOR

Time Overcurrent Relay 51

Impedance Relay 21

Acceleration Relay 18

Supervised Instantaneous Overcurrent 50

4) PHASE UNBALANCE

Negative sequence Overvoltage Relay 47

Current Balance Relay 46

5) LOSS OF VOLTAGE

Undervoltage Relay 27

6) EXCITATION

Loss of Excitation Relay 40

Overexcitation Relay 76

Rotor Winding Temperature Relay 49Power Factor Relay 55

7) SYNCHRONISM

Out-of-step Relay 78

Underpower Relay 37

Reverse Power Relay 32


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GENERATOR PROTECTION AGAINST SYSTEM DISTURBANCES

As the generator is synchronized to the power system, it is responsive to

disturbances, which occur on the system. As shown in fig. E-1, certain protective devices

are installed to protect against these conditions. One typical example is that of frequency.Large steam turbines are designed to operate within a very narrow range of speed i.e.

between 49.5 and 50.5 Hz.

High frequency can occur as a result of load rejection, perhaps as a consequence

of tripping transmission lines or load feeders. However the turbine governor will

normally control the turbine speed and maintain frequency close to normal. In case the

governor loses control, the turbine is fitted with an over speed trip, which is set to

operate at 110 per cent, say 55 Hz.

Low frequency can occur as a result of system overload. If the turbine generator

operates below 49.5 Hz, serious vibration and consequent damage may occur to the

large, low-pressure turbine blades. The turbine is permitted to operate at low frequency

only for very short periods of time typically:

48.5 -- 49.5 Hz - 60 Minutes accumulated

47.5 -- 48.5 Hz - 10 Minutes accumulated

A frequency relay (81) is installed to alarm or trips.

In practice, in a large interconnected power system, the frequency rarely falls

outside normal limits. However, such an extreme situation can occur, if the power system

becomes disconnected into separate areas or islands, so that each generation of group of

generators is supplying its own block of load. In some areas we will have too much

generation available, hence the frequency will initially rise. In other areas there will be

insufficient generation, and if load shedding is not rapidly initiated, the generator will become overloaded. The consequences will be:

1) A fall in frequency;

2) A fall in voltage;

3) A rise in stator current.

The voltage regulator will increase excitation on the generator in order to maintain

line voltage, and this may lead to overheating in the rotor. To protect the rotor,

overcurrent protection is sometimes installed in the excitation circuit. This relay is set to

alarm the operator.


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The stator winding may be protected from overheating by the installation of an

extremely inverse time overcurrent relay (50/51) set to operate just before the stator

winding short time thermal limit is reached. To present this relay operating during

normal operation, a combined instantaneous element is usually connected as a permissivefor the time overcurrent contacts. This will prevent operation of the unit below 115 per

cent of maximum rated current.

During a cold start-up, several hours are required to bring a steam turbine

generator unit up to speed. During this low speed period of the generator voltage at this

low frequency may be high enough to overexcite the main transformer primary. To avoid

this problem, an overvoltage relay (59F) may be installed to compare voltage and

frequency; this is known as a volts-bertz relay. This relay will operate at about 115% of

rated voltage when frequency is normal. At low frequency the voltage trip point will be

proportionally lower. This relay will also protect the stator insulation against overvoltage

at normal frequency.

Protection against closing the breaker out of phase is provided by connecting a

directional time overcurrent relay (67). When power flows into the generator, this relay

will operate and trip.

Another type of relay, which is often installed, is a synchronizing relay. This type

will not allow the breaker to close unless the phase angle is within a determined range

(usually 10 degrees either side of synchronism).

Operation of the generator is subject to the following limits:

1) Minimum excitation;

2) Overheating of the stator winding due to overload; and3) Overheating of the rotor winding due to excessive excitation on current.

The limits of generator operation are indicated by the units capability curve. A

typical units curve is shown in Fig.E-2. This shows the combination of megawatts and

mega vars that can be produced by the generator at different power factors. Positive vars

are vars supplied by the generator. Negative vars are fed into the generator from the

power system.

We cannot maintain the same MVA at lower power factor, due to the temperature

limit of the rotor winding. The capability of the generator is reduced at low lagging poor

factor.


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On the leading power factor side, very low excitation current may cause the rotor

to fall out of step, due to loss of magnetic torque. This is the steady-state stability limit.

There is yet another limit beyond this the overheating of stator iron which results

from excessive flow of capacitive currents. Usually the excitation to a dangerous level.

What would happen if the generator suffered a complete loss of field perhaps due

to a defect in the excitation circuit? In this situation, remember, the generator is still

connected to the power system, and is still delivering megawatts because it is still being

driven by its prime mover. However, it will no longer supply vars. On the contrary, it

will draw var from then system in order to maintain excitation. The power factor will

move to, say, 0.5 leading. So the generator will continue running and producing power as

an induction generator. However, this will probably lead to low voltage at the generator.

However, this will probably lead to low voltage at the generator terminals, and, more

importantly, serious overheating will occur in the stator iron. If the field cannot be

restored promptly, the unit should be down. The loss of field relay (40) may be used for

alarm or to initiate tripping of the unit.

Earlier loss of field relays worked by measuring current in the excitation circuit.

When this fall below a pre-set level, the relay operated after a time delay.

Nowadays, loss of field is detected by measurement on the generator high voltage

side. One method is to use a mega var meter set to operate when the imported (that is

negative) mega vars reach a high level, implying that the unit is operating as an induction

generator.

A more common method is to install an impedance relay, which compares the

state of voltage and current. The impedance characteristic is shown in Fig. E-3.


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FIGURE


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Following are the various protections recommended for the generator and generator

transformer protection:

Type of Fault ANSI Device No. Protection FunctionsGENERATOR STATOR Short Circuits 87 g

87 GT

21 G

51/27 G

Generator differential

Overall differential

Minimum impedance (or alternatively Over

Current / Under voltage)Asymmetry

Stator Overload

Earth Fault Stator

46 g

51 G

64 G1

64 G2

Negative sequence

Overload

95% Stator earth fault

100% Stator earth faultLoss of excitation 40 G Loss of excitationOut of step 98 G Pole SlipMonitoring 32 G / 37 G Low forward power/reverse power

(double protection for large generators)Blade fatigue 81 G Minimum frequencyInter turn fault 59 G / 87 GT Over voltage or over currentMag. Circuits 99 G Over fluxing volt/Hz.Higher Voltage 59 G Over VoltageAccidental Energisation 27/50 G Dead machineMonitoring 60 G PT fuse failureGENERATOR ROTOR:

Rotor Ground 64 F Rotor earth faultGENERATOR TRANSFORMER Short Circuits 87 GT

51 GT

87 T

Overall differential

Ove

ksm project

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