Protection of Generators 1

ABSTRACT

This project titled “STUDY OF PROTECTIONS OF 60 MW

TURBO-GENERATORS OF CAPTIVE POWER PLANT IN VSP”

covers the complete electric protections of Turbo generator in a Thermal

Power Plant of Visakhapatnam Steel Plant. The main objective of the

project is to study the various protections provided for the alternator and

the necessity of each.

The study also covers verification of the existing settings used for

different protective relays by actually calculating the fault currents under

various conditions.

Normally the faults in Generator can occur either inside like stator,

rotor or external to it in the bus-ducts of feeders. While internal faults in a

generator should be cleaned as fast as possible to minimise the damage of

the core where as the external faults should be sustained for a

considerable period to enable the respective down-stream protections to

act and isolate the same from the generator. In case of downstream

protections not clear a fault, a backup is to be provided in generator

scheme to isolate the same. Proper coordination between these

protections are also to be verified.

1

Chapter-1

INTRODUCTION

An electrical power system should ensure the availability of electrical

energy without interruption to every load connected to the system. Among all the

systems, electrical energy is quite transferable to the consumers by transmission

and distribution systems. The power supply to the consumer should not disturbed

and the agencies which supplies the power without interruption. As industrial

processes and plants have become more complex and extensive, the demand for

improved reliability of electrical power supplies has also increased. The potential

costs of outage time following a failure of the power supply or plant have

accordingly risen dramatically as well. If at all any fault happens in the system the

fault system has to be isolated so that other system should not be disturbed. This

is a challenge to the engineers to supply the uninterrupted power to the

customers. To attain this, logical and fast acting protective equipment is

required.

The protection analysis of generator is studied under existing system. The

protection analysis is very important for long period running of generator free

from mal-trippings. It is noteworthy that generator capacity increased from 10

MW to 500 MW. So the cost of the machine also increases naturally. If the

analysis of the relay settings is not proper the machine, may not trip during fault

condition due to which winding may burn or its insulation may get deteriorate as

high currents circulated in the stator windings of the machine. The adequate

protection of the machine depends upon choosing of suitable relays and also

their settings.

2

The protective equipment design depends upon the nature of faults

occurring in the power system. If at all fault happens in the balanced system with

huge power loss at fault point with unbalanced nature of the power system it is

difficult and laborious to calculate the fault currents and voltages. So, power

system engineers established some special methods to calculate fault conditions

such as Symmetrical & Unsymmetrical in nature. Among all the faults,

symmetrical three phase fault is most severe fault and LG fault is most occurring

fault.

The Power Grid Corporation’s Sub-station adjacent to Ukkunagaram is

connected to Vijayawada by a 400 kV line. It is also being connected to Jaipur,

Orissa (Eastern Grid) through DC back to back arrangement of 500 MW capacity

and by 400 kV AC double circuit line. Power is stepped down through a 315

MVA, 400/220 kV auto transformer at Power Grid Corporation Sub-station and is

fed to the adjacent AP TRANSCO switching station. This switching station is also

connected to Bommuru and Gajuwaka sub-stations by 220 kV double circuit lines.

Bommuru sub-station is connected to generating stations at Vijayawada, Lower

Sileru, Vijjeswaram, Kakinada and Jegurupadu. Gajuwaka sub-station is

connected to Upper Sileru. One 1,000 MW TPP has been set up near

Visakhapatnam as Simhadri NTPC. This plant is connected to Kalpaka switching

station. Power is supplied to VSP from AP TRANSCO switching station over two

220 kV lines on double circuit towers. Power is received at the Main Receiving

Station (MRS) located near Main gate and further distributed to various units

within the plant.

3

BASIC REQUIREMENTS OF PROTECTIVE RELAYING

A well designed and efficient protective relaying should have:

SPEED:-

Protective relaying should disconnect a faulty element as quickly as

possible. 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.

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.

RELIABILITY :-

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.

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.

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.

4

Chapter-2

INTRODUCTION TO VSP

Steel comprises one of the most important inputs in all sectors of

economy. Steel industry is both a basic and a core industry. The economy of

any nation depends on a strong base of Iron and steel industry in that country.

History has shown that countries having a strong potentiality of Iron and Steel

Production have played a prominent role in the advancement of civilisation in the

world. Steel is such a versatile commodity that every object we see in our day to

day life has used steel either directly or indirectly. To mention few it is used for

such a small item as nails, pins, needles etc. through surgical instruments,

agricultural implements, boilers, ships, railway materials, automobile part etc., to

heavy machines, structures etc. The great investment that has gone into

fundamental research in Iron and steel technology has helped both directly and

indirectly many modern fields of today’s science and technology. It would seen

very painful to image the fate today’s civilisation had steel not been there.

The per capital consumption of steel in India during 1970’s was around 10

kg compared to about 700 kg obtaining in many advanced countries, over 800 kg

in Japan. This was very low. Viewing in the backdrop of Indian population which

was standing at about 800 Million, even a 10 kg of increase in steel consumption

would need the setting up of a 8 Million ton of steel per year production capacity.

Keeping this end in view the Government of India had cleared the decision of

setting up of a shore based integrated steel plant at Visakhapatnam.

5

BACKGROUND

The decision of Government of India to set up an integrated steel plant at

Visakhapatnam was announced by the then Prime Minister Smt. Indira Gandhi in

the Parliament on 17th April 1970. Visakhapatnam Steel Plant (VSP), a public

sector undertaking, is a subsidiary of Rashtriya Ispat Nigam Limited. It is one of

the most sophisticated modern plants. The foundation stone for which was laid

by the former Prime Minister Smt. Indira Gandhi in the year 1972. It is

strategically located on the coast of Bay of Bengal in the state of Andhra

Pradesh. An integrated steel plant with the state-of-the-art technology.

Overcoming the perils of recession in the steel market it has turned into a profit

making organization with proper strategic decisions. The management of late

has been geared up to implement new technologies to meet the challenges of

completion and the dumping imports of steel. Visakhapatnam is in a

geographically advantageous position. Midway between Kolkata and Chennai. It

is easily accessible to major business centers.

A professionally organized steel plant, VSP has been the recipient of the

ISO 9001, ISO 18001, and the ISO 14000 certifications. Armed with these quality

standards and an advocate to the TQM it has achieved the six-sigma and 5S

level of efficiency. The management of the plant is organized into production,

marketing, finance, personnel, purchase and other auxiliary departments.

The plant is designed to produce 3.0 Million tones of liquid steel. It

possesses the state-of-the-art technology and a strong well trained manpower of

17,000 employees. The organization is house of technology where international

levels of efficiency are being pursued in terms of productivity and specific energy

6

consumption. For the financial year 2003-04 the organization recorded a

turnover of Rs. 6174 crores. VSP achieved a turnaround to post a net profit of

nearly Rs. 1521 crores. The net profit estimated for financial year 2005-06 is

16,000 Crores. In addition to a wide range of steel products there are other by-

products which are produced by VSP like tar, pitch and the noted fertilizer –

Pushkala. VSP because of its geographical advantage and standard products

has carved a niche for itself. Nearly 40% of the South Indian domestic market

has been captured for the steel products. International customers are from the

countries of China, Singapore, Russia, Nepal, Sri Lanka, USA, Japan, UAE etc.

VSP TECHNOLOGY : STATE-OF-THE-ART

7 Metre tall Coke oven batteries with coke dry quenching

Biggest Blast furnaces in the country

Bell less top charging system in Blast furnace

100% slag granulation at the BF cast house.

Suppressed combustion – LD gas recovery system.

100% continuous casting of liquid steel.

“Tempcore” and “Stelmor” cooling process in LMMM & WRM.

Extensive waste heat recovery systems i.e. BPTS, GETS, ECS.

Comprehensive pollution control measures.

ACHIEVEMENT AND AWARDS

1) Indira Priya Darshini Vrikshamitra for massive afforestation efforts

of RINL : 1992-93.

2) Award from Ministry of heavy industries for achieving MoU targets

for 2000-01.

3) National Energy conservation award-2002 -- First Prize in

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Integrated sector.

4) Certificate of appreciation for achieving OHSAS 18001 from

department of Factories, Govt. of Andhra Pradesh.

5) Award from Andhra Pradesh Pollution Control Board for significant

work "on cleaner production technologies".

6) Rolling shield on Environment protection.

7) II prize for RINL's stall at the International Trade fair.

8) Successful re-certification of ISO 9001 : 2000.

9) Quality circle & Suggestion related awards.

10) Udyog excellence Gold Medal award for excellence in Steel

Industry.

11) Best Management award from Govt. of Andhra Pradesh.

12) Best safety award from Govt. of Andhra Pradesh.

13) ISO 14001 accreditation.

14) OHSAS 18001 accreditation.

15) Best Taxpayer award from Govt. of Andhra Pradesh.

16) Award .from Andhra Pradesh Pollution Control Board (APPCB),

Govt of A.P. for best efforts in Rainwater harvesting.

17) Awarded 1st Prize by the Govt of India "Indira Gandhi Rajbhasha

Shield" for propagation of Official language (Hindi).

18) RINL was awarded "Best Enterprise Award" by WIPS in 2001.02

(forum of women in public sector) for making excellent contribution

in harnessing the growth and development of women and impacting

their lives significantly in and around the Public Sector.

19) VSP has bagged the "Greentech Safety Silver Award" for the year

2002.03 in Steel Sector for implementing best safety standards.

20) Vizag Steel's global rating has gone up to 67th position in 2002

from 68th in 2001 amongst World's largest steel producing

companies.

21) VSP has won the Rolling Shield for "Ecological Protection"

instituted by the Ministry of Information and Broadcasting.

22) Best HRD practices Award by ISTD for the year 2002-03.

23) Special award (being hat trick) for the lowest specific energy

consumption among integrated steel plant in India.

8

Chapter-3

INTRODUCTION TO THERMAL POWER PLANT

Power requirement of VSP is met through captive generation as well as

supply from ABSEB grid. The captive capacity of 270 MW is sufficient to meet all

the plant needs in normal operation time we have 3 units of each 60 MW and one

unit of 67.5 MW capacity.

BOILERS

Thermal Power Plant has 5 Boilers each of 330 T/hr. steam capacity at

101 KSCA and 540O C. The boilers are of BHEL make, capable of firing

combination of fuels namely, Coal, Coke Oven Gas, Blast Furnace Gas and Oil.

Normally 4 Boilers are kept in full load operation to produce 247.5 MW of power,

supply steam to 2 Turbo Blowers and process needs. Boiler’s outlet flue gas is

passed through Electro Static Precipitators to control air pollution.

TURBO GENERATORS

Thermal Power Plant has 4 Turbo Generators, three of 60 MW capacity

each and the fourth 67.5 MW . Special features of the turbo sets are :-

i) Electro Hydraulic Turbine Governing System.

ii) Central admission of steam to reduce axial thrust.

iii) Forced air cooled generators

Power is generated and distributed at 11 kV for essential category loads.

Excess power from TG-1, 2 and 3 is transferred to 220 kV Plant Grid through

9

step up/down transformers. All the Power Generated from TG-4 at 11 kV is

stepped up through a 220 kV transformer and transferred to plant grid.

TURBO BLOWERS

VSP has 2 Blast Furnaces. To meet the blast air requirement, 3 Turbo

Blowers, each of 6067 NM3 /min capacity, are installed at TPP. These blowers

are of axial type and are the largest blowers installed in India. The blowers are

provided with suction filters, pre-coolers and inter- coolers.

AUXILIARIES OF TPP

These include coal conveyors, cooling towers & pump house No-4 for

cooling water system, pump house for ash water, ash slurry , fire water and fuel

oil & HSD air compressor station, emergency Diesel Generators, electric switch

gear for power distribution, ventilation and air conditioning equipment etc. The

entire power generated at Back Pressure Turbine Station and Gas Expansion

Turbine Station is transmitted over 11 kV cables to power plant, stepped up

through a 220 kV transformer at LBSS5 and transferred to plant grid.

CHEMICAL WATER TREATMENT PLANT

Chemical Water Treatment Plant located in TPP zone produces high purity

De-mineralised Water and Soft Water. There are six streams of De-mineralising

units each capable of producing 125 cubic meters per hour each. Two softening

units of 125 m3/hr. each. DM water is supplied to TPP, Steel Melt Shop, CDCP

Boilers at Coke Ovens, and Rolling Mills. Soft water is supplied to Chilled water

plant-I, II and SMS mould cooling.

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CHILLED WATER PLANT NO-2

Chilled Water Plant No-2 located in TPP zone is having nine Chillers, each

having a chilling capacity of 337 M3/hr. The chilled water is supplied to TPP,

Blast Furnace and Sinter Plant for air conditioning purpose at 70 C. The return

water temperature is 160 C.

COKE DRY COOLING PLANT (CDCP) BOILERS

In VSP, hot coke produced in the Coke Oven Batteries is cooled by

circulating Nitrogen in Coke Dry Cooling Plant. The hot circulating gas is passed

through Waste Heat Boilers in which steam is produced at 40 KSCA pressure

and 4400 C temperature. There are three Coke Dry Cooling Plants, four Waste

Heat Boilers. Boiler is of 25 T/Hr Capacity.

BPTS & CHILLED WATER PLANT NO-1

The 40 KSCA steam generated in CDCP Boilers is utilised for driving 2

Nos. of 7.5 MW Back Pressure Turbines for generation of Power. The 2.5 Ata

exhaust steam is utilised for production of Chilled water in CWP-1. The 7 Ata

extraction steam is used for process requirements of CO & CCP zone . The

CWP-1 has 5 Chillers installed, each capable of cooling 337 M3/hour BPTS and

CWP-1 are housed in a single located near Battery No-3 of CO&CCP zone.

GAS EXPANSION TURBINE STATION (GETS)

Both the Blast Furnace of VSP are designed to operate at a high top

pressure of 2.5 Kg/cm2. The high pressure BF Gas is cleaned in Gas cleaning

plant and expanded in Gas Expansion Turbines driving electric generators. The

BF Gas after passing through the Turbine is fed to gas distribution net work and

11

is used as heating fuel in TPP & other units of VSP. Each Blast Furnace is

connected to a Gas Expansion Turbine of 12 MW capacity 7.5 MW of power is

generated by each of the turbine at full production level. GETS is located in BF

zone, between the two furnaces.

TPP ELECTRICS

There are 3 Generators present in the TPP. The capacity of each

generator is 60 MW, Generation of the voltage level 11kV, total generation is 180

MW out of this 30-40 MW are consumed by the Blast furnace, Steel Melting

Shop, Coke oven, Rolling mills and 30-32 MW is consumed by the TPP

auxiliaries. Remaining 110 MW is connected to LBSS5. In addition to this there

is one generator (TG-4) rated at 67.5 MW, 11 Kv, 90 MVA. There are two Gas

Expansion Turbines (GETS) and two Back Pressure Turbines (BPTS) rated 12

MW & 7.5 MW respectively. From this 20 MW is supplied to LBSS5. LBSS1

consumes a load of 40 MW. The remaining 157.5 MW is supplied to Main

Receiving Station (MRS). At this station LBSS2 consumes a load of 50 MW for

Blast furnace and Air separation plant. LBSS3 consumes a load of 5 MW for

LBSS3 MMSM and WRM. LBSS4 consumes a load of 30 MW. Township

consumes a load of 5 MW. There are two APSEB tie lines connected to MRS for

exporting and importing depending upon the conditions.

GSB-1

Generator Switch Board-1 is a 11 kV, 4500 Amps, 3 section board, located

at 0 meters in TPP. Each 60 MW generator is connected to each section of the

board. The bus is provided with a bus coupler and the bus coupler 4500 Amps

reactor between section-1 & 2 as well as between section-2 & 3. To sections-1 &

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3 of GSB-1 350 MVA, 220 kV/11 kV are connected. Out of these 3 transformers,

two are always in operation and the other one is standby. The 220 kV side of

transformers are connected to sub-station called Load Block Sub-Station-5

(LBSS). Power is evacuated through LBSS-5 transformers-1, 2 and 3 and is

distributed through the 220 kV network of the plant. All the critical loads of the

steel plant i.e. Water supply, Blast furnace, Steel Melt shop and Coke oven are

connected to GSB through 11 kV cables. In addition all the TPP auxiliaries are

also connected to GSB-1. All outgoing feeders are connected to GSB through

link-reactor and Minimum Oil Circuit Breaker (MOCB) in addition to earthing

switches.

Turbo Generators of TPP normally operate in parallel with state grid. All

three generators of each 60 MW are connected to Generator switch board. This

GSB-1 is a 11 kV 4500 Amps, 3 selection board located at 0 Mt level in AA’ bay.

All category-1 loads of the steel plant are connected to GSB-1 through 11 kV

cable. Power is evacuated through 50/63 MVA (11kV/220kV) transformer 1, 2 & 3

which are connected to Section-1 and 3 of the GSB-1. Synchronisation facility

exists for any of the incoming generator, 50/63 MVA Transformer-1, 2, 3.

ISLAND OPERATION SCHEME :-

A scheme has been envisaged at TPP to get isolated form the grid in case

of system disturbance or low frequency condition with ABB make relay type FCX

103b relay with following settings:

df/dt 2 cycles/sec rate of fall below 50 HZ

1st stage 47.5 HZ FOR 0.5 SEC.

over frequency 51.5 HZ with time delay 150 MS

2nd stage 46.9 HZ with 1 sec. delay

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POWER REQUIREMENT

Integrated Steel Plants are major consumers of electricity, with specific

consumption of power at around 600-650 kWh/Ton of liquid steel. The estimated

annual power requirement of Visakhapatnam Steel Plant, at full level of

production in each shop (corresponding to 3.0 MT of liquid steel), is 1932 million

kWh. This corresponds to an average demand of 221 MW. The estimated

energy consumption and average demand of major shops is given below:

SHOPAnnual Energy(106 kW Hrs.)

Average Demand (MW)

RMHP 35 4.0

CO & CCP 171 19.5

SINTER PLANT 254 29.0

BLAST FURNACE 210 24.0

SMS & CCM 126 14.5

LMMM 100 11.5

WRM 118 13.5

MMSM 100 11.5

CRMP 35 4.0

TPP 310 35.0

ASP 258 29.5

COM. STATION & CWP 131 15.0

AUXILIARY SHOPS 20 2.5

WATER SUPPLY 15 2.0

TRAFFIC & OTHERS 7 1.0

TOWNSHIP 28 3.0

LOSSES 14 1.5

TOTAL 1932 221.0

14

SOURCES OF POWER


supply from APSEB grid. The captive capacity of 270 MW is sufficient to meet all

the plant needs in normal operation time. In case of partial outage of captive

generation capacity due to breakdown, shutdown or other reasons, the short fall

of power is availed from ABSEB grid. Turbo Generators of VSP normally operate

in parallel with state grid. Excess generation over and above plant load is

exported to APSEB.

The agreement with APSEB provides for a contract demand of 150 MVA

and permit export of power. Tariff for import, export, demand charges, penalties

etc. are stipulated. For purpose of billing, import and export energy is separately

metered at Main Receiving Station.

POWER DISTRIBUTION IN POWER PLANT

220 kV & 11 kV LOAD BLOCK SUBSTATION-5 (LBSS-5)

Integrated Steel Plants are major consumers of electricity, with specific

consumption of power at around 600-650 kWh/Ton of liquid steel. The estimated

annual power requirement of Visakhapatnam Steel Plant, at full level of

production in each shop (corresponding to 3.0 MT of liquid steel), is 1932 million

kWh. This corresponds to an average demand of 221 MW.


supply from AP TRANSCO grid. The captive capacity of 270 MW is sufficient to

meet all the plant needs in normal operation time. In case of partial outage of

captive generation capacity due to breakdown, shutdown or other reasons, the

short fall of power is availed from ABSEB grid. Turbo Generators of VSP

15

normally operate in parallel with state grid. Excess generation over and above

plant load is exported to AP TRANSCO.

The agreement with AP TRANSCO provides for a contract demand of 150

MVA and permit export of power. Tariff for import, export, demand charges,

penalties etc. are stipulated.

LBSS-5 is located outdoor and it consists of 220 kV, 1250 A, 3 phase main

bus-1, main bus-2 and transfer bus. LBSS-5 is having 12 bays i.e. three nos. for

the three tie lines, two nos. of LBSS-1 lines, 3 nos. for the three 50/63 MVA

transformers, one no. for 30/40/50 MVA transformer, one for 90 MVA

transformer, one for Bus coupler & one for bypass. In case of any difficulty in

taking into service any of the 220 kV circuit breakers of transformer or lines as

the case may be, bypass breaker can be taken into service in lieu of the defective

breaker by charging the transfer bus. Both 220 kV main bus-1 & main bus-2 can

be paralleled & transformer feeders (T1, T2, T3, T4 & T5) can be either

connected to Bus-1 or Bus-2 or distributed between Bus-1 & Bus-2 depending on

operational/maintenance requirement. All 220 kV circuit breakers are SF6

breakers. Synchronising facility exists only for tie lines ML1, ML2, ML3, Bypass

and Bus coupler breakers at Control & Relay panel of LBSS-5 located in ECR.

The loads (lines or transformers) can be transferred form Bus-1 to Bus-2 and vice

versa live through ‘On Load Bus Transfer scheme (OLBT). A typical single line

diagram is enclosed.

SYNCHRONISATION

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The operation of connecting an alternator in parallel with another alternator

or with common bus-bars is kwon as Synchronizing. Generally, alternators are

used in a power system where they are in parallel with many other alternators. It

means that the alternator is connected to a live system of constant voltage and

constant frequency. Often the electrical system to which the alternator is

connected, has already so many alternators and loads connected to it that no

matter what power is delivered by the incoming alternators, the voltage and

frequency of the system remains the same. In that case, the alternator is said to

be connected to infinite bus-bars. It is never advisable to connect a stationary

alternator to live bus-bars, because, stator induced emf being zero, a short circuit

will result. For proper synchronization of alternators, the following three

conditions must be satisfied.

(1) The terminal voltage or effective voltage of the incoming

alternator must be the same as bus-bar voltage.

(2) The speed of the incoming machine must be such that its

frequency (=PN/120) equals the bus-bar frequency.

(3) The phase of the alternator voltage must be identical with the

phase of the bus-bar voltage. It means that the switch must be

closed at (or very near) the instant of the two voltages have correct

phase relationship.

Synchronisation facility exists for any of the incoming generators, 50/63

MVA Transformer 1, 2 & 3, Bus couplers and Bus couplers with reactors at 11

KV. The synchronising operation is to be carried out using synchronising trolley

in ECR.

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Chapter-4

THEORETICAL REVIEW OF RELAYS

TYPES OF PROTECTIONS: -

Two types of protection:

1. Primary protection

2. Backup protection

Primary protection :-

Primary protection is the first line of defence and primary relays clear faults

in the protected section as fast as possible. 100% reliability is not guaranteed for

protective scheme and also for associated CT’s, PT’s and CB’s. Therefore some

sort of backup protection must be provided.

Backup protection :-

Backup relays operate if the primary relays fail and cover not only the local

station but the next one also and have a time delay long enough for the primary

relays to operate if they can.

ELECTROMAGNETIC RELAYS

These are earliest form of relays devices used for power system protection

and till now they are the most widely used variety relays. All these devices

depend upon electromagnetic interaction and have mechanical movement

associated with them which Actually make or break pairs of contacts indicating

18

relays operations. The electromagnetic interaction is manifested either in form of

force of attraction or torque of rotation which has produced different classes of

electromagnetic relays

ATTRACTED ARMATURE TYPE

The type includes plunger, hinged armature relays. These are the simplest

type which respond to AC as well as DC. The principle of this type is that an

electromagnetic force is produced by the magnetic flux operating quantity . If this

force exceeds the restraining force , the relay operates . the electromagnetic

force is given by F = KI2 ------------ (1)

INDUCTION RELAYS

Induction type relays are the most widely used for protective relaying

purposed involving A.C. quantities. Torque is produced in these relays when one

alternating flux reacts with the current induced in the rotor by another alternating

flux displaced in time and space but having the same frequency. These relays

are classified depending on the type of rotor. If the rotor is a disc, is known a

induction disc relay. If the rotor is a cup, the relay called as induction cup relay.

The Actuating force is given by F = K 1 2 Sin ---- (2)

In the abnormal conditions or fault conditions the corresponding relay

contact operation which in turn operates the tripping relay type VAJHM called

master trip relay or Generator lock out relay which will trip generator circuit

breaker. Alarm annunciation will come on control desk.

The following are the relay details which are used for generator in TPP.

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DESCRIPTION Reference Make Type

Under voltage relay 27A/27B EE VAG 31

Generator under voltage relay 27 G EE VAG 21

Generator reverse power relay 32 G EE WCD11

Generator low forward power relay 37GA/GB EE WCD13

Generator field failure relay 40 G EE YCGF11

Generator negative sequence current relay

46 G EE CTN31

Definite time O/C relay with 1st high set unit

50T/51G EE CTU62

Voltage control over current relay 51 V EE CDV62

Generator over voltage relay 59G1/59G2 EE CAGM22

Voltage neutral displacement relay 64V EE VDG14

Directional inverse time over current relay

67RYB EE CDD21

Directional inverse time earth fault relay

67W EE CDD21

VT fuse failure relays 68A/68E/68PM EE VAGM-61

Generator out of step relay 78G EE ZTO

Generator differential relay 87G EE CAG34

Generator restricted E/F relay 64G EE CAG14

Generator under frequency relay 81G E FTG11

Sensitive E/F & Instantaneous O/C relay

64S/50X ABBRX1428X RX12 21

GENERATOR DIFFERENTIAL RELAY

This relay is used to protect against stator faults as shown in below figure.

A differential relay is defined as the relay that operates when the vector

difference of two or more similar electrical quantities exceeds a predetermined

value.

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External & Internal circuit connections of generator differential relay

When there is no fault in the generator winding for through faults the

current in pilot wires fed from CT connections are equal. The difference in

current I1 -I2 is zero. When there is a fault inside the protected winding, the

balance is disturbed and difference current (I1- I2) flows through operating coil of

the relays.

Relay settings:

Plug setting Range

R Y B

0.1 0.1 0.1 0.05 to 0.2

CTR- 4500/1A

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GENERATOR RESTRICTED EARTH FAULT RELAY

Connection diagram is shown in below figure. In circulating current

protection schemes, the sudden and often asymmetrical growth of the system

current during external fault conditions can cause the protective current

transformers to go into saturation resulting in a high unbalance current. To

ensure stability under these conditions, a voltage operated, high impedance

relay is used. It is a attracted armature relay.

External & Internal circuit connections of generator stator earth fault relay

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NEGATIVE PHASE SEQUENCE RELAY

Negative phase sequence currents to star resulting from unbalanced

loading produce a field rotating at twice synchronous speed with respect to the

rotor and hence induce double frequency currents in the rotor. These currents

are very large and result in severe over heating of the rotor.

It is necessary to limit the time for which negative phase sequence

currents can flow in a steam generator. The time for which a generator may be

allowed to operate with unbalance stator currents without danger of permanent

damage is obtained from expression:

I22 T = K

Where ‘K’ is the constant depending up on the type of the machine.

The E.E. make scheme suitable for this application employs a type CDN

relay which comprises an induction disc tripping unit having an adjustable inverse

time current ( I22 T ).

The relay settings I2s.

A B C Range K3

10% 10% 10% 7.5% 1

10% 1.78

15% 4

20% 7.1

30% 16

23

GENERATOR VOLTAGE RESTRAINED OVER CURRENT RELAY

Fault conditions cause a greater drop in busbar voltage than normal over

load, and this fact has been utilised in voltage restrained over current relay. The

relay has two operating characteristics viz. An over load characteristics

determine by the operation of instantaneous under voltage unit monitoring the

generator voltage. Under over load conditions, when the generator voltage is

usually near normal the instantaneous under-voltage unit is energised and the

short across the resistor in the shading coil circuit is removed. Below figure

shows the internal connections diagram.

External and internal connections of Voltage restrained over current relay

24

The relay operates on a long IDMT under fault conditions, when the

generator voltage falls to the setting of the under voltage unit, the resistor is short

circuited and the torque on the disc increased by 2.5 times so that the normal

setting currents are 0.4 times those marked on the plug board, and the relay

operates in Accordance with the fault characteristic.

Both characteristic of the relay as shown in figure.

Time / current characteristics of Voltage restrained over current relay

TIME IN SECS. – NORMAL VOLTS

100 50 30 20 15 10 8 7 6 5 4

0.4 0.6 0.8 1 1.5 2 3 4 5 6 8 10 15 20

0.4 0.6 0.8 1 1.5 2 3 4 5 6 8 10 15 20

30 20 10 8 6 5 4 3.5 3 2.6 2.2

TIME IN SECS. - LOW VOLTS

Plug setting - R Y B

25

1.5 1.5 1.5

Time multiplier - 1 1 1

CTR - 4500/1A

GENERATOR DIRECTIONAL OVER CURRENT RELAY

When fault current can flow in both directions through the relay location, it

is necessary to make the response of the relay directional by the introduction of

directional control elements. Directional over current relays are combination of

directional and inverse over current relay units as shown in figure.

GENERATOR DIRECTIONAL OVER CURRENT RELAY

26

Plug Setting:

R Y B Range

1 1 1 0.5 -2A

CTR - 4500/1A

PSM :

1 1.3 2 2.5 3 4 5 6 7 8 9 10 12 14 16 18 20

30 20 15 10 8 6 5 4 3.5 3 2.8 2.6 2.4 2.2

GENERATOR DIRECTIONAL EARTH FAULT RELAY

The relay is used for directional earth fault protection for generator.

Typical external and internal connection shown in figure above. Since the current

may be derived from any phase, in order to obtain directional response it is

necessary to obtain a related voltage. Such a voltage is the residual voltage of

the system. Which is vector sum of the individual phase voltage of the system.

The voltage developed across secondary terminals will be vector sum of the

phase to ground voltage.

This will be zero for balanced phase voltage, but for simple earth fault

conditions will be equal to the depression of the faulted phase voltage. In all

cases the residual voltage is equal to three times the zero sequence voltage drop

and is Displaced from the residual current. The residual are applied to the

directional element of the earth fault relay. The relay inverse time characteristic

scale is shown in figure below.

27

Relay settings:

Plug setting - 0.1 Range

TMS - 0.1 0.1 to 0.4

CTR- 4500/1A

PSM :

1 1.3 2 2.5 3 4 5 6 7 8 9 10 12 14 16 18 20

30 20 15 10 8 6 5 4 3.5 3 2.8 2.6 2.4 2.2

DEFINITE TIME/INSTANTANEOUS OVER CURRENT RELAY

This relay protects the generator against short circuit faults. This relay

contains definite time over current element and instantaneous high set elements.

The relays areas particularly suitable on systems where there is a wide variation

in source impedance.

Relay settings : Definite time over current relay.

Range

Plug setting - 0.95 0.5 - 2 Amps.

Time setting - 10 sec. 6- 60 sec.

Instantaneous setting - 4

GENERAL FIELD FAILURE RELAY

Under current relays connected in the field circuit have been extensively

but the most selective type of loss-excitation relay is a directional-distance type

operating from the A.C. current and voltage at the main generator field. The

enclosed figure shows loss-of-excitation characteristics and the operating

characteristics of one type of loss-of-excitation relay on R-X-diagram when the

28

excitation is lost, the equivalent generator impedance traces a path from the first

quadrant into a region of the fourth quadrant that is entered only when excitation

is severally reduced or completely lost. By encompassing this region within the

relay characteristic, the relay will operate when the generator first starts to slip

poles and will trip the field breaker and will trips the field breaker and disconnect

the generator from the system.

R-X diagram

The satisfactory application of YCAF field failure relay, requires full

knowledge of the operating conditions. i.e. the maximum rotor angle at which the

Machine can operate within the stability limit. In general practice is to use an

offset setting equal to half the Machine transient resistance and a circle diameter

equal to synchronous reactance of the Machine ‘Xs’ for rotor angle upto 900. The

external connection diagram of YCGF field failure relay is shown in figure.

29

General field failure relay

The relay movement is a high speed induction cup unit with operating,

restraint and bias windings adjusting the ohmic values of the diameter and offset

of the relay characteristic circle is provided by the combination of plug board and

potentiometer settings K1 K2 K3 K4 and K5. K1 is in the restraint circuit and

provided line adjustment of characteristics circle diameter. K2 potentiometer is in

the polarising circuit and should be set to coincide with offset taps K1 to establish

maximum torque angle at an offset settings. The characteristics offset is

obtained by injecting voltage from the current circuit through transactor. Taps for

this transactor are brought out to plug board K3 K4 and combinations of two plug

settings provide adjustment of offset settings. K5 allows course adjustment of the

characteristics circle diameter by selection of taps on an auto-in the resistance

circuit.

30

The setting values are

K1 - 0.9

K2 - 10

K3 - 0

K4 - 10

K5 - 170

Generally they are set K2 = K3 + K4 i.e. offset taps.

The diameter of the characteristics circle is K1 x K5 ohms.

X1 = 0.25 = 25%

MACHINE TRANSIENT IS SECONDARY OHMS

= 25 x 11 2 4500/1100 x 75 11000/110

= 18.15 .

Required offsetting = X 1 d = 18.15 2 2

= 9.05

We have put offset for 10.

Xs = 2 = 200%

MACHINE SYNCHRONOUS REACTANCE IN SECONDARY OHMS

= 200 x 11 2 x 4500/1 = 145.2 100 75 1100/110

Required circle diameter Xs = 145.2

As per relay design we have put setting

K5 - 170 and K1 -09

K1 x K5 = circle Dia

170 x 0.9 = 153

31

EARTH FAULT PROTECTION IN GENERATOR FIELD WINDINGS WHICH ARE NORMALLY UNEARTHED FIRST ROTOR EARTH FAULT

A single earth fault is not in itself dangerous since it does not cause any

fault current, but a second fault effectively short circuits or all parts of the field

system and the unbalancing of the magnetic force caused there by may be

sufficient to spring the shaft and make it eccentric. If the condition were allowed

to persist, however it might lead to reverse mechanical damage. The figure

shows the method detection using the principle of negative potential biasing,

where by an earth fault any where in the field circuit can be detected. The d.c

supply injection establishes a small bias on the alternator field winding circuit so

that are points are negative with respect to earth.

Earth fault relay

The rectified output of a transformer fed from the station L.V supply

provides a biasing potential approximately 30v. This is connected with positive

terminal through a current limiting resistor and the secondary winding of

transductor to the positive pole of the field circuit. When a fault occurs current

32

flows in the bias circuit and the dc winding of the transductor. This results in

saturation of the transductor core which reduces the impedance of the A.C.

winding thus allows the relay ‘A’ to operate. Thus giving alarm for first rotor

earth fault. The relay will not operate if A.C. auxiliary supply fails. Under this

conditions i.e. when auxiliary supply fails the relay ‘B’ which will be in picked up

condition when the auxiliary supply is available will drops off when auxiliary

supply fails. This then n.c Contacts of the relay ‘B’ will get through for “A.C.

failure” alarm circuit.

Technical data - Field ckt voltage - 0-450 v d.c

Bias ckt voltage - 240v 10% 5Hz

Bias voltage - 30v d.c

Sensitivity - 1ma DC 10%

Setting of the first rotor earth fault is 1mA. The circuit diagram of first

rotor earth fault is shown in enclosed figure.

SECOND ROTOR EARTH FAULT RELAY

When a single earth fault is detected in the d.c field circuit of a Machine,

the Machine has to be taken out of service at the first opportunity. This is

because if allowed to run with an E/F on the rotor a subsequent second earth

fault can cause severe damage to the Machine. However a relay which can

detect such a second rotor earth fault and trip out the Machine can make it

possible to run the Machine even with single earth fault with out any such risks

thus helping to preserve the generation Capacity.

The heart of the second rotor earth fault detection scheme is a very

sensitive transductor element. The AC winding of the transductor is connected in

33

series with a rectified AC voltage relay ‘A’. The DC winding of the transductor on

the other hand is connected in series with field earth fault circuit.

Under normal conditions i.e. when no DC current flowing the AC winding

and the transductor presents a high impedance and the AC winding and the

transductor presents a high impedance and the AC voltage applied is mostly

dropped across this winding. Hence the relay ‘A’ drops i.e. remains de-

energised.

When second rotor earth fault occurs a DC current flows through the

transductor DC winding which causes the impedance of the AC winding to reduce

considerably by driving the transductor core into saturation. Hence the applied

voltage is fully available across the relay and the latter operates.

The selector switch SW1 on the generator panel will have 4 (four)

positions as follows.

1. First rotor earth fault.

2. Balance

3. Test

4. Second rotor earth fault.

Normally the selection of switch will be put in position 1 in which the first

rotor earth fault relay will be in service. If the occurrence of the first rotor earth

fault of any one machine, the selector switch of the machine is put in position-2.

Connecting the coarse control potentiometer across the field winding circuit of

the affected machine.

Simultaneously the milli-ammeter is also inserted in the circuit. As can be

seen from the figure, the portions of the field winding on either side of the first

34

rotor fault and the coarse control potentiometer forms a DC bridge with mill-

ammeter connected across a pair of nodes. By adjusting the coarse/fire control

and the range selector switch of the milli-ammeter the bridge is balanced to mull

point. The selector switch is kept in position-3 in which milli-ammeter is replaced

by relay, but the relay trip circuit is isolated. After making sure that the relay does

not pick-up then the SW1 is turned to position-4. Thereby putting the second

rotor earth fault relay complete in service.

Protection against prime mover failure (Need of low forward power

relay and reverse power relay ): The effect of prime mover failure is to cause the

machine to motor by taking power from the system which may result in severe

mechanical damage and, in addition will impose a heavy motoring load on the

generator. The reverse power relay is normally used for two applications as a

reverse power relay to trip the generator when the machine starts motoring and

as a reverse power interlock device to prevent the possibility of a turbo-generator

set overspeeding should a steam valve fail to close completely after the

generator circuit breaker has opened on a fault.

Use a low forward power interlock instead of reverse power interlock. So

that it is not necessary the delay the generator circuit breaker tripping till the set

has actually started motoring. When low interlock with normally closed contact

as soon as the power supplied by the generator falls below O – 5 % of rated

power the low power relay resets and completes the tripping circuits to the

generator circuit breaker.

35

GENERATOR SENSITIVE EARTH FAULT PROTECTION

This protection is especially provided in the generator to sense any earth

leakage near to the neutral. Question may come why separate earth fault relays

is used. Reason is that other earth fault relay senses earth current of higher

magnitude. But when there is a earth fault near to the neutral it may not be

possible for the machine voltage to drive that much higher current. So one earth

fault relay having very low current operating unit is used for sensitive earth fault

relay. Since it is a low current setting relay it may mal operate. To protect it from

mal operation we have used another relay which will sense earth fault current in

earthing transformer neutral which will ensure that definitely some earth fault is

present. This may operate generator terminal earth fault also. To prevent to do

so we have put over current relay 50 X which will sense over current and prevent

the earth fault relay to operate.

RELAY SETTINGS : 64GS - 2.5 ma

CTR - 4500 / 1A

50 X - R Y B

2.5A 2.5A 2.5A

50NX1 (From earthing transformer # 1 neutral CT)

setting - 0.05 Amps

CTR - 200/1

PARTIAL DIFFERENTIAL PROTECTION SCHEME

This relay is meant for to protect the GSB-1 Bus sections. There are three

sections. For each section one relay is provided. Partial differential relay

contains two separate relays. One is IDMT/INST over current relay for phase

36

faults and other one is IDMT earth fault relay for earth faults. The partial

differential relay will act when either of those two relays are acted.

IDMT/INST over current relay:

Plug setting Range TMS

IDMT R Y B

1 1 1 0.5-2 1

Instantaneous setting - 4

IDMT earth fault relay :

Plug setting Range TMS

0.1 0.1 - 0.4 0.2

GENERATOR OUT OF STEP RELAY :-

A CAG 19 relay serves as an over current starter and this is set at

between 50 and 20% of nominal current. Based on 5 A CTs below the current no

operation can occur. Both character look into the source and consequently

ignore all conditions of load other than those which produce a reversal of power

flow such as would occur with a condition of pole slip or power swing exceeding

90O. The timer is incorporated so that discrimination can be made between a

power sourcing and a pole slip condition. A trip condition can only occur if the

timer has timed out before the fault moves into the blinder operate region. If the

fault never reaches the operate region of the blinder or moves between the

directional and blinder characteristics in a time less than the time setting than no

operation will occur. The relay consists of a directional unit and a reverse reach

blinder based on the YTG Mho type static relay measurement technique and

incorporates these components.

37

It is used to protect synchronous motors against the effects if pole slipping

caused by excessive load or insufficient field excitation, pole slipping in the

generator slowing down and losing synchronism. The ZTO relay consists of a

directional unit with a variable lag angle (1) setting between 50 to 75O, and a

blinder unit also with a variable lag angle (2) setting between 50 to 75O which

has reverse resistive reach setting between 0-25 ohms and 32 ohms.

Since it can deal only with a pole slip condition emanating from are

direction. The ZTP relay is limited to applications in close proximity to a

generator.

R1 = 0.90 to 1 R2 = 0.55 to 1 R3 = 0.5 to 32

The relay setting :

K1 = 0.92

K2 = 0.82

K3 = 8

t = 50

1 = 75O

2 = 75O

38

Chapter-5

FAULT CALCULATIONS AND RESPONSE OF

RELAYS FOR DIFFERENT FAULTS

We are having three generators of 60 MW capacity each and one

generator of 67.5 MW capacity. Our generators are floating neutral type. As our

planet is captive power plant generators are connected to 11 KV Bus (GSB1).

The 11 KV is stepped upto 220 KV through two 220KV/11KV power transformers.

The 220 KV sub station is names as LBSS-5. We earthed our system at GSB-1

through three earthing transformers. Out of three transformers two transformers

are taken to service. Our plant auxiliary supply and emergency loads are

connected to this GSB-1 Bus.

We are having separate 220 KV station it is names as main receiving

station. It receives the APSEB supply through line AL1 and AL2 our 220 KV

substation (LBSS5) is having tie connection with MRS. Four sub-stations located

at different places are fed through MRS and catering the loads our steel plant.

Figure shows the single line diagram of total power system in V.S.P.

NAME PLATE DETAILS OF TG1, TG2 & TG3

MW rating : 60 MW

MVA rating : 75 MVA

Rated power factor : 0.8

Rated voltage : 11000 Volts

Rated current : 3936 Amps

39

Rotor voltage : 300 Volts

Rotor current : 596 Amps

Connection : Star

Speed : 3000 rpm

Coolant : Air

Winding type : Double layer

Positive sequence impedance : 18.8%

Negative sequence impedance : 19.7%

Zero sequence impedance : 12.9%

NAME PLATE DETAILS OF TG4

MW rating : 67.5 MW

MVA rating : 84.375 MVA

Rated power factor : 0.8

Rated voltage : 11000 Volts

Rated current : 4429 Amps

Rotor voltage : 307 Volts

Rotor current : 624 Amps

Connection : Star

Speed : 3000 rpm

Coolant : Air

Winding type : Double layer




NAME PLATE DETAILS OF TRANSFORMERS T1 & T2

Rated KVA : ON AN 50000

ON AF 63000

Rated voltage : HV 220 kV

LV 11 kV

HV LV

Rated current : ON AN 131.2 A 2624.6 A

40

ON AF 165.3 A 3307.0 A

Frequency : 50 Hz




All the values are based on 50 MVA

NAME PLATE DETAILS OF TRANSFORMERS T3.

Rated KVA : 9000

Rated voltage : HV 220 kV

LV 11 kV

Rated current : HV 236.2 A

LV 4723.8 A

Frequency : 50 Hz

Impedance voltage : 11.6%

FAULT CALCULATIONS

Choose a system base of 100 MVA and base voltage as 11 kV at all

Generators. The impedance voltages on 100 MVA base are as below.

GENERATORS TG-1, TG-2 & TG3

(MVA)b, new (KV2)b, old

We have Z(p.u.) new = Z(pu) old x ------------------ x ------------------(MVA)b, old (KV2)b, new

100 112

Positive sequence impedance or reactance, XG1 (pu) = 0.188 x ------- x ------ 75 112

= 0.25067 pu

100 112

Negative sequence impedance or reactance, XG2 (pu) = 0.197 x ------- x ------ 75 112

41

= 0.2626 pu

100 112

Zero sequence impedance or reactance, XG0 (pu) = 0.129 x ------- x -------- 75 112

= 0.172 pu

Generator TG-4

100 112

Positive sequence impedance or reactance, XG1 (pu) = 0.2145 x --------- x ------ 84.375 112

= 0.2542 pu

100 112

Negative sequence impedance or reactance, XG2 (pu) = 0.2092 x ------- x ------ 84.375 112

= 0.2479 pu

100 112

Zero sequence impedance or reactance, XG0 (pu) = 0.1157 x ---------- x -------- 84.375 112

= 0.13713 puREACTORS

Reactance of reactors = 0.2 (positive)

= 0.2 (negative)

= 1.0 (Zero sequence)

(KV2)b 112

Base reactance, Xb = ------------ = -------- = 1.21 (MV)b 100

0.2Positive sequence reactance, of reactor = -------- = 0.1653 pu

1.21

0.2Negative sequence reactance, of reactor = -------- = 0.1653 pu

42

1.21

1.0 Zero sequence reactance, of reactor = -------- = 0.82645 pu

1.21

TRANSFORMERS T1 & T2

100 112

Positive sequence impedance or reactance, XT1 (pu) = 0.2267 x --------- x ------ 50 112

= 0.4534 pu

100 112

Negative sequence impedance or reactance, XT2 (pu) = 0.2158 x ------- x ------ 50 112

= 0.4316 pu

100 112

Zero sequence impedance or reactance, XT0 (pu) = 0.1679 x ------- x -------- 50 112

= 0.3358 pu

TRANSFORMER T3

100 112

Positive sequence impedance or reactance, XT1 (pu) = 0.116 x --------- x ------ 90 112

= 0.1289 pu

EARTHING TRANSFORMER

Here the 11 kV bus is earthed using zigzag transformer earthing through a 14 resistance.

Actual value 14Its pu value = --------------------- = ------- = 11.57025 pu

Base value 1.21

43

3 FAULT OR SYMMETRICAL FAULT CALCULATIONS

For Fault at F1 :

The Thevenin’s equivalent circuit for fault at F1 is obtained by exciting the

passive Thevenin’s network at the fault point by negative of pre-fault voltage and

is as shown in figure enclosed.

On reducing the circuit in the enclosed figure we get

The equivalent impedance of circuit at F1 = j 0.12007 pu

1.0Fault current, If = ----------------- = - j 8.32863 pu

j 0.12007

If (pu) = 8.32863

If = 8.32863 x base value of current

100 x 106

Base value of current = ------------------------ = 5248.6388 Amps3 x 11 x 103

Fault current, If = 8.32863 x 5248.6388 = 43.71897 kA

The contribution of each generator to the fault are given as:

Current contributed by Generator-1, IG1 (pu) = 3.99

IG1 = 3.99 x 5248.6388 = 20.942 kA


IG2 = 1.9803 x 5248.6388 = 10.394 kA

44

If = 43.71897 kA

IG1 = 20.942 kA


IG3 = 1.27623 x 5248.6388 = 6.6985 kA


IG3 = 1.0821 x 5248.6388 = 5.6797 kA

The relays which are operated for 3 fault at F1 is:

Partial differential IDMT/Instantaneous over current relay:

The relay settings are :

R Y B TMS

1 1 1 0.45

and instantaneous over current setting = 1.3 x 4 = 5.2 Amps

The 3- symmetrical fault current is, If = 43.7139 kA

Fault current when referred to CT secondary = 43.7139 / 4500 = 9.7

So, for 3- fault partial differential instantaneous over current relay

operates.

For the IDMT relay, for PSM = 9.7, time in seconds = 3.1 sec. (from the scale).

Actual operating time = 3.1 x 0.45 = 1.395 sec.

The IDMT relay will act with 1.395 seconds time delay if instantaneous over

current relay fails to act.

45

IG2 = 10.394 kA

IG3 = 6.6985 kA

IG4 = 5.6797 kA

For this fault i.e. for 3- fault on GSB1, differential relay of Generator-1 will

not act since it is a through fault.

If the fault occurs before the generator circuit breaker then the differential

relay will operate to open the generator circuit breaker. The fault current remains

same if the fault occurs either on GSB-1 section or before Generator circuit

breaker or before Power transformer circuit breaker. But the relays operating at

different faults are not same.

If the fault occurs before generator circuit breaker the differential relay will

act instantly and trip the generator circuit breaker. So the remaining system will

not be effected. In the same way if the fault occurs before the power transformer

circuit breaker the transformer differential relay will act and trip the transformer

circuit breaker only, so remaining system will be unaffected.

For Fault at F2 :

The Thevenin’s equivalent circuit for fault at F2 is obtained by exciting the

passive Thevenin’s network at the fault point by negative of pre-fault voltage and

is as shown in figure enclosed.

On reducing the circuit in the enclosed figure we get

The equivalent impedance of circuit at F2 = j 0.10699 pu

1.0Fault current, If = ----------------- = - j 9.346745 pu

j 0.10699

If (pu) = 9.346745

Fault current, If = 9.346745 x 5248.6388 = 49.0577 kA

46

If = 49.0577 kA



IG1 = 2.22213 x 5248.6388 = 11.6632 kA


IG2 = 3.989x 5248.6388 = 20.9364 kA


IG3 = 2.22213 x 5248.6388 = 11.6632 kA


IG3 = 0.91355 x 5248.6388 = 4.7949 kA

The relays which are operated for 3 fault at F2 is:

Partial differential IDMT/Instantaneous over current relay:

The relay settings are :

R Y B TMS

1 1 1 0.45

and instantaneous over current setting = 1.3 x 4 = 5.2 AmpsThe 3- symmetrical fault current is, If = 49.0577 kA

47

IG1 = 11.6632 kA

IG2 = 20.9364kA

IG3 = 11.6632 kA

IG4 = 4.7949 kA

Fault current when referred to CT secondary = 49.0577 / 4500 = 10.9

So, for 3- fault partial differential instantaneous over current relay

operates.

For the IDMT relay, for PSM = 10.9, time in seconds = 2.9 sec. (from the scale).

Actual operating time = 2.9 x 0.45 = 1.31 sec.

The IDMT relay will act with 1.31 seconds time delay if instantaneous over

current relay fails to act.

For this fault also the differential protection scheme of Generator-2 will not

act since it is a through fault.

UNSYMMETRICAL FAULT CALCULATIONS

Here we have discussed the most occurring unsymmetrical fault i.e. L-G

fault only. The 3 sequence diagrams are enclosed.

FOR L-G FAULT OR EARTH FAULT AT F1

From the sequence diagrams

Equivalent positive sequence impedance at fault point F1 = Z1 = j 0.131922 pu

Equivalent negative sequence impedance at fault point F1 = Z2 = j 0.121268 pu

Equivalent zero sequence impedance at fault point F1 = Z0 = 17.3652+ j 0.413 pu

For L-G fault, the positive, negative and zero sequence networks are connected

in series.

EO 1.0IR1 = IR2 = IRO = ----------------- = --------------------------------

Z1+Z2+Z0 17.3652 + J 0.66618

= 0.057544 -2.197O

Fault current If = 3 x IR1 = 3 x 0.057544

= 0.172632 pu

If = 0.17263 x 5248.6388 = 906.083 A

48



IG1 = 0.09084 x 5248.6388 = 476.7863 A


IG2 = 0.047182 x 5248.6388 = 247.6413 A


IG3 = 0.03461 x 5248.6388 = 181.6554 A

The relay which is operated is:

Fault current If = 906.083 A

Fault current when referred to CT secondary = 906.083 / 4500

= 0.201352

Partial differential protection for GSB-1 Section-1

Plug setting = 0.1 A

Time multiplier setting = 0.3

Plug setting multiplier = 0.201352 / 0.1

49

If = 906.083 A

IG1 = 476.7863 A

IG2 = 247.6413 A

IG3 = 181.6554 A

= 2.0135

From the scale for PSM = 2.0135, time in sec = 10 sec.

The time required for the relay to operate for the earth fault at F1 = 10 x 0.3

= 3 Sec.

Partial differential IDMT earth fault relay (51N) of GSB-1 Section-1 will act with

operating time = 3 Sec.

Here the fault is at GSB-1 Section-1 so the restricted earth fault relay will not

operate since it is a through fault.

If the earth fault is at generator terminals then for the same fault current i.e.

If = 906.083 Amps

the current contribution of each Generator are:

IG1 = 476.7863 A

IG2 = 247.6413 A

IG3 = 181.6554 A

IG1 when referred to CT secondary = 476.7863 / 4500 = 0.106 A

The restricted earth fault relay setting = 0.05 Amps

Restricted earth fault relay of Generator-1 will operate for Earth faults at

Generator-1 terminals only.

FOR L-G FAULT OR EARTH FAULT AT F2

From the sequence diagrams

Equivalent positive sequence impedance at fault point F2 = Z1 = j 0.11368 pu

Equivalent negative sequence impedance at fault point F2 = Z2 = j 0.1179 pu

Equivalent zero sequence impedance at fault point F2=Z0=17.5537+j 0.413225 pu

For L-G fault, the positive, negative and zero sequence networks are connected

in series.

EO 1.0

50

IR1 = IR2 = IRO = ----------------- = -------------------------------- Z1+Z2+Z0 17.3553 + J 0.644805

= 0.0575796 -2.128O

Fault current If = 3 x IR1 = 3 x 0.0575796

= 0.172738 pu

If = 0.172738 x 5248.6388 = 906.64 A



IG1 = 0.04720235 x 5248.6388 = 247.748 A


IG2 = 0.07833 x 5248.6388 = 411.1432 A


IG3 = 0.04720235 x 5248.6388 = 247.748 A

The relay which is operated is:

Fault current If = 906.64 A

Fault current when referred to CT secondary = 906.64 / 4500

= 0.2015

51

If = 906.64 A

IG1 = 247.748 A

IG2 = 411.1432 A

IG3 = 247.748 A

Partial differential protection for GSB-1 Section-1

Plug setting = 0.1 A

Time multiplier setting = 0.3

Plug setting multiplier = 0.2015 / 0.1

= 2.015

From the scale for PSM = 2.015, time in sec = 10 sec.

The time required for the relay to operate for the earth fault at F1 = 10 x 0.3

= 3 Sec.

Partial differential IDMT earth fault relay (51N) of GSB-1 Section-1 will act with

operating time = 3 Sec.

Here also the fault is at GSB-1 Section-2 so the restricted earth fault relay will

not operate since it is a through fault.

If the earth fault is at second generator terminals then for the same fault current

i.e.

If = 906.64 Amps

the current contribution of each Generator are:

IG1 = 247.748 A

IG2 = 411.1432 A

IG3 = 247.748 A

IG2 when referred to CT secondary = 411.1432 / 4500 = 0.0914 A

The restricted earth fault relay setting = 0.05 Amps

Restricted earth fault relay of Generator-2 will operate for Earth faults at

Generator-2 terminals only.

Chapter-6

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CONCLUSION

The very purpose of protection system is to monitor the

unwanted conditions and when such conditions arise, to remove the

fault in the shortest time possible, leaving unaffected are operational.

In this project we have studied analysed the protection provided

to 60 MW Turbo generator in Thermal Power Plant of

Visakhapatnam Steel Plant. In this we have analysed both

symmetrical and unsymmetrical faults and response of different

relays for these faults with the existing settings.

In the present age, microprocessor based relays have come to

market. These relays provide a spectrum of information in a single

relay. Practically it has been observed that these relays are good

from the study and analysis point of view where as for reliability and

dependability, the Electro Mechanical and Electro Magnetic Relays

are still superior to microprocessor based relays. Since

Electromechanical relays operate only with actual electrical signals.

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Protection of Generators 1

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