Protection of Generators

5/28/2018 Protection of Generators

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rotection of generators

A Mini Project Report Submitted in the partial fulfillment

Of the requirement for the award of

BACHELOR OF TECHNOLOGY IN

ELECTRICAL & ELECTRONICS ENGINEERING

Submitted by

R.Venkatesh - 096Q1A0213

K.V.S.Kishore - 096Q1A0252

S.Dorababu - 106Q5A0205

A.V.V.Vikas - 096Q1A0201J.Ashok - 096Q1A0212

K.N.V.Prasad - 096Q1A0220

Under Esteemed guidance of

Mr. SURESH

(External guide)

M r .P. Koteswararao

(I nternal guide)

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

KAKINADA INSTITUTE OF ENGINEERING AND TECHNOLOGY II

KORANGI

2009-2013


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ABSTRACT

Generators and auxiliary transformers are subjected to a number of

possible hazards for which protection should be provided. The main objective of

generator protection is to detect faults and abnormal conditions as fast as possible

and in case of fault to avoid extending the damage to a minimum. Protection

scheme for Generator depend on many factors, such as size and importance of the

machine, type of prime mover, grounding, protection philosophy etc

This report is aimed to study the Protection of Generators in a Combined

Cycle Power Plant (GVK-Jegurupadu). GVK is using the combined cycle principle

for generation of power. Here air and gas combustion mixture is used to drive the gas

turbine which in turn drives the gas turbine generator(GTG). In order to avoid

wastage of energy of flue gases they are used for converting water in to steam and

this is used to drive the steam turbine. Steam turbine acts as a governor to steam

turbine generator(STG). The plant consist of two phases. Total capacity of plant is

463MW (235MW of phase -1 and 228MW of phase-2).

This plant is using the latest technology that is available for the

operation. This plant is completely automated using the concept of digital control

system (DCS)

Signature of the internal guide: Signature of the HOD:

Submitted by: R.Venkatesh (096Q1A0213)

K.V.S.Kishore (096Q1A0252)S.Dorababu (106Q5A0205)

A.V.V.Vikas (096Q1A0201)

J.Ashok (096Q1A0212)

K.N.V.Prasad (096Q1A0220)


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ACKNOWLEDGEMENT

We express our deep sense of gratitude to Ch. Vinod Kumar, Head

of the Department of Electrical and Electronics Engineering, KIET+ for havingkeen interest at every stage of development of our project work and to

P.Koteswara rao for guiding us in every aspect.

We are also deeply indebted to C.V.S MURTHY,Principal, KIET+

for providing the necessary facilities during the execution of this mini project.

We thank Mr. N. Srinivasarao (GM) and HR team of GVK

JEGURUPADU POWER GENERATION LIMITED for their help in completion

of this project.


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CONTENTS

1. Introduction to power plants in india2. Introduction to GVK power plant3. Introduction to combined cycle power plant4. Operation and operating modes5.Need for protection of generators6. Protection schemes of generators7. Conclusion


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INTRODUCTION TO POWER PLANTS IN INDIA

The electricity sector in India supplies the world's 6th largest energy

consumer, accounting for 3.4% of global energy consumption by more than 17%of global population. The Energy policy of India is predominantly controlled by

the Government of India's, Ministry of Power, Ministry of Coal and Ministry of

New Renewable Energy and administered locally by Public Sector

Undertakings (PSU s).

About 64.75% of the electricity consumed in India is generated by

thermal power plants, 21.73% by hydroelectric power plants, and 2.78 % by

nuclear power plants and by Renewable Energy Sources. More than 50% of India'scommercial energy demand is met through the country's vast coal reserves.

INTRODUCTION ON GVK ENERGY POWER PLANT

INTRODUCTION:

GVK is 235MW of phase -1 and 228MW of phase-2 combined cyclepower plant located at Jegurupadu, east Godavari district. The combined cycle unit

is multi- shaft power train with three gas turbine,one steam turbine of phase-1 and

one gas turbine,one steam turbine of phase-2. The basic operation mode of the

plant is to supply electrical power to grid. The facility operates with natural gas

and is suitable for fuel oil as back-up. The plant is capable of running at maximum

continuous rating (MCR) and a part load as well as continuous and in two shift

operation. In a daily ambient temperature the minimum is 13.9C and the

maximum is 49C. The design ambient air temperature is 29C. The performanceguarantee for the combined cycle plant is based on the gross electrical output.

Water which is required for the plant is available from two sources; supplies such

as tube wells. This report is aimed to study the operation of combined cycle power

plant (ccpp). GVK is using the combined cycle principle for the generation of

power. In this plant gas from GAIL is being used as the fuel input. Here this input


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is combining chamber. Here this input is combining with atmospheric air and

combustion these places inside the combustion chamber. Here flue gases are

produced and wastage of flues are used to drive the gas turbine generator (GTG).

In order to avoid wastage of flue gases they are using these flues for converting

water in to steam and this is used the drive the steam turbine generator (STG).

The power supplied by is in the form of three-phase 50-hertz AC current, at a

nominal voltage 220kv of phase-1 and 400kv of phase-2.They power is generated

power as per the power contracts and is supplied to the APTRANSCO department.

This plant is using the latest technology that is available for operation. This plant is

completely automated using the concept of digital control systems.

SALIENT FEATURES:

Capacity : 235MW of phase-1

228MW of phase-2

Location : Jegurupadu, East Godavaridistrict

Type of station : combined cycle power plant

Gas supply : GAIL

PROCESS DESCRIPTION:

The power plant uses natural gas as primary fuel and naphtha as alternative fuel

with HSD as start-up fuel to produce power. Gas authority of India Limited

(GAIL) supplies Natural gas. Raw water is drawn from reservoir.

The exhaust gas from the turbine is led to heat recovery steam generator. The dual

pressure steam generated is the steam turbineThe air water-cooled generator is

coupled on the cold end side of the GT and ST generator at high-pressure side of

ST and are arranged in parallel feeding two step up transformers. The gas turbine

equipped with duel fuel hybrid burner i.e. capable of firing either Natural gas,

liquid fuel (Naphtha/HSD) or Mix fuel operation. For natural gas operation with


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premix burner no water injection is repudiation to match the required emission

level. However liquid fuel operation will require water injection to reduce the

NOX emission levels.

The ambient air shall be filtered in three stages and led to the compressor ofthe gas Turbine. Gas Turbine is provided with two silo combustion chambers

(inconal alloy) respectively. The compressed air enters the combustion chambers

through the annular space provided on the outer side of the casing cone. The air

and fuel mixture is burnt in silo combustion chambers. The hot gases then flow

through the turbine with its four stages.

After expanding in the turbine the flue gases shall be directly led in to the

HRSG. Steam is generated in the HRSG by heat transfer from the water the flue

gases to the water/steam. From the HRSG the super heated steam is expands. Thesuperheated LP steam is also led to the steam turbine as well. The expanded steam

is condensed in a water cooled condenser and returned by the condensate pump to

Deaerator via condensate preheater thus completing the cycle.

In order to make best use of the thermal energy of the steam; the pressure at

the exhaust end shall be optimized. For this purpose a surface type water-cooled

condenser is provided and cooling is achieved by cooling water circulated through

a forced cooling tower. The air as well as the non-condensable gases entering the

ST and the circuit is extracted from the steam space of the condenser by the

vacuum pump.

The condensate is preheated to extract residual heat in the condensate

preheater. It also equipped with circulation pumps and LP heater for maintaining

desired inlet temperature to mitigate cold end corrosion (Dew point corrosion).

From the feed water storage tank (Deaerator) the feed water is delivered to

HP and LP drum using 2*100% HP and LP feed water pumps respectively. One

pump is in operation at full load.

The HP&LP bypass operation shall be in operation when the steam is not

utilized in the steam turbine.The power generated by the gas turbine and the steam

turbine generators are stepped up to 220KV& 400KV level and evacuated by

APTRANSCO through overhead transmission lines.


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INTRODUCTION TO CCPP

The most efficient way for utilities to use gas or oils is in a combined-cycle

system, which combines two means of producing electricity. Hot gases from the

combustion chamber spin the gas turbine and the generator to make electricity. The

system then pipes the still-hot exhaust gases leaving the combustion turbine to a

'waste heat' steam boiler where their heat produces steam. The steam turns a

turbine, connected to a second generator, to produce electricity. Condensers

convert the steam to water that returns to the boiler to repeat the cycle.

Combined cycle power plants are highly efficient, flexible, reliable, cost-effective

and environmentally friendly solutions to generate electrical power.

In combined cycle power plants (CCPPs) a gas turbine generator generates

electricity while the waste heat from the gas turbine is used to make steam to

generate additional electricity via a steam turbine.

In other words: The output heat of the gas turbine flue gas is utilized to generate

steam by passing it through a heat recovery steam generator (HRSG), so it can be

used as input heat to the steam turbine power plant. This combination of two power

generation cycles enhances the efficiency of the plant. While the electrical

efficiency of a simple cycle plant power plant without waste heat utilizationtypically ranges between 25% and 40%, a CCPP can achieve electrical efficiencies

of 60% and more. Supplementary firing further enhances the overall efficiency.

The high fuel utilization factor of the plant contributes to low lifecycle costs.

Together with an outstanding operational flexibility, CCPPs can provide a tailor-

made solution for your power needs.


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OPERATION AND OPERATING MODES

The Combined cycle power plant (ccpp) is basically designed to produce

electric power for the grid. In normal operation mode, the GT and ST are in

operation.The GT should be running at 100% load for best efficiency.

POWER GENERATION:

The overall plant output is controlled by means of gas turbine.The steam

turbine always generates the power, which is made available by the waste heat

from the gas turbine. Therefore, control simplify involves varying the quantity of

fuel supplied to the gas turbine and changing the gas turbine combustion airflow

rate in the upper load range by variable inlet guide vanes.

The steam cycle of power plant operates with sliding steam pressure. The

pressure of the HP steam decreases proportionally to the combined cycle load


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down to approximately 50%. At combined cycle loads above 50%,the steam

turbine opens are fully open. At loads below 50%, the steam turbine control valves

throttle to maintain a constant steam pressure. The result is an optimum efficiency

of combined cycle power plant.

NORMAL OPERATION:

In normal operation,the set point of the by-pass valves is slightly above

the actual live steam pressure.with combined cycle loads over 50%,the set point

will automatically be raised. However, the speed of the set point adjustment is

limited. Thus the pressure gradient is always kept with in the allowable limits for

the HRSG drums even with fast loads increases of the gas turbine.

The pressure of the LP/HP steam decreases proportionally to the combined

cycle load down to approximately 80% of combined cycle load, the IP and LP

control valves of the steam turbine maintains a constant pressure.

BASE LOAD (FULL LOAD OPERATION):

The load of the GT is limited to 100%, which is accomplished through the

gas turbine control system. Combined cycle plant operation at 100% is defined

with the GT running at 100% load (=base load) and the whole steam led through

the ST. Base load operation of the GT is given by turbine inlet temperature at the

pre-set base load temperature and the variable inlet guide vanes fully

opened(max.flow). This definition applies toall ambient conditions.

PART LOAD OPERATION:

For high block efficiency at GT part load, the VIGV and fuel flow control

keep the turbine exhaust temperature at a high level, down to approximately 60%

GT load.


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For low load operation, HP,IP&LP steam pressure are kept at their

respective fixed pressure levels. If these pressure levels are exceeded, the

steamturbine is operated in sliding pressure mode.

The electrical output of the power plant is controlled only by means of thegas turbine. The GT gross load is controlled by the operator by entering manually a

GT active gross load set point from the DCS in the control room.

CONTROL SYSTEM

GENERAL DESCRIPTION:

The facility is equipped with an overall plant process control system basedon decentralised control system technology. This system enables safe and reliable

operation, control and supervision of the process with a high degree of automation.

The DCS system provides functions such as:

- Signal conditioning, annunciation, recording- Operation, monitoring and supervision- Open and closed loop control, sequence logic, protection-

Data communication, plant management applications

CONTROL LOCATIONS:

The plant will be operated and supervised from the main control room.

All information required for remote operation and supervision of the GT, the ST

and the other remote operable equipment is available in the main control room.

The gas turbine is provided with its own safety system and governor, based

on EGATROL technology. Necessary information for remote operation, control

and monitoring are transferred to the overall plant control system.

The steam turbine is provided with its own safety system and governor.

Necessary information for remote operation, control and monitoring is transferred

to the overall plant control system.


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Autonomous systems are equipped with their own local control

systems.These systems are provided with local control panels allowing for full

local operation, control and monitoring.The necessary information for remote

monitoring and/or control is transferred to the overall plant control system

LIST OF AUTONOMOUS SYSTEMS:

Autonomous system Remote control Remote monitoring

Condenser tube cleaning

system

No Yes

HV breakers for GT and

ST generators

synchronisation

Yes Yes

Waste water system No Yes

Cooling water dosing

system

No Yes

Generator step up and

unit transformer

Yes Yes

Compressed air supply

system

No Yes

Static starting device Yes Yes

synchronisation Yes Yes

Electrical protection

system

Yes Yes

Automatic voltage

regulator

Yes Yes

Fire protection system No Yes

Stand by diesel generator Yes Yes


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TURBINE

The turbine converts the thermal energy in the combustion gas into

mechanical energy which is transmitted through the compressor to the generator

which produces electrical energy to the grid.

INTRODUCTION TO GAS TURBINE:

The gas turbine and generator delivers electric power to a grid in single cycle

operation. Alternatively, it can deliver electrical power to a grid and its exhaust

gasses to a heat recovery steam generator (HRSG) in combined cycle operation.

The major components of the gas turbo set are the air intake system;

compressor; combustor; turbine; exhaust gas system generator and exciter. This

section will briefly describe the function of these components.

GAS TURBINE IN POWER PLANT:

The major components of the gas turbine are described below:

The casing, which holds encloses the turbine and its stationary blade carrier,is a made of globular cast iron.

The rotor is made of discs welded together to form a single shaft with thecompressor rotor.

Turbine bladingconsists of stationary and rotating blades. The former aremounted in a blade carrier attached to the casing, while the rotating blades


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are attached to the rotor. The conversion of heat energy into mechanical

energy takes place within the turbines blading.

The ring shaped stationary vane carrier holds the stationary vanes in theirrespective grooves. The carrier which hangs inside the turbine casing is

made of material which can expand with changes in temperature.

FUELSUSED INGAS TURBINE:

Natural Gas from GAIL Naphtha (HPCL)


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FEATURESOFGASTURBINES

The gas turbines have the following opearational features. They are

1. The gas turbines produce a large amount of useful work from the relativelysmall input

2. The maintainance cost is less and the mechanical life is long when compared toapiston driven engine.

3. The start up time to a full load for a gas turbine is in minutes VS for a steamturbine

4. Gas turbines can operate utilizing various types of fuels. But generally naturalgas is been used in it.

5. Atmospheric air is typically the working fluid for the gas turbine and doesnotrequire any coolant for basic power generation.

AIR INTAKE SYSTEM

The air intake system draws in ambient air, which passes through filtering

and sound damping systems. It is then forwarded to the combustor where it is used

in combustion and cooling functions.

3-stage filter are used where required by ambient conditions. An anti-icing

device is installed in cold weather climates to prevent the formation of ice at the

compressor inlet.

The major components of an air intake system with standard 2-stage filter

are described below.

LOUVERS:

Ambient air is drawn in through louvers which prevent rain water and large

foreign objects from entering the air intake duct.

FILTER:

The air then passes through a 2-stage filter which removes dirt and

contaminants contained in the air.


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SILENCER:

The air, flowing at relatively high speed, then passes through a sound

damper or silencer, which lessens the noise level.

SAFETY FLAP VALVES:

Located downstream from the filter housing, the safety flap valves open

automatically if the filters become clogged and negative pressure within the

housing becomes too high. This action prevents damage to the filter housing, air

intake elbow and manifold.

MANHOLE:

A manholeis provided for cleaning and inspection purposes.

COMPRESSOR:

Air from the air intake system is forwarded to the compressor where it is

compressed by the combined effect of rotating blades and stationary vanes. Atthe

compressor outlet the compressed air is directed through a diffuser to the

combustion chamber and into the hot gas path for cooling purposes. Another part

of the compressed air is branched off for sealing purposes at those locations where

the rotor passes through the casing.The intermediate shaft is attached to the

compressors rotor coupling, at the cool end of the gas turboset. The intermediate

shaft performs the following functions:

Transfers mechanical power of gas turbines rotor to the generator rotor. Transfers the slow rotational motion of barring device through a latch wheel

to the compressor and generator rotors to prevent the turbine shaft from

bending during cool down period.

Transfers the actual rotational speed of the rotor to the control systemthrough a gear wheel by non-contact rotational speed measurement.

Transfer the phase angle to the control system by non-contact Proximitermeasurement.


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The major components of compressor are described below:

VARIABLE INLET GUIDE VANES:

VIGV modulate the air flow from the air intake structure through the

compressor.

BLOW-OFF VALVES:

These expel excess air to atmosphere during gas turbine start-ups and shut-

downs. This prevents air turbulence with in the compressor and consequent

stressing of the blades.

DIFFUSER:

It is a ring shaped device located at the compressors outlet. After thecompressed air flows through the diffuser, it is deflected by guide vanes into the

turbine housing where it cools the turbinesstationary vane carrier before passing

on the combustor.


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BLADES:

Rotating and fixed blades compress the air which flows into the compressor

from the air intake system.

BEARINGS:

One journal and one thrust bearing is located in the air intake section hold

and guide the compressors rotor in the radial and axial direction respectively. Oil

from the lube oil system lubricates and cools the bearings.

COMBUSTOR:

The combustor is annular (ring) type is device placed around the shaft

between the compressor and the turbine, it is here that the combustion process

takes place. Combustion is a chemical reaction between oxygen in the pressurized

air and combustible components(carbon and hydrogen) in the fuel. when the


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mixture is ignited in the combustor, the hot, pressurized combustion gas is

forwarded to the turbine where it is expanded through the blading.

Because of the heat developed during combustion, the combustor must be

cooled with air from the compressor. This happens in a counter-flow manner, thatis, the combustion gas flow and the cooling air flow move in opposite directions.

The main components of the annular combustor are described below:

A ring shaped housing, which encloses and holds all components of the

combustor, is suspended with in the turbine housing.

A heat shield segment and its support protect the primary zone from direct

flame radiation. Its back side is cooled by air from the compressor.

A front segment and its support protect the primary zone in which combustion

takes place. It includes a cover plate and a support for the burners. The cover plate

has holes through which the combustion air from the compressor reaches the

compressor.

STEAMTURBINESYSTEMS

The steam turbine extract the energy of pressurized superheated steam as

mechanical movement. An ideal steam turbine is considered to be anisentropic

process,or constant entropy process, in which the entropy of the steam entering the

turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is

truly isentropic, however, with typical isentropic efficiencies ranging from 20%-

90% based on the application of the turbine. The interior of a turbine comprises

several sets of blades, or buckets as they are more commonly referred to. One set

of stationary blades is connected to the casing and one set of rotating blades is

connected to the shaft. The sets intermesh with certain minimum clearances, with

the size and configuration of sets varying to efficiently exploit the expansion of

steam at each stage.
http://en.wikipedia.org/wiki/Isentropic_processhttp://en.wikipedia.org/wiki/Isentropic_processhttp://en.wikipedia.org/wiki/Isentropic_processhttp://en.wikipedia.org/wiki/Isentropic_process


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In a steam turbine nozzles apply pressurized supersonic steam to a set of

curved blades mounted on a rotor. Each blade whips the steam back in the opposite

direction, simultaneously allowing the steam to expand a little. A stationary blade

then redirects the steam towards the next set of blades toward the exhaust end with

the gap between acting like a nozzle. The process repeats in successive stages untilthe steam is exhausted at nearly atmospheric pressure. The moving blades are

mounted radially on the rotor, while the stationary blades are mounted to the case

of the turbine. Turbines always consist of a number of stages, with each stage

being carefully optimized for the pressure and volume of steam that it contacts.

Because high pressure steam exhausted through a nozzle into the air travels so fast,

the turbine, in order for it to be efficient, must rotate very fast. This requires that

the rotor and its blades be well balanced to protect it against vibrations, and creates

difficulties with the seals around the rotor. The centrifugal force experienced bythe blade is so strong that it must be carefully designed and made out of the

strongest available materials to prevent it from failing catastrophically.

MAJOR COMPONENTS OF STEAM TURBINE:

Casings Blading Blade carriers with stationary blades Welded disc rotor with rotating blades Dummy piston Rotor coupling Gland seals Bearings Stop and control valves Drain lines


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CROSS SECTION OF STEAM TURBINE:

CLASSIFICATION OF STEAM TURBINE PRESSURE:

HP high pressure >105bar

IP intermediate pressure ~105bar

LP low pressure ~26bar


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HP turbine casings are of double designed type, based on steam pressure and

temperature at which the turbine will operate and the application for which the

turbine was designed.

LP blading, which is a mixture of impulse and reaction blading, operates in aslightly different environment. Steam passing through this section on its way to the

condenser, expands from superheated steam to the point of saturation.

NEED FOR PROTECTION OF GENERATORS

Generators, unit transformers and auxiliary transformers are subject to a number of

possible hazards for which protection should be provided. The main objective of

generator protection is to detect faults and abnormal conditions as fast as possible

and in case of fault to avoid extending the damage to a minimum.

Protection scheme for Generator and unit transformers depend on many factors,

such as size and importance of the machine, type of prime mover, grounding,

protection philosophy etc, thereto, two main subjects must be taken into

consideration at the design stage:

Proper overall concept with co-ordinated main and backup protectionfunctional strategy. Selection should be based more on generators

importance than on its rating

Depending on power plant layout and its functional concept the Hardware should

allow a tailored solution with a minimum of components.


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PROTECTION SCHEMES OF GENERATORS

The following are the main protection schemes adopted for our generator.

1. Generator Differential Protection

2. Loss of Field or Loss of Excitation Protection

3. Negative Sequence or Current Unbalance Protection

4. Over Fluxing or Over Excitation Protection

5. Over Current Protection

6. Stator Earth Fault Protection

7. Rotor Earth Fault Protection

8. Restricted Earth Fault Protection

9. Backup Impedance Protection

10. Low Forward Power Protection

11. Reverse Power Protection

12. Pole Slip Protection

13. Pole Discrepancy Protection

14. Local Breaker Back Protection

15. Bus Bar Protection

16. Over Frequency Protection

17. Under Frequency Protection

18. Over Voltage Protection


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1.GENERATOR DIFFERENTIAL PROTECTION:

Setting : 0.5 Amp Time : Instantaneous

It is one of the important protections to protect generator winding against internal

faults such as phase-to-phase and three phase-to-ground faults. This type of fault is

very serious because very large current can flow and produce large amounts of

damage to the winding if it is allowed to persist. One set current transformers of

the generator on neutral and phase side, is exclusively used for this protection. The

differential protection can not detect turn-to-turn fault and phase to ground within

one winding for high impedance neutral grounding generator such as ours. Upon

the detection of a phase-to-phase fault in the winding, the unit is tripped with out

time delay.

Relays acted : a. Flag operation at Protection panel.

b. Acting of Master relay

c. Indication at Annunciation Panel.

Consequences : a. Tripping of 220KV breaker

b. Tripping of Field breaker

c. Stop command to Turbine thro Mark-IV

Status : a. Unit is at coasting down.

Once the differential protection operated, the unit can not be taken into service

unless the generator winding is thoroughly examined by the maintenance staff of

any internal faults


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2.LOSS OF FIELD OR EXCITATION PROTECTION :

Setting : K1-2, K2-1, K3-2 Trip after 2 Sec.

When the synchronous machine with excitation, is connected to the grid, it

generates reactive power along with active power to the grid and the rotor speed is

same as that of grid frequency. Loss of field or loss of excitation results in loss of

synchronism between rotor flux & stator flux. The synchronous machine operates

as an induction machine at higher speed and draws reactive power from the grid.

This will result in the flow of slip frequency currents in the rotor body as well as

severe torque oscillations in the rotor shaft. As the rotor is not designed to sustain

such currents or to withstand the high alternating torques which results in rotor

overheating, coupling slippage and even rotor failure.

A loss of excitation normally indicates a problem with the excitation system. Some

times it may be due to inadvertent tripping of filed breaker, open or short circuit of

field winding or loss of source to the exciter. If the generator is not disconnected

immediately when it loses excitation wide spread instability may very quickly


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develop and major system shutdown may occur.

When loss of excitation alarm annunciates at annunciation panel, the machine may

probably be running with less excitation at leading MVAR power. Increase the

excitation on the machine until it reaches on lagging MVAR power. The machinetrips on the same protection along with alarm resynchronize the machine and try to

stabilize at required MVAR power. If not possible, trip the machine immediately

and inform to the maintenance staff for thorough checking of the Automatic

Voltage Regulator (AVR) and its associated parts.









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3.NEGATIVE SEQUENCE OR CURRENT UNBALANCE PROTECTION:

Setting : Alarm75% of 12s Time - 5 Sec.

Trip75% of 12s Time - 300 Sec.

When the machine delivering the equal currents in three phases, no unbalance or

negative phase sequence current is produced as the vector sum of these currents is

zero, when the generator is supplying an unbalanced load to a system, a negative

phase sequence current is imposed on the generator. The system unbalance may be

due to opening of lines, breaker failures or system faults. The negative sequence

current in the stator winding creates a magnetic flux wave in the air gap which

rotates in opposite direction to that of rotor synchronous speed. This flux induces

currents in the rotor body, wedges, retaining rings at twice the line frequency.

Heating occurs in these areas and the resulting temperatures depend upon the level


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and duration of the unbalanced currents. Under these conditions it is possible to

reach temperatures at which the rotor material no longer contain the centrifugal

forces imposed on them resulting in serious damage to the turbine-generator set.

Any machine as per design data will permit some level of negative sequence

currents for continuous period.

An alarm will annunciate at annunciation panel if negative sequence currents

exceeds a normal level. Reduce the MVAR power on the machine if necessary

load also and keep the machine for some time till the alarm vanishes at

annunciation panel. If the machine trips on the Negative sequence protection never

take the machine into service until the temperatures on the rotor parts settle down

to its lower value. Resynchronize the machine to the grid after considerable time

under grid & feeder parameters are within limits. If the unit trips again on the same

protection, stop the machine after consideration time so as to cool down the rotor

parts and inform to the maintenance staff for thorough examination of the system.






Status : a. Unit is at FSNL.


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4. OVER FLUXING OR EXCITATION OR VOLTS PER HERTZ

PROTECTION:

Setting : Alarm1.17 Time - 10 Sec.

Trip1.17 Time - 30 Sec.

Per unit voltage divided by per unit frequency commonly called Volts/Hertz is a

measurable quantity that is proportional to flux in the generator or step-up

transformer cores. Moderate over fluxing (105-110%) increases core loss resulting

in increase of core temperatures due to hysterics & eddy currents loss. Long term

operation at elevated temperatures can shorten the life of the stator insulation.

Severe over fluxing can breakdown inter-laminar insulation followed by rapid

local core melting. Over fluxing normally can be caused by over speed of the

turbine or over excitation during Off-line condition, and load rejection or AVR

mal-functioning during On-line condition.

If alarm annunciation panel, Increase/Reduce the speed of the turbine to ratedgenerator speed (3000RPM) and reduce the generator voltage to rated during Off-

line condition. Reduce the MVAR power on the generator during On-line

condition. If the machine trips on over fluxing protection during On-line, Keep the

machine at FSNL till the grid parameters stabilize and resins. Again the machine

trips on the same stop the machine for examination of the AVR & Governor


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systems by maintenance staff.









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5.OVER CURRENT WITH VOLTAGE RESTRAINT PROTECTION:

Setting : Alarm85% Time - 10 Sec.

Trip100% Time - 0.5 Sec.

Normally generators are designed to operate continuously at rated MVA,

frequency and power factor over a range of 95 to 105% rated voltage. Operating

the generator at rated MVA with 95% voltage, 105% stator current is permissible.

Operating of the generator beyond rated KVA may result in harmful stator over

current. A consequence of over current in winding is stator core over heating and

leads to failure of insulation.

If alarm annunciates at annunciation panel, Reduce the stator current to the below

the rated by reducing the MVAR power on the machine. When the trips on thesame protection, Resins the machine after keeping the machine at FSNL for some

time, and keep the stator current below the rated.






Status : a. Unit is at FSNL.


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6. STATOR EARTH FAULT PROTECTION:

Setting : 70% Time - 5 Sec.

Normally the generator stator neutral operates at a potential close to ground. If a

faulty phase winding connected to ground, the normal low neutral voltage could

rise as high as line-to-neutral voltage depending on the fault location. Although a

single ground fault will not necessarily cause immediate damage, the presence of

one increases the probability of a second. A second fault even if detected by

differential relay, may cause serious damage. The usual method of detection fault

is by measuring the voltage across the secondary of neutral grounding transformer

(NGT). Here are two over lapping zones to detect stator ground faults in a high

impedance grounded generator system, the two zones are put together cover 100%

stator winding for earth faults. A fundamental frequency neutral over voltage relaycovers about 0-95% of the stator zonal winding for all faults except those near the

neutral. Another third harmonic neutral under voltage relay covers remaining 96-

100% of the stator zone 2 winding on neutral side.









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7.ROTOR EARTH FAULT PROTECTION :

Settings : Less than 80K ohm

Any rotor field winding of the generator is electrically isolated from the ground.

Therefore the existence of one ground fault in the field winding will usually not

damage the rotor. However the presence of two or more ground faults in the

winding will cause magnetic and thermal imbalance plus localized heating and

damage to the rotor metallic parts. The rotor earth fault may be caused due to

insulation failure of winding or inter-turn fault followed by localized heat.







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8.RESTRICTED EARTH FAULT PROTECTION:

Settings : 0.1 Amp. Time : Instantaneous

It is Status : a. Unit is at coasting down.

similar to generator differential protection in working. It protects the high voltage

winding of 11/220KV power transformer against internal faults. One set current

transformers of the power transformer on neutral and phase side, is exclusively

used for this protection. The protection can not detect turn-to-turn fault within one

winding. Upon the detection of a phase-to-phase or phase-to-ground fault in the

winding, the unit to be tripped without time delay.






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Once the restricted earth fault protection operated, the unit can not be taken into

service unless the transformer winding is thoroughly examined by the maintenance

staff for any internals faults.

9.BACKUP IMPEDANCE PROTECTION:

Settings ; K1-3, K2-0.71 Time1.5 Sec.

As in name implies, it is used to protect the generator from supplying the over

loaded or faulty system. It is backup protection of the generator over current

protection. In measures ratio of the voltage and current supplied by the generator

and initiates trip signal when the measured impedance is less than the preset value.

If the machine trips on the Backup protection, never take the machine into serviceuntil the temperatures of the generator settle down to its lower value.

Resynchronize the machine to the grid after considerable time when grid & feeder

parameters are within limits.



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Status : a. Unit is at coasting down


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10.LOW FORWARD POWER PROTECTION:

Setting : 0.5% Time : 1 Sec.

The generator will not develop output power when turbine input is less than the no

load losses and motoring action develops on the turbine. The generator is able to

generate power, usually 55 to 10% of generator capacity, within pre-determined

time after closing of 220KV breaker.





Status : a. Unit is at FSNL with potential.


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The unit trips on the low forward protection, Resins the machine and increase input

power to the turbine as quickly as possible within low forward power time setting.

Even after two to three attempts, the machine is tripping on the same protection;

probably the governor of turbine is faulty. Inform to maintenance staff for

rectification of the same

11.REVERSE POWER PROTECTION:

Setting : 0.5% Time - 2.0 Sec.

It is backup protection to the low forward protection. Motoring of a generator will

occur when turbine output is reduced such that it develops less than no-load losses

while the generator is still on-line, the generator will operate as a synchronousmotor and driving the turbine. The generator will not be harmed by synchronous

motoring and a steam turbine can be harmed through over heating during

synchronous motoring if continued long enough. The motoring of the turbine

output can be detected by reverse power protection. The avoid false tripping due to

power swings a time delay is incorporated before tripping signal is generated.








The unit trips on the reverse power protection. Resins the machine and increase the

input power to the turbine as quickly as possible within low forward power time

setting. Even after two to three attempts, the machine is tripping on the same

protection; probably the governor of turbine is faulty. Inform to maintenance staff

for rectification of the same.


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12.POLE SLIP OR OUT-OF-STEP PROTECTION:

Setting : 6.9 ohm.

When a generator loses synchronism, the resulting high current peaks and off-

frequency operation may cause winding stresses, pulsation torques and mechanical

resonances that have the potential danger to turbine generator. Therefore, to

minimize the possibility of damage, it is generally accepted that the machine

should be tripped without time delay preferably during the first half-slip cycle of

the loss of synchronism condition. The electrical center during loss-of-synchronous

conditions can occur in the generator as a result of increased impedance of the

generator while decrease system impedance. The protections normally applied in

the generator zone such as back-up impedance, loss of excitation etc., will not

protect a generator during loss of synchronism under normal generator conditions.








The unit trips on the Pole slip protection, Resynch the machine after stabilization

of the grid parameters

13.POLE DISCREPANCY PROTECITON:

Setting : 0.5 Sec.

If One or two poles of generator breaker fail to close during synchronization, all

poles of the breaker trip on this protection. It may be due to mechanical failure of


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the breaker un equal distribution of closing signal to the breaker from protection

system.

Relays acted : a. Flag operation at 220KV Breaker panel.

b. Indication at Annunciation Panel.

Consequences : a. tripping of 220KV breaker

Status : a. Unit is at FSNL with potential.

The generator breaker trips on the pole discrepancy protection, Resynch the

generator. Even after two to three attempts, the machine is tripping on the same

protection, probably the generator breaker is faulty. Inform to maintenance staff for

rectification of the same.


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14.LOCAL BREAKER BACKUP PROTECTION:

Setting : 25% Time : 0.8 Sec.

For most of the faults, the generator breaker involves tripping the generator from

the system. Failure of the breaker to open probably results in loss of protection and

other problems such as motoring action or single phasing, If one or two poles of

the generator breaker fail to open due to mechanical failure in breaker mechanism,

the result can be a single phasing and negative phase sequence currents inducted on

the rotor. The LBB protection is energized when the breaker trip is initiated after a

suitable time interval if confirmation of the confirmation of breaker tripping from

three poles is not received. The energized tripping signal from LBB protection will

trip all 220KV generator breakers and all 220KV feeder breakers through Bus-bar

protection.


b. Acting of Master relay for all units.



b. Tripping of Field breaker of all units.

Status : a. all Units are at FSNL.

Once the LBB protection operated, the entire station is in dark. First restore all

essential services to all units such as lube oil system and turning gear etc., from

battery backup and. Checkup the faulty 220KV breaker and isolate the breaker

from the system by opening the both side of the isolators.

After restoring all services from station supply, Close 220KV feeder breakers first

and take all units into service one after the other duly co-coordinating with theDE/LD.

Since it involves complex operation, it is necessary to get help from maintenance

staff for restoring the normally in the station. Never attempt to close the faulty

220KV generator in panic as it causes permanent damage to the generator and

transformer.


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15.BUS BAR PROTECTION:

Setting : 0.8 Amp.

There are mainly three protection zones namely called generator zone, bus duct

transformer zone, 220KV breakers zone. The protection of generator zone and bus

duct & transformer zone are covered in previous schemes. All 220KV breakers atswitchyard will come under Bus-Bar protection. Functioning of this scheme is

similar to the generator differential protection or generator-transformer differential

protection. It measures all incoming currents from the generators at 220KV side

and all outgoing currents in 220KV feeders, and initiates trip signal if it detects any

deviation more than the preset value as the algebraic sum of all currents at 220KV


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bus must be less than the preset value. It isolates all 220KV generator breakers and

all 220KV feeder breakers connected to 220KV bus.


b. Acting of Master relay for all units.



b. Tripping of Field breaker of all units.

Status : a. all Units are at FSNL.

Once the Bus-Bar protection operated, the entire station is in dark. First restore allessential services to all units such as lube oil system and turning gear etc., from

battery backup and 6.6/0.44KV Stage II reserve power supply. Checkup the

entire 220KV switch yard for any wire snapping or equipment damage.

After restoring all services from station supply, Close 220KV feeder breakers first

and take all units into service one after the other duly co-ordinating with the

DE/LD.

Since it involves complex operation, it is necessary to get help from maintenance

staff for restoring the normalcy in the station. Never attempt to restore the 220KV

supply at switch yard in panic unless the entire system is thoroughly examined and

satisfy yourself as it causes permanent damage to the equipment or injury/death to

the person working at switch yard.


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16.OVER FREQUENCY PROTECTION:

Setting : 52 Hz Time - 2 Sec.

For a generator connected to a system, abnormal frequency operation is a result ofa severe system disturbance. The generator can tolerate moderate over frequency

operation provided voltage is within an acceptable limits. The machine operated at

higher speeds at which the rotor material no longer contain the centrifugal forces

imposed on them resulting in serious damage to the turbine-generator set. The

abnormal over frequency on the machine may be due to improper speed control


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adjustment or disoperation of the speed controller or severe grid disturbance or

sudden load through off.








The unit trips on the over frequency protection, Resins the machine. Even after two

to three attempts, the machine is tripping on the same protection; probably the

governor of turbine is faulty. Inform to maintenance staff for rectification of the

same.

17.UNDER FREQUENCY PROTECTION:


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Setting : 48 Hz Time : 2.0 Sec.

For a generator connected to a system, under frequency operation is a result of a

severe system disturbance. The generator can tolerate moderate under frequency

operation provided voltage is within an acceptable limits. The machine operated atlower higher speeds causes severe over fluxing in the generator-transformer. The

abnormal under frequency on the machine may be due to improper speed control

adjustment or disoperation of the speed controller.


b. Indication at Annunciation Panel

Consequences : a. NIL

Status : a. Unit is at lower speed with potential.

Increase governor speed until machine reaches full speed. Even after two to three

attempts, the machine are running at lower speed, probably the governor of turbine

is faulty. Inform to maintenance staff for rectification of the same.

18.OVER VOLTAGE PROTECTION :


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Setting : a. 110% Time - 2.0 Sec.

b. 120% Time - 0.3 Sec.

Generator voltage is at present value under normal operating conditions as selected

by operator in AVR. If it parts from preset value, May be due to AVR mal-

functioning or a system disturbance. Severe over voltage can cause over fluxing

and winding insulation failure. The over voltage protection can be considered as a

backup to the Volts-per-Hertz protection.






Status : a. Unit is at FSNL without potential.

Raise the generator voltage slowly with manual mode in AVR and keep generator

voltage within the limits of normal voltage. If it is unable to control the generator

voltage, trip the field breaker and inform to the maintenance staff for rectification

of the AVR.


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CONCLUSION:

In this mini project we had linked up the project work as

practical oriented with full pledge involvement with Operations & Maintenance

team. We had studied the working of generator and its protection schemes atGVKenergypower plant. And we also studied how the electrical power generated

& exported in the combined cycle power plant.

This project completely discussed about the protection of generators

in combined cycle power plant. As new source of natural gases are found and with

the increasing demand for electrical power, plants like this are of significance

importance. To increase the efficiencies of the plant, combined cycle principle is

adopted. In this modified plant the exhausts of gas stage are used for stream stage,

so the operational parameters have different considerations at varies operational

times

Protection of Generators

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