1

ABSTRACT

The power project is located near village Palatana in south Tripura. The power project is located on Udaipur-

Kakraban Road. The location is at approximate 230 29’59.2” N and 91026’13.7”E.

The total land under acquisition is 195.35 acres. The site is accessed by road and rail as follows.

Nearest important town and distance: Udaipur -9km

Nearest Port and distance: nearest seaport in Indian Territory is Kolkata

Nearest important railway station & distance: Agartala in Tripura

Nearest airport and distance: Agartala in Tripura (60 km)

Nearest Highway: Udaipur-Kakraban state Highway

This power plant is owned by OTPC (ONGC Tripura Co. Pvt. Ltd). And it is designed, erected and

commissioned by BHEL (Bharat Heavy Electricals Ltd).The output power of the plant is 2 X 363.3 (726.6)

MW in total. It is a combined cycle power plant. It has both a Gas Turbine fired by natural gas coupled to

generator, and a Heat Recovery Steam Generator (HRSG) utilizing the waste heat of gas turbine to run Steam

Turbine which in turn is coupled to generator.

The hot exhaust gas from the gas turbine at about 625 0 C (design temp 640 0 C) enters HRSG where steam is

produced. This steam runs steam turbine coupled with generator and produces Electricity. Output of gas turbine

is 232.3 MW at 40 0 C ambient condition and that of steam turbine generator is 131 MW at 40 oC. There are

two units, so the total power produced is 726.6 MW. The efficiency of CCGT power plant is more than 50%

whereas other open cycle plants have a maximum efficiency of about 35%. Starting power is fed from 132 KV

switch yard, control room and outgoing power is through 400 KV switch yard. This 400 KV double circuit line

is connected to Bongaigaon where from it is connected to national grid.

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CHAPTER 1

POWER PLANT INTRODUCTION

Fig 1: OTPC Overview

The natural gas based combined cycle gas turbine (CCGT) power project also known as Palatana Power Project

with a capacity of 726.6 MW at Palatana in Udaipur, Tripura is set up by ONGC

Tripura Power Company Limited (‘OTPC’), a joint venture company, promoted by Oil & Natural Gas

Corporation, Government of Tripura and Infrastructure Leasing & Financial Services (IL&FS). The project

comprises of two units (1+1) of capacity 363.3 MW each (231 MW Gas Turbine +132.3 MW steam turbine)

with an aggregate plant capacity of 726.6 MW.

Oil and Natural Gas Corporation Ltd. (“ONGC”), a fortune 500 company of the Government of India owns

significant gas reserves in the North eastern state of Tripura. However due to low industrial demand in the north

eastern regions, lack of adequate road/ rail infrastructure these natural gas reserves are yet to be commercially

developed. The economic viability of transportation of gas to other parts of the country where gas is deficit is

limited by the complexities of logistics, vast distances from the load centres in northern and western part of

India and attendant costs. Thus to optimally utilize the gas reserves available in Tripura , ONGC proposed to

initially develop a 726.6 MW thermal power plant , close to its gas fields in Tripura along with an associated

power transmission project from its project site to Bongaigaon in Assam.

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The Ministry of Power (MoP) has allocated the power from the project to the NER beneficiary states. The

generation project combined with the transmission project and the upstream gas supply project is slated to bring

investments of around 9000 crores in this region.

The entire project is broadly divided into two distinct project components namely:

GENERATION PROJECT- 726.6 MW combined cycle gas turbine (CCGT) power plant at

Palatana, Tripura, based on ONGC’s gas supply. The engineering, procurement and construction contract of Rs.

2200 crores for this project has been awarded to BHEL on August 11, 2008.

The first unit of 363.3MW of Palatana CCGT power project was declared under commercial operational w.e.f

00.00hrs of 04.01.2014 and the second unit of 363.3 MW on 00.00hrs of 24.03.2015.

TRANSMISSION PROJECT – The implementation of generation project essentially requires evacuation of

power from the generation site to the north eastern region beneficiary state (NER). There were no high voltage

440/220KV transmission lines in Tripura. Hence, the power evacuation project entailed the construction of a

400 KV AC transmission line from Palatana (generation project site) to 400KV receiving substation of Power

Grid Corporation of India Ltd.

(PGCIL) at Bongaigaon in Upper Assam over a distance of 650 Km.

4

CHAPTER 2

OPERATION OVERVIEW

2.1 OPERATIONAL FLOWCHART

Fig 2: Flowchart

In the Combined Cycle power plant or combined cycle gas turbine, a gas turbine generator generates electricity

and waste heat is used to make steam to generate additional electricity via a stream turbine. The use of liquid

fuels, usually diesel, is common as alternate fuels.

In gas turbine, turbine is driven by hot gases which are the products of combustion. Natural gas acts as a fuel

whose pressure is increased in Gas Booster Compressor (GBC) and then it is fed to the combustion chamber

where it is burned with air and then this high pressure & high temperature gas is used to turn the gas turbine

which drives an alternator. The heat is recovered from exhaust gas of gas turbine by HRSG and then this heat

is used to make steam of water in a boiler. This steam is used to turn the steam turbine which also drives another

alternator. The exhaust smoke is coming outside through chimney. Then the exhaust steam is fed to the

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condenser (in condenser a vacuum pump is present, it creates a vacuum in the condenser so that the exhaust

steam can enter into condenser from steam turbine easily). If vacuum pump is not present, then when the steam

enter into the condenser, that time a back-thrust creates, as a result steam again goes back to the turbine which

is not wanted, and then to cool this hot water, it is fed to the cooling tower and Condenser extraction pump

(CEP) is used to store it in the Condensate Storage Tank (CST). De-aerator is near in the CST in which

uncondensed steam in the cool water is removed then this cool water is some heated by water pre-heater which

is near in the CST & De-aerator, then this water is fed to the boiler through Boiler Feed Pump (BFP) and then

this cycle is continuing.

The assembly consists of the following components:

Load commutated inverter(LCI), Diesel engine, Gas booster compressor(GBC), air filter, combustion chamber,

gas turbine, alternator, exciter, HRSG, boiler, steam turbine, condenser, De-aerator, water pre-heater.

The generator voltage is 15.75KV which is stepped up to 400KV by the step-up transformer. From there the 3

phase current is supplied to switchyard for transmission. The generation power of gas turbine generator (GTG)

in this plant is 232.3MW & steam turbine generator (STG) is 131MW.

2.2 DESCRIPTION OF COMPONENTS/STAGES

A gas turbine comprising of compressor, combustor and turbine needs input temperature to the turbine to be

very high. However the incoming gas from ONGC underground pipeline is at 14 kg per sq. cm and temperature

at 25 oC. Moreover, the incoming gas needs to be conditioned before being sent to the gas booster compressor

(GBC) to prevent damage and failure of expensive equipments. The incoming gas from ONGC underground

pipeline is passed through a pipe to Gas Scrubber via a series of instruments to indicate, transmit and record

pressure, temperature and flow of gas through it.

2.2.1 Gas Scrubber:-

Though the pipeline is carrying “dry gas,” some water and hydrocarbon liquids may be present in the gas stream

as the gas moves through the pipeline. Thus it is passed through a set of two vessels called gas scrubbers to

separate out any moisture or condensate from the gas before it enters the Gas Booster Compressors. Liquids

exit the bottom of the vessel and the scrubbed natural gas exits the top. For unit1 there are two gas scrubbers

and similar is the case for unit2.

The condensate is then collected in the drain tank from both the scrubbers and stored for disposal. There are 3

level transmitters fixed on the stand pipe of the gas scrubbers to detect the level of condensate material. Also,

on the drain tank there is ultrasonic level meter to detect the level of waste condensate. To empty the gas

scrubber at any time, the valves at output side are closed and gases are expelled into the atmosphere through

flare header.

The condensate free gas is then collected from both the scrubber and sent through the common pipeline to a pair

of filter separator collector arrangement.

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2.2.2 Filter-Separator Skid:-

Filter Separator can play a vital role in helping keep contaminants from damaging the valuable natural gas

compressors and turbine in the gas downstream. It removes small solids such as dust and rust as well as lube

oil and glycol, thus allowing to produce cleaner natural gas outputs to be sent to the gas compressors.

The natural gas stream first passes through filter tubes and elements that capture solid particles and cause liquids

to coalesce into larger droplets, which are then captured by the wire mesh or vane mist extractor. The resulting

stream is typically free of solid particles. Captured dust particles are eventually discharged to drain condensate

tank by an automatic level control valve. A differential pressure transmitter and indicator which measure the

difference between high pressure and low pressure side of the filter, is used to detect the health of the filter.

This is because precipitate collected creates obstruction in the flow of gases in the filter which reduces the

pressure of outgoing gases as compared to the incoming gases. Hence, DP measures this difference and gives

indication of filter’s health. The DP indication implies the following:

0.2 to 0.3 – Filter is in good health

0.8- Alarm started and indication given to replace Filter

1- The GBC stops.

The condensate and precipitate free gas is then sent to the gas booster compressor after initial gas conditioning.

Fig 3: - Filter Separator

2.3 GAS BOOSTER COMPRESSOR

Gas transmission through pipelines may result in pressure drop because of friction losses. Booster compressors

are used to restore the pressure drop from these losses as well as raise the pressure of gases to an adequate level

i.e. 32 kg/ sq cm so as to expel it on the gas turbine. The plant has 3 GBC (2 working and 1 standby). A lot of

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redundancy can be seen in the power plant to ensure that generation of power never stops. The Gas booster

compressors are variable speed motor driven having an energy requirement of 4MW each. Thus GBCs increase

the pressure on the gas and transport it to the gas turbine. The incoming gas at 25 deg centigrade and 14 kg/sq

cm pressure is expelled on the GBC. There is a thrust bearing arrangement to keep the GBC arrangement fixed.

The gas is not directly expelled onto the GBC because direct starting may cause surging or stalling. Hence, the

gas is passed through the suction valve to GBC and circulated through anti surge valve and back to GBC. Also

to maintain the temperature of the inlet gas it is passed through the bypass cooler.

GBC arrangement has an associated gear box and motor chamber with it. The motor rotates it to compress it to

an output temperature of 120 deg centigrade and pressure of 32 kg/ sq cm.

The GBC output is then sent to absolute filter section from where it is sent to performance heater if gas turbine

load is more than 40 MW.

Fig 4:- Gas Booster Compressor

2.4 COALESCENT FILTER

The compressed gas from the two GBCs is collected and passed through the pipe to a set of two coalescent

filter sections. These are primarily used to remove liquid and particulate contaminants from compressed gas,

providing protection for critical equipment and components. Coalescing filtration is a continuous process

whereby oil and water aerosols and fine droplets run together to form larger heavier droplets that are

gravitationally drained away. The liquid contaminants may originate from compressor lube oils, condensed

moisture, carbonized oils etc. Solid contaminants such as rust particles, dust, welding flash etc determine the

useful life of the filter because they become permanently trapped in the element, creating a barrier that restricts

flow and increases

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differential pressure drop. The output gas from the coalescing filter section goes to the final gas

scrubber else to the performance heater if gas turbine load is 40MW.

2.5 PERFORMANCE HEATER

The Performance heater is used to heat the gas from GBC to raise it to a temperature of 180 deg

centigrade before sending it to the gas turbine. Incoming hot water from IP boiler is passed through

thin tubes in the performance heater and the gas flows over the pipes. Thus, heat is extracted from the

hot water and transferred to the gas to raise its temperature. There are two performance heater which

works together. The feed water after passing through the two performance heaters goes to the

condensate/hot well. The gas is then sent to the Final gas scrubber before being sent to the Gas

Turbine.

2.6 GAS TURBINE GENERATION

The incoming gas from the GBC at 32 kg/sq cm pressure and 120 oC or 180oC is expelled to the gas

turbine. The gas turbine converts heat energy from the combustion of natural gas to mechanical

energy driving the turbine blades. It is based on the principle of Brayton’s Cycle where compressed

air is mixed with fuel and burned under constant pressure conditions. The resulting hot gases are

allowed to expand through a turbine to perform work. In a Gas turbine about two thirds of this work

is spent in driving the air compressor, the rest one third is the produced energy.

The gas turbine mainly consists of three main sections:-

• The compressor-which draws air into the engine, pressurizes it, and feeds it to the combustion

chamber at speeds of hundreds of miles per hour.

• The combustion system-typically made up of a ring of fuel injectors that inject a steady stream

of fuel into combustion chambers where it mixes with the air. The mixture is burned at

temperatures of more than 1200 deg centigrade. The combustion produces a high temperature,

high pressure gas stream that enters and expands through the turbine section. There are 18

chambers forming the combustion system. Combustion is initiated by means of discharge from

two high voltage electrode spark plugs in chamber 1 and 2. At the time of firing, one or both

sparks of these plugs ignite a chamber. The remaining chambers are ignited by crossfire through

the tubes that interconnect the reaction zones of the remaining chambers. 4 fire scanners are

present in chamber 15, 16, 17, 18 that detects the presence of flames to be transmitted to the

control system.

• The turbine is an intricate array of alternate stationary and rotating aero foil-section blades. As

hot combustion gas expands through the turbine, it spins the rotating blades. The rotating blades

perform a dual function: they drive the compressor to draw more pressurized air into the

combustion section, and they spin a generator to produce electricity.

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Fig 5:- 3D view of 9FA GT

2.7 HEAT RECOVERY STEAM GENERATION

The heat is recovered from exhaust gas of gas turbine by HRSG and then this heat is used to make

steam of water in a boiler. This steam is used to turn the steam turbine which also drives another

alternator. The exhaust smoke is coming outside through chimney. The steam is condensed in the

condenser and recycled to where it was heated; this is known as Rankine Cycle.

Fig 6: HRSG

The components of HRSG are as follows:

HP , IP and LP Economizer- Economizers are heat exchange devices that pre heat the incoming

feed water, up to but not normally beyond the boiling point of that fluid. Economizers are so named

because they can make use of the heat energy in flue gases that are hot, but not hot enough to be used

in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. It is

constructed of modules consisting of rows of plain/spiral fine tubes welded top and bottom headers.

Heat transfer is achieved by single pass flow on gas side and multi-pass flow on water side.

Compress or 3 stage turbine blades

Combustion Chamber

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HP, IP and LP Evaporator- This heat transfer section is arranged next to the superheater in the

direction of gas flow. In the case of H.P. section the heated feed water from the economizer enters

the steam drum and this boiler water from the steam drum flows to the common feeder through the

down comer and is distributed to the bottom header of the evaporator module through the feeders.

While the L.P. section, the feeder water from the boiler drum is distributed to the evaporator module

after descending through the down comers.

Steam water mixture generated in the evaporator tubes, due to the heat transfer from the fuel gas flows

back to the drum through the riser tubes. Saturated steam is separated by centrifugal separators and

final scrubbers placed in the drum. The steam flows to the superheater, while the saturated water is

recirculated through the down comers. Heat transfer is achieved by single pass flow on both gas and

steam side.

HP, IP and LP Superheater- It is the first heat transfer in the in the direction of gas flow section,

arranged next to the evaporator. A superheater is a device used to convert saturated steam or wet

steam into dry steam used in steam engines. The saturated steam is piped from the boiler drum area

into tubes inside an area of the furnace known as the super heater, which has an elaborate set up of

tubing where the steam vapour picks up more energy from hot flue gases outside the tubing and its

temperature is now superheated above the saturation temperature. The superheated steam is then

piped through the main steam line to the valves before the high pressure turbine.

2.8 STEAM TURBINE

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and

converts it into rotary motion. , the steam turbines are split into three separate stages, the first being

the High Pressure (HP), the second the Intermediate Pressure (IP) and the third the Low Pressure

(LP) stage. The Cold reheat steam from the High Pressure turbine falls in temperature and pressure

which is again reheated and sent to the Intermediate pressure turbine. The hot reheat steam is

conducted to the intermediate pressure turbine where it falls in both temperature and pressure and

exits directly to the long-bladed low pressure turbines and finally exits to the condense

Fig 7:- Steam Turbine

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2.9 CONDENSATE CYCLE

Condenser is a device or unit used to condense the exhaust gases from the LP turbine to its liquid

state, typically by cooling it.

The exiting steam from the LP steam turbine, now a little above its boiling point, is brought into

thermal contact with cold water (pumped in from the cooling tower) in the condenser where it

condenses rapidly back into water. In condenser a vacuum pump is present; it creates a vacuum in the

condenser so that the exhaust steam can enter into condenser from steam turbine easily). If vacuum

pump is not present, then when the steam enter into the condenser, that time a backthrust creates, as

a result steam again goes back to the turbine. The condensed steam is stored in condenser also known

as Hotwell.

Two Condensate Extraction Pumps (1 working and 1 standby) are used to pump the water from the

Hotwell to Deaerator through the gland steam condenser. The pumps are operated when there is a

certain amount of water present in the Hotwell. Initially the discharge valve of the pump is closed

and minimum recirculation line is kept open to limit the inrush current.

Cooling Tower- Cooling towers are heat removal devices used to transfer process waste heat to the

atmosphere. Cooling towers either use the evaporation of water to remove process heat and cool the

working fluid or in the case of closed circuit dry cooling towers rely solely on air to cool the working

fluid

Fig 8:- Cooling Towers

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2.10 FEED WATER SYSTEM

Deaerator- A deaerator is a device that is widely used for the removal of oxygen and other non-

condensable gases from the feed water which is to be steam-generating boiler. The condensed water

is then passed by a feed pump through a deaerator. The oxygen present in the water is removed in the

de-aerator for preventing the water from becoming corrosive. From the deaerator, the water is stored

in the Feed Water Storage Tank.

Fig 9:-Deaerator

2.11 Boiler Feed Pump

There is a common header from this tank from where two boiler feed pumps (BFP) are connected;

one for HP and IP Boiler and the other for LP Boiler. The BFP’s are assisted with suction valves and

discharge valves on its either sides. There is a HP BFP discharge header for Feed station and the same

is replicated in the LP side also. The water from the HP discharge header is fed to the boiler though

3 Flow Control Valves (FCV). The 3 valves follow differential opening principles. The first and

second CV provides 100% outflow (1 working and 1 standby) while the third opens 30%. The third

CV is a bypass valve for allowing entire flow of water in the drum.

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CHAPTER 3

CONTROL SYSTEM

A distributed control system (DCS) is a dedicated control system for the power plant wherein control

elements are distributed throughout the system. In DCS, a hierarchy of controllers is connected by

communications network for command and monitoring. A DCS typically uses custom designed

processors as controllers. The Control System used for

Gas Turbine- Mark VI Control System .Steam Turbine- MaxDNA System

3.1 MaxDNA DCS

MaxDNA is a fully integrated control system designed for control of steam turbine. MaxDNA DCS

system consists of a modern Windows XP-based human machine interface maxVUE, maxTools.

These systems are connected to the DPU (Digital Processing Unit) and maxPAC I/O system.The

transmitters used in the field are transducers which sense the change in physical environment and

converts it into corresponding electrical signals which is generally current signal ranging from 4 mA

to 20 mA. These signals are sent to the Junction Box (JB) and through signal cables. These cables are

used for bidirectional communication. From JB the cables are taken to Control Equipment Room

(CER). In CER, there is a Terminal Board (TB) where input lines from JB enter the 16 channel

module. These are connected via input lines to different I/O modules according to the signal they

carry like analog input module, Digital Input module, RTD module, Thermocouple module etc. The

front side of the terminal board is in the form of racks which is generally 4 in number. The first rack

is the primary rack while below is the hot backup rack. Whenever a critical card in the primary rack

fails the backup takes over the entire primary rack. The CAT -5 cables connects the DPUs. The first

module in the rack is DPU (Digital Processing Unit) and the second module is kept empty. The rest

modules contain I/O cards. Signal goes from DPU to different cards. The I/O cards are connected to

the DPU by back plane.

3.2 SPEEDTRONIC Mark VI

SPEEDTRONIC Mark VI turbine control is the control system used for GE turbine. It is designed as

a complete integrated control, protection and monitoring system for turbine. The heart of the control

system is Control Module which is available as 13 or 21 slot standard VME card rack. Inputs are

received by the control module through termination blocks. Each I/O card contains a DSP processor

to digitally filter the data before conversion to 32 bit floating point format. In addition to the I/O

cards, the Control Module contains an internal communication card, a main processor card and

sometimes a flash disk card. Each card takes one slot except for the main processor which takes two

slots. I/O data is transmitted on the VME backplane between the I/O cards and the VCMI is used for

internal communications. Mark VI control systems are available in Simplex and Triple Redundant

forms. The name Triple Module Redundant (TMR) is derived from the basic architecture with three

completely separate and independent control modules, power supplies.

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CHAPTER 4

VARIOUS PORTIONS OF ELECTRTICAL SINGLE LINE DIAGRAM

4.1 GENERATOR: The generator at this project is hydrogen cooled three phase synchronous

generator with 2 pole cylindrical rotor with a terminal voltage of 15.75 KV. Total 4 generators are

in this plant:

• One pair of Gas Turbine Generator (GTG)

• One pair of Steam Turbine Generator (STG)

Stator: The stator body is of totally enclosed air/gas tight construction. The stator winding is star

connected with all the phases and neutral ends brought out through bushings. The terminal of the

phases and neutral of the GTG & STG are brought out by IPB

Rotor: The rotor winding is provided with a complete damper winding with short circuit ring on

both sides. Rotor earth protection scheme is used for alarm at the first stage and further tripping at

the second stage.

4.2 TURBINE: Here two types of turbine are used, steam turbine & gas turbine. Gas turbine at

Palatana site is of type: Fr-9FA-an advanced class Gas Turbine fully controlled by MK-VI control

system. Steam turbine here is divided by three portion, IP portion & LP portion.

4.2.1 EXCITATION SYSTEM: The excitation used in the gas turbine is static excitation. In

steam turbine brushless excitation is used, in which for excitation, firstly a PMG (permanent magnet

generator) used, whose magnetic field is permanent & constant and the armature is rotating and the

is produced AC voltage is then fed to the AVR (automatic voltage regulator) in which SCR is present.

This SCR convert this AC voltage into DC and then this DC is excites the main field system of STG.

4.2.2 INITIAL STARTING SYSTEMS: Usually, every gas turbine requires a starting

device. In this case, generator works motor up to 85% speed by a process, called cranking, by which

the generator is rotated as a motor to overcome the inertia of the gas turbine. And once it reaches this

speed, it works as generator. LCI gives the voltage of 2KV in the cranking process. After the

cranking, the machine is run as generator. This cranking process is done in the gas turbine, not in the

steam turbine, because the velocity of steam is higher than the velocity of gas, so there is no need to

overcome the inertia of the steam turbine.

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4.3 BUSDUCT: A bus duct is combine arrangement of numbers of bus bars made of hollow

copper/aluminum tube or bars that conducts huge amount of current .Types of bus duct use d in this

plant are of three types :

1. INSULATED PHASE BUS DUCT (IPB):-IPB is a method of construction for circuits

carrying very large currents, typically between a generator and its step-up transformer in a power

plant. In an IPB each phase current is carried on a separate conductor enclosed in separate conductor

enclosed in separate grounded metal housing.

2. SEGREGATED PHASE BUSDUCT (SPB):-SPB is designed with a conductor enclosed

in common enclosure and with metallic barrier between each three phases.

Fig 10: SPB

3. NON-SEGREGATED PHASE BUSDUCT (NSPB):-NSPB are similar to SPB but

without a metallic barrier between them.

Fig 11: NSPB

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4.4 TRANSFORMER

Fig 12: Transformer

A. HT TRANSFORMER:

HT transformers used in this power plant are:

-Gas Turbine Generator Transformers (GTGT)-one per unit (230 MVA)

-Steam Turbine Generator Transformers (STGT)-one per unit (150 MVA)

-Station transformers (ST)

-Interconnecting transformers

-Unit Auxiliary Transformers (UAT)

All of the HT transformers except the interconnecting transformer are located in the

transformer yard situated and running along –side the power house .The interconnecting

transformers are situated within the 400KV switchyard which will interconnect the 400KV

transmission line to132 KV switchyard.

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The interconnecting transformer is 400/132 KV ,125MVA autotransformers with on load tap

changer of +10% and -10% in steps of 1.25 %.It will step-down the 400KV to 132 KV once

the project starts generating power.

The rating of the generator transformers are15.75/400 KV.GTG & STG transformers are

provided with on load tap changer in the range of -10% to +10% in1.25% equal steps on HV

side .

4.4.1TRANSFORMER TANK:

The transformer tank is welded and fabricated from good commercial grade low carbon steel of

adequate thickness to withstand without any deformation continuous internal gas pressure of 0.35

kg/cm2over normal oil pressure .The transformer also withstand complete vacuum under standard

atmospheric conditions .The core is built with low loss, high permeability, and cold rolled silicon

steel lamination. One set of winding temperature indicators with necessary current transformer,

heating coil and a detector element and one set of oil temperature indicator are mounted locally. Each

of the above indicators is provided with necessary contacts for alarm and trip and transducers with

4.20mA output for remote logging of winding/oil temperature. Buchholz relay is provided with two

floats and two pairs of electrically separate contacts for alarm and trip. Separate oil surge relay is

provided for OLTC.

4.4.2 BUSHING:

Fig 13: Bushing

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All bushings are homogeneous, non-porous, porcelain type, uniformly glazed and free from blisters,

burns etc. Neutral and phase bushing CTs are furnished as required with its secondary leads wired up

to the terminal blocks .The terminal for CT secondary leads have provision for sorting. The

arrangement is such that the CT can be removed from the transformer without removing the tank

cover.

4.4.3 MARSHALLING BOX:

A sheet steel weather proof Marshalling box is mounted at the tank of transformer and is

accommodated with all auxiliary devices except those which are located directly on the transformer.

All terminal boxes for external cable connections are located in this box.

4.4.4 ON LOAD TAP CHANGERS:

Fig 14: OLTC

The OLTC gear is designed to complete successfully tap changes for the maximum current to which

transformer can be loaded. Devices are incorporated to prevent tap change when the through current

is in excess of the safe current that the tap changer can handle .The OLTC gear can withstand through

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fault currents without injury .The OLTC is provided with the Local – Manual/Local –

Electrical/Remote Electrical modes of operation.

4.4.5 OFF CIRCUIT TAP CHANGERS:

Off circuit tap changers switch is provided in 3-phase ,hand operated ,by an external handle with

position markings and pad locking facility and mechanical stops to prevent over cranking beyond

exterminate positions.

B. LT TRANSFORMERS:

LT transformers used in this power plant are of two types with a step down capacity of:

-6.6/0.433 KV auxiliary transformers

-415/415 V Lightning Transformers

The rating of LT transformers are decided considering the maximum possible load at any instant. HV

sides of auxiliary transformers are delta connected and LV sides are star connected. HV sides are

connected with off circuit tap changer in the range of +5% to -5% in 2.5% equal steps .HV side

suitably cable connected and LV side for non-segregated phase bus duct.

Auxiliary transformers is mineral oil filled .The ratings of all LT transformers (6.6/0.433 KV) are

limited to 2MVA each .These transformers are dry type.

Lightning transformers is 415/415V ratio , dry type , air cooled, suitably sized to limit the fault current

to less than fault rating of the MCB’s used downstream .Primary are delta connected and secondary

are star connected .Primary are provided with off circuit lap changer of +5% to -5% in steps of 2.5%.

Transformers are designed for over fluxing withstand capability of 110% continuous and 125% for

at least 1 minute.

4.5 GTG (Gas Turbine Generator)

It is a three phase generator. The star side of this generator is grounded by NGT & NGR because, by

the NGT we can step down the negative voltage level(which has to grounded)to a small value and

NGR is used to resist the current in the neutral wire which is to be grounded. From the delta side of

the GTG, 15.75 KV is produced, which is stepped up to 400KV by GTGT (gas turbine generator

transformer). The generation power of gas turbine generator in this plant is 232.3 MW. Between GTG

& GTGT a tap has been taken for UAT which is used for for the initial starting purpose, which has

stepped down the generator voltage 15.75 KV to 6.6 KV. A cranking switch is also used for initial

starting purpose of the GTG. In this plant there are two GTG. One is for unit 1 & the other for unit 2.

The entire portion of unit 1 and unit 2 are same.

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Fig 15: Gas turbine generator

4.6 STGT (Steam Turbine Generator Transformer)

Fig 16: STGT

It is 150 MVA transformer and it steps up the voltage from 15.75 to 400KV, which is then fed to the

switch yard. The vector group of it is YNd1, i.e. -30 degree phase displacements. It is a delta star

transformer. The primary side is delta and secondary side is star. The star side is solidly grounded.

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4.7 SWITCHYARD: Switchyard is basically an assembly of switchgears in a particular enclosed area.

Switchgears are collection of protective and switching equipments. Switchyard is an essential part of the

generating power station.

Fig 17: 400 KV Switchyard

4.7.1Current transformer:

Fig 18: Current transformer

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It is used for metering and protection purpose. It steps down the current to a measurable quantity thus providing

display of the current parameters in the control panel. CT used here are dead tank type. CT of generator

transformer has 4-core secondary side and its ratio in phase 2 are

800/1- used for bus differential protection

150/1- used for GT overall protection and reference

4.7.2 Potential Transformer(EMVT):

Fig 19: PT

It is also used for metering and protection purpose. It steps the voltage to a known ratio for measuring

effectiveness.

Surge arrestors: 120 KV, 10 KA nominal discharge current, gapless, metal oxide type surge arrestors

is used for 132KV systems. For 400 KV system lightning arrestors is of 360 KV, 20 KA nominal

charge current. Arrestors are suitable for installation in effectively earthed system. A leakage current

23

detector as an integral part of discharge counter is provided. Surge arrestors have adequate thermal

discharge capacity for severe switching surges. Grading resistors, capacitors and rings is provided for

uniform voltage distribution between the units making up the arrestors.

4.7.3 Isolators:

Fig 20: Isolator

It is an off load device used for isolating a circuit from another. It is manually operated during

maintenance or replacement of faulty parts. It is placed on both the sides of a circuit breaker.

Type of isolator used here is air break isolator.

4.7.4 Bus Coupler:

It is a circuit breaker assisted by an isolator used for transferring power from one bus to another.

4.7.5Insulator/ bushing:

These are made of homogeneous vitreous porcelain, the glazing of which are of uniform brown or

dark brown colour. Insulators are designed to avoid excessive concentration of electrical and stresses

in any section or across leakage surfaces.

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Bus Bar arrangement: there are two 400 kV three phase bus bars in the 400KV switchyard and also

there are two 132 KV three bus bars in the 132 KV switch yard. The bus bar used in this is called one

and a half Circuit Breaker system since here for every two circuits 3 CBs are used.

4.7.6 Sequential Event Recorder (SER)

SER is to be used for the switchyard signals. These signals as a minimum contains all protective

relay, lock out relay and breaker auxiliary contacts. These are half wired to SER cabinet and are to

be located in switchyard control room.

4.7.7 SCADA (supervisory control & data acquisition system):

This is a content of the control panel. The 400/132 KV switch yard is to be provided with a

dedicated microprocessor based SCADA system for supervisory control and acquisition of status

indications, alarms and measured values.

4.7.8 SWITCHGEAR: The apparatus used for switching, controlling and protecting the

electrical circuits and equipments is known as Switchgear. It essentially consists of switching and

protecting devices such as switches, fuses, circuit breakers, relays etc.

HT SWITCHGEAR

It is a 6.6 KV board. The HT switchgear boards:

1CA BOARD

1CB BOARD

0CA BOARD

2CA BOARD

2CB BOARD

4.7.9 POWER AND CONTROL CONNECTION:

For 6.6 KV switchgear, the incoming power is coming through Bus Duct.

4.7.10 CONTROL AND INDICATION:

Breaker control supply is of 220 volt DC. For river water pump house battery of 24/48 V DC is

selected. All switchboards are provided with redundant DC control supply with auto change over

facilities. Breaker spring charging motor is suitable for 220V DC. Isolating switch fuse unit of

adequate capacity is provided in all switchgear for incoming supplies. Electric insulation is provided

as Breaker ‘ON’-Red Lamp; Breaker ‘OFF’-Green Lamp; Breaker Auto Trip Amber Lamp, i.e.,

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amber lamp indicates the breaker tripped condition in SERVICE position; Breaker Trip coil healthy

white lamp;DC fail –Blue lamp. All indication lamps is of cluster LED type. Breakers are controlled

from remote as per plant operation requirement. Breaker signals, operation, protection relay oriented

condition and necessary current, voltage and power signals is routed to DCS in addition to local

insulation. All switchgear is provided with one DC fail indication lamp for each DC incomers. Further

a common DC alarm contact is wired up for remote annunciation top indicate failure of DC supply to

any breaker panel.

4.7.11 CURRENT TRANSFORMER:

CT secondary is rated for 1A. For each fault protection core balance CT’s are provided.

4.7.12 VOLTAGE TRANSFORMER:

HT switchgear has a 3 phase voltage transformer with the HCR type primary and secondary fuses .for

incoming and tie feeders also,voltage transformer is provided . the VT’s are of drawn type and

provided with secondary fuses .Secondary voltage of the VT is 110 V AC. Fuse failure relay is

provided on the secondary side of the entire VT to monitor failure of LV fuses.

4.7.13 MEASURING INSTRUMENTS:

All the measuring instruments are digital multi functioning type and is provided with suitable ports

for remote hook up to DCS.All the incomer and transformer feeders are provided with digital meter

to read voltage, current, frequency, power factor, power and energy. All the outgoing feeders have

the facility to read voltage and current. All the meters in a switchboard is daisy chained for remote

hook up and wire break for monitoring of these analog signals is provided.

4.7.14 PUSH BUTTONS:

All the push buttons are of heavy duty,spring return type suitable for flush monitoring on the sheet

cubicle doors. The push buttons have two number of ‘NO’ and two numbers of ‘NC’contacts .The

continuous current and breaking capacity of the contacts are adequate for duty involved.

4.7.15 CONTROL/SELECTOR SWITCHES:

Circuit breaker control switches are of 3 positions (Trip/Neutral/Close) with spring return to

‘neutral’. Local/Remote selectors switches have make before break failure on its contacts. The

selector switches has 4 positions.

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4.7.16 RELAYS:

Fig 21: Relay

Microprocessor based multi-functioning numerical relays with facilities such as programmable

scheme logic ,comprehensive protection ,measurement of electrical quantities ,post fault analysis etc

are provided for the protection of Switchgear incomers , tie feeders ,outgoing motor and transformer

feeders. All the protective relays are of draw out type. All the relays have built in testing facilities.

Small auxiliary relays may be of non draw out type. Each feeder has necessary auxiliary relays, timers

etc to meet the circuit requirements. Auxiliary relays with flag are provided in case of SF 6 breakers.

For transformer feeder self reset auxiliary relays with hand reset false indicator is provided for contact

multiplication of:

Transformer Buchholz alarm and trip contacts for main tanks

Transformer winding temperature indicator alarm and trip contacts.

Transformer oil temperature indicator alarm and trip contacts.

Transformer pressure relief trip contacts.

Transformer oil level low alarm contacts.

27

Under voltage relays are provided in the bus PT circuit. For dry type winding transformers windings,

temperature alarm and trip alone is applicable and provided. All protection/supervision/trip relays

have necessary communication with the DCS system.

4.7.17 LT SWITCHGEAR:

The LT switchgear boards are:

• 415 V Power Control Centres(PCC)

• 415 V Motor Control Centres (MCC)

• AC/DC Distribution boards

• Normal AC/Emergency DC lightning Distribution Board.

• 415 V Bus ducts with support structures

The PCC, MCC, DB provide power, control, indication and protection for 415/240 V drives/feeders

in the plant. All PCC has two bus sections with separate incomers and bus couplers. Each incomer is

rated for feeding the entire PCC and corresponds to the transformer rating with margin. Normally the

two incoming breakers are ON with the bus coupler breaker in OFF condition. Necessary auto

changeover scheme and manual changeover schemes are provided with necessary control indication

and interlocks. Momentarily paralleling facility with check synchronizing relay is provided in the,

manual changeover scheme. The unit PCC is extended on one end with circuit breaker to provide

emergency section which is fed from the emergency DG. Interconnection facility between the

emergency switchgear fed by the EDG set is provided. Adequate interlocking is used in the operation

of incoming and bus coupler circuit breaker to prevent continuous parallel operation of the two

transformers. The PCC uses air circuit breaker. MCC incomer and bus coupler up to 630A rating uses

fuse switch unit. Beyond 630 A an air CB is used. The outgoing motor feeders of MCC are of switch

fuse, contactor and protective relays. All AC motor starters are suitable for direct on line starting.

Motors rated 90KW and above have breaker controls. Motors less than 90KW rating uses switch fuse,

contactor and O/L relays.220V DC system is underground .DCDB is suitably sized for DC system

fault current for one sec duration. The PCC/MCC incomer uses an ammeter and voltmeter with

selector switches and power supply indicating lamps(R,Yand B).The motor feeders uses three

indicating lamps for ON,OFF and TRIP and push buttons for START (in test position) ,STOP and

RESET.

4.7.18 LT BUSDUCT: LT bus ducts are installed to connect the LV side of the transformer to

the PCC. The bus duct is totally enclosed, discontinuous type non- segregated with aluminum

conductors. Bus conductors and their support insulators are designed to withstand while carrying the

short circuit current of 50 KA for 1sec.

4.7.19 AIR CIRCUIT BREAKERS: Circuit breakers are fully drawn out, 3 pole indoor,air

break and trip free type. CB’s of all the PCC incoming, bus coupler and motor feeder breakers have

protective relays. Outgoing PCC feeders also have relays.CB’s are provided with motor operated

spring charged stored energy closing mechanism. Manual spring charging and closing mechanism is

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also provided. One open-close-open operation of the CB is done after failure of failure of power

supply to the motor.

The breaker has the following positions with positive indication for each of them

• Service position- both power and control circuits are connected

• Test position- power circuit is disconnected and the control circuit is connected.

• Disconnected position-power and control circuits are disconnected

• The earth connection remains connected in TEST position. The earth connection make before

the main power/control contacts make and break after the main power /control contacts break

. The breaker is suitable for operation from both remote as well as local through dust proof,

heavy duty type push buttons while under SERVICE position. In TEST position the breaker

is operated only from a local push button. Anti pumping feature is used to prevent automatic

re-closure of the breaker after tripping in the event of sustained fault. Breaker does not close

unless the spring is fully charged.

• INSTRUMENT TRANSFORMER: The CT ‘S & VT”S are insulted type with primary and

secondary terminals marked indelibly. Facility is provided for short circuiting and grounding

each CT secondary. The CT’s are capable of withstanding the peak momentary short circuit

and the symmetrical current. The neutral side of the CT is normally earthed through a link.

CT secondary is rated for 1A. VT’s have fuses on both primary and secondary. For PCC bus

VT’s are housed in a separate compartment. VT’s secondary is rated for 110V AC.

• CONTROL TRANSFORMER: The control transformers are dry type. The transformer is

complete with switch and fuse/MCB. One control transformer is provided in MCC module to

control supply to respective feeders.

• FUSES: The fuses are of HRC link type with a breaking capacity of 80 KA. Visual indication

is provided to indicate the fuse failure. The motor fuse rating and characteristics are suitably

chosen so they do not operate during starting period of the motor.

• CONTACTORS: The main motor contactors are direct-on-line, air break triple pole AC3

duty for AC motors, with minimum two NC and two NO auxiliary contacts.

• THERMAL OVERLOAD RELAY: These are of three elements, ambient temperature

compensated, time lagged, bi metal thermal overload relays. The relays are manually reset

type with one changeover contacts. Resetting of the relays is possible with compartment door

closed, with the resetting knob at the front.

• PROTECTION RELAYES: These relays are microprocessor based numerical relays with

multifunction, selective relay scheme logic, post fault analysis, communication and diagnostic

features. Indicating devices are so located that it can be seen when the relay has operated,

without opening door.

• MEASURING INSTRUMENTS: All measuring instruments are moving iron spring control

industrial type. Breaker /contactor starter signals, protection relay operated condition and

necessary current /voltage signals are routed to DCS in addition to local indication. Ammeters

are provided for all the motors above 30KW and for other important motors that are specially

required. Provision for remote and local ammeter is also given in the PCC/MCC panel.

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• CONTROL SWITCH/SELECTOR SWITCH: Breaking control switches have

NeutralClosed-Trip positions. Voltmeter and ammeter in the incomers/feeders are provided

with a selector switch.

4.7.20 UAT (unit auxiliary transformer):

The rating of Unit Auxiliary Transformers are 15.75/6.6 KV. UAT is of 3 phases, 2 winding

type and cooling are with mineral oil (ON AN/ON AF). The transformers are provided with off circuit

tap changer on HV side in the range of -5% to +5% in 2.5% equal steps. Voltage generate from GTG

is 15.75 KV; from here this voltage is fed to the GTGT & UAT. In GTGT, voltage is stepped up to

400KV, and in UAT voltage is stepped down to 6.6KV. It is a delta-star with secondary grounded

through a register i.e. NGR, to limit the earth fault current to 300A for 10 seconds. The vector group

of it is Dyn 11 i.e. +30 deg phase displacement and its power rating is 16MVA. UAT is used to

starting of all the devices when power starts to generate in the plant. And then there is no need to take

power from outside i.e. station transformer. Before power is generating, to start all of the machines,

we have to take power from station transformer. UAT is charged 2 switchgear board of unit 1 i.e.

1CA & 1CB and 2 switchgear boards of unit 2 is i.e. 2CA & 2CB which are 6.6KV switchboard

through SPB. The 2 CA &2CB board of unit 2 is same as the 1CA & 1 CB board of unit 1. In this

power plant auxiliary power consumption is about 7%.

A. UNIT SWITCHBOARD 1CA:

It is 6.6KV 1250A board, power is coming first from ST through Tie from 0CA board and when

power generation has starts to produced, and then in this board power is coming from UAT through

SPB. From this board, power is going to BFP(4000kW), CEP(600KW), CW pump(1150KW),

DMCW pump(280KW), Tie to 6.6KV station switchboard-0CA(sec-1) through segregated phase bus

duct, static excitation(2000KVA), LCI(7MVA), spare motor feeder(4000KW), spare transformer

feeder(1600KVA),STG PMCC(1600KVA, 6.6/.433KV), GT PMCC(1600KVA, 6.6/.433KV), CT

PMCC(1600KVA, 6.6/.433KV). when in any switchgear board, if power has come through a TIE in

that board, then TIE PT indicates that power is coming or not. If TIE PTis working, then it denotes

that power is coming in this board through TIE. Again if in any switchgear board any incoming line

is connected, then LINE PT board through that incomer. Bus PT indicates that particular board is

charged or not, i.e. in each board one BUS PT is present. In a board LINE PT is always connected

before BUS PT, because if power is coming from incoming line i.e. if incoming line is working, then

the board is charged

• GT PMCC (gas turbine power motor control centre): It is a 415V, 2500A board. In this

section power is coming from 1CA board &1CB board i.e. 6.6KV bus-bar through a step down

transformer which is stepped down voltage level from 6.6KV to 415V, and then this 415V is

connected to GT PMCC through NSPB. In this GT PMCC, there are two normal sections & one

emergency section. Two normal sections are, one for 1CA & other for 1CB board. A bus coupler is

used between two normal sections. For emergency section, power is coming out from diesel engine

power control centre (DG PCC) is used.

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• CT PMCC (cooling tower power motor control centre): It is a 415V, 2500A board. In this section

power is coming from 1CA board &1CB board i.e. 6.6KV bus-bar through a step down transformer

which is stepped down voltage level from 6.6KV to 415V, and then this 415V is connected to CT

PMCC through NSPB. In this CT PMCC, a bus coupler is used for 1CA & 1CB.

• STG PMCC (steam turbine generator power motor control): It is a 415V, 2500A board. In this

section power is coming from 1CA board &1CB board i.e. 6.6KV bus-bar through a step down

transformer which is stepped down voltage level from 6.6KV to 415V, and then this 415V is

connected to CT PMCC through NSPB. In this, there are two normal sections & two emergency

sections. Two normal sections are, one for 1CA & other for 1CB board. . A bus coupler is used

between these sections. For two emergency sections, power is coming out from diesel engine power

control centre (DG PCC) is used.

ST MCC (steam turbine motor control centre): It is a 415V, 400A board. In this section power is

coming from two lines of 415V, 2500A STG PMCC, one line from normal section A, and other from

normal section B. In this centre, fuse is used as a bus coupler and also as protection purpose of this

board, because of lower current.

HRSG MCC (heat recovery steam generator motor control centre): It is a 415V, 250A board. In

this section power is coming from two lines of 415V, 2500A STG PMCC, one line from normal

section A, and other from normal section B. In this centre, fuse is used as a bus coupler and also as

protection purpose of this board, because of lower current.

• LCI (load commutated inverter): In this section power is coming out from 6.6KV 1CA board

through a transformer which is a double winding secondary. The function of this is to give the voltage

in the time of cranking. The voltage is required in the time of cranking is 2KV. The LCI transformer

is not grounded, because there is no distributed system in LCI. It is used only for cranking.

• STATIC EXCITATION SYSTEM: Static excitation station means the use of D.C. system for

the excitation of field. For static excitation of the field of GTG, we have to use a step down

transformer, which steps down the voltage 6.6kv to 415V. The voltage of 6.6kv coming from 1CA

board to this transformer. This 415V is then converted to D.C. by the use of thyristor and then fed to

the field system of GTG. After rectification this 415V is changed to 260V (full load excitation

voltage), because in the process of rectification (rectification waveform in thyristor) some voltage is

lost. The GTG field current is 2200A.

B.UNIT SWITCH BOARD 1CB: The board is similar to the 1CA board, except in this 1CB board

there are two CW PUMP and tie to SEC 2 of 6.6KV station switchboard is connected to SEC 1 of 0

CA through line.

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4.7.21 DG PCC (Diesel engine generator power control centre)

In this project there are two number of emergency diesel engine generator set. Its rating are suitably

sized for safe shut down of the units in the event of simultaneous failure of grid supply and also to

meet emergency loads such as emergency drives, lightning, control and indications etc. it will come

into operation in the event of total power failure in the station. DG PCC is a 415 V centre; there are

two sections in the centre. Power is coming from two DG for two sections which are 750 kVA, 415

V, 3 phase diesel engine generator. One section i.e. SEC-A is for unit-1 and SEC-B is for unit -2.

These DG’s are used for emergency purpose.

4.7.22 STATION TRANSFORMER:

Fig 22: Station transformer

Station transformers are of 3 phases, 2 winding mineral oil cooled type, HV sides of the transformers

are connected to 132 KV switchyard. These transformers are of OLTC of -12.5% and +7.5% in steps

of 1.25%. There are two ST in this plant. By those we bring current from outside for initial starting

purpose. These are Star-star transformer and step down transformer and the primary of these

transformers is solidly grounded and secondary is grounded through a resistor because the outside

voltage is stepped down to 6.6KV from 132kV and power of these transformers is 25MVA. These

32

are YNyn0 type transformer. These transformers are charged to 0CA board through segregated phase

bus duct (SPB)

UNIT switchboard OCA: it is 6.6KV 2500A board. Power is coming out from ST through SPB.

In this board, there are two sections, one for ST-1 and another for ST-2. From this board, power is

going to two gas compressor motor in sec-1(3840KW) but in sec-2, there is one gas compressor motor

service switchgear (2000KVA,3000A,6.6K/.433KV),raw water switchgear(1600KVA,6.6/.433

KV,2500A),administrative building PMCC (1600,2500 A,6.6/.433KV), gas station

PMCC(1600,2500A,6.6/.433KV),spare transformer feeder(2000KVA,6.6/.433KV), spare motor

feeder(3840 KW). In SEC-2, the entire portion is same as SCE-1. There are 4 ties connected to 0CA,

in sec-1 two ties is connected, one for 1CA & another for 2CA, and in sec-2, again two ties are

connected. One is for 1CB & another for 2CB.

A bus coupler is connected in between sec-1 and sec-2.

1. GAS STN PMCC (gas station PMCC): It is a 1600KVA, 2500A, 6.6/.433KV board. Power

comes in this board through step down transformer, which steps down the voltage 6.6 kV to

415 V. from transformer power is fed to this board through NSPB. In this centre power is

coming from both sec-1 & sec-2 through transformer

2. RAW WATER SWITCHGEAR: It is a 1600KVA, 2500A, 6.6/.433KV board. Power comes

in this board through step down transformer, which steps down the voltage 6.6KV to 415 V.

from transformer power through NSPB. In this centre power is coming from both sec-1 and

sec-2 through transformer. Power goes to ETP MCC,fire water MCC,

DM plant MCC, hydrogen plant MCC

i. ETP MCC (effluent treatment plant MCC): it is a 400 V, 400 A board. In this board power

is coming from both part of raw water switchgear. In this centre, fuse is used as a bus coupler

and also as a protection purpose of this board of lower current.

ii. FIRE WATER MCC: It is a 400V, 400 A board. In this board power is coming both part of

raw water switchgear. In this centre, fuse is used as a bus coupler and also as a protection

device because of low power.

iii. DM PLANT MCC: It is a 400 V, 630 A board. In this board power is coming from both part

of raw water switchgear. In this centre, fuse is used as a bus coupler and also as protection

purpose of thus board, because of lower power.

iv. HYDROGEN PLANT MCC: It is a 400 V, 250A board. In this board power is coming from

both part of raw water switchgear. In this centre, fuse is used as a bus coupler and also as

protection purpose of this board, because of lower current.

3. ADMINISTRATIVE BUILDING PMCC: It is a 1600KVA, 2500A,6.6/.433 KV board. Power

comes in this board through step down transformer, which steps down the voltage 6.6 KV to 415 V.

From transformer power is fed to this board through NSPB. In this centre power is coming from both

sec-1 and sec-2 through transformer. From this centre power goes to training hostel DB (Distribution

Board)(415 V ,125 A) and security hostel DB (415V,125 A)

33

4. STNSER. SWGR (Station Service Switchgear): It is a 2000 KVA, 3000A,6.6/.433 KV

switchgear. Power comes in this board through step down transformer, which steps down the

voltage6.6 KV/415V.From transformer power is fed to this board through NSPB. In this centre power

is coming from both sec-1 and sec-2 through transformer. From this centre power goes to AC MCC

(415 V, 630 A), ventilation MCC-1(415 V, 630 A) and ventilation MCC-2 (415 V, 630 A),

Miscellaneous MCC-1(415 V, 400 A) and Miscellaneous MCC -2(415 V, 400 A).

4.7.23 STG (steam turbine generator): It is a three phase generator. The star side of this

generator is grounded by NGT and NGR because, by the NGT we can step down the negative voltage

level to small value which has to be grounded and NGR is used because to resist the current in the

neutral wire which has to be grounded. From the delta side of the STG , 15.75KV is produced, which

is stepped up to 400KV by STGT (steam turbine generator transformer). The generation power of

steam turbine generator (STG) is 131 MW. There is no UAT and cranking switch in the STG. The

STGT and the bus-bar system of STG is same as GTG.

4.7.24 STGT (steam turbine generator transformer): It is 150 MVA transformer and it

steps up the voltage fro 15.75 KV to 400 KV, which is then fed to the switchyard .The vector group

of it is YNd1, i.e.,-30 degrees phase displacement. It is a delta star transformer. The primary side is

delta and secondary side is star. The star side is solidly grounded because in this side voltage is high.

Since, when the voltage is high there should be solid ground.

4.7.25 SYNCHRONIZATION

Synchronization is needed when load sharing is needed, i.e. when there are two sources of power is

present, and then synchronization is needed. Synchronization means that two source of power can

share the same load if they fulfilled the following conditions:

1) Phase sequence of the voltage of power plant should be same as that of central grid.

2) Frequency of both the source should be same.

3) The generated voltage of power plant should be equal to the voltage of central grid.

In a synchronizer, if there is a rotation in clockwise, it means that this power plant has leading

frequency to that of central grid frequency.

DC systems:

The system comprises of that DC batteries and the chargers which is to be located in the power

house. Batteries:

- One set of 220 V batteries for each of the module (one GTG and one STG)

- One set of 125 V batteries for gas turbine

- One set of 48V batteries for telephone and other system required

- One set of 220 V batteries for station loads

- One set of battery for switchyard loads

34

- One set of 48V batteries for PLCC

- One set of 24/48 V DC for River water pump house

The battery will be stationary lead acid type (vented type) capable of high discharge performance

sized to meet the load duty cycle requirements for a minimum period of one hour. The battery is to

be rated for 100% loads. The battery is to be designed for maximum durability during all service

conditions including high rate of discharge and rapid fluctuation of load. The battery will be

permanently connected to the load in parallel with a charger and supplies the load during emergency

conditions when AC supply will be lost. DC system is tooperated as underground system.

Fig 23: Battery Bank

4.7.26 Chargers:

- Two nos. of float cum boost chargers for battery

- One no. of discharge resistors for each set of battery completely assembled in 2 mm thick cubicle

fitted with bi-directional wheels for discharge cycle testing of the different batteries

- Common discharge resistors with necessary tapping covering the discharge characteristics of more

than one battery

35

The float cum boost charger is to be suitable for float charging as well as boost charging the battery.

Each battery charger is to be capable of float charging the battery while supplying the station normal

DC load. The design is such that, in case the load excesses the charger capacity, the excess load

current is to be supplied by the battery. The other float cum boost charger will be normally in float

mode and will be cut in to the circuit automatically takeover the functions of other in case of its

failure. On failure of station AC failure both float cum boost chargers will go out of service and

battery will take over to supply station emergency loads automatically, in the event of power supply

fail. Interlocks are to be provided such that when boost charger is selected in boost mode it will be

disconnected from DC load. The battery charger is to be solid state using silicon rectifiers with full

wave, fully controlled bridge configuration. The battery charger will be for 450 V+-10%, 3 phase, 50

Hz, incoming supply and at least on charger per module is to be fed from emerge. The charger is to

be self protecting against all AC and DC transients and steady atate abnormal currents and voltages.

The battery charger is to be equipped with:

- Si type, rectifier circuits

- Dry type, double wound with Cu conductor rectifier transformers

- Fully rated blocking diode with redundancy, for reliability so that failure of one diode will not affect

the system

- Heavy duty, load break type incoming isolation switches operated by handle with padlocking facility

in ON and OFF position

- Air break type incoming AC contractor with thermal overload relays

- HRC and fast acting semi conducting fuses for float cum boost charger

- LED type indicating lamp

- Manual/ auto control for selection of float and boost charger

- Voltage setters for setting the output of float/boost charger independently

- Current limit setters

- Alarm acknowledgements , reset and test button

- Solid state surge protectors on both AC & DC side

4.7.27 UPS SYSTEM:

UPS catering to one each of the modules will be used (One GTG+ one HRSG and one STG constitute

a module). The UPS system will have adequate capacity to supply clean power to the instrumentation

control system for a period of 60 min. in case of main power failure. Each UPS system comprises of

2 sets of static type redundant converter-cum-chargers and inverters, each capable of supplying 100%

load, connected in parallel, two Battery banks with support time of 60 min. and an output AC power

distribution board.

36

Each UPS system consists of one by –pass regulating transformer along with surge suppression

equipment, static switches, servo controlled stabilizer, manual by-pass arrangement and other power

devices to supply power at constant output voltage in case of failure of both inverter system. In the

event of mains supply failure the floating bank is connected to the DC input of each inverter and

maintains continuity of AC power output without interruption through inverters. The changes over

from inverter to by-pass transformer are not more than 5 min. Battery charger is of automatic type

having provision for boost and float charging of battery bank.

However, provision will be made for manual charging of battery banks, if required. Inverters of UPS

system synthesizes AC wave form through pulse width modulation with at least 12 pulses/half cycle

to take care of the dynamic condition(like switching on and off to the connected electrical loads). The

output frequency is controlled by a quartz crystal oscillation.

37

CHAPTER 5

PROTECTION

There will be one set of relay panel for GTG and STG, STGT, GTGT and UAT, plant electrical control

system and control panels as required for offsite areas. All protective type relays will be of Numerical

type suitable for flushing mounting. Typical metering and protection for generator and unit auxiliary

transformers will be enclosed. For generator protection redundant numerical relays will be used to form

two separate groups. Further transformer (GTG, UAT and ST) have main protection and back up

protection in separate relays. Necessary auxiliary relays with flag indication will be provided for fault in

transformer for Buchholz/surge protection, winding temperatures/ oil temperature/ oil level/pressure

relief. All these relays have hand reset flags or other means for visual indication of their operation.

Supervision relays will also be provided to monitor each trip relay/DC supply.

A. GENERATOR PROTECTION (GTG & STG):

1. UNDER VOLTAGE TRIP: A 2 stage under voltage protection element, configurable as either

phase to phase or phase to neutral measuring is provided to back up the automatic voltage regulator.

2. OVER VOLTAGE TRIP: A 2 stage overvoltage protection element, configurable as either phase

to phase or phase to neutral measuring is provided to back up the automatic voltage regulator.

3. GENERATOR DEFINITE TIME OVERLOAD TRIP

4. UNDER FREQUENCY TRIP

Turbine abnormal frequency protection is provided to protect the turbine blade from potential damage

due to prolonged under/over frequency operation of the generator. Up to six frequency bands can be

programmed, each having an integrating timer to record the time spend within the band.

5. GENERATOR OVER FREQUENCY TRIP: see no. 4

6. GENERATOR NEGATIVE SEQUENCE TRIP: Negative phase sequence thermal overload

protection is provided to protect against unbalanced loading which can cause overheating in the

rotor. Both the alarm and the trip stages are provided.

7. GENERATOR OVER FLUXING TRIP:

A five stages over fluxing (V/Hz) element is provided to protect the generator ,orconnected

transformer against over protection.

8. GENERATOR OUT OF STEP POLE SLIPPING TRIP:

A lens shaped impedance characteristic is used to detect loss of synchronization (pole slipping)

between the generation and the power system. Two zones are created by a reactance line which is

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used to distinguish whether the impedance centre of the pole slip is located in the power system or in

the generator. Separate counters are used to count pole slips in the 2 zones.

9. GENERATOR BACKUP IMPEDANCE TRIP:

10. GENERATOR FIELD FAILURE PROTECTION TRIP

A two stage offset mho definite time impedance element is provided to detect failure of the machine

excitation. A power factor alarm element is available to offer more sensitive protection.

11. 95% GENERATOR STATOR EARTH FAULT TRIP

Residual overvoltage protection is available for stator earth fault protection where there is an isolated

or high impedance earth. The residual voltage can be measured from a broken delta VT ,from the

secondary winding of a distribution transformer earth at the generator neutral or can be calculated

from the three phase to neutral voltage measurements.

12. GENERATOR DEAD MACHINE PROTECTION TRIP:

A voltage supervised over current scheme is provided for dead machine/generator unintentional

energisation at standstill. Protection is to detect if the machine circuit breaker is closed accidentally

when the machine is not running.

13.100% GENERATOR STATOR EARTH FAULT TRIP

A 3rd harmonic voltage element is provided to detect earth fault close to the generator star point, this

element combine with the standard stator earth fault protection provides 100% stator earth fault

protection. 100% stator earth fault protection can also be provided by a low frequency injection

method. There are 2 stages of definite time under resistance protection and 1 stage of definite time

over current protection. An external 20 Hz generator and band pass filter is required with this function.

14. GENERATOR ROTOR EARTH FAULT TRIP

Rotor earth fault protection can be provided by low frequency injection method. There are 2 stages

of definite time under resistance protection. An external injection, coupling and measurement unit is

required with this function.

15. GENERATOR REVERSE POWER TRIP

Two definite time stages of power protection and each stage can be independently configured to

operate as reverse power(RP),overpower(OP) or low forward power (LFP) protection. The direction

of the power measured by the protection can be reversed by selecting the operand mode, generating

/motoring. The relays provide a standard 3 phase power protection element and also a single phase

power protection element which can be used with a dedicated metering class CT using the sensitive

current input.

16. GENERATOR LOW FORWARD POWER TRIP WITH AND WITHOUT TURBINE TRIP

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17. GENERATOR DIFFERENTIAL RELAY TRIP

Phase segregated generator differential protection is provide for high speed discriminative protection

for all fault types.The protection can be selected as biased or high impedance or interturn

18. OVERALL DIFFERENTIAL TRIP: See no.17

19. GENERATOR TRANSFORMER BACKUP OVERCURRENT EARTH FAULT TRIP

Four definite time stages of negative phase sequence over current protection are provided for remote

backup protection for both phase to earth and phase to phase to phase faults. Each stage can be

selected to be either non-directional, directional forward or directional reverse.

20. GENERATOR TRANSFORMER BACKUP OVERCURRENT TRIP

A voltage dependent over-current (controlled or restrained)or under impedance protection is provide

for back up protection of phase faults.

21. GENERATOR TRANSFORMER RESTRICTED EARTH FAULT TRIP

Restricted earth fault is configurable as high impedance or a biased low impedance element. This can

be used to provide high speed earth fault protection] and is mainly applicable to small machines where

differential protection is not possible or for transformer applications.

22. GENERATORTRANSFORMER LV/HV WTI TRIP,OTI TRIP,SUDDEN PRESSURE

RELAY TRIP, BUCCHOLZ TRIP: 10 RTDs are provided to monitor the temperature accurately in

the windings of the machine. Each RTD has an instantaneous alarm and definite time trip stage.

23. UAT DIFFERENTIAL TRIP

24. UAT EARTH FAULT PROTECTION TRIP

25. UAT HV OVERCURRENT TRIP

26. UAT LV REF TRIP: See no.22

27. UAT-1 LV/HV WTI TRIP, OTI TRIP, SUDDEN PRESSURE RELAY TRIP,BUCCHOLZ

TRIP

28. GAS TURBINE TRIP TO GTG-1

29. MAIN AND MIN CB LBB TRIP, BUS BAR 1 AND TRIP FROM SWITCHYARD

B. STG PROTECTION: -

1. DRUM LEVEL HIGH TRIP

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2. GAS TURBINE TRIP TO STEAM TURBINE

3. EPB TRIP

4. FIRE PROTECTION CHANNEL A& B

5. TURBINE & GENERATOR SHAFT VIBRATION HIGH TRIP

6. CONTROL OF EMERGENCY SUPPLY OFF TRIP

7. GENERATOR TRIP (GRP) TO TURBINE

8. TURBINE & BEARING TEMPERATURE HIGH TRIP

9. LUBE OIL PRESSURE LOW TRIP

10. LIVE STREAM TEMPERATURE LOW TRIP & HIGH TRIP

11. HP EXHAUST TEMP. HIGH TRIP

12. LP EXHAUST TEMP. HIGH TRIP

13. IP EXHAUST TEMP. HIGH TRIP

14. IP INLET STREAM HIGH TRIP & LOW TRIP

15. CONDESSER PRESSURE HGH TRIP

16. AXIAL DISPLACEMENT HIGH TRIP

17. LUBE OIL LEVEL LOW TRIP

18. DP ACROSS HP TURBINE LOW TRIP

C. TRANSFORMER PROTECTION (generator transformer /station transformer

/interconnecting transformer feeder protection):

Generator transformer protection has also given in the above .Triple pole, high speed, biased

differential numerical protection relay are to be used. This relay also offer restricted earth fault

protection. A triple pole high set instantaneous over current relay is to be provided. Single pole IDMT

relay is used for over fluxing protection with an adjustable setting to match the V/F characteristics of

transformer. A multifunction numerical 3 phase earth fault over current relay, circuit breaker fail

protection, output latching relay, selective relay scheme logic, event records, disturbance records and

communication facilities is to be used for back up protection of transformer feeders. Main and backup

protection is offered in separate relays.

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D. SWITCHYARD PROTECTION SYSTEM:

The switchyard protection system comprises of all the protective relays, auxiliary relays, timers etc.

Each feeder will e housed in simplex type of relay panel meant for each switchyard panels. All

auxiliary relays, timers, supervision and monitoring relays, auxiliary voltage transformer and

auxiliary current transformer will be connected to DC power supply.

E. BUS BAR PROTECTION:

Bus bar protection is of triple pole, high speed, and high impedance bus differential comprehensive

protection scheme. Operating time of relay is less than 15ms. The scheme incorporates all required

auxiliary relays and lockout tripping relays with adequate number of contacts for each zone .The

scheme is to be designed to have a built in cheque feature. Necessary CT switching of the zone

switching logic/circuitry is in built with the relay. Lock out relays one per feeder for tripping the

feeder on operation of bus bar protection and breaker fail protection is to be provided.

F. VARIOUS FEEDER PROTECTIONS:

LINE FEEDER:

High speed, multi function numerical, non switch distance protection is used to protect

100% of the line and with switch on to fault tripping, loss of load protection, current reversal

protection and power blocking. Multifunction numerical feeder management relays providing

integrated feeder protection and auto reclose having directional phase over current and earth fault

protection, under frequency and over frequency protection for back up protection are to be used.

Breaker fail protection to tri all the EHV breakers connected to the associated bus bar section in case

of failure of the line to trip on main and back up protection are to be provided . The relay will also

initiate the transfer trip over the communication channel of the respective transmission line.

TRANSFORMER FEEDERS:

• Short circuit protection on all of the three phases

• Over current protection on all of the three phases

• Instantaneous earth fault protection

• Hand reset lockout relays

• Standby IDMT earth fault protection

INCOMER FROM TRANSFORMERS AND TIE FEEDERS:

• Backup over current protection on all of the three phases

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• Backup earth fault protection connected to the neutral CT

• Earth fault protection Hand reset lockout relay

MOTOR FEEDER:

• Comprehensive digital motor protection

• Time delay under voltage trip

• Thermal overload protection

• Short circuit protection

• Locked rotor protection

• Unbalance load/single phasing

• Earth fault

• Loss of load

• Differential protection of motor rated 1250 KW & above

BREAKER OPERATION FEEDER:

• Short circuit protection

• Negative phase sequence over current

• Thermal overload

• Instantaneous earth fault

• IDMT over current protection

• IDMT earth fault protection

• Restricted earth fault protection (only for transformer incomers)

Apart from protection relays, each electrically operated breaker is provided with anti-pumping, trip

annunciation, lock –out relay and trip circuit supervision relay.

CONTROL PANELS:

Control panels will be provided in control room for auxiliary power supply system of 6.6 KV AND 415 V

systems up to PCC/MCC. The panels also include control switches with indicating lamps for incomer, bus

coupler of 6.6 KV switchgear and 415 V PMCC. For remote areas conventional control panels will be

provided. Indicating lamps are of filament type with series resistor or LED type. Following will be

monitored through DCS on CRT:

• Status (ON,OFF and TRIP) of all motors operated through CRT

• Status (ON, OFF and TRIP) of UPS and DC system

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• Operation of all protection relays Generator, generator transformer and UAT

Operation of transformer protection mounted on transformers for all HT and LT

transformers.

• Excitation system

• Trip circuit healthy for all breaker operated feeder

For DCS controlled feeders, ready to start feedback will be made available from switchgear/MCC. Main

controlled room will accommodate electrical auxiliary control panel, GTGT, STGT protection panels,

RTCC panels and UAT protection panels, generator control and protection panels, generator AVR panels

and UPS.

44

CHAPTER 6

DISCUSSION

In case of power generation, OTPC is the combination of steam, gas and combined cycle plant. I went to ONGC

Tripura Power Company ltd. for my internship program. I visited steam power plant, I observed how water is

collected, purified and then boiled to produce steam.

There are several switch gear and control rooms to control the overall system of producing steam and power

generation. Various types of relays used for protective purposes that are also controlled in control room.

I visited gas turbine of OTPC. There I have seen how fresh air and natural gas supplied by ONGC are used as fuel

to burn. After burning, produced hot gas used to rotate the turbine as well as power generation.

For protective measures relays are also used and controlled in switch gear room. After gas turbine, I visited

combined cycle power plant (CCPP). Here the exhausted hot gas is being used to boil water for producing steam.

I visited the distribution section of OTPC. In sub-station, stepped up or down of voltages is being done using

transformers and power is distributed.

Different types of isolators are being used for maintenance purpose of transmission lines.

The authorities in OTPC were very concerned about all kinds of safety. The friendly environment in OTPC

encouraged me to co-operate with each other. I learned a lot and obtained practical knowledge from my internship

at OTPC, which will help me in my future life.

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CHAPTER 7

CONCLUSION

This information of sheet summaries the process of the CCGT Power Generation & Transmission of

ONGC Tripura Company Ltd.

This project is a natural gas based power plant that utilizes the natural gas resources available in

surplus in the state of Tripura. The power plant is a Combined Cycle Gas Turbine (CCGT) Power

Plant with gross capacity 726.6 MW. It is based on ONGC’s gas supply and is implemented through

a Single Purpose Vehicle (SPV) called ONGC-Tripura Power Company Ltd. (OTPC) incorporated

on 27/09/2004. OTPC is promoted by GoT (Govt. Of Tripura), Infrastructure Leasing & Financial

Services Ltd. (IL&FS) and Oil and Natural Gas Corporation (ONGC).

The project consists of two units of capacity 363.3 MW Combined Cycle Gas Baised Turbine

(CCGT). OTPC sales its generated power to the North Eastern States viz. Assam, Tripura, Meghalaya,

Manipur, Arunachal Pradesh, Nagaland, Mizoram that are connected through NEWNE grid and

13.5% to the any entity in the country via regional grid.

The project would supply clean power to the national transmission grid wherein the present grid mix

is relatively skewed towards power generation from coal-based plants. Consequently, implementation

of the project would lead to a reduction in Carbon-dioxide emissions by feeding clean power into the

grid.

During my tenure of training, I have learnt to integrate my effort in alliance with the need

of the organisation, thereby increasing the productivity of the organisation and consequently

“Make Tomorrow Brighter”.