thdc summer training

7/21/2019 THDC SUMMER TRAINING

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Abhilash Rwaat

8/2/201

SUMMER TRAINING REPORT | ABHILASH RAWAT

THDCINDIALTD. Tehri Hydro Power Plant


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Tehri Hydro Power Plant 2014

TABLE OF CONTENT

SERIAL

NUMBER

TOPICS PAGE

NUMBER

1 Submission Details 2

2 Acknowledgement 3

3 Introduction 4

4 THDC India Limited Projects 5

5 Salient Features Of Tehri Dam 6

6 Location Of Equipments (Elevation Wise) 7

7 Hydro Generator

A. Rating

B. Layout &Structural Detail

C. Stator

D. Rotor

E. Upper &Lower Bracket

F. Bearing & Breaking System

G. Cooling System

H. Fire Extinguishing System

12

8 Generator Transformer

A. Rating

B. General Construction Features

C. Transformer Cooling System

D. Generator Transformer Protection

16

9 Conclusion 20


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SUBMISSION DETAILS

Name of the Organization Tehri Hydro Development Corporation Ltd.

Location of Powerhouse Tehri-Garhwal, Uttarakhand, India

Mentor Shri Arun Kumar,Senior Manager (Operation)

Duration June 23st, 2014 to August 2th, 2014

Title Of Report Summer Training Report Tehri HPP

Name Abhilash Rawat

College THDC Institute Of Hydropower & Technology

University Uttarakhand Technical University

Discipline Electrical Engineering

Roll.no 110970105001


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Acknowledgemen

I am very grateful to Shri Sajeev R, AGM(Operation) Tehri HEP, THDC

India Limited for guiding and providing me information about thevarious equipments at Tehri Hydro Power Plant.

I would like to acknowledge my deep gratitude to Shri Arun Kumar, Senior

Manager(Operation)Tehri HEP for his guidance which helped me to

understand the role of a Electrical engineer as well as for explaining

the functions of various equipments required for the smooth operation

of Hydro Power Plant

I would like to thank HR Department, Rishikesh, THDC India Limited for

giving me opportunity of Summer Training. I am also indebted to

acknowledge the Executives of Tehri Power House who gladly shared

their vast knowledge about generation of electricity.

I would like to thank my Institution, THDC Institute of Hydropower

Engineering & Technology, Tehri, Uttarakhand and the Faculty members

for providing opportunity and valuable guidance rendered during summertraining.

I also extend my heartfelt thanks to my family and well wishers.

Abhilash Rawat

THDC Institute Of Hydropower & Technology


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INTRODUCTION

The Tehri Dam is a multi-purpose rock and earth-fill embankment dam at the confluence of Bhagirath

and Bhilangna River at Tehri in Uttarakhand, India. Tehri Hydro Power Projects comprises of thre

phases. Phase 1: Tehri HEP (4X250MW) was completed in 2006, Phase 2: Koteshwar HEP

(4X100MW) was completed in 2012. The Tehri Dam withholds a reservoir for irrigation, municipa

water supply and the generation of 1,400 MW of hydroelectricity.

Third phase: (4X250MW) Pumped storage Plant of hydroelectricity generation is under

construction. This dam is worlds 3rd

largest rock fill dam. The complex will afford irrigation to an

area of 270,000 hectares (670,000acres), irrigation stabilization to an area of 600,000 hectares

(1,500,000acres), and supply of 270 million gallons of water per day to the industrialize areas of

Delhi, Uttar Pradesh and Uttarakhand.

4x250 MW Tehri HPP stage -1 comprises an underground powerhouse of size 196x20x61.7m and

is designed to accommodate four no. 250MW generating units coupled to vertical Francis turbines

working under a net head of 188 m along with its mechanical and electrical auxiliaries. A separate

cavern of size 161x17x28.9 m has been designed to accommodate eight numbers of 15.75/420kv

, three phase Generator Transformer at EL 605 m out of which four numbers for Tehri HPP stage-1

has been installed and four no. of Generator Transformer for future PUMP STORAGE PLANT

(PSP). 7 Bays of 420kV Gas Insulated Switchgears and associated equipment have been installed

at EL 618m. Provisions for future extension for PSP has been made for 04 numbers incoming bays

and 01 no. outgoing bays besides a provision for one starting bus to be added in future for back to

back starting of PSP , if required. Two circuits of 420kv Gas Insulated Bus Ducts (GIB) have been

extended to the Port yard from where two nos. 400kv overhead lines of length 182 & 184 km each

are taking off to Meerut Sub Station(Power Grid) for evacuation of the power.


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THDCIL Project At A Glance

Sl No. Projects/State Installed

Capacity(MW)

Remarks River/Basin

PROJECT UNDER OPERATION

i. Tehri Dam & HPP

Uttarakhand

1000 Commissioned in

2006-07

Bhagirathi

ii. Koteshwar HEP

Uttarakhand

400 Commissioned in

2011-12

Bhagirathi

Total 1400

PROJECT UNDER CONSTRUCTION

i.

ii.

Tehri PSPUttarakhand

Vishnugad

Pipalkoti HEP,

Uttarakhand

1000

444

First pump storage schein central sector

ROR scheme on

Alaknanda River

Bhagirathi

Alaknanda

Total 1444


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Salient Features

a) Geographical Features:

1. LOCATION

State : Uttarakhand

District : Tehri Garhwal

Dam : At the confluence of river Bhagirathi and Bhilangana

Power House : Bhagirathipuram

River : Bhagirathi

2. DAM

Type : Earth & Rock filled

Top Level : EL-839.5 meter

Height : 260.5meter

Width at river bed : 1125meter

Length at TOP : 592.25 meter

Width at top : 25.5 30.5 meter

3. RESERVOIR

Max. water level : El-835.5 meter

Normal reservoir level : EL-830 meter,

Min. draw down level : EL-740 meter

Gross storage : 3540 MCM

Dead storage : 925MCM

Water spread at FRL : 42 Sq.km

Water spread at MDDL : 18 Sq.km

4. HYDROLOGY

Catchment area : 7511 Sq. Km

Max. observed discharge : 3800Cumecs


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Location of Major Equipmen

Elevation wise

E.L. means Elevation with reference to sea level (E.L. 0 meters)

Power House:

E.L.- 578 m:

Waste Water Clarification Unit- Waste water disposal system. Sludge dewatering Tank, Lime

dosing pump and tank, Alum dosing tank and pump, polyelectrolyte, Agitator 1,2,3,4, Air

Compressor, Sludge dewatering pump-1,2, Filter feed pump-1,2, Backwash Pump-1,2.

E.L.- 584 m:

Cooling Unit- Filtration system, Ejector, Generator and Turbine cooling water system & cubicle

(ejector and filter control) for all 4 units, Generator Transformer cooling system, Generator rotor

jacking, Brake dust collecting fan, turbine head cover drain, turbine leakage unit, oil cooling pump

Draft tube manhole, Drainage Leakage valve control cabinet, Hydraulic Service Seal Valve, Main

Inlet Valve, MIV Leakage Unit, Filtration Unit.

Drainage Pump Station-I(DPS-I)-: Pump number 1 and 2

E.L.-591.6 m:

Dewatering Pump Station- Pump number 1, 2 and 3

Runner Seal, Shaft Seal water connection point, Spiral casing for each unit, Turbine Guide Bearing

cooling system.

E.L.-595.5 m:

Generator Lower Bracket Barrel: Stator air cooler, Turbine oil cooler, Emergency closing valve,

Rotor braking system and rotor lifting unit, Generator Thrust bearing.

Oil Pressure Unit-(MIV) Air oil vessel, Air vessel, two induction motors attached with pumpimmersed underneath sump oil section to maintain oil and air pressure for vessels.

Turbine Generator coupling chamber: Lower Guide bearing, servo motors for opening and

closing of wicket gates


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E.L.-600 m

Excitation floor:, RTU excitation, Independent Excitation Transformer, Static Excitation System, Se

Excitation Transformer, Unit Auxiliary Transformer, Unit Auxiliary switchgear, Voltage Transforme

Cabinet, Transformer cooling water system.

17.5 KV Isolated Phase Bus duct: LAVT cubicle, Dynamic Braking Cubicle, Neutral Groundin

Transformer

Oil Pressure Unit-(Turbine): Air oil vessel, Air vessel, two induction motors attached with pump

immersed underneath sump oil section to maintain oil and air pressure for vessels.

Mechanical Turbine Governor:actuator cubicle (hydro-mechanical cabinet).

Compressor Room: Two air compressors pump number 1 & 2, works in two stages.

E.L.-605 m:

Machine Hall: Unit Control Board (UCB), Generators Turbine Unit upper cabinet with name plate

details, MIV vault opening for all 4 units, Dewatering Pump station service switchgear and

Transformer, Overhead Travelling crane ( E.L.-626.10m)

Transformer Hall: Power Transfer Vault total 8 in numbers, 4 for Generator Transformer of Tehri

HEP, 4 for Generator Transformer of Tehri PSP, 4 numbers of Generator Power Transformer of all

units of HEP, Generator Transformer Cooler control cabinet.

Station Service Transformer tapped to unit # 4.

E.L.-618 m:

GAS INSULATED SWITCHGEAR (GIS): Transformer bushing, PDM (Partial Discharge

Maintenance), Circuit Breakers, Isolators, Earthing Switches, Gas Insulated Bus duct (GIB) of 840m

tunnel till IFB building, 4 Bus Bars-BB11,BB12,BB21,BB22; Bus coupler.

E.L.-706m:

Butterfly Valve Chamber:The Butterfly Valve Chamber houses 4 nos. butterfly valves of 5000mm

size. Penstock Assembly Chamber is an underground cavern parallel to BVC, Fire Fighting System,Drainage Pump Station no.3 (DPS-III).

Station Control Area (Control Building):It is an underground multi-storey building having lowest floor

at 600.0m and top floor at El. 622.0m.


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E.L.-605 m:Mechanical Maintenance Department, Hydro Mechanical Department

E.L.-608 m:Station Relay Room, CCR( UPS room), Computerised Control System room ( CCS

Monitoring System, Control Panels and Computers, Main Mosaic Mimic Board.

E.L.-611 m:Video Conference Hall, Conference Hall, Visitors Room.

E.L.-615 m:Air Conditioning and Air Ventilation Room, Electrical Maintenance Group Room, Cabl

Room.

E.L.-618 m:Station Control Area Service Switchgear(Auxiliary supply L.T. room), 11KV switchgear

panel- main bus bar, Battery Bank Ist and IInd bus bars DC, SCA Transformers, L.T. Panel (415V),

Machine Testing Laboratory and Library.

Power House:

E.L.- 584 m:

Cooling Unit- Cooling water system is provided in the power house for cooling of generating units

and generator transformers. It is provided fully independent open circuit cooling system for each

unit, and double-loop cooling water system for each transformer. Cooling water for users is taken

from the penstock and the draft tube. Water for ejector operation is fed from the penstocks, while the

drawn-in water is taken from the draft tubes and passes through the deaerators to get the water free

of air bubbles accumulated there during passing the runner chambers.

Main Inlet Spherical Valve (MIV): The spherical valve is of the horizontal shaft type and comprises:

split body, split rotor, rotor trunnions, service and maintenance seals, bearings, lock and othercomponents and parts. The spherical valve gets closed both when there is no flow in the penstock

(i.e. when the wicket gates are closed) and under flow conditions by the counterweights attached to

the levers. The spherical valve is opened by two swinging servomotors operated by the valve oil

pressure system. The spherical valve is to be opened after filling up the spiral case with water and

pressure differential across the valve maximum 0.3 MPa.

Draft Tube Manhole:

Hydro Turbine: Four numbers of Vertical Francis Turbine are installed in power house. Each

machine has a rated capacity of 255 MW and can be operated in the head range of 122.6-230.1 m.

The unique feature of the design of Francis turbine is that single runner is capable of operation

under large head variation of over 100.0m.The direction of rotation is anti-clock wise when viewed

from the top.


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E.L.-591.6 m:

Dewatering Pump Station: Dewatering system is provided in the power house for dewatering o

penstock, spiral case and draft tube between penstock / main inlet valve and draft tube gate fo

access to turbines and MIVs for inspection and maintenance. In emergency the pumps of the system

can be used for dewatering the flooded areas of the power house.

E.L.-595.5 m:

Oil Pressure Unit-(MIV): Oil pressure system for servomotors of each valve consists of oil pumping

unit equipped with two 11.2 ltr/sec, 55 KW oil pumps, sump tank, control cubicle and other

necessary equipment and auxiliaries. For the opening of the valve is provided with oil pressure of 60

kgf/square cm. This oil pressure is supplied by oil pumps. The pressure oil system consists of oil

sump, oil pumping unit, valves, fittings, instrumentation and automatic operation. Two oil pumping

set are mounted on oil sump, one acting as main and other as standby. The pump shall be of

variable displacement type suitable for auto start/stop operation with the help of pressure switches.

E.L.-600 m:

Excitation System: Each generator is equipped with two modern static excitation systemsknown as parallel/self excitation system and independent excitation system. Parallel/self excitation

system is meant for excitation of the generator in generation mode as well as in synchronous

condenser mode.

The excitation system is intended for providing the following functions:

Start up, field flashing and switching to the system by precise synchronizing.

Operation of the generator at loads varying from no load to maximum load for the generator.

Operation in the synchronous condenser mode both with inductive and capacitive load.

Field forcing at the set voltage response and de excitation at disturbances in the power grid

causing voltage rise or drop in the system.

Rotor field suppression by the field circuit breaker at protection operation with simultaneous

inversion of the rectifying unit.

Operation on joint control maintaining even distribution of the reactive load betweenmachines.

Limiting the ratio of ceiling field current to nominal field current by two (2) per unit without time

delay as well limiting of over load by the time-inverse characteristics.

Limiting minimum field current with set point depending on generator active power in the

mode of VAR load demand from the grid.

Protection of the generator for loss of excitation/asynchronous run.


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Governor: A governor is incorporated in the turbine generator to ensure constant speed. It sensechange in speed & then controls wicket gate opening automatically through servomotors so as t

oppose changes in turbine speed. Turbine governors are equipment for the control and adjustment o

turbine power output and grid load as fast as possible.

The governor consists of the following units:

1. Electronic Unit

2. Hydro mechanical unit.

Governor for each unit consists of a digital programmable electronic part for control and

regulating function and a hydraulic part act as a power amplifying servo unit. The governor

performs the following tasks for the regulating and control of the generating unit:

Speed Frequency control

Power Control

Governing during synchronous condenser mode operation

The electronic governor has provision for both automatic operation as well as manual control

through the hydraulic actuator. The governor comes into action when the turbine start signal is

given from the unit control board/control room. It then regulates the speed of the generating unit

in operation. The governor is of proportional differential (PD) action type. The governor has high

sensitivity, quick response to speed/load changes.

Hydraulic actuator unit of governor consists of oil pumping unit, oil pressure accumulator, oil

leakage unit, amplifier and solenoid operated distributing valve etc. the hydraulic actuator unit has

following features:

Two numbers remotely controlled (from control room) shut down devices for normal and

emergency closure of wicket gates.For emergency closure both solenoid operated valvesare fed from two independent 220 V DC sources.

Independent emergency solenoid valve which operates in emergency conditions and send

the pressurized oil directly to the wicket gate servomotors by passing the governor.

Control devices for the manual operation allowing opening and closing of wicket gates to any

position.

Wicket gate maximum opening limiting device.

Transmitter for wicket gate and limiter position.

Direct indication of gate limit and position on actuator unit


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HYDRO GENERATOR

Hydro generator (CB 870/300-28) is a vertical shaft, alternating current, 3-phase synchronou

machine of semi umbrella type with combined thrust and lower guide bearing located below the roto

and upper guide bearing above the rotor. Guide bearing are arranged in the oil bath of upper an

lower brackets and thrust bearing in oil bath of lower bracket. The generator is designed for nomina

output of 250 MW and maximum continuous output of 275 MW at 0.9 lagging power facto th

rotational speed of the machine is 214.3 rpm and runaway speed is 410 rpm.

Key parameters of hydro generator are as follows:

1- Nominal output 278/250 MVA/MW

2- Maximum continuous output 305.8/275 MVA/MW

3- Power factor 0.9 lagging

4- Frequency 50_+3% Hz5- Rated terminal voltage between phases 15.75 _+5%

6- Nominal speed(Rotational speed) 214.3 rpm

7- Runaway speed(over speed) 410.3 rpm

8- Rotational direction Counter clockwise If looked from top

9- Fly wheel effect 24000 t.m^1/2

10-Inductive reactance, unsaturated: Relative

i) Synchronous, by direct axis, Xd 1.0

ii) Transient, by direct axis, Xd 0.29

iii) Sub Transient, by direct axis, Xd 0.129

iv) Negative phase sequence 0.2

v) Zero phase sequence 0.09

11-Maximum efficiency under rated load with pf=0.9 98.23%

12-Voltage across slip ring with rated load and 120

Degree C temperature of field winding 300V

13-Field current rated load 1600A

14-Field voltage ratio under field forcing 2.5 p.u.

15- Voltage rise after 100% load rejection with pf=0.9 28%

16-Total weight of generator 1300 t

17-Weight of rotor 700 t


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General lay out and structural details:

Hydro generator has umbrella- type vertical design with two guide bearing arranged in oil baths o

upper and lower bracket and thrust bearing in oil bath of lower bracket. Hydro generator sha

consists of rotor hub, thrust bearing hub and shaft extension. Hydro generator rotor gob is directljoint to turbine shaft.

STATOR

Stator frame is form construction made of steel sheet rolled metal. It consists of horizontal shelves,

vertical rip and sheeting. Stator frame has a ring shape and consists of four sections, which are

welded at site. There are a number of apertures in stator frame for winding terminals and air coolers.

Stator core is assembled of varnished laminations, stamped of 3432-type coiled cold rolled

electrical steel with 35mm thickness. Core is arranged to stacks by height, with ventilation ducts

between them for provision of effective stator ventilation.

Stator winding is wave bar type.

Stator winding consists of 696 Rebel bars.

Main insulation is thermosetting Monolit- type, made of F-class materials with epoxy binding

agent. Fastening of winding bars is provided by slot wedges with use of flexible polyurethane gasket

between the upper bar and slot wedge.

ROTOR

Rotor of hydro generator consists of rotor hub, thrust bearing hub and shaft extension with planted

upper guide bearing hub, spider, stacked rim, twenty-eight poles. Rotor rim is assembled of

segments, stamped of steel with 4mm thickness.Pole cores are assembled of punched steel sheet

with 2 mm thickness. This sheet has insulating coating in order to decrease the losses. They are

pressed by steel pole end plates with pressing studs. Pole coils are made of copper band. Turn to

turn insulation is glass type with epoxy binding agent (class F materials).The main insulation of pole

core is glass textolite (Class F material). Poles are fastened to rotor rim with T-shape tales and

counter wedges. During installation of poles on rotor rim, the pole coil should be drawn in to pole

shoe with springs.

UPPER BRACKETS

Upper brackets supports to stator. It has central part and twelve arms. Oil bath of upper guide

bearing is located in central part of bracket. Ten oil coolers and guide bearing are built into central

part. Upper flooring of generator is disposed on top of bracket arms. It consists of support beams

and steel sheets. Felt gaskets are installed between flooring sheets and support beams in order to

decrease the noise in machine hall. Upper hood with brush rocker is installed on central part of


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bracket. There are twelve support jacks on upper bracket arms supported into foundation fo

suppression of horizontal displacement.

LOWER BRACKET

Lower bracket is radial type welded construction with central part and twelve arms, which support

to foundation. Oil bath of lower guide and thrust bearing is located in central part of bracket. Twelv

oil coolers are built in to it. Lower bracket is the support for unit shaft. Bracket is installed on it. There

are twelve support jacks on lower bracket arms for suppression of radial displacement. Sole plates o

lower bracket fasten to foundation by means of studs and anchor plates.

BEARINGS

Upper and lower guide and thrust bearings are lubricated without use of separate installed oil

circulation pumps. Upper guide bearing is located in central part of upper bracket above stator and

rotor of hydro generator. Thrust bearing has rotating disk (runner) with polished bottom surface andtwelve fixed pads on rigid supports. Friction surface of pads is coated with linings on TEFLON basis.

Thrust bearing pads are installed by means of flexible disk supports on bolts with spherical heads.

These bolts allow adjusting the height of supports and providing the equal load on each pad.

Support bolt with spherical head allow the pads to self adjust into operation position. Thrustbearing runner has the insulation against shaft current. Lower guide bearing and thrust bearing arelocated in oil bath in central part of lower bracket under rotor. Twelve oil coolers are installed in oil

bath of lower guide and thrust bearing. Cooling water circulates by U-shape tubes of these coolers.

Guide bearings consist of set of pads located around the hubs fastened on shaft.Upper and guide bearing pads have the same design. Guide bearings contain twelve pads. Friction

surfaces of bearings pads are coated with -83 babbit. Self adjusted pads have the insulation

against the shaft current. Oil baths of bearing are covered with special seal shields, which prevent

the penetration of oil vapor and splashes into generator.

BRAKING SYSTEM

Electrical and Mechanical (auxiliary) braking is used for regular braking. When hydro generator is

disconnected from grid, turbine wicket gate is closed and rotation speed is reduced down to 50% of

rated value, short-circuiting of main terminals of stator winding and current supply into rotor winding

from brake thyristor converter occurs. When rotation speed reduced down to 10% of rated value,

the mechanical breaking is automatically switched on. In case of electrical braking system failure, or

electrical damage of generator, the mechanical breaking is automatically switched on, when rotation

speed will become 10% of rated value. Twenty four pneumatic brakes provide the mechanical

breaking. They are installed under rotor rim and operated by compressed air with 0.8 MPa pressure.


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COOLING SYSTEM

Cooling system of generator active parts is of air-type with cooling of heated air in air coolers wit

water cooling. Air circulation through generator is carried out by closed cycle inside generator pit b

means of crowding action of rotor arms, ventilation ducts in rotor rim, centrifugal ventilators on ensurfaces of rim and poles. Baffles are installed below and above rotor for ordering of air motion an

decrease of ventilation losses. Heated air from stator winding end zone and ventilation ducts in stato

core is going to stator frame and cooling by passing through air coolers, which are installed on stato

frame sheeting. Then it come out to generator pit and again is drawn into rotor.

FIRE EXTINGUISHING SYSTEM

Fire extinguishing is carried out with water, which is supplied by two collectors to end parts of

winding and spraying by 450 special nozzles. Water flow through one nozzle is about 0.08 l/sec;total water flow is 34 l/sec. switching on of water supply into collectors is performed by fire

extinguishing control unit. Fire extinguishing control unit provides both manual and automaticswitching on water supply; possibility of check of automatic water supply system. It has the pressure

gauge, which indicates the presence of water pressure in fire extinguishing pipe line and electric

contact pressure gauge, which indicates the presence of water pressure in fire extinguishing

collectors.

Automatic switching on water supply is carried out by pneumatic driven valve. Pneumatic valve is

controlled by electric signal, which is supplied by fire alarm detector(four VMX 1000 F 90

temperature detectors and four IMX 1001 E type smoke detectors installed on wall of generator pit) ,

or from control board.


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306 MVA GENERATOR TRANSFORMER

Generator transformer is an oil-filled, three-phase, double-winding, delta-star group 11 connected

transformer:

Key parameters of Generator Transformer are:

1- Rating 306 MVA

2- Rated voltage of H.V. winding 420 kV

3- Rated voltage of L.V. winding 15.75 kV

4- Rated current in H.V. winding with tap changer in normal position 420.6 A

5- Rated current in L.V. winding with operating cooling system 11217.1 A

6- HV-LV impedance at nominal tap 14.5%10%

7- Type of cooling OFWF

8- Permissible oil temperature rise above ambient temperature(+30C) in normal operation 40C.

9- Permissible winding temperature rise 53C.

10-Full weight of transformer with oil 284.6 t

11-Oil weight 54.47 t,

12-Volume 62600 l

13-HV insulation level SI 1050 LI 1300- AC 38

14-LV insulation level LI 95 AC 38

15-Core and winding weight 179100 Kg

16-Flow rate of each oil pump 2530 Lpm

17-Flow rate of each oil pump 2300 Lpm

GENERAL CONSTRUCTION FEATURESThe tank cover mounts the H.V. and L.V. bushings. The 420 kV bushings HV are of special oil-SF 6construction. Flange connection consists of two parts. One is standard transformer bushing; theother is connection device from SF6 switchgear side intended for connection with switchgearequipment. Main bushings of incomers consist of resin-impregnated base with grading layer formonitoring. On the 15.75 kV side the oil-filled L.V. bushings with porcelain insulation parameters24kV/12.5 kA are installed.The conservator is mounted above the tank cover on a separate support structure. The pipeconnecting the conservator to the tank contains the Buchholtz relay. The Buchholz relay is isolated

from the conservator with an isolating gate valve and with an isolating butterfly valve from thetransformer tank.At variation of load and temperature of the transformer the oil level varies (breathes). In order toavoid ingress of moisture with air into the transformer during breathing, the silica gel dryer isinstalled. Additional protection of oil is achieved by provision of enclosure made of air-filled oil-resistant nitrite rubber. The enclosure is floating on the oil surface of the conservator following theoil level. The enclosure is connected with the dryer, therefore, the air increasing in it is always dry.Such system prevents from direct contact of transformer oil with ambient air thus decreasingpotential of oil contamination due to pollution, increase of oxygen content.


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Oil level behind the enclosure in the dryer is measured by oil gauge of magnetic type. The gaugedriving magnet turns following the float and by its magnetic field makes the driven magnet anindicator disk with contacts turn. At decrease of level to El.65 mm the indicator reaches the Emptyposition and the signal Low Level Alarm is given. The signal will be also given if the enclosure ha

been torn and fell down. Oil temperature in the transformer is measured by an alcohol thermometeinstalled on the tank cover. Brass bulb thermometers with visual readings are installed in speciapockets arranged in the hot spots of the cooling system.Pipelines from the pump sets can be closed with gate valves located below the conservator. Thpipes can be cross-connected by opening the bypass. Provision is made for a valve on the tancover, plugs on the conservator and pipe inlet from the cooler for air release from the top points. Oilfilling pipe with gate valve is connected to the conservator. The transformer windings have insulationlevels designed for operation on the system with effective neutral grounding (networks with largeground fault currents). In this connection, the windings have incomplete insulation of the neutraland the transformer must always operate with solidly grounded neutral.

Transformer cooling system

OFWF(Oil Forced Water Forced) cooling system comprises two water/oil coolers (one main and one

standby). Hot oil is taken from the upper part of the transformer and forced by the pump through

water/oil cooler for cooling and goes to the lower part of the transformer. Two oil pumps (pump

No.1, pump No.2) are mounted before the cooler on the oil inlet side The transformer cooling

system is of two loops configuration. Water from ejectors AT el-584 m passes through automatic

filters to the water-to-water coolers of the primary loop where it cools the clean water of the second

loop and thereafter it drains to down stream. Water of the secondary loop is fed from the clean water

system. Clean water is conveyed to the make up tank.

Water of the secondary loop is handled by two pumps located at El.600.00 m in bus duct galleries.

Normally one pump operates; the other pump goes into operation automatically as stand by. Water

flows in the primary and secondary loops are controlled by the flow meter with remote indication.

Water temperature of the secondary loop is monitored by the electro contact thermometer. Design

water flow in the primary loop is 250m3/hr. The rate of water flow in the secondary loop is 135m3/hr.

Check temperature of water at the heat exchanger outlet: raw water maximum 30C, clean water

maximum 35C. There are two oil coolers in the system; one of them is standby. The oil cooler

should reduce oil temperature by 10-15C and maintain temperature of the upper oil strata at 50-

55

C.


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GENERATOR TRANSFORMER PROTECTION

INTERNAL FAULT PROTECTION

The following devices protect the transformer against internal faults:

Differential current protection against all kinds of short circuits in the windings anbushing. Protection 87T is connected to the current transformers located in GIS-I and tthe current transformers of the 17.5 kV busduct. Protection 87OA is wired to the currentransformers in GIS-I and to the current transformers in the generator neutral;

Buchholtz relay protection against faults inside the transformer tank accompanied withgas evolution

Restricted ground fault protection 87N against internal faults connected to the currenttransformers on H.V. side and transformer neutral.

All protections operate to trip unit circuit breakers, circuit breaker in 11 kV switchgear and

incomer circuit breaker 415 V to actuate the C/B failure protection, to discharge the generatorfield and to stop the unit

Buchholtz relay protection is a sensitive protection against faults inside the transformer tank

which result in evolution of gas and oil surge from the tank into the conservator. Gas is evolved

during de-composition of oil and solid insulating material, due to action of the electric arc and

excessive heating and failure of the core. The sensitive elements react to accumulation of gas in

the upper part of the relay and to oil surges from the tank into the conservator during intensive

gas evolution. The protection responds also to extreme oil level drops in the conservator.

Buchholtz relay has two sensitive elements upper and lower. The contacts of the upper

sensitive element operate alarm; those of the lower element actuate switching off the transformer

stop.

EXTERNAL FAULT PROTECTION

Over current protection 51GT: The protection is activated when generator is incondenser mode and when generator is switched off, in the mode of station servicesfed from 420 kV. It is connected to the current transformer on H.V. side

Ground fault protection on the 420 kV side.

Over excitation protection (24) is connected to CT in the busduct and to phase-to-phasevoltage of the secondary winding of TV5.

Insulation monitoring on 15.75kV side (59N) wired to the open delta of TV5 actuatesfailure signal;

Winding high temperature protection (26X) comprises temperature detectors installed in thetransformer tank.


20/20


CONCLUSION

The vocational training at Tehri HEP(4X250 MW) helped me in improving my practical knowledge an

awareness of Synchronous Generator & Transformer to a large extent. Here I came to know abou

the technology used in power generation. Besides this, I observed and experienced the machiner

and equipments used in the power generation. I also learnt about how the electrical equipment are

being operated and how they tackle and various problems under different circumstances. I could sa

that the summer training at Tehri HEP(4X250MW) is of great experience for me and it really helpe

me in developing my knowledge about generator & transformer and other equipment used in power

generation.

thdc summer training

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