Narora Atomic Power Plant Vocational Training Report

ACKNOWLEDGEMENT

As I continue with the report it would be only fair to acknowledge the support and guidance I received throughout the training period at Narora Atomic Power Station which led to the successful completion of my training. I am indebted to Mr. Anurag Varshney, SO/O Finance Department, Operations for because of his guidance I got this golden opportunity to carry out the field work at this esteemed Power Station. Thanks are due to Mr. D.S. Chaudhary, Station Director and Mr. G.D. Sharma, Training Superintendent for allowing me the opportunity to work in the organization. I would like to extend my heartfelt gratitude towards Mr. Khagesh

Chandra Rakesh, Mr. Rahul Pandey and other officers who were

instrumental in providing this training facility. I would also acknowledge

the inspiration, guidance and motivation I received during these 4

weeks. I would be unfair if I do not acknowledge the resourcefulness of the

Electrical Department and the guides Mr. S.K. Katiyar, Mr. D. Prasad,

Mr. Mukesh Yadav, Mr. R.K. Sharma and Mr. Harish Sharma, for it was

because of their untiring and dedicated efforts that I was able to gain

much insights into the functioning of Power Industry and moreover my

knowledge about the subject was improved. Last but not the least I would thank my parents and also my friends at

NAPS for being a constant source of support.

Index

S.No. Description Page No. 1 Introduction 1

2 Layout of NAPS 2

3 Some Important Data about NAPS 3

4 Principle of Nuclear Reactor 4

a) Turbine Generator Cycle 5

b) Reactor Cycle 5

c) Moderator System 6

d) Reactor Fuel 6

5 Shutdown Systems 7

6 Electrical Systems 8

a) Turbo Generator 8

b) Principle Components 9

7 Cooling of Generator Set 10

8 Generator Excitation System 11

a) Static Excitation System 11

b) Components of Static Excitation System 12

9 Generator Transformer 14

10 Station Unit Transformer 15

11 Power Supply Classifications at NAPS 16

12 Control Power Supply System 18

13 Diesel Generator Set 20

14 Power Motor Generator Set 21

15 Switchyard 23

a) Switchyard Equipments 24

16 Motor Control Centre 28

17 Electrical Protection Systems 29

a) Circuit Breakers 29

b) Protective Relay 31

1. Types of Relay 32

18 Trainee’s Training Experience at NAPS 34

19 Conclusions 36

~ 1 ~

Introduction

The Narora Atomic Power Station is situated at the banks of river Ganga in

Bulandshahr district of Uttar Pradesh. It is 60 KMs away from Aligarh which is

the closest substantial population centre.

The Narora Atomic Power Station (NAPS) is a twin unit module of 220MWe

each of pressurized heavy water reactors. The reactors use natural uranium

available in India as fuel & heavy water produced in the country as moderator

& coolant. The station is connected to high voltage network through five 220

kV lines, one to Moradabad, one to Harduaganj, one to Simbhaoli, one to

Khurja and one to Atrauli. It is designed for base load operation as a

commercial station.

The foundation stone of the plant was laid on 4th January 1974 and it was

dedicated to the nation as Nuclear Power Plant in the early 90s, ever since it

has stood as an epitome of safe and secure Power Source.

With the synchronization of the Narora Atomic Power Station with northern

grid through five lines of 220kV, it has occupied an important place on the

power map of the India. With this, yet another important milestone in the

Indian nuclear program has been achieved, as NAPS is an effort towards

standardization of PHWR units & a stepping-stone to the 500MWe units. A

significant & unique feature of this project has been the evolution of the

design suitable for seismic sites.

Narora Atomic Power Plant is the fourth Atomic Power Plant installed in India

preceded by Tarapur in Maharashtra, Rawatbhata in Rajasthan and Kalpakkam

in Tamil Nadu. One peculiarity about NAPS is that it is the first indigenous

Power Plant of the country.

~ 2 ~

Layout of NAPS

NAPS have the following main parts:-

1. Administration Building

2. Overhead Water Tank

3. Canteen

4. Natural Draft Cooling Towers (NDCT).

5. 220KV Switch Yard

6. Stack Tower

7. Service Building

8. Supplementary Control Room

9. Reactor Building

10. Purification Building

11. Turbine Building

12. Pump House

13. Station Training Centre

~ 3 ~

SOME IMPORTANT DATA OF NAPS

DATA SPECIFICATION

Transmission Lines 1. Moradabad

2. Atrauli

3. Khurja

4. Simbhaoli

5. Harduaganj

Single Line

Single Line

Single Line

Single Line

Single Line

Stack Height 142 Meters

NDCT Height 128 Meters

NDCT Top Diameter 58 Meters

NDCT Base Diameter 107 Meters

NDCT Throat Diameter 53 Meters

Steam Flow 1314 Ton/hr

PHT Flow 12700 Ton/hr

Steam Pressure 40-48 kg/cm2

PHT Pressure 87.0 kg/cm2

CCW Flow 39000 Ton/hr

Coolant Tubes 306

No. of Fuel Bundles in one channel 12

Fuel Bundle UO2 – Weight 15kgs

No. of Bundles in a core 3672

Condenser Pressure 680 mm of Hg

RB Design Pressure 1.25 kg/cm2

Station Load 18 – 20 MW

Generator Power 220 MW

Grid Voltage 220 kV

ISO-14001 certification 19th AUGUST 1999

~ 4 ~

PRINCIPLE OF NUCLEAR REACTOR

A Nuclear Power reactor is only a source of heat, the heat being produced

when the uranium atom splits (fission). Nuclear fission is a nuclear reaction in

which the nucleus of an atom splits into smaller parts (lighter nuclei). The

fission process often produces free neutrons and photons (in the form of

gamma rays), and releases a very large amount of energy even by the energetic

standards of radioactive decay. The heat produces steam, which drives the

turbo-generator & produces electricity. Natural uranium, the fuel used in this

reactor, consist of two types (isotopes) of uranium namely U-235 and U-238 in

the ratio of 1:139. It is the less abundant i.e. U-235 isotope that fissions and

produces energy. When a U-235 atom is struck by a slow (or thermal) neutron,

it splits into two or more fragments. Splitting is accompanied by tremendous

release of energy in the form of heat, radioactivity & two or three fast

neutrons. These fast neutrons, which fly out of the split atom at high speeds,

are made to slow down with the help of moderator (heavy water). So that they

have high probability to hit other 92U235 atoms which in turn releases more

energy & further sets of neutrons and fission. Attainment of self-sustained

fission of uranium atoms is called a ‘Chain Reaction’. At this stage the reactor is

said to have attained “criticality”.

The basic nuclear reaction is as follows:

𝑈92 235 + 𝑛0

1 𝑆𝑟38 94 + 𝑋𝑒54

140 + 2 𝑛0 1 + (Heat Energy) + γ

(Natural Uranium Oxide)

𝑈92 235 + 𝑛0

1 𝐵𝑎56 141 + 𝐾𝑟36

92 +3 𝑛0 1 + (Heat Energy) + γ

~ 5 ~

TURBINE GENERATOR CYCLE

REACTOR CYCLE Heavy Water is used in the Reactors as moderator and as coolant for the

Reactor fuel. The two functions are separate, each having its own closed

circulating system. The fuel coolant system is called the Primary Heat Transport

System, and is a high pressure, high temperature circuit. The moderator and

reflector circuit is called the moderator system, and is a low pressure, low

temperature circuit. The Pressure tubes & Calandria Tubes are insulated from

each other in the Reactor core by Carbon di-oxide Gas in the annular space

between the calandria tubes and the coolant tubes. Figure shown above is a

simplified schematic diagram of the Reactor Cycle. Heavy water at 293 0C

enters the Steam Generator tubes to raise steam from Demineralized Water in

shell side, for the turbine and returns back to the Reactor at 249 0C. The

working pressure, which is the mean of the pressure, in the Reactor inlet &

outlet headers is 87.0 kg/cm2.

~ 6 ~

Moderator System

The moderator system is a Heavy Water with Cover Gas as Helium. Calandria is

always kept full of heavy water up to 96% Level. Remaining volume is covered

by Helium Gas, which acts as Cover Gas to avoid downgrading of Moderator

D2O. Moderator is used to slow down the speed of fast neutron. Moderator

(D2O) system circulating pump take suction from bottom of calandria and

discharge back to calandria through moderator heat exchanger for maintaining

moderator temp. Working pressure and temperature of moderator system are

8kg/cm 2 and 63oC respectively.

In order to avoid escape & loss of Heavy Water from PHT / Moderator System,

a high standard of integrity is maintained by using multiple seals & leakage

collection system in the liquid phase. D2O Vapour recovery Dryer Systems is

used for the vapour phase collection.

Reactor Fuel

Fuel from the reactor is in the form of bundles 49.53 cm long & 8.17cm dia. &

each bundle consists of 19 hermetically sealed zinc alloy tubes containing

compact & sintered pallets of natural uranium. Twelve such bundles are

located in each fuel channel.

~ 7 ~

Shutdown System

NAPS have two diverse & independent shut down system, one of them is fast

acting & other is slow acting.

1. Primary Shutdown System

The system is meant to shut down the Reactor whenever any operating

parameter crosses a set limit. The system operates automatically & can also be

operated manually. The system has 14 rods of cadmium sandwiched in

stainless steel as neutron absorbing element. Any trip signal actuates the

mechanical drum assembly and the criticality is reduced to sub criticality in a

span of 2.3 seconds.

2. Secondary Shutdown System

The Secondary Shutdown system comes into action when the primary

shutdown system fails to operate. It is provided as a backup protective system.

It consists of 12 liquid poison tubes which remain empty during normal course

of operation. But during operation the system enables the filling of tubes with

a neutron absorbing liquid. The principle is such that four when liquid filled

tanks are pressurized than the liquid rises up in liquid tubes located inside

reactor. It makes the reactor sub-critical in 1.4 seconds.

3. Automatic Liquid Poison Addition System

The primary and secondary shut down systems are unable to maintain the

state of sub criticality for long enough therefore an additional system known as

Automatic Liquid Poison Addition System is employed. Liquid poison is added

in the moderator. This poison will absorb the neutrons and thus will interrupt

chain reaction. Poison can be added either manually or automatically.

~ 8 ~

Electrical Systems

The electrical system deals with generation of electrical energy from heat

energy obtained from nuclear reaction and its subsequent transmission and

utilization.

Turbo Generator

TECHNICAL SPECIFICATION

1. Active Power - 237.7 MW

2. Power factor - 0.90

3. Total Power - 264.0 MVA

4. Stator - 16.5kV, 9240 A

5. Rotor - 326 V, 2755 A

6. RPM - 3000 RPM

7. Short circuit ratio - 0.58

8. Response time - 50ms

9. Efficiency at full load - 98.6%

10. Frequency - 50 Hz

11. Connection - 3 phase

12. Coolant - a) DM water, b) Hydrogen

13. Insulation - Class B

14. Production - 1991-92

15. Made by - BHEL – Haridwar

~ 9 ~

Principal Components:

Stator

The stator is the stationary part of the generator. It is made up of stacked

laminations of Cold Rolled Grain Oriented Silicon Steel. All these laminations

are insulated from each other. The core is provided with number of ducts both

in the plane of the core and in perpendicular plane to facilitate rapid cooling.

The stator is wound for three phase windings and is star connected.

Rotor

It is made up of Chromium-Nickel steel. Field winding conductors are placed in

rotor slots and are connected to form a series winding. A D.C voltage is applied

to the field winding to provide necessary excitation.

Damper windings are also provided. It is used to damp out the oscillations

produced due to abrupt change of load.

Slip Ring

Slips rings are made of copper, brush gear is provided in the generator shaft to

inject excitation current from the static rectifier unit to the rotating main field.

The slip rings are provided with inclined holes for self ventilation.

Principal of Operation of Turbo Generator

The electric generator is based on the principal of faraday laws of electromagnetic Induction discovered by Michael Faraday in 1831.

When the magnetic flux linked by a conductor changes, an EMF is induced in it.

Magnitude of EMF is directly proportional to the rate of change of flux.

~ 10 ~

Cooling Of Generator Set

Stator Cooling

Generator stator windings are cooled by DM water passing through the hollow

conductor. DM water is used for stator winding cooling purpose because of it

has:

Low viscosity

No fire hazards

Better heat removal capacity

Non conducting

Rotor Cooling

Stator core, rotor winding and core are cooled by hydrogen present in the

stator and rotor air gap. Two axial shaft fans mounted on both end of rotor

body are provided to circulate hydrogen gas in the independent and

symmetrically closed circuit.

Gas coolers are mounted in the stator body for hydrogen cooling. Hydrogen

gas is used for generator because of its

High heat conductivity

Less density

High heat removing capacity

Low voltage loss across it

~ 11 ~

Generator Excitation System

The excitation systems are basically classified as:-

i) DC Excitation System: It utilizes generator as source of power driven by

motor or shaft of main generator. It can be self or separately excited.

ii) AC Excitation System: It uses AC machine as source of power. Usually the

exciter is on the same shaft as turbine generator. The AC output is rectified by

either controlled or non controlled rectifiers.

iii) Static Excitation System: In static Excitation system, all components are

stationary. It supplies DC current directly to the field of the main generator

through slip rings.

Static Excitation System

Static excitation for 235MW is preferred because of following reasons:

Fast response time

High reliability

Interchangeability of part during operation

Very low maintenance

Less space requirement

~ 12 ~

Components of Static Excitation System

1. Excitation transformer

2. Controlled Rectifier Bridge

3. Automatic Voltage regulator

4. Field breaker

Excitation Transformer

Three single phase transformer rated at 833KVA, 16.5KV/332V are connected in delta to the 16.5KV system through tap of bus duct from main generator. The LV side of the transformer is connected in star and feeds the input to the rectifier.

Controlled Bridge Rectifier

There are total of 4 three phase thyristor based rectifier bridges to convert the AC into DC. These bridges are fed from excitation transformer and are connected in parallel at the output. Three bridges are used to convert AC into Dc during normal operation while the fourth one is used as a backup in case any one of the bridges fails. The control of firing pulses is given through AVR cubicle.

Automatic Voltage Regulator (AVR)

Control signals are generated here for rectifier. The AVR derives its input from the PT and CT of the generator and controls the excitation for varying the machine terminal voltage and reactive overflow in addition to this basic function of AVR in voltage regulator.

~ 13 ~

The AVR incorporates the following addition feature:-

1. Rotor Current Limiter

AVR protect rotor from overloading and the excitation system from suffering

voltage in excess of the ceiling voltage.

2. Stator Current Limiter

This limiter monitors the stator current limits the excitation in case there is

stator over load.

3. Load Angle Limiter

It monitors the load angle and ensures that generator does not enter unstable

region.

Field Breaker The field breaker is of air blast type. In this breaker provision is provided to

discharge the energy stored in the field though a non-linear resistance

whenever the breaker is open the means of a special contact of the breaker

when classes before the field breaker opens.

~ 14 ~

Generating Transformer Technical Specification 1) Capacity - 265 MVA

2) LV side Voltage - 16.5KV

3) HV Side Voltage - 235KV

4) Power factor - 0.9

5) Impedance - 0.14 pµ

6) Coolant - Oil Natural Air forced

The HV voltage of 235 KV is about 6.8 % above 220 KV .The 14 % impedance

specified will result in voltage drop of about 7 % at full load and 0.9 pf. Thus

the full load voltage drop in transformer is almost neutralized by higher ratio

specified.

OFF load tap changers are provided for GTs as the plant has to work normally

as a base load station in the grid. A range of +/-10 % in steps of 2.5 % has been

provided for varying the output voltage of transformer.

~ 15 ~

Station Unit Transformer

Technical Specification

1) Normal load of SUT specified - 20.8 MVA

2) Type of transformer - Outdoor, 3 phase core type

3) Rated voltage - 220/6.6 KV

4) Frequency - 50 Hz

5) Winding impedance % (HV-LV) - 9%.

The transformer is specified with a voltage rate of 220/6.6 KV. The HV voltage

corresponds to the voltage of the HV buses of the main output system. The L.V.

voltage of 6.6 KV is the no load voltage of the LV side on load the voltage drop

in the SUT will reduce the terminal voltage to 6.6 KV with the proper selection

of tap.

The star/star connections for HV/LV winding were chosen in order to obtain

proper vector matching of 6.6 KV unit and station system. The SUT is also

specified with an unloaded tertiary. The tertiary has a power rating of about

1/3 of the main winding. The tertiary winding in delta is provided so as to

provide ground path to the harmonics.

The transformer is specified with an onload tap changer to maintain steady

voltage at the 6.6 KV bus. The on load tap changer has range of +/- 12%.

Insteps of 1- 5%. Here only two SUT are available for initial start up to supply

power to station auxiliary when unit is shut down. SUT take supply from 220 KV

grid & feeds power to station auxiliaries. The capacity of UT is 31.5MV.

~ 16 ~

Power Supply Classification at NAPS

Each load within the station has been classified according to degree of

reliability required for its supply. There are four classes of power supply at

NAPS.

Power Voltage Nature Source

Class-I 250V DC Uninterrupted Battery bank

Class-II 415V AC Uninterrupted Power MG set

Class-III 415V AC Interruptible Diesel generator

Class-IV 415V AC & 6.6 kV interruptible Grid supply & TG

Set

Class IV Power

Power for the class IV station service is normally available from two sources.

These are the unit transformer, which are directly connected to the generator

output terminals, and the start up transformer, which is connected to the 220

kV bus system of the station.

Class IV supply is arranged in two voltages viz. 6.6 kV and 415 volts. Motors

loads above 200 kW are fed at 6.6 kV whereas motors below 200kW rating are

fed at the medium voltage of 415 volts.

Class III Power

This system feeds to those loads which can be interrupted shortly. These loads

are required to run even when Reactor is shut down. System is normally

charged from 6.6 KV system and when the 6.6 KV supply fails DGs

automatically start and recharge the system.

~ 17 ~

The class III supply system consists of two main buses P and Q. Bus P is fed

from 6.6 KV switchgear ( UT side) through a 2000 KVA transformer and Q is fed

from 6.6KV switchgear (UT side) through another 2000 KVA transformer.

Emergency diesel generators, one each, are connected to these class III buses

to restore supply in 30 to 60 sec.

There is a tie between two main buses P and Q. This tie is connected via two

breakers in series to take care of the eventually of failure class III supply to the

affected Bus.

Class II Power

Class-II Bus-S and Bus-T are kept constantly charged by two power generator

sets to convert 250 V DC to 415 V AC. As the motor of MG set is driven by 250V

DC from class-I power batteries, the class-II is also uninterrupted power supply.

Class-II may also be tied to class-III if any MG set becomes unavailable. This

condition calls continuous DG set running. Potential loss on any class-III or any

class-II buses initiate emergency transfer i.e. all DG's start to charge any dead

bus. The batteries can feed class II loads for about 30 minutes mean while class

III power supply must be restored.

Class I Power

The class I power supply system consists of two main buses U and V each is fed

from 500 KW ACVR, which is fed from class III buses P and Q respectively. Each

bus has a 2200 AH battery bank connected to it. The normal supply is from

class III system through ACVRs and the battery bank.

Battery bank has 2250 AH, discharge capacity for 30 minutes with the end of

discharge voltage of 204 volts.

~ 18 ~

Control Power Supply System

Class I: Control Power Supply

250V DC control power supply consist of two DC Battery banks which supplies

power to one bus each. In normal condition the battery bank is kept charge by

Class III Power Supply through ACVR. When Class III supply fails this battery

bank supplies the power.

Control power supply is used for auto tripping circuit, auto closing circuit and

test circuit. This has been separated from Class–I Power Supply System keeping

in view all time, availability and reliability.

Class II: Control power Supply (415V AC)

Inverters have been used to convert DC to AC. There are four Invertors out of

these, three operate continuously and the 4th remains as standby operating at

no load.

Source of supply to DC Motors are as below:

1. Bus-U (250V DC) CL-1 Power Supply Bus for INV-1

2. Bus-V (250V DC) CL-1 Power Supply Bus for INV-2

3. Bus-W (250V DC CL-1) Control Supply Bus for INV-3

4. Bus-X (250V DC CL-1) Control Supply Bus for INV-4 This shows four buses. Either bus may be tied to standby Bus-X when any

inverter trips. 240V AC is also derived from 415V AC (Inverter Output).

240V AC buses are called cells. There are eleven cells. Three cells for 240V AC

& remaining seven cells for 48V DC. 48V DC is used for logic circuits. 48V DC is

obtained by stepping down voltage from 240 AC to 48V DC and then by

rectifying.

~ 19 ~

Automatic Transfer Scheme

6.6 KV Bus-D and Bus-E are fed by UT while Bus-F and Bus-G are fed by SUT. In

case either incoming breaker trips on protection, potential on corresponding

buses will be lost and loads may trip on under voltage. To avoid this both bus

sections have been provided with a CB to close within 80 ms. This avoids

tripping of Reactor on less than two PCP trip. This scheme is known as Auto

transfer.

Emergency Transfer Scheme

When potential is lost on either Bus-P or Bus-Q or Bus-S or Bus-T or any ACVR

trip all 3 DG start automatically DG-1 synchronizes on auto to Bus-P & DG-2 to

BU-Q. This is called - EMTR. This scheme operates for three conditions:

1. Class-III under voltage

2. Class-II under voltage

3. ACVR failure

It is used to restore class –III supply and to maintain class –II supply.

Power Line Communication Apart from other modes of communication like telephone system, wireless etc., communication can also be established through the transmission line, which is known as Power Line Carrier Communication (PLCC). This system provides direct and independent communication between main plant and other substations and load dispatch center of U.P. State Electricity Board (UPSEB) grid. This will be exclusively used for communication in relation to Power System Operation and control. The carrier communication system is coupled to the 220KV power lines through coupling Capacitor Voltage Transformers (C.V.T’S).

~ 20 ~

Diesel Generator Set

Technical Specification

Rated continuous output - 1450 kW

Overload capacity for 8 hrs. - 1650 kW

Overload capacity for 2 hrs. - 1750 kW

The DG set are capable of parallel operation of Class IV Power Supply. Whenever there is a loss of Class IV Supply, the DG set is set into action which restores the power within one minute. Hence it is also called short interruption supply. The DG set is grounded through Neutral Grounding Resistor (NGR) of 0.5 ohms to limit the grounding current to 480 A.

Diesel Engine

Technical Specification

1. Rating - 2600 BHP

2. RPM - 1000rpm

3. No. of strokes - 4 stoke

4. Power factor - 0.8 lag

5. Engine cylinder - 16 cylinders

6. Excitation - Static Excitation

7. Pole - 6 Pole

8. Excitation voltage - 58.5 V

9. Excitation current - 326 A

10. Connection - star, 3 phase

The diesel engine is started by air motor. For cooling, oil and heavy water are used. Speed is controlled by the governing system. In the generator, lap wound type of stator winding is used. The field winding of generator is excited by 48 V D.C. voltage through the slip ring. The rotor is rotated and an E.M.F is produced in the stator winding. If the generator generates 60% of the voltage then the field winding is excited by generated voltage.

~ 21 ~

Power Motor Generator Set

AC Machine

Rated terminal voltage - 425V Rated continuous output at 0.8 pf - 325kVA Over load rating for 30 min - 360kVA Insulation class - F Locked rotor current on the - 1250A Base load of - 140kW

DC Motor

Rated normal terminal voltage - 258V Maximum working terminal voltage - 300V Minimum working terminal voltage - 200V Rating - 290 kW No load armature current - 60A Full load speed - 1000rpm Armature current at rated voltage - 1350 A

Pony Motor

Rating - 37 kW, 1000 rpm Rated voltage - 415V, 50Hz Current - 67 V

~ 22 ~

Techo Generator

Output - 100V at 1000rpm The motor generator set is meant for uninterrupted power supply (415V, 3phase, 50 Hz) to important auxiliaries. The D.C. motor of this MG set is supplied from class-1-250V DC supply, 500kW ACVR from class 3 supply and 2200 AH, 250V DC batteries. On loss of class IV supply, the class III system will also loose supply. DG set starts and restores class III supply. During this period the 250V DC batteries will continue to supply the MG set.

Starting The PMG is started with the help of pony motor with resistance control on Bus1. Initially the dc machine acts as a dc generator, when the terminal voltage across the generator equals the supply voltage (250V DC), circuit 0breakers are closed after which it acts as a dc motor. This in turn rotates the alternator.

Grounding In order to reduce the ground fault, the machine is grounded through a neutral grounding resistor of 0.4 ohm. This will restrict ground fault current to 600A or less.

~ 23 ~

SWITCHYARD

~ 24 ~

Technical Specification

1. Type - Outdoor 2. Nominal Voltage - 220kV 3. Max. Operating voltage - 240kV 4. Basic impulse levels:-

a) For transformer winding - 950 kV (peak)

b) For other equipments - 1050kV 5. Three phase fault level - 10000 MVA 6. Short time current rating - 23.6 kA/sec For all equipment 7. Minimum creep age distance - Total-5600 mm;

for insulation and Bushing- 2800mm. 8. Number of strain /Suspension/ Insulation /string - 254 X 140 fog type 9. Specified current rating for:

a) Main bus bar - 2000A

b) Bus coupler bay bus - 2000A

c) Bay bus of other element - 750A

~ 25 ~

Switchyard Equipments

Circuit Breakers

There are 11 C.B used in switchyard. The circuit breakers are of air blast Type

Following C.B are used in switchyard as per follows:

CB No. USED

CB -1 Bus coupler CB -2 Generated transformers - (Unit#1) CB -3 Start up transformer - (Unit#1) CB -4 One line - Moradabad CB -5 SUT - (Unit#2) CB -6 One line - Shimbholi CB -7 One line - Khurja CB -8 One line - Atrauli CB -9 GT - (Unit#2) CB -10 Transfer Bus CB -11 One line - Harduaganj There is a centralized compressed air system for feeding air to the circuit. A

ring main system with two feed points and in the piping to facilitates isolation

of any breaker circuit without disturbing air connection to the other circuits.

~ 26 ~

Isolators There are 44 isolators used in switchyard. The isolators are pneumatically

operated type and are capable of remote control from control room. These

isolators are worked only on off time condition. Grounding Switch is provided

on the line isolators. These grounding switches are mechanically interlocked

with the main isolators.

Lightening Arresters

Specifications:

Rating - 198 kV

Discharge current - 10 kA

Impulse spark over voltage - 550 kV peak

Switching surge spark over - 420/453 kV Power

Frequency spark over - 1.5 times rated Voltage

Reset Voltage - 205 kV

Lighting arrestors are provided on all five lines at their entry into switchyard

and also near the HV terminals of the power transformer. The arrestors are of

heavy-duty station type manufactured by M/s WS Insulators.

~ 27 ~

Capacitive Voltage Transformer (CVT)

CVT’s are provided on all 3 phases of the 220 kV lines. These CVT's serve the

dual function viz. VT for the line protection and coupling capacitor for carrier

communication. Each main bus bar is provided with one set of electromagnetic

VT for the purpose of metering synchronizing and feeding other protection

circuit. One single phase cut is provided for synchronizing bus.

Current Transformer

Current transformers are used for measurement of large current flowing in a

power line of AC supply. It is connected in series with phase wire. It has

secondary winding and the conductor whose current is to measured acts as

primary. 5 core CT's are provided for each of the elements of the switchyard.

Carrier Communication

Carrier communication facilities are provided for communication between

NAPS control room and Grids substations connected to NAPS. Phase coupling

has been envisaged for single circuit lines. Wave traps are provided on phases

associated with the ‘’communication”. Wave traps block the high frequency

carrier waves due to its high impedance and pass the power frequency signal.

CVTs having high capacitance pass the carrier frequency for PLCC.

Synchronizing Arrangements & Remote Controls

The remote controls for the switchyard circuit are provided on the control

room of NAPS. Synchronizing facilities are available for synchronizing any

element to the line bus bars. Emergency and synchronizing control is done

from control room.

All isolators can also be controlled remotely from the central control room.

However the grounding switches have to be operated manually at the

switchyard.

~ 28 ~

Motor Control Centre The motor control Center (MCC) is an assembly of panel from where motor starters for different motors in the station are grouped and controlled from control room or field. The centralized system of motor control through MCC’s in contrast with the distributed starter scheme affords the following advantages:

a) Grouping of large number of motor starter used in the station makes maintenance and operations easier.

b) Control cabling length and installation costs are reduced. This is especially true where centralized control system is used - such as in NAPS where most of the equipments are controlled from control room - or from location near to the load. Motors below 90 kW capacity are fed from the MCC of the associated class of the systems (Class IV, III or II).

Circuit diagram of Motor Control Centre (MCC)

~ 29 ~

Electrical Protection System The objective of a protection scheme is to keep the power system stable by

isolating only the components that are under fault, whilst leaving as much of

the network as possible still in operation.

Circuit Breakers

A circuit breaker is an automatically operated electrical switch designed to

protect an electrical circuit from damage caused by overload or short circuit.

Its basic function is to detect a fault condition and interrupt current

flow.

Circuit Breaker Specification

Rated voltage

Rated current

Rated Frequency

Rated making capacity

Rated breaking capacity

Short time current rating

Insulation level

Number of poles

Arc Formation

When a fault occurs, heavy current flows through the contacts of the circuit

breaker. At the instant, when the contacts begin to separate, the contact area

decreases rapidly and large fault current causes increased current density and

hence produces a rise in temperature. The heat produced in the medium

between contacts is sufficient to ionize the air or the oil. The ionized medium

acts as conductor and an arc is struck between the contacts.

~ 30 ~

220 KV & 6.6 KV System

Both 220 KV and 6.6 KV system has air blast type circuit breakers. These

breakers employ a high pressure air blast as an arc quenching medium. The

contacts are opened in a flow of air blast established by the opening of blast

valve. The air blast cools the arc and sweeps away the arcing products to the

atmosphere. This rapidly increases the dielectric strength of the medium

between contacts and prevents from re-establishing the arc. Consequently, the

arc is extinguished and flow of current is interrupted.

6.6 KV breakers are indoor type with compressed air as medium for operating

and quenching the arc during the process of interruption. Operating air

pressure for ABCB is 16 kg/𝑐𝑚2 and for 220 kV air blast circuit breaker is 31.7

kg/𝑐𝑚2.

415V System

Air circuit breakers are used in 415V system. These breakers are used in 415V

class IV (Bus J, K, L and M) and class II (BUS S and T). 415V breakers are used

for controlling motor loads from 90KW to 200KW. The breakers are

continuously rated for 1300A, 2000A and 3750A and symmetrical making

capacity of 50 kA.

250V DC System

DC circuit breakers employ high resistance method for arc extinction. Air circuit

breakers are used with arc splitters and arc chute to lengthen the arc.

The Switchgear for MG set, ACVR and supply breakers to power board is rated

for 2500A.The bus section breakers are rated for 1000A and feeder breakers

are rated for 630A. All 2500A breakers are electrically operated while 1000A

and 630A breakers are manual breakers.

~ 31 ~

Protective Relays

It is a protective device which detects abnormal condition in the power

system and initiates corrective action in order to bring the system to its

normal state.

It processes the input mostly voltage and current from the system and

issues a trip signal when a fault is detected within its jurisdiction.

Functional Characteristics of Relays

Selectivity

Relay should select the faulty section and protect that section only and must

not disturb the healthy circuit.

Sensitivity

Relay should be able to detect the smallest fault and system abnormality.

Speed

Relay should have a proper speed of operation. It should clear the fault before

it damages the system.

Reliability

The protection should not fail to operate in the event of faults in the protected

zone.

~ 32 ~

Types of Relay

Instantaneous Over Current Relay

It is applied for phase fault protection of Motor feeders, Transformers feeders

etc.

Earth Fault Relay Type

It is basically an over current relay used for earth fault protection of motor

feeders and transformer feeder. It provides time delayed over current

protection.

Definite Time Over Current Relay

The relay is used for time grade over current protection for feeders and stalling

protection for motors.

Under Voltage Relay

If under voltage occurs below the set point of relay, it drops and DC relays

picks up to give trip signal for breaker.

Instantaneous Differential Relay

It is basically a 3 phase over current relay designed for more sensitive

application. The way the relay will be connected in the circuit gives it the name

differential. The relay is past action and sensitive. It is used for short circuit

protection for big motor generators.

~ 33 ~

Fuse Failure Relay

It is used for detecting the failure or inadvertent removal of voltage

transformer, secondary fuses and prevention of incorrect tripping of circuit

breaker, for example - failure of PT secondary fuse in distance protection can

result in tripping of the feeder.

Directional Inverse Time Over Current Relay

Relays will operate for current flowing in either direction. Directional over

current relays operate only in one particular direction of power flow as

desired.

Transformer Differential Relay

It is used in phase to phase fault and ground fault protection of power

transformer.

~ 34 ~

Trainee’s Training Experience at NAPS My training experience at NAPS was quite fruitful and beneficial as it was

a golden opportunity for me to visit Narora Atomic Power Station from

inside which would not have been possible any other time as for security

reasons.

From Day One itself we were exposed to Industrial Working Procedures

like visiting the Electrical Workshop and seeing specially the gigantic

circuit breaker. Exploring various parts of it made me understand many of

its concepts better.

The experience we had at the field training was also very vibrant. Starting

from Turbine Building Visit to visiting individual component section gave

an actual feeling of how huge machineries are handled and maintained.

Although this was my first training at Power Station Industry and so this

training experience was more intriguing.

The Lectures started with Alternator, its specifications at NAPS, its

working, cooling, its capability curve and its protection. Watching the

huge 265 MVA generator amidst blatant noise was itself an experience.

Next we were introduced about the electrical protection practices at

NAPS where all types of protection schemes were given lecture on.

One of the best moments was visiting the switch yard. The best place to

clear all doubts one has is to visit the switch yard and understand it’s

working. To see installed CVTs, main busses, CTs, Lightening arrestors,

transformers at one place in service condition- what more one could ask

for as an Electrical Engineering student.

~ 35 ~

Later in the field visits we visited PMG sets, DG sets, Battery Section etc. I

must admit it was my first experience here to see the inside of DC motor

and alternator. To see how the machines are wound and how actually slip

rings, commutator look like in actual and how it is different from book

diagrams gave a real glimpse of vastness of electrical engineering.

One of the major difference which I saw here and wasn’t in the another

Power Plant where the strictness and alertness of CISF security. I really

appreciate the way security is beefed up owing to its strategic

importance.

Not to leave the wonderful subsidized canteen which was a harbinger of

new energy whenever we were given short tea breaks.

All in all, my experience at NAPS was full of learning and understanding

Power System concepts and had its twists and turns which were beautiful

in their own way.

I just wished I had some pictures standing beside the humongous NDCT

Tower as a souvenir of what transpired in the one month training I

undertook.

~ 36 ~

Conclusions

The nuclear power has come of age with comprehensive capabilities in all

aspects of nuclear power and is poised for a large expansion program.

The challenge is to pursue the three-stage program, develop and

commercially deploy technologies for utilization of thorium and ensure

the country’s long term energy security.

At present nuclear reactors has an increasingly important role to play in

the generation of electricity and in the other areas such as defense.

Needless to say, when pursuing such a program, it is paramount

importance that health and safety of the plant personnel and member of

the public are fully ensured.

The pressurized heavy water reactor, which will be the main source of the

nuclear power in India for present as well as future, have several safety

features. This Design provides redundancy in protective and safety

system and adopts the concept of defense in depth. The double

containment feature provides an added level of safety level.

Operation of nuclear power station is characterized by the strict

adherence to a set of prescribed limits and guidelines. The operation

personnel are carefully selected, trained and qualified. Environmental

releases and exposure of personnel are routinely monitored so as to

ensure that they are within stipulated limits. The regulation authorities

critically review the design and procedure for manufacture, construction

and operation, prior to issue of appropriate licenses. Experience with

Narora atomic power station has demonstrated that the pressurized

heavy water reactor system are capable of operation with high reliability

while ensuring safety of plant personnel and the surrounding population,

and with the with minimal impact on environment.