summer vacation report
Published on: Mar 3, 2016
Transcripts - naps narora
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CARRIED OUT AT
NUCLEAR POWER CORPORATION OF INDIA LIMITED
(A GOVT. OF INDIA ENTERPRISE)
Mr. GAURAV SHARMA (Training Superintendent)
Mr. KHAGESH C. RAKESH, SO/E (STC)
III Year B.Tech, Electrical & Electronics Engineering
Aligarh College of Engineering & Technology
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I wish to express my profound sense of respect and gratitude to
Shri.D.S.CHOUDHARY (Station Director), Narora Atomic Power Station. Narora to
kindly allow me to carry out vocational training at NAPS.
I also express my gratefulness to Shri.GAURAV SHARMA (Training Supt.), Shri
K.C.Rakesh, SO/E (STC), NAPS .Narora for constant inspiration, motivation and
invaluable support I got from them.
I am thankful to the Engineers and officers of Control Maintenance Unit , Sh. H.C GAUR
(SO/F), Sh. G. S. RAWAT (SO/C), Sh. RAJESH SHARMA (SO/D), Sh. D.V. SINGH,
and Sh. ANUJ KUMAR & I am also thankful to the Engineers and officers of EMU,
Sh.R.K.PANDEY(SO/E), Sh.S.K.KATIYAR, Sh.DASHRATH PRASAD, Sh.MUKESH
BABU, Sh.HARISH SHARMA whose co-operation, full constant source of their ideas
and valuable interaction helped me a lot in completing the project. Finally I thank to
security, administration and other staff of the station for their valuable co-operation
during the training period enabling me to achieve the goal.
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S.NO. DESCRIPTION PAGE NO.
1.0 Introduction to NAPS 4
1.1 NAPS Plant layout 7
1.2 Some important data of NAPS 8
1.3 Nuclear Power stations in India 9
1.4 Principle of Nuclear Reactor 10
1.5 Nuclear Reaction 13
1.6 Heavy water and its usage 16
1.7 Moderator System 16
1.8 Primary heat transport system 16
1.9 Reactor fuel 16
1.10 Shut down system 17
1.11 Steam cycle 17
1.12 Main Control Room 19
2.0 Important measurement at NAPS 20
2.1 Pressure measurement 20
2.2 Temperature measurement 21
2.3 Level measurement 21
2.4 Flow measurement 22
3.0 ELECTRICAL SYSTEM 23
3.1 Turbo generator 23
3.2 Generating transformer 28
3.3 Station unit transformer 28
4.0 Power supply classification at NAPS 29
4.1 Diesel generator set 31
4.2 Power motor generator set 32
5.0 Switchyards 34
6.0 Motor control centre 38
7.0 Circuit breakers 40
8.0 Electrical protection system 40
8.1 Protective Relays &Types of Relay 42
9.0 Trainee’s Training experience at NAPS 44
10 Conclusion 46
INTRODUCTION TO NAPS
One of the chief aims of the Department Of Atomic Energy is the development of
Nuclear Energy for economic generation as an alternative source of electric power
when in due course the conventional sources will be exhausted in the country.
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Petroleum prices are escalating. The amount of coal required for 400MWe power
generations of the order of 5 x 106
KGs per day. Whereas a Nuclear Power Station of
the same capacity needs only 200 kg of Atomic Fuel per day. Transportation of coal of
such magnitude over long distance is not economical.
NARORA, a small ancient village, is situated on the bank of Holy River Ganga in the
district Bulandshaher in Uttar Pradesh. The plant is about 60 km from Aligarh, which is
the nearest population center. 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
The NAPS is a twin unit module of 220MW 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. A NAPP is the fourth Nuclear Power Station in the
country after Tarapur in Maharashtra, Rawatbhata in Rajasthan & Kalpakkam in
TamilNadu. A NAPS is the first indigenous Nuclear Power Plant of India. The station
has two pressurized heavy water reactors with installed capacity of 220MWe, each
using natural uranium as fuel. The station is connected to high voltage network
through five 220 KV lines, one to Moradabad, one to Atrauli, one to Simboli, & two to
Khurja. It is designed for base load operation as a commercial station.
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TOP VIEW OF NARORA ATOMIC POWER PLANT
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Reactor Building houses the Reactor Primary Heat Transport System, Moderator
System, Reactivity System, Fuel Handling System & some of the Auxiliaries. The Turbo-
Generator & its associated conventional equipment, Emergency Diesel Sets, Control &
Power MG Sets, Station Batteries, Electrical Switch Gear Compressors, Chillers and
Main Control Room are located in Turbine Building. Both the units share common
facilities such as Service Building, Spent Fuel Storage Bay (SFSB) & other auxiliary
devices such as Heavy Water Upgrading & Waste Management Facility. NAPS have
two natural draughts cooling & two induced draught cooling towers. NAPS have the
following main parts: -
SOME IMPORTANT DATA OF NAPS
1. Administrative Building 5. Switchyard 9. Reactor Building
2. Domestic Water Head
6. Stack 10. Purification Building
3. Canteen 7. Service Building 11. Turbine Building
4. NDCT 8. Supplementary Control
12. Pump House
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Transmission Lines Five
220 KV Narora – Moradabad Single Line
220 KV Narora –Atrauli Single Line
220 KV Narora – Simboli Single Line
220 KV Narora – Khurja Double 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
NUCLEAR POWER STATIONS IN INDIA
Following are the India’s Atomic Power Stations operating as on today:-
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S.NO. PLACE CAPACITY
1. TAPS (1 & 2), Tarapur 2x160MW
2. TAPS (3 & 4), Tarapur 2x540MW
3. RAPS (1), Rawatbhata 1x100MW
4. RAPS (2),Rawatbhata 1x200MW
5. RAPS ( 3 & 4), Rawatbhata 2x220MW
6. RAPS ( 5 & 6), Rawatbhata 2x220MW
7. MAPS ( 1 & 2), Kalpakkam 2x220MW
8. NAPS ( 1 & 2), Narora 2x220MW
9. KAPS ( 1 & 2), Kakrapara 2x220MW
10. KGS (1 & 2), Kaiga 2x220 MW
11. KGS (3 & 4), Kaiga 2x220 MW
12. KUDANKULAM (1), Tamilnadu 1x1000MW
Following are the power stations under construction: -
S.NO. PLACE CAPACITY
1. KAPS ( 3 & 4 ), Kakrapara 2X700 MW
2. RAPS ( 7 & 8 ), Rawatbhata 2X700 MW
3. KUDANKULAM ( 2 ), Tamilnadu 2X1000 MW
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PRINCIPLE OF NUCLEAR REACTOR
A Nuclear Power reactor is only a source of heat, the heat being produced when the
uranium atom splits (fission). 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”.
Heavy Water is used in the Reactors as moderator & reflector for the neutrons 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 below is a simplified schematic diagram of the Reactor
Cycle. Heavy water at 293 0
C enters the Steam Generator tubes to raise steam from
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Demineralized Water in shell side, for the turbine and returns back to the Reactor at
C. The working pressure, which is the mean of the pressure, in the Reactor inlet &
outlet headers is 87.0 kg/cm2
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.
PHWR SIMPLIFIED FLOW DIAGRAM
Moderator (D2O) system circulating pump take suction from bottom of calandria &
discharge back to calandria through moderator heat exchangers for maintaining
moderator temperature. Moderator inlet to calandria is at its middle point from two
opposite sides. Working pressure and temperature of moderator system are 8 kg/cm2
C respectively with a cover gas pressure of 0.25 kg/cm2
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
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TURBINE AND STEAM
At rated power, 1.33 x 106
kg/hour of saturated steam at 39.7 kgf/cm2 pressure is
provided by Four Steam Generator to supply to the Turbine. The Turbine rated at 220
Mew, is a tandem compound machine with one high-pressure cylinder and one low-
pressure cylinder with double flow. From the outlet of the HP cylinder, the steam at a
pressure of 5.6 kg/cm2
passes to a pair of moisture separator and then to a pair of
reheater, where steam is heated up to 233o
C for admission to the low-pressure cylinder.
Steam exhausted from the L.P. turbine is condensed in a single pass condenser
capable of maintaining a vacuum of 680 mm of Hg with a NDCT cooled water
temperature of 320
C. The feed water is heated in six stages up to 1710
C and sent to
the Steam Generators. The Steam Re-heater drain is returned separately to Steam
SCHEMATIC OF PHWR INCLUDING TURBINE GENERATOR CYCLE
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The basic atomic energy we get from the process called nuclear fission of the 92U235
atom with the thermal neutron. The typical fission reaction is as follows: -
+ 2 0n1
+ y (Heat Energy) + γ
(Natural Uranium Oxide)
+ 3 0n1 +
y (Heat Energy) + γ
The fission process releases energy. The transformation of this energy is as
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shown below: -
Nuclear Fission Energy
(Formation of steam)
(Operating Turbine & Generator)
(With the help of generator)
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HEAVY WATER AND ITS USAGE
Heavy water is used as both the moderator and coolant. Heat energy is transported by
coolant from reactor to the vertical, integral U-tubes in shell type of heat exchangers,
which functions as a boiler to produce steam and drives Turbo Generator. The heavy
water (D2O) is identical to the ordinary water (H2O) as far as the chemical properties are
concerned. However, in physical properties there are minor variations (boiling point
C and freezing point 3.82o
C). The deuterium (D2) in heavy water is an isotope of
hydrogen (H2) having one neutron and one proton in its nucleus. The absorption cross-
section of heavy water for neutron is far less than the ordinary water, which helps in
The moderator system is a heavy water and helium system. Calandria is always full of
moderator up to 96% and remaining volume is covered by helium gas, which acts as
cover gas. 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 temperature.
Working pressure and temperature of moderator system are 8Kg/cm2 and 63o
PRIMARY HEAT TRANSPORT SYSTEM
The fuel coolant system is called the primary heat transport system and is a high
pressure and high temp circuit. The coolant transports this heat to the four steam
generators. Four pumps maintain the circulation through pressure tubes around the fuel
bundles, each having a capacity of 3560m3
/hr. The coolant temperature at inlet & outlet
of a reactor are 249o
C & 293.4o
C respectively at a pressure of 101 kg/cm2
& 87 Kg/cm2
respectively. A pressuring pump maintains the system pressure using automatic feed &
bleed principle instrument relief valves & suitable regulating & protective system action
limit the system pressure to 93.91 Kg/cm2
. PHT is constantly pressurized at 87Kg/cm2
to keep it in liquid form at 295oC. High pressure, high temperature heavy water areas
have been separated from high pressure, high temperature light water areas for
recovery of high isotopic purity heavy water.
Fuel from the reactor is in the form of bundles 49.53 cm long & 8.17cm dia & each
bundle consists of 19 hermetically sealed zircalloy tubes containing compact & sintered
pallets of natural uranium. Twelve such bundles are located in each fuel channel.
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SHUT DOWN SYSTEMS
NAPS are provided with two diverse & independent shut down system, one fast acting &
other slow acting. The primary shutdown system has shutoff mechanism at 14 locations
in the reactor. Each of the 14 mechanisms has cadmium sandwiched steel as neutron
absorbing element. Normally these rods are parked outside core during power operation
& fully in on a trip signal. The rods are held out of reactor core by rope & drum
arrangement. These rods drop in the core under gravity whenever a trip signal is
received, & make the reactor sub-critical in less than 2.3 sec.
The secondary shutdown system is a fast acting back up system to the primary
shutdown system. This system provides sufficient reactivity worth by promptly filling
twelve vertical tubes in the reactor core with a neutron absorbing liquid (Lithium
Pentaborate deca –hydrate). 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 sec.
Both the shutdown systems are backed by Automatic Liquid Poison addition system
injecting controlled quantities of boron into the moderator after receiving the appropriate
signal to ensure guaranteed sub-criticality of the reactor for prolonged periods.
Whenever there is total blackout of the station & automatic liquid poison system is not
available, addition of poison, under gravity, to moderator is incorporated.
The turbine is an impulse reaction type, designed for saturated steam duty, revolving at
3000 rpm with steam condition of 39.71 kg/cm2
pressure & 0.26% wet & 250.3o
C at inlet
to stop valves. The turbine each rated at 235 Mew is a tandem compound machine with
one high-pressure cylinder and one low-pressure cylinder with double flow. From the
outlet of the high pressure cylinder the steam passes to a pair of moisture separator and
then to a pair of reheater where steam is heated up to 233o
C and 5.6/cm2 for admission
to the low-pressure cylinder. Steam exhausted from the L.P turbine is condensed in a
single pass condenser. Steam is extracted from suitable stages of the turbine top
provide for 6 stage regenerative feed heating, with a final feed water temperature of
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5&6 H.P Turbine
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MAIN CONTROL ROOM
Control Room as the name suggests, what it is? It is a place from where every
instrument device etc. can be controlled in either field or Reactor Building. Control
Room is the most important place in any nuclear power plant. It is a place, which has
full control over the reactor and all its peripheral. It has separate system wise, panels for
both units. On any error in any device or system an audiovisual indication is produced in
the control room. It also has fuel-handling panel from where staff members can see the
calandria channel by CCTV cameras and refueling is done from this panel only.
To present the operator with the desired information in a compact, overall fashion and
reduce the large number of recorders, meters and annunciating windows used in the
earlier plants, a computer based operator information system is introduced in NAPS
called as control room computer system (CRCS). This system is designed as purely
informative system, with no control features being included in the system. The
information is presented in any desired format and alarm annunciations are provided by
color CRT displays. The standardization input signal is used in CRCS. The input signal
is represented as 0.5-4.5V. At zero signal reading is 0.5Vand at full signal reading is
4.5V.The representation of row signal from sensors is represented into the standard
form by Signal Conditioning Modules (SCM). There are separate SCM for different type
of signal. These SCM are separate for both units and are located in the Control Room
Computer system itself. Moreover all the paperless recorders installed at NAPS are
connected to a common computer through LAN, where all the data is stored and
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IMPORTANT MEASUREMENT AT NAPS
It encompasses monitoring and control of various plant parameters principle of
redundancy, diversity testability and maintainability are given prime consideration. A
high degree of automation is aimed at to promote reliability. The system is design to
confirm to fail safe criteria. All visual indication to control, which may require intervention
during operation, is located in a single central control room. The safety systems are
generally triplicates, the safety function being achieved by 2 out of 3 logic’s. Each
channel is totally independent of other channels with separate sensor, signal handing
equipment, cable routes and power supplies. In channel temperature monitoring
system, only two channels are used with coincident logic of 2 out of 2 to reduce the
power. The instrumentation for the control and protection system is kept separate and
independent of each other. A computer based operator information system is used for
data information display for operators. If any default occurs in the system it will inform
the operator by audio-visual window annunciates system, which is provided on control
room panels to cover certain essential parameters.
Pressure is one of the important variable encountered in the nuclear power plant,
because uncontrolled it can lead to severe damages and loss of efficiency. Following
are the some important examples of pressure measurement.
Primary heat transport is pressurized to prevent vapor flashing and thus power
Moderator pump suction pressure is controlled to ensure that the helium-circulating tank
will not flood under certain conditions.
Steam pressure is controlled to ensure economic efficiency and power control. Pressure
is actually the measurement of force acting on area of surface. The units of
measurement are either in pounds per square inch (PSI) in British units or Pascal (Pa)
in metric. One PSI is approximately equal to 7000 Pa.
Common pressure detectors are Diaphragm, strain gauge, and bourdon tubes,
differential pressure transmitters.
Scale of pressure measurement is:-
1) Gauge pressure
2) Absolute pressure
3) Vacuum scale, usually stated in inches of mercury below atmospheric
P Gauges=P absolute – P atm
P vacuum=P atm – P absolute
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The primary sensor used in the temperature measurement is thermocouple (T/C),
bimetallic strip and resistance temperature detector (RTD).
Thermocouple: - A T/C consists of two pieces of dissimilar metals with their ends joined
together. When heat is applied to the junction, a voltage, in the range of mille-volts
(mV), is generated. A thermocouple is said to be self-powered.
Resistance temperature detector: - The resistance of device various as the temperature
increases. RTD is one of the most accurate temperature sensors. Not only does it
provides good accuracy it also provide excellent stability and repeatability.
The measurement of level in nuclear power plant assume sample significance specially
in heavy water system as can be seen from the below given example:
Moderator level is measured and control for reactivity adjustment.
Dump tank sump level is a measure to ensure a low level will not lead to dump
Storage tank level is a direct indication of heavy water contents; a drop in level could be
only because of leakage.
Boiler drum level is measured and controlled to provide adequate heat sink for the
reactor and to match the requirement of steam to turbine.
Ultrasonic method of level measurement: -The ultrasonic operates on the principle of
sonar. Sound waves are sent out to the free surface of liquid under test and are
reflected back to the receiving unit, level changes are accurately measured by detecting
the time intervals taken for the waves to travel to the surface and back to the receiver.
The longer the time interval, the farther away is the liquid surface, which in turn is an
indication of level measurement.
The ultrasonic gauge needs physical contact with the material. It is non-disturbance
technique. It can be used for solid and liquid material level measurement.
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The flow measurement is very important in process industries because it establishes
definite ratios and quantities of process materials for production quantity control. In
nuclear plants, flow measurement is critical for cooling loops such as calandria spray
flows; adjuster rod coolant flow is measured in selected channels only. In case of other
cooling loop such as bleed cooler, moderator heat exchanger, a low process water flow
would result in high outlet temperature. In moderator circulation system, a low flow
could point towards cavitations of pumps. In turbine generator steam, flow directly
depends on the load. Flow can be measured by venturi tube
Venturi Tube: -The flow of liquid through the venturi tube establishes the pressure
differential, which can then be measured and related to the flow rate. In this tube, a
large pressure recovery can be made, and flows can travel through it with much higher
velocity without the turbulence, which destroys the accuracy of orifice plates.
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The electrical system deals with generation of electrical energy from heat
Energy obtained from nuclear reaction and its subsequent transmission and
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
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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.
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.
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
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
N = No. of turns
dφ/dt = Rate of change of flux
E = Induced EMF
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Cooling Of Generator Set
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
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
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Generator Excitation System
The excitation system 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
Components Of Static Excitation System
1. Excitation transformer.
2. Controlled Rectifier Bridge.
3. Automatic Voltage regulator.
4. Field breaker
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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 fail. The control of firing pulses is given through
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. The AVR incorporates the following
Rotor Current Limitor
AVR protect rotor from overloading and the excitation system from suffering
voltage in excess of the ceiling voltage
Stator Current Limitor
This limiter monitors the stator current limits the excitation in case there is
stator over load
Load Angle Limitor
It monitors the load angle and ensures that generator does not enter
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.
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1) Capacity - 265 MVA
2) LV side Voltage - 16.5KV
3) HV Side Voltage - 235KV
4) power factor - 0.9
5) Impedance - 0.14 pu
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
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.
Station Unit Transformer
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%.
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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
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
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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.
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
250 V DC from class-I power batteries, the class-II is also uninterrupted
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.
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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.
Diesel Generator Set
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.
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
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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. The air is stored in air receiver by
the compressor and this air gives six starts to D.G. set. For cooling, oil and
heavy water are used. The cooling arrangement forms closed path and cool
by the jacket water. 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
Power Motor Generator Set
Rated terminal voltage - 425V
Rated continuous output at 0.8 p.f - 325kVA
Over load rating for 30 min - 360kVA
Insulation class - F
Locked rotor current on the - 1250A
Base load of - 140kW
Rated normal terminal voltage - 258V
Maximum working terminal voltage - 300V
Minimum working terminal voltage - 200V
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Rating - 290 kW
No load armature current - 60A
Full load speed - 1000rpm
Armature current at rated voltage - 1350 A
Rating - 37 kW, 1000 rpm
Rated voltage - 415V, 50Hz
Current - 67 V
Output - 100V at 1000rpm
The motor generator set is meant for uninterrupted power supply (415V, 3
phase, 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.
The PMG is started with the help of pony motor with resistance control on
Bus 1. Initially the dc machine acts as a dc generator, when the terminal
voltage across the generator equals the supply voltage (250V DC), circuit
breakers are closed after which it acts as a dc motor. This in turn rotates the
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.
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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
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
There are 11 C.B used in switchyard. The circuit breakers are of air blast
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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-Khurja
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
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.
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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.
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
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 facilities are provided for communication between
NAPS control room and Grids substations connected to NAPS. Phase coupling
has been envisaged for single circuit lines and inter circuit has been
envisaged for double circuit line to Khurja. Wave traps are provided on
phases associated with the ‘’communication”. Wave traps block the high
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frequency carrier waves due to its high impedance and pass the power
frequency signal. CVTs having high capacitance pass the carrier frequency
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
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
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).
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Circuit diagram of Motor Control Centre (MCC)
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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
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.
220 KV & 6.6KV System
Both 220 KV and 6.6KV 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
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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.
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 r.m.s.
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.
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
• 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
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issues a trip signal when a fault is detected within its jurisdiction.
Functional Characteristics Of Relays
Relay should select the faulty section and protect that section only and must
not disturb the healthy circuit.
Relay should be able to detect the smallest fault and system abnormality.
Relay should have a proper speed of operation. It should clear the fault
before it damages the system.
The protection should not fail to operate in the event of faults in the
Types of Relay
Instantaneous Over Current Relay
It is applied for phase fault protection of Motor feeders, Transformers
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
Definite Time Over Current Relay
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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.
Intantaneous 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.
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
Transformer Differential Relay
It is used in phase to phase fault and ground fault protection of power
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Trainee’s Training Experience at
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
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. At the end of Generator
lecture some of my doubts regarding reactive power and capability curves
were thoroughly addressed. 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 its
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.
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
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 upowing to its strategic importance.
Not to leave the wonderful subsidised canteen which was a harbinger of new
energy whenever we were given short tea breaks.
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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
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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 (Plutonium
based atomic weapons) where radio nuclides are required. 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 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.