1. Summer Training Report
on
Coal Fired Electrical Power Systems
of NTPC BGTPP

2. About NTPC
Past:
 Bongaigaon Thermal Power Station was the biggest plant situated at Salakati in the
district of Kokrajhar in Assam. It consisted of four coal fired units of 60 MW each.
 Major equipment like boiler, turbine, auxiliaries like BFP, CEP, PA fans, bowl mills
etc. were supplied by BHEL.
 Since commissioning none of the units could deliver the desired level of generation
for various reasons, the station had to be shut down.
Present:
 The alternative to the above was NTPC's proposal to set up a new 750 MW (3 X 250
MW) Thermal Power Station at the same location.
 It is one of the coal-based plant in India. The coal is derived from Makum coal
mines at Margherita, Assam and also from Eastern Coalfields Limited.

3. Various Components of the
Functional Diagram
 Coal Handling Plant(CHP): HP Turbine
1. Wagon Tripler
2. LP Turbine
 Conveyor Belt Condensor
 Stacker Reclaimer CEP
 Coal Storage BFP
 Crusher FRS
 Mills Bunker HP Heater
 Boilers LP Heater
 Boiler Drums Deaerator
 FD fans Ash Handling Plant
 PA fans FGD
 ID fans ESP
 Economizer RAH
 OAC

4. Some Important Definitions
Coal Handling Plant
NTPC BgTPP meets its coal requirement from Margherita and Eastern Coalfields
Limited (ECL).
Objectives of Coal Handling Plant are:
 Transportation and Handling of Coal.
 Arrangement for transferring of coal from Coal- wagons to coal-bunker or coal stock yard.
 Supply of Crushed coal to bunker

5. Boiler in a Steam Power Plant
In Thermal Power Plant Boiler, Coal is burnt to heat up the water. The
fuel is burnt inside the Boiler, whereas the water which is heated. runs
through tubes on the surface of the Boiler.
Components:
 Boiler furnace
 Boiler Drum
 Economizer
 Super Heater
 Pre-heater
 Igniters
 Boiler Feed Pump

6. Mechanism

7. Turbine
A steam turbine is a mechanical device that extracts thermal energy from
pressurized steam, and converts it into useful mechanical work.
Components of a steam turbine are:
 Casing
 Rotor
 Blades
 Sealing System
 Stop & Control Valves
 Couplings & Bearings
 Barring Gear
Turbines are of three categories, according to pressure level:
 High Pressure Turbine(HPT)
 Intermediate Pressure Turbine(IPT)
 Low Pressure Turbine(LPT)

8. 400 KV SWITCHYARD
It is a switching station which has the following credits:
 Main link between generating plant and Transmission system.
 Step-up and/or Step-down the voltage levels.
 Switching ON/OFF Reactive Power Control devices.
Switchyard Equipments and Switchgears:
1. Transformers
2. Switchgear
3. Load Switches
4. Circuit Breakers
5. Earth switches
6. Isolators

9. Transformers
Generator transformer (GT): The generator is connected to this
transformer by means of isolated bus oil filled duct. This transformer is
used to step up the generating voltage of 16.5 KV to grid voltage
normally 420 kV.
Unit Auxiliary Transformer (UAT):
-Draws its input from the main bus duct connecting generator to the
generator transformer.
-Used for the working of large devices.
- Uses the generated 16.5 kV to convert into 6.6 kV in stage I and 21kv
into 11kv & 6.6kv in stage II & stage III.

10. Station Transformer (ST): It is used to feed the power to the auxiliaries during the
start-up. This transformer normally rated for the initial auxiliary load requirements
of unit. In physical cases this load is of order of 60% of the load at full generating
capacity. It is also provided with on load tap changer to cater to the fluctuating
voltage of the grid.
ICT (Inter Connecting Transformer): Inter connecting transformer (ICT) are used to
connect 400KV switchyard to 132KV switchyard. They are autotransformers i.e.
they can be used for both stepping up and stepping down voltage.
CPT (Construction Power Transformer): This is the transformer which gives the
output for construction in which the voltage required is 433V.
Instrument transformers:
Instrument transformers are used for stepping-down the electrical parameter
(Voltage or Current) to a lower and safe value for Metering and Protection purposes.
CTs are single phase oil immersed type. Secondary current is generally 1A, but also
5A in certain cases.

11. Cooling of transformer
In oil immersed transformer heat is dissipated by THERMO SIPHON ACTION.
The purpose of using Oil is:
Cooling: Provides a better cooling and helps in exchanging heat .
Insulation: A non conductor of electricity, hence a good insulator.
The Various Types of Cooling used:
 AN – Air Natural
 ON – Oil Natural
 AF – Air forced
 OF – Oil forced
 ONAF – Oil natural Air forced
 OFAN - Oil forced Air natural
 OFAF – Oil forced Air forced


12. Transformer Accessories:
 Conservator
 Breather
 Radiator:
Nature of Transformer Faults
A transformer generally suffers from following types of
transformer fault:
 Over current due to overloads and external short circuits.
 Terminal faults.
 Winding faults.
 Incipient faults.

13. Differential Protection for
Standard Transformers:
In order to correctly apply transformer differential protection it is necessary to
properly compensate for:
♦ Current magnitude compensation
♦ Phase angle shift compensation
♦Zero sequence current compensation
 Oil Temperature Indicator or OTI:
This device is used to measure the top oil temperature.
 Winding Temperature Indicator or WTI:
This device measures the LV and HV winding temperature.
 Pressure Release Valve :
The relief valve (RV) is a type of valve used to control or limit the pressure in
a system or vessel.

14. Switchgear
This covers the equipments which are associated with switching and protection. These
equipments are used to control, regulate and measure electrical quantities. They are:
 Isolators
 Switching Isolators
 Circuit Breakers
 Protective Relays.
 Load Break Switches
 Earth Switches

15. Isolators:
They can break an electric circuit when the circuit is to be
switched on no-load only. These are normally used in various
circuits for the purpose of isolating a certain portion when
required for maintenance etc. There are two types of isolators used:
 Pentograph Isolator
 Double break Isolator with Earth Switch
Switching isolators:
They are capable of
 Interrupting transformer magnetized currents
 Interrupting line charging current
 Load transfer switching

16. Circuit Breakers
They can break or make the circuit on load and even on faults. The equipment is most
important and is heavy duty equipment mainly utilized for protection of the various circuits
and operation at load. Tripping coils are used to trip the circuit breakers and Closing coils are
used to activate the circuit breakers. Both are operated by spring motors.
Load break switches
These are those interrupting devices which canmake or break circuits at 8 times the rated
current. These are normally installed on the same circuit or on the circuits which are backed up
by circuit breakers.
Earth switches
These are devices which are normally used to earth aparticular system to avoid accident, which
may happen due to induction on account of live adjoin circuits. These do not handle any
appreciable current at all.
Lightning arrestors:
Station type“lightening arresters” are provided at theterminals of the transformers for
protection against lightening or any surges developing in the system, the practice is also to
install lightening arresters at the incoming terminals of the line. Shielding of substation from
direct lightening stroke is provided through earth wires located at structures‘peaks’. Recently
masts are also used for the purpose of shielding substation.

17. TRANSMISSION LINE PROTECTION
Distance Relays
 Proportional to the distance to the fault, (hence the name 'distance relay').
 Independent of the fault current levels.
 Employed to provide protection to transmission lines connected in a network.
 Simple to apply.
 High speed relays.
 Can be easily modified to work as unit schemes.
 Suitable for high speed reclosing.
 Made to respond to the impedance between the relay location and the point where
fault is incident.
Application of distance relays:
Zsecy = Zpri/Impedance ratio
(where, Impedance ratio = P.T.Ratio/C.T.Ratio)

18. Principle:
 It compares the currents and voltages at the relaying point with current providing the
operating torque and the voltage provides the restraining torque. Thus, it’s a voltage
restrained over-current relay.
 The equation at the balance point in a simple impedance relay is:
K1V2 = K2I2 or V/I = K3
where, K1, K2 and K3 are constants.
Types of Distance Relays:
 Impedance relay
 Reactance relay
 Mho relay
 Modified impedance relay

19. GENERATOR and its PROTECTION
Turbo-generator:
Turbo generator is basically a Synchronous Alternator. The stator houses the armature windings and
the rotor houses the field windings. D.C. voltage is applied to the field windings through static
excitation system. When the rotor is rotated the lines of magnetic flux cut through the stator
windings. This induces an e.m.f. in the stator windings.
Excitation System:
For this type of excitation system thyristor rectifier is used to directly control the generator filed
current and it can response with faster system by controlling the exciter field current. Thyristor
excitation fed system is necessary to provide slip rings and brushes to connect to the machine
rotor.This function is important to alert and response to the error or faulty for system stability due to
critical application. But it have disadvantage because this system not been developed commercially
and not common system for generator at around the world. It also difficult to providing the reliably
control signals to them from stationary equipment.
Generator Components:
 Rotor
 Rotor winding
 Stator core
 Stator windings

20. Generator Cooling
 Rotor Cooling System: The rotor is cooled by means of gap pickup cooling,
wherein the hydrogen gas in the air gap is sucked through the scoops on the
rotor wedges and is directed to flow along the ventilating canals milled on the
sides of the rotor coil, to the bottom of the slot where it takes a turn and comes
out on the similar canal milled on the other side of the rotor coil to the hot
zone of the rotor. to rotation of the rotor, a positive suction as well as discharge
is created due to which a certain quantity of gas flows and cools the rotor.The
conductors used in the rotor windings are hollow which is done to have
internal cooling of the rotor.
 Stator Cooling System: The stator is cooled by distillate which isfed from one
end of the machine by Teflon tube and flows through the upper bar and
returns back through the lower bar of another slot. The stator winding is
cooled in this system by circulating DM water through hollow conductors. The
DM water should be at 40°C. As it is a closed loop the water that comes out of
the generator is again cooled and demineralized. Water passes through lower
bars along the length to the other end returns through the upper bars of
another slot and drain into drain header.

21. Generator Protection
Generator Earth fault protection:
 Types of Generator grounding:
 Ungrounded system
 Effectively grounded system
 Low impedance grounded system
 High impedance grounded system
95% Stator Earth fault protection:
 Results in voltage shift of generator neutral w.r.t ground
 Detected by voltage relay connected across grounding resistor or from the generator terminal
through open delta VT
 Protect approx. 95% of stator winding
 Sensitive inverse-time over voltage relay across the grounding resistor at the secondary
winding of NGT.
100% Stator Earth Fault protection:
 An undetected fault near the neutral end of the generator will bypass the grounding
transformer and the ground fault relaying. If a second ground fault occurs in the same phase
near the generator terminal it will be undetectable and unrestricted by the grounding
impedance.

22. Generator Differential Protection:
 Covers phase faults.
 Earth fault not covered due to high impedance earthlingresulting in low earth fault current.
Effects of phase fault:
 High currents
 Potential damage to machines
 Expensive outage/repair due to damage.
Types of differential protection:
 High impedance differential protection.
 Biased differential protection.
Types of Inter turn fault protection:
 Current based protection system
 Voltage based protection system
Inadvertent Energisation During Standstill Protection:
 Starts as Induction motor.
 Significant Inrush current (phase current 3-4pu) and prolonged period of acceleration(10-
60 sec)
 Severe thermal stress in stator and rotor windings.
 Turbine Bearing and end blade damage.