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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: ♦ C urrent magnitude compensation ♦ P hase 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: K 1 V 2 = K 2 I 2 or V/I = K 3 where, K 1 , K 2 and K 3 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.