Introduction to power station protections

1. WELCOME

2. Introduction to
protection system of
Thermal stations
By
N.RENGARAJAN
Chief Manager/Elect.
Thermal Power Station-I Expansion
Neyveli Lignite Corporation Ltd, Neyveli

3. PROTECTION SYSTEM
 Sensitive to abnormal condition
 Protects equipment against major damage
 Protects the personnel
 Isolates the faulty equipment
 Prevents cascade tripping
 Improves system reliability

4. PROTECTION SYSTEM
 Instrument transformers
 Relays
 Circuit breakers
 Auxiliary supply
 Fault recorders
 Event loggers

5. Instrument Transformers
 To transform currents or voltages to a value easy to
handle for relays and instruments.
 To insulate the metering & protection circuits from the
primary high voltage system.
 To facilitate standardization of the instruments and
relays to a few rated currents and voltages.

6. Current transformers

7. Typical rating details of CT
 CT Ratio (e.g: 2000 / 1A)
 Accuracy
(e.g: Class: 1, 10P10, PS)
 Burden (e.g: 10 VA)
 Knee point voltage
(e.g: Vkp = 500V and / or
Vk / 2 < 30 mA)
 Insulation level (e.g: 1.1 KV)

8. CT ERROR
Variation of error with current

9. Voltage transformers

10. Rating Plate of Voltage transformers
 PT/VT Ratio
(e.g: 33/√3KV / 110/√3V /110/3V)
 Accuracy (Class: 1 / 3P)
 Burden (e.g: 100 VA/50VA)
 Insulation level (e.g: 70/170 KV)
 Frequency (50Hz)
 Type (Cast Resin)

11. Variation of error with voltage
 Non-linear characteristic of the exciting voltage

12. Hierarchy of Relays
 First Generation (Electro-mechanical) Relays
 Second Generation (Static) Relays
 Third Generation (Programmable) Relays
 Fourth Generation (Smart) Relays

13. Electro-mechanical Relays

14. Applications
 Over current with IDMT characteristics
 Instantaneous Short Circuit
 Short Circuit / Earth fault with DTL
 Under Voltage / Over Voltage
 Differential current relays
 Distance protection relays

15. Design of IDMT Relay

16. Mechanism & Settings

17. IDMT Characteristics

19. Static Relays

20. Design of Static relay

21. Advantages
 Low burden on CT and PT
 No mechanical inertia
 Fast operation and long life.
 Low maintenance
 Quick reset action.
 Greater sensitivity.
 Unconventional characteristics are possible.
 The low power consumption.

22. Programmable Relays and
Smart Relays

23. Block diagram

24. Multiple Characteristics

25. Advantages
 Numeric Setting
 Password protection
 User selectable input settings
 Man-Machine Interface (MMI)
 Self Checking
 Remote Communication
 Time Synchronisation

26. GENERATOR PROTECTIONS
 GENERATOR
 GENERATOR TRANSFORMER
 UAT’S
 EXCITATION TRANSFORMER
 SWITCH-YARD & LINE BACK UP
26

27. GENERATOR SINGLE LINE DIAGRAM

28. GENERATOR DATA SHEET
 ANSALDO ENERGIA, ITALY
 283.5 MVA, PF 0.85, 15.75 KV,10392 A, 3 PHASE AC, 50 HZ
 STATIC EXCITATION 2725 A, 346 V DC
 STAR, IP 55,CLASS –F, DUTY S1
 3000 RPM, OVER SPEED 3600 RPM- 2 MIN.
 HYDROGEN COOLED
28

29. System Conditions
 Short circuits
 Overloads
 Loss of load
 Unbalanced load
 Loss of synchronism

30.  Mixture of mechanical and electrical problems.
Faults include :-
 Insulation Failure
 Stator
 Rotor
 Excitation system failure
 Prime mover / governor failure
 Bearing Failure
 Excessive vibration
 Low steam pressure

31. Generator Protections
 Earth faults on stator and generator connections
 Phase faults on stator and generator connections
 Inter-turn faults on stator
 Backup protection :- External Earth faults
External Phase faults
 Failure of prime mover
 Loss of field
 Unbalanced loading
 Rotor earth faults and inter-turn faults
 Overload
 Failure of speed governing system
 Sudden loss of load

32. 32
GENERATOR PROTECTIONS
 OVERLOAD
 SHORT CIRCUIT
 EARTH FAULT
 INTER TURN FAULT
 DIFFERENTIAL
 LOSS OF EXCITATION
 NEGATIVE SEQUENCE
 BACK UP IMPEDENCE
 OVER/UNDER VOLTAGE
 OVER/UNDER
FREQUENCY
 OVER-FLUXING
 LOW FORWARD/
REVERSE POWER
 OUT OF STEP

33.  Most probable result of stator winding insulation failure is a phase-
earth fault
 Desirable to earth neutral point of generator to prevent dangerous
transient over voltages during arcing earth faults
 Easy to troubleshoot and measure the fault level
 Damage resulting from a stator earth fault will depend upon the
earthing arrangement
Effects of Earthing

34. Solid Earthing :
Method of Earthing
 Fault current is high
 Rapid damage occurs
 burning of core iron
 welding of laminations
 Used on LV machines only
Resistance/Reactance Earthing :  Fault current is low
 Damage is limited
 Used on MV/HV machines
Neutral Grounding Transformer :  Fool proof Stator E/F protection
 95% and 100% S.E/F
 Used on large machines

35. Small, inter-laminar short grew into a major melt zone that triggered two other
melt areas caused by intense over-fluxing of the magnetic circuit in the stator core

36. The top portion of the rotor had rubbed on the stator.

37. 37
GENERATOR PROTECTION
CLASSIFICATIONS
CLASS – A
 FAULTS OF SERIOUS NATURE REQUIRING TOTAL SHUT
DOWN
CLASS- B
 FAULTS OF LESS SERIOUS NATURE NOT REQUIRING
TURBINE TRIP
CLASS- C
 FAULTS OF LEAST SERIOUS NATURE TO PERMIT HOUSE
LOAD OPERATION

38. 38
CLASS-A PROTECTIONS
 REVERSE POWER
 OVER VOLTAGE
 STATOR E/F 100% & 95%
 GENERTOR DIFF.
 OVER ALL DIFF.
 OVER-FLUXING
 EXCITATION SYSTEM LOCK OUT
 OVER FREQUENCY-II
 RE/F OF GT & UAT
 DIFFERENTIAL OF GT & UAT
 O/L TRIP OF UAT
 ROTOR E/F-2
 PRV/BUCH.
 LBB/BBP
 EMERGENCY TRIP

39. 39
CLASS-B PROTECTIONS
 LOSS OF EXCITATION
 U/F - STAGE 2
 NPS - STAGE 2
 GEN. & GT O/C
 OUT OF STEP-STAGE-2
 STAND BY E/F-GT
 TEMP. TRIP OF GT
 UNDER IMP-2
 O/C TEMP. TRIP OF
EXCITATION TR.

40. 40
CLASS-C PROTECTIONS
 UNDER IMP. STAGE-1
 NPS STAGE-1
 OUT OF STEP STAGE-1
 GENERATOR O/L
 UNDER FREQUENCY STAGE-1
 GENERATOR U/V

41. ANY QUESTIONS ?

42. THANK YOU