Generator protection by bhushan kumbhalkar


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Main equipment in the power plant is Generator. It's cost is much higher than any other equipment so we will have to protect the generator from all the possible faults and errors.

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Generator protection by bhushan kumbhalkar

  1. 1. Introduction:  In a generating station the generator and transformer are the most expensive equipments and hence it is desirable to employ a protective system to isolate the faulty equipment as quickly as possible to keep the healthy section in normal operation and to ensure uninterruptable power supply.  The basic electrical quantities those are likely to change during abnormal fault conditions are current, voltage, phase angle and frequency . Protective relays utilizes one or more of these quantities to detect abnormal conditions in a power system.  Protective system cost is 4-5%of the total cost
  2. 2. SWITCHGEAR  Switchgear is a general term covering a wide range of equipments concerned with switching and protection. Eg: Circuit breaker, Isolator, Earth switch etc.
  3. 3. DESIRABLE PROTECTION ATTRIBUTES Reliability  Selectivity Speed Simplicity  Economics
  4. 4. PROTECTION ZONES 5 GEConsumer&Industrial Multilin 1. Generator or Generator-Transformer Units 2. Transformers 3. Buses 4. Lines (transmission and distribution) 5. Utilization equipment (motors, static loads, etc.) 6. Capacitor or reactor (when separately protected) Unit Generator-Tx zone Bus zone Line zone Bus zone Transformer zone Transformer zone Bus zone Generator ~ XFMR Bus Line Bus XFMR Bus Motor Motor zone
  5. 5. MAIN EQUIPMENT FOR SWITCHGEAR OPERATION  Current transformer  Potential transformer  Relay  Circuit breaker
  6. 6. 7 GEConsumer&Industrial Multilin VP VS Relay • Voltage (potential) transformers are used to isolate and step down and accurately reproduce the scaled voltage for the protective device or relay • VT ratios are typically expressed as primary to secondary; 14400:120, 7200:120 • A 4160:120 VT has a “VTR” of 34.66 Voltage Transformers
  7. 7. 8 GEConsumer&Industrial Multilin • Current transformers are used to step primary system currents to values usable by relays, meters, SCADA, transducers, etc. • CT ratios are expressed as primary to secondary; 2000:5, 1200:5, 600:5, 300:5 • A 2000:5 CT has a “CTR” of 400 Current Transformers
  8. 8.  Alarm  Act at Abnormal condition.  Disconnect.  Fast operation.  Use system supply.
  9. 9. Simple electromechanical relay
  10. 10. 1. Reed relay 2. Latching relay 3. Solid state relay 4. Solid state contact relay 5. Ratchet relay 6. Coaxial relay 7. Overload protection relay 8. Forced guided contact relay 9. Buchholz relay
  11. 11. •It like a fuse •It is a switch •Interrupt the faulty part •Operation
  12. 12. 1.Voltage class 2.Current rating 3.Type of circuit breaker
  13. 13. 1.Air breaker circuit breaker 2.Miniature circuit breaker 3.Air blast circuit breaker 4.SF6 circuit breaker 5.Low oil circuit breaker 6.Vaccum circuit breaker
  15. 15. PLANT LAYOUT
  16. 16. GE N UA T AV R 220 kv bus 220 kv HVCB 6.6 KV CB NGT 10.5 KV 220 KV GT EXT TR SER TR 415 V AC LA Single line Diagram of generator connection
  18. 18. 500 MW TG ON TEST BED
  19. 19. NATURE OF FAULTS IN GENERATOR  Insulation failure.  Tends to deteriate with rising temp.  Insulation failure may cause inter-turn fault, ph to ph or earth fault.  Bring winding in to direct contact with core plates.  Any failure to restrict earth fault may result into core plate damage.  Insulation of rotor winding is also important.
  20. 20. Fault Occur In Generator • Stator Fault • Rotor fault • Abnormal Running Condition 1) Unbalanced Loading 2) Over loading 3) Over Speed 4) Over Voltage 5) Failure of Primer Mover 6) Loss Of Excitation 7) Excessive vibration 8) Difference in expansion between rotating and stationary parts 9) Loss of synchronism
  21. 21. PROTECTION APPLIED TO GENERATOR  Relays to detect faults outside generator  Relays to detect faults in side generator  Over speed protections.  Temp measuring device for bearings, stator winding, Oil temp.
  22. 22. EQUIPMENT GROUNDING  Prevents shock exposure of personnel  Provides current carrying capability for the ground-fault current  Grounding includes design and construction of substation ground mat and CT and VT safety grounding
  23. 23. SYSTEM GROUNDING  Limits overvoltages  Limits difference in electric potential through local area conducting objects  Several methods  Ungrounded  Reactance Coil Grounded  High Z Grounded  Low Z Grounded  Solidly Grounded
  24. 24. SYSTEM GROUNDING 25 GEConsumer&Industrial Multilin 1. Ungrounded: There is no intentional ground applied to the system-however it’s grounded through natural capacitance. Found in 2.4-15kV systems. 2. Reactance Grounded: Total system capacitance is cancelled by equal inductance. This decreases the current at the fault and limits voltage across the arc at the fault to decrease damage. X0 <= 10 * X1
  25. 25. SYSTEM GROUNDING 26 GEConsumer&Industrial Multilin 3. High Resistance Grounded: Limits ground fault current to 10A-20A. Used to limit transient overvoltages due to arcing ground faults. R0 <= X0C/3, X0C is capacitive zero sequence reactance 4. Low Resistance Grounded: To limit current to 25-400A R0 >= 2X0
  26. 26. SYSTEM GROUNDING 27 GEConsumer&Industrial Multilin 5. Solidly Grounded: There is a connection of transformer or generator neutral directly to station ground. Effectively Grounded: R0 <= X1, X0 <= 3X1, where R is the system fault resistance
  28. 28. Stator protection: Stator faults include the following- i. Phase-to-earth faults ii. Phase-to-phase faults iii. Inter-turn faults From these phase faults and inter turn faults are less common ,these usually develop into an earth faults. This causes- • Arcing to core • Damage of conductor and insulation
  30. 30. Stator inter-turn fault protection: • Inter-turn fault on the same phase of the stator winding cannot be detected by transverse differential protection as it does not disturb the balance between the currents in neutral and high voltage CTs. • For protection against inter-turn faults the following protection schemes are used. (1)Cross differential protection. (2)Residual voltage protection.
  31. 31. mmmmmm mmmmmm mmmmmm mmmmmm mmmmmm Loading resistor Over voltage relay With time delay STATOR EARTHFAULT RELAY
  32. 32. exciter P.B Field wdg Voltage relay ROTOR E/F RELAY
  33. 33. Rotor earth fault protection: • DC injection method or AC injection method. • The dc or ac voltage is impressed between the field circuit and ground through a sensitive overvoltage relay and current limiting resistor or capacitor(in case of ac). • But dc source is generally used as over-current relay in case of dc is more sensitive than ac. • A single earth fault in rotor circuit will complete the path and the fault is sensed by the relay.
  34. 34. Rotor earth fault protection AC Injection method
  36. 36. Rotor temperature alarm • It is provided in large generators. • It indicates the level of temperature but not the actual hot spot temperature. • The relay measures the temperature by measuring the resistance .(as shown in fig)
  37. 37. GENERATOR PROTECTION • Abnormal Operating Conditions The "Wild" Power System G Exciter Loss of Field Loss of Field Overexcitation Overexcitation Overexcitation Open Circuits Loss of Synchronism Inadvertent Energizing, Pole Flashover Abnormal Frequency Abnormal Frequency Breaker Failure Reverse Power Over Power
  38. 38. Loss of excitation protection: When the excitation of generator is lost it operate as a Induction generator. It derives excitation from the system and supply power at leading power factor. Which may cause-  A fall in voltage & so loss of synchronism & system instability.  Over heating of rotor due to induction current on it. A protection having MHO characteristic is used to detect loss of field.
  39. 39. Differential protection of generator:
  40. 40. Differential protection using balancing resistor:
  41. 41. Modified differential protection
  42. 42. Modified differential protection: • Generally protection is made for 80 to 85% of the winding. • If any fault occurs near the neutral point then the fault current is very small and relay does not operate. • Modified differential protection scheme is used to over come this. • Two phase elements (PC and PA) and balancing resistor(BR) is connected in star and the earth relay(ER) is connected between the star point and neutral pilot wire.
  43. 43. External fault back-up protection
  44. 44. External fault back up protection: • Over-current and earth-fault protection is provided for back-up protection of large sized generators protected by differential protection. • Induction type IDMT relay is used for this purpose.
  45. 45. STEAM VALVE C.B TRIP Protective relay Reverse power relay Reverse power relay scheme
  46. 46. REVERSE POWER PROTECTION  Failure of the prime mover of a generator set ,will keep the set running as a synchronous compensator, taking the necessary active power from the net work and could be detrimental to to the safety of the set, if maintained for any length of time. The amount of power taken will depend on the type of prime mover involved. It ranges from 5% to 25%.
  47. 47. m m 46 mm Zc ZA A B C Ia Ib Ic VZC VZA POSITIVE SEQ Ia IbIc VZC VZA VZA+VZC X Y NEGATIVE SEQUENCE Negative phase sequence protection
  48. 48. Negative phase sequence protection: • Unbalance may cause due to single phase fault or unbalanced loading and it gives rise to negative sequence current . • This current in rotor causes rotor overheating and damage to the rotor. • This can be protected by negative sequence current filter with over current relay.
  49. 49. Negative phase sequence protection:
  50. 50. Excite r FUSE T1 T2 FUS E TRIP SHUNT FILED WDG Field failure protection
  51. 51. FIELD FAILURE PROTECTION  Loss of generator field excitation under normal running conditions may arise due to any of the following condition. 1. Failure of brush gear. 2. unintentional opening of the field circuit breaker. 3. Failure of AVR. When generator on load loses it’s excitation , it starts to operate as an induction generator, running above synchronous speed. cylindrical rotor generators are not suited to such operation , because they do not have damper windings able to carry the induced currents, consequently this type of rotor will overheat rather quickly.
  52. 52. Over voltage protection:  Overvoltage protection is required in case of hydro- electric or gas turbine generators but not in case of turbo generators. Over voltage may be caused due to-  Transient over voltage in the transmission line due to lightening.  Defective operation of the voltage regulator.  Sudden loss of load due to line tripping. The protection is provided with an over voltage relay. It is usually of induction pattern with an IDMT Characteristic
  53. 53. Overcurrent protection: • Overloading of the machine causes overheating in the stator winding. • This can be prevented by using over-current relay with time delay adjustment. • But overheating not only depends on over-current but also the failure of the cooling system in the generator. • So temperature detector coils such as thermistors or thermocouples are used at various points in stator winding for indication of the temperature.
  54. 54. GENERATOR PROTECTION Name Input Protecting to Differential protection Differential Current Stator core and winding Stator earth fault Voltage Stator core and winding Over current Current Stator core and winding Over voltage Voltage Stator core and winding Interturn short circuit Current Stator core and winding Rotor Earth Fault Current Rotor winding Over and under frequency Frequency Turbine protection Reverse power flow Voltage and current Turbine protection Loss of excitation Voltage and current Power System Protection Back up protection for lines Voltage and current Generator protection