7th Sem

Electrical Engineering Department

Batch – B1 (2014 Batch)

Abhishek Choksi

(140120109005)

Switch Gear & Protection (2170908)

ALA Presentation

On

“Generator Protection”

Prepared By: Guided By:

Prof. Grishma Pipaliya

Gandhinagar Institute Of Technology

Generator Protection

Introduction

• In a generating station the generator and transformer are the mostexpensive equipments and hence it is desirable to employ protectivesystem to isolate the faulty equipment as quickly as possible to keepthe healthy section in normal operation and to ensure uninterruptablepower supply.

• The basic electrical quantities those are likely to change duringabnormal fault conditions are current, voltage, phase angle andfrequency . Protective relays utilizes one or more of these quantities todetect abnormal conditions in a power system

Generator protection

• Stator protection Differential

• Restricted earth fault

• Negative sequence current

• Loss of load

• Rotor earth fault

• Over speed

• Over voltage

• Loss of field

• Back –up over current

Differential protection

• CTs are provided at each end of the generator winding which is to beprotected.

• When there is no fault the differential current (I1-I2) through the relayis zero. So the relay will not operate.

• When the fault occurs the balance is disturbed and differential current(I1-I2) flows through the operating coil of the relay causing relayoperation and the trip circuit of the circuit breaker is closed

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 verysmall 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) isconnected in star and the earth relay(ER) is connected between the starpoint and neutral pilot wire.

Restricted or balanced earth fault protection:

• In case of small size generators star point is not available because it ismade inside the generator and grounded through some low resistancethen percentage differential relay for ground fault is provided and isknown as restricted earth fault protection.

• This scheme can be used only for ground faults but not for phase faults

Stator protection:

Stator faults include the following-

• Phase-to-earth faults

• Phase-to-phase faults

• Inter-turn faults

• From these phase faults and inter turn faults are less common ,theseusually develop into an earth faults. This causes-•Arcing to core

• Damage of conductor and insulation

Stator inter-turn fault protection

• Inter-turn fault on the same phase of the stator winding cannot bedetected by transverse differential protection as it does not disturb thebalance between the currents in neutral and high voltage CTs.

• For protection against inter-turn faults the following protectionschemes are used.

• (1) Cross differential protection.

• (2)Residual voltage protection.

Cross differential protection:

• Used in case of hydro- electricgenerator having double windingarmature.

• As shown in figure relay Rcprovidesprotection against phaseto ground and phase to phasefault.

• The relay R1 provides protectionagainst inter- turn faults.

Residual voltage protection:

• These method is used in case of alternators those don’t have parallelstator windings.

• During normal operation VRES=VRN+VBN+VYN=0.

• In case of fault VRES is not zero and this residual voltage operates therelay

Rotor faults

• Faults in the rotor circuit may be either earth faults or between theturns of the field winding .

• Field circuits are normally operated un-earthed. So a single earth faultwill not affect its operation.

• But when a second fault arises then field winding is short circuited andproduce unsymmetrical field system which leads to unbalanced forceson rotor and results in excess pressure and bearing and shaft distortion.

Rotor earth fault protection:

• The rotor earth fault protection is done by “dc injection method or acinjection method”.

• The dc or ac voltage is impressed between the field circuit and groundthrough a sensitive overvoltage relay and current limiting resistor orcapacitor(in case of ac).

• But dc source is generally used as over-current relay in case of dc ismore sensitive than ac.

• A single earth fault in rotor circuit will complete the path and the faultis sensed by the relay.

Loss of excitation protection:

• When the excitation of generator is lost it operate as a Inductiongenerator. It derives excitation from the system and supply power atleading power factor. Which may cause-

• A fallin voltage & solos of synchronism & system instability.

• Over heating of rotor due to induction current on it. A protectionhaving MHO characteristicis used to detect loss of field

Over voltage protection:

• Overvoltage protection is required in case of hydro- electric or gasturbine 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 ofinduction pattern with an IDMT Characteristi

Overload protection:

• Overloading of the machine causes overheating in the stator winding.

• This can be prevented by using over-current relay with time delayadjustment.

• But overheating not only depends on over-current but also the failureof the cooling system in the generator.

• So temperature detector coils such as thermistors or thermocouples areused at various points in stator winding for indication of thetemperature.

Reverse power protection:

• When prime-mover fails machine starts motoring and draws electricalpower from the system and this is known as inverted operation .

• The generator can be protected from inverted operation by usingsingle-element directional power relay(reverse power relay) whichsenses the direction of power flow.

Negative phase sequence protection:

• Unbalance may cause due to single phase fault or unbalanced loadingand 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 overcurrent relay.

Conclusion

• Protective relays are used to detect electrical faults and to alarm,disconnects or shutdown the faulted apparatus to provide personnelsafety and equipment protection.

• A protective relay does not prevent the appearance of faults rathertakes action only after a fault has occurred in the system.

References

• Electrical power system by C.L.Wadhwa

• Electrical power by J.B.Gupta

• www.wikipedia.com