In a generating station the generator and transformer are the most expensive equipments and hence it is desirable to employ 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.

1. 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

2. Generator Protection

3. Introduction
• In a generating station the generator and transformer are the most
expensive equipments and hence it is desirable to employ 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

4. 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

5. Differential protection
• CTs are provided at each end of the generator winding which is to be
protected.
• When there is no fault the differential current (I1-I2) through the relay
is 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 relay
operation and the trip circuit of the circuit breaker is closed

8. 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.

10. Restricted or balanced earth fault protection:
• In case of small size generators star point is not available because it is
made inside the generator and grounded through some low resistance
then percentage differential relay for ground fault is provided and is
known as restricted earth fault protection.
• This scheme can be used only for ground faults but not for phase faults

12. 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 ,these
usually develop into an earth faults. This causes-
• Arcing to core
• Damage of conductor and insulation

13. 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.

14. Cross differential protection:
• Used in case of hydro- electric
generator having double winding
armature.
• As shown in figure relay Rc
providesprotection against phase
to ground and phase to phase
fault.
• The relay R1 provides protection
against inter- turn faults.

15. Residual voltage protection:
• These method is used in case of alternators those don’t have parallel
stator windings.
• During normal operation VRES=VRN+VBN+VYN=0.
• In case of fault VRES is not zero and this residual voltage operates the
relay

16. Rotor faults
• Faults in the rotor circuit may be either earth faults or between the
turns of the field winding .
• Field circuits are normally operated un-earthed. So a single earth fault
will not affect its operation.
• But when a second fault arises then field winding is short circuited and
produce unsymmetrical field system which leads to unbalanced forces
on rotor and results in excess pressure and bearing and shaft distortion.

17. Rotor earth fault protection:
• The rotor earth fault protection is done by “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.

19. 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 fallin voltage & solos 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

20. 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 Characteristi

21. Overload 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.

22. Reverse power protection:
• When prime-mover fails machine starts motoring and draws electrical
power from the system and this is known as inverted operation .
• The generator can be protected from inverted operation by using
single-element directional power relay(reverse power relay) which
senses the direction of power flow.

23. 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.

25. Conclusion
• Protective relays are used to detect electrical faults and to alarm,
disconnects or shutdown the faulted apparatus to provide personnel
safety and equipment protection.
• A protective relay does not prevent the appearance of faults rather
takes action only after a fault has occurred in the system.

26. References
• Electrical power system by C.L.Wadhwa
• Electrical power by J.B.Gupta
• www.wikipedia.com