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Generator single line diagram

Generator Protections are broadly classified into three types: Class A, B and C. Class A covers all electrical protections for faults within the generating unit in which generator field breaker, generator breaker and turbine should be tripped. Clas B covers all mechanical protections of the turbine in which turbine will be tripped first and following this generator will trip on reverse power / low forward power protections.

Generator protection functions and test methods
Generator protection functions and test methods

Class C covers electrical protection for faults in the system in which generator will be unloaded by tripping of generator breaker only. The unit will come to house load operation and the UAT will be in service.

Various protections of this class are:

  1. 220 KV (HV side of Generator Transformer) busbar protections.
  2. Generator Transformer HV side breaker pole discrepancy.
  3. Generator negative phase sequence protection
  4. Generator Transformer over current / Earth fault protection
  5. Reverse power protection without turbine trip

Generator protection functions:

I – For insulations failures

  1. Differential
  2. Inter-turn fault
  3. Stator Earth Fault (95% & 100%)
  4. Rotor Earth fault (2 Stage)

II – For abnormal running conditions

  1. Loss of excitation (field failure)
  2. Unbalanced loading (negative phase sequence)
  3. Pole sleeping
  4. Overfrequency / Overspeed
  5. Overvoltage
  6. Reverse/Forward power
  7. Impedance/Over current back-up protection, etc…

III – For Generator transformer protections

  1. Differential protection
  2. Bias test
  3. 2nd harmonics restrained.
  4. REF protection
Generator protection single line diagram
Generator protection single line diagram

I – For insulation failures protection

1. Generator differential protection (87 G)

It is unit type protection, covering the stator winding for phase to phase faults due to breakdown of insulation between stator phase windings. This relay is not sensitive for single line to earth faults as the earth fault current is limited due to the high neutral earthing resistance.

If CTs of identical ratios are used on neutral and line side of generator, an operating current setting of 20% it can be adopted. It is instantaneous in operation and it trips the generator breaker (Class – A) to eliminate the system in – feed to the fault along with field breaker and turbines.

For all machines of ratings 10 MVA and above, this protection shall be provided.


Generator differential protection (87 G)
Generator differential protection (87 G)


  • Pickup Value of Differential Current: 0.10 I/InO
  • T I-DIFF> Time Delay: 0.00 sec
  • Pickup Value of High Set Trip: 2.0 I/InO
  • T I-DIFF>> Time Delay: 0.00 sec

2. Interturn fault protection of the stator winding (64GIT)

Formerly, this type of protection was considered unnecessary because breakdown of insulation between points on the same phase winding, contained in the same slot, and between which a potential difference exists, will very rapidly change into an earth fault, and will be detective by either the differential protections or the stator earth fault protection.

An exception is the generator designed to produce a relatively high voltage in comparison to its output and which therefore contains a large number of conductors per slot.

With the size and voltage output of generators increasing, this form of protection is becoming essential for all generating units.


Interturn fault protection of the stator winding (64GIT)
Interturn fault protection of the stator winding (64GIT)

The recommended relay is the high impedance relay having a setting range of 10-40% of rated current.


  • L-E Voltage of Faulted Phase Uph Min 110 V
  • L-E Voltage of Unfaulted Phase Uph Max 110 V
  • Uen> Earth Displacement Voltage 10 V
  • T-DELAY TRIP Uen/3U0 0.50 sec
Title:Generator protection functions and test methods – VASUMURUGAN. R.
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Generator protection functions and test methods
Generator protection functions and test methods

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  1. Mahmoud Faisal Mohammad
    Aug 17, 2022

    very helpful to understand, thanks

  2. Rupa v dave
    Apr 01, 2020

    I won’t to idea hi voltage protection &neutral can be fase and 440 volt and can be damage of my PCB and compressor pl. guide me

  3. muzaffar khan
    Mar 10, 2020

    great help in understanding basics…

  4. Dadar-ool
    Mar 10, 2020

    I am writing a review on the topic “Methods for identifying K3 and providing automatic reclosure of cable overhead lines.” I need articles, textbooks, and, in general, literature. I ask you to help!

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