Generator protection is different and complex as compared to other equipment and machines due to the reason that it is connected with 3 other systems simultaneously, a DC exciter circuitry for providing DC to field winding, a prime mover for rotating rotor and is synchronized with the grid. Also, generating systems consist of auxiliaries like heat water pumps and exhaust fans, etc. which are supplied power through the generator itself that is why it is never preferred to completely turn off the generator as it would be a time taking task to start the generator again. Also, it is not preferred to have a backup generator for auxiliaries as this would change the short circuit rating.

When it is required to cut the generator with the grid due to maintenance or fault, then the following steps should be taken:

  • Slowing boiler firing
  • Fuel supply is minimized
  • Running the generator at baseload

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Types of Sub

faults & causes
faults & causes

Abnormal conditions in generator:

It is further classified into two parts:

  • Mechanical
  • Electrical​


Loss of prime mover

Problem: In this condition, the generator tries to compensate its losses causing a lot of heat.
The Machine starts drawing small active power from the grid but continues to supply reactive power.

​​Solution: A reverse power relay can be used, It consists of 2 coils, a voltage coil (PT) and a current coil (CT), both are energized by the same phase. They both induce magnetic fields in an Aluminum disk which under normal operation is restricted by stoppers but as the power flows in opposite direction, the Aluminum disk also rotates in the reverse direction and a trip mechanism gets activated.​

If you haven’t checked out our previous blog on the Synchronous Condenser, then please click here.

Over speeding

Problem​: Over speeding results in an increase of frictional losses.

Solution: Governor which can vary the speed by changing frequency and fuel supply is used to overcome this scenario.


Unbalance loading

Problem​: An unbalance loading results in negative sequence current. The negative sequence current rotates in the opposite direction as compared to the rotor, this results in the induction of current in the core having a double frequency. Due to which eddy current and hysteresis losses are increased resulting in a high amount of heating in the generator. (Rotor overheating)

Solution: A negative sequence relay can be used, It consists of 4 impedances and a relay operating coil between them, 3 CTs (1 from each phase) feeds current to these impedances, all impedances are chosen such that in case of negative sequence current, input from 2 CTs cancel out each other and remaining CT input energizes the relay coil and initiates tripping mechanism.

Loss of excitation

​Problem​: There are two scenarios which may occur:

1. The grid can compensate Q, in this case there would be large heating in rotor and stator.

2. If the grid fails to provide Q, then there would be a chance of blackout.

​Solution: MHO relay used to monitor field current to prevent loss of excitation.

An alarm can also be used here, and an effort can be made to restore the excitation.

Faults in generator:

Stator faults:

  1. Phase & earth faults
  2. Longitudinal differential protection scheme​
  3. Differential protection scheme using a balance resistor
  4. Modified differential protection
  5. Balanced earth fault protection
  6. Inter turn faults

To learn the detail of each faults with example and diagrams, click on “Stator Faults”.

Rotor faults:

Fig 6: DC Injection Method of Rotor Earth Fault Protection in Alternator
Fig 6: DC Injection Method of Rotor Earth Fault Protection in Alternator
Fig 6: DC Injection Method of Rotor Earth Fault Protection in Alternator

In DC injection method, a DC voltage relay is connected with the positive terminal of a DC exciter, the negative terminal of the relay is connected with an external DC power source which is usually fed by an auxiliary transformer through bridge rectifier circuitry, the positive terminal of the bridge rectifier is grounded. In the case of rotor fault, the positive potential of the external DC source appears across the terminals of relay and the protective relay operates.

You may check our previous blog focusing on fault analysis in power systems


This concludes our topic of Generator protection; we hope our blog made it easier for you to understand this topic. Feel free to suggest or ask us any questions you might have in the comments section.

Hiring a professional electrical engineer to conduct an Arc Flash Analysis and Short Circuit Study is a great way to ensure the safety of your facility and workers against unwanted incidents.

AllumiaX, LLC is one of the leading providers of Power System Studies in the northwest. Our matchless services and expertise focus on providing adequate analysis on Arc Flash, Transient Stability, Load Flow, Snubber Circuit, Short Circuit, Coordination, Ground Grid, and Power Quality.

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