Over Fluxing or Over Excitation Protection 24 G.
Over Fluxing relay is used to protect the generator against overheating and the winding insulation failure. Over fluxing protection is a popular protection in transformers, which is mainly used in power transformer.
Principle of Over fluxing relay operation:
All you know about the emf generation in the alternator/transformer’s winding, consider E- emf, Φ- magnetic Flux, f- frequency of the generator. Generally, frequency f is depending upon the synchronous speed of the generator..
The generated emf E is
The flux is directly proportional to EMF and indirectly proportional to frequency. Coming to the generator the EMF is depending on the Excitation of field winding and the frequency is depending on the speed of the alternator. The relay is designed to monitor the ratio of voltage/emf to frequency. Increasing the ratio of v/f is called over fluxing.
Cause of Over fluxing or Over Excitation:
Moderate over fluxing (105-110%) increases core loss resulting in increase of core temperatures due to hysterics & eddy currents loss. If they continue for Long term, which decrease the life of the stator insulation. Severe over fluxing can breakdown inter-laminar insulation followed by rapid local core melting. Over fluxing normally can be caused by over speed of the turbine or over excitation during Off-line condition, and load rejection or AVR mal-functioning during On-line condition.
Overcome of Over fluxing or Over Excitation:
- Reduce the speed of the turbine to rated generator speed, if the speed exceeds the rated speed. If the speed is not in control then ask Speed governor maintenance engineer to reduce speed or trip the turbine
- Decrease the Transformer tapping, which reduce the generator voltage.
- Reduce the generator MVR load.
- If all above is not working means, then trip the grid breaker.
Note: Do not open generator breaker
ANSI Code for Over Excitation relay: 24-G => G- Generator
Relays Under Over Excitation:
- 24 relay opted
- AVR trip
- Mater trip 86M grid relay (Not generator master trip) opted-
- Tripping of 110kV grid breaker.
- Tripping of generator breaker in an extreme condition