What is high or over frequency shutdown?

Over frequency is a condition where the frequency of the electricity generated by the generator is too high. The level at which the frequency is determined to be too high will be preset in the generator control panel.

Some control panels will also have the "over frequency runaway" set. This is simply a second higher level  tripping point, which indicates the generator has gone past the standard generator trip point.

Why is the generator is shutting down on high frequency?

On a generator, the diesel engine drives the AC alternator. The frequency of the electrical output is proportional to the speed of the diesel engine. The speed of the engine is maintained by a governor, but as load is applied to the generator and remove the load, the speed will also change. Sharp increases and decreases will cause the largest changes in speed. In many instances, if you are removing load from a generator beyond its suggested capacity, then the over frequency alarm will display. If you apply load to quickly, you will generate an under-frequency alarm.

How to stop the over frequency generator alarm?

Firstly, run the generator with no load. If the frequency is stable, it is unlikely to be a generator fault. Apply / remove the load from the generator, if the alarm continues you have two options:

1. Reduce the load removed from them generator, split it into smaller steps.
2. Change the over frequency protections in the control panel to a higher level.
3. Increase the size of the generator to cope with the application / removal of load.

Note: If you change the under / over frequency protections in the control panel, you may expose your equipment to an electrical frequency for which it was not design and damage may occur.