Earth fault activates underfrequency trip Easergy P3F30

[Macho]

Hello.

The grid I am talking about is 3 phase lane to lane 6kV with isolated ground through peterson coil.

On line to earth fault Easergy P3F30 also detects underfrequency (frequency changes randomly)

Is here a way to avoid underfrequency 81U-1 trip while earth fault is active ?

We also use ROCOF 81 (rate of change of frequency) to block underfrequency 81U-1 trip but it does not always work. Sometimes frequency change does not reach ROCOF 81 value thus not blocking underfrequency protection.

We do not have this problem with other protection devices.

[Chunmei_Yang · Accepted solution]

One solution try to use earth fault protection such as 50n/51N or 59N to extend the under frequency protection time delay to avoid this instantaneous maloperation during earth fault condition. 

[Macho]

We had extended underfrequency protection time delay. No problems since then.