Generator field ground protection alarm and trip setpoints

[Doug W.]

For resistance based field ground protection, most literature says to alarm at 20 Kohms and trip at 5 Kohms. Does anyone know of a basis or any literature that may help with how these values were arrived at?

 

[pwrengrds]

The older Basler and ABB field ground fault relays were fixed at about 5k ohms.

Typically I have not seen them set up to trip, just alarm. It's an ungrounded system, it's not a good idea to operate for extended periods grounded, it won't cause damage, a second separate ground is what causes the damage.

It's more of a warning that it's time to clean the brush rigging. I usually set them up a high alarm at 20k and low alarm at 5k, gives some time to get the maintenance done before causing issues.

David

 

[Doug W.]

Thanx David for the response. That is what I have typically seen recommended as well, the 20K and 5K values. I was just trying to find any basis for how these values were arrived at? It was probably suggested way back when by someone and was just carried through in subsequent documentations over time. I'm not saying the values do not make sense or may not be perfectly fine in most cases, but it would be nice to have some history to go on.