Ground Fault Protection

[CuriousElectron]

Greetings,
Why is additional zero-sequence CT's and GFS devices are often recommended for WYE grounded systems? For solidly grounded WYE system, the ground fault current is typically high. Are these additional GFP sensing devices are put in to detect high impedance ground faults, such as arcing faults?

Thanks,
EE

 

[RRaghunath]

In solidly grounded systems, zero-sequence CTs are provided only in overheadline (OHL) feeders.
Earth faults in OHL feeders often tend to be high impedance type, yielding very little current and thus making it difficult to detect the fault.

 

[che12345]

Dear Mr. CuriousElectron
Q1. "...Why is additional zero-sequence CT's and GFS devices are often recommended for WYE grounded systems? For solidly grounded WYE system, the ground fault current is typically high. Are these additional GFP sensing devices are put in to detect high impedance ground faults, such as arcing faults? "
A1. Irrespective of whether it is a LV or MV solidly grounded WYE system, the ground fault current varies from very low to very high level. The [type of ground fault] and there fore the [ground fault current level] is totally "unpredictable" However, it is advisable to limit it at low level in order to stop it from further deterioration, which may result to a more serious three-phase fault.
A2. In LV solidly grounded WYE system, it is very often set at <10%, but not exceeding 100A; whichever is lower; with a time delay say 0.5s.
A3. There are different opinion/practice between IEC and ANSI/NEC.
Che Kuan Yau (Singapore)