Sensitive earth fault protection - medium voltage 2

[RalphChristie]

I've got a protective issue question, I'll appreciate it if anyone can help me or point me in some direction.

With a high impedance grounding system (secondary of trsf grounded through resistors) in a medium voltage system, on what kind of earth fault protection does IEC insist? (I'm looking for any IEC-standard, but information from an IEEE-viewpoint will also be fine) Is earth fault protection on the residual connection (3 CT's) enough, or is sensitive earth fault protection (Balancing core / ring CT) essential? The question is related to feeders of overhead lines, permanent cables and trailing cables.

Many thanks

Regards




Ralph

 

[dpc]

This depends on the resistance used. For a true high-resistance grounding system, line-to-ground fault current will be only a few amps. Traditionally, voltage relays have been used instead of current relays, assuming the grounding is done via a distribution transformer. The newer digital relays can provide a very sensitive current detection scheme as well.

For a low-resistance grounding system (50 A to 400 A), standard ground overcurrent detection methods can be used.

I am speaking from a practical application viewpoint for US installations. I know nothing about IEC requirements.

 

[RRaghunath]

High impedance earthed systems typically have fault currents limited to <10A. However, I know high impedance installations where the design max. fault currents are as high as 25A.

With regard to protection, if the current are low, current based protection is not reliable and hence residual overvoltage is sensed and annunciated.

In case the currents are as high as say 25A, core balance CTs can be included in each of the outgoing feeders and trip of the respective feeder issued on sensing the earth fault.

Trust the above is helpful.

 

[gordonl]

I haven't tried to protect a high impedance grounded system but it could have challenges. Ie: discriminating between charging current during faults and ground fault current, at MV this could become a factor depending on the size of the system. Selectivity could also be a challenge.