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GFI on ungrounded MV system

GFI on ungrounded MV system

GFI on ungrounded MV system

(OP)
I'm currently reviewing a submittal that indicates residual-type ground fault detection to be provided for an existing ungrounded 12,470-volt system that currently has no ground fault indication.  I'm rejecting this because I do not believe this can work.

My first preference would be detection of zero-sequence voltage.  Unfortunately, it is probably not possible to install PT's anywhere on this system due to space restraints.

Therefore, I'm considering zero-sequence current detection.  I'm still not convinced this will work, but have not been able to find any real documentation either way.

Any thoughts?

RE: GFI on ungrounded MV system

I agree with you that trying to monitor zero sequence current probably isn't going to work.  There just won't be enough ground fault current.  

RE: GFI on ungrounded MV system


pb--you have an intersting but not unsolvable dilemma.  Neither zero-sequence overvoltage or overcurrent relaying is directional, so it can be kinda’ tricky what should trip [or alarm.]  A limitation of residual-current sensing in ungrounded systems is the lack of reliable, predictable fault current.  

Voltage-based fault detection is well understood and accepted in petrochem operations.  It is most often applied on 5kV-class systems, but it has been studied for 15kV-class systems and would seem a phenominal improvement for modification of ungrounded MV systems  IIRC, JR Dunki-Jacobs studied it in the 1970s; published in IEEE/IAS transactions.

There are an inherent set of three phase-to-ground-connected capacitors in the form of cable shields and winding-to-core insulation.  To avoid damage, capacitance has to be damped, an it can be done most cost-effectively with effectively parallel resistive means.  A starting point is ~2-5 amperes per MVA of serving transformer capacity.  

One approach is to install transformers {rated for continuous phase-to-phase voltage} connected in a grounded-wye/broken-delta configuration with a damping resistor on the “break” in the delta-wired PT secondaries.  

Relay is usually a '59G' device that responds to relatively low fundamental voltage [id est, ~0.02-0.15 p-u] but is third-harmonic voltage restrained.  

An alternaitive for a wye-source arrangement may be an appropriately rated wyepoint-to-ground resistor, directly connected, or via a 1ø transformer.  
  

RE: GFI on ungrounded MV system

It is not generally feasible to utilize zero sequence current detection for ground fault relaying on ungrounded systems.  However, there will be some current flow during a ground fault due to the distributed capacitance of the system.  Schweitzer Engineering Labs (SEL) has an interesting approach to this problem that is discussed in a very recent application guide on their website.  Go to www.selinc.com and look for Application Guide AG2002-06.  I don't think this idea is really new, but I haven't seen it for awhile and it's much more feasible with the sensitivity of the newer digital relays than it was with the old electromechanical relays.

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