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 Post subject: Ground Fault Current Flow Characteristics
PostPosted: Tue Dec 12, 2017 10:25 am 

Joined: Mon Aug 24, 2015 10:24 am
Posts: 28
I had a technical question about ground fault current. In Easypower software, ground fault current appears to flow when I run a phase-to-ground fault simulation, even if there is no ground conductor in the circuit. The following is the rationale I’ve come up with as to a reason that current still flows. Does this seem correct to you or do you have any additional thoughts or references to share?

It is true that the zero-sequence impedance is greatly affected by the presence of a ground wire or not. Without a ground wire, ground current will flow but will be reduced dramatically. It will flow without a ground wire because all conduit has finite impedance to current and will carry some current via the skin effect back to the source. Additionally, the zero-sequence impedance of the phase cables is the combined parallel combination of various return paths and the ground wire is one (not the only) of those paths. The basic parallel paths equation demonstrates that the smallest impedance parallel path will dominate the total ground impedance (i.e. in the case where Z1<<Z2, (Z1 x Z2) / (Z1 + Z2) ~= Z1). The smaller impedance therefore acts as the driver and makes for more ground current flow than one may think.

Specifically in Easypower, the ground fault current equation includes zero sequence impedance using a Z0/Z1 ratio as a multiplier. This Z0/Z1 multiplier is explained in detail on page 4 of 8 of the attached reference by Kaufmann. As Page 5 of the attached PDF explains, the return ground-fault will flow along the skin of the conduit, and will flow whether the conduit is metallic or not. When there is no ground wire, the effects of conduit materials are insignificant, i.e. metallic vs. non-metallic will result in similar Z0 levels. On the other hand, when there is a ground wire, the conduit material does affect the cable impedance. Again this has to do with parallel impedance paths of varying magnitudes.

I reviewed circuit impedance in Easypower with and without a ground conductor. The without ground circuit zero impedance (R0 = 0.50, X0 = 0.34 --> Z0 = 0.60) is much higher than the with ground circuit zero impedance (R0 = 0.11, X0 = 0.07 --> Z0 = 0.13) but nevertheless the without ground circuit has finite impedance, hence current will flow.

Does the above description strike you are accurate, inaccurate, only part of the story??

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 Post subject: Re: Ground Fault Current Flow Characteristics
PostPosted: Wed Dec 13, 2017 4:59 am 
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Location: North Carolina
Your system conductors are insulated to ground even if “ground” means Earth. This is the definition of a capacitor (two conductors separated by an insulator). Particularly as voltage gets above 10 kV, the leakage current gets to be significant (measureable and noticeable) in ungrounded or high resistance systems even though R0 may be high. The ratio is due to the fact that the result is an RC (or usually more like an LC) circuit from EE101 class which has a time constant, BODE diagram, etc., but gets treated as first order for power systems.

So it’s not necessarily leakage but just simple capacitive coupling. In an arcing fault even in high resistance or ungrounded systems you get additional arcs to the enclosure to create a 3 phase arcing fault with about the same fault magnitude as grounded systems.

This is assuming no Earth electrode is in use. The Earth path is inversely proportional to distance so even if there are no ground rods connected to equipment sitting on Earth or concrete pads, eventually it doesn’t matter and after a few miles Earth impedance is much less than a ground wire...hence utility ungrounded or multigrounded systems have very low impedance to the substation after a couple miles.

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 Post subject: Re: Ground Fault Current Flow Characteristics
PostPosted: Wed Dec 13, 2017 10:27 am 

Joined: Mon Aug 24, 2015 10:24 am
Posts: 28
Thanks Paul - always detailed and helpful commentary you provide.

Happy Holidays!

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