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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File comment: Kaufmann reference on grounding.
GE-More_Specific_Eqpt_Gnd.pdf [3.84 MiB]
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