Hi Electrical Folks,
I am establishing the worst case ground fault current to perform GPR and Step and Touch calculations for a new 15kV substation. IEEE 80 indicates that LLG faults should be considered in addition to SLG faults. However, my industrial application is low resistance ground. In the case of a SLG fault my currents are quite low (25-800A) and clearing time relatively fast (<0.2s), but of course if a double line to ground fault were to occur and send the bulk of its current through the earth return path then the impact on GPR and absolute fault current would be severe (~18kA of supply current). The substation is located at ~5km from the plants main 15kV switchroom in a rural area and service will consist of 3-wire supply without shield or neutral/ground conductor. For clarity, the new substation consists of a 15kV/0.6kV delta/wye transformer (HRG on secondary neutral), with its MV junction box fed from underground by 3c armoured cable from a terminal pole a distance away from the substation grounds.
I've come to my own initial findings, and wanted to confirm with the experts here. I consider the likelihood of an effective cross-country LLG fault to be so low that it should not be considered for this design application. Instead only SLG fault current should be considered. My reasoning is that if a fault were to develop on one phase at either end of this service, then the low currents and fast response time, controlled transient overvoltage and high system insulation level, would all summate to limit the fault extent to SLG. I just can't imagine a case where a SLG fault would physically evolve into a LLG fault which must flow cross country in my application.
I'd really appreciate comments on this assumption! If any other scenarios come to mind please also suggest those too.
Thanks very much,
Justin
I am establishing the worst case ground fault current to perform GPR and Step and Touch calculations for a new 15kV substation. IEEE 80 indicates that LLG faults should be considered in addition to SLG faults. However, my industrial application is low resistance ground. In the case of a SLG fault my currents are quite low (25-800A) and clearing time relatively fast (<0.2s), but of course if a double line to ground fault were to occur and send the bulk of its current through the earth return path then the impact on GPR and absolute fault current would be severe (~18kA of supply current). The substation is located at ~5km from the plants main 15kV switchroom in a rural area and service will consist of 3-wire supply without shield or neutral/ground conductor. For clarity, the new substation consists of a 15kV/0.6kV delta/wye transformer (HRG on secondary neutral), with its MV junction box fed from underground by 3c armoured cable from a terminal pole a distance away from the substation grounds.
I've come to my own initial findings, and wanted to confirm with the experts here. I consider the likelihood of an effective cross-country LLG fault to be so low that it should not be considered for this design application. Instead only SLG fault current should be considered. My reasoning is that if a fault were to develop on one phase at either end of this service, then the low currents and fast response time, controlled transient overvoltage and high system insulation level, would all summate to limit the fault extent to SLG. I just can't imagine a case where a SLG fault would physically evolve into a LLG fault which must flow cross country in my application.
I'd really appreciate comments on this assumption! If any other scenarios come to mind please also suggest those too.
Thanks very much,
Justin
