Question

Transient overvoltages due to restriking or intermittent ground faults can and do develop substantial overvoltages on ungrounded electrical systems with respect to ground. There have been many documented cases within industry where multiple equipment failures (e.g.-motors) over an entire 480 V system have occurred while trying to find and locate a ground fault. Measured line-to-ground voltages of 1200 V or higher in these instances have been reported. In all instances, the cause has been traced to a low-level intermittent arcing ground fault on an ungrounded system. The mechanism explaining how this occurs is best explained in conjunction with Fig. 4. Under worst case conditions, there is an energy exchange between the system inductance and the shunt capacitance to ground resulting in significant voltage escalation with respect to ground. 2

Answer 1

In underground system the capacitive reactance is more than the inductive reactance or almost equivalent to the inductive reactance of the system. In such case the voltage leads current by 90 degree and when the line to ground fault occurs the circuit breaker break its contact at current zero position but at this instant the voltage is at its peak and hence Arching occurred at the contacts which leads up to the half cycle of the fault current the two healthy phases in this case are acted as return path for fault current. Again at the zero crossing instant of fault current the voltage is at its negative peak but due to inductance and intertia of the system (which gets charged by fault current) aided the voltage peak and result to a higher voltage(almost double or more) in the system which in turn increases fault current. This leads to LLG fault or LLLG fault and causes system failure. The initiative voltage of LG fault is much small leads to low level intermittent Arcing but due to restriking the voltage(due to energy exchange between system inductance and capacitance) leads to very high value and causes severe fault. The figure 4 shows how the voltage level increases after every half cycle.