We use high-resistance (10,000 Ohms) grouding on 6kV system. Grounding is made with EVT.
Upon an earth fault, fault current through ZCT is supplied from the capacitance of the system (mainly cables) plus 6.6kV/1.73/10000=0.38A. With this arrangement, ZCT trip point is set so low, 0.2A to 1A...
We typically do not ground the control transformer secondary. Expectation is that a ground fault does not lead to stoppage of a motor. We, however, need to make sure that the secondary control wiring insulation resistance is high enough. During maintenance period, we check the insulation...