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Earth-fault detection and localization in isolated industrial MV network – comparison of directional overcurrent protection and signal injection method

Neutral earthing directly affects systems’ behaviour regarding the maximum level of earth- fault current and overvoltages. Isolated neutral is used where continuity of power supply is essential, which is often the case in industrial power utilities. Although the system can remain in service during a single-phase earth-fault, locating the fault should be done as quickly as possible, without disrupting the operation of the loads. Directional earth-fault protection (67N) can be used to indicate the faulty feeder. An alternative method is low- frequency (LF) current injection combined with sensors for detecting the injected signal, which is a known technique used in LV systems. In this paper, the application of 67N protection and the LF current injection method are tested on the same MV system with inaccessible neutral point. Inductive voltage transformers (IVTs) are used for signal injection. Both methods are discussed, and their limitations are compared based on simulation results obtained using the model of an isolated MV industrial plant. The model includes relay protection devices and IVTs for signal injection.

Isolated network ; Earth-fault ; Directional earth-fault protection ; Low-frequency signal injection

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