Application Notes; Insulated Systems; Figure 208: Current Distribution In An Insulated System With C Phase Fault - GE MiCOM P40 Technical Manual

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APPLICATION NOTES

When insulated systems are used, it is not possible to detect faults using standard earth fault protection. It is

possible to use a residual overvoltage device to achieve this, but even with this method full discrimination is not

possible. Fully discriminative earth fault protection on this type of system can only be achieved by using a SEF

(Sensitive Earth Fault) element. This type of protection detects the resultant imbalance in the system charging

currents that occurs under earth fault conditions. A core balanced CT must be used for this application. This

eliminates the possibility of spill current that may arise from slight mismatches between residually connected line

CTs. It also enables a much lower CT ratio to be applied, thereby allowing the required protection sensitivity to be

more easily achieved.

The following diagram shows an insulated system with a C-phase fault.

The protection elements on the healthy feeder see the charging current imbalance for their own feeder. The

protection element on the faulted feeder, however, sees the charging current from the rest of the system (IH1 and

IH2 in this case). Its own feeder's charging current (IH3) is cancelled out.

With reference to the associated vector diagram, it can be seen that the C-phase to earth fault causes the

voltages on the healthy phases to rise by a factor of √3. The A-phase charging current (Ia1), leads the resultant A

phase voltage by 90°. Likewise, the B-phase charging current leads the resultant Vb by 90°.

P446SV-TM-EN-1

IH1 + IH2 + IH3

IR3 = IH1 + IH2 + IH3 - IH3

IR3 = IH1 + IH2

Chapter 13 - Current Protection Functions