Ground Fault Protection

[timm33333]

I am trying to figure out that which of the two ground fault protection methods is better (either "ZSCT - zero sequence" method or the "residual" method.) I heard that ZSCT is more sensitive and is better for LV. But at MV and HV, the ZSCT saturates and so residual method should be used for MV and HV. Is it correct? Thanks for help!

 

[7anoter4]

See: IEEE Std 242-2001 IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems
Chapter 8 Ground-fault protection 8.3.5 Sensing, relaying, and trip devices. 8.4 Frequently used ground-fault protective schemes

 

[che12345]

Dear Mr. timm33333

Q. " I am trying .... which of the two ground fault protection methods is better (either "ZSCT" or the "residual" method.) I heard that ..."
A1. In general, for [ground fault protection], irrespective of whether LV or MV; the "ZSCT" is better than the "residual" method. It is more sensitive and therefore can detect lower zero-sequence current.
A2. The disadvantage is that the "ZSCT" is an [extra external] item which involved [extra cost and room to house it].
A3. The "residual" method is widely used in LV and MV. For [LV with four CTs] and for [MV with three CTs] which are (already existing) for over-current/short-circuit protection. Therefore, no extra item of "ZSCT" which involved extra cost and room to house it.
a) In LV with four CTs, the "residual" connection is done on the CT wiring. The "residual" current is connected to, very often a separate EF relay.
b) In MV with three CTs, the residual summation is done within the OC protection relay. Therefore an external "ZSCT" which is an [external item and additional cost] is not widely used.
Che Kuan Yau (Singapore)

 

[timm33333]

Thanks for the information. IEEE 242 Section 8.4.1 states that residual ground fault is usually used on MV (not in LV), and that residual function should be used as 51N and it should not be used as 50N. I am not sure what it means, because I have seen 50N a lot in the industry.

Also IEEE 242 section 8.5.2 shows the coordination of the downstream feeder ground fault protection (trips at 0.1 sec) with the upstream main ground fault protection (trips at 0.3 sec). This makes sense. But should we also coordinate the downstream "ground fault" protection with the upstream "phase" protection?