Grounding Cable Type

[ctpt]

A 6.6kV system is fed by 6.6kV generators, the generator neutrals are ungrounded. However, the 6.6kV bus is grounded by a grounding transfomer. The grounding transformer is primary star and secondary open delta with a resistor connected at the open delta to limit the gound fault current to 30A. To get the ground fault current flowing obviously the star primary has to be grounded. My question is whether the neutral cable from grounding transformer primary star point to ground should be a normal gounding cable or it has to be a power cable with insulation suitable for primary voltage ie. 6.6kV.

 

[jghrist]

If the cable is connected to ground, then it can be a normal grounding cable, or bare copper.

 

[RRaghunath]

With resistor across open delta, the 6.6kV system may not qualify as 'effectively grounded'.
If true (to be verified through calculation as per IEEE), then the neutral potential could rise close to phase potential during single phase-to-ground fault.
It means that the grounding cable needs to be 6.6kV insulated cable (may be same specification as the phase cable).

 

[davidbeach]

I think the question is about the cable between wye-point and ground on the wye side of the transformer. Since the cable is connected to ground, there should not be a need for more insulation than normal 600V insulation.

There is also no information in the post to indicate what voltage the delta is wound for, so it isn't possible to answer a question regarding the cable connecting the resistor to the broken delta (NOT open delta).

 

[ctpt]

Yes, I was talking about the cable from the primary wye side neutral to ground. The 30A ground fault current value through this cable is driven by the resistor put at the secondary delta corner. One would assume that the voltage drop across the resistor during fault would keep the primary neutral point to ground potential. But I am not sure and it seems there are mixed views in the forum too.
By the way, the secondary voltage is 120V.

 

[davidbeach]

If the single winding voltage on the secondary of the grounding transformer is 120V, you need to expect up to 360V across the resistor.

Also, with this grounding method, during a ground fault on the high side, you will be seeing as much as full phase-to-phase voltage from the unfaulted phases to ground.