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Cable insulation rating for a delta winding.

Cable insulation rating for a delta winding.

Cable insulation rating for a delta winding.

(OP)
Hello,

If I have a 415V/11kV star/delta transformer what insulation rating would be required for the 11kV cable if I had to choose between a 6.35/11kV and 12.7/22kV cable?

RE: Cable insulation rating for a delta winding.

According to IEC60502-2 [and IEC60183] 4.1 "Rated voltages" systems are divided into three categories:
A-any phase-to-ground fault is cleared within 1minute.
B-the fault clearing time should not exceed 1 h
C-more then 1h
Table 1 recommends for category A and B-cable 12 kV maximum voltage 6 kV [phase-to-neutral] that means 6*sqrt(3)=10.39 kV rated cable voltage.
For category C 8.7 kV [15 kV rated cable].
That means if you are in case A or B an 11 kV rated cable is suitable. In C case you need a minimum 15 kV rated cable.

RE: Cable insulation rating for a delta winding.

(OP)
Thanks.

I am having some trouble understanding you. So Category A is for LG fault, is this also the case for B&C or is this any fault, i.e. L-L? Unfortunately I don't have access to any of the standards you have listed. The cable I was referring to is a 3 core cable. It is a short cable run (400m) from the Delta secondary of a 415V/11kV star:delta that supplies a 415V switchboard via a 11kv/415V step down kiosk transformer.

So if the fault is cleared within 1 hour, then the 6.35/11kV cable is fine to use but if it is greater than 1 hour then 12.7/22kV would be used.

I know the practice for a solidly earthed star 11kV system is for s 6.35/11kV cable to be used but for an impedance earthed star system it would be 11/11kV or 12.7/22kV in my case. If there is a 11kV L-G fault how will this be detected with the delta winding assuming no differential protection just overcurrent / short circuit relay? Assuming that the 11kV cable is earthed at both ends will it be the upstream (415V - star system) protection that will clear the earth fault or will the 11kV relay clear the fault (overcurrent)?


RE: Cable insulation rating for a delta winding.

If 11 kV source is a 415V/11kV star:delta transformer, and there is no separate earthing transformer, then you have an ungrounded system and you need to use a cable rated for full Ø-Ø voltage.

RE: Cable insulation rating for a delta winding.

As jghrist said, the medium voltage side of your transformer is an ungrounded system. You may control the integrity of the phases with respect the Ground [Earth] employing a zig-zag transformer [connected with a NGR-or not] - for instance-[or else].
However, since the cable insulation shield [or screen] is grounded [at least at one end] if one of the live phases is connected to the ground then the ground potential will rise up to this phase potential and the potential difference between other phase conductor and shield will be phase-to-phase voltage [1.73 times more].If the fault will continue not more than 1 hr. [case B] then an insulation rated 6 kV will withstand 11 kV. In case C the insulation could break down.

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