HRG of Station Service Transformer supplying MCC? 2

[CuriousElectron]

Greetings,

I have a question - suppose I have a station service transformer, 15kV delta connected on the primary and 480/277V Y connected on the secondary, supplying an MCC. The MCC is a 3 phase, 4 Wire system, but supplies only three phase, 3 wire loads. From design perspective, would there still be a benefit of high resistance grounding the neutral point of the Y-connected secondary of the transformer, if the service continuity is desired? My thought is that for 3 phase, 3 wire load, if one phase faults to ground at the load side, EGC would carry the ground fault current back to the switchgear ground bus and from MCC the fault current would flow back into the transformer.

I understand there are also other considerations, like safety and arc flash factors, but it may make sense to have delta connected secondary if service continuity is desired like in this scenario.

Am I on the right path with my reasoning?

Thank you,
EE

 

[dpc]

The ground fault current has to flow to the transformer neutral whether solidly grounded or resistance grounded. The HRG allows uninterrupted operation if one phase is grounded. it also reduces risk of arcing faults and reduces risk of through fault damage to the transformer. If you are trying to compare HRG wye system to ungrounded delta, then without question the HRG system is preferable since it controls overvoltage transients during ground faults.

But you also said the MCC is a 4-wire system. In an HRG system, there can be no line-neutral loads. Ground fault current will flow back to transformer neutral by whatever paths are available. Most of it should flow in the equipment grounding conductor.

An HRG system requires that qualified staff be available to trace down any ground faults - just like ungrounded delta systems.

Cheers,

Dave

 

[Kiribanda]

From outside you may see that an HRG system is similar to a DELTA ungrounded system.
But operational wise it is not so. Operating an ungrounded system invites various
problems such as transient over voltages (not very severe since the voltage is only 480V, but still...)
detrimental to the cables. Therefore, from operation point, an HRG system is much much better
than an ungrounded system (DELTA).

 

[jraef]

If there are going to be VFDs anywhere on the MCC or fed from it, you do not want an HRG or Delta system, all VFDs are designed around expecting a solidly grounded Wye system. Some brands offer ways of adapting them for HRG or Delta, but you lose some of the protections that are built in.


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