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High Voltage High Resistance Grounded Systems and Measuring VT's in switchgear

Grobs41
We have a 33KV/3.3KV 10MVA Dyn1 transformer with the secondary start point earthed through a high resistance ground unit. This HRG lowers the ground fault on the 3.3KV system to around 5amps.

The transformer is connected via short lengths of cables to a new 3.3KV switchboard incomer. My question relates to the measuring VT's on the incomer. The measuring VT's are star/star connected 3.3KV/110V. Both primary and secondary winding star points on these VT's are solidly grounded to earth.


As the primary winding of the VT is solid grounded, my thoughts are as this provides a parallel earth path to the HRG unit, and any earth fault on the 3.3KV system would flow via the VT due to the low impedance, rather than through the HRG unit. The fuses on the VT would likely operate quickly and the fault path be directed back through the HRG unit.


Should the VT primary winding star point be therefor left unearthed?

  • 0
    Hello, Grobs. We need a little more information.


    You say the transformer is connected to the switchboard by short lengths of cable. Does this include a neutral connection, i.e. from the transformer star point?


    Is the load on the switchboard always going to be balanced (e.g. motors) or is anything supplied that could possibly cause an imbalanced load?


    I presume that the measuring VTs have star points connected just to earth, as you say, and not to any neutral conductor.
  • 0
    How much current do the VTs take - not as much as 5A surely ?
  • 0
    Grobs41:

    Should the VT primary winding star point be therefore left unearthed?

     


    There seems to be a bit of confusion here. The two transformers are performing completely different functions. If you think of it slightly differently and consider the main transformer as a generator and the VT as a load you will view it differently. When you have an earth fault, the current feeding in to the fault is driven by the generator (main transformer) and not by the load (VT) as the instrumentation it is connected to is not generating any power. Whether the VT neutral is solidly earthed, resistance earthed or left unearthed probably makes little difference in the event of a fault. From a safety point of view it is probably better earthed.


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