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34.5kV wye-grounded / 13.8 wye-ungrounded 3 phase Transformer

34.5kV wye-grounded / 13.8 wye-ungrounded 3 phase Transformer

34.5kV wye-grounded / 13.8 wye-ungrounded 3 phase Transformer

(OP)
I work for a utility which uses wye wye solidly grounded distribution transformers for 99.9% of our customers, though we have one customer who has requested to have their parallel 10MVA transformers with the secondaries ungrounded. We will be installing 34.5kV/13.8kV 2 - 10MVA transformers. I was wondering what could be the potential risks with this configuration, also if you have experience with this configuration could you give me some pointers on any issues you had/have with these transformers? Thanks in advance for your time and assistance.

RE: 34.5kV wye-grounded / 13.8 wye-ungrounded 3 phase Transformer

We have requests for this type of connection from customers who are seeking to avoid transferring the utility ground potential rise (GPR) into their site. The incoming connection arrives in utility substation with its own earth grid, and no connections to the earth grid of the customer's actual site (which includes a separate earth grid). The utility substation transformer is unearthed on the transformer secondary, with an earthing transformer installed in customer's site with a resistance earthing system to limit the GPR during an earth fault on the LV side of the utility transformer. Any earth faults on the incoming HV side of the utility transformer will result in a GPR on the utility substation earth grid, but it will not be transferred onto the customer earth grid. Is this the situation that you are faced with?

I would either make sure that there is not a situation where you might end up with no earth on the transformer secondary (e.g., downstream circuit breaker open, removing the earthing transformer from the transformer secondary system), or that you have some other method of detecting an earth fault (e.g., 3V0 protection). Care needs to be taken with regards to any connections - including LV, protection, control and communications circuits - into the customer site.

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