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Voltage Transformer Primary Connection Details

Voltage Transformer Primary Connection Details

Voltage Transformer Primary Connection Details

(OP)
Can anyone provide or point me toward installation guidelines describing the details of making up a grounded-WYE primary connection using three voltage transformers (in this case 72.5kV primary)? In particular I think the three VTs should be connected together with conductor, and then the combination be connected to ground at one point. This as opposed to grounding each VT, and relying on ground to connect them together. I'm looking for confirmation of this, and guidelines for conductor size. Thanks.

RE: Voltage Transformer Primary Connection Details

Personally I would keep the connection from the VT to the grounding bar on the structure as short and direct as possible, and connect each VT case direct to the grounding bar. The conductor size between the VT and the grounding bar would be governed by the maximum earth fault current and fault duration. I would expect the main ground connections from each of the VT structures to be connected solidly together and to the main substation grounding grid.
Regards
Marmite

RE: Voltage Transformer Primary Connection Details

Assuming outdoor single-bushing 72.5 kV VTs:

If they are mounted on a single steel structure, then I'd run copper wire horizontally along the structure connect the three phases, then run copper down the structure (both legs if the structure has two legs) to the ground grid.  If the VTs are on single phase columns, then ground each individually to the ground grid.

I would use whatever the standard ground stinger size is for the substation, which is determined by the available fault current, but is often #2/0 or #4/0 copper minimum for durability even if not needed for fault current.

I don't think there is any standard that says you even need to run copper wire for grounding the VTs.  Using the steel structure as the grounding conductor is technically acceptable.  The VT bases are bolted to the steel and the steel is grounded.
 

RE: Voltage Transformer Primary Connection Details

My $0.02.  Consider testing requirements for the VT.  One has to be able to isolate H0, in order to test the primary insulation.  Having some kind of removable link makes this possible.

RE: Voltage Transformer Primary Connection Details

If you're speaking to equipment ground (which on single-bushing VTs is generally also the H2 ground), then it doesn't really matter...either running ground to each unit separately or running a common ground conductor and tying that to ground will work.

If you're speaking to the grounding of the secondary windings, they should be ground at one and only one point. So if the secondary windings are connected in a grounded-wye configuration, all of the X3 terminals (assuming IEEE/CSA) should be connected together and then tied to ground.

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