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Installing wye transformer in a high-leg delta circuit.

Luca Toniolo

Jan 30, 2024
Looking for some guidance on proper grounding setup for a machine.
The building main circuit has a high-leg delta transformer, which supplies 3 phases ABC at 240V and a center-tap ground which reads 120v with phase A and C, and 208v to B
Machine needs 400v a neutral and a ground.
I have a multi-tap 3 phase transformer wye type with neutral N, I have connected only the three phases from the main, and I can get proper voltage out, I get 400v between the three phases which I will call RST, and I get proper voltages from N to each RST, all 220V.
The first grounding approach I tried, is grounding the machine to the transofrmer, and then route it to the main ground (the one that reads 208v to B), everything seems to work, but I have a suspicioud 70v from the ground to the neutral, which I read should not be present, what could I do to remove this voltage?
What exact transformer do you have? Nameplate diagrams are good.

Assuming it's isolating and wye secondary, you need to ground the neutral on the transformer secondary. If this assumption is wrong, you should *not* do that.
My guess is its a wye autotransformer, and you can’t bond the two (delta wye), neutrals/ grounds. They are differing potentials. Like he is detecting.
You need a typical delta-wye transformer. That way the wye secondary is a separate source, and you can ground the wye secondary, because it is isolated from the primary.

If you have high-leg power, there is no way to get a grounded neutral that is balanced to all three phase wires unless there is an isolation between the source and load. A standard transformer with primary and secondary provides that.

An "autotransformer" has no isolation, the wires are continuous from input to output. It can only provide a voltage change, but no true isolation. So the neutral point of the resulting 3 phase cannot be grounded.

Gaochun EDZ seems to be in Jiangsu china. So I am surprised you have a problem. The 230V 3 phase (line-to-neutral) should be 400V line-to-line. But it seems that your power is different from the usual.