Two Winding Transformer

Three winding transformer is used for supplying power to 12-pulse VFD.

The neutral-to-ground voltage you are measuring is due to the harmonics introduced by the VFD in to the upstream source. This is common with VFDs and the Technical manual should explain you more in detail.

Your neutral is probably unearthed and the insulation point of view, there shouldn't be any concern as the transformer winding neutral is rated to the same level as the winding line terminal ('uniformly insulated').

Right Sir,

But neutral is earthed, still getting voltage.

We have two transformers for PCC, both charged with bus coupler off. Both transformer neutral are earthed (both neutral connected together in PCC). Even one of transformer made off , still voltage persists.

Do you have a circuit like this?

There are many ways to do this. Does it use 12, or 18 diodes?

More importantly, is your AC ground tied to your DC ground? If it is, then you may be shorting out certain diodes. In fact they are probably blown out. That is all it takes depending on where you pick ground on the AC side.

If you take a simple full wave bridge, tie one end of the secondary coil of a transformer to ground and then tie the DC + or - to ground, you are shouting out at least one diode. Once that diode is blown, you start to see unpleasant voltage shifts.

So, exactly what do you have?

In my opinion, the voltage between neutral and grounding it is measured far from transformer and close to VFD. So, the neutral in a three-phase cable could present a built-up voltage- if only one end is grounded- induction from the live conductors.

It may also be due to the fact that you are using a digital voltmeter. They have such a high imput impedence that they can measure voltage in the air (so to speak)

If your neutral truely is connected to earth ground, it should be close to 0 ohms. If that is the case, then a reading of 52 volts would require hundreds if not thousands of amps.

In my opinion, it is not necessary. Let's say a 240 sqr.mm [500 MCM] single-core low voltage cables, laid in a single row R-S-T-N the neutral in 1000 ft. distance at 300 A/phase will get a built-up voltage of 54 V.