VT Primary Earthing

Please note that the VT is connected in parallel with the line (i.e. each Phase/Ground).

Regards,

Obviously to reduce the insulation cost. If the VT Primary is not earthed, then the primary winding has to be insulated for the ful line-to-line volatge (11kV in this case). But, if the VT Primary is earthed - AND, WHEN IT IS CONNECTED TO AN "EARTHED SOURCE" - then the primary winding insulation can be rated only for the phase voltage (approx. 6.35kV, in this case).The difference in insulation cost can be considerable.

But, please note that to have this advantage, the 11kV SYstem MUST be Earthed. In an unearthed system this advantage is lost.

You are right regarding the cost difference in the insulation; however, this difference is not large as you expect.

The VT primary winding is connected directly to the power circuit either between two phases or between one phase and ground. This depends upon the rating of the transformer and requirements of the application.

Voltage transformers are usually connected phase to ground.

During a disturbance in a three-phase network system, the voltage across the voltage transformer may sometimes be increased (due to Ferroresonance) above the nominal rated voltage. Using a damping resistor or VT guard in the residual voltage secondary (Open Delta-winding), can considerably reduce the risk for ferroresonance.

There is an additional factor that can reduce or totally eliminate the risk of ferroresonance which is the over-voltage factor "Vf".

IEC specifies the voltage factors as: 1.9 for systems not being solidly earthed & 1.5 for systems with solidly earthed neutral.

The duration is specified to be 30 seconds if automatic fault tripping is used during earth faults, in other cases 8 hours.

Regards,

I am surprised at your answer. Ferro-resonance will be there even when VT Primary is earthed. Earthing or not of the VT Primary does not help avoid ferro-resonance. Also, over-voltage factor is more related to power frequency over-voltages during earth-fault conditions. Again, this has no bearing on ferro-resonance. I think you have much too deviated from the question.

On the contrary, my previous comment was in line with the same context; I think you did not get the point. In my previous post I did not mention (unearthed VT); where I have stated that the VTs are normally connected between Phase and ground.

The rest of the post illustrates a case that might happen to a VT due to ferroresonance phenomenon, in case it has been connected in Phase/ground scheme in unearthed neutral network, which is more susceptible to ferroresonance. This phenomenon would have an impact on the VT, as the core will be overheated and cause failure in the insulation. Which could be mitigated by applying a damping resistor across the Open-Delta winding of the VT, and/or using a VT guard system.

The voltage factor is determined by the maximum operating voltage which, in turn, is dependent on the system earthing conditions. Where the multiplying factor to be applied to the rated primary voltage, to determine the maximum voltage at which a transformer must comply with the relevant thermal requirements for a specified time and with the relevant accuracy requirements.

As a conclusion, the voltage transformers operating in unearthed networks must always be protected against ferroresonance.

Hope that the above would clear the surprise and misunderstanding...

Regards,