Tertiary winding-Grounding 1

[Prot123]

Hi There,
I've a query regarding use of tertiary winding for station service.
We have an application where tertiary winding needs to be used for station service. Basically tertiary will feed into a station service transformer. My question is do we need to ground the corner of tertiary delta or not? If corner grounded, will ground fault between tertiary and station service cause zero sequence current to flow? or we restrain from grounding the delta tertiary and rely on 59N to detect ground fault between tertiary and station service? Just wanted to know the usual practice and if there are any standards on this.Thank you.

 

[KSS227]

Delta tertiary generally not grounded. Power utilities uses separate BCPU which can sense the OC & EF faults. For this you may need to add ISO, CB, CT, VT in the tertiary before connecting station trafo.

 

[cuky2000]

Tertiary winding on auto-transformer best work if is ungrounded since not only provide power to the station service transformer but also acts as stabilizing winding providing a path for zero-sequence current and 3rd harmonic current in order to stabilize
the neutral point of the fundamental frequency voltages.

Here are some operation suggested to consider:
* Surge arrester rated for voltage LL should be specified to control potential overvoltage.
* In case the station service transformer is disconnected while the transformer is energized, it is recommended to ground one of the corners of the delta.
* If a MV insulated cable is used to connect the station service transformer, it is recommended the cable insulation should be rated for LL.
* An alarm scheme is should be considered to detect a line-to-ground fault and alert a maintenance crew to solve the issue.

Below is an example collected from similar work for one of the US utility that we hope help in your case.

 

[FPelec]

IMHO it does not make much sense to ground a corner of the delta winding, for this application.
If you need to provide a reference to ground for the MV system connected to the tertiary winding, e.g. to prevent ferroresonance issues, you could install a neutral grounding transformer and connect the neutral through an high value resistance (my first guess should be about 1000 Ohm).

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[prc]

Another way is to use three single phase electromagnetic voltage transformers with primary in star with neutral grounded and one secondary in broken delta to detect grounding of any phase.