Transformers parallel work 1

[Batu]

We have a 60/80 MVA onan/ Onaf 154 / 34.5 kV Step Up transformer with Dyn1 vector group and 8.22% uk.
Our other transformer will be a 100 MVA onan 154 / 34.5 kV
step up YNd1 8.22% uk.

My problem is the vector groups. I cannot be sure if they can work in parallel. Some says it's ok some says it's not..What I know is they should be the same..

I know this is the place where I can get the real answer..

Thank you in advance..
Batu

 

[digitrex]

The first transformer should be originally used as a step-down (not step-up).

Phase angle shift from HV to LV is OK. I think the two transformers can be paralleled but you have to consider the following "what if's":

1. If the new YNd1 transformer trips, will the other transformer(80MVA) get overloaded?

2. If the new YNd1 transformer trips, will you lose the 154kV system's neutral grounding?

3. Are you going to maintain the 34.5kV neutral of Dyn1 transformer grounded? How does it influence your 34.5kV system neutral grounding if the Dyn1 transformer trips?

 

[digitrex]

Another "What if":

4. What if the transformer tap changer steps are in different value?

 

[busbar]

It may be a good idea to do lowside phasing-voltmeter checks before closing the tie. Else, a “dynamic’ test of your relaying can be needlessly disconcerting and stressful.

As implied by others, with the assortment of winding types, [zero-sequence] protective-relaying response will vary depending on switching.

If there are OLTCs on the two units, do they have a parallel-operation feature? Excessive loop-var circulation my occur if taps are not adjusted to limit this.

Consider also the fault-duty increase with a closed tie.

 

[Batu]

First of all thanks for the responses..
Digitrex you are right. I find out that both of the transformers are step-down..

The parallel work will occur only during transferring ( For maintenance or etc. ) which will last 3 minutes approx...

The other problem is since the load curves are different uk may not be the same during the transferring too..


Sorry for taking your extra time..
Batu

 

[jbartos]

Suggestion: The paralleled transformer vector groups should be the identical to establish the transformer compatibility for the paralleling. The protective relaying should also be compatible. The system grounding is normally implemented on one of the parallel transformer at a time. If the transformer nameplate data are forwarded to the transformer tech support, then the manufacturer recommendation might be followed. The transformer paralleling should be avoided because of potential hazard in case of the single failure. There will always be some circulating currents flowing, which may be posing a problem in time if these circulating currents increase.