Parallel Operation

[satnam123]

Hi,
Could you kindly advise me about the thread, which had a very long discussion about the transformer connections by WAROSS and lot of other people contributed.
Thanks

 

[waross]

satnam123
I am sorry to say that I made some statements which were in error. Specifically in regards to running transformers in parallel with different tap settings.
With that warning, the thread is thread238-143892
Why don't you ask a specific question and we will be happy to help.
respectfully

 

[satnam123]

Thanks Waross, Here is my question.
I was looking for your thread as that gives a lot of info about transformers and in regard to errors , people who know only they make mistakes. The idea of this question is to get the concept and I found many things I could not understand so I will ask in stages

Q1 I was looking at a single line diagram and the rating mentioned was 12/15 MVA OA/FA 220/6.6 KV
OLTC, Z=8.5 % and the NGR= 381 ohm 10 A 10 Sec.

When FA comes in to play, Is it automatic?
NGR- what does 10 A and 10 sec stands for.?

Most of the utility transformers in substations are solidly grounded, so when we do a short circuit study, how do we decide that we have to use a resistance grounded transformer as the fault current is higher than certain value.

Q2 At same location I found a switchgear, which had a NGR with an open fused disconnect rated 10 A, 30sec. I have never seen a resistance grounded switchgear.

Thanks

 

[DanDel]

OA/FA refers to the temperature ratings of the transformer. In this case these are ambient cooled liquid insulation (OA) and fan-cooled (FA). The fan cooled rating indicates what is acceptable when the fans are cooling the transformer, assuming that the temperature never increases beyond the 'rise' rating, which is the maximum temperature above ambient that the transformer can operate at any time. The FA rating is only applicable if the fans are running. Typically, the fans have controls which turn them on at a certain temerature set point, so it may or may not be 'automatic', depending on these controls and their setpoints.

The NGR ratings indicate that the resistor can handle 10A for 10 seconds safely (which means that the ground protection should be set to trip significantly before 10 seconds).

Your short circuit study must use the NGR in your transformer model, which limits the maximum GF current, if it is present. Multiple transformers may be modeled, solid or resistance grounded, but the maximum GF current through a NGR will not exceed its rating just because another solidly grounded transformer is present, even if paralleled.

Any NGR, by definition, should be connected between the transformer neutral and the ground point. The location of the NGR is typically at the transformer. Some switchgear systems have high-resistance neutral grounding, which is often located in the switchboard.

 

[waross]

Another point on the OA/FA ratings. Transformers may be rated OA/FA but supplied without fans. A number of years later, when the load has risen to the OA rating the fans will be added to increase the rating.
respectfully

 

[satnam123]

Hi
thanks for earlier responses.

I was looking at a SLG done 10 yrs ago and the guy had
a 4.16 KV/600 V delta/STAR 2000 KVA transformer feeding an MCC and some HVAC auxiliaries.
The guy had the trasnsformer high resistance grounded.
Why would somebody ground a 600 V 3 ph transformer high resistance grounded. I do a lot of commercial building stuff but I have not come across 600 V sec t/f
high resistance grounded.

Thanks

 

[DanDel]

Simply put, LV systems with high-resistance grounding are used where delta ungrounded systems were used years ago, in an effort to keep the machines running with limited outages or nuisance trips.

 

[satnam123]

Thanks Dendel,

Just a quick question on your comment, delta systems are ungrounded these days also. If this was done in old days, how do we counteract this situation these days. I would appreciate a comment.
Thanks

 

[davidbeach]

High resistance grounded wye systems provide the very low ground fault currents typical of capacitively grounded (otherwise known as ungrounded) delta systems without the transient overvoltages associated with arcing faults on capacitively grounded systems.

 

[DanDel]

satnam123, I was suggesting that ungrounded delta systems are not in use to the extent that they were years ago. The HR grounded system today is used instead of the ungrounded delta system for more safety and flexibility.
What exactly is it that you want to counteract?

 

[satnam123]

What I meant was, if we are doing the same thing today like
feeding an MCC and HVAC aux. with a 4.16KV/600 V transformer,we would not ground the sec. with a high resistance. So, the advantrages we could have with a High res. secondary neutral, how do we acheive that now.
Thanks

 

[DanDel]

You basically have two choices: HR grounding, or solid grounding. The advantage of HR grounding is that a single GF will not cause an outage. You cannot achieve that advantage with a solidly grounded system.