One phase current less during enrgization 3

[sasalu]

Hi All,

I have attached screen shot of Transformer inrush current during energization, we suspect this is due to some problem in the primary circuit and checking the possibilities. Sometimes transformer gets energized without any unbalance in current among phases. Due to unbalance in current Transformer HV SEF Protection is operating after 1 sec delay. My query is because of this unbalance transformer phase current inrush current last for more than 1 sec and to avoid nuisance tripping during energization can we enable second harmonic blocking in the SEF function to block the tripping.

Channel details:
1-A Phase
2-B Phase
3-C Phase
4-N Measured Trf Neutral current.

 

[Kiribanda]

Your SEF protection should be wired as a residual scheme. Therefore, when only two phases are energized,
it is normal that SEF protection works because of zero seq. current flowing in the residual connection.
In my opinion SEF has nothing to do with second harmonic blocking. Therefore, you should find why only
two phases were energized and find out the root cause.

 

[beyond86]

can we enable second harmonic blocking in the SEF function to block the tripping.

I wouldn't do that. It's obviously that you have problems in primary side. Check transformer (resistance of phase especially), check primary connection of phase C.

 

[sasalu]

Hi Kribanada and beyond86,
Thanks for your feedback we are suspecting problem in the isolator side of the transformer bay no problem in the CT circuit, Because after trials of isolator open/close problem is shifting to other phase which seeing less current before it was not. In the offline we checked all isolator contacts and are making properly but during breaker closing operation something happening which we not able to find out. Until finding the problem to avoid multiple tripping during energization can we increase the time delay or some sort of blocking the protection. During energization one phase seeing inrush current of 64.0 A.
During the load condition thermoscanning done for all isolator contacts and found to be ok. My another query is phase seeing less current is getting energized or not.

 

[RRaghunath]

Why do you need a sensitive earth fault protection for transformer primary??
Transformer inrush current is known to have 3rd harmonic component and that is what seems to be causing SEF pickup. The inrush is known to last a few seconds sometimes.
So, review whether you need a sensitive pick up or it can be increased?
If you must keep the Sensitive pickup, I agree blocking the protection based on the presence of 2nd harmonic component can help. This is widely used for preventing maloperation of transformer differential protection during energisation.

 

[sushilksk]

Does low inrush current is oberved always in C-phase, or it is changing randomly with every attempt? Is there any controlled switching device?

 

[davidbeach]

It's more useful to look at the inrush with all three phase currents on one trace rather than as three separate traces. But that looks like pretty normal inrush. Inrush is a pseudo random event and every inrush will be different from every other, but they will all exhibit those kinds of waveforms. Which phase has what magnitude will vary.

Only pseudo random in that if you know point on wave of the interruption, remnant flux, and point on wave of the reenergization you can get very close in predicting what the inrush will look like. Multiple transformers deenergized simultaneously and then reenergized simultaneously will produce an inrush current that looks that from a single transformer of the combined size.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Kiribanda]

I donot agree to increase the time delay of SEF (STAND BY EARTH FAULT) protection
because your transformer is energized with two phases. Therefore, my advice is first
resolve the bkr/ isolator contact issue and make sure that the transformer is
energized with a balanced three phase supply.

 

[davidbeach]

It looks like a perfectly normal close. One phase being much less then the others is a typical result.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[beyond86]

I don't agree that it looks like normal. When we energize transformers, vector sum of phase current must be about zero.

 

[davidbeach]

That’s a nice fantasy. Look at actual events. Always weird, always some measured imbalance. Yeah, perhaps they should be balanced. But don’t hold your breath looking for one that actually is. Normal close, move on.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[beyond86]

Record of energizing of transformer 110\10 with solid grounded neutral at 110 side.

 

[EnzoAus]

It looks like a perfectly normal close. One phase being much less then the others is a typical result.

David, in suggesting the close looks normal, are you assuming CH4 is C phase or the neutral as per the OP's post? His screenshots show negligible C phase current after a full second whilst the neutral seems to approximate the sum of A and B phase.

 

[davidbeach]

One phase low is not uncommon. Neutral is the sum of the three in this case. The point is that energization is not a balanced event by any means.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[davidbeach]

And what beyond86 posted on 26 Jun 22 05:19 isn't really a transformer energization event, it's a load energization event. There may very well be a transformer between the measuring point and the load, but the load dominates the resulting waveform. Try it again with the low-side disconnect open and you'll get a very different picture.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[thermionic1]

These are definitely just energizing events here.

500MVA 345/138 Auto w/ Tert Inrush - Synchronous Close breaker, tuning on sync close not perfect yet. 345 & 138 Voltages

2500KVA 13.2 D / 4.16 floating Wye Inrush causing 51N Trip.

 

[davidbeach]

Yep. The first one is a really good example. A raw version of the second one would look much more like the first one in that each phase would be predominantly either positive or negative. The 60Hz (assuming) fundamental filtering makes it look much more balanced than it really is. Once the filtering is applied there can be false residual in the relay quantities that doesn't exist in an unfiltered version of the original.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[beyond86]

Ok, I will try again.