660 MW Synchronous Generator tripping on through fault and when back Chrging GT of other Unit

http://www.pecplc.com/index.php/articles/nuisance-generator-differential-protection-trips-in-real-time-situations

Clever detective work. I had thought of aggressive field forcing to promote system stability would be present on this equipment, but had not realized it could be the root of the problem. Also, not everyone realizes that energizing a step up transformer from the high side can result in much higher, like double, the typical magnetizing current for a transformer.

Ofcourse there is a valid point that when large power transformer is energized there is a huge magnetizing current specially if charged when voltage waveform is at 0 degree(Doubling effect- flux becomes 2 times). But we can't control that angle there is CONTROL SWITCHING DEVICE(CSD). But in normal case(without CSD) it is not possible that everytime the transformer is backcharged , the voltage waveform is at 0 degree. If angle is not 0 degree each time then why Generator of other Unit is tripping everytime when we backcharging the GT

I would like some clarification. I understand that you are talking about large transformers like GSU (generator step up) but I thought a transformer is a transformer. I was always led to believe that a transformer did no "know" primary or secondary. I work with small transformers (typically 1 KVA) and sometimes use a transformer designed for 480 volt primary, 120 volt secondary with secondary connected to 120 volt outlet to get 480 volt for a small test project. Would magnetizing current be much different (obviously 4 times greater amps at 120 volt than 480 volt) in this application? " Also, not everyone realizes that energizing a step up transformer from the high side can result in much higher, like double, the typical magnetizing current for a transformer ". Thank you. I am not an engineer but I muddle through some engineering work.

As I mentioned earlier, I have read somewhere that transformers can be designed as step up or step down. The typical 12x magnetizing current can be 25x if the windings are energized opposite to the design. I have no citation for this, it came up during problems I was analyzing with a large (for paper mills) power system. I’d be interested in the reason for this, as I had always assumed before that the magnetizing current was the same factor for either direction. I suppose that you could design a generator step up transformer to have lower magnetizing current from the secondary side, to allow for black start capability.

It seems you have polarity problem with one of the CTs (Line side or neutral side). Check the CT connections against the relay manual wiring diagram.

After correcting, don't forget to conduct stability test, for confirmation.

Also the stabilising resistor rating (inadequate) if it is high impedance differential and the bias settings in case of Biased differential protection can cause this problem.

If there is a polarity problem, it should not be stable in normal condition. It is stable now and ruuning at full load ie.660MW. It trips on Generator differential Protection only when there is EF in 400kV line or when GT of other Unit is backcharged.

Are you talking of GT Overall differential protection or Generator differential protection??

More details like the protection settings, relay model etc. will help understand the problem better.

You haven't mentioned what you did when faced with the problem. Your initial findings will also be helpful, if you care to share.

The CTs are the same ratio, so not combination protection. I think there is an extra zero in the ratio, though?

The equipment & supplier feedback would be interesting information, but as Solar Eagle found, it could have little to do with the protective equipment at all.

It appears at this point they are just putting the equipment back in service, so are now looking for suggestions to keep this from happening again, not a common event, but annoyingly costly....

Ratio is correct i.e. 30000/5A. 22574 A is rated current of 660MW Generator. No one neither Relay supplier nor the Generator supplier is found anything worth. They have not found the cause yet.

Thanks, I never did the math. I guess I assumed a much higher generator voltage, dismissing the possibility of the existence of a 25kV class 25,000 Ampere rated continuous current generator circuit breaker. I would have assumed a generator-unit transformer connection, except you mention back-charging the other generator transformer, which should not be possible with a unit connection.

I am talking about Generator differential Protection only. At this plant Generator is not part of Overall differential Protection. Generator diff settings are as follows-:

Is1=500mA , K1= 0 %

Is2= 5A , K2= 150%

Alstom Relay P345 Model

CTR =30000/5 both side

We have found nothing abnormal. One thing that is ovbious is high magnetizing inrush current while back charging the GT of other Unit. Still we did not get any point when it is tripping on Generator differential Protection when there is EF in Lines

Is2 setting of 5A holds the key for maloperation I suppose. Alstom manual recommends 1.2A for Is2.

How it matters is that the slope K1 is zero and continues for upto 5A, the set value of Is2. This means right up to 5A differential current the bias curve is non-existent which could make the protection less stable.

Alstom Manual states that Is2 should be 1.2In. In my case In is 3.76 A (CTR is 30000/5). So 1.2×3.76 = 4.512A. We have set at slightly higher side i.e. Is2= 5A. Is it still possible that Is2 plays a culprit ?

Sorry I missed the fact that your CT is with 5A secondary. In that case, Alstom recommended values for Is1 and Is2 are 1A and 6A (20% and 120% respectively).

You can try these values and see whether it helps.

https://www.acronymfinder.com/~/search/af.aspx?Acronym=CSCTPP&string=exact

Chhabra Super Critical Thermal Power Plant