Transformer Diff. Tripping

[salem]

We had a case where a transformer(20 MVA) differential protection tripped under motor starting (3.3 MW motor), what could be the reason for that.the transformer was loaded at around 16 MVA at the time of motor starting.

 

[RalphChristie]

Most probably CT-saturation. Do a mag-curve on each CT and compare the results.

Can you give a little more info on your system? Old or new installation, type of CTs, type of relay, etc, etc. It is quite difficult to give a meaningful answer with almost no information.

regards
Ralph

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[RRaghunath]

To me it sounds like CT polarity problem. Have you done the stability check for the differential protection.

Also, you may like to check if the transformer REF protection is integrated in to the same differential protection and if so, the neutral CT polarity as well as the REF protection stability check.

 

[salem]

the ct's polarity are OK, also the system is stable among nearly full load of the transformer

 

[slavag]

Hi Salem.
Please see attached link
thread238-190808
In additional:
1. Are you see "0" in diff current meas (I'm hope it's digital relay)?
2. Are you have only one gounding point of CT star point?
3. What is protection setting?
4. What is a relay (maybe we can help).
5. Are you have DR printout?
Regards.
Slava

 

[stevenal]

Stability under steady load proves nothing. Stability during a through fault is what you want. But since you have stability under load, you can use this fact to examine the relay quantities while the transformer is loaded. If these are static relays, you may want to upgrade.

 

[oldfieldguy]

Could be a CT saturation thing, but probably not...

What manufacturer and model of differential relays are you using?

I had a client with a couple of transformers in service for ten years using GE STD relays. He thought everything was fine until a bird hit an overhead feeder and subjected one of his transformers to a through fault, and he got an incorrect differential operation.

It turned out that his relays had the tap screws in the wrong position for the last ten years, but a poor selection of CT ratios kept the current under normal operating conditions to a a level below the pickup of the differential element. The demise of the bird briefly raised the current into the pickup range of the relay, and the incorrect tap screw postions took the transformer down.

It's worth a look...


old field guy

 

[rcw retired EE]

I have seen several problems as described by old field guy. Transformers run well for years until a heavy load or a through fault uncovers the relay setting or CT connection problem. Load current doesn't always trip

With a typical motor inrush current of 600% and a load of 16 MVA you system could have seen current about equivalent to 32-36 MVA ((6 x 3.3) + 16)or 180% of transformer rating.

Incorrect settings or wiring problems are the most likely cause, but it could also be many other things such as: a loose wire lead, CT saturation, unusual zero sequence currents during motor starting from surge suppression capacitors.

Keep looking and good luck!