Transformer Differential Protection trip Fault

[emadshaaban1]

Hello
Step-down transformer (20kv /6.6KV ) 22MVA CT ration 2000/1, was tripped by (Sepam 2000) differential Protection Unit .
I've measured the Transformer insulation resistance for both primary and secondary side ,all results was great 7 G-ohm.
I've primary injected current in CTs ,all CT secondary currents was correct as ratio .
IF i have restored the Transformer with out load it keeps energized with out trip .
If I have restored the Transformer with load it take 1day and trip by differential unit .

what would the problem be? How can I full be sure the CT are okay ?

Emad Shaaban
Tripoli Libya

 

[DTR2011]

Have you looked at the relay event reports? Specifically, the relay should be able to record the Idiff. When transformer is energized, have you measured the currents at the relay? I have seen this before and most likely it is either a incorrect connection or CT ratio. As the load increases, the Idiff becomes greater than the setting and causes a trip. The Sepam relay should have this information available in event reports.

 

[JG2828]

Like DTR2011 said. I don't have experience with the SEPAM 2000, but according to the spec sheet it records the fault data in standard comtrade format. If you can upload the comtrade file from one of the trips we would be able to provide a better guess of what is happening.

http://notronco.com/dnl/catalogue/schneider/MGC/AC0401EN.pdf

 

[3I0]

Never used a Sepam 2000, but I have seen similar happen where the relay was programmed with the wrong transformer winding vector group.

If the event records don't turn up anything too suspicious, that's where I'd look first.

 

[unclebob]

Is this a new unit or an existing one that never triped before?

 

[emadshaaban1]

Dear all
thank you for your instruction ,
the transformer had been in service since 2004 . I am not sure a bout the CTs although I make current injection to check the ration ,I want to be sure 100% that CTs are okay ,what other tests that I can do (like saturation test!!)?

Emad Shaaban
Tripoli Libya

 

[davidbeach]

Post a COMTRADE event file from one of the trips. There's a how bunch of possible things that can be rapidly ruled out once the event report can be reviewed.

 

[oldfieldguy]

Very carefully verify your set-up - CT ratios, winding groups, connections to the relay, programming, etc.

I have seen improperly set up relays stay in service for years before a misoperation caused an investigation that located problems.

old field guy

 

[jghrist]

If this a new installation or if there have been recent wiring changes, I would suspect CT wiring errors.

 

[slavag]

Emad, lot of options for false trip.
Comtrade file ans event list from relay are needed
Transformer in operation 12 years, Im not believe in some mistakes.
Please make stability test with 400V. or disconnect trip from 87T and check stability with load
From my point of view it is some problem with CT wiring, some bad connection.