Temperature causing differential trip
Temperature causing differential trip
(OP)
Hello
We have a transformer that have bushing CTs at the primary and pole mounted CTs on the secondary. The transformer is an old one with an oil circulating pump (OFAF). When the pump control developed problem the temperature of the transformer started rising. Before the temperature of the transformer could reach the set trip value, the transformer tripped on differential. Since then the transformer could not carry much load until the cooling problem was rectified.
Is there any relationship between differential and temperature?
We have a transformer that have bushing CTs at the primary and pole mounted CTs on the secondary. The transformer is an old one with an oil circulating pump (OFAF). When the pump control developed problem the temperature of the transformer started rising. Before the temperature of the transformer could reach the set trip value, the transformer tripped on differential. Since then the transformer could not carry much load until the cooling problem was rectified.
Is there any relationship between differential and temperature?






RE: Temperature causing differential trip
Never seen something like this before and personally I can't see that there will be any relationship between the two.
I think there can be a difference between the output of a CT at ambient temperature and the output at a high temperatures. If it is true, there will be a difference between the differential spill current during different temperatures. Thus for the same load the output from the secondary CTs will be constant (temperature does not change) but the output from the primary CTs will differ (between ambient temperature and the high oil temperature)
Maybe scottf or someone with CT-experience can comment on it.
To what kind of temperature do you refer: Top oil or winding temperature and what is the setpoint?
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RE: Temperature causing differential trip
Ain't there a loose conection inside the transformer that became worse during high temperatures?
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
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RE: Temperature causing differential trip
Different temperature CTs, and I also wondered if the difference would be enough to make a difference. I am also thinking that increased resistance in a CT winding would be the equivalent to adding more resistance in series with the CT. That should not be a problem as long as the burden is not too great.
Like RalphChristie, I'm waiting to see what other comments there are.
It's also possible that without oil circulation, damage was done to the transformer, and it's tripping on differential fault because you have a differential fault. It may be wise to look at the oil for obvious signs of degrading.
yours
RE: Temperature causing differential trip
RE: Temperature causing differential trip
I think getting the oil sample tested at this time for DGA should give a plenty good indication of the problem atleast tell u what the oil is like. If it is a bad primary joint causing the differential trip, believe me it is not good for the transformer. (u know that as well).
Yeah, i cant think of anything else. There are so many grey areas though like CT condition etc. Now, u say that until the cooling problem was not resolved, the transformer could not be loaded as before. So does that mean that the unit has been fully loaded since and has had no problems?
Thanks
RE: Temperature causing differential trip
THX
RE: Temperature causing differential trip
We have a bucholz relay but there was no bucholz alarm nor trip.
RE: Temperature causing differential trip
I think DGA should be done ASAP. It could be really bad if the fault really exists. I am confused though, by the fact that the heating would accentuate an actual fault inside a transformer.
The CT could be at fault. Check for CT Output varation WRT top oil temperature and also check the Differential and restraining currents to the relay.
I think more input from you should help.
Thanks
RE: Temperature causing differential trip
Are you sure that someone did not wire the Buchholz relay to the same shunt trip as the differential relay? This could have been done for economy reasons or because and isolation relay had to be bypassed causing aliasing of trip signals. On something this old you can expect just about any kind of martian-rigging. ( I have updated some racial slurs for the space age. )
RE: Temperature causing differential trip
Not likely, differential relays wont detect turn to turn faults. Maybe phase to phase or phase to ground.
Check the setting of the 87 its probably 15-20%. Temperature difference won't be causing a mismatch that could cause a trip at this setting. If it was me I would have the transformer tested as soon as possible. First test the CT's and differential relay.
RE: Temperature causing differential trip
Since this problem only occured when the transformer was overheated, you probably will have a hard time finding it at a lower temperature.
I recently had an infant mortaility of an Omron G7J power relay. A volt-ohmmeter said that the coil and diode bridge for the coil were good but it just did not want to run on 120 volts. Some problems only occur when something is energized and deenergized tests will not find the problem.
I have encountered quite a few strange problems that fall into the small percentage of electrical problems that are just like witchcraft.
RE: Temperature causing differential trip
RE: Temperature causing differential trip