Temperature causing differential trip

[khadisa]

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?

 

[RalphChristie]

khadisa:

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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[RalphChristie]

Oh, now that I think again of it:

Ain't there a loose conection inside the transformer that became worse during high temperatures?

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[waross]

My first though was the same as RalphChristie;
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

 

[GTstartup]

Typical differential setting is 15%, you wont get that with temperature difference, if anything at all. Most likely it's a real fault that shows up on higher temperatures.

 

[amps21]

Like waross said,

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

 

[amps21]

I forgot yesterday about your buchholz relay (if u have one). Have u gotten any alarm, gas collection etc. A hot joint that drops voltage to that (causing diff. trip) extent can surely evolve gasses.

THX

 

[khadisa]

amps21

We have a bucholz relay but there was no bucholz alarm nor trip.

 

[amps21]

OK does that mean the Buchholz did not have any gas what so ever. If you had not checked this physically, rightaway, it is possible that the gas got dissolved in oil due to conservator oil pressure.

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

 

[mc5w]

Thermal expansion and contraction of the windings could be causing a turn-to-turn short to come and go.

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. )

 

[GTstartup]

Thermal expansion and contraction of the windings could be causing a turn-to-turn short to come and go.

.

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.

 

[mc5w]

A turn-to-turn short could be say 10% to 25% of the winding if the short occurs between different layers. Or, thermal expansion could cause a partial ground fault that goes away when the transformer cools.

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.

 

[amps21]

The current due to turn 2 trun fault locally is very high compared to what shall be visible @ the terminals. It all depends on the kind of fault. But this fault to happen and extinguish and then happen again will cause gas generation for sure and finally devestate the unit. So i think, checking the unit for gasses is very important.

 

[patrick7]

I think the key may lie in the fact that this is an OLD transformer. The diff fault could be real. I would think that the higher temp has an intermitant short occuring either within the winding or at a connection point. I would treat this as a real fault, perform a doble test on the transformer as well as gas analysis. compare the previous doble report with this one and that will give you a good idea what is hapening. Primary concern is for safety. you dont want this fault to continue unchecked and become catastropic.