Nuisance tripping in Sepam T40

[Emad Shaaban]

The Transformer feeders in our plant are running by Sepam T40s at both primary and secondary sides.
During the last 8 years we encountered issues that happened quite several times ,in which the Sepam of Primary record an external trip and open its SWG..
This configured external trip input is triggered by low side Sepam output ,which is activated by earth fault.
The confusion is that the low side Sepam has NOT any evidence of fault ..
We have done all of functional tests for both Sepam and all are as required.

Furthermore, one of these incidents included more than 10 Sepam to trip with same faults in the same time with out a real faults recorded by the down stream sides .

The only doubts we have have are these faults are because of feedback cable interference voltages ..
But this still illogic because it happens with number of feeders going to different substitutions
Does any body have other explanations?
How can we properly proof or test if cable interference is the reason ,considering that it’s a sudden issue not repeated ?

 

[thermionic1]

So T40 is basically a feeder type protection, yes? No differential (87T)?

Your suspicion of the cable from LV relay to HV relay may be a problem. On the LV side, are there mechanical contacts or some kind of hybrid transistor / mechanical? I've seen instances where he hybrid contacts feeding a higher impedance opto can cause issues with "phantom voltage". I've seen resistors placed in parallel with the opto to help alleviate the problem, or just switch to mechanical contacts and be done with the problem. Do you have any issues with DC grounds as well?

Are your feedback cables long? Are they screened?

 

[Emad Shaaban]

Hi
I thank you for your response
78T is done using Sepam 2000 d22
Both LV and HV sides use T40 and the external trip coming directly from LV side Sepam (soft contact)
Can you explain what u mean by DC ground ?
Yes the external trip cables that come from LV side are 2km long and it’s a multi core screened cable

 

[thermionic1]

LV side Sepam (soft contact). Soft contact as in NOT regular mechanical relay contacts and more like a transistor? I'd look into that.

DC Ground is inadvertent grounding of the control circuit, momentary or otherwise

 

[System Protection]

I agree with thermionic1, that is the first thing I'd check.

Here is the NERC lessons learned overview of the issue.

https://www.nerc.com/pa/rrm/ea/Lessons%20Learned%20Document%20Library/LL20130703_Use_Loading_Resistors_when_applying_solid_state_contacts.pdf

I've seen it quite of few times.


I've also seen a phantom trip issue that similar to yours, but I could never figure out the cause as it abruptly and randomly stopped happening.
I was able to capture it on an oscilloscope; it was a transient voltage spike on the trip bus of a breaker. It was not enough to trip the breaker but it was enough to pick up a relay input (which then tripped the breaker). A delay on the pickup of the input would have worked as a "bandaid".

 

[Emad Shaaban]

Thanks for all of you

I have already found an evedance of the phantom voltage that lasts less than 30ms as seen in the I24..
Now to connect a load resistor ,how can I determine the proper size f this resistor .

 

[JJ_Roy]

I had a similar issue with a dry contact from a transformer bucholtz or beta relay. We put loading resistors across the input at the opto board and it solved the problem.

I remember making a simple circuit calculation using the opto board input impedance and pick up voltage to determine the proper loading resistor size. You just need to make sure the parallel resistance of the loading resistor and opto board don't make the voltage too low to be seen by the opto input.