We are facing problem of frequent tripping in Unit Station Transformer -2 during heavy rain. During checking, not found any abnormalities.
1.Fault UST2 OTI Trip Observation- * Direct OTI trip initiated without Alarm
* Oil temperature is normal
* Tripping contacts are not physically operated
* OTI trip contacts are operated frequently as per in Disturbance record Action taken-Spare Core of another cable used for OTI Trip contact
2. UST2 PRD Trip

* PRV contacts are not physically operated
* PRV trip contacts are operated frequently as per in Disturbance record New cable used for all contacts

There is a false contact occurs during / after heavy raining and high humidity, condensation might take place. Control terminals in the marshalling box need to be checked / replaced, as well as wiring also need inspection in particular at entry points
.

thanks sir. we will do it and make inspection on regular basis.

Make sure for rainy conditions the marshalling box is properly closed and weather protected as moist condition also leads to tracking and may simulate tripping inadverently.Please also make sure the tripping impulse from OTT/WTT/BT/MOG/PRD etc are driving a mechanical hand reset type VAJH type relay and the output contacts of this relays are used for trip ckt initiation.
If these are substituted by numerical relay binary inputs thena lso there will be a problem of spurious tripping .
Make sure the grounding of the multicore control cable if sheathed the grounding should be at TRPP panel end only,if armoured cable is used the armour grounding at the panel end only.

Sekhar sir what is the reason of grounding the multicore control cable?

I am assuming that the trip circuit is floating DC (ungrounded). If so, the moisture could be causing a "sneak" circuit, otherwise known as a "hot short" in the tripping circuit, which essentially bypasses the sensing relay contacts and actuates the tripping relay coil. I would check the cabling between the sensing relay contacts and the trip relay coil and the cabling on the hot side of the sensing relay contacts for insulation problems.

Tony's point that the DC should be ungrounded is very important. I would measure the DC Positive to ground and the DC negative tp ground. These values should be fairly close ( 65+ or 65- on a 125VDC battery). If you find a large difference in these voltages, you likely have a ground or a high resitance ground on your DC. Eliminating this ground would likely resolve the problem and remove the sneak circuit that occurs when moisture is present.

On a separate note - I have seen this type of problem when moisture accumulated in the CT cabinets or CT junction boxes (For slip on CTs) The moisture shunts some of the CT current to ground and causes a differential operation. I realize this is not the problem you are experiencing but would be worht checking the CT's and CT wiring to see that they are also staying dry

Bushing contamination and dust can be become the culprit by high humidity and rain in a process called scintillation. As the humidity decreases, surface currents caused by the concentration of the electric field on the contaminated area dry the contaminant into a Dry Band. Dry Bands produce small arcing phenomena which can be detected by our technology, and is a very effective predictor of potential flash-over situations. Flash-overs often clean away the dry bands temporarily. However, the flash-over residue reduces the hydrophobic qualities of the insulator surface that promotes continued contamination and flash-over. There is a helpful video at www.exacterinc.com/Videos. Let me know if this is helpful information.

You have said that the tripping problems occur during heavy rains. This is a very common problem if your distribution system downstream of your substation are predominantly overhead. During dry season the insulation condition (insulation level) of your downstream feeders and components of the feeder lines are normally higher but during rainy season or when the air is wet the insulation level degrades significantly. An insulating component say a pin insulator if dirty and wet may flash over. It may trip you circuit breaker at the substation, but when you inspect your lines later you cannot even notice the problem. The flash over was a self clearing fault. It faulted but it clears the fault itself, incidentally that momentary fault tripped your substation breaker.

Hi Ankush, hope you have already find the fault & solved it. If need Transformer oil Purifier to strengthen the Transformer oil's dielectric strength, pls let me know. I will provide it for you. Thanks. oilpurifier-kung@hotmail.com

Check your bushing for posible contamination. It is possible, too that the bushing/s is starting to fail. HV bushing is one of the weakest component in HV system. Bushing flashover causes about 10% of all failures. It is advisable to conduct proper tests since some bushing just explodes without any prior indication of failure.

This is a clear case of control cable fault . The cable connecting Transformer Marshalling box & the relay panel. During rains when the cable trench or the duct is filled with water, it may induce this false tripping.

We also had the same type of problem, false tripping due to OSR and PRV, latter we found that all these tripping are due to water ingress so we put sealant and now its working fine.