Buccholz Relay 2

[nandrade]

I have a 160 MVA 138KV/13.8 Kv transformer. I see that the valve between the tank and conservator, is 20% open. The Company who installed the transf. left it that position, because the transformer shutdown when energizated, by the buccholz relay operation. (with the valve open 100%, normal position). That was related for the technician who are worked since that date.

Reciently i saw and think, that the sudden trip of the buccholz, is more sensible. Our Buccholz relay is calibrate by factory for operate at 1m/s of sudden rise of oil to the conservator.

Question. What is the correct calibration for the buccholz for this type of transformer. (oil volumen: 40.000 liters)

 

[2short]

The buchholz relay is factory calibrated. You should perform regular functional tests to verify that the contacts will still operate properly and actuate whatever function you want, such as an alarm or a trip. When I examine the old spare buchholz relay in my office, I see that there is a hinged plate connected to the lower float. There is a small hole in the hinged plate to allow for the slow flow of oil back and forth between the tank of the transformer and the conservator during normal expansion and contraction of the the oil that results from changes in load or ambient temperature. The sizes of the plate and the hole are fixed and cannot be changed without changing the operation of the relay.

On the older buchholz we have installed on our 30 year old transformers, there is a test button under a protective threaded cap. When the cap is removed and the button pressed half way down, the upper float in the buchholz relay will travel downward simulating low oil level, and should alarm only. When the button is depressed fully, the lower float in the buchholz relay will travel downward simulating either low-low level or sudden pressure, and should trip.

It is important to keep the valves on both sides of the buchholz relay fully opened to allow for the free flow of oil between the tank and the conservator during normal operation of the transformer. Last year, when we took our unit off line, we de-gassed one of our GSU transformers. Being sensitive to recent events with mispositioned valves on one of our transformers, the maintenance people doing the degas verified three times that the isolation valve was open. Only problem was that the arrow on the handwheel of the valve was backwards, and we actually closed the valve. Several days later, after we went back to full power and the ambient temperature rose, the oil expanded sufficiently that the pressure relief device actuated and dumped a significant amount of oil on the ground. (Turns out about half of the handwheel arrows on similar valves on that transformer and its companions were incorrect.) When we opened the improperly closed isolation valve, we got an actuation because the pressure buildup caused the oil to flow into the conservator so rapidly that the relay actuated. The rapid oil flow probably pushed the hinged plate and depressed the lower float and actuated the relay. Fortunately the maintenance foreman had suggested that we cut out the trip signal, or we would have dumped 1100Mw!

 

[nandrade]

I made the functional test. I noted that when i fully pulldown the button, it didn`t reset, keeping closed. I had to flow dry air to reset it. I see that is a problem, and i buy a new relay to replace the older.

I think that it could be the reason for the 20% open valve (i have only one, not two between tank and conservator), this palet was too sensible for the oil flow.

Do you know something else to commissioning the installation of a new buccholz Relay. Im afraid when we put the new relay, and try to operate the transformador with 100% valve open, we`ll have a problem.

 

[ScottyUK]

nandrade,

There should not be any high velocity oil movement in the pipe from the conservator to the transformer tank. If there is, there is something desperately wrong with your transformer. We use a Buchholz relay of the type described on some pretty big transformers - 400MVA - and do not experience any problem with oil movement causing false operation of the relay.

Problems I have seen with these relays are:

The reed switches degrade over time, causing the contacts to become non-conductive. Operating the switch with a Megger or similar connected across the terminals usually helps get the switch conducting again.

The floats inside the relay develop leaks and don't float properly, causing the relay to operate. This can't be rectified easily without drilling the float and expelling the oil before resealing it.



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If we learn from our mistakes,
I'm getting a great education!

 

[040358]

There are three velosity settings in buchholz relay. The normal velosity setting is 1 m/s. The most senitive setting is 0.75 m/s and least sensitive setting is 1.5 m/s. Your setting is OK. If the buchholz didn't reset then you have to release the air through the small valve from where you perform functional test (alarm and trip) by means of air pump.