Buchholz relay operation
This technical article provides guidance on the possible causes of Buchholz relay operation and recommends actions following the receipt of a Buchholz surge trip or gas collection alarm. It covers both operational situations and situations that arise during the commissioning of new transformers.
Buchholz relays installed on the transformer main tank have two elements: Gas Collection and oil surge element described in more detail below. We will also try to investigate possible reasons for the operation of both elements.
- Gas Collection Element
- Oil Surge Element
- Possible Reasons for Operation of the Gas Collection Element
- Possible Reasons for Operation of the Oil Surge Element
- Actions to be Taken Following a Buchholz Alarm or Trip Operation
The gas collection element collects any gas escaping from the transformer and closes a contact when the gas volume reaches a certain limit. If gas continues to be produced after this limit is reached, it will escape out of the relay into the conservator and no further operation will take place.
The gas collection element operates when a certain volume of gas is collected. Therefore it gives no direct indication of the rate of gas production.
Primarily, this element is pre-set to operate and close contact at a certain oil velocity through the relay. This will correspond to the oil flow expected during an arcing fault where oil surges from the main tank towards the conservator.
However, this element will not operate on the gas collection because the relay is designed to accumulate only a set volume of gas before allowing all excess gas to pass on to the conservator. This releases the excess gas before it can reach the level of the surge element float, allowing an oil surge event to be discriminated from a gas collection event.
Secondly, if for any reason the oil level in the relay should drop (for example if the oil is escaping from the transformer and the oil level drops below the level of the relay), then this element will also operate.
If gas is actually found in the relay following an indicated operation, then the following possible causes need to be considered.
If the oil level has dropped below the Buchholz relay, then first the alarm and then the trip elements will operate. This condition will also be confirmed by the oil level indicator on the conservator. Care needs to be taken to ensure that a residual mark on the sight glass is not obscuring the fact that the oil level has actually dropped.
However, it is possible that oil has escaped from explosion vents or damage to the tank following a major internal failure. The latter possibilities should be investigated before concluding that a benign slow leak is a cause.
The energy released from electrical discharges in the oil, causes the oil to decompose into hydrocarbon gases. Normally these gases are absorbed into the oil, but if the activity is severe then these gases can form bubbles.
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Overheating of the windings, either caused by an internal fault or overloading, can lead to the decomposition of the oil and gas production. Overheating of steel parts is possible as a result of overfluxing either from an excessive voltage or geomagnetically induced DC currents (solar flares).
Some other faults, such as core to earth insulation failure, will also cause gas production. All these conditions need to be investigated before a transformer is returned to service.
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This commonly occurs where oil filling is attempted without applying a vacuum, or with only partial vacuum applied. In contrast, proper vacuum oil filling procedures ensure that wherever trapped air bubbles or air spaces do form during an oil fill, that these rarefied gas volumes must reduce to very small air bubbles when they are finally compressed under static oil head following the release of vacuum.
These residual gas bubbles are usually fine enough to be easily absorbed by the de-gassed oil.
Any need for repetitive bleeding of air puts the integrity of the vacuum oil fill into doubt. In all cases, adhere to the OEM instructions when vacuum oil-filling the transformer.
Gas collected in the Buchholz relay of a transformer should always be thoroughly investigated. The gas should never be assumed to be just air and dismissed, even after recent oil filling activity.
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This is very unlikely to occur because all parts of the oil circuit are normally under positive pressure. The only potential area where air may be drawn in is at the inlet side of an oil pump. If this is occurring, it is a very dangerous condition for the transformer as air bubbles in the winding will cause failure.
For those circumstances where there is no gas found in the relay and there is no possibility that any gas could have leaked out of the relay or petcock connection pipework, then the following causes should be considered.
If the operating element of the Buchholz relay is a mercury tilt switch, then these switches are susceptible to momentary operation if they experience very high levels of vibration or more likely a significant mechanical shock.
This explanation for Buchholz operation should only be considered if the contact operation was momentary and coincided with an identifiable event and no gas was found in the relay.
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It is possible for wiring or protection relay faults to cause spurious operation of the alarm circuit. This possibility should only be considered once the causes set out above have been dismissed.
The following causes of oil surge element operation need to be considered:
A power arc within the tank will produce gas quickly enough to cause an oil surge. This surge can cause the surge element to operate. Normally, disruptive damage of this type will result in sufficient gas being produced to operate the gas collection element as well.
Surges, produced by this type of failure, are also likely to have operated pressure relief devices.
Low oil level operation of the oil surge element arises for the same reason discussed above for the gas collection element.
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Some designs of the transformer are susceptible to creating an oil surge through the Buchholz pipe when the pumps are started or stopped. The phenomenon is due to the slight flexibility of the coolers causing volume changes, which in turn cause oil flows to and from the conservator.
Generally pumps sucking oil from the coolers will cause a surge on startup and pumps delivering oil to the cooler (normally mounted at a high level) will cause a surge at shut down.
Pump starting or stopping will not result in gas production so it is important to check that no gas is present in the Buchholz before considering this cause as a possibility. Excessive air in the coolers can increase the magnitude of the oil surge on pump start, bleeding the coolers can help to solve the problem, but again it must not be assumed that this is the cause of the Buchholz trip unless the DGA is clear.
On very old transformers with a bursting disk type of pressure relief system situated above the level of the oil in the conservator, a failure of the disk for some reason, such as being knocked with a ladder during maintenance, can allow oil surges to occur when the pumps start.
The Buchholz relay is designed to be mounted in sloping pipework inclined 3º from the horizontal to facilitate gas collection. This means the gas oil surge element is designed to operate with the Buchholz mounted in a slightly inclined attitude, and furthermore, is calibrated to operate for unidirectional expulsion of oil.
Accordingly, it is very important to mount the Buchholz relay correctly with its indicating arrow on the side of the relay pointing towards the conservator. Inadvertent reversal of the relay (during maintenance) could make the oil surge element unduly sensitive to pump start or pump stop.
Vibration or shock may affect the surge element in the same way as the gas collection element. Again, this cause should only be considered if the contact operation was momentary and coincided with an identifiable event and no gas was found in the relay.
After the Buchholz alarm, the transformer must be switched out as soon as possible. If the transformer cannot be immediately switched out, then it must be considered to be at risk of failure and a minimum 20m exclusion zone established preferably by evacuating the compound. On no account must the transformer be approached for oil sampling, or for any other reason, until it is known to be de-energized.
If the transformer is not switched out immediately, investigation actions that do not require the transformer to be approached can be carried out. On transformers with a tap changer, no further tap changing operations should be carried out.
When the alarm signal is given, the color of the gas should be observed through the inspection-windows. The gas may be released or samples can be taken for analysis.
It should be noted that:
- Whitish gas: it is caused by electric arcing in contact with paper, cotton, and silk
- Yellowish gas: it is caused by wood and cardboard
- Greyish gas: it is caused by a breakdown of the magnetic circuit
- Black gas: it is caused by free arcing in the oil
Note that there may be air in the transformer during commissioning or after an operation of oil refilling. In similar cases, the alarm is only temporary and should end in a short period of time.
Once the transformer has tripped, or been switched out, a decision has to be made as to the next actions. This section is intended to help guide that decision. At all times the precautionary principle must be applied. Until it can be conclusively proved otherwise, the transformer must be assumed to have suffered an internal failure and should therefore not be re-energized.
The following investigation actions are required to gather the information needed to make a decision on the next operational action. The list may not be exhaustive for a particular situation.
- Take oil samples from the main tank, all selectors, and any other oil-filled compartments containing high voltage parts (except diverter switches) and send them for urgent laboratory DGA.
- Check for the presence of gas in all Buchholz relays and record the volume of any gas found.
- Sample any collected free gas and send the sample for laboratory analysis.
- Check all pressure relief devices, either check for the operation of the button indicator in spring operated types or check for rupture of the diaphragm in explosion vent types.
- Check for leakage of oil particularly around pressure relief devices but also at tank joints.
- Check for signs of distortion of the tank including tap changers.
- Check and record oil levels in all conservator tanks. Care is needed to ensure that tide marks are not disguising the fact that the conservator is empty.
- Check and record the level of oil in bushings.
- Check and record the level of oil in tap changer diverters.
- Check electrical protection for operation.
- Check fault recorders for operation.
- Check and record tap-position and times of tap changer operation from the data logger.
- Note the position of cooler and tap changer controls including maximum temperature indications on WTI dials and whether the electrical protection on any of the auxiliary motors is tripped.
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Source: Guide for Transformer Maintenance by Working Group A2.34 – C. Rajotte ( CA) – (Convener), TF Leaders: M. Foata (CA), P. Jarman (UK), F. Larese (FR), P. Lorin (CH), B. Pahlavanpour (UK), J.P. Patelli (FR), J. Velek (CZ), R. Willoughby (AU), R. Barrento (PT), P. Boman (US), I. Diaconu (RO), A. Drobyshevski (RU), Y. Ebisawa (JP), T. Fagarasan (RO), N. Fantana (DE), H. Gago (ES), J. Gebauer (DE), P. Gervais (CA), M. Krüger (AT), G. Lawler (IR), R. Maina (IT), C. Moldoveanu (RO), P. Mueller (CH), D. Olan (CA), L Paulhiac (FR), M. Pena (BR), E. Perez-Moreno (ES), S. Quintin (ES),V. Samoilis (GR), F. Simon (FR), A. Shkolnik (IS), B. Sparling (CA), P. Warczynski (PL)