Mechanical Detection of Faults in Transformer

Substation Transformer | Source: Nynas

 

The transformer still needs a mechanical component or systems for protection especially when the primary component such as CT and PT cannot detect the source of the fault. For example, a turn-to-turn fault can cause considerable current in the shorted turn, while current in the remaining winding remains relatively unchanged. Since there is little or no change in the current monitored by the CTs, there is no differential current to operate the relays. 


"The turn-to-turn fault will evolve into a ground fault, giving the protective relays the necessary change in current to operate. Some transformer faults go undetected". 


There are two methods of detecting transformer faults other than electric measurements. These methods are 


  • Accumulation of gases due to slow decomposition of the transformer insulation or oil. These relays can detect heating due to high-resistance joints or due to high eddy currents between laminations. 
  • Increases in the tank oil or gas pressures caused by internal transformer faults.

To protect the transformer from bad effects against this type of fault, there are mechanical components necessary to install in the device other than PTs and CTs. 


Gas accumulator relay

This type of relay, commonly known as the Buchholz relay, is applicable only to transformers equipped with conservator tanks and with no gas space inside the transformer tank. The relay is placed in the pipe from the main tank to the conservator tank and is designed to trap any gas that may arise through the oil. It will operate for small faults by accumulating the gas over a period of time or for large faults that force the oil through the relay at a high velocity. 


This device is able to detect a small volume of gas and accordingly can detect arcs of low energy. The accumulator portion of the relay is frequently used for alarming only; it may detect gas that is not the result of a fault, but that can be evolved by gassing of the oil during sudden reduction of pressure. This relay may detect heating due to overall heating, high-resistance joints, high eddy currents between laminations, low- and high-energy arcing, or accelerated aging caused by overloading.



Buchholz Relay


Gas detector relay

The gas detector relay can be used only on conservator transformers, either conventional or sealed. The relay will often detect gas evolution from minor arcing before extensive damage occurs to the windings or core. This relay may detect heating due to overall heating, high-resistance joints, high eddy currents between laminations, low- and high-energy arcing, or accelerated aging caused by overloading. Essentially, the gas detector relay is a magnetic-type liquid-level gage with a float operating in an oil-filled chamber. 


Pressure relays

When a high current passes through a shorted turn, a great deal of heat is generated. This heat, along with the accompanying arcing, breaks down the oil into combustible gases. Gas generation increases pressure within the tank. A sudden increase in gas pressure can be detected by a sudden-pressure relay located either in the gas space or under the oil. The sudden-pressure relay usually operates before relays sensing electrical quantities, thus limiting damage to the transformer. 


Type of Pressure Relays

  • Sudden Oil Pressure Relay
  • Sudden Oil/ Gas Pressure Relay
  • Sudden Gas Pressure Relay
  • Static Pressure Relay


Sudden-oil-pressure relay

The sudden-oil-pressure relay is applicable to all oil-immersed transformers and is mounted on the transformer tank wall below the minimum liquid level. Transformer oil fills the lower chamber of the relay housing, within which a spring-backed bellows is located. The bellows is completely filled with silicone oil, and additional silicone oil in the upper chamber is connected to that in the bellows by way of two small equalizer holes.


In the event of small rises in oil pressure—due to changes in loading or ambient, for example—the increased pressure is also transmitted to the silicone oil. However, instead of operating the piston, this pressure is gradually relieved by oil escaping from the bellows into the upper chamber by way of the equalizer holes. The bellows then contract slightly. The pressure bias on the relay is thus relieved by this differential feature. Relay sensitivity and response to a fault are thus independent of transformer operating pressure.


Sudden-gas/oil-pressure relay

A more recent version of the above relays utilizes two chambers, two control bellows, and a single sensing bellow. All three bellows have a common interconnecting silicone-oil passage with an orifice and ambient temperature-compensating assembly inserted at the entrance to one of the two control bellows. An increase in transformer pressure causes a contraction of the sensing bellows, thus forcing a portion of its silicone oil into the two control bellows and expanding them. 

An orifice limits the flow of oil into one control bellows to a fixed rate, while there is essentially no restriction to flow into the second control bellows. The two control bellows expand at a uniform rate for a gradual rate of rising in pressure, but during high rates of transformer pressure rise, the orifice causes a slower rate of expansion in one bellows relative to the other. The dissimilar expansion rate between the two control bellows will cause a mechanical linkage to actuate a snap-action switch, which initiates the proper tripping. 


Related Article: Guide for Protective Relay Applications to Power Transformers: Fuse Protection


Sudden-gas-pressure relay

The sudden-gas-pressure relay is applicable to all gas-cushioned, oil-immersed transformers and is mounted in the region of the gas space. It consists of a pressure-actuated switch, housed in a hermetically sealed case and isolated from the transformer gas space except for a pressure-equalizing orifice. The relay operates on the difference between the pressure in the gas space of the transformer and the pressure inside the relay. 

An equalizing orifice tends to equalize these two pressures for slow changes in pressure due to loading and ambient temperature change. However, a more rapid rise in pressure in the gas space of the transformer due to a fault results in the operation of the relay. High-energy arcs evolve a large quantity of gas, which operates the relay in a short time. The operating time is longer for low-energy arcs. 


Static pressure relay

The static pressure relay can be used on all types of oil-immersed transformers. It is mounted on the tank wall under oil and responds to the static or total pressure. These relays, for the most part, have been superseded by the sudden-pressure relay, but many are in service on older transformers. However, because of their susceptibility to operation by temperature changes or external faults, a majority of the static pressure relays that are in service are connected for alarm only. 



Reference: 

  • IEEE Guide for Protective Relay Applications to Power Transformers | Download
  • Publisher: IEEE
  • Committee: Sponsor Power Systems Relay Committee of the IEEE Power Engineering Society

1 comment:

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