Buchholz Relay, 8 Answer You Should Know

Buchholze Relay
Buchholze relay

Buchholz relay is a gas detection, dielectric failure sensitive safety device for the high-power electrical systems.

It is commonly used for the protection of oil immersed transformers that are filled with oil and are likely to get electrical surges over time.

These surges may end the transformer into a potential security threat of life and infrastructure near the transformers.

Buchholz relay, itself is inside a container filled with oil and this container and its oil is practically separate from the internal part of the transformer and its oil respectively.

What type of relay is Buchholz relay?

Buchholz Relay is Gas Detection relay. or Gas actuated relay.

Buchholz relay working principle

There are 2 parts of a Buchholz relay. The first one is the upper and other is the lower one. The upper part of the Buchholz relay is a float. This float has a hinge attached to it and allows the float a vertical motion up and down its mean position depending upon the level of the oil inside the Buchholz relay container.

The movement of the float inside the container effects the alignment of the mercury switch that is fixed on itself (on the float). The lower part of the Buchholz relay has a mercury switch and a baffle plate (a plate that regulates the level of oil inside the container of Buchholz relay).

In the case of Buchholz relay the regulation of oil by the baffle plate effects the level of oil inside the whole container of the Buchholz relay not just the lower part.

The location of this baffle plate is in front of the inlet from the tank side of the container. When a high-pressure fluid enters from that side, it changes the alignment of the mercury switch attached to itself.

Change in the alignment of the mercury switch triggers the alarm. There is another part that can be considered as the whole description of is the topic. This is the upper part of the relay on the outer side of it. It has gas filled within.

This part is serves as the outlet of for some gases that are produced in result of some failures inside the transformer.

Uses of Buchholz relay

Buchholz relay is a gas activated relay which depend on the gas produced inside a transformer in case of faults. Its uses are:

  • A sensor that tells that in which part of the transformer oil level has decreased due to leakage. If the transformer has an oil leakage buchholz relay would activate.
  • A detector of short circuit of internal circuitry of the transformers.
  • A detector of gas production inside the tank of transformers, we can make gas analysis to get an idea about the fault type inside the transformer.

Is the Buchholz Relay an over current relay?

No, Buchholz relay is not an over-current relay. It does not sense over- currents in the circuitry of a transformer. It detects the gas production in the tank.

If an overcurrent sweeps the transformer, the Buchholz relay will not sense it, however it will react if the oil temperature rises due to the over current or if the insulation of the windings gets burned down due to the over-current. The gasses produced in the result will rise up to the tank and then travel to the Buchholz relay, setting the alarm on.

Why is Buchholz relay always inclined to 5-7 degrees?

Buchholz relay is mounted on a pipe / pole. On this pole its angle is 5-7 degrees (sometime 0-5 degrees) to keep the fluid flow inside the relay nice and continuous (not discontinuous in case of gas release).

The main purpose of selection of this range of angles for the Buchholz relay is to make it convenient for both types of fluids, the heavier and the lighter ones.

If an angle is selected, that makes the turn from the tank to the entrance of Buchholz relay to sharp, the heavier fluids will not be able to descend (or flow down). This usually happens when the angle approaches to 0 degrees from the value of 7 degrees (or 5 degrees).

In the other case, it will be too difficult for the gaseous fluids to rise up. This happens when the angle approaches to 7 degrees (or 5 degrees) from the value of 0 degrees. So, the main purpose is to catch the gases in and let the oil flow freely.

Why are mercury switches used in the Buchholz Relays?

Mercury switches are used to either trip or set an alarm in case if any faulty situation. When internal windings get out order, they produce heat and that heat evaporates the oil inside the tank.

This oil vapors rise up in to the Buchholz relay. This whole process rises the fluid level inside the Buchholz relay. Due to this, mercury switch gets tripped and the power supply of the transformer is cut immediately. This is to prevent any more melting of the windings inside the tank.

Why Buchholz relay is called a gas-actuated relay?

Buchholz relay is a gas actuated relay because the functionality of the Buchholz relay totally depends upon the level of fluid (specifically gas) inside the container.

If the level of the fluid increases, the Buchholz relay gets tripped . It pauses all the functionalities until the transformer is repaired.

What type of fault does Buchholz relay employ for?

Buchholz relay is used to cater with inner faults of the transformer (or other oil immersed power devices). Specifically, in the oil immersed transformers, the Buchholz relay is really handy. If there are any short circuits, they are dealt by Buchholz relays.

Short circuit may occur due to inter-turn faults in the windings. Short circuits might occur due to the incipient faults (the faults that occur inside the transformer during impulse testing) and also due to the core faults (the faults in the core laminations and bolts laminations).

How many Buchholz relays are used in a transformer?

Generally, a transformer has up to 4 Buchholz relays. But it is totally dependent on the requirement of the power supply of the transformer.

The transformers used for the small businesses and offices that has almost 1000 computers, 10-15 ovens, 50 Air conditioners and 2 water pumps that calculates up to 8kW. The transformer may need only one Buchholz relay.

I have seen a transformer of 2.5KVA with only one Buchholz relays. I also have seen smaller transformers with one Buchholz relays.

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