Buchholz Relay

The Buchholz relay protects the transformer from internal malfunctions. The gas-actuated relay is what it is. The conservator and the main tank are separated by the Buchholz relay. Transformers with ratings more than 500 KVA use this kind of relay. Economic considerations prevent it from being utilized in tiny transformers.

Table of Contents

• Definition of Buchholz Relay
• Why Is Buchholz Relay essential for transformers?
• Construction of Buchholz Relay
• Working Method of Buchholz Relay
• When Does a Buchholz Relay Operate?
• What is the difference between a gas-actuated relay and a Buchholz relay?
• Advantages of Buchholz relay
• Limitations of Buchholz Relay
• Conclusion

Definition of Buchholz Relay

An electrical transformer protection device is a Buchholz relay. A Buchholz or gas-actuated relay is placed between the conservator tank and the main tank for electrical transformers of the conservator type. Buchholz relays have three functions, but gas-actuated relays only have two. Max Buchholz is the inventor of the Buchholz relay, hence its name.

Why is Buchholz Relay essential for transformers?

The purpose of a Buchholz relay is to safeguard transformers from internal malfunctions. Impulsive breakdown of the insulating oil or just the transformer oil can result in short circuit defects, such as inter-turn faults, incipient winding faults, and core faults. When it detects such issues, the Buchholz relay will activate the alarm circuit.

Construction of Buchholz Relay

The two hinges of the Buchholz relay are positioned inside the metal chamber. The hose that connects the conservator and main tank is attached to this metallic chamber. Together with the mercury switch, the hinged one is positioned in the upper part of the metallic chamber. The alarm is activated by this mercury switch. The mercury switch and the other float are positioned in the lower part of the metallic chamber. The tripping circuit is activated by the mercury switch. One baffle plate and two floats are often used when installing Buchholz relays. Within the relay housing, the floats are positioned high and low.

Construction of Buchholz Relay
Source: circuitglobe.com

A spindle that passes vertically through the relay housing is connected to the baffle plate. Transformers with conservators alone can employ Buchholz relays. It is inserted into the conduit that runs between the transformer tank and the conservator. It is made up of a chamber filled with oil. Within the oil-filled chamber are two hinged floats, one at the top and one at the bottom, each of which is connected to a mercury switch. An external alarm circuit is linked to the mercury switch on the top float, while an external trip circuit is linked to the mercury switch on the lower float.

Working Method of Buchholz Relay

The arc within the main tank is caused by an internal transformer malfunction. The thermal effect causes the transformer's oil to begin heating. Few of the gas vapors are collected in the upper section of the main tank as it ascends. The amount of oil in the transformer tank begins to drop as a result of the evaporation. The relay alerts the staff when the mercury switch trips inside the metallic chamber. The transformer's supply cuts out, and it is unplugged from the system for upkeep. The test cock on the relay is used to release the chamber's pressure. The lower mercury switch, which is positioned inside the metallic chamber, tilts slightly when a serious fault develops inside the transformer, closing the tripping circuit. As a result, the main circuit is cut off from the transformer.

Buchholz Relay Arrangement
Source: circuitglobe.com

The Buchholz relay operates in a relatively straightforward manner. The fault currents in the transformer generate heat if a small malfunction happens. Gas bubbles occur as the transformer oil breaks down. Through the pipeline, these gas bubbles travel in the direction of the conservator. In the relay chamber, these gas bubbles gather and displace oil in proportion to the amount of gas gathered. The mercury switch shuts off the alarm circuit's contacts when the oil displacements tilt the hinged float at the top of the chamber. Through the glass on the chamber's walls, one can see how much gas has been collected. The gas samples are collected and examined. The gas's color reveals the type of fault that occurred, while its quantity shows the problem's severity. The float at the chamber's bottom is unaffected by minor flaws because there aren't enough gases generated to run it.

A significant amount of gas will evolve, the float at the bottom of the chamber will be tilted, and the trip circuit will be closed when severe faults such as phase-to-earth faults and tap shifting gear faults occur. This trip circuit will isolate the transformer and activate the circuit breaker.

When Does a Buchholz Relay Operate?

The Buchholz relay functions under three situations:

This occurs whenever the transformer oil level drops, or whenever gas bubbles appear inside the transformer as a result of a serious malfunction. If transformer oil flows quickly from the main tank to the conservation tank or vice versa,

What is the difference between a gas-actuated relay and a Buchholz relay?

Gas-actuated relays do not detect the transformer liquid level; instead, they function by measuring the amount of gas produced by minor and major electrical faults. Buchholz relays function according to the transformer insulating liquid level, the quantity of gas produced by minor and major electrical faults, and other factors.

Advantages of Buchholz relay

• Buchholz relays help prevent serious defects by indicating inter-turn faults and problems caused by core heating.
• By examining the air samples, it is possible to identify the type and extent of the issue without having to disassemble the transformer.

Limitations of Buchholz Relay

The Buchholz relay has the following drawbacks.

• Only oil-immersed transformers employ the relay.
• Only faults below the oil level can be detected by it.
• The connected cables are not protected by this relay. As a result, cables are protected separately.
• The relay has a high response time.
• The relay has a minimum operation time of 0.1 seconds.

Conclusion

Every oil-immersed transformer with a rating greater than 500 kVA uses a Buchholz relay, a type of gas-and-oil actuated protective relay. From an economic perspective, Buchholz relays are not offered with ratings less than 500 kVA. General transformer problems may cause one or more relay operators to trip or sound an alarm. All alerts and trips should be thoroughly examined, regardless of the cause of the issue. Before the transformer is used again, the cause of any trips must be found. With a conventional multimeter, the resistance can be measured to determine the state of the electrical switches themselves. However, the Buchholz relay, which is an essential guard for the transformer, is often considered necessary to ensure its proper functioning.