What is Buchholz Relay? Working, Diagram & Construction

Buchholz Relay is a gas actuated relay. It is connected to the conservator as shown in Fig. 1.

Fig. 1: Location of Buchholz Relay.

Construction of Buchholz Relay

Fig. 2 shows constructional details of Buchholz relay. It consists of a domed vessel. There are two mercury switches. The upper mercury switch is attached to a float. This float is hinged as shown. The lower mercury switch is attached to the flap and the mechanism is hinged as shown in Fig. 2

Alarm circuit contacts are brought to upper mercury switch and trip circuit contacts are brought to lower mercury switch. The upper mercury switch operates during incipient faults and lower mercury switch operates in case of severe faults.

Fig. 2: Buchholz Relay Constructional Details

Working of Buchholz Relay

Under normal working conditions, the position of mercury switches is as shown. It is seen that the alarm circuit contacts and trip circuit contacts are not bridged by liquid mercury in the switch. In case of incipient faults, the heating oil occurs, the oil decomposes and gases are formed. The majority portion of gases is hydrogen (70%). This gas is accumulated at the topside near domb. Due to gas pressure, the float comes down and mercury level shifts closing the alarm circuit contacts. Thus alarm circuit is complete and alarm sounds.

In case of severe faults, gas formation is rapid and large amount of gas rushes from transformer to Buchholz relay. During its travel, the gas produces sufficient pressure on the flap, the flap turns in anticlockwise direction, the mercury switch also tilts and mercury level inside the mercury switch shifts, closing the trip circuit contacts. Thus, trip signal is produced causing operation of circuit breaker.

The Buchholz relay is used for transformer having rating greater than 750 kVA because of economy reasons.

Merits of Buchholz Relay

1. Simple in construction.
2. Fault detection is faster.
3. Operation is reliable.

Demerits of Buchholz Relay

1. Only applicable for oil immersed transformer.
2. Only the faults below oil level can be detected.