Buchholz relays

Construction

The relays structurally consist of two main sections, i.e. main housing and upper housing which are both made of corrosion-resistant aluminum alloy and covered with electrostatic powder paint. These sections are also resin treated to seal possible micropores.

Main Housing

Depending on the model, screw or flange type connections are available. On each side there are graduated inspection glass windows which enable the volume of gas to be read off and the contact system to be examined. The inspection windows have also additional metal protection covers against external hazards.

Upper Housing

Upper housing holds all the inside mechanisms and is also fitted with a cable terminal box, a breather cock and a test button for the mechanical release of the floats to test the alarm circuits. Cable terminal box incorporates the base mounted electrical connection parts and earth terminal and has a cover for external hazard protection. Breather cock is generally used to exhaust the air in the relay and to take gas samples. The inside mechanism comprises upper and lower contact systems for alarm and tripping positions. Both contact systems include:

• a float made of oil resistant, closed cellular type special plastic foam
• a magnet and a mechanical or mercury contact

Additionally, the lower contact system is also fitted with a deflector plate for oil flow sensing. Single float Buchholz relays have only one contact system together with the deflector plate.

Operation Principle

During normal operation the relay is completely filled with oil keeping the floats in their top limit or rest position. The contact mechanisms in the relays respond to:

• slight faults causing a slow evolution of gas in the transformer
• serious faults creating immediate oil surge
• oil leakages

Slight Faults

When a slight or incipient fault occurs in the transformer, the small bubbles of gas, which pass upwards towards the oil conservator tank, are trapped in relay housing, thus causing its oil level to fall. As a result the upper float rotates on its hinge and operates the alarm switch, thus operating an external alarm device.

Serious Faults

When a serious fault, core insulation break-down, short circuits etc., occurs in the transformer, the gas generation is violent and causes the oil to rush through the Buchholz Relay to the oil conservator tank. In the relay, this oil surge impinges on the deflector plate fitted on the lower float and causes the rotation of the float itself, thus operating the tripping contact and disconnecting the transformer.

Oil Leakage

An oil leak in the transformer causes the oil level in the relay to fall, thus operating first the alarm (upper) float and then the tripping (lower) float. The ingress of air into the transformer, arising from the defects in the oil circulation system, operates the alarm float.