Q1Power System Protection

Working of Buchholz Relay

The Buchholz relay is a protective device specific to oil-immersed transformers (and, in some designs, oil-immersed on-load tap changers) that are fitted with a separate, elevated conservator tank connected to the main transformer tank via a sloped pipe — the Buchholz relay is mounted within this connecting pipe, positioned so that any gas generated within the main tank, or any surge of oil displaced by an internal fault, must pass through the relay's own housing on its way to (or from) the conservator.

Construction

The Buchholz relay housing contains two hinged floats (or, in some designs, a float and a separate baffle-plate-actuated element), each carrying a mercury switch (in traditional designs) or a magnetically-actuated reed switch (in more modern designs) that closes an electrical alarm or trip circuit when that particular float tilts/lowers sufficiently. The upper float is positioned to respond to a slow accumulation of gas within the relay housing (gas rising from the main tank and collecting at the top of the relay chamber, displacing oil and causing the upper float to descend as the oil level within the housing falls), while the lower element (typically a hinged flap or vane, rather than a simple float, positioned directly in the flow path between the main tank and conservator) is designed to respond specifically to a sudden, rapid surge of oil flow through the connecting pipe, such as would occur from a major internal fault.

Working - Response to Minor (Incipient) Faults

A minor or incipient internal fault within the transformer — such as a small, localized overheating (hot spot) in the core laminations or winding insulation, or a very early-stage, low-energy partial discharge/arcing condition — causes slow, gradual decomposition of the surrounding insulating oil and/or solid insulation, generating small quantities of gas (typically hydrogen and various hydrocarbon gases) that rise slowly through the oil and accumulate within the main tank, and subsequently pass upward through the connecting pipe into the Buchholz relay housing. As this gas accumulates within the relay housing, it progressively displaces the housing's oil, causing the oil level to fall and the upper float to descend, eventually closing its mercury/reed switch contact and initiating an alarm signal (rather than immediate tripping), alerting operators to the developing (but not yet severe) fault condition, allowing time for the collected gas to be sampled and analyzed (via dissolved gas analysis) to help diagnose the nature and severity of the developing fault, and for a planned outage to be arranged for inspection and repair before the condition potentially escalates into a more severe fault.

Working - Response to Major (Severe) Faults

A major internal fault — such as a winding-to-winding or winding-to-core short circuit — releases a large amount of energy very rapidly, causing violent, near-instantaneous vaporization of a substantial volume of surrounding oil and generating a sudden, high-velocity surge of oil and gas that is forced rapidly through the connecting pipe toward the conservator (since the sudden internal pressure rise displaces oil violently through the only available path, the connecting pipe passing through the Buchholz relay). This rapid oil surge strikes and displaces the lower hinged flap/vane element within the Buchholz relay housing (which is specifically designed and positioned to respond to oil flow velocity/surge rather than to slow gas accumulation), causing it to tilt and close its own separate switch contact, which is wired directly to initiate immediate tripping of the transformer's circuit breaker(s), isolating the transformer from the system before the fault can escalate into a catastrophic failure (such as tank rupture or fire).

Overall significance: the Buchholz relay's dual-response capability — a slow-acting alarm function for gradually-developing incipient faults (allowing preventive, planned intervention before serious damage occurs) combined with a fast-acting trip function for sudden, severe internal faults (providing rapid isolation to limit fault damage) — makes it one of the most valuable and widely-used protective devices specifically for oil-immersed transformers, providing a form of sensitivity to internal insulation and localized-heating conditions that electrical protection schemes based purely on current/voltage measurement (such as differential protection, discussed elsewhere in this paper) generally cannot detect, since Buchholz protection responds directly to the physical/chemical consequence of the developing fault (gas generation and oil displacement) rather than to its indirect electrical current signature.