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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Which relay provide signal before the fault?
Buchholz relay
Over current relay
Under current relay
None of above
Buchholz relay
Quick Summary: The Buchholz relay is a gas-actuated protective device installed in oil-immersed transformers that detects incipient (incipient or slow-developing) faults before they escalate into major failures. It acts as an early warning system by responding to the accumulation of gases or pressure surges caused by localized heating, insulation degradation, or arcing within the transformer tank.
The Buchholz relay is a gas-actuated protective device installed in oil-immersed transformers that detects incipient (incipient or slow-developing) faults before they escalate into major failures. It acts as an early warning system by responding to the accumulation of gases or pressure surges caused by localized heating, insulation degradation, or arcing within the transformer tank.
Vgas∝∫Iarc2dt — represents the volume of gas produced proportional to the energy of an incipient arcing fault
The relay consists of a container with two hinged floats. As insulation breakdown occurs, the oil decomposes, releasing gases that rise and collect in the upper chamber, causing the upper float to tilt and close the alarm contact. If a severe fault occurs suddenly, the surge of oil hits the lower float, closing the trip contact to isolate the transformer immediately.
Used exclusively in oil-immersed transformers.
Installed in the pipe between the main tank and the conservator.
Capable of detecting inter-turn faults and core heating.
Acts as both an alarm for minor faults and a trip mechanism for major faults.
Detects incipient faults at early stages.
Prevents major damage to the transformer.
Simple, rugged design.
Can only be used on oil-filled transformers.
Cannot detect faults in the auxiliary circuits.
False alarms can be triggered by vibration or seismic activity.
Power transformers (> 500 kVA).
Oil-filled distribution transformers.
The Buchholz relay is named after Max Buchholz, who invented the device in 1921.
Option B (Overcurrent relay) operates only when current exceeds a threshold, which happens during a fault, not before.
Option C (Undercurrent relay) is generally used for loss-of-load detection or specific pilot wire protection schemes.
A is correct — The Buchholz relay is unique because it detects gaseous decomposition products from oil insulation before a major fault fully develops.
Remember that Buchholz relays are exclusively for oil-immersed units; for dry-type transformers, we use thermal sensors or differential relays instead.