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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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Buchholz relay is used in protection of
Transformer
Alternator
Induction motor
None of above
Transformer
Quick Summary: The Buchholz relay is a gas-actuated protective device specifically designed for oil-immersed transformers rated above 500 kVA. It is installed in the connecting pipe between the main transformer tank and the conservator tank to detect internal faults.
The Buchholz relay is a gas-actuated protective device specifically designed for oil-immersed transformers rated above 500 kVA. It is installed in the connecting pipe between the main transformer tank and the conservator tank to detect internal faults.
Vgas∝Ifault2⋅t — Represents the volume of gas generated relative to fault energy
During an internal fault (like insulation failure or arcing), oil decomposes and releases gases (e.g., hydrogen, methane). These gases travel towards the conservator and get trapped in the Buchholz relay, causing a float to drop and operate an alarm or trip circuit depending on the fault severity.
It provides inherent protection against incipient faults (slow-developing faults).
The relay operates in two stages: alarm for minor faults and trip for major faults.
It is incapable of detecting external faults (e.g., short circuits outside the transformer tank).
Detects faults at a very early stage (incipient faults).
Simple construction and installation.
Provides both alarm and trip signals.
Applicable only for oil-immersed transformers.
Cannot detect faults in the connecting cables or bushings.
Requires a minimum oil level to function.
Oil-filled power transformers
Oil-filled distribution transformers
On-load tap changers
Buchholz relays do not operate on dry-type transformers as they lack insulating oil.
Option B (Alternator) uses differential protection (Merz-Price), not Buchholz relays.
Option C (Induction motor) uses thermal overload and earth fault relays.
A is correct — The Buchholz relay is exclusively used for the protection of oil-immersed transformers against internal faults.
In exams, remember that Buchholz relay = Transformer; Differential Relay = Alternator/Large Transformers; Impedance Relay = Transmission Lines.