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What is the function of BUCHHOLZ relay in oil-filled transformers?

Buchholz protection is normally provided on all transformers fitted with a conservator.
The Buchholz relay is contained in a cast housing which is connected in the pipe to the conservator, as in figure shown.

A typical Buchholz relay will have two sets of contacts. One is arranged to operate for slow accumulations of gas, the other for bulk displacement of oil in the event of a heavy internal fault.
An alarm is generated for the former, but the latter is usually direct-wired to the circuit breaker trip relay.
The device will therefore give an alarm for the following fault conditions, all of which are of a low order of urgency: 
a. hot spots on the core due to short circuit of lamination insulation
b. core bolt insulation failure
c. faulty joints
d. interturn faults or other winding faults involving only lower power
e. loss of oil due to leakage

When a major winding fault occurs, this causes a surge of oil, which displaces the lower float and thus causes isolation of the transformer.
This action will take place for:
i. all severe winding faults, either to earth or interphase
ii. loss of oil if allowed to continue to a dangerous degree
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