Theory and Procedure of the Experiment - 6



The Buchholz relay is one of the important protective devices for oil immersed power transformer, which will operate based on the oil or gas pressure. It detects two types of faults i.e. minor and major fault. Minor faults comprise of faults in core laminations, over heating in windings, bad connections, low oil levels etc. In minor faults the alarm circuit will be actuated to switch on the buzzer. In case of major faults like internal short circuit between phase and earth, phase to phase fault, insulation break down etc., the trip circuit will be closed due to enormous amount of gas bubbles.

Buchholz relay is a gas actuated relay used for protecting oil immersed transformer against all types of internal faults and makes use of the fact that fault produces over current and overheating that decomposes oil, thus generating gases.


      The Buchholz relay comprises hinged float and mercury switch assembly for both the alarm and trip circuits. The entire assembly is in an oil proof case which has two glass windows. When the oil level is reduced from the desired level, the float switch moves down that will touch the contact. In case of major faults, the gases generated in transformer tank due to decomposing of oil rush towards conservator tank through Buchholz relay. These gases pressurize the oil and reduce the oil level in buchholz relay and the float switch go down to close the trip circuit as shown in the below figure. While reducing the oil level, the alarm will get activated. If the pressure is higher in the transformer tank the trip circuit will be activated to close the mercury switch and trip the power to transformer.


    Valve V1 - Buchholz Relay to transformer tank
    Valve V2 - Conservator to Buchholaz Relay
    Valve V3 - Reservoir to transformer tank
    Valve V4 - Oil release in the Buchholaz Relay (left side)
    Valve V5 - Air release in the Buchholaz Relay(right side)

    1. Keep all the front panel switches in OFF condition.
    2. If any pressure is present in the relay, release it first by using Valve V5 and also use the hand valve which is present on the reservoir to release the pressure from the remaining part.
    3. The main power supply chord is connected at the back side of buchholz relay setup.
    4. Close all Valves except V2.
    5. Fill the oil in the buchholz relay by using foot pump by little opening of the Valve V5 (only if air lock is present in the relay).
    6. Switch ON the power supply.
    7. Switch ON the buzzer toggle switch.
    8. Open the Valve V1 slightly and decrease the oil level manually below the particular level by carefully seeing the oil level in the buchholz relay. Now the alarm contact will be closed and the buzzer will operate in control panel.
    9. After getting the alarm indication, close the Valve V1.
    10. First release the Valve V1 and V3 and collect the oil in to the resevoir then follow the steps 2-8 once again.
    11. After completion of experiment open all the Valves including hand Valve except V4 and V5 to release the air pressure inside all the tanks.