Why there is only single float in the OSR attached to OLTC of a Transformer?

         Oil surge relay (OSR) is a single float Buchholtz relay fixed to OLTC and its conservator tank. The purpose of OSR is similar to the Buchholtz relay and acts whenever sudden oil rush towards OLTC conservator tank happens due to flashover in OLTC tank. Generally, Buchholtz relay connected to the main tank contains two floats i.e. upper and lower, electrically connected to alarm and trip circuits. Apart from low oil level condition, upper float acts when gasses accumulate due to minor internal faults in the Transformer and lower float acts when sudden rush in oil towards to conservator tank is sensed due to major internal faults in the Transformer. In the OSR connected to OLTC, only single float is provided which acts due to oil rush caused from OLTC internal fault. If the upper float, which acts due to accumulation of gasses, is also provided in the OSR, many times gasses that are normally generated in the frequent operation of OLTC trips the Transformer without any internal fault. Hence only single float is provided in the OSR connected to OLTC to avoid unwanted interruption of supply to the Transformer and ensures safety tripping when any internal fault occurs or low oil level condition in the OLTC tank.

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3 thoughts on “Why there is only single float in the OSR attached to OLTC of a Transformer?

  1. Hi,What is the direction of Oil surge relay fixing. towards to conservator or towards to main oil tank?.
    While fixing time the isolation valves direction also should be similar with oil surge relay?

    thanks .

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  2. The Buchholz Relay is a gas operated relay. Meaning it operates when gas enters it. Not oil. When a major internal fault occurs, due to sparking gases are formed which rush towards the conservator. These gases hit the lower float in the relay which in turn trip the machine. Oil doesn’t operate the relay.

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