GAS ACCUMULATOR AND GAS DETECTOR RELAYS

Some transformer faults go undetected. A turn-to-turn fault can cause considerable current in the shorted turn, while current in the remaining winding remains relatively unchanged. Since there is little or no change in the current monitored by the CTs, there is no differential current to operate the relays.

Eventually, the turn-to-turn fault will evolve into a ground fault, giving the protective relays the necessary change in current to operate. There are two methods of detecting transformer faults other than by electric measurements.

These methods are:

a) Accumulation of gases due to slow decomposition of the transformer insulation or oil. These relays can detect heating due to high-resistance joints or due to high eddy currents between laminations.

b) Increases in tank oil tank pressures caused by internal transformer fault.

Gas accumulator relay

This type of relay, commonly known as the Buchholz relay, is applicable only to transformers equipped with conservator tanks and with no gas space inside the transformer tank. The relay is placed in the pipe from the main tank to the conservator tank and is designed to trap any gas that may rise through the oil.

It will operate for small faults by accumulating the gas over a period of time or for large faults that force the oil through the relay at a high velocity. This device is able to detect a small volume of gas and accordingly can detect arcs of low energy.

The accumulator portion of the relay is frequently used for alarming only; it may detect gas that is not the result of a fault, but that can be evolved by gassing of the oil during sudden reduction of pressure. This relay may detect heating due to overall heating, high-resistance joints, high eddy currents between laminations, low- and high-energy arcing, or accelerated aging caused by overloading.

Gas detector relay

The gas detector relay can be used only on conservator transformers, either conventional or sealed. The relay will often detect gas evolution from minor arcing before extensive damage occurs to the windings or core.

This relay may detect heating due to overall heating, high-resistance joints, high eddy currents between laminations, low- and high-energy arcing, or accelerated aging caused by overloading. Essentially, the gas detector relay is a magnetic-type liquid-level gage with a float operating in an oil-filled chamber.

The relay is mounted on the transformer cover with a pipe connection from the highest point of the cover to the float chamber. A second pipe connection from the float chamber is carried to an eye-level location on the tank wall.

This connection is used for removing gas samples for analysis. The relay is equipped with a dial graduated in cubic centimeters and a snap action switch set to give an alarm when a specific amount of gas has been collected.