Pressure Relief devices/valves are typically classified in the Transformer industry as safety or protection devices transformer pressure relief device valve for liquid-immersed transformers open in case of an internal fault and guarantee that the occurred overpressure can properly release. Pressure relief device valves for transformers close with decreasing pressure back to a completely closed and leak-tight position. Pressure relief device valves for transformers can be installed in any position or inclination on the cover or the wall of a transformer. Upon request, a transformer can be equipped with several pressure relief valves. The pressure relief valves are tested for operating pressure. The application of these devices is to limit a sudden rise in oil pressure, inside a free-breathing oil-cooled Transformer tank.

When the oil-immersed transformer internal fault such as short circuit, the tank produces a large amount of gas，so that the pressure inside the tank increased sharply, the pressure relief valve can release the pressure generated within 2 milliseconds, and when the pressure down to the valve to close the pressure, the valve is closed reliably, so as to protect the oil-immersed transformer tank.

The working principle of the transformer pressure relief valve is very simple. If pressure arises inside a transformer and exceeds a pre-set pressure limit, the pressure safety valve opens its valve clap, which is held by a spring and releases the internal pressure until it declines. After the decrease of the pressure, the pressure valve clap moves back to its original position and closes completely. Normally, the pressure relief device will be mounted on top of the transformer. Due to internal faults, it is suggested to have such pressure relief valves to protect the transformer and release arising pressure quite suddenly.

What should we do if the PRV trips in a transformer?

Pressure Relief Valve (PRV) trips when you are having too much gas pressure within the transformer oil conservator. Gas pressure increases in the transformer for two reasons. (1) The transformer is overloaded and (2) there is some fault in the transformer.

When the transformer is overloaded the winding heats up and the oil expands in that heat. Oil travels upward into the oil conservator. This makes the room for gas in the conservator lesser and thus the pressure increases.

When there is earth fault or phase to phase fault within the transformer there will be enormous heat and because of this methane, ethane, acetylene, etc gases will be formed out of paper insulation and oil in quick succession. Subsequently, the PRV will trip.

So the action plan will be to understand whether there is a fault in the transformer or it is overloaded. See the current readings for understanding the loading condition and if it is overloaded, decrease the load. If there is a fault in the transformer, then you do not have any option but to go for repair.