Pressure Relief Devices_ MBT Oil-Immersed Transformer

Pressure Relief Devices

A pressure Relief Device is one of the important accessories installed on the transformer. This device is known as the last line of defense of transformers against excessive internal pressure. pressure relief device does not operate properly and pressure is not sufficiently relieved within a few milliseconds, the tank can be broken, spreading flaming oil over a wide area. Let’s discover more about them.


The pressure Relief device is designed as a protection device for oil-immersed transformers in case of an internal fault. This device closes with decreasing pressure back to a completely closed and leak-tight position. It can be installed in any position or inclination on the cover or the wall of a transformer. To comply with the customers’ request, several pressure relief valves can be installed on the top of the transformer. The pressure relief valves are checked for operating pressure. The function of these devices is to limit a sudden rise in oil pressure, inside a free-breathing oil-cooled Transformer tank.


When some internal faults such as short circuits occur, a large amount of gas is produced inside the tank. Therefore, the pressure inside the tank increases sharply, the pressure relief valve can reduce the pressure generated within 2 milliseconds, and when the pressure is down to the valve to close the pressure, the valve is closed reliably, to protect the transformer tank.

The working principle

The working principle is very simple. If pressure arises inside the transformer and exceeds the preset pressure limit, the pressure relief valve opens the valve clap, which is spring held, and releases the internal pressure until the pressure is reduced. After depressurization, the pressure valve will return to its original position and close completely. Normally the pressure relief device will be installed on top of the transformer. Due to internal faults, such pressure relief valves should be available to protect the transformer and release the pressure that arises quite suddenly.

What is the solution if the pressure relief device trips in a transformer?

When there is too much gas pressure inside the transformer, Pressure Relief Valve (PRV) trips. There are 2 main reasons to make gas pressure increase in the transformer as below:

-    The transformer is overloaded.

-    There are some faults in the transformer.

When the transformer is overloaded, the temperature of the winding increases and the oil expands in that heat. Oil moves upward into the oil conservator. This makes room for gas in the conservator lesser and thus the pressure increases.


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


Therefore, if there is a fault in the transformer or it is overloaded, you should analyze the root cause and handle it soon to avoid unexpected accidents. If your transformer is overloaded, please decrease the load. If there is a fault in the transformer, then you do not have any option but to go for repair.

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