Sudden Pressure Relay

Internal arcing in an oil-filled power transformer can instantly vaporize surrounding oil, generating gas pressures that can cause catastrophic failure, rupture the tank, and spread flaming oil over a large area. This can damage or destroy other equipment, in addition to the transformer, and present extreme hazards to workers. The relay is designed to detect a sudden pressure increase caused by arcing. This relay is very sensitive and will operate if the pressure rises even slightly. If a very small pressure change occurs caused by a small electrical fault inside the tank, this relay will alarm. The relay is set to operate before the pressure relief device. The control circuit should de-energize the transformer and provide an alarm. The relay will ignore normal pressure changes such as oil-pump surges, temperature changes, etc.
Modern sudden pressure relays consist of three bellows (see Picture 1) with silicone sealed inside. Changes in pressure in the transformer deflect the main sensing bellows. Silicone inside acts on two control bellows arranged like a balance beam (one on each side). One bellows senses pressure changes through a small orifice. The opening is automatically changed by a bimetallic strip to adjust for normal temperature changes of the oil. The orifice delays pressure changes in this bellows. The other bellows responds to immediate pressure changes and is affected much more quickly. Pressure difference tilts the balance beam and activates the switch. This type of relay automatically resets when the two bellows again reach pressure equilibrium. If this relay operates, do not re-energize the transformer until you have determined the exact cause and corrected the problem.

Picture 1: Sudden Pressure Relay, Section


Old style sudden pressure relays have only one bellows. A sudden excessive pressure within the transformer tank exerts pressure directly on the bellows, which moves a spring-loaded operating pin. The pin operates a switch that provides alarm and breaker trip. After the relay has operated, the cap must be removed and the switch must be reset to normal by depressing the reset button.
Once every 3 to 5 years, the sudden pressure relay should be tested according to manufacturer’s instructions. Generally, only a squeezebulb and pressure gauge (5 psi) are required. Disconnect the tripping circuit and use an ohmmeter to test for relay operation. Test the alarm circuit and verify that the correct alarm point is activated. Use an ohmmeter to verify the trip signal is activated or, if possible, apply only control voltage to the breaker and make sure the tripping function operates. Consult the manufacturer’s manual for your specific transformer’s detailed instructions.

Testing Suggestion

Picture 2: Photograph of a Sudden Pressure Relay


Picture 2 shows an example relay. Inspect the isolation valve to ensure it is open. With the transformer offline and under clearance, functionally test the sudden pressure relay by slowly closing the isolating valve. Leave it closed for a few seconds and reopen the valve very suddenly; this should activate the alarm. If the alarm does not activate, test the relay. If the relay fails to function, replace it with a new one.