Why Pressure Relief Device(PRD) for Oil Filled Transformer?
The winding of the transformer is housed in the transformer oil tank. The oil is used in the oil-filled transformer for insulation as well as for cooling purposes. In case of a fault inside the transformer caused by a sudden and severe short circuit, the transformer oil gets heated up, and enormous gases are evolved in the transformer tank, which causes a substantial increase in the internal pressure and develops the pressure in the transformer tank. If the increased pressure is not vented outside of the transformer tank, there may be a potential hazard of transformer tank explosion. In this case, the PRD of the transformer is used to protect the transformer.
The transformer tank is not designed to sustain pressure as it is not considered a pressure vessel, and if the gas pressure is not vented outside of the transformer tank, the tank can explode. Therefore, to release the excess pressure of the transformer tank, a pressure relief device or PRD of the transformer is installed for the oil-filled transformer to protect the transformer under severe fault conditions.
A Pressure Relief device (PRD of Transformer) is also called a Pressure Relief Valve(PRV). A (PRV) is a type of valve used to release stored gas in various equipment in order to maintain an optimal pressure level. PRV opens gradually as pressure builds up in order to release the necessary amount of pressure.
Construction of PRD
The Major parts of the pressure relief devices (PRD of Transformer) are the spring, disk, lip, indicator, etc. An overview of the PRD is shown below.
PRD Working Principle
The Pressure relief device has a spring-loaded mechanism that vents the excessive pressure built inside the transformer. The PRD of the transformer automatically resets when the pressure inside the tank drops down to pre-set pressure. The PRD allows the rapid release of excessive pressure that is built during severe internal faults. The pressure relief device is fitted on the top of the main tank. The PRD of the transformer has inverse time characteristics. It operates faster in case of severe faults.
A bright color-coded mechanical indicator moves with the valve disc during the operation of the pressure relief device. The pin is held in position by an ‘O’ ring in the pin bushing. This pin is clearly visible from ground level, which gives an indication that the device has operated.
The indicator pin can be reset manually by pushing it downward. The pressure relief device is provided with a shielded weatherproof alarm /Trip and is operated by the movement of the valve disc.
The nominal working pressure of the PRD is 20 kPa to 175 kPa, and it can be set in step of 10 kPa. Tolerances are -5 to +7 kPa for 20-90 kPa and +/- 10 kPa for 100-175 kPa.
Mounting of PRD of Transformer
The transformer’s pressure relief device (PRD) is mounted on top of the tank, as shown below.
