What is function of Pressure Relief Device (PRD)?


When a transformer has a serious electrical malfunction, pressure relief devices (PRDs) are its last line of defense. Because PRDs are intended to relieve pressure inside the transformer tank, that they are not applicable to transformers without a tank.

Purpose of PRD

During a significant electrical failure, a high temperature arc is formed, which causes the surrounding insulating liquid to decompose and evaporate. This quick rise in volume within the transformer tank causes a sudden increase in tank pressure. The pressure must be reduced to avoid a tank rupture. PRDs allow pressure to be released.

Types of PRD

PRDs are commonly divided into two types: those that open and close and those that open and stay open. Overall, the re-closing types resembles to be more popular in today’s market.

Re-Closing PRD

  • Transformer PRDs are identical to normal spring-loaded safety relief valves (SRVs).
  • A spring holds a huge metal plate linked to a central shaft in place. Spring tension is estimated to be overcome at a specific pressure (set point).
  • When the tank pressure rise above the PRD’s fixed pressure, the spring will compress and the plate will shift to the open position.
  • The higher the tank pressure, the greater the spring compression. Once the tank pressure is lowered, the spring tension will automatically return the plate to its closed position.
  • A rod linked to a colorful indicator normally directs personnel that the PRD has been actuated; this is useful because personnel are unable to be present at the time of actuation.
  • Aside from the local visual display, the PRD will almost definitely be linked to the alarm monitoring system and the transformer tripping circuit.
  • The PRD lift pressure must be appropriately calculated to ensure proper operation. PRDs should be checked annually. PRD testing is mainly done by hand.

Non-Re-Closing PRD

  • This form of PRD is no longer popular because current technology advancements have rendered its design obsolete.
  • Older designs used a relief pin & diaphragm arrangement.
  • In the case of high tank pressure, the relief pin would break, relieving the pressure.
  • The tank remained open to the atmosphere until the PRD was replaced.
  • Relief pins are designed to break at certain pressures and cannot be fixed. Each pin is tagged with its breaking strength & lifting pressure.
  • The broken pin must be replaced with a pin with the exact same settings as the damaged pin, or the unit will fail severely (tank rupture may occur before the PRD lifts).