The standard pressure relief for most distribution transformers is a pressure relief valve (or PRV). This valve operates automatically with a self-actuated spring. It can also be operated manually with the pull ring.
Main Function of the PRV valve in a Transformer
Pressure Relief devices/valves are typically classified in the Transformer industry as safety or protection devices. The transformer pressure relief device valve for liquid-immersed transformers opens in case of an internal fault and guarantees that the occurred overpressure can properly release.
Automatic Resealing Mechanism
Pressure relief device valves for transformers close with decreasing pressure back to a completely closed and leak-tight position.
Installation Flexibility
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.
Testing and Application
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.
Response to Internal Faults
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.
Working Principle of the Pressure Relief Valve
The working principle of the transformer pressure relief valve is very simple. If pressure arises inside a transformer and exceeds a preset 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.
Mounting Position and Safety Importance
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.
Purpose of the PRV valve in a transformer:
• Protection:
PRVs protect the transformer’s tank from rupturing due to excessive pressure buildup.
• Overpressure Prevention:
They release pressure during fault conditions (like short circuits or overloads) or due to thermal expansion of the insulating oil.
• Fault Indication:
Some PRVs have indicators (like a flag or a limit switch) that signal when the valve has operated, alerting personnel to potential issues.
How the PRV valve in the transformer works:
1. Pressure Rise:
When the pressure inside the transformer tank increases beyond a set point (cracking pressure), the PRV activates.
2. Valve Opens:
A spring-loaded mechanism within the PRV opens, releasing the excess pressure.
3. Pressure Release:
The valve allows oil or gas to escape, lowering the pressure inside the tank.
4. Resealing:
Once the pressure returns to a safe level, the valve automatically reseals, preventing further loss of oil or gas.
Types and Features of PRV Valve in Transformers:
• Automatic and Manual Operation:
Some PRVs can be operated manually (e.g., using a pull ring) in addition to their automatic function.
• Flow Rate:
The flow rate of the PRV is designed to match the size and potential pressure increase of the transformer.
• Material and Design:
PRVs are often made of materials like stainless steel and aluminum and may have specific designs depending on the type of transformer fluid used.
• Directional Cover:
Some PRVs have a directional cover to safely expel hot oil away from the transformer.
Importance:
• Safety:
PRVs are a crucial safety device for transformers, preventing catastrophic failures and potential hazards.
• Reliability:
They contribute to the overall reliability and longevity of the transformer by preventing damage from overpressure.
• Maintenance:
Regular inspection and maintenance of PRVs are essential to ensure they function correctly and reliably.
Conclusion
The 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. The transformer is overloaded and 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. Therefore, periodic maintenance of the PRV valve in transformer is necessary.
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Reference
1- What is transformer pressure relief valve main function and How to check
https://www.transformer-home.com/news/what-is-transformer-pressure-relief-valve-main-function-and-how-to-check/
2- Transformer Pressure Relief
https://www.maddox.com/resources/articles/pressure-relief-fault-detection-devices
