Pressure Relief Device Operation and its Types

Introduction

A pressure relief device (PRD) is a safety feature in transformers. Its job is to release the insulating fluid if the pressure inside the transformer suddenly gets too high. This is crucial because it prevents the transformer from exploding, which could cause serious harm and damage nearby.

Having a PRD also helps protect the transformer from getting severely damaged. It does this by lowering the internal pressure and controlling the oil leakage.

One great thing about a PRD is that it can quickly detect faults, no matter how many windings the transformer has, where the tap changer is positioned, or if there are instrument transformers.

PRD Types

Re-closing Pressure Relief Device

Re-closing Pressure Relief Devices (PRDs) are equipped with a spring-loaded mechanism to release insulating fluid when pressure builds up inside a transformer. Once the pressure drops, they automatically seal back. They come with visual indicators and alarm systems to alert personnel. These PRDs are preferred today because they are faster, more sensitive, and more reliable than non-re-closing PRDs.

Advantages of re-closing PRDs

Advantages of re-closing PRDs include their ability to prevent oil leakage and tank rupture, ease of testing and maintenance, independence from transformer parameters, and adjustability to different pressure settings.

Disadvantages of re-closing PRDs

However, there are some disadvantages. They require careful calculation of spring tension and lift pressure, can be affected by temperature changes, and may not be suitable for certain transformer types

Non-re-closing PRD

Non-re-closing PRDs, on the other hand, have a relief pin and diaphragm that permanently release pressure. They are considered outdated and less efficient compared to re-closing PRDs.

Advantages of non-re-closing PRDs

Advantages of non-re-closing PRDs include simplicity, affordability, capability to handle large pressure surges, and easy replacement.

Disadvantages of non-re-closing PRDs

However, they also have drawbacks. They leave the tank open to the atmosphere, necessitate exact replacement of the relief pin, lack sensitivity to minor faults, and lack indicator or alarm systems.

Construction of PRD

A Pressure Relief Device (PRD) for distribution transformers serves the same purpose as it does for power transformers: it releases insulating fluid if there's a sudden and uncontrolled increase in pressure inside the transformer. While the principle remains the same, the structure of a distribution transformer PRD can vary based on the transformer's type and size.

For instance, Hitachi Energy offers a PRD for distribution transformers with the following components:

1. A cylindrical body with a flange for mounting onto the transformer tank.
2. A spring-loaded valve that opens when the pressure inside the tank exceeds a preset limit.
3. A diaphragm that seals the valve, preventing oil leakage during normal conditions.
4. A rupture pin is designed to break the diaphragm and release pressure permanently during severe faults.
5. A visual indicator displaying the PRD's status (normal, operated, or ruptured).
6. An electrical contact that sends a signal to the control system when the PRD activates or ruptures.

Operation of PRD

The Pressure Relief Device (PRD) functions to alleviate pressure buildup within the transformer caused by minor or major faults. It operates by opening a valve when pressure inside the tank reaches a certain level, which allows excess fluid to be released.

This valve is activated by a spring mechanism that responds to the pressure within the tank. Depending on the type of PRD, the valve may either re-close automatically once pressure levels decrease or remain permanently open.

The PRD's effectiveness lies in its rapid and sensitive response to pressure changes, enabling it to discharge insulating fluid within milliseconds. Additionally, its large opening area facilitates a high flow rate of fluid.

By reducing internal pressure and limiting fluid loss, the PRD serves to prevent catastrophic events such as transformer explosions or oil leaks.