Transformer Pressure Relief & Fault Detection Devices

You may be familiar with the terms pressure relief and fault detection. They are sometimes used interchangeably. But, each plays a different role in transformers. Here, we’ll clarify these differences and the devices used for each. Let’s start with a look at how they work.

The sealed airspace inside a transformer tank expands and contracts with temperature fluctuations. This causes changes in the airspace’s pressure. As the oil heats, it expands and compresses the airspace, causing a rise in airspace pressure. When the oil cools, it contracts. This increases the airspace volume and reduces the airspace pressure. 

Difference Between Pressure Relief and Fault Detection

Normal changes in internal pressure occur gradually. They are primarily related to transformer loading and ambient temperature fluctuations. These pressure changes require what is called pressure relief.

Abnormal events, like internal faults, cause a different type of pressure change. These changes are more sudden. To detect these types of events, fault detection devices are used.

Now let's take a look at both pressure relief and fault detection devices.

Transformer Pressure Relief

Tank designs have a certain withstand pressure. This is the amount of positive pressure they tolerate before permanently deforming. The minimum value is 7psig per IEEE C57.12.39. For pressure levels above 7psig, a pressure relief device is needed. These devices operate automatically when tank pressure reaches a certain level. There are many types of pressure relief devices on the market. We’ll look at the two most common for distribution transformers:

• Pressure relief valves (PRV)
• Pressure relief devices (PRD)

Note: There are different philosophies surrounding transformer pressure relief implementation which go beyond the scope of this article. We have provided the IEEE Tank Pressure Coordination standard values under the PRV section below for reference (See IEEE Std. C57.12.39). The below values differ from the CSA. 

Pressure Relief Valve (PRV)

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 (pictured below).

Automatic Cracking Pressure Operation

The valve’s spring compresses when tank pressure is too high. The PRV’s cracking pressure is defined by IEEE standards to be 10 PSIG +/- 2 PSIG. The compressed spring opens the valve to expel the excess pressure. 

Note: CSA standard requires a PRV cracking pressure of around 7psig.

Reseal Pressure

Once the excess pressure is removed, the valve automatically reseals at 6 PSIG. This prevents any excess gas from leaking out of the tank.

Flow Rate

Each PRV comes with a specified flow rate—for example, 50 SCFM at 15 PSIG, 20 deg. C. The flow rate of a PRV depends on the size of the transformer. Smaller transformers require a lower flow rate than larger ones.

Fluid Application

Certain transformer fluids require particular types of PRVs. For example, natural esters require a different PRV design than mineral oil. This is due to the difference in how each fluid oxidizes. This is important to consider when retrofilling a mineral oil filled transformer with natural esters. 

Pressure Relief Device (PRD)

Small vs. Large Pressure Relief

Pressure relief devices (PRDs) perform the same basic function as a PRV. But, they are usually used on larger transformers. It is uncommon to see a PRD on a transformer smaller than 1,500 kVA. But, it is perfectly acceptable to use them on smaller units when needed. PRDs are usually mounted on the top of the transformer tank. 

Note: “Pressure relief device” is a generic industry term used for both PRVs and PRDs. PRVs are generally the go to for smaller kVAs.

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For padmounts the PRD is usually in a tamper proof enclosure on the top of the tank. 

Monitoring

One benefit of the cover mounted PRD is that it can monitor–both locally and remotely. Each PRD has a physical indicator (or semaphore). This provides a physical sign the device has operated. This semaphore can either be a yellow button on the top or an arm that raises when pressure is released.

Alarm contacts

A PRD can also have alarm contacts for remote electronic monitoring. The contact performs a similar function to the semaphore. The contacts allow remote monitoring and a time specific indication for the device’s operation.

Now let's look at fault detection devices.

Transformer Fault Detection

A low-impedance arcing fault in transformer oil creates a sudden pressure rise in the tank. As the arc passes through the insulating fluid, it rapidly breaks it down into component gasses. This makes a gas bubble. This bubble results in a very sudden pressure change inside the tank. Fault detection devices detect this sudden change or shock wave of pressure in fault conditions.

Unlike pressure relief devices, fault detection devices monitor the rate of pressure change in the tank. They don’t operate under slower pressure rises from overloads or outside temperature changes. Examples of fault detection components are:

• Internal fault detectors (IFD sensors)
• Sudden pressure relays (SPRs).

Internal Fault Detector (IFD)

An Internal Fault Detector (IFD) is a common safety device on polemount and padmount transformers. It has a brightly colored indicator that pops out after an internal fault occurs. You can see this indicator from the ground. It alerts utility line workers that the transformer has faulted and must be replaced before energization. This protects line workers from re-energizing faulted transformers. (e.g. catastrophic tank failure)

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IFDs are used on both 1-ph and 3-ph units. They work well on polemounts, padmounts, and substation style transformers. The IFD sensor also has an integrated standard pressure relief valve that meets IEEE or CSA standards. This provides a two-in-one device for fault detection and pressure relief.

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Sudden Pressure Relay (SPR)

A sudden pressure relay (or rapid pressure rise relay, RPR) is a safety device generally installed on larger transformers. Typically, power class units above 10 MVA. They offer both alarm and trip functions for monitoring and protection. SPRs sense low level faults which may be missed by a relay monitoring only electrical parameters. It is a very fast operating protection device.

The most recent designs use a seal-in auxiliary relay with the SPR. This seal-in relay protects against switching surges and has a manual reset. For best results, the seal-in relay should be included with the SPR. The SPR comes in two options—"in gas” or “under oil”. Both function the same way. But they are separate devices, designed for two different mediums.

Conclusion

It's important to understand the difference between pressure relief and fault detection. Generally, pressure relief is required, whereas fault detection is optional. Even so, many utilities are now requiring fault detection, like IFDs on their transformers. This protects linemen working in the field to restore power to the grid after a fault. The right pressure relief and fault detection system depends on the transformer type, use, and needs of the project. 

Maddox provides a wide range of these components on our transformers. Fill out the form below to get a quote for your transformer.