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Transformer gauges, indicators and monitoring devices

Transformers have monitoring devices such as gauges, sample valves, and test ports to help identify issues in the transformer before they become critical.

Written by:
Ben Gulick & Mac Spiller

May 22, 2023

Liquid temperature gauge on padmount transformer

Transformers are a critical part of the electrical grid, and it is important to make sure they are operating properly while in service. To accomplish this, transformers are fitted with monitoring devices to ensure they are behaving correctly under normal service conditions. There are many types and configurations of monitoring devices out there; we’ll cover the most common ones here.

Common Transformer Gauge Types

Liquid Level Gauge

Maddox padmount transformer liquid level gauge

The liquid level gauge monitors the oil or fluid level inside your transformer. The fluid inside padmount and substation transformers insulates the windings and cools the transformer while in operation. It is important to make sure the fluid remains at the right level throughout the life of the transformer. A low oil level could indicate a leak or problem that needs attention.

(Learn how to troubleshoot a transformer oil leak)

Typical oil level gauges have an internal float that sits on top of the oil. High and Low level indicators mark the position of the float as it rests on top of the oil. The face of the gauge will also have a 25℃ temperature reference marker as well since the level of the oil is affected by the outside temperature (sometimes a lower oil level just means it’s cold outside).

Liquid Temperature Gauge

Maddox padmount transformer liquid temperature gauge

The liquid temperature gauge provides the temperature of the oil inside the transformer tank. Ambient outside temperature and transformer loading are the two factors that influence the oil temperature of the transformer.  As the load on the transformer increases, the oil temperature rises. Some types of oil temperature gauges come with the option for a thermal which allows the gauge to be easily replaced in the field. Learn all about transformer temperature ratings.

The red pointer on the temperature gauge is the maximum level indicator which shows the historical high point of the gauge’s day-to-day movement. Since temperature readings vary with load and ambient temperature, it is key to have an indication of the peak operating temperature to ensure the unit is not being operated above its insulation rating.

Pressure Vacuum Gauge

Maddox padmount transformer pressure vacuum gauge

The pressure vacuum gauge indicates the internal transformer tank pressure. As the transformer oil warms and cools inside the tank, the gauge dial will fluctuate in the positive and negative directions. Ideal tank pressure during normal operation is between 2 and 5 psi.

Since the gauge reading will vary with temperature, sometimes the gauge may indicate negative pressure or vacuum. While oil samples should not be taken with the tank under vacuum, a negative reading does not necessarily indicate a problem. Some transformers are vacuum filled with hot oil during the manufacturing process. When the unit is filled and pressurized with nitrogen at the factory, it is set to the standard 2 to 5 psi, but after the oil in the tank cools down closer to the outside temperature, the gauge pointer moves back toward the zero mark. Once the unit is energized and reaches normal operating temperatures, however, the pressure gauge will move back to the positive location on the gauge.

In some cases, the pressure gauge may read zero. A scenario showing the absence of positive or negative pressure may be concerning as a neutral reading may indicate a leak somewhere in the tank or a faulty gauge. Hard-to-see pinhole leaks can arise from flying rocks or road debris during transit from the factory, or at the construction site during installation. To ensure a good service life, it is key to make sure the transformer holds pressure before energizing.

Common Valves & Fittings

Drain & Sample Valve

Maddox padmount transformer drain and sample valve

The drain and sample valve is one of the most common accessories included with liquid filled transformers. The main part of the valve body is a globe type valve that regulates the flow of oil drained from the transformer tank. During a transformer’s life, it may be necessary to drain a portion or all the fluid inside the transformer for maintenance purposes. In addition, a sampling device is commonly included on the inlet or outlet end. This allows for regular samples to be taken of the oil, such as a Dissolved Gas Analysis or DGA test, while the transformer is in service.

Nitrogen Test Port

Maddox padmount transformer nitrogen test port

All liquid filled transformers with a sealed tank have a nitrogen blanket that fills the tank’s headspace. This nitrogen pressurizes the tank (typically between 2 and 5 psi) and provides a seal for protecting and preserving the insulating fluid in the transformer from contaminants such as oxygen and moisture. For maintenance purposes, a nitrogen test port can be added to the unit. This port can be used to recharge the nitrogen blanket inside the tank as well as relieve pressure for any routine maintenance which may require it.

Nitrogen blanket in a Maddox padmount transformer oil tank

Pressure Relief Devices

A pressure relief device (PRD) is a type of valve included with all sealed tank transformer designs which helps to relieve excess tank pressure. Transformer tanks can build up pressure for a variety of reasons including simple overheating to internal faults. When this occurs, the excess pressure needs a way out to prevent the tank from deforming or rupturing. The PRD operates by venting the excess pressure from the transformer tank to the atmosphere before it reaches a critical level that could cause the transformer tank to rupture.

There are several sizes and types of pressure relief devices. The size of the transformer will typically dictate whether a small or large pressure relief device is required. A smaller wall-mounted pressure relief device, or pressure relief valve (PRV), is used for most distribution transformers. For designs with a higher tank volume, a large cover-mounted pressure relief device may be selected.

Pressure relief devices with a substation transformer large cover mounted pressure relief device on the left and a small padmount wall mounted pressure relief device on the right.

A typical PRD consists of a spring-loaded valve that is installed on the transformer tank. The valve is designed to open when the pressure inside the tank exceeds a set point. When the pressure reaches the set point, the spring is compressed, and the valve opens to allow the excess pressure to escape. In the case of a cover-mounted PRD, the escaping gas or oil typically produces a loud noise, which serves as an audible warning to operators that the transformer is experiencing an overpressure condition. Many cover mounted PRDs will come with a semaphore, or flag, (shown in the above photo on the left) which shows if the device has operated in the field. Both the small and large pressure relief devices are self-resetting (meaning they don’t have to be manually adjusted or replaced after a successful operation).

Alarm Contacts

Maddox substation transformer gauges with alarm contacts: liquid level gauge, liquid temperature gauge, and pressure vacuum gauge.

Alarm contacts are outfitted on transformer devices such as gauges to provide additional monitoring and service functionality–such as the operation of cooling fans. They can give an indication of the condition of a transformer and provide an early warning of potential problems. Alarm contacts are often included with the liquid level, liquid temperature and pressure vacuum gauges; they are contact points set at particular levels on the gauge faceplate. When the pointer of the gauge reaches one of the set contact points, it opens or closes a circuit like a switch (depending on how the control circuit is wired). A normally open (NO) contact will close a circuit when it operates, and a normally closed (NC) contact will open a circuit when it operates. If the oil level, temperature, or pressure exceeds a predetermined threshold, the contacts will close or open the control circuit they are connected to–often triggering an alarm. In the case where an alarm contact is wired to control an alarm, the operation of the contact will alert the operator of the transformer to a potential problem so that it can be addressed before it causes damage to the transformer or other equipment.

A transformer liquid temperature gauge with alarm contacts.

Transformer Remote Monitoring

A diagram showing several substation transformers with remote monitoring connected to a monitoring station

Many customers desire to keep a close eye on their transformers without having to travel on site for a visual inspection. One simple and affordable way to accomplish this is to install gauges with alarm contacts. Gauges can be fitted with multiple contacts. Each contact is configured for a specific spot on the gauge, so that when the gauge indicator reaches that spot, a switch is triggered. Typically, the alarm contacts on the gauges are wired so that when one of the set points on the gauge is switched, it sends a signal through a circuit that communicates remotely with a program or application that alerts the customer. Remote monitoring is common for solar transformers.

Alarm contacts are commonly used to activate fans on transformers with an extended forced air rating. Transformers are often fitted with fan packages to keep the transformer cool and provide extra kVA/capacity for periods of peak loading. The fans are not designed to run continuously, but only for limited time periods. This can be accomplished with a temperature gauge with alarm contacts. The contacts are configured on the gauge at defined temperature levels where the fans need to turn on and off. When the pointer of the gauge reaches the temperature level with one of the contacts, the circuit controlling the fans is switched on or off. There are multiple contact configurations available for specific fan operation schemes.

A substation transformer with remote monitoring device attached to alarm contacts on transformer gauges


All these monitoring devices play a crucial role in ensuring the proper functioning of transformers. These devices help identify issues before they become critical and potentially cause costly downtime. It is important to regularly check and maintain these monitoring devices to keep transformers running safely and efficiently. At Maddox Industrial Transformer, we understand the importance of monitoring devices and offer a variety of options to suit our customers' needs.

Maddox padmount transformer loaded on truck

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