The Ultimate Buyer's Guide for Purchasing transformer test system

If you're working with power infrastructure (specifying, sourcing, or maintaining equipment), then you need to understand everything about electrical transformers. From long-distance transmission to local power distribution, transformers are crucial components in power systems, helping to control voltage, protect equipment, and keep systems running efficiently. 

If you want to learn more, please visit our website.

This guide is designed for electrical distributors, manufacturer reps, and engineering teams making high-stakes decisions about power infrastructure. You'll learn how transformers work, how to select and maintain them, how emerging technology is reshaping their design, and more.

What is an electrical transformer?

To understand what a transformer does, let’s start with the basics: a transformer is a static electrical device that transfers energy between two or more circuits using electromagnetic induction. It doesn’t generate power; it simply adjusts voltage levels so power can be safely and efficiently used or transmitted. It “transforms” the voltage, hence the name.

If you’ve ever plugged a laptop into a wall outlet or seen a substation along a highway, you've encountered a transformer at work. The transformer’s job is to raise or lower voltage levels as needed, stepping up the voltage for long-distance transmission or stepping it down for safe use in homes, offices, or industrial equipment.

The simplest form of a transformer includes a core made of laminated steel and two windings of insulated wire, called the primary and secondary coils. When AC flows through the primary winding, it generates a magnetic field that causes voltage to appear in the secondary winding.

This core concept leads directly to how different transformer designs are engineered for specific applications.

Types of electrical transformers

Not all transformers are created equal. Their design, size, cooling method, and use case determine how well they perform in specific environments.

Common transformer categories

• Power transformers: Found in transmission networks, these are built to handle high voltages (typically above 33 kV). They're large, liquid-cooled units designed for continuous, high-load operation. They can be step-up or step-down in power flow (see below).
• Distribution transformers: These reduce voltage from transmission levels to end-user levels, usually below 33 kV. They're widely used in commercial, residential, and industrial settings.
• Step-up transformers: These raise voltage levels and are often used at power generation facilities to prepare electricity for transmission across long distances. These are called generator step-up transformers, or GSUs.
• Step-down transformers: These reduce voltage, often to achieve safe, usable voltage levels near the point of utilization. 

Common transformer designs

• Pad-mounted transformers: Ground-level units enclosed in steel cabinets, these are common in urban areas and industrial parks where underground service is required.
• Pole-mounted (overhead) transformers: Mounted on utility poles, these serve rural or low-density areas with overhead lines or locations where installing underground service conductors would be challenging, such as old but dense urban areas.
• Substation transformers: Heavy-duty transformers intended for use in a substation yard, typically found in areas inaccessible to the general public. 

Learn more: Types of transformers

Key components of electrical transformers

Now that we’ve covered types of electrical transformers, let’s examine what makes them work. 

Core and windings

Transformer cores and coils handle the voltage conversion that moves electricity efficiently from one point to another. The engineering behind these components is critical. Poor design means energy loss through heat, which reduces efficiency and shortens equipment lifespan. Better core and coil design equals less waste, better performance, and longer-lasting equipment.

• Core: Made from laminated silicon steel or amorphous steel to channel magnetic flux efficiently and reduce energy loss due to eddy currents.
• Windings: Typically, copper or aluminum coils that carry current. Winding configuration impacts the voltage ratio and load capacity.

Learn more: Transformer cores and coils

Protective and functional elements

• Insulation: Materials like paper and oil separate conducting parts, preventing arcing and ensuring safe operation.
• Enclosure/tank: Shields internal parts from weather, mechanical damage, and environmental contaminants.
• Cooling system: Dissipates heat. Can be air-based (dry-type transformers) or liquid-based (oil-immersed transformers).
• Bushings: High-voltage feed-throughs that prevent flashover.
• Tap changer: Allows operators to fine-tune voltage output without opening the enclosure.
• Transformer gauges: Provide real-time measurements on pressure, temperature, and oil levels within the tank.

Learn more: What are transformer bushings, and what’s their function?

Transformer gauges

Reliable transformers require accurate monitoring. Whether using analog dials or digital telemetry, gauges serve as the first warning system for impending issues. They monitor key parameters, such as temperature, pressure, and liquid levels, allowing maintenance teams to ensure the transformers are operating as they should. There are two primary ways to install transformer gauges:

• External gauges: For padmount transformers, external gauges are mounted outside of the main cabinet and are housed in a designated, accessible, lockable, weatherproof box. This provides a layer of protection and allows for safe and easy manual access and inspection. For substations, external gauges are mounted directly to the transformer and are exposed.
• Internal gauges: For padmount transformers, internal gauges are installed inside the low-voltage side of the transformer cabinet, generally in areas that are not easily accessible. This keeps them protected from the elements, but can make maintenance and monitoring more difficult and dangerous. For substations, internal gauges are installed in weatherproof boxes or ATCs. Smart transformers often have internal gauges integrated into their systems, delivering real-time data via SCADA and other platforms.

Deciding whether to use internal or external gauges depends on several factors. To learn more, visit our complete guide on external vs. internal transformer gauges.

How transformers work

The way transformers work is based on the principle of electromagnetic induction. An AC current in the primary winding generates a magnetic field, which fluctuates with the current. A voltage is generated in the secondary coil, and the difference in the number of turns between the primary and secondary windings decides whether the voltage increases or decreases.

How well the core and windings are designed is a major factor in the transformer’s efficiency. Losses primarily occur through resistive heating in the windings, eddy currents in the core, and magnetic hysteresis. High-quality materials and precision engineering help reduce these inefficiencies.

Learn more: How do transformers work?

Understanding step-up vs. step-down transformers

Step-up and step-down transformers convert voltage to the levels devices need for safe, efficient operation. At the heart of this voltage transformation is the turn ratio:

• Step-up transformers: Have more turns in the secondary than the primary coil. These increase voltage and are often used right after power generation (GSUs).
• Step-down transformers: Have fewer turns in the secondary coil than the primary coil. They decrease voltage, often for local distribution and equipment use.

These applications are foundational to the transformer deployment strategy. Knowing which one you need depends on your energy flow direction and end-user voltage requirements.

Learn more: Understanding step-up vs. step-down transformers

The role of harmonics in electrical transformers

Modern electrical systems are full of devices that draw power in non-linear ways. Think of variable frequency drives (VFDs), computers, LED lighting, and other electronic equipment. Unlike traditional loads that draw a smooth, sinusoidal current, these non-linear loads distort the normal waveform. This distortion is called harmonic distortion, and it introduces frequencies into the system that aren’t supposed to be there, which bog down the overall system by taking away from the clean, usable power.

When harmonics pass through a transformer, they cause several issues. The most immediate problems are excess heat and vibration, which can degrade insulation, shorten equipment life, and reduce overall efficiency. Harmonics also increase core losses and cause extra stress on the winding and magnetic components.

To handle these conditions, K-rated transformers are specially designed to withstand the additional heating caused by harmonic currents. The “K” rating (such as K-4, K-13, or K-20) indicates how much harmonic content the transformer can handle without being damaged. These transformers come with features like enhanced insulation systems, lower internal losses, and optimized impedance design to reduce the impact of distorted waveforms.

Ignoring harmonics in your design can lead to premature transformer failure, higher maintenance costs, and power quality issues throughout your system. That’s why it’s critical to assess the types of loads your transformer will serve and plan accordingly, especially in commercial or industrial settings where non-linear loads are common.

Fusing options for padmount transformers

Fuses are small but essential components that help keep padmount transformers safe. They play the role of a safety switch, which means they cut off power when something goes wrong (like a short circuit or an overload), which prevents damage to the transformer and the equipment connected to it.

However, even though they all function in a similar way, not all padmount transformers are exactly the same. Bayonet fuses, for example, are placed inside the electrical transformer and are easy to swap out without shutting down the whole system. Current-limiting fuses are often utilized when a fuse that will step in fast during serious faults to keep the damage to a minimum is needed. They are often used in conjunction with bayonets. Bayonet assemblies can take a number of different fuse links. Dual-sensing fuse links are a popular choice if you want to go for something a bit more advanced that can keep an eye on both heat and current to catch problems early.

Learn more: Fusing options for padmount transformers

Fluid options for liquid-cooled electrical transformers

In liquid-cooled transformers, the fluid inside the tank serves two main purposes: it cools the internal components and insulates them electrically. These fluids flow around the transformer windings and core, carrying away heat and preventing electrical arcs between internal parts. Without the right cooling and insulation, transformers can overheat, short-circuit, or break down early. 

Choosing the right fluid is just as important as selecting the right transformer, especially if you want to meet safety codes and environmental regulations. 

Mineral oil is the traditional and most widely used fluid in transformers. It’s affordable, has good insulating and cooling properties, and performs reliably in most standard applications. However, it’s also flammable, which can pose risks in indoor installations or areas where fire safety is a major concern.

For environments where fire risk must be minimized, such as hospitals, data centers, or densely populated urban areas, silicone fluid offers a safer alternative. It’s more fire-resistant and chemically stable at high temperatures. While it comes with a higher price tag, the added safety and reduced maintenance can make it a worthwhile investment in critical installations.

Natural esters, also known as vegetable-based or bio-based fluids, are becoming increasingly popular for their environmental benefits. They’re biodegradable, derived from renewable resources, and have a high fire point, which improves both safety and sustainability. These fluids are especially well-suited for environmentally sensitive sites, such as near water sources or in protected natural areas, and are often chosen to help meet green building standards or reduce environmental liability.

Transformer safety standards: IEEE, NEMA, and NEC requirements

To ensure your transformer system runs reliably, stays efficient, and passes inspections, you must stay on top of transformer safety standards. These guidelines and regulations help engineers and facility managers design, install, and maintain transformers that hold up under pressure and meet legal requirements.

Here are two major standards every transformer project should follow and why they matter.

IEEE C57 Series

This is the go-to standard for the design and performance of power transformers. The IEEE C57 series covers everything from insulation aging to how well a transformer can handle short-circuit conditions. It also includes testing procedures that ensure transformers can stand up to real-world demands, not just lab conditions. If you're looking at long-term reliability and lifecycle performance, this is the framework to build on.

NEC Article 450

Once you’re ready to install a transformer, the National Electrical Code (NEC) comes into play, specifically Article 450. This section covers how transformers should be grounded, protected from overloads, and safely installed. It’s a must-follow guide for contractors and inspectors, and it ensures that your installation meets both safety and legal standards.

Electrical transformer applications

Let’s connect technical specs to real-world use. Transformer applications break down by voltage and purpose:

• Low-voltage (<1 kV): Office buildings, elevators, and home appliances
• Medium-voltage (1–35 kV): Factories, hospitals, and commercial complexes
• High-voltage (>35 kV): Transmission lines, wind farms, and substations

These categories determine not only transformer specs but also installation type and regulatory scrutiny.

How to specify electrical transformers for projects

Transformer specs must match both technical and environmental constraints:

• Define your load profile: How much power do you need now, and how much will you need in 5 years? Bear in mind that while typical loads are measured in active/usable power (watts “W”), transformers are measured in apparent power (volt-amperes “VA”). A lot goes into the conversion of the two (such as power factor), but a quick rule of thumb is that your usable power is around 85% of your apparent power. So if you need 1MW of power, your transformer should be sized around 1.17MVA.
• Confirm primary/secondary voltages and phase configuration. For a 3-phase transformer, this will typically be either Delta or Wye connection. 
• Consider impedance, cooling method, and insulation class.
• Factor in site conditions: temperature, humidity, and altitude.

Collaborate with engineering teams early to avoid redesigns or capacity bottlenecks.

Electrical transformer configuration and installation

Getting the configuration and installation right is essential for a reliable, safe transformer system.

Three-phase vs. single-phase

Your system's configuration depends on the type of load and the environment. In industrial and commercial environments, three-phase transformers are the standard. They’re designed to handle higher loads and provide balanced power across systems, which improves efficiency and stability. 

For lighter applications, such as in homes or rural areas, single-phase transformers are more common. They’re simpler, more cost-effective, and ideal for locations where the power demand isn’t as high.

Installation methods

Where and how you install your transformer also impacts performance, safety, and cost.

• Pad-mounted transformers are installed at ground level in locked, weatherproof enclosures. They’re ideal for urban and high-density areas because they’re safe, tamper-resistant, and easier to maintain.
• Pole-mounted transformers are attached to utility poles and are quicker and cheaper to install, especially when space or underground access is limited. They’re usually seen in rural or suburban settings.
• Substation transformers are larger, custom-built units found in power distribution stations. They include dedicated protection and control equipment and are engineered to meet specific voltage and capacity needs.

Installation best practices

Regardless of the installation type, it is essential to keep installation best practices in mind. Grounding should always meet National Electrical Code (NEC) and local code requirements to ensure safety and system stability. Outdoor installations must use weather-rated enclosures to protect against moisture and corrosion. It’s also important to verify that all cables, breakers, and connectors are properly sized and rated for the system’s load.

Learn more: Safe harbor energy regulations in the U.S.

Haoshuo supply professional and honest service.

Maintenance best practices for electrical transformers

Keeping a transformer in good shape isn’t complicated, but it does require consistency. Regular, targeted maintenance not only prevents unexpected failures but can also extend the life of a unit by decades.

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1. Start with the oil: Transformer oil not only cools the system, it also insulates it. Over time, that oil breaks down or becomes contaminated. Regular oil analysis can reveal early signs of problems, like moisture, dissolved gases, or particles that indicate insulation aging or arcing. Catching these signs early lets you plan repairs before things get expensive.
2. Don’t skip the visuals: A quick look can tell you a lot. Bushings, those porcelain or polymer parts where wires connect, are especially worth checking. Look for cracks, discoloration, or oil leaks. These may seem small, but they often point to deeper internal stress.
3. Use the right tools: Infrared thermal imaging is a non-invasive way to scan for hot spots while the transformer is energized. It’s one of the easiest ways to catch loose connections or overloaded components early. On the electrical side, tools like a megger or winding resistance meter help test insulation quality and winding health, both crucial for long-term performance — however, this part of maintenance is typically performed by a servicer, not the buyer of a transformer.
4. Think ahead: Proactive maintenance helps avoid outages while also staying compliant with safety and performance standards. Most regulations now require regular condition monitoring and recordkeeping, so staying on top of maintenance is part of staying audit-ready.

Common electrical transformer failures

Even with regular maintenance, electrical transformers can still fail, but the root causes are usually predictable. Understanding them makes it easier to prevent the worst-case scenarios.

Load-related stress

Running a transformer near or over its rated load for long periods can overheat components and stress the insulation. Unbalanced loads where one phase carries more current than the others are just as dangerous since they create uneven heating that wears things down faster.

Cooling system issues

If the radiator fans stop working or the oil isn’t circulating properly, internal temperatures can spike. Once that happens, insulation begins to degrade quickly, and failure becomes a matter of time.

Aging insulation

Insulation naturally wears down with age and use, especially in harsh environments. Contaminants like moisture or oxygen can speed up the process. That’s why oil testing is so important: it gives you a window into the health of the insulation without taking the unit offline.

Physical damage

Sometimes, the damage starts before the transformer is even turned on. Improper handling during shipping or installation can crack internal components or misalign parts. That’s why careful inspection before energizing a new or relocated unit is always a smart move.

How to avoid these problems

Most failures can be traced back to poor planning or skipped maintenance. Specifying quality equipment, following proper installation steps, and committing to regular checks go a long way. When transformers are treated as long-term assets, rather than set-it-and-forget-it machines, their reliability improves dramatically.

The future of electrical transformers

Transformers are changing fast to keep up with how we use and produce electricity today. Demands like integrating more renewable energy, smarter maintenance, and planning for growing power needs are driving changes that make transformers more reliable and efficient. Let’s look at the main trends shaping their future.

Intelligent monitoring

Many of today’s transformers come with built-in sensors that monitor important things like oil level, temperature, and load continuously. These sensors send real-time information so operators can spot problems early. Using smart alerts, they can fix issues before a breakdown happens, which saves time and money.

Load growth planning

Cities and towns are growing, and so is the demand for electricity. When designing transformers, engineers now choose models that can easily handle more power as the area expands. This means new buildings, electric cars, or factories won’t overload the system, and upgrades won’t cause major disruptions.

Grid modernization

The power grid is becoming more flexible and less centralized. Transformers are getting smarter and more eco-friendly to match this change. They help the grid respond quickly to changes and use energy more efficiently, which supports a cleaner and more reliable power supply.

Where to buy electrical transformers

Transformer Testing

The purpose of this article is to provide a list of the standard battery of tests performed on new and remanufactured transformers, while also providing an introductory explanation of the purpose and scope of the most common routine factory and diagnostic field tests. Factory testing is performed according to IEEE C57.12.00 and IEEE C57.12.90 standards for liquid-immersed distribution, power, and regulating transformers and IEEE C57.12.01 and IEEE C57.12.91 standards for dry-type distribution transformers and power transformers.

Test Classifications

(as defined in IEEE C57.12.80-):

Routine Tests

Tests made for quality control by the manufacturer on every device or representative samples, or on parts or materials as required, to verify during production that the product meets the design specifications” (Section 3.393).

IEEE C57.12.00 & IEEE C57.12.90 (Liquid-Immersed)
IEEE C57.12.01 & IEEE C57.12.91 (Dry-Type)

Design Tests

Those tests made to determine the adequacy of the design of a particular type, style, or model of equipment or its component parts to meet its assigned ratings and to operate satisfactorily under normal service conditions or under special conditions if specified, and to demonstrate compliance with appropriate standards of the industry. Syn: type test (IEC)” (Section 3.99).‍

IEEE C57.12.00 & IEEE C57.12.90 (Liquid-Immersed)
IEEE C57.12.01 & IEEE C57.12.91 (Dry-Type)

Other Tests

Tests so identified in individual product standards that may be specified by the purchaser in addition to design and routine tests (Examples: impulse, insulation power factor, audible sound.)” (Section 3.311).

IEEE C57.12.00 & IEEE C57.12.90 (Liquid-Immersed)
IEEE C57.12.01 & IEEE C57.12.91 (Dry-Type)

Transformer Turns Ratio (TTR)

Every two-winding transformer has a ratio. The ratio is the relationship between the number of turns on the primary and secondary windings of a transformer. To understand the basic function of a transformer, you could think of it as a ratio box. No matter what you put into it, it will always produce a result proportionate to the ratio. An example of a 1:1 ratio would be where the input and output voltages are the same (for every 1 turn on the primary winding, you would have 1 corresponding turn on the secondary). For a 2:1 ratio, the secondary (output) voltage is half of the primary (input) voltage—for every two turns on the primary winding, you have one corresponding turn on the secondary side, and so on. If you applied 10 volts to the primary of a transformer with a 2:1 ratio, the result would be 5 volts on the secondary; if you put 20 volts into the same transformer, you would get 10 volts out. This predetermined relationship between the primary and secondary windings for any given transformer is called the calculated ratio. The turns ratio test (TTR) is performed to confirm that the unit’s tested ratio lies close enough to the calculated value per IEEE standards

To find the calculated ratio, divide the rated primary phase voltage by the rated secondary phase voltage as depicted on the nameplate of the transformer. When determining the calculated ratio for a transformer, it is important to refer to the coil (phase) voltage—the phase voltage determines the number of turns at the transformer coils. For a delta-connected winding, the phase voltage is the same as the line-to-line voltage, but for a wye-connected winding, the line and phase voltages are different. For a wye winding, the coil or phase voltage is represented by the second smaller number (line-to-neutral) and is obtained by dividing the phase-to-phase voltage by the square root of 3 and is written as follows: Y/ . For example, a transformer that is Y/ - 480 Y/ 277 would have a calculated ratio of 27.51, whereas a transformer with a delta primary, such as D - 480 Y/ 277, would have a calculated ratio of 47.65.

When a transformer is built at the factory, the actual ratio at the coils will differ slightly from the calculated value due mainly to the fact that you cannot have partial turns. IEEE standards allow a 0.5% variance above and below the calculated value for tested ratios. This standard is used by Maddox, and it is the same standard employed by field testing companies and associations such as NETA. Maddox performs a standard TTR test on all used units when they are brought into inventory and again after the remanufacturing process is complete. Test values are provided for all tap positions on remanufactured units.

Winding Resistance

A winding resistance test helps evaluate the condition and quality of the current-carrying path of the windings in a transformer. For new factory-built padmount transformers, this test is only required for sizes above 2,500 kVA (IEEE C57.12.00), but Maddox utilizes the winding resistance test for all remanufactured medium-voltage units. Winding resistance provides essential diagnostic information, which can aid in evaluating whether a unit is suitable for repair or remanufacturing. Issues such as loose internal connections, faulty tap changers, open circuits, and broken conductor strands or crimp connections may be identified with this test. In the case of a delta connection, measurements are made phase-to-phase (H1-H2, H2-H3, H1-H3). With a wye connection, measurements may also be made phase-to-phase (H1-H2, H2-H3, H1-H3), as well as phase-to-neutral (H1-H0, H2-H0, H3-H0).

This test is measured in ohms, and the value is typically low (tenths or hundredths of a decimal). Keep in mind that when this test is performed (especially in the field), it is typically done at the bushings of the transformer. As a result, measurements will include any components in the current-carrying path of the windings, such as tap changers, fuses, switches, and cable leads, which may affect test results. A questionable reading may not always indicate a problem with the coils themselves. For instance, if one phase of a transformer has a significantly longer section of internal bus work between the bushing and the winding lead it connects to, you may see a higher measured value across that particular phase. In this case, the test data would not indicate a problem, but a simple fact inherent to the mechanical design of the unit.

The same type of variance, however, could show up on a phase with a loose or frayed cable connection at the tap changer, where the variant reading would indicate a mechanical problem that would need to be addressed during the remanufacturing process. For this reason, the proper performance of a winding resistance test requires a proficient mechanical knowledge of the internal workings of the transformer, as well as an aptitude for evaluating the available test data. Results between phases that fall within 5% of each other are generally considered acceptable (IEEE Std 62-, p. 7, section 6.1.1).

Insulation Resistance (Megger)

While insulation resistance (or megger) testing is not recognized by IEEE for determining pass/fail criteria on newly manufactured transformers (discussed in the following paragraph), it is a useful supplementary test for units that have spent some time in service out in the field. As the title indicates, the purpose of this test is to determine the quality of the insulation on a piece of electrical equipment. Insulation resistance testing is used on a variety of electrical apparatus, such as conveyors, motors, fans, refrigerators, HVAC systems, and cables. In this test, we are measuring the resistive capability of the insulation material within the transformer between windings and core ground, commonly measured in megohms. The test is performed by applying a specified DC voltage through the conductor(s) of the transformer. Over time, the insulation may age or degrade from factors such as overheating, external physical stress, or moisture. This degradation can lead to a reduced capability of the insulation to withstand the required operating voltages of the transformer.

Transformers do not see the kind of mechanical and physical stressors common to motors and cables in a raceway. Due to the design of distribution class transformers, test results can yield inconclusive values and may puzzle a field technician who is used to meggering stand-alone cables in a raceway. Core grounding methods and components such as switches, fuses, and tap-changing devices may also affect the results of insulation resistance testing on transformers. For this reason, insulation resistance tests for transformers should be treated as supplementary to the battery of standard routine tests outlined under IEEE C57.12.00, and not the be-all and end-all of determining a good transformer from a bad one. Insulation resistance is performed on the high-side windings to low-side windings, high-side windings to ground, and low-side windings to ground.

Workers performing Megger on a dry-type transformer.

Impedance Voltage (Positive Sequence), Load Loss

With an impedance test, we are measuring the losses in the transformer: the watts/power wasted or lost during electrical operation. The quality of construction along with the type of materials used in the building of the transformer’s coil assembly play a role in determining the results of this test. Unlike the insulation and winding resistance tests, which serve as supplementary evaluations for distribution class transformers, the impedance/load loss test yields concrete results that can be taken at face value. This test may be used to confirm the design values for a given unit where a certain number of losses is requested by a customer on a new factory-built unit.

IEEE lists standardized impedances for distribution class transformers above 500 kVA at 5.75% (+/-7.5%), but sometimes, customers will request something different. This can be largely accomplished with the design itself. The impedance (%IZ) of a transformer is affected by the resistive (%IR) and reactive (%IX) components of a transformer. It is during this test that the reactive and resistive components of impedance are identified, as well as the resulting X/R ratio of a unit. For remanufatured transformers, these values will be determined by how the unit was originally manufactured at the factory. Impedance/load loss testing is a standard routine factory test (IEEE C57.12.00), which is required for all new factory-built padmount distribution class transformers, and it is performed on all transformers that are repaired or remanufactured by Maddox. The tolerance from the specified value for two-winding transformers is ±7.5%; for zigzag units, autotransformers, and transformers with three or more windings, the tolerance is ±10% (IEEE C57.12.00-, p. 64, 9.2).

Excitation, No-Load Loss

With an excitation test, we are testing the flow of magnetic flux in the transformer core. If the words magnetic flux sound a bit too technical, think about a simple magnet with two ends or poles (one south and the other north). If you sprinkled iron shavings on a table near the magnet, you would see the iron shavings line up in long oval loops springing from one end of the magnet to the other (these invisible phenomena are referred to as magnetic lines of flux). These magnetic fields are all around us, and it is this same principle which is behind the invention of the directional compass. A transformer’s ability to produce these lines of flux is what we are after here in the excitation test. To perform this test, voltage is applied to the low side of the transformer windings with the high-side windings open, which allows the amount of magnetic flux required for operation to flow through the core.

Another way to think of excitation is to think of it as the amount of work required to start the transformer. The quality of the core and assembly and its construction influence how much excitation is needed during energization. Imagine trying to roll a car with a dead battery down the street to a nearby parking lot. You would have to do some amount of work to get the car going, which would require a bit more heaving and grunting in the beginning; this extra heaving and grunting would merely be spent in getting the object out of its stationary state. In the same way, a poorly built (or damaged) core assembly will require more “heaving” and “grunting” when the transformer is energized. The additional work required at startup is what we refer to as inrush current. The excitation current and the associated no-load loss are the power that keeps the core energized during normal operation. The quality, orientation, and construction of the laminated core steel in a transformer will determine the exciting current. For new distribution class transformers, the DOE has set minimum efficiencies in distribution class transformers up to kVA.

Excitation/no-load loss testing is another standard factory routine test for new factory-built transformers (IEEE C57.12.00), and it is an essential part of the repair and remanufacturing process at Maddox. The presence of a higher-than-normal exciting current often can lead to the discovery of internal problems, such as shorted turns, a damaged core assembly, or a faulty tap changer. Some of these issues can be fixed in the repair process. This test is also used as a baseline for determining the viability of a unit for repair or remanufacturing, and it is performed on all 3-phase distribution class transformers.

Learn more about transformer cores in this article.

Loss testing being performed.

Phase Relation

The phase relation test confirms the angular displacement and phase sequence between windings in a 3-phase transformer. In layman's terms, it confirms the coils are connected correctly inside the transformer tank. For example, if you apply a voltage across H1 and H2 at the primary winding, you would expect to measure a corresponding voltage across X1 and X2 at the secondary winding. Let’s say for the sake of this example that when you applied a voltage across H1 and H2, you instead found the corresponding voltage across X2 and X3 on the low winding. In this case, the primary and secondary windings would be out of sequence, and the internal winding connections would need to be fixed accordingly.

With two-winding transformers, the coils may be connected in delta or wye. For a 3-phase transformer connected delta on the primary and wye on the secondary, a 30-degree phase shift will typically be present, which can be either leading or lagging. For a transformer connected delta on the primary and delta on the secondary, there is typically a zero-degree (or no) phase shift (the same scenario exists where a wye connection is on both the primary and secondary side). The phase relation test confirms that the internal connection of the coils matches the vector grouping diagram on the nameplate of a given transformer. This information is vital to the proper operation of an electrical system—especially where multiple units are tied together. Verifying proper phasing is a basic part of the repair and remanufacturing process. This test is another routine test required for all new factory-built transformers (IEEE C57.12.00), and it is performed on all units brought into the Maddox facility for repair and remanufacturing.

Leak Test

Every liquid-filled transformer is tested to verify the transformer tank will hold and maintain pressure when put into service by adding 5 PSI of pressure to the tank and leaving it for 24 hours. A visual inspection then is made to verify that no fluid leaks exist around any gaskets or seals, and that no pressure has left the tank by checking the pressure gauge when possible. In the case where a radiator was repaired or replaced, additional care is taken to verify a successful repair that will hold up to usual service conditions in the field.

Liquid-filled transformer awaiting shipment.

Applied Potential

The applied potential test is currently not part of the standard battery of tests for remanufactured transformers. It is a routine test per IEEE C57.12.00, and applied potential is performed on all new factory-built Maddox transformers. The purpose of this test is to ensure the integrity of the insulation system in a given unit by putting the insulation under short-term stress via an overvoltage. For this test, a voltage is applied and gradually increased between the windings being tested, with the starting voltage being no more than one-quarter of the full value. The duration of the test is one minute at the specified test voltage, as outlined in IEEE C57.12.90. This test is often omitted in the field for in-service transformers due to the difficulty of achieving the required test voltage levels. Applied potential testing is designed to fail an insulation system that is already compromised; it is generally agreed that it will not result in damage or failure when performed on a unit with good insulation.

Induced Potential Test

The induced potential test is another overvoltage test, like the applied potential test. It is also a routine IEEE test performed on all newly manufactured transformers. Like applied potential testing, this particular test is currently not included among the list of tests performed on remanufactured transformers. To perform this test, a voltage “greater-than-rated volts per turn to the transformer” (IEEE C57.12.90-, p. 62) is applied and gradually increased for a designated period of time, depending on the frequency at which the test is performed (the frequency supplied must be raised to prevent over-excitation of the core, as the applied test voltage is significantly higher than the rated voltage). The formula for establishing the minimum test frequency is set forth in section 10.7.2 of IEEE C57.12.90.

Impulse Test

Impulse testing is another test that is only performed on newly manufactured units at the factory. The purpose of this test is to analyze a transformer's ability to withstand large voltage surges, as would be common in a typical electrical system. During normal service conditions, transformers are often exposed to sudden high-voltage spikes resulting from lightning or the operation of switches. Along with induced and applied potential tests, we are again testing the dielectric strength of the insulation system in the transformer. In the case of lightning, the voltage wave can take a variety of forms. For this reason, the impulse test is designed to imitate both the form of the wave and the succession in which the various wave shapes may occur. For class I power transformers, these are one reduced full wave, one full wave, two chopped waves, and two full waves. For pad-mounted distribution transformers, these are one reduced wave and one full wave.

Insulation Power Factor Test

Power factor testing is most commonly associated with larger class I and II power transformers, and it is a standard routine test for power units per IEEE C57.12.00. It is important to note that this test is not recognized by IEEE as an accepted method for determining pass/fail criteria on distribution class transformers. The test code laid out in IEEE C57.12.90 also notes the difficulty and potential problems associated with attempting to establish absolute values to apply across the board for this test on distribution class transformers (See Notes 1, 2 & 3 of Table 4, IEEE C57.12.90-, p. 67). Although test results can be difficult to interpret at times, power factor testing provides a diagnostic benefit when comparing test data for a single unit over a period of time. For example, if a more recent set of test results contrasts significantly with data from an earlier test for the same unit, this could alert a technician to the possibility of an issue that may need attention. The use of an initial stand-alone test to establish pass/fail criteria in distribution class transformers, however, is not advisable. Due to the physical construction and the presence of other ancillary components within the path of the test voltage, such as tap changers and switches, results can vary widely for distribution class transformers.

While power factor testing is not performed under the standard battery of tests required by IEEE C57.12.00 for smaller transformers (generally below 10 MVA), it is required by NETA on all distribution class units. It’s important to note that the test values suggested by NETA for insulation power factor testing exist in lieu of the absence of an agreed-upon standard (See Table 100.3, NETA Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems). The values established by NETA do not remove the extant difficulties expressed under IEEE C57.12.90, but rather act as a general guide for technicians performing this test in the field. When field test values fall outside what is recommended by NETA, the next course of action should be to perform the remaining battery of standard tests for field commissioning and evaluate the results. A field technician may need to sign off on his end for any values that do not fall within NETA’s recommendations, and it may be necessary for Maddox to add clarification or verify that the unit’s test results are within acceptable limits according to factory standards. Insulation power factor testing is a standard test for all new and remanufactured class I power transformers supplied by Maddox.

Additional notes:

• Field testing conditions: testing should never be conducted when the tank is under vacuum; the dielectric strength of the insulating material is severely reduced under negative pressure (IEEE Std 62-, p.6, section 5.3.4).
• Testing voltages: for windings in a grounded wye configuration where the insulation is graded or reduced for applications on grounded systems, the test voltage should be applied based on the lowest insulation level of the tested winding (IEEE Std 62-, p.6, section 5.3.3).
• IEEE tolerances for quoted losses: sometimes, the specified no-load and load loss values may be slightly lower than the actual tested values after production on new transformers. This is not uncommon, since an exact value is difficult to attain every time. For this reason, IEEE allows a certain tolerance for specified losses. For specified losses, the no-load losses may not exceed 10%, and the total losses may not go above 6% (IEEE C57.12.00-, p.62, 9.3).

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