Harmonic Mitigating Transformers

A harmonic mitigating transformer (HMT) is specifically engineered to reduce current and voltage distortion caused by non-linear loads such as variable frequency drives (VFDs), rectifiers, and computer equipment. Unlike standard K-Factor transformers, which are designed to tolerate harmonics, an HMT actively cancels certain harmonic components—particularly triplen (3rd, 9th, 15th)—through phase-shifting and winding configuration. Rex Power Magnetics HMTs improve power quality, enhance system efficiency, and reduce heating in conductors and equipment, making them ideal for modern facilities with high electronic load density.

An HMT uses phase-shifted secondary windings—typically ±15°, ±30°, or other engineered configurations—to produce currents that are out of phase with harmonic components generated by non-linear loads. When multiple transformers feed a common panel, these phase shifts cause triplen harmonics to cancel in the line conductors, significantly reducing total harmonic distortion (THD). The winding geometry, core design, and internal shielding are optimized to minimize losses and suppress both current and voltage harmonics without requiring external filters or active components.

A K-Factor transformer is designed to withstand harmonic currents without overheating, while a harmonic mitigating transformer is designed to reduce them. HMTs use phase-shifting techniques and specialized winding connections (such as zig-zag, delta, or dual secondary configurations) to achieve harmonic cancellation. This proactive approach improves overall power quality by lowering neutral currents, reducing upstream transformer loading, and minimizing interference with sensitive electronics. Rex Power Magnetics offers both designs to match application needs based on harmonic analysis results.

HMTs are used in facilities with significant non-linear or electronic loads—such as data centers, commercial buildings, hospitals, educational institutions, and industrial plants using drives or PLCs. They are installed in power distribution systems supplying equipment like VFDs, servers, and LED lighting systems. In multi-transformer systems, they are configured in pairs or groups with specific phase displacements to achieve harmonic cancellation across large sections of the network. This results in improved voltage stability, reduced heating, and enhanced reliability of electrical infrastructure.

Unless designed for special service conditions / environments, below are the standard service conditions for dry type distribution transformers:

• Ambient Temperature: -40°C to + 30°C (max peak +40°C)
• Relative Humidity: less than 70%
• Altitude: up to 1000m (3300 ft.) above seal level

To ensure proper operation, avoid installing transformers in environments with excessive moisture, extreme temperatures, or direct sunlight. Maintain recommended clearances and keep all ventilation panels unobstructed.

Each transformer is built with high-grade silicon steel cores and precision-wound copper or aluminum conductors. The core and coil assemblies are dipped in polyester resin, which provides mechanical rigidity, improved dielectric strength, and moisture resistance. The winding configurations are designed for specific phase-shifted performance and optimized for thermal management under harmonic loading. Electrostatic shielding can be included to further minimize common-mode noise. This construction ensures long life, low vibration, and high electrical performance in continuous-duty environments.

Rex Power Magnetics offers harmonic mitigating transformers in a variety of phase-shift configurations, including 0°, ±15°, and ±30° displacements. These shifts are selected based on load profile and the number of transformers operating in parallel. For example, a +15° and –15° pair can effectively cancel 3rd, 9th, and 15th harmonics when supplying balanced three-phase loads. Custom winding connections and vector group designs are also available to achieve optimized harmonic cancellation for specific systems.

Yes. Rex Power Magnetics designs custom HMTs to meet specific harmonic profiles, voltage levels, and mechanical requirements. Customization options include unique phase-shift angles, impedance values, kVA ratings, and enclosure types. Engineering teams can also incorporate electrostatic shielding, multiple secondary outputs, or other design features to achieve targeted harmonic cancellation levels. Each transformer is individually tested to ensure compliance with harmonic mitigation performance standards and long-term operational reliability.

Any transformer which is not installed and energized immediately should be stored in a dry, clean space having a uniform temperature to prevent condensation on the windings. Dry type transformers with resin dipped or epoxy vacuum impregnated coils can be stored at ambient temperatures as low as -50C. Transformers with encapsulated or epoxy cast coils should not be stored at ambient temperatures below -20C to prevent cracking of the epoxy. Preferably, transformers should be stored in a heated building having adequate air circulation and protected from cement, plaster, paint, dirt, and water or other gases, powders, and dust. The floor on which the transformer is being stored should be resistant to the upward migration of water vapor. Precautions should be taken to prevent storage in an area that water could be present, such as roof leaks, windows, etc. Condensation or absorption of moisture can be greatly reduced by keeping the transformer enclosure 5⁰C-10⁰C above ambient temperature. This can be easily achieved by the installation and energization of space heaters (optional). If the transformer is not furnished with internal space heaters, then external, portable heaters can be used. Note: Lamps or heaters should never come in direct contact with the transformer coil insulation.

It is not advisable to store a dry type transformer outdoors, but in the case that it is unavoidable, protective measures should be taken to prevent moisture and foreign debris from entering the transformer enclosure. The plastic wrapping supplied during shipment should be left in place, and a suitable drying agent such as silica gel packs should be used. The unit should also be checked periodically for indications of condensation on the windings, coil support blocks, core, core clamping system and bus/cables.

In dry-type transformers, the surrounding air plays a critical role in their operation. Generally, low ambient temperatures do not negatively impact an energized transformer, as the no-load losses typically generate enough heat to maintain proper conditions, even in environments as cold as -40°C. However, transformers stored at low temperatures present two primary concerns:

• Insulation Brittleness: At low temperatures, the insulation in the coils may become brittle. Expanding conductors when a cold transformer is loaded, or contracting conductors during cold storage, may cause cracks in the insulation, leading to internal faults.
• Condensation Risk: Low temperatures can cause condensation inside the transformer enclosure, as well as on the transformer coils. Energizing a transformer with condensation present on the coils can lead to internal faults and insulation damage.

Rex Power Magnetics recommends testing transformers (megger testing), warming them to above 0°C, or following a drying-out procedure if moisture is suspected. Refer to Rex Power Magnetics’ cold start procedures to ensure safe energization in cold conditions. Energizing a transformer with compromised insulation due to moisture can cause damage and potential safety hazards.

Cold Start Procedure

The minimum required clearances of a dry type transformer to walls, floors or other equipment must adhere to the local electrical code requirements.

In the absence of such requirements, Rex Power Magnetics recommends that dry type transformers be mounted so that there is an air space of no less than 150mm (6”) between the enclosures, and between the enclosure and any adjacent surface except floors. When the adjacent surface is a combustible material, the minimum permissible separation between the transformer enclosure and the adjacent surface should be 300mm (12”). Where the adjacent surface is the wall on which the transformer is mounted, the minimum permissible separation between the enclosure and the mounting wall should be 6mm (0.25”) so long as the surface is of a non-combustible material.

Temperature rise refers to average increase of temperature of the transformer windings at full load above the ambient temperature. when operating at full load. In addition to the average temperature rise of the windings, transformers also experience a “hot spot” temperature, which refers to the highest temperature point in the windings.

For example, a transformer with a 220°C insulation system may be designed with a 150°C average temperature rise and a 30°C hot spot allowance. This means that Above a 40C ambient, the total absolute temperature will not exceed 220°C. Transformers with lower temperature insulation systems (180°C or 200°C) will be designed with lower temperature rises (115° or 130°C) and hot spots so they can be installed in the same ambient temperature and still not exceed the temperature rating of the insulation system.

The table below shows the maximum average winding temperature rise, maximum hot spot temperature rise and maximum winding temperature for the most common insulation classes. Note that these are based on a max average ambient of 30°C during any 24-hour period and a maximum ambient of 40°C at any time.

Customers occasionally specify a transformer of a particular insulation class to be designed with an average temperature rise below the average temperature rise values shown in the table above. The benefits of doing so include:

• Longer Transformer Life: Lower temperature rise means the transformer can operate at a lower overall temperature, extending its life expectancy.
• Handling Higher Ambient Temperatures: Transformers with lower temperature rise ratings can operate safely in higher ambient temperatures without exceeding their insulation limits.
• Increased Overload Capacity: These transformers can handle continuous or short-term overloads without overheating, making them ideal for environments where transformers may be subject to occasional overloading.

The life expectancy of a dry-type transformer is primarily determined by the insulation system and the operating temperature. According to IEEE Std. C57.96, the deterioration of insulation is directly related to the time and temperature the transformer experiences during operation. Insulation materials degrade faster at higher temperatures, so the transformer’s life expectancy is closely tied to how well it is kept within its design temperature limits.

In most transformers, the highest temperature occurs at a specific point in the windings, known as the hot spot. This area undergoes the most significant wear over time, making it the primary factor in determining the transformer’s ageing process.

All of Rex Power Magnetics’ dry-type transformers are designed using UL-listed insulation systems with a maximum hot spot temperature that ensures a design life of at least 30 years under standard operating conditions (continuous rated load, typical ambient temperatures, and no sustained overloads). Transformers designed with reduced temperature rise can extend this design life expectancy to over 50 years, as operating at lower temperatures slows the insulation’s ageing process.

Factors That Affect Life Expectancy:

• Temperature: As discussed, the most significant factor is the operating temperature of the transformer. Operating continuously at higher temperatures reduces the expected lifespan.
• Humidity and Condensation: Humid environments or condensation can affect the insulation material and lead to premature failure. Proper storage and maintenance help mitigate this risk.
• Short Circuit Events: Sudden surges or short circuits can damage internal components and shorten the transformer’s life.
• Overloading: Continuous overloading beyond the transformer’s rated capacity generates excess heat, which accelerates insulation degradation.
• Environmental Conditions: Extreme environments, such as exposure to dust, moisture, or chemicals, can also lead to earlier-than-expected failure.

By following proper installation and maintenance practices, such as avoiding overloading and ensuring the transformer operates within its designed ambient temperature, you can significantly extend its lifespan. Rex Power Magnetics’ high-quality transformers are built for durability, ensuring reliable performance for decades under standard conditions.

It`s normal for new transformers to release some harmless odors from the varnish impregnation used in the coils for a week or two after energization. Older Transformers can also release some odor if loaded to a higher level than they have experienced previously in their history.

Rex Power Magnetics’ ventilated distribution transformer terminals are rated 90°C. Conductors with at least a 90°C insulation rating at or below their 90°C ampacity rating should be utilized.