VPI/VPE Power Transformers

A VPI (Vacuum Pressure Impregnated) or VPE (Vacuum Pressure Encapsulated) power transformer is a dry-type transformer designed for medium-voltage distribution systems. The windings are vacuum-processed with resin or varnish to create a moisture-resistant, mechanically rigid insulation system. These transformers are used where safety, low maintenance, and environmental protection are priorities. Rex Power Magnetics VPI/VPE transformers deliver reliable performance for commercial, institutional, and industrial power systems up to 44 kV class.

Each transformer is built using high-grade, grain-oriented silicon steel cores and precision-wound copper or aluminum conductors. The windings are vacuum-processed with polyester or epoxy resin under controlled pressure cycles to ensure complete penetration of insulation voids. This results in a mechanically strong, moisture-resistant insulation system. After impregnation or encapsulation, the windings are oven-cured and assembled within ventilated or sealed enclosures. The design provides low losses, quiet operation, and long-term dielectric stability.

VPI/VPE dry-type transformers eliminate the use of oil or other liquid dielectrics, removing fire and spill hazards. They are non-flammable, environmentally safe, and require minimal maintenance. The resin-treated windings provide exceptional resistance to moisture, dust, and chemical contamination. Their compact design and low noise levels make them ideal for indoor installations, basements, and mezzanines where oil-filled units are impractical or prohibited by code.

In VPI construction, the windings are impregnated with polyester or epoxy varnish under vacuum and pressure, then oven-cured to bond the insulation layers. This provides excellent mechanical strength and protection against contaminants.

In VPE construction, multiple vacuum-pressure cycles are used with thicker resin coatings, creating a more robust, moisture-resistant finish. VPE offers superior protection in high-humidity or corrosive environments. Rex Power Magnetics manufactures both types to meet application-specific environmental and dielectric requirements.

These transformers are used in medium-voltage distribution systems for commercial buildings, hospitals, universities, manufacturing plants, and renewable energy installations. They are also common in mining operations, marine systems, and data centers where high reliability and environmental tolerance are essential. Rex Power Magnetics VPI/VPE transformers provide dependable voltage transformation for switchgear, motor control centers, and substation applications.

Shielded isolation transformers include a copper electrostatic shield between the primary and secondary windings, which helps reducing capacitive coupling between the windings, attenuating the transfer of high-frequency noise and transients from one system to the next. The primary advantages include superior noise suppression, improved protection against high-voltage spikes, and reduced risk of common-mode interference. The disadvantage is that shielded transformers can be more costly and larger in size compared to their non-shielded counterparts. In applications where high-frequency noise or voltage transients are a concern, the added investment in a shielded transformer is often justified.

Rex Power Magnetics VPI/VPE transformers are designed and tested in accordance with IEEE C57.12.01, IEEE C57.12.91, CSA C9, and CSA C22.2 No. 47 standards. All units are CSA-certified, UL-recognized, and manufactured under an ISO 9001-certified quality management system. Routine tests include dielectric withstand, turns ratio, impedance, insulation resistance, and temperature rise verification. Type and special tests are available on request to validate performance for critical installations.

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.

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.

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.

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 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.

Voltage regulation refers to the voltage drop on the secondary side of the transformer from no load to full load. The voltage drop is a function of the percentage loading and the power factor of the load. Low voltage distribution isolation transformers generally have a voltage regulation which will range between 1-3% at full load for load with a power factor of 1, and 2-5% at full load for load with a power factor of 0.8.

Yes. Outdoor-rated VPI/VPE transformers are supplied with NEMA 3R, 4, or 4X enclosures, offering protection against rain, dust, and corrosion. The sealed insulation system prevents moisture ingress, while weatherproof housings ensure safe operation under varying temperature and humidity conditions. Stainless steel or epoxy-coated enclosures can be specified for coastal, marine, or chemical plant environments. Proper ventilation and clearance ensure optimal cooling and performance in outdoor installations.

Yes. Rex Power Magnetics designs and manufactures custom VPI/VPE transformers to meet specific electrical, mechanical, and environmental requirements. Options include special voltage ratios, kVA ratings, impedance levels, tap arrangements, enclosure materials, and winding configurations. Custom thermal or seismic designs can be provided for specialized installations. Every custom unit is fully tested to the same standards as catalog models, ensuring reliability and compliance with customer specifications.