VPI/VPE Power Transformers

The standard rating for power transformers is 60 Hz, with 50/60 Hz available as an option. Frequency enters the design through the core flux: for a given voltage and turns count, flux density is inversely proportional to frequency, so a core designed at 60 Hz that is energized at 50 Hz runs at 20% higher flux density. On a core already near its economic flux limit, that pushes the steel toward saturation, raising no-load loss, magnetizing current, noise and inrush. A 50/60 Hz unit is therefore designed with the core sized for 50 Hz, meaning more steel or more turns, and it then operates conservatively at 60 Hz. As a result, the transformer is larger and heavier than a 60 Hz-only design of the same kVA. Furthermore, DOE 2016 and NRCan 2019 efficiency requirements are only defined at 60 Hz transformers and do not apply for a transformer rated for 50 Hz operation.

The 50/60Hz option is needed mainly for marine duty and offshore units built in Canada for vessels or platforms with 50 Hz generation, where the transformer may also run on 60 Hz shore power, and for equipment exported to 50 Hz regions.

Voltage regulation is the drop in secondary voltage from no load to full load, and on a medium-voltage power transformer it is dominated by the reactive component of the impedance rather than the resistive one. A formula accurate enough for specification work is regulation ≈ %R × cos φ + %X × sin φ, where φ is the load power factor angle. Take a 2,000 kVA Rex VPI/VPE unit with a nameplate impedance of 5.75%, of which the resistive part, from load loss divided by rated kVA, is about 1.0%; the reactive part is then √(5.75² − 1.0²) ≈ 5.66%.

At unity power factor the regulation is about 1.0% plus a small second-order term, roughly 1.2% in total. At 0.8 lagging it becomes 1.0 × 0.8 + 5.66 × 0.6 ≈ 4.2%. The lesson is that a specification asking for tight regulation on a plant with induction motors and few capacitors is really a request for lower impedance, which raises the secondary fault current. On a Rex VPI/VPE transformer the standard ±2 × 2.5% primary taps compensate for the average drop by raising the no-load secondary voltage, but taps are set de-energized and cannot follow load swings; where the load is cyclical, accept the regulation or apply power factor correction. The impedance in transformers article gives the full derivation.

The standard Rex VPI/VPE design uses a 220°C insulation system with a 150°C average winding rise. IEEE C57.12.01 allows a 30°C hot-spot allowance above the average rise, so at a 40°C ambient the hottest point in the winding sits at 40 + 150 + 30 = 220°C, exactly at the system limit. There is no margin, which is why 40°C is the standard maximum ambient.

Specifying the optional 115°C rise on the same 220°C system changes the arithmetic. The hot-spot rise becomes 115 + 30 = 145°C, so the winding reaches 220°C only at an ambient of 75°C. The practical benefit is either a higher permissible ambient at full load or, more commonly, continuous operation at rated load with the hot spot roughly 35°C below the limit, which extends insulation life several times over. An 80°C rise design gives a hot-spot rise of 110°C and a larger margin still, and is usually chosen for continuous full-load duty in data centres or for a future fan-cooled rating.

One caveat applies. Ambient above 40°C also affects the enclosure, the terminal temperature rating and any accessories, so Rex should be told the true site maximum on the Technical Data Sheet rather than relying on winding margin alone. The transformer insulation classes and thermal management in dry-type transformers articles cover the relationships.

The transformer terminals Rex VPI/VPE transformers are rated at 90°C and conductors with at least a 90°C insulation rating should be used, sized for 75°C ampacity per the Canadian Electrical Code and the NEC.

For example, a 2,000 kVA, 600 V secondary carries 1,925 A at full load, and with the 125% continuous-load factor the conductors need about 2,400 A of ampacity. At the 75°C column that is roughly seven parallel 500 kcmil copper conductors per phase; at the 90°C column it is six. The bus stub must have enough lug positions for whichever count applies, which is why Rex asks for cable size and quantity on the Technical Data Sheet and offers bus duct entry as an alternative at these currents..

The standard Rex configuration is terminal bus stubs suitable for cables, with the stubs located in the top section of the enclosure.. This arrangement is typical when the transformer will be cable connected with cable entering from the side or below. Although the bus stubs are “pointed” upward, cables entering below can connect without having to loop above the transformer. « Stubs down » is the mirror arrangement for bottom cable entry from a trench, and Rex can route the stubs to either end of the enclosure.

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.

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.

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.

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

• Ambient Temperature: -30°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.

The Transformer Cold Start Procedure exists because a VPI/VPE coil that has been below freezing has two vulnerabilities: the resin and layer insulation are brittle at low temperature, so a sudden load and the conductor expansion it causes can crack them; and any moisture that condensed on the cold coil surfaces during a temperature swing is a dielectric hazard the instant voltage is applied. The procedure is about temperature and dryness, not about the electrical connections.

The procedure requires using external heat and periodic insulation resistance tests to ensure the transformer is dry prior to energizing. Where the unit cannot be warmed, the procedure allows energization at no load, using the no-load losses to warm the core and coils gradually, followed by staged application of load. The low-ambient temperature considerations article covers the physics behind each step.

Anti-condensation strip heaters are an optional feature on the VPI/VPE Power Transformer. They are low-watt-density resistance heaters mounted low in the enclosure so that warm air rises past the coils, and their purpose is to hold the coil surface a few degrees above the dew point when the transformer is de-energized; the product page’s storage guidance puts the target at 5°C to 10°C above ambient. They are supplied from an external 120 V or 240 V single-phase source, not from the transformer’s own secondary, because they are needed precisely when the transformer is deenergized.

Control is by a thermostat or a humidistat, or both, wired so the heaters run when the enclosure is cool or humid and switch off in normal operation. Heaters should be specified for any unit that will be stored on site before commissioning, any outdoor Type 3R or 4X installation, any standby or seasonally loaded transformer that spends long periods de-energized, and any temporary power unit that moves between sites. The heater rating and terminal block location are on the Rex approval drawing, and the transformer maintenance checklist article includes heater checks.

Three separate grounding functions meet at a VPI/VPE power transformer and should not be confused. The first is the core ground. The laminated core and its clamping frame are bonded to the enclosure at a single point inside the transformer, so that the core is at ground potential but no circulating current can flow through it; it is made at the factory and checked at commissioning. The second is equipment bonding. The enclosure is bonded to the plant grounding electrode system with a conductor sized to the primary overcurrent device under CEC Section 10 or NEC 250.

The third is system grounding of the secondary, which makes the transformer a separately derived system. A wye secondary neutral is connected to ground either solidly, through a neutral grounding resistor, or via a grounding transformer if the secondary is delta. The system bonding jumper between neutral and enclosure, and the grounding electrode conductor, must be made at one point only, at the transformer or at the first secondary disconnect, never both, or neutral current will circulate on the bonding path and defeat ground fault relaying. For a resistance-grounded secondary Rex can mount the NGR and its monitoring relay on the enclosure; the Neutral Grounding Transformer page covers the delta-secondary case. The transformer grounding and bonding article works through the code requirements.

Neutral Ground Resistors (NGR) are a common option on the VPI/VPE transformer, meaning the resistor, its enclosure and, where required, the neutral current transformer and monitoring relay are mounted with the transformer and wired to the neutral bushing. Resistance grounding limits ground-fault current to a value large enough for relays to detect selectively but small enough to prevent arc damage to the coil and enclosure at the fault point, and it is standard practice for industrial 600 V and 4.16 kV systems in Canada, particularly in mining where the CEC and provincial mining regulations require it.

The resistor is sized from the line-to-neutral voltage and the chosen ground-fault current. For a 2,000 kVA, 600 V wye secondary the line-to-neutral voltage is 347 V; a 5 A scheme needs 347 ÷ 5 ≈ 69 Ω, and a 10 A scheme about 35 Ω. For a 4.16 kV wye secondary the line-to-neutral voltage is 2,400 V, and a 400 A low-resistance system needs 2,400 ÷ 400 = 6 Ω. The chosen current must exceed the system’s capacitive charging current by a comfortable margin to avoid transient overvoltages, and the zero-sequence CT and ground relay must be coordinated with it. The neutral grounding reactors article explains resistance grounding, and the mining page describes the regulatory context.

A Kirk-Key interlock is a trapped-key scheme, offered as an option on power transformers. The principle is that a single key can exist in only one place at a time. In the simplest form, the key is trapped in a lock on the primary switch or breaker while the switch is closed; it can be released only by opening and locking the switch open. That same key then fits the lock on the transformer enclosure’s access panel, and turning it to open the panel traps it there, so the switch cannot be re-closed until the panel is closed and the key returned.

The scheme enforces isolation without depending on a procedure being followed or a padlock being applied, and it satisfies the requirement in CSA Z462 and NFPA 70E for establishing an electrically safe work condition before opening a compartment that exposes medium-voltage conductors. On a VPI/VPE unit the interlocked panel is normally the one giving access to the tap links and primary terminals; secondary compartments with low-voltage stubs are often left on ordinary fasteners. Rex coordinates the lock series and key codes with the switchgear supplier as part of the bus coordination option, so the interlock is ordered with the switchgear. The transformer commissioning article lists the interlock function test among the pre-energization checks.

In an energy storage installation the VPI/VPE transformer steps the inverter’s 480 V or 600 V output up to the 13.8 kV or 27.6 kV collector bus on discharge, and steps the same bus down to charge the battery. The transformer is indifferent to direction, but three design parameters need attention.

1. Taps become a compromise. The ±2 × 2.5% primary taps are set at commissioning to give the correct low-voltage terminal voltage at the expected collector-bus voltage; with power flowing both ways, the drop across the transformer impedance reverses sign, so the inverter terminal voltage will be higher on discharge and lower on charge by roughly the regulation figure at rated current. Most inverters tolerate that band, but the tap should be set at the mid-point of the two conditions rather than optimized for one.
2. Regulation is set by the specified impedance, and the inverter manufacturer usually states a preferred range, often around 5% to 6%, that balances fault contribution against voltage swing; put that number on the Technical Data Sheet.
3. Protection must accommodate a source on both sides: differential or overcurrent relaying needs current transformers at both windings, the inverter’s limited fault contribution means primary-side fuses may not clear a secondary-side fault quickly, and the inverter’s harmonic output must be checked against the transformer rating.

The step-up dry-type transformers article covers the specification essentials.

Yes. Rex supplies VPI/VPE transformers for temporary power substations that move between construction sites, mine developments and utility outage support. The VPI epoxy resin bond gives the coils good resistance to handling shock, but the specification should state the application so that Rex addresses four details.

1. Anti-vibration mounting: Neoprene isolation pads installed between the core-and-coil frame and the enclosure base are required to minimize road vibration that is transmitted to the transformer core & coil.
2. Bracing: The core & coil is braced with thicker gauge core clamps and stabilizing structural steel “A” frames to ensure that the transformer can handle the mechanical stress associated with regular transport and tilting.
3. Enclosure: Type 3R is the minimum for outdoor use, and a Type 4X stainless enclosure is advisable for units that will see road salt or coastal sites.
4. Terminations: cam-lock or quick-connect terminal chambers may be required based on how the temporary skid will be used on site.

In addition to these design considerations, customers should consider anti-condensation strip heaters, a Kirk-Key interlock and thermometers when specifying a transformer for skid or trailer mounting. Special procedures during and after moving a transformer and prior tot energization at a new site, and consideration of the ambient temperatures that the transformer will experience during transit are also critical to ensure the full life expectancy of the transformer.

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.

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.

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 -30°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.

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.

A power transformer’s lifetime energy cost is dominated by losses, and the way to compare two transformers is a total owning cost evaluation that converts no-load and load losses to a capitalized value. The owner supplies an A factor, the value of one kilowatt of no-load loss over the evaluation period, and a B factor, the value of one kilowatt of load loss adjusted for the expected load profile; the evaluated cost is purchase price plus A × NLL plus B × LL.

Rex can provide estimated or even guaranteed no load and load losses at the quotation stage so the evaluation can be done numerically. The role of transformers in energy efficiency article explains the loss components.

Insulation aging in a dry-type transformer follows an Arrhenius relationship: the rate roughly doubles for every 8°C to 10°C increase in hot-spot temperature. The Rex design basis of at least 30 years assumes the hot spot at its rated 220°C continuously, which is conservative because few transformers run at 100% load, 24 hours a day, at 40°C ambient. Under a real load profile the hot spot is below the limit most of the time and the aging rate is correspondingly lower.

The useful consequence for selection is that a cyclical load with a short daily peak ages the transformer far less than the peak alone suggests. A VPI/VPE coil has a thermal time constant of tens of minutes, so a two-hour peak at 110% of rating following a night at 50% load may not raise the hot spot above the continuous rating at all. Conversely, a steady 100% load in a 40°C plant uses the full design life, and the same load at 50°C ambient roughly halves it. Rex applies the IEEE C57.96 loading guide for dry-type transformers to evaluate a specific profile and can advise whether a standard 150°C rise unit, a 115°C rise unit, or a fan-cooled AFN rating best fits the duty. The role of design, environment and load in transformer lifespan article expands on the aging model.

Low secondary voltage under load on a VPI/VPE power transformer has three common causes, and a few measurements separate them. First, record the primary voltage at the transformer terminals and the secondary voltage at both no load and full load. If the primary itself is low, the problem is upstream, usually a utility feed at the bottom of its band. The transformer cannot correct for a low source except through its taps.

If the primary is at nominal and the secondary is low at no load, the tap setting is wrong. Rex VPI/VPE units ship with ±2 × 2.5% primary taps, normally on the nominal position; if the nameplate shows nominal and the no-load ratio is still off, check that all three phases are on the same tap link. Moving one tap step below nominal on the primary raises the secondary by 2.5%. If the primary is nominal, the no-load secondary is correct and only the loaded voltage is low, the cause is regulation, and the drop should agree with the value calculated from nameplate impedance and load power factor; a much larger drop points to a loose or corroded secondary connection, which a thermographic scan under load will show as a hot lug. The transformer tap changers article describes tap operation, and the Maintenance, Operating and Installation Manual covers the tap link procedure.

Every Rex VPI/VPE transformer ships with a certified test report from the IEEE C57.12.91 production tests, and the commissioning tests repeat a subset to confirm nothing has changed in transit or installation. Turns ratio: the field TTR on each tap should agree with the nameplate ratio within 0.5%, the IEEE tolerance, and all three phases should agree with each other within the same band. Winding resistance: a low-resistance ohmmeter reading on each phase should agree with the factory value corrected for temperature and be balanced between phases within about 2%. Insulation resistance: a megger test at the voltage the Maintenance, Operating and Installation Manual specifies for each winding, primary to ground, secondary to ground and primary to secondary, should give readings of the same order as the factory report after temperature correction; a much lower reading points to moisture, and the cold start procedure applies.

Applied and induced potential tests are not normally repeated; if the owner requires a dielectric test, 75% of the factory value is the usual limit. The transformer commissioning article and the Documentation page provide the checklist and reference forms.

Because its cooling ducts are open to room air, a VPI/VPE transformer needs cleaning that a cast coil unit largely does not, and the interval depends on the environment: annually in a clean electrical room, and every three to six months in a dusty plant or a mine. The transformer is de-energized, isolated, locked out with the Kirk-Key released, and the enclosure panels removed. Dry, oil-free compressed air at low pressure, in the order of 200 kPa or less, is blown through the vertical ducts between coil layers and between coil and core from the top down, with a vacuum cleaner at the bottom to collect debris; high-pressure air can drive dust deeper into the ducts and damage the resin surface.

While the panels are off, the inspection covers the items on the transformer maintenance checklist: torque of bus and tap connections, discolouration of the resin or insulation at the coil ends, cracking of the resin surface, condition of coil supports and core clamps, the core ground lead, enclosure gaskets and filters on Type 12 units, fan operation and bearing noise on AFN and AF units, thermometer calibration and set points, and strip heater operation. A thermographic scan of the terminals under load before the outage and an insulation resistance test after cleaning complete the record. The Maintenance, Operating and Installation Manual gives the full procedure.

Provision for future fans is the lowest-cost way to reserve a fan-cooled rating on a VPI/VPE unit without installing the fans on day one, and it is the option to select when plant load is expected to grow in a second phase. The provision changes the transformer itself, not only the enclosure. The core and coil are designed so that winding temperature rise with fans running stays within the 220°C system limit at the AF rating, which affects duct sizes and coil geometry and must be built in from the start; a standard ANN unit cannot be given a full fan rating afterwards. The enclosure is fitted with fan mounting brackets and the thermometer is specified with the extra contact that will start the fans. If the transformer is supplied with primary and or secondary bus work, the bus is sized for the higher future current as well.

The dual rating, such as 2,000/2,667 kVA ANN/future AFN, appears on the nameplate from the outset so that switchgear and cable can be sized for the eventual load. The thermal management in dry-type transformers article explains the ANN and AFN classes.

The Technical Data Sheet on the VPI/VPE Power Transformer page is the interactive form under the Transformer Configurator heading; it is completed online and the Download PDF button produces the sheet that accompanies the quotation request. Rex catalogue products such as control and low-voltage distribution transformers are ordered by part number through the Part Number Builder, but a medium-voltage power transformer is an engineered item and the data sheet takes the place of a part number.

The fields that drive the design, and should not be left blank, are the kVA rating and any fan rating, primary and secondary voltages and connections, BIL for each winding, frequency, impedance and its tolerance, temperature rise, conductor material, taps, enclosure type and finish, terminal configuration and cable entry direction, altitude and ambient range, sound level if lower than standard, and the accessories from the option list. The notes field is the place for the switchgear the transformer will couple to, seismic requirements, loss evaluation factors and any specification clauses Rex must take exception to. On order, Rex issues an approval drawing package showing dimensions, weights, terminal locations, cable entry, the nameplate and a wiring diagram for accessories, and manufacturing begins after the customer signs it off. The custom vs standard transformers article explains where the catalogue ends and the engineered design begins.

The Rex VPI/VPE transformer is CSA Certified and UL Listed, built under an ISO 9001:2015 quality system, and referenced to CSA C22.2 No. 47, CSA C9, IEEE C57.12.01 and IEEE C57.12.91. The CSA mark is the certification that Canadian provincial inspectors, including the Electrical Safety Authority in Ontario, require before a transformer may be connected, and it is issued against CSA C22.2 No. 47 as the product safety standard for air-cooled transformers, with CSA C9 covering performance requirements. The UL Listing is the equivalent evidence for an installation under the NEC, and is what an American authority having jurisdiction looks for on the nameplate.

Copies of the current certificates are on the Certifications page, and the Rex Power Magnetics contact page reaches the engineering group for marking questions.

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.

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.

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.