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Frequently Asked Questions

What is a Zig-Zag Grounding Transformer?

A Zig-Zag grounding transformer is a special three phase autotransformer with no secondary or output. It is used to establish a neutral on a 3 wire system. There are several reasons why a zig zag autotransformer may be used include ground fault protection, neutral grounding (solid or via a resistor), and harmonic filtering.

What is an Isolation Transformer?

All transformers other that autotransformers are Isolation Transformer. This is due to the fact that the primary and secondary windings are electrically isolated from each other (they are not physically connected to each other). The transformation in voltage and current between primary and secondary windings occurs as a result of the shared magnetic field in the core (Mutual Inductance).

What is an Autotransformer?

An autotransformer is a transformer that has only one winding per phase, part of which is common to both the primary and secondary circuits. Transformers wired in a “Buck-Boost” configuration are autotransfomers. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. An autotransformers can be used in either a step-up or step-down application unlike isolation transformers. Autotransformers can also be used as part of a reduced voltage starter to reduce motor inrush current (see Motor Starting Autotransformers)

What are the standard service conditions for a dry type distribution transformer?

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: < 70%
  • Altitude: ≤ 1000m ASL (3300 ft.)

Avoid rain, moisture, high temperature, intense heat or direct sunlight. Maintain the minimum recommended clearances and ensure that all ventilated panels are clear of obstructions.

What is temperature rise?

Temperature rise refers to average increase of temperature of the transformer windings at full load above the ambient temperature. These is also a “hot spot” temperature, which refers to the hottest temperature at any specific point in the transformer winding. For example, distribution transformer with a 220°C insulation system would typically be designed with a 150°C average temperature rise and a + 30°C hot spot allowance. 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 C130°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. See table below with the maximum average winding temperature rise, maximum hot spot temperature rise and maximum winding temperature for the most common insulation ratings. 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.

Insualation Class Average Winding
Temperature Rise
Hot Spot
Temperature Rise
Maximum Winding
Class 180 115°C 140°C 180°C
Class 200 130°C 160°C 200°C
Class 220 150°C 180°C 220°C

Customers often request lower than the average temperature rise values shown in the table above. The lower average temperature rise can offer the following benefits:

  • The transformer can be installed in higher ambient temperatures without overheating beyond what the insulation is designed for.
  • The transformer can be expected to have a longer operational life with lower MTBF.
  • The transformer can handle continuous overloads and higher short time overloads without overheating beyond what the insualation is designed for.

What are the storage requirements for a dry type transformer?

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.

What is required clearance around a dry type distribution transformer?

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.

What do I need to consider for transformers energized at low ambient temperatures?

In Dry Type Transformers, the air inside and surrounding the enclosure are a critical part of the transformer’s function. Low ambient temperatures generally do not adversely impact an energized transformer, as the energization (no-load) losses typically generate enough heat to maintain appropriate conditions, in low ambient temperatures as low as -40 °C.

There are two main issues with energizing transformers which have been stored at low ambient temperatures:

  • The insulation in the coils can become brittle at low temperatures. The expansion of the conductors after loading a transformer from a cold start, or the contraction of the conductor during storage at lower temperatures can lead to a crack in the insulation between turns or between layers, leading to an internal fault.
  • Low ambient temperatures can lead to condensation forming within the transformer enclosure, as well as on and inside the transformer coils. Energizing a transformer with condensation on the coils can result in an internal fault and damage to the insulation.

Rex's installation, operation and maintenance manuals recommends that transformers be tested (meggered), brought above 0°C and/or go through a dry-out process if moisture is suspected to be present. Refer to REX's cold start procedure when energizing a transformers below 0 °C.Damage and injury can result from energizing a transformer which has had its insulation system compromised due to moisture.

What is the terminal temperature rating of ventilated dry type distribution transformers?

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.

What is the odor emitting from my new transformer? 

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.

What is the life expectancy of dry type transformers 

The minimum life expectancy of a dry type transformers is primary dependent on the insulation life vs. temperature relationship, designated as minimum life expectancy in IEEE Std C57.12.56. Other factors such as humidity/condensation, short circuit events, sustained overloads, and other external factors can cause a dry type transformer to fail prematurely, however those are outside the normal operating conditions of the transformer and are not considered in the design life expectancy.

Aging or deterioration of insulation is a function of time and temperature and described in detail in IEEE Std. C57.96. Since, in most transformers, the temperature is not uniform, the part that is operating at the highest temperature will ordinarily undergo the greatest deterioration. Therefore, aging studies consider the aging effects produced by the highest temperature.

Based on the insulation life expectancy curves outlined in IEEE Std C57.96, all of Rex Power Magnetics’ dry type transformers utilize a UL listed insulation system with a maximum hot spot temperature that will provide a design life expectancy of a minimum of 30 years under continuous rated load, and standard ambient temperature conditions. For transformer with a reduced average and hot spot temperature rise, this design life expectancy exceeds 50 years.

Can a transformer be connected in reverse (back fed)?

In general, dry type transformers can be connected in reverse (back fed) however there are several precautions that should be considered:

  • Compensated Windings: Control transformers and distribution transformers below 3kVA are typically designed with an overvoltage on the secondary to compensate for voltage regulation at full load. Reverse feeding these transformers may lead to a lower than expected output voltage.
  • Inrush current: The inrush current when energizing a transformer from the intended secondary terminals will be significantly higher than on the primary side as a multiple of the rated current. This high inrush current can cause nuisance tripping of the protective breaker and special considerations may need to be made.
  • Voltage Taps: Given that there are typically no voltage adjustment taps on the secondary side, the transformer cannot be adjusted to account for higher or lower than nominal incoming voltages. In order to not damage the insulation or overexcite the core, the input voltage should not exceed the nominal rated voltage. Under voltage conditions are ok, and the taps on the primary winding can be used to boost the output voltage
  • Grounding: When the secondary (wye) of a delta-wye transformer is energized instead of the primary (delta), then the wye side of the transformer is not a separately derived service. As such, the neutral should not be connected to building ground nor should it be bonded to the transformer enclosure

Always review applicable codes and standards and consult with the local authority having jurisdiction before reverse feeding transformers.