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What does Dry-Type Self-Cooled Force Air-Cooled Transformer Class AA FA mean?

A dry-type transformer that has a self-cooled rating with cooling obtained by the natural circulation of air and a forced-air cooled rating with cooling obtained by the forced circulation of air. This is sometimes referred to as fan-cooled. Fan cooling can increase a transformer’s kVA rating by 25% to 50% depending on the type and size of the transformer.

  • Why is the insulation rating for some distribution transformer set at 220°C and for others the rating is 200°C?

      Most standard ventilated distribution transformers use a 220°C insulation system. This insulation system provides a 150°C temperature rise over ambient, a 30°C hot spot and is meant to be installed in a 40°C ambient temperature.

      However, if a transformer is wound using copper wires, a few of the smaller frame sizes (15kVA and 30kVA three phase, 15kVA and 25kVA single phase) utilize a 200°C insulation with a 130°C temperature rise over ambient, a 30°C hot spot and is meant to be installed in a 40°C ambient temperature.

      The differences in these smaller kVA sizes using copper wire are the result of U.L. ratings of the wire’s insulation temperature rating. Copper wire has a significantly smaller diameter than an equivalent ampacity aluminum wire.

      Small copper wire with 220°C insulation is not always available, so insulation systems are limited to 200°C (Note: U.L. does not cover 200°C insulation systems for units greater than 1.2kV, they will use 180°C insulation systems).

      Small control and potted transformers have insulation systems well below 220°C because of the resin used. As a result, transformer manufactures generally rate transformers using copper conductors 30kVA and below as having a 200°C insulation system.

      Manufacturers compensate for this by building the transformers to run cooler at full load and as a result have a lower 130°C temperature rise and can operate in a 40°C environment.

      Because both units can be operated in a 40°C ambient, we say copper transformers 30kVA and below with a 130°C temperature rise are equivalent to larger units with a 150°C temperature rise.

  • What are the Advantages and Disadvantages to Using a Fan-Cooled Transformer?

      Advantages:

      • Smaller size; fans may add some height but may reduce width and depth
      • Lower costs for larger units (generally above 1000kVA) to add fans instead of conductor and core
      • Potentially better low-load efficiencies

       

      Disadvantages:

      • Increased complexity and maintenance
      • Increased cost as fan packages may cost more than just adding material in smaller units
      • Additional energy losses and noise when fan motors are operated in higher loads

  • Can transformers be operated above a 1000m/3300′ altitude?

      There are two main considerations for operating transformers at altitudes above 1000m/3300′. Current standards state designs must be valid to these heights. Above this height, the density of air no longer works as effectively to remove heat. As a result the functional kVA of the transformer must be reduced at higher altitudes, typically about .3% for every 100m/330′. The second issue is the dielectric constant of air is reduced at higher altitudes. Dry type transformers use air gaps as an important component of the electrical insulation properties. At higher altitudes, this lower insulation values, typically in medium voltage BIL levels. Ideally, if transformers will be installed above 1000m, inform the manufacturer and the design can be adjusted to meet all requirements at the higher altitudes.

  • Can I increase the kVA rating of an existing transformer?

      The most common way to increase the available kVA rating of an existing transformer is to add additional fan cooling. This typically requires modifications including raising the transformers core, adding fans and fan brackets, a motor power supply and controls to start fan cooling when the transformer’s components reach a preset temperature. Fans should never be added without contacting and following specific instructions from the manufacturer. Low temperature rise transformers (115C and 80C rise with 220C insulation) can maintain higher loads in lower ambient transformers. Always follow proper ventilation and clearance instructions.

  • What is the ambient temperature specs for a standard transformer?

      A standard transformer with 220°C insulation and a 150°C temperature rise, will be rated to run full load in an average 30°C ambient environment over 24 hours with a maximum 40°C ambient temperature.

      Other magnetics may have lower or higher ambient ratings depending on the design and application.

  • What is ANSI C57.12.91?
  • What is ANSI C57.12.51?

      IEEE Standard for Ventilated Dry- Type Power Transformers, 501 kVA and Larger, Three-Phase, with High- Voltage 34.5 kV to 601 V and Low- Voltage 208Y/120 V to 4160 V covering General Requirements. The current standard was updated in 2008.

      This standard is intended to set forth characteristics relating to performance, limited electrical and mechanical interchangeability, and safety of the equipment described, and to assist in the proper selection of such equipment. Specific rating combinations are described in the range from 750/1000 to 7500/10 000 kVA inclusive, with high-voltage 601 to 34 500 volts inclusive and low-voltage 208Y/120 to 4160 volts inclusive. Part I of this standard describes certain electrical and mechanical requirements and takes into consideration certain safety features of 60-Hz, two-winding, three-phase, ventilated dry-type transformers with self-cooled ratings 501 kVA and larger, generally used for step-down purposes. Part Il describes other requirements or alternatives which may be specified for some applications and lists forced-air-cooled ratings for certain sizes.

  • What does Dry-Type Self-Cooled Transformer Class AA mean?
  • What does Dry-Type Self-Cooled Future-Forced-Air-Cooled Transformer Class AA FFA mean?

      Per 1.2.9.4 of NEMA ST-20, a dry-type transformer that has a self-cooled rating with cooling obtained by the natural circulation of air and which contains the provision for the addition of forced-air-cooling equipment at a later date.

  • What does Dry-Type Self-Cooled Force Air-Cooled Transformer Class AA FA mean?

      A dry-type transformer that has a self-cooled rating with cooling obtained by the natural circulation of air and a forced-air cooled rating with cooling obtained by the forced circulation of air. This is sometimes referred to as fan-cooled. Fan cooling can increase a transformer’s kVA rating by 25% to 50% depending on the type and size of the transformer.

  • What does Dry-Type Forced-Air-Cooled Transformer Class AFA mean?
  • What is Temperature Rise?
  • What are Low Temperature Rise Transformers?

      All transformers have operating losses, and heat is the product of these losses. Hammond low temperature rise transformers are designed with reduced 115°C or 80°C full load operating temperature rises. These units decrease total operating losses by 20% and 35% respectively, compared with the standard 150°C rise operating system. Hammond low temperature rise transformers provide greater efficiency under normal operating conditions, and overload capability without harm to their service life or reliability.

  • What is a Fan Cooled transformer?
  • What is NEMA ST 20?
  • What is Coil Hot-Spot Temperature?

      It is the absolute maximum temperature present in the transformer. This number is equal to the sum of the ambient temperature, temperature rise and a variable.

      T Hot Spot = T ambient + T rise + (10-20) °C.

  • What is meant by “Class” in insulation?

      The insulation rating is the maximum allowable winding (hot spot) temperature of a transformer operating at an ambient temperature of 40°C. Insulation systems are classified by the temperature rating. The following table summarizes the different insulation systems available.

      Insulation Rating

      Insulation Class

      Average Winding Temperature Rise

      Hot Spot Temperature Rise

      Maximum Winding Temperature

      Class 105 A 55 degree C 65 degree C 105 degree C
      Class 150 or 130 B 80 degree C 110 degree C 150 degree C
      Class 180 F 115 degree C 145 degree C 180 degree C
      Class 200 N 130 degree C 160 degree C 200 degree C
      Class 220 H 150 degree C 180 degree C 220 degree C
               
      Note: the maximum acceptable temperature rise based on an average ambient of 30 degree C during any 24 hour period and a maximum ambient of 40 degree C at any time.

  • What is ANSI C57.12.01?
  • Do any performance issues arise during high ambient temperatures?

      Temperatures which exceed the rated ambient temperatures for which the insulation system is designed can cause insulation damage and premature failure. This can often occur in hotter
      environments or in rooms which have inadequate ventilation. Care should be taken in installing stacked transformers because the top transformer may use air that has been heated by the lower unit. Damage from high ambient temperatures often does not cause an immediate failure but can cause damage that results in a failure weeks, months or years later.

      High ambient temperatures can be mitigated several ways:

      • Order a transformer designed with a lower temperature rise.
      • Use fan cooling, this is typically an economical solution when a unit exceeds 500-1500kVA.
      • Place the transformer in a temperature controlled location.
      • Properly ventilate the location that the transformer is located in.

      Never try to use cooling fans directly on a transformer or blow across a transformer’s windings.

      Manufacturers use special fans, specific locations, and cooling patterns to cool transformers. Improper placement of airflow could cause disruption of the convection airflow and cause the transformer to overheat.

  • Do any performance issues arise during low ambient temperatures?

      Generally low ambient temperatures do not affect an energized transformer. No-load losses on an energized transformer typically generate enough heat to operate effectively in temperatures to -20°C or lower.

      The main issue with lower temperatures is when the unit is not energized. Extremely low temperatures or if the transformer heats up too quickly may cause welds and insulation to become brittle and crack, especially if the transformer experiences any mechanical stresses.

      More importantly, low temperatures can cause moisture (dew, frost) to form on the unit. This can be absorbed into the insulation system and not be apparent.

      If ambient goes below -30°C, special designs and cold start procedures may be necessary. Care should be taken to store transformers in dry areas with temperature control. Installation manuals typically suggest that transformers be tested (meggered), brought above 0°C and/or go through a dry-out process if moisture is suspected to be present.

      Damage and injury can result from energizing a transformer which has had its insulation system compromised by moisture.

  • Why Are There Physical Clearance (Distance) Requirements on the Nameplate?

      A ventilated transformer’s physical clearance requirements are designed to provide adequate clearance for airflow cooling. Generally, the larger the transformer the more airflow and clearance is needed. The more important areas are the front and back of ventilated transformers where the air may enter in the bottom and exit at the top. Since the sides of distribution transformers generally don’t have ventilation openings, side clearance is less important. There must also be no obstructions that limit airflow into the bottom vents and top clearance from ceilings must also be maintained.

      All smaller clearances should be reviewed by the manufacturer to verify they are adequate. Also note that electrical codes require minimum front panel clearances to allow safe and easy access to the wiring area. Units supplied with factory installed wall-mounting brackets may also have the back closer to the wall than the nameplate requirements; this is acceptable.  These statements may not apply to non-ventilated and/or potted transformers which don’t have ventilation slots and/or may have zero clearance when mounted to wall suing supplied brackets.

  • What is Temperature Class?

      It is the maximum temperature that the insulation can continuously withstand.

      The classes of insulation systems in a transformer are rated as follows:

      Class 105°C

      Class 150°C

      Class 180°C

      Class 220°C

  • Are “Low Temperature Rise” transformers more efficient?

      One common misconception is that low temperature rise units are more efficient.

      While they usually have better efficiency at full load, it doesn’t guarantee that this will be the case at lower loads.

      Efficiency regulations typically use an average load of 35% to 50% when specifying efficiency. A low temperature rise transformer with more mass and surface area, but running with a low load may be less efficient and produce more heat than a standard transformer, while still maintaining a low overall temperature rise.

      Core loss for low temperature rise units are higher than a transformer with the same kVA rating, but a higher rise (present whenever unit is energized).

      Transformer temperature rise and efficiency should be regarded as two separate issues. If a high efficiency unit is desired, the TP1 (DOE 10 CFR Part 431), C802.2 (Canada) or NEMA Premium® energy efficient specifications should be noted. These regulations are all compatible with low temperature rise transformers.

  • What is the maximum surface temperature of a transformers terminals wire leads or connections points?

      Per NEMA ST-20 (2014):

      • Transformers <= 10 kVA can be up to 50C above ambient with a typical maximum ambient of 25C.
      • Transformers > 10 kVA can be up to 35C above ambient with a typical maximum ambient of 40C.
      • Transformers with lower than standard temperature rises will have lower maximum connection point temperatures.

  • What is the base temperature rise of a transformer?

      The base temperature rise of a transformer is the maximum temperature rise at the expected full load capacity. Transformers can be built to run cooler than the base temperature rise, these are typically referred to as lower temperature rise transformers which can either operate in higher ambient temperatures or have additional service factor.

      • 105C Insulation System: 55C Base Temperature Rise
      • 150C Insulation System: 80C Base Temperature Rise
      • 180C Insulation System: 115C Base Temperature Rise
      • 220C Insulation System: 150C Base Temperature Rise

      The Hot Spot Allowance is added the expected ambient temperature and full load temperature rise to get the total expected temperature rise of a transformer.

  • What is the lowest temperature transformers can be stored in?

      Dry-type transformers with the exception of Cast Coil (HPS Endura Coil) can be stored to temperatures of -50 degrees C. Cast Coil Transformers can only be stored to temperatures of -20C. If a standard cast coil transformer is stored below -20C, the epoxy coils can crack. Transformers which are energized before 0C can be run in ambient temperatures to -40C.

      There are two main concerns with low temperatures occuring during storage when the units are not energized. Energized transformers have load and no-load losses which will keep the core and coils operational to -40C:

      • Contraction and Expansion of the core and coil during low temperatures can crack or damage the insulation.
      • Cold temperatures can cause condensation to form on the transformer which can result in short circuits and insulation damage.

      The minimum ambient temperature designed for is -40C based on the Environment Canada data Extreme Minimum Temperatures for Wiarton, ON (-36.4C on 18-Jan-1977) and Lucknow, ON (-36.7C on 05-Feb-1918). If the temperature of a transformer’s core and coil is below -25C (typically during unenergized storage), please consult HPS for the cold start procedure before any energization. If the temperature of a transformer is below 0C, please follow the HPS Cold Start Procedure. It is recommended that the transformer be meggered to make sure it has a minimum value of 100 Megaohm before energization after storage.

  • What is the lowest temperature transformers can be energized?

      Dry-type transformers with the exception of Cast Coil (HPS Endura Coil) can be stored to temperatures of -50 degress C. Cast Coil Transformers can only be stored to temperatures of -20C. If a standard cast coil transformer is stored below -20C, the epoxy coils can crack. Transformers which are energized before 0C can be run in ambient temperatures to -40C.

      There are two main concerns with low temperatures occur during storage when the units are not energized. Energized transformers have load and no-load losses which will keep the core and coils operational to -40C:

      • Contraction and Expansion of the core and coil during low temperatures can crack or damage the insulation.
      • Cold temperatures can cause condensation to form on the transformer which can result in short circuits and insulation damage.

      The minimum ambient temperature designed for is -40C based on the Environment Canada data Extreme Minimum Temperatures for Wiarton, ON (-36.4C on 18-Jan-1977) and Lucknow, ON (-36.7C on 05-Feb-1918). If the temperature of a transformer’s core and coil is below -25C (typically during unenergized storage), please consult HPS for the cold start procedure before any energization. If the temperature of a transformer is below 0C, please follow the HPS Cold Start Procedure. It is recommended that the transformer be meggered to make sure it has a minimum value of 100 Megaohm before energization after storage.