What is transformer rating based on?

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What is transformer rating based on?
What is transformer rating based on? (on photo: Transformer produced by Japanese transformer manufacturer Kitashiba Electric Co., Ltd.)

Temperature & Insulation

In the U.S., transformers are rated based on the power output they are capable of delivering continuously at a specified rated voltage and frequency under “usual” operating conditions without exceeding prescribed internal temperature limitations.

Insulation is known to deteriorate with increases in temperature, so the insulation chosen for use in transformers is based on how long it can be expected to last by limiting the operating temperature.

The temperature that insulation is allowed to reach under operating conditions essentially determines the output rating of the transformer, called the kVA rating.

Standardization has led to temperatures within a transformer being expressed in terms of the rise above ambient temperature, since the ambient temperature can vary under operating or test conditions.

Transformers are designed to limit the temperature based on the desired load, including the average temperature rise of a winding, the hottest-spot temperature rise of a winding, and, in the case of liquid-filled units, the top liquid temperature rise. To obtain absolute temperatures from these values, simply add the ambient temperature.

Standard temperature limits for liquid-immersed power transformers are listed in Table below.

Standard limits for Temperature Rises Above Ambient

Temperature riseTemperature
Average winding temperature rise65°C **
Hot spot temperature rise80°C
Top liquid temperature rise65°C

** The base rating is frequently specified and tested as a 55°C rise.

The normal life expectancy of a power transformer is generally assumed to be about 30 years of service when operated within its rating. However, under certain conditions, it may be overloaded and operated beyond its rating, with moderately predictable “loss of life.”

Situations that might involve operation beyond rating include emergency rerouting of load or through-faults prior to clearing of the fault condition.

Outside the U.S., the transformer rating may have a slightly different meaning. Based on some standards, the kVA rating can refer to the power that can be input to a transformer, the rated output being equal to the input minus the transformer losses.

Power transformers have been loosely grouped into three market segments based on size ranges.

These three segments are:

  1. Small power transformers: 500 to 7500 kVA
  2. Medium power transformers: 7500 to 100 MVA
  3. Large power transformers: 100 MVA and above

Note that the upper range of small power and the lower range of medium power can vary between 2,500 and 10,000 kVA throughout the industry.

It was noted that the transformer rating is based on “usual” service conditions, as prescribed by standards. Unusual service conditions may be identified by those specifying a transformer so that the desired performance will correspond to the actual operating conditions.

Unusual service conditions include, but are not limited to, the following: high (above 40˚C) or low (below –20˚C) ambient temperatures, altitudes above 1000 m above sea level, seismic conditions, and loads with total harmonic distortion above 0.05 per unit.

Reference: Electric Power Transformer Engieneering – Leo L. Grigsby
(Get this book from Google Books)

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About Author


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on