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Transformer Losses Standards
Oil transformer, sealed type (without conservator)

Unlike many countries around the world, Europe has no mandatory standard on energy efficiency of distribution transformers. The two main documents which describe losses in transformers are: the European Standard EN 50464-1, which has superseded the harmonised document HD428 for oil cooled transformers, and the harmonised document HD538 for dry type transformers, which is still valid (or their various country equivalents, e.g., DIN, etc.). Data from these norms is given in the Annex.

Despite the fact that there are no mandatory standards in Europe, there are some procurement procedures (internal standards of electricity distribution companies) which are highly demanding in Benelux, Germany, Austria, Switzerland and Scandinavia.

Most of the electricity distribution companies in these countries buy transformers at C 1 [C’ minus 30%] (HD 428) or AoBk (new 50464) standards. ENDESA in Spain purchases HD 428 CC’ for 400 kVA units. EdF has introduced a certain purchasing policy which specifies no load losses between Co and Eo and load losses between Dk and Bk.

The mix of losses is focused on low no-load losses for small ratings and low load losses for higher ratings. Also tolerance of losses has changed recently. More often utilities reduce the tolerance of losses to, e.g., 0% instead of 15%.

Efficiency standards outside Europe may be expressed in terms of electrical efficiency, at a certain load level, or in terms of maximum values for no-load and load loss. Some examples follow below. Australia “recalculated” the American 60 Hz efficiency NEMA TP-1 standard – which has never become mandatory in USA at federal level – to 50 Hz and also interpolated linearly the efficiencies for ratings which are different from those used in the USA. New Zealand follows the Australian regulations for distribution transformers as a matter of policy.

In China, the standards are regularly upgraded since1999 with S7 and then S9 having been replaced by the current standard S11, which defines allowable levels for no-load and load losses slightly below Europe’s AC’ level.

S11 will soon be replaced by S13 which is expected to specify lower loss levels.

The Indian Bureau of Energy Efficiency (BEE), classifies distribution transformers in the range from 25 up to 200 kVA into 5 categories from 1 Star (high loss) to 5 Stars (low loss). 5 Stars represents worldclass performance. 3 Stars is being proposed as a minimum efficiency standard, and is being widely followed by utilities.

Japan has a different type of distribution system, with the last step of voltage transformation much closer to the consumer. The majority of units are pole mounted single phase transformers. The driver for setting up minimum efficiency standards was the Kyoto commitment.

Transformers, together with 17 other categories of electrical equipment, should meet minimum efficiencies. In the case of transformers, the efficiency is defined at 40% load. Target average efficiency has been defined for the year 2006 (oil) or 2007 (dry type), based on the best products on the market in 2003.

SOURCE: Strategies for development of Energy Efficient Distribution Transformers

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Edvard Csanyi

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 facilities. Professional in AutoCAD programming.


    Jun 03, 2021

    What is the maximum allowable loss in % at 50% and 100% capacity utilization in case of two winding transformers and auto transformer above 33kv ? What will be the acceptable standardization that is required to be followed.

  2. Ryan Max
    Dec 16, 2015

    Until 2500 kVA losses and no load losses expressed in IEC, for bigger units is there any information about losses in IEC?

  3. Maizlan Maizan
    Oct 08, 2015


    Can you explain in details between these 2 different Tesla :-

    Tx 11KV/0.433KV = 0.75 Tesla & 1.55 Tesla

  4. Asif Eqbal
    Mar 07, 2012

    Is there any IEC or IEEE for transformer losses with respect to voltage rating and MVA capacity? Among dry type and oil type which will have higher losses?

    • Edvard
      Mar 07, 2012

      Of course, it’s a standard IEC 60076-1.

    • anwar
      Nov 09, 2015

      what is the standard and optimum No load and Full load loss of a transformer of 30-40 MVA rating

  5. Lucille
    Jan 04, 2012

    I’m interested in standards used in China. Do you have some references about it?

  6. Edvard
    Jan 07, 2011

    Depending on the size of the transformer, a “NEMA TP1 compliant” transformer can save $100-300 each year at an electricity rate of $0.075 cents per kWh. A typical large commercial facility will have 6-10 low voltage transformers, and thus can save between $600 and $3,000 per year.

    From “Siemens Power Distribution & Control”

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