Transformer Efficiency

Existing Standards

The required standards for transformer efficiency in North America are NEMA Standard Publication TP-1-2002 (USA) and CSA Publication C802.2-00 (Canada). The measurement and calculation methods, required by these standards, accurately determine a transformer’s losses and energy efficiency when supplying linear loads. The method used to determine the total losses requires the summation of excitation losses and impedance losses. These losses are determined by performing open-circuit and short-circuit tests.

The Reality

Unfortunately, modern electrical distribution systems typically supply a high percentage of nonlinear electronic loads, particularly in 120/208-volt systems. As a result, transformer losses increase and energy efficiencies decrease. The level of deterioration is a function of harmonic voltage magnitudes at a transformer’s primary terminals, load-generated harmonic current magnitudes at its secondary terminals and their phase relationships. There are, unfortunately, no recognized standards or measurement methods for determining transformer losses or efficiency under nonlinear loading.

Misleading Claims

A number of manufacturers now claim transformer efficiencies that meet or exceed the requirements of NEMA TP-1-2002 or CSA Publication C802.2-00 under severe nonlinear loading. These claims are often misleading since:

  • There is no recognized standard guide for determining energy efficiency of a transformer under nonlinear loading or a standard test method for measuring its energy consumption under nonlinear loading.
  • The manufacturer’s published "Power-In – Power-Out Measurement Method," which boasts ±0.3% revenue class instrumentation accuracy, in reality, can produce an efficiency measurement error of ±1.34%.
  • The US Department of Energy does not evaluate or endorse the performance of any manufacturer’s transformers since (i) they lack the statutory authority to do so, and (ii) there are no established guidelines or standards.

The Solution

It is clear that the public would be better served if there were guides and standard methods for determining the efficiency of transformers under nonlinear loading. Unfortunately, it appears unlikely that NEMA, CSA, or the DOE will develop such documents or methods in the near future.

In an effort to resolve the technical issues and to advance transformer technology in general, Power Quality International has expended considerable resources to develop:

  • A "Guide for Determining Energy Efficiency for Distribution Transformers Under Nonlinear Loading." This standard requires the same efficiencies as NEMA TP-1-2002 and CSA C802.2-00 but under nonlinear loading (per IEEE Std. 519-1992, Table 4.3 harmonic current magnitudes).
  • A "Standard Test Method for Measuring the Energy Consumption of Distribution Transformers under Nonlinear Loading," which is based on a Voltage and Current Differential, Loss Measurements Method.
  • A "Voltage and Current Differential, Loss Measurement Method Instrument," which limits the measurement error to ±0.033% when measuring the efficiency of a transformer under linear or any nonlinear load condition.

To encourage the development of a national guideline and standard, and to validate their measurement method and instrumentation, PQI has offered its R&D documentation and a "Voltage and Current Differential, Measuring Loss Measurement Instrument" to the Department of Energy’s Oak Ridge National Laboratory for evaluation and verification.

Since it is highly unlikely that any manufacturer of K-Rated or harmonic mitigating transformers would be willing or able to acquire a nonlinear load bank of sufficient capacity to verify their higher kVA ratings (>112.5kVA), PQI developed an IEEE Std. C57.110-1998 compliant computer program.

This sophisticated software is able to calculate a transformer’s nonlinear losses and efficiencies based on its NEMA TP2 linear losses or efficiencies. These calculations can be made for any nonlinear load condition. The computer program, which has been independently verified, is the basis for The PQI Calculator™.

The program has also been offered to the DOE for evaluation and verification.


The PQI Calculator Information Checklist

Downloadable interactive PDF which captures required data for input into The PQI Calculator

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