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.
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.
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:
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:
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.
Downloadable interactive PDF which captures required data for input into The PQI Calculator
Specs and Drawings
Search by Industry
Search by Product
Please feel free to call us Monday through Friday, 9:00am to 5:00pm EST, at 888.539.7712.