TRANSFORMER EFFICIENCY

The highest standards for transformer efficiency in North American are found in NEMA Standard Publication TP 1-2002, CSA Publication C802.2-00 and EPA’s Energy Star requirements. The measurement and calculation methods, required by these standards, accurately determine a transformer’s losses and energy efficiency when it supplies linear resistive and/or inductive loads. The method used to determine total losses requires the summation of no-load losses and load losses. These losses are determined by performing an open-circuit and a short-circuit tests.

Unfortunately, modern electrical distribution systems typically supply a high percentage of non-linear 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 for determining a transformer losses or efficiency under these non-linear conditions.

Several manufacturers now claim transformer efficiencies that meet or exceed the requirements of NEMA TP 1-2002 but under severe non-linear load conditions. One manufacturer has even published their test methods and claims Department of Energy validation and endorsement. At best, these claims are misleading since:

1. There is no recognized standard guide for determining energy efficiency of a distribution transformer or a standard test method for measuring its energy consumption, under non-linear load conditions,
    

2. The manufacturer’s published ‘Power In – Power Out’ Measurement Method, which boasts +/- 0.3% revenue class instrumentation accuracy, in reality, can produce an error of +/- 1.5% when measuring the efficiency of a 75kVA transformer under non-linear loading. As a result, a claimed efficiency of 98% may, in fact, be only 96.5% [1] and
    

3. The 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.

It is clear that the public would be better served if there were guides and standard methods for determining the efficiency of transformers under non-linear load conditions. Unfortunately, it appears unlikely that NEMA, CSA or the EPA will develop such documents in the foreseeable future.

In an effort to resolve the technical issues and to advance transformer technology in general, the e-Rated Transformer Corporation has expended considerable resources to develop:

(i) A Guide for Determining Energy Efficiency for Distribution Transformers under Non-Linear Load Conditions. This standard requires the same efficiencies as NEMA PT 1-2002.

(ii) A Standard Test Method for Measuring the Energy Consumption of Distribution Transformers under Non-Linear Load Conditions, which is based on a Voltage & Current Differential Measurements Method. [1]

(iii) A Voltage & Current Differential Measuring Method Instrument, which limits the measurement error to +/- 0.04%, when measuring the efficiency of a transformer under any non-linear ambient and/or load condition. [1]

To encourage the development of a national guideline and standard, and to validate our measurement method and instrumentation, e-Rated Transformer Corporation has offered its R&D documentation and a VAD Measuring Method Instrument to the Department of Energy’s Oak Ridge National Laboratory for evaluation and verification.

Since it is highly unlikely that any manufacturer of harmonic mitigating transformers would be willing or able to acquire a Non-Linear Load Bank, with sufficient capacity to verify their higher kVA ratings, PQI employs the e-Rated Transformer Engineering Program™. With reference to Figures 1, 2 and 3, this sophisticated computer program will calculate a transformer’s non-linear losses and efficiencies, based on its easily measured NEMA TP 1 linear losses and efficiencies. These calculations can be made for any non-linear load condition. This computer program, which has already been independently verified, has also been offered to the DOE for evaluation and verification.

75kVA Conventional Transformer vs. 75kVA Distribution TransFilter ™
Linear & Non-Linear Losses
Figure 1


75kVA Conventional Transformer vs. 75kVA Distribution TransFilter™
Linear Load Efficiency
Figure 2

75kVA Conventional Transformer vs. 75kVA Distribution TransFilter™
Non-Linear Load Efficiency
Figure 3
 

PQI’s objective is to offer an accurate and meaningful alternative with respect to the measurement of transformer energy efficiency under any non-linear load condition and to develop harmonic mitigating transformers that meet or exceed the efficiency requirements of NEMA TP 1-2002, CSA Publication C802.2-00 and EPA Energy Star®.

[1]  For information on substantiating IEEE Transactions, please call Power Quality International, Inc. at (888) 539-7712.