Transformer Testing under 'Realistic'
non-linear load conditions.

The Harmonics & Energy (H&E) Lab at MIRUS International Inc. provides the unique ability to test various types of transformers under ‘real world’ non-linear load conditions. It allows for demonstration of the effectiveness of Mirus’ Harmony™ and ULLTRA™ Series transformers in both saving energy and improving power quality. It also allows for validation of the calculated energy savings produced by Mirus’ EValuator™ Energy Savings and Payback Analysis program.

Figure 1: H&E Lab showing Non-linear Load Bank

The H&E Lab is equipped with a Non-linear Load Bank that is representative of 1-phase power electronic equipment such as computers, telecom equipment, broadcasting equipment and many other modern workplace loads. Several 3-phase Variable Frequency Drives (VFD’s) are fed with 1-phase power such that their front-end diode bridge rectifier will draw current with a waveform and harmonic spectrum similar to that of a very high K-factor, 1-phase non-linear load. The H&E Non-linear Load Bank has the capability of loading up to a 225 kVA transformer to its full load rating. Larger transformers can be loaded proportionately (ie. 500 kVA to 45% load). This is believed to be the largest 120V phase-to-neutral non-linear load bank of any transformer manufacturer, including all other HMT manufacturers.

The H&E Lab metering console consists of two Power Measurement Ltd., ION 7500 revenue class digital power meters synchronized to measure concurrently. The ION 7500 has 0.1% accuracy on power measurements and measures harmonics up to the 63rd. One meter is used to connect to the transformer primary and the second meter is connected to the transformer secondary. Highly accurate current transformers are used to ensure measurement accuracy and repeatability.

A sample of the typical load profile of the H&E Lab Non-linear Load Bank is shown in Figure 2. In this example, a 45 kVA transformer was operated at both 100% and 50% loading. At full load, secondary current was 129A with a K-factor of just over 9 and current total harmonic distortion (Ithd) of 81%. At 50% load, the K-factor increased to over 13 with Ithd > 90%.

Figure 2: Sample of H&E Lab Non-linear Load Bank current profile on testing of a 45kVA transformer at 100% and 50% loading.

Figure 3 demonstrates how testing in the H&E Lab can be used to verify the calculated transformer efficiencies of the EValuator™ Energy Savings Analysis program to ensure that its predicted energy savings are realized. The graph shows both calculated and tested values for a 45 kVA Harmony-1E HMT and a 96.5% linear efficiency K13 Delta-Wye transformer. As is evident, the measured efficiencies quite closely match the calculated efficiencies especially in the normal loading range of 20% to 65%. And even though the K13 transformer used in this analysis had reasonably high efficiency under linear loading (96.5%), it had significantly lower efficiency when compared with a Harmony-1E HMT under non-linear loading.

Figure 3: ‘EValuator’ calculated efficiencies vs H&E Lab tested efficiencies on 45kVA transformers

Utilizing EValuator™ to compare these two transformers with K9 load profile and 50% loading operating 12 hrs/day at $0.07 /kwhr produced an attractive payback of less than 3 years.