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