Operating Instructions

58 Siemens Industry, Inc.
Power Factor Convention
Power Factor is the ratio of a signed number (true power) and an unsigned number (apparent power). This
discrepancy has led to some customer confusion. The MD-xxHD meter allows users to select between two
conventions (ANSI and IEEE). In the IEEE convention the sign of PF follows the sign of power itself. In the
ANSI convention a “+” PF indicates a lagging current (inductive load) while a “–“ PF indicates a leading
current (capacitive load). The sign relationships are shown below for these conventions in each electrical
quadrant.
Total Harmonic Distortion
The MD-xxHD meter reports overall harmonic content in power (% THD) based on its measurement of
Power, Var and Apparent Power as illustrated in the figure below. This method cannot indicate the
harmonic number or distribution but provides the overall harmonic content.
In many circumstances users are interested in the harmonic content of current. In cases where the
voltage is very sinusoidial the THD measurement is a good estimate of both power and current. However,
if the voltage waveform is distorted the reported THD in power can be misleading. ViewPoint HD can
provide additional analysis of harmonic content in voltage and current by sampling the MD-xxHD raw data
and performing digital signal processing on the signal. Using this method the individual harmonics levels
can be observed. The results are presented in graphical form.