Rockwell Automation 1404-M4_M5_M6_M8 Powermonitor 3000 Master Module Installation Instructions User Manual
Page 17
Powermonitor 3000 Master Module 17
Publication 1404-IN007F-EN-P - November 2009
Phase Error
Phase shift between the primary to secondary signals is another source of inaccuracy
introduced by the user-supplied PTs and CTs. Phase shift is generally not of concern for
simple voltage or current measurements. When these signals are combined, for instance when
calculating line to line voltage or phase power, the effect of phase shift can become
significant. The difference in phase error among different transformers causes measurement
errors. If all the PTs and CTs introduced a 5° phase shift, there would be no error in the
measured quantities. If, on the other hand, the PTs had a phase error of 1° and the CTs had a
phase error of 6°, there would be a 5° phase error in the power calculation. This would show
up as power factor and reactive power (VAR) errors. Phase errors cannot be corrected by
adjusting the power monitor configuration since the errors change based on varying
conditions of the power system.
A typical PT phase error varies from ±1°…±0.25°, depending on the PT’s accuracy class.
Applying higher than rated voltage increases the phase error and may saturate the transformer
and cause even larger errors.
The phase error in a CT increases as its current decreases, and is lowest when the current is
greater than 80% of the CT rating. Because significant phase error can occur when CT
current is less than 20% of rated current, CTs sized for protection do not perform well when
used for metering.
The phase error of both PTs and CTs are also affected by the power factor of the load on the
secondary. For best accuracy, loads should be resistive, with PT loads as high as possible and
CT loads as low as possible.
Bandwidth Error
For fundamental 50 or 60 Hz measurements, bandwidth error has no affect on accuracy.
However, for waveforms with significant harmonic content, the PTs and CTs you supply may
attenuate higher harmonics. Most instrument quality PTs have a flat frequency response out
to 3 kHz, or the 50th harmonic on a 60 Hz system. Current transformers, especially older,
existing units, tend to be less linear, with a flat response only out to 300 Hz, or the fifth (60
Hz) harmonic. Wide-band instrument CTs are available for improved frequency response.
Bandwidth error cannot be corrected by adjusting the power monitor configuration.
In addition, operation of either the PTs or CTs at extremely low frequencies may also cause
saturation and resulting magnitude and phase errors.
For more detailed information on instrument transformer accuracy and power measurement,
refer to Bulletin 1403 Powermonitor II Tutorial, publication