Rockwell Automation 1404-M4_M5_M6_M8 Powermonitor 3000 Installation Instructions, PRIOR to Firmware rev. 3.0 - Manual (Page 18)

Publication 1404-IN007D-EN-P - October 2004
18 Powermonitor 3000
For a PT the Ratio Error increases as the transformer’s load current
increases, so its total load impedance should be as high as possible.
Conversely, a CT’s Ratio Error increases as the voltage supported by
the transformer secondary increases, so its total load impedance,
including the impedance of the wire connecting the CTs to the
metering device, should be as low as possible. This is why #12 AWG
or larger is usually recommended for wiring CTs with a 5 amp
secondary rating.
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 five-degree phase shift, there would be no error in
the measured quantities. If on the other hand the PTs had a phase
error of one degree and the CTs had a phase error of six degrees,
there would be a five-degree phase error in the power calculation.
This would show up as power factor and reactive power (VAR) errors.
Phase errors can not be corrected by adjusting the Powermonitor 3000
configuration since the errors change based on varying conditions of
the power system.
A typical PT phase error varies from ±1° to ±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.