Rockwell Automation 1404-M4_M5_M6_M8 Powermonitor 3000 Master Module Installation Instructions - Manual (Page 16)

16 Powermonitor 3000 Master Module
Publication 1404-IN007F-EN-P - November 2009
System Accuracy Considerations
User-supplied potential transformers (PTs) and current transformers (CTs), as well as wiring
from the CTs to the power monitor, may reduce the accuracy of your power monitor system.
The quality of the power monitor’s measurements can be no better than the quality of the
signals presented to its input terminals. It is your responsibility to select transformers that are
adequate for the desired metering accuracy.
ANSI/IEEE C57.13, Requirements for Instrument Transformers, defines three classes of
transformer accuracy: class 1.2, class 0.6, and class 0.3. The application should dictate the
transformer accuracy class used.
PTs and CTs may introduce errors in three areas: ratio errors, phase errors, and bandwidth
errors.
Ratio Errors
The voltage ratio of a PT is the number of primary turns of wire divided by the number of
secondary turns. Manufacturing tolerances may cause the ratio to be slightly different than the
design specifies, causing an error affecting the voltage input to the power monitor.
Likewise, the current ratio of a CT is a function of the ratio of the number of turns of wire on
the primary and secondary. Some error in this ratio is quite common in commercial grade PTs
and CTs.
Other errors include magnetic core losses, winding impedance, and the burden, or load, on
the transformer secondary. The combination of these errors is known as Ratio Error. You
may compensate for Ratio Error, if known, by adjusting the Basic Configuration entries for
PT and CT primary or secondary voltages.
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 4 mm
2
(12 AWG) or larger is usually recommended for wiring
CTs with a 5 A secondary rating.