Phase Current Measurement Cmmxu - Hitachi Relion 650 Series Technical Manual

Section 12
Monitoring
CVMMXN function, is using these values the zero clamping will influence the subsequent supervision
(observe the possibility to do zero point clamping within measurement supervision, see
section
Compensation facility
In order to compensate for small amplitude and angular errors in the complete measurement chain
(CT error, VT error, IED input transformer errors and so on.) it is possible to perform on-site
calibration of the power measurement. This is achieved by setting the complex constant which is
then internally used within the function to multiply the calculated complex apparent power S. This
constant is set as amplitude (setting parameter PowAmpFact, default value 1.000) and angle (setting
parameter PowAngComp, default value 0.0 degrees). Default values for these two parameters are
done in such way that they do not influence internally calculated value (complex constant has default
value 1). In this way calibration, for specific operating range (for example, around rated power) can
be done at site. However, to perform this calibration it is necessary to have an external power meter
with high accuracy class available.
Directionality
CTStartPoint defines if the CTs earthing point is located towards or from the protected object under
observation. If everything is properly set power is always measured towards protection object.
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Practically, it means that active and reactive power will have positive values when they flow from the
busbar towards the protected object and they will have negative values when they flow from the
protected object towards the busbar.
In some application, for example, when power is measured on the secondary side of the power
transformer it might be desirable, from the end client point of view, to have actually opposite
directional convention for active and reactive power measurements. This can be easily achieved by
setting parameter PowAngComp to value of 180.0 degrees. With such setting the active and reactive
power will have positive values when they flow from the protected object towards the busbar.
Frequency
Frequency is actually not calculated within measurement block. It is simply obtained from the pre-
processing block and then just given out from the measurement block as an output.
12.1.8.3

Phase current measurement CMMXU

The Phase current measurement (CMMXU) function must be connected to three-phase current input
in the configuration tool to be operable. Currents handled in the function can be calibrated to get
better than class 0.5 measuring accuracy for internal use, on the outputs and IEC 61850. This is
achieved by amplitude and angle compensation at 5, 30 and 100% of rated current. The
compensation below 5% and above 100% is constant and linear in between, see
176
"Measurement supervision").
Busbar
IED
P Q
Protected
Object
IEC09000038-1-en.vsd
Internal IED directionality convention for P & Q measurements
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Figure 92.
Technical Manual