accurate. The averaged data are provided primarily as a convenience or
for system control purposes; it is not intended for billing purposes.
9.9
Voltage and Current
Voltage and current are measured in a similar fashion to power, using
overlapping 1024-point measurements. In this case, however, the cross
product is replaced with the square of the voltage or current samples.
The square root of the resulting sum is proportional to the rms voltage or
current value during the measurement interval. This value is corrected for
the CT and/or PT correction factors before further use. The 933A uses
the resulting data to correct the energy measurements, as described above.
It also provides data to the host system by averaging it over one-second
intervals.
9.10
CT and PT Compensation
To correct for the inaccuracies of the CT’s and PT’s used in the metering
setup, CT and PT compensation may be enabled. Since the system voltage
is relatively constant, the PT compensation factor is a single, complex (i.e.,
magnitude and phase, or real and imaginary) correction factor.
CT compensation is more complicated than PT compensation. Due to
the fact that magnetizing currents in CT’s are not exactly proportional
to the load current, a matrix is used. This allows the entry of several
different compensation factors measured at different current levels. The
933A interpolates between the numbers in this table (also complex) to
determine the correction factor to be used.
Correction for energy is performed using the (complex) product of the
PT and CT factors. Correction for voltage or current is performed using
the magnitude of the appropriate factor. Correction for phase angle is
made using the phase of the appropriate component, i.e. the arctangent of
the complex value. The actual calculation performed may be different
than this description, due to computational considerations (a complex
multiplication is far faster than a trigonometric operation such as an
arctangent, for example); however, the end result will be as described.
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