114
Functional Description
are the most accurate. The averaged data are provided primarily as a convenience or for system
control purposes; it is not intended for billing purposes.
11.9
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 1133A 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.
11.10
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 1133A
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.
11.11
Transformer Compensation
There are two different types of transformer compensation. They are used to correct for the losses
in a transformer when primary-side metering is used to meter the energy delivered to a customer
at the secondary of the transformer.
Copper compensation is used to correct for the
I
2
R
losses in the transformer windings due
primarily to their (non-zero) resistance.
As you would expect, this effect is primarily active
(resistive), although there may be minor reactive effects, and it is proportional to the current
squared. This factor allows the user to correct for these losses. It is a complex factor, providing
both Watts and VARs correction, and is proportional to current squared; i.e. so many watts and
VARs are to be subtracted from the registered amounts per ampere squared of load current.
Iron losses (also called core losses) are due to magnetizing currents (the small amount of current
required to generate the flux in the core, which is unrelated to the load current) and eddy current
losses in the core material. These are approximately proportional to the square (watts) or 4th
