User's Manual

Operation: Base Metrology
18 CENTRON® OpenWay™ Meter Technical Reference Guide
Volt-amperehour (VAh) Measurement
The CENTRON OpenWay meter measures either Vectorial or RMS volt-amperes using
arithmetic phase summation. The arithmetic method of measurement ensures that the
resulting VAh value contains as much of the harmonic information as possible.
Volt-ampere values are calculated by multiplying the RMS voltage value times the
coincident RMS current value.
The voltage and current values from each phase are squared and then stored in their
respective accumulators. At the end of one second, each accumulator contains the sums of
the square of the voltages or currents for each phase. The contents of these accumulators
are passed to the consumption routine where they are averaged (divided by the sample
count) and the square root is taken, yielding the RMS voltage and RMS current for each
phase.
Every second the RMS voltage and the RMS current for each phase are multiplied together
to establish a VA-second value for each phase. These values are scaled and corrected.
The total VAhour value is calculated by adding the VA-second quantities for each phase and
dividing the total by 3600. This value is added to the appropriate register. If the harmonics
on the Voltage waveform differ from the harmonics on the Current waveform, then the
harmonic energies will fall out of the Watthour and Varhour calculation, and thus the VA
Vectorial measurement, but they will not fall out of the VA Arithmetic measurement.
The VA Vectorial and VA Arithmetic measurements will also differ when there is
imbalanced power. Imbalanced power is generated when the phases of the service are not in
balance with one another.
Demand Calculations
To calculate demand, the selected quantities are accumulated over a programmable time
period (1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, or 60 minutes) depending on the programmed
demand interval length. At the end of the interval, the accumulated values are stored in
separate demand storage registers and the accumulating registers are cleared. Incremental
values for the next demand interval are then accumulated.
The maximum demand in a billing period is determined by comparing the demand values
for the most recently completed interval to the respective readings presently stored in the
peak demand registers. If the previous demand is greater than the value in the
corresponding peak demand register, the lower value (the maximum demand recorded so
far) is replaced. If the previous demand is less than the value in the corresponding peak
demand register, the maximum demand value remains unchanged. This update process is
carried out when a demand interval is completed, when a power outage occurs, when Test
Mode is initiated, or when a Real Time Rate change occurs.
The CENTRON OpenWay meter demand calculations are performed using one of two
possible methods: block or rolling. The demand method is selected when the register is
programmed.
Draft