9.7
Signal Sampling
Each of the signals, representing the scaled voltage and current inputs have
their own ADC. Each signal is sampled simultaneously at a rate of 10,240
samples per second. If connected to an external timing source (e.g. GPS)
it provides the sample clock to the 933A, it is synchronized to UTC-USNO
(GPS), allowing measurements of phase angle across a power grid to be
compared directly.
9.8
Power and Energy
Power and energy are determined by making twenty separate measure-
ments per second of the cross product of voltage and current for each
phase. Each measurement uses 1024 samples (i.e., it takes data from a 100
millisecond window), yielding a 50% overlap.
Apparent power (VA) and reactive power (VAR) are determined from
the results of the voltage and current magnitude measurements (see
next section), using standard identities. The active power measurements
and reactive power measurements are then compensated for PT and CT
corrections (see below) using a complex multiplication, and corrections are
performed for transformer iron and copper losses, if enabled.
At this point, two different things are done with the resulting measurement
data.
First, a determination is made of the quadrant in which this
data should be registered (Wh delivered or received, VARh delivered or
received.) The results of each measurement cycle (20/sec) are then added
to the proper set of registers. These registers are stored periodically, and
accumulation restarted from zero. The user may configure the unit for
different intervals to register energy.
The second thing that is done with the data is to determine the actual
power level. This number will be displayed on the front panel (as watts
or VARs), and it will be returned via the serial interface if a simple
request for “power” is made. This result is calculated by averaging the
20 power measurements made during each second. Therefore, the update
rate for this quantity is once per second.
This data is not registered
separately depending on the quadrant, as the energy data is; therefore, it
is theoretically possible, if the direction of power flow changes periodically,
that the sum of measurements reported over the serial interface may
gradually depart from the registered energy data. This is due to the loss
of information in the averaging process; the registered data are the most
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