Operation
23
The integer output variable
nvoElecKWH
, rolls over to zero when its count reaches 50,000—at 50,000
kWH. This permits measurements across the roll over point. For example, in most home metering, only the
three least significant digits are reported on the bill and if a measurement is smaller than the previous
measurement, then it is assumed that a thousand kW boundary was crossed. So if the June measurement
was 945 kW and the July measurement was 23 kW, then the energy consumed between measurements was
1023 kW – 945 kW = 78 kW.
nvoElecWH_f
is a floating point variable that never rolls over and that has a
resolution of one part in 8 million of the current measurement value.
The energy value may be retrieved either by polling, or by binding the network variable, and setting
nciWHMaxSendT
to control the interval between updates. The energy is computed every 10 seconds, but
any update rate between 10 seconds and 18 hours may be used. Set
nciWHMaxSendT
to zero seconds to
disable automatic updates.
4.2.2 Zeroing Energy
To zero the energy, make an override request of the energy object. Send
nviRequest
to the WattNode with
the
object_id
field set to 1 (Energy object), and the
object_request
field set to 6 (OVERRIDE). Energy will
reset to zero, and then continue accumulating. The energy is also reset to zero whenever
nciGain
or
nciCTAmps
are changed (see the next section). Whenever energy is zeroed,
nvoEngyClrT
will be set to the
current time and date. If the WattNode is being used for billing or sub-metering, see
3.3.1 Authentication
on information to prevent unauthorized zeroing.
4.2.3 Energy Configuration
The output energy, power and demand measurements all depend on the calibration of the WattNode and on
the full-scale current rating of the CTs. The Energy object contains these variables, and must be configured
even if the energy measurement is not being used. The WattNode is calibrated during assembly, and should
never need adjustment. If the WattNode is being used for billing or sub-metering see
3.3.1
Authentication
on information to prevent unauthorized changes to the configuration.
The full-scale current rating of the CTs is set with the variable
nciCTAmps
. The type of this network
variable is SNVT_amp, which ranges from 0 to 3276.7 amps in steps of 0.1 amps. This must be correctly
set during installation. The full-scale rating of the CTs are printed on label of the CTs. If more than one CT
is used, they must all have the same full-scale rating. See section
1.2 Current Transformers
for more
details on selecting CTs and their full-scale current rating.
If you wish to calibrate the WattNode with CTs attached for better accuracy, or if the WattNode is being
recalibrated as part of a maintenance schedule, then the variable
nciGain
changes the gain of the WattNode.
The WattNode is recalibrated by connecting it to an accurate power standard, setting the power standard
for the nominal operating voltage of the WattNode, and setting the current to the full-scale rating of the
CTs. Then compare the power reported by the WattNode to the power produced by the power standard. If
they are different, then adjust
nciGain
until they are within tolerance (0.5%). The variable
nciGain
is a
SNVT_muldiv, and contains two fields,
multiplier
and
divisor
. Each is an unsigned integer that ranges
from 0 to 65,535. Normally, the divisor is left at 65535, and the multiplier adjusted. This allows the value
to be adjusted to within 0.01%.
As a calibration example, suppose the WattNode is connected to a 10 amp split-core CT and attached to an
accurate power standard. The power standard is set to supply 115 VAC and 10 amps for a power of 1150
watts. If the WattNode’s power output indicates 1139 watts, then the reading is 0.96% too low. To correct
this, multiply
nciGain.multiplier
by 1150 / 1139. The general formula is:
ower
indicatedP
truePower
multiplier
nciGain
multiplier
nciGain
⋅
=
.
.
The factory setting for
nciGain
is printed on the back label of the WattNode so that
nciGain
may be
restored if its value is changed or lost.
