Operation
29
4.4.2 Demand Output
The demand power and the peak demand are each available in three different units:
Demand
Network Variables
Peak Demand
Network Variables
Resolution Range
nvoDemandW
nvoPkDemandW
0.1 watt
0 to 6553.5 watts
nvoDemandKW
nvoPkDemandKW
0.1 kilowatt
0 to 6553.5 kilowatts
nvoDemand_f nvoPkDemand_f
1.2e-7
×
measurement
0 to 1.0e38 watts
Table 4.5: Demand Output Network Variables
Any or all of these output units may be used, depending on the application. These variables are updated
every time a demand interval or subinterval ends. The
nvoDemand_f
and
nvoPkDemand_f
variables are
floating point variables that have a resolution of one part in 8 million of the current measurement value.
Demand measurement starts whenever any of the demand configuration variables are changed, when the
demand object is enabled with the
nviRequest
network variable. If the time of day,
nviTimeSet,
is updated
by a monitoring computer or a clock module on the network then the demand measurement intervals will
remain synchronized to the time of day. If several WattNodes need to make synchronized demand
measurements, then they should all be disabled. Then, at the desired start time for the demand interval, they
should all be enabled. As
nviTimeSet
is regularly updated in all the WattNodes, then they will all stay
synchronized.
To protect against losing data during a power failure, the WattNode updates non-volatile memory every
thirty seconds with the current demand measurement data. If power fails, then when power returns, the
WattNode will restore the demand measurement and continue from where it was when power failed
(though up to 30 seconds of data may be lost). As soon as
nviTimeSet
is updated, the WattNode will
resynchronize to the correct time of day, allowing multiple WattNodes to stay synchronized even through
power failures.
The WattNode will lose synchronization if
nviTimeSet
and its internal time differ by the larger of 30
seconds or 5% of the demand subinterval period. The synchronization, however, only considers the
minutes and seconds within an hour, but not the hour itself. So if
nviTimeSet
disagrees by one hour and ten
seconds from the internal clock, then the demand interval will be changed by ten seconds and remain
synchronized. If the WattNode does lose synchronization, it will continue to measure demand, but the
demand period start times may no longer be linked to the time of day. If power fails for long enough that
the WattNode does lose synchronization, then the monitoring computer can resynchronize any WattNodes
on the network.
Synchronizing the demand measurements—say to the hour—may be inadvisable for billing, as the
customer could arrange to split periods of high demand across two demand intervals, resulting in lower
demand for each of the two intervals. Using several subintervals largely eliminates the possibility of timing
high load periods to reduce the peak demand. The overlapping demand intervals, ensures that the high load
period will be largely centered in at least one interval.
4.4.3 Peak Demand
The peak demand measurement is updated every time a new demand measurement is made. It is cleared
whenever the demand configuration changes, or by making an override request of the Demand object. To
use the override technique for zeroing the peak demand,
nviRequest
must be sent with the
object_id
field
set to 3 (Demand object), and the
object_request
field set to 6 (OVERRIDE). The peak demand is stored in
non-volatile memory, so that its value is not lost during power outages. If the WattNode is measuring peak
demand for billing or sub-metering, see
3.3.1 Authentication
on information to prevent unauthorized
zeroing.
