Allen-Bradley Powermonitor 3000 User Manual page 35

Allen-Bradley HMIs
Configurable Energy Counter Rollover
You may configure the number of digits at which energy values roll
over to zero. The parameter range is 4 to 15 digits. Configure this
setting in Advanced Device Configuration using the Display Module or
by writing to Table A.6. This setting allows you to optimize the energy
counter rollover for use with applications that support a limited
number of significant digits. For instance, the Display Module
supports a resolution of five significant digits. The Trend Log, which is
used for automatic data re-population in some energy logging
applications such as RSEnergyMetrix, supports twelve significant digits
with eight digits of precision.
Demand Calculation
A typical industrial utility bill includes not only an energy (or kWh)
charge but also a Demand charge. Demand is equal to the average
power level during a predefined time interval. Some power providers
may base demand on current, VA, or VARs instead of kW. This interval
continuously repeats and is typically between five and 30 minutes in
length. The formula for kW demand is shown below:
Demand
T = Demand interval duration
t = Time at beginning of interval
P(t) = Power as a function of time
Usually, a utility rate tariff includes a peak demand charge,
determined by the peak demand that occurs during a specified period,
which may be one month, one year, or some other duration. As a
result, only one occurrence of a high demand level can have a
long-term effect on your utility bill. The peak demand value indicates
to the utility the reserve capacity they need to satisfy your short-term
power requirements. The peak demand charge helps to pay the utility
for maintaining this instantaneous capacity.
The Powermonitor 3000 computes demand levels for watts, VA, amps,
and VARs, and provides three different methods for projecting
demand.
The utility may provide a pulse that indicates the end of each demand
interval. The utility updates the demand value at the end of each
interval and maintains the highest value obtained during any interval.
Powermonitor 3000 Operations
t + T
1
∫
• P t ( ) t d
-- -
=
T
t
Publication 1404-UM001D-EN-E - October 2004
3-9