Allen-Bradley Powermonitor 3000 User Manual page 36

3-10 Powermonitor 3000 Operations
Publication 1404-UM001D-EN-E - October 2004
This method is known as thermal demand. You may set up a
Powermonitor 3000 to determine its demand interval from the utility
pulse. To accomplish this, connect the utility pulse to status input #2
and make the appropriate settings in the Advanced Device
Configuration.
If the utility does not provide a demand interval pulse, you won't be
able to synchronize with the utility to control your demand. In this
case, you may use the sliding window method. This method breaks
the demand interval into many sub-intervals and updates the demand
value at the end of each sub-interval. For example a five-minute
interval might be divided into five one-minute sub-intervals. The
demand for each one-minute interval is calculated and at the end of
five minutes the average value of the sub-intervals is computed to
obtain a demand value. At the end of the sixth minute, the value for
sub-interval one is discarded and a new demand value computed
based on sub-intervals two through six. In this way a new five-minute
demand value is obtained every minute. The maximum value is then
maintained as the peak demand. This method approximates the actual
demand the utility measures.
How can you minimize your peak demand in order to reduce your
utility demand penalty charges? One way is to measure the power
being used and project the demand level at the end of the interval.
This method permits you to reduce power consumption when the
projected demand reaches a predetermined threshold, thus preventing
the final demand from exceeding the desired level.
Projected Demand Calculation
Select the best projection method for your system by comparing the
projected values from each method with the actual demand at the end
of the interval. The three methods of projecting demand are described
below.
Instantaneous
The Powermonitor 3000 computes instantaneous demand by
substituting the elapsed interval duration for the total interval duration
(T) in the demand equation. It is therefore identical to the standard
computation except it integrates the power only over the elapsed
interval duration and calculates the average value over the elapsed
duration. The modified equation thus becomes: