Rockwell Automation 1426 PowerMonitor 5000 Unit - Manual (Page 63)

Rockwell Automation Publication 1426-UM001F-EN-P - November 2013 63
Metering Chapter 4
Demand power factor is calculated by using the following formula.
Demand Calculation
Demand is equal to the average power level during a predefined time interval.
This interval continuously repeats and is typically 15 minutes but can be between
5 and 30 minutes in length. The power monitor computes demand levels for
watts, VA, amps, and VARs, and provides two different methods for projecting
demand. The formula for real power (kW) demand is shown below.
T = Demand interval duration
T = Time at beginning of interval
P(t) = Power as a function of time
If your electric utility provides a pulse that indicates the end of each demand
interval, the power monitor can be set up to determine its demand interval from
the utility pulse.
Some electric service providers 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 15 minute interval can be divided into 15 one-minute sub-
intervals. Each minute, the following occurs:
• The demand for the sub-interval is calculated and stored.
• The average value of the most recent fifteen sub-intervals is computed to
obtain a demand value.
• Sub-interval values older than fifteen minutes are discarded.
Projected Demand Calculation
Projected demand calculates an instantaneous (default) or first-order projection
of demand at the end of a demand interval. 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 methods of projecting demand are
described below.
kWDemand
kVADemand
---------------------------------
Demand
1
T
--- P t
( ) t
d
t
t T +
( )  Ч
=