value after billing demand interval expired. The PM295 allows the user to adjust a
thermal time constant in intervals of 1 to 3600 seconds with 0.1 second steps to
match the power utility requirements. The following formula is used to define a
thermal time constant for your application:
τ =
−
t
ln
100
100 S%(t)
where
- thermal time constant, sec
t - billing demand interval, sec (demand interval × number demand intervals)
S%(t) - the level that the thermal demand pointer will attain at the end of billing
demand interval, expressed in percentage of the steady-state value
To approximate meters with S%(t) = 63%, the thermal time constant is considered
to be equal to the billing demand interval, that is,
= t, taken in seconds. For
example, using a 15-minute billing demand interval, the thermal time constant is 900
seconds, and using a 30-minute demand interval - 1800 seconds.
For thermal demand meters with S%(t) = 99%, the thermal time constant will be
195.4 seconds using a 15-minute billing demand interval, and 390.9 seconds using a
30-minute billing demand interval.
4.3.5 Accumulated and Predicted Demands
The PM295 offers two evaluated parameters for power load control using predicted
changes in power demand: accumulated demand, and predicted sliding window
demand. They are calculated for all total powers.
Accumulated demand is the average power consumption over the fixed demand
period, which is continually updated as power is being consumed during the demand
period. Actually, accumulated demand is calculated from the consumed energy being
integrated in the block interval demand storage register (as described in Section
4.2.3) that is divided by the constant demand period. Thus, at any point during the
demand period, accumulated demand represents the value proportional to the
consumed energy converted to kW, kvar or kVA units. At the beginning of each
demand period, accumulated demand is reset to zero. When power is being
consumed, accumulated demand grows up to the maximum value, which is
evaluated to block interval demand at the end of the demand period.
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