Honeywell EXCEL 5000 User Manual page 96

ALPHABETIC REFERENCE
74-5577–33 (US)
EN2B-0184 GE51 R0518 (Europe)
The algorithm uses the measured energy values (Z 1 Z 10 , and Z 11 ) to calculate
current power consumption (Parameter P1), possible power consumption
(Parameter P4), and finally the power value to be shed or restored by the XFM 36-
1/S/R loads (P4 minus P1). (This algorithm does not use display Parameters P2 and
P3. Therefore, their values remain zero at run-time.)
Current power consumption (P1) is calculated by taking the energy (kWh) used in
the last 1/10 of the window (Z 10 - Z 9 ) and dividing by the time (in hours) of that 1/10
of the window (t 10 - t 9 ).
Possible power (kW) consumption (P4) is the average power allowed in the next
1/10 of the window (t 11 - t 10 ). It is determined by first calculating the remaining
energy in kWh that may be consumed in the next 1/10 of the window without
exceeding the demand limit setpoint. To calculate the remaining energy, subtract
the energy (kWh) already used in the previous 9/10 of the window (Z 10 - Z 1 ) from
the amount of energy allowed for a whole window (limit setpoint P13 or P14 [kW]
times window [hours]). To determine the possible power (the rate of consumption
that will result in consuming the remaining energy), the remaining energy is divided
by the time [in hours] of 1/10 of a window.
Example 1: Window = 1/4 hour (15 minutes)
Limit setpoint = 600 kW
Z 1 = 45 kWh
Z 9 = 165 kWh
Z 10 = 181 kWh
Datapoint user address IA___Energy_Intv indicates the current energy used so far
within the sliding window (equal to Z 10 minus Z 1 in the previous diagram). The
General Functions subsection explains the other Datapoint user addresses used in
this algorithm.
The power value to be switched = P4 - P1 = 560 kW - 640 kW = -80 kW.
Therefore, a -80 will be sent out to the Po outputs of XFM 35 to have the XFM 36-
1/S/Rs shed 80 kW of loads.
Parameter 6 provides a safety margin between the power limit setpoint (Parameter
13 or 14) and the actual operating setpoint. The value of Parameter 6 is subtracted
from the value of Parameter 13 or 14 (whichever is in effect) before calculating the
possible power P4. Therefore, P6 has a greater effect on the value of the kW to be
shed than if it were subtracted from the P4 - P1 calculation.
Example 2: Refer to Example 1, except for the following: P6 = 40 kW
NOTE: This example shows that a value of 40 for P6 causes more than a 40 kW
difference in the shed signal to XFM 36-1/S/R. It is not trying to suggest
that P6 introduces instability or wild shedding and restoration of loads.
Obviously, if the P6 value were in the algorithm for a period of time, the
system would settle out with the lower demand setpoint.
Example 2 demonstrates a principle that is inherent to the Sliding Window algorithm.
All the necessary shedding (and restoring) to stay below the limit setpoint, must
always occur in the last calculation interval (in XFM 35, the last 1/10 of the window
[measurement interval]).
Where there are enough sheddable loads (at least 25 percent of the total load),
shed and restore activity should be stable and under control. However, when there
is a lot of activity, (changes in the kW consumption) and only a small percentage of
loads are sheddable, control may be lost. This can be attributed to the inability to
shed loads that may be held ON by the minimum on-time function or the RM
override input of XFM 36-1/S/R.
Select the Sliding Window algorithm by setting Parameter P9 to the value 1.
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EXCEL CARE CONTROL ICONS