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11-38 • E2 RX/BX/CX I&O Manual 026-1614 Rev 4 5-JAN-2013
11.11.6 How Demand Control Uses
Load Shedding
Demand Control uses three parameters to perform load
shedding: the current KW input value, the current integral
error (described in
Section 11.11.2, Demand Monitoring),
and the active KW setpoint. The active setpoint is chosen
from four different setpoint values depending on the sea
-
son (summer or winter) and occupancy (occupied or unoc-
cupied).
The Demand Control application is programmed with
three different “modes” of operation. Under normal condi
-
tions, Demand Control operates in the first mode, then
proceeds to the second or third mode if the demand is not
adequately lowering.
Mode 1: KW Input Is Greater Than Setpoint
Mode 1 is usually the first stage of demand shedding
used by Demand Control. It begins when the KW input
rises above the setpoint.
When this occurs, Demand Control begins by shedding
one level at a time, starting with First Shed #1, and con
-
tinuing sequentially through all First, Rotational, and Last
levels (as shown in
Figure 11-21).
The Demand Control application will continue to shed
levels in this sequence until the KW goes below the set
-
point minus a user-defined hysteresis value and the inte-
gral error is less than zero. When both these conditions are
met, the shed levels will be restored in last-shed-first-
restore order.
Mode 2: KW Input Has Been Greater Than Setpoint
for 1/4 Demand Window And Is Still Going Up
Mode 2 begins when the KW input has been above the
setpoint for an amount of time equal to 1/4 the demand
window duration, and the KW input has not begun to
come down. In short, if Mode 1 is not working to lower
demand, Mode 2 will take over.
When Mode 2 begins, Demand Control sheds two lev-
els at a time, using the same sequence of priorities as
Mode 1. Demand Control continues to shed in this manner
until the KW goes below the setpoint minus a user-defined
hysteresis value
and the integral error is less than zero.
When both these conditions are met, the shed levels will
be restored in last-shed-first-restore order.
Mode 3: Integral Error Approaching Zero
If the integral error is getting too close to zero, this sig-
nifies the demand window may soon be above the power
company’s demand setpoint. When this occurs, Demand
Control switches to Mode 3.
Mode 3 simply calculates the amount of KW differ-
ence between the current KW input and the setpoint, and
sheds enough levels to meet or exceed the KW shedding
requirement. Demand Control does this by reading the
KW levels of all applications (which have been defined by
the user when the applications were set up) in all the prior
-
ity levels that have not yet been shed. It then sheds as
many levels as necessary to bring the KW input value
down (thus saving the demand window from exceeding
the setpoint).
Note that though each application has a user-defined
Application KW parameter that represents how much total
power the application uses, the actual amount of KW used
by the application may be significantly lower (especially if
it is OFF when Demand Control sheds it). Demand Con
-
trol compensates for this fact by assuming shedding an
application will only lower the KW input value by 75% of
the application’s designated KW value. For example, if a
priority level contains a single AHU with a user-assigned
KW value of 250, Demand Control will only expect to
save 75% of this value (187.5 KW) by shedding it.
Example of a Mode 3 operation: A Demand Control
application reads an integral error near zero and a KW
input 300 KW above the setpoint. Mode 3 begins by deter
-
mining how many levels it will need to shed to bring the
KW input down to the setpoint.
First, since Demand Control assumes it will only save
75% of an application’s KW rating by shedding it, the
application determines that it must shed 400 KW worth of
levels to achieve the setpoint.
Second, Demand Control looks at all the applications
in the next few priority levels (assume for purposes of this
example that all First and Rotational Shed levels are
already shed, and there are six defined Last Shed levels).
The applications in the remaining levels are as follows:
Last Shed 1: Total 80 KW
Last Shed 2: Total 90 KW
Last Shed 3: Total 75 KW
Last Shed 4: Total 80 KW
Last Shed 5: Total 95 KW
Last Shed 6: Total 75 KW
Mode 3 will immediately shed stages 1, 2, 3, 4, and 5
for a total of 420KW. If 75% of this total is actually saved
by this action, the total KW reduction will be 315 KW,
which will bring the input 15KW below the setpoint.
After Mode 3 makes its emergency adjustment,
Demand Control will recalculate. If the KW input is still
above the setpoint and the integral error is still near zero, it
will make another Mode 3 adjustment. If the KW is below
the setpoint but not below the setpoint minus the user-
defined hysteresis value, it will go back to operating in
Mode 1 (shedding levels sequentially). If the KW is below
the setpoint minus the hysteresis value and the integral
error is below zero, Demand Control will begin restoring