Emerson E2 User Manual download pdf (Page 179)

Demand Control Software Overview • 11-37

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

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Emerson E2 User Manual Page 179

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Emerson E2 User Manual Page 179

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