Emerson E2 How Demand Control Uses Load Shedding, Mode 1 KW Input Is Greater Than Setpoint

11.11.6How 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

11-38 • E2 RX/BX/CX I&O Manual 026-1614 Rev 4 5-JAN-2013