Emerson E2 Load Shedding, Shedding Levels, Priority Levels, Definition, First Shed

As mentioned in Section 11.11.1, Introduction to Demand Limit Control, all KW input values are gathered together and averaged together in a demand window. The demand setpoint is then subtracted from this average. The result of this calculation is called the integral error (or integral) of the demand window. The integral error repre- sents how far below or above the setpoint the input has been for the entire demand window.

Under non-demand conditions, the integral error will be below zero, since the average KW or watt-hour value will be below the setpoint.

The current value of the integral error and the current value of the KW input are what determine when the Demand Control application will begin load shedding.

11.11.3 Load Shedding

11.11.3.1 Definition

Load shedding is the process of curbing the amount of kilowatt usage within one or more target applications. Most applications that drive mechanical or electronic devices, such as lights, heaters, air conditioners, compres- sors, etc., are configured with special inputs that, when active, “shed” the application.

Physically, when a device is “shed,” its control set- points are altered by a specified amount in a direction that will save energy. For instance, an HVAC unit with heating and cooling stages that is commanded to shed will have its heating setpoints lowered and its cooling setpoints raised. This will cause the HVAC unit to operate at a lower capac- ity, thus saving energy.

Table 11-13lists a number of E2 RX-300 and BX-300 applications and how load shedding affects them.

Application	Shedding Effect
All HVAC applica-	Raises cooling setpoints, lowers
tions (RT-100s, Multi-	heating setpoints
Flex RTUs, AHUs,
Heat/Cools)
Sensor Control and	Raises or lowers Cut In/Cut Out
Analog Input I/O	setpoints (whichever direction
Modules	saves energy)
Refrigeration Circuits	Raises case temperature setpoints
Suction Groups	Raises suction setpoint
Condenser	Raises discharge setpoint
Analog Output I/O	Raises or lowers PID setpoint
Modules	(whichever will save energy)
Digital Output I/O	Control output is locked OFF.
Modules

Table 11-13- Shedding Effect on Applications

11.11.4 Shedding Levels

Some applications have a single input (usually called DEMAND SHED) that, when ON, sheds the application. Each application is programmed with a specific “demand bump” value that is added or subtracted from the control setpoints when this input comes ON.

Other applications, notably HVAC applications, have two inputs: a primary and a secondary (usually called PRI DEMAND SHED and SEC DEMAND SHED). Each input has its own “demand bump” value, so that when the primary is ON, the primary demand bump is in effect, and when the secondary is ON, the secondary demand bump is in effect. When both of these inputs are ON, the demand bump values are added/subtracted together and applied to the setpoint(s). As a result, two levels of shedding are pos- sible for a single application.

Example: An air conditioner with a cooling setpoint of 70°F has a primary demand bump value of 2 and a sec- ondary demand bump value of 2. In a typical Demand Control application setup, the PRI DEMAND SHED input would be one of the first inputs to go into shed; when this occurs, the bump value is added to the cooling setpoint, raising it to 72°F.

The SEC DEMAND SHED would be set up to shed later (if necessary) as a second level of shedding for the air conditioner. When this input is turned ON, the setpoint is raised by the secondary demand bump value. Since the PRI DEMAND SHED input is still ON, both values are added to the setpoint, raising the setpoint to 74°F.

11.11.5 Priority Levels

Each application that you wish to set up for shedding must be assigned a priority level in the Demand Control application. When a Demand Control application sheds loads, it does so by shedding all applications that are in the first priority level, and then continuing to shed higher pri- ority levels until the demand is at an acceptable level or until all levels are shed.

Priority levels are simply outputs that connect to the demand shedding inputs of one or more applications. When a level is shed, the output comes ON and all appli- cations connected to the output are shed.

There are three types of priority levels:

1. First Shed

First Shed levels are the first ones to be shed when demand shedding begins. The Demand Control applica- tion first begins shedding by activating level #1 of the First Shed levels. As needed, each level thereafter is shed in succession (#2, #3, and so on to #20).

Applications that are part of the First Shed priority lev- els should be non-vital applications that would have no

Demand Control Software Overview • 11-35