Emerson E2 - Manual (Page 177)

Demand Control Software Overview • 11 - 35
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 11-13 lists a number of E2 RX-300 and BX-300
applications and how load shedding affects them.
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
Application Shedding Effect
All HVAC applica-
tions (RT-100s, Multi-
Flex RTUs, AHUs,
Heat/Cools) Raises cooling setpoints, lowers
heating setpoints
Sensor Control and
Analog Input I/O
Modules Raises or lowers Cut In/Cut Out
setpoints (whichever direction
saves energy)
Refrigeration Circuits Raises case temperature setpoints
Suction Groups Raises suction setpoint
Condenser Raises discharge setpoint
Analog Output I/O
Modules Raises or lowers PID setpoint
(whichever will save energy)
Digital Output I/O
Modules Control output is locked OFF.
Table 11-13 - Shedding Effect on Applications