Emerson E2 User Manual download pdf (Page 178)

11-36 • E2 RX/BX/CX I&O Manual 026-1610 Rev 13 14-SEP-2011

significant effect on the system if it were to be shed for a

prolonged period of time. Examples: motor room exhaust

fans, satellite HVAC units and/or lights in low-occupancy

rooms.

2. Rotational Shed

The Rotational Shed levels are shed in sequence only if

all defined First Shed levels have already been shed and

the Demand Control application needs more shedding to

lower demand.

Unlike the First Shed priority levels, Rotational Shed

levels do not always begin shedding by activating level #1.

Instead, it shares the burden of being the first to activate

among all the Rotational Shed levels. If, during one

demand condition, Rotational Shed level #1 is the first of

all the Rotational Shed levels to activate, it “rotates” first

activation status to the next level in sequence (in this case,

level #2). Thus, the next time a demand condition occurs,

level #2 will activate first, and further levels will activate

in sequence until it reaches level #1, which will be the last

to activate.

Applications that are Rotational Shed priority should

be more vital to system performance than the ones config-

ured as First Shed priority levels. In addition, all applica-

tions assigned to Rotational Shed levels should all be of

relatively equal importance, since they each will spend

about the same total amount of time in shed.

Also, if you will be shedding a small amount of loads

that are relatively equal in priority, you may choose to

define them all under Rotational Shed levels, leaving the

First Shed levels blank. Demand Control would then skip

directly to the Rotational Shed levels during load shed-

ding, and all loads would share shed time equally.

3. Last Shed

The Last Shed priority levels are shed in sequence only

if all First Shed and Rotational Shed levels have been shed

and the Demand Control application still requires demand

reduction.

Like the First Shed priority levels, level #1 is always

the first Last Shed level to be activated, followed by #2,

#3, #4, and so on up to #20.

Applications that are assigned to this priority should be

the most vital applications to system performance. These

applications will be shed only as a last resort, and there-

fore will spend the least amount of time in shed.

A diagram of how Demand Control cycles through all

of the three different types of priority levels are shown in

the diagram in Figure 11-21.

Other Notes About Priority Levels

All priority levels, whether they be First, Rotational, or

Last, should be shedding relatively the same number of

kilowatts. When the Demand Control application is shed-

ding in a non-emergency situation, it usually sheds one or

two levels at a time. Therefore, Demand Control would do

a much better job of reducing demand quickly if each acti-

vated level were reducing the KW by an equivalent

amount.

Also, each priority level has three important user-

definable parameters that affect the level’s time in shed:

• Minimum Shed Duration - the minimum amount of

time a level must remain in shed. Demand Control

will not restore the level until this minimum time

has passed.

• Maximum Shed Duration - the maximum amount of

time a level will remain in shed. When the level has

been shed longer than this duration, it will automat-

ically be restored regardless of the current need for

demand shedding.

• Minimum Restore Duration - the minimum amount

of time a level must be restored from shed before it

may be shed again. Demand Control will not re-

shed this level until this minimum time has passed.

Figure 11-21

- Shedding Priority Levels

NOTE: If a level is in RESTORE but its mini-

mum restore duration has not been met, levels

of the next highest priority category or catego-

ries will not be shed until the lower priority lev-

els are all in shed.

2212

3313

4414

5515

6616

8818

9919

10 1020

7717

1819

FIRST SHED

LAST SHED

ROTATIONAL SHED

LEVELS ROTATE

IN THIS DIRECTION

FOR NEXT DEMAND

CONDITION

BEGIN

SHEDDING

26512042

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