Emerson E2 operation manual Rotational Shed, Last Shed, Other Notes About Priority Levels

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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.

BEGIN

SHEDDING

1	11
2	12
3	13
4	14
5	15
6	16
7	17
8	18
9	19
10	20

FIRST SHED

2

3

4

5

20	19	18	17
1				16
				15
ROTATIONAL SHED				14
				13
				12
6				11

710

8 9

LEVELS ROTATE

IN THIS DIRECTION

FOR NEXT DEMAND

CONDITION

1	11
2	12
3	13
4	14
5	15
6	16
7	17
8	18
9	19
10	20

LAST SHED

26512042

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.

Figure 11-21- Shedding Priority Levels

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.

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.

Demand Control Software Overview • 11-37