Emerson E2 Demand Control, Output Light Dimming, Introduction to Demand Limit Control

longer than the programmed proof delay, the Proof cell turns its Proof output ON to signify a failure has occurred.

The Proof cell will deactivate the Proof output if the proof failure has been cleared longer than the programmed proof latch time.

11.10.9 Output Light Dimming

The dimming feature allows the lighting output to be varied based on a light level sensor. Dimming capability can work with both Standard Control and Alternate Con- trol configuration. For setup, the user can:

•Specify the light levels for the minimum and maxi- mum light output.

•Specify the ramp speed that the indoor lighting out- put changes.

11.11 Demand Control

Demand Control applications are used by E2 to keep the total power usage of the building environmental sys- tem below a specific demand setpoint dictated by the power company. Demand Control applications achieve this by reducing power consumption in one or more appli- cations in the other E2 applications as necessary until the total KW is below the setpoint. This process is called load shedding.

The Demand Control application is designed to calcu- late power usage in much the same way power companies do, which ensures that it will be able to meet the power company’s demand limit accurately. The Demand Control application is also designed with a large amount of flexi- bility in the way load shedding is prioritized, which allows users to maximize power economization with a minimal amount of interference into the normal operations of the building’s HVAC systems.

11.11.1Introduction to Demand Limit Control

Power companies supply power to consumers at a fixed rate per kilowatt hour until a pre-defined level of energy consumption is reached. This level is called the demand limit. When the demand limit is exceeded, the rate is greatly increased as a penalty for high power demand by the consumer. Typically, once the demand limit is exceeded, the increased rate is charged for the remain- der of the year.

To determine if a consumer has reached the demand limit, the power company arbitrarily monitors a con- sumer’s energy consumption for a fixed period of time (usually anywhere from 15 to 30 minutes with most power companies, although it may be as short as 5 minutes). This

monitoring period is called the demand window. Demand windows are always the same length of time, but they may be measured at any time the power company chooses to.

Since Demand Control applications have no way of determining when the power company will start a demand window, they simply assume that the demand window can begin at any time. As new values of the KW input are gathered, Demand Control applications “roll” their demand windows forward and recalculate their load shed- ding algorithms as if the power company were measuring the average power of the new window.

For example, if the power company’s demand window lasts for 15 minutes, the Demand Control applications will keep a constant real-time average of all recorded KW val- ues from the previous 15 minutes. If a Demand Control application determines from its calculations that the cur- rent KW usage might cause its demand limit to be exceeded for the current or a future demand window, it will begin shedding loads to bring the KW usage down. A graphic example of the “rolling demand window” used by the Demand Control application is shown in Figure 11-20.

Figure 11-20- Example of “Rolling Demand Window”

In short, the primary goal of a Demand Control appli- cation is to make sure that every possible interval of time that might be used by the power company as a demand window will be under the set demand limit. This ensures the demand setpoint will never be exceeded.

11.11.2 Demand Monitoring

The E2 either monitors power using an analog KW transducer (which sends the current KW usage via analog signal), or monitors energy consumption using a digital watt-hour transducer (which pulses to signify a fixed amount of watt-hours have been used).

11-34 • E2 RX/BX/CX I&O Manual 026-1610 Rev 13 14-SEP-2011