Demand Meter Object - Metasys - LIT-12011288 - Software Application - Metasys Building Automation System - 6

DLLR Technical Bulletin

Product
Building Automation Systems > Building Automation Systems > Metasys Building Automation System
Document type
Technical Bulletin
Document number
LIT-12011288
Version
6
Revision date
2013-10-15

If you select Demand Limiting, an analog or digital meter is required to determine the instantaneous energy rate for the facility. Load Rolling does not require a meter. The meter should reside on the same engine as the DLLR object.

If you use an analog meter, pulses can be counted by either the N1 Pulse Counter object or the Counter Input object. These pulses can be converted to a rate for DLLR using the Pulse Meter or N1 Pulse Counter.

Note: We do not recommend using an N1 Migration object for the meter, unless you can guarantee that the NCM Service Time is always less than 60 seconds. The Service Time attribute is located on the Diagnostic tab of the NCM object. Optionally, you can rewire the meter to a field controller that reports directly to the NAE or NCE.

If you use a digital meter, no conversion is necessary. The meter value represents the energy rate that can be sent to an Analog Input object and fed to the DLLR object.

When using a power meter, do the following:

  • Consider the meter constant. A meter that provides at least 100 counts per minute is preferred. Higher counts per minute result in greater accuracy and less erratic meter value changes, which helps prevent unnecessary shedding. If a meter with lower than 100 counts is used, the algorithm may detect a large variation in readings and consequently apply a safety margin that sheds more loads than desired. If this type of meter is used, use the Error Limit attribute to reduce the safety margin and thus reduce the amount of load shed. See Error Limit.

  • Ensure a permanent communication path between the meter and DLLR object; therefore, the meter and DLLR object should reside on the same engine.

  • Calculate a new rate with the meter object exactly once every 60 seconds.

  • Verify that the new rate is the average power over the just-elapsed minute and displays in the meter object Present Value attribute.

When you define the meter object (or when DLLR restarts), DLLR sends a Reset command to the meter object in case it needs to synchronize to a pulse counter object. DLLR then signs up for the Present Value attribute of the meter object to get the energy rate. This rate is shown in the Energy Rate attribute of the DLLR object (Figure 1). The value is used in determining how much load to shed or release this minute for the Demand Limiting strategy.

If the DLLR object Meter Object attribute changes, DLLR cancels the signup to the old meter object, sends a Reset command to the new meter object, and then signs up for the Present Value attribute of the new meter object.

The DLLR object contains two attributes that allow the user to specify the energy units that display in the DLLR reports: Rate Units and Consumption Units. These attributes can be set even if no meter is defined, which would be the case when only the Load Rolling strategy is being used, which does not require a meter.

The default unit type for the meter is kilowatts; however, other types of units, such as BTUs, can be set for meters that report other types of data.

DLLR determines the reliability of the meter reading, so a high or low reading is not misinterpreted as a meter error. The meter is an object-reference attribute in the DLLR object.