Error Limit - 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

The Error Limit attribute is an important attribute in the Sliding Window Alg Advanced section of the DLLR object UI that prevents unnecessary shedding when you select a demand meter with fewer than 100 pulses per minute. This situation often occurs if you limit demand at night when most equipment is turned off. This attribute limits the safety margin (Smoothed Absolute Error) to a percentage of the Active Demand Limit. That is, when the energy rate changes by more than a few percent per minute (which is common for meters with fewer than 100 pulses per minute), the safety margin used by the algorithm can grow to where more load is shed than may be needed. This attribute allows you to reduce the size of the safety margin and thereby avoid shedding too aggressively.

Figure 1 shows trend graphs of typical energy usage that compare load shedding with three different Error Limits: 10%, 5%, and 0%. Graph 1 shows load shedding when the default value of 10% is used. This Error Limit is appropriate for most metering situations, but it can be fine-tuned once you understand your energy usage. For example, if you receive occupant comfort complaints and you anticipate no serious energy spikes, you may wish to adjust the Error Limit to 5% to reduce the amount of load that is normally shed while still retaining a safety margin (Graph 2). Be aware that if you reduce the Error Limit too much, the target may be exceeded, as Graph 3 indicates.

Note: Since adjusting the Error Limit may increase the possibility of exceeding the demand limit target, we recommend that you test the Error Limit while the DLLR object is in Monitor Only mode before committing to a value.
Figure 1. Comparing 10%, 5%, and 0% Error Limits