18 August 1998
Condenser water supply temperature.
OA dry bulb temperature.
OA relative humidity.
Program output. Condenser water supply temperature setpoint.
Application notes. A dedicated local loop controller may be implemented.
12. DEMAND LIMITING PROGRAM. Demand limiting is accomplished by shedding electrical loads or
starting sources of auxiliary power such as standby generators to prevent electrical demand from
exceeding a peak value (target). This technique is used to reduce electrical costs where electric demand
is a cost factor in the utility rate schedules. Peak demand values are established by the utility company
using fixed demand intervals, sliding window intervals, and/or time of day schedules. The strategy to be
utilized in UMCS is the sliding window interval. Many complex schemes exist for reducing peak demand
billings; however, all schemes continuously monitor power demand and calculate the rate of change of
the demand value in order to predict future peak demand using prediction techniques. When the
predicted peak approaches preset limits, predetermined auxiliary power sources must be started and
predetermined scheduled electrical loads within pre-established groups must be shut off or power-limited
on a prescheduled priority basis to reduce the connected load before the peak is exceeded. Within a
particular group, the order in which a load is shed must be changed by the program so that after a load
has been the first to be shed in a group, it is moved to last in the group and another load becomes first.
The most commonly shed loads are non-critical HVAC and other utility systems. Design requirements for
this applications program will be indicated by the letter X adjacent to the demand limiting step listing on
the appropriate database table, as shown in Chapters 8 and 9.
a. Field hardware requirements. The hardware requirements are:
UMCS inputs from utility system.
(a) Equipment status (differential pressure switch, auxiliary contact, flow switch, chiller
current) - one for each piece of equipment
Instantaneous kilowatts (kW) demand for each metered point.
UMCS outputs to utility system.
Start/stop control signal to each load to be shed.
Analog control signal or set point adjustment.
b. Software I/O requirements. The software requirements are:
Day of week/holiday.
Time of day.
Chiller percent capacity.