Demand - GE Multilin 850 Instruction Manual

MONITORING

Demand

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Current Demand is measured on each phase, and on three phases for real, reactive, and
apparent power. Setpoints allow the user to emulate some common electrical utility
demand measuring techniques for statistical or control purposes.
The relay is not approved as, or intended to be, a revenue metering instrument. If used in a
peak load control system, the user must consider the accuracy rating and method of
measurement employed, and the source VTs and CTs, in comparison with the electrical
utility revenue metering system.
The relay can be set to calculate Demand by any of three methods.
• Thermal Exponential: This selection emulates the action of an analog peak recording
Thermal Demand meter. The relay measures the quantity (RMS current, real power,
reactive power, or apparent power) on each phase every second, and assumes the
circuit quantity remains at this value until updated by the next measurement. It
calculates the Thermal Demand equivalent based on:
-kt
d(t) = D(1 - e )
Where:
d = demand value after applying input quantity for time t (in minutes),
D = input quantity (constant),
k = 2.3/thermal 90% response time.
Figure 4-119: Thermal Demand Characteristic (15 min response)
The 90% thermal response time characteristic defaults to 15 minutes. A setpoint
establishes the time to reach 90% of a steady-state value, just as with the response
time of an analog instrument. A steady-state value applied for twice the response
time will indicate 99% of the value.
• Block Interval: This selection calculates a linear average of the quantity (RMS current,
real power, reactive power, or apparent power) over the programmed Demand time
interval, starting daily at 00:00:00 (i.e. 12 am). The 1440 minutes per day is divided into
the number of blocks as set by the programmed time interval. Each new value of
Demand becomes available at the end of each time interval.
• Rolling Demand: This selection calculates a linear average of the quantity (RMS
current, real power, reactive power, or apparent power) over the programmed
Demand time interval, in the same way as Block Interval. The value is updated every
minute and indicates the Demand over the time interval just proceeding the time of
update.
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS