Schweitzer Engineering Laboratories SEL-421-4 - Page 1031

7.7
Date Code 20171006 Instruction Manual SEL-400 Series Relays
Metering
Demand Metering
Rolling Demand
Rolling demand is a sliding time-window arithmetic average. Rolling demand
measurement is similar to a step-sampled A/D conversion system. Figure 7.4
shows the rolling demand response for a step input for a demand meter time con-
stant of 15 minutes (DMTC := 15). The relay divides the DMTC period into three
5-minute intervals and averages the three DMTC subinterval samples every
DMTC period. Table 7.4 lists the rolling demand response for four DMTC peri-
ods shown in Figure 7.4. Rolling demand metering response is at 100 percent
(1.0 per unit) of the full applied value after a time equal to the fourth DMTC
period (see (d) in Figure 7.4).
Figure 7.3 Thermal Demand Metering
Step
Current
Input
Thermal
Demand
Meter
Response
(EDEM := THM)
1.0
1.0
0.9
0.5
0
0.5
0
0
5
10
15
0
510
15
Time
(minutes)
DMTC Period
Time
(minutes)
R
C
—
+
Table 7.4 Rolling Demand Calculations
DMTC Period
(see Figure 9.18)
1/3 DMTC
Interval
(minutes)
Interval
Sample
(per unit)
Rolling
Demand
Tota l
Rolling
Demand
Calculation
Rolling
Demand
Response
(per unit)
(a) –5 to 0 0 0 0 / 3 0
(b) 0 to 5 1 1 1 / 3 0.33
(c) 5 to 10 1 2 2 / 3 0.67
(d) 10 to 15 1 3 3 / 3 1.00