Schweitzer Engineering Laboratories SEL-751 - Page 283

4.157
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
averages the three latest five-minute totals because setting DMTC = 15
(15/5 = 3). The rolling demand meter response is updated every five minutes,
after a new five-minute total is calculated.
The following is a step-by-step calculation of the rolling demand response
example in Figure 4.91 (bottom).
Time = 0 Minutes. Presume that the instantaneous current has been at zero
for quite some time before “Time = 0 minutes” (or the demand meters were
reset). The three five-minute intervals in the sliding time-window at “Time = 0
minutes” each integrate into the following five-minute totals:
Rolling demand meter response at “Time = 0 minutes” = 0.0/3 = 0.0 per unit.
Time = 5 Minutes. The three five-minute intervals in the sliding time-win-
dow at “Time = 5 minutes” each integrate into the following five-minute
totals:
Rolling demand meter response at “Time = 5 minutes” = 1.0/3 = 0.33 per unit.
Time = 10 Minutes. The three five-minute intervals in the sliding time-win-
dow at “Time = 10 minutes” each integrate into the following five-minute
totals:
Rolling demand meter response at “Time = 10 minutes” = 2.0/3 = 0.67 per unit.
Five-Minute Totals Corresponding Five-Minute Interval
0.0 per unit –15 to –10 minutes
0.0 per unit –10 to –5 minutes
0.0 per unit –5 to 0 minutes
0.0 per unit
Five-Minute Totals Corresponding Five-Minute Interval
0.0 per unit –10 to –5 minutes
0.0 per unit –5 to 0 minutes
1.0 per unit 0 to 5 minutes
1.0 per unit
Five-Minute Totals Corresponding Five-Minute Interval
0.0 per unit –5 to 0 minutes
1.0 per unit 0 to 5 minutes
1.0 per unit 5 to 10 minutes
2.0 per unit