Schweitzer Engineering Laboratories SEL-787, Instruction Manual, Page: 144 / 564

4.66
SEL-787 Relay Instruction Manual Date Code 20081022
Protection and Logic Functions
Demand Metering
Figure 4.43 Voltage V
S
Applied to Series RC Circuit
In the analogy:
Voltage V
S
in Figure 4.43 corresponds to the step current input in
Figure 4.42 (top).
Voltage V
C
across the capacitor in Figure 4.43 corresponds to the
response of the thermal demand meter in Figure 4.42 (middle).
If voltage V
S
in Figure 4.43 has been at zero (V
S
= 0.0 per unit) for some
time, voltage V
C
across the capacitor in Figure 4.43 is also at zero
(V
C
= 0.0 per unit). If voltage V
S
is suddenly stepped up to some constant
value (V
S
= 1.0 per unit), voltage V
C
across the capacitor starts to rise toward
the 1.0 per unit value. This voltage rise across the capacitor is analogous to the
response of the thermal demand meter in
Figure 4.42 (middle) to the step
current input (top).
In general, as voltage V
C
across the capacitor in Figure 4.43 cannot change
instantaneously, the thermal demand meter response is not immediate either
for the increasing or decreasing applied instantaneous current. The thermal
demand meter response time is based on the demand meter time constant
setting DMTC (see
Table 4.23 ). Note in Figure 4.42 , the thermal demand
meter response (middle) is at 90 percent (0.9 per unit) of full applied value
(1.0 per unit) after a time period equal to setting DMTC = 15 minutes,
referenced to when the step current input is first applied.
The SEL-787 updates thermal demand values approximately every second.
Rolling Demand Meter Response
The response of the rolling demand meter in Figure 4.42 (bottom) to the step
current input (top) is calculated with a sliding time-window arithmetic average
calculation. The width of the sliding time-window is equal to the demand
meter time constant setting DMTC (see
Table 4.23 ). Note in Figure 4.42 , the
rolling demand meter response (bottom) is at 100 percent (1.0 per unit) of full
applied value (1.0 per unit) after a time period equal to setting
DMTC = 15 minutes, referenced to when the step current input is first applied.
The rolling demand meter integrates the applied signal (e.g., step current)
input in five-minute intervals. The integration is performed approximately
every second. The average value for an integrated five-minute interval is
derived and stored as a five-minute total. The rolling demand meter then
averages a number of the five-minute totals to produce the rolling demand
meter response. In the
Figure 4.42 example, the rolling demand meter
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 4.42 (bottom).
V
S
V
C
+
+
—
—
R
C