Schweitzer Engineering Laboratories SEL-700G Series, Инструкция по эксплуатации

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4.183
Date Code 20170814 Instruction Manual SEL-700G Relay
Protection and Logic Functions
Other Settings
Thermal Demand Meter Response
The response of the thermal demand meter in Figure 4.122 (middle) to the
step current input (top) is analogous to the series RC circuit in Figure 4.123.
Figure 4.123 Voltage V
S
Applied to Series RC Circuit
In the analogy:
➤
Voltage V
S
in Figure 4.123 corresponds to the step current input
in Figure 4.122 (top).
➤
Voltage V
C
across the capacitor in Figure 4.123 corresponds to
the response of the thermal demand meter in Figure 4.122
(middle).
If voltage V
S
in Figure 4.123 has been at zero (V
S
= 0.0 per unit) for some
time, voltage V
C
across the capacitor in Figure 4.123 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 4.121 (middle) to the step
current input (top).
In general, as voltage V
C
across the capacitor in Figure 4.123 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 4.51). Note in Figure 4.122 that 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-700G
updates thermal demand values approximately every second.
Rolling Demand Meter Response
The response of the rolling demand meter in Figure 4.122 (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 4.51). Note in Figure 4.122 that
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 (for example, 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 4.122 example, the rolling demand
meter averages the three latest five-minute totals because setting DMTC = 15
V
S
V
C
+
+
—
—
R
C