8-22
Breaker Monitor, Metering, and Load Profile Functions
Date Code 20001006
SEL-351 Instruction Manual
Thermal Demand Meter Response (EDEM = THM)
The response of the thermal demand meter in Figure 8.11 (middle) to the step current input (top) is
analogous to the series RC circuit in Figure 8.12.
Figure 8.12: Voltage V
S
Applied to Series RC Circuit
In the analogy:
Voltage V
S
in Figure 8.12 corresponds to the step current input in Figure 8.11 (top).
Voltage V
C
across the capacitor in Figure 8.12 corresponds to the response of the thermal
demand meter in Figure 8.11 (middle).
If voltage V
S
in Figure 8.12 has been at zero (V
S
= 0.0 per unit) for some time, voltage V
C
across
the capacitor in Figure 8.12 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 8.11 (middle) to the step current input (top).
In general, as voltage V
C
across the capacitor in Figure 8.12 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 8.3). Note in Figure 8.11, the thermal demand meter response
(middle) is at 90% (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-351 Relay updates thermal demand values approximately every 2 seconds.
Rolling Demand Meter Response (EDEM = ROL)
The response of the rolling demand meter in Figure 8.11 (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 8.3). Note in
Figure 8.11, the rolling demand meter response (bottom) is at 100% (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.
Summary of Contents for SEL-351
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