Schweitzer Engineering Laboratories SEL-734 - Page 53

4.7
Date Code 20090730 Instruction Manual SEL-734 Meter
Metering
Demand Metering
Thermal Demand Meter Response (EDEM=THM)
The thermal demand meter response in Figure 4.3 to the step current input is
analogous to the series RC circuit in Figure 4.4.
Figure 4.4 Voltage V
S
Applied to Series RC Circuit
In the analogy:
➤ Voltage V
S
in Figure 4.4 corresponds to the step current input
in Figure 4.3 (top).
➤ Voltage V
C
across the capacitor in Figure 4.4 corresponds to
the response of the thermal demand meter in Figure 4.3
(middle).
If voltage V
S
in Figure 4.4 has been at zero (V
S
= 0.0 per unit) for some time,
voltage V
C
across the capacitor in Figure 4.4 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 4.3 (middle) to the step current input (top).
In general, because voltage V
C
across the capacitor in Figure 4.4 cannot
change instantaneously, the thermal demand meter response to increasing or
decreasing applied instantaneous current is also not immediate. The thermal
demand meter response time is based on the demand meter time constant
setting DMTC (see Table 4.6). Note in Figure 4.3, 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 energy input is first applied.
The SEL-734 updates thermal demand values approximately every second.
Rolling Demand Meter Response (EDEM=ROL)
The response of the rolling demand meter in Figure 4.3 to the step current
input is calculated with a sliding time-window arithmetic average. The width
of the sliding time-window is equal to the demand meter time constant setting
DMTC (see Table 4.6). Notice in Figure 4.3, 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 demand meter subintervals (setting DMSI; see Section 9: Settings).
See Table 4.2 for a listing of possible DMTC intervals and DMSI subintervals.
The integration is performed approximately every second. The average value
for an integrated DMSI-minute subinterval is derived and stored as a
C
V
c
V
s
R
+
–
Courtesy of NationalSwitchgear.com