Schweitzer Engineering Laboratories SEL-587Z, Instruction Manual, Page: 62 / 276

3.30
SEL-587Z Instruction Manual
Date Code 20020903
Relay Elements
Demand Ammetering
Figure 3.27 Response of Thermal Demand
Ammeter to a Step Input (setting DMTC = 15 minutes).
The response of the thermal demand ammeter in Figure 3.27 (bottom) to the
step current input (top) is analogous to the series RC circuit in Figure 3.28 .
Figure 3.28 Voltage V
S
Applied to Series RC Circuit.
In the analogy:
Voltage V
S
in Figure 3.28 corresponds to the step current input
Figure 3.27 (top).
Voltage V
C
across the capacitor in Figure 3.28 corresponds to the
response of the thermal demand ammeter in Figure 3.27 (bottom).
If voltage V
S
in Figure 3.28 has been at zero (V
S
= 0.0 per unit) for some
time, voltage V
C
across the capacitor in Figure 3.28 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 ammeter in
Figure 3.27 on page 3.30 (bottom) to the
step current input (top).
Step
Current
Input
Thermal
Demand
Meter
Response
Inst
ant
aneous Curr
ent (per unit
)
Thermal Demand Curr
ent (per unit
)
1.0
1.0
0.5
0.5
0.9
0
0
0
0
5
5
10
10
15
15
Time
(minutes)
Time
(minutes)
DMTC = 15 minutes
+
+
–
–
Vc Vs C
R