Sel 551 - Figure 3.24 Response of Thermal Demand Ammeter to a Step Input (Setting DMTC = 15 Minutes)

3.42
SEL-551 Relay Instruction Manual Date Code 20110408
Relay Elements and Logic
Demand Ammetering
The SEL-551 provides demand and peak demand ammetering for the
following values:
These demand and peak demand values are thermal demand values. Thermal
demand ammetering is explained in the following discussion.
Thermal Demand
Ammeter Operation
The example in Figure 3.24 shows the response of a thermal demand ammeter
to a step current input. The current input is at a magnitude of zero and then
suddenly goes to an instantaneous level of 1.0 per unit (a “step”).
Figure 3.24 Response of Thermal Demand Ammeter to a Step Input (Setting
DMTC = 15 minutes)
Thermal Demand Ammeter Response
The response of the thermal demand ammeter in Figure 3.24 (bottom) to the
step current input (top) is analogous to the series RC circuit in Figure 3.25.
Currents
I
A,B,C,N
Input currents (A primary)
I
G
Residual ground current (A primary; IG = 3I0 = IA + IB + IC)
3I
2
Negative-sequence current (A primary)
Thermal Demand
Meter Response
Thermal Demand Current (per unit)
0
051015
0.5
0.9
1.0
Time
(Minutes)
DMTC = 15 minutes
Step Current
Input
Instantaneous Current (per unit)
0
051015
0.5
1.0
Time
(Minutes)