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DMC3S - Manual - 08 - 2021
Voltage memory
To increase reliability in all fault conditions, the protection has a voltage memory.
This makes it possible to determine the direction of the current even in case of a fault in the immedi-
ate vicinity of the voltage transformers. In this situation the protection uses a fictitious polarisation
voltage signal which corresponds to the memorised voltage from the cycle preceding the fault.
The memorised voltage is used as a reference for one second after all three phase-to-phase voltag-
es drop to a value of less than 2%
U
n
.
Memory-F67.ai
1s
T
0
U
12
U
12
≤
1%
U
n
&
1s
T
0
U
23
≤
1%
U
n
1s
T
0
U
31
U
<
≤
1%
U
n
U
23
U
31
Input U
31
Input U
23
Input U
12
U
23
0.01
U
N
Start time
I
L1
I
L1
* cos
I
PD
>
Start
0.02 s
1 s
Voltage reference
(memory voltage)
Memory
U
23
Memory
U
31
Memory
U
12
Memory Timer
Voltage memory - protection 67
Closing on short circuit
1 s (voltage memory)
Example U23, IL1
