and the rated power system frequency, that is, the memory voltage is used for 15 periods at
rated system frequency and for a proportionally reduced number of periods as the frequency
deviates from rated power system frequency.
As long as the reference voltage U
ref
is greater than 0.5% of the rated voltage, it is used to
determine fault direction.
In this case, a forwards fault satisfies the condition:
-90° < arg Z
ref
< +90°
A reverse fault satisfies the condition:
+90° < arg Z
ref
< -90°
Z
ref
is the impedance measured by the protection, which contains a component of the source
impedance Z
S
in addition to the line impedance Z
L
. The operating characteristic has to be
mathematically transformed in order to make the influence of the source impedance visible. If
the reference voltage is less than 0.5% of the rated voltage, direction is not taken into account
for the phase-to-ground loop and tripping is blocked. In the case of the phase-to-phase loops,
tripping is either enabled or blocked, depending on the setting of the parameter
MemDirMode.
5.1.7.3
VT supervision
GUID-66C20810-64C5-419C-9A32-29BBE2A3DA00 v1
The purpose of the VT supervision function is to monitor the VT leads with respect to
asymmetrical short-circuits and open-circuits. An MCB can be included for three-phase VT
short-circuits and arranged to block the protection via a separate optocoupler input.
The input variables monitored by the VT supervision function are the three voltages U
L1
, U
L2
,
and U
L3
and the three currents I
L1
, I
L2
, I
L3
. The zero-sequence (U
0
, I
0
) and negative-sequence
(U
2
, I
2
) components are calculated for both the three-phase voltage and three-phase current
systems:
3U
0
= U
L1
+ U
L2
+ U
L3
3U
2
= U
L1
+ U
L2
× a2 + U
L3
× a
3
0.5
1/120
2
a
j
3I
0
= I
L1
+ I
L2
+ I
L3
3I
2
= I
L1
+ I
L2
× a2 + I
L3
× a
Section 5
1MRK 505 406-UEN B
Bay protection functions
64
Bay protection functions REB500
Technical manual
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