Method of operation
7SV512
V1
26
C53000---G1176---C91
4.2.2 Current flow monitoring
Each of the phase currents and an additional plausi-
bility current (see below) are filtered by two-stage
numerical filter algorithms so that only the funda-
mental frequency is used for further evaluation. The
filters are designed such that the occurrence and
the disappearance of a sinusoidal current is de-
tected within less than a half a.c. period.
Particular features apply for recognition of the in-
stant of interruption. With sinusoidal currents, cur-
rent interruption is detected after approx. 5 to 10 ms.
With aperiodic d.c. current components in the fault
current and after interruption (e.g. with linear current
transformers) or if the current transformers are satu-
rated by the d.c. component in the fault current, it
can take up to one a.c. cycle, with extreme condi-
tions, before the disappearance of the primary cur-
rent is reliably detected.
Four currents are monitored and compared with the
set threshold. Besides the three phase currents, a
fourth current threshold is provided in order to de-
tect a plausibility current, normally the earth current
(residual current). This does not have any direct in-
fluence on the basic function of the breaker failure
protection but it allows a 2-out-of-4 comparison to be
made (refer Figure 4.4). This means that the current
detection signal L
K
> associated with a phase cur-
rent I
L
K
can only be generated when current is pres-
ent in at least one of the other phases, or an earth
(residual) current is detected.
With a phase-to-phase fault, current flows in at least
two poles of the breaker; with a single-phase fault,
an earth current is always present (residual sum of
the phase currents) if only one pole carries current.
Thus, on the one hand, detection of the fault current
and start of the timer is guaranteed. On the other
hand, the 2-out-of-4 logic provides high security
against false operation.
Three times the negative sequence current of the
symmetrical current components can be taken as
plausibility current in case the earth current is not
available. This negative sequence current is calcu-
lated by 7SV512 (Figure 4.5). The negative se-
quence current I
2
is calculated according to its defi-
nition equation:
3
¡
I
2
=
I
L1
+ a
2
¡
I
L2
+ a
¡
I
L3
where
a = e
j120
&
&
&
&
&
&
>
=1
>
=1
>
=1
I
L1
I
L2
I
L3
I---BF
I---BF
I---BF
I
pl
I---BF
L1>
L2>
L3>
Figure 4.4
Current flow monitoring with 2-out-
of-4 check
I
L1
I
L2
I
L3
I
E
3
¡
I
2
I
pl
PLAUS 1POL
NEG.SEQ. CURRENT
RESIDUAL CURRENT
Figure 4.5
Plausibility current
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