The neutral-point current is permanently assigned to the measuring input for end
a (T14).
A further amplitude matching factor is calculated and displayed by the P634 for
each end added. All currents involved must be referred to a common reference
value as this application entails protection of an electric node in a zero-sequence
system. This common reference current value is calculated based on the settable
reference power and the nominal voltage for end a. The amplitude matching
factors then result from the ratio of the primary transformer current values for
the relevant end and the common reference current value.
Reference current: I
ref
=
S
ref ,prim
3 ⋅ V
nom,prim,a
Amplitude matching factor:
k
am,x
=
I
nom,CT ,prim,x
I
ref
for x = End a, b, c, d
with
S
ref, prim
= (019 031) REF_1: Reference power Sref
V
nom, prim a
= (019 017) MAIN: Vnom prim. end a PSx
I
nom, CT, prim a
= (019 020) MAIN: Inom C.T.prim.,end a
I
nom, CT, prim b
= (019 021) MAIN: Inom C.T.prim.,end b
I
nom, CT, prim c
= (019 022) MAIN: Inom C.T.prim.,end c
I
nom, CT, prim d
= (019 026) MAIN: Inom C.T.prim.,end d
The reference current and matching factors for the transformer ends are
displayed at the P634.
As before, the P634 checks that the reference current and matching factors
remain within the permitted ranges. The permitted range for the reference
current can be looked up in the operating system. The following rule applies to
the matching factors:
●
The largest matching factor must be ≤ 16.
●
The value for the second largest matching factor must come to ≥ 0.5.
There are no restrictions concerning further possible matching factors.
Should the P634 calculate a common reference current value or matching factors
not satisfying the above conditions then an alarm will be issued and the P634 will
automatically be blocked.
The measured values are multiplied by the matching factors and they are then
available for further processing. Consequently, all threshold values and
measured values always refer back to the relevant reference current
rather than to the transformer nominal current or the nominal current
of the device.
As shown below, the differential current is formed from the sum of all phase
currents involved and the neutral-point current:
I
diff,N
= |k
am,a
·I
N,a
+ k
am,b
·I
N,b
+ k
am,c
·I
N,c
+ k
am,Y
·I
a,Y
|
= |k
am,a
·∑{I
A,a
,I
B,a
,I
C,a
} + k
am,b
·∑{I
A,b
,I
B,b
,I
C,b
} + k
am,c
·∑{I
A,c
,I
B,c
,I
C,c
} + k
am,d
·∑
{I
A,d
,I
B,d
,I
C,d
} + k
am,Y
·I
a,Y
|
The calculation of the restraining current depends on the set operating mode.
If the operating mode is set to “Low imped. / sum(IP)”, the restraining current is
calculated from the sum of all involved ends:
I
R,N
= |k
am,a
·I
N,a
+ k
am,b
·I
N,b
+ k
am,c
·I
N,c
+ k
am,d
·I
N,d
|
= |k
am,a
·∑{I
A,a
,I
B,a
,I
C,a
} + k
am,b
·∑{I
A,b
,I
B,b
,I
C,b
} + k
am,c
·∑{I
A,c
,I
B,c
,I
C,c
} + k
am,d
·∑
{I
A,d
,I
B,d
,I
C,d
}|
P634
3 Operation
3-150 P634/EN M/R-42-A // P634‑311‑653
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