2.19 Monitoring function
315
7SA522 Manual
C53000-G1176-C155-3
Scanning
Fr eq uen cy
The sampling frequency and the synchronism of the analog-digital converters is con-
tinuously monitored. If any deviations cannot be removed by remedied synchroniza-
tion, then the processor system is restarted.
Measurement Value
Acquisition –
Currents
Up to four input currents are measured by the device. If the three phase currents and
the earth fault current from the current transformer star point or a separated earth
current transformer of the line to be protected are connected to the device, their digi-
tised sum must be zero. Faults in the current circuit are recognised if
I
F
= |I
L1
+ I
L2
+ I
L3
+ k
I
·I
E
| > Σ,7+5(6+2/'·I
N
+ Σ,)$&725·Σ | I |
Factor k
I
(address ,,SK&7) takes into account a possible different ratio of a sep-
arate I
E
–transformer (e.g. cable core balance current transformer). Σ,7+5(6+2/'
and Σ,)$&725 are setting parameters. The component Σ,)$&725 · Σ | I | takes
into account the allowable current proportional ratio errors of the input transducers
which are particularly prevalent during large fault currents (Figure 2-138). The dropout
ratio is about 97 %. Σ | I | is the sum of all currents:
Σ | I | = |I
L1
| + |I
L2
| + |I
L3
| + |k
I
·I
E
|
This malfunction is signaled as ´)DLOXUHΣ,µ (FNo. 162).
Note
Current sum monitoring can operate properly only when the residual current of the
protected line is fed to the fourth current input (I
4
) of the relay.
Figure 2-138 Current sum monitoring
Measured Value Ac-
quisition –
Voltages
Four measuring inputs are available in the voltage path: three for phase–earth voltag-
es as well as one input for the displacement voltage (e-n voltage of an open delta con-
nection) or a busbar voltage. If the displacement voltage is connected to the device,
the sum of the three digitized phase voltages must equal three times the zero se-
quence voltage. Errors in the voltage transformer circuits are detected when
U
F
= |U
L1
+ U
L2
+ U
L3
+ k
U
·U
EN
| > 25 V.
The factor k
U
allows for a difference of the transformation ratio between the displace-
ment voltage input and the phase voltage inputs (parameter 8SK8GHOWD). The
dropout ratio is about 97%.
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