Siemens SIPROTEC 7SA522 Manual page 315

Scanning
Frequency
Measurement Value
Acquisition –
Currents
Measured Value Ac-
quisition –
Voltages
7SA522 Manual
C53000-G1176-C155-3
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.
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 = |I + I + I + k
F L1 L2 L3 I
(address ,,SK &7) takes into account a possible different ratio of a sep-
Factor k
I
–transformer (e.g. cable core balance current transformer). Σ, 7+5(6+2/'
arate I
E
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
| + |I | + |I
L1 L2
This malfunction is signaled as ´)DLOXUH Σ ,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
Figure 2-138 Current sum monitoring
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 = |U + U + U
F L1 L2 L3
The factor k allows for a difference of the transformation ratio between the displace-
U
ment voltage input and the phase voltage inputs (parameter 8SK  8GHOWD). The
dropout ratio is about 97%.
| > Σ, 7+5(6+2/'·I
·I
E
| + |k ·I |
L3 I E
+ k ·U | > 25 V.
U EN
2.19 Monitoring function
+ Σ, )$&725·Σ | I |
N
) of the relay.
4
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