Siemens SIPROTEC 4 7VK61 - Page 132

Measured Value Acquisition - Currents
Up to four input currents are measured by the device. If the three phase currents and the earth current from
the current transformer starpoint or a separated earth current transformer of the line to be protected are
connected to the device, their digitized sum must be zero. Faults in the current circuit are recognized if
Ι
F
= |
Ι
L1
+ Ι
L2
+ Ι
L3
+ k
Ι
·Ι
E
| > ΣI THRESHOLD + ΣI FACTOR·Σ | Ι |
Factor k
Ι
(address 221 I4/Iph CT) takes into account a possible different ratio of a separate Ι
E
transformer
(e.g. cable core balance current transformer). ΣI THRESHOLD and ΣI FACTOR. are setting parameters.
The ΣI FACTOR Σ | Ι | part takes into account permissible current-proportional transformation errors of the
transformer, which can occur in the case of high short-circuit currents.Figure 2-49). Σ | Ι | is the sum of all
currents:
Σ | Ι | = |Ι
L1
| + |Ι
L2
| + |Ι
L3
| + |k
Ι
·Ι
E
|
This fault is signaled as
Failure Σ I (no. 162).
i
i
NOTE
Current sum monitoring can operate properly only when the residual current of the protected line is fed to
the fourth current input (Ι
4
) of the relay.
[stromsummenueberwachung-020313-kn, 1, en_GB]
Figure 2-49 Current sum monitoring
Measured Value Acquisition Voltages
Four measuring inputs are available in the voltage path: three for phase-to-earth voltages and one input for
the displacement voltage (e-n voltage of open delta winding) 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
sequence 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 displacement voltage input and
the phase voltage inputs (address 211 Uph / Udelta).
This fault is signaled as Fail Σ U Ph-E (no. 165).
i
i
NOTE
Voltage sum monitoring is only effective if an external displacement voltage is connected to the displace-
ment voltage measuring input.
Functions
2.7 Monitoring Functions
132 SIPROTEC 4, 7VK61, Manual
C53000-G1176-C159-5, Edition 05.2018