2.28 Measurement Supervision
167
7UM61 Manual
C53000-G1176-C127-3
Measurement Value
Acquisition – Cur-
rents
In the current paths there are three input transformers; the digitized sum of the trans-
former currents of one side must be almost zero for generators with isolated starpoint
during earth-fault-free operation. A current circuit fault is detected if
I
F
= | I
L1
+ I
L2
+ I
L3
| > ΣI THRESHOLD · I
N
+ ΣI FACTOR · I
max
ΣI THRESHOLD and ΣI FACTOR are programmable settings. The component ΣI
FACTOR · Imax takes into account the admissible current-proportional transformation
error of the input transformer, which can occur particularly when high fault current
levels are present (see the following figure). The dropout ratio is about 95 %.
This malfunction is signaled as “Failure Σ I”.
The current sum monitoring is not executed if the starpoint was set as 273 at Power
System Data 1 (address low-resist.).
Figure 2-62 Current sum monitoring
Measured Value Ac-
quisition Voltages
Four measuring inputs are available in the voltage path: If three of them are used for
phase-earth voltages, and one input for the displacement voltage (e–n voltage from
the broken delta winding or neutral transformer) of the same system, a fault in the
phase-earth voltage sum is detected if
| U
L1
+ U
L2
+ U
L3
+ k
U
· U
E
| > SUM.thres. U + SUM.Fact. U x U
max
Here SUM.thres. U and SUM.Fact. U are parameter settings, and U
max
is the
highest of the phase-earth voltages. Factor k
U
considers the transformation ratio dif-
ferences between the displacement voltage input and the phase voltage inputs (pa-
rameter k
U
= Uph / Udelta Address 225). The SUM.Fact. U x U
max
component
considers admissible voltage-proportional transformation errors of the input transduc-
ers, which can be especially large in the presence of high voltages (see the following
figure).
This malfunction is reported as “Fail Σ U Ph-E”.
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