12.3 Analog Input Circuits
The analog inputs are fed through an analog preprocessing feature (anti-aliasing
filtering) to a common analog-to-digital converter. In conjunction with the self-
monitoring function, the CT/VT supervision function that is available for the
device’s general functions can detect deviations in many cases. However, it is
still necessary to test from the device terminals in order to make sure that the
analog measuring circuits are functioning correctly.
The best way to carry out a static test of the analog input circuits is to check the
primary measured operating data using the operating data measurement
function or to use a suitable testing instrument. A “small” measured value (such
as the nominal current in the current path) and a “large” measured value (such
as the nominal voltage in the voltage path) should be used to check the
measuring range of the A/D converter. This makes it possible to check the entire
dynamic range.
The accuracy of operating data measurement is <1%. An important factor in
evaluating device performance is long-term performance based on comparison
with previous measurements.
In addition, a dynamic test can be used to check transmission performance and
the phase relation of the current transformers and the anti-aliasing filter. This
can best be done by measuring the trigger point of the first zone when there is a
two-phase ungrounded fault. For this test, the value of the short-circuit current
should be such that a loop voltage of approximately 2 V is obtained at the
device's terminals with the set impedance. Furthermore, a suitable testing
instrument that correctly replicates the two-phase ungrounded fault should be
used for this purpose.
This dynamic test is not absolutely necessary, since it only checks the stability of
a few less passive components. Based on reliability analysis, the statistical
expectation is that only one component in 10 years in 1000 devices will be
outside the tolerance range.
Additional analog testing of such factors as the impedance characteristic or the
starting characteristic is not necessary, in our opinion, since information
processing is completely digital and is based on the measured analog current
and voltage values. Proper operation was checked in conjunction with type
testing.
12 Maintenance P634
P634/EN M/R-42-A // P634‑311‑653 12-5
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