Megger VLF SIN-45 - Page 40

Operating Modes
40
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Description
Frequency of the VLF test voltage (0.01 Hz to 0.1 Hz).
A setting of 0.1 Hz is definitely recommended, since all the experience
documented in the relevant technical literature or in the corresponding
standards refer to this frequency as the diagnostic frequency.
By measuring at different frequencies, a tanδ spectrum can furthermore
be shown for the test object. This spectrum can provide further information
on the condition of the test object.
If the capacitance of the connected test object does not permit
(see page 13) a measurement using 0.1 Hz and an automatic
frequency adjustment is performed at the start of the
measurement, the evaluation criteria that are independent of
frequency should be looked more closely. These include,
amongst others, the deviation of the absolute tanδ values
between the phases of a cable system and the change of the tanδ
with increasing voltage (Δtanδ).
Number of the voltage levels (1 to 6) that the test voltage runs through in
the course of a test.
The first voltage level is 0.5Uo. The voltage is increased by 0.5Uo with
every additional voltage level. The sixth voltage level would accordingly be
3Uo.
One requirement for the automatic evaluation (see page 43) of the test
results is that the measurements were taken in respect of at least 3
voltage levels.
Once the value is confirmed, a calculation of the respective voltage values
of the individual levels is carried out while taking the nominal voltage into
account and displayed on the screen.
To avoid possible breakdowns, it is recommended that already
heavily aged cables are not be measured at voltage levels greater
than 2Uo (≤1.5Uo is even safer).
Number of tanδ measured values (5 to 20) per voltage level.
At least 8 measured values per voltage level should be recorded in order
to obtain a calculated tanδ mean value that is statistically meaningful. The
higher the number of values, the more reliable the calculated mean value.
However, the stress placed on the test object also increases accordingly.
As the goal is a non-destructive diagnosis, the number of measured
values, especially in the case of high test voltages, should be kept limited
(recommended are 8 to 10 values).
Insulation type of the cable under test.
The chosen insulation type determines the criteria used for the automatic
evaluation of the measurement results. In case of a mixed cable system
(Mixed option), evaluation is performed according to the criteria defined
for PILC cables.
Standard according to which the recorded measurement results are to be
automatically evaluated.