Tettex MIDAS 2881, Operating Instructions Manual

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132 Applications Guide
Difference between the two higher currents
should be less than 10% Difference between the two higher currents should
be less than 5%
In general, if there is an internal problem, the differences between the phases with the higher current
will be greater. When this happens, other tests should also show abnormalities, and an internal
inspection should be considered. The results, as with all others, should be compared with factory and
prior field tests.
14.3 Rotating Machines
Approximately 70% of breakdowns of big rotating machines are ascribed to problems of the winding
insulation. These insulation problems are mainly caused by voids which allow localized ionization
processes (partial discharge). As a consequence the temperature can increase locally (hot spots) until
a breakdown occurs which results in a short-circuit.
The main purpose of capacitance and dissipation factor tests on rotating machines is to assess the
extent of void formation within the winding insulation. The measurements will also reveal potential
problems due to deterioration, contamination, or moisture penetration.
Test Levels
For evaluating the extent of insulation deterioration caused by ionization a power factor (dissipation
factor) tip-up test can be used. In this test, the dissipation factor is measured at two different voltages,
the first low enough so that no ionization occurs (normally 25 percent of rated line-to-ground voltage),
the second at rated line to ground voltage or slightly above it. The tip-up value is obtained by
subtracting the value of the dissipation factor measured at the lower test voltage from that measured
at the higher test voltage.
When the dissipation factor increases significantly above a certain voltage, it is evident that ionization
is active and producing some loss.
Sometimes it might be helpful to perform the tip-up test at more than only two voltages. For example if
the first measurements show an abnormal tip-up a second test sequence with 20% , 40%, 80%, 100%
and 125% of the rated voltage can be executed. This can give the a deeper insight into the aging
mechanism of the tested insulation.
14.3.1 Test Procedure
An overall measurement on a rotor or stator winding will proof the insulation condition between the
winding and ground. If the connection between the winding phases and neutral can be conveniently
opened the inter-winding or phase-to-phase insulation can also be measured.
When a tip-up test is made on a complete phase winding, the average dissipation value is measured.
Therefore an isolated section having an abnormally high tip-up may be completely masked.
The temperature of the windings should be above and never below the ambient temperature to avoid
the effects of moisture condensation on the exposed insulating surface. Temperature measurements
when using temperature correction (if data available) should be based on that at the winding surface.
Prolonged exposure to high humidity conditions before testing should be avoided because such
exposure may result in moisture absorption in the insulating materials. It is desirable to make tests on
the winding insulation shortly after shutdown.
The figure below shows the specific connections between the test set and a typical generator. It is
assumed that the connection between the winding phases and neutral can be opened.