Two methods are used to make power
(dissipation) factor and capacitance measurements. The first is the
grounded specimen test (GST), where current, watts, and capacitance
of all leakage paths between the energized central conductor and all
grounded parts are measured.
Measurements include the internal core
insulation and oil as well as leakage paths over the insulator
surfaces. The use of a guard circuit connection can be used to
minimize the effects of the latter.
The second method is the ungrounded
specimen test (UST), where the above quantities are measured between
the energized center conductor and a designated ungrounded test
electrode, usually the voltage or test tap.
The two advantages of the UST method
are that the effects of unwanted leakage paths, for instance across
the insulators, are minimized, and separate tests are possible while
bushings are mounted in apparatus.
Standards recommend that power factor
and capacitance measurements be made at the time of installation, a
year after installation, and every three to five years thereafter. A
significant increase in a bushing’s power factor indicates
deterioration of some part of the insulating system.
It may mean that one of the insulators,
most likely the air-end insulator, is dirty or wet, and excessive
leakage currents are flowing along the insulator. A proper reading
can be obtained by cleaning the insulator.
On the other hand, a significant
increase of the power factor may also indicate deterioration within
the bushing. An increase in the power factor across the C1 portion,
i.e., from conductor to tap, typically indicates deterioration within
the core.
An increase across the C2 portion of a
bushing using a core, i.e., from tap to flange, typically indicates
deterioration of that part of the core or the bushing oil. If power
factor doubles from the reading immediately after initial
installation, the rate of change of the increase should be monitored
at more frequent intervals.
If it triples, then the bushing should
be removed from service. An increase of bushing capacitance is also a
very important indicator that something is wrong inside the bushing.
An excessive change, on the order of 2
to 5%, depending on the voltage class of the bushing, over its
initial reading probably indicates that insulation between two or
more grading elements has shorted out. Such a change in capacitance
is indication that the bushing should be removed from service as soon
as possible.
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