Tettex MIDAS 2881, Руководство по эксплуатации

Страница: 128 / 164

118 Applications Guide
14.1.3 Measuring Data Interpretation
Condenser Bushings
The dissipation factor and capacitance recorded are compared with one or more of the following:

Nameplate data.

Results of prior tests on the same bushing.

Results of similar tests on similar bushings.
Dissipation factors for modern condenser bushings are generally in the order of 0.5% after correction
to 20°C. They should be within twice the nameplate value. Increased dissipation factors indicate
contamination or deterioration of insulation.
Capacitances should be /- 5 .. +/- 10% of nameplate value, depending upon the total number
of condenser layers. Increased capacitance indicates the possibility of short-circuited condenser
layers. Decreased capacitance indicates the possibility of a floating ground sleeve, or open or poor
test tap connection.
Negative dissipation factors accompanied by small reductions in capacitance or charging current are
experienced occasionally, and may result from unusual conditions of external surface leakage or
internal leakages resulting from carbon tracks.
On bushings equipped with taps, the measurement on C1 is supplemented by a Tap-Insulation test on
C2. Test potential may have to be reduced from 2.5 kV depending upon the tap rating. The dissipation
factor of tap insulation is normally not corrected for temperature. Dissipation factors recorded for tap
insulation are generally on the order of 1%. Results should be compared with those of earlier tests or
with results of tests on similar bushings.
Capacitances recorded for tests on potential taps should also be checked against nameplate values, if
available. Decreased capacitance indicates the possibility of a floating ground sleeve, or poor test tap
connection.
Dry-Type Porcelain Bushings
Bushings of this design may be used in circuit breakers or transformers, or as roof or wall bushings.
They are not equipped with special test electrodes or facilities, so that the only test applicable is the
Overall method, conductor to mounting flange.
The test results are analysed and graded on the basis of comparison of results among similar .
bushings and with results recorded for previous tests. Abnormally high losses and dissipation factor
result from:

Cracked porcelain

Porous porcelain which has absorbed moisture (not common in modern porcelain)

Losses in the secondary insulations, such as varnished cambric

Corona around the centre conductor.

Conducting paths over the insulation surfaces to ground.

Improper use or bonding of resistance coatings or glazing on internal porcelain surfaces.
Cable-Type Bushings
Overall dissipation factor and Hot-Collar losses are relatively high because of inherently high losses in
the cambric insulation. Test results should be compared among similar bushings and with those
recorded for previous tests. Abnormally high losses can result from moisture entering the top of the
bushing and contaminating cambric and compound, migration of oil into the compound through a
bottom seal, cracked porcelain, etc.
Hot-Collar Test