| dc.description.abstract | In this study, the electrical insulation characteristics of
woods locally available in Kenya were investigated, with the aim of
finding out whether- any of the woods could be used for high
voltage low frequency insulation purposes.
The thirteen woods studied were Bluegum. Meru oak, Mahogany,
Elgon Teak. Muhugu, Pine, Cypress. Cedar, Mvuli. Rosewood, Mukui,
Podo and Camphor. In order' to choose the most appropriate wood,
dielectric strength and loss tangent measurements were first
carried out for dry wood and wood impregnated with transformer oil
( for 30 minutes, 1 hour, 3 hours and 5 hours ). The first four woods, in terms of high dielectric strength and low dielectric Ioss
tangent, were picked for further study using results from this
initial study. In the further study the effect of a varnish
coating on the dielectric strenqth and loss tangents
investigated. The treated wood that maintained a high dieIectric
strength was considered to be the most appropriate for application
as an insulator. The shape of the wood for application at 11kv was
optimized by calculating field values numerically, making plots of
surface electric fields and by practical measurement of the
flashover voltages and leakage current of the insulator shapes.
The effects of impregnating the wood with transfformer oill and
varnishing dry wood, on the flashover voltage and leakage current of
the untreated dry wood insulator, were also investigated. Lastly,
the effectiveness of the varnish coating in keeping moisture away
from the insulator', under constant hIgh humidity (96%) and temperature ( 40 C ), was investigated.
After a consideratIon of the dielectric strengths and loss
tangents of tile dry and impregnated wooos, the first four woods
starting with the best, in terms of the hiqhest dieIectric strenqth
attained, were Muhugu, Elgon Teak, Podo and Cypress. After
varnishing, Muhugu had the highest dielectric strength ( 235kv/cm
across its grains ). Muhugu is more expensive and difficult to
machine. It chips off more easily and is less abundant than Elgon Teak which had the second hiqhes dielectric strenghth 200kv/cm across its grain ). So, Elgon Teak was considered to be more advantageous to use for insulating purposes. When a vanishing coat was applied to the dry wood insulator, the leakage current decreased from 54uA to 51,uA ( dimensions of the insulator as in
Table 7.6, shape ( b ) ). while the fIashover volta'ge decreased
from 51kV to 49kv. Impregnating the insulator with trans tormer oil lowered the flashover voltage from 51kV to 49kV and increased
leakage current from 54,uA to 60,uA. The oiI varnish coating of the
insulator cracked after 2 months of exposure to high humidity and
temperature, and moisture got into the wood.
In conclusion, it was observed that: All dry woods studied
have a higher dielectric strength across the grains than along grains. Effective impregnation of wood with transformer oil is only possibIe with thin specimens ( a cross-section cut acrose the thick specimens showed that oil seeped to only a small depth from the surface of the insulator) and has the effect of increasing the dieIectric strength of wood and lowering its dieIectric loss tangent. CorreIat ion of surface electric field vaIues and flashover voltages of different insulators profiles, shows that the
higher the electric field along the surface of an insulator ( for a given set of eIectrodes and applied voltage ) the lower the flashover voltage of the insulator. With circular eIectrodes, the concave insulator was found to have the best shape in terms of
highest flashover voltage. least volume leakage current and economy
of insulator material. Oil varnishes are not effective in
keeping out moisture rom wood exposed to high temperatures and
humidities for long periods. Elgon leak is the most appropriate
wood for high voltage low frequency insulation purposes.
Future work in this line should:
( a ) Employ artificial seasoning which allows even thick woods be dried without it developing any cracks.
( b ) Use much higher voltages in the measurement of dielectric loss tangent.
( c ) Investigate the mechanical properties of woood and the effect of mechanical loads on the electrical insulation properties of wood
( d ) Investigate the effects of electric and cyclic thermal loading on the Longterm dielectnc strenqth of wood
( e ) Investigate prediction of flashover' voltages from surface field calculations.
( f ) Study the effectiveness of silcone varnishes an keeping out
moisture from wood
( g ) Investigate prediction of the flashover voltages of a given
insulator shape from the values of surface electric fieIds calculated. | en |
