15.19 Electricity Distribution

The electricity distribution system comprises towers and lines to move the electricity from generator to user, with sub-stations containing transformers to change voltages and switches to interconnect lines. Where it is necessary to connect a metal element at a voltage to another at zero volts, such as connecting a line to a tower, an insulator is required to separate the two, capable of withstanding the difference in voltage. Ceramics are used extensively for these purposes due to the stability of their electrical, thermal and chemical properties under harsh weather conditions.

Large high-tension porcelain insulators can be made from conventional siliceous clay, and used to be made by coiling. Now they are moulded and finished on a lathe. Firing of larger structures can be for up to 4 days at 1200 ºC. Porosity has to be kept below about 4% to maintain a high dielectric strength. High voltage bushings and transmission line insulators are designed as a stack of “sheds” or re-entrant structures to increase the length of the path any surface breakdown would have to transverse, and maintains dry sections in wet weather.

Ceramic insulating bushings: various examples,
a 1200 volt test bushing and bushings on
a large transformer - courtesy Trench.
Earlier versions, image courtesy of the Potteries
Museum and Art Gallery, Stoke-on-Trent

High voltage switches (circuit breakers), that can be up to 400 kV in the UK or 800 kV elsewhere, have to break (disconnect) immense current flows when a fault develops on a line or transformer, which results in severe arcing. An important property of the ceramic insulation is the ability to withstand this arcing, so it does not degrade leading to dielectric breakdown. Other characteristics needed to protect against electrical arcing are to have a high softening temperature, high resistance to thermal shock and high dielectric strength. Heat dissipation is also important because the heat produced during arcing can cause the insulator to fracture. The high compressive strength and rigidity of porcelain reduces deformation under service conditions even at elevated temperatures, so it is well suited to this application.

Medium voltage circuit breaker “vacuum bottle”
and cross section showing internal bellows
allowing contacts to be separated
- courtesy ERL Switchcraft

Pollution can create conductive films, and critical bushings in a high voltage sub-station may have automatic washing facilities to remove pollutants. Semiconductor glazes are used to reduce corona discharge at the surface in high electric fields such as near the conductor/insulator interface of bushings. They modify the surface conductivity, which smoothes any voltage gradients. Various other special coatings may also be applied to reduce flashovers and reduce radio interference from discharges. Glass ceramics are used now at medium voltages (10-30 kV) as it is easier to make the ceramic/metal interface.

15.20 Miscellaneous

Collection of industrial ceramics

To illustrate the diverse applications of ceramics in industry in addition to those above, it is informative to look at some of the wide range of components produced in the past by Royal Worcester Industrial Ceramics in addition to those for the textile and wire industries. They produced grinding balls, pump plungers and impellors, fishing tackle rings, abrasion and corrosion resistant sensors especially for the steel industry (e.g. feed hopper levels), corrosion resistant metering floats for the chemical industry, mill linings, sanding nozzles, thermocouple tubes, heating element supports and spacers, potentiometer and coil formers. Other applications are because of ceramic’s resistance to high levels of radio frequency radiation such as in welding and food conveyor supports in RF ovens. For the aircraft industry they produced high temperature fire seals, insulation spacers and instrument bases.