GETTING IT RIGHT

It has been said that manufacturing of electrical porcelain is as much an art as a science. The raw materials are mined from the earth with very minor processing other than crushing and screening to obtain a desired particle size. It is up to the porcelain manufacturer to filter out impurities and adjust the mix to minimize bloat and other defects. Shrinkage between the shaping stage and the fired stage is another variable that is difficult to control and results in general tolerance of about three percent on as fired dimensions.

Glazing also has variables that are difficult to control, but as long as the glaze is not too thick or too thin for regular glaze the process is fairly straightforward. This is not the case for semiconducting glaze.

The thickness of the semiconducting glaze must be carefully controlled and the conductivity is particularly sensitive to the firing curve. For this reason, manufacturers prefer to accept a large tolerance on the conductivity. If the conductivity is not within the specified limits after the first firing then it can be fired again up to two more times to bring the conductivity into specification. This extra handling and firing is costly and therefore it is tempting to specify a tolerance as high as ±50%.

This temptation must not be given into for two reasons. First, there is the issue of thermal runaway. This places a very practical upper limit on the conductivity. Too high of a conductivity will compromise the safety margin between the operating voltage and the voltage where thermal runaway may occur. We design our SCG insulators to have a thermal runaway voltage that is at least twice the nominal operating voltage when the conductivity of the insulator is at its maximum tolerance.

The second reason concerns only stacking station post insulators. If a conductivity tolerance of ±50% is adopted then the conductivity of individual sections in the stack can vary by as mush as three times. For example, if the nominal conductivity at operating voltage is 1.0 mA and a 50% tolerance is employed, then it becomes possible for one section to have a conductivity as high as 1.5 mA while another section could be as low as 0.5 mA. This is a ratio of three to one.

Obviously, the voltage across the section with the lower conductivity will be 50% greater than it normally would be and the voltage across the section with the higher conductivity will be half of what it should. Now if the section with the lower conductivity is at the energized end of the stack then the voltage distribution across the stack will be less linear than if semiconducting glaze was not used at all. Providing for a linear voltage distribution across the insulator is an important contribution of semiconducting glaze that improves the contamination and RIV performance.

Return to the top