Major areas of concern today with polymer insulators are their service life in contaminated environments and water droplet corona mitigation. In the mid 1990’s MacLean Power Systems started a design initiative to improve all suspension insulator’s performance in these areas. The MPS patented stacked shed design emerged and is now used on all suspension Apex Insulators.
Silicone insulators are successful in solving insulator pollution problems, but with traditional designs replacement of the units may be required after a relatively short period of time. Ceramic insulators are generally incapable of long term performance without extensive maintenance and replacement in these harsh environments. Rubber erosion of generic silicone insulators can occur on the high voltage or line end of the insulator. With rubber erosion hydrophobicity is compromised, high leakage currents and dry band arcing may develop. And in the worst cases electrical tracking can occur resulting in a flashover or mechanical failure of the rod. Specially designed stacked sheds placed adjacent to one another are designed to combat these effects The shed profile is changed significantly in the high stress region, resulting in much greater surface area and lower leakage current density compared to conventional designs.
Tracking and erosion tests prescribed by IEC and ANSI standards are good indicators of insulator performance in most environments, but experience is the only way to evaluate performance in severely contaminated environments. For this reason the MPS stacked shed design has been put through long-term field trials in several locations around the world including the natural aging tests at the Koeberg Insulator Test Station in Cape Town, South Africa. In 1997 a two year test began comparing the unique stacked shed insulator to a conventional insulator design at Koeberg. The measured leakage current on the standard design was ten times that of the stacked shed insulator at the end of the extensive test. The rubber erosion difference on the two is clearly visible at the high voltage end and even on the exposed sheath further up the insulator.
Based on the results the South African utility, ESKOM, ranked the standard design as unacceptable for service in the Koeberg environment, whereas the stacked shed design was accepted. Other in-service evaluations have been performed in Taiwan and Mexico, as well as other places heeding similar results.
Aside from aging of the rubber, water droplet corona can have great adverse affects on insulator performance. EPRI has shown that water droplets on a hydrophobic surface such as silicone can elongate under electrical field influence. This results in an enhancement of the electric field at the tip of that droplet. As that field grows in strength, discharges can occur and is considered an aging mechanism on non-ceramic insulators. Water droplet corona is a concern primarily on the sheath of the insulator where the tip of that elongated droplet is in direct contact with the sheath. Droplet elongation on the shed results in a tip direction away from the insulator and is not damaging. The stacked shed insulator design eliminates the exposed sheath in this high stress area. Whereas a conventional shed design has a great deal of exposed sheath in the high stress area.
The patented stacked shed design can only be found on Apex Insulators and is offered in varying configurations such as twelve line stacked sheds and three tower stacked sheds for a 765 kV application. All MPS suspension insulators come with a three stacked shed standard to give the product a long life capability in any environment with improved corona protection.