Comparing Performance of Helical & Concentric Shed Profiles on HV Bushings

Bushings

Bushings with polymeric insulators are available in several variants based on different manufacturing methods and composition of the housing material. These insulators can be molded in sections with a concentric shed profile and then glued together. Alternatively, they can be extruded in a helical-shaped continuous process. Similarly, the silicone material used can either be liquid silicone rubber (LSR) or high temperature vulcanizing (HTV) rubber. LSR is typically used for concentric-shaped insulators while HTV silicone rubber can be used in both types.

Fig. 1: Wall bushing and transformer bushing at HVDC substation both use helical shaped housings made with HTV material.

Many of the HVDC bushings used for decades have been of the type enclosed in a composite insulator comprised of a filament wound tube with a custom HTV silicone rubber material extruded over it in a helical profile. The advantage of such a process, according to suppliers who offer such a design, is that it better allows optimizing shed profile as well as size of insulator and also helps promote maximum seal integrity. Based on successful service experience in these numerous HVDC applications, similar concepts have since become one of the alternatives increasingly being selected for AC bushing applications.

Bushings
Fig. 2: Bushings on same transformer use both concentric (background) and helical-shaped insulators (foreground).

Existing relevant standards for these products prescribe only minimum requirements to ensure performance under normal service conditions. But these components must also function safely for their entire service life in more severe environments, such as under continuous high UV or high pollution. Given this, additional testing has been performed by some manufacturers beyond the standard requirements for the types of composite insulators described above. These additional tests have included:

• High altitude field test at 4000 meters;
• UV testing extended to 3000 hours;
• Custom designed 5000-hour acid fog test;
• 1000-hour salt fog test with DC voltage;
• 5000-hour multiple stress testing.

Other investigations in regard to long-term service experience in different environments have also been performed and published. For example, a paper presented at the 2019 INMR WORLD CONGRESS in Tucson compared resistance to biofouling of different types of silicone material.

Objective of Present Study

The objective of a more recent investigation into performance of different types of composite insulator designs has been to address frequently asked questions related to performance of transformer bushings fitted with such insulators under periods of heavy rain and complete loss of hydrophobicity. A common perception is that all HTV-silicones are the same as far as hydrophobicity recovery in such situations. At the same time, few publications have addressed the question of performance of helical-profile composite insulators.

Test Objects

Tests were performed on three commercially available, completely dry RIP type transformer bushings for 300 kV system voltage. Two of the bushings were fitted with helical insulators with different shed profiles but the same HTV silicone material. The third bushing was fitted with a concentric insulator using a different HTV silicone material (see Fig. 3). All other aspects of the bushings were identical.

1. Transformer bushing #1 with helical shed profile 1;
3. Transformer bushing #2 with helical shed profile 2;
4. Transformer bushing # 3 with concentric shed profile 3.

Bushings
Fig. 3: Composite insulators and different shed profile dimensions.
Profile 1 and 2 made from same HTV silicone, whereas Profile 3 made from a different type, though still HTV.

Bushing Ratings

• Rated highest voltage for equipment (Um): 300 kV;
• Power-frequency withstand voltage (1-minute dry): 560 kV;
• Rated lightning impulse withstand voltage (BIL): 1175 kV;
• Rated switching Impulse withstand voltage (SIL): Wet 850 kV;
• Arcing distance: 2300 mm.

Attend the 2022 INMR WORLD CONGRESS to listen to a lecture by Technical Specialist, Roger Hedlund, of Hitachi Energy in Sweden. Mr. Hedlund will present findings of a recent test program comparing performance and hydrophobicity recovery of different types of composite housings on HV bushings exposed to heavy rain.