Designing & Testing Solid-Core Bushings for Long Service Life

Bushings, HV/HP Testing

Bushings are devices enabling one or several conductors to pass safely through an earthed partition such as a wall or tank, in most cases that of a HV power transformer. Although not the costliest component of a transformer, bushings nonetheless have an important influence on its service reliability.

The following characteristics are required for a bushing to perform efficiently during over its service life:

• Good thermal dissipation behavior to avoid overheating and preventing ageing phenomena during operation and overload. Moreover, the conductor must also be designed to meet the thermal short time current that can be superimposed to continuous operation at rated current;

• Reliable internal and external electrical insulation under rated voltage and service overvoltage, under all environment conditions and especially under pollution;

• Sufficient mechanical strength, taking into account conductor and external connection toward short circuit forces, seismic events, wind and ice.

The main bushing technologies available in regard to alternative internal insulation are oil-impregnated paper (OIP), resin-impregnated paper (RIP), resin-impregnated synthetic (RIS) and SF6. Each offers advantages but also disadvantages.

Fig. 1: Factory acceptance test of oil impregnated paper bushing (1100 kV, 2500A).

OIP technology, consisting of a condenser core wound using kraft paper and subsequently treated and impregnated with oil, offers good heat exchange due to oil’s thermal properties and also the possibility to have oil circulation to cool the conductor. Moreover, analysis of dissolved gases is a proven diagnostic tool allowing for preventive maintenance. On the other hand, OIP bushings have insulation class A up to 105°C, lower than that of RIP or RIS bushings.

While the OIP solution has the longest service experience and is still the most widely used, its main disadvantage is from a safety and environmental point of view. There is possible leakage and high flammability risk in case of accidents, even if rare. Porcelain housings are used for most such applications and, in case of an internal fault, explosion could occur (again rare), with dangerous material projected at high velocity towards people and equipment. This risk is prevented when composite housings are used in place of porcelain. In fact, the transition to silicone composite housings has been motivated by factors such as reduced risk, better pollution performance and easier handling from a weight point of view.

Fig. 2: Resin impregnated paper bushing, 420 kV.

RIP and RIS technologies have higher mechanical stability. Although their heat exchange behavior is less proficient than for oil-impregnated paper bushings, this is compensated for by the possibility to operate these at higher temperatures because of the different thermal insulation class E up to 120°C. Additional advantages of RIP and RIS type bushings are that they are not flammable materials, meaning less problems from the explosion risk point of view, while allowing use of a ceramic or composite housing. Composite housings are used in most cases because of their hydro-repellent silicone material, improving performance under pollution. A disadvantage of RIP and RIS technologies is the present higher costs: dry technology is in general more expensive than an OIP solution with relative cost increase depending on voltage range.

Fig. 3: Resin impregnated paper bushing 550 kV with porcelain external insulation.
Fig, 4: Resin impregnated paper bushing 530 kV hybrid for HVDC.

Gas is a traditional technology using sulfur hexafluoride as insulation medium inside the bushing. SF6 is widely used in bushings in GIS and GIL installations, in dead-tank circuit breakers and in DC applications for transformer and trough wall-bushings. In regard to converter transformers bushings, SF6 in combination with RIP provides a reliable hybrid solution up to 530 kV to reduce space in a valve hall. SF6 can also be used up to the highest voltage wall bushing applications.

Fig. 5: Wall bushing, 800 kV 6000A.
Fig. 6: Wall bushings, 250 kV & 420 kV, 2500A.

Attend the 2022 INMR WORLD CONGRESS in Berlin where bushing experts, Armando Pastore and Dr. Laura de Fina with GE Grid Solutions in Italy will review the latest developments in alternative bushing technologies. They will also explain how solid core bushings can now be designed to offer excellent pollution performance as well as long service life.