Bushings are a very important part for a great deal of HV equipment. Without bushings, actual large power transformers are unthinkable. Throughout history they have shared their development with transformers and other representatives of HV technology.

Initially, bushings were a kind of solid type bushings often made from porcelain with an inserted conductor. Similar bushings are still frequently used in distribution networks for system voltages up to 52 kV due to their simplicity, reliability and low price. However, for only a slightly higher voltage they become impractical because of a very steep increase in size. Instead, for higher voltages a condenser bushing type is used today. Inside condenser type bushings, there are conductive electrodes that are used for radial and axial electrical field grading. One of the oldest descriptions of the condenser type bushing dates back to 1906. What was then described as an “innovation” was in fact a 200 kV condenser type bushing used as part of an HV test transformer. Soon after, in 1908, the production of capacitive graded bushings started: first the coarse graded and afterwards fine graded bushings in RBP technology. This solution was limited because of a relatively high level of partial discharge (PD) generated in small air pockets, which was inevitable for this technology.

OIP bushings were introduced around 1944 to fulfil the needs for higher voltages and lower PD level. Then around 1950, the first RIP bushing was produced. Today, OIP and RIP bushings are produced up to the highest AC and DC voltages – approximately up to 1200 kV and 35 kA.

Some time around 1990, a silicone rubber on a glass fibre epoxy tube was introduced as a bushing upper envelope with an aim to eliminate some disadvantages of a porcelain upper envelope that had been in use until then.

Recent bushing developments include applying silicone sheds directly on the RIP body (around 2005) and replacement of paper with inorganic material (RIS bushings, around 2012).

Looking into the future, it should be stated that actual power transformers with solid and liquid high temperature materials have allowable top oil temperature greater than the highest temperature allowed for all HV bushing types listed in standards, so HV bushings for higher temperatures should be developed.

Bushings are among the most frequent transformer failure cause. According to data from various research studies and electric power utilities, they cause from 5 to 50 % of the total number of transformer failures. Bushing failures are the most common cause of transformer fires that can lead to huge collateral and ecological damages at the switchyard. Bushings are a transformer’s crucial part and one transformer can have more than 10 bushings. A failure of any of them has a transformer failure as a consequence. A bushing explosion can damage the transformer in many ways. Upper porcelain envelope burst launches fragments of it at an enormous speed with destructive power. The burst of the bushing’s lower part damages a transformer in such a way that the conductive and burned debris of the condenser body pollutes its active part. Cleaning transformer’s active part from bushing fragments is a difficult job with doubtful results.

Bushings are normally mounted on the hottest part of the transformer and they are exposed to both the highest and the lowest temperatures as well as the atmospheric conditions. Animal attacks (rodents, birds, monkeys, etc.) to the silicone shed can seriously affect bushing properties. This, combined with mechanical forces, results in huge demands on the bushing insulation and sealing system. It should also be mentioned that the electrical field strength in the bushings HV condenser body is among the highest in HV technology. HV bushings are thin and long and therefore a fragile structure, sensitive to mechanical forces due to switchyard connection, short circuits, earthquakes or vandalism.

In terms of preventing bushings as well as transformer failure, bushings are subjected to periodic (off-line) and continuous (on-line) condition diagnostics. A lot of methods, traditional or recently developed, are used with various effectiveness. Nowadays, with expansion of bushing monitoring systems (continuous diagnostics), new possibilities emerge to improve transformer service reliability and availability. However, new challenges appear as well, as can be seen from the papers in this edition.

Antun Mikulecky, PhD, Guest Editor