Bushings are devices that allow high voltage conductors to pass through the earthed walls of transformers, switchgear and substation structures. An integral part of this function involves meeting all the electrical, thermal and mechanical requirements of the application.
For example, bushings must provide reliable electrical insulation both internally (against breakdown) and externally (against flashover) of the conductor exposed to the rated voltage and also to periodic service overvoltages – even under contaminated conditions.
Another key requirement is providing the mechanical strength needed to support the conductor as well as all external connections, including under short circuit and possible seismic forces. Moreover, the bushing must have the proper thermal design to avoid any overheating of its elements and to prevent the onset of ageing phenomena inside the insulation, both at rated current and during short circuit events.
The following technology review, taken from past INMR issues and also incorporating a recent contribution by Professor Stanislaw Gubanski of Chalmers University of Technology in Gothenburg, Sweden, discusses alternative bushing designs. It also looks to future trends as determined by changing market needs and competitive factors.
The bushing is in many ways similar to an insulator or surge arrester in that it is a comparatively low cost component ensuring the safe operation of a comparatively high value asset. For example, while bushings account for less than 5 percent of the cost of a typical power transformer, their catastrophic failure can lead to total loss of the transformer and possibly other expensive apparatus as well.
However, in contrast to insulators that contain no potentially explosive internal medium, certain types of bushings can pose a threat not only to the substation and its personnel but also to the safety of nearby communities. For example, the catastrophic failure of a 110 kV oil-impregnated paper bushing at a substation in New Zealand during the 1980s was so violent that shards of porcelain as well as oil and debris, were launched into homes in an adjoining residential area.
Fortunately, occurrences such as this – while dramatic – are also very rare. In fact, the bushing is a ‘workhorse’ that typically ranks among the most reliable components in any modern power network.
The basic principle in all bushing design is relatively straightforward: it consists of a cylindrical conductor surrounded by an insulating solid cylinder that is mechanically fixed to the earthed barrier. The distribution of electric field inside such a construction, however, is highly non-uniform in terms of both axial and radial components. The highest stress concentration appears at the so-called ‘triple junction’ between the earthed wall, the insulating cylinder and the gaseous or liquid medium outside the bushing body. This localized high concentration of stress can trigger the onset of partial discharges.
These discharges are often referred to as ‘gliding discharges’ since they have a strong capacitive coupling to the bushing’s internal conductor and therefore proceed along the insulating cylinder’s surface. They can lead to tracking along the bushing and even result in flashover.
Initiation of gliding discharges as well as their subsequent development becomes easier when the unit capacitance of the insulation (i.e. across its thickness) is greater. Therefore, the voltage level for their ignition and propagation (virtually equal to flashover voltage) is determined by this parameter. This stands in contrast to other types of discharges, where the typical controlling parameter is electrode separation distance.
Because of such considerations, the best way to increase a bushing’s flashover withstand voltage is by improving the electric field distribution along its surface. This can be achieved in a number of ways although, in the case of higher voltage levels, the most effective means is through capacitive control for AC applications and resistive control for DC applications.
Capacitive control is based on inserting metallic screens into the solid insulation of the bushing, essentially forming a system of in-series connected capacitors whose magnitude depends on their geometrical arrangement. Perhaps the most frequently used and effective solution is when series capacitances are maintained at equal levels. The impact of modifying field distribution in this way is illustrated in Figure 1.
Inserting metallic screens during manufacture of a bushing can be demanding and at times labor-intensive, although modern condenser core winding equipment has made this task increasingly automated. In the case of paper insulated bushings, metallic foils are inserted between the different paper layers. Choosing the appropriate radius and length of these screens then allows for the series capacitance desired.
Optimal resistive control of electric field distribution in the case of DC bushings usually involves covering the critical region near the electrode with semi-conducting layers. The aim here is to increase resistance with increasing distance from the earthed electrode.