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 conce con centr ntrati ation on app appear ears s at the soso-ca calle lled d ‘tr ‘tripl iple e jun juncti ction’ on’ between between the ea earth rthed ed wa wall, ll, the insula ins ulatin ting g cy cylin linder der and the gas gaseou eous s or li liuid uid med medium ium ou outsi tside de the bus bushin hing g bod body y. Th This is locali!ed 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 trac"ing along the bushing and even result in flash over. #nitiation of gliding discharges as well as their subseuent development becomes easier when the unit capacitance of the insulation $i.e. across its thic"ness% is greater. Therefore, the voltage level for their ignition and propagation $virtually eual to flash over voltage% is determined by this parameter. This stands in contrast to other types of discharges, where the typical controlling parameter is electrode separation distance. &ecause of such considerations, the best way to increase a bushing’s flash over 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 mea ns is thr throug ough h cap capaci acitiv tive e con contro troll for '( app applic licati ations ons an and d res resist istive ive con contro troll for )( applications. (apacitive control is based on inserting metallic screens into the solid insulation insulation of the bushing, essentially essentially forming a sy syst stem em of in ser erie ies s con onn nec ecte ted d cap apac acit ito ors who hos se magnit mag nitude ude dep depend ends s on the their ir geo geomet metric rical al arr arrang angeme ement. nt. *erhaps the most freuently used and effective solution is when series capacitances are maintained at eual levels. The impact impact of mod modify ifying ing field dis distri tribut bution ion in thi this s way is illustrated in +igure . #nserting metallic screens during manufacture of a bushing can be demanding and at times labor-intensive, although modern condenser core winding euipment has made this tas" increasingly automated. #n the case of paper insulated bushings, metallic foils are inserted between the different paper layers. (hoosing the appropriate radius and length of these screens then allows for the series capacitance desired. ptimal resistive control of electric field distribution in the case of )( bushings usually involves covering the critical region near the electrode with semiconducting layers. The aim here is to increase resistance with increasing distance from the earthed electrode.
