105. Modeling of 420 kV Capacitively Graded SF6 Gas to Air Bushing for Very Fast Transient Overvoltages

In a gas insulated substation (GIS), Very Fast Transient Overvoltages (VFTO) are generated due to switching operations and ground faults. The switching operations may be due to the operation of disconnectors or circuit breakers. These fast transients are associated with frequency components of the order of hundreds of MHz. Each switching operation produces several high frequency transients and may cause internal faults i.e., layer to layer faults or minor faults in a capacitively graded bushing, which is one of the important terminal equipment for GIS. This internal fault may progressively degrade bushing insulation strength due to associated partial discharge (PD) phenomena. In the present study, EMTP (Electromagnetic Transient Program) and PSPICE (Simulation Program with Integrated Circuit Emphasis) models have beenused to calculate the voltage distribution across the layers of 420 kV capacitively graded bushing for an impulse waveform of 1 ns rise time, which simulates the VFTO. For this simulation, equivalent electrical network of bushing has been considered. Finally, the effect of different equivalent layers of the bushing modeling circuits on transient voltage distribution is analysed. In non-condenser bushings, the electrical stress distribution is not uniform through insulation or along its surface. Concentration of stress in the insulation may give rise to partial discharge (PD) and reduce service life. Further, high axial stress may result in tracking and surface flashover. To overcome the above problem, electrical stresses are generally controlled by means of capacitively graded mechanism. In this design, the insulation thickness is divided into a number of capacitors by using concentric conducting layers. The parameters which control the stresses to safe level are such as radius of high voltage conductor, radius of outer layer, length of first layer, length of last layer and system voltage. SF6 gas-to-air capacitively graded bushing is one of the most important components as it connects Gas Insultated Switchgear (GIS) to an overhead transmission line. Very Fast Transient Over-voltage (VFTO) are generated during switching operations of disconnector switch (DS) or circuit breaker (CB) in such substations.