126. 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 transents 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 bushng insultation strength due to associated partal discharge (PD) phenomena. In the present study, EMTP (Electromagnetic Transient Program) and PSPICE (Simulation Program with Integrated Circuit Emphasis) models have been used to calculate the voltage distribution across the layers of 420 kV capacitively graded bushing for an mpulse 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 insultation 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 grade bushing is one of the most important components as it connects Gas Insultated Switchgear (GIS) to an overhead transmission line. Very Fast Transient Over-voltages (VFTO) are generated during switching operations of disconnector switch (DS) or circuit breaker (CB) in such substations. VFTO generated in GIS must be considered as an important factor in the design of primary and secondary equipment due to the following: Flash over to ground at the switching contacts and secondary breakdowns in GIS; Dielectric strength is reduced under VFTO, if non-uniform electric field is formed by the particles (mainly metallic); Effect on components such as bushing and transformer; Transient Enclosure Voltage (TEV) on external surface of the sheath. This may cause flash over to near by grounded objects; Failure of Electronic control circuits connected to GIS due to electromagnetic interference (EMI) generated by VFTO. The VFTO generated internal to GIS, propagates to the overhead transmission line through bushing. These fast transient overvoltages are associated with frequency components in the range of hundreds of MHz. The dimensions are of the orer of few meters for Extra High Voltage (EHV) and Ultra High Voltage (UHV) class capacitively graded bushing. For the high frequencies of VFTO, the voltage distribution across the layers of graded bushing may not be uniform. Moreover, the non-uniformity of distribution depends on termnal equipment of bushing. Further, continuous application of these oscillating, aperiodic voltage transients to the capacitively graded bushing, may cause discharge inside the bushing. This phenomenon may progressively degraded insulation strength of the bushing. In the present study, an equivalent electrical network is developed for a 420 kV SF6 gas-to-air capacitively graded bushing to simulate transient voltage distribution across its layers. For this purpose, an impulse waveform with a rise time of 1 ns has been considered as the source. The effect of different equivalent layers modeling circuits on non-uniform voltage factor has also been analysed as part of the study. Finally, the optimal bushing model for the evaluation of transient behaviour is discussed.