Simulation Based Design of A 275kV High Voltage Bushing

Abstract

Modern bushing designs should aim at reducing the electrical stress it is subjected to, by so doing, decreasing the likelihood of partial discharge (PD) occurring. PD is the localised dielectric breakdown of small portion of an electrical insulation system under HV stress, which does not bridge the space between two conductors. In this paper, a 275kV bushing was designed using the double-sided capacitive field grading methods (radial grading and axial grading). The calculated bushing design was then modelled in COMSOL Multiphysics 4.3 (CMPH) and simulated using the electrostatic equations. The simulation results were then compared to the theoretical calculated design results. Both radial and axial grading methods allowed calculating the length and spacing between foils. These methods consider that the capacitance and voltage drop between foils is constant. The radial grading method provides a bushing design with a constant radial electric field by changing the spacing between foils. On the other hand, the axial grading methods provides a bushing design with constant axial electric field by keeping constant the difference between foil length on the air and in the oil side.  In the radial grading design, the calculated and simulated radial electric field had a difference of 2.87% while there was a 4% difference in the simulated and calculated voltage drop between foils. In the axial grading design the calculated and simulated values for the voltage drop between foils was within a 32% error while the radial electric field stress was within a 20% error between the calculated and simulated value.The bushing design was improved by increasing the number of foils, which will reduce the voltage and the radial field stress. Also, reducing the length of the grounded bushing edge to be smaller than foil 21 will reduce radial field stress at the corner of the grounded bushing edge. Lastly, the foil design was modified by removing foils in the centre of the bushing and having them just along the top and bottom edges. This design would reduce the material usage per bushing thus decreasing the manufacturing cost.