Laminated Bus Insulation Design and Assessment Via Partial Discharge Test for Medium-voltage Converter Application
The laminated bus has been widely used in power electronics for decades. By arbitrarily sweep-ing the bus insulation thickness, its parasitic inductance increases significantly with the insula-tion. The buss weight, size, and cooling performance suffer from the increase of insulation; therefore, the thinner the insulation is, the better overall performance the bus can have. The insu-lation thickness should have the lowest boundary, which can guarantee to block certain voltage across it with enough lifetime. Since the insulation design and an assessment of the laminated bus have not been fully studied, currently, a large security margin has to be applied. This results in very thick insulation, especially for the medium and high voltage converters. Then again, ex-cessive Partial Discharge (PD) inside the defects of the adhesive layers or along surfaces can still degrade the insulation; thus, an overdesigned insulation may not help a great deal. Focusing on insulation design and assessment, two major representative structures in the bus are analyzed in this paper, as shown in Fig. 1. Based on the electric field simulation results along various ideal insulation systems, some basic guidelines can be concluded. There is one domi-nant difference between the ideal insulation systems and the real ones; defects in the adhesive layer are the main reason for accelerated insulation degradation. Therefore, these defects are fully estimated via PD tests among coupons with different material and fabrication processes. Part of these PD test results are shown in Fig. 2. inally, comprehensive guidelines on both the design and experimental evaluation of the laminat-ed bus insulation system will be provided in this paper, with a combined consideration on the electric field management, material selection, and fabrication process.