High-voltage withstand tests are essential for electric equipment like power transformers, electric cable lines, gas insulated switchgears etc. The partial discharge (PD) measurement for such equipment under applied high voltage is also required at the same time. Pulse Width Modulation (PWM)-based variable frequency power supply (VFPS) finds wide applications in the series resonance high-voltage testing circuits. However, the steep rising and falling voltage edges in the PWM process will produce large amount of partial discharge interference in a wide spectrum range, which will confuse the real partial discharge signal of the device and lead to the wrong estimation of the insulation status. This paper proposes a new VFPS based on the highefficiency linear power amplifier without PD interference. The topology and principle of the VFPS are given and discussed. A prototype rated at 232 V/35 A was designed and constructed to verify the feasibility of the proposed VFPS. Physical experiments were carried out on the prototype.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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In this paper, numerical techniques for simulation of magnetic domain structures of ferromagnetic materials using micromagnetics with large cells are investigated to save the computation cost. The exchange energy is modified to be applicable to large cells considering the angle between two neighboring cells. The demagnetizing energy is also modified to take account of erroneous magnetic surface charges which appear around the domain walls as a result of discretization. The method is then applied to a simple model and the result shows that an ideal vortex magnetic domain structure and domain wall motion can be represented.