Major issues of neutral beam (NB) injector power supplies are high-speed switching, regulation and transmission of dc ultra-high voltage (UHV) and suppression of surge energy input to the beam source at high voltage breakdown. A gate turn-off thyristor inverter type power supply where the control is performed on the low voltage ac side was designed for the ITER NB. Based on the remarkable progress of a high power IEGT (injection enhanced gate transistor), the design of the inverter has been modified to increase efficiency and compactness using such new elements. The power loss in the inverter is reduced to 0.5 MW from 1.6 MW by adopting the IEGT inverter system at the same frequency of 150 Hz. Further, power supply performance has been studied for higher frequency operation of the inverter at 300 Hz. It was confirmed that the voltage regulation was stable even when the filter capacitor was reduced to 1/5 of the original value by circuit simulation. For the transmission line of the dc UHV with intermediate voltages, a disc shape multi-conductor bushing with a transmission line test chamber has been developed. The dimensions of the bushing are 1.8 m (diameter) and 140 mm (thickness) at the edge. Electric fields at the conductor surface and insulator surface were designed to be lower than 5 kV mm−1 and 7 kV mm−1, respectively. An electric field at the bottom of the ground potential outer conductor was designed to be lower than 1.2 kV mm−1 to prevent particle levitation which triggers breakdown. The prototype transmission line has passed the lightning impulse test up to 1300 kV. A dc UHV up to 1175 kV was successfully sustained for 300 s. To prevent electric damage of the beam source at high voltage breakdown, core snubbers using Fe-based nanocrystalline soft magnetic materials have been adopted to dissipate the surge energy.