Asymmetrical nine-phase voltage step-up/down diode rectifiers with 18-step input currents

A novel concept of asymmetrical nine-phase diode rectifiers with 18-pulse performance is extended to voltage step-up or -down rectifier circuits, in which each diode conduction interval in the main rectifier is 80° and one in the two auxiliary rectifiers is 20°. The asymmetrical nine-phase diode rectifier requires differential delta-connected autotransformers having a 31.7 % VA rating of the DC output power without voltage stepping-up or -down, although a conventional symmetrical one requires a 51.4 % rating. A theoretical analysis for the idealized circuit gives that the proposed rectifier can be applied to the dc output range of 80-125% by the use of less than 50 % rating transformers. Experimental results for distortionless and distorted AC power supplies showed that the proposed rectifier can produce the resulting line-currents with significantly reduced harmonics. It will be well suited to a front-end of inverter-based applications because of its simple, robust structure, and a smaller transformer VA rating and smaller transformer losses.