Analysis of new step-up and step-down 18-pulse direct asymmetric autotransformer-rectifiers

This paper presents four new step-up and step-down 18-pulse direct asymmetric autotransformer rectifiers. The converters are designed to operate with variable frequency power supplies (400-800Hz), while meeting tight harmonic, power factor and output voltage requirements. The analysis takes into consideration the total number of windings used to achieve the step-up and step-down voltage gain-which determines the autotransformer structure and performance-the relative size of the windings as well as the total KVA ratings, and the existence of secondary-side series windings-which impact the commutation process of the diode bridges. A complete modeling procedure of the autotransformers is additionally presented with closed form expressions for all winding currents. Experimental results obtained with laboratory prototypes are used to validate the correct operation and high-performance of the proposed autotransformer rectifier units, where the polygon and delta-polygon autotransformer rectifiers showed the best results.