Non-characteristic harmonic analysis of double 12-pluse series converters based on modulation theory

This paper describes a calculating strategy of non-characteristic harmonics due to a UHVDC system operating on a non-ideal condition. The switching functions of a converter system are derived including the effects of an unbalanced ac supply and unbalanced ac side impedance of the transformer. The paper describes how the functions can be used in the calculation of non-characteristic harmonics in UHVDC systems. A new model for simulating the double 12-pulse series converters have been developed with PSCAD/EMTDC. This new model can study the unbalanced ac supply, such as the magnitude and the phase unbalanced, and the unbalanced three-phase impedances of the transformer. It is obtained that the 2^nd, 10^th, 14th harmonic appear in double 12-pulse series converter in UHVDC system and the result is accorded with the calculating strategy.