Comparison of space-vector-modulated direct and indirect matrix converters in low-power applications

Matrix converters are frequency converters which do not contain a direct current link circuit with passive components, unlike conventional frequency converters. Thus, matrix converters may provide a solution for applications where large passive components are not allowed or a purely semiconductor-based solution provides an economically more efficient result than conventional frequency converters. However, the lack of a link circuit may also be a drawback in non-ideal operation conditions and the direct structure also places restrictions on converter capability. This thesis concerns two space-vector-modulated matrix converter topologies. The topologies are a direct and indirect matrix converter, which are compared in low-power applications. The comparison is based on the space vector analysis and modelling of the matrix converter topologies concerned. The comparison is confirmed both in the simulations and in the experimental tests with the prototypes built. In addition to the comparison, the thesis also contains the space vector form analysis of the matrix converter operation under distorted supply and load conditions. The space vector theory is used for the explicit presentation of matrix converter modulation. In addition, some space vector modulation methods are compared from the point of view of the power losses and the output common-mode voltages produced. The space vector approach is used to analyse the migration of supply voltage distortion to output and its reflection back to the supply side. Three computationally simple methods for mitigation of the distortion migration are also compared. These new analytical results are confirmed by simulations and measurements: the distortion migration can be mitigated but not removed totally and the mitigation also increases the complexity of control and decreases supply current quality. The comparison of direct and indirect matrix converter topologies is based on the analyses of their non-ideal characteristics. Their voltage transfer characteristics, semiconductor power losses and supply current qualities are studied. The analyses are again tested either computationally or in the simulations and experiments. In addition to the passive RL load, the applications also contain a space-vector-controlled cage induction machine drive and a permanent magnet synchronous machine drive. The research shows that the two topologies are similar in the ideal case but after non-ideal characteristics are introduced, the direct topology shows better characteristics in nearly all loading situations tested, i.e. the direct matrix converter has more linear voltage transfer characteristics, lower power losses and higher supply current quality than the indirect matrix converter. Index terms: Frequency converter, matrix converter, space vector modulation