Space Vector Modulation for Low Switching Frequency Current Source Converters With Reduced Low-Order Noncharacteristic Harmonics

High-power current source converters (CSCs) are usually implemented with gate turn off thyristors (GTOs) or integrated gate commutated thyristors (IGCTs) that present a maximum switching frequency of a few kilohertz. Space vector modulation (SVM) offers a very elegant way of generating CSC gating signals online with increased gain and reduced switching frequency. However, for very low switching frequency, SVM results in low-order (5th and 7th) noncharacteristic harmonics complicating the design of the input filter. The reduction of the magnitude of these harmonics has been sought mostly through new sequences of space vectors (states) that present better performance for different ranges of modulation index and power factor. Moderate improvement can be obtained by calculating the statespsila on times for the reference vector in the middle of an SVM cycle. This paper proposes calculating the statespsila on times as the reference vector rotates. Simulation results show that this approach results in a significant reduction in the harmonic distortion of these two components, which, for a selected sequence of states, can be limited to 0.3% of the fundamental component as the modulation index varies from 0.05 to 1.0. Experimental results obtained with a digital signal processor development kit are also provided to show the superior performance of the proposed techniques.