Hybrid low switching frequency modulation strategy with high dynamic response for high-power voltage source converter

For high-power converters, the low switching frequency (SF) modulation represented by selective harmonic elimination pulse width modulation (SHE-PWM) is crucial to reduce the switching loss and enhance the efficiency with a good steady-state performance. However, since the advantages of SHE-PWM mainly come from the fixed switching angle distribution limited by non-linear equations, switching angles should be adjusted slowly. This requires that the bandwidth of proportional integral (PI) controllers be limited hugely, resulting in a poor dynamic response. As the most popular control to improve the dynamic response, finite-control-set model predictive control (FCS-MPC) has a worse steady-state performance under low SF because of large errors caused by a long prediction period and harmonics caused by space vector PWM (SVPWM). Therefore, a hybrid modulation strategy with an acceptable complexity is proposed in this article, which combines SHE-PWM and FCS-MPC. PI + SHE-PWM is adopted in a steady state with low SF, while MPC + SVPWM is used in a transient state with high SF. By studying smooth switching from the modulation level and control level, the proposed method can eliminate the current surge when switching between PI + SHE-PWM and MPC + SVPWM. Experiments verify that the proposed method have a good steady-state performance and high dynamic response simultaneously under overall low SF.