A Real-Time Selective Harmonic Elimination Based on a Transient-Free Inner Closed-Loop Control for Cascaded Multilevel Inverters

Applying the selective harmonic elimination (SHE) technique to grid-connected-cascaded modular multilevel inverters has been widely discussed in the literature. However, due to the difficulties of solving high-order nonlinear transcendental equations, the SHE technique cannot be implemented in real time so its applications are limited. This paper presents a technique to convert the nonlinear equations to a specific control system and use a look-up table, integral controllers and a decoupling controller to help the system converge to a zero-error steady state. An inner instantaneous observer is introduced to directly extract the harmonic spectrum of the output voltages without applying FFT algorithm to output step voltage waveforms. It can not only simplify software design but also eliminate the phase delay generated by conventional spectrum extracting algorithm. The proposed technique can achieve an accurate switching angle control and a fast response. Simulation and experimental results verified that the system can achieve zero-error steady state within one line period.