A real-time selective harmonic compensation (SHC) based on asymmetric switching angle modulation and current feedback control for cascaded modular multilevel inverters

With asymmetric switching angle modulation technique, a cascaded modular multilevel inverter (CMI) is able to not only control real power flow, but also compensate reactive power and selective harmonic currents with staircase waveforms and the minimum switching loss. To achieve these functions simultaneously, the references of a fundamental active current, a reactive current and all concerned harmonic currents must be generated. The existing techniques employed in selective harmonic elimination calculate solutions offline and store them in a DSP. It would take extremely large memory space because of multi-dimensional data in asymmetric switching angle modulation. This paper proposes a technique to transform a transcendental equation set into a decoupled multi-input multi-output (MIMO) nonlinear controlled system. Hence calculating solutions is transformed from solving the trigonometric equation set to achieving the desired response of the controlled system. Both DC model and small signal model are developed for the system. The integral controller, decoupling controller and a feedforward loop with a much smaller look-up table (LUT) than conventional approaches are designed to ensure the system to converge to a zero-error steady state. A current spectrum feedback loop with FFT block is applied to improve the system´s robustness.