An adaptive modulation scheme for fundamental frequency switched multilevel inverter with unbalanced and varying voltage sources

Stepped modulation operates at fundamental frequency allows the use of low speed switching devices and mitigates the switching loss and EMI concerns suffered by many other hard-switched inverters adopting carrier-based pulse-width modulation (PWM) such as space vector PWM and sinusoidal PWM. Half-height (HH) method is one of the simplest ways to compute switching angles of stepped sinusoidal output. Similar approaches have been utilized in digital-analog signal conversion and synthesis. However, there is limited research on HH-based modulation for power inverters. In this paper, a computational modulation scheme based on HH method was studied. The proposed modulation scheme is capable for real-time computation with most present microprocessors. This paper will demonstrate the implementation of the proposed modulation scheme at balanced voltage and the techniques of adaption to varying and unbalanced sources. Capability of having minimum total harmonic distortion (THD) under equal voltage sources of HH method was verified. The performance of the proposed modulation scheme was examined using numeric calculation and simulation with a 7-level cascade H-bridge inverter and the results are presented.