Small-Signal Characterization of Selective Harmonics Elimination (SHE) Modulation and its Impact on Passivity-based Impedance Analysis

Selective harmonics elimination (SHE) is employed to effectively mitigate specific low-order harmonics in low switching-frequency converters by precisely manipulating switching angles within a line cycle. SHE relies on a steady-state solution, raising questions about its dynamic performance and impact. This paper focuses on the small-signal characterization of SHE and its influence on converter impedance. It was observed that SHE has an amplifying—and uncontrolled—effect on certain high-frequency points, particularly pronounced when utilizing input voltage feedforward control due to its direct perturbation path. This leads to the converter terminal impedance exceeding 90°, violating the Passivity Criterion. A potential solution to mitigate this impact is to update the angle and modulation index of the SHE schemes at a slower rate or to avoid the use of feedforward terms. Alternatively, switching to SPWM-type modulation schemes optimized for low switching frequency operation may be considered.