Passive Filters for Grid-Tied Low-Frequency Switching Cascade H-Bridge Inverters

Low-frequency switching in multilevel inverters significantly minimizes switching losses and electromagnetic interference, and have applications in off-line power supplies, uninterruptible power supplies and motor drives. However, its widespread use is restricted by the inherent high harmonic distortion. Increasing the number of voltage levels and using selective harmonic elimination techniques can reduce total harmonic distortion and filtering requirements. This paper evaluates the effectiveness of four passive filters in mitigating harmonic content in low-frequency switching, grid-tied, symmetrical cascaded H-bridge inverters through numerical simulations. Output transfer functions are analyzed and guidelines for selecting filter components are proposed. The considered filters, L, LCL, LLCL, and LCL-LC, behave similarly at low frequencies, comparable to an L filter. The L filter is reliable and simple but offers less attenuation. The LLCL filter, similar to the L filter at high frequencies, can target specific harmonics with an LC trap, despite a resonance peak which can be reduced with passive or active damping. The LCL and LCL-LC filters act as third-order low-pass filters at high frequencies but may require complex control and calculation methods for current reference.