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Variable switching frequency pulse width modulation (VSF PWM) has been proposed in the past to optimize the total harmonic distortion (THD) performance and switching losses of an inverter. Nevertheless, such VSF PWMs give rise to switching harmonics which vary across a wide frequency spectrum, where the lower frequencies could resonate with the filter while the upper frequency could exceed the maximum switching frequency. This complicates VSF PWM design, restricting their practical uses. In this paper, a confined band VSF PWM (CB-VSF PWM) for a single-phase inverter with an LCL filter is presented. By confining the switching frequencies within a practical band, the design of the filter becomes easier while maintaining low current THD with low switching losses. The choices of the upper and lower bands of the proposed VSF are defined and the design criterion is introduced. It is found that the proposed method allows reduction in switching losses as well as reduction in current THD when the effect of the LCL filter is taken into consideration. The performance analysis is validated through simulation and experimental results in a 1 kW inverter. [ABSTRACT FROM AUTHOR]

Recently, the use of LLCL filters for grid inverters has been suggested to give better harmonic attenuation than the commonly used L and LCL filters, particularly around the switching frequency. Nevertheless, this filter is mainly designed for constant switching frequency pulse width modulation (CSF PWM) methods. In variable switching frequency PWM (VSF PWM), the harmonic components are distributed across a wide frequency band which complicates the use of a high order filter, including LCL and LLCL filters. Recently, a confined band variable switching frequency (CB-VSF) PWM method has been proposed and demonstrated to be superior to the conventional constant switching frequency (CSF) PWM in terms of switching losses. However, the applicability of LLCL filters for this type of CB-VSF PWM has not been discussed. In this paper, the authors study the suitability of an LLCL filter for CB-VSF PWM and propose design guidelines for the filter parameters. Using simulation and experimental results, it is demonstrated that the effectiveness of an LLCL filter with CB-VSF PWM depends on the parameters of the filters as well as the designed variable frequency band of the PWM. Simulation results confirm the performance of the suggested LLCL design, which is further validated using a lab scale prototype. [ABSTRACT FROM AUTHOR]