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Based on the comparisons of several inverter configurations in the aspects of efficiency, response, volume, current distortion, and price, in this article, we present a feasible three-phase four-wire (3Φ4W) grid-connected hybrid-frequency parallel inverter system (HbFPIS) with ripple compensation to achieve low current distortion and fast response. An HbFPIS consists of a low-frequency-high-power inverter (LFHPI) and a high-frequency-low-power inverter (HFLPI) connected in parallel. An LFHPI design results in higher power output with lower frequency (LF) switching devices, such as insulated gate bipolar transistors (IGBTs) being more cost-effective. An HFLPI design can reduce switching loss by lowering the power requirement of high-frequency (HF) switching devices while increasing dynamic response. With the direct digital control and ripple compensation, LF current ripples are attenuated by the HFLPI to a smaller level, remaining mainly HF current ripples and, thus, resulting in smaller filter size and weight. The design of an HbFPIS ensures that the system has high power output with cost-effective devices, fast response, and smaller output filters. Ripple attenuation, current tracking capability, and inverter stability are analyzed in the article. Low current distortion and fast current tracking performance are validated by the 300-kW simulated and the 10-kW experimental results of a 3Φ4W HbFPIS in grid-connected and active power filter modes. [ABSTRACT FROM AUTHOR]

This paper presents a sinusoidal pulsewidth modulation (SPWM) based direct digital control with average-voltage model (AVM) and division–summation process for paralleled three-phase three-wire (3φ3W) grid-connected converters to reduce circulating currents. With the direct digital control, plant of each single-phase converter can be derived directly, and a controller is then designed to cover the variation effects of filter inductance and grid voltage. Based on SPWM and AVM over a switching cycle, each 3φ3W converter can be decomposed into three single-phase converters and each converter is controlled individually to track its sinusoidal reference current tightly, reducing circulating currents significantly. Equal current sharing is therefore also achieved. The control laws for achieving grid connection are derived in detail. In the design and implementation, the inductances corresponding to various inductor currents are measured offline and stored in the controller for scheduling loop gain cycle by cycle. Experimental and simulated results from a three-converter system confirm the analysis and discussion of the proposed control approaches. [ABSTRACT FROM AUTHOR]