Abstract
Discontinuous conduction mode (DCM) for inverters requires a small size of inductor, and can realize soft switching for switches. However, the current stress under the DCM for two-level (2L) circuit is relatively high and there are still no linear models for the control. In this paper, T-type three-level (3L) circuit is used to construct the LCL-type grid-connected inverter (GCI). And the inverter-side inductor works under mixed conduction mode (MCM) combined by DCM when the load current is small and under trapezium conduction mode (TPCM) when it is high, so as to realize the soft switching in all switching cycles and reduce the current stress. In order to realize effective closed-loop control for the GCI under MCM, a linear charge quantity model (CQM) which takes the charge quantity output of the bridge as the control variable is proposed in this paper. Based on the proposed model, a direct charge control (DCC) method with a PID regulator working at fixed switching frequency is further proposed. Finally, the great steady-state and dynamic performance of the proposed model and control method is verified by the simulations and experiments.
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