Direct Charge Control for Mixed Conduction Mode Grid-Connected Inverter

Discontinuous conduction mode (DCM) for inverters can lower the size of filter inductors and can realize the soft switching for switches. However, the current stress under the DCM is high. Aiming to solve this problem, this article uses a T-type three-level circuit to construct an LCL-type grid-connected inverter, which introduces the trapezium conduction mode (TPCM) with the zero-voltage switching to lower the current stress when the grid current is large. Both the DCM and the TPCM can work at a fixed switching frequency, but it is difficult to design controllers using the conventional average state model. To address this challenge, this article proposes a charge quantity model (CQM) that is applicable to any inverter topology and conduction mode, where the output charge quantity of the bridge is used as a control variable. The proposed model represents a linear two-order model, which can greatly simplify controller design. Based on the CQM, a direct charge control method working at a fixed switching frequency is developed, which combines a PID regulator and a feedforward controller of the grid voltage. The excellent steady-state and dynamic control performances of the proposed model and control method are verified by simulations and experiments.