Modular Model-Predictive Control With Regulated Third-Harmonic Injection for Zero-Sequence Stabilized Inverter

A modular model-predictive control (MPC) method for a nonisolated three-phase
$LCL$
grid-connected converter is proposed with zero-sequence voltage control and regulated third-harmonic injection capabilities. The control method is proposed with the combination of a modified three-phase
$LCL$
converter topology to stabilize the common-mode voltage. The control mechanism consists of two layers: 1) the central-level control of
$dq$
-sequence output current control cascaded with 2) the local-level control of zero-sequence voltage three-phase inductor current/capacitor voltage MPC. No grid-side inductance value is needed for the two-layer cascaded control structure, which both improves the dynamic performance from the inner MPC loop and avoids the MPC parametric error caused by the uncertainty of grid-side inductance. With the zero-sequence MPC, the common-mode voltage and leakage current can be attenuated without a bulky isolation transformer. A regulated third-harmonic injection (RTHI) method is designed based on the zero-sequence voltage control and the proposed modified converter topology to improve the dc bus utilization and avoid the overmodulation issue. Compared with the conventional third-harmonic injection methods, no extra harmonics are injected to the grid, and there is no need for a specialized grid total harmonic distortion optimization algorithm with the proposed zero-sequence-controlled RTHI. The stability and robustness of the proposed RTHI method are also improved with the zero-sequence MPC regulation. In addition, the modular concept of the local MPC module enables the extensibility for the power converter control with a flexible number of phases. The dynamic performance, leakage current attenuation, dc bus utilization, and grid-connection power quality of the proposed MPC method with RTHI have been validated experimentally.