Model-Predictive Dual-Control Loop With Improved Current-Limiting Capability for Grid-Forming Inverter Under Grid Faults

Current-limiting capability is crucial for fault ride-through of grid-forming (GFM) inverters. Most current-limiting schemes for GFM inverters are implemented within classical linear controllers, which cannot guarantee optimal performance in case of emergencies like faults. Additionally, the inherent cascaded structure limits the bandwidth. The advanced model-predictive control (MPC) has been developed for power converters thanks to nonlinear objectives and constraints handling ability. One of the well-known MPCs, i.e., the finite-control-set MPC (FCS-MPC) has been employed to prevent overcurrent, which roughly includes a nonlinear penalization for current magnitude violation in the cost function. In this case, the cost function of FCS-MPC will go to infinity during faults at the expense of the voltage and current reference tracking ability, and thus, the power quality gets worse. Besides, the weighting factor design is usually a nontrivial task for MPC. To maintain the high bandwidth benefit of MPC and improve the power quality during faults, a model-predictive dual-control loop (MP-DCL) is proposed in this article. The proposed method involves the outer-voltage MPC loop to generate the optimal current reference. With the current-limiting factor applied, such a constrained reference will be tracked through the proposed inner-current MPC loop. The proposed MP-DCL expresses simple design benefits and ensures the GFM system recovers from fault to normal conditions smoothly with low overshoots and without oscillation. Experimental results verify the effectiveness of the proposed strategy through numerous comparisons with state-of-the-art solutions.