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The synchronization method’s ability is one of the fundamental guarantees for the stability and accuracy of injected current control for grid-connected converter-based distribution resources. These synchronization methods, such as phase locked loop (PLL), are generally based on Park transformation and grid voltage regulation, which may affect an unstable phenomenon under distortion and unbalanced grid voltage conditions and result in more computational complexity. In this present paper, analogous to the traditional voltage-oriented control strategy (VOC) in the synchronous rotating frame (dq0-frame) based on PLL and Parks transformation, an improved voltage-oriented control strategy (IVOC) without synchronization methods and Parks transformation is proposed for grid-connected four-leg inverters (GC-FLVSI) to achieve accurate current control with high-quality performance in the dq0-frame. This proposed strategy is not only used for controlling the GC-FLVSI but also to provide the module of GC-FLVSI in the dq0-frame based on the instantaneous active and reactive powers theory (DPC). The proposed IVOC strategy has the same properties and identical performance as the traditional VOC when the grid phase angle is correctly detected by any synchronization method, with the advantages of both traditional DPC and VOC at the same time. In order to validate the superiority and excellent dynamic and steady-state performances of the proposed IVOC strategy in comparison with the traditional VOC strategy, some simulation scenarios using MATLAB/Simulink under different operations and grid conditions have been performed and presented.

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This paper proposes an enhanced voltage-oriented control strategy (EVOC) based on super twisting sliding mode control (STSMC) for a grid-connected four-leg source voltage inverter (GC-FLVSI) in the synchronous rotating frame (dq0–frame) without using a phase-locked loop (PLL) and Park's transformation. The proposed strategy is used not only to control the dq0–axes FLVSI inject currents but also to derive the mathematical model of the GC-FLVSI in the dq0–frame based on the direct instantaneous power control theory (DPC) and to eliminate the impact of PLL and Park's transformation. The principle of the proposed EVOC strategy is analyzed in detail. The STSMC is used for accurate dq0–axes FLVSI inject current control, and it can result in sinusoidal currents with high quality, high robustness against parametric variations, and low chattering with easy implementation. Finally, the superiority of the proposed EVOC-STSMC strategy in terms of complexity, response, steady-state errors, robustness, total harmonic distortion (THDs) mitigation, neutral current reduction, and robustness against parameter variations is verified through comparative analysis with the EVOC strategy based on the PI controller (EVOC-PIC).