A Feedback-Based Flexible Compensation Strategy for a Weak-Grid-Tied Current-Controlled Converter Under Unbalanced and Harmonic Conditions

Abstract

Due to ever-increasing power demand and high renewable energy penetration, the power system's harmonics and voltage unbalancing have become significant power quality issues. In this aspect, because of the multifunctioning ability of the power electronic converters, the grid-connected power electronic converter-based distributed generations are expected to provide desired compensation whenever required. Especially in a weak grid environment, the compensation draws more attention due to constricted coupling between the voltage and current at the point of common coupling as a consequence of high grid impedance. Therefore, this article presents a feedback-based flexible compensation strategy for a weak-grid-tied distributed generation (DG) to enhance the compensation of harmonics and voltage unbalance in the system. The main uniqueness of the proposed work is to seamlessly embed a flexible harmonic loop with a fundamental current control loop for achieving three control targets: voltage harmonic compensation, DG line current harmonic rejection, and voltage unbalance compensation. Achieving control targets require voltage feedback and no need to measure nonlinear load current. In order to execute flexible harmonic compensation (i.e., from voltage harmonic compensation to DG line current harmonic rejection) through the harmonic loop, the proposed strategy generates a harmonic reference from voltage feedback and inherent current feedback. The delay introduced by the harmonic compensators is accurately compensated distinctly to improve the performance of the harmonic loop. The voltage unbalance compensation is achieved through the fundamental current control loop by improving the fundamental positive sequence voltage and decreasing negative sequence components concurrently. To enhance the flexibility between two different targets (i.e., flexible harmonic compensation and voltage unbalance compensation), the harmonic and unbalanced compensations are executed on an individual target basis, subjected to the converter's maximum current and system voltage constraints. The proposed compensation strategy offers better harmonic suppression and voltage unbalance compensation than conventional methods. Because it exactly acts upon modulation signals and directly regulates sequence components and voltage harmonics. Furthermore, the proposed control's detailed design and stability analysis is presented, and its superiority is validated through simulations and real-time experimentations.