Decoupled fractional-order harmonic filter for power quality enhancement of grid-connected DG units

The global expansion of power electronics interfaced distributed generator (DG) units has led to the incremen-tation of nonlinear loads in the distribution systems, which may impose negative effects on the power quality. To maintain the desired power quality while actively compensating the harmonics in the presence of frequency deviations, this work proposes a decoupled closed-loop fractional-order current controller that incorporates the harmonic alleviation with the DG power generation function. Accordingly, an optimal fractional-order proportional-integral-derivative (FOPID) is designed to control the DG unit fundamental current, while compensating the local nonlinear load harmonic current (LHC) effects. A noteworthy contribution of this paper is that the optimal control scheme delivers the desired performance without the need for LHC detection and effectively reduces the operational cost. Another contribution is the implementation of an optimal FOPI-based control scheme in the outer power control loop to extract the fundamental current reference with no phase-locked loop (PLL) components. This offers comprehensive solutions with several improvements, such as eliminating the power tracking steady-state error under fundamental current tracking errors and preventing harmonic disturbances from being imposed on the harmonic control by the power control part. The feasibility and effectiveness of the proposed power control scheme are validated in comparison with the conventional PI approach.