The use of power converters is very important in maximizing the power transfer from renewable energy sources such as wind, solar, or even a hydrogen-based fuel cell to the utility grid. A LCL filter is often used to interconnect an inverter to the utility grid in order to filter the harmonics produced by the inverter. Even though there is an extensive amount of literature available describing LCL filters, there has been a gap in providing a systematic design methodology. Furthermore, there has been a lack of a state-space mathematical modeling approach that considers practical cases of delta and wye connected capacitors showing their effects on possible grounding alternatives. This paper describes a design methodology of a LCL filter for grid-interconnected inverters along with a comprehensive study of how to mitigate harmonics. The procedures and techniques described in this paper may be used in small-scale renewable energy conversion systems and may also be retrofitted for medium and large-scale grid connected systems.

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This paper comprehensively analyzes the stability of a grid-interfacing inverter with LCL-filter in the discrete domain, where the LCL-filter, along with the controller, are modeled in a polar coordinate. System open-loop and closed-loop poles are analytically studied and expressed in the z-domain.Through the poles movement and distribution analysis, the relationship between system stability and the ratio of resonance frequency over sampling frequency is mathematically revealed and calculated as well as the system control gain limit. Moreover, this paper demonstrates that grid-voltage feedforward regulator would significantly alter the inverter stability in a weak power system. By means of Jury stability criterion, the stability status under different filter resonance frequency is given. The selection of resonance frequency and filter parameters makes a considerable difference on system behavior. Finally, to improve the robustness against grid inductance variation, a conservative design recommendation of filter parameters and control gain is given. Through the tests on a laboratory-scale prototype, the theoretical analysis is validated by experimental results.

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Abstract-The paper presents a robust continuous nonlinear model predictive control (CNMPC) for a grid-connected photovoltaic (PV) inverter system. The objective of the proposed approach is to control the power exchange between the grid and a photovoltaic system, while achieving unity power factor operation. As the continuous nonlinear MPC cannot completely remove the steady-state error in the presence of disturbances, the nonlinear disturbance observer-based control is adopted to estimate the offset caused by parametric uncertainties and external perturbation. The stability of the closed-loop system under both nonlinear predictive control and disturbance observer is ensured by convergence of the output-tracking error to the origin. The proposed control strategy is verified using a complete laboratoryscale PV test-bed system consisting of a photovoltaic emulator, a boost converter, and a grid-tied inverter. High performance with respect to DC-link voltage tracking, grid current control, disturbance rejection, and unity power factor operation has been demonstrated.

This paper presents the utilization of harmony search algorithm (HSA) to optimally design the proportional-integral (PI) controllers of a grid-side voltage source cascaded converter with two additional loops for smooth transition of islanding and resynchronization operations in a distributed generation (DG) system. The first loop is the frequency control loop which is superimposed on the real power set point of the cascaded controller of voltage source converter to minimize the frequency variation during the transition from the grid mode to islanding mode. The second loop is the resynchronization loop which reduces the phase shift of the AC voltages of the DG with the utility grid AC voltages during islanding operation leading to a successful grid reconnection event. The response surface methodology (RSM) is used to build the mathematical model of the system dynamic responses in terms of PI controllers' parameters. The effectiveness of the proposed PI control scheme optimized by the HSA is then compared to that optimization by both genetic algorithm and conventional generalized reduced gradient techniques. The HSA code is built using MATLAB software program. The validity of the proposed system is verified by the simulation results which are performed using PSCAD/EMTDC.