The Control of Grid-Connected Inverters in Microgrids

Microgrids based on renewable power generation are under increasing develop-ment all over the world. Grid-connected inverters form an indispensable interface between the microgrids and power grid, to deliver the renewable energy into the grid by controlling the injected current. Inductor-capacitor-inductor (LCL) filters have been widely adopted to attenuate the high-frequency harmonics generated by the in-verters. However resonance of the LCL filters significantly affects the system control performance in terms of stability, transient response, grid synchronization, and power quality. This thesis carries out comprehensive stability analyses and proposes novel current control methods for studying and improving the performance of LCL-filtered grid-connected inverters. Firstly, a systematic study is carried out on the relationship between the time de-lay and stability of single-loop controlled grid-connected inverters that employ in-verter current feedback (ICF) or grid current feedback (GCF). The ranges of time delay for system stability are analyzed and deduced in the continuous s-domain and discrete z-domain. It is found that in the optimal range to achieve the maximum bandwidth and ensure adequate stability margins, the existence of a time delay weakens the stability of the ICF loop, whereas a proper time delay is required to maintain the stability of the GCF loop. The present work explains, for the first time, why different conclusions on the stability of ICF loop and GCF loop have been drawn in previous studies. To improve system stability, a linear predictor based time delay reduction method is proposed for ICF, while a time delay addition method is used for GCF. A controller design method is then presented that guarantees adequate stability margins. The study of the delay-dependent stability is validated by simula-tion and experiment. Secondly, three current control methods (the single-loop control based on ICF, that based on GCF, and a dual-loop control with capacitor current feedback (CCF) active damping) are compared by investigating their LCL resonance damping mech-anism. The virtual impedance introduced by each method is identified, which com-prises frequency-dependent resistance (positive or negative) and reactance (inductive or capacitive). The reactance shifts the LCL resonance frequency while a positive resistance provides damping to the resonance and hence stabilizes the system. Using the virtual impedance, the system stability is analyzed. The stable range of sampling frequency for the above methods is deduced, as well as the gain boundaries of the controllers. The simple and intuitive stability analysis approach by means of virtual impedance can be extended to other single- or dual-loop control methods. The study facilitates the analysis and design of control loops for grid-connected inverters with LCL filters, and it has been verified by experiment. Thirdly, a pseudo-derivative-feedback (PDF) current control is, for the first time, applied to three-phase LCL-filtered grid-connected inverters, which significantly im-proves the transient response of the system to a step change in the reference input through the elimination of overshoot and oscillation. A complex vector method is ap-plied to the modeling of three-phase LCL-filtered inverters in a synchronous rotating frame (SRF) by taking cross-couplings into consideration. Two PDF controllers with different terms in an inner feedback path are developed for an ICF system and a GCF system, respectively. For the ICF system, a simple PDF controller with a proportional term is used. Compared with a proportional-integral (PI) controller, which can only reduce the transient overshoot by decreasing controller gains, the PDF controller is able to eliminate the transient overshoot and oscillation over a wide range of controller parameters. For the GCF system, a PDF controller with a proportional term and a second-order derivative is developed. Active damping is achieved with only one feedback variable of the grid current, and simultaneously the system transient response is improved. Both theoretical analysis and experimental results verify the advantages of the PDF control over PI control methods. Fourthly and finally, a direct grid current control method without phase-locked loop (PLL) is proposed to attenuate low-order current harmonics in three-phase LCL-filtered grid-connected inverters. In comparison with conventional indirect or direct controllers which need PLL and are difficult to achieve satisfactory harmonic attenuation performance, the proposed method is able to satisfactorily mitigate the harmonic distortion, and at the same time reduce control complexity and computation burden because PLL is avoided. It is found that the direct grid current control is necessary to effectively suppress the current harmonics caused by the distortion in grid voltage. Active damping is achieved with an inner ICF loop, which is found to be superior to the widely used CCF damping in improving system stability. A sys-tematic controller design procedure is proposed to optimize the system performance. Experimental results confirm the improved harmonic attenuation ability of the pro-posed method in comparison to that of conventional control methods