Investigations Of The Improvements Of The Power Quality Of Micro-grid Based On Three-phase Grid-connected Multi-function Inverters

Micro-grid system can solve the problems of the operation of distribution network with high distributed generations' penetrations effectively.However,power quality problems such as harmonics and unbalanced voltages are aroused due to the injections of a large amount of power electronic devices,intermittent distributed generations and nonlinear loads in micro-grid.The stability of the operation of micro-grid system can also be affected.In this paper,to solve the power quality problems in micro-grid effectively,the mechanism of productions and control schemes of the power quality problems are investigated systematic,and the following works are done:In order to restrain the high frequency harmonic currents in micro-grid,an improved multi-tuned(IMT)filter and its corresponding inner current loop controller is proposed.The topology of IMT filter with good high frequency harmonic attenuation performance is proposed.Then,based on the control characteristic of the proposed filter,an inner current loop controller is proposed to restrain the resonance peak of the IMT filter.To better suppress the low frequency harmonic currents and the negative sequence currents in micro-grid,two control schemes for improving the performance of three-phase multi-function grid-connected inverters are proposed: 1)An improved power quality detection block for multi-function grid-connected inverters with good transient disturbance rejection capability is proposed.2)By combining active disturbance rejection controller and repetitive controller,an inner current loop controller with both fast dynamic response and high tracking accuracy is proposed.A nonlinear phase locked loop based on linear active disturbance rejection controller(NLADRC-PLL)is proposed.The proposed NLADRC-PLL has both fast dynamic response and good filtering performance.Based on the good performance of NLADRC-PLL,a droop control scheme and a low-voltage ride through control scheme for three-phase multi-function grid-connected inverters are proposed.The proposed control schemes can improve the applicability of three-phase multi-function grid-connected inverters in the complex operating environment of micro-grid.By applying a switching controller for the power quality detection block,the power quality compensation function and the power generation function are kept non-interfering.By optimally calculating the reference currents in the proposed low-voltage ride through control scheme,power ripples of the three-phase multi-function grid-connected inverters during unbalanced faults are effectively restrained.To improve the overall power quality of micro-grid,a power quality comprehensive compensation strategy for micro-grid is proposed.By calculating the harmonic currents and negative sequence currents produced by the equivalent current source of each node in micro-grid,the compensation harmonic currents and negative sequence currents required by each three-phase multi-function grid-connected inverters are calculated.Then,a method for optimally allocating the compensation volume in real time is proposed based on Entropy-Shapely quadratic weight correction.In this paper,the strategies for restraining high frequency harmonic currents,low frequency harmonic currents,and negative sequence currents are investigated.Then,based on three-phase multi-function grid-connected inverters,control schemes for the operation of micro-grid and a power quality comprehensive compensation strategy are respectively proposed.The investigations of improving the power quality of micro-grid are proposed not only in device level and system level,but also in low and high frequency level.Thus,an integrated system to comprehensively improve the power quality of micro-gird is proposed.