The Research Of Power Quality And Control Technology In Microgrid

In this paper, the problems that distributed generation and the operation mode of microgrid bring to the microgrid power quality are analyzed. And in-depth research is done on the current major improvement measures to microgrid power quality. The power electronic converters are essential for the microgrid’s power transmission and transformation cannot be operated without them. The three-phase converters are widely used due to their mature control technology, high efficiency and better output. Therefore, weather the three-phase converter’s structure and control performance is good or bad directly affects the microgrid power quality. In addition, the structure of some devices which are to be improving power quality is based on the three-phase converter, so the three-phase converter shows the importance in microgrid.Secondly, as the most commonly used three-phase rectifier a starting point, its slow-scale instability phenomenon is studied. When analysising this unstable phenomenon using existing small signal model, there are limitations in the small signal model, and, the scope of application of three-phase small signal model is not determined. By strict mathematical deducing a new time domain model which can accurately predict the three-phase converter stability boundary is put forward in this paper, then, the Jacobian matrix of the closed-loop control system is calculated. So, the slow-scale instability phenomenon of the three-phase converter is explained in detail using Lyapunov rule. And a large amount of data from comparing the new time domain model, the small signal model and the simulation model points out the reason for the small signal model is limited, the applicable scope and the role of delay inertial link. On the basis of new time domain model the root cause of the different unstable phenomenon between single-phase converter and three-phase converter is also explained. In addition, this paper analyzes the circuit and control parameters’ effect on the stability of the converter. And, the double pole-double zero control method which has been used in the single-phase converter is applied to the three-phase rectifier for the first time, then, the good steady state and transient effects in simulation confirm this control method’ effectiveness.Thirdly, the harmonic pollution and three-phase unbalance in the power quality problems are paid special attention,and, the root and the control strategy of which are analyzed. A midpoint capacitance type three-phase four wire active filter is presented to improve the two power quality problems. By comparing the current harmonic detection and non harmonic detection control method, the latter is selected for its more advantage. And in view of the three-phase unbalance problem, a simple but effective control method is put forward, through increasing the feedback of zero sequence current in the control method, the three-phase unbalance is significantly improved, and the neutral line current is suppressed. Finally, a simulation model of the three-phase four wire active filter for harmonic compensation and improve unbalanced is set up in Matlab/Simulink, and, the simulation result confirm the feasibility and remarkable improvement to power quality of the proposed control strategy in microgrid.