Research On Dynamic Voltage Restorer To Improve Power Quality In Microgrid

To a certain extent, microgrid could supply power for the main grid, reduce energy consumption and improve power system reliability and flexibility to meet diverse demands of loads. But microgrid generally locates close to the load and is at the end of the distribution network so it is vulnerable to voltage sags, transients and other disturbances. In addition, due to the development of modern industrial high-technology, the high power quality is to be required by end users. Therefore, microgrid power quality problems could not be ignored.The paper mainly concerns about the power quality problems of microgrid. By understanding the basic structure of microgrid and analyzing microgrid power quality issues, voltage sags are the severe problem in microgrid. A dynamic voltage restorer (DVR) based on photovoltaic (PV) generation/battery units is proposed and it could operate in dynamic voltage restorer, uninterruptible power supply and micro-source mode. Using energy management system to control the switches signals, the effective switching between modes could be achieved.The main content and conclusions are as follows:(1) Based on the basic concepts of microgrid, the power quality in microgrid is analyzed, and voltage sags is known as the most important problem.(2) Voltage sags problem is studied including definition, source, harm and mitigation measures. And DVR is the powerful equipment that is commonly used for voltage sags and swells mitigation. As series voltage compensation equipments, the principle of DVR is illustrated and the importance of storage unit is put forward. The detection methods are studied and simulated in Matlab, and the results demonstrate that the dq detection method is effective to detect voltage disturbance.(3) A PV/battery hybrid generation unit is brought in as an energy storage unit of DVR. On account of the effect of temperature and irradiance, PV model is built in Matlab to illustrate the output characteristics changes. The structure of the unit is introduced. In order to achieve the system energy flow, the charge control of battery by PV array is specified.(4) A DVR based on PV/battcry generation units is proposed to improve voltage quality in a microgrid. Using energy management system (EMS), the proposed apparatus could be operated in dynamic voltage restorer mode, uninterrupted power supply mode and micro-source mode, and be switched smoothly among them. The detection method, compensation and control strategy used in the paper are illustrated. According to the operation modes switching, it indicates that the energy of the storage has an immediate influence on the normal operation of DVR. Therefore, the control flows of battery charge/discharge are put forward so that it is useful to improve the compensation ability of DVR.(5) Based on the structure of the proposed system, the microgrid simulation model, a PV/battery hybrid generation unit and the whole system simulation model are established. Then the output voltage of the energy storage unit and battery charging process with constant current control are analyzed. Meanwhile microgrid voltage sags, swells and outages are also discussed.The simulation results found that the combination of the battery and PV unit, could reduce the DC voltage fluctuation of the PV generation unit, dual-stage DC/DC converters could increase DC voltage. When the battery terminal voltage decreases, the battery charging in constant current mode by the grid could achieve energy. In grid-connected or islanded operation mode, voltage dips and voltage wells, the proposed DVR could operate as dynamic voltage restorer to compensate voltage dips or swells effectively and improve the microgrid voltage quality. As the power supply is interrupted, the operate mode will switch to the uninterruptible power supply to improve the load power supply reliability. In addition, the proposed DVR could also be used as a micro-source in microgrid to coordinate with the microgrid to supply power for load, so the power supply from main grid can be reduced and it is favorable for energy conservation and emission reduction. The effectiveness of the three operation modes could improve the utilization rate of DVR.