Research Of Voltage Control About DC Microgrid Based On Hybrid Energy Storage

In recent years, with the demand for energy and environmental issues becoming increasingly prominent, the distributed generation dominated by clean energy has received wide attention, and microgrid has produced. Most microgrids are AC microgrids. DC microgrid system having advantages of simple structure, less energy conversion, power supply of high quality, is batter than AC microgrid. With the development of distribution systems, DC microgrid system has received more and more people’s attention. Because of the intermittency and randomness of renewable energy sources’output power, energy storage systems have become an important part of DC microgrid voltage regulator, and are of great significance. With high energy density, lithium battery is widely used in the energy storage device. In order to maintain the balance of instantaneous power of microgrid system, stabilizing DC bus voltage, energy storage systems will absorb or emit power frequently, while frequent power charge/discharge will seriously affect the life of the lithium battery. Supercapacitor have advantages of high power density, long life, etc. Studying on control strategy of hybrid energy storage systems to stabilize DC bus voltage has gradually attracted more scholarly attention.Based on the analysis that stable operation of microgrid requires energy storage, this paper presented a control strategy of energy storage automatic charge and discharge for DC microgrid, and then proved the feasibility of the control strategy in simulation. In order to optimize the working process of the lithium batteries, this paper used lithium battery-supercapacitor-level cascade to form a hybrid energy storage system, and presented a control strategy of hybrid energy storage systems in DC microgrid based on voltage droop method. The control strategy takes advantage of the unable mutation characteristics of ultracapacitor voltage to achieve the smooth control of battery current, takes the segmented control of voltage to reduce the number of charging and discharging of lithium batteries and extend its lifetime. Experiment verifies the validity of the control strategy of hybrid energy storage. The details are as follows:First, this paper introduced the status, development prospects and problems of DC microgrid; highlighted the importance of hybrid energy storage in DC microgrid; analyzed the existing structure and control strategy of hybrid energy storage; built a DC micro grid model based on hybrid energy storage, and analyzed its operation mechanism. In order to study the cause of bus power fluctuations, this paper model the photovoltaic and wind turbine power; in order to use supercapacitors and lithium batteries more rationally, this paper study their respective charging/discharging mechanism. In addition, Boost converter /Buck converter and DC/DC converter are studied in this paper, which are important interface circuit for DC micro grid.Secondly, based on the analysis of DC bus voltage stability and the characteristics of voltage fluctuation, this paper proposed a new storage control strategy about automatic charge and discharge regulator. The control strategy takes the segmented control of voltage. The energy storage system doesn’t work when the DC bus voltage stables within a specified range, and stables bus voltage when it is unstable. The system can automatically switch work mode according to bus voltage without upper control. Hysteresis control prevents switches working mode errors.Aiming at stabilizing the DC bus voltage and optimizing the working process of the batteries, this paper presents a control strategy of hybrid energy storage systems in DC microgrid based on voltage droop method. The control strategy makes use of the ultracapacitor compensating the high-frequency components of bus power according to the DC bus voltage, indirectly compensates low-frequency components of bus power through the energy supplement of the batteries on the ultracapacitor, and takes advantage of the unable mutation characteristics of ultracapacitor voltage to achieve the smooth control of battery current. The control system can automatically switch work mode according to the conditions of DC bus voltage, ultracapacitor voltage and the state of charge of battery.Finally, this paper verified the control strategies for hybrid energy storage on the dSPACE. The experiments show that the control strategy can automatically adjust the output of batteries and ultracapacitor, maintain light fluctuation of DC bus voltage around rating, effectively reduce the frequency of battery charging and discharging and extend its using life.