Key Issues Research On Power Conversion System For Battery Energy Storage Applied In Microgrids

Although it is recognized that the technology of renewable energy generation is one of the most effective ways to deal with energy problem in the world,their inherent power fluctuation and intermittence can cause additional impact to the traditional power system,leading to deterioration of power supply quality.Microgrid with energy storage is an appropriate structure for large-scale renewable energy application.Power conversion system is the core part in battery energy storage system based on power electronics technology,which plays a decisive role in the whole system performance.This paper takes power conversion system as the main research object.This paper mainly carry out its research work in detail from the view of power fluctuations,multimode switching,operation under weak grid,high efficiency and high reliability demands.Including:1.The IGBT driving circuit in the power conversion system is studied,and a digital driving scheme which can adapt to the fluctuation of power and voltage is presented.FPGA is the core component in digital driver aiming to optimize the dynamic feature in switching process,thereby improve the converter's electromagnetic compatibility,and enhance the reliability of the converter.An improved active clamp technique is proposed to decrease the voltage spikes during turning off.Moreover,the threshold voltage is adjusted flexibly to avoid wrong actions.The driving circuit is designed and manufactured,and its effect is tested experimentally.2.Two novel modulation methods are proposed for three NPC power conversion system,which are suitable for three-phase three-wire and three-phase four-wire threelevel converters.These methods can reduce the algorithm delay,eliminate dead time,enhance the converter DC bus voltage utilization,reduce the number of switching actions,and finally enhance the reliability and efficiency of converter.The simulation and experimental verification of these methods are carried out,and the results can confirm its feasibility.3.The system configuration and basic principle of double stage power conversion system are analyzed.And several topologies of isolated DC-DC converters are compared.A modulation method based on mode switching is proposed,which can reduce the power circulation and realize the zero current soft switch in some switches.Then a bus voltage optimization method is proposed to increase the overall efficiency.The bus voltage is selected to the maximum efficiency point.The test platform is built and the measured results of DAB circuit and double stage power conversion system are given respectively.4.This paper studies the stability problems of power conversion system under weak grid.The impedance model of energy storage micro-source based on LCL filter is established.it can be concluded that the micro-source is unstable when it is connected to weak grid.A new control method based on virtual resistance is proposed,which can improve the operation characteristics of the converter under weak grid.Experimental prototype are designed and tested to verify the effect of control method.5.This paper analyzes the microgrid involving energy storage sources as main power supply.An improved virtual synchronous generator control strategy of power conversion system is given to simulate the operation of large synchronous generators in power systems and provide flexible voltage and frequency support for microgrids.At the same time,this method is able to solve the drawbacks of traditional method,including power fluctuations and current harmonic under strong grid.It is also able to achieve the seamless transition between on-grid and off-grid conditions.The experimental platform is built up to verify the feasibility of control method.