Research On Charging And Discharging System With Bidirectional Energy Flow For Battery

With the aggravation of energy and environmental problems, the application of energy storage station, electric vehicle and new energy has gradually increased. Currently, batteries are also in wide used. In the process of using a battery, large number of battery charging devices are required. Several charge-discharge tests are usually carried out on the battery before use. Therefore, a battery charging and discharging device which has high performance and does not pollute the power grid is required. At present, the most widely used high power battery charging device is realized by a phase control rectification of thyristors. In this kind of device, the current waveform distortion in ac side is obvious, the power factor is low, and the pollution of power grid is much. It has other disadvantages such as low automation, complex operation, and poor reliability. It also breaks down easily. In addition, the discharging device consumes electric energy in the form of thermal energy due to resistances in the power conversion device. This causes a great waste of energy. According to the existing problems of battery charging and discharging device, a battery charging and discharging system with bidirectional energy flow is studied in this thesis. The main work of this thesis can be summarized as the following:(1) Selection of topological structure and research of control strategy for the systemThe topological structure of the charging and discharging device with bidirectional energy flow is studied. The pre-stage adopts a three-phase PWM rectifier which could convert alternating current into direct current and feed the direct current back into the power grid. It could also ensure a low current total harmonics distortion (THD) and will not introduce reactive current into the power grid. The last stage adopts a Buck-Boost bidirectional DC/DC converter which could step down the high voltage output of PWM rectifier while charging the battery. It can also increase the voltage of the battery and feed the energy back into the power grid by the PWM rectifier. The device can be designed to charge the battery as well as make the battery discharge. The combined control strategy of pre and post stages is studied according to the method of charging and discharging. The battery constant current, constant voltage, charge control and discharge control are achieved.(2) Establishment of mathematical model and accomplishment of control algorithm for the systemThe mathematical model of three-phase PWM rectifier based on synchronously rotating dq reference frame is built up. Three phase phase-locked loop based on single synchronously rotating reference frame is designed to ensure the accurate control of line side power factor. The control algorithm based on current feed-forward decoupling is studied. The mathematical model of Buck-Boost converter is built up with the method of state-space averaging principle, and PID controller parameters are designed to make the system meet the requirement of battery charging and discharging.(3) Software and hardware design of the systemHardware circuit of the system is designed, such main circuits of PWM rectifier and Buck-Boost converter, current and voltage detecting circuit, driving circuit and protection circuit, etc. The main component parameters of the system are calculated. The software program is compiled using CCS software and each control algorithm is achieved. The experimental platform is built combined with the hardware circuit of the system.(4) Experimental measurement and simulationConverter models of the system are built up with PLECS. Control algorithms are verified by simulation respectively and the validity of core control algorithm is proven. The experimental platform is used to make the practical test. Simulation and experimental results show that the system could ensure the power factor of current in ac side which is approximated to 0.97(approximated to-0.97 during discharge) for charge or discharge. The current in ac side is sine wave and THD is less than 5%. This will reduce harmonic and reactive pollution to power grid. In the process of discharging, it could feed electric energy back into the power grid and save energy. In addition, for energy storage station, an integration of charging and discharging processes is achieved. Therefore, two sets of devices are not required. This lowers its production cost.