Research On Control Strategy For Parallel Inverters In Islanded Micro-grid

With the rapid development of global economy,the demand for electric power is increasing,but the traditional power grid is difficult to meet the accelerated expansion of electricity.The gradual increase of renewable energy has put forward new requirements for distributed generation,so the micro-grid technology that is developed in recent years has been widely concerned and studied.The efficient and stable operation is the prerequisite and key to take a variety of advantages of micro-grid,which can not be separated from a satisfactory and effective control method.The emphasis and difficulty of the micro-grid technology mainly focus on the control of converter and coordinated control of internal system.Based on the current domestic and dynamics of micro-grid,the paper introduces the concept of AC micro-grid,its topology and key control methods from two facets of both inverter and micro grid.The control effect of micro-grid control mainly lies inverter.On the basis of discussion on the difference between grid connected operation and islanded operation,a variety of control technologies of inverter are described and compared.Because the multi inverter technology in micro-grid can effectively enhance the power supply reliability and flexibility of system,so the principle structure and application range of some main control strategies are summarized.The research of multi-inverter in micro-grid focuses mainly on the rational distribution of load power,frequency and voltage stability,harmonic suppression and so on.Although the manifestation of these problems is different,the key of them is to balance the energy between each micro source.Aiming at this problem,the paper firstly takes a single inverter for example,and concentrates on the droop control strategy and its parameter selection.Then the paper takes a parallel inverter model as an example.After deeply analyzing the power distribution mechanism of parallel inverter in low voltage micro-grid,the disadvantages of traditional droop control are pointed out as follows: the active power can only be distributed by strict matching of droop coefficient.Due to the influence of line impedance,reactive power is difficult to be precisely distributed.The control method can cause the steady-state deviation of system frequency and common AC bus voltage.The sufficient condition to realize an errorless reactive power distribution is that the total impedance of each micro source must be inversely proportional to its corresponding capacity,but the matching condition is almost impossible because of the influence of measurement errors.To solve these problems one by one,the paper establishes a secondary control strategy for micro-grid.The performance of reactive power distribution is improved by using virtual impedance configuration in the first level of traditional droop control.However,reactive power distribution error still exist solely depending on the first level control,so the paper introduces the second level regulation control of reactive power.At the same time,the deviation of frequency and voltage caused by the first level control is also compensated in the second level control to improve the power supply quality.Finally,the Matlab/Simulink software platform is used to simulate two micro source systems under different working conditions,and the results show the effectiveness of the strategy.According to the theoretical analysis and simulation model,an islanded micro-grid experiment platform is built in the laboratory,and the main circuit and control circuit of hardware is designed.Since TMS320F2812 is the core of control circuit,the flow chart of each subroutine are designed.In addition,the software is written and debugged with hardware on CCS3.3 software platform.By analyzing experimental results,it comes to a conclusion that the control strategy can stabilize the system and is worth researching.