Research On Islanding Detection Technology For Multi-grid Connected Inverter Based Distributed Generation

Due to the continuous decrease of fuel energy and it’s increasingly serious damage to the environment, human sights on solar energy, wind energy and other clean and huge reserves of renewable energy, distributed generation technology based on renewable energy is an important direction for future development of the power industry. The phenomenon of islanding is a basic problem in distributed generation system. The so-called islanding refers to the main grid is tripped for breakdown or overhaul, the distributed generation system remains independent running status connected with the local load. After appearance, islanding will threaten the safety of power maintainer and power equipment, affect action programs of protection switch of distribution system, and damage to electrical equipment when reclosing. Therefore, islanding detection is a mandatory protection function of distributed generation device.At present, islanding detection Technologies of grid connected inverter based distributed generation system include two categories, the passive and active islanding detection, but it is mainly for the detection algorithm and parameter optimization of the two kinds of detection methods in single grid connected inverter. With the development of economy and technology, multi-inverter DG systems will be used more extensively, and the researchers have paid more attention to the islanding detection methods of multi-inverter systems. Therefore, the researches of this paper focus on the islanding detection technologies of multi grid connected inverter based DG systems, the specific contents are as follows.First of all, to start the research on some basic content of grid conn c contents are as follows.ected inverter based islanding detection technology in related to this paper, mainly include:(1) The islanding detection method based on grid connected inverter are described, local passive and active methods are analyzed, and emphasized the principle and the advantage and disadvantage of the active frequency drift methods which are used widely.(2) The performance evaluation Methods of islanding detection based on Qf×fo frame are analyzed, the calculating methods of non-detection zones (NDZs) boundary curve for active frequency drift method, Sandia frequency shift method and slip mode frequency shift method are given. (3) A small signal model of DG system is established, the constant current control strategy and APS islanding detection method are used to analyze the inverter control strategies with a positive feedback type islanding detection for it’s effects on the stability of the system, and gives the solution to the conflicts between maintaining system stability and improving islanding detection performance.Secondly, the existing islanding detection methods and their detection performance under the condition of multi grid connected inverter based DG system is studied. carries out the theoretical analysis and simulation verification for the performance of active frequency drift method, Sandia frequency shift method and sliding mode frequency shift method this several active frequency shift type multi grid connected inverter islanding detection methods; NDZ problem and the dilution effect of active frequency shift type multi grid connected inverter islanding detection methods is studied; The principle and performance of the impedance measurement method, the Correlation method and negative sequence current perturbation method for the multi grid connected inverter islanding detection are introduced.For the problem of NDZ and dilution effect of active frequency shift type multi grid connected inverter islanding detection methods, puts forward the strategies to solve these two problems, namely, using digital correlation theory to detect the phase difference between the voltage at the point of common coupling and the output current of the inverter, and carries out the adaptive adjustment for relative parameters of active frequency shift type islanding detection algorithm according to the detected phase difference, finally the proposed strategy has been verified through simulation and experiment. On the basis of the improved algorithm, the low current distortion SFS islanding detection method based on phase locked loop for multi grid connected inverter is put forward to further reduce the inverter output current distortion rate. For this method, the current distortion rate is firstly analyzed when the Sandia frequency shift method is used, the corresponding THD value of current of this method is Calculated in detail for two cases of voltage frequency has deviation and hasn’t deviation; And then introduces the structure and principle of virtual two-phase second-order generalized integrator based phase-locked loop which is used in the improved SFS islanding detection method of multi grid connected inverter; Finally, analyzes the principle, parameter settings and improvement for the low current distortion phase locked loop based islanding detection method, and the simulation and experimental results are presented. Finally, the multi grid connected inverter islanding detection method based on wavelet transform and neural network is developed. For this method, firstly introduces the basic theory of wavelet transform and neural network; Then a detailed analysis of the design principle of MATLAB simulation for this islanding detection method and simulation results of this method under six different states in case of single and multi grid connected inverter are given; finally, analyses the DSP implementation process of Mallat algorithm of wavelet transform and BP neural network algorithm for this islanding detection method, and gives the experimental results in islanding and non islanding state.