Research On The Control Strategy Of Interface Converter In AC/DC Hybrid Micro Grid

In this paper,the virtual synchronous generator(VSG)technology and its application in parallel system of AC/DC hybrid micro grid is researched,and an improved VSG model and control strategy is proposed.Firstly,according to structure characteristics of the low voltage AC/DC hybrid micro grid,the improved VSG model combined of DC sub micro grid and feeder equivalents synchronous generator is established,and a VSG control strategy with virtual resistance compensation and virtual coordinate transformation is accordingly put forward.Then,by adding the reference correction of voltage and power to VSG power loop,the improved VSG can simulate the three times frequency regulation operation mode of the synchronous generator and response to the dispatch of micro grid manage control system.Moreover,the virtual impedance method is used to solve the problem that the VSG control strategy is difficult to be utilized in the complex power supply network.Finally,the accuracy and effectiveness of the improved VSG control strategy are verified by simulation analysis.According to the operation characteristics of interlinking converter in AC/DC hybrid micro grid,an improved inverse-droop control strategy which is capable of simulating three times frequency regulation operation mode of synchronous generators in power system is proposed.The improved inverse-droop control principle and the corresponding three times frequency regulation methods of micro grid are analyzed in detail.Second regulation is performed by adding an integral element to the inverse-droop control loops so that the no-deviation regulation for AC bus voltage and frequency can be realized;third regulation is performed by correct power reference values in the control loops which can response to power dispatch of the manage control system so that the optimal power assignment can be realized.The accuracy and effectiveness of the proposed control strategy are verified by simulation analysis in DigSILENT/PowerFactory.For the pre-synchronization method of the virtual synchronous generator(VSG),only the case of a single VSG pre-synchronization and grid connection has been studied and analyzed.If the pre-synchronization of parallel VSGs is realized by controlling one of them,the method based on PLL can not meet the requirements of the fast and stable performance,as the use of the PI regulator may cause the dynamic circulation and power oscillation of parallel VSGs.In this paper,an open-loop pre-synchronization method based on virtual power loop is proposed.Firstly,the function of added virtual power loop and the open-loop pre-synchronization method by giving virtual power steps are analyzed.Moreover,the calculation of phase difference and pre-synchronization time are researched.Then,in view of the active circulation in the pre-synchronization process of parallel VSGs operation,the theoretical analysis is carried out by using the small signal model and the parameters of two power loops are designed respectively.Finally,the pre-synchronization process of different methods is compared by simulation and experiment.The results show that the process is smooth and stable on this paper's method and this paper's method applies to the pre-synchronization of parallel VSGs.