| The multi-terminal HVDC power system and DC grid based on voltage source converter(VSC) plays an important role in the large-scale renewable energy grid and smart grid construction and the power flow in a DC grid is determined by the resistance of each branch. According to the basic circuit principle and the control principle of converter station, to a certain topology of dc system, the trend of N-1 circuits can be controlled by N independent converter stations, for more than N-1 circuits in the system cannot effectively adjusted, so the optimal allocation of the dc grid current cannot only realize by the control of converter station. Even in some operating conditions overload of resection occurs uncontrollable circuits, endanger the safety of the DC system and connected AC system operating. Therefore, it is usually by adding extra tide controller to improve the power flow distribution in meshed of DC system, realize the whole dc power grid effective adjustment of the internal tide.In this paper, the DC power flow controller type was selected through the review and the summary of the controller topology structure, control mode and control effectiveness that had been studied domestic and abroad. Combined with the external characteristics of the DC power flow controller, the control parameters were introduced into the power flow calculation, and the analysis method based on current sensitivity was put forward to study the influence of DC power flow controller on the power flow distribution in DC grids. With the Zhoushan 5-terminal DC project as an example, the simulation software and the sensitivity analysis methodology has been done for the branch current value changes before and after the adjustment of the DC power flow controller.However the new nodes caused by the introduction of the controller may result in the increase of order number and change of elements in the node admittance matrix and the Jacobin matrix, leading to the impacts on the calculating speed and efficiency comparing to the conventional power flow calculation. The method of equivalent power injection to the power flow calculations of DC grids with the power flow controller was introduced. The new nodes were transferred to the both ending nodes of the branch with the controller in the form of additional injection power, preventing the large amount calculations. Through the equivalent power injection method and simulation software, the power flow calculation results were contrasted when DC grid containing one or more power flow controllers.In order to give full play to the control effect of the DC power flow controller, reduce the risk of overload resection of DC grids even converters considering the operation of economic factors, this paper discusses the installation position and number of DC power flow controller. In view of the DC grid in normal condition and N-1 condition, this paper proposed the principle of the optimal installation location of DC power flow controller based on the current sensitivity method and the combination of DC power line-loss in DC grid.The research in this paper can provide technical reference for installation and operation of DC power flow controller. It can also enhance the stability of DC grid and improve its economy, and has the vital significance in giving full play to the advantage of DC power flow controller. |
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