Research On Voltage Regulation And Power Flow Controller Location Method Of DC Grid

With the decrease of traditional fossil energy and the serious problem that it causes, a development of new clean energy has become the only way for sustainable development of human beings. DC grid technology which based on VSC-MTDC is an effective way to solve the consumption problem when large-scale renewable energy connecting to grid.DC grid contains redundant mesh, multi-point voltage droop control operates more flexible and reliable, but also more complicated.This paper makes research on voltage regulating method and DC power flow controller locating method for a DC grid which operates under multiple point voltage droop control strategy.A DC grid makes its primary voltage regulation according to the P-U characteristic curve. The influence of the curve slope to the controlling system and DC grid has been studied. A mathemetical model of voltage droop controller has been built. The transfer function of the controller is obtained by input-output method. By analyzing the root locus of the controlling system characteristic equation which is changed with DC voltage response coefficient(inverse of the slope), a reasonable range of the parameter is obtained.To solve the problem that a fixed slope coefficient can not response the power flower change flexibly and an overload may even occur to the convertor station, this paper proposes an adaptive control strategy that the slope can change according to the power margin of the convertor. The range of the slope is also used as dynamic limiter amplitude. The controller can allocates unbalanced power reasonably according to the actual situation, and is applicable to the case of two-way flow of power. It increases the flexibility of the converter station responding to the power change and ensures the DC voltage stability mean while.For the case that DC voltage bias occurs after primary voltage control, this paper analyses the causes of this phenomena. The loss estimation formula of convertor station is deduced. A DC grid power flow calculation method which based on Gauss-Seidel iteration is put forward. Combining two aspects mentioned above, a DC grid secondary voltage regulating method which aims at eliminating the system operation bias is proposed in this paper.A mathematical network equation of DC grid including DC power flow controller(DCPFC) which operates under multi-point voltage droop control strategy is established. Sensitivity of branch current to controller ratio is deduced. This paper proposes a DCPFC locating index based on sensitivity mean and variance, since then, a locating method to determine the best placement of DCPFC is put forward. The entropy theory has been introduced into DC grid; improved weighted power flow entropy in DC grid is proposed in order to conquer the limitation of power flow entropy in emphasizing the overload situation. Meanwhile, set the improved weighted power flow entropy as the objective to optimize the power flow of DC grid, and the transformer ratio of the DCPFC is obtained at the same time.For the study of DC grid primary and secondary voltage regulation, simulation models are built in PSCAD/EMTDC and simulation results are analyzed; for the study of locating DCPFC, C and MATLAB language are programmed to realize the method.