Voltage Control And Its Stability In DC Distribution

With the development of social economy and the progress of power electronics technology, flexible DC technology based on VSC converter has been widely applied in the power transmission field because of its own advantages. However, its application to the power distribution is still in its infancy. Voltage stability is an important index of AC distribution. Compared with AC distribution, DC distribution often contains more power electronics devices, which are vulnerable to the voltage fluctuation. Therefore, maintaining the voltage stability has significance to the operation of DC distribution. The research object of this thesis is DC distribution, whose coordination control,as well as local control was not only summarized, but also modeled, and then static and transient voltage stability were analyzed based on that.In terms of static voltage stability, based on the VSC substation local control linearized small-signal analysis model, droop control small-signal analysis model was deduced; and furthermore linearized equivalent impedance strategy was devised, by which the whole DC distribution can be simplified to the system comprised of one VSC and several linearized equivalent impedances one by one so as to carry out root locus analysis to discuss the small-signal stability of DC distribution quantitatively. In terms of transient voltage stability, a new AC current limitation segment was proposed between the inner and outer loop of local control, which ensures VSC transmit the maximum power via limiting the current reference of current inner control even in the case of significant DC voltage drop, in order to promote the recovery of DC voltage; and moreover, an energy function of DC distribution was deduced with the Lyapunov direct method in order to analyze the transient stability of DC distribution quantitatively and conveniently on the condition of DC voltage drop. Finally the validity of static and transient DC distribution voltage stability analysis was verified with PSCAD/EMTDC digital simulation software.