Research On Internal Model Decoupling Control Of VSC-HVDC System

With the development of direct current transmission technology,VSC-HVDC(Voltage Source Converter Based High Voltage Direct Current)as a new generation of direct current transmission technology is widely applied in many fields such as wind power grid connection,island power supply,urban power distribution network capacity expansion and transformation,etc.Compared with the LCC-HVDC(Line Commutated Converter High Voltage Direct Current).VSC-HVDC can improve the power quality of the power grid and increase the transmission capacity,with higher reliability,economy and flexibility.The VSC-HVDC system is a multiple input and multiple output system.Due to the weak inertia of DC transmission line,there are different degrees of cross coupling between the controlled variables of each converter station.The traditional PI double closed-loop decoupling control strategy only solves the coupling problem of active and reactive power components in a single converter station,without considering the coupling problem between different converter stations.In this paper,the research on the inter station decoupling control of VSC-HVDC system is carried out:Firstly,the structure and control principles of the double-ended VSC-HVDC system are studied,and the small-signal mathematical models of the rectifier station and the inverter station are derived respectively.The global small-signal model of the double-ended VSC-HVDC system is built in MATLAB software.According to the small-signal model,By analyzing the complex coupling phenomenon between the controlled variables of the double-ended converter stations,and it is found that the disturbance of one terminal converter station will be transmitted quickly through the DC line to the other end of the converter station.Secondly,in order to restrain the coupling between the converter stations and improve the independent operation ability of the converter stations,the structure and parameters of the decoupling controller between the internal model stations are designed based on the internal model control principle and the small-signal mathematical model of the double-ended VSC-HVDC.It is easy to debug and realize the decoupling between stations and the robustness of the system.In order to maintain the steady-state gain of the original system to the maximum extent,the equilibrium singular value perturbation approximation method is used to reduce the order of the designed controller,so as to facilitate the engineering application.Finally,the inter station decoupling control effect of IMC is verified in MATLAB simulation platform,and the VSC-HVDC inter station decoupling control system is built in PSCAD electromagnetic transient simulation platform to simulation the active power disturbance and reactive power disturbance of rectifier station,DC voltage disturbance and reactive power disturbance of inverter station,and the results verify that the designed inter station decoupling controller can effectively cut the complex interaction between weak double-ended VSC-HVDC converter stations reduces the mutual interference between different converter stations and improves the independent operation ability of VSC-HVDC converter stations.