Design Of Inter-Stations Decoupling Controller In VSC-HVDC System

With the development of power electronic devices,VSC-HVDC(Voltage Source Converter based High Voltage Direct Current)transmission technology has been gradually matured,and has good application prospects in wind power grid-connected,asynchronous grid interconnection,large-scale urban DC distribution and island power supply projects.However,due to the weak inertia of DC line,there is a complex coupling relationship between converter stations.The frequently-used PI double closed-loop controller realizes the decoupling between active and reactive power control variables in single station,ignoring the influence of coupling between converter stations in the system.Firstly,according to the working principle of VSC converter,the mathematical model of converter is obtained in dq rotating coordinate system.The mathematical models of converter AC side,converter DC side and DC line are deduced through the topological structure and operation state of VSC-HVDC system,and the global small signal mathematical model of the control object of VSC-HVDC system is obtained.The classical PI double closed-loop control strategy is simulated to verify the accuracy of the small signal model of the control object.Secondly,the disadvantage of diagonal PI double closed-loop controller is pointed out that it is difficult to realize decoupling between stations.A new structure of decoupling controller between stations is proposed.The controlled variables of the other converter station are introduced into the controller of each converter station to realize decoupling.According to the control objective,the controller parameters are solved by H_∞ mixed sensitivity algorithm.After analyzing the characteristics of the object model,the appropriate mixed sensitivity weight function is set to calculate the H_∞ decoupling controller between stations.Then,the controller is reduced by using the balanced truncation method after analyzing the Hankel singular value of the state matrix of the controller and choosing the appropriate reduced order.The controller is further reduced by using the dipole cancellation method and the dominant pole method of classical control theory,and the reduction controller is easy to be realized in engineering.Finally,the control performance of the controller is analyzed on the platform of MATLAB,and the step response of the traditional PI control strategy is compared with that of the Bird diagram and small signal model.It is proved that the H_∞ decoupling controller has better decoupling effect.Active power disturbance and short circuit fault of DC line are experimented on the electromagnetic transient model of PSCAD,and verify that the H_∞ decoupling controller designed in this paper can effectively weaken the interaction between two converter stations.