Research On Robust And Optimization Control Strategy Of VSC-HVDC System

Compared with the traditional LCC-HVDC (current source converter-based high voltage direct current,CSC-HVDC) transmission system, VSC-HVDC (voltage source converter-based high voltagedirect current, VSC-HVDC) transmission system has obvious advantages in many respects, such as supplying power to passive network, independent control of active and reactive power, non-synchronous power grid interconnectio and et al. So it gains widenregardsand application all over the world. Especialy in our country, VSC-HVDC transmission project has been developed rapidly in recent years, successively with the Zhoushan,Nanao,Xiamen, Dalian and Zhang-Bei projects into operation.But there are still a lot ofproblems to be urgently settled on the VSC-HVDC control system.The converter station control of VSC-HVDC is mostly based on the traditional PI control which has weak robustness.The master-slave control and voltage droop control are mainly employed in the sysmetic control of VSC-HVDC, however both of these two methods can not take into account the reliability and control accuracy at the same time.This paper mainly aimed at improving the robustness, dynamic and accuracy of the VSC-HVDC control system. Some achievements have been obtained as follows.A robust control strategy based on quantitative feedback theory (QFT) is proposed.To improve the robustness of converter station control and make it easy for the engineers to understand, on the basis of studying the d-q vector decoupling double loop control structure, applying the QFT, this paper designs a kind of dual loop robust control strategy for VSC-HVDC system. Thesimulation results in PSCAD show that, the control effect of the proposed control strategy is better than that of the PI control under such conditions, significant changes in converter station, the AC side short-circuit and voltage leap.A discrete mathematical model and a nonlinear robust control strategy for VSC-HVDC systems which are suitable for computer sampling are proposed. Because VSC-HVDC is a multi-input and multi-output strong nonlinear system, nonlinear feedback linearization method is used to establish the discrete linearizedmathematical model of VSC-HVDC system.Then the control strategy is designed based on the discrete sliding mode control method. The experimental results in MATLAB/SIMULINK shows that, the proposed control strategy canmake the VSC-HVDC system remain stable under suchperturbation conditions, reverse/step changeof converter operation instructions and the converter parameters change.A nonlinear control strategy of VSC-HVDC system based on passive control and slide mode control is proposed.Aiming at the nonlinear characteristics of VSC-HVDC system, a kind of passive control strategy based on PCHD model is proposed, which can make the VSC-HVDC system have better static and dynamic performance. But the strategy depends on the accurate model of the system parameters. When the system suffered disturbance, The variation of operating parameters of the system will lead to the poor robustness of the designed controller.In order to solve this problem, the sliding mode control is added to the proposed passive control strategy. The experimental results in MATLAB/SIMULINK show thatthe control effect of the proposed control strategy makes the VSC-HVDC system operate well under such conditions, significant changes in reverse/step changeof converter operation instructions,the AC side short-circuit andthe converter parameters change.A kind of control method which can improve the control accuracy of multi terminal VSC-HVDC system is put forward.The reference values of power and voltage of existing multi VSC-HVDC system based on droop control are directly set to expect ratings. But under this control method, there is always a big deviation between the actual value and the expected value. This paper analyzes the reasons for the existence of deviation. That is, this setting does not consider the effect of the DC line resistance and converter loss. Depending on the droop characteristic, it will cause a large power deviation. This paper proposes that use the steady-state operating point as the control reference value of the converter. In this method, the operation results in PSCAD show that this method can effectively reduce the deviation between the actual value and the expected value and improve the control precision of the multi-terminal system. At the same time, to ensure that the steady state operation points can be calculated quickly and accurately, a kind of node-currentpower flow calculation method based with the consideration of droop control of AC/DC hybrid system is proposed.The experimental results in MATLAB show that the proposed method is more rapid, accurate and robust than the existing method based on node-power.