Research On Control Strategies Of VSC-HVDC System Under Unbalanced Condition

VSC-HVDC is a new type of HVDC transmission technology which is based on voltage source converter. Besides the advantages of traditional HVDC transmission, it also can control active power and reactive power independently, supply power to small isolated distance load directly, connect scattered power supply, improve power quality, etc. In recent years, research institutions and scholars have been researching and discussing the VSC-HVDC model and control strategy under balanced condition extensively. However, in the actual operation, balanced electric power grid is a special case, When AC system fault unbalanced, it may cause system fault protection movement and threat the safety of converter seriously, it may even burn device. Therefore the control strategy for unbalanced VSC-HVDC system is directly related to its function and security, so it has a great importance for the engineering. This paper researched the control strategy under unbalanced VSC-HVDC system. The main contents are as follows:First of all, this paper introduced the development of VSC-HVDC technology and its technical characteristics, then this paper introduced the application field of VSC-HVDC technology and its engineering application around the world, on the basis, this paper analyzed the necessity of VSC-HVDC project construction.Secondly, this paper introduced the basic working principle of VSC-HVDC, expounded the system’s main composition and how each part works, researched and established the mathematic model of VSC-HVDC under balanced conditions.Thirdly, this paper researched the control strategy of current decoupling controller and power controller when VSC-HVDC system is under balance conditions, and then concluded the main control system structure of VSC converter. In PSIM simulation environment, this paper built VSC-HVDC simulation model system under balanced condition. The experimental results show that the system has good steady and dynamic performance under various conditions.Fourthly, this paper analysis and simulated the negative influences to VSC-HVDC system when it’s connected to unbalanced grid., researched the detection methods of positive and negative sequence components, designed synchronous phase and positive and negative sequence components detection system under unbalanced condition, The simulation based on PSIM verified that this method solved the delay and wave problem of conventional detection method.Fifthly, in order to solve the problems of unbalanced VSC-HVDC system, this paper researched control strategy of VSC-HVDC under unbalanced condition. This paper established positive and negative sequence mathematical model of VSC converter when the AC system fault unbalanced, designed state feedback linearization current decoupling controller based on nonlinear system feedback linearization theory. And finally, this paper designed active power balance control system and negative sequence voltage compensation control system under unbalance condition. Digital simulation based on PSIM demonstrated the designed controller has good performance in a variety of asymmetric fault conditions.