Wide Area PSS And Coordinated Control Of VSC-HVDC

With the development of the national economy, the scale of power system is expanding and the structure is getting complicated increasingly. The traditional single transmission mode has been replaced by the AC/DC hybrid transmission form. The regional interconnection power grid is capable in improving energy utilization rate and improving the quality of electric energy, while at the same time, it also brings new challenges to the power grid stability. The occurrence of low-frequency oscillations in power system will lead to system splitting. Therefore, to improve the stability of power system has important practical value.Voltage source converter based high-voltage direct current (VSC-HVDC) technology has unique technical advantages by adopting turn off devices and pulse width modulation technology. There is plenty of VSC-HVDC project have been put into operation, thus, it is significant to study the control system of VSC-HVDC in depth. This paper derives differential-algebraic equations of VSC-HVDC system and the mathematical model of VSC-HVDC controllers. The dynamic characteristics of VSC-HVDC system is analyzed by MATLAB simulation program.The particle swarm optimization (PSO) algorithm is widely used in the optimization of complex problems, while the PSO algorithm itself has some drawbacks. This paper try to improve the performance of classical PSO algorithm by employing the cloud model and introducing the migration thought, so is the particle swarm optimization based on cloud model (CPSO) composed. The optimal algorithm of CPSO can improve the convergence speed and avoid falling into the local optimum in the meanwhile.The wide area PSS can be designed by using the wide area signals provided by WAMS, which has obvious suppression effect for interval oscillation. This paper introduces the basic theory of PSS and the structure of wide area PSS; describes the designing of the excitation system composed of wide area PSS. There will be problems such as the packet-loss and communication delays during grid communication. The prediction method of least squares support vector machine (LS-SVM) is introduced in this paper, which can reduce the influence of continuous packet loss happens during the communication process of WAMS effectively. In addition, the Time-varying delays can be converted into fixed time delays by employing LS-SVM forecasting and curve smoothly fitting method, so as to improve controlling performance. Wide area PSS is cable for improving power system stability, and so is the VSC-HVDC system. Though wide area PSS and VSC-HVDC system both help in a general improvement of power system stability, there’s few academic tries to combine the researches of wide area PSS and VSC-HVDC. Coordinated design and optimization of wide area PSS and VSC-HVDC control system is employed and the effect of the control strategy on power system stability was explored in this paper. Firstly, the control system of VSC-HVDC including the inner loop controller, the outer loop controller and the additional damping controller is designed. Secondly, CPSO is employed to optimize the parameters of the wide area PSS and VSC-HVDC controller. Four-machine two-area testing system employed VSC-HVDC system is used to test the performance of the coordinated design and control system. The simulation result shows that the coordinated design of wide area PSS and VSC-HVDC control system is able to improving the damping and the stability of power system efficiency.