Studies On Voltage Stability Of AC/DC System Based On Improved Continuation Method And Bifurcation Theory

Since the seventies in twentieth century, voltage collapse incidents occurred frequently. This has given rise to serious concerns on voltage stability at home and abroad. Voltage instability has been widely considered to be one of the main threats to security and stability of power system. HVDC has obvious advantages on long distance, high capacity transmission and interconnection of power system. It will play an important role in power transmission from west China to east China and Nation-wide interconnection. According to the plan, in the next two decades, it will appear more than 20 DC lines in China. This will bring new challenges in voltage stability. In this paper, after analyzing static voltage stability of AC/DC power system by improved continuation method based on flow equations, the dynamic voltage stability was studied by bifurcation theory of nonlinear dynamics based on differential algebraic model. And several types of bifurcation points closely related with the voltage stability were discussed in depth. The work of this thesis is shown below:1. Based on analyzing the merits and drawbacks of the uniform and alternating iteration method, an advanced AC/DC power flow algorithm is presented, which considers the boundary constraint of the DC variables. The power flow equations of the AC and DC system are all solved by Newton-Laphson method. It makes the Jacobi matrix of DC system reversible. The reason why the element of correctional Jacobi matrix of AC system emerges imaginary part during the course of calculation was analyzed, and gave the way to adjust the tap of converter transformer in folw calcaulation.2. The improved continuation method was presented based on local parametrization. This method would not increase the dimension of Jacobi matrix during the correction, and the adaptive step size strategy was used, so the computational efficiency is raised. In the process of solving the static voltage stability margin, the coordinated control among the input of reactive power compensation device, the adjustment of the converter transformation ratio and the switch of the DC control modes was studied in detail. On this basis, from the initial direction of load increasement, the iterative method was used to find the most dangerous load growth direction, and the closest power limit point of AC/DC power system was seeked.3. The specific steps and techniques of continuation method for seeking the Hopf bifurcation (HB), singularity induced bifurcation (SIB), saddle node bifurcation (SNB) and limit induced bifurcation (LIB) points was dicussed.During the calculation, the critical eigenvalue was gained by eigenvalue sensitivity analysis, so the bifurcation detection functions were more versatile. On this basis, the influence on bifurcation valuses of DC control modes, the scaling parameters and the integration parameters of DC controllers, the short circuit ratios of converter station were analyzed. And the impact of VDCOL on systam stability was studied.4. The types of LIB were analyzed, and discussed the reason why the SIB would not occure while the hard limit was encountered. While considering the excitation voltage and DC controller limitations, we found that the excitation voltage limit value or the product of transformer ratio and the cosine value of the extinction angle on inverter station should be larger than a critical value while LIB was occurred and the formula for critical value of LIB in defferent types was derivated. The sensitivities of critical value with respect to parameters were calcaulated, from which some bifurcation points were eliminated and the stable region was augmented.5. A method of Multi-parameter bifurcation analysis is presented for the differential algebraic model of AC/DC power system. Although the dimension of the system equations is increased, the Jacobi matrix has an extremely sparse structure. Therefore the method is tractable for large systems. For single-infeed HVDC power system, choosing the reference voltage to the AVR, the parameter of DC subsystem and the the limit of converter transformer ratio as controllable parameter separately, the influence of these parameters on dynamic load margin and bifurcation types are studied. For multi-infeed HVDC power system, under typical DC control mode, the impedance of connection line between DC subsystems and more than one reference DC voltage interaction together influencing on system bifurcations are analyzed.This project is supported by the Key Project of the National Eleventh-Five Year Research Programme of China (No 2006BAA02A17).