Research Of Probabilistic Small Signal Stability Of AC/DC Hybrid System With DFIG Wind Farm

The High-voltage direct current transmission technology is widely used in interconnection of regional power grid at present.The interaction between AC and DC system has an effect on the small signal stability of the power system.The added dc system complicates the structure of the power system and the modeling of small signal stability.It is difficult to describe the system steady state quantitatively with the modal analysis method.At the same time,with the rapidly development of wind power in recent years,the appearance of large-scale wind power grid is common.on the one hand the shafting oscillation of DFIG do damage to the system stability,and the mechanism and inhibition of shafting oscillation are not enough for the complexity of the fan control.On the other hand,the random fluctuation of wind power output increases the uncertainty of the system operation mode,and there is limitation in the study of small signal stability under certain operation mode.Probability method is adopted to study the small signal stability under multi-operating points of ac/dc hybrid system with wind farm in this paper,containing the shaft oscillation,low-frequency oscillation.The major work is as follows:The models of ac/dc hybrid system containing typical dc system,synchronous generator,excitation system and prime mover,network and load are established with the idea of modularity.The problem of modal analysis modeling in ac/dc system is solved by using the insert modeling method(PMT).Considering the two random factors of synchronous machine output change and load fluctuation,the probabilistic power flow of the system obeying an arbitrary distribution arbitrary distribution can be calculated by combining central mixed moment and the cumulative amount.The probability distribution of the characteristic root and the damping ratio is calculated by combining the probabilistic power flow and the established model.The influence of integration of DC systems on the small-signal stability of AC system can be studied with the calculation of the participation factor of characteristic root and correlation ratio of low-frequency oscillation mode,To provide additional damp for the AC system with the quick and controllable dc power,the AC power modulation controller(DCM)using AC transmission line power as input signal is designed.The control effect of the controller is compared with the power system stabilizer(PSS)designed by the central frequency method.According to the control effect,the principle of coordination between DCM PSS parameter coordination is proposed.The model of AC/DC power system with doubly-fed wind farm is formed on the basis of the original system by using the plug-in modeling technology.Considering the wind power output fluctuation,synchronous machine output change and load fluctuation at the same time,The shafting oscillation and low frequency oscillation are studied based on five machine two regional system.A detailed mechanism analysis for the weak damping of shafting oscillation is carried out,and a shaft stabilizer whose input signal is slip radio is designed to suppress the shafting oscillation.The differences improvement of the three stabilizers of the shafting stabilization controller,DCM and PSS between the low frequency oscillation and the shafting oscillation are studied,According to the control effect,a method of parameter design for three kinds of controllers is proposed for improving the small signal stability of the power system.