Research On SSCI Caused By Grid-integration Of Doubly Fed Wind Power Generation Via Series Compensated Lines

Doubly Fed Induction Generators(DFIG) are being more and more widely used in wind generation area with its constant frequency operation under different rotor speed and flexibility on control. With the expansion of the scale of wind farms and wind farms being far away from load center, high voltage transmission line with series compensated capacitors(referred to as the series compensated line) is used to transport large-scale wind power. This may cause sub-synchronous oscillation aroused by the sub-synchronous control interaction(SSCI), leading to a large number of generators closing down. This study analyses the SSCI problem caused by doubly fed wind power generation system when connected to power grid via series compensation by using the method of eigenvalue analysis and transfer function analysis.Firstly, this paper establishes doubly fed wind power generation system model, which includes the mathematical model of shafting, doubly fed induction generators, transmission lines and double fed generator control system. At the same time, this paper briefly introduces the control principle of doubly fed wind power generation system, including the maximum wind power tracking and controller of the back-to –back converter.Secondly, by liberalizing the system state equations, this paper calculates the eigenvalues and eigenvectors of the coefficient matrix, getting the oscillation frequencies, damping ratios and relative factors of different oscillation modes of the system. Research shows that the oscillation mode of SSCI is caused by the state variables of rotor side converter controller, generator and line. The increase of the proportionality coefficient and integration coefficient of current loop controller and the series compensated capacitors will exacerbate the oscillation of SSCI and the proportional coefficient have a greater impact on the system damping than the integral coefficient. But the increase of line resistance will increase the damping of system and inhibits SSCI. What’s more, the parameters of rotor-side converter’s outer loop controller have little effect on SSCI.Thirdly, this paper builds a transfer function model, in which sub-synchronous turbulent current functions on generator and inherent branch of the line via rotor side inverter controller, concerning about electromagnetic transient process of DFIG and lines. By changing system parameters like the proportional coefficient of rotor side inverter controller, wind speed and degree of line series capacitor compensation, the change of model oscillation is observed. The results reveal that the increase of controller proportional coefficient, the decrease of the wind speed and the raise of the series compensated capacitor will aggravate the sub-synchronous oscillation caused by SSCI. Meanwhile the comparison of the transfer function model and system simulation model verifies the consistency. Lastly, by conducting pole analysis to the transfer function model, results show that the increase of controller proportional coefficient, the decrease of the wind speed and the raise of the series compensated capacitor can lead to the movement to the right side, proving the rationality of the model.