Study Of Low Frequency Oscillation In Power Systems With Large-Scale Integration Of Wind Power

With the growing interconnection among regional power grids, the scale of the modern power systems becomes larger and larger. In practical operations, power oscillations between the inter-area or intra-area grids happened more frequently than before due to lack of damping. Low frequency oscillation has become the primary factor affecting the power system dynamic stability and the transfer capacity. Meanwhile, the development of renewable technology makes the energy source diversified. Recent years, the wind power, as one of the most potential renewable energies, is increasingly integrated into the grid. Therefore, the study of low frequency oscillations of the power system with the large-scale wind power integration is an import issue.This dissertation studies several key questions such as the small signal stability of a wind turbine system, the influence mechanism of the power system with wind power integration, the forced oscillation source induced by a wind turbine system, its impact on tie-line power oscillations as well as mitigation of the oscillations. Several achievements and important conclusions have been gained in modelling and mechanism.1、The small signal stability of a direct-driven permanent magnet synchronous generator based wind turbine system (PMSG) is investigated. Firstly, A PMSG model suitable for small signal stability analysis is presented. The modal properties of a grid-connected PMSG wind turbine system are studied and the mechanism of the modal is revealed. Then the comprehensive impacts of the shaft model, the control strategies, the parameters of the wind turbine and the controllers, the operation points and lengths of the transmission line on the modal characteristic of the system are investigated by the eigenvalue analysis method. Meanwhile, the corresponding nonlinear model is set up in the PSCAD/EMTDC. This work offers a good fundamental of the wind turbine model.2、The influence mechanism of power systems with wind power integration is investigated. Firstly, a two-machine system model is developed to represent a power system integrated with large-scale wind power. The linearized state-space representation of the two-machine system is derived to reveal theoretically how the PMSG-based wind turbine affect the electro-mechanical oscillation mode of the grid from the viewpoint of eigenvalue. The theoretical analysis is validated by numerical modal analysis of two-machine system and nonlinear time-domain simulations in two-area four-generator system. During the validation, a total least square estimation of signal parameters via rotational invariance techniques (TLS-ESPRIT) method is applied to identify oscillation modes.3、The possibility of a PMSG based variable speed wind turbine system forming a low frequency disturbance source is discussed. Firstly, the aerodynamic model considering wind shear and tower shadow effects (TSWS) is adopted, and then the characteristic of mechanical torque fluctuation induced by TSWS is analyzed theoretically. The relationships are derived among the equivalent wind speed, mechanical torque fluctuation and the electrical torque fluctuation. The simulation is done to validate the analytical results and show the impact of different operation points and fluctuation components. This work illuminates the reason and condition for a wind turbine forming a resonant low frequency oscillation source.4、In the background of a wind farm may operate combined with an energy storage system, a control strategy of flywheel energy storage system (FESS) is proposed with the aim to eliminate forced oscillation power. Firstly, a reference power calculation model is designed with the inputs as active power of the wind farm and the rotor speed of a flywheel. The simulations are accomplished in Matlab/Simulink and the results show that the output of the FESS can compensate the the 3p oscillation power from a wind farm and therefore reduce the risk of a resonant low frequency oscillation induced by a wind farm.5、The impact of wind power fluctuations induced by TSWS on tie-line power of a weakly interconnected power grid is investigated. Considering the mutual influence of wind power and power systems, a frequency domain model of an interconnected power system with a wind farm is developed. The transfer function, which relates the tie-line power variation to the mechanical power variation of a wind turbine and the expression of the maximum magnitude of tie-line power oscillations are derived to identify the resonant condition and evaluate the potential risk. The effects of the parameters on the resonant magnitude of the tie-line power are also discussed. The frequency domain analysis reveals that TSWS may excite large tie-line power oscillations. Furthermore, the results of the theoretical analysis are validated through time domain simulations conducted in the two-area four-generator system and the Western Electric Coordinating Council 127 bus system, with PSCAD/EMTDC and DSATools, respectively.