Power System Low-frequency Oscillation Caused By Open-loop Modal Resonance And Impact Of Wind Power Integration To Power System

Modern power system are characterized by interconnection of regional power grids and long distance power transmission.The local-area and inter-area low frequency power oscillations often occurred in real power systems.The small-signal stability has been a key factor threatening the security and stability of interconnected power grids.There are still some phenomena of low frequency power oscillations that could not be explained by existing mechanisms,such as negative damping mechanism and mode resonance mechanism.Therefore,it is urgent to develop the mechanism of power system low frequency power oscillation.Meanwhile,large-scale wind power have been interconnected to the power system.However,there is no conforming conclusion or effective research method in the study of the impact of grid-connected power system on power system low frequency power oscillation.Therefore,a series of work are carried out in this dissertation focused on the mechanism and the analytical method of power system low-frequency power oscillation.The contents of this dissertation are as follows:(1)The open-loop modal resonance is applied to study the dynamic interaction between a synchronous generator and multi-machine power system.The one-input one-output transfer functions of the synchronous generator and the multi-machine power system are built,and the distribution of corresponding closed-loop oscillation modes is deduced in mathematics.The results demonstrated that the closed-loop oscillation modes are distributed symmetrically about the resonance point,and closed-loop mode at the right side demonstrated the decreased stability of the power system.At the status of open-loop modal resonance,the synchronous generator has strong interaction with the multi-machine power system.The simulations of test systems and two real power grids in China are made to verify the theory.The open-loop modal resonance can explain some phenomenon of power system low-frequency power oscillation with unknown reasons yet.(2)The open-loop modal resonance between interconnected systems is applied to study the low frequency power oscillation at the inter-area transmission lines.Two-input-two-output open-loop transfer function models of two power system are built.The theory reveals that the inter-area power oscillation can be caused by open-loop mode resonance between two interconnected power systems.The theory successfully explain an accident of inter-area low frequency power oscillation occurred in a main regional power system of China.The inter-area power oscillation would be damped if the resonance of open-loop modes disappears by appropriate controls.(3)The factor decomposition method is proposed to estimate the impact of wind power on the power system low-frequency power oscillation.The power system small-signal stability is affected by grid-connected wind power through two factors,that is,the changing power flow and the dynamic interaction with system.The constant power displacing method and damping torque method are utilized to calculate the impact of the two factors respectively.On the basis of that,a model reduction method to calculate the oscillation modes of power system connected with wind power is proposed.(4)The impact of grid-connected wind farm on power system small-signal stability is estimated by residues.An aggregation method to build the model of grid-connected wind farm for studying power system low frequency power oscillation is presented.When the aggregation conditions are satisfied,the entire wind farm can be represented by the model of a single wind turbine,which reduces the computational burden and complexity of the small-signal stability analysis and ensures the accuracy of the power system oscillation modes.The creative achievements made in the dissertation and their significance are as follows.(1)The open-loop modal resonance theory is applied to analysis the dynamic interaction of a synchronous generator and power system,explaining the phenomenon which cannot be analyzed with traditional theories.The open-loop modal resonance theory enriches the analysis theory of power system low-frequency power oscillation and provides a new direction for the analysis of low frequency power oscillation,which has great significance in practical guiding.(2)The open-loop modal resonance between interconnected systems is applied to provide a new mechanism explanation for the low frequency power oscillation at inter-area transmission lines,and lays a theoretical foundation for the damping control of inter-area power oscillation.(3)The factor decomposition method is proposed to provide a refinement analysis method for the study of the impact of wind power connected to the power system,which could be applied on other active devices.(4)An aggregation method to build the model of grid-connected wind farm for studying power system low frequency power oscillation is proposed.The new method could not only reduce the calculation burden and complexity,but also grantee the correctness in calculating power system oscillation modes.