Study On The Characteristics Of Sub/Super Synchronous Oscillation And Propagation In Large Scale Wind Power Transmission System

Wind power penetration continues to increase as a pillar of China’s new energy generation structure system,particularly in the three-northern regions of China,forming a new structure of large-scale wind power transmission system containing wind-fire bundling and HVDC transmission.The geographical location and grid structure result in a weak local AC grid,and wind farm accessibility is extensively concentrated,triggering a sequence of oscillatory instability events.The new form of oscillations included or produced by wind power grid connection differ greatly from traditional low-frequency oscillations and sub-synchronous resonance,as well as the phenomena of power oscillations propagating and spreading across a broad range in the grid.When the frequency of the propagated oscillation in the grid is near to the frequency of the shaft system mode of thermal power units,the oscillation triggers the shaft system torsional vibration and seriously compromises the grid’s safe and stable operating.The current research on the novel oscillations focuses on the sub/super synchronous frequency band,and no unanimous conclusion on the morphological characteristics of the sub/super synchronous oscillation are formed.As for the large-scale wind power transmission system where the oscillation propagation phenomena occur,there is still debate over the features of oscillation propagation in complicated AC and DC grids.This system includes many components,including wind farms,thermal power plants,AC grids,and HVDC transmission lines,etc.As a result,it is important to concentrate on the morphological characteristics of sub-synchronous oscillations in various scenarios,examine the major factors of affecting oscillation propagation,define the path that oscillations propagate in the system,and investigate the precise distribution of oscillation power.Finally,theoretical guidance for the detection and suppression of sub-synchronous oscillations must be provided.In response to the aforementioned issues,this dissertation focuses on the morphological properties of the new oscillations caused by the grid connection of wind turbines and the wide-area propagation of oscillations.The main research contents and results of this dissertation are as follows.1.To address the morphological characteristics of the new sub/super synchronous oscillations triggered by the grid connection of wind turbines,the relevant recorded waveform data of the occurred sub-synchronous oscillation events are analyzed,and the grid connection models of wind farms based on doubly-fed wind turbines and direct-drive wind turbines are built respectively,and the influence of system parameters on the morphological characteristics of the oscillations is analyzed in depth.The oscillation pattern is influenced by grid parameters,converter control parameters and other conditions,and the oscillation frequency has a wide range of time-varying characteristics.The mechanism involves the dynamic interactions of wind turbines,control of converter,series complementary lines,weak AC grids and HVDC transmission lines.2.For the issue of influencing factors in the propagation of sub-synchronous oscillations,a typical scenario of oscillation propagation phenomenon in the power grid in the sub-synchronous oscillation event in Hami,Xinjiang is analyzed.A maximum information coefficient correlation algorithm is proposed to construct an oscillation evaluation system based on the need to selecting the parameters in the system with high correlation with oscillation propagation.The correlation analysis findings suggest that the current of AC lines and the impedance characteristics of AC lines at sub-synchronous frequency are the key influencing factors in sub-synchronous oscillation propagation in AC lines.And the key influencing factor of oscillation propagation in DC lines is the impedance characteristics of AC lines at sub-synchronous frequency.3.The analytical expression of sub synchronous power is derived based on the equivalent circuit model theory,which can be completely determined by the key influencing factors of oscillation propagation,to address the issue of the propagation path of sub synchronous oscillations in complex grids.The numerical properties of subsynchronous power in various modes are investigated,and an approach for estimating the propagation path of sub-synchronous oscillations is proposed.Sub-synchronous power models of single-oscillation mode and multi-oscillation mode are created.The approach calculates the sub synchronous power of each component of the system in a single oscillation mode,defines the sub synchronous power source and load in accordance with the sign of power,and then determines the oscillation propagation path through the source-load power flow Following this,the viability of the above approach is confirmed in an equivalent grid-connected system composed of three wind farms,and the accuracy is examined in an equivalent simulation system in Hami,Xinjiang.4.The sub synchronous power is divided into AC and DC components in order to address the quantitative calculation of the sub synchronous oscillation propagation in the grid.Based on their respective analytical expressions,the current distribution coefficient and the impedance distribution coefficient are defined,and a calculation formula for the sub synchronous power distribution coefficient is derived to complete the quantitative calculation of the sub synchronous power distribution in each propagation line of the system.This dissertation completely explain the propagation mechanism of subsynchronous oscillation in complex AC-DC grids by combining the previous contents: 1)The propagation path of sub-synchronous oscillation is determined by the power flow between the sub-synchronous power source and load of single oscillation mode.2)The sub-synchronous power in each line of the propagation path is divided according to the proportion of the distribution coefficient.The larger distribution coefficient indicates more obvious sub-synchronous oscillation phenomenon in the line.3)The subsynchronous power distribution coefficient of thermal power units is larger when the oscillation frequency is close or equal to the frequency of the shaft system.It results in a large amount of sub-synchronous power flowing to the units and excite the shaft system torsional oscillation under the corresponding mode,which has a significant impact on the operation of the thermal power units.