Study On The Influence Of Large-Scale Wind Farms Connected To The Grid On The Damping Characteristics Of The Subsynchronous Oscillation Of Thermal Power Unit

With the introduction of the "dual carbon" goal,the installed capacity of new energy sources such as wind power and photovoltaics in China will further expand.Combined with the imbalanced distribution of energy and load centers in China,we need to vigorously develop ultra-high voltage and long-distance power transmission mode.At present,the wind-fire bundling power transmission form has become one of the main ways to solve the problem of wind power consumption in China.However,the direct-drive wind turbine itself has the problem of sub-synchronous oscillation.Therefore,it is of great significance to study the influence of direct-drive wind farms connected to the grid on nearby thermal power plants.The complex torque coefficient method is one of the main methods to study the subsynchronous oscillation problem of thermal power units,which can quantitatively analyze the system subsynchronous oscillation characteristics.This paper focuses on the wind-fire bundling system based on direct-drive wind turbines,and analyzes the influence of the grid connection on the damping characteristics of the subsynchronous oscillations of the thermal power plants.The main contents are as follows:(1)The basic principle of the complex torque coefficient method is introduced,and then based on this principle,the state space equation of the electrical part of the wind-fire bundling system is written and the analytical expression of electrical damping is obtained.Finally,the test signal method is used to verify the correctness of the theoretical derivation.In the process,considering the influence of inverter-side switching devices,it will cause large errors to obtain the electrical damping curve in the time domain.Therefore,the grid-side inverter is equivalent to the controlled source model,and the correctness of the simplified model is verified by scanning the harmonic impedance of the direct drive wind turbine.This provides a theoretical and model basis for the following content.(2)The two concepts of oscillating mode influence factor and electrical damping extreme value are proposed,and it is deduced that the extreme value of electrical damping is related to the eigenvalue and residue.The influence factor of the oscillation mode determines whether the electrical damping curve has a sudden change,and the electrical damping extreme value determines the magnitude of the sudden change of the electrical damping curve at the frequency.Starting from the analysis of the causes of subsynchronous oscillations in the traditional AC power grid,the relationship between the grid-connected oscillation mode of the direct-drive wind turbine and the electrical damping curve of the unit to be studied is analyzed by analogy.The analysis results show that,under certain control parameters,the grid connection of direct-drive wind turbines will cause the electrical damping curve to show negative damping near the modal frequency.(3)Analyze the influencing factors of the extreme point of electrical damping,and propose a thermal power unit subsynchronous oscillation suppression scheme based on the optimization and adjustment of the direct drive wind turbine’s control parameters.Based on the derivation of the aforementioned electrical damping extreme value,it can be seen that the amplitude of the electrical damping extreme point is determined by the real part of the characteristic root of the corresponding oscillation mode and the real part of the residue.Therefore,the system structure and the direct drive wind turbine control parameters are analyzed.The analysis results show that the direct drive wind farm may cause the negative frequency band of the electrical damping curve under the condition of low output.The increase in the number of grid-connected wind turbines and the weakening of the strength of the connected AC grid will worsen the negative damping amplitude at the corresponding frequency;in the wind turbine control parameters,increasing the voltage outer loop proportional coefficient and the current inner loop proportional coefficient can suppress the negative damping.This paper verifies the correctness of the analysis results by combining with time domain simulation.