Study On Characteristics And Mitigation Of Sub-synchronous Oscillation In AC Power Grid With Large Scale Wind Power

Developing new energy is the important measures to ensure energy security,climate change,wind power,photovoltaic(pv)is the main way of new energy in our country,in the northwest,north China,northeast China construction must kilowatt wind/photovoltaic base,using uhv dc sent out is the dominant form of renewable energy development and utilization in our country.In recent years,the wind power bases located in China's "three north" and other regions have repeatedly appeared obvious sub-synchronous oscillation phenomenon.The frequency range,amplitude and other main characteristics of sub-synchronous oscillation of wind turbine and thermal power turbine are quite different.Therefore,it is very necessary to study the sub-synchronous oscillation characteristics of AC power grid with typical wind turbine bases and its suppression technology.In view of the problems existing in the application of single synchronous oscillation suppression mode,a multi-level synchronous oscillation suppression system combining the fan side,wind field side and power grid side is proposed.In view of the existing power grid side parallel access type sub-synchronous oscillation suppression device of injection energy dilution effect,the side of the power grid is proposed series sub-synchronous oscillation suppression technology,demonstrates the series sub-synchronous oscillation suppression device,the feasibility of the grid side and study both trends of control functions series type time synchronous device key technology,The mathematical model,control strategy,operation mode,signal extraction and processing methods of the serial suppressor are systematically studied.Taking the typical power grid in Hami region of Xinjiang as the research scene,the parameter design and the simulation verification of the application effect of the serial sub-synchronous oscillation suppression device are carried out,so as to establish the "three lines of defense" of the mechanical-field-network suppression system.The effectiveness and feasibility of mechanical-field-network multi-level suppression system for sub-synchronous oscillation caused by wind power and other new energy sources are demonstrated theoretically.The above research results have strong pilot research value and guiding significance for improving the operational reliability of new energy power generation system,reducing the off-grid probability of units,ensuring the smooth transmission of new energy power and improving the consumption level of new energy,and have strong guiding significance in the later engineering application.The main research contents are as follows:Through the study,the sub-synchronous oscillation simulation and analysis model of Hami power network in Xinjiang is obtained,and the characteristics and propagation rules of the sub-synchronous oscillation of Hami power network are mainly revealed.The electromagnetic transient model of large-scale wind power grid-connected in Hami region of Xinjiang,including key nodes such as Hami-Dunhuan-Yandun-Shazhou 750 kV main grid frame,important 220 kV station and Tianshan converter station,is established,and sub-synchronous oscillation is deduced and reproduced.On this basis,the influence rules of AC system strength,wind speed,number of connected units,power grid operation mode and changes of network frame caused by faults on sub-synchronous oscillation frequency are obtained:The dominant frequency of sub-synchronous oscillation decreases with the weakening of the system strength,the decrease of the wind speed,the increase of the number of grid-connected units or the change of the system operation from large mode to small mode,and the influence of wind speed on the frequency of sub-synchronous oscillation is more obvious.The sub-synchronous oscillation frequency has broadband and time-varying characteristics,and the oscillation frequency ranges from 8Hz to 40Hz,and the oscillation amplitude ranges from 0.1A to 90 A,and the oscillation frequency occurs more frequently between 17 Hz to 32 Hz.In this paper,a "three-line" sub-synchronous oscillation suppression system of mechanic-field-network is proposed,and a joint suppression defense line including active sub-synchronous oscillation suppression of network side is constructed.The simulation models of fan converter,parallel compensation device and series compensation device are established,and the mechanism and characteristics of sub-synchronous suppression are analyzed.The equivalent impedance of fan converter can be improved to avoid sub-synchronous oscillation with power grid to the maximum extent.The shunt compensator injects sub-synchronous suppression current into the power grid in the form of current source,which is distributed according to the sub-synchronous impedance of the power grid.The series compensation device is connected to the power grid in the form of voltage source and has the function of blocking the sub-synchronous current of the power grid.Based on this,the sub-synchronous additional damping suppression system of fan converter,wind field parallel type and grid series type was constructed,forming "three lines of defense" for the joint suppression of sub-synchronous oscillation at fan side,wind field side and grid side.In this paper,a trispectral interpolation FFT algorithm based on four-term Rife-Vincent(?)self-convolution window is proposed,and the frequency extraction error is controlled within 0.1%.The amplitude extraction error is less than 1%.The phase extraction reduction error is less than 1%.A filter design method combining low-pass and working frequency band resistance is proposed to improve the detection response speed and amplitude accuracy under the time-varying frequency.A digital filter design method of zero phase shift compensation for the center frequency of frequency band is proposed to improve the phase precision to some extent.The phase shift generated in the 8 Hz-40 Hz synchronous oscillation frequency band of Hami Power Grid has lead and lag,so it is difficult to realize the phase compensation in the full frequency band.The signal extraction method based on sliding window FFT is explored.The self-convolution budget is introduced into the original sliding window algorithm to build the Rife-Vincent self-convolution window,and the tri-spectral interpolation algorithm with higher accuracy is tried to be used.A broadband signal extraction algorithm suitable for sub-synchronous oscillation is proposed,which is beneficial to the accurate analysis of later data.The design scheme of the main parameters of the network side series sub-synchronous oscillation suppressant is proposed.The main parameters of the 750kV/444MVA prototype are calculated and verified by RTDS experiment.In order to optimize the capacity of converter valve,the principle of determining the transformer ratio in series first,and then calculating the series number of converter valve is put forward.Converter valve is established direct current voltage frequency fluctuation more calculation model,put forward the trends of both the network side of the regulating series sub-synchronous oscillation suppression device main parameter design method,design the main parameters of 750 kv/444 mva prototype,compared to traditional power frequency parameters method,capacitance converter module parameters about two-thirds reduction,is of great significance to reduce the cost of device;According to the requirements of Hami network secondary synchronous suppression and power flow regulation,the technical and economic comparison between the centralized and distributed installation schemes is made.On this basis,a series sub-synchronous oscillation suppression device is designed,and the closed-loop test of the control system is completed based on RTDS.The voltage fluctuation is less than 10%,thus verifying the effectiveness of the design method and the suppression strategy.