Research On Frequency-Coupling Admittance Modeling And Broadband Oscillation Suppression Method Of Direct-Drive Wind Farms

Energy is important in the progress of human civilization,which is closely related to people’s livelihood and economy.Developing the renewable energy is the effective way to implement General Secretary Xi Jinping’s policy of“four revolutions and one cooperation”for national energy security,which is of great significance for realizing sustainable development.However,with the increasing proportion of direct-drive wind turbines in the power system,the broadband oscillation problem of direct-drive wind farms connected to the grid will become more serious.These broadband oscillations will cause equipment damage and trigger protection,which not only critically restricts the efficient consumption of renewable energy,but also threatens the stably operation of the grid,and causes huge economic losses to the related companies and important loads.Therefore,it is necessary to research the small-signal admittance modeling and stabilizing control method of the direct-drive wind farms.This dissertation focuses on the frequency-coupling admittance modeling method for directly-driven wind turbines and wind farms,the frequency-coupling admittance measurement technology and equipment,the root cause of the broadband oscillation and the oscillation suppression method of the directly-driven wind farm under the weak grid.It was supported by the National Key Research and Development Program of China and other projects.The critical problems have been settled,such as frequency-coupling admittance modeling of the directly-driven wind farm and broadband oscillation issues under the weak grid.It is helpful for integrating the large-scale renewable energy resource.The main innovations are shown as follows:(1)Considering the impacts brought by the different control methods,the frequency-coupling admittance models of the current closed-loop controlled,direct power controlled and phase-locked loop based on linear active disturbance rejection controller(LADRC-PLL)controlled wind turbines are built.The admittance characteristics of the mentioned wind turbines are analyzed.In the current closed-loop controlled wind turbine,the admittance amplitude of non-diagonal elements is similar to that of diagonal elements in the low-frequency area,and far smaller than that of diagonal elements in the high-frequency area.In the direct power controlled wind turbine,the admittance amplitude of non-diagonal elements is smaller than that of diagonal elements in the low-frequency area,and the admittance amplitude of non-diagonal elements is similar to that of diagonal elements in the high-frequency area.The admittance characteristics of the LADRC-PLL and the current closed-loop controlled wind turbines in sub/supersynchronous frequency are different.When the wind turbine adopted LADRC-PLL,the frequency range with negative resistance within the low-frequency area of the wind turbine is reduced and its negative resistance is weakened,which is conducive to reducing the risk of system stability problems.(2)It is found that the phase of off-diagonal elements in the admittance matrix is changed with time,which presents a pseudo-time-varying admittance characteristic when the traditional frequency-coupling admittance measurement method is applied.The essential reason is that the phase obtained by FFT analysis is the phase of the signal at the started sampling time,not the initial phase of the signal;the numerator and denominator terms in off-diagonal elements are different due to the frequency inconsistency,the influence of the started sampling time on the phase cannot be offset.Therefore,this paper proposes a frequency-coupling admittance measurement method for directly-driven wind turbines based on phase correction,which can measure the frequency-coupling admittance characteristics accurately.Considering the influence of spectrum aliasing,frequency leakage,and fence effect,the 35kV/lMVA frequency-coupling admittance measurement device is designed and an outdoor prototype which is convenient for transportation and hoisting is developed.The frequency-coupling admittance measurement of high voltage resistance and Static Var Generator(SVG)verifies the accuracy and effectiveness of the developed prototype.(3)Considering the influence of transformer and collector line in wind farm,the frequency-coupling admittance models of current closed-loop controlled,direct power controlled and LADRC-PLL controlled directly-driven wind farm are established and analyzed respectively,and the correctness of the models is verified by simulation measurement.Based on the frequency-coupling admittance model,the broadband oscillation mechanisms of current closed-loop controlled,direct power controlled,LADRC-PLL controlled directly-driven wind turbines and wind farms connected to the weak grid are analyzed,respectively.The dominated admittance element in frequency area with negative resistance is easy to interact with the weak grid,and the weaker the grid is or the larger the control bandwidth is,the worse the system stability is.The stability of the wind farm is dominated by wind turbines,and the impact of the collector line on the system stability is relatively small;the admittance of transformer will have a certain impact on the system stability margin and the transformer should not be ignored when analyzing the system stability.Among these three control strategies,the LADRC-PLL controlled wind turbine and wind farm have stronger adaptability to the weak grid,and LADRC-PLL has stronger robustness to its control parameters and better dynamic performance.(4)Considering the influence of DC voltage outer-loop,phase-locked loop,current inner-loop and grid impedance,the small-signal transfer function model of the wind turbine is established.It is revealed that the grid impedance has adverse effect on the system stability through DC voltage outer-loop and phase-locked loop.In order to suppress this adverse effect,a dual axis voltages feed-forward oscillation suppression method for the grid-connected inverter of wind turbine is proposed.The d-axis voltage feed-forward at point of common coupling is used to suppress the influence of the grid impedance on system stability through DC voltage outer-loop;the q-axis voltage feed-forward at point of common coupling is used to eliminate the influence of the grid impedance on system stability through phase-locked loop.After applying the proposed method,the positive resistance components of diagonal element admittance in frequency-coupling admittance model of wind turbine and wind farm are increased,the system damping is enhanced,the risk of system oscillation is reduced,and the system stability is improved.