Research On Analytical Model And Resonance Feature Of Time-varying Interharmonics In The Wind Power System Based On Doubly-fed Induction Generator

A new energy power system is gradually taking shape,which utilizes the complex AC/DC transmission in the main grid and has the feature of multi-source and multi-conversion.In response to demands that consuming clean energies,renewable energies that connected to the grid are increasing,and power electronic devices that flexible and controllable are becoming more popular.As a result,power quality phenomena and instability problems that random and time-varying are becoming more prominent and complicated.In this context,the wind power system,the main source of new energy in China,is prone to frequent engineering cases.Subsynchronous interharmonics and their oscillation events lead to the off-grid of many doubly-fed induction generators(DFIG)and the damage of equipment,resulting in voltage fluctuation and flicker,which have affected the quality power supply and reliable operation of the power system and caused serious economic losses.The above problems have not been completely solved,but with the wide access of renewable energies and smart conversion equipment,the potential risks have increased.Thus,this paper determines the theme to focus on the time-varying characteristics of interharmonics and their relevant resonance in the wind power system based on DFIG.In order to solve the problem of analysis and evaluation on interharmonics and subsynchronous resonance that are random and time-varying in the new energy power system,and then to provide theoretical support for the suppression of power quality and small disturbance instability,the following studies are carried out.Aiming at the time-varying interharmonics in the wind power generation system based on DFIG in recent years,considering the disturbance voltage component generated by the rotor side converter(RSC)on the inverter side and according to the voltage equation and flux linkage equation of DFIG,the analytical model of the stator interharmonic current in DFIG caused by the coupling of the disturbance component and the slip frequency is established,by coordinate transformation and mathematical derivation.The time domain simulation verifies the accuracy of the model.Based on the proposed analytical model,the transition law of RSC disturbance component in DFIG and the time-varying characteristics of induced stator interharmonic current with wind speed are analyzed.The results show that the interharmonic current induced by the RSC disturbance voltage on the stator side of DFIG has time-varying characteristics both on the frequency and root-mean-square.The frequency range of the subsynchronous interharmonic current is likely to match the natural frequency of the shafting of the thermal power plant,causing its subsynchronous oscillation,which seriously threatens the safety and stable operation of the generator and the power system.Aiming at the time-varying subsynchronous resonance in the series-compensated transmission system based on DFIG,in the three-phase static coordinate,considering the disturbance phase sequence and using both the small signal analysis and the harmonic linearization method,the method by increasing the related factor gradually is adopted.Thus,an input impedance model of DFIG in the frequency domain is established,which consists of the mutual inductance for excitation,RSC and phase-locked loop(PLL).Based on this,the topology of equivalent circuit is given,and the physical meaning of the phase-locked loop is explained.The analysis shows that the influence of the phase-locked loop on the input impedance of DFIG is equivalent to a voltage-controlled voltage source(VCVS)connected in series with the rotor-side controller branch.The stability criterion of the series-compensated transmission system based on DFIG is introduced from the perspective of impedance.The effects of excitation,disturbance phase sequence and PLL parameters on the input impedance and stability are analyzed.Then,the time-varying characteristics of the input impedance are revealled in deep,and the physical mechanism of the variation of subsynchronous resonance(SSR)frequency with time is clarified.Combining with a practical engineering,the further discussion is focused on the mechanism of the SSR phenomenon in the multi-machine system of Guyuan.The studies implies that because of the variation of wind speed with time,the input impedance of DFIG also varies with time,which makes the frequency and damping time-varying in the futher.For a series-compensated transmission system based on DFIG,the higher wind speed will result in the higher SSR frequency and the stronger damping,and the increase of the series compensation degree will widen the frequency range of the time-varying SSR.The time domain simulation verifies the correctness of the model and derived conclusions.Aiming at the problem about quantization on the time-varying characteristics of the subsynchronous resonance in the DFIG-based series-compensated transmission system,in two cases that ignoring and considering the influence of phase lock loop,based on the established input impedance model in the frequency domain and the least square fitting respectively,the mathematical relationship between wind speed and resonant frequency and damping characteristics are established,and after introducing the probability distribution of wind speed,two probability evaluation methods constructed by Monte Carlo simulation and Kernel smoothing density estimation are proposed to quantify the time-varying SSR of a DFIG-based series-compensated transmission system.In a typical case,the influence of the wind speed and series compensation degree on the probability density distribution of the resonant frequency and damping characteristics is investigated,so is that on their cumulative probability distribution.The time-varying characteristics of the resonant frequency and damping are quantitatively described,and the variation characteristics of their bandwidth are revealed.The resulting divergent SSR frequency implies that its bandwidth is within a limited range,and the probability density of frequency on the bounds are small and in the medium great.The resulting resonant frequency and damping characteristics jointly demonstrate that,the wind speed affects the probability distribution of resonant frequency and damping within the interval,while the series compensation degree determines the interval position of in the axis.Aiming at the time-varying SSR of the DFIG-based series-compensated transmission system in Guyuan,a 35kV/10MVA subsynchronous resonance dynamic suppressor(SSR-DS)is developed based on the parallel voltage source converter(VSC)technology.From the perspective of the controlled source and equivalent impedance,the general and simplified form of equivalent circuit to analysis the principle why the parallel VSC can suppress the subsynchronous resonance in doubly-fed wind farms is given.The suppression effect is illustrated by the response equation of subsynchronous current.The results show that the key of SSR-DS to suppress subsynchronous resonance is that the damping control link changes the order and coefficient of the free response equation of subsynchronous current,which changes the oscillation mode and the system damping in the futuer.Then based on the analysis of the structure and implementation scheme of SSR-DS,the control strategy is designed according to the proposed adjunction method of damping current in this paper.The optimal parameters,damping effect and dynamic performance are studied by the digital-physical closed loop simulation experiment.The experimental results show that the proposed SSR-DS can intensively solve the time-varying SSR in the large wind farm group.