Investigation On High Frequency Resonance Damping Method For Grid Connected DFIG Wind Power System

With the rapid development of worldwide renewable energy power generation,the status of wind power generation system(WPGS)is increasingly important in energy supply,in which DFIG based WPGS is the mainstream type of wind power market,due to its advantages of small converter capacity and flexible operation control.The wideband resonances caused by the interactions between WPGS and power grid,have become a key issue which affects stbale operation of WPGS.The HFR(High Frequency Resonance)in grid connected WPGS is one of the wideband resonances,the corresponding damping method is an urgently needed technique for the grid connection of WPGS.Proposing the high-performance damping method can significantly improve the safty and stability of WPGS,and provide the important technical support for the sustainable development of WPGS.This paper aims at the HFR of grid connected WPGS,makes a deep research for the HFR damping technique from the following three aspects,the adaptability for different grid status,the adaptability for the different WPGS parameters,the damping method for the HVDC(High Voltage Direct Current)connected WPGS.The specific research contents and contributions are presentd as follows,1.For the sence where parallel compensation capacitors switches and the background harmonics exist in power grid,the existing damping methods can be invalid.Aiming at this failure,this paper analyzes the characteristics of HFR with considering parallel compensation capacitor variation and background harmonics,then investigates the impedance requirements for DFIG which can cope with resonance frequency variation and is compatible with grid harmonic suppression,thereby an VFR(Virtual Variable-Frequency Resistance)is proposed.Furthermore,Based on the dual-converters characteristics of DFIG system,a multi-objective Collaborative control scheme of‘resonacne-harmonics’for RSC(Rotor Side Converter)and GSC(Grid Side Converter)is proposed.Through the analysis for the reshaped DFIG impedance under the different ratio of generator side impedance and GSC side impedance,a multi-objective allocation principle for RSC and the GSC is studied.With the allocation principle,resonance damping and harmonics suppression can be achieved effectively and flexibly.2.The variation of DFIG parameters can degrade the performance of the existing damping methods,for this issue,this paper proposes an HFR damping conception based on the H_∞optimal control.Firstly,an modeling approach for generalized controlled object model of RSC and GSC is established,the weight coefficient design principle for the generalized controlled object model is studied as well.With aforementioned model and principle,the HFR damping method based on H_∞robust optimization is accomplished.The H_∞damping controller have the adaptability for parameters variation,but when it is applicated in the different DFIGs with significant parameters,it can be invalid if its parameetrs is not be redesigned.Therefore,an impedance reshaping scheme for DFIG system based on voltage feedforward is designed,this method can reshape DFIG impedance without perceiving system parameters,more importantly,the impedance reshape effect is irrelevant to system parameetrs.Then,taking into account both damping performance and the fundamental control performance,the design procedure of the feedforward controller is investigated.The voltage feedforward based damping controller improv the practicability of HFR damping,since it can be applied in different units without parameters re-tuning.3.Aiming at the HFR of the VSC-HVDC connected WPGS,based on the output impedance model of VSC-HVDC at wind fram side,the resonacne mechanism of the wind farm connected VSC-HVDC is researched.Then,the influences on system stability caused by control delay of VSC-HVDC are analyzed,which indicates that the control delay of VSC-HVDC is the key factor which cause the HFR,thereby a DEDC(Delay Elimination based Damping Controller)is proposed.The perforamcne evaluation for the interconnected system indicates that the system stabilization of HVDC connected WPGS is achieved by the DEDC.Furthermore,for the scenario where a wind farm is connected to the VSC-HVDC via a long-distance transmission line,the wind farm impedance considering the effect of transmission line is established.Then,the mechanism of HFR in this scenario is studied,and the influence on system stability caused by VSC-HVDC control delay and the capacitive effect of transmission lines is analyzed as well.Based on the theoretical analysis,an hybrid impedance reshaping strategy combining DEDC with parallel resonance passive damping is proposed,which can significantly improve the opearaion capability pf VSC-HVDC connected wind farms.