Study On Impact Of Weak-infeed Wind Farm On Transmission Line Relay Protection And Key Technology

In the context of the green-oriented transition of energy,wind power has become an essential part of the modern power system.The geographic location span between wind resources and load center determines the wind power development path of “centralized construction,long-distance delivery” in China.As the first defensive line of the power system,relay protection must guarantee the reliable and stable operation of transmission lines connected to wind farms,thereby realizing the reasonable allocation and efficient utilization of wind resources.In this dissertation,the doubly fed induction generator weak-infeed source(DFIGWIS)is selected as the research object.Based on the DFIGWIS’s fault characteristics,the impact of the wind farm on the transmission line relay protection is studied and corresponding countermeasures are proposed.Considering the low-voltage ride-through(LVRT)modes under crowbar activation and improved excitation control,through quantitative analysis of the DFIGTWIS’ fault current and equivalent sequence impedance,two unconventional fault characteristics of the DFIGTWIS compared to synchronous power sources,i.e.,the uncertain weak-infeed and ephemeral frequency offset feature of the fault current and the amplitude and phase difference between equivalent positive-and negative-sequence impedance,are unveiled.Based on the DFIGTWIS’s unconventional fault characteristics,the impact mechanism of the wind farm on current differential protection,distance protection,and directional pilot protection is studied.It is found that the weak-infeed and frequency offset feature of the fault current would affect the phase-shift tolerance capability and sensitivity of differential protection,as well as the startup performance and the fault resistance coverage ability of distance protection.The unbalanced sequence impedance feature would affect the accuracy of the superimposed components-based and sequence components-based phase selection methods,and also increase the malfunction risk of the superimposed component-based directional elements(SCBDEs)and consequently affect the operation behavior of directional pilot protection.A differential protection criterion with the concave restraining region based on dynamic time warping(DTW)virtual phase difference is proposed.Based on the weak-infeed characteristic of the wind farm,the restraining angle that determines the boundary of the restraining zone is optimally designed as a power function with the current amplitude ratio as the main variable.And then a novel differential criterion with a concave restraining region that has high compatibility of phase-shift tolerance capability and sensitivity requirement is obtained.In addition,the DTW algorithm is used to construct the virtual phase angle difference and to indirectly reflect the real phase difference of the currents at two line terminals.In this way,the phase extraction error of the Fourier algorithm on the frequency offset current is eliminated on the one hand,and the strict synchronization requirements are loosened and the sensitivity to the current amplitude ratio is improved on the other hand.A compensated distance protection based on generalized amplitude ratio and phase zone division is proposed.Dynamic compensation is adopted to eliminate the enlarged additional impedance in the measured impedance and to improve the fault resistance coverage ability of distance protection.Two compensated phase-selection methods use the local quantities to compensate for the superimposed components and sequence components,and then identify the faulty phase based on the order of the generalized amplitude and the redevised faulty zone on the complex plane.The proposed phase-selection methods weaken the impact of unbalanced impedance and ensure the accurate selection of fault loops and impedance compensation methods under different fault types.Moreover,the predictive starting element improves the startup reliability of distance protection on the wind farm side.A directional pilot protection based on identification reconstruction and current decay rate is proposed.By introducing the parameter identification and fault point voltage reconstruction-based unbalanced fault direction discrimination criterion and the current decay rate difference-based balanced fault direction discrimination criterion,and then combining them with the traditional SCBDEs with a specially designed logic,the proposed directional pilot protection scheme can eliminate the operation-rejection risk for the in-zone faults on the premise of not abating the security for the out-of-zone faults.Based on the MATLAB/Simulink platform,a simulation model of the transmission line connected to a DFIGWIS-based wind farm is established to verify the characteristics of fault current and equivalent sequence impedance as well as the proposed protection schemes.The simulation results show that the phase-shift tolerance capability and sensitivity of the concave restraining region-based differential criterion,the reliability of each element in the compensated distance protection scheme,and the accuracy of the directional pilot protection scheme in distinguishing the in-zone and out-of-zone faults have been improved.The proposed schemes can alleviate the impact of the DFIGWIS’s unconventional fault characteristics on transmission line protection.