Research On Protection Under The Influence Of The Dfig Wind Farm Weak-feed Characteristic Considering The Change Of Wind Speed

There are obvious differences between the DFIG(Doubly fed Induction Generator)and the traditional synchronous generator,from the microcosmic structure and working principle to the macroscopic grid-connected mode.Moreover,the working state of the wind farm is also affected by wind speed and other factors.In addition,the large-scale and long-distanceconnection of wind farms put forward a severe test to the nowadays power system.Under this background,this paper classifies the wind farm into groups and clusters considering the influence of wind speed and DFIGs arrangement.Based on this model,the main research object is the pilot Directional protection in the pilot protection of the transmission line of the DFIG and power-grid connected system.On the basis of deducing the transient characteristics of the wind farms,a new principle of transmission line Directional Pilot Protection for a weak-fed wind farm is proposed.First of all,this paper utilizes Jensen Wake Algorithm to calculate the actual capture wind speed of each DFIG,combines the distance information from the bus in order to form the clustered index.Then,different clustering schemes are calculated by using the K-MEANS algorithm while the best clustering scheme under specific wind speed and direction is determined by Contour Coefficient.Acquiring the cluster scheme allows the multi-machine-tosingle-machine equivalent.The clustering and equivalent results equip further model discussion with the effect of wind speed.Secondly,based on the concept of similar external effect,transforming the time-varying DFIG fault network to the form of equivalent calculated sequence impedances,by which method the fault characteristics of DFIG could be analyzed by means of equivalent impedances.In this part,the changing process of DFIG fault sequence impedance is studied.It is summarized that the steady phase angle of the positive sequence impedance is at-90°～-180° and the steady phase angle of the negative sequence impedance is at 33°～ 90°.The great difference between the positive and negative sequence impedance is a clear demonstration of the weakfed characteristic.Further,the weak-feed characteristic of the DFIG wind farm is studied and defined by calculating the sequence impedance and the fault current characteristics.It is found that the weak-fed characteristic is mainly reflected in three aspects: the weak-fed of short-circuit fault current,the great differences between the positive and the negative sequence impedance and the serious fluctuation of the positive sequence impedance.Also,the effect of wind speed on the weak-fed rate is studied,and the conclusion is drawn that the effect of wind-speed on the weak feed rate is a three-section effect.Thirdly,due to the complex transient characteristics of DFIG,the directional pilot protection Element of pilot protection in the transmission line is studied.It is found that the inadaptability of the traditional protection method is universal after the DFIG wind farm is connected,especially in the power-frequency-fault-component based directional protection elements.To be specific,it is possible that the directional component with a power-frequencyfault component will exceed the traditional impedance measurement range when the wind power is connected.The excess of angle range means the sensitivity and selectivity of protection are no longer reliable and protection components may cause malfunction or movement resistance.In view of the above problems,in this paper,an adaptive directional comparative protection based on the weak-fed index is proposed: 1)additional criteria are proposed to distinguish the weak-fed side from the system-side based on the weak feed index in order to solve the problem of a power-frequency-fault component in phase angle measurement during the faulted channel.2)the weak-fed degree of negative sequence fault current is used as an action criterion operating the low-voltage shutdown signal function on the side of DFIG wind farm according to the weak-fed characteristic.Finally,the simulation model of the DFIG wind power system is built on Matlab / Simulink platform.The clustering and equivalent method is simulated.The transient characteristics of a single equivalent wind turbine are studied and the simulation of adaptive directional pilot protection based on a weak-fed index is carried out.The simulation results verify the rationality of the analysis in this paper and the effectiveness of the improvement measures.