| Doubly fed wind power generation systems are widely used in distribution networks in recent years for their flexible control strategy,small capacity of active converters and active and reactive power independent control.The fault characteristics of DFIG Systems are affected by various factors such as fault control strategy,speed and port parameters.The composition and attenuation characteristics of the short-circuit current are greatly changed,and the fault characteristics need to be studied deeply.In the event of a failure,the original protection may mal-operate or refuse to operate.Thus,the new principle of distribution network protection for DFIG Systems has become the focus of current research.With the increase of wind power access capacity,if a large number of DFIG Systems are simultaneously cut off in the event of grid failure,which will seriously affect the stable operation and power supply reliability of power grid.So the new grid protocol requires the wind farm according to the access system program and the electrical characteristics of their own units can run through the fault.At present,the commonly used crowbar protection can inhibit the rotor overcurrent,but does not meet our requirements for fault crossing,that is,in the event of failure to the grid to send reactive power to support the system voltage,and thus face new challenges.For the distribution network with DFIG Systems,the main research contents and contributions are shown as follows:(1)The expression of the three-phase short-circuit current of DFIG Systems is derived.The transition process of rotating speed,power and crowbar during symmetrical fault of the power grid is analyzed.The transient process of the stator and rotor side flux linkage is also analyzed,and the expression of the short-circuit current is deduced.The correctness of the expression is verified by comparison between theory and simulation.At the same time,the characteristics of the speed frequency current are analyzed.(2)A harmonic detection and extraction method of power system based on continuous wavelet transform and discrete wavelet transform is proposed.The two kinds of wavelet transforms are used in combination.The frequency of the rotating frequency is detected by continuous wavelet transform.The frequency of the harmonics is obtained.The discrete wavelet transform is used to segment the frequency and the frequency of the rotational frequency is extracted.The simulation results show that this method has high accuracy for the harmonics and inter-harmonics with close frequency.(3)The principle of line protection for distribution network based on the frequency of current is proposed.The characteristics of the frequency current during fault are analyzed for the distribution network with DFIG Systems.It is found that only the side of the line is detected when the line frequency is decaying the AC current.In the case of the fault,the protection on both sides of the line is detected or the frequency is not detected.Based on this,the protection principle is put forward and the protection scheme is constructed.The protection scheme has the advantage of only current,only the transmission logic,no need for simultaneous sampling.The simulation results show that the principle of the protection can distinguish the fault between the different positions,the different transition resistance,the different speed and the different crowbar resistance.(4)A hybrid fault crossing control strategy for DFIG Systems is proposed.Based on the requirement of wind power fault crossing in China,this paper proposes a hybrid fault control strategy based on reactive power output priority strategy and combined with crowbar method.The strategy has the advantages of two types of fault crossing control strategy,which can not only effectively limit the rotor current,but also can continue to feed enough reactive power to help voltage recovery,in line with China’s fault on the guide and procedures,satisfy the requirements of industrial applications. |
