Design Of Low-voltage Fault Protection Circuit For Doubly-fed Wind Power Generation System

Recently,the wind power industry has developed rapidly,but the grid connected to the wind farm is usually far away from the load center,therefore,it is weak and susceptible to fluctuations.the stability of the operation cannot be guaranteed,once the voltage of the grid drops sharply,largescale disconnection of the wind turbine will occur,the safe of the grid operation is threatend seriously.At present,for the doubly-fed wind power generation system,when the power grid fails,its normal work is not the most important research direction,and we has also issued relevant regulations for fault crossing of wind farms.Therefore,it is a great significance thing to study the fault traversing capability of wind turbines.This paper designs a protection scheme for a doublyfed wind power generation system in the event of a power grid failure leading to a voltage dip,which enables a double-fed turbines to be protected under low voltage conditions,and the system is not subject to rotor-side over-current and DC-side over-voltage and keep running with network.First,the basic principle of the doubly-fed wind power generation system is studied and its mathematical model is analyzed,including model of three-phase stationary coordinate system and the synchronous rotating coordinate system.Then the improved control strategy based on the vector control is studied,based on the theoretical analysis,it is simulated when using improved control strategy at the slight drop of the grid voltage(20%)and the moderate drop of the grid voltage(50%),the comparison and analysis of the waveform when it is used control strategy or not,from the simulation results,the control strategy can suppresses the overcurrent protection of the rotor side and the overvoltage protection of the DC side.Then the hardware protection circuit is designed,including the rotor side active protection circuit with parallel dynamic resistance,the DC side protection circuit containing the energy storage system device,and built models in MATLAB.The waveforms of rotor-side current and DC-side voltage are observed when the voltage of the grid is severely dropped(80%).It is compared that the difference between the traditional protection circuit and the protection circuit adopted in this paper.The simulation results show that the hardware protection circuit in this paper has more advantages and can suppress the rotor-side over-current and the DC-side over-voltage better.Finally,a reactive power compensation device based on the static synchronous compensator was designed,choose voltage source bridge circuit which is more efficient.In the MATLAB,three states were established: voltage drop slightly(20%),voltage drop moderately(50%),and grid voltage drop severely(80%),comparing and analyzing the waveforms of reactive power at the grid point voltage before and after installation of reactive power compensation device and its output reactive power.The results show that the device can effectively provide reactive compensation for the system,but the compensation capability is limited when the fault is serious.This article has designed a set of low voltage protection circuit which suit for doubly-fed wind power generation system,and simulate in MATLAB,verify the effectiveness of the protection scheme,It can provides protection for safe and reliable operation of double-fed wind power systems and power grids.