Impact Of LVRT Of DFIG On Distance Protection Of Transmission Line

The wind turbine generator and the synchronous generator have totally different structures and operation principles,which leads to the different transient characteristics during the fault.The traditional relay protection designed for the synchronous generator will be affected for the power systems with the wind power integration.To guarantee the safe and stable development of the wind power generation,it is urgent to address the issues concerning the relay-protection with the wind power integration.In this thesis,the doubly-fed induction generator(DFIG)is chosen as the research object.Based on the characteristics of the fault current,the mechanisms of the impact that the wind power integration imposes on the distance protection are revealed and a new protection scheme is proposed.The main studies are listed as follows:(1)The positive and negative sequence mathematic models of the DFIG and the equivalent circuits are established within the dual dq coordinates.Based on the vector control scheme,the control of the rotor-side converter(RSC)and grid-side converter(GSC)for the positive and negative sequence components is modeled.(2)With the Crowbar or the series dynamic braking resistor(SDBR)applied to the DFIG under the asymmetrical grid faults,analytical expressions of the positive and negative sequence fault currents are derived considering the stator flux transients,the stator resistance,and the GSC current.(3)Based on the extraction principle of the power-frequency phasor,the influence of the fault current of the DFIG on the Fourier algorithm is derived,and the transient characteristic of the measured impedance is analyzed.With the Crowbar or the SDBR,the impact of the weak-infeed characteristic of the DFIG on the capability of the distance protection to operate against the transition resistance is analyzed.(4)The phase angle characteristics of the positive and negative sequence superimposed impedances of the DFIG are analyzed based on which it is verified that the simplification algorithm to the voltage phase at the point of failure is not applicable to the system with the wind power integration.A novel method to calculate the voltage phase at the point of failure based on the measured voltage and current at the protection installation point and the line impedance is proposed,which is applied to the single phase ground distance protection based on the comparison of the voltage phase.