Study On Pilot Protection For Transmission Line In Wind Farm

In recent years,the penetration rate of wind power has continued to increase.As an important line connecting the power grid and wind farms,transmission lines have shown an important position in the integration of wind power.Affected by the characteristics of wind power faults,the traditional relay protection devices on the transmission line have many adaptability problems.Based on the analysis of the amplitude and phase characteristics of the short-circuit current of the wind farm,the sensitivity of the pilot current differential protection on the transmission line is studied and new adaptive differential protection schemes with higher sensitivity for wind farm transmission lines are put forward.Based on the derivation of the fault current expressions of the doubly-fed wind turbine(DFIG)under different low voltage ride-through strategies,the phase characteristics and amplitude characteristics of the short-circuit current of the wind farm are analyzed.It is found that during the low voltage ride-through process of the DFIG,the phase of the short-circuit current is affected by the fault conditions and the control strategy and has the characteristics of a large fluctuation range.At the same time,affected by the weak feed characteristics of the wind farm,the currents and their sequence components measured by the protections on both sides in the transmission line have a large difference in amplitude.The influence of the short-circuit current amplitude characteristics and phase characteristics of wind farms on the sensitivity of the pilot current differential protection is analyzed on the amplitude and phase plane.It is found that the sensitivity of the pilot current differential protection on the transmission line is reduced,even in extreme cases the protection may refuse to move due to the large difference in current amplitude on both sides,and the phase angle difference may exceed 90°.Aiming at the sensitivity drop and refusal of the pilot current differential protection on the transmission line of the wind farm,this paper constructs the amplitude factor and the virtual current factor as the adaptive braking coefficient to achieve the improvement of the sensitivity of the differential protection.Taking advantage of the differences in the amplitudes of the currents and its sequence components on both sides of the transmission line under the internal faults,the full amplitude factor,the positive sequence fault component factor and the negative sequence component factor are constructed as the adaptive braking coefficients,and by adding the comprehensive optimization module constitutes a comprehensive optimal adaptive differential protection scheme based on the amplitude factor,which realizes the complementary advantages between different amplitude factors,and greatly improves the sensitivity of the differential protection.By strengthening the negative sequence component and weakening the positive sequence and zero sequence components,a virtual current factor is constructed as an adaptive braking coefficient,which constitutes an adaptive differential protection scheme based on the virtual current factor,which overcomes the problem of performance degradation of pilot current differential protection on the transmission line.A model of doubly-fed wind farm connected to the grid through transmission line is built on the MATLAB/Simulink platform,and the performance of the proposed protection scheme is simulated.The results show that the proposed protection scheme has strong sensitivity on the transmission line of wind farms,which shows good performance and practicality.