Research On AC Line Protection Of Offshore Wind Power Grid Connection System

Offshore wind farms mostly adopt AC pooling,high-voltage flexible DC transmission and AC access to the power grid.Among them,both offshore AC collection lines and onshore AC access power grid lines are connected to power electronic equipment.The fault current waveform of power electronic equipment is severely distorted and exhibits non power frequency characteristics,resulting in the decline of the sensitivity of the AC line main protection based on the fault power frequency characteristics,or even the rejection of operation,which will seriously threate the safe operation of the offshore wind power grid connection system.Therefore,it is urgent to study the fault characteristics of flexible DC converter stations and wind turbine converters,and propose new protection principles suitable for onshore single ended power electronics access power grid lines and offshore dual ended power electronics collection lines,respectively.The fault characteristics of different power electronic power supplies in the offshore wind power grid connection system and the mechanism of degradation of traditional differential protection performance are revealed.Based on the differences in physical and control structures between flexible DC converter stations and wind turbine converters,the full time scale fault characteristics of flexible DC converter stations and wind turbine converters are analyzed.The mechanism of degradation in the operational performance of traditional differential protection on onshore single ended power electronic access power grid lines and offshore dual ended power electronic collection lines is revealed,and the operational boundaries of differential protection is clarified.It provides theoretical support for proposing new protection principles.A new line protection based on waveform difference for onshore single ended power electronic access to power grids is proposed.Using the Canberra distance to quantitatively depict the difference in fault current waveforms between onshore flexible DC converter stations and AC power grids,a new protection approach based on comparison of short-circuit current waveform characteristics is proposed.It breaks through the barriers of traditional differential protection based on comparison of determined fault power frequency characteristics,improves the operational performance of line protection.The simulation results verify the effectiveness of the proposed protection in the onshore single end power electronic connection to the power grid line.A new line protection based on time frequency characteristics difference for offshore dual ended power electronic collection lines is proposed.Based on different physical structures and control modes,the differences in fault response characteristics between offshore flexible DC converter stations and wind turbine converters were analyzed.A two-dimensional time-frequency domain fault feature extraction method for fault current based on wavelet transform is proposed.The difference in timefrequency characteristics of fault currents between offshore flexible DC converter stations and wind turbine converters has been identified.A protection criterion based on time-frequency energy coefficient has been constructed,and a new protection approach based on differences in time-frequency characteristics has been proposed.The simulation results verify the effectiveness of the proposed protection on the offshore dual terminal power electronic collection lines.