Global offshore wind farm is developing towards large-scale

clustered and long-distance deep offshore. The transmission and grid connection of large-scale offshore wind farm with a distance greater than 100 kilometers has become a major focus of offshore wind energy development. A technical and economic evaluation method for systematically evaluating various grid-connected methods is established by analyzing transmission limit and loss of cable and other electrical equipment

using the uniform annual value method and considering equipment investment cost

maintenance cost

loss cost and cable selection. Taking a 400 MW offshore wind farm as an example

the technical and economic comparison of high voltage alternating current(HVAC)

high voltage direct current(HVDC)and fractional frequency transmission system(FFTS)is carried out. The demonstration shows that FFTS by reducing the frequency can not only realize the growth of the ampacity of cable

but also significantly reduce the charging current in the AC cable. Therefore

the transmission distance of FFTS increases remarkably

which indicates that FFTS has both technical and economic advantages on large-scale and long-distance offshore wind farm transmission and connection system.