Based on the structure of wind turbines and its grid-connection principle, an improved low voltage ride through control scheme with "DC chopper + reactive compensation" of PMSG is proposed. An improved low voltage ride through control scheme with DC chopper and series dynamic braking resistor of DFIG is adopted. The grid-connection simulation models of PMSG and DFIG which have the capability of low voltage ride through are built in the PSCAD/EMTDC. Based on " Technical rule for connecting wind farm to power system", the low voltage ride through capability are verified with voltage sag generator (VSG) for PMSG and DFIG. Combined with the accessing scheme of Dabancheng wind farm group, a simulation example of wind farm groups which all the PMSG and DFIG have the capability of low voltage ride through is built, the characteristics of wind farm groups during fault ride-through in condition of different fault types of tie line and voltage drop at wind farm outgoing line as well as voltage drop at tie line between the groups are researched. Results indicate that the voltage drop of grid-connected large scale wind farms outgoing line have influence on voltage and frequency for the adjacent wind farm and power system, the whole wind power system can be restored to steady state operation after the failure. Research results contribute to analyze the running characteristic of grid-connected large scale wind farms and improve the receptivity ability for wind power.