Based on the two-machine parallel model

a method of collector network equivalence for wind farm is presented

which is suitable for low voltage ride through(LVRT)simulation. The equivalence of line reactance is based on the principle that the output reactive power of wind farm at the point of common coupling(PCC)before and after the transformation should be consistent during the grid voltage dip

and the equivalence of line resistance is based on the principle that the active power losses of collector network should be equal. By the impedance transformation of collector network

the wind farm can be made equivalent to a single-machine model connected to the PCC through an equivalent impedance. The line impedance equivalent method proposed is applicable to the equivalence of collector networks with tree like

radial or hybrid topology

and the calculated parameters of line impedance are constant which is convenient for actual application. Simulation results show that the equivalent model has high precision during a power system fault.This work is supported by National High Technology Research and Development Program of China(863 Program)(No. 2011AA05A104)and State Grid Corporation of China(No. 522727140006

No. 52272714000Y).