The topology of doubly fed induction generators (DFIGs) has a greater impact on optimal reactive power dispatch (ORPD) problem within wind farm. The reactive power limits of DFIG are analyzed, DFIGs are regarded as continuous reactive power sources participating reactive power compensation within wind farm and the impact of on-load tap changing transformers (OLTCs) tap on DFIG terminal voltage is considered. The optimal reactive power dispatch model within a wind farm is formulated with an objective of minimizing active power loss. Finally, the problem of optimal reactive power dispatch within HornsRev1 wind farm in Denmark is formulated and a particle swarm optimization (PSO) algorithm is proposed to get the optimal solution. The test results demonstrate the security and economy of the proposed method in achieving optimal solution.