With the development of wind power, wind farms are required to provide reactive power to the power system. For permanent magnet synchronous generator (PMSG) based large wind farms (WF), it may be an economical way to generate reactive power using the power electronic devices inside each wind turbine (INT). In this paper, an optimal reactive power dispatch of PMSG WF is proposed to minimize the power loss. Both the losses inside WTs and the losses of transmission system are all considered. Particle swarm optimization (PSO) algorithm is adopted to find the reactive power references of each WT which makes the total loss of WF minimal. A WF with 25 5 MW PMSG WTs arranged in 5 rows and 5 columns is used in the case study. And two traditional reactive power dispatch strategies are compared comprehensively with the proposed strategy at different scenarios, the results have shown that the proposed strategy obtains lower power loss than the other two traditional strategies in all the studied cases. (C) 2019 Elsevier Ltd. All rights reserved.