During several large-scale wind-turbine-generator (WTG) dropout accidents, part of the WTGS were tripped off because the system voltage exceeded the overvoltage protection value of WTGs. One of the problems exposed in the WTG dropout accidents is that some of the relay protections are not properly set up. In this paper, the principles of overvoltage relay protection of WTGs and capacitor banks are investigated, then a method of calculating the voltage drop on WTG’s transformers and power collection lines is given; finally, several principles to set overvoltage protection of WTGs and capacitor banks are proposed.