Abstract: This paper introduces a kind of reactive automatic compensating controller based on PLC and three-phase AC parameter acquisition module.This controller can automatically switch the capacitors banks according to the actual load conditions of mobile substation low voltage system,it can improve the power factor effectively.This device has many protection functions such as the over-current,overload and unbalanced three-phase.In addition to protective capacitor,it also can be used as supplement of mobile substation low voltage protection.