The embodiment of the utility model provides an auto -change over device and electric power system of var control mode of equipment of being incorporated into power networks relates to the electric power system control technical field that is incorporated into the power networks, avoids in the reactive compensation ability of the equipment of being incorporated into the power networks that can needn't leave unused when the alternating -current system is unusual going up more / the frequent switching of between lower limit mode and normal operating mode. This method includes: when the voltage of monitoring point is greater than upper limit threshold voltage, get into upper limit operational mode more, voltage when the monitoring point is less than upper limit threshold voltage ring width stagnates to when the current output reactive power of the equipment that is incorporated into the power networks satisfies predetermined condition, withdraw from upper limit operational mode more, when the voltage of monitoring point is less than lower limit threshold voltage, get into lower limit operational mode more, voltage when the monitoring point is greater than lower limit threshold voltage + ring width stagnates to when the current output reactive power of the equipment that is incorporated into the power networks satisfies predetermined condition, withdraw from lower limit operational mode more.