For long distance ultrahigh voltage AC transmission lines，it is quite necessary to adopt a controllable shunt reactor which can selfregulate its capacity according to load to reduce the line loss

to balance reactive power and to control voltage. This paper presents the protection research process

the protection configuration and the main protection principles of the controllable reactor of the high impedance type. The difference between the controllable shunt reactor protection and common shunt reactor protection is also described. The protection device has been used in the controllable shunt reactor demonstration project of a 500 kV switching station in Xinzhou

Shanxi Province.