This paper analyzes the overvoltage level of series capacitor which possibly occurs at various conditions

and based on this

determines the protection level Upl of overvoltage protector in the series capacitor compensation device.Taking the design electric field of interpolar medium of high voltage pp film shunt capacitors as criteria

obtains the suitable dielectric working electric field of the series capacitor at different load current and protection level of transmission line fault.It is suggested that when the line with the series capacitor compensation device operates in the duty of（N-1）

the fault load current shall not exceed 1.4pu

so as to avoid high design cost of the series compensation device due to extremely low interpolar medium working electric field of the capacitor.