Series capacitor compensation technology is one of the key technologies in EHV and UHV long distance transmission. With wide application of the series compensation device and rapid development of the power grid

transmission lines with high series compensation degree may well appear which will affect the protection of transmission lines and even the main protection performance. The operation characteristics of the main protection are studied by referring to engineering practice

and the influence of the transition resistance

system operation mode and power angle on the current differential protection is analyzed. According to the simulation results

countermeasures for improving the sensitivity of differential current protection are presented. This work is supported by National Natural Science Foundation of China (No. 51177058

No. 51277084).