I. Introduction
Unrepeatered ultra-long haul systems are widely used in ultra high voltage (UHV) power grid constructions, which are beneficial to desert, depopulated, poor environment areas. In China, the existing ±800 kV direct current (DC) UHV transmission line is about 2000 km long, ±1100 kV DC UHV transmission line has been exceeding 3000 km, and ±1000 kV alternating current (AC) UHV transmission line is more than 600 km. Due to the limitation of optical communication transmission distance, signal repeater stations can only be used between two converter stations. The goal of unrepeatered transmission systems is to reduce the number of repeater stations by extending communication distances between stations, thus offer a cost-effective solution.