Wireless Current Sensor (WCS) is used to assess the key equipment performance by monitoring the current flow of the parallel arresters applied for overvoltage protection in HVDC convertor stations. Once the current deviation between parallel arresters exceed 5%, some of the arresters must be replaced early in order to avoid the unscheduled shutdown of the HVDC transmission caused by the damaged equipment. The online monitoring system consists of WCS, DCU and software platform. Since 2016, the system has been put into field operation in more than 9 UHVDC Projects in China.