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The control and protection system is very important in HVAC/DC transmission system. It is hoped that AC system and DC system can act as back-up protection for each other. This paper suggests that converter transformer can be used as DC system back-up protection in HVAC/DC system and DC line auto-restart function is added in rectifier control of CIGRE HVDC benchmark model in PSCAD/EMTDC. The variational regulation of voltage and current of converter transformer on the side of network is probed by means of PSCAD/EMTDC if the control process of DC line auto-restart fails. According to the features of the changes of voltage and current, low current protection can be used as converter transformer back-up protection to protect DC line so as to optimize and consummate transformer protection. The simulation results prove that low current protection can operate reliably. And it can be realized easily due to simple logic.