A high voltage direct current transmission system, comprising: a converter module structure having a plurality of stories, whereby the stories are each provided with: - an insulation support part for electrical insulation between the structure and a sub-module; - a fixing base mounted on the insulation support part; - a movable base slideable along the fixing base by means of rails and rollers for insertion/extraction of a sub-module; - a sub-module mounted on the movable base; whereby a replacement base is further provided for allowing the submodule to be withdrawn outside the structure for replacement such that the sub-module is seated therein after being withdrawn; wherein the replacement base includes: - a base plate formed with side walls at opposite sides in a width direction thereof; - a guide rail provided at each of the opposite sides in the width direction of the base plate by extending in parallel to each other so as to guide the insertion/extraction movement of a sub-module; - a connection guide part provided at each of opposite sides of a rear end of the base plate allowing the rear end of the base plate to be easily guided to a front end of the fixing base when the replacement base is placed in the structure; - a rail connection guide to guide the fixing rails of the fixing base to be matched with the guide rail of the replacement base; - a support member connected to the front end of the replacement base at a first end thereof and connected to the structure at a second end thereof so as to support the replacement base to be horizontal; - a pair of hooks (68,69) configured to hook the replacement base to the insulation support part; whereby, in use, to allow a sub-module to be replaced: the replacement base is configured to be mounted to the structure, the front end of the fixing base is configured to be inserted between the connection guide parts, the replacement base is configured to be prevented from moving to the back as well as to the front of the structure by means of the hooks (68,69), and the fixing rail is configured to meet the guide rail, allowing the movable base to slide out. a converter module structure having a plurality of stories, whereby the stories are each provided with: - an insulation support part for electrical insulation between the structure and a sub-module; - a fixing base mounted on the insulation support part; - a movable base slideable along the fixing base by means of rails and rollers for insertion/extraction of a sub-module; - a sub-module mounted on the movable base; - an insulation support part for electrical insulation between the structure and a sub-module; - a fixing base mounted on the insulation support part; - a movable base slideable along the fixing base by means of rails and rollers for insertion/extraction of a sub-module; - a sub-module mounted on the movable base; whereby a replacement base is further provided for allowing the submodule to be withdrawn outside the structure for replacement such that the sub-module is seated therein after being withdrawn; wherein the replacement base includes: - a base plate formed with side walls at opposite sides in a width direction thereof; - a guide rail provided at each of the opposite sides in the width direction of the base plate by extending in parallel to each other so as to guide the insertion/extraction movement of a sub-module; - a connection guide part provided at each of opposite sides of a rear end of the base plate allowing the rear end of the base plate to be easily guided to a front end of the fixing base when the replacement base is placed in the structure; - a rail connection guide to guide the fixing rails of the fixing base to be matched with the guide rail of the replacement base; - a support member connected to the front end of the replacement base at a first end thereof and connected to the structure at a second end thereof so as to support the replacement base to be horizontal; - a pair of hooks (68,69) configured to hook the replacement base to the insulation support part; - a base plate formed with side walls at opposite sides in a width direction thereof; - a guide rail provided at each of the opposite sides in the width direction of the base plate by extending in parallel to each other so as to guide the insertion/extraction movement of a sub-module; - a connection guide part provided at each of opposite sides of a rear end of the base plate allowing the rear end of the base plate to be easily guided to a front end of the fixing base when the replacement base is placed in the structure; - a rail connection guide to guide the fixing rails of the fixing base to be matched with the guide rail of the replacement base; - a support member connected to the front end of the replacement base at a first end thereof and connected to the structure at a second end thereof so as to support the replacement base to be horizontal; - a pair of hooks (68,69) configured to hook the replacement base to the insulation support part; whereby, in use, to allow a sub-module to be replaced: the replacement base is configured to be mounted to the structure, the front end of the fixing base is configured to be inserted between the connection guide parts, the replacement base is configured to be prevented from moving to the back as well as to the front of the structure by means of the hooks (68,69), and the fixing rail is configured to meet the guide rail, allowing the movable base to slide out.