A HVDC back-to-back station is normally used to create an asynchronous interconnection between two AC networks.
There are several back-to-back stations in operation in the world. In these installations both the rectifier and the inverter are located in the same station and are normally used in order to create an asynchronous interconnection between two AC networks, which could have the same or different frequencies.
A back-to-back station is normally somewhat simpler than a converter station for a transmission project. The direct voltage level can be selected without consideration to the optimum values for an overhead line and a cable, and is therefore normally quite low, 150 kV or lower. The only major equipment on the DC-side is a smoothing reactor. The control equipment can also be simplified, as there is no need for a telecommunication link between the two converters.