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A back-to-back HVDC arrangement is used when two asynchronous AC systems need to be interconnected for bulk power transmission or for AC system stabilization reasons. In an HVDC back-to-back station there are no overhead lines or cables separating the rectifier and the inverter, hence the transmission electrical losses on the DC side can be neglected. In a back-to-back HVDC, the DC current can be kept high and the DC voltage low. The low DC voltage means that the air clearance requirement is low, which is in favor of a compact design of the valve housings. This enabled the modular back-to-back HVDC concept to be developed. The modular back-to-back converter station can be made very compact, thus requiring a minimum of open space. It can even be designed to fit into an existing right-of-way of a typical 400 kV AC line. In environmentally sensitive areas this is also considered a great advantage from a permitting point of view. Our ISO 14001 certification states that the design of the equipment and the construction at the site are performed in an environmentally acceptable way. The Argentinean and Brazilian networks have been interconnected via back-to-back HVDC with a rating of 1100 MW. A second interconnection of another 1100 MW is under construction. The back-to-back HVDC converter stations are of the modular type with capacitor commutated converters.