High-voltage direct current transmission is used for the low-loss transmission of electrical current over long distances. Here, the alternating current generated is first converted into direct current by a rectifier and then transmitted at a very high voltage (at least 220 kV). For the subsequent connection to the conventional alternating current grid, the direct current must be converted back into alternating current using an inverter. HVDC lines are increasingly being used for offshore wind farms or to transmit electricity to metropolitan areas (e.g. in China).