Most HVDC links are point-to-point connections. The link can be monopolar or bipolar. The monopolar HVDC link consists of a return path eitherthrough the ground or through the sea. This method is mostly used for power transmission using cables, because it saves the cost of laying one cable. When the ground resistance is too high, a metallic return path is preferred instead of using the ground. The bipolar HVDC link consists of two poles: one with positive polarity and the other one with negative polarity and with their neutral points grounded. In steady-state operation the current flowing in each pole is the same, and hence no current flows in the grounded return. The poles may be operated separately. If one of the poles malfunctions, then the other pole can transmit power by itself with the ground return (Figure 1).The back-to-back system is used for providing an asynchronous connection between two AC systems. The rectifier and the inverter are located in the same station.
HVDC technology has become a mature technology since the Gotland HVDC transmission link between the island of Gotland in the Baltic Sea and the Swedish mainland, with a rating of 20 MW, 200 A and 100 kV, went into operation in 1954. Until now HVDC links of 2000 MW and more have been designed and build for voltages in the ±500 to 600 kV range, but these voltage levels are not sufficient for distances around 2000 km from the giant power stations build in China and India. For these lines, 800 kV technology is applied. The power transmission link in China, between Xiangjiaba and Shanghai, operates at ±800 kV and is capable of delivering 6400 MW over a distance of 2071 km.