This paper aims at exploring the HVDC transmission systems and also presents a comprehensive investigation on one of the concerned issues, which is the contribution of HVDC LightT to short circuit currents. Different AC network conditions, load conditions and fault types are considered under different operation conditions and control modes.

A comprehensive investigation on the issue regarding the contribution of HVDC LightT to short circuit current has been performed. The studies lead to the following conclusions. The HVDC LightT, in contrast to the conventional HVDC that does not contribute any short circuit current, may contribute some short circuit current.

Description of HVDC Light

The possible maximum short circuit current contribution is determined by the SCR. It is inversely in proportional to the SCR and it occurs when the transmission system is operating at zero active power.

The amount of contribution depends on control modes, operation points and control strategies. With the reactive power control mode, the short circuit current contribution will be limited due to the current order limit decreasing with the voltage. The contribution to the short circuit current is irrelevant to the fault location if the fault current is evaluated in per unit with the base value equal to the 3-ph fault current at the corresponding fault location and without HVDC LightT connected.

If the HVDC LightT contributes a higher short-circuit current, the voltage dip due to distant fault is possibly reduced and thereby the connected electricity consumers may suffer less from disturbances.

The HVDC technology is used in transmission systems to transmit electric bulk power over long distances by cable or overhead lines. It is also used to interconnect asynchronous AC systems having the same or different frequency. Figure 2.1 shows a simpli?ed schematic picture of an HVDC system, with the basic principle of transferring electric energy from one AC system or node to another, in any direction. The system consists of three blocks: the two converter stations and the DC line. Within each station block there are several componentsinvolved in the conversion of AC to DC and vice versa.

The traditional HVDC system is built with line commutated current source converters, based on thyristor valves. The operation of this converter requires a voltage source like synchronous generators or synchronous condensers in the AC network at both ends. The current commutated converters can not supply power to an AC system which has no local generation. The control of this system requires fast communication channels between the two stations.

A HVDC transmission line costs less than an AC line for the same transmission capacity. However, the terminal stations are more expensive in the HVDC case due to the fact that they must perform the conversion from AC to DC and vice versa. On the other hand, the costs of transmission medium (overhead lines and cables),land acquisition/right-of-way costs are lower in the HVDC case.