30-01-2009, 03:16 PM
Voltage Source Converter (VSC) based HVDC transmission systems are available in market for power rated between a few MVA to a few hundreds of MVA. The new system results in many application opportunities and new applications in turn bring up new issues of concern. 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 Light? to short circuit currents. Different AC network conditions, load conditions and fault types are considered under different operation conditions and control modes. Results from simulations with complete functions as in real systems are presented and discussed. The associated control and protection strategy is also discussed. Finally a comparison is made between the HVDC Light? and conventional HVDC, the SVC and the STATCOM regarding the impact on the short circuit current. A comprehensive investigation on the issue regarding the contribution of HVDC Light? to short circuit current has been performed. The studies lead to the following conclusions. The HVDC Light?, in contrast to the conventional HVDC that does not contribute any short circuit current, may contribute some short circuit current. 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. Hence, it is comparable to the STATCOM as long as the maximum short circuit current contribution is concerned. 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. With the AC voltage control mode, the short circuit current contribution will be increased with the decreasing of active power, if the current order limit is not changed. If the current order limit is decreasing with voltage, the short circuit current contribution will be small even if the load level is low. 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 Light? connected. Under the same load and operation condition, the 1-ph fault current is usually smaller than the 3-ph fault current. Finally, it should be noticed that in associated with higher short-circuit current the voltage stability and performance is likely to be improved. If the HVDC Light? 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.