Interaction between HVAC - HVDC system: Impact of line length on transient stability

Amid the increasing need to implement more HVDC transmission schemes within the next two decades in South Africa, which is aimed at strengthening the existing HVAC transmission network and to enable the transfer of increased bulk power across the country, this paper seeks to highlight some of the issues that the power industry may be faced in terms of power system stability. In this paper, the transient stability analysis of a power system made of four generating plants and hybrid HVAC-HVDC transmission line is investigated. The length of the HVAC line will be fixed at a pre-specified value (i.e., 500km), while the HVDC line length is varied from 100 km to 3,000 km. DC line faults and AC line 3-phase faults are applied to the system to determine its resilience to transient instabilities at the different HVDC line lengths. The simulation results show that the use of longer HVDC transmission line in parallel with a HVAC transmission line improves the stability of the system after a DC transmission line fault occurs in the system. However, the HVDC line length does not have a significant impact on the stability of the system after a 3-phase fault occurs on the HVAC transmission line.