An investigation into the impact of HVDC schemes on Eskom HV network's transient stability.
Compared to high voltage alternating current (HVAC) lines, the response of HVDC schemes to system faults is faster and more controllable, thus making it more tolerable to system faults. Due to the inherent advantages of HVDC schemes over HVAC lines when it comes to bulk power transmission over long distances, it has become a common practice for power utilities to integrate HVDC schemes into their networks. The main objective of this research is to investigate the impacts that HVDC schemes may have on the transient rotor angle and voltage stability of Eskom’s main transmission system (MTS) network. Three test systems were used to carry out this research, namely: a two machines infinite busbar network (test network 1), a 30 machine 24 busbar network (test network 2) and Eskom’s MTS network (test network 3), with the emphasis being on test network 3. HVDC schemes may be used to improve the system synchronizing torque by making use of robust, fast, state-of-the-art control techniques with good communication systems, for DC power modulation at the converters. The HVDC controllers are used for temporary increment or decrement of DC power during system faults and transient periods. This restores the balance between the acceleration power gained by the generator during the fault and the system retarding power applied onto the generator. Previous research shows that the ability of a HVDC scheme to improve the transient stability of an AC system largely depends on the pre-disturbance conditions of the network and the robustness of the HVDC scheme controllers used. In this study, carried out using the DIgSILENT PowerFactory software tool, HVDC schemes of various configurations have been integrated at strategic locations of Eskom’s MTS network and time-domain dynamic simulation studies were carried out to determine how the HVDC scheme affects the network transient stability. This was done by assessing the obtained results on system fault levels, critical fault clearing times (CCTs), minimum and maximum voltage violations and thermal limit violations, against the requirements as stipulated in the South African Grid Code-The Network version. It was found that the integration of the LCC-HVDC scheme causes an improvement in the CCT of the generators, which indicates an improvement of machine transient rotor angle stability. In terms of transient rotor angle stability, the integration of the LCC-HVDC scheme meets the South African Grid Code (SAGC) requirements. The HVDC scheme does not have a significant impact on the network fault levels.