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Feasibility Study of Adding a Third VSC based HVDC Terminal on an Existing Point-Point LCC based HVDC Transmission System

Advantages of Voltage Source Converter (VSC’s) over Line Commutated Converter (LCC’s) urges to have at least one VSC based converter station in a multi terminal hybrid HVDC (MTDC) system. A hybrid HVDC system optimally combines the benefits of LCC (HVDC Classic) and VSC (HVDC Light). The resultant system offers advantages of bulk power transmission, independent control of active and reactive power, capability of interconnection with weak AC network and renewable energy sources, black start capability in absence of grid and no commutation failure issue at VSC inverter. This has made the hybrid HVDC ideally suitable for applications wherein the existing HVDC Classic needs an upgradation to dispatch higher power demands of the utilities. In this scenario, a third VSC terminal may be added to the existing point-to-point LCC HVDC transmission system to handle the large-capacity power transmission over long distances and to connect to the newer load centers or local power generation from distributed energy sources developed in its vicinity. Such a multiterminal arrangement may renew the utilization of existing classic HVDC transmission link with additional operating flexibility, thus saving essential investment cost. In this paper, the study results for a hybrid HVDC system will be presented, where a half-bridge cell based modular multi-level converter (HBMMC) is added as a third station to an existing point to point LCC link with hybrid HVDC breaker (HHB) used as fault clearing or fault ride through technical solution (FRT device). The steady state and dynamic performance of this multiterminal system is analyzed, and the simulation is performed in PSCAD/EMTDC. This paper will illustrate the advantages of the three-terminal hybrid HVDC. It will also address the new challenges due to addition of the third VSC terminal on the point to point LCC HVDC. Solution for mitigating the potential problems is presented. The proposed solutions are also verified by simulating AC and DC faults and the typical simulation results are presented in this paper. The results show that the multi-terminal hybrid HVDC system is able to handle different AC and DC faults with desired performances.