On Techno-economic Assessment of a Multi-terminal VSC-HVDC in AC Transmission Systems

Abstract: The Voltage Source Converter based Multi-Terminal high voltage Direct Current transmission system (VSC-MTDC) is currently considered as an attractive technical option for increased transmission capacity and improved controllability and flexibility of an electric power grid, thanks to its unique performance characteristics, and the possibility of using the extruded XLPE cables. In the planning process of embedding a VSC-MTDC system into an ac transmission grid, one must assess the potential values in terms of improved energy efficiency, i.e., reduction of power losses and generation costs. This economic assessment will be a determining feasibility factor of such an option. Towards this end, this thesis proposes a Mixed ac/dc Optimal Power Flow model (M-OPF) where the traditional ac OPF model is extended to incorporate a detailed steady-state VSC-MTDC system model. In the M-OPF model, the power flow equations of both ac and VSC-MTDC systems are solved simultaneously, and the fundamental technical limits of VSC (the maximum VSC valve current and the maximum dc voltage) are used as operation constraints of VSC stations. A cost-benefit approach using the M-OPF model as the calculation “engine” is proposed to determine the preferred VSC-MTDC alternatives in the transmission expansion planning process. In this approach, the operational benefits from using VSC-MTDC systems are evaluated against their investment costs to derive the Benefit-to-Cost Ratios (BCR) which reflect the cost-effectiveness of the alternatives. Case studies are conducted using the Nordic 32-bus system to investigate the capability of the VSC-MTDC system in reducing the generation costs and the power losses. The results of these case studies have revealed that the magnitudes of these reductions are very much dependent on the configuration and location of the embedded VSC-MTDC system. The reduction of the generation costs is mainly due to the fact that the VSC-MTDC system enhances transmission capacity by its capability of providing controlled reactive power support and mitigating heavily loaded ac transmission lines. The reduction of the power losses is a trade-off result between the reduced transmission losses by using VSC-MTDC system and the extra power losses introduced by VSC stations. A long-term transmission expansion planning exercise is also conducted using the same study cases to identify the preferred VSC-MTDC alternatives based on the calculated BCR values of all VSC-MTDC alternatives. The study results show that the alternatives with high benefits may not necessarily have high BCRs when the investment costs are considered.