Benefit Analysis of a Hybrid HVAC/HVDC Transmission Line: a Swiss Case Study

The conversion of existing multi-circuit HVAC transmission infrastructure to pure HVDC or hybrid AC/DC overhead lines is becoming a predominant measure to accelerate grid reinforcements. As the visual impact is identical, acceptance is assumed to be higher compared to a new construction. This study investigates operational benefits of a future hybrid AC/DC line in the Swiss transmission network. A voltage source converter based HVDC (VSC-HVDC) system model is developed for Optimal Power Flow (OPF) and integrated in the AC network. Three 2025 network configurations are created to demonstrate different advantages of the hybrid line. Benefits of two corridor options are evaluated by comparison against the pure AC base case. It is demonstrated that the hybrid conversion is beneficial since it can lead to reduction in the AC line loading, lower operating costs and increased utilization of the network infrastructure by enabling higher transit flows. However, efficiency of the transmission system is reduced as a consequence of high converter losses.