Converting AC Distribution Lines to DC to Increase Transfer Capacities and DG Penetration

Transfer capacities of urban distribution networks need to be increased to fulfill the increasing load demands and to accommodate distributed generation (DG). However, there are limited spaces to build new substations and lines, and curtailment of DGs may happen due to voltage violation during DG outputs fluctuation. This paper proposes and analyzes various methods to convert some existing ac MV lines to dc lines in order to form a hybrid ac/dc distribution network, based on which transfer capacities of lines can be increased, and flexible power shift can be achieved through a voltage source converter between two lines. The increases of transfer capacities are quantified. Optimal operation to fully utilize the increased capacities is achieved, in which losses are minimized in day-ahead scheduling, and node voltages are regulated real-time within security ranges based on limited measurement. Not only reactive power but also real power optimization are designed to maximize load supply and DG accommodation. A sensitivity method is proposed considering relatively large r/x ratio of an MV distribution network, which is effective for the real-time voltage regulation. Simulations are performed to verify the proposed method.