Optimal Power Flow Control in Parallel Operating AC and DC Distribution Links

DC links with back to back voltage source converters are usually built alongside the existing medium voltage ac distribution grids for infrastructure reinforcement. The distribution network operators need to run multiple of such parallel ac and dc links between two substations at optimal efficiency. This article shows that the active power steering capability of the dc link can be used to dynamically vary the share of power flow in the ac link such that the system operating efficiency is maximized for varying power demand, grid voltage, dc-link voltage, converter efficiency, link length, and conductor area. The algorithm developed based on the derived exact and estimated solution for this parallel ac-dc link power sharing ratio is proved through simulations on a 10 kV, 30 MVA system. The concept is validated using experiments on a scale-down lab model. Using case-study with adapted measured substation data of hourly average power demand profile for one year, it is shown that annual energy saving potential in the range of 8-92 MWh can be achieved with varying link length between 10-20 km for 5-20 kV grid voltage and 185-630 mm$^2$ conductor area if the proposed optimal power flow control is used.