Restructuring medium voltage distribution grids: Parallel AC-DC reconfigurable links

Energy transition will inevitably lead to greater electrification. For example, it is anticipated that electrical energy demand will rise by at least 2-3 times by 2050 with increasing share of electric vehicles and heat pumps. This will translate to significant increase in power demand on the existing medium voltage distribution grid, resulting in structural challenges on such predominantly radial ac networks. Dispersed and variable renewable energy resources further introduce power mismatches with local regions of excess generation and consumption. Under such a scenario, Distribution Network Operators (DNOs) must explore solutions to restructure the grid infrastructure with the goal of capacity reinforcement, improved controllability and efficient power redirection.

In this thesis, dc based technologies are proposed to realize the grid transition from purely ac to hybrid ac-dc networks to address the anticipated challenges posed by energy transition. Refurbishing the existing ac links to operate under dc conditions is shown to enhance the power transfer capacity by approximately 50\,\% within the studied constraints at higher energy efficiency. Reconfigurability between such parallel operating ac and dc links can further increase the achievable capacity gains during (n-1) contingencies, which relates to the capacity maintained with a single component failure in the system. Further, dc interlinks are introduced to weakly mesh the radial ac distribution networks for efficiently redirecting the power to minimize local demand mismatches, prevent branch overloads and increase availability of the grid.

The relevant component engineering aspects such as converter design as well as cable insulation performance under ac and dc conditions are developed to support the underlying assumptions. Control challenges such as mitigation of common mode current specific to parallel ac-dc link systems are explored. The concept of optimal active power steering capability of the dc link while supporting the full reactive power demand are developed mathematically and demonstrated using an experimental set-up of the proposed system.

In future, therefore, it is suggested that dc technologies will play a important role in restructuring the medium voltage ac distribution grids for achieving higher flexibility, controllability and inter-connectivity with enhanced capacity and efficiency. The proposed concepts of this thesis can be extended to integrate renewable energy resources directly to the embedded dc links, making the system multi-terminal and thus transition towards a universal dc grid.