Towards an Original Theory of Power Flow Control in Meshed AC/DC Hybrid Transmission Networks for the Nigerian Grid Modernisation Programme

This research develops an original theory of power flow control in meshed alternating current and direct current hybrid transmission networks, specifically motivated by the engineering challenges of the proposed Nigerian grid modernisation programme that includes a mix of new AC transmission lines, DC links between the major load centres, and flexible AC transmission system devices. The Nigerian transmission network's current radial and weakly meshed topology limits power flow controllability and resilience, and the planned introduction of high-voltage direct current links between Abuja, Lagos, and Kano creates a novel hybrid AC/DC meshed network for which existing power flow theory and control methods are inadequate. The theoretical contributions proceed in four steps: derivation of a unified power flow formulation for hybrid AC/DC meshed networks with voltage source converter HVDC links, eliminating the artificial interface constraints of existing sequential AC/DC power flow methods; development of optimal power flow theory for hybrid networks that exploits the continuous controllability of VSC-HVDC links as internal decision variables to achieve global loss minimisation and congestion relief beyond what is possible with AC control alone; formulation of a multi-period security-constrained optimal power flow incorporating unit commitment and N-1 contingency constraints specific to the Nigerian grid's generation adequacy conditions; and derivation of distributed control laws for real-time power flow regulation in the hybrid network using a passivity-based multi-agent control architecture. All theoretical results are validated through simulation on a representative Nigerian hybrid AC/DC network model. Keywords: hybrid AC/DC network, power flow control, HVDC, optimal power flow, Nigeria.