Frequency and voltage control in mixed AC and DC transmission networks

Abstract

The construction of a European Super Grid, where existing AC systems are connected through High Voltage Direct Current (HVDC) corridors, is one contribution to accommodating a high penetration of renewable energy by diversifying the energy mix and trading surpluses from some regions. This raises questions over the dynamics of mixed AC and DC systems that need to be thoroughly studied. This thesis examines the level of coupling of AC networks interconnected via HVDC and the provision of system services between AC networks via DC links. Since areas such as the North Sea may see interconnected DC links, control of such multi-terminal DC grids (MTDC) in the case of converter outages is also pursued. To form realistic case studies, a number of mixed AC and DC transmission networks were modelled and validated, in particular a dynamic equivalent model of the Great Britain transmission system coupled via voltage source converter (VSC) HVDC to the Scandinavian transmission network. To answer questions over the interaction of the dynamics of modular multi-level converters (MMC) with AC system dynamics, reduced dynamic models of such converters were developed taking an energy balancing approach. The impacts that two-level or modular multi-level VSCs produce on the dynamics of the hosting AC grids were investigated through perturbation studies. The displacement of conventional generation by converter-interfaced sources sees a decrease in frequency response provision and inertia and calls for alternative provision. Several methods for enhancing the frequency recovery (post outage) of AC grids were designed for and applied to VSCs of HVDC interconnectors. Supplementary droop control was found to improve the frequency nadir after a loss of in-feed event at the cost of passing some of that loss to the adjacent AC systems via the HVDC interconnection. Identifying and exploiting the overload capability of MMC enabled the sharing of primary reserves and improvement of the nadir frequency by allowing extra power to be transferred through HVDC links. Proposals to use the internal energy storage capability of MMCs for inertia provisions were investigated but the results show that the scope for this is very limited. It is important that where MTDC grids are envisaged, the power flow control is able to deal with converter station outages and not cause large disturbance to the DC or AC networks. Deficiencies with proposed methods were identified and a new proposal for coordinated control with multiple bus masters was made and control performance improvements demonstrated.