Control of AC/DC systems for improved transient stability and frequency support provision

Abstract

In this thesis, control of future AC/DC systems for improved system dynamic performance is studied. The objective is to determine mechanisms for providing AC network services (e.g. frequency support, damping, etc.) through coordinated control of HVDC power converters and FACTS devices while considering increased levels of wind generation. In particular, this work addresses some of the concerns associated with the stability of the future Great Britain (GB) transmission network as it evolves to support low carbon generation scenarios and the use of DC grids to integrate offshore renewable resources and form a sub-sea interconnection across Europe. The contributions of this thesis are in two main areas: emergency control for power system stabilization and exchange of frequency support across a DC grid. Fast control of FACTS devices and HVDC links can be exercised as a post-fault corrective action to maintain system stability without the need of constraining pre-fault transfer levels. This work employs a model predictive control (MPC) scheme that relies on system wide-area measurements to preserve the system stability after critical contingencies. MPC can explicitly account for system constraints and changing operating conditions and is therefore suited for on-line applications and power electronic actuators with limited short-term overload capability. The effectiveness of the proposed approach is demonstrated using time domain simulations on representative equivalent models of the future GB transmission network. A detailed analysis of the dynamic behaviour and stability issues associated with the GB transmission grid have also been presented. In the DC grid context, this thesis investigates the provision of frequency services considering frequency droop loops in the control of the converters. The interaction between onshore AC systems and a DC grid is analyzed through an extended steady-state formulation. A methodology for providing frequency response from offshore wind farms connected through a DC grid is also proposed. The performance of this scheme is illustrated both analytically and also through simulation results.