Publication
Hybrid multiterminal AC/DC networks are gaining interest nowadays in both domains of microgrids and high-voltage direct current (HVDC) networks as they have the potential to enhance the share of distributed generation within future power grids and support large-scale power transfer capabilities. However, effective grid-aware control of these networks, especially in real-time, requires computationally efficient and exact models, which are not available in the existing literature.
In this respect, this PhD thesis proposes a set of computationally efficient tools for steady-state analysis and real-time, grid-aware control of hybrid AC/DC networks: (1) a linear state estimation (SE) process, (2) a unified power flow model, and (3) a linearised optimal power flow (OPF) framework based on the concept of sensitivity coefficients (SCs).
The proposed methods extend standard AC power system theory and integrate the DC network and the AC-DC interfacing converters (IC) in a unified approach. While the DC network can be modelled similarly to a standard AC power system, the existing literature does not include the ICs in a holistic manner. The modelling equations of the ICs depend on their operational modes, such as power mode, voltage mode (AC or DC), or grid-forming operation, and have to account for the unbalanced AC grid conditions. Therefore, to fully capture the interactions between the AC and DC grids, the IC's non-linear behaviour needs to be modelled accurately.
The proposed SE, power flow, and SC-based OPF models accurately capture these interactions by treating the hybrid AC/DC network in a unified manner. The linear state estimation method is formulated as a recursive Kalman Filter using a linear and non-approximated measurement model based on the complex modulation index of a transformer-like IC model. The unified power flow model incorporates the AC network, DC network, and ICs, which may operate in various operational modes. In addition to the standard AC node types, Slack, PV, and PQ, six new node types are introduced to represent the DC buses and the relevant ICs. The linearised OPF leverages an efficient computation of the power flow SCs. These SCs are used in the OPF formulation to represent the non-convex grid constraints linearly.
The three proposed methods are rigorously validated through numerical time-domain simulations on low-voltage hybrid AC/DC microgrids and HVDC networks. In addition to these numerical validations, the real-time SE process and the linearised SC-based OPF framework are experimentally validated on a low-voltage, 26-bus AC/DC microgrid developed as part of this PhD thesis. The hybrid network consists of an existing AC microgrid that is extended with a DC network and interfaced via four ICs capable of operating in various operational modes. In addition to the above mentioned use cases, an elaborate framework for the islanding and resynchronisation of hybrid AC/DC networks has been developed. The framework leverag