Control and optimization of modern power networks : hybrid AC/DC networks and inverter-based microgrids

Electrical power is essential to modern society, and is necessary for innumerable applications from lighting, heating, household appliances, to large-scale machinery, communication and transportation. Ensuring a reliable, efficient and sustainable electrical power system is therefore crucial. At present, the generation, transmission and distribution of electrical power is being rapidly transformed by advances in technology and sustainability initiatives. Some of the most important new trends include: an increasing amount of distributed generation; widespread use of power electronic converters for generation, energy storage, and loads; and increasing use of DC transmission, forming hybrid AC/DC networks. This raises new challenges and opportunities for the power system, particularly in terms of its control and optimization. In this thesis, we will consider some of these challenges for the control and optimization of contemporary and future power systems, focussing especially on hybrid AC/DC networks and converter-based microgrids. Hybrid AC/DC networks are an effective solution for future power systems, due to the increasing number of converter-based loads and distributed energy resources. The hybrid AC/DC network allows the advantages of both AC and DC networks to be combined. By using high-efficiency converters to connect the AC and DC grids, the overall efficiency of the network can be improved. However, the interconnection of AC and DC networks via interlinking converters (ILCs) to form a hybrid network brings new technological challenges, one key area being the control of such a network. The network, and especially the interlinking converter, must be controlled to ensure that the DC and AC subsystems coordinate to stabilize the network and allocate power appropriately. This is an area which has attracted considerable recent interest due to the non-triviality of the control design. In this thesis, we discuss two main contributions to the literature on hybrid AC/DC networks: the first is to present primary and distributed secondary control schemes for hybrid AC/DC networks. Using a simple model for the interlinking converter, valid for slower timescales, we prove stability and optimality. We demonstrate our proposed controllers with realistic simulations, and show improved transient performance and more accurate power allocation compared to the traditional dual-droop approach. The second part of our work considers dynamic models of the ILCs for slightly faster timescales, and proposes a passivity framework for hybrid AC/DC grids. The use of passivity allows the derivation of decentralized conditions through which the stability of the network can be guaranteed. We discuss how an appropriate ILC control design can facilitate an appropriate power allocation in the network and demonstrate the proposed design with advanced simulations. The second major area of interest is converter-based AC microgrids. Distributed generation and battery storage are being integrated into the power network at an increasing rate, and these are connected to the network via power converters. To enhance efficiency and reliability, it is envisioned that distributed generation, storage and loads will form microgrids which are able to operate autonomously if necessary. Autonomous operation poses challenges in terms of frequency and voltage regulation, requiring grid-forming inverters to coordinate the regulation of the frequency and voltage. A further challenge is to achieve an appropriate steady-state power allocation between multiple sources in the network. This thesis presents several contributions in this area. Firstly, we present a passivity framework to obtain decentralized conditions for assuring stability, and consider its applicability to grid-forming inverter control schemes in the literature. Taking existing control schemes, we assess their passivity properties numerically and discuss how these can be improved. We then design a state feedback controller which: satisfies the passivity condition for stability; allows disturbances and load changes to be regulated by contributions from multiple inverters; and allows a wide range of performance specifications to be achieved. Again, realistic simulations are used to demonstrate the proposed control approach. We also demonstrate that the passivity framework is applicable for general resistive-inductive-capacitive models of the transmission lines with an arbitrary number of states and length-varying parameters. We then present work on optimizing low voltage distribution networks with controllable battery energy storage. This is a highly relevant problem as distribution networks around the world are experiencing an uptake of energy storage systems. The proposed formulation in this chapter allows the incorporation of various important features that have not been addressed in the literature. Previous literature on convexified optimal power flow had not investigated the case of unbalanced distribution networks, especially regarding unbalance constraints, power loss in the neutral wire, and meshed network configurations. These effects are practically important in many distribution networks but existing methods were not able to incorporate this into a convex formulation. The proposed method is then used to demonstrate optimized dispatch of energy storage systems in a suitable four-wire unbalanced distribution test network. All our analytical results are network independent and have been verified with realistic case studies in MATLAB/Simulink.