Optimal design and operation of hybrid AC/DC microgrids towards more resilient energy systems

Humanity has recently embarked on an ambitious journey to decarbonise the world in an effort to reverse the adverse effects of the practices deployed the past two centuries, which have led us to a serious climate change. Regulation and policy have been introduced by countries worldwide to encourage, promote and compensate these efforts, including the UK’s recent announcement pledging to be the first major economy eliminating their contribution to carbon emissions. A comprehensive framework for coordinating actions to reduce CO2 emissions is expected to be put forward addressing the challenges arising, including a significant transformation of the energy system, a restructuring of the energy market and a new way of engaging with the wider public to making what is conceptually significant, perceptually prominent. Within this context, the concepts of smart grids and, in a smaller scale, microgrids have arisen to assist these decarbonisation efforts. Microgrids have a great potential to actively contribute to grid health status, however the current practices in network design and operation hinder the capabilities of these types of networks. In fact, they do not allow microgrids to realise their potential and enable a paradigm shift in delivering resilience and security of supply from redundancy in network assets and preventive control to a more intelligent operation at the distribution level through corrective control actions. This thesis proposes innovative design and operational models for microgrids, and particularly hybrid AC/DC microgrids, that optimise the total system cost while satisfying pre-specified resilience targets. The modelling framework introduced comprises of a tailor-made genetic algorithm (i.e. for optimal sizing) combined with a detailed AC optimal power flow (OPF) that captures the technical characteristics of both the AC and DC subgrids with an extensive set of technologies considered. The proposed approach, being able to capture technical characteristics such as voltage and frequency through a detailed power flow algorithm, provides accurate solutions and therefore can meaningfully address operational challenges of microgrids. Its capability to additionally capture contingencies ensures that the proposed sizing solutions are suitable both during normal operation and transient states. Finally, the genetic algorithm provides convergence of the model with relative computational simplicity, which is why it has been particularly developed for the needs of this thesis. An innovative Dynamic Stability Constrained OPF is proposed as an extension that incorporates differential equations, such as the swing equation, characterising the operation of power systems. This is achieved via appropriate conversion of the equations to numerically-equivalent algebraic equations. This novel aspect will enable optimal decisions to be taken considering stability properties, which are undeniably necessary in the context of energy systems with renewable penetration of above 50% and are proven to significantly impact the system cost. Resilience is also central to this thesis, hence it is discussed and limitations of current research typically confusing resilience with reliability are identified. Subsequently, a definition to help the industry merge towards a common understanding and a way of quantifying resilience (particularly relevant to microgrids) are proposed. As a last step, this thesis identifies the imminent digitalisation energy systems are undergoing and utilising its merits, it introduces a strategy for interaction between distribution networks (incorporating microgrids as one type of resource) and transmission systems with the focus being on exchange of voltage support services. The operational models developed in this thesis could prove to be useful towards optimising the portfolio of assets to provide the services required.Open Acces

Enhanced DC voltage control strategy for fault management of a VSC-HVDC connected offshore wind farm

This paper proposes a DC voltage control strategy for fault management taking into advantage the operation of the master controller located in the offshore AC substation platform. The issue resolved via the proposed controller relates to over-voltages caused in the HVDC links when the power transfer onshore is disrupted due to faults occurring at the AC side of the onshore grid. The control strategy presented in this paper proposes an effective way of maintaining the DC over-voltage within safety limits via reducing the connected wind farm power output. The operation of the aforementioned control strategy requires small computational power and no communication

Provision of voltage ancillary services through enhanced TSO-DSO interaction and aggregated distributed energy resources

The electrical energy generated from renewable energy resources connected to transmission and distribution systems and the displacement of synchronous generators continues to grow. This presages a paradigm-shift away from the traditional provision of ancillary services, essential to ensure a robust system, from transmission-connected synchronous generators towards provision from synchronous and non-synchronous generation (including distribution-connected resources). Given that the available resources at the disposal of system operators are continuously increasing, the flexibility for operating the network can be enlarged. In this context, this paper introduces a dedicated voltage ancillary services strategy for provision of reactive power. A main feature of the proposed strategy is that it is technology-neutral, unlike existing ones that are focused on synchronous generators. The system need for voltage stability is placed at the core of this strategy, which is translated into a requirement for reactive power provision. The proposed strategy achieves, through the combined utilization of distributed generation and traditional resources, to defer the investments in reactive compensating equipment. Dynamic and transient studies are conducted to demonstrate the technical benefits of the strategy, while its practical feasibility is also validated through hardware-in-the-loop testing

Centralised busbar differential and wavelet-based line protection system for multi- terminal direct current grids, with practical IEC-61869-compliant measurements

This paper presents a method for discriminative detection of DC faults on VSC-powered multi-terminal HVDC transmission systems using two fundamental guiding principles, namely instantaneous current-differential and travelling waves. The proposed algorithm utilises local voltage and current measurements from all transmission lines connected to a DC busbar, and current measurement from the DC side of the converter. The scheme operates at a sampling frequency of 96 kHz which conforms with IEC 61869-9. No long distance communication is involved while measurements and signal exchange within DC substations are enabled by the utilisation of IEC 61850. Performance is assessed firstly through detailed transient simulation, using verified models of modular multi-level converters, hybrid DC circuit breakers and inductive DC-line terminations. Furthermore, practical performance and feasibility of the scheme is evaluated through laboratory testing, using the real time Opal-RT hardware prototyping platform. Simulation and experimental results demonstrate that the proposed protection algorithm can effectively, and within a very short period of time (i.e. less than 1 ms), discriminate between busbar and line faults (internal faults), while remaining stable during external faults. Additionally, it has been demonstrated that IEC 61869-9 is suitable for enabling fast DC protection schemes incorporating travelling waves

Impact of active distribution networks on transient stability

The increasing penetration of distributed energy resources (DERs) has turned passive distribution networks into active, in turn affecting the dynamic behavior of the system. It is essential to investigate to what extent transmission system dynamics are affected by these changes. This paper aims at identifying the impact of active distribution networks (ADNs) on transient stability. The studies consider two main scenarios. In the first, same initial conditions are considered in an attempt to investigate the effect of ADN dynamics alone. In the second, prefault operating conditions are also varied in a realistic manner through generation of representative daily profiles for load and renewable generation. The impact of fault, ADN, and critical SG location is investigated. In addition, the response of DER control and protection to short and longer faults highlights how different aspects of transient stability are affected

Digital architecture for monitoring and operational analytics of multi-vector microgrids utilizing cloud computing, advanced virtualization techniques, and data analytics methods

Microgrids are considered a viable solution for achieving net-zero targets and increasing renewable energy integration. However, there is a lack of conceptual work focusing on practical data analytics deployment schemes and case-specific insights. This paper presents a scalable and flexible physical and digital architecture for extracting data-driven insights from microgrids, with a real-world microgrid utilized as a test-bed. The proposed architecture includes edge monitoring and intelligence, data-processing mechanisms, and edge–cloud communication. Cloud-hosted data analytics have been developed in AWS, considering market arrangements between the microgrid and the utility. The analysis involves time-series data processing, followed by the exploration of statistical relationships utilizing cloud-hosted tools. Insights from one year of operation highlight the potential for significant operational cost reduction through the real-time optimization and control of microgrid assets. By addressing the real-world applicability, end-to-end architectures, and extraction of case-specific insights, this work contributes to advancing microgrid design, operation, and adoption

Trust & Fair Resource Allocation in Community Energy Systems

The energy sector faces numerous challenges, including rising electricity costs and inconsistent services due to network overload, often requiring the involvement of a central network operator to address these issues. However, a user-centric approach that prioritizes demand-side management, exemplified by decentralized Community Energy Systems (dCES), presents a promising solution to energy distribution and supply network challenges. dCES can be conceptualized as a small-scale, dynamic distribution network seamlessly integrated into the broader framework of the Smart Grid. In this paradigm, prosumers play an active role, as they must contribute to and draw from a shared energy resource pool, with the overarching goal of avoiding depletion. Specifically, various individuals with different energy consumption patterns and preferences work together to solve collective action problems, i.e., blackouts. Motivated firstly by fair resource allocation, and secondly by the idea that trust is a crucial factor for successful collective action among diverse individuals, we developed a suitable Multi-Agent System (MAS) for dCES to prevent resource depletion. Our experimental results show that introducing trust into dCES can lead to successful collective action, resulting in stable energy networks

Analysis of a wind farm with doubly fed induction generator based wind turbines interconnected to the grid

123 σ.Η αυξανόμενη χρήση της αιολικής ενέργειας στη σημερινή εποχή σε συνδυασμό με τη ραγδαία ανάπτυξη των τεχνολογιών ανεμογεννητριών έχει προκαλέσει το έντονο ενδιαφέρον για τη βέλτιστη αξιοποίηση του ανέμου σε ευρύ φάσμα ταχυτήτων. Στο πλαίσιο αυτό, βασική απαίτηση των αιολικών συστημάτων αποτελεί η αδιάλλειπτη παροχή μέγιστης ισχύος στο ηλεκτρικό δίκτυο ακόμα και σε συνθήκες διαταραχών. Στόχος της παρούσας διπλωματικής εργασίας είναι η ανάπτυξη κατάλληλου μοντέλου για την ανάλυση και τον έλεγχο ασύγχρονης γεννήτριας διπλής τροφοδότησης, που είναι πλέον πολύ διαδεδομένη στα αιολικά συστήματα. Αρχικά, περιγράφονται τα επιμέρους υποσυστήματα της ανεμογεννήτριας που μελετήθηκε. Συγκεκριμένα, γίνεται αναφορά στο αεροδυναμικό μέρος, αναλύεται μαθηματικά η ασύγχρονη γεννήτρια διπλής τροφοδότησης και επεξηγείται η λειτουργία του back – to – back μετατροπέα με λεπτομερή ανάλυση των επιμέρους τμημάτων του. Στη συνέχεια, εξετάζεται το σύστημα ελέγχου και ο τρόπος λειτουργίας του. Στο σημείο αυτό, περιγράφεται η τεχνική του διανυσματικού ελέγχου και αναλύεται ο έλεγχος της ανεμογεννήτριας τόσο από την πλευρά του δρομέα όσο και από την πλευρά του δικτύου. Ακόμη, περιγράφεται η λειτουργία του συστήματος ανεύρεσης σημείου παροχής μέγιστης ισχύος με δεδομένη χαρακτηριστική καμπύλη. Παρουσιάζονται τα χαρακτηριστικά αιολικού πάρκου και μελετώνται διάφορες περιπτώσεις λειτουργίας ώστε να αξιολογηθεί η εναρμόνισή του με τις τεχνικές απαιτήσεις του διαχειριστή του ελληνικού συστήματος. Στο πλαίσιο αυτό, προσομοιώνονται περιπτώσεις κανονικής λειτουργίας αλλά και διαταραχών στην πλευρά του δικτύου. Επιπλέον, συγκρίνονται οι αποκρίσεις του συστήματος της παρούσας εργασίας με εκείνες αντίστοιχου συστήματος της βιβλιογραφίας, προκειμένου να επιβεβαιωθεί η ακρίβεια των αποτελεσμάτων του προτεινόμενου μοντέλου. Τέλος, διερευνάται η επίπτωση στις αποκρίσεις του συστήματος ελέγχου με τυπικά κέρδη ανάδρασης ολοκληρωτικού σφάλματος με εκείνες που προκύπτουν μετά από βελτιστοποίησή τους με τη μέθοδο των Ziegler – Nichols.The fast development of wind energy in the current era combined with the rapid improvement of wind turbine technology has attracted the interest to optimize their use in wide wind speed ranges. In this context, an essential requirement for wind turbine systems is the uninterrupted supply of maximum power to the grid even under fault conditions. The aim of this thesis is the development of appropriate simulation models for analysis and control of doubly fed asynchronous generators, which are now widely used in wind systems. In a first step, the subsystems of the studied wind turbines are described. More specifically, the aerodynamic part of a wind turbine is examined, the doubly fed asynchronous generator is analyzed and the operation of the back – to – back converter is explained while the constitutive parts are detailed. In a second step, the control and its operating characteristics are examined. At this point, the vector control technique is described while the control actions of the wind turbine are analyzed both at the rotor side and at the grid side. The maximum power point tracking system (MPPT) based on a specific optimum power-speed characteristic is also described. In the followings, a wind farm is simulated under various operating conditions in order to assess fulfillment of the technical requirements of the Greek system administrator. In this context, both normal operating conditions as well as fault cases at the grid side are examined. Moreover, the response results of the studied grid interconnected wind farm system are compared to the corresponding ones of a system found in the literature, in order to validate the proposed model accuracy. Finally, the impact on the controller efficiency is examined when tuning the PI feedback gains by using the Ziegler – Nichols optimization technique.Αναστάσιος Δ. Ουλής-Ρούση

Impact of active distribution networks on transient stability

The increasing penetration of distributed energy resources (DERs) has turned passive distribution networks into active, in turn affecting the dynamic behavior of the system. It is essential to investigate to what extent transmission system dynamics are affected by these changes. This paper aims at identifying the impact of active distribution networks (ADNs) on transient stability. The studies consider two main scenarios. In the first, same initial conditions are considered in an attempt to investigate the effect of ADN dynamics alone. In the second, prefault operating conditions are also varied in a realistic manner through generation of representative daily profiles for load and renewable generation. The impact of fault, ADN, and critical SG location is investigated. In addition, the response of DER control and protection to short and longer faults highlights how different aspects of transient stability are affected