Electrical and Electronic Engineering - Theses
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ItemFlexibility and Grid Services from Distributed Multi-energy System( 2019)The energy system is in the transition towards a low carbon future. Large-scale renewable energy resources (RES) and distributed energy resources (DER) are replacing conventional generators, which places great challenges on the tra-ditional energy systems. The intermittent and uncontrollable nature of RES and lack of visibility of DER create more imbalances of supply and demand, which increase the demand for frequency control. On the other hand, the withdrawal of synchronous generators which are traditional grid services providers, further reduces system security. Additional flexibility and new grid services providers need to be sought in order to successfully integrate these emerging technolo-gies while maintaining the reliability and security of the system. Although the significance of consumer participation is seen as the “the heart of the transition”, the flexibility from consumers, DER, and other energy vectors and sectors, is yet untapped. This thesis aims to study the potential and possible ways of exploiting flexibility from such distributed multi-energy sys-tems (DMES), so as to aid the integration of RES. In this context, a comprehen-sive, integrated techno-economic modeling framework is developed, in order to identify, quantify and optimize the flexibility from DMES and develop new rel-evant business cases. This includes, a high-resolution multi-energy demand model to understand the “building block” of the energy system analysis, a mul-ti-markets, multi-services co-optimization model for optimal DMES operation, and a business case assessment model and an investment model for planning DMES under uncertainty to support new business cases. The power of the abovementioned contributions is demonstrated through various realistic case studies.
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ItemAdvanced management of residential battery energy storage in future distribution networks( 2019)In recent years, Australia and many other countries have seen a sharp rise in the number of residential solar photovoltaic (PV) system installations. This trend has also driven the adoption of residential battery energy storage (BES) systems, as they allow households to use their excess PV generation at other times. However, as it currently stands, most commercially available residential BES systems are controlled for the sole benefit of their owner, i.e., to reduce grid imports. But given their controllability, there exists a large opportunity for these systems to a) help mitigate the well-established PV issues in electricity distribution networks (e.g., voltage and thermal issues), or b) be used in the provision of services to the whole power system (e.g., frequency or energy services). For a), this thesis first assesses the performance of off-the-shelf (OTS) BES systems and demonstrates their inability and limitations to provide support in mitigating PV impacts. While overcoming these limitations has been the focus of several studies over the past years, there is need for practical and scalable, yet effective BES control strategies that provides benefits to both the customers and the distribution network to be developed. In this context, a decentralised control of BES systems is proposed which dramatically reduces PV impacts on the distribution network with minimal effects on households. As for b), in this thesis, the impacts of the widespread provision of services from residential BES systems on distribution networks are assessed. It is demonstrated that if left unconstrained, it can lead to severe issues. In this context, a distribution system operator (DSO) framework is proposed, using a three-phase AC optimal power flow-based approach, to ensure network integrity and fairness. The results show that it is not only possible to achieve this for multiple service providers, but it also unlocks much larger volumes of services compared to those achieved with the currently adopted fixed export limits of 5kW per phase. To assess the network performance as realistically as possible, this thesis uses an integrated medium voltage (MV) – low voltage (LV) network to consider the interactions of multiple voltage levels and fully capture the effects of DER (and the resulting reverse power flows). More specifically, a 22kV (i.e., MV) real feeder is used from Victoria, Australia, whereas the 400V (i.e., LV) circuits are modelled based on regional network design principles. Furthermore, the analyses utilise real smart meter demand, generation, and pricing data from the same region. The power flow assessments consider high granularity time-series data using three-phase four-wire unbalanced power flows. Finally, a Monte Carlo approach is adopted in the assessments in this thesis to cater for the uncertainties related to demand, generation, and locational aspects of distribution networks.
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ItemControl of electronically-coupled distributed energy resources in microgrids( 2017)The current power grid is going through a paradigm shift due to large scale integration of renewable energy sources. The safe, secure and reliable operation of renewable energy sources integrated power grid mostly depend on how well the renewable energy sources are integrated to the grid, controlled, managed and monitored. One of the emerging means for the integration of renewable energy sources to the grid is to treat them as distributed energy resources (DERs). DERs can be considered as smaller power sources comprising renewable energy sources such as solar photovoltaic (PV), wind turbine etc. along with combined heat and power (CHP) or cogeneration systems, micro turbines and energy storages. DER technologies are commonly installed at end user premises in the view to supply all or a portion of the customer's electric power demand. The inclusion of energy storage and backup generators such as micro turbines, CHP etc., enables DER units to inject power to the grid. With the growing number of DER units in smart distribution networks, control and management of the overall system have emerged as one of the major challenges. The successful implementation of the future smart grid with high penetration of DER units relies heavily on an efficient and reliable communication infrastructure. In the view to ensure a successful integration of renewable distributed energy resources, in this thesis, we propose different control strategies to address different issues in control of DERs. These control strategies are proposed for electronically coupled distributed energy resources which operate in grid-connected or islanded microgrids. We first study the adverse effects of unbalanced and/or harmonically polluted local loads on the performance of microgrids. We propose a multi-input muti-output control strategy for dispatchable power electronic based DERs in islanded and grid-connected operation modes. The controller is designed based on the $dq$ model of the DER unit, and internal model control principle, incorporating the theory of integral control and repetitive controller, is used to mitigate the effect of unbalanced and harmonically polluted loads. Furthermore, we propose a robust control method to enhance power sharing between dispatchable electronically-coupled DERs in an islanded microgrids. The controller is robust against nonlinear load disturbances. Other control objectives such as minimizing control effort and transient response behavior are also taken into account while designing the controller. \\ We also study the impact of sensor faults and erroneous measurements on the performance of DER system both in islanded and microgrid systems. Simulation results show that sensor faults can adversely affect the performance of DERs. A fault tolerant control is then proposed to detect and estimate the sensor faults or erroneous data measurements. The estimated faults are used to compensate for bad data. Results show that the proposed control strategy is effective in fault tolerant control of grid-connected and islanded DERs. Finally, we show that stable design of controllers for islanded and grid-connected modes does not necessarily mean that the DER system remains stable during switching between one mode of operation to another. To tackle this problem, we propose a controller design strategy using switched linear system theory. Using the proposed method, not only robustness and transient behavior of DER system in each mode are guaranteed, but also a stable operation during switching from islanded controller to grid-connected (and vice versa) is ensured. The effectiveness of the proposed controllers are demonstrated through various comparative simulation results carried out on benchmark microgrid systems.