Increased Observability in Electric Distribution Grids : Emerging Applications Promoting the Active Role of Distribution System Operators

Abstract

This thesis addresses supervision and control of horizontally integrated electric power systems, in which Distribution System Operators (DSOs) assume an active role. Focus lies on the technical possibilities emerging from the expanding Information and Communication Technology (ICT) and monitoring infrastructure in distribution grids. Strong emphasis is placed on experimental verifications of the investigated concepts wherever applicable. Electric grids are changing, and so are the roles of system operators. The interest in sustainable energy and the rapidly increasing number of Distributed Energy Resources (DERs) throughout the grid entail a range of challenges: vertical integration schemes, with a few bulk generators supplying mostly passive customers, are toppled; renewables are highly volatile; and the grid’s resilience to load changes is weakened due to fewer direct-coupled synchronous machines. The DSO is in a central position to all these developments. Their awareness of generation and consumption flexibility of customers and DERs brings DSOs in the position to be more actively involved in grid management. Several technical applications, emerging from the premise of increased observability in distribution grids, that promote this active role are explored in this work. Grid models are fundamental to power grid planning and operations, but quality and coverage of distribution grid models are typically less pronounced compared to transmission networks. The expanding monitoring and ICT infrastructure across the Low and Medium Voltage levels provides new, non-invasive opportunities to enhance topological knowledge, which is demonstrated on two different approaches. First, available measurements are used for estimating the π- parameters of lines and cables. The methodology handles noisy, unsynchronized data compromised by systematic measurement errors, and has been validated on generated and real data obtained from SYSLAB. Second, ICT allows for extending the model scope without a central topology storage. The presented binary connectivity extension approach preserves details of the local neighbourhood, which is augmented with information from remote areas that is gradually reduced in detail. Reduction performance is evaluated numerically as well as on the IEEE 906-bus European Low Voltage test feeder. Supervisory Control And Data Acquisition (SCADA) systems monitor and control the power grid, and enable human operators in control centres to interact with it. The rising number of data sources and faster grid dynamics are a challenge to static SCADA architectures, which is tackled by modern ICT means of distributed data acquisition, processing, and exchange. For that purpose, a Multi Agent System (MAS)-based framework for prototyping distributed applications was developed. Software agents represent power system elements, offering high-level application interfaces and flexible low-level data acquisition mechanisms, and a Common Information Model (CIM)-like abstraction layer in between. A distributed Situational Awareness (SA) concept is subsequently designed and tested on models of SYSLAB and the Power Networks Demonstration Centre (PNDC), focusing on two features. Related to topology reduction covered earlier, the first is dynamic gathering and representation of topological information distributed throughout the grid. The second feature is live data visualisation on top of the topologies, in which power balances and other data are queried on demand. Improved models and SA benefit the operation of interconnected power systems as a whole, exemplified on Load Frequency Control (LFC). While LFC will likely remain the Transmission System Operator (TSO)’s responsibility, DSOs play a major role due to their local awareness. The presented LFC approach consequently uses direct state observations to determine area imbalances, and actively involves primary devices in frequency control to achieve tuning-free secondary control. Stability proofs and performance studies with Automatic Generation Control (AGC) are conducted analytically, in simulations, and in SYSLAB, on a three-area power system. Considering the option of DSOs performing balancing tasks, a consecutive load mobility approach accounts for the fact that DSO areas are not necessarily self-sufficient. Missing power is covered by neighbouring areas to confine problems in a small region, which is demonstrated in SYSLAB on a two-area, grid-connected setup. The results highlight the advantages of harnessing the information obtained directly from the distribution grid for real-time supervision and control. Better models contribute to improved SA of DSOs, and distributed data exchange mechanisms allow the coordination of a multitude of DERs. Together, the proposed solutions will strengthen the DSO’s position to cope with current and future challenges. Regulatory issues concerning data ownership and privacy, as well as questions on the hierarchies between TSOs, DSOs, and third-parties like aggregators, are yet to be settled by the corresponding authorities, and are therefore not covered in the frame of this work. Nevertheless, with the current speed of development of technical solutions and regulations, near-complete observability of the electric grid will be achieved in the foreseeable future. Harnessing the increased observability already benefits the unbundling of electricity markets, and is imperative to ensure security of the grid.