Pitfalls of Zero Voltage Values in Optimal Power Flow Problems

The existence of strictly positive lower bounds on voltage magnitude is taken for granted in optimal power flow problems. Nevertheless, it is not possible to rely on such bounds for a variety of real-world network optimization problems. This paper discusses a few issues related to 0 V assumptions made during the process of deriving optimization formulations in the current-voltage, power-voltage and power-lifted-voltage variable spaces. The differences between the assumptions are illustrated for a 2-bus 2-wire test case, where the feasible sets are visualized. A nonzero relaxation gap is observed for the canonical multiconductor nonlinear power-voltage formulation. A zero gap can be obtained for the branch flow model semi-definite relaxation, using newly proposed valid equalities.Comment: 5 pages. Submitted to 2023 IEEE PES General Meetin

Optimal Power Flow in Four-Wire Distribution Networks: Formulation and Benchmarking

In recent years, several applications have been proposed in the context of distribution networks. Many of these can be formulated as an optimal power flow problem, a mathematical optimization program which includes a model of the steady-state physics of the electricity network. If the network loading is balanced and the lines are transposed, the network model can be simplified to a single-phase equivalent model. However, these assumptions do not apply to low-voltage distribution networks, so the network model should model the effects of phase unbalance correctly. In many parts of the world, the low-voltage distribution network has four conductors, i.e. three phases and a neutral. This paper develops OPF formulations for such networks, including transformers, shunts and voltage-dependent loads, in two variable spaces, i.e. current-voltage and power-voltage, and compares them for robustness and scalability. A case study across 128 low-voltage networks also quantifies the modelling error introduced by Kron reductions and its impact on the solve time. This work highlights the advantages of formulations in current-voltage variables over power-voltage, for four-wire networks.Comment: 10 pages, submitted to Power Systems Computation Conference 202

Combined Unbalanced Distribution System State and Line Impedance Matrix Estimation

To address the challenges that the decarbonization of the energy sector is bringing about, advanced distribution network management and operation strategies are being developed. Many of these strategies require accurate network models to work effectively. However, distribution network data are known to contain errors, and attention has been given to techniques that allow to derive improved network information. This paper presents a novel method to derive line impedance values from smart meter measurement time series, with realistic assumptions in terms of meter accuracy, resolution and penetration. The method is based on unbalanced state estimation and is cast as a non-convex quadratically constrained optimization problem. Both line lengths and impedance matrix models can be estimated based on an exact nonlinear formulation of the steady-state three-phase network physics. The method is evaluated on the IEEE European Low Voltage feeder (906 buses) and shows promising results

Making Distribution State Estimation Practical: Challenges and Opportunities

In increasingly digitalized and metered distribution networks, state estimation is generally recognized as a key enabler of advanced network management functionalities. However, despite decades of research, the real-life adoption of state estimation in distribution systems remains sporadic. This systematization of knowledge paper discusses the cause for this while comparing industrial and academic experiences and reviewing well- and less-established research directions. We argue that to make distribution system state estimation more practical and applicable in the field, new perspectives are needed. In particular, research should move away from conventional approaches and embrace generalized problem specifications and more comprehensive workflows. These, in turn, require algorithm advancements and more general mathematical formulations. We discuss lines of work to enable the delivery of tangible research.Comment: 10 page

Battery Energy Storage Systems and Distribution Grid Support

Inextricably, generation of electricity is balanced with respect to consumption. Conventionally, electricity is generated in response to demand. The integration or renewable energy sources poses challenges to the operation of the power system and increases the demand for flexibility. Part of the solution is to improve the flexibility of generators and consumers. Next, storage technology can be used to match supply and demand in time, by storing the electric energy in the meanwhile. Finally, the grid also represents a source of flexibility. By connecting generators and consumers through the transmission and distribution infrastructure, matching of supply and demand is improved. This dissertation deals with the complementarity of power grids and storage.The storage of electricity represents a combination of three functions: consuming electricity, accumulating the energy in some form, and finally generating electricity again. Pumped hydro storage is by large the most common storage technology in the grid today. Other storage technologies are often discussed. Secondary batteries convert electrical to chemical energy and back. Such batteries are used in battery energy storage systems, which can be designed and operated to perform services to support the grid.This dissertation first discusses a set of mature battery storage technologies, and a number of feasibility issues of such systems in the distribution grid is discussed. Secondly, optimization models for storage systems are proposed, and results are calculated for case studies of realistic distribution grids. A first model deals with the combination of storage with photovoltaic generation, to mitigate the impact on the medium-voltage grid. Next, generation-source neutral approaches are developed. Here, battery storage is used to mitigate voltage and power issues in low-voltage distribution grids. Results frame the importance of matching choices of battery and inverter technologies, sizing, services and location of battery storage over long time horizons.nrpages: 233status: publishe

Energy Storage Systems In The Residential Low Voltage Grid

Battery energy storage systems (BESS) in the low voltage grid help to postpone or avoid investment in new cables and transformers. BESS allow for a more flexible operation of a grid which is already in place, but that has difficulty coping with load increase and injection of local renewable energy sources. Such BESS of course must be operated in the most cost effective manner. Therefore, it is important that such systems are installed where they are most effective in solving problems.status: publishe

(Li-ion) batterijen & hun problemen

An overview is given of the fundamentals of batteries, as well as the problems encountered with specific Li-ion batteries.status: publishe