In this paper, a steady-state multi-terminal voltage source converter high voltage direct current (VSC MTDC) model is introduced. The proposed approach is extended to include multiple AC and DC grids ...with arbitrary topologies. The DC grids can thereby interconnect arbitrary buses in one or more non-synchronized AC systems. The converter equations are derived in their most general format and correctly define all set-points with respect to the system bus instead of the converter or filter bus, which is often done to simplify calculations. The paper introduces a mathematical model to include the converter limits and discusses how the equations change when a transformerless operation is considered or when the converter filter is omitted. An AC/VSC MTDC power flow is implemented using MATPOWER to show the validity of the generalized power flow model.
The restructuring of energy markets has increased the concern about the existing interdependency between the primary energy supply and electricity networks, which are analyzed traditionally as ...independent systems. The aim of this paper is focused on an integrated formulation for the steady-state analysis of electricity and natural gas coupled systems considering the effect of temperature in the natural gas system operation and a distributed slack node technique in the electricity network. A general approach is described to execute a single gas and power flow analysis in a unified framework based on the Newton-Raphson formulation. The applicability of the proposed approach is demonstrated by analyzing the Belgian gas network combined with the IEEE-14 test system and a 15-node natural gas network integrated with the IEEE-118 test system.
We consider the problem of deriving an explicit approximate solution of the nonlinear power equations that describe a balanced power distribution network. We give sufficient conditions for the ...existence of a practical solution to the power flow equations, and we propose an approximation that is linear in the active and reactive power demands of the PQ buses. For this approximation, which is valid for generic power line impedances and grid topology, we derive a bound on the approximation error as a function of the grid parameters. We illustrate the quality of the approximation via simulations, we show how it can also model the presence of voltage controlled (PV) buses, and we discuss how it generalizes the DC power flow model to lossy networks.
This letter proposes a linear load flow for three-phase power distribution systems. Balanced and unbalanced operation are considered as well as the ZIP models of the loads. The methodology does not ...require any assumption related to the R/X ratio. Despite its simplicity, it is very accurate compared to the conventional back-forward sweep algorithm.
Advances in the development of electric vehicles, along with policy incentives, will see a wider uptake of this technology in the transport sector in future years. However, the widespread adoption of ...electric vehicles could lead to adverse effects on the power system, especially for existing distribution networks. These effects would include excessive voltage drops and overloading of network components, which occur mainly during periods of simultaneous charging of large numbers of electric vehicles. This paper demonstrates how controlling the rate at which electric vehicles charge can lead to better utilization of existing networks. A technique based on linear programming is employed, which determines the optimal charging rate for each electric vehicle in order to maximize the total power that can be delivered to the vehicles while operating within network limits. The technique is tested on a section of residential distribution network. Results show that, by controlling the charging rate of individual vehicles, high penetrations can be accommodated on existing residential networks with little or no need for upgrading network infrastructure.
This paper considers unbalanced multiphase distribution systems with generic topology and different load models, and extends the Z-bus iterative load-flow algorithm based on a fixed-point ...interpretation of the AC load-flow equations. Explicit conditions for existence and uniqueness of load-flow solutions are presented. These conditions also guarantee convergence of the load-flow algorithm to the unique solution. The proposed methodology is applicable to generic systems featuring i) wye connections; ii) ungrounded delta connections; iii) a combination of wye-connected and delta-connected sources/loads; and iv) a combination of line-to-line and line-to-grounded-neutral devices at the secondary of distribution transformers. Further, a sufficient condition for the nonsingularity of the load-flow Jacobian is proposed. Finally, linear load-flow models are derived, and their approximation accuracy is analyzed. Theoretical results are corroborated through experiments on IEEE test feeders.
This paper applies a probabilistic power flow (PPF) algorithm to evaluate the influence of photovoltaic (PV) generation uncertainty on transmission system performance. PV generation has the potential ...to cause a significant impact on power system reliability in the near future. A cumulant-based PPF algorithm suitable for large systems is used to avoid convolution calculations. Correlation among input random variables is considered. Specifically correlation between adjacent PV resources are considered. Three types of approximation expansions based on cumulants, namely the Gram-Charlier expansion, the Edgeworth expansion, and the Cornish-Fisher expansion, are compared, and their properties, advantages, and deficiencies are discussed. Additionally, a novel probabilistic model of PV generation is developed to obtain the probability density function (PDF) of the PV generation production based on the environmental conditions. The proposed approaches with the three expansions are compared with Monte Carlo simulations (MCS) with results for a 2497-bus representation of the Arizona area of the Western Electricity Coordinating Council (WECC) system.
MATPOWER is an open-source Matlab-based power system simulation package that provides a high-level set of power flow, optimal power flow (OPF), and other tools targeted toward researchers, educators, ...and students. The OPF architecture is designed to be extensible, making it easy to add user-defined variables, costs, and constraints to the standard OPF problem. This paper presents the details of the network modeling and problem formulations used by MATPOWER, including its extensible OPF architecture. This structure is used internally to implement several extensions to the standard OPF problem, including piece-wise linear cost functions, dispatchable loads, generator capability curves, and branch angle difference limits. Simulation results are presented for a number of test cases comparing the performance of several available OPF solvers and demonstrating MATPOWER's ability to solve large-scale AC and DC OPF problems.
This paper proposes a unified load flow (LF) model for AC-DC hybrid distribution systems (DSs). The proposed model can be applied in hybrid DSs with mixed configurations for AC/DC buses and AC/DC ...lines. A new classification of DS buses is also introduced for LF analysis. A set of generic LF equations has been derived based on comprehensive analysis of the possible AC-DC hybrid system configurations. Three binary matrices, which are used as a means of describing the configuration of the AC and DC buses and lines, have been employed in the construction of the unified power equations. These matrices enable a single configuration at a time to be activated in the power equations. The proposed LF model is generic and can be used for both grid-connected and isolated hybrid DSs. The new model has been tested using several case studies of hybrid DSs that include different operational modes for the AC and DC distributed generators. As a means of evaluating the effectiveness and accuracy of the proposed model, the LF solution was compared to the solution produced by PSCAD/EMTDC. A comparison of the results reveals the efficacy of the proposed model.
Alternating current (ac) power flow (PF) presents difficulties for power system analysis and optimization due to its nonlinearity. Progress has been made to approximately linearize ac PF in recent ...decades. However, few studies have reported the simultaneous accurate approximation of reactive power and transmission losses. To bridge this gap, this paper investigates the linear approximation of ac PF considering the accuracy of the reactive load flows and transmission losses. Using the logarithmic transform of voltage magnitudes, a linear PF (LPF) model involving tap changers and phase shifters is derived from the approximation analysis of general branch flows. Transmission power loss and loss-concerned complex branch flow are also formulated. Cold-start and warm-start LPF calculation methods associated with injection compensation are also developed. Numerical simulations are performed to compare the proposed models and several state-of-the-art LPF models using 25 practical-scale test systems. The simulation results demonstrate the advantages of the proposed model over the other models for approximating voltage magnitudes, branch flows, and power losses. The effectiveness of using proper compensation injection in improving the solution accuracy is also verified.
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