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Concept# Power-flow study

Résumé

In power engineering, the power-flow study, or load-flow study, is a numerical analysis of the flow of electric power in an interconnected system. A power-flow study usually uses simplified notations such as a one-line diagram and per-unit system, and focuses on various aspects of AC power parameters, such as voltages, voltage angles, real power and reactive power. It analyzes the power systems in normal steady-state operation.
Power-flow or load-flow studies are important for planning future expansion of power systems as well as in determining the best operation of existing systems. The principal information obtained from the power-flow study is the magnitude and phase angle of the voltage at each bus, and the real and reactive power flowing in each line.
Commercial power systems are usually too complex to allow for hand solution of the power flow. Special-purpose network analyzers were built between 1929 and the early 1960s to provide laboratory-scale physical models of power s

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Sherif Alaa Salaheldin Fahmy, Mario Paolone, Quentin Walger

The problem of securely reconnecting active distribution networks (ADNs) - e.g. microgrids - to their upstream grids at the point of common coupling (PCC) has been extensively discussed by the existing literature. The latter is commonly referred to as resynchronization and has to be done with care in order to avoid large transient current flows resulting from differences of nodal voltage phasors at both sides of the PCC. The active resynchronization process can be split into two tasks: the PCC-control and the synchrocheck. The PCC-control refers to the process used to steer the PCC nodal voltage at the ADN's side (i.e. downstream) towards the PCC nodal voltage at the upstream-grid's side (i.e. upstream). The synchrocheck refers to the algorithm used to check the synchronization (i.e. phasor alignment within tolerances) of the upstream and downstream PCC nodal voltages. Methods for PCC-control and synchrocheck presented in the literature commonly ignore the ADN's operational constraints and rely on the assumption of a balanced system. In this respect, the contribution of this paper is twofold. First, an approximated optimal-power-flow is proposed to control ADNs' resources in order to rapidly steer their PCC downstream nodal voltages close to their non-controllable upstream counterparts. Second, an Interpolated-Discrete-Fourier-Transform (IpDFT)-based synchrocheck that verifies the alignment of all three-phases of both upstream and downstream nodal voltages at the PCC, is proposed. The algorithms associated to both contributions are experimentally validated on the CIGRE-low-voltage-benchmark-microgrid at the Distributed Electrical Systems Laboratory (DESL) at the ecole Polytechnique Federale de Lausanne (EPFL) where the results of the developed synchrocheck are further benchmarked against the Schneider Electric's Micom P143 grid relay.

Sherif Alaa Salaheldin Fahmy, Mario Paolone

With the increasing need of real-time regulation in power systems, grid-aware-control-frameworks are relying more often on sensitivity coefficients (SCs) to formulate and efficiently solve optimal control problems. As known, SCs are the derivatives of controlled quantities (e.g. nodal voltages at PQ nodes and branch currents) with respect to control variables i.e. nodal active and reactive power injections at PQ nodes, nodal voltage magnitudes and nodal active power injections at PV nodes and nodal voltage magnitudes and phase-angles at slack nodes. In a real control application, the knowledge of the system state, coming from a state-estimation process, allows for the direct computation of SCs without the need of a load-flow. Algorithms for this computation have been already proposed in the literature for PQ nodes’ nodal voltage SCs under the assumption of constant nodal power injections. The aim of this paper is to propose an analytical derivation of all node types (i.e. PQ, PV and slack) nodal voltage SCs for power grids with generic topologies, number of phases and voltage-dependent nodal power injection models. The paper also includes an exhaustive list of all other SCs that can be directly computed using nodal voltage SCs, a computational complexity analysis of the proposed method and a numerical benchmarking.

Andrey Bernstein, Niek Johannes Bouman, Benoit Jacques Louis Marie Cathiard, Andreas Martin Kettner, Jean-Yves Le Boudec, Mario Paolone, Lorenzo Enrique Reyes Chamorro, Enrica Scolari

The existing approaches to control electrical grids combine frequency and voltage controls at different time-scales. When applied in microgrids with stochastic distributed-generation, grid quality of service problems may occur, such as under- or overvoltages as well as congestion of lines and transformers. The COMMELEC framework proposes to solve this compelling issue by performing explicit control of power flows with two novel strategies: (1) a common abstract model is used by resources to advertise their state in real time to a grid agent; (2) subsystems can be aggregated into virtual devices that hide their internal complexity in order to ensure scalability. While the framework has already been published in the literature, in this paper we present the first experimental validation of a practicable explicit power-flow primary control applied in a real-scale test-bed microgrid. We demonstrate how an explicit power-flow control solves the active and reactive power sharing problem in real time, easily allowing the microgrid to be dispatchable in real time (i.e. it is able to participate in energy markets) and capable of providing frequency support, while always maintaining the quality of service.

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