Improving the Performance of Low Voltage Networks by an Optimized Unbalance Operation of Three-Phase Distributed Generators

This work focuses on using the full potential of PV inverters in order to improve the efficiency of low voltage networks. More specifically, the independent per-phase control capability of PV three-phase four-wire inverters, which are able to inject different active and reactive powers in each phase, in order to reduce the system phase unbalance is considered. This new operational procedure is analyzed by raising an optimization problem which uses a very accurate modelling of European low voltage networks. The paper includes a comprehensive quantitative comparison of the proposed strategy with two state-of-the-art methodologies to highlight the obtained benefits. The achieved results evidence that the proposed independent per-phase control of three-phase PV inverters improves considerably the network performance contributing to increase the penetration of renewable energy sources.Ministerio de Economía y Competitividad ENE2017-84813-R, ENE2014-54115-

Ancillary Services in Hybrid AC/DC Low Voltage Distribution Networks

In the last decade, distribution systems are experiencing a drastic transformation with the advent of new technologies. In fact, distribution networks are no longer passive systems, considering the current integration rates of new agents such as distributed generation, electrical vehicles and energy storage, which are greatly influencing the way these systems are operated. In addition, the intrinsic DC nature of these components, interfaced to the AC system through power electronics converters, is unlocking the possibility for new distribution topologies based on AC/DC networks. This paper analyzes the evolution of AC distribution systems, the advantages of AC/DC hybrid arrangements and the active role that the new distributed agents may play in the upcoming decarbonized paradigm by providing different ancillary services.Ministerio de Economía y Competitividad ENE2017-84813-RUnión Europea (Programa Horizonte 2020) 76409

Sistema de recarga de vehículos eléctricos: revisión tecnológica e impacto en el sistema eléctrico

El cambio climático está principalmente provocado por las emisiones de gases de efecto invernadero producidas por la actividad humana. Se estima que alrededor de un 50% de dichas emisiones son debidas al transporte por carretera y a la generación eléctrica. La electrificación del sector transporte mediante la utilización de vehículos eléctricos que sustituyan a los tradicionales de combustión interna es clave para contribuir a la descarbonización de la sociedad. Sin embargo, esta sustitución tecnológica tiene importantes implicaciones que han de ser tenidas en cuenta antes de su despliegue masivo. El objetivo de este artículo es realizar una revisión de la tecnología de recarga de vehículos eléctricos y su impacto en la red eléctrica.The climate change is mainly generated by the greenhouse gas emissions due to the human activity. It is estimated that around a 50% of these emissions comes from road transportation and electricity generation. The transportation sector electrification, by means of electric vehicles replacing traditional ones based on internal combustion engines, is key to achieve a decarbonized society. This technological replacement, however, has important implications that has to be considered before a massive rollout. The obMective of this paper is to review the charging technology of electric vehicles and its impact on the power system

Reduced Reference Frame Transform: Deconstructing Three-Phase Four-Wire Systems

This paper proposes a new reference frame, namely Reduced Reference Frame (RRF), specially suited for unbalanced three–phase four–wire systems which improves the performance of the classical Fortescue, Clarke and Park transformations. The RRF transfomation allows to represent any unbalanced three–phase sinusoidal magnitude with just two components even if the zero–sequence component is present. For doing so, the RRF transformation takes into account that the abc space–vector trajectory of the transformed three–phase sinusoidal magnitude is always within a plane. The geometric properties of this trajectory are considered for outlining a general classification of the transformation depending on the input voltages and an adequate reference frame within the plane. Then, a step–by–step procedure for computing the transformation matrix is detailed. Once the voltages and currents are transformed into the RRF, it is proposed a power theory which allows to compute the instantaneous active and reactive powers. The paper includes two simulations and an experimental validation through a real–time application to highlight the benefits of the proposal. The paper closes with the main conclusions and some future research lines where this transformation can be applied.Spanish Ministry of Economy and Competitiveness ENE2017-84813-R EASY-RES 76409

Optimal Coordinated Operation of Distributed Static Series Compensators for Wide-area Network Congestion Relief

Relieving network congestions is a critical goal for the safe and flexible operation of modern power systems, especially in the presence of intermittent renewables or distributed generation. This paper deals with the real-time coordinated operation of distributed static series compensators (DSSCs) to remove network congestions by suitable modifications of the branch reactance. Several objective functions are considered and discussed to minimize the number of the devices involved in the control actions, the total losses or the total reactive power exchanged, leading to a non-convex mixed-integer non-linear programming problem. Then, a heuristic methodology combining the solution of a regular NLP with k-means clustering algorithm is proposed to get rid of the binary variables, in an attempt to reduce the computational cost. The proposed coordinated operation strategy of the DSSCs is tested on several benchmark systems, providing feasible and sufficiently optimal solutions in a reasonable time frame for practical systems

Distributed consensus-based secondary control of multi-terminal DC railway systems

Electrification of the transportation sector is one of the main pillars of the future decarbonized society. In this context, high-speed railway corridors are expected to reduce as much as possible their environmental impact while maintaining the safety and quality of service with minimum investments. In pursuing those objectives, the multi-terminal DC railway system may become the preferred paradigm. In contrast to conventional AC railway electrification schemes, the multi-terminal DC railway system requires the adoption of a control algorithm to establish suitable references for the power converters. This paper elaborates on a new consensus-based secondary control strategy, which can be implemented in a distributed fashion. In this approach, each power converter controller receives information from its nearest neighbors to adjust its operating point. A 600-km railway system, subject to realistic train traffic, is used to compare the performance of the proposed control strategy with that of purely local controllers. Numerical simulations evidence that the proposed distributed control scheme provides a compromise solution in terms of voltage drops and equalization of power converter loading, surpassing the performance of conventional state-of-the art controllers

Experimental realisation of an AC-link shunt-series power flow controller

This is an open access article published by the IET under the Creative Commons Attribution License(http://creativecommons.org/licenses/by/3.0/)Power electronic-based interties in the distribution system are considered an important element for the integration of distributed energy resources. They can provide a series of network services such as active and reactive power control, voltage regulation and harmonic and imbalance compensation that facilitate the integration of these new resources. Despite dc links are usually proposed for this purpose, it is also possible to interconnect radial distribution feeders by means of ac links with direct ac/ac power conversion. This study presents the experimental validation of a current control loop based on feedback linearisation for an ac-link shunt-series power flow controller based on a vector switching matrix converter. The experimental results demonstrate the effectiveness of the proposed control in both steady-state and transient conditions

Experimental Assessment of a Centralised Controller for High-RES Active Distribution Networks

This paper assesses the behaviour of active distribution networks with high penetration of renewable energy sources when the control is performed in a centralised manner. The control assets are the on-load tap changers of transformers at the primary substation, the reactive power injections of the renewable energy sources, and the active and reactive power exchanged between adjacent feeders when they are interconnected through a DC link. A scaled-down distribution network is used as the testbed to emulate the behaviour of an active distribution system with massive penetration of renewable energy resources. The laboratory testbed involves hardware devices, real-time control, and communication infrastructure. Several key performance indices are adopted to assess the effects of the different control actions on the system’s operation. The experimental results demonstrate that the combination of control actions enables the optimal integration of a massive penetration of renewable energy.Ministerio de Economía y Competitividad ENE2015-69597-RMinisterio de Economía y Competitividad PCIN-2015-043Ministerio de Economía y Competitividad ENE2017-84813-

A model-less control algorithm of DC microgrids based on feedback optimization

This work addresses the problem of the optimal real-time control of a DC microgrid without relying on its corresponding network model. The main goal of such a controller is to keep the nodal network voltages within the regulatory limits while offering current sharing capability between the different controllable generators powering the DC microgrid. The proposed model-less methodology is based on feedback optimization, which takes advantage of the available real-time measurements to update the setpoints of the DC generation assets. The optimal control variables are determined in an iterative manner by applying a primal–dual saddle-point method, which guarantees appropriate convergence features. The paper details both centralized and distributed implementations which are compared through simulations. The results evidence a good dynamic performance and an optimal steady-state operation as the proposed control algorithm converges to the solution provided by a conventional model-based Optimal Power Flow.Universidad de Sevilla VI PPIT-USUnión Europea - Horizonte 2020 No 764090Ministerio de Economía ENE2017-84813-RCentro para el Desarrollo Tecnológico Industrial HySGrid+ CER-2019101

Short-term energy recovery control for virtual inertia provision by renewable energy sources

The proliferation of Converter-Interfaced Renewable Energy Sources (CIRES), which are inertia-less, and the gradual decommissioning of synchronous generation have posed several challenges to the electric power system. This has motivated a complete a shift in the CIRES design and its corresponding control philosophy. Integrating Energy Storage Systems (ESS) within CIRES enables the implementation of different operating modes allowing them to provide ancillary services (AS) in a similar way to the synchronous generation. In order to tackle with those short-term response AS, such as virtual inertia, fast ESS (FESS) solutions with high power-to-energy ratio, particularly flywheels and supercapacitors, are preferred. In spite of several control algorithms have been proposed to provide such fast AS, very little research effort has been paid on the proper FESS energy recovery after the AS provision. This task is particularly challenging, since supercapacitors must be operated at a certain state of charge to guarantee that the required AS can be provided within its operational limits. This paper aims to fill this gap by proposing a new energy recovery control scheme for supercapacitors after the provision of short-term AS, such as virtual inertia. The proposed control is validated via simulations which clearly highlights its adequate performance.Horizonte 2020 (Unión Europea) 764090Ministerio de Economía y Competitividad (MINECO). España ENE2017-84813-R