Application of a simplified thermal-electric model of a sodium-nickel chloride battery energy storage system to a real case residential prosumer

Recently, power system customers have changed the way they interact with public networks, playing a more and more active role. End-users first installed local small-size generating units, and now they are being equipped with storage devices to increase the self-consumption rate. By suitably managing local resources, the provision of ancillary services and aggregations among several end-users are expected evolutions in the near future. In the upcoming market of household-sized storage devices, sodium-nickel chloride technology seems to be an interesting alternative to lead-acid and lithium-ion batteries. To accurately investigate the operation of the NaNiCl2 battery system at the residential level, a suitable thermoelectric model has been developed by the authors, starting from the results of laboratory tests. The behavior of the battery internal temperature has been characterized. Then, the designed model has been used to evaluate the economic profitability in installing a storage system in the case that end-users are already equipped with a photovoltaic unit. To obtain realistic results, real field measurements of customer consumption and solar radiation have been considered. A concrete interest in adopting the sodium-nickel chloride technology at the residential level is confirmed, taking into account the achievable benefits in terms of economic income, back-up supply, and increased indifference to the evolution of the electricity market

Use of rod compactors for high voltage overhead power lines magnetic field mitigation

In the last decades, strengthening the high voltage transmission system through the installation of new overhead power lines has become critical, especially in highly developed areas. Present laws concerning the human exposure to electric and magnetic fields introduce constraints to be considered in both new line construction and existing systems. In the paper, a technique for passive magnetic field mitigation in areas close to overhead power lines is introduced, fully modelled and discussed through a parametric analysis. The investigated solution, which basically consists in approaching line conductors along the span making use of rod insulators, is applicable on both existing and under-design overhead lines as an alternative to other mitigating actions. Making use of a 3-dimensional representation, the procedure computes both positions of phase conductors and forces acting on insulators, towers, conductors and compactors, with the aim of evaluating the additional mechanical stress introduced by the compactors. Finally, a real case study is reported to demonstrate and quantify the benefits in terms of ground magnetic field reduction achievable by applying the proposed solution, in comparison to a traditional configuration. Furthermore, using compactors to passively reduce the magnetic field is simple to be applied, minimally invasive and quite inexpensive as regards to alternative mitigating actions

Effects of energy storage systems grid code requirements on interface protection performances in low voltage networks

The ever-growing penetration of local generation in distribution networks and the large diffusion of energy storage systems (ESSs) foreseen in the near future are bound to affect the effectiveness of interface protection systems (IPSs), with negative impact on the safety of medium voltage (MV) and low voltage (LV) systems. With the scope of preserving the main network stability, international and national grid connection codes have been updated recently. Consequently, distributed generators (DGs) and storage units are increasingly called to provide stabilizing functions according to local voltage and frequency. This can be achieved by suitably controlling the electronic power converters interfacing small-scale generators and storage units to the network. The paper focuses on the regulating functions required to storage units by grid codes currently in force in the European area. Indeed, even if such regulating actions would enable local units in participating to network stability under normal steady-state operating conditions, it is shown through dynamic simulations that they may increase the risk of unintentional islanding occurrence. This means that dangerous operating conditions may arise in LV networks in case dispersed generators and storage systems are present, even if all the end-users are compliant with currently applied connection standards

Generalised transformer modelling for power flow calculation in multi-phase unbalanced networks

Low voltage systems are unbalanced networks where a significant share of the users is single-phase connected, so a multi-phase system needs to be considered in order to assess the mutual influence of the different phases. The presence of single-phase unevenly distributed users, leads to unbalances in the power flow on the three phases. This issue is emphasised considering the presence of local single-phase generators. This study presents a generalised method for transformers modelling in any multi-conductor grid representation in order to allow the analysis on unbalanced networks such as low-voltage distribution systems. The method, based on an incidence matrix approach, is proposed to represent any network object involving mutual connections among the phases, once the impedances for each single-phase equivalent circuit are known. Some application examples validate the approach and illustrate how to numerically realise the model

A Novel Unidirectional Smart Charging Management Algorithm for Electric Buses

The difficulty of controlling the charging of electric buses (EBs) and their effects on network demand are discussed in this study. The solutions suggest a call for worldwide, complex infrastructures that manage EVs and EBs equally. Additionally, the Distribution Network (DN) must be prepared for an increased prevalence of reverse power flow caused by widespread distributed renewable generation. This paper focuses exclusively on EBs since they have higher capacity and predictable charging patterns, which makes them more significant for the DN in the context of a transition to complete vehicle electrification and technologies that are mature enough to be hosted. The proposed algorithm employs the Day-Ahead Energy Market (DAEM) in the Smart Charging (SC) to forecast the network operating circumstances. Additionally, the technique makes it possible to facilitate distributed photovoltaic (PV) generation, allowing network demand to be referenced depending on net demand. It also identifies an appropriate individual charger current per vehicle and per-time-step with load-levelling or peak-shaving as its primary goal. The final real demand demonstrates that a coarse correction of the demand is possible. According to the analysis of the DN voltage profile and associated line losses, the ideal node position location of the CS is dependent on PV penetration

Maintenance of a high-voltage overhead transmission line: Sustainability and noise impact assessment

Overhead high-voltage lines are a common choice for power transmission, but their planning, installation and management are often challenging tasks because of the surrounding public interest and of their importance as critical infrastructures. This is particularly true in the case of industrial installations requiring a high continuity of service. The working group formed by the University of Brescia (UniBS), the University of Padova (UniPD), and Torino Transmission Operating Area (AOT) of Terna Rete Italia S.p.A. (Terna) has studied an innovative solution featuring a remotely-operated switchgear mounted directly on the trellis holding the conductors. This strategy reduces visual impact, land use and vulnerability of the system to weather adversity, but noise exposure of the population requires appropriate study. This work introduces a new technical solution, discusses its benefits, and assesses the audible noise impact of the improved transmission line, considering the combined effect of switchgear action and corona discharge around the conductors. The sound emission data are fed as input into a sound propagation software enabling evaluating the noise perceived by people living in the neighborhoods. A mitigation solution is proposed and analyzed

INFLUENCE OF AVERAGE POWER FACTOR MANAGEMENT ON ACTIVE DISTRIBUTION NETWORKS

ABSTRAC

Centralized OPF in Unbalanced Multi-Phase Neutral Equipped Distribution Networks Hosting ZIP Loads

The Optimal Power Flow (OPF) model for low voltage active Distribution Networks (DNs), which are equipped with neutral conductors, requires an explicit representation of both phases and neutral conductors in its formulation to obtain complete information about the state variables related to these conductors. In this regard, a centralized OPF relaxation based on semi-definite programming is presented in this paper for neutral-equipped DNs hosting ZIP loads and neutral-ground impedance, and contain a significant level of unbalance. The major restriction in the development of an OPF model for these networks is the coupled power injection across the conductors which is successfully handled by deriving the explicit active and reactive power injections for each conductor through a network admittance matrix-based approach. The shortcomings of existing voltage magnitude-based technique for the modelling of ZIP loads are comprehensively reported and a novel complex voltage variable-based approach is proposed which successfully incorporates ZIP loads in the developed multi-phase OPF relaxation. For the handling of constant current load, a modelling approach based on the first-order-Taylor series is introduced as well. Furthermore, the impact of the application of Kron reduction approach on the global optimal solution of single- and multiple-point grounded DNs is discussed in detail. Three metrics, eigenvalue ratio, power mismatch and cumulative normalized constraint violation, are utilized to evaluate the exactness of proposed relaxation. Simulations, carried out on several medium and low voltage DNs, show that the proposed relaxation is numerically exact under several combinations of ZIP load parameters and a reasonable range of grounding impedance value for both time-varying and extreme system loading scenarios irrespective of the degree of unbalance in a network

Integration of Lithium-Ion Battery Storage Systems in Hydroelectric Plants for Supplying Primary Control Reserve

The ever-growing diffusion of renewables as electrical generation sources is forcing the electrical power system to face new and challenging regulation problems to preserve grid stability. Among these, the primary control reserve is reckoned to be one of the most important issues, since the introduction of generators based on renewable energies and interconnected through static converters, if relieved from the primary reserve contribution, reduces both the system inertia and the available power reserve in case of network events involving frequency perturbations. In this scenario, renewable plants such as hydroelectric run-of-river generators could be required to provide the primary control reserve ancillary service. In this paper, the integration between a multi-unit run-of-river power plant and a lithium-ion based battery storage system is investigated, suitably accounting for the ancillary service characteristics as required by present grid codes. The storage system is studied in terms of maximum economic profitability, taking into account its operating constraints. Dynamic simulations are carried out within the DIgSILENT PowerFactory 2016 software environment in order to analyse the plant response in case of network frequency contingencies, comparing the pure hydroelectric plant with the hybrid one, in which the primary reserve is partially or completely supplied by the storage system. Results confirm that the battery storage system response to frequency perturbations is clearly faster and more accurate during the transient phase compared to a traditional plant, since time delays due to hydraulic and mechanical regulations are overpassed. A case study, based on data from an existing hydropower plant and referring to the Italian context in terms of operational constraints and ancillary service remuneration, is presented

Il controllo delle reti attive di distribuzione

Within the energy landscape, both in the national and international context, a drastic revolution in the electrical power system management strategy is in progress. The ongoing trend leads to a gradual decentralization of energy production, which tends to be allocated close to the load in medium or small scale plants connected to Medium Voltage (MV) and Low Voltage (LV) distribution networks. In this perspective, the Distributed Generation (DG) is one of the emerging technologies, presenting significant benefits in particular related to the ability to exploit locally available energy resources like renewables or cogeneration. However, an increased penetration of DG plants in present distribution networks introduces some unavoidable implications in the way the networks are nowadays operated. The current connection and management methods, in fact, may induce heavy restrictions on the generable power or significant investments finalized to a complete modernization of the network. Increasing the DG penetration level, distributors necessary need to adopt new techniques for an appropriate control and regulation of the distribution systems, in order to maximize the DG penetration and simultaneously ensure adequate standards in terms of reliability, safety and quality of supply. The distribution networks, initially thought and now performed for a purely passive use, will be gradually translated into intelligent (smart), reliable, sustainable and economic systems. New operational strategies based on an active management of distribution networks are called to face and solve some structural problems induced by local generators, for example the voltage monitoring and regulation at each node both through set point references and price signals, the network and users controllability in normal, emergency and fault conditions and the protection effectiveness in presence of bidirectional power flow. In this context, new procedures and algorithms able to achieve these objectives, maximizing the system benefits introduced by DG plant connection and minimizing the problems resulting from the randomness of the active and passive users behavior, have been developed. Since the achievement of a proper monitoring of the network operational state will probably be a distributor cost, innovative tools to ensure a greater level of accuracy in the estimation of parameters associated with different plants on the system are required. Two solutions are developed to be used in alternative or integrated way, basing on load modeling, discrete communications from remote generation units and on-line voltage measurements optimally allocated into the field. At the same time, two strategies for the management of active distribution networks have been developed. The first, applicable in a short-term prospect, is based on a set point remote regulation activated on DGs in order to maximize their active power injections and at the same time to optimize the voltage regulation and the local compensation of reactive exchanges, aiming to achieve a global reduction of distribution losses. The latter, focused on a medium-long perspective, is based on a transparent computation of a price signal in a local active and reactive energy market, aimed to obtain a technical and economic optimization of the behaviors of active and passive price sensitive users. Island operation and economic evaluations of network reinforcement and regulation costs are obtainable implementing the developed tools.All’interno del panorama energetico nazionale e internazionale è in corso un graduale mutamento della strategia di gestione del sistema elettrico. La tendenza avviata conduce ad una progressiva decentralizzazione della produzione, che tende ad essere allocata in impianti di taglia medio-piccola posti in prossimità dei carichi e allacciati alle reti di distribuzione in Media Tensione (MT) e Bassa Tensione (BT). In questo contesto, la Generazione Distribuita (GD) è una delle realtà impiantistiche emergenti, in quanto prospetta notevoli vantaggi e possibilità, legati in particolare alla capacità di sfruttamento di risorse sparse sul territorio, sia di tipo rinnovabile che a fonte tradizionale (a limitato impatto ambientale), per lo più realizzate in assetto cogenerativo. Tuttavia, l’aumento della penetrazione di GD nelle attuali reti di distribuzione introduce alcune inevitabili implicazioni nelle modalità di esercizio delle reti stesse. L’attuale criterio di allacciamento e gestione, infatti, comporterebbe necessariamente pesanti limitazioni alla potenza iniettabile oppure ingenti investimenti per un completo ammodernamento della rete. Si rende dunque necessaria l’adozione di nuove tecniche di controllo e gestione della rete di distribuzione finalizzate a massimizzare la penetrazione di GD garantendo contestualmente standard adeguati in termini di affidabilità, sicurezza e qualità del servizio di connessione e di fornitura dell’energia elettrica. La rete di distribuzione, inizialmente pensata e attualmente esercita per un esercizio puramente passivo, dovrà quindi trasformarsi gradualmente in un sistema intelligente (smart), affidabile, sostenibile ed economico. Le principali problematiche che si riscontrano nella realizzazione di una gestione attiva della rete di distribuzione in presenza di GD sono rappresentate dalla necessità di controllare la tensione ad ogni nodo, di verificare il funzionamento degli impianti di rete e di utenza, di adottare nuove modalità di protezione operanti anche in presenza di bidirezionalità nei flussi di potenza e nei contributi alle correnti di guasto e infine di monitorare gli impianti connessi al sistema di distribuzione, in particolar modo le unità GD. Per raggiungere tali obiettivi sono necessarie strategie di monitoraggio e gestione delle risorse distribuite, al fine di massimizzare i benefici al sistema introdotti dall’allacciamento di impianti GD e allo stesso tempo minimizzare le problematiche conseguenti alla aleatorietà del comportamento delle utenza attive e passive connesse. Poiché il corretto monitoraggio delle condizioni operative della rete sarà probabilmente onere del distributore, tale soggetto dovrà adottare strumenti in grado di garantire un maggiore livello di accuratezza nella stima dei parametri fondamentali associati ai diversi impianti del sistema. Sono proposte due soluzioni, sviluppabili in modo alternativo o integrato, basate sull’utilizzo di tecniche di modellazione del carico, di comunicazioni discrete da parte delle unità di produzione e di misurazioni on-line ottimamente allocate provenienti dal campo. Al contempo, sono state sviluppate due strategie per la supervisione e la gestione delle reti: la prima, applicabile sul medio-breve periodo, si basa sull’invio di set-point agli impianti al fine di massimizzare la produzione iniettabile in rete e ottimizzare al contempo la regolazione della tensione e la compensazione locale degli scambi reattivi; la seconda, incentrata in un’ottica di medio-lungo periodo, basata sull’ottimizzazione del comportamento delle utenze attive e passive che vengono rese sensibili ad un segnale di prezzo nodale a mezzo di meccanismi di mercato locale, con la possibilità di esercire e auto sostenere in isola porzioni del sistema di distribuzione