Guest editorial: energy storage in smart grids

Energy storage systems and smart grids cooperation is now crucial and will encourage collaborative engagement by different players in the energy market, offering sophisticated management and control approaches. Therefore, new advances and innovative solutions for such cooperation are of preeminent importance. Moreover, electric mobility should also be considered in this scenario. Electric vehicles (EVs) can be seen as distributed energy storage systems that generally need to be charged but, in some cases, can be adopted to support the grid thanks to vehicle-to-grid (V2G) and vehicle-for-grid (V4G) modes, offering advantages of controlled operation with active or reactive power. So, energy storage systems can be distinguished into aggregated and distributed structures, and they can be based on different technologies, such as batteries, fuel-cells, and super-capacitors. Furthermore, the integration of new renewable energy solutions with energy storage systems in smart grids should also be promoted. Power and energy management are crucial for the upcoming challenges and novel opportunities in smart grids. In line with this trend, this Special Issue aims to present further research on and developments in energy storage systems in smart grids, including power electronics converters, novel modulation schemes, energy and power management strategies, advanced battery packs and Battery Management Systems (BMSs). Novel renewable energy solutions are also of interest, as well as their cooperative and strategic integration with storage and EV chargers. Advanced EV services, such as V2G and V4G in the perspective of contributions to improve power quality, can also be considered. Three main topics can be found in this Special Issue: power electronic converters, converter and machine modelling and lithium-ion battery packs. In the following section, the papers accepted per each topic will be rapidly summarized along with their main achievements

Efficiency comparison of a dc-dc interleaved converter based on SiC-MOSFET and Si-IGBT devices for EV chargers

The charging process is one of the main factors for the widespread dissemination of electric mobility, therefore, the use of optimized power electronics converters is of utmost importance. In addition to innovative topologies, the use of emerging technologies of semiconductors is also crucial. In this context, using a three-phase interleaved dc-dc topology, a comparison between the use of SiC-MOSFET and Si-IGBT is presented in this paper, mainly in terms of operating efficiency. Two cases have been presented: 1) with the same inductor, where only power device losses have been considered; 2) with the same inductor current ripple, where different inductors have been considered and the analysis included also the inductor design and losses. The simulations were carried out in LTspice simulation tool on realistic dynamic models of power switch modules obtained from the manufacturer’s experimental tests. The results validate the use of SiC-MOSFET for the three-phase interleaved dc-dc topology showing lower losses for both the power devices and inductor and, most important, prove the advantages of its use in terms of efficiency for a wide range of operating powers.This work has been supported by FCT - Fundacao para a Ciencia e Tecnologia with-in the Project Scope: UID/CEC/00319/2019, and by the FCT Project newERA4GRIDs PTDC/EEI-EEE/30283/2017

Analytical evaluation of output current ripple amplitude in three-phase three-level inverters

Nowadays, three-phase multilevel inverters are widely employed in medium and high-power applications, increasing the power ratings, improving the output voltage quality and reducing the conducted electromagnetic interferences. Despite of numerous pulse-width modulation (PWM) techniques have been developed for multilevel inverters, a detailed analysis of the output current ripple amplitude has not been reported yet. In this study, the peak-to-peak current ripple distribution over a fundamental period is analysed in details specifically for three-level three-phase voltage source inverters for both motor-load and grid-connected applications. In particular, the peak-to-peak amplitude of the current ripple is determined analytically as a function of the modulation index. The centred PWM strategy is considered in all the developments, implemented either by carrier-based or space vector (SV) PWM methods. With this modulation, the dc bus utilisation is maximised in a simple and effective way, and a nearly-optimal behaviour is obtained to minimise the current ripple rms. The results obtained in different cases and sub-cases identified in the proposed analytical approach are verified by experimental tests with reference to three-phase three-level neutral-point clamped configuration

Efficiency, Cost, and Volume Comparison of SiC-Based and IGBT-Based Full-Scale Converter in PMSG Wind Turbine

Power electronics, as an enabling technology in most renewable energy systems, is gaining attention as the penetration of renewable energy sources increases. Wide-bandgap power electronics are of particular interest due to their superior voltage blocking capabilities and fast switching speeds. They can viably be considered in the renewable energy sources, especially as the penetration of wind energy is expected to increase by a great extent in the upcoming years. In this paper, a comparison of Silicon Carbide-based and Silicon-based wind energy conversion systems has been performed, as it is crucial in understanding the benefits of adopting wide-bandgap-based solutions at a commercial level. For this analysis, a 2 MW permanent magnet synchronous generator-based wind conversion system with a bidirectional full-scale frequency converter comprised of two back-to-back inverters is considered. The efficiency, cost, and total volume of the passive components comparison have been conducted for Silicon- and Silicon Carbide-based converters. The comparison presented is a fair comparison, meaning that the converters are designed with modules of the same power ratings. Wind energy systems are compared both for the same switching frequency (low switching frequency suitable for IGBT modules) and also considering a Silicon Carbide-MOSFET-based converter working at high switching frequencies. The comparison is performed in PLECS simulation tool, using the PLECS libraries for different modules obtained from the manufacturers’ experimental data. The results show the benefits of using the Silicon Carbide-based converter when it comes to volume reduction in the passive components and provide insights to what is missing in order to achieve overall system volume and cost savings

Evaluation of current ripple amplitude in three-phase PWM voltage source inverters

Determination of current ripple in three-phase PWM voltage source inverters (VSI) is important for both design and control purposes, since this is the most popular conversion topology for energy conversion systems. In this paper the complete analysis of the peak-to-peak current ripple distribution over a fundamental period is given for three-phase VSIs. In particular, peak-to-peak current ripple amplitude is analytically determined as a function of the modulation index. Minimum, maximum, and average values are also emphasized. Although the reference is made to continuous symmetric PWM, being the most simple and effective solution to minimize the current ripple, the analysis could be easily extended to either discontinuous or unsymmetrical modulation, both carrier-based and space vector PWM. The analytical developments for all the different subcases are verified by numerical simulations

Analysis of Peak-to-Peak Current Ripple Amplitude in Seven-Phase PWM Voltage Source Inverters

Multiphase systems are nowadays considered for various industrial applications. Numerous pulse width modulation (PWM) schemes for multiphase voltage source inverters with sinusoidal outputs have been developed, but no detailed analysis of the impact of these modulation schemes on the output peak-to-peak current ripple amplitude has been reported. Determination of current ripple in multiphase PWM voltage source inverters is important for both design and control purposes. This paper gives the complete analysis of the peak-to-peak current ripple distribution over a fundamental period for multiphase inverters, with particular reference to seven-phase VSIs. In particular, peak-to-peak current ripple amplitude is analytically determined as a function of the modulation index, and a simplified expression to get its maximum value is carried out. Although reference is made to the centered symmetrical PWM, being the most simple and effective solution to maximize the DC bus utilization, leading to a nearly-optimal modulation to minimize the RMS of the current ripple, the analysis can be readily extended to either discontinuous or asymmetrical modulations, both carrier-based and space vector PWM. A similar approach can be usefully applied to any phase number. The analytical developments for all different sub-cases are verified by numerical simulations

Evaluation of current ripple amplitude in five-phase PWM voltage source inverters

Multiphase systems are nowadays considered for various industrial applications. Numerous PWM schemes for multi-phase voltage source inverters with sinusoidal outputs have been developed, but no detailed analysis of the impact of these modulation schemes on the output peak-to-peak current ripple amplitude has been reported. Determination of current ripple in five-phase PWM voltage source inverters is important for both design and control purposes. This paper gives the complete analysis of the peak-to-peak current ripple distribution over a fundamental period. In particular, peak-to-peak current ripple amplitude is analytically determined as a function of the modulation index, and a simplified expression to get its maximum value is carried out. Reference is made to centered symmetrical PWM, being the most simple and effective solution to maximize the dc bus utilization, leading to a nearly-optimal modulation to minimize the rms of current ripple. However, the analysis can be easily extended to either discontinuous or asymmetrical modulation, both carrier-based and space vector PWM. The analytical developments for all the different sub-cases are verified by numerical simulations

Studio dell'effetto dei tempi morti negli inverter multifase a tensione impressa

Questa attivit\ue0 di ricerca si inquadra nell'ambito dello studio degli inverter a tensione impressa, con particolare riferimento alle non linearit\ue0 dei convertitori ed all'estensione ai sistemi multifase (n>3). Come noto, con riferimento ad un ramo completo con doppio transistor-diodo, il cosiddetto \u201ctempo morto\u201d corrisponde all\u2019intervallo di tempo durante il quale entrambi i transistor sono mantenuti in stato di interdizione onde evitare un possibile corto circuito sull\u2019alimentazione dovuto alla disuniformit\ue0 ed alla aleatoriet\ue0 dei tempi di commutazione degli interruttori. Durante tale intervallo di tempo, la tensione di polo dipende dal verso della corrente di uscita, che interessa l\u2019uno o l\u2019altro diodo a seconda della propria polarit\ue0. I tempi morti costituiscono una delle principali cause di non linearit\ue0 nei convertitori a tensione impressa, ed in letteratura si trovano svariate tecniche di compensazione volte a ridurre la distorsione della tensione di uscita. Una completa compensazione risulta tuttavia impossibile, data l'impossibilit\ue0 pratica di conoscere con esattezza i tempi di commutazione degli interruttori. Se pur noto ed ampiamente trattato in letteratura, lo studio dell\u2019effetto dei tempi morti \ue8 tuttavia pressoch\ue9 limitato ai chopper ed agli inverter monofase e trifase. A seguito della recente diffusione delle applicazioni degli inverter multifase, prevalentemente nel settore degli azionamenti ad alta potenza e/o affidabilit\ue0, l\u2019Unit\ue0 di Bologna ha intrapreso un\u2019attivit\ue0 di ricerca per l\u2019estensione dell'analisi dei tempi morti a questa tipologia di convertitori. In particolare, si sono d\u2019apprima considerati gli effetti dei tempi morti in termini di distorsione armonica sulle tensioni di uscita [1]. L\u2019analisi \ue8 stata successivamente estesa con l\u2019impiego dei vettori di spazio multipli per meglio evidenziare gli effetti in termini di campi armonici [2]. Un ulteriore approfondimento ha riguardato il caso di correnti di uscita ad elevato contenuto armonico [3], causa di un\u2019ulteriore deviazione rispetto alla trattazione disponibile in letteratura. Gli sviluppi analitici sono stati validati da simulazioni in ambiente Matlab, e le prime verifiche sperimentali sono tuttora in fase di realizzazione, in collaborazione con il gruppo di ricerca dei proff. A. M\ufctze e R. Seebacher della Technical University of Graz (Austria)

Implementation of carrier-based optimized centered PWM in three-phase three-level inverters

Carrier-based modulation is widely adopted since it offers some benefits compared to space-vector modulation in both two-level and multilevel inverters, such as inherent simplicity, flexibility, reduced computational times, and possibility of implementation on industrial DSPs. Among the numerous types of PWM techniques, the centered PWM (CPWM) is nearly optimal modulation, since it minimizes the RMS of output current harmonics. Despite of CPWM is easy to implement in two-level inverters, the complexity increases in case of three-level and multilevel inverters. In this paper a simplified straight procedure to implement carrier-based optimized CPWM for three-level inverter is presented, equivalent to centered space vector modulation. The proposed method has been implemented by numerical simulations, considering the comparison of resulting modulating signals with the other methods existing in literature and verifying the pulse centering in the output phase voltage waveforms

Simplified implementation of optimised carrier-based PWM in three-level inverters

Carrier-based modulation has been widely adopted since it offers some benefits compared to space-vector modulation in both two-level and in multilevel three-phase inverters, such as inherent simplicity, flexibility, reduced computational times and easy implementation on industrial DSPs. Among the numerous types of pulse width modulation (PWM) techniques, the centred PWM (CPWM) is nearly optimal modulation since it minimises the RMS of the output current harmonics. Despite optimised CPWM being easy to implement in twolevel inverters, the complexity increases in the case of three-level and multilevel inverters. A straightforward and simplified procedure to implement optimised CPWM in carried-based modulation for the three-level inverter is proposed and numerically verified