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Advanced Control of Power Converters for Efficient Use of Distributed Energy Resources in Future Smart Microgrids
Full text linkThis work proposes a vision on the growing scenario of Smart Grids, where a pervasive
introduction of distributed generators and innovative power management schemes forces a deep
review of the power distribution network establishment. Despite a potentially general value of the
proposed approach, the analysis is focused on low voltage microgrids as the most flexible and open
architectures to demonstrate the feasibility of the network renewal. In recent years, distribution
network has experienced a massive introduction of Distributed Generators (DGs). So far, this has
been done based on a hierarchical government of the network, where utilities set regulations and
keep control of the connections from the top of the structure. The final user has a passive role in the
management, even participating to generation, owning a DG unit. This is basically due to historical
reasons: the distribution architecture has been built starting from a centralized controller driven
paradigm, and all the updates have been done in the same direction. This approach has been valid
until the electrical generation was supplied by a limited number of large plants: huge amounts of
power with all the related control challenges, but static generation infrastructure.
The upcoming scenario is instead a generation that is strongly decentralized toward distributed
energy resources. At the same time, development of Information and Communication Technologies
(ICT) is constantly rising. The combination of these two phenomena has the potential to change
completely the power distribution system, both in the architecture and in the roles of players in the
energy market.
From an architectural point of view, distributed generation units could become a combination of
renewable sources such as solar, wind, small hydro turbines, fuel cells etc. and traditional sources
like gas or diesel generators required for supply continuity of sensible loads. Coupled to the primary
energy sources, Energy Storage (ES) can be used to introduce a degree of freedom in energy
management: large storage units like batteries or flywheels can absorb the excess of generation, to
provide energy during peak demand times, or perform more complex optimizations. Small energy
sources, like super capacitors, can be used to improve power quality during transients such as
voltage sags or frequency variation transients. These generation and storage units shall be equipped
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with an intelligent digital controller able to measure all the required local variables and perform
local control of power converters and also capable to communicate in a bidirectional way, for
example via Power Line Communication, with other generators. The power topology of these
conversion units can be generally represented as a first conversion stage, dependent on the kind of
energy source, and a second stage that is a current controlled inverter. Such devices are named
“Energy Gateways” (EG) and represent the basic elements of future Smart Grids in the vision
presented in this work.
To limit the system complexity, a microgrid is considered: the microgrid is connected to the main
low voltage distribution network and presents a number of distributed generators, all equipped with
EG. In the proposed approach control of the micro grid is distributed, without centralized
controllers: the microgrid internal behaviour can be optimized, as well as the microgrid behaviour
seen by the main grid, based on EG to EG communication and control architecture. Considered the
degrees of freedom of the resulting system, different optimizations could be performed.
In more details, the optimization considered in this work is the distribution loss minimization and
it is obtained by properly controlling converters active and reactive power references in a distributed
way. In particular, the normal overrating of power converters opens the possibility of injecting
distributed reactive power, that can be locally delivered to the loads, reducing the absorption from
the main grid and the consequent losses and voltage drops. Also the active power can be partially
controlled, depending on the availability of energy storage or controllable generation units.
Different techniques have been proposed: stand-alone Energy Gateways with power references
control based on local measurement only, without need for communication, or distributed solution
where each EG communicates with the surrounding EGs, computes its optimum local power
reference and leaves the control to another EG with a Token Ring logic: iterating the local
optimization, the system converges to a global optimum of the loss. The optimization is a
constrained optimization, depending on power availability, limited by converters power ratings and
by distributed generators and storages, and the effects of these constraints are taken into account in
the distributed optimization analysis. The analysis have been performed first analytically and then by
simulation, developing a specific set of Matlab scripts that gives a flexible tool to define and test
every microgrid and its generators and loads, over which the distributed control algorithms have
been developed. Some of the major limits of these techniques have been addressed, among them the
need for transmission of synchrophasor with tight real-time requirements. At the end, a sub-optimum
controller is proposed, expecting to overcome the over mentioned limits that will be developed in the
continuation of this research activity.
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In parallel to this first topic, a second more specific problem has been investigated, representing
another aspect of the Smart Grid paradigm. As part of a visiting period with the PEMC (Power
Electronics Machines and Control) group at the University of Nottingham, the combination of a
STATCOM with energy storage and an engine powered synchronous generator has been studied to
smooth the frequency variations during sudden load changes. This represents one of the applications
of limited energy storage with high dynamic capabilities, like supercapacitors and is of interest both
as retrofit of existing synchronous generators, that could experience disconnection problems in a
more dynamic environment represented by the Smart Grid, and as a way of ensuring power quality
in a microgrid fed by a backup generator. The first application requires a radical increase in the
power size of the system, while the second one is more centred on Smart microgrids: independently
on the control approach, a microgrid has to be designed to support islanded operation, i.e. to be able
to feed loads even when disconnected from the main power network, and one of the options for this
backup supply of the microgrid is the use of synchronous generators as emergency voltage sources,
typically with diesel engines as prime movers. This second scenario has been considered and studied
and an innovative control technique has been proposed able to ride through step changes in the load
demand without changes in the prime mover speed and therefore without large changes in the
voltage frequency. This has been achieved through fast active power injection from the energy
storage as soon as the load transient is detected. From a qualitative point of view, the new load
power demand is supported by the storage until the synchronous generator prime mover overcomes
the related transient. This technique has been investigated in simulation and validated in a 10kVA
experimental setup, representing a simple isolated microgrid with a single generator and resistive
loads, confirming the effectiveness of the proposal.Questo lavoro ha l’obiettivo di analizzare alcuni aspetti del nuovo scenario delle Smart Grid,
dove la vasta introduzione di generazione distribuita e di tecniche innovative di gestione dell’energia
sta forzando una profonda revisione dell’attuale rete di distribuzione. Nonostante i temi affrontati
abbiano validità generale, l’analisi presentata si concentra sulle microreti in bassa tensione. Questo
perché la bassa tensione, a maggior ragione se limitata a una specifica area identificata da una
microrete, è l’architettura più flessibile ed aperta all’introduzione di soluzioni innovative.
Recentemente, la rete di distribuzione ha visto l’introduzione di un gran numero di generatori
distribuiti, introduzione finora basata su una rigida struttura gerarchica all’interno della rete, dove i
gestori mantengono il completo controllo sull’installazione e sulla gestione degli impianti. Questo è
legato soprattutto a ragioni storiche: la rete di distribuzione si basa su un paradigma di controllo
centralizzato, dove i flussi di potenza sono unidirezionali. La generazione avviene in un numero
limitato di centrali e la potenza viene distribuita ai carichi, in un’architettura rigida e totalmente
controllata.
Lo scenario emergente delle Smart Grid propone invece una generazione fortemente
decentralizzata, basata su un gran numero di sorgenti di energia distribuite, anche di potenza medio
bassa. Allo stesso tempo, il settore dell’ICT (Information and Communication Technology) è in
continua crescita. La combinazione di generazione distribuita e ICT ha la potenzialità di cambiare
completamente il sistema di distribuzione dell’energia, sia da un punto di vista architetturale che del
ruolo delle parti nel mercato dell’energia.
Da una prospettiva architetturale, la generazione distribuita potrebbe evolvere verso una
soluzione ibrida tra sorgenti rinnovabili quali fotovoltaico, eolico, celle a combustibile ed
idroelettrico e fonti tradizionali a combustione, quali turbine a gas e generatori diesel, queste ultime
in grado di garantire continuità di alimentazione ai carichi più sensibili in qualsiasi condizione.
Inoltre, un accumulo energetico (Energy Storage, ES) può essere usato per introdurre un grado di
libertà aggiuntivo nella gestione dell’energia: batterie o flywheels possono assorbire la generazione
in eccesso, per fornire energia durante i picchi di carico. Accumuli energetici di ridotta capacità,
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quali i supercondensatori, possono invece aumentare la qualità della potenza fornita ai carichi,
essendo in grado di rispondere velocemente a transitori della microrete, quali variazioni di frequenza
e buchi di tensione. Queste unità di generazione ed accumulo devono essere interfacciate alla rete
attraverso un convertitore di potenza, equipaggiato con un controllore digitale in grado di misurare e
controllare variabili locali per il corretto funzionamento del convertitore stesso, ma anche in grado di
comunicare in modo bidirezionale con altri controllori, ad esempio attraverso PLC (Power Line
Communication). La struttura di conversione risultante è stata definita in questo lavoro “Energy
Gateway”, (EG), e rappresenta l’elemento portante delle future Smart microgrids nella visione
proposta da questo lavoro.
Per limitare la complessità del sistema, l’analisi proposta è focalizzata sulle microreti: una
microrete è connessa alla rete di distribuzione tradizionale a bassa tensione, e presenta al suo interno
un certo numero di generatori distribuiti, tutti equipaggiati con EG. Nell’approccio proposto, il
controllo della microrete è distribuito, senza la presenza di un controllore centrale che gestisce tutte
le sorgenti di energia. Il comportamento interno della microrete può così essere gestito da interazioni
tra le diverse sorgenti, allo scopo di mostrare al punto di connessione con la rete tradizionale un
certo comportamento equivalente desiderato, oppure per ottimizzare il funzionamento della
microrete secondo parametri prestabiliti.
Entrando nel dettaglio, l’ottimizzazione considerata in questo lavoro è la minimizzazione delle
perdite di distribuzione all’interno della microrete, ed è ottenuta controllando opportunamente i
riferimenti di potenza attiva e reattiva dei convertitori con un approccio distribuito. In particolare, il
normale sovradimensionamento dei convertitori permette di iniettare nella microrete potenza reattiva
distribuita, che viene fornita ai carichi localmente. In questo modo si riduce l’assorbimento dalla rete
di distribuzione principale, così riducendo perdite e cadute di tensione. Anche l’iniezione di potenza
attiva può essere parzialmente controllata, a seconda della disponibilità di energia accumulata negli
EG o di sorgenti di energia totalmente controllabili (i.e. gas turbines, diesel generators, fuel cells).
Questo lavoro propone diverse soluzioni per minimizzare le perdite di distribuzione di una
microrete: Energy Gateways con riferimenti di potenza controllati usando come informazioni solo
variabili misurate localmente, senza nessuna comunicazione, oppure soluzioni distribuite dove gli
Energy Gateways comunicano e ognuno di essi esegue una minimizzazione locale delle perdite,
basandosi solo su informazioni ricevute dai generatori vicini. Iterando l’ottimizzazione locale, la
microrete converge al minimo globale delle perdite di distribuzione. L’ottimizzazione è analizzata
considerando i vincoli imposti dai limiti di dimensionamento dei convertitori degli Energy
Gateways. L’analisi è stata sviluppata prima analiticamente e successivamente in simulazione,
sviluppando un codice Matlab per definire la microrete e testare le diverse soluzioni di
ottimizzazione distribuita.
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In parallelo alla minimizzazione delle perdite è stato sviluppato anche un secondo argomento, che
rappresenta un altro aspetto del paradigma delle Smart Grid. Durante il periodo all’estero speso
presso il PEMC (Power Electronics Machines and Control) group all’università di Nottingham, è
stata studiata la combinazione tra uno STATCOM con accumulo energetico e un generatore
sincrono mosso da un motore diesel, allo scopo di minimizzare la variazione di frequenza del
generatore, che si verifica durante variazioni a gradino del carico. L’applicazione è importante sia
come adattamento di impianti di generazione esistenti, che potrebbero trovarsi in situazioni di
criticità in una rete in cui si verificano frequenti variazioni di carico, e sia come tecnica con cui
garantire stabilità in frequenza in una microrete alimentata da un singolo generatore diesel di backup
in mancanza della rete principale. Questo secondo caso è stato investigato nel dettaglio, in quanto
più orientato alle microreti: ogni microrete deve prevedere un backup energetico per garantire il
funzionamento in isola, e il generatore diesel è una delle possibili soluzioni. In questo lavoro è stata
proposta una tecnica di controllo innovativa, capace di superare transitori di carico con variazione
trascurabile della frequenza generata. L’obiettivo è stato raggiunto grazie ad un’iniezione di potenza
attiva attraverso lo STATCOM durante il transitorio di carico. Da un punto di vista qualitativo, lo
STATCOM e il suo accumulo energetico sopperiscono al maggior o minor carico nell’intervallo di
tempo necessario al motore diesel per aggiornare la sua potenza di uscita. La tecnica è stata prima
testata in simulazione e successivamente validata su un prototipo sperimentale che ha ricreato una
microrete elementare con generatore sincrono e STATCOM da 10kVA e carichi resistivi,
confermando l’efficacia della soluzione proposta
Optimization of Micro-Grid Operation by Dynamic Grid Mapping and Token Ring Control
No full textIn residential micro-grids, the distributed energy resources are interfaced with the grid by electronic power processors. If these processors perform cooperatively, full exploitation of energy sources can be achieved together with distribution loss reduction and voltage stabilization. The paper shows that this is possible without central controllers, by taking advantage of the local measurement, communication and control capability. In particular, it is shown that micro-grid operation can be optimized by applying token ring control and grid mapping techniques, which only require communication capability between neighbor nodes via power line
Cooperative control of smart micro-grids based on conservative power commands
No full textSmart micro-grids offer a new and wide application domain for power electronics. In fact, every distributed energy resource (DER) includes an electronic power processor (EPP) capable to control the active and reactive power flow from/to the distribution grid. If such EPPs perform cooperatively, they have the capability to fully exploit every local energy source while improving both power quality and distribution efficiency. This is of particular relevance in low-voltage residential micro-grids, where a plethora of small DERs may be active at the same time and the co-ordination of their operation can greatly improve the micro-grid performance. A simple and effective solution to achieve cooperative operation of EPPs is described in the paper. It makes use of a control method which requires power data exchange within the micro-grid and provides quasi-minimum distribution losses and local voltage support
Convergence analysis and tuning of a sliding-mode ripple-correlation MPPT
Full text linkThe development of fast Maximum Power Point Tracking (MPPT) algorithms for photovoltaic (PV) systems with high bandwidth and predictable response to irradiation transients is attractive for mobile applications and installations under fast changing weather conditions. This paper proposes the convergence analysis of a sliding-mode version of the MPPT based on ripple correlation control (RCC). The contribution of the paper is a dynamic model, useful to derive a set of design guidelines to tune the sliding-mode RCC-MPPT and achieve a desired dynamic performance under irradiation transients, without a dedicated commissioning phase. The research is based on sliding control theory and it includes both the chattering phenomena analysis and a discussion on the effects of reactive parasitic elements in the PV module. The proposed analysis and design have been validated by Matlab simulations first and then with experimental tests on a 35 W panel with a boost converter charging a 24 V battery. The results support the effectiveness of the proposed modelling procedure and design guidelines, showing good agreement between the model prediction and the experimental transient response
Improving Power Quality and Distribution Efficiency in Micro-Grids by Cooperative Control of Switching Power Interfaces
No full textSmart grids offer a wide application domain for power electronics. In fact, every distributed energy resource (DER) includes an electronic power processor (Switching Power Interface, SPI) which controls the currents drawn from the grid and can be driven to optimize the power flow, improve voltage stability and increase distribution efficiency. For these aims, such distributed SPIs must perform cooperatively. This is true also in low-voltage residential micro-grids, where the number of active DERs and the generated power may vary during daytime, thus requiring dynamic adaptation of SPI operation. To achieve this goal different approaches can be adopted, depending on the available communication capability. This paper discusses various control solutions applicable in absence of supervisory control, e.g., in residential micro-grids, where communication is possible between neighbor units only (surround control) or is not available at all (plug & play control)
Improving Power Quality and Distribution Efficiency in Micro-Grids by Plug & Play Control of Switching Power Interfaces
No full textSmart grids offer a wide application domain for power electronics. In fact, every distributed generator (DG) includes an electronic power processor (Switching Power Interface, SPI) which controls the currents drawn from the grid and can be driven to optimize the power flow, improve voltage stability and increase distribution efficiency. For these aims, such distributed SPIs must perform cooperatively. This is true also in low-voltage residential micro-grids, where the number of active DGs and the generated power may vary during daytime, thus requiring dynamic adaptation of SPI operation. To achieve this goal different approaches can be adopted, depending on the available communication capability. This paper discusses various control solutions applicable in absence of supervisory control, e.g., in residential micro-grids, where communication is possible between neighbor units only (surround control) or is not available at all (plug & play control)
Distributed cooperative control of low-voltage residential microgrids
No full textSmart micro-grids offer a new and challenging application environment for modern power electronics. In fact, smart micro-grids are populated by a plethora of distributed energy resources (DERs), which interface with the distribution grid by means of electronic power processors (EPPs). This scenario enables the distributed control of the active and reactive currents flowing in the power lines, with a tremendous potentiality to improve the grid performance in terms of distribution efficiency, power sharing, voltage stabilization, hosting capacity and demand response. This paper presents a simple and powerful approach to cooperative control of distributed EPPs, which only requires narrowband communication among neighbouring units. The proposed scalable and flexible distributed control architecture provides prompt response to power transients and efficient operation of the micro-grid in both grid-connected and islanded conditions
A generalized method to analyze the small-signal stability for a multi-inverter islanded grid with droop controllers2013 15th European Conference on Power Electronics and Applications (EPE)
No full textSurround Control of Distributed Energy Resources in Micro-Grids
No full textSmart micro-grids offer a new and wide application domain for power electronics. In fact, every distributed energy resource (DER) includes an electronic power processor (EPP) to control the power exchange with the grid. If such distributed EPPs perform cooperatively, they have the capability to fully exploit all renewable power sources and energy storage units while improving the power quality and transmission efficiency. In fact, EPPs control phase and shape of the currents injected into the grid, thus they can provide, in addition to dynamic control of the power flow between DERs and grid, also voltage stabilization and harmonic damping. This is of particular relevance in low-voltage residential micro-grids, where number of active DERs and generated power are unpredictable and may vary during the daytime. A simple and effective solution to achieve cooperative operation of EPPs is described in the paper. It is based on a control approach which requires communication between neighbor units only (surround control) and holds even in presence of multiple feeding points from the utility. Moreover, it allows independent and stable operation of the EPPs while providing voltage support and reduction of distribution losses. Accordingly, residential micro-grids may perform efficiently, adapt to supply and load variations and switch automatically from grid-connected to islanded operation
Distribution loss minimization by token ring control of power electronic interfaces in residential microgrids
No full textSmart micro-grids offer a new application domain for power electronics. In fact, every Distributed Energy Resource (DER) includes an Electronic Power Processor (EPP) to control the power exchange with the grid. If such distributed EPPs perform cooperatively, all the available energy sources and energy storage units can be fully exploited, resulting in reduced power consumption from the utility, high power quality, and increased hosting capability by the utility. The paper shows that even in low-voltage meshed micro-grids, where the electrical distribution pattern is complex and sources and loads may vary during daytime, such cooperative operation can be achieved by a proper selection of the local control algorithms and by allowing narrowband communication capability among neighbor EPPs. In particular, the paper describes a token ring control approach which allows full exploitation of the micro-grid capabilities with marginal investment in the ICT infrastructure
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