The growing presence of renewable energy sources, in particular the photovoltaic source and the rise of interest towards DC technology is marking a turning point in the electrical distribution systems, leading to the spread of DC microgrids. These systems are usually supplied by renewable energy sources (RES) and by storage devices capable of guaranteeing a support to the microgrid’s load demand. This thesis concerns the growing role played by the DC microgrid for the modern low voltage electrification distribution networks. An entire 400 V DC microgrid feeding two DC loads requiring a total power of 31.5 kW, supplied by three parallel converters connecting the microgrid with a photovoltaic array, a lead-acid battery and a supercapacitor, tied to the AC power system has been modelled and built in MATLAB/SIMULINK environment. The main focus of this thesis is the control strategy of such a microgrid allowing it to work properly in terms of DC bus voltage regulation, which must be equal to 400 V having a maximum voltage drop of 4 %, both in grid-tied and in stand-alone configurations. The control method proposed stems from ANSI\ISA 95 standard aiming at the smart control of the microgrid and it is named as hierarchical control. The hierarchical control implemented in this work is composed by three levels. The primary control aims at regulating the parallel converters through the droop-method allowing the load sharing between them, thanks to the emulation of a virtual conductance. This method shows the fast dynamics of the supercapacitor with respect to the battery in supplying the loads. Moreover, the droop control guarantees the stabilization of the common DC voltage at around 400 V (allowed deviations of 2 %) without using any communication channels. On the other hand the PV array converter has not the primary control, but it is controlled through the MPPT algorithm. The other two controls, the secondary and the tertiary, also known as supervisory controls need communication signals to be exchanged among the components of the grid to ensure a supervisory control of the microgrid. The secondary control is liable of the regulation of the stand-alone configuration of the microgrid and it can compensate the voltage deviations (around 2%) caused by the primary droop control. The tertiary control, instead, takes action, thus replacing the secondary one, when the microgrid is tied to the AC grid. The tertiary control, therefore, has also the target to manage the power flow between the AC and the DC networks. Finally in the tertiary control it is also implemented an individual control of the three-phase converter which stands for the connectivity of the AC main power system with the DC grid. The control of the interface converter is done through a vector decoupling controlled space vector sinusoidal pulse width modulation (SVPWM). This technique requires a double closed feed-forward decoupling method which is able not only to regulate the DC output voltage of the converter to the desired value of 400 V, but it also allows a unity power factor along the AC lines through the independent control of the active and reactive power withdrawn by the AC network. Simulation results are shown at the end of this thesis in order to verify not only the regulation at 400 V of the DC bus voltage along the DC microgrid obtained through the hierarchical control operating under different external and internal disturbances and by taking into consideration the microgrid’s variable size, but also the simulations here provided guarantee an optimal control of the interface converter in terms of unity power factor and DC bus voltage regulation.
La rilevante domanda e la sempre piu’ crescente necessita’ di fonti rinnovabili, prima tra tutte in Italia, il fotovoltaico, sta costituendo un cambiamento nel tradizionale metodo della trasmissione e distribuzione dell’energia elettrica. Inoltre I vantaggi nell’uso della corrente continua nelle tecnologie moderne stanno determinando una ampia diffusione nella ricerca scientifica delle cosidette microreti in corrente continua. Queste, infatti, sono solitamente alimentate da fonti di energia distribuite, tra le quali le fonti energetiche rinnovabili e le fonti di accumulo. In questa tesi verra’ studiata nel dettaglio una microrete in corrente continua funzionante a 400 V mostrandone i principali vantaggi. E’ stato quindi realizzato sulla piattaforma di MATLAB\SIMULINK un modello di una microrete in corrente continua che alimenta due carichi anch’essi in corrente continua che richiedono una potenza totale di 31.5 kW. Tale rete e’ alimentata principalmente da tre convertitori in parallelo che connettono un impianto fotovoltaico, una batteria al piombo e un supercondensatore, rispettivamente alla microrete. Questa a sua volta puo’ essere alimentata o meno dalla principale rete di distribuzione in corrente alternata. Il principale obiettivo di questa tesi, tuttavia, consiste nel mostare una efficiente tecnica di controllo di tale microrete che consenta una ottima regolazione della tensione sul bus principale di 400 V sia quando la microrete e’ connessa alla rete di distribuzione in corrente alternata (grid-tied configuration) sia quando la microrete deve operare senza il supporto della rete in alternata (stand-alone configuration). La strategia di controllo qui adottata si chiama controllo gerarchico e deriva dallo standard ANSI\ISA 95 che ha lo scopo di assicurare una certa flessibilita’ ed efficienza di controllo alla microrete. Il controllo gerarchico implementato in questa tesi e’ costituito da tre livelli piramidali. Il controllo primario consente una logica distribuzione del carico tra i convertitori in parallelo tramite il droop-control. Questa tecnica serve ad emulare, a regime, l’inerzia di un generatore sincrono attraverso una conduttanza virtuale posta all’uscita di ogni convertitore. Inoltre il controllo primario stabilizza la tensione di 400 V all’uscita di ogni convertitore con una tolleranza del 2 % senza l’impiego di canali di comunicazione tra i vari convertitori. Il convertitore del sistema fotovoltaico non e’ controllato dal droop control, ma dal controllo MPPT. Gli altri due controlli, il secondario e il terziario, conosciuti anche come controlli di supervisione, invece, richiedono l’utilizzo di segnali di comunicazione tra i vari componenti della rete al fine di garantire un controllo ottimale dei parametri elettrici, sopratutto per quanto riguarda la regolazione della tensione continua sul bus principale della microrete. Il controllo secondario si occupa della microrete nella configurazione “stand-alone” e riesce a compensare le deviazioni di tensione, intorno al 2 % della tensione nominale di 400 V, sul bus principale della microrete dovuti al controllo primario. Il controllo terziario, invece, sostituisce il secondario quando la microrete viene esercita nella configurazione “grid-tied”. Quindi il controllo terziario viene qui utilizzato anche per la gestione dei flussi di potenza tra la rete AC e la rete DC. Nella configurazione “grid-tied”, infatti, e’ implementato un controllo del convertitore trifase di interfaccia tra le due reti menzionate sopra. La strategia di controllo di tale convertitore modulato con la space vector modulation (SVPWM) si chiama tecnica di disacoppiamento vettoriale, la quale, attraverso l’utilizzo di due anelli in feed-forward della corrente e’ in grado non solo di ottenere un fattore di potenza unitario sulla rete AC in seguito al disaccopiamento delle dinamiche di corrente del convertitore, ma e’ anche in grado di garantire un valore costante e regolato a 400 V di tensione continua all’uscita del convertitore. Nel capitolo sei di questa tesi verranno mostrati i risultati ottenuti attraverso diverse simulazioni fatte in ambiente MATLAB\SIMULINK. Le simulazioni verificheranno non solo la corretta regolazione della tensione continua di 400 V sul bus prinicipale della microrete ottenuta in seguito al controllo gerarchico testato in presenza di diverse perturbazioni interne ed esterne e considerando anche le dimensioni variabili della microrete, ma queste simulazioni riusciranno anche a dimostrare la concretezza del controllo implementato sul convertitore di interfaccia tra rete AC e rete DC in termini di unita’ del fattore di potenza e di regolazione della tensione continua a 400 V.
Hierarchical control of a DC low voltage microgrid supplied by Distributed Energy Resources (DERs)
FAGGIO, ANTONIO
2014/2015
Abstract
The growing presence of renewable energy sources, in particular the photovoltaic source and the rise of interest towards DC technology is marking a turning point in the electrical distribution systems, leading to the spread of DC microgrids. These systems are usually supplied by renewable energy sources (RES) and by storage devices capable of guaranteeing a support to the microgrid’s load demand. This thesis concerns the growing role played by the DC microgrid for the modern low voltage electrification distribution networks. An entire 400 V DC microgrid feeding two DC loads requiring a total power of 31.5 kW, supplied by three parallel converters connecting the microgrid with a photovoltaic array, a lead-acid battery and a supercapacitor, tied to the AC power system has been modelled and built in MATLAB/SIMULINK environment. The main focus of this thesis is the control strategy of such a microgrid allowing it to work properly in terms of DC bus voltage regulation, which must be equal to 400 V having a maximum voltage drop of 4 %, both in grid-tied and in stand-alone configurations. The control method proposed stems from ANSI\ISA 95 standard aiming at the smart control of the microgrid and it is named as hierarchical control. The hierarchical control implemented in this work is composed by three levels. The primary control aims at regulating the parallel converters through the droop-method allowing the load sharing between them, thanks to the emulation of a virtual conductance. This method shows the fast dynamics of the supercapacitor with respect to the battery in supplying the loads. Moreover, the droop control guarantees the stabilization of the common DC voltage at around 400 V (allowed deviations of 2 %) without using any communication channels. On the other hand the PV array converter has not the primary control, but it is controlled through the MPPT algorithm. The other two controls, the secondary and the tertiary, also known as supervisory controls need communication signals to be exchanged among the components of the grid to ensure a supervisory control of the microgrid. The secondary control is liable of the regulation of the stand-alone configuration of the microgrid and it can compensate the voltage deviations (around 2%) caused by the primary droop control. The tertiary control, instead, takes action, thus replacing the secondary one, when the microgrid is tied to the AC grid. The tertiary control, therefore, has also the target to manage the power flow between the AC and the DC networks. Finally in the tertiary control it is also implemented an individual control of the three-phase converter which stands for the connectivity of the AC main power system with the DC grid. The control of the interface converter is done through a vector decoupling controlled space vector sinusoidal pulse width modulation (SVPWM). This technique requires a double closed feed-forward decoupling method which is able not only to regulate the DC output voltage of the converter to the desired value of 400 V, but it also allows a unity power factor along the AC lines through the independent control of the active and reactive power withdrawn by the AC network. Simulation results are shown at the end of this thesis in order to verify not only the regulation at 400 V of the DC bus voltage along the DC microgrid obtained through the hierarchical control operating under different external and internal disturbances and by taking into consideration the microgrid’s variable size, but also the simulations here provided guarantee an optimal control of the interface converter in terms of unity power factor and DC bus voltage regulation.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/115865