The thesis deals with grid-connected parallel inverters. It tries to find a valid alternative to the most common topologies. The initial scenario represents two parallel inverters, grid-connected, each through a transformer on the AC side. In addition, they are connected on the DC side through an equivalent parasitic capacitance, due to PV panels. In this situation a problem arises: a current flows in the capacitance and can even reach high values (tens of Ampere). It is called zero-sequence circulating current (ZSCC). Anyway, the solution with transformers allows to reduce that current but introduces disadvantages. A transformer often represents encumbrance, weight, costs and problems of transportation. Thus, an alternative solution should be found to reduce the ZSCC. In literature, some studies have already been carried out. A study demonstrates that, thanks to the synchronization of the triangular carrier signals of the two inverters, the ZSCC reduces in amplitude. This solution results valid but it is very difficult to synchronize two different waveforms due to the practical differences in the instruments and, even if it was possible, it would be a problem when the number of the inverters increases. Other papers suggest solutions depending on the type of modulation adopted for the inverters: Sinusoidal Pulse Width Modulation (SPWM) or Space Vector Modulation (SVM), in case of both 2Level or 3Level inverters. According to these studies, the aim is to work on the modulation of the inverters so to reduce the ZSCC, instead of using transformers, thus to increase the whole efficiency of the system. The modifications vary depending on the modulation: for SPWM, it will act on the triangular carrier signal while for SVM both on the triangular carrier signal and on the hexagon of the SVM itself. Good results have been obtained, with reduction of ZSCC depending on each considered case. The structure consists of 6 chapters: • Chapter 1 State of the art. In the first chapter there is an introduction of what the current technology presents, which are the problems and where they come from. Some solutions, already present in literature, are described. • Chapter 2 Case study. Here, it focuses on the actual model and solutions as the main object of the thesis. It is reported how the equivalent scheme is and its Simulink model. An introduction of the techniques is carried out. Then, there is a detailed analysis of the modified modulations. • Chapter 3 Test results. According to previous chapter, results of each simulation are reported, and a comparison is done to understand what differs between them and which can be more advantageous than another. • Chapter 4 Further modulation techniques. In addition to previous modulations, new ones are studied. It tries to look for more ways to get to the same goals. • Chapter 5 Laboratory tests. In this section, some laboratory results are presented based on a similar technique. • Chapter 6 Conclusions. The last chapter contains a summary of all the results obtained during the work and some comments on their behaviour and significance. There also are opinions on which future work could be.
La tesi tratta l’argomento di inverter in parallelo collegati alla rete. Si cerca di trovare una valida alternativa alle topologie più comuni. Lo scenario iniziale rappresenta due inverter in parallelo, collegati alla rete, ciascuno attraverso un trasformatore sul lato AC. Inoltre, sono collegati sul lato DC attraverso una capacità parassita equivalente, dovuta ai pannelli fotovoltaici. In questa situazione sorge un problema: una corrente scorre nella capacità e può raggiungere anche valori elevati (decine di Ampere). È nota come corrente di circolazione di sequenza omopolare (ZSCC). Ad ogni modo, la soluzione con trasformatori consentirebbe di ridurre tale corrente ma introduce degli svantaggi: un trasformatore rappresenta spesso ingombro, peso, costi e problemi di trasporto. Pertanto, si dovrebbe trovare una soluzione di diverso genere per ridurre la ZSCC. In letteratura, alcuni tentativi sono già stati effettuati. Uno studio dimostra che, grazie alla sincronizzazione delle portanti triangolari dei due inverter, la ZSCC si riduce in ampiezza. Questa soluzione risulta valida, ma è molto difficile sincronizzare due diverse forme d'onda a causa delle differenze tra gli strumenti e, anche se fosse possibile, sarebbe un problema quando il numero degli inverter aumenta. Altri studi suggeriscono soluzioni basate sul tipo di modulazione adottata per gli inverter: Sinusoidal Pulse Width Modulation (SPWM) o Space Vector Modulation (SVM), sia per inverter 2Livelli che 3Livelli. L'obiettivo posto è di lavorare sulla modulazione degli inverter in modo da ridurre la ZSCC, anziché utilizzare i trasformatori, in modo da aumentare l’efficienza totale del sistema. Le modifiche variano a seconda della modulazione: per SPWM, si agirà sulla portante triangolare mentre per SVM sia sulla portante triangolare che sull'esagono del SVM stesso. In generale si sono ottenuti risultati positivi, con relativa riduzione di ZSCC a seconda dei casi. La struttura è composta da 6 capitoli: • Capitolo 1 State of the art. Il primo capitolo presenta un'introduzione sull’attuale tecnologia, quali sono i problemi e da dove provengono. Alcune soluzioni, già presenti in letteratura, sono descritte. • Capitolo 2 Case study. Di seguito, ci si concentra sul modello e sulle soluzioni che rappresentano il fulcro del lavoro. Viene riportato lo schema equivalente, il suo modello Simulink e un'introduzione alle tecniche. Successivamente, è presentata un'analisi dettagliata delle modulazioni modificate. • Capitolo 3 Test results. Continuando, vengono riportati i risultati di ciascuna simulazione e viene effettuato un confronto per capire cosa differisce e quale situazione sia più vantaggiosa. • Capitolo 4 Further modulation techniques. Oltre alle modulazioni precedenti, ne vengono studiate di nuove. Si cerca di trovare altre soluzioni per il conseguimento degli stessi risultati. • Capitolo 5 Laboratory tests. In questa sezione, sono presentati alcuni risultati di laboratorio sulla base di una tecnica simile a quelle analizzate in precedenza. • Capitolo 6 Conclusions. L'ultimo capitolo contiene un riepilogo di tutti i risultati ottenuti durante il lavoro e alcuni commenti sul loro comportamento e significato. Sono date anche opinioni su quale potrebbe essere il lavoro futuro.
Operation of transformerless inverters, parallel connected, with modified SPWM and SVM
NARDONE, ARCANGELO
2016/2017
Abstract
The thesis deals with grid-connected parallel inverters. It tries to find a valid alternative to the most common topologies. The initial scenario represents two parallel inverters, grid-connected, each through a transformer on the AC side. In addition, they are connected on the DC side through an equivalent parasitic capacitance, due to PV panels. In this situation a problem arises: a current flows in the capacitance and can even reach high values (tens of Ampere). It is called zero-sequence circulating current (ZSCC). Anyway, the solution with transformers allows to reduce that current but introduces disadvantages. A transformer often represents encumbrance, weight, costs and problems of transportation. Thus, an alternative solution should be found to reduce the ZSCC. In literature, some studies have already been carried out. A study demonstrates that, thanks to the synchronization of the triangular carrier signals of the two inverters, the ZSCC reduces in amplitude. This solution results valid but it is very difficult to synchronize two different waveforms due to the practical differences in the instruments and, even if it was possible, it would be a problem when the number of the inverters increases. Other papers suggest solutions depending on the type of modulation adopted for the inverters: Sinusoidal Pulse Width Modulation (SPWM) or Space Vector Modulation (SVM), in case of both 2Level or 3Level inverters. According to these studies, the aim is to work on the modulation of the inverters so to reduce the ZSCC, instead of using transformers, thus to increase the whole efficiency of the system. The modifications vary depending on the modulation: for SPWM, it will act on the triangular carrier signal while for SVM both on the triangular carrier signal and on the hexagon of the SVM itself. Good results have been obtained, with reduction of ZSCC depending on each considered case. The structure consists of 6 chapters: • Chapter 1 State of the art. In the first chapter there is an introduction of what the current technology presents, which are the problems and where they come from. Some solutions, already present in literature, are described. • Chapter 2 Case study. Here, it focuses on the actual model and solutions as the main object of the thesis. It is reported how the equivalent scheme is and its Simulink model. An introduction of the techniques is carried out. Then, there is a detailed analysis of the modified modulations. • Chapter 3 Test results. According to previous chapter, results of each simulation are reported, and a comparison is done to understand what differs between them and which can be more advantageous than another. • Chapter 4 Further modulation techniques. In addition to previous modulations, new ones are studied. It tries to look for more ways to get to the same goals. • Chapter 5 Laboratory tests. In this section, some laboratory results are presented based on a similar technique. • Chapter 6 Conclusions. The last chapter contains a summary of all the results obtained during the work and some comments on their behaviour and significance. There also are opinions on which future work could be.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/139192