Lectura de Tesi Doctoral de Cristian Verdugo

Títol de la tesi: Photovoltaic PowerConverters for Large Scale Applications
Autor: Cristian Verdugo Retamal
Programa: Joint Doctoral Programme in Electric Energy Systems
Dissertation submitted to the PhD Doctorate Office of the Universitat Politècnica de Catalunya in partial fulfillment of the requirements for the degree of Doctor of Philosophy by the
UNIVERSIDAD DE MÁLAGA
UNIVERSIDAD DE SEVILLA
UNIVERSIDAD DEL PAÍS VASCO/EUSKAL ERRIKO UNIBERTSITATEA
UNIVERSITAT POLITÈCNICA DE CATALUNYA
Director: Pedro Rodríguez Cortés
Co-director: José Ignacio Candela García

Data de lectura: Divendres, 23 de de juliol · 2:00-5:00pm
Enllaç de la videotrucada: https://meet.google.com/nka-mdfj-wqn

Resum de la tesi:

Photovoltaic energy systems require power electronics interfaces to convert the energy generated and transfer it to the electrical grid. Depending on the power level, grid connected PV systems can be grouped into four types of configurations: centralized, string, multistring and ac-module, which can be used based on the application and power rating of the PV installation. Most of large scale applications are based on centralized configurations with inverters of two or three voltage levels connected to hundreds of PV arrays. However, with the development of high power multilevel converters, new possibilities have come out to implement multilevel converters in PV systems with higher efficiency and power quality. One of the main challenges of using multilevel converter in PV applications are the appearance of leakage currents and high floating voltages in the PV panels. To solve this issue, high or low frequency transformers are required to provide galvanic isolation. The Cascaded H-Bridge converter with high frequency transformers in the dc side has been widely studied as a promising solution for large scale applications. However, the implementation of high frequency transformers require a second conversion stage. In an effort to integrate ac transformers to reduce the number of conversion stages by using medium or low frequency components, cascaded transformer multilevel inverters (CTMI) have been proposed. Such configurations are connected in series through the secondary windings of the transformers, satisfying required isolation requirements and providing winding connection options for symmetrical and asymmetrical configurations. Based on such approach, this PhD dissertation presents the Isolated Multi-Modular Converter, characterized by a cascaded configuration of two arms connected in parallel. The objective of the proposed configuration is to enable the integration of a multilevel converter with galvanic isolation and the capability of operating at different power levels.
First, an introduction of multilevel converters with dc and ac galvanic isolation is reviewed. This approach defines the basis of the converter proposed. Then, a mathematical analysis of the IMMC for a single and three phase configuration is presented. This study demonstrates that the IMMC can be represented based on two electrical circuits whose purpose is to emulate the dynamic response of the output and circulating currents based on the current arms. Furthermore, the relationship between the power flow and the current arms is also revealed to define later the current references in the control loops. As the IMMC requires a dedicated control strategy to regulate the energy and the current signals, a central control architecture is addressed. This approach is essential for regulating the total energy per arm, giving rise to a proper converter operation. In order to provide a stable response in dynamic and steady state, the well-known crossover frequency and phase margin technique is used to determine the control parameters in the inner and outer control loops. As PV panels can operate at different conditions, they are prone to generate different power levels. Therefore, all power modules connected in series must provide a high flexibility to allow possible power imbalance scenarios. In this regard, two control strategies embedded in each module are evaluated. Through the amplitude and quadrature voltage compensation techniques, the converter demonstrates its functionality under power imbalances, while fulfilling the required control objectives. Moreover, by combining both control strategies, it is possible to extend the operating range, enabling higher levels of power imbalances. The work presented in this PhD dissertation is supported by simulation results. Additionally, a complete experimental setup was built to endorse the conclusions. Simulation and experimental results include balance and imbalance power scenarios, demonstrating a flexible multilevel converter for PV applications.