Resumen de Photovoltaic power converter for large scale applications
Cristian Andrés Verdugo Retamal
Most of large scale photovoltaic systems are based on centralized configurations with voltage source converters of two or three output voltage levels connected to photovoltaic panels. With the development of multilevel converters, new possible topologies have come out to replace current configurations in large scale photovoltaic applications, reducing filter requirements in the ac side, increasing the voltage level operation and improving power quality.
One of the main challenges of implementing multilevel converter in large scale photovoltaic power plants is the appearance of high leakage currents and floating voltages due to the significant number of power modules in series connection. To solve this issue, multilevel converters have introduced high or low frequency transformers, which provide inherent galvanic isolation to the photovoltaic panels. The Cascaded H-Bridge converter (CHB) with high frequency transformers in a second conversion stage has provided a promising solution for large scale application, since it eliminates the floating voltage problem and provides an isolated control stage for each dc side of the power modules. In an effort to integrate ac transformers to reduce the requirement of a second conversion stage, Cascaded Transformer Multilevel Inverters (CTMI) have been propose for photovoltaic applications. These configurations use the secondary side of the ac transformers to create the series connection, while the primary side is connected to each power module, satisfying isolation requirements and providing different possibilities of winding connections for symmetrical and asymmetrical configurations.
Based on the requirements of multilevel converters for large scale photovoltaic applications, the main goal of this PhD dissertation is to develop a new multilevel converter which provides galvanic isolation to all power modules, while allowing an independent control algorithm for their power generation. The configuration proposed is called Isolated Multi-Modular Converter (IMMC) and provides galvanic isolation through ac transformers. The IMMC comprises two group of series connected power modules referred as arm, which are electrically interconnected in parallel. The power modules are based on three-phase voltage source converters connected to individual group of photovoltaic panels in the dc side, while the ac side is connected to three-phase low frequency transformers. Therefore, several isolated modules can be connected in series.
Because the power generated by photovoltaic panels may be affected by environmental conditions, power modules are prone to generate different power levels. This scenario must be covered by the IMMC, thus providing high flexibility in the power regulation. In this regard, this PhD dissertation proposes two control strategies embedded in each power module, whose role is to control the power flow based on the dc voltage level and the current arm information. The Amplitude Voltage Compensation (AVC) regulates the amplitude of the modulated voltage, while the Quadrature Voltage Compensation (QVC) regulates its phase angle by introducing a circulating current flowing through the arms. Additionally, it is demonstrated that combining both control strategies, the capability to withstand power imbalances increases, providing a higher flexibility.
The IMMC was modelled and validated via simulation results. Besides, a control algorithm was proposed to regulate the total power generated. A downscale experimental setup of 10kW was built to endorse the analysis demonstrated via simulation.
This study considers an IMMC connected to the electrical grid, which operates in balance and imbalance power scenarios, demonstrating the complete flexibility of the converter to be implemented in large photovoltaic applications.
