Análise e implementação do inversor cc/ca monofásico zeta cuk integrado aplicado a sistemas fotovoltaicos

Abstract

This work presents the study, implementation, and analysis of a new topology called the Integrated Zeta-'Cuk Inverter (IZCI), based on the combination of the conventional Zeta and 'Cuk converter structures. This topology can be applied both in standalone systems and as an interface converter between a photovoltaic (PV) energy generation system and the single-phase alternating current (AC) power grid. The structure features six power switches that operate differently during the positive and negative half-cycles. Designed to operate in discontinuous conduction mode (DCM), the topology has three operating stages for each switching period. By analyzing the operation of these switches, it is possible to model the topology using the equivalent circuits of the conventional DC/DC converters it is composed of, due to the similarity in their static and dynamic behavior. To achieve this, the Generalized Averaged Network Modeling technique is applied. Based on the theoretical analysis, the passive components of the converter were designed. Furthermore, the transfer function was derived and used in the two implemented control models: a Multi-Resonant Proportional-Integral (PI-MR) controller for standalone mode, and a Proportional-Integral (PI) controller designed to regulate the output current when the topology operates as a grid-connected interface converter. A phase-locked loop (PLL) system is employed to ensure synchronization between the IZCI output current and the grid voltage. Additionally, the Perturb and Observe (P&O) algorithm is implemented to ensure maximum power point tracking (MPPT) of the photovoltaic array. Through computer simulations using Simulink®, the functionality and main characteristics of the converter operating in standalone mode and as a PV-grid interface were verified. Finally, experimental results from the practical assembly of the converter on a test bench are presented. By implementing the appropriate algorithms and using a DC source that simulates the behavior of a photovoltaic array, results were obtained that confirm the functionality of the proposed topology. The experimental results for the standalone mode aligned with the simulation outcomes, with the converter successfully synthesizing a 60 Hz sinusoidal AC voltage with controlled amplitude. The total harmonic distortion (THD) of the output voltage remained below the standard limits, with a value of 2% at nominal power. For a load with nonlinear current characteristics, the THD was 3.7%. Lastly, the results for the grid-connected operation of the converter are presented, showing the expected performance: injecting a sinusoidal current into the grid with low harmonic distortion (4% at nominal power) and a unity power factor. Although the converter's efficiency was 90.3% at nominal power with the applied semiconductors, there is a prospect of increasing this value to approximately 97.5%.