THD Reduction of Inverters in Photovoltaic Power Systems

Abstract

Photovoltaic power systems are quite common in modern consumer power systems. Alongside with the power sources, converters for photovoltaic energy systems made their own evolution path that was defined by two main requirements that have to be fulfilled: maximization of the power source usage and provision of the output voltage THD ratio on the levels defined by international standards. In modern power systems z-/quasi-z- source inverters are commonly used due to their ability to maintain system operation in maximum power point of the solar panel without the implementation of the additional devices. THD ratio of these topologies exceeds the limits defined by international standards therefore it has to be reduced. This article is devoted to determination, comparison, and discussion of THD reduction methods for the PV power system’s inverters. In the practical part of the article three inverter topologies, quasi-z-source, three-level cascaded quasi-z-source and three-level cascaded inverter which output cell’s voltages is defined by the desirable output voltage’s decomposition results achieved by orthogonal transformation were developed and simulated in Simulink environment as the models that represent different approaches on quasi-z-source THD ratio reduction. As a result of simulation output voltages waveform spectra and THD ratio for each model were obtained. According to them, a three-level cascaded z-source inverter in which output cell’s voltages are defined by the desirable output voltage’s decomposition results achieved by orthogonal transformation could provide the lowest THD ratio among considered devices. In conclusion to the study based on the advantages and drawbacks of each THD reduction method, outlined in the discussion section, a combination of cascaded multilevel topology implementation with size-reduced passive output filter was chosen as a solution that will simultaneously allow to achieve proper values of THD ratio, reduce the size of an output filter and avoid excessive sophistication of inverter’s control system. As potentially perspective fields of research were defined modernization of control algorithms, stabilization of the multilevel inverters cell’s output voltage of and determination of the methods that contain a mathematical determination of the cell’s output voltage in their basis implementation possibility in cascaded multilevel z-/quasi-z-topologies. That could result in their wider implementation and popularization in further converter devices that might lead to future complete elimination of the passive filters from the upcoming schematics due to lack of the additional output voltage filtering necessity. Ref.19, fig.7

Keywords

• z-source inverters
• renewable power systems
• cascaded multilevel inverters
• THD ratio
• output voltage quality