Transformerless Inverter Topologies for Single-Phase Photovoltaic Systems:A Comparative Review

In photovoltaic (PV) applications, a transformer is often used to provide galvanic isolation and voltage ratio transformations between input and output. However, these conventional iron-and copper-based transformers increase the weight/size and cost of the inverter while reducing the efficiency and power density. It is therefore desirable to avoid using transformers in the inverter. However, additional care must be taken to avoid safety hazards such as ground fault currents and leakage currents, e.g., via the parasitic capacitor between the PV panel and ground. Consequently, the grid connected transformerless PV inverters must comply with strict safety standards such as IEEE 1547.1, VDE0126-1-1, EN 50106, IEC61727, and AS/N ZS 5033. Various transformerless inverters have been proposed recently to eliminate the leakage current using different techniques such as decoupling the dc from the ac side and/or clamping the common mode (CM) voltage (CMV) during the freewheeling period, or using common ground configurations. The permutations and combinations of various decoupling techniques with integrated voltage buck-boost for maximum power point tracking (MPPT) allow numerous new topologies and configurations which are often confusing and difficult to follow when seeking to select the right topology. Therefore, to present a clear picture on the development of transformerless inverters for the next-generation grid-connected PV systems, this paper aims to comprehensively review and classify various transformerless inverters with detailed analytical comparisons. To reinforce the findings and comparisons as well as to give more insight on the CM characteristics and leakage current, computer simulations of major transformerless inverter topologies have been performed in PLECS software. Moreover, the cost and size are analyzed properly and summarized in a table. Finally, efficiency and thermal analysis are provided with a general summary as well as a technology roadmap.</p

Adaptive proportional resonant controller for single-phase grid-connected PV inverter based on grid impedance estimation technique

Ph. D ThesisPhotovoltaic PV systems have shown a significant growth in recent years driven by the increased efficiency and reductions in the cost of PV modules. Today, distribution generation based PV systems have a major contribution to the total electricity production worldwide. However, in areas with high penetration of PV system connected to the grid, interaction may arise between the grid and PV system. This research focuses on the effect of grid operating conditions on the performance of grid-connected PV inverter systems. A simulation model of the system under investigation has been developed to evaluate the impact of frequency deviation, grid voltage distortion and grid impedance variation on the harmonic performance of the injected current as well as the voltage at the point of the common coupler. Proportional resonance PR controller is employed to regulate the current produced from the PV inverter due to its reputation in tracking sinusoidal signals. The obtained simulation results demonstrate that the harmonic performance of the grid current and PCC voltage can be significantly influenced by the change of grid operating conditions. In particular, grid impedance variation can result in a shift in the system resonance frequency leading to distortion in network current and voltage. To adapt the PR controller to the grid impedance variation, a novel adaptive PR controller which takes into account the change in grid impedance is proposed. The adaptive consists of a high-order digital band-pass filter and chain of statistical signal processing technique. Simulation results show that the harmonic performance of grid current and PCC voltage can be enhanced and the proposed APR controller is robust against impedance variation. Finally, the proposed control method is experimentally implemented and the obtained results validate the effectiveness of the proposed control structure.Libyan Ministry of Higher Education and Scientific Research, and the general electricity company of Libya (GECOL