كاهش تزريق جريان مستقيم در اينورترهاي فتوولتاييك متصل به شبكه بدون ترانسفورمر

Single-phase transformerless grid-connected photovoltaic inverters have been widely employed in distributed generation systems due to their high efficiency, reduced weight, an‎d lower cost. However, one of the main challenges associated with these structures is the unintentional injection of DC components into the grid an‎d the resulting increase in current harmonic distortion. Even a very small DC offset error in the current measurement path can lead to the appearance of a DC component in the grid current, generation of the second-order harmonic, an‎d an increase in total harmonic distortion (THD), thereby violating grid interconnection stan‎dards. In this thesis, the effects of DC offset error on the grid current an‎d DC-link voltage are first analyzed theoretically, demonstrating that this error leads to the emergence of a DC component an‎d a second-order harmonic in the injected current. Subsequently, the DC offset compensation algorithm proposed in the reference paper, which is based on indirect extraction of the offset through the second harmonic component, is implemented an‎d simulated on a 1-kW single-phase transformerless photovoltaic inverter with an LC filter an‎d more realistic system parameters. Simulation results show that, in the uncompensated case, the DC component of the grid current is approximately 0.231 A an‎d the current THD is about 8.49%. After activating the DC offset compensation algorithm, the DC component is reduced to 0.053 A an‎d the second harmonic is significantly suppressed; however, due to the remaining low-order harmonics, particularly the third an‎d fifth harmonics, the THD remains relatively high at around 7.80%. To overcome this limitation, a combined control structure consisting of an improved DC offset compensation algorithm an‎d a multi-resonant proportional–resonant (MR-PR) controller is proposed in this research. By incorporating resonant branches tuned to the third an‎d fifth harmonic frequencies, the MR-PR controller enables targeted suppression of the dominant low-order harmonics. Simulation results obtained in MATLAB/Simulink indicate that, with the proposed structure, the third an‎d fifth harmonics are effectively reduced, the grid current THD is decreased to 0.29%, an‎d the DC component of the current is reduced to a negligible value of approximately 0.001 A. The results of this thesis demonstrate that effective an‎d stable elimination of DC offset in single-phase grid-connected inverters requires an integrated control approach that simultaneously addresses DC offset compensation an‎d low-order harmonic control. The proposed MR-PR controller combined with the DC offset compensation algorithm provides an efficient an‎d reliable solution for improving power quality in transformerless photovoltaic inverters.

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