The second-order generalized integrator (SOGI) has been widely used to implement grid synchronization for grid-connected inverters, and from grid voltages, it is able to extract the fundamental components with an output of two orthogonal sinusoidal signals. However, if there is a dc offset existing in the grid voltages, the general SOGI's performance suffers from its generated dc effect in the lagging sine signal at the output. Therefore, in this paper, a mixed second- and third-order generalized integrator (MSTOGI) is proposed to eliminate this effect caused by the dc offset of grid voltages. A detailed theoretical analysis on the proposed MSTOGI is presented to reveal the mechanism of eliminating the dc offset. After that, the MSTOGI is applied to a phase-locked loop (PLL) and thereby establish an MSTOGI-PLL which is more adaptable to various grid conditions and power quality. Moreover, a frequency-adaptive control scheme is added to the proposed MSTOGI-PLL to eliminate the phase difference between the PLL output and the grid in grid-connected applications where the grid frequency may vary. Finally, the experimental results from a laboratory prototype are given to demonstrate and verify the effectiveness of the proposed MSTOGI-PLL in terms of steady-state performance, dynamic response, and frequency adaptability.

The grid-connected inverters may experience excessive current stress in case of unbalanced grid voltage fault ride through (FRT), which significantly affects the reliability of the power supply system. In order to solve the problem, the inherent mechanisms of the excessive current phenomenon with the conventional FRT solutions are discussed. The quantitative analysis of three-phase current peak values is conducted and a novel current-limited control strategy is proposed to achieve the flexible active and reactive power regulation and successful FRT in a safe current operation area with the aim of improving the system reliability under grid faults. Finally, the simulation and experiments of traditional and proposed FRT solutions are carried out. The results verify the effectiveness of the proposed method.

Leakage current reduction of the single-phase transformerless cascaded H-bridge PV inverter is investigated in this paper. The high-frequency common-mode loop model of a typical single-phase cascaded H-bridge PV system is established. Based on the model, the main factors that affect the leakage current are discussed. The reason why the typical single-phase cascaded H-bridge inverter fails to reduce the leakage current is explained. In order to solve the problem, a cascaded topology based on the H5 inverter is presented, along with a new modulation strategy, which can ensure that the stray capacitor voltage is free of high-frequency components. In this way, the leakage current can be effectively reduced. Finally, a prototype with TMS320F28335DSP + XC3S400FPGA digital control is built. The performance tests of cascaded H-bridge and the proposed topologies are carried out. The experimental results verify the effectiveness of the proposed solution.

Leakage current reduction is crucial for the transformerless photovoltaic inverters. The conventional three-phase current source H6 inverter suffers from the large leakage current, which restricts its application to transformerless PV systems. In order to overcome the limitations, a new three-phase current source H7 (CH7) inverter is proposed in this paper. Only one additional Insulated Gate Bipolar Transistor is needed, but the leakage current can be effectively suppressed with a new space vector modulation (SVM). Finally, the experimental tests are carried out on the proposed CH7 inverter, and the experimental results verify the effectiveness of the proposed topology and SVM method.

Versatile single-phase voltage source inverters with unipolar voltage pulse and leakage current elimination capability have been extensively investigated for transformerless PV systems in the literature. However, the innovative current source inverters with leakage current elimination capability are not well explored. In this brief, a novel single-phase current source H5 (CH5) inverter is proposed. Only one extra IGBT is needed, but the leakage current can be significantly suppressed with a novel space vector modulation. Finally, the experimental tests are carried out on a single-phase CH5 inverter and the experimental results verify the effectiveness of the proposed topology and space vector modulation.

Three-phase cascaded inverter with leakage current reduction for the transformerless PV system is investigated in this paper. The common-mode loop model of the conventional three-phase cascaded H4 grid-connected inverter is established. The relationship among the leakage current, common-mode voltage, and differential mode voltage is analyzed. The inherent reason that the conventional cascaded H4 grid-connected inverter fails to reduce the leakage current is clarified. In order to solve the problem, a novel three-phase cascaded H5 grid-connected inverter and its modulation strategy are proposed. It can significantly reduce the leakage current. Finally, the performance tests of the cascaded H4 and H5 grid-connected inverters are carried out. The results verify the effectiveness of the proposed solution.