The leakage current reduction is one of the most important issues for transformerless PV systems. The conventional Z-source three-phase PV inverter failed to suppress the leakage current in an effective way. In order to solve the problem, a new modulation strategy for the Z-Source four-leg transformerless PV inverter is proposed in this paper. The common mode loop model is presented firstly. And then the common mode voltage behavior and the effect of factors on the leakage current are discussed. A new modulation strategy is proposed to achieve the step-up function and constant common mode voltage. Therefore, the leakage current can be suppressed effectively. Finally, the proposed strategy is digitally implemented and tested on the TMS320F28335DSP + XC3400FPGA controlled hardware platform. The experimental results verify the effectiveness of the proposed solution.

To diagnose short-circuit faults of single or double switches in current source converter, a method based on output currents is proposed. The method only needs the feedback currents and their corresponding reference signals that can be obtained from the main control system. This diagnosis algorithm need calculate the average values of the feedback currents and the average absolute values of the current reference signals in one power frequency cycle. Diagnosis variables can be get from ratio of the two values. Then, three detection thresholds are set to judge and locate the short-circuit power switches. The method can be implemented in the main control program directly, without additional voltage sensors and other auxiliary detection devices. Meanwhile, the method is simple, easy to implement in practice. The effectiveness of proposed algorithm is validated with experiments.

Leakage current reduction is one of the important issues for transformerless PV systems. Many interesting solutions have been reported to reduce the leakage current for three-phase PV inverters. However, most of them are limited to two-level inverters. Moreover, there is a potential risk of the over-current phenomenon. In order to solve the problem, a Z-source three-level four-leg inverter with a new modulation strategy is proposed in this paper. Firstly, the mathematical modeling of the three-level four-leg Z-source inverter for leakage current reduction is established for the first time. Secondly, a new carrier-based modulation strategy is proposed by utilizing the effective large, medium, small, and zero vectors, instead of the invalid vectors, to achieve the constant common mode voltage, as well as the leakage current suppression. Finally, the proposed solution is carried out on the TI (Texas Instruments) TMS320F28335 DSP + Xilinx XC3400 FPGA digital control hardware platform. The experimental results verify the effectiveness of the proposed solution.

Three-phase current source converter tends to be attractive in future industrial applications. In practice, the current source converter is not able to work with the negative dc link current. Therefore, the discontinuous dc link current will occur if the current source converter operates in case of light load, which will distort the grid current. Many interesting solutions have been reported to deal with these problems. However, most of them are only valid in the discontinuous current mode (DCM) but not suitable in other conditions such as continuous current mode (CCM). To solve this problem, a generalized solution is proposed in this paper. Firstly, the operation principle of current source converter is analyzed. Secondly, a new solution is proposed by using the new calculation of duty cycles, where the current source converter can operate in two cases. And then, the proposed solution is compared with the traditional one. The THD of grid current in traditional solution is 23.34%. However, it is reduced to 3.90% with the proposed solution. Finally, the proposed solution is carried out on the TMS320F28335 DSP + XC3400 FPGA digital control hardware platform. The experimental results verify the effectiveness of the proposed solution.

For single-phase current source converters, there is an inherent limitation in DC-side low-frequency power oscillation, which is twice the grid fundamental frequency. In practice, it transfers to the DC side and results in the low-frequency DC-link ripple. One possible solution is to install excessively large DC-link inductance for attenuating the ripple. However, it is of bulky size and not cost-effective. Another method is to use the passive LC branch for bypassing the power decoupling, but this is still not cost-effective due to the low-frequency LC circuit. Recently, active power decoupling techniques for the current source converters have been sparsely reported in literature. However, there has been no attempt to classify and understand them in a systematic way so far. In order to fill this gap, an overview of the active power decoupling for single-phase current source converters is presented in this paper. Systematic classification and comparison are provided for researchers and engineers to select the appropriate solutions for their specific applications.

This paper proposes a new three-switch single-phase Z-source inverter (ZSI) based on a CUK converter, which is named a CUK-based ZSI. This topology has characteristics of buck‒boost capability and dual grounding. In addition, the voltage gain of proposed inverter is higher than those of the single-phase quasi-Z-source and semi-Z-source inverters. Aside from that, a simple control method is presented to achieve the linear voltage gain. The operational principle of the proposed topology is described. Finally, a performance evaluation is carried out and the test results verify the effectiveness of the proposed solution.