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Different from the conventional photovoltaic (PV) inverters, a three-phase PV solid-state transformer (SST) based on the cascaded H-bridge (CHB) topology can be regarded as consisting of three single-phase CHB inverters in essence. During the low-voltage ride through (LVRT), some phases of three-phase PV SST may inversely absorb active power from ac grid affected by negative-sequence voltages, resulting in that the dc bus voltages of all H-bridges in these phases continue to rise, and that the system will shut down due to overvoltage protection. To deal with this problem, in this article, we study the mechanism of active power backflow during LVRT, analyzes and compares the main methods for suppressing active power backflow, and then proposes an LVRT control strategy based on the zero-sequence voltage compensation, which not only ensures that all modules in three-phase PV SST transmit the almost same active power but also can avoid effectively active power backflow problem even if the total output active of PV array is relatively low. The validity of the proposed method is verified by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

A nonlinear pulse width modulation-controlled single-phase boost mode photovoltaic grid-connected inverter with limited storage inductance current is proposed in this paper. The circuit topology, control strategy, steady-state principle characteristic, and design criterion for the key circuit parameters of this kind of inverter are investigated in depth, and important conclusions are obtained. The inverter's regenerating energy duty ratio 1–D which decreases with the decline of the grid-connected voltage is real time adapted by sampling and feeding back the inverting bridge modulation current, and the average value of the modulation current in each switching cycle tracks the reference sinusoidal signal to get high-quality grid-connected current. The active control of the energy storage inductance current and the balance of the voltage step-up ratio are realized by adding a bypass switch connected in parallel with the energy storage inductance and using two kinds of switching pattern namely boost pattern and freewheeling pattern. The theoretical analysis and experimental results of the 1 kVA 110 VDC/220 V50 Hz photovoltaic grid-connected inverter prototype show that it has the advantages such as single-stage boost conversion, high conversion efficiency, high quality of grid-connected current waveform, low value of energy storage inductance, simple control, etc. [ABSTRACT FROM AUTHOR]