Wenzhao Liu

Flexible Control Strategy for Grid-Connected Inverter Under Unbalanced Grid Faults Without PLL

Power oscillation and current quality are the important performance targets for the grid-connected inverter under unbalanced grid faults. First, the inherent reason for the current harmonic and power oscillation of the inverter is discussed with a quantitative analysis. Second, a new control strategy is proposed to achieve the coordinate control of power and current quality without the need for a phase-locked loop (PLL) or voltage/current positive/negative sequence extraction calculation. Finally, the experimental tests are conducted under unbalanced grid faults, and the results verify the effectiveness of the propose method.

Flexible Power Regulation and Current-Limited Control of the Grid-Connected Inverter Under Unbalanced Grid Voltage Faults

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.

Enhanced power quality and minimized peak current control in an inverter based microgrid under unbalanced grid faults

The microgrid inverter experiences the power oscillations and current harmonics in case of the unbalanced grid voltage faults. However, there is a trade-off between power oscillations and current harmonics should be considered in three phase three wire inverter systems during the conventional fault ride through control. In order to solve this problem, a novel control strategy is proposed to enhance the output current quality while mitigating the active and reactive output power oscillations. Moreover, a simple current-limited control strategy can be achieved without the necessity of the voltage/current positive/negative sequence extraction. Finally, the simulation tests of the conventional and proposed control solutions are carried out. The results verify the effectiveness of the proposed strategy.

A power sharing method based on modified droop control for modular UPS

An average power sharing control strategy for parallel operation of voltage source inverter (VSI) based modular Uninterruptible Power Systems (UPSs) is proposed in this paper. The presented method is based on a modified droop control. The proposed method conquers the drawback of conventional droop plus virtual impedance control, and at the same time some challenges in the real applications have been considered. Such as the mismatch of output impedance of the parallel modules, the different calibration accuracy of the sample circuit, and the offset of the voltage reference from the control unit. Simulation has been presented in order to verify the effectiveness of the proposed control methodology under these three different conditions. The simulation results demonstrate that a better power sharing performance is obtained and successfully prevent of negative power which can protect the DC bus.