Abstract
Grid impedance variation and voltage harmonics may cause instability of the grid-connected inverter system and reduce the grid current quality under weak grid. Switching losses and electromagnetic interference of inverter can be reduced by using lower switching frequency. However, it is a challenging task to achieve stable operation of LCL-type grid-connected inverter using the active damping method at the low switching frequency. In this paper, an improved control strategy is proposed for the dual-frequency inverter, which contains a power inverter and an auxiliary converter. For the proposed control strategy, the ability to suppress the influence of the grid voltage harmonics and the robustness against grid impedance variation are analyzed. In order to reduce the switching losses, the electrical energy is delivered to the power grid through the power inverter at a low switching frequency. By using the switching ripple compensation method and the grid voltage feedforward scheme, the switching current ripple caused by the power inverter and the current harmonics caused by the grid voltage harmonics can be suppressed. Since the filters used in the proposed inverter are all L type filters, the system stability is guaranteed under weak grid. The simulation and experimental results are provided to verify the effectiveness of the proposed grid-connected inverter system under weak grid.
Highlights
In recent years, the demand for renewable energy sources such as wind energy, microhydro energy, and solar energy, has grown rapidly, and more distributed generation systems (DGS) are connected to the utility grid [1]–[3]
The output impedance can be increased by the grid voltage full feedforward scheme, but its suppression effect of the current harmonics is weakened by the system delay which is inevitable for the digital control system [4]
Compared with the LCL-type inverter, the dual-frequency inverter with the proposed control strategy has the following advantages: 1) The LCL filter is not used in the proposed inverter system, which can improve the system stability under weak grid
Summary
The demand for renewable energy sources such as wind energy, microhydro energy, and solar energy, has grown rapidly, and more distributed generation systems (DGS) are connected to the utility grid [1]–[3]. The current control bandwidth of the inverter is limited by the low resonant frequency of the LCL filter [32], [33], which reduces the ability to suppress the influence of the grid background harmonics [5], [6]. In order to improve the efficiency and grid current quality, Yang et al [35] proposed the dual-frequency single-phase grid-connected inverter under stiff grid, which consists of parallel-connected power inverter and auxiliary converter. In order to improve the grid current quality, stability, and efficiency of grid-connected inverter under weak grid, a novel control strategy is proposed and its performance is analyzed and evaluated in this paper. 5) The current harmonics caused by the grid voltage harmonics can be more effectively suppressed by the auxiliary converter with a high switching frequency (60 kHz). According to [44], the influence of the PLL is ignored in the following analysis
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