提高LCL型并网逆变器对弱电网适应能力的虚拟阻抗方法
A Virtual Impedance Method to Improve the Performance of LCL-type Grid-connected Inverters Under Weak Grid Conditions
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摘要: 随着分布式电源并网功率等级的增加及接入位置的广泛分布,电网越来越表现出弱电网的特性,即含有较大的电网阻抗和丰富的电网电压背景谐波。其中,电网阻抗会改变控制环路的增益,影响其控制性能,甚至可能导致系统不稳定,而电网电压背景谐波会引起并网电流的畸变,使其无法满足并网电流谐波标准。由于逆变器的输出阻抗模型能够同时反映控制系统对电网阻抗的鲁棒性及对电网电压的抗扰性能,因此,该文通过揭示它们之间的关系,提出采用虚拟阻抗的方法对输出阻抗进行校正,并给出虚拟阻抗的实现过程和参数的设计方法。采用该方法可使得逆变器在电网阻抗宽范围变化时仍然能够稳定工作,同时保证并网电流满足谐波标准。以单相LCL型并网逆变器为例进行实验验证,实验结果验证了控制策略的有效性。Abstract: Under the high penetration of distributed power generation systems, the grid can not be treated as the ideal sinusoidal voltage source any more. Instead, it contains a large set of grid impedance values and varieties of harmonics, which features as the weak grid. When the inverter is operating under the weak grid condition, the stability problem can be aroused by the grid impedance, and the injected grid current is greatly distorted by the harmonics voltages. The output impedance of the inverter gave an insight into the robustness and the harmonic rejection ability of the control system, which is investigated in this paper. Based on which, the virtual impedance method was proposed to shape the characteristics of inverter output impedances. The implementation and parameter design of the virtual impedances were also analyzed. Thus, the robustness and harmonic rejection ability could be improved at the same time. Experimental results from a 6 kW LCL-type single-phase grid-connected inverter verify the effectiveness of the proposed method.
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