并网变流器的频率耦合阻抗模型及其稳定性分析

Frequency-coupled Impedance Model and Stability Analysis of Grid-connected Converter

可再生能源并网系统中,如光伏、风电以及无功补偿等并网均采用电压源型变流器。变流器与交流电网之间的相互作用会引发新的振荡问题,阻抗模型分析成为研究此类问题的重要方法。多数阻抗模型往往忽略了变流器外环控制和频率耦合的影响,适用于变流器高频动态特性研究,但在分析变流器中低频动态(如次同步振荡)方面不够精确。为弥补这一缺陷,文中针对典型电压源型并网变流器提出了一种频率耦合阻抗模型,同时考虑了互补频率耦合效应和外环控制,采用详细电磁模型时域仿真与辨识验证了阻抗模型推导的正确性。进一步构建并网系统的整体阻抗模型,分析变流器并网系统的稳定性,时域仿真结果证实了所提模型在中低频率振荡分析中的有效性。

Voltage source converters are used for grid connection of photovoltaic energy, wind energy and reactive power compensation in the renewable energy generation system. An emerging kind of oscillation has been observed due to the interactions among converters and AC power grids. In this case, the impedance model analysis has become one of the most promising methods for solving this type of oscillation. However, most of the existing impedance models are focusing on the high-frequency dynamics of converter, while neglecting the impacts of converter outer-loop control and frequency coupling. Therefore, these models are inaccurate when analyzing the medium and low frequency dynamic characteristics (such as the subsynchronous oscillation) of converters. To overcome these defects, this paper proposes a frequency-coupled impedance model of voltage source converter. Both outer-loop control and complementary frequency coupling are considered in this model. And the correctness of the model derivation is verified by the time-domain simulation and identification of detailed electromagnetic model. Based on the proposed impedance model, the overall impedance model of the grid-connected system is further derived to evaluate the system stability. The time-domain simulation has also demonstrated the effectiveness of the frequency-coupled impedance model in the medium and low frequency oscillation analysis.