考虑频率耦合的组串式光伏逆变并网系统建模及其稳定性分析



摘要

近年来，随着光伏和风电等新能源大规模地接入电网，一种被称为次同步控制作用（SSCI）或“次同步交互（SSI）”的新型“超/次同步振荡（SSO/SSR）”问题给电网的安全稳定运行带来了新的挑战。阻抗分析法是分析此类问题的一种有效手段。但是由于目前并网逆变系统的阻抗建模过程通常忽略了并网逆变器内部的频率耦合效应及其与前级变换器的相互影响，造成整个并网系统的阻抗模型精度不高的现象，进而影响稳定分析结果的准确性。

为此，本文以两级组串式光伏并网系统为研究对象，提出一种考虑频率耦合和最大功率点跟踪（Maximum Power Point Tracking，MPPT）控制策略影响的新型阻抗建模方法，并分析并网系统内部不同频率耦合因素对系统总体阻抗特性及其稳定性的影响。

首先，分析两级式与单级式光伏并网逆变系统的优劣，再对两级组串式光伏并网逆变系统的拓扑及内部频率耦合产生机理进行分析。然后，利用模糊控制与PID控制的等效原理拟合出一类模糊MPPT控制的等效传递函数，再采用谐波线性化小信号建模方法建立了一种考虑频率耦合影响且包含MPPT环节的组串式光伏并网系统整体阻抗模型，并通过参数设计的原理对系统所需参数进行计算。最后，在 MATLAB/Simulink 中搭建其仿真模型，分析了三种频率耦合因素以及是否包含MPPT环节对并网逆变系统阻抗特性和稳定性的影响，再通过RT-LAB硬件在环仿真实验中验证了所提方法的有效性和先进性。

结果表明，与非计及耦合阻抗模型相比，考虑频率耦合且包含MPPT环节的阻抗模型更为精确能够更加准确地预测系统潜在的谐振点。也证明了三种耦合因素对并网系统的影响。

关键词：阻抗建模；并网逆变器；光伏；频率耦合；最大功率点跟踪；

Impedance Modeling and stability analysis of String PV Grid-connected Inverter System considering frequency coupling

ABSTRACT

In recent years, such as photovoltaic and wind power have been connected to the electricity grid on a large scale , a new type of Sub-/Super synchronous Oscillation/ Resonance(SSO/SSR) problem, known as Sub-Synchronous Control Interaction (SSCI) or Sub-Synchronous Interaction (SSI), brings a new challenge to the safe and stable operation of power grid. The impedance analytical method is an effective method to analyze such problems.However, because the impedance modeling process of grid-connected inverter system,the frequency coupling effect inside the grid-connected inverter and its interaction with the preceding stage converter are usually ignored, as a result,the accuracy of impedance model of the whole grid-connected system is not high, which affects the accuracy of stability analysis results.

Therefore, a new impedance modeling method considering the influence of frequency coupling and Maximum Power Point Tracking (MPPT) control strategy is proposed in this paper. The influence of different frequency coupling factors within the grid-connected systemon the overall impedance characteristics and stability of the system is analyzed.

Firstly, the advantages and disadvantages of the two-stage and single-stage grid-connected photovoltaic inverter systems are analyzed,and then the topology and internal frequency coupling mechanism of the two-stage group series grid-connected photovoltaic inverter system are analyzed.Then, by using the equivalence principle of fuzzy control and PID control fitting out a kind of fuzzy MPPT control equivalent transfer function,and the harmonic linearized small signal modeling method established a considering frequency coupling effect and includes link of MPPT of photovoltaic (pv) grid system as a whole set of string type impedance model, and through the principle of parameter design for calculating parameters needed by the system.Finally, the simulation model is built in Matlab /Simulink,and the influence of three frequency coupling factors andwhether the MPPT link is included on the impedance characteristics and stability of the grid-connected inverter system is analyzed. Finally, the effectiveness and advance of the proposed method are verified by RT-Lab hardware loop simulation experiments.

The results show that, compared with the non-coupled impedance model,the impedance model considering frequency coupling and including MPPT is more accurate and can predict the potential resonance points more accurately.

KEY WORDS：impedance model；Grid-connected inverter；Photovoltaic (PV) ；Frequency coupling；Maximum power point tracking