| The development of renewable energy distributed generation conforms to the development strategy of energy conservation and emission reduction.The power electronic devices in the distributed power supply realize the power conversion function.When a distributed power source is connected to the distribution network via a power transmission line,the power flow characteristics of the system are changed,and the harmonic oscillation characteristics along the transmission line are also changed.In this paper,the suppression of harmonic oscillation amplification on the transmission line in a distributed power grid-connected system is studied to improve the power supply security of the grid-connected system.Based on the analysis of the uniform transmission line model,the harmonic oscillation characteristics along the transmission line with different end loads are obtained for the background harmonics of the distribution network.By establishing the Norton equivalent model and the Thevenin equivalent model of grid-connected inverters with CCM and VCM respectively,harmonic source characteristics and output impedance characteristics of the inverter with different control methods are studied.Combined with the different harmonic source characteristics of the inverters,the characteristics of the harmonic oscillation along the transmission line in the distributed power grid-connected system are studied.For the small-capacity CCM inverter grid-connected system with the power transmission line,the harmonic oscillation characteristics of the transmission line are studied,and the impedance matching characteristics of the R-APF control scheme are analyzed to reveal its limitations of the harmonic suppression.So a composite R-APF harmonic suppression strategy is proposed.The control and implementation scheme of the composite R-APF are constructed,and the control gain characteristics are analyzed.The simulation and experimental results show that this scheme can effectively suppress the abnormal amplification of harmonics along the transmission line.Considering that the distribution characteristics of the transmission line affect the resonance mechanism of the grid-connected inverter,and the composite R-APF has the function of impedance reconstruction.It can be find through the research that the composite R-APF can also suppress the resonance of the inverter itself.For the large-scale distributed power grid-connected system based on a transmission line,considering the high permeability of distributed power sources and the propagation interaction between grid-connected inverter harmonics and distribution network background harmonics along the transmission line,the symmetrical propagation characteristics of double-end harmonic sources along the transmission line are studied.By analyzing the harmonic suppression mechanism of frequency-dividing control R-APF and the limitations of its harmonic suppression effect,a dual frequency-dividing control R-APF harmonic suppression strategy is proposed based on the optimization of selective harmonic detection.The analysis shows that the strategy has symmetry and can achieve the same harmonic attenuation effect on the propagation of the double-end harmonic sources on the transmission line.Simulations and experiments have verified the effectiveness of this strategy for harmonic suppression under conditions of different line lengths and different harmonic sources,as well as strong robustness to changes of line parameters.For the transmission line-based microgrid grid-connected system,considering the propagation interaction among the VCM-type grid-connected inverter harmonics,the nonlinear load harmonics existing on PCC,and the distribution network background harmonics along the transmission line,a novel I-APF is proposed,which simulates an infinitely long transmission line by using the combined model of a controlled current source and a controlled voltage source.Theoretical analysis shows that the optimal installation location of the novel I-APF is the distribution network side of the transmission line.At this time,the harmonic attenuation for the propagation of the background harmonics of the distribution network is approximately 0.For inverter harmonics and the nonlinear load harmonics on the microgrid side,the novel I-APF achieves impedance matching and harmonic amplication suppression.Simulations and experiments have verified the different harmonic propagation suppression effects of the novel I-APF on different harmonic sources,the effectiveness of different harmonic source operating conditions,and the robustness to the changes of line parameter. |
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