## 4 Conclusions  
PV systems are integrated into the distribution networks through power inverters, which are nonlinear power electronic devices that inherently inject harmonics into the grid. Their current harmonic emission characteristics are not only determined by their general circuit topologies and control strategies, but are also affected by the background supply voltage distortions and the device operating powers. To investigate the harmonic integration impact of PV systems, a suitable harmonic source model capable of accurately representing the harmonic emission characteristics of PV systems operating under real grid conditions is required. Accordingly, this paper firstly reviews the conventional harmonic source modelling techniques and their applicability for grid-tied PV systems, with a special focus on the three most widely used frequency-domain modelling approaches, including CCSM, HNM and HAMM. Although HAMM has the best accuracy among all the three methods, its conventional derivation approach relies on the harmonic sensitivity tests with the PV system operating at constant power, and this can only be performed under laboratory conditions. To improve the applicability of HAMM for grid-tied PV systems operating under varying grid conditions and powers, this paper further analytically investigates the harmonic generation and grid interaction mechanisms of rooftop PV systems and obtains the following conclusions: (1) PV systems mainly generate odd harmonics; (2) the generated harmonic current spectrums of PV systems are mainly determined by the PWM strategy being applied, the supply voltage distortions and the device operating power. To further investigate the power dependency of harmonics emitted from PV systems, a fully controllable testbed is set up and based on this, the harmonic sensitivity tests are applied to two different single-phase PV systems operating at selected power levels, with the  
Figure 21: Comparison of voltage harmonics at PCC (phase C) for the application of three harmonic modelling approaches (CCSM, HNM and HAMM).  
Figure 20: Comparison of voltage harmonics at PCC (phase B) for the application of three harmonic modelling approaches (CCSM, HNM and HAMM).  
corresponding derived HAMs. It is noticed that a strong linear correlation exists between HAMs at adjacent powers. Based on this, a HAM derivation and self-tuning approach is proposed to derive the HAMs at different powers from the infield measurements of a grid-tied PV system. According to the accuracy comparison among CCSM, HNM and HAMM, it turns out that HAMM achieves the highest accuracy in the application of harmonic source modelling of PV systems. Finally, this paper proposes a co-simulation approach to compare the HPF results implemented with CCSM, HNM and HAMM, and it is found that using CCSM for the harmonic emission representation of PV systems may overestimate the supply voltage distortion level at PCC. The proposed HAM derivation approach greatly improves the infield implementation capability of HAMM, and has good compatibility with commercial HPF solvers.  
Conceptualization, X.X. and H.G.; methodology, X.X.; software, X.X.; validation, X.X., A.D. and Q.L.; formal analysis, A.D.; investigation, Q.L.; resources, X.X.; data curation, X.X.; writing--original draft preparation, X.X. and A.D.; writing--review and editing, X.X.; visualization, X.X.; supervision, X.X. and H.G.; project administration, X.X.; funding acquisition, H.G. All authors have read and agreed to the published version of the manuscript.  
This work was partially supported by the Innovation and Entrepreneurship Program of Jiangsu Province, Grant No. JSSCBS20210538.  
Not applicable.  
Data availability Statement: Data are contained within the article.  
The authors declare no conflict of interest.