Characterisation and assessment of the harmonic emission of grid-connected photovoltaic systems

This paper presents and discusses the results of several measurements carried out on photovoltaic systems connected to LV-grid, from the harmonic distortion viewpoint. The PV-system inverters are characterised by different ratings, in the range of 850 W up to 3 kW, and different structures, including low-frequency and high-frequency transformers. The harmonics emission and the total harmonic distortion (THD) were measured over a period of several days having different solar radiation and by this different generated power. The results indicated a strong dependency between the THD and the generated power, in such a way that the THD is always high when the generated power is low. The influence of network impedance measuring signal and of the modulation of the pulse width modulation (PWM) is analysed and discussed. Procedures to assess the connection of the PV-systems to the grid as per international standards are presented. The results indicated that, in practical cases, the measured harmonic emission may exceed the typical values obtained in laboratory tests.