On Harmonics in Low-voltage Networks

Abstract: On the road to reducing global warming, the use of renewable energy sources and efficient use of electricity are among the key aspects. The increasing number of energy efficient appliances such as LED lamps, booming demand for electric vehicles (EVs), and growing penetration of distributed energy resources such as photovoltaic (PV) systems in low-voltage (LV) networks are expected to affect the power quality in the entire electric power system and specifically in the LV network, where such new devices are connected. Waveform distortion, mainly expressed by the harmonic components, is one of the topics within power quality that is highly affected by the introduction of such new devices. However, there is a lack of publications discussing the existing level of voltage harmonics in LV networks or addressing the origin and transfer of harmonics in LV and MV distribution networks. This highlights the need for more research in this field.To evaluate the ability of the network to host new sources of harmonics, the existing harmonic voltage and current levels as well as the impact of these new sources on those levels should be investigated. Harmonic levels are determined by emission from harmonic sources, the propagation from other harmonic sources, and the aggregation between the contributions from different sources. Studies on harmonic emission from a variety of different individual devices under different conditions have already been carried out. However, limited knowledge is available about the harmonic aggregation and propagation in LV networks. This study aims to improve the understanding about the behaviour of harmonics in LV networks covering both aggregation and propagation.In the first part of this work, the impact of the MV network and remote LV loads on the harmonic voltage in the LV network are examined. Simulation results have revealed that for frequencies below the resonant frequency of the local LV network the harmonic voltage levels mainly are determined by aggregated emission of the whole distribution network (both LV and MV) rather than by the emission from local LV loads. Furthermore, a graphical method is introduced for harmonic propagation studies, using measurements but without the need for accurate synchronized measurements.In the second part of this work, the aggregated emission from a group of EV fast chargers is examined. A stochastic method, based on Bayesian statistics and harmonic correlation, was used to include uncertainties in harmonic hosting capacity calculation for an EV charging station equipped with fast chargers. The impact of MV network and remote LV loads on harmonic hosting capacity is investigated. It is also shown that harmonic hosting capacity studies are needed; and details of the distribution network must be included to get an accurate estimation of the harmonic hosting capacity.Finally, an alternative method for time aggregation of harmonic phase angle is proposed in this work.In general, this work contributes to reducing the research gaps recognized in harmonic analysis in the LV networks considering propagation and aggregation utilizing both simulation and measurement.