On Transfer Functions for Power Quality Studies in Wind Power and Solar PV Plants

Abstract: As part of the decarbonisation of the energy system, wind and solar power are expected to play an important role. However, together with their growth, new challenges appear in the electric power system; this requires further research, development, and often studies before connection is possible. Two of those challenges, both impacts on the quality of power, are the subject of this dissertation: harmonic distortion and voltage unbalance.The growing use of solar power for electricity generation, especially in distribution systems, will result in increased voltage unbalance due to single-phase photovoltaic inverters (PVIs). Regulation on power quality and potential impact on other equipment, place a limit to the number of PVIs that can be connected to the system, the so-called “hosting capacity”. To include different uncertainties in the planning stage, a stochastic method based on the transfer-impedance matrix is proposed for quantifying this hosting capacity, with respect to voltage unbalance. The method has been illustrated by applying it to two typical Swedish low-voltage networks.Wind power plants (WPPs) consist of a collection grid and a number of wind turbines. These are known to be a harmonic source as power electronic devices are used to connect them to the power system. Earlier studies have shown that the actual emission at harmonic frequencies is low, but that the main issue is related to the spread of harmonics through the collection grid, especially the role of resonances. Regulation setting emission limits and the potential adverse impact of harmonics on equipment make that studies are needed to predict harmonic voltages and currents in and around a WPP. These studies are based on measurements performed on individual turbines under certain operating conditions. The main issue related to this determination is distinguishing the emission originating within the WPP (primary emission) from the emission originating elsewhere (secondary emission). A critical review has been performed on methods used for harmonic emission determination (i.e. distinguishing between primary and secondary emission) in WPPs. It was concluded that this determination cannot be solved without making assumptions. Transfers functions are independent of the emission from the individual turbines and can be obtained with less assumptions. These transfer functions have been used to estimate the spread of harmonics through a WPP and towards the public grid. Transfer functions were shown to be a suitable tool to quantify amplifications due to resonances and identify which harmonic orders can be an issue.Furthermore, information on the different transfer functions allows the selection of proper mitigation methods. This application of transfer functions has been illustrated for a specific advanced mitigation method: the use of inverter control techniques to emulate a “virtual resistor”. In this way it is possible to damp resonances without increasing fundamental-frequency losses.The ultimate aim of harmonic studies is to avoid interference between the grid and equipment connected to it, in this case between the power-electronics in the wind turbine and other equipment. However, these studies rarely address actual cases of interference, instead of this, measured or calculated harmonic voltages and/or currents are compared with limits set in regulations. These and regulations differ strongly between countries and even between individual network operators. A comparative study of regulatory methods has been performed presenting their advantages and disadvantages from the viewpoint of the network operator and from the viewpoint of the owner or operator of the WPP.