On Waveform Distortion in Modern Low-Voltage Installations with Multiple Nonlinear Devices

Sammanfattning: The continuing society quest for more comfort combined with the need to minimize global environmental impacts is constantly introducing new technologies into our daily lives. Among the recent developments, the advances in energy-efficient lighting and renewable energy technologies have enabled a maturity level in cleaner electricity production and efficient use of energy. Aligned with these trends, more recently we are experiencing faster progress towards the electrification of the transport system. All these developments have been largely driven by advancements in power electronic technologies which ultimately introduces a significant number of nonlinear loads in the form of power converters into the low-voltage (LV) installations and networks for electricity distribution.The overall aim of this thesis is to investigate how these nonlinear loads (individually and together) impact the current waveform distortion in modern LV installations. The work addresses several issues related to the electrical interactions between the distribution grid and different nonlinear loads, such as LED lamps, power factor correction (PFC) converters, PV inverters, and electric vehicle chargers.As a first part, the influence of the network impedance is examined. A method combining analytical impedance network modelling with a probabilistic approach for the customer side equipment was developed to address the uncertainties associated with harmonic resonances in public LV networks. It was found that the main resonance is mainly due to the transformer inductance and the total customer capacitance, while cable capacitances and customer inductances have a small impact. Additionally, it was found that increasing PV penetration shifts the harmonic resonances to lower frequencies, but also decreases the impedance magnitude.The second part includes the examination of the so-called nonlinear interaction phenomenon. A methodology has been developed and applied to quantify the extent of nonlinear interaction between devices in the same LV installation. It was observed that the interaction of different power electronic devices creates nonlinearity deviation, changing the current harmonics emission mainly for low order harmonics. The harmonic phase angle is the most affected harmonic characteristic. Additionally, linked to the first part, it was observed that changes in the network impedance and voltage source waveform have a significant impact on the nonlinear interaction.As a third part, the current zero-crossing waveform distortion has been analysed with a focus on control instabilities. Prior measurements of multiple devices fitted with power-factor controller were compared with a simulation model and instabilities were evaluated. Results from this work have confirmed that zero-crossing distortion increases proportionally with the number of devices. In addition, it was found that the network impedance plays an important role in defining the stability-criteria of these devices.Results shown in this thesis have revealed the harmonic interdependency and its consequences in different frequency ranges: harmonics and supraharmonics. Understanding the details of these new scenarios becomes of fundamental importance to mitigate future power quality issues and ensure the functioning of equipment in modern LV installations. This work presents several findings and a comprehensive discussion serving as a guideline for future work on interaction analysis and its consequences for devices in the LV network.