Analysis of harmonics generated by power converters in power networks: impacts and mitigation techniques

Abstract

Power electronics applications have been expanded vastly and widely in different electrical networks such as industrial, residential, and commercial grids. This expansion is due to the ability of power electronics technology to improve efficiency and performance of energy conversion systems. Power converters have different topologies suitable for different energy sources and loads. The utilization of renewable energy resources and the demand for energy-efficient appliances make the power converters widely used in different applications such as Adjustable Speed Drives (ASD) and solar inverters. ASDs constitute a significant share of loads in distribution networks. It is estimated that motor drives are the main consumer of global electrical energy in low voltage distribution networks. Hence, to improve the efficiency and energy conservation of these systems, new topologies, control and fast switching devices have been increasingly utilized in motor drive applications, which enhances the energy efficiency of the motor. However, one of the main drawbacks of the ASDs is the generation of harmonics in power networks. These harmonics lead to overheating and power loss in equipment and distribution transformers and cause grid instability issues.Hence, in this thesis, a comprehensive analysis and modelling of harmonics generated by power converters are performed. The current harmonics (magnitude and phase-angle) of power converters are investigated at the unit and system levels. Understanding harmonics’ behaviour led to proposing novel and cost-effective harmonics mitigation techniques that can reduce the harmonics emission of power converters. The concept of the proposed techniques is based on using Electronic Inductor in a AC-DC converters to mitigate or cancel specific harmonics at multi-unit motor drive systems. The proposed techniques can ensure that the harmonic emission of the motor drives is complied with international standards such as IEC 61000-3-12.To mitigate current harmonics at a point of common coupling (PCC), its magnitude and phase-angle need to be known. However, installing power quality meters to measure the current harmonics is costly and may not be practical at a system level. Thus, cost-effective estimation techniques are proposed for current harmonics and grid impedance at a PCC. The method is based on voltage measurement at the PCC, where conventional AC-DC drives are connected. After estimating the current harmonic phase angle, the optimum current phase-angle for the other connected AC-DC drive with current control ability is obtained.Moreover, high penetration of nonlinear loads in the distribution network can create voltage harmonics at the PCC due to the interaction between their high level of current harmonics and the network impedance. This voltage distortion could then impact other sensitive loads such as power electronic-based devices. One of these devices is the conventional three-phase diode rectifier with a passive filter, which is a common topology for three-phase ASD systems. Thus, part of this thesis investigates the impact of grid power quality (voltage harmonics) on the operation of power converters. To evaluate the effect of voltage harmonics, a mathematical formulation of the drive voltage and current harmonics based on grid voltage harmonics is proposed.Pulse Width Modulated (PWM) voltage generated by power converters can generate high-frequency harmonics at its switching frequency. The switching frequency of converters in high-power applications is mainly between 2 and 9 kHz. Thus, these high- frequency harmonics circulate in transformers winding, which can affect transformer operating temperature and lifetime. Although the existing standards mainly consider the impact of harmonics up to 2 kHz on transformer losses, the importance of also using higher frequency harmonics in calculating transformer core loss and temperature rise is investigated in this thesis as per IEEE standard conclusions.