Supraharmonic Emissions in Future Electricity Networks with Solar PV Systems

The world is experiencing a shift from centralised to decentralised power generation as a pivotal strategy for achieving net-zero emission targets. As a result, solar photovoltaic (PV) systems have become the leading distributed energy resource (DER) connected to the grid. The integration of new DER systems presents several technical challenges, one of which is the power quality (PQ) issues introduced by associated power electronic interfaces. Conventional PQ studies have concentrated on voltage and current waveform distortion in the frequency range of 0–2 kHz, however, special attention has been paid to conducted emissions above 2 kHz over the past decade since their systematic knowledge is not completely developed yet. Waveform distortion in the 2–150 kHz frequency range is referred to as “supraharmonic emissions” or “high-frequency harmonic emissions”.This research study aims to identify the supraharmonic emission characteristics from rooftop solar PV systems containing identical PV inverters and their propagation characteristics within low voltage (LV) networks, focusing on different supraharmonic phenomenon appearing within the 2-25 kHz frequency range.The first part of this study is aimed at characterising supraharmonic emissions based on measurements of an LV installation with a rooftop solar PV system, consists of six (6) identical inverters. There are three measurement methods stipulated in international regulations, i.e. IEC 61000-4-7 (informative), IEC 61000-4-30 (informative), and CISPR 16-1-1 (normative). However, those regulations do not include prescriptive measurement and analysis techniques for the entirety of this range (2-150 kHz). Thus, the measurement methodology in IEC 61000-4-7 which is intended for emissions in the 2-9 kHz range, is utilised to analyse measurements up to 25 kHz in this study maintaining the consistency between 2-9 kHz and 9-25 kHz ranges.Two supraharmonic emission phenomena have been identified in two distinct frequency ranges: (a) resonance related emissions (2-5 kHz) due to the front-end filter of PV inverters, and (b) switching frequency emissions (15-17 kHz) due to the high frequency switching PWM pulses. Simultaneous measurements at different locations of the same LV installation showed that the emissions associated with resonance propagates within the installation, however, they do not propagate into the substation level where electricity is supplied to the installation. In addition, three (3) methods have been proposed to represent the phase angle for grouped supraharmonic emissions to be incorporated with harmonic summation techniques, which will assist in understanding the phase angle diversity of supraharmonic emissions.The second part of the study is aimed at analysing supraharmonic emissions based on a generic MATLAB/Simulink model. The model is developed using the commonly adopted voltage oriented control (VOC) under synchronous reference frame, often referred to as direct-quadrature (d-q) reference frame, identifying the boundary up to which low-frequency harmonic models are capable for supraharmonic analysis. The model is capable of representing the resonance and switching frequency supraharmonic emission phenomena identified, and is validated based on the measurements used in the first part of the study. The impact of carrier waveform phase shift in PWM signal generation is investigated within a multiple solar PV inverter system, which can be used to mitigate switching frequency harmonic flow into the grid.The study uniquely delves into characterising these emissions using advanced methods, such as phase angle representation, and improving understanding of the complex interactions that occur at the micro-level of inverters. Thus, the results from this research study are of significant importance to characterising and developing systematic knowledge on supraharmonic emissions from rooftop solar PV inverter systems.