Analysis and Modelling of Power-Dependent Harmonic Characteristics of Modern PE Devices in LV Networks

Abstract

This paper presents results of experimental and analytical evaluation of power-dependent harmonic emission of three common types of modern low voltage (LV) power electronic (PE) devices. After a detailed analysis of comprehensive test results, based on both existing and new waveform distortion indices, the development of component-based models of PE devices is discussed. The paper demonstrates the importance of including PE devices' controls for accurate modelling of their characteristics over the entire range of operating powers. Most of the analysed PE devices exhibit strong power-dependent changes of characteristics, additionally influenced by supply voltage conditions, which are important for the analysis of both existing networks and future "smart grids". ).testing of EVBCs, despite the fact that the power during the charging cycle is variable, and will typically reduce during the latter stages. Test procedures for power-generating LV PE devices do consider variable power operation, with [3] and [5] specifying test points at 100% and 50% (and 25% in [3]) of P rated for PVIs. However, the actual output of any PVI can be, and often is lower, dependent on ambient conditions. Across the operating range, the performance of the internal control circuits of a PE device may be compromised, altering the device's characteristics. This may have a negative impact on the supply grid when such equipment is connected in large numbers. This paper presents a detailed experimental-based characterization of a number of SMPS', EVBCs and PVIs across their entire operational ranges and for different supply conditions. Two new indices are introduced for the analysis, which allow separate assessment of the contributions from the low-frequency (LF) harmonics and all other waveform distortions to the device's total operating current. This is of particular importance for the analysis of modern PE devices, for which LF harmonics might not be the most significant part of the total waveform distortion (e.g. [8]). The analysis of measurements is supported by the development of componentbased (circuit-based) models, capable of correctly reproducing the harmonic characteristics over the entire operating range.This paper shows that mostof tested PE devices exhibit distinctive power-dependent performance changes. The impact of realistic supply voltage conditions, i.e. distorted voltage and source impedance, on device characteristics is also considered and is shown to have considerable impact on certain types of equipment. Particular attention is given to the increased distortion in low and very low power operating modes (defined as below 30% and 10% of P rated ), as this issue has received limited attention in literature and is not fully captured by existing indices. When operating in very low power mode, some of the measured PE devices enter unstable operating regions or disconnect (i.e. 'trip' via their internal protection). These effects signify the importance of PE devices' controls when evaluating and modelling power-dependent changes in...

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“…Generally, electric loads can be studied using static models based on fundamental frequency, such as ZIP and exponential models, which are mathematical representations that relate the active and reactive powers consumed as a function of the supply voltage root mean square (RMS) value [7]. Multifrequency models such as the Norton equivalent model may also be used, which are predominantly applicable to loads with switched-mode power supplies (SMPSs), such as appliances and lighting loads, and converters, such as battery chargers or photovoltaic inverters [8].…”

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