High penetration of PV systems in low voltage distribution networks: investigation of power quality challenges and mitigation

Abstract

World electricity demand is continuously increasing and fossil fuel supplies are not sustainable. Solar Photovoltaic (PV) energy is one of the emerging resources around the world, which produces emission free electricity. Nowadays, the advancements in rooftop solar PV technology, government subsidies, decreasing capital cost and feed-in-tariffs have promoted installation in residential and commercial applications. The exponential uptake in widespread integration of PV systems in existing low voltage (LV) distribution networks is raising additional new challenges in terms of power quality, stability and protection. In LV distribution networks, poor power quality (PQ) is the most serious concern. Characteristically, LV distribution networks are not designed for significant back-feed of power generation to the main grid. Also, these networks are unbalanced in nature due to asymmetry in system impedances and single-phase loads. This together with a large number of small-scale PV system integrations in LV networks can cause poor PQ challenges in terms of voltage quality and harmonics. PV systems can themselves generate harmonics, due to the usage of power electronic inverters. In addition, the augmentation of power electronics based appliances; the loads are becoming voltage sensitive and nonlinear in nature. The proliferation of widespread PV penetrations and a multitude of nonlinear load characteristics can have a stringent impact on the network harmonic levels. Therefore, the main objective of this research is to investigate and understand the impacts of high PV systems penetration on PQ of the distribution network and aim to alleviate them. In the first part of this thesis, the investigation of voltage quality challenges in the LV distribution network with high PV penetration are discussed. In this research, various voltage quality issues such as voltage rise, unbalance, fluctuations/flicker and sag/swell issues have been explored. Primarily, the analysis of results has been carried out through PSCAD simulations in various case studies. For this purpose, an IEEE-13 bus unbalanced distribution network is considered as a test system. Furthermore, to evaluate the severity of voltage quality issues in real-time grid connected PV systems, field measurement based investigations have been performed. Practical field tests have been conducted at two different sizes of 1.5 MW and 3.3MW PV systems located at the University of Queensland (UQ), St Lucia and Gatton campuses respectively. The impact of dynamic variations in solar irradiation has also been taken into account for the analysis. Further, a data clustering technique is also applied to estimate the probability of voltage flicker severity in the networks. Measurement results show that voltage quality concerns in the 1.5MW PV system are insignificant compared to the 3.3MW system. In the second part of this thesis, the characteristics of harmonic emissions from PV inverters and their aggregations during various operating conditions are assessed. The simulation results are validated with the field measurement data collected by various PQ analysers connected at the UQ PV site. Analysis revealed that individual voltage and current harmonic magnitudes are additive in nature due to increased PV system penetration. In addition, a comprehensive analysis has been performed in several different cases studies with high penetration of different PV inverter technologies to evaluate the severity of harmonic propagation and resonance issues on the distribution network. This analysis has also considered the harmonic distortions associated with various power electronic based nonlinear loads. Further, comparative studies have been performed with real-time harmonic measurements, which are obtained using online JAVA programs. The study has highlighted the PV system harmonic contributions on distribution transformer K-factor. Results confirm that the total harmonic distortions (THD) of voltage and current are exceeding the IEEE limits when the number of PV systems increases in the network. Moreover, the impacts of PV controller performance due to solar irradiation variations on the incidence of grid harmonic resonance have been presented. Furthermore, this research has suggested a novel solution to overcome the above PQ issues. The concept of adopting the PV inverter as a virtual DSTATCOM named as Solar-DSTATCOM has been proposed. Also, a new control strategy for the PV inverter has been developed to provide independent phase voltage regulation and load reactive power and harmonic compensation, which could eliminate issues in the unbalanced distribution network. Initially, the Solar-DSTATCOM controller has been verified in a PSCAD simulation environment. Further, different case studies have been performed on the IEEE-13 bus network for PQ issues compensation. In addition, the proposed Solar-DSTATCOM control system has been tested and verified in controller hardware-in-the-loop simulation environment, which combines the real-time digital simulator and dSPACE DS1103 hardware board. Detailed investigations are carried out for various different case studies, which include daytime, night-time operations, the impact of dynamic load profiles and finally harmonic analysis. The analysis has revealed that Solar-DSTATCOM exceptional performance in the hardware environment has enhanced the grid PQ by providing voltage regulation, reactive power compensation and power factor correction. The harmonic emissions are well within the limits.