Browsing by Author "Hassan, Faridul"

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    Adaptive Synchronization based Control Strategies for Harmonic Mitigation in Photovoltaic-Integrated Single-Phase Systems under Weak Grid Conditions
    (Department of Electrical Engineering, NITS, 2025) Hassan, Faridul
    Environmental degradation and the rapid depletion of fossil fuel reserves have emerged as pressing global challenges. The combustion of fossil fuels remains a major contribu tor to greenhouse gas emissions, particularly carbon dioxide (CO2), which significantly accelerates climate change. In response, there is a worldwide shift toward the adop tion and advancement of sustainable green energy technologies. This transition has prompted a large-scale deployment of renewable energy (RE) sources particularly pho tovoltaic (PV) system and their increasing integration into the power grid. The modern power system has evolved into a more complex and dynamic grid to accommodate these changes. To improve energy reliability and sustainability, PV systems are often cou pled with energy storage units and connected to the utility network. However, if not appropriately managed, the integration of such systems can pose risks to grid stability and may even lead to operational failures. Moreover, this paradigm shift has resulted in the widespread utilization of power electronic converters. Furthermore, the inherent intermittency of PV sources, combined with the extensive use of nonlinear power elec tronic converters/devices and loads, has introduced several power quality issues. These include the presence of harmonics, interharmonics, DC offsets, grid voltage sags/swells, and frequency fluctuations, all of which can deteriorate voltage and current waveforms across the grid. In single-phase grid-connected photovoltaic (GCPV) systems, it is essential to compen sate for the harmonics while simultaneously injecting high-quality of PV power into the grid, thereby improving the overall power quality. Numerous control strategies have been explored in the literature to ensure reliable and efficient operation of single-phase grid-connected PV systems. Among these strategies, the synchronization unit plays a critical role in enabling stable grid interfacing for all grid-connected power converters. Among various synchronization techniques phase-locked loops (PLLs) and frequency locked loops (FLLs) being the most widely adopted. The implementation of these methods requires two orthogonal signals, an in-phase component and a quadrature phase component, both separated by 90◦. While in three-phase systems these signals can be easily generated using Clarke’s transformation, single-phase systems face chal lenges in generating the required quadrature component. Traditionally, transport delay methods have been used to obtain the quadrature signal; however, PLLs based on such techniques often perform poorly under distorted grid conditions, DC offsets, and vari ations in grid voltage parameters. Quadrature signal generation can also be achieved using derivative-based or integral based techniques, each offering distinct advantages and limitations. Derivative-based methods exhibit strong capability in rejecting DC-offsets and faster dynamic response. However, they tend to amplify higher-order harmonics when the input grid voltage is distorted, which can adversely affect system performance, therefore it required pre filtering stage. Conversely, integral based approaches offers effective filtering charac teristics. Yet, these methods are susceptible to issues such as accumulation and over lapping errors over time. The second-order generalized integrator (SOGI) has emerged as a widely accepted solution for QSG and also used for implementing synchroniza tion schemes such as PLLs and FLLs, and for control strategies in single-phase grid connected photovoltaic (GCPV) systems. In the present work, both derivative-based and generalized integrator-based quadrature signal generation (QSG) together with synchronization techniques are investigated and modified for improved performance. First, a comprehensive advancement in derivative based QSG and synchronization techniques for single-phase grid-connected systems is presented. An improved derivative-based PLL (IDPLL) is developed, incorporating two improved derivative-based QSG (IDQSG) blocks to eliminate gain tuning require ments and inter-loop dependencies, thereby simplifying the design and significantly enhancing dynamic performance. Furthermore, a frequency-adaptive enhanced deQSG (AEdeQSG) is proposed by introducing a tuneable feedback gain (γ) for bandwidth control and employing a mapping-based fractional-order method to achieve frequency adaptability. The proposed methods effectively address challenges such as amplitude amplification, harmonic distortion, and unequal quadrature signal amplitudes under varying grid frequency conditions. Following this, the generalized integrator-based QSG approach is further explored in this work. An improved SOGI-QSG, referred to as iSOGI-FLL, is proposed. This enhanced design integrates an Enhanced Dynamic and Harmonic/DC Rejection (ED HDCR) loop to improve the performance of the conventional SOGI under conditions involving low-order harmonics and DC offset in the grid voltage. The modification en sures better dynamic response and superior harmonic attenuation. A control strategy built upon the iSOGI-FLL is developed, aiming to deliver more accurate quadrature signals, current references, and synchronization signals under various non-ideal grid conditions, enabling seamless operation of grid-connected photovoltaic (GCPV) sys tems. Additionally, the modern complex grid exhibits weak grid characteristics—being heav ily distorted with lower-order harmonics, interharmonics/subharmonics, and DC offset, and is highly prone to disturbances. In such scenarios, the standard second-order gen eralized integrator (SOGI) fails to effectively attenuate these harmonics. Therefore, to address weak grid conditions and to extract more accurate fundamental components with improved dynamic performance, two solutions based on the generalized integrator complex filter (GICF), also known as the complex coefficient filter (CCF), have been proposed. First, an enhanced generalized integrator-based complex filter (eGICF)-FLL is introduced. This structure enables precise extraction of the fundamental voltage and current components, resulting in accurate synchronization and reference signal genera tion, particularly under weak grid conditions. Second, a higher-order complex filtering method, termed the enhanced second-order complex coefficient filter (eSO-CCF), is developed for fast and accurate extraction of fundamental components of quadrature signals in single-phase system applications. The eSO-CCF significantly improves fre quency selectivity, enhances the accuracy of fundamental component extraction, and exhibits robust DC offset rejection. In contrast to traditional second-order and fourth order GI filters, the eSO-CCF effectively isolates the fundamental frequency from low order harmonics, interharmonics, and DC components. To evaluate its performance, the proposed filter is rigorously benchmarked against widely used methods such as SOGI, FOGI, CCF, and SO-CCF in both simulation and experimental settings. A comparative analysis based on the frequency spectrum of the generated quadrature signal (QSG) is also presented. Building upon this foundation, a single-phase enhanced second-order complex co efficient filter-based frequency-locked loop (1Φ-eSO-CCF-FLL) is designed and im plemented. This advanced synchronization strategy is utilized for the control of a single-stage grid-connected PV system integrated with a battery energy storage sys tem (BESS). The proposed control scheme ensures accurate frequency tracking while significantly improving power quality, even under weak grid conditions characterized by severe distortion, including lower-order harmonics, interharmonics, and DC offsets. To assess the effectiveness of the proposed approach, encompassing fundamental com ponent extraction, quadrature signal generation, synchronization signals estimation and frequency-adaptive control : a 5 kW experimental hardware prototype of a single-phase grid-connected photovoltaic (PV) system integrated with a battery energy storage sys tem (BESS) was developed. At the core of the setup is a Semikron-based converter stack configured as a voltage source converter (VSC), which utilizes an IGBT switch (SKM100GB125DN) and SKYPER-32 Pro gate drivers. The AC side of the system in cludes essential interfacing and protection components such as fuses, a grid interfacing inductor, a ripple filter, and other auxiliary circuitry. Grid connection is established through an NHR 9410 grid simulator. On the DC side, a PV emulator (AMETEK TerraSAS) is interfaced via a DC-DC boost converter. The VSC performs multiple functions, as it converts the DC output from the PV emulator into AC for injection into the grid or for supplying local AC loads connected at the point of common coupling (PCC). A custom-fabricated rectifier is also included on the load side for testing pur poses. Additionally, it supports harmonic compensation and reactive power injection to improve power quality. For BESS integration, a dual active half-bridge (DAHB) converter is connected at the DC side of the VSC. A high-frequency transformer is coupled to the AC side of the DAHB through an external inductor, both of which are designed and built in-house. To mitigate high-frequency switching noise, a film capaci tor is placed at the DAHB output. An ITech battery emulator (IT6012B-500-80) is used to emulate battery behavior and support charging operations. The control strategy is implemented on a field-programmable gate array (FPGA)-based controller (WAVECT WCU300), built on the Xilinx Zynq SoC XC7Z020 platform. A HIOKI power quality analyzer is used to record power quality data. Resulting waveforms and performance behavior are recorded using a mixed-signal oscilloscope (MSO58, 350 MHz, 6.25 GS/s). Additionally, the entire system—including the PV interface, VSC control, and BESS integration—is modeled and simulated in the MATLAB/Simulink environment and val idated through hardware experimentation. The proposed approaches, such as IDPLL, AEdeQSG, iSOGI, eGICF, and eSO-CCF-based fundamental component extraction, synchronization and for grid-connected PV integrated with BESS—have all been vali dated on a developed hardware test setup.