Author: Li, Zilin
Title: Advanced control of grid-forming inverters under uncertain operating conditions in AC microgrids
Advisors: Chan, K. W. Kevin (EE)
Cheng, K. W. Eric (EE)
Degree: Ph.D.
Year: 2022
Subject: Microgrids (Smart power grids)
Electric power systems -- Control
Hong Kong Polytechnic University -- Dissertations
Department: Department of Electrical Engineering
Pages: xiv, 166 pages : color illustrations
Language: English
Abstract: While coal, oil, and natural gas contribute more than 60% to global electricity generation, they also have brought hazardous issues due to the depletion of fossil fuels and global warming. Fortunately, renewable energy sources (RES) are promising to replace those fossil fuel energy sources for electricity generation. After intensive research in the past two decades, Microgrid has emerged as a feasible and attractive paradigm to accommodate a high penetration of RES-based distributed generation units (DGs). While there are AC, DC, and hybrid AC/DC microgrids, this thesis would focus on AC microgrids because most existing power systems and end-user loads are AC. Microgrids can operate in both grid-connected and islanded modes, often with the help of grid-forming (GFM) control strategies in the grid-interface inverters of DGs. In this thesis, the P-f & Q-V droop control is adopted to realize GFM functionalities because of its impressive performance in mimicking the characteristics of synchronous generators. Meanwhile, a microgrid could contain various DC-DC and/or DC-AC power converters with diverse control strategies. The modeling and control of power converters and microgrid stability analysis are therefore comprehensively reviewed and studied.
In future microgrids, GFM inverter-based DGs are expected to be the dominant power source and would inevitably encounter real-world operation uncertainties that have to be considered in the control of GFM inverters to ensure their safe and stable operations. Nevertheless, issues related to operation uncertainties have not yet been fully explored in existing studies. This thesis is therefore motivated to develop advanced control strategies for GFM inverters to cope with those issues.
One of such uncertainties comes from the intermittent power generation of RES. Although energy storage systems could flatten power generation profiles, their state-of-­charges (SoCs) would be time-varying and depend on various factors. The available power capacity of a GFM inverter would therefore fluctuate but was assumed constant in most existing literature. The time-varying available power could pose a significant challenge to preserve the power balance between the generation and load sides of an inverter, and even lead to instability. Therefore, to address the issues of time-varying power capacity, an advanced control scheme based on adaptive droop and virtual impedance is proposed for droop-controlled inverters in this thesis. The adaptive droop helps the inverter avoid voltage collapse or frequency instability caused by power deficiency as droop gains would be adjusted online according to the actual power capacity. An output impedance model of the inverter is also developed using the harmonic linearization method to investigate the impacts of droop gains on system stability. It reveals that large droop gains could induce small negative resistance around the fundamental frequency, resulting in system instability. An adaptive virtual impedance is therefore designed to counteract the negative resistance and enhance system stability. The proposed adaptive droop and virtual impedance scheme has been thoroughly validated by MATLAB/Simulink simulations and laboratory experiments, showing that inverters could be maintained at good performance when the available power is varied within 5% to 100% of the rated value.
Grid faults are also common operation uncertainties that would severely threaten the security of microgrids. A GFM inverter, acting as a voltage source, is sensitive to grid voltage variations. Limited by the thermal capacity of semiconductor switchers, GFM inverters cannot withstand high overcurrents as synchronous generators do and could be damaged during grid faults. Meanwhile, GFM inverters could also suffer from synchronization instability and insufficient power capacity. In order to cope with these challenges, a new strategy, namely positive-and negative-sequence limiting (PNSL) with stability enhanced P-f droop control (SEPFC), is proposed for droop-controlled inverters to deal with grid faults. The strategy is based on instantaneous saturators and can be easily implemented in the inverter's primary controller without requiring any fault detection or change of inner loop controllers. During grid faults, the strategy enables the inverter to preserve its voltage source characteristics. While PNSL is responsible for limiting output currents and active power by modifying voltage references, SEPFC ensures synchronization stability. Equipped with PNSL-SEPFC, the inverter's stability is studied mathematically using eigenvalue analysis so as to derive the design guidelines for the control parameters. PSCAD/EMTDC simulations based on a four-DG microgrid and hardware experiments demonstrate the effectiveness of the proposed PNSL-SEPFC under symmetrical and asymmetrical grid faults. The excellent performance of the PNSL-SEPFC is further highlighted by benchmarking with several existing methods.
PNSL-SEPFC could be further improved under asymmetrical grid faults as the healthy phase voltage could drop to an unacceptable level and zero-sequence components have not yet been considered. Meanwhile, three-phase four-wire (3P4W) GFM inverters are promising alternatives to interface distributed generation units to low-voltage microgrids. Therefore, based on PNSL-SEPFC, an enhanced fault ride-through (FRT) strategy is proposed. It further considers zero-sequence components and introduces adaptive virtual negative-and zero-sequence resistances. The strategy not only has excellent over-current limiting capability but also allows seamless transitions between normal and grid fault conditions. In order to investigate the impact on inverter voltages and currents, the inverter's sequence networks are developed to analyze the proposed FRT strategy in terms of the virtual negative-and zero-sequence resistances, grid short-circuit ratio (SCR), fault types, and fault impedances. The virtual negative-and zero-sequence resistances are then designed to be adaptive to the inverter's maximum voltage amplitude, so as to ensure both the healthy phase voltage quality and faulty phase current limiting. The complete FRT strategy in a GFM inverter has been verified by both MATLAB/Simulink simulations and laboratory experiments, fully showing the capabilities of the GFM inverter to operate safely and stably with desirable FRT performances under various asymmetrical faults and grid SCRs. 
Rights: All rights reserved
Access: open access

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