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Decentralized Schemes for Grid-Forming Inverter System Efficiency Improvement and Online-Inverter Detection

  • Author / Creator
    Sheykhi, Ali
  • Parallel grid-forming (GFM) inverters are used in many modern power system applications. Therefore, improving their system efficiency is of paramount importance
    for energy savings. The droop control method has been conventionally used to share
    the power among inverters proportional to their power ratings. However, the droop
    strategy does not guarantee an efficient power sharing especially at light loads, where
    the low power demand is divided among inverters, forcing them to supply a low power
    at a low efficiency according to their efficiency curve. Addressing the aforementioned
    issue while maintaining the advantages of parallel inverters used in residential areas
    is the focus of this thesis.
    A nested-loop control system for a single-phase inverter operating in standalone
    mode is introduced. The control system includes current, voltage, and harmonic
    control loops, and is designed based on the linear quadratic tracking (LQT) optimal
    control method. The designed controllers exhibit a satisfactory performance in tracking the reference signals and rejecting the unwanted harmonics in case of linear and
    nonlinear loads. Moreover, a total harmonic distortion (THD) of 1.65% is obtained
    when a nonlinear load is connected to the system.
    A decentralized modified droop strategy is also proposed to improve the system
    efficiency of parallel grid-forming inverters. The main idea is to control the power
    sharing among the inverters such that the output power of each inverter is maintained
    within a proper range with respect to the inverter efficiency curves. Hence, an optimal number of inverters supply the loads efficiently, and the unnecessary inverters do
    not share any power. In contrast to the existing methods, the proposed method
    does not have a single point of failure while it features modularity, flexibility, and
    reliability. Additionally, the reliability of the system is enhanced by extending the
    lifetime of inverters with higher power ratings which are considered as valuable assets
    of the system. Furthermore, a stability verification of the system within the designed
    operating range of the inverter no-load frequencies is conducted to ensure a stable
    operation.
    As in very-light load situations none of the inverters can operate within the proper
    power range with a high efficiency, an online-inverter detection (OID) method is
    proposed such that each inverter detects the online inverters and the unnecessary
    inverters stop injecting power to obtain a higher system efficiency. As the proposed
    OID is decentralized and modular, more inverters can be added to the system without
    adjusting the settings of installed inverters. Detailed derivations, simulations, and
    experimental results are presented to validate the effectiveness of the schemes.

  • Subjects / Keywords
  • Graduation date
    Spring 2022
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/r3-b965-3683
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.