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Stability Improvement and Protection of Grid-following Bidirectional Three-phase Voltage-sourced Converters under Unbalanced Grid Conditions

Bahmani, Mehran | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54946 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Mokhtari, Hossein; Karimi, Houshang
  7. Abstract:
  8. With the increasing expansion of modern DC loads and utilizing energy storage systems along with distributed renewable energy resources, grid-following bidirectional voltage-sourced converters (GFBVSCs) with fast dynamic performance are required. Due to the presence of single-phase loads and asymmetrical short circuit faults, unbalanced grid voltage conditions are available in the distribution system. Under unbalanced conditions, an oscillating component with a frequency of twice the grid angular frequency appears on the DC side. Removing double-frequency ripple in the DC link voltage without third harmonic current injection and reactive power injection to the grid based on the grid codes to support the grid voltage under fault conditions are other control objectives of the GFBVSC.DC link voltage control is one of the main purposes of GFBVSC control. According to the non-linear dynamics of the converter output filter, instability occurs in rectifying mode in the use of conventional controllers. Faster dynamic performance and ability to operate under unbalanced conditions further reduce the system stability margin in rectifying mode. The focus of this study is on solving the mentioned problem. A system state-space model is presented to consider the system dynamics. Based on this model, a robust control structure in the single synchronous reference frame is proposed to provide the fast dynamic performance of GFBVSC under unbalanced conditions. Also, it takes the advantages of ripple-free DC link voltage, non-distorted output current, and reactive power injection ability to support the grid in fault conditions. The control structure comprises three parallel control loops that exploit an optimal multi-variable control approach to achieve the mentioned objectives. Accordingly, to consider the tracking and disturbance rejection objectives in each control loop, a single-input multi-output (SIMO) output controller is employed. It is followed by a full-state feedback controller optimally designed based on Linear Quadratic Regulator (LQR) approach, providing robust system performance against uncertainties. Simulation and experimental results demonstrate the outperformance of the proposed controller in meeting control objectives and robust performance of the controller against varying the system parameters.Due to the use of the multi-variable control approach in the control structure, there is no conventional current control loop in the proposed controller. Therefore, in order to protect the converter in the event of short circuit faults in the lack of the current control loop, a current limiting strategy is proposed. Accordingly, two complementary methods are proposed: i) DC link capacitor virtual energy control for limiting the output currents under symmetrical/asymmetrical short circuit fault conditions, and ii) a current amplitude compensating coefficient for considering the impact of non-linear equations of the system on the reference generation process. In the development of the proposed current limiting strategy, two well-defined technical requirements are also included, i) ripple-free DC link voltage under unbalanced conditions, and ii) reactive power injection to support the grid under short circuit fault conditions according to the grid codes. Furthermore, the impact of non-linear equations of the system applied by output filter instantaneous power is precisely considered. Simulation results verify the effectiveness of the proposed current limiting strategy
  9. Keywords:
  10. Voltage Sourced Converter ; Nonlinear System ; Asymmetrical Short Circuit Fault ; Unbalanced Condition Voltage ; Current Limiting Strategy ; Optimal Robust Control ; Removing Double-Frequency Ripple ; Grid Codes ; Fast Dynamic Performance

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