| Author: | Chen, Xinquan |
| Title: | Transient and fault-ride-through control solutions for enhancing system protection and dynamics via grid-forming converters |
| Advisors: | Bu, Siqi (EEE) Kocar, Ilhan (EEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | 146 pages : color illustrations |
| Language: | English |
| Abstract: | With the increasingly ambitious objectives of reaching carbon neutrality, a substantial amount of renewable energy sources, such as wind and solar, are connected to the power system. These resources are typically connected to the grid through power-electronic inverters; hence collectively known as Inverter-Based Resources (IBRs). As an advanced IBR control approach, grid-forming (GFM) converter technology has gained significant attention in recent years, and it is expected to play an important role in future power systems. Being a voltage-source converter (VSC), the behavior of a GFM-IBR is mainly controlled by its embedded controller, which typically includes outer-loop and inner-loop control blocks and current limiters. Therefore, it is critical to investigate how the integration of GFM and its various control strategies affect the operation of the power system. During unbalanced grid conditions, GFM controls alter the magnitude and angle of the negative sequence current contributed by IBRs, differing from grid-following (GFL) controls and synchronous generators. This behavior may violate fault ride-through (FRT) requirements and cause maloperation of certain protection elements. First, this thesis clarifies the fundamentals and impacting factors of GFM inverters in the negative sequence system using dynamic and static analytical models. From the perspective of system dynamics, a decoupled-sequence dynamic model for GFM-IBR is proposed to study the internal oscillatory modes in negative sequence systems, providing crucial insights into the behavior of 2ω negative-sequence components within the GFM BPSC system under unbalanced perturbations. For the perspective of steady-state fault conditions, the equivalent negative-sequence impedance of GFM-IBR is estimated, considering balanced positive sequence control (BPSC) and positive and negative sequence control (PNSC) strategies. Comparative studies using the IEEE PSRC D29 system reveal that the magnitude and angle of negative sequence impedance are primarily determined by the inner voltage control parameters under the GFM BPSC strategy. Hence, the GFM inverter may provide sufficient negative sequence reactive current injection. When the inverter's current capacity is insufficient due to fault proximity, IBRs should rely on PNSC strategies to regulate the negative sequence system response appropriately. GFM control behavior during unbalanced grid faults can impact system stability and protection. Per IEEE 2800 standard, IBR should be capable of FRT capability. There is a need for the regulation of positive and negative sequence currents to comply with recent grid codes when unbalanced faults occur. For full converter-based GFM-IBRs, a PNSC structure for GFM-IBRs is proposed to regulate sequence components independently, with flexibility in utilizing different power synchronization loops and current limiters. To prevent overcurrent, an enhanced current saturation-based (CS-based) current limiting method (CLM) is developed for IEEE 2800 compliance, including reactive current injection and full utilization of current capacity during unbalanced faults. Another solution is the virtual impedance-based (VI-based) CLM, which updates the positive sequence internal voltage vectors to retain inner voltage dynamics. To improve the grid synchronization and voltage support capability, this thesis proposes a three-mode switching GFM control solution for Type-IV wind turbine generators (WTGs). Once the FRT start-up triggers, WTGs operate in the restricted mode. A sequence-domain αβ voltage synchronization loop (VSL) is activated to enhance frequency stability and maintain synchronism, while the CS-based CLM is used to inject optimal reactive currents for low-voltage ride-through. After fault clearance, the VI-based CLM is employed to eliminate transient components for WTG restoration in the transition mode. For doubly fed induction generator-based (DFIG-based) WTGs, this thesis proposes asymmetrical FRT controls for the GFM-DFIG based on the mechanism for forming the grid voltage. Firstly, internal voltage vectors are designed for the assessment of asymmetrical FRT capabilities. Then a PNSC is proposed to support the sequence components of internal voltage vectors for the GFM-DFIG. On this basis, an asymmetrical FRT control structure is proposed, incorporating negative-sequence reactive current injection and two types of positive-sequence control schemes: the current saturation-based method and the virtual impedance-based method. Additionally, a simplified calculation method for transient voltages is utilized to eliminate the impacts of transient flux leakage. All the proposed FRT control solutions are validated by using the EPRI benchmark system. To facilitate rapid and accurate short-circuit fault analysis of the system with IBR, a comprehensive and efficient solver incorporating phasor domain short-circuit models of GFM- and GFL-IBRs is proposed for steady-state fault calculations considering various CLMs. The proposed approach is validated by comparing it with detailed EMT modeling and simulations using a modified IEEE 39 bus system with multi-IBRs. The proposed solver platform enables system operators to perform rapid and accurate short-circuit computations and protective relay studies in power systems with high penetration of IBRs, facilitating the assessment of FRT strategies and compliance with grid codes. To evaluate the performance of AC protective relays in a system with GFM-IBRs, the impacts of GFM-IBR on the apparent impedance measured by distance relays are elaborated analytically during unbalanced faults in a two-bus system. The analysis provides the first explanation for why GFM-IBR may alter the trajectory of the apparent impedance, potentially causing maloperation of distance protection. EMT simulations using the EPRI benchmark system are conducted to validate the analytical findings and provide in-depth investigations. In a modified IEEE PSRC D29 system with IBRs, the performance of distance protection is investigated under varying impact factors, including K factors, fault proximity, and fault resistance. In addition, the performance of the negative sequence components-based protective relays, i.e., the instantaneous negative sequence overcurrent (50Q), directional negative sequence overcurrent (67Q), and fault-identification (FID) elements, are tested in a modified IEEE PSRC D29 system with IBRs with various FRT solutions, to identify potential maloperation issues. The conclusions of the thesis shed light on the merits and drawbacks of GFM-IBRs under unbalanced grid conditions. The suggestions and prospects for future IBR accommodation are inspired. |
| Rights: | All rights reserved |
| Access: | open access |
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