Design-Oriented Transient Stability Analysis of PLL-Synchronized Voltage-Source Converters (1811.08926v2)

Abstract: Differing from synchronous generators, there are lack of physical laws governing the synchronization dynamics of voltage-source converters (VSCs). The widely used phase-locked loop (PLL) plays a critical role in maintaining the synchronism of current-controlled VSCs, whose dynamics are highly affected by the power exchange between VSCs and the grid. This paper presents a design-oriented analysis on the transient stability of PLL-synchronized VSCs, i.e., the synchronization stability of VSCs under large disturbances, by employing the phase portrait approach. Insights into the stabilizing effects of the first- and second-order PLLs are provided with the quantitative analysis. It is revealed that simply increasing the damping ratio of the second-order PLL may fail to stabilize VSCs during severe grid faults, while the first-order PLL can always guarantee the transient stability of VSCs when equilibrium operation points exist. An adaptive PLL that switches between the second-order and the first-order PLL during the fault-occurring/-clearing transient is proposed for preserving both the transient stability and the phase tracking accuracy. Time-domain simulations and experimental tests, considering both the grid fault and the fault recovery, are performed, and the obtained results validate the theoretical findings.

• The paper introduces a phase portrait method to assess transient stability in PLL-synchronized VSCs during significant grid disturbances.
• The paper finds that increasing the damping ratio in second-order PLLs may not ensure stability, while a first-order PLL guarantees transient stability when equilibrium points exist.
• The paper proposes an adaptive PLL that switches between configurations during faults, validated through comprehensive time-domain simulations and experimental tests.

Design-Oriented Transient Stability Analysis of PLL-Synchronized Voltage-Source Converters

The paper at hand presents a critical examination of the transient stability associated with Phase-Locked Loop (PLL) synchronized voltage-source converters (VSCs). In power systems with high penetration of renewable energy, VSCs play a pivotal role in interfacing these resources with the grid. However, unlike traditional synchronous generators, VSCs do not follow physical synchronization laws, making their stability dynamics complex and dependent on control algorithms, notably the widely-used PLL.

Key Contributions

1. Transient Stability Analysis Using Phase Portraits: The research explores the transient stability—or synchronization stability—of VSCs under significant grid disturbances using a phase portrait approach. This enables the investigation of the synchronizing effects of the synchronous reference frame PLL (SRF-PLL).
2. Damping Ratio and Stability: Through quantitative analysis, the paper reveals that merely increasing the damping ratio of a second-order PLL might not ensure stability under severe faults. Contrarily, a first-order PLL consistently guarantees transient stability provided that equilibrium points exist.
3. Adaptive PLL Proposal: An adaptive PLL is proposed, which switches between first-order and second-order configurations during disturbances to maintain stability and phase-tracking accuracy, enhancing the robustness of VSCs in grid fault situations.
4. Time-Domain Simulations: The concept is validated through comprehensive time-domain simulations and experimental tests simulating grid faults and subsequent recoveries. The results bolster theoretical predictions, showcasing the merits of adaptive PLLs in ensuring stability.

Implications

The findings of this paper offer profound implications for the design and control of VSCs in power systems with significant renewable energy penetration. The adaptive PLL strategy provides a proactive approach to modulate between control algorithms to retain grid stability under diverse fault conditions. This adaptability could become crucial as power systems face more complexity with the integration of decentralized energy sources.

Practical and Theoretical Impact

In practical terms, the recommendations can influence the design protocols for VSC systems to improve their fault-ride-through capabilities, a key metric in grid codes. Theoretically, the paper advances the understanding of VSC dynamics, pointing to the limitations of existing PLL configurations and ushering in new control perspectives for grid synchronization processes.

Future Directions

Future research could focus on optimizing adaptive PLLs in real-time applications, accounting for more complex grid scenarios, such as harmonics and non-linear loads. Further investigation into the interaction nuances among multiple VSCs with varying PLL configurations in a microgrid environment could provide a more comprehensive stability framework.

In conclusion, this paper significantly contributes to the body of knowledge on VSC transient stability by introducing a novel adaptive PLL control mechanism. This research serves as a foundational reference for enhancing the resiliency and stability of VSCs in modern power systems dominated by renewable energies.