High-order delayed signal cancellation-based PLL under harmonically distorted grid voltages

Quasi type-1 phase-locked loop (QT1-PLL) has become very popular in recent times for grid-connected converters owing to its simple structure and fast dynamic response. By means of the use of a half-cycle moving average filter (MAF), this PLL can completely eliminate all the nominal frequency odd-order harmonics. However, the performance deteriorates when the grid experiences frequency drift. To address this issue, high-order non-adaptive MAF with the same window length has been proposed in the literature. Although it improves the off-nominal frequency performance, the filtering induced phase-lag remains the same. To address this issue, high-order delayed signal cancellation technique is considered in this study. The proposed technique demonstrates similar filtering performance as high-order MAF, however, by using lower window length. This makes the proposed technique fast responsive with lower memory requirement compared to similar other techniques in the literature. Detailed small signal modelling and associated parameter tuning method are established to facilitate the implementation of the proposed method inside current controllers of grid-connected converters. Comparative experimental results are provided with QT1-PLL and third-order QT1-PLL (TQT1-PLL) to illustrate the suitability and performance enhancement by the proposed method. These results show that in addition to the superior filtering capability of the proposed PLL, its settling-time is less than one cycle of nominal frequency under most grid conditions. Consequently, in terms of enhancing the dynamic response of the QT1-PLL, the proposed HDSC-PLL is superior to the TQT1-PLL

Issues and Challenges of Grid-Following Converters Interfacing Renewable Energy Sources in Low Inertia Systems : A Review

The integration of renewable energy sources (RESs) is a key objective for energy sector decision-makers worldwide, aiming to establish renewable-rich future power grids. However, transitioning from conventional systems based on synchronous generators (SGs) or systems with a low RESs share presents challenges, particularly when accompanied by decommissioning large central generation units. This is because the reduction in inertia and system strength, traditionally provided by SGs, can lead to a loss of essential system support functions like voltage and frequency. While current converter technologies attempt to compensate for the grid support provided by SGs by enhancing converter capabilities, they still heavily rely on the presence of SGs to function effectively. These converters, known as grid-following (GFL) converters, depend on the grid to operate in a stable and secure manner. As the penetration of RESs increases, the efficacy of GFL converters diminishes, posing stability challenges in low inertia systems and limiting the integration of RESs. Therefore, it is crucial to reassess the existing GFL converter technologies, control mechanisms, and grid codes to understand their status and future requirements. This will shed light on the advancements and limitations of GFL converters, enabling greater RESs integration and grid support independent of SGs. This paper aims to provide an up-to-date reference for researchers and system operators, addressing the issues and challenges related to GFL converter technologies, control systems, and applications in low inertia systems. It serves as a valuable resource for facilitating the transition towards future systems with 100% RESs penetration scenarios.© 2024 The Authors. This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/fi=vertaisarvioitu|en=peerReviewed