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Impedance-based stability analysis has been widely adopted for voltage source converters (VSCs). Considering unknown controller parameters, impedance measurement based on frequency scan is always required for stability evaluation, which endures complicated implementation and can only be conducted under small amount of stable operating conditions. To solve this problem, a novel impedance profile prediction method for grid-connected VSCs has been proposed. A combined structure of stacked autoencoder (AE) and principal component analysis (PCA) is firstly proposed to extract VSC admittance feature under stable operating points, and a comprehensive VSC admittance set can be further predicted through searching on an enlarged feature space with unstable scenarios included. The stability can then be evaluated on the predicted VSC admittances with a stability boundary derived. Simulations and experiments prove the effectiveness of the proposed method., Part of ISBN 9798350316643QC 20240603

This paper derives closed-form solutions for grid-forming converters with power synchronization control (PSC) by subtly simplifying and factorizing the complex closed-loop models. The solutions can offer clear analytical insights into control-loop interactions, enabling guidelines for robust controller design. It is proved that 1) the proportional gains of PSC and alternating voltage control (AVC) can introduce negative resistance, which aggravates synchronous resonance (SR) of power control, 2) the integral gain of AVC is the cause of sub-synchronous resonance (SSR) in stiff-grid interconnections, albeit the proportional gain of AVC can help dampen the SSR, and 3) surprisingly, the current controller that dampens SR actually exacerbates SSR. Controller design guidelines are given based on analytical insights. The findings are verified by simulations and experimental results., QC 20240408

This paper investigates initial undershoots during the power response of grid-forming (GFM) converters. At first, the small signal model of the active power control loops (APC) is established. Based on the analytical model, the locations of the closed-loop RHP zero is utilized to indicate the existence and the peak of the initial undershoots. Subsequently, the impacts of the virtual impedance (VI) control and operating points on the initial power undershoots are comprehensively investigated. It is found that the VI control helps to dampen the oscillation but only has minimal influence on the RHP zero. In addition, the larger active power reference and smaller grid voltage leads to stronger undershoots. Finally, simulation results are presented to verify the accuracy of the established model and analysis., Part of ISBN 9798350316445QC 20240201

This paper analyzes the current reference generation of inverter-based resources (IBRs) during asymmetrical faults. It is revealed that the existing current generation methods are insufficient in maximizing the fault current contribution under different current priority modes. In this paper, a novel current reference generation approach is proposed to address this problem while simultaneously ensuring compliance with converter current limitations and grid codes. Electromagnetic transient (EMT) simulations are given to verify the proposed method., Part of proceedings ISBN 9798350316445QC 20240201

With the increasing integration of inverter-based resources (IBRs) into power systems, the fault characteristics of the power system can be altered, potentially affecting the performance of the traditional phase selection element (PSE). This paper first analyzes the fault characteristics of IBRs, then provides a theoretical examination of how these unique fault characteristics affect the performance of PSE. It is revealed current reference generation of IBRs will significantly impact the efficiency of PSE by altering the proportion of active current. Electromagnetic transient (EMT) simulations are given to verify the theoretical analysis., Part of proceedings ISBN 9798350327007QC 20240213

Impedance-based method has been increasingly adopted to assess the stability of inverter-based resources (IBRs). To get a better view of the state-of-the-art and challenges for implementing the impedance-based dynamic analysis, a survey with general/specific questions has been initiated by IEEE Task Force on Frequency-Domain Modeling and Dynamic Analysis of High-Voltage Direct Current (HVDC) and Flexible AC Transmission System (FACTS). The feedbacks are collected from universities, national labs, transmission system operators (TSOs), power plant developers, as well as IBR vendors. It is interesting to note that while many common understandings have been established in practices, certain gaps still exist among different stakeholders. This article intends to bridge this gap by sharing a summary of the survey, including questionnaires, responses from different stakeholders, and the analysis of survey results. The challenges for different stakeholders using impedance-based method are identified, which shed a light on the future research work., QC 20231115

Current grid code has explicit requirement on the fault current injection profile of grid-following voltage-source converters (GFL-VSCs), i.e., the output reactive current of GFL-VSC needs to be proportional to voltage- magnitude deviation at the point of connection (PoC) until its current limitation is reached. This article points out that such way of injecting the fault current may result in transient overvoltage (TOV) at the PoC during fault recovery, and the rate of change of current reference of GFL-VSC is critical to the duration of TOV. Moreover, a method for estimating the maximum magnitude of TOV is presented, which provides an intuitive insight for the TOV phenomenon., Part of ISBN 9788957083505QC 20231010

Grid Forming (GFM) inverters are valuable assets for enabling sustainable power systems. Network Frequency Perturbation (NFP) plot could serve as a valuable tool for evaluating the behaviour of a GFM inverter's active power control (APC) loop. NFP plot has been recommended in the recent grid code modification by National Grid ESO as a document to be submitted by a grid forming (GFM) plant operator for compliance testing purposes. In a grid connected GFM inverter, it is well known that the APC and reactive power control (RPC) loops are inherently coupled with each other. However, state of the art guidelines on producing and interpreting an NFP plot have completely disregarded the influence of this coupling effect in an attempt to simplify the analysis. In this paper, a systematic analysis is presented which identifies the conditions under which such a simplification could be valid, or could otherwise introduce significant influence on the NFP plot. Simulation results are presented validating the theoretical analysis., Part of ISBN 9798350328233QC 20231010

Maintaining synchronization with the grid during fault ride-through (FRT) is essential for grid-connected voltage source converters (VSCs). However, improper current references can lead to a loss of synchronization during FRT due to the nonexistence of stable equilibrium points (SEPs). While the mechanism of SEPs has been explained, the development of control schemes ensuring their existence remains challenging due to the difficulty of obtaining real-time grid parameters during FRT. To address this issue, this article proposes a novel current reference control scheme that only requires prefault grid parameters. By analyzing the changes in grid parameters caused by faults and characterizing the correspondence between the during-fault and prefault grid parameters, a voltage-dependent current reference control scheme is proposed. Since no real-time grid parameter estimation is needed during FRT, the proposed control scheme is practical and easy to implement. Simulation studies using DIgSILENT PowerFactory demonstrate that the proposed control scheme enables the VSC to maintain synchronization with the grid under the conditions of short-circuit ratio equaling 1.3 and grid voltage dropping to 0.1 p.u. Moreover, experiments further verify the effectiveness and feasibility of the proposed control scheme., QC 20230925

Grid-forming (GFM) capability requirements are increasingly imposed on grid-connected voltage-source converters (VSCs). Under large grid disturbances, GFM-VSCs need to remain stable while providing GFM services. Yet, such objectives, as pointed out in this article, inherently lead to conflicting requirements on the dynamics of the internal voltage source (IVS) of GFM-VSCs, i.e., the fast IVS dynamics is needed to avoid the loss of synchronism with the grid, whereas the slow IVS dynamics is preferred for maintaining GFM capability. To tackle this challenge, an adaptive fast/slow IVS control is proposed, which switches GFM-VSC between fast and slow IVS dynamics based on system needs. The proposed method enhances the transient stability of GFM-VSC while maximizing its capability of providing GFM service. Further, the approach is robust to different grid strengths and different types of grid disturbances. The experimental results verify the theoretical findings and the effectiveness of the proposed control method., QC 20230904

Multi-terminal HVDC projects are increasingly developed in recent years to enhance the flexibility of energy transmission. Differing from the point-to-point HVDC systems, it is more challenging to design the multi-terminal HVDC system and ensure stable interoperability among converter systems, especially when the converters are manufactured from different vendors. Although impedance-based stability analysis allows for analyzing such systems based on black-box models, it is still difficult to utilize those black-box models to identify the root cause of potential instability. To tackle this challenge, this paper proposes a multi-level sensitivity analysis approach using frequency-domain sensitivity functions based on the impedance-based stability criterion. The proposed method differs from the classical sensitivity analysis based on state-space or transfer-function models, as it is purely based on black-box impedance models. Case studies on a four-terminal HVDC system are carried out for stability and sensitivity analysis based on the impedance measurement in PSCAD, through which the most sensitive HVDC station can be identified. The proposed theory and the analyzed results are finally validated by electromagnetic transient simulations., QC 20230324

Grid-forming (GFM) inverters are recognized as a viable solution to increase the penetration of renewable energy in bulk power systems. However, they are physically different from synchronous generators in terms of overcurrent capability. To protect the power semiconductor devices and support the power grid under severe symmetrical disturbances, the GFM control systems should be able to achieve the following requirements: current magnitude limitation, fault current contribution, and fault recovery capability. Various current-limiting control methods are reported in the literature to fulfill these goals, including current limiters, virtual impedance, and voltage limiters. This paper presents an overview of those methods. Emerging challenges that need to be addressed, including temporary overcurrent, unspecified output current vector angle, undesired current saturation, and transient overvoltage, are pointed out. Comparative simulations are conducted to demonstrate the performance of different methods under grid voltage drops and phase jumps. Finally, open issues of current-limiting control methods for GFM inverters, including transient stability assessment, voltage source behavior under overcurrent conditions, and windup of voltage controllers, are shared., QC 20230112