With the massive deployment of converter-based power generation, low-frequency oscillations (LFOs) such as subsynchronous oscillations (SSOs) caused by converter-grid interactions have garnered widespread attention. Accurate fieldmeasurement of low-frequency impedance for grid-connected converters is essential for the stability assessment. However, it is challenging for high-voltage and power converter systems, due to the high cost of impedance measurement equipment. To address the issue, a three-step equivalent transformation of the external voltage excitation source to a virtual excitation source is proposed, which facilitates imposing the excitation signals directly into the control system of the converter under test. Such equivalence is mathematically proven, demonstrating that the virtual excitation source can ensure an accurate self-measurement of the lowfrequency impedance of grid-connected converters. Finally, the effectiveness of the virtual excitation source injection method is validated through experimental tests.

The work proposes the Zero Harmonic Distortion Converter (ZHD) as a grid-connected grid-forming (GFM) converter due to its unique sinusoidal characteristic without capacitive filters, low parts count, and off-the-shelf power circuitry with a simple open-loop control structure that does not contribute to possible control interaction and resonances in grid-connected mode. These characteristic are reached by a harmonic cancellation in a three-winding transformer along with harmonic elimination using the Selective Harmonic Elimination Pulse Width Modulation (SHE PWM). The ZHD GFM converter is a compelling alternative for GFM battery energy storage systems (BESS) in medium-voltage applications.

In grid-connected inverter systems, frequency-domain passivity theory is increasingly employed to analyze grid-inverter interactions and guide inverter control designs. However, due to difficulties in meeting sufficient passivity-based stability conditions at low frequencies, passivity theory often falls short of achieving stable system specifications. This article introduces an extended frequency-domain passivity theory. By incorporating a weighting matrix, an extended stability condition is derived. Compared to conventional passivity-based stability conditions, the proposed theory significantly reduces conservativeness and is more suited for analyzing grid-inverter interactions and guiding inverter control design. Theoretical analyses, numerical examples, and experimental results are provided to validate the effectiveness of the proposed methods.

This paper presents a model-based analysis of the active rate of change of frequency (RoCoF) response power in a grid-forming (GFM) system at both unit and plant levels. Each unit response is first analytically decomposed into three components: ramp-up/down, constant, and damped (oscillatory/exponential). It is then revealed that plant topology and variations in cable lengths among units can cause unit response power imbalances through power synchronization loops, leading to unit interactions. These interactions shape the dynamic characteristics of the damped component, at the unit level-affecting overshoot and settling time-while having only a minor effect at the plant level.

This paper investigates the phenomenon of reverse active power in grid-forming voltage source converters during grid voltage recovery events. The study demonstrates that this issue originates from two primary aspects: the operating point during the fault steady state and the control dynamics. The operating point analysis reveals that maintaining a certain level of active power transmission during the fault steady state is necessary to keep the operating point within the positive-power region; otherwise, the operating point may shift into the negativepower region. Furthermore, the control dynamics cause the phase angle of the modulation voltage vector to rotate clockwise, which reduces the power angle and exacerbates the power reversal issue. Theoretical findings are validated through both simulation and experimental results.

The Ejina electric power system, located in the remote western reaches of Inner Mongolia, China, features high penetration of variable renewable energies, and relies on a single-circuit, 442 km radial 220 kV overhead line for connection to the main grid. This configuration poses stability challenges in grid-connected mode due to low system strength, and a high risk of blackout under the islanded operation mode due to the lack of an internal voltage source to establish the voltage and frequency for the entire region. To tackle these challenges, a 25MW/25MWh grid-forming battery energy storage system (GFM-BESS), together with the advanced energy management system (EMS) and high-speed stability control system are installed in the Ejina electric power system. The voltage source feature of the GFM-BESS guarantees the stability of Ejina electric power system in both grid-connected and islanded operation modes, as well as fulfilling N-1 security criteria. The outcomes of this real-world project demonstrate the feasibility of utilizing the GFM-BESS to stabilize the wide-area, remote/islanded electric power system with extremely high penetration (or even 100% penetration) of variable renewable energies. This paper intends to perform a detailed elaboration of this pioneering project, including system and functionality descriptions, as well as real-field testing results under various operating scenarios, which hopefully can shed a light on the extended application of GFM technology in the future bulk power system with high penetration of variable renewable energies.

Active damping is crucial for the stable operation and dynamic performance of grid-forming (GFM) voltage source converters (VSCs). This paper presents a comparative analysis of active damping methods used in the synchronization control of GFM-VSCs. These methods are classified into frequency-difference-based, active-power-based, and q-axis-voltage-based damping approaches. The comparison is based on three key metrics: reference tracking performance, inertial response capability, and stability impact. The advantages and drawbacks of each approach are analyzed, and practical design guidelines are proposed, including hybrid configurations that integrate power and voltage feedforward strategies. The effectiveness of the comparative evaluation is validated through both simulation and experimental results.

This paper presents a theoretical comparison of power swing characteristics between conventional synchronous generator (SG)-based systems and grid-following-voltage source converter (GFL-VSC) systems. Unlike conventional SG-based systems where the swing dynamics are characterized by the change of physical angle between different power sources, the swing characteristics in GFL-VSC systems are influenced by the control dynamic of the VSC, and the loss of synchronization is characterized by the divergence of the output angle of the phase-locked loop, which is control-dependent. Building on these insights, this paper further explores the performance and efficacy of impedance-based swing-detecting schemes at GFL-VSC systems. To validate the correctness of the comprehensive analysis, different simulations are carried out using the PSCAD/EMTDC platform, both in single-machine and multi-machine systems.