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.

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.

Synthetic inertia plays a pivotal role in the frequency response of grid-forming (GFM) converters. A small inertia factor provides insufficient inertia power to resist frequency variations during grid contingencies. By contrast, a large inertia factor allows GFM converters to mimic the dynamic response of synchronous generators but could compromise grid synchronization during extreme frequency events, e.g., frequency disturbances from 47 to 52 Hz or phase jumps up to 60°. This happens when a current-limiting GFM converter fails to track a output power reference governed by the power synchronization loop (PSL). Unlike conventional inertia designs that use fixed inertia factors, this article proposes a frequency domain inertia design method for GFM converters based on a modified bandwise PSL. The equivalent inertia factor of GFM converters can be defined as a transfer function by leveraging the equivalence between the high-frequency droop power component and the low-frequency inertia power component, such that the inertia factor is frequency-dependent and varies across different frequency ranges. The proposed inertia design solution combines the advantages of conventional low- and high-inertia design, providing large inertia factor in low-frequency range to mitigate frequency disturbances and small inertia factor in high-frequency range to facilitate power and frequency recovery. The dynamic response of GFM converters adopting the proposed design is experimentally verified and also demonstrated in an accompanied video file under different scenarios including network transients, such as grid phase jumps, grid voltage sags and frequency excursions, as well as operational events of power step change and load reduction.

Throughout the past few years, various transmission system operators (TSOs) and research institutes have defined several functional specifications for grid-forming (GFM) converters via grid codes, white papers, and technical documents. These institutes and organisations also proposed testing requirements for general inverter-based resources (IBRs) and specific GFM converters. This paper initially reviews functional specifications and testing requirements from several sources to create an understanding of GFM capabilities in general. Furthermore, it proposes an outlook on the defined GFM capabilities, functional specifications, and testing requirements for offshore wind power plant (OF WPP) applications from an original equipment manufacturer (OEM) perspective. Finally, this paper briefly establishes the relevance of new testing methodologies for equipment-level certification and model validation, focusing on GFM functional specifications.

Grid-forming (GFM) inverters are anticipated to play an essential role in facilitating the integration of renewable energy in bulk power systems. The fault response of GFM inverters and its impact on traditional protection schemes are ongoing research topics. Distance protection is today one of the most commonly applied protection schemes and depends on multiple system preconditions for reliable operation—many of which may no longer hold in systems with a high penetration of inverters. This paper investigates the impacts of GFM inverters on distance protection, with the main objective of providing an improved understanding of the topic. Important interoperability issues are highlighted with simulation results and elaborated upon based on the theory behind the distance relay model and the behaviours of GFM inverters during faults. The simulations consider numerous fault types and two GFM inverters with different current-limiting control techniques in their fault-ride through strategies. Results indicate several challenges that state-of-the-art distance relays may face with GFM inverters.

Data-driven approach is promising for predicting impedance profile of grid-connected voltage source converters (VSCs) under a wide range of operating points (OPs). However, the conventional approaches rely on a one-to-one mapping between operating points and impedance profiles, which, as pointed out in this article, can be invalid for multiconverter systems. To tackle this challenge, this article proposes a stacked-autoencoder-based machine learning framework for the impedance profile predication of grid-connected VSCs, together with its detailed design guidelines. The proposed method uses features, instead of OPs, to characterize impedance profiles, and hence, it is scalable for multiconverter systems. Another benefit of the proposed method is the capability of predicting VSC impedance profiles at unstable OPs of the grid-VSC system. Such prediction can be realized solely based on data collected during stable operation, showcasing its potential for rapid online state estimation. Experiments on both single-VSC and multi-VSC systems validate the effectiveness of the proposed method.

The fundamental frequency of ac power-electronic-based power systems may deviate from its nominal value, and it is highly affected by converter control dynamics. To capture the dynamics of fundamental frequency, two impedance modeling methods for voltage-source converters (VSCs) are reported, with respect to the selection of system reference frame. The first method is to model VSCs in a reference frame with the nominal frequency, while the second method models VSCs in a reference frame with varying fundamental frequency, and hence, the fundamental frequency is represented as an additional terminal variable in the impedance model. This article mathematically proves that the two impedance models are essentially equivalent, provided that the frequency dynamics is accounted in the modeling of control delay and power stage of VSCs in the second method. This equivalence is demonstrated for both grid-following (GFL) and grid-forming (GFM) VSCs. Stability predictions based on two methods are further compared based on an interconnected GFM and GFL VSC system. The results are also found to be identical. Finally, experiments validate the correctness of the theoretical analysis.

This paper explores the damping effect of a grid-forming (GFM) converter on the small-signal stability of a grid-following (GFL) converter during weak grid configurations. The paralleled operation of GFL and GFM converters is considered in this work. It reveals that the stabilising effect of the GFM converter is proportional with its active power production, and the theoretical upper limit to this stabilising effect is set by the steady-state stability limit of the GFM converter. Electromagnetic transient simulations verify the small-signal stability analysis and the theoretical findings.

Surge current capability of paralleled silicon carbide (SiC) metal-oxide-semiconductor-field-effect transistors (mosfets) operating in both first and third quadrants is required in various applications. The surge current distribution in paralleled SiC mosfets during third quadrant operation needs further investigations. This article, therefore, establishes a source-drain resistance model of SiC mosfets under different gate bias in surge current range, which reveals the current "competition mechanism" between the MOS-channel path and the body diode path under surge current conditions. It then investigates the influence of device parameters discrepancy on surge current distribution in paralleled SiC mosfets. It finds out that the discrepancy of body diode parameters has significant influences on surge current distribution under different gate biases, while the parameter discrepancy of MOS-channel has much smaller impact on surge current distribution, even with positive gate bias. The conclusions of this article are supported with simulation and experimental results.