Grid-forming Converters Research Articles

Power matching based current limitation method for grid forming converter under large disturbances

Grid forming (GFM) converter mimics the operational characteristics of the synchronous generator, serves as a voltage source that provides voltage and frequency support to the power system. However, due to its voltage source nature, the current of the GFM converter is highly determined by the external system, increasing the risk of overcurrent under large disturbances and potential damage to the converter. The conventional current reference limitation suffers from synchronization instability, which causes the converter to lose synchronization. To address this issue, this paper proposes a novel power matching based current limitation method, which maintains the synchronization of the GFM converter and also avoids overcurrent issues. Specifically, when a large disturbance occurs, considering the maximum output power of the GFM converter, the reference power of the converter is corrected using the frequency to match the output power in real time. Therefore, the output power of the converter is always its maximum value, with the current constrained to the maximum level allowed, avoiding overcurrent and instability. Finally, the superiority of the proposed method is verified by experiment and simulation.

Voltage dip propagation in renewable‐rich power systems utilizing grid‐forming converters

AbstractThe growing integration of converter‐interfaced renewable energy sources (RESs) utilizing Grid‐Following (GFL) converters has displaced conventional synchronous generators (SGs) in central generation units. This shift presents challenges, including diminished system inertia, lower fault levels, and implications for system strength and network resilience. The propagation of voltage dips, particularly during disturbances like system Short Circuit (SC) faults, is adversely affected by the increased penetration of such RESs. This is attributed to the limited support capability of these sources and their distinct SC response compared to SGs. In response to these challenges, Grid‐Forming (GFM) converters emerge as a promising technology equipped with advanced functionalities that emulate SG operation. Consequently, they hold potential for mitigating the effects of voltage dip propagation in renewable‐rich power systems. This study aims to assess the impact of employing GFM converters in renewable‐rich power systems on voltage dip propagation across the network. The authors’ investigation begins by examining the SC response of GFM converters and comparing it with the responses of traditional GFL converters and SGs. The paper proceeds to analyze voltage dip propagation, considering various penetration scenarios involving RESs based on GFL and GFM converters. The IEEE 9‐BUS test system, implemented in the DIgSILENT PowerFactory software, serves as the basis for these evaluations. Through extensive simulations and analysis, the authors’ research provides valuable insights into the effectiveness of GFM converters in enhancing the network's response to voltage dips.

Nonlinear Backstepping Control of Grid-Forming Converters in Presence of Grid-Following Converters and Synchronous Generators

Grids in near future are going to host synchronous generators (SGs), legacy grid-following converters (GFLCs), and relatively new grid-forming converters (GFCs), where the latter two technologies support converter-interfaced generations. This change is going to introduce new challenges – one of them is the dc-link voltage collapse issue in GFCs following the outage of a generator. As a solution, this paper proposes a nonlinear backstepping control strategy that guarantees Lyapunov stability of a reduced-order model of such systems. To this end, first a reduced model of a 6-bus system with one GFC, one GFLC, and one SG is considered to develop fundamental understanding of such systems and derive a stabilizing control strategy. Then, based on certain observations, the simplified model is extended to a multimachine system, which in turn helps developing the nonlinear control strategy in presence of bounded uncertainties. Further, a dc-link voltage tracking control is introduced to ensure controllable frequency support from GFCs. The proposed controller is decentralized in nature and requires limited communication only if neighboring buses have GFCs and/or GFLCs. Detailed phasor models of the 6-bus system and the modified 68-bus New England-New York system with 2 GFCs and 2 GFLCs are used to demonstrate the effectiveness of the proposed control strategies.

Interaction Between Grid-Forming Converters With AC Grids and Damping Improvement Based on Loop Shaping

Grid-forming converters (GFC) are a promising technology for improving the grid inertia and frequency stability via voltage source converters (VSC). However, GFCs could make the ac-side oscillations more complicated since it mimics the dynamics of synchronous generators (SG), and the ac-side oscillations could spread to the dc systems. This paper investigates the mechanism of the interactions between GFCs and ac grids, and proposes an enhanced GFC to improve the damping performance on oscillations. By separating the system model into GFC-related transfer functions (TF) and the ac grid-related TFs, the interaction and stability margin can be investigated by the phase difference of these TFs at the cross-gain frequency (CGF). A lead compensator is designed to reshape the phase characteristic of GFC-related TF for enhancing the damping performance of the GFC. The effectiveness of the enhanced GFC is tested according to the simulations on DIgSILENT/PowerFactory.

Experimental Validation of Virtual Inductance for Synchronization of Grid-Forming Converters

Grid-forming converters provide a voltage and frequency to the grid and can thus be used to enable the temporary islanded operation of distribution grid structures, e.g. during contingencies. When operating several grid-forming converters in parallel, they maintain synchronization by a supplementary controller, as e.g. a conventional droop controller. However, the droop control is designed for grids with a high line reactance to resistance ratio, which is not valid at the low voltage level. As a consequence, grid-forming converters might not be able to synchronize or maintain synchronization to the grid. This paper presents test results from a laboratory setup with a real low voltage microgrid validating the inability of two grid-forming converters to synchronize. To overcome this issue, a virtual output inductance, which effectively increases the line inductance, is implemented in the converter controllers and validated in the test setup. The measurement results prove that the additional inductance diminishes the synchronizing interactions and enables the successful synchronization of the converters. Furthermore, it is shown that the tuning of the inner converter control loops can significantly impact the effectiveness of the virtual inductance during transients.

External Inertia Emulation to Facilitate Active-Power Limitation in Grid-Forming Converters

External Inertia Emulation to Facilitate Active-Power Limitation in Grid-Forming Converters

Analysis and Weakening of Sequence Impedance Coupling in Grid-Forming Converters

Analysis and Weakening of Sequence Impedance Coupling in Grid-Forming Converters

A Novel Limited Grid-Forming Strategy to Support Power System Stability under Primary Energy Source Limitations

The available literature on grid-forming converters often assumes an unlimited primary energy source, for example by postulating a constant DC voltage source. However, this assumption is not justified in real-world scenarios as soon as primary energy source limitations are reached. Therefore, it is imperative for grid-forming control strategies to take such limitations into account and ensure that they support robust system stability even under primary source limitations. In this work, a novel concept of limited grid-forming control is introduced that supports the stability of the network, even under primary energy source power as well as energy constraints. Moreover, the control concept also allows for a seamless transition between full grid-forming and limited grid-forming behavior without any control mode switching. The proposed scheme is tested on a modified IEEE 9-bus test system with one synchronous machine replaced by a droop-based grid-forming controlled battery system incorporating the proposed control scheme. The simulation of a couple of dynamic events provides results that demonstrate promising performance, thereby substantiating the feasibility of the proposed control scheme.

How Many Grid-Forming Converters Do We Need? a Perspective From Small Signal Stability and Power Grid Strength

How Many Grid-Forming Converters Do We Need? a Perspective From Small Signal Stability and Power Grid Strength

Resiliency-Guided Grid-Forming Converter Control of Distributed Solar-Powered Electric Vehicles

Resiliency-Guided Grid-Forming Converter Control of Distributed Solar-Powered Electric Vehicles

Transient Stability Analysis of Grid-Forming Converter in Current Limiting Mode Based on Hamiltonian Theory

Transient Stability Analysis of Grid-Forming Converter in Current Limiting Mode Based on Hamiltonian Theory

On the Destabilizing Mechanism of Nonuniform Damping in Grid-Forming Converters Penetrated Multi-Machine System

On the Destabilizing Mechanism of Nonuniform Damping in Grid-Forming Converters Penetrated Multi-Machine System

Passivity and Decentralized Stability Conditions for Grid-Forming Converters

Passivity and Decentralized Stability Conditions for Grid-Forming Converters

Reactive Power Characteristics and FRT Strategy for Grid-Forming Converters with Virtual Impedance-Based Current Limiting

Reactive Power Characteristics and FRT Strategy for Grid-Forming Converters with Virtual Impedance-Based Current Limiting

Selective Harmonic Compensation in SPC Grid-Forming Converters for Improving Power Quality in Weak Grid

Selective Harmonic Compensation in SPC Grid-Forming Converters for Improving Power Quality in Weak Grid

Simultaneous Voltage and Current Harmonic Control of Grid-Forming Converters Considering Both Islanded and Grid-Connected Modes

Simultaneous Voltage and Current Harmonic Control of Grid-Forming Converters Considering Both Islanded and Grid-Connected Modes

Analysis of Stability Stationary Point for Virtual Parameters of Grid-forming Converter Integrated Power Systems Based on Small-signal Model

Analysis of Stability Stationary Point for Virtual Parameters of Grid-forming Converter Integrated Power Systems Based on Small-signal Model

P3R: Partial Power Postregulated Grid-Forming Converter for Prosumer DC Buildings

P³R: Partial Power Postregulated Grid-Forming Converter for Prosumer DC Buildings

Analysis and Parameters Design of Grid-Forming Converter for Enhancing the Stability of Photovoltaic Storage System Under Weak Grid

Analysis and Parameters Design of Grid-Forming Converter for Enhancing the Stability of Photovoltaic Storage System Under Weak Grid

Power Supporting Control of Grid-forming Converters under Grid Voltage Sags for Transient Stability Enhancement and Overcurrent Limitation

Power Supporting Control of Grid-forming Converters under Grid Voltage Sags for Transient Stability Enhancement and Overcurrent Limitation