Protection in Inverter-Dominated Grids: Fault Behavior of Grid-Following vs. Grid-Forming Inverters and Mixed Architectures—A Review

With ever increasing renewable energy sources, such as wind and solar, being interconnected to power systems, the grid strength, as measured by existing short-circuit ratio (SCR) measures, will become weaker. The high penetration of renewable generation has posed new challenges to the stability of power grids, and grid-forming (GFM) inverters have been introduced as an effective solution to improve power system stability under these conditions. However, the impact of grid strength (e.g., SCR and X/R ratio) on the stability of GFM and legacy grid-following (GFL) inverters is still not well studied, especially because no hardware test/evaluation work has been carried out. To fill this gap, this paper conducts a comprehensive hardware test of two commercial inverters (which can operate in either GFM or GFL control) under varying grid strengths (SCR and X/R) to gain a comprehensive understanding of how grid strength can impact the stability of GFM and GFL inverters. This comprehensive evaluation using commercial inverters reveals that both X/R and SCR affect the voltage stability of GFM and GFL inverters, but they exhibit different trends under varying grid impedances. X/R affects the voltage stability more than SCR; reducing X/R has a negative impact on the GFM inverter’s stability, but it has a positive impact on the GFL’s stability (this indicates that the GFM inverter might have better stability with transmission systems with a higher X/R ratio, and GFL inverters might have better stability with distribution systems with a lower X/R ratio); and under the same X/R with varying SCRs, the GFM inverter’s point of interconnection (POI) voltage decreases with a lower SCR, whereas the GFL inverter’s POI voltage increases with a lower SCR.

Grid-forming (GFM) inverter control techniques are attracting increased attention for interconnecting inverter-based resources (IBRs) to the electric grid because of their demonstrated performance advantages over existing grid-following (GFL) IBRs in weak system scenarios and low-inertia power systems. GFL inverters require a stiff voltage and frequency source for their operation. In contrast, GFM inverters can independently control both voltage and frequency. However, due to their inherent behavior as voltage sources, GFM inverters do not naturally possess the ability to limit fault current during grid transients such as short circuit faults. In this work, a virtual impedance-based fault current limiting approach is presented for droop-controlled grid-forming inverters. This approach has been tested and validated using electromagnetic transient simulations. A transient stability model of the GFM inverter has been developed and used to study the performance of the IEEE 2 Area system with GFM inverters displacing traditional generators. The results of this study demonstrate that GFM inverters can successfully restrain fault currents within allowed limits while retaining their original stability characteristics.

Inverter-based resources (IBRs) possess dynamics that are significantly different from those of synchronous-generator-based sources and as IBR penetrations grow the dynamics of power systems are changing. This article discusses the characteristics of the new dynamics and examines how they can be accommodated into the long-standing categorizations of power system stability in terms of angle, frequency, and voltage stability. It is argued that inverters are causing the frequency range over which angle, frequency, and voltage dynamics act to extend such that the previously partitioned categories are now coupled and further coupled to new electromagnetic modes. While grid-forming (GFM) inverters share many characteristics with generators, grid-following (GFL) inverters are different. This is explored in terms of similarities and differences in synchronization, inertia, and voltage control. The concept of duality is used to unify the synchronization principles of GFM and GFL inverters and, thus, established the generalized angle dynamics. This enables the analytical study of GFM-GFL interaction, which is particularly important to guide the placement of GFM apparatuses and is even more important if GFM inverters are allowed to fall back to the GFL mode during faults to avoid oversizing to support short-term overload. Both GFL and GFM inverters contribute to voltage strength but with marked differences, which implies new features of voltage stability. Several directions for further research are identified, including: 1) extensions of nonlinear stability analysis to accommodate new inverter behaviors with cross-coupled time frames; 2) establishment of spatial–temporal indices of system strength and stability margin to guide the provision of new stability services; and 3) data-driven approaches to combat increased system complexity and confidentiality of inverter models.

Sustainable energy technologies have become promising solutions to global warming and climate change. Operation of the electric power grid has been dominated in the past by synchronous generation, wherein conventional sustainable energy inverters are designed simply to feed maximum real power into the grid. With the increasing penetration of renewable energy, it is undergoing a rapid shift toward the power generation of inverter-based resources (IBRs). As a result, transitioning to a power grid with more IBRs requires introducing advanced inverter technology that can respond to various disturbances in frequency and voltage occurring on the grid. The grid-forming (GFM) inverter equipped with an energy storage system featuring frequency and voltage support functionalities is vital for the stability of the micro power grid system. The GFM inverter also offers PV output smoothing, low voltage ride through, and low frequency ride through. In addition, the GFM inverter functions as a voltage source inverter to supply energy under off-grid state when the main grid is in fault conditions. A SiC-based 30 kVA GFM inverter is presented with a 3-phase 3-level neutral-point-clamped (NPC) topology for high-frequency operation to achieve high efficiency and power density. Designchallenges in gate driver design, PCB layout, and thermal consideration are addressed. The performances of the designated GFMinverter are measured and tested under on-grid and off-grid operations to verify the functionalities of the advanced inverter, as well.

Electric power generation is quickly transitioning toward nontraditional inverter-based resources (IBRs). Prevalent devices today are solar PV, wind generators, and battery energy storage systems (BESS) based on electrochemical packs. These IBRs are interconnected throughout the power system via power electronics inverter bridges, which have sophisticated controls. This paper studies the impacts and benefits resulting from the integration of grid forming (GFM) inverters and energy storage on the stability of power systems via replicating real events of loss of generation units that resulted in large load shedding events. First, the authors tuned the power system dynamic model in Power System Simulator for Engineering (PSSE) to replicate the event records and, upon integrating the IBRs, analyzed the system dynamic responses of the BESS. This was conducted for both GFM and grid following (GFL) modes. Additionally, models for Grid Forming Static Synchronous Compensator (GFM STATCOM), were also created and simulated to allow for quantifying the benefits of this technology and a techno-economic analysis compared with GFM BESSs. The results presented in this paper demonstrate the need for industry standardization in the application of GFM inverters to unleash their benefits to the bulk electric grid. The results also demonstrate that the GFM STATCOM is a very capable system that can augment the bulk system inertia, effectively reducing the occurrence of load shedding events.

The modern paradigm of power electronics dominated grid (PEDG) has facilitated integration of grid forming (GFM) and grid following (GFL) inverters in the power systems. Addressing the potential disturbances in the PEDG will demand frequent reconfigurations including dynamic adjustment of these grid-interactive inverters' power contributions. However, such dynamic adjustments in GFM-GFL inverters' power contributions may in turn introduce stability issues. The optimal ratio of GFM to GFL inverters in a PEDG has not been well studied in literature with regards to system resiliency and reliable power delivery. This paper provides an analytical approach to determine the optimal ratio of GFM and GFL inverters to minimize their adverse dynamic interactions when subjected to severe disturbances. The main objective of this paper is to provide a comprehensive dynamic interaction-based analysis for a fleet of GFM and GFL inverters within a grid cluster and investigate the impact of inertia level emulated by these GFM inverters on the PEDG resiliency. The provided analysis is validated via several case studies in large-scale simulation representing a grid cluster with multiple GFM and GFL inverters.

With the large-scale integration of renewable energy sources, power electronic components within power grids have surged. Traditional synchronous generator-based power generation is gradually transitioning to renewable energy generation integrated with grid-following (GFL) and grid-forming (GFM) inverters. Furthermore, power grid topology structures are evolving from traditional radial and ring-type configurations toward meshed-type architectures. The impact of grid topology structures on the stability of hybrid systems combining GFL and GFM inverters urgently requires systematic investigation. This paper establishes state-space models of GFM and GFL inverters under three typical grid topology structures and then compares the small signal stability of hybrid systems. First, mathematical models of inverters and transmission lines are established, and a full-order state-space model of the system is accordingly derived. Second, key stability indicators, including eigenvalues, damping ratio, participation factors, and sensitivity indices, are obtained by analyzing the system state matrix. Finally, simulation models for these grid topology structures are implemented in MATLAB/Simulink R2022b to validate the influences of grid topology structures on the stability related to inverters. The results demonstrate that GFL inverters are highly sensitive to grid topology structures, whereas GFM inverters are more influenced by their synchronization control capabilities. Smaller GFL inverters connection impedances and larger GFM inverters connection impedances can enhance system stability.

Low inertia has become a great concern for the power grid with high inverter-based resources (IBR), such as solar and wind power generation systems. Therefore, synchronous condensers have been used to enhance the inertia level of the system. However, only a limited number of synchronous condensers can be installed due to their high costs. Thus, grid-forming (GFM) inverters have been proposed as a promising alternative method to provide frequency support with simulated inertia to achieve 100% renewable penetration in the future. In order to provide a cost-effective adoption strategy for frequency support, a comparative study between synchronous condensers and GFM inverters has been performed through extensive simulation studies. The simulation results show that both strategies can reduce the rate-of-change-of-frequency (RoCoF) and enhance the frequency stability, and the combination methods with both synchronous condenser and GFM inverters can achieve a cost-effective transition plan for the bulk grid.

This paper summarizes Electromagnetic Transient (EMT) simulation studies using PSCAD/EMTDC undertaken to evaluate the capability and suitability of commercially available large scale Grid Forming Inverters (GFMI) to dampen oscillations in a real bulk power transmission network. Faults and a range of grid voltage oscillation frequencies are tested on GFMI and synchronous condenser (SC) models using single source equivalent network model and comparisons of transient, post fault and oscillatory rejection tests are presented. A critical credible fault in the West Murray Zone (WMZ) was simulated on a wide‐area EMT model of the Australian National Electricity Market (NEM) to show the effectiveness of GFMI in providing system strength services and improving damping of network sub‐synchronous control interactions (SSCI). Two scenarios were examined: Direct replacement of existing centralized synchronous condensers in the WMZ of the NEM, and a decentralized distribution of GFMI in the transmission network (treated as expansion or repowering solution for existing grid following inverter equipped solar farms). Simulation results show that commercially available GFMI are a viable option for improving system strength in a practical transmission system with a high proportion of Inverter Based Resources (IBR).

As the integration of inverter-based resources (IBRs) is rapidly increasing in regard to the existing power system, switching from grid-following (GFL) to grid-forming (GFM) inverter control is the solution to maintain grid resilience. However, additional overcurrent protection, especially during fault transition, is required due to limited overcurrent capability and the high magnitude of spikes during fault recovery in IBRs, specifically in the GFM control mode. Furthermore, the power system stability should not be compromised by the employment of additional fault ride through (FRT) schemes. This article presents the design and implementation of an adoptive fault ride through (FRT) scheme for grid-forming inverters under symmetrical fault conditions. The proposed adoptive FRT scheme is comprised of two cascaded power electronic-based circuits, i.e., fault current ride through and a spikes reactor. This adoptive FRT scheme optimizes the fault variables during the fault time and suppresses the fault clearing spikes, without affecting system stability. A three-bus inverter-based grid-forming model is used in MATLAB/Simulink for the implementation of the proposed scheme. Further, a conventionally used FRT scheme, which includes fault current reactors, is simulated in the same test environment for justification of the proposed adoptive scheme. The adoptive FRT scheme is simulated for both time domain and frequency domain to analyze the response of harmonic distortion with the suppression of the fault current. Moreover, the proposed scheme is also simulated under the GFL mode of IBRs to justify the reliability of the scheme. The overall simulation results and performance evaluation indices authenticate the optimal, fault tolerant, harmonic, and spike-free behavior of the proposed scheme at both the AC and DC side of the grid-forming inverters.

As renewable power generation is increasingly integrated with the grid, challenges arising from the removal of synchronous generation are being highlighted in research and in practice. The reduction of grid inertia due to large scale integration of inverter-based resources (IBR) has impacts on system strength and grid stability during transient events. Grid-Forming (GFM) inverters are gaining popularity for their ability to replicate the dynamics of synchronous generation. Serving as a voltage source coupled through a reactance, GFM inverters can regulate the voltage and frequency in a similar fashion to a bulk synchronous generator and are able to complete black start operations as well as operate independently, setting the grid frequency and voltage. There are several GFM control methods, three of which are implemented and analysed in this paper; droop control, dispatchable virtual oscillator (dVOC) and matching control. The MATLAB simulation results of each control method under steady state, and dynamic conditions under grid disturbances and faults in the DC and AC side of the converter is presented together with their comparative analysis.

As renewable power generation is increasingly integrated with the grid, challenges arising from the removal of synchronous generation are being highlighted in research and in practice. The reduction of grid inertia due to large scale integration of inverter-based resources (IBR) has impacts on system strength and grid stability during transient events. Grid-Forming (GFM) inverters are gaining popularity for their ability to replicate the dynamics of synchronous generation. Serving as a voltage source coupled through a reactance, GFM inverters can regulate the voltage and frequency in a similar fashion to a bulk synchronous generator and are able to complete black start operations as well as operate independently, setting the grid frequency and voltage. There are several GFM control methods, three of which are implemented and analysed in this paper; droop control, dispatchable virtual oscillator (dVOC) and matching control. The MATLAB simulation results of each control method under steady state, and dynamic conditions under grid disturbances and faults in the DC and AC side of the converter is presented together with their comparative analysis.

As modern power systems are experiencing exceptional changes with increasing penetrations of inverter-based resources (IBRs), system restoration using IBRs has received attention. Using local grid-forming (GFM) assets near consumers, engineered to establish grid voltages in the absence of a stiff grid, i.e., bottom-up restoration, a distribution system could obtain high system resilience by not relying on the bulk power system restoration, which requires significant human intervention and procedure. This paper studies the technical feasibility of the novel approach with detailed electromagnetic transient (EMT) simulations. To thoroughly evaluate the potential of GFM inverters and the technical challenges in IBR-driven black start, a detailed three-phase inverter model is developed, including negative-sequence control for voltage balance and a phase-by-phase current limiter to sustain momentary overloading during the black start. To examine dynamic aspects of the black-start process, the EMT simulation also models transformer and motor dynamics to emulate their inrush and startup behaviors as well as network dynamics. In addition, active involvement of grid-following distributed energy resources is also studied to facilitate the black-start process. By allowing multiple GFM inverters to collectively black start without leader-follower coordination, we demonstrate that a system can achieve high resilience even with a fraction of assets lost. Two test cases of inverter-driven black start, using two and one GFM inverters, respectively, for a heavily unbalanced 2-MVA distribution feeder are demonstrated. Takeaways for further study and field deployment are provided.

Grid-following and grid-forming inverters are integral components of microgrids and for integration of renewable energy sources with the grid. For grid following (GFL) inverters, which need to emulate controllable current sources, a significant challenge is to address the large uncertainty of the grid impedance. For grid forming (GFM) inverters, which need to emulate a controllable voltage source, large uncertainty due to varying loads has to be addressed. This article presents a generalized control framework by leveraging the voltage-current duality in the plant dynamic model of GFL and GFM inverters. The modeling of uncertainties is also generalized under the control framework by quantifying the uncertainties in grid impedance parameters and the uncertainties in equivalent loading parameters for GFL and GFM inverters, respectively. Based on the generalized control framework, a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\mu}$</tex-math></inline-formula> -synthesis-based robust control design methodology is proposed for both GFL and GFM inverters. The control objectives, while designing the proposed optimal controllers, are reference tracking, disturbance rejection, and harmonic compensation capability with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${i})$</tex-math></inline-formula> sufficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${LCL}$</tex-math></inline-formula> resonance damping under large variations of grid impedance uncertainty for GFL inverters and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${ii})$</tex-math></inline-formula> with enhanced dynamic response under large variations of equivalent loading uncertainty for GFM inverters. A combined system-in-the-loop, controller hardware-in-the-loop, and power hardware-in-the-loop based experimental validation on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {10}$</tex-math></inline-formula> -kVA microgrid system with two physical inverter systems is conducted in order to evaluate the efficacy and viability of the proposed controllers.

Reliable power system operation with 100% inverter-based resources (IBRs) is an unsolved and challenging problem. One of the most challenging factors is ensuring power system stability after N-1 contingencies. This paper presents a promising solution using an operator support system (OSS) to enable stable operation of power system with up to 100% IBR generation. The OSS consists of two components. First is dynamic security assessment to evaluate the system resiliency, and identify critical N-1 contingencies that could endanger the system. The second component, as the key technology behind the OSS, is dynamic security optimization (DSO). The DSO optimizes the control parameters of generators and inverters to improve the stability of the system towards the identified N-1 contingencies. The key to system with 100% IBRs, as emphasized in many recent studies, is to establish the grid frequency reference using grid-forming (GFM) inverters. We show through high-fidelity Electro-Magnetic-Transient (EMT) simulations of the future generation models of Hawai‘i Island system with 100% IBR capacity that a system with 100% IBRs can be operated stably with the help of GFM inverters, and appropriate controller parameters can be found by DSO for the inverters. The DSO is verified via 28 critical N-1 contingencies of Hawai‘i Island system identified by Hawaiian Electric. The simulation results verify the effectiveness of DSO, and show significant stability improvement from DSO.