Grid-forming inverter

Overview

Grid-forming inverters (GFM) represent a distinct class of inverter-based resources (IBR) designed to autonomously establish voltage and frequency references for the electrical grid. Unlike traditional grid-following devices that rely on an existing grid signal to synchronize their output, GFM units mimic the physical attributes of synchronous generators. This capability allows them to provide essential inertia and short-circuit strength, which are critical for maintaining system stability as the penetration of variable renewable energy generators increases. Inverter-based resources, also known as converter-interfaced generation (CIG) or power electronic interface sources, include technologies such as battery storage and supercapacitors. These devices lack the intrinsic electromechanical behaviors of rotating machines, meaning their operational features are almost entirely defined by control algorithms. This reliance on software presents specific challenges to system stability; for example, a single software fault can potentially affect all devices of a certain type during a contingency event. Consequently, IBRs are sometimes referred to as non-synchronous generators.

The design of inverters for IBRs generally adheres to established industry standards, including IEEE 1547 and NERC PRC-024-2. These standards help ensure interoperability and reliability across different manufacturers and grid conditions. Grid-forming technology addresses the limitations of grid-following inverters, which typically require a strong grid signal to function effectively. In contrast, GFM inverters can operate in "black start" conditions, helping to restore power after a total grid collapse. By emulating the synchronous generator's behavior, GFM units contribute to the grid's ability to withstand disturbances and maintain balance between supply and demand. This is particularly important in grids with high shares of asynchronous connections, where the total rotating mass of traditional generators may be reduced. The operational status of these technologies is currently active, with ongoing deployments aimed at enhancing grid resilience and facilitating the integration of diverse energy sources.

How do grid-forming inverters differ from grid-following devices?

Grid-forming inverters (GFM) and grid-following inverters (GFL) represent two distinct control philosophies for inverter-based resources (IBRs). As defined by the provided grounding, IBRs are asynchronously connected to the electrical grid via electronic power converters, lacking intrinsic behaviors and relying entirely on control algorithms for their features. This reliance on software presents specific challenges to system stability, such as the potential for a single software fault to affect all devices of a certain type during a contingency. The distinction between GFM and GFL lies in how these algorithms interact with the grid’s voltage and frequency.

Operational Principles

Grid-following inverters operate as current sources. They require an existing grid voltage and frequency to synchronize with, effectively "following" the grid. They inject current based on the difference between the reference voltage and the grid voltage. In contrast, grid-forming inverters operate as voltage sources. They establish the grid voltage and frequency, "forming" the grid conditions for other devices to follow. This allows GFM inverters to provide virtual inertia and voltage support, addressing the stability challenges associated with increasing IBR penetration.

The design of these inverters generally follows standards such as IEEE 1547 and NERC PRC-024-2. These standards help define the operational parameters for IBRs, including battery storage and supercapacitors, which are key components in modern power electronic interface sources. The choice between GFM and GFL depends on the specific requirements of the electrical grid and the desired stability characteristics.

Classification of grid-forming devices

Classification of inverter-based resources (IBRs) is critical for understanding their impact on grid stability, as these devices lack the intrinsic behaviors of synchronous generators and are defined by control algorithms (per IEEE 1547 and NERC PRC-024-2 standards). ENTSO-E categorizes IBRs into three main classes based on their voltage support capabilities and autonomous operation features.

ENTSO-E Classification of IBRs

Class 1 devices, typically photovoltaic systems, provide basic reactive power support proportional to active power output (Q(U) characteristic). They operate primarily in grid-following mode, requiring a strong voltage source to synchronize. Class 2 devices are divided into subcategories: 2A (wind turbines with synchronous generators), 2B (wind turbines with doubly-fed induction generators), and 2C (wind turbines with full-power converters). These classes offer enhanced voltage support and frequency response capabilities.

Class 3 devices, primarily battery energy storage systems (BESS), exhibit the most advanced grid-forming characteristics. They can operate autonomously, establishing voltage and frequency references independently, similar to synchronous condensers. This capability is crucial for maintaining stability as IBR penetration increases, since a single software fault can affect all devices of a certain type in a contingency.

Protection functions and ride-through standards

Grid-forming inverters and other inverter-based resources (IBRs) must comply with rigorous protection functions and ride-through standards to maintain grid stability. The design of these inverters generally follows the IEEE 1547 and NERC PRC-024-2 standards, which define how IBRs respond to grid contingencies. These standards address specific challenges arising from the lack of intrinsic behaviors in IBRs, where features are almost entirely defined by control algorithms. A single software fault can affect all devices of a certain type in a contingency, highlighting the importance of standardized protection functions.

IEEE 1547 Standards

The IEEE 1547 standard outlines requirements for interconnection and interoperability of IBRs with the electrical grid. It includes provisions for overvoltage and overcurrent protections, which are critical for preventing damage to the inverter and the grid. Overvoltage protection ensures that the inverter can handle voltage spikes without tripping unnecessarily, while overcurrent protection prevents excessive current flow that could lead to thermal stress or mechanical failure. Additionally, IEEE 1547 addresses DC unbalance, which can occur in three-phase systems and affect the performance of the inverter.

NERC PRC-024-2 Standards

The NERC PRC-024-2 standard focuses on protective relay settings and performance for IBRs. It includes requirements for phase jump protections, which help the inverter to maintain synchronization with the grid during sudden phase shifts. This is particularly important for grid-forming inverters, which play a key role in maintaining grid frequency and voltage stability. The standard also specifies momentary cessation and reconnection timers, which dictate how long an IBR should remain disconnected from the grid during a fault and when it should reconnect.

Momentary Cessation and Reconnection Timers

Momentary cessation refers to the brief period during which an IBR disconnects from the grid in response to a fault. The duration of this cessation is critical for minimizing the impact on grid stability. Reconnection timers determine when the IBR should reconnect to the grid after the fault has been cleared. These timers are designed to ensure that the IBR does not reconnect too quickly, which could cause additional stress on the grid, or too slowly, which could delay the restoration of power. The specific values for these timers are defined in the IEEE 1547 and NERC PRC-024-2 standards.

In summary, the protection functions and ride-through standards for grid-forming inverters are essential for ensuring the reliability and stability of the electrical grid. The IEEE 1547 and NERC PRC-024-2 standards provide a framework for designing and operating IBRs, addressing key challenges such as overvoltage, overcurrent, DC unbalance, phase jump protections, and momentary cessation and reconnection timers.

What are the vulnerabilities of inverter-based resources?

Inverter-based resources (IBRs) lack the intrinsic physical behaviors of synchronous generators, making their operational features almost entirely defined by control algorithms. This dependency on software introduces specific challenges to system stability as IBR penetration increases. A critical vulnerability is the potential for cascading failures where a single software fault can affect all devices of a certain type during a contingency. This homogeneity means that unlike mechanical systems with varied tolerances, electronic converters may react identically to grid disturbances, potentially leading to mass disconnections if the firmware logic is not robust against simultaneous stressors.

Impact on Weak Grids

High IBR penetration significantly impacts weak grids, which are characterized by lower short-circuit ratios and greater sensitivity to frequency and voltage fluctuations. Because IBRs are asynchronously connected to the electrical grid via electronic power converters, they do not inherently provide the rotational inertia that stabilizes traditional grids. As the share of variable renewable energy generators and energy storages such as batteries and supercapacitors grows, the grid’s ability to absorb shocks diminishes. The stability of these systems relies heavily on the precise timing and response of the power electronic interface source, making them more susceptible to oscillations and voltage collapses compared to grids dominated by synchronous machines.

Standardization and Firmware Risks

To mitigate these vulnerabilities, the design of inverters for IBRs generally follows established standards such as IEEE 1547 and NERC PRC-024-2. These standards aim to harmonize the behavior of converter-interfaced generation (CIG) to ensure compatibility across different manufacturers and grid conditions. However, the reliance on standardized control algorithms also amplifies firmware update risks. If a critical bug exists in a widely adopted firmware version, a single update or a specific grid event can trigger widespread failures. The lack of intrinsic mechanical diversity means that software patches must be meticulously tested to prevent systemic instability, highlighting the need for rigorous validation of control logic in the era of power electronic interface sources.

Case study: The Blue Cut fire incident

The August 16, 2016 Blue Cut fire incident in Southern California serves as a critical case study for inverter-based resource (IBR) stability challenges. During this event, the grid experienced the sudden loss of 1200 MW of photovoltaic (PV) power. This significant drop in generation highlighted the vulnerabilities inherent in converter-interfaced generation (CIG) systems, where device features are defined by control algorithms rather than intrinsic mechanical behaviors.

A primary technical failure identified was the performance of frequency estimation algorithms within the inverter controls. IBRs, often termed non-synchronous generators, rely on these algorithms to maintain synchronization with the electrical grid. In the aftermath of the Blue Cut fire, it was observed that a single software fault could affect all devices of a certain type simultaneously during a contingency. This uniformity in response, while efficient under normal conditions, can exacerbate system instability when penetration levels are high. The incident demonstrated that without robust algorithmic resilience, the asynchronous connection via electronic power converters can lead to cascading failures.

Following the incident, the North American Electric Reliability Corporation (NERC) issued recommendations to address these stability issues. The design of inverters for IBRs generally follows the IEEE 1547 and NERC PRC-024-2 standards. The Blue Cut fire analysis reinforced the need for strict adherence to these standards, particularly regarding frequency response and voltage support. NERC emphasized that control algorithms must be robust enough to handle rapid changes in grid conditions, such as the sudden loss of significant generation capacity. This event underscored the importance of diverse control strategies to prevent single-point software faults from impacting the entire fleet of inverters.

Applications and future developments

Grid-forming inverters are increasingly deployed in battery energy storage systems (BESS) to provide essential inertia and voltage support, addressing the stability challenges posed by high penetrations of inverter-based resources. As noted in the grounding, IBRs lack intrinsic behaviors and rely entirely on control algorithms, meaning a single software fault can affect all devices of a certain type in a contingency. GFM technology mitigates this by emulating synchronous machine dynamics, allowing BESS units to act as primary stabilizers rather than passive followers. This application is critical for maintaining grid frequency and voltage during transient events, reducing the reliance on traditional synchronous condensers.

SuperFACTS and Synchronous Condensers

Research into SuperFACTS (Flexible AC Transmission Systems) explores integrating GFM capabilities into advanced transmission infrastructure. SuperFACTS devices aim to combine the benefits of synchronous condensers—such as rotational inertia and short-circuit power—with the rapid response times of power electronics. Synchronous condensers remain a key component in hybrid grids, providing mechanical inertia that IBRs traditionally lack. However, GFM inverters can replicate these characteristics through virtual synchronous machine (VSM) controls, offering a more compact and efficient alternative. The integration of GFM into SuperFACTS research focuses on enhancing grid resilience by allowing inverters to actively shape grid voltage and frequency, similar to how synchronous condensers do, but with faster control loops.

Economic Trade-offs of Oversizing

Implementing GFM functionality often requires oversizing semiconductor components to handle higher current and voltage stresses during grid contingencies. This oversizing increases the capital cost of the inverter but can reduce the need for additional grid infrastructure, such as synchronous condensers or reactive power compensators. The economic trade-off involves balancing the initial investment in larger semiconductors against the long-term operational savings and enhanced stability. Oversizing ensures that the inverter can maintain grid-forming capabilities even under extreme conditions, such as a sudden loss of generation or a significant load shift. This approach is particularly relevant for systems adhering to IEEE 1547 and NERC PRC-024-2 standards, which define the performance requirements for inverters in the electrical grid.

See also

• Fukushima nuclear power plant accident and comprehensive health risk management
• Combined heat and power
• History of nuclear energy in Bolivia
• Pumped Storage Hydropower Project
• Feed in tariffs for solar panels

References

1. "Inverter-based resource" on English Wikipedia
2. Grid-Forming Inverters: A Key Enabler for the Future Power System
3. Grid-Forming Inverters: Technical Overview and Standardization Status
4. Grid-Forming Inverters: A New Era for Power Electronics
5. Grid-Forming Inverters: Definitions, Requirements, and Applications