Why Grid Forming Inverters Have Become Critical Infrastructure
Grid forming inverters are power electronics devices that autonomously establish voltage magnitude and frequency on an electrical network — a capability that was previously the exclusive domain of large rotating synchronous generators. As solar photovoltaic, wind, and battery storage displace thermal generation, the synthetic inertia and voltage-source behaviour provided by grid forming inverters has shifted from a research curiosity to a grid-stability necessity recognised by system operators worldwide.
The fundamental distinction from conventional grid-following inverters is architectural. A grid-following inverter uses a phase-locked loop (PLL) to synchronise with an existing voltage waveform and injects current as a controlled current source — it is inherently dependent on a pre-existing grid voltage. A grid forming inverter, by contrast, behaves as a controlled voltage source, synthesising its own voltage reference and thereby capable of operating in islanded microgrids, riding through severe faults, and providing the inertial response that high-renewable grids increasingly require.
According to IRENA, inverter-based resources are projected to supply the majority of global electricity generation within this decade, making the stability services provided by grid forming technology no longer optional. Standards bodies including IEC are actively developing technical requirements for grid forming behaviour, while grid operators in Europe, North America, and Australia have begun mandating or incentivising grid forming capability in new grid-scale installations.
A grid forming inverter autonomously establishes voltage magnitude and frequency on an electrical network without relying on an external phase-locked loop (PLL), enabling operation in both grid-connected and islanded modes and providing synthetic inertia to support frequency stability in high-renewable power systems.
Grid following inverters synchronise to an existing grid voltage using a PLL and inject current as a controlled current source. Grid forming inverters act as controlled voltage sources that independently set voltage magnitude and frequency — enabling islanded operation, synthetic inertia provision, and stable performance in weak grid conditions where grid following inverters may lose synchronisation.
The Four Control Algorithms Competing for Dominance
Patent filings analysed reveal four primary control algorithm families in active development, each with distinct technical trade-offs that make them suited to different deployment contexts. The choice of control architecture has direct implications for inertia provision, fault ride-through capability, and interoperability with existing grid infrastructure.
Virtual Synchronous Machine (VSM) / Virtual Synchronous Generator (VSG)
The most widely filed approach programs the inverter’s control law to emulate the electromechanical dynamics of a synchronous generator. Implementations include a swing equation module that replicates rotor inertia, an automatic voltage regulator (AVR) for terminal voltage control, and governor dynamics for frequency response. Hitachi Energy’s patent filings (WO2024193866A1, WO2024193867A1, US20240322556A1) describe VSM control systems with supervisory switching logic that transitions between VSM and PLL modes based on monitored voltage magnitude or active power output thresholds. GE Infrastructure Technology (US20240204536A1, EP4489241A1) adds an internal electromotive force (EMF) model and feedforward control signal to improve stability specifically in weak grid conditions. Siemens Gamesa Renewable Energy (WO2024149558A1) applies VSM control to offshore wind with an additional power oscillation damping (POD) module.
Droop Control and Its Droop-Less Evolution
Droop control adjusts output frequency proportionally to active power deviation and output voltage proportionally to reactive power deviation, providing a decentralised mechanism for load sharing among parallel inverters without requiring communication. Sungrow Power Supply (CN119154617A) has patented an adaptive variant that determines droop coefficients dynamically from historical power and frequency data to achieve smooth power transfer. Enphase Energy’s droop-less inverter (US20240388085A1) eliminates steady-state frequency and voltage deviations by replacing conventional droop with virtual resistance and virtual reactance calculations — a significant departure from the established paradigm. The National Renewable Energy Laboratory (US20230361590A1) has documented a hierarchical three-layer framework combining primary droop control with secondary voltage and frequency restoration and tertiary optimal power dispatch, addressing the known limitation that pure droop control introduces permanent frequency offset.
“Enphase Energy’s droop-less grid forming inverter eliminates steady-state frequency and voltage deviations by replacing conventional droop coefficients with virtual resistance and virtual reactance calculations — a significant departure from the established paradigm.”
Virtual Oscillator Control (VOC)
Virtual oscillator control uses a nonlinear oscillator model to generate voltage and frequency references, providing inherent synchronisation without a PLL. The National Renewable Energy Laboratory (US20230101221A1) has filed foundational patents on nonlinear oscillator-based grid forming control for high-renewable microgrids. Enphase Energy (US20240421657A1) has extended VOC implementation with a specific signal-processing chain: measured output current is multiplied by a virtual conductance to obtain a virtual current, which is subtracted from a reference current and passed through a virtual admittance (comprising virtual capacitance and virtual inductance) to produce the reference voltage. Huawei Digital Power Technologies (CN116345519A) has patented a multi-strategy control system capable of switching between VSG, droop control, and VOC based on real-time grid operating mode — treating the three approaches as complementary rather than competing.
Power Synchronisation Control (PSC)
Power synchronisation control adjusts the inverter’s output voltage angle based on the difference between measured and reference active power, achieving grid synchronisation without a PLL. Enphase Energy (US20240356357A1) has documented a PSC implementation that measures output active power, determines the phase-angle difference relative to a reference, and adjusts the output voltage angle accordingly. SolarEdge Technologies (US20240055857A1) has filed a related approach using a power synchronisation loop (PSL) specifically designed for solar PV applications in weak grid conditions.
Map the full grid forming inverter patent landscape across all control algorithm families with PatSnap Eureka.
Explore Patent Data in PatSnap Eureka →Who Is Filing: Assignee Landscape and Strategic Positioning
Enphase Energy is the most prolific single filer in the analysed dataset, with at least six distinct grid forming inverter patent publications in 2024 covering the full spectrum of control approaches: virtual oscillator control (US20240421657A1), droop-less control (US20240388085A1), virtual impedance reactive power control (US20240421660A1), virtual frequency-based active power control (US20240421664A1), power synchronisation control (US20240356357A1), and current limiting via virtual impedance (US20240421656A1). This breadth of filing across multiple algorithm families suggests a strategy of comprehensive IP coverage rather than commitment to a single technical approach.
Enphase Energy filed at least six distinct grid forming inverter patent publications in 2024, covering virtual oscillator control, droop-less control, virtual impedance reactive power control, virtual frequency-based active power control, power synchronisation control, and current limiting — more than any other single assignee in the analysed dataset.
European and Japanese industrial groups are concentrated in VSM-based approaches. Hitachi Energy’s three published patent families (WO2024193866A1, WO2024193867A1, US20240322556A1) all centre on VSM control with supervisory switching, reflecting the company’s heritage in high-voltage power electronics and HVDC systems. Siemens AG (WO2024256153A1, WO2024251488A1, WO2023143983A1) spans VSM-based grid forming for utility-scale converters, DER grid connection, and microgrid islanding — demonstrating a vertical strategy from component to system level.
Chinese assignees — Sungrow, Huawei Digital Power, BYD, China Southern Power Grid, and academic institutions including Tsinghua University, Southwest Jiaotong University, Shandong University, and North China Electric Power University — collectively represent a significant portion of the filing volume, particularly in VSG-based control for photovoltaic systems, energy storage, and hybrid microgrids. This reflects the scale of China’s renewable energy deployment and the domestic policy push for grid-friendly inverter standards documented by IEA.
Application Domains: From Offshore Wind to Vehicle-to-Grid
Grid forming inverter patent filings span at least five distinct application domains, each presenting different technical requirements that drive variation in control architecture. The breadth of application is itself a signal of the technology’s maturity: what began as a research topic for microgrid islanding has expanded into offshore wind, utility-scale storage, solar PV, and bidirectional EV charging.
BYD Co., Ltd. filed a patent (CN119125823A, published December 2024) for a VSG-controlled bidirectional grid forming inverter for vehicle-to-grid (V2G) applications that provides inertia emulation and grid stability support from an electric vehicle battery, supporting bidirectional power flow between the EV and the grid.
- Solar photovoltaic: Sungrow (CN114977270A, CN119154617A) and SolarEdge Technologies (US20240055857A1) have filed grid forming control patents for PV systems, with Sungrow’s VSG-based approach enabling participation in primary frequency regulation. Huawei Digital Power has filed a dedicated photovoltaic grid forming control patent (CN119050011A).
- Offshore wind: Siemens Gamesa Renewable Energy (WO2024149558A1) has filed a VSM-based grid forming control system for offshore wind farms that incorporates a power oscillation damping (POD) module specifically for fault and weak grid conditions — a requirement driven by the long cable connections and reduced grid strength characteristic of offshore installations.
- Utility-scale energy storage: China Southern Power Grid (CN118316088A) has patented a VSG-based grid forming control method for energy storage inverters that replicates synchronous generator rotor motion and stator voltage equations, with governor and excitation regulation modules for coordinated grid services.
- Vehicle-to-grid (V2G): BYD (CN119125823A) has filed a grid forming inverter patent for V2G applications using VSG control to provide inertia emulation and bidirectional power flow between EV batteries and the grid — one of the first such filings explicitly combining grid forming capability with V2G functionality.
- Microgrids: Siemens AG (WO2023143983A1) and Tsinghua University (CN117638978A) have filed patents for microgrid grid forming control, with Tsinghua’s filing addressing hybrid AC/DC microgrids using coordinated VSG control on the AC side and voltage droop on the DC side.
- Distributed energy resources (DER): Siemens AG (WO2024251488A1) and the National Renewable Energy Laboratory (US20230361590A1) have addressed the specific challenge of connecting DERs to the grid with grid forming control that maintains stability across both grid-connected and island operating modes.
Identify white spaces and competitive threats in grid forming inverter applications with PatSnap Eureka’s AI-powered patent analysis.
Analyse with PatSnap Eureka →The Technical Frontier: Hybrid Modes, Current Limiting, and Adaptive Control
The most technically sophisticated recent filings address three challenges that have historically limited the deployment of grid forming inverters in utility-scale applications: seamless mode switching between grid forming and grid following operation, current limiting during faults, and adaptive control that responds to changing grid conditions without manual reconfiguration.
Hybrid Grid Forming / Grid Following Architectures
Multiple assignees have filed patents for inverters capable of switching between grid forming and grid following modes without discontinuity. Hitachi Energy’s supervisory control architecture (WO2024193866A1, WO2024193867A1) switches between VSM and PLL control based on monitored voltage magnitude or active power output thresholds. Fronius International (US20240356352A1) and Delta Electronics (US20240162701A1) have filed similar hybrid mode-switching systems. The commercial rationale is clear: grid forming behaviour is required during grid disturbances and islanded operation, while grid following mode may be preferred during normal grid-connected operation for compatibility with existing grid codes that were written assuming current-source inverter behaviour.
Unlike grid following inverters, which inherently limit current through their current-source control structure, grid forming inverters — acting as voltage sources — are vulnerable to overcurrent during faults. At least five distinct patent filings in the analysed dataset address current limiting specifically: Enphase Energy (US20240421656A1), GE Infrastructure Technology (EP4489241A1), Hitachi Energy (US20240322556A1), Siemens AG (WO2024251488A1), and Huawei Digital Power Technologies (EP4391319A1). All five employ virtual impedance as the limiting mechanism, inserting a calculated impedance into the voltage reference path to reduce current without abandoning grid forming behaviour.
Adaptive and Emergency Control
Sungrow’s adaptive droop coefficient patent (CN119154617A) determines control parameters dynamically from historical power and frequency data, enabling the inverter to adjust its behaviour as grid conditions evolve. North China Electric Power University (CN117096926A) has filed a self-adaptive virtual inertia and damping coefficient adjustment strategy that modifies parameters based on real-time frequency deviation and rate of change of frequency (ROCOF). Huawei Digital Power Technologies (CN118572762A) has filed a patent for event-triggered emergency control parameter switching that activates a dedicated emergency control parameter set when a grid emergency event is detected from sampled grid data — a capability relevant to the increasingly frequent grid disturbances associated with high-renewable systems.
At least five patent filings in the grid forming inverter dataset — from Enphase Energy, GE Infrastructure Technology, Hitachi Energy, Siemens AG, and Huawei Digital Power Technologies — address overcurrent protection using virtual impedance inserted into the voltage reference path, making virtual impedance current limiting the de facto standard approach for fault protection in grid forming inverters as of 2024–2025.
Multi-Inverter Coordination
As grid forming inverters are deployed in arrays — whether in utility-scale solar farms, battery storage facilities, or offshore wind substations — coordinating power sharing among parallel units without circulating currents becomes critical. ABB Schweiz (EP4270689A1) has filed a VSM-based grid forming control method with a dedicated multi-inverter power-sharing algorithm for microgrid and weak grid applications. Shandong University (CN117175605A) has patented a global communication-coordinated power sharing approach for parallel grid forming inverters based on improved VSG, achieving accurate load distribution without circulation current. These multi-inverter coordination patents represent an emerging sub-field within the broader grid forming landscape, consistent with the system-level deployment patterns documented by IEEE in its power systems standards work.