Applying grid-forming converters to enhance stability in current and future power systems

Power grids are undergoing a structural transformation as the energy transition accelerates. Large, synchronous generators driven by gas and steam turbines have historically provided the backbone of grid stability through inertia, short-circuit strength, and voltage support. However, these machines are steadily being displaced by inverter-based renewable resources, namely wind and solar PV.  

Inverter-based resources contribute significantly less fault current than synchronous generators and do not inherently provide inertia to the grid. As their penetration increases, system strength declines and voltage and frequency become more sensitive to disturbances. In several power systems, these effects are no longer theoretical but are already being observed. 

This is occurring at a time when electricity demand is surging, driven by AI data center build out, electrification of industrial processes, and the proliferation of EVs. Amid the shift, the need is growing for converters that leverage advanced functionalities, such as grid‑forming control, to actively establish voltage and frequency references, and provide inertia‑emulation capabilities to support grid stability. 

To understand the distinction between grid-following and grid-forming control, it is first necessary to understand the roles that active and reactive power play in maintaining stable power system operation 

Active power (P) controls the balance between generation and load, ensuring that system frequency remains within acceptable limits by matching the real energy produced by generators with the energy consumed by loads at every moment. Even small imbalances can cause frequency to rise or fall, signaling a mismatch that, if left uncorrected, can propagate across the system and trigger protective actions or generator trips.  

Reactive power (Q), by contrast, does not transfer usable energy but is essential for maintaining voltage levels and supporting the electromagnetic fields required for power transfer through transmission and distribution equipment. Adequate reactive power ensures that voltage remains within operational limits along transmission lines and at load centers, enabling power to flow efficiently and preventing excessive losses or equipment overheating. Without proper management of both active and reactive power, power systems can experience frequency deviations, voltage instability, increased stress on generators and network assets, and in severe cases, cascading outages that threaten overall grid reliability. 

A grid-following converter operates by locking onto the grid’s existing voltage waveform using a phase-locked loop (PLL), which allows it to synchronize its output current with the system. Active power is controlled by adjusting the portion of current that is aligned with the grid voltage. When this in-phase current increases, more real power is delivered from the DC source (e.g., battery, solar PV array, HVDC link, etc.) into the grid. Reactive power is managed separately by controlling the current component that is 90 degrees out of phase with the voltage, enabling the converter to inject or absorb VARs for voltage support.  

Grid-following converters operate reliably when connected to a strong network with a well-defined, stable voltage reference. However, in grids with lower short-circuit ratios or weaker voltage stiffness, interactions between the PLL and network dynamics can influence system response, potentially resulting in oscillations or temporary loss of synchronization during large disturbances.  

Grid-forming converters employ a fundamentally different control strategy. Instead of synchronizing to an external voltage waveform, the converter establishes and regulates its own voltage magnitude and frequency, effectively acting as a controlled voltage source. Grid-forming control strategies (e.g., droop, virtual synchronous machine/VSM, etc.) allow the converter to respond to system imbalances by adjusting power output in a manner analogous to generator governor and excitation responses. Because the converter directly regulates voltage and frequency, it can support system stability during disturbances and energize the grid from a de-energized state (i.e., black-start).  

Modern power systems benefit from a combination of grid-forming and grid-following control devices. Together, they can coexist within the same grid, allowing the network to integrate high levels of renewable generation while maintaining stability, efficiency, and dynamic performance across a wide range of operating conditions. 

Historically, stability and voltage control in AC power systems have been provided by traditional synchronous generators and/or dedicated synchronous condensers.   

Synchronous machines are mechanically coupled to the grid frequency and provide rotational inertia, which helps to slow the rate of change of frequency (RoCoF) following disturbances. In addition, they enable dynamic reactive power support and voltage regulation and contribute substantial short-circuit current, strengthening the grid and supporting reliable operation. However, as many conventional generators retire and are replaced by solar PV and wind, these stabilizing characteristics are diminishing and thus need to be replicated through grid-forming converter control strategies. 

Several types of power electronic platforms can be engineered to deliver grid-forming functionality. How and where these solutions are deployed depends on the unique requirements of the application. 

• HVDC - The primary purpose of HVDC converters is to act as the interface between AC power systems and DC transmission lines, enabling controllable, efficient transfer of large amounts of power over long distances (i.e., between two AC grids, from offshore wind farms, etc.) HVDC converters based on Line-Commutated Converter (LCC) technology rely on the voltage waveform of the connected AC system for commutation, which means they inherently operate in a grid-following mode and require a strong grid to function properly. In contrast, HVDC systems based on Voltage Source Converter (VSC) technology use self-commutated semiconductor devices that allow precise control of the AC voltage waveform and all four quadrants of the P-Q diagram enabling precise and modular control strategies to support the specific needs of the grids of the future (see Figure below). 

• HVDC-VSC converter stations with grid-forming control respond to disturbances in the connected AC grid by dynamically adjusting active power to provide frequency and inertial support, as well as reactive power support to stabilize the system voltage. They can also energize sections of the AC network from a shutdown state, providing voltage and frequency references that allow other generation sources to start and progressively restore the system. 
• HVDC grid-forming converters are not a recent innovation. Hitachi has over 20 years of proven operating experience covering a wide range of powers, voltages, and converter topologies and applications. Beginning in 2006 with the first grid-forming strategy enabling black-start capability for interconnectors, to its further expansion to offshore wind applications, including interconnectors embedded in extremely weak AC networks.

• STATCOMs – STATCOMs are FACTS (Flexible AC Transmission Systems) devices designed to regulate bus voltage by dynamically injecting or absorbing reactive current. The converter synthesizes a controllable AC voltage from a DC capacitor. By adjusting the magnitude of this internal voltage relative to the grid voltage, STATCOMs control the direction and magnitude of reactive power flow.
  
  STATCOMs can be designed to operate in either grid-following or grid-forming control mode. A STATCOM with grid-forming functionality behaves as an ohmic-inductive impedance connected in shunt, where its damping can be tuned by correct selection of the virtual impedance. Installations using grid-following control can be upgraded to provide grid-forming functionality. However, doing so requires system studies to determine appropriate control settings, followed by recommissioning the unit to install and validate the updated control software.
• Enhanced STATCOMs - Conventional STATCOMs do not have energy storage capacity apart from the DC capacitors and therefore cannot provide sustained active power support. Enhanced STATCOMs address this by integrating an energy storage system into the DC link. With an energy buffer available, the Enhanced STATCOM can inject or absorb active power to emulate traditional synchronous generator behavior through synthetic inertia algorithms and can sometimes provide support with fast frequency response (FFR). Because active and reactive power is controlled independently, the device can deliver coordinated P–Q support during disturbances, improving damping response and enhancing stability in weak or low-inertia systems.  
• Battery Energy Storage Solutions (BESS) - Battery systems are increasingly being deployed with grid-forming converters. Similar to Enhanced STATCOMs, BESS can provide both reactive power support and active power balancing. However, with greater energy storage capacity, they can sustain active power injection or absorption for longer durations than Enhanced STATCOMs, providing synthetic inertia, FFR, and frequency regulation on multi-second or even multi-minute timescales. Grid-forming BESS can also utilize VSM control, which emulates the electromechanical dynamics of synchronous generators to provide stable voltage and frequency references, enabling seamless integration and enhanced stability support in weak or low-inertia grid conditions. Additionally, they can energize the local grid independently from a utility grid and assist other plants in coming online (i.e., black-start).
• Static Frequency Converters (SFCs) - SFCs equipped with grid-forming control operate as controlled voltage sources that can establish and regulate voltage and frequency on one or both sides of a frequency conversion interface. Unlike STATCOMs, which are primarily designed for reactive power compensation and voltage support, SFCs are built to transfer active power continuously between asynchronous systems (e.g., 50/60 Hz networks or dedicated rail traction grids) while simultaneously providing voltage and frequency control. Compared to BESS and Enhanced STATCOMs, SFCs do not possess stored energy. Their active power capability is tied to the interconnected systems rather than an internal energy reservoir. As a result, grid-forming SFCs strengthen weak or isolated networks by providing synthetic inertia, fault current contribution, and stable frequency reference control, but their sustained active power support depends on upstream supply rather than stored energy.

In response to the growing demand for grid stabilizing technologies, Hitachi Energy has launched Grid-enSure® - an integrated portfolio of solutions designed to facilitate the transition to a more sustainable, resilient, and adaptable energy system. 

Built on decades of industry expertise and Hitachi Energy’s leadership in power electronics and advanced control systems, Grid-enSure® brings together both existing and next-generation technologies, including HVDC, STATCOMs, Enhanced STATCOMs, SFCs, and BESS. Within the portfolio, grid-forming control can be provided by the following solutions:
 

• HVDC Light® is Hitachi Energy’s advanced VSC-HVDC technology, engineered to efficiently transmit large amounts of power over long distances underground, underwater, or via compact cable routes where traditional AC systems are limited or uneconomical.
  
  Launched in 1997, the technology is highly compact and extends from lower tens of megawatts up to around 3,000 MW and ±640 kV, with very low electrical losses and fast active/reactive power response. VSC control enables features such as rapid power modulation, black-start capability, voltage support, and improved fault ride-through performance, making it ideal for:
  • Connecting offshore wind farms to onshore grids
  • Underground power links
  • Providing shore power supplies to islands and offshore oil & gas platforms
  • Connecting asynchronous grids
  • City center in-feeds

• SVC Light® STATCOM is based on the same technology platform as the HVDC Light®. It utilizes a modular VSC equipped with advanced semiconductors for switching and is available for system voltages up to 69 kV and converter ratings over -/+ 400 Mvar. For higher voltages, a step-down transformer is used to connect SVC Light to the grid. The STATCOM provides a symmetrical operating range. 
• SVC Light® Enhanced is Hitachi Energy’s state-of-the-art Enhanced STATCOM solution that extends traditional reactive power compensation with short-term active power contribution . The SVC Light® Enhanced builds on proven SVC Light® technology. The primary difference is the addition of supercapacitors for active power support and fully controllable inertial response on a millisecond timescale. The grid-forming capabilities of the MACH™ Control System makes the Enhanced STATCOM highly adaptable to rapidly evolving grid conditions.

• BESS – Hitachi Energy has more than 30 years of experience in BESS, with ~100 grid-forming systems deployed worldwide. Delivered as an end-to-end solution from grid interface to battery modules, the BESS is designed to actively support grid stability with ancillary services. Leveraging control strategies such as VSM, these systems establish stable voltage and frequency references while providing synthetic inertia, oscillation damping, and FFR. Stored energy allows for sustained active power support beyond the initial transient response, with the specific duration determined on a project-by-project basis. 
• SFC Light and PCS6000 – Hitachi Energy has more than 40 years of experience in high power, medium-voltage SFCs for various applications, including AC rail power supply, hydro pumped storage power plants, and grid interties. The PCS6000 is a three-level converter featuring a common DC link. Its design minimizes the number of semiconductors and the required capacitance, leading to fewer components and a reduced probability of failure. The use of integrated gate-commuted thyristor (IGCTs) enables low losses, high efficiency, and maximizes converter lifetime. In addition to the PCS6000, Hitachi Energy also offers the SFC Light, which is a highly flexible modular multi-level converter (MMC) designed for high availability and low losses.  Together, the SFCs have combined for more than 1.5 GW of installed capacity.

Beyond being a feature of individual converter-based assets, grid-forming control supports coordinated operation across multiple technologies within the Grid-enSure portfolio. By allowing STATCOMs, BESS, HVDC, and SFC to interact as voltage-forming elements of a unified system (analogous to synchronous generators in conventional grids) Hitachi Energy can deliver integrated solutions that combine the strengths of each technology to address application-specific grid stability challenges in an optimized and interoperable manner.

More than two decades ago, Hitachi Energy proved something the industry once thought impossible: a VSC-based HVDC system capable of forming voltage and frequency to black-start an AC grid. Long before “grid-forming” became a widely used term, HVDC Light® had already demonstrated - in real-world projects - its ability to establish voltage, control frequency, and energize key grid infrastructure independently.

A major milestone came in Estonia in 2007, where field tests showed a VSC-HVDC station could rebuild a stable AC system from zero. It gradually established voltage without transformer inrush, maintained tight frequency control, and supplied auxiliary power to start a 250 MW thermal unit. The demonstrations covered the full restoration sequence - energizing transmission lines, transformers, and reactors, then synchronizing and loading a large generator - all managed smoothly by the converter .

Traditional black-start methods rely on diesel generators, complex mechanical systems, and carefully staged switching. In contrast, VSC HVDC offers a more controlled and flexible approach. It can rapidly adjust both active and reactive power across its full operating range, maintaining stable frequency even with minimal load. Because the converter directly synthesizes voltage, it can energize equipment smoothly without harmful transients.

These early demonstrations, completed well before grid-forming became a formal requirement, laid the foundation for over 20 years of development. Today’s HVDC Light® systems build on the same core capabilities: establishing voltage, setting frequency, stabilizing the grid, and accelerating system restoration while delivering high-performance power transmission.

In short, modern grid-forming HVDC is built on proven field experience - starting with the first demonstrations that showed a converter can safely and reliably restart a grid, energize major equipment, and bring large generators online.

Reference: HVDC with Voltage Source Converters; A Powerful Standby Black Start Facility, IEEE PES T&D conference in Chicago, USA, April 21-24, 2008

Germany’s power system is undergoing a significant transformation as more renewables enter its energy mix. With the progressive closure of conventional power plants, system strength is declining in certain grid areas. Recognizing this shift, German transmission system operators (TSOs) announced in 2020 that all newly installed grid-connected converters must be capable of grid-forming operation, including the provision of substantial reactive power support. 

In response, Amprion, one of Germany’s four TSOs, enlisted Hitachi Energy to build a series of SVC Light® STATCOM installations, the first of which was commissioned in 2023 at the Opladen substation in Leverkusen, north of Cologne. 

The installation consists of a VSC branch rated at ±300 Mvar. Its primary objective is to supply dynamic reactive power independently of the local short-circuit level, which, under certain operating scenarios, can become extremely low. 

Extensive validation of the grid-forming functionality was performed through electromagnetic transient (EMT) simulations and Hitachi Energy’s virtual MACH™ software-in-the-loop testing. Using the MACH™ control platform, the project team verified the STATCOM’s voltage-source behavior under a wide range of contingencies.

Additional stress scenarios were also evaluated, including variations in grid short-circuit ratios. The grid-forming control exhibited inherent stabilization characteristics, responding immediately to abrupt voltage amplitude changes and angular shifts without requiring oversized hardware components.

As penetration of inverter-based resources in power grids continues to increase, stability can no longer be taken for granted. The traditional foundations of system strength (e.g., rotating mass, inherent inertia, and high fault current) are steadily diminishing. In their place, advanced power electronics and intelligent control must assume a far more active role in managing disturbances and preventing cascading failures.

Grid-enSure® is Hitachi Energy’s response to this dilemma. Grid-enSure covers the entire spectrum of power electronics technologies needed to support modern grids, including BESS, traditional and enhanced STATCOMs, SFCs, and HVDC. If required, any of these systems can be designed with grid-forming functionalities, providing addd support in weak or low-inertia systems.

The breadth of the Grid-enSure® portfolio enables Hitachi Energy to approach grid stability challenges from a truly technology-agnostic perspective. Rather than prescribing a single solution, the company can evaluate the specific characteristics of each network—its strength, generation mix, operational constraints, and future expansion plans—and then deploy the most appropriate combination of technologies. 

This flexibility ensures that grid operators are not forced into one-size-fits-all approaches. Instead, they gain access to tailored stability solutions that are optimized for the unique technical and operational requirements of their systems.

For more information on grid-forming solutions, including STATCOMs, BESS, HVDC, and SFCs, visit the Grid-enSure webpage.