Rethinking grid stability: The case for modern power electronics

As the global power mix increasingly shifts away from synchronous fossil-based generation to a higher share of wind and solar PV, the electric grid’s inherent stabilizing effect is diminishing.

A grid dominated by inverter-based resources behaves very differently than one built around synchronous machines. Wind and solar resources do not naturally provide the same “invisible” services as large rotating generators driven by gas or steam turbines. The consequence of this being that frequency and voltage become more sensitive to disturbances, particularly in weak grids, remote regions, and/or islanded networks.

Maintaining stability in low-inertia systems requires precise active and reactive power support on sub-millisecond timescales, enabling the grid to effectively “ride through” disturbances. Power electronic-based solutions are ideal for delivering this type of shaped, near-instantaneous power response.

Hitachi Energy has been a pioneer in the development of power electronics-based grid solutions, including HVDC converters, STATCOMs, Enhanced STATCOMS, battery energy storage systems (BESS) and static frequency converters (SFCs). Grid-enSure® brings these technologies together within an integrated portfolio. Combined with the company’s expertise in advanced control systems, Grid-enSure® is specifically designed to enhance grid resiliency and flexibility, facilitating the transition to a more sustainable and adaptable energy system.

At its core, technologies within the Grid-enSure® portfolio are meant to address the stability gaps created as traditional thermal power plants are replaced by intermittent renewables. They do so by combining advanced controls, high-speed semiconductor switching, and application-specific engineering to deliver the services that low-inertia power systems increasingly require, including:
 

• Grid-forming control - An increasingly important capability of converters operating in weak grids is grid-forming control. Traditional grid-following converters require an existing voltage waveform to synchronize to. As a result, they perform reliably in networks that have a strong existing voltage and frequency reference. However, in low-inertia grids, this dependence can create stability challenges, especially during disturbances.
  
  In contrast, grid-forming converters establish and regulate their own voltage magnitude and frequency. They behave like controlled voltage sources, creating a stable reference that other generating resources can follow. This capability allows the converters to support system stability during transient events, such as a fault, generator trip or reduction in renewable output, thereby strengthening the network and even helping to energize portions of the grid from a de-energized state (i.e., black-start).
• Synthetic (i.e., virtual) inertia - Inertia is the only inherent stabilizing effect in a power grid dominated by synchronous generation. When a large generator trips or demand suddenly changes, the rotating mass of synchronous machines naturally resists rapid changes in frequency. This limits the rate of change of frequency (RoCoF) and gives primary generator controls time to respond.
  
  As synchronous generation is displaced, that natural buffer is reduced. Low-inertia systems can experience higher RoCoFs and deeper frequency nadirs. In severe cases, protection systems may trip additional resources, increasing the risk of cascading instability.
  
  Synthetic inertia addresses the gap by using power electronic devices and advanced controls to emulate the inertial response of rotating machines. Rather than relying on  physical rotating masses, power electronics detect rapid frequency changes and then inject or absorb active power to slow the initial frequency movement. This occurs on sub-millisecond timescales, effectively reshaping the earliest system dynamics following a disturbance.
• Fast frequency response (FFR) - Whereas synthetic inertia acts on the earliest frequency dynamics by responding to the RoCoF, FFR is a rapid active power response deployed after a defined frequency threshold or event trigger. It is designed to arrest the frequency decline before under-frequency load shedding or other emergency actions are needed.
  
  FFR has become increasingly important as grids with high renewable penetration experience more pronounced frequency excursions. In recent years, it has emerged as a critical complement to traditional stability mechanisms and provides a level of protection in the initial moments following a contingency.
  
• Voltage support – While wind and solar PV can replace the active power provided by conventional synchronous generators, their reactive power support capabilities are limited, which poses issues when it comes to maintaining nominal voltage during faults, load swings, and transmission stress.
  
  Modern power electronic devices are ideal for addressing this problem, as they can inject or absorb reactive power rapidly, precisely, and locally. Technologies such as STATCOMs, Enhanced STATCOMs, HVDC voltage source converters (VSCs), BESS and SFCs can respond in milliseconds to voltage fluctuations, helping to stabilize the grid during a disturbance. Unlike passive compensation devices, power electronic solutions are highly controllable and can be tuned to the specific needs of the grid, allowing utilities to incorporate high shares of renewables without risking voltage collapse and widespread service disruptions.
• Interoperability – In multi-node systems like multi-terminal HVDC (MTDC) networks, HVDC VSCs are required to operate in parallel. This is also the case with SFCs in modern electric rail networks. In these applications, interoperability (i.e., the ability of equipment from one or multiple vendors to communicate, coordinate, and operate together effectively through defined interfaces and functionalities) is critical.
  
  Hitachi Energy has extensive real-world experience addressing the complexities associated with coordinating converter operation in MTDC and modern electric rail networks. Particularly within the context of HVDC, the company is playing a leading role in advancing the frameworks necessary for true multi-vendor interoperability through the InterOpera¹ project, along with participation in other industry initiatives.  

Grid stability challenges are rarely solved by a single technology. Each grid has its own characteristics, and operator requirements can vary significantly depending on regional infrastructure. A renewable-heavy transmission system may only require dynamic voltage support, whereas an islanded or weak grid may require additional active power injections. 

The Grid-enSure® portfolio is unique in this sense, in that it allows operators to address stability challenges based on system-specific needs rather than predefined technology choices. By combining power electronic technologies with advanced control strategies, solutions can be configured to deliver stability services tailored to the characteristics of each system (e.g., grid strength, generation mix, operational constraints, disturbance behavior, interconnection requirements, future expansion plans). 

The MACH™ control and protection system serves as the operational intelligence behind Hitachi Energy’s power electronic solutions.

A key strength of MACH™ is its modular architecture, which enables the platform to be scaled across different application segments and converter valve technologies. Its upgradable architecture also provides asset owners with a path to extend the life of their initial investment by improving functionality and strengthening system reliability through partial or full upgrades.

These characteristics are critical in a grid environment defined by higher renewable penetration, weaker system conditions, and growing demand for controllability.

The holistic approach used by Hitachi Energy reflects the reality of modern grid planning. The challenge is not simply to add more equipment, but rather to understand which stability services are missing, where they are needed, what response times are required, and how best to coordinate operation within the constraints of legacy technologies and existing infrastructure.   

By necessity, grid operators are risk averse. When faced with stability issues, they historically have opted for established grid planning approaches and technologies.

However, as the energy transition progresses and grids evolve, companies must embrace the mindset that the challenges faced today (and in the future) cannot be solved solely with yesterday’s technologies. Collectively, the industry needs to begin looking beyond traditional assumptions and consider more innovative solutions that challenge the status quo.

In many regions, frequency excursions and voltage instability are no longer a hypothetical problem that can be dealt with down the road. They are a present and material risk to grid stability.

Addressing these challenges requires not only advanced technologies, but also an understanding of how stability services interact at system level. In practice, this means evaluating grid conditions, defining the required responses, and determining how technologies should be selected, combined, and coordinated to maintain reliable operation and security of supply under dynamic conditions. 

Ultimately, the future grid will not be secured by energy alone. It will require controllability, speed, system awareness, and the ability to provide essential stability services precisely when and where they are needed. Grid-enSure® is designed for that future by giving operators the tools they need to transition from a grid that passively inherited stability from conventional generation to one that actively engineers it through power electronics, advanced controls, and integrated system design.

For more information on Hitachi Energy’s power electronic-based grid solutions, visit the Grid-enSure® webpage.