Bridging the inertia gap: how power electronics can help stabilize modern grids

Today, most conversations around the transformation of power grids are framed in terms of electricity generation replacement. When a large fossil-based power plant is retired, planners may install an equivalent amount of renewable generation capacity, such as wind or solar PV, to replace the lost energy production. From a power supply perspective, the system remains unchanged. However, the behavior and underlying dynamics of the grid are dramatically altered. 

Conventional synchronous generators driven by gas or steam turbines have historically contributed more to the grid than just megawatts. By virtue of their physical design and rotating mass, these assets provide a range of ancillary services that are essential for reliable network operation. One of the most important is system inertia, which is the only inherently instantaneous stabilizing force in a power grid. 

As penetration of inverter-based resources continues to grow, grid operators and system planners are faced with the problem of maintaining high levels of inertia with less contribution from conventional synchronous machines. This article discusses how Hitachi Energy is addressing this challenge through its Grid-enSure® portfolio and highlights the evolving role of power electronics in modern power grids.

Inertia in a power grid refers to the system’s ability to resist changes in frequency. Grid frequency reflects the real-time balance between electricity generation and consumption. When generation exceeds demand, the rotors of synchronous generators accelerate, causing frequency to rise. Conversely, when generation falls short of demand, the rotors decelerate, leading to a drop in frequency. 

Power system inertia is the combined effect of the rotating mass of all synchronous machines connected to the grid. Large gas and steam turbines, in particular, store substantial kinetic energy. This stored energy acts as a buffer during disturbances and is naturally released or absorbed, thus limiting the rate of change of frequency (RoCoF). Systems with higher inertia experience more gradual frequency deviations, giving control systems valuable time to respond and restore the balance between generation and demand. 

The picture below illustrates the RoCoF during an event (i.e., disturbance) in systems with different levels of inertia (Hsys). As shown, the system with the highest inertia has the smallest RoCoF and the least severe frequency nadir, which is the lowest absolute frequency level reached after the disturbance.  

To compensate for the loss of physical inertia as synchronous machines are replaced, advanced power electronic devices can be controlled to provide synthetic (i.e., virtual) inertia.

Synthetic inertia emulates the inertial response of rotating machines by detecting rapid frequency changes and injecting or absorbing active power. It is a RoCoF-driven, continuously modulated response designed to slow the initial acceleration or deceleration of frequency, providing the system with critical time for fast frequency response (FFR) and primary control actions to initiate.

Synthetic inertia mimics the physical effect of rotating mass  and behaves like a machine-level function, implemented inside the converter control with sub-millisecond reaction times. It reshapes the earliest system dynamics by reducing RoCoF, whereas FFR provides a fast active power step after a specified frequency level has been reached. 

Synthetic inertia is not inherently less reliable than traditional mechanical inertia. In fact, modern power electronic technologies equipped with grid-forming capabilities, including STATCOMs, HVDC-VSC converters, and BESS can provide fast, accurate, and tunable inertial response that outperforms the fixed behavior of synchronous machines under many grid conditions.

In BESS equipped with grid-forming converters, synthetic inertia is provided by discharging stored energy within the battery. In most cases, the storage capacity is sufficient for active power injection over multiple hours. This allows BESS to support the grid well beyond the initial disturbance, participating in both FFR and frequency regulation. 

Grid-forming STATCOMs are also effective providers of virtual inertia. In a conventional STATCOM, active power injection is limited to DC capacitors and the inertial response is not material. In an Enhanced STATCOM, the implementation of supercapacitors extends these capabilities. Similar to BESS, Enhanced STATCOMs are well suited for providing inertial support to weak grids and, if required, can be designed with enough storage capacity to inject power on timescales that are relevant for FFR. 

HVDC voltage source converters (VSCs) can deliver synthetic inertia through two primary mechanisms. The first is to release energy inherently stored within the converter and DC link to instantly stabilize the connected AC system. The second is to use energy from the remote terminal of the HVDC link (e.g., another synchronous grid or offshore wind installation). 

In the second case, if the remote terminal is connected to a “strong” power system, the available energy for stability support is effectively unconstrained, meaning there is no need to pre-allocate HVDC transfer capacity specifically for inertia. As a result, the converter can offer continuous and flexible frequency support across all relevant timescales while operating at its rated power, without sacrificing transmission capability. In practice, the only true limitation is the power transfer rating of the HVDC link itself.

Some wind farms equipped with advanced control systems can also contribute synthetic inertia to power grids by leveraging the kinetic energy stored in the rotating blades of turbines. However, this is typically not a reliable approach for inertial support, as there is no guarantee sufficient energy will be available from the turbines at all times. 

Overall, modern power electronic devices with grid-forming capabilities offer a highly flexible approach to providing virtual inertia to power grids with high penetration of wind and solar PV, complementing the well-established role of synchronous generators and condensers. 

Grid-enSure® is Hitachi Energy’s response to the evolving challenges facing today’s power grids. 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 to actively recreate the stabilizing behaviors provided passively by conventional power plants.

Several technologies within the Grid-enSure® portfolio are capable of providing inertial support, including grid-forming BESS, SVC Light® (conventional STATCOM), SVC Light® Enhanced (Enhanced STATCOM) and HVDC Light®. These solutions leverage high-speed semiconductor switching and microsecond-level control to inject or absorb active power and respond to frequency deviations by emulating the inertial behavior of synchronous machines.  

A salient example of Grid-enSure® technology at work is the Dalrymple BESS installation in South Australia, which Hitachi Energy developed in partnership with ElectraNet. The 30 MW / 8 MWh installation operates in grid-forming mode, marking a significant milestone. For the first time in the network, essential stability services — including inertia and system strength are provided by a battery system rather than conventional synchronous generators. In normal operation, the Dalrymple BESS enhances reliability for the Lower Yorke Peninsula by acting as a stabilizing asset within the broader grid.

In the event of a network outage, the Dalrymple BESS transitions to islanded operation, supplying power to the region in coordination with the nearby Wattle Point Wind Farm and distributed rooftop solar. In this mode, the BESS establishes and regulates system frequency, effectively acting as the grid reference that all other generation follows.

Through its grid-forming controls, the BESS emulates the inertial and stabilizing behavior of a synchronous machine, allowing the islanded network to operate indefinitely without any fossil fuel-based generation. Advanced supervisory controls ensure that wind and solar output are continuously managed to maintain system stability within defined operating limits.

When the main grid is restored, the Dalrymple BESS once again demonstrates the flexibility of power-electronics-based inertia. It actively adjusts the islanded system frequency to match that of the grid and then autonomously resynchronizes, enabling a smooth reconnection without disruption. 

This project (and others) highlights how BESS, when paired with grid-forming converters and advanced controls, can move beyond simple energy storage to become foundational assets for delivering synthetic inertia and ensuring stability in increasingly inverter-dominated power systems.

As modern power systems continue to incorporate higher shares of renewables, technologies like BESS, STATCOMs, and HVDC-VSCs with grid-forming control will play an increasingly important role in maintaining frequency stability. By emulating the inertial behavior of synchronous machines, these devices help ensure that the fundamental stabilizing mechanisms of the grid remain intact, even as the underlying generation mix evolves. 

The breadth of the Grid-enSure® portfolio allows Hitachi Energy to address grid stability challenges through a genuinely technology-agnostic lens. Instead of relying on a one-size-fits-all solution, each network is assessed based on its unique characteristics (e.g., system strength, generation mix, operational limitations, and future growth plans), enabling the deployment of one or multiple technologies depending on specific grid requirements. 

Ultimately, when properly designed and validated to ensure deterministic behavior, modern power electronic converters can match – and in many cases exceed – the performance of conventional synchronous machines when it comes to inertia and short-duration frequency support, positioning them as a cornerstone of resilient, future-ready power systems.

For more information on Hitachi Energy’s grid-forming technologies, visit the Grid-enSure webpage.