Applying modern power electronics for voltage support in renewable-dominated grids

Apparent power (i.e., the total amount of power flowing in an AC grid) is made up of two components: active power (P) and reactive power (Q).

Active power is the useful energy delivered to electrical consumers, while reactive power is essential for maintaining voltage by sustaining the electromagnetic fields required for energy transfer.

Voltage levels are primarily governed by the local balance of reactive power. When reactive power supply is adequate, voltages remain stable and close to nominal values. However, if there is a deficit (i.e., due to heavy loading, long transmission distances, or predominantly inductive demand), voltage declines. Excess reactive power, on the other hand, can lead to overvoltage conditions. This behavior is driven by the influence of reactive power on voltage drops across network impedance, particularly inductive reactance in transmission systems.

The coupling between P and Q becomes more pronounced under stressed system conditions. As active power transfer increases along transmission corridors, reactive power losses also rise, increasing the requirement for local voltage support. Without sufficient local compensation, voltage can deteriorate rapidly under high transfer conditions.

These challenges are further amplified by the growing penetration of renewables. Wind and solar PV installations are often located in remote regions with limited local demand, requiring large-scale power transfer to distant load centers. 

Unlike synchronous generators, renewable plants interface with the grid through power electronic converters whose reactive power capability is defined by their P–Q capability curve and operating point. Solar PV at nominal active power typically has little or no remaining capacity for reactive power support. Wind generators can provide reactive power within their converter limits, but there is typically limited available headroom if they are operating at full output. 

Voltage stability refers to the ability of a power system to maintain acceptable voltage levels under normal conditions and after disturbances. If voltage deviates too far from its nominal value, issues can arise, including:

• Cascading failures, potentially leading to widespread blackouts 
• Equipment malfunction or damage due to undervoltage or overvoltage 
• Increased losses and reduced system efficiency 

In extreme cases, insufficient reactive power support can lead to voltage collapse, where voltage drops uncontrollably across a region. This phenomenon has been responsible for several major blackouts, including the famous 2003 Northeast blackout in North America, where inadequate reactive power support and poor system visibility led to cascading outages affecting over 50 million people. 

More recently, the 2025 Iberian Peninsula blackout highlighted the growing complexity of maintaining voltage stability in modern grids with high shares of renewable energy. The blackout was not attributed to a single failure, but rather to a combination of interacting factors that ultimately led to system instability. 

According to a report from ENTSO-E, the disturbance was triggered by an uncontrolled and rapid rise in system voltage under conditions involving multiple concurrent stresses. A central issue identified in the investigation was insufficient and poorly coordinated voltage support across the network . The report highlighted that voltage control settings among local generators were not fully aligned with transmission system requirements, and in some cases relied on manual intervention, slowing the system’s response to changing conditions. 

Critically, available reactive power resources were unable to counteract the sudden voltage increase, leaving the grid vulnerable to overvoltage conditions. This lack of effective voltage support, combined with limited operating margins in the Spanish grid, contributed to cascading disconnections and ultimately a widespread blackout across Spain and Portugal, which impacted tens of millions of people and cost billions of dollars in economic damages. 

Voltage regulation in traditional power systems has relied on a combination of electromechanical and passive devices that inherently support or control reactive power. 

Synchronous generators and dedicated synchronous condensers have historically played a central role, as they can naturally supply or absorb reactive power through their excitation systems, providing continuous and flexible voltage control. In addition, utilities have deployed capacitor banks and reactors to manage localized voltage levels – with capacitors injecting reactive power to boost voltage, and reactors absorbing it to prevent overvoltage conditions.

Transformer-based solutions have also been essential. On-load tap-changing transformers allow operators to adjust voltage levels across different parts of the network without interrupting service, helping maintain acceptable voltage profiles under varying load conditions. For more dynamic needs, Static Var Compensators (SVCs) have been widely used to provide fast-acting reactive power support, responding to voltage fluctuations in near real time.

However, as power systems integrate increasing levels of renewable energy sources, conventional synchronous generation is being displaced, reducing the availability of naturally occurring reactive power and diminishing the voltage support and inertia that synchronous machines provide. As a result, maintaining stable voltage levels is becoming more challenging, driving the need for more advanced, power electronics-based solutions to actively manage reactive power and ensure grid stability.

Modern power electronics have become a cornerstone for maintaining voltage stability in today’s evolving electric grids. Their role is becoming even more critical as renewable generation is increasingly developed in geographically remote areas far from major load centers and in regions with comparatively low local demand. As previously discussed, this creates a growing need to transmit large amounts of power over long distances to demand hubs, where maintaining voltage stability becomes more challenging.

Power electronics are especially important in weak or remote grids, where low system strength makes voltage magnitude highly sensitive to disturbances. In regions with high renewable penetration, they provide essential reactive power and voltage control capability to support stable operation. 

They are also widely used in industrial applications, where large and often highly variable loads – such as electric arc furnaces in steel plants, mining operations, or LNG facilities with electric drives – can create additional challenges for grid code compliance and voltage management.

 Power electronic converters enable grid operators to:

• Inject or absorb reactive power dynamically, responding almost instantaneously to system needs 
• Regulate terminal voltage through high-speed control loops 
• Maintain voltage stability during disturbances 

All the power electronics-based solutions within Hitachi Energy’s Grid-enSure® portfolio can provide the type of dynamic and predictable reactive support required by today’s grids. These technologies include: 

• HVDC voltage source converters (VSCs) like the HVDC Light® regulate voltage by actively controlling reactive power at the point of connection to the AC grid. Unlike conventional HVDC systems that rely on line-commutated converters, VSCs use fully controllable semiconductor devices to synthesize AC voltages with adjustable magnitude and phase. By modulating the phase angle between the converter voltage and the grid voltage, the VSC can inject or absorb active power, which primarily influences system frequency and power flow, while modulation of the converter’s AC voltage magnitude directly controls reactive power, thereby supporting local voltage levels. This decoupled P-Q control allows an HVDC-VSC to provide fast and precise voltage support even in weak or low-inertia grids. 
• The SVC Light® STATCOM is based on the same VSC technology as the HVDC Light® and is connected to the grid via a coupling transformer, allowing rapid modulation of the AC voltage magnitude relative to the grid voltage. By controlling the output voltage of the STATCOM, the system can either supply reactive current when the grid voltage is low (raising the voltage) or absorb reactive current when the voltage is high (lowering the voltage). Fast response times (milliseconds) enable it to stabilize voltage fluctuations caused by load changes, short circuits, or the intermittent behavior of renewable generation, making it particularly effective in weak or low-inertia networks.
  
  One of Germany’s four TSOs, Amprion, enlisted Hitachi Energy to build a series of SVC Light® STATCOM installations. The first of these 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. 
• Enhanced STATCOMs like the SVC Light® Enhanced combine a traditional STATCOM with active power contribution in the form of supercapacitors, giving it both active and reactive power support capabilities. This dual capability enhances grid stability by enabling additional ancillary services, such as inertial support and fast frequency response (FFR), if required. 
• Similar to HVDC-VSCs and Enhanced STATCOMs, battery energy storage systems (BESS) offer both active and reactive power injection and absorption, enabling voltage regulation while simultaneously offering inertial and frequency support. By dynamically supplying active power, BESS can help stabilize frequency deviations and emulate inertia via virtual synchronous machine (VSM) control, making them increasingly popular in grids with high renewable penetration. 
• Static frequency converters (SFCs) are solid-state electronic devices that change AC power from one frequency and voltage to another. They are applicable in several applications, including electric rail networks and grid interties. In electric rail systems, traction loads are inherently dynamic, which can compromise power quality and cause energy losses if not properly managed. Hitachi Energy’s PCS6000 and SFC Light have a proven track record of mitigating these issues by delivering precisely controlled reactive power, balancing the load, and maintaining voltage stability. Their ability to respond rapidly to fluctuating train loads ensures reliable operation and minimizes disturbances to the wider grid.

Crucially, a converter’s contribution to voltage stability in a grid is determined by whether it operates in grid-following or grid-forming control mode.

Grid-following converters operate by synchronizing to the existing AC voltage waveform of the grid. They measure voltage and frequency at the point of connection and inject active and reactive power based on these measurements. 

For voltage regulation, grid-following converters rely heavily on external voltage sources. Their reactive power output can be rapidly adjusted through control algorithms, allowing dynamic voltage support during disturbances, but they cannot independently establish or maintain grid voltage if the system is weak or islanded. As a result, the converters excel in well-established, strong grid environments where voltage and frequency references are reliably available.

In contrast, grid-forming converters can actively establish the voltage waveform at their terminals, effectively behaving like a virtual synchronous machine. GFM converters generate both the amplitude and phase of the voltage, allowing them to provide voltage stiffness and stabilize the grid even in weak or isolated networks. They can respond to reactive power imbalances autonomously, regulate voltage at multiple connection points, and support system recovery during faults. 

As power systems continue to evolve toward higher shares of renewable and inverter-based generation, the challenge of maintaining voltage stability is becoming more complex and more critical. The traditional reliance on synchronous machines for reactive power and voltage control is giving way to a new paradigm centered on power electronics and advanced control strategies.

To ensure reliable operation, future grids will require:

• Greater deployment of dynamic reactive power compensation devices 
• Widespread adoption of grid-forming converter technologies 
• Enhanced system planning and coordination to manage voltage stability in low-inertia environments 

Grid-enSure® represents Hitachi Energy’s answer to this challenge. The portfolio spans the full range of power electronics technologies essential for supporting today’s dynamic power systems. Where necessary, these systems can incorporate grid-forming capabilities, offering enhanced support in weak or low-inertia networks.

By leveraging the full breadth of Grid-enSure®, Hitachi Energy takes a technology-neutral approach to grid stability. Rather than defaulting to a single solution, the company assesses each network’s unique characteristics (e.g., grid strength, generation mix, operational limitations, and planned growth) to determine the most effective combination of technologies.

This ensures that grid operators receive customized stability strategies optimized for the specific technical and operational demands of their systems, delivering both resilience and flexibility in an evolving energy landscape. 

For more information on voltage support with power electronics solutions, visit the Grid-enSure webpage.