The race to add more renewable generation is well under way, but a less visible challenge is fast becoming just as important: keeping the grid stable. As conventional power stations retire and inverter-based renewables take their place, essential services such as inertia, voltage support and short-circuit strength can no longer be taken for granted. In this Q&A, Thomas Hildinger, Principal Expert, Generators at Voith, explains why hydropower is uniquely placed to fill that gap, and why technologies such as synchronous condensers are becoming indispensable to the next phase of the energy transition.
Why is grid stability becoming one of the defining challenges of the energy transition?
The energy transition is fundamentally reshaping power systems. While the expansion of renewables – especially wind and solar – is essential, it also introduces new challenges for system stability. Wind and solar generation are inherently variable/non-dispatchable and are typically connected to the grid via power electronics, thus providing limited contributions to inertia and short-circuit power. As a result, the traditional stabilising effects of large rotating machines are proportionally declining.
This means that maintaining the balance between generation and consumption, ensuring stable frequency, voltage, and system strength, has become just as critical as producing clean energy. In this context, grid stability is no longer a secondary consideration, but a core requirement for a reliable and secure energy supply.
What changes are you seeing in the technical requirements placed on power systems?
We are seeing a clear shift in technical requirements toward ancillary services that were previously provided implicitly by conventional generation (hydro and fossil). Transmission system operators now increasingly require:
- Additional inertia to stabilise frequency.
- Dynamic reactive power to regulate voltage.
- Higher short-circuit capacity to ensure proper functioning of protection systems.
At the same time, grids must become more flexible and responsive to rapid fluctuations caused by the non-dispatchable renewable generation. This leads to more stringent grid codes and higher expectations for dynamic performance and fault ride-through capability.
How has the role of hydropower evolved?
Hydropower has evolved from being mainly a source of renewable energy to becoming a provider of essential grid services. Thanks to its synchronous, usually salient-pole machines, hydropower inherently provides significant contribution to inertia, voltage control, and system strength. Furthermore, hydropower plays a key and often well‑proven role in black start capability. Unlike other energy sources that may require hours to begin generating electricity, hydropower can achieve this within minutes. Today, thanks to its flexibility, hydropower plays a strategic role in stabilising power systems, particularly as intermittent renewable sources grow. In many cases, hydropower units can even operate in synchronous condenser mode, further supporting grid stability without generating active power. Furthermore, pumped storage projects are acting as large-capacity clean “water batteries”.
What is driving the resurgence of synchronous condensers?
The renewed interest in synchronous condensers is primarily driven by three factors:
- The retirement of conventional thermal power plants which previously provided inertia and short-circuit power
- The rapid growth of grid-following inverter-based renewable generation.
- Increasing requirements for grid stability and system strength.
As power systems become more complex and non-dispatchable renewable penetration increases, synchronous condensers are being recognised again as a reliable and proven solution to address these gaps.
How do synchronous condensers differ from static technologies?
The key difference lies in their physical nature. Synchronous condensers are rotating machines, while static technologies such as STATCOMs or SVCs are based on power electronics. This gives synchronous condensers three unique advantages: they provide real, rotating-mass, physical inertia; they contribute significantly with high short-circuit powe; and they offer high overload capability and robustness during faults.
Static systems are very fast and effective for voltage control, but they cannot replicate the combined inertia, overload capacity and fault-level support provided by synchronous machines
Why is short-circuit power becoming increasingly important?
Short-circuit power is essential for the correct operation of protection systems and for maintaining voltage stability during faults. However, it is often overlooked because, in the past, it was naturally provided by both hydro and fossil conventional power plants.
With the shift toward non-dispatchable renewable energy, the contribution of short-circuit power has declined significantly, making grids more vulnerable. In weaker grids, insufficient short-circuit strength can lead to instability (even blackout in extreme cases), low sensitivity in protection arrangements, or even prevent new generation from being connected.
Can existing assets be repurposed?
Yes, this is a significant opportunity. Many existing hydropower plants, fossil power stations, and even retired generators can be converted into synchronous condensers. This allows plant owners to:
- Extend the lifetime of their assets.
- Create new revenue streams through ancillary services.
- Support grid stability without major greenfield investments.
There are examples worldwide of such conversions; however, technical and economic feasibility has to be investigated on a case-by-case basis.
Where is demand growing most rapidly?
Demand for grid stability solutions is growing globally, particularly in regions with high non-dispatchable renewable penetration and ongoing grid modernization.
Strong growth is observed in:
- Europe and North America, driven by decarbonisation and the retirement of thermal power plants.
- Asia-Pacific, due to rapid infrastructure expansion.
- Emerging markets like Brazil, India, and Colombia, where electrification and renewable integration are accelerating.
Another important factor is the geography of the power system. In more peripheral regions – areas located far from major generation or load centers, often with weak and radial power systems – the need for synchronous condensers tends to be even greater. This is because electricity, especially from intermittent renewable sources such as wind and solar, often must travel long distances over transmission lines before reaching consumers.
Over these long distances, the system can become weaker, with lower inertia, reduced short-circuit power, and greater sensitivity to generation fluctuations. The intermittent nature of wind and solar power can make it more difficult to maintain stable voltage and frequency, particularly at the ends of transmission lines.
In this context, synchronous condensers play a key role by strengthening the grid, providing voltage support, inertia, and short-circuit power exactly where the system is most vulnerable. This ensures that electricity can be delivered reliably to the end of the transmission grid.
What concerns do utilities raise most often?
Utilities and grid operators most commonly highlight:
- Declining system inertia.
- Voltage instability in weak grid areas.
- Insufficient short-circuit power.
- The challenge of maintaining reliability while integrating renewables.
They are also increasingly concerned about meeting stringent regulatory requirements while ensuring system resilience and avoiding large-scale outages.
How will the ancillary services market evolve?
The market for ancillary services, such as reactive power, inertia and voltage support, is expected to grow significantly over the next decade.
As power systems evolve, these services will become more explicitly valued and procured, rather than being an implicit by-product of generation. Technologies that can provide multiple services simultaneously, such as synchronous condensers, will play an increasingly important role.
What will a stable, low-carbon power system look like?
A stable, low-carbon power system will be highly diversified, combining different sources of renewable generation with reliable grid-support technologies. It will be characterised by:
- A high share of renewable energy.
- Strong grid infrastructure with sufficient inertia and system strength.
- Advanced digital control systems to manage complexity.
In this system, hydropower and synchronous machine technologies will continue to play a critical role. They provide the physical stability needed to complement wind and solar power generation and ensure that electricity is delivered safely, reliably, and continuously.
