By Richard Martin | The Strategic Code
The debate over renewable energy is often framed around costs, deployment speed, and climate urgency. Proponents point to falling prices for wind and solar power, expanding storage options, and international policy momentum. Critics emphasize intermittency, land use, and materials. Yet one crucial element tends to get overlooked in the headlines: the role of synchronous inertia in keeping modern power grids stable.
Even in a grid that maximizes wind and solar, synchronous inertia remains indispensable. It is not an arbitrary engineering choice or a relic of the fossil fuel era. It is a physical necessity, deeply tied to the laws of motion and electromagnetism, that ensures a grid can withstand disturbances without collapsing. Without it, a power system risks fragility, cascading failures, and vulnerability to both natural disruptions and deliberate attacks.
This essay explains why synchronous inertia is central to grid stability, how renewable generation changes the picture, and what the implications are for both advanced and developing economies.
The Grid as One Machine
At its most fundamental, an alternating current (AC) grid is not a collection of separate devices but a single synchronous machine. Every large generator, whether in a hydroelectric dam, a nuclear plant, a coal-fired plant, or a combined-cycle gas turbine (CCGT), is kinetically linked to a rotating mass that spins at a rate proportional to grid frequency.
- In North America, that frequency is 60 hertz (Hz), meaning the magnetic field rotates 60 times per second.
- In Europe and much of Asia, it is 50 Hz.
- Generators achieve this by spinning at precise multiples. For example, a two-pole turbine at 60 Hz spins at 3,600 revolutions per minute (RPM).
All generators in a synchronous grid must remain in lockstep. When they do, the entire system behaves as a single giant machine, spread across thousands of kilometres.
The secret stabilizer of this system is inertia: the stored kinetic energy in all those spinning rotors. If a sudden imbalance occurs, for example if a power plant trips offline or a transmission line fails, the frequency begins to sag. Inertia resists the change, buying precious seconds for governors, relays, and control systems to react. Engineers call this moderating effect the limitation of the rate of change of frequency (RoCoF).
Why Inertia Matters
Inertia is not optional. It is a built-in property of rotating machines. Without it, disturbances would cause frequency to change almost instantaneously. This would leave no time for protective devices to act, and the grid could fragment.
- With inertia, frequency changes gradually, giving the system breathing room.
- Without inertia, frequency can spike or crash in milliseconds, overwhelming defences.
Hospitals, telecom networks, factories, and transport systems all depend on this hidden stabilizer. It is why power engineers describe the grid as “a big flywheel,” because that is essentially what it is.
Renewables and the Inertia Gap
Here is where renewable energy complicates the picture.
- Solar photovoltaic (PV) panels produce electricity via the photoelectric effect. They generate direct current (DC) with no moving parts. To feed into the AC grid, the DC must pass through an inverter, which synthesizes a sinusoidal waveform.
- Fuel cells and other electrochemical devices also produce DC with no rotational mass.
- Wind turbines are rotational, but they do not spin at grid frequency. They operate at variable speeds for efficiency, and their power output is decoupled from the grid by converters.
The result is that renewables add energy, but not inertia. As their share rises, the grid’s rotational inertia declines.
Synthetic versus Physical Inertia
Inverter-based resources can be programmed to provide synthetic inertia or grid-forming services. These mimic the effect of inertia by detecting frequency changes and adjusting output almost instantly.
This works, but with caveats.
- Synthetic inertia depends on software. It is a control algorithm, not a law of motion.
- It is vulnerable. Cyberattacks, coding errors, or firmware failures could compromise performance.
- It lacks brute force. Physical inertia cannot be switched off. It is always there, automatically resisting change.
An analogy helps. Physical inertia is like gravity; it is inherent in the physics of the system. Synthetic inertia is like central planning; it can work, but it is fragile, subject to errors, and dependent on constant management.
Buying Inertia Back
The reality of declining inertia is not theoretical. It arises most acutely in regions that have pursued renewables most aggressively.
- Australia’s National Electricity Market (NEM) has so much wind and solar power that grid operators now procure “inertia services” explicitly. Synchronous condensers, spinning machines that do not generate power but provide rotational mass, are being installed at high cost to stabilize the system.
- The United Kingdom has developed a dedicated Stability Pathfinder program, contracting both new synchronous condensers and repurposed synchronous generators to provide inertia, short-circuit strength, and voltage support. These units run without producing energy, yet remain critical to keeping the system secure.
- Europe relies on a combination of hydroelectricity in the Nordics, nuclear power in France, and gas or coal units elsewhere to anchor the ENTSO-E (European Network of Transmission System Operators for Electricity) system. Even Denmark, famous for its high wind share, depends on those continental rotors to keep its grid balanced.
The lesson is clear: as renewable penetration rises, systems must either keep synchronous machines online or integrate non-generating inertia to replace what has been lost. Inertia cannot be wished away.
Coal’s Continuing Role
Coal is often presented as an outdated relic of the 20th century, destined to vanish under the advance of renewables and cleaner fuels. Yet in practice, coal remains crucial in both developing and advanced economies for reasons that go beyond cheap energy. It supplies synchronous inertia, fault current, and firm baseload capacity at scale.
- China, which has added more renewable capacity than any other country, continues to expand its coal fleet. The reason is not just energy security but grid stability. Coal plants provide the inertia and frequency response needed to anchor a rapidly growing, renewables-heavy system.
- India, Southeast Asia, and parts of Africa face similar pressures. Coal plants can be built quickly, run continuously, and stabilize grids that would otherwise be fragile.
- Even advanced economies in Europe and North America still rely on legacy coal units during stress events, often not for energy but for their stabilizing characteristics.
This underscores a hard truth. Coal is not disappearing because it still fulfills a role that renewables and batteries cannot fully replicate, the provision of real, physical synchronous inertia. Any serious path to decarbonization must grapple with this, either by building nuclear and gas turbines at scale, or by investing heavily in non-generating synchronous machines.
The Real Baseload Constraint
Much of our electricity demand is inherently rotational. Motors drive pumps, compressors, conveyors, fans, and industrial processes. By most estimates, most global baseload demand involves rotational machinery. These loads expect a stable, synchronous backbone.
Without it, motors overheat, trip offline, or fail prematurely. Grid instability is not just about lights flickering. It is about industrial civilization grinding to a halt.
This is why baseload adequacy cannot be measured simply in annual megawatt-hours (MWh). It must be measured in firm power delivered continuously with stable frequency. Inertia is what makes that possible.
The Strategic Implications
Advanced Economies
Advanced industrial regions, including Europe, North America, and East Asia, can afford to push renewables aggressively because they already have legacy synchronous fleets. Nuclear plants, coal-fired generators, hydroelectric dams, and gas turbines keep inertia in the system. Grid-forming inverters and synchronous condensers can supplement this base. The result is a hybrid architecture:
- AC synchronous cores,
- high-voltage direct current (HVDC) overlays for long-distance interconnection,
- inverter-based renewables at the edges.
Developing Economies
The situation is very different in the Global South. Demand is growing rapidly, and infrastructure is thin. These countries cannot wait decades for nuclear buildouts or massive hydroelectric projects. For them, coal and gas remain attractive because they provide not just energy, but instant inertia and frequency stability.
This is why, despite policy pressure, coal plants are still being built in Asia and Africa. They can be online quickly, run continuously, and anchor fragile grids. The challenge is to find pathways, such as carbon capture or methane reforming, that decarbonize without sacrificing stability.
The Future Grid: Hybrid, Not Pure
A maximally solarized or wind-heavy grid is possible in theory, but not in practice without synchronous support. The realistic path forward is hybrid:
- nuclear, hydroelectricity, and gas turbines for firm power and synchronous inertia,
- coal in some regions, where demand growth or system fragility requires it, at least until alternatives are scaled,
- wind and solar for supplemental, variable generation,
- storage technologies, including batteries, pumped hydro, and hydrogen, to smooth imbalances,
- HVDC supergrids to link regions and balance variability,
- grid-forming inverters to bridge the gap where inertia is thin.
This model preserves resilience while cutting emissions. It recognizes that physics, not ideology, defines the limits of power systems.
Conclusion: Physics Is Non-Negotiable
The deeper lesson is that inertia is not arbitrary. It is what keeps civilization’s most complex machine, the electrical grid, synchronized, stable, and resilient.
Renewables can and must grow. But pretending that photovoltaic panels and wind turbines alone can carry an industrial grid misses the point. Without synchronous inertia, the system becomes fragile, complex, and vulnerable. With it, renewables can integrate smoothly into a robust backbone.
Australia, the United Kingdom, and Europe are already facing this reality. Their renewable-heavy systems must now purchase inertia outright, installing non-generating machines simply to stabilize the grid. China continues to build coal for the same reason, not just for energy, but for inertia. This is not a failure of renewables, nor proof that coal wins. It is a reminder that physics never went away.
The real takeaway is this: the future grid is not 100 percent renewable or 100 percent fossil, but a hybrid of renewables, storage, and synchronous generation. Physics demands it, and resilience depends on it.
About the Author
Richard Martin equips leaders to achieve strategic alignment through nested hierarchical action, harnessing initiative for maximal effectiveness with minimal friction.
www.thestrategiccode.com
© 2025 Richard Martin
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