What is a grid-forming inverter and why does the grid need inertia?

A grid-forming (GFM) inverter establishes and holds the grid's own voltage and frequency, using a virtual synchronous generator (VSG) algorithm that mimics the swing equation of a spinning rotor. It gives wind, solar and batteries synthetic inertia. The grid needs inertia because heavy rotors release kinetic energy automatically when frequency dips, buying operators a few seconds to respond.

Start with what inertia is. A conventional synchronous generator — steam or gas turbine — has an enormous rotor mass storing large kinetic energy at speed. When grid frequency falls, meaning load exceeds generation, those heavy rotors spontaneously give up some of that kinetic energy and briefly 'advance' extra output, buying the system operator a few seconds to bring reserves online. That automatic stabilising mechanism is system inertia. It is the grid's shock absorber, and it requires no command to act.

Wind and solar do not supply that buffer. Wind turbines connect through power-electronic converters, so the rotor is decoupled from the grid and contributes essentially no inertia; solar PV has no rotor at all and contributes zero. Once renewable penetration passes a certain level, system-wide equivalent inertia falls sharply — any disturbance, such as a large unit tripping or a big load switching in, produces a frequency excursion that is both larger and faster, narrowing the window left for protection systems. This is the core technical reason the 2016 South Australia blackout drew global attention.

GFM answers that gap directly. Conventional inverters are grid-following (GFL): they must first 'see' grid voltage and frequency and then track them, a passive follower. A GFM inverter instead takes the lead, setting and holding voltage and frequency itself — when frequency drops the algorithm releases more active power, the electronic equivalent of a heavy rotor shedding kinetic energy, and it withdraws that support when frequency rises. Early wind turbines were all grid-following, simply injecting power and relying on other plants to hold frequency; a new generation of wind farms is being fitted with GFM so that a 100% renewable system can be as stable as a fossil-driven one.

The proof has come on real grids. Tesla's 100 MWh battery at Hornsdale in South Australia was upgraded in 2018 into the world's first large-scale GFM deployment; its millisecond response far outpaces the minute-scale response of thermal generators and has effectively filled the inertia gap under high renewable penetration. Grid operators in the UK, Ireland and Denmark now treat GFM as a prerequisite for connecting large offshore wind, and manufacturers including Enercon, Vestas and GE Vernova are making GFM control standard on new products. Singapore poses a distinct version of the problem: it is an island system with no large interconnectors to push surplus midday solar thousands of kilometres away, so inertia falls faster as PV rises and synchronous output declines. The Energy Market Authority (EMA) has made grid stability a procurement criterion for large storage, with GFM capability a key requirement, and the planned battery systems on Jurong Island and Tekong Island will serve as a proving ground for GFM on a tropical island grid.

From the grid's point of view, a GFM device behaves like a synchronous generator — even though it contains not one gram of rotating mass.

This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第八章 8.3 + 第十五章 15.5