The energy transition is reshaping power grids at a pace that exposes structural weaknesses that were less apparent when conventional generation dominated the system. As large power plants retire and renewable penetration increases, system operators must manage a network with lower inherent inertia and a growing share of inverter-based resources. This reduced inertia poses a significant challenge to grid stability: frequency variations caused by momentary imbalances become larger and more frequent.
Expert report identifies causes of Iberian blackout
Traditional power systems rely on large rotating machines that convert mechanical energy into electrical energy. When a generator trips, the kinetic energy stored in the remaining connected machines provides the inertia that slows the frequency decline, buying time for the system to respond to the lost active power. The progressive replacement of large synchronous generators therefore confronts transmission system operators (TSOs) with a growing challenge: ensuring grid reliability and system stability.
One well-proven solution is the synchronous condenser (SC), a large rotating electrical machine connected to the grid. Unlike motors that drive loads or generators powered by turbines, synchronous condensers are controlled through excitation regulation to either generate or absorb reactive power as needed, enabling real-time voltage regulation. Their rotating mass also adds inertia, supporting frequency stability, while their stored rotational energy supplies short-term fault current, strengthening the grid's short-circuit capacity.
However, TSOs increasingly require additional functionality from inverter-based resources, accelerating adoption of concepts such as smart grids, grid-forming and black-start. In practice, this means battery energy storage units and their inverters must evolve from passive energy reservoirs into active grid participants.
Two modes with very different capabilities
From the grid's perspective, a power converter can operate in two modes: grid-following, in which it synchronises with an existing grid, and grid-forming, in which it can create and regulate the grid itself whenever necessary. The plant controller determines the appropriate operating mode based on grid conditions and can command the converter to switch seamlessly between the two in as little as 100 microseconds, enabling an immediate response to changing system conditions. In these modes, the equipment can provide several grid-support functions, also known as ancillary services.
In grid-following mode, the power converter behaves as a current source: the grid sets the voltage, and the inverter control stays synchronised to it. Functions such as frequency or voltage regulation are achieved by controlling active and reactive power injection. This operating mode is sufficient for many grid-connected applications and performs effectively when a strong grid reference already exists.
PV grows up: from generation to integrated energy
Without large rotating shafts, a conventional grid-following inverter provides no inherent inertial response, and while fast frequency response can be added through control software, it is measurement-based and therefore inherently delayed. Just as important, and often overlooked, is voltage. As synchronous machines are displaced, the grid loses not only inertia but also short-circuit strength and reactive-power support at precisely the locations where these are needed most. This is what makes a grid "weak," and it is the condition under which conventional inverter control begins to struggle.
Grid-forming mode represents a different operating philosophy. Here, the converter behaves as a controlled voltage source, establishing and regulating local voltage and frequency references while adapting to external setpoints. Grid-forming converters can provide many of the same ancillary services as their grid-following counterparts, but they also perform under weak-grid conditions and can run islanded networks where required.
Irena: 24/7 case for renewables no longer theoretical
One common implementation is the virtual synchronous machine (VSM) control strategy, in which inverter controls emulate selected dynamic characteristics of synchronous generators. The key distinction lies in behaviour during disturbances: grid-following converters react to grid conditions, whereas grid-forming converters actively support and stabilise the grid. Through advanced control strategies, grid-forming units can emulate inertial and damping responses and deliver faster active power support during frequency events. As renewable penetration grows, these capabilities are becoming increasingly important for maintaining secure and resilient grid operation.
Black-start: the ultimate resilience capability
Beyond supporting the grid during normal operation and disturbances, grid-forming capability enables an even more critical function: black-start, the ability to energise and restore a power system from a fully de-energised state without relying on an existing grid voltage reference. The function is closely linked to islanded grid-forming operation, in which the converter independently establishes voltage and frequency and progressively rebuilds the local network. To perform black-start successfully, the converter station requires an independent auxiliary power source for essential equipment, including controls, power supplies, communication links and cooling infrastructure. This auxiliary source may come from a UPS, a dedicated backup unit or an external station service supply.
Once black-start has succeeded, the converter operates in grid-forming mode and other power converters connected to the same grid can begin grid-following operation. Historically, black-start capability has been associated primarily with large hydroelectric or gas turbine plants. As those assets retire or run less frequently, the question of who restores the grid after a major blackout becomes urgent, and battery storage is increasingly the answer, provided the inverter and control architecture can support the full sequence reliably. (Daniel Gerber/hcn)
