Why frequency is the grid’s key number
A frequency of 50 Hz means that, at this very second, generation in the system equals demand. If demand jumps or a plant trips, frequency falls; if there is more generation than needed, it rises. The transmission system operator — Moldelectrica in Moldova — has to bring it back to 50 Hz as fast as possible.
To do that, the operator relies on reserves: capacity that balancing service providers keep on standby. Reserves work like a relay race, each layer taking over from the one before.
Four layers of reserve
FCR — frequency containment reserve. Responds automatically within seconds of a deviation. Its job is to stop frequency from falling or rising further, not to restore it.
aFRR — automatic frequency restoration reserve. Takes over from FCR after about 30 seconds. It is driven by the operator’s automatic controller, which continuously sends assets a setpoint — how much power to add or remove.
mFRR — manual frequency restoration reserve. Activated on the dispatcher’s command within minutes when a deviation persists. It frees aFRR for the next event.
RR — replacement reserve. The slowest layer: it replaces reserves already in action so the system is ready for the next surprise.
What is happening in Moldova
In March 2022 Moldova’s power system, together with Ukraine’s, was synchronised in an emergency with the European ENTSO-E grid. Operating in sync with Europe raises the bar for reserves. According to ANRE and Moldelectrica data cited by Renergy, aFRR in Moldova is practically zero today, and an estimated 246 MW of storage is needed for balancing: 72 MW of aFRR and 174 MW of mFRR. Moldelectrica is an observer in ENTSO-E, and full membership depends, among other things, on reserves.
Where batteries come in
A turbine needs time to ramp up or down. A battery switches from charging to discharging in a fraction of a second and follows a setpoint precisely. That makes battery energy storage (BESS) the best-suited source for FCR and aFRR.
The first step has been taken: on 11 June 2026 Moldelectrica activated balancing services from hybrid PV + storage plants at Rădeni (50 MW) and Negureni (40 MW) for the first time, up to 25 MW.
Who can become reserve
Not just batteries. Solar plants with storage, flexible industrial load, CHP and gas engines can also provide reserve — on their own or within an aggregated portfolio. To take part, an asset passes the operator’s prequalification tests.
Blackfield is not a balancing service provider; how we run storage on the day-ahead market is on the arbitrage and aggregation page. Terms are in the glossary.
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