System imbalance / FRCE

System imbalance is the net mismatch between scheduled and actual electricity flows in an LFC area or block at any moment. The Frequency Restoration Control Error (FRCE), defined in Article 3(2)(31) of the System Operation Guideline (Regulation (EU) 2017/1485), is the control signal that drives aFRR and mFRR activation: FRCE = Area Control Error (ACE) for an LFC area, or the frequency deviation if the area covers the whole synchronous area. Estonia is one LFC area within the Baltic LFC block since 9 February 2025.

FRCE (Frequency Restoration Control Error) is the control signal that drives aFRR and mFRR activation. It is defined in Article 3(2)(31) of the System Operation Guideline (Commission Regulation (EU) 2017/1485) as the control error of the Frequency Restoration Process — equal to the Area Control Error (ACE) of an LFC area, or to the frequency deviation if the LFC area covers the whole synchronous area.

How it is computed

Inside an LFC area: ACE = ΔP + K·Δf, where ΔP is the deviation between scheduled and actual cross-border power exchange, K is the LFC area's frequency-bias factor (MW per Hz), and Δf is the frequency deviation from 50 Hz. A positive FRCE means the area is exporting more than scheduled (or under-frequency); negative FRCE means the opposite. aFRR is dispatched continuously by a closed-loop controller that drives FRCE toward zero; mFRR is called manually when FRCE persists.

SO GL targets

SO GL Article 128 sets target FRCE quality for each synchronous area. For Continental Europe and Nordic: time outside the Level 1 range must be below 30% of year intervals; time outside Level 2 below 5%. Numerical Level 1 / Level 2 values are defined in each LFC block's Operational Agreement and depend on block size and reserve dimensioning.

Baltic LFC block

Since 9 February 2025, Estonia, Latvia and Lithuania form the Baltic LFC block inside CESA. Each TSO operates its own LFC area and computes its own FRCE; the block-level FRCE drives joint reserve activation. Elering submits Estonia's aFRR demand to PICASSO and mFRR demand to MARI; IGCC nets demand against other CESA areas before any cross-border activation occurs.

Sources

System Operation Guideline (Reg. (EU) 2017/1485) · ENTSO-E: System Operation · Baltic Transparency Dashboard

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What is system imbalance?

System imbalance is the net mismatch between scheduled and actual electricity flows in an LFC area or block at any moment. When generation falls below schedule (or consumption exceeds it), the system is short and frequency drifts below 50 Hz. When generation exceeds schedule, the system is long. The TSO continuously activates aFRR and mFRR to bring the imbalance back to zero.

What is FRCE?

FRCE (Frequency Restoration Control Error) is the specific control signal defined in System Operation Guideline (Reg (EU) 2017/1485) Article 3(2)(31) that drives aFRR and mFRR activation. For an individual LFC area, FRCE equals the Area Control Error (ACE). For an LFC block (like the Baltic block: EE+LV+LT), FRCE is the aggregate signal across the constituent areas. Article 152 binds aFRR activation to FRCE.

What's the difference between system imbalance, FRCE and ACE?

System imbalance is the broad concept (the net schedule-vs-actual mismatch). ACE is the LFC-area-specific measurement combining cross-border interchange deviation and frequency-bias term. FRCE is the formal control signal, equal to ACE for an LFC area or to the frequency deviation if the area covers a whole synchronous zone. In practice, all three describe the same phenomenon at different abstraction layers.

How is Estonia's LFC structure organised?

Since 9 February 2025, Estonia is one LFC area within the Baltic LFC block (EE+LV+LT), which sits within the Continental European synchronous area. Each Baltic TSO computes its own area FRCE; the three combine into a Baltic-block FRCE that drives the Coordinated Balancing Area activation logic through MARI and PICASSO.