Eirgrid Fast Frequency Response And Battery Inverter Guide

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Key Takeaways

• Transmission system nominal frequency will for the purpose of this document be considered to be 50 Hz.
• A Frequency Event occurs when frequency deviates by 0.3 Hz, falling below 49.7 Hz or rising above 50.3 Hz.
• Providing units must maintain an under-frequency reserve trigger capability with an upper threshold of 49.80 Hz and an over-frequency response trigger lower threshold of 50.20 Hz.
• BESS inverters must deliver their expected active power response within 0.3 s of the transmission system frequency falling through the reserve trigger.
• Units must demonstrate an FFR trajectory capability of 300 mHz and maintain response across the primary FFR period from T+0 seconds to T+10 seconds.
• Step-based responding units must deliver output in at least 10 discrete steps with no individual step greater than 5 MW.
• Battery sites require real-time state of charge signals and high-speed monitoring equipment with a minimum data resolution of 20 ms.

The Role of Fast Frequency Response in Low-Inertia Grids

As Ireland integrates historic volumes of non-synchronous wind and solar generation, system inertia decreases. To prevent catastrophic Rate of Change of Frequency (RoCoF) following generator trips, EirGrid and SONI procure fast-acting reserve through the DS3 programme.

System dynamics dictate rigorous operating baselines:

• Nominal Frequency: Under the DS3 System Services Protocol, the nominal frequency will for the purpose of this document be considered to be 50 Hz.
• Significant Disturbance Definition: A disturbance constitutes an official event based on a deviation in Transmission System Frequency of 0.3 Hz which if exceeded denotes that a Frequency Event has occurred.
• Frequency Event Boundaries: Specifically, a Frequency Event is therefore deemed to have occurred if the Transmission System Frequency falls below 49.7 Hz or rises above 50.3 Hz.

Inverter Response Parameters and Trigger Windows

Battery energy storage system (BESS) inverters operate under parameterised frequency response curves programmed to trigger automatically without operator intervention:

• Under-Frequency Triggering: The Providing Unit must maintain the capability to operate at its Under-Frequency Reserve Trigger Capability, which shall have an upper threshold of 49.80 Hz.
• Over-Frequency Triggering: At times of excess generation, the Providing Unit must maintain the capability to operate at its OFR Trigger Capability, which shall have a lower threshold of 50.20 Hz.
• Activation Speed: The Providing Unit shall provide its Expected response within 0.3 s of the Transmission System Frequency falling through its Under-Frequency Reserve Trigger.
• Trajectory Capability: Inverters must be capable of dynamic modulation, operating with an FFR Trajectory Capability of 300 mHz in response to an Under-Frequency Reserve Trigger.
• Operating Period: The official FFR period is taken to be between T+0 seconds to T+10 seconds after Time Zero for a given Frequency Event, with energy recovery evaluated from T+10 to T+20 seconds.

Grid Synchronization and Operational Constraints

To guarantee grid stability during fast dispatch, battery inverters must satisfy strict operational parameters:

Compliance with these parameters ensures that battery storage assets provide robust synthetic inertia and frequency containment, safeguarding the Irish transmission network.