Frequency response mitigation
Start only after the design event and the failing metric are explicit: which contingency, and which of nadir, RoCoF, or settling frequency violates its limit.
Preferred action order
- Confirm the study setup is credible: largest credible loss, load-damping assumption, governor models actually enabled and responsive (deadbands, baseload flags), and the inertia of the dispatch being studied.
- Increase responsive headroom first: commit or back down units to create governor headroom, and enable fast frequency response from storage, IBRs, or demand response — MW that respond in the first seconds matter most.
- Raise inertia where headroom alone is insufficient: additional synchronous units, synchronous condensers (with flywheels where available), and must-run rules for low-inertia hours.
- Fix control settings: droop, deadband, and response-withdrawal behavior so primary response is sustained, not transient.
- Re-coordinate UFLS last — it is the backstop that protects the system when the fixes above fall short, not the fix itself.
Working rules
- Evaluate the worst inertia hour (high renewables, light load), not the average dispatch.
- Screen with system-level metrics, then confirm the critical cases in time domain (ANDES, PSS/E) before claiming the fix works.
- Track the response trajectory: a good nadir with rapid response withdrawal can still fail the settling-frequency requirement.
- Frequency containment and oscillation damping are different problems; route poorly damped responses to
dynamic-stability-mitigation.
Deliver
- The design event and the violated metric (nadir, RoCoF, settling frequency) with values.
- The mitigation sequence: headroom, fast response, inertia, control fixes.
- The first credible fix and the time-domain validation still needed.