section_path: Causes/Inter-area oscillations
section_name: Inter-area oscillations
paragraph_text: Inter-area oscillations
  
  In the 30 minutes before the blackout at 12:33:18 CEST, low-frequency oscillations of the frequency were measured (with phasor measurement units) and damped out. The 0.2 Hz oscillations appear to have been between the Iberian Peninsula and the rest of the European wide grid, as they were in opposite grid phase. Two inter-area oscillations appeared from 12:03:15 to 12:07:40 and from 12:19:01 to 12:22:03. The disconnection from the Central European system occurred at 12:33:19 CEST right after the Spanish system lagged more than 90°, causing instability on the transmission lines.
  
  There were also low frequency voltage oscillations and, after the blackout started, continued frequency oscillations were detected. Along with these, systems could have disconnected due to fast ROCOF (rate of change of frequency) above 1.5 Hz/s, which can be stressful on spinning turbines.
  
  Similar oscillations and disconnection happened in December 2016 and July 2021, though with less severe adverse effects. Then, the North-South "ambient" frequency was 0.25 Hz, while now it appears to be lower at 0.217 Hz, likely due to the recent addition of the Baltic states and Ukraine to the Continental Europe Synchronous Area (CESA).