2025 Iberia Blackout Report [pdf]

> The ultimate cause of the peninsular electrical zero on April 28th was a phenomenon of overvoltages in the form of a "chain reaction" in which high voltages cause generation disconnections, which in turn causes new increases in voltage and thus new disconnections, and so on.

> 1. The system showed insufficient dynamic voltage control capabilities sufficient to maintain stable voltage

> 2. A series of rhythmic oscillations significantly conditioned the system, modifying its configuration and increasing the difficulties for voltage stabilization.

If I understand it correctly (and like software, typical), it was a positive feedback-loop. Since there wasn't enough voltage control, some other station had to be added but got overloaded instead, also turning off, and then on to the next station.

Late addition: It was very helpful for me to read through the "ANNEX X. BRIEF BASICS OF THE ELECTRIC SYSTEM" (page 168) before trying to read the report itself, as it explains a lot of things that the rest of the report (rightly) assumes you already know.

https://www.boe.es/buscar/doc.php?id=BOE-A-2000-5204

7.1(b) seems to be saying that generators connected at 200kV adjust their reactive power generation/absorption in real time according to the voltage they observe, based on a lookup table provided by the grid operator.

This seems sort of sensible according to my limited understanding of the theory of AC grids. You can write some differential equations and pretend everything is continuous (as opposed to being a LUT with 11 steps or so), and you can determine that the grid is stable.

However, check out this shorter report from red eléctrica:

https://d1n1o4zeyfu21r.cloudfront.net/WEB_Incident_%2028A_Sp...

Apparently these 220kV plants are connected to the 400kV grid via transformers in substations that are not owned by the generator operators. And those transformers have “tap changers” that attempt to keep the 220kV secondary side at the correct voltage within some fairly large voltage range on the 400kV side. Won’t this defeat the voltage control that the 220kV generators are supposed to provide? If the grid voltage is high, then absorption of reactive power is needed [0], and the generators are supposed to determine that they need to absorb reactive power (which they can do), but if the tap changer changes its setting, then the generator will not react correctly to the voltage on the 400kV side.

In other words, one would like the generator to absorb reactive power according to P_reactive(primary voltage • 220/400), but the actual behavior is P_reactive(primary voltage • 220/400 • tap changer position), the tap changer position is presumably something like 400/primary voltage, and I don’t understand how the result is supposed to function in any useful way. Adding insult to injury, the red eléctrica repoet authors seem to be suggesting that a bunch of tap changers operators didn’t configure their tap changes well enough to even keep secondary voltages in range.

Does anyone with more familiarity with these systems know how they’re supposed to work?

[0] I can never remember the sign convention for reactive power.

- Fast Frequency Response (FFR), sub-second power adjustment following frequency table

- Frequency Containment Reserve (FCR), ~second power adjustment following frequency table

- Automatic Frequency Restoration Reserve (aFRR), ~second energy production following TSO setpoint signal

- Manual Frequency Restoration Reserve, ~minute energy production following TSO activation signals

My understanding is the primary failure in Spain was that 9 separate synchronous plants that had sold aFRR(?) to the TSO then failed to deliver, so when the TSO algorithms tried to adjust the oscillations, nothing happened. Everything else was kinda "as designed".