Despite initial challenges in collecting data from several DSOs and generation companies in Spain, the Expert Panel was able to confirm that all parties have now provided data.
Based on the data available so far, the Expert Panel said it was able to offer additional information on a number of issues, such as a more detailed description of the forced oscillations (ie, oscillations arising from an external force) observed around 12:05 pm in southern Spain and of the counter-measures taken to mitigate them. The Expert Panel also offered new preliminary information related to the generation trips that took place just before the blackout.
The Expert Panel is looking particularly at the cascading series of generation disconnections and voltage increases as the most probable trigger for the blackout. Such cascading voltage increases have never before been linked to a blackout in any part of the European power system, the Expert Panel notes. If confirmed, this high-voltage blackout mode will require a thorough analysis and investigation by all power system experts of the ENTSO-E community, the Panel asserts.
The exceptional character of the incident highlights the need to improve the resilience of the power system in case of cascading voltage increases, the Expert Panel argues, and ENTSO-E sees at least two important lines to consider:
- enhancement of voltage control management procedures and capabilities of all active actors of the electricity system to prevent such major voltage-related incidents in the future;
- assessment of how the system defence plans can better protect the European power system against this new type of phenomenon.
System conditions prior to the blackout
During the night of 27 to 28 April, the Iberian power system operated normally, without notable variations in the voltage profile. From approximately 09:00 CEST, the variability of the voltage in Spain started increasing but without significant variations until 10:30 CEST. From 10:30 CEST, more significant voltage variations started to occur. But the voltage in the transmission system remained below the upper operational limits.**
During the half hour preceding the blackout (12:00 to 12:30 CEST), two main periods of oscillations (power, voltage and frequency swings) were observed in the Continental Europe Synchronous Area (CE SA).
The first one took place from 12:03 to 12:07 CEST.
A preliminary analysis of the available information, says the Expert Panel, indicates that this was a local, forced oscillation (ie, induced by an external source, namely a power plant), with a dominant frequency of 0.64 Hz, primarily affecting the Spanish and Portuguese power systems. The forced oscillation also excited an inter-area mode East–Centre–West oscillation (0.21 Hz) at small amplitude.
Mode shapes (a technique applied to WAMS measurements that is able to show the “direction” of an oscillatory mode) applied to this first oscillation suggest that this was a local mode between two generation clusters in the Iberian Peninsula system, one in the north-east part of Spain and another in the south-west of Spain
and Portugal.
In order to damp these oscillations, the operators in the control rooms of the relevant TSOs took several mitigating measures (as defined in the established operation protocols):
- Lines were switched on to decrease system impedance and improve generator stability.
- Fixed power operation mode was set up on the HVDC link between Spain and France as this is an effective measure for mitigating oscillations.
- The power flow between Spain and France was reduced, as an additional countermeasure to decrease the Iberian centre of inertia angle relative to the rest of the Continental Europe power system.
In addition, shunt reactor manoeuvres were carried out to recover voltages, which reached low values briefly during the oscillations.
Several electricity producers connected to the Spanish transmission system confirmed that these oscillations were clearly visible at their generators.
The second oscillation in the half hour prior to the blackout occurred between 12:19 and 12:22 CEST. This was also an inter-area oscillation, with a dominant frequency of 0.21 Hz, corresponding to what the Expert Panel describes as the well-known East–Centre–West Continental mode.
The mode shapes of the second oscillation with its oscillation vectors aligned in the same direction, clearly demonstrate the characteristics of an inter-area oscillation, according to the Expert Panel. In fact, the Panel notes, the whole Iberian Peninsula oscillates in a coherent way and similar amplitude against the rest of the continental European grid.
This second oscillation was effectively mitigated through further countertrading measures, which again reduced power flows between Spain and France, and also with the coupling of internal power lines in the south of Spain.
Following the second oscillation, the voltage was within the range 390–420 kV, before increasing again, but still within the operational voltage range for the transmission network. At that moment, Spain’s international scheduled exchanges – all in the export direction – were 1000 MW to France, 2000 MW to Portugal and 800 MW to Morocco.
Sequence of events during the incident
Around the time when the net active power exchange position of Spain began to decrease, the voltage started to rise. The Expert Panel is examining if and to what extent the rise in voltage is related to the following possible causes (among others): (i) the reduction of reactive power absorption by generators which decreased their active power when operating with a fixed power factor; and/or (ii) the reduction of reactive power absorption by transmission lines, associated with their loading; and/or (iii) increased reactive power injection from distribution systems. This potential correlation could have been exacerbated, says the Expert Panel, by the fact that the reduction of active power (and hence of reactive power absorption) took place mainly in the southern part of the system, leading to a longer low-loaded transmission path for exports towards France.
According to the Expert Panel, the currently available preliminary data indicates the following sequence of events taking place after the increase of voltage between 12:32:00 and 12:32:57 CEST. Prior to this sequence of events, the voltage in the transmission network was below the upper operational limit, the Expert Panel notes.
Events at 12:32:57, 12:33:16 and 12:33:17 CEST
Loss of generation was experienced in the Granada, Badajoz and Sevilla regions, amounting to an estimated total of about 2200 MW.
The first event was due to the tripping of a generation transformer, resulting from a problem on the lower voltage side, in the area of Granada. This transformer connected different generation facilities (PV, wind and solar thermal) to the transmission grid and at the time was injecting 355 MW.
The Expert Panel notes that the tripping of this transformer and consequently the loss of infeed options also explains the disconnections of some of the generation units connected to the transformer due to over-frequency, with others tripping due to overvoltage, as reported by the operators of the power plants.
The second event included trips of PV and solar thermal facilities connected to two 400 kV transmission substations, in the Badajoz area, with a total interrupted injection of around 720 MW.
The third event included several trips, in different areas, in less than one second: wind farms in Segovia and Huelva; photovoltaics in Badajoz, Sevilla, Caceres and Huelva; solar thermal in Badajoz; and other generators in a variety of locations. This amounted to more than 1100 MW.
The causes of these three events are still under investigation, notes the Expert Panel.
As a result of these events a voltage increase was observed in Spain, leading to a similar increase in Portugal, while the frequency decreased.
Events between 12:33:18 and 12:33:21 CEST
Voltage in southern Spain increased sharply, and consequently also in Portugal.
The over-voltage triggered a cascade of generation losses that caused the frequency of the Spanish and Portuguese power system to drop.
Event at 12:33:19 CEST
The power systems of Spain and Portugal started to lose synchronism with the European system.
Events between 12:33:19 and 12:33:22 CEST
The automatic load shedding and system defence plans of Spain and Portugal, elaborated in accordance with Commission Regulation (EU) 2017/1485 (2 August 2017), were activated but were unable to prevent the collapse of the Iberian power system.
Event at 12:33:21 CEST
The AC overhead lines between France and Spain were disconnected by protection devices against loss of synchronism.
Event at 12:33:24 CEST
All system parameters of the Spanish and Portuguese electricity systems collapsed, and the HVDC lines between France and Spain stopped transmitting power.
Rate of Change of Frequency (RoCoF) in the moments before the blackout remained below 1 Hz/s up until 12:33:20.400, when the frequency was already around 49 Hz. After that, the value of RoCoF exceeded 1 Hz/s.
Restoration process
Following the blackout, each impacted TSO immediately activated its respective system restoration plans, elaborated in accordance with Commission Regulation (EU) 2017/2196 establishing a network code for dealing with an electricity emergency and restoration, as well as any other relevant procedures and protocols for restoring the voltage of the electricity system, the Expert Panel notes.
Power system restoration in some regions of the Portuguese and Spanish systems was facilitated by, among other things, activation of power system resources such as black-start capabilities in certain power plants, as well as by the existing interconnections with France and Morocco.
The rapid restoration of supply demonstrated the preparedness and efficiency of the affected TSOs, Red Eléctrica and REN, says the Expert Panel, with support from and collaboration with the French TSO RTE, as well as the Moroccan utility ONEE. According to the Expert Panel, this was made possible by the joint work and co-operation among TSOs developed over the years, both between the control centres and within ENTSO-E. In addition, the real-time monitoring of the status of the European electricity systems and co-ordination among TSOs made use of the European Awareness System platform, a tool developed by all TSOs within ENTSO-E.
*The joint Expert Panel was set up two weeks after the 28 April Iberian blackout. It was set up in line with Article 15(5) of Commission Regulation (EU) 2017/1485 (2 August 2017) establishing a guideline on electricity transmission system operation and the Incident Classification Scale (ICS) methodology. The ICS methodology is the framework for reporting and classifying incidents in the power system and for organising the investigation of such incidents.
When the incident is classified according to ICS methodology criteria as scale 3 – blackout – the Expert Panel is tasked to investigate the root causes, produce a comprehensive analysis, and make recommendations in a final report which will be published.
The Expert Panel consists of representatives from TSOs, the Agency for the Cooperation of Energy Regulators (ACER), National Regulatory Authorities (NRAs), and Regional Coordination Centres (RCCs).
The Panel is led by experts from TSOs not directly affected by the incident and includes experts from both affected and non-affected TSOs.
**420 kV is the upper voltage range limit for normal operation in Portugal. In Spain, the following limits apply in accordance with the official Operational Procedure 1.4: “Voltages at the nodes – under normal operating conditions, the voltage at the 400 kV level at the connection nodes will be between 390 and 420 kV. At the 220 kV level, the voltage will be between 205 and 245 kV. Eventually, maximum values of up to 435 kV and minimum values of up to 375 kV may occur at the 400 kV level. At the 220 kV level, voltages may eventually drop to 200 kV. Any installation directly connected to the transmission network must be able to withstand these values without damage or disconnection.”
