A power surge that caused a widespread blackout in Spain and Portugal was the most severe in Europe in the last 20 years, and the first of its kind, a report has found.
Damian Cortinas, president of the association of electricity grid operators Entso-e, said the incident was the first known blackout to be caused by overvoltage, which occurs when there is too much electrical voltage in a network.
This is new territory, Cortinas said, adding the role of Entso-e was not to apportion blame to any party over the cause.
April's outage caused significant disruption for nearly a day when it plunged areas into darkness, cutting internet and telephone connections and halting transport links.
The blackout affected large parts of Spain and Portugal, and briefly impacted southwestern France.
The report, released on Friday, focused on the condition of the power systems on the day of the outage and the sequence of events leading up to it.
A series of cascading overvoltages - an increase in the electrical supply voltage above the established norm - was behind the outage, it concluded.
Overvoltage can be caused by surges in networks due to oversupply or lightning strikes, or when protective equipment is insufficient.
According to the report, automatic defence plans were activated but could not prevent the power system shutting down.
It follows several separate investigations and reports by the Spanish government, as well as power and grid companies. The national energy watchdog and Spanish lawmakers are also conducting separate investigations.
The Spanish government believes the Entso-e report supports its own findings.
Sara Aagesen, minister for ecological transition, said it was completely in line with the results of an investigation it commissioned which concluded in June that both the national grid provider Red Eléctrica and private electricity companies were at fault.
Both Red Eléctrica and the private firms have insisted that they were not to blame. Redeia, owner of Red Eléctrica, has blamed the blackout on some coal, gas and nuclear power plants' failure to help maintain appropriate voltage.
Spanish utilities said it was caused by poor planning from grid operators.
The Entso-e report also said some important data was missing and that collecting complete, high-quality data proved very challenging for this investigation.
A final report, to be published in the first quarter of next year, will investigate the root causes of the overvoltage and the actions employed to control voltage in the system.
The outage triggered a broader debate about Spain's energy model.
The opposition suggested an increasingly heavy reliance on renewable energy could have been a factor in causing the blackout and the country's decreasing supply of nuclear energy meant a dependable backup was not available.
The government roundly rejected these theories, and the new report avoided taking sides when it came to the causes of the blackout.
The loss of power forced Madrid Open Tennis organisers to stop a match mid-play, with power plants automatically shutting down during the incident.
Buildings were plunged into darkness while transport systems failed, resulting in significant disruption across the capital.
Emergency workers responded to numerous calls for assistance, rescuing people trapped in elevators and halting routine hospital operations in response to the crisis.
