The blackout affected large parts of Spain and Portugal, and briefly affected 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.
