Explainer-What caused the Iberian power outage and what happens next?

By Susanna Twidale and Nina Chestney

LONDON (Reuters) -Spain's government and its grid operator have issued separate findings into the causes that led to the massive blackout across Spain and Portugal on April 28, which caused gridlock in cities and left thousands stranded on trains and in elevators across the Iberian Peninsula.

WHAT CAUSED THE BLACKOUT?

The Spanish government said in a report on Tuesday that Spain's grid operator Redeia miscalculated the correct mix of energy in the system. The government also blamed some conventional power plants, or thermal power plants using coal, gas and nuclear, for failing to help maintain an appropriate voltage level and as a result, the grid was unable to cope with a surge in voltage that triggered a cascade of power plant disconnections, ultimately leading to the outage.

Voltage - the force that drives electric current - must be kept within a safe range to maintain grid stability.

Redeia said on Wednesday that a surge in voltage was the immediate cause of the outage but blamed it on conventional power plants failing to control the voltage level. It pointed instead to anomalies in the disconnection of power plants on April 28 and an unexpected spike in electricity demand from the transport network.

Redeia rejected the claim that its energy mix miscalculation was a key factor.

WHAT SHOULD HAVE HAPPENED?

Grid operators typically use a raft of tools to ensure power systems have the right frequency, voltage and supply to keep them stable.

The government report said the number of generators the grid had available to provide voltage control on April 28 was lower than it had in previous weeks and that not all units that should have responded did so as expected.

The companies operating the plants that failed to ensure the proper voltage controls have not been named and Redeia's report also did not identify them.

Spain's Energy Minister Sara Aagesen told a news briefing in Madrid that the grid operator did not have enough capacity to regulate voltage.

However, Redeia's operations chief Concha Sanchez said on Wednesday that based on the grid's calculations the grid had planned adequate voltage support, but some plants did not respond as expected.

WHERE RENEWABLES TO BLAME?

No. Both the government and Redeia said renewable energy sources were not responsible for the blackout.

Spain is one of Europe's biggest producers of renewableenergy and has a high share of solar power, which accounted for 59% of the country's electricity at the time of the blackout.

"Had conventional power plants done their job in controlling the voltage there would have been no blackout," Redeia's Sanchez said.

DID THE FRENCH POWER LINK PLAY A ROLE?

At the time of the outage, Spain was also exporting power to France and Portugal.

Energy Minister Aagesen explained that at 12:03 p.m., an "atypical" oscillation was detected in the power system. In response, the grid operator implemented standard control procedures, including reducing electricity exports to France. While these actions successfully mitigated the oscillation, they also caused a secondary effect: an increase in voltage, according to the report.

WHAT HAPPENS NEXT?

The government said on Tuesday it will propose measures to strengthen the grid and improve voltage control. It also plans to better integrate the peninsula with the European grid, it said.

Redeia said it will issue its full report later on Wednesday. The government report will go to the European Network of Transmission System Operators which is doing its own inquiry.

Any parties found responsible for the blackout may be liable for losses incurred during the outage, subject to any legal action.

(Reporting by Susanna Twidale and Nina Chestney; additional reporting by Pietro Lombardi and Joan Faus in Madrid; Editing by Louise Heavens)