By Pietro Lombardi

MADRID (Reuters) -Spanish grid operator Redeia blamed power plants for the massive blackout that affected the Iberian peninsula in April, as it disputed a government report that said its failure to calculate the correct energy mix was a key factor.

REE-owner Redeia's own investigation discovered anomalies in the disconnection of power plants on April 28 even though voltage in the system was within legal limits, operations chief Concha Sanchez told a news briefing on Wednesday.

A combined-cycle plant that was supposed to provide stability to the system disconnected in the first seconds of the blackout when it should not have, while there was also an anomalous growth in demand from the transport network, she said.

"Based on our calculation, there was enough voltage control capabilities planned" by Redeia, she said. "Had conventional power plants done their job in controlling the voltage there would have been no blackout."

Aelec, which represents Spain's main electricity companies including Iberdrola and Endesa, said in a statement on Tuesday it agreed that voltage control was the main cause of the outage, but said that, as system operator, Redeia was ultimately responsible for controlling voltage.

The government's report released on Tuesday said Redeia's miscalculation was one of the factors hindering the grid's ability to cope with a surge in voltage that led to the outage that caused gridlock in cities across the Iberian peninsula and left tens of thousands stranded on trains overnight or stuck in lifts.

But Sanchez said the system was in "absolutely normal conditions" at noon just before the blackout and that adding another gas plant to the system to absorb additional voltage would have made no difference.

Redeia, which is partly state-owned, will release its own full report on the causes of the outage, its chair, Beatriz Corredor, told the same news briefing.

Corredor said she had absolute faith in Redeia's calculations and that the operator had complied with all procedures and rules.

(Reporting by Pietro Lombardi, writing by Inti Landauro and Charlie Devereux, editing by Andrei Khalip and David Evans)