*

REE did not have enough thermal power stations switched on

when

surge occurred

*

Government to publish extensive report on causes of

blackout on

Tuesday

*

Some power plants required by law to regulate the grid's

voltage

also failed to do so

*

Blackout on April 28 caused gridlock in cities and left

thousands stranded in trains and lifts

(Recasts lead, adds Aagesen comment)

MADRID, June 17 (Reuters) - Spain's power grid operator

REE miscalculated its power capacity needs on the day that a

surge in voltage caused a massive blackout across the Iberian

peninsula in April, a government investigation has concluded.

REE did not have enough thermal power stations switched on

during peak hours of April 28 when the surge caused a chain

reaction leading to the power outage, Spain's Energy Minister

Sara Aagesen said on Tuesday.

"The system did not have sufficient dynamic voltage control

capacity," Aagesen told a news briefing in Madrid.

REE "told us that they made their calculations and estimated

that (switching on more thermal plants) was not necessary at

this time. They only set it for the early hours of the day, not

the central hours."

The blackout that lasted for several hours caused massive

gridlock in cities and left thousands stranded on trains and in

elevators across the Iberian peninsula. An extensive report on

the investigation, which will be made public later on Tuesday,

found that some power plants that are required by law to

regulate the grid's voltage failed to do so in the moments

before the blackout.

Power plants "should have controlled voltage and, moreover,

many of them were economically remunerated to do so. They did

not absorb all the reactive power that was expected in a context

of high voltages," she added.

Europe electricity grids' frequency or voltage is maintained

at 50 Hertz (Hz) to ensure stability. Even sight deviations can

lead to damage of equipment and infrastructure.

Keeping it stable requires electricity generation companies

to adjust their output to match demand and the grid operator

usually has tools to inform them when to do so by monitoring

continually the status of the frequency and knowing when demand

might be higher or lower. A drop in frequency shows demand has

exceeded supply and a rise shows supply is not matching demand.

The investigation found no evidence of a cyberattack, she

added.