LONDON, April 29 (Reuters) - The hunt has begun for the

cause of a massive blackout across Spain, Portugal and parts of

France that halted trains, bank machines and traffic lights, in

one of Europe's biggest ever power system collapses.

CYBER ATTACK?

Portugal's Prime Minister Luis Montenegro has said there was

no indication of a cyberattack, but both countries are still

looking at all hypotheses.

EXPLANATIONS SO FAR

Spanish grid operator Red Electrica, in a

statement on Monday night, pointed to a "strong oscillation in

the power flow, which triggered "a very significant loss of

generation."

This loss of generation went beyond what the electrical

systems are designed to handle and the Spanish grid was

disconnected from the European system.

The electrical system then collapsed, leading to voltage

losses in the supply points of both the Spanish and Portuguese

peninsular electrical systems, Red Electrica said.

WHAT CAUSES POWER OUTAGES?

The most common cause of an unplanned power cut which

disables electricity on a large scale is extreme weather such as

storms, lightning strikes or high winds.

They can also happen when there are faults at power

stations, power distribution lines, substations or other parts

of the electricity transmission system or grid.

Most power cuts typically last between a few minutes to

several hours.

WHAT IS THE IBERIAN POWER MIX?

Spain is one of Europe's biggest producers of renewable

energy and Monday's shutdown has already sparked debate about

whether the volatility of supply from solar or wind made its

power systems more vulnerable to such an outage.

Red Electrica data shows solar photovolatic (PV) energy was

providing almost 59% of Spain's electricity at the time of the

blackout, while wind power was providing nearly 12%, nuclear

almost 11% and combined cycle gas plants 5%.

Red Electrica data also shows that within just five minutes

on Monday, between 1230 and 1235 local time (1030-1035 GMT),

solar PV generation dropped from more than 18 GW to just 8 GW.

WHAT FACTORS COULD BE INVOLVED?

A source with direct knowledge of the sector said that at

the time of the outage the Spanish grid was running with very

little "inertia", which is the energy moving in a large rotating

mass like a generator or in some industrial motors.

Inertia helps to stabilize the grid by slowing down the rate

of frequency change when there's a sudden drop or rise in demand

or generation.

"In those conditions (when there is little inertia) if

there's a drop in production for whatever reason, the grid loses

(more) inertia and everything fails. And in a blackout, you need

to rebuild inertia before bringing things back online, which

takes a few hours," the source said, requesting anonymity.

Victor Becerra, professor of power systems engineering, at

the UK's University of Portsmouth, said the Iberian outage

showed the complexity of managing modern energy systems

particularly as they integrate increasing levels of intermittent

renewable energy, such as wind and solar.