EXPLAINER-What could be behind the Iberian power outage?

By Nina Chestney

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?

Spanish grid operator Red Electrica REDE.MC ruled out a cyber attack as the cause but Spain's High Court said it would open an investigation to determine the cause.

Spain's Prime Minister Pedro Sanchez said on Tuesday that even if Red Electrica had ruled out a cyber attack on its systems, it did not mean one could not have happened.

EXPLANATIONS SO FAR

Red Electrica said it had identified two incidents of power generation loss, probably from solar plants, in southwestern Spain that caused instability in the electric system and led to a breakdown of its interconnection with France. The electrical system collapsed, affecting both the Spanish and Portuguese systems.

Spain was exporting power to France and Portugal at the time of the outage. Exports to France were close to the available net export capacity until 1000 local time. According to Red Electrica data, exports to France stopped at 1235 local time from 868 MW beforehand.

The European Union will initiate a thorough investigation of the power outages in Spain and Portugal on Monday, EU Energy Commissioner Dan Jorgensen said on Tuesday.

The Spanish grid is interconnected with those of France, Portugal, Morocco and Andorra.

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. The weather at the time of Monday's collapse was fair.

Power outages can also happen when there are faults at power stations, power distribution lines, substations or other parts of the system.

Electricity flow between systems in Europe is maintained at 50 Hertz (Hz) to ensure stability. If that level varies, backup systems disconnect assets such as power generation sources from the grid to protect them from damage.

SPAIN'S ENERGY MIX

Spain is one of Europe's biggest producers of renewable energy, relying on wind and solar for 43% of the total, well above the global average, data from think tank Ember shows.

Solar photovoltaic (PV) accounted for 59% of Spain's electricity at the time of the blackout, wind nearly 12%, nuclear almost 11% and combined cycle gas turbine (CCGT) plants 5%, Red Electrica data showed. This compares with solar PV being 50%, wind 3%, nuclear nearly 15% and CCGT nearly 11% on the same day last year.

However, in a span of just five minutes, between 1230 and 1235 local time (1030-1035 GMT) on Monday, solar PV generation plunged by more than 50% to 8 gigawatts (GW) from more than 18 GW, the data showed. The cause is unknown.

WHAT FACTORS COULD BE INVOLVED?

Leading up to the outage, experts and industry sources said the Spanish grid was running with very little inertia, which is energy stored in a large rotating steam or gas turbine driving and rotating generators which acts as a buffer as it can quickly be used to compensate for sudden changes in demand or supply.

Solar, by contrast, provides little rotating mass to the grid.

Gas-fired power generation accounted for around 5% of electricity generation on Monday. Coal generation is being phased out in Spain by 2025. The largest coal plant was shuttered last year.

HOW IS POWER RESTORED?

The process of restoring power after a major cut is known as a "black start". It involves gradually restarting power plants individually and reconnecting them to the grid.

As Spain tried to get more power back on Monday it turned more gas and hydropower plants online and increased power imports from France and Morocco.

EXCESS RENEWABLES?

Monday's collapse has sparked debate about whether the volatility of solar and wind supply has made Spain's power systems more vulnerable to such an outage.

Prime Minister Sanchez said on Tuesday there was not a problem of excess renewable energy, adding that demand at the time of the blackout was relatively low and that there was ample supply.

EU Commissioner Jorgensen said the cause of the blackout cannot be reduced to any particular source of energy.

Still, the rapid development of renewables in Europe has flooded grids during periods of heavy sunlight and weak demand, sometimes driving wholesale electricity prices down to zero or negative and forcing solar farms to curtail their input.

Analysts expect an increase in negative price hours in Spain and Portugal in 2025 due to further expansion of solar farms