COLUMN-Blackout risk from solar power needs remedy: Wynn

LONDON Mon Dec 17, 2012 9:53am EST

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LONDON Dec 17 (Reuters) - European countries may be responding too slowly to the risk that they could cause blackouts across the power grid because they set solar installations to cut out suddenly if wider power supply outstrips demand.

At present, about half of solar panels in Europe would disconnect instantly if their local network frequency deviates from managed levels by a certain amount, due to settings designed to protect home appliances from damage.

The danger now is of blackouts when frequencies rise too high as a result of power generation exceeding demand.

Such deviations would require an unusual chain of events, but these did occur in 2003 and 2006, and the new, higher levels of solar power could deepen imbalances into blackouts, according to the European Network of Transmission System Operators for Electricity (ENTSOE).

The problem concerns the lower voltage distribution network that links transformers and households.

"Presently the impact of automatic disconnection criteria for generation units operating in low voltage grids causes concern with respect to the security for the entire power system of ENTSOE Continental Europe," the group reported in April.

"There is currently no severe risk for system security due to existing disconnection rules during normal operation and in case of normal type of contingencies. However, this risk should be further reduced by adaptation of the technical standards."

For grid purposes, Continental Europe is defined as all of Europe excluding Britain, Ireland, Scandinavia and the Baltic Republics.

The solution is to modify solar equipment called inverters to allow a more incremental shutdown, and to ensure such protection is fitted as standard in new installations.

Some countries are planning retrofits, including Germany and Italy, but some have no plans in place including France, the Czech Republic, Greece, Slovakia and Belgium, according to ENTSOE as of April (See Chart 1)

The German programme is expected to run from 2012 to 2015, to upgrade more than 300,000 solar panels and cost 65-175 million euros, but has been delayed pending regulations. An Italian retrofit programme is pending regulatory approval.

There appears to be no coordinated regional regulatory overview to monitor the progress of the retrofit process.

The risk is illustrative of the teething troubles and hidden costs facing the European Union's drive towards more renewable power, which requires expensive back-up generation and a strengthening and widening of grid networks.

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Chart 1: goo.gl/CPDPB (page 11)

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PROBLEM

The aim of original solar installation settings was to prevent frequencies reaching dangerous levels in local networks.

However, local generation and in particular roof-top solar panels have reached a capacity where a spontaneous shutdown would pose a serious network problem.

Peak generation during a sunny day in Continental Europe now exceeds by about an order of magnitude the immediate response operators can call upon to balance a sudden loss of solar power.

The European grid is managed at a frequency of 50 hertz, and many solar installations are programmed to shut instantly at 50.2 or 50.3 hertz.

Frequency changes when there is an imbalance between system load (demand) and power generation, and that happens especially during the daily ramping up and down of demand in the early morning and evening.

Fortunately, that does not coincide with periods when solar panels are generating at a maximum.

One recent major deviation in central Europe occurred late in the evening of mid-December last year.

"We could observe a progressive increase of the frequency from 50.05 to 50.16 Hz in half an hour between 23:30 and 23:55," said ENTSOE in the report "Assessment of the system security with respect to disconnection rules of photovoltaic panels", published earlier this year.

According to Eurelectric, the electric industry trade body, frequency is varying more than it used to.

Another potential trigger for an emergency deviation is the failure of a major power export link out of central Europe, which would lead to a jump in network frequency.

TRIGGER

ENTSOE reported two examples where frequencies exceeded the 50.2 solar threshold in the last decade, in Italy in 2003 and central Europe in 2006.

The Italian blackout was the result of a disconnection of the country from the continental European grid.

The 2006 event was exacerbated by the uncontrolled disconnection and reconnection of wind farms in response to changing frequency, ENTSOE said.

"With the present increase of PV (solar photovoltaic power) the same event would have evolved into a total black out," it said of the 2006 imbalance.

The risk is made more serious because of the increase in solar capacity, which far exceeds the instant reserves available in central Europe to balance the grid.

Primary control is a fast, autonomous first line of defence against disturbances where online conventional power generators respond to variations in frequency.

The Continental Europe zone can call on a primary response of 3,000 megawatts (MW) within about 30 seconds of a network emergency, according to the Union for the Coordination of the Transmission of Electricity (UCTE).

That compares with about 35,000 MW of installed renewable power capacity which has "unsuitable over frequency disconnection criteria", over the same area, according to ENTSOE.

"According to a recent inventory several thousand megawatts in-feed by photovoltaic panels could be interrupted spontaneously in case of over frequency above 50.2 Hz," ENTSOE calculates.

Under secondary control, grid operators can call on much bigger reserves within five to 10 minutes, as online power plants are ramped up ("spinning reserves") and, beyond that, they can start up flexible, previously offline gas-fired engines and power plants ("standing reserves").

But the subsequent, uncontrolled reconnection of thousands of megawatts of solar power, as grid operators contained the original outage, could provoke further instability - called a yo-yo effect - underlining how the problem needs addressing. (Reporting by Gerard Wynn; Editing by Anthony Barker)

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