System operator admits errors -- Why wind was unplugged

Deficiencies in the procedure for balancing electricity supply and demand in Spain lay behind the unnecessary curtailment of hundreds of megawatt of wind production during a bout of high winds in November last year, admits the country's transmission system operator, Red Eléctrica de España (REE). The reason for taking so much wind energy offline without cause lay in faulty communication and understanding between REE's central power system control and local wind production dispatch centres.

Both REE and the national wind association, Asociación Empresarial Eólica (AEE), say the procedural errors have since been identified and remedied. "Thanks to the experience, the risk of future curtailments has been reduced," says AEE's Alberto Ceña.

A post mortem of the circumstances in which the curtailment occurred has revealed that at least 800 MW of wind capacity -- and up to 1280 MW -- was shut down unnecessarily out of a total 2680 MW that was unplugged when unexpectedly high winds caused a dramatic ramp up in wind power output during an early morning when national electricity consumption was at an unusually low ebb. Demand at the time was about 20 GW, with the wind plant left online only serving about 4.8 GW of that (Windpower Monthly, December 2008).

Before ordering the wind plant offline, REE had reduced hydro production to zero and cut supply from four combined cycle gas stations, totalling 1500 MW. More thermal generation could not be taken off-line, says REE, if security of supply was to be maintained: Spain was about to start its day and the wind was likely to drop. There was no choice but to temporarily curtail wind production until demand picked up. But to turn off as much as 2680 MW was a miscalculation, agree all the concerned parties.

Miscalculations aside, REE insists curtailing at least 1400 MW was vital. Unexpected gales had caused a non-scheduled wind power surge of 5650 MW. "I've never seen anything like it," says Miguel de la Torre of REE's Control Centre for Renewable Energy (CECRE). Wind power was ramping up at 1400 MW an hour, he says, difficult to manage when demand was so low. The rapid ramp up also triggered a requirement for extra reserve capacity to cover for the possibility of the wind taking off as suddenly as it arrived. As it was, only 7517 MW of wind was online out of more than 16 GW of capacity because the winds were too strong for turbines to run safely.

Though REE had only meant to cut around 1400 MW of wind, nearly double that volume was kicked offline, largely due to a big omission in its calculations. All Spanish wind capacity is controlled by CECRE, either directly or through local dispatch control centres (DCC), providing REE with real-time monitoring capability and emergency override control. Each DCC should transmit plant operation data to REE every 12 seconds.

"But communications in remote outposts, often via satellite, can be hampered by innumerable hazards, especially extreme weather," says De la Torre. Due to the wild winds of November 2, signals were taking more than ten minutes to arrive from numerous centres.

CECRE normally phones the DCC to resolve any problems. "But the fast rising wind ramp was an emergency case," says De la Torre. Accordingly, REE resorted to an internal rule allowing it to disconnect wind capacity not communicating properly. As a result, coinciding with a first general curtailment order, CECRE also ordered the disconnection of every DCC with communication problems. Unknowingly to CECRE, those centres accounted for 800 MW of wind generation -- as it discovered on analysing the incident later.

REE's rule had presupposed communication problems to be isolated cases and that unplugging the culprits would have minimum system impact. Wrong. Unbelievably to many wind operators, CECRE had incorporated no formula for estimating just how much it is unplugging in such cases. "That has been part of the learning curve. We now have the formula in place," says De la Torre.

But the excess curtailment did not stop at 800 MW. CECRE's national wind curtailment orders are shared by each plant in proportion to the power it is producing at the time. The order to reduce production was therefore shared not across 7517 MW of online capacity, but across that figure minus the 800 MW already offline.

AEE believes REE's series of errors and the poor communication between control centres account for most of the 1280 MW that was curtailed without good reason. Ceña is pushing for more hands-on involvement with REE in balancing wind and in making sure rules are adequate and applied correctly.

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