Storm Atiyah: 8th December 2019

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Storm Atiyah: 8th December 2019



On Sunday December 8th the first named winter storm of the 2019/2020 season tracked from the North West of Ireland. The eye of the depression transitioned from Iceland, across Scotland and then into the North Sea. The strongest winds associated with this system were on the right hand edge of the direction the system was moving in (i.e. to the South West of the centre of depression).


Source: Met Eireann


Weather Warnings Issued

On the day before arrival of the storm, Saturday 7th December, Met Eireann published an Orange wind warning for the Western seaboard of the Island and Yellow warning for the rest of the country.

On Sunday 8th at 10am, Met Eireann then upgraded the warning in Kerry to a status Red. This is the most serious warning in the scale.

This Red warning in Kerry was valid from 4pm to 7pm on Sunday evening, with the peak of the storm expected to hit the West coast overall at that time.

The South West, where the winds were forecasted to be strongest, is an area that is one of most densely populated with wind farms on the island. But more of that later.


Met Eireann Storm Atiyah Map
Source: Met Eireann


IWEA Wind Farm Map
Source: IWEA


Impact on the Power System

Day Ahead wind forecasts were high across the whole day (light green line). The out-turn wind generation was much lower throughout the day. The vast majority of this shortfall was due to curtailment of wind farms by the system operators to ensure the security of the power system during periods of high wind penetration.


Wind (Forecast vs Actual)
Wind (Forecast vs Actual)
SNSP Limit:

The Transmission System Operators (TSO) implement a limit on the amount of “non-synchronous” generation on the power system at any one time. This limit is a function of overall consumer demand and is described in the following:


Limit on the amount of “non-synchronous” equation

In the early part of this decade, this limit was set at 50%. However, following years of improvements in operations and power system infrastructure by the TSOs, this limit has steadily increased over recent years and is now 70%.

In Ireland, the vast majority of non-synchronous generation is provided by wind farms. Therefore, if interconnector flows were ignored, a simplified version of the SNSP limit could be described as the following:

Simplified version of the SNSP limit


Adding the out-turn consumer demand and another line which is 70% of demand to the previous chart we can approximately observe which parts of the shortfall in wind generation was due to curtailment and those that are related to forecast error:


Wind (Forecast vs Actual) 2
Wind (Forecast vs Actual)


It is obvious in the chart above that the wind generation tracks 70% of Demand [red dashed line] for most of the day. In fact, this was a new record for the amount of demand served by wind on the island of Ireland.

However, there is a clear shortfall in wind generation over the evening peak between 4pm and 8pm. The deviation away from the 70% limit indicates that the shortfall is likely not due to curtailment.


Wind Cut-Out Speeds

During periods of very high winds it can become unsafe for a turbine to continue to generate and the turbine will automatically implement a braking system and reduce power output. The wind speed above which this occurs is known as the “Cut Out Speed”. It can vary by turbine type but is typically 25 m/s.

The shortfall of generation observed in the evening peak can be explained by a number of wind farms in the South West of the country experiencing these cut-out events due to the extraordinarily high wind speeds at the peak of the storm. An example of this can be seen in the plot below based on a number of wind farms.


Wind Capacity Factor vs Wind Speed in Kerry
Wind Capacity Factor vs Wind Speed in Kerry


The reduction in power correlates with the spike in mean wind speed between 17:00 and 19:00, which corresponds to time of the shortfall in wind across the overall power system.


Impact on the Power Market
System Imbalances

The chart below displays the Net Imbalance Volume [NIV] of the balancing market throughout the day.

  • Negative NIV values indicate the system is ‘Long’ and there is excess power than needed.
  • Positive NIV values indicate the system is ‘Short’ and requires more power to meet the deficit.


Net Imbalance Volume
Net Imbalance Volume


In the overnight period, the system was Long, dropping to as low as -831 MW at 04:30.

Throughout the day time the system was relatively Short, with the NIV spiking to 1.3 GW at 18:00 – which is the peak of the storm. A significant portion of this high NIV is related to the shortfall in wind production due to the cut-out wind speeds observed in the South West.







Due to the high wind forecast, Day Ahead prices cleared quite low throughout the overnight period, dipping as low as -11.86 €/MWh at 05:00.

The imbalance price was set throughout most of the night at 0 €/MWh by wind farm dispatch down actions. Notable exceptions were at 00:30 and 09:00 where prices dropped as low as – 130 €/MWh and – 247 €/MWh respectively.



Day Ahead prices were relatively low in the daytime period, peaking at 69.72 €/MWh at 17:00 during the evening demand peak.

Imbalance prices were higher than Day Ahead throughout much of the day due to the ‘Short’ NIV seen in the previous chart. Interestingly, the price at 18:00 when the NIV was 1.3 GW short was only 31 €/MWh higher than the Day Ahead price.

In most conditions, a NIV of + 1.3 GW during the evening peak in December should be cause for imbalance prices to outturn far higher. However, given the amount of wind on the system, any dispatchable generators on the system were at relatively low operating levels. At the time of the evening peak, the shortfall was met by these generators increasing their output and Turlough Hill providing the remainder.



For EirGrid and SONI, reaching 70% of non-synchronous penetration on the system is a remarkable achievement.  The Irish system is the first energy system in the world to achieve these levels which are critical to achieving a decarbonised energy system.

The swings in power prices during the day from lows of – €247MWh to highs of €90/MWh demonstrates the importance of having access to a 24/7 trading desk with a deep understanding of weather, forecasts and trading, particularly at times of unusual weather patterns such as storms and extended cold periods.


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