Higher Levels of Back-Up for Ireland's Renewables Based Grid

The grid operator in Ireland has always required back up generation, known as operating reserves, in the event that a power station fails which could cause a widespread blackout. Usually, these are powered by fast acting fossil fuel plant which can be switched on in an instant. But Ireland's transition to a wind powered based grid has not resulted in less back-up, but more back-up generation which obviously has an impact on the ability of wind to reduce emissions.

The first table below is from a few years ago and shows four different types of operating reserve with a minimum requirement of 110MW during the day and 75MW during the night.

The next table is from 2020 and shows that Reserves have increased to 155MW during the day and 150MW during the night - a 40% increase during the day and a doubling during the night.

 In a 2007 report, prepared for Eirgrid, titled "Wind Variability Management Studies (P.Meibom et al)" , Danish scientists and University researchers concluded that:

 "Generally, the demand for replacement reserves increases with
increasing wind power capacity installed.

The occurrence of high demands for replacement reserves is
mainly driven by a high number of simultaneous forced outages that happens
simultaneously to relatively high wind power. The value of these peaks tends to increase with increasing wind power capacity installed."

In another 2005 study by R. Doherty and M. O’Malley of UCD Dept of Electronic and Electrical Engineering, titled “A new approach to quantify reserve demand in systems with significant installed wind capacity” it was stated that :

The methodology is applied to a model of the all Ireland electricity system, and results show that as wind power capacity increases, the system must increase the amount of reserve carried or face a measurable decrease in reliability [i.e. increase the risks of a blackout - blog note].

So this was well known, that as you increase wind energy, the grid becomes more unstable, and more back-up reserves are needed. It's becoming increasingly clear that Ireland has already reached it's limit on wind generation, where the benefits are more than offset by the costs.

Eirgrid Increase Spinning Reserves - Irish Energy Blog Vindicated

To maintain a stable grid, Eirgrid have always ensured that there is enough "spinning reserve" (or back up generation) running or available to provide power at short notice in the event that a power station trips (i.e. suddenly has an outage) or there is a sudden change in demand. 

For many years, the minimum for this reserve requirement was set at 440MW, made up of four different types of reserve with different reaction times, of 110MW each. The quickest can deliver in less than five seconds, the slowest in less than five minutes, with the latter capable of lasting much longer than the former. The remaining reserves can react within 15 and 90 seconds. 

The two fastest reacting reserves are called Primary and Secondary Operating Reserve and are provided by units already running on the system that can change their output quickly to deal with unexpected events. The two slower reacting reserves are called Tertiary Operating Reserves and are provided by units both already on the system and that can start quickly at short notice. For additional security, there are also replacement reserves that can start from 20 minutes to four hours and are provided by fast acting offline generators such as open gas cycle turbines (basically jet engines).

The question then arises - if wind energy is inherently variable, just as demand is, or unpredictable just as a generator outage is, what impact does it have on reserve requirements in the event that it unexpectedly rises or falls ? In otherwords, does it contribute to an increase in unexpected events that can't be forecast by the grid operators ? If the answer is yes, then more reserves will be required, likely in the form of fast acting fossil fuel generators (and with a consequence increase in emissions.) Consider that the single largest generator that can fail in the system at any one time is about 500MW compared to combined wind energy capacity of 3,000MW which if acting in unison (as it usually does) is six times the size.

In 2014, the SEAI issued their  "Quantifying Ireland's Fuel and CO2 Savings from Renewables" report based on the contribution from wind energy during 2012. 

This is what they concluded in relation to the potential impact of wind generation on reserve requirements :

Future planned increases in wind capacity will influence the reserve requirements, particularly tertiary reserve requirements. The All-Island grid study showed that additional reserve requirement in hypothetical 2020 scenarios is related to the amount of wind installed but that the largest contributing factor remains the loss of the largest conventional unit. Wind power does not necessarily require larger amounts of primary and secondary reserve, when the characteristics of the wind are taken into account in the calculation of reserve requirements. The relative electrical isolation of the All-Island system means that the reserve levels consider the need for a high degree of generator flexibility, while additional rules ensure a sufficient number of units remain online to ensure frequency and voltage stability. Reserves allow the electricity system to respond to unexpected events but the ability of the system to incorporate variability and uncertainty due to renewable electricity generation is primarily determined by system flexibility. 

At present, renewable electricity generation on the All-Island system does not influence the quantity of reserve required.

In essence, the SEAI are claiming there will be little impact on reserves from wind power but with caveats thrown in about it impacting mainly tertiary reserve and careful use of words like "at present" which would indicate that future levels of wind energy are not being examined in their report anyway. 

In the same year, I wrote an article for this blog challenging the SEAI's report and in particular their omission of the impact of wind energy on reserves. I argued that higher levels of wind energy would indeed lead to an increase in reserves and based my argument on research done by both Danish and UCD researchers (in 2007 and 2005) : 

There should be enough spinning reserves to cover an outage of the largest unit in combination with a fast decrease of the current wind power production. However, the capacity of the largest online unit changes dynamically. (Doherty and O’Malley 2005) further demonstrate the dependency of the demand for TR1 [Reserve Type 1] from the installed wind power capacity. 

"Generally, the demand for replacement reserves increases with increasing wind power capacity installed. 

The occurrence of high demands for replacement reserves is mainly driven by a high number of simultaneous forced outages that happen simultaneously to relatively high wind power or load forecast errors. The value of these peaks tends to increase with increasing wind power capacity installed."  - Wind Variability Management Studies (P.Meibom et al)"

My article has now been vindicated four years later as a recent Eirgrid document shows that they have increased the minimum levels of spinning reserve required for the Irish grid.

Previous Operating Reserve Requirements

New Operating Reserve Requirements, Summer 2018

This means the minimum reserve now is 540MW (135 * 4), up 100MW, with at least half of that coming from units already running on the system. The reason for this increase is due to DS3 System Services Contracts. DS3 services, as explained before on this blog, are services required by conventional and other generators to facilitate high levels of wind energy. 

From the onset, the integration of wind generation presented a range of challenges previously unseen in the power sector. Through collaboration with the Regulatory Authorities and the wider electricity industry, DS3 has developed a number of innovative and progressive solutions. 

The cost of which may well become significant :

This means that some types of service providers could be available and eligible for payments for every hour of the year assuming they are not forced out or scheduled out for maintenance, even if the service is not required from those providers for all of these hours. The scale of overall payments will therefore increasingly depend on the portfolio of service providers and the expected availability of individual service providers.

2014 Irish Energy Blog article : http://irishenergyblog.blogspot.com/2014/12/seais-quantifying-savings-from.html