Blog: ‘Dispatch Down’ and the fight against climate change

03 Sep 2019

For most people it might be an obscure technical term but as IWEA’s Justin Moran explains Dispatch Down may be responsible for almost a quarter of a million tonnes of CO2 emissions in 2019.

What is Dispatch Down?

Put simply, ‘Dispatch Down’ occurs when EirGrid, as the transmission system operator, instructs a renewable electricity generator to produce less electricity than it can or even to shut down entirely.

If a wind farm receives this instruction even though everything might be working fine and the wind is blowing exactly how you would want it, all or some of the turbines need to turned out of the wind or shut down.

This can be mystifying to a member of the public who can see the wind is blowing but the blades on the wind turbine across the road are standing still.

EirGrid’s first priority is to ensure the safe and secure operation of the transmission system. They don’t Dispatch Down lightly but in Quarter 1 of 2019 almost 8 per cent of wind generation was lost because wind farms were told to stop generating.

EirGrid does this for two reasons, either because there are constraints on the system or because of curtailment. We’re going to explain both.

What is a constraint?

Just for a moment, forget about power lines and cables and think of our electricity system as being like Ireland’s road network. Some roads are bigger than others and in better condition.

There are roads notorious for traffic congestion and roads that often seem to have no one on them. A road can even be closed for maintenance or repair.

The electricity network is a lot like that. The network of overhead lines, underground cables and substations is the road network along which Ireland’s electricity travels from where it is generated in a wind farm or a fossil fuel plant to where it is needed – our homes, factories and businesses.

There are wind farms all over Ireland but the majority are along our southern and western coasts where wind conditions are best. This means on a very windy day a lot of electricity is being generated but what happens if there is so much electricity – both wind and fossil fuel – that there is not enough capacity on the power lines to transport it where it must go?

When this happens there is a constraint on the transmission system, similar to a traffic jam on the roads. There is no problem producing the electricity, there’s just no way to transport it.

To ensure the safe operation of the system one or more generators are instructed to shut off or produce less power to ease the bottleneck. This is done to ensure that the transmission lines don’t overload and become damaged.

We say when this happens to a wind farm – or a fossil fuel generator – that it has been ‘constrained’ and this is the most common form of Dispatch Down.

What is curtailment?

Constraints, as explained above, take place in a particular part of the network but curtailment occurs because of the challenges of incorporating renewable electricity onto the transmission system.

The best known form of curtailment is the SNSP limit. Ireland’s electricity system, like most other systems in the world, operates at a frequency of 50 hertz. With fossil fuel generators this is very straight forward as they generate electricity at the same frequency.

But wind isn’t like that; wind energy is ‘non-synchronous’ which means the frequency at which wind generates electricity is not ‘synchronised’ with the system at 50 hertz.

Ensuring that our frequency levels stay steady is probably the single most important priority in managing the electricity system so EirGrid have put in place the ‘System Non-Synchronous Penetration’ SNSP limit to ensure that the volume of wind energy is manageable.

The limit is currently 65 per cent which means that even on a very wind day when wind could provide 70 or 80 per cent of Ireland’s electricity it is not allowed to do so and wind farms are dispatched down until they hit the 65 per cent limit.

The wind energy that is lost when this limit is exceeded is called ‘curtailed wind energy’ or in other words, curtailment has occurred.

Impact of dispatch down

Dispatch down has two main negative impacts on the energy system. For the wind farm operator if they have been told to shut down their wind farm or to generate less power, they have less to sell on the electricity market.

If a wind farm is in an area with high levels of constraint over a long period this can have real financial consequences as, in almost all circumstances, the wind farm is not paid for the losses due to Dispatch Down.

But more important than any one individual wind farm is that when wind energy is dispatched down it is replaced by fossil fuels. In the first quarter of 2019 more than 305,000 MWh of wind energy, which equates to 7.7 per cent of total wind generation, was lost.

Taking the best-case scenario and assuming this power was then provided with natural gas instead of coal or peat means an additional 61,000 tonnes of CO2 were emitted for the first three months of the year.

If those results are replicated in the remaining three quarters it would mean nearly a quarter of a million tonnes of additional CO2 in 2019. Put simply, reducing dispatch down means more wind energy and less CO2 emissions.

Fixing Dispatch Down

EirGrid, ESB Networks and the Irish Wind Energy Association actively work together to try and find ways to reduce dispatch down.

One of EirGrid’s biggest achievements in recent years has been figuring out new ways to operate the electricity system safely and this has allowed them to raise the SNSP limit.

Five years ago the limit was at 50 per cent; it’s now at 65 and is due to reach 75 per cent by the end of 2021. The higher the limit, the more wind power we can safely use at times when there is a lot of wind.

Reducing the number of constraints on the system means reinforcing the electricity grid. This can mean building new overhead lines or underground cables to help move electricity across Ireland or constructing interconnectors to link us to other electricity systems like the proposed Celtic Interconnector to France or Greenlink to Britain.

It could require us to build battery storage units that help maintain the system or to ‘uprate’ the existing transmission system where the overhead lines you see by the side of the road are replaced with newer lines that can carry more power.

None of these solutions are easy but EirGrid and ESB Networks are world leaders in the integration of renewable energy and Ireland’s wind industry has built a network of wind farms that now provide 30 per cent of our power needs.

If we are serious about decarbonising our electricity system and achieving our objective to provide 70 per cent of our electricity from renewables by 2030, we’re going to need our best engineers working together to reduce, even eliminate, dispatch down.