Why do we see wind turbines stopped if there is enough wind?

You are not the first person to ask why you have sometimes seen a number of wind turbines stopped and you will not be the last. In fact, it is happens quite often when you are driving along the road alongside a wind farm and you notice that a lot of the wind turbines are not working.

A logical conclusion is that they are stopped because there is not enough wind. And that is certainly one of the reason for this to happen. But some people are intrigued when this happens but there is plenty of wind. There are a number of reasons why a wind turbine may be stopped. Here are the most common reasons according to the Asociación Empresarial Eólica (AEE).

Reasons why wind turbines may be stopped

Wind turbines may be stopped because there is not enough wind, since this is an intermittent resource. But the strange this is that, even though this might sound like a contradiction, too much wind also causes wind turbines to stop.

Anything in excess of 25 m/s (90 km/hr) is dangerous for the wind turbine so it opts to shut down. The connection speed is generally from 3 m/s (19.8 km/hr). This is the speed at which electricity starts to be generated.

Another reason for shutting down a wind turbine is to undertake preventive or corrective maintenance. The former involves scheduled shutdowns to inspect all the components. This is normally done every 6 or 12 months, when less wind is expected so a lower amount of potential energy is wasted. Corrective maintenance is undertaken when there is a breakdown and repairs need to be carried out.

Adverse weather conditions are another of the main reasons for these shutdowns. As we have already explained, too much wind can be dangerous, but also if the weather is extremely cold ice may accumulate on the blades and the turbine needs to be shut down. This is because it may cause an overload and malfunctioning.

If we take a look at the different regulations, we will find more reasons. For example, there is a regulation that, depending on the location of the wind turbine, obliges control of excess noise as a protective measure when the turbines are near inhabited areas, so the turbines may be shut down in accordance with the time of day.

Another regulation obliges the most up-to-date wind farms to have systems for the detection and monitoring of birds. This ensures safe passage for certain species of protected migratory birds. When a risk of collision is detected, the wind turbines are shut down until the birds have left the area.

Finally, we should speak about electrical conditions outside the established range. Wind farms may receive an order from a control centre to shut down, whenever there is exceptional non-compliance with energy quality parameters. 

Technical restrictions in the network, offloading and curtailments

As well as the above, there are other reasons with nothing to do with the wind farm that may cause the wind turbines to be shut down. This is the case with technical restrictions. Wind farms are just one part of an electrical system that needs to be controlled and supervised throughout the day.

The electricity produced in wind farms needs to be transported to the points of consumption. But the lines for transport and distribution do not have infinite capacity. There are in fact times and situations when a number of factors coincide to prevent full advantage being taken of the wind energy generated, so a number of turbines may need to be shut down and even whole wind farms. Here are some examples:

• Periods when there is low demand for electricity but high-level renewable generation. This could be a national holiday when all industry is shut down, a season when temperatures are very mild or one-off periods of low demand such as the holiday period.
• Areas with a weak electricity grid. Some parts of Spain do not yet have enough capacity to take up and channel all the power generated. In this case, Red Eléctrica de España (REE) sends out orders to shut down some installations.

In accordance with the Integrated National Energy and Climate Plan (PNIEC in Spanish), by 2030 Spain will need to increase current installed wind capacity by 70% and photovoltaic capacity by 240% to ensure that 74% of the electricity consumed comes from renewable sources. As we are assured by the AEE, this means that "installing more wind farms in areas where the wind is suitable but which have an electricity grid that is saturated may only worsen the problem of technical restrictions to renewables".

So, according to the Corporate Wind Energy Association, "we need to meet the technical challenge that faces all international electricity systems in order to be able to incorporate more renewable energy and establish more competitive prices for electricity".