Gearbox in wind turbines

Why do you need a gearbox in a wind turbine?

The short answer is that you don’t need one – if you are using a direct drive WTG. But even if the solution without gearbox is used by several manufacturers (e.g. the Goldwind 2.5 PMDD, Enercon models, etc.) the majority of makers decided to include this technology.

Purpose of the gearbox is to increase the rpm (revolutions per minute). The blades rotate very, VERY slowly. It is also important to mention that the longest the blade, the lower is the tip speed of the blade: you do not want to increase it to avoid generating noise and to lower the loads on the blade itself.

In order to reach the correct rotational speed and generate power at the frequency needed by the grid you will need to use a gearbox between the main shaft (connected to the blades) and the secondary, “high speed” shaft linked to the rotor in the generator. The conversion ratio depend on the WTG model, but can be around 1:100.

The gearbox must survive over 20 years with very high, cyclical loads. Torque can be extreme during emergency shutdown, and is usually high during start ups. The failure of a gear box is a very big problem, as you will have a long production downtime and you will need a crane to disassembly the broken component and install the new one.

Additionally, gearboxes should be as silent as possible, have low vibrations and dissipate quickly the heat produced by the internal mechanisms. Therefore lubrication systems and vibration absorber mechanism are crucial in their design.

Gearboxes are usually built using planetary gearing system, and are equipped with several  auxiliary system. For instance, it is possible to analyse the density of particles dissolved in the lubricant oil and the way the gearbox vibrate to detect problems and predict possible failures.


Wind farm earthing and optical fiber cables

In this post of many, many years ago I explained how wind farm trenches are usually built.

In addition to the medium voltage cables, in the trenches usually there are also 2 other type of cables:

  • Earthing cables
  • Optical fiber cables

The earthing cables are usually made of copper and they are used to dissipate fault currents, coming usually from lightning or short circuits.

Typically the earthing cables connect all the wind turbines with the substation. In the turbine side, they are usually connected with an earthing bar inside the tower.

Additionally, there is also a second earthing system inside the foundation connecting the earthing bar with the steel rebars inside the concrete.

This system usually grant low earthing resistance (<10 Ω) in the majority of cases. In specific situations (for instance, wind turbines in rock with high resistivity) it can be necessary to use additional measure to lower the resistance, for instance using several auxiliary copper rings around the foundation.

The optical cables bring all the information recorded in the wind turbine and the met mast to the SCADA system. Usually there is a software installed in a specialized server in a separated room of the wind farm substation.

From there, the information reach the stakeholders via an internet connection (usually there are remote control centers).

The fiber optic cable usually have from 4 to 16 fibres and, due to the distance, they are usually single mode. The topology of the optic fiber cables connection depend on the redundancy asked to the system. If you really want to avoid the risk of losing control of the turbine in case the optical fiber is damaged somehow, then the correct solution would be a redundant ring topology where a WTG is connected to the SCADA system following 2 different paths.


Wind derivatives: is hedging the risk the next step for wind energy?

Weather derivatives are not a new product. The first contract were traded over the counter in the ‘90s, with the Chicago Mercantile Exchange (CME) introducing from 1999 a broad set of products like futures and options that are widely traded today.

They are a financial product that can help hedging the risk associated with the inherent variability of weather.

They are not like an insurance. With an insurance, you know that one or more events (for instance, a hurricane damaging your wind farm) will trigger a payment if certain contractual conditions are met.

Derivatives are more “continuatives”. Simplifying a lot, you can get money if a certain index is above (or below) a certain threshold in a given time frame.

For instance, a common contract traded in the CME is linked to the (cumulated) difference between the actual temperature and 18⁰C. Basically, if the weather is warmer than usual you will have a payoff: this will lower the business risk of companies whose activity is linked to cold weather (for instance, selling products to household heating).

In the previous example the index underlying the derivative is the temperature. In the case of wind energy, derivatives can be built around 2 different concept – wind speed (as measured at the met mast or in a meteorological station) or wind power (that is, hedging the actual production of the wind farm).

A second categorization would be the typology of contract built around the chosen index. At least theoretically, all the standard structures are possible – e.g. futures, options, floors and other types of cash settlements.

I’m writing this post because I’ve noticed that, in countries where the wind energy has a high penetrations, wind derivatives are not a mental experiment – they are already a reality: for instance in Spain there is a specific market for them, and this second product cover the highly developed German market.

My first impression is that wind conditions are very local – therefore it can be hard to find an off the shelf financial product considering a wind index that match the conditions of the area where the wind farm is operation. Possibly these products are more useful for an utility (trying to hedge the risk on a nationwide level) than for a small energy producer.