# Constraints to medium voltage power cables in wind farms

(This post has been updated in July 2020 with the help of my friend and colleague Kamran)

I already discussed in two other posts how the wind farm cable trenches are usually built and how the medium voltage cables are made.

However, a more comprehensive explanation should include on how the medium voltage cables are dimensioned.

The power produced by the wind turbine is usually evacuated to the substation using a medium voltage (MV) cables connection.

This cables are usually buried. This solution is slightly more expensive but it offer much more protection to the cables than an overhead line (that is, a line where the cables are hanging from poles). A buried circuit is also less problematic from the environmental point of view. I do however see every now and then projects with overhead MV systems.

The reason for evacuating the power generated by the wind turbines with a medium voltage system is purely economic. A low voltage solution would have very high power losses (“Joule losses”) in the cables – basically the resistance of the conductor would create too much heat.

A higher voltage will decrease the current flowing in the cables and the related losses.

However, high voltage equipment is very expensive. Therefore, the medium voltage solution is a reasonable compromise, an optimum balance between the losses in the cables and the cost of the equipment.

So, what are the key constraints in the design of a MV system for a wind farm?

In many projects there is some freedom on the MV level used in the wind farm – that is, you can design a MV system using one of the standard voltage levels (such as 20 kV, 33 kV, 34.5 kV, etc.)

In situations where there is no specific requirement from the owner of the wind farm (or from the owner of the grid) the smartest choice is usually to select the MV level most commonly used in the country where the wind farm will be built.

Sometime the number of feeders in the substation is defined upfront: this limit the number of different circuits that can be designed to connect the wind turbines.

Another relevant limitation is the maximum allowable cable cross section: the bigger the cable diameter, the bigger the bending radius. For this reason cross section over  630 mm are very unusual.

For a given cable section you will try to maximize the amount of current transported without exceeding the thermal capacity. Under standard operation conditions such limit is usually around 90°C and the thermic behaviour of the cable will be obviously strongly influenced by the thermal resistivity of the surrounding material. Other relevant parameters are the depth of the cable, the temperature of the air and of the surrounding material, if there are other circuits nearby, etc.

Other significant project constraints that must be checked are the allowable current (how many amperes are transmitted by the cable – to be sure that the capacity of the cable is not exceeded) and the voltage drop between the 2 sides of the circuit (usually it should be less than 1.5%).

You will also try to minimize the power drop. A rule of thumb is that the losses should be less than 2%, but some wind farms have more aggressive requirements.

Last but not least, additional checks are performed to ensure the survival of the cable in emergency conditions – for instance in case of short circuits.

The short circuit currents are checked against the cable ratings under a variety of possible faults (phase to phase, phase to ground, etc.). Cables are designed to survive a higher temperature for a very short periods (seconds, or fraction of a second).

# How to measure wind resource

Met mast, weather vane and anemometer

I have to  start with a disclaimer – I’m not a specialist in wind resource analysis.

However, through the year I’ve seen several time this process so I think I can summarize it with a reasonable level of accuracy (and obviously if you spot a mistake, please let me know).

The wind resource assessment is done for several reason: to define the most suitable wind turbine given the local meteorology, to define the layout of the wind farm and, above all, to calculate the expected energy production of the wind farm – which is obviously a key input in the calculation of the profitability of the project.

To calculate the wind resource you will need to measure several variables:

• Wind speed
• Wind direction
• Wind shear
• Wind turbulence
• Air density
• Roughness of the area

This variables are usually measured installing one or more meteorological mast (“met mast”) in the area where the wind farm is planned  - obviously with the exception of the roughness, which is assessed by a specialist keeping into account the topography and the vegetation of the area.

This activity is called “site measurement campaign”.

The met mast is a tower made of steel (or, more unusually, in concrete) where the measuring equipment is installed. Ideally the met mast should have the same high of the wind turbines that are going to be installed in the area – however, to save money sometimes shorter masts are used.

The equipment installed on the met mast include usually the following:

• Anemometers (usually there are several anemometers at different heights)
• Weather vane (to record the direction of the wind)
• Barometer
• Thermometer

All the information collected is safely stored in an element called “data logger”. Auxiliary elements in a met mast are solar panels, a protective lightning rod on top, anti-vandalism fence and obviously the foundation.

Ideally, at least 1 year of data should be recorded. However longer measurements (2 to 5 years) have less uncertainties and capture better the seasonal and intraday variability of the wind in the area.

After the site measurement campaign the wind resource assessment start.

The first step is to “clean” all records before processing, removing errors that can occur due to malfunctioning of the instruments.

After, several key parameters are defined:

• Mean speed
• Wind rose
• Wind speed distribution
• Wind shear
• Wind turbulence
• Air density, pressure, temperature

The last step is to use this parameters to estimate electrical power production. There are quite a lot of commercial software in the industry, being some of the most widely used WAsP, WindPRO and OpenWind .

This software will try to optimize the wind farm layout to maximize energy production considering certain limitations – for instance, they will leave a distance of at least 6 rotors from one wind turbine to the other in the direction parallel to the wind.

Finally, when the layout is defined, the software will combine the power curveof the WTG with the wind speed distribution of the site to have the power output.

# Wind sector management – how to put more wind turbines in the same area

Wind sector management - image curtesy of wasp.dk

In many project my colleagues from the wind and site department (the people who calculate the best wind turbine model and the optimal layout in a wind farm) are forced to put quite a lot of wind turbines in a reduced space.

Each of these wind turbines generate a “wake effect” – basically, they create turbulence in the wind.

These turbulences can affect other turbines nearby, increasing loads. This is not good, because higher loads usually means more problems due to component failures.

Wind sector management it’s a solution to this problem – basically, when the wind is blowing from a certain direction some turbines are automatically shut down.

There are basically 2 alternatives: you can shut down the turbine upstream (the one creating the turbulence) or the one downstream (the one suffering the increased loads).

Stopping one or more wind turbines will obviously result in a loss of production. However, the guys in wind and site often found that, even considering these losses, the global output of the wind farm is higher in a densely packed wind farm with wind sector management then in a configuration without it.

In the market there are also more advanced solutions that, instead of stopping completely the wind turbines, change only some parameters of the WTGs. For instance the optimization algorithm could decide to change the speed of the rotor or the pitch of the blade.

Wind sector management is one of the curtailment that a wind farm can have. Other typical restrictions are linked to environmental issues (noise, shadow flickering, birds or bats) or to requirements coming from the grid.

# Dynamic loads on wind turbines

I discussed in another post the relevance of resonance analysis in the design of steel tower for wind turbines and the different technical solutions currently in the market.

But what are the loads that could induce resonance in the wind turbine?

There are several different type of dynamic loads:

Unbalanced rotor. This basically means that the blades have not the same weight. This problem can happen for several reasons: accumulation of snow, ice or simply variance in the production of the blade itself in the factory. The consequence is that the centre of gravity will move with the rotation of the blades inducing a centrifugal force in the system.

Tower shadow effect. It is also known as dam effect – basically, the tower affect the speed of the wind nearby. This will affect the blade passing in front of the tower, unbalancing the rotor.

Wind shear. The wind will usually have a different speed increasing from the bottom of the rotor to the top.

Errors in the configuration of the turbine. In this category I include any type of asymmetry generated by design mistakes, assembly errors, software errors, etc. For instance, a difference in the pitch of the blades could create a dynamic load.