Tag Archives: earthing

WTG earthing system

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A wind farm earthing system is designed for two different functions: to protect human beings and the components in case of power systems fault, and to protect them in case of a lightning hitting the structure.

Wind turbines have a considerably high possibility of being struck by a lightning during their lifetime. For this reason it is essential to give to the electrical current a low impedance path.

The main characteristics of an earthing system are:

  • Ensure that living beings in the vicinity of the earthing system are not exposed to dangerous potentials
  • Retain system voltages within reasonable limits in case of faults
  • Provide a sufficiently low impedance

The main factors to be considered designing a normal earthing system are soil resistivity, level of lightning protection required and power system earth fault level. In the case of wind turbines the earthing system is designed to be independent from the soil condition.

In modern wind farms all the WTGs are interconnected with a copper cable buried directly in the ground. It is called a multi-earthed system, and it reduces step voltage to a tolerable level in order to protect living beings from injuries.

Equipotentialisation of turbines and surrounding soil by means of a meshed earthing system reduces step voltage to a tolerable level in order to protect living beings from injury. This is obtained by connecting the entire interconnected foundation earthing (all the steel reinforcement bars in the foundation) to the earth interconnection wires between the wind turbines.

Basically the copper cable coming from the nearest turbine through the trench is tied to the reinforcement bars of the foundation. It goes all around the lower and upper reinforcement, and than is bond to the same earthing bar of all the electrical components of the WTG, such as the MV transformer and the switchgears.

Using this solution the soil conditions around the wind turbines and the resistance to remote earth of the earthing system are not a concern because all fault currents are handled by direct connections of known dimensions and lengths.

Cable trenches - the nervous system of the wind farm

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Cable trenches construction probably doesn’t looks like the most exciting things in life, but including for them there are things that is better to know to avoid mistakes.

They are used to connect the WTGs together with medium voltage cables, and they bring the earth and the optical fibres cables as well.

The standard construction sequence is as it follows:
After the opening of the trench, on the bottom a layer of sand 10 cm thick is spread, placing above it the medium voltage cables, the ground cable and the optical fiber cable.

Subsequent to the positioning of the cables another layer 30 cm thick of fine sand, washed and compacted properly, is extended.

Above this second layer it will be placed, throughout the entire length of the trench, a HDPE cover tile marking the presence of underground cables.

The cover tiles will be placed directly above the filling that covers the cables. It has a function of mechanical protection of the cables, signaling the proximity of the cables in case of reopening of the trench.

On this tape will be extended another layer 50 cm thick, using native material coming from the excavations free of stones, branches and roots, properly compacted in layers of no more that 20 cm in thickness of loose material before compaction.

After completing this layer it will be positioned a PVC warning tape informing of the existence of medium voltage electrical cables under it.

Finally, the backfilling of the trench will be completed with a layer of 30 cm using material coming from the excavations, free of stones, branches and roots, compacted properly. The last layer of backfill will be done with topsoil coming from the excavation and previously stored properly, in order to recover the natural environment of the area as soon as possible.

In the following pictures the main elements of the operation are shown.