The electrical installation in general of a wind farm must be suitable for very large areas, the extensions of which sometimes exceed tens of square kilometers. Between one turbine and another there can be tens or even hundreds of meters, so the connection between generators involves high costs; And when we talk about these connections, we are referring to all aspects, from telecommunications, electrical interconnections, and also physical ground systems. Communications have been solved with fiber optic networks and long-range radios, electrical interconnection is addressed with medium voltage underground networks, but ground systems can be approached in various ways based on some very popular standards such as IEEE 80 [1], IEEE 81 [2] and more deeply on IEEE 2760-2020 [3].
From the infrastructure of a wind farm, the meshes surrounding the distribution cables can be made available for use as part of the physical ground system, as well as the derived neutral cables in the transformers of the substations. However, it will be necessary to have underground cables exclusively dedicated to earth, installed in specially conditioned trenches and earth meshes made up of cables, rods, screeds and joints. These meshes must exist under and/or around each wind turbine, each substation, and each interconnection point. Determining all these elements requires a design stage, which is a special challenge in wind farms.
Steps for the design of an earthing system. We can quickly mention them as follows:
- Site Plan
- Resistivity Measurements
- Resistivity Models
- Conductor sizing
- Evaluation of safety criteria (contact and step voltages)
- Design of local grids for each turbine and substations
- Design of land interconnections
- Calculation of maximum currents and voltages in the event of faults
- Determination of buffer zones and interaction with other substations
- Installation and relevant testing.
Design tools. Some standards such as IEEE 80 can be applied to the design of physical ground systems, however, due to the large number of points to consider in a wind farm, the ideal would be to resort to software specially designed for these tasks.
Environmental and geotechnical considerations. Soil resistivity can be vulnerable to climatological variants such as temperature, rainfall, humidity and other effects caused by seasonal variations, in addition to other characteristic variables of the soil such as pH, salinity, moisture retention capacity, hardness, etc. These variations can also affect the conductivity of the wires, especially if bare copper rods and wires are used. The terrain variants can be very diverse throughout a wind farm, since plains, hills, rocky soil, swampy soil, among others, can coexist in the same park.
Soil resistivity measurements. The previous point explains why it should not be generalized (although it is usually done for economic reasons) with a single sample of soil resistivity, it is recommended to make independent measurements, being the ideal to measure around each turbine, as well as in each substation, interconnection points and even intermediate points between the trenches where the ground cables are installed.
Resistivity models. Starting from the fact that each soil measurement can be very different, it is recommended to make multiple resistivity models considering several layers, and even model the same points for different times of the year. The aim is to have the best possible dimensioning of the physical earth, at each point without using additional material, but always complying with the criteria for the protection of personnel (limit values of contact and passage voltages).
Fault currents and splitting factor. Relevant studies must be carried out to obtain ground fault currents from one to 3 phases, evaluating such faults at high, medium and low voltage. In the same way, the factor of division of the currents when a fault occurs, and determine with software the distribution of these throughout the earth system.
Materials for the earthing system. Consider the material for the cables, which can start from the copper or aluminum integrated in the meshes of the distribution cables, as well as specially dedicated cables that can go bare, rods, screeds, etc. Pure copper, copper-clad steel, and occasionally galvanized or stainless steel are usually used. Joints should be considered, which can be welded or by compression. Finally, the additional material that usually accompanies meshes and trenches, which can be special cement, certain types of sand and chemicals such as bentonite.
Effects of interconnection with other substations. Both distribution and transmission substations, as well as those dedicated to the wind farm, can have a lot of interaction with each other, so in the design stage the data of the already existing infrastructure must be considered, and use its short-circuit values, the coordination of certain protections and the previous design of the ground system to be able to define a suitable final model.
References:
[1] IEEE Std 2760-2020 TM, IEEE Guide for Wind Power Plant Grounding System Design for Personnel Safety.
[2] IEEE Std 80TM, IEEE Guide for Safety in AC Substation Grounding.
[3] IEEE Std 81TM, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System.
[4] IEC 61400-24, Wind Turbines, Part 24; Lightning Protection.
[5] IEEE Std 142TM, IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems (IEEE Green Book).
[6] IEEE Std 386TM, IEEE Standard for Separable Insulated Connector Systems for Power Station Distribution System above 600 V.
[7] IEEE Std 387TM, IEEE Standard for Qualifying Permanent Connections Used in Substation Grounding.
