Engineering consulting firm Hatch Goba has introduced a new approach to integrated earthing for wind energy facilities, using insulated cables to connect the individual wind turbine groundmats to the substation groundmat, taking the equivalent circuit of the entire wind energy facility (WEF) into account.
Hatch Goba delivered a presentation at the International Council on Large Electrical Systems’, or Cigre’s, international symposium, which was held in Cape Town from October 26 to 30, in collaboration with the International Electrotechnical Commission (IEC).
In the presentation – titled Safe Groundmat Design for Grid Connection Substations at Wind Energy Facilities – Hatch noted that the combination of high soil resistivity and a high fault current results in an unsafe potential rise in the wind farm area, as well as the transfer of dangerous potential to metallic structures and underground services in the WEF.
The presentation elaborated on a case study that described how an integrated grounding system can prevent these unsafe conditions, especially where soil resistivity is extremely high. This system was tailored for the specific site conditions encountered in the case study.
Wind farms, similar to other power generation installations, have three main components: the internal medium-voltage distribution network, or collector network; the medium- or high-voltage step-up substation; and the high-voltage evacuation lines.
The grid connection substation for a wind farm is normally located at the centre of the WEF and the electrical system must be appropriately earthed to provide a low- impedance connection between the electrical equipment and the general mass of the ground.
“This will provide reference potential for electrical equipment to ensure the effective operation of protective devices and to prevent excessive overvoltage and potential gradients during faults and normal operations, which may damage equipment or threaten human life,” explained Hatch Goba Africa, Europe and the Middle East regional director Philip König,
The WEF in Hatch Goba’s case study comprised 31 wind turbines spread across local community land, delivering a total output of 93 MW or 3 MW per wind turbine. The ground integration of the WEF required the upgrade of an existing upstream substation to 132 kV, the establishment of a new 132/33 kV substation, and interconnecting power line infrastructure between the substations.
König noted that each turbine in the WEF was connected to underground collector strings, which were terminated at the medium- voltage side of the new grid connection substation. The substation is equipped with two 132/33 kV, 50 MVA transformers.
“The WEF is located in an area with very high soil resistivity due to loamy and rocky soil conditions, which was confirmed by an investigation. “Using a bare-copper earthing system would have resulted in high potential gradients around the grid connection substation and unsafe touch potentials within the inhabited village area next to the substation,” highlighted König.
The Cigre/IEC event, which focused on the development of electricity infrastructure in sub- Saharan Africa, was cosponsored by Hatch Goba, together with other multinationals such as automation technologies developer ABB.
This is the second time the symposium has been held in Africa; it was previously held in Cairo, Egypt, in the 1980s. The symposia, which focus on specific subjects of topical interest, are held biennially in various countries where Cigre is present.