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Titlegrounding design for personal safety of a large scale wind power plant
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49



The red x’s in Figure 4.9 represent fault locations that will each need to be evaluated separately

(i.e. for this system 10 faults would need to be evaluated. Because each turbine will have a local

ground grid connected to the generator step-up unit (GSU) only the MV fault will need to be

considered for GPR at the turbine. Any fault on the LV side will travel back to the GSU nearly

exclusively via the ground grid, with very little if any contributing to the GPR by flowing into the

ground. The typical LV voltage at turbines is 690 V while the typical MV voltage is currently

nearly always 34.5kV (Currently work is being done to increase this voltage however). Typical

values for the HV transmission voltage vary based on the size and strength of the grid and the

cost of construction. Common numbers to see include 115 kV, 230 kV, and 345 kV.

4.4.2 Fault Current Split Factor (SF):

As mentioned in Chapter 1, upon being injected the fault current will split into two main

paths: the portion of the fault that travels through the intended return paths (the bare ground

conductor, and concentric neutrals) and the portion that travels through the soil. The ratio of that

soil current (i.e. the amount of the fault current that will contribute to Ground, Touch, and Step

potentials) to the total fault current is defined as the Split Factor (SF). This split factor is

ultimately calculated by paralleling the neutral return paths with that of the soil return path and

conducting a current division calculation. Figure 4.10 illustrates this simplified concept.



Here each line represents a return current path. These paths are composed of line parameters

defined by the material characteristics, and installation geometry. In addition to this all lines are

surrounding be the appropriate soil structure. A fault can then be placed at one end and the

various divisions of current can be calculated.

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