The split factor is used to take into account the fact that not all the fault current uses the earth as a return path. Some of the parameters that affect the fault current paths are:
1. Location of the fault
2. Magnitude of substation ground grid impedance
3. Buried pipes and cables in the vicinity of or directly connected to the substation ground system 4. Overhead ground wires, neutrals, or other ground return paths
The most accurate method for determining the percentage of the total fault current that flows into the earth is to use a computer program such as EPRI’s SMECC, Substation Maximum Earth Current Computation. This program and similar programs, however, require an involved data collection effort.
For the purposes of this Bulletin, the graphical method will be used.
Two types of graphs will be presented:
1. 100 percent remote, 0 percent local fault current contribution
2. 25, 50, and 75 percent local, which corresponds to 75, 50, and 25 percent remote fault current contribution
Figures 9-6 through 9-21 are for 100 percent remote, 0 percent local fault current contribution. These represent typical distribution substations with delta-wye grounded transformers, switching stations, and transmission stations with no local sources of zero-sequence current. Each of these graphs contains a number of curves representing various combinations of transmission lines and feeders at the substation.
Figure 9-6: Approximate Split Factor Sf 100% Remote, 1 Transmission Line, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C1. Copyright © 1998. IEEE. All rights reserved.
Figure 9-7: Approximate Split Factor Sf 100% Remote, 1 Transmission Line, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C2. Copyright © 1998. IEEE. All rights reserved.
Figure 9-8: Approximate Split Factor Sf 100% Remote, 2 Transmission Lines, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C3. Copyright © 1998. IEEE. All rights reserved.
Figure 9-9: Approximate Split Factor Sf 100% Remote, 2 Transmission Lines, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C4. Copyright © 1998. IEEE. All rights reserved.
Figure 9-10: Approximate Split Factor Sf 100% Remote, 4 Transmission Lines, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C5. Copyright © 1998. IEEE. All rights reserved.
Figure 9-11: Approximate Split Factor S f 100% Remote, 4 Transmission Lines, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C6. Copyright © 1998. IEEE. All rights reserved.
Figure 9-12: Approximate Split Factor S f 100% Remote, 8 Transmission Lines, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C7. Copyright © 1998. IEEE. All rights reserved.
Figure 9-13: Approximate Split Factor S f 100% Remote, 8 Transmission Lines, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C8. Copyright © 1998. IEEE. All rights reserved.
Figure 9-14: Approximate Split Factor S f 100% Remote, 12 Transmission Lines, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C9. Copyright © 1998. IEEE. All rights reserved.
Figure 9-15: Approximate Split Factor S f 100% Remote, 12 Transmission Lines, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C10. Copyright © 1998. IEEE. All rights reserved.
Figure 9-16: Approximate Split Factor S f 100% Remote, 16 Transmission Lines, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C11. Copyright © 1998. IEEE. All rights reserved.
Figure 9-17: Approximate Split Factor S f 100% Remote, 16 Transmission Lines, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C12. Copyright © 1998. IEEE. All rights reserved.
Figure 9-18: Approximate Split Factor S f 100% Remote, No Feeder, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C13. Copyright © 1998. IEEE. All rights reserved.
Figure 9-19: Approximate Split Factor Sf 100% Remote, No Feeder, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C14. Copyright © 1998. IEEE. All rights reserved.
Figure 9-20: Approximate Split Factor S f 100% Remote, No Transmission Line, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C15. Copyright © 1998. IEEE. All rights reserved.
Figure 9-21: Approximate Split Factor S f 100% Remote, No Transmission Line, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C16. Copyright © 1998. IEEE. All rights reserved.
Figures 9-22 through 9-27 are for 25, 50, and 75 percent local, which correspond to 75, 50, and 25 percent remote fault current contribution. This represents typical transmission substations or generating stations with X number of transmission lines (feeders are considered transmission lines on this graph). These stations contain local sources of zero-sequence current such as autotransformers and grounded-wye generator step-up transformers. The greater the local contribution, the lower the earth current since the locally contributed fault current usually has a direct conductive path to the system neutral.
Figure 9-22: Approximate Split Factor Sf 75% Remote, No Feeder, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C17. Copyright © 1998. IEEE. All rights reserved.
Figure 9-23: Approximate Split Factor Sf 75% Remote, No Feeder, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C18. Copyright © 1998. IEEE. All rights reserved.
Figure 9-24: Approximate Split Factor Sf 50% Remote, No Feeder, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C19. Copyright © 1998. IEEE. All rights reserved.
Figure 9-25: Approximate Split Factor Sf 50% Remote, No Feeder, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C20. Copyright © 1998. IEEE. All rights reserved.
Figure 9-26: Approximate Split Factor Sf 25% Remote, No Feeder, Low Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C21. Copyright © 1998. IEEE. All rights reserved.
Figure 9-27: Approximate Split Factor Sf 25% Remote, No Feeder, High Line Ground Resistance. Ref. IEEE Std. 80, Draft 13, June 19, 1998,
Figure C22. Copyright © 1998. IEEE. All rights reserved.
In calculating the number of transmission lines and feeders, only those that have either overhead shield wires or solidly grounded neutrals should be counted. If the number of lines falls between the given curve values, use the curve with the lower number of lines.
To use the graphs, the approximate tower impedance of the transmission lines and feeders should be known, and the value for the grid resistance has to be calculated. Since the design has not yet been started, an approximate value can be calculated by using Equation 9-6:
Equation 9-6 Rg = ρ πA
4
Where:
Rg = Substation ground resistance in Ω
ρ = Soil resistivity in Ω-m (calculated in Section 9.3.2 ,ρa(av1) ) A = Area occupied by the ground grid in m2 (calculated in Section 9.4)
Since the soil resistivity and the ground grid area are the two most important variables controlling the substation ground resistance, Equation 9-6 yields a sufficiently accurate answer to be used for the X-coordinate on the graphs. Other equations for Rg are presented in IEEE Std. 80. Once the appropriate graph has been chosen with all the available information, the intersection of the substation ground
resistance and the appropriate curve yields the value for the split factor on the Y-axis. Note that this value is given as a percentage and should be converted to decimal notation before using in an equation.