THE FAULT CURRENT DIVISION FACTOR: 101 FOR IMPROVED GROUND GRID DESIGN

Fault Current Division Factor

During an earth fault in the substation, be it local or remote, the earth is used as a path for the fault current to reach the neutral of the power system. Part of this earthing grid current flows to the earth, the ratio between the fault current amplitude and the portion of fault current which flows from the grounding grid to the surrounding earth is called as Fault Current Division Factor.

From the previous discussion, it is necessary to know the fault current division factor for calculating the maximum grounding grid current which is flowing into the earth through the grounding grid. However, as the short-circuit current path and its distribution are influenced by many factors like, the short-circuit fault type and location, the system structure and parameters, the grounding resistances of substation and tower footings, parameters of the phase lines and overhead ground wires, system increases in the future within 5–10 years etc, it is very difficult to calculate the fault current division factor accurately.

The basic idea is to represent the grid components by models, then constitute a network from these models and finally get the analysis solution of the network. In the following section there is a brief introduction on how to use a simplified equivalent method to calculate the fault current division factors of faults inside and outside a substation.

Fault Current Division Factor inside a Local Substation

When a short-circuit occurs inside a local substation, as shown in Figure below, in the total ground short circuit current I_F, the component I_S flows directly through the grounding grid not through the earth back to the neutral of the transformer in the substation. The component I_B1 flows back to the neutral of the transformer through the overhead ground wire and tower grounding devices, and only the residual portion of current I_G flows back to the neutral of the transformer through the earth.

I_G is called the grounding grid current which flows into the earth through the grounding grid, and it can be calculated using the following equation

Where, K_f1 = I_B1 / (I_F – I_S) = I_B1 / I_T , which is the fault current division factor of the overhead ground wire and tower grounding devices.

Analysis shows that the fault current division factor can be calculated by

Where, Z_G0 is the zero-sequence self resistance of overhead ground wires between two towers and can be calculated using the following equation

   (Ω/km)

where r_d is the phase line resistance per unit length (in Ω/km), n is the number of split conductors in every phase line, a_m is the equivalent conductor radius, D_g is the distance between the ground wire and its equivalent mirror and D is the geometric mean distance between two split-conductors, which is equal to n times the evolution of the product of the distances between n split-conductors.

Z_M0 in equation above is the zero sequence mutual inductance of the overhead ground wires between two towers

 (Ω/km)

where D_bx is the geometric mean distance between the phase line and ground wire or neutral line. β in Equation can be calculated by:

Where,

Where R_T is the tower footing grounding resistance.

When there are plenty of towers such that s>5 the 1^st equation can be simplified into

Generally, the tower footing grounding resistance RT is much larger than the substation grounding resistance R. Under the assumption that R=0, the above equation can be further simplified as

The result calculated by the above equation thus tends towards the safer side.

The fault current division factor of the substation grounding system will be

Fault Current Division Factor Outside a Local Substation

When a grounding short-circuit occurs outside a local substation, as shown in Figure, for the total ground short-circuit current I_F, I_S is the part which flows back to the transformer neutral, the component I_B1 of I_S flows back through the overhead ground wire connected with the grounding grid and the component I_G flows back through the grounding grid. I_G can be calculated by

where K_f2 is the fault current division factor of overhead ground wires.

Analysis shows that the fault current division factor K_f2 for a fault outside a substation can be calculated by

Calculations show that, when a short-circuit fault occurs at a place approximately 10–20 spans from a substation, the first term of the above equation is much smaller than the second term, so the above equation can be simplified to

In grounding design, generally, first use Equations

and

to calculate the short-circuit currents I_G of short-circuit faults inside and outside a local substation, flowing from the grounding grid to the earth, and then take the large one as the grounding grid current for the grounding grid design.