Computation of ground resistances and assessment of ground grid safety at 161/23.9-kV indoor-type substation

This paper computes ground resistances and assesses ground grid safety at three 161/23.9-kV indoor-type substations in the system of the Taiwan Power Company. For computing the ground resistance, a method based on two kinds of two-layer models (i.e., grid buried in the lower-layer of two-layer soil, two-layer soil simplified with one-layer structure of the Sverak's and empirical equations) is used and compared with the method proposed by the one-layer model of the Schwarz's equation in IEEE Standard 80, 2000 edition. As a result, the ground resistance obtained by the two-layer soil simplified with one-layer structure of the empirical equation is near the measurement. For assessing the ground grid safety, the ground potential rise is computed based on the calculated ground resistance and is compared with the minimum touch voltage. As a result, the design of the ground grid at all three 161/23.9-kV indoor-type substations is unsafe under a one-layer model of the Schwarz's equation for humans with 50-kg body weight.

I. Nomenclature

One-layer soil ground resistance of grid conductors (in ohms).

One-layer soil ground resistance of ground rods (in ohms).

One-layer soil mutual ground resistance between the grid conductors and ground rods (in ohms).

One-layer soil substation ground resistance (in ohms).

Uniform soil resistivity (in ohms–meters).

Total length grid conductors (in meters).

Diameter of grid conductor (in meters).

Depth of ground grid conductor (in meters).

Total area enclosed by ground grid .

Quantity of ground rods placed in area A.

Length of ground rod at each location (in meters).

Radius of ground rod (in meters).

Length of area occupied by the ground grid (in meters).

Width of area occupied by the ground grid (in meters).

Measured soil resistivity data obtained at different spacing in the Wenner four-pin method (in ohms-meters).

Number of measurements.

Two-layer soil model reflection factor.

Apparent soil resistivity (in ohms-meters).

Upper-layer soil resistivity (in ohms-meters).

Lower-layer soil resistivity (in ohms-meters).

Depth of the reflection boundary (in meters).

Two-layer soil ground resistance of grid conductors (in ohms).

Coefficient of burying.

Correction term due to lower-layer (in ohms).

Single mesh length in x direction (in meters).

Single mesh length in y direction (in meters).

Area shape factor.

The peripheral length of the grid (in meters).

Depth of upper layer (in meters).

Two-layer soil ground resistance of ground rods buried in lower layer (in ohms).

Ground resistance of a single ground rod (in ohms).

Ground resistance of the solid sleeve surface area S (in ohms).

Effective radius of the rodbed (in meters).

Two-layer soil substation ground resistance (in ohms).

Tolerable touch voltage for human with 50-kg body weight (in volts).

Tolerable touch voltage for human with 70-kg body weight (in volts).

Duration of shock current (in seconds).

Reduction factor for derating the nominal value of the surface layer soil resistivity.

GPR

Ground potential rise (in volts).

Maximum grid current that flows between ground grid and surrounding earth (in amperes).

Maximum ground fault current (in amperes).

Current division factor.

Current decrement factor.