DESIGN OF EARTH GRID OR EARTH MAT, ELECTRODE FOR A TYPICAL SUBSTATION 101 EASY EXPLANATION

Data Required for the Design of Earth Mat of a Substation

(i) Soil resistivity model for the site of the substation or soil resistivity test data where earth grid or earth mat has to be constructed.

(ii) Electrical circuit single line diagram for the station where earth grid or earth mat has to be constructed.

(iii) Layout map of the station showing the locations of buildings, roads, trenches, railway line etc. and the fencing line or boundary line of the station

(iv) Layout of the equipment in the station where earth grid or earth mat has to be constructed.

(v) Single line to earth fault current on the buses in the station, where earth grid or earth mat has to be constructed.

(vi) If there is local generation then contribution of local generation to the fault current

(vii) Earth wires connected to station earth grid or not

(viii) Magnitude of Grid current – if it is not available then the fraction of the total single line to earth fault current contributed by various transmission lines:

• Number of transmission line and feeders leaving/entering the station, where earth grid or earth mat has to be constructed.

• For each aerial line/feeder configuration, number and size of phase conductors and earth wire/s (configuration means typical distances between phase conductors and earth wire/s and from earth)

• GMR of earth wire and its resistance per km

• Length of each transmission line and feeder up to next station

• Average span length

• Average tower footing resistance

• Average resistivity of soil along the right of way of lines/feeders

(ix) Preferred material and preferred shape or size of conductors of electrode conductors if any

(x) Time of operation of fault clearance to be used

(i) for determining size of conductor and

(ii) for determining permissible values of step and touch voltages

(xi) Preferred depth of laying the earth electrode if any

(xii) Specified depth of crushed rock or gravel layer in the station

(xiii) Specified resistivity of crushed rock or gravel

(xiv) Any restrictions on spreading gravel outside the fence or making fence inaccessible from outside if necessary

(xv) Type of fence or the boundary wall etc

Data for Design Calculations of Earth mat or Earth Grid

(a) Soil resistivity data for the Earth mat / Earth grid

Soil resistivity has to be measured at a number of locations in the switchyard area corresponding to 10 different electrode spacings with Wenner method. From the measured data, measured average apparent soil resistivity, for each electrode spacing, has to be determined and presented as in Table below.

The average of ten measurements is 50.0 Ω-m. The percent difference between the average value and the minimum and the maximum measured values is -30% and, +30% these numbers are such that the soil at station site may be assumed to be uniform soil of resistivity 50 Ω-m.

(b) Single line circuit diagram

Single line circuit diagram of the lines interconnecting the substation to the electric power system of the area where earth grid or earth mat has to be constructed is shown in Figure. The power system feeding the transmission lines is represented by equivalent generators at the far end of transmission lines.

There are four 220 kV lines and four 132 kV lines connecting the station to the electric power system. The 220 kV buses and 132 kV buses are connected through two, 100 MVA, 220/132 kV grounded wye/delta transformers.

Length of 220 kV lines between bus No. 1 and 3 is 38 km each and that of lines between bus No. 2 and 3 is 2 km each. The line conductors are ACSR Zebra and earth wire is 7/3.66 mm steel wire.

The span length is 250 m and tower footing resistance is 10 Ω. Length of each of the four 132 kV lines is 25 km, phase conductor is ACSR Panther and earth wire is 7/3.66 mm steel.

The symmetrical earth fault current at the station for a single line to earth fault at 220 kV bus is 31500 A. Contribution of fault current of each of 38 km line is 3666 A, that of each 2 km line is 9351 A and of each 132 kV line is 1369 A.

(c) Station layout showing fence

Station size is 105 m × 75 m. The detailed layout and layout of electrical equipment of the station is not reproduced in this example. Configuration of earth conductors is shown in Fig. 11.7.

Conductors are spaced at 7.5 m each. The fence is shown 2 m inside the outermost conductor of the earth grid electrode.

(d) Configuration of transmission line conductors

Configuration of phase conductors and shield/earth wire of 220 kV and 132 kV transmission line conductors is shown in Figure. Similar towers are assumed for both the lines. Magnitude of various distances marked on Figure is given in Table below.

(e) Single line to earth fault current and other available data

Symmetrical earth fault current I_f = 31500 A

Duration of fault duration for sizing conductor t_s = 1 sec

Duration of shock for determining allowable body current t_s = 0.5 sec

Available grounding area for the earth grid or earth mat A_c = 105 × 75 m²

Crushed rock resistivity r_s = 3000 Ω.m

Thickness of crush rock h_s = 0.1 m

Depth of laying of grid h = 0.6 m

Number of incoming lines (220 kV) with shield wire = 4

Number of outgoing lines (132 kV) with shield wire = 4

Tower footing resistance of incoming line = 10 Ω

Tower footing resistance of outgoing line = 10 Ω

Fence location = 2 m away towards inside from peripheral conductor

Design Calculations with Empirical Formulae for Earth mat or Earth grid

The soil model has been determined from the measured soil resistivity data.

The magnitude of single line to earth fault current and fault current contributions of various lines have been determined from the network fault study. Since there is no generation at the substation, all of the fault current supplied by 220 kV and 132 kV lines, is returned through earth and earth wires.

Approximate earth resistance R_g = ρ/(4πr) = 0.25 Ω, where r =√( 105 × 75/π). This value is used along with data of four, single-circuit, 220 kV lines each with one shield wire and four single circuit 132 kV lines also each with one earth wire, to calculate grid current with program gridi.

Approximate magnitude of grid current is found to be 19461 A.

Thus, the four important data for earth grid/ earth mat design are:

(i) Average soil resistivity = 50 Ωm

(ii) Fault current = 31500 A

(iii) Grid current = 19461 A = 20000 A

(iv) Size of grid = 105 m x 75 m

(a) Size of earth conductor

Cross-section area Ac = 12.15I√tf = 12.15.31500 √l = 382.7 mm2

With an allowance of 15.0% for corrosion, area of conductor = 440.1 mm2

If round conductor is chosen, MS conductor size = d = 25.0 mm diameter = 0.025 m

If rectangular conductor is chosen, conductor size =10 mm x 45 mm MS strip

(b) Permissible dangerous voltages

K = (ρ – ρ_S) / (ρ + ρ_S) = (50 – 3000) / (50 + 3000) = -0.96721

Magnitude of Cs as per approximation of IEEE Std. 80 is

C_S = 1 – {a(1- ρ/ ρ_S)} / {2h_S + a}, where a = 0.09

C_S =1 – {0.09(1- 50/ 3000)} / {2*0.1 + 0.09} = 0.6961

The corresponding permissible voltages are:

E_S = ( 1000 + 6 ρ_S C_S) 0.116 / √t_s = ( 116 + 0.7 * 3000 * 0.6961 ) / √0.5  = 2215.79 V

E_T = ( 1000 + 1.5 ρ_S C_S) 0.116 / √t_s = ( 116 + 0.17 * 3000 * 0.6961 ) / √0.5 = 676.98 V

In areas where there is no gravel, with surface soil resistivity as 50 Ω m, and Cs = 1.0, permissible magnitude of Es and Et is 213.26 V and 176.35 V, respectively, since

E_S = ( 1000 + 6 ρ_S C_S) 0.116 / √t_s = ( 1000 + 6 *50 * 1) 0.116 / √0.5 =213.26 V

E_T = ( 1000 + 1.5 ρ_S C_S) 0.116 / √t_s = ( 1000 + 1.5 *50 * 1) 0.116 / √0.5 =176.35 V

(c) Preliminary layout of earth conductors and earth resistance

Preliminary layout of earth conductors is shown in Figure

To start with grid conductors are placed at regular spacing = D = 7.5 m.

The number of conductors is as follows:

Number of conductor along the length (long conductors) = 11

Number of conductor along with width (short conductors) = 15

In preliminary layout of grid electrode 25 vertical rods are placed as shown in Figure

Thus, Number of vertical ground rods N_r = 25

Length of each vertical ground rod = L_r =3m

L_R = total length of vertical earth rods = 75 m

The total length of horizontal conductors = L_c = (105 × 11 + 75 x 16) = 2280 m

Other values are as below :

L_p = 360m

L_x = 105 m

L_y = 75m

D_m = 129.03 m

L_t = L_c + L_r = 2355 m

A = 7875 m2

The earth resistance with IEEE formula is obtained as :

R_G =

R_G =  = 0.2695 Ω

(d) Grid current

With new value of earth resistance the grid current is calculated as 18900 A. However the rounded off value of 20000 A is used in further calculations. This takes into account a factor for future growth of fault current.

(e) Actual maximum mesh voltage and step voltage

Actual maximum mesh voltage and step voltage are calculated by using IEEE Std. 80 formulas.

K_ii = 1

K_h = (l+h)^0.5 = 1.2649

n_a = 2 Lc/ Lp = 2 . 2280/360 = 12.6667

n_b = [Lp / (4√A)0.5 = [360 / (4√7875) 0.5 = 1.007

n_c = 1 (for square and rectangular grid electrode)

n_d = 1 (for square and rectangular and L- shaped grid electrode)

n = n_{a .} n_b. n_c. n_d  = 12.7557

The mesh voltage is calculated from the expression

Em = ρ.K_mK_imI_G /L_m

Effective length for E_t= L_m =  = 2398.9 m

K_m =  = 0.61122

K_im = K_is =0.644+ 0.148 n = 2.531845

E_m = 645.11 V

The step voltage is calculated from the expression

E_s = ρ K_s K_is I_G / L_s

 =0.34697

Effective length for E_s is L_s = 0.75 L_c+ 0.85 L_r = 1773.75 m

E_s= 495.27 V

EPR = R_g × I_G = 0.2695 × 20000 = 5390 V

(f) Safety analysis

The maximum mesh voltage that may occur in the grid electrode is 645.11 V, this is less than

the permissible touch voltage of 676.98 V on gravel and is safe. The maximum step voltage

that may occur near a corner of grid electrode is 495.27 V, it is also less than its permissible

value of 2215.79 V on gravel. The permissible magnitude of step voltage if earth surface is

not covered with gravel is 213.26 V, which is less than 495.27 V. It is therefore necessary

that gravel layer of 100 mm thickness be spread and maintained to a distance of 1 m outside

the fence. The IEEE method does not calculate the touch voltage from outside the fence. This

information is presented in tabular form in Table below

(g) Effect of increase in depth of outermost conductor

If the depth of the outermost conductor is increased, it decreases the factor K_s. Thus, the actual step voltage is reduced. In this example if the depth of outermost conductor is increased from 0.6 m to 2 m, calculation of factor K_S is modified with 2h = 2 × 2 = 4 as

 = 0.16129

This makes E_s= 230.23 V

This value is still larger than 213.26 V, the permissible value of step voltage on natural soil. But it demonstrates the effect of increase of depth of burial of the outermost conductor of grid electrode on actual step voltage.