# Design and Analysis of Ground Grid System For Substation using E-TAP Software and FDM code in MATLAB

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#### Design and Analysis of Ground Grid System For Substation using E-TAP Software and FDM code in MATLAB

Design and Analysis of Ground Grid System For Substation using E-TAP Software and FDM code in MATLAB

Deep B. Desai

B.E. Electrical (2014-2018)

AbstractThe paper depicts about design of ground grid system for 66/11KV rectangular substation. Although it is designed by IEEE Std 80-2000, it also illustrates step potential, touch potential and absolute potential in normal and faulty condition by E-TAP intelligent software version 12.6.0. Furthermore, its graph design is analyzed by FDM (Finite Difference Method) via MATLAB code. Some of the factor which are useful in ground grid study are described below,

1. Fault current magnitude and duration

2. Geometry of grounding mat

3. Soil resistivity

4. Human factors Such as

-Body resistance -Standard assumption on physical condition of the individual

Keywords Ground grid system, FDM (Finite Difference Method) MATLAB code, E-TAP version 12.6.0 ground grid design, Substation grounding

1. INTRODUCTION

In substation earthing system is essential not only for providing the protection for people working in the vicinity of earthed facilities and equipment against danger of electric shock but also maintain proper function of electrical system. Reliability and security are to be taken in consideration as well as adherence to statutory obligations (IEEE and Indian standards on electrical safety [1-2] and environmental aspects). This paper is concerned with earthing practices and design for particular 66/11KV outdoor AC substation for power frequency in the range of 50 Hz. DC substation GIS and lightening effects are not covered in this paper. Moreover, the software output also has been seen for crosschecking of theoretical calculation. Furthermore, the analysis of software graph is done by FDM (Finite Difference Method) which is one of the mathematical solutions and it shows that how the graph generates in software.

2. EARTHING SYSTEM FOR SUBSTATION

1. Components of earthing system

An effective substation earthing system typically consists of earth rods, connecting cables from buried earthing grid to metallic parts of structures and equipment, connections to earthed system neutrals, and the earth surface insulating covering material briefly discussed in [1,3]. Current flowing into the earthing grid from lightening arrester operation impulse or switching surge flashover of insulators and line to ground fault current from the bus or connected

transmission lines all cause potential differences between earthed points in the substation. Without a properly designed earthing system, large potential differences can exist between different points within the substation itself. Under normal circumstances, it is the current constitutes the main threat to personal.

2. Practical Design Problem

1. All dimensions are in mm unless otherwise specified.

2. Earth mat & earth electrode location shown are indicative. Only minor modification if any may be carried out at site.

1. Wherever earthing conductor infringes with cable trench it shall be land at 300mm below the trench.

2. Main grid conductor shall lad at minimum 600mm below figure.

3. minimum distance of 6000mm shall be maintained between any two adjacent earth electrodes.

4. wherever earthing grid infringes with foundation, grid conductor shall be diverted suitable at site.

5. Every equipment/structure shall be connected to grid by two distinct earth connections.

6. Every alternate fence post shall be earthed with 25x3mm galvanized iron flat.

7. Gate shall be earthed with 25x3mm galvanized iron flat.

8. BMKs junction for current transformer and potential transformer shall be earthed at two points with 25x3mm galvanized iron flat.

9. For standard earthing drawings of various equipment refer figure.

3. MATHEMATICAL SOLUTION

1. Given Data: –

1. Fault duration tf=1s

2. Current division factor (sf)=1

3. Soil resistivity ()=5.23m

4. Crush rock resistivity(wet) ()=3000m

5. Thickness of crushed rock surfacing (hs)=0.1m

Etoucp0 =

(RB + 1.5sCs)0.116

ts

6. Depth of grid burial (h)=0.3m

7. Available grounding area (A)=48mÃ—18m

8. Fault current (If)=25000A

2. Solution According to IEEE Std 80-2000

• Conductor size

=476.746V . (7)

(RB + 1.5sCs)0.157

Etouch70 =

ts

=646V . (8)

Here step and touch potential foe 50kg body weight.

104

k0 + T m

Amm2 = I/(TCAP t ) ln(K +T

) (1)

c r r 0

Amm2 =25000/80

=312 2

Amm2 = (d2)/4 d2 = (500*4)/

= 25.23mm .. (2)

Diameter of vertical ground rod = 25.23 Ã—1.3 =32.799 mm (considering 30% corrosion allowance) Commercially rod of 32.799 mm diameter is not available so rod equivalent to 40.00 mm diameter is considered.

• Touch and Step Criteria Reflection factor (K)

s

k = ( )

• Initial Design Grid burial depth h= 0.6m,

Total length of buried conductor ()=(18*9) +(48*4)

=192+162=354m

Total length of buried conductor

(Lc)=26*3=78m L = Lc + Lt

=354+78

=432m (9)

Area (A)=48m*18m

=864m2 .. (10)

• Determination of Grid resistance

+ s

=-0.996

Here from the value of the reflection factor according to the

1

Rg = [(

Lt

1

) + (

20A

1

)(1 + ( )

1 + p0

A

graph of K hs

Value of hs is taken 0.1m.

Surface layer darting factor

=0.0885 . (11)

• Maximum Grid Resistance

Ig

Cs = ((1

0.09(1( ))

s

)

2h +0.09

Sf =

3I0

s

=0.693 (3)

Step potential for 50kg and 70kg

Estep50 = Ib(RB + 6sCs) . (4)

(RB+6sCs)0.116

IG = Df Ig

IG = Df Sf 3I0

=25000A (12)

• Ground Potential Rise(GPR)

GPR = Ig Rg

=2208.756V (13)

Estep50 =

ts

• Mesh voltage

=1557.98V . (5)

n = na

nb

nc

nd

Estep70 =

(RB + 6sCs)0.157

ts

2Lc

na = ( )

L

p

=5.3

=2115.41V .. (6) Touch potential for 5kg and 70kg

Etoucp0 = Ib(RB + 1.5sCs)

Lp

nb =

4A

=1.056

nc = nd=1

n= 5.3*1.056*1*1=5.7 (14)

h

kh = 1 +

h

o

=1.140 Kii= 1

• Output in E-TAP

1

) [ln

(D2)

D + 2h

+ (

4

) ( )

km = (2

(16hd)

kii

+ (

) ln(

8Dd 4d 8

)]

=0.6537

kh (2n 1)

KI = 0.644 + 0.148n

= 1.487

LC is the total length of the conductor in the horizontal grid in m

Lp is the peripheral length of the grid in m A is the area of the grid in m2

Lx is the maximum length of the grid in the x direction in m Ly is the maximum length of the grid in the y direction in m Dm is the maximum distance between any two points on te grid in m

• Mesh Voltage (Em) & Step Voltage (ES)

Em =

IakmKi

Lr

Lc + [1.55 + 1.22

]Lr

x2 + y2

= 351.84 V < Etouch (15)

1 1

1

1

n2

• Absolute Potential Profile (GPR)

Where

Ks = [2h + D + H + D (1 0.5 )]

=0.630

D = spacing between conductor of the grid in m h = depth of burial grid conductor in m

n = number of parallel conductors in one direction

• Step Potential

= 0.75

+ 085

=368.85 V < Estep (16)

Here as shown above and computed mesh voltage and Step voltage both are much less than the step potential criteria and touch potential criteria. So, obtained design is safe.

4. E-TAP GRAPH AND FDM MATLAB PROGRAM

• Touch Potential

• Absolute Potential Calculation using FDM (MATLAB Code) 

MATLAB PROGRAMME: v1=2208.0;

v2=0.0; v3=0.0; v4=2208.0; ni=200;

nx=25; ny=10;

v=zeros(nx,ny); for i=2:nx-1

v(i,1)=v1;

v(i,ny)=v3; end

for j=2:ny-1 v(1,j)=v4;

v(nx,j)=v2; end

v(1,1)=0.5*(v1+v4);

v(nx,1)=0.5*(v1+v2);

v(1,ny)=0.5*(v3+v4);

v(nx,ny)=0.5*(v2+v3); for k=1:ni

for i=2:nx-1 for j=2:ny-1

v(i,j)=0.25*(v(i+1,j)+v(i-1,j)+v(i,j+1)+v(1+j-1)); end

end end

diary FDM.out

[v(1,1), v(6,2), v(12,4), v(24,9)] [ [1:nx, 1:ny] v(i,j) ] diary off

OUTPUT

ans =

1.0e+003 * [2.2080 2.2001 2.1735 0.8082]

5. CONCLUSION

This paper has focus on ac substation grounding design and also compare theoretical as well as software output and analyzed mathematically calculation for generation of graph in software program by FDM coding in MATLAB.

By the calculation it has been proved that calculated step and touch potential must be less than tolerable limit of step and touch potential to made safe design for grounding system for substation. If any of the conditions or both conditions are not satisfied then varying different parameter in the calculation (i.e. soil resistivity, depth of buried rod, increasing number of earth pit in substation, use different material for rod, etc.) and recalculate whole design until both the conditions are not satisfied.

6. ACKNOWLEDGMENT

I hereby declare that all the given data, information and calculation are true and calculated from real example. The diagram of substation which is shown above is rear.

7. REFERENCE

1. N.M. Tabatabaei, S.R. Mortezaeei; Design of Grounding System in Substation by ETAP Intelligent Software , IJTPE Journal, March 2010, Page(s):45-49

2. Chae-Kyun Jung; Jong-kee choi; Ji-won Kang, A study on effect of grounding system on transient voltage in 154kV Substation, IEEE Conference Publications, 2009, Page(s): 1- 6

3. Ladanyi, J. Survey of the equivalent Substation earth current and earth impedance for transformer Station, IEEE Conference Publications, 2011, Page(s): 1-5

4. IEEE Std. 80-2000, IEEE Guide for Safety in AC Substation Grounding, IEEE: Institute of Electrical and Electronic Engineers, Inc., New York, 2000, Page(s) 1- 192

5. Hadi Sandhue -chapter 15 Numerical Method Page(s):- 660- 727