TABLE OF CONTENTS

Table of Contents

I. Introduction

a. Importance of Substation Grounding

b. Typical Shock Situations

II. Design Equations

a. Tolerable Voltage Limits

b. Conductor Sizing

c. Ground Grid Resistance

d. Maximum Grid Current

e. Ground Potential Rise

f. Mesh Voltage

g. Step Voltage

h. Evaluation of Calculated Parameters

i. Use of Design Equations

III. Special Danger Points

a. Substation Fence

b. Operating Handles

c. Metallic Cable Sheaths

d. Surge Arrester Grounding

e. Grounding of Lightning Shield

IV. Working Example

Annex A

References

I. Introduction

a. Importance of Substation Grounding

The substation grounding system is an essential part of the overall electrical system. The proper grounding of a substation is important for the following two reasons:

1. It provides a means of dissipating electric current into the earth without exceeding the operating limits of the equipment.

2. It provides a safe environment to protect personnel in the vicinity of grounded facilities from the dangers of electric shock under fault conditions.

The grounding system includes all of the interconnected grounding facilities in the substation area, including the ground grid, overhead ground wires, neutral conductors, underground cables, foundations, deep well, etc. The ground grid consists of horizontal interconnected bare conductors (mat) and ground rods. The design of the ground grid to control voltage levels to safe values should consider the total grounding system to provide a safe system at an economical cost.

b. Typical Shock Situations

Figure 1 and Figure 2 show the five voltages a person can be exposed to in a substation. The following definitions describe the voltages.

Figure 1. Shock Situations

Figure 2. Situation of transferred potential.

Ground potential rise (GPR):The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. GPR is the product of the magnitude of the grid current, the portion of the fault current conducted to earth by the grounding system, and the ground grid resistance.

Mesh voltage:The maximum touch voltage within a mesh of a ground grid.

Metal-to-metal touch voltage:The difference in potential between metallic objects or structures within the substation site that can be bridged by direct hand-to-hand or hand-to-feet contact.

Step voltage:The difference in surface potential experienced by a person bridging a distance of 1 m with the feet without contacting any other grounded object.

Touch voltage:The potential difference between the ground potential rise (GPR) and the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure.

Transferred voltage:

A special case of the touch voltage where a voltage is transferred into or out of the substation, from or to a remote point external to the substation site. The maximum voltage of any accidental circuit must not exceed the limit that would produce a current flow through the body that could cause fibrillation.

II. Design Equations

a. Tolerable Voltage Limits

where;

ρs = resistivity of surfacing layer

ts = duration of shock in seconds

Cs = reduction factor

hs = crushed rock layer thickness, meters

ρ = earth resistivity, ohm-meter

Estep50 = the limit of step voltage for a 50 Kg person

Etouch50 = the limit of touch voltage for a 50 Kg person

b. Conductor Sizing

The size of grid wire is determined as follows:

where;

I = current in kA

A = conductor current in sq mm

Tm = maximum allowable temperature in degrees Celsius

Ta = allowable temperature in degrees Celsius

Tr = ref temperature for material constant, degrees C

α0 = thermal coefficient at 0 degrees C

αr = thermal coefficient at Tr, degrees C

ρr = conductor resistivity at Tr, μΩ/cm3

K0 = 1/ α0

tS = duration of current flow

TCAP = thermal capacity factor. J/cm3/0C

c. Ground Grid Resistance

By Sverak’s equation;

where;

Rg = resistance of ground grid

ρ = soil resistivity

L = total length of buried conductor

A = area of the grid

h = depth of buried conductor, meters

d. Maximum Grid Current

Ig = (If) (Sf) (Df)

Where;

Sf = fault division factor (Refer to IEEE Std 80-2000 Table C.1)

Ig = grid current

If = maximum line to ground fault current

Df =decrement factor

tf =fault clearing time

Ta =X/(120R)

e. Ground Potential Rise

GPR = (Rg)(Ig)

If GPR is less than the touch voltage no further calculations are required.

f. Mesh Voltage

The general equation for mesh voltage can be expressed as follows:

Where;

Em = mesh voltage, volts

ρ = soil resistivity, ohm-meter

Km = geometric factor

IG =Grid current

Ki =Spacing factor for mesh voltage

Lm =Effective length for mesh voltage

where;

D = spacing between parallel conductors, meters

h = depth of ground grid conductors in meters

d = diameter of ground grid conductor in meters

Kii = 1 for grids with ground rods along the perimeter, or for grids with ground rods in the grid corners, as well as both, both along the perimeter and throughout the grid area

n =effective number of parallel rods

Where; h0 = 1 m (reference depth of grid)

n=nanbncnd where;

Ki = 0.664 + 0.148n

Since the grounding is a combination of grid wires and rods, the effective length of grounding conductor is:

Where;

Lr =Length of each ground rod

Lx =Total length of conductors along the X-axis

Ly =Total length of conductors along the Y-axis

LR =Total length of ground rods

Lc =Total length of conductors w/o ground rods

g. Touch Voltage

Where;

Es = step voltage, volts

ρ = soil resistivity; equal to 100 ohm-m

Ks = geometric factor; equal to 2.33

Ki = corrective factor

IG = maximum grid current; equal to 6837.4A

Ls = Effective length of buried conductor, m

Ls = 0.75LC+0.85LR

h. Evaluation of Calculated Parameters

A safe and effective grounding grid design has been achieved if the following are met:

1. Rg<5 ohms for distribution substation and <1 ohm for transmission substation

2. The calculated design mesh voltage of the grid should be less than the calculated tolerable step voltage.

3. The calculated design touch voltage of the grid should be less than the calculated tolerable touch voltage.

Revisions to the design of the grounding grid should be done if the three items are not complied.

i. Use of Design Equations

The design equations above are limited to a uniform soil resistivity, equal grid spacing, specific buried depths, and relatively simple geometric layouts of the grid system. It may be necessary to use more sophisticated computer techniques to design a substation ground grid system for non-uniform soils or complex geometric layouts.

III. Special Danger Points

a. Substation Fence Grounding

The grounding of the substation fence is critical because the fence is generally accessible to the public. The substation grounding design should be such that the touch potential on both sides of the fence is within the calculated tolerable limit of touch potential.

The substation fence should be connected to the main ground grid by means of an outer grid conductor installed a minimum of 0.91 meter (3 feet) (approximately one arm’s length) outside the substation fence. Connections to the outer grid conductor should be made at all corner posts and at line post every 12.92 to 15.24 meters (40 to 50 feet). The gateposts should be securely bonded to the adjacent fence. It is recommended that all gates swing inward and be designed and installed to prevent an outward swing. If gates are installed with an outward swing, then the ground grid should extend a minimum of 0.91 meter (3 feet) past the maximum swing of the gate.

b. Operating Handles

Equipment operating handles are a special circumstance because of the higher probability for coincidence of adverse factors, namely, the presence of a person contacting grounded equipment and performing an operation that can lead to electrical breakdown.

If the grounding system is designed conservatively for safe mesh potentials, then the operator should not be exposed to unsafe voltages. However, because of the uncertainty inherent in substation grounding design, a metal grounding platform (mat), connected to the operating handle should be placed where the operator should stand on it to operate the device regardless of whether the operating handle is insulated.

Considerations involved in the switch grounding platform ground conductor include the following:

1. Proper grounding calculations and grid design should result in acceptable touch and step potential voltages without the additional grounding platform grounding. However, since the operation of the switch places the operator directly at risk when a substation fault occurs, additional precautions are needed. This includes adding switch grounding platforms and a 3 to 6-inch layer of clean crushed rock that covers the entire area inside the substation fence and extends 3 to 4 feet outside the substation fence to reduce the risk of electric shock.

2. Switch grounding platform grounding is added to minimize the voltage between the switch operator’s hands and feet in the event of a fault at the switch during manual operation. The grounding platform should be connected to the operating handle by a copper cable that connects to the operating handle and the grounding platform as shown in Figure 3.

Figure 3. Typical switch grounding.

c. Metallic Cable Sheaths

Metallic cable sheaths in power cable have to be effectively grounded to prevent dangerous voltages resulting from insulation failure, electrostatic and electromagnetic induction, flow of fault current in the sheath, and voltage rise during fault current flow in the substation ground system to which the sheaths are connected. Cable sheaths should be grounded at two or more locations: at cable terminations and at splices and taps. Control cable shields are not intended to carry significant current and thus should only be grounded at one end. Where any cable sheath may be exposed to excessive ground current flow, a parallel ground cable should be run and connected to both ends.

d. Surge Arrester GroundingSurge arresters are designed to pass surge energy from lightning and switching transients to ground and so are frequently subjected to abnormal current flow to ground. They have to be reliably grounded to ensure protection of the equipment they are protecting and to minimize high potential gradients during operation.

The surge arrester grounds should be connected as close as possible to the ground terminals of the apparatus to be protected and have as short and direct a

path to earth as practical. Arrester leads should be as free from sharp bends as practical. The tanks of transformers and steel or aluminum structures may be considered as the path for grounding arresters, provided effective connections can be made and secure multiple paths are available.

e. Grounding of Lightning ShieldA shielding system cannot effectively protect substation equipment unless adequately grounded. Multiple low impedance connections from the shielding system to the substation ground grid are essential. It is beneficial to use at least two separate connections to ensure continuity and reliability. Whenever non-conducting masts or supports are used, install separate ground cables to establish a direct connection from the shield system to the substation ground system.

IV. Working Example

Technical data of the substation:

Lot dimension:Length = 24 meters

Width = 18 meters

Maximum ground fault current = 6837.4 Amps at the low side

X/R Ratio = 40

Number of incoming lines: 1 – 69 KV line

Number of distribution neutrals: 4

Data gathered for the design computation:

Soil resistivity = 100 ohm-m

Resistivity of gravel surfacing = 5000 ohm-meter

Thickness of crushed rock surfacing = 0.1 m

Fault duration = 0.3 second

The grounding design is based on the effect of shock on a 50 Kg person.

The grid wires will be buried at 1.5 meters below the surface of the earth fill

DESIGN COMPUTATIONS

1. The area to be occupied by the gridArea = 24 m x 18 m

= 432 sq. m.

2. Determination of minimum grid-wire sizeMaximum ground fault current is a single line to ground fault = 6837.4 A

From the formula of minimum conductor size, the size of grid wire is determined

as follows:

where;

I = current in kA

A = conductor current in sq mm

Tm = maximum allowable temperature in degrees Celsius

Ta = allowable temperature in degrees Celsius

Tr = ref temperature for material constant, degrees C

α0 = thermal coefficient at 0 degrees C

αr = thermal coefficient at Tr, degrees C

ρr = conductor resistivity at Tr, μΩ/cm3

K0 = 1/ α0

tS = duration of current flow

TCAP = thermal capacity factor. J/cm3/0C

For a hard drawn copper with 97% conductivity.

Tm = 1084 0C

Ta = 30 0C

Tr = 20 0C

αr = 0.00381 @20 0C

ρr = 1.7774 μΩ/cm3

K0 = 242

ts = 0.3 sec (assumed shock duration)

TCAP = 3.422 J/cm3/0C

Solution:

A = 25.64 sq mm

Wire size to be used is 100 sq mm or 4/0 AWG hard drawn copper for added ruggedness.

3. Touch and step potential

where;

ρs = resistivity of surfacing layer

ts = duration of shock in seconds

Cs = reduction factor

hs = crushed rock layer thickness, meters

ρ = earth resistivity, ohm-meter

Estep50 = the limit of step voltage for a 50 Kg person

Etouch50 = the limit of touch voltage for a 50 Kg person

According to IEEE Std 80-2000

The step and touch voltages are calculated based on the effect of shock on a 50-Kg person, hence,

Estep50 = 4633 Volts

Etouch50 = 1317 Volts

4. Initial design of grid

In the initial design of grounding, it will be assumed that the grid wires will be equally spaced at 6 meters both ways. Thus, the number of parallel conductors along the lot length is;

NL = (18/6) +1

= 4

The number of parallel wires along the lot width is;

NW = (24/6) +1

= 5

The design thus becomes a 4 x 5 grid. Assuming that the total number of ground rods is 31, each, 3 meters long, the total length of the buried conductor is;

L = (4 x 18) + (5 x 24) + (31 x 3)

= 285 meters

5. Determination of grid resistance

By Sverak’s equation;

where;

Rg = resistance of ground grid

ρ = soil resistivity

L = total length of buried conductor

A = area of the grid

h = depth of buried conductor, meters

The computed grid resistance is;

Rg = 2.4 ohms

6. Maximum grid current

Ig = (If) (Sf) (Df)

Where;

Sf = fault division factor (Refer to IEEE Std 80-2000 Table C.1)

Ig = grid current

If = maximum line to ground fault current

Df =decrement factor

For a 1 incoming transmission line and 4 feeder neutrals, the fault division factor is (See Table 1 of Annex A for IEEE Std 80-2000 Table C.1):

The decrement factor is:

;

Where;

tf =fault clearing time

Ta =X/(120R)

Therefore;

Ig = (If) (Sf) (Df)

= (6837.4) (0.272) (1.163)

=2,162.85A

7. The ground potential rise

The ground potential rise is the product of symmetrical grid current and the grid resistance.GPR = (Rg)(Ig)

= (2,162.85) x(2.4)

= 5,190.84 volts

From the above result, the GPR exceeds the touch voltage. Therefore, some design corrections have to be made.

8. The mesh voltage

The general equation for mesh voltage can be expressed as follows:

Where;

Em = mesh voltage, volts

ρ = soil resistivity, ohm-meter

Km = geometric factor

IG =Grid current

Ki =Spacing factor for mesh voltage

Lm =Effective length for mesh voltage

where;

D = spacing between parallel conductors, meters

h = depth of ground grid conductors in meters

d = diameter of ground grid conductor in meters

Kii = 1 for grids with ground rods along the perimeter, or for grids

with ground rods in the grid corners, as well as both, both along the perimeter and throughout the grid area

n =effective number of parallel rods

Where; h0 = 1 m (reference depth of grid)

Kh = 1.2247

n=nanbncnd =4.5950 where;

Therefore,

Km = 0.79769

Ki = 0.664 + 0.148n

=1.344

Since the grounding is a combination of grid wires and rods, the effective length of grounding conductor is:

Where;

Lr =Length of each ground rod

Lx =Total length of conductors along the X-axis

Ly =Total length of conductors along the Y-axis

LR =Total length of ground rods

Lc =Total length of conductors w/o ground rods

Therefore;

Lm =347.496 meters

Therefore, the mesh voltage is;

Em = 657.386 Volts

9. Touch voltage criterion

The calculated mesh voltage, which is 657.386 volts, is lower than the touch voltage, which is 1317 volts.

10. Step voltage

Where;

Es = step voltage, volts

ρ = soil resistivity; equal to 100 ohm-m

Ks = geometric factor; equal to 2.33

Ki = corrective factor

IG = maximum grid current; equal to 6837.4A

Ls = Effective length of buried conductor, m

= 0.41155

Ls = 0.75LC+0.85LR

=223.05 meters

Es = 528.395Volts

11. Step Voltage Criterion

The calculated step voltage of 528.395 volts is lower than the Estep50 which is 4633 volts limit.

12. ConclusionA safe substation grounding design has been achieved.

Annex A

Table 1. IEEE Std 80-2000 Table C.1 Approximate equivalent impedances of transmission lineoverhead shield wires and distribution feeder neutrals

Table 1. IEEE Std 80-2000 Table C.1 Approximate equivalent impedances of transmission lineoverhead shield wires and distribution feeder neutrals (continued)

References

1. IEEE Std 80-2000. IEEE Guide for Safety in AC Substation Grounding (Body Part 1).

2. IEEE Std 80-2000. IEEE Guide for Safety in AC Substation Grounding (Body Part 2).

3. IEEE Std 80-2000. IEEE Guide for Safety in AC Substation Grounding (Annexes Part 1).

4. IEEE Std 80-2000. IEEE Guide for Safety in AC Substation Grounding (Annexes Part 2).

5. RUS Bulletin 1724E-300. Design Guide for Rural Substations