Factors That Affect Substation Ground Grid Design

Jerry Johnson POWER Engineers, Inc.

Hailey, Idaho, USA (208) 788-3456

Presented at the POWER Engineers Substation Conference

September 1999

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Table of Contents

ABSTRACT....................................................................................................................... 1

1.0 INTRODUCTION...................................................................................................... 1

2.0 BASIC GROUNDING CONSIDERATIONS.......................................................... 1

3.0 SOIL RESISTIVITY ................................................................................................. 3

4.0 FAULT CLEARING TIMES.................................................................................... 6 4.1 EFFECTS ON THE HUMAN BODY............................................................................. 6 4.2 HIGH SPEED FAULT CLEARING .............................................................................. 6 4.3 EFFECTS ON TOLERABLE STEP AND TOUCH POTENTIALS....................................... 7

5.0 FAULT CURRENT ................................................................................................... 8 5.1 CALCULATION OF SF .............................................................................................. 9 5.2 DECREMENT FACTOR DF...................................................................................... 10

6.0 SUMMARY .............................................................................................................. 11

REFERENCES................................................................................................................ 11

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Factors that Affect Substation Ground Grid Design

Abstract This paper discusses three major factors that influence substation grounding system

design: 1) Soil Resistivity, 2) Fault Clearing Time and 3) Ground Fault Current. Each

must be considered along with short-comings of the analysis software and alternative

designs to develop a safe and the most economic solution.

1.0 Introduction As available fault currents increase on today’s electrical power grid, interest in substation

grounding system design also increases. Personnel safety is primary, but the economics

are also a key factor. Engineers do not wish to “over design” grounding systems, but they

do want to design systems that protect personnel and equipment while providing an

optimized economic solution.

This paper discusses how each of these three factors can affect the design of a substation

grounding system.

2.0 Basic Grounding Considerations Line-to-Ground faults occurring in or near a substation cause a current to flow from the

energized line through the buried ground grid in the station back to the source. This

current flow causes the grid to “rise in potential” above remote areas that are considered

to be at zero potential. The flow of current also causes the potential (voltage) or Ground

Potential Rise (GPR) to vary at different points in the substation. This can produce a

potential difference or Step Voltage between the feet of an individual standing on the

surface of the soil. This current flow can also cause a potential difference between metal

structures and various points on the surface of the soil. This difference results in a Touch

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Voltage between the hands of an individual touching a structure and the individual’s feet

on the surface of the soil as illustrated in Figure 1.

Figure 1: Basic Shock Situations The human body can withstand considerable voltage for a short period of time. For a

worker to be safe within a substation, this value must be below the level that will cause

the heart to fibrillate. The values of maximum allowable step and touch potentials for a

person weighing 50kg or 110lbs is defined as:

Estepts

=+(1000 6Cs )0.116sρ Eq 1 [1].

Etouchts

=+(1000 1.5Cs )0.116sρ Eq 2 [1].

Where: Cs = the reduction factor for a high resistivity layer of crushed rock. ρs = the resistivity of the surface material in Ω-m ts = the duration of the shock current in seconds.

Usually the ground grid design will require a layer of high resistivity crushed rock placed

on the surface of the substation to act as an insulator between a person’s feet and the

substation grid and to raise the tolerable voltages. When the station grid is designed, this

feature can be used to decrease the amount of buried conductor used or to increase the

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safety margin in the design. However, the cost of the crushed rock layer should be

considered since high resistivity rock may be difficult to find in certain areas.

3.0 Soil Resistivity The potential differences within a station result from the ground current flowing from the

grid conductors into the surrounding soil (earth). All soils have some resistance to

electric current flow which is measured as “resistivity”. The electrical resistivity of the

various layers of soil have a great influence on the resulting step and touch potentials

within the substation.

The Canadian Electrical Association conducted an extensive study of various types of

soil under a variety of conditions [2]. The study found that the resistivity of the soil

varied with soil type, density, moisture content, temperature and state (frozen or

unfrozen). Figure 2 shows how a clay soil was found to vary with temperature and state

for three different moisture contents.

Figure 2: Soil Resistivity Variation with Moisture and Temperature As Figure 2 inicates, the resistivity rises linearly on the log scale as the temperature

drops; but after passing the frozen state it rises rapidly with the falling temperatures. The

percent moisture also has a strong influence on the resistivity. The higher the moisture

content, the higher the resistivity becomes when the soil is in the frozen state and lower

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in the unfrozen state. It is important for the engineer/designer to determine moisture

content and temperature the soil can experience in the area of the substation when the

station grid is being designed. They also need to keep in mind the frost depth and

whether the ground rods penetrate unfrozen soil year round.

Figure 3 shows the ground grid that was used in the analysis. The following parameters

were used:

Station Size 180 ft by 80 ft Conductor Burial Depth 18 inches Conductor Size 4/0 AWG Copper Grid Mesh Size 20 ft by 20 ft Fault Current 10,000 Amperes Fault Clearing Time 0.25 seconds Soil Resistivity 25 Ω-m Insulating Layer 4” of 3000 Ω-m Crushed Rock

Figure 3: Substation Grid Layout Analysis of the grid was done using Safe Engineering Services, CDEGS Program [3]

using a uniform soil model of 25 Ω-m and 100 Ω-m. The values were taken from Figure

2, Curve 2 for temperatures of 5°C (41°F) and -5°C (23°F). The results of the case runs

show that for the 25 Ω-m case, the station meets IEEE 80 standards for both step and

touch potentials as illustrated in Figures 4 and 5. However, for the 100 Ω-m case,

allowable touch voltages are exceeded throughout the station grid as shown in Figure 6.

180 ft

80 ft

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Figure 4: Step Potential Plot for 25 Ω-m Soil Case

Figure 5: Touch Potential Plot for 25 Ω-m Soil Case

Figure 6: Touch Potential Plot for 100 Ω-m Soil Case

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Possible solutions would be to add additional conductor and/or ground rods to lower the

potential differences to allowable values. Alternative means to achieve safety include

equipment platforms electrically connected to the equipment or 6” buried mesh around

the equipment.

4.0 Fault Clearing Times

4.1 Effects on the Human Body The effects of an electrical current passing through vital parts of the human body depend

on the magnitude, duration and frequency of the current. The most serious consequence

from exposure is ventricular fibrillation, resulting in stoppage of blood circulation.

The human body is very susceptible to the effects of current at power frequencies (50Hz

and 60Hz). Currents of approximately 0.1mA can be lethal. The most common effects of

electrical shock on the body are perception, muscular contraction, unconsciousness, heart

fibrillation, respiratory nerve blockage and burning [1].

4.2 High Speed Fault Clearing Another means to reduce the dangerous fault circumstances is to modify the fault clearing time. High speed fault clearing has two main advantages: 1. The probability of shock is significantly reduced by a fast clearing time in contrast to

faults that persist for several minutes.

2. Experience as well as tests show that the chance for serious injury or death is reduced

if the duration of current through the body is very brief. The allowable current values

may be based on the primary relaying or protective devices, or in some cases, that of

the backup relaying.

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IEEE 80 states that a good case can be made for using the primary protective device

clearing time to calculate the maximum allowable step and touch potentials. This is due

to the low combined probability that the primary relay malfunctions will coincide with all

other adverse factors that are needed for an accident [1].

4.3 Effects on Tolerable Step and Touch Potentials Equations 1 and 2 show the relationship of the fault clearing times to the allowable values

of step and touch potentials. Since ts is in the denominator of each equation, the smaller

the clearing time the larger the allowable values of step and touch. Using fault clearing

values of 0.25sec. and 1.0sec. in the previous 25 Ω-m case analyzed, shows that the

allowable touch potential is exceeded for the 1.0 sec. case along the perimeter of the

station as shown in Figure 7.

Figure 7: Touch Potential Plot for 25 Ω-m Soil Case 1.0 second Clearing Time

To design the station to meet backup relaying contingencies, additional ground conductor

(smaller grid spacing) would be required for the substation to meet IEEE 80 standards for

touch potential.

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5.0 Fault Current The determination of the maximum grid current to be used in substation grounding

design has been receiving a lot of attention. For many years, engineers have been using

the maximum line to ground fault values in their designs. This approach is being looked

at more closely. Computer analysis and other techniques can be used to determine the

value for the maximum grid current. This can result in a more cost-effective substation

ground grid that still meets IEEE 80 standards [1].

Another reason for determining a more accurate value of the grid current is the increasing

magnitudes of fault currents. These increasing currents have a direct relationship to

increasing the GPR making it difficult and expensive to protect communication circuits

[4].

The symmetrical grid current that flows between the ground grid and the surrounding

earth is defined as:

Ig = SfIf Eq 3 [1].

Where: Ig = symmetrical grid current in Amps.

Sf = current division factor relating the magnitude of fault current to that of the grid current.

If = rms value of the symmetrical ground fault current in Amps. The design value of the maximum grid current is then defined as:

IG = CpDfIg Eq 4 [1]. Where: IG = maximum grid current in Amps.

Cp = projection factor for the increase in fault current during the station life-span. Cp = 1 is for zero future growth.

Df =Decrement factor based on the fault clearing time, ts.

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5.1 Calculation of Sf

Computer programs are available to calculate the current division factor Sf. These

programs accurately calculate the actual maximum ground fault current flowing in the

grid, but they require a considerable amount of data to be input into the program. In many

cases, an approximation can be used to estimate the value of Sf. In some situations this

approximation value is sufficient for designing the grounding system.

Southern Company Services and Georgia Power Company developed a set of curves,

which can be used to approximate Sf [4]. The information required to use these graphs is

the number of transmission and distribution lines at the substation and the substation grid

resistance. A typical graph is shown in Figure 8.

Figure 8: Percent Grid Current Versus Substation Grid Resistance

For the case run previously, if one transmission line serves the station and two

distribution feeders exit the station and the calculated grid resistance is 0.3 Ω (from the

output of the CDEGS Program), Figure 8 can be used to determine Sf . This value is

approximately 75% or 0.75. Using this value of Sf , the symmetrical grid current Ig is

equal to 0.75*10,000 or 7,500Amps.

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5.2 Decrement Factor Df The maximum grid current IG, is the maximum asymmetrical ac current that will flow

between the station grid and the surrounding soil. This current is defined by Equation 4

and includes the symmetrical current Ig as well as a correction factor for the dc

component, the transient and subtransient ac components. Both the ac transient and

subtransient as well as the dc component decay exponentially. The design of the station

ground grid must take into account the asymmetrical current; therefore a Decrement

Factor, Df is derived to take into account these asymmetrical components. The decrement

factor can be computed by using Equation 5.

Df Tatf

e tf Ta= + − −1 1( ) Eq 5 [1].

Where: tf = fault clearing time in seconds. Ta = equivalent system subtransient time constant in seconds. Typical values of the decrement factor are provided by IEEE 80 for an assumed X/R ratio

of 20 and are shown in Table 1.

Table 1: Typical Decrement Factors

Fault Duration (sec.) Cycles (60Hz ac) Decrement Factor 0.008 0.5 1.65 0.1 5 1.25 0.25 15 1.10

0.5 or longer 30 or more 1.0 Using the typical value of the decrement factor for the fault clearing time of 0.25 sec., a

growth projection factor of 15% and data previously calculated yield IG as:

IG = (1.15)(1.10)(7,500) IG = 9,487A

This value would be used in the final design of the substation grid instead of the 10,000

Ampere value used previously.

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6.0 Summary The soil’s electrical resistivity can vary with the temperature and moisture content. These

variables need to be taken into account in the design and analysis to ensure the station

grid meets IEEE 80 standards for the varying soil conditions.

The human body can withstand exposure to high current levels for only a short period of

time. High-speed fault clearing is essential to minimize the exposure time to the levels of

fault currents available in most substations. Quick clearing of faults also allows higher

permissible touch and step voltages.

The maximum grid current must also be considered. Much of the station’s ground fault

current may be carried out of the station by overhead static wires or system neutral wires.

Calculation of the current division factor is important, so that the station’s grid is not over

designed.

Many factors are involved in the design of a substation ground grid. These factors

determine the extent and amount of ground conductor required for the substation grid

design. When these factors are optimized, an economic station ground grid that meets or

exceeds IEEE standard can be designed.

References 1. “IEEE Guide for Safety in Substation Grounding”, IEEE Standard 80-1986, Institute

of Electrical and Electronic Engineers, Inc. New York, 1986. 2. “Earth Resistivities of Canadian Soil”, Research Report 143 T 250, Canadian

Electrical Association, July 1988. 3. CDEGS User Manual, Safe Engineering Services, Montreal Canada, 1998. 4. Garrett, D. L., Myers, J. G. and Patel, S. G., “Determination of Maximum Substation

Grounding System Fault Currents Using Graphical Analysis”, IEEE Transactions on Power Delivery, Vol. PWRD-2, No. 3, pp. 725-732, July 1987.