Design, Modeling, and Analysis of IEEE Std 80 Earth Grid Design Refinement Methods Using ETAP

Abstract

The design of a compliant, safe, and reliable substation earth grid is not a straightforward process; in most cases, it requires some additional measures to be taken due to various constraints that differ from environment to environment, such as soil resistivity, a high fault level, a limited surface area, construction budget, etc. The IEEE Std 80-2013 proposes various refinement methods that can be applied to address different situations. For this study, the current limiting method, current diversion method, and touch and step voltage increment method were applied using the Electrical Transient Analysis Program (ETAP). A power system was designed, where a fault current generated by the supply transformers and back-fed by the power system’s motors was exported to the earth grid. Using this fault current, various simulations were conducted to assess the performance of the earth grid. The analysis results show that the application of the current limiting method using neutral earthing resistors has a great impact on the design of the earth grid as this method significantly reduces the fault current injected into the earth grid. Furthermore, by applying the current diversion method, the amount of fault current injected into the earth grid is reduced by a fair amount, which improves the performance of the earth grid. Lastly, increasing the tolerable limits of touch and step voltages by reducing the fault clearance times significantly improves the compliance of the earth grid as the clearance time is reduced. From this study, it is therefore concluded that, by implementing the refinement methods depending on the design requirements and feasibility of the application, one can improve the compliance state of an earth grid.

1. Introduction

The substation earth grid is one of many key role players that ensure a continuous reliable supply of a distribution substation. The primary function of the earth grid is to provide a low resistive path to earth for fault currents that pose a danger to the substation equipment, its reliability, and people in the vicinity of the substation. Various design elements such as soil resistivity, fault level, available surface area, surface material, etc., are taken into consideration when designing an earth grid to ensure that it complies with the design standards as set out by IEEE Std 80-2013. However, in some cases, these design elements are constrained, and it is not possible to meet the design requirements without refining the earth grid using the predetermined refinements methods proposed in IEEE Std 80-2013.

The design of substation earth grids under such unfavorable design and environmental conditions can prove to be difficult. This is also true for maintaining existing earth grids to remain within the tolerable safety limits as required by IEEE Std 80. Often, the design input parameters, such as soil resistivity, available surface area and system fault level, as well as construction budget restrictions, are the main causes of the limitations that lead to flawed designs that endanger people and equipment.

In this paper, the IEEE Std 80 earth grid design refinement methods that can be used to improve an earth grid design were modeled, simulated, and analyzed for a 20 MVA, 11/6.6 kV substation using ETAP. An 80 m × 70 m earth grid made of 120 mm² annealed copper conductors with 12 in the x-axis direction and 10 in the y-axis direction was used. It also has 12 earth electrodes installed on the perimeter of the earth grid. This earth grid was superimposed on the power system model to accurately import system data and efficiently conduct simulations.

2. Literature Review

The IEEE Std 80-2013 substation earth grid design specification [1] outlines various refinement methods that can be employed during the design phase of earth grids to improve design compliance or methods that can be applied on brownfield projects to improve the integrity of existing earth grids. These methods include decreasing the total grid resistance, using closer grid spacing, diverting a greater part of the fault current to other parts, limiting the total fault current, barring access to energized areas, and increasing the tolerable touch and step voltage [1]. From these methods, some are easily applicable and more feasible to employ than others. For this reason, this research study only assessed the three commonly used refinement methods in the medium-voltage and low-voltage industry, and these methods are limiting the fault current, increasing the tolerable touch and step voltages, and the fault current diversion method.

2.1. Fault Current Limitation Method

There are various methods used to earth the neutral point of the secondary transformers in power systems, and each of these methods plays a critical role in the design and performance of substation earth grids. The commonly used earthing methods include solid earthing and impedance earthing [2]. In the solid earthing method, the secondary transformer neutral point is directly connected to the earth, whereas in the impedance earthing method, which is further divided into resistive or reactance earthing, the neutral point is connected to the earth via resistive or reactive elements [2,3]. In some rare cases, the secondary transformer’s neutral is left unearthed, such as in systems using the IT earthing system [2].

Even though there are various earthing methods to choose from, there are advantages and disadvantages presented by each method that should be considered before choosing and implementing an earthing method.

Table 1. Advantages and disadvantages of each method and how they compare against each other.

Method	Advantages	Disadvantages
Solid	Low overvoltage occurrence risk The usage of equipment with normal phase-earth insulation levels	The power system must trip on the occurrence of the first fault It results in extremely high fault currents High risk of exposing personnel to dangerous touch voltages when the fault occurs It requires additional reliable time coordination protection to clear the fault as quickly as possible
Resistive	Significantly reduces fault currents Dampens elevated voltage levels Allows for functionality of protection devices without any requirements Requires less maintenance	It trips on the first fault
Reactance	Limits fault currents Arching grounds are avoided Does not dissipate high levels of heat	Protection devices require a higher operating fault current compared to resistive It trips on the first fault May cause elevated voltage levels under fault conditions
High-impedance	Provides continuous power; it only trips on the second fault (provided that the fault level would not endanger personnel and equipment)	High risk of elevated voltages; requires reinforcement of equipment insulation It requires insulation monitoring It is difficult to implement protection schemes after the occurrence of the first fault

compares the advantages and disadvantages of each method obtained from findings by Prévé, Cressal Resistors, and Electrical Desk [2,4,5].

From the information in Table 1, it is observed that the resistive neutral earthing method is the most beneficial in terms of safety, application, reliability, and cost. For this study, the resistive neutral earthing method will be modeled and analyzed in Section 3: Earth Grid Design Criteria. In practice, the resistive earthing method is applied using devices called neutral earthing resistors (NERs), which are commonly used in medium-voltage installations to limit fault current that flows through the neutral point of a transformer or generator to safe, predetermined maximum values when under transient conditions to limit the damage to the transformer and other critical power system equipment [6]. NERs are application-based devices that are selected and sized to suit their intended purpose as per end-user needs. Their sizing is based on the anticipated heat dissipation from the fault current on the resistor elements [3,7].

Generally, for the protection of the power system, earth fault relays are employed and set at a specific sensitivity that will trigger the relay to operate, but, in cases where stringent earth fault protection is required of the MV/HV system, the NERs are added not only for the protection of equipment but also for that of personnel working in the vicinity of the substation [8].

2.2. Increasing the Tolerable Safety Limits of Touch and Step Voltage

2.2.1. Fault Clearance Time Variation

Since power systems consist of varying system voltage levels, for each of these voltage levels, the maximum permissible fault clearance time varies as well. The higher the system voltage gets, the quicker the fault needs to be cleared and, as a result, a low maximum fault clearance time is set by the system operators to protect the stability of the power system [9]. The fault clearance time consists of various factors that add together to determine the minimum time for the fault to be cleared from the system, which also depends on the type of relays (high speed or low speed), contacts, and circuit breakers used, including the relay fault detection response time, trip relay/circuit breaker operating time, and arc extinction [9,10]. The fault clearance time is a critical parameter in both power system analysis and earth grid design as, along with the system load, the transient stability of a fault is mainly controlled by the fault clearance time [11].

Fault Clearance Time Variation—Acceptable Operating Ranges

Additionally, since power systems have unique protection philosophies and differ in size in terms of voltage levels (132/11/6.6/0.55 kV), they will have different parameters/settings, such as trip times, that are expected to accurately operate when required to allow for both maximum protection and a sufficient protection coordination time to avoid undesired trips [12]. The general norm for good practice of an earth grid design is to have a maximum fault current duration of 0.5 s as stated in IEEE Std 80-2013 [1]; this was adapted from a study conducted by Biegelmeier and Lee [13] that provides evidence that, when the time of exposure to the current is in the region of 0.06 s–0.3 s, the heart will not be susceptible to ventricular fibrillation. In addition, since high ground gradients resulting from fault currents are normally infrequent and the shocks are also infrequent and occur in a very short period, it would therefore not be practical to design against currents that are below the fibrillation threshold; hence, the use of a fault current time of 0.5 s is acceptable as the worst-case scenario [1].

2.2.2. Surface Material Usage

In addition to the fault clearance time, the surface material used has a significant impact on the tolerable limits of touch and step voltage. A study conducted by Dladla et al. [14] showed that, by varying the types of surface materials, the performance of the safety limits of touch voltage and step voltage in an earth grid is greatly affected. The study showed that, by using crusher-run granite stones, the touch and step voltage are relatively low compared to when other available surface materials are used. The study further showed that, by varying the depth at which the surface material is buried, the safety limits are also affected, the deeper the applied surface material is buried, the higher the magnitude of the safety limits, and, vice versa, the shallow the applied surface material is buried, the lower the magnitude of the safety limits; however, care must be taken to ensure that they are adequately and solidly laid below the surface without the risk of being washed off by heavy rains or the movement of heavy machinery [14].

2.2.3. Diversion of the Fault Current

The fault current diversion method is adapted from the concepts of the fault current path and its distribution in power systems, especially in transmission substations with overhead transmission lines. From the maximum earth grid current I_G Equation (3) defined in Section 3: Earth Grid Design Criteria and existing literature [1,15,16], it is acknowledged that not all the fault current generated by the power system returns to the system through the earth grid: various elements, such as the fault type, fault location, and impedance properties of the power system’s neutral conductors, as well as the overhead ground lines, dissipate a margin of the generated fault current [17]. This is supported by a study conducted by Garrett [15] that suggests that the fault current division factor is greatly influenced by the overall earth grid resistance and tower footing.

Considering the proportional relationship between resistance and current, it can be deduced that, by diverting a portion of the fault current away from the substation’s earth grid, the total grid current will be marginally reduced. From the IEEE Std 80 specification, it is evident that this will directly reduce the ground potential rise of the earth grid. In another study conducted by Dladla et. Al. [18], it is stated that, with a lower ground potential rise, the odds of the tolerable touch voltage exceeding the ground potential rise and making the earth grid design compliant are greatly increased. With that in mind, determining the correct and practical current diversion factor is of paramount importance.

Generally, computer simulations are conducted to determine the accurate amount of fault current diverted from the earth grid, as computers are capable of accurately using numerical algorithms that consider a variety of data, and to apply first principle equivalent circuit components for the conductors connected to the earth grid [17]. Additionally, other factors, such as the location of the fault and the total earth grid resistance, are also considered by the computer simulation tools [2,17]. The commonly used simulation tools in the industry for substation earth grid design and analysis include ETAP, CDEGS, and SafeGrid.

2.3. Earth Grid Design Considerations for Microgrids

As civilization has advanced, urban areas have developed and industrial areas have significantly grown as well, whereas the electricity generation and distribution infrastructure has not been expanded linearly to meet the electricity demand. This expansion and growth of the consumers has presented developments in the form of integrating external power sources of AC and DC microgrids that aid in the electricity demand to the existing power system grid [19]. In his study on microgrids, Lasseter defined microgrids as a composite network of cluster loads, energy storage systems, and distributed generation units in a local distribution network [20].

Since the microgrids are interfaced with existing power systems and have critical equipment such as transformers, the return path for fault currents is the earth mass; for this reason, the microgrids are not exempted from the design and operation compliance requirements of earth grid designs. The safety and operational requirements ofzare applicable for microgrids, as well as all the necessary measures as prescribed in a traditional substation, and earth grid design guidelines must be adhered to as there is a risk of exposure to dangerous elevated voltages [21,22].

Various earthing methods are applicable for microgrids depending on the type of system and configuration used in the respective AC and DC systems. For AC grids, the earthing systems used are the terra-neutral (TN), terra-terra (TT), isolated terra (IT), and high-resistance grounding (HRG) [23]. The TN system is further divided into different grounds, i.e., TN-S, TN-C, and TN-C-S, with S denoting separate and C denoting combined [23].

Studies conducted by [2,23] highlight the importance of AC power system grid earthing configurations integrated with a DC microgrid system, where the different studies show that high neutral voltage fluctuations result in AC grid TN networks with an isolated earthing configuration in the microgrid’s DC bus due to its common mode voltage generated by converters [23]. For this reason, amongst others, high-resistance grounding (HRG) is highly recommended instead of isolated grounding configurations [2]. In addition, it is critical that, when designing the earth grid for microgrids, the inherent fault level of the power system as well as the fault level generated by the microgrids is considered to ensure the adequate sizing of earth grid elements such as conductors, earth electrodes, connection bars, and risers.

3. Earth Grid Design Criteria

The substation earth grid design IEEE Std 80-2013 specification gives a comprehensive design procedure that should be followed for the design of a compliant and efficient earth grid through its design flowchart shown in Figure 1 adapted from IEEE Std 80-2013 [1]. The flowchart outlines the key parameters that should be considered when designing an earth grid, and it sets out the design criterion that must be met for a substation earth grid to be considered compliant and safe for application.

From the IEEE Std 80-2013 earth grid design flowchart, the key design parameters that were assessed for this study that play an essential role in the refinements discussed in Section 2: Literature Review are the touch voltage, step voltage, ground potential rise, and fault current, which are critically associated with steps 3, 6, 7, 9, and 10 of the flowchart.

The main purpose of the refinement methods is to improve the design parameters of the safety limits of touch voltage, step voltage, and ground potential rise as described by Equations (1), (2), and (4). The touch voltage is defined by IEEE in [1] as the difference between the GPR and the potential of the surface that they are standing on when they are in contact with any conductive objects connected to the earth grid, and step voltage is the surface potential difference felt by a person with a 1 m distance between their feet when they are not in contact with any conductive objects connected to the earth grid [1]. Lastly, the ground potential rise is defined as the product of the maximum grid current and overall grid resistance [24], and is the highest electrical potential a substation’s earth grid can achieve concerning a distant earthing point believed to be at remote earth’s potential [1]. Concerning the GPR, another critical parameter is the maximum grid current, which is described by Equation (3), and it is directly proportional to the magnitude of the GPR along with the earth grid resistance of the substation.

$$E_{step70 }=(1000+6C_s· {\rho }_s) \frac{0.157}{\sqrt{t_s}}$$

(1)

$$E_{touch70 }=(1000+1.5C_s· {\rho }_s) \frac{0.157}{\sqrt{t_s}},$$

(2)

where:

C_s is the surface layer derating factor;

ρ_s is the resistivity of the surface material;

t_s is the duration of the shock current.

$$I_G=3I_0{D}_f{S}_f$$

(3)

where:

3I₀ is the system fault current;

D_f is the decrement factor;

S_f is the current division factor.

$$GPR=I_G{R}_g$$

(4)

where:

I_G is the maximum grid current;

R_g is the earth grid resistance.

The power system parameters consist of two 20 MVA, 11/6.6 kV transformers, and each transformer feeds a switchboard consisting of a 370 kW motor and two lumped loads with a total loading of 400 kVA. Under the N-1 operating condition, the energized transformer will feed twice the normal load. For simulation purposes, the supply transformers were initially solidly earthed. The substation earth grid was superimposed onto the power system to import the actual power system short-circuit study results directly onto the earth grid. These results were then used for the design of the earth grid. Figure 2 shows the earth grid superimposed onto the power system consisting of solidly earthed supply transformers.

The ETAP short-circuit study (based on IEC-60909) was conducted for the power system to assess the fault level generated by the supply transformers and back-fed to the supply by the 370 kW motors. The simulation shows that a steady state fault level of 21.877 kA is generated by the transformers and motors, contributing an additional fault level of 0.28 kA, resulting in a total peak fault current of 22.759 kA, which will be seen by the busbars and, subsequently, the substation earth grid through the neutral of the transformers. Figure 3 shows the short-circuit study results and Figure 4 shows the dialogue plane of the fault current value being exported to the earth grid study parameters.

Using the imported fault current and the predetermined input parameters of the substation under study, the input date was tabulated as shown in

Table 2. Earth grid design input parameters.

No.	Design Parameters Description	Value
1	Maximum fault current generated by the power system, 3I0	22.759 kA
2	Shock duration, ts	0.4 s
3	Earth grid conductor sizing fault current duration, tc	0.4 s
4	Decrement factor fault current duration, tf	0.4 s
5	The resistivity of the surface layer, ρs	4267.2 Ωm
6	The thickness of the surface layer, hs	0.2 m
7	Earth grid depth, ho	5 m
8	Soil resistivity of the studied area, ρ	50 Ωm
9	Depth at which the earth grid conductors are buried, h	1 m
10	The sum of earth grid conductor lengths in the x-axis, Lx	960 m
11	The sum of earth grid conductor lengths in the y-axis, Ly	700 m
12	The spacing between the parallel conductor (x-axis/y-axis), D	8.9 m/6.4 m
13	The length of each earth electrode, Lr	3 m
14	The sum of earth electrodes, nR	12
15	The sum of earth grid conductors in the x-axis, Nx	12
16	The sum of earth grid conductors in the y-axis, Ny	10
17	The calculated size of the earth grid conductors	120 m2
18	The earth grid’s surface area (80 m × 70 m), A	5600 m2
19	The combined length of all buried earth grid conductors (x-axis/y-axis) (960 m + 700 m), Lc	1660 m
20	The combined length of all earth electrodes installed (3 m × 12 m), LR	36 m
21	The combined length of the earth grid conductors and earth electrodes, LT	1696 m

. Amongst other parameters, the earth grid consists of an 80 m × 70 m surface area, 50 Ωm soil resistivity, 12 × 10 earth grid conductors laying in the x-axis and y-axis, respectively, and 12 earth electrodes buried 3 m deep into the earth mass.

Using the input data tabulated in Table 2, the earth grid was modeled and simulated in ETAP’s Ground Grid interface, and the isometric earth grid design is shown in Figure 5. The design was then simulated, and the results are shown in Figure 6, with the values of the safety limits of touch voltage, step voltage, and ground potential rise calculated.

Using this earth grid, the refinement methods that are discussed in Section 2: Literature Review were assessed to evaluate their impact and effectiveness on the substation earth grids.

4. Refinement Methods

The design specification of substation earth grids IEEE Std 80-2013 states various refinements that can be employed to improve the design of earth grids to comply with the design specifications. These include improving the soil resistivity readings, modifying the available surface area orientation, extending the earth grid, using different surface material, limiting the fault current and fault duration, etc. From these various earth grid refinement methods stipulated in the design specification, for this study, the refinement methods that were analyzed to assess their impact on the design and performance of the refined earth grid are the limiting the fault duration method, tolerable touch and step voltage increment method, and fault current diversion method. All these methods were applied on the earth grid superimposed onto a 20 MVA, 11/6.6 kV power system using ETAP.

4.1. Current Limitation Method

The first method tested was the current limitation method, where the maximum fault current generated by the power system’s transformers is limited to a certain value through employing the current limitation applications discussed in Section 2: Literature Review. For this study, the resistive neutral earthing method was employed using neutral earth resistors (NERs), where the NERs limit the fault current passing through the transformer’s neutral conductor to the ground using resistive elements. The power system model designed in Section 3: Earth Grid Design Criteria was updated from having a solid neutral to the substation earth grid to having a neutral earth resistor connected between the neutral point and the substation earth grid. Figure 7 shows the supply transformer’s secondary winding settings, with the grounding configuration changed to resistor, and the desired value to which the fault current was reduced to is specified as 400 A. This value was then varied between 500 A, 800 A, and 1000 A based on the commonly used NER values in the industry. Figure 8 shows the updated power system model with the NER on the neutral of the transformers.

The short circuit study was then simulated on the power system for each of the NER values, and the results were then imported to the ground grid design interface, where the earth grid study was conducted for each fault level to assess its impact on the earth grid’s performance. Figure 9 shows the steps and results of importing the short circuit results to the substation earth grid model. This process was then repeated for all the predetermined fault currents, and the imported results are shown in Figure 10.

The simulation was then conducted for each fault level resulting from the variation in the NER size, and the simulation results are tabulated in

Table 3. Application of the current limitation and its impact on the safety limits.

Variation in Current Limitation
Fault Current Limit (A)		Group Potential Rise (V)	Touch Voltage (V)		Step Voltage (V)
NER Application?	Fault Current (kA)		Calculated	Tolerable	Calculated	Tolerable
No NER used	22.759	7562.2	1122.5	1144.3	503.1	4026.9
Yes—1000 A	1.093	349.7	51.9	1144.3	23.3	4026.9
Yes—800 A	0.876	280.2	41.6	1144.3	18.6	4026.9
Yes—500 A	0.549	175.6	26.1	1144.3	11.7	4026.9
Yes—400 A	0.439	140.4	20.8	1144.3	9.3	4026.9

, showing the impact of limiting the fault current through the NER and its impact on the safety limits of touch voltage, step voltage, and ground potential rise.

Table 3 shows the results of implementing the current limitation method using the neutral earthing resistor as the power system’s supply transformer secondary grounding method of connection to the earth grid as opposed to the traditional method of solid grounding, where the neutral of the transformer is connected directly to the earth grid. From the simulation results, it is observed that the fault current injected into the earth grid is significantly reduced; however, for the application of NERs, care must be taken to ensure that the specified value of the NERs does not affect and influence the operation and sensitivity of the power system’s earth fault protection relays. The protection settings of the relays must be set in such a manner that the relay will still be able to detect and clear undesired earth fault currents within the power system.

4.2. Increasing the Tolerable Safety Limits of Touch and Step Voltage

The second method tested was the variation in the fault current duration to assess its impact on the tolerable safety limits of touch and step voltage. In power systems protection, prompt fault clearance is one of the fundamental requirements for the protection of people and equipment, preventing total system failure, ensuring the reliability of unaffected areas, and limiting damage to equipment. In earth grid design, fault duration plays a significant role concerning the touch and step voltages as it determines the level of severity when a person and/or equipment are exposed to elevated voltages: the longer the time of exposure, the more severe the damage.

For this study, the durations of the same fault current of I_f = 22.579 kA were varied between 300 ms and 500 ms to assess the impact of the small-time margins that exist between these times and how critical it is for power systems to have reliable and highly effective protection schemes to ensure that a fault is cleared promptly at all times. The initial input fault current duration settings of the substation earth grid were varied from 300 ms to 500 ms in incremental scales of 50 ms as shown in Figure 11.

The results of the simulations are tabulated in

Table 4. Application of the fault duration and its impact on the safety limits.

Variation in Current Limitation
Fault Duration (A)	Group Potential Rise (V)	Touch Voltage (V)		Step Voltage (V)
		Calculated	Tolerable	Calculated	Tolerable
0.3	7654	1136.2	1321.3	509.2	4649.9
0.35	7601.7	1128.4	1223.3	505.7	4305
0.4	7562.2	1122.5	1144.3	503.1	4026.9
0.45	7531.3	1118	1078.8	501	3796.6
0.5	7506.6	1114.3	1023.5	499.4	3601.8

, which shows the impact of the fault current duration on the performance of the safety limits of touch voltage, step voltage, and the ground potential rise.

From the simulation results tabulated in Table 4, it is observed that, by varying the fault current duration, the safety limits of the earth grid are greatly influenced. The results show that, the sooner the fault is cleared, the higher the tolerable threshold of the safety limits. At a fault duration of 0.3 s, the touch and step voltage tolerable voltages are 1321.3 V and 4649.9 V, respectively, with calculated values of 1136.2 V and 509.2 V. When the fault duration is gradually increased, it is observed that the tolerable thresholds decrease; for instance, at a fault duration of 5 s, the touch and step voltage tolerable voltages are 1023.5 V and 3601.8 V, with calculated values of 1114.3 V and 499.4 V. This variation of 2 s results in a tolerable voltage difference of 297.8 V and 1048.1 V for the touch and step voltage, respectively.

4.3. Fault Current Diversion

In power systems, it is highly advisable to design for the worst-case scenario (N-1 contingency); similarly, in the substation in earthing systems, the norm is adopted. However, even though it is highly advisable to design the earth grid for the worst-case scenario, where the maximum fault current that can be generated by the power system’s transformers is dissipated into the earth grid [1], in some cases, it may be necessary to implement the current diversion method to divert a portion of the fault current away from the earth grid to improve the design and safety compliance of the earth grid. In practice, this is computed by applying Kirchoff’s current law to obtain the current division factor between the substation earth grid resistance and the input impedance of each circuit (conductors or overhead lines) [15].

For this study, the fault current diversion method was applied on an earth grid superimposed on a 20 MVA, 11/6.6 kV power system. The earth grid was initially modeled and simulated as shown in Section 3: Earth Grid Design Criteria to analyze the impact of fault current diversion, where the initial settings of the earth grid’s current division factor input data as shown in Figure 12 were varied from 100% to 60% in decremental scales of 10%.

The simulation results of the fault current division are shown in

Table 5. Application of the current diversion and its impact on the safety limits.

Variation in Current Limitation
Current Division Factor (A)	Group Potential Rise (V)	Touch Voltage (V)		Step Voltage (V)
		Calculated	Tolerable	Calculated	Tolerable
100	7562.2	1122.5	1144.3	503.1	4026.9
90	6806	1010.3	1144.3	452.8	4026.9
80	6049.8	898	1144.3	402.5	4026.9
70	5293.5	785.8	1144.3	352.2	4026.9
60	4537.3	673.5	1144.3	301.8	4026.9

, where the impact and influence of the current diversion on the performance of the safety limits of touch voltage, step voltage, and ground potential rise are tabulated.

From the simulation results tabulated in Table 5, it is observed that, by diverting a percentage of the fault current directed to the earth grid, the calculated safety limits are proportionally affected. The results show that, as the percentage of the fault current injected into the earth grid reduces, the calculated touch and step voltages decrease; at a 100% division factor, which is the worst-case scenario, the touch and step voltages are 1122.5 V and 503.1 V, respectively; and when the percentage is gradually reduced to 60%, the touch and step voltages are proportionally reduced to 673.5 V and 301.8 V, respectively, yielding voltage differences of 449 V And 201.3 V for the touch and step voltages, respectively, for the 40% current division difference.

4.4. Optimized Earth Grid Using the Refinement Methods

To assess which refinement method would yield the best solution for problematic earth grid design conditions without altering the available earth grid surface area and input soil data, the results with the lowest tolerable thresholds for each method were subjected to the ETAP’s Ground Grid Optimization tool. For the current division method, the results obtained from the 60% division factor simulation were used. For the fault duration method, the results obtained from the fault duration of 0.3 s were used, and, lastly, from the current limitation method, results obtained using the 400 A-rated NER were used for the optimization comparison.

Table 6. Comparison of optimized earth grids based on the refinement methods and their impact on the safety limits.

Optimized Earth Grids Based on Refinement Methods
Refinement Method	Group Potential Rise (V)	Touch Voltage (V)		Step Voltage (V)
		Calculated	Tolerable	Calculated	Tolerable
Current Division	4841.4	1131	1144.3	312.8	4026.9
Fault Duration	7784.1	1313	1321.3	518	4649.9
Current Limitation	153.9	47.9	1144.3	9.9	4026.9

shows the results of the optimized earth grid using each of the refinement methods and how they compare against each other.

5. Conclusions

In this study, various substation earth grid design refinement methods adapted from the IEEE’s standard IEEE Std 80-2013 Guide for Safety in AC Substation Grounding were modeled, simulated, and analyzed. The methods assessed included the current limiting method, the current diversion method, and the touch and step voltage increment method. From the studies, it was observed that each of these methods influenced the design parameters and performance of the substation earth grid. From the current limiting method, the use of neutral earth resistors was implemented, and the results show that the NERs significantly reduce the fault current being injected into the substation earth grid. As a result, the ground potential rise is reduced, which is a critical design element tested for earth grid compliance. The current diversion method proved effective when applied as it partially reduced the fault current injected into the earth grid. This was achieved by applying the separation factor in the simulation; in practice, however, it is computed by applying Kirchoff’s current law to obtain the current division factor between the substation earth grid resistance and the input impedance of each circuit (conductors or overhead lines) [15].

Lastly, the touch and step voltage increment method was implemented by varying the fault current clearance time. The simulation results show that reducing the fault clearance time greatly increases the tolerable limits of the touch and step voltages. This method could be suited for all power systems with reliable high-speed protection relays.

Additionally, using the results of the refined earth grid designs, optimal design analyses were conducted for all the methods. The simulation results show that, out of all the methods applied, the current limiting method was the most effective and easy-to-apply method. By limiting the fault current, all the other design parameters are greatly impacted as the earth grid becomes capable of complying with both the design criteria as set out in IEEE Std 80, with far fewer earth grid conductors and electrodes in size and quantity, and the required surface area for the earth grid. As a result, this would greatly reduce the procurement and construction costs associated with the design and application of substation earth grids.

This study, therefore, concludes that, when designing a substation earth grid with limited design options, it is essential to first assess which of the design refinement methods would be applicable for that specific case. As a starting point, the current limiting method and the touch and step voltage increment methods are the best options to be explored first.