Safety Performance Evaluation of Fence Grounding Configurations in High Voltage Installations Zacharias G. Datsios1, Pantelis N. Mikropoulos2, Andreas Teneketzoglou3 and Dimitrios Tzikas4 High Voltage Laboratory, School of Electrical & Computer Engineering, Faculty of Engineering, Aristotle University of Thessaloniki, Thessaloniki 541 24, Greece 1 zdatsios@auth.gr, 2pnm@eng.auth.gr, 3ante@ee.auth.gr, 4dctzikas@auth.gr
Abstract- The design of the grounding configuration for the metal fence of a high voltage installation is important as the outside perimeter of the fence is accessible to the general public. In this work the safety provided by several fence grounding techniques commonly used in high voltage installations is evaluated for a 150/20 kV air insulated substation, a 400 kV step-up GIS substation and a large scale photovoltaic power station with the aid of grounding analysis software. A safe and cost-efficient fence grounding design depends on ground fault characteristics, soil conditions, installation area, distance between the fence and grounding grid as well as on the size and geometry of the latter. Index Terms--fence grounding, photovoltaic power stations, safety, substations, touch and step voltages.
I.
INTRODUCTION
A well designed grounding system of a high voltage installation should afford protection to persons against the danger of critical electric shock [1], [2]. Thus, the grounding system design should ensure that, in case of the most dangerous ground fault, the arising touch and step voltages are limited to values lower than the corresponding allowable voltage limits. When a ground fault occurs, the grounding grid, therefore also all grounded metal structures, are elevated to the same potential, called ground potential rise (GPR). Metal structures which are not connected to the grounding grid attain a potential due to coupling through the ground and, therefore, may pose a threat to persons. Hence, safety should also be evaluated for such structures located inside or in the vicinity of the installation area, e.g. security lighting and CCTV posts, pipes, rails and fences. Most commonly, high voltage installations such as substations, power plants and industrial facilities are protected by fences. The design of the grounding configuration for the metal fence of a high voltage installation is essential as the outside perimeter of the fence is accessible to the general public; this is even more so where the installation is located in a residential or urban area. Furthermore, it is well known that metal fences with large lengths could transfer high potentials to areas away from the grounding grid of the installation. This is common where the grid only covers small parts of the whole installation area, e.g. power plants and industrial facilities; in such cases measures against dangerous potentials transferred through the fence should be considered as well. Several papers have been published regarding the safety provided along fences against
potentials arising due to ground faults [3]-[5] as well as due to coupling effects [6]-[10]. According to the IEEE Std 80-2000 [1] and its recent revision [2], grounding of a metal fence which encloses an installation having energized electrical conductors or equipment can be achieved either by connecting the fence to the installation grounding grid or by utilizing a separate grounding configuration. If the latter design is selected, it must be ensured that the fence cannot be accidentally connected to the grounding grid, e.g. via metal pipes or cable sheaths. Otherwise, in case of a ground fault the fence will attain the ground potential rise of the grid and high touch and step voltages could appear along its length. In this paper, fence grounding techniques commonly used in high voltage installations are evaluated for a 150/20 kV air insulated substation, a 400 kV GIS step-up substation and a large scale photovoltaic power station. These installations differ in area, ground fault characteristics and distance between fence and grounding grid. The safety provided by several fence grounding techniques is assessed with the aid of grounding analysis software [11]. It is shown that a safe and cost-efficient fence grounding design depends on ground fault characteristics, soil conditions, installation area, distance between the fence and grounding grid as well as on the size and geometry of the latter. II. 150/20 kV AIR INSULATED SUBSTATION The 150/20 kV air insulated substation under study (Fig. 1) encompasses an area of approximately 11500 m2. The most dangerous ground fault in this substation is a 150 kV single phase ground fault with a symmetrical current, If, of 30 kA. The fault current division factor, Sf, is 0.8. The decrement factor, Df, is calculated 1.0313 for a fault duration, tf, of 0.5 s and a X/R ratio of 10. Hence, the maximum grid current, IG, is calculated 24.75 kA. Based on soil resistivity measurements, in accordance with [12] and [13], it was found that a uniform soil model with a resistivity of 87.1 ﾎｩm represents satisfactorily the actual soil conditions in the installation area. Table I shows the allowable touch, Etouch70, and step, Estep70, voltage limits for the evaluated substation calculated according to the IEEE Std [1], [2], as influenced by surface material thickness, hs, and resistivity, ﾏ《. These limits, referring to a shock current duration of 0.5 s and body weight of 70 kg, are also retained for the area beyond the outer boundaries of the substation, which is located in a rural area.
Fig. 1. Grounding grid for the 150/20 kV air insulated substation; solid red line: fence, dashed red line: fence (alternative design), green lines: insulating fence sections, circles: ground rods. TABLE I ALLOWABLE TOUCH AND STEP VOLTAGE LIMITS FOR THE 150/20 kV SUBSTATION CALCULATED ACCORDING TO THE IEEE STD [1], [2] 0.1 ρs (Ωm) without 4000 5000 6000 7000 8000 9000 10000
Etouch70 (V) 251 1150 1379 1609 1839 2069 2298 2528
Estep70 (V) 338 3933 4852 5771 6689 7608 8527 9446
hs (m) 0.15 Etouch70 Estep70 (V) (V) 251 338 1253 4348 1510 5373 1766 6397 2022 7422 2278 8447 2534 9472 2791 10496
0.2 Etouch70 (V) 251 1315 1587 1859 2130 2402 2674 2946
Estep70 (V) 338 4593 5681 6768 7856 8943 10031 11118
Fig. 1 shows the actual substation grounding grid, designed based on safety of people against the danger of critical electric shock due to ground faults. The grid covers an area of ~7000 m2 and comprises ~3200 m of horizontal tinned copper conductors with a cross section of 120 mm2 as well as 31 copper-clad steel rods with a diameter of 17 mm and a length of 3 m. It is buried at a depth of 0.6 m and its maximum mesh size is 5x6 m. The auxiliary building is grounded through a foundation ground electrode connected to the grounding grid. The entire substation area is covered with a high resistivity surface material (ρs=4000 Ωm, hs=0.1 m). A chain-link fence with outward-inclined barbed wire on top surrounds the substation. The fence has a total length of ~390 m and its distance from the grounding grid varies between 1.2 and 23.7 m (Fig. 1). Three different fence grounding techniques are evaluated in what follows: fence connected to a separate ground electrode, fence connected to the grounding grid and segmented fence grounding. A. Fence connected to a separate ground electrode Fence connection to separate ground electrodes is typically employed when the fence is at a distance from the grounding grid. The minimum value of this distance is often imposed by utility specifications. In case of separately grounded fence it is important that the fence remains electrically isolated from
the grounding grid; therefore, care should be taken to avoid unintended connections which could lead to hazardous touch and step voltages around the fence. As an additional measure, the part of the fence located below incoming overhead transmission and distribution lines can be isolated from the remaining fence by using insulating fence sections. However, it is extremely difficult to achieve safety in case of phase conductor falling on the isolated fence section; the entire fault current flows through the grounding configuration of this short section. Furthermore, according to IEEE Std [1], [2], it is unusual for phase conductors to fall on the fence, especially for the short and dead-ended last line spans. Nevertheless, the isolating practice may prevent the transfer of dangerous potentials to the remaining part of the fence. The fence of the 150/20 kV substation is assumed to be appropriately connected to a separate horizontal ground electrode installed at a depth of 0.6 m beneath the fence. Details on grounding and equipotential bonding of different fence parts can be found in [14]. The ground resistance of the substation grid is computed 0.458 Ω. For the most dangerous ground fault, the GPR of the grid and the fence potential are 11.34 kV and 7.32 kV, respectively. The maximum touch voltage along the fence is 2.47 kV, arising inside the substation area where the fence is very close to the grid (point A in Fig. 1). Relatively high touch voltages outside the fence are obtained in limited areas far away from the grid (maximum: 1.75 kV, point B in Fig. 1), due to an increased difference between fence and surface potentials. The average touch voltage along the fence at profiles parallel to the fence line at a distance of 1 m from the latter is computed 0.45 kV. Step voltages exceeding the allowable limit without surface material application (Table I) are observed in limited areas near fence corners (points A, B, C in Fig. 1); the step voltage attains a maximum value of 1.22 kV within the substation area, specifically at point A in Fig. 1. Safety around the fence could generally be achieved by extending the application of the surface material up to a distance of 1 m beyond the fence. To prevent scattering of the granular surface material due to the elements of nature, a rather short (~0.2 m) concrete wall should be constructed at a distance of 1 m outside the fence. In small areas around points A, B and C in Fig. 1, where touch voltages higher than the allowable limit (1.15 kV, Table I) are observed, a surface material of higher resistivity should be used. For the areas around points B and C a ground mat could be installed, instead, under the surface material to equalize the surface potential up to 1 m outside the fence. An alternative design for a separately grounded fence could be the installation of the fence ground electrode at a distance of 1 m beyond the fence. This leads to slightly lower fence potential (7.11 kV). However, the maximum touch voltage outside the fence remains the same and the corresponding value inside the fence at point A in Fig. 1 increases to 2.9 kV; by displacing the fence ground electrode, the fence potential is reduced whereas the surface potential remains high in this area due to the close proximity to the grounding grid.
D. Evaluation of fence grounding techniques Fig. 2 shows touch voltages computed along profiles 1 and 2 parallel to the fence as shown in Fig. 1. From Fig. 2(a) it can be observed that extending the grounding grid outside the fence results in much lower touch voltages along profile 1. The maximum touch voltage along profile 2 (Fig. 2(b)) is obtained at the fence corner designated as point B in Fig. 1 for all evaluated fence grounding techniques. Segmented fence grounding shows the lowest value for maximum touch voltage among fence grounding techniques. Table II summarizes grounding analysis results on fence grounding techniques for the 150/20 kV substation. From Tables I and II it can be deduced that safety is achieved for all fence grounding configurations with the application of surface material up to a distance of 1 m beyond the fence. However, as a concluding remark, the fence grounding technique resulting in the safest, yet cost-efficient, design is segmented fence grounding. 3.0
2.0
Separate grounding Separate grounding, 1 m beyond Grounding to the grid
2.5
1.6
2.0
Touch voltage (kV)
C. Segmented fence grounding Segmented fence grounding should be considered when the distance of the fence from the substation grounding grid varies widely. A trial and error procedure is usually employed to determine where the fence should be segmented into electrically isolated parts to ensure safety. This is not a difficult task for relatively small installations; for larger installations a systematic procedure can be used as proposed in [4]. It is important that insulating joints or sections isolating two metal fences should be able to withstand the maximum potential difference arising between these fences; otherwise, flashover may occur transferring dangerous potentials from one fence to another. Furthermore, the length of such sections should be long enough, to avoid accidental bridging between fences and coupling through the ground. For the evaluated 150/20 kV substation the fence could be segmented at its section being in closest proximity to the grounding grid, that is, at the section near the ultimate transmission tower (Fig. 1). To achieve a more efficient grounding design, this part of fence is moved within the grid area, leaving the tower outside the substation (Fig. 1, dashed red line segment). This fence section is grounded to the grid whereas the remaining part of the fence is connected to a separate ground electrode, buried under the fence (depth: 0.6 m). Insulating fence sections 6 m in length (Fig. 1, green line segments) are used to isolate the two different fence parts. The maximum touch voltage along the fence section grounded to the grid is 1 kV, considerably lower than the corresponding values obtained for the previously examined fence grounding techniques. The average touch voltage along the fence decreases by ~11% when compared to the case of separately grounded fence; the maximum touch voltage is
1.49 kV (point B in Fig. 1). Safety is achieved by extending the application of the surface material up to a distance of 1 m outside the fence; in a limited area around point B a higher resistivity surface material (ρs≥5000 Ωm, hs≥0.15 m) is required.
Touch voltage (kV)
B. Fence connected to the substation grounding grid Grounding the fence to the substation grid is a practice commonly used when the fence is located close to the grid. When using this technique, the fence potential is equal to the GPR of the grid and, therefore, touch and step voltages near the fence may be high. The fence of the evaluated 150/20 kV substation is at its longest length located away from the substation grid (Fig. 1); consequently, extending the grid up to the fence is not a cost-effective design. Nevertheless, this technique is assessed in this subsection. The simple interconnection of the fence ground electrode with the grounding grid using a number of horizontal ground electrodes installed at 10 or 20 m intervals, yields maximum touch voltages higher than that obtained for the case of separately grounded fence. Extending the grounding grid up to 1 m beyond the fence line, using 5x6 m meshes, results in an average touch voltage along the fence of 0.63 kV. The touch voltages at points B and C (Fig. 1) are 1.98 kV and 1.31 kV, respectively; the former, being the maximum touch voltage arising on the fence, is much lower than the corresponding values for the case of separately grounded fence. However, this technique, requiring ~1530 m of additional ground conductors, is certainly not cost-efficient.
1.5
1.0
0.8
0.4
0.5
0.0
1.2
(a) 0
(b)
0.0 10
20
30
40
Separate grounding Separate grounding, 1 m beyond Grounding to the grid Segmented grounding
0
10
20
30
40
50
Profile 2 length (m)
Profile 1 length (m)
Fig. 2. Touch voltage computed along profiles (a) 1 and (b) 2 (near points A and B in Fig. 1) parallel to the fence at a distance of 1 m from the latter for all evaluated fence grounding techniques. TABLE II GROUNDING ANALYSIS RESULTS FOR THE 150/20 kV SUBSTATION Fence grounding technique Separately grounded fence Separately grounded fence, 1 m beyond Fence connected to the grounding grid Segmented fence grounding
Rg (Ω)
Fence GPR potential (kV) (kV)
0.458 11.34 0.459 11.36 0.378 9.36 0.457 11.31
Etouch,max (kV)
inside: 2.47 outside: 1.75 inside: 2.9 7.11 outside: 1.76 inside: 1.31 9.36 outside: 1.98 inside: 1 7.12 outside: 1.49 7.32
Etouch,ave* Estep,max (kV) (kV) 0.45
1.22
0.45
1.20
0.63
0.72
0.40
0.78
*
Average touch voltage along the fence, computed at profiles parallel to the fence line at a distance of 1 m from the latter.
III. 400 kV STEP-UP GIS SUBSTATION The 400 kV GIS substation (Fig. 3) covers an area of ~4500 m2. The most dangerous ground fault in this substation is a 400 kV single phase ground fault (If=40 kA, tf=0.5 s, Sf=0.7). The decrement factor, Df, is calculated as 1.0427 for a X/R ratio equal to 13.7. Thus, the maximum grid current, IG,
Initial grounding design Final grounding design
600 500
Step voltage (V)
Profile 1
700
400 300 200 100 0
0
5
10
15
20
Profile 1 length (m)
600
Initial grounding design Final grounding design
le ofi Pr
2
Step voltage (V)
500 400 300 200 100 0 0
5
10
15
20
Profile 2 length (m)
Fig. 3. Grounding grid for the 400 kV step-up GIS substation; red line: fence, empty circles: ground rods; green and blue circles represent respectively removed and additional ground rods in the final design. Graphs show step voltages computed along profiles 1 and 2.
is 29.2 kA. Based on soil resistivity measurements [12], [13], it was found that a uniform soil model with a resistivity of 33.5 Ωm represents satisfactorily the actual soil conditions. Fig. 3 shows the substation grounding grid, designed based on safety of people against the danger of critical electric shock due to ground faults. The grid (maximum mesh size: 8x8 m) covers the whole substation area and it is installed at a depth of 0.5 m. It comprises ~1300 m of tinned copper conductors (185 mm2) and 27 copper-clad steel rods 19 mm in diameter and 3 m in length. The building housing the GIS buses and switchgear is grounded through a foundation ground electrode connected to the grounding grid. The entire substation area is covered with a high resistivity surface material (ρs=5000 Ωm, hs=0.2 m). The allowable touch and step voltage limits with and without surface material application were respectively calculated according to the IEEE Std [1], [2] as Etouch70=1.58 kV, Estep70=5.67 kV and Etouch70=0.23 kV, Estep70=0.27 kV. The substation is protected by a chain-link fence with outward-inclined barbed wire on top. As the grounding grid covers the whole substation area, the fence is grounded to the grid. To reduce touch voltages, the fence (~280 m in length) is installed within the grid area at a distance up to 1.5 m from the periphery conductors. The ground resistance is computed 0.223 Ω and, hence, the GPR is 6.51 kV. The maximum touch voltage is 1.38 kV (point A in Fig. 3), lower than the corresponding limit with surface material application. However, outside the boundaries of the substation, specifically up to ~4 m from the fence, step voltages with a maximum of 0.65 kV exceed the allowable limit without surface material application. As the application of surface material outside the substation area is not possible, other measures should be considered to reduce the arising hazardous step voltages. It is noteworthy that high step voltages outside the grounding grid are commonly found in installations covering a limited area when high fault currents are dissipated to the ground.
It is well known that touch and step voltages near the grid periphery depend on mesh geometry and size in this area. Touch voltages can be mitigated by reducing the mesh size at grid perimeter, increasing consequently the surface potential. Step voltages outside the fence can be reduced by increasing the burial depth of the outer grid conductors; this, however, leading to lower surface potential, causes higher touch voltages. An additional measure to reduce step voltages is to install deep rods at the perimeter of the grid, reducing also touch voltages. For the evaluated GIS substation safety against dangerous step voltages outside the substation area could be achieved by burying the peripheral grid conductors at a depth of 1.5 m instead of 0.5 m. Furthermore, it is necessary to increase the length of the rods at the grid perimeter from 3 to 9 m and install three additional rods as illustrated in Fig. 3. The ground resistance and GPR are estimated 0.204 Ω and 5.96 kV respectively. The maximum touch voltage is found 1.14 kV, lower than in the initial design. As shown in Fig. 3, the maximum step voltage is reduced to 0.27 kV, approximately equal to the allowable voltage limit. IV. PHOTOVOLTAIC POWER STATION Large scale photovoltaic (PV) power stations cover large areas and typically deliver power to the medium voltage distribution network. In many actual cases the distance between the fence and the installation grounding grid varies widely; this depends on property area covered and on the provision of future expansion. In case of a ground fault, high potentials appearing on the fence could be transferred in areas away from the grid, posing therefore a threat to persons. Hence, cautious treatment of fence grounding should be applied in large scale PV power stations to ensure safety. In this work fence grounding of a 3 MWp PV power station (Fig. 4) is investigated. The total property area is ~98300 m2 whereas the 12 PV panel array groups cover ~58000 m2. As detailed in [15], the most dangerous ground fault is a 20 kV single phase ground fault (If=1 kA, tf=0.5 s, Sf=1). The decrement factor, Df, was calculated 1.0127 for a X/R ratio equal to 4. Hence, the maximum grid current, IG, was found 1012.7 A. A two-layer soil model was derived for the installation area (upper and lower layer resistivities: 2796 and 7250 Ωm, respectively, upper layer thickness 4.45 m). The safe grounding grid design for this PV power station has been presented in [15]. The grid comprises the concreteencased part of the steel piles supporting the PV panel arrays as well as copper-clad steel ground conductors buried next to the arrays at a depth of 0.5 m interconnecting all metal support structures. The total number of piles is 3766 and the total length of ground conductors is ~1.7 km. The substations and the auxiliary building are grounded via a foundation ground electrode along with a ring electrode surrounding them at a distance of 1 m from their boundaries. According to common practice, surface material in large scale PV power stations is used only in certain areas to achieve safety. Hence, the allowable touch and step voltage limits without surface
A
variation of the fence potential; the maximum touch voltage is only slightly reduced (1.33 kV, point A in Fig. 4). Such a reduction in touch voltage cannot justify the high cost of installation of a horizontal ground electrode under the fence.
2
Insulating section Auxiliary Building
B Array Group 1
Array Group 2
Array Group 7
Substation 1
Array Group 3
Array Group 8
Substation 2
Array Group 4
Array Group 9
Array Group 10
95.2 m 4m
Array Group 5
Substation 3
Array Group 6
Array Group 11
Array Group 12
.5 40 m
Insulating section
Fig. 4. PV power station under study; red line: fence, green lines: insulating fence sections.
material application were calculated according to the IEEE Std [1], [2] as Etouch70=1.15 kV, Estep70=3.95 kV. The grounding analysis conducted in [15] for the PV power station did not consider fence grounding. Actually, the PV power station area is protected by a chain-link fence with outward-inclined barbed wire on top and a total length of ~1300 m; the distance between the fence and the grid varies from 4 m to 95.2 m (red line, Fig. 4). The concrete-encased parts (length: 1.1 m, post and concrete diameters: 5 cm and 23 cm, respectively) of the metal fence posts (426 in number) were considered as ground electrodes. Concrete resistivity was taken equal to 90 立m. Four different separately grounded fence configurations were evaluated: grounding through fence posts only, grounding through a horizontal ground electrode, segmented fence grounding through fence posts only and through a horizontal ground electrode. A. Fence grounded through fence posts The ground resistance and GPR of the grid are computed 11.15 立 and 11.29 kV, respectively, and the fence potential is 8.32 kV. The touch voltage along the fence, computed at profiles parallel to the fence line at a distance of 1 m from the latter, has an average value of 0.51 kV. The maximum touch voltage is 1.42 kV (point A, Fig. 4) whereas the maximum step voltage is 0.71 kV (point B, Fig. 4). It can be deduced that safety can be achieved by applying surface material in a small area near the main substation where the touch voltage limit is exceeded. B. Fence grounded through a horizontal ground electrode Fence connection to a horizontal ground electrode buried at a depth of 0.5 m beneath the fence causes an insignificant
C. Segmented fence grounding, fence posts only The fence was divided in two parts as shown in Fig. 4 using a trial and error method. Fences 1 and 2 have lengths of ~550 m and ~720 m and distances from the grounding grid from 4 up to 24.5 m and 10 up to 95.2 m, respectively. They are electrically isolated via insulating fence sections (length: 5 m) and grounded through the concrete-encased part of fence posts. The potentials arising on fences 1 and 2 are 10.33 kV and 6.46 kV, respectively. The maximum touch voltage along the fences is 1.89 kV, found at the beginning of fence 2 located near the main substation (Fig. 4), exceeding the allowable limit. This maximum touch voltage can be mitigated by dividing fence 2 in more isolated sections, as suggested in [3], increasing, however, installation cost. D. Segmented fence grounding, horizontal ground electrode In addition to fence posts, horizontal ground electrodes buried at a depth of 0.5 m underneath fences 1 and 2 are installed. This results in a 27% reduction in maximum touch voltage when compared to the case of segmented grounding using fence posts only. However, the maximum touch voltage along the fences still exceeds the allowable limit. E. Evaluation of the fence grounding techniques Fig. 5 shows the touch voltage computed along a profile parallel to the fence as shown in Fig. 4. It can be observed that the segmentation of the fence results in high touch voltages before and after fence isolation points. Furthermore, the touch voltage approaches zero at the points where the surface potential becomes equal to the fence potential. Table III summarizes the results for the evaluated fence grounding techniques. Safety can be achieved for all techniques with the application of surface material in limited areas around the fence near the main substation area. Fence grounding through the concrete-encased part of fence posts is the most cost-efficient method. 2.0 Fence posts only
1.8
Electrode Segmented, fence posts only
1.6
Touch voltage (kV)
Main Substation
Fence 1
Profile
Fe nc e
Segmented, electrode
1.4
1.2 1.0 0.8 0.6 0.4 0.2 0.0
0
10
20
30
40
50
60
70
80
90
Profile length (m)
Fig. 5. Touch voltage along a profile parallel to the fence at a distance of 1 m from the latter (Fig. 4) for all evaluated fence grounding techniques.
TABLE III GROUNDING ANALYSIS RESULTS FOR THE PV POWER STATION Fence grounding technique Fence posts Electrode Segmented, fence posts Segmented, electrode
Rg (Ω)
GPR Fence Etouch,max Etouch,ave* Estep,max (kV) potential (kV) (kV) (kV) (kV)
11.15 11.29 11.08 11.22
8.32 8.34 Fence 1: 10.33 11.79 11.94 Fence 2: 6.46 Fence 1: 10.30 11.75 11.90 Fence 2: 6.49
1.42 1.33
0.51 0.38
0.71 0.72
1.89
0.30
0.55
1.38
0.17
0.63
*
Average touch voltage along the fence, computed at profiles parallel to the fence line at a distance of 1 m from the latter.
V. CONCLUSIONS Several grounding techniques commonly used for metal fences of high voltage installations have been evaluated for a 150/20 kV air insulated substation, a 400 kV GIS step-up substation and a large scale photovoltaic power station. These installations differ in area, ground fault characteristics and distance between the fence and grounding grid. Generally, the fence of a high voltage installation can be grounded either to the grounding grid or to separate ground electrodes. These two techniques can also be combined with the aid of insulating fence sections between different parts of the fence. A safe and cost-efficient fence grounding design depends on ground fault characteristics, soil conditions, installation area, distance between the fence and grounding grid as well as on the size and geometry of the latter. Fence connection to separate ground electrodes can be used when the fence is located outside the grounding grid area. The fence potential in case of a fault current flowing through the grid is lower than the GPR and decreases as the distance between the fence and grid increases. Precautions should be taken against possible metal-to-metal touch voltages and inadvertent connections between grounded objects and the fence. A minimum separation distance between the fence and grid ensuring safety for a separately grounded fence cannot be specified. The fence is always connected to the grounding grid when located within the grid area. This can also be applied when the fence throughout most of its length is located close to the grid. In both cases dangerous touch voltages may arise if the grid does not extend beyond the fence line. In installations covering small areas, such as GIS substations, it may be extremely difficult to limit step voltages in the nearby areas outside the substation property even when soil resistivity is low. In this case a combination of burying deeper the peripheral conductors and installing deep ground rods at the grid perimeter may reduce step voltages to acceptable levels without increasing touch voltages along the fence. Segmented fence grounding can be applied if the fence is in close proximity or within the grounding grid in some areas and far away from the grid in other areas. The insulating fence sections used to electrically isolate parts of the fence
should be long enough to prevent bridging of the isolated fence parts and to limit the coupling through the ground. Depending on the location and number of isolation points, this grounding technique may not always result in a safer design when compared to continuous fence grounding. For large scale photovoltaic power stations covering vast areas, the safety provided by the fence grounding configuration should always be evaluated despite the typically low values of ground fault current. The most costefficient technique is fence grounding using fence posts as ground electrodes. The use of high resistivity surface material in limited areas around the fence could also be necessary to achieve safety. REFERENCES [1] [2] [3]
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