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Scientific Paper / Artículo Científico |
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https://doi.org/10.17163/ings.n28.2022.01 |
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pISSN: 1390-650X / eISSN: 1390-860X |
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CONSIDERATIONS IN THE DESIGN OF ELECTRICAL
SUBSTATIONS, INCLUDING THE EFFECT OF POTENTIAL GRADIENT ON SURROUNDING
METALLIC STRUCTURES |
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CONSIDERACIONES EN EL DISEÑO DE SUBESTACIONES ELÉCTRICAS, INCLUYENDO EL EFECTO DEL GRADIENTE DE POTENCIAL EN LAS ESTRUCTURAS MÉTALICAS CIRCUNDANTES |
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Wilo Chiliquinga1 |
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Received: 23-05-2022, Received after review: 13-06-2022,
Accepted: 22-06-2022, Published: 01-07-2022 |
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Abstract |
Resumen |
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For
designing and studying an electrical substation grounding system [GS], a
simple remote substation is considered according to the safety procedures
indicated in the IEEE 80 Standard. Buried metallic materials or nearby
metallic structures permanently endanger human life when electrical faults
occur. Scenarios related to the design of electrical substations that
consider the transfer of electrical potentials that can occur between GS and
buried metallic materials in their vicinity are presented, the behavior of
potential transfer is evaluated, values of transferred voltages are
calculated, and the main variables that influence the transferred voltage
levels are identified. The simulations are performed with the CYMGRD program
specific for GS calculations. Its analysis generates actual results in the
potential transfer that must be considered by the GS design engineer, which
leads to avoiding designing isolated substations without taking into account
existing elements that may affect the substation contour. |
Para el diseño y el estudio de un sistema de puesta a tierra de subestaciones eléctricas [GS], se considera una subestación remota simple según los procedimientos de seguridad indicados en la norma IEEE 80. Los materiales metálicos enterrados o las estructuras metálicas cercanas ponen en peligro permanente la vida humana cuando se producen fallos eléctricos. Se presentan escenarios relacionados con el diseño de subestaciones eléctricas que consideran la transferencia de potenciales eléctricos que puede producirse entre la GS y los materiales metálicos enterrados en sus proximidades, se evalúa el comportamiento de la transferencia de potencial, se calculan los valores de las tensiones transferidas y se identifican las principales variables que influyen en los niveles de tensión transferidos. Las simulaciones se realizan con el programa CYMGRD específico para el cálculo de GS. Su análisis genera resultados reales en la transferencia de potencial que deben ser considerados por el ingeniero de diseño de GS, lo que lleva a evitar el diseño de subestaciones aisladas sin tener en cuenta los elementos existentes que pueden afectar al contorno de la subestación. |
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Keywords: Electrical substation, grounding systems,
ground grid, potential transfer, step and touch voltage, and buried metal
structures. |
Palabras clave: Subestación eléctrica, sistemas de puesta a tierra, red de tierra, transferencia de potencial, tensión de paso y de contacto, y estructuras metálicas enterradas. |
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1Master of Electricity Program,
Universidad Politécnica Salesiana, Quito, Ecuador. 2,*Research
Group on Smart Grids GIREI, IUS-RECI Electricity Grids and Smart Cities,
Universidad Politécnica Salesiana,
Cuenca, Ecuador. Corresponding author ✉: pablo121075@icloud.com
Suggested citation: Chiliquinga, W. and Robles, P. “Considerations in the design of electrical substations, including the effect of potential gradient on surrounding metallic structures”. Ingenius, Revista de Ciencia y Tecnología. N.◦ 28, (julio-diciembre). pp. 9-24. 2022. doi: https://doi.org/10.17163/ings.n28.2022.01. |
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1.
Introduction The GS is an essential factor in
human safety and maintenance of electrical inputs that make up a substation,
considering its cost and durability [1]. In GS, ground fault currents must
safely dissipate to return the ground to their sources; so that the
protection devices can quickly disconnect the supply and eliminate the fault.
However, fault currents flowing through the ground grid also flow through
other buried metallic objects, including grounding systems connected to other
facilities not affected by the faults [2], [3], [4], [5]. Although the GS may
be physically isolated from each other, they are electrically linked through
unwanted coupling, transferring dangerous electrical potentials from the
meshes with fault currents to the non-energized passive meshes of other GS
with a risk of electrocution for the personnel present. in those areas [6],
[7], [8], [9]. Poor grounding in the oil and gas industry contributes to
unnecessary downtime, but lack of good grounding is also dangerous and increases
the risk of equipment failure leading to instrumentation errors, problems
harmonic distortion, and power factor problems [10], [11, 12]. Oil and gas
pipelines are large and sophisticated structures protecting against
electrical discharge, especially corrosion. For the design of the GS, cathodic protection [CP] must be included, in addition to
the electrical effects that can occur when these two systems are together GS,
and CP [13], [14], [15], [16], [17]. This article aims to
present case studies on the problems introduced by the presence of metallic
structures and adjoining protection systems, among others, in the transfer of
potential gradients [GPG] in passive GS. The design
procedures described in the standards related to GS of electrical substations
in urban areas and oil stations allow the calculation of safe levels of step
and touch voltages within the substation area, but adjacent GPG is not taken
into account [18], [19], [20], [21], [22], [23], [24]. The type of material
used can be a decisive factor in human electrocution. Inside the substations,
the touch voltage [MTV] and step voltage [MSV] are less dangerous due to the
high resistivity surface layer [25]. However, this layer does not extend
outside the substations, where the transferred touch and step voltages can be
harmful and much more if there are adjoining buried metallic structures not
connected to the GS [26], [27], [28]; [29]. 1.1.
Related Works The potential gradient generated
[30], [31]in oil refining complexes in the event of a failure in an
electrical substation causes its transfer to the process areas, causing
damage to the instrumentation system, [32, 33]. |
Metallic
parts such as water and gas pipes, rails, and building foundations can modify
the distribution of electrical potential in the area, depending on the
structural topology, which triggers the effect of the GPG, [8]. The
GS must consider the conductors directly involved in the protected
installation, and any other, connected or not, can interact with the whole GS
(Figure 1).
Figure 1. Equi-
potential contour of a mesh of one GS The pipelines carrying harmful
products are protected against corrosion, usually by layers of coating materials
integrated with active cathodic protection systems
[34, 35]. The current flow, typically adopted for large or long structures,
force the pipe to behave as a cathode, thus providing corrosion protection of
its exposed parts when the coating fails. However, buried lines with cathodic protection, close to the grounding networks of
electrical substations, allow the possibility of bonding and reduce the risk
of metal-to-metal contact voltages. This bonding connection, necessary for
the safety of operating personnel, can compromise the CP’s effectiveness. To
avoid corrosion of the CP and bonding with the mesh, mineral salts, which
ionize, forming a solid electrolyte with a pH varying from 8 to 10, must be
considered. In electrical
substations in urban areas, metal parts that can modify potential electrical
distribution are |
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contemplated [36],
[37], [38]. If they are attached to the main ground connection terminal of buildings,
as recommended by the standards, it allows it to function as a connection
between the GS, this type of grounding is known in the literature as Ufer grounding, which considers that the metal
encapsulated with concrete acts as an effective grounding electrode. However,
they are not part of the GS and are buried in their environment, maintaining
direct contact with the soil; they modify the potential profile on their
surface [39]. The GS is solid to
limit ground potential gradients to levels that avoid endangering human
safety and proper equipment operation under normal and fault conditions.
[40], [16]. What will allow us to comply with the normal conditions and the
fault conditions of a substation? Additionally,
provide the means to dissipate electrical ground fault currents. [41], or
prevent human beings inside the substation and in its adjoining areas from
being exposed to electrification or electrocution. Failure to pay attention
to safety in the design will cause potential gradients along the analyzed
surface to endanger humans [42]. Events caused by atmospheric discharges:
High ground-fault current circulation concerning the grounding system area nad is remote resistance [43]. Soil resistivity and its
distribution of ground currents can produce high GPG at points on the
substation surface under design [44]. Presence of the
human being so that his body forms part of a circuit between two points with
different potentials. |
The absence of sufficient contact resistance
or other resistance in series limits current flow through the body. The
duration of the failure is a function of the impact on the human being. Due
to these conditions, the low accident rate is due to a low probability of
coincidence of the favorable conditions mentioned above. Table 1 shows the
nomenclature of the variables used to analyze the exposed case studies
present in the electrical substations versus surrounding metallic structures.
Table 2 summarizes related works of the recommendations proposed by several
authors concerning the design of grounding systems and the behavior of the
potential gradient that occurs in surrounding metallic structures. Table 1. Nomeclature & Description
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Table 2. Summary of articles related
to grounding systems
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2. Materials and
methods 2.1. Problem Formulation Figure 2 presents a flow chart of the
electrical substation design methodology, including the effect of the
potential gradient on the surrounding metallic structures, where the
designer’s expertise allows obtaining results according to the threshold
contact voltages and the threshold step voltage.
Figure 2. Methodology used for the design Case studies are presented, which
will allow analyzing the transfer of potential. The simulation of the
scenarios is carried out with specialized CYMGRD software using the finite
element method (FEM) developed by EATON; it allows the interpretation of soil
resistivity measurements, elevation of earth potential, and evaluation of
dangerous points in any area of additional interest, generates a visual
representation of the results of the analysis on the potential of the mesh.
The proposed scenarios are according to the type of surface layer used and
the potential transfer analysis; the input data presented correspond to the
following: Table 3. General Notation & Descriptions
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The present study
focuses on the variation of the surface layer, the physical location of the mesh,
and the different locations of the surrounding metallic structures.
Figure 3. Top view of GS case study 1 In the case of asymmetric
grids, the analysis study is similar; taking the stratigraphy of the terrain,
the exposed potential gradients transfer study metric is applied for any grid
configuration. Parameter values of
the GS are taken based on the mesh indicated in figure 3 so that the
importance of step and touch voltage does not exceed the maximum allowed
values; the use of copper rods or electrodes is not considered for these
simulations. For the design and
simulation of the scenarios, the bodyweight of a 50kg person is taken. Assuming the most
sensitive case that can occur, the optimal mesh conductor for this
configuration is a copper conductor of 20.3776 mm2, equivalent to 2/0 AWG for
the simulations of the scenarios, a 4/0 AWG copper conductor is taken up,
where the IEEE 80 standard suggests considering the effects of corrosion
present by the PH of the soil stratigraphy, [22]. 2.1.1.
Case Study 1 Figure 3 shows a substation
grounding mesh; its dimensions correspond to 50×50 meters with 10-meter
grids; the perimeter conductor GS at 1 m depth in uniform soil with a
straight metallic cylindrical tube at 2 m depth. It is analyzed such
as a) effects are produced by the transfer of potentials in the mesh when
having the presence of an underground metal perimeter fence, and b) the
effects are due to potential transfers caused by the presence of a metallic
pipe buried under the GS. |
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For the analysis,
different configurations of the GS and the underground pipeline. Potentials are
analyzed in an asymmetric mesh; a case presented depending on the topology
and the facilities offered by its on-site construction.
Figure 4. Top view of asymmetrical
L-type mesh 2.1.2.
Case Study 2 In Figure 5, rails located at a
distance "d" from the mesh are added; for study case 2, the transfer
of potential is reflected in changes of touch, step, GPG, and Rg voltages that occur in the directions will be analyzed
indicated.
Figure 5. Top view Mesh- Rails |
2.1.3.
Case Study 3 The mutual interference of nearby
ground grids is analyzed to assess the potentially dangerous circumstances in
sites protected by the primary grid, including proximity to an adjoining one
that dissipates the ground fault current in the surrounding ground, mainly in
urban areas, a) mesh 1 Main and Mesh 2 Offline, b) mesh one primary and mesh
two connected, c) mesh one primary and Mesh 2 are connected to other GS. Two meshes of the
grounding system are considered, with similar topology and technical
parameters. Adjacent edges of the meshes are spaced a distance d meters
apart.
Figure 6. Nearby grounding grids 2.2.
Interferences
between GS and cathodic protection The standards and recommended
practices for the design of cathodic protection
systems establish the need to interconnect metallic structures with each
other. However, this equipotential bonding can compromise cathodic
protection and safety effectiveness. GS traditionally built with copper
electrodes, due to their stability characteristics over time, present
problems concerning cathodic protection: a copper
ground mesh connected to the structure under cathodic
protection can drain a considerable amount of the protection current. It may
be impossible to polarize the steel structure correctly in specific
scenarios. If the cathodic protection is no longer
adequate, corrosion is at risk due to a galvanic coupling between copper and
steel. 2.3.
Impact of groud faults on pipelines and possible solutions. Destructive electrical arcs can be
prevented by bonding the GS to the pipe. However, such a connection would
cause the cathodic protection system to drain; this
is solved by inserting an ISP or a polarization cell in said connection. The
fault current impressed on the pipe must be safely dissipated to earth
employing uncoupled intentional sacrificial anodes, e.g., magnesium or zinc
materials, connected and installed along the pipe with a low resistivity. These
sacrificial anodes would also facilitate the dissipation to the ground of
currents |
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conducted to the pipeline from
locations remote from the substation area. Instead of bonding conductors,
installing decoupling devices, such as insulator surge protectors between
grounding electrodes and pipelines, is the best compromise to safeguard
safety and functionality during ground faults. Decouplers
minimize the impact of ground faults on channels while preserving the
effectiveness of the cathodic protection. Human safety depends
on the energy absorbed before the fault is cleared and the system is
deactivated; it is suggested to establish step voltage and touch voltage
limits which are called thresholds, depending on the material used as the
surface layer and its reduction factor, Dalziel and Lee set constants related
to the electrical discharge energy tolerated. 3.
Results and discussion 3.1.
Analysis of
Results 3.1.1.
Effects of
perimeter mesh earth conductor The underground metallic pipe is
not considered; the contour and profile graphs of the potential gradient of
the cases are made, considering the finished floor as the asphalt ρs = 10000Ω − m.
Figure 7. Touch voltage, insulated
mesh- -ρs asphalt The underground
metallic pipe is not considered; the contour and profile graphs of the potential
gradient of the cases are made, considering the finished floor as the asphalt
(Figure 7 and 8). Ground mesh and perimeter fence without connection see
(Figure 9 and 10). |
A summary of the simulations with
the conditions set out for study case 1, with surface layers of concrete,
gravel, and asphalt is presented. At ρs = 200Ω − m concrete,
the touch and step threshold voltages correspond to 457.82 V and 730.83 V.
Figure 8. Insulated mesh potential profile -ρs asphalt Table 4. Mesh and perimeter fence without connection
Figure 9. Touch voltage,
mesh and fence without asphalt -ρs connection Changing for a finished gravel
floor, ρs = 5000Ω − m,
the threshold voltages of touch and step vary to 2489.47 V |
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and 8857.39 V, with the change to
asphalt
Figure 10. Potential mesh
and fence profile without asphalt-ρs connection
Figure 11. Modified mesh and mesh connected to fence According to the
simulations carried out with different values of the surface layer, the
threshold touch voltages [TTV] and threshold step voltage [TSV] are directly
related to the surface layer’s resistivity. The change of material in the
surface layer to gravel or asphalt allows the touch voltages, step voltage,
GS, and Rg to vary in favor of the safety of the
human being when the perimeter fence is implanted. The surface layer of
concrete allows the MTV to exceed the values of TTV and TSV, which suggests
improving the architecture of the mesh. Based on the mesh and perimeter fence
indicated in figure 3, the modifications |
made to the
architecture of the mesh to reduce the MTV and MSV when there is a surface ρs =200 Ω-m, suggest a) attach the
mesh to the perimeter fence, b) modify the mesh, increasing more loops on the
outer parts. See figure 11. In case 1, with the architecture change, the TTV
and TSV correspond to 457.82V and 730.83V, the Rg
= 0.34Ω does not vary, and the MTV and MSV, as well as the GS, are
shown in Table 5. Table 5. Mesh and Perimeter Mint
Figure 12. Metric with mesh architecture change 3.1.2.
Influence of
the perimeter mesh The results allow us to observe the
effects of the perimeter fence on the MTV, MSV, GPG, and Rg:
If the earth conductor of the perimeter fence encloses the substation mesh,
its effects on the MTV of the mesh decrease up to 48%, and the GS and Rg decrease by 27%. The most typical configurations
used to decrease the touch voltage are observed in figure 11; in table 5, the
MTV and MSV decrease in case 1. In case 2, with the modified mesh, the MTV
decreases, and MSV increases concerning the values indicated in table 3 when
there is a mesh and fence without a connection. If the GS increases, the Rg increases; this is logical since the GS is
proportional to Rg, according to its
formula, GP G = Ig.Rg. When
the GS increases, the peak touch voltage decreases. In case 1, mesh and fence
connected, the maximum voltage value decreases by 8%, especially where the
mesh and rail are not connected. In case 2, modified mesh, the maximum touch
voltage decreases by 16% for the case in which the mesh is present and the
fence is offline. |
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3.1.3.
The buried
metal pipe effects For the analysis of the effects of
the pipe on the ground grid, the fence indicated in figure 2 is not taken into
account; the touch, step, and GS voltage simulations are carried out on the
grid and direction 1. Metal pipe of a fixed diameter, buried at different
depths. See Table 5. A metal pipe is buried at a fixed depth and variable
diameters. See Table 6. An example of item a is simulated, with a surface
layer of asphalt for the case of a 400 mm diameter pipe case and a depth of 2m,
Table 6.
Figure 13. Touch voltage, mesh and
asphalt-ρs pipe
Figure 14. Potential direction profile 1 - mesh and pipe - ρs asphalt A summary of the
simulations carried out with pipes of different diameters, depths, and
surface |
gravel, and
asphalt are present. ρs =
200Ω − m [concrete], constant diameter pipe, and
underground metallic line at depths of 2, 3, 4, and 5 m, TTV, and TSV
correspond to 457.82 V and 730.83 V when changing the scenario For the
surface layer of gravel [ρs = 5000Ω
− m], the TTV and TSV vary between 2489.47 V and 8857.39 V, for
the case of asphalt as a surface layer being its ρs = 10000Ω − m, the TTV.
TSV equal 4605.76 V and 17322.6 V, Rg is
set to 0.47 Ω. The maximum touch, step, and PG voltages present a
variation presented in Table. Table 6. Constant pipe diameter and
variable depth
ρs = 200Ω − m [concrete],
pipe diameter variation at 0.8, 0.6, 0.4, 0.2m at constant depth, TTV
and TSV at 457.82V and 730.83V, yes gravel with ρs = 5000Ω − m is used as a
surface layer, TTV and TSV indicate values in 2489.47 V and 8857.39V. For the
scenario of using asphalt as the surface layer, knowing that its ρs = 10000Ω − m, the result
corresponds to 4505.76 V and 17322.6 V, indicating that Rg
= 0.47Ω. The MTV, MSV, and PG present a variation presented in Table 7.
Figure 15. Metric mesh and underground metallic pipe |
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Figure 16. Display of values Table 8 In the presence of
an underground metal pipe that crosses under the mesh, with a constant
diameter and variable depth, the MTV and MSV decrease the deeper it is, while
the Rg and GS do so on a smaller scale.
If its diameter is variable and its depth constant, the MTV and MSV increase
when the pipe diameter decreases, and the Rg
and GS increase to a lesser extent. In figure 3, different scenarios are
proposed, keeping constant the parameters of the mesh, diameter, and variable
depth, the objective of these configurations allows to determine which
strategy is more favorable for the decrease of TTV and TSV that can be
generated on the surface area of the pipeline when a fault occurs in the
electrical system. 3.1.4.
Analysis of
results of underground metallic pipelines The voltages MTV [457.82 V] and
MSV [730.83 V] of case 1 indicated in table 1 are taken as reference, where
it is recommended to install more ground mesh conductors parallel to the
underground pipe that passes below; this helps to decrease the TTV and TSV in
the |
Figure 17. Asymmetrical mesh touch voltage - ρs concrete When inside the grid
area, the buried metal pipe decreases the TTV and TSV; in real situations,
this does not happen because the pipes always enter and leave the ground grid
area. When a metal pipe leaves or is outside the mesh area, it causes elevation
of TTV and GPG in regions outside the mesh; this is logical since by
increasing the length of the pipe, the increase in potential will approach
the rise in network potential. When a fault occurs in the electrical system,
a current is generated in the pipe; this increases when the pipe is further
away. The scenarios presented with underground metallic pipes clearly show
the danger of transferred potentials. For GS designs in electrical
substations, corrective measures must be taken when metallic structures are
nearby; the designer must know their influence with acceptable precision;
otherwise, they may apply erroneous or unjustified measures. According to sound engineering practices, corrective procedures are in place to minimize the transfer of potential when metallic structures are buried. Joining the metallic structure to the main mesh, however, the consequences of the said procedure is carefully verified due to the possible transfer of the increase in the potential of the networks [reverse situation]. Provide denser meshes for the grid over the buried pipe. These |
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meshes can, in some cases, act as
a protective mesh by reducing the magnitude of the touch voltage. Install insulating
flanges on underground metal piping at suitable locations. The optimal
solution in the corrective methods to be used will depend on several factors,
such as the properties of the soil, the location of the metallic structure,
and the fault current, among others. 3.1.5.
Mesh Type L An GS study of an asymmetric mesh
is presented, see figure 4, for this case, a surface layer of resistivity ρs = 200Ω − m [concrete]
is considered, obtaining the values shown in Table 8. Table 8. Asymmetric mesh type L
Figure
18. Mesh
potential contour - without rails In the exposed case, it is
observed that with the surface layer of concrete, the MTV exceeds the MSV; in
figure 16, the procedures to improve the TTV are the same as those exposed in
symmetric or asymmetric meshes. 3.1.6.
Analysis of
results Scenario 2 A mesh and nearby rails are
considered, the configuration of the mesh indicated in figure 3 is used, the
study is made based on what is shown in figure 5, and a surface layer of
asphalt is considered. 3.1.7.
Case 2.1
Ground grid without considering nearby rails An analysis of the potential contour
of the ground grid is made without considering the rails; figure 18 shows the
potential distribution on the ground surface when a fault condition occurs. |
3.1.8.
Case 2.2
Ground Grid considers rails without connection to the grid It is proposed to analyze the
potential contour of the ground grid considering the rails at a distance
[d=30m] without connection; this is a common situation in electrical
substations and power plants, the simulation of the rails is done through
steel conductors 90 mm diameter aluminum-clad separated at a distance of 1.5
m and a length of 50 m. Figure 19. 3.1.9.
Case 2.3 Earth
grid considering the rails connected to the grid
Figure 19. Mesh potential contour -
with rails
Figure 20. Touch vo The potential
contour of the ground grid, considering the rails connected to the ground
grid employing 4/0 AWG copper cables at 5 points uniformly distributed on the
rails, is presented as a case study. See figure 20. Figures 17, 18, and 19
show the change in potential curves when the rails are installed; the most |
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important differences can be seen in the distribution of potentials in the surroundings since high GS are generated to be considered. Figures 19 and 20 show that the TTV values increase in the area between the rails and the mesh when they are not connected; they are joined with the mesh to reduce these potentials. Figures 21 and 22.
Figure 21. Mesh potential contour with
rail connection Table 9. Variable diameter and constant depth pipe - ρ concrete
Figure 22. Voltage of touch screen and
connected rails If the rails are not
connected to the mesh, the TTV and TSV values are high, see Table 9, item 2,
if they |
Rg values.
Another way to reduce the potentials in the rails when they are not connected
to the mesh is to insulate the union joints, see Table 9, item 8; in the same
way, the rails can be grounded using metal rods or copper-weld.
Figure 23. Display of values table 6 3.1.10.
Analysis of
Results Scenario 3 Mesh 1 and mesh two are
considered; Figure 6, the mesh configuration indicated in figure 3 is used with
a surface layer of gravel and a short-circuit current of 10 kA. 3.1.11.
Mesh 1 and
Mesh 2 No Connection When an electrical
fault occurs in mesh 1, it affects mesh two due to the difference between the
potentials of the ground surface and the potential in mesh 2; the simulation
for the two meshes is done by varying their distance, starting from 10 up to
120 meters; obtaining TTV= 2489.47 V and TSV=8857.39 V.
Figure 24. Touch and step voltage on the measuring axis of the meshes. Mesh 1 and Mesh 2 without connection |
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Table 10. Voltages touch step Rg GPG − Insulated mesh
The MTV generated by
mesh 1 in mesh 2 grow as they move away, reaching their maximum value of
2464.27 V at a distance of 120m; otherwise, the MSV decreases as they
move away. In meshes 1 and 2, the Rg
values are maintained at all separation distances, while in Mesh 2, the
GPG decreases as they move away. On the line of the measurement axis, a graph
of the touch and step voltage is plotted for insulated grids 1 and 2, for a
distance d=30m, Figure 24. 3.1.12.
Mesh 1 and
Mesh 2 are Connected If meshes 1 and 2
are connected, the leak fault currents are the same, and the potentially
dangerous voltages are generated due to their geometric symmetry. The meshes
may be intentionally or unintentionally attached through various metal
installations such as cables, pipes, or metal rails. Table 11. Voltages touch, step, Rg, GPG.Mesh 1 and 2
joined
The union of the two meshes is considered using an isolated cable with the same characteristics as those that make up the respective meshes, obtaining similarity between MTV and MSV in both meshes, see table 10. In an electrical failure, the voltage values are reflected in mesh 2. It indicates the need for an exhaustive analysis; before intentionally connecting mesh 1 with mesh 2, it is necessary to check if the security measures applied in the Mesh 2 installations can build up to dangerous voltages and ground currents. On the other hand, the parallel connection of the grounding system meshes decreases the ground currents and the TTV associated with mesh 1. The |
TTV and TSV metrics
at a distance d=30m are shown in Figure 25.
Figure
25. Voltages touch and step - on the measuring axis of mesh 1 and mesh 2
connected 3.1.13.
Mesh 1 and
Mesh 2 without Connection- Mesh 2 Underground. Distribution source
transformer stations are often located close to surrounding buildings in
urban areas. On the other hand, GS’s are often intentionally or
unintentionally interconnected; therefore, they tend to have a very low Rg. It is essential to assess potentially
dangerous voltages that may appear on the ground grid of an adjacent building
closest to the substation.
Figure 26. Touch and step
voltages on the measuring axis - Mesh 1 and Mesh 2 isolated - Mesh 2
grounded. Table 12. Voltages touch,
step, Rg, GPG Mesh 1 - Mesh 2 grounded
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The potential
distribution on the ground surface of grid 1, where the source distribution substation
is located, and grid 2, which represents the ground grid of the building
closest to the substation, is calculated. The calculations are made assuming
that the ground mesh of the building is connected to the ground meshes of the
surrounding buildings; it is assumed that the meshes of the structures have
the same characteristics and technical parameters equal to mesh 1. The
simulation for the two meshes is done by varying their distance, starting
from 10 to 60 meters, obtaining TTV=2489.47 V and TSV=8857.39V. In the
situation in urban areas, the MTV in the neighboring building for d = 30
presents high values; however, the stepped voltages in the space between the
substation and the adjoining installation are higher voltages for the grids
connected. The transfer of dangerous potentials and the technical parameters
of the mesh and the ground depend mainly on the value of the fault current. A
considerable distance between nearby meshes does not guarantee a decrease in
the transfer of dangerous potentials. In electrical substations, the
resistivity value of the surface layer is an important parameter to take into
account; its value directly influences the TTV and TSV values of the
substation and, therefore, the transferred potential gradients. 4.
Conclusions Based on the scenarios presented
in this study with variables of resistivity of the surface layer and
different configurations, the variation of the GPG has been determined against
several strategies. Technical parameters that appear in GS designs have been
related and evaluated, observing how these influence the GPG. This article
presents the simulation and analysis of real potential transfer scenarios
between electrical substations and metallic structures. Based on them,
results have been obtained that reflect the values of dangerous transfer
voltages to metallic structures near a substation, exceeding the TTV and TSV
allowed in a GS. Measures and procedures taken into account to reduce
transfer voltages in the design and construction of GS are indicated. The values of
short-circuit currents, soil resistivity, distances, and location between
nearby metal structures, among other design parameters, influence the
transfers of electrical potentials between a substation and adjoining metal
structures. It is essential to consider the GPG generated in a GS outside the
substation area, the interference effects of potentials generated by the
existence of metal structures, and the GS close to the substation. In nearby grids, an analysis is
made of the potentially
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protected by them. In designs of cathodic protection systems for pipelines in oil
stations, interferences between GS’s connected to the substation grid with cathodic protection systems are avoided as much as
possible. Otherwise, the problem should be deepened, proposing suitable
solutions to this interference, which would prevent compromising the
effectiveness of the cathodic protection system.
Detailed scenario analysis is
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circumstances are necessary for installing substations in urban areas. The
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