FIELD OF INVENTION
The invention generally relates to a grounding system of High
voltage substation in a compact area such as gas insulated
substation (GIS) or highly integrated substation (HIS) and
particularly relates to a system of minimising ground resistance
and surface potential of the grounding system both during normal
and fault condition in an area where soil resistivity is high and
grounding grid area is limited.
BACKGROUND
Ground resistance of high voltage substations must be as low as
possible for safe grounding of their equipment both during normal
and fault conditions. The design of grounding system with low
resistance and low surface potentials becomes difficult at locations
where soil resistivity is high and grounding grid area is limited.
This is possible mostly for installation of compact substations
such as gas insulated substations (GIS) or highly integrated
substations (HIS). There are concrete foundations besides a
horizontal grounding grid and vertical grounding rods in the
ground of substation.

In most of the substations these foundations may not be
connected to grounding grid.
Even when connected, they offer high resistance as they are not
intended for discharging fault current. Hence foundations may
result in irregularities of current discharge path. In compact
substations, available grounding area is less and at the same time
foundations occupy significant percentage of total area. Limiting
ground potential rise and other surface potentials practically is a
challenging task. Currently, to overcome these problems, more
number of vertical grounding rods to greater depth has been used.
However, this is not an economical and reliable solution beyond
certain extent.
The concrete foundations in substations are located under power
transformers, gas insulated switchgear and under a control panel,
protection relays and various auxiliary services. To overcome the
above, it is proposed to connect concrete foundations to grounding
grid and allow contribution in fault current discharge.

For this purpose a special conductor system concealed in concrete
foundation is proposed along with steel reinforcement of the
concrete foundation as an integral part of the grounding system as
part of this invention.
The grounding system of a high voltage substation comprises of
horizontal grounding grid and vertical grounding rods. The
performance parameters of grounding system are ground
resistance, touch potential, step potential and ground potential
rise (GPR). The performance of grounding grid depends on
different parameters such as resistivity of soil, resistivity of
crushed rock/high resistive coating, area of grounding grid, depth
of the grid, mesh size of grid, number of grounding rods, area of
ground conductors etc. Layer of crushed rock (gravels, high
resistance layer) is often spread above the grounding grid to
increase the contact resistance between the soil and feet of the
personnel in substations. The grounding system is conventionally
extended over the entire substation switchyard and some times
beyond its surrounding fence.

Grounding grid with cross elements ensures safe surface
potentials. However, it has relatively less impact on lowering the
ground resistance. The cross elements facilitate multiple paths for
the fault current and provide redundancy in the case of conductor
breaks. Surface potentials have been effectively controlled by
deeper grounding rods, introduction/increase of ground wells and
by replacing the surface with low resistivity soil or clay.
The horizontal grounding grid is generally located at 0.5 to 0.8 m
below the ground and is connected to the vertical grounding rods.
The depth to which the rods are inserted into the ground and their
quantity are based on soil resistivity and area of grounding grid.
Low resistance of grounding grid provides low potential which
appear on the grounding system in the event of a short circuit or a
lighting stroke.
The following surface potentials have to be limited to allowable
levels for better performance of grounding system:

1. Ground potential rise (GPR) is the maximum voltage that a
substation ground grid attains relative to a distant grounding
point or potential of remote earth. GPR is product of ground
resistance (R) and grid current (1G).
2. Step Voltage (Vs) is the difference in potential experienced by
a person bridging a distance of 1 m with feet, without
contacting any other grounding object.
3. Touch potential (Vt) is the potential difference between the
GPR and the surface potential at the point where a person is
standing while at the same time having contact with a
grounded structure.
OBJECTS OF THE INVENTION
One object of the present invention is to minimize resistance and
surface potentials of grounding system for gas insulated
substation installation by concealing current carrying conductors
in concrete foundation.

Another object is to optimize the concrete foundation design to
meet substation fault current requirement.
Still another object of invention is to optimize the number of
grounding rods and concrete foundations for compact substations.
Yet another object of invention is the design of the conductor
system, concealed in the concrete foundation, meeting functional
requirements.
One further object is better utilization of concrete columns for
improvement of performance of grounding system of compact
substations.
Still one more object of invention is to minimize the irregularities
in current discharge path which often occur in grounding grid.
Another object is to improve suitability of compact substations at
high soil resistivity locations by means of improved grounding grid
through a novel conductor system for a GIS installation proposed
on a concrete structure.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
The invention is described with the help of Figures 1 to 4, where:
Figure 1: Shows the conventional grounding system.
Figure 2: Shows the grounding system with concrete
foundations.
Figure 3: Shows details of concrete foundation.
Figure 4: Shows invented grounding grid scheme with conductor
system enclosed in concrete foundation.
DESCRIPTION OF THE INVENTION
Fig 1. Shows grounding grid [01] along with the grounding rods
[02]. Here, grounding grid has been installed at a depth of "h" from
the ground plane. The length of grounding rod [02] is Lg. The
length and width of grid are "L" and "W" respectively. "D" is grid
spacing. NL and Nw are number of nodes on length and width side
grounding grid [01] respectively. The cross section of grid
conductors [03] is decided by the fault current. The grounding
rods [02] are connected to grounding grid strategically.

Concrete being hygroscopic, attracts moisture. Buried in soil, a
concrete block [07] behaves as a semi-conductive medium with a
resistivity at about 30 to 300 ohm-m.
Due to the large contact surface areas between the concrete and
the soil, low resistance can be achieved.
The resistance offered by the grounding rods and the conductor
system concealed in concrete foundation [07] are comparable.
More clearly, proposed concept is particularly effective in high
resistive soil as grounding electrode [05] concealed in concrete has
lower or comparable resistance than a similar electrode buried in
the soil/ earth. In the present invention, the horizontal grounding
grid is firmly connected to the conductor system [05] concealed in
a concrete foundation [07]. Fig. 2 shows the proposed grounding
system with concrete foundations. Here conductor system
concealed in concrete foundation [05] connected to horizontal
grounding grid [01]. Fig. 3 shows the internal structure of
conductor system [05] concealed in a concrete foundation [07].

Here a conductor system comprising of four or nine or sixteen sets
of conductors depending on fault current rating and size of
concrete foundation are required at substation. The resistance
offered by each foundation arrangement decreases with increase of
its volume. In order to minimize thermal losses, multiple
conductors formed as a strand [06] is used instead of a single
conductor.
The material used for the strand shall have good corrosive
resistant properties i.e. good current carrying properties compared
to conventional steel conductor.
A plurality of multiple strand of conductors (set), firmly connected
and integrated as single unit, is arranged in the form of a matrix
which has the strands of conductors [06] positioned at the nodes
of the matrix. The stands are connected to each other forming a
combination similar to a mini grounding grid comprising of
horizontal and vertical conductors.

In compact substations most of the area is covered by foundations
which support GIS building, power transformers, yard isolators,
cable terminations etc. In such substations, the proposed design
helps in achieving required performance parameters with limited
number of vertical grounding rods [02]. Based on available
concrete foundations the equivalent number of vertical grounding
rods and depth to which grounding rod to be inserted can be
optimized significantly to achieve economical design of grounding
system. Fig. 4 shows the grounding system proposed in the
patent.
The conductor system [05] is designed in such a way that the
cross section of conductor for carrying current shall be much
higher than the cross section of grounding grid conductor [03].
Conductor system concealed in concrete foundation [07] is
optimized by considering the following:

1. Resistance offered by conductor concealed in concrete
foundation [07] shall be comparable to the resistance offered
by vertical grounding rods [02]. At the same time the current
passed through conductor concealed in foundation shall be
well below that which passes through the grounding rods.
This shall be ensured for all practical conditions like
variation in soil resistivity, concrete resistivity, operational
temperature etc.
2. The conductor system is formed in such a way that the cross
section of conductor for carrying current shall be much
higher than the cross section of grounding grid conductor.
The size of the conductor system further depends on
parameters such as soil resistivity, substation grounding
area, fault current, size of foundation etc.
3. Touch and step potentials shall be below allowable level
during fault current discharged through grounding system
with optimal substation area.

SUMMARY OF THE INVENTION;
The main objective of the present invention is to minimize
resistance of the grounding grid and thereby surface potentials of
grounding system of the compact substations. The invention is
based on use of novel conductor system concealed in concrete
columns of substation foundation.

WE CLAIM:
1. A method of reducing ground potential rise (GPR), touch
and step potential and ground resistance of High Voltage
Substations, in particular gas insulated substation (GIS)
and highly integrated substation (HIS) located in area of
high soil resistivity and having limited grounding grid area
comprises of the following steps:
• providing a modified conductor system [05] embedded in
concrete foundation [07] of the individual equipment of the
substation;
• connecting the steel reinforcement of the concrete
foundation[07] of individual equipment of the substation
to the said modified conductor system [05] and
• connecting the modified conductor system [05] and the
steel reinforcement of the concrete foundation of individual
equipment [07] to the horizontal grounding grid[01] of the
substation, characterized in that the grounding grid so
connected minimizes ground potential rise (GPR), touch
and step potential and ground resistance of the high
voltage substation under normal and fault conditions.

2. The method as claimed in claim 1, wherein internal
structure of the conductor system [05] concealed in the
foundation [07] comprises a plurality of multiple strand
of conductors [06] forming a set and firmly connected
and integrated as a single unit, arranged in the form of a
matrix having said strand of conductors positioned at the
nodes of the matrix, and such nodes of the matrix
connected to each other in a combination resembling a
mini grounding grid.
3. The method as claimed in claim 1, wherein the number of
conductors in a set of multiple conductors vary according
to fault current rating of a particular substation and size
of said concrete foundations.
4. The method as claimed in claim 1, wherein the
conductors used in the conductor system [05] have good
corrosion resistance properties.
5. The method as claimed in claim 1, wherein the conductors
of the conductor system [05] have better current carrying
properties than the conventional steel conductors.

6. The method as claimed in claim 1, wherein the
conductors of the said conductor system have
substantially higher cross section as compared to that of
the grounding grid conductor.
7. The method as claimed in claim 1, wherein resistance
offered by the conductor system [05] concealed in
concrete foundation shall be comparable to the
resistance offered by the grounding rods [02] for all
variations in soil resistivity, concrete resistivity and
operational temperature.
8. The method as claimed in claim 1, wherein current
passed through said conductor system is well below that
which passes through the grounding rods [02] for all
variations in soil resistivity, concrete resistivity and
operational temperature.
9. The method as claimed in claim 1, wherein the horizontal
grounding grid [01] is located at a depth of 0.5 to 0.8 m
below the ground and is connected to the vertical
grounding rods [02].

10. The method as claimed in claim 1, wherein depth to which the
grounding rods [02] are inserted and their quantity are
reduced significantly by using conductor system [05]
concealed in foundation [07] for discharge of current.

The task of controlling various parameters of grounding system of
high voltage substation particularly in gas insulated substation for
highly integrated substation having limited ground grid area and
located in high resistance soil zone have been successfully solved
in this invention. The steel reinforcement used in the concrete
foundation of various equipment of the substation has been used
as an integrated part of the grounding system.