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Techno-economic Reasons for Construction of
Electrical Substations on Step-like Ground Surface
and Related Open Issues
K Paramesha
Karnataka Electricity Regulatory Commission
Bangalore, 560001, India
kparamesha@gmail.com
Abstract — This paper is intended to analyze the reasons
behind the Construction of Electrical Substations by the
utilities on Step-like ground surfaces. Due to urbanization
and increased cost of land, the Electrical Power
Transmission utilities are finding it very difficult to get
suitable uniform levelled land for construction of
substations. This has made the utilities to use the available
land area and design tailor made substation layouts. This
paper deals with the technical and economic aspects of
such problems and that for the commonly adopted solution
– construction of substation on step-like ground. Two of
the already constructed substation will be taken for
illustration. Further, the open problem associated with the
grounding for such substations will be enumerated giving
the direction for the future work.
Keywords— Substation grounding, Construction on steplike ground, cost of levelling.
I. INTRODUCTION
Electrical power plays a major role in the economic
development of the nation as the modern society is totally
dependent on the electrical power for their day-to-day needs.
This requirement has made power companies to adhere to
stringent reliability and quality standards so as to maintain
continuous power supply to the end consumers. Practically,
the generation and utilization centres do not co-exist.
Typically, the generation availability would be far away from
the loads and the generated power needs to be carried to the
load centres through long transmission lines [1]. Transmission
of power from the point of generation to the load centres is
facilitated by high voltage transmission lines and the loads are
catered through different voltage substations as per the
requirement [1].
Electrical substation plays a key role in power transmission
system and houses equipments for transferring and controlling
the electrical energy across different voltage levels. The
reliability and availability of substation is most important for
providing continuous power supply to the consumers. The
Electrical substation mainly contains power transformers,
978-1-4799-6042-2/14/$31.00 ©2014 IEEE
Vishwanath Hegde
Department of Electrical Engineering
Malnad College of Engineering
Hassan, 573 201, India
hegde_mce@rediffmail.com
switch gears, control and protective relays, current and voltage
transformers, lightning arrestors, incoming and outgoing lines
and associated bus bar arrangements.
An elaborative earthing arrangement will be necessary at the
substation for several reasons: (i) during an earth fault in the
transmission system, the fault current finds its path through
soil towards the substation where the neutral grounding is
employed. The convergence of this large magnitude current
raises the potential and potential gradient in the soil posing
serious threat to the personnel at the substation. Therefore, an
elaborate grounding/earthing arrangement and earth mat
design [3] is necessary to limit the magnitude of step and
touch potential to safe levels and (ii) every equipment in the
Electrical substation needs to be earthed properly to facilitate
operation of the protective devices to quickly isolate the faulty
section.
During the monopolar operation of the HVDC line, earth
serves as a return conductor and very good substation earthing
is necessary not only to ensure safe operation but also to
minimize the energy loss. It may be noted here that the present
work is mainly on HVAC systems and therefore, above issue
is not relevant and hence not discussed.
The earth mat is interconnected to individual earth
connections of all equipments. The chemical treatment around
the earth mat strips would be applied for better bonding of
earth around the earth strips.
The typical dimensions of substations constructed by utilities
with minimum number of terminal bays are as follows:
a)
b)
c)
d)
400 kV substation: 400 m X 450 m
220 kV substation: 150 m X 200 m
110 kV substation: 60 m X 80 m
66 kV substation: 40 m X 60 m
The actual dimension of the substation can be larger when
higher number of terminal bays are employed.
The typical substation earth mat is designed as per the
standards to keep touch and step potentials within the safe
limits [3]. The size and spread of earth mat in a substation
depends on the earth resistance and area available for
construction of earth mat and also the method of design. In a
substation constructed in an area of 40 m X 60 m dimension,
typically, three fourth of the area would be covered by earth
mat.
At present, availability of land with uniform surface (levelled
land) to the extent stated above is becoming very difficult to
find. The main reasons are urbanization, increasing cost of
land and non-availability of suitable size of land. This has
forced the utilities to construct substations in the available
land itself with tailor made and modified layouts.
The objective of the present work is to study the technoeconomic factors which force the utilities to construct
substations in a non-uniform un-levelled terrain trimmed into
different levels (steps).
II. PRESENT WORK
As indicated earlier, because of cost of land and urbanization,
the land available for construction of the substations are
becoming very scarce to the extent of required dimensions.
The land which are found to be available many a times have
non-uniform level (as shown in Fig. 1).
Now, if the substations are to be constructed on a levelled land
from the topography of land shown in Fig.1, the construction
cost of the substation by levelling of the land would become
higher as explained in the case studies taken up below.
Fig. 1. Non–uniform land for substation construction
2.1 Case studies
Step 1
Step 2
Step 3
(Level difference = 1.54 Mtrs)
Fig. 2. side view of a substation constructed on step-like ground (involving
three steps Thallak, KPTCL)
The cutting and filling involved for a Techno-economical
construction of above substation in step like land surface is
given in the table I.
Table I- Cutting and filling Requirement in the substation area
220 kV Yard
Control Room,
Road and 100
MVA
Transformer
area
66 kV Yard
Total
Economical
Level
RL-M
100.76
99.22
Cutting
in
Cu-M
4179
1746
Filling
in
Cu-M
4151
1728
97.88
2991
8916
2958
8837
From the above table, a total of 8916 cubic meter and 8837
cubic meter of cutting and filling has to be carried out for a
substation constructed in step like land surface. A simple
calculation of cutting and filling for normal dry soil for the
above would be Rs. 14.91 Lakh (with only Rs. 84 per cubic
meter of cutting and filling charges [2]). But, the same
substation, if decided to be constructed by levelling the
surface into a uniform single level surface, the requirement of
cutting and filling would be almost doubled with a cost of Rs.
29.82 Lakhs. This would have cost impact along with
associated technical issues discussed later.
Case study 1
Typical substations constructed in step levels by the utilitiy in
order to retain the originality of the soil and to reduce the cost
of project are discussed below:
Case study 2
A 220/66kV substation is constructed in a place called Thallak
in Karnataka, wherein there are three different levels in the
substation area as shown in Fig. 2.
The cutting and filling details in the substation was planned as
14785 cubic meter and 19799 cubic meter respectively. Here
the case is somewhat different, which calls for more filling
A 220/66 kV substation is constructed in a place called
Chikkamagalur in Karnataka which has two different levels in
the substation area as shown in Fig. 3.
compared to cutting, that means the soil has to be brought
from outside for filling with a cost double that of cutting cost
i.e Rs. 164 per cubic meter [2]. This clearly shows that, when
the entire land for the substation is to be levelled to have a
uniform levelled land surface, the cost of cutting and filling
along with the cost of extra soil to be filled would cost more
than double along with other technical issues discussed later.
2.2 General technical issues
Apart from the economic issue, there exists a few technical
issues with land filling and it will be dealt here. Even after a
careful land filling, the originality of the earth would be lost
even though Proctor density as per standard is maintained after
leveling [4]. After cutting and filling is carried out for
leveling, the filled up soil would have problems associated
with soil conductivity because, the original soil density would
have been lost.
The other problem associated with the levelled land by above
process could be, running away of the loose soil if proper
retaining walls at intermediate levels are not provided to check
the displacement of soil, which would once again add to the
cost of civil work. Also, the filled soil in the substation yard
may yield when the movement of heavy equipment like power
transformer may take place, if sufficient care is not taken
while making provision for roads in the substation yard.
It can therefore be concluded that both cost and importantly
the other related technical issues do not favour construction of
substation by levelling the uneven ground.
Fig. 4 . Substation with three levels
The construction of substations involve cutting and filling of
the land to install equipment in different level in the form of
step like formation and the involved civil works as against the
total approximate cost of substation is given in table II.
Table II. Typical cost of different Voltage substations and the civil cost in
KPTCL
400 kV
Approximate
Cost
of
substation in Rs.
Crores:
90
Approximate
Cost of Civil
works in Rs.
Crores
12 to 14
220 kV
35
1 to 1.5
2.3 Alternative approach
110 kV
5 to 8
0.5 to 0.6
As an alternative, the substations layout can be designed
retaining the major ground level, which invariably calls for
different bays/sections placed at different ground levels. This
will not only retain the soil stability but also would lead to
reduced civil works. As practical examples, Figs. 3 & 4
presents two of the working KPTCL substations which are
constructed at two to three steps owing to non-availability of
levelled land.
66 kV
5 to 8
0.5 to 0.6
Voltage Class of
Substation
III. ISSUES PERTAINING TO GROUNDING
While the construction of substation in step-like terrain is a
practically employed solution, a closer look into the substation
grounding indicates several open issues. The associated
technical challenges, which need suitable quantitative
answers, are:
1. The earth termination resistance at different points in the
substation: As such, the international standards including
IEEE deals with analytical and empirical formulas for
substation earthing resistance, step and touch potentials
only for the case of construction on levelled earth [3 - 9].
Therefore there is an urgent need to reliably scrutinise and
quantify these quantities for substation constructed on
step-like land surface.
Fig. 3. Substation with two levels
2. Ground grid interconnections between earth mats laid in
different levels and their impact on current distribution
during the fault. Variation of the step and touch potential
between the different levels for different injection point
needs a careful adjudication.
[6]
3. Finally, considering the practical demand for construction
of substation at step-like land surface, it is necessary to
search for critical issues with the grounding and possibly
develop general guidelines for a safe ground grid design
methods.
In other words, more research efforts are necessary for
handling this problem of imminent practical interest.
IV. SUMMARY AND CONCLUSIONS
Availability of levelled land for the construction of power
substation is becoming increasingly difficult. Both from
economic point of view, as well as, due to associated technical
issues, levelling the ground surface is not suggested. This has
forced the utilities to construct substations in the available
land with tailor made and modified layouts, which invariably
results in bays at different ground levels. The associated
techno-economic issues are explained in this paper.
Further, in the present work, a very critical technical issue
pertaining to substation constructed on step-like land surface
is brought out. It is noted that the prevailing
international/national standards do not address the earthing
design for substation constructed on step-like land surface.
Therefore it is stated that there is an urgent need to
quantitatively evaluate the earthing resistance, step and touch
potentials for such substation earthing. At present the research
is in progress to find out the answers to the issues of practical
importance discussed in the present work.
ACKNOWLEDGMENT
Authors thank Prof. Udaya Kumar of Indian Institute of
Science, Bangalore, for his valuable discussions and
suggestions.
Authors also thank the authorities of Karnataka Power
Transmission Corporation Limited (KPTCL) for the images
and information on the substations constructed in step like
land surface.
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