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Study of Transferred Earth Potentials in Electrical Distribution
SubstationsPresented by,Buba, Sani DahiruGS34084
Supervisory Committee MembersDr. Wan Fatinhamamah Wan Ahmad -
ChairProf. M. Z. A. Ab. Kadir, Prof. Chandima Gomes, Dr. Jasronita
Jasni and Dr. Miszaina Osman (UNITEN)
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Presentation outline
Research ObjectivesObjective 2Objective 3Papers in progress
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Research Objectives The main objective of the study is to
determine the separation distance between MV and LV earthing
systems in a distribution substation to mitigate transfer of
potentials. In order to achieve the main objective, the following
minor objectives are implemented,
Determine the soil models at selected sites for installation of
distribution substations.
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Research Objectives (contd.)Estimate the short circuit fault
current liable to imposed at each substation as a result of bolted
three phase (symmetrical fault) and single line to ground
(unsymmetrical fault).Determine the grid resistance (Rg), earth
potential rise (EPR), step and touch voltages at each
substation.Determine the separation distance between the MV and LV
earthing system in each substation to mitigate transfer of earth
potentials.
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Objective 2In order to estimate the fault current in a
distribution network, the following steps were taken,One line
diagram indicating the network components was drawn.Suitable
calculation method, e.g. ohmic, p.u or MVA method was chosen.The
fault current was calculated using appropriate formula and the
results tabulated.
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Site locationsSite 1- located at near Surau College 14 UPMSite
2- located behind the Canteen, Faculty of EngineeringSite 3-
located adjacent to School of Graduate Studies Site 4- located at
Bukit Expo near Taman Pertanian Universiti (TPU) UPM.
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Calculation of line impedance (Z)Following site visit to TNB
main switching station (SSU 1 and 2), it was found that,
distribution substations in UPM are connected using 240mm2
Aluminium 3-core armoured cable with,R=0.161 at 50Hz, X=0.089Z= (R
+ jX) * LZ=(0.161 + j0.089) * LWhere L is the length of cable
line
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MVA method procedureUtility short circuit MVA at 25kA capacity,
Transformer short circuit MVA
III. Cable short circuit MVA
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MVA method procedure (contd.)IV. Calculation of total MVA
V. Calculation of fault current
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Objective 3In order to determine the safety criteria for each
substation, an earth grid needs to be designed manually or using
CDEGS.Soil model is required to determine the burial depth of earth
grid.
- Earth grid design procedureGrid dimension (LxB) depends on
available area to obtain an Rg
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Earth grid design data (Site 1)Based on IEEE Std. 80:2000,Grid
dimension 70mx70m, grid area 4,900m2Available fault current 5.316kA
@75% split factor (3.987kA)Burial depth 1.2mGrid geometry 21x21
parallel conductors @3.5m spacing
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Earth grid design procedure (contd.)Based on TNB code of
practice,Grid dimension 14.8mx13.6m, grid area 201m2Available fault
current 5.316kA @ 75% split factor (3.987kA)Burial depth 1.2mGrid
geometry 6mx5m parallel conductors @2.5m spacing
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SESCAD Design Data
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SESCAD Design Data
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SESCAD Design Data
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SESCAD Design Data
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SESCAD Design Data
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Results (IEEE Std.)
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Results (TNB code of practice)
- Discussion (IEEE Std.)Following IEEE recommendation to obtain a
grid resistance of
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Discussion (TNB Code of practice)Based on TNBs code of practice,
land sizes are allocated for substation installation based on
substation capacity, e.g. 30mx30m for 33/11kV substations and
14.8mx13.6m for 11/0.43kV substations [TNB Electricity Supply
Application Handbook, 2007].It is difficult to achieve recommended
grid resistance due to limited land.Results in the design of unsafe
substations, which could be hazardous to personnel and general
public.
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Discussion TNB code of practice (contd.)
The values of grid is resistance and hence EPR are often high
especially in areas with high resistivity.
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Contribution Due to the fact that soil resistivity varies from
one place to the other as shown in this study, allocation of land
for installation of distribution substation based on voltage level
is not the best practice from safety point of view.It is
recommended that a data base of soil resistivity values for
different locations be established such that land allocation is
based on soil resistivity and not voltage level.
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Papers in writing processImpact of Addition of Earth Rods to
Different Grid Configurations on Grid Resistance
Impact of Buried Conductor Length on the Resistance of
Substation Earth Grid, to be submitted to the Journal of Electric
Power Components and Systems soon.
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Thank You
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