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)

Presentation outline

Research ObjectivesObjective 2Objective 3Papers in progress

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.

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.

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.

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.

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

MVA method procedureUtility short circuit MVA at 25kA capacity, Transformer short circuit MVA

III. Cable short circuit MVA

MVA method procedure (contd.)IV. Calculation of total MVA

V. Calculation of fault current

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

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

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

SESCAD Design Data

SESCAD Design Data

SESCAD Design Data

SESCAD Design Data

SESCAD Design Data

Results (IEEE Std.)

Results (TNB code of practice)

Discussion (IEEE Std.)Following IEEE recommendation to obtain a grid resistance of

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.

Discussion TNB code of practice (contd.)

The values of grid is resistance and hence EPR are often high especially in areas with high resistivity.

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.

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.

Thank You

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