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1 Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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Presented by A.Kalyanasundaram
Larsen & Toubro Limited Power Transmission & Distribution
Chennai.
26-Jul-2019
OPTIMIZATION IN GROUNDING SYSTEM DESIGN USING DEEP
VERTICAL ELECTRODE CONTACTING WITH GROUNDWATER TABLE
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
Introduction Earth Resistivity Measurements – Wenner 4 Point Method Ground Potential Rise, Step & Touch Potential, Resistance Water Table & Water Conductivity Tolerable Step & Touch potential Case Study Analysis by CYMGRD
Parameters considered for grounding design Two Layer Earth resistivity Modelling using CYMGRD Software Surface Potential contour Step & Touch Potential profile Plot Grid Resistance
Field Measurement Surface Potential Step & Touch Potential Grid Resistance
Comparison between software results and field measurements
Grounding system Resistance Step and touch potential
Conclusion
TOPICS
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Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
Grounding System design is very Important aspect for ensuring Safety of System & Operating Personnel.
Soil resistivity is the most important parameter in governing the design of earthing system.
Often high soil resistivity being encountered in the field and coupled with Limited Substation Plot area leading to the problem of attaining the safe Touch and Step potentials within the permissible limits as defined in IEEE-80.
To overcome this difficulty, Usage of Deep-well electrode and establishing contact with ground water table is one of the methods suggested in IEEE-80.
Therefore, it is necessary to find an economical solution in Grounding Design using the above concept.
A clear methodology/procedure in Two layer modelling of Grounding Grid using water table in Grounding analysis software such as CYMGRD for providing economical solution is discussed here using case study
It can be validated through the field test, by measuring Grounding Grid Resistance, Step & Touch Potential to confirm that the measured values are within acceptable limits as per software calculated values.
INTRODUCTION
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Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
Wenner 4 Point Method as per IEEE-81
Earth Resistivity Measurement
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Where, C1, C2 : Current Electrodes P1,P2 : Potential Electrodes a : Spacing between Electrodes (in Mtr) b : Depth of Electrode below ground (in Mtr) R : Measured Resistance value (in Ohms) ρ : Soil Resistivity (in Ohm-Mtr)
Eq- (1)
Eq- (2)
1. Four Electrodes shall be driven in straight line 2. Depth of Electrode (b) shall be less than a/10 then b is assumed as 0 as per IEEE-81. Then Eq-(2) can be used.
Earth Resistivity shall be measured at different electrode spacing at desired location as per specification
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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Surface Potential Profile in Substation during Ground Fault
Reference: IEEE-80
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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WATER TABLE AND WATER CONDUCTIVITY
Water table is detected using conventional Bore-hole method.
Water table varies with seasonal conditions like Spring, Summer, Autumn and Winter.
During summer season the water table level goes to the lower depth, while during rainy period the water level tends to rise towards the ground surface.
To obtain a conservative design, Summer season water table level is taken into consideration.
Once the presence of water table is detected, the water is taken out for sample and sent to Chemical Laboratory to check for its electrical conductivity property. Based on result, the water’s electrical resistivity can be calculated.
It is recommended to use two bore holes at diagonally opposite in the Substation Plot corners, in order to ensure that the contact of deep-well electrode with the water table would be ensured during all seasons.
Where, ρ : Soil Resistivity (in Ohm-Mtr) C : Water Conductivity (S/Mtr)
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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TOLERABLE STEP AND TOUCH POTENTIAL
Formula for Tolerable Touch voltage and Step voltage Where, Cs : Surface layer derating Factor ρ : Soil Resistivity (in Ohm-Mtr) ρs : Surface material Resistivity (in Ohm-Mtr) Hs : Height of Surface layer Thickness ( in Mtr) Ts : Shock Dutation ( In seconds)
Note : The body weight either 50kg or 70kg shall be considered as client specification.
Reference : IEEE - 80
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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CASE STUDY ANALYSIS USING CYMGRD SOFTWARE
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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Parameters considered for grounding design
S.No. Description Value
1 Symmetrical Ground fault current for Grid System (If) 50kA
2 Shock Duration (ts) 0.5 sec
3 Fault clearing Duration considered for conductor sizing (tc) 1 sec
4 Split Factor or Fault current Division Factor (Sf) 0.45
5 Symmetrical Grid current (Ig)=Sf *If 50KA*0.45=22.51KA
6 Body Weight 70kgs
7 Surface material Resistivity -Gravel in wet condition (ρs) 5000 ohm-m
8 Surface Layer Thickness (hs) 0.1 m
9 Surface Layer Derating Factor (Cs) 0.706414
10 Main Grounding Mesh -Lead Sheathed stranded copper conductor 240 sq.mm
11 Depth of Burial of Main Grounding conductor 0.8m
12 Water Table level 28.3 m
13 Diameter of Deep driven Rods of Copper clad steel rod 15mm
14 Length of Copper Clad steel Rod 31m
15 Total No. of Deep driven Rods 10 Nos
16 Water Conductivity measured at Lab ( C) 0.512 S/m
17 Calculated Water Resistivity (ρ) 1.953 ohm-m
PARAMETERS CONSIDERED FOR GROUNDING DESIGN
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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TWO LAYER SOIL RESISTIVITY MODELLING USING CYMGRD SOFTWARE
Measured Value of Soil Resistivity Soil Resistivity Modelled in CYMGRD
Upper layer Resistivity is 270 ohm-m as per soil resistivity report Upper layer thickness of 28.3m based on water table level after necessary margin. Lower layer Resistivity is taken as 1.953 ohm-m based on water Resistivity value
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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MODELLING OF GRID LAYOUT IN CYMGRD SOFTWARE
SUBSTATION PLOT LAYOUT GRID LAYOUT MODELLED IN CYMGRD
GRID LAYOUT IN 3D MODELLED IN CYMGRD
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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SURFACE POTENTIAL CONTOUR , STEP & TOUCH POTENTIAL PROFILE PLOT
POTENTIAL CONTOUR PLOT SURFACE POTENTIAL PLOT POTENTIAL PROFILE PLOT
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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CASE STUDY FIELD MEASUREMENT
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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STEP VOLTAGES MEASURED AT FIELD
STEP VOLTAGES MEASURED AT FIELD
STEP VOLTAGE : MEASURED VALUE < PERMISSIBLE VALUE. HENCE SAFE
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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TOUCH VOLTAGES MEASURED AT FIELD
TOUCH VOLTAGES MEASURED AT FIELD TOUCH VOLTAGE : MEASURED VALUE < PERMISSIBLE VALUE. HENCE SAFE
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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MEASURED RESISTANCE VALUE
GRID RESISTANCE MEASURED AT FIELD
GRID RESISTANCE : MEASURED VALUE < PERMISSIBLE LIMIT HENCE SAFE
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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CASE STUDY COMPARISON BETWEEN SOFTWARE
ANALYSIS AND FIELD MEASUREMENTS
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
COMPARISON BETWEEN SOFTWARE RESULTS AND FIELD MEASUREMENTS
S.No Description Unit Value
Max. Permissible step voltage Volt 4927.42
Attainable step voltage (Software calculations) Volt 82.19
Attainable step voltage (Field Test) Volt 54.13
Max. Permissible Touch voltage Volt 1398.38
Attainable touch voltage ( software calculations) Volt 926.32
Attainable touch voltage (Field Test) Volt 156.33
Max. Permissible Ground Resistance Ohm 0.25
Ground Resistance (Software Calculations) Ohm 0.07
Ground Resistance (Field Test) Ohm 0.19
Summary for Step voltage, Touch voltage and Resistance
A
B
C
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
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CONCLUSION
Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
CONCLUSION
It is seen that when high resistivity Soil encountered in limited Substation Plot area, it is difficult to achieve the Grounding Grid Resistance, Safe Step & Touch Potential within acceptable limit as per IEEE-80, by using conventional methods.
IEEE-80 – 2013, clause 9.5 “Design in Difficult Conditions” recommends using deep driven ground rods and drilled ground wells.
Configuration like Deep-well Ground electrode having contact with ground water, which acts as current sink, as water resistivity is much lower due to the presence of dissolved minerals in water, is one of the viable solutions for effective grounding system.
This advantage is explored and analysed through Computer Ground Grid modelling using CYMGRD software and calculated the attainable Step & Touch Potential and Ground Grid resistance, within limits as per IEEE-80 / specification.
From the case study it is noted that after implementation of the Grounding Grid, necessary Field test like Ground Grid Resistance, Step & Potential were measured and compared against computed result and found that the measured values are within limits as per IEEE-80 / Specification.
This solution would work satisfactorily, when consistent water table level is ensured.
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Power Transmission &Distribution Sensitivity: LNT Construction Internal Use
THANK YOU
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