OPTIMIZATION IN GROUNDING SYSTEM DESIGN USING DEEP

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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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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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


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Surface Potential Profile in Substation during Ground Fault

Reference: IEEE-80


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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)


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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


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CASE STUDY ANALYSIS USING CYMGRD SOFTWARE


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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


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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


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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


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SURFACE POTENTIAL CONTOUR , STEP & TOUCH POTENTIAL PROFILE PLOT

POTENTIAL CONTOUR PLOT SURFACE POTENTIAL PLOT POTENTIAL PROFILE PLOT


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CASE STUDY FIELD MEASUREMENT


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STEP VOLTAGES MEASURED AT FIELD

STEP VOLTAGES MEASURED AT FIELD

STEP VOLTAGE : MEASURED VALUE < PERMISSIBLE VALUE. HENCE SAFE


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TOUCH VOLTAGES MEASURED AT FIELD

TOUCH VOLTAGES MEASURED AT FIELD TOUCH VOLTAGE : MEASURED VALUE < PERMISSIBLE VALUE. HENCE SAFE


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MEASURED RESISTANCE VALUE

GRID RESISTANCE MEASURED AT FIELD

GRID RESISTANCE : MEASURED VALUE < PERMISSIBLE LIMIT HENCE SAFE


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CASE STUDY COMPARISON BETWEEN SOFTWARE

ANALYSIS AND FIELD MEASUREMENTS


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


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CONCLUSION


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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THANK YOU

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OPTIMIZATION IN GROUNDING SYSTEM DESIGN USING DEEP