POWER SYSTEM GROUNDING

Presented

By

H.JAYAKUMAR

WHY GROUNDING IS REQUIRED?WHY GROUNDING IS REQUIRED?

GROUNDINGGROUNDING PLAYS A VITAL ROLE IN POWER PLAYS A VITAL ROLE IN POWER SYSTEM OPERATION. EFFECTIVE GROUNDING SYSTEM OPERATION. EFFECTIVE GROUNDING IS NECESSARY FOR THE PROPER OPERATION IS NECESSARY FOR THE PROPER OPERATION AND CO-ORDINATION OF PROTECTION AND CO-ORDINATION OF PROTECTION SCHEMES IN THE POWER SYSTEM AS WELL SCHEMES IN THE POWER SYSTEM AS WELL AS FOR PERSONNEL SAFETY.AS FOR PERSONNEL SAFETY.

PURPOSE OF EARTHING:PURPOSE OF EARTHING:

• PROTECTION OF INSTALLATION.

• IMPROVEMENT IN QUALITY OF SERVICE.

• SAFTY OF PERSONNEL.

GENERAL REQUIREMENTS:GENERAL REQUIREMENTS:

• LOW VALUE OF EARTH RESISTANCE.

• ACCEPTABLE SURFACE POTENTIAL GRADIENTS.

RULES/GUIDELINES:RULES/GUIDELINES:

• INDIAN ELECTRICITY RULES 1956.

• IS: 3043/1966, 1987 : CODE OF PRACTICE

FOR EARTHING.

• BS : CP 1013:1965

• CBI & P RECOMMENDATIONS

• CIGRE APPLICATION GUIDE

• ANSI / IEEE STD.80/ 1961,1967,1986.

TYPES OF EARTHING:TYPES OF EARTHING:

• SYSTEM EARTHING.

• PROTECTIVE EARTHING.

SYSTEM EARTHING EFFECTIVELY SYSTEM EARTHING EFFECTIVELY EARTHED SYSTEMEARTHED SYSTEM

(Xo / X1) < 3

(Ro / X1) < 1• Xo= ZERO SEQUENCE REACTANCE.• X1= POSITIVE SEQUENCE REACTANCE.• Ro= ZERO SEQUENCE RESISTANCE.• UNDER THE FAULT CONDITIONS THE VOLTAGES

OF THE OTHER TWO HEALTHY PHASES WILL NEVER BE MORE THAN 80% OF THE LINE TO LINE VOLTAGE OF THE SYSTEM. THIS IS ALSO CALLED CO-EFFICIENT OF EARTHING.

OBJECTIVE OF SOIL RESISTIVITY OBJECTIVE OF SOIL RESISTIVITY MEASUREMENTS:MEASUREMENTS:

• Estimating the ground resistance of a proposed sub-station or transmission tower.

• Estimating potential gradients including step and touch voltages.

• Computing the inductive coupling between neighbouring power and communication circuits.

• Designing cathodic protection systems.• Geographical surveying.

FACTORS AFFECTING THE FACTORS AFFECTING THE SOIL RESISTIVITY:SOIL RESISTIVITY:

• Type of the soil.

• Moisture.

• Dissolved salt in water.

• Temperature.

• Grain size and its distribution.

• Seasonal variation.

• Artificial treatment.

EQUIPMENT USED FOR MEASUREMENT EQUIPMENT USED FOR MEASUREMENT OF SOIL RESISTIVITY:OF SOIL RESISTIVITY:

• EARTH RESISTANCE METER(EARTH TESTER) Confirming to IS 9223:1979

(Frequency shall be 60 to 90 Hz and voltage 30 to 250V).

• METROVATT WEST GERMANY.

• EVERSHED & VIGNOL [EMI THORN OR MEGGER] UK.

EQUIPMENT USED FOR MEASUREMENT OF EQUIPMENT USED FOR MEASUREMENT OF SOIL RESISTIVITY:SOIL RESISTIVITY:

METHOD OF MEASURING SOIL METHOD OF MEASURING SOIL RESISTIVITYRESISTIVITY

• Geological information and soil samples

• Variation of depth method

• Two point method

• Four point method

- Equally spaced Wenner arrangement

- Unequally spaced or Schlumberger –

Palmer arrangement

WENNER FOUR ELECTRODE WENNER FOUR ELECTRODE METHOD:METHOD:

ρρ= 4= 4ππaR /[1+(2a/√a2+4b2)-(a/√a2+b2)]aR /[1+(2a/√a2+4b2)-(a/√a2+b2)]• ρ= Soil resistivity in ohm-meter.• a= spacing between adjacent electrode-meter.• R= earth tester reading-ohms.• B= depth of burial of electrode-meters.• When ‘a’ is large compared to ‘b’ the above formula be reduced to • ρ = 2πaR.• ALTERNATIVE ELECTRODE CONNECTION:• ELECTRODE RESISTIVITY

ARRANGEMENT FORMULA• C P P C ρ = 2πaR1

P C C P• C C P P ρ = 6πaR2

P P C C• C P C P ρ = 3πaR3

P C P C

The soil is said to be homogenous when the average value The soil is said to be homogenous when the average value

of ‘of ‘ρρ’ lies within 30 percent’ lies within 30 percent • CASE STUDY:

SL.NO a R ρ

Meters Ω Ohm-Meters

1 2 4.33 54.412

2 3 2.60 49.009 min

3 4 2.35 59.062

4 5 1.986 62.392

5 6 1.652 62.279

6 7 1.462 64.301

7 8 1.305 65.593 max

ρ Average 59.579

130% of ρ Ave = 1.3 x 59.579 = 77.453

70% of ρ Ave = 0.7 x 59.579 = 41.705

Minimum & Maximum value lies within 41.705 and 77.453

The Soil is Homogeneous.

SURVEY OF SOIL RESISTIVITY MEASUREMENT:SURVEY OF SOIL RESISTIVITY MEASUREMENT:CASE STUDY:CASE STUDY:

• SOIL RESISTIVITY MEASUREMENT:• Site of 120 MW Diesel Power Plant at Yelahanka,

Bangalore.Location Yelahanka Site Size 92.12m x 73.15m

Date of Measurement 23.01.91

Type of soil Agriculture Land

Condition of Ground Dry

Earth tester used ET3/2

A1 D1 B1

D2

C1 C2

B2 A2

Average of the above 37.99 ohm-metersAverage of the above 37.99 ohm-metersMinimum Resistivity Encountered 27.33 ohm-metersMinimum Resistivity Encountered 27.33 ohm-metersMaximum Resistivity Encountered 49.02 ohm-metersMaximum Resistivity Encountered 49.02 ohm-meters

Conclusion: since the maximum and minimum resistivity lies within 30% of Conclusion: since the maximum and minimum resistivity lies within 30% of the average value the soil is a homogeneous one the average value the soil is a homogeneous one

Inner- Electrode

Spacing

Megger Reading in ohms

(Soil Resistivity = 2πaR in ohm-m)*

2.5m 5m 7.5 10m 15m

Location A1-A2 2.0 Ω

(31.42)

1.03 Ω

(32.36)

0.94 Ω

(44.30)

0.72 Ω

(45.32)

0.52 Ω

(49.02)

Location B1-B2 1.74 Ω

(27.33)

0.94 Ω

(29.53)

0.83 Ω

(39.11)

0.62 Ω

(38.96)

0.52 Ω

(49.02)

Location C1-C2 1.74 Ω

(27.33)

1.03 Ω

(32.36)

0.61 Ω

(28.74)

0.61 Ω

(38.33)

0.51 Ω

(48.07)

Location D1-D2 2.45 Ω

(38.48)

1.06 Ω

(33.30)

0.83 Ω

(39.11)

0.63 Ω

(39.58)

0.51 Ω

(48.07)

LIMITATIONS OF WENNERS LIMITATIONS OF WENNERS METHOD:METHOD:

In the case of homogeneous soil for 8 times the spacing, the megger reading shall be one-eighth of R (megger reading), which may be beyond the lower limits of the megger reading. In order to overcome the above Schlumberger- palmer method is used.

UNEQUALLED SPACED OR UNEQUALLED SPACED OR SCHLUMBERGER – PALMER ARRANGEMENT:SCHLUMBERGER – PALMER ARRANGEMENT:

ρρ = [ = [ππc (c + d) x R] / dc (c + d) x R] / d

SL c d R ρ

NO Meter Meter Ω Ohm-Meters

1 2 20 7.76 53.633

2 4 16 4.20 65.973

ρ 59.803

When c=d, The above formula will reduced to 2πaR

Avg

OBJECTIVE OF GROUND OBJECTIVE OF GROUND RESISTANCE MEASUREMENTS:RESISTANCE MEASUREMENTS:

• Verify the adequacy of new grounding system.

• Detect changes in an existing grounding system.

• Determine hazardous step and touch voltages.

• Determine ground potential rise (GPR) in order to design protection for power and communication circuits.

METHODS OF EARTH METHODS OF EARTH RESISTANCE MEASUREMENT:RESISTANCE MEASUREMENT:

• Fall-of-potential method.

• E.B. Curdts 61.8% method.

• Slope method.

• IS: 3043 alternative method.

FALL-OF-POTENTIAL METHOD:FALL-OF-POTENTIAL METHOD:

Thumb Rule: Remote current electrode shall be at a minimum distance of 10 times the depth of the burial of the earth electrode being tested or 10 times the diagonal distance in case of earth mat.

Case Study of Fall of Potential MethodCase Study of Fall of Potential MethodREMOTE ELECTRODE(C2)DISTANCE=90MREMOTE ELECTRODE(C2)DISTANCE=90M

P2 Megger P2 Megger

Meters Reading Ohms Meters Reading Ohms

05 14.2 45 23.5

10 16.7 50 23.9

15 19.0 55 24.1

20 20.2 60 24.8

25 21.0 65 26.2

30 22.0 70 29.8

35 22.7 75 38.6

40 23.1 80 60.7

• Maintenance purpose annually only one reading may be taken with remote electrode at 90m & potential electrode 45m from the station ground to verify the value of 23.5Ω (Previous year result).

E.B.CURDTS 61.8% METHODE.B.CURDTS 61.8% METHODThis method is applicable to single ground electrode. Potential electrode

P2 is placed at 61.8% of remote current electrode C2 from the station ground.

CASE STUDY:CASE STUDY: C2 P2 Rg

Metres Metres Ω

40m 24.72 18.5

60m 37.08 18.7 Max

80m 49.44 18.6

120m 74.16 18.3 Min

Average Rg 18.525

105% of Rg average = 19.45Ω,

95% of Rg average = 17.60Ω.

All values lies within ±5% of Rg Average.

This method is acceptable.

THE SLOPE METHOD OF TESTTHE SLOPE METHOD OF TESTWas established by Dr. G.F. Tagg. The following is the summary of the paper

published in IEE 1970.(Vol. No. 177, No. 11) This technique shall be used when testing earth electrode systems which covers a large area. This method is useful when the position of the centre of the earthing system is either unknown or inaccessible(e.g. if the system is beneath the floor of a building). This method yields results of greater accuracy than those detailed above. The procedure is as follows:

a) The terminals C1 & P1 on the instruments are connected to the earth electrode.

b) Connect terminal C2 to a current electrode inserted in the ground 50m more or away. The distance from the earth electrode to the current electrode is EC.

c) The potential electrode connected to terminal P2, is inserted at several positions between the earth and current electrodes, starting from near the earth electrode.(The electrodes must be in a straight line). At each position the resistance is measured and the earth resistance curve is plotted from the results e.g., (as shown in fig) atleast 6 readings are needed. Drawing the curve will show up any incorrect points which may be either rechecked or ignored.

d) From the curve the equivalent reading to potential electrode position 0.2EC, 0.4EC & 0.6EC can be found. These becomes R1, R2 & R3 respectively.

e) Calculate the slope co-efficient µ. Where µ=R3-R2 R2-R1Which is the measure of the change of slope of the earth resistance curve. From the table shown in the

next page, obtain the value of PT /EC for this value of µ.

PT is the distance to the potential electrode at the position where the true resistance would be measured. Multiply the value of PT /EC by EC to obtain the distance P2. From the curve, again read off the value of resistance that correspond tothis value of PT. The value obtained is earth system resistance. It is important to note that: a) If the value of µ obtained is not covered in the table, then the current electrode will have to be

moved further away from the earthing system. b) If it is required, further sets of test results can be obtained with different values of EC, or different

directions of the line of EC.

From the results obtained of resistance for various values of the distance EC a curve may be plotted.

This shows how the resistance is decreasing asymptotically as the distance chosen for EC is increased. The curve indicated that the distances chosen for EC in tests(1) and (2) were not large enough; and that those chosen in tests(3) and (4) were preferable because they would give the more correct value of the earth resistance.

(c) It is unreasonable to expect an accuracy of readings of more than 5%, 10% is often adequate bearing in mind that this sort of variation could easily occur with varying soil moisture conditions or non-homogeneous soils.

Chart for use with slope method is in Annexure II.

ANNEXURE –IIANNEXURE –IIChart for use with slope methodChart for use with slope method

µ 0 1 2 3 4 5 6 7 8 90.40

0.41

0.42

0.43

0.44

0.45

0.46

0.47

0.48

0.49

0.6432

0.6418

0.6403

0.6389

0.6374

0.6360

0.6346

0.6331

0.6317

0.6302

6431

6416

6402

6387

6373

6359

6344

6330

6315

6301

6429

6415

6400

6386

6372

6357

6343

6328

6314

6300

6428

6413

6399

6384

6370

6358

6342

6327

6312

6298

6426

6412

6397

6383

6369

6354

6340

6325

6311

6297

6425

6410

6396

6382

6367

6353

6338

6324

6310

6295

6423

6409

6395

6380

6366

6351

6337

6323

6308

6294

6422

6408

6393

6379

6364

6350

6336

6321

6307

6292

6420

6406

6392

6377

6363

6348

6334

6320

6305

6291

6419

6405

6390

6376

6361

6347

6333

6318

6304

6289

0.50

0.51

0.52

0.53

0.54

0.55

0.56

0.57

0.58

0.59

0.6288

0.6273

0.6258

0.6242

0.6227

0.6212

0.6197

0.6182

0.6166

0.6151

6286

6271

6256

6241

6226

6210

6195

6180

6165

6150

6258

6210

6255

6239

6224

6209

6194

6179

6163

6148

6283

6268

6253

6238

6223

6207

6192

6177

6162

6147

6282

6267

6252

6236

6221

6208

6191

6176

6160

6145

6280

6265

6252

6235

6220

6204

6189

6174

6159

6144

6279

6264

6248

6233

6218

6203

6188

6172

6157

6142

6277

6262

6247

6232

6217

6201

6186

6171

6156

6141

6276

6261

6245

6230

6215

6200

6185

6169

6154

6139

6274

6259

6244

6229

6214

6198

6183

6168

6153

6138

0.60

0.61

0.62

0.63

0.64

0.65

0.66

0.67

0.68

0.69

0.70

0.71

0.72

0.6136

0.6120

0.6104

0.6087

0.6071

0.6055

0.6039

0.6023

0.6006

0.5990

0.5974

0.5957

0.5940

6134

6118

6102

6086

6070

6053

6037

6021

6005

5989

5973

5955

5938

6133

6117

6100

6084

6068

6052

6036

6019

6003

5987

5971

5953

5936

6131

6115

6099

6083

6066

6050

6034

6018

6002

5985

5969

5952

5935

6130

6113

6097

6081

6065

6049

6032

6016

6000

5984

5967

5950

5933

6128

6112

6096

6079

6063

6047

6031

6015

5998

5982

5964

5948

5931

6126

6110

6094

6076

6061

6045

6029

6013

5997

5980

5964

5947

5930

6125

6108

6092

6076

6060

6044

6027

6011

5995

5979

5962

5945

5928

6123

6107

6091

6074

6058

6042

6026

6010

5993

5977

5960

5943

5926

6121

6105

6089

6073

6057

6040

6024

6008

5992

5976

5959

5942

5924

µ 0 1 2 3 4 5 6 7 8 90.73

0.74

0.75

0.76

0.77

0.78

0.79

0.5923

0.5906

0.5889

0.5871

0.5854

0.5837

0.5820

5921

5904

5887

5870

5853

5835

5818

5920

5902

5885

5868

5851

5834

5817

5918

5900

5883

5866

5849

5832

5815

5916

5899

5882

5865

5847

5830

5813

5914

5897

5880

5863

5846

5829

5812

5912

5895

5878

5861

5844

5827

5810

5911

5894

5877

5859

5842

5825

5808

5909

5892

5875

5858

5841

5824

5806

5907

5890

5873

5858

5839

5822

5805

0.80

0.81

0.82

0.83

0.84

0.85

0.86

0.87

0.88

0.89

0.5803

0.5785

0.5766

0.5748

0.5729

0.5711

0.5692

0.5674

0.5655

0.5637

5801

5783

5764

5746

5727

5709

5690

5672

5653

5635

5799

5781

5762

5744

5725

5707

5688

5670

5851

5633

5797

5779

5760

5742

5723

5705

5686

5668

5650

5631

5796

5777

5759

5740

5722

5703

5685

5666

5648

5629

5794

5775

5757

5738

5720

5701

5683

5664

5646

5627

5792

5773

5755

5736

5718

5699

5681

5662

5644

5625

5790

5772

5753

5735

5716

5698

5679

5661

5642

5624

5788

5770

5751

5733

5714

5696

5677

5659

5640

5622

5786

5768

5749

5731

5712

5694

5675

5657

5638

5820

0.90

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

0.5618

0.5598

0.5578

0.5557

0.5537

0.5517

0.5497

0.5477

0.5456

0.5436

5616

5596

5578

5555

5535

5515

5495

5475

5454

5434

5614

5594

5574

5553

5533

5513

5493

5473

5452

5432

5612

5592

5572

5551

5531

5511

5491

5471

5450

5430

5610

5590

5570

5549

5529

5509

5489

5469

5448

5428

5608

5588

5588

5547

5527

5507

5487

5467

5446

5426

5606

5586

5565

5545

5525

5505

5485

5464

5444

5424

5604

5584

5563

5543

5523

5503

5483

5462

5442

5422

5602

5582

5561

5541

5521

5501

5481

5460

5440

5420

5600

5580

5559

5539

5519

5499

5479

5458

5438

5418

µ 0 1 2 3 4 5 6 7 8 91.30

1.31

1.32

1.33

1.34

1.35

1.36

1.37

1.38

1.39

0.4652

0.4618

0.4583

0.4549

0.4515

0.4481

0.4446

0.4412

0.4378

0.4343

4649

4614

4580

4546

4511

4477

4443

4408

4374

4340

4645

4611

4577

4542

4508

4474

4439

4405

4371

4336

4642

4607

4573

4539

4505

4470

4436

4402

4367

4333

4638

4604

4570

4535

4501

4467

4432

4398

4364

4330

4635

4601

4566

4532

4498

4463

4429

4395

4360

4326

4631

4597

4563

4529

4494

4460

4426

4391

4357

4323

4628

4594

4559

4525

4491

4457

4422

4388

4354

4319

4625

4590

4556

4522

4487

4453

4419

4384

4350

4316

4621

4586

4553

4518

4484

4450

4415

4381

4347

4312

1.40

1.41

1.42

1.43

1.44

1.45

1.46

1.47

1.48

1.49

0.4309

0.4267

0.4225

0.4183

0.4141

0.4099

0.4056

0.4014

0.3972

0.3930

4305

4263

4221

4178

4136

4094

4052

4010

3968

3926

4301

4258

4216

4174

4132

4090

4048

4005

3964

3921

4296

4254

4212

4170

4128

4086

4044

4001

3959

3917

4292

4250

4208

4166

4124

4082

4040

3997

3955

3913

4288

4246

4204

4162

4120

4077

4035

3993

3951

3909

4284

4242

4200

4157

4115

4073

4031

3989

3947

3905

4280

4237

4195

4153

4111

4069

4027

3985

3943

3900

4275

4233

4191

4149

4107

4065

4023

3980

3938

3896

4271

4229

4187

4145

4103

4061

4018

3976

3934

3892

1.50

1.51

1.52

1.53

1.54

1.55

1.56

1.57

1.58

1.59

0.3888

0.3840

0.3791

0.3740

0.3688

0.3635

0.3580

0.3523

0.3465

0.3405

3883

3835

3786

3735

3683

3630

3574

3517

3459

3399

3878

3830

3781

3730

3677

3624

3569

3511

3453

3393

3874

3825

3778

3724

3672

3619

3563

3506

3447

3386

3869

3820

3771

3719

3667

3613

3557

3500

3441

3380

3864

3818

3766

3714

3662

3608

3552

3494

3435

3374

3859

3811

3760

3709

3656

3602

3546

3488

3429

3368

3854

3808

3755

3704

3651

3597

3540

3482

3423

3362

3850

3801

3750

3698

3646

3591

3534

3477

3417

3355

3845

3796

3745

3693

3640

3586

3528

3471

3411

3349

µ 0 1 2 3 4 5 6 7 8 91.00

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

1.09

0.5416

0.5394

0.5371

0.5349

0.5327

0.5305

0.5282

0.5260

0.5238

0.5215

5414

5391

5369

5347

5325

5303

5280

5258

5235

5213

5412

5389

5367

5345

5322

5300

5278

5255

5233

5211

5409

5387

5365

5344

5320

5298

5276

5253

5231

5209

5407

5385

5362

5340

5318

5296

5273

5251

5229

5206

5405

5383

5360

5338

5316

5293

5271

5249

5229

5204

5403

5380

5358

5336

5313

5291

5269

5247

5224

5202

5400

5378

5356

5333

5311

5289

5267

5244

5222

5200

5398

5376

5354

5331

5309

5287

5264

5242

5219

5297

5396

5374

5351

5329

5307

5284

5262

5240

5217

5295

1.10

1.11

1.12

1.13

1.14

1.15

1.16

1.17

1.18

1.19

0.5193

0.5168

0.5143

0.5118

0.5093

0.5068

0.5042

0.5017

0.4992

0.4967

5190

5165

5140

5115

5090

5065

5040

5015

4990

4965

5188

5163

5137

5113

5088

5062

5037

5012

4987

4962

5185

5160

5135

5110

5085

5060

5035

5010

4985

4960

5183

5158

5132

5108

5083

5057

5032

5007

4982

4957

5180

5155

5130

5105

5080

5055

5030

5005

4980

4955

5178

5153

5127

5103

5078

5052

5027

5002

4977

4952

5175

5150

5125

5100

5075

5050

5025

5000

4975

4950

5173

5148

5122

5098

5073

5047

5022

4997

4972

4947

5170

5145

5120

5095

5070

5045

5020

4995

4970

4945

1.20

1.21

1.22

1.23

1.24

1.25

1.26

1.27

1.28

1.29

0.4942

0.4913

0.4884

0.4855

0.4826

0.4797

0.4768

0.4739

0.4710

0.4681

4939

4910

4881

4852

4823

4794

4765

4736

4707

4678

4936

4907

4878

4849

4820

4791

4762

4733

4704

4675

4933

4904

4875

4846

4817

4788

4759

4730

4701

4672

4930

4901

4872

4843

4814

4785

4756

4727

4698

4669

4928

4899

4870

4841

4812

4783

4754

4725

4696

4667

4925

4896

4867

4838

4809

4780

4751

4722

4693

4664

4922

4893

4864

4835

4808

4777

4748

4719

4690

4661

4919

4890

4861

4832

4803

4774

4745

4718

4687

4658

4916

4887

4858

4829

4800

4771

4742

4713

4684

4655

Earth resistancecurve

Resistance

ArbitraryPosition of E electrode 0.2 EC 0.4EC 0.6EC Position of

C electrode

Position of P electrode measured from E.

Field MeasurementField MeasurementArea of the 11kV S/S is 6.25Mx17.5M and the diagonal distance is 18.58M.• Measurement No. 1• Current Electrode at a distance of 49M from the Earth electrode (CE) • Megger reading: 0.2CE(09.8M) = 0.402Ω, R1

• 0.4CE(19.6M) = 0.534Ω, R2

• 0.6CE(29.4M) = 0.884Ω, R3

• µ = R3 -R2 = 0.884-0.534 =2.65

• R2 -R1 0.534-0.402

• From the chart (Annexure 1) µ obtained (2.65) is not covered (max 1.69)in the table, the current electrode will have to be moved further away from the earthing system.

• Measurement No.2• Current Electrode at a distance of 80M from the Earth electrode (CE)

• Megger reading: 0.2CE(16.0M)=0.273Ω, R1

• 0.4CE(32.0M)= 0.354Ω, R2

• 0.6CE(48.0M)=0.541Ω, R3

• µ = R3 -R2 = 0.541-0.354 =2.31

• R2 -R1 0.354-0.273

• From the chart (Annexure 1) µ obtained (2.31) is not covered (max 1.69)in the table, the current electrode will have to be moved further away from the earthing system.

IS : 3043 ALTERNATIVE METHOD:IS : 3043 ALTERNATIVE METHOD:

CASE STUDY:CASE STUDY:• Two suitable direction at 90 degree apart at one corner of the

fence are first selected. The potential electrode and current electrode are placed in these direction 250 to 300 metres away from the fence at the same distance. A reading is taken under these conditions. The current electrode is then moved in 30m. Steps until the same readings are obtained for three consecutive locations. This procedure is termed as locating the remote current electrode distance.

SL Spacing in Metres Megger Reading

NO P2 C2 Ω

1 270 270 0.026

2 270 300 0.039

3 270 330 0.039

4 270 360 0.039

• The current electrode is then left in the last foregoing position and the potential electrode is moved out in 30m. Step until three consecutive reading are obtained without a change in value. The last reading then corresponds to the true value of earth resistance.

SL Spacing in Metres Megger Reading

NO P2 C2 Ω 1 270 360 0.039 2 300 360 0.040 3 330 360 0.040 4 360 360 0.040

Resistance of the Grounding System = 0.040Ω i.e. 40 milliohms.

GROUND RESISTANCE ROD GROUND RESISTANCE ROD AND PIPE ELECTRODE:AND PIPE ELECTRODE:

• Depth of Buriel as per IS:3043 1966 & 1987

is minimum 2.75 meters • Rg = (100ρ / 2πL) loge(4L / d) OHMS

WHERE,

Rg = GROUND RESISTANCE

ρ = SOIL RESISTIVITY IN OHM-M

L = LENGTH IN CM

d = DIAMETER OF ROD OR PIPE.

Ex:- ρ = 100Ω-m

d = 4 cm

L = 250 cm

Then, Rg = 35.15 ohms

When d is 2.5 times

Rg = 29.31 ohms.

GROUND RESISTANCE OF GROUND RESISTANCE OF PLATE ELECTRODE:PLATE ELECTRODE:

• Rg = (ρ/A) x (√ π/A)

• ρ= Soil resistivity in ohm-meter.• A= Area of both sides of Plate in metre Sq.• Size of plate shall be min.600mm x 600mm, max.

1200mmx1200mm depth min.1.5 m as per IS:3043 1966 & 1987

• The minimum thickness shall be:

a) Cast iron 12mm

b) Galvanized iron 6.3mm

c) Copper 3.15mm

GROUND RESISTANCE OF GRID GROUND RESISTANCE OF GRID (EARTH MAT) :(EARTH MAT) :

• Depth of burial 0.5 to 1 meter • LAURIENT AND NIEMANN • Rg = ρ/4r + ρ/L = ρ/4 (√ π/A)+ ρ/L WHERE, Rg = GROUND RESISTANCE ρ = Average ground Resistivity in OHM-M r = Radius of a circle having the same area as that occupied by the Grid L = Total buried length of buried conductors (Both horizontal and Vertical) in meters A = Area of Grid (Earth Mat)

• The fault current carrying capacity of the conductor is designed for one minute since the primary breakers are fast acting and trips in

0.25 secs and the back up protection trips in 0.5 secs.

GROUND RESISTANCE VALUEGROUND RESISTANCE VALUE

• Large stations : 0.5 Ohm• Major stations : 1.0 Ohm• Other stations : 2.0 Ohm

CORROSION• The average loss in weight of specimen

150mmx125mmx3mm buried for 12 years in no case exceeds :

• Copper : 0.2 % per year• GI : 0.5 % per year• MS : 2.2 % per year

REQUIREMENT OF AN REQUIREMENT OF AN EMBEDDING MATERIALEMBEDDING MATERIAL

• It should have high electrical conductivity which should be constant, unaffected by changes in temperature & moisture;

• It should permanently remain once embedded and should not be either dissolved in or swept away by water;

• It should have high swelling property to absorb water and retain the same over long periods of time;

• It should not cause or accelerate the corrosion of the ground electrode material, such as steel;

• It should be easily applicable;• It should not cost much in relation to the total cost of

grounding installation.

BENTONITE BENTONITE • One of the most suitable substances for

chemical treatment of soils which fulfills most of the above requirements is a clay known as bentonite.

Bentonite

Calcium based Sodium based

BENTONITE Contd..BENTONITE Contd..

• Bentonite contains Na o (Soda), K o (Potash), Cao (Lime), Mgo (Magnesia) & other mineral salt that ionize forming a strong electrolyte with :

a) pH : 8-10

b) ρ : 2.5 Ohm-m at 300 % moisture

c) Swell index by volume : ≥ 8

d) Quantity required for Pipe electrode as per

IS:3043-1987 (2.75 m long 100mm Id 13

mm thick) : 45 Kg

2 2

HAZARDS DEPEND ON:HAZARDS DEPEND ON: • Frequency

• Magnitude

• Duration

Frequency:

• Humans vulnerable to the effects of ac at 50-60 Hz

• Slightly larger currents at low frequencies and DC

• Higher currents at high freq: (3000-10,000 Hz)

• Hundreds of amperes for lightning surgesContd.,

Magnitude:

Most common physiological effects in the order of increasing magnitude:

• Perception (1mA)

• Let-go currents (1-6 mA)

• Muscular contraction (9-25 mA)

• Ventricular fibrillation (100 mA)

• Burning

SAFE BODY CURRENTS:SAFE BODY CURRENTS: • Dalziel’s & LEE and recommended as per AIEE80/1963

I = 0.165/√ t Amps

IEEE -80/1976

I = 0.116/√ t Amps

IEEE -80/1986

I = 0.116/√ t Amps

I = 0.157/√ t Amps

CBI&P (India):

I = 0.155/√ t Amps

k

Where,t=Duration of the shock 8 ms to 3 secs

k50

k70

k

k

POTENTIAL GRADIENTSPOTENTIAL GRADIENTS

STEP VOLTAGE• The potential difference between two points on the

earths surface, separated by a distance of one pace, that will be assumed to be one meter, in the direction of maximum potential gradient.

TOUCH VOLTAGE• The potential difference between a grounded metallic

structure and a point on the earths surface separated by a distance equal to the normal maximum horizontal reach, approximately one meter.

G.P.R FOR EHV/UHV STATIONS IN INDIAG.P.R FOR EHV/UHV STATIONS IN INDIA

Limits of G.P.R is not prescribed in India. Following recommendations made by CIGRE:

The G.P.R should be limited to A value such that-• Touch voltages at places outside the fence and frequently

visited by public should not be more than 650 volts• The transferred voltage between sheaths and the conductor of

cables should be limited to 60% of 1 minute power frequency test voltage of the cable.

• Transferred voltage between the overhead ground wire and a conductor of an overhead line should be 60% of 1 minute power frequency test voltage of line arrestors.

Contd.,

TRANSFER POTENTIAL

SYSTEM VOLTAGE FROM 110 TO 400 kV

High Reliability Station

Tripping time 0.2 Sec and Never exceeds 0.5 Sec 650 V

Other Stations

430 V

A survey conducted by IEEE indicates that the maximum G.P.R permitted by various Electrical utilities varies from 1 to 20 kV

MAINTENANCEMAINTENANCE• Maintenance of earth-electrodes at power stations and

substations

• Records should be kept of the initial resistance of power station and substation earth-electrode systems and of subsequent tests carried out. Such tests should be made periodically to ensure that no serious increase in resistance occurs. Normally an annual test should suffice, but local circumstances may justify an increase or reduction in the intervals between tests in the light of experience. Where possible earth-electrodes should be arranged in groups so that they can be disconnected in turn from the general earthing system for test while leaving sufficient electrodes in service to provide an effective connection with earth.

MAINTENANCE Contd..MAINTENANCE Contd..• In urban substations it is frequently impossible to make any effective

check on the resistance to general mass of earth of the electrode system because of the connection to the metallic sheaths of numerous high voltage and low voltage cables. In such cases, the earthing connections within the substations should permit periodical and visual inspection.

• In rural substations, particularly those connected to overhead high voltage and low voltage lines, greater reliance needs to be placed on the electrode system, and it is important that there should be facilities for testing the resistance of the electrode to general mass of earth. Normally annual measurements shall be carried out but local circumstances in the light of experience may justify increase or decrease in this interval but it should not be less than once in two years.

PRECAUTIONS TO BE TAKEN DURING PRECAUTIONS TO BE TAKEN DURING RESISTANCE MEASUREMENTSRESISTANCE MEASUREMENTS

• There is a possibility of lethal potential existing between a station ground and a remote ground if a system fault involving the station ground occurs while ground resistance is being measured. The use of rubber gloves is advisable while making connections to the test electrodes. Under no circumstances should the two hands or other parts of the body of the testing personal should be allowed to complete the circuit between the points of possible high potential difference.

• An isolated lightning arrester ground should never be tested with the arrestor in service because of the possible high potential gradient around the ground connection.

• Since the resistivity of the upper soil layers is greatly influenced by the weather, a day test should be chosen which is free from extreme weather conditions.

SUPPLIERS OF EARTH TESTERSSUPPLIERS OF EARTH TESTERS M/s Ashida Electronics

Ram Rajya, Kopri Thane – 400 603, Maharastra

M/s Agarwal Electronics201, Shivashakthi Indus. Estate,Near Shreyas Talkies, LBS MargGhatkopar (w), Bombay – 400 086

M/s Cambride Instruments and Engg.company23/2, Maulana Abdul kalam azad Road,Howrah – 711 001, West Bengal

M/s Conin Prakriti InstrumentationREGD.Off & Facty.16, Ragendra Nagar Industrial Estate,P.O.Mohan nagar, Ghaziabad 201 007

M/s Continental Electronics Naroda177/4, Industrial Estate,Naroda -382 380, Dist.AhmedabadGujarat

M/s Erricson EngineersUnigue House, 11th Floor,25, Parsee Bazaar streetBombay-400 023 Contd.,

M/s Indotech Devices Pvt Ltd.,

H.O & works, A-75 Sector V

P.B.19, Noida 201 301

Dist.Ghaziabad, U.P

Instrument Techniques Pvt Ltd.,

Regd. Off & works B-2

Co-operative Industrial Estate,

Balanagar, Hyderabad – 500 037

Nippen Electric Instruments

Company Sett & of

12-A, Marol Maroshi Road,

Andheri (E), Bombay – 400 059

M/s Sharmasons sakova Instruments Pvt Ltd.,

P.B.No.6251, 2, OLF Industrial Area,

Najafgarh Road,

New Delhi 110 015

M/s W.J. Alcock & Co. Pvt Ltd.,

7, K.S.Roy Road,

Calcutta – 700 001

M/s Yokins Instruments

Yokin Trade Lines Admn.Office

B-3/40 Ashok Vihar Phase II, Delhi 110 052

BENTONITE SUPPLIERSBENTONITE SUPPLIERSM/s Amar minerals Pvt Ltd., Netivali Baug, KalyanDist.Thane, Maharastra

M/s Amarjyot IndustriesNo.45, Dr.Meisheri Road,Near Sandhurst road,Railway Stn.BombayWorks:A-108, MIDC, Dombivili, Dist.Thana

M/s Industrial Minerals & Chemicals Co.Pvt Ltd.,No.125, Narayan Dhauru street,Nagaevi, Bombay – 400 003Works: Kurla Road, chakala Andheri Bombay-58Works: Chinchpokli cross lane, Bomaby 400 027

M/s International Minerals & Chemicals Co.No.54-D, Govt.Industrial Estate,Bombay-400 067

M/s Ambika Minechem IndustriesNo.129/131, Kazi Sayed street4th Floor, Bombay – 400 003

Contd.,

M/s Mysore Agencies

23/1, 3 rd Cross, Lalbaugh Road,

Bangalore – 560 027

M/s Baroda Mineral Grinding Industries

National highway, Naroda Ahmedabad

M/s J.D.Jones & Co.Pvt Ltd.,

C/5, Gillander House,

No.8, Netaji Subhash Road,

Calcutta 700001

M/s Metamine Industries

Rathnagarmata Road,

Kapadvanj, Dist.Kaira

Gujarat

M/s Somanath Minerals & Chemicals

No.72, Chitra Industrial Estate,

Bhavnagar 364 004 (Gujarath)Contd.,

M/s Sunders India Corporation

Near Indian Airlines Corporation

Diwanpara Road, Bhavnagar – 364 001

M/s Manjunatha Pulverisers

Sabuvani Buildings

N.R.Road,

Bangalore – 560 002

M/s Mysore City & Mineral Industries

No.1032, IV Block, Rajaji nagar,

Bangalore 560 010

M/s Mine Chemical Industries,

Peenya Industrial Estate

Phase III, Bangalore

M/s Binto Plast Pvt Ltd.,

Industrial Estate,

Jodhpur – 342 001

Contd.,

M/s United Engineering Corporation

APSRTC Complex,

Vishakapatnam

M/s Ashapura Group of Industries

“ VIBHA” 55/3 13th H Main

HAL 2nd Stage Indira nagar,

Bangalore – 560 008

H.O.81/82, Mittal Court C

Nariman point,

Bombay – 400 021

M/s OSWAL MINERALS

No.6, 2nd Main Road,

Rama chandrapuram

Bangalore – 560 021

Remote electrode at: 60M 80M• 02.5 0.164 05 0.190 • 05.0 0.174 10 0.204 • 07.5 0.183 15 0.222• 10.0 0.190 16 0.218 R1 • 12.0 0.205 R1 20 0.227 • 12.5 0.206 25 0.239• 15.0 0.210 30 0.242• 17.5 0.221 32 0.247 R2 • 20.0 0.225 35 0.258• 22.5 0.236 40 0.274• 24.0 0.240 R2 45 0.290• 25.0 0.246 48 0.306 R3• 27.5 0.261 50 0.307• 30.0 0.274 55 0.320• 32.5 0.296 60 0.330• 35.0 0.316 65 0.345• 36.0 0.329 R3 70 0.434• 37.0 0.343 75 0.859• 37.5 0.346• 40.0 0.376 µ = R3-R2/R2-R1• 42.5 0.406 = 0.329-0.240/0.240-0.205 0.306-0.247/0.247-0.218• 45.0 0.438 = 2.5 = 2.03• 47.5 0.493• 50.0 0.563

THANK YOU