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POWER SYSTEM GROUNDING

POWER SYSTEM GROUNDING. Presented By H.JAYAKUMAR. WHY GROUNDING IS REQUIRED?.

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POWER SYSTEM GROUNDING

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  1. POWER SYSTEM GROUNDING Presented By H.JAYAKUMAR

  2. WHY GROUNDING IS REQUIRED? GROUNDINGPLAYS A VITAL ROLE IN POWER SYSTEM OPERATION. EFFECTIVE GROUNDING IS NECESSARY FOR THE PROPER OPERATION AND CO-ORDINATION OF PROTECTION SCHEMES IN THE POWER SYSTEM AS WELL AS FOR PERSONNEL SAFETY.

  3. PURPOSE OF EARTHING: • PROTECTION OF INSTALLATION. • IMPROVEMENT IN QUALITY OF SERVICE. • SAFTY OF PERSONNEL.

  4. GENERAL REQUIREMENTS: • LOW VALUE OF EARTH RESISTANCE. • ACCEPTABLE SURFACE POTENTIAL GRADIENTS.

  5. 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.

  6. TYPES OF EARTHING: • SYSTEM EARTHING. • PROTECTIVE EARTHING.

  7. SYSTEM EARTHING EFFECTIVELY EARTHED 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.

  8. OBJECTIVE OF SOIL RESISTIVITY 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.

  9. FACTORS AFFECTING THE SOIL RESISTIVITY: • Type of the soil. • Moisture. • Dissolved salt in water. • Temperature. • Grain size and its distribution. • Seasonal variation. • Artificial treatment.

  10. EQUIPMENT USED FOR MEASUREMENT 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.

  11. EQUIPMENT USED FOR MEASUREMENT OF SOIL RESISTIVITY:

  12. METHOD OF MEASURING SOIL RESISTIVITY • 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

  13. WENNER FOUR ELECTRODE METHOD:

  14. ρ= 4πaR /[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

  15. The soil is said to be homogenous when the average value of ‘ρ’ 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.

  16. SURVEY OF SOIL RESISTIVITY MEASUREMENT:CASE STUDY: • SOIL RESISTIVITY MEASUREMENT: • Site of 120 MW Diesel Power Plant at Yelahanka, Bangalore. A1 D1 B1 D2 C2 C1 B2 A2

  17. Average of the above 37.99 ohm-metersMinimum Resistivity Encountered 27.33 ohm-metersMaximum Resistivity Encountered 49.02 ohm-metersConclusion: since the maximum and minimum resistivity lies within 30% of the average value the soil is a homogeneous one

  18. LIMITATIONS OF WENNERS 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.

  19. UNEQUALLED SPACED OR SCHLUMBERGER – PALMER ARRANGEMENT:

  20. ρ = [πc (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 ρ59.803 When c=d, The above formula will reduced to 2πaR Avg

  21. OBJECTIVE OF GROUND 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.

  22. METHODS OF EARTH RESISTANCE MEASUREMENT: • Fall-of-potential method. • E.B. Curdts 61.8% method. • Slope method. • IS: 3043 alternative method.

  23. 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.

  24. Case Study of Fall of Potential MethodREMOTE 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).

  25. E.B.CURDTS 61.8% METHOD This method is applicable to single ground electrode. Potential electrode P2 is placed at 61.8% of remote current electrode C2 from the station ground.

  26. 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.

  27. THE SLOPE METHOD OF TEST Was 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: • The terminals C1 & P1 on the instruments are connected to the earth electrode. • 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. • 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.

  28. 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-R1 Which 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 to this value of PT. The value obtained is earth system resistance. It is important to note that: • 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. • 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.

  29. 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.

  30. ANNEXURE –IIChart for use with slope method

  31. Earth resistance curve Resistance Arbitrary Position of E electrode 0.4EC Position of C electrode 0.2 EC 0.6EC Position of P electrode measured from E.

  32. Field Measurement Area 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 -R10.534-0.402 • 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.

  33. 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 -R10.354-0.273 • 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.

  34. IS : 3043 ALTERNATIVE METHOD:

  35. 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

  36. 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.

  37. GROUND RESISTANCE ROD 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.

  38. GROUND RESISTANCE OF 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

  39. GROUND RESISTANCE OF GRID (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.

  40. GROUND 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

  41. REQUIREMENT OF AN EMBEDDING 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.

  42. 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

  43. BENTONITE Contd.. 2 2 • 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

  44. 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 surges Contd.,

  45. 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

  46. SAFE BODY CURRENTS: k • 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 k50 k70 k Where, t=Duration of the shock 8 ms to 3 secs

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