Importance of Grounding in Power System. Presented by Mr. H Jayakumar Ex- Joint Director CPRI

Transcription

1 Importance of Grounding in Power System Presented by Mr. H Jayakumar Ex- Joint Director CPRI

2 OBJECT OF EARTHING Prime Object of Earthing is to Provide a Zero Potential Surface in and around and under the area where the electrical equipment is installed. Earthing is essential at every stage of electricity generation, transmission and utilization

3 IMPORTANCE OF EARTHING Personal Safety Protection of Equipment : Prevent or at least minimize damage to equipment as a result of heavy fault current and lightning thus improve the reliability of equipment Protection of System : Improve the reliability of power supply.

4 STANDARDS FOLLOWED IS: 3043 : 1966, 1987 reaffirmed 2006 Code of Practice for Earthing. Indian Electricity rules 1956 ( as ammended up to 2000) IS: ( reaffirmed 2005 )Protection of Buildings and allied Structures against lightning Code of Practice. Manual on Earthing of AC Power Systems : CBIP Publication No.302 : 2007 BS: 7430 : 1991, Code of Practice for Earthing.( formerly CP 1013: 1966) British Standard Institution London 1992 IEEE :80 : 2000( Revision of IEEE Std 80: 1986) Guide for Safety in AC Substation Grounding IEEE :142 :2007(Revision of IEEE Std 142 :1991) Grounding of Industrial and Commercial Power System. IEEE 1100 : 2005 (Revision of IEEE Std 1100 : 1999) Powering and Grounding Electronic Equipment

5 1. Plate Earthing TYPE OF EARTHING R g = ρ A π A ohms 2. Pipe / Rod Earthing R g = 100ρ log 2πl e 4l d ohms Doubling the Diameter Decrease the Resistance by 10 to 12% Doubling the Length Decrease the Resistance by 40%

6 3. Strip Earthing TYPE OF EARTHING R g = 100ρ log 2πl e 2 2l wt ohms 4. Combination of 1 & 3 and 2 & 3 5. Mat Earthing R g ρ = + 4 r ρ L ohms

7 TYPE OF EARTHING R = Station ground resistance in ohms r = is the radius of a circle having the same area or that occupied by grid in meters L = Total buried length of conductors in meters ρ = Resistivity of the soil (assumed uniform) in ohm-m A = Area of both sides of the plates in m² L = Length of the rod or pipe (in cm) d = diameter of rod or pipe (in cm) l = length of the strip (in cm) w = depth of burial of the electrode in cm t = width (in the case of strip) or twice the diameter (for round conductor) in cm In the above formula no where type of material used for grounding is mentioned. So, as far as grounding is concerned material is immaterial since the ground resistance is the resistance offered to the nearest ground hence, ground (Soil Resistivity) plays a very important role.

8 Type of Earth Tester Which type of earth tester should be used for measuring the Soil Resistivity and Ground Resistance Digital meter or Analog meter? Digital meter of reputed make. Ground resistance is directly proportional to the soil resistivity, hence measurement of soil resistivity by accurate meter of reputed make is very important. In grounding the value of soil resistivity is most important for estimating the ground resistance and surface potentials (mesh, step and touch). Make Model Megger DET 2/2 UK Fluke 1623 & 1625 USA / UK Chauvin Arnoux 6460, 6462 & 6470 FRANCE / UK Kew (Kyoritsu) 4106 JAPAN

9 Comparison of Analog & Digital Earth Tester Why there is always a difference in the measured resistance values if measured with different earth testers even if all are calibrated ones? This is not true in case of reputed make digital earth testers. This may be true when compared with analog to digital earth testers of reputed make. PARAMETERS ANALOG METER DIGITAL METER Accuracy 5% of full scale and shall be effective 2% of reading in entire range hence above 25% of full scale. At 25% of very accurate and high resolution reading absolute error is 20%. Not accurate and low resolution Voltage 250V hand cranking Micro processor based 30 to 50 V automatic reading Frequency Fixed (60 to 90 HZ) Variable. In automode it selects test frequency with least amount of noise High Spike resistance Do not indicate Displays the high spike resistance. Open circuit Do not indicate Displays current circuit open/potential circuit open

10 Measurement of Soil Resistivity How to check the soil resistivity? Minimum how may readings should be taken and at what points for a correct measurement of soil resistivity value? a) Equally spaced or Dr. F.Wenner Arrangement Ρ = 2 Π AR Ω-M

11 Measurement of Soil Resistivity b) Unequally spaced or Schelumberger Palmer Arrangement Ρ = Π C ( C+d) * R / d Ω-M

12 Measurement of Soil Resistivity A set of readings taken with various probe spacings (2m, 5 m, 10 m, 25 m, 50m) gives a set of resistivity which, when plotted against spacing, indicates whether there are distinct layers of different soil or rock and gives an idea of their respective resistivities and depth. This has to be repeated in all directions of the site i.e., horizontal, vertical and diagonal. The mean value is calculated. In case the values lies within +/- 30% of mean value (70% to 130%) the soil is considered to be uniform (Homogenous soil).

13 CASE STUDY Ρ = 2 Π AR Ω-M A= Distance between adjacent electrode (M), R = Earth tester reading SL.NO a R ρ Meters Ω Ohm-Meters min max ρ Average % of ρ Ave = 1.3 x = % of ρ Ave = 0.7 x = Minimum & Maximum value lies within and The Soil is Homogeneous.

14 Measurement of Ground Resistance What is the principle for measuring the earth resistance value? The fall of potential method: Consisting of injecting a known alternating current through the electrode under test and an auxiliary current probe and plotting the ratio of V/I = R as a function of probe spacing x. The potential electrode is moved away from the ground under test in steps of 10%. A value of resistance is obtained at each step. The resistance is plotted as a function of distance in ohms at which this plotted curve appears to level out is taken as the resistance value of the ground under test. This level out will be potential electrode at 61.8% of distance between ground electrode under test and auxiliary electrode. This was established by E.B.Curdt s for small hemispherical electrode

15 Measurement of Ground Resistance

16 Measurement of Ground Resistance

17 Measurement of Ground Resistance Distance between station earth (P1C1) and potential electrode (P2) mtrs Earthing tester reading Ω Earth Pit No 1 Earth Pit No:2 Earth Pit No (20% R1) (40%R2) (60%R3) (61.8%of30) Remote Electrode(C2) from station ground (C1P1) 30m 30m 30m Result: Fall of Potential Method Ave. Of = Ave. Of =6.242 Average of =

18 Measurement of Ground Resistance E.B Curd s Method (R3-R2) (R3-R2) (R3-R2) Slope Method µ= (R2-R1) (R2-R1) (R2-R1) From the chart Value = Remote electrode (RE)30m = 60.13% of RE(30) 57.85% of RE(30) m m Value = 8.69Ω 6.40Ω - Analysis of Result Fall of Potential 8.545Ω 6.242Ω 17.46Ω E.B Curd ts 8.780Ω 6.470Ω 17.40Ω Slope Method 8.690Ω 6.400Ω - Remote electrode (C2) at 30 mtrs from earth pit (C1P1). Earth Pits are conventional type back filled 2Ft all around the electrodes. Earth Pit No1: Located at A & B colony 50mm dia 3mtrs Long GI Pipe. Earth Pit No2: Located in 33KV Substation 100mm dia 3mts Long GI Pipe. Earth Pit No3: Near DG Set New 60mm dia 2mtrs Long GI pipe.

19 Calculation of Earth Resistance of Multiple Electrodes How to calculate the resultant earth resistance value of the earth electrodes if more than one pits are required to be installed? Multiple electrodes in parallel yield lower resistance to ground than a single electrode. Multiple rods are commonly used to provide the low grounding resistance required by high capacity installations. Adding a second rod does not however provide a total resistance of half that of a single rod, unless the two are several rod length apart. A useful rule is that grounding systems of 2-24 rods placed one rod length apart in a line, hollow triangle, circle or square will provide a grounding resistance divided by number of rods and multiplied by the factor F. Multiplying Factors for Multiple Rods: No.of Rods F

20 Current carrying Capacity of Earth Electrode How to calculate current carrying capacity of an earth electrode? Current caring capacity of an earth electrode depends on 1. The total surface area of the electrode in contact with earth 2. Resistivity of the soil and 3. Duration of fault in seconds. The formula for current caring capacity (Current density) is: Current density = 7.57 X 10³ ρt Amp / Sq-m ρ = Resistivity of the soil (assumed uniform) in ohm-m. t = Duration of fault in seconds.

21 Geometry & No. of Electrodes Required 7. How does earth electrode geometry affects the earthing system? The total surface area coming in contact with earth is the criteria. 8. How to calculate the number of earth electrodes required for any particular application? This depends on the fault level the calculation is as follows: Calculation of No.of Plates required: For Example: The total surface area = 0.6 x 0.6 x 2 sq.m = 0.72 sq.m Fault current = 6 kilo amperes Duration of Fault = 1 sec. Soil Resisitivity = 100 ohm - m Current density = 7.57 X 10³ Amp / Sq-m ρt = 7.57 X 10³ = 757 Amp /Sq-m 100X1 One Plate will carry 757 X 0.72 = Amperes

22 Geometry & No. of Electrodes Required To carry 6 kilo amperes No.of plates required = 6000 / = 11 Nos. Calculation of No.of Pipes required: For Example: The total surface area of a 3 mtr long 80 mm dia = π x 0.08 x 3 sq.m = sq.m Fault current = 6 kilo amperes Duration of Fault = 1 sec. Soil Resisitivity = 100 ohm - m Current density = 7.57 X 10³ ρt Amp / Sq-m = 7.57 X 10³ = 757 Amp /Sq-m 100X1 One Pipe will carry 757 X = Amperes To carry 6 kilo amperes No.of plates required = 6000 / = 10.5 = 11 nos.

23 Minimum Accepted Earth Resistance What is the minimum accepted earth resistance value for different applications? Are these resistance values mentioned in any standard? References: US AID INDIA Book The earth resistance shall be as low as possible and shall not exceed the following limits Power stations (generating station) 0.5 ohms EHT Sub-station 1.0 ohms 33 KV Stations 2.0 ohms D/t Structure 5.0 ohms Tower Foot resistance 10.0 ohms Page 92 Modernisation of power distributions Same as above. IEEE standard chapter 4 page 164 Resistance in the 1 ohm to 5 ohms range are generally found suitable for industrial plant sub-station and buildings and large commercial installations. Lightning arrestors ground resistance for protection of buildings and allied structures Less than 10 ohms Clause Page 32 IS 2309 : 1989.

24 Precaution to Be Taken For Measuring Ground Resistance 10.Precaution to be taken while measuring ground resistance. Precautions to be taken during resistance measurement. a. Avoid taking measurement during cloudy day. b. There is a possibility of lethal potential existing between a station ground & 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 electrode. Under no circumstances should the two hands or other part of the body of the testing personal should be allowed to complete the circuit between the points of possible high potential difference. c. An isolated lightening arrester ground should never be tested with the arrester in service, because of the possible high potential gradients around the ground connection. d. Since the resistivity of the upper soil layers is greatly influenced by weather, a day test should be chosen which is free from extreme weather conditions. 11.How many times is year we have to check the Earthing point. IS: 3043: Clause (page76): Normally annual measurement of earth resistance of substations 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. This shall be compared with the internal record. Allthough resistance to ground will change seasonally and over time any increase of the resistance > 20% or more should be investigated and corrective action taken to lower the resistance.