The objective of a grounding system are: 1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential.

Transcription

1 GROUNDING SYSTEMS Part 1 Professor Ahdab Elmorshedy 1

2 Reference: High Voltage Engineering Theory and Practice, Text Book, Marcel Dekker Inc. NY, USA, Mazen Abdel-Salam, Hussein Anis, Ahdab Elmorshedy, Roshedy Radwan. Grounding Systems, Chapter (13 13). Overvoltages on Power Systems, Chapter (14). High-Voltage Cables Chapter (12 12). 2

3 The objective of a grounding system are: 1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential. 2. To assure correct operation of electrical/electronic devices. 3. To prevent damage to electrical/electronic apparatus. 4. To dissipate lightning strokes. 5. To stabilize voltage during transient conditions and to minimize the probability of flashover during transients. 6. To divert stray RF energy from sensitive audio, video, control, and computer equipment. 3

4 A safe grounding design has two objectives: 1. To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service. 2. To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock. 4

5 The PRIMARY goal of the grounding system throughout any facilities is SAFETY. Why ground at all? PERSONNEL SAFETY FIRST EQUIPMENT PROTECTION SECOND 5

6 What are the three main types of grounding? The three main types are: EQUIPMENT GROUNDING (SAFETY) SYSTEM GROUNDING LIGHTNING/SURGE GROUNDING 6

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8 The soil resistivity is the single most important factor affecting the resistance of the ground system. Most soils behave both as a conductor of resistance R, and as a dielectric. 8

9 For high frequency and steep-front waves penetrating a very high resistive soil, the earth may be represented by a parallel connection of resistance R, capacitance C, and a gap. For low frequencies and dc the charging current is negligible comparing to the leakage current, and the earth can be represented by a pure resistance R. 9

10 Soil Characteristics Soil type. Soil resistivity varies widely depending on soil type, from as low as 1 Ohm-metermeter for moist loamy topsoil to almost 10, Ohm-meters meters for surface limestone. Moisture content is one of the controlling factors in earth resistance because electrical conduction in soil is essentially electrolytic. 10

11 The resistivity of most soils rises abruptly when moisture content is less than 15 to 20 percent by weight, but is affected very little above 20 percent. The moisture alone is not the predominant factor influencing the soil resistivity. If the water is relatively pure, it will be of high resistivity and may not provide the soil with adequate conductivity. 11

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13 The influence of moisture content 13

14 The influence of temperature 14

15 Typical variations in soil resistivity as a function of moisture, temperature and soil content 15

16 Resistivities of different solutions 16

17 The soluble salts, acids or alkali presented in soil influence considerably the soil resistivity. The most commonly used salting materials are sodium chloride (common salt), copper sulfate and magnesium sulfate. Different types of salts have varying depletion rates; consequently, different types may be combined to produce the optimum depletion and conditioning characteristics. 17

18 The temperature effect on soil resistivity is almost negligible for temperatures above the freezing points. When temperature drops below water freezing point, the resistivity increases rapidly. Compactness and granularity affects soil resistivity in that denser soils generally have lower resistivity. 18

19 The two factors - moisture and salt content -are the most influential ones on soil resistivity for a given type of soil. Therefore the chemical treatment of soil surrounding ground rods is preferable and in some cases the only economically sound solution in obtaining low impedance of the ground system. 19

20 TYPES OF GROUND ELECTRODES Ground electrodes must penetrate into the moisture level below the ground level. They consist of a metal (or combination of metals) which do not corrode excessively for the period of time they are expected to serve. 20

21 Because of its high conductivity and resistance to corrosion, copper is the most commonly used material for ground electrodes. Other popular materials are hot-galvanized steel, stainless steel and lead. Ground electrodes may be rods, plates, strips, solid section wire or mats. 21

22 Three types of copper rods are available. Solid Copper-Copper -clad steel rod (copper shrunk onto the core)-copper bonded steel core (copper is molecularly bonded to nickel plated steel rod) Solid copper rods not prone to corrosion, but are expensive and difficult to drive into hard ground without bending. 22

23 A steel cored copper rod is used for this reason, however those rods that are simply clad are prone to the cladding tearing away from the core when driven in rocky ground, or when bent. This exposes the internal steel core to corrosion. The most cost effective solution is the molecularly bonded steel cored copper rod. 23

24 There are three components grounding electrode resistance: affecting (1) The resistance of the electrode which is negligible. (2) the resistance of the electrode-to to-soil interface area which is negligible (3) the resistance of the body of immediately surrounding the electrode. earth The main part of any electrode resistance is that of the body of earth surrounding the electrode. 24

25 GROUND RESISTANCE OF AN ELECTRODE Grounding point electrode The equations for the resistance of any complex system of ground electrodes can be developed from the fundamental principles. The starting point for such a development is the use of a buried metallic electrode with a hemispherical base of radius r. It is assumed that the hemispherical base is completely buried in the soil. 25

26 When a current I enters the ground through such an electrode, due to its hemispherical base, the current flows radial outward as shown in the sketch below. dr = ρdx ρ 2πx 2 A Hemispherical electrode 26

27 Current flow from a hemisphere in a uniform earth 27

28 Ifρ is the resistivity of the soil, the resistance offered by a hemispherical shell of thickness dx at a radial distance x from the electrode is given by R r 1 = a ρdx 2πx Hence, the resistance encountered by the ground electrode up to the depth of r 1 is ρ 1 1 R = 2π a r1 2 28

29 If r 1 is made, the total resistance of the ground electrode will be r ρ 1 R = 2πa This is the maximum resistance of the ground electrode. The general equation of the resistance shown above can be modified to a suitable form. If a current I enters the ground electrode, the potential drop up to the shell radius of r 1 will be given by 29

30 R = 1 a ρ = r a R π ρ = = 1 1 r a IR IR V = r a R π

31 The general equation for the electrode resistance: ρ R = 2πC Where C is the electrostatic capacitance of the electrode and its image above earth. Q V sphere = 4 πε 0 r Q V r in cm = C of a sphere in cgs units. = C = 4πε 0 r 31

32 At very high values of r 1, the resistance value will approach the value R, beyond which true ground can be assumed to be present. The figure below shows the plots of R, V and Vabs, the absolute potential with respect to the true ground against the radial distance r 1 from the electrode. 32

33 V = IR = IR a 1 r 1 Vabs = IR a r 1 Vabs = Iρ a 2πa r 1 Vabs = I ρ π 2 r 1 33

34 Step and touch voltages 34

35 Step and touch voltages 35

36 Step and Touch voltages near a grounded structure: E electrostatic stress (voltage gradient) E x = ρ i = ρi π 2 x 2 V r 1 = ρi ρi 1 1 dx = 1 2πx 2 2π a r1 r a 36

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38 Step potential Step potential is the voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the electrode. A person could be at risk of injury during a fault simply by standing near the grounding point. 38

39 Touch potential Touch potential is the voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the energized object and a point some distance away. The touch potential could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it. 39

40 Driven rods 40

41 Resistance of driven rods: The Ground Resistance (R) of a single rod, of diameter (d) and driven length (l) driven vertically into the soil of resistivity (ρ), can be calculated as follows: ρ 8l = ln 1 2 πl d where: ρ Soil Resistivity in ohm-m l Buried Length of the electrode in m d Diameter of the electrode in m The rod is assumed as carrying current uniformly along its rod. Examples (a) a)20mm rod of 3m length and Soil resistivity 50Ω-m..R= R=16.1 Ω (b) b)25mm rod of 2m length and Soil resistivity 30Ω-m..R= R=13.0 Ω R 41

42 Earth resistance shells surrounding a vertical earth electrode 42

43 The resistance of a single rod is not sufficiently low. A number of rods are connected in parallel. They should be driven far apart as possible to minimize the overlap among their areas of influence. 43

44 It is necessary to determine the net reduction in the total resistance by connecting rods in parallel. The rod is replaced by a hemispherical electrode having the same resistance. 44

45 The interfacing hemisphere 45

46 Equivalent hemisphere ρ ρ 8l = 1 ln 2πr 2πl d eq r eq = l 8 l ln d 11 R for large power station = 0.5 ohm R for small power station = 5 ohm R for towers = ohm R for labs = 2 ohm 46

47 Rod Electrodes in Parallel If the desired ground resistance cannot be achieved with one ground electrode, the overall resistance can be reduced by connecting a number of electrodes in parallel. These are called arrays of rod electrodes. 47

48 The combined resistance is a function of the number and configuration of electrodes, them, resistivity. their the separation dimensions between and soil Rods in parallel should be spaced at least twice their length to utilize the full benefit of the additional rods. 48

49 If the separation of the electrodes is much larger than their lengths and only a few electrodes are in parallel, then the resultant ground resistance can be calculated using the ordinary equation for resistances in parallel. In practice, the effective ground resistance will usually be higher than this. 49

50 Typically, a 4 rod array may provide an improvement of about 2.5 to 3 times. An 8 rod array will typically improvement of may be 5 to 6 times. give an 50

51 The multiple driven rod electrode The driven rod is an economical and simple means of making an earth connection but its resistance is not sufficiently low. A number of rods are connected in parallel. They should be driven far apart as possible to minimize the overlap among their areas of influence. It is necessary to determine the net reduction in the total resistance by connecting rods in parallel. The rod is replaced by a hemispherical electrode having the same resistance. 51

52 The method consists of assuming that each equivalent hemisphere carries the same charge. Calculate the average potential of the group of rods. From this and the total charge the capacity and the resistance can be calculated. 52

53 Rods too close 53

54 Two ground electrodes Equivalent hemisphere 54

55 Two driven rods: V 1 = ( I / 2) ρ + 2πr ( I / 2) ρ = 2πd Iρ 1 4π r + 1 d Actual value Re qactual = V I = ρ 1 4π r + 1 d Ideal value Re qideal = 4 ρ πr Overlapping coefficient (screening) η = ideal actual = Re q ideal Re q actual 1 55

56 Three electrode system Four electrode system 56

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