Industrial and Commercial Power Systems Topic 7 EARTHING

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1 The University of New South Wales School of Electrical Engineering and Telecommunications Industrial and Commercial Power Systems Topic 7 EARTHING

2 1 INTRODUCTION Advantages of earthing (grounding): Limitation of touch and step potentials to prevent electric shock Equi-potential bonding of exposed metal conductors to prevent electric shock Limitation of over-voltages on equipment for prevention of damage Fast operation of electrical protection and limitation of earth fault damage

3 General requirements: low impedance path (resistance and reactance) to earth conductor of local supply system and thence to earth of supply at main substation items need earthing (eg metal casings) must be connected to main earth by conductors of low enough impedance to provide equipotential bonding between all equipment items. Avoid earth loops. earth potential rise (EPR) associated with any fault current must be limited to safe levels earth conductors capable of handling fault current without thermal or mechanical damage

4 2 METHODS OF POWER SYSTEM EARTHING

5 Methods of earthing Unearthed system Solidly earthed system Resistance earthed system Reactance earthed system Use of an earthing transformer

6 Advantages of Solidly earthed systems High fault current and fast protection operation Better personnel and equipment safety Earth fault current easy to detect Unearthed systems Line to earth fault will not interrupt supply thus improves reliability Low fault current (limited by capacitance) will not cause damage

7 Disadvantages of Solidly earthed systems Line to earth fault causes loss of supply Line to ground fault current may be high enough to cause damage Unearthed systems Line-line fault current may be low and may not trip protection May have prolonged arc faults Line-line voltage imposed on phase insulation if earth fault occurs Overvoltages may cause increased insulation stress if there is an earth fault Finding faults may be difficult Regular maintenance is required

8 3 PERSONNEL PROTECTION

9 Standards AS/NZS :2002 : Effects of current on human beings and livestock - General aspects In USA and Canada: IEEE Std Dalziel s electrocution equation

10 Dalziel s electrocution equation: Defines a specific energy (I 2 t) as determining factor for potential electrocution (fibrillation). Values based on tests carried out on humans and animals.

18 Code of Practice, Electrical Safety Act 2002 IEC approach (also Australia): Special locations: A1 (<67kV) or A2 (>66kV) Frequented locations: B1 (<67kV) or B2 (>66kV) IEEE

19 4 TYPICAL SITUATIONS IN SUBSTATIONS

20 Possible hazardous situations which can occur when there is an earth surface potential rise (EPR) in vicinity of an earth electrode due to fault current flowing to earth through that earth electrode.

21 Step potential : voltage difference between a person's feet when spaced 1m apart. Touch potential: voltage difference between exposed metal object, connected directly to earth electrode, and ground surface potential where feet are placed (usually distance of 1m is used). Grid (mesh) potential: maximum possible touch voltage in an earth grid area. Transferred potential: voltage difference between earth surface potential and exposed metal object connected to remote earth (effectively at true earth potential of zero volts).

22 Potential hazards due to Earth Potential Rise from a fault.

23 5 EARTHING FOR EQUIPMENT PROTECTION - THERMAL EFFECTS

24 earth faults are most common type of electrical fault. heating of fault current cause significant damage. arcing earth fault, or high impedance arcing fault is potentially the most damaging. causes: (i) contaminated insulation (ii) physical damage to insulation, (iii) transient or continuous over-voltages. typical arc voltages in LV systems: ~ 100V. arcing fault currents << bolted-fault current.

25 6 SYSTEMS OF EARTHING IN LOW-VOLTAGE INSTALLATIONS

28 Direct earthing system: system relies on current flow through the ground thus requires low earth resistivity and good earth electrode. Not always possible. MEN system is preferred as it utilizes supply utilities neutral to provide an additional earth return path.

29 MEN system: earth connections to neutral at consumer s installation and along route to supply substation neutral provides the return path while in direct earth system the metallic path is provided by water pipes, cable sheaths or by special earthing connections if provided balancing of load to utilize phase current cancellation in return neutral to minimize voltage drop

30 neutral conductor must be earthed at substation and at other locations as necessary to ensure that total impedance between neutral and earth does not exceed 10 ohms conductors used to earth neutral conductor of distribution system must have a cross-section area of at least 20% that of the smallest size of neutral used in system

31 CMEN system: Common Multiple Earthed Neutral extension of MEN system high voltage and low voltage equipment is bonded (via a neutral conductor) to a single common earth impedance to ground of this interconnected system of earthing is very low, typically 1 ohm or less.

32 7 TYPES OF EARTHING SYSTEMS IN CONSUMER S INSTALLATIONS

33 TN systems TT systems IT systems TN-C TN-S TN-C-S In practice, only TT and TN systems are commonly used.

34 1st letter (I or T) gives relationship of supply to earth T (terra): direct connection of one point of supply system to earth I (insulation): all live parts of supply isolated from earth or one point connected to earth through an impedance

35 2nd letter (T or N) gives relationship of exposed conductive parts of the general installation to earth T (terra): direct connection of exposed conductive parts to earth, independent of earthing of supply system N (neutral): direct connection of exposed conductive parts to earthed point of supply (neutral point).

36 TN systems: one point directly earthed, exposed conductive parts connected to that point by protective conductor (PE) TN-S system: separate neutral (N) and PE throughout TN-C system: N and PE combined into a single conductor throughout TN-C-S system: N and PE combined into a single conductor in a part of the system

37 TN-S TN-CS TN-C

38 TT system: one point directly earthed, exposed conductive parts connected to earth via separate earth electrode, no direct connection between live parts and earth, exposed conductive parts connected to earth

39 IT system: no direct connection between live parts and earth, exposed conductive parts connected to earth

40 8 EARTHING FOR COMPUTER SYSTEM AND EQUIPMENT

41 Earthing systems for data links

42 9 EARTH RESISTANCE OF BURIED ELECTRODES

43 Earth resistance determined by: shape of electrode(s) extent of electrode(s) electrical resistivity of the soil

47 10 EQUIVALENT HEMISPHERE MODEL OF AN EARTH ELECTRODE

48 most common form of earth electrode is a driven rod or pipe or a complex distributed mesh in the ground. not simple for analytic calculation determine equivalent hemisphere and then used for potential distribution calculations

49 Example:

50 11 USE OF EQUIVALENT HEMISPHERE MODEL TO CALCULATE TOUCH, STEP, TRANSFERRED POTENTIALS

51 V(r) = ground potential w.r.t earth electrode

53 12 EARTH RESISTANCE OF CLOSELY-SPACED IDENTICAL ELECTRODES

54 separate earth electrodes buried close to one another earth potential fields around these electrodes when current flows to earth will interact earth resistance of combined system higher than that of parallel combination isolated electrode system

56 13 MEASUREMENT OF EARTH RESISTANCE

57 Three-electrode method

58 Fall-of-potential method potential probe remote current probe

60 14 EARTH RESISTIVITIES

61 not a good conductor as compared with metals. variable depending upon physical nature and chemical composition heavily influenced by moisture content and dissolved salts.

62 Rule of thumb Mud (compressed coal): Wet soil: Moist soil: Dry soil: Rock: 1 Ωm, 10 Ωm, 100 Ωm, 1000 Ωm, Ωm.

64 15 ELECTRIC SHOCK EFFECTS

65 body resistance foot contact resistance ρ = soil resistivity Equivalent circuits for touch and step potentials

66 Assume bare feet or conducting footwear:

67 Tolerable touch and step potentials

69 Thank you

70 Unused slides

71 In distribution substations, the buried grid system will determine the earth resistance. The maximum recommended resistance is 5Ω or even 1Ω.

79 Step voltage means the prospective or open circuit voltage that may appear between any two points (1 metre apart) on the surface of the ground. Touch voltage means the prospective or open circuit voltage that may appear between any point of contact with conductive parts (that are located within 2.4 metres of the ground) and any point on the surface of the ground with a horizontal distance of one metre from the vertical projection of the point of contact with the conductive part.

80 Transferred earth potential Although a line may be "earthed " there may be a dangerous voltage or potential between the line and the earth point, including a concrete pole or a steel tower, at the work site. For example, where a line is under access at a remote substation, a fault not directly associated with the line under access can cause a dangerous voltage rise on the substation earth grid. That voltage rise is transferred through the line to the work site, where it can create a hazard to workers on the site.

In 1924, South Australian James Stobie invented a steel and concrete pole to carry electricity and telephone lines, due to SA s lack of suitable timber.

81 In 1924, South Australian James Stobie invented a steel and concrete pole to carry electricity and telephone lines, due to SA s lack of suitable timber. A particular impetus for their invention was the need for a reliable supply of poles for the expansion of electricity into the countryside. Stobie Poles have other advantages too they re termite proof and have a life span of around 50 years. All Stobie Poles end at the SA border, with most other States preferring wooden telegraph poles.

82 Ref: AS : Electromagnetic compatibility

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