LIGHTNING PROTECTION

Transcription

1 The University of New South Wales School of Electrical Engineering and Telecommunications Industrial and Commercial Power Systems Topic 8 LIGHTNING PROTECTION

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3 Aim is to protect: People Buildings and Contents against damaging effects of lightning strikes.

4 Lightning is very common event. Worldwide, some 30 lightning flashes occur in every second on average. Frequency of occurrence of lightnings and thurderstorms varies significantly with location. Severity of lightning storms also varies with location. Local topographical features may cause variations in occurrence of ground flashes. Tall objects (building rooftop, tree top, overhead lines) tend to attract lightning flashes to themselves and thus shielding surrounding area from direct strikes.

5 Distribution of worldwide lightning strikes (flashes/km 2 /yr) [Source: NSSTC]

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8 2 PHYSICS OF LIGHTNING

9 Storms Collision among ice crystals and water droplets Charge separation

10 Lightning = sudden discharge of electricity between differently charged regions. Cloud flash Ground flash, less common. Downward leader (stepped leader). Upward leader Return stroke

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12 Lightning protection systems (LPS) are designed to ensure that lightning terminates on an air terminal (lightning rod) instead of on some other parts of building.

13 Ground flash consists of a sequence of highamplitude short-duration current impulses (strokes). Currents are uni-directional, and usually negative (negative charge injected into struck object). Stroke considered as generated from a current source, i.e. current waveshape and magnitude not affected by characteristics of ground termination.

14 Characteristics of ground flashes [Table B1, AS/NZS1768:2007]

15 Potentials during a lightning flash to earthed conductor.

16 Modes of entry of lightning impulses [Fig 5.1, AS1768:2007]

17 Principal effects of lightning discharge to object: Electrical. Thermal Mechanical

18 Cause death or serious injury in various ways : Direct strikes to person causing heart failure, brain damage, suspension of breathing, burns, etc. Asphyxiation or injury due to fires or structural damage Side flashes Electric shock from step, touch, or transferred voltages

19 3 ELEMENTS OF A LIGHTNING PROTECTION SYSTEM

20 Protection system components: Air terminals. Down conductors Earth termination network Equi-potential bonding Over-voltage protection

21 4 STANDARDS ON LIGHTNING PROTECTION

22 AS/NZS 1768:2007 Lightning Protection. Provide guidelines for protection of people, buildings and structures, and sensitive electronic equipment. Applicable to conventional lightning protection systems (LPS) and surge protective devices (SPD).

23 4.1 Risk assessment & management Risk management used to determine whether protection is needed and if so selection of adequate protection measures to reduce risk to below a tolerable level. Risk R is defined as probability of loss occurring over a one-year period.

24 Table 2.1, AS/NZS1768:2007

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26 Damage due to lightnings:

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29 4.2 Protection of structures Note: common to consider PL III as standard.

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33 Typical LPS using metal in or on a building [Fig 4.4 AS1768:2007]

34 Using horizontal and vertical air terminals [Fig 4.5 AS1768:2007]

35 4.3 Voltage calculation See Appendix D of AS1768:2003

36 Idealised lightning stroke currents. [Fig D1, AS/NZS1768:2007]

37 Approximate breakdown strength of air [Fig D2, AS/NZS1768:2007]

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44 4.4 Earthing and bonding

45 Methods of equipotential bonding: Bonding of services [Fig E1, AS/NZS1768:2003]

46 Combined utilities enclosure [Fig E2, AS/NZS1768:2003] (Preferred method for new buildings)

47 Common bonding network (CBN) [Fig E3, AS/NZS1768:2003] (Building with properly bonded reinforced concrete floor)

48 Ring earth [Fig E4, AS/NZS1768:2003]

49 4.5 Transient waveshapes See Appendix F of AS/NZS1768:2003 Majority of transients encountered in practice can be classified in terms of three standard waveshapes: 1.2/50µs unidirectional pulse. 8/20µs unidirectional pulse. 0.8µs/100kHz ring wave.

50 Standard uni-directional waveshape [Figure F1, AS/NZS 1768:2003]

51 0.5µs/100kHz ring wave (open-circuit voltage) [Figure F2, AS/NZS 1768:2003]

52 Recommended application for waveshapes of Figs F1 and F2 [Table F1, AS/NZS 1768:2003]

53 Typical voltage/time tolerance of computing equipment. [Figure F3, AS/NZS 1768:2003]

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55 5 OVERVOLTAGE PROTECTION IN LOW-VOLTAGE SYSTEMS

56 Crowbar devices Air spark gaps or gas discharge tubes SCR and triacs Clamping devices Metal oxide varistors (MOV) Avalanche diodes (Zener diodes) Switching and rectifier silicon diodes Isolators Opto-isolators Isolation transformers Common-mode filters

57 Surge diverter protection for electricity supply circuits

58 Low-pass filter to reduce rate of voltage rise. Multi-stage protection for telephone and signalling circuits.

59 Combination units.

60 Floating computer common (separate earth).

61 Appendix OTHER DESIGN METHODS FOR LIGHTNING PROTECTION

62 1. Cone of Protection Method

63 Volume protected by a catenary wire air termination.

64 Volume protected with vertical rod near building s edge.

65 2 Faraday Cage Method Also called mesh method, comprised of a series of horizontal air terminals such as copper tape which are bonded to vertically descending down-conductors. Minimum mesh sizes (IEC Standard):

66 3 Collection Volume Method An improved Electrogeometric model developed by Eriksson. Allows for computation of parabolic-like lightning collection volumes for all potential strike points on a building.

67 Thank you