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Analytical design of lightning protection system for communication tower sites
108880J ST Arif108882R DT Dissanayake108883V AHGR Fernando108885E NUL Gunawardhana108886H IMSK Jayarathna108888P GWASB Muhandiramge108895J KC Wijesinghe
Group B
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Lightning protection scenarios of communication tower sites; human hazardsand equipment damage
On the selection and installation of surge protection devices in a TT wiring system for equipment and human safety
Chandima Gomes a, Arturo Galvan Diego b
a Centre of Excellence on Lightning Protection, University Putra Malaysia, Malaysiab Instituto de Investigaciones Eléctricas, Cuernavaca Morelos, Mexico
Chandima GomesCentre of Excellence on Lightning Protection, University Putra Malaysia, Malaysia
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Abstract
Comprehensive analysis of lightning protection systems – sample 48 sites
Having an Air Termination, insulated/bonded down conductor to tower structure?
Importance of integrated distributed equipotentialized grounding system with correct SPD placements instead of achieving low ground resistance?
Introducing concrete embedded grounding systems for problematic locations!.
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Introduction
Communication towers are all-metal structures
Mostly located in the highest geography in the area
Prime target of lightning Property damage due to lightning is
high
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Introduction
Antennas
Equipment room
Radio cables
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Introduction
Tower related hazards in various stages; The attachment process – direct strike Passage of lighting current to ground
level – side flashes Discharging to the ground – ground
potential rise
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Methodology
48 communication tower sites in Sri Lanka have been investigated during 5 year period from 2003 to 2008 Checked the nature and installation features of
1. Air-termination system.2. Current path to ground level.3. Grounding system.
Taken quantitative measurements of1. Ground resistance of the system.2. Average soil resistivity of the site.
Collected confirmed lightning related damage records for the previous 1–3 years.
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Observations
Types of towers Lightning protection components
Air terminations Down conductors Grounding systems Grounding system configurations
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Observations – Types of towers
The 48 towers are all-metal, self supported structures
Height 40m to 100m Have 3 legs or 4 legs The cross sectional area of re-bars
are over 150mm2
Selected in similar contours of isokeraunic level (annual thunder days) – 120 to 140 days per year (high lightning density belt)
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Observations – Lightning protection components
Air termination Franklin Rod type (Non active air
termination) ESE (Early Streamer Emission) (Active
termination)
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Observations - Air terminations
Franklin rod ESE
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Observations – Air termination
Metal rod that covers all antenna structures in the tower within a cone of vortex angle 45° (18 sites).
Metal rod that does not cover all antenna structures in the tower within a cone of vortex angle 45° (06 sites).
ESE (Early Streamer Emission) rod of which the physical height covers all antenna structures in the tower within a cone of vortex angle 45° (13 sites).
ESE rod of which the physical height does not cover all antenna structures in the tower within a cone of vortex angle 45° (08 sites).
Other types of air-termination of which the physical height cover all antenna structures in the tower within a cone of vortex angle 45° (lightning prevention type) (01 site).
No air termination (02 sites).
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Observations – Down conductors
Single copper tape strapped to one of the tower legs: 33.
Single copper tape taken from the middle of the tower (typically strapped to the ladder): 03.
Two copper tapes strapped to tower legs: 02.
Insulted down conductor (and insulated air-termination) that is isolated from the tower material: 04.
No down conductor:06.
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Observations – Grounding system
Only the down conductors are grounded: 04.
The down conductors and the four legs are grounded and integrated: 39.
The down conductors and the four legs are grounded but not integrated: 05.
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Observations – Grounding system
Range of the grounding resistance; 0–2 Ω: 04. 2–10 Ω : 22. 10–20 Ω : 09. 20–100 Ω : 06. 100–1000 Ω : 02. Resistance could not be measured due to inaccessibility: 03. Resistance could not be measured due to lack of soil: 02.
Range of the ground resistivity; 0–15 Ω m: 07. 15–100 Ω m: 20. 100–1000 Ω m: 12. 1000–10,000 Ω m: 04. Resistivity could not be measured due to inaccessibility or lack of
soil: 05.
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Observations – Grounding system configuration
Only down conductors are grounded (4 sites) - a deep driven copper tube has been integrated with three radials that runs for 3–4 m from the deep driven rod (at a depth of about 0.5 m).
Four legs are grounded and integrated (39 sites) - The ring conductor was connected to either radials or deep driven rods/plates.
Legs are grounded but not integrated (5 sites)
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Observations – Grounding system configuration
4 legs of the towerDown
conductor
Option 1
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Observations – Grounding system configuration
Option 2
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Observations – Grounding system configuration
Option 3
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Observations – Grounding system configuration
Option 4
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Observations – Grounding system configuration
Option 5
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Information and discussion
Damage to equipment installed on the tower
Damage and injuries at ground level Model design
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Damage to equipments installed at tower
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Damage to equipments installed at tower
In all cases the damaged structure was within the cone of protection with a vortex angle of 45°
Only in one case, the possibility of direct strike to the damaged structure is justified
In the other cases there are strong evidences to conclude that the damaged objects have been subjected to arcing from the down conductor.
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Damage to equipments installed at tower
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Insulated down conductor
Insulation has been ripped off from the down conductor with burning marks
Insulation damage is higher at the top of the tower
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Insulated down conductor… Arcing marks on the
tower and the damages on the insulation of the down conductor – possible side flashing
The damaged antennas are also close to the down conductor and there are evidents of side flashing.
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Why…?
Consider a lightning Peak current 50kA Max. current derivative 50kA/µs
Traveling in a down conductor (IEC 62305-3) Resistance 3x10-3Ω/m Inductance 1.5µH/m Tower height 50m
IR = 0.75kV & MVdtdiL 75.3
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Why…?
There is a very high potential difference (MV range) between the down conductor and the tower or antenna structures which are essentially at ground potential
To prevent spark-over through insulation breakdown, such condition requires, 11.25 cm thick insulation covering of cross-linked
polyethylene (assuming 1.2/50 µs voltage impulse) or
2 m of air-separation even at level IV protection according to IEC 62305-3 (2006).
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Insulated down conductor…
No commercial product provide such insulation
Implementation of such insulation/isolation has many practical constraints
It is not commercially viable
Damage is due to the insulated down conductor
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Damage to equipments installed at tower
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Damage to equipments installed at tower…
For two cases, the down conductor (bare Cu tape) was loosely in contact with the tower
The thick painting has preventing the tape from getting into a electrical contact with the tower
Marks of side flash from down conductor to antennas
Same as early case Last case – direct strike
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Damage to equipments installed at tower…
Possibility of side flash from down conductor does not depend on; Ground resistance Integration of down conductor to the grounding system of
tower legs Coverage of the air termination
No damage to tower installed objects in following cases Towers with no air-termination. Towers of which air-termination does not cover all the
equipment installed on the tower, within a cone of protection with 45° vortex angle.
Towers with no down conductors.
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Damage and injuries at ground level
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Damage and injuries at ground level
Damages to the equipments in the Base Transceiver Station (BTS) – (102 occasions in 27 sites)
Personal injuries (13 cases in 8 sites) Step potential Direct contact with equipments during
lightning The 5 sites located in the rocky area
has the largest number of damages (42 equipment & 5 personal)
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Damage and injuries at ground level…
50% of the cases are in sites with earth resistance bellow 10Ω
No damages to sites with earth resistance between 70Ω & 100 Ω for the period of 3 years. (but these sites have distributed integrated earthing system)
Damage count is high for sites NOT having distributed integrated earthing system
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Damage and injuries at ground level…
In the 9 cases of step potential, site has no ring conductor Grounding resistance is high or cannot
be measured In the 4 cases of electric shock,
Potential at the grounding bar can be 20-30kV
Result momentary paralysis, fall unconscious
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Damage and injuries at ground level…
In 21 cases no SPDs installed In 6 cases SPDs installed incorrectly and
wiring is incorrect (in the TT system) More than one point connected to external
ground. Routing the grounding wire/tape in the same
cable tray with other signal lines. SPD only at the power entrance. No SPDs for the data lines. Inappropriate specifications of the installed
SPDs.
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Damage and injuries at ground level…
What is the proper way of installing SPDs in a TT system…?
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TT wiring system
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SPD
surge protective devices (SPDs) is twofold. As a transient propagates in a line, SPDs
should switch from high impedance mode to low impedance mode for a short duration allowing the transient to pass into earth.
After that switched back to the high impedance mode.
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Types of SPD connection in the TT system
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Zonal concept of SPDs
Finite impedance of SPDs; Impedance of a SPD is not zero In a transient, a large current travel through the SPD Will give a voltage difference That voltage can still be harmful to the equipment
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Selection of SPDs
Impulse current Maximum peak current that can
be handled by an SPD Zone 1 SPDs have a high impulse
current Low in Zone 2 and lower in Zone 3
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Selection of SPDs
Voltage protection level Minimum let-through voltage that will
appear across the terminals of the SPD at transient
Higher impulse current, higher the let-through voltage
Response time Maximum continuous operating
voltage (MCOV)
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Installation of SPDs
Wrong SPD installation with input & output lines in the same conduit in parallel
Wrong Correct
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Wrong installations
Routing the grounding wire/tape in the same cable tray with other signal lines.
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Model design
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Model design – designing of a grounding system for a tower on a hard rock
The towers were on a hard rock The access to the nearest large mass
of soil required a metal extension of nearly 400m.
Drawing of a Cu tape up to soil is not economical and will give a voltage difference in MV level
TT wiring system A special design is done…
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Special design…
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Special design…
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Special design…
Site encircled by a concrete beam with a perimeter about 80m
Reinforced with steel bars welded together A gutter is made at the top Waste water and collected rain water is diverted to the
gutter Hump is made to facilitate vehicle movements Tower legs, all mettle parts are connected to the earth ring
of GI tapes covered with concrete, welded to the steel bars All the shielding layers and metal sheaths of the cable
bunch and the cable tray should be terminated at the bulkhead.
There should be no electrical connection between the metal parts inside the building and the bulkhead
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Special design…
A class I (50kA) arrester installed at the main grounding bar (Zone 1)
Power to all equipments through class III arrester (10kA) (Zone 3)
Data lines are also connected through SPDs SPDs are grounded to the common grounding system No cable phones are recommended to the building. Neighbors supplied from the same transformer is
protected by SPDs No separate down conductor. Air termination is
electrically bonded to the tower A warning sign is placed asking not to stay out side
the boundary in thunder storms
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Special design… Advantages
Galvanized steel is used without using Cu – low cost, no threat of stealing
The foundation can be used for the fencing
Total design is low cost The suggested design was
implemented in 2007 to a site No damage for 3 years…
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DC resistance Vs Transient inductance…?
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Distributed grounding system…
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Special design…
When concrete is wet it is a very good conductor
Large mass of concrete will act as a large capacitor which will absorb the chargers of lightning
As the contact area to the rock is high, it will act as large number of parallel paths to discharge lightning to the rock
Properly installed SPD system equalized the potentials and save equipments
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Conclusions
No strong evidence on necessity of an air – termination
No strong evidence on necessity of a separate down conductor for all-metal towers
Not recommended to use insulated down conductors
The tower premises should be provided with distributed and integrated grounding system incorporated with a properly coordinated system of surge protective devices
Integration of earthing systems is very necessary; however such integration should accompany the installation of proper SPDs.
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Conclusions
The installation of SPDs have to be done in zonal approach
The proposed model grounding system can be used for tower sites in solid rocks
Inappropriate cable routing, multiple grounding references, inadequate surge protection systems and laps in routine maintenance should be avoided
Transient equipotentialization is more suitable for the safety, instead of attempting to achieve low ground resistance
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?
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Thank You…