Lightning Protection Scenarios of Communication Tower Sites_final

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

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

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!.

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

Introduction

Antennas

Equipment room

Radio cables

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

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.

Observations

Types of towers Lightning protection components

Air terminations Down conductors Grounding systems Grounding system configurations

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)

Observations – Lightning protection components

Air termination Franklin Rod type (Non active air

termination) ESE (Early Streamer Emission) (Active

termination)

Observations - Air terminations

Franklin rod ESE

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).

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.

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.

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.

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)

Observations – Grounding system configuration

4 legs of the towerDown

conductor

Option 1

Observations – Grounding system configuration

Option 2

Observations – Grounding system configuration

Option 3

Observations – Grounding system configuration

Option 4

Observations – Grounding system configuration

Option 5

Information and discussion

Damage to equipment installed on the tower

Damage and injuries at ground level Model design

Damage to equipments installed at tower

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.

Damage to equipments installed at tower

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

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.

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

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).

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

Damage to equipments installed at tower

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

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.

Damage and injuries at ground level

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)

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

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

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.

Damage and injuries at ground level…

What is the proper way of installing SPDs in a TT system…?

TT wiring system

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.

Types of SPD connection in the TT system

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

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

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)

Installation of SPDs

Wrong SPD installation with input & output lines in the same conduit in parallel

Wrong Correct

Wrong installations

Routing the grounding wire/tape in the same cable tray with other signal lines.

Model design

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…

Special design…

Special design…

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

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

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…

DC resistance Vs Transient inductance…?

Distributed grounding system…

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

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.

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

?

Thank You…