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7/28/2019 Oisd Paper
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Principles of Lightning Protection
For Oil and Gas Installations
A.J. Surtees, BSc, PhD, MBA, FICD, CEng, MIEE, SMIE (Aust)
Technical Manager
ERICO Inc. USA
MECHANISMS OF LIGHTNING DAMAGE
There are two basic mechanisms by which lightning may
enter and cause damage at a site. The first is a direct strike
where the building or surrounding structures receive a direct
lightning discharge. The effect is a large increase in the local
earth potential with subsequent damage to equipment
connected to outside services via such means as power
feeders, telephone subscriber lines, data or control cables and
physical pipelines.
The second mechanism is due to magnetic or capacitive
induction where a distant strike may induce large voltage
transients into power lines, communications lines and the
pipeline. The US IEEE standard 587 describes typical peak
amplitudes of voltage and current in power and
communications lines.
The mechanism of coupling to a buried pipeline is different.
The pipe, being insulated presents a high resistance to
ground and over its length a large capacitance to ground.
The earth potential rise associated with a ground strike in thevicinity of the pipe, is capacitively coupled to the pipe. This
potential is transferred in both directions from the point of
the strike, resulting in local earth potential rises at metering
stations and other facilities housing such equipment as
monitoring and telemetry electronics. It is this mechanism
which is responsible for damage to the sensitive metering
and electronic equipment. Thus, a strike some hundreds of
kilometres from a particular site may be responsible for local
damage.
The waveform of the transient induced into the pipe is
modified by the pipe capacitance. This means that protection
methods which may be effective against the standard 8/20spulse must be reviewed. Our experience is that the pulse rise
time is slowed, consequently protection networks involving
series inductive elements are largely ineffective.
A GENERIC APPROACH TO LIGHTNING
PROTECTION
Global Lightning Technologies has developed a generic Six
Point Plan for the protection of structures or facilities. The
concept behind the plan is that it prompts the user into
considering a holistic approach to lightning protection, one
embracing all aspects of potential damage, from the more
obvious direct strike to the more subtle mechanisms of
differential earth potential rises and voltage induction at
service entry points.
1. Capture the li ghtni ng stri ke at a preferred and know
point. This involves the use of an effective air terminal(s) on
the structure or vessel to be protected. In the design of area
protection, it is important to realise that many points on the
structure will be competing for the downward lightning
leader by launching upward interception streamers. Theeffectiveness of a lightning terminal is the measure of its
response time in launching such a streamer. The earlier the
streamer launch with respect to extraneous emission points
on the surrounding structure, the better the terminal will be
in ensuring it will take the strike (at a known point) and
prevent random striking or bypasses to adjacent uncontrolled
points.
2. Convey the li ghtning energy to ground in a safe manner
via a known route. This involves the use of a dedicated
down conductor, capable of withstanding the full energy of
the lightning discharge and conveying this to the grounding
system with minimal danger of side flashing to adjacentearth points. The ability of the down conductor to screen
adjacent equipment from the large electro magnetic impulse
associated with the discharge current, which may reach
energy levels as high as 250kA 8/20s, is also a measure of
its effectiveness in reducing damage by induction.
3. Ensure a low impedance earthi ng system to dissipate the
lightni ng discharge. The need to understand the
characteristics of an earthing system under impulse
conditions (associated with the higher Fourier spectral
components of the lightning discharge), is crucial if an
effective earth system is to be designed. An effect earth
system is one in which the potential rise of the surroundingearth is minimised and the rate of potential fall off from the
injection point is maximised.
4. Elimi nate earth l oops. Ensuring that a single point
earthing policy is adopted and that equipotential earth
bonding is used throughout the installation will help
eliminate common damage caused by differential earth
potentials.
5. Protect service entr y points of power f eeders. This
involves the installation of voltage clamping devices capable
of handling the large energy content (kA rating) of the over
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voltage surge, as well as reducing the extremely fast rising
edge (dv/dt and di/dt) of this transient.
6. Protect service entr y points of data or control li nes. This
involves the installation of high speed protective barriers. For
effective voltage limiting, a hybrid circuit is usually
employed where both speed of activation and energy
handling capacity are optimised in a multi-stage protective
module.
POINT 1. CAPTURE THE LIGHTNING STRIKE
AT A PREFERRED POINT
The potential of a direct strike to a communications tower
servicing a remote telemetry link, an offshore exploration
platform or even an elevated fuel storage tank may be high
with the resultant danger of equipment damage or even fire
as witnessed recently at the Cilacap refinery in Indonesia.
The design of an effective air terminal to protect such
structures requires some understanding of the mechanism of
the lightning discharge.
THE THUNDERCLOUD ORCUMULO-NIMBUS
Lightning is a natural phenomenon which develops when the
upper atmosphere becomes unstable due to the convergence of a
warm, solar heated, vertical air column on the cooler upper air
mass. These rising air currents carry water vapour which on
meeting the cooler air usually condense giving rise to convective
storm activity. Pressure and temperature are such that the
vertical air movement becomes self sustaining, forming the basis
of a Cumulo-nimbus cloud formation with its centre core capable
of rising to more than 15,000 metres.
To be capable of generating lightning, the cloud needs to be 3-4
km deep. The taller the cloud the more frequent the lightning.The centre column of the Cumulo-nimbus can have drafts
exceeding 120 km/hr creating intense turbulence with violent
wind shears and consequential danger to aircraft. This same up
draught gives rise to an electric charge separation which
ultimately leads to the lightning discharge.
The surface of the earth is initially negatively charged to the
order of 5 x 105 C, giving rise to an electric field intensity of
approximately 0.13 kVm-1. The lower atmosphere takes on an
opposing positive space charge. As rain droplets carry charge
away from the cloud, from the earth the storm cloud takes on the
characteristics of a dipole with the bottom of the cloud negatively
charged and the top of the cloud positively. It is known from
waterfall studies that fine precipitation acquires a positive
electrical charge. Larger particles acquire a negative charge. The
up draught of the Cumulo-nimbus separates these charges by
carrying the finer or positive charges to high altitudes. The
heavier negative charges remain at the base of the cloud and the
surface of the earth starts to accumulate positive charge. This
gives rise to the observed phenomenon where more than 90% of
cloud-to-ground discharges occur between a negatively charged
cloud and positively charged earth (negative lightning).
THE LIGHTNING DISCHARGE
The separation of electrical charge within a cloud allows electric
potential to increase to a point where a neutralising discharge
must occurs. The method by which this discharge takes place can
take on one of five different mechanisms:-
Cloud-to-cloud
Cloud-to-air
Intra-cloud discharges
Ground-to-cloud discharges and
Cloud-to-ground strikes.
Approximately 50% of all lightning discharges are cloud-to-
ground strikes. Ground-to-cloud discharges are extremely rare
and generally only occur from high mountain tops or tall man
made structures.
Cloud-to-ground discharges are further subdivided into positive
and negative leader discharges, of which about 90% are of the
negative category.
MECHANICS OF THE LIGHTNING STRIKE
The development of a cloud-to-ground discharge is a two staged
sequence, with one process being initiated from the cloud while a
second process is simultaneously being initiated from the groundor earth bound structures. Both mechanisms rely on an excessive
electron build up with subsequent ionisation and avalanche into
an electric current flow.
The Cloud Initiated Discharge - Leader
As a cloud accumulates charge, the electric field builds up to
the point where the air starts to breakdown forming an
ionised discharge called apilot streamer. This initial
discharge rapidly traverses about 30-50 meters towards the
ground. The presence of wind shear tends to blow away the
ionised air, halting the progression momentarily until
additional negative charge accumulates at the tip of the
column and air breakdown again occurs, allowing theionisation process to advance a further 30-50 metres. This
more intense discharge is generally known as thestepped
leader. The process repeats itself in a series of discrete steps
with a time interval of roughly 50 s. The course taken by
each of the steps in the leaders propagation towards the
ground is determined by the path through the air which
ionises more easily. This gives rise to the characteristically
zigzag nature of the cloud-to-ground discharge and the
branching of the lightning into many "fingers" in an attempt
to reach ground.
Being a highly ionised column, the tip of the leader is at
essentially the same potential as the charged cell from which ithas originated. As this tip approaches the ground, the potential
gradient further increases accelerating local ground ionisation.
At this point the potential difference between the leader and the
earth may be as high as 107 V, resulting in local air breakdown.
A ground originating discharge then begins to move up towards
the leader, intercepting at some tens of meters above ground
level.
The Earth Bound Discharge - Streamer
At the ground level, a point discharge such as a sharp metal
protrusion serves to enhance the electric field intensity as the
leader tip approaches, to the point where electrons are
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accelerated sufficiently to cause ionisation as they collide with
gas molecules. As the kinetic energy of the electrons is less than
this ionising potential, additional electrons are released and an
avalanche discharge results. To start the process, an initial
liberated electron is required. This can come from the natural
field intensification due to the presence of a charged cloud or
from deliberated introduced means such as a radioactive source
or spark gap. Such techniques of enhancing the emission of free
ions is the basis of early streamer emission, enhanced-ionisation
air terminals.
Once the electric field strength exceeds about 2 kVm-1 the
number of liberated free ions becomes adequate to cause a
current to flow which weakens the electric field. This current is
known as the upwardstreamer currentand can reach
magnitudes of several tens of amperes and can take the form of a
faintly luminescent discharge emanating from sharp protrusions.
Time captured photograph has shown that this upwards steamer
channel can reach several hundreds of meters as it propagates to
meet the descending leader.
The Main Discharge Or Return Stroke
Once the ionised channel has been completed by the junction of
the streamer and the leader, the build up of positive charge in the
earth flows upwards along the ionised discharge channel to
neutralise the large negative charge in the cloud giving rise to
what is known as the return stroke. Alternatively, the process
can be described using conventional electron flow as, electrons
migrating from the negative cloud to the positive earth. This is
characterised by a rapidly increasing electric current whose rate
of rise is typically 1010 amperes/sec.
Peak currents averaging around 30 kA appear typical with
minimum currents being about 3 kA. Maximum discharges
exceeding 200kA have been recorded.
It is also possible to have consecutive discharges down the same
channel. This occurs when the initial discharge neutralises the
localised charge cell in the cloud that initiated the stroke. Nearby
charge cells then flash across to the ionised channel and use it to
discharge to ground. In this manner up to 16 discharges have
been observed using the one channel.
The average energy released in a discharge is 55 kWhr, a
significant amount of energy by modern generation standards.
The danger of the discharge lies in the fact that all the energy is
expended in only 100-300 microseconds and that the peak
discharge current is reached in only 1 to 2 microseconds.
The following parameters are typical of the return stroke:-
Upward speed of return stroke is typically one-third to one-
half the speed of light near the ground and decreases as it
approaches the cloud.
Total time between ground and cloud < 100s
Peak current in first return stroke about 30kA
Time to peak < 10ms
Leader channel is heated to 30 000K
All charge contained in leader and branches is deposited to
ground down same channel.
Subsequent restrikes take on the following parameters:
Peak currents from 20 to 400kA
Time between return strokes 3 to 100ms
Number of return strokes 1 to 15, average of 4
Rising times even faster, typically a few nanoseconds.
These are some of the parameters which make lightning difficult
to control. The need to ensure that the lightning discharge is
effectively captured using a well designed early streamer
emission terminal is the key to such control.
THE DYNASPHERE EARLY STREAMER EMISSION AIR
TERMINAL
The result of many years of theoretical and ongoing field
research is the DYNASPHEREEarly Streamer Emission
Terminal. This product provides the design engineer with an air
termination relatively free of space charges which is capable of
creating photo-ionisation and which concentrates electric field to
release free electrons on the approach of a lightning leader.
The Dynasphere is a passive air terminal which requires no
external power source, relying solely on the energy contained in
the approaching leader for its dynamic operation. This
remarkable terminal has the ability to concentrate only thatelectric field which occurs in the millisecond time slots as the
leader charge approaches the ground.
The principle of operation relies on the capacitive coupling of the
outer sphere of the terminal to the approaching leader charge,
which in turn raises the voltage of the spherical surface. This rise
in voltage produces a field concentration across the insulated air
gap between the outer sphere and central grounded finial. As the
leader continues to approach the voltage on the sphere rises until
a point is reached where the air gap breaks down. This
breakdown creates local photo-ionisation and the release of
excess free electrons. These then accelerate under the intensified
field to initiate an avalanche condition and the formation of astreamer current begins.
Unlike the Dynasphere, pointed rods and other types of
enhancement terminals tend to create a corona space charge
above the emission point which serves to reduce the electric field
there by inhibiting streamer initiation. Also, unlike other air
terminals using battery or corona generated discharges, the
Dynasphere is radio-quiet only producing a spark discharge as
the leader approaches.
CONVENTIONAL APPROACH TO CALCULATION OF THE
PROTECTION RADIUS
In 1901, the British Lightning Committee formed to address theprotection which a Faraday rod would afford. After much debate,
it was resolved that this would be the area falling under a 450
cone drawn from the tip of the rod. Experience over the years has
highlighted many deficiencies associated with this method,
where lightning has bypassed air terminals and struck within the
safe area.
The deficiencies associated with this approach are depicted in
Figure 1. It was for this reason that the rolling sphere concept ofprotection for taller structures was introduced in the late 1970s.
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Figure 1. Lightning enters the "cone of protection" (Franklin
Rod method of protection) due to the inability of the
structure to launch a streamer.
COLLECTION VOLUME METHOD
The over simplification of the cone of protection approach
has prompted research into a more scientific approach to the
calculation of the effective protection radius afforded by a
lightning terminal. One such model is that of the Collection
Volume.
The derivation of the collection volume design concept can be
understood by considering the approach of the downward
leader. The charge Q distributed along this leader causes rapid
increase in electric field between it and ground points. When a
critical field value is reached, the ground point launches an
upward intercepting leader. The distance at which this occurs is
called the Striking Distance. The critical electrical field is
dependent on both leader charge and distance from the ground
point. Figure 2 shows how it is possible to form striking
distance hemispheres around an isolated ground point. The
greater the leader charge, the greater the striking distance.
Unfortunately, this simplistic approach of creating striking
distance hemispheres is not infallible in practice. Regard must be
taken of the relative velocities of the approaching leaders.
Figure 2 shows how it is possible to reach critical electric field
and launch an upward leader. If the downward leader is near the
periphery of the sphere, its velocity may carry it onward to
intercept another upward leader. Therefore, it is possible for the
downward leader to enter a striking distance hemisphere without
interception.
To cater for this the model requires that a limiting parabola be
placed on the hemisphere. This parabola is derived from velocity
factors and completes our collection volume. It can now be
stated that a downward leader entering such a volume is
theoretically assured of interception by the ground point
concerned. Figure 5 also shows how collection volumes become
larger with increased leader charge. That is, the larger the
magnitude of the current stroke, the larger the collection volume.
The collection volume model assumes that all points on the
structure are potential strike points and as such exhibit their
own natural collection volumes or attractive radius.
Figure 2. Collection volume and hemisphere bounded by
limiting parabola defined by charge on approaching leader.
THE BENJI CAD PROGRAM
A proprietary computer program has been developed by
Global Lightning Technologies Pty. Ltd. which evaluates the
protection radius afforded by an air terminal under different
conditions of leader intensity. Known as BENJI after thefounder of lightning research, Benjamin Franklin, the
program compares the protection radius produced by the air
terminal to the attractive radii produced by the electric field
intensification of competing points on the structure (corners
and edges, antennae, equipment, masts etc). The program
then optimises the placement, and number of air terminals,
to ensure that all these competing points lie within the
protective radius afforded by the ESE terminals.
The technology upon which the ESE air terminal and
computer model are based, follows that which is included in
the Australian / New Zealand Standard NZS/AS1768-1991
Lightning Protection, Appendix A.
POINT 2. SAFELY CONVEY THE LIGHTNING
ENERGY TO GROUND
Once the lightning discharge has been captured it is necessary to
covey this energy to the ground in a safe and controlled way.
Complications such as side flashing to adjacent conductors,inductive coupling of the large electromagnetic pulse onto nearby
signal lines and control of the excess energy content, need to be
considered. To this end ERICO has developed a special insluated
down conductor which comprises carefully selected dielectric
materials to create a capacitive balance and ensure insulation
integrity under high impulse conditions. In addition, a
conductive outer sheath allows electrostatic bonding of the
building through cable securing saddles. This ERICORE down
conductor evolved after extensive studies of potential voltage rise
in structures due to lightning injection.
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Figure 3. ERICORE insulated downconductor
The construction of the ERICORE also serves to reduce the
mutual inductance. A value of inductance of 1.6H/m is
normally regarded as quite small. However when a current is
impressed which is rising at the rate of 100kA/s, the effect of the
voltage developed due to this inductance (Ldi/dt) becomes
dominant. As an example, a single 60 metre down conductor
will rise to a value in excess of 1MV with an average discharge.
It is for this reason that a number of conductors are frequently
specified with standard protection methods.
Some of the practical benefits of ERICORE:-
It provides the design engineer with the ability to select
the most convenient lightning route to ground. The down
conductor can utilise air ducts etc. and be located remote
from electrical and sensitive electronic equipment.
The lightning is contained in the 50mm copper core
conductor and is oblivious to impedance irregularities in
the structure. The risk of side-flashing is reduced.
The structure carries only that minimal current which is
due to capacitive coupling to the main conductor.
Accordingly, voltages across concrete and reinforcing
members remain small. This leads to the conclusion that
no special bonding techniques are required.
By constraining the lightning injection energy to the core
of the cable, the amount of radiated field is reduced and
induction to adjacent cables, such as RF feeders on
telecommunications towers or data lines, is reduced.
POINT 3. ENSURE A LOW IMPEDANCE
GROUNDING SYSTEM
The importance of ensuring that the grounding system affords a
low earth impedance and not simply a low resistance must be
understood. A spectral study of the energy content associated
with the lightning impulse reveals both a high frequency and low
frequency component. The high frequency is associated with the
extremely fast rising front (typically < 10 s to peak current) of
the lightning impulse while the lower frequency component
resides in the long, high energy, tail or follow-on current in the
impulse. The grounding system appears to the lightning impulse
as a transmission line where wave propagation theory with the
normal rules of reflection and group velocity, apply.
Measurement of earth resistance with conventional low
frequency instruments may not provide results which are
indicative of the earth systems true effectiveness under
lightning discharge conditions. ERICO has developed an
Earth System Analyser in which a fast pulse is injected into
the earth test point to simulate the performance under
lightning impulse conditions. The peak current and voltage
amplitude within an effective measurement interval of
approximately 500ns is measured and used to calculate the
effective impedance.
The magnitude of the current pulse is programmable
according to the local conditions. The measurement window
lies with current pulses of 1 to 5A, 10 to 250V (peak) and
impedance range of 1.5 to 250 ohms. By effectively gating
off any pulses returning to the instrument after about 500ns,the instrument can be used to isolate distant grounds in a
complex system (greater than 75m away), allowing only the
earth-under-test to be measured without the need for
disconnection.
The instrument is also capable of providing repetitive pulses
at 30 second intervals to allow remote tracing of pulse
currents, their magnitude and flow direction.
POINT 4. ELIMINATE EARTH LOOPS
The current associated with a direct strike is typically 30kA
but may be as high as 270kA and exhibits a rise time ofmany thousands of amperes per second. When this current is
discharged through the lightning protection system, the
potential of the local earth system, with respect to the general
mass of the earth, will rise to a high level. The actual
calculation of this earth potential rise depends not only upon
the resistance of the earth grid but because of the high rates
of rise involved, also upon the inductance of the discharge
path.
Ignoring inductive effects, a simple calculation shows that
for a good earth resistance of 1 ohm and a discharge
current of 30kA, the earth potential rise will be 30kV. Given
that this rise can never be entirely eliminated, the aim in anywell designed lightning protection system is to equalise the
potential gradient to ensure that all equipment rises
uniformly in potential. This process is known as Earth
Potential Equalisation or EPE, and is achieved by bonding all
separate grounds points into a common ground system.
In practice, bonding involves connecting together all metallic
masses at a site with suitable conductors to ensure that they
are at the same electrical potential. Once bonded, this
common electrical potential must be matched to that of the
earth mass itself via a suitable connection to the grounding
system. This approach ensures that during a voltage
transient, all equipment within the site will rise and fall
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together as the surge current flows and potentially hazardous
voltages will not develop across the equipment.
Not only does this offer protection to the equipment housed
within, but it also ensures that personnel do not come into
contact with hazardous voltages when touching two pieces of
separate equipment - the touch potential.
Effective bonding design usually entails the adoption of a
single point earthing approach, in which all equipment
within the shelter is connected to a master bus bar which is
in turn bonded to the external grounding system at one point
only. In addition, the respective ground points for all
services, be they AC mains, telephone, data, coax feeders,
control signals or RF cables, should enter the shelter at a
common point - usually via an aluminium gland plate which
is itself also securely bonded to the external grounding
system.
POINT 5. PROTECT SERVICE ENTRY POINTS
OF POWER FEEDERS
This involves the installation of voltage clamping devicescapable of handling the large energy content (kA rating) of
the over voltage surge, as well as reducing the extremely fast
rising edge (dv/dt and di/dt) of this transient.
PRINCIPLES OF POWERLINE PROTECTION
The need for transient protection on power supplies is
becoming more apparent to the general community as failure
of equipment due to power lines transients becomes more
prevalent. This need is recognised by inclusion of new
requirements in modern surge protection standards such as
IEEE/ANSI C62.41, AS1768-1991, IEC 61643, IEC1024 and
BS6651. The question that requires further thought is how
bestto provide point of entry protection against transientswhich are conveyed along power circuits.
Experience has shown that failure of equipment due to
lightning induced surges can be attributed to two basic
mechanisms - over voltages with their excess energy content
and the extremely fast rise times associated with the
lightning impulse. Primary over voltage protection is usually
provided by Metal Oxide Varistors (MOV) or Spark Gap
devices (SG). Secondary protection is provided by low pass
power filtering which serves to reduce the peak let-through
voltage and the dv/dt of the impressed surge.
A number of devices and technologies are available for theprotection of the mains power entering a facility, all of which
require a decision as to the effectiveness offered. Typically
the devices available fall into two broad categories -shunt
protection or series hybrid protection.
SERIES HYBRID (FILTER) VS. SHUNT PROTECTION
Shuntprotection is the most basic form of protection
comprising over-voltage clamping devices which act to divert
the energy from a transient surge down to earth. Series
hybrid protection combines the energy diverting
characteristics of a shunt protector with a low pass filter.
This serves to further reduce the extremely fast rate of rise of
the voltage transient, further reducing the potential for
damage to the equipment. Such devices are usually known as
Power Filters or Surge Reduction Filters (SRF).
Field experience over the last fifteen years has shown that
simple shunt protection is generally adequate for the more
robust types of equipment such as lighting, air conditioning
and motor plants (pumps), but is inadequate at providing a
safe level of protection for equipment using semiconductor
electronics. Where such equipment is connected to the
mains, SRFs must be used to limit both the magnitude and
rate of rise of the voltage transient.
SHUNT PROTECTION
Shunt protection devices are referred to under a variety of
names including Transient Voltage Surge Suppressors
(TVSS), Surge Protective Devices (SPD) or sometimes
simply as Arrestors. They usually employ Metal Oxide
Varistors (MOV), air gaps, Silicon Avalanche Diodes (SAD)
or a combination of these.
The parameters which specify the performance of surge
suppressors are the voltage level to which they will clamp a
typical transient (let-through or residual voltage) and the
maximum peak current that can be diverted (peak kA rating).
The let-through voltage of a device is the maximum clamped
voltage appearing across the device when a surge is diverted.
The most common test waveform used to specify the
transient performance of surge suppressors is the 8/20s
current pulse - Figure 4 and Figure 5. This is usually
generated from a 6kV 1/50s charged capacitor source.
Larger voltages are generally used in the generation of very
large 8/20s pulses. The peak current of the 8/20s
waveform which should be considered for the testing of point
of entry protection devices, is specified in ANSI C62.41 as10kA (Category C3). For critical locations or exposed sites
this figure is often increased.
Figure 4. ANSI C62.41 Category A test pulse - 0.5s 100kHz
open circuit voltage ring wave.
Figure 5. ANSI C62.41 Category B test pulse - 1.2/50s
unidirectional open circuit voltage waveform, and resultant
8/20s current waveform
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In addition to thesingle shotcapability of a surge suppressor,
consideration needs to be given to the effects of multiple
strike lightning. As mentioned earlier, statistical results from
various lightning detection and tracking systems indicate that
over half of all lightning flashes consist of more than one
strike. The subsequent re-strikes follow the same lightning
channel and generally exhibit a faster rate of rise of current
than the initial strike. These multiple strikes are separated by
some tens to hundreds of milliseconds. The effect of such
multi-pulses on surge suppressors is to cause a cumulative
heating effect which is suspected, rather than the energy in
the strike itself, as being the cause of many of the recorded
suppressor failures. At present there is no standard for testing
under multi-pulse effects, but research carried indicates that
Metal Oxide Varistors actually fail under applied multipulse
currents which may only be 75% of the single shot rating of
the device.
ARC GAP DIVERTERS
Air gap arrestors are designed to arc over when transient
over-voltages occur and then extinguish when the transient
has passed. They are generally capable of diverting large
surge currents. The voltage across these devices when an arc
has formed is very low (typically tens of volts). However, the
voltage required to cause the arc to form is high, typically
>3kV The let through voltage seen across such devices is
characterised by a voltage spike reaching several thousands
of volts with a steep leading edge (high dv/dt).
The main problems with the use of such devices stems from
the large voltage required to force them into conduction. For
example, co-ordination problems with secondary protection
can arise where the secondary protection operates at a lower
voltage level than the arc gap arrestors. Also if a transient
occurs which is below the strike voltage of the arc gap, then
no protection is provided.
Extensive research by ERICO Inc. has revealed novel
techniques of pre-triggering spark gaps at voltages well
below their inherent 2kV striking voltage. This research is
on-going with the view to producing shunt protectors with
extremely low let-through voltages (
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T
100kA
30kA
10kA
3kA
40mmMOV
60mmMOV
120kAMOVTEC
1kA
10 100
Number of impulses
Impulse
magnitude
1 1000 10000
Figure 7. Estimate of MOV life - typical MOV specifications
showing the relationship between the magnitude and number
of surges which can be safely diverted
MOV devices are generally rated for the maximum energy
absorption in one impulse, however as illustrated in Figure 7,
a large number of smaller impulses will also cause failure of
the MOV.Increasing the single shot rating of the protective
devices will significantly increase the lifetime for smaller
impulses.
POINT 6. PROTECT SERVICE ENTRY POINTS
OF DATA OR CONTROL LINES
The protection of data or signal line circuits generally
requires a protection circuit capable of extremely fast
activation (to ensure clamping at the typical tens of volts
used in most signalling protocols), whilst at the same time
being able to handle significant surge energies.
For such purposes, a hybrid type circuit employing a number
of different components is usually used. Certain of thesecomponents are able to with stand large amounts of current
but are slow to activate (there by allowing the transient
voltage to rise to levels significantly higher than their
clamping voltage), whilst others are extremely fast (allowing
no overshoot and clamping at the precise voltage required)
but are only able to withstand tiny amounts of energy before
self-destructing.
The following are some of the most commonly encountered
components used in various signal line protection barriers:-
Gas Arresters - These devices are available in either a 2
or 3 leg configuration. They are made of a ceramic tubewhich is filled with an inert gas. When a certain potential
difference exists between any two of the legs, the arrester
fires. The breakdown voltages vary from 70V up to 15kV.
The three leg version enables the surge to be clamped to
ground irrespective of which line the surge was present
on. Gas arresters are comparatively slow to activate (often
several microseconds) so should be used in combination
with faster devices for optimum protection.
Metal Oxide Varistors - MOVs are voltage limiting
devices which clamp the voltage rise of an impressed
surge once the clamping threshold is exceeded. MOVs
are faster than gas arrestors at conducting, however on
their own they are still not fast enough to limit a voltage
transients to a safe level for typical data circuits operating
somewhere in the 5-30V region.
Solid State Devices such as Silicon Avalanche Diodes -
These are special diodes with extremely fast turn on
characteristics, typically in the picosecond domain. This
means that their clamping threshold is extremely well
defined with very little overshoot. They are available in a
range of clamping voltages - 7.5, 12, 15, 30, 36 ... 200V
making them an ideal protection component for data line
protection.
GA are able to handle significantly more energy than SADs
however, as they are slow to turn on, the let-through voltage
rises to well above the turn on threshold and easily exceeds
the safe operating limit of most signal/control circuits. It is
for this reason that a hybrid circuit comprising the above
three components (designated a level 3 protector) is typically
used in the design of data protection units.
With any signal or power transmission system employingtwo lines and a separate earth, two types of transient can
occur:-
ADifferential Mode transient where the voltage surge
appears across the two lines independent of their potential
with respect to earth, and
A Common Mode transient where the voltage surge is
common between each line and earth.
The selection of any data line protector should ensure that
both Common and Differential modes are eliminated.
PROTECTION PRACTICES SPECIFIC TO OIL
AND GAS INSTALLATIONS
Modern pipeline systems incorporate a range of sophisticated
computing, instrumentation and communications equipment.
Unfortunately, pipelines act as very efficient collectors of
lightning energy, exposing this equipment to a high level of
risk of damage due to lightning activity. It is essential,
therefore, that these systems be equipped with adequate
transient protection to ensure the correct operation of the
pipeline system.
Lightning activity and metallurgic effects are particularhazards for long, buried pipelines, producing dangerously
high voltages along the pipe. For the safety of personnel and
for the protection of equipment, it is essential to ensure that
these hazards are safely dissipated to earth.
Experience has shownthat the most extensive damage is
generally sustained by equipment electrically connected to
the pipe. This is to be expected with the pipe acting as a most
efficient collector of lightning energy. The range of pipeline
surge protection devices marketed by ERICO Lightning
Technologies is shown in Error! Reference source not
found.
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Figure 8. Part of ERICO's range of Cathodic
Protection systems
It is common practice to sectionalise pipelines by the
inclusion of insulated joints of the flange or monolithic type.Cathodic Protection (CP) voltage is applied to these pipeline
sections.
At metering and scrubber stations, an earthing system is
generally installed and the out of ground pipe work between
the insulated joints connected to it. Without protection,
induced transient pipeline voltages can easily breakdown the
insulated joint resulting in a permanent low resistance path
to earth. Flange type insulated joints are particularly
susceptible.
There are two common methods of protecting insulated
joints. Special cell can be used as an electrolytic switchblocking voltages in the cathodic protection range while
shunting hazardous voltages to ground.
Such cells require regular maintenance to check electrolyte
levels as well as careful installation to minimise lead
inductance for effective lightning protection.
A second method involves the use of gas arresters electrically
connected across the insulated joints. When a transient
voltage exceeds the breakdown voltage of the arrester, the
gas within the arrester ionises and creates a low impedance
path to shunt the surge energy to ground. The arrester is self
restoring reverting to a virtual open circuit. Providing the
surge rating of the arrester is not exceeded, it will exhibit
almost unlimited life. Figure 9 illustrates an explosion proof
100kA 8/20s insulated joint protector.
Figure 9. IJP unit with surge rating of 100kA and
housed in an explosion proof enclosure.
CATHODIC PROTECTION
Modern switched mode power supplies used for cathodic
protection are electronically controlled and regulated for
maximum efficiency and operational accuracy. The CP
power supply is connected between the pipe and a buriedearth system located perhaps 50-100 metres from the pipe
and known as a ground bed. Input sensing comes from the
pipe and a reference ground electrode.
In an unprotected system surge currents will flow from the
pipe to points of different potentials such as the ground bed
and reference cell. There may also be flash overs within
equipment cubicles caused by the cubicle being connected to
yet another earth, for example the station earth system.
Protection of electronic circuitry requires multistage
protectors with the primary protector being a high energy
absorption device such as a gas arrester.
As mentioned previously, gas arresters are relatively slow to
operate and can allow dangerously high voltages to pass
before fully conducting. For this reason secondary protectors
such as MOVs and pulse rated clamping diodes are typically
used.
All cabling inputs and outputs from electronic CP power
supplies require protection with hybrid devices designed to
divert surge energy to a common ground. This should be the
station earth mat, to which all metalwork should be bonded.
As with all transient protection, the philosophy is to create
an equi-potential plane which will uniformly rise in potential
with respect to true earth.
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Figure 10. Block diagram of a hybrid lightning protection
unit for a dual CP system
Under normal operating conditions, the protector is
transparent to the CPU and electrical isolation between the
ground bed, the reference earth and the station earth mat, is
maintained. In the event of a surge on the pipe, the protector
acts to clamp the potential difference between the anode
ground bed, the two pipe terminals and the reference earth to
levels that will not cause damage to the CPU. A heavy duty
gas arrester is used to divert the transient energy to ground
via the station earth mat. The protector has been designed for
use with CP power supplies rated at 20V/5A.
Installation Considerations
CPU protectors of this kind should be installed with a
minimum of lead length between the unit and the CPU.
Under fast rise time transient conditions, cable inductance
becomes significant and high transient voltages can be
developed across long leads. It is essential that the protector
be grounded to the station earth mat.
Figure 11. Wiring arrangement for the CPU Protector
provided by ERICO.
PIPELINE POTENTIAL CLAMPING
Traditionally, large electrolytic capacitors have been used to
protect pipelines from these hazards. However this method is
not ideal, particularly in areas of high ambient temperature
where the de-ratings applied to capacitors make them
virtually useless.
The Pipeline Potential Clamp (PPC) is connected between
the pipeline and ground. A high energy gas discharge tube
at the front end of the unit protects the pipe and associated
equipment from lightning and other high energy transients,
diverting the energy to ground. Inductive filtering and Metal
Oxide Varistors act as a secondary protection stage,
dampening the fast rise time and keeping voltages to safe
levels. A series of diodes are used to protect the pipe from
AC voltages and telluric effects, clamping the pipe-to-ground
potential to within safe limits.
INTRINSIC SAFETY
The use of lightning protection units in certified hazardous
areas requires some consideration. Providing LPUs or
transient barriers contain no energy storage component and
may be certified as simple apparatus, then they can be
connected in intrinsically safe circuits on the hazardous side
without upsetting the integrity of the IS circuit. Intrinsically
safe barriers incorporating lightning protection on their input
circuitry are now becoming available.
Figure 12. A typical gas pipeline metering station showing
the location of lightning protection units for insulated joints,
cathodic protection and pipe connected transducers.
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GENERIC APPROACH TO THE PROTECTION OF EQUIPMENT
IN HAZARDOUS AREAS
All equipment located within hazardous areas which is
connected to support equipment lying outside the area (ie.
located within the safe area) should be protected using
Intrinsically Safe Barriers (ISB). It is preferable that the
brand of barriers used also incorporate lightning transient
protection. The barriers must be installed outside the
confines of the hazardous area ie. in the safe area - Figure 13
LP E
Figure 13. Typical installation layout of an intrinsically safe
area
Typical equipment requiring protection includes:
Pressure regulators, digital links
Pressure release valves, analogue links
Pressure transducers, 4-20mA links
Flow computers - RS485 links
Insulated flanges
Two options exist for protection of non-certified equipment
(RTU, modem, charger, batteries, telecom lines etc.) needed
to support the instrumentation in the zone 1 area. These
options depend on where the additional equipment is
located:-
Support equipment not certified as intrinsically safe and
located within the zone 1 area, should be installed inside
intrinsically safe enclosures. Feeders from this equipment to
the safe area should be protected with ISB devices.
Support equipment installed outside of the hazardous area,
should also be protected with Universal Transient Barriers
(UTB) to ensure that any impressed currents are maintained
to safe levels.
When installing lightning protection barriers, it is important
to ensure that the total loop resistance of the ISB, UTB and
RTU does not exceed the maximum specified loop resistance.
Equipment installed within the safe area should be
referenced to a single point Safe area Earth System (SES).
All ISBs installed should be referenced to the SES. The SES
should be bonded to the IES where distances permit (
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Appendix
PARTIAL REFERENCE LIST OF VOLATILE STORAGE FACILITIES
PROTECTED BY ERICO INC.
TASMANIA
Berriedale Sewage Plant, Ellis Point,Berriedale 1
Comalco Aluminium Powder Plant,Bell Bay 1
Prince of Wales Sewage Plant, DerwentPark Road 1
Sewerage Treatment Plant, Burnie 1
WESTERN AUSTRALIA
Industrial Plant Kwinana 1
NEW SOUTH WALES
Blue Circle Southern Cement - Berrima Plant 1Clarence Colliery Waste Water
Treatment Tank 1Coal Storage - Hunter Valley 1Griffith City Council -
Sewage Treatment Plant 2Hunter Valley Open Cut Mine 1Hunter Valley Colliery 2000T CoalStorage Bin 1
Hunter Valley Open Cut No.1 1Newvale Colliery 1Sydney Water - North Head Sewage
Treatment Plant 1Tower Colliery - Wilton 1Waterboard -
Rouse Hill Sewerage Treatment Plant 3Westcliff Colliery Borehole No 1 1Westcliff Colliery - Methane Drainage 6Wingeecarribee Shire Council -
Moss Vale Sewage Treatment Plant 3
QUEENSLAND
Airport Treatment Plant 1OK Tedi Mining, PNG 1QCL Cement Silos, Gladstone &Townsville 2Qld. Cement & Lime, Townsville 1Qld. Cement & Lime Silo, Gladstone 1Racecourse Mill-Sugar Refinery, Mackay 1Surge Bins - Abbot Point Coal 2Thallanga Mine Site 1White Mining, North Goonyella Mine 1
INDONESIA
Kaltim Prima Coal 5Kawasan Industri Gresik (Jawa Timur) 3Rumah Sakit Semen Gresik 2Semen Gresik (Persero) Proyek Tuban 2Treatment Centre of Industrial Bureau 1
TAIWAN
Chung Chou Sewerage Treatment Bldg 1
THAILAND
Egat Mah Moh, Lampang 1Rayong Wire, Rayong 3Sane Chemical, Chonburi. 1TGCI Factory, Saraburi 5The Siam Cement Office, Nakornratchasima 1