British Journal of Applied Science & Technology
3(1): 107-122, 2013
SCIENCEDOMAIN international
www.sciencedomain.org
Lightning Risk Analysis of a Power Microgrid
R. W. Y. Habash1*, V. Groza1, T. McNeill1 and I. Roberts1
1
School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa,
Ontario, Canada.
Authors’ contributions
This work was carried out in collaboration between all authors. Authors RWYH and VG
designed the study, performed the statistical analysis, wrote the protocol, and wrote the first
draft of the manuscript. Authors TM and IR managed the analyses of the study. Author
RWYH managed the literature searches. All authors read and approved the final manuscript.
st
Research Article
Received 1 November 2012
st
Accepted 21 December 2012
th
Published 29 January 2013
ABSTRACT
Aims: This paper provides an in-depth description of lightning risk analysis and related
protection standards as an introductory guideline to alert microgrid (MG) designers and
provide basic understanding of the lightning phenomena as well as designing effective
protection techniques.
Study Design: Computer-simulated models for protecting MG components have been
developed in order to obtain data and check the validity of the proposed solutions.
Place and Duration: This study was carried out in Ottawa, Ontario, Canada during the
period of January 2011 to January 2012.
Methodology: Models are developed using the graphical environment of MATLAB and
PSCAD corresponding to a proposed MG at the University of Ottawa campus. As part of
lightning risk management, two simplified lightning preventive techniques are considered: a
MG and related distribution network taking into account the presence of transformers and
the surge transfer through transmission lines within the MG environment.
Conclusion: It is concluded that: (1) placing one or more shielding wires on a rooftop is an
inexpensive yet reliable way to provide lightning protection for a MG environment; and (2)
right surge arrester needs to be chosen for each application in order to have sufficient
operating voltage and a surge current voltage low enough to keep the MG transformers
and distribution lines safe.
Keywords: Lightning; risk assessment; protection techniques; microgrid.
____________________________________________________________________________________________
*Corresponding author: E-mail: rhabash@site.uottawa.ca;
British Journal of Applied Science & Technology, 3(1): 107-122, 2013
1. INTRODUCTION
Lightning is an atmospheric arc discharge of a large current which forms as a result of a
natural build-up of electrical charge separation in storm clouds where convection and
gravitational forces combine with an ample supply of particles to generate differential
electrostatic charges. When these charges achieve sufficient strength to overcome the
insulating threshold of the local atmosphere then lightning may occur. In thunderstorms, this
process results in an accumulation of positive charges towards the top of clouds and an
accumulation of negative charges in the cloud base region. The built-up electrical potential is
neutralized through an electrical discharge within or between clouds (in-cloud lightning), or
between the cloud and ground (cloud-to-ground or CG lightning), which is the most common
lightning in what regards to protection of electrical installations such as power plants,
substations, and wind turbine systems, is CG lightning [1].
Lightning effects are derived from direct strikes to structures and from the induced voltage
caused by the electromagnetic (EM) field associated to the return stroke current [2]. Energy
spectrum of the lightning current is very wide; lightning current varies from 2 kA (probability
85 – 90%) up to 200 kA (probability 0.7–1.0%) [3]. Peak currents may exceed 200 kA with
10/350 μs wave shape [4], but these values are rarely seen.
In general, lightning may produce surge currents and over voltages causing isolation
breakdown in equipment, dangerous step and touch voltages or ignition processes in
presence of flammable materials [5]. If the power equipment is not protected the overvoltage will cause burning of insulation. Thus it results into complete shutdown of the power
[6]. Also, lightning strikes to power stations may cause several effects in the station vicinities,
including the soil potential rise, current and voltage transference through nearby grounded
electrical systems, induced voltages [7] on overhead distribution lines. These effects may be
transferred to consumer service entrances that are connected to the system.
Assessment of the risk of damage due to lightning is a guide that may provide valuable
reference to determine the level of lightning protection. In the context of lightning, protection
means ensuring that direct lightning strikes are intercepted by protective masts and wires
and not by the plant conductors or other equipment.
The use of advanced models, suitably implemented into a computer program, is required for
the accurate calculation of lightning-induced voltages at different observation points of
complex distribution networks such as MGs with different voltage levels [8]. Several
publications have already gone into the shielding of high-voltage transmission lines against
lightning [9-12], substations [13-15], and transformers [16]. There are despite of similarities,
several differences between protection of transmission lines, substations, and transformers
as far as exposure to direct strokes is concerned due to nature of various components.
However, lightning protection of MGs combines all the above systems and their
corresponding techniques.
This paper provides an in-depth description of lightning risk analysis and related protection
standards. Two simplified lightning preventive techniques are considered in this article: MG
and distribution network taking into account the presence of distribution transformers and the
surge transfer through transmission lines within the MG environment.
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2. STANDARDS FOR LIGHTNING PROTECTION
Until recently, the International Electrotechnical Commision (IEC) Standards usually adopted
for lightning protection was IEC 61024-1 series [17-20] for lightning protection system (LPS),
while IEC 61312 series [20-22] for protection against lightning electromagnetic pulse (LEMP)
and IEC 61622 TR2 [23] for risk assessment. In 2006, all these standards were substituted
by complete set of standards (IEC 62305-1 to 4) [24-27] providing the general principles of
protection against lightning, risk management, protection measures against physical
damages to structures and life hazard, and protection measures against damages to
electrical and electronic systems within structures. These standards provide the general
principles to be followed in designing the protection of a structure and services entering the
structure.
IEC 62305-1 introduces terms and definitions, lightning parameters, damages due to
lightning, basic criteria for protection and test parameters to simulate the effect of lightning
on lightning protection systems (LPS) components.
IEC 62305-2 [25] gives the risk assessment method and its evaluation. It requires a risk
assessment to be carried out to determine the characteristics of any lightning protection
system to be installed. In order to perform the risk management proposed in [25] the CG
lightning frequency per kilometer square and per year is needed. This parameter could be
achieved with a network of appropriate sensors connected to a computer which is
responsible to validate and record data events.
IEC 62305-3 [26] is focused on protection measures to reduce physical damages as well as
injuries of living beings due to touch and step voltages.
IEC 62305-4 [27] considers the protection against LEMP of electrical and electronic systems
within the structures. While, IEC 61643-1 [28] is focused on surge protective devices
connected to low-voltage power distribution systems.
3. RISK ANALYSIS
Lightning risk incorporates three major processes. First is lightning hazard evaluation (LHE)
which is based on lightning occurrence frequency, peak values of lightning currents, and
energy of lightning. Second is lightning risk assessment (LRA) taking into account
calculation of reduction of damage and assessment of lightning damages, their occurrence
frequency and reduction of loss or damage. The third process is lightning risk management
(LRM) including determination of the best measures to protect human life, services, and
equipment.
LRA is a tool applied to lightning safety for various structures including power systems. The
essentials of risk assessment incorporate LHE including classification of hazards,
probabilities of occurrences, and urgency of mitigation actions. LRM is to establish a rational
scheme to avoid an unfavorable event. There are two main elements to LRM: detection and
prevention. In general, detectors play an important role since it is an integral part of
protection. Additional risk management efforts include the use of conventional lightning
mitigation techniques. LRM establishes a rational scheme to prevent lightning damage [28].
LRM is a process which consists of LHE, LRA, and LRM. LHE, the first process, is to
evaluate the severity of lightning, which differs from region to region, considering not only the
number of lightning but also other factors such as the peak values of lightning currents and
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the energy of lightning strokes. The Iso-keraunic level (IKL) has been used as an index of
lightning severity in an area. However, the IKL level does not necessarily coincide with the
number of lightning strokes on the ground [28]. Furthermore, in spite of the assumption
adopted in the IEC documents, the number of occurrences of damage by lightning of some
kind on facilities is not proportional to the number of lightning [29]. In engineering, risk is the
anticipated as follows [28]:
R   N i  Ci  (1  Pi )
(1)
where R is the total risk of an object; Ni is the number of damage occurrences of the ith kind;
Ci is the loss when the ith damage occurred on the object; Pi is a risk reduction factor, which
is 0 if no lightning protection is done and 1 if the perfect lightning protection is carried out.
Once LRA is made, the best protection scheme is established by considering the cost of
protection schemes. The third phase of LRM is a process to determine the best policy taking
the lightning risk, the loss due to damage and the cost of protection schemes into
consideration.
Total number of damage occurrences in a facility Dt is the sum of the number of damage
occurrences by direct lightning Dd, number of damage occurrences to transmission and/or
distribution systems Dl, and number of damage occurrences by the induced lightning to
distribution lines or low-voltage circuits of the customer facility and overvoltage through
grounding systems Dg. The number of each damage occurrence is calculated as follows
[28]:
Dt  Dd  Dl  Dg
Dt  N d  Pd  N l  Pl  N g  Pg
(2)
where Nd is the number of direct lightning hits to the system, Nl is the number of induced
lightning on the transmission and/or distribution lines, and Ng is the number of lightning that
generate an overvoltage on the grounding systems, Pd is the occurrence probability of
damage by the direct lightning hits to the systems, Pl is occurrence probability of damage
due to induced lightning from transmission and/or distribution lines, and Pg is the occurrence
probability of damage due to grounding system. If we let the loss to be L, the lightning risk of
a customer facility is obtained as follows:
Rc  Dt  L
(3)
In the lightning risk components, the number of lightning is considered to be proportional to
the ground flash density of the region. The number of direct lightning hits to distribution
systems Nd may be estimated using electro-geometric models such as the ArmstrongWhitehead model [29]. It is therefore possible to use these results to estimate the number of
direct lightning hits that cause damage on a customer facility. An empirical equation has
been derived, relating the density of flash to ground and the number of storms per year, as
follows:
S D  0.2T 1.6
(4)
2
Where SD is strike density per km per year and T represents thunder-storm days per year.
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According to risk analysis, the level of lightning protection and insulation level (P) of
electrical equipment may be determined from [30]:
P  1
Ra
R
(5)
-5
where R is risk of lightning disaster in the region; Ra is the allowed risk (1×10 ).
4. PROTECTION TECHNIQUES
Lightning can affect facilities including power plants and MGs in two ways, namely direct and
indirect strikes. Direct lightning flash strikes part of the power system directly, injecting large
impulse currents. The major indirect effect of lightning is the voltage induced on the power
system by the rapidly changing magnetic flux associated with the high di/dt of the lightning
current. There are various approaches that provide sufficient protection against direct and
indirect lightning strikes. However, the purpose of a lightning protection system is to give
lightning currents a lower impedance alternative path to ground around the building or object
being protected.
4.1 Air Terminals
The air terminal concept which is most popular techniques of lightning protection that
incorporate sharp Franklin [30] rods, horizontal and vertical conductors (Faraday Cage)
evolving into the “Cone of Protection” and the “Rolling Sphere” techniques for design of
lightning protection. Such a lightning protection system consists of collectors (air terminals)
to intercept lightning strokes, conductors to conduct surge currents to ground, and the earth
interface for dissipation of surges to earth. These collector/diverter systems encourage the
termination of strikes in close proximity to the “protected” area by providing some form of
termination points (collector or air terminals) deployed in a location and manner that actually
increases the risk of a strike to that area [31].
There are two basic approaches to providing sufficient protection: lightning masts, at some
distance from the MG, with sufficient height to provide an effective cone of protection, and
lightning conductors above the MG. Neither can provide absolute protection against lightning
strikes; however, the likelihood of a strike attaching to the MG will be decreased by several
orders of magnitude if properly designed system is installed.
4.2 Surge Protective Device (SPD)
The SPD or surge arrestor is a device that will ideally conduct no current under normal
operating voltages (for example, have an extremely high resistance) and conduct current
during overvoltage's (i.e. have a small resistance). SPDs are used to limit the surge voltage
magnitude to a level that is not damaging to transformers, switchgear or other service
entrance equipment [32]. SPDs limit surge voltages by diverting the current from the surge
around the insulation of the power system to the ground. There are four different classes of
SPDs; station, intermediate, distribution, and secondary. The functions of a lightning arrester
are: 1) to act like an open circuit during normal operation of the system, 2) to limit the
transient voltage to a safe level with a minimum delay and fitter, and 3) to bring the system
back to its normal operation mode when transient voltage is suppressed [32]. Technically,
the purpose of installing SPDs is to provide equipotential bonding during transient conditions
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between live and earthed parts of the electrical system and equipment and therefore to
protect it from undesired transient overvoltage and to divert lightning current to the ground.
The selection of the SPD depends on the expected lightning current that it should discharge
and on the overvoltage category of the equipment that is to be protected.
Most of the transformers are protected with surge arresters. The residual voltage of the
arrester plays a very important role in protecting the transformers. By selecting arresters with
residual voltages as low as possible, a far better protection can be achieved. If the lightning
surges are severe, it may even blast the arrestors. Some surges may enter to the distribution
transformers from high voltage side to the ground through the tank through oil insulation and
consequently reduces the insulation resistance of the transformer.
5. MICROGRID MODEL SIMULATION
MG is defined as a power system composed of distributed energy resources (DER) that can
operate co-ordinately as an electrical generator to provide maximum electrical efficiency with
a minimum incidence to loads in the local power grid [33]. MGs operate mostly
interconnected to the higher voltage distribution network, but they can also be operated
isolated from the main grid, in case of faults in the upstream network. Any electrical
generator from different types of energy, renewable or not renewable, can work as a DER if
they are integrated as an independent or as a collective unit. A typical MG has the same size
as a low voltage distribution feeder and will rarely exceed a capacity of 1 MVA and a
geographical span of few hundred meters. Fig. 1 shows a schematic diagram of the
proposed MG.
MG design involves installing apparatus, protective devices and equipment. In the event of a
lightning strike, the expensive equipment such as power transformers and inverters may get
severely damaged and slow down the activity of the system. Furthermore, insulation
flashovers or outages can occur. Therefore, various techniques are implemented to protect
MGs.
Computer-simulated models of a MG have been developed to carry out tests in order to
obtain data and check the validity of proposed solutions. Models are developed using the
graphical environment of MATLAB and Power System Computer-Aided Design (PSCAD)
corresponding to the proposed MG environment.
The University of Ottawa is considering a MG (photovoltaic system on roof top of the Sport
Complex (SC) building). Decisions about whether or not this system requires lightning
protection should be based upon risk. In this paper, we have developed a LRM plan that
may help decide whether lightning protection is warranted. The procedure should include an
effective protection scheme that, for the SC building may be expected from a lattice of
shielding conductors strung some distance above the SC building.
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Fig. 1. Microgrid components
5.1 Microgrid Model
The electro-geometric model Aliabad and Vahidi [34], a well-known analysis technique used
for lightning shielding design has been implemented to design an effective protection
scheme using MATLAB. Various equations may be used to calculate the striking distances
as shown in Table 1 [35].
Table 1. Lightning strike equations for the electro-geometric model
Model
Love
Formula
Darveniza
S  2 I s  30 1  e 0.147 I s
S  10 I s0.65
Whitehead
S  9 .4 I
Suzuki


0.67
s
S  3.3I s0.78
Eriksson
S  0.67h 0.6 I s0.74
Rizk
S  1.57 h 0.45 I s0.69
For the purpose of this simulation, Love’s equation is used, where Is represents the return
current of the lightning in kA. When using a shield wire, the protective zone offered by the
wire is the arc at the top of the shield wire of radius rs until it intersects the striking distance
to ground rg with the center at the intersections, arcs that are created by striking distance to
the object to be protected, rc. Any object that is under the arc or in the zone is then
protected. The shield zone offered by one shield wire is show in Fig. 2. Values of a and y
shown in Fig. 2 are computed using relations in Eq. (6).
a  rc2  rg  h   rc2  rg  y 
2
2
2
2
y  rg  rc2   rc2  rg  h   a 


rs  k s rg ks = 1 (for Love’s equation)
(6)
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To compute the protective zone for multiple shield wires, the technique Aliabad and Vahidi
[34] takes into account the number of shield wires and computes the protective region. For
various numbers of shield wires, MATLAB program was run and the results are shown in Fig
3 shows the protective zone of one shield wire. With one shield wire (Fig. 3a); a length of
about 70 m can be protected. The maximum protective height is 20 m, which is the height of
the wire. The protective region does not cover the full length of the MG under consideration.
In addition, there is only a small volume under the protective zone compared to the size of
the plant. Fig. 3b shows the protective zone of two shield wires. With two shield wires, a
length of about 150 m can be protected.
Fig. 2. Protection provided by a single shield wire for the electro-geometric model.
(c)
(d)
Fig. 3. Proposed protective zones. (a) One-shield wire, (b) Two-shield wires, (c) Threeshield wires, (d) Four-shield wires.
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However, there is a separation in the protective region and a distance of 60 m separates two
“sub”-protective regions. Using two shield wires is much better than using one shield wire
because the protective region of one shield wire is doubled in this case. However, using two
shield wires would not be ideal because there is a gap in the protective zone. As a result, a
large portion of the MG remains unprotected. Fig. 3c shows the protective zone of three
shield wires. In this case, a length of 200 m can be protected which is suitable for the SC
building. Unlike the case with two shield wires, there is no separation in the protective zone
of three shield wires. Therefore, the protective region of three shield wires is much better
than that of two shield wires. However, the height of the protective region is still relatively
low. The minimum protective height offered by the protective zone of three shield wires is 5.5
m. The height of the plant was assumed to be around 20 m. Since 5.5 m is much less than
20 m, a large portion of the SC building is still subject to damage from lightning strike. Thus,
the protection of the plant should be improved. Fig. 3d shows the protective zone of four
shield wires. The protective zone for this case resembles that of the case with three shield
wires. However, there is now another dip in the top of the protective region. Once again, the
protective zone covers a distance of 200 m, which is sufficient for protecting the length of the
SC building. Furthermore, the protective zone of four shield wires is an improvement on the
protective zone of three shield wires because the minimum protective height is higher. For
the case with four shield wires, the minimum protective height is about 14 m.
Therefore, a much larger region in the upper portion of substation can be protected.
However, the protective region can be further improved because part of the top of the
substation remains unprotected.
In conclusion, using two shield wires is much better than using one shield wire because
larger protective zone, but this is not ideal because there is a gap in the protective zone. The
protective region of three shield wires is much better than that of two shield wires but the
height of the protective region is still relatively low. The protective zone of four shield wires is
an improvement on the protective zone of three shield wires because the minimum
protective height is higher. The case with more shield wires is the best because it will protect
the full length of the SC building and the protective zone has the highest minimum protective
height.
5.2 Distribution Transformer Models
The effects of lightning strikes upon distribution transformers within the MG environment
were simulated using PSCAD. In particular, the level of over-voltage at the distribution
transformers was investigated in order to detect and avoid voltage levels that would damage
these transformers.
The lightning discharging model used for these simulations is shown in Fig. 4, where, i0
represents lightning current, i represents the current flowing the stricken object, Z0
represents the lightning channel surge impedance (usually 300 Ω), and Z represents the
impedance between breakdown lightning strike point and the ground.
Bruce and Godle proposed lightning current waveform double exponential function as shown
in equation (7) [36]. The amplitudes of the two exponentials forming the double exponential
waveform were positive and negative 21 kA, which represents a low to average lightning
stroke current. The rise time of the positive exponential was 1.2 µs, and the fall time of the
negative exponential was 50 µs [37]. Because lightning has a very sharp rise time (1.2 µs),
the energy imposed by lightning influences behaves as high-frequency (HF) energy.
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
i0 (t )  kI 0 e t  e  t

(7)
Where, I0 is the peak of lightning current (generally, kA to hundreds kA), and i0(t) is the
instantaneous lightning current, α and β represent wave-head and wave-tail attenuation
quotients of lightning current, respectively, and k represents the waveform correction index.
Fig. 4. Lightning discharge model.
In order to simulate the effects of lighting on transformers connected to a MG, a model of
these transformers is needed. Models include not only the winding resistance and selfinductance but also have ground capacitance, mutual inductive and capacitive coupling
between the two winding and the inter-turn capacitances within each winding. As lightning is
a HF phenomenon, modeling its effects requires a different transformer model than the
traditional non-ideal transformer model for low-frequency (60 Hz) operation. In addition, HF
modeling is essential in the design of power transformers to study impulse voltage and
switching surge distribution [38]. Three HF transformer models have been implemented in
this study as shown in Fig. 5. These models include Pi model, Piantini model, and Model 3
[39-41].
In a well-known purely capacitive Pi model (Fig. 5a), the transformer is represented by the
capacitances C1 (between primary and earth), C2 (between secondary and earth), and C12
(between primary and secondary). The Piantini model and Model 3 (Fig.5b and Fig.5c)
consist of winding impedances, shunt elements, and capacitances within windings.
When the lightning discharging model was connected to each of these transformer models
the primary voltage of each reached a similar dangerous level of around 6000 kV as seen in
Fig. 6. The Pi and Model 3 show primary voltages that attenuate much faster than that of the
Piantini model due to coupling to the secondary side as shown in Fig. 7. The Piantini model
shows an attenuated and highly oscillatory secondary voltage while the other two models
show almost the same voltage as at the primary. The Pi model and Model 3 each contain a
small capacitor (less than one nanofarad) between the primary and secondary terminals that
shorts the primary to the secondary for HF such as those in lightning. Confirming the
conclusion of Yu et al. [42], the Piantini model best reflects the observed results of a real
transformer. The Pi and Model 3 transformers show primary voltages that attenuate much
faster than that of the Piantini model due to coupling to the secondary side as shown in Fig.
7. The Piantini model shows an attenuated and highly oscillatory secondary voltage while
the other two models show almost the same voltage as at the primary.
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(a)
(b)
(c)
Fig. 5. Transformer models (a): Pi. (b) Piantini. (c) Model 3.
Fig. 6. Transformer model primary voltages under lightning strike.
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(a)
(b)
Fig. 7. Secondary voltage under lightning strike. (a) Piantini model, (b) Pi and Model 3.
The overvoltages at the primary side of the transformer were investigated. Each overvoltage
waveform shows a spike waveform and then a constant value almost the same as the
discharge voltage of the surge arrester. Basing on Fig. 1, transformer A is at 35 kV/10 kV
and transformer B is at 10 kV/220 V. Transmission lines of various lengths; generator: 3 km,
load 1: 1 km, load 2: 2 km, and load 3: 5 km. Table 2 shows the load testing results with
peak transformer secondary voltage (Vs) and peak load voltage (VL) for a variety of
transmission line lengths and types of load before and after installing surge arrestors. The
results show that the overvoltage at line terminals is lower than the voltage at secondary
terminal of the transformer. It is evident from Table 2 that longer transmission line attenuates
the lightning overvoltage and delay reflections. The transformer secondary voltages and the
load voltages are low enough to not damage a 10 kV transformer but could harm delicate
loads.
Assuming a lightning stroke to the primary side of the MG transformer that produces roughly
about 6000 kV overvoltage, the proposed solution to this problem is to apply a surge arrestor
across the primary in order to suppress overvoltage to safe levels (for example, 75 kV). The
IEEE model may be adopted for the surge arrester [43]. By using the surge arrester
parameters, the secondary voltage will be reduced to 123 kV, as shown in Table 3 and Fig.
8, but this is too high and will still damage the transformer. It is better to put a surge arrestor
on the secondary side as well, but as Table 3 shows the secondary voltage is too low for the
surge arrester to have any effect. Accordingly, an arrester with better current-voltage
characteristics would be ideal in order to avoid damages to low-voltage systems. In fact,
each application requires an arrester to maintain sufficient operating voltage and a surge
current low enough to keep a transformer safe.
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Table 2. Simulation load testing results
Transferred Voltage (kV)
1 km
30 km
Vs
VL
Vs
No Load: Ideal Transformer
21.2
29.6
15.9
No Load: Non-ideal Transformer
106
189.0
106.0
Resistance (50 Ω): Ideal Transformer
21.7
2.9
15.9
Resistance (50 Ω): Non-ideal Transformer
106
19.0
106.0
Capacitance (0.001 µF): Ideal Transformer
22.6
17.7
15.9
Capacitance (0.001 µF): Non-ideal Transformer
106
96.0
106.0
Inductance (0.1 mH): Ideal Transformer
26.5
19.0
15.9
Inductance (0.1 mH): Non-ideal Transformer
106
103.0
106.0
VL
50 km
Vs
VL
5.4
15.9
2.1
27.4
106.0
10.1
0.55
15.9
0.21
2.70
106.0
1.0
3.9
15.9
1.6
27.3
106.0
10.1
1.9
15.9
0.64
10.4
106.0
3.32
Table 3. Microgrid overvoltages
kV
No Arrester
With Arrester
Vs
6146
123
VL
3.3
0.099
V1
0.508
0.015
V2
0.606
0.018
V3
0.534
0.016
(a)
(b)
Fig. 8. (a) A microgrid lightning strike. (b) With surge arrester.
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5.3 Distribution Line Model
There are numerous potential solutions to improve the lightning performance of distribution
lines, but none of them provide absolute protection. A shield wire will prevent most of the
flashes from striking the phase conductors, but the ground potential rise caused by the
current flow through the pole ground impedance will lead to back flashovers in most of the
cases. In order to mitigate the effects of direct strikes, the shield wire should not only be
grounded at every pole, but the ground resistances should be less than 10 Ω if the critical
flashover overvoltage is less than 200 kV [44]. In the case of an unshielded overhead line,
an effective protection against direct strokes can be achieved only with the installation of
surge arresters on all the phases of every pole [45-47].
6. CONCLUSION
This article provides an overview of lightning and related protection standards with an
approach to incorporate the three major processes of lightning risk: LHE, LRA, and LRM.
MATLAB and PSCAD were used to simulate major scenarios of protection for proper LRM in
a MG environment. The electro-geometric model has been implemented to design an
effective protection scheme using MATLAB. Responses to lightning-induced transients of
three transformer models have been analyzed to asses surge transfer. The models were
validated with the simulation results using PSCAD. From the results, it is concluded that: (1)
placing one or more shielding wires on the rooftop of the MG is an inexpensive yet reliable
way to provide lightning protection for a large power installation; and (2) right surge arrester
needs to be chosen for each application in order to have sufficient operating voltage and a
surge current voltage low enough to keep MG transformers and distribution lines safe.
COMPETING INTERESTS
Authors have declared that no competing interests exist.
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© 2013 Habash et al.; This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.
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