Lightning Risk Analysis of a Power Microgrid

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*Corresponding author: E-mail: [email protected];

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

1School 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.

Received 1st

November 2012 Accepted 21

st December 2012

Published 29th

January 2013

ABSTRACT

Aims: This paper provides an in-depth description of lightning risk analysis and relatedprotection standards as an introductory guideline to alert microgrid (MG) designers andprovide basic understanding of the lightning phenomena as well as designing effectiveprotection techniques.Study Design: Computer-simulated models for protecting MG components have beendeveloped 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 theperiod of January 2011 to January 2012.Methodology: Models are developed using the graphical environment of MATLAB andPSCAD corresponding to a proposed MG at the University of Ottawa campus. As part of lightning risk management, two simplified lightning preventive techniques are considered: aMG and related distribution network taking into account the presence of transformers andthe 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 aninexpensive 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 sufficientoperating voltage and a surge current voltage low enough to keep the MG transformersand distribution lines safe.

Keywords: Lightning; risk assessment; protection techniques; microgrid.

Research Article



British Journal of Applied Science & Technology, 3(1): 107-122, 2013

108

1. INTRODUCTION

Lightning is an atmospheric arc discharge of a large current which forms as a result of anatural build-up of electrical charge separation in storm clouds where convection and

gravitational forces combine with an ample supply of particles to generate differentialelectrostatic charges. When these charges achieve sufficient strength to overcome theinsulating threshold of the local atmosphere then lightning may occur. In thunderstorms, thisprocess results in an accumulation of positive charges towards the top of clouds and anaccumulation of negative charges in the cloud base region. The built-up electrical potential isneutralized 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 commonlightning 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 voltagecaused by the electromagnetic (EM) field associated to the return stroke current [2]. Energyspectrum 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 with10/350 μ s wave shape [4], but these values are rarely seen.

In general, lightning may produce surge currents and over voltages causing isolationbreakdown in equipment, dangerous step and touch voltages or ignition processes inpresence of flammable materials [5]. If the power equipment is not protected the over-voltage 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 groundedelectrical systems, induced voltages [7] on overhead distribution lines. These effects may betransferred 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 valuablereference to determine the level of lightning protection. In the context of lightning, protectionmeans ensuring that direct lightning strikes are intercepted by protective masts and wiresand 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]. Severalpublications have already gone into the shielding of high-voltage transmission lines againstlightning [9-12], substations [13-15], and transformers [16]. There are despite of similarities,several differences between protection of transmission lines, substations, and transformersas 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 protectionstandards. Two simplified lightning preventive techniques are considered in this article: MGand distribution network taking into account the presence of distribution transformers and thesurge 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 adoptedfor 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 substitutedby complete set of standards (IEC 62305-1 to 4) [24-27] providing the general principles of protection against lightning, risk management, protection measures against physicaldamages to structures and life hazard, and protection measures against damages toelectrical and electronic systems within structures. These standards provide the generalprinciples to be followed in designing the protection of a structure and services entering thestructure.

IEC 62305-1 introduces terms and definitions, lightning parameters, damages due tolightning, basic criteria for protection and test parameters to simulate the effect of lightningon lightning protection systems (LPS) components.

IEC 62305-2 [25] gives the risk assessment method and its evaluation. It requires a riskassessment to be carried out to determine the characteristics of any lightning protectionsystem to be installed. In order to perform the risk management proposed in [25] the CGlightning frequency per kilometer square and per year is needed. This parameter could beachieved with a network of appropriate sensors connected to a computer which isresponsible to validate and record data events.

IEC 62305-3 [26] is focused on protection measures to reduce physical damages as well asinjuries of living beings due to touch and step voltages.

IEC 62305-4 [27] considers the protection against LEMP of electrical and electronic systemswithin the structures. While, IEC 61643-1 [28] is focused on surge protective devicesconnected 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, andenergy of lightning. Second is lightning risk assessment (LRA) taking into accountcalculation of reduction of damage and assessment of lightning damages, their occurrencefrequency 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, andequipment.

LRA is a tool applied to lightning safety for various structures including power systems. Theessentials of risk assessment incorporate LHE including classification of hazards,probabilities of occurrences, and urgency of mitigation actions. LRM is to establish a rationalscheme to avoid an unfavorable event. There are two main elements to LRM: detection andprevention. In general, detectors play an important role since it is an integral part of protection. Additional risk management efforts include the use of conventional lightningmitigation 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 toevaluate the severity of lightning, which differs from region to region, considering not only thenumber 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 thenumber of lightning strokes on the ground [28]. Furthermore, in spite of the assumptionadopted 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 theanticipated as follows [28]:

)1( iii P C N R (1)

where R is the total risk of an object; N i is the number of damage occurrences of the i th kind;C i is the loss when the i th damage occurred on the object; P i is a risk reduction factor, whichis 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 takingthe lightning risk, the loss due to damage and the cost of protection schemes intoconsideration.

Total number of damage occurrences in a facility Dt is the sum of the number of damageoccurrences by direct lightning Dd , number of damage occurrences to transmission and/or distribution systems Dl , and number of damage occurrences by the induced lightning todistribution lines or low-voltage circuits of the customer facility and overvoltage throughgrounding systems Dg . The number of each damage occurrence is calculated as follows[28]:

g g l l d d t

g l d t

P N P N P N D

D D D D

(2)

where N d is the number of direct lightning hits to the system, N l is the number of induced

lightning on the transmission and/or distribution lines, and N g is the number of lightning thatgenerate an overvoltage on the grounding systems, P d is the occurrence probability of damage by the direct lightning hits to the systems, P l is occurrence probability of damagedue to induced lightning from transmission and/or distribution lines, and P g is the occurrenceprobability 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:

L D R t c (3)

In the lightning risk components, the number of lightning is considered to be proportional tothe ground flash density of the region. The number of direct lightning hits to distributionsystems N d may be estimated using electro-geometric models such as the Armstrong-Whitehead 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 hasbeen derived, relating the density of flash to ground and the number of storms per year, asfollows:

6.12.0 T S D (4)

Where SD is strike density per km2

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]:

R

R

P a

1 (5)

where R is risk of lightning disaster in the region; R a is the allowed risk (1×10-5

).

4. PROTECTION TECHNIQUES

Lightning can affect facilities including power plants and MGs in two ways, namely direct andindirect strikes. Direct lightning flash strikes part of the power system directly, injecting largeimpulse 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 lightningcurrent. There are various approaches that provide sufficient protection against direct andindirect lightning strikes. However, the purpose of a lightning protection system is to givelightning currents a lower impedance alternative path to ground around the building or objectbeing protected.

4.1 Air Terminals

The air terminal concept which is most popular techniques of lightning protection thatincorporate 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 earthinterface for dissipation of surges to earth. These collector/diverter systems encourage thetermination 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 actuallyincreases the risk of a strike to that area [31].

There are two basic approaches to providing sufficient protection: lightning masts, at somedistance from the MG, with sufficient height to provide an effective cone of protection, andlightning conductors above the MG. Neither can provide absolute protection against lightningstrikes; however, the likelihood of a strike attaching to the MG will be decreased by severalorders 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 normaloperating voltages (for example, have an extremely high resistance) and conduct currentduring overvoltage's (i.e. have a small resistance). SPDs are used to limit the surge voltagemagnitude 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 surgearound 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 thetransient voltage to a safe level with a minimum delay and fitter, and 3) to bring the systemback 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 toprotect 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 dischargeand 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 thearrester plays a very important role in protecting the transformers. By selecting arresters withresidual voltages as low as possible, a far better protection can be achieved. If the lightningsurges are severe, it may even blast the arrestors. Some surges may enter to the distributiontransformers from high voltage side to the ground through the tank through oil insulation andconsequently 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 canoperate co-ordinately as an electrical generator to provide maximum electrical efficiency witha 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 operatedisolated from the main grid, in case of faults in the upstream network. Any electricalgenerator 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 sizeas a low voltage distribution feeder and will rarely exceed a capacity of 1 MVA and ageographical span of few hundred meters. Fig. 1 shows a schematic diagram of theproposed MG.

MG design involves installing apparatus, protective devices and equipment. In the event of alightning strike, the expensive equipment such as power transformers and inverters may getseverely damaged and slow down the activity of the system. Furthermore, insulationflashovers or outages can occur. Therefore, various techniques are implemented to protectMGs.

Computer-simulated models of a MG have been developed to carry out tests in order toobtain data and check the validity of proposed solutions. Models are developed using thegraphical 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 SportComplex (SC) building). Decisions about whether or not this system requires lightningprotection should be based upon risk. In this paper, we have developed a LRM plan thatmay help decide whether lightning protection is warranted. The procedure should include aneffective 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 usedfor lightning shielding design has been implemented to design an effective protectionscheme using MATLAB. Various equations may be used to calculate the striking distancesas shown in Table 1 [35].

Table 1. Lightning strike equations for the electro-geometric model

Model Formula

Love 65.010 s I S

Darveniza s I

s e I S 147.0

1302

Whitehead 67.04.9 s I S

Suzuki 78.03.3 s I S

Eriksson 74.06.067.0 s I hS

Rizk 69.045.057.1 s I hS

For the purpose of this simulation, Love’s equation is used, where I s represents the returncurrent of the lightning in kA. When using a shield wire, the protective zone offered by thewire is the arc at the top of the shield wire of radius r s until it intersects the striking distanceto ground r g with the center at the intersections, arcs that are created by striking distance tothe object to be protected, r c . Any object that is under the arc or in the zone is thenprotected. 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).

2

22

2 yr r hr r a g c g c

2

222

ahr r r r y g cc g

g s s r k r k s = 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 Fig3 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, alength 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) Three-shield 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 wirebecause the protective region of one shield wire is doubled in this case. However, using twoshield 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 threeshield wires. In this case, a length of 200 m can be protected which is suitable for the SCbuilding. Unlike the case with two shield wires, there is no separation in the protective zoneof 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 relativelylow. The minimum protective height offered by the protective zone of three shield wires is 5.5m. The height of the plant was assumed to be around 20 m. Since 5.5 m is much less than20 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 shieldwires. However, there is now another dip in the top of the protective region. Once again, theprotective zone covers a distance of 200 m, which is sufficient for protecting the length of theSC 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 thesubstation remains unprotected.

In conclusion, using two shield wires is much better than using one shield wire becauselarger protective zone, but this is not ideal because there is a gap in the protective zone. Theprotective region of three shield wires is much better than that of two shield wires but theheight of the protective region is still relatively low. The protective zone of four shield wires isan improvement on the protective zone of three shield wires because the minimumprotective height is higher. The case with more shield wires is the best because it will protectthe full length of the SC building and the protective zone has the highest minimum protectiveheight.

5.2 Distribution Transformer Models

The effects of lightning strikes upon distribution transformers within the MG environmentwere simulated using PSCAD. In particular, the level of over-voltage at the distributiontransformers was investigated in order to detect and avoid voltage levels that would damagethese transformers.

The lightning discharging model used for these simulations is shown in Fig. 4, where, i 0represents lightning current, i represents the current flowing the stricken object, Z 0represents the lightning channel surge impedance (usually 300 Ω), and Z represents theimpedance between breakdown lightning strike point and the ground.

Bruce and Godle proposed lightning current waveform double exponential function as shownin equation (7) [36]. The amplitudes of the two exponentials forming the double exponentialwaveform were positive and negative 21 kA, which represents a low to average lightningstroke current. The rise time of the positive exponential was 1.2 µs, and the fall time of thenegative 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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t t eekI t i

00 )( (7)

Where, I 0 is the peak of lightning current (generally, kA to hundreds kA), and i 0(t ) is the

instantaneous lightning current, α and β represent wave-head and wave-tail attenuationquotients 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 self-inductance but also have ground capacitance, mutual inductive and capacitive couplingbetween the two winding and the inter-turn capacitances within each winding. As lightning isa HF phenomenon, modeling its effects requires a different transformer model than thetraditional non-ideal transformer model for low-frequency (60 Hz) operation. In addition, HFmodeling is essential in the design of power transformers to study impulse voltage andswitching surge distribution [38]. Three HF transformer models have been implemented inthis 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 thecapacitances C 1 (between primary and earth), C 2 (between secondary and earth), and C 12

(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 modelsthe primary voltage of each reached a similar dangerous level of around 6000 kV as seen inFig. 6. The Pi and Model 3 show primary voltages that attenuate much faster than that of thePiantini model due to coupling to the secondary side as shown in Fig. 7. The Piantini modelshows an attenuated and highly oscillatory secondary voltage while the other two modelsshow almost the same voltage as at the primary. The Pi model and Model 3 each contain asmall 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 theconclusion of Yu et al. [42], the Piantini model best reflects the observed results of a realtransformer. The Pi and Model 3 transformers show primary voltages that attenuate muchfaster 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 whilethe 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 overvoltagewaveform shows a spike waveform and then a constant value almost the same as thedischarge voltage of the surge arrester. Basing on Fig. 1, transformer A is at 35 kV/10 kVand 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 withpeak 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. Theresults show that the overvoltage at line terminals is lower than the voltage at secondaryterminal of the transformer. It is evident from Table 2 that longer transmission line attenuatesthe lightning overvoltage and delay reflections. The transformer secondary voltages and theload voltages are low enough to not damage a 10 kV transformer but could harm delicateloads.

Assuming a lightning stroke to the primary side of the MG transformer that produces roughlyabout 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). TheIEEE 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 thesurge arrester to have any effect. Accordingly, an arrester with better current-voltagecharacteristics 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 surgecurrent low enough to keep a transformer safe.




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Table 2. Simulation load testing results

Transferred Voltage (kV)

1 km 30 km 50 km

Vs VL Vs VL Vs VL

No Load: Ideal Transformer 21.2 29.6 15.9 5.4 15.9 2.1No Load: Non-ideal Transformer 106 189.0 106.0 27.4 106.0 10.1Resistance (50 Ω): Ideal Transformer 21.7 2.9 15.9 0.55 15.9 0.21Resistance (50 Ω): Non-ideal Transformer 106 19.0 106.0 2.70 106.0 1.0Capacitance (0.001 µF): Ideal Transformer 22.6 17.7 15.9 3.9 15.9 1.6Capacitance (0.001 µF): Non-ideal Transformer 106 96.0 106.0 27.3 106.0 10.1

Inductance (0.1 mH): Ideal Transformer 26.5 19.0 15.9 1.9 15.9 0.64Inductance (0.1 mH): Non-ideal Transformer 106 103.0 106.0 10.4 106.0 3.32

Table 3. Microgrid overvoltages

kV Vs VL V1 V2 V3

No Arrester 6146 3.3 0.508 0.606 0.534

With Arrester 123 0.099 0.015 0.018 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 distributionlines, 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 thecurrent flow through the pole ground impedance will lead to back flashovers in most of thecases. In order to mitigate the effects of direct strikes, the shield wire should not only begrounded at every pole, but the ground resistances should be less than 10 Ω if the criticalflashover 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 anapproach 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 aneffective protection scheme using MATLAB. Responses to lightning-induced transients of three transformer models have been analyzed to asses surge transfer. The models werevalidated 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 reliableway 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 asurge 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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4. Berger K. Novel observations on lightning discharges: results of research on MountSan Salvatore, J. Franklin Institute. 1967;283(6):478-525.

5. Gallego LE, Duarte O, Torres H, Vargas M, Montaña J, Pérez E, Herrera J, Younes C.Lightning risk assessment using fuzzy logic, J. Electrostatics. 2004;2-4:233-9.

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