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E-mail address: [email protected].

SDI Paper Template Version 1.6 Date 11.10.2012 1

2

Lightning Risk Analysis of a Power Microgrid 3

4

R. W. Y. Habash*, V. Groza, T. McNeill, and I. Roberts 5

6 School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa, Ontario, 7

Canada. 8 9 10 .11

ABSTRACT 12

13

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. 14 Keywords: Lightning, risk assessment, protection techniques, microgrid. 15

16

1. INTRODUCTION 17

18 Lightning is an atmospheric arc discharge of a large current which forms as a result of a 19 natural build-up of electrical charge separation in storm clouds where convection and 20 gravitational forces combine with an ample supply of particles to generate differential 21 electrostatic charges. When these charges achieve sufficient strength to overcome the 22 insulating threshold of the local atmosphere then lightning may occur. In thunderstorms, this 23 process results in an accumulation of positive charges towards the top of clouds and an 24 accumulation of negative charges in the cloud base region. The built-up electrical potential is 25 neutralized through an electrical discharge within or between clouds (in-cloud lightning), or 26 between the cloud and ground (cloud-to-ground or CG lightning), which is the most common 27 lightning in what regards to protection of electrical installations such as power plants, 28 substations, and wind turbine systems, is CG lightning [1]. 29

Lightning effects are derived from direct strikes to structures and from the induced 30 voltage caused by the electromagnetic (EM) field associated to the return stroke current [2]. 31 Energy spectrum of the lightning current is very wide; lightning current varies from 2 kA 32


(probability 85 – 90 %) up to 200 kA (probability 0.7–1.0 %) [3]. Peak currents may exceed 33 200 kA with 10/350 µs wave shape [4], but these values are rarely seen. 34

In general, lightning may produce surge currents and over voltages causing isolation 35 breakdown in equipment, dangerous step and touch voltages or ignition processes in 36 presence of flammable materials [5]. If the power equipment is not protected the over-37 voltage will cause burning of insulation. Thus it results into complete shutdown of the power 38 [6]. Also, lightning strikes to power stations may cause several effects in the station vicinities, 39 including the soil potential rise, current and voltage transference through nearby grounded 40 electrical systems, induced voltages [7] on overhead distribution lines. These effects may be 41 transferred to consumer service entrances that are connected to the system. 42

Assessment of the risk of damage due to lightning is a guide that may provide valuable 43 reference to determine the level of lightning protection. In the context of lightning, protection 44 means ensuring that direct lightning strikes are intercepted by protective masts and wires 45 and not by the plant conductors or other equipment. 46

The use of advanced models, suitably implemented into a computer program, is required 47 for the accurate calculation of lightning-induced voltages at different observation points of 48 complex distribution networks such as MGs with different voltage levels [8]. Several 49 publications have already gone into the shielding of high-voltage transmission lines against 50 lightning [9-12], substations [13-15], and transformers [16]. There are despite of similarities, 51 several differences between protection of transmission lines, substations, and transformers 52 as far as exposure to direct strokes is concerned due to nature of various components. 53 However, lightning protection of MGs combines all the above systems and their 54 corresponding techniques. 55

This paper provides an in-depth description of lightning risk analysis and related 56 protection standards. Two simplified lightning preventive techniques are considered in this 57 article: MG and distribution network taking into account the presence of distribution 58 transformers and the surge transfer through transmission lines within the MG environment. 59 60 61

2. STANDARDS FOR LIGHTNING PROTECTION 62

63 Until recently, the International Electrotechnical Commision (IEC) Standards usually adopted 64 for lightning protection was IEC 61024-1 series [17-20] for lightning protection system (LPS), 65 while IEC 61312 series [20-22] for protection against lightning electromagnetic pulse (LEMP) 66 and IEC 61622 TR2 [23] for risk assessment. In 2006, all these standards were substituted 67 by complete set of standards (IEC 62305-1 to 4) [24-27] providing the general principles of 68 protection against lightning, risk management, protection measures against physical 69 damages to structures and life hazard, and protection measures against damages to 70 electrical and electronic systems within structures. These standards provide the general 71 principles to be followed in designing the protection of a structure and services entering the 72 structure. 73

IEC 62305-1 introduces terms and definitions, lightning parameters, damages due to 74 lightning, basic criteria for protection and test parameters to simulate the effect of lightning 75 on lightning protection systems (LPS) components. 76

IEC 62305-2 [25] gives the risk assessment method and its evaluation. It requires a risk 77 assessment to be carried out to determine the characteristics of any lightning protection 78 system to be installed. In order to perform the risk management proposed in [25] the CG 79 lightning frequency per kilometer square and per year is needed. This parameter could be 80 achieved with a network of appropriate sensors connected to a computer which is 81 responsible to validate and record data events. 82

IEC 62305-3 [26] is focused on protection measures to reduce physical damages as 83 well as injuries of living beings due to touch and step voltages. 84


IEC 62305-4 [27] considers the protection against LEMP of electrical and electronic 85 systems within the structures. While, IEC 61643-1 [28] is focused on surge protective 86 devices connected to low-voltage power distribution systems. 87 88 89

3. RISK ANALYSIS 90 91 Lightning risk incorporates three major processes. First is lightning hazard evaluation (LHE) 92 which is based on lightning occurrence frequency, peak values of lightning currents, and 93 energy of lightning. Second is lightning risk assessment (LRA) taking into account 94 calculation of reduction of damage and assessment of lightning damages, their occurrence 95 frequency and reduction of loss or damage. The third process is lightning risk management 96 (LRM) including determination of the best measures to protect human life, services, and 97 equipment. 98

LRA is a tool applied to lightning safety for various structures including power systems. 99 The essentials of risk assessment incorporate LHE including classification of hazards, 100 probabilities of occurrences, and urgency of mitigation actions. LRM is to establish a rational 101 scheme to avoid an unfavorable event. There are two main elements to LRM: detection and 102 prevention. In general, detectors play an important role since it is an integral part of 103 protection. Additional risk management efforts include the use of conventional lightning 104 mitigation techniques. LRM establishes a rational scheme to prevent lightning damage [28]. 105 LRM is a process which consists of LHE, LRA, and LRM. LHE, the first process, is to 106 evaluate the severity of lightning, which differs from region to region, considering not only the 107 number of lightning but also other factors such as the peak values of lightning currents and 108 the energy of lightning strokes. The Iso-keraunic level (IKL) has been used as an index of 109 lightning severity in an area. However, the IKL level does not necessarily coincide with the 110 number of lightning strokes on the ground [28]. Furthermore, in spite of the assumption 111 adopted in the IEC documents, the number of occurrences of damage by lightning of some 112 kind on facilities is not proportional to the number of lightning [29]. In engineering, risk is the 113 anticipated as follows [28]: 114

115 ∑ −××= )1( iii PCNR (1)

where R is the total risk of an object; Ni is the number of damage occurrences of the ith kind; 117 Ci is the loss when the ith damage occurred on the object; Pi is a risk reduction factor, which 118 is 0 if no lightning protection is done and 1 if the perfect lightning protection is carried out. 119

Once LRA is made, the best protection scheme is established by considering the cost of 120 protection schemes. The third phase of LRM is a process to determine the best policy taking 121 the lightning risk, the loss due to damage and the cost of protection schemes into 122 consideration. 123

Total number of damage occurrences in a facility Dt is the sum of the number of damage 124 occurrences by direct lightning Dd, number of damage occurrences to transmission and/or 125 distribution systems Dl, and number of damage occurrences by the induced lightning to 126 distribution lines or low-voltage circuits of the customer facility and overvoltage through 127 grounding systems Dg. The number of each damage occurrence is calculated as follows 128 [28]: 129

130

ggllddt

gldt

PNPNPND

DDDD

×+×+×=

++=

(2)

131 where Nd is the number of direct lightning hits to the system, Nl is the number of induced 132 lightning on the transmission and/or distribution lines, and Ng is the number of lightning that 133


generate an overvoltage on the grounding systems, Pd is the occurrence probability of 134 damage by the direct lightning hits to the systems, Pl is occurrence probability of damage 135 due to induced lightning from transmission and/or distribution lines, and Pg is the occurrence 136 probability of damage due to grounding system. If we let the loss to be L, the lightning risk of 137 a customer facility is obtained as follows: 138 139 LDR tc ×= (3)

In the lightning risk components, the number of lightning is considered to be proportional to 140 the ground flash density of the region. The number of direct lightning hits to distribution 141 systems Nd may be estimated using electro-geometric models such as the Armstrong-142 Whitehead model [29]. It is therefore possible to use these results to estimate the number of 143 direct lightning hits that cause damage on a customer facility. An empirical equation has 144 been derived, relating the density of flash to ground and the number of storms per year, as 145 follows: 146 147 6.1

2.0 TSD = (4)

Where SD is strike density per km2 per year and T represents thunder-storm days per year. 148

According to risk analysis, the level of lightning protection and insulation level (P) of 149 electrical equipment may be determined from [30]: 150

151

R

RP a−= 1

(5)

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

-5). 153

154

155

4. PROTECTION TECHNIQUES 156 157 Lightning can affect facilities including power plants and MGs in two ways, namely direct and 158 indirect strikes. Direct lightning flash strikes part of the power system directly, injecting large 159 impulse currents. The major indirect effect of lightning is the voltage induced on the power 160 system by the rapidly changing magnetic flux associated with the high di/dt of the lightning 161 current. There are various approaches that provide sufficient protection against direct and 162 indirect lightning strikes. However, the purpose of a lightning protection system is to give 163 lightning currents a lower impedance alternative path to ground around the building or object 164 being protected. 165 166

4.1 Air Terminals 167

168 The air terminal concept which is most popular techniques of lightning protection that 169 incorporate sharp Franklin [30] rods, horizontal and vertical conductors (Faraday Cage) 170 evolving into the “Cone of Protection” and the “Rolling Sphere” techniques for design of 171 lightning protection. Such a lightning protection system consists of collectors (air terminals) 172 to intercept lightning strokes, conductors to conduct surge currents to ground, and the earth 173 interface for dissipation of surges to earth. These collector/diverter systems encourage the 174 termination of strikes in close proximity to the “protected” area by providing some form of 175 termination points (collector or air terminals) deployed in a location and manner that actually 176 increases the risk of a strike to that area [31]. 177

There are two basic approaches to providing sufficient protection: lightning masts, at 178 some distance from the MG, with sufficient height to provide an effective cone of protection, 179


and lightning conductors above the MG. Neither can provide absolute protection against 180 lightning strikes; however, the likelihood of a strike attaching to the MG will be decreased by 181 several orders of magnitude if properly designed system is installed. 182

183

4.2 Surge Protective Device (SPD) 184 185 The SPD or surge arrestor is a device that will ideally conduct no current under normal 186 operating voltages (for example, have an extremely high resistance) and conduct current 187 during overvoltage's (i.e. have a small resistance). SPDs are used to limit the surge voltage 188 magnitude to a level that is not damaging to transformers, switchgear or other service 189 entrance equipment [32]. SPDs limit surge voltages by diverting the current from the surge 190 around the insulation of the power system to the ground. There are four different classes of 191 SPDs; station, intermediate, distribution, and secondary. The functions of a lightning arrester 192 are: 1) to act like an open circuit during normal operation of the system, 2) to limit the 193 transient voltage to a safe level with a minimum delay and fitter, and 3) to bring the system 194 back to its normal operation mode when transient voltage is suppressed [32]. Technically, 195 the purpose of installing SPDs is to provide equipotential bonding during transient conditions 196 between live and earthed parts of the electrical system and equipment and therefore to 197 protect it from undesired transient overvoltage and to divert lightning current to the ground. 198 The selection of the SPD depends on the expected lightning current that it should discharge 199 and on the overvoltage category of the equipment that is to be protected. 200

Most of the transformers are protected with surge arresters. The residual voltage of the 201 arrester plays a very important role in protecting the transformers. By selecting arresters with 202 residual voltages as low as possible, a far better protection can be achieved. If the lightning 203 surges are severe, it may even blast the arrestors. Some surges may enter to the distribution 204 transformers from high voltage side to the ground through the tank through oil insulation and 205 consequently reduces the insulation resistance of the transformer. 206 207 208

5. MICROGRID MODEL SIMULATION 209

210 MG is defined as a power system composed of distributed energy resources (DER) that can 211 operate co-ordinately as an electrical generator to provide maximum electrical efficiency with 212 a minimum incidence to loads in the local power grid [33]. MGs operate mostly 213 interconnected to the higher voltage distribution network, but they can also be operated 214 isolated from the main grid, in case of faults in the upstream network. Any electrical 215 generator from different types of energy, renewable or not renewable, can work as a DER if 216 they are integrated as an independent or as a collective unit. A typical MG has the same size 217 as a low voltage distribution feeder and will rarely exceed a capacity of 1 MVA and a 218 geographical span of few hundred meters. Fig. 1 shows a schematic diagram of the 219 proposed MG. 220

MG design involves installing apparatus, protective devices and equipment. In the event 221 of a lightning strike, the expensive equipment such as power transformers and inverters may 222 get severely damaged and slow down the activity of the system. Furthermore, insulation 223 flashovers or outages can occur. Therefore, various techniques are implemented to protect 224 MGs. 225

Computer-simulated models of a MG have been developed to carry out tests in order to 226 obtain data and check the validity of proposed solutions. Models are developed using the 227 graphical environment of MATLAB and Power System Computer-Aided Design (PSCAD) 228 corresponding to the proposed MG environment. 229

230


231 232

Fig. 1. Microgrid components. 233 234 235

5.1 Microgrid Model 236

The University of Ottawa is considering a MG (photovoltaic system on roof top of the Sport 237 Complex (SC) building). Decisions about whether or not this system requires lightning 238 protection should be based upon risk. In this paper, we have developed a LRM plan that 239 may help decide whether lightning protection is warranted. The procedure should include an 240 effective protection scheme that, for the SC building may be expected from a lattice of 241 shielding conductors strung some distance above the SC building. 242

The electro-geometric model [34], a well-known analysis technique used for lightning 243 shielding design has been implemented to design an effective protection scheme using 244 MATLAB. Various equations may be used to calculate the striking distances as shown in 245 Table 1 [35]. 246 247 Table 1. Lightning strike equations for the electro-geometric model 248 249

Model Formula

Love 65.010 sIS =

Darveniza ( )sI

s eIS147.0

1302 −+= Whitehead 67.0

4.9 sIS = Suzuki 78.0

3.3 sIS = Eriksson 74.06.0

67.0 sIhS = Rizk 69.045.0

57.1 sIhS = 250 For the purpose of this simulation, Love’s equation is used, where Is represents the return 251

current of the lightning in kA. When using a shield wire, the protective zone offered by the 252 wire is the arc at the top of the shield wire of radius rs until it intersects the striking distance 253 to ground rg with the center at the intersections, arcs that are created by striking distance to 254 the object to be protected, rc. Any object that is under the arc or in the zone is then 255 protected. The shield zone offered by one shield wire is show in Fig. 2. Values of a and y 256 shown in Fig. 2 are computed using relations in Eq. (6). 257

258


( ) ( )2

22

2yrrhrra gcgc −−−−−=

( )2

222

−−−−−= ahrrrry gccg

gss rkr = ks = 1 (for Love’s equation)

(6)

259

260 261

Fig. 2. Protection provided by a single shield wire for the electro-geometric model. 262 263 264

To compute the protective zone for multiple shield wires, the technique [34] takes into 265 account the number of shield wires and computes the protective region. For various 266 numbers of shield wires, MATLAB program was run and the results are shown in Fig 3 267 shows the protective zone of one shield wire. With one shield wire (Fig. 3a); a length of 268 about 70 m can be protected. The maximum protective height is 20 m, which is the height of 269 the wire. The protective region does not cover the full length of the MG under consideration. 270 In addition, there is only a small volume under the protective zone compared to the size of 271 the plant. Fig. 3b shows the protective zone of two shield wires. With two shield wires, a 272 length of about 150 m can be protected. However, there is a separation in the protective 273 region and a distance of 60 m separates two “sub”-protective regions. Using two shield wires 274 is much better than using one shield wire because the protective region of one shield wire is 275 doubled in this case. However, using two shield wires would not be ideal because there is a 276 gap in the protective zone. As a result, a large portion of the MG remains unprotected. Fig. 277 3c shows the protective zone of three shield wires. In this case, a length of 200 m can be 278 protected which is suitable for the SC building. Unlike the case with two shield wires, there is 279 no separation in the protective zone of three shield wires. Therefore, the protective region of 280 three shield wires is much better than that of two shield wires. However, the height of the 281 protective region is still relatively low. The minimum protective height offered by the 282 protective zone of three shield wires is 5.5 m. The height of the plant was assumed to be 283 around 20 m. Since 5.5 m is much less than 20 m, a large portion of the SC building is still 284 subject to damage from lightning strike. Thus, the protection of the plant should be improved. 285 Fig. 3d shows the protective zone of four shield wires. The protective zone for this case 286 resembles that of the case with three shield wires. However, there is now another dip in the 287 top of the protective region. Once again, the protective zone covers a distance of 200 m, 288 which is sufficient for protecting the length of the SC building. Furthermore, the protective 289 zone of four shield wires is an improvement on the protective zone of three shield wires 290


because the minimum protective height is higher. For the case with four shield wires, the 291 minimum protective height is about 14 m. 292

Therefore, a much larger region in the upper portion of substation can be protected. 293 However, the protective region can be further improved because part of the top of the 294 substation remains unprotected. 295

In conclusion, using two shield wires is much better than using one shield wire because 296 larger protective zone, but this is not ideal because there is a gap in the protective zone. The 297 protective region of three shield wires is much better than that of two shield wires but the 298 height of the protective region is still relatively low. The protective zone of four shield wires is 299 an improvement on the protective zone of three shield wires because the minimum 300 protective height is higher. The case with more shield wires is the best because it will protect 301 the full length of the SC building and the protective zone has the highest minimum protective 302 height. 303

304 305

306

307

308

309 310

311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334

(c) (d) 335

336

337 338

339

340

341

342

Fig. 3. Proposed protective zones. (a) One-shield wire. (b) Two-shield wires. (c) Three-shield wires. (d) Four-shield wires.


5.2 Distribution Transformer Models 343

344 The effects of lightning strikes upon distribution transformers within the MG environment 345 were simulated using PSCAD. In particular, the level of over-voltage at the distribution 346 transformers was investigated in order to detect and avoid voltage levels that would damage 347 these transformers. 348

The lightning discharging model used for these simulations is shown in Fig. 4, where, i0 349 represents lightning current, i represents the current flowing the stricken object, Z0 350 represents the lightning channel surge impedance (usually 300 Ω), and Z represents the 351 impedance between breakdown lightning strike point and the ground. 352

353 354

355 Fig. 4. Lightning discharge model. 356

357 358

Bruce and Godle proposed lightning current waveform double exponential function as 359 shown in equation (7) [36]. The amplitudes of the two exponentials forming the double 360 exponential waveform were positive and negative 21 kA, which represents a low to average 361 lightning stroke current. The rise time of the positive exponential was 1.2 µs, and the fall time 362 of the negative exponential was 50 µs [37]. Because lightning has a very sharp rise time (1.2 363 µs), the energy imposed by lightning influences behaves as high-frequency (HF) energy. 364

365 ( )tt

eekItiβα −− −= 00 )( (7)

366 Where, I0 is the peak of lightning current (generally, kA to hundreds kA), and i0(t) is the 367 instantaneous lightning current, α and β represent wave-head and wave-tail attenuation 368 quotients of lightning current, respectively, and k represents the waveform correction index. 369

In order to simulate the effects of lighting on transformers connected to a MG, a model of 370 these transformers is needed. Models include not only the winding resistance and self-371 inductance but also have ground capacitance, mutual inductive and capacitive coupling 372 between the two winding and the inter-turn capacitances within each winding. As lightning is 373 a HF phenomenon, modeling its effects requires a different transformer model than the 374 traditional non-ideal transformer model for low-frequency (60 Hz) operation. In addition, HF 375 modeling is essential in the design of power transformers to study impulse voltage and 376 switching surge distribution [38]. Three HF transformer models have been implemented in 377 this study as shown in Fig. 5. These models include Pi model, Piantini model, and Model 3 378 [39-41]. 379

In a well-known purely capacitive Pi model (Fig. 5a), the transformer is represented by 380 the capacitances C1 (between primary and earth), C2 (between secondary and earth), and 381 C12 (between primary and secondary). The Piantini model and Model 3 (Fig.5b and Fig.5c) 382 consist of winding impedances, shunt elements, and capacitances within windings. 383


384 (a) 385

386

387 (b) 388

389

390 (c) 391

392 Fig. 5. Transformer models (a): Pi. (b) Piantini. (c) Model 3. 393

394 395 When the lightning discharging model was connected to each of these transformer 396

models the primary voltage of each reached a similar dangerous level of around 6000 kV as 397 seen in Fig. 6. The Pi and Model 3 show primary voltages that attenuate much faster than 398 that of the Piantini model due to coupling to the secondary side as shown in Fig. 7. The 399 Piantini model shows an attenuated and highly oscillatory secondary voltage while the other 400 two models show almost the same voltage as at the primary. The Pi model and Model 3 401 each contain a small capacitor (less than one nanofarad) between the primary and 402 secondary terminals that shorts the primary to the secondary for HF such as those in 403 lightning. Confirming the conclusion of [42], the Piantini model best reflects the observed 404 results of a real transformer. The Pi and Model 3 transformers show primary voltages that 405 attenuate much faster than that of the Piantini model due to coupling to the secondary side 406 as shown in Fig. 7. The Piantini model shows an attenuated and highly oscillatory secondary 407 voltage while the other two models show almost the same voltage as at the primary. 408


409 Fig. 6. Transformer model primary voltages under lightning strike. 410

411

412 (a) 413

414 (b) 415

416 Fig. 7. Secondary voltage under lightning strike. (a) Piantini model. (b) Pi and Model 3. 417

418 419 The overvoltages at the primary side of the transformer were investigated. Each 420

overvoltage waveform shows a spike waveform and then a constant value almost the same 421 as the discharge voltage of the surge arrester. Basing on Fig. 1, transformer A is at 35 kV/10 422 kV and transformer B is at 10 kV/220 V. Transmission lines of various lengths; generator: 3 423 km, load 1: 1 km, load 2: 2 km, and load 3: 5 km. Table 2 shows the load testing results with 424 peak transformer secondary voltage (Vs) and peak load voltage (VL) for a variety of 425 transmission line lengths and types of load before and after installing surge arrestors. The 426 results show that the overvoltage at line terminals is lower than the voltage at secondary 427 terminal of the transformer. It is evident from Table 2 that longer transmission line attenuates 428 the lightning overvoltage and delay reflections. The transformer secondary voltages and the 429 load voltages are low enough to not damage a 10 kV transformer but could harm delicate 430 loads. 431 432


Table 2. Simulation load testing results 433 434

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 2.1

No Load: Non-ideal Transformer 106 189.0 106.0 27.4 106.0 10.1

Resistance (50 Ω): Ideal Transformer 21.7 2.9 15.9 0.55 15.9 0.21

Resistance (50 Ω): Non-ideal Transformer 106 19.0 106.0 2.70 106.0 1.0

Capacitance (0.001 µF): Ideal Transformer 22.6 17.7 15.9 3.9 15.9 1.6

Capacitance (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 0.64

Inductance (0.1 mH): Non-ideal Transformer 106 103.0 106.0 10.4 106.0 3.32

435 436

Assuming a lightning stroke to the primary side of the MG transformer that produces 437 roughly about 6000 kV overvoltage, the proposed solution to this problem is to apply a surge 438 arrestor across the primary in order to suppress overvoltage to safe levels (for example, 75 439 kV). The IEEE model may be adopted for the surge arrester [43]. By using the surge arrester 440 parameters, the secondary voltage will be reduced to 123 kV, as shown in Table 3 and Fig. 441 8, but this is too high and will still damage the transformer. It is better to put a surge arrestor 442 on the secondary side as well, but as Table 3 shows the secondary voltage is too low for the 443 surge arrester to have any effect. Accordingly, an arrester with better current-voltage 444 characteristics would be ideal in order to avoid damages to low-voltage systems. In fact, 445 each application requires an arrester to maintain sufficient operating voltage and a surge 446 current low enough to keep a transformer safe. 447 448 Table 3. Microgrid overvoltages 449 450 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 0.016 451 452

5.3 Distribution Line Model 453

454 There are numerous potential solutions to improve the lightning performance of distribution 455 lines, but none of them provide absolute protection. A shield wire will prevent most of the 456 flashes from striking the phase conductors, but the ground potential rise caused by the 457 current flow through the pole ground impedance will lead to back flashovers in most of the 458 cases. In order to mitigate the effects of direct strikes, the shield wire should not only be 459 grounded at every pole, but the ground resistances should be less than 10 Ω if the critical 460 flashover overvoltage is less than 200 kV [44]. In the case of an unshielded overhead line, 461 an effective protection against direct strokes can be achieved only with the installation of 462 surge arresters on all the phases of every pole [45-47]. 463


464 (a) 465

466 (b) 467

Fig. 8. (a) A microgrid lightning strike. (b) With surge arrester. 468 469

470

6. CONCLUSION 471 472 This article provides an overview of lightning and related protection standards with an 473 approach to incorporate the three major processes of lightning risk: LHE, LRA, and LRM. 474 MATLAB and PSCAD were used to simulate major scenarios of protection for proper LRM in 475 a MG environment. The electro-geometric model has been implemented to design an 476 effective protection scheme using MATLAB. Responses to lightning-induced transients of 477 three transformer models have been analyzed to asses surges transfer. The models were 478 validated with the simulation results using PSCAD. From the results, it is concluded that: (1) 479 placing one or more shielding wires on the rooftop of the MG is an inexpensive yet reliable 480 way to provide lightning protection for a large power installation; and (2) right surge arrester 481 needs to be chosen for each application in order to have sufficient operating voltage and a 482 surge current voltage low enough to keep MG transformers and distribution lines safe. 483

484

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