Lightning Risk Analysis of a Power Microgrid

Transcription

1 SDI Paper Template Version 1.6 Date Lightning Risk Analysis of a Power Microgrid R. W. Y. Habash*, V. Groza, T. McNeill, and I. Roberts School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa, Ontario, Canada. 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 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. 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

2 (probability %) up to 200 ka (probability %) [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. 2. STANDARDS FOR LIGHTNING PROTECTION Until recently, the International Electrotechnical Commision (IEC) Standards usually adopted for lightning protection was IEC series [17-20] for lightning protection system (LPS), while IEC series [20-22] for protection against lightning electromagnetic pulse (LEMP) and IEC TR2 [23] for risk assessment. In 2006, all these standards were substituted by complete set of standards (IEC 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 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 [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 [26] is focused on protection measures to reduce physical damages as well as injuries of living beings due to touch and step voltages.

3 IEC [27] considers the protection against LEMP of electrical and electronic systems within the structures. While, IEC [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 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 C 1 P ) (1) i i ( i where R is the total risk of an object; N i is the number of damage occurrences of the ith kind; C i is the loss when the ith damage occurred on the object; P i 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 D t is the sum of the number of damage occurrences by direct lightning D d, number of damage occurrences to transmission and/or distribution systems D l, 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 D g. The number of each damage occurrence is calculated as follows [28]: D = D t t d D = N d + D + D P d l g + N P + N l 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 that l g P g (2)

4 generate 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 damage due to induced lightning from transmission and/or distribution lines, and P g 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: R = D L (3) c t 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 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 has been derived, relating the density of flash to ground and the number of storms per year, as follows: 1.6 S D = 0.2T (4) Where S D is strike density per km 2 per year and T represents thunder-storm days per year. According to risk analysis, the level of lightning protection and insulation level (P) of electrical equipment may be determined from [30]: P = 1 where R is risk of lightning disaster in the region; R a is the allowed risk ( ). 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, Ra R (5)

5 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 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.

6 Fig. 1. Microgrid components. 5.1 Microgrid Model 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. The electro-geometric model [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 Formula Love S = 10I s Darveniza I S = 2I ( s s e ) Whitehead S = 9.4I s Suzuki S = 3.3Is Eriksson S = 0.67h I s Rizk S = h I 1 s For the purpose of this simulation, Love s equation is used, where I s 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 r s until it intersects the striking distance to ground r g with the center at the intersections, arcs that are created by striking distance to the object to be protected, r c. 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).

7 a = s r 2 c s g 2 2 ( r h) r ( r y) 2 g c g ( r h) y = r r r a g c c g r = k r k s = 1 (for Love s equation) 2 (6) Fig. 2. Protection provided by a single shield wire for the electro-geometric model. To compute the protective zone for multiple shield wires, the technique [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. 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

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

9 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, i 0 represents lightning current, i represents the current flowing the stricken object, Z 0 represents the lightning channel surge impedance (usually 300 Ω), and Z represents the impedance between breakdown lightning strike point and the ground Fig. 4. Lightning discharge model. 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. i 0 ( t) = ki 0 α ( t β e e t ) 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 attenuation quotients of lightning current, respectively, and k represents the waveform correction index. 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 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. (7)

10 (a) (b) (c) Fig. 5. Transformer models (a): Pi. (b) Piantini. (c) Model 3. 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 [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.

11 Fig. 6. Transformer model primary voltages under lightning strike (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 (V L ) 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.

12 Table 2. Simulation load testing results Transferred Voltage (kv) 1 km 30 km 50 km Vs V L Vs V L Vs V L No Load: Ideal Transformer No Load: Non-ideal Transformer Resistance (50 Ω): Ideal Transformer Resistance (50 Ω): Non-ideal Transformer Capacitance (0.001 µf): Ideal Transformer Capacitance (0.001 µf): Non-ideal Transformer Inductance (0.1 mh): Ideal Transformer Inductance (0.1 mh): Non-ideal Transformer 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. Table 3. Microgrid overvoltages kv Vs V L V 1 V 2 V 3 No Arrester With Arrester 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].

13 (a) (b) Fig. 8. (a) A microgrid lightning strike. (b) With surge arrester. 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 surges 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. REFERENCES 1. Glushakow B. Effective lightning protection for wind turbine generators, IEEE Trans. Energy Conv. 2007; 22(1): Silveira FH., Visacro S. Lightning effects in the vicinity of elevated structures, J. Electros. 2007;5-6: Bagdanavicius N., Nakvosas M., Drabatiukas A., Kilius S. Modelling of building lightning protection parameters. Sec. Int. Conf. Advances in Circuits, Electr. Micro- Electr. 2009; Berger K. Novel observations on lightning discharges: results of research on Mount San Salvatore, J. Franklin Institute. 1967;283(6): 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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