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C4-213
CIGRE 2018
Evolution of lightning protection of nuclear power plants: An overview of EDF's experience. P. DUQUERROY, C. TROUILLOUD Electricité de France France
SUMMARY In the last years in France, lightning protection design of NPP has undergone significant evolution. The initial design was based on “state of the art protection” applying general lightning standards to ensure a good protection level. For new projects (EPR), lightning was considered as a climatic hazard. That means that it was demonstrated that the safety functions of the plant may not be affected by lightning, whatever its effect. At each periodic review, this demonstration is applied to the existing NPP. This paper is a general overview of the new practical and scientific challenges in relation with this evolution. We first explain the nuclear regulation context and the consequences on lightning protection and safety analysis. And then, we have broken down the global problem into several elementary problems. For example, we distinguish overvoltages coming from HV network from overvoltages generated by local strikes, and lightning damages from lightning EMI without permanent damage. We also address separately the different types of lightning effects (contact voltages, physical damages, and EMI). At the beginning, we first have to characterise precisely the lightning stress level at local or regional level. For this end, beside scientific literature (Cigré 549), we analysed the last 10 years local data of the French LLS operated by Météorage to address Ng, Nsg, annual and seasonal variations, reference design events (high currents or severe storms) with probability of 10-4. Beyond the design basis level, we also address the extreme lightning and the design of protection against that level. We compared the LLS data with the lightning related incidents on French fleet to improve the understanding of lightning problems and better target the need and design of protection. We also present some numerical simulation methods used in the design process (overvoltage propagation from external HV network, step and touch voltages, inside buildings radiated magnetic field, overvoltages in cables), and the numerical and experimental validation program in agreement with French nuclear Authority. The design of lightning protection has to be first determinist. After the experience of standardized lightning Risk Management of the 15 past years, we also discuss the eventual technical interest of a probabilistic approach. Is it possible and useful to estimate some residual damage probability?
KEYWORDS Lightning, Nuclear Power Plant (NPP), Safety regulation, Modelling, Risk assessment, Probabilistic Safety Assessment (PSA).
[email protected]
1.
INTRODUCTION
In the last years in France, lightning protection design of NPP has undergone significant evolution. In the 70’s, the initial design was based on “state of the art protection” applying general lightning standards to ensure a good protection level. Lightning is now considered as a climatic hazard by the French nuclear regulation. And then, it is demonstrated that the safety functions of the plant may not be affected by lightning. This paper is a general overview of the new practical and scientific challenges in relation with this evolution.
2.
NUCLEAR LIGHTNING REGULATION
INB regulation [1] requires lightning to be taken into account in the safety demonstration. Thus, each effect of lightning is considered, and for each one, it is verified that it cannot affect the environment and that safety functions are always operable. Of course, general lightning standards may be used in this demonstration, but experience shows that they are rarely adapted.
3.
LOCAL LIGHTNING HAZARD CHARACTERIZATION
The comparison between lightning activity and lightning related events has shown clearly that local lightning may not be characterized only with ground flash density, or striking point’s density. It should at least consider the statistic distribution of current because of the great importance of higher currents in lightning hazard on large sites. Finally, we have a good idea of local lightning with 2 parameter: -
Local Nsg: The best way to evaluate local Nsg is to calculate its value with LLS data in the 10 last years. If we calculate the value with the 80 closer impacts, we have an accuracy of ±20%.
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Regional statistic distribution of maximum lightning current of striking points. The best way, if possible, is also to use data from LLS. There are significant differences in the proportion of higher currents depending on the region, and even greater differences with proportion of higher currents obtained from Cigré log-normal distributions. As Cigré log-normal curves tend to zero at infinity, which is not realistic, they may not be used to address high currents and it seems that the overestimation may be already of a factor 2 to 5 at 100 kA, which is not negligible. In reality, the difference is even higher because the LLS curves also overestimate the number of higher currents due to dispersion linked with correlation errors between current estimation and field measurement.
Fig. 1: Percentage of striking points over a lightning current value (kA). (Cigré curve is built with 90% negative first strokes, and 10% positive first strokes).
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Lightning Protection Level (LPL) and extreme values: Lightning protection depends also on lightning electrical parameters (Imax, di/dt, Q, W, shape of lightning current, etc.). The protection level is adapted to the function of the protected structure and equipment: -
Non-nuclear buildings (restaurant, administrative buildings, etc.). No additional requirements due to the location on a NPP. Anyway, they benefit from some site measures, such as the meshed earthing grid or measures against fire (supervision, firemen, storm alarm).
-
Nuclear buildings: Protection is based on electrical parameters of LPL I of standard IEC 62305-1 and rely as far as possible on natural structures. It is called the “basic design”.
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Nuclear safety hardened kernel: These equipment have been defined after the experience of Fukushima. If everything on site would be out of order, this “hardened kernel” are the equipment and structures that should not be damaged and that would be needed to ensure reactor cooling in this extreme situation. These equipment should withstand extreme values of climatic hazards and also extreme lightning, beyond “design basis”. That is why it is called “design extension”.
The choice of extreme lightning parameters for design extension is not so easy. The subject depends on the climate and is still the object of scientific work. The first and most significant parameter is peak lightning current. From our point of view, there are 3 ways to address extreme currents: -
Lightning current measurements
-
LLS data
-
Scientific studies
Lightning current measurements gives minimum values. Depending on the local climate, it is then difficult to evaluate the underestimation of the extreme currents. On the other side, LLS data significantly overestimate the maximum current roughly of about a factor 2, which seems to be the maximum possible estimation error. But this factor is also very difficult to assess because most studies rely on small currents of triggered lightning or instrumented towers. And it seems not so easy to extrapolate this accuracy to rather different phenomena, such as extreme lightning. Finally, there is the Cooray-Rakov estimation [2] of 300kA in temperate regions. We estimated the more realistic value is the scientific estimation of 300kA in temperate regions. And this current is also consistent with measured data and LLS data, taking into account their estimation errors. That is why we based the “design extension” on a maximum current value of 300kA. Climate change: We also have to maintain the installation during their lifetime with an acceptable level of safety, and consequently take into account possible climate evolution. We examined carefully the current knowledge of possible lightning evolution. It seems that several conclusions are contradictory. On one side, temperature rise should make atmosphere more stable with less lightning in average. On the other side, studies between temperature and lightning show a positive correlation, but decreasing with the time period of the studies. For longer time period, it is not sure that there is still a correlation between temperature rise and lightning increase. In the worst case, with a temperature rise of 4°C and a maximum correlation of 12% per °C, it could lead to a maximum of 48% increase. So there is a possibility of an average increase of ground flash density of roughly 0% to 50%. Anyway, even in the worst case, an increase of about 50% on the ground flash density is not really significant. On the 19 French sites, Nsg varies already between 0.2 and 2.7 and it does not affect significantly the lightning risk, in comparison with current distribution, or winter lightning. In this range of variation, Nsg has very little influence on risk, and limited variation of Nsg doesn’t justify to differentiate protection levels on each site and to take into account climate change possible influence.
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4.
CHOICE OF LIGHTNING PROTECTION LEVEL
4.1.
Basic design
The basic design protection level is fixed at LPL I of IEC 62305-1. Depending on collection area, ground flash density and current distributions, LPL I provides a lightning protection that should be exceeded only around once in 10 000 years. 4.1.
Design extension
Beyond basic design, an extreme lightning protection level is defined for equipment required in the event of an accident. Depending on lightning phenomenon, some electrical parameters beyond LPL I have been use in protection studies for extreme lightning flashes, such as: -
300 kA for peak current of first positive impulse
-
700 C for global charge transfer of a lightning flash
-
45 MJ for specific energy of a lightning flash
5.
NPP LIGHTNING PROBLEMS
As illustrated on Figure 2, there are 3 types of possible lightning problems on nuclear power plants: -
1: Strike on external HV connected lines: HV network is well protected against lightning and there are very few events of this type of lightning problems. But they are always analyzed very carefully because the consequences could possibly result in the loss of connection with HV network and consequently in a risk of loss of off-site power (LooP). The residual risk is linked to: The propagation of lightning overvoltages on MV and LV electrical distribution on the site. The protection design is based on insulation co-ordination calculation in worst configurations. The failure of protection (SPD) or insulation between different voltage levels. The analysis of failure modes has to demonstrate that consequences are acceptable.
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2: Strike on buildings on the NPP or on services connected to buildings, or in the vicinity, resulting in Permanent Damages (contact voltages, fire ignition, piercing, failure of equipment, etc.). They are classified in 3 types of lightning effects: Contact voltages (touch and step voltages): We never registered any lightning human injuries on French fleet. And as the few cases each year are almost exclusively wanderer, and that the nuclear site are highly equipotential with strict operating instructions, we can consider that this risk is highly negligible on nuclear sites. Physical damages: We also never registered any type of physical damages on French nuclear sites. And we demonstrated that lightning was a very negligible contribution in the fire ignition risk. Fire is also a “hazard” in nuclear regulation and every case of fire ignition is carefully registered and documented. Numerous cases are registered. Most of the events are only smoke and auto-extinguishing but none of these events were connected with lightning. It was easy to check in comparing the precise time of fire ignition with lightning activity from LLS. None of the fire events was matching with lightning data. And the amount of data was sufficient to be statistically confident about the results. So we can conclude that the natural lightning protection offered by the building structures make the residual risk of lightning fire ignition very negligible in face of other fire sources. Electrical and electronic damages due to overvoltages: In 30 years on 19 NPP sites (58 reactors and about 2000 buildings), we registered 51 events of this type.
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3: Strike on buildings on the NPP or on services connected to buildings, or in the vicinity, resulting in Dysfunction of electrical or electronic systems: EMI (Electro Magnetic Interference) without permanent failure. In 30 years on 19 NPP sites (58 reactors), we registered 226 events of this type.
Fig. 2: NPP lightning problems addressed by different standard comities.
As shown in Figure 2, each one of the 3 types of lightning problems is partially addressed by different standard comities. Consequently, it can be found in general standards some technical elements useful for analysis and modelling, but none of the existing standards addresses today the safety item. In the future, if the lightning protection of NPP was the subject of a specific standard, we think it should be in the scope of the IEC TC45 (nuclear I&C) to address all the different lightning problems and to address properly the difficult issue of the tolerable consequences for the plant. Obviously, lightning experts, or EMC experts alone are not qualified to evaluate what type of events or damages are tolerable or not on NPP. This is fully in the scope of process experts and safety experts under the control of nuclear Authority. The role of lightning experts is also very important in defining adequately accurate input data: local characteristics of lightning, good match and effectiveness of lightning protections, appropriate modelling, etc. That’s what we will speak about in the following…
6.
NPP LIGHTNING PROTECTION, DESIGN AND ASSESSMENT
In this part, we will distinguish the 3 types of lightning effects defined in the IEC 62305: -
Touch and step voltages
-
Physical damages
-
Electrical and electronic failures and dysfunction: In IEC 62305, there are only “electrical and electronic failures” because TC81 deals only with “permanent failures”. We added “dysfunctions” i.e. electromagnetic interferences (EMI) without permanent failure in the scope of TC77 because they are also important in NPP.
6.1.
Touch and step voltages
The protection measures rely mainly on: -
Meshed earthing grid
-
Meshed equipotential bonding (metallic structures, earthing cables, cable ducts, cables trays, shields, etc.
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Operating instructions
The sizing and validation of the earthing grid is based on numerical models. The calculation also evaluate the risk of short-circuits at power frequency (50 Hz), and the risk of lightning strikes. The methodology is based on IEC 60479.
Fig. 3: Step (link) and Touch (right) voltages modelling.
The operating experience (about 2000 reactor.year) reports no lightning injury. If we consider that almost every lightning injured people in a country are outside walkers (golfers, backpackers, etc.) and that a NPP is a highly equipotential site (validated by modelling), we consider that the residual risk is very negligible. 6.2.
Physical damages
The protection measures rely mainly on: -
Natural structures of buildings (metallic structures, or reinforced concrete buildings)
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LPS on weakest structures (e.g. cooling tower) or equipment outside buildings (siren alarm mast).
-
All the protection measures to prevent fire ignition (e.g. operating instructions), to limit fire extension (e.g. fire walls) and fire consequences (e.g. fire men, alarms, etc.).
The protection sizing is based on thermal effects calculation to define adequate protection characteristics (e.g. metal envelop thickness) in connection with lightning characteristics (standardized LPL or lightning characteristics beyond level I). The operating experience in 30 years (about 2000 reactor.year) reports no physical damage. And in the all-time world experience, very few physical damages are reported, and none due to lightning alone. 6.3.
Electrical and electronic systems failures
The protection measures rely mainly on: -
Meshed earthing grid
-
Meshed equipotential bonding (metallic structures, earthing cables, cable ducts, cables trays, shields, etc.
-
SPD on lines connecting important equipment to distant or tall structures.
-
Adapted EMC immunity level of equipment
All the lightning problems (damages and dysfunction) encountered on sites are of this type. The operating experience (about 2000 reactor.year) reports 51 electrical or electronic failures and 226 EMI (dysfunction without damages). EMI are most of time false alarm triggering and sometimes activation of safety procedures that could exceptionally lead to reactor shutdown. The objective of the protection improvement is to lower as much as possible the number of these events and to ensure and demonstrate that it cannot affect the safety strategy of the plant.
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The physical phenomena leading to lightning events are complex and varied. And to reach this goal, we have broken down the global problem into several elementary problems: -
The impulse magnetic field inside structures.
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The propagation of overvoltage coming from lightning strikes on external HV connected lines (adequacy with equipment insulation and analysis of consequences of protection failure or insulation failure).
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Overvoltage in buried cables connected to stroke structures inside the NPP (adequacy with equipment insulation).
We worked on these problems for about 10 years. And we tried to develop, improve and validate numerical models to calculate the electromagnetic stress on equipment related to lightning strikes. 6.3.1. Magnetic field inside building struck by lightning Magnetic field inside structures struck by lightning has been calculated with different software. First models were based on wired models using CDEGS software, based on the method of moments. It solves the equations in frequency domain, and time domain solution are obtained using inverse Fourier transformation. In cooperation with ONERA (the French Aerospace Laboratory), with injected impulse current in a building and noticed the wired model was not accurate enough. So we compared with other models: -
CST MWS solver based on TLM method; with equations solved in time domain.
-
LIRIC consisting in modeling the problem as an equivalent electrical circuit.
The experimental results and the comparison of the 3 models (cf. Fig. 4) were published in XII SIPDA 2013 [7].
Fig. 4: Comparison of numerical models.
These results led to the development of a “thin sheet model” using CST MWS software. The level and the dynamic of calculated magnetic field rise are much closer to experimental measurements [8]. The validated model has finally been transcribed into magnetic field maps depending on the height in the building. The maps are used to check the adequacy between the maximum impulse magnetic field and the immunity of sensitive equipment.
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Fig. 5: Examples of magnetic field maps at different levels.
6.3.2. Overvoltages coming from lightning strikes on HV external connected lines The lightning overvoltages coming from external HV lines propagates on the electrical distribution inside the NPP. They are limited by HV SPD, damped by propagation along cables and through HV/MV transformers. The overvoltages are calculated with EMTP-RV to check the insulation coordination and especially the adequacy of insulation level of safety motors. We have 2 main difficulties to overcome: -
The variation of modeling parameters: It is impossible to model all possible arrangement (motor location, lightning strike location, electrical network configuration, number of connected equipment, etc.). So we tried to find the worst cases through sensitivity analysis. We varied the parameters to study the influence of variations on the overvoltages, and to make the sizing more robust, and not depending on possible variations of electrical network.
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The classical components model (cables, motors, transformers) available in EMTP-RV are not validated for the precise application of lightning frequencies and industrial arrangements. So we are working on development and validation of HF models of nuclear components. For example, EMTP-RV models assume generally buried cables while NPP cables are installed on cable trays. So we evaluated the electrical parameters of cables arrangements per unit length through 3D Maxwell equation solving, and experimental data.
Fig. 6: Cables arrangement in cable tray.
6.3.3. Overvoltages in cables connected to buildings struck by lightning The calculation of overvoltages in cables connected to buildings struck by lightning is a very complicated problem because of the complexity of the installations, and the great number of meshed metallic conductors, cables, bonding, etc. Modelling lightning phenomenon is a great challenge
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because on one side, it is impossible to model 3D arrangement of every metallic conductor, and on the other side, an over simplification shows unacceptable overvoltages everywhere what are obviously not exact through experience analysis. We started to work on this subject in 2006 and presented first results in Cigré 2008.
Fig. 7: Two studied cases presented in 2008.
But this first step raised new questions and led to further developments. We studied also carefully the very interesting and totally different approach of the German standard KTA2206 [9]. This method of overvoltage calculation is dedicated to NPP and it is based on the lightning current division in earth cables, metallic reinforcement of galleries and cables shields. And it takes into account the number of cables connected which is the most influent parameter. The calculation method and its experimental validation are also very well documented through numerous publications [10][11][12][13][14]. The main limits of this method are the fixed hypothesis of the calculations. It supposes some construction rules and some predominance of inductive or resistive coupling depending on the configuration. It is sometimes difficult to use on existing NPP where reality may differ from calculation hypothesis. That is why we continued to work on this subject in the last years with the PhD of Luis Diaz. He developed a new 3D model of structure struck by lightning and connected cables. He also proposed a DoE (Design of Experiment) technique to take into account the variability of modeling parameters. His final report [15] brought recently some new important results and understandings. But it needs complements and experimental validations. And at the moment, the most reliable and validated method still appears to be the standard KTA 2206.
Fig. 8: Modelling configuration proposed by Luis Diaz in 2016.
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Fig. 9: Example of modelling proposed by Luis Diaz in 2016.
7.
ANALYSIS OF LIGHTNING DATA FROM LLS
7.1.
Lightning data
From 2006, EDF is in contract with Météorage, the operator of the national LLS in France. In 2017, we had already 10 complete years of lightning data (2007-2016) on all our 19 nuclear sites. On each site, the data set is collected in a circle of 3.4 km diameter, around the site center. The records of this lightning data base are lightning strokes. And since a few years, Météorage provides also the information to group the strokes using the same canal and striking point, and to group striking points in the same flash. So we are able to distinguish the first stroke in a striking point, and the first striking point in a flash. Finally, the total data set (for 19 sites and 10 years) consists of 4507 flashes, 6388 striking points, and 8873 strokes.
Fig. 10: Lightning strikes around CIVAUX in 2013 (left) and example of a flash with 2 striking points (right).
In 2016 and 2017, we used this lightning data around the sites to study the most accurately as possible local lightning activity and also the correlation with lightning related events.
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We studied the following aspects:
7.2.
-
Annual variation, Seasonal variation (possible cumulative effects with other seasonal climatic hazards),
-
Higher currents and Extreme storms (design reference level and design extension)
-
Local Ng, Nsg, and local current distributions (characterization of local lightning).
-
Lightning activity and lightning characteristics vs lightning related events (better understanding of lightning related events). Local lightning characterization
Around each site, we calculated the stroke density, the striking point’s density Nsg, and the flash density Ng. We estimated the accuracy of the results in agreement with Diendorfer [15] and the standard IEC 62858 [17]. We had enough data (80 flashes) to achieve the specified accuracy of ±20% for Ng around 18 sites in a circle of diameter between 1.2 km and 3.4 km depending on the volume of lightning data. On the 19th site, we only could achieve an accuracy of ±25% for Ng in the total zone (3.4 km diameter) due to the very low ground flash density. We also studied the local aspect of the calculation, in varying the diameter of the calculation zone. As we see on Fig. 11, the accuracy of ±20% (80 flashes) is marked with a red dot. Below this point, on a smaller zone, as expected, we notice that the uncertainty increases rapidly and results are not representative. On a larger zone, the accuracy increases but the ground flash density is less representative of local climate. And finally, the standard accuracy of ±20% with 80 flashes seems to be a good compromise. For example, for Cruas site, the ground flash density Ng is estimated at 1.36 with a confidence interval of [1.13:1.64] and the striking point density Nsg is estimated at 2.04 with a confidence interval of [1.69:2.45].
Fig. 11: Local ground flash density Ng and calculation accuracy (dotted lines) around Cruas NPP in a circle of diameter varying from zero to 3.4 km.
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We studied also the lightning current distributions. We need much more data to draw an acceptable current distribution than to calculate a flash density. And consequently, it was impossible to do at each site, at a very local level. So, we tried to define regional distributions rather than local distributions. We grouped the sites to compare the different distributions: -
All sites (19 French sites as the reference curve)
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Coastal sites (5 sites on the coast with very low Ng: Blayais, Flamanville, Gravelines, Paluel, Penly)
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Inland sites (14 sites: non-coastal sites)
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Plain sites (6 sites on a flat area in the center of France with low Ng: Belleville, Chinon, Civaux, Dampierre, Nogent, St-Laurent)
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Valley of Rhone (3 sites in the south of France with higher Ng: Bugey, Cruas, Tricastin)
For lower value of lightning currents, the distribution is mainly linked with the ground flash density of the region (Fig. 12a). But for higher lightning currents, we noticed differences in the proportion of high currents (Fig. 12b): -
First, the percentage of high currents is significantly higher for coastal sites, and the relative difference increases with current. On Fig. 12b, we notice that there is already 2 times more impacts over 100kA and about 5 times more around 180-200kA.
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Second, for other sites, we did not notice any significant differences between difference regions in France where NPP are installed. On Fig. 12b, all the other distributions seems more or less the same.
(a)
(b)
Fig. 12: Number of striking points per site over a current value in the range of 0-50kA (a) and Percentage of striking points over a current value in the range of 100kA-200kA (b).
Finally, for the 19 French NPP and their local climate, we can characterize lightning activity with 2 parameters: -
The “striking point” density Nsg (with a local value on each site)
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The “striking point” current distribution (with 2 distributions: Coastal and Inland)
Note: it is better to define lightning with “striking point density” than “flash density”, even if some strikes are subsequent strokes with much lower average current values, because this potential problem is well solved through the current distribution.
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7.3.
Lightning activity vs Lightning related problem
We compared the lightning activity as previously defined with lightning related problems. And we distinguished: -
Permanent failures of electrical or electronic systems
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EMI (Electro Magnetic Interference) Dysfunction without permanent failure.
We distinguished also: -
Coastal sites with lower Nsg and higher proportion of high lightning currents.
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Inland sites with higher Nsg and lower proportion of high lightning currents.
The comparison results are given in Table 1. We calculated the average number of lightning damages per site in 30 years, and we noticed that it is a little lower for inland sites than coastal sites (ratio of 0.8). On the other side, the number of striking points (or flashes) is 2.2 times higher. It seems that the ground flash density or striking point density are not good indicators of the risk of permanent failures. Looking at the 3 right columns, we notice that the damages seems rather very well connected to the number of high currents over 150-200kA. This is not so surprising because NPP are usually well protected against classical lightning currents because of the high density of metallic structures and equipotential bonding. Lightning currents is divided into numerous connected lines, metallic trays, structures, etc. and only a small part of the initial current circulates in one single connected line. So, most of time, only a very high lightning current value could endanger equipment insulation. This explanation is also in agreement with standard KTA 2206, and its calculation of overvoltages based on current division. Number of sites
Lightning damages (30years)
Lightning damages /site/30y
Flashes /site/y
Striking Points /site/y
>100kA /zone.10y
>150kA /zone.10y
>200kA /zone.10y
Coastal sites
5
16
3.20
0.57
0.79
3.80
1.40
1.00
Inland sites
14
35
2.50
1.23
1.73
3.64
1.36
0.50
All sites
19
51
2.68
1.06
1.48
3.68
1.37
0.63
0.8
2.2
2.2
1.0
1.0
0.5
Inland/Coastal
Table 1: Comparison between coastal sites and other sites for lightning damages.
We made the same analysis for EMI (dysfunction without permanent failures) and the results are very different (cf. Table 2). This time, we notice that the ratio of lightning related EMI is much closer to the ratio of striking points. And in this case, Nsg seems to be a better indicator of the risk. An explanation could be that EMI is not strongly linked with current value, but more linked with current derivative, or maybe with flash duration or flash multiplicity. The ratio of 1.7 is significantly lower than 2.2 and we tried to find an explanation. And finally, the most likely explanation is the limited influence of higher currents, because even if the current derivative is usually less, the zone of emission is much larger. And it could have a measurable influence.
Coastal sites
Number of sites
EMI (30years)
EMI /site/30years
Flashes /site/y
Striking Points /site/y
>100kA /zone.10y
>150kA /zone.10y
>200kA /zone.10y
5
40
8.0
0.57
0.79
3.80
1.40
1.00
Inland sites
14
186
13.3
1.23
1.73
3.64
1.36
0.50
All sites
19
226
11.9
1.06
1.48
3.68
1.37
0.63
1.7
2,2
2.2
1.0
1.0
0.5
Inland/Coastal
Table 2: Comparison between coastal sites and other sites for EMI (electromagnetic interference without permanent failure).
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We tested the previous result with a seasonal comparison between lightning activity and lightning related events. As it is well known that winter lightning exhibits significantly higher currents (we also noticed it clearly in our seasonal analysis of LLS data), we expected to find more damages in winter with the same number of lightning strikes. On Fig. 13 we compared the seasonal variation of: -
The number of lightning strikes.
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The number of lightning damages with permanent failure for all 19 sites. As there is little data in this category and some important monthly variation, we also proposed a smoothed curve (orange dotted line) to make the figure more readable.
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The number of EMI without permanent failure for all 19 NPP.
To make the comparison more easily, we also normalized all the curves to the same number of events to only focus on seasonal distribution along the year.
Fig. 13: Seasonal variation of permanent damages, EMI and lightning activity.
In winter period, the lightning damages with permanent failures are about 4 times more numerous than expected due to higher lightning current. And on the opposite, in summer, the number of damages is about 60% of the expected value. It means that the winter lightning is about 10 times more destructive. The EMI curve is much closer from the lightning strikes curve. And, just like observed previously on the “coastal/inland” analysis, we still have a small intensification of the risk in winter that can be attributed to a limited influence of higher currents. Finally, the 2 analysis (“coastal/inland” and “seasonal”) are really consistent and it gives more confidence in the conclusions.
8.
INTEREST AND LIMITS OF A PROBABILISTIC APPROACH
All the lightning sizing, studies, modelling presented in this paper are determinist. It means we define relevant lightning characteristics and the safety analysis establishes: -
If systems are vulnerable or not to lightning
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If vulnerable systems can be protected 100% or not
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If exceptional damage is acceptable or not.
So that at the end, all the safety function may not be affected by lightning thanks to their resistance, protection and redundancies.
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In the nuclear field, there is also a probabilistic approaches in Screening and PSA (Probabilistic Safety Assessment). Screening is a process that distinguishes items (ex: climatic hazards) that should be included or excluded from an analysis based on 7 defined criteria. A probability lower than 10-7 is an exclusion criteria. At the moment, the only standardized risk assessment is IEC 62305-2, and once validated in the field of industrial sites, it will remain some difficulties: -
It doesn’t take into account EMI (dysfunction without permanent damages)
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A safety function is spread over several buildings.
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The result is a probability of damage in one building. Experience shows that only very few equipment are affected within the numerous electrical and electronic equipment inside a struck building.
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It doesn’t take into account the influence of lightning current and the number of metallic links to the building
For all these reasons, it seems we are still very far from the evaluation of the fragility curves of safety functions related to lightning characteristics.
9.
CONCLUSIONS
In this paper, we presented the state of EDF’s practices and knowledge on NPP lightning protection, and explained the specificity of nuclear regulation. A specific methodology was developed because standard IEC 62305 is not adapted to a safety demonstration. For personnel safety and physical damages, a very high level of protection (far over LPL I) is obtained with natural structures, and meshed equipotential bonding. No damage has ever been noticed and the residual risk is very negligible. All damages and IEM are linked with electrical and electronic failures and dysfunctions. It seems the permanent failures are linked with current value, and that IEM are more in relation with ground flash density, or lightning parameters correlated with lightning density. We presented also the results of some numerical modelling to improve the protection of electrical equipment. The optimal lightning protection of NPP, and more generally of large sensitive industrial sites, is still a technical and scientific challenge. We worked on this subject for about 20 years and already noticed significant evolutions. The WG Cigré C4.43 is a good opportunity to confront different practices and go to an international agreements. In the next years and decades, we still expect some evolution and improvement of: -
Lightning knowledge (local lightning parameters, extreme values, climate change)
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LLS performances (lightning location, CG and IC discrimination, and electric characteristics evaluation especially for higher currents)
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Understanding of large industrial site lightning events with on-site measurements, LLS data and further analysis.
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Optimization of lightning protection.
BIBLIOGRAPHY [1]
Arrêté du 7 février 2012 fixant les règles générales relatives aux installations nucléaires de base (JORF n°0033 du 8 février 2012 page 2231 texte n°12).
[2]
V.Cooray, V.Rakov (2011): On the upper limits of peak current of first return strokes in negative lightning flashes, Atmospheric Research.
[3]
IEC 61662 (1995-04): Technical Report – Assessment of the risk of damage due to lightning.
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[4]
NFEN 62305-2 (2006-11): Protection against lightning – Part 2: Risk Management.
[5]
Lightning risk analysis on French Nuclear Power Plants (P.Duquerroy, P.Baraton, F.Audran), ICLP 2006 Kanasawa.
[6]
Lightning risk assessment evaluation on French nuclear power plants (P.Duquerroy, C.Miry, P.Seltner) ICLP 2014 Shanghai.
[7]
Evaluation of Lightning Induced Magnetic Fields Inside Reinforced Concrete Buildings (C.Miry, E.Amador, P.Duquerroy, E.Bachelier, D.Prost, F.Issac), XII SIPDA 2013 Belo Horizonte.
[8]
Protection against lightning of reinforced concrete buildings (C.Miry, E.Amador, P.Duquerroy, E.Bachelier, D.Prost, F.Issac), ICLP 2014 Shanghai.
[9]
KTA 2206 (German Standard) Design of nuclear power plants against damaging effects from lightning (2009-11).
[10]
A.Kern, J.Wiesinger, W.Zischank: « Calculation of the longitudinal voltage along metal tubes caused by lightning currents and protection measures » ISH 1991.
[11]
A.Kern, W.Zischank: « The effect of parallel wires on the longitudinal voltage drop along shielded cables » ICLP 1992.
[12]
F.Heidler, W.Zischank, J.Wiesinger, A.Kern, M.Seevers : « Inducted overvoltages in cable ducts taking into account the current flow into earth » ICLP 1998.
[13]
W.Zischank, F.Heidler, J.Wiesinger, A.Kern, M.Seevers : « Shielding effectiveness of reinforced concrete cable ducts carrying partial lightning currents » ICLP 1998.
[14]
W.Zischank, A.Kern, R.Frentzel, F.Heidler, M.Seevers : « Assessment of the lightning transient coupling to control cables interconnecting structures in large industrial facilities and power plants » ICLP 2000.
[15]
Lightning induced voltages In cables of power production centers (L.Diaz PhD Report 2016-11)
[16]
G.Diendorfer: « Some comments on the achievable accuracy of local ground flash density values » ICLP 2008 Uppsala.
[17]
IEC 62858: Lightning density based on lightning location systems (LLS) – General principles (2015-08).
[18]
ASAMPSA_E Report 5: Guidance document – Implementation of lightning hazard in extended PSA (2017).
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