Surv Geophys (2012) 33:973–989DOI 10.1007/s10712-012-9191-1Long-Term Power Transmission Failuresin Southeastern Brazil and...
974                                                            Surv Geophys (2012) 33:973–989coupling on the external geop...
Surv Geophys (2012) 33:973–989                                                           9752005; Pirjola 2005; Huttunen e...
976                                                           Surv Geophys (2012) 33:973–989with changing global cloud cov...
Surv Geophys (2012) 33:973–989                                                                              977Fig. 1 The ...
978                                                                  Surv Geophys (2012) 33:973–989                       ...
Surv Geophys (2012) 33:973–989                                                                979Fig. 3 Monthly averages o...
980                                                                   Surv Geophys (2012) 33:973–989         (a)         (...
Surv Geophys (2012) 33:973–989                                                                    981correlation was maint...
982                                                                     Surv Geophys (2012) 33:973–989Fig. 7 a Scatter dia...
Surv Geophys (2012) 33:973–989                                                                           983Fig. 8 Monthly...
984                                                                 Surv Geophys (2012) 33:973–989example. The daily trend...
Surv Geophys (2012) 33:973–989                                                            9854 DiscussionThe ISA.CTEEP 138...
986                                                                      Surv Geophys (2012) 33:973–989Fig. 11 The global ...
Surv Geophys (2012) 33:973–989                                                            987Fig. 12 The tropospheric     ...
988                                                                     Surv Geophys (2012) 33:973–989Friis-Christensen 19...
Surv Geophys (2012) 33:973–989                                                                          989Lanzerotti LJ (...
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Long term power transmission failures in southeastern brazil and the geophysical environment


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Long term power transmission failures in southeastern brazil and the geophysical environment

  1. 1. Surv Geophys (2012) 33:973–989DOI 10.1007/s10712-012-9191-1Long-Term Power Transmission Failuresin Southeastern Brazil and the Geophysical EnvironmentMagda A. S. Duro • Pierre Kaufmann • Fernando C. P. Bertoni • ´Emilio C. N. Rodrigues • Jose Pissolato FilhoReceived: 16 November 2011 / Accepted: 19 March 2012 / Published online: 21 April 2012Ó Springer Science+Business Media B.V. 2012Abstract High-voltage transmission networks represent a large electrical circuit justabove the ground subjected to a number of transient overcharges of various kinds, some ofwhich may lead to failures. Some failures might be related to anomalies of the geophysicalenvironment. We have analyzed one unprecedented long series of transmission grid fail- ˜ures (9 years) on high-voltage networks located in Sao Paulo state, southeastern Brazil,from 1998 to 2006, which includes an important fraction of the past solar activity cycle 23.Ninety-five distinct failure causes were given by the power line operator to explain thetransmission grid shut downs. Most failures were attributed to atmospheric discharges,corresponding to 1,957 failures out of a total of 4,572 for the whole period at 138 kV, and170 out of 763 at 440 kV, respectively. They correspond to less than one ten thousandth ofthe actual number of atmospheric discharges recorded in the same area, demonstrating thegrid’s high resilience to breakdowns due to lightning. A clear concentration of failures inthe region’s thunderstorm season has been found. A significant 67 and 77 % reduction inthe number of failure rates per year has been found for the 138 and 440 kV grids,respectively, for the period studied, in good correspondence with the decay in the sunspotnumbers. No obvious correlation was found between power failures and the planetaryindex of geomagnetic activity or major geomagnetic storms in the period, either on short oron long time scales. Assuming that the dependence of the electrosphere/ionosphere-groundM. A. S. Duro ˜Escola de Engenharia, Universidade Presbiteriana Mackenzie, Sao Paulo, BrazilP. Kaufmann (&) Á F. C. P. Bertoni ˜Escola de Engenharia, CRAAM, Universidade Presbiteriana Mackenzie, Sao Paulo, Brazile-mail: kaufmann@craam.mackenzie.brP. KaufmannCentro de Componentes Semicondutores, Universidade Estadual de Campinas, Campinas, SP, BrazilE. C. N. Rodrigues ˜ ´ ´ISA.CTEEP, Companhia de Transmissao de Energia Eletrica Paulista, Jundiaı, SP, BrazilJ. P. Filho ´ ¸˜Faculdade de Engenharia Eletrica e Computacao, Universidade Estadual de Campinas, Campinas, SP,Brazil 123
  2. 2. 974 Surv Geophys (2012) 33:973–989coupling on the external geophysical environment plays a major role in explaining thereduction in power failures as the solar cycle wanes, it is suggested that the increase inatmosphere conductivity caused by the larger cosmic ray flux then reduces the thresholdvoltage required to produce lightning strokes, so reducing their effectiveness in disruptinghigh-voltage power lines.Keywords Space weather Á Power transmission failures Á Atmospheric discharges ÁGeomagnetic storms Á Electrosphere Á Atmosphere conductivity Á Solar activity ÁSolar cycle1 IntroductionHigh-voltage electrical power transmission network systems are known to be vulnerable tovarious kinds of environmental perturbations (see Hoyt and Schatten 1997; Pirjola 2007;Thomson et al. 2010, and references therein). However, the quality of electricity trans-mission and distribution systems also depends on various other factors such as the man-agement of the high-voltage lines, the quality of replacement equipment, the skill ofoperational maintenance people, etc. Unpredictable disruptions are a serious concern forthe transmission network operators. It is well known that disturbances to the physical regimes in outer space, also known asspace weather, directly influence the upper atmosphere and may also impact the engi-neering systems on the Earth’s surface (Boteler et al. 1998; Lanzerotti et al. 1999; Hoyt andSchatten 1997; Pirjola et al. 2000; Thomson et al. 2010). However, power line interactionswith the geophysical environment regime are poorly known. One reason for this is the lackof long-term data sets concerning power transmission failures. Electrical power companiesoften do not make their records available, to avoid consumer claims for financial com-pensation. The absence of open data sources and restrictions to having access to networkgrid industries represent a problem to those who try to advance our understanding of theimpact of geophysical disturbances on electricity distribution systems (Thomson et al.2010). Another difficulty is that, even when such access is made possible, most powertransmission or distribution networks are physically modified, or technically improved, onrelatively short time scales (much shorter than a solar cycle period), so producing non-uniform and biased data samples. The Earth’s surface is somewhat shielded from the radiation present in space by theupper atmosphere that becomes ionized by solar EUV radiation and by the terrestrialmagnetosphere. This regime is disturbed with increasing solar activity, which enhancesionization in the ionosphere, and by increasing fluxes of charged particles penetrating theatmosphere at high latitudes and in the polar regions. Geophysical effects on the iono-sphere and magnetosphere have been described in numerous studies (see, for example,Kivelson and Russell 1995; Abdu et al. 2006; Eastwood 2008, and references therein).These disturbances have noticeable effects on engineering systems on the terrestrial sur-face (Hoyt and Schatten 1997; Lanzerotti 1983, 2001; Lanzerotti et al. 1999; Thomsonet al. 2007, 2010). Most studies on the influence of solar activity on terrestrial technological systems arerelated to geomagnetic disturbances. The latter generate geomagnetically induced currents,GICs, which are believed to be the principal transient anomaly influencing technologicalsystems such as power line transmission networks and pipelines (Hoyt and Schatten 1997;Boteler et al. 1998; Pirjola et al. 2000; Molinski et al. 2000; Molinski 2002; Kappenman123
  3. 3. Surv Geophys (2012) 33:973–989 9752005; Pirjola 2005; Huttunen et al. 2008). Boteler et al. (1998) presented a review ondocumented GIC-related disturbances in electrical systems on the ground over 150 years,suggesting that they were clustered in the years of peak solar activity (i.e., at times ofmaximum sunspot numbers, and when geomagnetic indices increase and geomagneticstorms become more frequent). This study, however, does not contain grid failures recorded continuously on a dailybasis for many years. One famous example of the impact that GICs have on power lines hasbeen for the March 13, 1989 large geomagnetic storm which caused the electric power‘‘blackout’’ over Quebec, Canada. A total of 83 % of the system was re-established after9 h, causing a loss of 25,000 MW in energy generation (Barnes et al. 1991; Stauning 2002;Bolduc 2002). One long period (1999–2005) comparison of the distribution of days through the yearwith large geomagnetic-induced currents, obtained from pipeline measurements at highlatitudes in Europe, suggests a decrease in GICs as the yearly solar sunspot numberdecreases, with the exception of the year of 2003. That is likely to be related to the‘‘Halloween’’ (October–December) highly disturbed period (Huttunen et al. 2008). How-ever, there are no studies on the failures of electrical power transmission systems that aremonitored continuously, although there are a number of studies that show single highlysuggestive examples of a one-to-one GIC association with geomagnetic storms (see, forexample, Kappenman 2005; Trivedi et al. 2007; Watari et al. 2009). Despite the infamous1989 Quebec electrical power failure, there are no obvious direct correlations between theoccurrence of GICs and actual power grid failures; a review of the effects of GICs on gridsin Finland indicates that the probability of a power failure caused by GICs is one in20 years (Elovaara 2007). In a recent review, Thomson et al (2010) summarized the principal ‘‘ten well known’’and ‘‘ten unknown’’ facts about GIC risks to power grids. Among the unknown facts, itshould be stressed the need for a better knowledge of solar event signatures and by-products in the interplanetary medium which exert a great influence on the Earth. On theother hand, there are only a few sparse reports on systematic analyses of actual gridfailures, irrespectively of their causes, obtained over a long period of time, on a givennetwork (Hoyt and Schatten 1997). For such studies, data for networks that remainunchanged for a sufficiently large number of years to provide a uniform data set should becompared with solar activity or with geophysical environment disturbances. These con-ditions are seldom available. Geomagnetic disturbances are not the only geophysical cause that impacts technologicalsystems on the ground. The significance of the ionosphere (as a part of the global atmo-spheric electric circuit) and its influence on technological systems has not been muchstudied. Equivalent circuits have been proposed to describe the complex global circuit(Rycroft et al. 2000; Harrison 2004; Tinsley and Yu 2004; Aplin et al. 2008). In a sim-plified equivalent electrical circuit, the fair weather regions of the electrosphere may berepresented by a resistance and a capacitor in parallel, with areas with thunderstorms actingas current generators. The above-quoted authors have shown that nearly 95 % of the ionosphere-to-groundresistance is below 10 km, and so stressing the importance of the troposphere in the globalcircuit. Certain changes in the conductivity of the troposphere might arise from externalinfluences on the upper atmosphere (Harrison 2004; Rycroft 2006). Cosmic ray fluxincreases as the solar cycle decreases are known to enhance the atmosphere conductivity,affecting cloud electricity regimes (Rycroft et al. 2000; Stozhkov et al. 2001a, b; Stozhkov2003; Rycroft 2006). An increase in atmospheric conductivity has been also associated 123
  4. 4. 976 Surv Geophys (2012) 33:973–989with changing global cloud coverage over the planet and to the increased occurrence oflightning (Svensmark and Friis-Christensen 1997; Stozhkov 2003). Such changes in thetroposphere might have a significant influence on technological systems on the ground. In this study, we have analyzed the first long and uninterrupted series data on disrup-tions of high-voltage transmission networks in the southeastern Brazil. The data wereconsistently reported for almost 9 years (January 1998–October 2006), that is, for themajority of the solar cycle 23 of solar activity. ˜2 Electrical Transmission Failures in Sao Paulo State, Brazil ˜ ´The ISA.CTEEP (Companhia de Transmissao de Energia Eletrica Paulista) is the principalprivate electric transmission company in Brazil. It accounts for 30 % of all energy pro-duced in the country and for 60 % of the energy consumed in southeastern Brazil.ISA.CTEEP has provided us with a complete report on power transmission failuresrecorded from January 1, 1998 to October 16, 2006, for all nine high-voltage transmission ˜networks operated at different voltages in Sao Paulo State. The nine grids are listed inTable 1. Figure 1 shows only the two longest grids selected for this study, at 138 and ˜440 kV, on a Sao Paulo State map; the other seven grids were not considered here becausethey are considerably shorter than the two extended grids. Figure 2 is a larger map, ˜showing the location of Sao Paulo State with respect to South America and to the SouthAmerican Geomagnetic Anomaly (SAGA), described by contours of total magnetic fieldstrength (NOAA 2010). The ISA.CTEEP power company reported 95 different failure causes during this period.Most failures are attributed to atmospheric discharges, 1,957 events out of a total of 4,572for the whole period for the 138 kV grid, and 170 out of 763 for the 440 kV grid. Theremaining failures, 2,615 at the 138 kV grid and 593 at the 440 kV grid, were distributedon 94 other different reported failure causes. The larger group corresponds to failures dueto ‘‘unknown causes.’’ They correspond to 868 failures (19 %) at the 138 kV grid and to 55(7.2 %) at 440 kV grid. However, according to the company personnel such failuresattributed to ‘‘unknown causes’’ are often derived from distinct identification criteria. Forexample, a failure cause labeled as ‘‘unknown’’ might be classified in another categoryafter a deeper investigation. In other words, the number of failures assigned to unknowncauses depends on how carefully searches were made for the causes. Such a search dependson the number of people hired to perform this job, which was different for different years.Table 1 ISA.CTEEP high-volt- Voltage (kV) Total extent (km)age transmission networks 20 50.00 34.5 25.00 69 1377.60 88 2828.70 138 10625.40 230 1589.80 345 422.10 440 7294.50 460 42.80123
  5. 5. Surv Geophys (2012) 33:973–989 977Fig. 1 The ISA.CTEEP 138 kV (black lines) and 440 kV (gray lines) high-voltage transmission networksover Sao Paulo State, Brazil. The geographic coordinates of Sao Paulo city are 23o 320 5100 S and 46o 380 1000 W ˜ ˜Therefore, the set of failures attributed by the company to unknown causes is severelyinfluenced by selection effects; therefore, they were not considered in detail in this study.The total number of failures (4,572 and 763 at 138 and 440 kV grids, respectively)attributed to unknown causes, however, is real and was included in the statistics. Thedescription of some of the remaining 93 reported failure causes are rather subjective. Theycorrespond to few cases in each category, such as caused by fire under the lines (145 casesat 138 kV grid; 92 cases at 440 kV grid), other lines interferences (176 at 138 kV and 44 at440 kV), failures in relay protection (108 at 138 kV and 22 at 440 kV), and so on. Thenumbers of failures on each of these categories were considered too small to have astatistical significance for this study. According to ISA.CTEEP company criteria, the failures attributed to ‘‘atmosphericdischarges’’ correspond to the presence of thunderstorm activity in the area where andwhen the shutdown occurred. Long-term monthly means of power failures attributed toatmospheric discharges are shown in Figs. 3 (for the 138 kV grid) and 4 (for the 440 kVgrid). Three trends are immediately observed:(a) The number of failures is considerably larger for the 138 kV grid(b) There are more failures during the rainy season, when thunderstorms are more frequent in the southeastern part of Brazil (October–March)(c) There is a significant reduction (by more than 50 %) in the number of failures attributed to atmospheric discharges (accumulated in the rainy season months) as time progresses. The effects (b) and (c) are evident for the 138 kV power grid and strong for the 440 kVgrid. 123
  6. 6. 978 Surv Geophys (2012) 33:973–989 ˜Fig. 2 The location of Brazil in South America and of Sao Paulo State close to the center of the SouthAtlantic Magnetic Anomaly (SAMA), on a NOAA (2010) magnetic map3 Failures Attributed to Atmospheric Discharges and the Geophysical Environment3.1 Relevant Solar Cycle IndicesThe main indices used to specify the geophysical environment are the solar sunspotnumber (R), the planetary magnetic index (Kp), the disturbance (storm time) magneticindex (Dst) and the cosmic ray fluxes at the ground (CR) (NOAA data services,1998–2006). Although there are causal relationships between solar activity, magneticindices, and cosmic ray fluxes, these relationships vary throughout the solar activity cycle.123
  7. 7. Surv Geophys (2012) 33:973–989 979Fig. 3 Monthly averages of 138 kV transmission failures attributed to atmospheric discharges. Theyexhibit a larger number of events during the summer months (October–March)Fig. 4 Monthly averages of 440 kV transmission failures attributed to atmospheric discharges. Theyexhibit a larger number of events during the summer months (October–March)3.2 Power Failures and Geomagnetic-Related IndicesThe power system failures analyzed for daily, monthly, or yearly time scales were notrelated to any of the geomagnetic indices, for the whole 9-year period. This is illustrated inFig. 5 (a) for monthly averages of Kp and Dst indices for the entire 9-year period and(b) for daily Kp and Dst indices for the year 2003, as an example. The daily trends for theother years (1998–2002 and 2004–2006) were basically similar. For 2003, the lack of 123
  8. 8. 980 Surv Geophys (2012) 33:973–989 (a) (b)Fig. 5 The occurrence of power failures on the 138 and 440 kV networks compared to the geomagneticindices Kp and Dst. Monthly averages are shown in (a) and daily values are shown as vertical bars in (b)123
  9. 9. Surv Geophys (2012) 33:973–989 981correlation was maintained during the exceptionally high solar and geomagnetic activity‘‘Halloween event’’ period (October–December 2003).3.3 Sunspot Number (R) and Cosmic Ray (CR) Indices3.3.1 Long Time Scale AnalysisThe yearly total numbers of power failures attributed to atmospheric discharges are shownin Fig. 6, top panel, compared to the R and CR indices (NOAA 1998–2006) during solarcycle 23. The steady decay in power line failures is very significant for both grids. Thedecay is of 67 % (from 316 failures 1998 to 104 failures in 2006) for the 138 kV grid and77 % (from 35 failures 1998 to 8 failures in 2006) for the 440 kV grid. The reduction in thenumber of failures follows approximately the reduction in sunspot number R. For the firstyear, 1998, however, there was an opposite trend, with high number of network failures fora high CR flux. Since there were no network failure data available for previous years, wemay tentatively attribute this discrepancy to a statistical probability. The scatter diagram of long-term power failures attributed to atmospheric discharges andthe sunspot number R is shown in Fig. 7a, b. The best fit straight lines exhibit correlationcoefficients of 0.78 for 138 kV and 0.67 for 440 kV data. The corresponding probability forsuch correlations coefficients to be accidental is less than 2 % and 5 %, respectively (Be-vington and Robinson 1992). The association is quite remarkable, suggesting that there mightbe a genuine physical connection between these two rather distinct processes.3.3.2 Power Failures on Monthly ScaleIn Fig. 8, we show, for the power grids failures, monthly average values of R and cosmicray flux monthly averages for the whole period studied here. It brings more detail to theFig. 6 Yearly average values of sunspot number (R) and cosmic ray flux (CR) (NOAA 1998–2006). Thetop panel shows the yearly number of power failures attributed to atmospheric discharges on the 138 kV(continuous line) and 440 kV (dashed line) electricity grid systems 123
  10. 10. 982 Surv Geophys (2012) 33:973–989Fig. 7 a Scatter diagram of 138 kV failures per year attributed to atmospheric discharges and the yearlyaverage sunspot numbers. b Scatter diagram of 440 kV failures per year attributed to atmospheric dischargesand the yearly average sunspot numbersinformation given in Fig. 6. With the decline of R, there is an enhancement in CR flux,exhibiting a jump after the ‘‘Halloween’’ period of exceptionally high activity in October2003. This result is very similar to the monthly R values versus monthly count of CoronalMass Ejections (CMEs), directly related to geomagnetic activity in the period 1996–2007(Thomson et al. 2010).3.3.3 Short Time Scale AnalysisWe show in Fig. 9 the daily values of R, and cosmic ray indices and the power failuresattributed to atmospheric discharges on the 138 kV network, for the year of 2003, as an123
  11. 11. Surv Geophys (2012) 33:973–989 983Fig. 8 Monthly average values of sunspot number (R) and cosmic ray flux (CR) (NOAA 1998–2006). Thetop panel shows the monthly power failures attributed to atmospheric discharges on the 138 kV (continuousline) and 440 kV (dashed line) gridsFig. 9 One year of daily values of sunspot number (R) and cosmic ray flux (CR) (NOAA 1998–2006) for2003. The vertical bars in the top panel exhibit daily power failures attributed to atmospheric discharges onthe 440 and 138 kV networks 123
  12. 12. 984 Surv Geophys (2012) 33:973–989example. The daily trends for the other years (1998–2002 and 2004–2006) were basicallysimilar and do not need to be shown here. The power grid failures are also concentrated during the thunderstorm months in allyears. They do not exhibit a closer association with R or CR shorter time variations.3.4 Power Failures and Lightning Detected in the Same RegionA complete survey on lightning occurrence detected by lightning sensor networks overBrazil, covering the period studied here, has been reviewed by Pinto et al. (2007). Theyindicate a significant cloud-to-ground lightning density over the southeastern region. ˜Furthermore, Pinto (2009, private communication) has indicated that, over Sao Paulo State,there were typically between 0.8 and 1.4 9 106 atmospheric discharges/year in the period1999–2006. Nearly 70 % of these were in the summer months (October–March), and thisagrees with the seasonal trend of power failures shown in Figs. 3, 4 and 8. The total number of atmospheric discharges detected per year in southeastern Brazil(provided by Pinto 2009, private communication) is compared to sunspot number R for thewhole period analyzed here (Fig. 10). There are no clear associations although a smallexcess of lightning at maximum R years (2000 and 2001) is apparent, as is anotherenhancement after 2004. On the other hand, we must remind ourselves that the total number of atmosphericdischarges detected by the sensor network is more than 4 orders of magnitude larger thanthe number of power failures attributed to atmospheric discharges. The two sets of data arenot readily comparable, due to their huge numerical difference. Furthermore, the powergrids are designed to respond to the smallest possible number of atmospheric dischargeswhile the lightning sensor networks are designed to respond to the largest possible numberof events. The absence of an association between thunderstorms and solar activity represented bysunspot number (R) was obtained for the decay phase of solar cycle 20 (1967–1976) (Freier1978). This study, however, might have restrictions for the comparison because it referredto a single city location (Minneapolis, Minnesota, USA) and involved a considerablysmaller number of thunderstorms (only tens per year).Fig. 10 The total number of cloud-to-ground flashes per year for 1999–2006, detected by the lightningsensor network in southeastern Brazil (Pinto 2009) (gray dashed line), compared to the sunspot number(black line) (1998–2006)123
  13. 13. Surv Geophys (2012) 33:973–989 9854 DiscussionThe ISA.CTEEP 138 and 440 kV networks in southeastern Brazil are remarkably robustand insensitive to lightning activity, responding to about only 0.25 % and of 0.02 % of alllightning recorded in the same region, respectively. Although the number of power failuresattributed to atmospheric discharges represent a tiny fraction of a percent of the totalnumber of atmospheric discharges actually detected in the same area for the same period,they reduce substantially (i.e., by 67 and 77 % at the 138 and 440 kV, respectively) as thesolar cycle progresses toward sunspot minimum. The larger number of failures for the higher activity phase of solar cycle 23 agrees withthe clustering of reported GIC-related disruptions in several solar maximum years (Boteleret al. 1998). However, it is quite remarkable that there was no evidence of a directrelationship between power transmission failures attributed to atmospheric discharges insoutheastern Brazil and various geomagnetic indices, both in the short term and on the longterm. The region is located at low geomagnetic latitudes, which may have some impli-cations for less effective geomagnetic storm effects (Tinsley 2000; Pirjola 2007; Thomsonet al. 2010, and references therein). Furthermore, it has been indicated that even at highlatitudes the probability of a power failure caused by GICs on networks is only one in20 years (Elovaara 2007). However, the long-term failure data for which this probabilityhas been derived have not been published. These associations cannot be further exploredbecause, to our knowledge, there are no long-term data series of actual power grid failuresavailable for other geographic locations which may be compared with ours presented here. ˜ The ISA.CTEEP power networks in Sao Paulo State are located near the center of theSouth Atlantic Geomagnetic Anomaly (SAMA), as shown in Fig. 2. The magnetic field isparticularly weak over this region and the energetic charged particles trapped in the VanAllen belts penetrate deeper into the atmosphere and are preferentially lost in this meridian.This effect may induce more currents in the ionosphere when solar activity is higher, andthus may have some effect on ionosphere-to-ground coupling. However, it is difficult todescribe how such coupling might cause power network failures. The main consequencesmight be an enhancement of GICs, which have not caused power failures. We have notedthat there are no similar systematic and long records of power network failures at othergeomagnetic latitudes which may be compared with ours. On the other hand, the electrical coupling provided by the global atmospheric electriccircuit between the ionosphere and the ground may play a role in explaining the corre-lations found in this study (Roble 1985; Rycroft et al. 2000; Rycroft 2006; Aplin et al.2008). The decrease in the number of power network failures as the sunspot numberdecreases might be a response to physical changes in the electrosphere with solar cycle. Asimplified equivalent electrical circuit for the electrosphere is shown in Fig. 11 (Rycroft2006). The clear atmosphere condition, at the right side of the Fig. 11, corresponds to aresistance and a capacitor in parallel. The cloudy thunderstorm areas correspond to acurrent generator, driving the circuit. A larger number of thunderstorms naturally implyenhanced chances for power grid failures, since the grids are in the generator’s circuit path.This effect was observed in practice for the power networks in southeastern Brazil, for thethunderstorm months October–March (Figs. 3, 4, 8). It is well known that there is an enhancement in the cosmic ray flux in the atmosphere asthe sunspot number decreases. This effect is suggested for the period surveyed in this study(Fig. 6, after 2002). The increased cosmic ray fluxes produce more ionization in theatmosphere, increasing its conductivity (Stozhkov 2003) (see Fig. 12a). The yearly averageof atmospheric currents measured over mid North America and in the polar regions 123
  14. 14. 986 Surv Geophys (2012) 33:973–989Fig. 11 The global ionosphere-ground equivalent electrical circuit (Rycroft 2006). The fair weatherregions, right side, correspond to a certain resistance in parallel to the capacitance between the twoconducting surfaces. The left side represents the thunderstorm generation regions, where the switches are toclose the circuit due to atmospheric discharges. Lightning below thunderclouds are relevant to thisdiscussionexhibited increases for solar cycle 20 (1978–1983) maximum by almost a factor of two(i.e., from 1 9 10-12 J/m2 to 1.8 9 10-12 J/m2) (Roble, 1985; Stozhkov et al. 2001a, b;Stozhkov 2003, and references therein). On the other hand, there is a suggested correlationbetween the global cloud cover and the number of atmospheric discharges with cosmic rayflux (Svensmark and Friis-Christensen 1997; Stozhkov et al. 2001a, b; Stozhkov 2003) (seeFig. 12b). The increase in conductivity caused by these two effects may reduce the elec-trostatic voltage threshold needed to produce atmospheric discharges, which in turn maybecome less apt to cause power network failures. In other words, in quiet Sun years thereare two opposite effects, both of which result in a larger number of atmospheric dischargeswhich are less efficient at disrupting our electrical power grid systems.5 Concluding RemarksThe ISA.CTEEP reports on failures in high-voltage transmission networks in southeasternBrazil offered us the unique opportunity to develop a comprehensive analysis, for a longperiod of time (1998–2006), of their possible coupling to the geophysical environment.Although from a climatology standpoint it would have been desirable to have longerperiods for analysis, these are not available for power networks, mainly because (a) mostcompanies do not open their data banks and (b) transmission networks undergo significanttechnical modifications from year to year, compromising the uniformity of their data. Our studies concentrated on the longest and most reliable 138 and 440 kV networks, ˜covering the Sao Paulo State of southeastern Brazil. Most of the power failures (i.e.,123
  15. 15. Surv Geophys (2012) 33:973–989 987Fig. 12 The tropospheric (a)ionization enhancement bycosmic ray fluxes and lightning(after Stozhkov et al. 2001a).a Yearly average increase in J, 1012 Am-2atmospheric current J(h) (open N, cm-2 s-1circles) and cosmic ray fluxN(h) at h = 8 km in the polarregion (black dots). b Theincrease in yearly number oflightning L detected in the USA(black dots) and the ionproduction rate q in the aircolumn (2–10 km) at middlelatitudes (open circles) Year (b)42.8 % for the 138 kV grid and 22.3 % for the 440 kV grid) are attributed to atmosphericdischarges, that is lightning. The power failures exhibit strong seasonal variations, withmaxima in the season with more thunderstorms in the area. The largest number of failures occurred at the peak of solar activity in cycle 23, andthen decreasing substantially as the sunspot number decreased, by 67 and 77 % for the 138and 440 kV grids, respectively. However, there was no association of power failures due toatmospheric discharges with enhancements in the magnetic activity indices Kp or Dst,neither on the short nor long term scales. This trend is similar to the yearly distribution oflarge GICs in a high northern latitude pipeline (Huttunen et al. 2008), except for 2003(containing the ‘‘Halloween’’ solar active period). The incidence of GICs, however, doesnot necessarily correlate with power transmission failures. Therefore, and similarly, thegeomagnetic effects that might have occurred in the ISA.CTEEP power grids were noteffective in causing failures. On the other hand, changes in the physical parameters of the electrosphere mightbecome significant and so be able to explain the observed decrease in power grid failures asthe solar cycle 23 progressed beyond solar maximum. Global atmospheric equivalentelectric circuits are rather complex (see, for example, Rycroft et al. 2000; Tinsley and Yu2004; Rycroft 2006; Aplin et al. 2008). The electrical resistance in the circuit becomesmost pronounced at tropospheric altitudes, below 10 km (Rycroft et al. 2000; Harrison2004; Aplin et al. 2008). Galactic cosmic rays produce ionization in the troposphere,enhancing the conductivity and possibly influencing cloud electrification (Svensmark and 123
  16. 16. 988 Surv Geophys (2012) 33:973–989Friis-Christensen 1997; Stozhkov et al. 2001a, b; Stozhkov 2003). These processes mayalso reduce the threshold voltage necessary to produce atmospheric discharges. Further-more, the suggested increase in both cloud cover and the number of atmospheric dischargeswith larger cosmic ray fluxes (Stozhkov et al. 2001a, b; Stozhkov 2003) may, in turn,increase the ion production and further enhance the troposphere conductivity. The netresult of the two opposite effects is the production of a larger number of atmosphericdischarges, but discharges that are less efficient at causing network failures. The dependence of the physical properties of the electrosphere on the external geo-physical environment is of major importance for understanding its impact on technologicalsystems on the ground, such as on high-voltage transmission networks. Substantialamounts of research are still required on cloud electrification processes, lightning occur-rence and threshold voltage regimes, and also on atmospheric ionization by cosmic raysand their relationship to cloud cover, both regionally and over the planet.Acknowledgments This research was partially supported by Brazilian agencies FAPESP and CNPq. Wethank anonymous referees for their very helpful comments and the editor in chief for assistance.ReferencesAbdu MA, Souza JR, Sobral JHA, Batista IS (2006) Magnetic storm associated disturbance dynamo effects in the low and equatorial latitude ionosphere. In: Tsurutani B, McPherron R, Gonzalez W, Lu G, Sobral JHA, Gopalswamy N (eds) Recurrent magnetic storms: corotating solar wind streams, Geophysical Monograph 167, AGU Books Board, pp 283–304Aplin KL, Harrison RG, Rycroft MJ (2008) Investigating earth’s atmospheric electricity: a role model for planetary studies. Space Sci Rev 137:11–27. doi:10.1007/s11214-008-9372-xBarnes PR, Rizy DT, McConnell BW (1991) Electric utility industry experience with geomagnetic dis- turbances. Oak Ridge National Laboratory—Power Systems Technology Program. Report ORNL-6665Bevington PR, Robinson DK (1992) Data reduction and error analysis for the physical sciences. McGraw- Hill, Boston, pp 256–257 ´Bolduc L (2002) GIC observations and studies in the Hydro-Quebec power system. J Atmosph Solar Terr Phys 64:1793–1802. doi:10.1016/S1364-6826(02)00128-1Boteler DH, Pirjola RJ, Nevanlinna H (1998) The effects of geomagnetic disturbances on electrical systems at the earth’s surface. Adv Space Res 22:17–21. doi:10.1016/S0273-1177(97)01096-XEastwood JP (2008) The science of space weather. Phil Trans R Soc A 366:4489–4500. doi: 10.1098/rsta.2008.0161Elovaara J (2007) Finnish experiences with grid effects of GICs. In: Jean Lilensten (ed) Space weather, astrophysics and space science library, vol 344, pp 311–326. doi: 10.1007/1-4020-5446-7_27Freier GD (1978) A 10-year study of thunderstorm electric fields. J Geophys Res 83:1373–1376. doi: 10.1029/JC083iC03p01373Harrison RG (2004) The global atmospheric electrical circuit and climate. Surv Geophys 25:441–484. doi: 10.1007/s10712-004-5439-8Hoyt DV, Schatten KH (1997) The role of the sun in climate change. Oxford University Press, New York, pp 143–152Huttunen KEJ, Kilpua SP, Pulkkinen A, Viljanen A, Tanskanen E (2008) Solar wind drivers of large geomagnetically induced currents during the solar cycle 23. Space Weather 6:S10002. doi:10.1029/ 2007SW000374Kappenman JG (2005) An overview of the impulsive geomagnetic field disturbances and power grid impacts associated with the violent sun–earth connection events of 20–31 October 2003 and a comparative valuation with other contemporary storms. Space Weather 3:S08C1. doi:10.1029/2004SW000128Kivelson MG, Russell CT (eds) (1995) Introduction to space physics. Cambridge University Press, Cam- bridge, p 568Lanzerotti LJ (1983) Geomagnetic induction effects in ground-based systems. Space Sci Rev 34:347–356. doi:10.1007/BF00175289123
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