Long-Term Power Transmission Failuresin Southeastern Brazil and the Geophysical Environment

Magda A. S. Duro • Pierre Kaufmann • Fernando C. P. Bertoni •

Emilio C. N. Rodrigues • Jose Pissolato Filho

Received: 16 November 2011 / Accepted: 19 March 2012 / Published online: 21 April 2012� Springer Science+Business Media B.V. 2012

Abstract High-voltage transmission networks represent a large electrical circuit just

above the ground subjected to a number of transient overcharges of various kinds, some of

which may lead to failures. Some failures might be related to anomalies of the geophysical

environment. 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 the

transmission 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, and

170 out of 763 at 440 kV, respectively. They correspond to less than one ten thousandth of

the actual number of atmospheric discharges recorded in the same area, demonstrating the

grid’s high resilience to breakdowns due to lightning. A clear concentration of failures in

the region’s thunderstorm season has been found. A significant 67 and 77 % reduction in

the 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 sunspot

numbers. No obvious correlation was found between power failures and the planetary

index of geomagnetic activity or major geomagnetic storms in the period, either on short or

on long time scales. Assuming that the dependence of the electrosphere/ionosphere-ground

M. A. S. DuroEscola de Engenharia, Universidade Presbiteriana Mackenzie, Sao Paulo, Brazil

P. Kaufmann (&) � F. C. P. BertoniEscola de Engenharia, CRAAM, Universidade Presbiteriana Mackenzie, Sao Paulo, Brazile-mail: kaufmann@craam.mackenzie.br

P. KaufmannCentro de Componentes Semicondutores, Universidade Estadual de Campinas, Campinas, SP, Brazil

E. C. N. RodriguesISA.CTEEP, Companhia de Transmissao de Energia Eletrica Paulista, Jundiaı, SP, Brazil

J. P. FilhoFaculdade de Engenharia Eletrica e Computacao, Universidade Estadual de Campinas, Campinas, SP,Brazil

123

Surv Geophys (2012) 33:973–989DOI 10.1007/s10712-012-9191-1

coupling on the external geophysical environment plays a major role in explaining the

reduction in power failures as the solar cycle wanes, it is suggested that the increase in

atmosphere conductivity caused by the larger cosmic ray flux then reduces the threshold

voltage required to produce lightning strokes, so reducing their effectiveness in disrupting

high-voltage power lines.

Keywords Space weather � Power transmission failures � Atmospheric discharges �Geomagnetic storms � Electrosphere � Atmosphere conductivity � Solar activity �Solar cycle

1 Introduction

High-voltage electrical power transmission network systems are known to be vulnerable to

various 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 of

operational maintenance people, etc. Unpredictable disruptions are a serious concern for

the transmission network operators.

It is well known that disturbances to the physical regimes in outer space, also known as

space 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 and

Schatten 1997; Pirjola et al. 2000; Thomson et al. 2010). However, power line interactions

with the geophysical environment regime are poorly known. One reason for this is the lack

of long-term data sets concerning power transmission failures. Electrical power companies

often 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 network

grid industries represent a problem to those who try to advance our understanding of the

impact of geophysical disturbances on electricity distribution systems (Thomson et al.

2010). Another difficulty is that, even when such access is made possible, most power

transmission or distribution networks are physically modified, or technically improved, on

relatively 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 the

upper atmosphere that becomes ionized by solar EUV radiation and by the terrestrial

magnetosphere. This regime is disturbed with increasing solar activity, which enhances

ionization in the ionosphere, and by increasing fluxes of charged particles penetrating the

atmosphere 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; Thomson

et al. 2007, 2010).

Most studies on the influence of solar activity on terrestrial technological systems are

related to geomagnetic disturbances. The latter generate geomagnetically induced currents,

GICs, which are believed to be the principal transient anomaly influencing technological

systems 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; Kappenman

974 Surv Geophys (2012) 33:973–989

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2005; Pirjola 2005; Huttunen et al. 2008). Boteler et al. (1998) presented a review on

documented 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 of

maximum sunspot numbers, and when geomagnetic indices increase and geomagnetic

storms become more frequent).

This study, however, does not contain grid failures recorded continuously on a daily

basis for many years. One famous example of the impact that GICs have on power lines has

been 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 after

9 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 year

with large geomagnetic-induced currents, obtained from pipeline measurements at high

latitudes in Europe, suggests a decrease in GICs as the yearly solar sunspot number

decreases, 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 are

monitored continuously, although there are a number of studies that show single highly

suggestive examples of a one-to-one GIC association with geomagnetic storms (see, for

example, Kappenman 2005; Trivedi et al. 2007; Watari et al. 2009). Despite the infamous

1989 Quebec electrical power failure, there are no obvious direct correlations between the

occurrence of GICs and actual power grid failures; a review of the effects of GICs on grids

in Finland indicates that the probability of a power failure caused by GICs is one in

20 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, it

should 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 the

other hand, there are only a few sparse reports on systematic analyses of actual grid

failures, irrespectively of their causes, obtained over a long period of time, on a given

network (Hoyt and Schatten 1997). For such studies, data for networks that remain

unchanged for a sufficiently large number of years to provide a uniform data set should be

compared with solar activity or with geophysical environment disturbances. These con-

ditions are seldom available.

Geomagnetic disturbances are not the only geophysical cause that impacts technological

systems 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 much

studied. 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 be

represented by a resistance and a capacitor in parallel, with areas with thunderstorms acting

as current generators.

The above-quoted authors have shown that nearly 95 % of the ionosphere-to-ground

resistance is below 10 km, and so stressing the importance of the troposphere in the global

circuit. Certain changes in the conductivity of the troposphere might arise from external

influences on the upper atmosphere (Harrison 2004; Rycroft 2006). Cosmic ray flux

increases 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; Stozhkov

2003; Rycroft 2006). An increase in atmospheric conductivity has been also associated

Surv Geophys (2012) 33:973–989 975

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with changing global cloud coverage over the planet and to the increased occurrence of

lightning (Svensmark and Friis-Christensen 1997; Stozhkov 2003). Such changes in the

troposphere 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 were

consistently reported for almost 9 years (January 1998–October 2006), that is, for the

majority 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 principal

private 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 failures

recorded 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 in

Table 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 because

they 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 South

American Geomagnetic Anomaly (SAGA), described by contours of total magnetic field

strength (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 4,572

for the whole period for the 138 kV grid, and 170 out of 763 for the 440 kV grid. The

remaining failures, 2,615 at the 138 kV grid and 593 at the 440 kV grid, were distributed

on 94 other different reported failure causes. The larger group corresponds to failures due

to ‘‘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 failures

attributed to ‘‘unknown causes’’ are often derived from distinct identification criteria. For

example, a failure cause labeled as ‘‘unknown’’ might be classified in another category

after a deeper investigation. In other words, the number of failures assigned to unknown

causes depends on how carefully searches were made for the causes. Such a search depends

on the number of people hired to perform this job, which was different for different years.

Table 1 ISA.CTEEP high-volt-age transmission networks

Voltage (kV) Total extent (km)

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 42.80

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Therefore, the set of failures attributed by the company to unknown causes is severely

influenced 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. The

description of some of the remaining 93 reported failure causes are rather subjective. They

correspond to few cases in each category, such as caused by fire under the lines (145 cases

at 138 kV grid; 92 cases at 440 kV grid), other lines interferences (176 at 138 kV and 44 at

440 kV), failures in relay protection (108 at 138 kV and 22 at 440 kV), and so on. The

numbers of failures on each of these categories were considered too small to have a

statistical significance for this study.

According to ISA.CTEEP company criteria, the failures attributed to ‘‘atmospheric

discharges’’ correspond to the presence of thunderstorm activity in the area where and

when the shutdown occurred. Long-term monthly means of power failures attributed to

atmospheric discharges are shown in Figs. 3 (for the 138 kV grid) and 4 (for the 440 kV

grid).

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 kV

grid.

Fig. 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

Surv Geophys (2012) 33:973–989 977

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3 Failures Attributed to Atmospheric Discharges and the Geophysical Environment

3.1 Relevant Solar Cycle Indices

The main indices used to specify the geophysical environment are the solar sunspot

number (R), the planetary magnetic index (Kp), the disturbance (storm time) magnetic

index (Dst) and the cosmic ray fluxes at the ground (CR) (NOAA data services,

1998–2006). Although there are causal relationships between solar activity, magnetic

indices, and cosmic ray fluxes, these relationships vary throughout the solar activity cycle.

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 map

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3.2 Power Failures and Geomagnetic-Related Indices

The power system failures analyzed for daily, monthly, or yearly time scales were not

related to any of the geomagnetic indices, for the whole 9-year period. This is illustrated in

Fig. 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 the

other years (1998–2002 and 2004–2006) were basically similar. For 2003, the lack of

Fig. 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)

Surv Geophys (2012) 33:973–989 979

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(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)

980 Surv Geophys (2012) 33:973–989

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correlation 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) Indices

3.3.1 Long Time Scale Analysis

The yearly total numbers of power failures attributed to atmospheric discharges are shown

in Fig. 6, top panel, compared to the R and CR indices (NOAA 1998–2006) during solar

cycle 23. The steady decay in power line failures is very significant for both grids. The

decay is of 67 % (from 316 failures 1998 to 104 failures in 2006) for the 138 kV grid and

77 % (from 35 failures 1998 to 8 failures in 2006) for the 440 kV grid. The reduction in the

number of failures follows approximately the reduction in sunspot number R. For the first

year, 1998, however, there was an opposite trend, with high number of network failures for

a high CR flux. Since there were no network failure data available for previous years, we

may tentatively attribute this discrepancy to a statistical probability.

The scatter diagram of long-term power failures attributed to atmospheric discharges and

the sunspot number R is shown in Fig. 7a, b. The best fit straight lines exhibit correlation

coefficients of 0.78 for 138 kV and 0.67 for 440 kV data. The corresponding probability for

such 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 might

be a genuine physical connection between these two rather distinct processes.

3.3.2 Power Failures on Monthly Scale

In Fig. 8, we show, for the power grids failures, monthly average values of R and cosmic

ray flux monthly averages for the whole period studied here. It brings more detail to the

Fig. 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

Surv Geophys (2012) 33:973–989 981

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information 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 October

2003. This result is very similar to the monthly R values versus monthly count of Coronal

Mass Ejections (CMEs), directly related to geomagnetic activity in the period 1996–2007

(Thomson et al. 2010).

3.3.3 Short Time Scale Analysis

We show in Fig. 9 the daily values of R, and cosmic ray indices and the power failures

attributed to atmospheric discharges on the 138 kV network, for the year of 2003, as an

Fig. 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 numbers

982 Surv Geophys (2012) 33:973–989

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Fig. 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) grids

Fig. 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

Surv Geophys (2012) 33:973–989 983

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example. The daily trends for the other years (1998–2002 and 2004–2006) were basically

similar and do not need to be shown here.

The power grid failures are also concentrated during the thunderstorm months in all

years. They do not exhibit a closer association with R or CR shorter time variations.

3.4 Power Failures and Lightning Detected in the Same Region

A complete survey on lightning occurrence detected by lightning sensor networks over

Brazil, covering the period studied here, has been reviewed by Pinto et al. (2007). They

indicate 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 period

1999–2006. Nearly 70 % of these were in the summer months (October–March), and this

agrees 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 the

whole period analyzed here (Fig. 10). There are no clear associations although a small

excess of lightning at maximum R years (2000 and 2001) is apparent, as is another

enhancement after 2004.

On the other hand, we must remind ourselves that the total number of atmospheric

discharges detected by the sensor network is more than 4 orders of magnitude larger than

the number of power failures attributed to atmospheric discharges. The two sets of data are

not readily comparable, due to their huge numerical difference. Furthermore, the power

grids are designed to respond to the smallest possible number of atmospheric discharges

while the lightning sensor networks are designed to respond to the largest possible number

of events.

The absence of an association between thunderstorms and solar activity represented by

sunspot number (R) was obtained for the decay phase of solar cycle 20 (1967–1976) (Freier

1978). This study, however, might have restrictions for the comparison because it referred

to a single city location (Minneapolis, Minnesota, USA) and involved a considerably

smaller 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)

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4 Discussion

The ISA.CTEEP 138 and 440 kV networks in southeastern Brazil are remarkably robust

and insensitive to lightning activity, responding to about only 0.25 % and of 0.02 % of all

lightning recorded in the same region, respectively. Although the number of power failures

attributed to atmospheric discharges represent a tiny fraction of a percent of the total

number 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 the

solar cycle progresses toward sunspot minimum.

The larger number of failures for the higher activity phase of solar cycle 23 agrees with

the clustering of reported GIC-related disruptions in several solar maximum years (Boteler

et al. 1998). However, it is quite remarkable that there was no evidence of a direct

relationship between power transmission failures attributed to atmospheric discharges in

southeastern Brazil and various geomagnetic indices, both in the short term and on the long

term. The region is located at low geomagnetic latitudes, which may have some impli-

cations for less effective geomagnetic storm effects (Tinsley 2000; Pirjola 2007; Thomson

et al. 2010, and references therein). Furthermore, it has been indicated that even at high

latitudes the probability of a power failure caused by GICs on networks is only one in

20 years (Elovaara 2007). However, the long-term failure data for which this probability

has been derived have not been published. These associations cannot be further explored

because, to our knowledge, there are no long-term data series of actual power grid failures

available 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 the

South Atlantic Geomagnetic Anomaly (SAMA), as shown in Fig. 2. The magnetic field is

particularly weak over this region and the energetic charged particles trapped in the Van

Allen 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, and

thus may have some effect on ionosphere-to-ground coupling. However, it is difficult to

describe how such coupling might cause power network failures. The main consequences

might be an enhancement of GICs, which have not caused power failures. We have noted

that there are no similar systematic and long records of power network failures at other

geomagnetic latitudes which may be compared with ours.

On the other hand, the electrical coupling provided by the global atmospheric electric

circuit 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 number

decreases might be a response to physical changes in the electrosphere with solar cycle. A

simplified equivalent electrical circuit for the electrosphere is shown in Fig. 11 (Rycroft

2006). The clear atmosphere condition, at the right side of the Fig. 11, corresponds to a

resistance and a capacitor in parallel. The cloudy thunderstorm areas correspond to a

current generator, driving the circuit. A larger number of thunderstorms naturally imply

enhanced 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 the

thunderstorm months October–March (Figs. 3, 4, 8).

It is well known that there is an enhancement in the cosmic ray flux in the atmosphere as

the 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 the

atmosphere, increasing its conductivity (Stozhkov 2003) (see Fig. 12a). The yearly average

of atmospheric currents measured over mid North America and in the polar regions

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exhibited 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 correlation

between the global cloud cover and the number of atmospheric discharges with cosmic ray

flux (Svensmark and Friis-Christensen 1997; Stozhkov et al. 2001a, b; Stozhkov 2003) (see

Fig. 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 may

become less apt to cause power network failures. In other words, in quiet Sun years there

are two opposite effects, both of which result in a larger number of atmospheric discharges

which are less efficient at disrupting our electrical power grid systems.

5 Concluding Remarks

The ISA.CTEEP reports on failures in high-voltage transmission networks in southeastern

Brazil offered us the unique opportunity to develop a comprehensive analysis, for a long

period of time (1998–2006), of their possible coupling to the geophysical environment.

Although from a climatology standpoint it would have been desirable to have longer

periods for analysis, these are not available for power networks, mainly because (a) most

companies do not open their data banks and (b) transmission networks undergo significant

technical 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.,

Fig. 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 thisdiscussion

986 Surv Geophys (2012) 33:973–989

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42.8 % for the 138 kV grid and 22.3 % for the 440 kV grid) are attributed to atmospheric

discharges, that is lightning. The power failures exhibit strong seasonal variations, with

maxima in the season with more thunderstorms in the area.

The largest number of failures occurred at the peak of solar activity in cycle 23, and

then decreasing substantially as the sunspot number decreased, by 67 and 77 % for the 138

and 440 kV grids, respectively. However, there was no association of power failures due to

atmospheric 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 of

large 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, does

not necessarily correlate with power transmission failures. Therefore, and similarly, the

geomagnetic effects that might have occurred in the ISA.CTEEP power grids were not

effective in causing failures.

On the other hand, changes in the physical parameters of the electrosphere might

become significant and so be able to explain the observed decrease in power grid failures as

the solar cycle 23 progressed beyond solar maximum. Global atmospheric equivalent

electric circuits are rather complex (see, for example, Rycroft et al. 2000; Tinsley and Yu

2004; Rycroft 2006; Aplin et al. 2008). The electrical resistance in the circuit becomes

most pronounced at tropospheric altitudes, below 10 km (Rycroft et al. 2000; Harrison

2004; Aplin et al. 2008). Galactic cosmic rays produce ionization in the troposphere,

enhancing the conductivity and possibly influencing cloud electrification (Svensmark and

N, c

m-2

s-1

J, 1

012

Am

-2Year

(a)

(b)

Fig. 12 The troposphericionization enhancement bycosmic ray fluxes and lightning(after Stozhkov et al. 2001a).a Yearly average increase inatmospheric current J(h) (opencircles) 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)

Surv Geophys (2012) 33:973–989 987

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Friis-Christensen 1997; Stozhkov et al. 2001a, b; Stozhkov 2003). These processes may

also reduce the threshold voltage necessary to produce atmospheric discharges. Further-

more, the suggested increase in both cloud cover and the number of atmospheric discharges

with 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 net

result of the two opposite effects is the production of a larger number of atmospheric

discharges, 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 technological

systems on the ground, such as on high-voltage transmission networks. Substantial

amounts of research are still required on cloud electrification processes, lightning occur-

rence and threshold voltage regimes, and also on atmospheric ionization by cosmic rays

and 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.

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