This document analyzes power transmission failures in southeastern Brazil over 9 years from 1998 to 2006. It finds that most failures were attributed to atmospheric discharges, with 1957 failures out of 4572 total for the 138kV grid and 170 out of 763 for the 440kV grid. There was a clear concentration of failures during the region's thunderstorm season. The study also found a significant 67% and 77% reduction in failure rates per year for the 138kV and 440kV grids respectively, corresponding to the decay in sunspot numbers over the solar cycle. No obvious correlation was found between failures and geomagnetic activity indices.

1. Surv Geophys (2012) 33:973–989
DOI 10.1007/s10712-012-9191-1
Long-Term Power Transmission Failures
in 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
Ó 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. Duro
˜
Escola de Engenharia, Universidade Presbiteriana Mackenzie, Sao Paulo, Brazil
P. Kaufmann (&) Á F. C. P. Bertoni
˜
Escola de Engenharia, CRAAM, Universidade Presbiteriana Mackenzie, Sao Paulo, Brazil
e-mail: kaufmann@craam.mackenzie.br
P. Kaufmann
Centro de Componentes Semicondutores, Universidade Estadual de Campinas, Campinas, SP, Brazil
E. C. N. Rodrigues
˜ ´ ´
ISA.CTEEP, Companhia de Transmissao de Energia Eletrica Paulista, Jundiaı, SP, Brazil
J. P. Filho
´ ¸˜
Faculdade de Engenharia Eletrica e Computacao, Universidade Estadual de Campinas, Campinas, SP,
Brazil
123

2. 974 Surv Geophys (2012) 33:973–989
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 Á Power transmission failures Á Atmospheric discharges Á
Geomagnetic storms Á Electrosphere Á Atmosphere conductivity Á Solar activity Á
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
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3. Surv Geophys (2012) 33:973–989 975
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
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4. 976 Surv Geophys (2012) 33:973–989
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-
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 42.80
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5. Surv Geophys (2012) 33:973–989 977
Fig. 1 The ISA.CTEEP 138 kV (black lines) and 440 kV (gray lines) high-voltage transmission networks
over 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 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.
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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 South
Atlantic Magnetic Anomaly (SAMA), on a NOAA (2010) magnetic map
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.
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7. Surv Geophys (2012) 33:973–989 979
Fig. 3 Monthly averages of 138 kV transmission failures attributed to atmospheric discharges. They
exhibit a larger number of events during the summer months (October–March)
Fig. 4 Monthly averages of 440 kV transmission failures attributed to atmospheric discharges. They
exhibit a larger number of events during the summer months (October–March)
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
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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 geomagnetic
indices Kp and Dst. Monthly averages are shown in (a) and daily values are shown as vertical bars in (b)
123

9. Surv Geophys (2012) 33:973–989 981
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). The
top 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
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10. 982 Surv Geophys (2012) 33:973–989
Fig. 7 a Scatter diagram of 138 kV failures per year attributed to atmospheric discharges and the yearly
average sunspot numbers. b Scatter diagram of 440 kV failures per year attributed to atmospheric discharges
and the yearly average sunspot numbers
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
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11. Surv Geophys (2012) 33:973–989 983
Fig. 8 Monthly average values of sunspot number (R) and cosmic ray flux (CR) (NOAA 1998–2006). The
top panel shows the monthly power failures attributed to atmospheric discharges on the 138 kV (continuous
line) 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) for
2003. The vertical bars in the top panel exhibit daily power failures attributed to atmospheric discharges on
the 440 and 138 kV networks
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12. 984 Surv Geophys (2012) 33:973–989
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 lightning
sensor network in southeastern Brazil (Pinto 2009) (gray dashed line), compared to the sunspot number
(black line) (1998–2006)
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13. Surv Geophys (2012) 33:973–989 985
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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14. 986 Surv Geophys (2012) 33:973–989
Fig. 11 The global ionosphere-ground equivalent electrical circuit (Rycroft 2006). The fair weather
regions, right side, correspond to a certain resistance in parallel to the capacitance between the two
conducting surfaces. The left side represents the thunderstorm generation regions, where the switches are to
close the circuit due to atmospheric discharges. Lightning below thunderclouds are relevant to this
discussion
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.,
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15. Surv Geophys (2012) 33:973–989 987
Fig. 12 The tropospheric (a)
ionization enhancement by
cosmic ray fluxes and lightning
(after Stozhkov et al. 2001a).
a Yearly average increase in
J, 1012 Am-2
atmospheric current J(h) (open
N, cm-2 s-1
circles) and cosmic ray flux
N(h) at h = 8 km in the polar
region (black dots). b The
increase in yearly number of
lightning L detected in the USA
(black dots) and the ion
production rate q in the air
column (2–10 km) at middle
latitudes (open circles)
Year
(b)
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
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16. 988 Surv Geophys (2012) 33:973–989
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. We
thank anonymous referees for their very helpful comments and the editor in chief for assistance.
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