Geomagnetic disturbances (GMDs) on the Earth originate at the Sun and can cause many different impacts on critical systems including power grids, metallic communications systems, railways, and pipelines, among others • The focus in this study was on high-voltage power systems, which are defined by the IEC as transmission grids operating at voltages above 100 kV (and usually at much higher voltages) • The emphasis on high voltage is because at the present time most of the power delivered to end users is concentrated in the delivery system from large power plants to the cities where the power is used • In order to understand the effect of GMDs on the power system, we need to understand the basic types of geomagnetic storms, the B-fields they create and how they may affect the power system

1. The Webinar video is available here:
https://attendee.gotowebinar.com/recording/7375054912215582214

2. 1
Comité d’Études C4 / Study Committee C4
Analyse Technique des Réseaux / System Technical Performance
Technical Brochure 780 Presentation:
Understanding of the geomagnetic storm
environment for high voltage power grids
Dr. William A. Radasky, Convenor
wradasky@aol.com
WG C4.32
29 January 2020

3. 2
PS1/Q1: G. BARBERA
(Argentina)
Members
W. RADASKY, Convenor US A. PINHEL BR
R. ADAMS AU Y. SAKHAROV RU
C. BALCH US E. SALINAS SE
Z. EMIN UK E. SAVAGE US
A. HALLEY AU P. SMITH UK
T. OHNSTAD NO J. VAN BAELEN BE
Members of WG C4.32

4. 3
Introduction - 1
• Geomagnetic disturbances (GMDs) on the Earth originate at the
Sun and can cause many different impacts on critical systems
including power grids, metallic communications systems,
railways, and pipelines, among others
• The focus in this study was on high-voltage power systems,
which are defined by the IEC as transmission grids operating at
voltages above 100 kV (and usually at much higher voltages)
• The emphasis on high voltage is because at the present time
most of the power delivered to end users is concentrated in the
delivery system from large power plants to the cities where the
power is used
• In order to understand the effect of GMDs on the power system,
we need to understand the basic types of geomagnetic storms,
the B-fields they create and how they may affect the power
system

5. 4
Introduction - 2
• In 2013 the international organization Cigré (Conseil
International des Grands Réseaux Électriques, in English – the
International Council on Large Electric Systems) began a
research study to understand the measured geomagnetic data
accumulated over the past 30 years
• The work was undertaken by Cigré Study Committee C4, which
formed a working group (C4.32) chaired by this author
• The focus of work was to examine measured magnetometer data
at different latitudes and longitudes and to determine the
importance of the “type” of geomagnetic disturbance and
whether there is a correlation between the “type” and the
likelihood of effects on the power grid
• Electrojet storms
• Sudden Impulse (SI) events
• Coronal Hole High Speed Stream (CHHSS) events

6. 5
Introduction - 3
• This presentation will summarize the work accomplished in WG
C4.32 beginning with a description of the three types of
geomagnetic storms
• The focus will be on large events, but the study does not
attempt to determine the worst case storm levels as high quality
digital data does not exist for enough time
• Also it is difficult to deal with the “standard” indexes published
by NOAA and other space weather groups that rank the level of
geomagnetic storms as they are not adequate for determining
impacts on power systems
• The results of this work were published in Technical Brochure
780 in October 2019

7. 6
Scope of CIGRE Study Committee C4
• The scope of SC C4 covers system technical performance
phenomena that range from nanoseconds to many hours. SC
C4 focuses on methods and tools for analysis of system
technical performance. SC C4 has been engaged in the
following topics:
• Power Quality
• EMC/EMI
• Insulation Coordination
• Lightning
• Power systems performance models and numerical analysis

8. 7
Power System Phenomena Considered in SC C4

9. 8
SOLAR ACTIVITY
PS1/Q1: G. BARBERA
(Argentina)

10. 9SOHO Image - June 9, 2002
Coronal Mass Ejection (from Active Sunspot)

11. 10
PS1/Q1: G. BARBERA
(Argentina)
Sunspot Cycle and Large Geomagnetic
Storm Events
Large Geomagnetic Storms can and do occur at anytime in the Sunspot
Cycle and not just around the Sunspot peaks

12. 11
Storms have Continent-
Wide Footprints
Field
Disturbances from
Electrojet Current
Couple to
Power Systems
Geomagnetic Storms and HV Power Grids

13. 12
Saturated
GIC in Transformers Leading to Saturation
The Root Cause of All Power Grid Problems
Normal

14. 13
ELECTROJET STORMS
PS1/Q1: G. BARBERA
(Argentina)

15. 14
Cigre Study Electrojet Data Locations
While data sources were all in Scandinavia, they are applicable at
the same geomagnetic latitudes throughout the world

16. 15
Geomagnetic Field Inclination

17. 16
PS1/Q1: G. BARBERA
(Argentina)
• The two horizontal vector components of the disturbed B field
were measured for all types of storms
• It was necessary to subtract the static geomagnetic fields in order to
examine the behavior of the orientation of the dynamic B-fields
• In many of the plots in the TB and in this presentation, the
magnitude, with some arbitrary DC offset removed, is plotted
• For E field calculations it is not appropriate to simply use the
magnitude for the B field to compute E as both components are
computed separately and then combined as the total magnitude of
the electric field
• In many cases the orientation of the disturbed B- and E-field vectors are
plotted
• Thank you to: http://wdc.kugi.kyoto-u.ac.jp/caplot/index.html for
most data evaluated in this project
Data Analysis of Measured Data

18. 17
Electrojet Data Summary

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PS1/Q1: G. BARBERA
(Argentina)
Comparison of Measured Electrojet Data (4 locations)

20. 19
PS1/Q1: G. BARBERA
(Argentina)
Comparison of E-Field Vector Rotations

21. 20
Cigré Electrojet Data ExampleComparison of Measured Electrojet Data (2 locations)

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SUDDEN IMPULSE STORMS
PS1/Q1: G. BARBERA
(Argentina)

23. 22
Sudden Impulse (SI) Geomagnetic Storms
• The SI is also produced by a coronal mass ejection (CME) at the
Sun which arrives at the Earth
• When there is a sudden increase of the solar wind dynamic
pressure, an electromagnetic field is generated that propagates
to the Earth’s surface appearing essentially as a plane wave
• While exposing the Sun facing side of the Earth, it also propagates to the
nighttime side with little attenuation
• The rapid time derivative of the SI can produce high peak E-fields
• Because of the exposure mechanism, SIs expose all latitudes of the Earth
simultaneously
• There have been reported cases of power system impacts at low
latitudes due to SIs
• Given the short duration (minutes) of an SI induced electric field
and the accompanying GIC, it appears that it is more likely to
create a blackout due to voltage collapse as opposed to
transformer damage

24. 23
Data Used in SI Study
• NOAA (Boulder, Colorado) provided a list of 167 Sudden Impulse
events from 1997 to 2005 (with 22 selected for closer examination)
• NOAA measurements used 1 minute time steps
• Data were downloaded for 28 sites, including:
• 10-second Norway data (thank you Magnar G. Johnsen of Tromso
Geophysical Observatory)
• 1-second data for Japan and China
• 1-minute data for North and South America and Africa
• (thank you http://wdc.kugi.kyoto-u.ac.jp/caplot/index.html for most of the
non-Norwegian data.)
• For Norway we had 10-second and 1-minute data, and E field
conversions for both were not greatly different

25. 24
SI Data Locations

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!
Comparison of an SI at Worldwide Locations

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Computed Horizontal Electric Fields-SI

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PS1/Q1: G. BARBERA
(Argentina)
Rotation of SI Horizontal E-field

29. 28
CORONAL HOLE STORMS
PS1/Q1: G. BARBERA
(Argentina)

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Coronal Hole Geomagnetic Storms
• The Coronal Hole term is used as it refers to a different type of
emission of charged particles from the Sun
• Sometimes these are also identified as coronal hole high speed
streams (CHHSS)
• Usually after the peak of the Sunspot maximum, active regions
on the Sun will develop multiple high-speed continuous
streams
• As the Sun rotates every ~27 days, these streams if Earth
directed, can return to expose the Earth
• This effect is similar to that of a rotating lawn sprinkler
Coronal Hole High Speed Stream Description

31. 30
Example of Coronal Hole Transformer Impacts
• The Ap index is used to characterize all daily geomagnetic storm
environments
• For an entire day (or for a 3 hour period) a value for Ap between 30 and
50 indicates a minor storm (green color in the next chart)
• Due to the rotation of the Sun and during a time usually after the peak
of the sunspot number, a pattern of minor storms can be observed if a
coronal hole is active
• In 1994, 5 Generator Step Up (GSU) transformers were damaged in the
Chicago area during a long period of coronal hole storms
• No measured currents were available on the damaged transformers, but
it was clear from nearby magnetic field measurements and a few GIC
measurements in the region, that long lasting quasi-dc currents were
being induced in the high voltage network producing saturation and
possibly hot spot heating
• The GIC levels were apparently not high enough to cause a voltage
collapse of the network
Exposure Characteristics of CHHSS

32. 31
Ap Index in 1994CHHSS Activity in 1994

33. 32
Cigré Coronal Hole Data – 4 MonthsMeasurements of CHHSS Activity in Early 1994

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Cigré Coronal Hole Data – 4 Days
!
April 1-4 CHHSS Activity

35. 34
COUPLING TO POWER
GRIDS
PS1/Q1: G. BARBERA
(Argentina)

36. 35
Ground resistivity versus depth for four example
ground models
Geo-electric field response of example
ground models
Layered-Earth Ground Conductivity
Models and Their Impact on Electric Fields

37. 36
Area Coverage at One Minute in Time

38. 37
A Simple 100 km Transmission Line and Transformer Connections to
Ground Illustrate GIC Flow Circuit Principles
Power Grid Design Trends and GIC Risk Factor
Geo-Electric Field
Apply a Uniform 1 V/km Geo-Electric
Field
Review Circuit
Resistance Attributes

39. 38
Resistance per Phase vs. kV Rating
Trend – ~Factor of 10 Decrease in R
and ~Factor of 10 Increase in GIC
-5 illustrates how the line resistances vary as a function of the transformer operating volta
h there is variation within a voltage class, the trend is clear that as higher voltages w
ed in more recent years, larger conductors (and conductor bundles) reduced the resistance
th of the transmission lines resulting in higher GICs.
Figure 7-5. The per phase resistance in ohms per kilometer as a function of
the operating voltage of the transmission line (based on U.S. line data).
e line data (the curve fit shown in Figure 7-5) and adding in the other smaller resistances
sformer windings and the grounding, one can compute the typical GIC as a function

40. 39
A/Phase Increases Due to Lower Line Resistances
a 100-km Long Line and an E-Field of 1 V/km
Trend: the higher the kV rating – the higher the GIC an

41. 40
DEVELOPMENT OF GIC
WAVEFORMS FOR
TRANSFORMER TESTING
PS1/Q1: G. BARBERA
(Argentina)

42. 41
Development of GIC Waveforms for Testing
• For each type of storm a worst-case GIC waveform was
developed using the maximum electric fields (found in this
study) and a low resistance power line typical for >500 kV
• The peak currents and time to decrease to a 10% value were:
• SI: 120 A/phase, 2 minutes
• Electrojet: 630 A/phase, 2 hours
• CHHSS: 290 A/phase, 6 hours (and repeating over several
consecutive nights
• Detailed GIC waveforms were provided to Study Committee A2
which is developing GIC test methods for large power
transformers

43. 42
Additional Information
• A chapter is provided to describe typical geomagnetic storm
warning capabilities that could be used by the power industry
• Another chapter deals with possible mitigation methods to
reduce or block GIC
• Annexes are provided dealing with
• An actual voltage collapse case due to an electrojet
geomagnetic storm
• How to set up a GIC measurement on a transformer
• An example of a relay trip during a Sudden Impulse storm

44. 43
Conclusions
• Cigre Working Group C4.32 made significant progress in
understanding the measurements of geomagnetic fields over
the past 30 years
• While electrojet storms are important, two other types of storms
have the potential to create serious problems on power grids
world wide
• Sudden Impulse storms do not vary significantly with latitude
and are therefore a threat to voltage stability at any location in
the world
• They also occur more often than other types of storms
• Coronal Hole storms have the ability to damage transformers
due to the long duration of the magnetic fields and also due to
their repetitive nature creating a “dose” effect
• The levels of GIC may be low enough not to trigger a voltage instability