IEEE Electrification Magazine - December 2015 - 5

Reports from North American power
utilities show lines tripping, voltage
sag, and other operational incidents,
coinciding with each magnetic substorm. At 07:45 UT [02:45 Eastern
Standard Time (EST)], the auroral electrojet had moved down to sit above
the Hydro Quebec power system in
Canada, and the rapid magnetic-field
change of a substorm onset caused
large GICs to flow across the system.
Transformer saturation led to high
harmonic levels that caused the tripping out of the static var compensators (SVCs) on the system. The
increased reactive power demand
from the transformers, coupled with
the lack of voltage support from the
tripped SVCs, caused a system shutdown. Subsequent phases of the magnetic storm continued to produce
lesser operational problems on many
other systems. On the evening of 13
March 1989, at 21:58 UT (16:58 EST),
the auroral electrojets had moved to

even lower latitudes, and a surge of
the electrojet currents produced disturbances down to midlatitudes. This
event is the likely cause of transformer damage discovered at sites on the
eastern seaboard of the United States
and in the United Kingdom.
The March 1989 event prompted a
surge of research into modeling GICs in
power systems that led to a better
understanding of how the geomagnetic
disturbances, ground structure, and system characteristics influence the size of
GICs that occur in different parts of a
network. The external, ionospheric, and
magnetospheric currents create magnetic-field variations that induce electric
currents in the Earth, creating a magnetic field that contributes to the magnetic
disturbance observed at the Earth's surface. Inside the Earth, the induced currents act to cancel the external magnetic-field variation, resulting in a decay of
the disturbance with depth characterized by the skin depth of the region. At

GICs frequencies, the skin depths range
from tens to hundreds of kilometers,
and the Earth resistivity down to these
depths, not just the surface soil, has to be
taken into account to determine the
electric fields produced during geomagnetic disturbances.
The geomagnetically induced electric fields can be used as input to a network model to calculate the GICs that
will occur. The GICs are driven by the
induced electromagnetic field (EMf)
produced by the magnetic field variations directly in the transmission lines
(figure 1). These can be represented in
the modeling by voltage sources in the
lines equal to the integrals of the electric fields along the length of each line.
Because of the low frequencies involved,
the network is represented by the
resistance values of its components.
Although simple in principle, one of
the biggest problems in this modeling
is finding the appropriate resistance
values to use. The voltage sources and

IEEE Electrific ation Magazine / d ec em be r 2 0 1 5

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Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2015

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