IEEE Electrification Magazine - December 2015 - 57

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there is almost no leakage flux from the core to tank

wall when the GIC is small and the core is not saturated

E ＝ 1 V/km

xx
the leakage flux increases as the GIC becomes larger

Huainan

Taizhou

Nanjing

and the core gradually becomes saturated

Suzhou

xx
the maximum amount of leakage flux happens when

the core is at full saturation
0-100 A
100-150 A
150-200 A
200-300 A

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when the transformer is operating under the influence

of GIC, the current in the winding will cause more leakage flux than it does under normal conditions.
Figures 8-10 show the distributions of core flux density as
well as the leakage flux of transformers with different load
rates. They show that when magnetic flux in the transformer
has reached its maximum amount, an increase in load current in the windings will lead to a reduction of flux density in
the yoke as more leakage flux goes through the space
between the yoke and the windings. Additionally, the phase
of the load current will also have an influence on the leakage
flux. When investigating the influence of leakage flux on
heating effect in magnetic metal components, the calculation result suggests that the temperature rise in the upper
and lower T beams of the core in a shell-type transformer is
most vulnerable to GIC influence. Specifically, GIC continuously over 20 A will cause local overheating in upper and
lower T beams, and the temperature rise will exceed 85°; GIC
continuously over 25 A will cause local overheating in the oil
tank; and GIC continuously over 30 A will cause local overheating in yoke clamping. Temperature rise is what causes
the destruction of a transformer.

Shanghai

Wannan
Zhezhong
Zhenan

300-400 A
Fuzhou
GIC Flowing from Neutral Point into Earth
GIC Flowing from Earth into Neutral Point

Figure 6. GIC in a UHV power grid under a northward geoelectric field
of 1 V/km.

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According to measured data of GIC in a 500-kV power

grid in mid- to low-latitude areas and calculated GIC
values in 750- and 1,000-kV power grids in these areas,
it is the transmission line structure and dc resistance
that determine the GIC level. With the scale of power
grids expanding, GIC is no longer a problem that only
concerns high-latitude areas but a global problem
since geomagnetic storms have the potential to cause
damage on power grids in different areas of the globe.
Areas with more developed economies, and, therefore,
larger power grids, are exposed to higher GIC risk. On
the other hand, as the technologies of large-scale
power grids continue to develop, higher voltage levels
mean higher GIC levels. This can be exemplified by
China's 1,000-kV UHV power grid, which is under construction. It has a transmission line resistance per unit
length of 0.007265 Ω/km and is considered to be a
power grid with high GIC risk. The GICs in some

Summary
So far, many achievements have been made in worldwide
research on GIC in power grids. While a large proportion of
them were achieved by research teams in high-latitude
countries in northern Europe and North America, the GIC
problem is different in different areas of the globe. To sum
up, the following are the main conclusions and suggestions of this article:

(a)

Zhebei

(b)

(c)

Figure 7. The core structure of the single-phase, five-pillar transformer. (a) The main transformer, (b) tap-changer transformer, and (c) compensation transformer.

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

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