IEEE Electrification Magazine - December 2015 - 56

Figure 3. Transmission lines in a 1,000-kV power grid (photo by Ke Xu).

Figure 4. Conductors on a 1,000-kV transmission line (photo by
Xiaomin Zou).

E ＝ 1 V/km

Huainan

Nanjing

Taizhou
Suzhou

0-100 A
100-150 A
150-200 A
200-300 A

Zhebei
Wannan

Shanghai

Zhezhong

300-400 A

Zhenan
Fuzhou

GIC Flowing from Neutral Point into Earth
GIC Flowing from Earth into Neutral Point
Figure 5. GIC in a UHV power grid under an eastward geoelectric
field of 1 V/km.

I E E E E l e c t r i f i cati o n M agaz ine / december 2015

influence of edge effect are exposed to higher risk in that
they suffer from larger GIC values.
To explain edge effect, we would like to use the GIC
calculation result with part of China's 1,000-kV UHV
power grid blueprint. The part used here is the grid that
has already been put into operation or is currently under
construction. The result is shown in Figures 5 and 6,
where different sizes of circles and thicknesses of lines
are used to represent different levels of GIC, and the hollow or solid circles represent whether GIC flows in or out
of the neutral point in a substation. These two figures
show GIC values in 1,000-kV transformers and transmission lines under the stimulation of a 1-V/km geoelectric
field that is either eastward or northward, where GIC
interaction with the 500-kV grid is ignored. The edge
effect can be demonstrated here. If we take the direction
of the geoelectric field as the reference direction in each
figure, then substations in the middle of a line have relatively smaller GIC, while substations located at the start
point or end point of the direction have larger GIC; also, a
larger number of transmission lines connected along the
direction leads to larger GIC in these substations. To be
specific, in Figure 5, where the reference direction is
pointed from west to east, the Huainan and Shanghai
substations have a larger GIC than everywhere else, with
values of 382.9 and 375.88 A, respectively, and we can see
in Figure 5 that they are located on the west and east
side of the grid. As with Figure 6, the Huainan and
Fuzhou stations have large GIC values of 206.48 and
248.81 A in their transformer neutral points, respectively.
They are located on the south and north sides of the grid,
coinciding with the start and end points of the reference
direction here. When compared with Figure 1, the network used in Figures 5 and 6 is smaller in scale and number of transmission lines, which explains the correspondingly smaller calculated GIC values.
Furthermore, in terms the structure of three-phase
transformers, core-type transformers are currently widely
used in the power grid, while shell type is quite rare. The
UHV main transformers, however, are designed with the
single-phase, four-pillar type or single-phase five-pillar
type, both of which are varieties of the shell type. Moreover, UHV transformers are equipped with a separated
tap-changer transformer and compensation transformer.
Figure 7 shows the core structure of a single-phase, fivepillar UHV transformer, where the core structures of the
main transformer, tap-changer transformer, and compensation transformer are presented from left to right. For
single-phase, five-pillar UHV transformers, the main
transformer and tap-changer transformer are both shelltype transformers.
According to the research on GIC-caused leakage flux
in 500-kV shell-type transformers (the structure of
which is almost identical to the UHV transformer in
Figure 7), as Dr. Bing Zhang pointed out in his dissertation (Zhang, 2010):

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