IEEE Electrification Magazine - December 2015 - 12

50
45
40
35
30
25
20
15
10
5
0

28

Second Harmonic
Third Harmonic
Fourth Harmonic
Fifth Harmonic

25

50

75

Idc = 10 A /Phase
Idc = 20 A /Phase
Idc = 50 A /Phase

27
26
25
24
23

0

Winding Hot-Spot Gradient Versus Time

29
Temperature (°C)

% Peak Harmonics
(% of Rated Load Current)

Peak Amplitude of Harmonics in
% of Rated Load Current Versus GIC

100 125 150 175 200

0

5

10 15 20 25 30 35 40 45 50
Time (min)

GIC (A /Phase)
Figure 6. The calculated hot-spot temperature winding rises of a fully
loaded 250-MVA single-phase autotransformer due to dc.

(a)

50
45
40
35
30
25
20
15
10
5
0

50

Sixth Harmonic
Seventh Harmonic
Eighth Harmonic
Ninth Harmonic
Tenth Harmonic
11th Harmonic

40
30
20
GIC (A)

% Peak Harmonics
(% of Rated Load Current)

Peak Amplitude of Harmonics in
% of Rated Load Current Versus GIC

10
0
-10
-20

0

25

50

75

100 125

150 175

200

GIC (A /Phase)
(b)

-30
-40
0:14:24 1:26:24 2:38:24 3:50:24 5:02:24 6:14:24
Time

Figure 5. The magnitudes of magnetizing current harmonics: (a) second to fifth harmonic and (b) sixth to 11th harmonic.

Figure 7. The measured GIC profile at neutral of a three-phase bank
of GSUs during a 7-h period.

much longer (30-45 min) than the duration of the high-peak
GIC pulses, and, correspondingly, only small increases in the
core temperatures would be experienced.
According to the February 2012 North American Electric Reliability Corporation Task Force Interim Report [1],
the most likely consequence from a severe GMD event is
a power outage and not widespread transformer thermal
failures. The combination of increased reactive power
absorption and injected harmonics into the system by
saturated transformer cores can lead to voltage instability and subsequent system collapse.

subsequent power-system collapse. Because of the short
duration of high-peak GIC pulses, even high levels of GICs
would not cause damaging overheating in transformers.

For Further reading
"Effects of geomagnetic disturbances on the bulk power system," North American Electric Reliability Corporation Special
Reliability Assessment Rep. NERC2012, Feb. 2012.
R. Girgis and K. Vedante, "Methodology for evaluating
the impact of GIC and GIC capability of power transformer
designs," presented at the IEEE PES Conf., Vancouver, BC,
Canada, July 2013.

cONcLUSIONS
High levels of GICs cause transformer core saturation,
resulting in a narrow high-peak pulse of magnetizing current. The magnitude of the magnetizing current and its
harmonics vary proportionally with the magnitude of GIC
flowing through the transformer windings.
The combination of increased reactive power absorption and injected harmonics into the system by saturated
transformer cores can lead to voltage instability and

12

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

biographies
Ramsis S. Girgis (ramsis.girgis@us.abb.com) is the R&D
manager at the ABB Power Transformer Plant, St. Louis, Missouri. He is a Life Fellow of the IEEE.
Kiran B. Vedante was a consulting R&D engineer at the
ABB Power Transformer Plant, St. Louis, Missouri. He is a
Senior Member of the IEEE.



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

IEEE Electrification Magazine - December 2015 - Cover1
IEEE Electrification Magazine - December 2015 - Cover2
IEEE Electrification Magazine - December 2015 - 1
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IEEE Electrification Magazine - December 2015 - Cover3
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