IEEE Electrification Magazine - December 2015 - 11

(a)

(b)

(c)

(d)

Figure 2. Core dc flux path in various core types: (a) core form, three phase, three limb; (b) core/shell form, one phase; (c) core form, three
phase, five limb; and (d) shell form, three phase, seven limb.

35
30
25

Magnetizing Current Versus Phase Angle
for Various GICs
GIC = 10 A /Phase
GIC = 15 A /Phase
GIC = 20 A /Phase

20
15
10
5
0

0 30 60 90 120 150 180 210 240 270 300 330 360
Phase Angle (°)

Figure 3. Magnetizing current pulse due to dc.

example, the temperature rise of the transformer windings
due to a 50-A GIC pulse of 2-min duration would only be
about 1 K versus the 5-K rise due to dc.
Another consequence of the unidirectional flux density
shift in the core is that significant increases in core losses are
experienced for the duration of the GIC pulse. The increase in
core losses results in an increase in the core hot-spot temperature. However, the core's thermal time constant is typically

% Peak Magnetizing Current Versus GIC
100
90
80
70
60
50

Peak of Magnetizing Current
(% of Rated Full Load Current)

Magnetizing Current (% Rated Current)

duration for medium-level GIC pulses is in the range of several to tens of minutes. Therefore, the thermal responses of
transformer windings and structural parts to GIC pulses will
need to be based on much shorter durations than the steady
state due to dc. Thus, referring to Figure 6, it can be shown
that the increase in the hot-spot temperatures of the windings and structural parts of a transformer due to GIC pulses
would be only a fraction of that calculated for dc. For

40
30
20
10
0

0

5

10 15

20

25

30 35

40

45

50

GIC (A /Phase)
Figure 4. Peak magnetizing current versus GIC/phase.

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

11



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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