IEEE Electrification Magazine - December 2015 - 40

adding a dc offset to the magnetic flux. One direct result
from this dc offset is the shift of the transformer operating
range. Altering the magnetizing current with a dc offset
may lead the transformer into half-cycle saturation
(Figure 6). Recent studies and operational experiences have
shown that the increase of magnetizing current magnitudes is approximately in proportion to the GIC current

Z
Y
X
(a)

Harmonics Amplitude (%)

100
90
80
70
60
50

Sixth
Seventh
Eighth
Ninth
Tenth

Fundamental
Second
Third
Fourth
Fifth

40
30
20
10
0
0.1

1
10
100
Direct Neutral Current (A /Phase)

1,000

(b)
80
Q (Mvar/Phase)

70
60
50
40
30
20
10
0
0

50 100 150 200 250 300 350 400 450
Direct Neutral Current (A /Phase)
(c)

Figure 7. A GIC impact study on shell-type transformer design:
(a) transformer magnetic-field analysis of a shell type transformer
(1/4 model), (b) GIC level versus harmonics, and (c) GIC level versus
var consumption.

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

magnitude. The saturation condition causes an increase in
var consumption by the transformer and the generation of
even and odd harmonics. If a transformer operates in saturation for long enough, the produced harmonics and abnormal
stray flux could enter the transformer internal clamping
structure, tank, and windings, potentially causing overheating and degradation of insulation, which may shorten the
asset's life span. Other negative impacts may include an
increase in dissolved gases and noise levels.
Of particular note is that transformer flux paths differ
with different transformer designs. Normally, singlephase, three-phase five-leg core form and three-phase
shell form transformers are considered more susceptible
than three-phase three-leg core-form transformers. And
more intuitively, higher GIC would be induced on higher
voltage levels, larger-capacity transformers, and longer
transmission lines that interconnect with transformers.
Industry regulation is still weak on the phenomena.
NERC Standard EOP-010-1, Geomagnetic Disturbance
Operations, is the first NERC standard that aims at mitigating the effects of GMD events by implementing
operating plans, processes, and procedures. DVP has
assisted with this standard. The IEEE Transformers
Committee is on track to publish a new guide, IEEE
C57.163, Guide for Establishing Power Transformer Capability While Under Geomagnetic Disturbances. This guide
will be the first industry document to identify GMD
issues and the impact to transformer designs. With the
recognition of industrial guidelines and the study of
risks associated with GMD phenomena in the DVP service territory, DVP is mainly focused on its extra-highvoltage (EHV) system-230 kV and above-yet a few
115-kV locations are also considered.
DVP currently has a fleet of more than 200 transformers serving its EHV transmission system. The majority is
composed of single-phase core- or shell-type transformers, but there are a few three-phase core-type transformers in service. Back in the 1990s, DVP's power transformer
technical specifications were enhanced considering overloading and overheating problems that can be experienced
during events. A more comprehensive contemplation of
GIC impact was initiated in 2008, when GIC signatures
with varying magnitudes and durations for DVP's new
EHV-rated GSU and transmission transformers have been
specified. An example of a specified GIC signature is as
follows: 15 A for 30 min, followed by 60 A for 10 min, followed by 15 A for 3 h, assuming that the transformer is
experiencing 105% excitation and 100% loading.
A close collaboration has been established between
DVP and its EHV transformer manufacturers to implement some practical design measures to increase GIC
capability. Some of the design measures taken have been
to apply nonmagnetic t-beam support members on shellform transformers and nonmagnetic yoke beams and
high-temperature-rated tie plate insulation on core-form
transformers. These design provisions have effectively

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