IEEE Electrification Magazine - December 2015 - 10

Flux Density Versus Phase Angle (20-A dc)

2.5

Bm, ac
Bm, (ac + dc)

2.0
1.5
Flux Density (T)

leakage flux changes, and this can cause higher circulating
currents and winding overheating in some old shell-form
designs, which can happen at relatively low levels of GICs.

Thermal Effects of dc on Power Transformers

1.0

Effects on Windings

0.5

In Figure 6, the calculated temperature gradients (above oil
temperature) of the hot spots in the windings of the same
250-MVA transformer are presented when the transformer
is fully loaded and at the same time subjected to three different levels of dc for a 50-min duration. The figure shows
that the steady-state winding hot-spot temperature is
reached within approximately 30 min from the time the
dc is applied. This is a typical thermal response, corresponding to a winding thermal time constant of 7-8 min.

0.0

-0.5
-1.0
-1.5
360

330

300

270

240

210

180

150

120

-30
0

-60

-90

-2.0
Phase Angle (°)
(a)
ac + dc

Effects on Tie Plates

Time

I
ac

ac + dc

ac
Time
(b)

In the absence of dc, the main core magnetic flux is confined mainly to the core cross-sectional area. As the core is
driven into magnetic saturation for a fraction of a cycle,
the permeability of the core material drops down into the
range of the permeability of the material of the tie plate,
which is mostly mild steel. As a result, some of the main
core flux flows into the tie plates, which have a finite
cross-sectional area. This causes higher eddy losses along
the length of the tie plates. As the level of dc increases, the
magnetic flux density in the tie plates increases linearly
until it reaches the magnetic saturation level of the tieplate material and then levels off.
The high level of leakage flux, which is also rich in harmonics, impinges on the tie plates at the top and the bottom of the windings, causing high localized eddy losses.
This component of losses increases approximately linearly with the level of dc. The total of these components of
losses causes the higher temperatures in the tie plates.

Thermal Effects of GICs on Power Transformers
flux density shift in the core caused by dc and (b) the part-cycle saturation of transformer cores.

The nature of the signature of the GIC is most critical in
accurately determining the thermal impact of GIC, rather
than dc, on transformers.

Thermal effects of GIcs on Power Transformers

Typical Signature of GICs

The high magnitudes of the magnetization current and the
associated current harmonics produce higher magnitudes
of leakage flux that is also rich in harmonics. This results in
much higher eddy and circulating current losses in the
windings and the structural parts of the transformer. This
causes corresponding increases in losses and temperatures.
Also, as the core saturates, part of the main flux strays to
the tie plates, tank, windings, etc., causing higher losses
and temperatures in these parts. However, as will be
explained later, because of the short duration of high-level
GIC pulses, the temperature rises of windings and structural parts are much smaller than those calculated for dc.
Additionally, as the core saturates, the pattern of the load

Figure 7 shows an example of a GIC signature that was
measured recently in the neutral of a three-phase bank of
single-phase generator step-up transformers (GSUs) at a
large generating station located in the northeastern United States. As demonstrated in Figure 7, the signature of
GIC is characterized by a large number of narrow consecutive pulses of low to medium levels over a period of hours
separated by a few high-peak pulses of less than a minute
to several minutes duration.

Figure 1. The effect of dc on transformer core magnetization: (a) the

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

Effect of GIC Pulses on Transformer Temperatures
As shown in Figure 7, the duration of high-peak GIC pulses is
in the tens of seconds to a few minutes. The corresponding

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