IEEE Electrification Magazine - December 2015 - 9

The magnitude of
dc flux density shift
in the core depends
on the magnetic
reluctance of the
dc flux path.

magnitude of the dc as well as the design of the transformer.
The flux density shift adds to the ac flux in one half cycle and
subtracts from it in the other half cycle, as shown in Figure 1(a).
When the dc flux is large enough, it leads to core peak flux densities in the magnetic core presaturation range in one half of the
cycle. As shown in Figure 1(b), the B-I characteristics of the transformer core materials are inherently very nonlinear. For higher
magnitudes of dc, the core provides an increasingly higher reluctance to the dc ampere-turns, thus resulting in further but smaller
increases in the flux density shift and a higher peak magnetizing
current pulse.

Factors Affecting the Magnitude of dc Flux Density Shift
The magnitude of the dc flux density shift in the core depends on the magnetic reluctance of
the dc flux path. Thus, the dc flux density shift in a three-phase, three-limb core form would
be the lowest of all core types. This is because this design offers an order of magnitude higher
magnetic reluctance to the dc ampere-turns (see Figure 2). Therefore, this core type is less
susceptible to core part-cycle saturation. As shown in Figure 2, all other core types offer much
less reluctance to the dc ampere-turns because the path of the dc flux is through the core,
which has orders of magnitude higher permeability. They are more susceptible to core partcycle saturation at lower levels of dc.
Also, while the design/operating flux density of the core and the type of core joints are
important factors in determining the final value of the peak of the sum of the ac and dc
flux density in three-phase core-form transformers with a three-limb core, this is not the
case for transformers with other core types.

Magnitude of Magnetizing Current Associated with GICs
Figure 3 presents an example of calculated magnetizing current pulses in a 250-MVA, 500-240-kV
single-phase autotransformer of core-form construction, when subjected to dc/GIC levels of 10,
15, and 20 A. As shown in the figure, the peaks of the current pulses in this case are 18, 27, and
37% of the full-load current, respectively. The average duration of this current pulse is only in the
range of 1/10th to 1/12th of a cycle.
Figure 4 presents the percentage magnetizing current drawn by the same 250-MVA
transformer. The magnitude of the magnetizing current varies proportionally with the
magnitude of the dc/GIC flowing through the transformer windings.
This magnetizing current will increase the effective reactive power absorbed by the
transformer. Consequently, there is a large increase in the reactive power demand for the
duration of the GIC flow in the bulk electric system. The additional reactive power demand
during a geomagnetic disturbance (GMD) event, if not offset by available resources, can
cause a reduction in system voltage to the point of encroaching on secure system limits. In
extreme cases where a severe GMD is coupled with multiple contingencies occurring over
a short period of time, voltage collapse could result. Thus, the reactive power loss resulting
from part-cycle saturation of transformer cores is one of the major concerns during GMDs.

image licensed by ingram publishing

Magnetizing Current Harmonics Associated with GICs
As stated previously, the magnetization current pulse injects higher-order odd and even
current harmonics into the power system to which the transformer is connected. Figure 5(a)
and (b) presents the calculated magnitudes of the magnetizing current harmonics versus
the magnitude of GICs for the same 250-MVA transformer. The figure shows that the
higher the level of GICs, the greater the magnitude of the harmonics. This is obviously
caused by the higher magnetizing current for higher GIC levels. Figure 5(a) and (b) also
shows that the harmonic content of the magnetizing current pulse associated with GICs
is characterized by magnitudes of harmonics that do not decrease significantly for higher-order harmonics. This is due to the short duration nature of the magnetizing current
pulse. These harmonics can cause electrical resonance, and differential relays misoperate.
As a result, the grid stability may be compromised.
IEEE Elec trific ation Magazine / d ec em be r 2 0 1 5

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