IEEE Electrification Magazine - December 2015 - 18

core-types are available under "Options > AC Power Flow
Model" and can be used by clicking "Reset All Scalings to
Default Values." A per-unit breakpoint of zero implies a
one-segment linear curve.
Note that the K values provided
here are generic and solely depend
on the transformer type. In reality,
K values may vary across individual
transformers, depending on factors
such as loading and age, even for
the same transformer type. While
performing time-domain GIC analysis, one may also have to consider
the time delay that might exist
between the transformer GICs and
reactive losses. For accurate GIC
studies, the actual K values of
transformers should be used. These
are best obtained from transformer
manufacturers or through transformer testing.
In this way, the GIC-induced
reactive power losses for all transformers can be found, either using
default or actual K parameters. The
next step is to evaluate the effect of the reactive losses on
system voltages. This can be done to ensure, for instance,
that no voltage criteria are being violated. The "Include GIC
in Power Flow" option in the main GIC Analysis Form is
enabled, and then the power flow is solved by selecting
"Tools > Single Solution Full Newton." This computes the
new ac state of the system with the GIC losses. Since voltage change contours are better visualized for large systems,
we now move on to the 20-bus system shown in Figure 7. A
uniform 8 V/km eastward electric field is applied.

The one-line diagram shows contours of changes in bus
voltages caused by the Q GIC of the system's transformers.
The purple arrows show the increase in reactive power
flows in the lines. The red regions
around the HV buses of the system
indicate a fall in voltages due to the
system's increased reactive losses.
The regions shown in blue experience an increase in voltage since
they are mostly generator buses
regulating the voltages of some of
the other buses, such as Bus 1.

The line lengths
calculated using
this method do not
account for the
actual geographic
route of a line,
but they are
a reasonable
engineering
approximation.

18

17

So far, we discussed the data fields
for elements such as substations
and transformers to build the dc
system model required for GIC
studies. The defaults and techniques for estimating values for
missing data fields covered in the
previous sections are particularly
helpful in large-system studies to
fill the gaps in missing/unavailable
data. In this section, certain additional considerations for large-system GIC studies are
covered.
Reliability standards for transmission planning (i.e.,
North American Electric Reliability Corporation (NERC) TPL007-1-Transmission System Planned Performance for Geomagnetic Disturbance Events) are being developed in an
effort to address the GMD issue. According to the proposed
standards, utilities will be required to perform GMD vulnerability assessments. This involves using a benchmark GMD
scenario, which comprises of nonuniform electric fields, to

0.10

7

15

16

Voltage Change

Substation 6

6

Substation 3

(p.u.)

Substation 2

Large-System Analysis

8

19
20

0.00

Substation 5
5

11
-0.10

2

3

12

Substation 4 4

1

Substation 1

Substation 8

21

13

14

Figure 7. Voltage-change contours for the 20-bus system, solved with GIC-induced reactive losses. The purple arrows show the increase in reactive
power flows.

18

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



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

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