IEEE Electrification Magazine - December 2015 - 20

CT

Neutral Bushing

Figure 10. A GIC blocking device installed at an actual HV transformer. (Photo courtesy of American Transmission Company.)

transformer as one having a blocking device. To understand this, use the 20-bus test case. Apply a 1-V/km eastward electric field, and then calculate the GICs. Next, open
the "Transformers" tab to view the results. Figure 11 shows
the results sorted by the highest effective GICs, with transformers 6-7 and 6-8, both in Substation 6, having the highest values. Also, the total reactive power losses in this case
amount to 228 Mvar.
Next, toggle the "GIC Blocked for Transformer Neutral"
field for both transformers to "YES" and solve the GICs.
The model assumes an infinite dc resistance in the neutrals of these transformers, and the effective GICs now
reduce to zero. This causes the total reactive losses in the
system to be reduced to 121.4 Mvar.

Figure 11. The transformer results for the 20-bus test system.

20

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

Figure 12. The current transformer used for monitoring GICs, located
on the neutral bushing of a transformer. (Photo courtesy of American
Transmission Company.)

The highest effective GIC criteria used in this example for
blocking transformers is a simple method and purely for
illustration. In reality, different scenarios would have to be
studied to make an informed decision regarding the placement of blocking devices. Factors such as the impact on
neighboring regions and their transformers should be considered. In this example, the effective GICs in nearby transformers 12-13 and 12-14 increase from 0.137 to 0.210 per unit after
blocking the neutrals of the transformers in Substation 6,
even though the overall reactive losses in the system reduce.

conclusions
GMD modeling has come a long way, with tools and models now available to perform large-system planning



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

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