Electric Power Generation, Transmission, and Distribution

entering the neutral of the transformer, while virtually no change is evident in the top oil readings.
Because the hot spot is confined to a relatively small area, standard bulk top oil or other over temperature
sensors would not be effective deterrents to use to alarm or limit exposures for the transformer to these
conditions.
Designing a large transformer that would be immune to GIC would be technically difficult and
prohibitively costly. The ampere turns of excitation (the product of the normal exciting current and the

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3 Core

1 Ph

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Reactive demand

(MVars)

GIC transformer neutral (A)

Transformer reactive demand

FIGURE 16.4 The exciting current drawn by half-cycle saturation conditions shown in Fig. 16.3 produces a
reactive power loss in the transformer as shown in the top plot. This reactive loss varies with GIC flow as shown.
This was measured from field tests of a three-phase bank of single-phase 500=230 kV transformers. Also shown in the
bottom curve is measured reactive demand vs. GIC from a 230=115 kV three-phase three-legged core-form
transformer. Transformer core design is a significant factor in estimating GIC reactive power impact.

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Exciting current (A)

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Harmonic order

Transformer harmonics

FIGURE 16.5 The distorted transformer exciting current shown in Fig. 16.3 has even and odd harmonic current
distortion. This spectrum analysis was half-cycle saturation conditions resulting from a GIC flow of 25 A per phase.