Combined Supply-Side Gradients |
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Some of the supply-side neutral return current leaves the neutral conductor to flow through the earth,
creating field gradients on the earth's surface
[Field Gd].
As we have seen, this current redistribution happens
automatically and unavoidably as a result of grounding installed on power
lines. Current flowing through these grounds
creates localized spreading gradients, typically at the base of poles
[Spreading Gd]. |
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These two gradients are present at the same time and their effects superimpose
onto each other. The illustration below shows what the resulting combination may look like.
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The pattern of the combined gradients incorporates features from both
components. The dominant feature depends on the relative strength
of the component gradients at any one point.
The ground rod gradient component is strongest near the pole and
prevails in the combined pattern next to the pole. However, the
ground rod gradient component drops off rapidly and the field gradient is all that
remains away from the pole.
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The strength of the gradient from each component depends on the amount of current present.
Since these currents, both the field current and the spreading current at the ground rod, can change
at any time independently of each other, the final combined gradient can be seen
as a dynamic pattern that changes with system loading and the resulting neutral return currents.
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While these gradients are generally too small to be perceived over small distances such as the step length of a cow (approximately five feet),
they may reach considerable levels over the distance of power lines several miles long.
Compared to stairs and terraces, these gradients are like a shallow
incline extending over great distances.
The terrain may look flat, but the incline may rise to hundreds
of feet over distances of a few miles.
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Because these gradients are small, they are easily overshadowed by other
gradient effects, especially near farmsteads where utilization-side gradients
[User Side Gd]
can be much stronger.
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