Differences Between Current Sources |
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We have seen how normal neutral return currents flowing to, from, or through the earth
cause voltage gradients in the earth and on its surface. We have also seen how
electrical currents split and intermingle on the conductive
network formed by the neutral conductors, grounding conductors, and the earth.
Each of these factors affects the shape and magnitude of voltage gradients.
The most significant factor affecting the resulting gradients, however, is the
difference between the load currents on the supply side (utility) and on the
utilization side (customer) of the transformer. As we learned earlier,
these are the two source currents from which the earth currents and earth
gradients originate.
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The term “load current” reminds us that currents flow only when an
appliance (electrical load)
is turned on. We have noted that whenever a customer appliance
is turned on, two distinct currents are set in motion: one on the supply
side of the transformer
and the other on the utilization side. Even though they are created by
the same electrical load, the levels (magnitudes) of these two currents
are not the same. Furthermore, there is a unique characteristic
called “load-balancing” that comes into play on the utilization side
and another set of load-current issues on the supply side.
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Load Current Comparison |
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There can be significant differences in the magnitude of the current level between the
supply side and the utilization side for the same electrical load. |
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Currents on the utilization side created
by a farmstead load, such as a light bulb or a motor being turned on, are
many times larger than the corresponding currents that these loads create on the supply side.
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Earth gradients associated with utilization current are often
much stronger than the gradients associated with the corresponding supply current.
They are also created on the farm, generally in closer proximity to confined livestock.
On a distribution system of 14,400 volts, the utilization-side current can be
as much as 60 times the size of the supply current.
Other factors tend to lower the net return current on the utilization side
and raise neutral return current levels on the source side. |
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Load Balancing on the Utilization Side |
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The three-wire, 120/240-volt power service is uniquely North American. Much of
the rest of the world uses low-voltage, three-phase systems similar to distribution systems.
The American system, also referred to as the Edison system, incorporates two
120-volt circuits that can be used separately or together as a 240-volt circuit.
The two 120-volt circuits have a common return wire in the neutral conductor.
If the two 120-volt circuits
are balanced (operate at all times with the same amount of electrical load),
then they appear as single 240-volt circuit with no current in the neural
wire. The neutral wire carries the unbalanced current between the two 120-volt circuits.
This is illustrated below.
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In the animation, click on either switch to turn its corresponding light bulb on.
The two light bulbs (loads) can be turned on separately or together.
Choose one of the “show” buttons above the figure
to view all the currents, the supply circuit currents only, or the utilization
circuit currents only. The animation viewing options allow you
to see different combinations of
currents to simplify the presentation and help focus on specific currents.
In reality, when an electrical load is turned on, all currents immediately begin to flow,
as illustrated by the “show all” option.
Explore the various load combinations and observe the
current paths and directions in each instance.
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The amount of load that is energized on either of the two 120-volt circuits is a random
value at any one time, because it depends on which appliances are turned on at that
instant and how they are distributed between the two circuits. Good electrical wiring
at a residence or farmstead will include an attempt to evenly distribute loads between
the two circuits. This will help in maintaining a state that is close to balance,
thus minimizing current on the neutral and reducing earth current and resulting
earth gradients. Continuous perfect balance, however, is a practical impossibility. |
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Some current will always flow on the utilization neutral conductor,
although this current will tend to be smaller than the overall load current.
The amount of neutral current is not tied to the load current itself,
but to the balance state. There may be more current on the
neutral when a single light bulb is lit than when ten bulbs are lit. |
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Although the current on the utilization neutral
may potentially be much greater than the current on the supply neutral,
the actual difference may be slight.
It may also change erratically as the load balance on the service wires
changes with the activation of different appliances. |
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Many Loads on the Supply Side |
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The utility's supply system, also called the distribution system,
is designed to provide power to many customers.
In densely populated areas, a single transformer may serve
many customers, causing the supply current at the transformer to rise with the number
of customers. |
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A similar process occurs on rural systems such as the one
we are examining here. Even when the transformer serves only a single customer,
the power line may continue down the road to more farms and more transformers.
The supply currents for those farms will accumulate on the supply conductors
and flow by the service transformer under examination. The neutral return current
that leads to earth gradients may also increase. |
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The overall current on the supply side of the transformer may then be higher than that
called for by the load at the farm. Some of this current will flow from the neutral
conductor into and through the earth, increasing the supply-side gradients.
The amount of neutral current from other sources
present near the transformer in question and whether this current flows
from the earth to the neutral or vice versa
depends on many factors, including the location of the transformer on the power line,
(e.g. main line versus a lateral line), the amount and location of the other
electrical loads, the number, quality, and location of system grounds,
and whether the other loads are single- or three-phase. |
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Conclusions |
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The electrical currents and therefore the earth gradients associated with any electrical system,
either utility- or customer-owned, are complex and dynamic. While a rigid relationship
exists within a transformer between
the supply current and the utilization current, this relationship tends to break down
farther from the transformer.
On the farmstead side, the prevalent use of two 120-volt circuits with a neutral
as the common conductor often
reduces the neutral current but introduces a random element
to the amount of neutral current present. On the supply side, there is
a random accumulation of additional supply and neutral currents from other farms down the road.
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