Three-phase transformer circuits
Since three-phase is used so often for
power distribution systems, it makes sense that we would need
three-phase transformers to be able to step voltages up or down. This is
only partially true, as regular single-phase transformers can be ganged
together to transform power between two three-phase systems in a variety
of configurations, eliminating the requirement for a special three-phase
transformer. However, special three-phase transformers are built for
those tasks, and are able to perform with less material requirement,
less size, and less weight from their modular counterparts.
A three-phase transformer is made of
three sets of primary and secondary windings, each set wound around one
leg of an iron core assembly. Essentially it looks like three
single-phase transformers sharing a joined core:
Those sets of primary and secondary
windings will be connected in either Δ or Y configurations to form a
complete unit. The various combinations of ways that these windings can
be connected together in will be the focus of this section.
Whether the winding sets share a common
core assembly or each winding pair is a separate transformer, the
winding connection options are the same:
- Primary - Secondary
- Y - Y
- Y - Δ
- Δ - Y
- Δ - Δ
The reasons for choosing a Y or Δ
configuration for transformer winding connections are the same as for
any other three-phase application: Y connections provide the opportunity
for multiple voltages, while Δ connections enjoy a higher level of
reliability (if one winding fails open, the other two can still maintain
full line voltages to the load).
Probably the most important aspect of
connecting three sets of primary and secondary windings together to form
a three-phase transformer bank is attention to proper winding phasing
(the dots used to denote "polarity" of windings). Remember the proper
phase relationships between the phase windings of Δ and Y:
Getting this phasing correct when the
windings aren't shown in regular Y or Δ configuration can be tricky. Let
me illustrate:
Three individual transformers are to be
connected together to transform power from one three-phase system to
another. First, I'll show the wiring connections for a Y-Y
configuration:
Note how all the winding ends marked with
dots are connected to their respective phases A, B, and C, while the
non-dot ends are connected together to form the centers of each "Y".
Having both primary and secondary winding sets connected in "Y"
formations allows for the use of neutral conductors (N1 and N2)
in each power system.
Now, we'll take a look at a Y-Δ
configuration:
Note how the secondary windings (bottom
set) are connected in a chain, the "dot'" side of one winding connected
to the "non-dot" side of the next, forming the Δ loop. At every
connection point between pairs of windings, a connection is made to a
line of the second power system (A, B, and C).
Now, let's examine a Δ-Y system:
Such a configuration would allow for the
provision of multiple voltages (line-to-line or line-to-neutral) in the
second power system, from a source power system having no neutral.
And finally, we turn to the Δ-Δ
configuration:
When there is no need for a neutral
conductor in the secondary power system, Δ-Δ connection schemes are
preferred because of the inherent reliability of the Δ configuration.
Considering that a Δ configuration can
operate satisfactorily missing one winding, some power system designers
choose to create a three-phase transformer bank with only two
transformers, representing a Δ-Δ configuration with a missing winding in
both the primary and secondary sides:
This configuration is called "V" or
"Open-Δ." Of course, each of the two transformers have to be oversized
to handle the same amount of power as three in a standard Δ
configuration, but the overall size, weight, and cost advantages are
often worth it. Bear in mind, however, that with one winding set missing
from the Δ shape, this system no longer provides the fault tolerance of
a normal Δ-Δ system. If one of the two transformers were to fail, the
load voltage and current would definitely be affected.
The following photograph shows a bank of
step-up transformers at the Grand Coulee hydroelectric dam in Washington
state. Several transformers (green in color) may be seen from this
vantage point, and they are grouped in threes: three transformers per
hydroelectric generator, wired together in some form of three-phase
configuration. The photograph doesn't reveal the primary winding
connections, but it appears the secondaries are connected in a Y
configuration, being that there is only one large high-voltage insulator
protruding from each transformer. This suggests the other side of each
transformer's secondary winding is at or near ground potential, which
could only be true in a Y system. The building to the left is the
powerhouse, where the generators and turbines are housed. On the right,
the sloping concrete wall is the downstream face of the dam:
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