[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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3.5.4 Three-Phase <strong>Transformer</strong> Connections<br />
Three-phase connections can be made either by using three single-phase transformers or by using a<br />
three-phase transformer. Advantages of the three-phase transformer is that it costs less, the weight is less,<br />
it requires less floor space, and has lower losses than three single-phase transformers. Circuit computations<br />
involving three-phase transformer banks under balanced conditions can be made by dealing with<br />
only one of the transformers or phases. It is more convenient to use line-to-neutral values, since transformer<br />
impedances can then be added directly to transmission-line impedances. All impedances must<br />
be converted to the same side of the transformer bank by multiplying them by the square of the voltage<br />
ratio.<br />
There are two basic types of three-phase transformers, core type and shell type. The magnetic circuit<br />
of the shell type is very similar to three single-phase transformers. This type of transformer has a return<br />
circuit for each phase of the magnetic flux. Consequently, the zero-sequence impedance is equal to the<br />
positive-sequence impedance. The conditions with respect to magnetizing currents and zero-sequence<br />
impedances are essentially the same as for single-phase transformers connected in the same way. The<br />
center-phase coil is usually wound in a direction opposite to that of the two outer phases in order to<br />
decrease the core yoke required between phases. This reversal of polarity is corrected when the leads are<br />
terminated.<br />
The magnetic circuit of each phase of a three-limb core-type transformer is mutually connected in<br />
that the flux of one phase must return through the other two phases. In this type of transformer, the<br />
total instantaneous magnetic flux in each core section due to the fundamental excitation current is zero.<br />
However, in the wye–wye-connected transformer, there are third-harmonic voltages in the phases caused<br />
by the third-harmonic current. These voltages and the resulting magnetic flux are all induced in the same<br />
direction. Since there is no return path for this flux in the core, the flux must return through the relatively<br />
low-reluctance path outside the core. The core-type transformer is occasionally manufactured with a<br />
five-limb core. In this case, the magnetic circuit and performance characteristics are similar to that of<br />
shell-type or single-phase transformers.<br />
Unbalanced system faults and loads can cause significant zero-sequence magnetic flux to occur for<br />
some three-phase connections. Unless a magnetic return path for this flux is provided, the flux returns<br />
outside the core and can cause eddy-current heating in other transformer conductive components, such<br />
as the tank. The existence of zero-sequence flux either within or outside the core depends on both core<br />
configuration and winding connections. Three-phase transformer-core assemblies do not usually provide<br />
full-capacity return legs for zero-sequence flux. Thus, if sufficient zero-sequence flux occurs, it will be<br />
forced to return outside the core.<br />
Three-phase transformer connections can be compared with each other with respect to:<br />
• Ratio of kVA output to the kVA internal rating of the bank<br />
• Degree of voltage symmetry with unbalanced phase loads<br />
• Voltage and current harmonics<br />
• <strong>Transformer</strong> ground availability<br />
• System fault-current level<br />
• Switching and system fault and transient voltages<br />
In some cases, there may also be other operating characteristics to determine the most suitable<br />
connection for each application.<br />
3.5.4.1 Double-Wound <strong>Transformer</strong>s<br />
The majority of three-phase transformer connections are made by connecting the individual phases either<br />
between the power-system lines, thus forming a delta connection, or by connecting one end of each phase<br />
together and the other ends to the lines, thus forming a wye (also referred to as star) connection. For<br />
these connections, the total rating of the internal windings is equal to the through-load rating. This<br />
accounts for the popularity of these connections. For all other double-wound transformer connections,<br />
the ratio is less than unity. For example, in the interconnected star or zigzag connection, the transformer<br />
is capable of delivering a load equal to only 86.6% of the internal winding rating. Since the cost of a<br />
transformer varies approximately with the three-quarter power of the internal kVA, the cost of an<br />
interconnected star or zigzag transformer is approximately 5% higher than for a similar double-wound<br />
transformer. All of these types of three-phase connections are shown in Figure 3.5.2.<br />
3.5.4.1.1 Wye–Wye Connections of <strong>Transformer</strong>s<br />
Joining together the terminals of similar windings with the same polarity derives the neutral of the wye<br />
connection. This neutral point is available and can be brought out for any desired purpose, such as<br />
grounding or zero-sequence current measurements and protection. For high-voltage transmission systems,<br />
the use of the wye-connected transformer is more economical because the voltage across the phase<br />
of each winding is a factor of 1.73 less than the voltage between the lines. If the neutral point is grounded,<br />
it is not necessary to insulate it for the line voltage.<br />
If the neutral is not grounded, the fault current during a system line to ground fault is insignificant<br />
because of the absence of a zero-sequence current path. If the neutral is grounded in the wye–wye<br />
transformer and the transformer is made with a three-limb core, the zero-sequence impedance is still<br />
high. As a result, the fault currents during a system line-to-ground fault are relatively low. For wye–wye<br />
transformers made of three single-phase units or with a shell-type or five-limb core-type, the zerosequence<br />
impedance is approximately equal to the positive-sequence impedance. The fault current during<br />
a system ground fault for this case is usually the limiting factor in the design of the transformer. In all<br />
types of wye-wye transformer connections, only the transformer positive-sequence impedance limits the<br />
fault current during a system three-phase system fault.<br />
With the wye connection, the voltages are symmetrical as far as the lines are concerned, but they<br />
introduce third-harmonic (or multiples of the third harmonic) voltage and current dissymmetry between<br />
lines and neutral. The third-harmonic voltage is a zero-sequence phenomenon and thereby is exhibited<br />
in the same direction on all phases. If the transformer and generator neutrals are grounded, thirdharmonic<br />
currents will flow that can create interference in telephone circuits. If the transformer neutral<br />
is not grounded, the third-harmonic voltage at the neutral point will be additive for all three phases, and<br />
the neutral voltage will oscillate around the zero point at three times the system frequency. Thirdharmonic<br />
voltages are also created on the lines, which can subject the power system to dangerous<br />
overvoltages due to resonance with the line capacitance. This is particularly true for shell-type threephase<br />
transformers, five-limb core-type transformers, and three-phase banks of single-phase transformers.<br />
For any three-phase connection of three-limb core-form transformers, the impedance to thirdharmonic<br />
flux is relatively high on account of the magnetic coupling between the three phases, resulting<br />
in a more stabilized neutral voltage. A delta tertiary winding can be added on wye–wye transformers to<br />
provide a path for third-harmonic and zero-sequence currents and to stabilize the neutral voltage. The<br />
tertiary in this application will be required to carry all of the zero-sequence fault current during a system<br />
line-to-ground fault.<br />
The most common way to supply unbalanced loads is to use a four-wire wye-connected circuit.<br />
However, the primary windings of the transformer bank cannot be wye-connected unless the primary<br />
neutral is joined to the neutral of the generator. In this case, a third-harmonic voltage exists from each<br />
secondary line-to-neutral voltage because the generator supplies a sinusoidal excitation current. The<br />
third-harmonic currents created by the third-harmonic voltages can be a source of telephone interference.<br />
If the primary neutral is not connected to the generator, single-phase or unbalanced three-phase loads<br />
in the secondary cannot be supplied, since the primary current has to flow through the high impedance<br />
of the other primary windings.<br />
3.5.4.1.2 Delta–Delta Connection<br />
The delta–delta connection has an economic advantage over the wye–wye connection for low-voltage,<br />
high-current requirements because the winding current is reduced by a factor of 1.73 to 58% of that in<br />
the wye–wye connection.<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC