Rectifier and transformer thereof

Details Rectifier and transformer thereof Rectifier and transformer thereof

2001-06-15

2002-05-28

Han, Jessica (Department: 2838)

Electric power conversion systems

Current conversion

Using semiconductor-type converter

C363S044000, C363S069000

Reexamination Certificate

active

06396723

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rectifier and transformer using this rectifier whereby three-phase AC is converted to DC with little harmonics.
2. Description of the Related Art
When converting three-phase AC to DC, the most typical method is to employ a single three-phase full-wave rectifier in which six rectifying elements are connected in a bridge configuration. In such a three-phase full-wave rectifier, DC voltage is output by changing over the rectifying elements so that they successively conduct at intervals of 60°. However, with this method, the rectified DC voltage contains a voltage ripple of large amplitude having a period of six times the power source frequency; this produces harmonics which cause various problems.
As a means of dealing with this, 18-pulse AC/DC converters have been proposed such as for example in Laid-open Japanese Patent Publication No. H. 4-229077. As shown in
FIG. 1
, this is characterized in that, for the power lines
1
r
,
1
s
and
1
t
, two transformers
2
and
3
are employed that output six-phase AC of equal voltage but offset in phase by +40° and −40°, respectively. An arrangement is adopted wherein three-phase full-wave rectifiers
4
and
5
are connected to the two transformers
2
and
3
through lines
1
r
′,
1
s
′ and
1
t
′ and
1
r
″,
1
s
″ and
1
t
″, while three-phase full-wave rectifier
6
is directly connected to power source lines
1
r
,
1
s
and
1
t
. The outputs of the three-phase full-wave rectifiers
4
,
5
and
6
are connected in parallel to DC lines
7
p
and
7
n.
FIG. 2
is a transformer vector diagram corresponding to FIG.
1
. R
1
, S
1
and T
1
correspond to the phases of the three-phase AC of the power source, their voltages being input to the three-phase full-wave rectifier
6
. In contrast, voltages corresponding to the vertices R
2
′, S
2
′, T
2
′ of the equilateral triangle obtained by rotating by +40° the equilateral triangle formed by the vertices R
1
, S
1
and T
1
are output from transformer
2
and input to three-phase full-wave rectifier
4
. Likewise, voltages corresponding to the vertices R
3
′, S
3
′ and T
3
′ of the equilateral triangle obtained by rotating by −40° the equilateral triangle formed by the vertices R
1
, S
1
and T
1
are output from transformer
3
and input to three-phase full-wave rectifier
5
.
Since the three-phase full-wave rectifier
4
or three-phase full-wave rectifier
5
conduct so as to fill in the valleys of the DC voltage ripple that is output through three-phase full-wave rectifier
6
in the 18-pulse transformer constructed in this way, the voltage ripple becomes small, and harmonics are reduced.
However, with this system, it is necessary that voltage of magnitude equal to the three-phase AC voltage of the power source should be output from the transformer, and the current flowing must also be uniform in order for the three-phase full-wave rectifiers to conduct equally. Consequently, the current that is rectified through the transformers is large at ⅔ of the total, and transformers are required which can withstand this current capacity. Effective miniaturization of the 18-pulse rectifier is therefore impeded by the fact that practically all of its capacity is accounted for by the transformers.
SUMMARY OF THE INVENTION
Accordingly, one object of present invention is to provide a novel 12- or 18-pulse rectifier and transformer using such a rectifier whereby performance equivalent to the above can be achieved using a transformer of even smaller size.
In order to achieve the above object, a rectifier according to the present invention comprises: a main three-phase full-wave rectifier that converts three-phase AC (R phase, S phase, T phase) into DC; a transformer that outputs AC of a total of 3(n−1) phases corresponding to the points that equally divide by n (n=2, 3) the arcs drawn in a transformer vector diagram in which an equilateral triangle is formed whereof the R phase, S phase and T phase are vertices, centered on each vertex and linking the remaining two points; and (n−1) auxiliary three-phase full-wave rectifier(s) that convert into DC the 3(n−1) phase AC that is output from the transformer, the output lines of the main three-phase full-wave rectifier and the (n−1) auxiliary three-phase full-wave rectifier(s) being connected in parallel.
With a rectifier constructed in this way, the output voltage from the transformer becomes lower than the power source voltage. Furthermore, the current flowing through the transformer in the DC line can be reduced to ¼ of the whole in the case of a 12-pulse rectifier and to ⅓ of the whole in the case of an 18-pulse rectifier, so transformer capacity can be reduced.
In a rectifier according to the present invention the transformer satisfies a transformer vector diagram obtained by adding to the equilateral triangle 3(n−1) straight lines extending parallel with the one side of the equilateral triangle which is furthest, on the sides of the equilateral triangle that are closest to the respective points obtained by the n equal divisions of the arc, in the transformer vector diagram.
With a rectifier constructed in this way, a transformer can be realized with a straightforward winding construction.
In a rectifier according to the present invention, in the transformer vector diagram, the transformer satisfies a transformer vector diagram expressed by the periphery of the 3(n+2)-gon formed by superimposing the 3(n−1)-gon formed with the n points of equal division of the arc as vertices on the equilateral triangle.
With a rectifier constructed in this way, the total number of turns of the winding becomes fewer than in the case of the transformer described above and the capacity becomes smaller, so further miniaturization of the transformer can be achieved.
In a rectifier according to the present invention, in the transformer vector diagram, the transformer satisfies a transformer vector diagram expressed by the hexagon formed by straight lines parallel with the side opposite the equilateral triangle and passing through the vertices of the equilateral triangle and straight lines parallel with the sides adjacent the equilateral triangle passing through the n points of equal division of the arc.
With a rectifier constructed in this way, a transformer of small capacity can be achieved with a simpler winding construction.
In a rectifier according to the present invention, reactors corresponding to the leakage inductance of the transformer are mounted on each phase of the power lines (R phase, S phase and T phase) between the branch point to the transformer and the main three-phase full-wave rectifier.
With a rectifier constructed in this way, the drop in output voltage into the auxiliary three-phase full-wave rectifiers resulting from the leakage inductance of the transformer is balanced by a lowering of input voltage to the main three-phase full-wave rectifier produced by the provision of the reactors, so the conduction angle of the main three-phase full-wave rectifier and auxiliary three-phase full-wave rectifiers can easily be adjusted.
In a rectifier according to the present invention, harmonic attenuators such as DC reactors are provided on the DC lines where the outputs of the main three-phase full-wave rectifier and the (n−1) auxiliary three-phase full-wave rectifiers are connected in parallel.
With a rectifier constructed in this way, the slight remaining voltage ripple in the DC that is output through the main three-phase full-wave rectifier and auxiliary three-phase full-wave rectifiers can be further reduced.
A transformer according to the present invention inputs three-phase AC (R phase, S phase and T phase) and, in a transformer vector diagram in which an equilateral triangle is formed whose vertices are the R phase, S phase and T phase, outputs AC of a total of 3(n−1) phases corresponding to

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