Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2001-12-18
2003-07-01
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S044000
Reexamination Certificate
active
06587360
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to balancing of the input voltage in a polyphase rectifying apparatus having a circuit structure in which single-phase rectifiers are star-connected.
2. Description of the Related Art
FIGS. 6 and 7
are block diagrams showing and explaining the structure of a conventional three-phase three-wire rectifying apparatus. In
FIG. 7
, reference numerals
21
,
22
, and
23
indicate single-phase rectifiers, and reference numerals
24
,
25
, and
26
indicate converters. According to such star-connected single-phase rectifiers, the input voltage of each single-phase rectifier is given by “line voltage/{square root over ( )}3”. Therefore, if the input source is AC 400V, the input voltage of each single-phase rectifier is approximately 230V, and accordingly, a 200V system can be used in the design of the single-phase rectifiers.
However, for such simple star-connected single-phase rectifiers, the input voltage is not balanced and the merit of reducing the input voltage to a 1/{square root over ( )}' level is not efficiently obtained. The reason for this will be explained below.
FIG. 8
is a diagram showing the general structure of a stabilized power supply which outputs a stabilized (i.e., fixed) output voltage. That is, the stabilized power supply is a “black box” for changing the impedance Zi (observed from the input) according to the output voltage Vo and internally converting and outputting the internal energy which depends on the impedance.
That is, when the output voltage decreases to below a specific level, the stabilized power supply decreases the impedance Zi observed from the input so as to transmit a larger amount of energy, thereby increasing the output voltage Vo. Conversely, when the output voltage increases above a specific level, the stabilized power supply increases the impedance Zi so as to decrease the transmitted energy, thereby decreasing the output voltage Vo.
FIG. 6
shows a generalized structure of the single-phase rectifiers which function as a stabilized power supply. That is, the single-phase rectifier is also regarded as a black box for changing the impedance Zin (observed from the input) according to its own output voltage. For convenience of explanation, it is assumed that the impedance Zic of the converter observed from the input is not changed. This is an ideal condition. In addition, each single-phase rectifier has the same characteristics, that is, it is assumed that the three functions “Ziu=f(Vou)”, “Ziv=f(Vov)”, and “Ziw=f(Vow)”, are the same. This is also an ideal condition.
Under these ideal conditions, if the output voltages Vou, Vov, and Vow of the single-phase rectifiers are the same, the impedances Ziu, Ziv, and Ziw observed from the input are the same. It is obvious that when the impedances Ziu, Ziv, and Ziw are all the same, the input voltages Viu, Viv, and Viw are also the same.
When such an ideal balanced state is slightly unbalanced, for example, when the output voltage Vou of the single-phase rectifier
11
slightly decreases, the impedance Ziu observed from the input is slightly decreased so as to stabilize the output voltage, as explained above. Accordingly, the input voltage Viu is also slightly decreased because the input voltages Viu, Viv, and Viw depend on the impedance ratio.
Here, the energy transmitted through the single-phase rectifier
11
is (Viu)
2
/Ziu. Therefore, when the input voltage Viu decreases, the transmitted energy also decreases, and when the transmitted energy decreases, the output voltage Vou also decreases. Accordingly, the impedance Ziu decreases again, thereby decreasing Viu, Vou, . . . , that is, such a decreasing operation (i.e., positive feedback) is repeated and the input voltage Viu finally decreases to a lower limit.
Therefore, the circuit shown in
FIG. 6
is essentially unstable, and if the output voltage is slightly changed due to a disturbance, this change is amplified. Therefore, even if the characteristics of the single-phase rectifiers and converters are equalized, it is impossible to actually maintain a balanced state of the input voltages.
As explained above, the problem of unbalanced input voltages is not solved only by simply providing single-phase rectifiers for each phase.
FIG. 9
shows a conventional circuit which employs a step-up chopper type active filter for each single-phase rectifier in
FIG. 6
, and
FIGS. 10A
to
10
C are diagrams showing the simulated characteristics of the circuit. In
FIG. 9
, resistors
127
,
128
, and
129
have been substituted for the converters on the assumption that the impedance observed from the input is the same for each phase.
When the resistors
127
,
128
, and
129
have the same resistance, the input voltages of the single-phase rectifiers are balanced as explained above. Here, it is assumed that the resistance of the resistor
127
is higher by 1% than the resistances of the other resistors so as to provide an imbalance between the converters.
FIGS. 10A
to
10
C show the results of a simulation for the case that the resistor
127
has a resistance 1% higher than the other resistors.
FIG. 10A
shows the waveform of the input voltage of each single-phase rectifier, that is, the waveforms which are measured by voltmeters
76
,
77
, and
78
. As clearly shown in
FIG. 10A
, the input voltages of the single-phase rectifiers are unbalanced. The input voltage of the phase corresponding to the resistor
128
is almost 0, and accordingly, the input voltages of the other phases are higher than the specific (i.e., suitable) level.
FIG. 10B
shows the waveform of the output voltage of each single-phase rectifier, that is, the waveforms which are measured by voltmeters
130
,
131
, and
132
. The output voltage of the phase corresponding to the resistor
128
is lower, while the output voltages of the other two phases are higher. Here, the specific output voltage is 400V, and the output voltages of the other two phases are considerably higher than 400V.
FIG. 10C
shows the waveform of the line current of each phase, that is, the waveforms which are measured by ammeters
73
,
74
, and
75
. The waveforms indicate that a function as an active filter is not active but a function close to condenser input is active.
Japanese Unexamined Patent Application, First Publication No. Hei 6-217551 discloses a technique for solving imbalances in the input voltage, in which a controller
30
(see
FIG. 5
of the publication) is provided. The input voltage of each phase is balanced by controlling the load of each single-phase rectifier by using the controller
30
.
In addition, PCT International Publication No. WO94/27357 discloses another conventional technique for solving imbalances in the input voltage, in which a virtual neutral point N is generated using resistive elements, and a control circuit for making the voltage between the connection point of the single-phase rectifiers and the virtual neutral point zero is provided. That is, the electric potentials of the connection point of the single-phase rectifiers and the virtual neutral point are the same, and which results in a balanced state.
PCT International Publication No. WO99/57800 also discloses another conventional technique for solving imbalances in the input voltage, in which means A
2
(see
FIG. 3
of the publication) for generating an artificial neutral point is provided, and the connection point NA
2
between the single-phase rectifiers is the artificial neutral point. Such means for generating an artificial neutral point is limited to a magnetic component which makes the sum of the vectors of the magnetic fluxes inside the system zero.
SUMMARY OF THE INVENTION
In consideration of the above circumstances, an object of the present invention is to provide a polyphase rectifying apparatus which substantially solves the instability of the single-phase rectifiers without adding a special circuit element (as added in the conventional systems), thereby balancing the input voltages of the single-phase
Haga Hiroyuki
Kikuchi Yoshihiko
Arent Fox Kintner & Plotkin & Kahn, PLLC
Nguyen Matthew
Shindengen Electric Manufacturing Co. Ltd.
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