Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2000-09-01
2001-10-23
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S037000
Reexamination Certificate
active
06307760
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power converter for generating variable-frequency power from electric power of AC power source.
2. Description of the Related Art
An inverter apparatus is widely used as a power converter for driving AC motors and, for large capacity use in particular, three level inverter apparatus is generally utilized.
A typical inverter apparatus comprises a DC portion of a converter for converting AC power source power into DC power, a DC portion of an inverter for converting DC power into AC power, and a DC bus of a DC link portion which interconnects the former two portions. The DC portion of the converter and that of the inverter have their respective smoothing capacitors, in such a way that if a resonance frequency of a resonance circuit constituted by capacitance of these two smoothing capacitors and inductance of the DC bus approaches a switching frequency of the converter or the inverter, a large resonance current flows through the resonance circuit, thus generating heat by the DC bus and the smoothing capacitors. To prevent such heating, a three level inverter apparatus described for example in Japanese Patent Publication No. 18435/1999 has such a configuration that the PWM switching frequency of its converter and inverter is set at not more than 1/{square root over ( )}2 of the primary resonance frequency of its DC link portion. Also, Japanese Patent Publication No. 46481/1999 describes such a configuration of a parallel multiple inverter apparatus in which a plurality of inverters is interconnected by a DC circuit that a resonance current may be suppressed by inserting a dumping resistor to both a positive-polarity potential bus and a negative-polarity potential bus of the three DC buses.
In the case of a large capacity inverter apparatus, the capacitance of its smoothing capacitor becomes large as well as the size of the apparatus itself, with a resultant increase in the magnitude of the inductance of the DC bus, thus decreasing the resonance frequency of the DC bus. In such a case, a prior art described in the above-mentioned Japanese Patent Publication No. 18435/1999 needs to decrease the PWM switching frequency correspondingly, which, however, distorts the output waveform of the inverter, thus problematically increasing vibration of the load motor.
Also, another prior art described in Japanese Patent Publication No. 46481/1999 has such a problem that a current flowing through resistors inserted to the positive-polarity and negative-polarity potential buses brings about a large loss, thus decreasing the conversion efficiency of the inverter apparatus.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to suppress a resonance current flowing through the DC bus of a three level inverter apparatus, thus improving the conversion efficiency and the operational stability.
This object can be achieved by a three level inverter apparatus which comprises a converter for converting AC power source power to DC power, an inverter for converting thus converted DC power to AC power, and a DC link for interconnecting these converter and inverter, in which the DC link has two first smoothing capacitors connected in series between a positive-polarity potential bus and a negative-polarity potential bus for storing DC power obtained from the converter and two second smoothing capacitors connected in series between these positive-polarity and negative-polarity potential buses for storing DC power to besupplied to the inverter, in such a configuration that an interconnection point of the above-mentioned two first smoothing capacitors and that of the above-mentioned two second smoothing capacitors are interconnected by an intermediate potential bus, which has in turn a resistor inserted therein for controlling resonance.
Since this three level inverter apparatus can use its resonance-suppressing resistor to decrease a resonance current flowing through a resonance path comprising the first and second smoothing capacitors and the intermediate potential bus, it is possible to suppress heating and efficiency degradation due to resonance even if the switching frequency of the converter or the inverter is close to the resonance frequency of that resonance path. Also, a current corresponding to a difference between a positive-polarity bus current and a negative-polarity bus current flows through the intermediate potential bus located at the DC neutral point, but it is rather small in magnitude as compared to the resonance current, thus causing only a small loss due to the resistors inserted with a resultant slight decrease in the efficiency of the inverter apparatus. Also, the above-mentioned object can be achieved by another three level inverter apparatus which comprises a converter for converting AC power source power to DC power, an inverter for converting thus converted DC power to AC power, and a DC link for interconnecting these converter and inverter, in which this DC link has two first smoothing capacitors connected in series between the positive-polarity potential bus and the negative-polarity potential bus for storing DC power obtained from the converter and two second smoothing capacitors connected in series between the above-mentioned positive-polarity and negative-polarity potential buses for storing DC power to be supplied to the above-mentioned inverter in such a configuration that an interconnection point of the above-mentioned two first smoothing capacitors and that of the above-mentioned two second smoothing capacitors are interconnected by an intermediate potential bus, with a reactor inserted to both the above-mentioned positive-polarity and negative-polarity potential buses for controlling resonance.
This three level inverter apparatus can use inductance of the reactor thus inserted, to change the resonance frequency of the resonance path constituted by the first and second smoothing capacitors and the positive-polarity and negative-polarity buses. Therefore, the gap between the switching frequency of the converter or the inverter and the resonance frequency of that resonance path can be made larger to suppress heating and efficiency degradation due to resonance.
REFERENCES:
patent: 5621628 (1997-04-01), Miyazaki et al.
patent: 5644483 (1997-07-01), Peng et al.
patent: 5726550 (1998-03-01), Inaniwa et al.
patent: 6111770 (2000-08-01), Peng
patent: 6191676 (2001-02-01), Gabor
patent: 11-18435 (1999-01-01), None
patent: 11-46481 (1999-02-01), None
Hitachi , Ltd.
Kenyon & Kenyon
Laxton Gary L.
Wong Peter S.
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