Modular multi-level adjustable supply with series connected...

Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter

Reexamination Certificate

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C363S065000

Reexamination Certificate

active

06236580

ABSTRACT:

BACKGROUND OF THE INVENTION
It is highly desirable to use modular multi-level converters to create a regenerative drive. The modular multi-level topography provides a design which can produce high output voltages while using lower voltage level switching devices. One such topography is shown in FIG.
1
and in U.S. Pat. No. 5,625,545 which is hereby incorporated by reference. In this design, a plurality of series connected inverters or cells form a three-phase output for variable frequency control of the motor. The series connected outputs of each cell are fed from a three-phase rectifier. Multiple three-phase windings of a single transformer feed the rectifiers in each cell. The cells as shown have a rectifying section, a capacitor storage section, and an inverter output section. Harmonic components on the input side are effectively canceled by the phase relationship in the transformer windings. This type of drive has been demonstrated to be highly efficient and provides low harmonic content on the input while providing a very low harmonic content variable frequency AC output to a three-phase motor.
FIG. 2
a
shows a cascaded multilevel inverter for static VAR compensation/generation applications. The cascade multi-level inverter consists of single-phase full bridges, in which each bridge has its own separate DC source. The inverter of
FIG. 2
a
can generate almost sinusoidal waveform voltages with each power semi conductor switching on and off only once per cycle. It can eliminate the need for transformers in a static VAR compensator. As shown in
FIG. 2
a,
multiple inverters have their respective outputs connected in series. As shown, each H-bridge inverter unit generates a quasi-square waveform. As a result, the staircase voltage waveform shown in
FIG. 2
b
is obtained. Switching phase angles are calculated off-line to minimize harmonics for each modulation index.
FIG. 3
a
shows a multicell voltage source inverter as could be used in an AC voltage motor drive. The series connection of cells shown in
FIG. 3
a
is also shown in more detail in
FIG. 3
b.
As seen in
FIG. 3
b,
the cells
1
through
3
have their output voltages VC
1
, VC
2
, VC
3
connected in series. As shown in
FIG. 3
b,
the input to each cell is a single-phase transformer and the three inputs to cells
1
through
3
can be connected in parallel across the line.
It is highly desirable to have a low-cost variable frequency drive or power supply which gives an output at medium voltage or higher while providing low harmonics to the line and load. In addition, it would be desirable if the device can transfer real power between the input and the load in both directions, and have controllable VAR demand (leading or lagging) at the input.
SUMMARY OF THE INVENTION
The invention provides for a power supply that can be used to supply three phase AC power in a controllable manner to a load. The preferred load can be any three phase device, and in particular an AC motor. The AC motor can be operated as a variable frequency device. The power supply operates from a three phase AC source. The output of the power supply is derived by three eighth AC legs each forming a single phase of a three phase output. Each of these legs are composed of cells which are capable of deriving an adjustable frequency output from a fixed frequency input. In standard preferred embodiments, it will be desirable that each of these phases have a cell which has an input side having a rectifier to convert the AC input power to DC voltage and currents. In addition, each cell will preferably have an inverter or output side which converts the DC supply from the input side to an adjustable output AC. The input of at least two of the cells in any given leg are connected in series. Similarly, at least two of the cells in a leg will have their respective output sides connected in series. As a result, the output voltage from each leg can be the sum of the series connected cell outputs. In a preferred embodiment, the cells will have a transformer in at least the input or output side. This provides for isolation and also for properly matching the desired input and output voltages. Some of the transformers, specifically at least one of the transformers that may be placed in the output side of a converter can boost the voltage of the output of such cell. As a result, the output voltage can be higher than that of other cells. Because the cells are in series and are controlled both on the input and output side, the respective waveforms utilized by both the input side and the output side can be controlled to reduce the harmonic content of the AC wavelength to the load and/or the harmonics imposed on the AC source. Each of the cells preferably will be a single phase unit, such that the series connected outputs will form one leg of a three phase output. Because of the arrangement, the rectifier and converters can be made having regenerative capability such that four quadrant operation can be achieved at a reasonable cost. In some embodiments, it will be desirable to utilize all of the controlled cells to be identical so as to achieve a reduction in cost by standardizing on a given piece of apparatus for module which makes up the overall system. Because of the modular style utilized in this drive, many variations are available which provide additional positive features for providing controllable AC power.


REFERENCES:
patent: 5625545 (1997-04-01), Hammond
patent: 5638263 (1997-06-01), Opal et al.
patent: 5986909 (1999-11-01), Hammond et al.
patent: 6014323 (2000-01-01), Aiello et al.
“Dynamic Performance and Control of a Static Var Generator Using Cascade Multilevel Inverters,” Fang Zheng Peng et al., IEEE Transactions of Industry Applications, vol. 33, No. 3, pp. 748-755, May/Jun. 1997.

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