Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2002-05-17
2004-02-10
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S081000, C318S803000
Reexamination Certificate
active
06690592
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the priority of German Patent Application Serial No. 101 24 197.6, filed May 17, 2001, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates, in general, to an inverter with line-side and load-side self-commutated pulse converters, and more particularly, to an inverter with a pulse converter that operates efficiently at different pulse frequencies on the line side and the load side.
FIG. 1A
is a schematic circuit diagram of a conventional commercially available inverter. The depicted converter
2
includes self-commutated pulse converters on the line side
4
and the load side
6
, which are electrically connected to one another through a DC intermediate circuit
8
. A line filter
12
is connected between the power mains
10
and the AC terminals of the line-side self-commutated pulse converter
4
. The line filter
12
on the line side becomes smaller and less expensive with increasing pulse frequency (operating frequency) f
pN
of the self-commutated pulse converter
4
. A load
14
is connected to the AC terminals of the load-side self-commutated pulse converter
6
. The line-side self-commutated pulse converter
4
is controlled so that the line current i
N
has a low harmonic content. The residual harmonics are filtered by the line filter
12
.
FIG. 1B
shows the line current i
N
, the line voltage u
N
and the input voltage u
E
of the line-side self-commutated pulse converter
4
for one phase during a line voltage period. The line-side pulse converter
4
can also be controlled to return energy to the mains
10
. The load-side self-commutated pulse converter
6
is controlled so as to generate an AC voltage with an adjustable amplitude and frequency from the constant DC voltage U
Z
that is applied to each phase to the input of converter
6
.
FIG. 1C
shows the load current i
L
, the output voltage u
L
and the voltage u
LG
of the corresponding fundamental oscillation over one period of the converter
6
. The current valves
16
and
18
on both the line side
4
and the load side
6
of the depicted conventional self-commutated pulse converters are hereinafter referred to as disconnectable or gate-turn-off insulated-gate bipolar transistors (IGBT), with the term disconnectable and gate-turn-off being used synonymously.
The conventional inverter
2
, which is suitable for different line conditions and/or can tolerate reverse voltages, has typically a line frequency f
N
that is substantially identical to the output frequency f
U
. In a well-designed self-commutated current converter
6
on the load side, the ratio between the pulse frequency f
pL
and the output frequency f
U
should not be smaller than a predetermined value. For example, the Siemens catalog DA 65.10-2000 entitled “Simovert Masterdrives Vector Control” lists in the diagram “Reduction Curves for the Motor-Side Inverter” in normal operation a pulse frequency f
pL
of, for example, 6 kHz which can be increased to a maximum pulse frequency p
Lmax
=16 kHz. However, when the pulse frequency f
pL
is increased to 16 kHz, the current rating is degraded by 50%, which also degrades by 50% the power available from the converter. The exemplary pulse frequency f
pL
=6 kHz is also used for the line-side self-commutated pulse converter
4
of the inverter
2
, although half the pulse frequency f
pN
would still be sufficient. This unnecessarily increases the cost of the line-side pulse converter
4
.
If the two self-commutated pulse converters
4
and
6
of the inverter
2
are both designed for the same lower pulse frequency f
pN
of the line-side self-commutated pulse converter
4
, then the load-side pulse converter
6
which requires a pulse frequency f
pL
=6 kHz would not be able to operate under full load. The diagram in the above-referenced Siemens catalog shows that the rated current at a pulse frequency f
p
=6 kHz is reduced by 25% in comparison to the current at a nominal pulse frequency of 3 kHz. At least the load-side self-commutated pulse converter
6
would have to be oversized, increasing its cost.
It would therefore be desirable and advantageous to provide an improved inverter to obviate prior art shortcomings.
SUMMARY OF THE INVENTION
According to one aspect of the invention, an inverter includes a first self-commutated pulse converter having an AC input side connected to a power mains and a DC output side, said first converter comprising first disconnectable converter valves of a first circuit topology and implemented as semiconductor switches operating at a first pulse frequency; at least one second self-commutated pulse converter having a DC input side and an AC output side connected to a load, said at least one second converter comprising second disconnectable converter valves of a second circuit topology and implemented as semiconductor switches operating at a second pulse frequency different from the first pulse frequency; and a DC intermediate circuit electrically connecting the DC output side of the first converter to the DC input side of the at least one second converter.
By no longer using the same semiconductor switches as disconnectable current rectifier valves of the line side and load side, the pulse converters can be optimized for the desired operating frequencies which can be considerably different. In this way, the pulse converter of the inverter can be adapted to the required pulse frequencies simply by changing the power components while staying within the same power class. This approach not only reduces cost, but also saves space.
According to one embodiment, the inverter has at least two load-side self-commutated pulse converters, which are connected to a common DC intermediate circuit, wherein each of the load-side pulse converters has different semiconductor switches that operate as disconnectable current rectifier valves and can be matched to the loads to be powered. The pulse frequencies (f
pL
) of the at least two load-side self-commutated pulse converters can be multiples of one another.
The different semiconductor switches for the disconnectable current rectifier valves of the line-side and load-side pulse converters are advantageously selected so that the semiconductor switches used at a high pulse frequency are optimized for small switching losses and the semiconductor switches used at a low pulse frequency are optimized for small forward losses.
According to yet another advantageous embodiment, the disconnectable converter valves of the self-commutated pulse converters are insulated gate bipolar transistors and self-blocking field-effect transistors and/or cascode circuit elements. For example, if the line-side current converter uses insulated gate bipolar transistor, the load-side converter employs self-blocking field-effect transistors or cascode circuit elements, and vice versa.
According to another embodiment, a decoupling diode with a low forward voltage is connected in series with each self-blocking field-effect transistor, and a free-wheeling diode for high pulse frequencies is connected antiparallel with the series connection, i.e., the free-wheeling diode is biased in the opposite direction of the decoupling diode. Alternatively, the cascode circuit element includes a self-blocking MOSFET and a self-conducting JFET which are connected as a cascode, with a free-wheeling diode connected antiparallel to the cascode, i.e., the free-wheeling diode is biased in the opposite direction of the cascode configuration.
REFERENCES:
patent: 4447868 (1984-05-01), Turnbull
Siemens-Catalog DA 65.10-2000, “Simovert Masterdrives Vector Control”, 1998/99.
Feiereisen Henry M.
Patel Rajnikant B.
Siemens Aktiengesellschaft
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