Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
1999-12-08
2001-05-08
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
Reexamination Certificate
active
06229722
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inverter system to obtain several kV high voltage outputs and more particularly to a multiple inverter system designed to obtain high voltage output using a plurality of unit inverters.
2. Description of the Related Art
So far, there are many needs for energy saving by variable speed drive operation of AC motors, particularly, existing high-voltage motors. In particular, a high-voltage driving system is demanded, which is applicable directly to existing high-voltage motors; e.g., 3 k system, 6 kV system in Japan and 4.2 kV system and 2.4 kV system in overseas.
A general method so far used to construct a power conversion system for obtaining high voltage is a method to connect secondary windings of a plurality of transformer in series as described in Section 3 of the technical report titled “Multiple-Power Converter and Application Technology thereof” published from The Institute of Electrical Engineers of Japan (July, 1995).
An example of a high-voltate 12-phase inverter system that is so far often used is shown in FIG.
1
.
This inverter system is composed of a rectifier
110
, which converts AC to DC, a DC smoothing circuit
120
comprising a reactor
121
and a capacitor
122
, inverter circuits
130
and
131
, which convert DC to AC of optional frequency, transformers
140
and
141
and a load
150
.
This circuit is in a structure that DC output of the rectifier
110
is commonly used, a plurality of inverter circuits
130
,
131
are provided for this DC voltage and the secondary side windings of the output transformers
140
,
141
are connected in series so as to obtain a desired high voltage.
A control circuit is composed of a speed command unit
162
, a transmitter (OSC)
163
, which decides the output frequency in the inverter circuits
130
,
131
, a distributor (RING)
164
, which distributes the signal from the transmitter
163
to semiconductor devices in the inverter circuits
130
,
131
, an amplifier
165
, a voltage control circuit (AVR)
166
, a phase shifter (PHC), which decides a gate signal phase of the rectifier
110
, a voltage detecting transformer
143
, which detects the output AC voltage of the output transformers
140
,
141
and the voltage detected by the voltage detecting transformer
143
is input to one of the input terminals of a comparator
134
via a reverse current preventing diode
144
, a command from the speed commanding unit
162
is input to the other input terminal of the comparator
145
and a deviation obtained by the comparator
145
is given to a voltage control circuit
166
.
A circuit shown in
FIG. 2
is in a structure to obtain a high-voltage by combining a plurality of mutually insulated inverter circuits
130
,
131
by the output transformers
140
,
141
, and excepting these elements, other component elements which are the same as those shown in
FIG. 1
are assigned with the same reference numerals used in FIG.
1
and their explanations are omitted.
This circuit is in such a structure that an inverter circuit is provided to each of the outputs from the rectifiers
110
,
111
and the secondary windings of the output transformers are connected in series so as to obtain a desired high-voltage.
In the case of the structure shown in FIG.
1
and
FIG. 2
, the output transformers
140
,
141
are required for the outputs of the inverter circuits
130
,
131
, respectively and therefore, an area needed for installing them becomes large. Furthermore, to make the output transformers
140
,
141
to be durable for the use from low frequency, there is such a defect that their external shape becomes larger than ordinary transformers of fixed frequency.
Further, a neutral point clamped 3 level inverter shown in
FIG. 3
has been developed and put in practical use in recent years. This inverter converts the AC output from an AC power source
11
into DC by a rectifier
12
and after smoothed by capacitors
13
,
14
, supplies AC output obtained from a 3 level inverter circuit using 3 sets of a circuit comprising self-turn-off semiconductor devices S
1
~S
4
composed of, for instance, a gate turn-off thyristor (GTO) and diodes D
1
~D
6
to a load motor
16
. Further, P, N indicate control buses and C indicates a neutral-point potential.
A multiple level inverter as shown in
FIG. 3
has an economical problem that the connection of semiconductor devices in series becomes necessary because the circuit voltage becomes equivalent to the output voltage and a size of the system becomes large because the dielectric strength becomes high.
For a conventional system in the structure as described above, there exist such problems as shown below. As techncal problems when comprising a high-voltage converter, the following matters are pointed out.
(1) If an inverter circuit is constructed without connecting semiconductor devices in series, an output transformer are required, which is not economical.
(2) If an inverter circuit is constructed by connecting semiconductor devices in series, an output transformer can be eliminated but the system may not become fully reliable because it becomes necessary to select semiconductor devices that are to be connected in series and the gate control becomes complicate.
(3) In the serially connected structure of semiconductor devices, the harmonic reduction is limited as a matter of course because the output side harmonic componet is decided by PWM switching frequency of semiconductor devices.
(4) If even one of a lot of semiconductor devices comprising the main circuit becomes defective, the continuous operation of the system becomes impossible and and it becomes a problem in a system demanded for the continuous operation.
Further, in particular, when the high-votage output obtained by connecting the ouput sides of a plurality of unit inverters in series is supplied to an AC load, there are problems as shown below.
FIG. 4
shows an example of a definite circuit using a this type of conventional multiple inverter system. The structure shown in
FIG. 4
will be described below. That is, this circuit is provided with a rectifier
2
, which converts AC voltage of an AC power source A
1
into DC voltae, a unit inverter, which converts DC power of the rectifier A
2
into AC power, connected to the rectifier A
2
in parallel with it via a smoothing capacitor A
3
, provided with four bridge connected semiconductor devices A
5
, A
6
, A
7
, A
8
of, for instance, IGBT and the like and a gate controller A
40
to give a firing command in the specified order to the semiconductor devices A
5
~A
8
comprising the unit inverter A
9
.
Although not shown in
FIG. 4
, a pluality of the unit inverters
9
including the same smoothing capacitor A
3
as the structure described above are provided, the input sides of the unit inverters A
9
are connected to the rectifier A
2
in parallel with it and the output sides of the unit inverters A
9
are connected in series, and an AC load A
10
that is, for instance, an induction motor is connected to the ouput side of the thus connected multiple inverter.
The unit inverter A
9
is provided with a bypass circuit described below to protect the unit inverter A
9
. The bypass circuit is connected between the buses of the input side of the AC load A
10
and is composed of a bypass switch A
41
comprising, for instance, a thyristor, a diode bridge comprising diodes A
42
, A
43
, A
44
, A
45
connected between the pypass switch A
41
and the AC load A
10
, a current detector A
46
to detect load current and a switch operating circuit A
47
which gives an ON command to the bypass switch A
41
when the current value detected by the current detector A
46
exceeds a specified value.
The byass circuit is also incorporated in other unit inverters (not shown) than the unit inverter A
9
.
In
FIG. 4
, when the unit inverter A
9
is in the normal state without causing a short-circuit, etc., the bypass switch A
41
is kept in the OFF state and it therefore performs nothing.
However, when the semiconductor devices A
Hirata Akio
Ichikawa Kosaku
Kawakami Kazuto
Satoh Kazuhiro
Banner & Witcoff , Ltd.
Kabushiki Kaisha Toshiba
Nguyen Matthew
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