Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2001-03-23
2002-06-18
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S056080, C363S097000
Reexamination Certificate
active
06407937
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an active overvoltage protection apparatus for a bidirectional power switch which has two back-to-back in series connected semiconductor switches in the “common collector mode” topology.
BACKGROUND OF THE INVENTION
A bidirectional power switch of the type described above is known from the publication “Novel Solutions for Protection of Matrix Converter to Three Phase Induction Machine”, printed in the Proceedings of the “IEEE Industry Applications Society Annual Meeting”, New Orleans, La., Oct. 5-9, 1997, pages 1447 to 1454. The circuit diagram of a such bidirectional switch in the “common collector mode” topology is illustrated in more detail in FIG.
1
. For comparison,
FIG. 2
shows a bidirectional power switch in the “common emitter mode” topology, which is likewise known from the aforementione mentioned publication. These two bidirectional power switches each have two semiconductor switches
4
and
6
, which are connected back-to-back in series. In
FIG. 1
, these two semiconductor switches
4
and
6
are connected back-to-back in series in such a manner that the two collector connections are electrically conductively connected to one another. In
FIG. 2
, the circuit formed by the two semiconductor switches
4
and
6
are connected back-to-back in series in such a manner that their emitter connections are electrically conductively connected. Since the emitter connections are linked, this circuitry is referred to as the common emitter mode. Insulated gate bipolar transistors (IGBT) are used as the semiconductor switches
4
and
6
, and each have a reverse diode. The internal topology can be seen at the accessible connections of the bidirectional power switch
2
. In the bidirectional power switch with the “common collector mode” topology shown in
FIG. 1
, the connections E
1
, E
2
, G
1
and G
2
are accessible on the power switch
2
. In contrast to this, in the bidirectional power switch
2
with the “common emitter mode” topology shown in
FIG. 2
, the connections C
1
, C
2
, G
1
and G
2
are accessible.
According to the aforementioned publication, bidirectional power switches are used in a matrix converter. A matrix converter is a self-commutating direct converter. This self-commutating direct converter is a voltage intermediate-circuit converter without an intermediate circuit. The bidirectional power switches are arranged in a 3×3 switch matrix in the matrix converter. This arrangement of the bidirectional power switches results in three input phases being connected to three output phases. The actuation of the semiconductor switches or of the semiconductor switch in the power switches of the 3×3 switch matrix in each case results in a current path being connected in a bidirectional manner, that is to say from the input to the output and vice versa. One phase of the matrix converter is an arrangement of three bidirectional power switches, which produces a connection from the three mains system phases to one output phase. An LC filter is also connected to the input connections of the matrix converter, and is linked on the input side to a three-phase mains system. This LC filter, which is also referred to as an input filter, isolates pulse-frequency oscillations from the mains system. The size of this filter depends on the pulse frequency of the matrix converter. This self-commutating direct converter offers the advantage that, by virtue of the topology, it can feed back into the mains system, and produces sinusoidal mains system currents by means of an appropriately designed control system.
In addition to the already mentioned embodiments of the bidirectional power switch, there is also a further embodiment, which can likewise be found in the aforementioned cited publication. This embodiment is a semiconductor switch which is integrated in a diode bridge.
Since the matrix converter has no passive freewheeling circuits such as a voltage intermediate-circuit converter, high reverse voltages occur across the semiconductor switches owing to the inductances which are present in the circuit, particularly in the case of pulse blocking generated on the basis of an EMERGENCY-OFF (switching off the actuation pulses of all the semiconductor switches). These overvoltages can also occur as a consequence of an incorrectly initiated commutation sequence or due to failure of the actuation of bidirectional power switches. The output circuit is always interrupted in these situations. The interruption in the output circuit in conjunction with the inductances which are present in the circuit causes the overvoltages, which may result in destruction of the semiconductor switches.
An overvoltage protection apparatus is known from the aforementioned publication “Novel Solutions for Protection of Matrix Converter to Three Phase Induction Machine”. This overvoltage protection apparatus has two 6-pulse diode bridges which are linked to one another on the DC-voltage side by means of a capacitor. On the AC-voltage side, the one 6-pulse diode bridge is connected to the input connections of the matrix converter. The other diode bridge is connected on the AC-voltage side to the output connections of the matrix converter. A resistor is connected electrically in parallel with the capacitor, and it discharges said capacitor. An LC filter is also connected to the input connections of the matrix converter, and is linked on the input side to a 3-phase mains system. This LC filter, which is also referred to as an input filter, isolates pulse-frequency oscillations from the mains system. The size of this filter depends on the pulse frequency of the matrix converter.
Any overvoltages which occur are rectified by the diode bridges, and are passed to the capacitor. This capacitor thus absorbs the commutation energy. This overvoltage protection apparatus, which is also the subject matter of U.S. Pat. No. 4,697,230, requires an initial charging circuit for the capacitor. This initial charging circuit is required to prevent overvoltages at twice the mains voltage from occurring when the matrix converter is switched on. Without initial charging, high peak currents likewise occur, which must be carried by the diodes in the diode bridge. The resistor is designed such that a predetermined amount of energy is discharged from the capacitor. This amount of energy depends on a predetermined difference between the mains system voltage and the capacitor voltage.
An overvoltage protection apparatus which has two 6-pulse diode bridges is also known from the publication “Performance of a two Steps Commutated Matrix Converter for AC-Variable-Speed Drives”, printed in the EPE '99 Proceedings, Lausanne, September 1999, pages 1 to 9. Each of these two diode bridges has a capacitor on the DC-voltage side. These two capacitors are electrically connected in parallel. A zener diode and a pulse resistor are electrically connected in parallel with these two capacitors and are used to limit the voltage across the capacitors to a predetermined value. Furthermore, each bidirectional power switch has a varistor and a back-to-back series connected zener diode, by means of which the overvoltages across the bidirectional power switch are limited.
In the publication “A Matrix Converter without Reactive Clamp Elements for an Induction Motor Drive System”, by Axel Schuster, printed in IEEE, 1998, pages 714 to 720, a number of varistors are provided as the overvoltage protection apparatus. A varistor is connected electrically in parallel with each semiconductor switch with each bidirectional power switch in the 3×3 switch matrix. These 18 varistors protect the 18 semiconductor switches of the nine bidirectional power switches against overvoltages.
When this overvoltage protection device is being used, the connection point of the two collector connections of the two back-to-back series connected semiconductor switches must be connected to the exterior in the bidirectional power switches in the common collector mode. It is also possible for the bidirectional power
Bruckmann Manfred
Springmann Walter
BakerBotts LLP
Siemens Aktiengesellschaft
Vu Bao Q.
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