Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific voltage responsive fault sensor
Reexamination Certificate
2001-02-07
2002-12-17
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific voltage responsive fault sensor
C361S111000, C361S056000
Reexamination Certificate
active
06496343
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an overvoltage protection apparatus for a matrix converter having nine bidirectional power switches arranged in a 3×3 switch matrix.
A matrix converter is a self-commutated direct converter. This self-commutated direct converter is a frequency converter without an intermediate circuit. The three input phases are connected to the three output phases by means of an arrangement of electronic power switches in a 3×3 switch matrix. Such a self-commutated direct converter offers the advantage that, by virtue of the topology, it has a feedback capability and produces sinusoidal mains currents by means of appropriately defined control. The bidirectional switches in the switch matrix can be semiconductor switches integrated in a diode bridge, or can be two back-to-back, series-connected semiconductor switches. The two back-to-back series-connected semiconductor switches in a bidirectional power switch of the switch matrix can be configured using either a common emitter mode topology or a common collector mode topology. An insulated gate bipolar transistor (IGBT) is preferably used as the semiconductor switch. The actuation of the semiconductor switches in the power switches of the switch matrix in each case produces a current path in a direction governed by the arrangement of the semiconductor switches. If both semiconductor switches are actuated, current is allowed to flow in both directions, such that a reliable electrical connection is produced between an input phase and an output phase. If only one semiconductor switch of the bidirectional power switch of the switch matrix is actuated, then this results in a connection in only one preferred current direction. One phase of the matrix converter is an arrangement of three bidirectional power switches, which produces a connection from the three main phases to a respective one of the output phases.
Since the matrix converter does not have any passive freewheeling circuits, as in the case of an intermediate-circuit voltage converter, the inductances which are present in the circuit can cause high reverse voltages to occur across the semiconductor switches, particularly in the event of a pulse interruption generated on the basis of an EMERGENCY OFF (disconnection of the actuation pulses from all the semiconductor switches). These overvoltages can also occur as a consequence of an incorrectly initiated commutation sequence or due to the failure of the actuation of bidirectional power switches. In the aforementioned situations, the output circuit is always interrupted. The interruption of the output circuit in conjunction with the inductances which are present in the circuit causes the overvoltages, which can lead to destruction of the semiconductor switches.
An overvoltage protection apparatus 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.
FIG. 1
shows a matrix converter
2
, with this known overvoltage protection apparatus
4
, in more detail. This overvoltage protection apparatus has two 6-pulse diode bridges
6
and
8
which are linked to one another on the DC-voltage side by means of a capacitor
10
. On the AC-voltage side, the 6-pulse diode bridge
6
is connected to the input connections R, S, T of the matrix converter
2
. On the AC-voltage side, the diode bridge
8
is connected to the output connections U, V, W of the matrix converter
2
. A resistor
12
is connected electrically in parallel with the capacitor
10
, and discharges the capacitor
10
. An LC filter
14
is also connected to the input connections R, S, T of the matrix converter
2
and is linked on the input side to a three-phase main system
16
. This LC filter
14
, which is also referred to as the input filter, keeps pulse-frequency harmonics away from the main system
16
. The size of this filter
14
depends on the pulse frequency of the matrix converter
2
.
The matrix converter
2
has nine bidirectional power switches
18
which are arranged in a 3×3 switch matrix. The bidirectional power switches
18
may be, firstly, a semiconductor switch
22
integrated in a diode bridge
20
and, secondly, two back-to-back series-connected semiconductor switches
24
and
26
. The two back-to-back series-connected semiconductor switches
24
and
26
are configured using the common emitter mode topology. An asynchronous machine
28
is connected to the output connections U, V, W of the matrix converter
2
.
Any overvoltages which occur are rectified by the diode bridges
6
,
8
and are passed to the capacitor
10
. This capacitor
10
thus absorbs the commutation energy. This overvoltage protection apparatus
4
, which is also the subject of U.S. Pat. No. 4,497,230, requires an initial charging circuit for the capacitor
10
. This initial charging circuit is required in order that no overvoltages at double or triple the main voltage occur when the matrix converter
2
is switched on. Such overvoltages can cause high surge currents, which must be accommodated by the diodes in the diode bridge
6
. The resistor
12
is designed such that a predetermined amount of energy is discharged from the capacitor
10
. This amount of energy is dependent on a predetermined difference between the main voltage and the capacitor voltage.
The cited publication also describes a further overvoltage protection apparatus. This further overvoltage protection apparatus is based on the principle that the existing diodes in the bidirectional power switches can also be used for an overvoltage protection apparatus in order to allow reduction of the number of diodes connected to the connections R, S, T and U, V, W of the matrix converter. The publication also discusses an overvoltage protection apparatus for a matrix converter
2
, which in each case has two back-to-back series-connected semiconductor switches
24
and
26
which serve as bidirectional power switches
18
. The publication discusses this overvoltage protection apparatus for a matrix converter
2
in which bidirectional power switches
18
each use a semiconductor switch
22
integrated in a diode bridge
20
. This overvoltage protection apparatus saves
6
diodes in comparison to the previously described overvoltage protection apparatus
4
.
An overvoltage protection apparatus is likewise known from the publication “Performance of a two Steps Commutated Matrix Converter for AC-Variable-Speed Drives” printed in the proceedings of the EPE '99, Lausanne, September 1999, pages 1 to 9. The apparatus described in this publication has two 6-pulse diode bridges. Each of these two diode bridges has a capacitor on the DC-voltage side. The 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 zener diode, which are used to limit the overvoltages across the bidirectional power switch.
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, discusses the use of a number of varistors as the overvoltage protection apparatus. A varistor is connected electrically in parallel with each semiconductor switch in each bidirectional power switch in a 3×3 switch matrix. These
18
varistors protect the
18
semiconductor switches in the nine bidirectional power switches against overvoltages.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an overvoltage protection apparatus having low component costs.
In accordance with the invention, this object is achieved by an overvoltage protection apparatus, comprising: (1) nine bidirectional power switches arranged in a 3×3 switch matrix; (2) firs
Mahlein Jochen
Springmann Walter
Baker & Botts LLP
Jackson Stephen W.
Kitov Z
Siemens Aktiengesellschaft
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