Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific voltage responsive fault sensor
Reexamination Certificate
2002-09-27
2003-08-05
Riley, Shawn (Department: 2838)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific voltage responsive fault sensor
C363S050000
Reexamination Certificate
active
06603647
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for controlling positive or negative freewheeling paths of the phases of a matrix converter having nine bidirectional power switches arranged in a 3×3 switch matrix, with each power switch having two back-to-back series-connected semiconductor switches.
A matrix converter is a self-commutated direct converter which allows a rigid three-phase network to be converted to a system with a variable voltage and frequency. The arrangement of the bidirectional power switches in a 3×3 switch matrix allows one of the three output phases of the matrix converter to be electrically connected to one input phase. One phase of the matrix converter comprises an arrangement of three bidirectional power switches, which are each connected on one side to an input phase and on the other side to an output phase. An arrangement such as this is also referred to as a 3×1 switch matrix. The matrix converter does not require an intermediate circuit. The self-commutated direct converter offers the advantage that its topology means that it allows feedback, and appropriately applied control allows sinusoidal network currents to be achieved.
The bidirectional power switches in the matrix converter each have two back-to-back series-connected semiconductor switches. Insulated gate bipolar transistors (IGBTs) are preferably used as the semiconductor switches, each of which having a back-to-back parallel-connected diode. Bidirectional power switches designed in this way are preferably used for converters for low and medium power levels. Actuation of these semiconductor switches in the bidirectional power switches in each case produces a current path in a direction which is governed by the arrangement of the semiconductor switches. If both the semiconductor switches in one bidirectional power switch are actuated, then this power switch is switched on bidirectionally, and current can flow in both directions. This results in a reliable electrical connection between an input phase and an output phase of the matrix converter. If only one semiconductor switch in a bidirectional power switch is actuated, then this switch is switched on unidirectionally, and an electrical connection is produced only for a preferred current direction between an input phase and an output phase of the matrix converter.
Optimum actuation allows sinusoidal network current consumption. In order to avoid loading the feeding network with pulse-frequency harmonics, the matrix converter also requires an input filter, comprising LC elements. Owing to the large number of semiconductor switches, the actuation process is very complex.
When the matrix converter is switched off, it is necessary to ensure that the output current tends to zero before all the switches can be switched off. It is advantageous for the current to be reduced to zero by means of a natural diode function, rather than by current regulation. This can be achieved by means of freewheeling in the matrix converter. Furthermore it is desirable for the matrix converter to be switched to a safe state at any time in the event of a fault, for example in the event of an overcurrent. This means that it is desirable to able to change from commutation control for normal operation to freewheeling control.
The object of the commutation logic for one phase of the matrix converter is to actuate the six semiconductor switches of the three bidirectional power switches in the respective output phases of the matrix converter in such a way that the switching commands for the triggering equipment are implemented correctly, allowing reliable operation of the converter. The control logic must in all circumstances prevent a short circuit of the supply voltages on the input side, or at the output, not resulting in an interruption in the load current. Otherwise, this would lead to destruction of at least one semiconductor switch as a result of an overcurrent or overvoltage.
In the case of voltage-controlled commutation logic, the phase that is intended to be commutated to and the polarity of the phase-to-phase input voltages are required. In this case, the polarity of the output current is not important, since there is always a path for both current paths both in the steady case and during commutation.
FIG. 2
shows, in the form of a graph, all the possible commutation steps. Overall, there are 22 different switching operations which can occur, depending on the commutation control. A “1” means that a semiconductor switch in a bidirectional power switch is switched on, with a “0” representing a switched-off semiconductor switch in a bidirectional power switch.
Once the matrix converter has been switched on, all the semiconductor switches in the nine bidirectional power switches are switched off. If it is intended to change to a steady state, a change must be made from the center point “OFF” to one of the three corner points “U”, “V” or “W”. A change can be made from one steady state to any other steady state. Depending on the polarities of the voltages, there are three different routes to change to a new steady state. While traveling from one steady state to the next, it is therefore impossible to reverse. This “one-way only regulation” is necessary in order to avoid undefined states and reactions.
It must be possible to switch off the matrix converter at any time, even during a commutation process. To allow this to be done, there must be a route from each state to the center point “OFF” in FIG.
2
. In order to avoid overvoltages and destruction of the semiconductor switches in the bidirectional power switches when switching off all the semiconductor switches, a device must be provided which allows current to continue to flow during the switching off process, dissipating the energy in the load.
This further current flow is made possible by means of a freewheeling path, which must be switched. If only one semiconductor switch is actuated in a bidirectional power switch with two back-to-back series-connected semiconductor switches, this means that the bidirectional power switch is closed unidirectionally. If its enabled current direction is in the opposite direction to the voltage which is applied to it, then this is referred to as freewheeling. If this unidirectionally closed bidirectional power switch allows a positive current flow, that is to say from the feed network to the load, then this freewheeling is referred to as positive freewheeling. If a negative current flow is allowed, then the freewheeling is referred to as negative freewheeling.
Four-stage, current-dependent commutation is known from the publication “A Matrix Converter without reactive clamp elements for an induction motor drive system”, A. Schuster, PESC 98, Japan, pages 714-720. This current-dependent commutation uses the polarity of the output current as a decision variable for the switching sequence of the four semiconductor switches which are involved in the commutation process in the two bidirectional power switches. Furthermore, this publication describes a switching-off strategy for the matrix converter, which can change from specific states to the freewheeling mode, following normal operation.
This approach has the disadvantage that a certain time delay may occur before the freewheeling mode is reliably reached. It is therefore not possible to switch off the matrix converter at any time. Nevertheless, additional protective measures are thus required for each semiconductor switch in the bidirectional power switches for this time period. This publication proposes the use of varistors as the protective measure, which are connected electrically in parallel with each semiconductor switch.
A method for commutation and for switching on a freewheeling path is known from the publication “A Matrix converter switching controller for low losses operation without snubber circuits”, R. Cittadini, J. J. Huselstein, C. Glaize, EPE 97, pages 4.199 to 4.203. Depending on the voltage which is applied to the commutation group, additional switches are switched on in a
Briesen Christian
Bruckmann Manfred
Kaiser Marco
Schierling Hubert
Simon Olaf
Feiereisen Henry M.
Riley Shawn
Siemens Aktiengesellschaft
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