Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2002-10-01
2004-03-02
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S127000
Reexamination Certificate
active
06700806
ABSTRACT:
The invention relates to an apparatus for conversion of three-phase power to DC power which, after filtering of switching-frequency spectral components, and even in the event of a single-phase failure, has a sinusoidal profile of the input current and the capability to produce an output voltage which is greater than or less than the peak value of the concatenated power supply system voltage, as is described in the precharacterizing clause of patent claim
1
, and to methods for controlling this apparatus.
According to the prior art at the moment, three-phase, pulse-controlled rectifiers with little reaction on the power supply system and without potential isolation generally have a step-up controller characteristic, that is to say the adjustable output voltage is limited at the lower end by the amplitude of the concatenated power supply system voltage. Direct use of such systems, for example for supplying a pulse-controlled inverter system with a variable intermediate-circuit voltage, is thus generally impossible. Furthermore, for systems such as these, the output capacitor must be initially charged during the course of a start-up procedure, and there is no capability for current limiting in the event of an output short circuit.
Known circuits based on step-down controllers admittedly have less implementation complexity, while being restricted to unidirectional power conversion, that is to say they have only one power semiconductor, which can be switched off, per phase, but the output voltage is limited in the upward direction by the concatenated power supply system voltage and, in the event of a single-phase failure, there is no capability to continue operation with a sinusoidal input current and a resistive fundamental frequency power supply system response.
AT 404.415 describes a unidirectional pulse-controlled converter circuit with a wide input voltage range and with a wide output voltage range for a given input voltage, whose implementation likewise requires only one active device which can be switched off per phase, and which allows operation with little reaction on the power system even in the event of a phase failure. However, this system is characterized by a high reverse voltage load on the active devices and has two coupling capacitors with a high current load and two magnetic energy stores per phase, so that it is not possible to take account of the general requirement for industrial electronic systems for a low level of complexity in the power section, or to take account of the requirement for a minimum number of passive components.
The object of the invention is thus to provide a three-phase, pulse-controlled rectifier system in which the power system current has a sinusoidal profile and which has an output voltage range which is not restricted by the concatenated power supply system voltage, in two-phase and three-phase operation, and which does not require any apparatus for initial charging of the output capacitor.
According to the invention, this is achieved by the characterizing features of patent claim
1
. Further advantageous refinements of the invention can be found in the dependent claims.
The rectifier system according to the invention may be regarded as being formed by an extension, according to the invention, of the basic structure of a unidirectional three-phase rectifier system, which corresponds to the prior art, with a stabilized output current and/or DC link circuit. The power section of a conventional unidirectional pulse-controlled rectifier system with a current output is formed, in the simplest case, by a voltage-stabilizing, input-side star or delta circuit of filter capacitors, by a three-phase bridge circuit whose conductance state can be controlled by arranging a power semiconductor, which can be turned off, in each phase, and by the current-stabilizing output inductance connected downstream from this bridge.
Each bridge arm of the controllable three-phase bridge has an identical structure and is formed by a positive output diode, which is connected on the cathode side to the output inductance and to whose anode the emitter of the electronic control switch (for example a power transistor) which can be switched off and the anode of a positive input diode, which is connected on the cathode side to the phase input terminal, is connected, and by a negative output diode, which is connected on the cathode side to the collector of the control switch and is connected on the anode side to the negative output busbar, and by a negative input diode, which branches off from the input phase terminal and is likewise connected on the cathode side to the collector of the power transistor. The second terminal of the output inductance is connected to the positive output busbar and, furthermore, an output capacitor which defines the output voltage is generally arranged between the positive and the negative output busbar. One control switch may be switched off in order to prevent current from flowing via the relevant bridge arm while, when the control switch is switched on, that bridge arm has identical characteristics to the bridge arm of a conventional diode bridge. The capability to control the conductance state which this provides may be used to reduce the output voltage of the system in comparison to conventional diode rectification, and in order to reduce the power supply system reactions. As more detailed analysis shows, with appropriate symmetrical-phase control, the output current is split between the input phases such that, after filtering of switching-frequency harmonics by means of the low-pass filter which is formed from the input-side capacitors and the internal power supply system inductance, this results in a sinusoidal power supply system current profile which is in phase with the power supply system voltage. However, the sinusoidal form of the current profile can be maintained only theoretically in the event of a single-phase failure, that is to say with the output inductance having very high inductance values, which are financially unacceptable. Furthermore, in this case, the output voltage range is restricted in the upward direction by the rectified mean value of the remaining concatenated power supply system voltage, that is to say, in some circumstances, it is impossible to maintain the output voltage setting for three-phase operation.
The fundamental idea of the invention is now to avoid these disadvantages by low-complexity integration of a step-up controller stage in the converter structure. For this purpose, a step-up controller diode which is oriented in the direction of the output, is connected between the positive output capacitor terminal and the second terminal of the output inductance and, branching off from the anode of this diode, the emitter of a step-up controller transistor is connected to the negative output voltage rail. The output inductance of the conventional converter carries out the function of the step-up controller inductance. If a voltage which is below the mean value of the maximum output voltage of the controllable diode bridge is intended to be formed at the output of the system, the step-up controller transistor remains switched off, and the operation of the system according to the invention then corresponds to that of a conventional pulse-controlled rectifier. A voltage value above the conventionally maximum achievable output voltage value is achieved by fully sinusoidally driving the three-phase bridge, with a corresponding step-up controller transistor duty ratio. The fundamental operation of the step-up controller in this case corresponds completely to that of a DC voltage/DC voltage step-up controller which is arranged between the diode bridge output and the output capacitor, and therefore does not need to be explained in any more detail.
For three-phase operation, the current in the output inductance is advantageously kept at a constant value. In the event of a single-phase failure, the control transistors for the remaining phases are switched on in those intervals in which the remaining concatenated
Birch & Stewart Kolasch & Birch, LLP
Delta Energy Systems (Switzerland) AG
Sterrett Jeffrey
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