Device and method for controlling an electric power converter

The control device of an electric power converter comprises a control device controlling turn-on of semi-conductor legs. The converter comprises DC voltage supply lines, an inverter connected between said lines and outputs. The control device comprises a processing unit to supply modulation signals of control signals of said inverter legs. The control device comprises a module for determining a general control component. The module for determining detects a detection signal representative of a highest current signal in absolute value. The detection signal is used to select a modulation signal on which an over-modulation is applied via the general control component. In the method, the detection signal is used to determine a general control component.

BACKGROUND OF THE INVENTION

Known electric power converters are used in particular in uninterruptible power supplies, and speed variators for electric motors or associated with power generators for coupling to a distribution system. Such a converter1represented inFIG. 1generally comprises DC voltage VDC lines L1and L2and an inverter2formed by power semi-conductor legs2A,2B,2C connected between the lines L1and L2to supply AC voltages VO on output to a load3or to an electric power distribution system. When the legs are controlled in high frequency, in particular in pulse width modulation, electric filters4can be fitted between the outputs of the legs2A,2B,2C and the load3or power system. A voltage and current measuring device62arranged on output lines61supplies signals Vo and Io to the control circuit. Often a rectifier5connected between AC inputs VI and the lines L1and L2supplies the DC voltage VDC. Capacitors C1and C2connected to the lines L1and L2perform filtering of the DC voltage VDC.

FIG. 2shows an example of a part of a processing unit7of a control circuit6to supply control signals of the legs. In this circuit, a regulator8enables three-phase modulation signals to be regulated and supplied according to reduced setpoints Cd, Cq, Co, in particular by a Park or Concordia transform in the dqo or αβo domains. These known transforms and rotations are generally computed by means of matrices respectively called Park and Concordia matrices. Signals MC1for each phase on output of the regulator are preferably used for intersective type modulation on a triangular high-frequency carrier signal enabling pulse width modulation. In the diagram ofFIG. 2, the regulator8supplies first three-phase modulation signals MC1, a module9determines signals of general control component OM comprising an over-modulation to be applied to the first signals MC1with operators10, a module11applies a reference voltage V2to said signals MC1by operators12, and a module13supplies a high-frequency signal designed to be modulated by modulation signals MC2modified by the operators10and12. Operators14combine the modulation signals MC2with preferably triangular high-frequency signals F1to supply control signals CVA, CVB and CVC of the inverter legs2A,2B,2C in pulse width modulation format. As the leg controls are preferably binary on-off commands, a conditioning circuit16shapes the control signals. The over-modulation signals OM are generated by the modulation signals MC1and by the type of over-modulation. The reference signal V2is generally representative of a DC voltage, for example half of the voltage VDC of the lines L1and L2.

In known converters, the module9determines signals of general control component OM according to modulation signals MC1and to a signal representative of a phase shift, for example an angle or a cosine φ between an output voltage Vo and current Io. An example of a module9, represented inFIG. 3, shows the use of a signal representative of phase shift to act on the lead or lag of the general control component signal. Output current Io and voltage Vo signals are applied to a module20to compute a signal representative of a phase shift which will be supplied to the module9for determining the general control component.

In known devices of the state of the art, the use of signals representative of a phase shift to determine the general control component does not enable efficient over-modulation management to be achieved. The phase shift signals are in fact no longer usable when the loads are not balanced and/or non-linear. A phase shift signal for all of the three phases leads to errors of appreciation. Furthermore the use of a phase shift signal no longer enables high-performance over-modulation to be applied if the current-voltage phase shift exceeds a certain value, for example an angle greater than ±Π/6.

SUMMARY OF THE INVENTION

The object of the invention is to provide a device and a method for control of an electric power converter and a converter comprising a device enabling efficient over-modulation even with unbalanced, dephased or non-linear currents.

In this specification, the terms “general control component” and “common control component” may be used interchangeably and refer to a specific signal or set of signals.

In a control device according to the invention, means for determining a common control component comprise means for detecting current flowing in the outputs of the legs of said converter to be used to select at least one modulation signal receiving an over-modulation. The means for determining a common control component determine the common control component according to the first modulation signals and according to the detection signal detected by the current detecting means.

Preferably, the detection signal is representative of an output line or of a phase in a three-phase power system.

Preferably, the current delivery means detect instantaneous current, quasi-instantaneous current, or current signals with a small lag or a very low integration. A small lag is a short delay caused by electronic circuitry.

In a preferred embodiment, the current detecting means detect a first detection signal representative of a highest current in absolute value to supply a first detection signal acting as an over-modulation reference, the means for determining a common control component comprising first processing means to supply a first common control component with over-modulation dependent on the first detection signal or on a modulation signal selected according to the detection signal.

Advantageously, the means for determining a common control component comprise means for controlling limiting of the common control component.

Preferably, the means for controlling limiting receive modulation signals to supply negative and positive limiting values defining a limiting zone.

In a particular embodiment, the current detection means detect a second high current lower than the highest first current in absolute value to supply a second detection signal acting as over-modulation reference, the means for determining a common control component comprising second processing means to supply a second general control component with over-modulation dependent on the second detection signal or on a second modulation signal selected according to the second detection signal.

According to a first alternative embodiment, the means for controlling limiting receive: a first signal representative of the first detection signal or of a modulation signal selected according to the first detection signal, a second signal representative of the second detection signal or of a modulation signal selected according to the second detection signal, and a control signal representative of a risk of over-modulation overshoot supplied by means for detecting an overshoot.

The means for controlling limiting supply a selected detection or modulation signal representative of the first detection signal if a risk of overshoot is not detected or representative of said second detection signal if a risk of overshoot is detected to determine a common control component.

According to a second alternative embodiment, the means for controlling limiting receive: a first signal representative of a first common control component dependent on said first detection signal, a second signal representative of a second common control component dependent on said second detection signal, and a control signal representative of a risk of over-modulation overshoot supplied by means for detecting an overshoot.

The means for controlling limiting supply a common control component representative of the first common control component if a risk of overshoot is not detected or of the second common control component if a risk of overshoot is detected.

An electric power converter according to the invention comprises: DC voltage supply lines, conversion means having at least three legs connected between said DC voltage lines and outputs to convert the DC voltage into output AC voltages, and control means for controlling turn-on of said legs of the conversion means, comprising at least one control device as defined above and current measuring means arranged on output conductor lines and connected to the control device to supply signals representative of currents to said means for determining a common control component to be used in determining the common control component.

According to the invention, a method for controlling an electric power converter comprises:

detecting a detection signal representative of a line in which a highest current in absolute value is flowing, selecting a modulation signal for application of an over-modulation, and determining a common control component according to signals resulting from detection of the detection signal, and from selection of a modulation signal.

Advantageously, the method comprises computing limit values of the common control component or of over-modulation.

Advantageously, the method comprises controlling limiting of the general control component according to the limit values.

In a particular embodiment, the method comprises:a first detection of a first detection signal representative of a line in which a highest current in absolute value is flowing,a first selection of a modulation signal for application of an over-modulation,a first determination of a first general control component according to signals resulting from the first detection of a first detection signal, and from the first selection of a modulation signal,a second detection of a second detection signal representative of a line in which a high current lower than the first highest current in absolute value is flowing,a second selection of a modulation signal for application of an over-modulation, anda second selection determination of a second general control component according to signals resulting from the second delection of a second delection signal, and from the second selection of a modulation signal.

Preferably, the method comprises: computing limit values of the general control component or of over-modulation, controlling limiting the first common control component to detect a risk of overshoot of the limit values, and supplying a signal of a common control component representative of the first common control component if a risk of overshoot is not detected or of the second common control component if a risk of overshoot is detected.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In a control device according to an embodiment of the invention, a module9for determining the general control component represented inFIG. 4comprises a processing module21for processing the modulation signals MC1for example of the modulation signals of three phases MC1A, MC1B and MC1C, and a detection module22for detecting current signals I123supplied by a measuring circuit40to detect a detection signal23of the highest current in absolute value. The object of this selection is notably to know on which line or which phase the current is the highest. Thus, a module24connected to the modules21and22determines a value of the general control component OM with over-modulation with the modulation signals and with signals representative of the detected output line having the highest current in absolute value.

InFIG. 5a control device of an electric power converter according to an embodiment of the invention comprises a control circuit6controlling turn-on of said legs2A,2B,2C of the converter2. The control circuit6comprises a processing unit7to supply modulation signals MC1, MC1A, MC1B, MC1C, MC2of control signals of said legs, and a module9for determining a general control component OM for processing an over-modulation. Said general control component OM is determined according to first modulation signals MC1, MC1A, MC1B, MC1C of the legs of the conversion means to process second modulation signals MC2, MC2A, MC2B, MC2C of the legs of the converter. In this device according to an embodiment of the invention, the module9for determining comprises a detection module22for detecting a signal23of current I123flowing in the outputs of the legs of said converter to be used in selection of at least one modulation signal receiving an over-modulation. The module9for determining determines said general control component OM according to the first modulation signals and according to said detection signal23detected by the detection module22.

The detection signal is preferably representative of an output line, or a phase in a three-phase power system. Detection is preferably performed in instantaneous current, quasi- instantaneous current, or current signals with a small lag or a low integration.

A converter generally comprises DC voltage supply lines L1and L2, a converter2having at least three legs2A,2B,2C connected between the DC voltage lines L1and L2and outputs to convert said DC voltage into output AC voltages VO, and a control circuit6controlling turn-on of the legs2A,2B,2C. In an embodiment of the invention, a converter also comprises at least one control device as defined above and current measuring sensors40arranged on output conductor lines61and connected to said control device to supply signals I123representative of currents to the module9for determining to be used in determining the general control component OM.

The detection module detects a first signal detection representative of a highest current in absolute value to supply a first detection signal acting as over-modulation reference. The module9for determining thus comprises, for example, processing modules to supply a first general control component OM with over-modulation dependent on said first detection signal. An embodiment of such a module is represented in the block diagram ofFIG. 6. A detection module22receives the current signals I123and detects the line or output where the current is the highest. It supplies a detection signal23to a selection module21to select a reference modulation signal60. This selection can be performed, for example, in the form of a scalar matrix or product. Then a module24determines a first general control component signal OM1according to the modulation and/or detection signals.

In this diagram, a module25for determining limiting values receives modulation signals MC1to supply negative and positive limiting values26defining, for example, a limiting zone. The signals26and the first general control component OM1are supplied to a module27controlling limiting of the general control component. On output of the module27, a general control component OM is limited without any risk of overshoot.

To improve control of limiting, the detection module detects a second detection signal23B of a high current lower than the first highest in absolute value current and supplies a second detection signal60B acting as over-modulation reference. In this case, the module for determining comprises second processing modules to supply a second general control component with over-modulation dependent on said second detection signal.

FIG. 7represents an example of a module for determining the general control component with two detections of detection signals enabling optimal over-modulation. A first detection module22A receives the current signals I123and detects the line or output where the current is the highest in absolute value. It supplies a first detection signal23A to a first selection module21A to select a first reference modulation signal. Then a module24A determines a first general control component signal OM1according to the first modulation and detection signals. A second detection module22B receives the current signals I123and detects the line or output where the current is the second highest in absolute value. It supplies a second detection signal23B to a second selection module21B to select a second reference modulation signal. Then a module24B determines a second general control component signal OM2according to the second modulation and detection signals. A module25receives modulation signals MC1and supplies negative and positive limiting values26defining a limiting zone.

In the diagram ofFIG. 7, a module27controlling limiting of the general control component receives a first signal representative of the first general control component OM1dependent on said first detection signal, a second signal representative of a second general control component OM2dependent on said second detection signal, and a control signal29representative of a risk of over-modulation overshoot supplied by an over-modulation overshoot detection module28. The overshoot detection module28enables the module27controlling limiting to supply a general control component OM representative of the first general control component OM1if a risk of overshoot is not detected or of the second general control component OM2if a risk of overshoot is detected.

The choice or replacement of signals to supply a different control component can also be made on other signals of the processing chains. For example, a module27for controlling limiting of the general control component can select a detection signal representative of said first detection signal23A if a risk of overshoot is not detected or representative of said second detection signal23B if a risk of overshoot is detected to determine a general control component OM.

FIG. 8shows an example of modulation signals MC1, MC1A, MC1B and MC1C standardized between −1 and +1.FIG. 9shows examples of limiting signals corresponding to a positive limiting signal30and to a negative limiting signal31. A limiting zone32is defined between these two signals.

An example of a module25for determining limiting values is represented inFIG. 10. It comprises a first module35giving a positive maximum value30of the modulation signals MC1A, MC1B, and MC1C and a second module36giving a negative maximum value31of the modulation signals MC1A, MC1B, and MC1C.

An example of a detection module22is represented inFIG. 11. It comprises a module37giving an absolute value of the current signals I123and a module38giving an indication of the maximum current signal.

FIG. 12shows a first method for controlling an electric power converter according to an embodiment of the invention comprising a converter2having at least three legs connected between DC voltage lines and outputs to convert said DC voltage into output AC voltages, and a control circuit6controlling turn-on of the converter legs and comprising processing means to supply modulation signals of control signals of said legs.

This method in particular comprises a step41of detection of a detection signal representative of a highest current of a line where a highest current absolute value is flowing and a step42of selection of a modulant signal for application of an over-modulation. In a step43, a general control component OM is determined according to signals resulting from detection of said detection signal and selection of a modulant signal.

To prevent risks of over-modulation overshoot, a step44enables limit values of the general control component or of the over-modulation to be computed. Then a step45comprises control of limiting of the general control component according to said limit values.

FIG. 13shows a second method for controlling an electric power converter according to an embodiment of the invention. In this method, a first detection51detects a first detection signal representative of a highest current or of a line where a highest current in absolute value is flowing, and a first selection selects a first modulant signal, at a step52, for application of an over-modulation. In a step53, a first determining of a general control component OM1is determined according to signals resulting from the first detection of a first detection signal, and from the first selection of a modulant signal. A second detection step54detects a second detection signal representative of a second highest signal or a line where a high current lower than the first highest current in absolute value is flowing. A second selection, in a step55, selects a second modulant signal for application of an over-modulation. In a step56, a second determining of a second general control component is determined according to signals resulting from the second detection of a second detection signal, and from the second selection of a modulant signal.

This method comprises computation of limit values of the general control component or of over-modulation in a step57, and control of limiting of the first general control component to detect a risk of overshoot of the limit values in a step58. Then a step59supplies a signal of a general control component OM representative of the first general control component OM1if a risk of overshoot is not detected or of the second general control component OM2if a risk of overshoot is detected.

FIG. 14Aillustrates limiting signals30and31of the general control component defined according to the first modulation signals MC1.FIG. 14Billustrates signals of the general control components OM1and OM2according to the risks of overshoot. Thus, for example, between the times t1and t2the component OM1is chosen in priority, then between the times t2and t3the component OM2is chosen due to a risk of overshoot of the limits by the component OM1. InFIG. 14Cthe general control component OM which will be applied to the modulation signals is the resultant of the components OM1and OM2selected for a maximum efficiency and according to the risks of overshoot. The operation of such a control device according to an embodiment of the invention is dynamic and adapts itself continually to the current nature and values.

FIG. 15Ashows modulation signals MC2A, MC2B, MC2C on which a control component OM is applied andFIG. 15Bshows examples of signals I123of currents IA, IB, IC measured on the output lines61of the converter. In this figure, over-modulation is of “Flat top” type consisting, in this case, in forcing turn-on of transistors of the legs when the current is at a maximum to reduce the losses, voltage regulation being performed in the other legs.

InFIGS. 15A and 15B, at a time t5, the current IA is the highest and over-modulation is on the modulation signal MC2A of the corresponding line. Then, at the time t6, the highest current is the current IC and the second highest current is the current IA. Over-modulation is still on the signal MC2A but it corresponds to the second highest signal to avoid a risk of limit overshoot. At a time t7, the highest current is IC and over-modulation switches to the signal MC2C corresponding to modulation of the leg of the same line or output as the current IC. Then at a time, t8the strongest current is IB and the second strongest signal of less high value than IB is the current IC. In this case, to avoid overshoot risks, over-modulation is on MC2C.

The figures show regular signals; but the invention also and especially applies to currents having forms, amplitudes and phase shifts that can be very different even for each phase.

Conversion devices according to embodiments can in particular be inverters, uninterruptible power supplies, speed variators, one-way or two-way power converters, or frequency converters.

The invention applies in particular to three-phase converters with three legs or four legs in particular when a neutral or common leg is used, but other converters having a different number of legs and/or phases can be concerned.

The invention applies in particular to converters in which the legs operate at two levels, but it also applies when the legs operate at multiple output voltage levels.

The semi-conductors of these converters are advantageously insulated gate bipolar transistors called IGBT but other types of semi-conductors can be used. The legs can comprise several semi-conductors connected in series and/or in parallel according to the electrical voltages, currents or powers used. For example, the input or output voltages can range from a few tens of volts to a thousand volts for low-voltage power system applications or have voltages of several thousand volts in particular in medium-voltage applications. The input or output currents can range from a few amperes to over a thousand amperes.

In an other technical language, the legs of the converter can be also named arms or stages.