BRIEF DESCRIPTION OF THE DRAWINGS
Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention given as non-restrictive examples only and represented in the accompanying drawings in which:
FIG. 1 represents a diagram of a converter in which the invention can be applied;
FIG. 2 represents a block diagram of a control circuit of a converter of known type;
FIG. 3 represents a diagram of a vector space determining control of the arms of a three-arm converter;
FIG. 4 represents a detail of a regulation triangle of a diagram of a vector space of FIG. 3;
FIGS. 5 and 6 represent diagrams of a control device according to an embodiment of the invention;
FIG. 7 represents a converter according to the invention with a control device according to FIGS. 5 and 6 to control power semi-conductor arms;
FIGS. 8A to 8C show voltage signals in power semi-conductor arms;
FIGS. 9 and 10 represent carrier signals with opposite slopes;
FIG. 11 represents carrier signals applicable to a control device according to the invention to control a converter with three voltage levels;
FIG. 12 represents regulation in an enlarged triangle to be used in a control device according to an embodiment of the invention;
FIG. 13 represents a control method according to an embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In an electric power converter control device, the converter arms are controlled according to a regulation mode defined in a vector space. FIG. 3 represents a diagram of a vector space able to determine control of the arms of a three-arm converter referenced on axes A, B and C. In this case, a reference vector VR, which is the image of the control voltages to be applied to the three arms, points in a triangle 40 of the vector space. A detail of a regulation triangle 40 of a vector space diagram is represented in FIG. 4. Apexes 41, 42 and 43 of the triangle 40 correspond to states of the power semi-conductors or to voltage states of the converter outputs during a regulation cycle. The notation of the apexes corresponds to the individual states of each output and to a global state of said outputs corresponding to the common mode voltage in reduced unit generated by the converter. Thus, during a regulation cycle equivalent to a chopping period, the outputs of the converter arms successively take set values corresponding to a common mode output voltage defined in the apexes of the triangle of the vector space surrounding the vector.
With a state-of-the-art device, the number of voltage variations per cycle and the voltage deviation or amplitude per cycle can be very high resulting in large common mode voltages and currents. For example, in the triangle 40 represented in FIG. 4, the global state of the output corresponding to the common mode voltage in reduced unit can take the values −2, −1, 0, 1, 2 and generate a large number of variations and a strong amplitude.
In a control device according to one embodiment of the invention, the number of variations and/or the amplitude of the outputs is reduced to reduce the common mode disturbances.
FIGS. 5 and 6 represent partial diagrams of a control device according to an embodiment of the invention. In FIG. 5, a regulation signal processing module 50 comprises a first module 51 for determining possible turn-offs for a reference vector VR1 that is the image in the vector space of the three control signals MCIA, MCIB, MCIC coming from the converter regulation module. Turn-off is preferably performed by over-modulation signals to be applied to modulation of the three arms, but other turn-off means can however be used. The turn-offs enable a semi-conductor arm to be turned off to have a positive, negative or intermediate voltage. A first turn-off selection module 52 selects turn-offs enabling double switching from among the possible turn-offs. Turn-off selection enables an arm of said converter to be turned off according to the turn-offs enabling double switching in other arms. Thus, when one arm is off and the other two arms are switching, their switching direction is complementary as are the corresponding voltage variations. Selected complementarity of the voltage variations greatly reduces common mode voltages and currents.
Advantageously, when there are at least two turn-off possibilities enabling double switching, a second turn-off selection module 53 selects one or more turn-offs suitable for intermediate voltage balancing. Turn-off selection enables an arm of said converter to be turned off also according to intermediate voltage balancing. For example, over-modulation signals FT carrying turn-off information can be applied directly to the modulation signals or via a general control component OM. In FIG. 5, a general control component OM_FT also comprises the over-modulation with turn-off signals applied to the modulation signals with operators 54. Over-modulation is preferably of the flat top type, i.e. by flat blocking of a modulation signal commanding turn-off of the arm. On output of the operators 54, modulation signals MDA, MDB and MDC comprise selected over-modulation signals to reduce the common mode voltages.
In FIG. 6, the control device comprises modules 60 and 61 supplying carrier signals to supply at least two carrier signals with opposite slopes S1 and S2.
The signals S1 and S2 can be applied directly to carrier signal modulation operators 63 and 64 to be modulated by the modulation signals MDA, MDB and MDC comprising the power arm or output turn-off over-modulation signals.
Preferably, the control device comprises a selection module 65 for selecting carrier signals to be applied to converter arm control. This module receives first carrier signals S1 and S2 with opposite slopes and distributes signals S3 to S8 corresponding to carrier signals with opposite slopes reconditioned according to the direction of slope of the signals, and the positive or negative voltages of the switching arms. Carrier signal selection also enables the switching dead times to be managed according to the switching direction, transistor-diode or diode-transistor. By having switching directions of identical types, reduction of the voltage variations and consequently of the common mode disturbances is even more efficient.
FIG. 7 represents a converter according to the invention with a control device according to FIGS. 5 and 6 to control power semi-conductor arms. The electric converter comprises:
- a DC power supply comprising a first positive voltage V1 line L1, a second negative voltage V2 line L2, and a third intermediate voltage V0 line L0,
- conversion means having at least three power semi-conductor arms 10, 11, 12 connected between said positive, negative or intermediate voltage lines and outputs to supply at least one output voltage, and
- a control device comprising a control circuit controlling turn-on of the power semi-conductor arms 10, 11, 12.
Each power semi-conductor arm comprises four transistors T10A to T10D, T11A to T11D and T12A to T12D coupled with diodes respectively D10A to D10D, D11A to D11D and D12A to D12D. The output voltages and currents depend on the on or off states of the transistors or of the diodes of the arms. Thus, turn-off of the outputs by over-modulation and the switching directions of the other arms are also determined according to modulation signals supplied by a regulating unit of the converter and the output currents.
A control circuit 67 comprises a processing unit to supply modulation signals of control signals of said arms, and also comprises modules for determining over-modulation signals. The control modulation signals MCI coming from the converter regulating unit are used for determining the over-modulation performing turn-off of a power output of an arm. A current measuring circuit 68 arranged on output conductor lines is connected to said control device to supply signals representative of current. These signals IS are used to define the carrier signals with opposite slopes to be applied to the semi-conductor arms.
FIGS. 8A to 8C show voltage signals in power semi-conductor arms. In FIG. 8A, a curve 70 illustrates turn-off switching of a switch or transistors connected between the positive line and an output, for example T10A or T10B. In FIG. 8B a curve 71 illustrates turn-on switching of a switch or transistors connected between the negative line and an output, for example T10C or T10D. Between the times t1 and t2 corresponding to the two switchings, there is a dead time DT. In FIG. 8C, curves 72 and 74 show different voltage variation times on the output of an arm. The output voltage variation can occur on turn-off of the switch T10A or T10B, shown by curve 72 or 73, or on turn-on of the switch T10C or T10D, shown by curve 74. At this time, the variation depends on whether the switch T10A or T10B is initially diode on or transistor on, and on the sign of the current flowing therein.
In FIG. 8C, curves 72 and 73 show different voltage variations on the output when switching takes place in the transistor-diode direction. These variations are dependent on the output current value. A curve 74 shows a voltage variation on the output when switching in the diode-transistor direction takes place after the time t2. This variation depends on the transistor switching control.
FIG. 9 represents examples of carrier signals with opposite slopes S1 and S2. In this figure, the carrier signals are signals in the shape of a saw-tooth comprising a first portion 80 with a very fast variation or stepped, and a second portion 81 having a slope 82 forming a ramp, the complementary double switching direction being preferably determined on the fast variation portions. For example, at the time t4 the signal S1 goes quickly from a high value to a low value whereas the signal S2 goes from a low value to a high value.
Advantageously, for better attenuation of the common mode voltage variations, the carrier signal selection module 65 selects the carrier signals according to priority switching of the same type, either transistor-diode or diode-transistor, in two arms which switch at the same time so as not to have a time lag due to the dead times. Preferably, when the choice is possible, the selection module 65 selects the carrier signals according to a priority switching in the diode-transistor direction in two arms which switch at the same time.
To be used with the converter arm modulation signals, the carrier signals are preferably standardized or calibrated according to the amplitude of the output voltages and of the DC supply voltages V1, V0, V2, at values respectively CV1, CV0, CV2. Thus in FIG. 10, at a time t5 the signal S1 goes quickly from a value CV0 to a value CV2 and the signal S2 goes quickly from a value CV2 to a value CV0. In FIG. 10, modulation signals MDB and MDC cross the carrier signals to command converter arms. In this figure, the modulation signal MDA is not represented for in this case it turns an arm off and does not take part in switching.
FIG. 11 represents carrier signals applicable to a control device according to the invention to control a converter with three voltage levels. In this figure, two saw-tooth carrier signal curves S3 and S4 are calibrated for power semi-conductor commands operating in globally positive voltages CV0 and CV1, and two saw-tooth carrier signal curves S7 and S8 are calibrated for power semi-conductor commands operating in globally negative voltages CV2 and CV0. The modulation signal MDA is blocked by over-modulation at the value CV1, and the carrier signals do not cross the modulation signal. On the negative part, the modulation signals MDB and MDC cross the carrier signals respectively S7 and S8. At the moment when the fast variations of the carrier signals occur, the converter output switchings are performed in complementary manner to greatly limit disturbances, in particular common mode disturbances.
To increase the possibilities of choice of modulation and to have additional degrees of freedom of control, in a processing device of control modulant signals according to an embodiment of the invention, modulation can preferably be performed on an enlarged modulation triangle. In FIG. 12, the carrier signal selection module selects the carrier signals according to a direction of rotation in an enlarged triangle 85 pointed by a vector VR in a control vector space. This enlarged triangle has a first apex 86 in a first initial triangle, a second apex 87 in a second initial triangle contiguous to the first triangle, and a third apex 88 common to the first triangle and to the second triangle. In this new enlarged triangle the directions of rotation illustrate the choices of carrier signals used.
The selection characteristics of the turn-on signals and of the carrier signals with opposite slopes enable an output variation with at most two voltage levels and/or an output variation with at most two voltage level variations to be obtained during a regulation cycle corresponding to a vector defined in a vector space.
FIG. 13 represents a flowchart of a control method according to an embodiment of the invention. During a regulation cycle, referenced by 100 at the beginning of the flowchart, two phases enable the type of turn-off and the power semi-conductor control orders to be determined. A first phase 101 determines an arm to be turned off and the turn-off position of said arm in a converter configuration with three voltage levels. Turn-off is preferably applied to modulation signals via an over-modulation signal. The over-modulation signal is advantageously carried by a general control component also carrying other regulation signals. A second regulation phase 102 selects carrier signals to obtain the control orders while at the same time optimizing the switching characteristics of the power semi-conductors of the converter arms which switch.
In the embodiment of FIG. 13, a step 103 determines the possible turn-offs enabling a converter arm to be turned off, and a step 104 makes a first selection of turn-offs enabling double switching from among the possible turn-offs. Preferably, these two steps 103 and 104 can be combined in a single step determining and directly selecting turn-offs enabling double switching from among the possible turn-offs. Selection of an over-modulation thereby enables an arm of said converter to be turned off according to the over-modulations enabling double switching.
A step 105 also makes a second selection of turn-offs suitable for intermediate voltage balancing to perform regulation. Thus, said selection selects the arm to be turned off according to the intermediate voltage balancing determined by the step 105. In a step 106, selection of the arm to be turned off is associated with an over-modulation signal.
The flowchart of FIG. 13 comprises a step 107 for supplying carrier signals to supply at least two carrier signals with opposite slopes, and modulation of said carrier signals with opposite slopes by modulation signals.
A step 108 selects carrier signals with opposite slopes to be applied to control of converter arms. In a particular cycle, selection of carrier signals enables carrier signals to be selected according to a direction of rotation in an enlarged triangle pointed by a vector VR of a control vector space.
Preferably, a carrier signal selection step 109 selects carrier signals according to priority switching of the same type, either transistor-diode or diode-transistor, in two arms which switch at the same time. Advantageously, a step 110 selects the diode-transistor direction when this is possible.
In this preferred embodiment, the method comprises supply of carrier signals of saw-tooth shape comprising a first portion with a very fast variation or stepped and a second portion having a slope forming a ramp, the direction of complementary double switching being determined on the fast variation portions. Thus, during a regulation cycle corresponding to a vector defined in a control vector space, a switching arm command causes an output variation having at most two voltage levels or at most two voltage level variations.
Conversion devices according to preferred embodiments are preferably variable speed drives, but they can also be in particular inverters, uninterruptible power supplies, uni-directional or two-directional power converters, or frequency converters.
The invention applies in particular to three-phase converters with three DC voltage levels with three or four arms, but other converters having a different number of arms and/or of phases can be concerned.
The functions of the control means can be performed by electronic circuits that can be analog, digital, and/or in programmed form to be implemented in microcontrollers or microprocessors.
The power semi-conductors of these converters are advantageously insulated gate bipolar transistors called IGBT but other types of semi-conductors can be used. The arms can comprise several semi-conductors connected in series and/or in parallel depending on the voltages, currents or electric powers used. For example, the input or output voltages can be 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 be from a few amperes to more than a thousand amperes.
In an other technical languages, the arms of the converter can be also named legs or branches.
Patent Application
20070263422
