Converter for symmetrical reactive power compensation, and a method for controlling same

Abstract

A converter for symmetrical reactive power compensation has phase legs whose associated phases of a three-phase AC voltage network can be connected and are interconnected in an insulated star connection. The first phase leg is devoid of sub modules. The second and third phase legs each has a phase module with series-connected bipolar sub modules. A control device controls phase module currents and determines voltages to be set at each phase module. A decoupling unit calculates correction voltages for each phase module as a function of a first connection voltage between the first and second phase legs, a second connection voltage between the second and third phase legs and a first and/or a second control voltage each derived from target currents and the phase module currents of the second or third phase legs. The voltages to be set are derived from the control voltages and correction voltages.

Claims

1. A converter for symmetrical reactive power compensation, the converter comprising: three phase paths including a first phase path, a second phase path and a third phase path, each connectable to an associated phase of a three-phase AC power supply, said three phase paths being connected to one another in an isolated star point circuit; said first phase path being a sub module-free phase path and said second and third phase paths each including a phase module with a series circuit of two-pole sub modules; each said sub module having an energy storage device and at least one power semiconductor and being actuatable such that the poles of said sub module have at least one positive sub module voltage, at least one negative sub module voltage or a voltage with a value zero dropping across them; a feedback control device for regulating phase module currents, said control device being configured to determine voltages to be set on each said phase module, and said control device including a decoupling unit configured to compute correction voltages for each said phase module in dependence on: a first supply voltage between the first and second phase paths; a second supply voltage between the second and third phase paths; and control voltages, each derived from a nominal current and a phase module current of said second and third phase paths, respectively; so that the voltages to be set are derivable from the control voltages and the correction voltages; said control device further including an averager, a subtractor, a frequency former and an adder; said averager configured to form an average of energy store voltages of an associated phase module; said subtractor having an input connected to an output of said averager and is configured to form a difference from the average of the energy store voltages and a DC voltage nominal value; said frequency former having an input connected to an output of said subtractor; and said adder having an input connected to an output of said frequency former.

2. The converter according to claim 1, wherein said sub modules are full-bridge circuits and the sub module voltage corresponds to an energy storage device voltage of said energy storage device.

3. The converter according to claim 1, wherein said energy storage devices are storage capacitors.

4. The converter according to claim 1, wherein said control device further comprises a control unit, connected to said decoupling unit, for actuating said sub modules.

5. The converter according to claim 1, wherein each of said phase modules in said second and third phase paths is connected to an associated phase of the AC power supply via a coupling inductance.

6. The converter according to claim 1, wherein said first phase path comprises a coupling inductance.

7. The converter according to claim 1, wherein said converter further comprises ammeters for measuring the phase module currents and voltage transformers for measuring the supply voltages.

Description

(1) The invention is explained in more detail below using the exemplary embodiments represented in FIGS. 1 to 4.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(2) FIG. 1 shows a schematic representation of a first exemplary embodiment of a converter according to the invention.

(3) FIG. 2 shows a schematic representation of an exemplary embodiment of a sub module of the converter.

(4) FIG. 3 shows a feedback control device of the converter of FIG. 1 in the form of a block diagram.

(5) FIG. 4 shows a schematic representation of a second exemplary embodiment of the converter according to the invention.

DESCRIPTION OF THE INVENTION

(6) Specifically, FIG. 1 shows a schematic representation of an exemplary embodiment of a converter 10 according to the invention for symmetrical reactive power compensation. The converter 10 has three phase paths 1, 2, 3. A first phase path 1 is connected via a connection point 101 to a first associated phase of an AC power supply. The AC power supply itself is not graphically represented in FIG. 1. A second phase path 2 is connected via a connection point 201 to an associated second phase of the AC power supply. A phase path 3 is connected via a connection point 301 to a third, associated phase of the AC power supply. The connection points 101, 201, 301 have supply voltages dropped between them, which are represented as arrows in FIG. 1, said arrows being denoted by the references U21, U13 and U32. The voltage U32 and the voltage U21 are measured by means of the voltage transformers 4 and 5, respectively.

(7) The converter 10 is connected to the AC power supply in isolable fashion by means of suitable switches, which are not represented here.

(8) The phase paths 1, 2 and 3 are connected to one another in a star circuit. The star point 6 of the star point circuit is of isolated design. This means that the star point 6 is not at a fixed, defined potential.

(9) The second phase path 2 has a phase module 7 that comprises a series circuit comprising two-pole sub modules 8. The third phase path 3 has a phase module 9 that comprises a series circuit comprising sub modules 8. In the example shown, the series circuits each comprise N sub modules 8, the number of sub modules being geared to the voltages that need to be applied in the phase module. In the exemplary embodiment of the converter 1 from FIG. 1, all the sub modules 8 are of the same design. A current through the phase module 7 is measured by means of an ammeter 11. A current through the phase module 9 is measured by means of a corresponding ammeter 12. The phase module 7 of the second phase path 2 is connected to the associated phase of the AC power supply via a coupling inductance 13. The phase module 9 of the third phase path 3 is connected by means of a coupling inductance 14 to that phase of the AC power supply that is associated with the third phase path 3. In the exemplary embodiment shown in FIG. 1, the phase paths 1, 2, 3 are connected to the AC power supply without a connecting transformer.

(10) The converter 10 further has a feedback control device 19, represented only schematically here, that is set up to use control outputs 191 and control outputs 192 to regulate the phase module currents i.sub.Aand i.sub.B.

(11) FIG. 2 shows the design of the sub modules 8 in more detail. The sub module 8 is in the form of a two-terminal network, the two terminals of the sub module 8 being denoted by X1 and X2 in FIG. 2. The sub module 8 of FIG. 2 is in the form of what is known as a full-bridge circuit or H-bridge circuit. It has two series circuits comprising power semiconductor switching units 15, which each consist of a parallel circuit comprising a power semiconductor switch that can be switched off and a diode reverse-connected in parallel therewith. Further, the sub module 8 comprises an energy store 16, which is in the form of a storage capacitor in the exemplary embodiment shown in FIG. 2. In this case, the storage capacitor is connected in a parallel circuit with respect to the two series circuits containing the power semiconductor switching units 15. Suitable actuation of the sub modules via the control outputs 191, 192 of the feedback control device 19, which is not shown in FIG. 2, allows each of the sub modules 8 to be actuated such that the two terminals X1 and X2 of the sub modules have a sub module voltage dropped across them that is the same as the voltage dropped across the capacitor 16, the voltage across the capacitor 16 but with the reverse polarity or a voltage having the value zero. As a result, a voltage profile that corresponds to a stepped AC voltage can be produced on each of the phase modules 7, 9 in a temporal sequence.

(12) It should be noted that in the exemplary embodiment of FIGS. 1 and 2, the currents through the second and third phase paths 2 and 3 can be regulated independently of one another, but the current through the first phase path 1 is dependent on the two currents through the phase paths 2 and 3. This needs to be taken into consideration when regulating the converter 10.

(13) FIG. 3 shows the design of an exemplary embodiment of the feedback control device 19 of the converter from the exemplary embodiment of FIGS. 1 and 2 in detail. As an aim to better understanding of the design, the individual components of the feedback control device are broken down in the form of blocks 100 to 700. The feedback control steps being executed in blocks 100 to 500 take place in parallel for the two phase modules of the second and third phase paths. To avoid repetition, these feedback control steps are described in detail below only for the phase module of the second phase path. An averager 100 adds the voltages UDC1 to UDCN of the energy stores 16 of the sub modules 8 of the phase module 7 and forms an average of these voltages. A subtractor 200 compares the difference DIFF between the average of the voltages ascertained in block 100 with a DC voltage nominal value U.sub.DCref and passes the result to a voltage regulator 2001, which delivers an active nominal current value at the output of the subtractor 200. A frequency former 300 converts the ascertained active nominal current value into an AC variable by producing the active nominal current from the active nominal current value. The active nominal current is an AC variable whose phase corresponds to the phase of the grid voltage in the AC power supply. An adder 400 is used to add a prescribed reactive nominal current irefA for the phase module 7 to the active nominal current. A unit 500 compares the nominal current I.sub.NOMA (The nominal current I.sub.NOMB is shown in the other phase path) computed from the active nominal current and the reactive nominal current irefA with a phase module current iA, measured by means of the ammeter 11, to form a current control error and passes the result to a current regulator 501, which computes, at its output, a control voltage Ustell2 for the phase module 7.

(14) An appropriate arrangement of blocks 100, 200, 300, 400 and 500 is provided in relation to the phase module 9, the prescribed reactive nominal current irefB being generally different than the reactive nominal current irefA. A decoupling unit 600 now determines correction voltages U.sub.cv2, U.sub.cv3 for the control voltages Ustell2, Ustell3. For the phase module 7, this involves the voltage U21 and the voltage U32 being added to the control voltage Ustell2 in block 600. The voltage Uconv2 to be set on the phase module 7 that is computed in this manner is forwarded to a module management unit (MMS) 18 in block 700, which module management unit accordingly provides, at its outputs 191 forming the outputs of the feedback control device 19, this voltage that is to be set in control signals for the individual sub modules 8 of the phase module 7.

(15) For the third phase module 9, the decoupling unit 600 computes the correction voltage U.sub.cv3 by adding the voltage U21 to the control voltage Ustell3. The sum of the control voltage Ustell3 and the voltage U21 is forwarded to a module management unit 18 for the phase module 9 by the decoupling unit 600. The module management unit 18 for the phase module 9 converts the voltage Uconv3 that is to be set into control signals for the sub modules 8 of the phase module 9 and provides them at the output 192.

(16) FIG. 4 represents a second exemplary embodiment of a converter 20 according to the invention. In FIGS. 1 and 4, parts that are the same or are of the same type are provided with the same reference symbols. To avoid repetition, the description of FIG. 4 discusses only the differences between the embodiments of FIGS. 1 and 4. The sub modules 8 of the converter 20 are also of the same design and correspond to the sub modules 8 of FIG. 2.

(17) In contrast to the converter 10 of the exemplary embodiment of FIG. 1, the converter 20 in the exemplary embodiment of FIG. 4 has a coupling inductance 17 in the first phase path 1. The coupling inductance 17 has the same inductive properties as the coupling inductances 14 and 13. In accordance with this difference, the feedback control device 19 or the feedback control method needs to be adapted to the converter 20. Blocks 100, 200, 300, 400, 500 and 700 of the feedback control device of FIG. 3 are unchanged. In the decoupling unit 600 for the converter 20, however, the correction voltage for the respective control voltage Ustell2 or Ustell3 for the phase module 7 or 9, respectively, needs to be computed differently than for the converter 10 of the exemplary embodiment of FIG. 1. The corresponding formulae are reproduced below:
Ustell2 is corrected to 2*Ustell2+Ustell3U21;1
Ustell3 is corrected to 2*Ustell3+Ustell2U32U21.2

LIST OF REFERENCE SYMBOLS

(18) 1, 2, 3 Phase path 101, 201, 301 Connection point 10 Converter 4, 5 Voltage transformer 6 Star point 7, 9 Phase module 8 Sub module 11, 12 Ammeter 13, 14, 17 Coupling inductance 15 Power semiconductor switching unit 16 Energy store 18 Module management system 151 Power semiconductor 19 Feedback control device 191, 192 Control output 100 Averager 200 Subtractor 300 Frequency former 400 Adder 501 Current regulator 600 Decoupling unit 700 Module management unit U13, U21, U32 Supply voltages X1, X2 Terminal of the sub module UDC1 . . . UDCN Energy store voltage UDCREF DC voltage nominal value Ustell2 Control voltage Ustell3 Control voltage Uconv2 Voltage to be set Uconv3 Voltage to be set irefA, irefB Reactive nominal current iA, iB Phase module current