Power conversion device

Abstract

To provide a power conversion device which is capable of preventing destruction of a switch element or lifetime reduction of a capacitor and improved in reliability.SOLUTION: A power conversion device comprises: a multilevel converter connected to each of power supply lines of a three-phase AC power source and each configured by connecting a plurality of unit converters in series; and control means. The control means detects a harmonic component of a current flowing to a load, calculates a compensation current which should flow to each power supply line in order to suppress the harmonic component, controls an output voltage of the multilevel converter so as to obtain the compensation current, and controls a value of the compensation current in such a manner that an actual value of a current flowing between each power supply line and the multilevel converter is settled at a threshold.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a power conversion device connected in parallel with a load to each power line of a three-phase AC power supply to which the load is connected.

Background Art

[0002] There is known a power conversion device such as an active filter that is connected in parallel with a load to each power line of a three-phase AC power supply to which a load such as an electrical device is connected and suppresses harmonic components included in the current flowing through the load.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When an imbalance occurs in the voltage of a three-phase AC power supply, an imbalance occurs in each phase current flowing between each power line and the power conversion device. When an imbalance occurs in each phase current flowing between each power line and the power conversion device, current concentrates on a specific phase switch element or capacitor in the power conversion device, which may lead to destruction of the switch element or reduction in the life of the capacitor.

[0005] An object of embodiments of the present invention is to provide a highly reliable power conversion device capable of preventing destruction of a switch element and reduction in the life of a capacitor.

Means for Solving the Problems

[0006] The power conversion device according to the embodiment includes a multilevel converter connected to each power line of a three-phase AC power supply, each of which is formed by connecting a plurality of unit converters in series, and a control means. first detection means for detecting a load current flowing through the load; second detection means for detecting an input current from each power line to the multilevel converter; It includes. The control means detects a harmonic component of the current flowing through the load, obtains a compensation current to be passed through each power line in order to suppress the harmonic component, controls the output voltage of the multilevel converter so as to obtain the compensation current, and controls the value of the compensation current so that the effective value of the current flowing between each power line and the multilevel converter falls within a threshold value. In particular, the control means includes: a harmonic detection unit that detects a harmonic component of the load current detected by the first current detection means; a compensation current calculation unit that calculates a compensation current to be passed through each power line in order to suppress the harmonic component detected by the harmonic detection unit; a gain multiplication unit that multiplies the compensation current calculated by the compensation current calculation unit by a gain; a voltage control unit that controls the output voltage of the multilevel converter required to obtain the compensation current multiplied by the gain multiplication unit; an effective value calculation unit that calculates effective values of currents flowing between each power line and the multilevel converter based on the compensation current calculated by the compensation current calculation unit or the input current detected by the second current detection means; and a gain control unit that compares each effective value calculated by the effective value calculation unit with a threshold value and controls the gain of the multiplication unit according to the comparison result. The gain control unit sets the gain to "1" when each effective value calculated by the effective value calculation unit is equal to or less than the threshold value, and sets the ratio of the threshold value to the maximum value among the effective values as the gain when any of the effective values calculated by the effective value calculation unit is greater than the threshold value.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

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Figure 8

Embodiments for Carrying Out the Invention

[0008] An embodiment of the present invention will be described with reference to the drawings. As shown in Fig. 1, a load such as an air conditioner 2 is connected to the R-phase, S-phase, and T-phase power supply lines (first, second, and third power supply lines) Lr, Ls, and Lt of a three-phase AC power supply 1. The air conditioner 2 includes a rectifier circuit 3 that rectifies the power supply voltages Er, Es, and Et of the power supply lines Lr, Ls, and Lt by a plurality of diodes connected in a bridge, a DC capacitor 5 to which the output voltage of the rectifier circuit 3 is applied via a DC reactor 4, an inverter 6 that converts the voltage of the DC capacitor 5 into an AC voltage of a predetermined frequency and outputs it, and a compressor motor 7 that operates by the output of the inverter 6, etc.

[0009] The power conversion device 10 of the present embodiment is connected to the power supply lines Lr, Ls, and Lt to which the air conditioner 2 is connected in a parallel relationship with the air conditioner 2.

[0010] The power conversion device 10 includes an initial charging circuit A, buffer reactors 11r, 11s, and 11t, clusters (first, second, and third clusters) 12r, 12s, and 12t in which one end is connected to the power supply lines Lr, Ls, and Lt via the initial charging circuit A and the buffer reactors 11r, 11s, and 11t and the other ends are interconnected (star connection), a detection unit (first detection means) 13 that is arranged at a position closer to the air conditioner 2 than the connection position of the initial charging circuit A in the power supply lines Lr, Ls, and Lt and detects the power supply voltages Er, Es, and Et and the currents (referred to as load currents) Ir, Is, and It flowing through the air conditioner 2, a detection unit (second detection means) 14 that is arranged in the energization path between the initial charging circuit A and the buffer reactors 11r, 11s, and 11t and detects the currents Irm, Ism, and Itm flowing between the power supply lines Lr, Ls, and Lt and the clusters 12r, 12s, and 12t, a detection unit 15 that detects the zero cross points of the line voltages Ers, Est, and Etr connected to the power supply lines Lr, Ls, and Lt, and a control unit 16 that controls the clusters 12r, 12s, and 12t according to the detection results of these detection units 13, 14, and 15. Note that a multilevel converter 12 is formed by these clusters 12r, 12s, and 12t. That is, the control unit 16 controls the multilevel converter 12.

[0011] The initial charging circuit A includes resistors Rr, Rs, Rt inserted in the current paths between the power lines Lr, Ls, Lt and the buffer reactors 11r, 11s, 11t, and switches Sr, Ss, St connected in parallel to these resistors Rr, Rs, Rt. The switches Sr, Ss, St are relay contacts or semiconductor switches whose opening and closing are controlled by the control unit 16. When the three-phase AC power supply 1 is turned on, they continue the off state until then to form a current path for capacitor charging through the resistors Rr, Rs, Rt, and turn on after a predetermined time has elapsed since the power-on to form a bypass current path for the resistors Rr, Rs, Rt. The predetermined time is the time required for the capacitors 25 of the respective unit converters 20r to 20t in the clusters 12r, 12s, 12t to be fully charged. Note that the resistors Rr, Rs, Rt may be replaced with a plurality of positive temperature coefficient thermistors.

[0012] The cluster 12r connected to the power line Lr is a so-called multi-series converter cluster formed by serially connecting (cascading) a plurality of unit converters (cells) 20r that each selectively generate and output a DC voltage of multiple levels (multi-level) by switching. By adding up the output voltages (cell output voltages) of the respective unit converters 20r, an AC voltage Vrm having a waveform close to a sine wave for reducing harmonics is generated and output.

[0013] Each unit converter 20r includes a pair of output terminals, switch elements 21, 22, 23, 24 each having a parasitic diode D, a capacitor (DC capacitor) 25 connected to the output terminals via these switch elements 21 to 24, a voltage detection unit 26 that detects the voltage (capacitor voltage) Vc of this capacitor 25 and notifies the control unit 16, etc. By selectively forming a plurality of current paths by turning on and off (opening and closing) the switch elements 21 to 24, a DC voltage of multiple levels (positive level, zero level, negative level) is generated and output. The switch elements 21 to 24 are semiconductor switch elements, and for example, MOSFETs or IGBTs are used.

[0014] The cluster 12s connected to the power line Ls is a so-called multi-series converter cluster formed by connecting in series a plurality of unit converters 20s, each of which selectively generates and outputs a plurality of levels of DC voltage by switching. By adding the output voltages (cell output voltages) of the respective unit converters 20s, an AC voltage Vsm having a waveform close to a sine wave for reducing harmonics is generated and output. The configuration of each unit converter 20s is the same as that of each unit converter 20r.

[0015] The cluster 12t connected to the power line Lt is a so-called multi-series converter cluster formed by connecting in series a plurality of unit converters 20t, each of which selectively generates and outputs a plurality of levels of DC voltage by switching. By adding the output voltages (cell output voltages) of the respective unit converters 20t, an AC voltage Vtm having a waveform close to a sine wave for reducing harmonics is generated and output. The configuration of each unit converter 20t is the same as that of each unit converter 20r.

[0016] The detection unit 15 detects the zero-crossing points of the line voltages Ers, Est, and Etr, and has three-phase zero-crossing detection circuits configured as shown in FIG. 2. The configurations of these three zero-crossing detection circuits are the same, and the configuration of the zero-crossing detection circuit for the line voltage Ers is shown in FIG. 2 as a representative.

[0017] The zero-crossing detection circuit for the line voltage Ers applies the line voltage Ers of the power lines Lr and Ls to the photodiode 33a of the photocoupler 33 via the diode 31 and the resistor 32, and applies a constant DC voltage V between the collector and emitter of the phototransistor 33b of the photocoupler 33 via the resistor 34. The voltage Vro generated between the collector and emitter of the phototransistor 33b is output as the zero-crossing detection signal. That is, as the line voltage Ers changes, the photodiode 33a repeatedly emits and extinguishes light. Due to the on and off of the phototransistor 33b corresponding to the light emission and extinction, as shown in FIG. 3, a zero-crossing detection signal Vro with a waveform in which the voltage changes between a high level and a low level at each zero-crossing point of the line voltage Ers is output. The time when Vro changes from a high level to a low level and the time when it changes from a low level to a high level are the zero-crossing points. Similarly, the zero-crossing detection circuit for the line voltage Est outputs a zero-crossing detection signal Vso, and the zero-crossing detection circuit for the line voltage Etr outputs a zero-crossing detection signal Vto.

[0018] In order to make the currents flowing through the power supplies, that is, the currents (Ir + Irm), (Is + Ism), and (It + Itm) described later, approach a sine wave as close as possible to the power supply voltages Er, Es, and Et, the control unit 16 detects the harmonic components of the load currents Ir, Is, and It detected by the detection unit 13, calculates the compensation currents (the compensation currents to be added to the load currents Ir, Is, and It) to be passed through the power lines Lr, Ls, and Lt in order to suppress the harmonic components, calculates the output voltages (AC voltages) Vrm, Vsm, and Vtm of the multilevel converter required to obtain the compensation currents, and controls the switching of each unit converter 20r to 20t in the multilevel converter 12 so that the output voltages Vrm, Vsm, and Vtm can be obtained. By supplying the AC voltages Vrm, Vsm, and Vtm from the multilevel converter 12 to the power lines Lr, Ls, and Lt, the harmonic components included in the load currents Ir, Is, and It can be suppressed. That is, the power conversion device 10 operates as a so-called active filter.

[0019] In particular, the control unit 16 controls (feedback controls) the value of the compensation current so that the effective values of the currents Irm, Ism, Itm flowing between the power lines Lr, Ls, Lt and the multilevel converter 12 are within a threshold value (a predetermined upper limit value). Hereinafter, the currents Irm, Ism, Itm are referred to as input currents to the multilevel converter 12.

[0020] As specific means for executing these controls, the control unit 16 includes a harmonic detection unit 41, a compensation current calculation unit 42, a gain multiplication unit 43, a voltage control unit 44, a coordinate conversion unit 45, an effective value calculation unit 46, and a gain control unit 47 shown in FIG. 4. The harmonic detection unit 41 detects the harmonic components Irh, Ish, Ith of the load currents Ir, Is, It detected by the detection unit 13. The compensation current calculation unit 42 calculates command values (referred to as compensation current command values) Id, Iq on the rotating coordinate axis of the compensation current (the compensation current to be added to the load currents Ir, Is, It) to be passed through the power lines Lr, Ls, Lt in order to suppress the harmonic components Irh, Ish, Ith detected by the harmonic detection unit 41. The gain multiplication unit 43 multiplies the compensation current command values Id, Iq calculated by the compensation current calculation unit 42 by the gain K respectively, and outputs the multiplication results as compensation current command values Idref, Iqref. The voltage control unit 44 calculates the output voltages (AC voltages) Vrm, Vsm, Vtm of the multilevel converter 12 necessary to generate input currents Irm, Ism, Itm that follow the compensation current command values Idref, Iqref output from the gain multiplication unit 43.

[0021] The control unit 16 controls the output voltages of the respective unit converters 20r to 20t in the multilevel converter 12 so that the output voltages Vrm, Vsm, Vtm calculated by this voltage control unit 44 can be obtained by the multilevel converter 12.

[0022] The coordinate conversion unit 45 converts the compensation current command values Id and Iq calculated by the compensation current calculation unit 42 into the input current command values Irm_ref, Ism_ref, and Itm_ref on the stationary coordinate axis by coordinate conversion. The effective value calculation unit 46 calculates the effective values Irm_rms, Ism_rms, and Itm_rms of the current input to the multilevel converter 12 at the current time, namely Irm, Ism, and Itm, based on the input current command values Irm_ref, Ism_ref, and Itm_ref obtained by the coordinate conversion unit 45.

[0023] The gain control unit 47 compares the effective values Irm_rms, Ism_rms, and Itm_rms calculated by the effective value calculation unit 46 with a predetermined threshold value Im, and controls the gain K of the multiplication unit 43 according to the comparison result. Specifically, when the effective values Irm_rms, Ism_rms, and Itm_rms are less than or equal to the threshold value Im, the gain K is set to "1" (K = "1"). When any of the effective values Irm_rms, Ism_rms, and Itm_rms is greater than the threshold value Im, the ratio of the threshold value Im to the maximum value of the effective values Irm_rms, Ism_rms, and Itm_rms (Im / maximum value of the effective value) is set as the gain K (less than "1").

[0024] Examples of the line voltages Ers, Est, Etr and the load currents Ir, Is, It when the power supply voltages Er, Es, Et are in an unbalanced state are shown in FIG. 5, and examples of the changes in the effective values Irm_rms, Ism_rms, Itm_rms of the input currents Irm, Ism, Itm associated therewith are shown in FIG. 6.

[0025] That is, when the power supply voltages Er, Es, Et are in an unbalanced state, the load currents Ir, Is, It become unbalanced. Since the multilevel converter 12 outputs the currents Irm, Ism, Itm for compensating the unbalanced load currents Ir, Is, It, as it is, the current will concentrate in a particular phase cluster. For example, as shown in FIG. 6, the effective value Ir_rms of the input current Irm will increase significantly and exceed the threshold value Im, which may cause damage to the switch elements 21 to 24 of each unit converter 20r in the cluster 12r and a reduction in the life of the capacitor 25.

[0026] Therefore, when the effective value Irm_rms of the input current Irm reaches the threshold value Im, by setting the gain K for the compensation current command values Id and Iq for harmonic suppression to a value less than the normal "1", the effective value Irm_rms of the input current Irm can be suppressed below the threshold value Im. Here, the gain K is set as the ratio (Im / maximum value of the effective values) between the threshold value Im and the maximum value among the effective values Irm_rms, Ism_rms, and Itm_rms. As a result, the current showing the maximum value among the effective values Irm_rms, Ism_rms, and Itm_rms will fall within the threshold value Im. On the other hand, the currents of the effective values that are not the maximum value are controlled to lower values as the gain K decreases. However, since each of the effective values Irm_rms, Ism_rms, and Itm_rms is decreased by the same ratio (gain K), the compensation current for harmonic suppression is reduced by the same ratio. And thereby, it is possible to prevent the destruction of the switch elements 21 to 24 of each unit converter 20r in the cluster 12r and the reduction of the life of the capacitor 25.

[0027] Note that FIG. 7 shows the changes in the capacitor voltages Vc when the capacitors 25 of each of the clusters 12r, 12s, and 12t are initially charged when the power supply voltages Er, Es, and Et are in an unbalanced state. Vcr is the average value of the capacitor voltages Vc in the multilevel converter 12r, Vcs is the average value of the capacitor voltages Vc in the multilevel converter 12s, and Vct is the average value of the capacitor voltages Vc in the multilevel converter 12t. In this example, the average capacitor voltages Vcr and Vcs change with substantially the same values, and the average capacitor voltage Vct changes with a value lower than the average capacitor voltages Vcr and Vcs. That is, the average capacitor voltages Vcr, Vcs, and Vct become unbalanced.

[0028] The control unit 16 calculates the line voltages Ers, Est, and Etr by performing the following calculations using the zero-crossing point signals Vro, Vso, Vto obtained by the detection unit 15, the average value Vcr of each capacitor voltage Vc detected by each voltage detection unit 26 of each unit converter 20r in cluster 12r, the average value Vcs of each capacitor voltage Vc detected by each voltage detection unit 26 of each unit converter 20s in cluster 12s, the average value Vct of each capacitor voltage Vc detected by each voltage detection unit 26 of each unit converter 20t in cluster 12t, the number N of each unit converter 20r in cluster 12r, the number N of each unit converter 20s in cluster 12s, and the number N of each unit converter 20t in cluster 12t. √2 is the square root of "2".

[0029] Ers = (Vcr - Vcs) × N / √2 Est = (Vcs - Vct) × N / √2 Etr = (Vct - Vcr) × N / √2 Then, the control unit 16 calculates the unbalance rate by performing the following calculations using the obtained line voltages Ers, Est, and Etr. Unbalance rate = (maximum value of the differences between the line voltages Ers, Est, Etr and the average voltage Eave) / (average voltage Eave) × 100% Here, the average voltage Eave is the average value of the line voltages Ers, Est, Etr, and is calculated as (Ers + Est + Etr) / 3. The unbalance rate is expressed as a percentage of the value obtained by dividing the largest value among the differences (absolute values) Eave - Ers, Eave - Est, Eave - Est between this average voltage Eave and the line voltages Ers, Est, Etr by the average voltage Eave.

[0030] As shown in the flowchart of FIG. 8, the control unit 16 calculates the unbalance rate of the power supply voltages Er, Es, and Et (S1), and determines whether the calculated unbalance rate is less than a set value, for example, less than 10% (S2).

[0031] When the unbalance rate is less than 10% (YES in S2), the control unit 16 continues the operation of the multilevel converter 12 for suppressing harmonics under the judgment that the effective values Irm_rms, Ism_rms, Itm_rms of the input currents Irm, Ism, Itm can be suppressed below the threshold value Im by the control of the gain K (S3).

[0032] When the unbalance rate is 10% or more (NO in S2), the control unit 16 stops the multilevel converter 12 and protects the multilevel converter 12 under the judgment that it is difficult to suppress the effective values Irm_rms, Ism_rms, Itm_rms of the input currents Irm, Ism, Itm below the above-mentioned threshold value Im.

[0033] In the above embodiment, the power conversion device having a configuration in which the other ends of the clusters 12r, 12s, 12t are interconnected (star connection) has been described. However, the present invention can be similarly implemented in a so-called delta connection power conversion device in which the clusters 12r, 12s, 12t are connected between the power lines Lr, Ls, Lt.

[0034] In addition, the above embodiments and modification examples are presented as examples and are not intended to limit the scope of the invention. These embodiments and modification examples can be implemented in various other forms, and various omissions, rewritings, and changes can be made without departing from the gist of the invention. These embodiments and modification examples are included in the scope of the invention in terms of the gist, and are included in the invention described in the claims and the equivalent scope thereof.

Description of Reference Numerals

[0035] 1...Three-phase AC power supply, Lr, Ls, Lt...Power lines, 3...Air conditioner (load), 10...Power conversion device, 12...Multilevel converter, 12r, 12s, 12t...Clusters, 16...Control unit, 20r, 20s, 20t...Unit converters, 21 - 24...Switching elements, 25...Capacitor

Claims

1. A power conversion device connected in parallel with a load to each power line of a three-phase AC power supply to which the load is connected, A multilevel converter connected to each of the power lines, each of which is formed by connecting a plurality of unit converters in series; Control means for detecting a harmonic component of a current flowing through the load, obtaining a compensation current to be passed through each power line to suppress the harmonic component, controlling the output voltage of the multilevel converter so as to obtain the compensation current, and controlling the value of the compensation current so that the effective value of the current flowing between each power line and the multilevel converter falls within a threshold value; First detection means for detecting a load current flowing through the load; Second detection means for detecting an input current from each power line to the multilevel converter; comprising The control means includes A harmonic detection unit that detects a harmonic component of the load current detected by the first current detection means; A compensation current calculation unit that calculates a compensation current to be passed through each power line to suppress the harmonic component detected by the harmonic detection unit; A gain multiplication unit that multiplies the compensation current calculated by the compensation current calculation unit by a gain; A voltage control unit that controls the output voltage of the multilevel converter necessary to obtain the compensation current multiplied by the gain multiplication unit; An effective value calculation unit that calculates an effective value of a current flowing between each power line and the multilevel converter based on the compensation current calculated by the compensation current calculation unit or the input current detected by the second current detection means; A gain control unit that compares each effective value calculated by the effective value calculation unit with a threshold value and controls the gain of the multiplication unit according to the comparison result; and includes When each effective value calculated by the effective value calculation unit is less than or equal to the threshold value, the gain control unit sets the gain to "1", and when any of the effective values calculated by the effective value calculation unit is greater than the threshold value, the gain control unit sets the ratio of the threshold value to the maximum value among the effective values as the gain. Power conversion device.

2. The control means calculates an unbalance rate of the voltage of the three-phase AC power supply, operates the multilevel converter when the calculated unbalance rate is less than a set value, and stops the multilevel converter when the calculated unbalance rate is greater than or equal to the set value. The power conversion device according to claim 1.

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