Power conversion device

Abstract

To provide a highly reliable power conversion device capable of preventing destruction of a switch element and shortening of life of a capacitor.SOLUTION: A power conversion device includes a multi-level converter connected to each power supply line of a three-phase AC power supply and each including a plurality of unit converters connected in series, and control means. The control means detects a harmonic component of a current flowing through a load, obtains a compensation current to be supplied to each power supply line in order to suppress the harmonic component, controls an output of the multilevel converter so as to obtain the compensation current, and controls the compensation current so that a peak value of the compensation current falls within 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 generates and outputs a compensation current for suppressing 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] The compensation current generated by the power conversion device includes a peak value that is more than twice the effective value, and the peak value may increase significantly due to the influence of power supply voltage distortion or the like. During this increase, an excessive surge voltage is applied to each switch element of the power conversion device, and furthermore, the ripple component of the current flowing through each capacitor of the power conversion device deteriorates, which may lead to destruction of each switch element and reduction of the life of each capacitor. In particular, when a modular multilevel converter, so-called MMC, is applied to an active filter, since each phase has a plurality of unit converters (cells) composed of switch elements and capacitors, the rate at which the current peak value in a specific phase increases due to power supply voltage distortion or the like becomes large, and the adverse effect appears more significantly than in a two-level converter.

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

Means for Solving the Problem

[0006] The power conversion device of the embodiment is connected to each power line of a three-phase AC power supply to which a load is connected in a parallel relationship with the load, and is connected to each of the power lines, and each is a multi-level converter formed by connecting a plurality of unit converters in series; detecting a harmonic component of the current flowing through the load, obtaining a compensation current to be supplied to each of the power lines in order to suppress the harmonic component, controlling the output of the multi-level converter so as to obtain the compensation current, and controlling the compensation current so that the peak value of the compensation current falls within a threshold value; a first detecting means for detecting a load current flowing through the load; a second detecting means for detecting a compensation current flowing between each power line and the multilevel converter; and comprises. The control means includes: a harmonic detecting section for detecting a harmonic component of the load current detected by the first current detecting means; a gain multiplying section for multiplying each of the harmonic components detected by the harmonic detecting section by a predetermined gain and outputting the multiplication result as a compensation current command value; a current control section for calculating an output voltage of the multilevel converter necessary for causing the multilevel converter to generate a compensation current that follows the compensation current command value obtained by the gain multiplying section, and controlling the switching of the multilevel converter so as to obtain the output voltage; and a gain control section for calculating a peak value of the compensation current command value obtained by the gain multiplying section every 1 / 6 of the power supply voltage cycle of the three-phase AC power supply, setting the gain to "1" when the calculated peak value is equal to or less than the threshold value, and setting the ratio of the threshold value to the peak value (= threshold value / peak value) as the gain (< "1") when the calculated peak value is greater than the threshold value.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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 (also referred to as a system power supply) 1. The air conditioner 2 includes a rectifier circuit 3 that rectifies the power supply voltages (also referred to as system 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 this rectifier circuit 3 is applied via a DC reactor 4; an inverter 6 that converts the voltage of this DC capacitor 5 into an AC voltage of a predetermined frequency and outputs it; a compressor motor 7 that operates by the output of this inverter 6, and the like.

[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 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 these 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 positions of the buffer reactors 11r, 11s, and 11t in the power supply lines Lr, Ls, and Lt and detects the power supply voltages Er, Es and the currents (referred to as load currents) Ir, Is flowing through the air conditioner 2; a detection unit (second detection means) 14 that detects the currents (referred to as compensation currents; also referred to as input currents to the clusters) Irm, Ism flowing between the power supply lines Lr, Ls, and Lt and the clusters 12r, 12s, and 12t through the buffer reactors 11r, 11s, and 11t; a detection unit 15 that is connected to the power supply lines Lr, Ls, and Lt and detects the phases θ of the power supply voltages Er, Es, and Et; 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.

[0011] The cluster 12r connected to the power line Lr is a so-called multi-series converter cluster formed by connecting in series (cascading) a plurality of unit converters (cells) 20r, each of which selectively generates and outputs 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.

[0012] This power conversion device 10 is a modular multi-level converter (MMC) having clusters each provided with a plurality of unit converters for each phase. Although the number of unit converters for each phase is generally three, it may be five or more.

[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, and a voltage detection unit 26 that detects the voltage (capacitor voltage) Vc of this capacitor 25 and notifies the control unit 16. 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 and the cluster 12t connected to the power line Lt also have the same configuration as the cluster 12r connected to the power line Lr. The cluster 12s connected to the power line Ls generates and outputs an AC voltage Vsm having a waveform close to a sine wave for reducing harmonics by adding up the output voltages (cell output voltages) of the respective unit converters 20s.

[0015] The cluster 12t connected to the power line Lt generates and outputs an AC voltage Vtm having a waveform close to a sine wave for reducing harmonics by adding up the output voltages (cell output voltages) of the respective unit converters 20t. The configurations of all the unit converters 20r to 20t are the same.

[0016] These clusters 12r, 12s, 12t constitute the multilevel converter 12.

[0017] The control unit 16 takes in the values of the load currents Ir and Is detected by the detection unit 13, and calculates and takes in the value of the load current It from the values of the load currents Ir and Is. The control unit 16 takes in the values of the compensation currents Irm and Ism detected by the detection unit 14, and calculates and takes in the value of the compensation current Itm from the values of the compensation currents Irm and Ism.

[0018] Then, in order to make the power currents Irmain, Ismain, Itmain flowing through the three-phase AC power supply 1 (also referred to as system currents) approach a sine wave synchronized with the power voltages Er, Es, Et as much as possible, the control unit 16 detects the harmonic components of the load currents Ir, Is, It, calculates the compensation currents Irm, Ism, Itm (the compensation currents to be added to the load currents Ir, Is, It) to be supplied to the power lines Lr, Ls, Lt to suppress the harmonic components, calculates the output voltages (AC voltages) Vrm, Vsm, Vtm of the multilevel converter required to obtain the compensation currents Irm, Ism, Itm, and controls the switching of each unit converter 20r to 20t in the multilevel converter 12 so that the output voltages Vrm, Vsm, Vtm are obtained. By supplying the AC voltages Vrm, Vsm, Vtm from the multilevel converter 12 to the power lines Lr, Ls, Lt, the harmonic components included in the load currents Ir, Is, It can be suppressed.

[0019] In particular, the control unit 16 controls (feedback controls) the values of the compensation currents Irm, Ism, Itm so that the peak value Iamp of the compensation currents Irm, Ism, Itm falls within the threshold value (a predetermined upper limit value) Is. As a specific means for executing this control, the control unit 16 includes a harmonic detection unit 30, a gain multiplication unit 40, a current control unit 50, a gain control unit 60, and a capacitor voltage control unit 70 shown in FIG. 2. The harmonic detection unit 30 performs a rotational coordinate transformation on the load currents Ir, Is, It based on the phase θ of the power supply voltages Er, Es, Et detected by the detection unit 15, and thereby obtains the d-axis current Id_dr on the rotational coordinate axis corresponding to the load currents Ir, Is, It. A rotational coordinate transformation unit 31, a low-pass filter (LPF) 32 that extracts the low-frequency component of the d-axis current Id_dr obtained by this rotational coordinate transformation unit 31, a subtraction unit 33 that extracts (detects) the harmonic component Idh of the d-axis current Id_dr by subtracting the output of this low-pass filter 32 from the d-axis current Id_dr obtained by the rotational coordinate transformation unit 31, a rotational coordinate transformation unit 34 that performs a rotational coordinate transformation on the compensation currents Irm, Ism, Itm based on the phase θ of the power supply voltages Er, Es, Et detected by the detection unit 15, and thereby obtains the q-axis current Iq_dr on the rotational coordinate axis corresponding to the compensation currents Irm, Ism, Itm, a low-pass filter (LPF) 35 that extracts the low-frequency component of the q-axis current Iq_dr obtained by this rotational coordinate transformation unit 34, and a subtraction unit 36 that extracts (detects) the harmonic component Iqh of the q-axis current Iq_dr by subtracting the output of this low-pass filter 35 from the q-axis current Iq_dr obtained by the rotational coordinate transformation unit 34.

[0020] The gain multiplication unit 40 multiplies the harmonic components Idh, Iqh detected by the harmonic detection unit 30 by a predetermined gain K respectively, and outputs the multiplication results as compensation current command values Id_ref, Iq_ref. The current control unit 50 calculates the output voltages (AC voltages) Vrm, Vsm, Vtm of the clusters 12r, 12s, 12t necessary for generating the compensation currents Irm, Ism, Itm that follow the compensation current command values Id_ref, Iq_ref obtained by the gain multiplication unit 40 in the multilevel converter 12, and performs pulse width modulation control (PWM control) on the on / off operations of the switch elements 21 - 24 of the unit converters 20r - 20t in the clusters 12r, 12s, 12t so that the output voltages Vrm, Vsm, Vtm are obtained.

[0021] The gain control unit 60 calculates the peak value Iamp of the compensation current command values Id_ref and Iq_ref obtained by the gain multiplication unit 40 every predetermined period (= 1 / 6 of the power supply voltage period T) based on the above phase θ, and compares the calculated peak value Iamp with a predetermined threshold value Is. Then, the gain K of the gain multiplication unit 40 is variably set according to the comparison result. Specifically, when the peak value Iamp is less than or equal to the threshold value Is, the gain K is set to "1" (K = "1"). When the peak value Iamp is greater than the threshold value Is, the ratio of the threshold value Is to the peak value Iamp (= Is / Iamp) is set as the gain K (less than "1").

[0022]

Number

[0023] The capacitor voltage control unit 70 monitors the capacitor voltages Vc of the capacitors 25 in each of the clusters 12r, 12s, and 12t, and outputs a predetermined adjustment value for these capacitor voltages Vc to the harmonic detection unit 30. This adjustment value is added to the subtraction unit 33 of the harmonic detection unit 30.

[0024] The gain control executed by the gain control unit 60 will be described with reference to the flowchart of FIG. 3. The gain control unit 60 counts the elapsed time t from the start of the power supply voltage cycle T based on the phase θ detected by the detection unit 15 (S1), and calculates the peak value Iamp of the compensation current command values Id_ref and Iq_ref (S2). When the count value t does not reach 1 / 6 of the power supply voltage cycle T (NO in S3), the gain control unit 60 compares the peak value Iamp calculated this time with the peak value Iamp calculated last time and stored in the internal memory (S4). When the peak value Iamp calculated this time is larger than the peak value Iamp calculated last time (YES in S4), the gain control unit 60 updates and stores the peak value Iamp calculated this time as the latest peak value Iamp in the internal memory (S5), and repeats the process from S1 above. When the peak value Iamp calculated this time is the same as or smaller than the peak value Iamp calculated last time (NO in S4), the gain control unit 60 does not execute the update storage in S5 and repeats the process from S1 above.

[0025] When the count value t reaches 1 / 6 of the power supply voltage cycle T (YES in S3), the gain control unit 60 compares the peak value Iamp calculated this time with the threshold value Is (S6). When the peak value Iamp is less than or equal to the threshold value Is (NO in S6), the gain control unit 60 sets the gain K to "1" (S7). When the peak value Iamp calculated this time is larger than the threshold value Is (YES in S6), the gain control unit 60 sets the ratio of the threshold value to the peak value Iamp (= Is / Iamp) as the gain K (< "1") (S8).

[0026] Subsequently, the gain control unit 60 clears the count value t (S9), clears the peak value Iamp in the internal memory (S10), and returns to the process from S1 above.

[0027] The waveforms of the power supply voltage Er, load current Ir, compensation current Irm, and power supply current Irmain when the gain K is "1" are shown in FIG. 4 together with the peak value Iamp. The waveforms of the power supply voltage Er, load current Ir, compensation current Irm, and power supply current Irmain when the gain K is less than "1" are shown in FIG. 5 together with the peak value Iamp.

[0028] The compensation currents Irm, Ism, Itm generated by the multilevel converter 12 include peak values that are more than twice the effective value, and these peak values may increase significantly under the influence of power supply voltage distortion or the like. During this increase, an excessive surge voltage is applied to the switching elements 21 to 24 of the multilevel converter 12, and furthermore, the ripple component of the current flowing into each capacitor 25 of the multilevel converter 12 deteriorates, which may lead to the destruction of each switching element 21 to 24 and the reduction of the lifespan of each capacitor 25.

[0029] Therefore, when the peak value Iamp of the compensation current command values Id_ref, Iq_ref exceeds the threshold value Is, by setting the gain K to a value less than the normal "1", the peak value Iamp of the compensation current command values Id_ref, Iq_ref can be suppressed to be equal to or less than the threshold value Is. Thereby, the destruction of each switching element 21 to 24 and the reduction of the lifespan of each capacitor 25 can be prevented.

[0030] In particular, since the ratio of the threshold value Is to the peak value Iamp (= Is / Iamp) is selected as the gain K, the compensation current command values Id_ref, Iq_ref can be set to an appropriate state without causing excessive suppression of the compensation current command values Id_ref, Iq_ref.

[0031] When the load is the air conditioner 2 including the rectifier circuit 3 of the diode bridge, peak values appear in the compensation currents Irm, Ism, Itm generated by the multilevel converter 12 every 1 / 6 of the power supply voltage cycle. By calculating the peak value Iamp of the compensation current command values Id_ref, Iq_ref every 1 / 6 of the power supply voltage cycle and variably setting the gain K, the peak value Iamp can be appropriately suppressed without response delay.

[0032] In the above embodiment, the power conversion device configured to interconnect (star connection) the other ends of the clusters 12r, 12s, 12t 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 supply lines Lr, Ls, Lt.

[0033] 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.

Explanation of Reference Numerals

[0034] 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

【Claim 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 the current flowing through the load, obtaining a compensation current to be supplied to each of the power lines in order to suppress the harmonic component, controlling the output of the multilevel converter so as to obtain the compensation current, and controlling the compensation current so that the peak value of the compensation current falls within a threshold value; first detection means for detecting a load current flowing through the load; second detection means for detecting a compensation current flowing between each of the power lines and the multilevel converter; comprising the control means includes a harmonic detection unit for detecting a harmonic component of the load current detected by the first current detection means; a gain multiplication unit for multiplying each of the harmonic components detected by the harmonic detection unit by a predetermined gain and outputting the multiplication result as a compensation current command value; a current control unit for calculating an output voltage of the multilevel converter necessary to generate a compensation current that follows the compensation current command value obtained by the gain multiplication unit and controlling the switching of the multilevel converter so as to obtain the output voltage; a gain control unit for calculating a peak value of the compensation current command value obtained by the gain multiplication unit every 1 / 6 of the power supply voltage cycle of the three-phase AC power supply, setting the gain to "1" when the calculated peak value is less than or equal to the threshold value, and setting the ratio of the threshold value to the peak value (= threshold value / peak value) as the gain (< "1") when the calculated peak value is greater than the threshold value; including a power conversion device.

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