Power conversion device

Abstract

To provide an electric power conversion system that can appropriately monitor a power supply voltage to continue operation properly.SOLUTION: An electric power conversion system is equipped with: electric power converters that are connected to power supply lines of a three-phase AC power supply, which are connected to the power supply lines through passive filters to perform electric power conversion by switching; a voltage detecting part that detects voltages between power-distribution paths extending from the power supply lines to the passive filters and predetermined positions in the electric power converters; a rectangular wave removing filter that removes rectangular wave components generated accompanying the switching from voltages detected by the voltage detecting part; and a control part that estimates values of the power supply voltages of the three-phase AC power supply on the basis of the voltages having the rectangular wave components removed therefrom by the rectangular wave removing filter and zero-phase voltages superposing on potentials at the predetermined positions in the electric power converters, and controls the switching of the electric power converters on the basis of the estimated results.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a power conversion device connected to each power line of a three-phase AC power supply.

Background Art

[0002] Power conversion devices such as multilevel converters and converters that are connected to each power line of a three-phase AC power supply and perform power conversion by switching are known.

[0003] This power conversion device needs to protect the power conversion device, such as monitoring the power supply voltage of the three-phase AC power supply and stopping the operation when an abnormality occurs in the power supply voltage.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The power supply voltage of a three-phase AC power supply changes according to the operating state of the power supply and the operating conditions of other electrical devices connected to the three-phase AC power supply. Accurately capturing this change has become an important issue.

[0006] An object of embodiments of the present invention is to provide a power conversion device that can accurately monitor the power supply voltage with a simple configuration.

Means for Solving the Problems

[0007] The power conversion device according to the embodiment is connected to each power line of a three-phase AC power supply, including a plurality of clusters each composed of a plurality of unit converters each consisting of a switching element and a capacitor, connecting one ends of these clusters to each other to form a neutral point, and the other ends of each of these clusters being connected to each of the power lines via a passive filter, and performing power conversion by switching multilevel converterand; a voltage detection unit that detects a voltage between the energization path from each of the power supply lines to the passive filter and the neutral point ; a rectangular wave removal filter that removes a rectangular wave component associated with the switching from the voltage detected by the voltage detection unit; and a zero-phase voltage that is superimposed on the potential of the neutral point in the power converter, based on the voltage that has passed through the rectangular wave removal filter and the zero-phase voltage, estimates the value of the power supply voltage of the three-phase AC power supply, and based on this estimation result, controls the switching of the multilevel converter ; and a control unit.

Brief Description of the Drawings

[0008]

Figure 1

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Best Mode for Carrying Out the Invention

[0009] [1] First Embodiment The first 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 power lines Lr, Ls, Lt of a three-phase AC power supply 1. The air conditioner 2 includes a rectifier circuit 3 that rectifies the power supply voltages Vr, Vs, Vt of the three-phase AC power supply 1 by a plurality of diodes connected in a bridge, a capacitor 5 to which the output voltage of this rectifier circuit 3 is applied via a DC reactor 4, an inverter 6 that is connected between both ends of this capacitor 5 and converts and outputs the DC voltage into an AC voltage of a predetermined frequency, a compressor motor 7 that operates by the output of this inverter 6, and the like. Note that the three-phase AC power supply 1 may be a commercial three-phase AC power supply or a three-phase AC power supply for self-generation.

[0010] A power conversion device 10 is connected to the power lines Lr, Ls, Lt to which this air conditioner 2 is connected in a relationship parallel to the air conditioner 2.

[0011] The power conversion device 10 includes an LC filter 11, series reactors 14r, 14s, 14t, a passive filter composed of this LC filter 11 and the series reactors 14r, 14s, 14t, and a multilevel converter (converter) 20 connected to power lines Lr, Ls, Lt via this passive filter. Further, the power conversion device 10 is disposed at a position on the air conditioner 2 side from the connection position of the LC filter 11 in the power lines Lr, Ls, Lt, and includes a detection unit 15 that detects currents Ir, Is, It (load currents) flowing through the air conditioner 2, a detection unit 16 that detects currents Icr, Ics, Ict flowing through the energization paths between the series reactors 14r, 14s, 14t and the multilevel converter 20, a voltage detection unit 17 that detects voltages Vr_n, Vs_n between each energization path from two of the power lines Lr, Ls, for example, the power lines Lr, Ls, to the passive filter (LC filter 11 and series reactors 14r, 14s, 14t) and a neutral point (predetermined position) A described later in the multilevel converter 20, a rectangular wave removal filter 18 that removes rectangular wave components accompanying the switching of the multilevel converter 20 from the voltages Vr_n, Vs_n detected by this voltage detection unit 17, and a control unit 30 that estimates the values and phases θ of the power supply voltages Vr, Vs, Vt of each phase of RST of the three-phase AC power supply 1 based on the voltages Vr_n, Vs_n that have passed through this rectangular wave removal filter 18 and a zero-phase voltage Vo superimposed on the potential of the neutral point A described later in the multilevel converter 20, and controls the switching of the multilevel converter 20 according to this estimation result and the detection results of the detection units 15, 16.

[0012] The LC filter 11 includes an LC circuit composed of a reactor 12r and a capacitor 13r for each phase of RST, an LC circuit composed of a reactor 12s and a capacitor 13s, and an LC circuit composed of a reactor 12t and a capacitor 13t.

[0013] The multilevel converter 20 includes clusters 21r, 21s, and 21t that selectively generate and output DC voltages of three or more levels for each phase of the power lines Lr, Ls, and Lt, and functions as an active filter that suppresses harmonic components of the current generated from the air conditioner 2, which is a load, and flowing through the rectifier circuit 3. Hereinafter, each of the clusters 21r, 21s, and 21t has the same configuration and is collectively referred to as the cluster 21.

[0014] The cluster 21r is a so-called multi-series converter cluster formed by connecting in series (cascading) a plurality (three) of unit converters (cells) 22r, each of which selectively generates and outputs a DC voltage of a plurality of levels (multilevel) by switching. An AC voltage Vcr0 is generated and output by adding up the output voltages (cell output voltages) Vcr of each unit converter 22r.

[0015] Similarly, in the cluster 21s, a plurality of unit converters 22s are connected in series, and an AC voltage Vcs0 is generated and output by adding up the output voltages Vcs of each unit converter 22s.

[0016] In the cluster 21t, a plurality of unit converters 22t are connected in series, and an AC voltage Vct0 is generated and output by adding up the output voltages Vct of each unit converter 22t. The output voltage of each cluster 21 has a waveform close to a sine wave for reducing harmonics.

[0017] One ends of these clusters 21 are interconnected (star-connected) to form a neutral point A, and the other ends of the clusters 21 are connected to the power lines Lr, Ls, and Lt via passive filters (LC filters 11 and connecting reactors 14r, 14s, 14t), respectively.

[0018] Since the configuration of each cluster 21 is the same, the configuration of each unit converter 22r in the cluster 21r is shown in FIG. 2 as a representative example.

[0019] Each unit converter 22r includes a pair of output terminals, switch elements Q1, Q2, Q3, Q4 each having a reflux diode D, a capacitor (DC capacitor) C connected to the output terminals via these switch elements Q1 to Q4, and a voltage detection unit Cx that detects the voltage Vcr of the capacitor C and notifies the control unit 30. It generates and outputs a plurality of levels of DC voltage by selectively forming a plurality of current paths by switching (on, off) of the switch elements Q1 to Q4. When using MOSFETs as the switch elements Q1 to Q4, the parasitic diodes of the MOSFETs can be used also as the reflux diodes D.

[0020] The control unit 30 sets an AC voltage command value for generating an AC voltage having substantially the same waveform as the AC voltage Vr of the three-phase AC power supply 1 in the cluster 21r, and compares the voltage levels of each carrier signal in the form of a triangular wave having different phases from each other with the same number as the number of each unit converter 22r and the voltage level of the AC voltage command value, and generates a driving signal (gate signal) for switching for the switch elements Q1 to Q4 of each unit converter 22r by pulse width modulation (PWM). When generating this driving signal, the control unit 30 ensures a dead time in the off state between the on and off of the switch elements Q1 and Q2 arranged in series with each other and between the on and off of the switch elements Q3 and Q4 arranged in series with each other in each unit converter 22r, respectively, for preventing short circuits.

[0021] In the case of switching by two-level modulation, each unit converter 22r selectively generates and outputs DC voltages of two levels of "positive level" and "negative level". In the case of switching by three-level modulation, each unit converter 22r selectively generates and outputs DC voltages of three levels of "positive level", "zero level", and "negative level".

[0022] The configurations and switching of each unit converter 22r of the cluster 21r described above are the same for the configurations and switching of each unit converter 22s of the cluster 21s, and are also the same for the configurations and switching of each unit converter 22t of the cluster 21t.

[0023] The voltage detection unit 17 detects the voltages Vr_n and Vs_n between each current path from the two power lines Lr and Ls to the passive filter and the neutral point A in the multilevel converter 20. Specifically, the voltage detection unit 17 includes a series circuit of resistors R1 and R2 connected between the current path from the power line Lr to the passive filter and the neutral point A, and a series circuit of resistors R3 and R4 connected between the current path from the power line Ls to the passive filter and the neutral point A, and detects the voltage Vr_n generated between the connection point of the resistors R1 and R2 and the neutral point A, and detects the voltage Vs_n generated between the connection point of the resistors R3 and R4 and the neutral point A.

[0024] As shown in FIG. 3, the rectangular wave removal filter 18 includes advance compensators 41 and 42 that compensate for the phase delay of the voltages Vr_n and Vs_n detected by the voltage detection unit 17, and low-pass filters (LPFs) 43 and 44 that remove the rectangular wave components associated with the switching of the multilevel converter 20 from the voltages Vr_n and Vs_n that have passed through the advance compensators 41 and 42. The voltages Vr_nm and Vs_nm that have passed through the low-pass filters 43 and 44 are sent to the control unit 30.

[0025] Based on the voltages Vr_nm and Vr_nm that have passed through the rectangular wave removal filter 18 and the zero-phase voltage Vo in the multilevel converter 20, and with the potential of the neutral point A of the multilevel converter 20 as the reference potential, the control unit 30 estimates the values and phases θ of the power supply voltages Vr, Vs, and Vt of the three-phase AC power supply 1. As this estimation means, the control unit 30 includes a phase-locked loop circuit (PLL circuit) shown in FIG. 4.

[0026] The subtraction units 51 and 52 capture the values of the power supply voltages Vr and Vs by subtracting the zero-phase voltage Vo (the zero-phase voltage superimposed on the potential of the neutral point A) calculated by the subsequent zero-phase voltage calculation unit 59 from the voltages Vr_nm and Vr_nm that have passed through the rectangular wave removal filter 18. The subtraction unit 53 captures the difference between the values of the power supply voltages Vr and Vs captured by the subtraction units 51 and 52 as the value of the power supply voltage Vt. Vr = Vr_nm - Vo, Vs = Vs_nm - Vo, Vt = -Vr - Vs Regarding the zero-phase voltage Vo, when the multilevel converter 20 stops its switching operation, a zero-phase voltage Vo with a value of 15% of the values of the power supply voltages Vr, Vs, Vt and a frequency three times the frequency of the power supply voltages Vr, Vs, Vt is superimposed on the potential of the neutral point A of the multilevel converter 20.

[0027] The values of the power supply voltages Vr, Vs, Vt captured by these subtraction units 51, 52, 53 become the estimation results of the control unit 30. In addition to this, the control unit 30 includes a coordinate conversion unit 54, a subtraction unit 55, a PI control unit 56, an addition unit 57, an integration unit 58, and a zero-phase voltage calculation unit 59 in order to improve the tolerance to power disturbances and to estimate the phase θ.

[0028] The coordinate conversion unit 54 obtains a d-axis voltage Vd corresponding to the values of the power supply voltages Vr, Vs, Vt and a q-axis voltage Vq with a level of "0 (zero)" by performing coordinate conversion on the values of the power supply voltages Vr, Vs, Vt captured by the subtraction units 51 to 53 according to the phase θ obtained by the subsequent integration unit 58. The subtraction unit 55 captures the deviation between the q-axis voltage Vq obtained by the coordinate conversion unit 54 and the target value "0". The PI control unit 56 obtains the frequency ω of the power supply voltages Vr, Vs, Vt by means of PI control with the deviation captured by the subtraction unit 55 as the input.

[0029] The addition unit 57 adds a reference frequency ωo corresponding to the power supply frequency of the three-phase AC power supply 1 to the frequency ω obtained by the PI control unit 56 in order to cope with the fact that the frequency ω obtained by the PI control unit 56 is still unknown at the start of operation. The integration unit 58 obtains the phase θ of the power supply voltages Vr, Vs, Vt by integrating the frequency ω that has passed through the addition unit 57. The obtained phase θ becomes the estimation result of the control unit 30.

[0030] The zero-phase voltage calculation unit 59 calculates a zero-phase voltage Vo that is superimposed on the potential of the neutral point A based on the phase θ obtained by the integration unit 58 and the d-axis voltage Vd obtained by the coordinate conversion unit 54. The calculated zero-phase voltage Vo is fed back to the subtraction units 51, 52.

[0031] With the above configuration, the values and phases θ of the power supply voltages Vr, Vs, and Vt can be accurately monitored. As a result, the appropriate operation of the multilevel converter 20 under the control of the control unit 30 can be continued.

[0032] Note that the control unit 30 estimates the values and phases θ of the power supply voltages Vr, Vs, and Vt based on the potential of the neutral point A where the zero-phase voltage Vo is superimposed. Therefore, electrical insulation between the control unit 30 and the multilevel converter 20 is not required. As a countermeasure against power disturbances, when there is a variation of a predetermined level or more in the estimated values of the power supply voltages Vr, Vs, and Vt, the control contents for each unit converter of the multilevel converter 20, such as the control gain, target voltage, target current, and memory for repetitive control, are changed according to the amount of variation in the estimated values of the power supply voltages Vr, Vs, and Vt, so that harmonics can be stably reduced.

[0033] [Cut-off frequency fx of low-pass filters 43 and 44] The LC filter 11 and the series reactors 14u, 14v, and 14w form an LCL resonance circuit for removing the switching frequency components of the pulse width modulation (PWM) control of the control unit 30 so that they do not flow out to the system on the three-phase AC power supply 1 side. The gain characteristics of this LCL resonance circuit are shown in FIG. 5. Since the multilevel converter 20 multilevels the output voltage for each phase by each cluster 21, even if the actual switching frequency (the switching frequency of the switch elements Q1 to Q4 in each unit converter) fs is lower than the resonance frequency (cut-off frequency) fc of the LCL resonance circuit, the equivalent switching frequency of the multilevel converter 20 is higher than the resonance frequency fc, which is "N × fs" (N is the number of stages of each unit converter in one cluster).

[0034] The cut-off frequencies fx of the low-pass filters 43 and 44 in the rectangular wave removal filter 18 are set between the resonance frequency fc of the LCL resonance circuit and the equivalent switching frequency "N×fs" of the multilevel converter 20. By providing the cut-off frequency fx to the low-pass filters 43 and 44, the control unit 30 can detect abnormalities in the power supply voltages Vr, Vs, and Vt when resonance occurs in the LCL resonance circuit. That is, the low-pass filters 43 and 44 can detect the occurrence of the resonance frequency fc of the LCL resonance circuit, which is lower than the cut-off frequency fx, without detecting the equivalent switching frequency "N×fs", which is higher than the cut-off frequency fx. These frequencies are, for example, the switching frequency fs of the switching elements Q1 to Q4 is 3 kHz. In this case, the equivalent switching frequency "N×fs" as the multilevel converter 20 is 9 kHz, the resonance frequency fc of the LCL resonance circuit is 5 kHz, and the cut-off frequency fx is set to 8 kHz.

[0035] [Advance compensation of the rectangular wave removal filter 18] Phase delays occur in the voltages Vr_n and Vs_n detected by the voltage detection unit 17 due to the effects of sampling and filter characteristics. As it is, there is a possibility that proper estimation by the control unit 30 cannot be performed.

[0036] Taking this into consideration, advance compensators 41 and 42 such as imperfect differentiation are provided in the rectangular wave removal filter 18 to compensate for the phase delays of the voltages Vr_n and Vs_n.

[0037] The values of the power supply voltages Vr, Vs, and Vt estimated by the control unit 30 when there are the advance compensators 41 and 42 are shown in FIG. 6. If there are no advance compensators 41 and 42, as shown in FIG. 7, distortion occurs in the values of the power supply voltages Vr, Vs, and Vt estimated by the control unit 30, and accurate values cannot be detected.

[0038] [Other controls of the control unit 30] The control unit 30 includes an estimating means for estimating the values of the power supply voltages Vr, Vs, and Vt based on the capacitor voltages (the detected voltages of the voltage detection unit Cx) in each unit converter of the multilevel converter 20. When the multilevel converter 20 is stopped, it compares the estimation result based on the capacitor voltage with the estimation result based on the voltages Vr_nm, Vs_nm, and the zero-phase voltage Vo that have passed through the rectangular wave removal filter 18. Note that the power supply voltages Vr, Vs, and Vt based on the voltages Vr_nm, Vs_nm, and the zero-phase voltage Vo that have passed through the rectangular wave removal filter 18 can be estimated regardless of whether the load 2 is operating and / or whether the power conversion device 10 itself is operating. However, the estimation of the power supply voltages Vr, Vs, and Vt based on this capacitor voltage can only be used when the multilevel converter 20 is stopped. When the multilevel converter 20 is stopped, the d-axis voltage Vd obtained by the coordinate conversion unit 54 corresponds to the values of the power supply voltages Vr, Vs, and Vt. Assuming that the average value of the voltages of the capacitors C in each unit converter within one cluster is Vc and the number of stages of each unit converter within one cluster is N, it is represented by the following formula. The values of the power supply voltages Vr, Vs, and Vt can be estimated from this d-axis voltage Vd.

[0039]

Equation

[0040] When the multilevel converter 20 is stopped, if there is no abnormality in the power supply voltages Vr, Vs, and Vt, the values of the power supply voltages Vr, Vs, and Vt estimated by the capacitor voltage Vc should be the same as the values of the power supply voltages Vr, Vs, and Vt estimated based on the voltages Vr_n, Vs_n, and the zero-phase voltage Vo that have passed through the aforementioned rectangular wave removal filter 18, respectively.

[0041] However, if there is an abnormality in any of the clusters 21 in the multilevel converter 20 or the unit converters 22 therein, especially if there is a high probability of a capacitor C in the unit converter 22 breaking down, a difference will occur between these two estimation results. Therefore, the control unit 30 calculates the difference in the estimated power supply voltage in each phase, and in all phases, if this difference is less than a predetermined threshold value, the multilevel converter 20 is considered normal and its startup is allowed. However, if the difference in any one phase is equal to or greater than the threshold value, the multilevel converter 20 is not started under the judgment that there is an abnormality somewhere in the multilevel converter 20. This prevents failures and damages caused by starting the multilevel converter 20 in a state where there is an abnormality.

[0042] [2] Second Embodiment The second embodiment of the present invention will be described. In the drawings, the same parts as those in the first embodiment are denoted by the same reference numerals, and detailed descriptions thereof are omitted.

[0043] As shown in FIG. 8, a converter (transformer) 60 is connected to the power lines Lr, Ls, Lt of the three-phase AC power supply 1 via a passive filter composed of an LC filter 11 and series reactors 14r, 14s, 14t, and a load, for example, an air conditioner 2 is connected to the positive output terminal P and the negative output terminal N of the converter 60. The air conditioner 2 includes an inverter 6 that converts the output voltage Vdc of the converter 60 into an AC voltage of a predetermined frequency and outputs it, and a compressor motor 7 that operates by the output of this inverter 6.

[0044] The converter 60 includes a switching circuit 61 that converts the power supply voltages Vr, Vs, Vt of the three-phase AC power supply 1 into a DC voltage by switching according to a drive signal from the control unit 30, and a DC capacitor (smoothing capacitor) 62 connected to the output terminal of this switching circuit 61.

[0045] A DC voltage detection unit 63 for detecting the output voltage Vdc of the converter 60 is connected between the positive output terminal P and the negative output terminal N of the converter 60. The DC voltage detection unit 63 serially connects resistors 63a and 63b having the same resistance value, and detects the voltage Vdc generated between the interconnection point of the resistors 63a and 63b and the negative output terminal N as the output voltage of the converter 60. This voltage Vdc is sent to the control unit 30.

[0046] The power conversion device 10 is constituted by the passive filter, the converter 60, the DC voltage detection unit 63, the voltage detection unit 17, the rectangular wave removal filter 18, and the control unit 30.

[0047] The switching circuit 61 of the converter 60 is, for example, as shown in FIG. 9, a two-level output type in which series circuits of switch elements T1 and T2 are provided for each phase of the power lines Lr, Ls, and Lt, and a DC capacitor 62 is connected to these three series circuits. Alternatively, as shown in FIG. 10, the switching circuit 61 includes a series circuit of switch elements T1 and T2 and a series circuit of switch elements T3 and T4 connected to the interconnection point of the switch elements T1 and T2. These series circuits are provided for each phase of the power lines Lr, Ls, and Lt, and a three-level output type in which DC capacitors 62a and 62b, which are a split of the DC capacitor 62, are connected to these series circuits. As shown in FIG. 11, a three-level output type including a series circuit of diodes D1 and D2 and a series circuit of switch elements T3 and T4 connected to the interconnection point of the diodes D1 and D2, with these series circuits provided for each phase of the power lines Lr, Ls, and Lt and DC capacitors 62a and 62b, which are a split of the DC capacitor 62, connected to these series circuits, may be used as the switching circuit 61.

[0048] The voltage detection unit 17 detects voltages Vr_n and Vs_n between each energization path from two of the power lines Lr, Ls, and Lt, for example, the power lines Lr and Ls, to the passive filter (LC filter 11 and associated reactors 14r, 14s, 14t) and the negative output terminal (predetermined position) N of the converter 60.

[0049] The rectangular wave removal filter 18 includes the same compensators 41 and 42 and low-pass filters (LPFs) 43 and 44 as in the first embodiment, and removes the rectangular wave components from the detected voltages Vr_n and Vs_n of the voltage detection unit 17.

[0050] Based on the voltages Vr_nm and Vr_nm that have passed through the rectangular wave removal filter 18 and the zero-phase voltage Vo in the converter 60, and with the potential of the negative output terminal N of the converter 60 as the reference potential, the control unit 30 estimates the values and phases θ of the power supply voltages Vr, Vs, and Vt of the three-phase AC power supply 1. As this estimation means, the control unit 30 includes the phase-locked loop circuit (PLL circuit) shown in FIG. 12.

[0051] The arithmetic unit 70 obtains a value that is half of the detected voltage (the output voltage of the converter 60) Vdc of the DC voltage detection unit 63. The subtraction units 51 and 52 subtract the output voltage "Vdc / 2" of the arithmetic unit 70 from the voltages Vr_n and Vr_n that have passed through the rectangular wave removal filter 18, and also subtract the zero-phase voltage Vo (the zero-phase voltage superimposed on the potential of the negative output terminal N) calculated by the subsequent zero-phase voltage calculation unit 59, thereby capturing the values of the power supply voltages Vr and Vs. The subtraction unit 53 captures the difference between the values of the power supply voltages Vr and Vs captured by the subtraction units 51 and 52 as the value of the power supply voltage Vt. Vr = Vr_n - (Vdc / 2) - Vo, Vs = Vs_n - (Vdc / 2) - Vo, Vt = -Vr - (Vdc / 2) - Vs The potential of the interconnection point of the resistors 63a and 63b corresponds to the midpoint potential of the DC capacitor 62, and the interconnection point of the resistors 63a and 63b acts as the virtual neutral point of the converter 60 which is a converter. The potential of the negative output terminal N viewed from this virtual neutral point is "-Vdc / 2", and the voltages Vr_n, Vr_n, and the zero-phase voltage Vo with the potential of the negative output terminal N as the reference include the voltage "Vdc / 2". Therefore, the voltage "Vdc / 2" is subtracted from the voltages Vr_n, Vr_n, and the zero-phase voltage Vo.

[0052] Regarding the zero-phase voltage Vo, it changes according to the operating conditions of the converter 60. For example, when sinusoidal modulation is applied to the switching control of the converter 60, the zero-phase voltage Vo becomes 0 (zero). When the switching operation of the converter 60 is stopped, or when the third-harmonic superposition method is applied to the switching control of the converter 60, a zero-phase voltage Vo with a value of 15% of the values of the power supply voltages Vr, Vs, Vt and a frequency three times that of the power supply voltages Vr, Vs, Vt is superimposed on the potential of the negative output terminal N.

[0053] The values of the power supply voltages Vr, Vs, Vt captured by these subtraction units 51, 52, 53 become the estimation results of the control unit 30. In addition to this, the control unit 30 includes a coordinate conversion unit 54, a subtraction unit 55, a PI control unit 56, an addition unit 57, an integration unit 58, and a zero-phase voltage calculation unit 59 in order to improve the tolerance to power disturbances and to estimate the phase θ. Since the configurations of the coordinate conversion unit 54 to the integration unit 58 are the same as those in the first embodiment, the description thereof is omitted.

[0054] The zero-phase voltage calculation unit 59 calculates a zero-phase voltage Vo that is superimposed on the potential of the negative output terminal N based on the phase θ obtained by the integration unit 58 and the d-axis voltage Vd obtained by the coordinate conversion unit 54. The calculated zero-phase voltage Vo is fed back to the subtraction units 51, 52.

[0055] With the above configuration, the values and phase θ of the power supply voltages Vr, Vs, Vt can be accurately monitored. As a result, the appropriate operation of the converter 60 under the control of the control unit 30 can be continued. That is, the control unit 30 controls the switching operation of the switching circuit of the converter 60 based on the estimated values of the power supply voltages Vr, Vs, Vt, thereby stably reducing harmonics. For example, the control unit 30 changes the control gain, target voltage, target current, memory for repetitive control, etc. in the control of the switching operation according to the estimated values of the power supply voltages Vr, Vs, Vt, and controls the timing and period of the switching operation.

[0056] Note that the control unit 30 estimates the values and phases θ of the power supply voltages Vr, Vs, and Vt based on the potential of the negative output terminal N to which the zero-phase voltage Vo is superimposed. Therefore, electrical insulation between the control unit 30 and the converter 60 is unnecessary.

[0057] [3] Third Embodiment The third embodiment of the present invention will be described. As shown in FIG. 13, the power conversion device 10 includes a converter 60 and a DC voltage detection unit 63 of the second embodiment instead of the multilevel converter 20 of the first embodiment. The converter 60 functions as an active filter that suppresses the harmonic components of the current flowing through the rectifier circuit 3 of the air conditioner 2. Other configurations and controls are the same as those of the first and second embodiments. Therefore, the description thereof will be omitted.

[0058] [4] Modification In the first embodiment, the case where the number of unit converters in each of the clusters 21r, 21s, and 21t is three for each phase has been described. However, the number can be set as appropriate, and three or more is desirable. Due to the relationship between the number of drive signals for each unit converter 22, the number is preferably odd.

[0059] The above-described embodiments and modifications are presented as examples and are not intended to limit the scope of the invention. These embodiments and modifications 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 modifications are included in the scope of the invention in terms of the gist and are included in the scope of the invention described in the claims and its equivalents.

Description of Reference Numerals

[0060] 1…Three-phase AC power supply, Lr, Ls, Lt…Power lines, 3…Air conditioner (load), 10…Power conversion device, 11…LC filter, 14u, 14v, 14w…Link reactors, 17…Voltage detection unit, 18…Rectangular wave removal filter, 20…Multi-level converter (converter), 30…Control unit, 60…Converter (converter), 63…DC voltage detection unit.

Claims

1. A power conversion device connected to each power line of a three-phase AC power supply, including a plurality of clusters each composed of a plurality of unit converters each consisting of a switching element and a capacitor connected in series, one ends of these clusters being interconnected to form a neutral point, the other ends of each of these clusters being connected to each of the power lines via a passive filter, and a multilevel converter that performs power conversion by switching; a voltage detection unit that detects the voltage between the energization path from each of the power lines to the passive filter and the neutral point; a rectangular wave removal filter that removes the rectangular wave component associated with the switching from the voltage detected by the voltage detection unit; a control unit that estimates the value of the power supply voltage of the three-phase AC power supply based on the voltage that has passed through the rectangular wave removal filter and the zero-phase voltage superimposed on the potential of the neutral point in the power converter, and controls the switching of the multilevel converter based on this estimation result; A power conversion device comprising:

2. A power conversion device connected to each power line of a three-phase AC power supply, including a converter including a switching circuit that is connected to each of the power lines via a passive filter and converts the power supply voltage of the three-phase AC power supply into a DC voltage, and a DC capacitor connected between the positive output terminal and the negative output terminal of the switching circuit; a voltage detection unit that detects the voltage between the energization path from each of the power lines to the passive filter and the negative output terminal of the converter; a rectangular wave removal filter that removes the rectangular wave component associated with the switching from the voltage detected by the voltage detection unit; a control unit that estimates the value of the power supply voltage based on the voltage that has passed through the rectangular wave removal filter and the zero-phase voltage superimposed on the potential of the negative output terminal of the converter, and controls the switching of the converter based on this estimation result; A power conversion device comprising:

3. The voltage detection unit detects the voltage between the two energization paths from two of the power lines to the passive filter and the neutral point among the respective power lines. The power conversion device according to Claim 1.

4. The voltage detection unit detects the voltage between the two energization paths from two of the power lines to the passive filter and the negative terminal among the respective power lines. The power conversion device according to Claim 2.

5. The control unit estimates the value and phase of the power supply voltage by subtracting the zero-phase voltage from the voltage that has passed through the rectangular wave removal filter, and controls the switching of the multilevel converter based on this estimation result. The power conversion device according to claim 1.

6. The control unit estimates the value and phase of the power supply voltage by subtracting the zero-phase voltage from the voltage that has passed through the rectangular wave removal filter, and controls the switching of the converter based on this estimation result. The power conversion device according to claim 2.

7. The rectangular wave removal filter is a low-pass filter having a cut-off frequency between the resonance frequency of the passive filter and the equivalent switching frequency of the multilevel converter. The power conversion device according to claim 1.

8. The rectangular wave removal filter is a low-pass filter having a cut-off frequency between the resonance frequency of the passive filter and the equivalent switching frequency of the converter. The power conversion device according to claim 2.

9. The rectangular wave removal filter includes an advance compensator that compensates for the phase delay of the voltage detected by the voltage detection unit. The power conversion device according to claim 7 or claim 8.

10. The control unit includes means for estimating the value of the power supply voltage based on the voltage of the capacitor in each unit converter, and when the multilevel converter is stopped, if the difference between the estimation result based on the voltage of the capacitor and the estimation result based on the voltage that has passed through the rectangular wave removal filter and the zero-phase voltage is equal to or greater than a threshold value, the multilevel converter is not started. The power conversion device according to claim 1.

11. The control unit includes means for estimating the value of the power supply voltage based on the voltage of the DC capacitor, and when the converter is stopped, if the difference between the estimation result based on the voltage of the DC capacitor and the estimation result based on the voltage that has passed through the rectangular wave removal filter and the zero-phase voltage is equal to or greater than a threshold value, the converter is not started. The power conversion device according to claim 2.

12. The control unit estimates the value of the power supply voltage with the potential of the neutral point or the negative terminal as the reference potential. The power conversion device according to claim 1 or claim 2.

13. The multilevel converter selectively generates and outputs DC voltages of three or more levels for each phase of each power line. The power conversion device according to claim 1.

14. The converter selectively generates and outputs DC voltages of three or more levels for each phase of the respective power lines. The power conversion device according to claim 2.

15. The multilevel converter functions as an active filter that suppresses harmonic components of the current flowing through the load connected to the power line. The power conversion device according to claim 1.

16. The converter functions as an active filter that suppresses harmonic components of the current flowing through the load connected to the power line. The power conversion device according to claim 2.

17. When there is a variation of a predetermined amount or more in the estimation result, the control unit changes the control content for each unit converter of the multilevel converter according to the amount of the variation. The power conversion device according to claim 1.

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