Power converter

Abstract

A power conversion device (101) comprises: an AC / DC converter that is connected to a three-phase AC power source (102), and that converts AC power to DC voltage; and a controller (107) that controls the AC / DC converter. The controller (107) includes: an AC current control unit that outputs an AC output voltage command for each phase on the basis of a current detection value detected by a current detector (104); a third harmonic superimposition unit that performs control to superimpose a frequency that is triple the fundamental frequency onto the AC output voltage command; a PWM unit that determines a switching pattern of a plurality of semiconductor switching elements by comparing a signal superimposed by the third harmonic superimposition unit with a carrier wave; and a zero-phase current calculation unit that calculates a zero-phase current on the basis of the current detection value. The AC current control unit excludes a frequency component of a predetermined range of the zero-phase current from the target of control on the basis of a zero-phase current value obtained by the zero-phase current calculation unit.

Description

Technical Field

[0001] The present disclosure relates to a power conversion device, particularly to an AC / DC converter.

Background Art

[0002] In a general AC / DC converter connected to a three-phase three-wire transformer, when connected to a commercial power supply, the secondary side of the transformer may be star-connected and its neutral point may be floating. In such a configuration, the voltage utilization rate of the AC / DC converter is improved by superimposing the third harmonic on the output voltage using the freedom of the zero-phase voltage (for example, Patent Document 1).

[0003] Also, in an AC / DC converter, when there is a path for the zero-phase current, the alternating current to be controlled by the AC / DC converter includes both a normal-mode current and a zero-phase current. At this time, since the paths through which the normal-mode current and the zero-phase current flow are different, and the impedances on each main circuit are also different, the stability of the alternating current control may be improved by separating them and configuring an independent control system (for example, Patent Document 2). Patent Document 2 targets a three-phase four-wire system in which the secondary side of the transformer is star-connected and its neutral point is grounded, and performs feedback control by separating the zero-phase current from the normal-mode current, and the zero-phase current is controlled to be zero.

[0004] Also, in Patent Document 3, by performing coordinate transformation, since the zero-phase current is eliminated during the transformation process, when the current after the transformation is used as the control target, as a result, the zero-phase current is ignored and only the normal-mode current is controlled.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] On the other hand, when considering a three-phase three-wire system, third-harmonic superimposition is performed on the zero-sequence voltage. As a result, a zero-sequence current of the third harmonic, corresponding to the impedance of the zero-sequence current path and the third-harmonic voltage, flows in principle. Therefore, if the command value of the zero-sequence current is set to zero, control interference may occur, potentially leading to unstable AC current control.

[0007] This disclosure aims to solve the above-mentioned problems and to provide a power conversion device capable of stable operation in AC current control. [Means for solving the problem]

[0008] A power converter according to one embodiment includes an AC / DC converter connected to a three-phase AC power supply and converting AC power to DC voltage, and a controller for controlling the AC / DC converter. The AC / DC converter includes a plurality of semiconductor switching elements provided between the positive and negative electrodes and driven by AC power, a DC capacitor provided in parallel with the plurality of semiconductor switching elements for holding the converted DC voltage, a filter circuit provided between the three-phase AC power supply and the plurality of semiconductor switching elements and composed of an AC filter reactor and an AC filter capacitor, a neutral point line connected to the DC capacitor via the neutral point of the AC filter capacitor and serving as a path for zero-sequence current, and a current detector for detecting the current of each phase between the filter circuit and the plurality of semiconductor switching elements. The controller includes an AC current control unit that outputs AC output voltage commands for each phase based on current detection values ​​detected by a current detector; a third harmonic superposition unit that performs control by superimposing a frequency three times the fundamental frequency onto the AC output voltage command; a PWM unit that determines the switching pattern of multiple semiconductor switching elements by comparing the signal superimposed by the third harmonic superposition unit with the carrier wave; and a zero-sequence current calculation unit that calculates the zero-sequence current based on the current detection values. The AC current control unit excludes frequency components within a predetermined range from the zero-sequence current from the control target based on the zero-sequence current value obtained by the zero-sequence current calculation unit. [Effects of the Invention]

[0009] A power converter in accordance with this disclosure is capable of stable operation in AC current control. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows an example of the circuit configuration of a power converter 101 according to Embodiment 1 of the present disclosure. [Figure 2] This figure illustrates the function of the controller 107 of the power converter 101 according to Embodiment 1 of the present disclosure. [Figure 3] This figure illustrates the superposition of third harmonics according to Embodiment 1 of this disclosure. [Figure 4]It is a block diagram showing an example of the function of the zero-phase current calculation unit 302 according to Embodiment 1 of the present disclosure. [Figure 5] It is a diagram for explaining the function of the ACR unit 301 according to Embodiment 1 of the present disclosure. [Figure 6] It is a diagram showing an example of the circuit configuration of the power conversion device 101# according to a modification of Embodiment 1 of the present disclosure. [Figure 7] It is a diagram for explaining the controller 107# according to Embodiment 2 of the present disclosure. [Figure 8] It is a diagram for explaining the configuration of the ACR units 701 and 702 according to Embodiment 2 of the present disclosure. [Figure 9] It is a diagram schematically comparing the current flow paths of the normal mode current and the zero-phase current according to Embodiment 2 of the present disclosure. [Figure 10] It is a diagram for explaining the configuration of the current control unit of the controller 107P according to Embodiment 3 of the present disclosure. [Figure 11] It is a diagram for explaining the configuration of the zero-phase current calculation unit 1110 of the power conversion device according to Embodiment 4 of the present disclosure.

Embodiments for Carrying Out the Invention

[0011] Embodiment 1. 1]FIG. 1 is a diagram showing an example of the circuit configuration of the power conversion device 101 according to Embodiment 1 of the present disclosure.

[0012] Referring to FIG. 1, the power conversion device 101 according to Embodiment 1 of the present disclosure is an AC / DC converter that converts input AC power into DC power and performs bidirectional power flow between the AC input and the DC output.

[0013] The power conversion device 101 is connected via a commercial power supply 102 that supplies power by a three-phase AC voltage and a transformer 103.

[0014] The primary side and the secondary side are electrically insulated from each other by the transformer 103. In this example, the side connected to the commercial power supply 102 is referred to as the primary side, and the side connected to the AC / DC converter side is referred to as the secondary side.

[0015] The power conversion device 101 includes current detectors 104a, 104b, 104c, AC voltage detectors 105a, 105b, 105c, a DC voltage detector 106, an AC filter reactor 112, an AC filter capacitor 113, semiconductor switching elements 108a_P, 108a_N, 108b_P, 108b_N, 108c_P, 108c_N, DC capacitors 110P, 110N, and a controller 107.

[0016] Regarding the AC side of the power conversion device 101, a filter circuit is constituted by the AC filter reactor 112 and the AC filter capacitor 113.

[0017] The current detectors 104a, 104b, 104c detect the current of each phase on the output side of the AC filter reactor 112.

[0018] The AC voltage detectors 105a, 105b, 105c detect the AC voltage of each phase transformed from the transformer 103.

[0019] The DC voltage detector 106 detects the DC voltage that is the output of the power conversion device 101. Based on the current value of each phase from the current detectors 104a, 104b, 104c, the AC voltage value of each phase from the AC voltage detectors 105a, 105b, 105c, and the DC voltage value of the DC voltage detector 106, the controller 107 controls so that an arbitrarily set DC voltage is obtained with respect to the input AC voltage.

[0020] The semiconductor switching elements 108a_P and 108a_N are connected in series with the positive and negative DC terminals 111P and 111N. The semiconductor switching elements 108b_P and 108b_N are connected in series with the positive and negative DC terminals 111P and 111N. The semiconductor switching elements 108c_P and 108c_N are connected in series with the positive and negative DC terminals 111P and 111N. In addition, the midpoints of each are connected to the AC terminals of each phase.

[0021] Furthermore, freewheeling diodes 109a_P, 109a_N, 109b_P, 109b_N, 109c_P, and 109c_N are connected in antiparallel to each semiconductor switching element 108a_P, 108a_N, 108b_P, 108b_N, 108c_P, and 108c_N, respectively.

[0022] DC capacitors 110P and 110N are connected in series between the positive and negative DC terminals 111P and 111N, and their midpoint is connected to the AC filter capacitor 113.

[0023] The controller 107 controls the input AC voltage to an arbitrarily set DC voltage by controlling the on / off state of semiconductor switching elements 108a_P, 108a_N, 108b_P, 108b_N, 108c_P, and 108c_N.

[0024] Examples of semiconductor switching elements include IGBTs (Insulated Gate Switches). Bipolar transistors and MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) are used.

[0025] The DC terminals 111P and 111N supply DC power by being connected to load devices or other power converters driven by DC voltage.

[0026] In this example, we will describe a case where a three-phase coupled reactor is used as the AC filter reactor 112, but it is also possible to configure it with three separate reactors for each of the three phases. The AC filter capacitor is configured in a three-phase star connection and has a neutral point. In the AC / DC converter according to this disclosure, it is necessary to configure a neutral point line 114 via the neutral point of the AC filter capacitor 113 to secure a path for the zero-sequence current, so the three-phase AC filter capacitor needs to be in a star connection rather than a delta connection.

[0027] Figure 2 is a diagram illustrating the function of the controller 107 of a power converter 101 according to Embodiment 1 of this disclosure. Referring to Figure 2, the control block of the controller 107 according to Embodiment 1 is shown, which generates a gate signal that controls the on / off state of a semiconductor switching element from AC current control.

[0028] The controller 107 includes an ACR unit 301, a zero-sequence current calculation unit 302, a third-harmonic superposition unit 303, a PWM unit 305, an inverter 306, and a gate drive device 307.

[0029] In this example, the ACR (Auto Current Regulator) unit 301 performs control based on the current detection value isens_R detected by the current detector 104a, so that the current command value iref_R calculated by the higher-level control system becomes the current command value iref_R.

[0030] Note that, although only one phase of the three phases is shown here as an example, the same principles apply to the other phases.

[0031] In this case, in addition to the current command value and current detection value, which are typical inputs to the ACR unit 301, the zero-sequence current calculation value calculated by the zero-sequence current calculation unit 302 is also used as an input.

[0032] The zero-sequence current calculation unit 302 calculates the zero-sequence current using the current detection values ​​for all three phases as input. When the three phase current detection values ​​are separated into the normal mode current component inormal_X(X:R,S,T) and the zero-sequence current component izero, they are expressed as shown in the following equation. The zero-sequence current component is a common value for all phases.

[0033] isens_R = inormal_R+izero isens_S = inormal_S+izero isens_T = inormal_T+izero Here, since the sum of the normal mode currents in the three phases is zero, the zero-sequence current izero is given by the following equation.

[0034] izero = (isens_R+isens_S+isens_T) / 3 The third harmonic superposition unit 303 adds the third harmonic voltage command value Vtriref to the output of the ACR unit 301 using an adder 304. This third harmonic voltage command value Vtriref is added to each phase as a zero-sequence voltage command, so a common value for all three phases is added. Subsequently, this becomes the input to the PWM unit 305, and pulse width modulation is performed, so a pulse signal for controlling the on / off state of the semiconductor switching element is output from the gate drive unit 307. Since the on / off states of the two semiconductor switching elements connected in series are inversely related, for example, by using an inverter 306 on the pulse signal of the upper semiconductor switching element, an on / off pulse signal for the lower semiconductor switching element can be obtained. At this time, a dead time may be inserted to prevent simultaneous on-up of the upper and lower semiconductor switching elements.

[0035] Figure 3 illustrates the superposition of third harmonics according to Embodiment 1 of this disclosure. Referring to Figure 3, the schematic waveforms of the modulated wave input to the PWM unit 305 are shown, including the modulated wave vref1 before third harmonic superposition, the modulated wave vref2 after third harmonic superposition, and the superimposed third harmonic vreftri. As shown in the figure, by superimposing a third harmonic with an appropriate phase, the peak value of the modulated wave, which is the output voltage command, is reduced, and the voltage utilization rate can be improved. Therefore, although the third harmonic appears in the phase voltage of the output voltage, it does not appear in the line voltage because the third harmonic is a zero-sequence voltage, and thus it is applicable to a three-phase three-wire power supply configuration.

[0036] On the other hand, in the case of a zero-sequence current path as shown in Figure 1, the potential of the neutral point line 114 changes due to the superposition of third harmonics, causing a third-harmonic current that depends on the impedance of the filter circuit to flow. Since this current flows in principle due to the superposition of third harmonics, if, for example, the current command value is such that the zero-sequence current is zero, it will be contrary to the AC current control. Generally, the three-phase AC current command value given to the ACR unit 301 only considers the normal mode component, so as a result the zero-sequence current component of the AC current command value becomes zero, and the current of the third-harmonic component becomes a deviation. Therefore, if the gain in the AC current control of the ACR unit 301 is set high, the AC current control may become unstable in an attempt to eliminate the deviation.

[0037] On the other hand, the zero-sequence current component can also be the resonant current of the AC filter reactor 112 and AC filter capacitor 113 of the filter circuit. Since this resonant current is a disturbance component, it is a current that should be actively reduced to zero in the AC current control unit.

[0038] Therefore, in the ACR section 301 of the power converter 101 according to this embodiment 1 in which third harmonic superposition is performed, it is preferable to control the zero-sequence current component so that it becomes zero by the AC current control unit, while excluding the third harmonic component, which in principle cannot be zero, from the control target, and including the resonant frequency component of the filter circuit as a control target.

[0039] Figure 4 is a block diagram showing an example of the functions of a zero-sequence current calculation unit 302 according to Embodiment 1 of the present disclosure. Referring to Figure 4, the zero-sequence current calculation unit 302 includes adders 501, 502, a multiplier 503, and an LPF unit 504.

[0040] The zero-sequence current detection value is obtained by multiplying the sum of the three-phase alternating currents by 1 / 3. Therefore, the sum of each alternating current is calculated using adders 501 and 502, and then multiplied by 1 / 3 using multiplier 503. Furthermore, in this embodiment 1, a Low Pass Filter (LPF) section 504 is provided to perform low-pass filtering.

[0041] This configuration removes frequency components higher than the set frequency. The low-pass filter processing is well-known, so its explanation is omitted. The zero-sequence current obtained in this manner, from which components higher than the set frequency have been removed, is used as the zero-sequence current compensation value and is output to the zero-sequence current calculation unit 302.

[0042] Figure 5 is a diagram illustrating the function of the ACR unit 301 according to Embodiment 1 of the present disclosure. Referring to Figure 5, the ACR unit 301 includes subtractors 601, 602 and a gain multiplier 603.

[0043] In typical AC current control, the subtractor 601 calculates the difference between the current command value iref and the current detection value isens, and the control is performed by multiplying this by a gain. However, since the zero-sequence current component of the current command value iref is zero, the control is performed so that the current detection value isens becomes zero even if third-harmonic currents are present.

[0044] In this embodiment 1, compensation for the zero-sequence current component is performed by subtracting the zero-sequence current compensation value izero_comp from the current detection value using the subtractor 602.

[0045] Here, since the zero-sequence current compensation value izero_comp excludes only high-frequency components by the LPF section 504 explained in Figure 4, third harmonics and lower frequency components are excluded from the current detection value isens.

[0046] Subsequently, the deviation from the current command value iref is calculated and input to the gain multiplier 603, which becomes the output of the ACR unit 301.

[0047] In some cases, a compensation value based on the AC voltage is added to the value after gain multiplication in a feedforward manner to improve controllability, but this has been omitted in this example.

[0048] Therefore, zero-sequence current components such as the third harmonic component are excluded from the current detection value isens, and thus these components are not subject to control. On the other hand, relatively high-frequency components such as the resonant frequency component of the filter circuit composed of the AC filter reactor 112 and the AC filter capacitor 113 are included in the current detection value isens and are therefore subject to control.

[0049] Therefore, it is controlled to become zero based on the current command value iref. As described above, by using the power converter 101 according to Embodiment 1 of this disclosure, even when there is a path through which zero-sequence current flows and the zero-sequence current contains a mixture of components that cannot be made zero by AC current control, such as third harmonic superposition and DC offset of current detection values, and components that are preferable to be made zero by AC current control, such as the resonant frequency component of the filter circuit, it becomes possible to effectively exclude the components that cannot be made zero from the control target, thereby improving the stability of AC current control in the power converter.

[0050] Figure 6 shows an example of the circuit configuration of a power converter 101# according to a modified example of Embodiment 1 of the present disclosure.

[0051] Referring to Figure 6, the case in which DC capacitor 110 is placed between the positive and negative electrodes instead of DC capacitors 110P and 110N is shown.

[0052] Furthermore, a case is shown in which the negative electrode line and the AC filter capacitor 113 are connected to form a neutral point line 114.

[0053] With this configuration, it is also possible to connect to a point at the same potential as the DC terminal 111N to form a zero-sequence current path within the power converter 101#.

[0054] Furthermore, this configuration eliminates the need to configure the DC capacitor 110 in series, thus reducing the number of components. On the other hand, the impedance of the zero-sequence current path is different, resulting in a unipolar applied voltage to the AC filter capacitor.

[0055] Even in this configuration, by using a power converter 101# according to a modified example of Embodiment 1 of this disclosure, even when there is a path through which zero-sequence current flows and the zero-sequence current contains a mixture of components that cannot be made zero by AC current control, such as third harmonic superposition and DC offset of current detection values, and components that are preferable to be made zero by AC current control, such as the resonant frequency component of the filter circuit, it becomes possible to effectively exclude the components that cannot be made zero from the control target, thereby improving the stability of AC current control in the power converter.

[0056] Embodiment 2. Figure 7 illustrates a controller 107# according to Embodiment 2 of this disclosure.

[0057] Referring to Figure 7, controller 107# differs from controller 107 in that it has ACR units 701 and 702, a zero-sequence current calculation unit 703, and an adder 704 instead of the ACR unit 301 and zero-sequence current calculation unit 302. The other configurations are the same as those described in Figure 2, so a detailed explanation will not be repeated.

[0058] In this example, the AC current control system separates and independently implements the current control system for the normal mode component and the current control system for the zero-sequence current.

[0059] The ACR unit 701 controls the normal mode component current, and the ACR unit 702 controls the zero-sequence current. The zero-sequence current calculation unit 703 calculates the zero-sequence current from the three-phase current detection values, but low-pass filtering that extracts only predetermined frequency components, as in Embodiment 1, is not performed at this stage. After each is controlled independently, the two controlled quantities are added together by the adder 704. Subsequent processing involves third-harmonic superposition and pulse signal generation using PWM, similar to Embodiment 1 described in Figure 2.

[0060] Figure 8 illustrates the configuration of ACR units 701 and 702 according to Embodiment 2 of the present disclosure. Referring to Figure 8, the ACR unit 701 includes subtractors 801 and 802 and a gain multiplier 803.

[0061] The ACR section 702 includes an HPF section 804 and a gain multiplier 805. The ACR unit 701, which performs current control of the normal mode component, will now be described.

[0062] The subtractor 801 of the ACR unit 701 eliminates the zero-sequence current component by subtracting the zero-sequence current detection value isero_sens, obtained by the calculation of the zero-sequence current calculation unit 703, from the current detection value isens.

[0063] The subtractor 802 then outputs the difference between the current command value iref, which is calculated using only the normal mode component, and the current command value iref.

[0064] The gain multiplier 803 takes the difference as input, amplifies it with a predetermined gain, and outputs it. On the other hand, the ACR unit 702 controls the zero-sequence current.

[0065] Specifically, the HPF section 804 performs high-pass filtering. This is to exclude frequency components below the third harmonic from the zero-sequence current control target.

[0066] The cutoff frequency of the HPF section 804 must be designed to sufficiently attenuate the third harmonic component while not attenuating the resonant frequency component of the filter circuit. Also, since the zero-sequence current command value is always zero, the output of the HPF section 804 directly becomes the input to the gain multiplier 805.

[0067] In this second embodiment, the current control system is separated into the normal mode component and the zero-sequence current component.

[0068] Figure 9 is a schematic comparison of the current paths between the normal mode current and the zero-sequence current according to Embodiment 2 of this disclosure.

[0069] Refer to Figure 9(A) to see the current path of the normal mode current. In this example, the pulse voltage generation sections 901a to 901c associated with the switching of the semiconductor switching element by the gate drive device 307 are shown in a simplified manner.

[0070] Specifically, the diagram shows a case where current flows from right to left as a current path to the AC filter reactor 903a in accordance with the generation of a pulse voltage by the pulse voltage generation unit 901a.

[0071] Furthermore, the diagram shows a case where current flows from left to right as a current path to the AC filter reactors 903b and 903c in accordance with the generation of pulse voltages by the pulse voltage generating units 901b and 901.

[0072] Referring to Figure 9(B), the path of the zero-sequence current is shown. This shows the case where the current path through the AC filter reactor 904, AC filter capacitor 905, and DC capacitor 906 is formed according to the generation of the pulse voltage from the pulse voltage generation unit 902.

[0073] Furthermore, since the paths of the zero-sequence currents are independent and essentially the same for each phase, we can simply consider the path for one phase.

[0074] As shown in Figures 9(A) and (B), the normal mode current flows through the AC filter reactor so that the sum of the three phases is always zero, while the zero-sequence current has the same value in each phase and mainly circulates over the power converter 101 via the AC filter capacitor and the neutral point line. There is also zero-sequence current via the stray capacitance of the transformer and other connected equipment, but it is very small compared to the zero-sequence current via the neutral point line described above and can be ignored. As described above, since the impedance of the paths through which the normal mode current and the zero-sequence current flow are different, it is possible to further stabilize the AC current control by performing gain design individually.

[0075] Embodiment 3. Figure 10 illustrates the configuration of the current control unit of controller 107P according to Embodiment 3 of this disclosure.

[0076] Referring to Figure 10, the controller 107P includes ACR units 1002 and 1005, an αβ conversion unit 1001, an inverse αβ conversion unit 1003, a zero-sequence current calculation unit 10004, and adders 1006, 1007, and 1008.

[0077] The controller 107P according to Embodiment 3 describes a method for separating the normal mode component current and the zero-sequence current from the current detection value detected by the current detector, using a method different from that shown in Figure 8.

[0078] The αβ conversion unit 1001 performs a Clark transformation, which is a three-phase / two-phase conversion, on the current detection values ​​isens_R, isens_S, and isens_T. This yields the current detection values ​​isens_α and isens_β on the α-β coordinate system. The relationship before and after the Clark transformation is expressed by the following equation.

[0079] isens_α = 2 / 3×(isens_R-0.5×(isens_S+isens_T)) isens_β = 1 / √3×(isens_S-isens_T) In other words, zero-sequence current components that take the same value in each phase are excluded from both isens_α and isens_β. Therefore, by controlling the ACR unit 1002, which takes the current command values ​​iref_α and iref_β on the αβ coordinate system as input, it becomes possible to control the current targeting only the normal mode current.

[0080] Furthermore, by inputting this into the inverse alpha-beta conversion unit 1003, the control is converted back to a three-phase control variable. On the other hand, the zero-sequence current, which was excluded by the αβ conversion unit 1001 mentioned above, is calculated by the zero-sequence current calculation unit 1004 based on the three-phase current detection values.

[0081] In order to control the zero-sequence current, the ACR section 1005 needs to exclude third-harmonic components and DC components from the control target, and therefore performs high-pass filtering in the HPF section as explained in Figure 8.

[0082] Therefore, the ACR unit 1005, which is the zero-sequence current control unit, is basically the same as the ACR unit 702 shown in Figure 8.

[0083] As described above, the two control quantities, which are separated and controlled independently, are added together phase by phase in adders 1006, 1007, and 1008. For subsequent processing, third-harmonic superposition and pulse signal generation by PWM are performed, similar to Embodiment 1 described in Figure 2.

[0084] As described above, by using the power conversion device according to Embodiment 3 of this disclosure, even when there is a path through which zero-sequence current flows and the zero-sequence current contains a mixture of components that cannot be made zero by AC current control, such as third harmonic superposition or DC offset of the current detection value, and components that are preferable to be made zero by AC current control, such as the resonant frequency component of the filter circuit, it becomes possible to effectively exclude the components that cannot be made zero from the control target. In addition, by constructing different current control systems for the normal mode component current and the zero-sequence current, which each have paths with different impedances, it becomes possible to design the control gain independently, thereby improving the stability of AC current control in the power conversion device.

[0085] Embodiment 4. Figure 11 is a diagram illustrating the configuration of the zero-sequence current calculation unit 1110 of a power converter according to Embodiment 4 of the present disclosure.

[0086] Referring to Figure 11, the zero-sequence current calculation unit 1110 includes adders 1101, 1102, and 1106, multipliers 1103 and 1107, a zero-order hold unit 1104, and a one-sampling delay unit 1105.

[0087] As explained above, the zero-sequence current detection value is obtained by adding the three-phase current detection values ​​using adders 1101 and 1102 and multiplying by 1 / 3 using multiplier 1103.

[0088] So far, we have discussed the third harmonic component, the DC component due to the DC offset of the current detector, and the resonant frequency component of the filter circuit as frequency components of zero-sequence current. However, there is also a switching frequency component associated with the switching of semiconductor switching elements.

[0089] The switching frequency component of this zero-sequence current is generally equal to the carrier frequency used in the PWM section and is a component that arises with switching. Therefore, it cannot be suppressed by AC current control and may become a disturbance in current control.

[0090] In this fourth embodiment, the value obtained by removing the switching frequency component from the zero-sequence current detection value is used for subsequent AC current control and zero-sequence current control.

[0091] Since the other components are the same as those described in Embodiments 1 to 3, a detailed explanation of them will be omitted. Here, the zero-sequence current detection value is calculated by taking the average of consecutive binary values ​​sampled at twice the frequency of the carrier wave.

[0092] This example demonstrates an example of average value calculation, where sampling is performed at a frequency twice the frequency of the carrier wave in the zero-order hold unit 1104. Furthermore, by holding the previous sampled value in the one-sampling delay unit 1105, performing a sum calculation in the adder 1106, and multiplying by 1 / 2 in the multiplier 1107, it is possible to calculate the average value of two consecutive values ​​sampled at twice the frequency of the carrier wave.

[0093] Other AC current control methods are the same as those described in Embodiments 1 to 3, so a detailed explanation will be omitted.

[0094] As described above, by using the power conversion device according to Embodiment 4 of this disclosure, even when there is a path through which zero-sequence current flows and the zero-sequence current contains a mixture of components that cannot be made zero by AC current control, such as third harmonic superposition and DC offset of the current detection value, and components that are preferable to be made zero by AC current control, such as the resonant frequency component of the filter circuit, it is possible to effectively exclude the components that cannot be made zero from the control target. In this power conversion device, since only the switching frequency component of the disturbing zero-sequence current can be removed by the sampling method, the resulting detection delay can also be suppressed to the same extent as the switching frequency, and the stability of AC current control is improved.

[0095] The embodiments disclosed herein should be considered illustrative and not restrictive in all respects. Furthermore, it goes without saying that there is no problem in suitably combining multiple embodiments, and it is desirable to further enhance the effects of each embodiment in order to make efficient and economical use of the power conversion device.

[0096] The scope of this disclosure is indicated by the claims rather than the above description, and all changes within the meaning and scope of the claims are intended. [Explanation of Symbols]

[0097] 101 Power converter, 102 Commercial power supply, 103 Transformer, 104a, 104b, 104c, 105 Current detector, 105a, 105b, 105c AC voltage detector, 106 DC voltage detectors, 107, 107P; controllers, 108a, 108b, 108c; semiconductor switching elements, 109a, 109b, 109c; freewheeling diodes, 110, 110N, 110P, 906; DC capacitors, 111N, 111P; DC terminals, 112, 903a, 903b, 903c, 904; AC filter reactors, 113, 905; AC filter capacitors, 114; neutral point lines, 301, 701, 702, 1002, 1005 ACR section, 305 PWM section, 303 third harmonic superposition section, 307 gate drive device.

Claims

1. A three-phase AC power supply is connected to an AC / DC converter that converts AC power to DC voltage, The system includes a controller that controls the AC / DC converter, The aforementioned AC / DC converter is A plurality of semiconductor switching elements are provided between the positive and negative electrodes and are driven by receiving the AC power, A DC capacitor is provided in parallel with the plurality of semiconductor switching elements to hold the converted DC voltage, A filter circuit is provided between the three-phase AC power supply and the plurality of semiconductor switching elements, and is composed of an AC filter reactor and an AC filter capacitor. A neutral point line is connected to the DC capacitor via the neutral point of the AC filter capacitor and forms the path of the zero-sequence current, The filter circuit and the current detector for detecting the current in each phase between the plurality of semiconductor switching elements are included. The aforementioned controller, An AC current control unit that outputs AC output voltage commands for each phase based on the current detection value detected by the current detector, A third-harmonic superposition unit that performs control by superimposing a frequency three times the fundamental frequency onto the AC output voltage command, A PWM unit that determines the switching pattern of the plurality of semiconductor switching elements by comparing the signal superimposed by the third harmonic superposition unit with the carrier wave, It includes a zero-sequence current calculation unit that calculates the zero-sequence current based on the current detection value, The AC current control unit excludes frequency components within a predetermined range from the zero-sequence current based on the zero-sequence current value obtained by the zero-sequence current calculation unit, A power converter in which the frequency components within the predetermined range are within a range of three times the frequency of the three-phase AC power supply or less.

2. The zero-sequence current calculation unit calculates a zero-sequence current compensation value by applying a low-pass filter to the zero-sequence current detection value calculated from the three-phase current detection value, thereby excluding frequency components above a predetermined range. The AC current control unit is The power conversion device according to claim 1, which outputs AC output voltage commands for each phase based on a reference current command value and a value obtained by subtracting the zero-sequence current compensation value from the current detection value.

3. The AC current control unit is A first AC current control unit outputs a first AC voltage output command for controlling the current of the normal mode component, excluding the zero-sequence current component, A second AC current control unit that outputs a second AC voltage output command for controlling the current of the zero-sequence current, The power conversion device according to claim 1, further comprising an adder for adding the first AC voltage output command and the second AC voltage output command.

4. The power conversion device according to claim 3, wherein the first AC current control unit and the second AC current control unit are set to different control gains designed according to the impedance of their respective current paths.

5. The power conversion device according to claim 3, wherein the first AC current control unit removes the zero-sequence current component from the detected current value by performing a Clarke conversion on the detected current value.

6. The power conversion device according to claim 3, wherein the second AC current control unit calculates a zero-sequence current value by applying a high-pass filter to the zero-sequence current to exclude frequency components below a predetermined range.

7. The power conversion device according to claim 1, wherein the zero-sequence current calculation unit takes the average value of two consecutive zero-sequence currents sampled at twice the frequency of the carrier wave used in the PWM unit as the zero-sequence current value.

8. The aforementioned AC filter capacitor has a neutral point by being connected in a star configuration with three phases. The power conversion device according to claim 1, wherein the neutral point is connected to the positive electrode or the negative electrode to form the neutral point line of the zero-sequence current.

9. The power conversion device according to claim 1, wherein the DC capacitor is composed of two capacitors connected in series, and the neutral point of the zero-sequence current is formed by connecting the midpoint of the capacitors to the neutral point.

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