Electric power conversion device and refrigeration cycle device

Abstract

An electric power conversion device (100) comprises: a rectification unit (12) that rectifies a power supply voltage; an inverter (15) that converts the rectification voltage rectified by the rectification unit (12) into a drive voltage; a detection unit (20) that detects a physical quantity correlated with a common mode current flowing through the inside of the electric power conversion device (100); a compensation current generation unit (30) that generates, on the basis of the physical quantity detected by the detection unit (20), a compensation current for cancelling the common mode current; and an injection unit (40) that injects the compensation current into the ground potential. The magnitude Z1 of the ground impedance of a load unit, including the inverter (15) and a load (60), and the magnitude Z2 of the ground impedance of the injection unit (40) are set with respect to a voltage V1 between the DC buses (71, 72) connecting the rectification unit (12) and the inverter (15), and an operating voltage V2 for operating the compensation current generation unit (30) so as to satisfy a first impedance constraint condition represented by the relation "V1 / Z1 < V2 / Z2".

Description

Power conversion device and refrigeration cycle device

[0001] The present disclosure relates to a power conversion device that converts a power supply voltage applied from an AC power supply into a drive voltage for a load, and a refrigeration cycle device.

[0002] When the refrigeration cycle device is an air conditioner, leakage current can be a problem. Leakage current is a high-frequency current that flows through stray capacitance between the compressor (load) and earth potential due to switching control of the inverter's switching elements. The leakage current that flows to earth potential returns to the refrigeration cycle device (source) via the ground point where the AC power supply is grounded, and is therefore also called "common mode current."

[0003] Japanese Patent Laid-Open No. 2006-129999 discloses a technique for monitoring a current corresponding to a common mode current leaking from a load, and when the instantaneous value, average value or peak value of the monitored current is greater than a threshold value, controlling a switch to close and outputting a compensation current for compensating for the common mode current.

[0004] JP 2013-158099 A

[0005] The common-mode current flowing through the power conversion device flows out from a common-mode voltage generated in a load section including an inverter and a load, which acts as a voltage source. The compensation current for compensating for the common-mode current varies depending on the earth impedance of the load section and the earth impedance of the injection section that injects the compensation current into the earth potential. Furthermore, these two earth impedances have different fluctuation characteristics with respect to frequency. Therefore, unless the earth impedance of the injection section is appropriately set in relation to the earth impedance of the load section, a sufficient compensation current cannot be supplied in the desired frequency band.

[0006] The present disclosure has been made in view of the above, and has an object to provide a power conversion device that can cause a sufficient compensation current to flow in a desired frequency band.

[0007] In order to solve the above-mentioned problems and achieve the object, a power conversion device according to the present disclosure is a power conversion device that converts a power supply voltage applied from an AC power supply into a drive voltage for a load, and includes a noise filter, a rectifier, a converter, a detector, a compensation current generator, and an injection unit. The noise filter reduces noise currents flowing into and out of the power conversion device. The rectifier rectifies the power supply voltage applied via the noise filter. The converter converts the rectified voltage rectified by the rectifier into a drive voltage. The detector detects a physical quantity correlated with a common mode current flowing inside the power conversion device via an earth potential that serves as a reference for the power supply voltage. The compensation current generator generates a compensation current to cancel out the common mode current based on the physical quantity detected by the detector. The injection unit injects the compensation current into the earth potential. In the power conversion device configured as described above, the magnitude Z1 of the earth impedance of the load section including the conversion section and the load, and the magnitude Z2 of the earth impedance of the injection section are set so as to satisfy the first impedance constraint condition expressed as "V1 / Z1<V2 / Z2" with respect to the voltage V1 between the DC buses connecting the rectification section and the conversion section, and the operating voltage V2 that operates the compensation current generation section.

[0008] The power conversion device according to the present disclosure has the advantage of being able to pass a sufficient compensation current in a desired frequency band.

[0009] FIG. 1 is a diagram showing a first example of the configuration of a power conversion device according to embodiment 1; FIG. 2 is a diagram showing a second example of the configuration of a power conversion device according to embodiment 1; FIG. 3 is a diagram used to explain a common mode voltage that may occur in a power conversion device according to embodiment 1; FIG. 4 is a diagram showing a specific example of a common mode voltage that may occur in a power conversion device according to embodiment 1; FIG. 5 is a diagram showing divisions of components in a control system when the power conversion device according to embodiment 1 is regarded as one control system; FIG. 6 is a diagram used to explain a loop transfer function when the power conversion device according to embodiment 1 is regarded as one control system; FIG. 7 is a diagram showing an example of setting the frequency characteristics of the earth impedance of the load section and the injection section in the power conversion device according to embodiment 1;

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A power conversion device and a refrigeration cycle device according to embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0011] Embodiment 1. Fig. 1 is a diagram showing a first configuration example of a power conversion device 100 according to embodiment 1. The power conversion device 100 according to embodiment 1 drives a load 60 by converting a power supply voltage applied from an AC power supply 50 into a drive voltage for a motor 61 connected to the load 60. When the power conversion device 100 is installed in a refrigeration cycle device, an example of the load 60 is a compressor or a fan.

[0012] A midpoint 54 of the AC power supply 50 is grounded to a ground potential 51. The ground potential 51 is the potential of a ground wire or ground surface (not shown) and serves as a reference potential for the power supply voltage. Although the AC power supply 50 is shown as a single-phase three-wire system in FIG. 1 , this is not limiting. The AC power supply 50 may be a single-phase two-wire system, a three-phase three-wire system, or a three-phase four-wire system. The power distribution system of the AC power supply 50 is determined according to the specifications of the refrigeration cycle apparatus.

[0013] The power conversion device 100 includes a noise filter 11, a rectifier 12, a smoothing capacitor 13, an inverter 15, and a canceller 80. The noise filter 11 is a passive filter composed of passive elements such as an inductor and a capacitor. The noise filter 11 is electrically connected to a ground potential 52. The noise filter 11 is disposed between an AC power supply 50 and the rectifier 12 and operates to reduce noise currents flowing into and out of the power conversion device 100. The rectifier 12 rectifies the power supply voltage applied from the AC power supply 50 via the noise filter 11. The smoothing capacitor 13 is disposed between a DC bus 71 and a DC bus 72, which are electrical wiring connecting the rectifier 12 and the inverter 15, and smoothes and holds the rectified voltage rectified by the rectifier 12. The inverter 15 is a conversion unit that converts the rectified voltage rectified by the rectifier 12 into a drive voltage for the load 60. Specifically, the inverter 15 includes a plurality of switching elements 14 and converts the rectified voltage rectified by the rectifier 12 into a drive voltage for the motor 61 to drive the motor 61 .

[0014] The canceller 80 also includes a detection unit 20, a compensation current generation unit 30, and an injection unit 40. The detection unit 20 includes a first coil 21 and a second coil 22 magnetically coupled to the first coil 21. The first coil 21 includes two windings 21a and 21b. The winding 21a is disposed on the power line 18, and the winding 21b is disposed on the power line 19. The power lines 18 and 19 are electrical wiring for electrically connecting the noise filter 11 and the rectifier unit 12. The second coil 22 includes two windings 22a and 22b, each of which is magnetically coupled to the winding 21b. The first coil 21 is a main circuit winding, and the second coil 22 is an auxiliary winding. The black circles in the figure indicate the polarity of the voltage generated in each winding. For example, if the voltage induced in winding 21b is higher on the black circle side, the voltage induced in windings 22a and 22b will be higher on the black circle side. Therefore, voltages of opposite polarities are always induced in windings 22a and 22b.

[0015] The compensation current generating unit 30 is divided into an adjusting circuit unit 38 and a semiconductor element circuit unit 39. The adjusting circuit unit 38 includes termination resistors 31a and 31b and adjustment resistors 33a and 33b. The semiconductor element circuit unit 39 includes voltage-driven elements 35a and 35b. The termination resistor 31a, adjustment resistor 33a, and voltage-driven element 35a are provided corresponding to the winding 22a, while the termination resistor 31b, adjustment resistor 33b, and voltage-driven element 35b are provided corresponding to the winding 22b. An example of the voltage-driven elements 35a and 35b is a metal-oxide-semiconductor field-effect transistor (MOSFET) as shown in the figure, but voltage-driven elements other than MOSFETs may also be used.

[0016] 1, the operating voltage for operating the semiconductor element circuit unit 39 is the bus voltage Vdc1. The bus voltage Vdc1 is the voltage between the DC buses 71 and 72.

[0017] The injection unit 40 includes a resistor 41 and a capacitor 42 connected in series to the resistor 41. The injection unit 40 is electrically connected to a ground potential 52. Note that in FIG. 1 , the connection order of the resistor 41 and the capacitor 42 in the injection unit 40 may be reversed from that shown in the figure. That is, the resistor 41 may be disposed on the side of the ground potential 52. Furthermore, the injection unit 40 does not need to include both the resistor 41 and the capacitor 42, and it is sufficient if it includes at least one of the resistor 41 and the capacitor 42. Furthermore, the injection unit 40 may include an inductor connected in series in addition to the resistor 41 and the capacitor 42.

[0018] Fig. 2 is a diagram showing a second configuration example of the power conversion device 100 according to the first embodiment. In the power conversion device 100 shown in Fig. 2, a control power supply 90 is added to the configuration of Fig. 1. The control power supply 90 is an operating power supply for the compensation current generating unit 30, and applies a control power supply voltage Vdc2 to the semiconductor element circuit unit 39. The other configuration is the same as or equivalent to that in Fig. 1, and the same or equivalent components are denoted by the same reference numerals, and duplicated explanations will be omitted.

[0019] In the section "Problems to be Solved by the Invention," it was explained that the compensation current for compensating for the common-mode current varies due to the influence of the earth impedance of the load section and the earth impedance of the injection section, but it also varies depending on the configuration of the canceller 80. In Fig. 1, the bus voltage Vdc1 is the operating voltage of the compensation current generating section 30, whereas in Fig. 2, the operating voltage is a constant control power supply voltage Vdc2 generated by the control power supply 90. For this reason, the earth impedance conditions for flowing an appropriate compensation current differ between Fig. 1 and Fig. 2. Details regarding the earth impedance conditions will be described later.

[0020] Next, a common-mode voltage generated in the load section including the inverter 15 and the load 60 will be described. FIG. 3 is a diagram illustrating a common-mode voltage that may occur in the power conversion device 100 according to the first embodiment. In FIG. 3, the motor 61 of the load 60 is represented by a circuit symbol for an inductance. Also, in FIG. 3, the motor 61 is assumed to be a three-phase motor, and the inverter 15 is assumed to be a three-phase inverter. When the inverter 15 drives the motor 61, a neutral point potential 132 of the motor 61 fluctuates. When the inverter 15 performs PWM (Pulse Width Modulation) operation, the neutral point potential 132 contains high-frequency components resulting from the PWM operation. Therefore, when a U-phase voltage 133U, a V-phase voltage 133V, and a W-phase voltage 133W are applied to the motor 61, a common-mode current 135 flows through the parasitic capacitance 62.

[0021] The potential difference between the neutral point potential 132 and the earth potential 63 is called the "common-mode voltage." In the case of a three-phase inverter, the common-mode voltage is calculated as (Vu + Vv + Vw) / 3, where Vu is the amplitude of the U-phase voltage 133U, Vv is the amplitude of the V-phase voltage 133V, and Vw is the amplitude of the W-phase voltage 133W.

[0022] 4 is a diagram showing a specific example of a common-mode voltage that may occur in the power conversion apparatus 100 according to the first embodiment. In FIG. 4, the horizontal axis represents time, and the vertical axis represents the amplitude of the common-mode voltage in terms of a pu value relative to the bus voltage Vdc1. Specifically, the upper part of FIG. 4 shows the waveform of the common-mode voltage when the modulation factor λ during PWM operation is a medium value (λ=0.5), and the lower part shows the waveform of the common-mode voltage when the modulation factor λ during PWM operation is a low value (λ=0.05).

[0023] A common-mode current flows every time there is a change in the common-mode voltage. As shown in the upper part of Figure 4, when the modulation factor λ is not extremely low, the voltage change width when the common-mode voltage changes is often one-third of the bus voltage Vdc1. On the other hand, as shown in the lower part of Figure 4, when the modulation factor λ is extremely low, the voltage change width when the common-mode voltage changes becomes the bus voltage Vdc1. In other words, the lower the modulation factor λ during PWM operation, the larger the common-mode current. When the voltage change width of the common-mode voltage is large, the common-mode current also becomes large. Therefore, the most stringent condition occurs when the voltage change width of the common-mode voltage becomes the bus voltage Vdc1.

[0024] Next, the operation of the power conversion device 100 according to the first embodiment will be described. When the inverter 15 performs a switching operation, the aforementioned common-mode voltage is generated in the load section including the inverter 15 and the load 60. This common-mode voltage causes a common-mode current to flow. The common-mode current flows inside the power conversion device 100 and therefore also flows through the power lines 18 and 19. The windings 22a and 22b of the detection section 20 detect a physical quantity correlated with the common-mode current flowing through the power lines 18 and 19. The detection voltage of the winding 22a is generated across the termination resistor 31a, and the voltage of the termination resistor 31a is applied to the voltage-driven element 35a via the adjustment resistor 33a. The detection voltage of the winding 22a is an example of a physical quantity correlated with the common-mode current.

[0025] When the voltage of termination resistor 31a exceeds the threshold voltage of voltage-driven element 35a, voltage-driven element 35a becomes conductive. When voltage-driven element 35a becomes conductive, connection point 37 between voltage-driven elements 35a and 35b is electrically connected to high-potential DC bus 71 via the conductive voltage-driven element 35a, and a compensation current flows from connection point 37 toward ground potential 52 via injection unit 40. This direction cancels out the common-mode current, and the compensation current flowing via injection unit 40 reduces the common-mode current.

[0026] Similarly, the detection voltage of winding 22b is generated across termination resistor 31b, and the voltage of termination resistor 31b is applied to voltage-driven element 35b via adjustment resistor 33b. When the voltage of termination resistor 31b exceeds the threshold voltage of voltage-driven element 35b, voltage-driven element 35b becomes conductive. When voltage-driven element 35b becomes conductive, node 37 is electrically connected to the low-potential DC bus 72 via the conductive voltage-driven element 35b, and a compensation current flows from ground potential 52 toward node 37 via injection unit 40. This direction is opposite to that when voltage-driven element 35a is conductive, but because windings 22a and 22b are connected with opposite polarity, it cancels out the common-mode current. Therefore, even when voltage-driven element 35b is conductive, the compensation current flowing via injection unit 40 reduces the common-mode current.

[0027] 1, the detector 20 detects a physical quantity correlated with the common mode current flowing between the noise filter 11 and the rectifier 12, and the compensation current generator 30 generates a compensation current based on the detected physical quantity, and is connected between the rectifier 12 and the inverter 15, which is a noise power supply source. Therefore, the detector 20 and the compensation current generator 30 form a feedback system when viewed from the inverter 15.

[0028] 1, the detection unit 20 and the compensation current generation unit 30 can generate compensation currents of both polarities, but common-mode current reduction is also possible with a compensation current of one polarity. Therefore, when a configuration is adopted in which a compensation current of one polarity is generated, one of the windings 22a, 22b of the detection unit 20 can be omitted. Furthermore, of the set of the termination resistor 31a, the adjustment resistor 33a, and the voltage-driven element 35a, and the set of the termination resistor 31b, the adjustment resistor 33b, and the voltage-driven element 35b in the compensation current generation unit 30, one set of the windings 22a, 22b corresponding to the omitted winding can be omitted.

[0029] As described above, in canceller 80 according to the first embodiment, detection unit 20 detects a physical quantity correlated with the common-mode current flowing inside power conversion device 100 via earth potential 51, which serves as a reference for the power supply voltage. Furthermore, compensation current generation unit 30 generates a compensation current for canceling out the common-mode current based on the physical quantity detected by detection unit 20. Then, injection unit 40 injects the compensation current output from compensation current generation unit 30 into earth potential 52.

[0030] Next, a description will be given of some key points in configuring the power conversion device 100 according to embodiment 1. Fig. 5 is a diagram showing classification of components in a control system when the power conversion device 100 according to embodiment 1 is regarded as one control system.

[0031] When the power conversion device 100 according to the first embodiment is regarded as a single control system, the components in the control system can be classified as shown in Fig. 5. As shown in Fig. 5, the entire components including the AC power supply 50, noise filter 11, rectifier 12, smoothing capacitor 13, inverter 15, and load 60, which are electrically connected via earth potentials 51 and 63, are classified as a controlled object. Furthermore, the detection unit 20 is classified as a detector, and the compensation current generation unit 30 and injection unit 40 are classified as a controller.

[0032] FIG. 6 is a diagram illustrating a loop transfer function when the power conversion apparatus 100 according to the first embodiment is regarded as one control system. FIG. 6 shows the configuration of a control system when the power conversion apparatus 100 according to the first embodiment is divided as shown in FIG. 5. The detector outputs a detected quantity. The detected quantity in the first embodiment is a physical quantity correlated with the common mode current. The controller outputs a controlled quantity using a controlled input quantity as an input signal. The controlled input quantity is generated based on the physical quantity. The controlled object is controlled by the controlled quantity and outputs a controlled object quantity. The controlled object quantity in the first embodiment is a compensation current. The control system executes these processes repeatedly.

[0033] As shown in Fig. 6, the open-loop transfer function is a transfer function that represents the relationship between the control input quantity and the detection quantity when the control system loop is disconnected between the detector and the controller. The open-loop transfer function is generally used to determine whether a control system is stable or unstable, but in the first embodiment, it is used to set the circuit constants of the injection unit 40. The details of the content will be described later.

[0034] Fig. 7 is a diagram showing an example of setting the frequency characteristics of the impedance to ground of the load unit and the injection unit 40 in the power conversion device 100 according to the first embodiment. The horizontal axis of Fig. 7 represents frequency, and the vertical axis represents the absolute value and phase of the impedance. The absolute value of the impedance is the magnitude of the impedance. In Fig. 7, the solid line represents the magnitude of the impedance to ground of the load unit, the dashed line represents the magnitude of the impedance to ground of the injection unit 40, and the dashed-dotted line represents the phase of the impedance to ground of the load unit.

[0035] The load's impedance to ground is mainly composed of parasitic capacitance 62 and the inductance of detection unit 20. For this reason, the absolute value of the load's impedance to ground has a resonant frequency of several MHz (approximately 3 MHz in the example of FIG. 7), and in the low-frequency range up to the resonant frequency, parasitic capacitance 62 becomes dominant and the impedance decreases capacitively, while in the high-frequency range above the resonant frequency, the inductance of detection unit 20 becomes dominant and the impedance increases inductively. Furthermore, the phase of the load's impedance to ground changes suddenly from -90° to +90° around the resonant frequency.

[0036] The impedance to ground of injection unit 40 is mainly composed of the resistance value of resistor 41, the capacitance value of capacitor 42, and the parasitic inductance present on the path to inject the compensation current. As shown in Figure 7, the characteristics of the impedance to ground of injection unit 40 are such that in the low frequency range, the capacitance value of capacitor 42 becomes dominant and the impedance decreases capacitively, in the mid frequency range, the drop is alleviated by the resistance value of resistor 41, and in the high frequency range, the parasitic inductance becomes dominant and the impedance increases inductively.

[0037] Based on the above, in the power conversion device 100 according to the first embodiment, the impedance to ground of the load section and the impedance to ground of the injection section 40 are set as follows relative to the common-mode voltage generated when the inverter 15 is in operation and the operating voltage that operates the compensation current generation section 30.

[0038] First, when the operating voltage for operating the compensation current generating unit 30 is the voltage between the DC buses 71 and 72, the impedance to ground of the injection unit 40 is set so as to satisfy the impedance constraint condition shown in the following equation (1) with respect to the impedance to ground of the load unit.

[0039] Z1>Z2...(1)

[0040] In the above equation (1), Z1 is the magnitude of the impedance to ground of the load section, and Z2 is the magnitude of the impedance to ground of the injection section 40.

[0041] The impedance constraint condition shown in the above formula (1) applies to the power conversion apparatus 100 having the configuration shown in Fig. 1 in which the compensation current generating unit 30 is connected between the DC buses 71 and 72, i.e., the operating voltage of the compensation current generating unit 30 is the bus voltage Vdc1. Note that the characteristics of the impedance to ground of the injection unit 40 shown in Fig. 7 are an example in which the circuit constants of the injection unit 40 are set so that the above formula (1) is satisfied in the low frequency range up to about 2 MHz and in the high frequency range from about 5 MHz onwards.

[0042] Furthermore, the impedance constraint condition shown in the following equation (2) is applied to the power conversion device 100 configured as shown in FIG.

[0043] V1 / Z1<V2 / Z2…(2)

[0044] In the above equation (2), Z1 is the magnitude of the ground impedance of the load unit, and Z2 is the magnitude of the ground impedance of the injection unit 40. Furthermore, V1 is the maximum voltage change width of the common mode voltage generated when the conversion unit is operating, and corresponds to the bus voltage Vdc1. Furthermore, V2 is the voltage value of the operating voltage that operates the compensation current generation unit 30, and corresponds to the control power supply voltage Vdc2.

[0045] If the power conversion device 100 according to embodiment 1 is configured to satisfy the impedance constraint condition shown in the above equation (1) or the above equation (2), it becomes possible to flow a sufficient compensation current even under severe conditions such as when the power conversion device 100 operates at a low modulation rate.

[0046] In this paper, the impedance constraint condition shown in the above equation (2) may be referred to as the "first impedance constraint condition," and the impedance constraint condition shown in the above equation (1) may be referred to as the "second impedance constraint condition."

[0047] 8 is a diagram illustrating a compensation band that is set as a frequency band in which compensation operation is actively performed by the compensation current generating unit 30 according to embodiment 1. The horizontal axis of Fig. 8 represents frequency, and the vertical axis represents the gain of the open-loop transfer function described in Fig. 6 .

[0048] When the refrigeration cycle apparatus is an air conditioner, there are frequency bands in which it is necessary to actively perform compensation depending on the characteristics of the load 60. In other words, there are frequency bands in which it is not necessary to actively perform compensation depending on the characteristics of the load 60. At least in the frequency bands in which it is necessary to actively perform compensation, the first or second impedance constraint condition described above must be satisfied. Furthermore, in frequency bands in which it is not necessary to actively perform compensation, it is not necessary to satisfy the first or second impedance constraint condition, and the first or second impedance constraint condition is not necessary.

[0049] 8 shows an example in which "band 1" with a lower limit frequency fr_min and an upper limit frequency fr_max is set as a compensation band in which compensation operation is actively performed. Note that while Fig. 8 shows an example in which the frequency characteristics are flat, such that the gain between the lower limit frequency fr_min and the upper limit frequency fr_max is G, the frequency characteristics do not necessarily have to be flat.

[0050] 8 also shows "band 2" and "band 3" as frequency bands where compensation operation is not actively performed. Band 2 is a band with a lower frequency than band 1, and band 3 is a band with a higher frequency than band 1. Fig. 8 shows an example in which the upper limit frequency in band 2 is f0_low and the lower limit frequency in band 3 is f0_high.

[0051] The upper limit frequency f0_low in band 2 and the lower limit frequency f0_high in band 3 are frequencies at which the gain of the open-loop transfer function is 0 dB. The lower limit frequency fr_min in band 1 may be set as the upper limit frequency f0_low in band 2. The upper limit frequency fr_max in band 1 may be set as the lower limit frequency f0_high in band 3.

[0052] Furthermore, there may be cases where the gain of the open-loop transfer function does not become 0 dB or less in bands 2 and 3. In such cases, a band-limiting filter or the like may be provided in the output path of the compensation current to forcibly set the gain of the open-loop transfer function to 0 dB or less. Note that the method using a band-limiting filter is just one example, and it goes without saying that other methods may also be used.

[0053] Furthermore, when a compensation band in which compensation operation is actively performed is set as shown in FIG. 8 , it is desirable that the power conversion device 100 be configured so that the first or second impedance constraint condition described above is satisfied in the set compensation band. The power conversion device 100 configured in this manner can ensure that a sufficient compensation current flows even under severe conditions, such as when the power conversion device 100 operates at a low modulation factor. Alternatively, the power conversion device 100 may be designed so that the first or second impedance constraint condition is satisfied only in the set compensation band. Such a design relaxes the constraints on the injection unit 40, thereby lowering the implementation hurdle for the injection unit 40.

[0054] 8 , when bands 2 and 3 are set in which compensation operation is not actively performed, the first or second impedance constraint described above may be unnecessary in bands 2 and 3, and the first or second impedance constraint may not be satisfied. Such a design can relax the constraints on the injection unit 40 while ensuring that a sufficient compensation current flows, thereby lowering the hurdle for implementing the injection unit 40. Furthermore, such a design can prevent a compensation current containing unintended distortion components from being output in bands 2 and 3, thereby preventing deterioration of electromagnetic noise due to increased distortion of the common-mode current.

[0055] As described above, the power conversion device according to embodiment 1 includes a noise filter, a rectifier, a converter, a detector, a compensation current generator, and an injection unit. The noise filter reduces noise currents flowing in and out of the power conversion device. The rectifier rectifies the power supply voltage applied via the noise filter. The converter converts the rectified voltage rectified by the rectifier into a drive voltage. The detector detects a physical quantity correlated with a common mode current flowing through the power conversion device via a ground potential that serves as a reference for the power supply voltage. The compensation current generator generates a compensation current for canceling out the common mode current based on the physical quantity detected by the detector. The injection unit injects the compensation current into the ground potential.

[0056] In the power conversion device configured as described above, the magnitude Z1 of the earth impedance of the load section including the conversion section and the load, and the magnitude Z2 of the earth impedance of the injection section are set so as to satisfy the first impedance constraint condition shown in the following equation (3) with respect to the voltage V1 between the DC buses connecting the rectification section and the conversion section, and the operating voltage V2 that operates the compensation current generation section.

[0057] V1 / Z1<V2 / Z2…(3)

[0058] Furthermore, in the power conversion device according to embodiment 1, when the operating voltage that operates the compensation current generating unit is the voltage between the DC buses, the earth impedance Z1 of the load unit and the earth impedance Z2 of the injection unit are set so as to satisfy the second impedance constraint condition shown in the following equation (4).

[0059] Z1>Z2... (4)

[0060] According to the power conversion device configured as above, it is possible to pass a sufficient compensation current even under severe conditions such as when the power conversion device operates at a low modulation rate.

[0061] Furthermore, in the power conversion device according to the first embodiment, when a compensation band is set as a frequency band in which the compensation current generating unit actively performs compensation operation, it is desirable that the first or second impedance constraint condition be satisfied at least in the compensation band. If the power conversion device is configured in this manner, it is possible to ensure that a sufficient compensation current flows even under severe conditions such as when the power conversion device operates at a low modulation factor.

[0062] Furthermore, the power conversion device according to the first embodiment may be designed so that the first or second impedance constraint condition does not have to be satisfied in frequency bands other than the compensation band. This design ensures that a sufficient compensation current flows while relaxing the constraints on the injection unit, thereby lowering the implementation hurdle for the injection unit. Furthermore, this design prevents a compensation current containing unintended distortion components from being output in frequency bands other than the compensation band, thereby preventing the deterioration of electromagnetic noise due to increased distortion of the common-mode current.

[0063] Furthermore, in the power conversion device according to the first embodiment, when the magnitude of the ground impedance of the injection unit exceeds the magnitude of the ground impedance of the load unit in a frequency band other than the compensation band, it is desirable to set the circuit constants of the injection unit so that the gain of the open-loop transfer function of the control system is 0 dB or less, so that compensation operation is not actively performed. The control system here refers to a control system in which the AC power supply, noise filter, rectifier, and converter are controlled objects, the detection unit is a detector, and the compensation current generation unit and injection unit are controllers. If the control system is configured in this manner, it is possible to prevent a compensation current containing unintended distortion components from being output in a frequency band other than the compensation band, and to prevent deterioration of electromagnetic noise due to increased distortion of the common-mode current.

[0064] Furthermore, in the power conversion device according to the first embodiment, a band-limiting filter may be provided in the output path of the compensation current so that compensation operation is not actively performed when the magnitude of the ground impedance of the injection unit exceeds the magnitude of the ground impedance of the load unit in a frequency band other than the compensation band. With the power conversion device configured in this manner, it is possible to prevent the output of a compensation current containing unintended distortion components in a frequency band other than the compensation band, thereby making it possible to prevent deterioration of electromagnetic noise due to increased distortion of the common-mode current.

[0065] Second Embodiment Fig. 9 is a diagram showing a configuration example of a refrigeration cycle apparatus 900 according to the second embodiment. The refrigeration cycle apparatus 900 according to the second embodiment includes the power conversion apparatus 100 described in the first embodiment. The refrigeration cycle apparatus 900 according to the second embodiment can be applied to products including a refrigeration cycle, such as air conditioners, refrigerators, freezers, and heat pump water heaters. In Fig. 9, components having the same functions as those in the first embodiment are denoted by the same reference numerals as those in the first embodiment.

[0066] The refrigeration cycle device 900 includes a compressor 315 incorporating the motor 61 in embodiment 1, a four-way valve 902, an indoor heat exchanger 906, an expansion valve 908, and an outdoor heat exchanger 910, which are attached via refrigerant piping 912.

[0067] Inside the compressor 315, a compression mechanism 904 that compresses the refrigerant and a motor 61 that operates the compression mechanism 904 are provided.

[0068] The refrigeration cycle device 900 can perform heating or cooling operation by switching the four-way valve 902. The compression mechanism 904 is driven by a motor 61 that is variably controlled in speed.

[0069] During heating operation, as shown by the solid arrow, the refrigerant is pressurized by the compression mechanism 904 and sent out, passes through the four-way valve 902, the indoor heat exchanger 906, the expansion valve 908, the outdoor heat exchanger 910 and the four-way valve 902 and returns to the compression mechanism 904.

[0070] During cooling operation, as shown by the dashed arrow, the refrigerant is pressurized by the compression mechanism 904 and sent out, and returns to the compression mechanism 904 through the four-way valve 902, the outdoor heat exchanger 910, the expansion valve 908, the indoor heat exchanger 906 and the four-way valve 902.

[0071] During heating operation, the indoor heat exchanger 906 acts as a condenser to release heat, and the outdoor heat exchanger 910 acts as an evaporator to absorb heat. During cooling operation, the outdoor heat exchanger 910 acts as a condenser to release heat, and the indoor heat exchanger 906 acts as an evaporator to absorb heat. The expansion valve 908 reduces the pressure of the refrigerant to expand it.

[0072] The configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, or different embodiments may be combined with each other. It is also possible to omit or modify parts of the configurations as long as they do not deviate from the gist of the invention.

[0073] REFERENCE SIGNS LIST 11 noise filter, 12 rectifier unit, 13 smoothing capacitor, 14 switching element, 15 inverter, 18, 19 power line, 20 detection unit, 21 first coil, 21a, 21b, 22a, 22b winding, 22 second coil, 30 compensation current generation unit, 31a, 31b termination resistor, 33a, 33b adjustment resistor, 35a, 35b voltage-driven element, 37 connection point, 38 adjustment circuit unit, 39 semiconductor element circuit unit, 40 injection unit, 41 resistor, 42 capacitor, 50 AC power source, 51, 52, 63 earth potential, 54 neutral point, 60 load, 61 motor, 62 parasitic capacitance, 71, 72 DC bus, 80 canceller, 90 control power source, 100 power conversion device, 132 neutral point potential, 133U U-phase voltage, 133V V-phase voltage, 133W W-phase voltage, 135 common mode current, 315 compressor, 900 refrigeration cycle device, 902 four-way valve, 904 compression mechanism, 906 indoor heat exchanger, 908 expansion valve, 910 outdoor heat exchanger, 912 refrigerant piping.

Claims

1. A power conversion device that converts a power supply voltage applied from an AC power supply into a drive voltage for a load, comprising: a noise filter that reduces noise current flowing in and out of the power conversion device; a rectifier that rectifies the power supply voltage applied through the noise filter; a converter that converts the rectified voltage rectified by the rectifier into the drive voltage; a detector that detects a physical quantity correlated with a common mode current flowing inside the power conversion device via an earth potential that is a reference for the power supply voltage; a compensation current generator that generates a compensation current for canceling out the common mode current based on the physical quantity detected by the detector; and an injection unit that injects the compensation current into the earth potential, wherein an impedance to earth of a load unit including the converter and the load and an impedance to earth of the injection unit satisfy a first impedance constraint condition shown in the following equation (1) with respect to a voltage between a DC bus connecting the rectifier and the converter and an operating voltage for operating the compensation current generator. V1 / Z1<V2 / Z2 (1) Z1: magnitude of the impedance to earth of the load section Z2: magnitude of the impedance to earth of the injection section V1: voltage value between the DC buses V2: voltage value of the operating voltage 2. The power conversion device according to claim 1, wherein when a compensation band is set as a frequency band in which the compensation current generating unit actively performs compensation operation, the first impedance constraint condition is satisfied at least in the compensation band.

3. The power conversion device according to claim 2, wherein the first impedance constraint condition is not satisfied in a frequency band other than the compensation band.

4. The power conversion device according to claim 1, wherein, when an operating voltage for operating the compensation current generating unit is a voltage between the DC buses, the impedance to earth of the load unit and the impedance to earth of the injection unit satisfy a second impedance constraint condition shown in the following formula (2): Z1>Z2 (2), Z1: magnitude of the impedance to earth of the load unit, Z2: magnitude of the impedance to earth of the injection unit.

5. The power conversion device according to claim 4, wherein when a compensation band is set as a frequency band in which the compensation current generating unit actively performs compensation operation, the second impedance constraint condition is satisfied at least in the compensation band.

6. The power conversion device according to claim 5, wherein the second impedance constraint condition is not satisfied in a frequency band other than the compensation band.

7. A power conversion device as claimed in claim 3 or 6, wherein when the magnitude of the earth impedance of the injection unit exceeds the magnitude of the earth impedance of the load unit in a frequency band other than the compensation band, the compensation operation is not actively performed, so that the circuit constants of the injection unit are set so that the gain of a loop transfer function of a control system in which the AC power supply, the noise filter, the rectification unit and the conversion unit are controlled as control objects, the detection unit is a detector, and the compensation current generation unit and the injection unit are controllers is 0 dB or less.

8. A power conversion device as claimed in claim 3, 6 or 7, wherein a band-limiting filter is provided in an output path of the compensation current so that the compensation operation is not actively performed when the magnitude of the impedance to earth of the injection section exceeds the magnitude of the impedance to earth of the load section in a frequency band other than the compensation band.

9. A refrigeration cycle device comprising the power conversion device according to any one of claims 1 to 8.

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