Power conversion apparatus

Abstract

To provide a technique for continuously driving a power converter until planned stop timing, while suppressing development of galvanic corrosion when resistance of coolant is decreased.SOLUTION: A power conversion apparatus 100 has a cooling mechanism cooling a power converter 4 by using coolant supplied from a cooling device 6. The power converter 4 includes a converter 42 and an inverter 45. The power conversion apparatus 100 controls DC voltage outputted from the converter 42 to follow a DC voltage reference value. The power conversion apparatus 100, when resistivity of coolant is less than a first threshold value, calculates another DC voltage reference value smaller than the DC voltage reference value based on the resistivity. Further, the power conversion apparatus 100 controls the converter to make the DC voltage close to the other DC voltage reference value.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a power conversion device having a cooling mechanism that cools a power converter using a refrigerant supplied from a cooling device. [Background technology]

[0002] Patent Document 1 discloses a semiconductor cooling device that cools a semiconductor device with pure water. Specifically, the semiconductor cooling device controls the electrical conductivity of the pure water by intermittently operating a pump that circulates the pure water to maintain it within a specified value. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-339537 Summary of the Invention [Problem to be solved by the invention]

[0004] Intermittent operation of the pure water (refrigerant) supplied from the cooling device may result in insufficient cooling of the power converter, including the inverter and converter. On the other hand, continuous operation of the cooling device to circulate the refrigerant may cause the refrigerant's conductivity to exceed a specified value, increasing the refrigerant's leakage current and rapidly progressing electrolytic corrosion in the metal heat sink used to cool the semiconductor. In this case, the cooling mechanism installed in the power converter may leak refrigerant due to pinholes or other issues, potentially causing the power converter to malfunction. Therefore, ions in the refrigerant are typically removed using ion exchange resins or other methods to maintain the refrigerant's conductivity below a specified value.

[0005] Furthermore, when a power converter used in production facilities such as a factory is in operation, the power converter cannot be stopped immediately, and it is desirable for the power converter to operate continuously until a scheduled shutdown time.

[0006] One object of the present disclosure is to provide a technology that can suppress the progression of electrolytic corrosion and enable a power converter to continue operating until the planned shutdown time, even if the resistivity (the inverse of the conductivity) of the refrigerant decreases for some reason. [Means for solving the problem]

[0007] A first aspect of the present disclosure relates to a power conversion apparatus having a cooling mechanism that cools a power converter using a refrigerant supplied from a cooling device. The power converter includes a converter and an inverter. The power conversion apparatus includes a control device that controls a DC voltage output from the converter to follow a DC voltage reference value. When the resistivity of the refrigerant is less than a first threshold, the control device calculates another DC voltage reference value that is smaller than the DC voltage reference value based on the resistivity. Furthermore, the control device controls the converter to follow the DC voltage to the other DC voltage reference value.

[0008] A second aspect of the present disclosure relates to a power conversion device having a cooling mechanism that cools a power converter using a refrigerant supplied from a cooling device. The power converter is a converter in which the outputs of multiple cell converters are connected in series. The power conversion device is equipped with a control device. When the resistivity of the refrigerant is equal to or lower than a threshold, the control device determines the number of series-connected cell converters to enable from among the multiple cell converters based on the resistivity. Furthermore, the control device controls the converter to enable the output of the cell converter corresponding to the number of series-connected cell converters. [Effects of the Invention]

[0009] According to a first aspect of the present disclosure, when the resistivity of the refrigerant is less than a first threshold, another DC voltage reference value smaller than the DC voltage reference value is calculated. Furthermore, the converter is controlled so that the DC voltage output from the converter follows the another DC voltage reference value. As a result, when the resistivity of the refrigerant decreases, the DC voltage is lowered, thereby reducing the leakage current of the refrigerant. Therefore, the power converter can be continuously operated while suppressing the progression of electrolytic corrosion. Therefore, the power converter is expected to operate continuously until the planned shutdown time while suppressing the progression of electrolytic corrosion.

[0010] According to a second aspect of the present disclosure, a power converter is a converter in which the outputs of multiple cell converters are connected in series. When the resistivity of the refrigerant is equal to or lower than a threshold, the number of cell converters to be enabled in series is determined. Furthermore, the converter is controlled to enable the outputs of the cell converters corresponding to the number of cell converters connected in series. As a result, when the resistivity of the refrigerant decreases, the number of enabled cell converters in series is reduced, thereby lowering the output voltage of the power converter and reducing the refrigerant leakage current. This makes it possible to continuously operate the power converter while suppressing the progression of electrolytic corrosion. Therefore, the power converter is expected to operate continuously until the planned shutdown time while suppressing the progression of electrolytic corrosion. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an explanatory diagram showing an overview of a water-cooled power conversion system according to a first embodiment. [Figure 2] 1 is a block diagram showing an example of the configuration of a control device according to a first embodiment. [Figure 3] FIG. 1 is an explanatory diagram showing a specific example of a DC voltage reference setting unit according to the first embodiment; [Figure 4] 4 is a flowchart illustrating an outline of a processing example of the control device according to the first embodiment. [Figure 5] 5 is a flowchart showing an example of processing by a current command unit according to the first embodiment. [Figure 6] FIG. 10 is an explanatory diagram showing an overview of a cooling power conversion system according to a second embodiment. [Figure 7] FIG. 10 is an explanatory diagram showing a specific example of a control device according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] A power conversion device according to an embodiment of the present disclosure will be described with reference to the accompanying drawings. Elements common to the various drawings will be designated by the same reference numerals, and duplicated descriptions will be omitted.

[0013] 1. First Embodiment Overview FIG. 1 is an explanatory diagram showing an overview of a water-cooled power conversion system 10. The water-cooled power conversion system 10 includes a power conversion device 100 and a cooling device 6. The power conversion device 100 is a device that converts electric power. The input side of the power conversion device 100 is connected to a three-phase AC power source 1 via a circuit breaker 2 and a transformer 3, and the output side of the power conversion device 100 is connected to an electric motor 5. The power conversion device 100 receives AC power (also referred to as first AC power) supplied from the three-phase AC power source 1. The power conversion device 100 outputs AC power (also referred to as second AC power) different from the first AC power to the electric motor 5 so that the electric motor 5 operates at a predetermined rotation speed. The power conversion device 100 also receives an AC voltage on the input side of a transformer 3 detected by a transformer 3a. The electric motor 5 is, for example, a three-phase AC motor.

[0014] The power conversion device 100 includes a power converter 4 and a control device 7. The power converter 4 includes a first current detector 41, a converter 42, a DC capacitor 44, a DC voltage detector 48, an inverter 45, and a second current detector 49. The first current detector 41 is a sensor that detects the input current of the converter 42 in the first AC power.

[0015] Converter 42 converts the first AC power into DC power and outputs the DC power to inverter 45 via a DC circuit. The DC circuit is a circuit in which a DC capacitor 44 and a DC voltage detector 48 are connected. DC capacitor 44 is a capacitor that smooths the DC voltage in the DC power to stabilize it. DC voltage detector 48 is a sensor that detects the DC voltage in the DC circuit.

[0016] The inverter 45 converts the DC power input from the converter 42 via the DC circuit into AC power (second AC power) of a predetermined voltage and frequency. The inverter 45 then outputs the second AC power to the electric motor 5 via a second current detector 49. The second current detector 49 is a sensor that detects the output current of the second AC power.

[0017] The control device 7 controls the converter 42 and the inverter 45. The control device 7 instructs the converter 42 and the inverter 45 to perform power conversion. The control device 7 (power conversion device 100) may be connected to, for example, a higher-level device 200. In this case, the higher-level device 200 instructs the control device 7 to perform power conversion in the converter 42 and the inverter 45. The control device 7 instructs the converter 42 and the inverter 45 to perform power conversion in accordance with the instruction from the higher-level device 200. The configuration of the control device 7 will be described in detail later.

[0018] The cooling device 6 supplies a refrigerant (also referred to as pure water) to the power conversion device 100. The cooling device 6 is connected to the converter 42 and the inverter 45, and cools the converter 42 and the inverter 45 with the refrigerant. That is, the converter 42 and the inverter 45 are each equipped with a cooling mechanism that allows the refrigerant supplied from the cooling device 6 to pass through.

[0019] The refrigerant circulates through a path that connects the cooling device 6, the converter 42, and the inverter 45 in a ring shape using cooling piping (including an inlet cooling piping and an outlet cooling piping). Specifically, the refrigerant that absorbs heat from the converter 42 and the inverter 45 passes through the outlet cooling piping and returns to the cooling device 6. The refrigerant that is returned to the cooling device 6 and warmed then passes through a flow meter 61 and enters a surge tank 62. A circulation pump 63 applies water pressure to the refrigerant that leaves the surge tank 62, sufficient for the refrigerant to circulate through the cooling piping. The refrigerant that leaves the circulation pump 63 is cooled by a heat exchanger 64. External cooling water is used to cool the refrigerant in the heat exchanger 64. The refrigerant cooled by the heat exchanger 64 passes through the inlet cooling piping and enters a cooling mechanism provided in the converter 42 and the inverter 45. The cooling mechanism may be, for example, a heat sink for the semiconductor that constitutes the converter 42 or the inverter 45, or a metal heat sink for cooling the converter 42 or the inverter 45. As a result, the converter 42 and the inverter 45 are cooled by the refrigerant supplied from the cooling device 6. Note that, to cool the refrigerant in the heat exchanger 64, cooling air may be used instead of external cooling water.

[0020] A portion of the refrigerant coming out of the heat exchanger 64 passes through a flow rate control valve 65 and enters an ion exchanger 66. The refrigerant from which ions have been removed in the ion exchanger 66 is combined with the refrigerant coming out of the flow meter 61 and returned to the surge tank 62. A resistivity meter 67 and a thermometer 68 are connected to the inlet cooling piping in the cooling device 6, and information on the resistivity Rc of the refrigerant, which is the output value of the resistivity meter 67, is input to the control device 7.

[0021] Consider a case where a cooling mechanism such as a metallic heat sink is at the same potential as the DC circuit of the power conversion device 100. In this case, the cooling mechanism and the cooling device 6 have different potentials, so at least a portion of the cooling piping connected between components with different potentials is made of an insulating material (e.g., a pipe made of Teflon (registered trademark) material, etc.).

[0022] Let us consider the cause of electrolytic corrosion. Electrolytic corrosion can occur due to leakage current when a refrigerant passes through metals with different potentials. Therefore, by removing ions from the refrigerant using an ion exchanger 66, the resistivity Rc of the refrigerant can be maintained at a predetermined value or higher, and the leakage current flowing through the refrigerant can be suppressed. This makes it possible to suppress electrolytic corrosion. According to this embodiment, even if the resistivity Rc of the refrigerant decreases for some reason, the progression of electrolytic corrosion can be suppressed. A specific example of how this can be achieved will be described in detail later.

[0023] 1-2. Control device configuration example 2 is a block diagram showing an example of the configuration of the control device 7 according to the first embodiment. The control device 7 has hardware that realizes various functions. The hardware includes a processing circuit capable of high-speed calculations. Examples of the processing circuit include an FPGA (Field Programmable Gate Array) and an ASIC (Application Specific Integrated Circuit). In addition to the processing circuit, the hardware may also include a storage device and a computing unit (e.g., a CPU) that executes a program stored in the storage device.

[0024] The control device 7 includes a converter control unit that controls the converter 42 and an inverter control unit that controls the inverter 45.

[0025] 1-2-1. Converter control section As shown in FIG. 2, the converter control unit includes a PLL unit 81, a coordinate conversion unit 82, a DC voltage reference setting unit 86, a current command unit 88, a current control unit 83, a voltage control unit 84, and a PWM control unit 85.

[0026] The PLL unit 81 is typically a PLL circuit. The PLL unit 81 receives the AC voltage (also referred to as the AC voltage detection value Vsfb) detected by the transformer 3a and detects the phase θ that serves as a reference for the AC voltage detection value Vsfb. The PLL unit 81 then outputs the detected value, that is, the phase θ, to the coordinate conversion unit 82 and the voltage control unit 84.

[0027] The coordinate conversion unit 82 receives the input current (input current detection value Ifb) and phase detection value of the converter 42 detected by the first current detector 41, and detects an active current and a reactive current based on the input current detection value Ifb and the phase detection value. Then, the coordinate conversion unit 82 outputs to the current control unit 83 an active current detection value Irfb which is the detection value of the active current and a reactive current detection value Iifb which is the detection value of the reactive current.

[0028] The DC voltage reference setting unit 86 receives information on the resistivity Rc of the refrigerant detected by the resistivity meter 67, calculates a DC voltage reference value Vdcb based on the resistivity Rc of the refrigerant, and outputs the calculated value to the current command unit 88.

[0029] The current command unit 88 receives a differential voltage obtained by using a differential detection circuit 87 to calculate the difference between the DC voltage reference value Vdcb and the DC voltage (also referred to as a detected DC voltage value Vdcfb) detected by the DC voltage detector 48. The current command unit 88 also receives the detected DC voltage value Vdcfb and the detected AC voltage value Vsfb. Based on the detected AC voltage value Vsfb, the detected DC voltage value Vdcfb, and the differential voltage, the current command unit 88 calculates a current reference value for causing the detected DC voltage value Vdcfb to follow the DC voltage reference value Vdcb, and outputs the current reference value to the current control unit 83. The current reference value includes an active current reference value Irb and a reactive current reference value Iib. Details of the processing by the current command unit 88 will be described later.

[0030] The current control unit 83 calculates a current command value ci so that the active current detection value Irfb and the reactive current detection value Iifb from the coordinate conversion unit 82 follow the active current reference value Irb and the reactive current reference value Iib, and outputs the current command value ci to the voltage control unit 84.

[0031] The voltage control unit 84 generates a voltage command value vi based on the phase detection value from the PLL unit 81 and the current command value ci from the current control unit 83 , and outputs the voltage command value vi to the PWM control unit 85 .

[0032] The PWM control unit 85 outputs a gate pulse signal modulated with a carrier frequency specified by the voltage command value vi to the converter 42. As a result, the converter 42 can convert the AC power (first AC power) input to the converter 42 into DC power by turning on / off a switching element in the converter 42 based on the gate pulse signal.

[0033] 1-2-2. Inverter control unit As shown in FIG. 2, the inverter control unit includes a speed detection unit 71, a speed control unit 76, a current control unit 73, a voltage control unit 74, and a PWM control unit 75.

[0034] The speed detection unit 71 receives as input the output current (also referred to as output current detection value Iofb) of the second AC power detected by the second current detector 49 and a voltage signal from the voltage control unit 74. Then, based on the output current detection value Iofb and the voltage signal, the speed detection unit 71 calculates a rotation speed feedback value rsf indicating the rotation speed of the electric motor 5 and outputs it to the speed control unit 76.

[0035] The speed control unit 76 receives a differential rotation speed drs, which is the difference between the rotation speed feedback value rsf from the speed detection unit 71 and the rotation speed command value rsb, using the difference detection circuit 72. The rotation speed command value rsb is output from, for example, the higher-level device 200. Then, the speed control unit 76 calculates a current command value based on the differential rotation speed drs so as to make the rotation speed feedback value rsf (the rotation speed of the electric motor 5) follow the rotation speed command value rsb, and outputs the current command value to the current control unit 73.

[0036] The current control unit 73 calculates a voltage command value so that the output current detection value Iofb follows the current command value, and outputs the voltage command value to the voltage control unit 74.

[0037] The voltage control unit 74 generates a voltage command value based on the command value from the current control unit 73 and outputs it to the PWM control unit 75 .

[0038] The PWM control unit 75 outputs a gate pulse signal modulated with a carrier frequency specified by the voltage command value to the inverter 45. As a result, the inverter 45 can convert the DC power input to the inverter 45 into AC power (second AC power) by turning on / off the switching element based on the gate pulse signal. Therefore, the inverter 45 can output the second AC power to the electric motor 5. Therefore, the power conversion device 100 drives the electric motor 5 so that the rotation speed feedback value rsf (the rotation speed of the electric motor 5) follows the rotation speed command value rsb from the upper device 200.

[0039] 1-2-3. Example of changing the DC voltage reference value The DC voltage reference setting unit 86 (control device 7) changes the DC voltage reference value Vdcb in accordance with the resistivity Rc of the refrigerant detected by the resistivity meter 67. For example, when the resistivity Rc of the refrigerant is equal to or greater than a first threshold value TH1, the DC voltage reference setting unit 86 outputs a DC voltage reference value Vdcb_max shown in Fig. 3. Then, the current command unit 88 calculates a current reference value (including an active current reference value Irb and a reactive current reference value Iib) based on a differential voltage obtained by the difference detection circuit 87, which is the difference between the DC voltage reference value Vdcb (in this case, the DC voltage reference value Vdcb_max) and the DC voltage detection value Vdcfb, the AC voltage detection value Vsfb, and the DC voltage detection value Vdcfb, so that the DC voltage detection value Vdcfb follows the DC voltage reference value Vdcb (in this case, the DC voltage reference value Vdcb_max).

[0040] As another example, when the resistivity Rc of the refrigerant is smaller than the first threshold value TH1, the DC voltage reference setting unit 86 outputs a DC voltage reference value Vdcb that is smaller than the DC voltage reference value Vdcb_max shown in Fig. 3. Then, the current command unit 88 calculates a current reference value (including an active current reference value Irb and a reactive current reference value Iib) based on a differential voltage obtained by subtracting the DC voltage reference value Vdcb (in this case, a value smaller than the DC voltage reference value Vdcb_max) obtained by the difference detection circuit 87 from the DC voltage detection value Vdcfb, the AC voltage detection value Vsfb, and the DC voltage detection value Vdcfb, so that the DC voltage detection value Vdcfb follows the DC voltage reference value Vdcb (in this case, a value smaller than the DC voltage reference value Vdcb_max).

[0041] The current command unit 88 operates to reduce the reactive current reference value Iib as much as possible, thereby suppressing the apparent current and reducing the conversion efficiency.

[0042] A method for changing the DC voltage reference value Vdcb will be described in detail with reference to Fig. 3. Fig. 3 is a diagram showing the input / output characteristics of the DC voltage reference setting unit 86, with the horizontal axis (X axis) representing the resistivity Rc of the refrigerant input to the DC voltage reference setting unit 86 and the vertical axis (Y axis) representing the DC voltage reference value Vdcb output from the DC voltage reference setting unit 86. When the resistivity Rc of the refrigerant is equal to or greater than R1, the DC voltage reference value Vdcb is set to a fixed value of the DC voltage reference value Vdcb_max. When the resistivity Rc of the refrigerant is equal to or less than R4, the DC voltage reference value Vdcb is set to a fixed value of the DC voltage reference value Vdcb_min. Note that the resistivity R1 is greater than the resistivity R4, and the DC voltage reference value Vdcb_max is greater than the DC voltage reference value Vdcb_min.

[0043] Here, consider the case where the resistivity Rc of the refrigerant is between resistivity R1 and resistivity R4. In this case, the DC voltage reference value Vdcb is a value on the line connecting points (R1, Vdcb_max) and (R4, Vdcb_min) represented by the X and Y axes. In other words, when the resistivity Rc of the refrigerant is between resistivity R1 and resistivity R4, the DC voltage reference value Vdcb changes in proportion to the resistivity Rc of the refrigerant. For example, as shown in FIG. 3, the DC voltage reference value Vdcb when the resistivity Rc of the refrigerant is resistivity R2 is defined as DC voltage reference value Vdcb1, and the DC voltage reference value Vdcb when the resistivity Rc of the refrigerant is resistivity R3 is defined as DC voltage reference value Vdcb2. In this case, the magnitude relationship among the resistivities R1 to R4 is R1>R2>R3>R4, and the magnitude relationship among the DC voltage reference values ​​Vdcb is Vdcb_max>Vdcb1>Vdcb2>Vdcb_min. The DC voltage reference value Vdcb_max is the rated value of the DC voltage, and the DC voltage reference value Vdcb_min is a reference value of the DC voltage at which the apparent current input to the converter 42 is equivalent to the allowable current of the converter 42, even when the PWM modulation factor in the converter 42 is maximized. The resistivity R1 is an example of a first threshold value TH1 of the resistivity Rc of the refrigerant, and the resistivity R4 is an example of a second threshold value (not shown) of the resistivity Rc of the refrigerant. The DC voltage reference value Vdcb_max is an example of a first DC voltage reference value (not shown), and the DC voltage reference value Vdcb_min is an example of a second DC voltage reference value (not shown).

[0044] When the DC voltage value of the portion of the cooling piping made of insulating material connected to the metal portion of the power converter 4 where the leakage current is greatest is Vdcb_max (rated voltage), the resistivity Rc of the refrigerant that results in a leakage current equal to or less than a predetermined value is resistivity R1. The predetermined leakage current means a value that can suppress electrolytic corrosion within a range that does not affect the expected life of the power conversion device 100 (power converter 4).

[0045] 3 may be stored in a storage device. In this case, the control device 7 sets the DC voltage reference value Vdcb corresponding to the resistivity Rc of the refrigerant based on the information.

[0046] 1-2-4. Example of an abnormality notification The control device 7 determines in the DC voltage reference setting unit 86 whether the resistivity Rc of the refrigerant is equal to or less than a specified value that is smaller than a threshold value (first threshold value TH1). The specified value is a value smaller than the resistivity R1. Alternatively, the specified value may be a value larger than the resistivity R4. If it is determined that the resistivity Rc of the refrigerant is equal to or less than the specified value, the control device 7 may issue a resistivity alarm (abnormality notification) to the host device 200, as shown in FIG. 2, indicating that the resistivity Rc of the refrigerant is in an abnormal state. The abnormal state includes, for example, a state in which the ion exchanger 66 provided in the cooling device 6 is deteriorated.

[0047] As another example, the control device 7 further includes a speed abnormality detection unit 77, as shown in FIG. 2. The speed abnormality detection unit 77 determines whether the differential rotation speed drs output from the difference detection circuit 72 is outside a predetermined range. If it is determined that the differential rotation speed drs is outside the predetermined range, the speed abnormality detection unit 77 may determine that the rotation speed of the electric motor 5 has become unable to follow the rotation speed command value rsb, and may issue a speed follow-up warning (abnormality notification) to the upper device 200, indicating that the rotation speed of the electric motor 5 is abnormal. This is because, when the DC voltage reference value Vdcb is lower than the rated DC voltage, there is a possibility that the rotation speed of the electric motor 5 will not be able to follow the rotation speed command value rsb depending on the load state of the electric motor 5.

[0048] 1-3.Example of control device processing Summary FIG. 4 is a flowchart briefly illustrating an example of processing performed by the control device 7 according to the first embodiment.

[0049] In step S100, the control device 7 acquires various pieces of information. Then, the process proceeds to step S110. The various pieces of information include at least information on the resistivity Rc of the refrigerant.

[0050] In step S110, the control device 7 determines whether the resistivity Rc of the refrigerant has changed. If it is determined that the resistivity Rc of the refrigerant has changed (step S110; Yes), the process proceeds to step S120. Otherwise (step S110; No), the process proceeds to step S130.

[0051] In step S120, the control device 7 (DC voltage reference setting unit 86) changes the DC voltage reference value Vdcb based on the resistivity Rc of the refrigerant, that is, calculates another DC voltage reference value Vdcb based on the characteristics of the refrigerant resistivity Rc and the DC voltage reference value Vdcb shown in Fig. 3. Thereafter, the process proceeds to step S130.

[0052] In step S130, the control device 7 controls the converter 42 so that the DC voltage output from the converter 42 follows the DC voltage reference value Vdcb (that is, another DC voltage reference value Vdcb).

[0053] 1-3-2. Current command processing example FIG. 5 is a flowchart showing an example of processing by the current instruction unit 88 in the control device 7 according to the first embodiment.

[0054] In step S200, the control device 7 acquires the differential voltage and the DC voltage detection value Vdcfb output from the difference detection circuit 87. After that, the process proceeds to step S210.

[0055] In step S210, the control device 7 calculates the active power command value P1 from the differential voltage and the DC voltage detection value Vdcfb, after which the process proceeds to step S220.

[0056] In step S220, the control device 7 sets a reactive power command value Q1. The reactive power command value Q1 is, for example, 0. Thereafter, the process proceeds to step S230.

[0057] In step S230, the control device 7 acquires the AC voltage detection value Vsfb, after which the process proceeds to step S240.

[0058] In step S240, the control device 7 calculates current reference values ​​(including an active current reference value Irb and a reactive current reference value Iib) based on the AC voltage detection value Vsfb. Specifically, the control device 7 converts the active power command value P1 and the reactive power command value Q1 into the active current reference value Irb and the reactive current reference value Iib. Thereafter, the process proceeds to step S250.

[0059] In step S250, the control device 7 determines whether or not the converter 42 satisfies a predetermined operating condition. If it is determined that the converter 42 satisfies the predetermined operating condition (step S250; Yes), the process proceeds to step S260. Otherwise (step S250; No), the process proceeds to step S280.

[0060] The predetermined operating condition includes controlling the power factor of the converter 42 to approach 1. In this case, when the DC voltage output from the converter 42 is the DC voltage detection value Vdcfb, this means that the modulation factor of the converter 42 is at its maximum (the AC voltage of the converter 42 is at its maximum) and the converter 42 can operate at an operating point with current reference values ​​consisting of the active current reference value Irb and the reactive current reference value Iib.

[0061] In step S260, the control device 7 determines whether the apparent current value of the current reference made up of the active current reference value Irb and the reactive current reference value Iib is equal to or less than the allowable current of the converter 42. If it is determined that the current reference value is equal to or less than the allowable current of the converter 42 (step S260; Yes), the process proceeds to step S270. Otherwise (step S260; No), the process proceeds to step S290.

[0062] In step S270, the control device 7 outputs a current reference value consisting of the active current reference value Irb and the reactive current reference value Iib as a current command value ci to the current control unit 83. Then, the process ends.

[0063] If it is determined that the converter 42 does not satisfy the predetermined operating conditions (step S250; No), this means that the required AC voltage cannot be output even when the modulation factor of the converter 42 is maximized due to a drop in the DC voltage, and therefore the converter 42 cannot operate with a power factor close to 1. Therefore, in step S280, the control device 7 adjusts the reactive current reference value Iib to be lowered while maintaining the active current reference value Irb. Specifically, the control device 7 updates the reactive current reference value Iib by subtracting a predetermined current value ΔI from the reactive current reference value Iib. Thereafter, the process returns to step S250. Here, the reactive current reference value Iib and the predetermined current value ΔI are positive in the leading phase direction of the current supplied from the converter 42 to the power source. In other words, by lowering the AC voltage of the converter 42 and operating it in the lagging phase direction, the power factor of the converter 42 deteriorates, but a predetermined active power can be ensured.

[0064] If it is determined that the apparent current value of the current reference made up of the active current reference value Irb and the reactive current reference value Iib is not equal to or less than the allowable current of the converter 42 (step S260; No), that is, if the current reference value is outside the allowable current of the converter 42, continuous operation of the converter 42 may cause a malfunction of the power converter 4. Therefore, in step S290, the control device 7 outputs an abnormality notification to the upper device 200 to the effect that the current reference value is outside the allowable current of the converter 42. Then, the processing ends.

[0065] 1-4.Effects According to the first embodiment, when the resistivity Rc of the refrigerant is less than the first threshold TH1, another DC voltage reference value Vdcb smaller than the DC voltage reference value Vdcb_max is calculated. Furthermore, the converter 42 is controlled so that the DC voltage output from the converter 42 follows the other DC voltage reference value Vdcb. As a result, when the resistivity Rc of the refrigerant decreases, the DC voltage can be lowered to reduce the leakage current of the refrigerant. Therefore, it is possible to continuously operate the power converter 4 while suppressing the progression of electrolytic corrosion. Therefore, the power converter 4 can continuously operate until the planned shutdown time while suppressing the progression of electrolytic corrosion.

[0066] 2. Second Embodiment 6 is an explanatory diagram showing an overview of a water-cooled power conversion system 10A according to embodiment 2. The water-cooled power conversion system 10A includes a power conversion device 100A and a cooling device 6. The cooling device 6 has the same configuration as in embodiment 1 described above, and therefore description thereof will be omitted here.

[0067] The power conversion device 100A is a device that converts power. The input side of the power conversion device 100A is connected to a three-phase AC power source 1 via an input transformer 111, and the output side of the power conversion device 100A is connected to an electric motor 5. The power conversion device 100A receives AC power (also referred to as first AC power) supplied from the three-phase AC power source 1. The power conversion device 100A outputs AC power (also referred to as second AC power) different from the first AC power to the electric motor 5 so that the electric motor 5 operates at a predetermined rotation speed.

[0068] The power conversion device 100A includes a power converter 4A and a control device 7A. The power converter 4A is a modular multilevel converter (MMC) that outputs three AC voltages and has the outputs of multiple cell converters 120 connected in series. The power converter 4A is configured with three series cell converters, each of which has four single-phase output cell converters 120 connected in series. Specifically, as shown in FIG. 6, the power converter 4A is configured with three parts: a first series cell converter in which the outputs of four cell converters 120 (120RA, 120RB, 120RC, and 120RD) that constitute the first phase are connected in series; a second series cell converter in which the outputs of four cell converters 120 (120SA, 120SB, 120SC, and 120SD) that constitute the second phase are connected in series; and a third series cell converter in which the outputs of four cell converters 120 (120TA, 120TB, 120TC, and 120TD) that constitute the third phase are connected in series. The cell converter 120 includes a diode rectifier 121 and a single-phase output inverter 122. The single-phase outputs of the inverters 122 are connected in series. The input transformer 111 has multiple insulated secondary windings and is connected to the AC inputs of the diode rectifiers 121 that make up each cell converter 120.

[0069] 6, the power converter 4A has a first series cell converter, a second series cell converter, and a third series cell converter connected in a Y connection. Specifically, a first terminal of the cell converter 120RA is connected to the neutral point of the power converter 4A, a second terminal of the cell converter 120RA is connected to a first terminal of the cell converter 120RB, a second terminal of the cell converter 120RB is connected to a first terminal of the cell converter 120RC, a second terminal of the cell converter 120RC is connected to a first terminal of the cell converter 120RD, and a second terminal of the cell converter 120RD is the first phase (R phase) output of the cell converter 120 and is connected to the electric motor 5.

[0070] The first terminal of the cell converter 120SA is connected to the neutral point of the power converter 4A, the second terminal of the cell converter 120SA is connected to the first terminal of the cell converter 120SB, the second terminal of the cell converter 120SB is connected to the first terminal of the cell converter 120SC, the second terminal of the cell converter 120SC is connected to the first terminal of the cell converter 120SD, and the second terminal of the cell converter 120SD becomes the second phase (S phase) output of the cell converter 120 and is connected to the electric motor 5.

[0071] The first terminal of the cell converter 120TA is connected to the neutral point of the power converter 4A, the second terminal of the cell converter 120TA is connected to the first terminal of the cell converter 120TB, the second terminal of the cell converter 120TB is connected to the first terminal of the cell converter 120TC, the second terminal of the cell converter 120TC is connected to the first terminal of the cell converter 120TD, and the second terminal of the cell converter 120TD becomes the third phase (T phase) output of the cell converter 120 and is connected to the electric motor 5. The neutral point of the power converter 4A is grounded.

[0072] The multiple cell converters 120 are equipped with a cooling mechanism that can pass the refrigerant supplied from the cooling device 6. The refrigerant circulates through a path that connects the cooling device 6 and the multiple cell converters 120 in a ring shape using cooling piping (including inlet cooling piping and outlet cooling piping). This allows the power converter 4 (power conversion device 100) to cool the multiple cell converters 120 using the refrigerant supplied from the cooling device 6. As in the first embodiment, if the cooling piping (including inlet cooling piping and outlet cooling piping) and the cooling mechanisms (heat sinks, etc.) provided in the multiple cell converters 120 have different potentials, or if there is a potential difference between the cooling piping and the cooling mechanism and the ground, at least a portion of the cooling piping is made of an insulating material.

[0073] The control device 7A controls the multiple cell converters 120 (MMC). Basically, similar to the first embodiment, the control device 7A controls the switching of each inverter 122 of the multiple cell converters 120 based on commands from a higher-level device (not shown) so as to drive the electric motor 5 at a predetermined rotational speed. The control device 7A also includes an inverter enable setting unit 90. The inverter enable setting unit 90 inputs information on the refrigerant resistivity, which is the output value of the resistivity meter 67. The inverter enable setting unit 90 determines whether the refrigerant resistivity is equal to or lower than a threshold value. If it is determined that the refrigerant resistivity is equal to or lower than the threshold value, the inverter enable setting unit 90 determines the number of cell converters 120 in series to be enabled from among the multiple cell converters 120, based on the refrigerant resistivity.

[0074] A method for determining the number of cell converters 120 in series will be described in detail with reference to FIG. 7. In the example shown in FIG. 7(A), the number of cell converters 120 in series is set to "4" when the refrigerant resistivity is equal to or greater than R1; the number of cell converters 120 in series is set to "3" when the refrigerant resistivity is less than R1 and equal to or greater than R2; the number of cell converters 120 in series is set to "2" when the refrigerant resistivity is less than R2 and equal to or greater than R3; the number of cell converters 120 in series is set to "1" when the refrigerant resistivity is less than R3 and equal to or greater than R4; and the number of cell converters 120 in series is set to "0" when the refrigerant resistivity is less than R4. The magnitude relationship between the resistivities R1 to R4 is R1>R2>R3>R4. For example, consider a case where the refrigerant resistivity decreases from equal to or greater than R1 to equal to or greater than R2 and less than R1. If the same resistivity as resistivity R1 is set as the threshold, the resistivity of the refrigerant will be below the threshold, so the inverter validity setting unit 90 determines the number of cell converters 120 in series corresponding to the resistivity of the refrigerant to be "3".

[0075] The inverter enable setting unit 90 then controls the power converter 4 to enable the output of the cell converter 120 corresponding to the series number. "Enable" means to operate the output of the inverter 122 in the cell converter 120 so as to contribute to driving the electric motor 5. A cell converter 120 that is controlled not to enable the output of the inverter 122, i.e., that disables the output of the inverter 122, is controlled to set the output of the inverter 122 in the cell converter 120 to zero. Note that the first series cell converter, second series cell converter, and third series cell converter are all controlled so that the series numbers of the enabled cell converters 120 are the same. It is also preferable to control the disabled cell converter 120 starting from the one closest to the neutral point.

[0076] Consider the output of a cell converter 120 to be disabled among the multiple cell converters 120. The control device 7 sets the output of the cell converter 120 to be disabled to zero. For example, as shown in (B) in FIG. 7, if an output short-circuiter 123 is provided on the output side of a series cell converter (e.g., first series cell converter, second series cell converter, third series cell converter) in which four cell converters 120 are connected in series, the control device 7 controls so that all switching elements constituting the inverter 122 are gate-blocked and the output short-circuiter 123 is closed.

[0077] Consider a case where an output short-circuiter 123 is not provided on the output side of the series cell converter. For example, when the series cell converter outputs a positive voltage, the control device 7 controls the switching elements of the inverter 122 (Q3 and Q4 shown in FIG. 7B) provided on the lower arm to block the gates, and controls the switching elements of the inverter 122 (Q1 and Q2 shown in FIG. 7B) provided on the upper arm to turn on. As another example, when the series cell converter outputs a negative voltage, the control device 7 controls the switching elements of the inverter 122 (Q1 and Q2 shown in FIG. 7B) provided on the upper arm to block the gates, and controls the switching elements of the inverter 122 (Q3 and Q4 shown in FIG. 7B) provided on the lower arm to turn on. This allows the output of the cell converter 120 to be disabled among the multiple cell converters 120 to be zero, even when an output short-circuiter 123 is not provided on the output side of the series cell converter.

[0078] According to the second embodiment, the power converter 4A is a converter in which the outputs of multiple cell converters 120 are connected in series. The forward converters constituting the cell converters 120 are diode rectifiers 121, making it impossible to control the DC voltage of the cell converters 120. However, when the refrigerant resistivity is equal to or lower than a threshold, the number of cell converters 120 to be enabled in series is determined. Furthermore, the converter is controlled to enable the output of the cell converter 120 corresponding to that number of cell converters. As a result, when the refrigerant resistivity decreases, the number of enabled cell converters 120 in series is reduced to lower the output voltage of the power converter 4. This reduces the difference between the ground potential and the ground potential of the cell converter 120 with the highest potential, thereby reducing the refrigerant leakage current. This allows the power converter 4A to operate continuously while suppressing the progression of electrolytic corrosion. Therefore, the power converter 4 can operate continuously until the scheduled shutdown while suppressing the progression of electrolytic corrosion. [Explanation of symbols]

[0079] 1...Three-phase AC power supply, 2...Circuit breaker, 3...Transformer, 3a...Transformer, 4, 4A...Power converter, 5...Motor, 6...Cooling device, 7, 7A...Control device, 10, 10A...Water-cooled power conversion system, 41...First current detector, 42...Converter, 44...DC capacitor, 45...Inverter, 49...Second current detector, 61...Flow meter, 62...Surge tank, 63...Circulation pump, 64...Heat exchanger, 65...Flow control valve, 66...Ion exchanger, 67...Resistivity meter, 68...Thermometer, 100...Power conversion device, 111...Input transformer, 120...Cell converter, 121...Diode rectifier, 122...Inverter, 123...Output short-circuit, 200...Host device

Claims

1. A power conversion device having a cooling mechanism that cools a power converter using a refrigerant supplied from a cooling device, the power converter includes a converter and an inverter; the power conversion device includes a control device that controls the DC voltage output from the converter so that it follows a DC voltage reference value; The control device If the resistivity of the refrigerant is less than a first threshold, calculating another DC voltage reference value that is smaller than the DC voltage reference value based on the resistivity; configured to control the converter to cause the DC voltage to follow the further DC voltage reference value. A power conversion device characterized by:

2. The power conversion device according to claim 1, After calculating the different DC voltage reference value, the control device calculating a current reference value for making the DC voltage follow the different DC voltage reference value based on the AC voltage input to the power converter, a differential voltage obtained by subtracting the DC voltage from the different DC voltage reference value, and the DC voltage; When the converter satisfies a predetermined operating condition and the current reference value is equal to or less than an allowable current of the converter, the converter is controlled using the current reference value as a current command value. A power conversion device characterized by:

3. The power conversion device according to claim 2, The predetermined operating condition includes controlling the power factor of the converter to approach unity. A power conversion device characterized by:

4. The power conversion device according to claim 2, the power conversion device is configured to drive the electric motor so that the rotation speed of the electric motor follows a rotation speed command value from a host device; The control device is configured to issue an abnormality notification to the host device indicating that the rotation speed of the electric motor is abnormal when a differential rotation speed obtained by subtracting the rotation speed from the rotation speed command value is outside a predetermined range. A power conversion device characterized by:

5. A power conversion device having a cooling mechanism that cools a power converter using a refrigerant supplied from a cooling device, the power converter is a converter in which outputs of a plurality of cell converters are connected in series, The power conversion device includes a control device, The control device When the resistivity of the refrigerant is equal to or less than a threshold value, the number of cell converters to be enabled in series among the plurality of cell converters is determined based on the resistivity; configured to control the converter to enable an output of the cell converter corresponding to the number of series A power conversion device characterized by:

Images