KR20080088358A - Power conversion equipment and power conversion method - Google Patents

Abstract

A power conversion apparatus and a power conversion method are provided to expand a detectable operation range by improving ground fault detection sensitivity according to an operation speed of a motor of a system. A power conversion apparatus includes a ground resistor(10), a current detector(11), and a ground fault detector(7). The ground resistor grounds a DC circuit unit between a converter(3) and an inverter(4). The current detector detects current flowing in the ground resistor. The ground fault detector detects a ground fault from an output current of the current detector. The ground fault detector has a ground fault detection level varying unit(73), a comparing unit(74), and a ground fault determining unit(75). The ground fault detection level varying unit varies a ground fault detection level according to an operation state of the power conversion apparatus. The comparing unit compares the detection level outputted from the ground fault detection level varying unit with the current flowing in the ground resistor. The ground fault determining unit determines a ground fault according to output of the comparing unit.

Description

Power conversion device and power conversion method {POWER CONVERSION EQUIPMENT AND POWER CONVERSION METHOD}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter and a power conversion method for controlling an electric power converter to drive an AC motor. Particularly, in the case where a ground fault occurs in the system, it is preferable to detect a system abnormality (ground fault) with high accuracy and early. The present invention relates to a power conversion device and a power conversion method.

Background Art Conventionally, as a power converter for driving an AC motor or the like, a method of converting electric power of an AC power source into a direct current by a converter and converting a direct current into an alternating current by an inverter has been used. A grounding resistor is connected to the DC circuit portion between the converter and the inverter, and the amount of current flowing through the grounding resistor changes when the system has a ground fault. Therefore, the ground fault was detected by comparing this current detection value with the ground fault detection level (refer patent document 1).

In addition, a technique has been proposed for detecting a short circuit based on a video detection signal detected by a video current transformer provided in a main circuit supplied to a load by an inverter or the like by varying the ground fault determination level according to the operating frequency of the inverter. (See patent document 2).

Moreover, in order to prevent the malfunction of a ground fault detection apparatus, when the load current of a transmission system exceeds a setting value, the technique which makes a detection sensitivity slow by making a detection setting value large is also proposed (refer patent document 3). Moreover, the technique which detects a ground fault by the combination of an image current and an image voltage in a ground fault direction relay is also proposed (refer patent document 4).

[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-212376

[Patent Document 2] Japanese Patent Application Laid-open No. Hei 8-306297

[Patent Document 3] Japanese Patent Application Laid-Open No. 5-260641

[Patent Document 4] Japanese Patent Application Laid-Open No. 2000-341851

The conventional ground fault detection circuit described in Patent Document 1 described above detects an alternating current flowing through the ground resistance, performs a filter process and a rectification process, compares the ground fault detection level with the detected signal, and flows to the ground current at the time of ground fault. It is judged that a ground fault occurred when the AC current exceeded the set level. Here, in a system for driving an electric motor using a converter and an inverter, a current flows through the ground resistance even in normal operation in which a ground fault does not occur due to the influence of parasitic capacitance present on the motor side and the transformer side.

For this reason, when a detection level is set so as to prevent erroneous detection with a current flowing even in a normal state, the ground fault detection sensitivity is deteriorated, and the operation range in which the ground fault can be detected is also narrowed. If the system is an operation pattern that repeats acceleration and deceleration of the motor, it is possible to catch an abnormality (ground fault) in the detectable area. However, in other systems, abnormality (earth fault) detection may be delayed or rendered impossible.

In the conventional ground fault detection technique described in Patent Document 2 described above, the ground fault determination level is only changed by the inverter frequency, and ground fault detection according to the operation pattern of the motor of the system cannot be performed.

In addition, in the conventional ground fault detection technique described in Patent Document 3 described above, since the detection sensitivity is slowed, the detection sensitivity cannot be increased in accordance with the operating speed of the motor of the system. In addition, in the conventional ground fault detection technique described in Patent Document 4 described above, since the detection of the video current and the video voltage is a constant level of detection, the operation range cannot be widened in accordance with the operating speed of the motor of the system, thereby increasing the detection sensitivity. I could not.

Fig. 10 shows the ground fault detection range and the ground fault detection sensitivity of the conventional example, and a ground fault detection level 102 is given a fixed value by adding a margin to the maximum value of the normal ground current peak value 101 in order to prevent false detection. Doing. In this case, it can be seen that when the operation range 106 capable of detecting the ground fault is narrow and the ground resistance value is large, it is impossible to detect 105 and the ground fault detection sensitivity deteriorates.

Therefore, an object of the present invention is to increase the detection sensitivity according to the operating speed of the motor of the system in the event of a ground fault in the system, and to improve the system abnormality (ground fault) by expanding the operating range detectable by the detection sensitivity. It is also desirable to provide a power converter and a power conversion method that can be detected early.

In order to solve the above problems and to achieve the object of the present invention, the power converter of the present invention, a ground resistor for grounding the DC circuit portion between the converter and the inverter, a current detector for detecting a current flowing through the ground resistor, and a current A ground fault detection device for detecting a ground fault from the output current of the detector, wherein the ground fault detection device includes a ground fault detection level variable unit for varying the ground fault detection level in accordance with an operating state of the power converter and a detection level output by the ground fault detection level variable unit. And a comparison section for comparing the current detection value flowing through the ground resistor and a ground fault determination section for determining the ground fault detection by the comparison output of the comparison section.

In addition, the power converter of the present invention includes a grounding resistor for grounding a DC circuit portion between a converter and an inverter, a current detector for detecting a current flowing through the grounding resistor, a ground fault detection device for detecting a ground fault from an output current of the current detector; And an inverter control device for controlling the output voltage of the inverter such that the current detection value flowing to the ground resistor during the normal operation of the power converter is small, and the inverter control device includes the amount of video component synthesized in the inverter output voltage command. And a video component adder for synthesizing the amount of the video component variable to the inverter output voltage command, wherein the ground fault detection device detects a ground fault that varies the ground detection level according to an operating state of the power converter. The level variable portion, the detection level outputted by the ground fault detection level variable portion, and the ground resistor Is provided with a comparator for comparing the current detection value, the fault determination unit for determining a ground fault detected by the comparison unit compares the output.

Moreover, the power converter of this invention detects a ground fault from the grounding resistor which grounds the DC circuit part between a converter and each inverter of several inverters, the current detector which detects the electric current which flows through a grounding resistor, and the output current of a current detector. And a ground fault detection device, wherein the ground fault detection device stores a relationship between an operation state of each inverter of the plurality of inverters during operation of the power converter and a current detection value flowing through the ground resistor, and each of the plurality of inverters. The ground fault detection level is varied in accordance with the operating state of the inverter, and the ground fault detection level variable part for setting an internal table or arithmetic expression based on the data stored in the storage unit, and the detection level and ground resistor outputted by the ground fault detection level variable part. Whether the ground fault detection is determined by the comparison unit comparing the current detection value flowing through the comparison unit with the comparison output of the comparison unit; Determination is equipped with parts.

In the power conversion method of the present invention, the DC circuit portion between the converter and the inverter is grounded by the ground resistor, the current flowing through the ground resistor is detected by the current detector, and the ground fault is detected from the output current of the current detector. Detected by the ground fault detection device, the ground fault detection level variable step of varying the ground fault detection level according to the operating state of the power converter, the detection level output from the ground fault detection level variable step, and the current detection value flowing through the ground resistor. And a ground fault determination step of determining ground fault detection by the comparison output of the comparison step.

In the power conversion method of the present invention, the DC circuit portion between the converter and the inverter is grounded by the ground resistor, the current flowing through the ground resistor is detected by the current detector, and the ground fault is detected from the output current of the current detector. By the inverter control device when the output voltage of the inverter is controlled by the inverter control device so that the current detection value flowing through the ground resistor during normal operation of the power converter is reduced. A video component variable step of varying the amount of the video component to be varied; and a video component adding step of synthesizing the amount of the video component variable to the inverter output voltage command. In the ground fault detection level variable step of varying the ground fault detection level, and the ground fault detection level variable step Comparing the detected level and the current detection value flowing through the resistor to ground force compared to including a ground fault determining step for determining a ground fault by comparing the output of the step of comparing step.

In the power conversion method of the present invention, the DC circuit portion between the converter and the inverter is grounded by the ground resistor, the current flowing through the ground resistor is detected by the current detector, and the ground fault is detected from the output current of the current detector. Is detected by the ground fault detection device, the storage step of storing the relationship between the operating state of each inverter of the plurality of inverters during operation of the power converter and the current detection value flowing through the ground resistor, and each inverter of the plurality of inverters The ground fault detection level is varied according to the operating state of the ground, and the ground fault detection level variable step of setting an internal table or arithmetic expression based on the data accumulated in the storage step, and the detection level and ground resistor output from the ground fault detection level variable step. Ground fault detection by the comparison step of comparing the current detection value flowing through Intended to include the fault determining step for determining.

According to the present invention, the ground fault detection level can be varied according to the level of the disturbance current (for example, the ripple component due to switching of the inverter and the image component due to the third harmonic superposition) in the operating range. Further, by performing the third harmonic superposition only in the operating region of the motor having a high output voltage, the disturbance current component accompanying the third harmonic superposition can be reduced, and the ground fault detection level can be varied according to the reduced disturbance current level.

According to the present invention, in the event of a ground fault in the system, the detection sensitivity can be increased in accordance with the operating speed of the motor of the system, and the system abnormality (ground fault) can be accurately and earlyly increased by expanding the operating range detectable by the detection sensitivity. Exhibits the effect of being detected.

Best Mode for Carrying Out the Invention The best mode for carrying out the invention is described below.

First, Embodiment 1 will be described.

1 is a configuration diagram of a power converter of a first example of the present embodiment. Reference numeral 1 denotes an AC power source, reference numeral 2 denotes a transformer for converting the voltage of the AC power source 1 to a desired voltage, and reference numeral 3 denotes a converter for converting AC power obtained through the transformer 2 into DC power.

Reference numeral 4 is an inverter for converting the DC power output from the converter 3 into desired AC power, and 5 is an AC motor driven by the power output from the inverter 4. Reference numeral 6 is an inverter control device for operating the inverter 4 so that the output torque or speed of the electric motor 5 satisfies desired characteristics, and reference 7 is a ground fault detection device for detecting a ground fault when a ground fault occurs.

Reference numeral 8 detects and outputs the speed of the electric motor 5 as a speed detector directly connected to the electric motor 5. Reference numeral 9 detects and outputs an output current of the inverter 4 as a current detector. The output signals of the speed detector 8 and the current detector 9 are input to the inverter control device 6, and the inverter control device 6 performs various arithmetic operations to provide signals for operating the inverter 4. Output

Reference numeral 10 denotes a ground resistor connected between the DC circuit between the converter 3 and the inverter 4 and the ground, and reference numeral 11 denotes a current detector which detects and outputs a current flowing through the ground resistor 10. The output signal of this current detector 11 is input to the ground fault detection device 7, the ground fault detection device 7 performs various arithmetic processing, detects a ground fault from the electric current which flows into the ground resistor 10, and determines it.

Next, the main operation of the inverter control device will be described. First, in the inverter control device 6, the speed command value output from the speed command generator 61 and the speed detection value output from the speed detector 8 are input to the speed controller 62. The speed controller 62 calculates and outputs the torque current command value so that the speed detection value coincides with the speed command value.

The torque current command value, the excitation current command value output from the excitation current command generator 63, and the inverter output current detection value output from the current detector 9 are input to the current controller 64. The current controller 64 calculates and outputs an inverter voltage command value so that the inverter output current detection value matches the current command value.

The inverter voltage command value is input to the image component calculator 65, and a voltage obtained by synthesizing the image component calculated by the image component calculator 65 with the inverter voltage command value is input to the pulse generator 66. The pulse generator 66 calculates and outputs a pulse signal for turning on and off the switching element of the inverter 4 so that the inverter output voltage of the inverter 4 matches the inverter output voltage command value. Although the converter 3 also has a converter control device, a description of its operation is omitted here.

Next, the detection operation of the ground fault detection device 7 according to the present embodiment will be described. In the ground fault detection device 7, the current value output from the current detector 11 is input to the filter circuit 71, and the filter circuit 71 performs signal processing in accordance with the set filter characteristics, and performs a signal processing. Outputs The current value after the signal processing is input to the rectifier circuit 72. The rectifier circuit 72 performs rectification on the input signal, and outputs the current value rectified by AC from DC.

The current value after the rectifying process and the ground fault detection level output from the ground fault detection level calculator 73 are input to the comparator 74. The comparator 74 compares the ground fault detection level with the current value after rectification processing, and outputs the result. The comparison result is input to the ground fault detection processor 75, and the ground fault detection processor 75 performs the judgment after the ground fault detection and the process after the ground fault detection based on the input comparison result. Here, the speed detection value output from the speed detector 8 is input to the ground detection level calculator 73, and the ground detection level according to the speed detection value is calculated and output by the set table or calculation formula.

Next, the effect by varying the ground fault detection level in this embodiment is demonstrated using FIG. FIG. 2 shows waveforms (a) in which the current value detected by the current detector 11 is filtered by the filter circuit 71 during normal operation without a ground fault, and waveforms (b) which are rectified by the rectifier circuit 72. ) Is an example. As shown in FIG. 2, even if a ground fault does not occur, a current flows into the ground resistor 10 through the parasitic capacitance 22 present on the motor 5 side and the parasitic capacitance 21 present on the transformer 2 side. Flows. The frequency component of the current includes a switching frequency of the inverter 4 as a power converter, a frequency of an image component, and the like.

3 shows an example in which the vertical axis takes the peak value of the waveform rectified by the rectifying circuit 72 in the state where the speed is applied to the horizontal axis and no ground fault occurs when the motor 5 is operated at each speed. will be. As shown in FIG. 3, when the switching frequency component 32 of the inverter 4 which is a power converter is changed in accordance with the operating state of the electric motor 5, or added to the speed command value from the speed command generator 61. When the magnitude of the video frequency component 33 calculated by the video component calculator 65 is changed, the peak value 31 of the current flowing through the ground resistance in the normal state changes in accordance with the operating state.

4 shows waveforms (a) in which the current value detected by the current detector 11 is filtered by the filter circuit 71 when a ground fault occurs on the motor 5 side, and waveforms rectified by the rectifier circuit 72 ( An example of b) is shown. As shown in FIG. 4, at the ground fault occurrence time 41, the current of the inverter output frequency component flows in addition to the frequency component shown in FIG.

Fig. 5 shows an example in which the vertical axis takes the ground current peak value of the waveform rectified by the rectifying circuit 72 in a state where a speed is applied to the horizontal axis and operated at each speed to cause a ground fault. In addition, since ground faults are rarely always caused by complete ground faults and are considered to be grounded through some resistance value, the ground faults are simulated by the ground fault resistor 23 in FIG. 1, and the current peaks for the respective ground fault resistance values are shown in FIG. 5. The value is shown. As shown in Fig. 5, the ground current peak value at the time of a ground fault with respect to the ground current peak value 51 at normal times also changes according to the operating state of the motor 5, and naturally, if the ground resistance value is large (54). The higher the intermediate current 53, the smaller the case 52, and the smaller the ground resistance value, the larger the ground current peak value.

FIG. 6 shows the effects of the present embodiment on the ground fault detection sensitivity and the ground fault detection range based on FIGS. 2 to 5. Here, the ground fault detection sensitivity detects the ground current peak value exceeding the ground fault detection level 82 up to the resistance values 83, 84, 85 of which the ground fault resistance of the ground fault resistor 23 shown in FIG. It indicates whether it is possible. Here, the ground fault detection range 86 indicates to what extent the ground current peak value can be detected in the range of the operating speed of the electric motor 5.

Therefore, in the present embodiment, the ground fault detection level 82 is varied in accordance with the operating speed of the electric motor 5 in response to the normal ground current peak value 81 as shown in FIG. In this case, the operation range 86 which can detect a ground fault in the case where the ground resistance is about 84 is wide, and a ground fault detection becomes possible even when the ground resistance is large (85). Here, the higher the operating speed of the electric motor 5, the lower the ground fault detection level 82, and the detection sensitivity is improved.

This has the effect of improving the ground fault detection operating range 86 and the ground fault detection sensitivity. By doing so, in the event of a ground fault in the system, the detection sensitivity and the detectable driving range are expanded, thereby causing a system fault. The ground fault can be detected with high precision and early.

Next, Embodiment 2 will be described.

Fig. 7 shows another embodiment, in which a multiplier 67 multiplying a coefficient by an image component calculated by an image component calculator 65 synthesized with an inverter voltage command value from the current controller 64 and an image for calculating the coefficient. The component variable coefficient calculator 68 is provided. Then, by varying the coefficient of the video component variable coefficient calculator 68 in accordance with the operating state of the electric motor 5, the video component calculated by the video component calculator 65 synthesized to the inverter voltage command value from the current controller 64. Is different from FIG. 1 in that the value of V is varied according to the operating state of the electric motor 5.

The purpose of synthesizing the video component calculated by the video component calculator 65 to the inverter voltage command value from the current controller 64 is to make the voltage waveform of the speed command value into a trapezoidal wave shape, and output from the inverter 4. To increase the voltage present. Thereby, a voltage margin can be produced | generated at the time of high voltage output of the inverter 4. As shown in FIG. In view of the above-mentioned purpose, the synthesis of the inverter component command value from the current controller 64 of the image component calculated by the image component calculator 65 corresponds to a high voltage when a voltage margin is required in the inverter 4. It is necessary to perform only at the time of the high speed operation of the electric motor 5, and it is not necessary to perform it at the time of the low speed operation of the electric motor 5 corresponding to the low voltage with sufficient voltage margin in the inverter 4.

On the other hand, as shown in Figs. 2 and 3, when the current value flowing in the ground resistor 10 is considered in normal operation, the image component calculator 65 is zeroed by setting the coefficient of the image component variable coefficient calculator 68 to zero. The current by the image component to be calculated is zero. Thereby, the current value flowing through the ground resistor 10 can be reduced in normal times.

Therefore, in Fig. 7, the operation state of the electric motor 5, for example, the operation speed of the electric motor 5 and the inverter 4 are synthesized with the inverter voltage command value from the current controller 64 in accordance with the voltage that can be output. The video component value calculated by the video component calculator 65 is varied by the coefficients of the video component variable coefficient calculator 68.

As a result, when the low speed operation of the motor 5 (the output of the inverter 4 is low voltage), the video component value calculated by the video component calculator 65 is zero, and the coefficient of the video component variable coefficient calculator 68 is zero. To zero. Then, the coefficient of the image component variable coefficient calculator 68 is applied to the image component value calculated by the image component calculator 65 only when the motor 5 needs high speed operation (the output of the inverter 4 is high voltage). By making the integer an integer, the image component values at a predetermined ratio were synthesized.

FIG. 8 is a view for explaining the effect of the present embodiment, and the ground current peak value 91 in normal operation is shown in FIG. 6 by varying the video component by the coefficient of the video component variable coefficient calculator 68. FIG. This can be reduced in comparison with the ground current peak value 81. Thereby, the detectable driving range 96 can be extended to a wide speed range compared with the detectable driving range 86 shown in FIG. In the case where the ground resistance value is about medium (84), the detectable operation range 96 is wide, and in the case where the ground resistance value is large (95), it can be detected, and the detectable operation range 96 can be widened. It can be seen that.

By doing in this way, with respect to the 1st example of FIG. 1, the ground fault detection range by the ground fault detection range 96 can be expanded, and the ground fault detection level 92 is reduced by reducing the ground current peak value 91 at the time of normal operation. ), There is an effect that can further improve the ground detection detection sensitivity. Therefore, when a ground fault occurs in the system, the system abnormality (ground fault) can be detected with high accuracy and early by expanding the detection sensitivity and the detectable operating range.

Next, Embodiment 3 will be described.

FIG. 9 is another embodiment example, and is different from FIGS. 1 and 7 in that a plurality of inverters 4a, 4b, 4c, ... are connected to the common converter 3. Further, a ground fault detection set value calculator 76 is provided in the ground fault detection device 7 to operate the operation states (motors 5a, 5b, 5c, ...) in the inverter control devices 6a, 6b, 6c,. Speed, the output voltages of the inverters 4a, 4b, 4c, ..., etc.) are input to the ground fault detection set value calculator 76 together with the current value flowing through the ground resistor 10 during normal operation.

In the ground detection setting value calculator 76, the ground resistor 10 during normal operation is supplied to the ground resistor 10 during normal operation in accordance with the operation state of each inverter 4a, 4b, 4c, ... during a test operation or during an operation operation. The current value flowing through is stored, and the ground fault detection level is calculated and output from the stored current value in each operation state.

The ground fault detection level calculated by the ground fault detection set value calculator 76 is input to the ground fault detection level calculator 73 together with the operation status signal, and is set in a table or arithmetic expression. In addition, the ground fault detection level calculator 73 inputs an operation pattern during the operation operation from the ground fault detection setting value calculator 76, and the ground fault detection level according to the driving pattern is calculated and output by the set table or arithmetic expression. The point differs from FIGS. 1 and 7. Here, the driving pattern includes various driving patterns based on a combination of driving speeds of the plurality of electric motors 5a, 5b, 5c, ... by the plurality of inverters 4a, 4b, 4c, ....

When a plurality of inverters 4a, 4b, 4c, ... are connected to the common converter 3, the parasitic capacitances 22a, 22b, 22c, ... on the side of each inverter 4a, 4b, 4c, ... Since the synthesized current flows through the ground resistor 10, it is difficult to vary the detection level.

Therefore, the ground current setting value calculator 76 stores the current flowing through the ground resistance 10 in the operation pattern of each inverter 4a, 4b, 4c, ... determined as the system, and the ground detection level calculator 73 stores the current. By setting the ground fault detection level according to each operation pattern, the detection level can be changed.

In this way, even in a system configuration in which the plurality of inverters 4a, 4b, 4c, ... are provided in the common converter 3, the ground fault detection range and the ground fault detection sensitivity can be improved in the same manner as in FIG. When a ground fault occurs in the system, the system abnormality (ground fault) can be detected with high accuracy and early by expanding the detection sensitivity and the detectable operating range.

It goes without saying that the present embodiment is not limited to the above-described embodiment and can be appropriately changed as long as it does not deviate from the gist of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The block diagram of the power converter of the 1st example which shows this embodiment.

FIG. 2 is a waveform diagram of ground current during normal operation, in which FIG. 2A is a waveform filtered by the filter circuit 71, and FIG. 2B is a waveform rectified by the rectifier circuit 72. FIG.

3 is a characteristic diagram of an operating speed of a ground fault detection current peak value in normal operation.

4 is a waveform diagram of ground currents during ground fault, FIG. 4A is a waveform filtered by the filter circuit 71, and FIG. 4B is a waveform rectified by the rectifier circuit 72;

5 is a characteristic diagram of an operating speed of a ground fault detection current peak value at ground fault.

6 is an explanatory diagram of ground fault detection range and ground fault detection sensitivity;

7 is a configuration diagram of a power conversion device of a second example, showing another embodiment.

8 is an explanatory diagram of a ground fault detection range and ground fault detection sensitivity in a second example;

9 is a configuration diagram of a power conversion device of a third example showing another embodiment.

10 is an explanatory diagram of a ground fault detection range and ground fault detection sensitivity in a conventional example.

<Explanation of symbols for main parts of the drawings>

1: AC power

2: transformer

3: converter

4: Inverter

5: electric motor

6: inverter control device

7: ground fault detection device

8: speed detector

9: current detector

10: grounding resistor

11: current detector

21: parasitic capacitance of the transformer

22: parasitic capacitance on the motor side

23: ground fault resistor

61: speed command generator

62: speed controller

63: Excitation Current Command Generator

64: current controller

65: image component calculator

66: pulse generator

71: filter circuit

72: rectifier

73: ground fault detection level calculator

74: comparator

75: ground fault detection processor

76: ground fault detection set value calculator

Claims (10)

A power converter having a converter for converting AC power into direct current, and an inverter for converting the direct current power output by the converter into alternating current power of a desired frequency and supplying the motor to the motor. A ground resistor for grounding a DC circuit portion between the converter and the inverter, a current detector for detecting a current flowing through the ground resistor, and a ground fault detection device for detecting a ground fault from an output current of the current detector, The ground fault detection device includes a ground fault detection level variable unit configured to vary a ground fault detection level according to an operating state of a power converter; A comparison unit for comparing the detection level output from the ground fault detection level variable unit with the current detection value flowing through the ground resistor; Ground fault determination unit for determining ground fault detection by the comparison output of the comparison unit Power conversion device comprising a. The method of claim 1, The ground fault detection device includes a storage unit that stores a relationship between an operating state at the time of operation of the power converter and a current detection value flowing through the ground resistor, And a table or a calculation formula set in the ground fault detection level variable unit based on the data accumulated in the storage unit in order to vary the ground fault detection level in accordance with the operating state of the power converter. The method of claim 1, And wherein the ground fault detection level varying unit lowers the ground fault detection level according to an operating speed of the power converter. A power converter having a converter for converting AC power into direct current, and an inverter for converting the direct current power output by the converter into alternating current power of a desired frequency and supplying the motor to the motor. A ground resistor for grounding a DC circuit portion between the converter and the inverter, a current detector for detecting a current flowing through the ground resistor, a ground fault detection device for detecting a ground fault from an output current of the current detector; An inverter control device for controlling the output voltage of the inverter such that the current detection value flowing to the ground resistor becomes small during normal operation of the power converter; The inverter control device, An image component variable section for varying an amount of the image component synthesized with the inverter output voltage command; An image component adder for synthesizing the amount of the image component varied in the inverter output voltage command, The ground fault detection device, A ground fault detection level variable unit for varying a ground fault detection level according to an operating state of the power converter; A comparison unit for comparing the detection level output from the ground fault detection level variable unit with the current detection value flowing through the ground resistor; Ground fault determination unit for determining ground fault detection by the comparison output of the comparison unit Power conversion device comprising a. The method of claim 4, wherein The ground fault detection device includes a storage unit that stores a relationship between an operating state at the time of operation of the power converter and a current detection value flowing through the ground resistor, And a table or an expression set in the ground fault detection level calculator based on the data stored in the storage unit in order to vary the ground fault detection level according to the operating state of the power converter. The method of claim 4, wherein And the image component variable unit increases the amount of the image component according to the driving speed of the power converter. A power conversion device including a converter for converting AC power into direct current and a plurality of inverters connected in common to the converter, for branching outputs of the converter to convert AC power of the plurality of desired frequencies to be supplied to an electric motor. as, A ground resistor for grounding a DC circuit portion between the converter and each inverter of the plurality of inverters, a current detector for detecting a current flowing through the ground resistor, and a ground fault detection device for detecting a ground fault from an output current of the current detector. and, The ground fault detection device, A storage unit that stores a relationship between an operating state of each inverter of the plurality of inverters during operation of the power converter and a current detection value flowing through the ground resistor; A ground fault detection level variable unit for varying a ground fault detection level in accordance with an operating state of each inverter of the plurality of inverters, and setting an internal table or arithmetic expression based on data stored in the storage unit; A comparison unit for comparing the detection level output from the ground fault detection level variable unit with the current detection value flowing through the ground resistor; Ground fault determination unit for determining ground fault detection by the comparison output of the comparison unit Power conversion device comprising a. A power conversion method in a power conversion apparatus for converting AC power into a direct current by a converter and converting the direct current power output by the inverter into an AC power having a desired frequency and supplying the same to an electric motor. When the DC circuit portion between the converter and the inverter is grounded by a ground resistor, a current flowing through the ground resistor is detected by the current detector, and a ground fault is detected by the ground fault detection device from the output current of the current detector, A ground fault detection level variable step of varying the ground fault detection level by the ground fault detection device in accordance with an operating state of a power converter; A comparison step of comparing the detection level output from the ground fault detection level variable step with the current detection value flowing through the ground resistor; Ground fault determination step for determining ground fault detection by the comparison output of the comparison step. Power conversion method comprising a. A power conversion method in a power conversion apparatus for converting AC power into a direct current by a converter and converting the direct current power output by the inverter into an AC power having a desired frequency and supplying the same to an electric motor. The DC circuit portion between the converter and the inverter is grounded by a ground resistor, the current flowing through the ground resistor is detected by the current detector, and a ground fault is detected by the ground fault detection device from the output current of the current detector. When the output voltage of the inverter is controlled by the inverter control device so that the current detection value flowing through the ground resistor in the normal operation of the power converter is small, By the inverter control device, An image component variable step of varying an amount of an image component synthesized with the inverter output voltage command; An image component addition step of synthesizing the amount of the image component varied in the inverter output voltage command, By the ground fault detection device, A ground fault detection level variable step of varying a ground fault detection level in accordance with an operating state of the power converter; A comparison step of comparing the detection level output from the ground fault detection level variable step with the current detection value flowing through the ground resistor; Ground fault determination step for determining ground fault detection by the comparison output of the comparison step. Power conversion method comprising a. A power conversion method in a power conversion apparatus for converting AC power into direct current by a converter, branching outputs of the converter by a plurality of inverters, converting the outputs of the converter into AC powers of the respective desired frequencies, and supplying them to an electric motor. When the DC circuit portion between the converter and the inverter is grounded by a ground resistor, a current flowing through the ground resistor is detected by the current detector, and a ground fault is detected by the ground fault detection device from the output current of the current detector, By the ground fault detection device, A storage step of storing a relationship between an operating state of each inverter of the plurality of inverters during operation of the power converter and a current detection value flowing through the ground resistor; A ground fault detection level variable step of varying a ground fault detection level in accordance with an operating state of each inverter of said plurality of inverters, and setting an internal table or arithmetic expression based on the data accumulated in said storage step; A comparison step of comparing the detection level output from the ground fault detection level variable step with the current detection value flowing through the ground resistor; Ground fault determination step for determining ground fault detection by the comparison output of the comparison step. Power conversion method comprising a.

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