JP2001218474A - Method and apparatus for detecting ground fault of inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a ground fault when an inverter is in a stop without switching the semiconductor bridges at the inverter output end. SOLUTION: The inverter switches the current of a direct-current bus (2), obtained by rectifying alternating-current power through diodes (d1 to d6) and filtering it through a capacitor C, through the plurality of sets of the semiconductor bridges (6) and outputs it as alternating-current power. When the inverter is in a stop, any ground fault at the inverter output end is detected by comparing the potential difference between at least one phase of the inverter output end and one pole of the direct-current bus with a reference voltage at a comparator (11).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ground fault detecting method and a ground fault detecting device at a load side output terminal of an inverter that drives an electric motor or the like.

[0002]

2. Description of the Related Art In general, when an output terminal of an inverter is grounded, it is necessary to detect the occurrence of a ground fault and to provide a protection function so that the inverter does not operate, in order to protect the inverter itself and the load operated by the inverter. It has been done for. For this reason, conventionally, a ground fault current flowing at the time of a ground fault has been detected to determine whether there is a ground fault. As such a conventional technique, for example, there is a technique disclosed in Japanese Patent Application Laid-Open No. 5-328740. According to the disclosed technology, the lower arm of the semiconductor bridge is turned on to detect a ground fault at the output terminal of the inverter. As a result, when a current flows in any of the phases, there is a high possibility of a ground fault current. Therefore, it is determined that a ground fault has occurred, and the upper and lower arms of the semiconductor bridge are turned off and the protection operation of the inverter starts. If no current is detected, it is determined that there is no ground fault, and the operation shifts to normal operation.

[0003]

As described above, according to the prior art, since it is necessary to detect the current of the inverter circuit, it is necessary to perform switching control so as to turn on the semiconductor bridge at the output terminal of the inverter. Therefore, a check for a ground fault was performed at the time of starting the operation of the inverter. If you want to detect a ground fault while the inverter is stopped, it can be detected by turning on at least the lower arm of the semiconductor bridge while the inverter is stopped.However, it is not possible to turn on the semiconductor bridge while the inverter is stopped. Not preferred for safety. If the resistance value at the time of the ground fault is small, an excessive current flows to the inverter and the load connected to the inverter at the time of the ground fault detection operation, which adversely affects the inverter and peripheral devices. Was. SUMMARY OF THE INVENTION It is an object of the present invention to provide an inverter ground fault detection method and apparatus which enable ground fault detection without switching the semiconductor bridge at the output terminal of the inverter while the inverter is stopped.

[0004]

In order to achieve the above object, a method for detecting a ground fault in an inverter according to the present invention is characterized in that an AC power supply is rectified by a diode, converted to DC, and then smoothed by a capacitor. In an inverter that outputs current as alternating current power by switching a current through a plurality of sets of semiconductor bridges, when the inverter is stopped, a potential difference between at least one phase of an inverter output terminal and one pole of the DC bus, a reference voltage, Are compared to detect a ground fault at the output terminal of the inverter. A specific ground fault detection device for that purpose is as described in claim 2, wherein the AC power supply is rectified by a diode and converted into DC, and then the current of the DC bus smoothed by a capacitor is switched by a plurality of sets of semiconductor bridges. In the inverter that outputs AC power, a means for detecting a potential difference between the inverter output terminal and one pole of the DC bus, a reference voltage generating means for generating a reference voltage, and one of the inverter output terminal and one of the DC buses A comparator that compares the potential difference between the poles with the reference voltage, generates an output when a potential difference exceeding the reference voltage is input, and detects a ground fault in any phase of the inverter output terminal. It is characterized by. Further, the invention according to claim 3 is characterized in that the potential difference detecting means according to claim 2 is a voltage dividing resistor connected between any phase of the inverter output terminal and one pole of the DC bus. And

[0005]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a ground fault detecting device for an inverter according to the present invention. In the figure, the case of a three-phase inverter is shown.
It can be a multi-phase type that exceeds phases. The three-phase AC power supplies u, v, and w have forward diodes d1, d2, d3.
And reverse diodes d4, d5 and d6 are connected. The positive electrode 1 of the forward diode is connected to the positive electrode 3 of the DC bus 2, and the negative electrode 4 of the reverse diode is connected to the negative electrode 5 of the DC bus 2. A smoothing capacitor C is connected between the positive electrode 1 of the forward diode and the negative electrode 4 of the reverse diode. A semiconductor bridge 6 for switching is connected to the right end of the DC bus 2. In this semiconductor bridge 6, the upper arm semiconductor 7aU and the lower arm semiconductor 7bU, the upper arm semiconductor 7aV and the lower arm semiconductor 7bV, and the upper arm semiconductor 7aW and the lower arm semiconductor 7bW are connected in series, respectively. , AC outputs U, V, W of each phase are derived as output terminals and connected to a load such as a motor. In such an inverter, according to the present invention, a series circuit of one voltage-dividing resistor 8 and the other voltage-dividing resistor 9 is provided between the output terminal U of the inverter and the negative electrode 5 of the DC bus 2. Is connected. Mutual connection point 10 between one voltage-dividing resistor 8 and the other voltage-dividing resistor 9
Is connected to the negative input of the comparator 11, and the reference voltage (Vref) 12 is connected to the positive input of the comparator 11. The output of the comparator 11 is connected to a controller (not shown) to control the semiconductor bridge to be turned off at the time of a ground fault or to shut off the power of the inverter. In this inverter, if the output terminal U is grounded and the neutral point of the power supply is grounded when the inverter is stopped without the semiconductor bridge 6 being fired, as shown by a dotted line in FIG. This is equivalent to the connection of the ground resistor 13 between the output terminal U and the neutral point of the power supply. Therefore, at the time of grounding, the output terminal U → the one voltage dividing resistor 8 → the connection point 10 → the other voltage dividing resistor 9 → the negative electrode 5 of the DC bus 2 → the reverse diodes d4, d5, d6 → the AC power supplies u, v , W → ground resistor 13 → output terminal U flows. Accordingly, at the connection point 10, a potential difference generated by one of the voltage-dividing resistors 8 and the other voltage-dividing resistor 9 is generated. When the potential difference is larger than the reference voltage Vref, the comparator 11 detects the potential difference. Then, it is determined that a ground fault has occurred, and the inverter is turned off via the controller. That is, according to the present invention, when the power is turned on before the operation of the inverter is started, the ground fault accident is automatically checked without fail. In this embodiment, although only one of the three phases of the inverter output terminals U, V, and W is detected, a load such as a motor is connected to the output terminal, so that only one phase is detected. The circuit can also detect ground faults in other phases.

FIG. 2 shows an example in which a potential difference is detected between the output terminal of the inverter and the positive terminal of the DC bus. FIG.
The same parts as those shown in FIG. In this example, a series connection circuit of one voltage-dividing resistor 8 and the other voltage-dividing resistor 14 is connected between the output terminal U of the inverter and the positive electrode 3 of the DC bus 2. A connection point 15 between one voltage-dividing resistor 8 and the other voltage-dividing resistor 14 is connected to the positive input of the comparator 11, and the negative input of the comparator 11 has a reference voltage (Vref).
12 are connected. Here, when the output terminal U of the inverter is grounded while the inverter is stopped, the output terminal U
This is equivalent to connecting the ground resistor 16 between the ground and the ground. In this case, inverter output terminal U → ground resistance 16 →
AC power supplies u, v, w → forward diodes d1, d2, d
3, a ground fault current flows through the path of → the positive pole 3 of the DC bus → the other voltage dividing resistor 14 → the connection point 15 → the one voltage dividing resistor 8 → the inverter output terminal U. At this time, if the divided potential difference between one of the voltage dividing resistors 8 and the other voltage dividing resistor 14 is larger than the reference voltage Vref, the comparator 11 detects the difference and judges that a ground fault abnormality has occurred. The inverter is turned off via the controller.

FIG. 3 shows an example of the detected voltage at the ground point as viewed from the negative side of the DC bus when a ground fault has occurred in the case of neutral point grounding. The ground potential 17 when the inverter output terminal is viewed from the negative side of the DC bus becomes a peak voltage 1 / √3 times the power supply voltage. In the figure, after turning on the inverter, 3
The case where a ground fault abnormality occurs after 0 ms is shown. At this time, the detection voltage 18 is a voltage obtained by dividing the ground potential 17 with a voltage dividing resistor. FIG. 4 shows an example of the detected voltage at the ground point viewed from the negative side of the DC bus when a ground fault occurs in the case of single-phase grounding. When the inverter output terminal is viewed from the negative side of the DC bus, the ground potential 18 has a peak voltage equal to the power supply voltage, but non-voltage sections 21 of several ms appear alternately in the voltage waveform. Here, if a ground fault occurs at the output terminal of the inverter 30 ms after powering on the inverter, the ground potential 1
9, the divided voltage 20 is obtained and the comparator 11
, A ground fault abnormality is determined and detected. However, if a ground fault abnormality occurs 20 ms after the power is supplied to the inverter, the ground potential 19 is 0
Since the divided voltage 20 is 0, the comparator 11 cannot immediately detect the ground fault abnormality, and a delay of several ms occurs. But,
Since the detection is performed when the inverter is stopped, such a delay does not cause any practical problem.

[0008]

As described above, according to the present invention, when the inverter is stopped, the potential difference between at least one phase of the inverter output terminal and one pole of the DC bus is compared with the reference voltage. As a result, a ground fault at the output terminal of the inverter is detected, so that a ground fault at the output terminal is automatically checked before starting the inverter, and a low-cost, highly reliable and highly safe ground fault detection function is provided. The effect of being able to provide an improved inverter is obtained.

[Brief description of the drawings]

FIG. 1 is a basic circuit diagram of an inverter including a device for detecting a ground fault by using a divided voltage between an inverter output terminal and a negative electrode of an inverter DC bus as a ground fault detecting device according to the present invention.

FIG. 2 shows another embodiment of the present invention, and is a basic circuit diagram of an inverter including a device for detecting a ground fault by a divided voltage between an inverter output terminal and a positive electrode of an inverter DC bus.

FIG. 3 is a voltage waveform diagram showing a ground potential at an inverter output terminal viewed from a negative side of an inverter DC bus and a divided voltage (detection voltage) between the inverter output terminal and the DC bus negative electrode in the case of neutral point grounding. It is.

FIG. 4 is a voltage waveform diagram showing a ground potential of an inverter output terminal viewed from a negative side of an inverter DC bus and a divided voltage (detection voltage) between the inverter output terminal and the DC bus negative electrode in the case of one-phase grounding. It is.

[Explanation of symbols]

 2 DC bus 3 Positive electrode 5 Negative electrode 6 Semiconductor bridge 8 One voltage dividing resistor 9 The other voltage dividing resistor 10 Connection point 11 Comparator 12 Reference voltage 13 Grounding resistance 14 The other voltage dividing resistor 15 Connection point 16 Grounding resistance 17 Ground potential 18 Divided voltage 19 Ground potential 1 Divided voltage

 ────────────────────────────────────────────────── ─── Continued on front page F term (reference) 2G014 AA04 AB29 AC19 5G004 AA02 AA05 AB02 BA01 DA02 5H007 AA12 AA17 CA01 CB05 CC01 DC02 DC05 FA12

Claims (3)

[Claims] 1. An inverter that rectifies an AC power supply with a diode, converts the rectified power into a direct current, and then switches the current of the DC bus smoothed by a capacitor with a plurality of sets of semiconductor bridges to output the power as AC power. Detecting a ground fault at the inverter output terminal by comparing a potential difference between at least one phase of the inverter output terminal and one pole of the DC bus with a reference voltage. Detection method. 2. An inverter that rectifies an AC power supply with a diode, converts the rectified DC power into DC power, and then switches the DC bus current smoothed by a capacitor with a plurality of sets of semiconductor bridges to output AC power. Means for detecting a potential difference between one pole of the bus, reference voltage generating means, and comparing a potential difference between the inverter output terminal and one pole of the DC bus with a reference voltage of the reference voltage generating means. do it,
A ground fault detecting device for an inverter, comprising: a comparator that generates an output when a potential difference exceeding a reference voltage is input and detects a ground fault in any phase of an inverter output terminal. 3. A means for detecting a potential difference between an inverter output terminal and one pole of a DC bus includes a voltage divider connected between any phase of the inverter output terminal and one pole of the DC bus. 3. The ground fault detecting device for an inverter according to claim 2, wherein the ground fault detecting device is a resistor.