JP5649473B2 - Protection device for power converter - Google Patents

Description

  The present invention relates to a protection device for a power conversion device, and more particularly to a protection device for a power conversion device that suppresses an overvoltage of a DC capacitor when a failure occurs in the power conversion device.

  In recent power converters, power converters equipped with a DC capacitor on the DC side are widely used. With the increase in voltage and current, the number of DC capacitors in series and in parallel increases, and the DC capacitor as a whole is increased. The proportion occupied by is also increasing. In such a power converter, when operating normally, a DC voltage obtained by converting an AC voltage on the power supply system side into a DC by a converter is applied to the DC capacitor and is not overcharged. However, the DC capacitor may be overcharged by the energy on the power supply system side due to a DC short circuit failure due to breakage of the switching element on the converter side or false ignition, and the rated voltage may be exceeded. The same applies to the inverter side, and the DC capacitor is overcharged by the energy on the motor side due to the DC short circuit failure due to breakage of the switching element on the inverter side or false ignition. At this time, if the failure state continues, there is a risk of causing an accident such as a burst of the DC capacitor and an arc discharge of the switching element. In order to prevent such damage from spreading, it is necessary to provide a protection circuit on the power supply system side and the motor side. Therefore, a proposal has been made to suppress the overcharge of the DC capacitor by consuming the energy caused by the short-circuit failure in the protection circuit. (For example, refer to Patent Document 1).

JP 2010-11612 (page 8-9, FIG. 9, FIG. 10)

  The conventional protection circuit shown in Patent Document 1 is provided with a three-phase rectifier diode and a thyristor on the input side of a three-level converter that supplies power to a DC capacitor from a power supply (system) side by a rectifier circuit, and power is supplied when a converter side fails. By applying a gate extinguishing signal to all the switching elements of the converter and starting the thyristor, energy from the power source (system) side is consumed by the protective device. The same applies to the motor side, and a three-phase rectifier diode and a thyristor are provided on the output side of the power conversion device configured by a three-level inverter that drives a DC motor by converting a DC voltage into an AC voltage. By applying a gate extinguishing signal to all the switching elements of the power converter and starting the thyristor, energy from the motor side is consumed by the protection circuit.

  However, the protection circuit of the conventional example does not consider the influence when the protection circuit is damaged. For this reason, when the protection circuit is damaged, there is a problem that the elements of the plurality of phases of the converter are damaged by the energy from the power source (system) side. The same applies to the motor side, and there is a problem that the elements of the plurality of phases of the inverter are damaged by the energy from the motor side.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide a protection device for a power conversion device in which the reliability of a protection circuit is improved.

In order to achieve the above object, a protection device for a power conversion device according to the present invention includes a three-level converter provided with a first short-circuit fault detection means for detecting an internal short-circuit fault using an AC power supply as an input, and the three-level converter. Positive and negative DC capacitors fed from the DC capacitor, a three-level inverter that drives an AC motor by converting a three-level DC voltage applied to the DC capacitor to an AC voltage, and an internal short circuit of the three-level inverter A protection device that performs overvoltage protection of a power conversion device having a second short-circuit failure detection means for detecting a failure, the first diode bridge circuit rectifying the input of the three-level converter, and the first diode A first thyristor which is provided in parallel with the DC output of the bridge circuit and ignites the thyristor when the first short-circuit fault detecting means is activated; The thyristor circuit, a second diode bridge circuit that rectifies the input of the AC motor, and a DC output of the second diode bridge circuit are provided in parallel, and the thyristor is activated when the second short circuit fault detecting means is activated. A second thyristor circuit configured to be ignited, and the first and second thyristor circuits are each formed by connecting a plurality of thyristors in series , and the first and second thyristor circuits The gate signal of each thyristor is output as a one-shot pulse shorter than a half cycle of the power supply frequency .

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the protection apparatus of the power converter device which improved the reliability of the protection circuit.

The circuit block diagram of the protection apparatus of the power converter device which concerns on Example 1 of this invention. 1 is a circuit configuration diagram of a thyristor gate driving device in Embodiment 1. FIG. 4 is a timing chart of thyristor gate control according to the first embodiment. FIG. 3 is a timing chart of thyristor gate control according to the first embodiment and illustrates an operation when a thyristor current is zero-crossed. 1 is a circuit configuration diagram of a diode failure detection circuit according to Embodiment 1. FIG. The circuit block diagram of the failure detection circuit of the diode of the protection apparatus of the power converter device which concerns on Example 2 of this invention.

Embodiments of the present invention will be described below with reference to the drawings.

  Hereinafter, the protection apparatus of the power converter device which concerns on Example 1 of this invention is demonstrated with reference to FIG. 1 thru | or FIG.

  FIG. 1 is a circuit configuration diagram of a protection device for a power conversion device according to a first embodiment of the present invention. In FIG. 1, a three-phase AC voltage supplied from an AC power supply (system) is supplied to a three-level converter 3 via an input transformer 1 and an input switch 2.

  The circuit configuration for one phase of the three-level converter 3 is shown in FIG. That is, switching elements Q1, Q2, Q3, and Q4 are connected in series from the positive potential side to the negative potential side of the DC output. Freewheel diodes D1, D2, D3, and D4 are connected in antiparallel to each switching element Q1, Q2, Q3, and Q4, respectively. Clamp diodes DC2 and DC3 are connected between the connection point of switching elements Q1 and Q2 and the neutral point of DC output, and between the connection point of switching elements Q3 and Q4 and the neutral point of DC output, respectively. As a result, the potential at the connection point is clamped to zero potential. Then, an AC input for one phase is applied to the connection points of the switching elements Q2 and Q3. A circuit similar to the above is provided for two more phases, so that a three-level DC output can be obtained.

  Positive and negative DC voltages converted by the three-level converter 3 are applied to DC capacitors 4P and 4N, respectively. Protective fuses 5P and 5N are connected in series to the DC capacitors 4P and 4N, respectively.

  The three-level DC voltage smoothed by the DC capacitors 4P and 4N is supplied to the three-level inverter 6. FIG. 1 shows a circuit configuration for one phase of the three-level inverter 6, but this configuration is basically a circuit configuration symmetrical to the three-level converter 3. That is, the internal configuration is the same, and the 3-level converter 3 outputs a 3-level direct current with an AC input, whereas the 3-level inverter 6 outputs a 3-phase alternating current with the 3-level DC input. Then, the AC motor 7 is driven by the AC output of the three-level inverter 6.

  A diode bridge circuit 8 that rectifies three-phase alternating current and a thyristor circuit 10 that can short-circuit the output of the diode bridge circuit 8 are connected to the input side of the three-level converter 3. Similarly, a diode bridge circuit 9 that rectifies three-phase alternating current and a thyristor circuit 11 that can short-circuit the output of the diode bridge circuit 8 are connected to the output side of the three-level inverter 6. Each arm constituting the diode bridge circuits 8 and 9 is connected in series with a diode that has a withstand voltage sufficient to continue rectification even when one element is damaged. Similarly, in the thyristor circuits 10 and 11, thyristors 10a and 10b and 11a and 11b each having a withstand voltage sufficient to continue the on / off operation are connected in series even when one element is damaged. Here, in FIG. 2, the diodes of the diode bridge circuits 8 and 9 and the thyristors of the thyristor circuits 10 and 11 are shown in two series, but may be three or more series.

  The positive potential side of the thyristor circuit 10 is connected to the positive potential side of the DC circuit via the resistor 11, and the negative potential side is connected to the negative potential side of the DC circuit via the resistor 13. Similarly, the positive potential side of the thyristor circuit 11 is connected to the positive potential side of the DC circuit via the resistor 12, and the negative potential side is connected to the negative potential side of the DC circuit via the resistor 14. These resistors are used to initially charge a starting auxiliary capacitor (not shown) provided in each of the thyristor circuits 10 and 11.

  In such a circuit configuration, when the switching element of the three-level converter 3 is damaged or erroneously ignited, this is detected by a first short-circuit fault detecting means (not shown), and all of the three-level converter 3 and the three-level inverter 6 are detected. A gate extinguishing signal is given to the switching elements of the thyristor 10 and a signal for firing the thyristors 10a and 10b is given. Similarly, when the switching element of the three-level inverter 6 is damaged or erroneously ignited, this is detected by a second short-circuit fault detecting means (not shown) and all the switching elements of the three-level converter 3 and the level inverter 6 are gated. A signal for igniting the thyristors 111 and 112 is provided along with an arc extinguishing signal.

  FIG. 2 shows a circuit configuration of the gate drive circuit 20 that drives the thyristors 10a and 10b of FIG. Between the anode and the cathode of the thyristor 10a, a voltage detection means 21 comprising a resistor 211, a resistor 212 connected in series with the resistor 211, and a parallel connection body composed of the LEDs 213 is provided. The thyristor 10b side is also provided with voltage detection means 21 having the same circuit configuration. These voltage detection means can also serve as a voltage dividing resistor that eliminates voltage imbalance due to leakage current of the thyristors 10a and 10b. In a state where the protective device does not operate, that is, in a normal operation state of the power converter, a DC voltage rectified by the diode bridge 8 is applied to both ends of the thyristors 10a and 10b. As described above, the voltage detection means 21 and 22 are in a state where the DC voltage is divided in the normal state. The voltage detection means 21 feeds back this normal state by the LED 213 connected in parallel with the resistor 212. If the thyristor 10a is damaged, the LED 213 is turned off (signal FBK1). Since the voltage detecting means 22 has the same circuit configuration, when the thyristor 10b is damaged, the LED of the voltage detecting means 22 is turned off (signal FBK2).

  The converter short-circuit signal in FIG. 2 is an abnormal signal given from the first short-circuit fault detection means. The converter short circuit signal and the LED output signals FBK1 and FBK2 are supplied to the re-ignition / thyristor failure detection logic circuit 23. The re-ignition / thyristor failure detection logic circuit 23 outputs a gate logic signal and a thyristor failure detection signal. The gate logic signal is amplified by the drive stage 24 and applied to the isolation transformer 25. One secondary output of the insulating transformer 25 is given to the gate of the thyristor 10a through the diode 261 and the resistor 271. The other secondary output of the isolation transformer 25 is given to the gate of the thyristor 10b via the diode 262 and the resistor 272.

  The internal configuration and operation of the re-ignition / thyristor failure detection logic circuit 23 will be described below.

  First, in a state where the protective device does not operate, that is, in a normal operation state of the converter 3, the converter short circuit signal is set to the low level. Since the one-shot circuit 231 is a one-shot circuit that operates at a rising edge, the one-shot circuit 231 is at a low level when the protective device does not operate. The latch circuit 232 performs an operation of latching the high level when the input becomes the high level. However, if the input of the latch circuit 232 is the low level, the output of the latch circuit 232 remains at the low level.

  The optical signals of FBK1 and FBK2 are given to receivers 233 and 234, respectively, and converted into electrical signals. If these electrical signals are at a high level when there is LED light emission, the protection device is not operating, that is, the three-level converter 3 is in a normal operation state, the thyristor 10a and the thyristor 10b are healthy, and both the LED 213 and the LED 223 emit light. If so, the receivers 233 and 234 are both at the high level, and the output of the AND circuit 235 having these as inputs is at the high level. As a result, the output of the OR circuit 236 becomes High level. The state in which the output of the OR circuit 236 becomes a high level can be output to a control circuit (not shown) as a normal state.

  Here, when either the thyristor 10a or the thyristor 10b, for example, the thyristor 10a is damaged, the LED 213 is turned off, and the FBK1 is turned off, the output of the receiver 233 becomes the low level and the output of the receiver 234 becomes the high level. The output of the circuit 235 is at a low level. As a result, in a normal operation state in which the output of the latch circuit 232 is at the low level, the output of the OR circuit 236 is at the low level and outputs a thyristor failure detection signal.

  Next, FIG. 3 shows a timing chart of each part of FIG. 2 when the protective device is operated by the converter short circuit signal of FIG.

  A high level is input to the converter short circuit signal, and the one-shot circuit 231 outputs a one-shot high level signal at this rising edge. The latch circuit 232 latches and outputs the High level. One input of the OR circuit 236 is an AND condition of the receivers 233 and 234. As will be described later, since the thyristors 10a and 10b are turned on, the voltages across these thyristors are both lowered to the on voltage level (several V), the LEDs 213 and 223 are turned off, and both FBK1 and FBK2 are at the low level. Although the output of the circuit 235 is at a low level, according to the output of the latch circuit 232, the OR circuit 236 can return a normal state (= high level) to a control circuit (not shown) without detecting a failure.

  Further, the converter short-circuit signal is given to the insulating transformer 25 via the OR circuit 239, the one-shot circuit 240, and the drive stage 24, and becomes a gate current via the diode 261 and the resistor 271 to turn on the thyristor 10a. . Similarly, the thyristor 10b can be turned on because a gate current is applied through the diode 261 and the resistor 271.

  In the above description, the latch circuit 232 performs an operation for masking the judgment that the thyristor voltage drops when the thyristor is fired, the LED does not shine, and a failure occurs.

  Next, a re-ignition operation assuming a case where the thyristor current is reduced to near zero during the protection circuit operation will be described with reference to FIG. When the converter short circuit signal is given to the gate logic circuit 23 at time t1, the thyristors 10a and 10b are turned on as described above. The operation up to time t2 is as described above. At time t2, when the thyristor current becomes zero and the thyristors 10a and 10b are turned off, the power on the power supply (system) side increases the voltage across the thyristors 10a and 10b via the diode bridge 10 of the protection circuit. As a result, the voltage detectors 21 and 22 of the thyristors 10a and 10b detect the voltage between the thyristors AK and turn on the LEDs 213 and 223, respectively. As a result, the receivers 233 and 234 both output High level signals because their inputs are both at the High level. This signal is input to one input terminal of the AND circuit 238 via the OR circuit 237, and if the output of the one-shot circuit 231 is in the high level period, the output of the AND circuit 238 becomes the high level, and the OR circuit One-shot circuit 240 is operated via 239. As a result, the thyristors 10a and 10b can be re-ignited.

  The one-shot operation of the one-shot circuit 231 is used for masking a thyristor failure as described above, but the one-shot period is used for a time for permitting re-ignition. When the period of crossing zero current is long, a one-shot period much longer than that in FIG. 3 or FIG. 4 may be used. Further, the one-shot circuit 240 determines the time for which the gate current of the thyristors 10a and 10b flows, and the period is preferably shorter than the time when the thyristor current is zero-crossed as shown in FIG. That is, a time shorter than a half cycle of the power supply frequency is preferable. The reason for this is that when a gate current is continuously supplied to the thyristor while a reverse pressure is applied between the thyristors AK when the zero current is reached, the element generation loss is prevented from rapidly increasing due to the transistor effect of the thyristor. It is to do.

  FIG. 5 is a circuit configuration diagram showing a diode failure detection circuit of the diode bridge circuit 8 of FIG. In FIG. 5, one end of the resistor 81 is connected to the intermediate connection point of the two diodes 8a and 8b, similarly one end of the resistor 82 is connected to the intermediate connection point of the diodes 8c and 8d, and one end of the resistor 83 is connected to the intermediate connection point of the diodes 8e and 8f. On the other hand, one end of the resistor 84 is an intermediate connection point between the two diodes 8g and 8f, one end of the resistor 85 is an intermediate connection point between the diodes 8i and 8j, and similarly one end of the resistor 86 is an intermediate connection point between the diodes 8k and 8l. The other ends of these resistors 81, 82, 83, 84, 85, 86 are connected to one end of a resistor 87, and the other end of the resistor 87 is grounded. Here, the resistance values of the resistors 81, 82, 83, 84, 85, and 86 are equal to each other.

  In the above configuration, when the power conversion device is operating normally, that is, when the protection circuit is not operating and is on standby, the voltage across the resistor 87 during diode rectification usually indicates 0V. This is because the circuit configuration is integrated by resistors from the middle points of the diodes connected in series.

  Here, consider a case where any diode is damaged. In this case, the voltage across the resistor 87 operates so that the neutral point is distorted, and a voltage for one phase appears. Thus, the voltage across the resistor 87 is divided to such an extent that it can be handled at the control level (illustration of this part is omitted), and the voltage passed through the average value circuit 88 is given to the level determination circuit 89. When this voltage exceeds a predetermined level, it is possible to assume that any of the diodes has failed.

  The average value circuit 88 of FIG. 5 is provided to facilitate level determination and prevent noise malfunction. Since the redundancy is achieved by connecting diodes in series in actual use, the average value circuit 88 may have a sufficiently long filter time.

  The motor side protection device corresponding to FIGS. 2 and 5 is the same as the power source (system) side protection device, and the description thereof will be omitted.

  Hereinafter, the protection apparatus of the power converter device which concerns on Example 2 of this invention is demonstrated with reference to FIG.

  FIG. 6 shows a modification of the diode bridge failure detection circuit of the first embodiment shown in FIG.

  In FIG. 6, voltage determination means 801 for detecting the anode-cathode voltage of each diode is provided by the number of diodes. If the corresponding diode is normal, the voltage determination unit 801 confirms that a voltage is generated at both ends of the diode in the power supply voltage cycle. The voltage determination unit 801 may be a circuit that drives an LED with a resistor, such as the voltage detection units 21 and 22 in FIG. In this example, the output of the voltage determination means 801 will be described as a high level (with light) if there is a predetermined voltage between the anode and cathode of the diode, and a low level (without light) if there is no voltage. The voltage determination means repeats the output of the High / Low level in the power supply voltage cycle. However, if the diode is damaged and no voltage is generated at both ends of the diode, the voltage determination means continues to output the Low level.

  The output of the voltage determination unit 801 is given to the up counter 802. The up counter 802 starts counting itself at a predetermined cycle at the Low level, and resets the count value when the High level is input. If the voltage determination unit 801 repeats the High / Low level, the count is within the set upper limit count value and the reset is repeated. If the diode is damaged, only the Low input is input and the up-count is not reset. Therefore, the up-counter reaches a predetermined upper limit count value, and at this time, the High level is output.

  The outputs of the individual up counters 802 are logically ANDed with the conditions of the determination start command by the AND circuit 803 provided correspondingly, and given to the OR circuit 804. Therefore, if the output of any of the up counters 802 is at a high level and the determination start command is at a high level, the OR circuit 804 outputs a high level. This becomes a diode failure signal.

  The determination start command condition is given here when the output voltage of the three-level inverter 6 on the motor side is low (due to the low output frequency, etc.), the voltage generated in the diode becomes small, and the voltage determination This is because the determination by the means 801 is difficult, so that the failure determination of the diode is started when the control of the three-level inverter 6 reaches a certain frequency or higher. In the case of the three-level converter 3, this phenomenon does not occur, so that the condition for the determination start command can be omitted.

  In addition, since the count cycle of the up counter 801 is redundant with the diodes connected in series, there is no problem even if it is 10 times or more the power cycle, and the failure may be detected at a slow cycle.

  In the circuit of the first embodiment shown in FIG. 3, it is possible to easily detect a diode failure in the case of two series. However, if the number of series increases, it becomes difficult to detect a diode failure. The circuit shown in FIG. 6 of this embodiment is suitable for accurately detecting the failure of the diode regardless of the number of series.

  Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  For example, the thyristor circuit 11 may be omitted by connecting the DC outputs of the diode bridge 8 and the diode bridge 9 in the first embodiment.

  In the first embodiment, in consideration of the failure probability of the thyristor and the diode, only the thyristor circuits 10 and 11 may have a thyristor in a serial configuration, and the diode bridge circuits 8 and 9 may have a configuration in which no diode is configured in series.

1 Input Transformer 2 Input Switch 3 Converter 4P, 4N DC Capacitor 5P, 5N Fuse 6 Inverter 7 AC Motor 8, 9 Diode Bridge 8a, 8b, 8c, 8d, 8e, 8f, 8g, 8h, 8i, 8j, 8k 8l Diode 10, 11 Thyristor circuit 10a, 10b, 11a, 11b Thyristor 12, 13, 14, 15 Resistor 20 Gate circuit 21, 22 Voltage detection circuit 23 Re-ignition / thyristor failure detection logic circuit 24 Drive stage 25 Pulse transformer 211, 212, 221, 222, 271, 272 Resistance 213, 223 LED
261, 262 Diode 231, 240 One shot circuit 232 Latch circuit 233, 234 Reception circuit 235, 238 AND circuit 236, 237, 239 OR circuit

Claims (11)

A three-level converter provided with first short-circuit fault detection means for detecting an internal short-circuit fault with an AC power supply as an input;
Positive and negative DC capacitors fed from the three-level converter;
A three-level inverter that converts the three-level DC voltage applied to the DC capacitor into an AC voltage to drive the AC motor;
A protection device that performs overvoltage protection of a power converter having second short-circuit fault detection means for detecting an internal short-circuit fault of the three-level inverter,
A first diode bridge circuit for rectifying the input of the three-level converter;
A first thyristor circuit provided in parallel with the DC output of the first diode bridge circuit and configured to ignite the thyristor when the first short-circuit fault detection means is activated;
A second diode bridge circuit for rectifying the input of the AC motor;
A second thyristor circuit provided in parallel with the DC output of the second diode bridge circuit and configured to ignite the thyristor when the second short-circuit fault detecting means is activated;
Each of the first and second thyristor circuits is formed by connecting a plurality of thyristors in series , and the gate signal of each thyristor of the first and second thyristor circuits is a one-shot shorter than a half cycle of a power supply frequency. A protective device for a power conversion device, wherein the protection device outputs the pulse .   2. The protection device for a power converter according to claim 1, wherein each of the first and second diode bridge circuits includes an arm in which a plurality of diodes are connected in series. Voltage detecting means for detecting an anode-cathode voltage of each thyristor of the first thyristor circuit;
2. The thyristor of the first thyristor circuit is determined to be normal when the detection voltage of the voltage detection means is not less than a predetermined value when the first short circuit failure detection means is not activated. Item 3. The power conversion device protection device according to Item 2. Voltage detecting means for detecting an anode-cathode voltage of each thyristor of the second thyristor circuit;
2. The thyristor of the second thyristor circuit is determined to be normal when the detection voltage of the voltage detection means is not less than a predetermined value when the second short circuit failure detection means does not operate. Item 3. The power conversion device protection device according to Item 2. The first diode bridge circuit includes an arm in which two diodes are connected in series,
The middle point of the two series diodes of each arm is connected to one end of a second resistor that is provided in common through the first resistor and the other end is grounded,
The protection device for a power converter according to claim 2, wherein when the voltage across the second resistor is equal to or higher than a predetermined value, it is determined that any of the diodes has failed. The second diode bridge circuit includes an arm in which two diodes are connected in series,
The middle point of the two series diodes of each arm is connected to one end of a second resistor that is provided in common through the first resistor and the other end is grounded,
The protection device for a power converter according to claim 2, wherein when the voltage across the second resistor is equal to or higher than a predetermined value, it is determined that any of the diodes has failed. Voltage detection means for monitoring the anode-cathode voltage of each of the thyristors connected in series in the first thyristor circuit;
After the first short circuit failure detecting means is actuated and the thyristor is fired, when any of the voltage detecting means detects a voltage equal to or higher than a predetermined value within a predetermined time, the thyristor is turned on again. The protection device for a power conversion device according to any one of claims 1 to 6, further comprising arc means. Voltage detecting means for monitoring the anode-cathode voltage of each of the thyristors connected in series in the second thyristor circuit;
After the second short circuit failure detecting means is actuated and the thyristor is fired, when any of the voltage detecting means detects a voltage higher than a predetermined value within a predetermined time, the thyristor is turned on again. The protection device for a power conversion device according to any one of claims 1 to 6, further comprising arc means. The voltage determination means for detecting the level of each diode in the first diode bridge circuit by detecting the voltage between the anode and the cathode, and the respective outputs of the voltage determination means are received and counted at a predetermined cycle at the low level input. And a counter for starting and resetting the count value by high level input,
The protection device for a power converter according to claim 2, wherein when the count value of any one of the counters reaches a predetermined upper limit value, it is determined that any one of the diodes has failed. The voltage determination means for detecting the level of each diode of the second diode bridge circuit by detecting the anode-cathode voltage and the outputs of the voltage determination means, respectively, receive a low level input and count at a predetermined cycle. And a counter for starting and resetting the count value by high level input,
3. The protection device for a power converter according to claim 2, wherein when the count value of any one of the counters reaches a predetermined upper limit value, it is determined that any one of the diodes has failed. 11. The protection device for a power converter according to claim 10 , wherein when the operating frequency of the three-level inverter exceeds a predetermined value, the diode failure determination is started.

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