JP2012110125A - Protection device for 3-level power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a protection device for a 3-level power converter which is capable of turning off an element quickly in a proper turning-off order to protect the 3-level power converter when a short circuit is detected.SOLUTION: In a protection device for a 3-level power converter, when receiving a short circuit detection signal from any one of short circuit detection devices 31a-31d, a gate signal control device 4 outputs an element driving signal for first turning switching elements 1a and 1d to an off state and then turning switching elements 1b and 1c to an off state. In a case that a short circuit is detected at gate driving devices 3a and 3d, the outer switching elements 1a and 1d are immediately turned off independently in the respective gate driving devices. On the other hand, in a case that a short circuit is detected at gate driving devices 3b and 3c, turning-off of the inner switching elements 1b and 1c is masked. In addition, when the switching elements 1a-1d are turned off at the time of detecting the short circuit, soft turning-off with a slower turning-off speed compared with a normal operation is performed.

Description

  The present invention relates to a protection device for a three-level power converter capable of outputting positive, zero, and negative three-level voltages.

FIG. 6 is a diagram illustrating a configuration of a protection device for a conventional three-level power converter.
In FIG. 6, reference numerals 101a to 101d denote semiconductor switching elements (IGBTs) constituting one phase of the power converter. This power converter can output positive, zero, and negative three-level voltages by changing the locations where the elements 101a to 101d are driven. For example, a positive voltage is output by turning on the elements 101a and 101b, a zero voltage is output by turning on the elements 101b and 101c, and a negative voltage is output by turning on the elements 101c and 101d.

Reference numerals 102a to 102d denote gate drive devices that drive the elements 101a to 101d, and each have a short circuit detection device and a short circuit protection device. Note that the short-circuit detection device and the short-circuit protection device are connected in the gate drive device, and when the short-circuit detection device detects a short circuit of the element, the short-circuit protection device that receives the detection signal blocks the ignition signal. Operates and shuts off the element.
Reference numerals 103a to 103d are signal lines for transmitting an ignition signal for driving the elements 101a to 101d to the gate driving devices 102a to 102d from the gate signal control device 104 that generates or controls the ignition signal. Furthermore, reference numeral 105 denotes a signal line for transmitting a signal detected by the short circuit detection device to the gate signal control device 104.

  The three-level power converter has a prohibition condition when the semiconductor switching element is shut off. For example, in the state where the elements 101a and 101b in FIG. 6 are conducting, when the element 101b is cut off first, the load current is connected to the reflux elements (elements 101c and 101d connected in parallel to the elements 101c and 101d). Processed through a freewheeling diode). At this time, since the element 101a is still conductive, the input side of the element 101b rises to a positive voltage. Furthermore, since a load current is passed through the elements 101c and 101d through the associated return element, a negative voltage is applied to the output side of the element 101b. As a result, there arises a problem that the combined voltage of the two DC power sources at the converter input is applied across the element 101b.

Further, even when the element 101c is shut off first in a state where the elements 101c and 101d are in conduction, the combined voltage of the two DC power supplies is applied to the element 101c, and the same problem occurs. .
Therefore, in order to prevent such a prohibition condition, it is necessary to protect the elements by preferentially blocking the elements 101a and 101d and then blocking the elements 101b and 101c when a short circuit is detected. .

However, in the short-circuit protection device shown in FIG. 6, when a short-circuit is detected, the device is immediately shut off, and no measures are taken to shut off the device in consideration of the shut-off order so as not to satisfy the forbidden conditions.
As a countermeasure, for example, there is a technique described in Patent Document 1. In this technique, when a short circuit is detected, the driving state of the elements is grasped and the order in which the elements are shut off is controlled.

  That is, as shown in FIG. 7, the gate signal control device 104 includes a gate drive on / off signal generator 104a and a logic circuit 104b, and an ignition signal from the gate drive on / off signal generator 104a to the gate drivers 102a to 102d. Is passed through the logic circuit 104b. Thereby, the logic circuit 104b grasps the drive states of the elements 101a to 101d. Further, the short circuit detection signals transmitted from the short circuit detection devices in the gate driving devices 102a to 102d through the signal lines 105a to 105d are input to the logic circuit 104b, whereby the logic circuit 104b grasps the short circuit state of the elements 101a to 101d. . And in this logic circuit 104b, the interruption | blocking order of element 101a-101d is judged based on the said drive state at the time of a short circuit detection, and the said short circuit state.

Then, the logic circuit 104b operates the short-circuit protection device in the gate driving devices 102a to 102d via the signal lines 106a to 106d based on the determination result of the interruption order, interrupts the ignition signal, and activates each element. Cut off. At this time, the normal firing signal from the gate drive on / off signal generator 104a is locked in the logic circuit 104b and is not transmitted to the gate drivers 102a to 102d.
In this way, measures are taken to shut off the element so as not to become a forbidden condition after detection of a short circuit.

Japanese Patent Laid-Open No. 10-66348

However, in the technique described in Patent Document 1, as shown in FIG. 7, a short circuit detection signal output from the gate drive circuits 102a to 102d and an ignition signal output from the gate drive on / off signal generation phase 104a. Are once passed through the logic circuit 104b, and the logic circuit 104b grasps the driving state and the short circuit state of each of the elements 101a to 101d, determines the blocking order, and starts the short circuit protection operation. Therefore, it takes time to detect the short circuit and shut off the element.
Therefore, an object of the present invention is to provide a protection device for a three-level power converter that can quickly shut down and protect elements in an appropriate shut-off order when a short circuit is detected.

In order to solve the above-described problem, a protection device for a three-level power converter according to claim 1 includes a first smoothing capacitor connected between a positive terminal and an intermediate terminal of a DC power supply, the intermediate terminal, and the intermediate terminal. A second smoothing capacitor connected between a negative electrode terminal of a DC power source, first to fourth switching elements sequentially connected in series between the positive electrode terminal and the negative electrode terminal; First to fourth freewheeling diodes connected in parallel to the fourth switching element, the anode side to the intermediate terminal, and the cathode side to the connection point between the first switching element and the second switching element A first clamp diode connected to the first switching diode, and a second clamp connected to the connection point between the third switching element and the fourth switching element on the anode side and to the intermediate terminal on the cathode side Comprising a diode, a, a second three-level power converter protective devices for obtaining a voltage output of the three-level by connecting the connection point to the AC output terminal and the switching element and the third switching element,
First to fourth driving means for controlling on / off of the first to fourth switching elements, and a control signal for performing on / off control of the first to fourth switching elements, Control means for outputting to the drive means, wherein the first to fourth drive means detect a short circuit of a switching element that is controlled to turn on and off by itself, and the short-circuit detection means performs a short circuit. Transmitting means for transmitting the detection to the control means, and when the control means is notified that the short circuit is detected from any of the transmission means, the first and first 4 is turned off, and then a control signal is output to turn off the second and third switching elements. The first and fourth driving means are short-circuited by their own short-circuit detecting means. Regardless of the control signal from the control means, the switching element that is controlled to turn on and off immediately is turned off, and the second and third drive means are short-circuited by their own short-circuit detection means. When a control signal for turning off a switching element that is controlled to turn on and off is input from the control means, the switching element is turned off.

As described above, when it is transmitted from the short-circuit detection means of any one of the first to fourth drive means that the control means has detected that the short-circuit has been detected, the control means first precedes regardless of the drive state or short-circuit state of each switching element. A control signal for turning off the outer first and fourth switching elements and then turning off the inner second and third switching elements is output. Therefore, unlike the conventional device, there is no need to take measures such as acquiring the drive state and short-circuit state of the switching element in the logic circuit and determining the shut-off order before starting the short-circuit protection operation. Therefore, the switching elements can be shut off in an appropriate order when a short circuit is detected, and the time from when the short circuit is detected until the switching elements are shut off and protected can be shortened.
Further, when a short circuit is detected by the first and fourth driving means, the switching element is cut off independently in the driving means that has detected the short circuit, so that the switching element in which the short circuit has occurred can be immediately cut off. .

A protection device for a three-level power converter according to a second aspect is the invention according to the first aspect, wherein the first to fourth driving means are in a normal operation in which a short circuit is not detected by the short circuit detecting means. Then, the switching element is turned off at a preset normal cutoff speed, and the switching element is turned off at a cutoff speed slower than the normal cutoff speed when a short circuit is detected by the short detection means. It is characterized by that.
As a result, when the switching element is interrupted when a short circuit is detected, it is possible to prevent a high voltage from being applied to the switching element and to prevent the switching element from being stressed.

Furthermore, the protection device for a three-level power converter according to a third aspect is the invention according to the second aspect, wherein the first to fourth driving means turn the switching element off when the short circuit is detected. The shut-off signal for turning off the switching element is attenuated at the normal shut-off speed until the shut-off start level of the switching element, and then attenuated at a shut-off speed slower than the normal shut-off speed to turn off the switching element. It is characterized by a state.
Thereby, stress to the switching element at the time of interruption | blocking can be prevented, shortening time until interruption | blocking of a switching element.

  According to the present invention, when the three-level power converter is stopped when a short circuit is detected, the switching elements can be quickly shut down and protected in a shut-off order that does not become a prohibition condition.

It is a whole lineblock diagram of a protection device of a 3 level power converter concerning the present invention. It is a block diagram which shows the inside of a U-phase arm bridge. It is a figure for demonstrating a soft interruption | blocking function. It is a timing chart which shows operation | movement of this embodiment. It is a timing chart which shows operation | movement of this embodiment. It is a block diagram of the protection apparatus of the conventional 3 level power converter. It is a block diagram of the protection apparatus of the conventional 3 level power converter.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
(Constitution)
FIG. 1 is an overall configuration diagram of a protection device for a three-level power converter according to the present invention.
In the figure, reference numeral 10 denotes a three-level power converter (hereinafter simply referred to as a power converter). This power converter 10 includes a DC power source 11 connected between a positive terminal P and a negative terminal N, a first smoothing capacitor 12 connected between a positive terminal P and an intermediate terminal O, and an intermediate terminal And a second smoothing capacitor 13 connected between O and the negative terminal N. That is, the first smoothing capacitor 12 and the second smoothing capacitor 13 are connected in series with the intermediate terminal O interposed therebetween.

  The power conversion device 10 has a three-phase configuration of a U-phase, a V-phase, and a W-phase, and includes a U-phase arm bridge 14U, a V-phase arm bridge 14V, and a W-phase arm bridge 14W. These phase arm bridges have the same configuration. And the power converter device 10 can output the voltage of three levels, positive, zero, and negative from the output lines 15U-15W of each phase.

Next, the internal configuration of the arm bridge 14 of each phase will be described. As described above, since the arm bridges of the respective phases have the same configuration, the internal configuration of the U-phase arm bridge 14U will be described here.
FIG. 2 is a configuration diagram showing the inside of the U-phase arm bridge 14U.
The arm bridge 14U includes first to fourth switching elements 1a to 1d and freewheeling diodes 2a to 2d connected in antiparallel to the switching elements 1a to 1d.

  The switching elements 1a to 1d are semiconductor switching elements, and are composed of, for example, IGBTs. The first to fourth switching elements 1a to 1d are sequentially connected in series. Among these, the series connection circuit of the switching elements 1a and 1b has one terminal connected to the positive potential line P1 and the other terminal connected to the positive potential line P1. It is connected to the U-phase output line 15U. In the series connection circuit of the switching elements 1c and 1d, one terminal is connected to the U-phase output line 15U and the other terminal is connected to the negative potential line N1.

With such a configuration, when switching elements 1a and 1b are simultaneously turned on, the potential of positive potential line P1 is output from U-phase output line 15U. When switching elements 1c and 1d are simultaneously turned on, U-phase output line 15U The potential of the negative potential line N1 is output. When switching elements 1b and 1c are simultaneously turned on, a zero potential is output from U-phase output line 15U.
Gate driving devices 3a to 3d are connected to the switching elements 1a to 1d, respectively. The gate drive devices 3a to 3d receive element drive signals, which are control signals for performing on / off control of the switching elements 1a to 1d, from the gate signal control device 4. Then, the gate drive devices 3a to 3d drive and control the switching elements 1a to 1d based on the input firing command signal.

Each of the gate drive devices 3a to 3d includes a short circuit detection device and a short circuit protection device. The short circuit detection devices 31a to 31d detect short circuit states of the switching elements 1a to 1d, respectively.
The short-circuit protection device of the gate driving device 3a is composed of a soft shut-off unit 32a. Further, the short circuit protection device of the gate drive device 3d has the same configuration as the short circuit protection device of the gate drive device 3a, and is composed of a soft shut-off unit 32d. On the other hand, the short circuit protection device of the gate driving device 3b is composed of a soft shut-off portion 32b and a shut-off mask portion 33b. Further, the short circuit protection device of the gate driving device 3c has the same configuration as the short circuit protection device of the gate driving device 3b, and includes a soft blocking part 32c and a blocking mask part 33c.

  When the short circuit detection devices 31a and 31d detect a short circuit state, the short circuit detection device 31a and 31d outputs a short circuit detection signal indicating that the switching device is in a short circuit state to the gate signal control device 4 via the signal line 7, and at the time of short circuit detection A shutoff signal for shutting off is output to the soft shutoff units 32a and 32d. When the short-circuit detection devices 31b and 31c detect the short-circuit state, the short-circuit detection signal is output to the gate signal control device 4 through the signal line 7 and the cut-off signal is output to the cut-off mask portions 33b and 33c.

When the short-circuit detection devices 33b and 33c detect a short-circuit state and output a cut-off signal, the cut-off mask portions 33b and 33c have a function of temporarily masking the switching elements 1b and 1c so as not to immediately enter the cut-off operation. .
When the blocking mask portions 33b and 33c are in a state of masking the blocking signal, switching is performed when an element drive signal for turning off the switching elements 1b and 1c is input from a gate drive on / off signal generating portion 41 described later. A shut-off signal is output to the soft shut-off parts 32b and 32c in order to shut off the elements 1b and 1c.
The soft blocking portions 32a to 32d have a function of blocking at a plurality of blocking speeds when blocking the switching elements 1a to 1d.

FIG. 3 is a diagram for explaining the soft cutoff function.
The soft shut-off function attenuates a shut-off signal that shuts off the switching elements 1a to 1d at a normal shut-off speed X until reaching the element shut-off start level Y point, and is slower than the normal shut-off speed X after the shut-off start level. Attenuating at a proper shutoff speed Z. In this way, two cutoff speeds are prepared, and the switching elements 1a to 1d are shut off by switching these cutoff speeds.
In the present embodiment, when the switching elements 1a to 1d are normally shut off, they are shut off at a constant shutoff speed X, and when the short circuit state is detected and shut off, the soft shutoff shown in FIG. 3 is performed. .

The gate signal control device 4 includes a gate drive on / off signal generator 41. The gate drive on / off signal generator 41 outputs element drive signals of the switching elements 1a to 1d to the gate drive devices 3a to 3d via the signal lines 6a to 6d.
That is, the gate drive on / off signal generator 41 outputs an element drive signal for turning on the switching elements 1a and 1b and turning off the switching elements 1c and 1d when outputting a positive potential. Further, when a zero potential is output, an element drive signal for turning off the switching elements 1a and 1b and turning on the switching elements 1c and 1d is output. Furthermore, when outputting a negative potential, an element drive signal for turning off the switching elements 1a and 1d and turning on the switching elements 1b and 1c is outputted.

In addition, when the three-level power converter 10 is normally stopped, the gate drive on / off signal generator 41 first outputs an element drive signal for turning off the outer switching elements 1a and 1d, and then switches on the inner switching. An element drive signal for turning off the elements 1b and 1c is output.
In the present embodiment, the gate drive on / off signal generator 41 is the same as the case of performing the normal stop operation described above even when a short circuit detection signal is input from any one of the short circuit detection devices 31a to 31d via the signal line 7. The element drive signal is output.

In FIG. 2, reference numerals 5a and 5b denote clamp diodes. The first clamp diode 5a has an anode side connected to the intermediate terminal O and a cathode side connected to a connection point between the switching elements 1a and 1b. The second clamp diode 5b has an anode side connected to a connection point between the switching elements 1c and 1d and a cathode side connected to the intermediate terminal O.
In FIG. 2, the switching elements 1a to 1d correspond to the first to fourth switching elements, the gate signal control device 4 corresponds to the control means, and the gate drive devices 3a to 3d respectively correspond to the first to fourth drives. Corresponding to the means, the short-circuit detection devices 31a to 31d respectively correspond to the short-circuit detection means, and the signal line 7 corresponds to the transmission means.

(Operation)
Next, the operation of this embodiment will be described.
FIG. 4 is a timing chart for explaining the operation of the present embodiment.
Here, the case where the output of the power converter device 10 is short-circuited in a state where the switching elements 1a and 1b are conducting will be described.
First, the case where the short circuit detection device 31a of the gate drive device 3a detects a short circuit state will be described. In this case, the short-circuit detection device 31a outputs a short-circuit detection signal to the gate drive on / off signal generator 41 and outputs a cutoff signal to the soft cutoff unit 32a. At the same time, the gate of the switching element 1a is connected to the signal line 6a so that the normal element drive signal input from the gate drive on / off signal generator 41 is not received. Switch to the connection.

  That is, when the short-circuit detection device 31a detects a short-circuit state at time t1 in FIG. 4, the short-circuit detection device 31a outputs a cutoff signal to the soft cutoff unit 32a. Thereby, the switching element 1a is interrupted at a moderate interrupting speed by the soft interrupting function (time t2). At time t1, the short circuit detection signal is turned on indicating that a short circuit state is detected, and this short circuit detection signal is input to the gate drive on / off signal generator 41 via the signal line 7.

  The gate drive on / off signal generation unit 41 inputs the on-state short-circuit detection signal, and at time t3, first, the element drive signals of the outer switching elements 1a and 1d are turned off. At this time, the element drive signals of the switching elements 1b and 1c are complementarily turned on. These element drive signals are output to the gate drive devices 3a to 3d via the signal lines 6a to 6d. At this time, the switching element 1a has already been turned off by the soft cutoff function, and the switching element 1d is originally in the off state. Therefore, the switching operation of the switching elements 1a and 1d is not performed by the element drive signal output at this time.

  At a time t4, which is a predetermined time after the time t3, the gate drive on / off signal generator 41 turns off the element drive signals of the switching elements 1b and 1c, and outputs them to the gate drive device via the signal lines 6b and 6c. Output to 3b and 3c. As a result, the switching elements 1b and 1c perform a blocking operation. At this time, the switching elements 1b and 1c are softly cut off by the soft cutoff function of the soft cutoff units 32b and 32c.

  Thus, when a short circuit state is detected by the short circuit detection device 31a, the gate drive device 3a is self-supporting and immediately shuts off the switching element 1a. On the other hand, the inner switching element 1b is blocked after receiving an element drive signal for turning off the switching element 1b from the gate drive on / off signal generator 41 (element blocking mask). When the short circuit state is detected by any of the short circuit detection devices 31a to 31d, the gate drive on / off signal generator 41 first shuts off the outer switching elements 1a and 1d and then shuts off the inner switching elements 1b and 1c. Since the element driving signal is output, the switching elements 1a to 1d can be surely shut off in the shut-off order so as not to be the prohibition condition at the time of short circuit detection.

Next, a case where the short-circuit detection device 31b of the gate drive device 3b detects a short-circuit state in a state where the switching elements 1a and 1b are conductive will be described.
FIG. 5 is a timing chart for explaining the operation of the present embodiment.
When the short circuit detection device 31b detects the short circuit state at time t11 in FIG. 5, the short circuit detection device 31b outputs a cutoff signal to the cutoff mask portion 33b. Thereby, the cutoff signal is masked by the cutoff mask part 33b, and at this time, the switching element 1b is not immediately cut off. At time t11, the short circuit detection signal is turned on indicating that a short circuit state is detected, and the short circuit detection signal is input to the gate drive on / off signal generator 41 via the signal line 7.

  The gate drive on / off signal generation unit 41 inputs the on-state short-circuit detection signal, and at time t12, first, the element drive signals of the outer switching elements 1a and 1d are turned off. At this time, the element drive signals of the switching elements 1b and 1c are complementarily turned on. These element drive signals are output to the gate drive devices 3a to 3d via the signal lines 6a to 6d. Thereby, the switching element 1a performs a cutoff operation. At this time, the switching element 1a is softly cut off by the soft cutoff function of the soft cutoff unit 32a.

  At a time t13, which is a predetermined time after the time t12, the gate drive on / off signal generator 41 turns off the element drive signals of the switching elements 1b and 1c and outputs them to the gate drive device via the signal lines 6b and 6c. Output to 3b and 3c. Thereby, the interruption | blocking mask part 33b cancels | releases a mask state, and performs the interruption | blocking operation | movement of the switching element 1b. That is, at time t13, the switching operation between the switching elements 1b and 1c is performed. At this time, the switching elements 1b and 1c are softly cut off by the soft cutoff function of the soft cutoff units 32b and 32c.

In this way, the gate driving device 3b is provided with the cutoff mask portion 33b to mask the cutoff signal. Therefore, when the short-circuit detection device 31b detects a short-circuit state, the switching element 1b is not immediately shut off. it can.
Therefore, when a short circuit occurs in the output of the power conversion device 10, the outer switching elements 1a and 1d are first shut off, and then the inner side is detected regardless of which short circuit detection device detects the short circuit state. The switching elements 1b and 1c can be cut off, and the prohibition condition can be prevented when a short circuit is detected.

Further, when the short-circuit detection devices 31a and 31d detect the short-circuit state, the gate drive devices 3a and 3d self-support and immediately perform the shut-off operation of the switching elements 1a and 1d. The time until protection can be shortened.
Furthermore, when the short circuit state is detected, the shut-off operation of the switching elements 1a to 1d can be soft shut-off.
When a short circuit is detected, an excessive current flows through the switching element. If the switching element is cut off as in normal operation, a high voltage is applied to the switching element instantaneously, and the switching element is stressed. . In this embodiment, since the interruption | blocking operation | movement of switching element 1a-1d is made into soft interruption | blocking at the time of short circuit detection, a switching element can be interrupted | blocked, without applying a stress.

(effect)
In the above embodiment, when a short circuit is detected in any one of the gate drive devices of each switching element, the gate signal control apparatus that has received the short circuit detection signal first turns off the outer switching element and then switches the inner switching element. An element driving signal that turns off the element is output. As described above, when a short circuit is detected, an element drive signal for performing the above-described blocking operation is output regardless of the drive state or short circuit state of the switching element. Therefore, it is not necessary to provide a logic circuit for determining the interruption order when a short circuit is detected. Further, the signal lines for transmitting the short circuit detection signal from the gate driving device to the gate signal control device can be combined into one. Therefore, simplification of control and miniaturization of the circuit can be realized.

In addition, when a short circuit is detected by the gate driving device of the outer switching element, the switching element is shut off by the gate driving device so that the switching element can be shut off immediately after the short circuit is detected.
Furthermore, when a short circuit is detected by the gate driving device of the inner switching element, the element driving signal for turning off the switching element from the gate signal control device without immediately shutting off the switching element by the gate driving device. Therefore, it is possible to reliably prevent a prohibition condition from being detected when a short circuit is detected.

Further, when the switching element is shut off at the time of detecting the short circuit, the soft shut-off which makes the shut-off speed gentler than that in the normal operation is performed, so that the switching element can be shut off without applying stress. Furthermore, at the time of soft shut-off, the shut-off signal is attenuated at the normal shut-off speed up to the shut-off start level of the switching element, and then the switching element is shut off by being attenuated at a shut-off speed slower than the normal shut-off speed. While shortening the time until the element is shut off, it is possible to prevent stress on the switching element at the time of shutting down.
(Modification)
In the above embodiment, the case where the present invention is applied to a three-level power converter that outputs a three-phase AC voltage has been described. However, the present invention is not limited to three phases.

  DESCRIPTION OF SYMBOLS 1a-1d ... Switching element, 2a-2d ... Freewheeling diode, 3a-3d ... Gate drive device, 4 ... Gate signal control device, 5a, 5b ... Clamp diode, 6a-6d ... Signal line (for ignition signal), 7 ... Signal line (for short-circuit detection signal), 10 ... 3-level power converter, 11 ... DC power supply, 12 ... first smoothing capacitor, 13 ... second smoothing capacitor, 14U-14W ... arm bridge, 15U-15W ... Output line, P ... positive terminal, N ... negative terminal, O ... intermediate terminal

Claims (3)

A first smoothing capacitor connected between the positive terminal and the intermediate terminal of the DC power supply;
A second smoothing capacitor connected between the intermediate terminal and the negative terminal of the DC power source;
First to fourth switching elements sequentially connected in series between the positive terminal and the negative terminal;
First to fourth freewheeling diodes connected in parallel to the first to fourth switching elements, respectively.
A first clamp diode having an anode side connected to the intermediate terminal and a cathode side connected to a connection point between the first switching element and the second switching element;
A second clamp diode having an anode side connected to a connection point between the third switching element and the fourth switching element, and a cathode side connected to the intermediate terminal;
A protection device for a three-level power converter that obtains a three-level voltage output by connecting a connection point between the second switching element and the third switching element to an AC output terminal,
First to fourth driving means for controlling on / off of the first to fourth switching elements, respectively;
Control means for outputting a control signal for performing on / off control of the first to fourth switching elements to the first to fourth driving means,
The first to fourth driving means include a short-circuit detecting means for detecting a short-circuit of a switching element that is controlled to turn on and off, and a transmission means for transmitting to the control means that a short-circuit is detected by the short-circuit detecting means. Each has
When it is transmitted from any of the transmission means that the short circuit has been detected, the control means first turns off the first and fourth switching elements, and then turns off the second and third switching elements. Output a control signal that
When the first and fourth driving means detect a short circuit by their own short-circuit detection means, the switching elements that are immediately turned on and off independently of the control signal from the control means are turned off,
When the second and third driving means detect a short circuit by their own short-circuit detecting means, the switching means receives a control signal for turning off the switching element that is controlled to turn on and off from the control means. 3 is a protection device for a three-level power converter.   In the normal operation in which the short circuit detection unit does not detect a short circuit, the first to fourth drive units turn off the switching element at a preset normal cutoff speed, and the short circuit detection unit detects the short circuit. 2. The protection device for a three-level power converter according to claim 1, wherein when the short circuit is detected, the switching element is turned off at a cutoff speed slower than the normal cutoff speed.   When the first to fourth driving means turn off the switching element at the time of detecting the short circuit, the first to fourth driving means send a cutoff signal for turning off the switching element to the normal start level of the switching element. 3. The protection device for a three-level power converter according to claim 2, wherein the switching element is turned off by being attenuated at a cutoff speed and then attenuated at a cutoff speed slower than the normal cutoff speed.

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