JP2725950B2 - Optical thyristor valve protection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protection device for preventing breakage of an optical thyristor due to a partial commutation failure of an optical thyristor valve in which a number of optical thyristors are connected in series.

[0002]

2. Description of the Related Art FIG. 6 is a partial block diagram of a conventional protection device for an optical thyristor valve disclosed in, for example, Hitachi Review, Vol. 67, No. 6, page 10 published in June 1985. It is a circuit diagram. In the figure, a number of optical thyristors 1
Are connected in series to form an optical thyristor valve. A snubber circuit in which a capacitor 2 and a resistor 3 are connected in series is connected between the anode A and the cathode K of each optical thyristor 1. In order to detect a forward voltage, for example, an LED 6 is connected in series with resistors 4 and 5 connected in series between both ends of the snubber circuit.
Is connected, and a zener diode 7 is connected between the connection point between the resistors 4 and 5 and the cathode K so that the current of the LED 6 does not become larger than a predetermined value. Some optical thyristors
In FIG. 1, a reverse voltage detecting element including a resistor 8, an LED 9, and a zener diode 10 is connected between the anode A and the above-described forward voltage detecting elements 4 to 7 for detecting a reverse voltage. The light output of each LED 6, 9 is a light guide (L
G) to the forward voltage detector (FVD) 30 and the reverse voltage detector (IVD) 31 respectively. Forward voltage detector 3
0 and the electric output of the reverse voltage detector 31 are input to the ignition control protection device 12 together with the ignition command from the control circuit 11. The ignition control protection device 12 includes an LED (not shown) for igniting the optical thyristor 1, and its light output is sent to each optical thyristor 1 by a light guide (LG). Are fired simultaneously. The above-described forward voltage detecting elements 4 to 7 constitute forward voltage detecting means together with the forward voltage detector 30, and the reverse voltage detecting elements 8 to 10 also constitute reverse voltage detecting means together with the reverse voltage detector 31.

[0003] The reverse voltage immediately after commutation is the reverse voltage detector 30.
When the reverse voltage time is insufficient, the ignition control and protection device 12 functions to protect the optical thyristor 1 from partial commutation failure due to insufficient commutation allowance angle. Optical output is sent to each optical thyristor 1,
All optical thyristors 1 conduct and commutation fails,
The optical thyristor 1 is protected.

If the reverse voltage time immediately after commutation is sufficient and the optical thyristor 1 is sufficient to turn off,
No forced firing is performed.

[0005]

Even the reverse voltage time immediately after commutation of the optical thyristor voltage V _AK 7 [0007] was sufficient, for example, at time t _3, it contains the surge voltage, the voltage rise When a voltage having a high rate is applied, the optical thyristor may be conductive because carriers remain.
When the junction temperature of the optical thyristor is high after the overcurrent is applied, the set predetermined reverse voltage time is not sufficient, and self-ignition occurs in a forward voltage applied state,
In some cases, re-conduction occurs with the application of a forward voltage. In such a case, the characteristics of the optical thyristor vary, and some of the optical thyristors are conductive and some are not conductive, resulting in variations in the conduction of the optical thyristor. In such a case, there is a problem that the entire voltage is applied to the non-conductive optical thyristor, and if this voltage exceeds the withstand voltage, the optical thyristor is damaged.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem. When the voltage applied to the optical thyristor is equal to or higher than a predetermined forward voltage increasing rate, or when the voltage applied to the optical thyristor is increased. It is an object of the present invention to provide an optical thyristor valve protection device that does not damage an optical thyristor even when a directional voltage is no longer detected due to self-firing of the optical thyristor or when an overvoltage is applied to the optical thyristor.

[0007]

According to a first aspect of the present invention, there is provided a protection device according to the first aspect of the present invention, wherein the reverse voltage time immediately after commutation is longer than a first predetermined time and the first predetermined time is applied from the start of reverse voltage application. If a voltage equal to or higher than a predetermined forward voltage increase rate is applied to the optical thyristor until the second predetermined time longer than the predetermined time, or the forward voltage applied to the optical thyristor is changed to the self-point of the optical thyristor. Means for stopping the detection output of the forward voltage detecting means of the optical thyristor when detection is stopped by arcing or when an overvoltage is applied to the optical thyristor, and forcing the optical thyristor to fire in response to the stopping means And means for causing this to occur.

The protection device according to a second aspect of the present invention is configured such that the reverse voltage time immediately after the commutation is longer than the first predetermined time and the second voltage is longer than the first predetermined time from the start of applying the reverse voltage. Means for stopping the detection output of the forward voltage detecting means of the optical thyristor when an overvoltage is applied to the optical thyristor during a predetermined time, and means for forcibly firing the optical thyristor in response to the stopping means And an overvoltage suppressing means connected in parallel to the optical thyristor for suppressing an overvoltage applied to the optical thyristor.

A third aspect of the present invention provides the protection device, wherein the reverse voltage time immediately after the commutation is longer than the first predetermined time and the second voltage is longer than the first predetermined time from the start of the reverse voltage application. When a voltage equal to or higher than a predetermined forward voltage increase rate is applied to at least one of the plurality of optical thyristors, or is applied to at least one of the plurality of optical thyristors up to a predetermined time. The forward voltage detection means of the optical thyristor when the detected forward voltage is no longer detected due to self-ignition of the optical thyristor, or when an overvoltage is applied to at least one of the plurality of optical thyristors. Means for stopping the detection output, and in response to the stopping means, all of the plurality of optical thyristors are sequentially forced as soon as the forward voltage of each optical thyristor is detected. It is provided with a means for arc.

[0010]

In the protection device according to the first aspect of the present invention, when a voltage higher than a predetermined forward voltage increase rate is applied to the optical thyristor by means for stopping the detection output of the forward voltage detecting means of the optical thyristor, or When the forward voltage applied to the optical thyristor is no longer detected due to the self-ignition of the optical thyristor, or when an overvoltage is applied to the optical thyristor, it is detected, and the means for forcibly firing the optical thyristor is stopped. , The optical thyristor is forcibly fired to prevent a voltage higher than the breakdown voltage from being applied to the optical thyristor.

[0011] In the protection device according to claim 2 of the present invention,
The case where an overvoltage is applied to the optical thyristor is detected by the means for stopping the detection output of the forward voltage detecting means of the optical thyristor, and the optical thyristor is forcibly fired in response to the stopping means by the means for forcibly firing the optical thyristor. The overvoltage suppressor suppresses the overvoltage applied to the optical thyristor so that a voltage higher than the withstand voltage is not applied to the optical thyristor.

In the protection device according to claim 3 of the present invention,
When a voltage equal to or higher than a predetermined forward voltage increase rate is applied to the optical thyristor by means for stopping the detection output of the forward voltage detecting means of the optical thyristor, or when the forward voltage applied to the optical thyristor is applied to the optical thyristor, When the detection is stopped by the self-firing or the case where an overvoltage is applied to the optical thyristor, all the optical thyristors are moved in the forward direction of each optical thyristor in response to the stopping means by the means for forcibly firing the optical thyristor. As soon as the voltage is detected, the forcible ignition is sequentially performed so that a voltage higher than the withstand voltage is not applied to the optical thyristor.

[0013]

[Embodiment 1] Hereinafter, Embodiment 1 of the present invention will be described with reference to FIG.
Will be described. In the figure, 1 to 11 are the same as those described with reference to FIG. Anode of each optical thyristor 1
A capacitor 13 and a resistor 14 are connected in series between A and the cathode K, and the connection point is connected to the base of a transistor 17 via a diode 15 and a zener diode 16. The collector and the emitter of the transistor 17 are connected in parallel with the Zener diode 7. These elements 13 to 17 constitute means for stopping the detection output of the forward voltage detection elements 4 to 7.

The ignition control protection device 12 includes a NOT circuit 12
a, differentiating circuit 12b, diode 12c, AND circuit 1
2d, an ignition device 12e, a margin angle insufficiency determiner 12f, and a one-shot multivibrator 12g.

The output side of the forward voltage detector 30 is a NOT circuit 1
2a, its inverted output is differentiated by a differentiating circuit 12b, and only a positive output is input to one input terminal of an AND circuit 12d by a diode 12c.

The output side of the reverse voltage detector 31 is provided with a margin angle shortage judging unit 12f and a one-shot multivibrator 12
g, and the judgment output of the margin angle insufficiency judging unit 12f is
It is input to the ignition device 12e of the optical thyristor 1. Further, the pulse output of the one-shot multivibrator 12g is input to the other input terminal of the AND circuit 12d, and A
The output of the ND circuit 12d is input to the ignition device 12e.

The output of the control circuit 11 is input to an ignition device 12e. During normal operation, the output of the control circuit 11 activates the ignition device 12e, and writes an optical output for ignition to each optical thyristor 1. The optical thyristor 1 is fed through a guide (LG) to fire the optical thyristor 1.

Next, the operation of the first embodiment will be described with reference to the waveform diagram of FIG. At time t ₁ immediately after the commutation, the reverse voltage starts to be applied to the optical thyristor valve, and continues to be applied until time t ₂ . In this embodiment, the reverse voltage time is the first predetermined time T ₁ during which the optical thyristor 1 can recover the forward blocking ability.
It is assumed that the forward voltage starts to be applied from time t ₂ , and a surge voltage is applied at time t _3, for example, and the forward voltage rise rate (hereinafter dv / dt) equal to or higher than the power supply commutation voltage.
) Is applied.

When the output of the reverse voltage detector 31 is input,
Margin angle shortage determination unit 12f is reverse voltage period from time t ₁ to time t ₂ because the first longer than the predetermined time T _1, without determining insufficient margin angle, thus put out an output signal to the ignition device 12e Absent. One-shot multivibrator 12g at the beginning the reverse voltage is applied at time t ₁ and outputs the second pulse of a predetermined time T ₂ to the AND circuit 12d. When the forward voltage starts to be detected at time t _{2 and} the output of the forward voltage detector 30 is input to the ignition control protection device 12, the inverter 12a outputs an inverted output of the input, and the differentiating circuit 12b outputs the inverted output. The output is differentiated, the diode 12c cuts the negative output, and inputs only the positive output to the AND circuit 12d.

The turn-off time for the optical thyristor 1 to recover the forward blocking ability varies depending on the peak value of the conduction current, the element junction temperature, and the dv / dt at the time of the forward recovery. First time from the reverse voltage time that can be turned off by dv / dt during normal operation
The predetermined time T ₁ is set. Normal dv / dt at time t ₃
When the above voltage is applied, the optical thyristor 1 in the turn-off process may not be able to recover the forward blocking ability due to re-conduction of a part of the optical thyristor 1 due to variation in element characteristics. Time t
₃ is a delay, if any, for example, about 1ms time from t ₁ to t _3, the optical thyristor 1 can sufficiently recover the forward blocking capability. The second predetermined time T ₂ are set to, for example, about 1ms as time to recover the forward blocking capability.

The time t _{3 in} FIG. 2 is within a time shorter than the second predetermined time T ₂ . When a large dv / dt is applied to the optical thyristor 1, the dv / dt is applied to the capacitor 13 and the resistor 14 in FIG.
And the voltage applied to the resistor 14 increases,
When this voltage becomes equal to or higher than the zener voltage of the zener diode 16, a current flows between the base and the emitter of the diode 15, the zener diode 16, and the transistor 17, the transistor 17 is turned on, and the current of the LED 6 is bypassed and stops flowing. The optical output of the forward voltage detection stops, and the output of the forward voltage detector 30 stops. In response to this, inverter 1
2a generates an output, which is differentiated by a differentiating circuit 12b, and a diode 12c outputs a positive pulse output. A
Since the output of the ND circuit 12d outputs the one-shot multivibrator 12g and the diode 12c is generated when the positive and firing command to the ignition device 12e from the AND circuit 12d at time t ₃ when the forward voltage detection has been stopped Will be issued. The optical output of the ignition device 12e is sent to each optical thyristor 1 via a light guide (LG), and the non-conductive optical thyristor is also made conductive so that a voltage higher than the breakdown voltage is not applied to the non-conductive optical thyristor. Thus, the optical thyristor can be prevented from being damaged.

In the first embodiment, the detection signal is obtained by detecting the dv / dt of the voltage applied to the optical thyristor using the capacitor 12 and the resistor 14. However, the differential function is obtained by using the operational amplifier. Dv / dt is detected, and when this output becomes larger than a predetermined value, the transistor is turned on,
By stopping the emission of the forward voltage detection LED, it is possible to detect that a dv / dt of a predetermined value or more is applied.
Since the recovery of the forward blocking capability of the optical thyristor also depends on the junction temperature and the conduction current, the allowable dv / dt value is changed according to the junction temperature and the conduction current value, and the forced thyristor is protected. The firing frequency can also be reduced.

Embodiment 2 FIG. Embodiment 2 of the present invention will be described with reference to FIG. In the figure, 1 to 11, 12b to 12
d, 12f, 12g, 14 to 17, and 31 are the same as those described with reference to FIG. In the second embodiment, the capacitor 2
A capacitor 18 and a resistor 19 are connected in series between the connection point of the resistor 3 and the cathode K of the optical thyristor 1, and between a connection point of the capacitor 18 and the resistor 19 and a connection point of the diode 15 and the zener diode 16. Is connected to a diode 20. The anode A of the optical thyristor 1 and the resistor 1
4 is connected to a resistor 13A. Note that these elements 13A to 13 constitute means for stopping the detection output of the forward voltage detection elements 4 to 7.

The ignition control protection device 12A is different from the circuit of FIG. 1 in that the one-shot multivibrator 1 is further connected to the AND circuit 12d and the output side of the margin angle insufficiency determiner 12f.
2h, an OR circuit 12i in which the one-shot multivibrator 12h and the output side of the control circuit 11 are connected, and OR
An AND circuit 12j to which the circuit 12i and one output side of a forward voltage detector 30A described later are connected is provided. The outputs of the AND circuit 12d and the AND circuit 12j are input to the ignition device 12e-1, and the differentiation circuit 12b Has a forward voltage detector 3
The other output side of 0A is connected. The ignition control protection device 12A constitutes means for sequentially forcibly firing the optical thyristor as soon as the forward voltage of the optical thyristor is detected.

FIG. 4 is a block diagram showing the configuration of the forward voltage detector 30A according to the second embodiment. The forward voltage detector 30A includes a plurality of phototransistors 21 each of which is connected to a power supply, each emitter is grounded via a resistor 22, and detects light of each LED 6 transmitted by a plurality of light guides LG.
A plurality of amplifiers 24 and NOT circuits 25 sequentially connected to the emitters of the respective phototransistors 21; an OR circuit 26 connected to the output side of the plurality of NOT circuits 25 and outputting an output F1; And an OR circuit 27 that is connected to the output side and outputs the output F2. In the forward voltage detector 30A, light sent from each LED 6 via the light guide LG is converted into a photocurrent by the phototransistor 21, and this photocurrent flows through the resistor 22 to the ground 23, thereby Is generated, and it can be detected that the forward voltage is applied to the optical thyristor 1.

The output of the control circuit 11 is input to the OR circuit 12i. During normal operation, the ignition device 12e-1 operates by the output from the control circuit 11, and the light output for ignition is supplied to each optical thyristor 1. The light is transmitted through a light guide (LG), and the optical thyristor 1 is fired.

Next, the operation of the second embodiment will be described with reference to the waveform diagram of FIG. At time t ₁ immediately after the commutation, the reverse voltage starts to be applied to the optical thyristor valve, and continues to be applied until time t ₂ . In this embodiment, the reverse voltage time is the first predetermined time T ₁ during which the optical thyristor 1 can recover the forward blocking ability.
It assumes the case where more and more time t ₂ is the forward voltage has begun to be applied.

[0028] When the reverse voltage detector 31 signal is output, margin angle shortage determination unit 12f is reverse voltage period from time t ₁ to time t ₂ because the first longer than the predetermined time T _1, margin angle It does not determine that it is insufficient, and therefore does not output a signal to the ignition device 12e-1. At the start of application of the reverse voltage at time t ₁ , the one-shot multivibrator 12g operates for the second predetermined time T ₂
Is output to the AND circuit 12d.

First, as shown by V _AK , ₁ , for example, a surge voltage is applied at time t ₃ , and the rate of increase of this applied voltage (d
v / dt) is greater than the predetermined value, the predetermined time T ₁
It can be assumed that the forward blocking ability cannot be restored in a slightly longer time. When a surge voltage is applied, a current flows through the capacitors 2 and 18, and a voltage applied to the resistor 19 increases. This voltage becomes equal to or higher than the zener voltage of the zener diode 16, and the diodes 21 and 16
, A base current flows through the transistor 17, the transistor 17 conducts, and no current flows through the LED 6,
The detection of the forward voltage is stopped at time t ₃ as shown in the output F3.

Further, assuming a state after an overcurrent flows through the optical thyristor 1, the state is such that the temperature of the junction is rising. Shortly before time t ₃ , the forward blocking capability cannot be restored in a reverse voltage time slightly longer than the first predetermined time T ₁ due to a rise in the temperature of the junction, and an element that re-energizes due to self-ignition may occur. Appears and an overvoltage is applied to the element having recovered the forward blocking ability. V _{AK, 2} indicates the overvoltage, the voltage applied to the resistor 14 by an overvoltage rises at time t ₃ equal to or greater than the Zener voltage of the Zener diode 16, diode 15, Zener diode 1
6. The transistor 17 is turned on, the current stops flowing to the LED 6, and the detection output of the forward voltage is stopped so as to indicate the output of the amplifier 24 as F3.

Similarly, when the temperature of the junction rises due to the overcurrent flowing through the optical thyristor 1 and the forward blocking ability cannot be recovered in a time slightly longer than the predetermined time T ₁ , the time t ₃ Then, the self-ignition occurs as shown by V _AK , ₃ and the forward voltage disappears, the current does not flow through the LED 6, and the detection output of the forward voltage stops as shown by F3.

As described above, when a voltage higher than a predetermined applied voltage increase rate is applied, when an overvoltage is applied, or when a forward voltage is no longer detected due to self-ignition, a forward voltage detection output is output. Stopped. Note that in either case here, at time t ₃ the detection of the forward voltage has been described as being stopped, this is merely for convenience of explanation, those actually to mention separately generated .

Next, even though the detection output of the forward voltage of the light thyristor in as shown in F3 is stopped at time t _3, as shown in F4, still forward voltage is applied to another optical thyristor May be. The output indicated by F1 is the inverted output of the output F3 and is input to the differentiating circuit 12c, and the output is as shown by 12b in FIG. As for this output, only the positive output is made effective by the diode 12b,
It becomes like c, and is inputted to one input terminal of the AND circuit 12d. The output of the one-shot multivibrator 12g is input to the other input terminal of the AND circuit 12d, and the output of the AND circuit 12d is as shown by 12d in FIG. The output 12d is directly input to the firing device 12e-1, which applies a light input for firing to each optical thyristor 1 and forcibly causes the optical thyristor 1 to fire. The thyristor 1 is prevented from being broken due to variation in conduction.

The output of the AND circuit 12d operates the one-shot multivibrator 12h, and the output of the one-shot multivibrator 12h is input to the OR circuit 12i. The output of the OR circuit 12i becomes positive and becomes A
This is an input of the ND circuit 12j. A positive signal is input to one input terminal of the AND circuit 12j, and is input to the ignition device 12e-1 simultaneously with detection of the forward voltage, and the optical input is applied again to each optical thyristor 1 to indicate the forward voltage. The optical thyristor 1 is forcibly fired. This is because, as described above, the optical thyristor which is not yet in the forward voltage state even if the optical input for forcibly firing the optical thyristor 1 is applied by directly operating the ignition device 12e-1 by the AND circuit 12d. 1
Cannot be fired, and it is necessary to apply an optical input only when a forward voltage appears.

Embodiment 3 FIG. In the embodiment described above, as means for stopping the detection output of the forward voltage, the forward voltage detection LE is used.
D6 and a capacitor 18 to a diode 20 are provided.
The current of ED6 was bypassed.
The current of No. 6 may be cut off using a switch element.

Embodiment 4 FIG. After detecting the overvoltage, the detection output of the forward voltage detection means is stopped, and the ignition device 12e-
Since the operation of the optical thyristor 1 requires time, there is a risk that the overvoltage exceeds the withstand voltage of the optical thyristor 1 during that time. Therefore, as shown by an imaginary line in FIG. 3, an overvoltage suppression element 32 of zinc oxide, which is a nonlinear resistor, is connected in parallel with the optical thyristor 1 so that the overvoltage is suppressed for a short time.
By firing, the optical thyristor 1 can be protected.

In order to distinguish whether the forward voltage is no longer detected due to the self-ignition of the optical thyristor 1 or the forward voltage is no longer detected due to a power supply voltage drop due to a system fault, the time at which the power supply voltage drops is determined. When the detection stop timings of the forward voltage overlap, it is also possible not to determine the self-ignition.

[0038]

As described above, the present invention relates to the case where the reverse voltage time immediately after commutation is longer than the first predetermined time, and the second predetermined time longer than the first predetermined time from the start of the reverse voltage application. Until the time, when a voltage equal to or higher than a predetermined forward voltage increase rate is applied to the optical thyristor, or the forward voltage applied to the optical thyristor is no longer detected by the self-ignition of the optical thyristor In the case, or when an overvoltage is applied to the optical thyristor, means for stopping the detection output of the forward voltage detecting means of the optical thyristor, and means for forcibly firing the optical thyristor in response to the stopping means are provided. Therefore, the reverse voltage time immediately after commutation is the first
Between the time when the reverse voltage is applied and the second time longer than the first time,
Means for stopping the detection output of the forward voltage detection means of the optical thyristor when an overvoltage is applied to the optical thyristor,
A means for forcibly firing the optical thyristor in response to the stopping means and an overvoltage suppressing element connected in parallel to the optical thyristor for suppressing an overvoltage applied to the optical thyristor are provided. Reverse voltage time is the first
Between the time when the reverse voltage is applied and the second time longer than the first time,
When a voltage higher than a predetermined forward voltage rise rate is applied to at least one optical thyristor of the plurality of optical thyristors, or when the forward voltage applied to at least one optical thyristor of the plurality of optical thyristors is higher than the predetermined voltage. Means for stopping the detection output of the forward voltage detecting means of the optical thyristor when the detection is stopped by the self-ignition of the optical thyristor or when an overvoltage is applied to at least one of the plurality of optical thyristors. And means for forcibly firing all of the plurality of optical thyristors in response to the stopping means, so that when a voltage higher than a predetermined forward voltage increase rate is applied to the optical thyristor, or When the forward voltage applied to the optical thyristor is no longer detected due to self-ignition of the optical thyristor, or The effect that overvoltage is applied to the lister can be detected using the forward voltage detection path, so that a new detection signal system becomes unnecessary, and in these cases, a protection device that does not damage the optical thyristor can be obtained. Play.

[Brief description of the drawings]

FIG. 1 is a circuit diagram partially showing a first embodiment of the present invention in a block diagram.

FIG. 2 is an operation waveform diagram of each unit for explaining the operation of the first embodiment of the present invention.

FIG. 3 is a circuit diagram partially showing a block diagram of a second embodiment of the present invention;

FIG. 4 is a circuit diagram showing a forward voltage detector according to Embodiment 2 of the present invention.

FIG. 5 is an operation waveform diagram of each section for explaining the operation of the second embodiment of the present invention.

FIG. 6 is a circuit diagram partially showing a block diagram of a conventional protection device for an optical thyristor valve.

FIG. 7 is an operation waveform diagram for explaining an operation of the conventional protection device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical thyristor 5, 14, 19 Resistance 6, 9 LED 7, 10, 16 Zener diode 13, 18 Capacitor 15, 20 Diode 12, 12A Ignition control protection device 17 Transistor 30, 30A Forward voltage detector 31 Reverse voltage detection Container 32 Overvoltage suppression element

Claims (3)

(57) [Claims] 1. An optical thyristor valve comprising a plurality of optical thyristors connected in series, comprising means for detecting a forward voltage and a reverse voltage of the optical thyristor, wherein a reverse voltage time immediately after commutation is a first predetermined time. If the time is shorter than the first time, the reverse voltage time immediately after the commutation is longer than the first predetermined time. And when a voltage equal to or higher than a predetermined forward voltage increase rate is applied to the optical thyristor during a period from the start of applying the reverse voltage to a second predetermined time longer than the first predetermined time, or Forward voltage detecting means for the optical thyristor when the forward voltage applied to the thyristor is no longer detected due to self-ignition of the optical thyristor, or when an overvoltage is applied to the optical thyristor And a means for forcibly firing the optical thyristor in response to the stopping means. 2. An optical thyristor valve comprising a plurality of optical thyristors connected in series, comprising means for detecting a forward voltage and a reverse voltage of the optical thyristor, wherein a reverse voltage time immediately after commutation is a first predetermined time. If the time is shorter than the first time, the reverse voltage time immediately after the commutation is longer than the first predetermined time. And when an overvoltage is applied to the optical thyristor from the start of the reverse voltage application to a second predetermined time longer than the first predetermined time, the detection output of the forward voltage detection means of the optical thyristor is changed. Means for stopping, means for forcibly firing the optical thyristor in response to the stopping means, and an overvoltage suppressing means connected in parallel to the optical thyristor for suppressing an overvoltage applied to the optical thyristor. A protection device for an optical thyristor valve, comprising a step. 3. An optical thyristor valve comprising a plurality of optical thyristors connected in series, comprising means for detecting a forward voltage and a reverse voltage of the optical thyristor, wherein a reverse voltage time immediately after commutation is a first predetermined time. If the time is shorter than the first time, the reverse voltage time immediately after the commutation is longer than the first predetermined time. And during a period from the start of application of the reverse voltage to a second predetermined time longer than the first predetermined time, the voltage applied to at least one of the plurality of optical thyristors is equal to or higher than a predetermined forward voltage increase rate. Is applied, or when the forward voltage applied to at least one of the plurality of optical thyristors is no longer detected by the self-ignition of the optical thyristor, Means for stopping a detection output of a forward voltage detecting means of the optical thyristor when an overvoltage is applied to at least one of the plurality of optical thyristors, and the plurality of optical thyristors in response to the stopping means Means for sequentially forcibly firing all of the optical thyristors as soon as the forward voltage of each optical thyristor is detected.