Thyristor forced commutation circuit

Abstract

To provide a forced commutation circuit for a thyristor which is configurable without using a large-sized capacitor and a large-sized reactor.SOLUTION: The present invention relates to a forced commutation circuit 100 which is connected in parallel with a thyristor. The forced commutation circuit includes a voltage source 300 and a self-arc extinguishing element Q1 which are connected in series. The voltage source 300 includes a capacitor which is charged by predetermined voltage. Voltage of the voltage source 300 is greater than voltage obtained by subtracting a forward drop voltage of a thyristor Th1 from a forward drop voltage of the forced commutation circuit 100, and voltage resistance of the capacitor included in the voltage source 300 is smaller than a maximum voltage applied to the forced commutation circuit 100.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This invention relates to a forced commutation circuit that turns off a thyristor by commuting the current flowing through the thyristor.

Background Art

[0002] FIG. 4 is a circuit diagram showing an example of a three-phase power conversion circuit. This power conversion circuit consists of a U-phase AC circuit 500U, a V-phase AC circuit 500V, and a W-phase AC circuit 500W. The U-phase AC circuit 500U, V-phase AC circuit 500V, and W-phase AC circuit 500W each include a pair of thyristors Th1a and Th1b connected in inverse parallel. Here, the thyristors Th1a and Th1b do not have a self-extinguishing function. Therefore, forced commutation circuits 600U, 600V, and 600W may be provided for the U-phase AC circuit 500U, V-phase AC circuit 500V, and W-phase AC circuit 500W to commute the current flowing through the thyristors Th1a and Th1b and turn off the thyristors Th1a and Th1b.

[0003] FIG. 5 is a circuit diagram showing the configuration of a forced commutation circuit 600, which is an example of this type of forced commutation circuit. Note that the forced commutation circuit 600 in FIG. 5 is a circuit for performing forced commutation of the thyristor Th1a, and the circuit for performing forced commutation of the thyristor Th1b has a similar configuration.

[0004] As shown in FIG. 5, the forced commutation circuit 600 includes a diode D0, an auxiliary thyristor Th2, a capacitor C0, and a reactor L0. Here, the diode D0 and the auxiliary thyristor Th2 are connected in series between the anode and cathode of the thyristor Th1a. Also, a series resonance circuit consisting of the capacitor C0 and the reactor L0 is connected in parallel to the auxiliary thyristor Th2.

[0005] In this configuration, the operation when thyristor Th1a is turned off while the main current Ia is flowing will be explained. When thyristor Th1a was off in the previous cycle, the circuit voltage charged capacitor C0 in the direction where the left side of the diagram is positive. In this case, first, the auxiliary thyristor Th2 is turned on. This causes the charge stored in capacitor C0 to discharge through the auxiliary thyristor Th2, and a current Ib flows through the auxiliary thyristor Th2. As a result, resonance occurs in the series resonant circuit consisting of capacitor C0 and reactor L0. Due to this resonance, capacitor C0 is charged in the reverse direction and then begins to discharge, and the polarity of current Ib is reversed. When the polarity of current Ib is reversed, the auxiliary thyristor Th2 is turned off. When the auxiliary thyristor Th2 is turned off, the resonant current of the series resonant circuit consisting of capacitor C0 and reactor L0 becomes the current Ic that flows through thyristor Th1a. This current Ic cancels out the main current Ia flowing through thyristor Th1a. As a result, thyristor Th1a is turned off. Subsequently, when the capacitor C0 is charged to a predetermined voltage by the current Ic, the current Ic stops. Such a forced commutation circuit for a thyristor is disclosed, for example, in Patent Document 1.

[0006] With the widespread use of self-extinguishing elements such as IGBTs (Insulated Gate Bipolar Transistors), circuits using thyristors are no longer used as the main switching element in power conversion circuits that perform high-frequency switching. However, because thyristors have characteristics such as low conduction loss and high surge current withstand capability, they are still used today as changeover switches where high speed is not required.

[0007] Incidentally, when using IGBTs as a changeover switch, since typical IGBTs do not have reverse voltage rating, it is necessary to connect two IGBTs in reverse series to form a changeover switch for AC applications. In that case, the forward voltage of the changeover switch will be 2V x 2 = 4V. In contrast, when using a thyristor as a changeover switch, the forward voltage will be around 1.3V.

[0008] Furthermore, while IGBTs can only handle surge currents up to twice their rated value, thyristors can handle surge currents up to about 10 times their rated value. Because thyristors have a high surge current withstand capability, they can withstand large currents that cannot be extinguished (such as short-circuit currents) until the fuse blows. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 62-12372 [Overview of the project] [Problems that the invention aims to solve]

[0010] Incidentally, in the conventional forced commutation circuit described above, in order to interrupt the main current Ia flowing through the thyristor Th1a, the currents Ib and Ic flowing through the capacitor C0 must be greater than the maximum value of the main current Ia. Furthermore, these large currents Ib and Ic must continue to flow through the capacitor C0 for the duration required to interrupt the thyristor Th1a, and the capacitor C0 must have the capacitance necessary for this. In addition, the capacitor C0 may be subjected to a voltage at least equivalent to the peak voltage of the power supply system (circuit voltage, which is the phase-to-phase voltage in the example of Figure 4), and a corresponding voltage withstand capability is required. Moreover, when a voltage is applied to the capacitor C0, resonance may occur with the reactor L0 or the wiring inductance. The peak of the resonant voltage can reach about twice the applied voltage. For this reason, the capacitor C0 becomes large.

[0011] Furthermore, the reactor L0 must have an inductance that achieves a resonant period longer than the time required to interrupt the thyristor Th1a. Also, the reactor L0 must have a current capacity greater than the maximum value of the main current Ia. Here, since the current flowing through the reactor L0 is a non-repetitive waveform, the winding cross-sectional area of ​​the reactor L0 can be smaller than the continuous rating, but the iron core must have a larger cross-sectional area to avoid saturation with respect to the peak current. Therefore, the reactor L0 becomes large.

[0012] This invention has been made in view of the circumstances described above, and aims to provide a forced commutation circuit for a thyristor that can be constructed without using large capacitors and large reactors. [Means for solving the problem]

[0013] A forced commutation circuit for a thyristor, according to one aspect of this invention, is a forced commutation circuit connected in parallel to a thyristor, and includes a series-connected voltage source and a self-extinguishing element, wherein the voltage source includes a capacitor charged to a predetermined voltage, the voltage of the voltage source is greater than the voltage obtained by subtracting the forward voltage drop of the thyristor from the forward voltage drop of the forced commutation circuit, and the withstand voltage of the capacitor included in the voltage source is less than the maximum voltage applied to the forced commutation circuit. [Effects of the Invention]

[0014] In this forced commutation circuit, the self-extinguishing element turns on, supplying the voltage from the voltage source to the thyristor via the self-extinguishing element. As a result, the thyristor's current is commutated through the path passing through the voltage source and the self-extinguishing element, causing the thyristor to turn off. After the thyristor turns off, the self-extinguishing element turns off, and the commutation operation ends. Since this forced commutation circuit does not utilize resonance, it does not require the large capacitors and large reactors that constitute a resonant circuit. [Brief explanation of the drawing]

[0015] [Figure 1] This is a circuit diagram showing the configuration of a forced commutation circuit, which is one embodiment of this invention. [Figure 2] This is a circuit diagram showing the first example of a voltage source for the forced commutation circuit. [Figure 3] This is a circuit diagram showing a second example of a voltage source for the same forced commutation circuit. [Figure 4] This is a circuit diagram showing an example of a three-phase power conversion circuit. [Figure 5] This is a circuit diagram showing an example of a conventional forced commutation circuit configuration. [Modes for carrying out the invention]

[0016] The embodiments of this disclosure will be described below with reference to the drawings. Figure 1 is a circuit diagram showing the configuration of a forced commutation circuit 100, which is one embodiment of this invention. In addition, to facilitate understanding of the forced commutation circuit 100, Figure 1 also shows the thyristor Th1 to which the forced commutation circuit 100 is applied, and the control circuit 200 that controls the forced commutation circuit 100, together with the forced commutation circuit 100.

[0017] As shown in Figure 1, the forced commutation circuit 100 is connected in parallel to the thyristor Th1. This forced commutation circuit 100 includes a voltage source 300, a reactor L, a self-extinguishing element Q1, and diodes DB, D1, and D2.

[0018] Here, the voltage source 300, the reactor L, the self - extinguishing element Q1, and the diode DB are connected in series between the anode and the cathode of the thyristor Th1. In the illustrated example, the self - extinguishing element Q1 is an IGBT (Insulated Gate Bipolar Transistor) with a diode connected in anti - parallel, the collector is connected to the reactor L, and the emitter is connected to the diode DB. The diode DB has its anode connected to the emitter of the self - extinguishing element Q1 and its cathode connected to the cathode of the thyristor Th1. This diode DB is a reverse - voltage blocking diode that prevents a reverse voltage from being applied to the self - extinguishing element Q1. Also, the reactor L serves to limit the current I2 (see FIG. 1) that flows when the self - extinguishing element Q1 is on. In this embodiment, the wiring of the path through which the current I2 flows functions as the reactor L.

[0019] Also, the diode D1 is connected in parallel to the voltage source 300. More specifically, the diode D1 has its anode connected to the anode of the thyristor Th1 and its cathode connected to the common connection point of the voltage source 300 and the reactor L. This diode D1 functions as a reverse - voltage blocking diode that prevents a reverse voltage from being applied to the voltage source 300. Also, the diode D2 has its cathode connected to the common connection point of the voltage source 300 and the reactor L and its anode connected to the common connection point of the reactor L and the self - extinguishing element Q1. This diode D2 functions as a reflux diode that allows the electrical energy stored in the reactor L to flow as a reflux current.

[0020] In this embodiment, the voltage source 300 is a capacitor charged to a predetermined voltage. The negative electrode of the voltage source 300 is connected to the first node a to which the anode of the thyristor Th1 is connected, and the positive electrode is connected to the second node b which is the common connection point of the reactor L, the diodes D1 and D2. The voltage of the voltage source 300 is greater than the voltage obtained by subtracting the forward voltage drop of the thyristor Th1 from the forward voltage drop of the forced commutation circuit 100. Specifically, the voltage of the voltage source 300 is greater than the voltage (in this example, 2V) obtained by subtracting the forward voltage drop of the thyristor Th1 (for example, 2V) from the forward voltage drop of the series circuit of the self-extinguishing element Q1 and the diode DB (for example, 4V). Also, the withstand voltage of the capacitor constituting the voltage source 300 is lower than the circuit voltage which is the maximum voltage applied to the forced commutation circuit 100. Here, the circuit voltage is determined based on, for example, the phase voltage of a polyphase circuit, or the lowest voltage value among the peak value, average value, and effective value of the phase voltage with respect to the neutral point.

[0021] Next, the operation of this embodiment will be described. When turning off the thyristor Th1, the control circuit 200 turns on the self-extinguishing element Q1. As a result, the voltage of the voltage source 300 is applied between the cathode and anode of the thyristor Th1 via the reactor L, the self-extinguishing element Q1, and the diode DB, and a current I2 flows through the path of the voltage source 300, the reactor L, the self-extinguishing element Q1, and the diode DB.

[0022] Here, when the self-extinguishing element Q1 is turned on, the current I2 may increase rapidly and damage the self-extinguishing element Q1. Therefore, in FIG. 1, the reactor L limits this rapid increase in the current I2. When the current I2 flows through the self-extinguishing element Q1, the main current I1 of the thyristor Th1 decreases accordingly. When the main current I1 becomes 0 and the state where the main current I1 is 0 continues for a predetermined time, the thyristor Th1 turns off.

[0023] The control circuit 200 turns on the self-extinguishing element Q1, and then, after the time required for the thyristor Th1 to turn off has elapsed, it turns off the self-extinguishing element Q1 and cuts off the current I2. At this time, the diode D2 allows the electrical energy stored in the reactor L to flow as a return current. In addition, the diode DB prevents the reverse voltage generated in the reactor L due to the cutoff of current I2 from being applied to the self-extinguishing element Q1.

[0024] Next, a specific example of the voltage source 300 will be described. Figure 2 is a circuit diagram showing the configuration of voltage source 300A, which is the first example of the voltage source 300. This voltage source 300A consists of a large-capacity electrolytic capacitor (often called an electrolytic condenser in Japan) 31 and a charging circuit 32 for charging this electrolytic capacitor 31.

[0025] The electrolytic capacitor 31 has its negative electrode connected to the first node a and its positive electrode connected to the second node b. In this embodiment, the charging circuit 32 may have a low output voltage and charge over a long period of time. Therefore, the charging circuit 32 may have a small power capacity. In the illustrated example, the charging circuit 32 is composed of a transformer 321 and a rectifier circuit 322 consisting of diodes 323 and 324. The transformer 321 is supplied with an AC voltage of 400V from an AC circuit equipped with a thyristor Th1. The transformer 321 steps down this 400V AC voltage and outputs it. The rectifier circuit 322 rectifies the output voltage of the transformer 321 and outputs a DC voltage of 10V to the electrolytic capacitor 31. Assuming the charging voltage of the electrolytic capacitor 31 is 10V, and that the electrolytic capacitor 31 can supply a current of 1000A for, for example, 1 second, and that it is charged in 100 seconds, the power required to charge the electrolytic capacitor 31 will be approximately 10V × 1000A × 1 second / 100 seconds = 100W.

[0026] Figure 3 is a circuit diagram showing the configuration of a second example of voltage source 300, voltage source 300B. This voltage source 300B has multiple (four in this example) capacitors C_1 to C_4, multiple (nine in this example) diodes D_1 to D_9, and a charging circuit 40 consisting of a diode 41 and a resistor 42. Note that in Figure 3, the number of capacitors is set to four to prevent the diagram from becoming too complex, but the number of capacitors may be five or more.

[0027] In Figure 3, the multiple diodes D_1 to D_9 include a first diode and a second diode. Here, the first diodes are diodes D_1 to D_6 that turn on when a voltage is applied between the first node a and the second node b such that the first node a is positive and the second node b is negative. The second diodes are diodes D_7 to D_9 that turn on when a voltage is applied such that the first node a is negative and the second node b is positive.

[0028] Here, when the first diodes D_1 to D_6 are turned on, they connect multiple capacitors C_1 to C_4 in parallel between the first node a and the second node b (see solid arrows). Also, when the second diodes D_7 to D_9 are turned on, they connect multiple capacitors C_1 to C_4 in series between the first node a and the second node b (see dashed arrows).

[0029] Therefore, in voltage source 300B, during the period when a voltage is applied between the first node a and the second node b such that the first node a is positive and the second node b is negative, multiple capacitors C_1 to C_4 are connected in parallel between the first node a and the second node b. The commutation operation then takes place during this period when a voltage is applied between the first node a and the second node b such that the first node a is positive and the second node b is negative.

[0030] Furthermore, in the voltage source 300B, during the period when a voltage is applied between the first node a and the second node b such that the first node a is negative and the second node b is positive, multiple capacitors C_1 to C_4 are connected in series between the first node a and the second node b. During this period, a DC voltage obtained by rectifying the AC voltage of the other phase is supplied to the second node b by the charging circuit 40, and this DC voltage charges the series-connected capacitors C_1 to C_4. The AC voltage of the other phase refers to the AC voltage supplied to the AC circuit of another phase of the AC circuit to which the thyristor Th1, to which the forced commutation circuit 100 is connected, belongs.

[0031] Here, let N be the number of capacitors in the voltage source 300B, C be the capacitance of one capacitor, and E be its breakdown voltage. In this case, if N capacitors are connected in series, the capacitance will be C / N and the breakdown voltage will be N·E. On the other hand, if N capacitors are connected in parallel, the capacitance will be N·C and the breakdown voltage will be E.

[0032] If the AC voltage of the other phase is 400V, a peak voltage of 400 × √2 = 566V will be applied to the capacitors connected in series. For example, if N=5, a voltage of 566V / 5 = 113V will be applied to one capacitor. Therefore, a capacitor with a higher voltage rating than this voltage is used, for example, a capacitor with a capacitance of 1000μF and a voltage rating of 160V. In this case, the capacitance when connected in series is 1000μF / 5 = 200μF, and the capacitance when connected in parallel is 1000μF × 5 = 5000μF.

[0033] To turn off the thyristor Th1, the voltage source 300B needs to maintain a voltage greater than the forward voltage drop across the thyristor Th1 (e.g., 2V) minus the forward voltage drop across the series circuit of the self-extinguishing element Q1 and diode DB (e.g., 4V) (2V in this example), which is the time required to shut off the thyristor Th1. In this case, the forward voltage drop across the series circuit of the self-extinguishing element Q1 and diode DB and the forward voltage drop across the thyristor Th1 are on the order of a few volts. Therefore, for the capacitor in parallel connection, a voltage of several hundred volts as in conventional circuits is not necessary; rather, a high capacity at a low voltage is desirable. In parallel connection, as in this example, it is sufficient to achieve a voltage rating of 160V and a capacitance of 5000μF.

[0034] In this example, a 400V AC voltage is used to charge five capacitors connected in series. In this case, the applied voltage per capacitor is approximately 100V, which is acceptable.

[0035] Thus, in this second example, a voltage of several tens of volts can be output from multiple capacitors connected in parallel for the duration required for the thyristor Th1 to shut off, and a voltage of several hundred volts can be used to charge multiple capacitors connected in series.

[0036] As described above, the forced commutation circuit 100 according to this embodiment includes a series-connected voltage source 300 and a self-extinguishing element Q1. The voltage source 300 includes a capacitor that is charged to a predetermined voltage. The voltage of the voltage source 300 only needs to be greater than the voltage obtained by subtracting the forward voltage drop of the thyristor Th1 from the forward voltage drop of the forced commutation circuit 100. The breakdown voltage of the capacitor included in the voltage source 300 may be less than the maximum voltage applied to the forced commutation circuit. Therefore, the forced commutation circuit 100 can be constructed without using large capacitors and large reactors.

[0037] Furthermore, in the forced commutation circuit 100, the reactor L is connected in series with the self-extinguishing element Q1. Therefore, the current flowing when the self-extinguishing element Q1 is ON is limited by the reactor L, protecting the self-extinguishing element Q1. This reactor L may also be the inductance of the wiring. In addition, the reactor L may be omitted.

[0038] Furthermore, in the forced commutation circuit 100, diode D1 is connected in parallel with the voltage source 300. This diode D1 functions as a reverse voltage blocking diode to prevent a reverse voltage from being applied to the voltage source 300.

[0039] Furthermore, in the forced commutation circuit 100, a diode D2 is connected in parallel to the reactor L. This diode D2 functions as a freewheeling diode that allows current to flow through the reactor L when the self-extinguishing element Q1 is off, preventing surge voltage from being applied to the self-extinguishing element Q1.

[0040] In a preferred embodiment, the forced commutation circuit 100 is a forced commutation circuit provided at the thyristor Th1 of an AC circuit including a thyristor, and has a voltage source 300A. This voltage source 300A includes an electrolytic capacitor 31 connected between a first node a corresponding to the negative terminal and a second node corresponding to the positive terminal of the voltage source 300A, and a charging circuit 32 for charging the electrolytic capacitor 31. The charging circuit 32 includes a transformer 321 that steps down the AC voltage supplied to the AC circuit, and a rectifier circuit 322 that charges the electrolytic capacitor 31 with a DC voltage obtained by rectifying the output voltage of the transformer 321. Therefore, according to this embodiment, a low-voltage, high-capacity capacitor can function as the voltage source 300A.

[0041] In a preferred embodiment, the forced commutation circuit 100 is a forced commutation circuit provided on the thyristor Th1 of each phase of a multi-phase AC circuit, each containing a thyristor, and has a voltage source 300B. The voltage source 300B includes a plurality of capacitors C_1 to C_4, a plurality of diodes D_1 to D_9, and a charging circuit 40. The plurality of diodes D_1 to D_9 include first diodes D_1 to D_6 which are turned on when a voltage is applied between a first node a corresponding to the negative terminal of the voltage source 300B and a second node b corresponding to the positive terminal of the voltage source 300B such that the first node a is positive and the second node b is negative, and second diodes D_7 to D_9 which are turned on when a voltage is applied between the first node a and the second node b such that the first node a is negative and the second node b is positive. When the first diodes D_1 to D_6 are turned on, they connect multiple capacitors C_1 to C_4 in parallel between the first node a and the second node b. When the second diodes D_7 to D_9 are turned on, they connect multiple capacitors C_1 to C_4 in series between the first node a and the second node b. The charging circuit 40 charges the multiple capacitors C_1 to C_4 connected in series with a DC voltage obtained by rectifying the AC voltage supplied to the AC circuit of the other phase during the period when the multiple second diodes D_7 to D_9 are turned on. In this embodiment as well, a low-voltage, high-capacity capacitor can function as a voltage source 300B. [Explanation of Symbols]

[0042] 100... Forced commutation circuit, Th1... Thyristor, 300, 300A, 300B... Voltage source, Q1... Self-extinguishing element, L... Reactor, DB, D1, D2, D_1~D_9, 41... Diode, C_1~C_4... Capacitor, 200... Control circuit, 31... Electrolytic capacitor, 32, 40... Charging circuit, 321... Transformer, 322... Rectifier circuit, 42... Resistor.

Claims

1. A forced commutation circuit connected in parallel to a thyristor, The circuit includes a voltage source, a self-extinguishing means, and a reverse voltage blocking means that prevents a reverse voltage from being applied to the self-extinguishing means, all connected in series. The aforementioned voltage source is charged to a predetermined voltage. The voltage of the voltage source is greater than the voltage obtained by subtracting the forward voltage drop of the thyristor from the forward voltage drop of the forced commutation circuit. A diode is connected in parallel to the voltage source to prevent a reverse voltage from being applied to the voltage source. A forced commutation circuit in which the voltage supplied to the voltage source is less than the maximum voltage supplied to the forced commutation circuit.

2. The forced commutation circuit according to claim 1, wherein a reactor that limits the current flowing when the self-extinguishing means is turned on is connected in series with the self-extinguishing means.

3. The forced commutation circuit according to claim 2, wherein the reactor is the inductance of the wiring.

4. The forced commutation circuit according to claim 2, wherein a diode is connected in parallel to the reactor.

5. A forced commutation circuit provided in a thyristor of an AC circuit including a thyristor, The voltage source includes an electrolytic capacitor connected between a first node corresponding to the negative terminal of the voltage source and a second node corresponding to the positive terminal of the voltage source, and a charging circuit for charging the electrolytic capacitor. The forced commutation circuit according to claim 1, wherein the charging circuit includes a transformer that steps down the AC voltage supplied to the AC circuit, and a rectifier circuit that charges the electrolytic capacitor with a DC voltage obtained by rectifying the output voltage of the transformer.

6. A forced commutation circuit provided in the thyristor of each phase of a multiphase AC circuit, each phase including a thyristor, The voltage source includes a plurality of capacitors, a plurality of diodes, and a charging circuit. The plurality of diodes include a first diode that turns on when a voltage is applied between a first node corresponding to the negative terminal of the voltage source and a second node corresponding to the positive terminal of the voltage source such that the first node is positive and the second node is negative, and a second diode that turns on when a voltage is applied between the first node and the second node such that the first node is negative and the second node is positive. When the first diode is turned on, it connects the plurality of capacitors in parallel between the first node and the second node. When the second diode is turned on, it connects the plurality of capacitors in series between the first node and the second node. The forced commutation circuit according to claim 1, wherein the charging circuit charges the plurality of capacitors connected in series with a DC voltage obtained by rectifying the AC voltage supplied to the AC circuit of the other phase during the period when the plurality of second diodes are ON.

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