JPH09233833A - Ac/dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the increase of a fault current caused by an arm short- circuit in an AC/DC converter and utilize the current capacity of the semiconductor switch (thyristor) of the AC/DC converter efficiently. SOLUTION: An AC/DC converter 2 which converts AC into DC or inversely converts DC into AC and a converter transformer which transforms an AC voltage value into a value suitable for the AC/DC converter 2 are provided. Current limiters 10-12 whose resistance values are normally almost zero by maintaining a superconducting state and, when currents exceeding the specified critical current are applied by the fault and the superconducting state is transferred to the normal conducting state, are increased are connected between the AC/DC converter 2 and the converter transformer. With this constitution, when an arm short-circuit fault occurs in the AC/DC converter 2, currents larger than the normal current flow and, if the currents exceed the critical current of the current limiters 10-12, the superconducting sate of the current limiters 10-12 is transferred to the normal conducting state and the resistance values of the current limiters 10-12 are increased, so that the increase of the fault currents can be suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC / DC converter for reducing an overcurrent flowing when a circuit is short-circuited and realizing an efficient AC / DC converter.

[0002]

2. Description of the Related Art The structure of a conventional AC / DC converter using a thyristor is shown in FIG. The AC / DC converter 2 is an arm (1U, 1V, 1W, 1X, 1) in which a required number of thyristors are connected in series.
(Y, 1Z) are connected in a bridge, and AC side terminals 4A, 4
The converter transformer 5 is connected to B and 4C. As is well known, a thyristor, which is one of semiconductor switches, has a blocking capability that allows a current to flow in only one direction and does not allow a reverse current to flow. Therefore, it is possible to apply an AC voltage from a transformer and give an ignition pulse to the gate of the thyristor from a control device (not shown) to make the arm conductive and perform a rectifying operation.

There is a phenomenon in which the armature is lost due to a failure of the thyristor during operation of the conversion device, resulting in a short circuit state, which is called an arm short circuit. In FIG. 9, when an arm short circuit occurs in the arm 1U while the arms 1V and 1Z are energized, a short circuit current i as shown in FIG. 10 flows through the normally energized 1V arm. This current is the current on the secondary short circuit of the transformer and is of a magnitude limited by the leakage impedance of the transformer. If the short-circuit current is large, the arm 1V may be destroyed due to an excessive increase in the junction temperature of the thyristor having a sound arm 1V. Arm short circuit current i
Is most likely to occur immediately after turn-off of the arm due to commutation, and increases as the control delay angle during operation decreases. The maximum value of the arm short-circuit current i is roughly given by the following equation (DC Technical Committee on DC Power Transmission "Commentary on DC Power Transmission Technology", Corona Publishing Co., Ltd., 1978, p72).

[0004]

## EQU1 ## i = 2.times.Idc / Z where Idc is the rated DC output current (pu), and Z is the leakage impedance (pu) of the transformer.

In the conventional transformer, the leakage impedance of the transformer is selected so that the healthy arm does not break due to overcurrent when the arm is short-circuited, that is, the short-circuit current i does not exceed the allowable current withstanding capacity of the thyristor. Conventionally, generally, a transformer having leakage impedance of about 0.16 pu to 0.2 pu is combined with a converter to form an AC / DC converter.

[0006]

As described above, conventionally, a combination of a converter and a transformer has been used so that the operating current level is operated in a range smaller than the current withstand capability of the thyristor. This leaves the problem that the power of the converter is not being used effectively. Therefore, an object of the present invention is to effectively utilize the capability of the converter by connecting a current limiting device using a superconducting wire.

[0007]

An AC / DC converter according to a first aspect of the present invention is an AC / DC converter for converting AC to DC or DC for AC, and AC voltage is converted to a value suitable for the AC / DC converter. Equipped with a transformer for converter, the superconducting state is normally maintained and the resistance value is maintained at almost zero, and when a current exceeds the predetermined critical current due to a failure and a conducting current flows, the state changes to the normal conducting state and the resistance value becomes large. The current limiting device is connected between the AC / DC converter and the converter transformer. As a result, when a short circuit occurs in the arm of the AC / DC converter, a larger current flows than in the normal state, and when this current exceeds the critical current of the current limiter, the current limiter shifts to the normal conduction state and the resistance value increases, It suppresses the increase of fault current. Therefore, since the fault current at the time of a fault can be suppressed, the rated current of the AC / DC converter at a normal time can be increased. In the same sense,
As the current rated value (conventional), a semiconductor switch with a small withstand current can be adopted, and the size can be reduced.

According to a second aspect of the present invention, there is provided an AC / DC converter in series with each arm of an AC / DC converter in which semiconductor switches are bridge-connected,
Normally, the superconducting state is maintained and the resistance value is kept at almost zero,
When a fault current occurs and an energizing current exceeds a predetermined critical current, the current limiter is connected to connect to the normal conducting state and increase the resistance value. As a result, similarly to the AC / DC converter of claim 1, when the arm short circuit of the AC / DC converter has a failure, the short-circuit current is prevented from increasing.

An AC / DC converter according to a third aspect of the present invention comprises an AC / DC converter for converting AC to DC or DC for AC, and a converter transformer for converting AC voltage to a value suitable for the AC / DC converter. The secondary side of the step-down transformer connected between the AC / DC converter and the converter transformer is normally maintained in a superconducting state and maintains a resistance value of substantially zero, and a predetermined value is generated due to a failure. A current limiter is connected to which the resistance value increases when the energizing current exceeds the critical current and transitions to the normal conducting state. As a result, similarly to the AC / DC converter of claim 1, when the arm short circuit of the AC / DC converter has a failure, the short-circuit current is prevented from increasing.

According to a fourth aspect of the present invention, in the AC / DC converter, the superconducting state is normally maintained at the DC side terminal of the AC / DC converter for converting AC into DC or reverse conversion of AC into AC, and the resistance value is kept substantially zero. When a fault current occurs and an energizing current exceeds a predetermined critical current, the current limiter is connected to connect to the normal conducting state and increase the resistance value. Thereby, claim 1
In the same manner as the AC / DC converter, the increase in the short circuit current is suppressed when the AC / DC converter has an arm short circuit failure.

An AC / DC converter according to a fifth aspect of the present invention comprises an AC / DC converter for converting AC to DC or DC to AC, and a DC capacitor installed at a DC side terminal of the AC / DC converter. In a series connection with a capacitor, the superconducting state is normally maintained and the resistance value is kept at almost zero.When a current flows in excess of a predetermined critical current due to a failure, it changes to the normal conducting state and the resistance value increases. To connect the vessels. Thereby, like the AC / DC converter of claim 1,
When the arm short circuit of the AC / DC converter fails, the short circuit current is prevented from increasing.

An AC / DC converter according to a sixth aspect of the present invention is the AC / DC converter according to any one of the first to fifth aspects, in which a lightning arrester is connected in parallel to the current limiting device. Thereby, when the arm short circuit of the AC / DC converter fails, the current flow exceeds the critical current of the fault current limiter and the fault current limiter transitions to the normal conducting state, which increases the resistance value and suppresses the increase of the fault current. At this time, the surge arrester suppresses the overvoltage generated at both ends of the current limiting device.

According to a seventh aspect of the present invention, there is provided an AC / DC converter according to any one of the first to sixth aspects, wherein a signal indicating that the fault current limiter has changed from a superconducting state to a normal conducting state is used.
The circuit breaker provided on the alternating current side of the transformer for the converter is opened when the current limiting device changes to the normal conducting state. With this, a fault in the AC / DC converter is detected when the fault current limiter transitions from the superconducting state to the normal conducting state, and this signal is used to open the breaker provided on the AC side of the transformer for the converter, causing a failure. To prevent the spread to other systems.

An AC / DC converter according to claim 8 is the AC / DC converter according to any one of claims 1 to 7, wherein a signal indicating that the fault current limiter has changed from a superconducting state to a normal conducting state is used.
The operation of the AC / DC converter is stopped when the current limiting device changes to the normal conducting state. Thereby, the fault of the AC / DC converter is detected by the current limiter transitioning from the superconducting state to the normal conducting state, and the operation of the AC / DC converter is stopped by using this signal to prevent the propagation of the fault.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a connection configuration diagram of an AC / DC converter according to a first embodiment of the present invention. 1U, 1V,
Reference numerals 1W, 1X, 1Y, and 1Z are arms formed by semiconductor switches, and bridge-connected to form the converter 2. Transformer 5 and converter AC terminals 4A, 4B and 4C
Current limiters 10, 11 and 12 using superconducting wires are connected between the two. The operation of the thus connected converter will be described.

First, a current limiting device using superconductivity has a structure in which, for example, a superconducting wire is wound into a coil and placed in liquid helium. In many cases, the coil is wound in a non-inductive manner so that the inductance when current is conducted becomes zero. By maintaining the temperature of the superconducting wire below a predetermined level with low temperature liquid helium, the superconducting state is maintained, and the resistance value is maintained at almost zero to energize. When the energizing current exceeds the critical current, the coil is quenched and the superconducting state is changed to the normal conducting state, and the resistance of the coil increases. The current limiter using superconductivity performs such an operation.

In the circuit configured as shown in FIG. 1, when an arm short circuit occurs in the arm 1U while the arms 1V and 1Z are energized, a short circuit current i as shown in FIG. 2 flows through the 1V arm and the 1U arm. Before the arm short circuit occurred, a current below the critical current of the current limiter (rated current of the converter) was flowing through the current limiters 10, 11 and 12, and the current limiter was energized in the superconducting state. Has almost zero resistance. When an arm short circuit occurs, the current flowing through the fault current limiter increases. Then, the short-circuit current exceeds the critical current of the fault current limiter and shifts to the normal conduction state, and the resistance value of the fault current limiter increases. Then, as shown in FIG. 2, the short-circuit current i increases as shown by the dotted line without the current limiter, but rapidly decreases with the current limiter as shown by the solid line. Because of this operation, the current flowing through the arm is always below the critical current of the fault current limiter. In other words, the maximum arm current of the converter will be determined by the critical current of the fault current limiter, so if the critical current of this fault current limiter is designed to be less than the current withstanding capacity of the semiconductor switch, the current that the semiconductor always energizes will be The current withstand capability can be approximated, and as a result, an efficient conversion device can be produced. For example, conventionally, about 1/10 or less of the withstand current of the semiconductor switch was set as the rated current of the converter, but according to the present invention, about 1/2 can be selected as the rated current. There is a commutation failure in the reverse converter, and in that case, a current higher than the rated current flows primarily, but the commutation failure is immediately recovered by the control operation. Since the overcurrent in that case is less than twice the rated current, it is not appropriate for the fault current limiter to operate due to such a transient phenomenon. Try not to exceed. For that reason, the rated current can be increased to about 1/2 of the current withstanding capacity as described above.

FIG. 3 is a connection configuration diagram of the AC / DC converter of the embodiment of claim 6 in the AC / DC converter of claim 1. In FIG. 3, lightning arresters 21, 22, and 23 are connected in parallel to current limiters 10, 11, and 12. As is well known, surge arresters, when the voltage applied to the arrester terminals exceeds a certain level,
The resistance value of the lightning arrester, which had been high impedance until then, becomes smaller, and the current applied to the lightning arrester suppresses the voltage applied to the terminals of the lightning arrester. The voltage at that time depends on the voltage-current characteristics of the arrester. Now, in the converter configured as shown in FIG. 3, when an arm short circuit occurs, the current limiter still operates due to the overcurrent, and the magnitude of the short circuit current is suppressed. However, at that time, an overvoltage is generated across the current limiter. If the overvoltage is left unattended, the problem of threatening the insulation of the converter or the transformer remains. The surge arrester connected in parallel suppresses the overvoltage. With this configuration, it is possible to suppress the overvoltage inside the circuit even when the current limiter operates.

FIG. 4 is a partial connection configuration diagram of the AC / DC converter according to the second embodiment. 1U is an arm of a converter, in which a current limiting device 10 using a superconducting wire is connected in series to form an arm 20 as a whole. The arms are bridge-connected to form an AC / DC converter. With this configuration, it is possible to suppress an overcurrent when the arm is short-circuited, and in this case also, an efficient conversion device can be provided.

A sixth aspect of the AC / DC converter of the second aspect.
Although not shown, the lightning arrester is connected in parallel to the current limiting device connected in series to 1U of FIG. The function is to suppress overvoltage between the arrester terminals, which is accompanied by suppression of overcurrent during operation of the fault current limiter. Also in this case, an efficient conversion device can be provided.

FIG. 5 is a connection configuration diagram of the AC / DC converter according to the third embodiment. 2 is a converter, 4A, 4B, 4C
Is a converter terminal and 5 is a transformer. Terminals 4A, 4B, 4
A high-voltage transformer 31, 32, 33 is connected between C and the transformer 5, and a current limiter 41 using a superconducting wire on the secondary side of the transformer,
The configuration is such that 42 and 43 are connected. This is because the current limiter of FIG. 1 is connected to the secondary side of the step-down transformer, so that the operation is the same as that of FIG. 1 and the same effect can be obtained. Moreover, with such a connection, the insulation of the fault current limiter against voltage to ground can be lowered, and the effect of facilitating the production of the fault current limiter can be obtained.

A sixth aspect of the AC / DC converter of the third aspect.
Although not shown, the lightning arrester is connected in parallel to the current limiter connected to the secondary side of the step-down transformer of FIG. The function is to suppress overvoltage between the arrester terminals, which is accompanied by suppression of overcurrent during operation of the fault current limiter. Also in this case, an efficient conversion device can be provided.

FIG. 6 is a connection configuration diagram of an AC / DC converter according to the seventh embodiment. Circuit breakers 61, 62, 63, transformer 5, current limiters 10, 11, 12 and converter 2 are connected in series, and current limiter 1
The normal conduction state transition signal 71 output from 0, 11, 12 is guided to the protective device 70, and the protective device 70 is connected so as to send the circuit breaker open signal 72 to the circuit breakers 61, 62, 63. When the arm short circuit occurs and the overcurrent flows, the current limiter shifts to the normal conduction state and suppresses the overcurrent as described above. When a short circuit occurs and the fault current limiter operates, it is an accident, and the converter must be stopped immediately to prevent the accident from spreading to the sound parts. With the configuration shown in FIG. 6, it is possible to immediately open the AC side circuit breaker when the current limiting device operates, and disconnect the faulty converter from the AC circuit. The effect is obtained.

FIG. 7 is a connection configuration diagram of an AC / DC converter according to the fourth embodiment. 5 is a transformer, 101 is a self-excited converter, 102P and 102N are DC terminals of the converter, 103 is a DC capacitor, and 110 is a current limiter. In FIG. 7, the converter 101
The current limiter 110 is connected in series to the circuit of the DC capacitor 103 and the DC capacitor 103. The self-excited converter is also a bridge-connected semiconductor switch, and when the arms are short-circuited or the upper and lower arms are simultaneously energized, a large discharge current flows from the DC capacitor. When the current limiter is connected to the converter as shown in FIG. 7, overcurrent is suppressed, and a converter with high efficiency can be provided.

A sixth aspect of the AC / DC converter of the fourth aspect.
Although not shown, it is a configuration in which a lightning arrester is connected in parallel to the fault current limiter of FIG. 7, and an overvoltage that appears at the fault current limiter terminal when the fault current limiter suppresses an overcurrent at the time of short circuit inside the converter. The effect that the lightning arrester can be suppressed is obtained.

FIG. 8 is a connection configuration diagram of the AC / DC converter according to the fifth embodiment. 5 is a transformer, 101 is a self-excited converter, 102P and 102N are DC terminals of the converter, 103 is a DC capacitor, and 110 is a current limiter. In FIG. 8, a current limiter 110 is connected in series with the DC capacitor 103. The self-excited converter is also a bridge connection of semiconductor switches,
When the arms are short-circuited or the upper and lower arms are simultaneously energized, a large discharge current flows from the DC capacitor. FIG.
When the current limiter is connected in series with the DC capacitor as described above, overcurrent is suppressed, and a converter with good efficiency can be provided.

Claim 6 in the AC / DC converter of Claim 5
Although not shown, the lightning arrester is connected in parallel to the current limiting device of FIG. The surge arrester can suppress the overvoltage appearing at the terminals of the current limiter when the current limiter suppresses the overcurrent at the time of a short circuit inside the converter.

Although not shown in the eighth aspect, in the apparatus of the first to seventh aspects, the operation of the AC / DC converter is stopped by using the normal conduction transition signal of the current limiting device. Since the overcurrent is suppressed when the current limiting device operates, the normal protection device may not operate, and in that case, there is a problem that the conversion device continues to operate despite an accident. By using a signal indicating that the fault current limiter has changed to the normal conduction state, it is possible to stop the conversion device when the converter has an accident.

[0029]

According to the present invention, it is possible to provide a conversion device that effectively utilizes the current withstand capability of a semiconductor switch.

[Brief description of drawings]

FIG. 1 is a connection configuration diagram of an AC / DC converter according to an embodiment of claim 1 of the present invention.

FIG. 2 is a current waveform when carrying out claim 1 of the present invention.

FIG. 3 is a connection configuration diagram of an AC / DC converter according to an embodiment of claim 6 of the present invention.

FIG. 4 is a connection configuration diagram of an AC / DC converter according to an embodiment of claim 2 of the present invention.

FIG. 5 is a connection configuration diagram of an AC / DC converter according to an embodiment of claim 3 of the present invention.

FIG. 6 is a connection configuration diagram of an AC / DC converter according to an embodiment of claim 7 of the present invention.

FIG. 7 is a connection configuration diagram of an AC / DC converter according to an embodiment of claim 4 of the present invention.

FIG. 8 is a connection configuration diagram of an AC / DC converter according to an embodiment of claim 5 of the present invention.

FIG. 9 is a connection configuration diagram of a conventional AC / DC converter.

FIG. 10 is a current waveform when the arm of the conventional AC / DC converter is short-circuited.

[Explanation of symbols]

 1U, 1V, 1W, 1X, 1Y, 1Z ... Arm 2 ... AC / DC converter 3P, 3N ... DC terminal 4A, 4B, 4C ... AC terminal 5 ... Transformer 10, 11, 12 ... Current limiter 20 ... Arm 21, 22, 23 ... Lightning arrester 31, 32, 33 ... Transformer 41, 42, 43 ... Current limiter 61, 62, 63 ... Circuit breaker 70 ... Protective device 71 ... Normal conduction transition signal 72 ... Circuit breaker open signal 101 ... Self-excited Converter 102P, 102N ... DC terminal 103 ... Capacitor 110 ... Current limiter

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication H02M 7/5387 9181-5H H02M 7/5387 Z

Claims (8)

[Claims] 1. An AC / DC converter for converting AC into DC or vice versa, and a converter transformer for converting AC voltage into a value suitable for the AC / DC converter, and in a superconducting state in a normal state. The current limiter for the AC / DC converter and the converter is maintained so that the resistance value is maintained at almost zero, and when a failure occurs, the current value exceeds the predetermined critical current and the current value changes to the normal conduction state and the resistance value increases. An AC / DC converter that is connected to a transformer. 2. The superconducting state is normally maintained and the resistance value is maintained at almost zero in series with each arm of the AC / DC converter in which the semiconductor switches are bridge-connected, and the conduction current exceeds a predetermined critical current due to the occurrence of a failure. An AC / DC converter, which is characterized by connecting a current limiting device that changes to a normal conducting state when flowing and has a large resistance value. 3. An AC / DC converter for converting AC into DC or DC for AC, and a converter transformer for converting AC voltage into a value suitable for the AC / DC converter. On the secondary side of the step-down transformer connected between the converter transformer, the superconducting state is normally maintained and the resistance value is kept substantially zero, and the current flow exceeds the predetermined critical current due to the occurrence of a failure. An AC / DC converter, which is characterized in that a current limiter is connected to which the resistance value increases when the current flows to the normal conduction state. 4. A superconducting state is normally maintained at a DC side terminal of an AC / DC converter for converting AC into DC or DC into AC, and a resistance value of the AC / DC converter is maintained at substantially zero. An AC / DC converter characterized in that a current limiter is connected to which a resistance value increases when a current flows over the current, causing a transition to a normal conducting state. 5. An AC / DC converter for converting AC into DC or vice versa, and a DC capacitor installed at a DC side terminal of the AC / DC converter. Is characterized by connecting a fault current limiter that maintains the superconducting state and keeps the resistance value at almost zero, and when a failure causes an energizing current to flow beyond a predetermined critical current, transitions to the normal conducting state and the resistance value increases. AC / DC converter. 6. The AC / DC converter according to claim 1, wherein a surge arrester is connected in parallel with the current limiting device. 7. The AC / DC converter according to claim 1, wherein a signal indicating that the fault current limiter has changed from a superconducting state to a normal conducting state is used, and the fault current limiter has changed to a normal conducting state. At this time, the AC / DC converter is characterized in that the breaker provided on the AC side of the converter transformer is opened. 8. The AC / DC converter according to any one of claims 1 to 7, wherein a signal indicating that the fault current limiter has changed from a superconducting state to a normal conducting state is used, and the fault current limiter has changed to a normal conducting state. At this time, the AC / DC converter is characterized in that the operation of the AC / DC converter is stopped.