Patent Description:
A DC distribution panel is provided between load apparatuses and a rectification device of a DC power supply or the like, in a DC power distribution system. The DC distribution panel has a function of branching DC power inputted from the rectification device into a plurality of feeders and supplying power to the load apparatuses connected to the respective feeders. The DC distribution panel may be provided with a plurality of circuit breakers corresponding to the plurality of feeders in order to prevent flowing of large short-circuit current when a failure such as short-circuit has occurred in the load apparatus.

One conventional example of such DC distribution panels includes a plurality of semiconductor circuit breakers provided correspondingly for respective feeders, circuit breaker capacitors provided near the semiconductor circuit breakers, and a distribution panel capacitor connected in parallel with the semiconductor circuit breakers (see, for example, Patent Document <NUM>).

In the conventional DC distribution panel, when short-circuit current flows through the feeder due to short-circuit in the load apparatus, the semiconductor circuit breaker is turned off, whereby the short-circuit current can be interrupted. In addition, since the conventional DC distribution panel has the circuit breaker capacitor, it is possible to inhibit application of overvoltage to the semiconductor circuit breaker due to short-circuit of the load apparatus.

Patent Document <NUM>, according to its abstract, relates to an electrical power distribution system (EPDS) for an aircraft. The EPDS may include a DC bus, a power source port, a solid state power controller (SSPC) of a first type interposed between the power source port and the DC bus, at least one load port and an SSPC of a second type interposed between the load port and the DC bus. Power input to the SSPC of the first type may be connected to a unidirectional solid state switching device (SSSD) of the SSPC of the first type. The SSPC of the first type may have forward and reverse current conducting capability and forward and reverse current blocking capability. Power input to the SSPC of the second type may be connected to a unidirectional SSSD of the SSPC of the second type. The SSPC of the second type may have forward and reverse current conducting capability and capability of blocking current from only one direction.

In the DC distribution panel, the load apparatuses are connected to the plurality of feeders on the output side. When short-circuit has occurred in one of the load apparatuses, reverse current might flow into the DC distribution panel from the feeder connected to the load apparatus where short-circuit has not occurred. If the reverse current flows into the DC distribution panel, the circuit breaker corresponding to the feeder where short-circuit has not occurred might perform an interruption operation. In the conventional DC distribution panel, short-circuit current of the feeder where short-circuit has occurred can be interrupted, but the reverse current from the feeder where short-circuit has not occurred cannot be interrupted. As a result, in the conventional DC distribution panel, there is a problem that the feeder where short-circuit has not occurred, i.e., the normal feeder is also interrupted.

The present invention has been made to solve the above problem, and an object of the present invention is to provide a DC distribution panel that, even when short-circuit current has occurred in one feeder, interrupts only the feeder where short-circuit current has occurred, and thus can continue operations of load apparatuses connected to normal feeders.

Therefore, there is provided a DC distribution panel according to independent claim <NUM>.

In the DC distribution panel according to the present invention, at least one of the plurality of circuit breakers includes the reverse current interruption unit for interrupting reverse current flowing through the positive electric path from the output terminal side to the input terminal side. Thus, only the feeder where short-circuit current has occurred is interrupted, and operations of the load apparatuses connected to normal feeders can be continued.

Embodiments <NUM>-<NUM> are also to be considered as embodiments of the present invention.

Hereinafter, a DC distribution panel according to embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the drawings, the same reference characters denote the same or corresponding parts.

<FIG> is a schematic diagram of a DC power distribution system to which a DC distribution panel is applied according to embodiment <NUM>. A DC power distribution system <NUM> of the present embodiment is composed of an AC/DC converter <NUM> for converting AC power outputted from an AC power supply <NUM> to DC power, a DC distribution panel <NUM> for distributing the converted DC power to three systems of feeders <NUM>, <NUM>, and <NUM>, and load apparatuses <NUM>, <NUM>, and <NUM> connected to the feeders <NUM>, <NUM>, and <NUM>. In the DC power distribution system <NUM> shown in <FIG>, one AC/DC converter <NUM> is provided. However, for the purpose of improving reliability or increasing the power capacity, two or more AC/DC converters may be used. In addition, although the feeders on the output side of the DC distribution panel <NUM> are three systems, two or more systems of feeders can be applied. The load apparatuses <NUM>, <NUM>, and <NUM> may be apparatuses driven by DC power, e.g., lighting equipment, air conditioning equipment, power generation equipment such as photovoltaic generation, or power storage equipment such as a battery.

<FIG> is a schematic diagram of the DC distribution panel according to the present embodiment. The DC distribution panel <NUM> of the present embodiment includes an input terminal <NUM>, circuit breakers <NUM>, <NUM>, and <NUM> respectively connected to branch electric paths branched from the input terminal <NUM> into three systems, and output terminals <NUM>, <NUM>, and <NUM> of the respective circuit breakers <NUM>, <NUM>, and <NUM>. The input terminal <NUM> is composed of a positive input terminal 15a and a negative input terminal 15b. The output terminal <NUM> is composed of a positive output terminal 71a and a negative output terminal 71b. Similarly, the output terminal <NUM> is composed of a positive output terminal 72a and a negative output terminal 72b, and the output terminal <NUM> is composed of a positive output terminal 73a and a negative output terminal 73b.

The electric paths inside the DC distribution panel are branched from the input terminal <NUM> into three systems. The positive input terminal 15a is connected to the positive output terminal of the output terminal via a positive electric path 16a, and the negative input terminal 15b is connected to the negative output terminal of the output terminal via a negative electric path 16b.

As shown in <FIG>, the circuit breaker <NUM> includes a short-circuit current interruption unit <NUM> and a reverse current interruption unit <NUM>. The reverse current interruption unit <NUM> is connected to the output terminal side of the short-circuit current interruption unit <NUM>. The short-circuit current interruption unit <NUM> has an interruption mechanism for interrupting the electric paths when excessive current flows through one or both of the positive electric path 16a and the negative electric path 16b, and as the interruption mechanism, for example, a mechanical-type interruption mechanism, a fuse, or the like can be used. The short-circuit current interruption unit <NUM> is provided for both of the positive electric path 16a and the negative electric path 16b, but may be provided for only one of them. The interruption mechanism of the short-circuit current interruption unit <NUM> is closed during a normal operation. When short-circuit has occurred in the feeder connected to the output terminal <NUM> of the circuit breaker <NUM>, the interruption mechanism is opened, thus functioning to interrupt excessive short-circuit current. Hereinafter, using the reverse current interruption unit <NUM> as a boundary, input-terminal-side parts of the positive electric path 16a and the negative electric path 16b are referred to as an input-side positive electric path and an input-side negative electric path, and output-terminal-side parts thereof are referred to as an output-side positive electric path and an output-side negative electric path.

The reverse current interruption unit <NUM> includes a semiconductor switching element <NUM>, a first diode <NUM> connected in antiparallel to the semiconductor switching element <NUM>, a second diode <NUM> connected in series to the semiconductor switching element <NUM>, and a capacitor <NUM> connected in parallel with the series connection of the semiconductor switching element <NUM> and the second diode <NUM>. The emitter of the semiconductor switching element <NUM> is connected to the input-side positive electric path 16a, and the collector thereof is connected to the output-side positive electric path 16a. The cathode of the first diode <NUM> is connected to the output-side positive electric path 16a, and the anode thereof is connected to the input-side positive electric path 16a. The cathode of the second diode <NUM> is connected to the input-side positive electric path 16a, and the anode thereof is connected to the input-side negative electric path 16b. The capacitor <NUM> is connected between the output-side positive electric path 16a and the output-side negative electric path 16b.

The reverse current interruption unit <NUM> is controlled by a control circuit <NUM>. The control circuit <NUM> includes a determination circuit <NUM> for determining whether or not reverse current has occurred on the basis of a current value detected by a current sensor <NUM>, and a gate drive circuit <NUM> for turning off the semiconductor switching element <NUM> of the reverse current interruption unit <NUM> when the determination circuit <NUM> determines that reverse current has occurred. The current sensor <NUM> is provided at a position for detecting current flowing through the positive electric path 16a. The current sensor <NUM> may be provided at a position for detecting current flowing through the negative electric path 16b.

Although the configuration of the circuit breaker <NUM> has been described above, the other circuit breakers <NUM> and <NUM> connected to the corresponding ones of the branch electric paths branched into three systems from the input terminal <NUM> also have the same configuration as the circuit breaker <NUM>.

Next, operation of the DC distribution panel <NUM> of the present embodiment will be described. First, a factor of occurrence of reverse current will be described.

<FIG> is a schematic diagram of the DC power distribution system formed by the DC distribution panel in which each circuit breaker has only the short-circuit current interruption unit. <FIG> is a configuration diagram showing the configuration of power storage equipment as an example of a load apparatus. The DC power distribution system shown in <FIG> is composed of the AC/DC converter <NUM> for converting AC power to DC power, the DC distribution panel <NUM> having the circuit breakers <NUM>, <NUM>, and <NUM> each having only the short-circuit current interruption unit <NUM>, and the load apparatuses <NUM>, <NUM>, and <NUM> respectively connected to the feeders <NUM>, <NUM>, and <NUM> of the output systems for the three circuit breakers. The power storage equipment shown in <FIG> is composed of a capacitor <NUM>, two semiconductor switching elements <NUM> connected in parallel to the capacitor <NUM>, and an inductance element <NUM> and a storage battery <NUM> which are connected to an intermediate point between the two semiconductor switching elements <NUM>. Input terminals 95a and 95b at both ends of the capacitor <NUM> are respectively connected to the positive output terminal and the negative output terminal of the DC distribution panel. It is assumed that, in the DC power distribution system shown in <FIG>, the power storage equipment shown in <FIG> is connected as the load apparatuses <NUM>, <NUM>, and <NUM>.

In <FIG>, it is assumed that the load apparatus <NUM> has failed and short-circuit has occurred. In this case, short-circuit current <NUM> flows from the AC power supply <NUM> through the AC/DC converter <NUM> and the DC distribution panel <NUM> to the load apparatus <NUM>. The short-circuit current <NUM> becomes a factor of causing problems such as failure expansion in the load apparatus <NUM> and failure of the feeder <NUM>. In order to prevent this, the short-circuit current is interrupted by the short-circuit current interruption unit <NUM> of the circuit breaker <NUM>.

However, as shown in <FIG>, due to sharp reduction in the impedance of the feeder <NUM> where the short-circuit has occurred, reverse currents <NUM> and <NUM> occur to flow from the capacitors <NUM> provided to the load apparatuses <NUM> and <NUM> that have not failed, through the insides of the circuit breakers <NUM> and <NUM> from the output terminal side to the input terminal side. The reverse currents <NUM> and <NUM> are superimposed on the short-circuit current <NUM>, thus flowing to the feeder <NUM>. Such current is also called sneak current. In a case where the reverse currents <NUM> and <NUM> are large, the reverse currents induce interruption in the short-circuit current interruption units <NUM> of the circuit breakers <NUM> and <NUM>. As a result, supply of power to the feeders <NUM> and <NUM> connected to the load apparatuses <NUM> and <NUM> that have not failed is interrupted, so that the entire DC power distribution system is stopped.

In order to inhibit such sneak current, the DC distribution panel <NUM> of the present embodiment has the reverse current interruption unit. With reference to <FIG> and <FIG>, operation of the DC distribution panel <NUM> of the present embodiment will be described. In each of the control circuits <NUM> of the circuit breakers <NUM> and <NUM> corresponding to the feeders <NUM> and <NUM> where short-circuit has not occurred, the determination circuit <NUM> determines whether or not excessive reverse current has occurred in the positive electric path 16a on the basis of the current value detected by the current sensor <NUM>, and if it is determined that excessive reverse current has occurred, an off command is given to the gate drive circuit <NUM> to turn off the semiconductor switching element <NUM>. In the DC distribution panel <NUM>, the reverse currents caused in the feeders <NUM> and <NUM> can be interrupted by turning off the semiconductor switching elements <NUM>. The determination circuit <NUM> has a current threshold stored in advance, and determines that excessive reverse current has occurred if the current value detected by the current sensor <NUM> is greater than the current threshold.

Further, in the DC distribution panel <NUM>, even in a state in which the semiconductor switching elements <NUM> of the circuit breakers <NUM> and <NUM> are off, since the first diode <NUM> connected in antiparallel are present, the circuit breakers <NUM> and <NUM> readily allow supply of power from the AC power supply to the load apparatuses <NUM> and <NUM> after the feeder <NUM> where the short-circuit has occurred is interrupted by the short-circuit current interruption unit <NUM> of the circuit breaker <NUM>.

In the DC distribution panel configured as described above, the reverse current interruption unit is provided for interrupting reverse current that flows through the positive electric path inside the circuit breaker from the output terminal side to the input terminal side. Thus, only the feeder where short-circuit current has occurred is interrupted, and operations of the load apparatuses connected to normal feeders can be continued.

Preferably, the current threshold for the determination circuit <NUM> is set to be smaller than the maximum current in a reverse bias safe operating area (RBSOA) of the semiconductor switching element <NUM>. By this setting, failure of the semiconductor switching element <NUM> can be assuredly prevented.

As the semiconductor switching element <NUM>, an insulated-gate bipolar transistor (IGBT), a gate commutated turn-off thyristor (GCT), a metal-oxide-semiconductor field-effect transistor (MOSFET), or the like may be used. It is noted that, during normal operation, current constantly flows through the semiconductor switching element <NUM>, and therefore a unipolar element such as MOSFET, which exhibits small voltage drop, is preferable.

In the present embodiment, the current sensor <NUM> is provided at a position for detecting current flowing through the positive electric path 16a. The current sensor at this position directly detects reverse current flowing through the positive electric path from the output terminal side to the input terminal side. However, current proportional to the reverse current flowing through the positive electric path also flows through the negative electric path from the input terminal side to the output terminal side, and therefore, the current sensor may be provided at a position for detecting current flowing through the negative electric path 16b.

The second diode <NUM> is used for the purpose of stabilizing voltage across the semiconductor switching element <NUM> and the first diode <NUM>, and therefore current hardly flows through the second diode <NUM>, as compared to the semiconductor switching element <NUM>. Therefore, the rated current of the second diode <NUM> may be smaller than the rated current of the semiconductor switching element <NUM> and the rated current of the first diode <NUM>. Using the second diode having small rated current enables size reduction and cost reduction of the DC distribution panel.

Preferably, the control circuit <NUM> is provided with a control power supply <NUM> for supplying power to the current sensor <NUM>, the determination circuit <NUM>, and the gate drive circuit <NUM>. As shown in <FIG>, the control power supply <NUM> can obtain power from the output-side positive electric path and the output-side negative electric path.

In consideration of a case where power might not be supplied to the DC distribution panel, it is preferable that elements that can be cooled naturally by air without the need of power are used as the semiconductor switching element <NUM>, the first diode <NUM>, and the second diode <NUM>.

<FIG> is a schematic diagram of a DC distribution panel according to embodiment <NUM>. The DC distribution panel of the present embodiment is configured such that an inrush current prevention unit is added to the DC distribution panel described in embodiment <NUM>.

As shown in <FIG> in embodiment <NUM>, a capacitor is present in the load apparatus. Further, the DC distribution panel has the capacitor <NUM> in the reverse current interruption unit <NUM>. In a state in which the capacitor of the load apparatus or the capacitor of the DC distribution panel has no electric charge stored therein, e.g., in a state in which the DC distribution panel is initially connected to the AC/DC converter and output of DC power from the AC/DC converter is started, the capacitor is charged and thus excessive current might flow in the DC distribution panel in a short period of time. Such current is called inrush current. The inrush current can cause failure of the semiconductor switching element <NUM>, the first diode <NUM>, or the like of the reverse current interruption unit <NUM>.

As shown in <FIG>, the DC distribution panel <NUM> of the present embodiment has an inrush current prevention unit <NUM> provided on the positive electric path 16a between the short-circuit current interruption unit <NUM> and the reverse current interruption unit <NUM>. In the inrush current prevention unit <NUM>, a resistor <NUM> and a bypass switch <NUM> are connected in parallel to the positive electric path 16a. In addition, the DC distribution panel <NUM> has a voltage sensor <NUM> for measuring the charge voltage of the capacitor <NUM> of the reverse current interruption unit <NUM>. The voltage sensor <NUM> outputs the charge voltage of the capacitor <NUM> to the determination circuit <NUM> of the control circuit <NUM>. Further, the control circuit <NUM> has a switch drive circuit <NUM> for controlling opening/closing of the bypass switch <NUM>. The switch drive circuit <NUM> is connected to the determination circuit <NUM>. The determination circuit <NUM> determines whether or not charging of the capacitor <NUM> is completed on the basis of the detection value of the charge voltage of the capacitor <NUM> sent from the voltage sensor <NUM>, and if it is determined that the charging is completed, the determination circuit <NUM> issues a command for closing the bypass switch <NUM>, to the switch drive circuit <NUM>.

Next, operation of the DC distribution panel will be described. When input of DC power is started in a state in which the bypass switch <NUM> of the inrush current prevention unit <NUM> is opened, current in the positive electric path 16a flows through the resistor <NUM> of the inrush current prevention unit <NUM>, to charge the capacitor <NUM>. The determination circuit <NUM> determines whether or not charging of the capacitor <NUM> is completed on the basis of the charge voltage of the capacitor <NUM> detected by the voltage sensor <NUM>, and if it is determined that the charging is completed, the determination circuit <NUM> issues a command for closing the bypass switch <NUM>, to the switch drive circuit <NUM>. The switch drive circuit <NUM> closes the bypass switch <NUM>. By such operation, inrush current when the capacitor is charged flows through the resistor <NUM>, and thus the current value of the inrush current can be reduced.

In the DC distribution panel configured as described above, as in embodiment <NUM>, the reverse current interruption unit is provided for interrupting reverse current that flows through the positive electric path inside the circuit breaker from the output terminal side to the input terminal side. Thus, only the feeder where short-circuit current has occurred is interrupted, and operations of the load apparatuses connected to normal feeders can be continued.

In addition, the DC distribution panel has the inrush current prevention unit, whereby the current value of inrush current that occurs when the capacitor is charged can be reduced. Thus, it is possible to avoid failure of the semiconductor switching element, the first diode, or the like due to excessive inrush current.

Here, the resistance value of the resistor <NUM> will be described. The resistance value of the resistor <NUM> is denoted by R, the rated current of the first diode <NUM> is denoted by Ir, and the charge voltage of the capacitor <NUM> is denoted by V. The charge voltage of the capacitor <NUM> also corresponds to the rated voltage of the feeder. In this case, it is preferable that the resistance value R of the resistor <NUM> satisfies the following expression.

In a case of using the resistor that satisfies the above expression, the value of current flowing through the first diode <NUM> becomes smaller than the rated current value of the first diode <NUM>, and thus failure of the semiconductor switching element <NUM> and the first diode <NUM> can be assuredly prevented.

Preferably, the bypass switch <NUM> is opened in a non-operated state. Therefore, preferably, a normally-open switch is used.

In the present embodiment, the inrush current prevention unit is provided on the positive electric path. However, the inrush current prevention unit may be provided on the negative electric path.

<FIG> is a schematic diagram of a DC distribution panel according to embodiment <NUM>. The DC distribution panel of the present embodiment is configured such that an inductance element is added to the DC distribution panel described in embodiment <NUM>.

As described in embodiment <NUM>, in the control circuit <NUM>, the determination circuit <NUM> determines whether or not excessive reverse current has occurred on the basis of the current value detected by the current sensor <NUM>, and if it is determined that excessive reverse current has occurred, an off command is given to the gate drive circuit <NUM> to turn off the semiconductor switching element <NUM>. However, in actuality, a delay time arises in the current sensor <NUM>, the determination circuit <NUM>, and the gate drive circuit <NUM>, and therefore it is impossible to immediately turn off the semiconductor switching element <NUM>. During the delay time, the reverse current increases. Where the rated voltage of the feeder is denoted by V, the inductance of the positive electric path inside the DC distribution panel is denoted by L, and the delay time is denoted by T, an increase amount ΔI of the reverse current can be represented by the following expression.

Accordingly, in a case where the inductance of the electric path inside the DC distribution panel is small, ΔI becomes great and thus the reverse current immediately increases in a short period of time, so that the semiconductor switching element <NUM>, the first diode <NUM>, and the like might fail.

As shown in <FIG>, the DC distribution panel of the present embodiment has an inductance element <NUM> provided on the positive electric path 16a between the short-circuit current interruption unit <NUM> and the reverse current interruption unit <NUM>. The inductance of the positive electric path inside the DC distribution panel is made great by the inductance element <NUM>. Since the inductance of the positive electric path inside the DC distribution panel is made great, the increase amount of the reverse current in the delay time is reduced. As the inductance element <NUM>, for example, a coil or a tortuous electric path may be used.

In addition, the DC distribution panel has the inductance element provided on the positive electric path therein, whereby the increase amount of the reverse current until the semiconductor switching element is turned off from when the reverse current has occurred can be reduced. Thus, it is possible to avoid failure of the semiconductor switching element, the first diode, or the like due to excessive inrush current.

Where the overcurrent setting value of the determination circuit <NUM> is denoted by loc and the maximum current in the reverse bias safe operating area (RBSOA) of the semiconductor switching element <NUM> is denoted by Imax, it is preferable that an inductance value L of the inductance element <NUM> satisfies the following expression. Here, the overcurrent setting value loc of the determination circuit <NUM> is set to a greater value than the rated current (the rated current in the regeneration direction) of the load apparatus in the direction of the reverse current.

In a case of using the inductance element that satisfies the above expression, it is ensured that the value of current flowing through the semiconductor switching element becomes smaller than the maximum current value in the reverse bias safe operating area of the semiconductor switching element, and thus failure of the semiconductor switching element <NUM> and the like can be prevented.

In the present embodiment, the inductance element is provided on the positive electric path. However, the inductance element may be provided on the negative electric path. In addition, the inductance element is provided between the short-circuit current interruption unit <NUM> and the reverse current interruption unit <NUM>, but may be provided at any position as long as the position is on the electric path inside the DC distribution panel.

In the DC distribution panel of embodiment <NUM>, all the three circuit breakers have the short-circuit current interruption units and the reverse current interruption units. In a DC distribution panel according to embodiment <NUM>, the circuit breaker corresponding to the feeder that does not need the reverse current interruption unit has only the short-circuit current interruption unit.

<FIG> is a schematic diagram of the DC distribution panel according to the present embodiment. In the DC distribution panel <NUM> of the present embodiment, among the three circuit breakers provided, the circuit breaker <NUM> has only the short-circuit current interruption unit <NUM> and does not have the reverse current interruption unit.

<FIG> is a configuration diagram of the load apparatus connected to the output terminal <NUM> of the circuit breaker <NUM>. The load apparatus shown in <FIG> is the same as the power storage equipment shown in <FIG> in the above embodiment, but has a reverse flow prevention diode <NUM> provided between the input terminal 95a and the capacitor <NUM>. The reverse flow prevention diode <NUM> is provided for preventing reverse flow of discharge current from the capacitor <NUM> to the feeder side. Such a reverse flow prevention diode can be used in an apparatus such as lighting equipment that does not have a regeneration function.

In a case where the load apparatus having such a reverse flow prevention diode is connected to the output terminal <NUM>, for example, even when short-circuit has occurred in the load apparatus connected to the output terminal <NUM>, reverse current does not flow into the DC distribution panel <NUM> from the load apparatus connected to the output terminal <NUM>. Therefore, the circuit breaker <NUM> corresponding to the output terminal <NUM> need not have the reverse current interruption unit.

In addition, the circuit breaker corresponding to the feeder where reverse current does not occur has only the short-circuit current interruption unit and does not have the reverse current interruption unit. Therefore, it is possible to realize the DC distribution panel with a small size and at low cost.

Although the disclosure is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations to one or more of the embodiments of the disclosure.

Claim 1:
A DC distribution panel (<NUM>) comprising:
an input terminal (<NUM>) including a positive input terminal and a negative input terminal;
a plurality of circuit breakers (<NUM>, <NUM>, <NUM>) having short-circuit current interruption units (<NUM>) provided to at least either of positive electric paths (16a) and negative electric paths (16b) respectively branched from the positive input terminal and the negative input terminal; and
output terminals (<NUM>, <NUM>, <NUM>) including positive output terminals and negative output terminals of the plurality of circuit breakers (<NUM>, <NUM>, <NUM>), wherein
at least one of the plurality of circuit breakers (<NUM>, <NUM>, <NUM>) includes a reverse current interruption unit (<NUM>) for interrupting reverse current (<NUM>, <NUM>) flowing through the positive electric path (16a) from the output terminal side to the input terminal side,
characterised in that the reverse current interruption unit (<NUM>) has
a semiconductor switching element (<NUM>),
a first diode (<NUM>) connected in antiparallel to the semiconductor switching element (<NUM>),
a second diode (<NUM>) connected in series to the semiconductor switching element (<NUM>), and
a capacitor (<NUM>) connected in parallel with the series connection of the semiconductor switching element (<NUM>) and the second diode (<NUM>),
wherein the emitter of the semiconductor switching element (<NUM>) is connected to the input-side positive electric path (16a), and the collector thereof is connected to the output-side positive electric path (16a), wherein the cathode of the first diode (<NUM>) is connected to the output-side positive electric path (16a), and the anode thereof is connected to the input-side positive electric path (16a), wherein the cathode of the second diode (<NUM>) is connected to the input-side positive electric path (16a), and the anode thereof is connected to the input-side negative electric path (16b), and wherein the capacitor (<NUM>) is connected between the output-side positive electric path (16a) and the output-side negative electric path (16b).