JPH06351160A - Dc connection controller and dc connection release controller - Google Patents

Abstract

PURPOSE:To make it possible to connect two DC power-supply units in parallel without stopping the DC power-supply unit in operation, by closing a first DC electro-magnetic contactor when a coupling command is generated from outside, and closing a second DC electro-magnetic contactor when a difference in voltage between DC power supply units lowers to a given value of below. CONSTITUTION:A coupling releasing electro-magnetic contactor 13 is closed when a control unit 18 receives a coupling command 17 from the outside. Then, a difference in voltage between DC/DC converters (DC power supplies) 11 and 12 is detected by the control unit 18 with a voltage detector 16, and an electro- magnetic contactor 14 used for short-circuiting a charging resistor is closed when the voltage difference is lowered to a given value or below. Consequently, the DC/DC converters 11 and 12 are connected at any time regardless of the operation or stoppage of DC/DC converter 11 or 12 without a larger current of charging through the electrolytic capacitors 19 and 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system in which the DC outputs of a DC power supply device such as a DC / DC converter are connected in parallel, and the DC outputs are controlled to be connected or opened regardless of whether the power supply is being started or stopped. The present invention relates to a DC connection opening control device.

[0002]

2. Description of the Related Art Conventionally, a DC power supply system for connecting the DC outputs of two DC / DC converters has adopted one of the following two methods.

1) System without parallel connection In a DC power supply system without parallel connection as shown in FIG. 5, load contactors 2 and 3 are respectively provided on the load side of the DC / DC converters 4 and 5 as in the AC output power supply system. And a power supply / reception contactor 1 is provided between both DC / DC converters 4 and 5, and when one DC / DC converter stops, the load contactor on the converter side is opened and the power supply / reception contactor 1 is turned on. The other D being activated
The power was supplied from the C / DC converter.

However, in the case of this DC power supply system which is not connected in parallel, there is a power failure between the time when one DC / DC converter is stopped and the time when the power supply / reception operation is completed. That is, there is a problem in that the redundancy for the uninterruption of the power supply system due to the parallel connection is lost.

2) Parallel Connection System In the parallel connection DC power supply system shown in FIG.
Even if any one of the C / DC converters 4 and 5 is stopped, power can be supplied as is from the remaining DC / DC converters, so that there is an advantage that operation can be continued without a power failure on the load side. is there.

However, in the conventional DC power supply system connected in parallel, since the DC power supply devices such as the DC / DC converters 4 and 5 are provided with the large-capacity electrolytic capacitors 6 and 7, respectively, on the output side. There is a slight voltage difference between the two power supplies (for example, one power supply unit is stopped and the other power supply unit is started, or the load capacity of one power supply unit is large and the load capacity of the other power supply unit is large. When the DC power supply is connected, a large current flows from the power supply with a high voltage to the power supply with a low voltage, which causes a large current to charge the electrolytic capacitor, resulting in welding of the circuit connection and shortening of the life of the electrolytic capacitor. May cause
In order to avoid this, when connecting in parallel, it is necessary to stop all power supply devices to be connected for the time being and then perform the connection operation.When connecting and disconnecting, the power supply to the load side is stopped. There was a problem that the service deteriorated.

[0007]

As described above, in the case of the conventional DC power supply system in which the conventional DC power supply devices are not connected in parallel, the period from the stop of one DC power supply device to the completion of the power supply / reception operation. Since there is a power failure, there is a problem that redundancy, which is a great advantage of the DC power supply system, is lost against the uninterruption of the power supply system by parallel connection.

Further, in the case of the conventional DC power supply system for parallel connection, since each DC power supply device has a large-capacity electrolytic capacitor on the load side, there is a slight voltage difference on the output side (for example, If one power supply unit is stopped and the other power supply unit is started, or if the load capacity of one power supply unit is large and the load capacity of the other power supply unit is small, these DC power supply units When connected, a large current that charges the electrolytic capacitor flows from the power supply side with a high voltage to the power supply side with a low voltage, which may cause welding of the circuit connection part and shorten the life of the electrolytic capacitor. When connecting the DC power supply, it is necessary to stop all the power supplies to be connected for the time being and then perform the connection operation. Service power supply is stopped to have a problem to decrease.

The present invention has been made in view of such conventional problems, and in a DC power supply system for parallel connection, all DC power supply devices to be connected are connected or connected or opened without stopping. An object of the present invention is to provide a DC connection control device and a DC connection open control device that can be operated.

[0010]

According to a first aspect of the present invention, there is provided a DC connection control device, one of a first DC electromagnetic contactor having two or more poles for connecting and opening two DC power supply devices, and one of the DC power supply devices. And a second direct current electromagnetic contactor for connecting and opening
DC electromagnetic contactor and the charging resistance for charging the electrolytic capacitors of the two DC power supply devices, which are provided in parallel with the second DC electromagnetic contactor, and the voltages on both sides of the second DC electromagnetic contactor. The first DC electromagnetic contactor is turned on in response to a voltage difference detector for detecting a difference and an external connection command between two DC power supply devices, and then the voltage difference detected by the voltage difference detector is a predetermined value. And a control unit for performing connection control for closing the second DC electromagnetic contactor when the following occurs.

According to a second aspect of the present invention, there is provided a DC connection opening control device having a first DC electromagnetic contactor having two or more poles for connecting and opening two DC power supply devices, one of the DC power supply devices and the first DC power supply device. A second direct current electromagnetic contactor for connecting and opening the electromagnetic contactor and a second direct current electromagnetic contactor provided in parallel;
A charging resistor for charging an electrolytic capacitor of each of the two DC power supply devices, a voltage difference detector for detecting a voltage difference between both sides of the second DC electromagnetic contactor, and a connection between two DC power supply devices from the outside. In response to the command, the first DC electromagnetic contactor is turned on, and then the second DC electromagnetic contactor is turned on when the voltage difference detected by the voltage difference detector becomes equal to or less than a predetermined value. In response to an opening command between the two DC power supply devices from the controller, the second DC electromagnetic contactor is opened, and thereafter, the opening control for opening the first DC electromagnetic contactor is provided. .

According to a third aspect of the present invention, there is provided a DC connection control device comprising:
A direct current electromagnetic contactor having two or more poles for connecting and opening the two direct current power supply devices, and a charging transistor for performing intermittent control of the charging current flowing through the electrolytic capacitors of the two direct current power supply devices via the direct current electromagnetic contactor, A current detector that detects the current flowing through the charging transistor and a DC electromagnetic contactor are turned on in response to an external connection command between two DC power supply devices, and then the current detection value of the current detector The control unit controls the connection so that the base current of the charging transistor is gradually increased to conduct the current while monitoring so as not to exceed a certain value.

According to a fourth aspect of the present invention, there is provided a DC connection opening control device having two or more poles of DC electromagnetic contactors for connecting and opening two DC power supply devices, and two DC power supply devices via the DC electromagnetic contactors, respectively. Charging transistor that controls the on / off state of the charging current flowing in the electrolytic capacitor, a current detector that detects the current flowing in the charging transistor, and a DC electromagnetic contactor that receives an external connection command between two DC power supply devices. Then, the current detection value of the current detector is monitored so that it does not exceed a certain value, while gradually increasing the base current of the charging transistor to make it conductive, and two direct currents from the outside. While receiving the open command between the power supply units and monitoring the current detection value of the current detector, gradually decrease the base current of the charging transistor until it falls below the specified value, and turn it off. , Then, in which a control unit that performs the opening control for opening the DC electromagnetic contactor.

[0014]

According to the first aspect of the present invention, when the two DC power supply devices are connected to each other, the control section first receives the connection command from the outside between the two DC power supply devices, and the control unit first operates the first DC power supply device. By turning on the electromagnetic contactor and then performing connection control to turn on the second DC electromagnetic contactor when the voltage difference detected by the voltage difference detector becomes a predetermined value or less, the first DC electromagnetic contact is performed. The charging resistor suppresses a large current flowing from the starting DC power supply unit into the electrolytic capacitor on the DC power supply unit side that was stopped when the power supply was turned on. It can be controlled so as to flow into the electrolytic capacitor on the side of the power supply device, and the two DC power supply devices can be connected in parallel without stopping the DC power supply device that is being activated.

According to another aspect of the present invention, when the two DC power supply devices are connected to each other, when the two DC power supply devices are connected to each other, the controller first receives a connection command from the outside to connect the two DC power supply devices. The direct current electromagnetic contactor is turned on, and then the second direct current electromagnetic contactor is turned on when the voltage difference detected by the voltage difference detector becomes a predetermined value or less. When the contactor is turned on, the charging resistor suppresses the large current flowing from the starting DC power supply unit into the electrolytic capacitor on the DC power supply unit that had been stopped until then, and the current gradually stops. It can be controlled to flow into the electrolytic capacitor on the side of the DC power supply, and even with a small capacity DC electromagnetic contactor, two DC power supplies can be connected in parallel without stopping the DC power supply during startup. But Kill.

When the two DC power supply units are opened, the control unit first opens the second DC electromagnetic contactor in response to an opening command from the outside between the two DC power supply units, and then the first DC power supply unit. By performing the opening control for opening the DC electromagnetic contactor of, first, by opening the second DC electromagnetic contactor, a breaking current flows between both DC power supply devices through the charging resistor,
Since the first DC electromagnetic contactor is opened after the current has been sufficiently reduced, a large breaking current does not flow at the time of opening, and the two DC power supply devices are opened even if mutual currents flow between them. The breaking current that flows at this time is small, and even a DC electromagnetic contactor with a small current capacity can be safely opened without stopping the DC power supply device.

According to another aspect of the present invention, there is provided a DC connection control device,
When connecting two DC power supply devices, the control unit turns on the DC electromagnetic contactor in response to an external connection command between the two DC power supply devices, and then the current detection value of the current detector By controlling the connection so that the base current of the charging transistor is gradually increased and conducting while monitoring so that it does not exceed a certain value, the DC power supply side that was stopped until then when the DC electromagnetic contactor was turned on A large amount of current that is about to flow into the electrolytic capacitor from the DC power supply device that is starting up can be suppressed by the charging transistor, and the current can be controlled to gradually flow into the electrolytic capacitor on the side of the DC power supply device that is stopped. It is possible to connect two DC power supply devices in parallel without stopping the DC power supply device that is being activated.

In the DC connection opening control device of the invention of claim 4, when connecting the two DC power supply devices, a DC electromagnetic contactor is turned on upon receiving a connection command from the outside between the two DC power supply devices. Then, while monitoring the current detection value of the current detector so that it does not exceed a certain value, the base current of the charging transistor is gradually increased to perform conduction control so that the DC electromagnetic contactor is turned on. The charging transistor suppresses the large current that would otherwise flow into the electrolytic capacitor on the side of the DC power supply unit that had been stopped until then from the DC power supply unit that was in operation, and the side of the DC power supply unit where the current was gradually stopped It can be controlled so that it flows into the electrolytic capacitor, and even if it is a small capacity DC electromagnetic contactor, two DC power supply devices can be connected without stopping the DC power supply device during startup. It can be connected to Parallel.

Further, when the two DC power supply devices are opened, the current detection value of the current detector is monitored while receiving an opening command from the outside between the two DC power supply devices until the current value falls below a predetermined value. By gradually lowering the base current of the charging transistor to shut it off, and then opening the DC electromagnetic contactor to open it, the breaking current is gradually reduced through the charging transistor, and then the DC current is sufficiently reduced before the DC Since the electromagnetic contactor is opened, a large breaking current does not flow at the time of opening, and even if mutual currents are flowing between the two DC power supplies, the breaking current flowing when opening both DC power supplies is small. Even a DC electromagnetic contactor with a small current capacity can be safely opened without stopping the DC power supply device.

[0020]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows a circuit diagram of a common embodiment of the invention of the DC connection control device of the invention of claim 1 and the invention of the DC connection open control device of claim 2, wherein DC / DC is used as a DC power supply device.
In order to connect the DC converters 11 and 12 in parallel, the two-pole connection opening electromagnetic contactor 13, the one-pole charging resistance short-circuiting electromagnetic contactor 14, and the charging resistance short-circuiting electromagnetic contactor 14 are connected in parallel. The charging resistor 15 provided, the voltage detector 16 for monitoring the voltage across the charging resistor 15, the electromagnetic contactor 13 for opening the connection upon receiving the connection command 17 from the outside, and then the voltage difference detector Electromagnetic contactor 1 for charging resistance short circuit when the voltage difference detected by 16 becomes less than a predetermined value
The control unit 18 performs connection control for turning on 4 and opening the charging contact short-circuiting electromagnetic contactor 14 in response to an opening command 17 from the outside, and then opening control for opening the connection opening electromagnetic contactor 13. It consists of And each DC /
The DC converters 11 and 12 are provided with electrolytic capacitors 19 and 20, respectively.

The operation of the DC connection control device and the DC connection opening control device having the above-mentioned configuration will be described. << Connection operation >> When the control unit 18 receives the connection command 17 from the outside, first the connection opening electromagnetic contactor 13 is turned on. To do.
At this time, for example, when the DC / DC converter 11 is starting and the DC / DC converter 12 is stopping, the charging current whose peak value is limited is stopped from the starting DC / DC converter 11 through the charging resistor 15. It flows to the DC / DC converter 12 side, and the electrolytic capacitor 20 is charged (steps S1 and S in the flowchart of FIG. 2).
2).

The control unit 18 controls the voltage detector 16 to control both D
The voltage difference between the C / DC converters 11 and 12 is detected, and when the voltage difference becomes a predetermined value or less, a charging resistor short circuit magnetic contactor 14
Is input (steps S4 and S5).

By the above operation, it is possible to connect the DC circuit without flowing a large charging current to the electrolytic capacitor 19 or 20, regardless of whether the DC / DC converters 11 and 12 are being started or stopped. Will be able to.

<< Opening Operation >> When the control unit 18 receives the opening command 17 from the outside, first, the charging resistor short-circuiting electromagnetic contactor 14 is opened. As a result, both DC / DC converters 1
Even if currents are flowing between 1 and 12, current will flow to the charging resistor 15 and be reduced (step S
6, S7).

Therefore, the control unit 18 monitors the detection voltage of the voltage detector 16 and opens the connection opening electromagnetic contactor 13 if it determines that the current has decreased to a predetermined value or less (steps S8 to S10). ).

By controlling the opening by the control unit 18, the current is sufficiently small when the connection opening electromagnetic contactor 13 is opened, and the breaking current is also small. Therefore, the connection opening electromagnetic contactor 13
Can safely operate even if the breaking current capacity is small, and the overall size of the device can be reduced.

Next, a common embodiment of the DC connection control device of the invention of claim 3 and the DC connection open control device of the invention of claim 4 will be described with reference to FIG. In the embodiment of the DC connection control device and the DC connection open control device of FIG. 3, the elements common to those of the device of FIG. 1 are designated by the same reference numerals, but the electromagnetic contactor 14 for charging resistance short circuit is further added. In place of the voltage detector 16, a current detector 22 is provided in place of the voltage detector 16.
Then, the control unit 23 receives the connection command 17 from the outside, turns on the electromagnetic contactor 13 for opening the connection, and then monitors the current detection value of the current detector 22 so that it does not exceed a certain value. In response to the connection control for gradually increasing the base current of the charging transistor 21 to make it conductive, and receiving the open command 17 from the outside, the base current of the charging transistor 21 is gradually reduced while watching the detection current of the current detector 22. It has a function of turning it off and then performing opening control for opening the connection opening electromagnetic contactor 13.

Next, the operations of the DC connection control device and the DC connection open control device having the above-mentioned configurations will be described.

<< Connection Operation >> When the control unit 23 receives the connection command 17 from the outside, first, the connection opening electromagnetic contactor 13 is opened.
Throw in. At the beginning of this charging, the charging transistor 2
In No. 1, no base current is given, and no current flows between both DC / DC converters 11 and 12 (steps S11 and S12 in the flowchart of FIG. 4).

However, after the connection opening electromagnetic contactor 13 of the control unit 23 is turned on, the base current of the charging transistor 21 is gradually increased while watching the detected current of the current detector 22 not to exceed a predetermined value. Finally, a saturation current is applied (steps S13 and S1).
4).

Thus, also in the case of this embodiment, a large charging current is applied to the electrolytic capacitor 19 or 20 regardless of whether the DC / DC converters 11 and 12 are being started or stopped by the above operation. It becomes possible to connect a DC circuit without flowing.

<< Opening Operation >> When the control unit 23 receives an opening command 17 from the outside, first the base current of the charging transistor 21 is gradually reduced and turned off while watching the detection current of the current detector 22, and both DCs are turned on. / DC converters 11 and 12
Even if currents are flowing between each other, the currents are gradually reduced and cut off (steps S15 to S17).

After that, the connection opening electromagnetic contactor 13 is opened to complete the opening operation (step S18).

Thus, also in the case of this embodiment, the control unit 2
By controlling the opening by 3, the current is sufficiently small when the connection opening electromagnetic contactor 13 is opened, and the breaking current is also small. Therefore, the connection opening electromagnetic contactor 13 can be safely operated even if it has a small breaking current capacity, and the overall size of the device can be reduced.

In the case of the second embodiment, the charging resistor 15 used in the first embodiment can be omitted, and the contactless charging transistor 21 is used instead of the charging resistor short-circuiting electromagnetic contactor 14. Therefore, maintenance can be reduced. In the case of the first embodiment, the charging resistor 1
5 and the charging resistor short-circuiting electromagnetic contactor 14 are used, when both DC / DC converters 11 and 12 have different load sizes, both DC / DC converters 11 and 1 are used.
A slight voltage difference may occur between the two, and a small inrush current may flow into the electrolytic capacitor at the moment when the charging resistor 15 is short-circuited. In the case of the second embodiment, however, the charging transistor 21 connects the two. Both DC / DC converter 1
Since the impedance of 1 and 12 decreases linearly,
Inrush current to the electrolytic capacitor can be completely suppressed.

[0036]

According to the DC connection control device of the first aspect of the present invention, when the two DC power supply devices are connected, the control unit first receives the connection command between the two DC power supply devices from the outside. When the first DC electromagnetic contactor is turned on and then the voltage difference detected by the voltage difference detector falls below a predetermined value, the second
Since the connection control for turning on the DC electromagnetic contactor is performed, when the first DC electromagnetic contactor is turned on, the DC power supply which has been stopped by the electrolytic capacitor on the side of the DC power supply device which has been stopped until then is being activated. A large current that is about to flow from the equipment can be suppressed by the charging resistance, and the current can be controlled to gradually flow into the electrolytic capacitor on the side of the DC power supply that is stopped, and the DC power supply that is starting can be stopped. Instead, two DC power supply devices can be connected in parallel.

According to the DC connection open control device of the second aspect of the invention, in addition to the invention of the first aspect, when the two DC power supply devices are opened, the two DC power supply devices are opened from the outside. In response to the command, the control unit first opens the second DC electromagnetic contactor and then performs the opening control to open the first DC electromagnetic contactor. When opened, a breaking current flows between the two DC power supplies through the charging resistor, and the first DC electromagnetic contactor is opened after the current is sufficiently reduced, so that a large breaking current does not flow at the time of opening. Even if currents flow between the two DC power supply devices, the breaking current that flows when the two DC power supply devices are opened can be reduced, and even a small-capacity DC electromagnetic contactor can be activated. Connect two DC power supplies in parallel without stopping the DC power supply of
Further, it can be opened, and even a small-capacity DC electromagnetic contactor can be safely connected and opened without stopping the DC power supply device, and the overall size of the device can be reduced.

According to the DC connection control device of the third aspect of the present invention, when the two DC power supply devices are connected, the control unit receives a DC connection command between the two DC power supply devices from the outside, and the control unit receives the DC electromagnetic contact. After that, the connection control is performed so that the base current of the charging transistor is gradually increased and conducted while monitoring the current detection value of the current detector so that it does not exceed a certain value. When the DC electromagnetic contactor is turned on, the charging transistor suppresses the large current that would flow into the electrolytic capacitor on the side of the DC power supply that had been stopped until then from the activating DC power supply. It can be controlled so as to flow into the electrolytic capacitor on the side of the DC power supply that is stopped, and two DC power supplies can be connected in parallel without stopping the DC power supply that is starting. It is possible.

According to the DC connection disconnection control device of the invention of claim 4, in addition to the invention of claim 3, when the two DC power supply devices are opened, the two DC power supply devices are externally opened. In response to the command, while monitoring the current detection value of the current detector, the base current of the charging transistor is gradually reduced and cut off until it becomes a predetermined value or less, and then the DC electromagnetic contactor is opened. Since the breaking current is gradually reduced through the charging transistor and the DC electromagnetic contactor is opened after the current is sufficiently reduced, a large breaking current does not flow at the time of opening. Even if currents flow between the two DC power supply devices, the breaking current that flows when the two DC power supply devices are opened can be reduced, and even a small-capacity DC electromagnetic contactor can be activated. 2 without stopping the DC power supply of
Two DC power supplies can be connected in parallel and opened, and even a DC electromagnetic contactor with a smaller current capacity can be safely connected without stopping the DC power supply,
It can be opened, and the overall size of the device can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a common embodiment of the invention of claims 1 and 2.

FIG. 2 is a flowchart showing the operation of the above embodiment.

FIG. 3 is a circuit diagram of a common embodiment of the invention of claims 3 and 4.

FIG. 4 is a flowchart showing the operation of the above embodiment.

FIG. 5 is a circuit diagram of a conventional example.

FIG. 6 is a circuit diagram of another conventional example.

[Explanation of symbols]

 11, 12 DC / DC converter 13 Electromagnetic contactor for connection opening 14 Electromagnetic contactor for charging resistance short circuit 15 Charging resistance 16 Voltage detector 17 Connection opening command 18 Control unit 19, 20 Electrolytic capacitor 21 Charging transistor 22 Current detector 23 Control unit

Claims (4)

[Claims] 1. A first direct current electromagnetic contactor having two or more poles for connecting and opening two direct current power supply devices, and connecting one of the direct current power supply device and the first direct current electromagnetic contactor, A second direct-current electromagnetic contactor to be opened; and the second direct-current electromagnetic contactor provided in parallel with the second direct-current electromagnetic contactor.
A charging resistor for charging an electrolytic capacitor of each of the two DC power supply devices, a voltage difference detector for detecting a voltage difference between both sides of the second DC electromagnetic contactor, and an external DC power supply device between the two DC power supply devices. A connection in which the first DC electromagnetic contactor is closed in response to a connection command, and then the second DC electromagnetic contactor is closed when the voltage difference detected by the voltage difference detector becomes a predetermined value or less. A DC-coupling control device comprising a control unit for controlling. 2. A first direct current electromagnetic contactor having two or more poles, which connects and opens two direct current power supply devices, and a connection between one of the direct current power supply devices and the first direct current electromagnetic contactor, A second direct-current electromagnetic contactor to be opened; and the second direct-current electromagnetic contactor provided in parallel with the second direct-current electromagnetic contactor.
A charging resistor for charging an electrolytic capacitor of each of the two DC power supply devices, a voltage difference detector for detecting a voltage difference between both sides of the second DC electromagnetic contactor, and an external DC power supply device between the two DC power supply devices. A connection in which the first DC electromagnetic contactor is closed in response to a connection command, and then the second DC electromagnetic contactor is closed when the voltage difference detected by the voltage difference detector becomes a predetermined value or less. The second DC electromagnetic contactor is opened in response to control and an opening command from the outside between the two DC power supply devices, and then the first DC electromagnetic contactor is opened.
DC connection opening control device comprising a control unit for performing opening control for opening the DC electromagnetic contactor. 3. A direct current electromagnetic contactor having two or more poles, which connects and opens two direct current power supply devices, and an interruption of a charging current flowing through an electrolytic capacitor of each of the two direct current power supply devices via the direct current electromagnetic contactor. A charging transistor that performs control, a current detector that detects a current flowing in the charging transistor, and a DC electromagnetic contactor are turned on upon receiving a connection command between the two DC power supply devices from the outside, and thereafter, A DC connection control comprising a control unit for performing connection control for gradually increasing the base current of the charging transistor to conduct the current while monitoring the current detection value of the current detector so that it does not exceed a certain value. apparatus. 4. A direct current electromagnetic contactor having two or more poles, which connects and opens two direct current power supply devices, and an interruption of a charging current flowing through an electrolytic capacitor of each of the two direct current power supply devices via the direct current electromagnetic contactor. A charging transistor that performs control, a current detector that detects a current flowing in the charging transistor, and a DC electromagnetic contactor are turned on upon receiving a connection command between the two DC power supply devices from the outside, and thereafter, A connection control for gradually increasing the base current of the charging transistor to make it conductive while monitoring the current detection value of the current detector so that it does not exceed a certain value, and between the two DC power supply devices from the outside. While monitoring the current detection value of the current detector in response to the opening command of the above, the base current of the charging transistor is gradually reduced and turned off until it falls below a predetermined value. And a control unit for performing an opening control for opening the DC electromagnetic contactor thereafter.