DESCRIPTION BIDIRECTIONM. BUCK BOOST DC-DC CONVERTER, RAILWAY COACH DRIVE CONTROL SYSTEM, AND RAILWAY FEEDER SYSTEM TECHNICAL FIELD [1] The present invention relates to a DC-DC converter that is used when direct-current voltage sources are connected to each other, and is applicable to, for example, an electric vehicle or the like on which a power storage device is mounted. BACKGROUND ART [2] Conventionally, a technology has been known, in which a power storage device such as a secondary battery or an electric double-layer capacitor is applied to a railway system, and kinetic energy of a vehicle is effectively used by storing surplus regenerative power generated when braking the vehicle and using the stored power when the vehicle is accelerated. In this case, a buck boost converter (hereinafter, a bidirectional buck boost DC-DC converter) capable of controlling power in bidirectional directions is used for connecting a direct-current wire and the power storage device (for example. Patent Document 1). [3] Patent Document 1: Japanese Patent Application Laid-open No. 2005-206111 r-N [4] However, the bidirectional buck boost DC-DC converter as described above cannot control current when a primary-side voltage is lower than a secondary-side voltage in the converter because of the circuit configuration. Therefore, the bidirectional buck boost DC-DC converter needs to be used under the condition that the primary-side voltage is always higher than the secondary-side voltage. [5] For avoiding such a problem, a DC-DC converter (hereinafter, a bidirectional buck boost DC-DC converter) is useful, which is capable of causing power to flow bidirectionally, from the primary side to the secondary side and from the secondary side to the primary side regardless of a magnitude relation between the primary-side voltage and the secondary-side voltage of the bidirectional buck boost DC-DC converter. The circuit configuration thereof is disclosed, for example, in Patent Document 2. [6] Patent Document 2: Japanese Patent Application Laid-open No. 2001-268900 DISCLOSURE OF INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [7] However, in the bidirectional buck boost DC-DC converter disclosed in Patent Document 2, an operation- pattern of switching elements is determined for each of four operation modes, i.e., a case of setting the primary- side voltage higher than the secondary-side voltage and a case of setting the primary-side voltage lower than the secondary-side voltage when power flows from the primary side to the secondary side, and a case of setting the primary-side voltage higher than the secondary-side voltage and a case of setting the primary-side voltage lower than the secondary-side voltage when power flows from the secondary side to the primary side. Therefore, for example, this technology does not consider a case where the primary- side voltage and the secondary-side voltage are the same and a case where the power flow is zero, so that it is impossible to continuously transmit between the operation modes. [8] Moreover, the conduction rate of each switching element is described to be controlled by volume, so that it is not considered to automatically control the power flow on instantaneous value basis. Thus, it is impossible to automatically control a direction and a magnitude of power from the primary side to the secondary side and from the secondary side to the primary side in the DC-DC converter to a desired value continuously on instantaneous value basis. [9] The present inven"cion is accomplished to solve such problems, and an object of the present invention is to provided a bidirectional buck boost DC-DC converter, railway coach drive system, and railway feeder system capable of causing power to flow bidirectionally from a primary side to a secondary side and from the secondary side to the primary side regardless of a magnitude relation between a secondary-side voltage and a primary-side voltage in a state where different direct-current voltage sources are connected to the primary side and the secondary side in the DC-DC converter and automatically controlling a direction and a magnitude of the power to a desired value continuously on instantaneous value basis. MEANS FOR SOLVING PROBLEM According to an aspect of the present invention, there is provided a bidirectional buck boost DC-DC converter in which a direct-current power is supplied bidirectionally between two direct-current voltage sources of a primary-side power supply and a secondary-side power supply. The bidirectional buck boost DC-DC converter includes a primary-side converting unit that is connected to an input/output terminal of the primary-side power supply and perform;s a power conversion operation on the primary-side power supply; a secondary-side converting unit that is connected to an input/output terminal of the secondary-side power supply and performs a power conversion operation on the secondary-side power supply; a coupling unit that connects the primary-side converting unit and the secondary-side converting unit to each other and that mediates supply and reception of a power between the primary-side converting unit and the secondary-side converting unit; and a control unit that detects at least a voltage of the primary-side converting unit on a power supply side and a voltage of the secondary-side converting unit on a power supply side from among the voltage of the primary-side converting unit on a power supply side, the voltage of the secondary-side converting unit on a power supply side, and a voltage at an arbitrary point between a positive-electrode-side connecting terminal and a negative- electrode-side connecting terminal of the coupling unit each connecting the primary-side converting unit and the secondary-side converting unit, detects at least one of a current flowing into/out of the primary-side converting unit, a current flowing into/out of the secondary-side converting unit, and a current flowing into/out of the coupling unit, controls so that a selected one of detected currents is consistent with a command value corresponding to the selected one of the detected currents, and that controls a power conversion operation on the primary-side converting unit and the secondary-side converting unit based on detected voltage of the primary-side converting unit on a power supply side, detected voltage of the secondary-side converting unit on a power supply side, the selected one of the currents, and a signal based on the command value corresponding to the selected one of the currents such that a direction and a magnitude of a power flowing bidirectionally between the primary-side power supply and the secondary-side power supply are controlled to be continuously variable on instantaneous value basis regardless of a magnitude relation between a voltage of the primary-side power supply and a voltage of the secondary- side power supply in a state where the two direct-current voltage sources are connected to each other. According to another aspect of the present invention, there is provided a railway coach drive control system including an inverter for drive control that feeds a power supplied from a wire to an electric motor as a drive power; a power storage device that stores a power supplied from the wire; and a bidirectional buck boost DC-DC converter that is provided between the wire and the power storage device and controls a power of the wire and the power storage device bidirectionally. The the bidirectional buck boost DC-DC converter including a primary-side converting unit that is connected to an input/output terminal of the primary-side power supply and performs a power conversion operation on the primary-side power supply; a secondary- side converting unit that is connected to an input/output terminal of the secondary-side power supply and performs a power conversion operation on the secondary-side power supply; a coupling unit that connects the primary-side converting unit and the secondary-side converting unit to each other and that mediates supply and reception of a power between the primary-side converting unit and the secondary-side converting unit; and a control unit that detects at least a voltage of the primary-side converting unit on a power supply side and a voltage of the secondary- side converting unit on a power supply side from among the voltage of the primary-side converting unit on a power supply side, the voltage of the secondary-side converting unit on a power supply side, and a voltage at an arbitrary point between a positive-electrode-side connecting terminal and a negative-electrode-side connecting terminal of the coupling unit each connecting the primary-side converting unit and the secondary-side converting unit, detects at least one of a current flowing into/out of the primary-side converting unit, a current flowing into/out of the secondary-side converting unit, and a current flowing into/out of the coupling unit, controls so that a selected one of detected currents is consistent with a command value corresponding to the selected one of the detected currents, and that controls a power conversion operation on the primary-side converting unir and the secondary-side converting unit based on detected voltage of the primary- side converting unit on a power supply side, detected voltage of the secondary-side converting unit on a power supply side, the selected one of the currents, and a signal based on the command value corresponding to the selected one of the currents such that a direction and a magnitude of a power flowing bidirectionally between the primary-side power supply and the secondary-side power supply are controlled to be continuously variable on instantaneous value basis regardless of a magnitude relation between a voltage of the primary-side power supply and a voltage of the secondary-side power supply in a state where the two direct-current voltage sources are connected to each other. According to still another aspect of the present invention, there is provided a railway feeder system that supplies a power to a vehicle by a direct-current power source connected to a wire and a rail. The railway feeder system including an inverter for drive control that feeds a power supplied from the wire to an electric motor as a drive power; a power storage device that stores a power supplied from the wire; and a bidirectional buck boost DC- DC converter that is provided between the wire and the power storage device and controls a power of the wire and the power storage device bidirectionally. The bidirectional buck boost DC-DC converter including a primary-side converting unit that is connected to an input/output terminal of the primary-side power supply and performs a power conversion operation on the primary-side power supply; a secondary-side converting unit that is connected to an input/output terminal of the secondary-side power supply and performs a power conversion operation on the secondary-side power supply; a coupling unit that connects the primary-side converting unit and the secondary-side converting unit to each other and that mediates supply and reception of a power between the primary-side converting unit and the secondary-side converting unit; and a control unit that detects at least a voltage of the primary-side converting unit on a power supply side and a voltage of the secondary-side converting unit on a power supply side from among the voltage of the primary-side converting unit on a power supply side, the voltage of the secondary-side converting unit on a power supply side, and a voltage at an arbitrary point between a positive-electrode-side connecting terminal and a negative- electrode-side connecting terminal of the coupling unit each connecting the primary-side converting unit and the secondary-side converting unit, detects at least one of a current flowing into/out of the primary-side converting unit, a current flowing into/out of the secondary-side converting unit, and a current flowing into/out of the coupling unit, controls so that a selected one of detected currents is consistent with a coininand value corresponding to the selected one of the detected currents, and that controls a power conversion operation on the primary-side converting unit and the secondary-side converting unit based on detected voltage of the primary-side converting unit on a power supply side, detected voltage of the secondary-side converting unit on a power supply side, the selected one of the currents, and a signal based on the command value corresponding to the selected one of the currents such that a direction and a magnitude of a power flowing bidirectionally between the primary-side power supply and the secondary-side power supply are controlled to be continuously variable on instantaneous value basis regardless of a magnitude relation between a voltage of the primary-side power supply and a voltage of the secondary- side power supply in a state where the two direct-current voltage sources are connected to each other. [0011] According to an aspect of the present invention, there is provided a bidirectional buck boost DC-DC converter in which a direct-current power is supplied bidirectionally between two direct-current voltage sources of a primary-side power supply and a secondary-side power supply. The bidirectional buck boost DC-DC converter includes a primary-side converting unit that is connected to an input/output terminal of the primary-side power supply and performs a power conversion operation on the primary-side power supply; a secondary-side converting unit that is connected to an input/output terminal of the secondary-side power supply and performs a power conversion operation on the secondary-side power supply; a coupling unit that connects the primary-side converting unit and the secondary-side converting unit to each other and that mediates supply and reception of a power between the primary-side converting unit and the secondary-side converting unit; and a control unit that detects at least a voltage of the primary-side converting unit on a power supply side and a voltage of the secondary-side converting unit on a power supply side from among the voltage of "che primary-side converting unit on a power supply side, the voltage of the secondary-side converting unit on a power supply side, and a voltage at an arbitrary point between a positive-electrode-side connecting terminal and a negative- electrode-side connecting terminal of the coupling unit each connecting the primary-side converting unit and the secondary-side converting unit, detects at least one of a current flowing into/out of the primary-side converting unit, a current flowing into/out of the secondary-side converting unit, and a current flowing into/out of the coupling unit, controls so that a selected one of detected currents is consistent with a command value corresponding to the selected one of the detected currents, and that controls a power conversion operation on the primary-side converting unit and the secondary-side converting unit based on detected voltage of the primary-side converting unit on a power supply side, detected voltage of the secondary-side converting unit on a power supply side, the selected one of the currents, and a signal based on the command value corresponding to the selected one of the currents, so that a direction and a magnitude of a power flowing bidirectionally between the primary-side power supply and the secondary-side power supply can be controlled to be continuously variable on instantaneous value basis regardless of a magnitude relation between a voltage of the primary-side power supply and a voltage of the secondary-side power supply in a state where the two direct-current voltage sources are connected to each other. BRIEF DESCRIPTION OF DRAWINGS [0012] [Fig. 1] Fig. 1 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a first embodiment. [Fig. 2] Fig. 2 is a diagram illustrating a configuration example of a control unit 30a according.to the first embodiment. [Fig. 3] Fig. 3 is a diagram illustrating a configuration example of a current-command converting unit 31a according to the first embodiment. [Fig. 4] Fig. 4 is a diagram illustrating a configuration example of a current-command adjusting unit 32a according to the first embodiment. [Fig. 5] Fig. 5 is a diagram illustrating a configuration example of a current control unit 33a according to the first embodiment. [Fig. 6] Fig. 6 is a diagram illustrating a configuration example of a modulation ratio command generating unit 34a according to the first embodiment. [Fig. 7] Fig. 7 is a diagram illustrating a configuration example of a gate-signal generating unit 35a according to the first embodiment. [Fig. 8] Fig. 8 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the first embodiment. [Fig. 9] Fig. 9 is a diagram illustrating a result of a simulation of operation waveform.s of the bidirectional buck boost DC-DC converter according to the first embodiment. [Fig. 10] Fig. 10 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the first embodiment. [Fig. 11] Fig. 11 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the first embodiment. [Fig. 12] Fig. 12 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the first embodiment. [Fig. 13] Fig. 13 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the first embodiment. [Fig. 14] Fig. 14 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the first embodiment. [Fig. 15] Fig. 15 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the first embodiment. [Fig. 16] Fig. 15 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a second embodiment. [Fig. 17] Fig. 17 is a diagram illustrating a configuration example of a control unit 30b according to the second embodiment. [Fig. 18] Fig. 18 is a diagram illustrating a configuration example of a current-command adjusting unit 32b according to the second embodiment. [Fig. 19] Fig. 19 is a diagram illustrating a configuration example of a primary-side capacitor-voltage upper-limit limiting operation-amount calculating unit 60 according to the second embodiment. [Fig. 20] Fig. 20 is a diagram illustrating a configuration example of a primary-side capacitor-voltage lower-limit limiting operation-amount calculating unit 61 according to the second embodiment. [Fig. 21] Fig. 21 is a diagram illustrating a configuration example of a secondary-side capacitor-voltage upper-limit limiting operation-amount calculating unit 62 according to the second embodiment. [Fig. 22] Fig. 22 is a diagram illustrating a configuration example of a secondary-side capacitor-voltage lower-limit limiting operation-amount calculating unit 63 according to the second embodiment. [Fig. 23] Fig. 23 is a diagram illustrating a configuration example of a primary-side switching-circuit- current upper-limit limiting operation-amount calculating unit 66 according to the second embodiment. [Fig. 24] Fig. 24 is a diagram illustrating a configuration example of a primary-side switching-circuit- current upper-limit limiting operation-amount calculating unit 67 according to the second embodiment. [Fig. 25] Fig. 25 is a diagram illustrating a configuration example of a secondary-side switching- circuit-current upper-limit limiting operation-amount calculating unit 68 according to the second embodiment. [Fig. 26] Fig. 26 is a diagram illustrating a configuration example of a secondary-side switching- circuit-current upper-limit limiting operation-amount calculating unit 69 according to the second embodiment. [Fig. 27] Fig. 27 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a third embodiment. [Fig. 28] Fig. 28 is a diagram illustrating a configuration example of a control unit 30c according to the third embodiment. [Fig. 29] Fig. 29 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a fourth embodiment. [Fig. 30] Fig. 30 is a diagram illustrating a configuration example of a control unit 30d according to the fourth embodiment. [Fig. 31] Fig. 31 is a diagram illustrating a configuration example of a current-command converting unit 31b according to the fourth embodiment. [Fig. 32] Fig. 32 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a fifth-embodiment. [Fig. 33] Fig. 33 is a diagram illustrating a configuration example of a control unit 30e according to the fifth embodiment. [Fig. 34] Fig. 34 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a sixth embodiment. [Fig. 35] Fig. 35 is a diagram illustrating a configuration example of a control unit 30f according to the sixth embodiment. [Fig. 36] Fig. 36 is a configuration diagram of a ^ bidirectional buck boost DC-DC converter according to a seventh embodiment. [Fig. 37] Fig. 37 is a diagram illustrating a configuration example of a control unit 30g according to the seventh embodiment. [Fig. 38] Fig. 38 is a diagram illustrating a configuration example of a modulation ratio command generating unit 34b according to the seventh embodiment. [Fig. 39] Fig. 39 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the seventh embodiment, /-N [Fig. 40] Fig. 40 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the seventh embodiment. [Fig. 41] Fig. 41 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the seventh embodiment. [Fig. 42] Fig. 42 is a diagram illustrating a result of a simulation of an operation waveform of the bidirectional buck boost DC-DC converter according to the seventh embodiment. [Fig. 43] Fig. 43 is a configuration diagram of a bidirectional buck boost DC-DC converter according to an eighth embodiment. [Fig. 44] Fig. 44 is a diagram illustrating a configuration example of a control unit 30h according to the eighth embodiment. [Fig. 45] Fig. 45 is a diagram illustrating a configuration example of a modulation ratio command generating unit 34c according to the eighth embodiment. [Fig. 46] Fig. 46 is a diagram illustrating a configuration example of a gate-signal generating unit 35b according to the eighth embodiment. [Fig. 47] Fig. 47 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the eighth embodiment. [Fig. 48] Fig. 48 is a diagram illustrating a.result of.a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the eighth embodiment. [Fig. 49] Fig. 49 is a diagram illustrating a result of a simulation of operation waveforms of the bidirectional buck boost DC-DC converter according to the eighth embodiment. [Fig. 50] Fig. 50 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a ninth embodiment. [Fig. 51] Fig. 51 is a diagram illustrating a configuration example of a control unit 30i according to the ninth embodiment. [Fig. 52] Fig. 52 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a tenth embodiment. [Fig. 53] Fig. 53 is a diagram illustrating a configuration example of a control unit 30j according to the tenth embodiment. [Fig. 54] Fig. 54 is a diagram illustrating a configuration example of a modulation ratio command generating unit 34d according to the tenth embodiment. [Fig. 55] Fig. 55 is a diagram illustrating a configuration example of a gate-signal generating unit 35c according to the tenth embodiment. [Fig. 56] Fig. 56 is a configuration diagram of a bidirectional buck boost DC-DC converter according to an eleventh embodiment. [Fig. 57] Fig. 57 is a diagram illustrating a configuration example of a control unit 30k according to the eleventh embodiment. [Fig. 58] Fig. 58 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a twelfth embodiment. [Fig. 59] Fig. 59 is a diagram illustrating a configuration example of a control unit 30m according to the twelfth embodiment. [Fig. 60] Fig. 60 is a diagram illustrating a configuration example of a current-command converting unit 31c according to the twelfth embodiment. [Fig. 61] Fig. 61 is a diagram illustrating a configuration example of a current-command adjusting unit 32c according to the twelfth embodiment. [Fig. 62] Fig. 62 is a diagram illustrating a configuration example of a current control unit 33b according to the twelfth embodiment. [Fig. 63] Fig. 63 is a configuration diagram of a bidirectional buck boost DC-DC converter according to a thirteenth embodiment. [Fig. 64] Fig. 64 is a diagram illustrating a configuration example of a control unit 30n according to the thirteenth embodiment. [Fig. 65] Fig. 65 is a diagram illustrating a configuration example of a current-command converting unit 31d according to the thirteenth embodiment. [Fig. 66] Fig. 66 is a diagram illustrating a configuration example of a current-command adjusting unit 32d according to the thirteenth embodiment. [Fig. 67] Fig. 67 is a diagram illustrating a configuration example of a current control unit 33c according to the thirteenth embodiment. [Fig. 68] Fig. 68 is a diagram illustrating an application example of a bidirectional buck boost DC-DC converter according to a fourteenth embodiment. [Fig. 69] Fig. 69 is a diagram illustrating an application example of a bidirectional buck boost DC-DC converter according to a fifteenth embodiment. EXPLANATIONS OF LETTERS OR NUMERALS [0013] la: primary-side converting unit secondary-side converting unit coupling unit primary-side power supply secondary-side power supply coupling reactor current detector connecting line voltage detector current detector lb Ic 2a 2b 3 6 7 10: switching circuit 11, 12: switching element capacitor voltage detector control unit 13 14 30 31 32 current-command converting unit current-command adjusting unit current control unit 34: modulation ratio command generating unit 35: gate-signal generating unit 280: wire 281: pantograph 282: inverter for drive control 283: electric motor 284: rail 285: bidirectional buck boost DC-DC converter 286: power storage device 287: direct-current power source 288: vehicle 289: system control device BEST MODE(S) FOR CARRYING OUT THE INVENTION [14] First embodiment. Fig. 1 is a configuration diagram of a bidirectional buck boost DC-DC converter according to the first embodiment. As shown in Fig. 1, a primary-side converting unit la is connected to input/output terminals 23a and 24a of a primary-side power supply 2a including a primary-side power supply impedance 21a and a primary-side power supply voltage source 22a, and is connected to a secondary-side converting unit lb that is connected to input/output -eminals 23b and 24b of a secondary-side power supply 2b including a secondary-side power supply impedance 21b and a secondary-side power supply voltage source 22b through a coupling unit Ic including a coupling reactor 3 and a connecting line 5. [15] The primary-side converting unit la includes a primary-side switching circuit 10a in which switching elements 11a and 12a are connected in series, a primary- side capacitor 13a that is connected in parallel with the primary-side switching circuit 10a, and a voltage detector 14a that detects voltage of the primary-side capacitor 13a. A secondary-side converting unit lb is configured in the same manner, so that the primary-side converting unit la is explained below. [16] A positive terminal of the switching element 11a on an upper-arm side of the primary-side switching circuit 10a is a first terminal 15a, negative terminals of the switching element 12a on a lower-arm side of the primary- side switching circuit 10a are a second terminal 16a and a fourth terminal 18a, the first terminal 15a is connected to the positive electrode side of the primary-side capacitor 13a, and the second terminal 16a is connected to the negative electrode side of the primary-side capacitor 13a. The fourth terminal 18a is connected to a fourth terminal 18b of a secondary-side switching circuit 10b that is configured in the same manner via the connecting line 5, a third terminal 17a that is a connecting point between a negative electrode side of the switching element 11a on the upper-arm side and a positive electrode side of the switching element 12a on the lower-arm side and a third terminal 17b of the secondary-side switching circuit 10b configured in the same manner are connected by the coupling reactor 3, and a first current detector 4 that detects a Current IL of the coupling reactor 3 is provided. [17] Voltage between an arbitrary point between the third terminal 17a of the primary-side switching circuit 10a and the third terminal 17b of the secondary-side switching circuit 10b and the connecting line 5 is a coupling unit voltage VL, and a voltage detector 6 is provided for detecting the coupling unit voltage VL. [18] In Fig. 1, the configuration is such that a value that is the voltage between the coupling reactor 3 and the connecting line 5 detected by the voltage detector 6 is utilized as the coupling unit voltage VL; however, for example, the coupling unit voltage VL can be voltage between the third terminal 17a of the primary-side switching circuit 10a and the connecting line 5 or voltage between the third terminal 17b of the secondary-side switching circuit 10b and the connecting line 5. [19] Furthermore, a primary-side capacitor voltage VI output from the primary-side converting unit la, a secondary-side capacitor voltage V2 output from the secondary-side converting unit lb, the coupling reactor current IL output from the coupling unit Ic, and the coupling unit voltage VL are input to a control unit 30a. The control unir 30a ourputs gate signals Gla, Gib, G2a, and G2b for controlling on/off of each of the switching elements 11a, lib, 12a, and 12b to the primary-side converting unit la and the secondary-side converting unit lb so that a power PL that flows in the coupling unit Ic from the primary side to the secondary side is consistent with a command value P*. [20] The command value P* corresponds to a signal or the like that, for example, is input from a control device that controls a power storage system including the DC-DC converter of the present invention and is an upper-level device of the control unit 30a of the DC-DC converter. [0C21] Current in the first terminal 15a and current in the second terminal 16a of the primary-side switching circuit 10a, current in the coupling reactor 3 and current in the connecting line 5, and current in a first terminal 15b and current in a second terminal 16b of the secondary- side switching circuit 10b are each have the same value but flow in the opposite directions. Therefore, the content of the present invention can be accomplished by detecting any one of them. In the whole explanation in the specification, it is assumed that the current in the first terminal 15a of the primary-side switching circuit 10a (hereinafter, referred to as a primary-side switching circuit current II), the current in the coupling reactor 3 (hereinafter, referred to as a coupling reactor current IL), and the current in the first terminal 15b of the secondary-side switching circuit 10b (hereinafter, referred to as a secondary-side switching circuit current 12) are detected. When the negative electrode side (a line from the primary-side input/output terminal 24a to the secondary- side input/output terminal 24b via the second terminal 16a and the fourth terminal 18a of the primary-side switching circuit 10a, the connecting line 5, and the fourth terminal 18b and the second terminal 16b of the secondary-side switching circuit 10b) in the circuit is grounded, the potential to the ground of the second terminal 16a-of the primary-side switching circuit 10a, the second terminal 16b of the secondary-side switching circuit 10b, and the connecting line 5 that is to be a ground potential is stable at a low level compared with the potential to the ground of the first terminal 15a of the primary-side switching circuit 10a, the first terminal 15b of the secondary-side switching circuit 10b, and the coupling reactor 3 that are high and fluctuates regularly. Therefore, the dielectric strength voltage required to the current detector can be low, and the current detector can obtain detection values with less noise by,providing the current detector on the negative electrode side of the circuit. [0022] Next, the configuration of the control unit 30a is explained. Fig. 2 is a diagram illustrating a configuration example of the control unit 30a according to the first embodiment of the present invention. As shown in Fig. 2, the control unit 30a includes a current-command converting unit 31a, a current-command adjusting unit 32a, a current control unit 33a, a modulation ratio command generating unit 34a, and a gate- signal generating unit 35a. [23] The current-command converting unit 31a generates a coupling-reactor base current command ILO* based on the power command P* and the coupling unit voltage VL. [24] The current-command adjusting unit 32a adjusts the coupling-reactor base current command ILO* input from the current-command converting unit 31a,, and generates a coupling-reactor current command IL*. The current control unit 33a generates a current difference' DIL based on the coupling-reactor current command XL* and the coupling reactor current IL. [26] The modulation ratio command generating unit 34a generates a primary-side modulation ratio command VREFl and a secondary-side modulation ratio command VREF2 based on the current difference DIL input from the current control unit 33a, the primary-side capacitor voltage VI, and the secondary-side capacitor voltage V2. [27] The gate-signal generating unit 35a generates the /-N ga-e signals Gla, Gib, G2a, and G2b for controlling on/off of each of the switching elements 11a, lib, 12a, and 12b based on the primary—side modulation ratio command VREFl and the secondary-side modulation ratio command VREF2 input from the modulation ratio command generating unit 34a. [28] In Fig. 2, the control unit 30a is configured so that the command value P* is input from outside; however, the configuration can be such that a signal corresponding to the coupling-reactor base current command ILO* or the coupling-reactor current command IL* is input from outside instead of the command value P*. In this case, the current-command converting unit 31a and the current-command adjusting unit 32a can be omitted. [002 9] The configuration examples of the current-command converting unit 31a, the current-command adjusting unit 32a, the current control unit 33a, the modulation ratio command generating unit 34a, and the gate-signal generating unit 35a are explained below. [30] Fig. 3 is a diagram illustrating a configuration example of the current-command converting unit 31a according to the first embodiment of the present invention. The configuration can be such that a low-pass filter or the like is inserted into input and output of a function block of a divider 4 0 or the like to remove unnecessary frequency components, although not shown. [31] As shown in Fig. 3, the current-command converting unit 31a generates the coupling-reactor base current command ILO* by dividing the command value P* by the coupling unit voltage VL by using the divider 40. [32] Fig. 4 is a diagram illustrating a configuration example of the current-command adjusting unit 32a according to the first embodiment of the present invention. The configuration can be such that a low-pass filter or the like is inserted into input and output of a limiter 70a to remove unnecessary frequency components, although not shown. [33] As shown in Fig. 4, the current-command adjusting unit 32a causes the limiter 70a in which the upper and lower limits are set by a current-command upper-limit limiting value ILMTH and a current-command lower-limit limiting value ILMTL to limit an upper limit and a lower limit of the coupling-reactor base current command ILO* generated by the current-command converting unit 31a, and outputs the value as the coupling-reactor current command IL*. [34] The function of the limiter 70a is explained. A signal that is obtained by limiting the upper and lower limits of the coupling-reactor base current command ILO* is used as the coupling-reactor current command IL*, so that the upper and lower limits of the actual coupling reactor current IL which is controlled to be consistent with the coupling-reactor base current command ILO* can be limited. The coupling reactor current IL is current that always flows in any of the switching elements 11a to 12b. Therefore, the current in the switching elements 11a to 12b can be limited by limiting the upper and lower limits of the coupling reactor current IL. [35] It is appropriate to set the current-command upper-limit limiting value ILMTH and the current-command lower-limit limiting value ILMTL to be equal to or lower than a current resistance of the switching elements 11a to 12b. With the above configuration of the current- command adjusting unit 32a, even if the coupling-reactor base current command ILO* calculated in the current—command converting unit 31a becomes too large with respect to the current resistance of the switching elements 11a to 12b in -he case, for example, where the excessive command value P* is input to the control unit 30a, it is possible to limit the coupling-reactor current command IL* within the current resistance of the switching elements 11a to 12b by the limiter 70a. [37] Thus, the actual coupling reactor current IL and therefore the current in the switching elements 11a to 12b can be limited within the current resistance thereof. Consequently, the switching elements 11a to 12b can be prevented from breakage because of overcurrent, so that the bidirectional buck boost DC-DC converter that is strong against disturbance such as excessive power command input can be obtained. [38] Fig. 5 is a diagram illustrating a configuration example of the current control unit 33a according to the first embodiment of the present invention. The configuration can be such that a low-pass filter or the like is inserted into input and output of a function block of a subtracter 200 or the like to remove unnecessary frequency components, although not shown. [39] As shown in Fig. 5, in the current control unit 33a, a deviation between the coupling-reactor current command IL* and the coupling reactor current XL generated in the current-command adjusting unit 32a is generated by the subtracter 200, which is input to a proportional- integral controller 201. In the proportional-integral controller 201, the current difference DIL is calculated by the following equation: DIL=(Kl+K2/s)x(IL*-IL) where Kl; proportional gain, K2; integral gain, and s; Laplace operator. . [40] Fig. 6 is a diagram illustrating a configuration example of the modulation ratio command generating unit 34a according to the first embodiment of the present invention. The configuration can be such that a low-pass filter or the like is inserted into input and output of a function block of an adder 211a or the like to remove unnecessary frequency components, although not shown. [41] As shown in Fig. 6, the secondary-side capacitor voltage V2 is divided by the primary-side capacitor voltage VI in a divider 210a of the modulation ratio command generating unit 34a to obtain a ratio V2/V1 between the secondary-side capacitor voltage V2 and the primary-side capacitor voltage VI. A limiter 213a limits the lower and upper limits of the ratio V2/V1 to zero and one to obtain,a value that is to be a primary-side base modulation ratio command VREFIA to the primary-side converting unit la. [42] The current difference DIL generated in the current control unit 33a is added to the primary-side base modulation ratio command VREFIA by the adder 211a to obtain the primary-side modulation ratio command VREFl as the modulation ratio command of the primary-side converting unit la. That is, the VREFl is expressed by VREF1=VREF1A+DIL. [43] On the other hand, the primary-side capacitor voltage VI is divided by the secondary-side capacitor voltage V2 in a divider 210b to obtain a ratio V1/V2 between the primary-side capacitor voltage VI and the secondary-side capacitor voltage V2. A limiter 213b limits the lower and upper limits of the ratio V1/V2 to zero and one to obtain a value that is to be a secondary-side base modulation ratio command VREF2A to the secondary-side converting unit lb. [44] A DIL2 that is obtained by inverting a sign of the current difference DIL generated in the current control unit 33a by a sign inverting circuit 212 is added to the secondary-side base modulation ratio command VREF2A by the adder 211b to obtain the secondary-side modulation ratio command VREF2 as the modulation ratio command of the secondary-side converting unit lb. That is, the VREF2 is expressed by VREF2=VREF2A+DIL2. [0044] Fig. 7 is a diagram illustrating a configuration example of the gate-signal generating unit 35a according to the first embodiment of the present invention. The configuration can be such that a low-pass filter or the like is inserted into input and output of a function block of a comparator 220a or the like to remove unnecessary frequency components, although not shown. [46] As shown in Fig. 7, the gate-signal generating unit 35a first generates a carrier signal CAR that takes a value of zero to one in a carrier signal generator 222. It is appropriate that the carrier signal CAR is, for example, a triangle wave or a sawtooth wave. Then, the comparators 220a and 220b, and inverting circuits 221a and 221b determine the gate signals Gla to G2b of each of the switching elements 11a to 12b by the following logic in accordance with a magnitude relation between the primary-side modulation ratio command VREFl and the secondary-side modulation ratio command VREF2 generated by the modulation ratio command generating unit 34a, and the carrier signal CAR, [48] If VREF1>CAR, the gate signal Gla to the switching element 11a is turned on and the gate signal G2a to the switching element 12a is turned off. Adversely, if VREFKCAR, the gate signal Gla to the switching element 11a is turned off and the gate signal G2a to the switching element 12a is turned on. [49] If VREF2>CAR, the gate signal Gib to the switching element lib is turned on and the gate signal G2b ■Lc rhe switching element 12b is turned off. Adversely, if VREF2CAR, the gate signal Gla to the switching element 11a of the primary-side converting unit la is turned on and the gate signal G2a to the switching element 12a of the primary-side converting unit la is turned off. At the same time, the gate signal G2b to the switching element 12b of the secondary-side converting unit lb is turned on, and the gate signal Gib to the switching element lib of the secondary-side converting unit lb is turned off. [141] If VREFCAR, the gate signal Gla to the switching element 11a of the primary-side converting unit la is turned off and the gate signal G2a to the switching element 12a of the primary-side converting unit la is turned on. At the same time, the gate signal G2b to the switching element 12b of the secondary-side converting unit lb is turned off, and the gate signal Gib to the switching element lib of the secondary-side converting unit lb is turned on. [161] If VREF