Patent Publication Number: US-9906154-B2

Title: Power conversion unit and power conversion device

Description:
TECHNICAL FIELD 
     The present invention relates to a circuit for power conversion. 
     BACKGROUND ART 
     In a power conversion device, the loss in a power semiconductor is reduced because the speed of a switching operation is increased by technological innovations in the field, of power semiconductors used in a power semiconductor module, which is a main component of the power conversion device. Thus, a cooler for cooling the power semiconductor module can be downsized. As a result, the power conversion device is downsized. In particular, a UPS (Uninterruptible Power Supply) having the power conversion device is installed in an urban neighborhood where land prices are high, and is used for a data center. Therefore, it is preferred that the footprint of the power conversion device be small. Further, when power semiconductor modules forming a power conversion circuit in the power conversion device are connected in parallel and used to output a rated current, it is preferred that the currents flowing in the individual power semiconductor modules be balanced. 
     There is a well-known power conversion device that balances the currents flowing in parallel-connected power semiconductor modules. According to Patent Literature 1, a plurality of power semiconductor modules are arrayed so that their lateral surfaces are parallel to each other with respect to the direction in which terminal sections provided for the power semiconductor modules are arrayed. The power semiconductor modules are connected by an alternating-current conductor and a multilayer board formed of a positive conductor and a negative conductor. Further, the currents flowing in the individual power semiconductor modules are balanced by forming a cut-out portion in the alternating-current conductor. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Laid-Open No. 2012-95472 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, if terminals are arrayed in the up-down direction of the power conversion device described in Patent Literature 1 because, for example, a reactor and other heavy parts to be connected to the power conversion device are disposed below the power conversion device, the power semiconductor modules are arrayed horizontally to increase the width of the power conversion device. This results in an increase in the footprint of the power conversion device. 
     Solution to Problem 
     In solving the above problem, according to an aspect of the present invention, there is provided a power conversion unit including a circuit connection section, a first power semiconductor module, a second power semiconductor module, and a capacitor. The circuit connection section includes a positive conductor having an external positive terminal, a negative conductor having an external negative terminal, and an alternating-current conductor having an external alternating-current terminal. The first power semiconductor module is connected to the positive conductor, the negative conductor, and the alternating-current conductor. The second power semiconductor module is connected to the positive conductor, the negative conductor, and the alternating-current conductor. The capacitor is connected to the positive conductor and the negative conductor. The first power semiconductor module includes a first direct-current terminal and a first alternating-current terminal. The first direct-current terminal includes a first positive terminal connected to the positive conductor and a first negative terminal connected to the negative conductor. The first alternating-current terminal is connected to the alternating-current conductor. The second power semiconductor module includes a second direct-current terminal and a second alternating-current terminal. The second direct-current terminal includes a second positive terminal connected to the positive conductor and a second negative terminal connected to the negative conductor. The second alternating-current terminal is connected to the alternating-current conductor. The external alternating-current terminal, the first power semiconductor module, the second power semiconductor module, the capacitor, and the external direct-current terminal including the external positive terminal and the external negative terminal are arrayed on a straight line extending in the longitudinal direction of the circuit connection section. The external alternating-current terminal is disposed at one longitudinal end of the circuit connection section, and the external direct-current terminal is disposed at the other longitudinal end of the circuit connection section. 
     Advantageous Effect of Invention 
     An aspect of the present invention makes it possible to reduce the footprint of a power conversion device including a plurality of parallel-connected power semiconductor modules. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a configuration of a UPS according to an embodiment of the present invention. 
         FIG. 2  illustrates a circuit configuration of a converter  11 . 
         FIG. 3  illustrates a circuit configuration of an inverter  12 . 
         FIG. 4  illustrates a circuit configuration of a step-up chopper  13 . 
         FIG. 5  illustrates a configuration of a power conversion unit  101 . 
         FIG. 6  is a perspective view illustrating the configuration of the power conversion unit  101 . 
         FIG. 7  is a right side view illustrating the configuration of the power conversion unit  101 . 
         FIG. 8  is an exploded perspective view illustrating a front configuration of the power conversion unit  101 . 
         FIG. 9  is an exploded perspective view illustrating a rear configuration of the power conversion unit  101 . 
         FIG. 10  is a perspective view illustrating a front configuration of a main circuit busbar assembly  151 . 
         FIG. 11  is a perspective view illustrating a rear configuration of the main circuit busbar assembly  151 . 
         FIG. 12  is an exploded perspective view illustrating the front configuration of the main circuit busbar assembly  151 . 
         FIG. 13  is an exploded perspective view illustrating the rear configuration of the main circuit busbar assembly  151 . 
         FIG. 14  is a diagram illustrating the paths of currents flowing from an external alternating-current terminal  154 T in the main circuit busbar assembly  151  to a negative fuse connection section  159 . 
         FIG. 15  is a diagram illustrating the paths of currents flowing from a positive fuse connection section  158  in the main circuit busbar assembly  151  to the external alternating-current terminal  154 T. 
         FIG. 16  is a perspective view illustrating a configuration of a power conversion section  2   a.    
         FIG. 17  is a front view illustrating the configuration of the power conversion section  2   a.    
     
    
    
     DESCRIPTION OF EMBODIMENT 
     An embodiment of the present invention will now be described with reference to the accompanying drawings. 
     A UPS (Uninterruptible Power Supply) will be described as the embodiment of the present invention. 
       FIG. 1  illustrates a configuration of the UPS according to the embodiment of the present invention. 
     The UPS  2  uses a continuous inverter power supply method that is capable of continuously supplying electrical power during power outage. The present invention is applicable to not only the continuous inverter power supply method but also the other power supply methods such as a continuous commercial power supply method. 
     During a normal operation, a commercial three-phase alternating-current power supply voltage  3  supplies electrical power to a load  4  through a converter  11  and an inverter  12 . The converter  11  converts commercial three-phase alternating-current power supply voltage  3  to a direct-current voltage  5  and supplies the direct-current voltage  5  to the inverter  12 . The inverter  12  converts the direct-current voltage  5  to three-phase alternating-current power  6 . This ensures that electrical power equivalent to normal commercial power is steadily supplied to the load  4  as control is exercised by the converter  11  and the inverter  12  even when the voltage of the commercial power supply  3  varies due, for instance, to an instantaneous voltage drop. 
     Meanwhile, in the event of power outage, a storage battery  14  supplies electrical power to the load  4  through the inverter  12  while the inverter  12  is activated. This enables the UPS  2  to uninterruptedly supply electrical power to the load  4 . In the present embodiment, the total voltage of the storage battery  14  is made sufficiently lower than the direct-current voltage applied to the inverter  12  in order to reduce the cubic volume of the UPS  2 . Thus, the UPS  2  according to the present embodiment includes a step-up chopper  13  that raises a low direct-current voltage, which is outputted by discharging the storage battery  14 , to a desired direct-current voltage  5  and outputs the raised direct-current voltage to the inverter  12 . If no limitation is imposed on the cubic volume of the UPS  2 , the UPS  2  may exclude the step-up chopper  13  and include a high-voltage storage battery  14  capable of supplying a desired direct-current voltage. 
     A filter  18  is connected between the commercial power supply  3  and the converter  11  in order to shape a sine wave. The filter  18  includes, for example, a reactor and a capacitor, which are provided for each phase. One end of the reactor is series-connected to an alternating-current terminal of the converter  11 . The capacitor is parallel-connected to the other end of the reactor. The filter  18  is a heavy part and therefore disposed directly below the converter  11 . A filter  19  is connected between an alternating-current terminal of the inverter  12  and the load  4 . The filter  19  includes, for example, a reactor and a capacitor, which are provided for each phase. One end of the reactor is series-connected to the alternating-current terminal of the converter  11 . The capacitor is parallel-connected to the other end of the reactor. The filter  19  is a heavy part and therefore disposed directly below the inverter  12 . A reactor  15  is connected between the storage battery  14  and an alternating-current terminal of the step-up chopper  13 . The reactor  15  is a heavy part and therefore disposed directly below the step-up chopper  13 . 
     In the subsequent description, the converter  11 , the inverter  12 , and the step-up chopper  13  are collectively referred to as the power conversion section  2   a.    
     The UPS  2  may further include a cooling mechanism that air-cools the power conversion section  2   a.    
     In compliance with an instruction, a bypass circuit  17  bypasses the power conversion section  2   a  and directly connects the commercial power supply  3  to the load  4 . A maintenance bypass circuit  16  is used for the maintenance of the power conversion section  2   a  and the bypass circuit  17 . In compliance with an instruction, the maintenance bypass circuit  16  bypasses the bypass circuit  17  and directly connects the commercial power supply  3  to the load  4 . 
       FIG. 2  illustrates a circuit configuration of the converter  11 . 
     Three-phase alternating-current power from the commercial power supply  3  is supplied to alternating-current terminals R, S, T of the converter  11 . In each of R, S, and T phases, the three-phase alternating-current power is rectified by a switching element  21  and rectifying element  23  of an upper arm, a switching element  22  and rectifying element  24  of a lower arm, and capacitors  120 , and then outputted to direct-current terminals P, N. In the present embodiment, an IGBT (Insulated Gate Bipolar Transistor) is used as the switching elements  21 ,  22 , and a diode is used as the rectifying elements  23 ,  24 . However, different types of elements may alternatively be used. 
       FIG. 3  illustrates a circuit configuration of the inverter  12 . 
     The direct-current voltage  5 , which is obtained by conversion in the converter  11  or the step-up chopper  13 , is supplied to the direct-current terminals P, N of the inverter  12 . In each of U, V, and W phases, the direct-current voltage  5  is then converted to the alternating-current power  6  by the switching element  21  and rectifying element  23  of the upper arm, the switching element  22  and rectifying element  24  of the lower arm, and the capacitors  120 , and outputted to alternating-current terminals U, V, W. Three-phase alternating currents outputted from the alternating-current terminals U, V, W are supplied to the load  4 . 
       FIG. 4  illustrates a circuit configuration of the step-up chopper  13 . 
     The output of the storage battery  14  is supplied to an input terminal Bat of the reactor  15 . While the switching element  22  of the lower arm is ON, energy is stored in the reactor  15  connected between the input terminal Bat and an alternating-current terminal C. Subsequently, when the switching element  22  of the lower arm turns OFF, the reactor  15  generates a counter-electromotive voltage to turn ON the rectifying element  23  of the upper arm. A voltage obtained by adding a direct-current voltage outputted from the storage battery  14  to the counter-electromotive voltage of the reactor  15  is developed at output terminals P, N of the step-up chopper  13 . Thus, a stepped-up direct-current voltage is outputted. 
     As described above, the converter  11  and inverter  12  included in the UPS  2  according to the present embodiment both have at least one basic circuit that includes power semiconductor modules  110 , the capacitors  120 , a positive fuse  131 , and a negative fuse  132 . Each of the power semiconductor modules  110  is a two-level half-bridge circuit that is formed by series-connecting the switching element  21  and rectifying element  23  of the upper arm to the switching element  22  and rectifying element  24  of the lower arm. A three or more multi-level conversion circuit may be used instead of the two-level half-bridge circuit. 
     In the present embodiment, the basic circuit is implemented by a power conversion unit  101 , and power conversion units  101  are combined to implement the converter  11 , the inverter  12 , and the step-up chopper  13 . This not only communizes the types of parts used for the power conversion section  2   a , but also facilitates the assembly and maintenance of the power conversion section  2   a.    
       FIG. 5  illustrates a configuration of the power conversion unit  101 . 
     In the power conversion unit  101 , the power semiconductor modules  110  are implemented by parallel-connecting a first power semiconductor module  111  and a second power semiconductor module  112 . The first power semiconductor module  111  and the second power semiconductor module  112  are of a 2-in-1 type and used to form the upper and lower arms, respectively. Further, the capacitors  120  are implemented by parallel-connecting a first capacitor  121  and a second capacitor  122 . In this manner, a plurality of power semiconductor modules and a plurality of capacitors are used to implement the power semiconductor modules  110  and the capacitors  120  in accordance with electrical power required for the power conversion unit  101 . 
     Further, in the power conversion unit  101 , the fuse  131  is series-connected to the positive ends of the power semiconductor modules  110  and capacitors  120 , and the fuse  132  is series-connected to the negative ends of the power semiconductor modules  110  and capacitors  120 . A second terminal  131   b  of the positive fuse  131  corresponds to the P terminals of the converter  11 , inverter  12 , and step-up chopper  13 . A second terminal  132   b  of the negative fuse  132  corresponds to the N terminals of the converter  11 , inverter  12 , and step-up chopper  13 . The fuses  131 ,  132  included in the power conversion unit  101  increase its reliability in the event of a short-circuit fault. In a situation where a circuit breaker is provided to isolate the power conversion unit  101 , either or both of the fuses  131 ,  132  may be omitted. 
     The power semiconductor modules  111 ,  112  each include the switching element  21  and rectifying element  23  of the upper arm and the switching element  22  and rectifying element  24  of the lower arm. An external alternating-current terminal  154 T is connected between the upper and lower arms of the power semiconductor modules  111 ,  112 . A gate terminal of the upper arm switching elements  21  of the power semiconductor modules  111 ,  112  is connected to a gate terminal  111   g . A gate terminal of the lower arm switching elements  22  of the power semiconductor modules  111 ,  112  is connected to a gate terminal  112   g.    
       FIG. 6  is a perspective view illustrating a configuration of the power conversion unit  101 . 
     Coordinates of the UPS  2  are subsequently defined by using an X-axis, a Y-axis, and a Z-axis. The Y-axis direction is a forward direction of the UPS  2 . The Z-axis direction is an upward direction of the UPS  2 . The X-axis direction is a leftward direction of the UPS  2 . The fuses  131 ,  132  are each provided with one terminal oriented in a rearward (−Y) direction and with the other terminal oriented in a forward (+Y) direction. Further, the fuses  131 ,  132  are disposed in the forward (+Y) direction with respect to the main circuit busbar assembly  151 . More specifically, a first terminal  131   a  of the positive fuse  131  and a first terminal  132   a  of the negative fuse  132  are oriented in the rearward (−Y) direction and connected with a mounting screw  139  to the main circuit busbar assembly  151 . Meanwhile, a second terminal  131   b  of the positive fuse  131  and a second terminal  132   b  of the negative fuse  132  are oriented in the forward (+Y) direction. When the above arrangement scheme is employed, the second terminal  131   b  of the positive fuse  131  and the second terminal  132   b  of the negative fuse  132 , which serve as the terminals for coupling the host power conversion unit  101  to another power conversion unit  101 , are positioned in the front of the UPS  2 . This provides increased front accessibility during assembly and maintenance, thereby providing improved workability. As mentioned earlier, the power conversion unit  101  includes a total of three external terminals, namely, the second terminal  131   b  of the positive fuse  131  and the second terminal  132   b  of the negative fuse  132 , which are connected to a unit coupling busbar assembly  161  for connecting to another power conversion unit  101 , and the external alternating-current terminal  154 T provided for the main circuit busbar assembly  151 . 
       FIG. 7  is a right side view illustrating the configuration of the power conversion unit  101 . 
     The power conversion unit  101  includes the power semiconductor modules  110 , the capacitors  120 , the fuses  131 ,  132 , and the man circuit busbar assembly  151  that electrically connects the foregoing parts. An air-cooling fin  113  is disposed on the rear surface (−Y direction) of the power semiconductor modules  110  to cool them. The power semiconductor modules  110 , the capacitors  120 , and the fuses  131 ,  132  are disposed in the order named along a downward (−Z) direction. A housing is designed with a cooling mechanism incorporated so that air for cooling the air-cooling fin  113  flows in the upward (+Z) direction along the rear surface of the main circuit busbar assembly  151 . This arrangement scheme is employed so that the air-cooling  113  is positioned leeward from the capacitors  120 , that is, upward (+Z) from the capacitors, in order to prevent the capacitors  120  and other parts from receiving heat from the air-cooling fin  113 . Further, the fuses  131 ,  132  having a high calorific value are disposed windward to be efficiently cooled. In the present embodiment, the air-cooling fin  113  is disposed in the −Y direction of the power semiconductor modules  110 . However, if, for instance, the air flow path is oriented in the −Y direction, the air-cooling fin  113  may be disposed in the +Z direction of the power semiconductor modules  110 . The direction in which the air-cooling fin  113  is disposed is determined in accordance with the air flow path. Moreover, as the power semiconductor modules  110  is positioned adjacent to the capacitors  120 , it is possible to reduce parasitic inductance formed on the main circuit busbar assembly  151 , which connects the power semiconductor modules  110  to the capacitors  120 . Thus, a surge voltage generated upon switching can be reduced. Additionally, the impedance between the power semiconductor modules  110  in the host power conversion unit  101  and the capacitors  120  in a neighboring power conversion unit  101  can be minimized. This makes it possible to make effective use of not only the capacitors  120  in the host power conversion unit  101  but also the capacitors  120  in another power conversion unit  101 . As a result, the capacitance of the capacitors that is used for one power conversion unit  101  can be reduced. In addition, the cubic volume of the power conversion unit  101  can also be reduced. 
     The power semiconductor modules  110  and the capacitors  120 , which have terminals protruding in the forward (+Y) direction, are disposed in the rearward (−Y) direction with respect to the main circuit busbar assembly  151 . This arrangement scheme ensures that all the terminals of the power semiconductor modules  110  and capacitors  120  are positioned forward. This makes it easy, for example, to inspect, install, or remove terminal sections during maintenance. 
       FIG. 8  is an exploded perspective view illustrating a front configuration of the power conversion unit  101 .  FIG. 9  is an exploded perspective view illustrating a rear configuration of the power conversion unit  101 . 
     In the present embodiment, the power semiconductor modules  110  are incorporated by parallel-connecting the power semiconductor module  111  and the power semiconductor module  112 , which are two-level half-bridge circuits (2 in 1). The number of parallel-connected power semiconductor modules in the power conversion unit  101  is preferably a minimum required number of power semiconductor modules that permit minimum electrical power provided by a model selected for the line-up of a UPS or other power conversion device using the power conversion unit  101 . The reason is that a desired amount of electrical power can be obtained by parallel-connecting the power conversion units  101  as for a model that requires a larger amount of electrical power. In consideration of the above, the present embodiment is configured so that two power semiconductor modules are connected in parallel. 
     The power semiconductor modules  111 ,  112  respectively include positive terminals  111   p ,  112   p , negative terminals  111   n ,  112   n , alternating-current terminals  111   ac ,  112   ac , and control terminals  111   d ,  112   d . The control terminals  111   d ,  112   d  include gate terminals  111   g ,  112   g , respectively. 
     The positive terminals  111   p ,  112   p  in the power semiconductor modules  110  are connected to a positive connection terminal  152   p  in the main circuit busbar assembly  151 . The negative terminals  111   n ,  112   n  in the power semiconductor modules  110  are connected to a negative connection terminal  153   n  in the main circuit busbar assembly  151 . The alternating-current terminals  111   ac ,  112   ac  in the power semiconductor modules  110  are connected to a connection terminal  154   ac  connected to the external alternating-current terminal  154 T. The positive terminals  111   p ,  112   p , the negative terminals  111   n ,  112   n , and the alternating-current terminals  111   ac ,  112   ac  are connected to the main circuit busbar assembly  151  by a welding or other joining method. These terminals may alternatively be connected to the main circuit busbar assembly  151  by using, for example, screws or clips. 
     In order to suppress the difference between the distance from the capacitors  120  to the positive terminal hip and negative terminal  111   n  of the power semiconductor module  111  and the distance from the capacitors  120  to the positive terminal  112   p  and negative terminal  112   n  of the power semiconductor module  112 , the positive terminal  111   p  and negative terminal  111   n  of one power semiconductor module  111  are positioned in a reverse manner from the positive terminal  112   p  and negative terminal  112   n  of the other power semiconductor module  112 . Further, the positive terminal  111   p  and negative terminal  111   n  in the power semiconductor module  111  face each other in proximity, and the positive terminal  112   p  and negative terminal  112   n  in the power semiconductor module  112  face each other in proximity. Thus, in the XZ-plane, the power semiconductor module  112  is positioned in a reverse manner from the power semiconductor module  111 . Using the arrangement scheme reduces the difference in the impedance between the power semiconductor modules  111 ,  112  and the capacitors  121 ,  122 . This provides an improved balance between the current flowing in the power semiconductor module  111  and the current flowing in the power semiconductor module  112 . 
     The positive terminal  121   p  and negative terminal  121   n  of the capacitor  121  are fastened with capacitor mounting screws  129  to a capacitor joint  156  provided for the main circuit busbar assembly  151 . Similarly, the positive terminal  122   p  and negative terminal  122   n  of the capacitor  122  are fastened with the capacitor mounting screws  129  to a capacitor joint  157  provided for the main circuit busbar assembly  151 . 
       FIG. 10  is a perspective view illustrating a front configuration of the main circuit busbar assembly  151 .  FIG. 11  is a perspective view illustrating a rear configuration of the main circuit busbar assembly  151 . 
     Capacitor connection sections  156   p ,  156   n  are attached to the rear surface of the capacitor joint  156 , and capacitor connection sections  157   p ,  157   n  are attached to the rear surface of the capacitor joint  157 . Capacitor mounting screw connection sections  156   pf ,  156   nf  are disposed opposite the capacitor connection sections  156   p ,  156   n  on the main circuit busbar assembly  151 , and capacitor mounting screw connection sections  157   pf ,  157   nf  are disposed opposite the capacitor connection sections  157   p ,  157   n  on the main circuit busbar assembly  151 . The positive terminal  121   p  and negative terminal  121   n  of the capacitor  121  are fastened to the rear surface of the capacitor joint  156  by using the capacitor mounting screws  129  on the front of the capacitor joint  156 . This ensures that the positive terminal  121   p  and negative terminal  121   n  of the capacitor  121  are in contact with the capacitor connection sections  156   p ,  156   n , respectively, and that the capacitor mounting screws  129  are in contact with the capacitor mounting screw connection sections  156   pf ,  156   nf . Further, the positive terminal  122   p  and negative terminal  122   n  of the capacitor  122  are fastened to the rear surface of the capacitor joint  157  by using the capacitor mounting screws  129  on the front of the capacitor joint  157 . This ensures that the positive terminal  122   p  and negative terminal  122   n  of the capacitor  122  are in contact with the capacitor connection sections  157   p ,  157   n , respectively, and that the capacitor mounting screws  129  are in contact with the capacitor mounting screw connection sections  157   pf ,  157   nf.    
     Fuse connection sections  158 ,  159  are attached to the front surface of the main circuit busbar assembly  151 . Fuse mounting screw connection sections  158   b ,  159   b  are disposed opposite the fuse connection sections  158 ,  159  on the main circuit busbar assembly  151 . The first terminal  131   a  of the positive fuse  131  is fastened to the front surface of the fuse connection section  158  by using a fuse mounting screw  139  on the rear of the fuse mounting screw connection section  158   b . This ensures that the first terminal  131   a  of the positive fuse  131  is in contact with the fuse connection section  158 , and that the fuse mounting screw  139  is in contact with the fuse mounting screw connection section  158   b . Further, the first terminal  132   a  of the negative fuse  132  is fastened to the front surface of the fuse connection section  159  by using the fuse mounting screw  139  on the rear of the fuse mounting screw connection section  159   b . This ensures that the first terminal  132   a  of the negative fuse  132  is in contact with the fuse connection section  159 , and that the fuse mounting screw  139  is in contact with the fuse mounting screw connection section  159   b.    
       FIG. 12  is an exploded perspective view illustrating the front configuration of the main circuit busbar assembly  151 .  FIG. 13  is an exploded perspective view illustrating the rear configuration of the main circuit busbar assembly  151 . 
     The main circuit busbar assembly  151  includes a positive conductor  152 , a negative conductor  153 , an alternating-current conductor  154 , and an insulator  155 . The positive conductor  152 , the negative conductor  153 , and the alternating-current conductor  154  are shaped like a flat plate. In order to reduce parasitic inductance generated between the power semiconductor modules  110  and the capacitors  120 , the positive conductor  152  and the negative conductor  153  are stacked via the insulator  155  so that their opposing faces are parallel and close to each other. This makes it possible to reduce current ripple and impedance in the main circuit busbar assembly  151  and reduce a surge voltage generated upon switching. Further, the insulator  155  covers a large portion of the stacked conductors, namely, the positive conductor  152 , the negative conductor  153 , and the alternating-current conductor  154 , in such a manner that the conductors are at a predetermined insulating distance from each other. In order to provide improved heat dissipation, the front and rear conductive surfaces of the main circuit busbar assembly  151  may be exposed while the predetermined insulating distance is maintained. The insulator  155  is formed, for instance, of highly-fluid insulative resin. 
     In the present embodiment, the alternating-current conductor  154 , the positive conductor  152 , and the negative conductor  153  are disposed in the order named along the rearward (−Y) direction. However, as far as the positive conductor  152  and the negative conductor  153  are disposed nearest each other, the parasitic inductance remains unaffected even if the conductors are disposed in a different order. The conductors are formed, for instance, by cutting, bending, or otherwise working a single conductive plate made of copper, aluminum, or other highly conductive material. As an example, the positive conductor  152  is described below. The positive conductor  152  is formed by initially cutting out an opening  152   h  through which all the terminals  111   p ,  111   n ,  111   ac ,  111   d ,  112   p ,  112   n ,  112   ac ,  112   d  of the power semiconductor modules  111 ,  112  penetrate while a portion serving as the connection terminal  152   p  for connecting to the positive terminals  11   p ,  112   p  is left in the plane of a single conductive plate. The connection terminal  152   p  is then formed by bending the portion left in the plane of the conductive plate, which serves as the connection terminal  152   p,  90 degrees in the forward (+Y) direction. The above-described forming method also applies to the negative conductor  153  and the alternating-current conductor  154 . 
     The positive conductor  152  includes two connection terminals  152   p  that are respectively connected to the positive terminals  111   p ,  112   p  of the power semiconductor modules  110 . The negative conductor  153  includes two connection terminals  153   n  that are respectively connected to the negative terminals  111   n ,  112   n  of the power semiconductor modules  110 . The alternating-current conductor  154  includes two connection terminals  154   ac  that are respectively connected to the alternating-current terminals  111   ac ,  112   ac  of the power semiconductor modules  110 , and the external alternating-current terminal  154 T to be connected to the outside. 
     At the capacitor joints  156 ,  157 , the negative terminals  121   n ,  122   n  of the capacitors  121 ,  122  are brought into contact with the capacitor connection sections  156   n ,  157   n  on the rear surface of the negative conductor  153 . Further, the capacitor connection sections  156   p ,  157   p , which protrude in the rearward (−Y) direction from the positive conductor  152 , are disposed so that the positive terminals  121   p ,  122   p  of the capacitors  121 ,  122  are brought into contact with a conductor in a virtual plane identical with the rear surface of the negative conductor  153 . The capacitor connection sections  156   p ,  157   p  are separate from the positive conductor  152  and connected to the positive conductor  152 , for instance, by brazing, soldering, or swaging. Further, the capacitor connection sections  156   p ,  157   p  can also be manufactured by a cutting method for creating the capacitor connection sections  156   p ,  157   p  protruding from a single conductive plate, or by a casting method such as die-casting. The capacitor connection sections  156   p ,  157   p  are provided with holes into which the capacitor mounting screws  129  are driven. The negative conductor  153  is provided with openings  156   h ,  157   h  that prevent the protruding positive capacitor connection sections  156   p ,  157   p  from coming into contact with the negative conductor  153 . 
     Further, at the capacitor joints  156 ,  157 , the capacitor mounting screws  129 , which secure the positive terminal  121   p  of the capacitor  121  and the positive terminal  122   p  of the capacitor  122 , are brought into contact with the capacitor connection sections  156   pf ,  157   pf  on the front surface of the positive conductor  152 . Moreover, the capacitor connection sections  156   nf ,  157   nf , which protrude in the forward (+Y) direction from the negative conductor  153 , are disposed so that the capacitor mounting screws  129 , which secure the negative terminal  121   n  of the capacitor  121  and the negative terminal  122   n  of the capacitor  122 , are brought into contact with a conductor in a virtual plane identical with the front surface of the positive conductor  152 . The capacitor connection sections  156   nf ,  157   nf  are separate from the negative conductor  153  and connected to the negative conductor  153  by the same method as for the capacitor connection sections  156   p ,  157   p . The capacitor connection sections  156   nf ,  157   nf  are provided with holes into which the capacitor mounting screws  129  are driven. The positive conductor  152  is provided with openings  156   h ,  157   h  that prevent the protruding negative capacitor connection sections  156   nf ,  157   nf  from coming into contact with the positive conductor  152 . 
     At the joint between the main circuit busbar assembly  151  and the fuses  131 ,  132 , the first terminal  131   a  of the positive fuse  131  is brought into contact with the fuse connection section  158  on the front surface of the positive conductor  152 . Further, the fuse connection section  159 , which protrudes in the forward (+Y) direction from the negative conductor  153 , is disposed so that the first terminal  132   a  of the negative fuse  132  is brought into contact with a conductor in a virtual plane (second virtual plane) identical with the front surface of the positive conductor  152 . The fuse connection section  159  is separate from the negative conductor  153  and connected to the negative conductor  153  by the same method as for the capacitor connection sections  156   p ,  157   p . The fuse connection section  159  is provided with a hole into which the fuse mounting screw  139  is driven. The positive conductor  152  is provided with an opening  159   h  that prevents the protruding negative fuse connection section  159  from coming into contact with the positive conductor  152 . 
     As the first terminal  131   a  of the positive fuse  131  and the first terminal  132   a  of the negative fuse  132  are brought into contact with the main circuit busbar assembly  151  in the same virtual plane, it is easy to mount the fuses  131 ,  132  and easy to handle the main circuit busbar assembly  151 . 
     Further, at the joint between the main circuit busbar assembly  151  and the fuses  131 ,  132 , the fuse mounting screw  139 , which secures the first terminal  132   a  of the negative fuse  132 , is brought into contact with the fuse connection section  158  on the rear surface of the negative conductor  153 . Moreover, the fuse connection section  158   b , which protrudes in the rearward (−Y) direction from the negative conductor  153 , is disposed so that the fuse mounting screw  139 , which secures the first terminal  131   a  of the positive fuse  131 , is brought into contact with a conductor in a virtual plane identical with the rear surface of the negative conductor  153 . The fuse connection section  158   b  is separate from the positive conductor  152  and connected to the positive conductor  153  by the same method as for the capacitor connection sections  156   p ,  157   p . The fuse connection section  158   b  is provided with a hole into which the fuse mounting screw  139  is driven. The negative conductor  153  is provided with an opening  158   h  that prevents the protruding positive fuse connection section  158   b  from coming into contact with the negative conductor  153 . 
     From up to down (in the −Z direction), the external alternating-current terminal  154 T, the power semiconductor module  111 , the power semiconductor module  112 , the capacitor  121 , the capacitor  122 , and the fuse connection sections  158 ,  159  are disposed in the order named. This makes it possible to reduce the horizontal size (X direction and Y direction) of the power conversion unit  101 . Further, the power semiconductor modules  111 ,  112  in the present embodiment have the same structure and are longer in Z direction than in X direction. The power semiconductor module  112  is positioned adjacent to and in the downward (−Z direction) of the power semiconductor module  111 . This reduces the X-direction size of the power conversion unit  101  as compared to the case where the two power semiconductor modules are arranged in a horizontal direction. On the surface (XZ plane) of the main circuit busbar assembly  151 , the longitudinal direction of the power semiconductor modules  111 ,  112  is oriented in the longitudinal direction of the main circuit busbar assembly  151 . If, for example, the distance from the alternating-current terminal  111   ac  in the power semiconductor module  111  to the positive terminal  111   p  and the negative terminal  111   n  and the distance from the alternating-current terminal  112   ac  in the power semiconductor module  112  to the positive terminal  112   p  and the negative terminal  112   n  are the terminal-to-terminal distance, the terminal-to-terminal distance is longer than the widths (X-direction length) of the power semiconductor modules  111 ,  112 . Further, the terminal-to-terminal distance in the present embodiment is greater than the width of the main circuit busbar assembly  151  (length in transverse direction, that is, X-direction length). This reduces the X-direction size of the power conversion unit  101 . 
     As the power conversion unit  101  is configured as described above, it is possible to reduce the footprint of a power conversion device, such as the UPS  2 . 
     As the power semiconductor modules  111 ,  112  are disposed in the Z direction and oriented in an opposite direction from each other, the path of a current flowing in the power semiconductor module  111  is different from the path of a current flowing in the power semiconductor module  112 . Therefore, the currents might be imbalanced. A configuration for avoiding such an imbalance is described below. 
     First of all, a case where a current flows from the alternating-current side of the main circuit busbar assembly  151  to the direct-current side will be described. 
       FIG. 14  is a diagram illustrating the paths of currents flowing from the external alternating-current terminal  154 T in the main circuit busbar assembly  151  to the negative fuse connection section  159 . 
     For zoning purposes, line segments h 1  to h 5  are defined with reference to the locations of the external alternating-current terminal  154 T existing in the longitudinal direction of the main circuit busbar assembly  151 , the alternating-current terminal  111   ac  of the first power semiconductor module  111 , the positive and negative terminals  111   p ,  111   n  of the first power semiconductor module  111 , the positive and negative terminals  112   p ,  112   n  of the second power semiconductor module  112 , the alternating-current terminal  112   ac  of the second power semiconductor module  112 , and the fuse connection section  158  or  159 . 
     Here, as a steady state, let us assume a situation where the IGBT of the lower arm in each of the power semiconductor modules  111 ,  112  is ON while a current flows to the external alternating-current terminal  154 T. In this state, the current flows from the external alternating-current terminal  154 T through the alternating-current conductor  154  to the IGBT of the lower arm in the power semiconductor modules  111 ,  112 , to the negative conductor  153 , and to the negative fuse connection section  159  in the order named. For purposes of explanation, the current path is divided into path  1  and path  2 . Path  1  is the path of the current flowing through the first power semiconductor module  111 . Path  2  is the path of the current flowing through the second power semiconductor module  112 . 
     Here, it is assumed that the positive conductor  152 , the negative conductor  153 , and the alternating-current conductor  154  have the same impedance per unit length, and that the impedance is Z C . Further, it is assumed that the power semiconductor modules  111 ,  112  have the same impedance, and that the impedance is Z M . In this instance, the impedance Z 1  of path  1  and the impedance Z 2  of path  2  are expressed by the following equations by using line segments h 1  to h 5 .
 
 Z   1   =h   1   Z   C   +Z   M +( h   3   +h   4   +h   5 ) Z   C   (1)
 
 Z   2 =( h   1   +h   2   +h   3   +h   4 ) Z   C   +Z   M +( h   4   +h   5 ) Z   C   (2)
 
     From Equations (1) and (2), the imbalance between the impedances Z 2 −Z 1  is expressed by the following equation.
 
 Z   2   −Z   1 =( h   2   +h   4 ) Z   C   (3)
 
     As indicated in Equation (3), the imbalance between the impedances of paths  1  and  2  in the steady state is determined by the imbalance between path lengths h 2 +h 4 . The imbalance between the path lengths arises because the power semiconductor module  112  is oriented in an opposite direction from the power semiconductor module  111 . 
     A current imbalance arises in accordance with the ratio of an impedance imbalance to the impedance of all paths. Therefore, the current imbalance can be reduced when the imbalance between the path lengths h 2 +h 4  is sufficiently smaller than the total path length. That is to say, the following relational expression is obtained.
 
 h   2   +h   4   &lt;&lt;h   1   +h   3   +h   5   (4)
 
     In other words, h 2 +h 4  is sufficiently smaller than a value obtained by subtracting h2+h4 from the distance between the external alternating-current terminal  154 T and the fuse connection sections  158 ,  159 . Here, as mentioned earlier, the first power semiconductor module  111  and the second power semiconductor module  112  need to be positioned close to each other. Therefore, h 3  needs to be short. Thus, Equation (4) signifies that h 1  and h 5  should be sufficiently longer than h 2  and h 4 . Consequently, when the external alternating-current terminal  154 T and the fuse connection section  158  or  159  are provided on both ends of the main circuit busbar assembly  151 , the currents flowing in the first power semiconductor module  111  and the second power semiconductor module  112  can be balanced in the steady state. It should be noted that h 2  and h 4  may be equal. 
     Meanwhile, if Equation (4) is not sufficiently satisfied due, for instance, to dimensional limitations on the overall structure of a power conversion device, the current imbalance becomes great. In such an instance, the imbalance between path lengths can be reduced by adjusting the conductor thickness of an imbalanced alternating-current conductor  154 . 
     Here, it is assumed that the positive conductor  152  and the negative conductor  153  are equal in thickness, and that the thickness of the alternating-current conductor  154  is n times the thickness of the positive conductor  152  and negative conductor  153 . The impedance Z 1  of path  1  and the impedance Z 2  of path  2  are expressed by the following equations. 
     
       
         
           
             
               
                 
                   
                     Z 
                     1 
                   
                   = 
                   
                     
                       
                         
                           h 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         Z 
                         C 
                       
                     
                     + 
                     
                       Z 
                       M 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             h 
                             3 
                           
                           + 
                           
                             h 
                             4 
                           
                           + 
                           
                             h 
                             5 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         Z 
                         C 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     Z 
                     2 
                   
                   = 
                   
                     
                       
                         
                           
                             h 
                             1 
                           
                           + 
                           
                             h 
                             2 
                           
                           + 
                           
                             h 
                             3 
                           
                           + 
                           
                             h 
                             4 
                           
                         
                         n 
                       
                       ⁢ 
                       
                         Z 
                         C 
                       
                     
                     + 
                     
                       Z 
                       M 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             h 
                             4 
                           
                           + 
                           
                             h 
                             5 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         Z 
                         C 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     From Equations (5) and (6), n for equalizing Z 1  and Z 2  is expressed by the following equation. 
     
       
         
           
             
               
                 
                   n 
                   = 
                   
                     1 
                     + 
                     
                       
                         
                           h 
                           2 
                         
                         + 
                         
                           h 
                           4 
                         
                       
                       
                         h 
                         3 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     As described above, when the ratio of the thickness of the positive conductor  152  and negative conductor  153  to the thickness of the alternating-current conductor  154  is equal to n, which is expressed by Equation (7), the impedance imbalance can be suppressed to avoid the imbalance between the current flowing in path  1  and the current flowing in path  2 . 
     Next, a case where a current flows from the direct-current side of the main circuit busbar assembly  151  to the alternating-current side will be described. 
       FIG. 15  is a diagram illustrating the paths of currents flowing from the positive fuse connection section  158  in the main circuit busbar assembly  151  to the external alternating-current terminal  154 T. 
     Here, as a steady state, let us assume a situation where the IGBT of the upper arm in each of the power semiconductor modules  111 ,  112  is ON while a current flows from the external alternating-current terminal  154 T. In this state, the current flows from the fuse connection section  158  through the positive conductor  152  to the IGBT of the upper arm in the power semiconductor modules  111 ,  112 , to the alternating-current conductor  154 , and to the external alternating-current terminal  154 T in the order named. For purposes of explanation, the current path is divided into path  3  and path  4 . Path  3  is the path of the current flowing through the first power semiconductor module  111 . Path  4  is the path of the current flowing through the second power semiconductor module  112 . In this instance, path  3  is equal in length to path  1 , and path  4  is equal in length to path  2 . Therefore, when the positive conductor  152 , the negative conductor  153 , and the alternating-current conductor  154  are equal in thickness, the conditions for the length of each zone are as expressed by Equation (4) above. Further, the conditions for a case where the alternating-current conductor  154  is different in thickness from the positive conductor  152  and the negative conductor  153  are as expressed by Equation (7) above. 
       FIG. 16  is a perspective view illustrating a configuration of the power conversion section  2   a.    
     The power conversion section  2   a  is disposed in a housing (not shown) of the UPS  2 . An open/close door (not shown), which is to be opened for the maintenance of the UPS  2 , is attached to the front surface of the housing of the UPS  2 , that is, positioned in the Y-axis direction of the power conversion section  2   a . Opening the open/close door makes it easy to access the front surface of the power conversion section  2   a.    
     The power conversion section  2   a  includes a plurality of power conversion units  101 , which are disposed in the X-axis direction. The converter  11  includes three power conversion units, which respectively correspond to the three phases of commercial power. Similarly, the inverter  12  includes three power conversion units  101 , which respectively correspond the three phases. 
     The step-up chopper  13  includes two power conversion units  101 , which are connected in parallel. The step-up chopper  13  may be formed of one power conversion unit  101 . If the rated power of the power semiconductor modules  110  included in the power conversion unit  101  is exceeded by electrical power required for the step-up chopper  13 , N power conversion units  101  are connected in parallel to increase the permissible power N-fold. For similar purposes, the converter  11  and the inverter  12  may each include, as needed, a plurality of parallel-connected power conversion units  101  per phase. 
     The power conversion units  101  in the power conversion section  2   a  are parallel-connected through the unit coupling busbar assembly  161 . The longitudinal direction of each of the power conversion units  101  is the Z direction, and the power conversion units  101  are disposed in the X direction. The longitudinal direction of the unit coupling busbar assembly  161  is the X direction, and the unit coupling busbar assembly  161  is disposed in the +Y direction of the power conversion units  101 . That is to say, the longitudinal direction of each of the power conversion units  101  crosses the longitudinal direction of the unit coupling busbar assembly  161 . Consequently, the power conversion units  101  can be efficiently disposed within a limited cubic volume. 
     The unit coupling busbar assembly  161  is assembled, by using mounting screws  169 , to the second terminal  131   b  of the positive fuse  131  and the second terminal  132   b  of the negative fuse  132 , which are mounted on the lower front surface of each power conversion unit  101 . 
       FIG. 17  is a front view illustrating the configuration of the power conversion section  2   a.    
     The external alternating-current terminals  154 T disposed on the upper ends of the power conversion units  101  respectively correspond to the R, S, and T terminals of the converter  11 , the U, V, and W terminals of the inverter  12 , and the C terminals (common) of the step-up chopper  13 . Two terminals are disposed on the left end of the unit coupling busbar assembly  161 . The upper one of these terminals corresponds to the P terminals of the converter  11 , inverter  12 , and step-up chopper  13 , and the lower one corresponds to the N terminals of the converter  11 , inverter  12 , and step-up chopper  13 . 
     When the configuration according to the above-described embodiment is employed to parallel-connect an appropriate number of power conversion units  101  in accordance with a desired amount of electrical power and the number of phases, various power conversion devices, such as the converter  11 , the inverter  12 , and the step-up chopper  13 , can be configured in an arbitrary manner. When the power conversion unit  101  is manufactured as a minimum configuration unit, the parts for various power conversion devices can be communized to achieve cost reduction. Further, when the communized power conversion units  101  are employed and disposed in parallel, the resulting layout is in good order. This increases the ease of unit-to-unit connection wiring and provides improved overall assemblability. Moreover, the terminals, capacitor terminals, and fuse terminals for the power semiconductor modules included in the power conversion unit  101  and the unit coupling busbar assembly  161  for connecting the power conversion units are on the front surface of the power conversion device. This improves front accessibility during assembly and maintenance, thereby providing increased workability. 
     A method of manufacturing the power conversion section  2   a  will now be described. First of all, a manufacturer disposes a plurality of power conversion units  101  in the +X direction and disposes the unit coupling busbar assembly  161  in the +Y direction with respect to the power conversion units  101 . Subsequently, the manufacturer connects a plurality of positive conductors  152 , which are respectively included in the power conversion units  101 , through a positive conductor  162  included in the unit coupling busbar assembly  161 , and connects a plurality of the negative conductors  153 , which are respectively included in the power conversion units  101 , through a negative conductor  163  included in the unit coupling busbar assembly  161 . In this manner, the power conversion section  2   a  is manufactured. Further, the manufacturer can manufacture, for example, the converter  11 , the inverter  12 , and the step-up chopper  13  by using the power conversion units  101 . Moreover, the manufacturer can manufacture the UPS  2  by connecting, for example, the storage battery  14 , the maintenance bypass circuit  16 , and the bypass circuit  17  to a plurality of alternating-current terminals  154 T, which are respectively included in the power conversion units  101 . 
     In the power conversion unit  101 , from up to down (in the −Z direction), external direct-current terminals including an external positive terminal and an external negative terminal, the power semiconductor module  111 , the power semiconductor module  112 , the capacitor  121 , the capacitor  122 , and the external alternating-current terminal  154 T may be disposed in the order named. In such an instance, the external direct-current terminals are disposed on the upper end of the main circuit busbar assembly  151 , and the external alternating-current terminal  154 T is disposed on the lower end of the main circuit busbar assembly  151 . In this case, heavy parts, such as the reactor connected to the external alternating-current terminal  154 T, are disposed below the power conversion unit  101 . Therefore, the distance between the external alternating-current terminal and the heavy parts can be shortened to reduce the impedance. 
     Terms used in this document will now be explained. A power conversion unit corresponds, for example, to the power conversion unit  101 . A positive conductor corresponds, for example, to the positive conductor  152 . A negative conductor corresponds, for example, to the negative conductor  153 . An alternating-current conductor corresponds, for example, to the alternating-current conductor  154 . A circuit connection section corresponds, for example, to the main circuit busbar assembly  151 . A capacitor corresponds, for example, to the capacitors  120 . A positive connection conductor corresponds, for example, to the positive conductor  162 . A negative connection conductor corresponds, for example, to the negative conductor  163 . Two two-level half-bridge circuits correspond, for example, to the power semiconductor modules  111 ,  112 . Two capacitors correspond, for example, to the capacitors  121 ,  122 . A positive fuse corresponds, for example, to the fuse  131 . A negative fuse corresponds, for example, to the fuse  132 . A power conversion device corresponds, for example, to the converter  11 , the inverter  12 , the step-up chopper  13 , the power conversion section  2   a , and the UPS  2 . A first power semiconductor module corresponds, for example, to the power semiconductor module  111 . A second power semiconductor module corresponds, for example, to the power semiconductor module  112 . An external positive terminal corresponds, for example, to the fuse connection section  158 . An external negative terminal corresponds, for example, to the fuse connection section  159 . An external alternating-current terminal corresponds, for example, to the external alternating-current terminal  154 T. A first direct-current terminal corresponds, for example, to the positive terminal  111   p  and the negative terminal  111   n . A first positive terminal corresponds, for example, to the positive terminal  111   p . A first negative terminal corresponds, for example, to the negative terminal  111   n . A first alternating-current terminal corresponds, for example, to the alternating-current terminal  111   ac . A second direct-current terminal corresponds, for example, to the positive terminal  112   p  and the negative terminal  112   n . A second positive terminal corresponds, for example, to the positive terminal  112   p . A second negative terminal corresponds, for example, to the negative terminal  112   n . A second alternating-current terminal corresponds, for example, to the alternating-current terminal  112   ac . A specific direction corresponds, for example, to the −Z direction. An array direction corresponds, for example, to the X direction. a corresponds, for example, to h 2 . b corresponds, for example, to h 4 . c corresponds, for example, to h 3 . 
     The present invention is not limited to the above-described embodiment, and various modifications may be made without departing from the spirit of the present invention. 
     REFERENCE SIGNS LIST 
       1 : Power conversion device, 
       2 : UPS (Uninterruptible Power Supply), 
       11 : Converter, 
       12 : Inverter, 
       13 : Step-up chopper, 
       101 : Power conversion unit, 
       110 : Power semiconductor modules, 
       111 ,  112 : Power semiconductor module, 
       113 : Air-cooling fin, 
       120 : Capacitors, 
       121 ,  122 : Capacitor, 
       131 ,  132 : Fuse, 
       151 : Main circuit busbar assembly, 
       152 : Positive conductor, 
       153 : Negative conductor, 
       154 : Alternating-current conductor, 
       154 T: External alternating-current terminal, 
       155 : Insulator, 
       161 : Unit coupling busbar assembly, 
       162 : Positive conductor, 
       162 T: External positive terminal, 
       163 : Negative conductor, 
       163 T: External negative terminal, 
       164 : Insulator.