Abstract:
An inexpensive and space-saving power conversion device is provided which can eliminate a high-cost and large reactor like a buffer reactor. A device includes multiple switching elements  21   u,    21   x  performing conversion between DC and AC upon switching, a unitary unit C including the switching elements  21   u,    21   x , and a capacitor  30 , and unit arms  10 P,  10 N each including at least one unitary unit C. The primary side of a transformer  40  is connected between the pair of unit arms  10 P,  10 N so as to suppress a short-circuit current by a leakage inductance component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT Application No. PCT/JP2012/080359, filed on Nov. 22, 2012, and claims priority to Japanese Patent Application No. 2011-256825, filed on Nov. 24, 2011, the entire contents of both of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present disclosure relate to a power conversion device that mutually converts power between AC and DC. 
     BACKGROUND ART 
     Power conversion devices that mutually convert power between AC and DC are applied to various applications. For example, a three-phase-two-level type is utilized as a converter that converts AC from a power system into DC, and an inverter that converts AC into DC to drive a motor. The three-phase-two-level is a technology of selecting six switches at two levels of ON and OFF, thereby performing power conversion between DC and three-phase AC. 
     A switching element that performs switching which is a change between ON and OFF is a semiconductor element. In general, an IGBT (Insulated Gate Bipolar Transistor) is utilized as a switching element. The three-phase-two-level inverter can be configured by six such switching elements at minimum. 
     The control technology of the three-phase-two-level inverter is, in general, a PWM control. The PWM control is a technology of controlling a pulse width to control a level of an output AC voltage. When, for example, an input AC voltage is Vdc, switching is performed between two values of +Vdc/2 and −Vdc/2 at a predetermined timing for each phase. Hence, the output waveform from the three-phase-two-level inverter can be a pseudo-AC waveform. 
     Meanwhile, a necessity for a large-scale DC power feeding of which power loss is less than an AC power feeding is increasing recently. For example, power feeding through seafloor cables, 50-Hz/60-Hz conversion, and a long-distance DC power feeding from a remote large-scale solar power generation system to a consumer location are getting attention. 
     According to such a DC power feeding, DC subjected to ON/OFF is an extremely high voltage like 300 kV. Conversely, the IGBTs utilized as the switching elements have a rating of substantially 6500 V. Hence, when a large number of such switching elements are connected in series to be utilized as a multi-level inverter, a voltage applied to each switching element can be decreased. 
     SUMMARY 
     Technical Problem 
     According to the above-explained power conversion devices, a capacitor is sometimes utilized as a voltage source that changes the output of the voltage in accordance with a switching. In this case, for example, a unitary unit which connects a DC capacitor in parallel with two switching elements is configured. According to such a unitary unit, when the one switching element is ON, the voltage corresponding to the DC capacitor is output, and when the other switching element is ON, the output voltage becomes zero. 
     It is necessary for the DC capacitor configuring the unitary unit to control the voltage value to be constant so as to allow appropriate charging/discharging. Hence, A back-flow current that refluxes a DC power source is required to always flow through the unitary unit. More specifically, each phase is required to be provided with a short-circuited path for charging/discharging. 
     However, in a three-phase power conversion devices, three phases are connected to the same DC power source. Hence, when the DC voltage resultant values of respective phases differ even slightly, a large short-circuit current flows between the phases, which may affect the devices. Even if the average value of the DC voltage resultant value of each phase is consistent, when the ON/OFF timing and the cycle are different, the same technical problem occurs. 
     In order to address this technical problem, a buffer reactor is inserted in each phase so as to prevent the short-circuit current from becoming excessively large. However, the use of this buffer reactor results in an increase in the size of the whole device, and an increase in costs. 
     It is an objective of embodiments of the present disclosure to provide a power conversion device which can eliminate a high-cost and large reactor like a buffer reactor, and which is inexpensive and space-saving. 
     Solution to Problem 
     To address the aforementioned technical problems, a power conversion device according to an embodiment employs the following structure. 
     (1) A plurality of switching elements that performs conversion between DC and AC upon switching. 
     (2) A plurality of unitary units comprising the switching elements and a capacitor. 
     (3) A plurality of unit arms comprising at least one of the unitary units. 
     (4) A transformer having a primary winding connected between a pair of the unit arms so as to suppress a short-circuit current by a leakage inductance component. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram illustrating an example structure of a power conversion device according to an embodiment; 
         FIG. 2  is a circuit diagram illustrating a unitary unit in  FIG. 1 ; 
         FIG. 3  is a circuit diagram illustrating an example structure of a power conversion device utilizing a reactor; 
         FIG. 4  is a circuit diagram having one phase simplified in  FIG. 1 ; 
         FIG. 5A  is a diagram illustrating a voltage waveform of a positive-side unitary unit, and  FIG. 5B  is a diagram illustrating a voltage waveform of a negative-side unitary unit; 
         FIG. 6A  is a diagram illustrating a voltage waveform of a primary winding of a positive-side transformer,  FIG. 6B  is a diagram illustrating a voltage waveform of a primary winding of a negative-side transformer, and  FIG. 6C  is a diagram illustrating a voltage waveform of a secondary winding of the transformer; 
         FIG. 7  is a circuit diagram illustrating an example case in which secondary windings of transformers are connected in parallel with each phase; 
         FIG. 8  is a structural diagram illustrating an example case in which the primary and secondary windings of a transformer in each phase are formed on a common iron core; and 
         FIG. 9  is a structural diagram illustrating an example case in which a three-phase transformer is applied. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     [A. Structure According to Embodiment] 
     [1. Entire Structure] 
     A structure according to this embodiment will be explained with reference to  FIGS. 1 and 2 . According to this embodiment, a power conversion device is connected between a three-phase AC system and a DC system, and performs conversion between AC and DC. This power conversion device has, for each three phase, unit arms  10 P,  10 N that are positive and negative phase arms. The unit arms  10 P,  10 N are connected to an AC system through transformers  40 P,  40 N. 
     [2. Unit Arm] 
     Each of the positive unit arm  10 P and the negative unit arm  10 N has N number of unitary units C connected in series. The unitary unit C is a chopper bridge unit converter to be discussed later.  FIG. 1  illustrates a case in which N=2, but it is fine if N≧1. 
     [3. Unitary Unit] 
     The chopper bridge unit converter that is the unitary unit C has, as illustrated in  FIG. 2 , a leg  20  and a capacitor  30  connected in parallel. In the leg  20 , two switching elements  21 U,  21 X having a self-extinguishing function are connected in series. Example switching elements  21 U,  21 X are IGBTs. The respective switching elements  21 U,  21 X are connected with diodes  22 U,  22 X in a reverse-parallel manner. Those diodes  22 U,  22 X are feedback diodes. 
     [4. Transformer] 
     Each of the transformers  40 P,  40 N is a single-phase transformer for insulation having a winding ratio of 1:1 between the primary winding and the secondary winding. The transformers  40 P,  40 N are separately provided for the positive side and the negative side for each phase. The positive side in the primary and secondary windings of the transformers  40 P,  40 N are indicated by a black dot. 
     [5. Connection Relationship of Each Portion] 
     One end of each unit arm  10 P of the positive side in each phase is connected to the positive side of the DC power source. The other end of each unit arm  10 P in each phase is connected to the positive side of the primary winding of the positive transformer  40 P. 
     One end of each unit arm  10 N of the negative side in each phase is connected to the negative side of the DC power source. The other end of each unit arm  10 N in each phase is connected to the positive side of the primary winding of the negative transformer  40 N. 
     The negative side of the primary winding of the positive transformer  40 P in each phase is connected with the negative side of the primary winding of the negative transformer  40 N. In addition, the negative sides of the primary windings of the positive transformers  40 P and the negative sides of the primary windings of the negative transformers  40 N are connected between the three phases of the U phase, the V phase, and the W phase. 
     The secondary winding of the positive transformer  40 P in each phase is connected with the secondary winding of the negative transformer  40 N in series. The negative sides of the secondary windings of the negative transformers  40 N are short-circuited in the three phases of the U phase, the V phase, and the W phase. The positive sides of the secondary windings of the positive transformers  40 P are connected to the AC side as connection ends of the U phase, the V phase, and the W phase. 
     [B. Operation According to Embodiment] 
     [1. Utilization of Leakage Inductance] 
     Actual transformers always have leakage fluxes due to a magnetic leakage. The leakage fluxes do not contribute to a transformer operation, but work as a leakage inductance of the primary and secondary windings. 
     According to this embodiment, as will be discussed later, when the DC circulation current flows, the primary windings of the transformers  40 P,  40 N that are insulation transformers become the path of the DC circulation current. Hence, by the leakage inductance components of the primary windings of the transformers  40 P and  40 N, like a reactor, a sharp increase of the DC circulation current is suppressed. 
     For example, as illustrated in  FIG. 3 , when a reactor L is installed and an insulation transformer T is utilized for an output to a system and for an insulation, the winding portion of the insulation transformer T is unavailable as a reactor. According to this embodiment, by employing the above-explained connection structure, the leakage inductance components of the transformers  40 P,  40 N at the primary side can serve as a reactor. 
     [2. Suppression of Harmonic Component] 
     In addition, according to a switching performed by semiconductor elements, distorted waveforms are generated. The harmonic components in the distorted waveforms affect the device. In order to address this problem, for example, a filter that absorbs generated harmonic components can be inserted. In general, such a filter can be realized by a reactor or a capacitor that suppresses harmonic components. 
     However, in order to decrease the harmonic components leaking to the power system to a level that does not affect the device, a large-capacity filter is necessary. Accordingly, a reactor and a capacitor necessary for the filter become large, resulting in cost increase and weight increase. 
     According to this embodiment, a multi-level conversion device is configured to have unitary units C connected in series and in multi stages. Hence, the output waveform can become further close to a sinusoidal wave, thereby suppressing harmonic components. 
     [3. AC Voltage Output Operation] 
     An output operation of an AC waveform according to this embodiment will be explained with reference to  FIGS. 4 to 6C . In  FIG. 4 , in order to simplify the explanation, the unitary unit C is indicated one each for the positive side and the negative side. First, as illustrated in  FIG. 4 , the neutral point of the DC power source is taken as a ground, and is also taken as a reference voltage. 
     Next, reference numerals are defined as follows: 
     Vu is a voltage at an AC output point as viewed from the ground. 
     Vdc is a positive/negative voltage of the DC power source. 
     Vc is a voltage of the capacitor  30  in the unitary unit C. 
     VuP is an output voltage of the unitary unit C connected to the positive power source side. 
     VuN is an output voltage of the unitary unit C connected to the negative power source side. 
     VuRef is an AC voltage instruction to be output calculated by an upper system. 
     In this case, the output voltage VuP of the positive unitary unit C is as follows.
 
 VuP=Vdc−Vu Ref  (Formula 1)
 
     The voltage waveform of this VuP is illustrated in  FIG. 5A . In addition, the waveform of a voltage VtrP1 of the primary winding at the positive transformer  40 P is illustrated in  FIG. 6A . 
     At this time, the output voltage Vu is output as follows.
 
 Vu=Vdc−VuP=Vdc −( Vdc−Vu Ref)= Vu Ref  (Formula 2)
 
     Conversely, the output voltage VuN of the negative unitary unit C is as follows.
 
 VuN=Vdc−Vu Ref  (Formula 3)
 
     The voltage waveform of this VuN is illustrated in  FIG. 5B . In addition, the waveform of a voltage VtrN1 of the primary winding at the positive transformer  40 P is illustrated in  FIG. 6B . 
     At this time, the output voltage Vu is output as follows.
 
 Vu=−Vdc+VuN=−Vdc +( Vdc−Vu Ref)=− Vu Ref  (Formula 4)
 
     The primary and secondary windings of the transformer  40 P are subtractive polarity, and the primary and secondary windings of the transformer  40 N are additive polarity. Hence, as illustrated in  FIG. 6C , a voltage Vtr2 synthesized at the secondary side is output. 
     [4. DC Charging/Discharging Operation] 
     When an AC load current is Iu, this Iu flows through the positive unitary unit C and the negative unitary unit C, respectively. At this time, the capacitor  30  of the positive unitary unit C performs charging/discharging with power PowerP expressed by the following formula.
 
Power P=VuP×Iu =( Vdc−Vu Ref)× Iu   (Formula 5)
 
     When VuRef and Iu are in the same phase, i.e., when operation is carried out at a power factor of 1, the calculated average value of PowerP in one AC cycle becomes a negative value. That is, when the above-explained output voltage control is performed, the average value of the capacitor voltage in the positive unitary unit C cannot be maintained to be constant, and thus the operation cannot be continued. 
     Likewise, as to the capacitor voltage of the negative unitary unit C, when the power factor is 1, PowerN has a positive average value at AC one cycle. Hence, it becomes difficult to maintain the average value of the capacitor voltage to be constant, and thus the operation cannot be continued. 
     In order to address this technical problem, DC charging/discharging current is allowed to flow through a path from the positive side of the DC power source, the positive unitary unit C, the positive transformer  40 P, the negative transformer  40 N, the negative unitary unit C, and to the negative side of the DC power source. This stabilizes the average value of the capacitor voltage. 
     More specifically, a correction value ΔVfcControl which controls the average value of the capacitor voltage to be constant is calculated through the following formula. Next, based on this correction value ΔVfcControl, the output voltages Vup, VuN of the positive and negative unitary units are corrected and output.
 
Δ Vfc Control= G ( s )×( VC ref− VCu _AVE)  (Formula 6)
 
where:
 
     VCref is a capacitor voltage instruction value of unitary unit C (a value set in advance); 
     VCu AVE is a capacitor voltage average value of U phase positive and negative whole unitary units; and 
     G(s) is a control gain, where s is a Laplace operator and proportional integral control is appropriate. 
     [C. Advantageous Effects of Embodiment] 
     According to this embodiment explained above, a sharp increase of the DC circulation current is suppressed by the leakage inductances at the primary side of the transformers  40 P,  40 N, thereby controlling the average value of the capacitor voltage of the unitary unit C to be constant. Hence, a compact power conversion device at low cost can be configured without installing a large and high-cost device like a buffer reactor. 
     Such a structure is advantageous when the structure is, in particular, configured as a multi-level conversion device. That is, the switching elements  21 U,  21 X have less necessary space than a reactor, etc., however when such switching elements are connected in a multi-stage manner, the necessary space increases according to an increased number of the switching elements. According to this embodiment, however, the space for a reactor can be saved. Therefore, even if the number of switching elements  21 U,  21 X to be connected increases, an increase in the size is avoidable. 
     [D. Other Embodiments] 
     The embodiment of the present disclosure is not limited to the above-explained form. 
     (1) For example, as illustrated in  FIG. 7 , in each phase of the aforementioned embodiment, the respective secondary windings of the transformers  40 P,  40 N may be connected in parallel. A series connection or a parallel connection is selected as needed in accordance with a DC system, an AC system, a load, etc., to be connected. 
     (2) In addition, according to the aforementioned embodiment, each phase is configured by the pair of transformers  40 P,  40 N. However, as illustrated in  FIG. 8 , the two transformers  40 P,  40 N may have a common iron core M. 
     That is, as illustrated in  FIG. 8 , the primary winding and the secondary winding for each phase are wound around the common iron core M. Two primary windings are provided for the positive side and the negative side. The one primary winding has an end connected to an end of the unit arm  10 P in each phase. The other primary winding has an end connected to an end of the unit arm  10 N in each phase. The neutral points of the two primary windings are connected together. 
     The negative sides of the secondary windings in each phase are connected together through the three phases. The positive side of the secondary winding in each phase is connected to an AC side as a connection end of the U phase, the V phase, and the W phase. 
     According to such a structure, in the figure, as is indicated as Icharge, when a short-circuit current flows, it becomes a DC charging/discharging current of a capacitor. DC magnetic fluxes generated by this DC charging/discharging current are canceled with each other. Accordingly, the saturated flux density can be reduced, enabling a further downsizing of the iron core M. Note that IuP and IuN in the figure are positive and negative input currents, and IuP+IuN is an output current. 
     Such a transformer for each phase can be thought as a combination of two single-phase transformers or can be thought as a single transformer having two primary windings. In addition, as explained above, regarding the secondary winding in each phase, two windings may be connected in parallel. 
     (3) Still further, according to the aforementioned embodiment, the transformers  40 P,  40 N are provided for respective three phases. However, as illustrated in  FIG. 9 , the aforementioned embodiment can be carried out through the winding structure of a three-phase transformer. For example, the following winding structure is employed in each leg of a three-phase/three-leg transformer. 
     That is, the three-phase transformer illustrated in  FIG. 9  has two primary windings in each phase. Ends Up, Vp, and Wp of the one primary winding in the three-phase transformer are connected to the ends of the arms  10 P in respective phases. Ends Un, Vn, and Wn of the other primary winding of the three-phase transformer are connected to the ends of the arms  10 N in respective phases. The neutral points of the two primary windings of the three-phase transformer in each phase are connected together. 
     The negative sides of the secondary windings of the three-phase transformer are connected together among the three phases. Positive ends Us, Vs, and Ws of the secondary winding of the three-phase transformer in each phase are connected to the AC side as the U phase, V phase, and W phase connection ends. 
     According to such a structure, an advantageous effect originating from the mutual cancelation of the DC magnetic fluxes in respective phases as explained above can be obtained. In addition, since it is configured by a three-phase/three-leg transformer, etc., a further downsizing can be accomplished. As explained above, as to the secondary winding of each phase, two windings may be connected in parallel. 
     (4) The aforementioned embodiment can perform a conversion from DC to AC and from AC to DC through the similar structure. That is, the power conversion device of the aforementioned embodiment can be utilized as an inverter and a converter. In addition, the AC system side of the power conversion device may be subjected to a delta connection, or may be subjected to a three-phase Y connection with a neutral point. 
     (5) The specific example of the present disclosure was explained in the present specification, but the specific example is merely presented as an example, and is not intended to limit the scope and spirit of the present disclosure. The present disclosure can be carried out in other various forms, and permits various omissions, replacements, and modifications without departing from the scope and spirit of the present disclosure. Such forms and modifications thereof are within the scope and spirit of the present disclosure, and are also within the equivalent range of the subject matter as recited in appended claims.