Abstract:
First and second capacitors are connected in series between positive/negative electrodes of a DC power source. A first terminal of a reactor and a first terminal of an electric power system are connected to the interconnection point between the first and second capacitors. A first switch section has first and second terminals respectively connected to the positive electrode of the DC power source and a second terminal of the reactor. A second switch section has first and second terminals respectively connected to the negative electrode of the DC power source and the second terminal of the reactor. A third switch section has first and second terminals respectively connected to the second terminal of the reactor and a second terminal of the electric power system. A fourth switch section has first and second terminals respectively connected to the second terminal of the reactor and the second terminal of the electric power system.

Description:
FIELD 
     The present invention relates to an interconnection inverter device which converts direct-current power fed from a direct-current power source to alternating-current power to interconnect to an electric power system. 
     BACKGROUND 
     An interconnection inverter device according to a conventional technique of this type includes, for example, one that is disclosed in Patent Literature 1 below. The interconnection inverter device disclosed in Patent Literature 1 is configured to include first power conversion unit which has a step-down function and is disposed on the input side of a reactor; second power conversion unit which has a step-up function and is disposed on the output side of the reactor; and third power conversion unit disposed on the output side of the second power conversion unit to convert a direct current to an alternating current and then output the resulting current to an electric power system. The device provides control to switch between the first power conversion unit and the second power conversion unit depending on the system voltage and the magnitude of an input voltage to the first power conversion unit. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2004-104963 
     SUMMARY 
     Technical Problem 
     However, the interconnection inverter device disclosed in Patent Literature  1  mentioned above always requires the first power conversion unit serving as a step-down circuit and the second power conversion unit serving as a step-up circuit, leading to an increase in circuit scale and loss. 
     Furthermore, this interconnection inverter device has to provide control to the first power conversion unit that is a step-down circuit when the input voltage is higher than the system voltage and to the second power conversion unit that is a step-up circuit when the input voltage is lower than the system voltage. This is combined with the control that is provided to third power conversion unit for converting a direct current to an alternating current, resulting in further complication in control. 
     The present invention was developed in view of the aforementioned problems. It is therefore an object of the invention to provide an interconnection inverter device which requires neither step-up circuit nor step-down circuit and can be controlled in a simplified manner. 
     Solution To Problem 
     In order to solve the aforementioned problems, an interconnection inverter device for converting direct-current power fed from a direct-current power source to alternating-current power to interconnect to an electric power system according to one aspect of the present invention is constructed in such a manner as to include: first and second capacitors which are connected in series to both ends of the direct-current power source and an interconnection point of which is connected to one end of the electric power system; a reactor with one end connected to the connection point between the first capacitor and the second capacitor; a first switch section with one end connected to a positive electrode of the direct-current power source and the other end connected to the other end of the reactor; a second switch section with one end connected to a negative electrode of the direct-current power source and the other end connected to the other end of the reactor; a third switch section with one end connected to the other end of the reactor and the other end connected to the other end of the electric power system; a fourth switch section with one end connected to the other end of the reactor and the other end connected to the other end of the electric power system; and a third capacitor connected to both ends of the electric power system in parallel thereto. 
     Advantageous Effects of Invention 
     The present invention advantageously provides an interconnection inverter device which is reduced in size, weight, and loss because step-up circuits or step-down circuits are obviated and control can be provided in a simplified fashion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating the configuration of an interconnection inverter device according to a first embodiment of the present invention. 
         FIG. 2  is a time chart illustrating a one-cycle switching operation of the interconnection inverter device of the first embodiment. 
         FIG. 3  is a view of the flow of current in the positive half cycle of a system voltage illustrated on the circuit diagram of  FIG. 1 . 
         FIG. 4  is a view of the flow of current in the negative half cycle of the system voltage illustrated on the circuit diagram of  FIG. 1 . 
         FIG. 5  is a view illustrating the configuration of an interconnection inverter device according to a second embodiment of the present invention. 
         FIG. 6  is a view illustrating the configuration of an interconnection inverter device according to a third embodiment of the present invention. 
         FIG. 7  is a time chart illustrating one-cycle switching operation of the interconnection inverter device of the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, an interconnection inverter device according to the embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. Note that the descriptions below will not limit the present invention. 
     First Embodiment 
       FIG. 1  is a view illustrating the configuration of an interconnection inverter device according to a first embodiment of the present invention. The interconnection inverter device shown in  FIG. 1  has a circuit configuration that is referred to as a current source inverter device. By way of an example, the interconnection inverter device is illustrated as including a direct-current power source  1  as a direct-current input, such as a solar battery module or a fuel cell, and converting direct-current power from the direct-current power source  1  to alternating-current power to interconnect to an electric power system  2 . The inverter device is made up of an inverter circuit  5  and a control section  6  for controlling the inverter circuit  5 . More specifically, the inverter circuit  5  is configured as follows. 
     In  FIG. 1 , a positive direct-current bus  4 P drawn from the positive electrode of the direct-current power source  1  is connected with a switching element S 1 , a diode D 1 , a diode D 3 , and a switching element S 3 , while a negative direct-current bus  4 N drawn from the negative electrode of the direct-current power source  1  is connected with a switching element S 2 , a diode D 2 , a diode D 4 , and a switching element S 4 . Both ends of the direct-current power source  1  are connected in series with a first capacitor that is a capacitor C 1  and a second capacitor that is a capacitor C 2 . The capacitor C 1  and the capacitor C 2  are connected to each other at a connection point, i.e., an intermediate point between the capacitors C 1  and C 2 , to which one end of a reactor L is connected. The other end of the reactor L is connected to both the point connected to the cathodes of the diodes D 1  and D 3  and the point connected to the anodes of the diodes D 2  and D 4 . Furthermore, one end of the switching element S 1  (the drain in  FIG. 1 ) is connected to the positive pole of the capacitor C 1  (the positive electrode of the direct-current power source  1 ), while the other end of the switching element S 1  (the source in  FIG. 1 ) is connected to the anode of the diode D 1 . One end of the switching element S 2  (the source in  FIG. 1 ) is connected to the negative pole of the capacitor C 2  (the negative electrode of the direct-current power source  1 ), while the other end of the switching element S 2  (the drain in  FIG. 1 ) is connected to the cathode of the diode D 2 . Furthermore, one end of the switching element S 3  (the source in  FIG. 1 ) is connected to the anode of the diode D 3 , while one end of the switching element S 4  (the drain in  FIG. 1 ) is connected to the cathode of the diode D 4 , with the other ends of the switching elements S 3  and S 4  connected to each other. Between the connection point of the switching elements S 3  and S 4  and the intermediate point between the capacitors C 1  and C 2 , the electric power system  2  and a capacitor C 3  serving as filter unit are connected in parallel to each other. 
     Note that the diodes D 1  to D 4  are interposed in order to block current which may be caused when the switching elements S 1  to S 4  are reverse biased. Furthermore, the switching element S 1  and the diode D 1  function as a first switch section for controlling a current flowing through the reactor L, while the switching element S 2  and the diode D 2  function as a second switch section for controlling a current flowing through the reactor L. The switching element S 3  and the diode D 3  function as a third switch section for controlling a current flowing through the electric power system  2 , while the switching element S 4  and the diode D 4  function as a fourth switch section for controlling a current flowing through the electric power system  2 . These switch sections are each controlled by the control section  6 . 
     A description will next be made to the operation of the interconnection inverter device according to the first embodiment with reference to  FIGS. 2 to 4 .  FIG. 2  is a time chart showing one-cycle switching operation of the interconnection inverter device according to the first embodiment.  FIG. 3  is a view of the flow of current in the positive half cycle of a system voltage illustrated on the circuit diagram of  FIG. 1 .  FIG. 4  is a view of the flow of current in the negative half cycle of the system voltage illustrated on the circuit diagram of  FIG. 1 . In  FIG. 2 , the system voltage V is the output voltage from the electric power system  2 , and the system current I is the current that flows into or out of the electric power system  2 . 
     Note that in  FIG. 2 , the system voltage V is defined to be positive when the connection end between the third switch section and the fourth switch section to which one end of the electric power system is connected has a positive potential. On the other hand, the system voltage V is defined to be negative when the intermediate point between the capacitors C 1  and C 2  to which the other end of the electric power system is connected has a positive potential. However, in this embodiment, it is assumed that the frequency of the electric power system is known (for example, a commercial frequency of 50 Hz or 60 Hz) and the information on the polarity of the system voltage is available without detecting the system voltage. Furthermore, the system current I is defined as positive when flowing from the fourth switch section to the electric power system  2 , while the system current I is defined as negative when flowing from the electric power system  2  to the third switch section. 
     As shown in  FIG. 2 , in the positive half cycle of the system voltage, the switching elements S 1  and S 3  are controlled to be OFF, the switching element S 4  is controlled to be ON, and the switching element S 2  is PWM (Pulse Width Modulation) controlled. Providing control in this manner allows a positive current to flow as indicated with a thick solid line K 2  of  FIG. 3  in the positive half cycle of the system voltage. On the other hand, in the negative half cycle the system voltage, the switching elements S 2  and S 4  are controlled to be OFF, the switching element S 3  is controlled to be ON, and the switching element S 1  is PWM controlled. Providing control in this fashion allows a negative current (current flowing in the direction opposite to that in  FIG. 3 ) to flow as indicated with a thick solid line K 4  of  FIG. 4  in the negative half cycle of the system voltage. 
     A description will next be made in more detail below to the operation in each of the positive and negative half cycles of the system voltage. First, in  FIG. 3  which shows the operation of the system voltage in the positive half cycle, when the switching element S 2  to be PWM-controlled is ON, the current flows as indicated with a thick broken line K 1 , allowing electric energy from the direct-current power source  1  (the capacitor C 2 ) to be stored as current energy in the reactor L. Note that with the system voltage V higher than a direct-current power source voltage E, the current that could otherwise start to flow from the electric power system  2  to the reactor L is blocked by the switching element S 3  (normally OFF in the positive half cycle) and the diode D 4 . On the other hand, when the switching element S 2  is switched from ON to OFF, the current energy stored in the reactor L is released and output to the electric power system  2  as indicated with a thick solid line K 2 . 
     The same holds true for the operation of the system voltage in the negative half cycle. In  FIG. 4 , when the switching element S 1  which is PWM controlled is ON, a current flows as indicated with a thick broken line K 3 , allowing electric energy from the direct-current power source  1  (the capacitor C 1 ) to be stored as current energy in the reactor L. Note that with the system voltage V higher than the direct-current power source voltage E, the current that could otherwise start to flow from the electric power system  2  to the reactor L (in the direction opposite to that in  FIG. 3 ) is blocked by the switching element S 4  (normally OFF in the negative half cycle) and the diode D 3 . On the other hand, when the switching element S 1  is switched from ON to OFF, the current energy stored in the reactor L is released and output to the electric power system  2  as indicated with a thick solid line K 4 . 
     In the operation of the system voltage during each of the positive and negative half cycles, PWM control is provided to tailor the system current I as a sine wave, in the case of which the PWM control is intended to increase the width of the ON pulse near the peak of the system voltage but decrease the width of the ON pulse near the zero crossing point. Furthermore, the system current I is smoothed by the operation of each of the reactor L and the capacitor C 3  and then supplied to the electric power system  2 . 
     Here, in each of the aforementioned operations, the electromotive force produced in the reactor L when the switching element S 1  is switched from ON to OFF can be greater than the peak voltage of the system voltage. Accordingly, even when the direct-current power source voltage E is less than the system voltage V, the interconnection inverter device of the first embodiment can supply the current energy stored in the reactor L to the electric power system  2  without providing a step-up circuit. Furthermore, even when the direct-current power source voltage E is greater than the system voltage V, a step-down circuit is not necessary. As such, the interconnection inverter device of the first embodiment provides the circuit configuration reduced in size and weight when compared with the interconnection inverter device disclosed in Patent Literature 1 above which requires the step-up circuit and the step-down circuit. 
     Furthermore, the interconnection inverter device disclosed in Patent Literature 1 unavoidably requires one diode and two switching elements through which the current energy stored in the reactor is supplied to the electric power system. However, in the interconnection inverter device of the first embodiment, only one diode and one switching element have to be passed through. Accordingly, the interconnection inverter device of the first embodiment provides a circuit configuration of reduced loss when compared with the interconnection inverter device disclosed in Patent Literature 1. 
     As described above, according to the interconnection inverter device of the first embodiment, the interconnection point of the capacitors C 1  and C 2  that are connected in series to both ends of a direct-current power source is connected with one end of the reactor L and one end of the electric power system, with both ends of the electric power system  2  connected with the capacitor C 3  in parallel. The interconnection inverter device includes the first switch section (S 1 , D 1 ) actuated when storing energy from the capacitor C 1  in the reactor L; the second switch section (S 2 , D 2 ) actuated when storing energy from the capacitor C 2  in the reactor L; the third switch section (S 3 , D 3 ) actuated when supplying the energy stored in the reactor L by the actuation of the first switch section (S 1 , D 1 ) to the electric power system  2 ; and the fourth switch section (S 4 , D 4 ) actuated when supplying the energy stored in the reactor L by the actuation of the second switch section (S 2 , D 2 ) to the electric power system  2 . In the positive half cycle of the system voltage, the switching elements S 1  and S 3  are controlled to be OFF, the switching element S 4  is controlled to be ON, and the switching element S 2  is PWM controlled. In the negative half cycle of the system voltage, the switching elements S 2  and S 4  are controlled to be OFF, the switching element S 3  is controlled to be ON, and the switching element S 1  is PWM controlled. This obviates a step-up circuit and a step-down circuit, thereby realizing an interconnection inverter device which can be controlled in a simplified manner. 
     Second Embodiment 
       FIG. 5  is a view illustrating the configuration of an interconnection inverter device according to a second embodiment of the present invention. The interconnection inverter device of the first embodiment shown in  FIG. 1  is configured on the precondition that the electric power system employs a known frequency and will not use information on the system voltage polarity. However, the interconnection inverter device of the second embodiment is configured as illustrated to include a system voltage detection section  8  for detecting the polarity of the system voltage and supply a value detected by the system voltage detection section  8  to the control section  6 . 
     The interconnection inverter device of the second embodiment can determine the polarity of the system voltage based on the value detected by the system voltage detection section  8 . Thus, even with the frequency of the electric power system being unknown, the inverter device can provide switching control to the first through fourth switch sections depending on the polarity of the system voltage. As a result, this makes it possible to provide improved energy efficiency for interconnection as compared to the interconnection inverter device of the first embodiment. 
     Third Embodiment 
       FIG. 6  is a view illustrating the configuration of an interconnection inverter device according to a third embodiment of the present invention. The interconnection inverter device of the second embodiment shown in  FIG. 5  is configured, as disclosed, to include the system voltage detection section  8  for detecting the polarity of the system voltage. In contrast, the interconnection inverter device of the third embodiment is configured, as illustrated, to further include a system current detection section  9  for detecting a current (system current) flowing into or out of the electric power system  2  and supply the value detected by each of the system voltage detection section  8  and the system current detection section  9  to the control section  6 . 
     A description will next be made to the operation of the interconnection inverter device according to the third embodiment with reference to  FIG. 7 .  FIG. 7  is a time chart showing one-cycle switching operation by the interconnection inverter device of the third embodiment. Note that the polarities of the system current I and the system voltage V are defined in the same manner as in the first embodiment. 
     As shown in  FIG. 7 , the interconnection inverter device of the third embodiment provides control of different types to the first through fourth switch sections depending on the sign of the system voltage V and the system current I, which will be more specifically described as follows. 
     (1) When the system voltage V has a positive polarity and the system current I has a negative polarity, 
     the switching elements S 2  and S 4  are controlled to be OFF, the switching element S 1  is controlled to be ON, and the switching element S 3  is PWM controlled; 
     (2) When the system voltage V has a positive polarity and the system current I has a positive polarity, 
     the switching elements S 1  and S 3  are controlled to be OFF, the switching element S 4  is controlled to be ON, and the switching element S 2  is PWM controlled; 
     (3) When the system voltage V has a negative polarity and the system current I has a positive polarity, 
     the switching elements S 1  and S 3  are controlled to be OFF, the switching element S 2  is controlled to be ON, and the switching element S 4  is PWM controlled; and 
     (4) When the system voltage V has a negative polarity and the system current I has a negative polarity, 
     the switching elements S 2  and S 4  are controlled to be OFF, the switching element S 3  is controlled to be ON, and the switching element S 1  is PWM controlled. 
     The interconnection inverter device of the third embodiment can determine the polarity of the system voltage and the system current based on the output from the system voltage detection section  8  and the system current detection section  9 . Thus, even when the frequency of the electric power system is unknown and there occurs a phase difference between the system voltage and the system current, the inverter device can provide switching control to the first through fourth switch sections depending on the polarity of the system voltage and the system current. As a result, this makes it possible to provide improved energy efficiency for interconnection as compared to the interconnection inverter device of the first and second embodiments. 
     Industrial Applicability 
     As described above, the interconnection inverter device according to the present invention can advantageously obviates a step-up circuit and a step-down circuit, and can be controlled in a simplified manner. 
     Reference Signs List 
       1  DIRECT-CURRENT POWER SOURCE 
       2  ELECTRIC POWER SYSTEM 
       4 P,  4 N DIRECT-CURRENT BUS 
       5  INVERTER CIRCUIT 
       6  CONTROL SECTION 
       8  SYSTEM VOLTAGE DETECTION SECTION 
       9  SYSTEM CURRENT DETECTION SECTION 
     C 1  to C 3  CAPACITOR 
     D 1  to D 4  DIODE 
     L REACTOR 
     S 1  to S 4  SWITCHING ELEMENT