Power conversion system, heat pump system and photovoltaic power generation system

Abstract

To provide a power conversion system having a function to supply power from a DC power supply such as a solar battery to a DC load and convert the power into an AC by a system interconnection inverter to make the AC reversely flow to a power system, in which operation efficiency of the DC load is improved according to the DC load.SOLUTION: A power conversion system 1 includes a DC / DC converter 10 to which a DC power supply 40 can be connected, a system interconnection inverter 30, and a DC link 20 that connects the DC / DC converter with the system interconnection inverter 30. Adjusting a DC link voltage Vdc according to a DC load 50 that can be connected to the DC link 20 within a half period of a system voltage Vac on an AC side of the system interconnection inverter 30 allows operation efficiency of the DC load to be improved.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a power conversion system, a heat pump system, and a solar power generation system. [Background technology]

[0002] For example, Patent Document 1 describes a power conversion device that converts DC power generated by a power generation device into AC power, the power conversion device including a boost converter circuit that boosts the output voltage of the power generation device, an inverter circuit that converts the output voltage of the boost converter circuit into AC power and connects it to a system, a step-down converter circuit that converts the output power of the boost converter circuit and supplies it to an internal load, and a controller, where the controller sets the output voltage of the boost converter circuit to a voltage value lower than the maximum value of the AC power of the system when the output power of the power generation device is supplied to the internal load via the boost converter circuit and the step-down converter circuit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2011 / 099280 Summary of the Invention [Problem to be solved by the invention]

[0004] In a power conversion system that supplies power from a DC power source such as a solar cell to a DC load while converting it to AC using a grid-connected inverter and allowing reverse power flow to the power grid, there is a demand for supplying an appropriate amount of power to the DC load and improving the operating efficiency of the DC load.

[0005] The present disclosure aims to improve the operating efficiency of a DC load depending on the DC load. [Means for solving the problem]

[0006] A power conversion system according to a first aspect includes a DC / DC converter to which a DC power source can be connected, a grid-connected inverter, and a DC link connecting the DC / DC converter and the grid-connected inverter, and within a half cycle of a grid voltage on the AC side of the grid-connected inverter, a voltage of the DC link is adjusted according to a DC load connectable to the DC link, thereby improving the operation efficiency of the DC load. A power conversion system according to a second aspect is the power conversion system according to the first aspect, in which the voltage of the DC link is adjusted according to a state of the DC load connectable to the DC link, thereby making it possible to make the voltage applied to the DC load appropriate. A power conversion system according to a third aspect is the power conversion system according to the second aspect, wherein the DC link is connectable with a DC load whose output varies depending on the voltage of the DC link, thereby making it possible to make the output of the DC load appropriate. A fourth aspect of the power conversion system is the power conversion system of the second aspect, wherein the DC link includes a power converter as the DC load, and a DC load can be connected to adjust the voltage of the DC link according to a state of power conversion of the power converter, thereby making it possible to make the voltage applied to the DC load including the power converter appropriate. A power conversion system according to a fifth aspect is the power conversion system according to the fourth aspect, wherein the DC link includes the power converter that operates a rotating electric machine at a variable speed as the DC load, and a DC load can be connected in which the voltage of the DC link is adjusted according to the operating state of the rotating electric machine. This makes it possible to make the voltage applied to the DC load including the power converter that operates the rotating electric machine at a variable speed appropriate. A power conversion system according to a sixth aspect is the power conversion system according to any one of the first to fifth aspects, in which the adjustment of the voltage of the DC link includes control including a period during which the voltage is adjusted according to the DC load connectable to the DC link and a period during which the voltage is adjusted to a voltage based on the grid voltage, within a half cycle of the grid voltage, thereby making it possible to prevent an excessive voltage from being applied to the DC load. A seventh aspect of the present invention relates to a power conversion system according to the sixth aspect, and further relates to a power conversion system according to the sixth aspect, wherein the voltage based on the grid voltage is a voltage greater than an absolute value of the grid voltage. This makes it possible to ensure a voltage required for a grid-connected inverter regardless of fluctuations in the voltage of a DC link. A power conversion system according to an eighth aspect is the power conversion system according to any one of the first to seventh aspects, wherein the grid-connected inverter is controlled in any one of PWM operation, PAM operation, and rectification operation according to the DC load connectable to the DC link, whereby the voltage applied to the DC load can be made appropriate by controlling the grid-connected inverter. A power conversion system according to a ninth aspect is the power conversion system according to the first aspect, wherein the DC load can be connected to the DC link via a switch circuit, thereby making it possible to prevent an excessive voltage from being applied to the DC load. A power conversion system according to a tenth aspect is the power conversion system according to the ninth aspect, wherein a capacitor having a larger capacitance than a capacitor provided in the DC link is provided on the input side of the DC load in parallel with the DC load, thereby making it easier to control the voltage of the DC link and the voltage of the DC load. A power conversion system according to an eleventh aspect is the power conversion system according to the ninth or tenth aspect, in which the switch circuit is controlled to be on while the voltage of the DC link is within a voltage range that is predetermined according to the DC load, thereby making it possible to prevent an excessive voltage from being applied to the DC load. A power conversion system according to a twelfth aspect is the power conversion system according to any one of the ninth to eleventh aspects, in which the switch circuit is controlled to be on while the voltage of the DC link is within a voltage range that is determined in advance according to the input voltage of the DC load, thereby making it possible to suppress an excessive current from flowing when the switch circuit is turned on. A power conversion system according to a thirteenth aspect is the power conversion system according to any one of the ninth to twelfth aspects, in which the DC / DC converter connects the DC power source connectable to the DC / DC converter and the DC load connectable to the DC link via a diode, whereby, when the diode is turned on, loss in a current path is reduced. A heat pump system according to a fourteenth aspect includes the power conversion system according to any one of the first to thirteenth aspects, and a heat pump device as the DC load, thereby eliminating the need to provide a separate DC / DC converter. A solar power generation system according to a fifteenth aspect includes the power conversion system according to any one of the first to thirteenth aspects, and a solar cell as the DC power supply. This makes it possible to realize a power generation system with a simple circuit configuration that can adjust the voltage applied to a DC load appropriately while connecting the solar cell to a power grid. [Brief description of the drawings]

[0007] [Figure 1] 1 is a diagram illustrating an example of a power conversion system according to a first embodiment. [Diagram 2] This figure explains the relationship between the DC link voltage and the AC voltage and AC current of the power grid when the grid-connected inverter is operated in parallel / reverse power flow. (a) is the case where the DC link voltage is 330 V, (b) is the case where the DC link voltage is 270 V, and (c) is the case where the DC link voltage is adjusted to boost the part lower than the AC voltage. [Diagram 3] FIG. 1 is a diagram illustrating a power conversion system including N sets of DC power supplies and DC / DC converters. [Figure 4] FIG. 1 is a diagram illustrating a power conversion system having M DC loads. [Diagram 5] FIG. 1 is a diagram illustrating a power conversion system including N pairs of DC power supplies and DC / DC converters, and M DC loads. [Figure 6] FIG. 2 is a diagram illustrating an operation mode of the grid-connected inverter. [Figure 7] FIG. 1 is a diagram illustrating an operation mode for supplying power to a DC load in a parallel / reverse power flow. [Figure 8] 1 is a diagram illustrating a DC link voltage target value according to a change in the DC link voltage target value based on a DC load, where (a) is a case where the DC link voltage target value based on a DC load is smaller than a lower limit of an output voltage of a DC / DC converter, (b) is a case where the DC link voltage target value based on a DC load is larger than the lower limit of an output voltage of a DC / DC converter but smaller than the absolute value of a peak voltage of an AC voltage, and (c) is a case where the DC link voltage target value based on a DC load is larger than the absolute value of a peak voltage of an AC voltage. [Figure 9] 13 is a diagram illustrating changes in a DC link voltage target value when a voltage margin is added, where (a) is a case in which the DC link voltage target value based on the DC load is smaller than the lower limit of the output voltage of the DC / DC converter, (b) is a case in which the DC link voltage target value based on the DC load is larger than the lower limit of the output voltage of the DC / DC converter but smaller than the sum of the absolute value of the peak voltage of the AC voltage and the voltage margin, and (c) is a case in which the DC link voltage target value based on the DC load is larger than the sum of the absolute value of the peak voltage of the AC voltage and the voltage margin. [Figure 10] FIG. 13 is a diagram for explaining an operation mode in which power is not supplied to a DC load in a parallel / reverse power flow. [Figure 11] FIG. 13 is a diagram for explaining a DC link voltage target value in an operation mode in which power is not supplied to a DC load in a parallel / reverse power flow. [Figure 12] FIG. 1 is a diagram for explaining an operation mode in which power is supplied to a DC load from a DC power source and a power grid in parallel / forward power flow. [Figure 13] FIG. 1 is a diagram for explaining an operation mode in which power is supplied from a power grid to a DC load in parallel / forward power flow. [Figure 14] FIG. 2 is a diagram for explaining an operation mode in which power is supplied from a DC power source to a DC load during parallel-off. [Figure 15]1 is a diagram illustrating a DC link voltage target value in an operation mode in which power is supplied from a DC power source to a DC load during parallel-off, where (a) is a case in which the DC link voltage target value based on the DC load is greater than a lower limit value of the output voltage of a DC / DC converter, and (b) is a case in which the lower limit value of the output voltage of the DC / DC converter is greater than a DC link voltage target value based on a DC load. [Figure 16] FIG. 1 illustrates an example of a power conversion system according to a second embodiment. [Figure 17] 1 is a diagram illustrating a DC link voltage target value according to a change in the DC link voltage target value based on a DC load, where (a) is a case where the DC link voltage target value based on a DC load is smaller than a lower limit of an output voltage of a DC / DC converter, (b) is a case where the DC link voltage target value based on a DC load is larger than the lower limit of an output voltage of a DC / DC converter but smaller than the absolute value of a peak voltage of an AC voltage, and (c) is a case where the DC link voltage target value based on a DC load is larger than the absolute value of a peak voltage of an AC voltage vac. [Figure 18] 1A and 1B are diagrams illustrating a means for suppressing an inrush current generated by a load switch, in which (a) shows a case where an inrush current occurs, and (b) shows a case where the inrush current is suppressed. [Figure 19] FIG. 13 is a diagram illustrating an example of a power conversion system which is a modified example of the power conversion system according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. [First embodiment] (Power Conversion System 1) FIG. 1 is a diagram illustrating an example of a power conversion system 1 according to a first embodiment. The power conversion system 1 includes a DC / DC converter 10, a DC link 20, and a grid-connected inverter 30. In the following, the DC / DC converter 10 will be referred to as the DC / DC converter 10, and the DC link 20 will be referred to as the DC link 20. The DC link 20 connects the DC / DC converter 10 and the grid-connected inverter 30. The output side of the DC / DC converter 10 and the input side of the grid-connected inverter 30 are the DC link 20. The DC link 20 is provided with a capacitor C1. The voltage between both terminals of the capacitor C1 is the voltage of the DC link 20, and the DC link voltage v dc It is written as follows.

[0009] A DC power supply 40 is connected to the input side of the DC / DC converter 10. In the following, direct current is written as DC, and the DC power supply 40 is written as DC power supply 40. Here, the DC power supply 40 is a solar cell (photovoltaic cell), and the output voltage of the DC power supply 40 is a DC power supply voltage v pv The DC / DC converter 10 is configured so that a DC power supply 40 can be connected thereto.

[0010] A DC load 50 is connected to the DC link 20. Hereinafter, the DC load 50 will be referred to as a DC load 50. Power is supplied to the DC load 50 from the DC link 20. The DC link 20 is configured so that the DC load 50 can be connected thereto.

[0011] The output side of the grid-connected inverter 30 is connected to a power grid 100 via a switch 60. The power grid 100 is a single-phase AC grid. In the following, AC is abbreviated as AC. The AC voltage, which is the system voltage of the power grid 100, is referred to as AC voltage v ac (instantaneous value) and the effective value of the AC voltage v ac rms The absolute value of the peak voltage of the AC voltage is AC peak voltage |v ac peak|. A reactor L2 is provided on one line between the grid-connected inverter 30 and the switch 60, and a reactor L3 is provided on the other line. A capacitor C3 is provided between the two lines between the grid-connected inverter 30 and the switch 60. The reactors L2, L3 and capacitor C3 act as a filter that removes high-frequency components generated by the operation of the grid-connected inverter 30. The capacitor C3 is a non-polar capacitor such as a ceramic capacitor or a film capacitor.

[0012] With the grid-connected inverter 30 sandwiched between them, the DC link 20 side is referred to as the DC side (direct current side), and the power grid 100 side is referred to as the AC side (alternating current side).

[0013] The DC / DC converter 10 is a voltage conversion circuit. The DC / DC converter 10 is connected to a DC power supply 40 and receives a DC power supply voltage v pv is used as an input voltage, and is converted to a different voltage to produce an output voltage. The circuit configuration of the DC / DC converter 10 is determined according to the input voltage of the DC / DC converter 10, the output power of the DC / DC converter 10, the specifications of the DC / DC converter 10, and the like. The DC / DC converter 10 can be configured as a boost circuit that boosts the input voltage to produce an output voltage, a step-down circuit that drops the input voltage to produce an output voltage, or a step-up / step-down circuit that boosts or drops the input voltage to produce an output voltage. The step-up circuit is sometimes referred to as a step-up chopper circuit, the step-down circuit as a step-down chopper circuit, and the step-up / step-down circuit as a step-up / step-down chopper circuit. The DC / DC converter 10 may have either a unidirectional or bidirectional power direction. However, when the DC power source 40 is only a storage battery or a capacitor, it is desirable that the DC / DC converter 10 to which the DC power source 40 is connected is bidirectional. The circuit configuration of the DC / DC converter 10 will be described later.

[0014] The grid-connected inverter 30 is a power conversion circuit that transfers power between the DC link 20 and the power system 100. The grid-connected inverter 30 performs forward power flow, reverse power flow, and parallel-off operations. Forward power flow converts AC power of the power system 100 into DC power and supplies it to the DC link 20. Reverse power flow converts DC power of the DC link 20 into AC power and supplies it to the power system 100. Parallel-off separates the DC link 20 from the power system 100, and the DC link 20 side and the power system 100 operate independently. The circuit configuration of the grid-connected inverter 30 will be described later.

[0015] The DC power supply 40 is a power supply that outputs DC power. The DC power supply 40 may be the above-mentioned solar cell, fuel cell, DC part of a PWM (Pulse Width Modulation) converter of a generator, a storage battery, a capacitor, or the like. The voltage and power supplied by the DC power supply 40 vary depending on the type and state of the DC power supply. The voltage and power supplied by the DC power supply 40 also vary depending on the operating state of the DC power supply. A capacitor C2 may be connected in parallel to the DC power supply 40. The capacitor C2 is a capacitor that converts the DC power supply voltage v pv The DC power supply 40 and the capacitor C2 may be combined together to form a DC power supply unit. The capacitor C2 is an electrolytic capacitor, a ceramic capacitor, a film capacitor, or the like. Furthermore, a storage battery or a capacitor may be connected in parallel to the DC power supply 40, either directly or via a charge / discharge control circuit. The storage battery or the capacitor suppresses fluctuations in the power of the DC power supply. The DC power supply 40 and the storage battery or the capacitor may be combined together to form a DC power supply unit.

[0016] The DC load 50 is a load that is operated by receiving DC power. A capacitor C4 may be provided in parallel with the DC load 50. The capacitor C4 suppresses fluctuations in the voltage supplied to the DC load 50. The capacitor C4 is an electrolytic capacitor, a ceramic capacitor, or the like. The DC load 50 and the capacitor C4 may be collectively referred to as a DC load (or a DC load section). Operating the DC load 50 may be expressed as operating the DC load 50. It may also be expressed as power being supplied to the DC load 50, or a voltage being applied to the DC load 50. The DC load 50 may be a heat pump device (heat pump system) used in air conditioners, water heaters, freezers, refrigerators, showcases, and the like, a ventilator, an EV (Electric Vehicle) charger, a heater, a DC motor, or the like. The DC load 50 may include a power converter that operates a rotating electric machine at a variable speed. The DC load 50 may be one whose output changes depending on the voltage of the DC link 20, which is the input voltage. Such DC loads 50 include heaters and DC motors that do not include a power converter and whose output changes depending on the input voltage. For example, a heater changes the amount of heat and temperature output depending on the input voltage, and a DC motor changes its rotation speed and torque depending on the input voltage. Supplying power to the DC load 50 is referred to as "supplying power to the DC load 50."

[0017] The switch 60 separates the AC side from the DC side. When the switch 60 is closed (on), the AC side and the DC side are connected, and the DC side and the AC side are operated in parallel. When the switch 60 is open (off), the AC side and the DC side are disconnected, and the DC side and the AC side are operated independently. By opening (off) the switch 60, the above-mentioned parallel-out is reliably performed. When the power conversion system 1 is operated in parallel, the switch 60 is closed (on).

[0018] DC link voltage v dc is the AC voltage v, as described below. ac Therefore, the capacitor C1 is adjusted to the DC link voltage v dcIt is preferable that the capacitance is set to a value that allows easy adjustment of the capacitance of the capacitor C1. For example, it is sufficient to suppress noise components such as ringing caused by the operation of the DC / DC converter 10 and the grid-connected inverter 30. The capacitor C1 is a ceramic capacitor, a film capacitor, or the like.

[0019] (Circuit configuration of DC / DC converter 10) The DC / DC converter 10 is a step-up / step-down converter. The DC / DC converter 10 includes switching elements Sw1 and Sw2, diodes D1 and D2, and a reactor L1. The switching elements Sw1 and Sw2 are, for example, IGBTs (Insulated Gate Bipolar Transistors), and are three-terminal elements including a collector terminal, an emitter terminal, and a control terminal (gate terminal). The diodes D1 and D2 are two-terminal elements including an anode terminal and a cathode terminal. The switching element Sw2 and the diode D2, which are connected in series, are connected in parallel to a DC power supply 40. The collector terminal of the switching element Sw2 is connected to the positive side of the DC power supply 40, and the emitter terminal of the switching element Sw2 is connected to the cathode terminal of the diode D2. The anode terminal of the diode D2 is connected to the negative side of the DC power supply 40.

[0020] The diode D1 and the switching element Sw1, which are connected in series, are connected in parallel to the DC link 20. The emitter terminal of the switching element Sw1 is connected to the negative side of the DC link 20, and the collector terminal of the switching element Sw1 is connected to the anode terminal of the diode D1. The cathode terminal of the diode D1 is connected to the positive side of the DC link 20. A reactor L1 is connected between a connection point (a connection point between the emitter terminal of the switching element Sw2 and the cathode terminal of the diode D2) where the switching element Sw2 and the diode D2 are connected, and a connection point (a connection point between the anode terminal of the diode D1 and the collector terminal of the switching element Sw1) where the diode D1 and the switching element Sw1 are connected. The anode terminal of the diode D2 and the emitter terminal of the switching element Sw1 are connected. The terminals do not have to be directly connected, and may be connected via wiring or the like. The switching elements Sw1 and Sw2 may be MOS transistors other than IGBTs.

[0021] The on / off of the switching elements Sw1 and Sw2 is controlled by a control signal supplied to the control terminals of the switching elements Sw1 and Sw2. The DC / DC converter 10 functions as a step-up circuit or a step-down circuit by a combination of on and off of the switching elements Sw1 and Sw2. The DC / DC converter 10 functions as a step-up circuit by holding the switching element Sw2 on and PWM controlling the switching element Sw1. The DC / DC converter 10 functions as a step-down circuit by holding the switching element Sw1 off and PWM controlling the switching element Sw2.

[0022] (Grid-connected inverter 30) The grid-connected inverter 30 includes switching elements Sap, San, Sbp, and Sbn, and diodes Dap, Dan, Dbp, and Dbn. The switching elements Sap, San, Sbp, and Sbn are the same as the switching elements Sw1 and Sw2 described above. The diodes Dap, Dan, Dbp, and Dbn are the same as the diodes D1 and D2 described above.

[0023] The switching elements Sap and San connected in series and the switching elements Sbp and Sbn connected in series are connected in parallel to the DC link 20. The collector terminal of the switching element Sap is connected to the positive side of the DC link 20, and the emitter terminal is connected to the collector terminal of the switching element San. The emitter terminal of the switching element San is connected to the negative side of the DC link 20. Similarly, the collector terminal of the switching element Sbp is connected to the positive side of the DC link 20, and the emitter terminal is connected to the collector terminal of the switching element Sbn. The emitter terminal of the switching element Sbn is connected to the negative side of the DC link 20. The diodes Dap, Dan, Dbp, and Dbn are so-called feedback diodes. The diodes Dap, Dan, Dbp, and Dbn are connected in inverse parallel to the switching elements Sap, San, Sbp, and Sbn of the same sign.

[0024] A connection point between the emitter terminal of the switching element Sap and the collector terminal of the switching element San is connected to one line of the power system 100 via a reactor L2 and a switch 60. A connection point between the emitter terminal of the switching element Sbp and the collector terminal of the switching element Sbn is connected to the other line of the power system 100 via a reactor L3 and a switch 60.

[0025] The switching elements Sap, San, Sbp, Sbn are controlled to be turned on and off by control signals supplied to control terminals of the switching elements Sap, San, Sbp, Sbn. In a forward power flow, the switching elements Sap, San, Sbp, Sbn are controlled so that power is supplied from the power system 100 to the DC link 20. In a reverse power flow, the switching elements Sap, San, Sbp, Sbn are controlled so that power is supplied from the DC link 20 to the power system 100. In a parallel-out state in which the switch 60 is open, the switching elements Sap, San, Sbp, Sbn are all controlled to be turned off.

[0026] Next, the control in the power conversion system 1 will be described. As shown in FIG. 1, the DC load 50 is connected to the DC link 20 and is powered by the DC link 20. The voltage applied to the DC load 50 is the DC link voltage v dc The DC link voltage v dc must be a voltage that allows the operation of the DC load 50 to be maintained properly. The DC load 50 may be powered from the DC power source 40, from the power grid 100 via the grid-connected inverter 30, or from both the DC power source 40 and the power grid 100. In any case, the DC link voltage v must be set to a value that allows the operation of the DC load 50 to be maintained properly. dc The DC link voltage v dc In addition, the DC link voltage v dc is adjusted.

[0027] FIG. 2 shows the DC link voltage v dc and the AC voltage v of the power system 100 ac and AC current i ac Here, the AC voltage effective value v ac rms is 200V. Figure 2(a) shows the DC link voltage v dc When the DC link voltage v is 330V, Fig. 2(b) shows dc When the DC link voltage v is 270V, Fig. 2(c) shows dc is the AC voltage v ac In Fig. 2(a), (b), and (c), the DC link voltage v dc , AC voltage v ac , AC current i ac The horizontal axis is time (seconds), and the vertical axis is the upper DC link voltage v dc , AC voltage v ac So, the voltage (V) and the AC current i ac In Fig. 2(a), (b), and (c), the AC current i acThe total current distortion rate (%) of the MOSFET is shown in Fig. 2 (a), (b), and (c) which were obtained by simulation.

[0028] AC voltage rms value v ac rms is 200V, the AC peak voltage |v ac peak | is approximately 283V. The DC link voltage v shown in Figure 2(a) dc 330V is the AC peak voltage |v ac peak In this case, the AC current i ac The total current distortion rate of AC current i is low at 3.0%. ac In this way, distortion is suppressed.

[0029] The DC link voltage v shown in Figure 2(b) dc 270V is the AC peak voltage |v ac peak In this case, the AC voltage v ac Peak voltage v ac peak In the vicinity, AC current i ac However, it is distorted due to the inclusion of high frequency currents. The total current distortion rate is high at 25.3%.

[0030] As shown in Figure 2(b), the AC current i ac The distortion occurs because of the AC peak voltage |v ac peak Therefore, as shown in Fig. 2(c), the DC link voltage v dc is the AC voltage v ac At the lower end, the DC link voltage v dc By boosting the AC current i ac The high-frequency current in the DC link voltage v is suppressed. The total current distortion rate is low at 3.0%, the same as in Fig. 2(a). As shown in Fig. 2(c), dc The DC link voltage v dc As shown in Figure 2(c), the DC link voltage v dc By adjusting the DC link voltage v in Figure 2(a), dc Compared to when the DC link voltage v dcdecreases, resulting in reduced losses.

[0031] 2(a), (b), and (c) do not explain the voltage applied to the DC load 50 powered by the DC link 20. For example, as shown in FIG. 2(a), dc If the DC link voltage v in FIG. 2(b) is 330V, an excessive voltage may be applied to the DC load 50. dc If 270V is a suitable voltage for powering the DC load 50, then, as mentioned above, the AC current i ac Therefore, as shown in Fig. 2(c), the DC link voltage v dc AC voltage v ac By adjusting the lower portion to boost, the period during which excessive voltage is applied to the DC load 50 is shortened, and the AC current i ac In the following, the voltage applied to the DC load 50 is referred to as a DC load voltage v dc load It is written as follows.

[0032] When the power conversion system 1 is operating in parallel / reverse flow, the DC link voltage v dc is the output voltage of the DC / DC converter 10. The DC link voltage v dc is controlled by the DC / DC converter 10. The DC link voltage v dc may be controlled by the output voltage of the DC / DC converter 10, or may be controlled by the output current (or output power) from the DC / DC converter 10. dc This is called output voltage control mode, and the DC link voltage v is controlled by the output current (or output power) from the DC / DC converter 10. dc The case where the output current is controlled is referred to as the output current control mode (output power control mode). The control of the power conversion system 1 will be described in detail below.

[0033] (Output voltage control mode) In output voltage control mode, the DC link voltage v dc is controlled according to equation (1).

[0034]

number

[0035] DC link voltage target value v * dc is the DC link voltage v dc This is the target value when controlling the DC link voltage v dc is the DC link voltage target value v * dc The DC link voltage target value v * dc changes in a period (sufficiently) shorter than a half period of the AC voltage (grid voltage) on the AC side of the grid-connected inverter 30. dc is the DC link voltage target value v * dc It may be assumed that this is the case.

[0036] DC link voltage target value v * dc is the DC link voltage target value v based on the DC load of 50 * dc load and the DC link voltage target value v based on the grid-connected inverter 30. * dc inv and the lower limit v of the output voltage of the DC / DC converter 10. conv lowlimit The DC link voltage target value v based on the grid-connected inverter 30 is set to the maximum value among the above. In the following, the above terms may be explained using only the symbols. The same applies to other cases. * dc inv is the voltage determined by the system voltage.

[0037] v * dc load is determined according to the configuration and state of the DC load 50. For example, v * dc load If the DC load 50 includes a power converter, v may be determined according to the state of power conversion of the power converter. *dc load For example, v may be determined according to the power conversion state such as the duty ratio, modulation rate, voltage utilization rate, output power, and output current of the power converter. * dc load This allows the voltage applied to the DC load 50 including the power converter to be adjusted appropriately.

[0038] When the DC load 50 includes a power converter that operates a rotating electric machine at a variable speed, v * dc load For example, v may be determined so as to be approximately proportional to the rotation speed of the rotating electrical machine. * dc load This makes it possible to adjust the voltage applied to the DC load 50, which includes a power converter that operates the rotating electric machine at a variable speed.

[0039] If the DC load 50 includes a storage battery such as an EV, v is set according to the type of storage battery, its state (SOC (State of Charge), SOH (State of Health)), charging time, etc. * dc load If the DC load 50 is a load whose output varies with the voltage, such as a heater or a DC motor, v may be determined according to the output. * dc load may be determined.

[0040] v * dc inv is the instantaneous value of the AC voltage v of the system connected to the grid-connected inverter 30. ac , and the operating state of the grid-connected inverter 30. For example, when the grid-connected inverter 30 is in parallel operation, the AC voltage v ac Absolute value of |v ac | v * dc inv When the grid-connected inverter 30 is in parallel / reverse power flow, the grid voltage v ac Absolute value of |v ac |Voltage margin v margin The value (|v ac |+v margin ) to v *dc inv v margin may be determined according to the operating state of the DC load 50. margin v for an operating state with small fluctuations in power consumption margin More specifically, in a DC load 50 including a power converter that operates a rotating electric machine at a variable speed, v margin v for when the rotating machine is running at a constant speed or when it is stationary (when the power converter is stopped) margin It may be determined to be larger.

[0041] v conv lowlimit is a value determined by the configuration of the DC / DC converter 10. For example, if the DC / DC converter 10 is a boost circuit, v conv lowlimit is the input voltage of the boost circuit. If the DC / DC converter 10 is a step-down circuit, v conv lowlimit In principle, this will be “0V.”

[0042] As shown in equation (1), the DC link voltage target value v * dc is the DC link voltage target value v based on the DC load of 50 * dc load and the DC link voltage target value v based on the grid-connected inverter 30. * dc inv and the lower limit v of the output voltage of the DC / DC converter 10. conv lowlimit The DC link voltage v dc (DC link voltage target value v * dc ) is the DC link voltage target value v based on the DC load of 50 * dc load In other words, the DC link voltage v dc is adjusted according to the DC load 50. As described above, the DC link voltage target value v based on the DC load 50 * dc load is determined by the state of the DC load 50. Therefore, the DC link voltage v dc (DC link voltage target value v *dc ) may be adjusted depending on the condition of the DC load 50.

[0043] v dc But, v * dc load If it is less than this, the proper voltage will not be applied to the DC load 50. dc But, v * dc inv If the AC current i ac Distortion occurs in the dc is the output voltage of the DC / DC converter 10, so v dc v conv lowlimit Cannot be set below this value.

[0044] v conv lowlimit V * dc load and v * dc inv If less than (v * dc load , v * dc inv >v conv lowlimit ), within a half cycle of the AC voltage, v * dc load But, v * dc inv Always greater than (v * dc load ≧v * dc inv ) then, v * dc v * dc load (v * dc =v * dc load ). * dc inv V * dc load Larger than (v * dc inv >v * dc load ), v * dc is adjusted to vary periodically within a half period of the AC voltage.

[0045] Under the conditions shown in equation (1), v* dc By adjusting the voltage, it is possible to operate the DC load 50 at a voltage that matches the DC load 50 while allowing power from the DC power source 40 to flow back to the power grid 100. * dc By adjusting the voltage, a voltage suitable for operation (appropriate voltage) is applied to the DC load 50. Prior art document 1 describes the provision of a boost converter circuit that boosts the output voltage of the power generation device, and a step-down converter circuit that converts the output power of the boost converter circuit and supplies it to the internal load. The power conversion system 1 shown in FIG. 1 has a simple circuit configuration by combining the boost converter circuit and the step-down converter circuit described in Prior art document 1 into one common circuit. Furthermore, the power conversion system 1 does not include the step-down converter circuit that supplies power to the internal load in Prior art document 1, and therefore can eliminate power loss that occurs due to a converter that adjusts the voltage applied to the DC load 50.

[0046] (Output current control mode (output power control mode)) The DC / DC converter 10 detects or estimates the input voltage and outputs a target value v * dc The DC / DC converter 10 may be configured to operate in response to a value equivalent to the output current in addition to these values. The value equivalent to the output current of the DC / DC converter 10 may be determined based on a value equivalent to the current (or power) flowing through the DC load 50 or a value equivalent to the current (or power) flowing through the grid-connected inverter 30. These values ​​may be detected values ​​detected by a sensor or the like, estimated values, or target values. By configuring the DC / DC converter 10 to operate in response to a value equivalent to the output current of the DC / DC converter 10, it is possible to reduce the output voltage v due to current fluctuations. dc It is possible to suppress fluctuations in the power supply voltage Vp, Vp-p, and Vp-p of the DC / DC converter 10. The detected value is a value that is actually measured, and the estimated value is a value that is estimated based on the configuration of the DC / DC converter 10. The target value is a value that the DC / DC converter 10 is desired to approach when it is operated.

[0047] In the output current control mode (output power control mode), the output current i conv (or output power P conv ) to the target value i * conv (or P * conv ) to control i * conv (or P * conv ) is determined according to the operating state of any one or a combination of the DC power source 40, the DC load 50, and the grid-connected inverter 30. For example, when the DC power source 40 is a solar cell, the output current from the solar cell during MPPT (Maximum Power Point Tracking) control is i * conv By controlling the DC / DC converter 10 in the output current control mode (output power control mode), it is possible to prevent a shortage of the current (power) for operating the DC load 50. By controlling the DC / DC converter 10 in the output current control mode (output power control mode), it is possible to prevent a shortage of the current (power) supplied from the DC side to the AC side of the grid-connected inverter 30.

[0048] (Power Conversion System 1A of Modified Example 1) As a first modification of the power conversion system 1, a power conversion system 1A including a plurality of pairs of DC power supplies 40 and DC / DC converters 10 will be described. 3 is a diagram illustrating a power conversion system 1A including N pairs of DC power supplies 40 and DC / DC converters 10. N is an integer equal to or greater than 2. When distinguishing between the N DC / DC converters 10, they are represented as DC / DC converters 10-1 to 10-N. When distinguishing between the N DC power supplies 40, they are represented as DC power supplies 40-1 to 40-N.

[0049] When the DC / DC converter 10 is controlled in the output voltage control mode, the DC link voltage target value v * dc is expressed by equation (2).

[0050]

number

[0051] It is desirable to control only one DC / DC converter 10 in the output voltage control mode. conv lowlimit It is preferable to control the DC / DC converter 10 having the largest output voltage in the output voltage control mode. conv lowlimit If a DC / DC converter 10 for which v is not the maximum is set to the output voltage control mode, it becomes necessary to adjust the output voltages of the other DC / DC converters 10 to this lower limit value, and accuracy becomes an issue. conv lowlimit If the DC / DC converter 10 with the largest lower limit is driven in the output voltage control mode, the other DC / DC converters 10 do not need to adjust their lower limit values.

[0052] When the power conversion system 1A of the first modification is controlled in the output current control mode (output power control mode), all the DC / DC converters 10 should be controlled in the output current control mode (output power control mode).

[0053] (Power Conversion System 1B of Modified Example 2) As a second modification of the power conversion system 1, a power conversion system 1B including a plurality of DC loads 50 will be described. 4 is a diagram illustrating a power conversion system 1B including M DC loads 50. M is an integer equal to or greater than 2. When distinguishing between the M DC loads 50, they are represented as DC loads 50-1 to 50-M.

[0054] When the DC / DC converter 10 is controlled in the output voltage control mode, the DC link voltage target value v * dc is expressed by equation (3).

[0055]

number

[0056] When the power conversion system 1B of the second modification is controlled in the output current control mode (output power control mode), all the DC / DC converters 10 should be controlled in the output current control mode (output power control mode).

[0057] (Power Conversion System 1C of Modified Example 3) As a third modification of the power conversion system 1, a power conversion system 1C including a plurality of pairs of DC power supplies 40 and DC / DC converters 10 and a plurality of DC loads 50 will be described. 5 is a diagram illustrating a power conversion system 1C including N pairs of DC power supplies 40 and DC / DC converters 10, and M DC loads 50. M and N are integers of 2 or more. When distinguishing between the N DC / DC converters 10, they are represented as DC / DC converters 10-1 to 10-N. When distinguishing between the N DC power supplies 40, they are represented as DC power supplies 40-1 to 40-N. When distinguishing between the M DC loads 50, they are represented as DC loads 50-1 to 50-M.

[0058] When the DC / DC converter 10 is controlled in the output voltage control mode, the DC link voltage target value v * dc is expressed by equation (4).

[0059]

number

[0060] When the power conversion system 1C of the third modification is controlled in the output current control mode (output power control mode), all the DC / DC converters 10 should be controlled in the output current control mode (output power control mode).

[0061] 1 shows a step-up / step-down converter as the DC / DC converter 10. The DC / DC converter 10 may be a known DC / DC converter such as a DAB (Dual Active Bridge) converter, a flyback converter, or a combination of these. In the case of only step-up, the DC / DC converter 10 may be a step-up converter, and in the case of only step-down, the DC / DC converter 10 may be a step-down converter.

[0062] (DC link voltage target value v * dc (How to determine the DC link voltage v dc is the output of the DC / DC converter 10. The DC link voltage v dc is the DC link voltage target value v * dc In the output voltage control mode, the DC / DC converter 10 acquires the states of the DC load 50 and the grid-connected inverter 30 and controls the DC link voltage target value v * dc In the case where a plurality of DC / DC converters 10 are provided, the DC / DC converter 10 in the output voltage control mode acquires the state of the DC load 50, the state of the grid-connected inverter 30, and the states of the other DC / DC converters, and determines the DC link voltage target value v * dc Determine.

[0063] A specific description will be given of a power conversion system 1 including one pair of a DC / DC converter 10 and a DC power supply 40, and one DC load 50. The DC / DC converter 10 acquires a DC link voltage target value v based on the DC load 50 from the DC load 50. * dc load and a DC link voltage target value v based on the grid-connected inverter 30 obtained from the grid-connected inverter 30. * dc inv From this, the DC link voltage target value v * dc The DC / DC converter 10 determines the voltage V from the DC load 50. * dc load You may get v *dc load v * dc load The state required to determine the above is, for example, the modulation factor of the power converter, and in the case where the power converter includes a rotating electric machine that is operated at a variable speed, the rotation speed and output power of the rotating electric machine, etc. The rotation speed and output power of the rotating electric machine, etc. may be detected values, estimated values, or target values. In addition, the DC / DC converter 10 receives the power from the grid-connected inverter 30, v * dc inv You may get v * dc inv v * dc inv The conditions required to determine the power system state are, for example, the AC voltage v ac And so on.

[0064] A separate controller controls the DC load from 50 to v * dc load or v * dc load The state required to determine the grid-connected inverter 30 is obtained, and v * dc inv or v * dc inv In this case, the controller operates the DC / DC converter 10 in the output voltage mode.

[0065] The controller may be configured as a computer (or a microcomputer) including a CPU, a ROM, a RAM, etc. The ROM includes a non-volatile rewritable memory. The controller operates by loading a program stored in the ROM into the RAM and executing it. The controller may be configured with an ASIC, etc.

[0066] In the power conversion systems 1B and 1C having a plurality of DC loads 50, v * dc load or v * dc loadIn the power conversion systems 1A and 1C including a plurality of DC / DC converters 10, the DC / DC converter 10 in the output voltage control mode may acquire the state of the other DC / DC converters 10. The state of the DC / DC converter 10 may be, for example, the lower limit value v of the output voltage of the DC / DC converter 10. conv lowlimit It is.

[0067] (Operation mode of grid-connected inverter 30) Up to this point, the grid-connected inverter 30 has been described in terms of parallel / reverse power flow, but the operation of the power conversion system 1 will now be described in detail, including the parallel / forward power flow case and parallel-off case. First, the operation mode of the grid-connected inverter 30 in the power conversion system 1 will be described. Fig. 6 is a diagram for explaining the operation modes of the grid-connected inverter 30. In Fig. 6, from the left side, a connection state with the power grid 100, an operation mode of the grid-connected inverter 30, and prerequisites are shown.

[0068] In the parallel / reverse power flow, the grid-connected inverter 30 performs a PAM (Pulse Amplitude Modulation) operation or a PWM operation. The PAM operation is an operation in which two of the four switching elements Sap, San, Sbp, and Sbn constituting the grid-connected inverter 30 (for example, the switching elements Sap and Sbn and the switching elements San and Sbp) are turned on and the other two switching elements are maintained off. ac This is an operation in which the on and off are switched (switched) near the zero crossing or when the operating mode is changed.

[0069] PAM operation is performed by adjusting the DC link voltage target value v * dc is the DC link voltage target value v based on the grid-connected inverter 30. * dc inv and the DC link voltage v dc and AC voltage v ac Absolute value of |v acThe absolute value of the difference between |(v dc -|v ac |)| is the allowable output voltage of the grid-connected inverter 30 and the grid voltage v ac Voltage difference Δv ac It is used when it is smaller.

[0070] The PWM operation is an operation in which the switching elements Sap, San, Sbp, and Sbn of the grid-connected inverter 30 are switched repeatedly.

[0071] In the parallel / forward power flow, the grid-connected inverter 30 performs full-wave rectification or PWM rectification. The full-wave rectification operation is an operation in which the switching elements Sap, San, Sbp, and Sbn are kept off and rectification is performed by the diodes Dap, Dan, Dbp, and Dbn in the grid-connected inverter 30. In addition, when the switching elements Sap, San, Sbp, and Sbn are monopolar elements that can pass a current in the reverse direction, the switching elements Sap, San, Sbp, and Sbn may be operated by synchronous rectification in which the switching elements Sap, San, Sbp, and Sbn are turned on in accordance with the timing at which the diodes Dap, Dan, Dbp, and Dbn become forward biased. The full-wave rectification operation and the synchronous rectification operation are examples of the rectification operation.

[0072] The PWM rectification operation is an operation of repeating switching of the switching elements Sap, San, Sbp, and Sbn of the grid-connected inverter 30. When controlling the quality of the forward flowing power with priority, the forward flowing current value and the phase difference between the current and the voltage are controlled. In addition, the DC link voltage v dc may be controlled with priority given to the above.

[0073] During parallel-off, the switching elements Sap, San, Sbp, and Sbn of the grid-connected inverter 30 are maintained in the off state.

[0074] (Operation mode of power conversion system 1) (1) Operating mode supplying DC load 50 in parallel / reverse flow An operation mode will be described in which power is supplied to a DC load 50 in a parallel / reverse power flow in which power is supplied from the DC side to the AC side. In the following, a case in which there is one DC / DC converter 10 and one DC load 50 will be described.

[0075] 7 is a diagram for explaining an operation mode for supplying power to a DC load 50 in parallel / reverse power flow. The upper side shows the power conversion system 1, and the lower side shows the operation of the DC / DC converter 10 and the operation of the grid-connected inverter 30. Here, the DC / DC converter 10 is driven in the output voltage control mode.

[0076] The flow of power is indicated by arrows in the power conversion system 1 shown in the upper part of Fig. 7. Power is supplied from a DC power source 40 to a power system 100 and a DC load 50. The DC link voltage target value v * dc is the DC link voltage target value v based on the DC load 50 as shown in equation (1). * dc load and the DC link voltage target value v based on the grid-connected inverter 30. * dc inv and the lower limit v of the output voltage of the DC / DC converter 10. conv lowlimit It is determined by the magnitude relationship with

[0077] v * dc load But, v * dc inv , and v conv lowlimit If it is greater than v * dc is the DC link voltage target value v based on the DC load of 50 * dc load It becomes. * dc V * dc inv Since the power supply voltage is larger than the grid-tied inverter 30, the grid-tied inverter 30 performs PWM operation to link the DC and AC sides.

[0078] v * dc inv But, v * dc load , and vconv lowlimit If it is greater than v * dc v * dc inv It becomes. * dc V * dc inv Therefore, the grid-connected inverter 30 performs PAM or PWM operation to link the DC side and the AC side.

[0079] v conv lowlimit But, v * dc load , and v * dc inv If it is greater than v * dc v conv lowlimit It becomes. * dc V conv lowlimit Therefore, the grid-connected inverter 30 performs a PWM operation to link the DC side and the AC side.

[0080] DC link voltage target value v based on DC load 50 * dc load varies depending on the type and operating state of the DC load 50. The DC link voltage target value v based on the DC load 50 * dc load The change in the DC link voltage target value v * dc An example in which the value of the variable θ changes will be described.

[0081] FIG. 8 shows the DC link voltage target value v based on the DC load 50. * dc load Changes in the DC link voltage target value v * dc FIG. 8(a) is a diagram illustrating a DC link voltage target value v based on the DC load 50. * dc load is the lower limit of the output voltage of the DC / DC converter 10, v conv lowlimit 8(b), the DC link voltage target value v based on the DC load 50 is * dc load is the lower limit of the output voltage of the DC / DC converter 10, v conv lowlimit is larger than the AC voltage vac Absolute value of peak voltage |v ac peak If | is smaller than , then Fig. 8(c) shows the DC link voltage target value v based on the DC load 50. * dc load is the AC voltage v ac Absolute value of peak voltage |v ac peak | is larger than the voltage (V). The vertical axis is voltage (V) and the horizontal axis is time (seconds). Time proceeds from time t0 to time t6. Time t0 is 0 seconds, time t3 is 0.01 seconds, and time t6 is 0.02 seconds. Figures 8(a), (b), and (c) show one cycle of 50 Hz AC (alternating current). The period from time t0 to time t6 is the period when the AC voltage v ac The period from time t0 to time t3 and the period from time t3 to time t6 are one period of the AC voltage v ac One period consists of two repeated half periods. In the following, only the half period from time t0 to time t3 will be explained. The other cases are similar.

[0082] DC link voltage target value v based on the grid-connected inverter 30 * dc inv AC voltage v ac Absolute value of |v ac | * dc inv =|v ac In the following, AC voltage v ac Absolute value of |v ac | is the absolute value of the AC voltage |v ac It is written as |.

[0083] In Fig. 8(a), (b), and (c), the DC link voltage target value v * dc The thick solid line indicates the DC link voltage target value v based on a DC load of 50. * dc load The dotted line indicates the lower limit of the output voltage of the DC / DC converter 10, v conv lowlimit is the dashed line, and the DC link voltage target value v based on the grid-connected inverter 30 is * dc inv (=AC voltage v ac Absolute value of |v ac|) is indicated by a thin solid line. Note that overlapping parts are indicated by shifting them.

[0084] Figure 8(a) shows the * dc load V conv lowlimit In FIG. 8(a), the time t1, t2, t4, and t5 are v conv lowlimit and v * dc inv (=|v ac |) intersects with

[0085] During the period from time t0 to time t1, v conv lowlimit But, v * dc load and v * dc inv (=|v ac v * dc v conv lowlimit It becomes. * dc inv (=|v ac |) is v * dc Therefore, the grid-connected inverter 30 connects the DC side and the AC side by PWM operation. During the period from time t1 to time t2, v * dc inv (=|v ac |) is v conv lowlimit and v * dc load v * dc v * dc inv (=|v ac |) will be used. * dc is |v ac Therefore, the grid-connected inverter 30 links the DC side and the AC side by PAM operation. The period from time t2 to time t3 is the same as the period from time t0 to time t1, so v * dc v again conv lowlimit The grid-connected inverter 30 connects the DC side and the AC side by PWM operation. In Fig. 8(a), v * dc load v * dc The DC load 50 is supplied with a voltage sufficient for operation (the correct voltage).

[0086] Figure 8(b) shows the * dc load V conv lowlimit is greater than |v ac peak In FIG. 8(b), at times t2, t3, t4, and t5, v * dc load and v * dc inv (=|v ac |) intersect. Times t2, t3, t4, and t5 are different from those in FIG. 8(a). Except where noted, the others are similar.

[0087] During the period from time t0 to time t1, v * dc load But, v conv lowlimit and v * dc inv (=|v ac v * dc v * dc load It becomes. |v ac | is v * dc Therefore, the grid-connected inverter 30 connects the DC side and the AC side by PWM operation. During the period from time t1 to time t2, v * dc inv (=|v ac |) is v conv lowlimit and v * dc load v * dc v * dc inv (=|v ac |) will be used. * dc is |v acTherefore, the grid-connected inverter 30 links the DC side and the AC side by PAM operation. The period from time t2 to time t3 is the same as the period from time t0 to time t1, so v * dc v again * dc load The grid-connected inverter 30 connects the DC side and the AC side by PWM operation.

[0088] During the period from time t0 to time t1 and from time t2 to time t3, the DC link voltage target value v * dc is the DC link voltage target value v based on the DC load of 50 * dc load and an appropriate voltage is supplied to the DC load 50. From time t1 to time t2, the DC load 50 is supplied with a DC link voltage target value v * dc load The DC link voltage target value v based on the grid-connected inverter 30 is larger than * dc inv (=|v ac In the half cycle of the AC voltage from time t0 to time t3, the DC link voltage v dc is controlled according to the DC load 50. In the period from time t1 to time t2, the DC link voltage v dc is a period during which the power supply voltage Vp is controlled to be a voltage determined by the system voltage. This prevents an excessive voltage from being applied to the DC load 50.

[0089] Figure 8(c) shows the * dc load ga |v ac peak | is greater than. During the period from time t0 to time t6, v * dc load But, v conv lowlimit and v * dc inv (=|v ac v * dcv * dc load It becomes. * dc is |v ac Since | is larger than the reference voltage, the grid-connected inverter 30 performs PWM operation to link the DC side and the AC side.

[0090] In the period from time t0 to time t6, an appropriate voltage is supplied to the DC load 50. Therefore, the operating efficiency of the DC load 50 is improved.

[0091] DC link voltage target value v based on DC load 50 * dc load Even if the DC link voltage target value v changes (fluctuations), as shown in Fig. 8(a), (b), and (c), * dc The DC link voltage target value v * dc By adjusting the DC link voltage v dc changes (fluctuations) to supply a voltage (power) that drives the DC load 50.

[0092] Next, the DC link voltage target value v based on the grid-connected inverter 30 is * dc inv AC voltage absolute value |v ac |Voltage margin v margin We will explain the case where the value is the sum of v * dc inv =|v ac |+v margin ). Figure 9 shows the voltage margin v margin DC link voltage target value v * dc load FIG. 9(a) is a diagram illustrating the change in the DC link voltage target value v based on the DC load 50. * dc load is the lower limit of the output voltage of the DC / DC converter 10, v conv lowlimit 9(b), the DC link voltage target value v based on the DC load 50 is * dc load is the lower limit of the output voltage of the DC / DC converter 10, v conv lowlimit is larger than the AC voltage vac Absolute value of peak voltage |v ac peak |Voltage margin v margin 9(c) shows that the DC link voltage target value v based on the DC load 50 is smaller than the value obtained by adding * dc load is the AC voltage v ac Absolute value of peak voltage |v ac peak |Voltage margin v margin The vertical and horizontal axes are the same as in Fig. 8.

[0093] DC link voltage target value v based on the grid-connected inverter 30 * dc inv (=AC voltage v ac Absolute value of |v ac |+Voltage margin v margin ) is indicated by a two-dot chain line. The rest is the same as in Figure 8.

[0094] Figure 9(a) shows the * dc load V conv lowlimit The time t1, t2, t4, and t5 are smaller than v * dc inv (=|v ac |+v margin ) and v conv lowlimit This is the time when the and intersect.

[0095] During the period from time t0 to time t1, v conv lowlimit V * dc load and v * dc inv v * dc v conv lowlimit It becomes. During the period from time t1 to time t2, v * dc inv V * dc load and v conv lowlimit Greater than. * dc v * dc inv It becomes. During the period from time t2 to time t3, v *dc v again conv lowlimit It becomes. During the period from time t0 to time t3, v * dc is |v ac Therefore, the grid-connected inverter 30 connects the DC side and the AC side by PWM operation.

[0096] Figure 9(b) shows the * dc load V conv lowlimit is greater than |v ac peak |+v margin The time t1, t2, t4, and t5 are smaller than v * dc load and v * dc inv (=|v ac |+v margin ) intersects with

[0097] During the period from time t0 to time t1, v * dc load V conv lowlimit and v * dc inv (=|v ac |+v margin ) is greater than v * dc v * dc load It becomes. During the period from time t1 to time t2, v * dc inv (=|v ac |+v margin ) is v * dc load and v conv lowlimit Greater than. * dc v * dc inv (=|v ac |+v margin ) The period from time t2 to time t3 is the same as the period from time t0 to time t1, so v * dc v again * dc load It becomes. During the period from time t0 to time t3, v * dc ga |v ac Therefore, the grid-connected inverter 30 links the DC side and the AC side by PWM operation.

[0098] Figure 9(c) shows the * dc load ga |v ac peak |+v margin This is the case when it is greater than . During the period from time t0 to time t3, v * dc load V conv lowlimit and v * dc inv v * dc v * dc load It becomes. During the period from time t0 to time t3, v * dc ga |v ac Therefore, the grid-connected inverter 30 links the DC side and the AC side by PWM operation.

[0099] DC link voltage target value v based on the grid-connected inverter 30 * dc inv AC voltage V ac Absolute value of |v ac |Voltage margin v margin The value (v * dc inv =|v ac |+v margin ) the DC link voltage v dc Even if the voltage fluctuates, the DC voltage required for the grid-connected inverter 30 is ensured, and waveform distortion in the output of the grid-connected inverter 30 is suppressed.

[0100] (2) Operation mode in which no power is supplied to DC load 50 in parallel / reverse flow In a parallel / reverse flow in which power is supplied from the DC side to the AC side, in an operation mode in which power is not supplied to the DC load 50 , all power from the DC power source 40 is reverse-flowed to the power grid 100 . FIG. 10 is a diagram for explaining an operation mode in which power is not supplied to the DC load 50 in the parallel / reverse power flow. The upper side shows the power conversion system 1, and the lower side shows the operation of the DC / DC converter 10 and the operation of the grid-connected inverter 30. Here, the DC link voltage target value v based on the grid-connected inverter 30 is * dc inv AC voltage v ac Absolute value of |v ac | * dc inv =|v ac The DC / DC converter 10 is driven in the output voltage control mode.

[0101] In the power conversion system 1 shown in the upper part of FIG. 10, the flow of power is indicated by arrows. Power is supplied from a DC power source 40 to a power system 100. A DC link voltage target value v based on a DC load 50 is * dc load The DC link voltage target value v * dc As shown in equation (1), the DC link voltage target value v based on the grid-connected inverter 30 * dc inv (=|v ac |) and the lower limit v of the output voltage of the DC / DC converter 10 conv lowlimit It is determined by the magnitude relationship with

[0102] v * dc inv V * dc load and v conv lowlimit If greater than v * dc v * dc inv (=|v ac v * dc ga |v ac Therefore, the grid-connected inverter 30 performs PAM operation to link the DC side and the AC side.

[0103] v conv lowlimit V * dc load and v * dc inv (=|v ac |) if v * dc v conv lowlimit It becomes. * dc V conv lowlimit Therefore, the grid-connected inverter 30 performs a PWM operation to link the DC side and the AC side.

[0104] FIG. 11 shows the DC link voltage target value v in the parallel / reverse power flow operation mode in which the DC load 50 is not supplied with power. * dc In FIG. 11, the times t1, t2, t4, and t5 are conv lowlimit and v * dc inv (=|v ac |) intersects with

[0105] During the period from time t0 to time t1, v conv lowlimit V * dc inv (=|v ac |) is greater than v. * dc v conv lowlimit The grid-connected inverter 30 performs a PWM operation to link the DC side and the AC side. During the period from time t1 to time t2, v * dc inv (=|v ac |) is v conv lowlimit greater than. Therefore, v * dc v * dc inv (=|v ac |) will be used. * dc is |v ac Therefore, the grid-connected inverter 30 performs PAM operation to link the DC side and the AC side.

[0106] (3) An operation mode in which power is supplied from the DC power source 40 and the power grid 100 to the DC load 50 in a parallel / forward flow mode. In a parallel / forward power flow in which power is supplied from the AC side to the DC side, in an operation mode in which power is supplied from the DC power source 40 and the power system 100 to the DC load 50, the DC power source 40 and the power system 100 supply power to the DC load 50 in parallel. 12 is a diagram for explaining an operation mode in which power is supplied from a DC power source 40 and a power system 100 to a DC load 50 in a parallel / forward power flow. The upper side shows the power conversion system 1, and the lower side shows the operation of the DC / DC converter 10 and the operation of the grid-connected inverter 30.

[0107] In the power conversion system 1 shown in the upper part of Fig. 12, the flow of power is indicated by arrows. Power is supplied to a DC load 50 from a DC power source 40 and a power system 100. As shown in the lower part of Fig. 12, the grid-connected inverter 30 performs full-wave rectification operation or PWM rectification operation. The operation of the grid-connected inverter 30 is appropriately selected according to constraints on the forward flow of power (current) (e.g., power factor, harmonic content rate, current peak, efficiency of the grid-connected inverter 30, etc.). The grid-connected inverter 30 rectifies a DC link voltage target value v * dc is regulated, the DC / DC converter 10 operates in an output current control mode.

[0108] (4) Parallel / forward flow operation mode in which the power grid 100 supplies power to the DC load 50 In a parallel / reverse power flow in which power is supplied from the AC side to the DC side, in an operation mode in which power is supplied from the power system 100 to the DC load 50, the DC power supply 40 does not supply power to the DC load 50. 13 is a diagram for explaining an operation mode in which power is supplied from the power system 100 to the DC load 50 in parallel / forward power flow. The upper side shows the power conversion system 1, and the lower side shows the operation of the DC / DC converter 10 and the operation of the grid-connected inverter 30.

[0109] The flow of power is indicated by arrows in the power conversion system 1 in the upper part of Fig. 13. Power is supplied from a power system 100 to a DC load 50. As shown in the lower part of Fig. 13, the grid-connected inverter 30 performs full-wave rectification operation or PWM rectification operation. The operation of the grid-connected inverter 30 is appropriately selected according to constraints on the forward flow of power (current) (for example, power factor, harmonic content, current peak, or efficiency of the grid-connected inverter 30). The DC / DC converter 10 is stopped.

[0110] (5) An operation mode in which the DC power source 40 supplies power to the DC load 50 during parallel-off. In an operation mode in which power is supplied from the DC power source 40 to the DC load 50 during parallel-off, power is not supplied from the power system 100 to the DC load 50 . 14 is a diagram illustrating an operation mode in which power is supplied from the DC power source 40 to the DC load 50 during parallel-off. The upper side shows the power conversion system 1, and the lower side shows the operation of the DC / DC converter 10 and the operation of the grid-connected inverter 30.

[0111] The flow of power is indicated by arrows in the power conversion system 1 shown in the upper part of Fig. 14. Power is supplied from a DC power source 40 to a DC load 50. As shown in the lower part of Fig. 14, the grid-connected inverter 30 is stopped, and the DC link voltage target value v based on the grid-connected inverter 30 is * dc inv The DC link voltage target value v * dc is the DC link voltage target value v based on the DC load 50 as shown in equation (1). * dc load and the lower limit v of the output voltage of the DC / DC converter 10. conv lowlimit The DC / DC converter 10 operates in an output voltage control mode.

[0112] v * dc load V conv lowlimit If greater than (v * dc load ≧v conv lowlimit ), v * dc is v *dc load and v conv lowlimit V * dc load If greater than (v conv lowlimit ≧v * dc load ), v * dc is v conv lowlimit become.

[0113] FIG. 15 shows the DC link voltage target value v in an operation mode in which the DC power source 40 supplies power to the DC load 50 during parallel off. * dc FIG. 15(a) is a diagram illustrating a DC link voltage target value v based on the DC load 50. * dc load is the lower limit of the output voltage of the DC / DC converter 10, v conv lowlimit 15(b) shows that the lower limit value v of the output voltage of the DC / DC converter 10 is conv lowlimit is the DC link voltage target value v based on the DC load of 50 * dc load For reference, the AC voltage v ac Absolute value of |v ac Indicates |.

[0114] As shown in Fig. 15(a), v * dc load V conv lowlimit If greater than (v * dc load ≧v conv lowlimit ) then, v * dc v * dc load It becomes. As shown in Figure 15(b), v conv lowlimit V * dc load If greater than (v conv lowlimit ≧v * dc load ) then, v * dc v conv lowlimit It becomes.

[0115] [Second embodiment] The power conversion system 2 of the second embodiment includes a load switch circuit 70 on a line that connects the DC load 50 to the DC link 20. The load switch circuit 70 is an example of a switch circuit.

[0116] Fig. 16 is a diagram illustrating an example of a power conversion system 2 according to the second embodiment. The power conversion system 2 includes a load switch circuit 70 in addition to the power conversion system 1 illustrated in Fig. 1. The power conversion system 2 includes a capacitor C5 instead of the capacitor C4 provided in parallel with the DC load 50 in the power conversion system 1.

[0117] The load switch circuit 70 includes a load switch Sx and a diode D3. The load switch Sx is provided on one of the lines connecting the DC load 50 to the DC link 20 (here, the line connected to the + side of the DC link 20). The diode D3 is connected with a reverse bias between the pair of lines connecting the DC load 50 to the DC link 20. The load switch Sx may be an element similar to the switching elements Sw1 and Sw2 described above.

[0118] The load switch Sx is controlled by a separate controller to regulate the DC link voltage v dc is the DC link voltage target value v based on the DC load of 50 * dc load In other words, the DC link voltage v dc In a period in which the DC link voltage v is in a voltage range that is determined in advance according to the DC load 50, the load switch Sx is controlled to be closed (on). dc may be closed (on) during a period in which the load switch Sx is in a voltage range that is predetermined according to the DC load 50. During a period in which the load switch Sx is closed (on), power is supplied from the DC link 20 to the DC load 50. During a period in which the load switch Sx is open (off), power is supplied from the capacitor C5 to the DC load 50. The diode D3 is a so-called feedback diode for the load switch Sx.

[0119] The capacitor C5 supplies power to the DC load 50 during the period when the load switch Sx is open (OFF). Therefore, the capacitor C5 needs to have a larger capacity than the capacitor C4 in the power conversion system 1. The capacitor C5 is preferably selected to have a larger capacity than the capacitor C1 in the DC link 20. When selected in this way, the DC link voltage v between the DC / DC converter 10 and the grid-connected inverter 30 dc It is easy to adjust the DC voltage v that supplies the DC load 50. dc load The capacitor C5 is an electrolytic capacitor, a film capacitor, or the like.

[0120] (Operation mode of power conversion system 2) An operation mode of the power conversion system 2 in parallel / reverse power flow for supplying power to the DC load 50 will now be described. FIG. 17 shows the DC link voltage target value v based on the DC load 50. * dc load Changes in the DC link voltage target value v * dc FIG. 17(a) is a diagram illustrating a DC link voltage target value v based on the DC load 50. * dc load is the lower limit of the output voltage of the DC / DC converter 10, v conv lowlimit 17(b) shows the DC link voltage target value v based on the DC load 50. * dc load is the lower limit of the output voltage of the DC / DC converter 10, v conv lowlimit is larger than the AC voltage v ac Absolute value of peak voltage |v ac peak 17(c) shows that the DC link voltage target value v based on the DC load 50 is * dc load is the AC voltage v ac Absolute value of peak voltage |v ac peak 17 is a diagram similar to FIG. 8 for the power conversion system 1, but shows the opening (OFF) and closing (ON) of the load switch Sx instead of the operation of the grid-tied inverter 30.

[0121] DC link voltage target value v based on the grid-connected inverter 30 * dc inv AC voltage V ac Absolute value of |v ac | (v * dc inv =|v ac |). DC load voltage v applied to DC load 50 dc load is indicated by a dashed dotted line.

[0122] Figure 17(a) shows the * dc load V conv lowlimit If v is smaller than * dc load <v conv lowlimit ). At times t1, t2, t4, and t5, * dc inv (=|v ac |) and v conv lowlimit This is the time when the and intersect.

[0123] During the period from time t0 to time t1, v conv lowlimit V * dc load and v * dc inv (=|v ac v * dc v conv lowlimit The load switch Sx is controlled to be closed (ON), the DC load 50 is supplied with power from the DC link 20, and the capacitor C5 is charged. During the period from time t1 to time t2, v * dc inv (=|v ac |) is v * dc load and v conv lowlimit v * dc v * dc inv (=|v ac The load switch Sx is controlled to be open (OFF), and the DC load 50 is powered by the discharge of the capacitor C5. The period from time t2 to time t3 is the same as the period from time t0 to time t1, and v * dc v again conv lowlimit The load switch Sx is controlled to be closed (ON), the DC load 50 is supplied with power from the DC link 20, and the capacitor C5 is charged.

[0124] If we ignore the voltage drop caused by the discharge of the capacitor C5 during the period from time t1 to time t2, as shown by the dashed line in FIG. 17(a), the DC load 50 has a current of v conv lowlimit In FIG. 8(a), during the period from time t1 to time t3, the DC load 50 is applied with a voltage of v * dc inv (=|v ac |) was applied. v * dc inv (=|v ac |) is v conv lowlimit By controlling the load switch Sx to be open (OFF) and closed (ON), it is possible to prevent the DC load 50 from being supplied with an excessive voltage. dc This improves the operating efficiency of the DC load 50.

[0125] Figure 17(b) shows the v * dc load V conv lowlimit is greater than |v ac peak | is smaller than v * dc inv (=|v ac |) and v * dc load This is the time when the and intersect.

[0126] During the period from time t0 to time t1, v * dc load V conv lowlimit and v * dc inv (=|v ac |) greater than. v * dc v * dc loadThe load switch Sx is controlled to be closed (ON), the DC load 50 is supplied with power from the DC link 20, and the capacitor C5 is charged. During the period from time t1 to time t2, v * dc inv (=|v ac |) is v * dc load and v conv lowlimit Greater than. * dc v * dc inv (=|v ac The load switch Sx is controlled to be open (OFF), the DC load 50 is powered from the capacitor C5, and the capacitor C5 is discharged. The period from time t2 to time t3 is the same as the period from time t0 to time t1, and v * dc v again * dc load The load switch Sx is controlled to be closed (ON), the DC load 50 is supplied with power from the DC link 20, and the capacitor C5 is charged.

[0127] If we ignore the voltage drop caused by the discharge of the capacitor C5 during the period from time t1 to time t2, then, as shown by the dashed line in FIG. 17(b), the DC load 50 has a current of v * dc load A DC load input voltage v is applied. dc load is always suitable for DC loads of 50 V * dc load In FIG. 8(b), during the period from time t1 to time t3, the DC load input voltage v dc load v * dc load Bigger v * dc inv (=|v ac By controlling the load switch Sx to open (OFF) and close (ON), it is possible to prevent an excessive voltage from being supplied to the DC load 50. The voltage v applied to the DC load 50 dc This improves the operating efficiency of the DC load 50.

[0128] Figure 17(c) shows the * dc load ga |v ac peak | is greater than. During the period from time t0 to time t3, v * dc load V conv lowlimit and v * dc inv (=|v ac |) greater than. v * dc v * dc load The load switch Sx is controlled to be closed (ON), and the DC load 50 is supplied with power from the DC link 20. DC load 50 DC load input voltage v dc load is always suitable for DC loads of 50 V * dc load The voltage applied to the DC load 50, v dc This improves the operating efficiency of the DC load 50.

[0129] At times t2 and t5 in FIG. 17(a) and (b), the load switch Sx is controlled to change from open (off) to closed (on) by the DC link voltage v dc may be performed while the voltage is within a predetermined voltage range according to the DC load 50. Note that while the voltage is within the predetermined voltage range, the load switch Sx may be kept open (off), or the load switch Sx may be controlled to change from closed (on) to open (off).

[0130] Next, a means for suppressing an inrush current (means for suppressing an inrush current) that occurs when the load switch Sx is switched from open (off) to closed (on) will be described. FIG. 18 is a diagram for explaining a means for suppressing an inrush current generated by the load switch Sx. FIG. 18(a) shows a case where an inrush current occurs, and FIG. 18(b) shows a case where the inrush current is suppressed. In FIG. 18(a) and FIG. 18(b), the upper part shows a voltage waveform, and the lower part shows a current waveform. The lower current waveform shows the current i flowing from the load switch Sx to the DC load 50. dc load The vertical axis is the current i dc load(au). Times t1, t2, t4, t5, t2', and t5' are times when the load switch Sx is switched between open (OFF) and closed (ON).

[0131] As shown in Figure 18(a), the DC load input voltage v dc load and the DC link voltage v dc When the load switch Sx is switched from open (off) to closed (on) in a state where there is a voltage difference between dc load In addition, an inrush current flows due to the charging of capacitor C5. The inrush current may increase circuit loss and shorten the lifespan of capacitor C5.

[0132] As shown in Figure 18(b), the DC load input voltage v dc load and the DC link voltage v dc When the load switch Sx is controlled to switch from open (off) to closed (on) while the voltage difference between the DC load input voltage v and the load switch Sx is within a predetermined voltage range (at times t2' and t5'), the inrush current caused by the charging of the capacitor C5 is suppressed. dc load and the DC link voltage v dc The voltage difference between the first and second terminals may be set based on the allowable magnitude of the inrush current.

[0133] As a means for suppressing the inrush current, the load switch Sx may be subjected to a chopping operation in which the load switch Sx is intermittently repeatedly opened (off) and closed (on) when the load switch Sx is switched from open (off) to closed (on). The chopping operation may be performed with a predefined chopping period, or may be performed with a chopping period determined by detecting the current of the load switch Sx. The operation of switching the load switch Sx from an open (off) state (off state) to a closed (on) state (on state) (turn-on operation) may be delayed by increasing the value of a resistor connected to the control terminal of the load switch Sx. A small-capacity reactor may be inserted between the load switch Sx and the capacitor C5 to suppress abrupt changes in the current.

[0134] (Modification of the power conversion system 2 of the second embodiment) A power conversion system 3, which is a modification of the power conversion system 2, will be described.

[0135] 19 is a diagram showing an example of a power conversion system 3 which is a modified example of the power conversion system 2 according to the second embodiment. The same components as those in the power conversion system 2 are given the same reference numerals and description thereof will be omitted.

[0136] The power conversion system 3 includes a DC / DC converter 11 instead of the DC / DC converter 10 of the power conversion system 2. The DC / DC converter 11 eliminates the switching element Sw2 and diode D2 in the DC / DC converter 10 and includes a diode D4. The power conversion system 3 includes a load switch circuit 71 instead of the load switch circuit 70 of the power conversion system 2. The load switch circuit 71 is another example of a switch circuit.

[0137] The diode D4 in the DC / DC converter 11 is provided in parallel with the series connection of the reactor L1 and the diode D1. The anode terminal of the diode D4 is connected to the terminal of the reactor L1 that is not connected to the diode D1, and the cathode terminal of the diode D4 is connected to the cathode terminal of the diode D1.

[0138] The load switch circuit 71 includes a load switch Sy and a diode D5. The anode terminal of the diode D5 is connected to the + side of the DC power supply 40. The collector terminal of the load switch Sy is connected to the + side of the DC link 20, and the emitter terminal is connected to the cathode terminal of the diode D5. The DC load 50 is connected between the - side of the DC power supply 40 and the emitter terminal of the load switch Sy (the cathode terminal of the diode D5).

[0139] The DC / DC converter 11 is a boost circuit that can output the input voltage as it is when not performing a boost operation. When performing a boost operation, a separate controller is used to switch the switching element Sw1 to control the DC link voltage vdc DC power supply voltage v pv When the boost operation is not performed, the switching element Sw1 is turned off and the DC / DC converter 11 is stopped. The output voltage of the DC power supply 40 (DC power supply voltage v pv ) is bypassed to the DC link 20. Diode D4 is a bypass diode. Diode D4 is switched to the off state during boost operation and to the on state when the DC / DC converter is stopped.

[0140] The load switch Sy may be an element similar to the load switch Sx in the load switch circuit 70 and functions similarly. dc is the DC link voltage target value v based on the DC load of 50 * dc load The load is controlled to close (ON) during the period close to the load, and open (OFF) during the other periods. dc load In addition, the DC load input voltage v dc load , DC power supply voltage v pv In this case, the load switch Sy is set to off. The diode D5 is connected to the output voltage of the DC power supply 40 (DC power supply voltage v pv ) to the DC load 50. The diode D5 is a bypass diode. The diode D5 is a diode that connects the DC load 50 and the DC power supply 40.

[0141] The power conversion system 3 includes diodes D4 and D5 which are bypass diodes. When the diodes D4 and D5 which are bypass diodes are in an on state, a current path with a small number of passive elements and active elements is formed, and loss in the current path is reduced.

[0142] Moreover, the means for suppressing the inrush current described in the power conversion system 2 can also be applied to the power conversion system 3.

[0143] (Application of three-phase AC to power systems) In the first and second embodiments, the power system 100 is described as a single-phase AC power system, but the present invention can also be applied to a three-phase AC power system. In a three-phase AC power system, the system voltage connected to the grid-connected inverter 30 is defined as AC voltage v u , v v , v w (instantaneous value). The DC link voltage target value v based on the grid-connected inverter 30 * dc inv is the AC voltage v u , v v , v w , and the operating state of the grid-connected inverter 30. For example, when in parallel with the power grid, the DC link voltage target value v * dc inv , AC voltage v u , v v , v w The maximum value of max(v u , v v , v w ) and the minimum value min(v u , v v , v w ) and the difference (max(v u , v v , v w )-min(v u , v v , v w )), in other words, the maximum value of the absolute value of the line voltage (instantaneous value) (max(|v u -v v |, |v v -v w |, |v w -v u For example, when the power flow is reversed in parallel with the power grid, the DC link voltage target value v based on the grid-connected inverter 30 may be determined as * dc inv The maximum absolute value of the line voltage (instantaneous value) plus the voltage margin v margin The value (max(|v u -v v |, |v v -v w |, |v w -v u |)+v margin) may be determined.

[0144] In the three-phase AC case, the DC link voltage target value v based on the grid-connected inverter 30 * dc inv is more than √3 / 2 (approximately 0.86) times the peak voltage of the line voltage. Therefore, the DC link voltage target value v * dc is the DC link voltage target value v based on the DC load of 50 * dc load The voltage range (v * dc =v * dc load On the other hand, in the case of single-phase AC, the DC link voltage target value v based on the grid-connected inverter 30 is limited. * dc inv Since the change in the DC link voltage target value v * dc is the DC link voltage target value v based on the DC load of 50 * dc load The voltage range (v * dc =v * dc load The voltage range in which this occurs is wide. Single-phase AC is more suitable for power systems than three-phase AC.

[0145] (Effects of the embodiment) The power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment include a DC / DC converter 10 to which a DC power supply 40 can be connected, a grid-connected inverter 30, and a DC link 20 connecting the DC / DC converter 10 and the grid-connected inverter 30, and within a half cycle of a grid voltage on the AC side of the grid-connected inverter 30, the voltage of the DC link 20 is adjusted according to a DC load 50 connectable to the DC link 20. The voltage of the DC link 20 is adjusted according to the DC load 50 connected to the DC link 20. This improves the operation efficiency of the DC load 50.

[0146] The voltage of the DC link 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may be adjusted according to the state of the DC load 50 that can be connected to the DC link 20. This allows the voltage applied to the DC load 50 to be adjusted to an appropriate value.

[0147] The DC links 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may be capable of connecting, as the DC load 50, a DC load whose output varies depending on the voltage of the DC links 20. This allows the output of the DC load 50 to be appropriate.

[0148] The DC link 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may include a power converter as a DC load 50, and may be connectable to a DC load in which the voltage of the DC link 20 is adjusted according to the state of power conversion of the power converter. This makes it possible to make the voltage applied to the DC load 50 including the power converter appropriate.

[0149] The DC link 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may include the power converter that operates the rotating electric machine at a variable speed as the DC load 50, and may be connectable to a DC load in which the voltage of the DC link 20 is adjusted according to the operating state of the rotating electric machine. This makes it possible to make the voltage applied to the DC load 50 including the power converter that operates the rotating electric machine at a variable speed appropriate.

[0150] The adjustment of the voltage of the DC link 20 in the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may have a control including a period during which the voltage is adjusted according to the DC load 50 connectable to the DC link 20 and a period during which the voltage is adjusted to a voltage based on the system voltage, within a half cycle of the system voltage. This makes it possible to prevent an excessive voltage from being applied to the DC load 50.

[0151] The voltage based on the system voltage of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may be a voltage greater than the absolute value of the system voltage. This ensures that the voltage required for the grid-connected inverter 30 is sufficient, regardless of fluctuations in the voltage of the DC link 20.

[0152] The grid-connected inverter 30 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may be controlled in any one of PWM operation, PAM operation, and rectification operation according to the DC load 50 that can be connected to the DC link 20. This allows the voltage applied to the DC load 50 to be appropriate by controlling the grid-connected inverter 30.

[0153] The DC link 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may be one to which the DC load 50 can be connected via the load switch circuits 70 and 71. This makes it possible to prevent an excessive voltage from being applied to the DC load 50.

[0154] In the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment, a capacitor C5 having a larger capacity than the capacitor C1 provided in the DC link 20 may be provided in parallel to the DC load 50 on the input side of the DC load 50. This makes it easier to control the voltage of the DC link 20 and the voltage of the DC load 50.

[0155] The load switch circuits 70 and 71 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may be controlled to be on while the voltage of the DC link 20 is within a voltage range that is predetermined according to the DC load 50. This makes it possible to prevent an excessive voltage from being applied to the DC load 50.

[0156] The load switch circuits 70 and 71 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may be controlled to be on while the voltage of the DC link 20 is within a voltage range that is determined in advance according to the input voltage of the DC load 50. This makes it possible to prevent an excessive current from flowing when the load switch circuits 70 and 71 are turned on.

[0157] The DC / DC converter 10 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may connect a DC power source 40 connectable to the DC / DC converter 10 and a DC load 50 connectable to the DC link 20 via a diode D5. With this, when the diode D5 is turned on, loss in the current path is reduced.

[0158] The heat pump system of the present embodiment includes power conversion systems 1, 1A, 1B, 1C, 2, and 3, and a heat pump device as a DC load 50. This eliminates the need to provide a separate DC / DC converter.

[0159] The solar power generation system of the present embodiment includes power conversion systems 1, 1A, 1B, 1C, 2, and 3, and a solar cell as a DC power supply 40. With this, a power generation system that can adjust the voltage applied to a DC load 50 appropriately while connecting the solar cell to a power grid 100 can be realized with a simple circuit configuration.

[0160] Although the embodiment has been described above, it will be understood that various changes in form and details are possible without departing from the spirit and scope of the claims. [Explanation of symbols]

[0161] 1, 1A, 1B, 1C, 2, 3...Power conversion system, 10, 11...DC / DC converter, 20...DC link, 30...Grid-connected inverter, 40...DC power supply, 50...DC load, 60...Switch, 100...Power system, 70, 71...Load switch circuit, C1, C2, C3, C4, C5...Capacitor, D1, D2, D3, D4, D5, Dap, Dan, Dbp, Dbp...Diode, L1, L2, L3...Reactor, San, Sap, Sbn, Sbp, Sw1, Sw2...Switching element, Sx, Sy...Load switch

Claims

1. a DC / DC converter to which a DC power supply can be connected; A grid-connected inverter; a DC link connecting the DC / DC converter and the grid-connected inverter, A voltage of the DC link is adjusted in accordance with a DC load connectable to the DC link within a half cycle of a system voltage on the AC side of the grid-connected inverter. Power conversion systems.

2. 10. The power conversion system of claim 1, wherein the voltage of the DC link is regulated according to a condition of a DC load connectable to the DC link.

3. The power conversion system according to claim 2 , wherein a DC load whose output varies depending on a voltage of the DC link can be connected to the DC link as the DC load.

4. 3. The power conversion system according to claim 2, wherein the DC link includes a power converter as the DC load, and a DC load can be connected to the DC link such that a voltage of the DC link is adjusted according to a state of power conversion of the power converter.

5. 5. The power conversion system according to claim 4, wherein the DC link includes, as the DC load, the power converter that operates a rotating electric machine at a variable speed, and a DC load in which a voltage of the DC link is adjusted depending on an operating state of the rotating electric machine can be connected.

6. 2. The power conversion system according to claim 1, wherein the adjustment of the voltage of the DC link includes control including, within a half cycle of the grid voltage, a period during which the voltage is adjusted in accordance with the DC load connectable to the DC link and a period during which the voltage is adjusted to be a voltage based on the grid voltage.

7. The power conversion system according to claim 6 , wherein the voltage based on the grid voltage is a voltage greater than an absolute value of the grid voltage.

8. The power conversion system of claim 1 , wherein the grid-tied inverter is controlled in one of a PWM operation, a PAM operation, and a rectification operation depending on the DC load connectable to the DC link.

9. The power conversion system according to claim 1 , wherein the DC load can be connected to the DC link via a switch circuit.

10. 10. The power conversion system according to claim 9, further comprising a capacitor having a capacitance larger than that of the capacitor provided in the DC link, the capacitor being provided on an input side of the DC load in parallel with the DC load.

11. The power conversion system according to claim 9 , wherein the switch circuit is controlled to be on while the voltage of the DC link is within a voltage range that is predetermined according to the DC load.

12. The power conversion system according to claim 9 , wherein the switch circuit is controlled to be on while a voltage of the DC link is within a voltage range that is predetermined according to an input voltage of the DC load.

13. The power conversion system according to claim 9 , wherein the DC / DC converter connects the DC power source connectable to the DC / DC converter and the DC load connectable to the DC link via a diode.

14. A power conversion system according to any one of claims 1 to 13; A heat pump device as the DC load; A heat pump system comprising:

15. A power conversion system according to any one of claims 1 to 13; A solar cell as the DC power source; A solar power generation system comprising:

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