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

Abstract

[Problem] To improve the operation efficiency of a DC load in accordance with the DC load. [Solution] This power conversion system comprises: a DC / DC converter to which a DC power supply is connectable; a system interconnection inverter; and a DC link connecting the DC / DC converter and the system interconnection inverter. The voltage of the DC link is adjusted in accordance with a DC load connectable to the DC link within a half period of a system voltage on the AC side of the system interconnection inverter.

Description

Power conversion systems, heat pump systems, and solar power generation systems

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

[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 grid, 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, wherein 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, the controller sets the output voltage of the boost converter circuit to a voltage value that is lower than the maximum value of AC power of the grid.

[0003] International Publication No. 2011 / 099280

[0004] In a power conversion system that supplies power from a DC power source such as a solar cell to a DC load, converts the power to AC using a grid-connected inverter, and then allows reverse power flow to the power grid, there is a demand for supplying appropriate 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.

[0006] A power conversion system according to a first aspect includes a DC / DC converter connectable to a DC power source, a grid-connected inverter, and a DC link connecting the DC / DC converter and the grid-connected inverter, and adjusts the voltage of the DC link according to a DC load connectable to the DC link within a half cycle of a grid voltage on the AC side of the grid-connected inverter. This improves the operating efficiency of the DC load. A power conversion system according to a second aspect is the power conversion system according to the first aspect, wherein the voltage of the DC link is adjusted according to a state of the DC load connectable to the DC link. This makes it possible to adjust the voltage applied to the DC load. 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 to a DC load whose output varies depending on the voltage of the DC link. This makes it possible to adjust the output of the DC load. A power conversion system according to a fourth aspect is the power conversion system according to the second aspect, wherein the DC link includes a power converter as the DC load, and is connectable to a DC load whose DC link voltage is adjusted according to a power conversion state of the power converter. This makes it possible to maintain an appropriate voltage applied to the DC load including the power converter. A power conversion system according to a fifth aspect is the power conversion system according to the fourth aspect, wherein the DC link includes a power converter that operates a rotating electric machine at a variable speed, and is connectable to a DC load whose DC link voltage is adjusted according to an operating state of the rotating electric machine. This makes it possible to maintain an appropriate voltage applied to the DC load including the power converter that operates the rotating electric machine at a variable speed. A power conversion system according to a sixth aspect is the power conversion system according to any one of the first to fifth aspects, wherein the adjustment of the DC link voltage includes control that includes, within a half cycle of the grid voltage, a period during which the adjustment is made according to the DC load connectable to the DC link and a period during which the adjustment is made to a voltage based on the grid voltage. This makes it possible to prevent an excessive voltage from being applied to the DC load. A power conversion system according to a seventh aspect is the power conversion system according to the sixth aspect, wherein the voltage based on the grid voltage is a voltage greater than the absolute value of the grid voltage.This ensures a voltage required for the grid-connected inverter regardless of fluctuations in DC link voltage. 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 one of PWM operation, PAM operation, and rectification operation depending on the DC load connectable to the DC link. This makes it possible to adjust the voltage applied to the DC load 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. This makes it possible to prevent 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. This makes it easier to control the DC link voltage and the DC load voltage. A power conversion system according to an eleventh aspect is the power conversion system according to the ninth or tenth aspect, wherein the switch circuit is controlled to be on while the DC link voltage is within a predetermined voltage range depending on the DC load. This makes 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, wherein the switch circuit is controlled to be on while the DC link voltage is within a predetermined voltage range depending on the input voltage of the DC load. This makes it possible to prevent 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, 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. This makes it possible to reduce loss in the current path when the diode is turned on. 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.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 source. 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 an electric power grid.

[0007] FIG. 1 is a diagram illustrating an example of a power conversion system according to a first embodiment. FIG. 2 is a diagram illustrating the relationship between a DC link voltage and the AC voltage and AC current of a power grid when a grid-connected inverter is operated in parallel / reverse power flow. (a) shows a case where the DC link voltage is 330 V, (b) shows a case where the DC link voltage is 270 V, and (c) shows a case where an adjustment is performed to boost a portion of the DC link voltage that is lower than the AC voltage. FIG. 3 is a diagram illustrating a power conversion system including N pairs of DC power sources and DC / DC converters. FIG. 4 is a diagram illustrating a power conversion system including M DC loads. FIG. 5 is a diagram illustrating a power conversion system including N pairs of DC power sources and DC / DC converters and M DC loads. FIG. 6 is a diagram illustrating operation modes of a grid-connected inverter. FIG. 7 is a diagram illustrating operation modes for supplying power to a DC load in parallel / reverse power flow. FIG. 8 is a diagram illustrating a DC link voltage target value due to a change in the DC link voltage target value based on the DC load. 1A shows a case where 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; 1B shows a case where 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 absolute value of the peak voltage of the AC voltage; and 1C shows a case where the DC link voltage target value based on the DC load is larger than the absolute value of the peak voltage of the AC voltage.

[0023] FIG. 1C illustrates a change in the DC link voltage target value when a voltage margin is added. 1A shows a case where 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; 1B shows a case where 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 1C shows a case where 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. 1D shows an operation mode in which power is not supplied to the DC load in parallel / reverse power flow. 1 is a diagram illustrating 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, a diagram illustrating an operation mode in which power is supplied to a DC load from a DC power source and a power grid in a parallel / forward power flow, and a diagram illustrating an operation mode in which power is supplied to a DC load from a power grid in a parallel / forward power flow.1A and 1B are diagrams illustrating an operation mode in which power is supplied from a DC power source to a DC load during parallel-off.

[0023] FIG. 1C 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. (a) shows a case in which the DC link voltage target value based on the DC load is greater than the lower limit of the output voltage of the DC / DC converter. (b) shows a case in which the lower limit of the output voltage of the DC / DC converter is greater than the DC link voltage target value based on the DC load. (c) shows a case in which the DC link voltage target value based on the DC load is greater than the absolute value of the peak voltage of the AC voltage. (d) shows a case in which the DC link voltage target value based on the DC load is greater than the lower limit of the output voltage of the DC / DC converter but less than the absolute value of the peak voltage of the AC voltage (VAC). (e) shows a diagram illustrating a means for suppressing an inrush current generated by a load switch. 10A shows a case where an inrush current occurs, and FIG. 10B shows a case where the inrush current is suppressed. It is a diagram showing an example of a power conversion system that is a modified example of the power conversion system according to the second embodiment.

[0008] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. [First Embodiment] (Power Conversion System 1) Fig. 1 is a diagram showing 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. Hereinafter, 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 constitute the DC link 20. The DC link 20 is provided with a capacitor C1. The voltage across 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. Hereinafter, direct current will be referred to as DC, and the DC power supply 40 will be referred to as the DC power supply 40. Hereinafter, the DC power supply 40 will be a photovoltaic cell, and the output voltage of the DC power supply 40 will be referred to as the 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.

[0011] The output side of the grid-connected inverter 30 is connected to the power grid 100 via a switch 60. The power grid 100 is a single-phase alternating current (AC) grid. Hereinafter, AC will be referred to as AC. The AC voltage, which is the system voltage of the power grid 100, will be referred to as AC voltage v ａｃ (instantaneous value) and the effective value of the AC voltage v ａｃ　ｒｍｓ and 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] The DC link 20 side of the grid-connected inverter 30 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 outputs a DC power supply voltage v ｐｖThe DC / DC converter 10 uses an input voltage of 10V as a power supply voltage and converts it to a different voltage to generate an output voltage. The circuit configuration of the DC / DC converter 10 is determined based on the input voltage of the DC / DC converter 10, the output power of the DC / DC converter 10, and the specifications of the DC / DC converter 10. The DC / DC converter 10 can be configured as a boost circuit that boosts the input voltage to generate an output voltage, a step-down circuit that lowers the input voltage to generate an output voltage, or a step-up / step-down circuit that boosts or lowers the input voltage to generate an output voltage. A boost circuit is sometimes referred to as a step-up chopper circuit, a step-down circuit as a step-down chopper circuit, and a step-up / step-down circuit as a step-up / step-down chopper circuit. The DC / DC converter 10 may supply power in either a unidirectional or bidirectional manner. However, if 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 be 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 exchanges 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 from the power system 100 into DC power and supplies it to the DC link 20. Reverse power flow converts DC power from the DC link 20 into AC power and supplies it to the power system 100. Parallel-off disconnects the DC link 20 from the power system 100, allowing the DC link 20 side and the power system 100 to 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 aforementioned 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 conditions 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 ｐｖThe DC power supply 40 and the capacitor C2 may be combined to form a DC power supply unit. The capacitor C2 may be 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 capacitor suppresses fluctuations in the power of the DC power supply. The DC power supply 40 and the storage battery or capacitor may be combined 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 may be 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 unit). 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, etc., a ventilation system, 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 have an output that 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 have a power converter and whose output varies depending on the input voltage. For example, a heater's output heat quantity and temperature vary depending on the input voltage, and a DC motor's rotation speed and torque vary depending on the input voltage. Supplying power to the DC load 50 is referred to as "power supply 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-off 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 As will be described later, ａｃ Therefore, the capacitor C1 is regulated within a half cycle of the DC link voltage v ｄｃ For example, it is sufficient if the capacitance can suppress noise components such as ringing that occur due to 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, insulated gate bipolar transistors (IGBTs), 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 series-connected switching element Sw2 and diode D2 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] A series-connected diode D1 and switching element Sw1 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 between the switching element Sw2 and the diode D2 (the connection point between the emitter terminal of the switching element Sw2 and the cathode terminal of the diode D2) and a connection point between the diode D1 and the switching element Sw1 (the connection point between the anode terminal of the diode D1 and the collector terminal of the switching element Sw1). The anode terminal of the diode D2 is connected to the emitter terminal of the switching element Sw1. Note that 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 states of the switching elements Sw1 and Sw2 are controlled by control signals 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 depending on the on / off combinations of the switching elements Sw1 and Sw2. The DC / DC converter 10 functions as a step-up circuit by keeping the switching element Sw2 on and PWM-controlling the switching element Sw1. The DC / DC converter 10 functions as a step-down circuit by keeping 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 series-connected switching elements Sap and San and the series-connected switching elements Sbp and Sbn 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 anti-parallel to the switching elements Sap, San, Sbp, and Sbn of the same sign.

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

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

[0026] Next, a description will be given of the control in the power conversion system 1. As shown in FIG. 1, the DC load 50 is connected to the DC link 20 and is supplied with power from 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 The DC load 50 may be supplied with power 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 ｄｃ is adjusted depending on the DC load 50 that can be connected to the DC link 20. ｄｃ In addition, the DC link voltage v ｄｃ is adjusted.

[0027] FIG. 2 shows the DC link voltage v when the grid-connected inverter 30 is operated in parallel / reverse power flow. 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 set to 200 V. Figure 2(a) shows the DC link voltage v ｄｃ When the DC link voltage v is 330V, ｄｃ When the DC link voltage v is 270V, ｄｃ is the AC voltage v ａｃ In Figures 2(a), (b), and (c), the DC link voltage v ｄｃ , AC voltage v ａｃ , and AC current i ａｃ The horizontal axis represents time (seconds), and the vertical axis represents the upper DC link voltage v ｄｃ , AC voltage v ａｃ Then, the voltage (V) and the AC current i ａｃ In Figures 2(a), (b), and (c), the AC current i ａｃ2(a), (b), and (c) show the overall current distortion rate (%) of the MOSFET. The results were obtained by simulation.

[0028] AC voltage effective value v ａｃ　ｒｍｓ is 200V, the AC peak voltage |v ａｃ　ｐｅａｋ | is approximately 283 V. The DC link voltage v 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 This prevents distortion from occurring in the image.

[0029] The DC link voltage v shown in FIG. ｄｃ 270V is the AC peak voltage |v ａｃ　ｐｅａｋ In this case, the AC voltage v ａｃ The peak voltage V ａｃ　ｐｅａｋ In the vicinity, AC current i ａｃ However, it is distorted by high frequency currents. The total current distortion rate is high at 25.3%.

[0030] As shown in FIG. 2(b), the AC current i ａｃ The distortion occurred 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 In the lower part, the DC link voltage v ｄｃ By boosting the AC current i ａｃ The high frequency current of the DC link voltage v is suppressed. The total current distortion rate is reduced to 3.0%, the same as in Fig. 2(a). As shown in Fig. 2(c), ｄｃ The DC link voltage v is changed (or varied) within a half cycle of the AC voltage. ｄｃ As shown in FIG. 2(c), the DC link voltage v ｄｃ When the DC link voltage v in FIG. ｄｃ Compared to when the DC link voltage is set to 330V, ｄｃ decreases, and losses are reduced.

[0031] 2(a), (b), and (c) do not explain the voltage applied to the DC load 50 supplied with power from the DC link 20. For example, as shown in FIG. 2(a), the DC link voltage v ｄｃ If the DC link voltage v in FIG. ｄｃ If 270V is a suitable voltage for powering the DC load 50, then, as described 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 voltage of 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 Hereinafter, 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 power 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 is referred to as an 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 power conversion system 1 is controlled is referred to as an output current control mode (output power control mode). Control of the power conversion system 1 will be described in detail below.

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

[0034]

[0035] DC link voltage target value v ＊ｄｃ is the DC link voltage v ｄｃ 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 half the 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 can 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 50 ＊ ｄｃ　ｌｏａｄ and the DC link voltage target value v based on the grid-connected inverter 30. ＊ ｄｃ　ｉｎｖ and the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10. In the following, the above terms may be explained using only the symbols. The same applies to other cases. The DC link voltage target value v ＊ ｄｃ　ｉｎｖ is the voltage determined by the grid voltage.

[0037] v ＊ ｄｃ　ｌｏａｄ is determined depending on the configuration and state of the DC load 50. For example, v is set to ＊ ｄｃ　ｌｏａｄ If the DC load 50 includes a power converter, v may be determined depending on the power conversion state of the power converter. ＊ ｄｃ　ｌｏａｄ For example, v may be determined according to the power conversion conditions 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 including the power converter that operates the rotating electrical machine at a variable speed.

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

[0040] v ＊ ｄｃ　ｉｎｖ is the AC voltage v which is the instantaneous value 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 | ＊ 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 It may be determined as follows. ｍａｒｇｉｎ may be determined depending on the operating state of the DC load 50. ｍａｒｇｉｎ v for an operating state with small fluctuations in power consumption ｍａｒｇｉｎMore 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 electrical 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] The value vconv_lowlimit is determined by the configuration of the DC / DC converter 10. For example, if the DC / DC converter 10 is a boost circuit, vconv_lowlimit is the input voltage of the boost circuit. If the DC / DC converter 10 is a step-down circuit, vconv_lowlimit is, in principle, 0 V.

[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 50 ＊ ｄｃ　ｌｏａｄ and the DC link voltage target value v based on the grid-connected inverter 30. ＊ ｄｃ　ｉｎｖ and the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10. ｄｃ (DC link voltage target value v ＊ ｄｃ ) is the DC link voltage target value v based on the DC load 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 state of the DC load 50 .

[0043] v ｄｃ But, v ＊ ｄｃ　ｌｏａｄ If it is less than 100 V, the appropriate voltage will not be applied to the DC load 50.ｄｃ But, v ＊ ｄｃ　ｉｎｖ If the AC current i ａｃ Distortion occurs in ｄｃ is the output voltage of the DC / DC converter 10, so v ｄｃ cannot be set below vconv_lowlimit.

[0044] vconv lowlimit is v ＊ ｄｃ　ｌｏａｄ and v ＊ ｄｃ　ｉｎｖ If it is smaller than (v ＊ ｄｃ　ｌｏａｄ , v ＊ ｄｃ　ｉｎｖ > vconv lowlimit), within a half cycle of the AC voltage, v ＊ ｄｃ　ｌｏａｄ But, v ＊ ｄｃ　ｉｎｖ The condition that is always greater than (v ＊ ｄｃ　ｌｏａｄ ≧v ＊ ｄｃ　ｉｎｖ ) then, v ＊ ｄｃ , v ＊ ｄｃ　ｌｏａｄ (v ＊ ｄｃ =v ＊ ｄｃ　ｌｏａｄ ). ＊ ｄｃ　ｉｎｖ Ga V ＊ ｄｃ　ｌｏａｄ Larger (v ＊ ｄｃ　ｉｎｖ >v ＊ ｄｃ　ｌｏａｄ ) under the condition that there is a period, v ＊ ｄｃ is adjusted to vary periodically within a half period of the AC voltage.

[0045] Under the condition 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 backward to the power grid 100. ＊ ｄｃBy adjusting the voltage, a voltage suitable for operation (appropriate voltage) is applied to the DC load 50. Prior Art Document 1 describes providing a boost converter circuit that boosts the output voltage of the power generation device and a buck 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 buck converter circuit described in Prior Art Document 1 into a single common circuit. Furthermore, because the power conversion system 1 does not include the buck converter circuit that supplies power to the internal load as in Prior Art Document 1, power loss caused by a converter that adjusts the voltage applied to the DC load 50 is eliminated.

[0046] (Output current control mode (output power control mode)) The DC / DC converter 10 controls the input voltage based on the detected or estimated value and the output voltage target value v ＊ ｄｃ The DC / DC converter 10 is configured to operate based on these values. In addition to these values, the DC / DC converter 10 may be configured to operate based on a value corresponding to the output current. The value corresponding to the output current of the DC / DC converter 10 may be determined based on a value corresponding to the current (or power) flowing through the DC load 50 or a value corresponding 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 based on a value corresponding to the output current of the DC / DC converter 10, it is possible to determine the output voltage v due to current fluctuations. ｄｃ 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 of the DC / DC converter 10 ｃｏｎｖ (or output power P ｃｏｎｖ ) to the target value i ＊ ｃｏｎｖ (or P ＊ ｃｏｎｖ ) is controlled. ＊ｃｏｎｖ (or P ＊ ｃｏｎｖ ) is determined depending on the operating state of any one or a combination of the DC power supply 40, the DC load 50, and the grid-connected inverter 30. For example, when the DC power supply 40 is a solar cell, the output current from the solar cell during MPPT (Maximum Power Point Tracking) control is determined as i ＊ ｃｏｎｖ By controlling 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 DC load 50. By controlling 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 grid-connected inverter 30.

[0048] (Power conversion system 1A of modified example 1) As modified example 1 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. FIG. 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 will be referred to as DC / DC converters 10-1 to 10-N. When distinguishing between the N DC power supplies 40, they will be referred to 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]

[0051] It is desirable to control only one DC / DC converter 10 in output voltage control mode. It is preferable to control the DC / DC converter 10 with the largest output voltage lower limit value vconv lowlimit in output voltage control mode. If a DC / DC converter 10 with a lower vconv lowlimit than the largest is set in output voltage control mode, the output voltages of the other DC / DC converters 10 must be adjusted to this lower limit value, which causes accuracy problems. On the other hand, if the DC / DC converter 10 with the largest vconv lowlimit is driven in output voltage control mode, the other DC / DC converters 10 do not need to adjust their lower limits.

[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 may be controlled in the output current control mode (output power control mode).

[0053] (Power conversion system 1B of modified example 2) As modified example 2 of the power conversion system 1, a power conversion system 1B including a plurality of DC loads 50 will be described. Fig. 4 is a diagram illustrating the 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 referred to 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]

[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 may be controlled in the output current control mode (output power control mode).

[0057] (Power Conversion System 1C of Modification 3) As Modification 3 of the power conversion system 1, a power conversion system 1C will be described which includes a plurality of pairs of DC power supplies 40 and DC / DC converters 10, and a plurality of DC loads 50. FIG. 5 is a diagram illustrating the power conversion system 1C which includes 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 greater. When distinguishing between the N DC / DC converters 10, they are referred to as DC / DC converters 10-1 to 10-N. When distinguishing between the N DC power supplies 40, they are referred to as DC power supplies 40-1 to 40-N. When distinguishing between the M DC loads 50, they are referred to 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]

[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 may 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 thereof. 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 Determination method of 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 ＊ ｄｃIn 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 calculates 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 the grid-connected inverter-based DC link voltage target value v ＊ 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 and v ＊ ｄｃ　ｌｏａｄ The state necessary to determine ＊ ｄｃ　ｌｏａｄ The conditions necessary for determining v are, for example, the modulation factor of the power converter, and if 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. The rotation speed and output power of the rotating electric machine may be detected values, estimated values, or target values. The DC / DC converter 10 receives v from the grid-connected inverter 30. ＊ ｄｃ　ｉｎｖ and v ＊ ｄｃ　ｉｎｖ The state necessary to determine ＊ ｄｃ　ｉｎｖ The conditions required to determine the AC voltage v of the power system are, for example, ａｃ And so on.

[0064] A separate controller controls the voltage from the DC load 50 to ＊ ｄｃ　ｌｏａｄ or v ＊ ｄｃ　ｌｏａｄ The state necessary for determining v is acquired from the grid-connected inverter 30. ＊ 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 microcomputer) including a CPU, ROM, 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 also be configured as an ASIC, etc.

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

[0067] (Operation Modes of the Grid-Connected Inverter 30) Up to now, the grid-connected inverter 30 has been described in the case of parallel / reverse power flow, but the operation of the power conversion system 1 will now be described in detail, including the case of parallel / forward power flow and parallel-out. First, the operation modes of the grid-connected inverter 30 in the power conversion system 1 will be described. Fig. 6 is a diagram illustrating the operation modes of the grid-connected inverter 30. In Fig. 6, from the left, the connection state with the power grid 100, the 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, in the case of the switching elements Sap and Sbn and in the case of 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 operation mode is changed.

[0069] PAM operation is performed by setting 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 ａｃ | Absolute value of the difference | (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 than

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

[0071] In the parallel / forward power flow mode, the grid-connected inverter 30 performs full-wave rectification or PWM rectification. In the full-wave rectification mode, the grid-connected inverter 30 maintains the switching elements Sap, San, Sbp, and Sbn off and rectifies the power using the diodes Dap, Dan, Dbp, and Dbn. If the switching elements Sap, San, Sbp, and Sbn are monopolar elements that allow current to flow in the reverse direction, the grid-connected inverter 30 may operate in synchronous rectification mode, in which the switching elements Sap, San, Sbp, and Sbn are turned on at the same time that the diodes Dap, Dan, Dbp, and Dbn become forward biased. The full-wave rectification mode and the synchronous rectification mode are examples of rectification modes.

[0072] The PWM rectification operation is an operation of repeatedly switching 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. ｄｃ may be controlled with priority given to

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

[0074] (Operation Modes of Power Conversion System 1) (1) Operation Mode for Supplying Power to DC Load 50 in Parallel / Reverse Power Flow An operation mode for supplying power to a DC load 50 in a parallel / reverse power flow in which power is supplied from the DC side to the AC side will be described. In the following, a case where there is one DC / DC converter 10 and one DC load 50 will be described.

[0075] 7 is a diagram illustrating an operation mode for supplying power to a DC load 50 in parallel / reverse power flow. The upper part shows the power conversion system 1, and the lower part 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 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 grid 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 value vconv_lowlimit of the output voltage of the DC / DC converter 10.

[0077] v ＊ ｄｃ　ｌｏａｄ But, v ＊ ｄｃ　ｉｎｖ , and if greater than vconv lowlimit, v ＊ ｄｃ is the DC link voltage target value v based on the DC load 50 ＊ ｄｃ　ｌｏａｄ It becomes. ＊ ｄｃ Ga V ＊ ｄｃ　ｉｎｖ Since the power supply voltage is larger than the power supply voltage, the grid-connected inverter 30 performs PWM operation to link the DC side and the AC side.

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

[0079] vconv lowlimit is v ＊ ｄｃ　ｌｏａｄ , and v ＊ ｄｃ　ｉｎｖ If it is greater than ＊ ｄｃbecomes vconv lowlimit. ＊ ｄｃ is vconv_lowlimit, the grid-connected inverter 30 performs 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 ＊ ｄｃ　ｌｏａｄ The change in the DC link voltage target value v ＊ ｄｃ An example in which the value of θ changes will be described.

[0081] FIG. 8 shows the DC link voltage target value v based on the DC load 50. ＊ ｄｃ　ｌｏａｄ DC link voltage target value v ＊ ｄｃ 8(a) is a diagram illustrating the DC link voltage target value v ＊ ｄｃ　ｌｏａｄ is smaller than the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10, the DC link voltage target value v ＊ ｄｃ　ｌｏａｄ is greater than the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10, but the AC voltage v ａｃ Absolute value of the peak voltage |v ａｃ　ｐｅａｋ 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 the peak voltage |v ａｃ　ｐｅａｋ | is larger than |. The vertical axis represents voltage (V) and the horizontal axis represents time (seconds). The time progression is 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 vａｃ One cycle is made up of two repeated half cycles. In the following, only the half cycle from time t0 to time t3 will be explained. The same applies to the other cases.

[0082] The 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 |). In the following, the 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 is represented by a thick solid line, and the DC link voltage target value v based on the DC load 50 ＊ ｄｃ　ｌｏａｄ is indicated by a dotted line, the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10 is indicated by a dashed line, and the DC link voltage target value v ＊ ｄｃ　ｉｎｖ (= AC voltage v ａｃ Absolute value of |v ａｃ |) are indicated by thin solid lines. Overlapping parts are indicated by shifting them.

[0084] FIG. 8(a) shows the ＊ ｄｃ　ｌｏａｄ In FIG. 8A, times t1, t2, t4, and t5 are the times when vconv_lowlimit and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) intersects with the

[0085] During the period from time t0 to time t1, vconv lowlimit is ＊ ｄｃ　ｌｏａｄ and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is greater than either. ＊ ｄｃis vconv lowlimit. ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is v ＊ ｄｃ Therefore, the grid-connected inverter 30 connects the DC side and the AC side through PWM operation. ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is vconv lowlimit and v ＊ ｄｃ　ｌｏａｄ greater than either of the above. ＊ ｄｃ , v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |). ＊ ｄｃ Ha|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 The grid-connected inverter 30 connects the DC side and the AC side by PWM operation. ＊ ｄｃ　ｌｏａｄ , v ＊ ｄｃ The DC load 50 is supplied with a voltage sufficient for operation (appropriate voltage).

[0086] FIG. 8(b) shows the ＊ ｄｃ　ｌｏａｄ is greater than vconv lowlimit, but |v ａｃ　ｐｅａｋ In FIG. 8B, at times t2, t3, t4, and t5, v ＊ ｄｃ　ｌｏａｄ and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) intersects with the time t2. Times t2, t3, t4, and t5 are different from those in FIG. 8A. The same applies to the others except where noted.

[0087] In the period from time t0 to time t1, v ＊ｄｃ　ｌｏａｄ is vconv lowlimit and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is greater than either. ＊ 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 through PWM operation. ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is vconv lowlimit and v ＊ ｄｃ　ｌｏａｄ greater than either of the above. ＊ ｄｃ , v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |). ＊ ｄｃ Ha|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 again v ＊ ｄｃ　ｌｏａｄ The grid-connected inverter 30 links 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 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 ａｃIn the half cycle of the AC voltage from time t0 to time t3, the DC link voltage v ｄｃ is controlled in accordance with 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 voltage is controlled to be a voltage determined by the grid voltage. This prevents an excessive voltage from being applied to the DC load 50.

[0089] FIG. 8(c) shows the ＊ ｄｃ　ｌｏａｄ ga|v ａｃ　ｐｅａｋ In the period from time t0 to time t6, v ＊ ｄｃ　ｌｏａｄ is vconv lowlimit and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is greater than either. ＊ ｄｃ , v ＊ ｄｃ　ｌｏａｄ It becomes. ＊ ｄｃ Ha|v ａｃ | is larger, the grid-connected inverter 30 performs PWM operation to link the DC side and the AC side.

[0090] During 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 Figs. ＊ ｄｃ The DC link voltage target value v ＊ ｄｃ By adjusting the DC link voltage v ｄｃ changes (fluctuations) and supplies 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 calculated. ＊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 9A is a diagram illustrating the change in the DC link voltage target value v based on the DC load 50. ＊ ｄｃ　ｌｏａｄ is smaller than the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10, the DC link voltage target value v ＊ ｄｃ　ｌｏａｄ is greater than the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10, but the AC voltage v ａｃ Absolute value of the peak voltage |v ａｃ　ｐｅａｋ |Voltage margin v ｍａｒｇｉｎ 9(c) shows that the DC link voltage target value v ＊ ｄｃ　ｌｏａｄ is the AC voltage v ａｃ Absolute value of the peak voltage |v ａｃ　ｐｅａｋ |Voltage margin v ｍａｒｇｉｎ The vertical and horizontal axes are the same as in FIG.

[0093] The 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 ) are indicated by two-dot chain lines. The rest is the same as in FIG.

[0094] FIG. 9(a) shows the ＊ ｄｃ　ｌｏａｄ is smaller than vconv_lowlimit. At times t1, t2, t4, and t5, ＊ ｄｃ　ｉｎｖ (= | v ａｃ |+v ｍａｒｇｉｎ) and vconv_lowlimit intersect.

[0095] During the period from time t0 to time t1, vconv lowlimit is v ＊ ｄｃ　ｌｏａｄ and v ＊ ｄｃ　ｉｎｖ greater than either of the above. ＊ ｄｃ is vconv lowlimit. In the period from time t1 to time t2, v ＊ ｄｃ　ｉｎｖ Ga V ＊ ｄｃ　ｌｏａｄ and vconv lowlimit is greater than v ＊ ｄｃ , v ＊ ｄｃ　ｉｎｖ In the period from time t2 to time t3, v ＊ ｄｃ becomes vconv lowlimit again. During the period from time t0 to time t3, v ＊ ｄｃ is |v ａｃ Therefore, the grid-connected inverter 30 links the DC side and the AC side by PWM operation.

[0096] FIG. 9(b) shows the ＊ ｄｃ　ｌｏａｄ is greater than vconv lowlimit, but |v ａｃ　ｐｅａｋ |+v ｍａｒｇｉｎ At times t1, t2, t4, and t5, v ＊ ｄｃ　ｌｏａｄ and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |+v ｍａｒｇｉｎ ) is the time when they intersect.

[0097] In the period from time t0 to time t1, v ＊ ｄｃ　ｌｏａｄ is vconv lowlimit and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |+v ｍａｒｇｉｎ ) is greater than either of the above. ＊ ｄｃ , v ＊ ｄｃ　ｌｏａｄIn the period from time t1 to time t2, v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |+v ｍａｒｇｉｎ ) is v ＊ ｄｃ　ｌｏａｄ and vconv lowlimit is greater than v ＊ 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 again v ＊ ｄｃ　ｌｏａｄ In 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] FIG. 9(c) shows the ＊ ｄｃ　ｌｏａｄ ga|v ａｃ　ｐｅａｋ |+v ｍａｒｇｉｎ In the period from time t0 to time t3, v ＊ ｄｃ　ｌｏａｄ is vconv lowlimit and v ＊ ｄｃ　ｉｎｖ greater than either of the above. ＊ ｄｃ , v ＊ ｄｃ　ｌｏａｄ In 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] The 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 power is not supplied to the DC load 50 in parallel / reverse power flow In a parallel / reverse power 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 reversely flowed to the power grid 100. FIG. 10 is a diagram illustrating an operation mode in which power is not supplied to the DC load 50 in parallel / reverse power flow. The upper part shows the power conversion system 1, and the lower part 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 | (v ＊ ｄｃ　ｉｎｖ =|v ａｃ |). The DC / DC converter 10 is driven in the output voltage control mode.

[0101] The flow of power is indicated by arrows in the power conversion system 1 shown in the upper part of Fig. 10. Power is supplied from a DC power source 40 to a power grid 100. A DC link voltage target value v based on a DC load 50 is ＊ ｄｃ　ｌｏａｄ is "0 V." 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 is ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) and the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10.

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

[0103] vconv lowlimit is v ＊ ｄｃ　ｌｏａｄ and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |), v ＊ ｄｃ becomes vconv lowlimit. ＊ ｄｃ is vconv_lowlimit, the grid-connected inverter 30 performs 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 mode in which the DC load 50 is not supplied with power. ＊ ｄｃ In FIG. 11, times t1, t2, t4, and t5 are the time periods when vconv_lowlimit and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) intersects with the

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

[0106] (3) Operation Mode in Parallel / Forward Power Flow in Which Power is Supplied to the DC Load 50 from the DC Power Source 40 and the Power System 100 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 to the DC load 50 from the DC power source 40 and the power system 100, the DC power source 40 and the power system 100 operate in parallel to supply power to the DC load 50. Fig. 12 is a diagram illustrating an operation mode in which power is supplied to the DC load 50 from the DC power source 40 and the power system 100 in a parallel / forward power flow. The upper part shows the power conversion system 1, and the lower part shows the operation of the DC / DC converter 10 and the operation of the grid-connected inverter 30.

[0107] The flow of power is indicated by arrows in the power conversion system 1 shown in the upper part of Fig. 12. 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 or PWM rectification. The operation of the grid-connected inverter 30 is appropriately selected depending on 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 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) Operation Mode in Parallel / Forward Power Flow in Which Power is Supplied from the Power System 100 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. Fig. 13 is a diagram illustrating an operation mode in which power is supplied from the power system 100 to the DC load 50 in a parallel / forward power flow. The upper part shows the power conversion system 1, and the lower part 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 shown in the upper part of Fig. 13. Power is supplied from the power grid 100 to the 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 selected appropriately depending on 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) Operation Mode in Which Power is Supplied from the DC Power Source 40 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 grid 100 to the DC load 50. Fig. 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 part shows the power conversion system 1, and the lower part 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 ＊ ｄｃ　ｉｎｖ There is no 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 value vconv_lowlimit of the output voltage of the DC / DC converter 10. The DC / DC converter 10 operates in the output voltage control mode.

[0112] v ＊ ｄｃ　ｌｏａｄ If vconv lowlimit is greater than v ＊ ｄｃ　ｌｏａｄ ≧vconvlowlimit),v ＊ ｄｃ is v ＊ ｄｃ　ｌｏａｄ and vconv lowlimit becomes v ＊ｄｃ　ｌｏａｄ If vconv lowlimit ≥ v ＊ ｄｃ　ｌｏａｄ ), v ＊ ｄｃ becomes vconv lowlimit.

[0113] FIG. 15 shows the DC link voltage target value v in the operation mode in which power is supplied from the DC power source 40 to the DC load 50 during parallel-off. ＊ ｄｃ 15(a) is a diagram illustrating the DC link voltage target value v ＊ ｄｃ　ｌｏａｄ is larger than the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10, FIG. 15B shows the DC link voltage target value v ＊ ｄｃ　ｌｏａｄ For reference, the AC voltage v ａｃ Absolute value of |v ａｃ Indicates |.

[0114] As shown in FIG. ＊ ｄｃ　ｌｏａｄ If vconv lowlimit is greater than v ＊ ｄｃ　ｌｏａｄ ≧vconv lowlimit), v ＊ ｄｃ , v ＊ ｄｃ　ｌｏａｄ As shown in FIG. 15(b), vconv lowlimit is ＊ ｄｃ　ｌｏａｄ If vconv lowlimit ≥ v ＊ ｄｃ　ｌｏａｄ ) then, v ＊ ｄｃ becomes vconv lowlimit.

[0115] Second Embodiment A power conversion system 2 according to a second embodiment includes a load switch circuit 70 on a line connecting a DC load 50 to a 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 shown 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 positive 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 control the DC link voltage v ｄｃ is the DC link voltage target value v based on the DC load 50 ＊ ｄｃ　ｌｏａｄ In other words, the DC link voltage v ｄｃ is in a voltage range predetermined 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 predetermined voltage range depending on 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 capacitance than the capacitor C4 in the power conversion system 1. The capacitor C5 is preferably selected to have a larger capacitance 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 and the DC voltage v 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 for supplying power to the DC load 50 in parallel / reverse power flow will be described. FIG. 17 shows the DC link voltage target value v ＊ ｄｃ　ｌｏａｄ DC link voltage target value v ＊ ｄｃ 17(a) is a diagram illustrating the DC link voltage target value v ＊ ｄｃ　ｌｏａｄ 17B shows the DC link voltage target value v based on the DC load 50 when vconv_lowlimit is smaller than the lower limit value vconv_lowlimit of the output voltage of the DC / DC converter 10. ＊ ｄｃ　ｌｏａｄ is greater than the lower limit value vconv lowlimit of the output voltage of the DC / DC converter 10, but the AC voltage v ａｃ Absolute value of the peak voltage |v ａｃ　ｐｅａｋ 17(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 the 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] The 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 |). The DC load voltage v applied to the DC load 50 ｄｃ　ｌｏａｄ is shown by a dashed dotted line.

[0122] FIG. 17(a) shows the ＊ ｄｃ　ｌｏａｄ is smaller than vconv lowlimit (v ＊ ｄｃ　ｌｏａｄ <vconv lowlimit). At times t1, t2, t4, and t5, ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) and vconv lowlimit intersect.

[0123] During the period from time t0 to time t1, vconv lowlimit is v ＊ ｄｃ　ｌｏａｄ and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is greater than either. ＊ ｄｃ 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. In the period from time t1 to time t2, v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |)is v ＊ ｄｃ　ｌｏａｄ and vconv lowlimit. ＊ ｄｃ , v ＊ ｄｃ　ｉｎｖ (= | v ａｃ The load switch Sx is controlled to be open (OFF), and the DC load 50 is supplied with power by discharging 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 ＊ ｄｃThe 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 Figure 17(a), vconv lowlimit is applied to the DC load 50 during the period from time t0 to time t3. ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) was applied. ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is greater than vconv lowlimit. 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 operation efficiency of the DC load 50.

[0125] FIG. 17(b) shows the ＊ ｄｃ　ｌｏａｄ is greater than vconv lowlimit, but |v ａｃ　ｐｅａｋ At times t1, t2, t4, and t5, v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) and v ＊ ｄｃ　ｌｏａｄ This is the time when the and intersect.

[0126] In the period from time t0 to time t1, v ＊ ｄｃ　ｌｏａｄ is vconv lowlimit and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is greater than. ＊ ｄｃ , v ＊ 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. ＊ｄｃ　ｉｎｖ (= | v ａｃ |)is v ＊ ｄｃ　ｌｏａｄ and vconv lowlimit is greater than v ＊ ｄｃ , v ＊ ｄｃ　ｉｎｖ (= | v ａｃ The load switch Sx is controlled to be open (OFF), the DC load 50 is supplied with power 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 again v ＊ 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 due to the discharge of the capacitor C5 during the period from time t1 to time t2, as shown by the dashed line in FIG. 17B, the DC load 50 is ＊ ｄｃ　ｌｏａｄ is applied. DC load input voltage v ｄｃ　ｌｏａｄ is always suitable for DC load 50 ＊ ｄｃ　ｌｏａｄ In FIG. 8B, 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), an excessive voltage is prevented from being supplied to the DC load 50. The voltage v applied to the DC load 50 ｄｃ This improves the operation efficiency of the DC load 50.

[0128] FIG. 17(c) shows the ＊ ｄｃ　ｌｏａｄ ga|v ａｃ　ｐｅａｋ In the period from time t0 to time t3, v ＊ ｄｃ　ｌｏａｄis vconv lowlimit and v ＊ ｄｃ　ｉｎｖ (= | v ａｃ |) is greater than. ＊ 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. The DC load input voltage v ｄｃ　ｌｏａｄ is always the v suitable for DC load 50 ＊ ｄｃ　ｌｏａｄ The voltage v applied to the DC load 50 ｄｃ This improves the operation efficiency of the DC load 50.

[0129] At times t2 and t5 in FIGS. 17(a) and 17(b), the load switch Sx is controlled to change from open (off) to closed (on) by the DC link voltage v ｄｃ is performed while the voltage is within a predetermined range depending on the DC load 50. Note that while the voltage is within the predetermined 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 that occurs when the load switch Sx is switched from open (off) to closed (on) (inrush current suppression means) will be described. Fig. 18 is a diagram for explaining a means for suppressing an inrush current that occurs due to 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 Figs. 18(a) and 18(b), the upper part shows a voltage waveform, and the lower part shows a current waveform. The lower current waveform represents the current i that flows from the load switch Sx to the DC load 50. ｄｃ　ｌｏａｄ The vertical axis represents the waveform of the current i ｄｃ　ｌｏａｄ (a.u.) 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 FIG. 18(a), the DC load input voltage v ｄｃ　ｌｏａｄ and DC link voltage v ｄｃWhen the load switch Sx is switched from open (off) to closed (on) in a state where there is a voltage difference between the load switch Sx and the DC load 50 (the state at times t2 and t5), the current i ｄｃ　ｌｏａｄ In this case, an inrush current flows due to the charging of the capacitor C5, which may increase the circuit loss and shorten the life of the capacitor C5.

[0132] As shown in FIG. 18(b), the DC load input voltage v ｄｃ　ｌｏａｄ and 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 input voltage v is within a predetermined voltage range (the state at times t2' and t5'), the inrush current caused by charging the capacitor C5 is suppressed. ｄｃ　ｌｏａｄ and DC link voltage v ｄｃ The voltage difference between the two may be set depending on the allowable magnitude of the inrush current.

[0133] Furthermore, as a means for suppressing inrush current, when the load switch Sx is switched from open (off) to closed (on), 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). The chopping operation may be performed with a predetermined chopping period, or may be performed 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 slowed down by, for example, 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 sudden changes in 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 denoted by the same reference numerals and will not be described.

[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 the diode D2 of 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 positive side of the DC power supply 40. The collector terminal of the load switch Sy is connected to the positive 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 negative 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 switches on and off the switching element Sw1 to control the DC link voltage v ｄｃ 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 ) to the DC link 20. The diode D4 is a bypass diode. The diode D4 is switched to the off state during the boost operation and to the on state when the DC / DC converter is stopped.

[0140] The load switch Sy functions in the same manner as the load switch Sx in the load switch circuit 70. The DC link voltage v ｄｃ is the DC link voltage target value v based on the DC load 50 ＊ ｄｃ　ｌｏａｄ The DC load input voltage v ｄｃ　ｌｏａｄ In addition, the DC load input voltage v ｄｃ　ｌｏａｄ , the DC power supply voltage v ｐｖ When the load switch Sy is set to 0, the diode D5 is connected to the output voltage of the DC power supply 40 (the 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] The inrush current suppression means described in the power conversion system 2 can also be applied to the power conversion system 3.

[0143] (Application to Three-Phase AC Power System) 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 set as an 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 connected 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 (max(|v ｕ -v ｖ |, |v ｖ -v ｗ |, |v ｗ -v ｕ For example, when the power flow is reverse in parallel with the power grid, the DC link voltage target value v ＊ ｄｃ　ｉｎｖ The voltage margin v is the maximum absolute value of the line voltage (instantaneous value). ｍａｒｇｉｎ The value (max(|v ｕ -v ｖ |, |v ｖ -v ｗ |, |v ｗ -v ｕ |) + v ｍａｒｇｉｎ ) may be determined.

[0144] In the case of three-phase AC, the DC link voltage target value v based on the grid-connected inverter 30 ＊ ｄｃ　ｉｎｖ is equal to or greater than √3 / 2 (approximately 0.86) times the peak voltage of the line voltage. ＊ ｄｃ is the DC link voltage target value v based on the DC load 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 ＊ ｄｃ　ｉｎｖSince the change in the DC link voltage target value v ＊ ｄｃ is the DC link voltage target value v based on the DC load 50 ＊ ｄｃ　ｌｏａｄ The voltage range (v ＊ ｄｃ =v ＊ ｄｃ　ｌｏａｄ The voltage range in which the voltage is constant is wide. Single-phase AC is more suitable than three-phase AC for power systems.

[0145] (Operational 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 the voltage of the DC link 20 is adjusted according to the DC load 50 connectable to the DC link 20 within a half cycle of the grid voltage on the AC side of the grid-connected inverter 30. The voltage of the DC link 20 is adjusted according to the DC load 50 connected to the DC link 20. This improves the operating efficiency of the DC load 50.

[0146] The voltage of the DC link 20 in 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 connectable to the DC link 20. This allows the voltage applied to the DC load 50 to be adjusted appropriately.

[0147] The DC link 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 according to the present embodiment may be configured to be connectable with a DC load 50, the output of which varies depending on the voltage of the DC link 20. This allows the output of the DC load 50 to be adjusted appropriately.

[0148] The DC link 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 according to the present embodiment may include a power converter as the DC load 50, and may be connectable to a DC load that adjusts the voltage of the DC link 20 depending on the power conversion state of the power converter. This makes it possible to adjust the voltage applied to the DC load 50 that includes the power converter.

[0149] The DC link 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 of the present embodiment may be connectable to a DC load 50 that includes the power converter that operates a rotating electric machine at a variable speed, and that adjusts the voltage of the DC link 20 depending on the operating state of the rotating electric machine. This makes it possible to adjust the voltage applied to the DC load 50 that includes the power converter that operates the rotating electric machine at a variable speed.

[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 be controlled to include, within a half cycle of the grid voltage, a period during which the voltage is adjusted in accordance with 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 grid voltage. This makes it possible to prevent an excessive voltage from being applied to the DC load 50.

[0151] The voltage based on the grid 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 grid voltage, thereby ensuring the voltage required for the grid-connected inverter 30 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 according to 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 adjusted appropriately by controlling the grid-connected inverter 30.

[0153] The DC link 20 of the power conversion systems 1, 1A, 1B, 1C, 2, and 3 according to the present embodiment may be connectable to the DC load 50 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 capacitance than the capacitor C1 provided in the DC link 20 may be provided on the input side of the DC load 50 in parallel with 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 according to 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 in accordance with 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 according to the present embodiment may be controlled to be on while the voltage of the DC link 20 is within a predetermined voltage range 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 according to the present embodiment may connect a DC power supply 40 connectable to the DC / DC converter 10 and a DC load 50 connectable to the DC link 20 via a diode D5. As a result, when the diode D5 is turned on, loss in the current path is reduced.

[0158] The heat pump system of this 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 this embodiment includes power conversion systems 1, 1A, 1B, 1C, 2, and 3, and a solar cell as a DC power supply 40. This allows a power generation system that can link the solar cell to the power grid 100 and adjust the voltage applied to the DC load 50 to an appropriate level with a simple circuit configuration.

[0160] Although the embodiments have been described above, it will be understood that various changes in form and details can be made without departing from the spirit and scope of the claims.

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

Claims

1. A power conversion system comprising: 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, wherein the voltage of the DC link is adjusted according to a DC load that can be connected to the DC link within a half cycle of a system voltage on the AC side of the grid-connected inverter.

2. The power conversion system of claim 1, wherein the voltage of the DC link is regulated in response 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 the voltage of the DC link can be connected to the DC link as the DC load.

4. 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 such that the voltage of the DC link is adjusted according to a power conversion state of the power converter.

5. A power conversion system as described in 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 can be connected in which the voltage of the DC link is adjusted according to the operating state of the rotating electric machine.

6. A power conversion system according to any one of claims 1 to 5, wherein the adjustment of the DC link voltage has control including a period during which the DC link voltage is adjusted in accordance with the DC load connectable to the DC link, and a period during which the DC link voltage is adjusted to a voltage based on the grid voltage, within a half cycle of 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 according to any one of claims 1 to 7, wherein the grid-tied inverter is controlled in one of PWM operation, PAM operation, and 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. 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 the input side of the DC load in parallel with the DC load.

11. The power conversion system according to claim 9 or 10, wherein 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 DC load.

12. The power conversion system according to any one of claims 9 to 11, wherein 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.

13. A power conversion system according to any one of claims 9 to 12, 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 heat pump system comprising: a power conversion system according to any one of claims 1 to 13; and a heat pump device as the DC load.

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

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