Power converter

Abstract

To provide a power conversion device capable of resolving imbalances of voltages among respective lines of a single-phase three-wire system and of simplifying a DC / AC inverter.SOLUTION: A power conversion device 1 comprises a DC / DC converter 2, a DC / AC inverter 3, and a control unit 4. The DC / AC inverter 3 comprises: a voltage-dividing circuit configured by a first capacitor C1 and a second capacitor C2 connected in series to divide an output voltage of the DC / DC converter 2; and an inverter circuit that converts a voltage divided by the voltage-dividing circuit into an AC voltage. The DC / DC converter 2 comprises a step-up / down circuit that performs step-up / down operations. The control unit 4 controls the step-up / down circuit so that a voltage V1 of the first capacitor C1 and a voltage V2 of the second capacitor C2 are equivalent to each other.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a power conversion device.

Background Art

[0002] Conventionally, as a power conversion device for solar power generation, for example, the one described in Patent Document 1 is known. As shown in FIG. 4, the power conversion device described in Patent Document 1 includes a capacitor C13, a boost chopper composed of an inductor L11, a switching element S11, and a diode D11, and a DC / AC inverter (two half-bridge inverters) composed of capacitors C11, C12 having equal capacitances connected in series and switching elements S12 to S15.

[0003] In the power conversion device described in Patent Document 1, the voltage input from the solar cell PV is smoothed by the capacitor C13 and then boosted by the boost chopper. The boost chopper outputs the boosted voltage to both ends of the capacitors C11 and C12 and divides the voltage equally by the capacitors C11 and C12. The DC / AC inverter can output a single-phase three-wire output by connecting the connection point of the capacitors C11 and C12 to the neutral wire of a single-phase three-wire system.

[0004] However, in the power conversion device described in Patent Document 1, due to differences in the specifications (for example, variations in power consumption and inrush current) of the load connected between R-N on the output side of the DC / AC inverter and the load connected between N-S, a voltage imbalance (unbalanced load) may occur between R-N and N-S. In this case, the divided voltages of the capacitors C11 and C12 do not become equal voltages, and a problem occurs in that the output voltage of the DC / AC inverter decreases on the side with a large load and increases on the side with a small load.

[0005] As a power conversion device that addresses the above problems, for example, the one described in Patent Document 2 is known. As shown in Figure 5, the power conversion device (DC / AC inverter) described in Patent Document 2 comprises a neutral line voltage adjustment circuit consisting of capacitors C21 and C22 with equal capacitance connected in series, switching elements S27 and S28 and a reactor L21, an inverter circuit consisting of switching elements S21 to S26 and a diode D21, a filter circuit and a control unit (CPU).

[0006] In the DC / AC inverter described in Patent Document 2, when the first DC voltage (voltage between the first DC voltage line DCL1 and the neutral line NL) is greater than the second DC voltage (voltage between the neutral line NL and the second DC voltage line DCL2), the control unit increases the on-duty ratio of switching element S27 and decreases the on-duty ratio of switching element S28. On the other hand, when the second DC voltage is greater than the first DC voltage, the control unit increases the on-duty ratio of switching element S28 and decreases the on-duty ratio of switching element S27.

[0007] With the above control, the DC / AC inverter described in Patent Document 2 can control the divided voltages of capacitors C21 and C22 to equal voltage even if a voltage imbalance occurs between the first AC voltage line ACL1 and the neutral line NL in a single-phase three-wire system, and between the second AC voltage line ACL2 and the neutral line NL. Furthermore, even if the switching elements S25, S26 and diode D21 are removed from the inverter circuit to form a half-bridge configuration, operation that can handle voltage imbalances is still possible.

[0008] However, the DC / AC inverter described in Patent Document 2 presents a problem in that the presence of a neutral wire voltage adjustment circuit and the presence of detection units and other components necessary to control the neutral wire voltage adjustment circuit lead to an increase in the size and complexity of the DC / AC inverter. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 9-65657 [Patent Document 2] Japanese Patent Publication No. 2021-93861 [Overview of the project] [Problems that the invention aims to solve]

[0010] The present invention has been made in view of the above circumstances, and its objective is to provide a power conversion device that can eliminate the voltage imbalance between each line of a single-phase three-wire system and that can simplify the DC / AC inverter. [Means for solving the problem]

[0011] To solve the above problems, the power conversion device according to the present invention is A DC / DC converter that receives a DC input voltage, A DC / AC inverter converts the output voltage output by the aforementioned DC / DC converter into an AC voltage and outputs it to a single-phase three-wire voltage line, Control unit and A power conversion device comprising, The aforementioned DC / AC inverter is A voltage divider circuit consisting of a first capacitor and a second capacitor connected in series, which divides the output voltage of the DC / DC converter, An inverter circuit that converts the voltage divided by the voltage divider circuit into the AC voltage, Equipped with, The DC / DC converter includes a buck-boost circuit that performs boost and buck operations. The control unit is characterized by controlling the step-up / step-down circuit so that the voltage of the first capacitor and the voltage of the second capacitor become equal.

[0012] In this configuration, the DC / DC converter includes a step-up / step-down circuit, and the control unit controls the step-up / step-down circuit so that the voltage of the first capacitor and the voltage of the second capacitor are equal. Therefore, the neutral line voltage adjustment circuit found in conventional DC / AC inverters is unnecessary. Consequently, this configuration makes it possible to eliminate voltage imbalances between each line of a single-phase three-wire system and to simplify the DC / AC inverter.

[0013] In the aforementioned power converter, The control unit can be configured to control the step-up / step-down circuit so that the first capacitor and the second capacitor are charged alternately.

[0014] In the aforementioned power converter, The aforementioned step-up / step-down circuit is A first step-up / step-down chopper section comprising a first switching element, a first coil, a first diode, and a second switching element, It includes a second step-up / step-down chopper section comprising the first switching element, the first coil, the second diode, and the third switching element, The first step-up / step-down chopper section charges the first capacitor, The second step-up / step-down chopper section can be configured to charge the second capacitor.

[0015] In the aforementioned power converter, The control unit can be configured to alternately charge the first capacitor by the first buck-boost chopper unit and charge the second capacitor by the second buck-boost chopper unit for each cycle of the switching period of the first switching element.

[0016] In the aforementioned power converter, The input voltage to the DC / DC converter can be configured to include the generated voltage of the solar cell. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a power conversion device that can eliminate the voltage imbalance between the lines of a single-phase three-wire system and can simplify the DC / AC inverter.

Brief Description of the Drawings

[0018] [Figure 1] It is a diagram showing a power conversion device according to an embodiment of the present invention. [Figure 2] It is a diagram showing the flow of current during the charging of the second capacitor in the DC / DC converter of the present invention, where (A) is a diagram when the first switching element is on, and (B) is a diagram when the first switching element is off. [Figure 3] It is a diagram showing the flow of current during the charging of the first capacitor in the DC / DC converter of the present invention, where (A) is a diagram when the first switching element is on, and (B) is a diagram when the first switching element is off. [Figure 4] It is a diagram showing a conventional power conversion device. [Figure 5] It is a diagram showing a conventional DC / AC inverter provided with a neutral line voltage adjustment circuit.

Embodiments for Carrying Out the Invention

[0019] Hereinafter, embodiments of the power conversion device according to the present invention will be described with reference to the accompanying drawings.

[0020] FIG. 1 shows a power conversion device 1 according to an embodiment of the present invention. The power conversion device 1 includes a DC / DC converter 2, a DC / AC inverter 3, a control unit 4, and terminals T1 to T5.

[0021] The power converter 1 converts the DC input voltage (the voltage generated by the solar cell PV) connected to terminals T1 and T2 into AC voltage and outputs it to single-phase three-wire voltage lines (first voltage line, neutral line, and second voltage line) connected to terminals T3 to T5. Terminal T1 is connected to the positive terminal of the solar cell PV, and terminal T2 is connected to the negative terminal of the solar cell PV. Terminal T3 is connected to the first voltage line, terminal T4 is connected to the neutral line, and terminal T5 is connected to the second voltage line.

[0022] The DC / DC converter 2 comprises a capacitor C3, switching elements S1 to S3 (FETs in this embodiment), a choke coil L1, and diodes D1 and D2, forming a buck-boost circuit that performs boost and buck operations.

[0023] Switching element S1 (corresponding to the "first switching element" of the present invention), choke coil L1 (corresponding to the "first coil" of the present invention), diode D1 (corresponding to the "first diode" of the present invention), and switching element S2 (corresponding to the "second switching element" of the present invention) constitute the first inverting chopper (corresponding to the "first boost-buck chopper section" of the present invention). On the other hand, switching element S1, choke coil L1, diode D2 (corresponding to the "second diode" of the present invention), and switching element S3 (corresponding to the "third switching element" of the present invention) constitute the second inverting chopper (corresponding to the "second boost-buck chopper section" of the present invention).

[0024] The DC / AC inverter 3 comprises a voltage divider circuit consisting of a first capacitor C1 and a second capacitor C2 with equal capacitance connected in series, an inverter circuit including switching elements S4 to S7 (e.g., IGBTs: insulated-gate bipolar transistors), and a filter circuit including choke coils L2 and L3 and capacitors C4 and C5.

[0025] Furthermore, semiconductor switches such as MOSFETs (metal-oxide-semiconductor field-effect transistors), such as SiC (silicon carbide)-MOSFETs and GaN (gallium nitride)-MOSFETs, can be used as switching elements S4 to S7. The same applies to switching elements S1 to S3 of the DC / DC converter 2.

[0026] In the DC / DC converter 2, capacitor C3 has its positive terminal connected to terminal T1 and its negative terminal connected to terminal T2. Switching element S1 has one end of its current path (drain) connected to the positive terminal of capacitor C3, and the other end of its current path (source) connected to the negative terminal of capacitor C3 via choke coil L1.

[0027] Diode D1 has its anode connected to the connection point between the choke coil L1 and the negative terminal of capacitor C3, and its cathode connected to the positive terminal of the first capacitor C1. Diode D2 has its anode connected to the anode of diode D1, and its cathode connected to the connection point X1 between the first capacitor C1 and the second capacitor C2.

[0028] Switching element S2 has one end of its current path (source) connected to the connection point between the other end of the current path (source) of switching element S1 and the choke coil L1, and the other end of its current path (drain) connected to the cathode of diode D2. Switching element S3 has one end of its current path (source) connected to the connection point between the other end of the current path (source) of switching element S1 and the choke coil L1, and the other end of its current path (drain) connected to the negative terminal of the second capacitor C2.

[0029] In the DC / AC inverter 3, the connection point X1 of the first capacitor C1 and the second capacitor C2 is connected to terminal T4, which is the neutral wire of a single-phase three-wire system. The positive terminal of the first capacitor C1 is connected to one end of the first leg, which is composed of switching elements S4 and S5, and to one end of the second leg, which is composed of switching elements S6 and S7. The negative terminal of the second capacitor C2 is connected to the other end of the first leg and to the other end of the second leg. The connection point X2 of the switching elements S4 and S5 that constitute the first leg is connected to terminal T3 via a choke coil L2, and the connection point X3 of the switching elements S6 and S7 that constitute the second leg is connected to terminal T5 via a choke coil L3.

[0030] The control unit 4 includes drive circuits for each of the switching elements S1 to S7 for turning them on and off, and a control circuit for sending control signals to each drive circuit. The control unit 4 may be composed of analog circuits, digital circuits such as a microcontroller or DSP, or a circuit that combines analog and digital circuits.

[0031] The control unit 4 controls the boost and buck operations of the DC / DC converter 2 so that the voltage V1 across the terminals of the first capacitor C1 and the voltage V2 across the terminals of the second capacitor C2 become equal. Figure 2 shows the current flow in the DC / DC converter 2 when the second capacitor C2 is being charged, and Figure 3 shows the current flow in the DC / DC converter 2 when the first capacitor C1 is being charged.

[0032] As shown in Figure 2(A), when switching element S1 is ON, switching element S2 is OFF, and switching element S3 is OFF, current flows from the solar cell PV through switching element S1 to the choke coil L1, and energy is stored in the choke coil L1. In the state shown in Figure 2(A), the discharge current of the first capacitor C1 and the discharge current of the second capacitor C2 flow to the inverter circuit side.

[0033] As shown in Figure 2(B), when switching element S1 is turned off, switching element S2 is turned off, and switching element S3 is turned on from the state in Figure 2(A), the energy stored in the choke coil L1 is released, and current flows through the path choke coil L1 → diode D2 → second capacitor C2 → switching element S3 → choke coil L1. In other words, the second inverting chopper, which consists of switching element S1, choke coil L1, diode D2, and switching element S3, charges the second capacitor C2.

[0034] The control unit 4 controls the on-duty cycle of the switching element S1 so that the voltage V2 across the second capacitor C2 approaches a predetermined target voltage. For example, if voltage V2 < target voltage, the control unit 4 increases the on-duty cycle to more than 0.5 to boost the voltage of the second inverting chopper, while if voltage V2 > target voltage, it decreases the on-duty cycle to less than 0.5 to buck the voltage of the second inverting chopper.

[0035] As shown in Figure 3(A), when switching element S1 is ON, switching element S2 is OFF, and switching element S3 is OFF (the same state as in Figure 2(A)), current flows from the solar cell PV through switching element S1 to the choke coil L1, and energy is stored in the choke coil L1. In the state shown in Figure 3(A), the discharge current of the first capacitor C1 and the discharge current of the second capacitor C2 flow to the inverter circuit side.

[0036] As shown in Figure 3(B), when the switching element S1 is turned off, the switching element S2 is turned on, and the switching element S3 is turned off from the state in Figure 3(A), the energy stored in the choke coil L1 is released, and current flows through the path choke coil L1 → diode D1 → first capacitor C1 → switching element S2 → choke coil L1. In other words, the first inverting chopper, which consists of switching element S1, choke coil L1, diode D1, and switching element S2, charges the first capacitor C1.

[0037] The control unit 4 controls the on-duty cycle of the switching element S1 so that the voltage V1 across the first capacitor C1 approaches the target voltage. For example, if voltage V1 < target voltage, the control unit 4 increases the on-duty cycle to more than 0.5 to boost the voltage of the first inverting chopper, while if voltage V1 > target voltage, it decreases the on-duty cycle to less than 0.5 to buck the voltage of the first inverting chopper.

[0038] The control unit 4 repeats the charging control shown in Figures 2 and 3. For example, for each cycle of the switching period of the switching element S1, the control unit 4 alternately performs charging control of the second capacitor C2 by the second inverting chopper (see Figures 2(A) and (B)) and charging control of the first capacitor C1 by the first inverting chopper (see Figures 3(A) and (B)).

[0039] According to the above control, even if a voltage imbalance (unbalanced load) occurs between the first voltage line and the neutral line and between the second voltage line and the neutral line of a single-phase three-wire system, the voltage V1 of the first capacitor C1 and the voltage V2 of the second capacitor C2 can be made equal by controlling the on-duty ratio of the switching element S1 when the first capacitor C1 is being charged and the on-duty ratio of the switching element S1 when the second capacitor C2 is being charged. Therefore, the power converter 1 according to this embodiment makes it possible to eliminate the voltage imbalance between each line of a single-phase three-wire system.

[0040] Furthermore, in the power conversion device 1 according to this embodiment, the neutral line voltage adjustment circuit found in conventional DC / AC inverters (for example, the DC / AC inverter described in Patent Document 2) is unnecessary, making it possible to simplify the DC / AC inverter 3.

[0041] Conventional power converters for solar cell PV systems use a boost chopper as the DC / DC converter, which only performs boosting operations. When the boost chopper alternately charges the first capacitor C1 and the second capacitor C2, the input voltage range from the solar cell PV is generally 50 to 450[V]. Therefore, the voltage across the voltage divider circuit consisting of the first capacitor C1 and the second capacitor C2 connected in series (V1 + V2) can reach a maximum of 900[V], which is twice 450[V]. In this case, the voltage becomes unnecessarily high compared to the voltage required by the DC / AC inverter 3, resulting in reduced power conversion efficiency or the need for components with higher voltage ratings in the DC / AC inverter 3.

[0042] In contrast, the power converter 1 according to this embodiment uses an inverting chopper that performs boost and buck operations as the DC / DC converter 2, so that an input voltage of 50 to 450 [V] can be boosted or bucked to a voltage (target voltage) instructed by the control unit 4. As a result, the DC / DC converter 2 can output the optimal voltage required by the DC / AC inverter 3, thus avoiding deterioration of power conversion efficiency or the need for components with high voltage tolerance in the DC / AC inverter 3.

[0043] Although embodiments of the power conversion device according to the present invention have been described above, the present invention is not limited to the above embodiments.

[0044] The power conversion device according to the present invention comprises a DC / DC converter to which a DC input voltage is input, a DC / AC inverter that converts the output voltage output by the DC / DC converter into an AC voltage and outputs it to a single-phase three-wire voltage line, and a control unit. The DC / AC inverter consists of a first capacitor and a second capacitor connected in series, and comprises a voltage divider circuit that divides the output voltage of the DC / DC converter, and an inverter circuit that converts the voltage divided by the voltage divider circuit into an AC voltage. The DC / DC converter includes a buck-boost circuit that performs boost and buck operations, and the control unit can be configured as appropriate, as long as it controls the buck-boost circuit so that the voltage of the first capacitor and the voltage of the second capacitor are equal.

[0045] For example, in the above embodiment, the control unit 4 alternately performs charge control of the first capacitor C1 and charge control of the second capacitor C2 for each cycle of the switching period of the switching element S1, but the interval of one cycle may be changed to multiple cycles.

[0046] In the above embodiment, the output voltage (generation voltage) of the solar cell PV is input to the DC / DC converter 2, but the DC output voltage output from a power generation device other than the solar cell PV or a DC power supply may also be input to the DC / DC converter 2. [Explanation of Symbols]

[0047] 1. Power converter 2 DC / DC converters 3 DC / AC Inverters 4. Control Unit

Claims

1. A DC / DC converter that receives a DC input voltage, A DC / AC inverter converts the output voltage output by the aforementioned DC / DC converter into an AC voltage and outputs it to a single-phase three-wire voltage line, Control unit and A power conversion device comprising, The aforementioned DC / AC inverter is A voltage divider circuit consisting of a first capacitor and a second capacitor connected in series, which divides the output voltage of the DC / DC converter, An inverter circuit that converts the voltage divided by the voltage divider circuit into the AC voltage, Equipped with, The DC / DC converter includes a buck-boost circuit that performs boost and buck operations. The control unit controls the step-up / step-down circuit so that the voltage of the first capacitor and the voltage of the second capacitor are equal, and so that the first capacitor and the second capacitor are charged alternately. A power conversion device characterized by the following features.

2. The aforementioned step-up / step-down circuit is A first step-up / step-down chopper section comprising a first switching element, a first coil, a first diode, and a second switching element, It includes a second step-up / step-down chopper section comprising the first switching element, the first coil, the second diode, and the third switching element, The first step-up / step-down chopper section charges the first capacitor, The second step-up / step-down chopper section charges the second capacitor. The power conversion device according to feature 1.

3. The control unit causes the first capacitor to be charged by the first buck-boost chopper unit and the second capacitor to be charged by the second buck-boost chopper unit alternately for each cycle of the switching period of the first switching element. The power conversion device according to feature 2.

4. The input voltage input to the DC / DC converter includes the generated voltage of the solar cell. A power conversion device according to any one of claims 1 to 3.

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