Power Conversion Device

Abstract

To simplify control software.SOLUTION: A power conversion device includes: a plurality of conversion cells each having an insulation type bidirectional DC / DC converter, a DC link, and an inverter connected to the converter via the DC link, and being connected in series via an AC output end of the inverter; and a control device for controlling the plurality of conversion cells. The control device controls the inverter so that when voltage of a system interconnected via the AC output end is normal, a DC voltage of a bus to which a DC output end connected to the DC link via the converter is connected becomes a predetermined value, and controls the converter so that a DC voltage of the DC link becomes a predetermined value when the voltage of the system is normal and when the voltage of the system is abnormal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a power conversion device. [Background technology]

[0002] Conventionally, an SST (Solid State Transformer) has been known as one type of power conversion device. The SST includes multiple cells, each of which has an isolated bidirectional DC / DC converter equipped with a high-frequency transformer and an inverter connected to one of the DC sides of the DC / DC converter (see, for example, Non-Patent Document 1). The DC output terminals of each cell are connected in parallel to a DC bus. The SST can be connected to a high-voltage grid without a step-up transformer by connecting the AC output terminals of each cell in series. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Voltage and Power Balance Control for a Cascaded H-Bridge Converter-Based Solid-State Transformer (IEEE Transactions on Power Electronics, Vol.28, No.4, pp.1523-1532) Summary of the Invention [Problem to be solved by the invention]

[0004] In the conventional control method for SSTs, in grid-connected operation, the inverter of each cell controls the DC voltage between the inverter and the isolated DC / DC converter (cell DC intermediate voltage), and the isolated DC / DC converter of each cell controls the DC output voltage (DC bus voltage).On the other hand, in stand-alone operation, the inverter of each cell controls the grid voltage on the AC output terminal side of each cell, and the isolated DC / DC converter of each cell controls the cell DC intermediate voltage.

[0005] In this way, in the conventional control method, the voltages controlled by the inverter and the isolated DC / DC converter are switched between grid-connected operation and stand-alone operation. As a result, the control software becomes bloated as the number of control switches increases, which may result in an increase in the cost of the control device, for example.

[0006] The present disclosure provides a power conversion device that can simplify control software. [Means for solving the problem]

[0007] In one aspect of the present disclosure, a plurality of conversion cells each having an isolated bidirectional DC / DC converter, a DC link, and an inverter connected to the converter via the DC link, the conversion cells being connected in series via AC output terminals of the inverter; a control device that controls the plurality of conversion cells, The control device controls the inverter so that the DC voltage of a bus to which the DC output terminal connected to the DC link via the converter is connected becomes a predetermined value when the voltage of the system interconnected via the AC output terminal is normal, and controls the converter so that the DC voltage of the DC link becomes a predetermined value when the voltage of the system is normal and when the voltage of the system is abnormal. [Effects of the Invention]

[0008] According to one aspect of the present disclosure, control software can be simplified. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a configuration example of a power conversion device according to an embodiment; [Figure 2] 1 is a diagram illustrating a control block common to an inverter control unit and a converter control unit in a control device for a power conversion device according to an embodiment; [Figure 3]FIG. 2 is a control block diagram illustrating a control method of an inverter control unit. [Figure 4] FIG. 3 is a control block diagram illustrating a first control method of a converter control unit. [Figure 5] FIG. 4 is a control block diagram illustrating a second control method of the converter control unit. DETAILED DESCRIPTION OF THE INVENTION

[0010] The following describes the embodiments. Note that "DC" and "AC" are abbreviations for "Direct Current" and "Alternative Current", respectively.

[0011] Fig. 1 is a diagram showing an example of the configuration of a power conversion device according to one embodiment. The power conversion device 1 shown in Fig. 1 is an SST having three series-connected conversion cells for each phase. Fig. 1 illustrates a case in which the SST is applied to a multi-source self-consumption renewable energy system equipped with a solar power generation device and a storage battery. The solar power generation device performs photovoltaic power generation (PV) using solar cells that convert light energy into electrical energy.

[0012] The solar power generation device is connected to a DC bus 13 via a DC / DC converter 11. The storage battery is connected to the DC bus 13 via a DC / DC converter 12. The DC bus 13 is connected to a DC output terminal of a power conversion device 1. The power conversion device 1 converts the DC of the DC bus 13 into AC and outputs it. The AC output terminal of the power conversion device 1 is connected to a grid 15 via an electric wire 14. An important load (not shown) that receives power supply from the electric wire 14 is connected to the electric wire 14.

[0013] One advantage of this self-consumption system is that it can function as an emergency power supply. When the grid 15 is normal, DC power generated by the PV is supplied to the power conversion device 1 via the DC / DC converter 11 and the DC bus 13, converted into AC power by the power conversion device 1, and output to the grid 15. On the other hand, when an abnormality occurs in the grid 15, such as a power outage, the grid 15 is disconnected by a switch (not shown), and the storage battery is utilized as an emergency power supply. The DC power output from the storage battery functioning as an emergency power supply is supplied to the power conversion device 1 via the DC / DC converter 12 and the DC bus 13, converted into AC power by the power conversion device 1, and supplied to important loads (not shown) connected to the power lines 14.

[0014] An operation mode in which the power conversion device 1 is connected to the grid 15 is called grid-connected operation, and an operation mode in which the power conversion device 1 operates as an emergency power source is called isolated operation.

[0015] The power conversion device 1 shown in FIG. 1 is an SST that bidirectionally converts DC on the DC bus 13 side to AC on the grid 15 side, or AC on the grid 15 side to DC on the DC bus 13 side. The power conversion device 1 includes three U-phase converter cells U1, U2, and U3, three V-phase converter cells V1, V2, and V3, and three W-phase converter cells W1, W2, and W3, and a control device 206 that controls the power conversion operations of these converter cells. The number of converter cells for each phase is not limited to three and may be one, two, or more. Since the converter cells for each phase have the same configuration, the following description will be given of the U-phase converter cell as a representative.

[0016] The multiple conversion cells U1, U2, and U3 are cell converters that bidirectionally convert DC on the DC bus 13 side to AC on the grid 15 side, or convert AC on the grid 15 side to DC on the DC bus 13 side. Each of the multiple conversion cells U1, U2, and U3 has DC output terminals r and s, an isolated bidirectional DC / DC converter 200, a DC link 300, an inverter 400, and AC output terminals p and q. Hereinafter, the isolated bidirectional DC / DC converter will also be simply referred to as a converter.

[0017] The DC output terminals r and s are, for example, output terminals connected to one DC output side (DC bus 13 side) of the converter 200. The high-potential side DC output terminal r is connected to a first bus P on the high-potential side of the DC bus 13, and the low-potential side DC output terminal s is connected to a second bus N on the low-potential side of the DC bus 13. The DC terminals r and s are connected to DC output terminals of a primary-side circuit 210a described below.

[0018] The AC output terminals p and q are output terminals connected to the other DC output side of the converter 200 (the side of the system 15), for example.

[0019] The multiple converter cells U1, U2, and U3 each have AC output terminals p and q, and are connected in series via the AC output terminals p and q. Each of the multiple converter cells U1, U2, and U3 has its first AC output terminal p connected to the second AC output terminal q of an adjacent converter cell, and its second AC output terminal q connected to the first AC output terminal p of the other adjacent converter cell. Of the multiple converter cells connected in series via the AC output terminals p and q, the first AC output terminal p of the converter cell located on the highest potential side (in this example, converter cell U1) is connected to the U-phase electric wire 14. On the other hand, of the multiple cells connected in series via the AC output terminals p and q, the second AC output terminal q of the converter cell located on the lowest potential side (in this example, converter cell U3) is connected to the neutral point 16.

[0020] The converter 200 boosts or lowers a DC voltage input from the DC bus 13 shared by the multiple conversion cells U1, U2, and U3, and outputs a predetermined DC voltage to the DC link 300. Alternatively, the converter 200 boosts or lowers the DC voltage input from the DC link 300, and outputs the predetermined DC voltage to the DC bus 13. The converter 200 includes, for example, a transformer 202, a primary side circuit 210a, and a secondary side circuit 210b. The primary side circuit 210a and the secondary side circuit 210b are magnetically coupled by the transformer 202.

[0021] For example, converter 200 is a DAB (Dual Active Bridge) converter having a primary-side full-bridge circuit provided on the primary side of transformer 202 and a secondary-side full-bridge circuit provided on the secondary side of transformer 202. The DAB converter transmits power between the primary side and the secondary side by applying a voltage to the leakage inductance of transformer 202 or to an external reactor connected in series to transformer 202. The transmitted power is controlled by the phase difference between an output voltage V1 output from two intermediate connection points of the primary-side inverter circuit (primary-side full-bridge circuit) and an output voltage V2 output from two intermediate connection points of the secondary-side inverter circuit (secondary-side full-bridge circuit).

[0022] The transformer 202 has a primary coil and a secondary coil that are magnetically coupled. The primary circuit 210a has a primary full-bridge circuit and a primary drive circuit. The secondary circuit 210b has a secondary full-bridge circuit and a secondary drive circuit.

[0023] The primary-side full-bridge circuit includes a primary-side first half-bridge circuit in which a primary-side first upper arm and a primary-side first lower arm are connected in series, and a primary-side second half-bridge circuit in which a primary-side second upper arm and a primary-side second lower arm are connected in series. The primary coil of the transformer 202 is connected between an intermediate connection point between the primary-side first upper arm and the primary-side first lower arm and an intermediate connection point between the primary-side second upper arm and the primary-side second lower arm.

[0024] The secondary full-bridge circuit includes a first secondary half-bridge circuit in which a first secondary upper arm and a first secondary lower arm are connected in series, and a second secondary half-bridge circuit in which a second secondary upper arm and a second secondary lower arm are connected in series. The secondary coil of the transformer 202 is connected between an intermediate connection point between the first secondary upper arm and the first secondary lower arm and an intermediate connection point between the second secondary upper arm and the second secondary lower arm.

[0025] A plurality of primary-side switch elements such as a first primary-side upper arm, a first primary-side lower arm, a second primary-side upper arm, and a second primary-side lower arm are driven by a primary-side drive circuit. A plurality of secondary-side switch elements such as a first secondary-side upper arm, a first secondary-side lower arm, a second secondary-side upper arm, and a second secondary-side lower arm are driven by a secondary-side drive circuit.

[0026] Specific examples of the primary side switch element and the secondary side switch element include semiconductor switching elements such as a metal oxide semiconductor field effect transistor (MOSFET) and an insulated gate bipolar transistor (IGBT).

[0027] The DC link 300 is connected to the DC output terminal of the secondary circuit 210b. The DC link 300 includes a DC capacitor 301.

[0028] The inverter 400 is a circuit connected to the converter 200 via the DC link 300. The inverter 400 converts the DC of the DC link 300 into AC and outputs it to the AC output terminals p and q, or converts the AC input from the AC output terminals p and q into DC and outputs it to the DC link 300.

[0029] The control device 206 controls a plurality of converter cells for each phase. The control device 206 has, for example, a memory and a processor (e.g., a CPU (Central Processing Unit)), and each function of the control device 206 is realized by the processor operating according to a program stored in the memory. Each function of the control device 206 may be realized by an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0030] Fig. 2 is a diagram illustrating a control block common to the inverter control unit and the converter control unit in a control device for a power conversion device of one embodiment. Fig. 3 is a control block diagram illustrating a control method for the inverter control unit. Fig. 4 is a control block diagram illustrating a first control method for the converter control unit. The control device 206 has an inverter control unit that controls the inverter 400 and a converter control unit that controls the converter 200.

[0031] The inverter control and converter control for each phase are performed using the same control method, so the control method for the inverter 400 and converter 200 of the converter cell U1 will be described below as a representative example.

[0032] In FIG. 2, the inverter control unit detects the U-phase system voltage Vu and the W-phase system voltage Vw, and performs DQ conversion on these detected values ​​using a DQ converter VD to generate a d-axis system voltage detection value Vd and a q-axis system voltage detection value Vq.

[0033] Next, we will explain the inverter control shown in Fig. 3. The inverter control differs in control content between grid-connected operation and isolated operation, and the inverter control unit switches the control method according to command information that switches between grid-connected operation and isolated operation.

[0034] First, inverter control during grid-connected operation will be described. During grid-connected operation, the inverter control unit controls the inverter 400 so that the DC voltage (DC bus voltage) of the DC bus 13 becomes a predetermined value. The inverter control unit performs PI adjustment calculations using a PI adjuster to calculate the difference between the voltage command of the DC bus 13 and the voltage detection value Vbus of the DC bus 13, and generates two-phase dq-axis current components Id and Iq. The inverter control unit converts the two-phase dq-axis current components Id and Iq into the DQ inverse converter VD -1 and generates the U-phase output current command Iuref. The DQ inverse converter is a computing unit that performs two-phase to three-phase conversion on two inputs.

[0035] The inverter control unit generates a U-phase output voltage command Vuref by adding the detected value of the grid voltage Vu to the result of proportional control performed by a proportional controller P on the difference between a U-phase output current command Iuref and a U-phase output current detection value Iu. The inverter control unit performs gate command calculation on the U-phase output voltage command Vuref to generate a gate signal for driving the inverter 400, thereby operating the inverter 400. Note that the DC / DC converter 11 controls the output voltage of the PV, and the DC / DC converter 12 controls the output voltage of the storage battery.

[0036] Next, inverter control during stand-alone operation will be described. During stand-alone operation, the inverter control unit controls the inverter 400 so that the U-phase system voltage becomes a predetermined value. The inverter control unit calculates a first value by adding the result of a PI adjustment calculation of the difference between the d-axis system voltage command and the d-axis system voltage detection value Vd to the U-phase system voltage command. The inverter control unit calculates a second value by a PI adjustment calculation of the difference between the q-axis system voltage command (=0) and the q-axis system voltage detection value Vq. The inverter control unit performs an inverse DQ transform on the first value and the second value to generate a U-phase output voltage command Vuref. The inverter control unit generates a gate command for driving the inverter 400 by performing a calculation to convert the U-phase output voltage command Vuref into a gate command, thereby operating the inverter 400.

[0037] Next, we will explain converter control shown in Fig. 4. Unlike inverter control, converter control has the same control content for grid-connected operation and stand-alone operation.

[0038] The converter control unit controls the converter 200 so that the DC voltage (DC intermediate voltage Edc) of the DC link 300 becomes a predetermined value, whether in grid-connected operation or stand-alone operation. The converter control unit performs a PI adjustment calculation using a PI adjuster on the deviation between a command for the DC intermediate voltage Edc and a detected value EdcU1 of the DC intermediate voltage Edc, thereby generating a command for the DC current Idc of the converter 200. The converter control unit performs a PI adjustment calculation using a PI adjuster on the difference between this DC current command and the detected value IdcU1 of the DC current Idc, thereby deriving a phase difference between the output voltages on the primary side (DC bus side) and secondary side (AC side) of the converter 200. The converter control unit performs a calculation to convert this phase difference into a gate command, thereby generating a gate command for driving the converter 200 and operating the converter 200.

[0039] In this way, when the voltage of the system 15 interconnected via the AC output terminals p and q is normal, the control device 206 controls the inverter 400 so that the DC voltage (DC bus voltage Vbus) of the DC bus 13 to which the DC output terminals r and s connected to the DC link 300 via the converter 200 are connected becomes a predetermined value. When the voltage of the system 15 is normal and when the voltage of the system 15 is abnormal, the control device 206 controls the converter 200 so that the DC voltage (DC intermediate voltage Edc) of the DC link 300 becomes a predetermined value.

[0040] In this way, in this embodiment, the voltage of the control target of the converter 200 is the same in both grid-connected operation and isolated operation, so that the control software can be simplified.

[0041] Fig. 5 is a diagram illustrating a second control method of the converter control unit. The second control method shown in Fig. 5 is a control method in which feedforward control of a DC intermediate voltage is added to the first control method shown in Fig. 4. This feedforward control unit multiplies the output voltage command Vuref and the output current command Iuref of the inverter 400 to calculate the AC output power of the converter cell U1, divides the calculated value of this AC output power by the DC intermediate voltage EdcU1 to calculate the DC current of the converter 200, and adds the calculated value of this DC current to the DC current command of the converter 200.

[0042] In this way, the control device 206 controls the converter 200 based on voltage information and current information at the AC output terminals p and q of the inverter 400 to compensate for fluctuations in the DC intermediate voltage Edc and suppress the DC intermediate voltage Edc. This makes it possible to improve the control response of the DC intermediate voltage Edc. Furthermore, suppressing fluctuations in the DC intermediate voltage Edc makes it possible to reduce the capacitance of the DC capacitor 301. Reducing the capacitance makes it possible to reduce the size of the DC capacitor 301, and ultimately to reduce the size of the power conversion device 1.

[0043] Although the embodiments have been described above, the technology of the present disclosure is not limited to the above-described embodiments, and various modifications and improvements are possible, such as combinations with or substitutions for part or all of other embodiments. [Explanation of symbols]

[0044] 1 Power conversion device 11,12 DC / DC converter 13 DC bus 14 Electric wire 15 strains 200 Isolated Bidirectional DC / DC Converter 202 Trans 206 Control device 300 DC Link 400 inverter p,q AC output terminal r,s DC output end U1,U2,U3,V1,V2,V3,W1,W2,W3 conversion cells

Claims

1. a plurality of conversion cells each having an isolated bidirectional DC / DC converter, a DC link, and an inverter connected to the converter via the DC link, the conversion cells being connected in series via AC output terminals of the inverter; a control device that controls the plurality of conversion cells, The control device controls the inverter so that the DC voltage of a bus to which the DC output terminal connected to the DC link via the converter is connected becomes a predetermined value when the voltage of the system interconnected via the AC output terminal is normal, and controls the converter so that the DC voltage of the DC link becomes a predetermined value when the voltage of the system is normal and when the voltage of the system is abnormal.

2. 2. The power conversion device according to claim 1, wherein when a voltage of the system is abnormal, the control device disconnects the system connected to the AC output terminal via an electric wire and controls the inverter so that the voltage of the electric wire becomes a predetermined value.

3. 3. The power conversion device according to claim 1, wherein the control device controls the converter so as to compensate for fluctuations in the DC voltage of the DC link and suppress the DC voltage of the DC link, based on voltage information and current information at the AC output terminal of the inverter.

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