JP2023084640A - Power conversion device - Google Patents

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

The present disclosure relates to power converters.

2. Description of the Related Art Conventionally, an SST (Solid State Transformer) is known as one of power converters. The SST includes a plurality of cells each having an isolated bidirectional DC/DC converter equipped with a high-frequency transformer and an inverter connected to one DC side of the DC/DC converter (see, for example, Non-Patent Document 1. ). The DC output of each cell is connected in parallel to the DC bus. The SST can be connected to a high-voltage system without a step-up transformer by connecting the AC output terminals of each cell in series.

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)

In the conventional control method of SST, in the case of grid-connected operation, the inverter of each cell controls the DC voltage (cell DC intermediate voltage) between the inverter and the isolated DC/DC converter, and the isolated DC of each cell The /DC converter controls the voltage of the DC output (DC bus voltage). On the other hand, in the case of self-sustaining operation, the inverter of each cell controls the voltage of the system on the AC output terminal side of each cell, and the isolated DC/DC converter of each cell controls the cell DC intermediate voltage.

Thus, in the conventional control method, the voltages to be controlled by the inverter and the insulated DC/DC converter are switched between grid-connected operation and isolated operation. Therefore, with an increase in control switching, the control software becomes bloated, and as a result, for example, there is a risk of an increase in the cost of the control device.

The present disclosure provides a power converter capable of simplifying control software.

In one aspect of the present disclosure,
a plurality of converters each having an isolated bi-directional DC/DC converter, a DC link, and an inverter connected to said converter via said DC link, connected in series via AC outputs of said inverters; a cell;
a control device that controls the plurality of conversion cells,
When the voltage of the system interconnected via the AC output terminal is normal, the control device increases the DC voltage of the bus connected to the DC output terminal connected to the DC link via the converter to a predetermined value. and controlling 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. An apparatus is provided.

According to one aspect of the present disclosure, control software can be simplified.

It is a figure which shows the structural example of the power converter device of one Embodiment. FIG. 3 is a diagram illustrating control blocks common to an inverter control unit and a converter control unit in a control device for a power conversion device according to one embodiment; 4 is a control block diagram illustrating a control method of an inverter control unit; FIG. FIG. 3 is a control block diagram illustrating a first control method of a converter control section; FIG. 5 is a control block diagram illustrating a second control method of the converter control section;

Embodiments will be described below. "DC" and "AC" are abbreviations of "Direct Current" and "Alternative Current", respectively.

FIG. 1 is a diagram illustrating a configuration example of a power converter according to one embodiment. A power conversion device 1 shown in FIG. 1 is an SST having three series conversion cells for each phase. FIG. 1 illustrates a case where SST is applied to a multi-source self-consumption type renewable energy system in which a photovoltaic power generation device and a storage battery are installed side by side. A photovoltaic power generation device performs photovoltaic power generation (PV) using a solar cell that converts light energy into electrical energy.

The photovoltaic power generation device is connected to a DC bus 13 via a DC/DC converter 11 . The storage battery is connected to a DC bus 13 via a DC/DC converter 12 . The DC bus 13 is connected to the DC output terminal of the power converter 1 . The power converter 1 converts the direct current of the direct current bus 13 into alternating current and outputs the alternating current. An AC output end 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 from the wire 14 is connected to the wire 14 .

As an advantage of this self-consumption type system, there is a function as an emergency power supply. When the grid 15 is normal, the DC power generated by the PV is supplied to the power converter 1 via the DC/DC converter 11 and the DC bus 13, converted into AC power by the power converter 1, and output to the grid 15. be done. On the other hand, in the event of an abnormality in the system 15 such as a power outage, the storage battery is used as an emergency power supply after the system 15 is disconnected by a switch (not shown). The DC power output from the storage battery that functions 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 the electric wire 14. It is supplied to a connected important load (not shown).

An operation mode in which the power converter 1 is connected to the grid 15 is called interconnected operation, and an operation mode in which the power converter 1 operates as an emergency power supply is called isolated operation.

The power converter 1 shown in FIG. 1 is an SST that bi-directionally converts direct current on the DC bus 13 side to alternating current on the grid 15 side, or converts alternating current on the grid 15 side to direct current on the DC bus 13 side. The power converter 1 includes three U-phase conversion cells U1, U2, and U3, three V-phase conversion cells V1, V2, and V3, three W-phase conversion cells W1, W2, and W3, and these a controller 206 that controls the power conversion operation of each of the conversion cells. The number of conversion cells for each phase is not limited to three, and may be one or two or more. Since the conversion cells for each phase have the same configuration, the U-phase conversion cell will be described below as a representative.

Each of the plurality of conversion cells U1, U2, and U3 is a cell converter that bi-directionally converts direct current on the DC bus 13 side to alternating current on the grid 15 side or converts alternating current on the grid 15 side to direct current on the DC bus 13 side. is. A plurality of conversion cells U1, U2, U3 respectively have DC output terminals r, s, an isolated bidirectional DC/DC converter 200, a DC link 300, an inverter 400, and AC output terminals p, q. Hereinafter, the isolated bidirectional DC/DC converter is also simply referred to as a converter.

The DC output terminals r and s are, for example, output terminals connected to one DC output side (the DC bus 13 side) of the converter 200 . The DC output terminal r on the high potential side is connected to the first bus P on the high potential side of the DC bus 13, and the DC output terminal s on the low potential side is connected to the second bus N on the low potential side of the DC bus 13. be done. DC terminals r and s are connected to a DC output end of a primary circuit 210a, which will be described later.

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

A plurality of conversion cells U1, U2, U3 have AC output ends p, q, respectively, and are connected in series via the AC output ends p, q. Each of the plurality of conversion cells U1, U2, and U3 has its own first AC output terminal p connected to the second AC output terminal q of one adjacent conversion cell, and its own second AC output terminal q is connected to It is connected to the first AC output terminal p of the other adjacent conversion cell. Among the plurality of conversion cells connected in series via AC output terminals p and q, the conversion cell located on the highest potential side (conversion cell U1 in this example) has a first AC output terminal p of the U phase. is connected to the electric wire 14 of the On the other hand, among the plurality of cells connected in series via the AC output terminals p and q, the second AC output terminal q of the conversion cell on the lowest potential side (conversion cell U3 in this example) is the neutral point. 16.

Converter 200 steps up or steps down a DC voltage input from DC bus 13 common to a plurality of conversion cells U1, U2, and U3, and outputs a predetermined DC voltage to DC link 300. FIG. Alternatively, converter 200 steps up or steps down the DC voltage input from DC link 300 and outputs a predetermined DC voltage to DC bus 13 . The converter 200 includes, for example, a transformer 202, a primary circuit 210a, and a secondary circuit 210b. The primary circuit 210 a and the secondary circuit 210 b are magnetically coupled by the transformer 202 .

For example, the converter 200 is a DAB (Dual Active Bridge) converter having a primary side full bridge circuit provided on the primary side of the transformer 202 and a secondary side full bridge circuit provided on the secondary side of the transformer 202. be. The DAB converter transmits power between the primary side and the secondary side by applying a voltage to the leakage inductance of the transformer 202 or to an external reactor connected in series with the transformer 202 . The transmitted power consists of the output voltage _V1 output from the two intermediate connection points of the primary-side inverter circuit (primary-side full-bridge circuit) and the secondary-side inverter circuit (secondary-side full-bridge circuit). is controlled by the phase difference with the output voltage _V2 output from the two intermediate connection points of .

The transformer 202 is a transformer that has a primary coil and a secondary coil, and the primary coil and the secondary coil are magnetically coupled. The primary side circuit 210a has a primary side full bridge circuit and a primary side drive circuit. The secondary side circuit 210b has a secondary side full bridge circuit and a secondary side drive circuit.

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 connected in series with the side second lower arm. The primary side coil of the transformer 202 is connected at an intermediate connection point between the primary side first upper arm and the primary side first lower arm and at an intermediate connection point between the primary side second upper arm and the primary side second lower arm. connected between

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

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

Specific examples of the primary side switch element and the secondary side switch element include semiconductor switching elements such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors).

DC link 300 is connected to the DC output end of secondary circuit 210b. DC link 300 has a DC capacitor 301 .

Inverter 400 is a circuit connected to converter 200 via DC link 300 . The inverter 400 converts the direct current of the direct current link 300 into alternating current and outputs it to the alternating current output terminals p and q, or converts the alternating current input from the alternating current output terminals p and q into direct current and outputs it to the direct current link 300 . .

Controller 206 controls multiple conversion cells for each phase. The control device 206 has, for example, a memory and a processor (eg, 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 FPGA (Field Programmable Gate Array) or ASIC (Application Specific Integrated Circuit).

FIG. 2 is a diagram illustrating control blocks common to the inverter control unit and the converter control unit in the control device for the power converter according to one embodiment. FIG. 3 is a control block diagram illustrating the control method of the inverter control unit. FIG. 4 is a control block diagram illustrating a first control method of the converter control section. Control device 206 has an inverter control unit that controls inverter 400 and a converter control unit that controls converter 200 .

Since inverter control and converter control for each phase are the same control method, the control method for inverter 400 and converter 200 of conversion cell U1 will be described below as a representative.

In FIG. 2, the inverter control unit detects a U-phase system voltage Vu and a W-phase system voltage Vw, DQ-converts the detected values by a DQ converter VD, and converts the d-axis system voltage detection value Vd and the q-axis A system voltage detection value Vq is generated.

Next, the inverter control shown in FIG. 3 will be described. Inverter control differs between grid-connected operation and self-sustained operation, and the inverter control unit switches the control method according to command information for switching between grid-connected operation and self-sustained operation.

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

The inverter control unit adds the detected value of the system voltage Vu to the result of proportionally controlling the difference between the U-phase output current command Iuref and the U-phase output current detection value Iu by the proportional controller P, Generate an output voltage command Vuref. The inverter control unit generates a gate signal for driving the inverter 400 by performing a gate command operation on the U-phase output voltage command Vuref to operate 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.

Next, inverter control during self-sustained operation will be described. In the self-sustained 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 the first value by adding the result of PI adjustment operation 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 performing a PI adjustment operation on the difference between the q-axis system voltage command (=0) and the q-axis system voltage detection value Vq. The inverter control unit performs DQ inverse conversion 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 and operates the inverter 400 by performing a calculation to convert the U-phase output voltage command Vuref into a gate command.

Next, converter control shown in FIG. 4 will be described. Unlike inverter control, converter control has the same control contents for grid-connected operation and isolated operation.

The converter control unit controls converter 200 so that the DC voltage (DC intermediate voltage Edc) of DC link 300 becomes a predetermined value in both grid-connected operation and isolated operation. Converter control unit generates a command for DC current Idc of converter 200 by performing a PI adjustment operation on a deviation between a command for DC intermediate voltage Edc and a detected value EdcU1 of DC intermediate voltage Edc. The converter control unit performs a PI adjustment operation on the difference between the DC current command and the detected value IdcU1 of the DC current Idc using a PI controller to control the primary side (DC bus side) and secondary side (AC side) of the converter 200. to derive the phase difference of the output voltages of The converter control unit generates a gate command for driving converter 200 and operates converter 200 by performing a calculation to convert this phase difference into a gate command.

In this way, when the voltage of the grid 15 interconnected via the AC output terminals p, q is normal, the control device 206 connects the DC output terminals r, s connected to the DC link 300 via the converter 200. The inverter 400 is controlled so that the DC voltage (DC bus voltage Vbus) of the DC bus 13 to be supplied becomes a predetermined value. Control device 206 controls converter 200 so that the DC voltage (DC intermediate voltage Edc) of DC link 300 becomes a predetermined value when the voltage of system 15 is normal and when the voltage of system 15 is abnormal. .

As described above, in the present embodiment, since the voltage to be controlled by the converter 200 is common between the grid-connected operation and the isolated operation, the control software can be simplified.

FIG. 5 is a diagram illustrating a second control method of the converter control section. The second control method shown in FIG. 5 is a control method in which feedforward control of the DC intermediate voltage is added to the first control method shown in FIG. 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 conversion cell U1, and divides the calculated value of the AC output power by the DC intermediate voltage EdcU1. Then, the DC current of converter 200 is calculated, and the calculated value of DC current is added to the DC current command of converter 200 .

In this way, based on the voltage information and current information of the AC output terminals p and q of the inverter 400, the control device 206 compensates for the variation in the DC intermediate voltage Edc and controls the converter so that the DC intermediate voltage Edc is suppressed. 200 control. This makes it possible to enhance the control response of the DC intermediate voltage Edc. Also, by suppressing fluctuations in the DC intermediate voltage Edc, the capacitance of the DC capacitor 301 can be reduced. By reducing the capacitance, the DC capacitor 301 can be miniaturized, and the power conversion device 1 can be miniaturized.

Although the embodiments have been described above, the technology of the present disclosure is not limited to the above embodiments. Various modifications and improvements such as combination or replacement with part or all of other embodiments are possible.

1 power conversion device 11, 12 DC/DC converter 13 DC bus 14 electric wire 15 system 200 insulated bi-directional DC/DC converter 202 transformer 206 control device 300 DC link 400 inverter p, q AC output terminals r, s DC output terminals U1 , U2, U3, V1, V2, V3, W1, W2, W3 conversion cell

Claims (3)

a plurality of converters each having an isolated bi-directional DC/DC converter, a DC link, and an inverter connected to said converter via said DC link, connected in series via AC outputs of said inverters; a cell;
a control device that controls the plurality of conversion cells,
When the voltage of the system interconnected via the AC output terminal is normal, the control device increases the DC voltage of the bus connected to the DC output terminal connected to the DC link via the converter to a predetermined value. and controlling 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. Device. 2. The power converter according to claim 1, wherein said control device controls said inverter so that the voltage of said system becomes a predetermined value when the voltage of said system is abnormal. 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 of the AC output terminal of the inverter. 3. The power converter according to claim 1 or 2.

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