Power converter

Abstract

To provide a power conversion device that can speed up the restart of conversion cells.SOLUTION: A power conversion device includes: a plurality of conversion cells each of which has an isolated bidirectional DC / DC converter, a DC link, and an inverter connected to the converter via the DC link, the plurality of conversion cells being connected in series via AC output terminals of the respective inverters; and a control device that controls the plurality of conversion cells, and the control device controls, when the converter stops, the inverter installed in the conversion cell equipped with the stopped converter so as to output zero voltage to the AC output terminals.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Conventionally, as one of power conversion devices, a Solid State Transformer (SST) is known. The SST includes a plurality of cells each having an insulated 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 terminals of each cell are connected in parallel to a 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.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The SST can continue operation (redundant operation) by bypassing a failed cell with a short-circuit switch. However, in this redundant operation, since the capacity of the power conversion device decreases, even when a conversion cell detects some abnormality and stops, if it is possible to operate, it is required to restart promptly.

[0005] One example of this is the overcurrent flowing through the high-frequency transformer of an isolated bidirectional DC / DC converter. Unexpected load fluctuations can cause the high-frequency transformer to become magnetized, resulting in an overcurrent. However, if there are no hardware failures such as component failures in the cell, a restart may be permitted.

[0006] In conventional methods, when an overcurrent occurs in an isolated bidirectional DC / DC converter, not only the isolated bidirectional DC / DC converter but also the inverter in the same conversion cell is shut down. When the inverter is also shut down, DC of the DC link between the isolated bidirectional DC / DC converter and the inverter Voltage increases 。

[0008] This disclosure is, Suppresses the rise in DC voltage of the DC link. To provide a power conversion device. [Means for solving the problem]

[0009] In one aspect of this disclosure, It comprises an isolated bidirectional DC / DC converter, a DC link, and an inverter connected to the converter via the DC link, and a plurality of conversion cells connected in series via the AC output terminal of the inverter, The system includes a control device for controlling the plurality of conversion cells, The control device, when the converter stops, controls the inverter mounted on the conversion cell containing the stopped converter. A reactive voltage with polarity that lags or leads the current flowing through it by 90 degrees. at the AC output terminal occurrence so The inverter A power converter is provided to control it. [Effects of the Invention]

[0010] According to one aspect of this disclosure, Suppresses the rise in DC voltage of the DC link. can. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows an example configuration of a power conversion device according to one embodiment. [Figure 2] This figure shows the rise pattern of the DC intermediate voltage when the inverter is stopped. [Figure 3] This figure shows the rise pattern of the DC intermediate voltage when the inverter is stopped. [Figure 4] This is a vector diagram of the inverter's output voltage during converter operation. [Figure 5] This is a vector diagram of the inverter's output voltage (zero voltage output) when the converter is stopped. [Figure 6] This is a vector diagram of the inverter's output voltage (reactive voltage output) when the converter is stopped. [Figure 7] This is a vector diagram of the inverter's output voltage (reactive voltage output) when the converter is stopped. [Modes for carrying out the invention]

[0012] The embodiments will be described below. Note that "DC" and "AC" are abbreviations for "Direct Current" and "Alternative Current," respectively.

[0013] Figure 1 shows an example of the configuration of a power converter according to one embodiment. The power converter 1 shown in Figure 1 is an SST equipped with three series-connected conversion cells for each phase. Figure 1 illustrates the case in which the SST is applied to a multi-source self-consumption type renewable energy system that includes a photovoltaic power generation system and a storage battery. The photovoltaic power generation system performs photovoltaic (PV) power generation using solar cells that convert light energy into electrical energy.

[0014] The solar power generation system is connected to the 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 the DC output terminal of the power converter 1. The power converter 1 converts the DC from the DC bus 13 to AC and outputs it. The AC output terminal of the power converter 1 is connected to the grid 15 via a power line 14. An important load (not shown) that receives power from the power line 14 is connected to the power line 14.

[0015] As an advantage of this self - consumption type system, it has a function as an emergency power source. During normal operation of the system 15, the 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 system 15. On the other hand, during an abnormality of the system 15 such as a power outage, after the system 15 is disconnected by a switch (not shown), the storage battery is utilized as an emergency power source. The DC power output from the storage battery functioning as an emergency power source 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 an important load (not shown) connected to the electric wire 14.

[0016] The operation mode when the power conversion device 1 is connected to the system 15 is called the connected operation, and the operation mode when the power conversion device 1 operates as an emergency power source is called the self - supporting operation.

[0017] The power conversion device 1 shown in FIG. 1 is an SST that bidirectionally converts the DC on the DC bus 13 side to AC on the system 15 side or AC on the system 15 side to DC on the DC bus 13 side. The power conversion device 1 includes three conversion cells U1, U2, U3 for the U - phase, three conversion cells V1, V2, V3 for the V - phase, three conversion cells W1, W2, W3 for the W - phase, and a control device 206 that controls the power conversion operations of each of these conversion cells. Note that the number of conversion cells in each phase is not limited to three and may be one or two or more. Also, since the conversion cells in each phase have the same configuration, the conversion cells of the U - phase will be described below as a representative.

[0018] The plurality of conversion cells U1, U2, U3 are each cell converters that bidirectionally convert the DC on the DC bus 13 side to AC on the system 15 side or AC on the system 15 side to DC on the DC bus 13 side. The plurality of conversion cells U1, U2, U3 each 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 will also be simply referred to as a converter.

[0019] The DC output terminals r and s are, for example, output terminals connected to one of the DC output sides (DC bus 13 side) of the converter 200. The high-potential DC output terminal r is connected to the high-potential first bus P of the DC bus 13, and the low-potential DC output terminal s is connected to the low-potential second bus N of the DC bus 13. The DC output terminals r and s are connected to the DC output terminals of the primary circuit 210a described later.

[0020] The AC output terminals p and q are, for example, output terminals connected to the other DC output side (system 15 side) of the converter 200.

[0021] Multiple conversion cells U1, U2, and U3 each have AC output terminals p and q, and are connected in series via these AC output terminals. Each of the multiple conversion cells U1, U2, and U3 has its first AC output terminal p connected to the second AC output terminal q of one adjacent conversion cell, and its second AC output terminal q connected to the first AC output terminal p of the other adjacent conversion cell. Of the multiple conversion cells connected in series via AC output terminals p and q, the first AC output terminal p of the conversion cell located on the highest potential side (conversion cell U1 in this example) is connected to the grid 15 via the interconnection reactor 17 (interconnection reactor 17u in the U-phase wire 14). On the other hand, of the multiple cells connected in series via 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 connected to the neutral point 16.

[0022] The converter 200 boosts or bucks the DC voltage input from the DC bus 13 common to multiple conversion cells U1, U2, and U3 and outputs a predetermined DC voltage to the DC link 300. Alternatively, the converter 200 boosts or bucks the DC voltage input from the DC link 300 and outputs a predetermined DC voltage to the DC bus 13. The converter 200 comprises, for example, a transformer 202, a primary circuit 210a, and a secondary circuit 210b. The primary circuit 210a and the secondary circuit 210b are magnetically coupled by the transformer 202.

[0023] For example, converter 200 is a DAB (Dual Active Bridge) converter having a primary full-bridge circuit provided on the primary side of transformer 202 and a secondary full-bridge circuit provided on the secondary side of transformer 202. The DAB converter transmits power between the primary and secondary sides by applying a voltage to the leakage inductance of transformer 202 or to an external reactor connected in series with transformer 202. The transmitted power is controlled by the phase difference between the output voltage V1 output from two intermediate connection points of the primary inverter circuit (primary full-bridge circuit) and the output voltage V2 output from two intermediate connection points of the secondary inverter circuit (secondary full-bridge circuit).

[0024] Transformer 202 is a transformer having a primary coil and a secondary coil, with the primary coil and secondary coil being magnetically coupled. Primary circuit 210a has a primary full-bridge circuit and a primary drive circuit. Secondary circuit 210b has a secondary full-bridge circuit and a secondary drive circuit.

[0025] The primary side full bridge circuit includes a primary side first half-bridge circuit in which the primary side first upper arm and the primary side first lower arm are connected in series, and a primary side second half-bridge circuit in which the primary side second upper arm and the primary side second lower arm are connected in series. The primary side coil of transformer 202 is connected between the intermediate connection point between the primary side first upper arm and the primary side first lower arm and the intermediate connection point between the primary side second upper arm and the primary side second lower arm.

[0026] The secondary full-bridge circuit includes a secondary first half-bridge circuit in which the secondary first upper arm and the secondary first lower arm are connected in series, and a secondary second half-bridge circuit in which the secondary second upper arm and the secondary second lower arm are connected in series. The secondary coil of transformer 202 is connected between the intermediate connection point between the secondary first upper arm and the secondary first lower arm and the intermediate connection point between the secondary second upper arm and the secondary second lower arm.

[0027] Multiple primary-side switch elements, such as the primary-side first upper arm, primary-side first lower arm, primary-side second upper arm, and primary-side second lower arm, are driven by the primary-side drive circuit. Multiple secondary-side switch elements, such as the secondary-side first upper arm, secondary-side first lower arm, secondary-side second upper arm, and secondary-side second lower arm, are driven by the secondary-side drive circuit.

[0028] Specific examples of primary and secondary switching elements include semiconductor switching elements such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors).

[0029] The DC link 300 is connected to the DC output terminal of the secondary circuit 210b. The DC link 300 has a DC capacitor 301.

[0030] The inverter 400 is a circuit connected to the converter 200 via the DC link 300. The inverter 400 converts the DC from the DC link 300 to 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 to DC and outputs it to the DC link 300.

[0031] The control device 206 controls multiple conversion cells for each phase. The control device 206 includes, for example, 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 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).

[0032] For example, when the voltage of the grid 15 connected via 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 is connected is a predetermined value. Furthermore, when the voltage of the grid 15 is normal and when the voltage of the grid 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 is a predetermined value.

[0033] Figures 2 and 3 show the DC intermediate voltage rise pattern when the inverter stops. When the converter 200 stops due to detection of a malfunction or the like, if the inverter 400, which is mounted on the same conversion cell as the stopped converter 200, is also stopped, the inverter 400 functions as a rectifier circuit as shown in Figures 2 and 3, regardless of the polarity of the U-phase current. As a result, the DC intermediate voltage Edc rises, which may cause damage to elements such as the DC capacitor 301.

[0034] Therefore, in order to avoid damage to elements due to this voltage rise, the control device 206 controls the inverter 400 in a predetermined manner when the converter 200 stops.

[0035] Figure 4 is a vector diagram of the inverter output voltage during converter operation. When converter 200 is in operation, the control device 206 controls inverter 400 so that the voltage (Vu1 + Vu2 + Vu3) output to the AC side as the U-phase system voltage Vsu is distributed among the cells.

[0036] Figure 5 is a vector diagram of the inverter output voltage (zero voltage output) when the converter is stopped. When the converter 200 of conversion cell U1 stops, the control device 206 controls the inverter 400 mounted on conversion cell U1 so that the inverter 400 mounted on conversion cell U1 that houses the stopped converter 200 outputs zero voltage to the AC output terminals p and q. For example, the converter 200 of conversion cell U1 may be temporarily stopped by the control device 206 if an abnormality such as an overcurrent is detected. Regarding the voltage output to the AC side, the control device 206 controls the inverters 400 of conversion cells U1, U2, and U3 so that the inverter output Vu1 of conversion cell U1 becomes zero voltage, and the remaining voltages are shared and output by the remaining conversion cells U2 and U3.

[0037] Zero voltage can be easily achieved by turning on switches Q1 and Q3, or switches Q2 and Q4, of inverter 400. Figures 4 and 5 use a rotational coordinate system called the dq axis coordinate system, where the d axis is defined as the direction of the grid voltage and the q axis is defined as the direction 90 degrees ahead of the d axis. Therefore, when power is supplied from power converter 1 to grid 15, the d-axis current is positive polarity, and in the case of a lagging power factor, the q-axis current is negative polarity.

[0038] Figures 6 and 7 are vector diagrams of the inverter output voltage (reactive voltage output) when the converter is stopped. When the converter 200 of the conversion cell U1 is stopped, the control device 206 controls the inverter 400 mounted on the conversion cell U1 that houses the stopped converter 200 to output an reactive voltage at the AC output terminals p and q with a polarity of 90 degrees lagging or leading the output current iu of the inverter 400. When the converter 200 of the conversion cell U1 is stopped, the inverter 400 of the conversion cell U1 outputs a voltage Vu1 that leads or lags the U-phase output current iu by 90 degrees. As a result, reactive power is input to the inverter 400, which prevents an increase in the DC intermediate voltage Edc.

[0039] As shown in Figure 1, the power converter 1 may be equipped with a short-circuit switch 18 between the AC output terminals p and q. When the control device 206 determines that the converter 200 cannot be restarted while the inverter 400 of the conversion cell U1 is outputting zero voltage, as shown in Figure 5, it determines that the conversion cell U1 has failed and turns on the short-circuit switch 18. By turning on the short-circuit switch 18 while the output voltage is zero, there are no transient fluctuations in the AC voltage of the grid 15, and disturbances to the grid 15 can be prevented.

[0040] Although embodiments have been described above, the technology of this disclosure is not limited to the embodiments described above. Various modifications and improvements are possible, such as combinations or substitutions with some or all of the other embodiments. [Explanation of Symbols]

[0041] 1. Power converter 11,12 DC / DC converters 13 DC bus 14 Electric wire 15 strains 16 Neutral point 17, 17u, 17v, 17w interconnected reactor 18 Short-circuit switch 200 Isolated Bidirectional DC / DC Converter 202 Transformers 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

[Claim 1] It comprises an isolated bidirectional DC / DC converter, a DC link, and an inverter connected to the converter via the DC link, and a plurality of conversion cells connected in series via the AC output terminal of the inverter, The system includes a control device for controlling the plurality of conversion cells, The control device controls the inverter such that, when the converter stops, an reactive voltage with a polarity that lags by 90 degrees or leads by 90 degrees relative to the current flowing through the inverter mounted on the conversion cell on which the stopped converter is mounted is generated at the AC output terminal.

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