Power converter and method for controlling the same

Abstract

To improve the power conversion efficiency of the whole power converter having a plurality of conversion cells each having an insulation-type bidirectional DC-DC converter.SOLUTION: The power converter includes: a plurality of conversion cells each having an insulative DC / DC converter, a DC link, and an inverter connected to the converter by the DC link, the conversion cells being connected in series by an AC output terminal of the inverter; and a controller for controlling the conversion cells. The controller controls the inverter so that the transmission powers of the conversion cells become uneven when the total transmission powers of the conversion cells are smaller than a predetermined power.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a power conversion device and a control method thereof. [Background technology]

[0002] Conventionally, an SST (Solid State Transformer) has been known as one type of power conversion device. The SST includes a number of cells, each of which includes an insulated 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 system 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] An isolated bidirectional DC / DC converter has a characteristic that its power conversion efficiency decreases when the transmission power becomes too small compared to the rated power. Therefore, in a power conversion device including multiple conversion cells each having an isolated bidirectional DC / DC converter, if a relatively small transmission power is equally shared among the multiple conversion cells, the power conversion efficiency of the entire power conversion device may decrease.

[0005] The present disclosure aims to improve the power conversion efficiency of an entire power conversion device including a plurality of conversion cells each having an isolated bidirectional DC / DC converter.

Means for Solving the Problem

[0006] The first aspect is an isolated DC / DC converter, a DC link, an inverter connected to the converter via the DC link, a plurality of conversion cells connected in series via an AC output terminal of the inverter, and a control device for controlling the plurality of conversion cells, and when the total transmission power of the plurality of conversion cells is lower than a predetermined power, the control device controls the inverter so that the transmission power of each of the plurality of conversion cells becomes uneven. It is a power conversion device.

[0007] The second aspect is the power conversion device of the first aspect, when the number of the plurality of conversion cells is N and the minimum value of the transmission power for the power conversion efficiency of one converter to be equal to or higher than a predetermined value is Pth, the predetermined power is N × Pth.

[0008] The third aspect is the power conversion device of the second aspect, when the total transmission power is P and n is an integer from 1 to (N - 1), the control device adjusts the output voltage of the inverters from the first to the (N - n)-th to V * ·cosφ / (N - n) when (N - n)Pth ≤ P < (N - n + 1)Pth, and adjusts the output voltage of the inverters from the (N - n + 1)-th to the N-th to V * ·sinφ / n.

[0009] The fourth aspect is the power conversion device of the third aspect, when P < Pth, the control device adjusts the output voltage of the first inverter to V * ·cosφ, and adjusts the output voltages of the inverters from the second to the N-th to " *Adjust to "sinφ / (N-1)".

[0010] A fifth aspect is a power conversion device according to any one of the first to fourth aspects, When the total transmission power is higher than the predetermined power, the control device controls the inverters so that the transmission power of each of the plurality of conversion cells is equalized.

[0011] The sixth aspect is A control method for a power conversion device including an isolated DC / DC converter, a DC link, and an inverter connected to the converter via the DC link, the power conversion device including a plurality of conversion cells connected in series via AC output terminals of the inverter, the control method comprising: When a total transmission power of the plurality of converter cells is lower than a predetermined power, the inverter is controlled so that the transmission power of each of the plurality of converter cells becomes unequal. Effect of the Invention

[0012] According to the present disclosure, the power conversion efficiency of the entire power conversion device can be improved. [Brief description of the drawings]

[0013] [Figure 1] 1 is a diagram illustrating a configuration example of a power conversion device according to an embodiment; [Diagram 2] 1 is a characteristic diagram showing an example of the relationship between the power conversion efficiency of an isolated bidirectional DC / DC converter and the power transmitted between the primary side and the secondary side. FIG. [Diagram 3] 2 is a vector diagram of voltage and current output from a power conversion device according to an embodiment of the present invention; [Figure 4] FIG. 2 is a functional block diagram illustrating an example of a control device. [Diagram 5] 13 is a voltage vector diagram of each conversion cell when each conversion cell equally shares the burden of transmitting power to the grid. [Figure 6] 13 is a voltage vector diagram of each conversion cell when the power transmitted to the grid is borne by some of the conversion cells. [Figure 7] 13 is a voltage vector diagram of each conversion cell when the transmission power to the grid is borne by only one conversion cell. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the embodiments will be described. Note that "DC" and "AC" are abbreviations for "Direct Current" and "Alternative Current", respectively.

[0015] Fig. 1 is a diagram illustrating an example of the configuration of a power conversion device according to an embodiment. The power conversion device 1 illustrated in Fig. 1 is an SST including three conversion cells connected in series. A DC bus 2 is connected to a DC output terminal of the power conversion device 1. The power conversion device 1 converts the DC of the DC bus 2 into AC and outputs the AC. The AC output terminal of the power conversion device 1 is connected to a system 4 via an interconnection reactor 3.

[0016] The power conversion device 1 includes a plurality of conversion cells U1, U2, and U3, and a control device 20 that controls the power conversion operation of each of these conversion cells. The number of conversion cells is not limited to three, and may be two or four or more. The plurality of conversion cells may have the same configuration. Fig. 1 shows the configuration of the conversion cell U1 as a representative example.

[0017] Each of the multiple conversion cells U1, U2, and U3 has DC output terminals r and s, an insulating bidirectional DC / DC converter 10, a DC link 11, an inverter 12, and AC output terminals p and q.

[0018] The DC output terminals r and s are output terminals connected to one of the DC output terminals of the isolated bidirectional DC / DC converter 10. The high-potential side DC output terminal r is connected to a first bus 2p on the high-potential side of the DC bus 2, and the low-potential side DC output terminal s is connected to a second bus 2n on the low-potential side of the DC bus 2.

[0019] The AC output terminals p and q are output terminals connected to the AC output terminals of the inverter 12, for example.

[0020] 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. The multiple converter cells U1, U2, and U3 each have a first AC output terminal p connected to a second AC output terminal q of one of the converter cells adjacent to itself, and a second AC output terminal q connected to a first AC output terminal p of the other converter cell adjacent to itself. Among 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) and the second AC output terminal q of the converter cell on the lowest potential side (in this example, converter cell U3) are connected to the grid 4 via an interconnection reactor 3.

[0021] The isolated bidirectional DC / DC converter 10 steps up or down the DC voltage input from the DC bus 2, and controls the DC link 11 so that the DC voltage Vdc is constant at a predetermined voltage.

[0022] For example, the isolated bidirectional DC / DC converter 10 is a DAB (Dual Active Bridge) converter having a primary-side full-bridge circuit 14 provided on the primary side of a transformer 13 and a secondary-side full-bridge circuit 15 provided on the secondary side of the transformer 13. The DAB converter transmits power between the primary side and the secondary side by applying a voltage to a leakage inductance of the transformer 13 or an external reactor connected in series to the transformer 13.

[0023] 2 is a characteristic diagram showing an example of the relationship between the power conversion efficiency of an isolated bidirectional DC / DC converter and the power Pa transmitted between the primary side and the secondary side. When the power Pa transmitted between the primary side and the secondary side exceeds a predetermined power value Pth, the isolated bidirectional DC / DC converter 10 can apply ZVS (Zero Voltage Switching), which can reduce switching loss, and can therefore operate with high power conversion efficiency. Prate represents the rated power value of the isolated bidirectional DC / DC converter 10.

[0024] 1, the inverter 12 is a circuit connected to the insulated bidirectional DC / DC converter 10 via a DC link 11. The inverter 12 converts the DC of the DC link 11 to AC and outputs it to AC output terminals p, q, or converts the AC input from the AC output terminals p, q to DC and outputs it to the DC link 11. The inverter 12 can output an AC voltage V having a phase difference θ with respect to an AC current I at any amplitude by a method called PWM (Pulse Width Modulation).

[0025] The inverter 12 has a bridge circuit including a plurality of switches Q1, Q2, Q3, and Q4. Examples of the switches Q1, Q2, Q3, and Q4 include semiconductor switching elements such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors).

[0026] The control device 20 controls a plurality of conversion cells for each phase of the power conversion device 1. The control device 20 has, for example, a memory and a processor (for example, a CPU (Central Processing Unit)), and each function of the control device 20 is realized by the processor operating according to a program stored in the memory. Each function of the control device 20 may be realized by an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0027] The control device 20 controls the isolated bidirectional DC / DC converter 10 so that the voltage of the DC link 11 becomes a constant DC voltage Vdc. On the other hand, the control device 20 controls the inverter 12 by the following control method.

[0028] 3 is a vector diagram of the voltage V and current I output from the power conversion device of one embodiment. The power P transmitted from the power conversion device 1 to the system 4 is calculated by using the voltage V output from the power conversion device 1, the current I output from the power conversion device 1 to the system 4, and the phase difference φ between the voltage V and the current I as follows: P = V I cosφ The power P represents the total transmission power transmitted from the multiple conversion cells U1, U2, and U3 to the grid 4. The voltage Vs represents the voltage of the grid 4. The voltage VL represents the voltage across the interconnection reactor 3. The voltage V corresponds to the sum of the voltages output from the inverters 12 of the multiple conversion cells U1, U2, and U3 (V=V1+V2+V3).

[0029] 4 is a functional block diagram showing an example of a control device 20. The control device 20 includes a current control unit 21, a voltage command unit 22, and a signal generation unit .

[0030] The current control unit 21 controls the current I flowing from the power conversion device 1 to the grid 4 to a predetermined current command value I by using feedback control such as proportional control or proportional-integral control. * The voltage command V * Generates the voltage command V * is a command value for the voltage V output from the power conversion device 1.

[0031] The voltage command unit 22 outputs a voltage command V * The individual voltage commands V1 of the multiple conversion cells U1, U2, and U3 are * ,V2 * ,V3 * Individual voltage command V1 * is the command value of the voltage V1 output from the inverter 12 of the converter cell U1. * is the command value of the voltage V2 output from the inverter 12 of the conversion cell U2. * is the command value of the voltage V3 output from the inverter 12 of the conversion cell U3.

[0032] The signal generating unit 23 generates voltages V1, V2, and V3 outputted to the AC output terminals p and q of each of the multiple conversion cells U1, U2, and U3 in accordance with an individual voltage command V1. * ,V2 * ,V3 * The signal generating unit 23 outputs gate signals for the inverters 12 of the multiple conversion cells U1, U2, and U3 so that the gate signals for the inverters 12 of the multiple conversion cells U1, U2, and U3 respectively match the gate signals for the inverters 12 of the multiple conversion cells U1, U2, and U3. The switches Q1, Q2, Q3, and Q4 in the respective inverters 12 are turned on or off in accordance with the gate signals generated by the signal generating unit 23.

[0033] [Table 1] Table 1 shows a method for calculating individual voltage commands by the voltage command unit 22. The calculation method is classified into the following three cases. In the following description, the isolated bidirectional DC / DC converter 10 may be abbreviated as DAB.

[0034] Case I shows the calculation of individual voltage commands when the power P transmitted to the grid 4 is equal to or greater than N×Pth (P≧N×Pth). Here, Pth is the minimum value of the transmitted power for one isolated bidirectional DC / DC converter 10 to operate with high efficiency, with the power conversion efficiency being equal to or greater than a predetermined value. N is the number of conversion cells in the power conversion device 1, and here, N=3. θ n V n * and the current command value I * is the phase difference between the two. n is an integer between 2 and N.

[0035] When the conditions of Case I are met, the voltage command unit 22 sets the individual voltage commands for the first to (N-1)th conversion cells as "V * / N", and adjusts each phase difference θ to φ. In this way, in case I where the power P is equal to or greater than N×Pth, the control device 20 controls the inverters 12 of each of the multiple converter cells U1, U2, and U3 so that the transmission power of each of the multiple converter cells U1, U2, and U3 is equal.

[0036] Figure 5 shows the voltage command V * and individual voltage command V1* , V2 * , V3 * shows a vector diagram. In Case I, individual voltage commands are calculated for a plurality of conversion cells U1, U2, U3 to equally share the power P. The voltage command unit 22 calculates, for all of the conversion cells U1, U2, U3, individual voltage commands V1 * , V2 * , V3 * to "V * / 3" and adjusts each phase difference θ1, θ2, θ3 to φ.

[0037] Case II shows the calculation of individual voltage commands when the power P is equal to or greater than Pth and less than N×Pth (Pth ≦ P < N×Pth). In Case II, the voltage command unit 22 calculates individual voltage commands that unevenly distribute the power P to each conversion cell so that the maximum number of isolated bidirectional DC / DC converters bears the transmission power conditions for high-efficiency operation. That is, in Case II, the voltage command unit 22 adjusts the transmission power of each of the N conversion cells so that the number of DABs (for example, DABs that achieve ZVS) for high-efficiency operation is maximized.

[0038] Condition 1 of Case II, '(N - 1)Pth ≦ P < N·Pth', indicates that if the power P is evenly shared among N conversion cells, all N DABs cannot operate with high efficiency, but if the power P is evenly shared among (N - 1) conversion cells, all (N - 1) DABs can operate with high efficiency. When Condition 1 of Case II is satisfied, the voltage command unit 22 adjusts the individual voltage commands for the first to (N - 1)th conversion cells to "V * ·cosφ / (N - 1)" and adjusts each phase difference θ to zero. On the other hand, when Condition 1 of Case II is satisfied, the voltage command unit 22 adjusts the individual voltage command for the Nth conversion cell to "V * ·sinφ" and adjusts the phase difference θ to π / 2. In this case, the control device 20 improves the power conversion efficiency of the entire power conversion device 1 by operating the first to (N - 1)th DABs with ZVS.

[0039] Condition 2 of Case II "(N-2)Pth≦P<(N-1)Pth" indicates that if the power P is equally shared among the (N-1) conversion cells, all of the (N-1) DABs cannot be operated with high efficiency, but if the power P is equally shared among the (N-2) conversion cells, all of the (N-2) DABs can be operated with high efficiency. When Condition 2 of Case II is satisfied, the voltage command unit 22 sets the individual voltage command for each of the first to (N-2) conversion cells as "V * On the other hand, when the condition 2 of the case II is satisfied, the voltage command unit 22 adjusts the individual voltage commands of the (N-1)th and Nth conversion cells to "V * ·sinφ / 2", and adjusts each phase difference θ to π / 2. In this case, the control device 20 operates the first to (N-2)th DABs in ZVS, thereby improving the power conversion efficiency of the entire power conversion device 1.

[0040] Therefore, the condition of Case II can be generalized as "(Nn)Pth≦P<(N-n+1)Pth" (n is an integer from 1 to (N-1)). In other words, when the generalized condition of Case II is satisfied, the voltage command unit 22 sets the individual voltage command of each of the first to (Nn)th conversion cells as "V * On the other hand, when the generalized condition of Case II is satisfied, the voltage command unit 22 adjusts each individual voltage command to "V * ·sinφ / n", and adjusts each phase difference θ to π / 2. In this case, the control device 20 operates the first to (Nn)th DABs in ZVS, thereby improving the power conversion efficiency of the entire power conversion device 1.

[0041] In this way, in case II where the power P is equal to or greater than Pth and less than 3×Pth, the control device 20 controls the inverters 12 of the converter cells U1, U2, U3 so that the transmission power of each of the multiple converter cells U1, U2, U3 becomes unequal.

[0042] Figure 6 shows the voltage command V* and the individual voltage commands V1 * , V2 * , V3 * The vector diagrams are shown. In Case II, individual voltage commands are calculated for the plurality of conversion cells U1, U2, U3 to unevenly bear the power P. For the first and second conversion cells U1, U2, the voltage command unit 22 adjusts the respective individual voltage commands V1 * , V2 * to "V * ·cosφ / 2" and adjusts the respective phase differences θ1, θ2 to zero. On the other hand, for the third conversion cell U3, the voltage command unit 22 adjusts the individual voltage command V3 * to "V * ·sinφ" and adjusts the phase difference θ3 to π / 2.

[0043] Case III shows the calculation of the individual power command when the power P is less than Pth (P < Pth). In Case III, since the power P to be transmitted is small and none of the conversion cells can operate with high efficiency, the voltage command unit 22 calculates the individual voltage command so that one conversion cell bears all of the power P. That is, when the conditions of Case III are satisfied, for the first conversion cell, the voltage command unit 22 adjusts the individual voltage command to "V * ·cosφ" and adjusts the phase difference θ to zero. On the other hand, when the conditions of Case III are satisfied, for the second to Nth conversion cells, the voltage command unit 22 adjusts the respective individual voltage commands to "V * ·sinφ / (N - 1)" and adjusts the respective phase differences θ to π / 2.

[0044] Figure 7 shows the vector diagrams of the voltage command V * and the individual voltage commands V1 * , V2 * , V3 * In Case III, individual voltage commands are calculated for the plurality of conversion cells U1, U2, U3 to unevenly bear the power P. For the first conversion cell U1, the voltage command unit 22 adjusts the individual voltage command V1 * to "V *On the other hand, the voltage command unit 22 adjusts the individual voltage command V2 for the second and third conversion cells U2 and U3. * ,V3 * "V * ·sinφ / 2", and the phase differences θ2, θ3 are adjusted to π / 2.

[0045] Therefore, according to the technology disclosed herein, each conversion cell unequally bears the total transmission power of the power conversion device so as to maximize the number of conversion cells that bear the transmission power required for highly efficient operation, thereby improving the power conversion efficiency of the entire power conversion device.

[0046] As described above, the embodiment has been described, but the above embodiment is presented as an example, and the present invention is not limited to the above embodiment. The above embodiment can be implemented in various other forms, and various combinations, omissions, substitutions, modifications, etc. can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the scope of the invention and its equivalents described in the claims.

[0047] For example, the primary-side bridge circuit or the secondary-side bridge circuit of the isolated bidirectional DC / DC converter is not limited to a full-bridge circuit, and may be a half-bridge circuit. [Explanation of symbols]

[0048] 1 Power conversion device 2 DC bus 3 Interconnection Reactor 4 systems 10. Isolated bidirectional DC / DC converter 11 DC Link 12 Inverter 13. Transformer 14 Primary side full bridge circuit 15 Secondary side full bridge circuit 20 Control device 21 Current control section 22 Voltage command unit 23 Signal generation department p,q AC output terminal r,s DC output terminal

Claims

1. a plurality of conversion cells each including an isolated 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 an AC output terminal of the inverter; A control device for controlling the plurality of conversion cells, The control device controls the inverter such that the transmission powers of the plurality of conversion cells are unequal when a total transmission power of the plurality of conversion cells is lower than a predetermined power.

2. Let N be the number of the plurality of conversion cells, and Pth be the minimum value of transmission power for making the power conversion efficiency of one converter equal to or greater than a predetermined value. The power conversion device according to claim 1 , wherein the predetermined power is N×Pth.

3. Let P be the total transmission power, and n be an integer from 1 to (N-1), When (N-n)Pth≦P<(N-n+1)Pth, the control device controls the output voltages of the first to (N-n)-th inverters to V ＊ ・Adjust the output voltage of the inverters from the (N-n+1)th unit to the Nth unit to V ＊ The power conversion device according to claim 2 , wherein the power conversion voltage is adjusted to sinφ / n.

4. When P<Pth, the control device controls the output voltage of the first inverter to V ＊ ・Adjust the output voltage of the inverters from the second unit to the Nth unit to "V ＊ The power conversion device according to claim 3, wherein the output of the power conversion circuit is adjusted to "sinφ / (N-1)".

5. The power conversion device according to claim 1 , wherein when the total transmission power is higher than the predetermined power, the control device controls the inverters so that the transmission power of each of the plurality of conversion cells is equalized.

6. A control method for a power conversion device including an isolated DC / DC converter, a DC link, and an inverter connected to the converter via the DC link, the power conversion device including a plurality of conversion cells connected in series via AC output terminals of the inverter, the control method comprising: a control method for controlling the inverter when a total transmission power of the plurality of conversion cells is lower than a predetermined power, so that the transmission power of each of the plurality of conversion cells is unequal.

Images