A dc transformer topology and method of controlling the same

Abstract

The application discloses a DC transformer topology structure and a control method thereof, which comprises a transformer, an inductor, a transformer low-voltage side circuit and a transformer high-voltage side circuit. The transformer low-voltage side circuit is a full-bridge structure and comprises four low-voltage side bridge arms and a first DC support capacitor. The low-voltage side bridge arm comprises a power switching device and a diode in anti-parallel connection with the power switching device. The transformer high-voltage side circuit is also a full-bridge structure and comprises two power switching device diode bridge arms, two diode bridge arms and a second DC support capacitor. The power switching device diode bridge arms and the diode bridge arms are asymmetrically arranged, and the second DC support capacitor is connected in parallel across the transformer high-voltage side circuit. The inductor is connected in series with the transformer. The power switching device can be saved, bidirectional power flow is realized, the high-voltage side power switching device does not need to transmit rated power, and a device with small capacity and low cost can be selected, so that the cost of the wind field DC transformer is greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of DC transformer technology, and in particular to a DC transformer topology and its control method. Background Technology

[0002] Wind power is the world's fastest-growing green energy technology. In recent years, the wind power industry has continued to grow rapidly, achieving large-scale development and application globally, and has entered a stage of stable development.

[0003] During wind power startup, a small amount of power is supplied from the grid in the reverse direction. After startup, the wind farm transmits a large amount of power to the grid in the forward direction. Therefore, wind farms based on DC aggregation require DC transformers capable of providing both forward and reverse power. Existing DC transformer topologies used in wind power aggregation can be divided into two categories: bidirectional power topologies and unidirectional power topologies. Unidirectional power topologies require additional high-voltage, small-capacity reverse power DC transformers to meet the wind farm's needs. Although the power of these transformers is relatively small, their voltage is high, and the number of power submodules is large, resulting in high cost. Bidirectional power topologies use large-capacity, fully controlled power semiconductor switching devices on both the high and low voltage sides. The forward and reverse power of this topology are equal, but the reverse power required by the wind farm is much smaller than the forward power. Therefore, bidirectional power topologies waste the capacity of the fully controlled power switching devices, leading to higher costs. Summary of the Invention

[0004] To address the aforementioned issues, this patent proposes a DC transformer topology and its control method. This eliminates the need for half of the fully controlled power switching devices on the high-voltage side, enabling bidirectional power flow. Furthermore, the fully controlled power switching devices on the high-voltage side do not require the transmission of rated power, allowing the use of smaller capacity and lower-cost devices. Based on this technology, the cost of DC transformers for wind farms can be significantly reduced.

[0005] In a first aspect, embodiments of the present invention provide a DC transformer topology, including a power submodule;

[0006] The power submodule includes a transformer, an inductor, a low-voltage side circuit of the transformer, and a high-voltage side circuit of the transformer.

[0007] The low-voltage side circuit of the transformer is connected to the DC output terminal of the wind farm, and the high-voltage side circuit of the transformer is connected to the high-voltage DC grid.

[0008] The low-voltage side circuit of the transformer is a full-bridge structure, including four low-voltage side bridge arms and a first DC support capacitor. The first DC support capacitor is connected in parallel across the two ends of the low-voltage side circuit of the transformer. Each low-voltage side bridge arm includes a power switching device and a diode. The power switching device and the diode are connected in anti-parallel.

[0009] The high-voltage side circuit of the transformer is a full-bridge structure, including two power switching device diode bridge arms, two diode bridge arms, and a second DC support capacitor. One diagonal of the full-bridge structure of the high-voltage side circuit of the transformer is configured with power switching device diode bridge arms, and the other diagonal is configured with diode bridge arms. The second DC support capacitor is connected in parallel across the two ends of the high-voltage side circuit of the transformer. Each power switching device diode bridge arm includes at least one power switching device and at least one diode, with the power switching device and the diode connected in anti-parallel. Each diode bridge arm includes at least one diode.

[0010] The inductor is connected in series with the transformer.

[0011] In some specific embodiments, the first and second terminals of the low-voltage side circuit of the transformer are respectively connected to the positive and negative terminals of the DC output terminal of the wind farm, and the first and second terminals of the high-voltage side circuit of the transformer are respectively connected to the positive and negative terminals of the high-voltage DC grid.

[0012] In some specific embodiments, the low-voltage side circuit of the transformer includes:

[0013] Power switching devices S1, S2, S3 and S4, diodes D1, D2, D3 and D4, and first DC support capacitor C1;

[0014] The diodes D1, D2, D3 and D4 are connected in antiparallel with the power switching devices S1, S2, S3 and S4, respectively;

[0015] The first terminal of power switch S1 is connected to the first terminal of the low-voltage side circuit of the transformer; the second terminal of power switch S1 is connected to the first terminal of power switch S2; the second terminal of power switch S2 is connected to the second terminal of the low-voltage side circuit of the transformer; the first terminal of power switch S3 is connected to the first terminal of power switch S1; the second terminal of power switch S3 is connected to the first terminal of power switch S4; the second terminal of power switch S4 is connected to the second terminal of power switch S2; the first terminal of the low-voltage side of the transformer is connected between power switch S1 and S2; and the second terminal of the low-voltage side of the transformer is connected between power switch S3 and S4.

[0016] The first and second terminals of the first DC support capacitor C1 are respectively connected to the two ends of the low-voltage side circuit of the transformer.

[0017] In some specific embodiments, the high-voltage side circuit of the transformer includes:

[0018] Power switching devices S5 and S8, diodes D5, D6, D7 and D8, and second DC support capacitor C2;

[0019] The diodes D5 and D8 are connected in antiparallel with the power switching devices S5 and S8, respectively.

[0020] The first terminal of power switch S5 is connected to the second terminal of diode D7, the second terminal of power switch S5 is connected to the second terminal of diode D6, the first terminal of diode D6 is connected to the second terminal of power switch S8, the second terminal of diode D7 is connected to the first terminal of the high-voltage side circuit of the transformer, the first terminal of diode D7 is connected to the first terminal of power switch S8, the second terminal of power switch S8 is connected to the second terminal of the high-voltage side circuit of the transformer, the first terminal of the high-voltage side of the transformer is connected between power switch S5 and diode D6, and the second terminal of the high-voltage side of the transformer is connected between power switch S8 and diode D7.

[0021] The first and second ends of the second DC support capacitor C2 are respectively connected to the two ends of the high-voltage side circuit of the transformer.

[0022] In some specific embodiments, the inductor is configured on the high-voltage side or low-voltage side of the transformer, or is the leakage inductance of the transformer.

[0023] In some specific embodiments, the power switching device diode bridge arm includes multiple power switching devices and multiple diodes, wherein the power switching devices and the diodes are connected in antiparallel, and the multiple power switching devices are connected in series; the diode bridge arm includes multiple diodes connected in series.

[0024] In some specific embodiments, the power switching device is a fully controlled power switching device.

[0025] In some specific embodiments, multiple power submodules are included, wherein the low-voltage side circuits of the transformers in the multiple power submodules are connected in parallel, and the high-voltage side circuits of the transformers in the multiple power submodules are connected in series.

[0026] Secondly, embodiments of the present invention provide a DC transformer topology control method, comprising the following steps:

[0027] When the wind farm side of the DC transformer topology is not energized and the DC grid side is energized, the power switching devices in the low-voltage side circuit of the DC transformer are kept off, and the power switching devices in the high-voltage side circuit of the DC transformer are switched between on and off to generate charging current to charge the low-voltage side circuit of the DC transformer until the low-voltage side circuit of the DC transformer reaches the rated voltage.

[0028] Turn off the power switching devices in the high-voltage side circuit of the DC transformer, and control the power switching devices in the low-voltage side circuit of the DC transformer using single active full-bridge control.

[0029] In some specific embodiments, the power switching devices in the high-voltage side circuit of the DC transformer are switched between on and off to generate a charging current, which charges the low-voltage side circuit of the DC transformer, including the following steps:

[0030] Turning on the power switching device in the high-voltage side circuit of the DC transformer causes the current in the inductor connected in series with the DC transformer to rise, and the current in the inductor charges the low-voltage side circuit of the DC transformer.

[0031] Turn off the power switching devices in the high-voltage side circuit of the DC transformer until the current in the inductor connected in series with the DC transformer drops to 0.

[0032] Repeat the steps above.

[0033] The beneficial effects of the above-described technical solutions provided in the embodiments of the present invention include at least the following:

[0034] A low-cost bidirectional asymmetric power wind farm DC transformer topology and control is proposed. This eliminates the need for half of the fully controlled power switching devices on the high-voltage side, enabling bidirectional power flow. Furthermore, the fully controlled power switching devices on the high-voltage side do not require rated power transmission, allowing for the use of smaller capacity and lower-cost devices. Based on this technology, the cost of wind farm DC transformers can be significantly reduced. Since the charging current drops to zero, there is no DC bias current in the transformer, preventing magnetic saturation caused by the asymmetric full-bridge circuit.

[0035] In some specific embodiments, the high-voltage side is configured with two fully controlled power switching devices forming an asymmetrical full-bridge circuit, thus supporting bidirectional power flow and belonging to a bidirectional power topology. Since only two fully controlled power switching devices are used, the number of power switching devices on the high-voltage side is reduced by half. In some specific embodiments, the fully controlled power switching devices and their anti-parallel diodes on the high-voltage side can have the same current-carrying capacity or different current-carrying capacities, i.e., the current-carrying capacity of the anti-parallel diode is greater than that of the fully controlled device. This configuration method can reduce the cost of fully controlled power switching devices. In some specific embodiments, the DC transformer topology is configured with parallel connections on the low-voltage side and series connections on the high-voltage side, forming a series-parallel combination topology for power submodules. This topology can improve both the high-voltage side voltage and the low-voltage side current, thereby meeting the requirements of high-voltage, high-capacity applications. In some specific embodiments, the power switching devices on the high-voltage side of the DC transformer topology are connected in series, forming a series combination topology for power switching devices. This topology achieves high voltage on the high-voltage side through series connection of devices, without requiring series or parallel connection of power submodules. This does not increase the number of isolation transformers or power switching devices on the low-voltage side. In some specific embodiments, series connection of power switching devices and series or parallel connection of power submodules are combined. This topology improves the withstand voltage level of a single power submodule on the high-voltage side by connecting power switching devices in series on the high-voltage side, thereby reducing the number of power submodules required. This reduces the number of isolation transformers and power switching devices on the low-voltage side. At the same time, the number of power switching devices connected in series on the high-voltage side is less, which is conducive to the implementation of series connection of power switching devices.

[0036] Other features and advantages of the invention will be set forth in the following description, or may be learned by practicing embodiments of the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and drawings.

[0037] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0038] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the technical solutions of the invention, but do not constitute a limitation on the technical solutions of the invention. In the drawings:

[0039] Figure 1 This is a topology diagram of a DC transformer according to Embodiment 1 of the present invention;

[0040] Figure 2 This is a diagram of another DC transformer topology in Embodiment 1 of the present invention;

[0041] Figure 3 This is a schematic diagram of the DC transformer topology application in Embodiment 1 of the present invention;

[0042] Figure 4 This is a topology diagram of a DC transformer according to Embodiment 2 of the present invention;

[0043] Figure 5 This is a diagram of another DC transformer topology in Embodiment 2 of the present invention;

[0044] Figure 6 This is a flowchart of a DC transformer topology control method according to Embodiment 3 of the present invention. Detailed Implementation

[0045] Exemplary embodiments of this disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of this disclosure are shown in the drawings, it should be understood that the technical solutions of the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0046] To address the problems existing in the prior art, embodiments of the present invention provide a DC transformer topology and its control method.

[0047] Example 1

[0048] Embodiment 1 of the present invention provides a DC transformer topology, the structure of which is as follows: Figure 1 As shown, it includes a power submodule;

[0049] The power submodule includes a transformer, an inductor, a low-voltage side circuit of the transformer, and a high-voltage side circuit of the transformer.

[0050] The low-voltage side circuit of the transformer is connected to the DC output terminal of the wind farm, and the high-voltage side circuit of the transformer is connected to the high-voltage DC power grid.

[0051] The low-voltage side circuit of the transformer is a full-bridge structure, including four low-voltage side bridge arms and a first DC support capacitor. The first DC support capacitor is connected in parallel across the two ends of the low-voltage side circuit of the transformer. Each low-voltage side bridge arm includes a power switching device and a diode. The power switching device and the diode are connected in anti-parallel.

[0052] The high-voltage side circuit of the transformer is a full-bridge structure, including two power switching device diode bridge arms, two diode bridge arms, and a second DC support capacitor. One diagonal of the full-bridge structure of the high-voltage side circuit of the transformer is configured with a power switching device diode bridge arm, and the other diagonal is configured with a diode bridge arm. The second DC support capacitor is connected in parallel across the two ends of the high-voltage side circuit of the transformer. Each power switching device diode bridge arm includes at least one power switching device and at least one diode, with the power switching device and the diode connected in anti-parallel. Each diode bridge arm includes at least one diode. The inductor is connected in series with the transformer.

[0053] In some specific embodiments, the low-voltage side circuit of the transformer is connected to the DC output terminal of the wind farm, and the high-voltage side circuit of the transformer is connected to the high-voltage DC grid, including:

[0054] The first and second terminals of the low-voltage side circuit of the transformer are respectively connected to the positive and negative terminals of the DC output terminal of the wind farm, and the first and second terminals of the high-voltage side circuit of the transformer are respectively connected to the positive and negative terminals of the high-voltage DC grid.

[0055] In some specific embodiments, the low-voltage side circuit of the transformer includes:

[0056] Power switching devices S1, S2, S3 and S4, diodes D1, D2, D3 and D4, and first DC support capacitor C1;

[0057] The diodes D1, D2, D3 and D4 are connected in antiparallel with the power switching devices S1, S2, S3 and S4, respectively;

[0058] The first terminal of power switch S1 is connected to the first terminal of the low-voltage side circuit of the transformer; the second terminal of power switch S1 is connected to the first terminal of power switch S2; the second terminal of power switch S2 is connected to the second terminal of the low-voltage side circuit of the transformer; the first terminal of power switch S3 is connected to the first terminal of power switch S1; the second terminal of power switch S3 is connected to the first terminal of power switch S4; the second terminal of power switch S4 is connected to the second terminal of power switch S2; the first terminal of the low-voltage side of the transformer is connected between power switch S1 and S2; and the second terminal of the low-voltage side of the transformer is connected between power switch S3 and S4.

[0059] The first and second terminals of the first DC support capacitor C1 are respectively connected to the two ends of the low-voltage side circuit of the transformer.

[0060] In some specific embodiments, the high-voltage side circuit of the transformer includes:

[0061] Power switching devices S5 and S8, diodes D5, D6, D7 and D8, and second DC support capacitor C2;

[0062] The diodes D5 and D8 are connected in antiparallel with the power switching devices S5 and S8, respectively.

[0063] The first terminal of power switch S5 is connected to the second terminal of diode D7, the second terminal of power switch S5 is connected to the second terminal of diode D6, the first terminal of diode D6 is connected to the second terminal of power switch S8, the second terminal of diode D7 is connected to the first terminal of the high-voltage side circuit of the transformer, the first terminal of diode D7 is connected to the first terminal of power switch S8, the second terminal of power switch S8 is connected to the second terminal of the high-voltage side circuit of the transformer, the first terminal of the high-voltage side of the transformer is connected between power switch S5 and diode D6, and the second terminal of the high-voltage side of the transformer is connected between power switch S8 and diode D7.

[0064] The first and second ends of the second DC support capacitor C2 are respectively connected to the two ends of the high-voltage side circuit of the transformer.

[0065] In some specific embodiments, the inductor is configured on the high-voltage side or low-voltage side of the transformer, or is the leakage inductance of the transformer.

[0066] In some specific embodiments, the power switching device diode bridge arm includes multiple power switching devices and multiple diodes, wherein the power switching devices and the diodes are connected in anti-parallel; the diode bridge arm includes multiple diodes connected in series.

[0067] In some specific embodiments, the power switching device is a fully controllable power switching device. A fully controllable power switching device can be controlled to both turn on and off via a control signal. Examples of fully controllable devices include gate turn-off thyristors (GTOs), high-power transistors (GTRs), power MOSFETs, and insulated-gate bipolar transistors (IGBTs).

[0068] In some specific embodiments, such as Figure 2 As shown, it includes multiple power sub-modules, in which the low-voltage side circuits of the transformers in the multiple power sub-modules are connected in parallel, and in which the high-voltage side circuits of the transformers in the multiple power sub-modules are connected in series.

[0069] In some specific embodiments, such as Figure 1The DC transformer topology shown has a full-bridge structure on the low-voltage side, consisting of four fully controlled power switching devices and their anti-parallel diodes, with a DC support capacitor on the DC side. The isolation circuit consists of a transformer and its series inductor, which can be placed on the high-voltage side, the low-voltage side, or directly replaced by the transformer leakage inductance. The high-voltage side consists of two fully controlled power switching devices and their anti-parallel diodes, and two diodes, forming an asymmetrical full-bridge circuit (asymmetrical means that the left and right arms of the full bridge are not symmetrical, because one diagonal of the full bridge is equipped with fully controlled power switching devices, while the other diagonal only contains diodes), with a DC support capacitor on the DC side.

[0070] Figure 1 The topology shown features an asymmetric full-bridge circuit on the high-voltage side, composed of two fully controlled power switching devices, thus supporting bidirectional power flow and constituting a bidirectional power topology. Because only two fully controlled power switching devices are used, the number of power switching devices on the high-voltage side is reduced by half. Figure 1 In this configuration, the fully controlled power switching device and its anti-parallel diode on the high-voltage side can have the same current-carrying capacity or different current-carrying capacities, meaning the anti-parallel diode has a greater current-carrying capacity than the fully controlled device. This configuration method can reduce the cost of the fully controlled power switching device.

[0071] In some specific embodiments, such as Figure 2 The DC transformer topology shown will have multiple Figure 1 The basic structure shown has the low-voltage side connected in parallel and the high-voltage side connected in series, which can form a series-parallel combination topology for power submodules. This topology can improve the high-voltage side voltage and the low-voltage side current, thereby meeting the needs of high-voltage, high-capacity applications.

[0072] In some specific embodiments, such as Figure 3 The DC transformer topology shown connects to a 500kV DC grid on the high-voltage side and to a 30kV wind farm DC collection bus on the low-voltage side. The rated voltage of the high-voltage side of the power submodules is 3300V. The fully controlled power switching devices are 6500V IGBTs, and the diodes are also 6500V diodes, with the diodes having a higher current-carrying capacity than the IGBTs. There are 151 power submodules on the high-voltage side. The low-voltage side also uses 6500V IGBTs and 6500V diodes, with the diodes having the same current-carrying capacity as the IGBTs. The number of power submodules on the low-voltage side is also 151. Figure 3 Compared to a traditional bidirectional power topology, the topology shown saves 151*2 = 302 6500V IGBTs on the high-voltage side. Each 6500V IGBT costs approximately 20,000 RMB. Figure 3 The topology shown saves approximately 6.04 million RMB in construction costs.

[0073] The above embodiments propose a low-cost bidirectional asymmetric power wind farm DC transformer topology, which eliminates the need for half of the fully controlled power switching devices on the high-voltage side and enables bidirectional power flow. Furthermore, the fully controlled power switching devices on the high-voltage side do not need to transmit rated power, allowing for the selection of smaller capacity and lower-cost devices. Based on this technology, the cost of wind farm DC transformers can be significantly reduced.

[0074] Example 2

[0075] Embodiment 2 of the present invention provides a DC transformer topology, the structure of which is as follows: Figure 4 As shown, it includes a power submodule;

[0076] The power submodule includes a transformer, an inductor, a low-voltage side circuit of the transformer, and a high-voltage side circuit of the transformer.

[0077] The low-voltage side circuit of the transformer is connected to the DC output terminal of the wind farm, and the high-voltage side circuit of the transformer is connected to the high-voltage DC power grid.

[0078] The low-voltage side circuit of the transformer is a full-bridge structure, including four low-voltage side bridge arms and a first DC support capacitor. The first DC support capacitor is connected in parallel across the two ends of the low-voltage side circuit of the transformer. Each low-voltage side bridge arm includes a power switching device and a diode. The power switching device and the diode are connected in anti-parallel.

[0079] The high-voltage side circuit of the transformer is a full-bridge structure, including two series-connected power switching device diode bridge arms, two diode bridge arms, and a second DC support capacitor. One diagonal of the full-bridge structure is configured with power switching device diode bridge arms, and the other diagonal is configured with diode bridge arms. The second DC support capacitor is connected in parallel across the two ends of the high-voltage side circuit. Each power switching device diode bridge arm includes multiple power switching devices and corresponding diodes. Each power switching device is connected in anti-parallel with its corresponding diode, and the multiple power switching devices are connected in series. Each diode bridge arm includes multiple diodes connected in series. The inductor is connected in series with the transformer.

[0080] In some specific embodiments, the low-voltage side circuit of the transformer is connected to the DC output terminal of the wind farm, and the high-voltage side circuit of the transformer is connected to the high-voltage DC grid, including:

[0081] The first and second terminals of the low-voltage side circuit of the transformer are respectively connected to the positive and negative terminals of the DC output terminal of the wind farm, and the first and second terminals of the high-voltage side circuit of the transformer are respectively connected to the positive and negative terminals of the high-voltage DC grid.

[0082] In some specific embodiments, the inductor is configured on the high-voltage side or low-voltage side of the transformer, or is the leakage inductance of the transformer.

[0083] In some specific embodiments, the power switching device is a fully controlled power switching device.

[0084] In some specific embodiments, such as Figure 4 As shown, it includes multiple power sub-modules, in which the low-voltage side circuits of the transformers in the multiple power sub-modules are connected in parallel, and in which the high-voltage side circuits of the transformers in the multiple power sub-modules are connected in series.

[0085] In some specific embodiments, such as Figure 4 The DC transformer topology shown will Figure 1 The basic structure shown involves power switching devices on the high-voltage side connected in series, forming a series-connected power switching device topology. This topology achieves high voltage on the high-voltage side through device series connection without requiring series or parallel connection of power submodules. Therefore, it does not increase the number of isolation transformers or the number of power switching devices on the low-voltage side, resulting in lower cost. Figure 2 The topology shown is lower. But Figure 4 The topology shown presents challenges in connecting high-voltage devices in series, and the low-voltage side current-carrying capacity is lower than that of the topology. Figure 2 The topology shown.

[0086] In some specific embodiments, such as Figure 5 The topology shown will Figure 2 and Figure 4 Combining the circuits shown, a combined topology of series-connected power switches and series-parallel power submodules can be obtained. This topology, through the series connection of high-voltage-side power switches, improves the withstand voltage level of a single power submodule on the high-voltage side, thereby reducing... Figure 2 The number of power submodules required by the topology is reduced, thereby decreasing the number of isolation transformers and low-voltage side power switching devices. Meanwhile, Figure 5 The number of power switching devices connected in series on the high-voltage side of the topology shown is relatively large. Figure 4 The topology shown is less complex, which facilitates the implementation of power switching devices in series.

[0087] The above embodiments propose a low-cost bidirectional asymmetric power wind farm DC transformer topology, which eliminates the need for half of the fully controlled power switching devices on the high-voltage side and enables bidirectional power flow. Furthermore, the fully controlled power switching devices on the high-voltage side do not need to transmit rated power, allowing for the selection of smaller capacity and lower-cost devices. Based on this technology, the cost of wind farm DC transformers can be significantly reduced.

[0088] Example 3

[0089] Embodiment 3 of the present invention provides a DC transformer topology control method, the process of which is as follows: Figure 6 As shown, it includes the following steps:

[0090] When the wind farm side of the DC transformer topology is not energized and the DC grid side is energized, the power switching devices in the low-voltage side circuit of the DC transformer are kept off, and the power switching devices in the high-voltage side circuit of the DC transformer are switched between on and off to generate charging current to charge the low-voltage side circuit of the DC transformer until the low-voltage side circuit of the DC transformer reaches the rated voltage.

[0091] Turn off the power switching devices in the high-voltage side circuit of the DC transformer, and control the power switching devices in the low-voltage side circuit of the DC transformer using single active full-bridge control.

[0092] In some specific embodiments, the power switching devices in the high-voltage side circuit of the DC transformer are switched between on and off to generate charging current, which charges the low-voltage side circuit of the DC transformer, including the following steps:

[0093] Turning on the power switching device in the high-voltage side circuit of the DC transformer causes the current in the inductor connected in series with the DC transformer to rise, and the current in the inductor charges the low-voltage side circuit of the DC transformer.

[0094] Turn off the power switching devices in the high-voltage side circuit of the DC transformer until the current in the inductor connected in series with the DC transformer drops to 0.

[0095] Repeat the steps above.

[0096] by Figure 1 The control method is illustrated using the topology shown. The wind farm side is unpowered, while the DC grid side is powered. Therefore, it is necessary to provide power to the wind farm, i.e., to achieve black start of the wind farm. Figure 1 For example, the control method at this time (black start control method) is as follows:

[0097] Power switching devices S1 to S4 are always kept off.

[0098] Step 1): Power switching devices S5 and S8 are turned on, and the current of inductor L1 increases under the action of voltage. Since the low-voltage side power switching devices S1 to S4 are turned off, it is equivalent to a diode full-bridge rectifier, and the current of inductor L1 is the low-voltage side charging.

[0099] Step 2): Power switching devices S5 and S8 are turned off, L1 freewheels, continuing to charge the low-voltage side, and the charging current gradually decreases to 0. Since the charging current decreases to 0, the DC transformer has no DC bias current and will not experience magnetic saturation due to the use of an asymmetrical full-bridge circuit;

[0100] Step 3): Repeat steps 1) and 2) to charge the low-voltage side until it is charged to the rated voltage.

[0101] Step 4): After black start is completed, switch to normal operation control, that is: S5 and S8 remain off, and S1 to S4 are controlled according to the Single Active Bridge (SAB) control law.

[0102] Those skilled in the art can change the above order without departing from the scope of protection of this invention.

[0103] The above embodiments propose a low-cost bidirectional asymmetric power wind farm DC transformer topology and control, which eliminates the need for half of the fully controlled power switching devices on the high-voltage side and enables bidirectional power flow. Furthermore, the fully controlled power switching devices on the high-voltage side do not need to transmit rated power, allowing for the selection of smaller capacity and lower-cost devices. Based on this technology, the cost of wind farm DC transformers can be significantly reduced. Since the charging current drops to zero, the transformer has no DC bias current and will not experience magnetic saturation due to the use of an asymmetric full-bridge circuit.

[0104] Any modifications, additions, and equivalent substitutions made within the scope of the principles of this invention shall still fall within the patent coverage of this invention.

Claims

1. A DC transformer topology, characterized in that, Includes power submodules; The power submodule includes a transformer, an inductor, a low-voltage side circuit of the transformer, and a high-voltage side circuit of the transformer. The low-voltage side circuit of the transformer is a full-bridge structure, including four low-voltage side bridge arms and a first DC support capacitor. The first DC support capacitor is connected in parallel across the two ends of the low-voltage side circuit of the transformer. Each low-voltage side bridge arm includes a power switching device and a diode. The power switching device and the diode are connected in anti-parallel. The low-voltage side circuit of the transformer includes: power switching devices S1, S2, S3, and S4; diodes D1, D2, D3, and D4; and a first DC support capacitor C1. Diodes D1, D2, D3, and D4 are connected in anti-parallel to power switching devices S1, S2, S3, and S4, respectively. The first terminal of power switching device S1 is connected to the first terminal of the low-voltage side circuit of the transformer; the second terminal of power switching device S1 is connected to the first terminal of power switching device S2; the second terminal of power switching device S2 is connected to the second terminal of the low-voltage side circuit of the transformer; the first terminal of power switching device S3 is connected to the first terminal of power switching device S1; the second terminal of power switching device S3 is connected to the first terminal of power switching device S4; and the second terminal of power switching device S4 is connected to the second terminal of power switching device S2. The first low-voltage side circuit of the transformer is connected between power switching devices S1 and S2, and the second low-voltage side circuit of the transformer is connected between power switching devices S3 and S4. The first and second terminals of the first DC support capacitor C1 are connected to the two ends of the low-voltage side circuit of the transformer, respectively. The high-voltage side circuit of the transformer is a full-bridge structure, including two power switching device diode bridge arms, two diode bridge arms, and a second DC support capacitor. One diagonal of the full-bridge structure is configured with power switching device diode bridge arms, and the other diagonal is configured with diode bridge arms. The second DC support capacitor is connected in parallel across the two ends of the high-voltage side circuit. Each power switching device diode bridge arm includes at least one power switching device and at least one diode, with the power switching device and the diode connected in anti-parallel. Each diode bridge arm includes at least one diode. The high-voltage side circuit of the transformer includes: power switching devices S5 and S8, diodes D5, D6, D7, and D8, and a second DC support capacitor C2. Diodes D5 and D8 respectively... The power switching devices S5 and S8 are connected in anti-parallel. The first terminal of power switching device S5 is connected to the second terminal of diode D7, the second terminal of power switching device S5 is connected to the second terminal of diode D6, the first terminal of diode D6 is connected to the second terminal of power switching device S8, the second terminal of diode D7 is connected to the first terminal of the high-voltage side circuit of the transformer, the first terminal of diode D7 is connected to the first terminal of power switching device S8, and the second terminal of power switching device S8 is connected to the second terminal of the high-voltage side circuit of the transformer. The first terminal of the high-voltage side of the transformer is connected between power switching device S5 and diode D6, and the second terminal of the high-voltage side of the transformer is connected between power switching device S8 and diode D7. The first and second terminals of the second DC support capacitor C2 are respectively connected to the two ends of the high-voltage side circuit of the transformer. The inductor is connected in series with the transformer.

2. The DC transformer topology as described in claim 1, characterized in that, The first and second terminals of the low-voltage side circuit of the transformer are respectively connected to the positive and negative terminals of the DC output terminal of the wind farm, and the first and second terminals of the high-voltage side circuit of the transformer are respectively connected to the positive and negative terminals of the high-voltage DC grid.

3. The DC transformer topology as described in claim 1, characterized in that, The inductor is configured on the high-voltage side or low-voltage side of the transformer, or it is the leakage inductance of the transformer.

4. The DC transformer topology as described in claim 1, characterized in that, The power switching device diode bridge arm includes multiple power switching devices and multiple diodes, wherein the power switching devices and the diodes are connected in antiparallel, and the multiple power switching devices are connected in series; the diode bridge arm includes multiple diodes connected in series.

5. The DC transformer topology as described in any one of claims 1 to 4, characterized in that, It includes multiple power sub-modules, wherein the low-voltage side circuits of the transformers in the multiple power sub-modules are connected in parallel, and the high-voltage side circuits of the transformers in the multiple power sub-modules are connected in series.

6. A method for controlling the topology of a DC transformer, applicable to the DC transformer topology as described in any one of claims 1-5, characterized in that, Includes the following steps: When the wind farm side of the DC transformer topology is not energized and the DC grid side is energized, the power switching devices in the low-voltage side circuit of the DC transformer are kept off, and the power switching devices in the high-voltage side circuit of the DC transformer are switched between on and off to generate charging current to charge the low-voltage side circuit of the DC transformer until the low-voltage side circuit of the DC transformer reaches the rated voltage. Turn off the power switching devices in the high-voltage side circuit of the DC transformer, and control the power switching devices in the low-voltage side circuit of the DC transformer using single active full-bridge control.

7. The DC transformer topology control method as described in claim 6, characterized in that, The power switching devices in the high-voltage side circuit of the DC transformer are switched between on and off to generate charging current, which charges the low-voltage side circuit of the DC transformer. The steps include: Turning on the power switching device in the high-voltage side circuit of the DC transformer causes the current in the inductor connected in series with the DC transformer to rise, and the current in the inductor charges the low-voltage side circuit of the DC transformer. Turn off the power switching devices in the high-voltage side circuit of the DC transformer until the current in the inductor connected in series with the DC transformer drops to 0. Repeat the steps above.

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