Wind farm dc transformer and black start method

Abstract

The application provides a wind farm DC transformer and a black start method. The DC transformer comprises a forward power circuit and a reverse power circuit connected with the forward power circuit. The forward power circuit is used for boosting DC power generated by a wind farm and transmitting the boosted DC power to an onshore converter station. The reverse power circuit is used for transmitting reverse power provided by the onshore converter station to the forward power circuit to provide start-up power for the wind farm. The application can meet the demand for reverse power in black start and greatly reduce the overall cost of the DC transformer.

Description

Technical Field

[0001] This invention belongs to the field of transformer technology, and specifically relates to a DC transformer for wind farms and a black start method. Background Technology

[0002] Offshore wind power is characterized by abundant resources, high utilization hours, no land occupation, and suitability for large-scale development, making it the latest frontier in global wind power development. Compared to onshore wind power, offshore wind power has 20% to 40% higher energy efficiency, and also offers advantages such as no land occupation, high wind speeds, less dust, large power generation, stable operation, and zero dust emissions. Furthermore, it reduces turbine wear and extends the service life of wind turbine units, making it suitable for large-scale development.

[0003] Offshore wind power DC transmission systems can significantly reduce cable usage and increase transmission distance, showing broad application prospects. These systems mainly consist of an offshore converter station (offshore step-up DC transformer, DC / DC), an onshore converter station (DC / AC), and submarine cables. Before starting an offshore wind farm, the wind farm side is not energized and requires reverse power from the onshore converter station to support its startup; this startup process is called "black start."

[0004] Black-start functionality requires the offshore step-up DC transformer to have the capability for reverse power flow, and the reverse starting power must be much smaller than the forward generating power. Existing main solutions are:

[0005] High-power forward unidirectional DC transformer + low-power reverse DC transformer;

[0006] This solution requires a low-power reverse DC transformer, which, although low in power, still has a high voltage, thus resulting in high cost.

[0007] Therefore, it is necessary to design a DC transformer for wind farms and a black start method to solve the above-mentioned technical problems. Summary of the Invention

[0008] To address the above problems, this invention proposes a DC transformer for wind farms, wherein the DC transformer comprises:

[0009] A forward power circuit and a reverse power circuit connected to the forward power circuit, wherein,

[0010] The forward power circuit is used to boost the DC power generated by the wind farm and transmit it to the onshore converter station.

[0011] The reverse power circuit is used to transfer the reverse power provided by the onshore converter station to the forward power circuit in order to provide startup power to the wind farm.

[0012] Furthermore, the forward power circuit includes a low-voltage side circuit, an isolation transformer T1, and a high-voltage side circuit, wherein,

[0013] The input terminal of the forward power circuit is connected to the wind farm, and the output terminal of the forward power circuit is connected to the low-voltage side of the isolation transformer T1.

[0014] The input terminal of the high-voltage side circuit is connected to the high-voltage side of the isolation transformer, and the output terminal of the high-voltage side circuit is connected to the onshore converter station.

[0015] Furthermore, the low-voltage side circuit includes a full-bridge power module and a DC support capacitor C2, wherein,

[0016] The DC support capacitor C2 is connected in parallel to the input terminal of the full-bridge power module, and the output terminal of the full-bridge power module is connected to the low-voltage side of the isolation transformer T1.

[0017] Furthermore, the output terminal of the full-bridge power module is connected to the low-voltage side of the isolation transformer T1 through a series resonant circuit.

[0018] Furthermore, the high-voltage side circuit includes a first full-wave rectifier and a DC support capacitor C3, wherein,

[0019] The input terminal of the first full-wave rectifier is connected to the high-voltage side of the isolation transformer T1, and the DC support capacitor C3 is connected in parallel to the output terminal of the first full-wave rectifier.

[0020] Furthermore, the reverse power circuit includes a second full-wave rectifier, a winding transformer T2, and a power feed coil T3, wherein,

[0021] The output terminal of the second full-wave rectifier is connected to the input terminal of the high-voltage side circuit. The input terminal of the second full-wave rectifier is connected to the low-voltage side of the winding transformer T2. The low-voltage side of the energy feed coil T3 is connected to the high-voltage side of the winding transformer T2. The high-voltage side of the energy feed coil T3 is connected to the output terminal of the high-voltage side circuit.

[0022] Furthermore, the reverse power circuit also includes a switch K1, which is connected in parallel to the low-voltage side of the power feed coil T3 to bypass the reverse power circuit.

[0023] On the other hand, the present invention also provides a black start method for a DC transformer in a wind farm, wherein the DC transformer is the aforementioned DC transformer, and the black start method includes:

[0024] The control onshore converter station generates an AC voltage with a set threshold at the output of the forward power circuit so that an AC ripple current is generated at the input of the reverse power circuit. The AC current is rectified by the reverse power circuit and then charges the DC support capacitor at the input of the forward power circuit.

[0025] After charging is complete, the control system stops generating a set threshold AC voltage at the output of the forward power circuit of the onshore converter station, and closes the bypass switch in the reverse power circuit so that after the reverse power circuit is bypassed, the output of the forward power circuit generates output power that can be delivered to the onshore converter station.

[0026] Furthermore, the forward power circuit includes a low-voltage side circuit, an isolation transformer, and a high-voltage side circuit, wherein,

[0027] The input terminal of the forward power circuit is connected to the wind farm, and the output terminal of the forward power circuit is connected to the low-voltage side of the isolation transformer.

[0028] The input terminal of the high-voltage side circuit is connected to the high-voltage side of the isolation transformer, and the output terminal of the high-voltage side circuit is connected to the onshore converter station.

[0029] Furthermore, the low-voltage side circuit includes a full-bridge power module and the aforementioned DC support capacitor, wherein,

[0030] The DC support capacitor is connected in parallel to the input terminal of the full-bridge power module, and the output terminal of the full-bridge power module is connected to the low-voltage side of the isolation transformer T1.

[0031] The present invention discloses a DC transformer for wind farms and a black start method. By adding a low-cost energy harvesting coil (i.e., energy feed coil) and a low-voltage, low-power rectifier circuit (i.e., a second full-wave rectifier) ​​to the DC transmission line, energy is harvested by utilizing a certain AC ripple current output from the onshore converter station during the black start process. This can meet the reverse power requirements of black start and significantly reduce the overall cost of the DC transformer.

[0032] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description, claims and drawings. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 A basic DC transformer topology according to an embodiment of the present invention is shown.

[0035] Figure 2 An embodiment of the present invention is shown, in which a DC transformer topology is based on a basic DC transformer, and a single diode is used on the low-voltage side of the winding transformer.

[0036] Figure 3 The illustration shows a combined structure of a DC transformer topology formed based on a basic DC transformer, according to an embodiment of the present invention.

[0037] Figure 4 A DC transformer topology according to an embodiment of the present invention is shown.

[0038] Figure 5 A flowchart of a black-start method for a DC transformer in a wind farm according to an embodiment of the present invention is shown. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Furthermore, in this invention, the terms “first,” “second,” and other similar terms are not intended to imply any order, quantity, or importance, but are merely used to distinguish different elements.

[0041] like Figure 1 The present invention provides a DC transformer for a wind farm, wherein the DC transformer comprises:

[0042] A forward power circuit and a reverse power circuit connected to the forward power circuit, wherein,

[0043] The forward power circuit is used to boost the DC power generated by the wind farm and transmit it to the onshore converter station.

[0044] The reverse power circuit is used to transfer the reverse power provided by the onshore converter station to the forward power circuit in order to provide startup power to the wind farm.

[0045] Currently, another solution for black-start functionality requiring an offshore boost DC transformer is a bidirectional high-power DC transformer. However, this bidirectional high-power DC transformer lacks a reverse high-power operating condition, leading to wasted power switching device capacity and high cost. This invention effectively solves this problem, and a detailed description of the invention follows.

[0046] In one embodiment of the present invention, the forward power circuit includes a low-voltage side circuit, an isolation transformer T1, and a high-voltage side circuit, wherein,

[0047] The input terminal of the forward power circuit is connected to the wind farm, and the output terminal of the forward power circuit is connected to the low-voltage side of the isolation transformer T1.

[0048] The input terminal of the high-voltage side circuit is connected to the high-voltage side of the isolation transformer, and the output terminal of the high-voltage side circuit is connected to the onshore converter station.

[0049] In one embodiment of the present invention, the low-voltage side circuit includes a full-bridge power module and a DC support capacitor C2, wherein the DC support capacitor C2 (the positive terminal of the DC support capacitor C2 serves as the DC+ port of the positive power circuit input terminal, and the negative terminal of the DC support capacitor C2 serves as the DC- port of the positive power circuit input terminal) is connected in parallel to the input terminal of the full-bridge power module, and the output terminal of the full-bridge power module is connected to the low-voltage side of the isolation transformer T1.

[0050] In one embodiment of the present invention, the full-bridge power module includes a first bridge arm and a second bridge arm connected in parallel, wherein,

[0051] The first bridge arm includes a fully controlled power switch device S1 and a fully controlled power switch device S2. A diode D1 is connected in anti-parallel to the fully controlled power switch device S1, and a diode D2 is connected in anti-parallel to the fully controlled power switch device S2. The cathode of the fully controlled power switch device S1 is connected to the anode of the fully controlled power switch device S2 through a connection point.

[0052] The second bridge arm includes a fully controlled power switch device S3 and a fully controlled power switch device S4. A diode D3 is connected in anti-parallel to the fully controlled power switch device S3, and a diode D4 is connected in anti-parallel to the fully controlled power switch device S4. The cathode of the fully controlled power switch device S3 is connected to the anode of the fully controlled power switch device S4 through a connection point.

[0053] The anode of the fully controlled power switch S1 is connected to the anode of the fully controlled power switch S3, and the cathode of the fully controlled power switch S2 is connected to the cathode of the fully controlled power switch S4.

[0054] The positive terminal of the DC support capacitor C2 is connected to the anode of the fully controlled power switching device S1, and the negative terminal of the DC support capacitor C2 is connected to the cathode of the fully controlled power switching device S2.

[0055] In one embodiment of the invention, the output terminal of the full-bridge power module is connected to the low-voltage side of the isolation transformer T1 via a series resonant circuit, specifically:

[0056] The connection point on the first bridge arm is connected to one end of the winding on the low-voltage side of the isolation transformer T1 through a series resonant circuit; the connection point on the second bridge arm is connected to the other end of the winding on the low-voltage side of the isolation transformer T1.

[0057] In one embodiment of the present invention, the series resonant circuit includes a series resonant inductor L1 and a series resonant capacitor C1, wherein one end of the series resonant inductor L1 is connected to one end of the series resonant capacitor C1, the other end of the series resonant inductor L1 is connected to a connection point on the first bridge arm, and the other end of the series resonant capacitor C1 is connected to one end of the winding on the low-voltage side of the isolation transformer T1.

[0058] In one embodiment of the present invention, the high-voltage side circuit includes a first full-wave rectifier and a DC support capacitor C3, wherein the input terminal of the first full-wave rectifier is connected to the high-voltage side of the isolation transformer T1, and the DC support capacitor C3 is connected in parallel to the output terminal of the first full-wave rectifier. Specifically:

[0059] The first full-wave rectifier includes two third bridge arms connected in parallel, wherein,

[0060] The third bridge arm includes an upper bridge arm and a lower bridge arm, which are connected by a connection point.

[0061] In the two third bridge arms, the two connection points are respectively connected to the two ends of the high-voltage side winding of the isolation transformer T1;

[0062] Both the upper and lower bridge arms are equipped with n series-connected diodes D5;

[0063] For the upper arm of one of the third bridge arms: among the n diodes D5, the cathode of the first diode D5 serves as one end of the upper arm of one of the third bridge arms, the anode of the nth diode D5 is connected to the connection point of one of the third bridge arms, and among the remaining diodes D5, the cathode of the next diode D5 is connected to the anode of the previous diode D5.

[0064] For the lower arm of one of the third bridge arms: among the n diodes D5, the cathode of the first diode D5 is connected to the connection point of one of the third bridge arms, the anode of the nth diode D5 serves as one end of the lower arm of one of the third bridge arms, and among the remaining diodes D5, the cathode of the next diode D5 is connected to the anode of the previous diode D5.

[0065] The structure of the other third bridge arm is the same as that of one of the third bridge arms;

[0066] One end of each of the two upper bridge arms is connected to the positive terminal of the DC support capacitor C3, and one end of each of the two lower bridge arms is connected to the negative terminal of the DC support capacitor C3 (this negative terminal can serve as the DC-port of the positive power circuit output).

[0067] In this embodiment, the two connection points of the first full-wave rectifier are respectively connected to the two ends of the winding on the high-voltage side of the isolation transformer T1.

[0068] In one embodiment of the present invention, the reverse power circuit includes a second full-wave rectifier, a winding transformer T2, and a power feed coil T3. The power feed coil T3 utilizes the reverse principle of a current transformer (CT), converting a small current on the high-voltage line into a large current on the secondary side of the coil to charge the low-voltage side. The power feed coil T3 functions as a current ratio converter and provides insulation. In this embodiment, the output terminal of the second full-wave rectifier is connected to the input terminal of the high-voltage side circuit, and the input terminal of the second full-wave rectifier is connected to the low-voltage side of the winding transformer T2. The low-voltage side of the power feed coil T3 is connected to the high-voltage side of the winding transformer T2, and the high-voltage side of the power feed coil T3 is connected to the output terminal of the high-voltage side circuit.

[0069] Specifically, the power feed coil T3 is configured as follows: a first connecting wire is led out from the positive terminal of the DC support capacitor C3 (one end of the first connecting wire can be used as the DC+ port of the positive power circuit output terminal). A portion of the first connecting wire is wound on the primary side of an iron core as the high-voltage side winding of the power feed coil T3, and a portion of a second connecting wire is wound on the secondary side of the iron core as the low-voltage side winding of the power feed coil T3.

[0070] In this embodiment, specifically for the second full-wave rectifier:

[0071] The second full-wave rectifier includes a fourth and a fifth bridge arm connected in parallel. The fourth bridge arm has diodes D5 and D6, with the anode of diode D5 connected to the cathode of diode D6 via a connection point. The fifth bridge arm has diodes D7 and D8, with the anode of diode D7 connected to the cathode of diode D8 via a connection point. The cathodes of diodes D7 and D5 are interconnected and then connected to the positive terminal of DC support capacitor C2. The anodes of diodes D6 and D8 are interconnected and then connected to the negative terminal of DC support capacitor C2. The two connection points of the second full-wave rectifier are respectively connected to the two ends of the low-voltage winding of winding transformer T2.

[0072] In one embodiment of the present invention, the reverse power circuit further includes a switch K1, which is connected in parallel to the low-voltage side of the power feed coil T3 (i.e., the two ends of switch K1 are respectively connected to the two ends of the winding on the low-voltage side of the power feed coil T3) to bypass the reverse power circuit. Additionally, in this embodiment, the two ends of the winding on the low-voltage side of the winding transformer T2 are respectively connected to the two ends of switch K1.

[0073] The above is Figure 1 The introduction of topology, in this invention, is based on the above. Figure 1 Based on the medium DC transformer, it is about to Figure 1 As a basic DC transformer, the intermediate DC transformer can be improved in several ways, such as:

[0074] In one embodiment of the present invention, such as Figure 2 As shown, to further reduce costs, in the reverse power circuit, the low-voltage side of the winding transformer T2 uses a half-wave rectifier circuit composed of a diode D5 to rectify the positive half-wave AC and charge the low-voltage DC side. That is, at this time, the second full-wave rectifier includes diode D5. The anode of diode D5 is connected to one end of the winding on the low-voltage side of the winding transformer T2, and the cathode of diode D5 is connected to the positive terminal of the DC support capacitor C2. At this time, the other end of the winding on the low-voltage side of the winding transformer T2 is connected to the negative terminal of the DC support capacitor C2.

[0075] In this embodiment, a bypass diode is added to the high-voltage side of the winding transformer T2 to complete the negative half-wave current flow. That is, a diode D1 is connected in parallel across the two ends of switch K1. The DC transformer in this embodiment is relatively... Figure 1 The topology shown saves two diodes, resulting in lower cost and suitability for applications with low reverse startup power.

[0076] In one embodiment of the present invention, such as Figure 3 As shown, will Figure 1The DC transformers described in the text are combined in series and parallel to form a combined DC transformer structure. This combined DC transformer structure has m forward power circuits. The high-voltage sides of the m forward power circuits are connected in series (i.e., the output terminals of the m forward power circuits are connected in series), and the low-voltage sides are connected in parallel (i.e., the input terminals of the m forward power circuits are connected in parallel). Specifically:

[0077] m DC support capacitors C3 are connected in series. The positive terminal of the first DC support capacitor C3 can lead out the first connecting line (one end of the first connecting line can be used as the DC+ port of the DC transformer output terminal of the combined structure), and the negative terminal of the m-th DC support capacitor C3 can be used as the DC- port of the DC transformer output terminal of the combined structure.

[0078] In the reverse power circuit: the cathodes of diode D7 and D5 are connected to each other and then connected to the positive terminals of m DC support capacitors C2; the anodes of diode D6 and D8 are connected to each other and then connected to the negative terminals of m DC support capacitors C2.

[0079] In m positive power circuits, the positive terminal of one of the DC support capacitors C2 serves as the DC+ port of the DC transformer input of the combined structure, and the negative terminal of the DC support capacitor C2 serves as the DC- port of the DC transformer input of the combined structure.

[0080] In this embodiment, since it is difficult to connect a large number of diodes in series, this embodiment uses a combination structure of series and parallel connection of forward power modules (forward power circuits) to reduce the voltage on the high-voltage side of each forward power module, that is, to reduce the number of diodes connected in series, while increasing the current carrying capacity on the low-voltage side.

[0081] On the other hand, the present invention also provides a black start method for DC transformers in wind farms, based on Figure 1 The black-start method for the DC transformer shown includes:

[0082] When all fully controlled power switching devices in the full-bridge power module of the forward power circuit are turned off, the bypass switch in the reverse power circuit is kept open, that is, the fully controlled power switching devices S1 to S4 are kept off, and the switch K1 is kept open.

[0083] The onshore converter station generates an AC voltage at a set threshold at the output of the forward power circuit, causing an AC ripple current to be generated at the input of the reverse power circuit. This AC current, after rectification by the reverse power circuit, charges the DC support capacitor at the input of the forward power circuit. Specifically:

[0084] The onshore converter station generates a certain AC voltage through the DC+ and DC- ports at the output of the forward power circuit. This AC voltage forms an AC path through the DC support capacitor C3, generating an AC ripple current. The AC current is induced to the secondary side through the power feed coil T3, and after being rectified by diodes D5 to D8 (D5 for simplified topology), it charges the DC support capacitor C2.

[0085] After charging is complete, the control system stops generating a set threshold AC voltage at the output of the forward power circuit of the onshore converter station, and closes the bypass switch in the reverse power circuit so that after the reverse power circuit is bypassed, the output of the forward power circuit generates output power that can be delivered to the onshore converter station.

[0086] Specifically:

[0087] After the DC support capacitor C2 completes charging, the onshore converter station stops outputting AC voltage at the DC+ and DC- ports on the high-voltage side (output of the forward power circuit). Switch K1 is closed to bypass the reverse power circuit. Consequently, fully controlled power switching devices S1 and S4 conduct complementaryly, as do S2 and S3. Power is then output to the onshore converter station via the resonant path formed by the series resonant inductor L1 and series resonant capacitor C1, the isolation transformer T1, and the high-voltage side full-wave rectifier bridge (i.e., the first full-wave rectifier). The DC transformer then completes its startup.

[0088] In this embodiment, the energy feeder coil T3 can only transform AC current. Therefore, during the black start-up phase, the DC side of the onshore converter station is required to output a certain AC harmonic current so that the energy feeder coil T3 can extract energy.

[0089] In addition, in this embodiment, based on Figure 1 DC transformers, such as Figure 4 As shown, the wind farm is an offshore wind farm. The offshore wind farm includes an AC-DC converter and a wind turbine. The DC+ and DC- ports of the input terminal of the forward power circuit are both connected to the AC-DC converter. The AC-DC converter is used to convert the AC power generated by the wind turbine into DC power, which is then transmitted to the forward power circuit for voltage boosting through the DC+ and DC- ports of the input terminal of the forward power circuit. After voltage boosting, it is transmitted to the receiving-end converter station (i.e., the onshore converter station) through the DC+ and DC- ports of the output terminal of the forward power circuit.

[0090] based on Figure 4 The present invention also provides the following examples:

[0091] The low-voltage side (input terminal of the forward power circuit) has a DC voltage of 30kV, and the high-voltage side (output terminal of the forward power circuit) has a DC voltage of 500kV. The high-voltage side series diodes (i.e., the diodes in the first full-wave rectifier) ​​are based on diodes with a rated voltage of 6500V. There is one series bridge arm, meaning there are two third bridge arms in the first full-wave rectifier. Each third bridge arm has an upper bridge arm and a lower bridge arm. Therefore, the first full-wave rectifier has four bridge arms. Each of these four bridge arms contains 151 series diodes D9, for a total of 4 * 151 = 604 diodes D9 across the four series bridge arms.

[0092] In the full-bridge power module of the low-voltage side forward power circuit (low-voltage side circuit), all fully controlled power switching devices are I GCTs, and the rated voltage of the I GCTs is 6500V. The rated voltage of all diodes is 6500V. Each fully controlled power switching device and its anti-parallel diode form a series bridge arm. There are a total of 4 bridge arms. Each bridge arm contains 9 I GCTs and 9 anti-parallel diodes. The four bridge arms contain a total of 4*9=36 I GCTs and 4*9=36 anti-parallel diodes.

[0093] Since the first full-wave rectifier has four bridge arms, each bridge arm contains nine diodes with a rated voltage of 6500V (for example, there are nine diodes each of D5, D6, D7 and D8), the four bridge arms contain a total of 4*9=36 diodes.

[0094] based on Figure 4 Compared to traditional bidirectional power DC transformer solutions, this new topology eliminates the need for fully controlled power switching devices on the high-voltage side, significantly reducing costs. In contrast, traditional solutions using small-capacity reverse power DC transformers require a separate high-voltage, small-capacity DC transformer, which necessitates not only fully controlled power switching devices on its high-voltage side but also a large number of such devices due to the high voltage. Figure 4 The proposed solution does not have the aforementioned problems, thus significantly reducing costs.

[0095] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A DC transformer for a wind farm, wherein, The DC transformer includes: A forward power circuit and a reverse power circuit connected to the forward power circuit, wherein the forward power circuit is used to boost the DC power generated by the wind farm and transmit it to the onshore converter station; the reverse power circuit is used to transfer the reverse power provided by the onshore converter station to the forward power circuit to provide starting power to the wind farm. The reverse power circuit includes a second full-wave rectifier, a winding transformer T2, and a power feed coil T3. The output terminal of the second full-wave rectifier is connected to the input terminal of the high-voltage side circuit, and the input terminal of the second full-wave rectifier is connected to the low-voltage side of the winding transformer T2. The low-voltage side of the power feed coil T3 is connected to the high-voltage side of the winding transformer T2, and the high-voltage side of the power feed coil T3 is connected to the output terminal of the high-voltage side circuit. The reverse power circuit also includes a switch K1, which is connected in parallel to the low-voltage side of the power feed coil T3 to bypass the reverse power circuit. The energy feeder coil T3 extracts energy based on the principle of a current transformer. The forward power circuit includes a low-voltage side circuit, an isolation transformer T1, and a high-voltage side circuit. The input terminal of the low-voltage side circuit is connected to the wind farm, and the output terminal of the low-voltage side circuit is connected to the low-voltage side of the isolation transformer T1. The input terminal of the high-voltage side circuit is connected to the high-voltage side of the isolation transformer T1, and the output terminal of the high-voltage side circuit is connected to the onshore converter station.

2. A DC transformer for a wind farm according to claim 1, wherein, The low-voltage side circuit includes a full-bridge power module and a DC support capacitor C2, wherein... The DC support capacitor C2 is connected in parallel to the input terminal of the full-bridge power module, and the output terminal of the full-bridge power module is connected to the low-voltage side of the isolation transformer T1.

3. A DC transformer for a wind farm according to claim 2, wherein, The output of the full-bridge power module is connected to the low-voltage side of the isolation transformer T1 via a series resonant circuit.

4. A DC transformer for a wind farm according to claim 1, wherein, The high-voltage side circuit includes a first full-wave rectifier and a DC support capacitor C3, wherein... The input terminal of the first full-wave rectifier is connected to the high-voltage side of the isolation transformer T1, and the DC support capacitor C3 is connected in parallel to the output terminal of the first full-wave rectifier.

5. A black-start method for a DC transformer in a wind farm, wherein, The DC transformer is the DC transformer as described in any one of claims 1-4, and the black start method includes: The control onshore converter station generates an AC voltage with a set threshold at the output terminal of the forward power circuit, so as to generate an AC ripple current at the input terminal of the reverse power circuit. The AC ripple current charges the DC support capacitor C2 at the input terminal of the forward power circuit after being rectified by the reverse power circuit. After charging is complete, the control system stops generating a set threshold AC voltage at the output of the forward power circuit of the onshore converter station, and closes the bypass switch in the reverse power circuit so that after the reverse power circuit is bypassed, the output of the forward power circuit generates the output power to be delivered to the onshore converter station.

6. A black-start method for a DC transformer in a wind farm according to claim 5, wherein, The forward power circuit includes a low-voltage side circuit, an isolation transformer T1, and a high-voltage side circuit, wherein... The input terminal of the low-voltage side circuit is connected to the wind farm, and the output terminal of the low-voltage side circuit is connected to the low-voltage side of the isolation transformer T1. The input terminal of the high-voltage side circuit is connected to the high-voltage side of the isolation transformer T1, and the output terminal of the high-voltage side circuit is connected to the onshore converter station.

7. A black-start method for a DC transformer in a wind farm according to claim 6, wherein, The low-voltage side circuit includes a full-bridge power module and the DC support capacitor C2, wherein... The DC support capacitor C2 is connected in parallel to the input terminal of the full-bridge power module, and the output terminal of the full-bridge power module is connected to the low-voltage side of the isolation transformer T1.

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