Dc transformer module with three-level bypass structure and control method thereof

Abstract

The application provides a direct-current transformer module with a three-level bypass structure and a control method thereof. The direct-current transformer module comprises a three-level bypass branch, a front-stage circuit, an isolation transformer, a resonance branch and a rear-stage circuit, the three-level bypass branch, the front-stage circuit, the isolation transformer and the rear-stage circuit are cascaded in sequence, and the resonance branch is connected in series between the front-stage circuit and the isolation transformer or between the isolation transformer and the rear-stage circuit. The front-stage circuit comprises 2N-1 bridge circuits connected in series, N is a positive integer greater than or equal to 2, and the direct-current side capacitor of the Nth bridge circuit is split into two. The three-level bypass branch is connected to the capacitor midpoint of the Nth bridge circuit through the midpoint of four series-connected switching devices.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronic applications, in particular to a DC transformer module with a three-level bypass structure and a control method thereof. BACKGROUND

[0002] As an important component device for realizing voltage conversion in a DC power grid, a DC transformer has attracted more and more attention from scholars in the field of DC power grids. In order to realize the conversion from medium / high voltage to low voltage, affected by the stress and cost of switching tubes, the DC transformer of this type of application usually adopts a structure of multiple modules in series input and parallel output (ISOP), and the modules generally adopt an isolated DC-DC converter based on a dual active bridge (DAB) or an LC resonance technology.

[0003] With the increase of the voltage level, the number of modules required by the DC transformer also increases, which leads to a decrease in the reliability of the system, and therefore a large number of redundant modules need to be provided in the system. Some scholars have proposed some new circuit topologies for the ISOP structure of the DC transformer. These circuits can reduce the number of modules used and the number of redundant modules of the system by increasing the voltage level of the series side, and also reduce the cost.

[0004] Representative new circuits include a three-level circuit and a T2DAB circuit. The three-level circuit can increase the voltage level by 1 times compared with the dual active bridge circuit, and the number of modules can be reduced by 1 / 2. The T2DAB circuit mentioned in the patent CN111082665A can increase the voltage level of the high-voltage side by at least 2 times compared with the dual active bridge circuit.

[0005] Although the new circuit can reduce the number of modules, it still cannot avoid the necessary bypass function. In the three-level circuit, the bypass function of the module can be easily realized by setting a three-level bypass bridge arm. However, for the T2DAB circuit, the patent CN111082665A does not consider the bypass problem. If the traditional half-bridge bypass method is used, the switching device stress of the half-bridge circuit is high, and the cost is also high. If the bypass bridge arm scheme of the three-level circuit is referred to, when the T2DAB circuit uses an even number of series front bridge circuits, after the capacitors of all the bridge circuits realize series voltage sharing, the T2DAB circuit can provide voltage clamping points for balancing the upper and lower voltages, and the three-level bypass bridge arm can be connected to reduce the switching device stress of the bypass branch. However, when the T2DAB circuit uses an odd number of bridge circuits, especially three, the T2DAB circuit cannot provide voltage clamping points similar to the three-level bridge arm, and if the three-level bypass bridge arm is forcibly connected, the switching tube voltage stress will still be high. SUMMARY

[0006] The embodiment of the present application provides a DC transformer module with a three-level bypass structure, comprising a three-level bypass branch and a front-stage circuit, the three-level bypass branch comprises a first semiconductor switch, a second semiconductor switch, a third semiconductor switch and a fourth semiconductor switch connected in series, a positive end of the first semiconductor switch is a positive end of the three-level bypass branch, the first semiconductor switch and the second semiconductor switch are connected to lead out a first terminal, the second semiconductor switch and the third semiconductor switch are connected to lead out a second terminal, the third semiconductor switch and the fourth semiconductor switch are connected to lead out a third terminal, and a negative end of the fourth semiconductor switch is a negative end of the three-level bypass branch; the front-stage circuit comprises at least 2N-1 bridge circuits connected in series, N is a positive integer greater than or equal to 2, wherein a positive end of a direct current side of a first bridge circuit is a positive end of the front-stage circuit, a negative end of a direct current side of a 2N-1 bridge circuit is a negative end of the front-stage circuit, a capacitor branch is connected in parallel to a direct current side of each bridge circuit, the capacitor branch of an N bridge circuit comprises at least two capacitors connected in series, a midpoint of the series capacitors is defined as a fourth terminal, the positive end of the three-level bypass branch is connected to the positive end of the front-stage circuit, the negative end of the three-level bypass branch is connected to the negative end of the front-stage circuit, and the second terminal is connected to the fourth terminal; the first terminal and the third terminal lead out as a first port of the DC transformer module.

[0007] According to some embodiments, the module further comprises an isolation transformer, the isolation transformer is a multi-winding transformer, including at least 2N windings, defining two ends of each winding as the A end of the winding and the B end of the winding, the connection mode includes at least one of the following: first, the bridge circuit is a half-bridge circuit including two semiconductor switches, the midpoints of all 2N-1 half-bridge circuits are connected to the A ends of the first 2N-1 windings of the isolation transformer, and the B ends of the first 2N-1 windings of the isolation transformer are connected in series through a first resonant branch; or second, the bridge circuit is a half-bridge circuit including two semiconductor switches, the B ends of the first 2N-1 windings of the isolation transformer are connected, and the midpoints of all 2N-1 half-bridge circuits are connected to the A ends of the first 2N-1 windings of the isolation transformer through a first resonant branch; or third, the bridge circuit is a half-bridge circuit including two semiconductor switches, the midpoints of all 2N-1 half-bridge circuits are connected to the A ends of the first 2N-1 windings of the isolation transformer, and the B ends of the first 2N-1 windings of the isolation transformer are connected in series through a first resonant branch; and the B end of the 2N-1 winding is connected in series with the B end of the first winding through a first resonant branch; or fourth, the bridge circuit is a half-bridge circuit including two semiconductor switches; the midpoints, positive terminals and negative terminals of all 2N-1 half-bridge circuits are connected to the first 2N-1 windings of the isolation transformer through a first resonant branch; or fifth, the bridge circuit is a half-bridge circuit including two semiconductor switches, and the series capacitor branch of all bridge circuits includes two series-connected capacitors; the midpoints of all 2N-1 half-bridge circuits and the midpoints of the series capacitor branches are connected to the first 2N-1 windings of the isolation transformer; or sixth, the bridge circuit is a full-bridge circuit including four semiconductor switches, and the AC ports of all 2N-1 full-bridge circuits are connected to the first 2N-1 windings of the isolation transformer through a second resonant branch; the first resonant branch includes a capacitor or a series-connected capacitor and inductor.

[0008] According to some embodiments, in the first, second and third connection modes, the B end of the N winding is further connected to the fourth terminal through a direct connection wire or a first resonant branch or a semiconductor switch.

[0009] According to some embodiments, the module further comprises a post-stage circuit, and the connection mode comprises at least one of the following: the post-stage circuit is a full-bridge circuit comprising four semiconductor switches, the second N winding of the isolation transformer is connected after the AC port of the post-stage circuit in series with a second resonant branch; the DC port of the post-stage circuit is the second port of the DC transformer module; or the post-stage circuit is a multi-phase half-bridge circuit comprising 2N-1 half-bridge circuits, the isolation transformer further comprises the second N to fourth N-2 windings, the midpoints of the half-bridge circuits of all the post-stage circuits are led out and connected to the A end of the second N to fourth N-2 windings of the isolation transformer after being connected to a second resonant branch in series, the B end of the second N to fourth N-2 windings of the isolation transformer is connected together, and the bridge arms of all the half-bridge circuits of the post-stage circuit are connected in parallel at both ends to be the second port of the DC transformer module; or the post-stage circuit is a multi-phase full-bridge circuit comprising 2N-1 full-bridge circuits, the isolation transformer further comprises the second N to fourth N-2 windings, the AC port of all the full-bridge circuits of the post-stage circuit is connected to the second N to fourth N-2 windings of the isolation transformer after being connected to a second resonant branch in series; and the DC port of all the full-bridge circuits of the post-stage circuit is connected in parallel to be the second port of the DC transformer module; wherein the second resonant branch comprises a direct connection wire, a capacitor, an inductor, or a series connection of a capacitor and an inductor, and the semiconductor switches of the post-stage circuit comprise half-controlled switches.

[0010] According to some embodiments, the first semiconductor switch, the second semiconductor switch, the third semiconductor switch, the fourth semiconductor switch of the three-level bypass branch, and the semiconductor switches of the pre-stage circuit are all fully-controlled switches.

[0011] According to some embodiments, the three-level bypass branch further comprises a first bypass switch connected between the first terminal and the third terminal of the three-level bypass branch.

[0012] The embodiments of the present application also provide a control method of a DC transformer module with a three-level bypass structure, comprising: turning on the first semiconductor switch and the fourth semiconductor switch of the three-level bypass branch; unlocking all the semiconductor switches of the pre-stage circuit, and detecting the voltage of the capacitor branch of the bridge circuit of all the pre-stage circuits, and the voltage of the first port and the voltage of the second port of the DC transformer module; controlling the total voltage of the capacitor branch of all the bridge circuits of the pre-stage circuit to be balanced based on the switch type of the post-stage circuit; and controlling the voltage of the two series capacitors of the N bridge circuit of the pre-stage circuit to be balanced based on whether the B end of the N winding of the isolation transformer is connected with the fourth terminal or not.

[0013] According to some embodiments, the control method for balancing the total voltage of the capacitor branches of the bridge circuits of the front-stage circuits based on the switching type of the rear-stage circuit comprises: if the rear-stage circuit is a half-controlled switch, controlling the duty cycle of the semiconductor switch of the front-stage circuit, controlling the power or the first port voltage or the second port voltage of the DC transformer module; and simultaneously fine-tuning the duty cycle of the semiconductor switch of the bridge circuit of the front-stage circuit, adjusting the transferred power of each winding of the isolation transformer, so that the total voltage of the capacitor branches of the bridge circuits of all the front-stage circuits is balanced; if the rear-stage circuit is a fully-controlled switch, unlocking all the semiconductor switches of the rear-stage circuit, and controlling the turn-on time difference between the semiconductor switches of the front-stage circuit and the semiconductor switches of the rear-stage circuit, controlling the power or the first port voltage or the second port voltage of the DC transformer module; and simultaneously fine-tuning the turn-on time difference, adjusting the transferred power of each winding of the isolation transformer, so that the total voltage of the capacitor branches of the bridge circuits of all the front-stage circuits is balanced.

[0014] According to some embodiments, the B terminal of the Nth winding of the isolation transformer is connected to the fourth terminal through a branch, and the control method for balancing the voltage of the two series capacitors of the Nth bridge circuit of the front-stage circuit comprises: if the B terminal of the Nth winding of the isolation transformer is connected to the fourth terminal through a direct connection wire or a first resonance branch or a semiconductor switch, further fine-tuning the duty cycle of the upper and lower switch tubes of the semiconductor switch of the Nth bridge circuit of the front-stage circuit, so that the voltage of the two series capacitors of the Nth bridge circuit of the front-stage circuit is balanced; if the B terminal of the Nth winding of the isolation transformer is not connected to the fourth terminal through a branch, controlling the first semiconductor switch and the second semiconductor switch of the three-level bypass branch to be complementary on, the third semiconductor switch and the fourth semiconductor switch to be complementary off, and adjusting the duty cycle of the first semiconductor switch and the fourth semiconductor switch, so that the voltage of the two series capacitors of the Nth bridge circuit of the front-stage circuit is balanced.

[0015] According to some embodiments, after the DC transformer module fails, the control method further comprises: locking all the semiconductor switches of the front-stage circuit and the rear-stage circuit; turning off the first semiconductor switch and the fourth semiconductor switch of the three-level bypass branch; turning on the second semiconductor switch and the third semiconductor switch of the three-level bypass branch; closing the first bypass switch, and the bypass ends.

[0016] Compared with the traditional two-level and three-level schemes, the technical scheme provided by the embodiments of the present application has higher high-voltage side voltage in the case of having a bypass function, can further reduce the number of designed DC transformer modules, and can make each module have a bypass capability after the ISOP system is composed in the case of only having three bridge circuits, thereby increasing the reliability of the whole system; and compared with the traditional half-bridge bypass scheme, the switching device stress and cost are reduced. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of a DC transformer module with a three-level bypass structure provided in an embodiment of this application.

[0019] Figure 2 This is one of the schematic diagrams of the first connection method of the DC transformer module with a three-level bypass structure provided in the embodiments of this application.

[0020] Figure 3 This is the second schematic diagram of the first connection method of the DC transformer module with a three-level bypass structure provided in the embodiments of this application.

[0021] Figure 4 This is one of the schematic diagrams of the second connection method of the DC transformer module with a three-level bypass structure provided in the embodiments of this application.

[0022] Figure 5 This is the second schematic diagram of the second connection method of the DC transformer module with a three-level bypass structure provided in the embodiments of this application.

[0023] Figure 6 This is a schematic diagram of the third connection method of the DC transformer module with a three-level bypass structure provided in the embodiments of this application.

[0024] Figure 7 This is a schematic diagram of the fourth connection method of the DC transformer module with a three-level bypass structure provided in the embodiments of this application.

[0025] Figure 8 This is a schematic diagram of the fifth connection method of the DC transformer module with a three-level bypass structure provided in the embodiments of this application.

[0026] Figure 9 This is a schematic diagram of the sixth connection method of the DC transformer module with a three-level bypass structure provided in the embodiments of this application.

[0027] Figure 10 This is one of the schematic diagrams of the first connection method of the subsequent circuit provided in the embodiments of this application.

[0028] Figure 11 This is the second schematic diagram of the first connection method of the subsequent circuit provided in the embodiments of this application.

[0029] Figure 12 is a second connection mode schematic diagram of the post-stage circuit provided by the embodiment of the present application.

[0030] Figure 13 is one of the third connection mode schematic diagrams of the post-stage circuit provided by the embodiment of the present application.

[0031] Figure 14 is another of the third connection mode schematic diagrams of the post-stage circuit provided by the embodiment of the present application.

[0032] Figure 15 is a control method flow schematic diagram of the DC transformer module with the three-level bypass structure provided by the embodiment of the present application. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some of the embodiments of the present application, but not all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0034] It should be understood that the terms “first”, “second” and the like in the claims, specification and drawings of the present application are used to distinguish different objects, and are not used to describe a particular sequence. The terms “include” and “contain” used in the specification and claims of the present application indicate the presence of the described features, whole, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, whole, steps, operations, elements, components and / or sets thereof.

[0035] Figure 1 is a schematic diagram of a DC transformer module with a three-level bypass structure provided by the embodiment of the present application.

[0036] As shown in Figure 1 , the DC transformer module with the three-level bypass structure includes a three-level bypass branch 10, a pre-stage circuit 20, an isolation transformer 30 and a post-stage circuit 40.

[0037] The three-level bypass branch 10 includes first, second, third and fourth semiconductor switches 101, 102, 103 and 104 connected in series. The positive terminal of the first semiconductor switch 101 is the positive terminal 105 of the three-level bypass branch. The first semiconductor switch 101 and the second semiconductor switch 102 are connected to lead out a first terminal 106. The second semiconductor switch 102 and the third semiconductor switch 103 are connected to lead out a second terminal 107. The third semiconductor switch 103 and the fourth semiconductor switch 104 are connected to lead out a third terminal 108. The negative terminal of the fourth semiconductor switch 104 is the negative terminal 109 of the three-level bypass branch.

[0038] The first, second, third and fourth semiconductor switches 101, 102, 103 and 104 of the three-level bypass branch and the semiconductor switches of the pre-stage circuit are all controllable switches.

[0039] The pre-stage circuit 20 includes at least 2N-1 bridge circuits connected in series, N being a positive integer greater than or equal to 2. The positive terminal of the direct current side of the first bridge circuit is the positive terminal of the pre-stage circuit, and the negative terminal of the direct current side of the 2N-1 bridge circuit is the negative terminal 109 of the pre-stage circuit. A capacitor branch is connected in parallel to the direct current side of each bridge circuit. The capacitor branch of the N bridge circuit includes at least two series-connected capacitors, and the midpoint of the series-connected capacitors leads out a fourth terminal.

[0040] The positive terminal 105 of the three-level bypass branch is connected to the positive terminal of the pre-stage circuit, the negative terminal 109 of the three-level bypass branch is connected to the negative terminal of the pre-stage circuit, the second terminal 107 is connected to the fourth terminal, and the first terminal 106 and the third terminal 108 lead out as the first port of the direct current transformer module.

[0041] The isolation transformer 30 is a multi-winding transformer including at least 2N windings, and the two ends of each winding are defined as the A terminal and the B terminal of the winding.

[0042] According to some embodiments, as shown in Figure 2 The bridge circuits of the pre-stage circuit 20 are half-bridge circuits including two semiconductor switches, the midpoints of all 2N-1 half-bridge circuits lead out the A terminals of the first 2N-1 windings of the isolation transformer 30, and the B terminal of the N winding is connected to the fourth terminal through 309 (a direct connection wire or a first resonance branch or a semiconductor switch). The B terminals of the first 2N-1 windings of the isolation transformer 30 are connected in series through a first resonance branch in turn.

[0043] The first resonance branch includes a capacitor or a series-connected capacitor and inductor.

[0044] When N = 2, the isolation transformer includes at least 4 windings, as shown in Figure 2As shown, the first winding 301, the second winding 302 and the third winding 303 are connected in series through a first resonant branch. The components 304, 305, 306, 307 and 308 each represent a possible first resonant branch.

[0045] According to some embodiments, as shown in FIG. 2, the bridge circuit of the pre-stage circuit 20 is a half-bridge circuit including two semiconductor switches. The midpoints of all 2N-1 half-bridge circuits are connected to the A ends of the first 2N-1 windings of the isolation transformer 30. The B ends of the first 2N-1 windings of the isolation transformer 30 are connected in series through a first resonant branch. Figure 3

[0046] According to some embodiments, as shown in FIG. 3, the bridge circuit of the pre-stage circuit 20 is a half-bridge circuit including two semiconductor switches. The B ends of the first 2N-1 windings of the isolation transformer 30 are connected. The midpoints of all 2N-1 half-bridge circuits are connected to the A ends of the first 2N-1 windings of the isolation transformer 30 through a first resonant branch. Figure 4

[0047] According to some embodiments, as shown in FIG. 4, the bridge circuit of the pre-stage circuit 20 is a half-bridge circuit including two semiconductor switches. The B ends of the first 2N-1 windings of the isolation transformer 30 are connected. The midpoints of all 2N-1 half-bridge circuits are connected to the A ends of the first 2N-1 windings of the isolation transformer 30 through a first resonant branch. Figure 5

[0048] According to some embodiments, as shown in FIG. 5, the bridge circuit of the pre-stage circuit 20 is a half-bridge circuit including two semiconductor switches. The midpoints of all 2N-1 half-bridge circuits are connected to the A ends of the first 2N-1 windings of the isolation transformer 30. The B end of the N winding is connected to the fourth terminal through a direct wire or a first resonant branch or a semiconductor switch. The B ends of the first 2N-1 windings of the isolation transformer 30 are connected in series through a first resonant branch, and the B end of the 2N-1 winding is connected in series with the B end of the first winding through a first resonant branch. Figure 6

[0049] According to some embodiments, as shown in FIG. 6, the bridge circuit of the pre-stage circuit 20 is a half-bridge circuit including two semiconductor switches. The midpoints, positive ends and negative ends of all 2N-1 half-bridge circuits are connected to the first 2N-1 windings of the isolation transformer 30 through a first resonant branch. Figure 7

[0050] According to some embodiments, as shown in FIG. 7, the bridge circuit of the pre-stage circuit 20 is a half-bridge circuit including two semiconductor switches. The midpoints, positive ends and negative ends of all 2N-1 half-bridge circuits are connected to the first 2N-1 windings of the isolation transformer 30 through a first resonant branch. Figure 8 ​​​​​As shown, the bridge circuit of the front-end circuit 20 is a half-bridge circuit including two semiconductor switches. All series capacitor branches of the bridge circuits include two capacitors connected in series. The midpoints of all 2N-1 half-bridge circuits and the midpoints of the series capacitor branches are led out and connected to the first 2N-1 windings of the isolation transformer 30.

[0051] According to some embodiments, when N=2, such as Figure 9 As shown, the bridge circuit of the front-end circuit 20 is a full-bridge circuit including four semiconductor switches. All 2N-1 AC ports of the full-bridge circuit are led out and connected in series with a second resonant branch, which is then connected to the first 2N-1 windings of the isolation transformer 30.

[0052] The second resonant branch includes a direct-connected wire, a capacitor, an inductor, or a capacitor and an inductor connected in series.

[0053] According to some embodiments, the subsequent circuit 40 is a full-bridge circuit containing four semiconductor switches. The AC port of the subsequent circuit 40 is connected in series with a second resonant branch and then connected to the 2Nth winding of the isolation transformer 30. The DC port of the subsequent circuit 40 is the second port of the DC transformer module.

[0054] The semiconductor switch in the subsequent circuit 40 is a semi-controlled switch. For example... Figure 10 As shown, the semiconductor switch is an IGBT. (As indicated...) Figure 11 As shown, the semiconductor switch is a diode.

[0055] According to some embodiments, such as Figure 12 As shown. The subsequent circuit 40 is a multiphase half-bridge circuit containing 2N-1 half-bridge circuits, and the isolation transformer 30 also contains the 2Nth to 4N-2th windings. The midpoints of the half-bridge circuits of all subsequent circuits 40 are led out, connected to a second resonant branch, and then connected to the A end of the 2Nth to 4N-2th windings of the isolation transformer 30. The B ends of the 2Nth to 4N-2th windings of the isolation transformer 30 are connected together. The two ends of the bridge arms of the half-bridge circuits of all subsequent circuits 40 are connected in parallel, forming the second port of the DC transformer module.

[0056] According to some embodiments, the subsequent circuit 40 is a multiphase full-bridge circuit containing 2N-1 full-bridge circuits, and the isolation transformer 30 further includes the 2Nth to 4N-2th windings. The AC ports of the full-bridge circuits of all subsequent circuits are connected in series with a second resonant branch, and then connected to the 2Nth to 4N-2th windings of the isolation transformer 30. The DC ports of the full-bridge circuits of all subsequent circuits 40 are connected in parallel, forming the second port of the DC transformer module.

[0057] The semiconductor switch in the subsequent circuit 40 is a semi-controlled switch. For example... Figure 13 As shown, the semiconductor switch is an IGBT. (As indicated...) Figure 14The semiconductor switch is a diode.

[0058] Optionally, the three-level bypass branch 10 further comprises a first bypass switch 110 connected between the first terminal 106 and the third terminal 108 of the three-level bypass branch.

[0059] Figure 15 is a control method flow diagram of a direct-current transformer module with a three-level bypass structure provided by the embodiments of the present application, comprising the following flows.

[0060] In S10, the first semiconductor switch and the fourth semiconductor switch of the three-level bypass branch are turned on.

[0061] In S20, all semiconductor switches of the front-stage circuit are unlocked, and the capacitor branch voltage of the bridge circuit of all front-stage circuits and the voltage of the first port and the voltage of the second port of the direct-current transformer module are detected.

[0062] In S30, based on the switch type of the rear-stage circuit, the total voltage of the capacitor branches of the bridge circuits of all front-stage circuits is controlled to be balanced.

[0063] If the rear-stage circuit is a half-controlled switch, the duty cycle of the semiconductor switches of the front-stage circuit is controlled, and the power or the voltage of the first port or the voltage of the second port of the direct-current transformer module is controlled. At the same time, the duty cycle of the semiconductor switches of the bridge circuit of the front-stage circuit is fine-tuned, and the transmission power of each winding of the isolation transformer is adjusted, so that the total voltage of the capacitor branches of the bridge circuits of all front-stage circuits is balanced.

[0064] If the rear-stage circuit is a fully-controlled switch, all semiconductor switches of the rear-stage circuit are unlocked, and the turn-on time difference of the semiconductor switches of the front-stage circuit and the semiconductor switches of the rear-stage circuit is controlled, and the power or the voltage of the first port or the voltage of the second port of the direct-current transformer module is controlled. At the same time, the turn-on time difference is fine-tuned, and the transmission power of each winding of the isolation transformer is adjusted, so that the total voltage of the capacitor branches of the bridge circuits of all front-stage circuits is balanced.

[0065] In S40, based on whether the B terminal of the Nth winding of the isolation transformer is connected to the fourth terminal through a branch or not, the voltage of the two series capacitors of the Nth bridge circuit capacitor branch of the front-stage circuit is controlled to be balanced.

[0066] If the B terminal of the Nth winding of the isolation transformer is connected to the fourth terminal through a direct connection wire or a first resonance branch or a semiconductor switch, the duty cycle of the upper and lower switch tubes of the semiconductor switches of the Nth bridge circuit of the front-stage circuit is further fine-tuned, so that the voltage of the two series capacitors of the Nth bridge circuit capacitor branch of the front-stage circuit is balanced.

[0067] If the B terminal of the Nth winding of the isolation transformer is not connected with the fourth terminal through a branch, the first semiconductor switch and the second semiconductor switch of the three-level bypass branch are controlled to be complementary on, the third semiconductor switch and the fourth semiconductor switch are controlled to be complementary off, and the duty cycles of the first semiconductor switch and the fourth semiconductor switch are adjusted so that the voltages of the two series capacitors in the Nth bridge circuit capacitor branch of the front-stage circuit are balanced.

[0068] After the DC transformer module fails, all semiconductor switches of the front-stage circuit and the back-stage circuit are blocked. The first semiconductor switch and the fourth semiconductor switch of the three-level bypass branch are turned off. The second semiconductor switch and the third semiconductor switch of the three-level bypass branch are turned on. The first bypass switch is closed, and the bypass is ended.

[0069] The resonant branch deployed according to the application can also implement resonant control on the circuit, and realize power transmission through frequency conversion or open-loop control. Thus, the topology of the DC transformer and the corresponding starting to normal operation and bypass control strategy can be designed according to the application.

[0070] The above has described the embodiments of the application in detail, and the principles and implementation manners of the application have been described by applying specific examples. The above embodiment descriptions are only used to help understand the method of the application and its core idea. Meanwhile, the changes or deformations made by the person skilled in the art according to the idea of the application, based on the specific implementation manners and application scope of the application, all belong to the protection scope of the application. In summary, the content of the specification should not be understood as a limitation of the application.

Claims

1. A DC transformer module with a three-level bypass structure, comprising: a three-level bypass branch, comprising a first semiconductor switch, a second semiconductor switch, a third semiconductor switch and a fourth semiconductor switch connected in series, a positive terminal of the first semiconductor switch being a positive terminal of the three-level bypass branch, the first semiconductor switch and the second semiconductor switch being connected to lead out a first terminal, the second semiconductor switch and the third semiconductor switch being connected to lead out a second terminal, the third semiconductor switch and the fourth semiconductor switch being connected to lead out a third terminal, a negative terminal of the fourth semiconductor switch being a negative terminal of the three-level bypass branch; a pre-stage circuit, comprising at least 2N-1 bridge circuits connected in series, N being a positive integer greater than or equal to 2, wherein a positive terminal of a direct current side of a first bridge circuit is a positive terminal of the pre-stage circuit, a negative terminal of a direct current side of a 2N-1th bridge circuit is a negative terminal of the pre-stage circuit, a capacitor branch of each bridge circuit is connected in parallel to a direct current side of the bridge circuit, the capacitor branch of the Nth bridge circuit comprises at least two capacitors connected in series, a midpoint of the series capacitors leading out a fourth terminal, the positive terminal of the three-level bypass branch being connected to the positive terminal of the pre-stage circuit, the negative terminal of the three-level bypass branch being connected to the negative terminal of the pre-stage circuit, the second terminal being connected to the fourth terminal, the first terminal and the third terminal leading out as a first port of the DC transformer module. 2.The module of claim 1, further comprising an isolation transformer, the isolation transformer being a multi-winding transformer comprising at least 2N windings, two terminals of each winding being defined as an A terminal of the winding and a B terminal of the winding, and the connection mode comprising at least one of the following: firstly, the bridge circuit is a half-bridge circuit comprising two semiconductor switches, midpoints of all 2N-1 half-bridge circuits are connected to lead out A terminals of the first 2N-1 windings of the isolation transformer, B terminals of the first 2N-1 windings of the isolation transformer are connected in series through a first resonant branch in turn; or secondly, the bridge circuit is a half-bridge circuit comprising two semiconductor switches, B terminals of the first 2N-1 windings of the isolation transformer are connected, midpoints of all 2N-1 half-bridge circuits are connected to lead out, and are connected to A terminals of the first 2N-1 windings of the isolation transformer through a first resonant branch in turn; or thirdly, the bridge circuit is a half-bridge circuit comprising two semiconductor switches, midpoints of all 2N-1 half-bridge circuits are connected to lead out A terminals of the first 2N-1 windings of the isolation transformer, B terminals of the first 2N-1 windings of the isolation transformer are connected in series through a first resonant branch in turn, and B terminal of the 2N-1th winding and B terminal of the first winding are connected in series through a first resonant branch. In the fourth type, the bridge circuit is a half-bridge circuit including two semiconductor switches; the midpoints of all 2N-1 half-bridge circuits are connected to the A ends of the 2N-1 windings of the isolation transformer via a first resonant branch; and the negative terminals of all 2N-1 half-bridge circuits are connected to the B ends of the 2N-1 windings of the isolation transformer. In the fifth type, the bridge circuit is a half-bridge circuit including two semiconductor switches; the DC side of all bridge circuits is connected in parallel with a capacitor branch including two series-connected capacitors; the midpoints of all 2N-1 half-bridge circuits and the midpoint of the series capacitor branch are led out and connected to the two ends of the 2N-1 windings of the isolation transformer. In the sixth type, the bridge circuit is a full-bridge circuit including four semiconductor switches; the AC ports of all 2N-1 full-bridge circuits are led out and connected to the 2N-1 windings of the isolation transformer. The first resonant branch includes a capacitor or a series-connected capacitor and inductor.

3. The module of claim 2, wherein, In the first, second, and fifth connection modes, the B end of the Nth winding is also connected to the fourth terminal via a direct connection wire or the first resonant branch or semiconductor switch.

4. The module of any one of claims 2 to 3, further comprising a post-stage circuit, and the connection mode of the post-stage circuit comprises at least one of the following: The post-stage circuit is a full-bridge circuit including four semiconductor switches; the AC ports of the post-stage circuit are connected to the 2Nth windings of the isolation transformer via a second resonant branch; and the DC ports of the post-stage circuit are the second ports of the DC transformer module. The post-stage circuit is a multi-phase half-bridge circuit including 2N-1 half-bridge circuits; the isolation transformer further includes 2Nth to 4N-2th windings; the midpoints of all half-bridge circuits of the post-stage circuit are led out and connected to the A ends of the 2Nth to 4N-2th windings of the isolation transformer via a second resonant branch; the B ends of the 2Nth to 4N-2th windings of the isolation transformer are connected together; and the bridge arms of all half-bridge circuits of the post-stage circuit are connected in parallel to serve as the second ports of the DC transformer module. The post-stage circuit is a multi-phase full-bridge circuit including 2N-1 full-bridge circuits; the isolation transformer further includes 2Nth to 4N-2th windings; one of the AC ports of all full-bridge circuits of the post-stage circuit is led out and connected to one end of the 2Nth to 4N-2th windings of the isolation transformer via a second resonant branch; the other AC port of all full-bridge circuits of the post-stage circuit is directly connected to the other end of the 2Nth to 4N-2th windings of the isolation transformer; and the DC ports of all full-bridge circuits of the post-stage circuit are connected in parallel to serve as the second ports of the DC transformer module. wherein The second resonant branch includes a capacitor or an inductor or a series-connected capacitor and inductor, and the semiconductor switches of the post-stage circuit include half-controlled switches.

5. The module of claim 1, wherein, The first, second, third, and fourth semiconductor switches of the three-level bypass branch and the semiconductor switches of the pre-stage circuit are fully-controlled switches.

6. The module of claim 1, wherein, The three-level bypass branch further comprises: a first bypass switch connected between the first terminal and the third terminal of the three-level bypass branch.

7. A control method of a DC transformer module with a three-level bypass structure, for controlling the module according to any one of claims 1-6, the module further comprising a post-stage circuit, a DC port of the post-stage circuit being a second port of the DC transformer module, the control method comprising: turning on the first semiconductor switch and the fourth semiconductor switch of the three-level bypass branch; unlocking all semiconductor switches of the pre-stage circuit, and detecting the capacitor branch voltage of all bridge circuits of the pre-stage circuit, and the voltage of the first port and the voltage of the second port of the DC transformer module; based on the switch type of the post-stage circuit, controlling the total voltage balance of the capacitor branches of all bridge circuits of the pre-stage circuit, comprising: if the post-stage circuit is a half-controlled switch, controlling the duty cycle of the semiconductor switches of the pre-stage circuit, controlling the power or the voltage of the first port or the voltage of the second port of the DC transformer module; at the same time, fine-tuning the duty cycle of the semiconductor switches of the bridge circuits of the pre-stage circuit, adjusting the transferred power of each winding of the isolation transformer, so that the total voltage of the capacitor branches of all bridge circuits of the pre-stage circuit is balanced; if the post-stage circuit is a fully-controlled switch, unlocking all semiconductor switches of the post-stage circuit, and controlling the time difference of turning on the semiconductor switches of the pre-stage circuit and the semiconductor switches of the post-stage circuit, controlling the power or the voltage of the first port or the voltage of the second port of the DC transformer module; at the same time, fine-tuning the time difference, adjusting the transferred power of each winding of the isolation transformer, so that the total voltage of the capacitor branches of all bridge circuits of the pre-stage circuit is balanced; based on whether the B terminal of the Nth winding of the isolation transformer is connected to the fourth terminal with or without a branch, controlling the voltage balance of the two series capacitors of the Nth bridge circuit capacitor branch of the pre-stage circuit, comprising: if the B terminal of the Nth winding of the isolation transformer is connected to the fourth terminal through a direct connection wire or a first resonance branch or a semiconductor switch, further fine-tuning the duty cycle of the upper and lower switch tubes of the semiconductor switches of the Nth bridge circuit of the pre-stage circuit, so that the voltage of the two series capacitors of the Nth bridge circuit capacitor branch of the pre-stage circuit is balanced; if the B terminal of the Nth winding of the isolation transformer is not connected to the fourth terminal with a branch, controlling the first semiconductor switch and the second semiconductor switch of the three-level bypass branch to be complementary on, and the third semiconductor switch and the fourth semiconductor switch to be complementary off, and adjusting the duty cycle of the first semiconductor switch and the fourth semiconductor switch, so that the voltage of the two series capacitors of the Nth bridge circuit capacitor branch of the pre-stage circuit is balanced.

8. The control method of claim 7, wherein, The three-level bypass branch further comprises a first bypass switch connected between the first terminal and the third terminal of the three-level bypass branch, and after the DC transformer module fails, the control method further comprises: locking all semiconductor switches of the pre-stage circuit and the post-stage circuit; turning off the first semiconductor switch and the fourth semiconductor switch of the three-level bypass branch; a second semiconductor switch and a third semiconductor switch of a three-level bypass branch are turned on; the first bypass switch is closed, and the bypass is ended.

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