A composite device with multi-port DC power flow and short-circuit current control function

Abstract

The application discloses a composite device topology with DC power flow and short-circuit current control functions suitable for a multi-terminal flexible DC power transmission system and a control method thereof, and belongs to the technical field of power generation, power transformation or power distribution. i The DC power transmission line is connected between the DC port of the sending terminal converter station and the DC port of the receiving terminal converter station. i The DC power transmission line is connected between the DC port of the sending terminal converter station and the DC port of the receiving terminal converter station. i The DC power transmission line is connected between the DC port of the sending terminal converter station and the DC port of the receiving terminal converter station. i The DC power transmission line is connected between the DC port of the sending terminal converter station and the DC port of the receiving terminal converter station. The power flow control part and the short-circuit current control part are connected in parallel with the adjustable voltage source in the DC power transmission line, two coupling inductors, two coupling inductor control switches, n bypass switches connected in parallel across the adjustable voltage source in the DC line, six IGBT switch tubes, three bidirectional switches and a fault transfer branch are used to realize bidirectional power flow regulation and short-circuit current limitation among n lines, and the defects of large loss and large current stress of the fault transfer branch of the existing composite device are overcome.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of composite device with multi-port direct current flow and short circuit current control function, mainly for multi-terminal direct current transmission occasion, it is related to power electronics, belongs to the technical field of power generation, transformation or distribution. BACKGROUND

[0002] Multi-Terminal Direct Current (MTDC) refers to a DC transmission network composed of three or more converter stations. This system not only enables multi-source power supply and multi-point power reception, but also exhibits higher flexibility compared to traditional two-terminal DC transmission systems. MTDC technology is considered one of the effective means to address the challenges of large-scale renewable energy grid connection, transmission corridor resource shortage, and high-capacity long-distance power transmission in China. In the MTDC system, converter stations are connected in series (Series Connection), parallel (Parallel Connection), or hybrid (Hybrid Connection) to meet the efficient transmission and flexible allocation of electric energy. Voltage Source Converter (VSC) is an advanced converter technology that has significant advantages over traditional Current Source Converter (CSC). In particular, during power flow reversal, VSC can quickly switch the direction of DC current without changing the voltage polarity, and it avoids the problem of commutation failure. Therefore, VSC is more conducive to the construction of Voltage Source Converter-based Multi-Terminal High Voltage Direct Current (VSC-MTDC), thereby further improving the flexibility and reliability of the transmission system.

[0003] Figure 1The equivalent circuit of a typical ring-type four-terminal VSC-MTDC system is shown. It is assumed that VSC1, VSC2 and VSC3 are the converter stations of three offshore wind farms, and they operate in the constant power mode, while VSC0 is the onshore converter station, which operates in the constant voltage mode, at this time, the power is transmitted from VSC1, VSC2 and VSC3 to VSC0. In view of the problems of DC power flow control and fault current suppression existing in the DC power transmission system, the DC power flow controller (DCPFC) and the DC current breaker (DCCB) are usually introduced into the DC power transmission system to enhance the power flow direction control ability of the transmission line and the regulation ability of the line transmission power in the DC power transmission network.

[0004] Since there is no problem of reactive power, reactance and phase angle in the DC power grid, the power flow regulation is mainly realized by changing the voltage and resistance value. Therefore, the existing DC power flow controller is mainly divided into two types of resistance type and voltage type. The resistance type controller scheme has simple structure, but its regulation ability is limited, it can only realize one-way power flow regulation, and the on-state loss is large. The voltage type controller includes three types of DC transformer, series adjustable voltage source and inter-line power flow controller. The DC transformer changes the line voltage by being connected in series in the line, and then realizes the power flow regulation, but this method needs to withstand the high voltage and large power at the system level, which increases the cost and the complexity of the system. Although the series adjustable voltage source does not need to withstand high voltage directly, it needs to interact with the external power source, which increases the complexity of the system. In order to simplify the system structure and avoid the use of additional power source, researchers have proposed the inter-line power flow controller, which can make the power flow transfer between different lines through a reasonable design scheme, so as to realize flexible power flow regulation, which improves the reliability of the system and reduces the complexity of the system.

[0005] In addition, the line impedance of the DC power grid is extremely low, so small voltage changes can cause large current changes, therefore, it is particularly important to develop a fast current limiting and short-circuit breaking device for the DC power grid. At present, the existing short-circuit current limiting device is mainly applied to the AC power grid, which mostly relies on the natural zero point of AC current to realize breaking. However, in the DC power grid, there is no natural zero point during short circuit, so the breaking process becomes extremely difficult, which is difficult to meet the high requirements of safety and reliability of the DC power grid. The current research mainly focuses on the hybrid DC circuit breaker based on power electronic technology, which can effectively cut off the short-circuit current through fast response and precise control. At the same time, there are also studies on limiting the peak value and rising rate of fault current by connecting a current limiting circuit in series in the power grid, so as to reduce the impact of short circuit on the power grid.

[0006] However, the current research and development of the device for direct current flow control and short circuit control are often independent of each other, and the composite control strategy of short circuit and flow is rarely involved. The separated research and application mode leads to the disadvantages of relatively high cost, large device size and inconvenience of unified control. The patent CN201710341926.0 proposes a composite device with direct current flow and short circuit control, however, the composite device can only be applied to the flow regulation between two ports, and has no specific expansibility. In addition, the device involving the composite control strategy of short circuit and flow needs more switching tubes to realize the flow control, and most of the fault current flows through the fault transfer branch during short circuit, and the loss caused by the multiple switching tubes increases the cost of the multi-terminal direct current transmission system. SUMMARY

[0007] The present application aims to solve the technical problems of the composite device with flow control and short circuit control functions in the current multi-terminal direct current transmission system, such as large loss and large current stress of the fault transfer branch, and to realize the coordinated management of flow and short circuit in the direct current grid through the integrated composite device, thereby improving the operation efficiency of the multi-terminal direct current transmission system, reducing the cost, and enhancing the safety and reliability of the system.

[0008] The present application adopts the following technical solutions to achieve the above-mentioned application purposes:

[0009] A composite device with multi-port direct current flow and short circuit current control functions, comprising: an i-th sub-module connected between a direct current transmission line of a sending terminal converter station and a direct current transmission line of a receiving terminal converter station, the i-th sub-module comprising: a flow control part connected in parallel with an adjustable voltage source in the i-th direct current transmission line and a short circuit current control part, i=1, 2, n, n is a positive integer, wherein,

[0010] The flow control part comprises: a first coupled inductance control switch, a first coupled inductance, a second coupled inductance control switch, a second coupled inductance and an i-th flow control switch, one end of the first coupled inductance and one end of the second coupled inductance are connected to the direct current port of the receiving terminal converter station, the other end of the first coupled inductance is connected to the current output end of the first coupled inductance control switch, the other end of the second coupled inductance is connected to the current input end of the second coupled inductance control switch, the current input end of the first coupled inductance control switch, the current output end of the second coupled inductance control switch and one end of the i-th bidirectional switch are connected to the negative electrode of the adjustable voltage source in the i-th direct current transmission line, the other end of the i-th bidirectional switch is connected to the positive electrode of the adjustable voltage source in the i-th direct current transmission line,

[0011] The short-circuit current control part comprises an i-th fault transfer branch, an i-th mechanical switch and an i-th short-circuit current limiting switch, one end of the i-th mechanical switch and one end of the i-th fault transfer branch are connected to the i-th sending end converter station DC port, the other end of the i-th mechanical switch is connected to the positive electrode of the adjustable voltage source in the i-th DC transmission line, the other end of the i-th fault transfer branch is connected to one end of the i-th short-circuit current limiting switch, and the other end of the i-th short-circuit current limiting switch is connected to the negative electrode of the adjustable voltage source in the i-th DC transmission line.

[0012] As a further optimization scheme of the composite device with the multi-port DC power flow and short-circuit current control function, the adjustable voltage source in the i-th DC transmission line is a capacitor.

[0013] As a further optimization scheme of the composite device with the multi-port DC power flow and short-circuit current control function, the adjustable voltage source in the i-th DC transmission line is connected in parallel with the i-th bypass switch, when the first coupling inductance control switch, the second coupling inductance control switch and each power flow control switch are all turned off and each bypass switch is closed, the power flow control part in each sub-module is bypassed.

[0014] As a further optimization scheme of the composite device with the multi-port DC power flow and short-circuit current control function, the i-th power flow control switch is a bidirectional switch composed of a first switch tube and a second switch tube connected in common.

[0015] As a further optimization scheme of the composite device with the multi-port DC power flow and short-circuit current control function, a first diode is connected in anti-parallel between the current input end and the current output end of the first switch tube, and a second diode is connected in anti-parallel between the current input end and the current output end of the second switch tube.

[0016] As a further optimization scheme of the composite device with the multi-port DC power flow and short-circuit current control function, the i-th short-circuit current limiting switch is a bidirectional switch.

[0017] As a further optimization scheme of the composite device with the multi-port DC power flow and short-circuit current control function, the i-th fault transfer branch is composed of a bidirectional switch and an arrester connected in parallel with the bidirectional switch.

[0018] As a further optimization scheme of the composite device with the multi-port DC power flow and short-circuit current control function, the switch tubes in the bidirectional switch, the first coupling inductance control switch and the second coupling inductance control switch are all insulated gate bipolar transistors.

[0019] The application provides a control method of a composite device with multi-port DC power flow and short-circuit control, comprising a power flow control strategy and a short-circuit current control strategy.

[0020] Power flow control strategy: When no fault occurs, the first coupled inductor control switch and each short-circuit current limiting switch are closed, and the power flow control device is put into operation. The control method is as follows: the DC current of the DC transmission line where two of the adjustable voltage sources are located is collected; a current reference value is set; the current reference value is subtracted from the collected DC current of the transmission line. The difference is passed through the PI link and a logical operation to obtain the PWM switching signal of the n line switches. This makes the n power flow branches operate in a complementary conduction state and maintains the DC transmission line current at a given value.

[0021] Short-circuit current control strategy: When a fault occurs, the power flow control device is immediately locked and the short-circuit control part is put into operation. The control method is as follows:

[0022] At time tf, a short-circuit to ground fault occurs on the i-th DC transmission line. After a period of time, at time t1, the composite device detects the fault and immediately closes the second coupled inductor control switch and each power flow control switch to lock the power flow control link. At the same time, it opens each short-circuit current limiting switch to protect the capacitor.

[0023] After a delay of Δt1, the fault transfer branch is activated and the fault current is transferred to the fault transfer branch;

[0024] After a delay of Δt2, the power flow control switch in the submodule connected to the DC transmission line with the short-circuit fault is disconnected;

[0025] After a delay of Δt3, the mechanical switch in the submodule of the DC transmission line where the short circuit fault occurs is disconnected, and the main part of the power flow control is cut off;

[0026] After a delay of Δt4, the bidirectional switches in each fault transfer branch are closed, the faulty line is disconnected, and the remaining lines resume normal operation.

[0027] The multi-terminal DC power transmission system includes the composite device, which is connected between the DC port of the sending-end converter station and the DC port of the receiving-end converter station.

[0028] The present invention adopts the above technical solution and has the following beneficial effects:

[0029] (1) The present invention proposes a composite device for a DC power grid with DC current control and DC short-circuit control, which realizes bidirectional current control between n transmission lines through two coupled inductors, two coupled inductor control switches and a current control switch. Compared with the existing composite device, the number of switch tubes is reduced, and the loss is reduced. When a DC line fault occurs, the capacitor serving as the adjustable voltage source is cut off from the line through the short-circuit current limiting switch, so that the fault current flows through the coupled inductor, and the fault current is suppressed before the circuit breaker is operated, thereby effectively reducing the current stress of the fault transfer branch. Then, the fault current is cut off in combination with the fault transfer branch.

[0030] (2) The composite device with DC power flow control and DC short circuit control for DC power grid is provided, two coupled inductors and two coupled inductor control switches are used, the two coupled inductors and the two coupled inductor control switches transfer line energy when the power flow control part is started, and suppress fault current when the short circuit current part is started, so as to reduce the peak value of the fault current, and the cost of the device is further reduced by using the multiplexing device.

[0031] (3) The multi-terminal DC power transmission system is provided, the composite device is used to realize DC power flow control, improve the operation efficiency of the multi-terminal DC power transmission system, reduce the cost, and enhance the safety and reliability of the system. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a four-terminal flexible DC power transmission system.

[0033] Figure 2 is a four-terminal flexible DC power transmission system comprising the composite device of the application.

[0034] Fig. 3(a) is a circuit topology schematic diagram of the composite device of the application; Fig. 3(b) is a circuit topology schematic diagram of the fault transfer branch in the composite device of the application; Fig. 3(c) is a circuit topology schematic diagram of the composite device in the embodiment of the application.

[0035] Figure 4 is a control strategy block diagram of the composite device of the application.

[0036] Figure 5 is a working mode diagram after the opening of switch Q 11 when the power flow control part of the composite device in the embodiment of the application is in forward operation.

[0037] Figure 6 is a working mode diagram after the opening of switch Q 21 when the power flow control part of the composite device in the embodiment of the application is in forward operation.

[0038] Figure 7 is a working mode diagram after the opening of switch Q 31 when the power flow control part of the composite device in the embodiment of the application is in forward operation.

[0039] Figure 8 is a working mode diagram after the opening of capacitor control switches BS1, BS2 and BS3 when the short circuit current suppression part of the composite device in the embodiment of the application exists in forward operation.

[0040] Figure 9is the working mode chart of the short-circuit current suppression part of the composite device in the embodiment of the application when the forward operation exists and after the fault transfer branch MB1, MB2, MB3 is removed.

[0041] Figure 10 is the working mode chart of the short-circuit current suppression part of the composite device in the embodiment of the application when the forward operation exists and after the switch Q 11 , Q 12 is removed.

[0042] Figure 11 is the working mode chart of the short-circuit current suppression part of the composite device in the embodiment of the application when the forward operation exists and after the fault transfer branch MB1, MB2, MB3 is removed.

[0043] Figure 12 is the line current waveform chart when the composite device executes dynamic power flow control in the embodiment of the application.

[0044] Figure 13 is the line current waveform chart when the composite device executes power flow inversion control in the embodiment of the application.

[0045] Figure 14 is the line current waveform chart when the composite device executes fault current clearing in the embodiment of the application.

[0046] Figure 15 is the sending end converter station port voltage waveform chart when the composite device executes fault current clearing in the embodiment of the application.

[0047] Label explanation in the figure: C i , the ith capacitor, S i , the ith bypass switch, T1, the first coupling inductance control switch, L1, the first coupling inductance, T2, the second coupling inductance control switch, L2, the second coupling inductance, Q i1 , the first switch tube in the ith power flow control switch, Q i2 , the second switch tube in the ith power flow control switch, MB i , the ith fault transfer branch, UFD i , the ith mechanical switch, BS i , the ith short-circuit current limiting switch, D i1 , the first diode in the ith power flow control switch, D i2 , the second diode in the ith power flow control switch, SA, the arrester. DETAILED DESCRIPTION

[0048] The technical solutions of the application will be described in detail below with reference to the accompanying drawings.

[0049] The direct current flow controller is suitable for a multi-terminal flexible direct current transmission system, and a topological diagram is shown in Fig. 3(a), which comprises an i capacitor C i , i = 1, 2, n as an adjustable voltage source i , which is connected in series in a direct current transmission line, and does not need an external power supply to provide power i , and the i bypass switch S i is connected between two stages of the capacitor C i1 . The adjustable voltage source is connected with a flow control part and a short-circuit current control part between two stages, the flow control part comprises a first coupled inductance control switch T1, a first coupled inductance L1, a second coupled inductance control switch T2, a second coupled inductance L2 and an i flow control switch, the i flow control switch is composed of a first switch tube Q i2 , a second switch tube Q i , and the first coupled inductance L1 is connected with the second coupled inductance L2 at one end to receive a direct current port of a sending terminal converter station, the other end of the first coupled inductance L1 is connected with a current output end of the first coupled inductance control switch T1, the other end of the second coupled inductance L2 is connected with a current input end of the second coupled inductance control switch T2, the current input end of the first coupled inductance control switch T1, the current output end of the second coupled inductance control switch T2 and one end of the i bidirectional switch are all connected with a negative electrode of the i adjustable voltage source, and the other end of the i bidirectional switch is connected with a positive electrode of the i adjustable voltage source; the short-circuit current control part comprises an i fault transfer branch MB i , an i mechanical switch UFD i and an i short-circuit current limiting switch BS i , one end of the i mechanical switch UFD i is connected with one end of the i fault transfer branch MB i to receive the i sending terminal converter station direct current port, the other end of the i mechanical switch UFD i is connected with one end of the i short-circuit current limiting switch BS i , and the other end of the i short-circuit current limiting switch BS i is connected with the negative electrode of the i adjustable voltage source. The first switch tube Q i1 is reversely connected in parallel between a current input end and a current output end with a first diode D i1 , the second switch tube Q i2 is reversely connected in parallel between a current input end and a current output end with a second diode D i2 , and the i short-circuit current limiting switch BS i is a bidirectional switch. As shown in Fig. 3(b), the i fault transfer branch MB i is composed of a bidirectional switch and an arrester SA connected in parallel with the bidirectional switch.

[0050] When the first coupling inductance control switch T1, the second coupling inductance control switch T2 and the first DC transmission line first switch tube Q i1 , the second DC transmission line second switch tube Q i2 are all turned off, and the first bypass switch S1, the second bypass switch S2 and the third bypass switch S3 are all closed, the power flow control part of the composite device is bypassed and does not participate in the DC power flow regulation; when all the DC transmission line currents of the DC power flow controller are forward operation, n complementary conduction DC power flow branches are included, each DC power flow branch is sequentially connected by the i capacitor C i , the i bidirectional switch BS i , the first coupling inductance L1, the first coupling inductance control switch T1, the first switch tube Q i1 , the second diode D i2 ; the energy between the capacitors connected in series in each DC transmission line is transmitted through the first coupling inductance L1 between the DC power flow branches; when all the DC transmission line currents of the composite device are reverse operation, each DC power flow branch is sequentially connected by the second coupling inductance L2, the second coupling inductance control switch T2, the first diode D i1 , the second switch tube Q i2 , the i capacitor C i and the i bidirectional switch BS i ; when the DC transmission line currents of the composite device are both forward operation and reverse operation, when the forward operation line transmits energy to the reverse operation line, the energy in the first coupling inductance L1 is transmitted to the second coupling inductance L2 by turning off the first coupling inductance control switch T1 and closing the second coupling inductance control switch T2, and then the energy is transmitted to the capacitor connected in series in the reverse operation line by the second coupling inductance L2; when a short-circuit fault occurs in the DC transmission line 1 of the composite device, the first coupling inductance control switch T1 and the second coupling inductance control switch T2 are closed, and the power flow control switch Q i1 -Q i2 , then the switches in the fault transfer branch MB1-MB n are closed and the first fault transfer branch MB1 is turned off after a short time, and finally the first fault transfer branch MB1 is turned off, so that the fault line is cut off.

[0051] The DC power flow controller has regulation and control capability to accurately control the size of the current on the three DC transmission lines; when the composite device shown in FIG. 3(c) is used, the four-terminal flexible DC power transmission system including the composite device of the present application is shown in FIG. 3(d), VSC0 is a constant voltage station, VSC1, VSC2 and VSC3 are constant power stations, and the composite device is connected in series at the DC port of VSC0 to realize DC power flow regulation and fault current suppression. Figure 2 The implementation process of the control strategy is shown in FIG. 4.Figure 4 The control strategy block diagram is shown clearly. In the control process, the system first collects the real-time currents I1 and I2 on the first and second DC transmission lines, respectively, and subtracts them from the given reference values I 1_ref , I 2_ref , respectively. The resulting difference is processed through a PI element to achieve accurate regulation of the error. The difference processed through the PI element is processed by a logic operation unit to generate PWM switching signals for the first switching tubes Q 11 , Q 21 , and Q 31 in the three DC transmission lines. The PWM switching signals ensure that the three DC current branches work in a complementary conduction state, thereby effectively maintaining the transmission line current at a given value. The three DC current branches working in a complementary conduction state means, for example, that when the first switching tube Q 11 in the first DC transmission line is turned on, the first switching tube Q 21 in the second DC transmission line and the first switching tube Q 31 in the third DC transmission line are all turned off.

[0052] The control characteristics of the composite device with DC current flow control and DC short-circuit current control for the DC power grid are analyzed below by taking two typical operating conditions as examples.

[0053] 1. Current flow control function

[0054] According to the current flow direction and line control requirements, the first coupled inductor control switch T1 is always closed. First, the first switching tube Q 11 in the first DC transmission line is turned on, and the energy in the first capacitor C1 is transferred to the first coupled inductor L1. The specific mode is shown in Figure 5 . Subsequently, the first switching tube Q 21 in the second DC transmission line is turned on, and the energy in the second capacitor C2 is transferred to the first coupled inductor L1. The specific mode is shown in Figure 6 . Finally, the first switching tube Q 31 in the third DC transmission line is turned on, and the energy in the first coupled inductor L1 is transferred to the third capacitor C3. The specific mode is shown in Figure 7 . In this way, the power P is transferred from the first adjustable voltage source V C1 , the first adjustable voltage source V C2 to the third adjustable voltage source V C3 , so that the line current is maintained at a given current reference value Iref.

[0055] 2. Short-circuit current suppression function

[0056] When a DC fault occurs in the first DC transmission line, the flow control switch Q 11 -Q 32The first coupled inductor control switch T1 and the second coupled inductor control switch T2 are turned on, and then the first capacitor control switch BS1, the second capacitor control switch BS2 and the third capacitor control switch BS3 are turned off. Figure 8 As shown in Figure 1, all fault currents now pass through the first coupled inductor L1 and the second coupled inductor L2, suppressing the current rise rate of the first DC transmission line. Simultaneously, the capacitors connected in series to each DC transmission line are removed from the line, ensuring reliable operation of the capacitors. Subsequently, the first fault transfer branch MB1, the second fault transfer branch MB2, and the third fault transfer branch MB3 are activated, as shown in Figure 1. Figure 9 After that, the power flow controller switch Q corresponding to the fault line, i.e., the first DC transmission line, is disconnected. 11 , Q 12 ,like Figure 10 Finally, the switch in the first fault transfer branch is closed to remove the fault line from the system, as shown in Figure 11 shown.

[0057] The technical solution of the present invention is described below through two specific examples:

[0058] Implementing the above example requires Figure 1 In the four-terminal ring network flexible DC transmission system shown in Figure 1, VSC0 operates in constant voltage mode, controlling V1 to 400 kV. VSC1, VSC2, and VSC3 operate in constant power mode, injecting P1 = 540 MW, P2 = 360 MW, and P3 = 180 MW into the system, respectively. The parameters of the DC transmission line are shown in Table 1.

[0059] Table 1 Line parameters of the four-terminal ring network DC transmission system

[0060] Line parameters Line 1 Line 2 Line 3 Line 4 Line 5 Length / km 200 150 200 100 100 Resistance / Ω 2 1.5 2 1 1 Inductance / mH 80 60 80 40 40

[0061] In Example 1, the device was put into use before t=4s, and the first DC transmission line current I1 and the second DC transmission line current I2 were controlled to 0.80kA and 1.50kA respectively. At t=4s, the reference value of I1 was changed to 0.50kA, while the reference value of I2 remained unchanged. Figure 12 The current waveforms of lines 1, 2, and 3 are given in Figure 13 The adjustable voltage source V C1 、V C2 、V C3 It can be seen from the above waveforms that the DC power flow controller of the present invention can achieve the predetermined control target, and the voltage of the adjustable voltage source can be stabilized at a constant value.

[0062] In example 2, before t=6s, the four-terminal DC power transmission system is in normal operation state, and the composite device is in operation for power flow control. At t=6s, a ground fault occurs in transmission line 1, the fault current suppression part of the composite device is put into operation, and the rising rate of the fault current is suppressed first, and then the fault line is cut off from the system. Figure 14 The current waveforms of lines 1, 2 and 3 are shown in FIG. 2, Figure 15 The port voltage waveforms of the sending-end converter station are shown in FIG. 3.

[0063] The basic principles, main features and advantages of the present application are shown and described above. It should be understood by those skilled in the art that the present application is not limited by the above specific examples, and the above specific examples and descriptions in the specification are only for further illustration of the principles and preparation effects of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of the present application is defined by the claims and their equivalents.

Claims

1. A composite device with multi-port DC power flow and short circuit current control functions, characterized in that, The first switch tube current input end and the current output end are reversely connected in parallel with a first diode, and the second switch tube current input end and the current output end are reversely connected in parallel with a second diode. a first DC port of a sending end converter station and a second DC port of a receiving end converter station i a first DC port of a sending end converter station and a second DC port of a receiving end converter station i a first DC port of a sending end converter station and a second DC port of a receiving end converter station i the first sub-module comprises a power flow control part and a short-circuit current control part connected in parallel with the adjustable voltage source in the DC transmission line i the first sub-module comprises a power flow control part and a short-circuit current control part connected in parallel with the adjustable voltage source in the DC transmission line i =1,2,……, n , n is a positive integer greater than 2, wherein The tidal current control part comprises: a first coupled inductance control switch, a first coupled inductance, a second coupled inductance control switch, a second coupled inductance and a i The tidal current control switch, one end of the first coupled inductance and one end of the second coupled inductance are connected to the DC port of the end conversion station, the other end of the first coupled inductance is connected to the current output end of the first coupled inductance control switch, the other end of the second coupled inductance is connected to the current input end of the second coupled inductance control switch, the current input end of the first coupled inductance control switch, the current output end of the second coupled inductance control switch and the i One end of the bidirectional switch is connected to the current output end of the first coupled inductance control switch i The negative pole of the adjustable voltage source in the DC transmission line is connected to the current output end of the second coupled inductance control switch i The other end of the bidirectional switch is connected to the current input end of the second coupled inductance control switch i The positive pole of the adjustable voltage source in the DC transmission line is connected to the current input end of the first coupled inductance control switch The short-circuit current control part comprises a first i fault transfer branch, a first i mechanical switch, and a first i short-circuit current limiting switch i The one end of the mechanical switch is connected to the first i The one end of the fault transfer branch is connected to the first i The DC port of the sending end converter station is connected to the first i The other end of the mechanical switch is connected to the first i The positive pole of the adjustable voltage source in the DC transmission line is connected to the first i The other end of the fault transfer branch is connected to the first i The one end of the short-circuit current limiting switch is connected to the first i The other end of the short-circuit current limiting switch is connected to the first i The negative pole of the adjustable voltage source in the DC transmission line is connected to the first 2. The composite device with multi-port DC power flow and short circuit current control functions according to claim 1, characterized in that, The first i The adjustable voltage source in a DC transmission line is a capacitor.

3. The composite device with multi-port DC power flow and short circuit current control functions according to claim 1, characterized in that, The first i A DC power transmission line has a first i When the bypass switches, the first coupling inductance control switch, the second coupling inductance control switch, and each power flow control switch are all off and the bypass switches are all closed, the power flow control part in each sub-module is bypassed.

4. The composite device with multi-port DC power flow and short circuit current control functions according to claim 1, characterized in that, The first i The tide flow control switch is a bidirectional switch composed of a first switch tube and a second switch tube connected in common collector.

5. The composite device with multi-port DC power flow and short circuit current control functions according to claim 4, characterized in that, The switch tube in the bidirectional switch and the first and second coupling inductance control switches are all insulated gate bipolar transistors.

6. The composite device with multi-port DC power flow and short circuit current control functions according to claim 5, characterized in that, The first i The short-circuit current limiting switch is a bidirectional switch.

7. The composite device with multi-port DC power flow and short circuit current control functions according to claim 6, characterized in that, The first i The failover branch is composed of a bidirectional switch and an arrester connected in parallel with the bidirectional switch.

8. The composite device with multi-port DC power flow and short circuit current control functions according to claim 7, characterized in that, The first switch tube current input end and the current output end are reversely connected in parallel with a first diode, and the second switch tube current input end and the current output end are reversely connected in parallel with a second diode.

9. The control method of the composite device with multi-port DC power flow and short circuit current control function according to any one of claims 1 to 8, characterized in that, The current of the two DC transmission lines is collected, the difference between the current of the two DC transmission lines and a reference value is subjected to PI regulation, and the PWM switch signal of each flow control switch is generated according to the signal after the PI regulation, so that each DC flow branch is complementarily conducted under the action of the PWM switch signal of each flow control switch. After detecting that a short-circuit fault occurs in the DC transmission line, the second coupling inductance control switch and each flow control switch are closed, each short-circuit current limiting switch is turned off, the flow control part is locked after being activated, the flow control switch in the sub-module connected to the DC transmission line where the short-circuit fault occurs is turned off after the fault current is transferred, the mechanical switch in the sub-module connected to the DC transmission line where the short-circuit fault occurs is turned off, the flow control part in the sub-module connected to the DC transmission line where the short-circuit fault occurs is removed after the fault line is cut off, and each fault transfer branch is removed. The composite device of any one of claims 1 to 8 is connected between the DC port of the sending end converter station and the DC port of the receiving end converter station.

10. A multi-terminal DC power transmission system characterised in that, ​

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