Composite device with multi-port direct-current power flow and short-circuit current control function

Abstract

The invention discloses a composite device topology suitable for a multi-terminal flexible DC power transmission system and having a DC power flow and short-circuit current control function and a control method thereof, and belongs to the technical field of power generation, power transformation or power distribution. The controller comprises an ith sub-module connected to an ith direct-current transmission line between a direct-current port of a sending-end converter station and a direct-current port of a receiving-end converter station, and the ith sub-module comprises a power flow control part and a short-circuit current control part which are connected in parallel with an adjustable voltage source in the ith direct-current transmission line, two coupling inductors, two coupling inductor control switches, n bypass switches connected in parallel to the two ends of an adjustable voltage source in a direct-current line respectively, six IGBT switch tubes, three bidirectional switches and a fault transfer branch are used for realizing bidirectional power flow adjustment and short-circuit current limitation between n lines; the defects that an existing composite device is large in loss and large in fault transfer branch current stress are overcome.

Description

Technical Field

[0001] The present invention relates to a composite device with multi-port DC power flow and short-circuit current control functions, which is mainly used in multi-terminal DC power transmission occasions, relates to power electronics technology, and belongs to the technical field of power generation, power transformation or power distribution. Background Art

[0002] Multi-Terminal Direct Current (MTDC) refers to a DC transmission network consisting of three or more converter stations. This system can not only realize multi-power supply and multi-point power reception, but also has higher flexibility than the traditional two-terminal DC transmission system. At the same time, MTDC technology is regarded as one of the effective means to effectively solve the challenges currently faced by my country, such as large-scale renewable energy grid connection, transmission corridor resource shortage, and large-capacity long-distance power transmission. In the MTDC system, converter stations are connected to each other 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), as an advanced commutation technology, has significant advantages over traditional current source converter (CSC). Especially when the power flow is reversed, VSC can realize the rapid switching of DC current direction without changing the voltage polarity, while avoiding the problem of commutation failure. Therefore, VSC is more conducive to forming a multi-terminal direct current transmission system based on voltage source converter (VSC-based Multi-Terminal High Voltage Direct Current, VSC-MTDC), thereby further improving the flexibility and reliability of the transmission system.

[0003] Figure 1The figure shows the equivalent circuit of a typical ring-type four-terminal VSC-MTDC system. Assume that VSC1, VSC2 and VSC3 are converter stations of three offshore wind farms respectively, and operate in constant power mode, while VSC0 is an onshore converter station, which operates in constant voltage mode. At this time, power is transmitted from VSC1, VSC2 and VSC3 to VSC0. In view of the difficulties of DC power flow control and fault current suppression in DC transmission systems, DC power flow controllers (DCPFC) and DC current breakers (DCCB) are usually used in DC transmission systems to enhance the power flow direction control capability of transmission lines in DC transmission networks and the ability to adjust line transmission power.

[0004] Since the DC grid does not have the reactive power, reactance and phase angle problems of the AC system, its power flow regulation is mainly achieved by changing the voltage and resistance value. Therefore, the existing DC power flow controllers are mainly divided into two categories: resistance type and voltage type. The resistance type controller has a simple structure, but its regulation ability is limited. It can only achieve unidirectional power flow regulation, and the conduction loss is large. The voltage type controller includes three types: DC transformer, series adjustable voltage source and line-to-line power flow controller. The DC transformer changes the line voltage by connecting in series in the line to achieve power flow regulation, but this method needs to withstand high voltage and high power at the system level, which increases the cost and system complexity. Although the series adjustable voltage source does not need to withstand high voltage directly, it needs to interact with an external power supply, which increases the complexity of the system. In order to simplify the system structure and avoid the use of additional power supplies, researchers have proposed a line-to-line power flow controller. This controller uses a reasonable design scheme to enable the power flow to be transmitted between different lines, thereby achieving flexible power flow regulation. This method not only improves the reliability of the system, but also reduces the complexity of the system.

[0005] In addition, the line impedance of the DC grid is extremely low, so a small voltage change may lead to a large current change. Therefore, it is particularly important to develop fast current limiting and short-circuit disconnection devices for DC grids. At present, the existing short-circuit current limiting devices are mainly used in AC grids, and most of them rely on the natural zero-crossing point of the AC current to achieve disconnection. However, in the DC grid, since there is no natural zero-crossing point during a short circuit, the disconnection process becomes extremely difficult, and it is difficult to meet the high requirements of the DC grid for safety and reliability. Current research mainly focuses on hybrid DC circuit breakers based on power electronics technology, which effectively cut off short-circuit currents through fast response and precise control. At the same time, there are also studies that limit the peak value and rise rate of the fault current by inserting a current limiting circuit in series in the grid, thereby reducing the impact of short circuits on the grid.

[0006] However, the current development and operation of devices for DC power flow control and short-circuit control are often independent of each other, and rarely involve composite control strategies for short circuits and power flows. This discrete research and application model leads to disadvantages such as relatively high costs, large equipment size, and inconvenience in unified control. Patent CN201710341926.0 proposes a composite device with DC power flow and short-circuit control. However, the composite device can only be used for power flow regulation between two ports and has no specific scalability. In addition, the device involving the composite control strategy of short circuit and power flow requires more switching tubes to achieve power flow control, and most of the fault current flows through the fault transfer branch during a short circuit, and the loss caused by multiple switching tubes increases the cost of the multi-terminal DC transmission system. Summary of the invention

[0007] The purpose of the present invention is to address the deficiencies of the above-mentioned background technology and to provide a composite device with multi-port DC power flow and DC short-circuit current control functions and a control method thereof, so as to solve the technical problems that the composite device with power flow control and short-circuit control functions in the current multi-terminal DC transmission system has large losses and large fault transfer branch current stress. The invention aims to achieve coordinated management of power flow and short circuit in the DC power grid through an integrated composite device, thereby improving the operating efficiency of the multi-terminal DC transmission system, reducing costs, and enhancing the safety and reliability of the system.

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

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

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

[0011] The short-circuit current control part includes: 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 DC port of the i-th sending-end converter station, 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 a composite device with multi-port DC power flow and short-circuit current control functions, the adjustable voltage source in the i-th DC transmission line is a capacitor.

[0013] As a further optimization scheme of a composite device with multi-port DC power flow and short-circuit current control functions, the i-th bypass switch is connected in parallel at both ends of the adjustable voltage source in the i-th DC transmission line. When the first coupled inductor control switch, the second coupled inductor control switch, and each power flow control switch are turned off and each bypass switch is closed, the power flow control part in each submodule is bypassed.

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

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

[0016] As a further optimization solution for a composite device having multi-port DC power flow and short-circuit current control functions, the i-th short-circuit current limiting switch is a bidirectional switch.

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

[0018] As a further optimization solution for a composite device with multi-port DC power flow and short-circuit current control functions, the switch tube in the bidirectional switch and the first coupled inductor control switch and the second coupled inductor control switch are all insulated gate bipolar transistors.

[0019] The present invention provides a control method for a composite device with multi-port DC power flow and short-circuit control, including: 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 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, and the difference is passed through the PI link and through logical operation to obtain the PWM switching signal of the n line switch tubes, so that the n power flow branches work in a complementary conduction state and the DC transmission line current is maintained 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 fault to ground occurs in the i-th DC transmission line. After a period of time, at time t1, the composite device detects the occurrence of the fault and immediately closes the second coupled inductor control switch and each power flow control switch to lock the power flow control link, and simultaneously opens each short-circuit current limiting switch to protect the capacitor;

[0023] After a delay of Δt1, the fault transfer branch is activated to transfer the fault current 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 where the short circuit fault occurs 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 cut off, and the remaining lines resume normal operation.

[0027] The multi-terminal DC power transmission system comprises 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 power flow control and DC short-circuit control. The device realizes bidirectional power flow control between n transmission lines through two coupled inductors, two coupled inductor control switches and a power flow control switch. Compared with the existing composite device, the number of switch tubes is reduced, and the loss is reduced. When a DC line fails, the capacitor serving as an 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. The fault current is suppressed before the circuit breaker is operated, effectively reducing the current stress of the fault transfer branch. The fault current is then cut off in combination with the fault transfer branch.

[0030] (2) The present invention proposes a composite device for a DC power grid with DC power flow control and DC short-circuit control, which reuses two coupled inductors and two coupled inductor control switches. The two coupled inductors and two coupled inductor control switches transmit line energy when the power flow control part is started, and suppress the fault current when the short-circuit current part is started, thereby reducing the fault current peak value. The cost of the device is further reduced by reusing components.

[0031] (3) The multi-terminal DC power transmission system proposed in the present invention realizes DC power flow control through the proposed composite device, thereby improving the operating efficiency of the multi-terminal DC power transmission system, reducing costs, and enhancing the safety and reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0033] Figure 2 The invention relates to a four-terminal flexible direct current power transmission system comprising the composite device of the invention.

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

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

[0036] Figure 5 The power flow control part of the composite device in the embodiment of the present invention is in forward operation and the switch Q is turned on. 11 The working mode diagram after .

[0037] Figure 6 The power flow control part of the composite device in the embodiment of the present invention is in forward operation and the switch Q is turned on. 21 The working mode diagram after .

[0038] Figure 7 The power flow control part of the composite device in the embodiment of the present invention is in forward operation and the switch Q is turned on. 31 Working mode diagram.

[0039] Figure 8 It is a working mode diagram of the short-circuit current suppression part of the composite device in the embodiment of the present invention after the capacitor control switches BS1, BS2, and BS3 are disconnected when there is forward operation.

[0040] Fig. 9It is a working mode diagram after activating the fault transfer branches MB1, MB2, and MB3 when the short-circuit current suppression part of the composite device in the embodiment of the present invention is in forward operation.

[0041] Fig.10 The short-circuit current suppression part of the composite device in the embodiment of the present invention has a forward operation and disconnects the switch Q 11 , Q 12 Working mode diagram.

[0042] Fig.11 It is a working mode diagram after the fault transfer branches MB1, MB2, and MB3 are removed when the short-circuit current suppression part of the composite device in the embodiment of the present invention is in forward operation.

[0043] Fig.12 1 is a line current waveform diagram when the composite device performs dynamic power flow control in an embodiment of the present invention.

[0044] Fig.13 1 is a line current waveform diagram when the composite device performs power flow reversal control in an embodiment of the present invention.

[0045] Fig.14 It is a line current waveform diagram when the composite device in the embodiment of the present invention performs fault current shedding.

[0046] Fig.15 It is a voltage waveform diagram of the port of the sending-end converter station when the composite device in the embodiment of the present invention performs fault current removal.

[0047] Description of the numbers in the figure: C i , the i-th capacitor, S i , the i-th bypass switch, T1, the first coupled inductor control switch, L1, the first coupled inductor, T2, the second coupled inductor control switch, L2, the second coupled inductor, Q i1 , the first switch tube in the i-th flow control switch, Q i2 , the second switch tube in the i-th flow control switch, MB i , the i-th failover branch, UFD i , the first mechanical switch, BS i , the i-th short-circuit current limiting switch, D i1 , the first diode in the i-th flow control switch, D i2 , the second diode in the i-th flow control switch, SA, and a lightning arrester. DETAILED DESCRIPTION

[0048] The technical solution of the invention is described in detail below with reference to the accompanying drawings.

[0049] The DC power flow controller of the present invention is applicable to a multi-terminal flexible DC power transmission system, and its topology diagram is shown in FIG3(a), including: an i-th capacitor C as an adjustable voltage source; i , i=1,2,n, the i-th capacitor C i are connected in series in a DC transmission line, and no external power supply is required. The i-th capacitor C i The i-th bypass switch S is connected in parallel between the two stages. i Each adjustable voltage source is connected between two stages with a flow control part and a short-circuit current control part; the flow control part includes: a first coupled inductor control switch T1, a first coupled inductor L1, a second coupled inductor control switch T2, a second coupled inductor L2 and an i-th flow control switch, wherein the i-th flow control switch is a first switch tube Q connected with a common collector. i1 , the second switch tube Q i2 The short-circuit current control part comprises: an i-th fault transfer branch MB; an i-th fault transfer branch MB1; an i-th fault transfer branch MB2; an i-th fault transfer branch MB3; an i-th fault transfer branch MB4; an i-th fault transfer branch MB5; an i-th fault transfer branch MB6; an i-th fault transfer branch MB7; an i-th fault transfer branch MB8; an i-th fault transfer branch MB9; an i-th fault transfer branch MB10; an i-th fault transfer branch MB11; an i-th fault transfer branch MB12; an i-th fault transfer branch MB13; an i-th fault transfer branch MB14; an i-th fault transfer branch MB15; an i-th fault transfer branch MB16; an i-th fault transfer branch MB17; an i-th fault transfer branch MB18; an i-th fault transfer branch MB19; an i-th fault transfer branch MB20; an i-th fault transfer branch MB21; an i-th fault transfer branch MB31; an i-th fault transfer branch MB18; an i-th fault transfer branch MB19; an i-th fault transfer branch MB21; an i-th fault transfer branch MB19; an i-th fault transfer branch MB11; an i-th fault transfer branch MB12; an i-th fault transfer branch i 、The first mechanical switch UFD i and the i-th short-circuit current limiting switch BS i , the first mechanical switch UFD i One end of the i-th failover branch MB i One end is connected to the DC port of the i-th sending converter station, and the i-th mechanical switch UFD i The other end is connected to the positive electrode of the ith adjustable voltage source, and the ith fault transfer branch MB i The other end is connected to the i-th short-circuit current limiting switch BS i One end, the i-th short-circuit current limiting switch BS i The other end is connected to the negative electrode of the i-th adjustable voltage source. i1 A first diode D is connected in reverse parallel between the current input terminal and the current output terminal. i1 , the second switch tube Q i2 A second diode D is connected in reverse parallel between the current input terminal and the current output terminal. i2 , the i-th short-circuit current limiting switch BS i is a bidirectional switch. As shown in Figure 3(b), the i-th fault transfer branch MB i It consists of a bidirectional switch and a lightning arrester SA connected in parallel with it.

[0050] When the first coupled inductor control switch T1, the second coupled inductor control switch T2 and the first switch tube Q of the i-th DC transmission line are i1 , the second switch tube Q of the i-th DC transmission line i2 When all the DC power flow controllers are in forward operation, they include n complementary conducting DC power flow branches, each of which is connected by the i-th capacitor C i , the i-th bidirectional switch BS i , the first coupled inductor L1, the first coupled inductor control switch T1, the first switch tube Q i1 , the second diode D i2 The first coupling inductor L1 is used to transmit the energy between the capacitors in series in each DC power transmission line. When all the DC power transmission line currents of the composite device are in reverse operation, each DC power flow branch is connected in series by the second coupling inductor L2, the second coupling inductor control switch T2, the first diode D i1 , the second switch tube Q i2 、ith capacitor C i and the i-th bidirectional switch BS i The composite device is connected in series in sequence; when the DC transmission line current is running in the forward direction and in the reverse direction, when the forward-running line transfers energy to the reverse-running line, the energy in the first coupled inductor L1 is transferred to the second coupled inductor L2 by turning off the first coupled inductor control switch T1 and closing the second coupled inductor control switch T2, and then the second coupled inductor L2 transfers energy to the capacitor connected in series in the reverse-running line; when a short-circuit fault occurs in the DC transmission line 1 of the composite device, the first coupled inductor control switch T1, the second coupled inductor control switch T2 and the power flow control switch Q are closed. i1 -Q i2 Then, close the failover branch MB1-MB n The switch in the circuit breaker opens and the first mechanical switch UFD1 is disconnected soon after, and finally, the first fault transfer branch MB1 is disconnected to realize fault line removal.

[0051] The DC power flow controller of the present invention has the ability to accurately control the magnitude 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 invention is as follows: Figure 2 As shown, VSC0 is a constant voltage station, VSC1, VSC2, and VSC3 are constant power stations, and a composite device is connected in series at the DC port of VSC0 to achieve DC power flow regulation and fault current suppression. The control strategy is implemented through Figure 4 The control strategy block diagram shown in the figure is clearly displayed. In this control process, the system first collects the real-time currents I1 and I2 on the first and second DC transmission lines, and compares them with the given reference values ​​I 1_ref ,I 2_ref The difference is subtracted and passed through the PI link to achieve accurate error adjustment. The difference processed by the PI link is processed by the logic operation unit to generate the first switch tube Q in the three DC transmission lines. 11 , Q 21 , Q 31 The PWM switching signal ensures that the three DC power flow branches work in a complementary conduction state, thereby effectively maintaining the transmission line current at a given value. The three DC power flow branches work in a complementary conduction state, for example, the first switch tube Q in the first DC transmission line 11 When turned on, the first switch tube Q in the second DC transmission line 21 , the first switch tube Q in the third DC transmission line 31 All turned off.

[0052] Taking two typical working conditions as examples, the control characteristics of the composite device with DC power flow control and DC short-circuit control used in the DC power grid are analyzed below.

[0053] 1. Flow control function

[0054] According to the current flow direction and line control requirements, the first coupled inductor control switch T1 is constantly closed, and the first switch tube Q of the first DC transmission line is first opened. 11 , the energy in the first capacitor C1 is transferred to the first coupled inductor L1. The specific mode is as follows Figure 5 As shown; then the first switch tube Q of the second DC transmission line is turned on 21 , the energy in the second capacitor C2 is transferred to the first coupled inductor L1. The specific mode is as follows Figure 6 As shown; finally, the first switch tube Q of the third DC transmission line is turned on 31 , the energy in the first coupling inductor L1 is transferred to the third capacitor C3. The specific mode is as follows Figure 7 As shown. In this way, the power P is realized from the first adjustable voltage source V C1 , the first adjustable voltage source V C2 Transfer 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 on the first DC transmission line, the current control switch Q is closed first. 11 -Q 32The first coupled inductor control switch T1 and the second coupled inductor control switch T2 are connected, and then the first capacitor control switch BS1, the second capacitor control switch BS2 and the third capacitor control switch BS3 are disconnected. Figure 8 As shown, at this time, all fault currents pass through the first coupled inductor L1 and the second coupled inductor L2, and the current rise rate of the first DC transmission line is suppressed. At the same time, the capacitors connected in series in each DC transmission line are removed from the line to ensure the 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 FIG. Fig. 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 Fig.10 Finally, the switch in the first fault transfer branch is closed to remove the faulty line from the system, as shown in Fig.11 shown.

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

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

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

[0060] Line parameters Line 1 Route 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 was kept unchanged. Fig.12 The current waveforms of lines 1, 2, and 3 are given in Fig.13 The adjustable voltage source V C1 、V C2 、V C3 It can be seen from the above waveform 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 transmission system was in normal operation, and the power flow control of the composite device was put into operation. At t = 6s, a short circuit to ground occurred in transmission line 1, and the fault current suppression part of the composite device was put into operation, first suppressing the rising rate of the fault current, and then cutting off the fault line from the system. Fig.14 The current waveforms of lines 1, 2, and 3 are given in Fig.15 The port voltage waveform of the sending-end converter station is given in .

[0063] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above specific embodiments. The above specific embodiments and the description in the specification are only for further illustrating the principles and preparation effects of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of the present invention 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: include: Connected between the DC port of the sending converter station and the DC port of the receiving converter station i The first i submodule, the i The submodules include: i The power flow control part and short-circuit current control part of the DC transmission line with an adjustable voltage source connected in parallel, i =1,2, n , n is a positive integer, where The power flow control part includes: a first coupled inductor control switch, a first coupled inductor, a second coupled inductor control switch, a second coupled inductor and a first coupled inductor. i A power flow control switch, one end of the first coupled inductor and one end of the second coupled inductor are connected to the DC port of the receiving converter station, the other end of the first coupled inductor is connected to the current output end of the first coupled inductor control switch, the other end of the second coupled inductor is connected to the current input end of the second coupled inductor control switch, the current input end of the first coupled inductor control switch, the current output end of the second coupled inductor control switch and the i One end of the bidirectional switch is connected to the i Negative connection of adjustable voltage source in DC transmission line, i The other end of the bidirectional switch is connected to the i Positive connection of adjustable voltage source in DC transmission line, The short-circuit current control part includes: i Failover leg, i Mechanical switches and i Short-circuit current limiting switch, i One end of the mechanical switch and the i One end of the failover leg is connected to the i DC port of the sending end converter station, i The other end of the mechanical switch is connected to i The positive pole of the adjustable voltage source in the DC transmission line, i The other end of the failover leg is connected to i Short-circuit current limiting switch, i The other end of the short-circuit current limiting switch is connected to the i Negative pole of adjustable voltage source in DC transmission line.

2. A composite device with multi-port DC power flow and short-circuit current control functions according to claim 1, characterized in that: The said i The adjustable voltage source in the DC transmission line is a capacitor.

3. A composite device with multi-port DC power flow and short-circuit current control functions according to claim 1, characterized in that: The said i The adjustable voltage source in the DC transmission line is connected to the i Bypass switch, when the first coupled inductor control switch, the second coupled inductor control switch and each flow control switch are turned off and each bypass switch is closed, the flow control part in each submodule is bypassed.

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

5. A composite device with multi-port DC power flow and short-circuit current control functions according to claim 4, characterized in that: A first diode is connected in reverse parallel between the current input terminal and the current output terminal of the first switch tube, and a second diode is connected in reverse parallel between the current input terminal and the current output terminal of the second switch tube.

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

7. A composite device with multi-port DC power flow and short-circuit current control functions according to claim 6, characterized in that: The said i The fault transfer branch consists of a bidirectional switch and a lightning arrester connected in parallel with it.

8. A composite device with multi-port DC power flow and short-circuit current control functions according to claim 7, characterized in that: The switch tube in the bidirectional switch, the first coupled inductor control switch, and the second coupled inductor control switch are all insulated gate bipolar transistors.

9. A control method for a composite device with multi-port DC power flow and short-circuit current control functions according to any one of claims 1 to 8, characterized in that: include: A power flow control strategy is to close the first coupled inductor control switch and each short-circuit current limiting switch, collect the currents of the two DC transmission lines, perform PI adjustment on the difference between the currents of the two DC transmission lines and the reference value, generate PWM switching signals of each power flow control switch according to the PI-adjusted signal, and each DC power flow branch is complementary-conducted under the action of the PWM switching signals of each power flow control switch; and, The short-circuit current control strategy, after detecting a short-circuit fault in a DC transmission line, closes the second coupled inductor control switch and each flow control switch, turns off each short-circuit current limiting switch, locks the flow control part and then activates each fault transfer branch, and after transferring the fault current, turns off the flow control switch in the submodule connected to the DC transmission line where the short-circuit fault occurs, turns off the mechanical switch in the submodule of the DC transmission line where the short-circuit fault occurs, removes the flow control part in the submodule of the DC transmission line where the short-circuit fault occurs, and then removes each fault transfer branch and cuts off the fault line.

10. A multi-terminal direct current transmission system, characterized in that: It comprises the composite device as described in any one of claims 1 to 8, and the composite device is connected between the DC port of the sending-end converter station and the DC port of the receiving-end converter station.

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