Multi-port direct-current circuit breaker with current limiting and power flow control functions and control method of multi-port direct-current circuit breaker

Abstract

The invention discloses a multi-port direct-current circuit breaker, which comprises a plurality of parallel circuits, a main circuit breaker, a lightning arrester, a mode switching switch, a current-limiting inductor, a tidal current capacitor, a current-limiting capacitor, a first thyristor, a second thyristor and a third thyristor, each line comprises a first current transfer switch, a first mechanical switch, a second current transfer switch, a second mechanical switch, an isolating switch and an electric reactor which are connected in sequence; the first current transfer switch is connected with a first direct-current bus, the second mechanical switch is connected with a second direct-current bus, and the isolating switch and the reactor are connected in series and then connected with the first mechanical switch and the second current transfer switch; the main circuit breaker and the lightning arrester are connected in parallel and then are connected in series with the first thyristor, the second thyristor and the current-limiting capacitor are connected in series, the third thyristor and the current-limiting inductor are connected in series and then are connected in parallel with the first thyristor, the mode switching switch and the tidal current capacitor are connected in series and then are connected in parallel with a plurality of lines, and the mode switching switch is connected with a first direct-current bus. The tidal current capacitor is connected with the second DC bus.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and more specifically, to a multi-port DC circuit breaker with current limiting and power flow control functions and its control method. Background Technology

[0002] There are two basic network construction methods for DC power grids: the first method uses a modular multilevel converter (MMC) based on half-bridge submodules plus a DC circuit breaker, which is suitable for DC power grids with any number of terminals; the second method uses an MMC with DC fault self-clearing capability, such as an MMC based on full-bridge submodules, which does not require a DC circuit breaker and is suitable for small-scale DC power grids with fewer than 10 terminals.

[0003] When using a grid configuration with half-bridge submodules (MMC) and DC circuit breakers, converter stations are typically required to continue operating during DC line faults and cannot be locked out. The faulty line is quickly disconnected by the DC circuit breaker, and the fault handling principles are similar to those of the AC grid. When using a grid configuration without DC circuit breakers, the relevant converters within the grid are locked out during DC line faults. After locking out, the fault current drops to zero and remains stable approximately 10ms. The faulty line is then isolated via a disconnecting switch, and subsequently, the relevant converters are unlocked to restore power. The time from the onset of the fault to the restoration of power is generally around 20ms, and the impact on the AC grid is usually within acceptable limits.

[0004] When a DC power grid is constructed using half-bridge submodules (MMC) and DC circuit breakers, the DC circuit breaker becomes a critical component. Currently, there are three main types of high-voltage DC circuit breakers: traditional mechanical circuit breakers based on conventional switches, solid-state circuit breakers based on pure power electronic devices, and hybrid circuit breakers combining both. Although feasible solutions have been developed, their high cost and large size make them difficult to widely use in power grids like AC circuit breakers. Therefore, DC circuit breakers remain a fundamental technological bottleneck in the development of DC power grids.

[0005] Meanwhile, due to the extremely rapid development of fault current in DC power grids, large smoothing reactors need to be installed at both ends of the line to suppress the rise of fault current. However, when selecting large smoothing reactors to achieve better current limiting effects, the dynamic characteristics of the flexible DC system will be affected. To resolve this contradiction, fault current limiters can be installed at both ends of the line to replace ordinary smoothing reactors. These limiters operate at low impedance during normal operation and switch to high impedance during faults.

[0006] Furthermore, in DC power grids where the number of transmission lines exceeds the number of converter stations, relying solely on converter stations is insufficient to effectively regulate power flow, resulting in inadequate degrees of freedom in power flow control. In such cases, a DC power flow controller is necessary to enhance regulation capabilities; this controller is also a key piece of equipment for the development of DC power grids.

[0007] The future DC grid will contain a large number of DC lines. If the entire system needs to be equipped with numerous separate DC circuit breakers, fault current limiters and power flow controllers, the cost will be high and the size will be huge, posing a huge economic and technical challenge to the development of DC grids. Summary of the Invention

[0008] The purpose of this invention is to overcome the defects and deficiencies of the prior art and provide a multi-port DC circuit breaker with current limiting and power flow control and its control method. The multi-port DC circuit breaker utilizes the similarity of topology to integrate the DC circuit breaking, fault current limiting and power flow control functions of multiple ports into one, so as to reduce the number of required components and thus significantly reduce the engineering cost.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A multi-port DC circuit breaker with current limiting and power flow control includes multiple parallel lines, a main circuit breaker, a surge arrester, a mode switching switch, a current limiting inductor, a power flow capacitor, a current limiting capacitor, a first thyristor, a second thyristor, and a third thyristor. Each line includes a first bridge arm, a second bridge arm, a disconnecting switch, and a reactor. The first bridge arm includes a first current transfer switch and a first mechanical switch, and the second bridge arm includes a second current transfer switch and a second mechanical switch.

[0011] The first terminal of the first current transfer switch is connected to the first DC bus, the second terminal of the first current transfer switch is connected to the first terminal of the first mechanical switch, the second terminal of the first mechanical switch is connected to the first terminal of the second current transfer switch, the second terminal of the second current transfer switch is connected to the first terminal of the second mechanical switch, and the second terminal of the second mechanical switch is connected to the second DC bus. After the disconnecting switch and the reactor are connected in series, the second terminal of the reactor is connected to the second terminal of the first mechanical switch and the first terminal of the second current transfer switch, respectively.

[0012] The main circuit breaker and surge arrester are connected in parallel and then connected in series with the first thyristor. The second thyristor and the current-limiting capacitor are connected in series and then connected in parallel with the first thyristor. The third thyristor and the current-limiting inductor are connected in series and then connected in parallel with the first thyristor. The mode switching switch and the power flow capacitor are connected in series and then connected in parallel with multiple lines. The first terminal of the mode switching switch is connected to the first DC bus, and the second terminal of the power flow capacitor is connected to the second DC bus.

[0013] The first terminal of the main circuit breaker and the first terminal of the surge arrester are respectively connected to the first DC bus. The second terminal of the main circuit breaker and the second terminal of the surge arrester are respectively connected to the anodes of the first thyristor, the second thyristor and the third thyristor. The first terminal of the current-limiting capacitor is connected in series with the cathode of the second thyristor. The first terminal of the current-limiting inductor is connected in series with the cathode of the third thyristor. The cathode of the first thyristor, the second terminal of the current-limiting capacitor and the second terminal of the current-limiting inductor are respectively connected to the second DC bus.

[0014] Furthermore, both the current transfer switch and the main circuit breaker use multiple insulated gate bipolar transistors connected in series and parallel.

[0015] Furthermore, the mode switching switch includes two structures: one is constructed using a third mechanical switch; the other is constructed by connecting a third current transfer switch and a third mechanical switch in series.

[0016] Furthermore, the first mechanical switch, the second mechanical switch, and the third mechanical switch are all ultra-fast mechanical switches.

[0017] Furthermore, the surge arrester adopts a metal oxide surge arrester, and the number of parallel lines is m, where m is an integer greater than or equal to 3.

[0018] A power flow control method for a multi-port DC circuit breaker with current limiting and power flow control, applied to any of the aforementioned multi-port DC circuit breakers with current limiting and power flow control, includes the following steps:

[0019] S11: When the system containing the DC circuit breaker is operating normally, the DC circuit breaker is in power flow control mode, and the mode switching switch and all mechanical switches of the lines are in the closed state.

[0020] S12: Collects the power flow capacitor voltage and the current of each line through an external voltage sampling circuit, and transmits the collected power flow capacitor voltage and the current of each line to the microcontroller.

[0021] S13: The microcontroller calculates the difference between the power flow capacitor voltage and its reference value, and generates a pair of complementary PWM signals through modulation to control the first current transfer switch and the second current transfer switch connected to the MMC line, and to control the first current transfer switch and the second current transfer switch of the relaxation line.

[0022] S14: The microcontroller calculates the difference between the current of the remaining lines and its reference value, and generates a pair of complementary PWM signals by modulation to control the first current transfer switch and the second current transfer switch of the remaining lines respectively.

[0023] A DC fault handling method for a multi-port DC circuit breaker with current limiting and power flow control is provided, which is applied to the multi-port DC circuit breaker with current limiting and power flow control described above. The DC fault handling method is used to realize rapid mode switching, current limiting and DC fault clearing.

[0024] Furthermore, the DC fault handling method includes the following steps:

[0025] S21: After the system where the DC circuit breaker is located detects a line fault, it controls the main circuit breaker and the first thyristor to conduct.

[0026] S22: After the main circuit breaker is fully turned on, the first current transfer switch controlling the normal line is turned on and the second current transfer switch is turned off, and the first current transfer switch controlling the faulty line is turned off and the second current transfer switch is turned on.

[0027] S23: After the above current transfer switch is fully turned on or off, the second mechanical switch controlling the normal circuit, the first mechanical switch controlling the faulty circuit, and the third mechanical switch controlling the mode switching are turned off.

[0028] S24: After the above mechanical switch is completely disconnected, the first thyristor is disconnected and the second and third thyristors are turned on. The negative voltage of the current limiting capacitor causes the current in the first thyristor to decrease to 0. The current limiting capacitor completes the initial current limiting. During the process of the current limiting capacitor being engaged, the negative voltage across the current limiting capacitor decreases and then the positive voltage increases to the system voltage. The current in the current limiting capacitor decreases to 0 and the current limiting inductor is fully engaged.

[0029] S25: After the current-limiting inductor is fully engaged, the main circuit breaker is opened, allowing the fault current to be transferred to the surge arrester. The fault current is dissipated through the surge arrester and quickly reduced to 0.

[0030] S26: After the line fault current drops to 0, the isolating switch controlling the faulty line is opened, completely clearing the line fault, and restoring normal power flow to the remaining normal lines.

[0031] Furthermore, in step S22, if the mode switching switch is composed of a third current transfer switch and a third mechanical switch connected in series, the third current transfer switch for mode switching also needs to be disconnected.

[0032] Furthermore, in step S22, if the mode switching switch is composed of only one third mechanical switch, then the mode switching switch does not need to be operated.

[0033] Compared with the prior art, the multi-port DC circuit breaker of the present invention, which has current limiting and power flow control, integrates the fault current limiting, DC circuit breaking and power flow control functions of multiple ports into one by utilizing the similarity of topology, reducing the number of required components and thus significantly reducing the engineering cost.

[0034] The power flow control method of the multi-port DC circuit breaker with current limiting and power flow control of the present invention can directly generate the corresponding line switching signal by detecting and controlling the power flow capacitor voltage and the current of each branch separately through a microcontroller. This solves the problem that the traditional control method needs to exhaustively enumerate an exponential number of switching states, and therefore cannot be adapted to multi-port power flow controllers.

[0035] The fault current limiting and DC fault handling method of the multi-port DC circuit breaker with current limiting and power flow control of the present invention can realize rapid switching from power flow control mode to DC circuit breaking mode, and complete rapid fault current limiting and DC circuit breaking, thus meeting the speed requirements of DC grid fault clearing. Attached Figure Description

[0036] Figure 1 A schematic diagram of the topology of a multi-port DC circuit breaker with current limiting and power flow control.

[0037] Figure 2 This is a schematic diagram of the simulation waveforms of the power flow capacitor voltage and line current of a multi-port DC circuit breaker under power flow control conditions.

[0038] Figure 3 This is a schematic diagram of the simulation waveforms of the line current and the main circuit breaker circuit of a multi-port DC circuit breaker under DC fault conditions.

[0039] Figure 4 This is a schematic diagram of the simulation waveforms of surge arrester current and surge arrester voltage in a multi-port DC circuit breaker under DC fault conditions. Detailed Implementation

[0040] The multi-port DC circuit breaker with current limiting and power flow control of the present invention and its control method will be further described below with reference to the accompanying drawings and specific embodiments.

[0041] Example 1

[0042] This embodiment discloses a multi-port DC circuit breaker with current limiting and power flow control, including multiple parallel lines, a main circuit breaker, surge arresters, a mode switching switch, a current-limiting inductor, a power flow capacitor, a current-limiting capacitor, a first thyristor, a second thyristor, and a third thyristor. The surge arresters are metal oxide surge arresters. The number of parallel lines is m, where m is an integer greater than or equal to 3. The m DC transmission lines are connected to the first DC bus and the second DC bus. Each line includes a first bridge arm, a second bridge arm, a disconnecting switch, and a reactor. The first bridge arm includes a first current transfer switch and a first mechanical switch, and the second bridge arm includes a second current transfer switch and a second mechanical switch.

[0043] The first terminal of the first current transfer switch is connected to the first DC bus. The second terminal of the first current transfer switch is connected to the first terminal of the first mechanical switch. The second terminal of the first mechanical switch is connected to the first terminal of the second current transfer switch. The second terminal of the second current transfer switch is connected to the first terminal of the second mechanical switch. The second terminal of the second mechanical switch is connected to the second DC bus. After the disconnecting switch and the reactor are connected in series, the second terminal of the reactor is connected to the second terminal of the first mechanical switch and the first terminal of the second current transfer switch, respectively.

[0044] The main circuit breaker and surge arrester are connected in parallel and then connected in series with the first thyristor. The second thyristor and the current-limiting capacitor are connected in series and then connected in parallel with the first thyristor. The third thyristor and the current-limiting inductor are connected in series and then connected in parallel with the first thyristor. The mode switching switch and the power flow capacitor are connected in series and then connected in parallel with multiple lines. The first terminal of the mode switching switch is connected to the first DC bus, and the second terminal of the power flow capacitor is connected to the second DC bus.

[0045] The first terminal of the main circuit breaker and the first terminal of the surge arrester are respectively connected to the first DC bus. The second terminal of the main circuit breaker and the second terminal of the surge arrester are respectively connected to the anodes of the first thyristor, the second thyristor and the third thyristor. The first terminal of the current-limiting capacitor is connected in series with the cathode of the second thyristor. The first terminal of the current-limiting inductor is connected in series with the cathode of the third thyristor. The cathode of the first thyristor, the second terminal of the current-limiting capacitor and the second terminal of the current-limiting inductor are respectively connected to the second DC bus.

[0046] The first, second, and third thyristors are thyristors with strong voltage and current carrying capacity, typically in the kV and kA range, such as T3441N52TOHXPSA1. The current transfer switch, also called a load transfer switch, is a switch located in the low-loss branch of a hybrid circuit breaker used to transfer fault current. Both the current transfer switch and the main circuit breaker use multiple insulated-gate bipolar transistors (IGBTs) connected in series and parallel.

[0047] Current transfer switches require a relatively small number of IGBTs and withstand lower voltages, typically a few to a dozen IGBTs. Therefore, in the attached diagram, only one IGBT is shown within a rectangle, as shown below. Figure 1 As shown in the diagram. The main circuit breaker requires a large number of IGBTs and withstands high voltages, typically over a thousand. Therefore, two IGBTs are drawn within a rectangle in the attached diagram and indicated by ellipses, as shown below. Figure 1 As shown, IGBTs all use collectors and emitters connected to the main circuit of the multi-port DC circuit breaker, and gates connected to the control circuit of the multi-port DC circuit breaker.

[0048] like Figure 1 As shown, in the multi-port DC circuit breaker topology, 1 is the first current transfer switch of line 1, 2 is the first mechanical switch of line 1, 3 is the disconnecting switch of line 1, 4 is the reactor of line 1, 5 is the second current transfer switch of line 1, 6 is the second mechanical switch of line 1; 7 is the first current transfer switch of line 2, 8 is the first mechanical switch of line 2, 9 is the disconnecting switch of line 2, 10 is the reactor of line 2, 11 is the second current transfer switch of line 2, and 12 is the second mechanical switch of line 2.

[0049] 13 is the first current transfer switch of line m, 14 is the first mechanical switch of line m, 15 is the disconnecting switch of line m, 16 is the reactor of line m, 17 is the second current transfer switch of line m, and 18 is the second mechanical switch of line m; 18 is the main circuit breaker, 20 is the surge arrester, 21 is the first thyristor, 22 is the current limiting capacitor, 23 is the second thyristor, 24 is the current limiting inductor, 25 is the third thyristor, 26 is the power flow capacitor, 27 is the third mechanical switch, and 28 is the third current transfer switch.

[0050] like Figure 1 As shown, the mode switching switch design employs two different structures. When economy is a priority, only a single third mechanical switch can be used, i.e. Figure 1 The mode switching switch 1 structure; when reliability is a priority, it can be constructed by connecting a third current transfer switch and a third mechanical switch in series, i.e. Figure 1 The mode switching switch 2 structure in the middle. The first mechanical switch, the second mechanical switch and the third mechanical switch are all ultra-fast mechanical switches.

[0051] The DC circuit breaker of the present invention includes multiple line ports, each port being connected to the first and second bridge arms of the line. The fault current limiting, DC circuit breaking and power flow control functions connected to m DC lines are integrated together to form a multi-port DC circuit breaker, which allows the DC fault breaking and power flow control of m DC lines to share some components, reducing the number of components and improving economy.

[0052] Example 2

[0053] This embodiment discloses a power flow control method for a multi-port DC circuit breaker with current limiting and power flow control, applicable to any of the multi-port DC circuit breakers with current limiting and power flow control described above. The power flow control method controls each controlled variable independently, including the following steps:

[0054] S11: When the system containing the DC circuit breaker is operating normally, the DC circuit breaker is in power flow control mode, and the mode switching switch and all mechanical switches of the lines are in the closed state.

[0055] S12: Collects the power flow capacitor voltage and the current of each line through an external voltage sampling circuit, and transmits the collected power flow capacitor voltage and the current of each line to the microcontroller.

[0056] S13: The microcontroller calculates the difference between the power flow capacitor voltage and its reference value, and generates a pair of complementary PWM signals through modulation to control the first current transfer switch and the second current transfer switch of the line connected to the Modular Multilevel Converter (MMC), and to control the first current transfer switch and the second current transfer switch of the relaxation line.

[0057] S14: The microcontroller calculates the difference between the current of the remaining lines and its reference value, and generates a pair of complementary PWM signals by modulation to control the first current transfer switch and the second current transfer switch of the remaining lines respectively.

[0058] The power flow control method of the multi-port DC circuit breaker with current limiting and power flow control of the present invention can directly generate the corresponding line switching signal by detecting and controlling the power flow capacitor voltage and the current of each branch separately through a microcontroller. This solves the problem that the traditional control method needs to exhaustively enumerate an exponential number of switching states, and therefore cannot be adapted to multi-port power flow controllers.

[0059] Example 3

[0060] This embodiment discloses a DC fault handling method for a multi-port DC circuit breaker with current limiting and power flow control, applicable to any of the aforementioned multi-port DC circuit breakers with current limiting and power flow control. The DC fault handling method is used to achieve rapid mode switching, fault current limiting, and fault clearing, and includes the following steps:

[0061] S21: After the system where the DC circuit breaker is located detects a line fault, it controls the main circuit breaker and the first thyristor to conduct.

[0062] S22: After the main circuit breaker is fully turned on, the first current transfer switch controlling the normal line is turned on and the second current transfer switch is turned off, and the first current transfer switch controlling the faulty line is turned off and the second current transfer switch is turned on.

[0063] If the mode switch is constructed using a third current transfer switch and a third mechanical switch connected in series, the third current transfer switch for mode switching must be disconnected. If the mode switch is constructed using only a third mechanical switch, no operation is required.

[0064] S23: After the above current transfer switch is fully turned on or off, the second mechanical switch controlling the normal circuit, the first mechanical switch controlling the faulty circuit, and the third mechanical switch controlling the mode switching are turned off.

[0065] S24: After the above mechanical switch is completely disconnected, the first thyristor is disconnected and the second and third thyristors are turned on. The negative voltage of the current limiting capacitor causes the current in the first thyristor to decrease to 0. The current limiting capacitor completes the initial current limiting. During the process of the current limiting capacitor being engaged, the negative voltage across the current limiting capacitor decreases and then the positive voltage increases to the system voltage. The current in the current limiting capacitor decreases to 0 and the current limiting inductor is fully engaged.

[0066] S25: After the current-limiting inductor is fully engaged, the main circuit breaker is opened, allowing the fault current to be transferred to the surge arrester. The fault current is dissipated through the surge arrester and quickly reduced to 0.

[0067] S26: After the line fault current drops to 0, the isolating switch controlling the faulty line is opened, completely clearing the line fault, and restoring normal power flow to the remaining normal lines.

[0068] The DC fault handling method of the multi-port DC circuit breaker with current limiting and power flow control of the present invention realizes rapid switching from power flow control mode to DC circuit breaking mode, as well as rapid clearing of DC faults, thus meeting the requirements of speed for DC grid fault clearing.

[0069] Application Example 1

[0070] The following simulation verification uses a single-pole 500kV test system. There are four DC transmission lines connected to the first and second DC buses. One line is directly connected to the MMC, and the other three lines are connected to the DC buses. The currents of the three lines are referred to as I1, I2, and I3, respectively. Figure 1 The multi-port DC circuit breaker shown is equipped with current limiting and power flow control.

[0071] Figure 2 The overall waveform simulation results of the power flow capacitor voltage and the currents of the three lines are given. When 1 s < t < 2 s, the system is in a steady state and no power flow control is performed, I1 = 1.93 kA, I2 = 1.92 kA, and I3 = 1.48 kA. When t = 2 s, the first set of current reference values ​​is given to the four-port DC circuit breaker with power flow control capability. At this time, the power flow control mode is started, and the power flow capacitor voltage U C The voltage is rapidly charged to the rated value of 40 kV, and the current of each line is controlled to I1 = 1.00 kA, I2 = 2.50 kA, and I3 = 1.81 kA.

[0072] At t = 3 s, the second set of current reference values ​​is given to the four-port DC circuit breaker with power flow control capability. At this time, the reference value of the current in line 2 is -0.45A. It can be seen that after a transient process of 0.31 s, the DC circuit breaker achieves tracking of the reverse current reference value, and the current in each line is controlled to I1 = 2.50 kA, I2 = -0.45 kA, and I3 = 2.51 kA. At t = 4 s, the third set of current reference values ​​is given to the four-port DC circuit breaker with power flow control capability. At this time, the reference value of the current in line 1 is 0. It can be seen that after a transient process of 0.13 s, the DC circuit breaker achieves tracking of the current reference value, and the current in each line is controlled to I1 = 0 kA, I2 = 3.00 kA, and I3 = 2.33 kA.

[0073] At t = 5 s, a fault occurs at the outlet of line 3. The DC circuit breaker switches to DC interruption mode, and the power flow capacitor C is bypassed by the main circuit breaker MB, causing the power flow capacitor voltage to drop rapidly to 0. When 5 s < t < 6 s, no power flow control is applied to the system. After the fault in line 3 is completely cleared, I1 = 2.60 kA and I2 = 2.72 kA. At t = 6 s, the fourth set of current reference values ​​is provided to the four-port DC circuit breaker with power flow control capability. At this time, the DC circuit breaker switches back to power flow control mode, and the power flow capacitor voltage is rapidly charged to the rated value of 40 kV. It can be seen that after clearing the DC fault, the DC circuit breaker of this invention can still effectively control the power flow of the remaining healthy line currents, controlling the currents of the remaining two lines to I1 = 2.00 kA and I2 = 3.32 kA.

[0074] Figure 3 The simulation results of local waveforms for the currents of the three lines and the main circuit breaker are presented respectively. Figure 4 The simulation results of local waveforms for the current and voltage of the metal oxide surge arrester are given. After the fault occurs in line 3 at t = 5 s, the current in each line rises rapidly. After a fault detection delay of 1 ms, the system detects the fault and sends a conduction signal to the main circuit breaker. The main circuit breaker and the first thyristor conduct 0.25 ms later. At this time, a disconnection command is sent to the first current transfer switch of the faulty line (i.e., line 3) and the second current transfer switch of the normal lines (i.e., the remaining lines). If the mode switching switch adopts the mode switching switch 2 structure, a disconnection signal also needs to be sent to the third current transfer switch at this time.

[0075] After 0.25 ms, the aforementioned current transfer switch opens, and the current flowing through it rapidly decreases to 0. At this time, a disconnection signal is sent to the first mechanical switch of the faulty line (i.e., line 3), the second mechanical switch of the normal line (i.e., the remaining lines), and the third mechanical switch for mode switching. After 2 ms, the aforementioned mechanical switch opens, the current transfer switch connected in series with it is no longer under voltage, and the power flow capacitor is completely deactivated.

[0076] At this point, a disconnect signal is sent to the first thyristor, and a conduction signal is sent to the second and third thyristors. The fault current is quickly transferred to the branch containing the current-limiting capacitor, and as the current-limiting capacitor gradually charges to the system voltage, it is gradually transferred to the branch containing the current-limiting inductor. When the fault current has completely transferred to the branch containing the current-limiting inductor, i.e., t = 5.0159s, a disconnect signal is sent to the main circuit breaker. After 0.25 ms, the main circuit breaker opens, the metal oxide surge arrester engages, and the fault current rapidly decreases to 0.

[0077] In summary, the multi-port DC circuit breaker of the present invention, which integrates fault current limiting, DC circuit breaking and power flow control functions of multiple ports by utilizing topological similarity, reduces the number of required components and thus significantly reduces engineering costs.

[0078] The power flow control method of the multi-port DC circuit breaker with current limiting and power flow control of the present invention can directly generate the corresponding line switching signal by detecting and controlling the power flow capacitor voltage and the current of each branch separately through a microcontroller. This solves the problem that the traditional control method needs to exhaustively enumerate an exponential number of switching states, and therefore cannot be adapted to multi-port power flow controllers.

[0079] The fault current limiting and DC fault handling method of the multi-port DC circuit breaker with current limiting and power flow control of the present invention can realize rapid switching from power flow control mode to DC circuit breaking mode, and complete rapid fault current limiting and DC circuit breaking, thus meeting the speed requirements of DC grid fault clearing.

[0080] The above description is a detailed description of the preferred embodiments of the present invention. However, the embodiments are not intended to limit the scope of the patent application of the present invention. All equivalent changes or modifications made under the technical spirit disclosed in the present invention should fall within the patent scope covered by the present invention.

Claims

1. A multi-port DC circuit breaker with current limiting and power flow control, characterized in that, It includes multiple parallel lines, main circuit breaker, surge arrester, mode switching switch, current limiting inductor, power flow capacitor, current limiting capacitor, first thyristor, second thyristor and third thyristor. Each line includes a first bridge arm, a second bridge arm, a disconnecting switch and a reactor. The first bridge arm includes a first current transfer switch and a first mechanical switch. The second bridge arm includes a second current transfer switch and a second mechanical switch. The first terminal of the first current transfer switch is connected to the first DC bus, the second terminal of the first current transfer switch is connected to the first terminal of the first mechanical switch, the second terminal of the first mechanical switch is connected to the first terminal of the second current transfer switch, the second terminal of the second current transfer switch is connected to the first terminal of the second mechanical switch, and the second terminal of the second mechanical switch is connected to the second DC bus. After the disconnecting switch and the reactor are connected in series, the second terminal of the reactor is connected to the second terminal of the first mechanical switch and the first terminal of the second current transfer switch, respectively. The main circuit breaker and surge arrester are connected in parallel and then connected in series with the first thyristor. The second thyristor and the current-limiting capacitor are connected in series and then connected in parallel with the first thyristor. The third thyristor and the current-limiting inductor are connected in series and then connected in parallel with the first thyristor. The mode switching switch and the power flow capacitor are connected in series and then connected in parallel with multiple lines. The first terminal of the mode switching switch is connected to the first DC bus, and the second terminal of the power flow capacitor is connected to the second DC bus. The first terminal of the main circuit breaker and the first terminal of the surge arrester are respectively connected to the first DC bus. The second terminal of the main circuit breaker and the second terminal of the surge arrester are respectively connected to the anodes of the first thyristor, the second thyristor and the third thyristor. The first terminal of the current-limiting capacitor is connected in series with the cathode of the second thyristor. The first terminal of the current-limiting inductor is connected in series with the cathode of the third thyristor. The cathode of the first thyristor, the second terminal of the current-limiting capacitor and the second terminal of the current-limiting inductor are respectively connected to the second DC bus.

2. The multi-port DC circuit breaker with current limiting and power flow control according to claim 1, characterized in that, Both the current transfer switch and the main circuit breaker use multiple insulated gate bipolar transistors connected in series and parallel.

3. The multi-port DC circuit breaker with current limiting and power flow control according to claim 2, characterized in that, The mode switching switch has two structures: one is composed of a third mechanical switch; the other is composed of a third current transfer switch connected in series with a third mechanical switch.

4. The multi-port DC circuit breaker with current limiting and power flow control according to claim 3, characterized in that, The first, second, and third mechanical switches are all ultra-fast mechanical switches.

5. The multi-port DC circuit breaker with current limiting and power flow control according to claim 2, characterized in that, The surge arrester is a metal oxide surge arrester, and the number of parallel lines is m, where m is an integer greater than or equal to 3.

6. A power flow control method for a multi-port DC circuit breaker with current limiting and power flow control, characterized in that, The application of the multi-port DC circuit breaker with current limiting and power flow control as described in any one of claims 1 to 5 includes the following steps: S11: When the system containing the DC circuit breaker is operating normally, the DC circuit breaker is in power flow control mode, and the mode switching switch and all mechanical switches of the lines are in the closed state. S12: Collects the power flow capacitor voltage and the current of each line through an external voltage sampling circuit, and transmits the collected power flow capacitor voltage and the current of each line to the microcontroller. S13: The microcontroller calculates the difference between the power flow capacitor voltage and its reference value, and generates a pair of complementary PWM signals through modulation to control the first current transfer switch and the second current transfer switch connected to the MMC line, and to control the first current transfer switch and the second current transfer switch of the relaxation line. S14: The microcontroller calculates the difference between the current of the remaining lines and its reference value, and generates a pair of complementary PWM signals by modulation to control the first current transfer switch and the second current transfer switch of the remaining lines respectively.

7. A DC fault handling method for a multi-port DC circuit breaker with current limiting and power flow control, characterized in that, Applied to any one of claims 1 to 5, the DC fault handling method is used to achieve rapid mode switching, current limiting, and DC fault clearing.

8. The DC fault handling method for a multi-port DC circuit breaker with current limiting and power flow control according to claim 7, characterized in that, Includes the following steps: S21: After the system where the DC circuit breaker is located detects a line fault, it controls the main circuit breaker and the first thyristor to conduct. S22: After the main circuit breaker is fully turned on, the first current transfer switch controlling the normal line is turned on and the second current transfer switch is turned off, and the first current transfer switch controlling the faulty line is turned off and the second current transfer switch is turned on. S23: After the above current transfer switch is fully turned on or off, the second mechanical switch controlling the normal circuit, the first mechanical switch controlling the faulty circuit, and the third mechanical switch controlling the mode switching are turned off. S24: After the above mechanical switch is completely disconnected, the first thyristor is disconnected and the second and third thyristors are turned on. The negative voltage of the current limiting capacitor causes the current in the first thyristor to decrease to 0. The current limiting capacitor completes the initial current limiting. During the process of the current limiting capacitor being engaged, the negative voltage across the current limiting capacitor decreases and then the positive voltage increases to the system voltage. The current in the current limiting capacitor decreases to 0 and the current limiting inductor is fully engaged. S25: After the current-limiting inductor is fully engaged, the main circuit breaker is opened, allowing the fault current to be transferred to the surge arrester. The fault current is dissipated through the surge arrester and quickly reduced to 0. S26: After the line fault current drops to 0, the isolating switch controlling the faulty line is opened, completely clearing the line fault, and restoring normal power flow to the remaining normal lines.

9. The DC fault handling method for a multi-port DC circuit breaker with current limiting and power flow control according to claim 8, characterized in that, In step S22, if the mode switching switch is composed of a third current transfer switch and a third mechanical switch connected in series, the third current transfer switch for mode switching also needs to be disconnected.

10. The DC fault handling method for a multi-port DC circuit breaker with current limiting and power flow control according to claim 8, characterized in that, In step S22, if the mode switching switch is composed of only one third mechanical switch, the mode switching switch does not need to be operated.

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