A positive unloading type true bipolar flexible direct current converter and a cooperative control method

Abstract

The present application relates to a kind of active unloading type true bipolar flexible DC converter and collaborative control method, including converter valve, distributed active unloading branch, DC fast mechanical switch and fault current transfer branch;Distributed active unloading branch includes thyristor, inductance, energy dissipation resistance and thyristor, is closed in normal operation, first guide thyristor to form RLC loop unloading when fault, current reaches threshold and then turns on thyristor bypass inductance and drags tail energy dissipation;DC fast mechanical switch is disconnected after fault and half-bridge submodule is blocked;Fault current transfer branch is closed in normal operation, is closed after DC fast mechanical switch is disconnected, is limited by voltage through diode D current limiting, and fault current is dissipated by energy dissipation resistance.Compared with prior art, the present application has the advantages of quickly blocking DC fault, reducing dependence on external DC circuit breaker, no additional steady-state loss, guaranteeing system operation efficiency and the like.

Description

Technical Field

[0001] This invention relates to the fields of high voltage direct current and power electronic converter technology, and in particular to an active unloading type true bipolar flexible DC converter and a cooperative control method. Background Technology

[0002] Circuitry systems for AC trunk lines or AC distribution networks (e.g., devices that transmit power between AC networks via high-voltage DC links) are key technologies for realizing high-capacity, long-distance power transmission and asynchronous AC grid interconnection. With the development of fully controllable power electronic devices, flexible DC converters are gradually becoming an advanced solution to replace traditional converters.

[0003] True bipolar flexible high-voltage direct current (HVDC) transmission systems typically employ a symmetrical structure with positive and negative poles, and a metallic return line or grounding line between the two poles to improve power supply reliability and power flow flexibility. Compared to unipolar or pseudo-bipolar systems, true bipolar structures have higher DC-side voltage levels and larger transmission capacities, but also exhibit more severe electrical transients during faults. When a pole-to-pole short circuit or a pole-to-metal return line short circuit occurs, the positive and negative poles form a strongly coupled short-circuit loop through a low-impedance metallic circuit. This results in a rapid rise and high peak value of the fault current, placing higher demands on the insulation and thermal stability of converter valves and DC equipment.

[0004] Currently, half-bridge submodules (MMCs) are widely used in engineering as the main topology for flexible DC converter valves. While half-bridge submodules are simple in structure, have low losses, and offer mature control, when a bipolar short circuit occurs on the DC side, the diodes within the submodule arms provide a passive conduction path for the fault current. The converter valve itself lacks DC fault self-blocking capability, allowing the fault current to continuously flow into the short-circuit point through the AC system and DC lines. In a true bipolar structure, due to the extremely low return path impedance, the rise rate and peak value of the fault current are significantly higher than in unipolar or pseudo-bipolar systems, further amplifying the inherent weaknesses of half-bridge MMCs.

[0005] To limit fault current and release excess energy within the system, existing technologies typically employ two types of measures: one is to install external breaking devices such as fast DC circuit breakers or mechanical switches on the DC line or bus side, protecting converter valves and DC equipment by disconnecting faulty branches in a very short time; the other is to add a centralized DC unloading device on the converter station side, dissipating DC bus energy centrally through resistors or other means during a fault. The former requires complex hybrid DC circuit breakers or high-current fast mechanical switches, resulting in high equipment costs and technical barriers; while the latter can reduce the breaking pressure of DC circuit breakers, the energy path from submodule capacitors and bridge arm inductors to the centralized unloading device is relatively long, and link inductance and parasitic parameters make it difficult to accurately control voltage and current transients, making it difficult to balance response speed and device stress under true double maximum current conditions.

[0006] In recent years, some scholars have introduced distributed energy-consuming units such as energy transfer branches and fault current transfer branches inside the converter valve to reduce reliance on external DC circuit breakers and centralized unloading devices. However, most of these improvements are designed based on the fault mechanisms of unipolar or pseudo-bipolar systems. Under the bipolar short-circuit condition of a true bipolar system, the positive and negative potentials are forcibly flattened by the metal loop, and the voltage across the bridge arm inductor approaches zero during the fault phase. The bridge arm inductor mainly releases energy through free oscillation, and its dynamic behavior differs significantly from that under the pseudo-bipolar assumption. If the topology and control strategy designed for pseudo-bipolar systems are still used, it will not only be difficult to fully utilize the characteristic of "natural zero terminal voltage of bridge arm inductor" in the true bipolar structure, but it may also lead to a mismatch between the unloading path and timing, making it impossible to effectively suppress fault current peaks and energy accumulation. Patent application CN113702870A discloses a fault location method for modular multilevel converters based on reconfigured unloading resistors, involving multilevel power electronic converter technology. This method splits the original DC-side unloading resistor of the submodule in two, and reconstructs the midpoint of the unloading resistor to connect to the AC-side output port of the submodule via a pair of anti-parallel optocouplers and a current-limiting resistor. However, it still has shortcomings such as limited functionality, disconnect from system fault protection requirements, lack of ability to cope with catastrophic short-circuit faults, and lack of system-level fault isolation capability.

[0007] Therefore, there is an urgent need for an integrated technical solution for bipolar short-circuit scenarios in true bipolar systems, capable of locally completing fault isolation and orderly unloading at the converter valve source side. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide an active unloading type true bipolar flexible DC converter and its collaborative control method suitable for true bipolar flexible DC transmission systems, which can quickly block DC faults, reduce dependence on external DC circuit breakers, have no additional steady-state losses, and ensure system operating efficiency.

[0009] The objective of this invention can be achieved through the following technical solutions: An active unloading type true bipolar flexible DC converter includes a converter valve, a distributed active unloading branch (AUL), a DC fast mechanical switch (UFD), and a fault current transfer branch (FCL). The converter valve is composed of multiple half-bridge sub-modules, and each half-bridge module has a distributed active unloading branch (AUL) connected in parallel across its capacitor. The distributed active unloading branch (AUL) includes a thyristor. ,inductance Energy-consuming resistor and thyristors thyristor With inductance Then connected in series with the energy-consuming resistor The thyristors are connected in series to form the main unloading channel. Parallel connection in inductor Both ends; AUL is off during normal operation, using conventional half-bridge MMC control logic; in case of a fault, the thyristor is turned on first. An RLC circuit is formed to unload the thyristor, and the thyristor is turned on after the current reaches the threshold. Bypass inductor The trailing effect dissipates energy. The DC fast mechanical switch UFD is connected in series between the converter valve and the DC line; it is closed during normal operation and disconnected after a fault occurs and the half-bridge submodule is locked. The fault current transfer branch is equipped with an FCL installed on the DC line side, including an anti-parallel diode D and a power dissipation resistor. During normal operation, the DC fast mechanical switch UFD is turned off and then closed. Current is limited by the voltage clamped by diode D, and fault current flows through the energy-dissipating resistor. Dissipation, combined with the distributed active unloading branch AUL and the natural dissipation of the bridge arm inductor, achieves zero fault current across the entire domain.

[0010] The present invention also provides a cooperative control method for the active unloading type true bipolar flexible DC converter as described above, comprising the following steps: A short-circuit fault is detected, and after a detection delay, all half-bridge sub-modules are bypassed, decoupling the capacitors of the half-bridge sub-modules from the main power circuit. After the capacitor of the half-bridge submodule is bypassed, when the fault current reaches the current protection threshold, all half-bridge submodules are locked out. After a blocking delay, the half-bridge submodule completes blocking, disconnects the DC fast mechanical switch UFD, and then turns on the distributed active unloading branch AUL and the fault current transfer branch FCL, wherein the thyristor is turned on first. Current flows through the energy-consuming resistor and inductor Energy is dissipated, and the thyristor is turned on when the current or voltage meets the switching conditions. Bypass inductor The fault current is processed by current tailing. The fault current decays exponentially through the fault current transfer branch FCL until the current in the entire domain returns to zero, thus completing the fault clearing and unloading of the half-bridge submodule.

[0011] Furthermore, the half-bridge submodule includes an IGBT device, and the latching half-bridge submodule is used to ensure that the IGBT device is within a safe operating range.

[0012] Furthermore, the energy-consuming resistor The parameters are selected to satisfy the overdamped or critically damped conditions, that is: in For submodule capacitors, Inductor The inductance value causes the unloading current to rise first and then decrease monotonically without ringing.

[0013] Furthermore, the thyristor The switching conditions are set based on a switching current threshold, which is the smaller value between the current safety margin and the energy percentage-related current value. Specifically, the switching current threshold is: in For current safety margin, As a percentage of energy, For submodule capacitors, Inductor inductance value, This represents the capacitor voltage of the submodule.

[0014] Furthermore, the inductor The parameters are selected to satisfy the minimum damping ratio. ,Right now in Inductor inductance value, For submodule capacitors, For unloading resistor, This is the minimum damping ratio.

[0015] Furthermore, the energy-consuming resistor The parameter selection satisfies the energy ratio constraint and the voltage and current rating constraint, i.e. in For the energy percentage target, For the instantaneous current before the UFD is disconnected, For allowable peak pressure, The equivalent resistance of the pole-to-pole circuit. This is the equivalent forward voltage drop of the diode.

[0016] Furthermore, during normal operation, the distributed active unloading branch AUL and the fault current transfer branch FCL are both in the off state, the DC fast mechanical switch is in the closed state UFD, and the main circuit modulation and circulating current control law of the converter valve is consistent with that of a conventional half-bridge modular multilevel converter.

[0017] Furthermore, after the DC fast mechanical switch UFD is disconnected, the fault current only flows briefly in the local RL loop formed by the AC side and the inductance of the converter valve bridge arm, and the energy of the bridge arm inductance is naturally dissipated through the valve internal damping and the equivalent impedance of the AC system.

[0018] Furthermore, after the fault current transfer branch FCL is activated, the inter-electrode voltage of the diode clamping branch is... ,in For energy-consuming resistors, This increases the equivalent forward voltage drop of the diode, raises the equivalent potential at the fault point, and limits the fault current.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. Rapidly interrupts DC faults and reduces reliance on external DC circuit breakers: This invention achieves rapid suppression of fault current and local energy dissipation by setting up distributed active unloading branches, DC fast mechanical switches, and segmented coordinated control of fault current transfer branches. It does not rely on the long-delay action of external DC circuit breakers, which greatly improves the safety of the true bipolar flexible DC transmission system in dealing with bipolar short-circuit faults.

[0020] 2. No additional steady-state losses, ensuring system operating efficiency: During normal operation, the distributed active unloading branch and the fault current transfer branch are both in the off state. The voltage drop of the DC fast mechanical switch is negligible. The modulation and circulating current control law of the main circuit of the converter valve is consistent with that of the conventional half-bridge MMC. The system efficiency is maintained at the level of the half-bridge sub-module, avoiding the introduction of additional losses due to the addition of branches. Attached Figure Description

[0021] Figure 1 This is a topology diagram of the FBAU-MMC of the present invention; Figure 2 This is the FBAU-MMC current loop diagram for the bypass stage of the present invention. In the diagram, gray lines indicate that the corresponding lines are in a non-conducting state, black lines indicate that the corresponding lines are in a conducting state, and red dashed lines with arrows indicate the direction of current. Figure 3 This is the current loop diagram of the FBAU-MMC active unloading branch inductor current limiting mode during the blocking phase of the present invention. In the diagram, the gray line indicates that the corresponding line is in a non-conducting state, the black line indicates that the corresponding line is in a conducting state, and the red dashed line with arrows indicates the current direction. Figure 4 This is a diagram of the true bipolar topology of the present invention; Figure 5 This is a simulation result of the DC current of the present invention. Figure 6 This is a simulation result diagram of the DC voltage of the present invention; Figure 7 This is a simulation result diagram of the active unloading branch current of the present invention; Figure 8 This is a simulation result diagram of the fault current transfer branch current of the present invention; Figure 9This is a simulation result diagram of the capacitor voltage of the upper bridge arm submodule of phase A of the present invention; Figure 10 This is a simulation result diagram of the bridge arm current of the upper bridge arm of the present invention; Figure 11 The figure shows the simulation results of the alternating current of this invention. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0023] Example 1 This embodiment provides an active unloading type true bipolar flexible DC converter (MMC), named FBAU-MMC, which includes a converter valve, a distributed active unloading loop (AUL), a DC fast mechanical switch (UFD), and a fault current transfer loop (FCL). Figure 1 As shown, the converter valve consists of multiple half-bridge submodules. Each half-bridge submodule has a distributed active unloading branch connected in parallel across its capacitor. The distributed active unloading branch includes a thyristor. ,inductance Energy-consuming resistor and thyristors thyristor With inductance Then connected in series with the energy-consuming resistor The thyristors are connected in series to form the main unloading channel. Parallel connection in inductor At both ends, a DC fast mechanical switch is connected in series between the converter valve and the DC line to disconnect the electrical connection between the valve side and the DC line after the submodule is locked, thus isolating the bipolar short-circuit loop topologically. The fault current transfer branch is located on the DC line side and includes an anti-parallel diode D and a power-dissipating resistor. It undertakes controlled discharge of DC fault current before and after the DC side is disconnected. When a bipolar short circuit occurs on the DC side, the control strategy sequentially completes fault detection, submodule bypass and interlocking, and DC fast mechanical switch disconnection. At a safe moment, it engages the distributed unloading branch and fault current transfer branch, so that the energy of the submodule capacitor is dissipated along the short path and the DC line current is smoothly attenuated in the transfer resistor.

[0024] When the system is operating normally, both AUL and FCL are off, UFD is closed and its voltage drop is negligible. Taking a single-pole converter valve as the research object, let the voltage between the DC bus and the metal return line of that pole be denoted as... (Inter-electrode voltage is) Its main circuit modulation and circulating current control laws are consistent with those of a conventional half-bridge MMC, and the voltages of the upper and lower bridge arms are... , The relationship between AC side voltage, DC voltage and circulating current satisfies (1) (2) Define the AC output current and bridge arm circulating current of this phase as follows: (3) (4) In the formula: , These are the upper and lower bridge arm currents, respectively. This refers to the phase current on the AC side of the MMC. This represents the circulating current component between the bridge arms. Assume the phase voltage at the MMC AC output port is: (5) In the formula: Modulation factor ( ), For the output angular frequency, The phase angle difference is given by equations (1) and (2), and neglecting the voltage drop across the bridge arm inductance when solving for the steady-state modulation relationship. The voltages of the upper and lower bridge arms are then obtained as follows: (6) (7) Under normal true bipolar operating conditions, the topology of this invention is consistent with conventional half-bridge MMC in terms of main circuit modulation and circulating current control, and only participates in coordinated control through AUL, FCL and UFD in the event of DC fault.

[0025] When a bipolar short circuit occurs on the DC side, the system operating state rapidly switches from the aforementioned steady-state modulation mode to the fault-blocking mode. This invention relies on an auxiliary branch composed of AUL, FCL, and UFD to form a phased, coordinated energy consumption and current transfer process, thereby achieving rapid DC fault blocking and orderly energy release on the converter valve source side.

[0026] Example 2 This embodiment provides a method for coordinated control of DC fault interruption and active unloading type MMC, including the following steps: 1) In A DC bipolar short circuit can occur at any time, such as Figure 2 As shown, the DC bus voltage is quickly clamped to near 0, and the submodule capacitor and AC power supply rapidly release energy to the fault point through a low-impedance path, causing the fault current to rise sharply in a short period of time.

[0027] 2) The detection delay, such as Figure 2 As shown, the system in All submodules are bypassed at all times to decouple the submodule capacitors from the main power circuit and suppress their continued power feeding to the fault point.

[0028] 3) After the submodule capacitor is bypassed, the AC power supply continues to feed current to the fault point. When the fault current reaches the current protection threshold, all submodules are locked to ensure that the device is within the Safe Operating Area (SOA) and to prevent the Insulated Gate Bipolar Transistor (IGBT) from breaking down.

[0029] 4) The locking delay, in When the time submodule latch-up is complete, the UFD is disconnected to prevent fault current backflow. Then, AUL and FCL are turned on, as shown below. Figure 3 As shown, AUL pilot circuit (current flows through) and Rapid energy dissipation is achieved, and conduction resumes only when the current or voltage meets the switching conditions. (bypass It performs rapid tailing processing, energy dissipation path.

[0030] After AUL is turned on, the submodule capacitor With series inductor Unloading resistor A closed RLC circuit is formed, where the inductor first limits the slope to suppress the inrush current, and then the resistor dissipates the capacitor energy, achieving rapid unloading. Ignoring the effects of branch parasitic capacitance and inductance, its mathematical model can be written as: (8) (9) In the formula: For AUL branch current, This represents the capacitor voltage of the submodule.

[0031] definition , , ,remember .have to: (10) In the formula: For submodule capacitors, To unload the inductor, For unloading resistor, For damping parameters and characteristic roots.

[0032] Pick When the critical damping is achieved, the unloading current first rises and then monotonically decreases without ringing. To limit the peak current in the first stage while also considering the unloading speed, a switching current threshold is set: (11) In the formula: For current safety margin, This represents the percentage of energy used.

[0033] when When the inductor is disconnected, the unloading branch enters the rapid tailing stage, and the unloading current at this time is: (12) In the formula: This refers to the time required to disconnect the inductor.

[0034] The AUL is approximated as a series RLC circuit consisting of a submodule capacitor, a load-relief resistor, and an inductor. To avoid underdamped oscillations, the target minimum damping ratio is chosen. ,have to: (13) After a bipolar short circuit occurs on the true bipolar DC side and the submodule bypass and interlocking are completed sequentially, the DC-side fast mechanical switch UFD is activated. The circuit is immediately disconnected, isolating the converter valve from the DC line and the metallic return topology. Thereafter, the fault current no longer closes through the DC line, and the residual current in the bridge arm inductor forms a local RL loop only between the AC side equivalent impedance and the bridge arm inductor. Ignoring the effects of short-term AC voltage fluctuations, this loop can be approximated as a first-order RL circuit, with the following dynamic equation: (14) In the formula: For bridge arm inductance, It is the sum of the series resistance of the bridge arm, the internal loss of the valve, and the equivalent damping of the AC system.

[0035] Therefore, the analytical solution for the bridge arm current is: (15) (16) The voltage across the bridge arm inductor is: (17) but and Synchronously decays to 0 according to an exponential law. In engineering design, based on... With respect to the allowable peak current of the bridge arm , Perform tuning and Verification ensures that the energy in the AC feed stage is dissipated naturally through the valve's internal damping and the AC system, eliminating the need for a dedicated energy transfer branch.

[0036] At this time, the DC line is cut off, and the fault current only flows briefly in the local RL loop formed by the AC side and the bridge arm inductor. The energy of the bridge arm inductor is naturally dissipated through the internal damping of the valve and the equivalent impedance of the AC system, and its terminal voltage decays to near zero within a few milliseconds.

[0037] 5) The system has undergone The energy decays, the submodule capacitor voltage decays to 0, and the short-circuit impact current returns to zero exponentially through the FCL branch.

[0038] exist After the UFD is disconnected and the FCL is engaged, the fault current is commutated to the energy-dissipating branch consisting of a diode and a resistor in a single phase. The inter-electrode voltage of the diode clamping branch is... This increases the equivalent potential at the fault point, limits the current, and... Energy is dissipated. After being connected, the FCL current satisfies: (18) In the formula: For FCL current, The equivalent inductance of the pole-to-pole circuit. The equivalent resistance of the pole-to-pole circuit. For energy-consuming resistors, This is the equivalent forward voltage drop of the diode.

[0039] The current in the fault transfer branch is: (19) In the formula: This represents the instantaneous value of the fault current when the UFD is disconnected. is the first-order time constant of the loop.

[0040] To quickly complete the attenuation of fault current, The parameter selection is subject to the following constraints: (20) In the formula: With an energy percentage target of [0.6, 0.9], this embodiment uses 0.75. For the instantaneous current before the UFD is disconnected, For the allowable peak pressure.

[0041] 6) Because the surplus energy of each branch is different, the time required for dissipation to be completed is also different. At this moment, the current across the entire domain returns to 0, at which point the fault is cleared and the submodule is unloaded.

[0042] like Figures 5-11 As shown, this embodiment was verified using the built PSCAD / EMTDC simulation platform, and the true bipolar topology used is as follows. Figure 4 As shown, considering the symmetry of the true bipolar MMC flexible DC transmission system, taking the rectifier side as an example, the detailed simulation parameters are shown in Table 1.

[0043] Table 1 exist When a permanent bipolar fault occurs in the system, the DC-side voltage is rapidly clamped to near zero potential, such as... Figure 6 As shown. After the fault occurs, the DC fault current rises rapidly. Detection and bypass of all submodules are completed 0.1ms after the fault. At this time, the fault current is approximately 5.9kA. Figure 5 As shown; after the submodule is bypassed, the AC power supply continues to discharge to the fault point, and the fault current continues to increase, surging to about 6.0kA approximately 0.13ms after the fault. At this time, all submodules are locked out, and after a 0.3ms lockout delay, the submodules are locked out, and the fault current reaches its peak value of approximately 6.44kA. At this point, the UFD is disconnected, and AUL and FCL are turned on.

[0044] In the collaborative energy consumption stage, such as Figure 7 As shown, the unloading current of AUL reaches its maximum value of 40kA 0.6ms after conduction, and the inductor of AUL is bypassed 3ms after the fault occurs. Then, a rapid tailing phase is performed. For example... Figure 8 As shown, the fault current transfer current of the FCL reaches its maximum value of 5.1kA approximately 0.12ms after conduction. Finally, the FCL decays to 0 70ms after the fault occurs, the FCL decays to 0 10ms after the fault occurs, and the global current decays to 0 70ms after the fault occurs.

[0045] Under DC pole-to-pole fault conditions, such as Figure 9 , Figure 10 and Figure 11 As shown, the submodule capacitor voltage, bridge arm current, and AC current of the upper arm of phase A are selected as observations to analyze the changes in internal quantities. Simulation results show that after the mechanical switch is quickly opened approximately 0.3 ms after the fault occurs, the bridge arm current rapidly decays to zero; after approximately 70 ms, the AUL current decays to zero, and the submodule capacitor voltage subsequently drops to near zero; the AC current rapidly decreases to zero and remains stable after the valve group is locked, without any circulating current amplification or power oscillation.

[0046] Simulation results show that under DC fault conditions, the FBAU-MMC can achieve controlled removal of the fault channel by sequentially completing fault detection, submodule blocking, DC-side current interruption, and coordinated energy dissipation. The peak fault current is effectively limited, and the clearing time remains on the order of milliseconds. The submodule capacitors dissipate energy through a distributed unloading circuit, avoiding the response lag caused by the long centralized DC unloading link. During normal operation, each controlled branch is in a shut-off or low-voltage-drop state, the UFD voltage drop is negligible, and the on-state and switching losses of the valve group are basically consistent with those of a conventional half-bridge MMC. No significant efficiency decrease caused by the addition of branches is observed.

[0047] This invention achieves rapid interruption and orderly energy release of DC faults through a short-link, phased collaborative energy dissipation mechanism, significantly reducing reliance on external DC circuit breakers. While possessing fault handling capabilities, it introduces almost no additional steady-state losses, maintaining system efficiency at the level of a half-bridge submodule. The selection and quantity of components are controlled within an acceptable range for engineering applications, which is beneficial for valve-side integration and thermal management. With clear timing and low control coupling, it helps improve the safety, reliability, and economy of converter valves under conditions such as pole-to-pole short circuits and single-pole grounding.

[0048] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. An active unloading type true bipolar flexible DC converter, characterized in that, The system includes a converter valve, a distributed active unloading branch (AUL), a DC fast mechanical switch (UFD), and a fault current transfer branch (FCL). The converter valve consists of multiple half-bridge sub-modules, with a distributed active unloading branch (AUL) connected in parallel across the capacitor of each half-bridge sub-module. The distributed active unloading branch (AUL) includes a thyristor. ,inductance Energy-consuming resistor and thyristors thyristor With inductance Then connected in series with the energy-consuming resistor The thyristors are connected in series to form the main unloading channel. Parallel connection in inductor Both ends; AUL is off during normal operation, using conventional half-bridge MMC control logic; in case of a fault, the thyristor is turned on first. An RLC circuit is formed to unload the thyristor, and the thyristor is turned on after the current reaches the threshold. Bypass inductor The trailing effect dissipates energy. The DC fast mechanical switch UFD is connected in series between the converter valve and the DC line; it is closed during normal operation and disconnected after a fault occurs and the half-bridge submodule is locked. The fault current transfer branch is equipped with an FCL installed on the DC line side, including an anti-parallel diode D and a power dissipation resistor. During normal operation, the DC fast mechanical switch UFD is turned off and then closed. Current is limited by the voltage clamped by diode D, and fault current flows through the energy-dissipating resistor. Dissipation, combined with the distributed active unloading branch AUL and the natural dissipation of the bridge arm inductor, achieves zero fault current across the entire domain.

2. A collaborative control method for an active unloading type true bipolar flexible DC converter as described in claim 1, characterized in that, Includes the following steps: A short-circuit fault is detected, and after a detection delay, all half-bridge sub-modules are bypassed, decoupling the capacitors of the half-bridge sub-modules from the main power circuit. After the capacitor of the half-bridge submodule is bypassed, when the fault current reaches the current protection threshold, all half-bridge submodules are locked out. After a blocking delay, the half-bridge submodule completes blocking, disconnects the DC fast mechanical switch UFD, and then turns on the distributed active unloading branch AUL and the fault current transfer branch FCL, wherein the thyristor is turned on first. Current flows through the energy-consuming resistor and inductor Energy is dissipated, and the thyristor is turned on when the current or voltage meets the switching conditions. Bypass inductor The fault current is processed by current tailing. The fault current decays exponentially through the fault current transfer branch FCL until the current in the entire domain returns to zero, thus completing the fault clearing and unloading of the half-bridge submodule.

3. The cooperative control method according to claim 2, characterized in that, The half-bridge submodule includes IGBT devices, and the latching half-bridge submodule is used to ensure that the IGBT devices are within a safe operating range.

4. The cooperative control method according to claim 2, characterized in that, The energy-consuming resistor The parameters are selected to satisfy the overdamped or critically damped conditions, that is: in For submodule capacitors, For inductance The inductance value causes the unloading current to rise first and then decrease monotonically without ringing.

5. The cooperative control method according to claim 2, characterized in that, The thyristor The switching conditions are set based on a switching current threshold, which is the smaller value between the current safety margin and the energy percentage-related current value. Specifically, the switching current threshold is: in For current safety margin, As a percentage of energy, For submodule capacitors, For inductance inductance value, This represents the capacitor voltage of the submodule.

6. The cooperative control method according to claim 2, characterized in that, The inductor The parameters are selected to satisfy the minimum damping ratio. ,Right now in For inductance inductance value, For submodule capacitors, For unloading resistor, This is the minimum damping ratio.

7. The cooperative control method according to claim 2, characterized in that, The energy-consuming resistor The parameter selection satisfies the energy ratio constraint and the voltage and current rating constraint, i.e. in For the energy percentage target, For the instantaneous current before the UFD is disconnected, For allowable peak pressure, The equivalent resistance of the pole-to-pole circuit. This is the equivalent forward voltage drop of the diode.

8. The cooperative control method according to claim 2, characterized in that, During normal operation, the distributed active unloading branch (AUL) and the fault current transfer branch (FCL) are both in the off state, the DC fast mechanical switch (UFD) is in the closed state, and the main circuit modulation and circulating current control law of the converter valve is consistent with that of a conventional half-bridge modular multilevel converter.

9. The cooperative control method according to claim 2, characterized in that, After the DC fast mechanical switch UFD is disconnected, the fault current only flows briefly in the local RL loop formed by the AC side and the inductance of the converter valve bridge arm. The energy of the bridge arm inductance is naturally dissipated through the valve internal damping and the equivalent impedance of the AC system.

10. The cooperative control method according to claim 2, characterized in that, After the fault current transfer branch FCL is activated, the inter-electrode voltage of the diode clamping branch is ,in For energy-consuming resistors, This is the equivalent forward voltage drop of the diode.

Images