A novel DC power distribution network hybrid current limiter and polarity self-recovery control method

Abstract

The application relates to the technical field of power distribution networks, and discloses a novel DC power distribution network hybrid current limiter and a polarity self-recovery control method, wherein the hybrid current limiter comprises: hybrid current limiters which are respectively arranged in series between current limiters and DC circuit breakers at two ends of a DC line, and each end hybrid current limiter comprises a current passing branch, a current limiting branch and a commutation branch; the current passing branch is used for providing a current passing path; the current limiting branch is used for limiting the increase of a fault current; and the commutation branch is used for performing current commutation of the current passing branch and the current limiting branch to block the passing of the fault current from the current passing branch; the application not only improves the fault current limiting capacity of the DC power distribution network, but also reduces the breaking stress of the DC circuit breaker and the investment cost of the DC power distribution network system, and realizes fast reclosing.

Description

Technical Field

[0001] This invention relates to the field of power distribution network technology, specifically to a novel hybrid current limiter for DC power distribution networks and a polarity self-recovery control method. Background Technology

[0002] In recent years, with the large-scale integration of distributed power sources and the rapid growth of DC loads, flexible DC distribution networks have experienced rapid development and widespread application due to their significant advantages in transmission capacity, control flexibility, and power quality. However, because DC distribution networks have low damping and low inertia physical characteristics, and lack the natural zero-crossing point found in AC power grids, once an inter-pole or single-pole ground fault occurs, the fault current will surge exponentially at an extremely high rate, reaching a huge steady-state peak within milliseconds. This not only seriously threatens the safe and stable operation of core equipment such as converter valves and cables, but also places extremely stringent requirements on the ultimate breaking capacity of DC circuit breakers responsible for fault isolation.

[0003] Currently, relying solely on increasing the breaking capacity of DC circuit breakers to cope with severe short-circuit faults not only faces significant technical bottlenecks but also incurs extremely high manufacturing costs. Therefore, connecting a DC fault current limiter in series with the DC circuit breaker to suppress the initial rise rate and peak value of the fault current has become the mainstream engineering consensus for improving the reliability of DC distribution network system protection. In existing technologies, while traditional inductive current limiters have a simple structure, their large inductance value generates a significant voltage drop and active power loss during normal operation of the DC distribution network system, severely degrading the dynamic regulation response characteristics of the DC grid; solid-state current limiters, on the other hand, suffer from high semiconductor static losses.

[0004] To overcome the aforementioned shortcomings, hybrid current limiters have been gradually proposed and applied. However, the existing hybrid DC current limiter topologies still have many limitations: on the one hand, during the transient process of fault current transfer and current-limiting inductor activation, the main switching devices often face extremely high turn-off stress and voltage spikes, which can easily lead to insulation breakdown and device damage; on the other hand, existing capacitor commutation technologies generally rely on complex external high-voltage charging equipment to maintain the capacitor state and lack an efficient integrated energy dissipation network, resulting in slow response to transient faults and fast reclosing, large equipment size, and high overall investment costs. Summary of the Invention

[0005] To address the aforementioned shortcomings in existing technologies, this invention provides a novel hybrid current limiter for DC distribution networks and a polarity self-recovery control method. This solves the problems of existing hybrid DC current limiters, such as high normal operating losses, impact on the dynamic response of DC distribution network systems, high main switch shutdown stress during faults, the need for external charging equipment for capacitor commutation, low integration of energy consumption networks, resulting in slow reclosing response, large size, and high cost.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A novel hybrid current limiter for DC distribution networks is disclosed. The hybrid current limiter is connected in series between the current-limiting reactor and the DC circuit breaker at both ends of the DC line. Each hybrid current limiter includes a current-carrying branch, a current-limiting branch, and a commutation branch. The commutation branch includes a first commutation unit, a second commutation unit, a third commutation unit, and a fourth commutation unit. A current-carrying branch is used to provide a current path; Current-limiting branches are used to limit the increase of fault current; The commutation branch is used to perform current commutation in the current-carrying branch and the current-limiting branch to block the flow of fault current from the current-carrying branch; The first commutation unit or the second commutation unit is used to perform circuit commutation of the current-carrying branch; The third or fourth commutation unit is used to perform current commutation of the current-limiting branch; When the DC distribution network system is operating normally, the current-limiting branch and the commutation branch are both in the off state, while the current-carrying branch is in the working state and normal current flows. When a single-pole ground fault occurs in a DC distribution network system, the fault current flows through the current-carrying branch briefly. When the fault current rises to 1.2 times the rated current, the first or second commutation unit is turned on, transferring the fault current in the current-carrying branch to the commutation branch. At the same time, the first or second commutation unit performs a discharge operation. After the discharge is completed, all the fault current in the first or second commutation unit flows into the current-limiting branch, and the current-limiting branch suppresses the growth of the fault current, thereby disconnecting the current-carrying branch. When the current-carrying branch is disconnected, the current in the current-limiting branch is transferred to the third phase unit or the fourth commutation unit, and zero-current turn-off is performed in the current-limiting branch. At the same time, the third commutation unit or the fourth commutation unit performs a discharge operation. After the third or fourth commutation unit completes its discharge, the fault current of the third or fourth commutation unit gradually decreases to zero, completing the turn-off operation of the DC circuit breaker and achieving isolation of the fault current.

[0007] Furthermore, the current-carrying branch includes an ultra-fast mechanical switch and a load transfer switch; the load transfer switch includes a first insulated-gate bipolar transistor valve group and a first diode valve group; the first insulated-gate bipolar transistor valve group includes a first insulated-gate bipolar transistor and a second insulated-gate bipolar transistor; the first diode valve group includes a first diode and a second diode; One end of the ultra-fast mechanical switch is the current input terminal, and the other end of the ultra-fast mechanical switch is connected to the collector of the first insulated gate bipolar transistor. The emitter of the first insulated-gate bipolar transistor is connected to the emitter of the second insulated-gate bipolar transistor; the collector of the first insulated-gate bipolar transistor is also connected to the cathode of the first diode, and the emitter of the first insulated-gate bipolar transistor is also connected to the anode of the first diode; the emitter of the second insulated-gate bipolar transistor is also connected to the anode of the second diode, and the collector of the second insulated-gate bipolar transistor is also connected to the cathode of the second diode, the cathode of the second diode being the current output terminal.

[0008] Furthermore, the current-limiting branch includes a second insulated-gate bipolar transistor valve group, a second diode valve group, and an inductor; the second insulated-gate bipolar transistor valve group includes a third insulated-gate bipolar transistor and a fourth insulated-gate bipolar transistor; the second diode valve group includes a third diode and a fourth diode; The emitter of the third insulated-gate bipolar transistor is connected to one end of an ultra-fast mechanical switch; The emitter of the third insulated-gate bipolar transistor is also connected to the collector of the fourth insulated-gate bipolar transistor. The collector of the third insulated-gate bipolar transistor is connected to the emitter of the fourth insulated-gate bipolar transistor. The collector of the third insulated-gate bipolar transistor is also connected to one end of an inductor, and the other end of the inductor is connected to the cathode of the second diode. The emitter of the third insulated gate bipolar transistor is also connected to the anode of the third diode, and the collector of the third insulated gate bipolar transistor is also connected to the cathode of the third diode. The collector of the fourth insulated-gate bipolar transistor is also connected to the cathode of the fourth diode, and the emitter of the fourth insulated-gate bipolar transistor is also connected to the anode of the fourth diode.

[0009] Furthermore, the first commutation unit includes a first resistor, a first thyristor, a third thyristor, a seventh thyristor, and a first capacitor; The cathode of the seventh thyristor is connected to one end of the ultra-fast mechanical switch; The anode of the seventh thyristor is connected to one end of the first capacitor; the anode of the seventh thyristor is also connected to one end of the first resistor, and the cathode of the seventh thyristor is also connected to the other end of the first resistor. The anode of the first thyristor is connected to the cathode of the third thyristor, the cathode of the first thyristor is connected to the anode of the third thyristor, and the anode of the third thyristor is also connected to the cathode of the second diode; the anode of the first thyristor is also connected to the other end of the first capacitor.

[0010] Furthermore, the second commutation unit includes a second resistor, a fifth thyristor, a sixth thyristor, an eighth thyristor, and a second capacitor; The anode of the eighth thyristor is connected to one end of the ultra-fast mechanical switch; The cathode of the eighth thyristor is connected to one end of the second capacitor; the cathode of the eighth thyristor is also connected to one end of the second resistor, and the anode of the eighth thyristor is also connected to the other end of the second resistor. The anode of the fifth thyristor is connected to the cathode of the sixth thyristor, the cathode of the fifth thyristor is connected to the anode of the sixth thyristor, and the anode of the sixth thyristor is also connected to the cathode of the second diode; the anode of the fifth thyristor is also connected to the other end of the second capacitor.

[0011] Furthermore, the third commutation unit includes a third resistor, a second thyristor, and a third capacitor; One end of the third resistor is connected to one end of the ultra-fast mechanical switch; The other end of the third resistor is connected to one end of the third capacitor, the other end of the third capacitor is connected to the anode of the second thyristor, and the cathode of the second thyristor is connected to one end of the inductor.

[0012] Furthermore, the fourth commutation unit includes a fourth resistor, a fourth thyristor, and a fourth capacitor; One end of the fourth resistor is connected to one end of the third resistor; The other end of the fourth resistor is connected to one end of the fourth capacitor, the other end of the fourth capacitor is connected to the cathode of the fourth thyristor, and the anode of the fourth thyristor is connected to the cathode of the second thyristor.

[0013] A novel polarity self-recovery control method for a hybrid current limiter in a DC distribution network includes the following steps: After the hybrid current limiter completes fault isolation for the DC interrupter and the total current inside the hybrid current limiter decays to zero, the residual voltage of the first capacitor of the first commutation unit or the second capacitor of the second commutation unit is sampled in real time. When the residual voltage polarity of the first capacitor or the second capacitor is opposite to its initial pre-charge polarity, the control DC distribution network system sends a trigger command to the third and seventh thyristors of the first commutation unit or the fifth and eighth thyristors of the second commutation unit to close the inductor of the current limiting branch and form a local inductor-capacitor underdamped resonant discharge circuit. Simultaneously, the first or second capacitor is reverse-discharged and recharged in the forward direction. After half a resonant cycle, the resonant discharge circuit current naturally crosses zero, and the third and seventh thyristors of the first commutation unit or the fifth and eighth thyristors of the second commutation unit are reliably turned off under the action of reverse voltage, so that the polarity of the first or second capacitor in the commutation branch is successfully reversed and restored to the pre-charge polarity, and finally the standby reset of the hybrid current limiter is completed.

[0014] The present invention has the following beneficial effects: 1. The novel hybrid current limiter and polarity self-recovery control method for DC distribution networks proposed in this invention improve the fault current limiting capability of DC distribution networks, reduce the breaking stress of DC circuit breakers and the investment cost of DC distribution network systems, and realize rapid reclosing; 2. The hybrid current limiter proposed in this invention utilizes the pre-charged capacitor of the commutation branch to accurately discharge and actively create an artificial zero crossing point, enabling the second insulated gate bipolar transistor valve group of the current limiting branch to exit operation under soft turn-off conditions with low current and low voltage, completely avoiding the current concentration impact caused by hard turn-off, thereby allowing the use of semiconductor devices with smaller withstand voltage and current capacity and lower cost, reducing the investment cost of power electronic components in the current limiter; 3. The hybrid current limiter proposed in this invention, for DC distribution network systems, achieves not only extremely small size and low cost constraints through active and rapid current limiting of a fully controlled insulated gate bipolar transistor, but also millisecond-level rapid fault clearing. 4. The hybrid current limiter proposed in this invention adopts an anti-parallel semiconductor valve group structure in both the current limiting branch and the commutation branch, and employs a pair of reverse capacitors, enabling the hybrid current limiter to have the ability to handle bidirectional power flow and bidirectional faults in the DC distribution network system equally. 5. The method proposed in this invention solves the problem in the prior art where the polarity reversal after capacitor discharge causes the current limiter to lose its continuous protection capability. It can passively achieve a complete reversal of the capacitor voltage polarity and efficient recovery of residual energy within a very short half-resonance cycle, so that the hybrid current limiter can quickly return to the initial pre-charge standby state, meeting the requirements of flexible DC distribution networks for rapid reclosing of instantaneous faults and continuous protection of secondary faults. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of a novel hybrid current limiter for DC distribution networks proposed in this invention; Figure 2 This is a schematic diagram showing the installation location of a novel hybrid current limiter for DC distribution networks proposed in this invention in a DC distribution network. Figure 3This is a schematic flowchart of a novel polarity self-recovery control method for a hybrid current limiter in a DC distribution network proposed in this invention. Figure 4 The equivalent circuit diagram of the first modular multilevel converter MMC1 in operation stage 1 when the hybrid current limiter proposed in this invention is connected to a DC distribution network system. Figure 5 The equivalent circuit diagram of the first modular multilevel converter MMC1 in operation stage 2 when the hybrid current limiter proposed in this invention is connected to a DC distribution network system. Figure 6 The equivalent circuit diagram of the first modular multilevel converter MMC1 in operation stage 3 when the hybrid current limiter proposed in this invention is connected to the DC distribution network system. Figure 7 The equivalent circuit diagram of the first modular multilevel converter MMC1 in operation stage 4 when the hybrid current limiter proposed in this invention is connected to the DC distribution network system. Figure 8 The equivalent circuit diagram of the first modular multilevel converter MMC1 in operation stage 5 when the hybrid current limiter proposed in this invention is connected to the DC distribution network system. Figure 9 This is a schematic diagram of the equivalent circuit of the polarity self-recovery control method of the hybrid current limiter proposed in this embodiment. Detailed Implementation

[0016] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0017] The specific embodiments of this invention are as follows: like Figures 1-2 As shown, a novel hybrid current limiter for DC distribution networks is disclosed. The hybrid current limiter is connected in series between the current-limiting reactor and the DC circuit breaker at both ends of the DC line. Each hybrid current limiter includes a current-carrying branch, a current-limiting branch, and a commutation branch. The commutation branch includes a first commutation unit, a second commutation unit, a third commutation unit, and a fourth commutation unit. The current-carrying branch provides a current path. The current-limiting branch limits the increase of fault current. The commutation branch performs current commutation in the current-carrying branch and the current-limiting branch to block the flow of fault current from the current-carrying branch.

[0018] The first or second commutation unit is used to perform circuit commutation of the current-carrying branch.

[0019] The third or fourth commutation unit is used to perform current commutation of the current-limiting branch, that is, to perform current commutation between the third and fourth insulated-gate bipolar transistors in the current-limiting branch.

[0020] When the DC distribution network system is operating normally, the current-limiting branch and the commutation branch are both in the off state, while the current-carrying branch is in the working state and allows normal current to flow.

[0021] When a single-pole ground fault occurs in a DC distribution network system, the fault current flows briefly through the current-carrying branch. When the fault current rises to 1.2 times the rated current, the first or second commutation unit is turned on, transferring the fault current in the current-carrying branch to the commutation branch. At the same time, the first or second commutation unit performs a discharge operation (discharge is performed by the first capacitor of the first commutation unit or the second capacitor of the second commutation unit; the first commutation unit operates when the fault occurs on the right side of the hybrid current limiter, and the second commutation unit operates when the fault occurs on the left side of the hybrid current limiter). After the discharge is completed, all the fault current in the first or second commutation unit flows into the current-limiting branch, and the current-limiting branch suppresses the increase of the fault current. After the current-limiting branch is turned on for a period of time, the current-carrying branch is disconnected (i.e., the ultra-fast mechanical switch of the current-carrying branch is disconnected).

[0022] When the current-carrying branch is disconnected, the current in the current-limiting branch is transferred to the third phase unit or the fourth commutation unit, and zero-current turn-off is performed in the current-limiting branch (that is, when the current flowing through the third and fourth insulated-gate bipolar transistors in the current-limiting branch is zero, zero-current turn-off is performed using the insulated-gate bipolar transistors). At the same time, the third commutation unit or the fourth commutation unit performs a discharge operation (discharge is performed by the third capacitor of the third commutation unit or the fourth capacitor of the fourth commutation unit, and the third commutation unit operates when the fault occurs on the right side of the hybrid current limiter, and the fourth commutation unit operates when the fault occurs on the left side of the hybrid current limiter).

[0023] After the third or fourth commutation unit completes its discharge, the fault current of the third or fourth commutation unit gradually decreases to zero, completing the turn-off operation of the DC circuit breaker and achieving isolation of the fault current.

[0024] In this step, Figure 1 The topology of the hybrid current limiter proposed in this invention is shown, including a DC branch, a current-limiting branch, and a commutation branch, used for current fault isolation in a faulty DC distribution network. This hybrid current limiter also possesses polarity self-recovery capability. Both the current-limiting branch and the commutation branch employ an anti-parallel semiconductor valve group structure (i.e.,... Figure 1 The first insulated-gate bipolar transistor and the first diode are connected in anti-parallel. S1. Anti-parallel second insulated-gate bipolar transistor and second diode S 2. The first and third thyristors are connected in anti-parallel, the sixth and fifth thyristors are connected in anti-parallel, and the second and fourth thyristors are connected in anti-parallel. Two pairs of reverse capacitors (the first capacitor and the third capacitor, and the second capacitor and the fourth capacitor) are also used, so that the hybrid current limiter has the ability to handle bidirectional power flow and bidirectional faults in the DC distribution network system.

[0025] The installation location of this hybrid current limiter in the DC distribution network is as follows: Figure 2 As shown, it is connected in series at both ends of the DC line and located between the current-limiting reactor and the DC circuit breaker; where Figure 2 AC power is used in the circuit; MMC1 and MMC2 are the first and second modular multilevel converters, respectively; FCL is a hybrid current limiter; FCL1 and FCL2 are the first and second hybrid current limiters, respectively. Figure 2 In the circuit, when a fault occurs on the line, that is, when the right side of the first hybrid current limiter FCL1 and the left side of the second hybrid current limiter FCL2 are faulted, the first and third commutation units of the first hybrid current limiter FCL1 are activated, and the second and fourth commutation units of the second hybrid current limiter FCL2 are activated, thereby achieving fault current isolation of the DC circuit breaker.

[0026] based on Figure 2 Regarding the installation location, the working principle of the hybrid current limiter proposed in this invention is as follows: First, the current in the current-carrying branch is transferred using the current-limiting branch and the commutation branch, giving the fast mechanical switch in the current-carrying branch time to turn off. Then, the pre-charged capacitor in the commutation branch (i.e., a capacitor containing voltage can be used for installation to meet the pre-charging requirements) is used to transfer the current between the third and fourth insulated-gate bipolar transistors in the current-limiting branch. This allows the second insulated-gate bipolar transistor valve group (the third and fourth insulated-gate bipolar transistors) in the current-limiting branch to exit operation under soft-turn-off conditions with low current and low voltage, completely avoiding the current concentration impact caused by hard turn-off. This allows the use of insulated-gate bipolar transistors and diodes with smaller withstand voltage and current capacity, and lower cost. At the same time, for DC distribution network systems, the active and rapid current limiting of the fully controlled insulated-gate bipolar transistor not only achieves the constraints of small size and low cost, but also achieves millisecond-level rapid fault clearing.

[0027] Specifically, the current-carrying branch includes an ultra-fast mechanical switch (UFD) and a load transfer switch (LCS); the load transfer switch includes a first insulated-gate bipolar transistor (IGBT) valve group and a first diode valve group; the first IGBT valve group includes a first IGBT transistor and a second IGBT transistor; the first diode valve group includes a first diode and a second diode.

[0028] One end of the ultra-fast mechanical switch is the current input terminal, and the other end of the ultra-fast mechanical switch is connected to the collector of the first insulated gate bipolar transistor.

[0029] The emitter of the first insulated-gate bipolar transistor is connected to the emitter of the second insulated-gate bipolar transistor; the collector of the first insulated-gate bipolar transistor is also connected to the cathode of the first diode, and the emitter of the first insulated-gate bipolar transistor is also connected to the anode of the first diode; the emitter of the second insulated-gate bipolar transistor is also connected to the anode of the second diode, and the collector of the second insulated-gate bipolar transistor is also connected to the cathode of the second diode, the cathode of the second diode being the current output terminal.

[0030] Specifically, the current-limiting branch includes a second insulated-gate bipolar transistor valve group, a second diode valve group, and an inductor. The second insulated-gate bipolar transistor valve group includes a third insulated-gate bipolar transistor and a fourth insulated-gate bipolar transistor; the second diode valve group includes a third diode and a fourth diode.

[0031] The emitter of the third insulated-gate bipolar transistor is connected to one end of an ultra-fast mechanical switch.

[0032] The emitter of the third insulated-gate bipolar transistor is also connected to the collector of the fourth insulated-gate bipolar transistor. The collector of the third insulated-gate bipolar transistor is connected to the emitter of the fourth insulated-gate bipolar transistor. The collector of the third insulated-gate bipolar transistor is also connected to one end of an inductor, and the other end of the inductor is connected to the cathode of the second diode.

[0033] The emitter of the third insulated-gate bipolar transistor is also connected to the anode of the third diode, and the collector of the third insulated-gate bipolar transistor is also connected to the cathode of the third diode.

[0034] The collector of the fourth insulated-gate bipolar transistor is also connected to the cathode of the fourth diode, and the emitter of the fourth insulated-gate bipolar transistor is also connected to the anode of the fourth diode.

[0035] Specifically, the first commutation unit includes a first resistor. First thyristor Third thyristor 7th thyristor First capacitor .

[0036] The cathode of the seventh thyristor is connected to one end of the ultra-fast mechanical switch.

[0037] The anode of the seventh thyristor is connected to one end of the first capacitor; the anode of the seventh thyristor is also connected to one end of the first resistor, and the cathode of the seventh thyristor is also connected to the other end of the first resistor.

[0038] The anode of the first thyristor is connected to the cathode of the third thyristor, the cathode of the first thyristor is connected to the anode of the third thyristor, and the anode of the third thyristor is also connected to the cathode of the second diode; the anode of the first thyristor is also connected to the other end of the first capacitor.

[0039] Specifically, the second commutation unit includes a second resistor. Fifth thyristor Sixth thyristor Eighth thyristor Second capacitor .

[0040] The anode of the eighth thyristor is connected to one end of the ultra-fast mechanical switch.

[0041] The cathode of the eighth thyristor is connected to one end of the second capacitor; the cathode of the eighth thyristor is also connected to one end of the second resistor, and the anode of the eighth thyristor is also connected to the other end of the second resistor.

[0042] The anode of the fifth thyristor is connected to the cathode of the sixth thyristor, the cathode of the fifth thyristor is connected to the anode of the sixth thyristor, and the anode of the sixth thyristor is also connected to the cathode of the second diode; the anode of the fifth thyristor is also connected to the other end of the second capacitor.

[0043] Specifically, the third commutation unit includes a third resistor. Second thyristor Third capacitor ; One end of the third resistor is connected to one end of the ultra-fast mechanical switch.

[0044] The other end of the third resistor is connected to one end of the third capacitor, the other end of the third capacitor is connected to the anode of the second thyristor, and the cathode of the second thyristor is connected to one end of the inductor.

[0045] Specifically, the fourth commutation unit includes a fourth resistor. Fourth thyristor Fourth capacitor ; One end of the fourth resistor is connected to one end of the third resistor.

[0046] The other end of the fourth resistor is connected to one end of the fourth capacitor, the other end of the fourth capacitor is connected to the cathode of the fourth thyristor, and the anode of the fourth thyristor is connected to the cathode of the second thyristor.

[0047] like Figure 3As shown, a novel polarity self-recovery control method for a hybrid current limiter in a DC distribution network is applied, comprising the following steps: Step 1: After the hybrid current limiter completes fault isolation for the DC interrupter and the total current inside the hybrid current limiter decays to zero, the residual voltage of the first capacitor of the first commutation unit or the second capacitor of the second commutation unit is sampled in real time.

[0048] This step involves the state detection and assessment of the DC distribution network system: after the hybrid current limiter completes fault isolation for the DC interrupter and the total current inside the hybrid current limiter decays to zero, the residual voltage of the first capacitor of the first commutation unit or the second capacitor of the second commutation unit in the commutation branch of the hybrid current limiter is sampled in real time. This allows for an accurate assessment of the current polarity reversal state and voltage deviation of the first or second capacitor, providing reliable closed-loop data support for the subsequent precise triggering and execution of the capacitor active recovery strategy.

[0049] Step 2: When the residual voltage polarity of the first capacitor or the second capacitor is opposite to its initial pre-charge polarity, the control DC distribution network system sends a trigger command to the third and seventh thyristors of the first commutation unit or the fifth and eighth thyristors of the second commutation unit to close the inductor of the current limiting branch, forming a local inductor-capacitor underdamped resonant discharge circuit. In this step, the resonant discharge circuit is reconfigured by determining whether the residual voltage polarity of the first capacitor of the first commutation unit or the second capacitor of the second commutation unit in the hybrid current limiter is opposite to its initial pre-charge polarity. If so, the DC distribution network system is controlled to send a trigger command to the third and seventh thyristors of the first commutation unit or the fifth and eighth thyristors of the second commutation unit to close the inductor of the current limiting branch, thus forming a local inductor-capacitor underdamped resonant discharge circuit. If not, no action is taken.

[0050] The purpose of the resonant discharge circuit reconfiguration is to utilize the residual energy within the capacitor after the commutation process, and to actively trigger the aforementioned devices to create a local underdamped LC resonant circuit between the capacitor and the current-limiting branch inductor. Taking advantage of the physical characteristic of capacitor voltage oscillation during resonance, the reverse residual voltage across the capacitor is flipped back to the initial pre-charge polarity direction within half a resonant cycle.

[0051] Step 3: Reverse discharge the first capacitor or the second capacitor and recharge it in the forward direction. After half a resonant cycle, the resonant discharge circuit current naturally crosses zero, and the third and seventh thyristors of the first commutation unit or the fifth and eighth thyristors of the second commutation unit are reliably turned off under the action of reverse voltage, so that the polarity of the first or second capacitor in the commutation branch is successfully reversed and restored to the pre-charge polarity, and finally the standby reset of the hybrid current limiter is completed.

[0052] This step involves passive polarity reversal and energy recovery, resonant self-turn-off, and state reset: First, under the action of the resonance mechanism, the first capacitor of the first commutation unit or the second capacitor of the second commutation unit is reverse-discharged and recharged in the forward direction, thereby forcibly reversing the terminal voltage of the capacitor to the initial standby polarity with extremely low energy loss and extremely short time delay. With the current naturally crossing zero and the thyristor automatically blocking, the resonance is perfectly interrupted, allowing the hybrid current limiter to quickly and completely return to the initial ready state, meeting the requirement of automatic reclosing in a very short time for the DC distribution network. Second, after half a resonance cycle of this charging process, the current in the current resonant discharge circuit naturally crosses zero, and the third and seventh thyristors or the fifth and eighth thyristors of the commutation branch are reliably turned off under the action of reverse voltage. At this time, the polarity of the first or second capacitor of the commutation branch is successfully reversed and restored to the required pre-charge polarity, thus completing the standby reset of the current limiter.

[0053] Therefore, the polarity self-recovery control method based on a hybrid current limiter proposed in this invention solves the problem in the prior art where polarity reversal after capacitor discharge leads to the loss of continuous protection capability of the current limiter. Moreover, this polarity self-recovery control method passively achieves complete reversal of capacitor voltage polarity and efficient recovery of residual energy within an extremely short half-resonance cycle, enabling the hybrid current limiter to quickly return to its initial pre-charge standby state, thus meeting the requirements of flexible DC distribution networks for rapid reclosing of instantaneous faults and continuous protection of secondary faults.

[0054] To verify the effectiveness of the novel hybrid current limiter for DC distribution networks proposed in this invention, the following experiment was conducted: The hybrid current limiter proposed in this invention is installed in, for example... Figure 2 In the DC distribution network system shown, when a line fault occurs, the right side of the first hybrid current limiter FCL1 and the left side of the second hybrid current limiter FCL2 also experience faults. Since the positions of the first hybrid current limiter FCL1 and the second hybrid current limiter FCL2 are symmetrically arranged, this embodiment only analyzes the working principle of the hybrid current limiter when the right side of the first hybrid current limiter FCL1 fails. For other scenarios, the working principle of the hybrid current limiter will be analyzed. Figure 2 The fault shown occurs on the left side of the second hybrid current limiter FCL2. The operating sequence of the current limiter is similar to that on the right side, the only difference being that when a fault occurs, the line current first drops to zero and then increases in the reverse direction; when the reverse current reaches 1.2 times the rated current, the system sends a shutdown signal to the LCS and to... S 2 and Provides a conduction signal. The triggered commutation branch is... and The subsequent processing steps are exactly the same for the branch in question, the only difference being that the corresponding symmetrical component will be activated to handle the reverse polarity. Therefore, the analysis process for a fault on the right side of the first hybrid current limiter FCL1 is as follows: The current-limiting capability of the first hybrid current limiter proposed in this invention is manifested through the introduction of a two-stage negative polarity capacitor pre-charging strategy. Firstly, the current in the current-carrying branch is transferred using the current-limiting branch and the commutation branch, providing turn-off time for the fast mechanical switch in the current-carrying branch. Secondly, the pre-charged capacitor in the commutation branch enables current transfer in the insulated-gate bipolar transistor of the current-limiting branch, forcing the main switching device in the current-limiting branch to achieve low-stress, low-current soft turn-off. Subsequently, as the capacitor is reverse-charged to the clamping voltage of the DC distribution network system, the commutation branch also enters a high blocking state, thereby achieving complete interruption of large DC fault currents. This precise coordination of transient timing ultimately creates an ideal arc-free, zero-current breaking window for the subsequent DC circuit breaker (DCCB).

[0055] Due to the rapid development of faults in DC distribution networks, to prevent prolonged overcurrent in power electronic devices, the current limiter must be able to isolate the fault before the first modular multilevel converter (MMC1) is blocked due to overcurrent. Within 10ms after the fault occurred, the first modular multilevel converter (MMC1) did not block; therefore, the equivalent circuit of the first modular multilevel converter (MMC1) in operation phase 1 is as follows: Figure 4 As shown. Figure 4 middle, For smoothing reactors, , , The equivalent capacitance, equivalent resistance, and equivalent inductance of the simplified first modular multilevel converter MMC1 are as follows:

[0056]

[0057]

[0058] In the formula, , , , These are the capacitance, arm reactance, number of arm sub-modules, and equivalent resistance of the first modular multilevel converter MMC1, respectively.

[0059] In operating phase 1, under normal conditions, the ultra-fast mechanical switch (UFD) and the load transfer switch (LCS) are turned on, and the current-carrying branch is a low-resistance direct-flow circuit. At this time, the loop current value is the steady-state value:

[0060] In the formula, This refers to the current during operating phase 1. The equivalent resistance of the line; The equivalent resistance of the line load; This is the DC bus voltage.

[0061] like Figure 5 The diagram shows stage 2 of the operation, where a single-pole ground fault occurs in the DC distribution network system. In the initial stage of the fault, the current limiter has not yet activated. The loop expression at this time is:

[0062] In the formula, The equivalent inductance of the line; This refers to the current during operating phase 2. For time.

[0063] Solving the above equation, we get:

[0064]

[0065]

[0066] In the formula, It is an exponential function; This is the steady-state value of the short-circuit current; The time when the fault occurred; The time constant of the DC distribution network system in phase 2 of operation; This is the inductance value of the smoothing reactor.

[0067] like Figure 6 The diagram shows the third stage of operation, where the current-limiting branch and the commutation branch are put into operation. S 2 (the fourth insulated-gate bipolar transistor and the fourth diode connected in parallel) are turned on, limiting the current inductor. The first thyristor in the commutation branch is connected in series between nodes A and B. T When phase 1 is turned on, the first capacitor in the commutation branch begins to discharge.

[0068] Based on this, , The equations are as follows:

[0069]

[0070]

[0071]

[0072]

[0073] In the formula, The system's equivalent inductance; The system's equivalent resistance; This refers to the current during operating phase 3; This represents the voltage of the line from node A to node B. For the current flowing through the inductor The current; The first capacitor of the commutation branch The voltage across 1; The first capacitor flowing through the commutation branch The current of 1; 1 is the first capacitor of the commutation branch.

[0074] Take the state vector, Solving for the state-space equations yields:

[0075] In the formula; This is a transpose operation; The first time derivative of the state vector for working phase 4; For work phase 4 at time The state vector; For DC distribution network system matrix; This refers to the external excitation vector of the DC distribution network system. Solving for:

[0076] in, The end time of Phase 2. The steady-state solution has a value of ;in , .

[0077] like Figure 7 The diagram shows working stage 4, when the first capacitor... When the discharge is complete and then reverse-charged, the commutation current decays rapidly. When the first capacitor... The discharge process is complete, naturally reaching the zero-crossing point, causing the first thyristor to... The commutator branch is shut off and reliably blocked. At this point, the fault current is completely transferred to the current-limiting branch, i.e. The loop expression at this point is:

[0078] Solving for: .

[0079] in, This is the end time of work phase 3; This refers to the current during operating phase 4.

[0080] At this stage, the inductor This stage is used solely to limit fault current. Its duration is the critical mechanical delay required for the UFD to achieve full dielectric isolation.

[0081] like Figure 8 The diagram shows working phase 5, which involves safely interrupting the current-limiting branch and avoiding interference with the circuit. S 2. This generates destructive hard-switching stress in the second thyristor. Triggered, and a shutdown signal is sent simultaneously. S 2. To make the flow through S The current of 2 returns to zero. S 2. After shutdown, all fault currents will switch to Branch circuit. The fault circuit expression at this time is:

[0082]

[0083] In the formula, For capacitor C 3 voltage; Take the state vector, Solving for the state-space equations yields:

[0084] In the formula, This is a transpose operation; The first time derivative of the state vector in working phase 5; For work phase 5 at time The state vector; The matrix for the DC distribution network system in phase 5 of the work; This refers to the external excitation vector of the DC distribution network system in phase 5 of the operation. Solving for: , .

[0085] when When the decay reaches zero, the third capacitor The discharge process is complete, naturally reaching the zero-crossing point, causing the second thyristor to... The controller shuts off and reliably blocks the commutation branch. Once the current flowing through the DC circuit breaker (DCCB) drops to a preset low current threshold, the controller sends a trip command to the circuit breaker. Under this low current condition, the DC circuit breaker (DCCB) completes the interruption operation, thereby achieving reliable isolation between the faulty line on the right side of terminal B and the DC bus.

[0086] also, Figure 9The equivalent circuit of the polarity self-recovery control method for the hybrid current limiter proposed in this invention is shown. The polarity self-recovery control method is as follows: First, condition monitoring and assessment are performed; once the DC circuit breaker has completed fault isolation and the current inside the hybrid current limiter has completely decayed to zero, the DC distribution network system is controlled to sample the residual voltage of the first capacitor of the first commutation unit or the second capacitor of the second commutation unit in real time. .

[0087] Secondly, the resonant discharge circuit is reconfigured. If the polarity of the residual voltage is detected to be opposite to that of the initially pre-charged capacitor, the DC distribution network system actively triggers the third and seventh thyristors or the fifth and eighth thyristors within the commutation branch to issue trigger commands. At this time, the conducting commutation capacitor... or With current-limiting inductor Together they form a local underdamped LC resonant discharge circuit.

[0088] Then, a passive polarity reversal and energy recovery are performed. The start time of the reversal process is set as... The initial voltage condition of the resonant discharge circuit is and initial current According to Kirchhoff's voltage law, the governing equation for this resonant switching circuit can be expressed as:

[0089]

[0090] In the formula, This is the equivalent parasitic resistance of the resonant discharge circuit. Because this resistance is extremely small, the resonant discharge circuit operates in a typical underdamped state. Commutation capacitor or Solving this second-order ordinary differential equation yields the transient time-domain expression for the capacitor voltage:

[0091] In the formula, , These are the sine and cosine functions, respectively. The attenuation coefficient is, and ; Let be the resonant angular frequency of the circuit, and .

[0092] Then, resonant self-turn-off and state reset are performed; after half a resonant cycle, the resonant current naturally crosses zero, and the thyristor achieves reliable turn-off under the action of the reverse recovery voltage, marking the end of the polarity reversal process. The time required for the commutation capacitor to reverse polarity... and the final positive recovery voltage after flipping They can be calculated separately as follows: ; ; At this point, the capacitor in the commutation branch has successfully restored its positive polarity. By employing the aforementioned passive self-recovery strategy, the capacitor in the commutation branch can complete polarity reset and transient energy recovery in a very short time. The residual voltage after the flip can be seamlessly transferred to the standby state, directly providing sufficient pulse energy for the next forced commutation operation of the DC distribution network system.

[0093] Specific embodiments have been used to illustrate the principles and implementation methods of this invention. The descriptions of the embodiments above are only for the purpose of helping to understand the method and core ideas of this invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this invention. Therefore, the content of this specification should not be construed as a limitation of this invention.

[0094] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A novel hybrid current limiter for DC distribution networks, characterized in that, The hybrid current limiter is connected in series between the current limiting reactor and the DC circuit breaker at both ends of the DC line, and each hybrid current limiter includes a current-carrying branch, a current-limiting branch, and a commutation branch; the commutation branch includes a first commutation unit, a second commutation unit, a third commutation unit, and a fourth commutation unit. A current-carrying branch is used to provide a current path; Current-limiting branches are used to limit the increase of fault current; The commutation branch is used to perform current commutation in the current-carrying branch and the current-limiting branch to block the flow of fault current from the current-carrying branch; The first commutation unit or the second commutation unit is used to perform circuit commutation of the current-carrying branch; The third or fourth commutation unit is used to perform current commutation of the current-limiting branch; When the DC distribution network system is operating normally, the current-limiting branch and the commutation branch are both in the off state, while the current-carrying branch is in the working state and normal current flows. When a single-pole ground fault occurs in a DC distribution network system, the fault current flows through the current-carrying branch briefly. When the fault current rises to 1.2 times the rated current, the first or second commutation unit is turned on, transferring the fault current in the current-carrying branch to the commutation branch. At the same time, the first or second commutation unit performs a discharge operation. After the discharge is completed, all the fault current in the first or second commutation unit flows into the current-limiting branch, and the current-limiting branch suppresses the growth of the fault current, thereby disconnecting the current-carrying branch. When the current-carrying branch is disconnected, the current in the current-limiting branch is transferred to the third phase unit or the fourth commutation unit, and zero-current turn-off is performed in the current-limiting branch. At the same time, the third commutation unit or the fourth commutation unit performs a discharge operation. After the third or fourth commutation unit completes its discharge, the fault current of the third or fourth commutation unit gradually decreases to zero, completing the turn-off operation of the DC circuit breaker and achieving isolation of the fault current.

2. The novel hybrid current limiter for DC distribution networks according to claim 1, characterized in that, The current-carrying branch includes an ultra-fast mechanical switch and a load transfer switch; the load transfer switch includes a first insulated-gate bipolar transistor valve group and a first diode valve group; the first insulated-gate bipolar transistor valve group includes a first insulated-gate bipolar transistor and a second insulated-gate bipolar transistor; the first diode valve group includes a first diode and a second diode; One end of the ultra-fast mechanical switch is the current input terminal, and the other end of the ultra-fast mechanical switch is connected to the collector of the first insulated gate bipolar transistor. The emitter of the first insulated-gate bipolar transistor is connected to the emitter of the second insulated-gate bipolar transistor; the collector of the first insulated-gate bipolar transistor is also connected to the cathode of the first diode, and the emitter of the first insulated-gate bipolar transistor is also connected to the anode of the first diode; the emitter of the second insulated-gate bipolar transistor is also connected to the anode of the second diode, and the collector of the second insulated-gate bipolar transistor is also connected to the cathode of the second diode, the cathode of the second diode being the current output terminal.

3. The novel hybrid current limiter for DC distribution networks according to claim 2, characterized in that, The current-limiting branch includes a second insulated-gate bipolar transistor (IGBT) valve group, a second diode valve group, and an inductor; the second IGBT valve group includes a third IGBT and a fourth IGBT; the second diode valve group includes a third diode and a fourth diode. The emitter of the third insulated-gate bipolar transistor is connected to one end of an ultra-fast mechanical switch; The emitter of the third insulated-gate bipolar transistor is also connected to the collector of the fourth insulated-gate bipolar transistor. The collector of the third insulated-gate bipolar transistor is connected to the emitter of the fourth insulated-gate bipolar transistor. The collector of the third insulated-gate bipolar transistor is also connected to one end of an inductor, and the other end of the inductor is connected to the cathode of the second diode. The emitter of the third insulated gate bipolar transistor is also connected to the anode of the third diode, and the collector of the third insulated gate bipolar transistor is also connected to the cathode of the third diode. The collector of the fourth insulated-gate bipolar transistor is also connected to the cathode of the fourth diode, and the emitter of the fourth insulated-gate bipolar transistor is also connected to the anode of the fourth diode.

4. The novel hybrid current limiter for DC distribution networks according to claim 2, characterized in that, The first commutation unit includes a first resistor, a first thyristor, a third thyristor, a seventh thyristor, and a first capacitor; The cathode of the seventh thyristor is connected to one end of the ultra-fast mechanical switch; The anode of the seventh thyristor is connected to one end of the first capacitor; the anode of the seventh thyristor is also connected to one end of the first resistor, and the cathode of the seventh thyristor is also connected to the other end of the first resistor. The anode of the first thyristor is connected to the cathode of the third thyristor, the cathode of the first thyristor is connected to the anode of the third thyristor, and the anode of the third thyristor is also connected to the cathode of the second diode; the anode of the first thyristor is also connected to the other end of the first capacitor.

5. The novel hybrid current limiter for DC distribution networks according to claim 2, characterized in that, The second commutation unit includes a second resistor, a fifth thyristor, a sixth thyristor, an eighth thyristor, and a second capacitor; The anode of the eighth thyristor is connected to one end of the ultra-fast mechanical switch; The cathode of the eighth thyristor is connected to one end of the second capacitor; the cathode of the eighth thyristor is also connected to one end of the second resistor, and the anode of the eighth thyristor is also connected to the other end of the second resistor. The anode of the fifth thyristor is connected to the cathode of the sixth thyristor, the cathode of the fifth thyristor is connected to the anode of the sixth thyristor, and the anode of the sixth thyristor is also connected to the cathode of the second diode; the anode of the fifth thyristor is also connected to the other end of the second capacitor.

6. The novel hybrid current limiter for DC distribution networks according to claim 3, characterized in that, The third commutation unit includes a third resistor, a second thyristor, and a third capacitor; One end of the third resistor is connected to one end of the ultra-fast mechanical switch; The other end of the third resistor is connected to one end of the third capacitor, the other end of the third capacitor is connected to the anode of the second thyristor, and the cathode of the second thyristor is connected to one end of the inductor.

7. The novel hybrid current limiter for DC distribution networks according to claim 6, characterized in that, The fourth commutation unit includes a fourth resistor, a fourth thyristor, and a fourth capacitor; One end of the fourth resistor is connected to one end of the third resistor; The other end of the fourth resistor is connected to one end of the fourth capacitor, the other end of the fourth capacitor is connected to the cathode of the fourth thyristor, and the anode of the fourth thyristor is connected to the cathode of the second thyristor.

8. A novel polarity self-recovery control method for a hybrid current limiter in a DC distribution network, characterized in that, The method applied to any one of claims 1-7 includes the following steps: After the hybrid current limiter completes fault isolation for the DC interrupter and the total current inside the hybrid current limiter decays to zero, the residual voltage of the first capacitor of the first commutation unit or the second capacitor of the second commutation unit is sampled in real time. When the residual voltage polarity of the first capacitor or the second capacitor is opposite to its initial pre-charge polarity, the control DC distribution network system sends a trigger command to the third and seventh thyristors of the first commutation unit or the fifth and eighth thyristors of the second commutation unit to close the inductor of the current limiting branch and form a local inductor-capacitor underdamped resonant discharge circuit. Simultaneously, the first or second capacitor is reverse-discharged and recharged in the forward direction. After half a resonant cycle, the resonant discharge circuit current naturally crosses zero, and the third and seventh thyristors of the first commutation unit or the fifth and eighth thyristors of the second commutation unit are reliably turned off under the action of reverse voltage, so that the polarity of the first or second capacitor in the commutation branch is successfully reversed and restored to the pre-charge polarity, and finally the standby reset of the hybrid current limiter is completed.

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