Hybrid DC circuit breaker based on self-charging oscillation-assisted commutation

Abstract

This invention discloses a hybrid DC circuit breaker based on self-charging oscillation-assisted commutation. The main current-carrying branch carries current bidirectionally during normal operation and interrupts fault current in case of a fault, achieving arc-free disconnection of the mechanical switch. One end of the current injection branch is connected to the side of the power electronic device furthest from the mechanical switch, and the other end is connected to the side of the mechanical switch furthest from the power electronic device to generate an oscillating current that cancels out the fault current, generates zero-current-point assisted commutation of the mechanical switch, and establishes the trigger voltage for the energy-consuming branch. The current injection branch includes a series-connected anti-parallel thyristor combination and a capacitor C. p and inductor L p One end of the commutation branch is connected to the side of the power electronic device away from the mechanical switch, and the other end is connected to the side of the mechanical switch away from the power electronic device. This provides a path for fault current transfer and oscillation to reverse the polarity of the capacitor voltage. The commutation branch includes several power electronic module structural units SM connected in series, which have bidirectional conduction and turn-off capabilities.

Description

Technical Field

[0001] This invention relates to the fields of power transmission, distribution and consumption in medium and high voltage DC systems, and in particular to a hybrid DC circuit breaker based on self-charging oscillation-assisted converter. Background Technology

[0002] DC transmission systems are characterized by low line impedance, small time constant, rapid short-circuit current rise rate, and high short-circuit current peak value. DC circuit breakers with fast breaking and current limiting functions have become key equipment for the safe operation of DC transmission systems, playing a crucial role in their engineering applications. Considering factors such as breaking time, manufacturing cost, and conduction losses, the DC circuit breakers used in DC power grid projects are mainly mechanical DC circuit breakers and hybrid DC circuit breakers.

[0003] Mechanical DC circuit breakers offer advantages such as low conduction loss, high breaking capacity, and low cost. Their key feature is creating a breaking environment similar to AC zero-point for the mechanical switch. However, the arc generated during disconnection in mechanical DC circuit breakers can easily damage the contacts, and the post-arc insulation recovery of the TIV (Through-Insulation Transmission) must be considered, potentially leading to arc reignition. In situations requiring repeated interruptions due to numerous faults, the performance requirements for the mechanical switch are high, making long-term reliable use impossible. Hybrid DC circuit breakers combine the advantages of mechanical and solid-state DC circuit breakers. They use a mechanical switch to carry normal load current while a power electronic switch interrupts fault current, significantly reducing the conduction losses of the DC circuit breaker while maintaining breaking capacity and operating speed.

[0004] However, since the key to hybrid DC circuit breakers lies in relying on power electronic devices to carry and ultimately interrupt large currents, these devices will be subjected to significant transient stress during turn-off. When the fault current is large, it may even cause the power electronic devices to exceed their safe operating range, leading to turn-off failure. This places high demands on the reliability and durability of the power electronic devices.

[0005] The information disclosed in the background section is only intended to enhance the understanding of the background of the present invention, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0006] To address the shortcomings or defects of the existing technologies, this paper combines the key technologies of traditional mechanical DC circuit breakers and traditional hybrid DC circuit breakers: by creating an "artificial current zero-crossing point" breaking environment for the switch through a pre-charge oscillation method, and by forcing fault current commutation through zero-current turn-off of power electronic devices, a hybrid DC circuit breaker based on self-charging oscillation-assisted commutation is provided. This enables the mechanical switch to break without arc under "zero voltage and zero current" conditions, while simultaneously achieving zero-current turn-off of power electronic devices, thus reducing the transient stress during turn-off.

[0007] The objective of this invention is achieved through the following technical solutions.

[0008] A hybrid DC circuit breaker based on self-charging oscillation-assisted commutation includes,

[0009] The main current-carrying branch carries current bidirectionally during normal operation and cuts off fault current in case of a fault, thereby achieving arc-free disconnection of the mechanical switch. The main current-carrying branch includes a series-connected isolating circuit breaker, a current-limiting inductor, a bidirectional current-carrying load transfer switch, and a mechanical switch. The load transfer switch is composed of multiple power electronic devices, and the mechanical switch is a circuit breaker or isolating switch.

[0010] The current injection branch connects one end to the side of the power electronic device furthest from the mechanical switch, and the other end to the side of the mechanical switch furthest from the power electronic device, to generate an oscillating current that cancels out the fault current, to generate the current zero-point auxiliary commutation of the mechanical switch, and to establish the trigger voltage for the energy-consuming branch. The current injection branch includes a series-connected anti-parallel thyristor combination and a capacitor C. p and inductor L p The anti-parallel thyristor combination includes a pair of anti-parallel thyristors T1 and T2;

[0011] The energy-consuming branch has one end connected to the side of the power electronic device furthest from the mechanical switch, and the other end connected to capacitor C. p and inductor L p The short-circuit current is cut off and energy is dissipated;

[0012] The commutation branch connects one end to the side of the power electronic device away from the mechanical switch and the other end to the side of the mechanical switch away from the power electronic device. It provides a path for the transfer of fault current and oscillation to reverse the polarity of the capacitor voltage. The commutation branch includes several power electronic module structural units (SMs) connected in series, which have bidirectional conduction and turn-off capabilities. The power electronic module structural units (SMs) are implemented by various topologies of different power electronic devices.

[0013] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the current injection branch controls the direction of fault current inflow by controlling the conduction of anti-parallel thyristors T1 and T2 in different directions, thereby achieving control over capacitor C. p Bidirectional controllable charging.

[0014] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the mechanical switch includes a break consisting of one or more breaks connected in series and parallel.

[0015] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the break point includes a vacuum break point or a gas break point.

[0016] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the gas break includes N2, air, and H2 breaks.

[0017] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the energy-consuming branch includes a metal oxide variable resistor (MOV).

[0018] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, before the system operates normally, the mechanical switch is closed, and then one of the anti-parallel thyristor combinations in the current injection branch is triggered. Then, the isolation circuit breaker is closed, and the DC system charges the capacitor in the current injection branch. After reaching the target voltage, the power electronic devices in the main current-carrying branch are turned on, and the system current is transferred to the main current-carrying branch. The current through the current injection branch will gradually decay to 0, and the thyristor in the current injection branch will turn off naturally. At this time, the DC system starts to operate normally, and the capacitor pre-charging is completed.

[0019] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, after the capacitor is pre-charged, the conducting current is injected into the other anti-parallel thyristor combination in the branch, and the commutation branch and the main current-carrying branch form a loop. The capacitor and inductor begin to oscillate. After half a cycle of oscillation, the current is zero, and the other anti-parallel thyristor combination in the current-injected branch is naturally turned off. At this time, the polarity of the capacitor voltage reaches its maximum reverse, and the polarity of the capacitor voltage is reversed.

[0020] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, under fault conditions, one of the anti-parallel thyristor combinations in the current injection branch is first triggered to inject an oscillating current in the opposite direction to the fault current into the main current-carrying branch. After the power electronic device in the main current-carrying branch is turned off, the fault current is transferred to the current injection branch. After the fault current reverse-charges the capacitor to the target voltage, the power electronic device in the commutation branch with the same direction as the fault current is turned on, and the fault current will naturally transfer to the commutation branch. At this time, the self-charging process of the fault current on the capacitor is completed.

[0021] In the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, under fault conditions, the oscillating current generated by the current injection branch resonance is injected into other branches, so that the current vector sum cancels out to zero, so that the mechanical switch can interrupt without arcing under zero voltage and zero current conditions, while the power electronic device can turn off at zero current.

[0022] The DC transmission system includes the aforementioned hybrid DC circuit breaker based on self-charging oscillation-assisted converter.

[0023] Beneficial effects

[0024] This invention creates a current zero-crossing point through an oscillating branch and achieves natural and forced commutation using power electronic devices. This enables arc-free breaking of the mechanical switch under "zero voltage and zero current" conditions, while simultaneously achieving zero-current turn-off of the power electronic devices. Compared to the fully controlled devices in traditional hybrid DC circuit breakers, the transient stress experienced by the fully controlled devices within the topology during turn-off is significantly reduced.

[0025] The above description is merely an overview of the technical solution of the present invention. In order to make the technical means of the present invention clearer and more understandable, so that those skilled in the art can implement it according to the contents of the specification, and in order to make the above and other objects, features and advantages of the present invention more obvious and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0026] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. Furthermore, the same reference numerals denote the same parts throughout the drawings.

[0027] In the attached diagram:

[0028] Figure 1 This is a schematic diagram of the structural topology of a hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to an embodiment of the present invention;

[0029] Figures 2(a) to 2(c) This is a schematic diagram of a partial example of the modular structure unit SM of a hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to an embodiment of the present invention.

[0030] Figure 3 A schematic diagram of the operation process for clearing short-circuit faults using a hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to an embodiment of the present invention;

[0031] Figure 4 This is a schematic diagram showing the changes in voltage across the circuit breaker, current in each branch, and control signal over time during the interruption of a short-circuit current using a hybrid DC circuit breaker based on self-charging oscillation-assisted commutation with a bidirectional breaking capability, according to an embodiment of the present invention.

[0032] The present invention will be further explained below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0033] Specific embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While specific embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0034] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0035] To facilitate understanding of the embodiments of the present invention, the following will provide further explanation and description with reference to the accompanying drawings and several specific embodiments, and the accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0036] like Figures 1 to 4 As shown, a hybrid DC circuit breaker based on self-charging oscillation-assisted commutation includes,

[0037] The main current-carrying branch carries current bidirectionally during normal operation and cuts off fault current in case of a fault, realizing arc-free disconnection of the mechanical switch. The main current-carrying branch includes a series-connected isolating circuit breaker, a current-limiting inductor, a bidirectional current-carrying load transfer switch and a mechanical switch. The load transfer switch LCS is composed of a small number of power electronic devices, and the mechanical switch is a circuit breaker or isolating switch.

[0038] The current injection branch connects one end to the side of the power electronic device furthest from the mechanical switch, and the other end to the side of the mechanical switch furthest from the power electronic device, to generate an oscillating current that cancels out the fault current, to generate the current zero-point auxiliary commutation of the mechanical switch, and to establish the trigger voltage for the energy-consuming branch. The current injection branch includes a series-connected anti-parallel thyristor combination and a capacitor C. p and inductor L p The anti-parallel thyristor combination includes a pair of anti-parallel thyristors T1 and T2;

[0039] The energy-consuming branch has one end connected to the side of the power electronic device furthest from the mechanical switch, and the other end connected to capacitor C. p and inductor L pThe short-circuit current is cut off and energy is dissipated;

[0040] The commutation branch connects one end to the side of the power electronic device away from the mechanical switch and the other end to the side of the mechanical switch away from the power electronic device. It provides a path for the transfer of fault current and oscillation to reverse the polarity of the capacitor voltage. The commutation branch includes several power electronic module structural units (SMs) connected in series, which have bidirectional conduction and turn-off capabilities. The power electronic module structural units (SMs) are implemented by various topologies of different power electronic devices.

[0041] In a preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the current injection branch controls the direction of fault current inflow by controlling the conduction of anti-parallel thyristors T1 and T2 in different directions, thereby achieving control over capacitor C. p Bidirectional controllable charging.

[0042] In a preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the mechanical switch includes a break consisting of one or more breaks connected in series and parallel.

[0043] In a preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the break point includes a vacuum break point or a gas break point.

[0044] In a preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the gas break includes N2, air, and H2 breaks.

[0045] In a preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the energy-consuming branch includes a metal oxide variable resistor (MOV).

[0046] In the preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, before the system operates normally, the mechanical switch is closed, and then one of the anti-parallel thyristor combinations in the current injection branch is triggered. Then, the isolation circuit breaker is closed, and the DC system charges the capacitor in the current injection branch. After reaching the target voltage, the power electronic devices in the main current-carrying branch are turned on, and the system current is transferred to the main current-carrying branch. The current through the current injection branch will gradually decay to 0, and the thyristor in the current injection branch will turn off naturally. At this time, the DC system starts to operate normally, and the capacitor pre-charging is completed.

[0047] In the preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, after the capacitor pre-charges, the conducting current is injected into the other of the anti-parallel thyristor combination in the branch, and the commutation branch and the main current-carrying branch form a loop. The capacitor and inductor begin to oscillate. After half a cycle of oscillation, the current is zero, and the other of the anti-parallel thyristor combination in the current-injected branch is naturally turned off. At this time, the polarity of the capacitor voltage reaches its maximum reverse, and the polarity of the capacitor voltage is reversed.

[0048] Furthermore, under fault conditions, the timing coordination of the power electronic devices within the topology can achieve self-charging of the capacitor by the fault current. This allows for the injection of an oscillating current, opposite in direction to the fault current, into the commutator branch under fault conditions, creating a current zero point when the power electronic devices in the commutator branch are turned off. The specific implementation method is as follows:

[0049] After a fault occurs, the thyristor A corresponding to the trigger current injection branch injects an oscillating current in the opposite direction to the fault current into the main current-carrying branch. After the power electronic device in the main current-carrying branch is turned off, the fault current is transferred to the current injection branch for the first time. At this time, the fault current reverse charges the capacitor, and the polarity of the capacitor voltage will change. After being reverse charged to the target voltage, the controllable power electronic device in the commutation branch with the same direction as the fault current is turned on, and the fault current is transferred to the commutation branch for the second time. At this time, the self-charging process of the capacitor by the fault current is completed.

[0050] To generate an oscillating current opposite to the fault current in the commutation branch, the polarity of the capacitor voltage needs to be changed. Another thyristor B in the current injection branch is turned on, forming a loop between the commutation branch and the current injection branch. The capacitor and inductor begin to oscillate, and the oscillating current in the commutation branch is in the same direction as the fault current. After half a cycle of oscillation, the current is zero, and thyristor B in the current injection branch naturally turns off, reversing the capacitor voltage polarity. When the mechanical switch opens to a sufficient insulation gap, it simultaneously triggers the anti-parallel thyristors A and B in the current injection branch to inject an oscillating current opposite to the fault current into the commutation branch. When the current is zero, the corresponding controllable power electronic device in SM is turned off.

[0051] Furthermore, the topology combines key technologies of traditional mechanical DC circuit breakers and traditional hybrid DC circuit breakers: by creating an "artificial current zero-crossing point" breaking environment for the switch through the pre-charge oscillation method, it realizes zero-current turn-off of power electronic devices and reduces the transient stress during turn-off; by forcing fault current commutation through zero-current turn-off of power electronic devices, it realizes arc-free breaking of mechanical switches under "zero voltage and zero current" conditions.

[0052] In a preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the metal oxide surge arrester includes a line-type metal oxide surge arrester, a gapless line-type metal oxide surge arrester, or a fully insulated composite-jacketed metal oxide surge arrester.

[0053] In a preferred embodiment of the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, the energy dissipation circuit includes one or more of the following devices: metal oxide surge arrester and removable surge arrester.

[0054] In one embodiment, the hybrid DC circuit breaker based on self-charging oscillation-assisted commutation includes four parts: a main current-carrying branch, a current injection branch, a commutation branch, and an energy-dissipating branch.

[0055] The main current-carrying branch can carry current bidirectionally during normal operation. In case of a fault, the fault current is transferred by the shutdown of power electronic devices, realizing arc-free disconnection of the mechanical switch. The current injection branch is used to generate an oscillating current that cancels out the fault current and injects it into other branches, creating a current zero point to assist commutation of the switch, and establishing the trigger voltage of the metal oxide variable resistor (MOV) in the energy-dissipating branch. The commutation branch provides a path for the transfer of fault current and can cooperate with the power electronic components of the current injection branch to oscillate and reverse the polarity of the capacitor voltage. The energy-dissipating branch is used to cut off the short-circuit current and dissipate energy.

[0056] The main current-carrying branch includes an ultra-fast mechanical switch, a small number of power electronic devices, a current-limiting inductor, and an isolating circuit breaker, all connected in series. The ultra-fast mechanical switch is connected in combination with the power electronic devices, while the current-limiting inductor and isolating circuit breaker are connected in series to the left of the power electronic devices.

[0057] The current injection branch includes capacitor C p Inductor L p It is connected in series with a pair of anti-parallel thyristors T1 and T2. The capacitor C... p When connected in combination with an anti-parallel thyristor, the inductor L p With capacitor C p Series connection.

[0058] The converter branch consists of several power electronic module structural units (SMs) connected in series.

[0059] The energy-consuming branch includes a metal oxide variable resistor (MOV).

[0060] The main current-carrying branch has a small number of power electronic devices that can be implemented by different topologies of different power electronic devices, and has bidirectional conduction and shutdown capabilities. In this example, it is a power electronic module structure unit (SM).

[0061] The modular structural unit (SM) can be implemented using various topologies of different power electronic devices, possessing bidirectional conduction and turn-off capabilities. Three example IGBT structures are shown below. Figures 2(a) to 2(c) The following is an example; other structures will not be listed here. The SM structure used in this example is shown in Figure 2(a). In application, different numbers of these structures are connected in series depending on the voltage level.

[0062] Before the DC system operates normally, the capacitor in the current injection branch is pre-charged through the timing coordination of the power electronic devices inside the main current-carrying branch and the current injection branch, without the need for separate charging equipment. The specific implementation method is as follows:

[0063] Before the system operates normally, the mechanical switch is first closed, then the thyristor T1 corresponding to the current injection branch is triggered, followed by the closing of the isolating circuit breaker RCB. The DC system begins charging the capacitor in the current injection branch. Once the target voltage is reached, the main current-carrying branch G1' is turned on, and the system current rapidly transfers to the main current-carrying branch. The current through the current injection branch gradually decays to zero, and the thyristor in the current injection branch naturally turns off. At this point, the DC system begins normal operation, and the capacitor pre-charging is complete.

[0064] By coordinating the current injection branch with the internal power electronic devices of the converter branch, the polarity of the capacitor voltage can be reversed. This is used to inject an oscillating current, opposite in direction to the fault current, into the main current-carrying branch under fault conditions, creating a zero-current point when the power electronic devices in the main current-carrying branch are turned off. The specific implementation method is as follows:

[0065] After the capacitor pre-charge is complete, in order to generate an oscillating current opposite to the fault current in the main current-carrying branch, the polarity of the capacitor voltage needs to be changed. Current is injected into another thyristor T2 in the current-injection branch, and the main current-carrying branch forms a loop with the current-injection branch, causing the capacitor and inductor to begin oscillation. After half a cycle of oscillation, the current is zero, and thyristor T2 in the current-injection branch naturally turns off. At this point, the capacitor voltage polarity reaches its maximum reverse polarity, turning off group G1, and the capacitor voltage polarity is reversed.

[0066] By coordinating the timing of the power electronic devices within the topology, the fault current can self-charge the capacitor. This allows an oscillating current, opposite in direction to the fault current, to be injected into the commutator branch during a fault, creating a zero-current point when the power electronic devices in the commutator branch are turned off. The specific implementation method is as follows:

[0067] After a fault occurs, the thyristor T1 corresponding to the trigger current injection branch injects an oscillating current in the opposite direction to the fault current into the main current-carrying branch. The main current-carrying branch G1' turns off at zero crossing, and the fault current is transferred to the current injection branch for the first time. At this time, the fault current reverse charges the capacitor, and the polarity of the capacitor voltage will change. After being reverse charged to the target voltage, the commutation branch G1 group is turned on, and the fault current is transferred to the commutation branch for the second time. At this time, the self-charging process of the capacitor by the fault current is completed.

[0068] To generate an oscillating current opposite to the fault current in the commutation branch, the polarity of the capacitor voltage needs to be changed. The current injection branch is activated by another thyristor, T2, forming a loop with the current injection branch. The capacitor and inductor begin to oscillate, and the oscillating current in the commutation branch is in the same direction as the fault current. After half a cycle of oscillation, the current is zero, and thyristor T2 in the current injection branch naturally turns off, reversing the capacitor voltage polarity. When the mechanical switch opens to a sufficient insulation gap, the anti-parallel thyristors T1 and T2 in the current injection branch are simultaneously triggered, injecting an oscillating current opposite to the fault current into the commutation branch. When the current is zero, the commutation branch G1 group is turned off.

[0069] Furthermore, based on the proposed hybrid DC circuit breaker with self-charging oscillation-assisted commutation, taking a DC system with a rated voltage of 20kV, a rated current of 2kA, and a maximum fault current of 10kA as an example, the operation process of clearing the right-side short-circuit fault by passing forward current through the DC circuit breaker is given as follows: Figure 3 As shown, g s For control signals of switching devices, the following steps are included:

[0070] 1) t < t0: Before the system operates normally, the mechanical switch K is closed first, then the thyristor T1 in the current injection branch is triggered, followed by the closing of the isolating circuit breaker RCB. The DC system begins to charge the capacitor in the current injection branch. After reaching the target voltage, the main branch G1' is turned on, and the system current quickly transfers to the main current-carrying branch. The current through the current injection branch will gradually decay to 0, at which point the DC system begins to operate normally.

[0071] When thyristor T2 is turned on, the main current-carrying branch and the current-injection branch form a loop, and the capacitor and inductor begin to oscillate. After half a cycle of oscillation, the current is zero, and thyristor T2 naturally turns off. At this time, the polarity of the capacitor voltage reaches its maximum in reverse, and the reversal of the capacitor polarity is completed.

[0072] 2) t0-t1: If a short-circuit fault occurs at time t0, the line current begins to rise continuously, and its rate of increase is limited by the current-limiting inductor. After a certain time delay, a trip signal is sent to the DC circuit breaker at time t1.

[0073] 3) t1-t2: After receiving the trip signal, the DC circuit breaker triggers thyristor T1 to inject an oscillating current in the main current-carrying branch that is opposite to the fault current. When the current crosses zero at time t2, the main branch G1' is turned off. At the same time, the fault current is transferred to the current injection branch.

[0074] 4) t2-t3: The fault current begins to charge the capacitor at time t2, and charges it to the target voltage at time t3. At the same time, the G1 group in the commutation branch SM is turned on. The target voltage is the capacitor voltage value required to generate a reverse oscillating current sufficient to interrupt the maximum fault current.

[0075] 5) t3-t4: The fault current transfers to the commutation branch at time t3, triggering thyristor T2, causing the capacitor and inductor to oscillate through the commutation branch. The oscillation ends at time t3', and the capacitor voltage polarity reverses. Simultaneously, the ultra-fast mechanical switch begins to drive contact separation, at which point the switch operates under arc-free tripping conditions with zero current and zero voltage. At time t4, the switch disconnects to a sufficient insulation gap to withstand the transient interruption voltage at time t4. When the oscillating current passes through the commutation branch, the current satisfies the following relationship:

[0076] I com =I f +I osc =i f (t4)+10kA (1)

[0077] 6) t4-t5: Thyristors T1 and T2 in the current injection branch are simultaneously triggered. The oscillating current generated by the capacitor and inductor will flow through the commutation branch and will be in the opposite direction to the fault current. At time t5, when the current in the commutation branch crosses zero, group G1 is turned off. At the same time, the fault current will be transferred to the current injection branch. The current flowing through the commutation branch during this stage satisfies the following relationship:

[0078] I com =I f -I osc =i f (t4)-10kA≤0 (2)

[0079] In equations (1) and (2), I com I is the commutator branch current, If is the varying DC system fault current, and I is the commutator branch current. osc It is the oscillating current.

[0080] 7) t5-t6: The fault current begins to charge the capacitor at time t5 until it reaches the MOV's trigger voltage, at which point the fault current shifts to the energy-dissipating branch. The current through the MOV will gradually decrease until it decays to zero at time t6.

[0081] 8) t6-t7: After the fault current is interrupted, there may be some leakage current in the system. The isolating circuit breaker will open at time t7 to clear the leakage current and isolate the main circuit of the circuit breaker from the power grid system.

[0082] Figure 4 The voltage V across the circuit breaker during the interruption of short-circuit current in the proposed hybrid DC circuit breaker based on self-charging oscillation-assisted commutation is given. CB The curves showing the relationship between the current I in each branch and the control signal over time.

[0083] Specifically, the voltage across the circuit breaker changes over time as follows: During normal system operation, the voltage across the circuit breaker is almost zero. At time t2, the fault current flows through the current injection branch, and the voltage across the circuit breaker is the change in capacitor voltage. When the fault current flows through the commutation branch, the voltage across the circuit breaker is almost zero. At time t5, the fault current is completely commutated to the current injection branch, and the voltage across the circuit breaker is now the capacitor voltage. The voltage across the circuit breaker begins to rise along with the capacitor voltage. At time t5', the MOV trigger voltage U is reached. MOV U MOV At approximately 1.5 times the DC system voltage, the MOV will consume energy to clamp the voltage across the circuit breaker to the DC system voltage.

[0084] Specifically, the time-varying characteristics of the circuit breaker current are as follows: Before time t0, the DC system operates normally, with a 2kA line operating current conducted by the main current-carrying branch; at time t1, the system begins to interrupt the fault current, triggering thyristor T1, and injecting oscillating current into the main current-carrying branch in the opposite direction to the fault current, resulting in a current I flowing through the mechanical switch. K The current begins to decrease; at time t2, the fault current completes its first transfer, flowing entirely through the current injection branch. After charging the capacitor in the current injection branch to the target voltage, the commutation branch G1 is turned on, and the current I flowing through the current injection branch... osc The current begins to decrease; at time t3, the fault current completes its second transfer, flowing entirely through the commutation branch. The ultra-fast mechanical switch K begins arc-free tripping, thyristor T1 turns off naturally, simultaneously triggering thyristor T2. The capacitor and inductor oscillate through the commutation branch, with the capacitor polarity reversed. At this time, the current I in the commutation branch... com For fault current and I osc The superposition; the oscillation ends at time t3', at which point I com Equal to the fault current; the ultra-fast mechanical switch tripping process takes approximately 2ms. At time t4, the insulation gap is broken to a level sufficient to withstand the transient interruption voltage, triggering thyristor T3. Oscillating current is injected into the commutator branch, in the opposite direction to the fault current. com The current begins to decrease, and when it reaches zero, group G1 is shut off. The fault current is then transferred back to the current injection branch. osc It rises rapidly. After reaching the MOV's trigger voltage, I...MOV It rises rapidly until time t5'. osc When the fault current drops to zero, all fault current is transferred to the energy-consuming branch, and the MOV consumes the remaining energy.

[0085] According to one aspect of the present invention, a DC transmission system is provided, comprising the aforementioned hybrid DC circuit breaker based on self-charging oscillation-assisted commutation.

[0086] Although embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments and application fields described above. The specific embodiments described above are merely illustrative and instructive, and not restrictive. Those skilled in the art can make many other forms based on the guidance of this specification and without departing from the scope of protection of the claims of the present invention, and all of these are within the scope of protection of the present invention.

Claims

1. A hybrid DC circuit breaker based on self-charging oscillation-assisted commutation, characterized in that, It includes, The main current-carrying branch carries current bidirectionally during normal operation and cuts off fault current in case of a fault, thereby achieving arc-free disconnection of the mechanical switch. The main current-carrying branch includes a series-connected isolating circuit breaker, a current-limiting inductor, a bidirectional current-carrying load transfer switch, and a mechanical switch. The load transfer switch is composed of multiple power electronic devices, and the mechanical switch is a circuit breaker or isolating switch. The current injection branch has one end connected to the side of the power electronic device away from the mechanical switch, and the other end connected to the side of the mechanical switch away from the power electronic device, to generate an oscillating current that cancels out the fault current, generate the current zero-point auxiliary commutation of the mechanical switch, and establish the trigger voltage for the energy-dissipating branch. The current injection branch includes a series-connected anti-parallel thyristor combination. 、 Capacitor C p and inductor L p The anti-parallel thyristor combination includes a pair of anti-parallel thyristors T1 and T2; The energy-consuming branch has one end connected to the side of the power electronic device furthest from the mechanical switch, and the other end connected to capacitor C. p and inductor L p The short-circuit current is cut off and energy is dissipated; The commutation branch connects one end to the side of the power electronic device away from the mechanical switch and the other end to the side of the mechanical switch away from the power electronic device. It provides a path for the transfer of fault current and oscillation to reverse the polarity of the capacitor voltage. The commutation branch includes several power electronic module structure units (SMs) with bidirectional conduction and turn-off capabilities connected in series. The power electronic module structure units (SMs) are implemented by various topologies of different power electronic devices. In the event of a fault, one of the anti-parallel thyristor combinations in the current injection branch is first triggered to inject an oscillating current in the main current-carrying branch that is opposite to the fault current. After the power electronic device in the main current-carrying branch is turned off, the fault current is transferred to the current injection branch. After the fault current charges the capacitor in reverse to the target voltage, the power electronic device in the commutation branch that is in the same direction as the fault current is turned on, and the fault current will naturally transfer to the commutation branch. At this time, the self-charging process of the capacitor by the fault current is completed.

2. The hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to claim 1, characterized in that, The current injection branch controls the direction of fault current inflow by controlling the conduction of anti-parallel thyristors T1 and T2 in different directions, thereby controlling the capacitor C. p Bidirectional controllable charging.

3. The hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to claim 1, characterized in that: The mechanical switch includes a break consisting of one or more breaks connected in series and parallel.

4. The hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to claim 3, characterized in that: The fracture surface includes a vacuum fracture surface or a gas fracture surface, and the gas fracture surface includes N2, air, and H2 fracture surfaces.

5. The hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to claim 1, characterized in that: The energy-consuming branch includes a metal oxide variable resistor (MOV).

6. The hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to claim 1, characterized in that: Before the system is in normal operation, the mechanical switch is closed, and then one of the anti-parallel thyristor combinations in the current injection branch is triggered. Then the isolating circuit breaker is closed, and the DC system charges the capacitor in the current injection branch. After reaching the target voltage, the power electronic devices in the main current-carrying branch are turned on, and the system current is transferred to the main current-carrying branch. The current through the current injection branch will gradually decrease to 0, and the thyristor in the current injection branch will turn off naturally. At this time, the DC system begins to operate normally, and the capacitor pre-charging is completed.

7. The hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to claim 6, characterized in that: After the capacitor is pre-charged, the conduction current is injected into the other anti-parallel thyristor combination in the branch, and the commutation branch and the main current branch form a loop. The capacitor and inductor begin to oscillate. After half a cycle of oscillation, the current is zero, and the other anti-parallel thyristor combination in the current injection branch is naturally turned off. At this time, the polarity of the capacitor voltage reaches its maximum reverse, and the polarity of the capacitor voltage is reversed.

8. The hybrid DC circuit breaker based on self-charging oscillation-assisted commutation according to any one of claims 1-7, characterized in that: In the event of a fault, the oscillating current generated by the resonance of the injected branch is injected into other branches, so that the current vector sum cancels out to zero. This allows the mechanical switch to disconnect without arcing under zero voltage and zero current conditions, while the power electronic devices turn off at zero current.

9. A DC transmission system comprising a hybrid DC circuit breaker based on self-charging oscillation-assisted commutation as described in any one of claims 1-8.

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