A DC circuit breaker and control method suitable for medium voltage DC systems

Abstract

The application discloses a DC circuit breaker suitable for a medium-voltage DC system and a control method, and relates to the field of DC circuit breakers. The auxiliary commutation module can not only commutate, but also replace part of the bidirectional power electronic switches in the traditional topological scheme to bear the static DC system voltage, so that the number of series connection of the fully-controlled power electronic switch devices in the bidirectional power electronic switches is reduced, the cost is lowered, and the technical economy of the hybrid DC circuit breaker is improved. Moreover, the auxiliary commutation module can selectively discharge the capacitor according to the current direction of the main branch, so that the current transfer of the main branch is quickly and reliably realized, the commutation speed of the fault current of the main branch is improved, the speed requirement of the arc extinguishing post-insulation strength recovery of the fast mechanical switch is lowered, and the overall reliability of the hybrid DC circuit breaker is improved.

Description

Technical Field

[0001] This invention relates to the field of DC circuit breakers, and in particular to a DC circuit breaker and control method suitable for medium-voltage DC systems. Background Technology

[0002] Medium-voltage flexible DC technology has unique advantages in new energy grid connection, flexible grid interconnection, intelligent power transmission and distribution, and improving power supply reliability. In particular, for new power systems, adopting power transmission and distribution schemes based on medium-voltage flexible DC technology will bring significant technical advantages and economic benefits.

[0003] When a DC line fault occurs in a flexible DC system, the fault current develops rapidly, exceeding the tolerance of power electronic devices within milliseconds. Since flexible DC systems based on half-bridge submodules (Modular Multilevel Converters, MMCs) cannot limit short-circuit current using converter blocking methods, DC circuit breakers must be used to clear DC-side faults in a very short time to ensure the safe and reliable operation of critical power electronic equipment in the DC grid.

[0004] Because DC systems lack a natural zero-crossing point for current, the mature arc-extinguishing technology used in AC circuit breakers cannot be directly applied. Furthermore, the need to absorb substantial inductive energy during fault current interruption makes DC circuit breakers a long-standing research hotspot in the field of DC fault protection. Currently, research on DC circuit breakers primarily focuses on DC breaking and overvoltage suppression, with the challenge of DC breaking being the biggest technical bottleneck hindering DC grid deployment. Additionally, the techno-economic viability of DC circuit breakers also restricts their application to larger-capacity DC power grids.

[0005] Medium-voltage high-capacity DC circuit breakers are key equipment for rapidly limiting and interrupting fault currents in medium-voltage DC systems. Hybrid DC circuit breakers, with their advantages of low on-state losses, flexible control, and moderate cost, have become the main development direction for medium-voltage DC circuit breakers. However, due to the new adaptability requirements of medium-voltage DC systems—including low cost, small footprint, and high breaking capacity—the technical approach of medium-voltage hybrid DC circuit breakers differs from that of high-voltage hybrid DC circuit breakers.

[0006] Therefore, it is of great significance to study a DC circuit breaker topology scheme suitable for medium-voltage DC systems to meet the various new adaptive requirements of medium-voltage DC systems for DC circuit breakers, and to improve the technical and economic efficiency of DC circuit breaker equipment while achieving reliable DC breaking and overvoltage suppression. Summary of the Invention

[0007] The purpose of this invention is to provide a DC circuit breaker and control method suitable for medium-voltage DC systems, which can improve the technical economy and overall reliability of hybrid DC circuit breakers.

[0008] To achieve the above objectives, the present invention provides the following solution:

[0009] A DC circuit breaker suitable for medium-voltage DC systems includes: a main branch, an auxiliary converter module, a bidirectional power electronic switch, and an energy-consuming branch;

[0010] The main branch is connected in series with the DC transmission line;

[0011] The input terminal of the main branch is connected to the input terminal of the auxiliary converter module, and the output terminal of the main branch is connected to the output terminal of the bidirectional power electronic switch and the output terminal of the energy-consuming branch, respectively.

[0012] The output of the auxiliary converter module is connected to the input of the bidirectional power electronic switch and the input of the energy-consuming branch, respectively.

[0013] Optionally, the main branch includes: a permanent magnet repulsion mechanism type fast mechanical switch.

[0014] Optionally, the auxiliary converter module includes: a first thyristor (2), a second thyristor (3), a third thyristor (4), a fourth thyristor (5), a pre-charge capacitor (6), and a bypass diode (7);

[0015] The cathode of the first thyristor (2) is connected to the cathode of the third thyristor (4), the negative terminal of the pre-charge capacitor (6), and the anode of the bypass diode (7), respectively. The anode of the first thyristor (2) is connected to the cathode of the second thyristor (3). One point on the connection line between the anode of the first thyristor (2) and the cathode of the second thyristor (3) serves as the input terminal of the auxiliary converter module.

[0016] The anode of the second thyristor (3) is connected to the anode of the fourth thyristor (5), the positive terminal of the pre-charge capacitor (6), and the cathode of the bypass diode (7), respectively.

[0017] The anode of the third thyristor (4) is connected to the cathode of the fourth thyristor (5); a point on the connection line between the anode of the third thyristor (4) and the cathode of the fourth thyristor (5) serves as the output terminal of the auxiliary converter module.

[0018] Optionally, the bidirectional power electronic switch includes: multiple power electronic switch modules connected in series;

[0019] The power electronic switch module includes a first diode (8), a second diode (9), a third diode (10), a fourth diode (11), a fully controlled power electronic switch device (12), and an anti-parallel diode (13);

[0020] The cathode of the first diode (8) is connected to the cathode of the third diode (10), the collector of the fully controlled power electronic switch device (12), and the cathode of the anti-parallel diode (13), respectively. The anode of the first diode (8) is connected to the cathode of the second diode (9). One point on the connection line between the anode of the first diode (8) and the cathode of the second diode (9) serves as the input terminal of the power electronic switch module.

[0021] The anode of the second diode (9) is connected to the anode of the fourth diode (11), the emitter of the fully controlled power electronic switch device (12), and the anode of the anti-parallel diode (13), respectively.

[0022] The anode of the third diode (10) is connected to the cathode of the fourth diode (11); a point on the connection line between the anode of the third diode (10) and the cathode of the fourth diode (11) serves as the output terminal of the power electronic switch module.

[0023] Optionally, the first thyristor (2), the second thyristor (3), the third thyristor (4), the fourth thyristor (5), and the fully controlled power electronic switching device (12) are all connected in parallel with a buffer circuit, a protection circuit, and a voltage equalization circuit.

[0024] Optionally, the energy-consuming branch includes a metal oxide varistor.

[0025] A control method for a DC circuit breaker suitable for a medium-voltage DC system includes:

[0026] When the DC system is running normally, the DC circuit breaker is in the conducting state, controlling the main branch to close, the auxiliary converter module and the bidirectional power electronic switch are both turned off, and the energy-consuming branch does not operate;

[0027] When a DC-side short-circuit fault occurs in the DC system, the main branch circuit is tripped, and the auxiliary converter module and the bidirectional power electronic switch are both turned on, so that the fault current is transferred from the main branch to the series branch formed by the auxiliary converter module and the bidirectional power electronic switch.

[0028] Once the insulation strength of the main branch is restored, the bidirectional power electronic switch is turned off to transfer the fault current to the energy-consuming branch. At the same time, the energy-consuming branch is controlled to consume the inductive energy in the DC system.

[0029] When the DC system current gradually decreases to the current threshold, the auxiliary converter module will automatically shut down.

[0030] Optionally, after a DC-side short-circuit fault occurs in the DC system, the main branch is tripped, and both the auxiliary converter module and the bidirectional power electronic switch are turned on, so that the fault current is transferred from the main branch to the series branch formed by the auxiliary converter module and the bidirectional power electronic switch, specifically including:

[0031] After a short-circuit fault occurs on the DC side of the DC system line, the fault current of the DC system continues to rise until the fault current rises to the current protection threshold, at which point a trip command is issued to the DC circuit breaker.

[0032] According to the tripping command, the moving and stationary contacts of the permanent magnet repulsion mechanism type fast mechanical switch of the main branch are separated, and the insulation gap between the moving and stationary contacts is gradually widened.

[0033] Based on the direction of the short-circuit current in the DC system, trigger signals are sequentially sent to the thyristors in the auxiliary converter module and the fully controllable power electronic switching devices in the bidirectional power electronic switch;

[0034] The pre-charge capacitor in the auxiliary converter module forms a closed loop with the permanent magnet repulsion mechanism of the main branch fast mechanical switch and discharges;

[0035] The permanent magnet repulsion mechanism type fast mechanical switch of the main branch extinguishes the arc after the current crosses zero, and the DC circuit breaker port bears the remaining negative voltage of the pre-charge capacitor;

[0036] The pre-charge capacitor in the auxiliary converter module continues to discharge along the short-circuit current direction until the pre-charge capacitor voltage discharges to 0V, at which point the bypass diode in the auxiliary converter module turns on.

[0037] Optionally, the step of sequentially sending trigger signals to the thyristors in the auxiliary converter module and the fully controllable power electronic switching devices in the bidirectional power electronic switch according to the direction of the DC system short-circuit current specifically includes:

[0038] Real-time monitoring of the direction of short-circuit current in the main branch;

[0039] If the direction of the short-circuit current is positive, the first thyristor (2) and the fourth thyristor (5) are turned on, and the fully controllable power electronic switching device in the bidirectional power electronic switch is turned on after a preset time.

[0040] If the short-circuit current is in the negative direction, the second thyristor (3) and the third thyristor (4) are turned on, and the fully controllable power electronic switching device in the bidirectional power electronic switch is turned on after a preset time delay.

[0041] Optionally, when the DC system current gradually decays to a current threshold, the auxiliary converter module automatically shuts down, and the process further includes:

[0042] Based on the voltage equalization circuits equipped in the auxiliary converter module and the power electronic switch module, the DC system voltage is distributed on the auxiliary converter module and the power electronic switch module according to the preset target.

[0043] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0044] This invention discloses a DC circuit breaker and control method suitable for medium-voltage DC systems. The auxiliary converter module can not only commutate the current, but also replace part of the bidirectional power electronic switches in the traditional topology to bear the static DC system voltage. This reduces the number of fully controlled power electronic switch devices in the bidirectional power electronic switches, lowers the cost, and improves the technical and economic efficiency of the hybrid DC circuit breaker.

[0045] The auxiliary converter module can selectively discharge capacitors according to the current direction of the main branch, thereby quickly and reliably transferring the current of the main branch. This not only improves the commutation speed of the fault current in the main branch, but also reduces the speed requirement for the recovery of insulation strength after arc extinction of the fast mechanical switch, thus improving the overall reliability of the hybrid DC circuit breaker. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 A schematic diagram of a DC circuit breaker topology suitable for medium-voltage DC systems is provided as an embodiment of the present invention;

[0048] Figure 2 A flowchart of a control method for a DC circuit breaker applicable to a medium-voltage DC system provided in an embodiment of the present invention;

[0049] Figure 3 A schematic diagram illustrating the process of a DC circuit breaker interrupting fault current provided in an embodiment of the present invention;

[0050] Figure 4 This is a schematic diagram of the conduction of the thyristor in the auxiliary converter module under forward short-circuit current according to an embodiment of the present invention;

[0051] Figure 5 This is a schematic diagram of the pre-charge capacitor discharging under forward short-circuit current provided in an embodiment of the present invention;

[0052] Figure 6 This is a circuit diagram showing the bypass diode conducting under forward short-circuit current, provided in an embodiment of the present invention.

[0053] Figure 7 This is a schematic diagram of the topology of power electronic switches in a traditional hybrid DC circuit breaker;

[0054] Figure 8 This is a schematic diagram illustrating the nonlinear characteristics of an MOV and the voltage characteristics of a power electronic switching device. Figure 8(a) in the diagram is a schematic diagram of the nonlinear characteristics of the MOV. Figure 8 (b) in the diagram is a voltage diagram of a power electronic switching device. Detailed Implementation

[0055] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0056] The purpose of this invention is to provide a DC circuit breaker and control method suitable for medium-voltage DC systems, which can improve the technical economy and overall reliability of hybrid DC circuit breakers.

[0057] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0058] The purpose of this invention is to provide a DC circuit breaker and control method suitable for medium-voltage DC systems. The DC circuit breaker of this invention includes a novel auxiliary converter module based on a thyristor bridge, which not only transfers the mechanical switching current of the main branch to the power electronic switching branch, but also reduces the number of fully controlled power electronic switching devices connected in series. This topology meets the various new adaptability requirements of medium-voltage DC systems for DC circuit breakers, and while achieving reliable DC breaking and overvoltage suppression, further improves the technical and economic efficiency of DC circuit breaker equipment.

[0059] This invention provides a DC circuit breaker suitable for medium-voltage DC systems, such as... Figure 1 As shown, it includes: a main branch, an auxiliary converter module, a bidirectional power electronic switch, and a power dissipation branch. The main branch is connected in series in the DC transmission line. The input terminal of the main branch is connected to the input terminal of the auxiliary converter module, and the output terminal of the main branch is connected to the output terminals of the bidirectional power electronic switch and the power dissipation branch, respectively. The output terminal of the auxiliary converter module is connected to the input terminals of the bidirectional power electronic switch and the power dissipation branch, respectively.

[0060] The main branch is generally located at the outlet of the converter valve. The main branch includes: a permanent magnet repulsion mechanism type fast mechanical switch.

[0061] Depending on the repulsion mechanism, fast mechanical switches used in hybrid DC circuit breakers can be mainly divided into two categories: electromagnetic repulsion mechanism type and permanent magnet repulsion mechanism type. Electromagnetic repulsion mechanisms offer faster opening speeds and faster insulation recovery at the mechanical switch contacts, but they are more expensive and require a larger footprint. For traditional hybrid DC circuit breakers, considering the slower transfer speed of fault current in the main branch, electromagnetic repulsion type fast mechanical switches are typically used to meet the insulation recovery speed requirements after arc extinction. Permanent magnet repulsion mechanisms have slower opening speeds and slower insulation recovery at the mechanical switch contacts, but they are less expensive and require less space. Therefore, using permanent magnet repulsion mechanism type fast mechanical switches instead of electromagnetic repulsion mechanism type fast mechanical switches in the main branch not only saves costs but also reduces the footprint.

[0062] The auxiliary converter module includes: a first thyristor 2, a second thyristor 3, a third thyristor 4, a fourth thyristor 5, a pre-charge capacitor 6, and a bypass diode 7.

[0063] The cathode of the first thyristor 2 is connected to the cathode of the third thyristor 4, the negative terminal of the pre-charge capacitor 6, and the anode of the bypass diode 7, respectively. The anode of the first thyristor 2 is connected to the cathode of the second thyristor 3. A point on the connection line between the anode of the first thyristor 2 and the cathode of the second thyristor 3 serves as the input terminal of the auxiliary converter module. The anode of the second thyristor 3 is connected to the anode of the fourth thyristor 5, the positive terminal of the pre-charge capacitor 6, and the cathode of the bypass diode 7, respectively. The anode of the third thyristor 4 is connected to the cathode of the fourth thyristor 5. A point on the connection line between the anode of the third thyristor 4 and the cathode of the fourth thyristor 5 serves as the output terminal of the auxiliary converter module.

[0064] The bidirectional power electronic switch includes multiple power electronic switch modules connected in series. Each power electronic switch module includes a first diode 8, a second diode 9, a third diode 10, a fourth diode 11, a fully controlled power electronic switch device 12, and an anti-parallel diode 13. The cathode of the first diode 8 is connected to the cathode of the third diode 10, the collector of the fully controlled power electronic switch device 12, and the cathode of the anti-parallel diode 13, respectively. The anode of the first diode 8 is connected to the cathode of the second diode 9. A point on the connection line between the anode of the first diode 8 and the cathode of the second diode 9 serves as the input terminal of the power electronic switch module. The anode of the second diode 9 is connected to the anode of the fourth diode 11, the emitter of the fully controlled power electronic switch device 12, and the anode of the anti-parallel diode 13, respectively. The anode of the third diode 10 is connected to the cathode of the fourth diode 11. A point on the connection line between the anode of the third diode 10 and the cathode of the fourth diode 11 serves as the output terminal of the power electronic switch module.

[0065] Fully controlled power electronic switching devices include, but are not limited to, IGBTs, IGCTs, and IEGTs.

[0066] The gates of thyristors are all connected to drive control signals; the gates of fully controllable power electronic switching devices are all connected to drive control signals.

[0067] The first thyristor 2, the second thyristor 3, the third thyristor 4, the fourth thyristor 5, and the fully controlled power electronic switching device 12 are all connected in parallel with a buffer circuit, a protection circuit, and a voltage equalization circuit.

[0068] Common buffer circuits include RC circuits, RCD circuits, and MOV-C circuits, which are connected in parallel across the device.

[0069] The protection circuit refers to the MOV, which is connected in parallel across the two ends of the device;

[0070] A voltage equalization circuit refers to a voltage equalization resistor connected in parallel across the two ends of a device.

[0071] The power-consuming branch includes: Metal Oxide Varistor (MOV).

[0072] This invention also provides a control method for DC circuit breakers applicable to medium-voltage DC systems, such as... Figure 2 As shown, it includes the following steps:

[0073] Step 1: When the DC system is running normally, the DC circuit breaker is in the conducting state, controlling the main branch to close, the auxiliary converter module and the bidirectional power electronic switch are both turned off, and the energy-consuming branch does not operate.

[0074] The DC circuit breaker is in the ON state and is carrying the rated DC current. The main branch fast mechanical switch is in the OFF state, the thyristors in the auxiliary converter module are in the OFF state, the fully controlled power electronic switching devices in the power electronic switch module are in the OFF state, and the MOV in the energy-consuming branch is not operating.

[0075] Step 2: After a DC-side short-circuit fault occurs in the DC system, the main branch circuit is tripped, and the auxiliary converter module and the bidirectional power electronic switch are both turned on, so that the fault current is transferred from the main branch to the series branch formed by the auxiliary converter module and the bidirectional power electronic switch.

[0076] When a short-circuit fault occurs on the DC side of the DC system, the DC circuit breaker receives a tripping command and operates.

[0077] First, the main branch fast mechanical switch is opened, and its moving and stationary contacts are continuously pulled apart, and the insulation gap at the break point gradually increases.

[0078] Secondly, forced commutation is achieved by controlling the auxiliary converter module. The main branch fast mechanical switch current crosses zero and extinguishes the arc. The fault current is transferred from the main branch to the series branch formed by the auxiliary converter module and the bidirectional power switch.

[0079] Step 3: After the insulation strength of the main branch is restored, control the bidirectional power electronic switch to turn off, so that the fault current is transferred to the energy-consuming branch. At the same time, control the operation of the energy-consuming branch to consume the inductive energy in the DC system.

[0080] Step 4: When the DC system current gradually decreases to the current threshold, the auxiliary converter module will automatically shut down.

[0081] When the DC system current gradually decreases to tens of milliamperes, the DC circuit breaker completes the clearing of the DC side fault in the system.

[0082] At this time, based on the voltage equalization circuits equipped with the auxiliary converter module and the power electronic switch, the DC system voltage will be distributed on the auxiliary converter module and the power electronic switch according to the predetermined target.

[0083] In a static state, both the thyristor and the power electronic switching device are in a blocking state, and the system voltage is distributed across each device according to the magnitude of the voltage-equalizing resistor connected in parallel with each device, based on a preset value.

[0084] Reference Figure 3 The process of a DC circuit breaker interrupting fault current is now described in detail according to the time sequence:

[0085] Before time t0, the main branch of the DC circuit breaker carries the normal system current, and the pre-charge capacitor voltage is maintained at the preset rated value.

[0086] At time t0, a short circuit fault occurs in the DC system line.

[0087] t0~t1: The short-circuit fault continues to develop, and the fault current of the DC system rises rapidly.

[0088] At time t1, the current in the DC system rises to the protection threshold, and the control and protection system issues a trip command to the DC circuit breaker.

[0089] t1~t2: This process is the inherent opening time of the fast mechanical switch, which refers to the time from when the fast mechanical switch receives the opening command until the moving and stationary contacts just separate.

[0090] At time t2, the transmission mechanism of the fast mechanical switch of the main branch starts to operate, and its moving and stationary contacts officially separate.

[0091] t2~t3: The transmission mechanism of the fast mechanical switch continues to operate, and the insulation gap between its moving and stationary contacts gradually increases.

[0092] At time t3, the DC circuit breaker sends trigger signals sequentially to the corresponding thyristors in the auxiliary converter module and the fully controllable power electronic switching devices in the power electronic switch, according to the direction of the short circuit in the DC system.

[0093] The discharge direction of the capacitor is opposite to the direction of the short-circuit current in the main branch, such as... Figure 4 As shown.

[0094] Taking the forward short-circuit current as an example, the first thyristor 2 and the fourth thyristor 5 are turned on;

[0095] Taking reverse short-circuit current as an example, the second thyristor 3 and the third thyristor 4 are turned on.

[0096] The discharge direction of the capacitor is opposite to the direction of the short-circuit current in the main branch. The measurement system can monitor the direction of the short-circuit current in the main branch in real time, and the control and protection system can make judgments based on the direction of the short-circuit current to achieve selective capacitor discharge.

[0097] Taking forward short-circuit current as an example:

[0098] like Figure 5 As shown, the first thyristor 2 and the fourth thyristor 5 are first turned on, and after a very short delay, the power electronic switch is turned on. At this time, the pre-charge capacitor and the main branch mechanical switch form a closed circuit to discharge.

[0099] When the main branch current crosses zero, the fast mechanical switch extinguishes the arc, and the fault current is completely transferred to the series branch of the auxiliary converter module and the power electronic switch. At this time, the pre-charge capacitor continues to discharge along the current direction.

[0100] During the above process, the main branch fast mechanical switch is always subjected to the reverse negative voltage of the capacitor, which helps its insulation strength to recover quickly.

[0101] like Figure 6 As shown, when the voltage of the pre-charge capacitor discharges to zero, its parallel bypass diode conducts. During this stage, the main branch fast mechanical switch only bears the conduction voltage drop of the auxiliary converter module and the power electronic switch, and the distance between its moving and stationary contacts continues to increase, allowing the insulation strength to continue to recover.

[0102] The power electronic switch can be turned off when the insulation strength between the contacts of the fast mechanical switch is strong enough to withstand the operating overvoltage caused by the turn-off of the power electronic switch.

[0103] t3~t4: The capacitors in the auxiliary converter module form a closed loop with the main branch fast mechanical switch and discharge.

[0104] At time t4, the fast mechanical switch of the main branch extinguishes its arc after the current crosses zero, and the DC circuit breaker port bears the remaining negative voltage of the capacitor. This voltage helps the insulation of the fast mechanical switch to recover.

[0105] The two ends of the main branch are the ports of the DC circuit breaker.

[0106] t4~t5: The capacitors in the auxiliary converter module continue to discharge along the short-circuit current direction until the pre-charge capacitor voltage is discharged to 0V.

[0107] At time t5, the voltage of the pre-charge capacitor in the auxiliary converter module discharges from the preset value to 0V, and the bypass diode begins to conduct.

[0108] t5~t6: During this process, the power electronic switch continues to conduct, the port voltage of the DC circuit breaker is the conduction voltage drop of the power electronic switch, the distance between the moving and stationary contacts of the fast mechanical switch continues to increase and the insulation strength of its break gradually recovers until the insulation gap between its moving and stationary contacts is sufficient to withstand the transient overvoltage caused by the interruption fault current.

[0109] At time t6, the power electronic switch turns off the fault current, and the fault current is transferred to the MOV in the energy-consuming branch.

[0110] t6~t7: The MOV in the energy-consuming branch operates and consumes the inductive energy in the DC system. When the system current gradually decreases to tens of milliamperes, the thyristor turns off due to the increase in circuit impedance. Afterwards, according to the voltage equalization circuit equipped in the auxiliary converter module and power electronic switch, the DC system voltage will be distributed on the auxiliary converter module and power electronic switch according to the predetermined target.

[0111] The technical effects of this invention are as follows:

[0112] 1. This invention proposes a DC circuit breaker topology scheme suitable for medium-voltage DC systems, which overcomes many shortcomings of traditional hybrid DC circuit breakers. It has advantages such as low on-state loss, no need for water cooling equipment in the main and branch circuits, fast switching speed, high technical and economic efficiency, and small footprint, thus meeting the new adaptive requirements of medium-voltage DC systems for DC circuit breakers.

[0113] (1) A hybrid DC circuit breaker with a rated operating value of 80kV / 2kA and a maximum breaking current of 9kA has been designed using 4.5kV / 4kA press-fit IGBT devices. In its topology, a low-voltage power electronic switch is connected in series as an auxiliary switch on the main branch to promote commutation. However, since such auxiliary switches need to carry current for a long time, the topology of connecting the power electronic switch in series on the main branch has two drawbacks: first, the conduction loss of the power electronic switch is relatively large; second, water cooling equipment must be equipped to dissipate heat from the auxiliary switch.

[0114] The hybrid high-voltage DC circuit breaker with a rated capacity of 200kV / 2kA, the 500kV rectifier-type hybrid high-voltage DC circuit breaker with a breaking capacity of 25kA / 3ms, and the rectifier-type hybrid DC circuit breaker prototype with a capacity of 500kV / 3kA all continue the converter concept, connecting power electronic switches in series in the main branch as auxiliary converter switches to transfer fault current.

[0115] In the DC circuit breaker topology proposed in this invention, the main branch only contains fast mechanical switches, eliminating the need for series power electronic switches, resulting in low conduction losses and eliminating the need for water cooling equipment.

[0116] (2) Compared with mechanical DC circuit breakers, hybrid DC circuit breakers introduce power electronic switching devices, which makes their breaking speed faster. According to the "Common Technical Requirements for High Voltage DC Circuit Breakers for Flexible DC Transmission" (GBT38328-2019) and "NB / TNBT10441-2020 Hybrid High Voltage DC Circuit Breakers", hybrid DC circuit breakers need to interrupt the fault current within 3ms after receiving the breaking command. Meeting this standard can be defined as fast breaking speed.

[0117] (3) The most important advantage of this invention is its high technical and economic efficiency, which is reflected in two aspects: First, the auxiliary converter module can replace some of the fully controlled power electronic switching devices in the traditional topology scheme to bear the static DC system voltage, reducing the number of fully controlled power electronic switching devices in series, which not only reduces the cost, but also reduces the overvoltage level of the DC circuit breaker; Second, the main branch can use permanent magnet repulsion mechanism type fast mechanical switch instead of electromagnetic repulsion mechanism type fast mechanical switch, which not only saves costs, but also reduces the footprint.

[0118] Figure 7 This illustrates the topology of power electronic switches in a traditional hybrid DC circuit breaker. Explanation of symbols:

[0119] N is the number of fully controlled switching devices connected in series in the power electronic switch, L is the equivalent inductance of the DC system, and U DC For DC system voltage, i DC For DC system current, U res For the protection residual voltage of MOV (corresponding) Figure 8 U in RES ), U 1mA The reference voltage for the MOV is 1mA, V max The maximum withstand voltage of the power electronic switch, V DC R is the rated DC voltage that the power electronic switch can withstand. ap Let K be the charge rate of the MOV, and K be the residual voltage ratio of the MOV. The charge rate of the MOV is defined as: The residual pressure ratio of MOV is defined as:

[0120] Because hybrid DC circuit breakers need to suppress overvoltages generated during the interruption process and absorb the large amount of inductive energy in the DC system when interrupting fault currents, an MOV must be connected in parallel across its power electronic switch.

[0121] The DC system current equation during energy consumption can be described as follows:

[0122] In this process, only N·U needs to be satisfied. res >U DC At this time, the MOV can continuously consume the inductive energy in the DC system until the DC system current i DC Exceeding 0. Based on existing engineering experience, N·U res ≥1.5U DC At this time, the energy dissipation rate of the MOV can meet the overall requirements of the DC system for line fault clearing time. Since the overvoltage generated during the switching process cannot exceed the peak voltage that the power electronic switch can withstand, the aforementioned 1.5U... DC This determines the minimum number of fully controlled power electronic switching devices that can be connected in series.

[0123] In engineering applications, considering factors such as long-term aging of MOVs and their turn-off voltage overshoot, the charge rate R of existing MOVs is currently... ap The residual pressure ratio K(U) needs to be designed to be between 0.8 and 0.9. res / U 1mA The voltage needs to be designed between 1.6 and 1.9. This situation results in the rated DC voltage V that a single power electronic switching device in a hybrid DC circuit breaker must withstand. DC Approximately 50% of the devices have a maximum withstand voltage V. max For traditional hybrid DC circuit breaker topologies, under static withstand voltage conditions, the power electronic switches need to withstand the entire DC system voltage, i.e., N·V. DC =U DC As can be seen from the above explanation, the number of series-connected fully controlled power electronic switching devices required is redundant.

[0124] In summary, the transfer branch of a traditional hybrid DC circuit breaker is entirely composed of fully controlled power electronic switching devices connected in series. Due to the high redundancy of these devices, the technical and economic efficiency of the traditional topology is not ideal. The auxiliary converter module proposed in this invention can replace some of the fully controlled power electronic switching devices in the traditional topology to withstand the static DC system voltage. Therefore, the number of fully controlled power electronic switching devices required in the current solution is also reduced, improving the technical and economic efficiency of the hybrid DC circuit breaker.

[0125] 2. The auxiliary converter module can selectively discharge capacitors according to the current direction of the main branch, thereby quickly and reliably transferring the current of the main branch. This not only improves the commutation speed of the fault current in the main branch, but also reduces the speed requirement for the recovery of insulation strength after arc extinction of the fast mechanical switch, thus improving the overall reliability of the hybrid DC circuit breaker.

[0126] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple; relevant parts can be referred to the method section.

[0127] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A control method for a DC circuit breaker suitable for medium-voltage DC systems, characterized in that, The DC circuit breaker suitable for medium-voltage DC systems includes: a main branch, an auxiliary converter module, a bidirectional power electronic switch, and a power-consuming branch. The main branch is connected in series with the DC transmission line; The input terminal of the main branch is connected to the input terminal of the auxiliary converter module, and the output terminal of the main branch is connected to the output terminal of the bidirectional power electronic switch and the output terminal of the energy-consuming branch, respectively. The output of the auxiliary converter module is connected to the input of the bidirectional power electronic switch and the input of the energy-consuming branch, respectively. The control method includes: When the DC system is running normally, the DC circuit breaker is in the conducting state, controlling the main branch to close, the auxiliary converter module and the bidirectional power electronic switch are both turned off, and the energy-consuming branch does not operate; After a DC-side short-circuit fault occurs in the DC system, the main branch circuit is tripped, and both the auxiliary converter module and the bidirectional power electronic switch are turned on, causing the fault current to transfer from the main branch to the series branch formed by the auxiliary converter module and the bidirectional power electronic switch. Specifically, after a DC-side short-circuit fault occurs in the DC system line, the fault current in the DC system continues to rise until it reaches the current protection threshold, at which point a tripping command is issued to the DC circuit breaker. According to the tripping command, the moving and stationary contacts of the permanent magnet repulsion mechanism type fast mechanical switch in the main branch are separated, and the insulation distance between the moving and stationary contacts gradually increases. The short-circuit current in the current system sequentially sends trigger signals to the thyristors in the auxiliary converter module and the fully controlled power electronic switching devices in the bidirectional power electronic switch; the pre-charge capacitor in the auxiliary converter module and the permanent magnet repulsion mechanism type fast mechanical switch in the main branch form a closed loop and discharge; the permanent magnet repulsion mechanism type fast mechanical switch in the main branch extinguishes the arc after the current crosses zero, and the DC circuit breaker port bears the remaining negative voltage of the pre-charge capacitor; the pre-charge capacitor in the auxiliary converter module continues to discharge in the direction of the short-circuit current until the pre-charge capacitor voltage discharges to 0V, and the bypass diode in the auxiliary converter module conducts; Once the insulation strength of the main branch is restored, the bidirectional power electronic switch is turned off to transfer the fault current to the energy-consuming branch. At the same time, the energy-consuming branch is controlled to consume the inductive energy in the DC system. When the DC system current gradually decreases to the current threshold, the auxiliary converter module will automatically shut down.

2. The control method for a DC circuit breaker applicable to a medium-voltage DC system according to claim 1, characterized in that, The step of sending trigger signals sequentially to the thyristors in the auxiliary converter module and the fully controlled power electronic switching devices in the bidirectional power electronic switch, based on the direction of the short-circuit current in the DC system, specifically includes: Real-time monitoring of the direction of short-circuit current in the main branch; If the direction of the short-circuit current is positive, the first thyristor (2) and the fourth thyristor (5) are turned on, and the fully controlled power electronic switching device in the bidirectional power electronic switch is turned on after a preset time. If the short-circuit current is in the negative direction, the second thyristor (3) and the third thyristor (4) are turned on, and the fully controlled power electronic switching device in the bidirectional power electronic switch is turned on after a preset time delay.

3. The control method for a DC circuit breaker applicable to a medium-voltage DC system according to claim 2, characterized in that, When the DC system current gradually decays to the current threshold, the auxiliary converter module automatically shuts down, followed by: Based on the voltage equalization circuits equipped in the auxiliary converter module and the power electronic switch module, the DC system voltage is distributed on the auxiliary converter module and the power electronic switch module according to the preset target.

4. The control method for a DC circuit breaker applicable to a medium-voltage DC system according to claim 1, characterized in that, The main branch circuit includes: a permanent magnet repulsion mechanism type fast mechanical switch.

5. The control method for a DC circuit breaker applicable to a medium-voltage DC system according to claim 1, characterized in that, The auxiliary converter module includes: a first thyristor (2), a second thyristor (3), a third thyristor (4), a fourth thyristor (5), a pre-charge capacitor (6), and a bypass diode (7); The cathode of the first thyristor (2) is connected to the cathode of the third thyristor (4), the negative terminal of the pre-charge capacitor (6), and the anode of the bypass diode (7), respectively. The anode of the first thyristor (2) is connected to the cathode of the second thyristor (3). One point on the connection line between the anode of the first thyristor (2) and the cathode of the second thyristor (3) serves as the input terminal of the auxiliary converter module. The anode of the second thyristor (3) is connected to the anode of the fourth thyristor (5), the positive terminal of the pre-charge capacitor (6), and the cathode of the bypass diode (7), respectively. The anode of the third thyristor (4) is connected to the cathode of the fourth thyristor (5); a point on the connection line between the anode of the third thyristor (4) and the cathode of the fourth thyristor (5) serves as the output terminal of the auxiliary converter module.

6. The control method for a DC circuit breaker applicable to a medium-voltage DC system according to claim 5, characterized in that, The bidirectional power electronic switch includes: multiple power electronic switch modules connected in series; The power electronic switch module includes a first diode (8), a second diode (9), a third diode (10), a fourth diode (11), a fully controlled power electronic switch device (12), and an anti-parallel diode (13); The cathode of the first diode (8) is connected to the cathode of the third diode (10), the collector of the fully controlled power electronic switch device (12), and the cathode of the anti-parallel diode (13), respectively. The anode of the first diode (8) is connected to the cathode of the second diode (9). One point on the connection line between the anode of the first diode (8) and the cathode of the second diode (9) serves as the input terminal of the power electronic switch module. The anode of the second diode (9) is connected to the anode of the fourth diode (11), the emitter of the fully controlled power electronic switch device (12), and the anode of the anti-parallel diode (13), respectively. The anode of the third diode (10) is connected to the cathode of the fourth diode (11); a point on the connection line between the anode of the third diode (10) and the cathode of the fourth diode (11) serves as the output terminal of the power electronic switch module.

7. The control method for a DC circuit breaker applicable to a medium-voltage DC system according to claim 6, characterized in that, The first thyristor (2), the second thyristor (3), the third thyristor (4), the fourth thyristor (5), and the fully controlled power electronic switching device (12) are all connected in parallel with a buffer circuit, a protection circuit, and a voltage equalization circuit.

8. The control method for a DC circuit breaker applicable to a medium-voltage DC system according to claim 1, characterized in that, The energy-consuming branch includes: a metal oxide varistor.

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