Twelve-pulse current converter based on superconducting current limiter and control method of superconducting current limiter

Abstract

The invention discloses a twelve-pulse current converter based on a superconducting current limiter and a control method of the superconducting current limiter. The twelve-pulse current converter comprises a first current converter, a second current converter and a third current converter, wherein the first current converter comprises three first upper bridge arms and three first lower bridge arms; the second converter comprises three second upper bridge arms and three second lower bridge arms; the first transformer and the second transformer are connected with the three-phase alternating current bus; each first upper bridge arm and the corresponding first lower bridge arm are connected with the first transformer through one first superconducting current limiter; and each second upper bridge arm and the corresponding second lower bridge arm are connected with the second transformer through a first superconducting current limiter. The first superconducting current limiter comprises a bypass branch and a through-flow branch, and is used for responding to a control signal and selectively conducting one of the bypass branch and the through-flow branch; wherein the through-flow branch comprises a superconducting current limiting unit, and the superconducting current limiting unit is in a normal state under the condition that the actual current of the through-flow branch is greater than the critical current. According to the embodiment of the invention, the commutation failure suppression can be realized.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the priority of Chinese patent application 202311094766.6, filed on August 28, 2023, entitled “Scheme for suppressing commutation failure based on resistive superconducting current limiter”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application belongs to the technical field of power system protection, and in particular, relates to a twelve-pulse converter based on a superconducting current limiter and a control method of the superconducting current limiter. Background Art

[0004] Commutation failure is a common fault in HVDC transmission systems, mainly occurring in converters. A common method to suppress commutation failure is to adjust the converter station control strategy to adjust the converter station trigger angle or DC line current under fault conditions, thereby reducing the risk of commutation failure. Due to the limitations on the adjustment range and speed of the inverter angle, the above method has limited effect on suppressing commutation failure caused by severe short-circuit faults such as metallic grounding.

[0005] Another common method of suppressing commutation failure is to connect additional devices in series. Such devices are generally composed of a half-bridge / full-bridge circuit composed of capacitors or resistors and semi-controlled / fully-controlled power electronic devices, installed on the AC side of the inverter, and compensate the commutation voltage by switching capacitors or resistors when a fault occurs. However, this method cannot effectively take into account commutation failures caused by faults in the high-voltage direct current transmission system (including DC overcurrent caused by inter-pole coupling and converter valve failure), and the use of fully-controlled devices limits the line power. Therefore, in view of the defects of the above-mentioned existing commutation failure suppression methods, it is necessary to propose a new commutation failure suppression method. Summary of the invention

[0006] The embodiments of the present application provide a twelve-pulse converter based on a superconducting current limiter and a control method for the superconducting current limiter, aiming to improve the defects of the commutation failure suppression method in the related art.

[0007] In a first aspect, an embodiment of the present application provides a twelve-pulse converter based on a superconducting current limiter, comprising:

[0008] A first converter comprises three first upper bridge arms and three first lower bridge arms, each of the first upper bridge arms being connected to a positive electrode on a DC side;

[0009] The second converter comprises three second upper bridge arms and three second lower bridge arms, each second upper bridge arm is connected to each first lower bridge arm of the first converter, and each second lower bridge arm is connected to the negative pole of the DC side;

[0010] The first transformer and the second transformer are connected to the three-phase AC bus;

[0011] Six first superconducting current limiters, in the first converter, each first upper bridge arm and its corresponding first lower bridge arm are connected to an AC end of the first transformer through a first superconducting current limiter; in the second converter, each second upper bridge arm and its corresponding second lower bridge arm are connected to an AC end of the second transformer through a first superconducting current limiter.

[0012] The first superconducting current limiter includes a bypass branch and a through-current branch, and is used to select one of the bypass branch or the through-current branch to be turned on in response to a control signal; wherein the through-current branch includes a superconducting current limiting unit, and when the actual current of the through-current branch is greater than the critical current of the superconducting current limiting unit, the superconducting current limiting unit is in a normal state.

[0013] In some embodiments, the first superconducting current limiter includes a first superconducting current limiting unit, a first switching device, a second switching device, a third switching device, a fourth switching device, a first resistor, and a second resistor;

[0014] The first upper bridge arm and the first superconducting current limiter are connected at a first node, the first lower bridge arm and the first superconducting current limiter are connected at a second node, and the first transformer and the first superconducting current limiter are connected at a third node;

[0015] The first end of the first superconducting current limiting unit is connected to the first node, and the second end of the first superconducting current limiting unit is connected to the second node;

[0016] The first switch device and the first resistor are connected in series between the first node and the third node, and the second switch device and the second resistor are connected in series between the second node and the third node; wherein the first switch device is a unidirectional conductive device from the first node to the third node, and the fourth switch device is a unidirectional conductive device from the second node to the third node;

[0017] The third switch device is connected between the second node and the third node, and the fourth switch device is connected between the first node and the third node; wherein the third switch device is a unidirectional conductive device from the second node to the third node, and the fourth switch device is a unidirectional conductive device from the third node to the first node.

[0018] In some embodiments, the first switching device, the second switching device, the third switching device, and the fourth switching device are half-controlled switching devices.

[0019] In some embodiments, the twelve-pulse converter based on superconducting current limiter further comprises:

[0020] Two second superconducting current limiters are respectively connected to the DC side positive pole and the DC side negative pole; wherein each first upper bridge arm is connected to the DC side positive pole through one of the second superconducting current limiters, and each second lower bridge arm is connected to the DC side negative pole through another second superconducting current limiter.

[0021] In some embodiments, the second superconducting current limiter comprises:

[0022] The second superconducting current limiting unit and the fifth switch device connected in series are used to form a current branch when the fifth switch device is turned on;

[0023] The third resistor and the sixth switch device connected in series are used to form a bypass branch when the sixth switch device is turned on.

[0024] In some embodiments, the second superconducting current limiter further comprises:

[0025] a seventh switching device, connected in parallel with the fifth switching device;

[0026] The eighth switch device is connected in parallel with the sixth switch device; wherein the fifth switch device, the sixth switch device, the seventh switch device and the eighth switch device are unidirectional conductive devices, the conduction directions of the fifth switch device and the seventh switch device are opposite, and the conduction directions of the sixth switch device and the eighth switch device are opposite.

[0027] In some embodiments, the superconducting current limiting unit is composed of a plurality of superconducting coils connected in series and / or in parallel.

[0028] In some embodiments, the primary winding and the secondary winding of the first transformer are connected in a Y / Y manner, and the primary winding and the secondary winding of the second transformer are connected in a Y / Δ manner.

[0029] In a second aspect, an embodiment of the present application provides a control method for a superconducting current limiter, characterized in that the first superconducting current limiter of the twelve-pulse converter based on the superconducting current limiter according to the first aspect is applied, comprising:

[0030] When the current-passing branch is turned on, obtaining the voltage across the superconducting current-limiting unit;

[0031] When the voltage across the superconducting current limiting unit does not reach the first voltage threshold, the current branch is kept conducting;

[0032] When the voltage across the superconducting current limiting unit reaches a first voltage threshold, an abnormality detection is performed on the current-passing branch, and the conduction of the current-passing branch is switched to the conduction of the bypass branch in response to the abnormality detection result.

[0033] In some embodiments, performing abnormality detection on the flow branch and switching the flow branch to bypass branch in response to the abnormality detection result includes:

[0034] Obtaining the working current of the current-carrying branch;

[0035] When the current direction of the working current is opposite to the preset direction, the conduction of the through-current branch is switched to the conduction of the bypass branch;

[0036] When the voltage across the superconducting current limiting unit reaches a second voltage threshold, the conduction of the current branch is switched to the conduction of the bypass branch; wherein the second voltage threshold is an overheating voltage threshold of the superconducting current limiting unit, and the second voltage threshold is greater than the first voltage threshold;

[0037] When the first formula is not true, the conduction of the flow branch is switched to the conduction of the bypass branch; wherein the first formula is:

[0038]

[0039] Among them, γ min is the cut-off angle threshold, β is the trigger angle, i d is the DC current, u △f is the commutation voltage.

[0040] The twelve-pulse converter based on superconducting current limiter provided in the embodiment of the present application can control the first superconducting current limiter to switch between the flow state and the bypass state by setting a plurality of first superconducting current limiters on the first converter and the second converter of the twelve-pulse converter, so as to suppress the phase change failure. The first superconducting current limiter conducts the flow branch in the flow state, and the superconducting current limiting unit on the flow branch can quench when the current is greater than the critical current and enter the normal state, thereby showing a resistance characteristic. When a phase change failure occurs, the superconducting current limiting unit in the normal state of the non-target phase can limit the short-circuit current, so as to suppress the phase change failure. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solution of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0042] Figure 1 It is a schematic diagram of the module structure of a twelve-pulse converter based on a superconducting current limiter provided in one embodiment of the present application;

[0043] Figure 2 is a schematic diagram of the circuit structure of a first superconducting current limiter Sup1 provided in an embodiment of the present application;

[0044] Figure 3 is a schematic diagram of the module structure of a twelve-pulse converter based on a superconducting current limiter provided in another embodiment of the present application;

[0045] Figure 4 is a circuit structure diagram of a second superconducting current limiter Sup2 provided in an embodiment of the present application;

[0046] Figure 5 is a circuit structure diagram of a second superconducting current limiter Sup2 provided in another embodiment of the present application;

[0047] Figure 6 1 is a schematic diagram of the circuit structure of a twelve-pulse converter based on a superconducting current limiter provided in one embodiment of the present application;

[0048] Figure 7 is a current schematic diagram of a twelve-pulse converter based on a superconducting current limiter provided in an embodiment of the present application when the current branch is in a conducting state;

[0049] Figure 8 is a current schematic diagram of a twelve-pulse converter based on a superconducting current limiter provided in an embodiment of the present application when the bypass branch is in a conducting state;

[0050] Fig. 9 It is a schematic diagram of the circuit structure of the CIGRE Benchmark standard model provided in one embodiment of the present application.

[0051] Fig.10 It is a flow chart of a control method of a superconducting current limiter provided in one embodiment of the present application.

[0052] In the attached figure:

[0053] 10, first converter; 20, second converter; Tf1, first transformer; Tf2, second transformer; Sup1, first superconducting current limiter; Sup2, second superconducting current limiter; UBri1, first upper bridge arm; DBri1, first lower bridge arm; UBri2, second upper bridge arm; DBri2, second lower bridge arm. DETAILED DESCRIPTION

[0054] The features and exemplary embodiments of various aspects of the present application will be described in detail below. In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present application, rather than to limit the present application. For those skilled in the art, the present application can be implemented without the need for some of these specific details. The following description of the embodiments is only to provide a better understanding of the present application by illustrating the examples of the present application.

[0055] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the statement "include..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0056] Commutation failure is a common fault in HVDC transmission systems, mainly occurring in converters. A common method to suppress commutation failure is to adjust the converter station control strategy to adjust the converter station trigger angle or DC line current under fault conditions, thereby reducing the risk of commutation failure. Due to the limitations on the adjustment range and speed of the inverter angle, the above method has limited effect on suppressing commutation failure caused by severe short-circuit faults such as metallic grounding.

[0057] Another common method of suppressing commutation failure is to connect additional devices in series. Such devices are generally composed of a half-bridge / full-bridge circuit composed of capacitors or resistors and semi-controlled / fully-controlled power electronic devices, installed on the AC side of the inverter, and compensate the commutation voltage by switching capacitors or resistors when a fault occurs. However, this method cannot effectively take into account commutation failures caused by faults in the high-voltage direct current transmission system (including DC overcurrent caused by inter-pole coupling and converter valve failure), and the use of fully-controlled devices limits the line power. Therefore, in view of the defects of the above-mentioned existing commutation failure suppression methods, it is necessary to propose a new commutation failure suppression method.

[0058] In order to solve the above technical problems, the applicant has noticed the superconducting current limiter. The superconducting current limiter is one of the most effective devices for limiting the short-circuit current of the power grid, and has attracted people's attention due to its good ability to suppress line overcurrent. Utilizing the superconducting characteristics of superconductors, the impedance of the superconducting current limiter is almost zero during normal operation. When a fault occurs, the impedance automatically increases, quickly limiting the short-circuit current to a level acceptable to the circuit breaker or the system. Compared with capacitors, superconducting current limiters can be connected in series on both the AC and DC sides at the same time, playing a role in slowing down the voltage drop on the AC side and limiting the short-circuit current on the DC side; compared with resistors, superconducting current limiters can remain connected in series in the line during steady-state operation, and do not require full-control device control input.

[0059] In addition, the superconducting current limiter is not limited by the inverter angle adjustment range and speed, and can respond quickly to severe short-circuit faults. Moreover, the response speed of the superconducting current limiter is in the millisecond level. The above characteristics make the superconducting current limiter an ideal auxiliary device for suppressing commutation failure.

[0060] The embodiment of the present application provides a twelve-pulse converter based on a superconducting current limiter and a control method of the superconducting current limiter. The twelve-pulse converter based on a superconducting current limiter provided in the embodiment of the present application is first introduced below.

[0061] Figure 1 The module structure diagram of a twelve-pulse converter based on superconducting current limiter provided by an embodiment of the present application is shown. The twelve-pulse converter based on superconducting current limiter comprises a first converter 10, a second converter 20, a first transformer Tf1, a second transformer Tf2 and six first superconducting current limiters Sup1.

[0062] The first converter 10 includes three first upper bridge arms UBri1 and three first lower bridge arms DBri1, and each first upper bridge arm UBri1 is connected to the positive pole of the DC side. The second converter 20 includes three second upper bridge arms UBri2 and three second lower bridge arms DBri2, and each second upper bridge arm UBri2 is connected to the same node as the three first lower bridge arms DBri1 of the first converter 10, and each second lower bridge arm DBri2 is connected to the negative pole of the DC side. That is, the three first lower bridge arms DBri1 of the first converter 10 and the three second upper bridge arms UBri2 of the second converter 20 are connected to the same node.

[0063] The first transformer Tf1 and the second transformer Tf2 may be connected to the inverter-side AC bus.

[0064] Three of the six first superconducting current limiters Sup1 are connected to the first converter 10 , and the other three first superconducting current limiters Sup1 are connected to the second converter 20 .

[0065] In the first converter 10, three bridge arm groups are included, and each bridge arm group is composed of a first upper bridge arm UBri1 and its corresponding first lower bridge arm DBri1. Each first upper bridge arm UBri1 and its corresponding first lower bridge arm DBri1 are connected to an AC end of the first transformer Tf1 through a first superconducting current limiter Sup1. That is, the three bridge arm groups of the first converter 10 are connected to the three AC ends a, b, and c of the first transformer Tf1 through three first superconducting current limiters Sup1, respectively.

[0066] Similarly, the second converter 20 also includes three bridge arm groups, each bridge arm group consists of a second upper bridge arm UBri2 and its corresponding second lower bridge arm DBri2. Each second upper bridge arm UBri2 and its corresponding second lower bridge arm DBri2 are connected to an AC end of the second transformer Tf2 through a first superconducting current limiter Sup1.

[0067] The first superconducting current limiter Sup1 includes a bypass branch and a through-current branch. The first superconducting current limiter Sup1 can select one of the bypass branch and the through-current branch to be turned on in response to corresponding control signals. The through-current branch includes a superconducting current limiting unit, and the superconducting current limiting unit can be in a normal state when the actual current of the through-current branch is greater than the critical current of the superconducting current limiting unit.

[0068] When the superconducting current limiting unit is in the superconducting state during operation, it exhibits zero resistance characteristics externally; when the current in the line where the superconducting current limiting unit is located exceeds the critical current of the superconducting current limiting unit, the superconducting current limiting unit loses superconductivity and becomes normal, at which time it exhibits resistance characteristics externally. When the superconducting current limiting unit is in the normal state, the equivalent resistance can limit the short-circuit current, avoid excessive current, and effectively suppress commutation failure.

[0069] Take the first superconducting current limiter Sup1 connected to one of the bridge arm groups of the first converter 10 as an example. Before the commutation failure occurs, the first superconducting current limiter Sup1 can respond to the control signal and select the flow branch to conduct, so that the first superconducting current limiter Sup1 maintains the flow state. At this time, the superconducting current limiting unit is in a superconducting state in a liquid nitrogen environment, showing a zero resistance characteristic.

[0070] When a commutation failure occurs, since the loop current exceeds the critical current of the superconducting current limiting unit, the superconducting current limiting unit loses its superconducting current and is in a normal state, showing resistance characteristics to the outside. At this time, the controller can determine that the faulty phase of the commutation failure is the target phase, and control the first superconducting current limiter Sup1 through a control signal to select the bypass branch to conduct, and bypass the superconducting current limiting unit. At this time, the first superconducting current limiter Sup1 is in a bypass state.

[0071] After the superconducting current limiting unit is quickly bypassed, the resistance of the superconducting current limiting unit is small, and the time to recover from the normal state of the quench to the superconducting state is short. After determining that the superconducting current limiting unit has completed the quench recovery, the controller can control the first superconducting current limiter Sup1 to reselect the flow branch to conduct through the control signal, so that the first superconducting current limiter Sup1 maintains the flow state.

[0072] Since the fault phase current is still greater than the critical current of the superconducting current limiting unit, the superconducting current limiting unit quenches again. At this time, the controller can determine that the phase is not the target phase, and control the first superconducting current limiter Sup1 to maintain the conduction of the current branch, that is, the first superconducting current limiter Sup1 maintains the current state. When the current branch is turned on, the superconducting current limiting unit quenches, and the resistance characteristics exhibited in the normal state can limit the short-circuit current, thereby suppressing the commutation failure.

[0073] When the faulty phase becomes the target phase again, the first superconducting current limiter Sup1 is still in the current-passing state, and the superconducting current limiting unit is quenched. At this time, the controller can control the first superconducting current limiter Sup1 through a control signal to select the bypass branch to be turned on, bypass the superconducting current limiting unit, and switch back to the current-passing state after detecting that the superconducting current limiting unit has completed quench recovery.

[0074] In this embodiment, by setting a plurality of first superconducting current limiters Sup1 on the first converter 10 and the second converter 20 of the twelve-pulse converter, the first superconducting current limiter Sup1 can be controlled to switch between the flow state and the bypass state to achieve the suppression of commutation failure. The first superconducting current limiter Sup1 conducts the flow branch in the flow state, and the superconducting current limiting unit on the flow branch can quench when the current is greater than the critical current and enter the normal state, thereby showing resistance characteristics. When a commutation failure occurs, the superconducting current limiting unit in the normal state of the non-target phase can limit the short-circuit current to achieve the suppression of commutation failure.

[0075] Please refer to Figure 2 In some embodiments, the first superconducting current limiter Sup1 may include a first superconducting current limiting unit SupCoil1, a first switching device T1, a second switching device T2, a third switching device T3, a fourth switching device T4, a first resistor R1 and a second resistor R2.

[0076] The first upper bridge arm UBri1 and the first superconducting current limiter Sup1 are connected to a first node N1, the first lower bridge arm DBri1 and the first superconducting current limiter Sup1 are connected to a second node N2, and the first transformer Tf1 and the first superconducting current limiter Sup1 are connected to a third node N3.

[0077] A first end of the first superconducting current limiting unit SupCoil1 is connected to the first node N1, and a second end of the first superconducting current limiting unit SupCoil1 is connected to the second node N2.

[0078] The first switch device T1 and the first resistor R1 are connected in series between the first node N1 and the third node N3, and the second switch device T2 and the second resistor R2 are connected in series between the second node N2 and the third node N3. The first switch device T1 is a unidirectional conductive device from the third node N3 to the first node N1, and the second switch device T2 is a unidirectional conductive device from the second node N2 to the third node N3.

[0079] The third switch device T3 is connected between the second node N2 and the third node N3, and the fourth switch device T4 is connected between the first node N1 and the third node N3. The third switch device T3 is a unidirectional conduction device from the second node N2 to the third node N3, and the fourth switch device T4 is a unidirectional conduction device from the third node N3 to the first node N1.

[0080] The first superconducting current limiter Sup1 is equivalent to comprising two through-current branches and two bypass branches corresponding to the first upper bridge arm UBri1 and the first lower bridge arm DBri1 respectively.

[0081] The first switch device T1 and the first resistor R1 form a bypass branch corresponding to the first upper bridge arm UBri1, and the second switch device T2 and the second resistor R2 form a bypass branch corresponding to the first lower bridge arm DBri1. The first superconducting current limiting unit SupCoil1 and the third switch device T3 form a flow branch corresponding to the first upper bridge arm UBri1, and the first superconducting current limiting unit SupCoil1 and the fourth switch device T4 form a flow branch corresponding to the first lower bridge arm DBri1.

[0082] like Figure 7 and Figure 8 As shown, Figure 7 shows a schematic diagram of the loop current when the current branch of the first superconducting current limiter Sup1 is turned on, Figure 8 A schematic diagram of loop current when the bypass branch of the first superconducting current limiter Sup1 is turned on is shown.

[0083] In some embodiments, the first switch device T1, the second switch device T2, the third switch device T3 and the fourth switch device T4 are half-controlled switch devices, wherein the half-controlled switch devices may be thyristors and their corresponding derivative devices.

[0084] Please refer to Figure 3 In some embodiments, the twelve-pulse converter based on superconducting current limiter may further include two second superconducting current limiters Sup2.

[0085] The two second superconducting current limiters Sup2 can be connected to the DC positive electrode and the DC negative electrode respectively. In the first converter 10, each first upper bridge arm UBri1 is connected to the DC positive electrode through one of the second superconducting current limiters Sup2. In the second converter 20, each second lower bridge arm DBri2 is connected to the DC negative electrode through another second superconducting current limiter Sup2.

[0086] Please refer to Figure 4 In some embodiments, the second superconducting current limiter Sup2 may include a second superconducting current limiting unit SupCoil2, a fifth switching device T5, a third resistor R3 and a sixth switching device T6.

[0087] The second superconducting current limiting unit SupCoil2 and the fifth switch device T5 are connected in series. When the fifth switch device T5 is turned on, the second superconducting current limiting unit SupCoil2 and the fifth switch device T5 can form a current branch to connect the two ends of the second superconducting current limiter Sup2.

[0088] The third resistor R3 is connected in series with the sixth switch device T6. When the sixth switch device T6 is turned on, the third resistor R3 and the sixth switch device T6 can form a bypass branch to connect the two ends of the second superconducting current limiter Sup2.

[0089] It can be understood that, in the above-mentioned second superconducting current limiter Sup2, the control signal can drive the fifth switch device T5 or the sixth switch device T6 to turn on. Without taking into account the switching time of the switch device, at most one of the fifth switch device T5 and the sixth switch device T6 is in the on state. That is, at any time, when the second superconducting current limiter Sup2 operates normally, only one of the through-current branch and the bypass branch is in the on state.

[0090] Please refer to Figure 5 In some embodiments, the second superconducting current limiter Sup2 may further include a seventh switching device T7 and an eighth switching device T8.

[0091] The seventh switching device T7 is connected in parallel with the fifth switching device T5 , and the eighth switching device T8 is connected in parallel with the sixth switching device T6 .

[0092] It should be noted that the eighth switch device T8 and the sixth switch device T6 are connected in parallel, which means that the sixth switch device T6 and the third resistor R3 are connected in series, and the eighth switch device T8 and the fourth resistor R4 are connected in series and then connected in parallel.

[0093] The fifth switching device T5, the sixth switching device T6, the seventh switching device T7 and the eighth switching device T8 are unidirectional conductive devices. The conduction directions of the fifth switching device T5 and the seventh switching device T7 are opposite, and the conduction directions of the sixth switching device T6 and the eighth switching device T8 are opposite.

[0094] In the above embodiment, the second superconducting current limiter Sup2 can only realize unidirectional flow of current when the seventh switch device T7 and the eighth switch device T8 are not set. Therefore, the second superconducting current limiter Sup2 can only be set in a line where the current direction remains unchanged, that is, similar to the first superconducting current limiter Sup1, connected to the bridge arm of the converter. After the seventh switch device T7 and the eighth switch device T8 are set, the second superconducting current limiter Sup2 can realize bidirectional flow of current. For example, when the second superconducting current limiter Sup2 is in the flow state, the fifth switch device T5 and the seventh switch device T7 are turned on. At this time, if the current direction is the unidirectional conduction direction of the fifth switch device T5, the current flows through the fifth switch device T5. On the contrary, if the current direction is the unidirectional conduction direction of the seventh switch device T7, the current flows through the seventh switch device T7. Correspondingly, when the second superconducting current limiter Sup2 is in bypass state, the sixth switching device T6 and the eighth switching device T8 are turned on, and whether the current flows through the sixth switching device T6 or the eighth switching device T8 depends on the current direction in the line where the second superconducting current limiter Sup2 is located.

[0095] In some embodiments, the superconducting current limiting unit may be composed of multiple superconducting coils connected in series or in parallel. After the multiple superconducting coils form the superconducting current limiting unit, it is necessary to ensure that the superconducting current limiting unit does not lose supersonic power when the system is in steady state operation, and that the resistance of the superconducting current limiting unit is large enough to suppress the short-circuit current when a system failure occurs.

[0096] In some embodiments, the primary winding and the secondary winding of the first transformer Tf1 are connected in a Y / Y manner, and the primary winding and the secondary winding of the second transformer Tf2 are connected in a Y / Δ manner.

[0097] Please refer to Figure 6 , Figure 6 A schematic diagram of the circuit structure of a twelve-pulse converter based on a superconducting current limiter in one embodiment is shown.

[0098] like Figure 6 As shown, VT4_YOUT, VT6_YOUT, and VT2_YOUT are respectively the three first upper bridge arms Ubri1 of the first converter 10 , and VT1_YOUT, VT3_YOUT, and VT5_YOUT are respectively the three first lower bridge arms Dbri1 of the first converter 10 .

[0099] Similarly, VT4_DOUT, VT6_DOUT, and VT2_DOUT are respectively the three second upper bridge arms Ubri2 of the second converter 20 , and VT1_DOUT, VT3_DOUT, and VT5_DOUT are respectively the three second lower bridge arms Dbri2 of the second converter 20 .

[0100] SupBri_Yc, VT4_YIN, VT1_YIN are the first superconducting current limiters Sup1 connected to one of the bridge arm groups, SupBri_Yb, VT6_YIN, VT3_YIN are the first superconducting current limiters Sup1 connected to another bridge arm group, and SupBri_Ya, VT2_YIN, VT5_YIN are the first superconducting current limiters Sup1 connected to the third bridge arm group.

[0101] Taking the first superconducting current limiter Sup1 formed by SupBri_Yc, VT4_YIN, and VT1_YIN as an example, VT4_YIN is Figure 2 The first switch device T1, VT1_YIN is Figure 2 The second switch device T2 in SupBri_Yc includes Figure 2 The remaining switching devices.

[0102] Similarly, in the second converter 20, SupBri_Dc, VT4_DIN, VT1_DIN are the first superconducting current limiters Sup1 connected to one of the bridge arm groups, SupBri_Db, VT6_DIN, VT3_DIN are the first superconducting current limiters Sup1 connected to another bridge arm group, and SupBri_Da, VT2_DIN, VT5_DIN are the first superconducting current limiters Sup1 connected to the third bridge arm group.

[0103] As an optional implementation, Figure 6 As shown, SupDC_Y and SupDC_D are two second superconducting current limiters Sup2 connected to the positive pole of the DC side and the negative pole of the DC side respectively.

[0104] Please refer to Figures 6 to 8 , Figure 7 and Figure 8 Also shown are schematic diagrams of loop currents corresponding to the second superconducting current limiter Sup2 when the through-current branch is turned on and the bypass branch is turned on.

[0105] Please refer to Fig. 9 , Fig. 9 A schematic diagram of the circuit structure of a CIGREBenchmark standard model is shown. The shaded part indicates that the two converters are converters after adding the first superconducting current limiter Sup1 using the node configuration method. Compared with the converter without the first superconducting current limiter Sup1, the commutation failure generated by the improved converter during operation is significantly reduced. That is, in the above embodiment, by setting the first superconducting current limiter Sup1, the commutation failure of the AC / DC hybrid power grid can be effectively suppressed.

[0106] As another optional implementation, in a six-pulse converter, three first superconducting current limiters Sup1 may be provided accordingly. In addition, a second superconducting current limiter Sup2 may be connected in series at the positive and negative electrodes of the DC side, respectively. That is, a twelve-pulse converter may include six first superconducting current limiters Sup1 and two second superconducting current limiters Sup2, and a six-pulse converter may include three first superconducting current limiters Sup1 and two second superconducting current limiters Sup2.

[0107] The present application also provides a control method for a superconducting current limiter, which is applied to the first superconducting current limiter in the above embodiment, such as Fig.10 As shown, the control method of the superconducting current limiter includes:

[0108] S110, when the current branch is turned on, obtaining the voltage across the superconducting current limiting unit;

[0109] S120, when the voltage across the superconducting current limiting unit does not reach the first voltage threshold, maintaining the current branch circuit conductive;

[0110] S130, when the voltage across the superconducting current limiting unit reaches a first voltage threshold, performing abnormality detection on the current-passing branch, and switching the current-passing branch to the bypass branch in response to the abnormality detection result.

[0111] In this embodiment, the controller can determine whether the superconducting current limiting unit in a single first superconducting current limiter is quenched according to whether the voltage across the superconducting current limiting unit in the single first superconducting current limiter reaches the first voltage threshold. If the superconducting current limiting unit is in a superconducting state, the conduction of the current branch can continue to be maintained; if the superconducting current limiting unit is quenched, it is necessary to perform an abnormality detection on the current branch, and according to the abnormality detection result, the first superconducting current limiter is switched from the conduction of the current branch to the conduction of the bypass branch when an abnormality exists.

[0112] In S110, taking the controller controlling a single first superconducting current limiter as an example, when the current-passing branch of the first superconducting current limiter is turned on, the controller may obtain the voltage across both ends of the superconducting current limiting unit.

[0113] It should be noted that the superconducting current limiting unit does not exhibit resistance characteristics in the superconducting state, and the voltage difference between the two ends of the superconducting current limiting unit is approximately equal to zero. If the superconducting current limiting unit is quenched under overcurrent, the superconducting current limiting unit will show resistance characteristics to the outside, and the voltage between the two ends will increase.

[0114] In S120, the controller can determine the state of the superconducting current limiting unit by comparing the voltage across the superconducting current limiting unit with the first voltage threshold. That is, when the voltage across the superconducting current limiting unit does not reach the first voltage threshold, it can be determined that the superconducting current limiting unit is still in the superconducting state; and when the voltage across the superconducting current limiting unit reaches the first voltage threshold, it can be determined that the superconducting current limiting unit is quenched and is in a normal state.

[0115] When the superconducting current limiting unit is in the superconducting state, it means that the first superconducting current limiter in the converter is in the current-passing state, and the current in the line does not over-current, that is, no commutation failure occurs at this time, and the controller can continue to control the first superconducting current limiter to maintain the current-passing branch conductive.

[0116] In S130, when the superconducting current limiting unit is in a quenched state and in a normal state, it means that the first superconducting current limiter in the converter is in a current-passing state, and the current of the line where it is located is too large, resulting in the superconducting current limiting unit being quenched. At this time, the current-passing branch can be abnormally detected, and the conducting branch can be switched from the current-passing branch to the bypass branch according to the abnormal detection result.

[0117] As an optional implementation manner, the above S130 may include:

[0118] S210, obtaining the working current of the current-carrying branch;

[0119] S220, when the current direction of the working current is opposite to the preset direction, switching the conduction of the current-passing branch to the conduction of the bypass branch;

[0120] S230, when the voltage across the superconducting current limiting unit reaches a second voltage threshold, switching the conduction of the current branch to the conduction of the bypass branch; wherein the second voltage threshold is an overheating voltage threshold of the superconducting current limiting unit, and the second voltage threshold is greater than the first voltage threshold;

[0121] S240, when the first formula is not true, the conduction of the flow branch is switched to the conduction of the bypass branch; wherein the first formula is:

[0122]

[0123] Among them, γ min is the cut-off angle threshold, β is the trigger angle, i d is the DC current, u △f is the commutation voltage.

[0124] In this embodiment, when the superconducting current limiting unit loses its time limit, the method for performing abnormal detection on the current branch may be to perform abnormal detection on the current direction, to perform abnormal detection on whether the superconducting current limiting unit is overheated, or to perform abnormal detection on whether the parameter formula is established based on the electrical parameters of the line.

[0125] In S210, when the superconducting current limiting unit is in a quenched state and in a normal state, an abnormality detection method for the current branch may be to obtain the working current of the current branch and confirm whether an abnormality occurs according to the current direction of the working current.

[0126] In S220, after obtaining the working current, the current direction of the working current can be compared with the preset direction. If the current direction of the working current is opposite to the preset direction, it means that an abnormality has occurred in the current branch. At this time, the current branch needs to be switched to the bypass branch.

[0127] Correspondingly, if the current direction of the working current is the same as the preset direction, it means that there is no abnormality in the current direction of the working current.

[0128] In S230, the controller may also obtain a voltage across the superconducting current limiting unit and compare the voltage across the superconducting current limiting unit with a second voltage threshold, where the second voltage threshold is an overheating voltage threshold of the superconducting current limiting unit and is greater than the first voltage threshold.

[0129] If the voltage at both ends reaches the second threshold, it indicates that the superconducting current limiting unit may be overheated and needs to be switched from the conduction branch to the conduction branch.

[0130] In S240, the controller may also obtain the Echizen trigger angle, the DC current of the line, and the commutation voltage, and substitute these parameters into the first formula. If the first formula is not true, it may be determined that an abnormality has occurred in the flow branch, and the flow branch conduction is switched to the bypass branch conduction. The first formula may be:

[0131]

[0132] Among them, γ min is the cut-off angle threshold, β is the trigger angle, i d is the DC current, u △f is the commutation voltage.

[0133] The functional blocks shown in the above-described block diagram can be implemented as hardware, software, firmware or a combination thereof. When implemented in hardware, it can be, for example, an electronic circuit, an application specific integrated circuit (ASIC), appropriate firmware, a plug-in, a function card, etc. When implemented in software, the elements of the present application are programs or code segments that are used to perform the required tasks. The program or code segment can be stored in a machine-readable medium, or transmitted on a transmission medium or a communication link by a data signal carried in a carrier wave. "Machine-readable medium" can include any medium capable of storing or transmitting information. Examples of machine-readable media include electronic circuits, semiconductor memory devices, ROM, flash memory, erasable ROM (EROM), floppy disks, CD-ROMs, optical disks, hard disks, optical fiber media, radio frequency (RF) links, etc. The code segment can be downloaded via a computer network such as the Internet, an intranet, etc.

[0134] It should be noted that, in this article, the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0135] This article uses specific examples to illustrate the principles and implementation methods of this application. The above examples are only used to help understand the methods and core ideas of this application. The above are only preferred implementation methods of this application. It should be pointed out that due to the limitations of textual expression and the objective existence of infinite specific structures, ordinary technicians in this technical field can make several improvements, modifications or changes without departing from the principles of this application, and can also combine the above technical features in an appropriate manner; these improvements, modifications, changes or combinations, or the direct application of the concepts and technical solutions of this application to other occasions without improvement, should be regarded as the scope of protection of this application.

Claims

1. A twelve-pulse converter based on a superconducting current limiter, characterized in that: include: A first converter, comprising three first upper bridge arms and three first lower bridge arms, each of the first upper bridge arms being connected to a positive electrode on a DC side; A second converter, comprising three second upper bridge arms and three second lower bridge arms, each of the second upper bridge arms is connected to each of the first lower bridge arms of the first converter, and each of the second lower bridge arms is connected to a negative electrode of a DC side; The first transformer and the second transformer are connected to the three-phase AC bus; Six first superconducting current limiters, in the first converter, each of the first upper bridge arm and its corresponding first lower bridge arm are connected to an AC end of the first transformer through a first superconducting current limiter; in the second converter, each of the second upper bridge arm and its corresponding second lower bridge arm are connected to an AC end of the second transformer through a first superconducting current limiter; The first superconducting current limiter includes a bypass branch and a through-current branch, and is used to select one of the bypass branch or the through-current branch to be turned on in response to a control signal; wherein the through-current branch includes a superconducting current limiting unit, and when the actual current of the through-current branch is greater than the critical current of the superconducting current limiting unit, the superconducting current limiting unit is in a normal state.

2. The twelve-pulse converter based on superconducting current limiter according to claim 1, characterized in that: The first superconducting current limiter includes a first superconducting current limiting unit, a first switching device, a second switching device, a third switching device, a fourth switching device, a first resistor and a second resistor; The first upper bridge arm and the first superconducting current limiter are connected at a first node, the first lower bridge arm and the first superconducting current limiter are connected at a second node, and the first transformer and the first superconducting current limiter are connected at a third node; The first end of the first superconducting current limiting unit is connected to the first node, and the second end of the first superconducting current limiting unit is connected to the second node; The first switch device and the first resistor are connected in series between the first node and the third node, and the second switch device and the second resistor are connected in series between the second node and the third node; wherein the first switch device is a unidirectional conductive device from the first node to the third node, and the fourth switch device is a unidirectional conductive device from the second node to the third node; The third switching device is connected between the second node and the third node, and the fourth switching device is connected between the first node and the third node; wherein the third switching device is a unidirectional conductive device from the second node to the third node, and the fourth switching device is a unidirectional conductive device from the third node to the first node.

3. The twelve-pulse converter based on superconducting current limiter according to claim 2, characterized in that: The first switching device, the second switching device, the third switching device and the fourth switching device are half-controlled switching devices.

4. The twelve-pulse converter based on superconducting current limiter according to claim 1, characterized in that: The twelve-pulse converter based on superconducting current limiter also includes: Two second superconducting current limiters are respectively connected to the DC side positive electrode and the DC side negative electrode; wherein each of the first upper bridge arms is connected to the DC side positive electrode through one of the second superconducting current limiters, and each of the second lower bridge arms is connected to the DC side negative electrode through another second superconducting current limiter.

5. The twelve-pulse converter based on superconducting current limiter according to claim 4, characterized in that: The second superconducting current limiter comprises: A second superconducting current limiting unit and a fifth switch device connected in series, used to form a current branch when the fifth switch device is turned on; The third resistor and the sixth switch device connected in series are used to form a bypass branch when the sixth switch device is turned on.

6. The twelve-pulse converter based on superconducting current limiter according to claim 5, characterized in that: The second superconducting current limiter also includes: a seventh switching device, connected in parallel with the fifth switching device; An eighth switching device is connected in parallel with the sixth switching device; wherein the fifth switching device, the sixth switching device, the seventh switching device and the eighth switching device are unidirectional conducting devices, the conduction directions of the fifth switching device and the seventh switching device are opposite, and the conduction directions of the sixth switching device and the eighth switching device are opposite.

7. The twelve-pulse converter based on a superconducting current limiter according to any one of claims 1 to 6, characterized in that: The superconducting current limiting unit is composed of a plurality of superconducting coils connected in series and / or in parallel.

8. The twelve-pulse converter based on a superconducting current limiter according to any one of claims 1 to 6, characterized in that: The primary winding and the secondary winding of the first transformer are connected in a Y / Y manner, and the primary winding and the secondary winding of the second transformer are connected in a Y / Δ manner.

9. A control method for a superconducting current limiter, characterized in that: The first superconducting current limiter applied to any one of claims 1 to 8 comprises: When the current-passing branch is turned on, obtaining the voltage across the superconducting current-limiting unit; When the voltage across the superconducting current limiting unit does not reach a first voltage threshold, maintaining the conduction of the current branch; When the voltage across the superconducting current limiting unit reaches a first voltage threshold, an abnormality detection is performed on the current-passing branch, and the current-passing branch is switched to the bypass branch in response to the abnormality detection result.

10. The control method of the superconducting current limiter according to claim 9, characterized in that: The performing abnormality detection on the flow branch and switching the flow branch conduction to bypass branch conduction in response to the abnormality detection result comprises: Obtaining the operating current of the current-carrying branch; When the current direction of the working current is opposite to the preset direction, switching the conduction of the through-current branch to the conduction of the bypass branch; When the voltage across the superconducting current limiting unit reaches a second voltage threshold, the conduction of the current branch is switched to the conduction of the bypass branch; wherein the second voltage threshold is an overheating voltage threshold of the superconducting current limiting unit, and the second voltage threshold is greater than the first voltage threshold; When the first formula is not true, the conduction of the through-current branch is switched to the conduction of the bypass branch; wherein the first formula is: Among them, γ min is the cut-off angle threshold, β is the trigger angle, i d is the DC current, u △f is the commutation voltage.

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