A half-bridge MMC fault current suppression method and system

Abstract

The present application discloses a half-bridge MMC fault current suppression method and system, the method comprising: obtaining basic parameters of an MMC converter station to be suppressed, and calculating the fault current value before the MMC converter station is locked according to the basic parameters; after a fault occurs in the MMC converter station and before it is locked, selecting a current limiting reactor to suppress the fault current according to the fault current value, and selecting different virtual impedance controls to suppress the fault current according to the control mode type of the MMC converter station, wherein the virtual impedance control comprises: a constant voltage control mode and a constant power control mode; after the MMC converter station is locked, limiting the fault current by a superconducting current limiter until the fault current is less than the maximum breaking capacity of a DC circuit breaker, and then suppressing the fault current by a DC circuit breaker, thereby solving the technical problem of the prior art that the half-bridge MMC short-circuit fault current suppression capability is poor.

Description

Technical Field

[0001] The present application relates to the field of electric power technology, and in particular to a half-bridge MMC fault current suppression method and system. Background Art

[0002] Compared with AC distribution network, DC distribution network can realize the efficient and flexible access of distributed new energy, DC load and variable frequency load, and significantly improve the flexibility of operation control on the distribution side. Compared with AC grid fault, DC distribution network has small damping and fast fault current rise speed. In addition, since DC has no natural zero crossing point, higher requirements are put forward for the protection technology of DC distribution network.

[0003] In order to ensure the safety of key equipment in the DC distribution network, it is necessary to suppress the rapid rise of fault current in the DC distribution network. At present, the methods for suppressing fault current in the DC distribution network are mainly limited to primary current limiting devices such as current limiting reactors and superconducting current limiters, or reducing fault current by increasing virtual resistance through control means. However, there is a lack of fault current suppression methods for the fault characteristics before and after unlocking and locking of the half-bridge MMC converter station, which makes the short-circuit fault current suppression ability of the half-bridge MMC poor. Summary of the invention

[0004] The present application provides a half-bridge MMC fault current suppression method and system, which are used to solve the technical problem that the prior art has poor half-bridge MMC short-circuit fault current suppression capability.

[0005] In view of this, the first aspect of the present application provides a half-bridge MMC fault current suppression method, the method comprising:

[0006] Obtaining basic parameters of the MMC converter station to be suppressed, and calculating the fault current value before the MMC converter station is locked according to the basic parameters;

[0007] After a fault occurs in the MMC converter station and before it is locked, a current limiting reactor is selected according to the fault current value to suppress the fault current, and different virtual impedance controls are selected according to the control mode type of the MMC converter station to suppress the fault current;

[0008] After the MMC converter station is locked, the fault current is limited by the superconducting current limiter until the fault current is less than the maximum breaking capacity of the DC circuit breaker, and then the fault current is suppressed by the DC circuit breaker.

[0009] Optionally, the virtual impedance control includes: a constant voltage control mode and a constant power control mode.

[0010] Optionally, the virtual impedance voltage calculation formula of the constant voltage control method is:

[0011]

[0012] In the formula, I m is the current measurement value, Z v is the virtual impedance value, ΔU V is the virtual impedance voltage.

[0013] Optionally, the virtual impedance power calculation formula of the constant power control mode is:

[0014]

[0015] In the formula, I m is the current measurement value, Z v is the virtual impedance value, ΔP V is the virtual impedance power.

[0016] Optionally, limiting the fault current by a superconducting current limiter until the fault current is less than the maximum breaking capacity of the DC circuit breaker, and then suppressing the fault current by a DC circuit breaker specifically includes:

[0017] The freewheeling current of the MMC bridge arm is limited by a superconducting current limiter, and the limited fault current is calculated based on the freewheeling current calculation formula;

[0018] When the limited fault current is less than the maximum breaking capacity of the DC circuit breaker, the fault current is suppressed by the DC circuit breaker;

[0019] Wherein, the freewheeling current calculation formula is:

[0020]

[0021] In the formula, I 0_MMC is the bridge arm current at the MMC converter station locking moment, L bridge is the bridge arm inductance, R L is the resistance value of the discharge circuit, R SR is the quench resistance of the superconducting current limiter.

[0022] A second aspect of the present application provides a half-bridge MMC fault current suppression system, the system comprising:

[0023] An acquisition module, used for acquiring basic parameters of the MMC converter station to be suppressed, and calculating the fault current value before the MMC converter station is locked according to the basic parameters;

[0024] A first suppression module is used to select a current limiting reactor to suppress the fault current according to the fault current value after a fault occurs in the MMC converter station and before it is locked, and to select different virtual impedance controls to suppress the fault current according to the control mode type of the MMC converter station;

[0025] The second suppression module is used to limit the fault current through the superconducting current limiter after the MMC converter station is locked until the fault current is less than the maximum breaking capacity of the DC circuit breaker, and then suppress the fault current through the DC circuit breaker.

[0026] Optionally, the virtual impedance control includes: a constant voltage control mode and a constant power control mode.

[0027] Optionally, the virtual impedance voltage calculation formula of the constant voltage control method is:

[0028]

[0029] In the formula, I m is the current measurement value, Z v is the virtual impedance value, ΔU V is the virtual impedance voltage.

[0030] Optionally, the virtual impedance power calculation formula of the constant power control mode is:

[0031]

[0032] In the formula, I m is the current measurement value, Z v is the virtual impedance value, ΔP V is the virtual impedance power.

[0033] Optionally, the second suppression module is specifically configured to:

[0034] After the MMC converter station is locked, the MMC bridge arm freewheeling current is limited by the superconducting current limiter, and the limited fault current is calculated based on the freewheeling current calculation formula;

[0035] When the limited fault current is less than the maximum breaking capacity of the DC circuit breaker, the fault current is suppressed by the DC circuit breaker;

[0036] Wherein, the freewheeling current calculation formula is:

[0037]

[0038] In the formula, I 0_MMC is the bridge arm current at the MMC converter station locking moment, L bridge is the bridge arm inductance, R L is the resistance value of the discharge circuit, R SR is the quench resistance of the superconducting current limiter.

[0039] It can be seen from the above technical solutions that this application has the following advantages:

[0040] The present application provides a method for suppressing a half-bridge MMC fault current, including: obtaining basic parameters of an MMC converter station to be suppressed, and calculating the fault current value before the MMC converter station is locked according to the basic parameters; after a fault occurs in the MMC converter station and before it is locked, selecting a current limiting reactor to suppress the fault current according to the fault current value, and selecting different virtual impedance controls to suppress the fault current according to the control mode type of the MMC converter station, wherein the virtual impedance control includes: a constant voltage control mode and a constant power control mode; after the MMC converter station is locked, the fault current is limited by a superconducting current limiter until the fault current is less than the maximum breaking capacity of the DC circuit breaker, and then the fault current is suppressed by a DC circuit breaker. Compared with the prior art, the present application combines a primary current limiting device with a secondary control strategy to fully improve the short-circuit fault current suppression capability of the half-bridge MMC. It is not only applicable to a half-bridge MMC converter station with a fixed active power, but also to a half-bridge MMC converter station with a fixed voltage control, thereby solving the technical problem that the prior art has a poor ability to suppress short-circuit fault current of the half-bridge MMC. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A schematic flow chart of an embodiment of a half-bridge MMC fault current suppression method provided in an embodiment of the present application;

[0042] Figure 2 A schematic diagram of the structure of a typical half-bridge MMC converter station provided in an embodiment of the present application;

[0043] Figure 3 A structural diagram of virtual impedance control of a constant voltage controlled MMC converter station provided in an embodiment of the present application;

[0044] Figure 4 A schematic diagram of the structure of virtual impedance control of a fixed power control MMC converter station provided in an embodiment of the present application;

[0045] Figure 5 It is a structural schematic diagram of an embodiment of a half-bridge MMC fault current suppression system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0046] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0047] See also Figure 1, a half-bridge MMC fault current suppression method provided in an embodiment of the present application includes:

[0048] Step 101, obtaining basic parameters of the MMC converter station to be suppressed, and calculating the fault current value before the MMC converter station is locked according to the basic parameters;

[0049] It should be noted that the basic information of the target half-bridge MMC converter station should be obtained first, including the rated DC voltage, the number of bridge arm submodules, the MMC submodule capacitance, the bridge arm reactor inductance, the connection transformer inductance, the PI control parameters, the DC voltage control value, and the active power control value. Figure 2 In the half-bridge MMC converter station shown, the main current limiting primary devices include current limiting reactors, superconducting current limiters and DC circuit breakers. The basic secondary control strategies on the DC side include constant DC voltage control and constant active power control.

[0050] After the extremely short-circuit fault occurs, the short-circuit fault current rises rapidly before the MMC converter station is locked. The fault current components include the submodule capacitor discharge current and the AC side feed-in current. At this time, the use of current-limiting reactors can effectively reduce the fault current rise rate. Therefore, the fault current value before the MMC converter station is locked is calculated based on the basic parameters.

[0051] The calculation formula of fault current before blocking is:

[0052]

[0053] Among them, τ1 is the MMC capacitor discharge current decay time constant, U dc is the DC voltage before the fault, C0 is the submodule capacitance, n is the number of bridge arm submodules, L bridge is the inductance of the bridge arm reactor, L r is the inductance of the current limiting reactor, ω is the oscillation angular frequency of the discharge current, and I0 is the DC current output by the MMC at the moment before the fault.

[0054] Step 102: After a fault occurs in the MMC converter station and before it is locked, a current limiting reactor is selected according to the fault current value to suppress the fault current, and different virtual impedance controls are selected according to the control mode type of the MMC converter station to suppress the fault current;

[0055] It should be noted that after a fault occurs in the MMC converter station and before it is locked, a suitable current-limiting reactor is selected according to the fault current value calculated in step 101 to effectively reduce the rising rate of the fault current.

[0056] While using current limiting reactors, virtual impedance control can be used to suppress fault current. Depending on the control mode of the converter station, there are two control methods:

[0057] (1) Figure 3 As shown, the MMC converter station is controlled by constant voltage.

[0058] Figure 3 Middle U ref is the reference voltage, i ref is the reference current, i dc is the actual value of current, U m is the voltage measurement value, I m is the current measurement value, Z v is the virtual impedance value, ΔU V is the virtual impedance voltage, satisfying

[0059] (2) Figure 4 As shown, the MMC converter station is controlled by constant power.

[0060] Figure 4 In, P ref is the reference active power, P m is the power measurement value, ΔP V is the virtual impedance power, satisfying

[0061] Step 103: After the MMC converter station is locked, the fault current is limited by the superconducting current limiter until the fault current is less than the maximum breaking capacity of the DC circuit breaker, and then the fault current is suppressed by the DC circuit breaker.

[0062] Specifically, after the MMC converter station is locked, the freewheeling current of the MMC bridge arm is limited by the superconducting current limiter, and the limited fault current is calculated based on the freewheeling current calculation formula; when the limited fault current is less than the maximum breaking capacity of the DC circuit breaker, the fault current is suppressed by the DC circuit breaker.

[0063] It should be noted that for the half-bridge MMC converter station, after the converter station is locked, there will still be AC ​​side feed-in current and bridge arm freewheeling current, which will cause the DC fault to be unable to be isolated. At this time, due to the lockout of the converter valve, virtual impedance control cannot be performed, and the fault current suppression and fault removal are mainly achieved by the cooperation of the superconducting current limiter and the DC circuit breaker. The superconducting current limiter mainly considers limiting the bridge arm freewheeling current, and its calculation formula is:

[0064]

[0065] Among them, I 0_MMC is the bridge arm current at the converter station locking moment, L bridge is the bridge arm inductance, R L is the resistance value of the discharge circuit, R SR It is the superconducting current limiter quenching resistance.

[0066] After the fault current is limited to below the maximum breaking capacity of the DC circuit breaker, i MMC_bridge <I k0 , the fault can be cleared by the DC circuit breaker; where I k0 It is the maximum breaking current of DC circuit breaker.

[0067] The present embodiment provides a half-bridge MMC fault current suppression method, which fully improves the half-bridge MMC short-circuit fault current suppression capability by combining a primary current limiting device with a secondary control strategy. The method is not only applicable to a half-bridge MMC converter station with fixed active power, but also to a half-bridge MMC converter station with fixed voltage control, thereby solving the technical problem of the prior art that the half-bridge MMC short-circuit fault current suppression capability is poor.

[0068] The above is an embodiment of a half-bridge MMC fault current suppression method provided in the embodiments of the present application, and the following is an embodiment of a half-bridge MMC fault current suppression system provided in the embodiments of the present application.

[0069] See also Figure 5 , a half-bridge MMC fault current suppression system provided in an embodiment of the present application includes:

[0070] An acquisition module 201 is used to acquire basic parameters of the MMC converter station to be suppressed, and calculate the fault current value before the MMC converter station is locked according to the basic parameters;

[0071] The first suppression module 202 is used to select a current limiting reactor to suppress the fault current according to the fault current value after a fault occurs in the MMC converter station and before the MMC converter station is locked, and to select different virtual impedance controls to suppress the fault current according to the control mode type of the MMC converter station;

[0072] The second suppression module 203 is used to limit the fault current through the superconducting current limiter after the MMC converter station is locked until the fault current is less than the maximum breaking capacity of the DC circuit breaker, and then suppress the fault current through the DC circuit breaker.

[0073] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working process of the above-described system and module can refer to the corresponding process in the aforementioned method embodiment, and will not be repeated here.

[0074] The terms "first", "second", "third", "fourth", etc. in the specification of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein, for example. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0075] It should be understood that in the present application, "at least one (item)" means one or more, and "plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and / or B" can mean: only A exists, only B exists, and A and B exist at the same time, where A and B can be singular or plural. The character " / " generally indicates that the objects associated before and after are in an "or" relationship. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least one of a, b or c can mean: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c can be single or multiple.

[0076] In the several embodiments provided in the present application, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or units, which can be electrical, mechanical or other forms.

[0077] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0078] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0079] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, and the computer software product is stored in a storage medium, including a number of instructions to enable a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (full name in English: Read-Only Memory, English abbreviation: ROM), random access memory (full name in English: Random Access Memory, English abbreviation: RAM), disk or optical disk and other media that can store program codes.

[0080] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A half-bridge MMC fault current suppression method, characterized in that: include: Obtaining basic parameters of the MMC converter station to be suppressed, and calculating the fault current value before the MMC converter station is locked according to the basic parameters; After a fault occurs in the MMC converter station and before it is locked, a current limiting reactor is selected according to the fault current value to suppress the fault current, and different virtual impedance controls are selected according to the control mode type of the MMC converter station to suppress the fault current; After the MMC converter station is locked, the fault current is limited by the superconducting current limiter until the fault current is less than the maximum breaking capacity of the DC circuit breaker, and then the fault current is suppressed by the DC circuit breaker; Wherein, the virtual impedance control includes: a constant voltage control mode and a constant power control mode; The method of limiting the fault current by the superconducting current limiter until the fault current is less than the maximum breaking capacity of the DC circuit breaker and then suppressing the fault current by the DC circuit breaker specifically includes: The freewheeling current of the MMC bridge arm is limited by a superconducting current limiter, and the limited fault current is calculated based on the freewheeling current calculation formula; When the limited fault current is less than the maximum breaking capacity of the DC circuit breaker, the fault current is suppressed by the DC circuit breaker; Wherein, the freewheeling current calculation formula is: ； In the formula, is the bridge arm current at the MMC converter station locking moment, , is the bridge arm inductance, is the resistance value of the discharge circuit, is the quench resistance of the superconducting current limiter.

2. The half-bridge MMC fault current suppression method according to claim 1, characterized in that: The calculation formula of the virtual impedance voltage in the constant voltage control method is: ； In the formula, is the current measurement value, is the virtual impedance value, is the virtual impedance voltage.

3. The half-bridge MMC fault current suppression method according to claim 1, characterized in that: The virtual impedance power calculation formula of the constant power control method is: ； In the formula, is the current measurement value, is the virtual impedance value, is the virtual impedance power.

4. A half-bridge MMC fault current suppression system, characterized in that: include: An acquisition module, used for acquiring basic parameters of the MMC converter station to be suppressed, and calculating the fault current value before the MMC converter station is locked according to the basic parameters; A first suppression module is used to select a current limiting reactor to suppress the fault current according to the fault current value after a fault occurs in the MMC converter station and before it is locked, and to select different virtual impedance controls to suppress the fault current according to the control mode type of the MMC converter station; The second suppression module is used to limit the fault current through the superconducting current limiter after the MMC converter station is locked until the fault current is less than the maximum breaking capacity of the DC circuit breaker, and then suppress the fault current through the DC circuit breaker; Wherein, the virtual impedance control includes: a constant voltage control mode and a constant power control mode; The second suppression module is specifically used for: After the MMC converter station is locked, the MMC bridge arm freewheeling current is limited by the superconducting current limiter, and the limited fault current is calculated based on the freewheeling current calculation formula; When the limited fault current is less than the maximum breaking capacity of the DC circuit breaker, the fault current is suppressed by the DC circuit breaker; Wherein, the freewheeling current calculation formula is: ； In the formula, is the bridge arm current at the MMC converter station locking moment, , is the bridge arm inductance, is the resistance value of the discharge circuit, is the quench resistance of the superconducting current limiter.

5. The half-bridge MMC fault current suppression system according to claim 4, characterized in that: The calculation formula of the virtual impedance voltage in the constant voltage control method is: ； In the formula, is the current measurement value, is the virtual impedance value, is the virtual impedance voltage.

6. The half-bridge MMC fault current suppression system according to claim 4, characterized in that: The virtual impedance power calculation formula of the constant power control method is: ； In the formula, is the current measurement value, is the virtual impedance value, is the virtual impedance power.

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