Method and system for suppressing fault current in M3C converter valve

Abstract

The invention provides a fault current suppression method and system in an M3C converter valve, the arrangement scheme of bridge arm reactance and bridge arm CT is designed by considering the arrangement scheme of nine bridge arms of the converter valve in a valve hall, the method can effectively suppress the bridge arm fault current rise rate after a short-circuit fault between the M3C bridge arms, and the fault current suppression efficiency of the M3C converter valve is improved. And the fault can be quickly detected, so that the operation safety of the M3C is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of fault current suppression, and in particular relates to a method and system for suppressing fault current in an M3C converter valve. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Improving the transmission capacity of transmission lines has always been an important research area in power engineering. The main factors that limit the transmission power of AC transmission systems include thermal limits, stable power limits, etc. Since the invention of transformers, increasing voltage levels has become an important means to improve transmission capacity. However, in modern power systems, another important parameter of AC transmission, the optimization of frequency, has also attracted attention. When coal-fired gas turbine generator sets are the main power source, such units require a higher speed (frequency) to ensure their economy, so 50 / 60Hz has become the standard frequency of the power grid. In recent years, the prime movers of renewable energy sources such as wind power and hydropower, which have developed rapidly, have very low speeds and are suitable for generating low-frequency electricity. In addition, low-frequency transmission has the advantages of reducing reactance and improving transmission capacity. Therefore, the frequency selection problem of power generation and transmission related to renewable energy should be re-examined.

[0004] Especially for COSCO offshore wind power transmission, the existing transmission methods mainly include AC transmission and flexible DC transmission. The AC transmission method is affected by the cable capacitance current problem and is not suitable for long-distance transmission. The construction cost of the flexible DC transmission offshore converter station itself is relatively high. The low-frequency transmission system can realize the long-distance transmission of offshore wind power without the need for an offshore converter station. Modular multilevel matrix converters (M3C) is a new type of AC / AC conversion device. The two sides of the device can be used to connect two three-phase AC systems respectively to realize the conversion of different voltage amplitudes and frequencies on both sides. It is a low-frequency transmission solution with research and application potential.

[0005] The wiring diagram of a typical new energy flexible low-frequency transmission system is as follows: Figure 1 As shown, the basic structure of a typical modular matrix converter M3C is as follows Figure 2 As shown in the figure, it consists of 9 bridge arm branches, soft-start resistor and its bypass switch, power frequency transformer, low frequency transformer, grounding system, valve control, cooling system and other main parts. Each bridge arm branch contains several cascaded full-bridge sub-modules and bridge arm reactors.

[0006] At present, the low-frequency transmission system based on M3C adopts the same bridge arm structure as the full-bridge sub-module flexible transmission converter valve MMC. A high-speed CT is arranged on each bridge arm to quickly detect overcurrent and lock the converter valve in time. The bridge arm reactor can reduce the rising speed of the fault current. This configuration and protection method can quickly detect overcurrent and take protective measures for faults other than the converter valve body, such as transformer valve side faults. However, the nine bridge arms of M3C are generally arranged in parallel. When a valve top or valve bottom short circuit occurs between the bridge arms, such as Figure 3 As shown, on the one hand, there is no bridge arm reactance in the fault current loop, and the fault current rises very quickly, causing serious damage to the equipment; on the other hand, the fault point may occur between the bridge arm CT and the valve, so that the bridge arm CT cannot detect the occurrence of overcurrent and allows the fault to develop, causing serious consequences. Summary of the invention

[0007] In order to solve the above problems, the present invention proposes a method and system for suppressing fault current in an M3C converter valve. The present invention designs the arrangement of bridge arm reactance and bridge arm CT by considering the arrangement of 9 bridge arms of the converter valve in the valve hall. The method of the present invention can effectively suppress the rising rate of bridge arm fault current after a short circuit fault between M3C bridge arms, and is conducive to rapid detection of faults, thereby improving the operating safety of M3C.

[0008] According to some embodiments, the present invention adopts the following technical solutions:

[0009] A method for suppressing fault current in an M3C converter valve comprises the following steps:

[0010] When the nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the power frequency side are adjacent to each other, the bridge arm reactor is arranged on the power frequency side of each bridge arm;

[0011] When the nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the low-frequency side are adjacent to each other, the bridge arm reactor is arranged on the low-frequency side of each bridge arm.

[0012] As an optional implementation, the specific process of arranging the bridge arm reactor on the power frequency side of each bridge arm includes: configuring a single bridge arm CT to be arranged on the power frequency side of the bridge arm, configuring the valve control device with bridge arm current overcurrent protection, and locking the M3C converter valve when the valve control device detects that the bridge arm current exceeds the set value.

[0013] As an optional implementation, the specific process of arranging the bridge arm inductor on the power frequency side of each bridge arm includes: configuring two bridge arm CTs to be arranged on the power frequency side and the low frequency side of the bridge arm respectively, and configuring the bridge arm overcurrent protection and the differential protection within the bridge arm through valve control. When any one of the two bridge arm CTs detects that the current exceeds the set value, the overcurrent protection is triggered and the M3C converter valve is locked; when the difference between the two CTs exceeds a predetermined value and lasts for a certain period of time, the differential protection within the bridge arm is triggered and the M3C converter valve is locked.

[0014] As an optional implementation, the process of arranging the bridge arm reactor on the low-frequency side of each bridge arm includes: configuring a single bridge arm CT to be arranged on the low-frequency side of the bridge arm, configuring the valve control device with bridge arm current overcurrent protection, and locking the M3C converter valve when the valve control device detects that the bridge arm current exceeds the set value.

[0015] As an optional implementation, the process of arranging the bridge arm reactor on the low-frequency side of each bridge arm includes: configuring two bridge arm CTs to be arranged on the power frequency side and the low-frequency side of the bridge arm respectively, and configuring the bridge arm overcurrent protection and the differential protection within the bridge arm through valve control. When any one of the two bridge arm CTs detects that the current exceeds the set value, the bridge arm overcurrent protection is triggered and the M3C converter valve is locked; when the difference between the two bridge arm CTs exceeds a predetermined value and lasts for a certain period of time, the differential protection within the bridge arm is triggered and the M3C converter valve is locked.

[0016] As an optional implementation, when the valve hall area conditions permit, the bridge arm reactance is split into two, and the two split bridge arm reactances are respectively arranged on the power frequency side and the low frequency side of the bridge arm.

[0017] As an optional implementation, the two bridge arm reactors of the same bridge arm after splitting are evenly divided or distributed and adjusted according to the failure probability.

[0018] A fault current suppression system in an M3C converter valve comprises an M3C converter valve and a plurality of bridge arm reactors, wherein when nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the power frequency side are adjacent, the bridge arm reactor is arranged on the power frequency side of each bridge arm;

[0019] When the nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the low-frequency side are adjacent to each other, the bridge arm reactor is arranged on the low-frequency side of each bridge arm.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention considers the arrangement of 9 bridge arms of the converter valve in the valve hall and designs the arrangement of the bridge arm reactance and the bridge arm CT. The method of the present invention can effectively suppress the rising rate of the bridge arm fault current after the short circuit fault between the bridge arms of M3C, and is conducive to quickly detecting the fault, thereby improving the operation safety of M3C.

[0022] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0024] Figure 1 This is a typical wiring diagram of the new energy transmission system via flexible low-frequency AC;

[0025] Figure 2 It is a schematic diagram of a typical three-phase-three-phase modular multi-level matrix converter M3C topology structure;

[0026] Figure 3 This is a schematic diagram of the short-circuit fault current loop between the M3C bridge arms;

[0027] Figure 4 A schematic diagram of a bridge arm reactance and bridge arm CT arrangement scheme and a fault current loop of a method for suppressing fault current in an M3C converter valve of the present invention;

[0028] Figure 5 It is a schematic diagram of the arrangement of bridge arm reactance and bridge arm CT and the fault current loop of another method for suppressing fault current in an M3C converter valve of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0030] It should be noted that the following detailed descriptions are all illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0031] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0032] Embodiment 1

[0033] A method for suppressing fault current in an M3C converter valve is proposed. Considering the arrangement of nine bridge arms of the converter valve in the valve hall, the arrangement of bridge arm reactance and bridge arm CT is designed.

[0034] The modular matrix converter M3C includes several main parts, including 9 bridge arm branches, soft-start resistors and bypass switches, power frequency transformers, low frequency transformers, grounding systems, valve control, cooling systems, etc. Each bridge arm branch is composed of several cascaded full-bridge sub-modules and bridge arm reactors in series. Among them, the main function of the bridge arm reactor is to achieve power control on the power frequency side and the low frequency side, suppress fault current, suppress internal circulating current, suppress harmonics, etc. The value of the reactor is mainly calculated and determined based on the active and reactive operating range of the power frequency side and the low frequency side, and fault current suppression.

[0035] The modular matrix converter M3C is a prior art and will not be described in detail here.

[0036] This embodiment provides a method for suppressing fault current in an M3C converter valve, and the specific method is as follows:

[0037] According to the layout of the 9 bridge arms of the M3C converter valve in the valve hall, the layout of the bridge arm reactance and bridge arm CT is designed:

[0038] The three phases on the low-frequency side are denoted as a, b, and c, the three phases on the industrial frequency side are denoted as u, v, and w, the bridge arm connected between phase a on the low-frequency side and phase u on the industrial frequency side is denoted as au, and the nine bridge arms of M3C are denoted as au, av, aw, bu, bv, bw, cu, cv, and cw according to the phases of the industrial and low frequencies connected by the bridge arms.

[0039] Case 1: When the 9 bridge arms in the valve hall are arranged in a way that the three bridge arms connected to the same phase on the power frequency side are adjacent to each other, that is, the 9 bridge arms are arranged adjacent to each other in the order of au, bu, cu, av, bv, cv, aw, bw, cw, then the following arrangement scheme is adopted:

[0040] Arrangement scheme of bridge arm reactor: Arrange the bridge arm reactor on the power frequency side of each bridge arm;

[0041] Arm CT arrangement scheme: Configuration scheme 1): Configure a single arm CT to be arranged on the power frequency side of the arm, and configure the arm current overcurrent protection with valve control. When the valve control device detects that the arm current exceeds the set value I oc After that, the M3C converter valve is locked. Configuration scheme 2): Two bridge arm CTs are arranged on the power frequency side and low frequency side of the bridge arm respectively, and the valve control is configured with bridge arm overcurrent protection and bridge arm internal differential protection. When any of the two CTs detects that the current exceeds the set value I ocThen the bridge arm overcurrent protection is triggered and the M3C converter valve is locked; when the difference between the two CTs exceeds the set value I oc_diff If the voltage continues for a certain period of time, the differential protection in the bridge arm will be triggered to lock the M3C converter valve. The project can choose the configuration plan according to the budget.

[0042] Description: If Figure 4 As shown in (a), when a short circuit fault occurs between two adjacent bridge arms au and bu, the fault circuit flows through the two bridge arm reactors, effectively reducing the current rise rate and the amplitude of the fault current. Under the single CT configuration scheme, the fault current will flow through the bridge arm CT on the power frequency side. When the current exceeds the overcurrent setting value I oc Afterwards, the M3C converter valve will be directly locked; under the dual CT configuration scheme, the fault current will only flow through the bridge arm CT on the power frequency side (when the fault point is between the valve and the bridge arm CT on the low frequency side), or flow through the bridge arm CTs on both the power frequency side and the low frequency side at the same time. For the former, the differential current protection in the bridge arm will be triggered, thereby identifying the fault more quickly and taking locking protection action.

[0043] Case 2: When the 9 bridge arms in the valve hall are arranged in a way that the three bridge arms connected to the same phase on the low-frequency side are adjacent to each other, that is, the 9 bridge arms are arranged adjacent to each other in the order of au, av, aw, bu, bv, bw, cu, cv, and cw, then the following arrangement scheme is adopted;

[0044] Bridge arm reactor arrangement scheme: The bridge arm reactor is arranged on the low-frequency side of each bridge arm;

[0045] Arm CT arrangement scheme: Configuration scheme 1): Configure a single arm CT to be arranged on the low-frequency side of the arm, and configure the arm current overcurrent protection with valve control. When the valve control device detects that the arm current exceeds the set value I oc After that, the M3C converter valve is locked. Configuration scheme 2): Two bridge arm CTs are arranged on the power frequency side and low frequency side of the bridge arm respectively, and the valve control is configured with bridge arm overcurrent protection and bridge arm internal differential protection. When any of the two CTs detects that the current exceeds the set value I oc Then the bridge arm overcurrent protection is triggered and the M3C converter valve is locked; when the difference between the two CTs exceeds the set value I oc_diff If the voltage continues for a certain period of time, the differential protection in the bridge arm will be triggered to lock the M3C converter valve. The project can choose the configuration plan according to the budget.

[0046] Description: If Figure 4 As shown in (b), when a short circuit fault occurs between two adjacent bridge arms au and av, the fault circuit flows through the two bridge arm reactors, effectively reducing the current rise rate and the amplitude of the fault current. Under the single CT configuration scheme, the fault current will flow through the bridge arm CT on the low-frequency side. When the current exceeds the overcurrent setting value Ioc Afterwards, the M3C converter valve will be directly locked; under the dual CT configuration scheme, the fault current will only flow through the bridge arm CT on the low-frequency side (when the fault point is between the valve and the bridge arm CT on the low-frequency side), or flow through the bridge arm CTs on the power frequency side and the low-frequency side at the same time. For the former, the differential current protection in the bridge arm will be triggered, thereby identifying the fault more quickly and taking locking protection action.

[0047] In addition, if the valve hall area conditions allow, the bridge arm reactance L arm Split into two, L arm_PF and L arm_LF They are arranged on the power frequency side and low frequency side of the bridge arm respectively, such as Figure 5 As shown, where L arm =L arm_PF +L arm_LF , the two bridge arm reactors of the same bridge arm after splitting can be evenly divided or adjusted according to the fault probability. Figure 5 As shown, if a short circuit fault occurs between the low-frequency sides of two adjacent bridge arms au and bu, the fault circuit flows through the power frequency side reactors of the two bridge arms; if a short circuit fault occurs between the power frequency sides of two bridge arms cu and cv, the fault circuit flows through the low-frequency side reactors of the two bridge arms. In this way, no matter what kind of fault occurs between the bridge arms, there is always enough impedance in the fault circuit to suppress the fault current.

[0048] Embodiment 2

[0049] A fault current suppression system in an M3C converter valve comprises an M3C converter valve and a plurality of bridge arm reactors, wherein when nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the power frequency side are adjacent, the bridge arm reactor is arranged on the power frequency side of each bridge arm;

[0050] When the nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the low-frequency side are adjacent to each other, the bridge arm reactor is arranged on the low-frequency side of each bridge arm.

[0051] The bridge arm reactor is arranged on the power frequency side of each bridge arm, including configuring a single bridge arm CT arranged on the power frequency side of the bridge arm, and the valve control device is configured with bridge arm current overcurrent protection. When the valve control device detects that the bridge arm current exceeds the set value, the M3C converter valve is locked;

[0052] Alternatively, two bridge arm CTs are respectively arranged on the power frequency side and the low frequency side of the bridge arm, and the valve control is configured with bridge arm overcurrent protection and intra-bridge arm differential protection. When any of the two bridge arm CTs detects that the current exceeds the set value, the overcurrent protection is triggered and the M3C converter valve is locked; when the difference between the two CTs exceeds the preset value and lasts for a certain period of time, the intra-bridge arm differential protection is triggered and the M3C converter valve is locked.

[0053] The arm reactor is arranged on the low-frequency side of each arm, including: configuring a single arm CT to be arranged on the low-frequency side of the arm, configuring the arm current overcurrent protection on the valve control device, and locking the M3C converter valve when the valve control device detects that the arm current exceeds the set value;

[0054] Alternatively, the process of arranging the bridge arm reactor on the low-frequency side of each bridge arm includes: configuring two bridge arm CTs to be arranged on the power frequency side and the low-frequency side of the bridge arm respectively, and configuring the bridge arm overcurrent protection and the intra-bridge arm differential protection through valve control; when any one of the two bridge arm CTs detects that the current exceeds the set value, the bridge arm overcurrent protection is triggered, and the M3C converter valve is locked; when the difference between the two bridge arm CTs exceeds a predetermined value and lasts for a certain period of time, the intra-bridge arm differential protection is triggered, and the M3C converter valve is locked.

[0055] Embodiment 3

[0056] The difference from the above embodiments is that, without considering the suppression of fault current under various faults in the valve, the low-frequency power transmission system based on M3C adopts the same bridge arm structure as the full-bridge sub-module flexible transmission converter valve MMC, and a high-speed CT is arranged on each bridge arm to quickly detect overcurrent and lock the converter valve in time.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art within the spirit and principle of the present invention without creative labor shall be included in the protection scope of the present invention.

Claims

1. A method for suppressing fault current in an M3C converter valve, characterized in that: The following steps are involved: When the nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the power frequency side are adjacent to each other, the bridge arm reactor is arranged on the power frequency side of each bridge arm; When the nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the low-frequency side are adjacent to each other, the bridge arm reactor is arranged on the low-frequency side of each bridge arm.

2. A method for suppressing fault current in an M3C converter valve according to claim 1, characterized in that: The specific process of arranging the bridge arm reactor on the power frequency side of each bridge arm includes: configuring a single bridge arm CT to be arranged on the power frequency side of the bridge arm, configuring the bridge arm current overcurrent protection in the valve control device, and locking the M3C converter valve when the valve control device detects that the bridge arm current exceeds the set value.

3. A method for suppressing fault current in an M3C converter valve according to claim 1, characterized in that: The specific process of arranging the bridge arm reactor on the power frequency side of each bridge arm includes: configuring two bridge arm CTs to be arranged on the power frequency side and the low frequency side of the bridge arm respectively, and configuring the bridge arm overcurrent protection and the intra-bridge arm differential protection through valve control. When any one of the two bridge arm CTs detects that the current exceeds the set value, the overcurrent protection is triggered and the M3C converter valve is locked; when the difference between the two CTs exceeds the preset value and lasts for a certain period of time, the intra-bridge arm differential protection is triggered and the M3C converter valve is locked.

4. A method for suppressing fault current in an M3C converter valve according to claim 1, characterized in that: The process of arranging the bridge arm reactor on the low-frequency side of each bridge arm includes: configuring a single bridge arm CT to be arranged on the low-frequency side of the bridge arm, configuring the valve control device with bridge arm current overcurrent protection, and locking the M3C converter valve when the valve control device detects that the bridge arm current exceeds the set value.

5. A method for suppressing fault current in an M3C converter valve according to claim 1, characterized in that: The process of arranging the bridge arm reactor on the low-frequency side of each bridge arm includes: configuring two bridge arm CTs to be arranged on the power frequency side and the low-frequency side of the bridge arm respectively, and configuring the bridge arm overcurrent protection and the intra-bridge arm differential protection through valve control. When any one of the two bridge arm CTs detects that the current exceeds the set value, the bridge arm overcurrent protection is triggered and the M3C converter valve is locked; when the difference between the two bridge arm CTs exceeds a predetermined value and lasts for a certain period of time, the intra-bridge arm differential protection is triggered and the M3C converter valve is locked.

6. A method for suppressing fault current in an M3C converter valve according to claim 1, characterized in that: When the valve hall area conditions permit, the bridge arm reactance is split into two, and the two split bridge arm reactances are arranged on the power frequency side and the low frequency side of the bridge arm respectively.

7. A method for suppressing fault current in an M3C converter valve according to claim 1, characterized in that: The two bridge arm reactors of the same bridge arm after splitting are evenly divided or adjusted according to the fault probability.

8. A fault current suppression system in an M3C converter valve, characterized in that: It includes an M3C converter valve and a plurality of bridge arm reactors, wherein when the nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the power frequency side are adjacent to each other, the bridge arm reactor is arranged on the power frequency side of each bridge arm; When the nine bridge arms of the M3C converter valve are arranged in a manner that three bridge arms connected to the same phase on the low-frequency side are adjacent to each other, the bridge arm reactor is arranged on the low-frequency side of each bridge arm.

9. The fault current suppression system in an M3C converter valve according to claim 8, characterized in that: The bridge arm reactor is arranged on the power frequency side of each bridge arm, including configuring a single bridge arm CT arranged on the power frequency side of the bridge arm, and the valve control device is configured with bridge arm current overcurrent protection. When the valve control device detects that the bridge arm current exceeds the set value, the M3C converter valve is locked; Alternatively, two bridge arm CTs are respectively arranged on the power frequency side and the low frequency side of the bridge arm, and the valve control is configured with bridge arm overcurrent protection and intra-bridge arm differential protection. When any of the two bridge arm CTs detects that the current exceeds the set value, the overcurrent protection is triggered and the M3C converter valve is locked; when the difference between the two CTs exceeds the preset value and lasts for a certain period of time, the intra-bridge arm differential protection is triggered and the M3C converter valve is locked.

10. The fault current suppression system in an M3C converter valve according to claim 8, characterized in that: The arm reactor is arranged on the low-frequency side of each arm, including: configuring a single arm CT to be arranged on the low-frequency side of the arm, configuring the arm current overcurrent protection on the valve control device, and locking the M3C converter valve when the valve control device detects that the arm current exceeds the set value; Alternatively, the process of arranging the bridge arm reactor on the low-frequency side of each bridge arm includes: configuring two bridge arm CTs to be arranged on the power frequency side and the low-frequency side of the bridge arm respectively, and configuring the bridge arm overcurrent protection and the intra-bridge arm differential protection through valve control; when any one of the two bridge arm CTs detects that the current exceeds the set value, the bridge arm overcurrent protection is triggered, and the M3C converter valve is locked; when the difference between the two bridge arm CTs exceeds a predetermined value and lasts for a certain period of time, the intra-bridge arm differential protection is triggered, and the M3C converter valve is locked.

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