DC power distribution network fault analysis method based on MMC converter

Abstract

The invention belongs to the field of power fault detection, and discloses a DC power distribution network fault analysis method based on an MMC converter, which aims at a DC power distribution network and combines topological structures and control strategies of various MMC converters. Fault characteristics of the direct-current power distribution network with different topological structures and control strategies are analyzed through various means such as theoretical analysis, simulation and experimental verification, and a direct-current power distribution network fault analysis model is constructed on the basis, so that various fault models can be analyzed in details, the direct-current power distribution network fault can be accurately identified, and the fault diagnosis accuracy of the direct-current power distribution network is improved. The fault processing efficiency can be greatly improved, and the operation power supply reliability of a power system can be greatly improved.

Description

Technical Field

[0001] The invention belongs to the field of power fault detection, and in particular relates to a DC distribution network fault analysis method based on an MMC converter. Background Art

[0002] With the rapid development of power electronics technology in recent years, high-voltage flexible DC transmission and DC microgrids have emerged. Fault detection and location of DC distribution networks are the core of relay protection of DC distribution networks. DC distribution systems are different from AC distribution systems. They have "low damping" characteristics. After a DC fault occurs, the fault current is very large and the fault development process is extremely fast. It can usually endanger the safety of the entire DC distribution network within a few milliseconds. Therefore, the fault detection and location strategy of the DC distribution network is required to be able to quickly locate the fault line within a few milliseconds.

[0003] Modular multilevel converter (MMC) was proposed by German scholar R. Marquardt and others in 2001 as a new type of VSC topology. Compared with traditional two-level and three-level VSC, MMC composed of multiple sub-modules cascaded has the advantages of low switching loss, easy capacity expansion, low IGBT static and dynamic voltage equalization requirements, and high output voltage waveform quality. The fault characteristics of DC distribution network are complex, showing a multi-stage, strong transient process, lacking fundamental frequency components, rich harmonic content, and containing multiple attenuation components. The protection principles and fault distance measurement methods in AC systems are mostly based on fundamental frequency components, which are difficult to directly apply to flexible DC distribution networks. Therefore, a DC distribution network fault analysis method based on MMC converter is needed to meet the fault detection needs of DC distribution network. Summary of the invention

[0004] In order to overcome the deficiencies in the prior art, the present invention aims to provide a DC distribution network fault analysis method based on MMC converters. The method analyzes the fault characteristics of DC distribution networks with different topologies and control strategies by theoretical analysis, simulation, experimental verification and other means for DC distribution networks combined with various MMC converter topologies, and constructs a DC distribution network fault analysis model on this basis, so that various fault models can be analyzed in detail, and then the DC distribution network faults can be accurately identified, which significantly speeds up the identification of faulty lines, which can not only greatly improve the fault handling efficiency, but also greatly improve the power supply reliability of the power system operation.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is: A method for analyzing faults in a DC distribution network based on an MMC converter is implemented based on the following steps: S1. Comprehensively evaluate the radial, two-terminal and ring topologies of DC distribution networks from multiple aspects of economic and power supply factors; S2. Considering that the constant voltage control and constant power control on the DC side cannot adapt to the characteristics of low inertia, constant power load access and weak AC power grid of the DC distribution network, analyze various control methods to meet the control requirements of the DC distribution network, and determine the advantages and disadvantages of various control methods and their application scenarios; S3. Analyze and extract fault characteristics of various types of faults occurring in the DC distribution network based on the MMC converter on the AC side, the DC side, and the converter itself; S4. Construct a DC distribution network fault analysis model and conduct detailed analysis of various faults; S5. The established DC distribution network fault analysis model is used to analyze the operating characteristics, losses, and operation and maintenance requirements of DC distribution networks with various topologies, and the selection basis and operating characteristics of the DC distribution network topology are given to provide relevant basis for the application of DC distribution networks, fault analysis, relay protection, safe operation and maintenance.

[0006] Furthermore, the power supply and economic evaluation factors in step S1 include control strategy, power flow characteristics, power supply reliability, power supply range, power supply quality, voltage level, operational economy and social economy.

[0007] Furthermore, the control methods in step S2 include DC voltage control, power synchronization control, virtual synchronous machine control, virtual inertia control and virtual impedance control.

[0008] Furthermore, the present invention also includes step S6 of system protection control after fault analysis: by improving the converter topology, the MMC converter itself is used to achieve rapid clearing of the fault current.

[0009] Furthermore, step S6 specifically includes utilizing a hybrid MMC converter with fault blocking capability to realize rapid clearing of fault current by the converter itself, wherein the hybrid MMC converter includes a half-bridge submodule HBSM and a full-bridge submodule FBSM or a clamped dual submodule CDSM.

[0010] The beneficial effects of the present invention are: The present invention analyzes the fault characteristics of DC distribution networks with different topologies and control strategies by combining various MMC converter topologies with DC distribution networks through theoretical analysis, simulation, experimental verification and other means, and constructs a DC distribution network fault analysis model on this basis, so that various fault models can be analyzed in detail, and DC distribution network faults can be accurately identified. DETAILED DESCRIPTION

[0011] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not intended to limit the scope of use of the present invention.

[0012] A method for analyzing faults in a DC distribution network based on an MMC converter is implemented based on the following steps: S1. Based on the AC distribution network topology and existing research results at home and abroad, a comprehensive evaluation of the radial, two-terminal and ring topologies of the DC distribution network is conducted from multiple aspects, including control strategy, power flow characteristics, power supply reliability, power supply range, power supply quality, voltage level, operation economy and social economy; S2. Considering that the constant voltage control and constant power control on the DC side cannot adapt to the characteristics of low inertia, constant power load access, and weak AC power grid on the DC distribution network, control methods such as DC voltage control, power synchronization control, virtual synchronous machine control, virtual inertia control, and virtual impedance control are analyzed to meet the requirements of various DC distribution networks for control methods and determine the advantages and disadvantages of various control methods and their application scenarios.

[0013] S3. Analyze and extract the fault characteristics of the DC distribution network based on MMC converter when various types of faults occur on the AC side, DC side and the converter itself by using theoretical analysis, simulation and experiments; S4. Construct a DC distribution network fault analysis model and conduct detailed analysis of various faults; The present invention carries out an analysis based on the MMC converter control strategy, focusing on the DC energy storage capacitor pre-charging control based on the MMC converter, the converter's double closed-loop control, the bridge arm energy sharing control and the sub-module capacitor voltage stabilization control method.

[0014] S5. The established DC distribution network fault analysis model is used to analyze the operating characteristics, losses, and operation and maintenance requirements of DC distribution networks with various topologies, and the selection basis and operating characteristics of the DC distribution network topology are given to provide relevant basis for the application of DC distribution networks, fault analysis, relay protection, safe operation and maintenance.

[0015] S6. System protection control after fault analysis: mainly from the two aspects of topology improvement and control strategy optimization, the protection strategy after the MMC DC system fault is analyzed. In actual flexible DC projects, AC circuit breakers are often used to clear DC fault currents, and then DC switches are used to cut off the faulty lines. Although this method is simple and reliable, it has problems such as slow operation speed and low system power supply reliability. The present invention improves the converter topology and uses the MMC converter itself to quickly clear the fault current. Specifically, it includes: using a hybrid MMC converter with fault blocking capability to achieve rapid clearing of fault current by the converter itself, and the hybrid MMC converter includes a half-bridge submodule HBSM and a full-bridge submodule FBSM or a clamped twin submodule CDSM, etc.

[0016] The present invention aims to comprehensively analyze the DC distribution network topology, converter structure and operation control strategy, and construct a fault characteristic analysis model of the DC distribution network based on MMC converters and its dynamic relationship with various factors. This allows for detailed analysis of various fault models, and thus enables accurate identification of DC distribution network faults.

[0017] Obviously, the embodiments described above are only a part of the embodiments of the present application, rather than all the embodiments, wherein preferred embodiments of the present application are provided, but the patent scope of the present application is not limited. The present application can be implemented in many different forms. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive. Although the present application is described in detail with reference to the aforementioned embodiments, for those skilled in the art, it is still possible to modify the technical solutions recorded in the aforementioned specific implementation methods, or to perform equivalent replacement of some of the technical features therein. Any equivalent structure made using the contents of the present application specification, directly or indirectly used in other related technical fields, is similarly within the scope of patent protection of the present application.

Claims

1. A DC distribution network fault analysis method based on MMC converter, characterized in that: The method is implemented based on the following steps: S1. Comprehensively evaluate the radial, two-terminal and ring topologies of DC distribution networks from multiple aspects of economic and power supply factors; S2. Considering that the constant voltage control and constant power control on the DC side cannot adapt to the characteristics of low inertia, constant power load access and weak AC power grid of the DC distribution network, various control methods are analyzed to meet the requirements of the DC distribution network for control, and the advantages and disadvantages of various control methods and their application scenarios are determined; S3. Analyze and extract fault characteristics of various types of faults occurring in the DC distribution network based on the MMC converter on the AC side, the DC side, and the converter itself; S4. Construct a DC distribution network fault analysis model and conduct detailed analysis of various faults; S5. The established DC distribution network fault analysis model is used to analyze the operating characteristics, losses, and operation and maintenance requirements of DC distribution networks with various topologies, and the selection basis and operating characteristics of the DC distribution network topology are given to provide relevant basis for the application of DC distribution networks, fault analysis, relay protection, safe operation and maintenance.

2. The DC distribution network fault analysis method based on MMC converter according to claim 1, characterized in that: The power supply and economic evaluation factors in step S1 include control strategy, power flow characteristics, power supply reliability, power supply range, power supply quality, voltage level, operational economy and social economy.

3. The DC distribution network fault analysis method based on MMC converter according to claim 1, characterized in that: The control methods in step S2 include DC voltage control, power synchronization control, virtual synchronous machine control, virtual inertia control and virtual impedance control.

4. The DC distribution network fault analysis method based on MMC converter according to claim 1, characterized in that: It also includes step S6 of system protection control after fault analysis: by improving the converter topology, the MMC converter itself is used to achieve rapid clearing of the fault current.

5. The DC distribution network fault analysis method based on MMC converter according to claim 4 is characterized in that: Step S6 specifically includes utilizing a hybrid MMC converter with fault blocking capability to enable the converter itself to quickly clear the fault current. The hybrid MMC converter includes a half-bridge submodule HBSM and a full-bridge submodule FBSM or a clamped dual submodule CDSM.