MMC-based DC system fault analysis method

Abstract

The invention belongs to the field of power fault analysis, and discloses an MMC-based DC system fault analysis method, which comprises the following steps of: constructing corresponding fault models of different types at different positions of a DC power distribution network on the basis of analysis on various fault characteristics of the DC power distribution network of an MMC converter when a fault analysis model is carried out; the method can significantly accelerate the discrimination of fault lines, can greatly improve the fault processing efficiency, can greatly improve the power supply reliability of the operation of a power system, actively promotes the state diagnosis technology of the power system, and improves the operation level of a power grid.

Description

Technical Field

[0001] The invention belongs to the field of power fault analysis, and in particular relates to a fault analysis method based on an MMC direct current system. Background Art

[0002] DC power distribution technology has many advantages over traditional AC power distribution technology, but it is still in the development stage and still faces many problems. The current development bottlenecks of DC power distribution technology mainly include the following three points: ① DC power flow control technology; ② DC voltage transformation technology; ③ DC fault detection, identification and isolation technology. Among them, fast detection and reliable isolation of DC faults are of great significance to ensure the safe and reliable operation of DC distribution networks.

[0003] Modular multilevel converter (MMC) was proposed by German scholar R. Marquardt et al. in 200 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 static and dynamic voltage equalization requirements of IGBT, and high output voltage waveform quality. The fault characteristics of DC distribution network based on MMC converter are different from those of AC distribution system and conventional high-voltage DC transmission system. Affected by nonlinear elements such as diodes, capacitors and fast control response in voltage source converters, 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 location methods in AC systems are mostly based on fundamental frequency components, which are difficult to be directly applied to flexible DC distribution networks. The protection principles of conventional high-voltage DC transmission systems are mostly aimed at two-terminal systems with smoothing reactor boundaries, and are also difficult to be directly used in DC distribution networks with complex topologies. The complex fault characteristics of DC distribution networks pose challenges to protection research.

[0004] Fault detection and location of DC distribution network is the core of relay protection of DC distribution network. DC distribution system is different from AC distribution system. It has the characteristic of "low damping". 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 DC distribution network is required to be able to quickly locate the fault line within a few milliseconds. The traditional fault protection method of AC distribution network is obviously not suitable for the protection of DC distribution network. How to deal with the contradiction between rapidity and reliability is a problem that needs to be solved in DC distribution network fault detection and location. Therefore, there is an urgent need for a fault analysis method based on MMC DC system. Summary of the invention

[0005] In order to address the deficiencies in the prior art, the present invention aims to provide a fault analysis method based on an MMC DC system, which can significantly speed up the identification of faulty lines, not only greatly improve the efficiency of fault handling, but also greatly improve the power supply reliability of the power system operation, actively promote the power system status diagnosis technology, and improve the power grid operation level.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: A fault analysis method based on MMC DC system, comprising the following steps: S1. DC distribution network based on MMC converter, combined with various MMC converter topologies and control strategies, extracts fault characteristics of DC distribution networks with different topologies and control strategies; S2. Construct a DC distribution network fault analysis model and conduct a detailed analysis of various power grid faults; based on the MMC converter with hybrid topology and active current limiting control strategy, consider the relevant fault influencing factors, derive and measure the evolution of the hybrid MMC DC bus voltage and current after the delay, and analyze the DC distribution network active current limiting control requirements for the DCCB breaking current.

[0007] Furthermore, the fault influencing factors that need to be considered when constructing the fault analysis model in step S2 include: the influence of the DC voltage feedforward controller in the active current limiting control strategy on the DC fault current peak, and the influence of different coordinated control strategies of the hybrid MMC and DCCB on the dynamic change process of the DC current of the fault line.

[0008] Furthermore, step S2 specifically includes: S2-1. Under the ideal condition of AC grid voltage balance, study and establish the switching function mathematical model of MMC suitable for transient analysis, as well as the mathematical model in dq rotating coordinate system suitable for control analysis; S2-2. Considering the fact that the MMC mathematical model in the stationary coordinate system is not conducive to the control system design, the instantaneous symmetrical components of the three-phase unbalanced quantity and their representation in the stationary coordinate system and the rotating coordinate system are calculated; S2-3. Construct the time domain model and frequency domain mathematical model of MMC when the AC system grid voltage is unbalanced; S2-4. Analyze the power balance relationship on both sides of the AC and DC of the MMC converter station and construct relevant fault analysis models.

[0009] Furthermore, in step S2, when performing the fault analysis model, it is necessary to build corresponding fault models of different types occurring at different locations in the DC distribution network based on the research of various fault characteristics of the MMC converter DC distribution network.

[0010] Furthermore, when constructing corresponding fault analysis models for different types of faults occurring at different locations in step S2, it includes: When various faults or abnormal operating conditions occur on the AC side, an effective parsing and analysis model for the AC side is established based on the control strategy of the MMC converter and the characteristics of various electrical signals on the AC and DC sides. When various faults or abnormal operating conditions occur on the DC side, an effective analysis model for the DC side is established based on the control strategy of the MMC converter and the characteristics of various electrical signals on the AC and DC sides. When the MMC converter itself encounters various faults and abnormal operating conditions, an effective parsing and analysis model of the MMC converter itself is established based on the output characteristics of the MMC converter and the characteristics of various electrical signals on the AC and DC sides.

[0011] The present invention has the following beneficial effects: The fault analysis method provided by the present invention, when conducting a fault analysis model, is based on the analysis of various fault characteristics of the MMC converter DC distribution network, thereby constructing corresponding fault models of different types occurring in different locations of the DC distribution network, which can significantly speed up the identification of faulty lines, not only greatly improve the fault handling efficiency, but also greatly improve the power supply reliability of the power system operation, actively promote the power system status diagnosis technology, and improve the power grid operation level. DETAILED DESCRIPTION

[0012] 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.

[0013] The present invention proposes a fault analysis method based on MMC DC system. The study of fault characteristics is the basis for carrying out research on protection, fault location, fault isolation, etc. The fault characteristics of DC distribution network are not only related to the topological structure of DC distribution network, but also to many factors such as the type and control mode of converter. The fault analysis method provided by the present invention is implemented based on the following steps: S1. DC distribution network based on MMC converter, combined with various MMC converter topologies and control strategies, the fault characteristics of DC distribution network with different topologies and control strategies are extracted.

[0014] The study of fault characteristics of DC distribution network is the basis of fault detection, location and isolation. At the same time, the use of different types of converters and the topology of DC distribution network also have a great impact on its fault characteristics.

[0015] S2. Construct a DC distribution network fault analysis model and conduct a detailed analysis of various power grid faults; based on the hybrid topology and the MMC converter with active current limiting control strategy, consider the impact of the DC voltage feedforward controller in the active current limiting control strategy on the DC fault current peak, and the impact of different coordinated control strategies of the hybrid MMC and DCCB on the dynamic change process of the DC current of the fault line, derive and measure the evolution of the hybrid MMC DC bus voltage and current after the delay, and analyze the DC distribution network active current limiting control on the DCCB breaking current size requirements. Specifically include: S2-1. Under the ideal condition of AC grid voltage balance, study and establish the switching function mathematical model of MMC suitable for transient analysis, as well as the mathematical model in dq rotating coordinate system suitable for control analysis; S2-2. Considering the fact that the MMC mathematical model in the stationary coordinate system is not conducive to the control system design, the instantaneous symmetrical components of the three-phase unbalanced quantity and their representation in the stationary coordinate system and the rotating coordinate system are calculated; S2-3. Construct the time domain model and frequency domain mathematical model of MMC when the AC system grid voltage is unbalanced; S2-4. Analyze the power balance relationship on both sides of the AC and DC of the MMC converter station and construct relevant fault analysis models.

[0016] Among them, when conducting fault analysis models, it is necessary to build corresponding fault models for different types of DC distribution networks at different locations based on the research of various fault characteristics of MMC converter DC distribution networks. Specifically, it includes: When various faults or abnormal operating conditions occur on the AC side, an effective parsing and analysis model for the AC side is established based on the control strategy of the MMC converter and the characteristics of various electrical signals on the AC and DC sides. When various faults or abnormal operating conditions occur on the DC side, an effective analysis model for the DC side is established based on the control strategy of the MMC converter and the characteristics of various electrical signals on the AC and DC sides. When the MMC converter itself encounters various faults and abnormal operating conditions, an effective parsing and analysis model of the MMC converter itself is established based on the output characteristics of the MMC converter and the characteristics of various electrical signals on the AC and DC sides.

[0017] The present invention aims to provide a fault analysis method based on MMC DC system, which can significantly speed up the identification of faulty lines, not only greatly improve the efficiency of fault handling, but also greatly improve the power supply reliability of power system operation, actively promote the power system status diagnosis technology, and improve the power grid operation level.

[0018] Obviously, the embodiments described above are only a part of the embodiments of the present application, rather than all the embodiments. The above embodiments provide preferred embodiments of the present application, but do not limit the patent scope of the present application. 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 above embodiments, for those skilled in the art, it is still possible to modify the technical solutions recorded in the above-mentioned 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 fault analysis method based on MMC DC system, characterized by: The following steps are involved: S1. DC distribution network based on MMC converter, combined with various MMC converter topologies and control strategies, extracts fault characteristics of DC distribution networks with different topologies and control strategies; S2. Construct a DC distribution network fault analysis model and conduct a detailed analysis of various power grid faults; based on the hybrid topology and the MMC converter with active current limiting control strategy, consider the relevant fault influencing factors, derive and measure the evolution of the hybrid MMC DC bus voltage and current after the delay, and analyze the DC distribution network active current limiting control requirements for DCCB breaking current.

2. The MMC DC system fault analysis method according to claim 1, characterized in that: The fault influencing factors that need to be considered when constructing the fault analysis model in step S2 include: the influence of the DC voltage feedforward controller in the active current limiting control strategy on the DC fault current peak, and the influence of different coordinated control strategies of the hybrid MMC and DCCB on the dynamic change process of the DC current of the fault line.

3. The MMC DC system fault analysis method according to claim 2, characterized in that: Step S2 specifically includes: S2-1. Under the ideal condition of AC grid voltage balance, study and establish the switching function mathematical model of MMC suitable for transient analysis, as well as the mathematical model in dq rotating coordinate system suitable for control analysis; S2-2. Considering the fact that the MMC mathematical model in the stationary coordinate system is not conducive to the control system design, the instantaneous symmetrical components of the three-phase unbalanced quantity and their representation in the stationary coordinate system and the rotating coordinate system are calculated; S2-3. Construct the time domain model and frequency domain mathematical model of MMC when the AC system grid voltage is unbalanced; S2-4. Analyze the power balance relationship on both sides of the AC and DC of the MMC converter station and construct relevant fault analysis models.

4. The MMC DC system fault analysis method according to claim 3, characterized in that: In step S2, when performing the fault analysis model, it is necessary to build corresponding fault models of different types occurring at different locations in the DC distribution network based on the research of various fault characteristics of the MMC converter DC distribution network.

5. The MMC DC system fault analysis method according to claim 4, characterized in that: When constructing the corresponding fault analysis models for different types of faults occurring at different locations in step S2, it includes: When various faults or abnormal operating conditions occur on the AC side, an effective parsing and analysis model for the AC side is established based on the control strategy of the MMC converter and the characteristics of various electrical signals on the AC and DC sides. When various faults or abnormal operating conditions occur on the DC side, an effective analysis model for the DC side is established based on the control strategy of the MMC converter and the characteristics of various electrical signals on the AC and DC sides. When the MMC converter itself encounters various faults and abnormal operating conditions, an effective parsing and analysis model of the MMC converter itself is established based on the output characteristics of the MMC converter and the characteristics of various electrical signals on the AC and DC sides.