DC side fault analysis method of DC power distribution system

Abstract

The invention belongs to the field of power system fault analysis, and aims to provide a DC side fault analysis method for a DC power distribution system, and the method comprises the steps: employing different DC power distribution network topological structures based on an MMC converter DC power distribution network, and carrying out the fault analysis of the DC side when various types of faults occur at the DC side of the DC power distribution network. The method can accurately analyze and process the interelectrode short-circuit fault and the single-pole grounding fault of the DC side of the DC power distribution network, can remarkably accelerate the discrimination of fault lines, can greatly improve the fault processing efficiency, and can greatly improve the operation power supply reliability of a power system.

Description

Technical Field

[0001] The invention belongs to the field of power system fault analysis, and in particular relates to a method for analyzing a DC side fault of a DC power distribution system. Background Art

[0002] Modular multilevel converter (MMC) is a new VSC topology. Compared with traditional two-level and three-level VSC, MMC composed of multiple cascaded submodules has the advantages of low switching loss, easy capacity expansion, low static and dynamic voltage balancing requirements for IGBTs, and high output voltage waveform quality. The fault characteristics of DC distribution networks based on MMC converters are different from those of AC distribution systems and conventional high-voltage DC transmission systems.

[0003] The DC distribution network adopts a small current grounding method. When a single-pole grounding fault occurs, there is a small short-circuit current, and it can continue to operate with load. The fault characteristics of the electrical quantity are not obvious, making it difficult for the system to quickly identify the fault line. If the system operates with a fault for a long time, the equipment will work under overvoltage for a long time, which can easily break down the insulation part of the power equipment and shorten its life; it may also cause large-scale power outages; it may even cause serious accidents such as power equipment burning and power cable explosion, which poses a huge threat to the stable operation and reliable power supply of the power grid. When a single-pole grounding occurs, through a reasonable grounding method, the AC side and the converter will not feed the fault current to the fault point, and the system can continue to transmit power in an asymmetric operation mode, but the rise of the non-fault pole voltage poses a challenge to the insulation of the system; in addition, the strong transient nature of its fault current poses a challenge to the design of the protection scheme. When an inter-pole short-circuit fault occurs, the fault current rises rapidly, and the rising rate can reach the kA level within ms, which poses a serious threat to the safety of the converter and then threatens the safe operation of the entire DC distribution network. Therefore, it is very important to analyze the characteristics of line faults and study the corresponding protection strategies.

[0004] Therefore, a DC side fault analysis method for a DC distribution system is needed to effectively identify and analyze the single-pole grounding fault line selection under the small current grounding mode of the DC distribution network. Summary of the invention

[0005] In order to address the deficiencies of the prior art, the present invention aims to provide a method for analyzing faults on the DC side of a DC power distribution system. The method adopts different DC distribution network topologies based on the MMC converter DC distribution network. When various types of faults occur on its DC side, the method can accurately analyze and process the inter-pole short-circuit faults and single-pole grounding faults on the DC side of the DC distribution network, which can significantly speed up the identification of faulty lines, greatly improve the fault handling efficiency, and greatly improve the power supply reliability of the power system operation.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: A method for analyzing a DC side fault in a DC power distribution system is implemented based on the following steps: S1. Based on the DC distribution network of MMC converter, the characteristics of inter-pole short-circuit fault and single-pole grounding fault on the DC side of the DC distribution network are extracted according to the main equipment and fault types in the DC distribution network protection system; S2. Construct the inter-pole short-circuit fault analysis model and the single-pole grounding fault analysis model on the DC side, and conduct detailed analysis on various power grid faults; S3. Using the constructed single-pole grounding fault analysis model, in the transient process of a single-pole grounding fault in the DC distribution network, the transient correlation coefficient rule is used to select the fault feeder or fault bus and determine the fault type; S4. Analyze the performance and fault losses of various MMC submodules in the DC power distribution system from the aspects of fault isolation speed, AC source blocking capability, and control strategy; S5. From the perspectives of investment cost and additional power loss, combined with case analysis, the economy and failure rate of various sub-modules are analyzed and compared to provide a basis for the subsequent selection of MMC converter sub-modules.

[0007] Furthermore, in step S2, based on the component-type protection strategy of the DC distribution network partitioning, a pole-to-pole short-circuit fault analysis model based on the integration of the DC distribution network is constructed.

[0008] Furthermore, the DC distribution network partition includes an AC side, a DC side, a converter, and a load.

[0009] Furthermore, step S3 specifically includes: S3-1. Using the modular multilevel converter MMC to inject a sinusoidal detection signal, considering the transition resistance and the single-pole ground fault distance and the MMC response speed, the optimal frequency of the detection signal is obtained, and the amplitude and injection time of the detection signal are obtained; S3-2. Analyze the response characteristics of the line to the detection signal, and then obtain the conclusion that the zero-mode impedance of the faulty line is inductive and the zero-mode impedance of the non-faulty line is capacitive; S3-3. For single-pole grounding faults in DC distribution networks, the transient correlation coefficient rule is used to select the fault line, and the faulty feeder or faulty bus is selected and the permanent fault is identified.

[0010] Furthermore, the transient correlation coefficient rule in step S3-2 compares the correlation of transient currents of the positive and negative poles of each feeder during the transient process of a single-pole grounding fault in the DC distribution network, and selects the fault line based on the correlation.

[0011] Furthermore, the protection system of the DC distribution network includes a measuring device, a relay, an outlet circuit breaker, and an isolation device.

[0012] Furthermore, the protection system of the DC distribution network divides the system into three areas: AC side protection, converter protection, and DC transmission line protection according to each major device and fault type.

[0013] The beneficial effects of the present invention are: The present invention plays a key guiding role in the processing of single-pole grounding faults and inter-pole short circuits under the low-current grounding mode of the DC distribution network, greatly speeds up the identification of faulty lines, and can 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.

[0014] Based on the analysis and research of existing MMC converters and their control strategies, the present invention analyzes and studies the performance of various MMC submodules from multiple aspects such as fault isolation speed, AC source blocking capability, and control strategy, and verifies them through simulation. In addition, from the perspectives of investment cost and additional power loss, combined with case analysis, the economic efficiency of various submodules is analyzed and compared. The research results obtained can provide a theoretical basis and reference for the selection of MMC converter submodules in engineering practice. DETAILED DESCRIPTION

[0015] 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.

[0016] A method for analyzing a DC side fault in a DC power distribution system is implemented based on the following steps: S1. Based on the DC distribution network of MMC converter, the characteristics of inter-pole short-circuit fault and single-pole grounding fault on the DC side of the DC distribution network are extracted according to the main equipment and fault types in the DC distribution network protection system.

[0017] The protection system of the DC distribution system includes measuring devices, relays, outlet circuit breakers, isolation equipment, etc. The DC distribution network protection system divides the system into three areas: AC side protection, converter protection, and DC side protection (DC transmission line protection) according to the main equipment and fault types.

[0018] S2. Construct the inter-pole short-circuit fault analysis model and the single-pole grounding fault analysis model on the DC side, and conduct detailed analysis on various power grid faults.

[0019] The inter-pole protection strategy of DC distribution network is an important guarantee for quickly and reliably removing faults, reducing the scope of fault impact, and ensuring the safe and stable operation of distribution network. It is a very important part of DC distribution network research.

[0020] The present invention is based on the component protection strategy of DC distribution network partition (such as AC side, DC side, converter, load), and constructs an inter-pole short-circuit fault analysis model based on the integration of DC distribution network. The present invention proposes protection schemes under different operation modes, different topologies, and different power levels, and determines the protection configuration, protection principle, protection setting, and protection action strategy of the DC distribution network.

[0021] S3. Using the constructed single-pole grounding fault analysis model, in the transient process of a single-pole grounding fault in the DC distribution network, the transient correlation coefficient rule is used to select the fault feeder or fault bus and determine the fault type. Specifically, it includes: S3-1. Using the modular multilevel converter MMC to inject a sinusoidal detection signal, considering the transition resistance and the single-pole ground fault distance and the MMC response speed, the optimal frequency of the detection signal is obtained, and the amplitude and injection time of the detection signal are obtained; S3-2. Analyze the response characteristics of the line to the detection signal, and then obtain the conclusion that the zero-mode impedance of the fault line is inductive and the zero-mode impedance of the non-fault line is capacitive. The transient correlation coefficient rule compares the correlation of the transient currents of the positive and negative poles of each feeder during the transient process of a single-pole grounding fault in the DC distribution network, and selects the fault line based on the correlation.

[0022] S3-3. For single-pole grounding faults in DC distribution networks, the transient correlation coefficient rule is used to select the fault line, and the faulty feeder or faulty bus is selected and the permanent fault is identified.

[0023] S4. Analyze the performance and fault losses of various MMC submodules in the DC power distribution system from the aspects of fault isolation speed, AC source blocking capability, and control strategy; S5. From the perspectives of investment cost and additional power loss, combined with case analysis, the economy and failure rate of various sub-modules are analyzed and compared to provide a basis for the subsequent selection of MMC converter sub-modules.

[0024] The present invention plays a key guiding role in the processing of single-pole grounding faults and inter-pole short circuits under the low-current grounding mode of the DC distribution network, greatly speeds up the identification of faulty lines, and can 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.

[0025] Based on the analysis and research of existing MMC converters and their control strategies, the present invention analyzes and studies the performance of various MMC submodules from multiple aspects such as fault isolation speed, AC source blocking capability, and control strategy, and verifies them through simulation. In addition, from the perspectives of investment cost and additional power loss, combined with case analysis, the economic efficiency of various submodules is analyzed and compared. The research results obtained can provide a theoretical basis and reference for the selection of MMC converter submodules in engineering practice.

[0026] Obviously, the embodiments described above are only a part of the embodiments of the present application, rather than all the embodiments. The preferred embodiments of the present application are given in the embodiments, but they 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 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 specification of this application, directly or indirectly used in other related technical fields, is similarly within the scope of patent protection of this application.

Claims

1. A method for analyzing DC side faults in a DC power distribution system, characterized in that: The method is implemented based on the following steps: S1. Based on the DC distribution network of MMC converter, the characteristics of inter-pole short-circuit fault and single-pole grounding fault on the DC side of the DC distribution network are extracted according to the main equipment and fault types in the DC distribution network protection system; S2. Construct the inter-pole short-circuit fault analysis model and the single-pole grounding fault analysis model on the DC side, and conduct detailed analysis on various power grid faults; S3. Using the constructed single-pole grounding fault analysis model, in the transient process of a single-pole grounding fault in the DC distribution network, the transient correlation coefficient rule is used to select the fault feeder or fault bus and determine the fault type; S4. Analyze the performance and fault losses of various MMC submodules in the DC power distribution system from the aspects of fault isolation speed, AC source blocking capability, and control strategy; S5. From the perspectives of investment cost and additional power loss, combined with case analysis, the economy and failure rate of various sub-modules are analyzed and compared to provide a basis for the subsequent selection of MMC converter sub-modules.

2. The DC side fault analysis method of a DC power distribution system according to claim 1, characterized in that: In step S2, based on the component-type protection strategy of the DC distribution network partition, an inter-pole short-circuit fault analysis model based on the integration of the DC distribution network is constructed.

3. The DC side fault analysis method of a DC power distribution system according to claim 2, characterized in that: The DC distribution network partition includes an AC side, a DC side, a converter, and a load.

4. The DC side fault analysis method of a DC power distribution system according to claim 1, characterized in that: Step S3 specifically includes: S3-1. Using the modular multilevel converter MMC to inject a sinusoidal detection signal, considering the transition resistance and the single-pole ground fault distance and the MMC response speed, the optimal frequency of the detection signal is obtained, and the amplitude and injection time of the detection signal are obtained; S3-2. Analyze the response characteristics of the line to the detection signal, and then obtain the conclusion that the zero-mode impedance of the faulty line is inductive and the zero-mode impedance of the non-faulty line is capacitive; S3-3. For single-pole grounding faults in DC distribution networks, the transient correlation coefficient rule is used to select the fault line, and the faulty feeder or faulty bus is selected and the permanent fault is identified.

5. The DC side fault analysis method of a DC power distribution system according to claim 4, characterized in that: The transient correlation coefficient rule in step S3-2 is to compare the correlation of the transient currents of the positive and negative poles of each feeder in the transient process of a single-pole grounding fault in the DC distribution network, and select the fault line based on the correlation.

6. The DC side fault analysis method of a DC power distribution system according to claim 1, characterized in that: The protection system of the DC distribution network includes a measuring device, a relay, an outlet circuit breaker, and an isolation device.

7. The DC side fault analysis method of a DC power distribution system according to claim 6, characterized in that: The protection system of the DC distribution network divides the system into three areas: AC side protection, converter protection, and DC transmission line protection according to each major equipment and fault type.