A feature quantity selection method for T-type transformer multi-type fault identification

Abstract

This application provides a feature quantity selection method for multi-type fault identification of T-type transformers, comprising: obtaining initial feature discrimination quantities of the T-type DC transformer; determining the three-phase power system fault type of the T-type DC transformer based on the initial feature discrimination quantities; selecting corresponding fault identification feature quantities and completing fault type determination based on the three-phase power system fault type; and selecting fault location feature quantities of the T-type DC transformer and completing fault location based on the determined corresponding fault type. The method proposed in this application, by selecting fault identification feature quantities and fault location feature quantities, achieves rapid discrimination and accurate location of multiple types of faults in T-type DC transformers. It can not only effectively distinguish between symmetrical and asymmetrical faults, but also further identify specific fault types and their locations. It has the advantages of clear criteria, simple implementation, and high reliability, and can significantly improve the sensitivity and selectivity of fault protection.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to a method for selecting feature quantities for multi-type fault identification of T-type transformers. Background Technology

[0002] T-type DC transformers, as key equipment in high-voltage DC power grids, have become the preferred solution for DC aggregation and DC step-up due to their advantages of low cost, high efficiency, and small construction scale. However, their characteristics, such as direct electrical coupling between high and low voltage ports, short fault propagation paths, and rapid fault current development, expose the system to severe fault risks during operation. The fault types of T-type DC transformers are complex and diverse, including internal faults occurring in submodules and bridge arms, as well as external faults such as inter-pole and pole-to-ground short circuits at the ports. Different types of faults have different electrical characteristics, evolution patterns, and severity, placing extremely high demands on the speed, selectivity, and reliability of fault detection and protection systems.

[0003] Achieving rapid and reliable identification of various fault types in T-type DC transformers is a primary prerequisite for constructing an effective protection system and ensuring the safe and stable operation of the DC power grid. The core of fault identification lies in selecting characteristic quantities that can sensitively reflect and effectively distinguish different fault states. Currently, the systematic selection method for fault characteristic quantities for the specific topology of T-type DC transformers is still imperfect. Existing research mostly focuses on the mechanism analysis of single or a few specific faults, lacking a fault characteristic quantity selection framework that can comprehensively consider internal component faults, internal branch faults, and external port faults, and clearly distinguish various types of faults.

[0004] The lack of an effective and systematic fault characteristic quantity selection strategy will lead to unclear protection logic, slow decision-making, and even misjudgment or failure to operate under complex fault conditions, which seriously hinders the engineering application of T-type DC transformers. Summary of the Invention

[0005] This application provides a feature quantity selection method for multi-type fault identification of T-type transformers. To solve the above-mentioned technical problems, this application adopts the following technical method: Firstly, this application provides a method for selecting feature quantities for multi-type fault identification of T-type transformers, including: Obtain the initial characteristic discrimination value of the T-type DC transformer; Based on the initial feature discrimination value, the fault type of the three-phase power system of the T-type DC transformer is determined; Based on the fault types of the three-phase power system, the corresponding fault identification feature quantities are selected and the fault type is determined. Based on the determined fault type, fault location feature quantities of the T-type DC transformer are selected and fault location is completed.

[0006] Optionally, the initial feature discrimination quantity includes the DC component of each of the three-phase bridge arm currents and the fundamental frequency component of each of the three-phase bridge arm currents.

[0007] Optionally, the three-phase power system fault types include three-phase symmetrical faults and three-phase asymmetrical faults. Determining the three-phase power system fault type of the T-type DC transformer based on the initial feature discrimination quantity includes: If the DC components of the current in each of the three-phase bridge arms are the same, and the sum of the fundamental frequency components of the current in each of the three-phase bridge arms is zero, the T-type DC transformer experiences a three-phase symmetrical fault; otherwise, the T-type DC transformer experiences a three-phase asymmetrical fault.

[0008] Optionally, the step of selecting corresponding fault identification feature quantities and completing fault type determination based on the fault type of the three-phase power system, for three-phase symmetrical faults, includes: The fault port voltage change rate, fault port voltage, and bridge arm DC current change rate directly connected to the fault port are selected as fault identification feature quantities to distinguish between three-phase common point short-circuit faults and external port short-circuit faults in three-phase symmetrical faults. If the rate of change of the fault port voltage is greater than the threshold of the rate of change of the fault port voltage, the fault port voltage is less than the threshold of the fault port voltage, and the rate of change of the bridge arm DC current is greater than the threshold of the bridge arm DC current, and all three conditions are met simultaneously, it is determined that the T-type DC transformer has an external port short circuit fault. If any one of the above three conditions is not met, the T-type DC transformer is determined to have a three-phase common point short circuit fault.

[0009] Optionally, the step of selecting corresponding fault identification feature quantities and completing fault type determination based on the fault type of the three-phase power system, for three-phase asymmetrical faults, includes: The bridge arm current sequence component is selected as the fault identification feature quantity to distinguish between single-phase ground fault and two-phase short circuit fault in three-phase asymmetrical faults. The bridge arm current sequence component includes positive sequence current, negative sequence current and zero sequence current. If the amplitudes of the positive sequence current, the negative sequence current and the zero sequence current are equal and all are not zero, it is determined that a single-phase ground fault has occurred in the T-type DC transformer. If the sum of the positive sequence current and the negative sequence current is zero, and the zero sequence current is zero, it is determined that a two-phase short circuit fault has occurred in the T-type DC transformer.

[0010] Optionally, the steps for selecting fault location features of a T-type DC transformer and completing fault location based on the determined fault type include: The inductance current of the bridge arm and the valve section current of the bridge arm are selected as the fault location characteristic quantities of the three-phase common point short circuit fault. If a three-phase common point short-circuit fault occurs in a T-type DC transformer, extract the bridge arm valve section current and bridge arm inductor current corresponding to the fault location feature quantity. Based on the bridge arm valve segment current and the bridge arm inductor current, the bridge arm current difference characterization is calculated. It is determined that the bridge arm current difference is less than the bridge arm current difference threshold. If so, the fault is determined to have occurred at a common point of the bridge arm; If not, the fault is determined to occur inside the valve section of the bridge arm.

[0011] Optionally, based on the determined fault type, the fault location feature quantity of the T-type DC transformer is selected and the fault location is completed; the steps for single-phase ground faults and two-phase short-circuit faults include: The inductance current of the bridge arm and the valve section current of the bridge arm are selected as the fault location characteristic quantities for single-phase ground fault and two-phase short circuit fault. If a single-phase ground fault or a two-phase short circuit fault occurs in a T-type DC transformer, extract the bridge arm valve section current and bridge arm inductor current corresponding to the fault location feature quantity. Based on the bridge arm valve segment current and the bridge arm inductor current, the bridge arm current difference characterization is calculated. Determine whether the bridge arm current difference indicator is less than the bridge arm current difference indicator threshold; If so, the fault is determined to have occurred at a common point of the bridge arm; If not, the fault is determined to occur inside the valve section of the bridge arm.

[0012] Secondly, this application also provides a computer-readable medium storing computer program code that, when executed by a processor, implements the method described in the first aspect.

[0013] This application has the following beneficial effects: The method proposed in this application achieves rapid identification and accurate location of multiple types of faults in T-type DC transformers by selecting fault identification feature quantities and fault location feature quantities. It can not only effectively distinguish between symmetrical and asymmetrical faults, but also further identify specific fault types and their locations. It has the advantages of clear criteria, simple implementation and high reliability, and can significantly improve the sensitivity and selectivity of fault protection. Attached Figure Description

[0014] Figure 1 A flowchart illustrating a feature quantity selection method for multi-type fault identification of a T-type transformer, provided in an embodiment of this application; Figure 2Equivalent circuit diagram of a three-phase common point short-circuit fault for a T-type DC transformer provided in this application embodiment; Figure 3 Equivalent circuit diagram of external port short-circuit fault of T-type DC transformer provided in the embodiments of this application; Figure 4 A circuit diagram of a single-phase ground fault in a T-type DC transformer provided in this application embodiment; Figure 5 A circuit diagram of a two-phase short-circuit fault in a T-type DC transformer provided in this application embodiment; Figure 6 Simulated waveforms of high-voltage port voltage and high-voltage bridge arm current when a three-phase common point short-circuit fault and a high-voltage port short-circuit fault occur, as provided in the embodiments of this application. Figure 6 (a) Simulated waveforms of the high-voltage port voltage when a three-phase common point short-circuit fault and a high-voltage port short-circuit fault occur. Figure 6 (b) Simulation waveforms of the high-voltage bridge arm current when a three-phase common point short-circuit fault and a high-voltage port short-circuit fault occur; Figure 7 The simulation waveform diagram of the low-voltage bridge arm current sequence component when the corresponding fault occurs is provided in the embodiment of this application; Figure 7 (a) Simulated waveform of the low-voltage bridge arm current sequence component when a two-phase short-circuit fault occurs. Figure 7 (b) Simulation waveform of the low-voltage bridge arm current sequence component when a single-phase ground fault occurs; Figure 8 The simulation waveform of the high-voltage bridge arm current when the corresponding fault occurs is provided in the embodiments of this application; Figure 8 (a) is a simulation waveform diagram of a single-phase ground fault in the high-voltage bridge arm between valve sections. Figure 8 (b) is a simulation waveform diagram of a single-phase ground fault in a high-voltage bridge arm at the common point. Detailed Implementation

[0015] To facilitate understanding by those skilled in the art, the present application will be further described below in conjunction with embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present application.

[0016] To solve the above technical problems, such as Figure 1 As shown, this application proposes a feature quantity selection method for multi-type fault identification of T-type DC transformers, including: Step S101: Obtain the initial characteristic discrimination value of the T-type DC transformer; When a fault occurs in a T-type DC transformer, the first step is to determine the symmetry of the fault, that is, to determine whether the fault is a three-phase symmetrical fault or a three-phase asymmetrical fault.

[0017] During the operation of a T-type DC transformer, the currents of each phase arm exhibit good symmetry under normal operating conditions. When a fault occurs in the system, if the changes in the three-phase arm currents are basically the same, it can be considered a three-phase symmetrical fault; if there are significant differences in the changes in the currents of each phase, it can be determined as a three-phase asymmetrical fault. Therefore, the initial characteristic discrimination quantity of the T-type DC transformer is obtained here. This initial characteristic discrimination quantity includes the DC component and the fundamental frequency component of each phase arm current.

[0018] Step S102: Based on the initial feature discrimination quantity, determine the three-phase power system fault type of the T-type DC transformer; The three-phase power system fault types in this step generally include three-phase symmetrical faults and three-phase asymmetrical faults. Using the initial characteristic discrimination quantity obtained in the previous step, and based on the aforementioned analysis, the above discrimination can be achieved. When a three-phase symmetrical fault occurs in the system, that is, the DC components of the currents in each of the three phases are the same, and the sum of the fundamental frequency components of the currents in each of the three phases is zero, as shown in the following formula: (1) in, I X_DC,y (y=a,b,c) means y Phase bridge arm X The DC component of the current I X_AC,y (y=a,b,c) means y Phase bridge arm X The fundamental frequency component of the current.

[0019] If the sum of the DC component of the current in each of the three-phase bridge arms and the fundamental frequency component of the current in each of the three-phase bridge arms does not satisfy the condition of formula (1), then the T-type DC transformer is determined to have a three-phase asymmetrical fault. After the fault symmetry is determined, the corresponding fault type identification process is entered according to the determination result.

[0020] Step S103: Based on the fault type of the three-phase power system, select the corresponding fault identification feature quantity and complete the fault type determination; Three-phase symmetrical faults in T-type DC transformers mainly include three-phase common point short-circuit faults and external port short-circuit faults. Three-phase asymmetrical faults in T-type DC transformers mainly include single-phase ground faults and two-phase short-circuit faults. Therefore, it is necessary to determine the type of fault. The determination process is as follows: The equivalent circuit diagrams for three-phase common point short-circuit faults and external port short-circuit faults are shown in Figures 2 and 3, respectively. U X_DC,y (X=H,L, y=a,b,c) represents y Phase bridge arm X DC component of voltage in cascaded submodulesu X_AC,y (X=H,M,L, y=a,b,c) represents y Phase bridge arm X The AC voltage component of the cascaded submodules u X_DC,y (X=H,M,L, y=a,b,c) represents y Phase bridge arm X DC component of voltage in cascaded submodules I g,ab This is the short-circuit current between phase A and phase B. I g,bc This is the short-circuit current between phase B and phase C. L X,y (X=H,L,M, y=a,b,c) represents y Phase bridge arm X Bridge arm inductance, U L This indicates the low-voltage side port voltage. U H Indicates the voltage at the high-voltage side port. R L For the high-voltage side load resistor, I g This is the short-circuit current. R g This is the resistance of the short-circuit branch.

[0021] The two most significant fault characteristics of three-phase common-point short-circuit faults and external port short-circuit faults are the port voltage and its rate of change, and the direction of the high- and low-voltage bridge arm currents. When an external port short-circuit fault occurs, the fault port voltage drops sharply to zero, and the bridge arm current directly connected to the fault port flows towards the port. Conversely, when a three-phase common-point short-circuit fault occurs, the fault port voltage changes very little, while the bridge arm current directly connected to the fault point flows towards the common point; this current flow direction is completely opposite to that of an external port short-circuit fault. Based on these characteristics, sufficient characteristic quantities can be selected to effectively distinguish between the two types of faults.

[0022] For three-phase common-point short-circuit faults and external port short-circuit faults, the fault port voltage and its rate of change, and the port current direction can be selected as fault characteristic quantities. Since the voltage change caused by a port short-circuit fault is much greater than the voltage and current change caused by a three-phase short-circuit fault, under the premise of determining whether the fault is symmetrical, the fault port voltage change rate, the fault port voltage, and the DC current change rate of the bridge arm directly connected to the fault port are selected as fault identification characteristic quantities to distinguish between three-phase common-point short-circuit faults and external port short-circuit faults in three-phase symmetrical faults. That is: if the fault port voltage change rate is greater than the fault port voltage change rate threshold, the fault port voltage is less than the fault port voltage threshold, and the bridge arm DC current change rate is greater than the bridge arm DC current change rate threshold, and all three conditions are met simultaneously, as shown in the following formula, an external port short-circuit fault is determined to have occurred in the T-type DC transformer: (2) Where, d U P / d t The rate of change of the fault port voltage, (d U P / d t ) thres The threshold for the rate of change of voltage at the fault port. U P The voltage at the faulty port. U P_thres The fault port voltage threshold. I B For the DC current of the bridge arm directly connected to the fault port, d I B / d t The rate of change of DC current in the bridge arm directly connected to the fault port, (d I B / d t ) thres The threshold value for the rate of change of DC current in the bridge arm directly connected to the faulty port. d I B / d t >0 indicates that the fault current flows from the port to the common point.

[0023] Conversely, a three-phase common point short-circuit fault will satisfy equation (3): (3) In the formula, d I B / d t >0 indicates that the fault current flows from the common point to the port. It can be seen that the two characteristic quantities are mutually exclusive. Therefore, it can also be understood that when any one of the three conditions in formula (2) is not met, it is determined that a three-phase common point short circuit fault has occurred in the T-type DC transformer.

[0024] The equivalent circuit diagrams for single-phase-to-ground short-circuit faults and two-phase-to-phase short-circuit faults are as follows: Figure 4 and Figure 5 As shown. The key to identifying these two types of faults lies in identifying the number of phases that have failed and whether a ground fault has occurred. For determining the number of faulty phases, the arm current can be directly used as a selectable fault characteristic quantity. If only one phase fails, the arm current of the faulty phase will be significantly greater than that of the other two phases; if two phases fail, the arm currents of two faulty phases will be significantly higher than that of one non-faulty phase. T-type DC transformers inherently possess a three-phase AC system. When an asymmetrical fault occurs, the sequence component behaves similarly to that of a traditional three-phase AC transmission system. Therefore, the arm current sequence component can be selected as a fault identification characteristic quantity to distinguish between single-phase ground faults and two-phase short-circuit faults in three-phase asymmetrical faults, thereby determining the number of faulty phases and thus identifying the fault type. Therefore, for single-phase ground faults and two-phase short-circuit faults, i.e., three-phase asymmetrical faults, the arm current sequence component can be selected as the fault characteristic quantity.

[0025] After selecting the bridge arm current sequence component as the characteristic quantity, the fault can be identified based on the fault characteristic quantity. The characteristic quantity of a single-phase ground fault is shown in equation (4): (4) in, That is, the positive sequence current generated at the fault point of phase a. That is, the negative sequence current generated at the fault point of phase a. This refers to the zero-sequence current generated at the fault point of phase a. It can be seen that when a single-phase ground fault occurs, the positive-sequence, negative-sequence, and zero-sequence components of the short-circuit current have equal amplitudes, and the zero-sequence current is always present, which is a typical characteristic of this fault. Therefore, when the amplitudes of the positive-sequence current, negative-sequence current, and zero-sequence current are equal and all are not zero, it can be determined that a single-phase ground fault has occurred in the T-type DC transformer.

[0026] The characteristic quantities of a two-phase short-circuit current fault are shown in equation (5): (5) It is evident that when a two-phase short-circuit fault occurs, the zero-sequence component of the short-circuit current is zero, which is a typical characteristic of this fault. This characteristic is significantly different from a single-phase ground fault: a single-phase ground fault exhibits zero-sequence current, while a two-phase short-circuit fault does not, consistent with the rule in three-phase AC transmission systems that "ground faults generate zero-sequence current, while unground faults do not generate zero-sequence current." Therefore, when the sum of the positive-sequence current and the negative-sequence current is zero, and the zero-sequence current is zero, it can be determined that a two-phase short-circuit fault has occurred in the T-type DC transformer.

[0027] Step S104: Based on the determined corresponding fault type, select the fault location feature quantity of the T-type DC transformer and complete the fault location.

[0028] After confirming the fault type through the above steps, the location of the fault cannot yet be determined. Therefore, it is necessary to select fault characteristic quantities for fault location. Since the location of external port faults can be determined without localization, the fault location in this step is designed for internal faults. That is, when it is confirmed that the fault is one of the following three types: three-phase common point short circuit, single-phase ground short circuit, or two-phase short circuit, the following method is used to accurately locate the fault location. The specific location process is as follows: To accurately locate faults in T-type DC transformers, characteristic quantities can be selected based on current consistency and used for fault location. The core of this approach lies in monitoring and comparing the waveform characteristics of the bridge arm inductor current and the valve section (i.e., cascaded submodule) current to construct a key criterion that clearly distinguishes the fault location. The principle is that under normal operating conditions, the inductor current flowing through a bridge arm must include all cascaded submodules connected to that arm in its flow path; that is, the inductor current will entirely flow through the valve section. Therefore, the measured bridge arm inductor current should maintain a high degree of consistency with the current flowing through the valve section of that bridge arm in amplitude, phase, and waveform.

[0029] When a fault occurs inside the transformer, this inherent current consistency will be disrupted. Specifically, if a short-circuit fault occurs within a submodule or valve section, the fault will create a new low-impedance short-circuit path at the fault point, causing a sharp increase in the current of that faulty valve section. This results in a significant difference between the current and the bridge arm inductance current, thus disrupting current consistency. However, if the fault occurs at the common connection point of the three bridge arms within a phase, the fault effect will propagate to the entire bridge arm circuit. In this case, the paths of all currents flowing through the valve sections of the faulty bridge arm completely overlap with the paths of the bridge arm inductance current. Therefore, the transformer bridge arm inductance current and valve section current still maintain current consistency.

[0030] Based on this principle, the bridge arm inductor current and the bridge arm valve segment current, as well as their difference, can be selected as fault characteristic quantities for locating internal faults. If a bridge arm current difference characteristic is detected between one or several specific valve segment currents and the bridge arm current, and if this bridge arm current difference characteristic is greater than the bridge arm current difference characterization threshold, it can be determined that the fault occurs inside that valve segment; conversely, if the currents of all valve segments in the entire bridge arm change synchronously with the bridge arm current, it can be determined that the fault occurs at the common connection point of that bridge arm. Based on the above fault characteristic quantity selection method, the short-circuit fault characteristic quantity occurring at the common point can be defined as shown in equation (6): (6) in, I X_SM Indicates bridge armX Valve section current, I X_L Indicates the current of inductor X in bridge arm. I XD Indicates bridge arm X Current difference characterization, I XD_thres Indicates bridge arm X The current difference characterizes the threshold. Equation (6) shows that when the fault occurs at the common point, the difference between the bridge arm inductor current and the bridge arm valve segment current will not exceed the threshold, and the current consistency is still maintained.

[0031] The characteristic quantities of short-circuit faults occurring inside the valve section are shown in equation (7): (7) Equation (7) shows that when the fault occurs inside the valve section, the difference between the bridge arm inductor current and the bridge arm valve section current will exceed the threshold, and the current consistency will be disrupted.

[0032] Experimental verification: To verify the effectiveness of the method proposed in this application, a Simulink simulation model was built for verification. The main variable parameters of the implementation case are shown below: Table 1. Main parameters of the simulation model of the bridge arm reuse type high voltage DC transformer ; Combination Figures 6 to 8 It can provide a visual explanation of the effectiveness of the selected feature quantities. Among them, Figure 6 This is used to demonstrate the changing patterns of fault characteristic quantities under three-phase common point short-circuit faults and high-voltage port short-circuit faults. Figure 6 (a) Reflects the characteristics of voltage changes at the high-voltage port during the two types of faults. Figure 6 (b) Reflecting the changing characteristics of the corresponding high-voltage bridge arm current, it can be seen that the voltage drop at the port under a short-circuit fault is much greater than that under a three-phase phase-to-phase short-circuit fault, and the direction of change of the high-voltage bridge arm current is opposite under the two faults. Therefore, the fault symmetry can be determined by judging the voltage drop at the port and the direction of change of the bridge arm current. Figure 7 Used to demonstrate the changing characteristics of the bridge arm current sequence components under three-phase unbalanced faults. Figure 7 (a) shows the distribution of the positive sequence, negative sequence, and zero sequence components of the fault branch current during a two-phase short-circuit fault. Figure 7 (b) indicates the variation characteristics of the three-sequence components during a single-phase ground fault. The former has the same amplitude of the three-sequence components and is not 0, while the latter has a zero-sequence component amplitude of 0. Therefore, the fault type can be distinguished by judging the relationship of the zero-sequence component amplitude. Figure 8 Used to reflect the differences in current characteristics at different fault locations. Figure 8 (a) shows the high-voltage bridge arm current waveform during a single-phase ground fault inside the valve section. Figure 8 (b) shows the corresponding waveform when a single-phase-to-ground short-circuit fault occurs at the common point. In the former, the bridge arm inductor current and the valve segment current are consistent, while in the latter, the bridge arm inductor current and the valve segment current change differently. Therefore, the fault location can be determined by judging the consistency between the bridge arm inductor current and the valve segment current. The results show that the selected fault characteristic quantities can clearly distinguish different types of faults and faults occurring in different locations, which has guiding significance for the research of fault identification and location strategies.

[0033] In summary, the method proposed in this application, by selecting fault identification features and fault location features, enables rapid identification and accurate location of multiple types of faults in T-type DC transformers. It can not only effectively distinguish between symmetrical and asymmetrical faults, but also further identify specific fault types and their locations. It has the advantages of clear criteria, simple implementation, and high reliability, and can significantly improve the sensitivity and selectivity of fault protection.

[0034] In some embodiments, this application also provides a computer system including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described method embodiments.

[0035] This application also provides a computer-readable storage medium for storing a computer program. This computer-readable storage medium can be applied to a computer device, and the computer program causes the computer device to execute the corresponding processes in the methods described above in the embodiments of this application; for brevity, further details are omitted here.

[0036] The above embodiments are preferred implementations of this application. In addition, this application can be implemented in other ways. Any obvious substitutions without departing from the concept of this technical solution are within the protection scope of this application.

[0037] To facilitate understanding by those skilled in the art of the improvements made by this application compared to the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this application.

Claims

1. A method for selecting feature quantities for multi-type fault identification of T-type transformers, characterized in that, include: Obtain the initial characteristic discrimination value of the T-type DC transformer; Based on the initial feature discrimination value, the fault type of the three-phase power system of the T-type DC transformer is determined; Based on the fault types of the three-phase power system, the corresponding fault identification feature quantities are selected and the fault type is determined. Based on the determined fault type, fault location feature quantities of the T-type DC transformer are selected and fault location is completed.

2. The method according to claim 1, characterized in that, The initial feature discrimination quantity includes the DC component of the current of each of the three-phase bridge arms and the fundamental frequency component of the current of each of the three-phase bridge arms.

3. The method according to claim 2, characterized in that, The three-phase power system fault types include three-phase symmetrical faults and three-phase asymmetrical faults. Determining the three-phase power system fault type based on the initial feature discrimination quantity includes: If the DC components of the current in each of the three-phase bridge arms are the same, and the sum of the fundamental frequency components of the current in each of the three-phase bridge arms is zero, the T-type DC transformer experiences a three-phase symmetrical fault; otherwise, the T-type DC transformer experiences a three-phase asymmetrical fault.

4. The method according to claim 3, characterized in that, The steps for selecting corresponding fault identification features and determining the fault type based on the three-phase power system fault type include: The fault port voltage change rate, fault port voltage, and bridge arm DC current change rate directly connected to the fault port are selected as fault identification feature quantities to distinguish between three-phase common point short-circuit faults and external port short-circuit faults in three-phase symmetrical faults. If the rate of change of the fault port voltage is greater than the threshold of the rate of change of the fault port voltage, the fault port voltage is less than the threshold of the fault port voltage, and the rate of change of the bridge arm DC current is greater than the threshold of the bridge arm DC current, and all three conditions are met simultaneously, it is determined that the T-type DC transformer has an external port short circuit fault. If any one of the above three conditions is not met, the T-type DC transformer is determined to have a three-phase common point short circuit fault.

5. The method according to claim 3, characterized in that, The steps for selecting corresponding fault identification features and determining the fault type based on the three-phase power system fault type, specifically for three-phase asymmetrical faults, include: The bridge arm current sequence component is selected as the fault identification feature quantity to distinguish between single-phase ground fault and two-phase short circuit fault in three-phase asymmetrical faults. The bridge arm current sequence component includes positive sequence current, negative sequence current and zero sequence current. If the amplitudes of the positive sequence current, the negative sequence current and the zero sequence current are equal and all are not zero, it is determined that a single-phase ground fault has occurred in the T-type DC transformer. If the sum of the positive sequence current and the negative sequence current is zero, and the zero sequence current is zero, it is determined that a two-phase short circuit fault has occurred in the T-type DC transformer.

6. The method according to claim 4, characterized in that, The steps for determining the corresponding fault type, selecting fault location feature quantities of the T-type DC transformer, and completing fault location for a three-phase common point short-circuit fault include: The inductance current of the bridge arm and the valve section current of the bridge arm are selected as the fault location characteristic quantities of the three-phase common point short circuit fault. If a three-phase common point short-circuit fault occurs in a T-type DC transformer, extract the bridge arm valve section current and bridge arm inductor current corresponding to the fault location feature quantity. Based on the bridge arm valve segment current and the bridge arm inductor current, the bridge arm current difference characterization is calculated. It is determined that the bridge arm current difference is less than the bridge arm current difference threshold. If so, the fault is determined to have occurred at a common point of the bridge arm; If not, the fault is determined to occur inside the valve section of the bridge arm.

7. The method according to claim 4, characterized in that, The fault location feature quantity of the T-type DC transformer is selected based on the corresponding fault type determined by the judgment, and the fault location is completed; the steps for single-phase ground fault and two-phase short circuit fault include: The inductance current of the bridge arm and the valve section current of the bridge arm are selected as the fault location characteristic quantities for single-phase ground fault and two-phase short circuit fault. If a single-phase ground fault or a two-phase short circuit fault occurs in a T-type DC transformer, extract the bridge arm valve section current and bridge arm inductor current corresponding to the fault location feature quantity. Based on the bridge arm valve segment current and the bridge arm inductor current, the bridge arm current difference characterization is calculated. Determine whether the bridge arm current difference indicator is less than the bridge arm current difference indicator threshold; If so, the fault is determined to have occurred at a common point of the bridge arm; If not, the fault is determined to occur inside the valve section of the bridge arm.

8. A computer-readable medium, characterized in that, The system contains computer program code that, when executed by a processor, implements the method as described in any one of claims 1 to 7.

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