Phase selection method and related device based on double-end weakly fed ac system sequence component under multi-frequency injection

Abstract

The application provides a kind of based on the phase selection method of double-end weakly fed ac system sequence component under multi-frequency injection and related device, the phase selection method includes: the amplitude information of negative sequence current and positive sequence voltage at new energy side protection installation is collected, amplitude information is used as starting criterion, the switching of harmonic injection mode is realized;On the basis of harmonic injection, the fault sequence network of double-end weakly fed ac system under specific positive and negative sequence harmonic is analyzed, the phase change law of sequence component voltage under different fault types is confirmed, and the phase difference is used to construct the phase selection criterion;The sequence component phase selection scheme based on multi-frequency signal injection is proposed.The application adapts to different system characteristics, ensures the correct action of phase selection element, has strong transition resistance tolerance and high sensitivity.

Description

Technical Field

[0001] This invention relates to the field of protection for MMC transmission lines connected to new energy sources, specifically a method and related device for phase selection based on a model of a dual-terminal weak-feed AC system using harmonic injection. Background Technology

[0002] Driven by energy transition and the increasing maturity of power electronics technology, the large-scale integration of renewable energy into the power system is an inevitable trend. The fault characteristics of renewable energy differ from those of conventional power grids, posing adaptation challenges for power frequency protection. Traditional protection systems are susceptible to failure to operate or maloperation due to the influence of power electronic devices connected at both ends of the line. Existing research primarily addresses the inadequacy of traditional phase selection elements in single-ended weak-feed systems, focusing on sequence component phase selection elements under power frequency conditions. This research improves control strategies and protection criteria. However, when constructing the criteria, the renewable energy system at the receiving end is considered a synchronous machine system, a stable and strong power source. When the receiving end is also a controlled weak power source, the underlying principles of the constructed criteria also suffer from adaptability issues. Summary of the Invention

[0003] To address the shortcomings of the existing technologies, this invention provides a sequence component phase selection method and related device for a dual-terminal weak-feed AC system based on multi-frequency injection, in order to solve the problem of unreliability of traditional protection discrimination methods caused by frequency offset characteristics and harmonics under new energy access.

[0004] The technical solution provided by this invention: a sequence component phase selection method for a dual-terminal weakly fed AC system based on multi-frequency injection, comprising:

[0005] The amplitude information of negative sequence current and positive sequence voltage at the protection installation point on the new energy side is collected, and the amplitude information is used as the start-up criterion to realize the switching of harmonic injection mode;

[0006] Based on harmonic injection, the fault sequence network of a double-ended weakly fed AC system under specific positive and negative sequence harmonics is analyzed, the phase change law of the sequence component voltage under different fault types is confirmed, and the phase selection criterion is constructed by utilizing the phase difference.

[0007] A sequence component phase selection scheme based on multi-frequency signal injection is proposed.

[0008] Furthermore, the acquisition of amplitude information of negative sequence current and positive sequence voltage at the new energy side protection installation point, and the use of amplitude information as a start-up criterion, includes:

[0009] When a system fault occurs, the changes in the positive-sequence voltage and negative-sequence current content at the protection installation point on the new energy side are used as the initiation criterion for control strategy switching (DN).

[0010] , (1)

[0011] In the formula, DN is the start signal for control switching, and "∪" represents logical "OR"; S n S p These are the positive and negative order discrimination signals, respectively; U + I - These represent the positive and negative sequence component amplitudes of the voltage at the new energy side protection installation point; ε=0.15, and the above variable U + To use the system rated voltage U N As the base value, the per-unit value, I - This is the actual value;

[0012] The energy side injects harmonic signals by changing the command value. The specific changes are as follows:

[0013] , (2)

[0014] In the formula i d i q d in the power frequency synchronous rotating coordinate system q The shaft currents, their maximum values ​​are i dmax i qmax The minimum values ​​are respectively i dmin i qmin i pv is the actual output current phasor of the new energy side; M and h are the amplitudes of the fundamental current and harmonic current, respectively. The values ​​of M, h, and h are related to the frequency of the injected harmonics. All of these are parameters to be tuned;

[0015] At this point, based on the injection frequency The generation frequency is and Two harmonics of different frequencies.

[0016] Furthermore, for any injected k-th harmonic, the corresponding three-phase phase relationship is as follows:

[0017] , (3)

[0018] In the formula: , , These represent the three-phase voltages A, B, and C, respectively. This refers to the amplitude of the harmonic voltage. The fundamental angular frequency; To inject harmonic multiples;

[0019] Mathematical analysis shows that:

[0020] For h = 3k + 1 (k = 0, 1, 2…):

[0021] , (4)

[0022] Its phase relationship is the same as that of the power frequency fundamental frequency, and it has positive sequence characteristics.

[0023] For h = 3k + 2 (k = 0, 1, 2, ...):

[0024] , (5)

[0025] The phase relationship is determined to be ACB, which has a negative order characteristic.

[0026] For h=3k (k=1,2,3…):

[0027] (6)

[0028] The three phases exhibit in-phase characteristics and zero-sequence characteristics.

[0029] By utilizing the relationship between the frequency of the injected harmonic signal and the sequence component, the required sequence electrical quantity signal is injected to complete the subsequent protection structure.

[0030] Furthermore, based on harmonic injection, the lower-order component network of the harmonic injection signal was analyzed, and the phase variation law of the sequence component voltage under different fault types was confirmed, including:

[0031] 1) Single-phase ground fault

[0032] A phase-to-ground short-circuit fault occurs on the line. Based on the injected higher-order positive-sequence and negative-sequence harmonics, an equivalent composite sequence network diagram based on the harmonic signal is constructed, and its boundary conditions are as follows:

[0033] , (7)

[0034] In the formula: , and These represent the positive harmonic sequence, negative harmonic sequence, and zero-sequence voltages measured at the protection installation location, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence harmonic currents measured at the protection installation location, respectively. and These represent the line impedances on the new energy side and the MMC side at harmonic frequencies, respectively. and These represent the equivalent impedance of the system at the harmonic frequencies; the subscripts '0', '1', and '2' represent the positive-sequence, negative-sequence, and zero-sequence components, respectively. ,

[0035] Further derivation yields the relationship between the positive, negative, and zero-sequence harmonic voltages on the new energy side as shown in equation (8):

[0036] , (8)

[0037] Substituting the boundary conditions for a single-phase ground fault, we obtain equation (9):

[0038] , (9)

[0039] At this point, the phase relationship can be expressed as:

[0040] , (10)

[0041] In equation (10), except for R f The phases of all other terms are close to 90°, and the impedance amplitudes of transmission lines, transformers, and synchronous machines are all relatively large under the injection of higher harmonics.

[0042] Substituting into equation (10), we get:

[0043] , (11)

[0044] Substituting the data for each phase, we can obtain: Approximately 180°, and Approximately 0°,

[0045] Based on this, and considering the differences in system parameters and the range of transition resistance variation, with a certain margin, the phase variation range of the positive-sequence, negative-sequence, and zero-sequence voltages in a phase-A ground fault is as follows:

[0046] , (12)

[0047] 2) Phase-to-phase fault

[0048] A phase-to-phase fault occurred on the line (BC phase-to-phase). Due to the phase-to-phase fault, R... f The voltage is relatively small, and with the increase of line impedance under high-frequency harmonics, Rf can be ignored. In this scenario, the positive-sequence voltage and negative-sequence voltage of the harmonics at the protection installation point can be expressed as Equation (13):

[0049] , (13)

[0050] The boundary conditions for the BC fault can be expressed as equation (14):

[0051] , (14)

[0052] At this point, substituting the values ​​will yield the phase angle relationship. near ,

[0053] The phase variation range of the positive and negative sequence voltages for interphase faults (BC) is as follows:

[0054] , (15)

[0055] 3) Phase-to-phase fault

[0056] When a BCG phase-to-ground short-circuit fault occurs on the line, the sequence harmonic voltages at the protection installation point are expressed as equation (16):

[0057] , (16)

[0058] Substituting the BCG fault boundary conditions, it can be expressed as equation (17): , (17)

[0059] Based on equation (17), we derive = , = ,

[0060] At this point, considering the effect of the transition resistance and taking a certain margin, the phase change range of the positive sequence, negative sequence, and zero sequence voltages in the BCG phase-to-phase grounding fault is derived as follows:

[0061] , (18)

[0062] 4) Three-phase fault

[0063] When a three-phase fault occurs in the system, regardless of whether harmonics are injected into the converter, there are no negative sequence components or zero sequence components at the protection installation point. Only positive sequence loops exist in the fault network. This characteristic can be used to select the fault phase.

[0064] Furthermore, a sequence component phase selection scheme based on multi-frequency signal injection is proposed, including:

[0065] For three-phase faults, the fault type is determined by whether only positive-sequence components are present. For asymmetrical faults, firstly, harmonic injection is initiated when the starting criterion is met; secondly, the fault type is determined by the phase difference between the sequence voltage components of the injected harmonics at the protection installation point; finally, based on... and The range is used to determine the specific fault phase, and α and β are defined as the phase difference, where α = , β= .

[0066] Further steps to confirm the phase selection scheme are as follows:

[0067] 1) Determine the fault type based on the values ​​of negative sequence current and positive sequence voltage at the protection installation location, i.e.:

[0068] , (19)

[0069] In the formula: The system's rated voltage.

[0070] The phase selection element that satisfies equation (19) will determine that an asymmetrical grounding fault has occurred in the system;

[0071] 2) If the amplitude criterion in step one is not met, then the amplitude of the negative sequence voltage is compared, i.e.:

[0072] , (20)

[0073] if If the amplitude satisfies equation (20), then the system experiences a three-phase symmetrical fault; otherwise, the system experiences an interphase fault.

[0074] The specific fault phase is determined based on the values ​​of α and β.

[0075] Another technical solution provided by this invention: a sequence component phase selection device for a dual-terminal weak-feed AC system based on multi-frequency injection, realizing the above-mentioned phase selection method, comprising:

[0076] The data acquisition module is used to collect the amplitude information of negative sequence current and positive sequence voltage at the new energy side protection installation point, and uses the amplitude information as a start-up criterion to realize the switching of harmonic injection mode;

[0077] The fault phase discrimination module is used to identify the fault phase based on the phase relationship between positive sequence and negative sequence, and zero sequence and negative sequence under different fault types, based on harmonic injection, and to complete the fault phase selection.

[0078] The phase selection scheme output module obtains the phase selection scheme based on the phase change law of the sequence component voltage under different fault types.

[0079] Another technical solution provided by the present invention is a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the steps of the above-described sequence component phase selection method for a dual-ended weakly fed AC system based on multi-frequency injection.

[0080] Another technical solution provided by the present invention is a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the above-described sequence component phase selection method for a dual-ended weakly fed AC system based on multi-frequency injection.

[0081] Compared with the prior art, the present invention has the following technical effects:

[0082] This invention adopts the concept of active detection protection. By injecting a stable harmonic sequence power supply, it constructs protection criteria by utilizing the fault boundary conditions and the phase relationship of harmonic sequence components under different fault types. This method can complete the operation under complex working conditions and different fault types. Attached Figure Description

[0083] Figure 1 This is a flowchart of the present invention;

[0084] Figure 2 This is a topology diagram of a large-scale photovoltaic system connected directly to the power system via flexible grid connection, as shown in an embodiment of the present invention.

[0085] Figure 3 Diagram of the control loop and topology for new energy sources;

[0086] Figure 4 The diagram shows the composite sequence network of harmonic faults during a phase A ground fault.

[0087] Figure 5 This is a composite sequence network diagram of phase-to-phase fault harmonic faults (BC phase-to-phase).

[0088] Figure 6 This invention presents the phase selection results when different types of faults occur in the double-ended weak feeder line. Detailed Implementation

[0089] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0090] This invention provides a sequence component phase selection method for a dual-terminal weak-feed AC system based on multi-frequency injection, comprising:

[0091] Step 1. Analyze the working principle of the limiting module and the harmonic injection method of the limiting module on the new energy side;

[0092] Step 2. In the initial stage of the fault, the characteristics of different fault types are utilized, combined with the system's control strategy. The new energy side adopts negative sequence current suppression, but it cannot be completely suppressed to 0. Therefore, the positive sequence voltage drop and the magnitude of the negative sequence current are used as the harmonic injection initiation criteria. In this paper, the positive sequence voltage drops to 0.9pu and the negative sequence current is taken as the actual value of 0.15. By collecting the amplitude information of the negative sequence current and positive sequence voltage at the protection installation point on the new energy side, the information is used as the initiation criteria to realize the switching of the harmonic injection mode.

[0093] Step 3. Based on harmonic injection, analyze the fault sequence network of a double-ended weakly fed AC system under specific positive and negative sequence harmonics, summarize the phase change law of sequence component voltage under different fault types, and construct phase selection criteria using the phase difference;

[0094] Step 4. After a fault, determine whether the sequence component voltage is a faulty phase by observing the phase change pattern under different fault types.

[0095] In step 1, the energy side injects harmonic signals by changing the command value. Specifically, the changes are as follows:

[0096] (1)

[0097] In the formula i d i q d in the power frequency synchronous rotating coordinate system q The shaft currents, their maximum values ​​are i dmax i qmax The minimum values ​​are respectively i dmin i qmin i pv is the actual output current phasor of the new energy side; M and h are the amplitudes of the fundamental current and harmonic current, respectively. The values ​​of M, h, and h are related to the frequency of the injected harmonics. All of these are parameters to be tuned;

[0098] At this point, based on the injection frequency It can generate a frequency of and Two harmonics of different frequencies.

[0099] Furthermore, in step 2, when a system fault occurs, negative sequence current suppression is adopted on the new energy side. Since the voltage drop is deeper during symmetrical faults and negative sequence current is generated during asymmetrical faults, the change in positive sequence voltage and negative sequence current content at the new energy side protection installation point is used as the initiation criterion for the limiting control switching. N :

[0100] (2)

[0101] In the formula, D N For the start signal controlling the switching, "∪" represents the logical "OR"; S n S p These are the positive and negative order discrimination signals, respectively; U + I -These represent the positive and negative sequence component amplitudes of the voltage at the protection installation point on the new energy side. Considering that the negative sequence component only occurs when an asymmetrical fault occurs in the system, the ε value can be relatively small. To ensure sensitivity, this paper takes ε = 0.15. The above variable U... + To use the system rated voltage U N As the base value, the per-unit value, I - This is the actual value. Harmonic injection is initiated when the activation criterion is met.

[0102] For any injected k-th harmonic, the corresponding three-phase phase relationship is as follows:

[0103] , (3)

[0104] In the formula: , , These represent the three-phase voltages A, B, and C, respectively. This refers to the amplitude of the harmonic voltage. The fundamental angular frequency; To inject harmonic multiples,

[0105] Mathematical analysis shows that:

[0106] For h = 3k + 1 (k = 0, 1, 2…):

[0107] , (4)

[0108] Its phase relationship is the same as that of the power frequency fundamental frequency, and it has positive sequence characteristics.

[0109] For h = 3k + 2 (k = 0, 1, 2, ...):

[0110] , (5)

[0111] The phase relationship is determined to be ACB, which has a negative order characteristic.

[0112] For h=3k (k=1,2,3…):

[0113] (6)

[0114] The three phases exhibit in-phase characteristics and zero-sequence characteristics.

[0115] By utilizing the relationship between the frequency of the injected harmonic signal and the sequence component, the required sequence electrical quantity signal is injected to complete the subsequent protection structure.

[0116] Furthermore, in step 3, based on the harmonic injection method in step 1 and the harmonic injection initiation method in step 2, a fault sequence network of a double-ended weakly fed AC system under specific positive and negative sequence harmonics is proposed. The phase change law of the sequence component voltage under different fault types is summarized, and a phase selection criterion is constructed using the phase difference. Here, the system under injected positive and negative sequence harmonics is specifically analyzed:

[0117] 1) Single-phase ground fault

[0118] Taking a phase-A ground fault as an example, when a phase-A ground fault occurs on a line, based on the injected higher-order positive-sequence and negative-sequence harmonics, an equivalent composite sequence network diagram based on the harmonic signal is constructed, and its boundary conditions are obtained as follows:

[0119] , (7)

[0120] In the formula: , and These represent the positive harmonic sequence, negative harmonic sequence, and zero-sequence voltages measured at the protection installation location, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence harmonic currents measured at the protection installation location, respectively. and These represent the line impedances on the new energy side and the MMC side at harmonic frequencies, respectively. and These represent the equivalent impedance of the system at the harmonic frequencies; the subscripts '0', '1', and '2' represent the positive-sequence, negative-sequence, and zero-sequence components, respectively. ,

[0121] Further derivation yields the relationship between the positive, negative, and zero-sequence harmonic voltages on the new energy side as shown in equation (8):

[0122] , (8)

[0123] Substituting the boundary conditions for a single-phase ground fault, we obtain equation (9):

[0124] , (9)

[0125] At this point, the phase relationship can be expressed as:

[0126] , (10)

[0127] In equation (10), except for R f The phases of all other terms are close to 90°, and the impedance amplitudes of transmission lines, transformers, and synchronous machines are all relatively large under the injection of higher harmonics.

[0128] Substituting into equation (10), we get:

[0129] , (11)

[0130] Substituting the data for each phase, we can obtain: Approximately 180°, and Approximately 0°,

[0131] Based on this, and considering the differences in system parameters and the range of transition resistance variation, with a certain margin, the phase variation range of the positive-sequence, negative-sequence, and zero-sequence voltages in a phase-A ground fault is as follows:

[0132] , (12)

[0133] 2) Phase-to-phase fault

[0134] Taking a phase-to-phase fault (BC) on a line as an example, because R... f The voltage is relatively small, and with the increase of line impedance under high-frequency harmonics, Rf can be ignored. In this scenario, the positive-sequence voltage and negative-sequence voltage of the harmonics at the protection installation point can be expressed as Equation (13):

[0135] , (13)

[0136] The boundary conditions for the BC fault are expressed as equation (14):

[0137] , (14)

[0138] At this point, substituting the values ​​will yield the phase angle relationship. near ,

[0139] The phase variation range of the positive and negative sequence voltages for interphase faults (BC) is as follows:

[0140] , (15)

[0141] 3) Phase-to-phase fault

[0142] Taking a BCG phase-to-ground short-circuit fault as an example, the sequence harmonic voltages at the protection installation point are expressed as equation (16):

[0143] , (16)

[0144] Substituting the BCG fault boundary conditions, it can be expressed as equation (17): , (17)

[0145] Based on equation (17), we derive = , = ,

[0146] At this point, considering the effect of the transition resistance and taking a certain margin, the phase change range of the positive sequence, negative sequence, and zero sequence voltages in the BCG phase-to-phase grounding fault is derived as follows:

[0147] , (18)

[0148] 4) Three-phase fault

[0149] When a three-phase fault occurs in the system, regardless of whether harmonics are injected into the converter, there are no negative sequence components or zero sequence components at the protection installation point. Only positive sequence loops exist in the fault network. This characteristic can be used to select the fault phase.

[0150] For three-phase faults, the fault type is determined by whether only positive-sequence components are present. For asymmetrical faults, firstly, harmonic injection is initiated when the starting criterion is met; secondly, the fault type is determined by the phase difference between the sequence voltage components of the injected harmonics at the protection installation point; finally, based on... and The range is used to determine the specific fault phase, and α and β are defined as the phase difference, where α = , β= .

[0151] The steps to confirm the phase selection scheme are as follows:

[0152] 1) Determine the fault type based on the values ​​of negative sequence current and positive sequence voltage at the protection installation location, i.e.:

[0153] , (19)

[0154] In the formula: The system's rated voltage.

[0155] The phase selection element that satisfies equation (19) will determine that an asymmetrical grounding fault has occurred in the system;

[0156] 2) If the amplitude criterion in step one is not met, then the amplitude of the negative sequence voltage is compared, i.e.:

[0157] , (20)

[0158] if If the amplitude satisfies equation (20), then the system experiences a three-phase symmetrical fault; otherwise, the system experiences an interphase fault.

[0159] Establish as attached Figure 2 The large-scale double-ended weakly fed system shown is used as a simulation model. Symmetrical and asymmetric faults are assumed to occur at the system output.

[0160] When AG, ABCG, ABG, and AB faults occur, the positive-sequence voltage and negative-sequence current signals of the photovoltaic-side transmission line are processed. The amplitude of these signals is used to determine whether injection is necessary. Then, the phase signals of the positive-sequence and negative-sequence harmonic voltages of the photovoltaic-side transmission line are processed, and a 20ms data window is used to calculate the phase relationship between positive and negative sequence, and between zero and negative sequence, to determine the faulty phase. The simulation results are shown in the table below.

[0161] Table 1. Judgment of different types of faults in the outgoing line of the double-ended weak feeder system.

[0162] AG 19.86 136.55 AB / 101.78 ABG 97.72 254.48

[0163] Establish as attached Figure 2 The large-scale double-ended weak feeder system shown is used as a simulation model. A ground fault is set at the point where the AG (Automatic Guided Vehicle) is grounded through different transition resistors at the fault point of the system's outgoing line.

[0164] When an AG fault occurs, the positive-sequence voltage and negative-sequence current signals of the photovoltaic-side transmission line are processed, and their amplitudes are used to determine whether injection should be performed. Then, the 7th harmonic current and voltage signals of the photovoltaic-side transmission line are processed, and the phase relationship between positive and negative sequence, and between zero and negative sequence is calculated using a 20ms data window to determine the faulty phase. The simulation results are shown in the table below.

[0165] Table 2. Judgment of AG grounding fault through transition resistor when the outgoing line of the double-ended weak feeder system is located.

[0166] 0.01 136.55 19.89 100 155.64 21.47 300 176.65 25.62

[0167] In this embodiment of the invention, a sequence component phase selection system for a dual-terminal weakly fed AC system based on multi-frequency injection is also provided, which can be used to implement the above-mentioned sequence component phase selection method for a dual-terminal weakly fed AC system based on multi-frequency injection. Specifically, the system includes:

[0168] The data acquisition module is used to collect the amplitude information of negative sequence current and positive sequence voltage at the new energy side protection installation point, and uses the amplitude information as a start-up criterion to realize the switching of harmonic injection mode;

[0169] The fault phase discrimination module is used to identify the fault phase based on the phase relationship between positive sequence and negative sequence, and zero sequence and negative sequence under different fault types, based on harmonic injection, and to complete the fault phase selection.

[0170] The phase selection scheme output module obtains the phase selection scheme based on the phase change law of the sequence component voltage under different fault types.

[0171] The module division in this embodiment of the invention is illustrative and represents only one logical functional division. In actual implementation, other division methods may be used. Furthermore, the functional modules in the various embodiments of the invention can be integrated into a single processor, exist as separate physical entities, or be integrated into a single module. The integrated modules described above can be implemented in hardware or as software functional modules.

[0172] This invention also provides a computer device, which includes a processor and a memory. The memory stores a computer program, which includes program instructions. The processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions in the computer storage medium to achieve corresponding method flows or corresponding functions. The processor described in this invention can be used for the operation of a phase selection method for a dual-terminal weak-feed AC system model identification based on harmonic injection.

[0173] In embodiments of the present invention, a storage medium is also provided, specifically a computer-readable storage medium (Memory). This computer-readable storage medium is a memory device within a computer device used to store programs and data. It is understood that the computer-readable storage medium here can include both the built-in storage medium of the computer device and extended storage media supported by the computer device. The computer-readable storage medium provides storage space containing the terminal's operating system. Furthermore, this storage space also contains one or more instructions suitable for loading and execution by a processor. These instructions can be one or more computer programs (including program code). It should be noted that the computer-readable storage medium here can be high-speed RAM or non-volatile memory, such as at least one disk storage device. The processor can load and execute one or more instructions stored in the computer-readable storage medium to implement the corresponding steps of the harmonic injection-based dual-terminal weak-feed AC system model identification and phase selection method in the above embodiments.

[0174] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0175] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0176] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0177] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0178] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A sequence component phase selection method for a dual-terminal weakly fed AC system based on multi-frequency injection, characterized in that, include: The amplitude information of negative sequence current and positive sequence voltage at the protection installation point on the new energy side is collected, and the amplitude information is used as the start-up criterion to realize the switching of harmonic injection mode; Based on harmonic injection, the fault sequence network of a double-ended weakly fed AC system under specific positive and negative sequence harmonics is analyzed, the phase change law of the sequence component voltage under different fault types is confirmed, and the phase selection criterion is constructed by utilizing the phase difference. A sequence component phase selection scheme based on multi-frequency signal injection is proposed.

2. The sequence component phase selection method for a dual-terminal weakly fed AC system based on multi-frequency injection as described in claim 1, characterized in that, The acquisition of amplitude information of negative sequence current and positive sequence voltage at the protection installation point on the new energy side, and the use of amplitude information as a start-up criterion, includes: When a system fault occurs, the changes in the positive-sequence voltage and negative-sequence current content at the protection installation point on the new energy side are used as the initiation criterion for control strategy switching (DN). ，（1） In the formula, DN is the start signal for control switching, "∪" represents logical "OR"; S n S p These are the positive and negative order discrimination signals, respectively; U + I - These represent the positive and negative sequence component amplitudes of the voltage at the new energy side protection installation point; ε=0.15, the above variable U + To use the system rated voltage U N As the standard value, I - This is the actual value; The energy side injects harmonic signals by changing the command value. The specific changes are as follows: ，（2） In the formula i d i q d in the power frequency synchronous rotating coordinate system q The shaft currents, their maximum values ​​are i dmax i qmax The minimum values ​​are respectively i dmin i qmin i pv is the actual output current phasor of the new energy side; M and h are the amplitudes of the fundamental current and harmonic current, respectively. The values ​​of M, h, and h are related to the frequency of the injected harmonics. All of these are parameters to be tuned; At this point, based on the injection frequency The generation frequency is and Two harmonics of different frequencies.

3. The sequence component phase selection method for a dual-terminal weakly fed AC system based on multi-frequency injection as described in claim 1 requires determining the frequency of the injected harmonics based on the characteristics of the injection frequency, characterized in that... For any injected k-th harmonic, the corresponding three-phase phase relationship is as follows: ，（3） In the formula: , , These represent the three-phase voltages A, B, and C, respectively. This refers to the amplitude of the harmonic voltage. The fundamental angular frequency; To inject harmonic multiples; Mathematical analysis shows that: For h = 3k + 1 (k = 0, 1, 2…): ， （4） Its phase relationship is the same as that of the power frequency fundamental frequency, and it has positive sequence characteristics. For h = 3k + 2 (k = 0, 1, 2, ...): ， （5） The phase relationship is determined to be ACB, which has a negative order characteristic. For h=3k (k=1,2,3…): （6） The three phases exhibit in-phase characteristics and zero-sequence characteristics. By utilizing the relationship between the frequency of the injected harmonic signal and the sequence component, the required sequence electrical quantity signal is injected to complete the subsequent protection structure.

4. The sequence component phase selection method for a dual-terminal weakly fed AC system based on multi-frequency injection as described in claim 1, characterized in that, Based on harmonic injection, the lower-order component network of the harmonic injection signal was analyzed, and the phase variation law of the order component voltage under different fault types was confirmed, including: 1) Single-phase ground fault A phase-to-ground short-circuit fault occurs on the line. Based on the injected higher-order positive-sequence and negative-sequence harmonics, an equivalent composite sequence network diagram based on the harmonic signal is constructed, and its boundary conditions are as follows: ， （7） In the formula: , and These represent the positive harmonic sequence, negative harmonic sequence, and zero-sequence voltages measured at the protection installation location, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence harmonic currents measured at the protection installation location, respectively. and These represent the line impedances on the new energy side and the MMC side at harmonic frequencies, respectively. and These represent the equivalent impedance of the system at the harmonic frequencies; the subscripts '0', '1', and '2' represent the positive-sequence, negative-sequence, and zero-sequence components, respectively. R f Represents transition resistance; Further derivation yields the relationship between the positive, negative, and zero-sequence harmonic voltages on the new energy side as shown in equation (8): ， （8） Substituting the boundary conditions for a single-phase ground fault, we obtain equation (9): ， （9） At this point, the phase relationship can be expressed as: ， （10） In the formula: This represents the phase relationship between the positive and negative sequences. This represents the phase relationship between the zero sequence and the negative sequence; In equation (10), except for R f The phases of all other terms are close to 90°, and the impedance amplitudes of transmission lines, transformers, and synchronous machines are all relatively large under the injection of higher harmonics. Substituting into equation (10), we get: ， （11） Substituting the data for each phase, we can obtain: Approximately 180°, and Approximately 0°, Based on this, and considering the differences in system parameters and the range of transition resistance variation, with a certain margin, the phase variation range of the positive-sequence, negative-sequence, and zero-sequence voltages in a phase-A ground fault is as follows: ， （12） 2) Phase-to-phase fault A phase-to-phase fault occurred on the line (BC phase-to-phase). Due to the phase-to-phase fault, R... f The voltage is relatively small, and with the increase of line impedance under high-frequency harmonics, Rf can be ignored. In this scenario, the positive-sequence voltage and negative-sequence voltage of the harmonics at the protection installation point are expressed as Equation (13): ， （13） The boundary conditions for the BC fault are expressed as equation (14): ， （14） At this point, substituting the values ​​will yield the phase angle relationship. near , The phase variation range of the positive and negative sequence voltages for interphase faults (BC) is as follows: ， （15） 3) Phase-to-phase fault When a BCG phase-to-ground short-circuit fault occurs on the line, the sequence harmonic voltages at the protection installation point are expressed as equation (16): ， （16） Substituting the BCG fault boundary conditions, we get Equation (17): ， （17） Based on equation (17), we derive = , = , At this point, considering the effect of the transition resistance and taking a certain margin, the phase change range of the positive sequence, negative sequence, and zero sequence voltages in the BCG phase-to-phase grounding fault is derived as follows: ， （18） 4) Three-phase fault When a three-phase fault occurs in the system, regardless of whether harmonics are injected into the converter, there are no negative sequence components or zero sequence components at the protection installation point. Only positive sequence loops exist in the fault network. This characteristic can be used to select the fault phase.

5. The sequence component phase selection method for a dual-terminal weakly fed AC system based on multi-frequency injection as described in claim 4, characterized in that, A sequence component phase selection scheme based on multi-frequency signal injection is proposed, including: For three-phase faults, the fault type is determined by whether only positive-sequence components are present. For asymmetrical faults, firstly, harmonic injection is initiated when the starting criterion is met; secondly, the fault type is determined by the phase difference between the sequence voltage components of the injected harmonics at the protection installation point; finally, based on... and The range is used to determine the specific fault phase, and α and β are defined as the phase difference, where α = , β= .

6. The sequence component phase selection method for a dual-terminal weakly fed AC system based on multi-frequency injection as described in claim 5, characterized in that, The steps to confirm the phase selection scheme are as follows: 1) Determine the fault type based on the values ​​of negative sequence current and positive sequence voltage at the protection installation location, i.e.: ， （19） In the formula: The system's rated voltage. The phase selection element that satisfies equation (19) will determine that an asymmetrical grounding fault has occurred in the system; 2) If the amplitude criterion in step one is not met, then the amplitude of the negative sequence voltage is compared, i.e.: ， （20） if If the amplitude satisfies equation (20), then the system experiences a three-phase symmetrical fault; otherwise, the system experiences an interphase fault. The specific fault phase is determined based on the values ​​of α and β.

7. A sequence component phase selection device for a dual-terminal weak-feed AC system based on multi-frequency injection, implementing the phase selection method as described in any one of claims 1-6, characterized in that, include: The data acquisition module is used to collect the amplitude information of negative sequence current and positive sequence voltage at the new energy side protection installation point, and uses the amplitude information as a start-up criterion to realize the switching of harmonic injection mode; The fault phase discrimination module is used to identify the fault phase based on the phase relationship between positive sequence and negative sequence, and zero sequence and negative sequence under different fault types, based on harmonic injection, and to complete the fault phase selection. The phase selection scheme output module obtains the phase selection scheme based on the phase change law of the sequence component voltage under different fault types.

8. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the steps of the sequence component phase selection method for a dual-terminal weak-feed AC system based on multi-frequency injection as described in any one of claims 1 to 6.

9. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the steps of the sequence component phase selection method for a dual-terminal weak-feed AC system based on multi-frequency injection as described in any one of claims 1 to 6.

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