Phasor differential protection method and system based on regulation and control characteristics of low-frequency converter

Abstract

The invention discloses a phasor differential protection method and system based on regulation and control characteristics of a low-frequency converter, and belongs to the technical field of relay protection. The method comprises the following steps: calculating negative-sequence current and positive-sequence voltage at two sides of a protected line based on three-phase voltage and current phasors; judging whether the drop depth of the positive sequence voltage meets a three-phase fault criterion, if so, not adjusting a braking coefficient, and if not, adjusting the braking coefficient through the amplitude ratio of the negative sequence current, and calculating a self-adaptive braking coefficient based on the regulation and control characteristics of the low-frequency converter based on the adjusted braking coefficient; and constructing a phasor differential protection criterion based on the adaptive braking coefficient, obtaining fault current vectors at two sides of the protected line, and starting phasor differential protection for the protected line when the fault current vectors meet the differential protection criterion. According to the method, protection sensitivity and reliability can be considered, and the sensitivity of main protection of the flexible low-frequency power transmission line is improved.

Description

Technical Field

[0001] The present invention relates to the field of relay protection technology, and more specifically to a phasor differential protection method and system based on the control characteristics of a low-frequency converter. Background Art

[0002] The flexible low-frequency AC transmission (LFAC) system, based on the modular multilevel matrix converter (M3C), is an emerging power transmission technology primarily targeted at scenarios such as medium- and long-distance offshore wind power grid integration and oilfield power supply. Its core goal is to reduce cable charging current and line losses by lowering the AC transmission frequency (typically 16.7Hz or 20Hz). This, combined with the M3C modular multilevel structure, enables efficient and flexible power transmission and control.

[0003] The existing project's primary low-frequency line protection still uses phasor differential protection based on Kirchhoff's current theorem. While this protection is stable and reliable, it suffers from insufficient sensitivity due to the fault current limiting strategies of the low-frequency system's M3C AC-AC converter and wind turbine converter. Summary of the Invention

[0004] To address the above problems, the present invention proposes a phasor differential protection method based on the control characteristics of a low-frequency converter, comprising:

[0005] Obtaining three-phase voltage and current phasors at protection installation points on both sides of the protected line, and calculating negative-sequence current and positive-sequence voltage on both sides of the protected line based on the three-phase voltage and current phasors;

[0006] determining whether the drop depth of the positive sequence voltage satisfies a three-phase fault criterion; if so, not adjusting the braking coefficient; and if not, adjusting the braking coefficient according to the amplitude ratio of the negative sequence current, and calculating an adaptive braking coefficient based on the control characteristics of the low-frequency converter based on the adjusted braking coefficient;

[0007] Based on the adaptive braking coefficient, a phasor differential protection criterion is constructed, and the fault current vectors on both sides of the protected line are obtained. When the fault current vector meets the differential protection criterion, the phasor differential protection is started for the protected line.

[0008] Optional phase failure criteria are as follows:

[0009]

[0010] in, and are the positive sequence voltages on the m and n sides of the protected line, UN is the rated voltage.

[0011] Optionally, the adjusted braking coefficient is calculated as:

[0012]

[0013] Among them, K NC is the adjusted braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

[0014] Optionally, a formula for calculating an adaptive braking coefficient based on the control characteristics of the low-frequency converter is as follows:

[0015]

[0016] Among them, K NC is the adjusted braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

[0017] Optional, phasor differential protection criteria, including:

[0018]

[0019] in, and are the fault current phasors on the m and n sides of the protected line, i qd is the minimum operating current, K adp is the adaptive braking coefficient.

[0020] On the other hand, the present invention further proposes a phasor differential protection system based on the control characteristics of a low-frequency converter, comprising:

[0021] An acquisition unit is used to obtain three-phase voltage and current phasors at protection installation points on both sides of the protected line, and calculate the negative sequence current and positive sequence voltage on both sides of the protected line based on the three-phase voltage and current phasors;

[0022] a calculation unit, configured to determine whether the drop depth of the positive-sequence voltage satisfies a three-phase fault criterion; if so, not adjust the braking coefficient; if not, adjust the braking coefficient according to the amplitude ratio of the negative-sequence current; and calculate an adaptive braking coefficient based on the control characteristics of the low-frequency converter based on the adjusted braking coefficient;

[0023] A starting unit is used to construct a phasor differential protection criterion based on the adaptive braking coefficient, and obtain the fault current vectors on both sides of the protected line. When the fault current vector meets the differential protection criterion, the phasor differential protection is started for the protected line.

[0024] Optional three-phase fault criteria are as follows:

[0025]

[0026] in, and are the positive sequence voltages on the m and n sides of the protected line, U N is the rated voltage.

[0027] Optionally, the adjusted braking coefficient is calculated as:

[0028]

[0029] Among them, K NC is the adjusted braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

[0030] Optionally, a formula for calculating an adaptive braking coefficient based on the control characteristics of the low-frequency converter is as follows:

[0031]

[0032] Among them, K adp is the adaptive braking coefficient, K NC is the adjusted braking coefficient, K s is a fixed braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

[0033] Optional, phasor differential protection criteria, including:

[0034]

[0035] in, and are the fault current phasors on the m and n sides of the protected line, i qd is the minimum operating current, K adp is the adaptive braking coefficient.

[0036] In yet another aspect, the present invention further provides a computing device comprising: one or more processors;

[0037] a processor for executing one or more programs;

[0038] When the one or more programs are executed by the one or more processors, the above-described method is implemented.

[0039] In another aspect, the present invention further provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed, the method described above is implemented.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The present invention proposes a phasor differential protection method based on the control characteristics of a low-frequency converter, comprising: obtaining three-phase voltage and current phasors at protection installation points on both sides of a protected line; calculating the negative-sequence current and positive-sequence voltage on both sides of the protected line based on the three-phase voltage and current phasors; determining whether the drop depth of the positive-sequence voltage meets the three-phase fault criterion; if so, not adjusting the braking coefficient; if not, adjusting the braking coefficient based on the amplitude ratio of the negative-sequence current; and calculating an adaptive braking coefficient based on the control characteristics of the low-frequency converter based on the adjusted braking coefficient; constructing a phasor differential protection criterion based on the adaptive braking coefficient, and obtaining the fault current vector on both sides of the protected line. When the fault current vector meets the differential protection criterion, phasor differential protection is activated for the protected line. This invention balances protection sensitivity and reliability, improving the sensitivity of primary protection for flexible low-frequency transmission lines. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a flow chart of the method of the present invention;

[0043] Figure 2 This is a topology diagram of an offshore wind power flexible low-frequency transmission system according to an embodiment of the method of the present invention;

[0044] Figure 3 This is a fault sequence current characteristic diagram of the M3C converter at the constant V / f side according to an embodiment of the method of the present invention;

[0045] Figure 4 This is a fault sequence current characteristic diagram of a wind turbine converter and a fixed PQ side M3C converter in an embodiment of the method of the present invention;

[0046] Figure 5 This is a sensitivity verification diagram of the optimization solution proposed for the intra-area BC fault in the embodiment of the method of the present invention;

[0047] Figure 6 This is a sensitivity verification diagram of the optimization solution proposed for the intra-area AN fault in the embodiment of the method of the present invention;

[0048] Figure 7 This is a reliability verification diagram of the optimization solution proposed for out-of-zone faults in an embodiment of the method of the present invention. DETAILED DESCRIPTION

[0049] Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to provide a thorough and complete disclosure of the present invention and to fully convey the scope of the present invention to those skilled in the art. The terminology used in the exemplary embodiments shown in the accompanying drawings is not intended to limit the present invention. In the accompanying drawings, identical elements are denoted by the same reference numerals.

[0050] Unless otherwise specified, the terms used herein (including technical terms) have the meanings commonly understood by those skilled in the art. In addition, it is understood that terms defined in commonly used dictionaries should be understood to have the same meanings as those in the context of the relevant fields, and should not be understood as idealized or overly formal meanings.

[0051] Example 1:

[0052] The present invention proposes a phasor differential protection method based on the control characteristics of low-frequency converters, such as Figure 1 Shown, including:

[0053] Step 1: Obtain three-phase voltage and current phasors at protection installation points on both sides of the protected line, and calculate the negative sequence current and positive sequence voltage on both sides of the protected line based on the three-phase voltage and current phasors;

[0054] Step 2: determining whether the drop depth of the positive-sequence voltage meets the three-phase fault criterion; if so, not adjusting the braking coefficient; if not, adjusting the braking coefficient according to the amplitude ratio of the negative-sequence current; and calculating an adaptive braking coefficient based on the control characteristics of the low-frequency converter based on the adjusted braking coefficient;

[0055] Step 3: Based on the adaptive braking coefficient, construct a phasor differential protection criterion, and obtain the fault current vectors on both sides of the protected line. When the fault current vector meets the differential protection criterion, start the phasor differential protection for the protected line.

[0056] Among them, the phase fault judgment criteria are as follows:

[0057]

[0058] in, and are the positive sequence voltages on the m and n sides of the protected line, U N is the rated voltage.

[0059] The calculation formula of the adjusted braking coefficient is:

[0060]

[0061] Among them, K NC is the adjusted braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

[0062] The calculation formula of the adaptive braking coefficient based on the control characteristics of the low-frequency converter is as follows:

[0063]

[0064] Among them, K adp is the adaptive braking coefficient, K NC is the adjusted braking coefficient, K s is a fixed braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

[0065] Among them, the phasor differential protection criteria include:

[0066]

[0067] in, and are the fault current phasors on the m and n sides of the protected line, i qd is the minimum operating current, K adp is the adaptive braking coefficient.

[0068] The present invention will be further described below with reference to specific cases:

[0069] The specific case process is as follows:

[0070] As attached Figure 2 The system shown in the figure first obtains the three-phase voltage and current phasors at the protection installation points on both sides of the protected line m and n. as well as

[0071] Calculate the negative sequence current on both sides of the line

[0072]

[0073] Where, is the rotation factor.

[0074] Calculate the positive sequence voltage on both sides of the line

[0075]

[0076] During a three-phase symmetrical short-circuit fault, the negative-sequence current amplitude in the bilateral fault current is very small and cannot be used for braking adjustment. First, the positive-sequence voltage drop depth is used to determine whether there is a three-phase fault:

[0077]

[0078] If the equation (3) is satisfied, the braking coefficient is not adjusted. If the equation (3) is not satisfied, the asymmetric fault braking coefficient adjustment logic is entered.

[0079] Calculate the negative sequence current amplitude ratio braking adjustment coefficient K NC :

[0080]

[0081] In formula (4), Measure the larger and smaller negative sequence currents on both sides of the line:

[0082]

[0083] The improved criterion for phasor differential protection is:

[0084]

[0085] Where, and Represent the fault current phasors on the m and n sides of the transmission line, i qd is the minimum operating current, K adp is the adaptive braking coefficient based on the control characteristics of the low-frequency converter:

[0086]

[0087] Where K S It is a fixed braking coefficient, usually selected as 0.7 to 0.9.

[0088] In the flexible low-frequency transmission system, the V / f side (voltage-frequency control side) adopts a low-frequency grid control strategy to build a stable 20Hz power grid. This control mode belongs to active grid control and does not specifically suppress sequence components (such as negative sequence and zero sequence). It mainly ensures the stable output of voltage and frequency. The PQ side (constant power control side) is designed as a low-frequency grid following control mode, which is responsible for adjusting the active and reactive power output of the low-frequency side of this station. The M3C converter and low-frequency wind turbine converter on the PQ side use a negative sequence suppression strategy to make their output low-frequency negative sequence current approach zero. Therefore, compared with the V / f side, the negative sequence impedance on the PQ side / wind farm side of the line can be approximately regarded as infinite, thereby effectively suppressing the impact of the negative sequence component on the system. When a fault occurs in the low-frequency transmission line, the V / f side converter does not suppress the negative sequence current, such as Figures 3 and 4 As shown, when a fault occurs within the zone, the significant difference in the negative sequence current amplitude on both sides of the line can be used to quickly reduce the braking coefficient, achieving sensitive action for the fault within the zone. When a fault occurs outside the zone, the negative sequence currents on both sides exhibit a ride-through characteristic, with amplitudes essentially similar. Using the original customized braking coefficient, the protection is reliable and does not trip falsely.

[0089] The present invention introduces the difference information of the negative sequence current amplitude on both sides into the braking criterion, and utilizes the significant difference that the negative sequence current of the low-frequency converter on the fixed V / f side is larger and the negative sequence current of the low-frequency converter on the fixed P / Q side and the wind turbine converter is smaller during the fault in the zone. The difference in the negative sequence current control of different low-frequency converters is mapped into the adaptive braking coefficient, and the equivalent braking coefficient is dynamically adjusted according to the negative sequence current amplitude ratio on both sides. The braking amount is greatly reduced in the event of a fault in the zone, and the protection sensitivity is improved. The reliability of the original differential protection can also be guaranteed in the event of a fault outside the zone. It can take into account both the protection sensitivity and reliability, and improve the sensitivity of the main protection of the flexible low-frequency transmission line.

[0090] In RTDS, taking a real project as an example, an electromagnetic transient model of the offshore wind power flexible low-frequency transmission system is built, and the fault point is set as shown in the attached figure. Figure 2 The offshore wind power converter adopts negative sequence suppression control strategy, and the onshore M3C AC-AC converter adopts constant V / f control. The fault occurrence time is 50ms. The existing differential protection action characteristics and the improved scheme action characteristics are plotted as shown in Figure 2. Figures 5 to 7 What I see.

[0091] Depend on Figures 5 and 6 It can be seen that the proposed improved scheme of phasor differential protection based on the difference of converter control characteristics can significantly reduce the braking current when the fault occurs within the zone, and is superior to the existing phasor differential protection in terms of sensitivity. Figure 7 As can be seen, the proposed improved phasor differential protection scheme based on differences in converter control characteristics performs on par with existing phasor differential protection for out-of-zone faults. While ensuring reliability, it significantly improves the sensitivity of existing differential protection. Furthermore, this protection scheme does not require additional CTs or an increased sampling rate, and its principle is easy to implement.

[0092] Example 2:

[0093] The present invention also proposes a phasor differential protection system 200 based on the control characteristics of a low-frequency converter, comprising:

[0094] The acquisition unit 201 is configured to obtain three-phase voltage and current phasors at protection installation points on both sides of the protected line, and calculate the negative-sequence current and positive-sequence voltage on both sides of the protected line based on the three-phase voltage and current phasors;

[0095] a calculation unit 202 configured to determine whether the dip depth of the positive-sequence voltage satisfies a three-phase fault criterion; if so, not adjusting the braking coefficient; and if not, adjusting the braking coefficient according to the amplitude ratio of the negative-sequence current, and calculating an adaptive braking coefficient based on the control characteristics of the low-frequency converter based on the adjusted braking coefficient;

[0096] The starting unit 203 is used to construct a phasor differential protection criterion based on the adaptive braking coefficient and obtain the fault current vectors on both sides of the protected line. When the fault current vector meets the differential protection criterion, the phasor differential protection is started for the protected line.

[0097] Among them, the three-phase fault judgment criteria are as follows:

[0098]

[0099] in, and are the positive sequence voltages on the m and n sides of the protected line, U N is the rated voltage.

[0100] The calculation formula of the adjusted braking coefficient is:

[0101]

[0102] Among them, K NC is the adjusted braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

[0103] The calculation formula of the adaptive braking coefficient based on the control characteristics of the low-frequency converter is as follows:

[0104]

[0105] Among them, K adp is the adaptive braking coefficient, K NC is the adjusted braking coefficient, K sis a fixed braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

[0106] Among them, the phasor differential protection criteria include:

[0107]

[0108] in, and are the fault current phasors on the m and n sides of the protected line, i qd is the minimum operating current, K adp is the adaptive braking coefficient.

[0109] The present invention can take into account both protection sensitivity and reliability, and improve the sensitivity of the main protection of the flexible low-frequency transmission line.

[0110] Example 3:

[0111] Based on the same inventive concept, the present invention also provides a computer device, which includes a processor and a memory, wherein the memory is used to store a computer program, the computer program includes program instructions, and the processor is used to execute the program instructions stored in the computer storage medium. The processor may be a central processing unit (CPU), or may be other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, which is suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions in the computer storage medium to implement the corresponding method flow or corresponding function, so as to implement the steps of the method in the above embodiment.

[0112] Example 4:

[0113] Based on the same inventive concept, the present invention also provides a storage medium, specifically a computer-readable storage medium (Memory), which is a memory device in a computer device for storing programs and data. It can be understood that the computer-readable storage medium here can include both built-in storage media in the computer device and, of course, extended storage media supported by the computer device. The computer-readable storage medium provides a storage space that stores the operating system of the terminal. In addition, one or more instructions suitable for being loaded and executed by the processor are also stored in the storage space. These instructions can be one or more computer programs (including program codes). It should be noted that the computer-readable storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk memory. The processor can load and execute one or more instructions stored in the computer-readable storage medium to implement the steps of the method in the above embodiment.

[0114] It will be understood by those skilled in the art that the embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Furthermore, the present invention may take the form of a computer program product implemented 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. The solutions in the embodiments of the present invention may be implemented in various computer languages, for example, the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0115] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowcharts and / or block diagrams, as well as combinations of processes and / or blocks in the flowcharts 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, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowcharts and / or block diagrams. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0116] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0117] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0118] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0119] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A phasor differential protection method based on the control characteristics of a low-frequency converter, characterized in that: include: Obtaining three-phase voltage and current phasors at protection installation points on both sides of the protected line, and calculating negative-sequence current and positive-sequence voltage on both sides of the protected line based on the three-phase voltage and current phasors; determining whether the drop depth of the positive sequence voltage satisfies a three-phase fault criterion; if so, not adjusting the braking coefficient; and if not, adjusting the braking coefficient according to the amplitude ratio of the negative sequence current, and calculating an adaptive braking coefficient based on the control characteristics of the low-frequency converter based on the adjusted braking coefficient; Based on the adaptive braking coefficient, a phasor differential protection criterion is constructed, and the fault current vectors on both sides of the protected line are obtained. When the fault current vector meets the differential protection criterion, the phasor differential protection is started for the protected line.

2. The phasor differential protection method according to claim 1, characterized in that: The three-phase fault judgment criteria are as follows: in, and are the positive sequence voltages on the m and n sides of the protected line, U N is the rated voltage.

3. The phasor differential protection method according to claim 1, characterized in that: The calculation formula for the adjusted braking coefficient is: Among them, K NC is the adjusted braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

4. The phasor differential protection method according to claim 1, characterized in that: The calculation formula of the adaptive braking coefficient based on the control characteristics of the low-frequency converter is as follows: Among them, K adp is the adaptive braking coefficient, K NC is the adjusted braking coefficient, K s is a fixed braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

5. The phasor differential protection method according to claim 1, characterized in that: The phasor differential protection criterion includes: in, and are the fault current phasors on the m and n sides of the protected line, i qd is the minimum operating current, K adp is the adaptive braking coefficient.

6. A phasor differential protection system based on the control characteristics of a low-frequency converter, characterized in that: include: An acquisition unit is used to obtain three-phase voltage and current phasors at protection installation points on both sides of the protected line, and calculate the negative sequence current and positive sequence voltage on both sides of the protected line based on the three-phase voltage and current phasors; a calculation unit, configured to determine whether the drop depth of the positive-sequence voltage satisfies a three-phase fault criterion; if so, not adjust the braking coefficient; if not, adjust the braking coefficient according to the amplitude ratio of the negative-sequence current; and calculate an adaptive braking coefficient based on the control characteristics of the low-frequency converter based on the adjusted braking coefficient; A starting unit is used to construct a phasor differential protection criterion based on the adaptive braking coefficient, and obtain the fault current vectors on both sides of the protected line. When the fault current vector meets the differential protection criterion, the phasor differential protection is started for the protected line.

7. The phasor differential protection system according to claim 6, characterized in that: The three-phase fault judgment criteria are as follows: in, and are the positive sequence voltages on the m and n sides of the protected line, U N is the rated voltage.

8. The phasor differential protection system according to claim 6, characterized in that: The calculation formula for the adjusted braking coefficient is: Among them, K NC is the adjusted braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

9. The phasor differential protection system according to claim 6, characterized in that: The calculation formula of the adaptive braking coefficient based on the control characteristics of the low-frequency converter is as follows: Among them, K adp is the adaptive braking coefficient, K NC is the adjusted braking coefficient, K s is a fixed braking coefficient, and The larger and smaller negative sequence currents are measured on both sides of the line, I N is the rated current.

10. The phasor differential protection system according to claim 6, characterized in that: The phasor differential protection criterion includes: in, and are the fault current phasors on the m and n sides of the protected line, i qd is the minimum operating current, K adp is the adaptive braking coefficient.

11. A computer device, characterized in that: include: one or more processors; a processor for executing one or more programs; When the one or more programs are executed by the one or more processors, the method according to any one of claims 1 to 5 is implemented.

12. A computer-readable storage medium, characterized in that A computer program is stored thereon, and when the computer program is executed, the method according to any one of claims 1 to 5 is implemented.

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