An adaptive distance protection method and protection device for flexible low-frequency transmission lines

Abstract

An adaptive distance protection method and device for flexible low-frequency transmission lines includes: extracting and processing sampled information of electrical quantities at the protection installation; utilizing the processed electrical quantity information to discriminate the control strategy of the converter providing the fault electrical quantity; employing a mho distance protection element based on positive-sequence voltage polarization when V / F control is determined; and employing a mho distance protection element based on measured voltage polarization when P / Q control is determined. This invention addresses the issue of abnormal distance protection behavior on flexible low-frequency transmission lines by utilizing electrical quantities at the protection installation to discriminate the converter control strategy providing the fault current and adaptively employing different types of distance protection elements to ensure proper operation of the distance protection.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system relay protection, relates to transmission line protection, and in particular to an adaptive distance protection method and protection device for a flexible low-frequency transmission line. Background Art

[0002] In scenarios where medium and long-distance line transmission and a high proportion of new energy are gathered, flexible low-frequency transmission systems (usually with a frequency of 20 Hz) have the advantages of electromagnetic induction voltage and current zero-crossing interruption, can strengthen electrical connections, increase line transmission power, and have flexible regulation capabilities. They are gradually becoming a powerful supplement to industrial frequency AC transmission and flexible DC transmission.

[0003] Distance protection measures the distance from the short-circuit point to the protection installation by calculating the ratio of the voltage at the start of the protected line to the line current. This method, which has a low correlation with system operating mode, has been widely used in power grids. Distance relays based on positive-sequence voltage polarization offer excellent directional characteristics, effectively identifying faults in the vicinity of the line, and are a key implementation method for distance protection.

[0004] Both sides of the low-frequency transmission line are equipped with power electronic devices and adopt different control strategies. On the low-frequency side of the two converter stations, one station is used for fixed low-frequency AC voltage control. It establishes and stabilizes the low-frequency AC system voltage through a phase-locked loop under self-generated low-frequency conditions. This station is called a V / F station. The other station is used to control the active and reactive power on the low-frequency side. This station is called a P / Q station. The positive-sequence d-axis component on the low-frequency side of the VF station is used to control the AC voltage amplitude, while the positive-sequence q-axis component and the negative-sequence d- and q-axis components are all controlled to zero. The positive-sequence d-axis component on the low-frequency side of the P / Q station is used to control the active power on the low-frequency side, while the positive-sequence q-axis component is used to control the reactive power on the low-frequency side. The negative-sequence d- and q-axis components are both controlled to zero.

[0005] When a flexible low-frequency transmission line fails, the P / Q control side still targets the original power transmission and implements a negative-sequence current suppression strategy. This significantly controls the positive-sequence current phase after the fault, causing distance impedance relays based on positive-sequence voltage polarization to lose their directionality and risk false tripping or failure to operate. However, the V / F control side exhibits the characteristics of a voltage source during the fault, with a lesser impact on distance protection. When flexible low-frequency transmission is used as a back-to-back connection between two power-frequency systems, the P / Q and V / F control strategies on both sides of the low-frequency line are swapped, making it impossible for protection to determine the operating mode on its own side through setting. Therefore, new distance protection methods suitable for low-frequency transmission lines are needed. Summary of the Invention

[0006] In order to solve the above problems existing in the prior art, the present invention provides an adaptive distance protection method for a flexible low-frequency transmission line.

[0007] The present invention specifically adopts the following technical solutions.

[0008] An adaptive distance protection method for a flexible low-frequency transmission line, characterized by comprising the following steps:

[0009] Step S1: collecting the voltage and current at the line protection installation location and extracting sequence components;

[0010] Step S2: using the voltage and current sequence component information to determine the converter control strategy that provides the fault current;

[0011] Step S3: According to the converter control strategy providing the fault current, phase-comparison distance relays with different polarization voltages are selected to perform adaptive distance protection of the transmission line.

[0012] The present invention further includes the following preferred embodiments.

[0013] In step S1, the three-phase voltage u of the line is collected A (t),u B (t),u C (t) and three-phase current i A (t), i B (t), i C (t), and through Fourier transform and phase sequence conversion, the positive sequence and negative sequence components of voltage and current are obtained respectively. and

[0014] Further,

[0015] In step S2, when When the condition is met, it is determined that the fault current collected by the protection is provided by the P / Q control side; otherwise, it is determined that the fault current collected by the protection is provided by the V / F control side, where C1 and C2 are the threshold values ​​of the positive sequence current and the negative sequence current, respectively.

[0016] Further,

[0017] The C1 and C2 are both 0.1I n Setting, I n is the rated current value of the flexible low-frequency transmission line.

[0018] Further,

[0019] In step S3, when it is determined that the converter control strategy providing the fault current is the V / F mode, a phase-comparison distance relay based on positive sequence voltage polarization is used to perform adaptive distance protection of the line:

[0020]

[0021] in, It is the phase of the positive sequence voltage at the protection installation; the working voltage To protect the phase of voltage and current at the installation, Z set It is the distance protection setting value.

[0022] When it is determined that the converter control strategy providing the fault current is the P / Q mode, a phase-comparison distance relay based on voltage polarization measurement is used to perform adaptive distance protection of the line:

[0023]

[0024] Among them, the working voltage To protect the phase of voltage and current at the installation, Z set It is the distance protection setting value.

[0025] The present application also claims protection for an adaptive distance protection device for a flexible low-frequency transmission line using the protection method, comprising a current and voltage acquisition module, a converter control strategy judgment module, a distance protection enabling module, a phase-comparison distance relay based on positive sequence voltage polarization, and a phase-comparison distance relay based on measured voltage polarization, characterized in that:

[0026] The current and voltage acquisition module acquires the voltage and current at the protection installation location;

[0027] The converter control strategy judgment module extracts the sequence components of the voltage and current components, and compares the positive sequence current component and the negative sequence current component with the threshold value respectively to determine the control strategy of the converter providing the fault current;

[0028] The distance protection enabling module enables or blocks the phase-comparison distance relay based on positive sequence voltage polarization and the phase-comparison distance relay based on measured voltage polarization according to the judgment result of the converter control strategy judgment module.

[0029] Further preferred.

[0030] In the converter control strategy judgment module, when When it is met, it is determined that the fault current collected by the protection is provided by the P / Q control side; otherwise, it is determined that the fault current collected by the protection is provided by the V / F control side, where C1 and C2 are the threshold values ​​of the positive-sequence current and the negative-sequence current, respectively; I1 and I2 are the positive-sequence current component and the negative-sequence current component, respectively.

[0031] When it is determined that the control strategy of the converter providing the fault current is the V / F mode, the distance protection enabling module triggers the phase-comparison distance relay based on the positive sequence voltage polarization to perform adaptive distance protection of the line;

[0032] When it is determined that the control strategy of the converter providing the fault current is the P / Q mode, the distance protection enabling module triggers the phase-comparison distance relay based on the measured voltage polarization to perform adaptive distance protection of the line.

[0033] A terminal comprising a processor and a storage medium; characterized in that:

[0034] The storage medium is used to store instructions;

[0035] The processor is configured to operate according to the instructions to execute steps of the adaptive distance protection method for the flexible low-frequency transmission line.

[0036] A computer-readable storage medium stores a computer program thereon, characterized in that when the program is executed by a processor, the steps of the adaptive distance protection method for the flexible low-frequency transmission line are implemented.

[0037] Compared with the prior art, the present invention has the following beneficial technical effects.

[0038] The present invention can solve the problem of incorrect operation of traditional distance protection methods when fault current is provided by a converter adopting a P / Q control mode, thereby improving the reliability of line protection and having great significance for ensuring the safe and stable operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 1 is a flow chart of an adaptive distance protection method for a flexible low-frequency transmission line according to the present invention;

[0040] Figure 2 It is the operating characteristic diagram of the distance relay based on positive sequence voltage polarization;

[0041] Figure 3 It is the operating characteristic diagram of the distance relay based on the polarization of the measuring voltage;

[0042] Figure 4 This is a schematic diagram of a 220kV voltage level line simulation system for a flexible low-frequency power transmission system according to an embodiment of the present invention;

[0043] Figure 5 yes Figure 4 The measured electrical quantity information of the N side of F1 fault;

[0044] Figure 6 yes Figure 4 The positive and negative sequence current amplitudes on the N side of the F1 fault;

[0045] Figure 7 This is a schematic diagram of the correct operation of the distance relay based on the polarization of the measured voltage;

[0046] Figure 8 yes Figure 4The measured electrical quantity information of the M side of the F2 fault;

[0047] Figure 9 yes Figure 4 The positive and negative sequence current amplitudes on the M side of the F1 fault;

[0048] Figure 10 The distance relay based on the measurement voltage polarization does not operate reliably. DETAILED DESCRIPTION

[0049] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, other embodiments obtained by ordinary technicians in this field without making creative efforts are all within the scope of protection of the present invention.

[0050] like Figure 1 As shown, the present application discloses an adaptive distance protection method for a flexible low-frequency transmission line, comprising the following steps:

[0051] Step 1: Extract the current i at the line protection installation location A (t), i B (t), i C (t) and voltage u A (t),u B (t),u C (t), where A, B, and C are phases and t is the sampling time. According to formula (1), the fundamental components of phase current and phase voltage are calculated respectively using Fourier algorithm.

[0052]

[0053] in, is the fundamental component, N is the number of sampling points per cycle, x(n) is the sampling value of the current point, a k 、b k are constants related to trigonometric functions.

[0054] According to formula (2), the sequence component of current is calculated.

[0055]

[0056] in, They are positive sequence current, negative sequence current, and zero sequence current respectively, and a is the conversion factor.

[0057] Step 2: Using the voltage and current sequence component information to determine the converter control strategy that provides the fault current;

[0058] If equations (3) and (4) are satisfied at the same time, it is judged to be P / Q control; otherwise, it is judged to be V / F control.

[0059]

[0060]

[0061] Where C1 and C2 are the thresholds of positive sequence current and negative sequence current, which can be taken as 0.1A.

[0062] Step 3: Based on the converter control strategy providing the fault current, phase-comparison distance relays with different polarization voltages are selected to perform adaptive distance protection of the transmission line;

[0063] like Figure 2 As shown in FIG, when V / F control is determined, the mho type distance protection element based on positive sequence voltage polarization of formula (5) is used.

[0064]

[0065] in, It is the phase of the positive sequence voltage at the protection installation; the working voltage To protect the phase of voltage and current at the installation, Z set It is the distance protection setting value.

[0066] like Figure 3 As shown in FIG, when it is judged to be P / Q control, the mho type distance protection element based on positive sequence voltage polarization of formula (6) is used.

[0067]

[0068] Among them, the working voltage To protect the phase of voltage and current at the installation, Z set It is the distance protection setting value.

[0069] The present application also seeks to protect an adaptive distance protection device for a flexible low-frequency transmission line based on the aforementioned protection method, comprising a current and voltage acquisition module, a converter control strategy judgment module, a distance protection enabling module, a phase-comparison distance relay based on positive sequence voltage polarization, and a phase-comparison distance relay based on measured voltage polarization;

[0070] The current and voltage acquisition module acquires the voltage and current at the protection installation location;

[0071] The converter control strategy judgment module extracts the sequence components of the voltage and current components, and judges the control strategy of the converter that provides the fault current based on the comparison of the positive sequence current component and the negative sequence current component with the threshold value; in the converter control strategy judgment module, when If it is satisfied, it is determined that the fault current collected by the protection is provided by the P / Q control side. Otherwise, it is determined that the fault current collected by the protection is provided by the V / F control side. Among them, C1 and C2 are the threshold values ​​of positive sequence current and negative sequence current respectively. and They are the positive sequence current component and negative sequence current component of the fault current respectively.

[0072] The distance protection enabling module enables or blocks the phase-comparison distance relay based on positive sequence voltage polarization and the phase-comparison distance relay based on measured voltage polarization according to the judgment result of the converter control strategy judgment module.

[0073] When it is determined that the control strategy of the converter providing the fault current is the V / F mode, the distance protection enabling module triggers the phase-comparison distance relay based on the positive sequence voltage polarization to perform adaptive distance protection of the line;

[0074] When it is determined that the control strategy of the converter providing the fault current is the P / Q mode, the distance protection enabling module triggers the phase-comparison distance relay based on the measured voltage polarization to perform adaptive distance protection of the line.

[0075] In combination with the above method, the following simulation example is provided for a flexible low-frequency power transmission system model:

[0076] like Figure 4 As shown, a 220 kV line model of a flexible low-frequency transmission system was constructed in RTDS. The converter connected to the M side of the line employed V / F control, while the converter connected to the N side employed P / Q control. Two fault points, F1 and F2, were set within and outside the zone. The simulation system components and line parameters are shown in Table 1. Line protection devices A and C, equipped with conventional mho-type distance protection based on positive-sequence voltage polarization, were connected to both sides of the line, respectively. Furthermore, line protection devices B and D, equipped with the improved distance protection proposed by the present invention, were connected to both sides of the line.

[0077] Table 1 Simulation system components and circuit parameters

[0078]

[0079] The above fault points were tested for various fault types, and the distance protection action results were shown in Table 2.

[0080] Table 2 Operation of distance protection backup section

[0081]

[0082] During a fault, current flows from the converter to the fault point. Therefore, when fault F1 occurs, the fault current collected by devices C and D on the N side is provided by the PQ-controlled converter on the N side. When fault F2 occurs, the fault current collected by devices A and B on the M side and devices C and D on the N side are both provided by the PQ-controlled converter on the N side. Table 2 shows the distance protection operation. When protection devices A and C on both sides of the flexible low-frequency transmission system are provided with fault electrical quantities from the PQ-controlled side, both fail to operate within the zone and malfunction outside the zone. Protection devices B and D operate correctly.

[0083] When a metallic phase-to-phase short circuit occurs at point F1, due to the P / Q control strategy of the N-side converter, the N-side measured voltage and the positive-sequence voltage are out of phase, causing the phase-comparison distance protection element based on the positive-sequence voltage polarization to lose its directionality and the N-side device C to fail to operate. However, the phase-comparison distance protection element based on the measured voltage polarization can correctly reflect the impedance relationship between the protection installation and the fault point. Through the N-side measured current, it can be calculated that its positive-sequence and negative-sequence currents are both less than 0.1I n ( Figure 6 ), it is determined that the converter control strategy providing the fault electrical quantity side is P / Q control, so the N-side device D adopts a phase-comparison distance protection element based on the measured voltage polarization, and the protection operates correctly ( Figure 7 ), fault removal.

[0084] When a metallic phase-to-phase short circuit occurs at point F2, due to the P / Q control strategy of the N-side converter, the measured voltage and current on the M side are both provided by the N side. The measured voltage and positive-sequence voltage are out of phase, causing the phase-comparison distance protection element based on the positive-sequence voltage polarization to lose its directionality and the M-side device A protection to malfunction. However, the phase-comparison distance protection element based on the measured voltage polarization can correctly reflect the impedance relationship between the protection installation and the fault point. Through the measured current on the M side, it can be calculated that its positive-sequence and negative-sequence currents are both less than 0.1I n ( Figure 9 ), it is determined that the converter control strategy providing the fault electrical quantity side is P / Q mode, so the M side device B adopts the phase-comparison distance protection element based on the measured voltage polarization, and the protection is reliable and does not operate ( Figure 10 ).

[0085] The present disclosure may be a system, method and / or computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0086] A computer-readable storage medium can be a tangible device that can hold and store instructions for use by an instruction execution device. A computer-readable storage medium can be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination thereof. More specific examples (a non-exhaustive list) of computer-readable storage media include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, such as a punch card or a raised structure in a groove on which instructions are stored, and any suitable combination thereof. As used herein, a computer-readable storage medium is not to be construed as a transient signal per se, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagating through a waveguide or other transmission medium (e.g., a light pulse through a fiber optic cable), or an electrical signal transmitted through an electrical wire.

[0087] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions to be stored in the computer-readable storage medium in each computing / processing device.

[0088] The computer program instructions for performing the operations of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, and conventional procedural programming languages ​​such as "C" language or similar programming languages. Computer-readable program instructions may be executed entirely on a user's computer, partially on a user's computer, as an independent software package, partially on a user's computer, partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., utilizing an Internet service provider to connect via the Internet). In some embodiments, an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), may be personalized by utilizing the state information of the computer-readable program instructions. The electronic circuit may execute the computer-readable program instructions, thereby realizing various aspects of the present disclosure.

[0089] 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, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. An adaptive distance protection method for flexible low-frequency transmission lines, characterized in that: The following steps are involved: Step S1: collecting the voltage and current at the line protection installation location and extracting sequence components; Step S2: Use the voltage and current sequence component information to determine the converter control strategy that provides the fault current. If it is satisfied, it is determined that the fault current collected by the protection is provided by the P / Q control side; otherwise, it is determined that the fault current collected by the protection is provided by the V / F control side, where: and are the positive sequence current component and negative sequence current component of the fault current, respectively. C1 and C2 are the threshold values ​​of the positive sequence current and negative sequence current, respectively. Step S3: According to the converter control strategy providing the fault current, phase-comparison distance relays with different polarization voltages are selected to perform adaptive distance protection of the transmission line; when it is determined that the converter control strategy providing the fault current is the V / F mode, a phase-comparison distance relay based on positive sequence voltage polarization is used to perform adaptive distance protection of the line; when it is determined that the converter control strategy providing the fault current is the P / Q mode, a phase-comparison distance relay based on measured voltage polarization is used to perform adaptive distance protection of the line.

2. The adaptive distance protection method for flexible low-frequency transmission lines according to claim 1, characterized in that: In step S1, the three-phase voltage u of the line is collected A (t),u B (t),u C (t) and three-phase current i A (t), i B (t), i C (t), and through Fourier transform and phase sequence conversion, the positive sequence and negative sequence components of voltage and current are obtained respectively. and 3. The adaptive distance protection method for flexible low-frequency transmission lines according to claim 1, characterized in that: The C1 and C2 are both 0.1I n Setting, I n is the rated current value of the flexible low-frequency transmission line.

4. The adaptive distance protection method for flexible low-frequency transmission lines according to claim 1, characterized in that: In step S3, the phase-comparison distance relay based on positive sequence voltage polarization performs adaptive distance protection of the line as follows: in, It is the phase of the positive sequence voltage at the protection installation; the working voltage To protect the phase of voltage and current at the installation, Z set It is the distance protection setting value.

5. The adaptive distance protection method for flexible low-frequency transmission lines according to claim 4, characterized in that: The adaptive distance protection of the line based on the phase-comparison distance relay measuring voltage polarization is as follows: Among them, the working voltage To protect the phase of voltage and current at the installation, Z set It is the distance protection setting value.

6. An adaptive distance protection device for a flexible low-frequency transmission line based on the protection method according to any one of claims 1 to 5, comprising a current and voltage acquisition module, a converter control strategy judgment module, a distance protection enabling module, a phase-comparison distance relay based on positive sequence voltage polarization, and a phase-comparison distance relay based on measured voltage polarization, characterized in that: The current and voltage acquisition module acquires the voltage and current at the protection installation location; The converter control strategy judgment module extracts the sequence components of the voltage and current components, and compares the positive sequence current component and the negative sequence current component with the threshold value respectively to determine the control strategy of the converter providing the fault current; The distance protection enabling module enables or blocks the phase-comparison distance relay based on positive sequence voltage polarization and the phase-comparison distance relay based on measured voltage polarization according to the judgment result of the converter control strategy judgment module.

7. The adaptive distance protection device for flexible low-frequency transmission lines according to claim 6, characterized in that: In the converter control strategy judgment module, when If it is satisfied, it is determined that the fault current collected by the protection is provided by the P / Q control side. Otherwise, it is determined that the fault current collected by the protection is provided by the V / F control side. Among them, C1 and C2 are the threshold values ​​of positive sequence current and negative sequence current respectively. and They are the positive sequence current component and negative sequence current component of the fault current respectively.

8. The adaptive distance protection device for flexible low-frequency transmission lines according to claim 7, characterized in that: When it is determined that the control strategy of the converter providing the fault current is the V / F mode, the distance protection enabling module triggers the phase-comparison distance relay based on the positive sequence voltage polarization to perform adaptive distance protection of the line; When it is determined that the control strategy of the converter providing the fault current is the P / Q mode, the distance protection enabling module triggers the phase-comparison distance relay based on the measured voltage polarization to perform adaptive distance protection of the line.

9. A terminal comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to execute the steps of the method according to any one of claims 1 to 5.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method according to any one of claims 1 to 5 are implemented.

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