Flexible direct current receiving end distance protection optimization method and system based on q-axis current injection

Abstract

The invention discloses a flexible direct current receiving end grid-connected line distance protection performance improving method and system based on q-axis current optimization injection, and belongs to the technical field of power system relay protection. According to the method, after a system fault is detected, equivalent positive sequence q-axis current and negative sequence q-axis current are injected into a current converter. According to the method, the action reliability and the transition resistance tolerance of the positive sequence voltage polarization phase comparison type distance protection are remarkably improved, and a technical guarantee can be provided for reliable operation of distance protection in a novel power system.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection technology, specifically relating to a technology for improving the distance protection performance of flexible DC receiving-end grid-connected lines. Background Technology

[0002] Traditional relay protection principles are based on synchronous power source characteristics to design operating criteria. However, the integration of power electronic equipment (such as MMCs and renewable energy sources) leads to system fault response characteristics that differ significantly from synchronous generators. During normal operation, negative sequence current is generally suppressed to zero to reduce device current stress, which is one of the main differences between its fault response characteristics and those of synchronous power sources. However, under the suppression of negative sequence current, positive sequence voltage polarized ratio-type distance protection is prone to failure to operate within the protection zone and maloperation outside the protection zone, especially when large-scale power electronic equipment is integrated into the system, severely impacting the performance of single-ended protection such as distance protection. Existing technologies face the challenge of positive sequence voltage polarized ratio-type distance protection easily failing to operate within the protection zone and maloperating outside the protection zone during faults in flexible DC receiving-end grid-connected lines. Optimizing the negative sequence loop during faults is one effective way to improve the operating performance of distance protection. Therefore, a technical method to improve the distance protection performance of flexible DC receiving-end grid-connected lines is urgently needed. Summary of the Invention

[0003] The purpose of this invention is to solve the problem that existing positive-sequence voltage polarization ratio phase-type distance protection is prone to failure to operate within the zone and false operation outside the zone when there is a fault in the flexible DC receiving-end grid-connected line. To address this issue, an optimization method and system for flexible DC receiving-end distance protection based on q-axis current injection is proposed. By injecting equal amounts of positive and negative sequence q-axis currents into the converter during the fault period, the negative sequence loop regulation is optimized, improving the operating performance and tolerance to transition resistance of the distance protection, while not affecting the system voltage ride-through characteristics (overvoltage / undervoltage). This provides technical support for the reliable operation of distance protection in new power systems.

[0004] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0005] In a first aspect, the present invention provides a method for improving the distance protection performance of flexible DC receiving-end grid-connected lines based on q-axis current optimization injection, characterized by comprising the following steps:

[0006] Fault detection steps: Detect the fault status of the flexible DC grid-connected system. When the positive sequence voltage drops below the preset voltage threshold, generate a start signal.

[0007] Current injection step: In response to the fault state, an optimized positive-sequence q-axis current and negative-sequence q-axis current are injected into the converter control system to improve the operating performance of the positive-sequence voltage polarization phase-to-phase distance protection. In response to receiving a distance protection operation signal or a strategy exit command, the injection of the positive-sequence and negative-sequence q-axis current is stopped.

[0008] In some embodiments, the current injection step further includes:

[0009] Startup processing: When the positive sequence voltage is detected to drop below a preset voltage threshold, a startup signal is generated;

[0010] Current reference value calculation and processing: Based on the remaining capacity of the modular multilevel converter (MMC) and system operating constraints, calculate the positive sequence q-axis current reference value i to be injected. d1(ref) and negative sequence q-axis current reference value i q2(ref) ;

[0011] Current control processing: Based on the current reference value, the MMC is controlled to inject corresponding positive and negative sequence q-axis currents, and the positive and negative sequence q-axis currents are injected in an open-loop control manner.

[0012] In some implementations, prior to the current reference value calculation process, the following steps are also included:

[0013] d-axis current adjustment steps: Set the positive sequence d-axis current reference value i q2(ref) With the following limit value i d1(ref)_lower_limit The comparison is performed, and if it is greater than the lower limit, it is limited to the lower limit to release the MMC capacity for q-axis current injection.

[0014] In some implementations, the lower limit value i d1(ref)_lower_limit It is set to 0.5 per unit.

[0015] In some implementations, the calculation of the positive-sequence and negative-sequence q-axis current reference values ​​includes:

[0016] Calculate the total q-axis current capacity Δi that the MMC can currently generate. q12(ref) ;

[0017] The total q-axis current capacity is evenly distributed between the positive and negative sequences, thus obtaining i q1(ref) = Δi q1(ref) i q2(ref) = Δi q2(ref) , and Δi q1(ref) =Δi q2(ref) = Δi q12(ref) / 2.

[0018] In some implementations, the total q-axis current capacity Δi q12(ref)Calculated using the following formula:

[0019] ;

[0020] Among them, i max This is the maximum allowable current of the converter.

[0021] In some implementations, the voltage threshold is 0.9 per unit.

[0022] Secondly, the present invention provides a system for improving the distance protection performance of a flexible DC receiving-end grid-connected line based on optimized q-axis current injection, comprising a fault detection module for detecting the fault state of the flexible DC grid-connected system; and a current injection control module configured to output optimized positive-sequence q-axis current reference values ​​and negative-sequence q-axis current reference values ​​to the converter control system in response to detecting the fault state, so as to control the injection of corresponding currents into the modular multilevel converter (MMC), thereby improving the operating performance of the positive-sequence voltage polarization phase-to-phase distance protection; and to stop injecting the positive-sequence and negative-sequence q-axis currents in response to receiving a distance protection operating signal or a strategy exit command, for implementing any one of the methods described above.

[0023] Thirdly, the present invention provides an electronic device, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor, when executing the computer program, implements a source-load spatiotemporal correlation modeling method based on an attention mechanism according to any one of claims 1 to 7.

[0024] Fourthly, the present invention provides a non-transitory computer-readable storage medium having a computer program stored thereon, wherein the computer program, when executed by a processor, implements a source spatiotemporal correlation modeling method based on an attention mechanism according to any one of claims 1 to 7.

[0025] Compared with the prior art, the beneficial effects and advantages of the present invention are as follows:

[0026] 1) Improve the reliability of distance protection operation by injecting positive and negative sequence q-axis current.

[0027] 2) Optimizing the current injection strategy can improve the adaptability of the protection system.

[0028] 3) The effects of the positive and negative sequence q-axis currents on the system voltage amplitude cancel each other out, preventing overvoltage or undervoltage problems;

[0029] 4) Current injection within the MMC capacity limit ensures safe and stable system operation, significantly improves the reliability of positive sequence voltage polarization ratio phase distance protection and its ability to withstand transition resistance, thus making it suitable for new power systems and providing reliable protection support for systems with a high proportion of power electronic equipment. Attached Figure Description

[0030] Figure 1 This is an overall flowchart of the flexible direct current receiving end distance protection optimization method based on q-axis current injection of the present invention;

[0031] Figure 2 A detailed flowchart of the current injection step;

[0032] Figure 3 Flowchart of the collaborative optimization injection strategy for MMC q-axis current;

[0033] Figure 4 This is a typical topology of a sea breeze-connected DC grid system;

[0034] Figure 5 This is a block diagram of the flexible DC receiving end distance protection optimization system based on q-axis current injection of the present invention. Detailed Implementation

[0035] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0036] Example 1: As Figure 1 The figure shows the overall flow of the flexible DC receiving end distance protection optimization method based on q-axis current injection of the present invention. The specific flow is as follows:

[0037] Specifically, the following steps are included:

[0038] Step 1: Fault detection; Detect the fault status of the flexible DC grid-connected system. When the positive sequence voltage drops below the preset voltage threshold, a start signal is generated.

[0039] Step 2, Current Injection Step: In response to the fault state, optimized positive-sequence q-axis current and negative-sequence q-axis current are injected into the converter control system to improve the operating performance of the positive-sequence voltage polarization phase-to-phase distance protection. In response to receiving a distance protection operation signal or a strategy exit command, the injection of the positive-sequence and negative-sequence q-axis current is stopped.

[0040] The fault detection in step 1 includes: after a fault is detected, when the positive sequence voltage drops below a set threshold, the q-axis current injection strategy is activated.

[0041] Among them, such as Figure 2 As shown, step 2, the current injection step, further includes:

[0042] Step 2.1, Start-up steps: When the positive sequence voltage is detected to drop below the preset voltage threshold, a start-up signal is generated; after a fault occurs, when the MMC detects that the positive sequence voltage has dropped below 0.9pu, q-axis current injection is performed: the start-up signal Signal is set to 1.

[0043] Step 2.2, Current Reference Value Calculation Steps: Based on the remaining capacity of the Modular Multilevel Converter (MMC) and system operating constraints, calculate the positive sequence q-axis current reference value i to be injected. d1(ref) and negative sequence q-axis current reference value i q2(ref) ;

[0044] This step further includes:

[0045] Considering the MMC capacity limit, i.e., the remaining capacity, the total q-axis current capacity Δi that can be increased is calculated according to equation (1). q12(ref) The expression is as follows:

[0046] (1)

[0047] Among them, i max This refers to the maximum allowable current of the MMC;

[0048] And according to equation (2), Δi is obtained by equal division. q1(ref) , Δi q2(ref) , i.e., Δi q1(ref) =Δi q2(ref) = Δi q12(ref) / 2. Therefore, the reference values ​​for the positive and negative sequence q-axis currents are i. q1(ref) = Δi q1(ref) i q2(ref) = Δi q2(ref) .

[0049] Based on the calculated reference values ​​of the positive and negative sequence q-axis currents, an open-loop control method is used to inject equal amounts of positive and negative sequence q-axis currents into the converter.

[0050] Before the current reference value calculation step in step 2.2, a d-axis current adjustment step is also included: adjusting the positive sequence d-axis current reference value i q2(ref) With the lower limit value i d1(ref)_lower_limit A comparison is made; if the value exceeds the lower limit, it is then capped at that lower limit to release the MMC capacity for q-axis current injection. Specifically, the positive-sequence d-axis current reference value i... d1(ref) The adjustment principle is expressed as follows:

[0051] (2)

[0052] In the formula, U pcc_ 1 represents the effective value of the positive sequence voltage at the current grid connection point, P * For active power instructions during normal operation, i max i is the maximum allowable current of MMC (effective value of phase current). max =n*I N I NThe rated current of the MMC is n, where n is a real number greater than 1, typically taken as 1.2 to 1.5. In this step, we take 1.2.

[0053] Equation (2) shows that the positive sequence d-axis current reference value i d1(ref) The adjustment principle is to adjust i d1(ref0) With lower limit value i d1(ref)_lower_limit Compare, if i d1(ref0) >i d1(ref)_lower_limit , let i d1(ref) =i d1(ref)_lower_limit ; if i d1(ref0) < i d1(ref)_lower_limit Then let i d1(ref) =i d1(ref0) ;

[0054] Furthermore, i d1(ref)_lower_limit The two determining principles include: 1) maximizing the distance protection withstand resistance capability, and 2) minimizing the change in the original active power state. Among these, i d1(ref) The lower limit of the allowable descent, i d1(ref)_lower_limit Set to 0.5 pu.

[0055] Step 2.3, Current control steps: Based on the current reference value, control the MMC to inject the corresponding positive sequence and negative sequence q-axis currents, and use an open-loop control method to inject the positive sequence and negative sequence q-axis currents.

[0056] like Figure 3 The diagram shows a typical implementation system structure, where positive and negative sequence q-axis current injection can be directly performed after a fault is detected. This illustrates the operation of two converter stations (MMCs) using different control modes in a typical flexible DC transmission system during a line fault. The system consists of two modular multilevel converters (MMCs) connected by a transmission line (line L). The MMC on the left uses a grid-connected control mode, while the MMC on the right uses a grid-following control mode. The "80%" mark (80% of the line length from the left MMC) is a key point for fault analysis; the "back-side line" refers to the upstream grid or load side of the grid-following control MMC. The essential difference between the two control modes under fault conditions is that the grid-connected control MMC acts as the system's "anchor," actively providing voltage support and enhancing the grid's resistance to disturbances. The grid-following control MMC is prone to instability when the grid voltage is severely distorted, requiring a protection system to quickly isolate the fault. This configuration is a typical solution adopted by modern power systems, especially those with a high proportion of new energy sources (such as wind farm grid connection), to improve stability. In this system, the grid-connected converter plays a crucial stabilizing role.

[0057] In summary, this invention provides a method for improving the distance protection performance of flexible DC receiving-end grid-connected lines based on optimized q-axis current injection. After a fault occurs, equal amounts of positive-sequence and negative-sequence q-axis current are injected into the converter, significantly improving the reliability of distance protection operation and its ability to withstand transition resistance. This is achieved by injecting positive-sequence q-axis current (i...) into the converter... q1 <0) and negative sequence q-axis current (i q2 Both <0) can shift the phase ratio result of distance protection towards the reliable action center, thereby improving the reliability of protection action; by injecting equal amounts of positive-sequence and negative-sequence q-axis current into the converter, the effects on the system voltage amplitude are the same but opposite in direction, and will not adversely affect the voltage ride-through characteristics (overvoltage / undervoltage); by comprehensively considering the requirements for improving distance protection performance, system overvoltage / undervoltage requirements, and MMC capacity constraints, a collaborative optimization injection strategy for MMC q-axis current during faults is proposed, providing theoretical guarantee and technical support for the reliability of distance protection in new power systems.

[0058] Example 2, as follows Figure 5 As shown, a system for improving the distance protection performance of a flexible DC receiving-end grid-connected line based on optimized q-axis current injection includes a fault detection module 100 for detecting the fault state of the flexible DC grid-connected system; and a current injection control module 200, configured to output optimized positive-sequence q-axis current reference values ​​and negative-sequence q-axis current reference values ​​to the converter control system in response to detecting the fault state, so as to control the injection of corresponding currents into the modular multilevel converter (MMC), thereby improving the operating performance of the positive-sequence voltage polarization phase-type distance protection; and to stop injecting the positive-sequence and negative-sequence q-axis currents in response to receiving a distance protection operating signal or a strategy exit command, for implementing the method described in any one of claims 1 to 7.

[0059] Furthermore, based on a similar inventive concept, Embodiment 3 of the present invention also provides an electronic device, including: a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor implements the above-described method when executing the computer program.

[0060] Furthermore, based on a similar inventive concept, Embodiment 4 of the present invention also provides a non-transitory computer-readable storage medium storing a computer program thereon, characterized in that the computer program implements the above-described method when executed by a processor.

[0061] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the invention.

[0062] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

[0063] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered to fall within the scope of protection of the present invention.

[0064] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings and technical solutions. These descriptions are merely preferred embodiments and are not intended to limit the scope or concept of the invention. To avoid unnecessary repetition, various possible combinations are not described separately. Any combination, modification, or improvement of the specific technical features described in the above embodiments, without contradiction, should fall within the scope of protection of this invention, provided it does not depart from the concept of the invention.

Claims

1. A method for improving the distance protection performance of flexible DC receiving-end grid-connected lines based on q-axis current optimization injection, characterized in that, Includes the following steps: Fault detection steps: Detect the fault status of the flexible DC grid-connected system. When the positive sequence voltage drops below the preset voltage threshold, generate a start signal. Current injection step: In response to the fault state, an optimized positive-sequence q-axis current and negative-sequence q-axis current are injected into the converter control system to improve the operating performance of the positive-sequence voltage polarization phase-to-phase distance protection. In response to receiving a distance protection operation signal or a strategy exit command, the injection of the positive-sequence and negative-sequence q-axis current is stopped.

2. The method according to claim 1, characterized in that, The current injection step further includes: Startup steps: When the positive sequence voltage is detected to drop below a preset voltage threshold, a startup signal is generated; Current reference value calculation steps: Based on the remaining capacity of the modular multilevel converter (MMC) and system operating constraints, calculate the positive sequence q-axis current reference value i to be injected. d1(ref) and negative sequence q-axis current reference value i q2(ref) ; Current control steps: Based on the current reference value, control the MMC to inject corresponding positive and negative sequence q-axis currents, and inject the positive and negative sequence q-axis currents using an open-loop control method.

3. The method according to claim 1, characterized in that, Prior to the current reference value calculation step, the following is also included: d-axis current adjustment steps: Set the positive sequence d-axis current reference value i q2(ref) With the following limit value i d1(ref)_lower_limit The comparison is performed, and if it is greater than the lower limit, it is limited to the lower limit to release the MMC capacity for q-axis current injection.

4. The method according to claim 3, characterized in that, The lower limit value i d1(ref)_lower_limit It is set to 0.5 per unit.

5. The method according to claim 3, characterized in that, The calculation of the positive-sequence and negative-sequence q-axis current reference values ​​includes: Calculate the total q-axis current capacity Δi that can be increased currently in the Modular Multilevel Converter (MMC). q12(ref) ; The total q-axis current capacity is evenly distributed between the positive and negative sequences, thus obtaining i q1(ref) = Δi q1(ref) i q2(ref) = Δi q2(ref) , and Δi q1(ref) =Δi q2(ref) = Δi q12(ref) / 2.

6. The method according to claim 5, characterized in that, The total q-axis current capacity Δi q12(ref) Calculated using the following formula: ; Among them, i max This is the maximum allowable current of the converter.

7. The method according to claim 1, characterized in that, The voltage threshold is 0.9 per unit.

8. A system for improving the distance protection performance of flexible DC receiving-end grid-connected lines based on q-axis current optimization injection, characterized in that, Includes a fault detection module for detecting fault status in the flexible DC grid-connected system; The current injection control module, in response to detecting the fault state, is configured to output optimized positive-sequence q-axis current reference values ​​and negative-sequence q-axis current reference values ​​to the converter control system to control the injection of corresponding currents into the modular multilevel converter (MMC), thereby improving the operating performance of the positive-sequence voltage polarization phase-to-phase distance protection. In response to receiving a distance protection operation signal or a strategy exit command, the module stops injecting the positive-sequence and negative-sequence q-axis currents, and is used to implement the method described in any one of claims 1 to 7.

9. An electronic device, comprising: The memory, the processor, and the computer program stored in the memory and executable on the processor, wherein the processor, when executing the computer program, implements a source-load spatiotemporal correlation modeling method based on any one of claims 1 to 7.

10. A non-transitory computer-readable storage medium having a computer program stored thereon, the computer program being executed by a processor to implement a source spatiotemporal correlation modeling method based on an attention mechanism according to any one of claims 1 to 7.

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