Method and system for improving applicability of current differential protection of flexible low-frequency network construction power transmission system

Abstract

The invention discloses a current differential protection applicability improving method and system for a flexible low-frequency network construction power transmission system. The method comprises the following steps: sampling three-phase current on two sides and extracting a fundamental frequency 20Hz phasor; calculating sequence network current by adopting a three-phase symmetric component method, and verifying the effectiveness of the sequence network by setting a threshold value; negative sequence and zero sequence effectiveness is dynamically judged based on sequence network current, and the maximum adjustment value of the braking coefficient is distributed; and calculating an auxiliary criterion value YF / YZ by using the sequence network current difference, comparing the auxiliary criterion value YF / YZ with a starting threshold value Ye to judge the credibility of an internal fault, and dynamically correcting a braking coefficient Kre according to the credibility of the internal fault. According to the method, the action sensitivity of protection in an internal fault is effectively improved, the problem of adaptability under a high-resistance fault is solved, and the reliability of current differential protection in a flexible low-frequency power transmission system is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of protection and control of flexible low-frequency grid systems, and specifically relates to a method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems by utilizing the characteristics of composite sequence grid current. Background Technology

[0002] Flexible low-frequency grid transmission systems are a valuable supplement to power frequency transmission systems and high-voltage direct current transmission technologies, offering unique advantages for medium- and long-distance power transmission. The ability to quickly and accurately identify faults within and outside the designated fault zone using electrical quantity information from both ends during a fault is a key requirement for AC main protection systems.

[0003] However, in low-frequency transmission systems with grids, the current characteristics during fault occurrences change because both the M3C grid-side and new energy source-side currents are controlled by power electronic converters. Unlike conventional transmission systems where current flows from both sides of the power network to the fault point, in this system, most of the current is output from the new energy source side, with only a small portion flowing to the fault point, while the majority flows through the fault point to the M3C grid-side, exhibiting a through-current characteristic. This characteristic results in the current amplitude ratio measured at the protection installation points on both sides of the fault phase being closer to 1.0, and the phase angle difference approaching 180°. Consequently, the distinction between faults within and outside the fault zone is not significant, leading to a significant reduction in the sensitivity coefficient and insufficient applicability of differential current protection for faults within the fault zone.

[0004] Currently, the applicability of conventional current differential protection in flexible low-frequency grid transmission systems for new energy sources is significantly reduced, with severe failure to operate within the protection zone, making it difficult to meet engineering application requirements. This is mainly because the current on both sides of the system is controlled by power electronic converters. When an asymmetrical fault occurs, the limiting effect of the converters leads to a smaller differential current and a larger braking current within the protection zone, resulting in a significant decrease in protection sensitivity and severe failure to operate. Therefore, it is urgent to propose a method to improve the applicability of the main protection for flexible low-frequency grid transmission lines. Summary of the Invention

[0005] To address the problem of in-zone failure of existing current differential protection in flexible low-frequency grid systems due to the influence of power electronic converter control strategies, this invention aims to provide a method and system for improving the applicability of current differential protection in flexible low-frequency grid transmission systems. Through a dynamic adjustment mechanism based on sequence network validity judgment and auxiliary criterion value calculation, the problem of in-zone failure of current differential protection in flexible low-frequency grid systems is solved, significantly improving protection sensitivity and applicability.

[0006] To achieve the above-mentioned objectives, the present invention proposes the following technical solution:

[0007] This invention proposes a method to improve the applicability of current differential protection in flexible low-frequency grid transmission systems, comprising the following steps:

[0008] The three-phase current values ​​at the protection installation points on the M3C grid side and the new energy side were sampled and obtained, and the fundamental frequency 20Hz current component was extracted.

[0009] The positive-sequence, negative-sequence, and zero-sequence current phasors at the protection installation points on both sides are calculated using the three-phase symmetrical component method. A sequence network current threshold e is set. When the amplitude of a certain sequence network current phasor is less than the threshold, the sequence network current is determined to be invalid and set to zero, so as to complete the verification and screening of the validity of the sequence network current.

[0010] Based on the filtered effective sequence network current phasors, the validity of the negative sequence and zero sequence networks is determined: if the negative sequence current at the protection installation point on the M3C side or the new energy side is not zero, the negative sequence network is determined to be valid, denoted as NF=1; otherwise, NF=0. If the zero sequence current is not zero, the zero sequence network is determined to be valid, denoted as NZ=1; otherwise, NZ=0. Subsequently, according to the validity identifiers NF and NZ, the maximum adjustment value T for the main protection braking coefficient is assigned to the negative sequence and zero sequence networks. FM and T ZM ;

[0011] Based on the valid sequence network current phasors, calculate the negative sequence auxiliary criterion value Y. F and zero-order auxiliary criterion value Y Z Set the startup threshold Y e By separately Y F Y Z With Y e By comparing the data, the credibility of the fault occurring within the protected area is determined, and the dynamic adjustment coefficient Y is calculated based on this credibility. FT and Y ZT ;

[0012] Using the dynamic adjustment coefficient Y FT Y ZT and the maximum adjustment value T FM T ZM The initial braking coefficient K1 is corrected to obtain the adjusted braking coefficient K. re ;

[0013] Using the corrected braking coefficient K re Perform current differential protection judgment and generate corresponding protection action commands.

[0014] In some implementations, the specific steps for ensuring the validity of the sequence network include:

[0015] If the negative sequence current amplitude at the M3C grid side or new energy side protection installation point is not zero, the negative sequence grid is deemed valid and marked as NF=1; otherwise, NF=0.

[0016] If the zero-sequence current amplitude at the M3C grid-side or new energy side protection installation point is not zero, the zero-sequence grid is deemed valid, and the identifier NZ=1 is recorded; otherwise, NZ=0.

[0017] Among them, the validity judgment is combined with the control scheme of the flexible low-frequency grid transmission system. If the sequence currents on both sides are zero, it is considered that the sequence network does not exist or is suppressed by the converter.

[0018] In some implementations, the step of allocating the maximum adjustment value uses the following formula:

[0019] ;

[0020] Wherein, K1 is the initial braking coefficient of the main protection, K min The minimum braking coefficient, determined by the system and current sensor specifications, is typically between 0.1 and 0.2. (T) FM and T ZM T represents the maximum adjustment value for the negative and zero order criteria, respectively. FM and T ZM These are the maximum adjustment values ​​for the negative and zero order criteria, respectively.

[0021] In some implementations, the auxiliary criterion value Y F and Y Z The calculation formula is:

[0022] ;

[0023] in, , , These represent the positive, negative, and zero-sequence network currents at the M3C network-side protection installation location, respectively. , , These are the positive, negative, and zero sequence network currents at the new energy side protection installation point, respectively.

[0024] In some implementations, the formula for the dynamically corrected braking coefficient is:

[0025]

[0026] The adjustment coefficient Y FT / Y ZT Calculated through the following tiered triggering mechanism:

[0027] .

[0028] In some implementations, the sequence current threshold e is set to 0.2% of the rated current value.

[0029] In some implementations, the activation threshold is determined by the system and differential protection elements, and Y is set according to the error range of the protection system and the sensor. e For commonly used current sensor threshold Y e ∈[0.1,0.2].

[0030] Secondly, this invention proposes a system for improving the applicability of current differential protection in flexible low-frequency grid transmission systems, used to implement any of the methods described above, including:

[0031] The data acquisition module is used to sample and acquire the three-phase current values ​​at the protection installation points on the M3C grid side and the new energy side;

[0032] The signal processing module is communicatively connected to the data acquisition module and is used to extract the fundamental frequency 20Hz current component from the three-phase current values ​​and to calculate the positive sequence, negative sequence, and zero sequence current phasors at the protection installation positions on both sides using the three-phase symmetrical component method.

[0033] The sequence network validity judgment module, communicatively connected to the signal processing module, is used to verify the validity of the decomposed sequence network current by setting a sequence network current threshold e. When the sequence network current amplitude is less than the threshold, the sequence network current is set to zero. Based on the sequence network current phasor, the validity of the negative sequence and zero sequence networks is judged, validity identifiers NF and NZ are generated, and the maximum adjustment value T of the negative sequence and zero sequence networks to the main protection braking coefficient is allocated. FM and T ZM ;

[0034] The dynamic adjustment module, which is communicatively connected to the sequence network validity determination module, is used to perform the following operations:

[0035] Based on the valid sequence network current phasors, calculate the negative sequence auxiliary criterion value Y. F and zero-order auxiliary criterion value Y Z Set the startup threshold Ye, and then set Y to... F Y Z With Y e By comparing the data, the credibility of the fault occurring within the protected area is determined, and the dynamic adjustment coefficient Y is calculated based on this credibility. FT and Y ZT ;

[0036] Using the dynamic adjustment coefficient Y FT Y ZT and the maximum adjustment value T FM T ZM The initial braking coefficient K1 is corrected to obtain the adjusted braking coefficient K. re ;

[0037] The protection execution module is communicatively connected to the dynamic adjustment module and is used to utilize the corrected braking coefficient K. re Perform current differential protection judgment and generate corresponding protection action commands.

[0038] Thirdly, the present invention proposes an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement any of the methods for improving the applicability of current differential protection in flexible low-frequency grid transmission systems.

[0039] Fourthly, this invention proposes a computer-readable storage medium storing a computer program that, when executed by a processor, implements the method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems as described in any one of the claims. 1) By introducing a dynamic adaptive mechanism based on composite sequence network characteristic analysis, the inherent defects of conventional protection in flexible low-frequency controlled power supply scenarios are effectively overcome, achieving significant technical progress. Its beneficial effects are specifically reflected in the following aspects:

[0040] Compared with the prior art, the significant beneficial effects of the technical solution of the present invention are:

[0041] 1) By dynamically calculating the sequence network auxiliary criteria and comparing them with the graded threshold, the possibility of faults in the zone can be accurately judged. Based on this, the protection action threshold is intelligently reduced, thereby completely solving the problem of protection failure to operate caused by the controlled characteristics of the system and greatly improving the sensitivity of fault action in the zone.

[0042] 2) By setting a threshold to intelligently ignore invalid sequence network information and dynamically allocating and adjusting weights according to fault characteristics, the protection method can not only effectively deal with metallic short circuits, but also stably adapt to complex high-resistance grounding faults with a transition resistance of up to 200Ω, demonstrating excellent reliability and wide adaptability.

[0043] 3) By using an intelligent hierarchical triggering mechanism, the adjustment logic of the "dead zone-linear zone-saturation zone" characteristics is introduced to ensure reliable operation without false triggering when there is a fault outside the zone, and to dynamically improve sensitivity when there is a fault inside the zone, thus balancing reliability and sensitivity.

[0044] 4) It can be implemented using existing protection devices, with low deployment costs and high practicality. Attached Figure Description

[0045] Figure 1 This is an overall flowchart of the method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to the present invention.

[0046] Figure 2 This is a block diagram of the system for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to the present invention.

[0047] Figure 3 This is a diagram illustrating the implementation process of the present invention.

[0048] Figure 4 This is a schematic diagram of the overall topology and control structure of a flexible low-frequency grid transmission system for new energy sources.

[0049] Figure 5 The equivalent circuit diagram of the system composite sequence network when a fault occurs in the protected area; (5a) Sequence network model of two-phase short circuit fault in the area, (5b) Sequence network model of single-phase short circuit to ground fault in the area, (5c) Sequence network model of two-phase short circuit to ground fault in the area.

[0050] Figure 6 The equivalent circuit diagram of the system composite sequence network when a fault occurs outside the protection zone is as follows: (6a) Sequence network model of two-phase short circuit fault outside the protection zone, (6b) Sequence network model of single-phase short circuit to ground fault outside the protection zone, and (6c) Sequence network model of two-phase short circuit to ground fault outside the protection zone.

[0051] Figure 7 The diagram shows the voltage and current waveforms at the installation points of the fault M3C and the new energy side protection.

[0052] Figure 8 Schematic diagram of the change in sensitivity coefficient of current differential protection before and after improvement (enhancement) for different metallic faults; (8a) single-phase ground fault, (8b) two-phase ground fault, (8c) two-phase phase-to-phase fault.

[0053] Figure 9 The protection sensitivity coefficient before and after the transition resistance for non-metallic faults was improved from 10, 20 to 200 ohms. Detailed Implementation

[0054] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0055] Example 1: As Figure 1 The diagram shown is a flowchart of the method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to the present invention. The method includes the following steps:

[0056] Step S1: Sample and obtain the three-phase current values ​​at the protection installation points on the M3C grid side and the new energy side. Use Fast Fourier Decomposition to extract the current component with a fundamental frequency of 20Hz. Let the three-phase currents at the M3C grid side protection installation points be... , , The three-phase currents at the new energy side protection installation point are respectively , , .

[0057] Step S2: Calculate the sequence currents (including positive, negative, and zero sequence current phasors) at the protection installation points on both sides using the three-phase symmetrical component method. Let the positive, negative, and zero sequence currents at the M3C network side protection installation point be denoted as follows: , , The positive, negative, and zero sequence currents at the new energy side protection installation point are respectively , , Based on the measurement error range of the selected current sensor, a sequence current threshold e is set. When the amplitude of a certain sequence current phasor is less than this threshold, the sequence current is deemed invalid and set to zero, thus completing the verification and screening of the sequence current's validity. Specifically, the validity of the decomposed sequence current is verified by setting the sequence current threshold e = 0.001. When the sequence current amplitude is less than the threshold e, the sequence current is set to zero, thus completing the verification and screening of the sequence current's validity. For example, when using an electronic power sensor, the sequence current threshold e is set to 0.2% of the rated current.

[0058] Step S3: Based on the filtered effective sequence network current phasors, determine the validity of the negative sequence and zero sequence networks: If the negative sequence current at the protection installation point on the M3C side or the new energy side is not zero, the negative sequence network is determined to be valid, denoted as NF=1; otherwise, NF=0. If the zero sequence current is not zero, the zero sequence network is determined to be valid, denoted as NZ=1; otherwise, NZ=0. Subsequently, according to the validity identifiers NF and NZ, assign the maximum adjustment value T of the main protection braking coefficient to the negative sequence and zero sequence networks. FM and T ZM ;

[0059] Combination such as Figure 3 The implementation process diagram shown is described in detail below:

[0060] Based on the sequence current values ​​on both sides, determine the validity of the negative-sequence and zero-sequence auxiliary criteria, denoted as NF and NZ. Combining the sequence current detection and the control scheme of the flexible low-frequency grid transmission system, determine whether a negative-sequence or zero-sequence sequence network exists under this fault. If the negative-sequence current amplitude at the protection installation point on the M3C grid side or the new energy side is not zero, the negative-sequence sequence network is considered valid, and NF=1 is recorded; otherwise, NF=0. If the zero-sequence current amplitude at the protection installation point on the M3C grid side or the new energy side is not zero, the zero-sequence sequence network is considered valid, and NZ=1 is recorded; otherwise, NZ=0. The validity determination is combined with the control scheme of the flexible low-frequency grid transmission system; if the sequence currents on both sides are zero, it is considered that the sequence network does not exist or is suppressed by the converter.

[0061] The validity judgment is based on the sequence network current distribution characteristics verified by simulation. Specifically, it is reflected in the difference between the composite sequence network model of faults within the zone and faults outside the zone. This difference is manifested in the asymmetry of the sequence network current distribution between the M3C network side and the new energy side when there is a fault within the zone, while the sequence network current distribution is close to symmetrical when there is a fault outside the zone.

[0062] Specifically, based on the detection results, the adjustment values ​​of the braking coefficient are assigned using negative-sequence and zero-sequence auxiliary criteria. Let the initial action coefficient of the main protection be K1, and the minimum action coefficient after adjustment be K. min To avoid the impact of measurement delay, line capacitor charging, etc. on the minimum action coefficient K min It must be greater than 0.15. The maximum adjustment value is K1-K. min Let the maximum adjustment values ​​of the negative-sequence and zero-sequence criteria for the main protection be T, respectively. FM and T ZM When both NF and NZ are 0, the auxiliary criterion does not activate and the main protection braking coefficient is not adjusted; when the negative-sequence and zero-sequence weighted values ​​NF and NZ are not simultaneously 0, the maximum adjustment value T of the negative-sequence and zero-sequence auxiliary criteria is... FM and T ZM The following formula is used:

[0063] (1);

[0064] Step S4: Calculate the negative sequence auxiliary criterion value Y based on the effective sequence network current phasors. F and zero-order auxiliary criterion value Y Z Set the startup threshold Y e By separately Y F Y Z With Y e By comparing the data, the credibility of the fault occurring within the protected area is determined, and the dynamic adjustment coefficient Y is calculated based on this credibility. FT and Y ZT ;

[0065] Combination such as Figure 3 The implementation process diagram shown is described in detail below:

[0066] The auxiliary criterion value Y F and Y Z The calculation formula is:

[0067] (2);

[0068] The formula is based on the following: Figure 6 and Figure 7 The differences in sequence network current distribution shown are summarized. During a fault within the region, Y... F / Y Z Significantly greater than 1, and close to 0 in cases of faults outside the zone. This characteristic is demonstrated by, for example... Figure 8The simulation verification of metallic faults is shown.

[0069] To avoid the influence of measurement errors, the auxiliary criterion activation threshold is set to Y. e Considering measurement errors and capacitor charging issues, Y e A value of 0.1-0.2 can be taken, by comparing Y. F With Y Z and Y e The magnitude relationship can be used to calculate the corresponding auxiliary criterion adjustment coefficient Y. FT and Y ZT Here, Q represents F or Z, corresponding to negative order and zero order respectively, and the general adjustment factor is set to Y. QT Adjustment coefficient Y FT and Y ZT The calculation is performed using the following hierarchical triggering mechanism, as shown in formula (6):

[0070] (3);

[0071] The tiered triggering mechanism here is manifested as follows:

[0072] Level 1 (no adjustment triggered): When the sequence current ratio Y Q Very small (Y) e ≥Y Q When the fault characteristics are not obvious, the adjustment coefficient is 0 and no adjustment is made.

[0073] Level 2 (Linear Adjustment Region): When Y... Q In the middle range (3Y) e >Y Q ≥Y e When Y is in the range of 0, the adjustment coefficient varies with Y. Q Linear increases allow for smooth, gradual adjustments.

[0074] Level 3 (Fully Triggered Adjustment): When Y Q Very large (Y) Q >3Y e When the fault characteristics are considered very clear, the adjustment coefficient is set to the maximum value of 1, and a full adjustment is performed. This constitutes an adaptive controller with dead zone, linear zone, and saturation zone characteristics, which can effectively filter out minor disturbances and errors (i.e., avoid false tripping) and provide the strongest adjustment force when the fault characteristics are clear (i.e., prevent failure to tripping), thereby robustly improving protection performance.

[0075] Step S5: Utilize the dynamic adjustment coefficient Y FT Y ZT and the maximum adjustment value T FM T ZM The initial braking coefficient K1 is corrected to obtain the adjusted braking coefficient K.re ;

[0076] Combination such as Figure 3 The implementation process diagram shown is described in detail below:

[0077] Based on the weighted values ​​and adjustment coefficients of each sequence network, the braking coefficient K after adjustment of the auxiliary criterion is calculated. re As shown in equation (4):

[0078] (4);

[0079] The adjusted braking coefficient is used to perform current differential protection judgment.

[0080] Step S6: Utilize the corrected braking coefficient K re Perform current differential protection judgment and generate corresponding protection action commands.

[0081] Combination such as Figure 3 The implementation process diagram shown is described in detail below:

[0082] According to the adjusted braking coefficient K re The operating conditions for the three-phase current differential main protection are as follows, taking phase A as an example:

[0083] (5)

[0084] Formula (5) shows that:

[0085] 1. Operating conditions: The protection will operate when the differential current of phase A is greater than the braking current.

[0086] 2. Braking (failure to operate) condition: When the differential current of phase A is less than or equal to the braking current, the protection will not operate (failure to operate).

[0087] Taking phase A as an example, this formula reflects the core criterion of current differential protection: by comparing the magnitude of the differential current and the braking current in real time, it determines whether to issue a trip command.

[0088] Example 2: Figure 2 As shown, this invention presents a system for improving the applicability of current differential protection in flexible low-frequency grid transmission systems, based on the method for improving the applicability of current differential protection in such systems. The system includes:

[0089] Data acquisition module 100 is used to sample and acquire the three-phase current values ​​at the protection installation points on the M3C grid side and the new energy side;

[0090] The signal processing module 200 is communicatively connected to the data acquisition module and is used to extract the fundamental frequency 20Hz current component from the three-phase current values ​​and to calculate the positive sequence, negative sequence, and zero sequence current phasors of the protection installation positions on both sides using the three-phase symmetrical component method.

[0091] The sequence network validity judgment module 300, communicatively connected to the signal processing module, is used to verify the validity of the decomposed sequence network current by setting a sequence network current threshold e. When the sequence network current amplitude is less than the threshold, the sequence network current is set to zero. Based on the sequence network current phasor, the validity of the negative sequence and zero sequence networks is judged, validity identifiers NF and NZ are generated, and the maximum adjustment value T of the negative sequence and zero sequence networks to the main protection braking coefficient is allocated. FM and S ZM ;

[0092] The dynamic adjustment module 400, which is communicatively connected to the sequence network validity determination module, is used to perform the following operations:

[0093] Based on the valid sequence network current phasors, calculate the negative sequence auxiliary criterion value Y. F and zero-order auxiliary criterion value Y Z Set the startup threshold Y e By separately Y F Y Z With Y e By comparing the data, the credibility of the fault occurring within the protected area is determined, and the dynamic adjustment coefficient Y is calculated based on this credibility. FT and Y ZT ;

[0094] Using the dynamic adjustment coefficient Y FT Y ZT and the maximum adjustment value T FM T ZM The initial braking coefficient K1 is corrected to obtain the adjusted braking coefficient K. re ;

[0095] The protection execution module 500 is communicatively connected to the dynamic adjustment module and is used to utilize the corrected braking coefficient K. re Perform current differential protection judgment and generate corresponding protection action commands.

[0096] This invention uses composite sequence current as an auxiliary criterion to reduce the action coefficient when a fault occurs within the current differential protection zone. This technology provides a highly reliable and easily deployable key protection solution for flexible low-frequency power transmission systems for new energy sources.

[0097] To verify the effectiveness of the method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems described in this invention, a simulation model of the flexible low-frequency transmission system was built on the PSCAD / EMTDC electromagnetic transient simulation platform. Taking a single-phase fault as an example, the installation locations of M3C and new energy protection after the fault are shown. Figure 4 The diagram shows the control and ride-through scheme structure of a flexible low-frequency transmission system. In the diagram, both the M3C and the renewable energy converter employ dual closed-loop control with positive and negative sequence decoupling. On the M3C grid-connected side, positive sequence control uses V / F grid-connected control, negative sequence control uses negative sequence voltage suppression, and an overvoltage ride-through strategy is implemented. Based on the current withstand capability of the M3C submodules and the transmission lines, the inner and outer loop limiting values ​​for positive and negative sequences are set. On the renewable energy side, a P / Q grid-connected control scheme is adopted, with negative sequence current suppression control. To prevent excessive voltage drop on the renewable energy side after a fault, which could lead to grid disconnection, internationally standard low-voltage ride-through control is used.

[0098] When a system fault occurs, the fault equivalent model will switch accordingly due to the fundamental differences in the control strategies on both sides and whether the current reaches the preset limit value. Analysis based on the symmetrical component method and fault boundary conditions shows that, as... Figure 5 , Figure 6 As shown, the current limiting values ​​of the converters on both sides have a crucial impact on the fault current distribution. Taking a single-phase ground fault in phase A as an example, , These represent the A-phase fundamental frequency current phasors extracted from the M3C grid side and the new energy protection installation location, respectively. , , The positive, negative, and zero-sequence network current phasors extracted from the M3C network-side protection installation location. , , The positive, negative, and zero-sequence network current phasors are extracted from the protection installation point on the new energy side. According to the symmetrical component method and the fault boundary conditions, the fault phase current phasors of M3C and the protection installation point on the new energy side after the fault are shown in Equation (6).

[0099] (6);

[0100] The differential current and braking current of phase A are calculated as shown in equation (2), where C0 is the zero-sequence current shunt coefficient, which is determined by the zero-sequence impedance on both sides of the fault point. The proportion is C0. The proportion is 1-C0.

[0101] (7);

[0102] The protection sensitivity after a fault is calculated using equation (7). Let the differential protection braking coefficient be K1, and the sensitivity of the conventional proportional braking type current differential protection be K. sen The protection sensitivity is calculated as shown in equation (8).

[0103] (8);

[0104] Typically, the positive sequence current of new energy sources The current is increased to the rated current or after a fault, and the amplitude limiting effect is generally up to 1.2 times the rated current. Due to the M3C negative sequence control limiting, the maximum current is typically no more than 0.3 times the rated current. Clearly, because the braking current is significantly larger than the differential current, the sensitivity of proportional braking type current differential protection decreases substantially.

[0105] To address the aforementioned issues, this invention utilizes the difference in the M3C grid-connected side and the new energy control structure and limiting value after a fault, and uses this difference to identify faults within and outside the zone, thereby reducing the fault braking coefficient within the zone and improving the applicability of differential protection in low-frequency grid-connected power transmission systems.

[0106] Simulation results show that the improved current differential protection scheme of the present invention can effectively improve the adaptability of current differential protection in grid-type flexible low-frequency transmission systems, effectively avoid the problem of conventional current differential protection failing to operate within the zone in flexible low-frequency grid-type transmission systems, thereby enhancing the stability and reliability of the system when facing complex fault conditions.

[0107] like Figure 7 The image shows the voltage and current waveforms measured at the M3C and new energy protection installation points after a single-phase fault, taking this as an example. Figure 8 As shown in the figure, a comparison is made between the effect of the present invention and conventional differential protection. It can be seen that, relying on the dynamic adjustment mechanism of composite sequence network weighting, the action sensitivity of the present invention is significantly improved during intra-zone faults. Figure 9 The figure shows a comparison of the current differential protection sensitivity of the conventional method and the method used in this invention when measuring metallic faults such as single-phase ground fault, two-phase-to-phase short circuit, and two-phase-to-ground fault in different fault locations, such as outside the M3C grid-side protection installation area, 10km, 20km to 90km of the line, and outside the new energy installation area.

[0108] Furthermore, to verify the applicability of the method under high-resistance fault conditions, a single-phase ground fault was used as an example. A fault point was set at the midpoint of the line, gradually increasing the grounding resistance from 10 ohms to 200 ohms. Simulation results show that, based on the dynamic correction process using auxiliary criteria, the differential protection action coefficient can be adaptively adjusted, and the trend of the corrected protection sensitivity is as follows: Figure 8 As shown.

[0109] Example 3: An electronic device proposed in this invention includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, it implements the improved current differential protection method applicable to flexible low-frequency power transmission systems.

[0110] Example 4: The present invention proposes a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the improved current differential protection method applicable to flexible low-frequency power transmission systems.

[0111] The foregoing has described specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired result. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired result. In some embodiments, multitasking and parallel processing are possible or may be advantageous.

[0112] This invention is not limited to the embodiments described above. The above description of specific embodiments is intended to illustrate and explain the technical solutions of this invention. The specific embodiments described above are merely illustrative and not restrictive. Without departing from the spirit and scope of the claims, those skilled in the art can make many specific modifications based on the teachings of this invention, and these modifications all fall within the scope of protection of this invention.

Claims

1. A method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems, characterized in that, Includes the following steps: The three-phase current values ​​at the protection installation points on the M3C grid side and the new energy side were sampled and obtained, and the fundamental frequency 20Hz current component was extracted. The positive-sequence, negative-sequence, and zero-sequence current phasors at the protection installation points on both sides are calculated using the three-phase symmetrical component method. Based on the measurement error range of the selected current sensor, a sequence network current threshold is set. When the amplitude of a certain sequence network current phasor is less than the threshold, the sequence network current is determined to be invalid and set to zero, thereby completing the verification and screening of the validity of the sequence network current. Based on the filtered effective sequence network current phasors, the validity of the negative sequence and zero sequence networks is determined: if the negative sequence current at the protection installation point on the M3C side or the new energy side is not zero, the negative sequence network is determined to be valid, denoted as NF=1; otherwise, NF=0. If the zero sequence current is not zero, the zero sequence network is determined to be valid, denoted as NZ=1; otherwise, NZ=0. Based on the validity identifiers NF and NZ, the maximum adjustment value T for the main protection braking coefficient is assigned to the negative sequence and zero sequence networks. FM and T ZM ; Based on the valid sequence network current phasors, calculate the negative sequence auxiliary criterion value Y. F and zero-order auxiliary criterion value Y Z Set the startup threshold Y e By separately Y F Y Z With Y e By comparing the data, the credibility of the fault occurring within the protected area is determined, and the dynamic adjustment coefficient Y is calculated based on this credibility. FT and Y ZT ; Using the dynamic adjustment coefficient Y FT Y ZT and the maximum adjustment value T FM T ZM The initial braking coefficient K1 is corrected to obtain the adjusted braking coefficient K. re ; Using the corrected braking coefficient K re Perform current differential protection judgment and generate corresponding protection action commands.

2. The method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to claim 1, characterized in that, The specific steps for ensuring the validity of the sequence network include: If the negative sequence current amplitude at the M3C grid side or new energy side protection installation point is not zero, the negative sequence grid is deemed valid and marked as NF=1; otherwise, NF=0. If the zero-sequence current amplitude at the M3C grid-side or new energy side protection installation point is not zero, the zero-sequence grid is deemed valid, and the identifier NZ=1 is recorded; otherwise, NZ=0. Among them, the validity judgment is combined with the control scheme of the flexible low-frequency grid transmission system. If the sequence currents on both sides are zero, it is considered that the sequence network does not exist or is suppressed by the converter.

3. The method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to claim 2, characterized in that, The step of allocating the maximum adjustment value adopts the following formula: ; Wherein, K1 is the initial braking coefficient of the main protection, K min T is the minimum braking coefficient. FM and T ZM T represents the maximum adjustment value for the negative and zero order criteria, respectively. FM and T ZM These are the maximum adjustment values ​​for the negative and zero order criteria, respectively.

4. The method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to claim 1, characterized in that, The auxiliary criterion value Y F and Y Z The calculation formula is: ; in, , , These represent the positive, negative, and zero-sequence network currents at the M3C network-side protection installation location, respectively. , , These are the positive, negative, and zero sequence network currents at the new energy side protection installation point, respectively.

5. The method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to claim 4, characterized in that, The formula for the dynamically corrected braking coefficient is: ; The adjustment coefficient Y FT and Y ZT Calculated through the following tiered triggering mechanism: 。 6. The method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to claim 1, characterized in that, The sequence current threshold e is set to 0.2% of the rated current value.

7. The method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems according to claim 1, characterized in that, The activation threshold is determined by the system and differential protection components, and Y is set according to the error range of the protection system and the sensor. e For commonly used current sensor threshold Y e ∈[0.1,0.2].

8. A system for improving the applicability of current differential protection in flexible low-frequency grid transmission systems, used to implement the method according to any one of claims 1-5, characterized in that, include: The data acquisition module is used to sample and acquire the three-phase current values ​​at the protection installation points on the M3C grid side and the new energy side; The signal processing module is communicatively connected to the data acquisition module and is used to extract the fundamental frequency 20Hz current component from the three-phase current values ​​and to calculate the positive sequence, negative sequence, and zero sequence current phasors at the protection installation positions on both sides using the three-phase symmetrical component method. The sequence network validity judgment module, communicatively connected to the signal processing module, is used to verify the validity of the decomposed sequence network current by setting a sequence network current threshold e. When the sequence network current amplitude is less than the threshold, the sequence network current is set to zero. Based on the sequence network current phasor, the validity of the negative sequence and zero sequence networks is judged, validity identifiers NF and NZ are generated, and the maximum adjustment value T of the negative sequence and zero sequence networks to the main protection braking coefficient is allocated. FM and T ZM ; The dynamic adjustment module, which is communicatively connected to the sequence network validity determination module, is used to perform the following operations: Based on the valid sequence network current phasors, calculate the negative sequence auxiliary criterion value Y. F and zero-order auxiliary criterion value Y Z Set the startup threshold Y e By separately Y F Y Z With Y e By comparing the data, the credibility of the fault occurring within the protected area is determined, and the dynamic adjustment coefficient Y is calculated based on this credibility. FT and Y ZT ; Using the dynamic adjustment coefficient Y FT Y ZT and the maximum adjustment value T FM T ZM The initial braking coefficient K1 is corrected to obtain the adjusted braking coefficient K. re ; The protection execution module is communicatively connected to the dynamic adjustment module and is used to utilize the corrected braking coefficient K. re Perform current differential protection judgment and generate corresponding protection action commands.

9. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems as described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it implements the method for improving the applicability of current differential protection in flexible low-frequency grid transmission systems as described in any one of claims 1 to 7.

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