Adaptive analysis method for current differential protection of alternating current line of new energy network flexible direct output system

Abstract

The invention discloses a current differential protection adaptability analysis method for an alternating current line of a new energy network flexible direct output system, and belongs to the technical field of power system relay protection. Establishing a system fault equivalent model based on a low-voltage ride-through control strategy of power electronic equipment at two sides of the outgoing line; according to different asymmetric fault types and a fault point boundary condition equation, deriving mathematical models of positive-sequence, negative-sequence and zero-sequence currents on the two sides of a fault phase line; calculating a differential current and a braking current, and establishing a function relationship between the differential current and the braking current and the VSG internal potential reference value, the transition resistance and the current sequence component ratio; and the action sensitivity of protection under different working conditions is judged by comparing the current differential protection ratio braking coefficient setting value with the current differential protection ratio braking coefficient setting value. The association rule of the current differential protection refusal risk and the low-pass control parameters under the high transition resistance fault is disclosed, a basis is provided for protection setting and control strategy optimization of a network construction flexible direct output system, and the problems that a protection adaptability analysis method is weak and the influence rule of the control parameters is not clear yet are solved.

Description

Technical Field

[0001] This application relates to the field of power system relay protection technology, and more specifically, to an adaptive analysis method for AC line current differential protection in flexible direct transmission systems of new energy economic grids. Background Technology

[0002] In the flexible direct transmission system of new energy power grids, both sides of the AC transmission line are equipped with power electronic equipment. After a fault, due to the influence of fault ride-through control, the weak-feed controlled characteristics of the fault current differ significantly from those of traditional synchronous machine systems. Existing protection systems, still based on the power frequency fault characteristics of synchronous machine systems, face serious challenges. Existing research has analyzed the impact of fault characteristics under the control response of dual-ended power electronic equipment on traditional protection. The literature “Zheng Liming, Jia Ke, Bi Tianshu, et al. AC-side fault analysis of a VSC-HVDC transmission system connected to offshore wind farms and the impact on protection [J]. Power System Protection and Control, 2021, 49(20): 20-32” studied the grid-connected wind turbine-V / f flexible DC system. Theoretical analysis shows that under the influence of the fault ride-through control strategy on both sides, the fault current amplitude is limited, the phase angle is controlled, the performance of distance protection and differential protection decreases, and may even fail to operate. The literature “Jia Jundi, Yang Guangya, Arne Hejde Nielsen. A review on grid-connected converter control for short-circuit power provision under grid unbalanced faults[J]. IEEE Transactions on Power Delivery, 2018, 33(2): 649-661” studies the fault characteristics of AC power grid under negative sequence current suppression strategy during asymmetrical faults. When negative sequence current is suppressed on both sides, almost no current flows through the fault point, and the differential protection fails to operate.The literature “Zheng Liming, Jia Ke, Yang Bin, et al. Singular value decomposition based pilot protection for transmission lines with converters on both ends[J]. IEEE Transactions on Power Delivery,2022,37(4):2728-2737” points out that under worst-case conditions, MMC blocking may occur when there is a fault at the near end of the flexible DC side, leading to the failure of traditional longitudinal protection to operate. For the scenario of new energy being transmitted through a network of flexible DC systems based on VSG control, existing research mainly focuses on system stability analysis and low-voltage control optimization, with less analysis on the fault current characteristics and protection adaptability on both sides of the AC line, and no in-depth analysis of the influence of low-voltage control parameters on traditional protection.

[0003] To address the aforementioned issues, existing technologies urgently need improvement. Summary of the Invention

[0004] The purpose of this application is to provide an adaptive analysis method and system for AC line current differential protection in a new energy grid-connected flexible DC transmission system. By studying the correlation between the operating characteristics of current differential protection and key low-voltage control parameters and current sequence components of the flexible DC converter station, the adaptability of current differential protection under different fault scenarios is revealed. This provides a basis for the protection setting and control strategy optimization of the grid-connected flexible DC transmission system, and solves the problems of relatively weak protection adaptability analysis methods and unclear influence laws of control parameters.

[0005] It has the advantage of enabling dynamic permission semantic adaptation and control of heterogeneous systems across organizations in emergency scenarios, improving the flexibility and adaptability of permission changes.

[0006] Firstly, this application provides an adaptive analysis method for AC line current differential protection in a new energy grid-connected flexible direct transmission system, including: Step 1: Establish an equivalent fault model for the flexible direct transmission system controlled by a virtual synchronous machine (VSG) for new energy, and determine the low-voltage ride-through control strategy and control parameters for the new energy converters on both sides of the transmission line and the flexible direct transmission end converter station; the low-voltage ride-through control strategy is an active voltage reduction and current limiting strategy after a fault, that is, at the moment of the fault, the VSG internal potential reference value Ef is switched to a preset current limiting value; the control parameters include the VSG internal potential reference value Ef; Step 2: Based on the different asymmetrical fault types of the AC transmission line, construct the corresponding fault point boundary condition equations, and in conjunction with the low voltage ride-through control strategy, establish mathematical models of the positive sequence, negative sequence, and zero sequence currents on both sides of the fault phase line respectively. Step 3: Based on the mathematical model, the differential current and braking current of the fault phase currents on both sides are calculated through symmetrical component transformation to obtain the functional relationship between the fault phase differential current and braking current; the functional relationship includes the VSG internal potential reference value, transition resistance and current sequence component ratio parameter. Step 4: Compare the setting value of the current differential protection ratio braking coefficient with the aforementioned functional relationship to determine the sensitivity of the current differential protection operation under different transition resistance and VSG internal potential reference values.

[0007] Preferably, the asymmetrical fault types include single-phase ground fault, two-phase phase-to-phase fault, and two-phase ground fault.

[0008] Preferably, the asymmetrical fault type is a single-phase ground fault; step two includes: Determine the boundary conditions for the series connection of positive and negative zero-sequence networks at the fault point; Under the condition that the negative sequence current is suppressed to zero on the wind turbine side, an equation is established between the negative sequence current component on the flexible DC side and the negative sequence current component flowing into the fault point. A mathematical model for deriving the fault phase currents on both sides of a line under a single-phase ground fault is presented; in this mathematical model, the zero-sequence current distribution coefficient is... C 0= Z r- / (3 R g + Z 0+ Z r- Negative sequence current distribution coefficient C 2 = (3) R g + Z 0) / (3 R g + Z 0+ Z r- ); Z r- For the negative sequence impedance on the flexible direct side, Z 0 represents the zero-sequence impedance, and Rg represents the transition resistance.

[0009] Preferably, the asymmetrical fault type is a two-phase interphase fault; step two includes: Determine the boundary conditions for the parallel connection of the positive and negative sequence networks at the fault points; Based on the VSG control strategy and symmetrical component transformation of the flexible DC converter station, a second mathematical model for the two-phase fault current on both sides of the line is established; in the second mathematical model, the positive and negative sequence current distribution coefficients are determined by the transition resistance and system impedance parameters.

[0010] Preferably, the asymmetrical fault type is a two-phase ground fault; step two includes: Determine the boundary conditions for the parallel connection of the positive and negative zero-sequence networks at the fault point; A third mathematical model is established for the fault phase currents on both sides of the line under a two-phase ground fault; the zero-sequence current distribution coefficient in the third mathematical model includes the zero-sequence impedance Z on the new energy side of the system. l0 With the zero-sequence impedance Z on the flexible straight side r0 The allocation relationship.

[0011] Preferably, in step three, calculating the differential current and braking current of the fault phase currents on both sides to obtain the functional relationship between the fault phase differential current and braking current includes: Calculate the differential current and braking current of the faulted phase at the protection installation points on both sides; where the differential current is the magnitude of the sum of the current phasors on both sides, and the braking current is the magnitude of the difference between the current phasors on both sides. Establish the ratio relationship between differential current and braking current, and determine it as a functional relationship between the differential current and braking current of the fault phase.

[0012] Preferably, in step four, determining the sensitivity of the current differential protection operation under different transition resistances and VSG internal potential reference values ​​includes: If the calculated result of the functional relationship is greater than or equal to the setting value of the current differential protection ratio braking coefficient, it is determined that the current differential protection can operate reliably under the fault scenario, and the protection scheme is applicable. If the calculated result of the function relationship is less than the setting value of the current differential protection ratio braking coefficient, it is determined that the current differential protection has a risk of failing to operate in this fault scenario, and the protection scheme is not applicable.

[0013] Secondly, this embodiment also provides an AC line current differential protection adaptability analysis system, including: The first module is used to establish a fault equivalent model of the flexible direct transmission system controlled by a virtual synchronous machine (VSG) for new energy, and to determine the low voltage ride-through control strategy and control parameters for the new energy converters on both sides of the transmission line and the flexible direct transmission end converter station. The low voltage ride-through control strategy is an active voltage reduction and current limiting strategy after a fault, that is, at the moment of the fault, the VSG internal potential reference value Ef is switched to a preset current limiting value. The control parameters include the VSG internal potential reference value Ef. The second module is used to construct the corresponding fault point boundary condition equations according to different asymmetrical fault types of AC transmission lines, and to establish mathematical models of positive sequence, negative sequence and zero sequence currents on both sides of the fault phase line in combination with the low voltage ride-through control strategy. The calculation module is used to calculate the differential current and braking current of the fault phase currents on both sides through symmetrical component transformation based on the mathematical model, and obtain the functional relationship between the fault phase differential current and braking current; the functional relationship includes the VSG internal potential reference value, transition resistance and current sequence component ratio parameter. The adaptive analysis module is used to compare the setting value of the current differential protection ratio braking coefficient with the functional relationship to determine the sensitivity of the current differential protection operation under different transition resistance and VSG internal potential reference values.

[0014] Thirdly, this embodiment also provides an electronic device, the electronic device comprising: one or more processors, and a memory for storing one or more computer programs; characterized in that the computer programs are configured to be executed by the one or more processors, the programs comprising steps for performing the adaptive analysis method for AC line current differential protection of the new energy network flexible direct transmission system as described in the first aspect.

[0015] Fourthly, this embodiment also provides a storage medium storing a computer program, which is loaded and executed by a processor to implement the steps of the adaptive analysis method for AC line current differential protection of the new energy network flexible direct transmission system as described in the first aspect.

[0016] As described above, this application provides an adaptive analysis method for AC line current differential protection in a new energy grid-connected flexible direct transmission system. By establishing an equivalent fault model of the new energy grid-connected flexible direct transmission system controlled by a virtual synchronous generator (VSG), the low-voltage ride-through control strategy and control parameters for the new energy converters and the flexible direct transmission terminal converter station on both sides of the transmission line are determined. Based on different asymmetrical fault types of the AC transmission line, corresponding fault point boundary condition equations are constructed. Combined with the low-voltage ride-through control strategy, mathematical models of the positive-sequence, negative-sequence, and zero-sequence currents on both sides of the faulty phase are established. Based on the mathematical models, the differential current and braking current of the faulty phase currents on both sides are calculated through symmetrical component transformation, obtaining the functional relationship between the faulty phase differential current and the braking current. The functional relationship includes the VSG internal potential reference value, transition resistance, and current sequence component ratio parameter. The setting value of the current differential protection ratio braking coefficient is compared with the functional relationship to determine the sensitivity of the current differential protection operation under different transition resistance and VSG internal potential reference values. This study reveals the correlation between the risk of current differential protection failure to operate and low-throughput control parameters under high transition resistance faults, providing a basis for protection setting and control strategy optimization of the grid-connected flexible direct transmission system. It also solves the problems of relatively weak protection adaptability analysis methods and unclear influence laws of control parameters. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart illustrating the steps of the adaptive analysis method for AC line current differential protection in a new energy network flexible direct transmission system disclosed in an embodiment of the present invention. Figure 2 This is a schematic diagram of the equivalent system fault model corresponding to different asymmetric fault types disclosed in the embodiments of the present invention; Figure 2 (a) is the fault equivalent model corresponding to a single-phase ground fault; Figure 2 (b) is the fault equivalent model corresponding to a two-phase interphase fault; Figure 2 (c) is the fault equivalent model corresponding to a two-phase ground fault; Figure 3 This is a graph showing the ratio of differential current to braking current of the fault phase in a single-phase ground fault as a function of transition resistance, as disclosed in an embodiment of the present invention. Figure 4 This is a graph showing the ratio of the differential current to the braking current of the fault phase in a two-phase interphase fault as a function of the transition resistance, as disclosed in an embodiment of the present invention. Figure 5 This is a schematic diagram illustrating the trend of the ratio of differential current to braking current of the fault phase in a two-phase ground fault as a function of transition resistance and VSG internal potential, as disclosed in an embodiment of the present invention. Figure 6 This is a schematic diagram of the A-phase current wave under different VSG internal potentials disclosed in the embodiments of the present invention; Figure 7 This is a schematic diagram of the operation of the current differential protection under different transition resistances in a single-phase grounding fault, as disclosed in an embodiment of the present invention. Figure 8 This is a schematic diagram of the operation of the current differential protection under different transition resistances in a two-phase interphase fault, as disclosed in an embodiment of the present invention. Figure 9 This is a schematic diagram of the three-phase current waveforms on the flexible DC side under a two-phase ground fault, as disclosed in an embodiment of the present invention. Figure 10 This is a schematic diagram of the operation of the differential current protection under a two-phase ground fault as disclosed in an embodiment of the present invention. Detailed Implementation

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which these embodiments belong; the terminology used herein and in the specification of the application is for the purpose of describing particular embodiments only and is not intended to limit these embodiments; the terms "comprising" and "having," and any variations thereof, in the specification of these embodiments and the foregoing drawings, are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification of these embodiments and the foregoing drawings are used to distinguish different objects, not to describe a particular order.

[0020] The implementation details of the technical solution in this embodiment are described in detail below: In new energy transmission systems via flexible DC-DC grids, both sides of the AC transmission line are equipped with power electronic equipment. After a fault, the fault current exhibits weak-feed controlled characteristics significantly different from those of traditional synchronous machine systems due to the influence of fault ride-through control. Existing protection systems, still based on the power frequency fault characteristics of synchronous machine systems, face significant challenges. However, for this typical transmission scenario of new energy transmission via flexible DC-DC grids based on VSG control, existing research mainly focuses on system stability analysis and low-fault ride-through control optimization, with limited analysis of the fault current characteristics on both sides of the AC line and the adaptability of the protection system. Furthermore, it lacks in-depth analysis of the impact of low-fault ride-through control parameters on traditional protection systems.

[0021] This application proposes an adaptive analysis method for AC line current differential protection in a new energy network flexible direct transmission system. First, starting from the fault point boundary conditions under different asymmetrical fault types, and combining the low-pass control strategy of the new energy converters on both sides of the transmission line and the converter station at the flexible direct transmission end, the analytical expression of the fault current on both sides of the fault phase line is obtained. Then, the analytical expressions of the differential current and braking current of the fault phase are further obtained. Based on this, combined with the setting value of the ratio braking coefficient in the current differential protection, the sensitivity of the traditional current differential protection is judged according to the ratio of the differential current to the braking current, and it is determined whether the protection scheme is still applicable.

[0022] like Figure 1 As shown, the method includes: Step 1: Establish an equivalent fault model for the grid-connected flexible direct transmission system controlled by a virtual synchronous machine (VSG) for new energy, and determine the low-voltage ride-through control strategy and control parameters for the new energy converters on both sides of the transmission line and the flexible direct transmission end converter station; the low-voltage ride-through control strategy is an active voltage reduction and current limiting strategy after a fault, that is, at the moment of the fault, the VSG internal potential reference value Ef is switched to a preset current limiting value; the control parameters include the VSG internal potential reference value Ef.

[0023] Step 2: Based on the different asymmetrical fault types of the AC transmission line, construct the corresponding fault point boundary condition equations, and in conjunction with the low voltage ride-through control strategy, establish mathematical models of the positive sequence, negative sequence, and zero sequence currents on both sides of the fault phase line.

[0024] Preferably, the asymmetrical fault types include single-phase ground fault, two-phase phase-to-phase fault, and two-phase ground fault.

[0025] In this embodiment, the equivalent system fault model differs for different types of asymmetric faults. For example... Figure 2 As shown. Figure 2 (a) is the fault equivalent model corresponding to a single-phase ground fault; Figure 2 (b) is the fault equivalent model corresponding to a two-phase interphase fault; Figure 2 (c) is the fault equivalent model corresponding to a two-phase ground fault.

[0026] In the single-phase ground fault equivalent circuit, the positive-sequence, negative-sequence, and zero-sequence networks are connected in series at the fault point. The positive-sequence network includes the equivalent potential sources of the renewable energy side and the flexible DC side, the converter internal impedance, and the line positive-sequence impedance; the negative-sequence network only includes the converter internal impedance of the flexible DC side and the line negative-sequence impedance (the negative-sequence current on the renewable energy side is suppressed to zero); the zero-sequence network includes the line zero-sequence impedance and the zero-sequence impedance of the grounding systems on both sides. A transition resistor Rg is connected in series between the fault point and ground, and the fault phase current flows through the three sequence networks and Rg to form a loop.

[0027] Equivalent circuit for a two-phase interphase fault. The positive-sequence network and negative-sequence network are connected in parallel at the fault point, while the zero-sequence network is not involved (there is no zero-sequence current path). The positive-sequence network includes the equivalent potential sources on both sides, the converter internal impedance, and the line positive-sequence impedance; the negative-sequence network only includes the internal impedance of the flexible DC-side converter and the line negative-sequence impedance. A transition resistor Rg is connected between the two faulty phases, and the positive-sequence and negative-sequence currents form a fault loop through Rg.

[0028] Equivalent circuit for a two-phase ground fault. The positive-sequence, negative-sequence, and zero-sequence networks are connected in parallel at the fault point. The composition of each network is the same as in (a). The positive-sequence network includes potential sources on both sides and positive-sequence impedance; the negative-sequence network only includes the negative-sequence impedance on the flexible DC side; and the zero-sequence network includes the line and the zero-sequence impedance of the systems on both sides. Transition resistors Rg are connected between the two faulted phases and ground. The currents from the three networks converge at the fault point and flow into the ground through Rg.

[0029] Preferably, the asymmetrical fault type is a single-phase ground fault; step two includes: Determine the boundary conditions for the series connection of positive and negative zero-sequence networks at the fault point; Under the condition that the negative sequence current is suppressed to zero on the wind turbine side, an equation is established between the negative sequence current component on the flexible DC side and the negative sequence current component flowing into the fault point. A mathematical model for deriving the fault phase currents on both sides of a line under a single-phase ground fault is presented; in this mathematical model, the zero-sequence current distribution coefficient is... C 0= Z r- / (3 R g + Z 0+ Z r- Negative sequence current distribution coefficient C 2 = (3) R g + Z 0) / (3 R g + Z 0+ Z r- ); Z r- For the negative sequence impedance on the flexible direct side, Z 0 represents the zero-sequence impedance, and Rg represents the transition resistance.

[0030] Specifically, in this embodiment, for a single-phase ground fault.

[0031] Taking a phase A ground fault via a transition resistor as an example, the positive and negative zero-sequence networks are connected in series at the fault point, and the current sequence components at the fault point are: (1) Since the negative sequence current is suppressed to zero on the wind turbine side, only the flexible DC side provides a path for the negative sequence current. The negative sequence current component on the flexible DC side is equal to the negative sequence current component flowing into the fault point. The boundary conditions of the fault point under a single-phase ground fault are: (2) In the formula: , These are the A-phase currents on the flexible DC side and the new energy side, respectively. , These are the positive and negative sequence components of the fault point current, respectively. , , These are the three-phase currents at the fault point (A, B, and C); , , These are the three-phase voltages of the fault point A, B, and C.

[0032] Combining equation (1) and the symmetrical component transformation matrix, the A-phase currents on both sides of the line can be expressed as: (3) In the formula: C 0 represents the zero-sequence current distribution coefficient under a two-phase ground fault. C 0= Z r- / (3R g + Z 0+ Z r- ); C 2 represents the negative sequence current distribution coefficient under a two-phase ground fault. C 2 = (3) R g + Z 0) / (3 R g + Z 0+ Z r- ). C l0 This represents the zero-sequence current allocation coefficient on the new energy side of the system. C l0 = Z r0 / ( Z r0 + Z l0 ); C r0 This represents the zero-sequence current distribution coefficient on the flexible DC side of the system. C r0 = Z l0 / ( Z r0 + Z l0 ).

[0033] Step 3: Based on the mathematical model, the differential current and braking current of the fault phase currents on both sides are calculated through symmetrical component transformation, and the functional relationship between the fault phase differential current and braking current is obtained; the functional relationship includes the VSG internal potential reference value, transition resistance and current sequence component ratio parameter.

[0034] Preferably, in step three, the calculation of the differential current and braking current of the fault phase currents on both sides to obtain the functional relationship between the fault phase differential current and braking current includes: calculating the differential current and braking current of the fault phase at the protection installation points on both sides; wherein the differential current is the amplitude of the sum of the current phasors on both sides, and the braking current is the amplitude of the difference between the current phasors on both sides; establishing the ratio relationship between the differential current and the braking current, and determining it as the functional relationship between the fault phase differential current and the braking current.

[0035] Specifically, for a single-phase ground fault, the differential current of the faulty phase at the protection installation points on both sides... I da Braking current I resa It can be represented as: (4) The ratio of the fault phase differential current to the braking current K a for: (5) Step 4: Compare the setting value of the current differential protection ratio braking coefficient with the aforementioned functional relationship to determine the sensitivity of the current differential protection operation under different transition resistance and VSG internal potential reference values.

[0036] Preferably, in step four, determining the sensitivity of the current differential protection operation under different transition resistances and VSG internal potential reference values ​​includes: if the calculated result of the functional relationship is greater than or equal to the setting value of the current differential protection ratio braking coefficient, then it is determined that the current differential protection can operate reliably in this fault scenario, and the protection scheme is applicable; if the calculated result of the functional relationship is less than the setting value of the current differential protection ratio braking coefficient, then it is determined that the current differential protection has a risk of failing to operate in this fault scenario, and the protection scheme is not applicable.

[0037] Specifically, for a single-phase ground fault, the ratio of the positive-sequence component to the negative-sequence component of the output current on the flexible DC side can be obtained from the analytical expression of the sequence components on the flexible DC side: (6) As can be seen from equation (5), K a The ratio of the positive-sequence to the negative-sequence components of the current on the flexible DC side is related to this value, which mainly depends on the transition resistance and the internal potential of the VSG. When the internal potential of the VSG is constant, as the transition resistance increases, K a First increase, then decrease, the change is as follows: Figure 3 As shown. When the transition resistance is small, the lower internal potential of the VSG can still meet the requirements. K a If the value is greater than the set value, the protection will operate reliably; however, when... R g When increased to a certain extent, the lower internal potential of the VSG will lead to K a If the voltage reference value of the flexible DC converter station is lower than the set value, that is, in the case of a high transition resistance fault, the current differential protection will face the risk of failure to operate.

[0038] Preferably, the asymmetrical fault type is a two-phase interphase fault; step two includes: Determine the boundary conditions for the parallel connection of the positive and negative sequence networks at the fault points; Based on the VSG control strategy and symmetrical component transformation of the flexible DC converter station, a second mathematical model for the two-phase fault current on both sides of the line is established; in the second mathematical model, the positive and negative sequence current distribution coefficients are determined by the transition resistance and system impedance parameters.

[0039] Specifically, in this embodiment, for a two-phase interphase fault.

[0040] Taking a phase-BC fault via a transition resistor as an example, the positive and negative sequence networks are connected in parallel at the fault point, and the boundary conditions at the fault point are: (7) Combining the VSG control strategy and symmetrical component transformation formula of the flexible DC converter station, the two-phase currents BC on both sides of the line can be expressed as follows: (8) (9) In the formula: a For rotation factor, Satisfying the relationship .

[0041] Differential current of the fault phase at both protection installation points , Braking current , They can be represented as: (10) (11) The ratio of the fault phase differential current to the braking current K b , K c for: (12) From the analytical expression of the sequence components on the flexible DC side, the ratio of the positive-sequence component to the negative-sequence component of the output current on the flexible DC side is: (13) As can be seen from equation (12), the differential currents of phase B and phase C have equal amplitudes but opposite phases. K b , K c It mainly depends on the magnitude of the transition resistance and the reference value of the internal potential of the flexible DC VSG, and decreases as the ratio of the positive-sequence component to the negative-sequence component of the current increases. Similar to grounding faults, when the internal potential reference value is constant, it decreases as the transition resistance increases. K b , K c The current differential protection faces the risk of failing to operate if it first increases and then decreases, meaning that in high transition resistance fault scenarios, the current differential protection may fail to operate. Figure 4 The following is an example of this embodiment regarding... K b , K c Graph showing the change in transition resistance.

[0042] Preferably, the asymmetrical fault type is a two-phase ground fault; step two includes: Determine the boundary conditions for the parallel connection of the positive and negative zero-sequence networks at the fault point; A third mathematical model is established for the fault phase currents on both sides of the line under a two-phase ground fault; the zero-sequence current distribution coefficient in the third mathematical model includes the zero-sequence impedance Z on the new energy side of the system. l0 With the zero-sequence impedance Z on the flexible straight side r0 The allocation relationship.

[0043] Specifically, in this embodiment, a two-phase ground fault is addressed.

[0044] Taking a two-phase (BC) ground fault via a transition resistor as an example, the positive and negative sequence networks are connected in parallel at the fault point, and the boundary conditions at the fault point are: (14) Combining the VSG control strategy and symmetrical component transformation formula of the flexible DC converter station, the two-phase currents BC on both sides of the line can be expressed as follows: (15) (16) Differential current of the fault phase at both protection installation points I db , I dc Braking current I resb , I resc They can be represented as: (17) (18) The ratio of the fault phase differential current to the braking current K b , K c They are respectively: (19) The ratio of the positive-sequence component to the negative-sequence component of the flexible DC side current is: (20) As can be seen from equation (19), when the line parameters are constant, the ratio of the fault phase differential current to the braking current is not only related to the ratio of the positive sequence component to the negative sequence component of the current, but also affected by the magnitude of the transition resistance. K b , K c. As the transition resistance and the reference value of the VSG internal potential change, as shown in... Figure 5 As shown.

[0045] Furthermore, for single-phase grounding faults, two-phase-to-phase faults, and two-phase-to-ground faults, this embodiment continues to illustrate how to analyze the fault current characteristics and protection adaptability on both sides of the AC line.

[0046] (1) Single-phase ground fault In scenarios where new energy is transmitted directly to the grid via VSG, an asymmetrical fault occurs on the AC line. A constant VSG internal potential setting will reduce the current differential protection's ability to withstand transition resistance, and may even lead to failure to operate. When a three-phase metallic ground fault occurs at 0.05s, the A-phase current waveform under different VSG internal potential values ​​is as follows: Figure 6 As shown. By Figure 6 It is evident that to meet the current limiting requirements of power electronic equipment, the internal potential of the VSG should not exceed 0.27 pu. Following the traditional active voltage reduction strategy, the reference value will be switched to this value after a fault. E f = 0.27pu.

[0047] Under this setting value, when a single-phase ground fault occurs on the line with different transition resistances, the operation of the current differential protection is as follows: Figure 7 As shown (taking a phase A ground fault as an example).

[0048] Depend on Figure 6 It can be seen that setting the current differential protection braking coefficient to 0.6 results in a ratio of fault phase differential current to braking current of 0.6. K The voltage gradually increases after the fault, reaching a stable level after approximately 20 ms. Simulation results for a phase-A ground fault on the transmitting line are as follows: Figure 5 As shown, when the transition resistance is 40Ω, after the fault... K a When the value increases to around 0.6, it remains stable, within the protection action boundary; however, when the transition resistance increases to 60Ω, K If the value is below 0.5, the protection will not activate.

[0049] (2) Two-phase fault When a two-phase phase-to-phase fault occurs on the line with different transition resistances, the operation of the current differential protection is as follows: Figure 8 As shown (taking a BC phase-to-phase fault as an example). The simulation results of a BC phase-to-phase fault occurring on the transmitting line are as follows. Figure 8 As shown, when the transition resistance is 40Ω, after the fault... K b The value reached around 0.6 and remained stable, indicating it was within the protection action boundary. K c It reaches around 0.8 and remains stable; however, when the transition resistance increases to 60Ω, K b , K cAll values ​​are less than 0.5, indicating that the protection fails to operate. This demonstrates that current differential protection may fail to operate during high transition resistance faults.

[0050] (3) Two-phase ground fault Depend on Figure 4 visible, K b , K c The voltage reference value is always greater than 0.6, meaning that under a two-phase high-resistance ground fault, the voltage reference value that meets the current-limiting requirement will not cause the differential protection to fail to operate. When calculating the internal potential, the reference value of the VSG internal potential only needs to meet the current-limiting requirement. In the simulation model, the fault type is set as two phases B and C grounded through a 100Ω transition resistor. At this time, the reference value of the VSG internal potential is selected as 0.2pu. The three-phase current on the flexible DC side and the operating quantity of the current differential protection are as follows: Figure 9 and Figure 10 As shown.

[0051] Depend on Figure 9-10 It is evident that when a two-phase ground fault occurs via a transition resistor, the three-phase current on the flexible DC side meets the current limiting requirements, while the current differential protection can correctly select the faulty phase, demonstrating good operating characteristics and verifying the correctness of the theoretical analysis.

[0052] Based on this, this embodiment focuses on the scenario of new energy being transmitted through a VSG-controlled flexible DC transmission system. Existing research mainly focuses on system stability analysis and low-pass control optimization, with limited analysis of the fault current characteristics and protection adaptability on both sides of the AC line, and lacks in-depth analysis of the impact of low-pass control parameters on traditional protection. This invention derives the correlation between the operating characteristics of current differential protection and key low-pass control parameters and current sequence components of the flexible DC converter station, revealing the adaptability of current differential protection under different fault scenarios. Theoretical analysis reveals that the ratio of fault phase differential current to braking current increases with the increase of the VSG internal potential reference value, and increases with the decrease of the ratio of positive sequence current to negative sequence current, exacerbating the risk of failure to operate under high transition resistance faults.

[0053] For AC transmission lines of new energy sources via VSG-controlled flexible DC transmission systems, this invention derives the correlation between the operating characteristics of current differential protection and key low-voltage control parameters and current sequence components of the flexible DC converter station, revealing the adaptability of current differential protection under different fault scenarios. Theoretical analysis shows that the ratio of fault phase differential current to braking current increases with the increase of the VSG internal potential reference value and increases with the decrease of the ratio of positive to negative current components. The risk of failure to operate is exacerbated under high transition resistance faults in single-phase grounding and two-phase interphase faults, while the differential protection operating characteristics are better under two-phase grounding faults. An electromagnetic transient simulation model of the new energy transmission system via VSG-controlled flexible DC transmission system was built in PSCAD / EMTDC, and simulation results under various fault scenarios verified the correctness of the method and conclusions.

[0054] Secondly, this embodiment also provides an AC line current differential protection adaptability analysis system, including: The first module is used to establish a fault equivalent model of the flexible direct transmission system controlled by a virtual synchronous machine (VSG) for new energy, and to determine the low voltage ride-through control strategy and control parameters for the new energy converters on both sides of the transmission line and the flexible direct transmission end converter station. The low voltage ride-through control strategy is an active voltage reduction and current limiting strategy after a fault, that is, at the moment of the fault, the VSG internal potential reference value Ef is switched to a preset current limiting value. The control parameters include the VSG internal potential reference value Ef. The second module is used to construct the corresponding fault point boundary condition equations according to different asymmetrical fault types of AC transmission lines, and to establish mathematical models of positive sequence, negative sequence and zero sequence currents on both sides of the fault phase line in combination with the low voltage ride-through control strategy. The calculation module is used to calculate the differential current and braking current of the fault phase currents on both sides through symmetrical component transformation based on the mathematical model, and obtain the functional relationship between the fault phase differential current and braking current; the functional relationship includes the VSG internal potential reference value, transition resistance and current sequence component ratio parameter. The adaptive analysis module is used to compare the setting value of the current differential protection ratio braking coefficient with the functional relationship to determine the sensitivity of the current differential protection operation under different transition resistance and VSG internal potential reference values.

[0055] Thirdly, this embodiment also provides an electronic device, the electronic device comprising: one or more processors, and a memory for storing one or more computer programs; characterized in that the computer programs are configured to be executed by the one or more processors, the programs comprising steps for performing the adaptive analysis method for AC line current differential protection of the new energy network flexible direct transmission system as described in the first aspect, which will not be repeated here.

[0056] Fourthly, this embodiment also provides a storage medium storing a computer program, which is loaded and executed by a processor to implement the adaptive analysis method steps of AC line current differential protection for new energy network flexible direct transmission system as described in the first aspect, which will not be repeated here.

[0057] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for adaptability analysis of AC line current differential protection in a new energy grid-connected flexible direct transmission system, characterized in that, include: Step 1: Establish an equivalent fault model for the flexible direct transmission system controlled by a virtual synchronous machine (VSG) for new energy, and determine the low-voltage ride-through control strategy and control parameters for the new energy converters on both sides of the transmission line and the flexible direct transmission end converter station; the low-voltage ride-through control strategy is an active voltage reduction and current limiting strategy after a fault, that is, at the moment of the fault, the VSG internal potential reference value Ef is switched to a preset current limiting value; the control parameters include the VSG internal potential reference value Ef; Step 2: Based on the different asymmetrical fault types of the AC transmission line, construct the corresponding fault point boundary condition equations, and in conjunction with the low voltage ride-through control strategy, establish mathematical models of the positive sequence, negative sequence, and zero sequence currents on both sides of the fault phase line respectively. Step 3: Based on the mathematical model, the differential current and braking current of the fault phase currents on both sides are calculated through symmetrical component transformation to obtain the functional relationship between the fault phase differential current and braking current; the functional relationship includes the VSG internal potential reference value, transition resistance and current sequence component ratio parameter. Step 4: Compare the setting value of the current differential protection ratio braking coefficient with the aforementioned functional relationship to determine the sensitivity of the current differential protection operation under different transition resistance and VSG internal potential reference values.

2. The adaptive analysis method for AC line current differential protection of the new energy network flexible direct transmission system according to claim 1, characterized in that, The asymmetrical fault types include single-phase ground faults, two-phase-to-phase faults, and two-phase-to-ground faults.

3. The adaptive analysis method for AC line current differential protection of the new energy network flexible direct transmission system according to claim 2, characterized in that, The asymmetrical fault type is a single-phase ground fault; step two includes: Determine the boundary conditions for the series connection of positive and negative zero-sequence networks at the fault point; Under the condition that the negative sequence current is suppressed to zero on the wind turbine side, an equation is established between the negative sequence current component on the flexible DC side and the negative sequence current component flowing into the fault point. A mathematical model for deriving the fault phase currents on both sides of a line under a single-phase ground fault is presented; in this mathematical model, the zero-sequence current distribution coefficient is... C 0 = Z r- / (3 R g + Z 0+ Z r- Negative sequence current distribution coefficient C 2 = (3 R g + Z 0) / (3 R g + Z 0+ Z r- ); Z r- For the negative sequence impedance on the flexible direct side, Z 0 represents the zero-sequence impedance, and Rg represents the transition resistance.

4. The adaptive analysis method for AC line current differential protection of the new energy network flexible direct transmission system according to claim 2, characterized in that, The asymmetrical fault type is a two-phase interphase fault; step two includes: Determine the boundary conditions for the parallel connection of the positive and negative sequence networks at the fault points; Based on the VSG control strategy and symmetrical component transformation of the flexible DC converter station, a second mathematical model for the two-phase fault current on both sides of the line is established; in the second mathematical model, the positive and negative sequence current distribution coefficients are determined by the transition resistance and system impedance parameters.

5. The adaptive analysis method for AC line current differential protection of the new energy network flexible direct transmission system according to claim 2, characterized in that, The asymmetrical fault type is a two-phase ground fault; step two includes: Determine the boundary conditions for the parallel connection of the positive and negative zero-sequence networks at the fault point; A third mathematical model is established for the fault phase currents on both sides of the line under a two-phase ground fault; the zero-sequence current distribution coefficient in the third mathematical model includes the zero-sequence impedance Z on the new energy side of the system. l0 With the zero-sequence impedance Z on the flexible straight side r0 The allocation relationship.

6. The method for adaptability analysis of AC line current differential protection in a new energy grid-connected flexible direct transmission system according to any one of claims 2-4, characterized in that, In step three, the calculation of the differential current and braking current of the fault phase currents on both sides to obtain the functional relationship between the fault phase differential current and braking current includes: Calculate the differential current and braking current of the faulted phase at the protection installation points on both sides; where the differential current is the magnitude of the sum of the current phasors on both sides, and the braking current is the magnitude of the difference between the current phasors on both sides. Establish the ratio relationship between differential current and braking current, and determine it as a functional relationship between the differential current and braking current of the fault phase.

7. The adaptive analysis method for AC line current differential protection of the new energy network flexible direct transmission system according to claim 6, characterized in that, In step four, determining the sensitivity of the current differential protection operation under different transition resistances and VSG internal potential reference values ​​includes: If the calculated result of the functional relationship is greater than or equal to the setting value of the current differential protection ratio braking coefficient, it is determined that the current differential protection can operate reliably under the fault scenario, and the protection scheme is applicable. If the calculated result of the function relationship is less than the setting value of the current differential protection ratio braking coefficient, it is determined that the current differential protection has a risk of failing to operate in this fault scenario, and the protection scheme is not applicable.

8. An adaptive analysis system for AC line current differential protection, characterized in that, include: The first module is used to establish a fault equivalent model of the flexible direct transmission system controlled by a virtual synchronous machine (VSG) for new energy, and to determine the low voltage ride-through control strategy and control parameters for the new energy converters on both sides of the transmission line and the flexible direct transmission end converter station. The low voltage ride-through control strategy is an active voltage reduction and current limiting strategy after a fault, that is, at the moment of the fault, the VSG internal potential reference value Ef is switched to a preset current limiting value. The control parameters include the VSG internal potential reference value Ef. The second module is used to construct the corresponding fault point boundary condition equations according to different asymmetrical fault types of AC transmission lines, and to establish mathematical models of positive sequence, negative sequence and zero sequence currents on both sides of the fault phase line in combination with the low voltage ride-through control strategy. The calculation module is used to calculate the differential current and braking current of the fault phase currents on both sides through symmetrical component transformation based on the mathematical model, and obtain the functional relationship between the fault phase differential current and braking current; the functional relationship includes the VSG internal potential reference value, transition resistance and current sequence component ratio parameter. The adaptive analysis module is used to compare the setting value of the current differential protection ratio braking coefficient with the functional relationship to determine the sensitivity of the current differential protection operation under different transition resistance and VSG internal potential reference values.

9. An electronic device, the electronic device comprising: One or more processors, a memory for storing one or more computer programs; characterized in that the computer programs are configured to be executed by the one or more processors, the programs including steps for performing the adaptive analysis method for AC line current differential protection of a new energy network flexible direct transmission system as described in any one of claims 1-7.

10. A storage medium storing a computer program; characterized in that, The program is loaded and executed by a processor to implement the steps of the adaptive analysis method for AC line current differential protection of a new energy network flexible direct transmission system as described in any one of claims 1-7.

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