Differential integral current protection method and system for berrigan line mode of flexible direct current line

Abstract

The application discloses a kind of based on the flexible direct current line differential protection method and system of berilong line mode differential integral current, by obtaining the geometric parameter of direct current line, calculating frequency variable polar domain impedance and admittance matrix, and selecting the line mode wave impedance, resistance and propagation coefficient under specific frequency;Real-time acquisition of the bipolar voltage and current at the two ends of line, after obtaining line mode by polar mode transformation, the line mode current of line reference point is accurately calculated using berilong model;Through band-pass filtering to suppress noise, and through optical fiber channel to realize the synchronization and compensation of two-end data;The sum of the reference point current at both ends is taken to generate the berilong line mode differential current, and then the line mode differential integral current is obtained by integrating the cumulative data window;Finally, the fault is determined by comparing the integral current with the action threshold value.The application solves the problem of single ground fault identification, and is suitable for reliable protection of flexible direct current line in new energy access scenario.

Description

Technical Field

[0001] This invention belongs to the field of relay protection technology for DC transmission systems, specifically relating to a line-mode differential integral current protection method and system based on the Berylon model for flexible DC transmission lines. Background Technology

[0002] Flexible DC systems have become an ideal solution for the collection and long-distance transmission of new energy power due to their advantages such as low loss, no limitation on transmission distance, and flexible control.

[0003] Flexible DC systems are mainly classified into true bipolar systems and pseudo bipolar systems according to their wiring methods. In pseudo bipolar systems, when a single-pole ground fault occurs, it will immediately lead to system voltage imbalance, causing the faulty pole to shut down and power transmission to be interrupted. The voltage to ground of the healthy pole will rise to twice its original value, severely damaging the insulation strength of the line. Therefore, if the main protection device in the system, which relies on single-ended electrical quantities, fails to operate and cannot accurately identify the single-pole ground fault, it may cause permanent damage to the system equipment, thereby triggering a chain reaction and leading to catastrophic consequences such as a complete network outage.

[0004] Differential current protection is selective and can effectively distinguish between faults inside and outside the fault zone, thus serving as backup protection for high-voltage direct current transmission systems. However, when a fault occurs outside the fault zone or the system power fluctuates, traditional differential protection is prone to maloperation due to the influence of the charging and discharging of distributed capacitances on the line. In engineering practice, long delay windows are often added to address this issue and avoid unbalanced currents. However, long delay windows not only prolong the protection's operating time but also mask critical transient processes during faults inside the fault zone. In particular, for pseudo-bipolar systems, when a single-pole ground fault occurs, the fault current lacks an effective flow path, causing the differential current to be approximately zero after the fault enters the steady-state stage, with a steady-state fault current only present in the initial stage of the fault. Therefore, traditional differential protection, which only focuses on the steady-state stage of the fault, cannot effectively identify single-pole ground faults in pseudo-bipolar systems. A protection method for identifying single-pole ground faults in pseudo-bipolar systems is urgently needed.

[0005] In summary, existing technologies suffer from drawbacks such as susceptibility to maloperation due to distributed capacitance, sacrifice of speed due to time delay settings, and insufficient ability to identify unipolar grounding faults in pseudo-bipolar systems. Therefore, a novel differential protection method capable of quickly and reliably identifying unipolar grounding faults in pseudo-bipolar systems is urgently needed. Summary of the Invention

[0006] To overcome the shortcomings of existing traditional current differential protection technology for flexible DC lines, this invention aims to provide a Berylone line-mode differential integral current protection method and system for flexible DC lines. The method involves acquiring the line's geometric parameters and the voltage and current at both ends, converting them to the line-mode domain via pole-mode transformation, and accurately calculating the line reference point current using the Berylone model. By synthesizing the calculated and filtered reference point currents at both ends, a Berylone line-mode differential current is generated. This differential current is further integrated over a time window, and finally, the protection criterion is determined based on the comparison between the integrated current value and the operating threshold.

[0007] To achieve the above-mentioned objectives, the present invention proposes the following technical solution: In a first aspect, the present invention proposes a method for differential integral current transfer protection of a flexible DC line using a Berylone line-mode, the method comprising the following steps: The first step is to calculate the system parameters: based on the geometric parameters of the DC line, calculate the polar impedance matrix of the frequency-dependent DC line. Z ( f and polar admittance matrix Y ( f ), and extract the line self-impedance. Z s ( f ), mutual impedance Z m ( f ), self-guided admittance Y s ( f ) and mutual admittance Y m( f Select a specific frequency; f 0, based on f Self-impedance at 0 Z s ( f 0) Mutual impedance Z m ( f 0) Self-guided admission Y s ( f 0) and mutual admittance Y m ( f 0), Calculate the line-mode wave impedance Z c (1) ( f 0) Linear resistance per unit length r 1( f 0) and linear mode propagation coefficient gamma 1( f 0); Using the midpoint of the route as a reference point rAccording to the linear mode propagation coefficient gamma 1( f 0) and DC line length l Calculate the traveling wave of the line mode from the reference point r Transmission time to both ends of the line tau 1; The second step is data acquisition and transformation: real-time sampling of the bipolar voltage at this end. u P ( t ), u N ( t ) and bipolar current i P ( t ), i N ( t The line-mode voltage at this terminal is obtained through pole-mode transformation. u 1( t and line mode current i 1( t ); The third step is to calculate and filter the reference point current: using the line-mode voltage at this terminal. u 1( t and line mode current i 1( t ), Calculate reference points based on the Berylon model r Linear current i r1 ( t );Will i r1 ( t The input is filtered by a bandpass filter to obtain the filtered local reference point line-mode current. i r1_filter_loc ( t ); The fourth step is to perform data communication and compensation: i r1_filter_loc ( t The data is transmitted to the other end via fiber optic channel, and the reference point line-mode current transmitted by the other end is received. After communication compensation, the reference point line-mode current of the other end is obtained. i r1_filter_rmt ( t ); Step 5: Perform differential current calculation: Calculate the line-mode current at the reference point on this end. i r1_filter_loc ( t ) and the line-mode current at the reference point at the opposite end i r1_filter_rmt ( t Summing and taking the absolute value yields the differential current of the Berylon line mode.i 1_diff ( t ); Step 6: Perform protection judgment and execution: [Regarding...] i 1_diff ( t Real-time sampling is performed and stored in a cumulative data window. The differential current values ​​within the window are summed to obtain the line-mode differential integral current. i 1_diff_itg ( t ), and will i 1_diff_itg ( t ) and preset action threshold value I set Comparison: If i 1_diff_itg ( t )> I set If so, it is determined that a fault has occurred inside the protected line and the protection action is triggered; if i 1_diff_itg ( t )< I set If the data window is not full, continue sampling; if i 1_diff_itg ( t )< I set The data window is full; clear the data window and prevent further action.

[0008] In some embodiments, in the first step, the polar impedance Z ( f and polar admittance Y ( f The frequency-varying parameter matrix is ​​obtained based on the geometric parameters of the DC line and is calculated using electromagnetic field theory and Carson's formula.

[0009] In some implementations, in the first step, a specific frequency f The selection range for 0 is from 100 Hz to 1000 Hz, so that the line-mode impedance is... Z c (1) and unit length line-mode resistance r 1 at a specific frequency f 0 and its adjacent frequency bands are approximately constant; The line mode impedance Z c (1) ( f The formula for calculating 0) is:

[0010] The unit length line-mode resistance r 1( f 0) The calculation formula is:

[0011] Among them, Re( z ) represents a complex number z The real part, Z s This is the line's self-impedance. Z m This represents the mutual impedance of the line.

[0012] The linear mode propagation coefficient gamma 1( f 0) The calculation formula is: .

[0013] In some implementations, in the first step, the transmission time tau The formula for calculating 1 is:

[0014] Among them, Im( z () represents the imaginary part of the complex number z. gamma 1( f 0) represents the linear mode at frequency f The propagation coefficient at 0.

[0015] In some implementations, the formula for calculating the polar mode transformation in the second step is:

[0016] in, u p ( t ), u N ( t ), i p ( t ), i N ( t () represent the time of the line. t The positive voltage sample value, negative voltage sample value, positive current sample value, and negative current sample value at each moment.

[0017] In some embodiments, in the third step, the reference point r Linear current i r1 ( t The formula for calculating ) is:

[0018] in, Z 1 represents the line-mode impedance. Z c (1) ( f 0), r 1 represents the line-mode resistance per unit length of the line. t This represents the current time.

[0019] The bandpass filter is a discrete-time filter, and its total output reference point line-mode current... i r1_filter_loc ( n The formula for calculating ) is:

[0020] in, i r1 [ n [This is the input of the bandpass filter at the current sampling time.] i r1 [ n -1] and i r1_filter _ loc [ n [-1] represents the input and output of the bandpass filter at the previous sampling time. i r1 [ n -2] and i r1_filter_loc [ n [-2] represents the input and output of the bandpass filter at the previous two sampling times; Bandpass filter parameters b 0、 b 1. b 2. a 1 and a 2 and sampling frequency F s and center frequency f m Yes, the calculation formula is as follows: , ,

[0021] in, w 0 represents the normalized center angular frequency, used to map the actual analog frequency to the digital discrete frequency domain. a This indicates a bandwidth adjustment parameter that incorporates the quality factor, controlling the passband width and steepness of the bandpass filter. bIndicates center frequency positioning parameters ( w The cosine value of 0 determines the specific location of the filter peak in the spectrum.

[0022] In some embodiments, in the fourth step, communication compensation is achieved by time compensation of the peer reference point line-mode current, and the compensated peer reference point line-mode current... i r1_filter_rmt ( t The formula for calculating ) is:

[0023] In the formula, i ’ r1_filter_rmt ( t ) represents the line-mode current at the reference point of the other end without communication compensation, obtained using the ping-pong method. t cs Δ t cs The calculation formula is:

[0024] in, This indicates the time interval between the signal's round trip via the optical fiber channel.

[0025] In some embodiments, in the fifth step, the Berylon line-mode differential current i 1_diff ( t The formula for calculating ) is:

[0026] In some embodiments, in the sixth step, the line-mode differential integral current... i 1_diff_itg ( t The formula for calculating ) is:

[0027] in, N This indicates the total number of data points in the cumulative data window. k Indicates the sampling time sequence number. i r1_diff ( k ) indicates the first k The instantaneous value of the line-mode differential current sampled at each sampling moment and stored in the cumulative data window; The action threshold I set The selection principle is: it must reliably avoid the maximum unbalanced differential current generated by faults outside the protected line area; The cumulative time length corresponding to the cumulative data window should not be less than the duration of the transient process caused by the charging and discharging of the DC line distributed capacitance, and the speed of the protection action should also be taken into account.

[0028] Secondly, the present invention proposes a Berylon line-mode differential integral current protection system for flexible DC lines, used to implement the method described in any one of the claims. The system includes a master-end protection system deployed at one end of the line and a slave-end protection system deployed at the other end of the line, which are connected through an optical fiber communication channel. The master-end protection system includes: The first parameter calculation module calculates the polar impedance of the frequency-dependent DC line based on the geometric parameters of the DC line. Z ( f and polar admittance Y ( f ) parameter matrix, and extract the line self-impedance Z s ( f ), mutual impedance Z m ( f ), self-guided admittance Y s ( f ) and mutual admittance Y m( f Select a specific frequency; f 0, based on f Self-impedance at 0 Z s ( f 0) Mutual impedance Z m ( f 0) Self-guided admission Y s ( f 0) and mutual admittance Y m ( f 0), Calculate the line-mode wave impedance Z c (1) ( f 0) Linear resistance per unit length r 1( f 0) and linear mode propagation coefficient gamma 1( f 0); Using the midpoint of the route as a reference point r According to the linear mode propagation coefficient gamma 1( f 0) and line length l Calculate the traveling wave of the line mode from the reference point r Transmission time to both ends of the line tau 1; The first data acquisition and processing module is used to sample the bipolar voltage at this end in real time. u P ( t ), u N ( t ) and bipolar current i P ( t ), i N ( t The line-mode voltage at this terminal is obtained through pole-mode transformation. u 1( t and line mode current i 1( t ); The first Berylon calculation and filtering module is connected to the data acquisition and processing module, and is used to utilize the line-mode voltage at this end. u 1( t and line mode current i 1( t ), Calculate reference points based on the Berylon model r Linear current i r1 ( t );Will i r1 ( t The input is filtered by a bandpass filter to obtain the filtered local reference point line-mode current. i r1_filter_loc ( t ); The first communication module is connected to the Berylon calculation and filtering module and is used to... i r1_filter_loc ( t The data is transmitted to the other end via fiber optic channel, and the reference point line-mode current transmitted by the other end is received. After communication compensation, the reference point line-mode current of the other end is obtained. i r1_filter_rmt ( t ); The differential calculation module, connected to the communication module and the Berylon calculation and filtering module, is used to calculate the local reference point line-mode current. i r1_filter_loc ( t ) and the line-mode current at the reference point at the opposite end i r1_filter_rmt ( t Summing and taking the absolute value yields the differential current of the Berylon line mode. i 1_diff ( t ); The integral judgment module, connected to the differential calculation module, is used to determine the integral judgment module.i 1_diff ( t Real-time sampling is performed and stored in a cumulative data window. The differential current values ​​within the window are summed to obtain the line-mode differential integral current. i 1_diff_itg ( t ), and will i 1_diff_itg ( t ) and preset action threshold value I set Comparison: If i 1_diff_itg ( t )> I set If so, it is determined that a fault has occurred inside the protected line and the protection action is triggered; if i 1_diff_itg ( t )< I set If the data window is not full, continue sampling; if i 1_diff_itg ( t )< I set The data window is full; clear the data window and prevent further action. The slave-end protection system includes: The system comprises a second parameter calculation module, a second data acquisition and processing module, a second Berylone calculation and filtering module, and a second communication module; each module of the slave-end protection system has the same function as the corresponding module in the master-end protection system, and is used to generate the slave-end reference point line-mode current. i r1 _ filter _ loc _ remote ( t It is transmitted to the master protection system via its communication module.

[0029] Compared with the prior art, the present invention has achieved the following beneficial technical effects: The beneficial technical effects achieved by this invention are mainly reflected in the following four aspects: 1. Overcame the challenge of identifying unipolar grounding faults in pseudo-bipolar systems. By using the Berylon model to accurately calculate the line reference point current and using the line-mode differential integral current as the core criterion, the transient characteristics of the initial stage of the fault can be sensitively captured and amplified, thereby achieving reliable identification of the slightest type of fault.

[0030] 2. Integration of two core technologies: 1) The Berylon model based on distributed parameter lines fundamentally separates and compensates for capacitive current, thereby reducing unbalanced current; 2) The introduction of an integral criterion, by integrating the differential current within the data window, smooths high-frequency transient fluctuations and accumulates fault characteristics, enabling the protection to reliably avoid external fault impacts and quickly respond to internal faults, thus significantly improving its ability to resist distributed capacitive current interference. 3. By scientifically setting the capacity of the cumulative data window, ensuring its duration is sufficient to cover the distributed capacitance transient process to guarantee reliability, and optimizing the window length to the shortest possible value to balance speed, this method avoids the drawback of traditional solutions sacrificing action speed due to long delay windows. It significantly shortens the overall protection action time, achieving an optimal balance between speed and reliability. 4. Simulation verification shows that the proposed method can stably and significantly identify internal and external faults in different fault locations (such as near end and mid point) and different transition resistances (including metallic grounding to high resistance grounding). Moreover, the current value after the fault is much higher than the fixed threshold. This provides a new differential protection scheme for flexible DC lines with strong anti-interference ability, fast action speed and reliable identification of single-pole grounding faults. This proves that the method has good applicability and robustness. Attached Figure Description

[0031] Figure 1 This is an overall flowchart of the Berylon line-mode differential integral current protection method for flexible DC lines according to an embodiment of the present invention.

[0032] Figure 2 This is a diagram of the protection system architecture corresponding to an embodiment of the present invention.

[0033] Figure 3 This is a schematic diagram of the testing system used in an embodiment of the present invention.

[0034] Figure 4 This is a schematic diagram of the physical parameters of the DC overhead line used in the test system of the present invention.

[0035] Figure 5 for Figure 3 The diagram shows the characteristic curve of the line-mode wave impedance of the DC overhead line in the test system as a function of frequency.

[0036] Figure 6 To correspond to different fault locations and transition resistance conditions, the Berylon line-mode differential integral current... i 1_diff_itg (t) Characteristic curves of changes over time before and after the fault. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0038] like Figure 1 The diagram shows the overall flowchart of the Berylon line-mode differential integral current protection method for flexible DC lines according to the present invention. The method specifically includes the following steps: The first step is to calculate system parameters, which specifically includes: Step 1: Calculate the polar impedance of the frequency-dependent DC line using electromagnetic field theory and Carson's formula, based on the DC line's geometric parameters. Z ( f and polar admittance Y ( f Two frequency-varying parameter matrices are used to extract the line self-impedance. Z s ( f ), mutual impedance Z m ( f ), self-guided admittance Y s ( f ) and mutual admittance Y m( f ).

[0039] Step 2: Based on a specific frequency f Calculate the line-mode impedance of a DC line using line parameters at 0. Z c (1) ( f 0) Linear resistance per unit length r 1( f 0) and linear mode propagation coefficient gamma 1( f 0). Specific frequency f The selection principle for 0 is: to make the line mode impedance Z c (1) With unit length line-mode resistance r 1 at frequency f 0 and its adjacent frequency bands can be approximated as constants that do not change with frequency.

[0040] Linear mode impedance Z c (1) ( f 0) The calculation formula is:

[0041] Linear resistance per unit length r 1( f 0) Calculation formula:

[0042] Among them, Re( z ) represents a complex number z The real part, Z s This represents the line's self-impedance. Z m Indicates the mutual impedance of the line; Linear propagation coefficient gamma 1( f 0) The calculation formula is:

[0043] Step 3: Based on the line length l and linear mode propagation coefficient gamma 1( f 0) Obtain the propagation time of the traveling wave in the line mode from the reference point to both ends of the line. tau 1. Transmission Time tau The formula for calculating 1 is:

[0044] Among them, Im( z () represents the imaginary part of the complex number z. gamma 1( f 0) indicates that the line mode is at a certain frequency. f The propagation coefficient at 0; The second step involves data acquisition and transformation, specifically including: Step 4: The protection devices at both ends of the line read the voltage and current sampling values ​​at their respective ends to obtain the current time. t bipolar voltage u P ( t ), u N ( t and bipolar current i P (t), i N (t); Step 5: Based on the bipolar voltage at this end of the line collected in Step 4 u P ( t ), u N ( t) and bipolar current i P (t), i N (t) Calculate the line-mode voltage u 1( t and line mode current i 1( t ). u 1( t ), i 1( t The formula for calculating ) is:

[0045] in, u p ( t ), u N ( t ), i p ( t ), i N ( t The numbers ) represent the time intervals of the line. t The positive voltage sample value, negative voltage sample value, positive current sample value, and negative current sample value at each moment.

[0046] The third step involves calculating and filtering the reference point current, specifically including: Step 6: Calculate the line mode impedance obtained in Step 2. Z c (1) ( f 0) Line-mode resistance r 1( f 0), the propagation time of the line mode traveling wave obtained in step 3 from the reference point to both ends of the line. tau Line-mode voltages obtained in steps 1 and 4 u 1( t ), line mode current i 1( t Calculate the line-mode current at reference point r. i r1 ( t ). i r1 ( t The formula for calculating ) is:

[0047] in, Z 1 represents the line mode impedance. Z c (1) ( f 0),r 1 represents the line-mode resistance per unit length of the line. t It indicates the time represented at the current moment.

[0048] Step 7: The local protection device will use the reference point line-mode current calculated based on the above steps. i r1 ( t The input is filtered by a bandpass filter to obtain the line-mode filtered current. i r1_filter_loc ( t Linear filter current i r1_filter_loc ( t ) Calculation formula in the discrete time domain i r1_filter_loc [ n ]for:

[0049] In the formula: i r1 [ n ]and i r1_filter_loc [ n [ represents the input and output of the bandpass filter at the current sampling time.] i r1 [ n -1] and i r1_filter _ loc [ n [-1] represents the input and output of the bandpass filter at the previous sampling time. i r1 [ n -2] and i r1_filter_loc [ n [-2] represents the input and output of the bandpass filter at the first two sampling times.

[0050] b 0、 b 1. b 2. a 1 and a 2 represents a parameter of the bandpass filter, the value of which is related to the sampling frequency of the bandpass filter. F s and center frequency f m The relevant calculation formula is as follows:

[0051]

[0052]

[0053]

[0054] in, w 0 represents the normalized center angular frequency, used to map the actual analog frequency to the digital discrete frequency domain. a This indicates a bandwidth adjustment parameter that incorporates the quality factor, controlling the passband width and steepness of the bandpass filter. b Indicates center frequency positioning parameters ( w The cosine value of 0 determines the specific location of the filter peak in the spectrum.

[0055] The fourth step involves data communication and compensation, specifically including: Step 8: Convert the bandpass filtered line-mode current obtained in Step 7. i r1_filter_loc ( t Sending data to and receiving data from the other end, and then compensating for the received data to obtain the final result. i r1_filter_rmt ( t ). Calculate the line-mode current at the reference point at the opposite end. i r1_filter_rmt ( t The formula for calculating ) is:

[0056] In the formula, i ’ r1_filter_rmt ( t ) represents the line-mode current at the reference point of the other end without communication compensation, Δ t cs The result was obtained using the "ping-pong method", Δ t cs The calculation formula is:

[0057] In the formula, The time interval is the time it takes for the input signal at this end of the line to be transmitted through the optical fiber channel, reflected at the other end of the line, and then obtained again.

[0058] The fifth step is to calculate the differential current, which includes: Step 9: Calculate the reference point line-mode current using the communication-compensated peer end obtained in Step 8. i r1_filter_rmt ( t ) and the line-mode current calculated at the reference point at this end i r1_filter_loc ( t The differential current of the Berylon line mode was obtained. i 1_diff (t Berylon line-mode differential current. i 1_diff ( t The formula for calculating ) is:

[0059] The sixth step is to perform protection judgment and execution, which specifically includes: Step 10: Calculate the Berylon line-mode differential current of the protected line in real time during each sampling period. i 1_diff ( t The discrete calculated value of the current is then stored in a predefined cumulative data window. The capacity of the cumulative data window is... S , S This is a pre-defined positive integer. It represents all currently stored data in the cumulative data window. N The calculated differential current values ​​are summed to obtain the line-mode differential integral current at the current moment. i 1_diff_itg ( t The linear-mode differential integral current is used to characterize the cumulative effect of the differential current within the time range corresponding to the cumulative data window. The integral current... i 1_diff_itg ( t ) and preset action threshold value I set Comparison: like i 1_diff_itg ( t (greater than) I set If so, it is determined that a fault has occurred inside the protected line, triggering a protection action command; like i 1_diff_itg ( t Less than I set And the number of data points in the cumulative data window N Less than capacity S If no fault is found inside the line, the process returns to step 4 to sample, store, and judge the differential current at the next sampling time.

[0060] like i 1_diff_itg ( t Less than I set And the number of data points in the cumulative data window N equal to capacity S If no fault is found inside the line, all data in the cumulative data window will be cleared, and the protection will not operate.

[0061] Threshold valueI set The selection principle is: it must reliably avoid the maximum unbalanced differential current generated when there is a fault outside the protected line area.

[0062] Step 10 involves accumulating the data window capacity. S The selection principle is as follows: the cumulative time length corresponding to the cumulative data window should not be less than the duration of the transient process caused by the charging and discharging of the DC line distributed capacitance, so as to ensure that the influence of transient unbalanced current can be covered; at the same time, in order to take into account the speed of protection action, the cumulative time length should be as small as possible while meeting the above coverage requirements. When the sampling frequency F s At 10 kHz, the cumulative data window capacity S The preferred value is 2000.

[0063] Example 2: Figure 2 As shown, the architecture of the Berylon line-mode differential integral current protection system for flexible DC lines of the present invention is illustrated, which is used to implement the method described in any one of Embodiment 1. The system includes a master-end protection system deployed at one end of the line and a slave-end protection system deployed at the other end of the line, which are connected through an optical fiber communication channel. The master-end protection system includes: The first parameter calculation module 1100 calculates the polar impedance of the frequency-dependent DC line based on the geometric parameters of the DC line. Z ( f and polar admittance Y ( f ) parameter matrix, and extract the line self-impedance Z s ( f ), mutual impedance Z m ( f ), self-guided admittance Y s ( f ) and mutual admittance Y m( f Select a specific frequency; f 0, based on f Self-impedance at 0 Z s ( f 0) Mutual impedance Z m ( f 0) Self-guided admission Y s ( f 0) and mutual admittance Y m ( f 0), Calculate the line-mode wave impedanceZ c (1) ( f 0) Linear resistance per unit length r 1( f 0) and linear mode propagation coefficient gamma 1( f 0); Using the midpoint of the route as a reference point r According to the linear mode propagation coefficient gamma 1( f 0) and line length l Calculate the traveling wave of the line mode from the reference point r Transmission time to both ends of the line tau 1; The first data acquisition and processing module 1200 is used for real-time sampling of the local bipolar voltage. u P ( t ), u N ( t ) and bipolar current i P ( t ), i N ( t The line-mode voltage at this terminal is obtained through pole-mode transformation. u 1( t and line mode current i 1( t ); The first Berylon calculation and filtering module 1300 is connected to the data acquisition and processing module and is used to utilize the line-mode voltage at its local end. u 1( t and line mode current i 1( t ), Calculate reference points based on the Berylon model r Linear current i r1 ( t );Will i r1 ( t The input is filtered by a bandpass filter to obtain the filtered local reference point line-mode current. i r1_filter_loc ( t ); The first communication module 1400 is connected to the Berylon calculation and filtering module, and is used to... i r1_filter_loc ( t The data is transmitted to the other end via fiber optic channel, and the reference point line-mode current transmitted by the other end is received. After communication compensation, the reference point line-mode current of the other end is obtained. ir1_filter_rmt ( t ); The differential calculation module 500, connected to the communication module and the Berylon calculation and filtering module, is used to calculate the local reference point line-mode current. i r1_filter_loc ( t ) and the line-mode current at the reference point at the opposite end i r1_filter_rmt ( t Summing and taking the absolute value yields the differential current of the Berylon line mode. i 1_diff ( t ); The integral judgment module 600 is connected to the differential calculation module and is used to determine the integral judgment module. i 1_diff ( t Real-time sampling is performed and stored in a cumulative data window. The differential current values ​​within the window are summed to obtain the line-mode differential integral current. i 1_diff_itg ( t ), and will i 1_diff_itg ( t ) and preset action threshold value I set Comparison: If i 1_diff_itg ( t )> I set If so, it is determined that a fault has occurred inside the protected line and the protection action is triggered; if i 1_diff_itg ( t )< I set If the data window is not full, continue sampling; if i 1_diff_itg ( t )< I set The data window is full; clear the data window and prevent further action. The slave protection system 2 includes: The system comprises a second parameter calculation module 2100, a second data acquisition and processing module 2200, a second Berylone calculation and filtering module 2300, and a second communication module 2400; each module of the slave-end protection system has the same function as the corresponding module in the master-end protection system, and is used to generate the slave-end reference point line-mode current. i r1 _ filter _ loc _ remote ( t It is transmitted to the master protection system via its communication module.

[0064] like Figure 3 The diagram illustrates a test system according to an embodiment of the present invention, specifically a ±400kV pseudo-bipolar DC transmission system established using the PSCAD / EMTDC simulation platform. The line length is 55.07km. Figure 4 The diagram shows the physical parameters of the line. A 20mH current-limiting inductor is installed at both ends of the line. The normal operating voltage of the line where the protection is located is shown. U dcN = 400kV. The frequency selected for calculating the frequency-dependent DC line parameters is 696.16Hz. The parameters of converter stations S1 and S2 are the same. Their bridge arm equivalent resistance, bridge arm inductance, submodule capacitance value and its initial capacitor voltage, and the number of submodules are 1.8W, 0.09H, 0.11F, 1.6kV and 250 respectively.

[0065] exist Figure 3 In the test system shown, the characteristic curves of the line-mode impedance and line-mode resistance of the DC line as a function of frequency are as follows: Figure 5 As shown. Taking protection A as an example, the line-mode differential integral current at protection A was tested under different transition resistances and different fault locations. i 1_diff_itg ( t The changes in the DC line at a specific frequency before and after the fault over time. f Line mode impedance at 0=707.1068Hz Z c (1) ( f 0) Linear resistance per unit length r 1( f 0) The propagation time of the line mode traveling wave from the reference point to both ends of the line. tau 1 are 18.9329 W / m and 2.4189×10, respectively. -4 W / m, 203.17ms, cumulative data window capacity S =2000. Testing revealed the following faults occurring on line MN: a ground fault on the positive metal wire outside the M-terminal zone, a short circuit fault between the metal wires outside the M-terminal zone, a ground fault on the positive metal wire inside the M-terminal zone, a short circuit fault between the positive metal wires inside the M-terminal zone, a ground fault on the positive metal wire at the midpoint of the line, and a transition resistance at the midpoint of the line. R f For 50ohm and 100ohm positive ground faults, the line-mode differential integral current at point A of the protection device. i 1_diff_itg ( t After the fault, the current tended to reach 3100A, 2530A, 11300A, 1670000A, 15900A, 3560A, and 3430A respectively over time, as shown in the curves.Figure 6 As shown. Select the reliability coefficient. K rel =1.1, the maximum unbalanced differential current outside the zone is 3100A, then the threshold value is... I set =1.1×3100=3410A, its threshold value is less than the line-mode differential integral current under the slightest fault in the region. It can be seen that the present invention can preserve the transient process in the early stage of the fault and can solve the problem that the traditional differential region cannot operate sensitively when a single-pole fault in a pseudo-bipolar flexible DC system is caused by the short-circuit current supplied by the line capacitance alone. At the same time, the proposed method is also applicable to other fault types, and has selectivity and a certain ability to withstand transition resistance.

[0066] In summary, this invention effectively suppresses the influence of transient currents caused by distributed capacitance in the line, significantly improving the speed and reliability of protection actions. In particular, it provides an effective technical solution for reliably identifying unipolar grounding faults in pseudo-bipolar systems. Furthermore, this invention overcomes the shortcomings of traditional differential protection, which is prone to false tripping due to distributed capacitance and cannot identify unipolar grounding faults in pseudo-bipolar systems. It possesses advantages such as independence from power supply characteristics, adaptive transient processes, and strong resistance to transition resistance, making it suitable for reliable protection of flexible DC lines in renewable energy integration scenarios.

[0067] It should be noted that although the technical solutions have been shown and described with reference to specific exemplary embodiments of the present invention, those skilled in the art should understand that the present invention is not limited to the above embodiments. Any substitutions, improvements, or additions made without departing from the spirit of the present invention and without creative effort shall fall within the scope of protection of the present invention.

Claims

1. A method for differential integral current transfer protection of a flexible DC line using a Berylon line-mode, characterized in that, The method includes the following steps: The first step is to calculate the system parameters: based on the geometric parameters of the DC line, calculate the polar impedance matrix of the frequency-dependent DC line. Z ( f and polar admittance matrix Y ( f ), and extract the line self-impedance. Z s ( f ), mutual impedance Z m ( f ), self-guided admittance Y s ( f ) and mutual admittance Y m( f Select a specific frequency; f 0, based on f Self-impedance at 0 Z s ( f 0) Mutual impedance Z m ( f 0) Self-guided admission Y s ( f 0) and mutual admittance Y m ( f 0), Calculate the line-mode wave impedance Z c (1) ( f 0) Linear resistance per unit length r 1( f 0) and linear mode propagation coefficient γ 1( f 0); Using the midpoint of the route as a reference point r According to the linear mode propagation coefficient γ 1( f 0) and DC line length l Calculate the traveling wave of the line mode from the reference point r Transmission time to both ends of the line τ 1; The second step is data acquisition and transformation: real-time sampling of the bipolar voltage at this end. u P ( t ), u N ( t ) and bipolar current i P ( t ), i N ( t The line-mode voltage at this terminal is obtained through pole-mode transformation. u 1( t and line mode current i 1( t ); The third step is to calculate and filter the reference point current: using the line-mode voltage at this terminal. u 1( t and line mode current i 1( t ), Calculate reference points based on the Berylon model r Linear current i r1 ( t );Will i r1 ( t The input is filtered by a bandpass filter to obtain the filtered local reference point line-mode current. i r1_filter_loc ( t ); The fourth step is to perform data communication and compensation: i r1_filter_loc ( t The data is transmitted to the other end via fiber optic channel, and the reference point line-mode current transmitted by the other end is received. After communication compensation, the reference point line-mode current of the other end is obtained. i r1_filter_rmt ( t ); Step 5: Perform differential current calculation: Calculate the line-mode current at the reference point on this end. i r1_filter_loc ( t ) and the line-mode current at the reference point at the opposite end i r1_filter_rmt ( t Summing and taking the absolute value yields the differential current of the Berylon line mode. i 1_diff ( t ); Step 6: Perform protection judgment and execution: [Regarding...] i 1_diff ( t Real-time sampling is performed and stored in a cumulative data window. The differential current values ​​within the window are summed to obtain the line-mode differential integral current. i 1_diff_itg ( t ), and will i 1_diff_itg ( t ) and preset action threshold value I set Comparison: If i 1_diff_itg ( t )> I set If so, it is determined that a fault has occurred inside the protected line and the protection action is triggered; if i 1_diff_itg ( t )< I set If the data window is not full, continue sampling; if i 1_diff_itg ( t )< I set The data window is full; clear the data window and prevent further action.

2. The Berylon line-mode differential integral current protection method for flexible DC lines according to claim 1, characterized in that, In the first step, the polar impedance Z ( f and polar admittance Y ( f The frequency-varying parameter matrix is ​​obtained based on the geometric parameters of the DC line and is calculated using electromagnetic field theory and Carson's formula.

3. The Berylon line-mode differential integral current protection method for flexible DC lines according to claim 1, characterized in that, In the first step, a specific frequency f The selection range for 0 is from 100 Hz to 1000 Hz, so that the line-mode impedance is... Z c (1) and unit length line-mode resistance R 1 at a specific frequency f 0 and its adjacent frequency bands are approximately constant; The line mode impedance Z c (1) ( f The formula for calculating 0) is: The unit length line-mode resistance r 1( f 0) The calculation formula is: Among them, Re( z ) represents a complex number z The real part, Z s This represents the line's self-impedance. Z m Indicates the mutual impedance of the line; The linear mode propagation coefficient γ 1( f 0) The calculation formula is: 。 4. The Berylon line-mode differential integral current protection method for flexible DC lines according to claim 1, characterized in that, In the first step, the transmission time τ The formula for calculating 1 is: Among them, Im( z () represents the imaginary part of the complex number z. γ 1( f 0) indicates that the line mode is at a certain frequency. f The propagation coefficient at 0.

5. The Berylon line-mode differential integral current protection method for flexible DC lines according to claim 1, characterized in that, In the second step, the formula for calculating the polar mode transformation is: in, u p ( t ), u N ( t ), i p ( t ), i N ( t The numbers ) represent the time intervals of the line. t The positive voltage sample value, negative voltage sample value, positive current sample value, and negative current sample value at each moment.

6. The Berylon line-mode differential integral current protection method for flexible DC lines according to claim 1, characterized in that, In the third step, the reference point r Linear current i r1 ( t The formula for calculating ) is: in, Z 1 represents the line mode impedance. Z c (1) ( f 0), r 1 represents the line-mode resistance per unit length of the line. t It indicates the time represented at the current moment. The bandpass filter is a discrete-time filter, and its total output reference point line-mode current... i r1_filter_loc ( n The formula for calculating ) is: in, i r1 [ n [This is the input of the bandpass filter at the current sampling time.] i r1 [ n -1] and i r1_filter _ loc [ n [-1] represents the input and output of the bandpass filter at the previous sampling time. i r1 [ n -2] and i r1_filter_loc [ n [-2] represents the input and output of the bandpass filter at the previous two sampling times; Bandpass filter parameters b 0、 b 1. b 2. a 1 and a 2 and sampling frequency F s and center frequency f m Yes, the calculation formula is as follows: ， ， in, w 0 represents the normalized center angular frequency, and 'a' represents the bandwidth adjustment parameter. b This indicates the center frequency positioning parameter.

7. The Berylon line-mode differential integral current transfer protection method for flexible DC lines according to claim 1, characterized in that, In the fourth step, communication compensation is achieved through time compensation of the peer reference point line-mode current. The compensated peer reference point line-mode current... i r1_filter_rmt ( t The formula for calculating ) is: In the formula, i ’ r1_filter_rmt ( t ) represents the line-mode current at the reference point of the other end without communication compensation, obtained using the ping-pong method. t cs Δ t cs The calculation formula is: in, This indicates the time interval between the signal's round trip via the optical fiber channel.

8. The Berylon line-mode differential integral current transfer protection method for flexible DC lines according to claim 1, characterized in that, In the fifth step, the Berylon line-mode differential current... i 1_diff ( t The formula for calculating ) is: 。 9. The Berylon line-mode differential integral current transfer protection method for flexible DC lines according to claim 1, characterized in that, In the sixth step, the line-mode differential integral current... i 1_diff_itg ( t The formula for calculating ) is: in, N This indicates the total number of data points in the cumulative data window. k Indicates the sampling time sequence number. i r1_diff ( k ) indicates the first k The instantaneous value of the line-mode differential current sampled at each sampling moment and stored in the cumulative data window; The action threshold I set The selection principle is: it must reliably avoid the maximum unbalanced differential current generated by faults outside the protected line area; The cumulative time length corresponding to the cumulative data window should not be less than the duration of the transient process caused by the charging and discharging of the DC line distributed capacitance, and the speed of the protection action should also be taken into account.

10. A Berylon line-mode differential integral current protection system for flexible DC lines, used to implement the method according to any one of claims 1-9, characterized in that, The system includes a master protection system deployed at one end of the line and a slave protection system deployed at the other end of the line, which are connected by an optical fiber communication channel. The master-end protection system includes: The first parameter calculation module calculates the polar impedance of the frequency-dependent DC line based on the geometric parameters of the DC line. Z ( f and polar admittance Y ( f ) parameter matrix, and extract the line self-impedance Z s ( f ), mutual impedance Z m ( f ), self-guided admittance Y s ( f ) and mutual admittance Y m ( f Select a specific frequency; f 0, based on f Self-impedance at 0 Z s ( f 0) Mutual impedance Z m ( f 0) Self-guided admission Y s ( f 0) and mutual admittance Y m ( f 0), Calculate the line-mode wave impedance Z c (1) ( f 0) Linear resistance per unit length r 1( f 0) and linear mode propagation coefficient γ 1( f 0); Using the midpoint of the route as a reference point r According to the linear mode propagation coefficient γ 1( f 0) and line length l Calculate the traveling wave of the line mode from the reference point r Transmission time to both ends of the line τ 1; The first data acquisition and processing module is used to sample the bipolar voltage at this end in real time. u P ( t ), u N ( t ) and bipolar current i P ( t ), i N ( t The line-mode voltage at this terminal is obtained through pole-mode transformation. u 1( t and line mode current i 1( t ); The first Berylon calculation and filtering module is connected to the data acquisition and processing module, and is used to utilize the line-mode voltage at this end. u 1( t and line mode current i 1( t ), Calculate reference points based on the Berylon model r Linear current i r1 ( t );Will i r1 ( t The input is filtered by a bandpass filter to obtain the filtered local reference point line-mode current. i r1_filter_loc ( t ); The first communication module is connected to the Berylon calculation and filtering module and is used to... i r1_filter_loc ( t The data is transmitted to the other end via fiber optic channel, and the reference point line-mode current transmitted by the other end is received. After communication compensation, the reference point line-mode current of the other end is obtained. i r1_filter_rmt ( t ); The differential calculation module, connected to the communication module and the Berylon calculation and filtering module, is used to calculate the local reference point line-mode current. i r1_filter_loc ( t ) and the line-mode current at the reference point at the opposite end i r1_filter_rmt ( t Summing and taking the absolute value yields the differential current of the Berylon line mode. i 1_diff ( t ); The integral judgment module, connected to the differential calculation module, is used to determine the integral judgment module. i 1_diff ( t Real-time sampling is performed and stored in a cumulative data window. The differential current values ​​within the window are summed to obtain the line-mode differential integral current. i 1_diff_itg ( t ), and will i 1_diff_itg ( t ) and preset action threshold value I set Comparison: If i 1_diff_itg ( t )> I set If so, it is determined that a fault has occurred inside the protected line and the protection action is triggered; if i 1_diff_itg ( t )< I set If the data window is not full, continue sampling; if i 1_diff_itg ( t )< I set The data window is full; clear the data window and prevent further action. The slave-end protection system includes: The system comprises a second parameter calculation module, a second data acquisition and processing module, a second Berylone calculation and filtering module, and a second communication module; each module of the slave-end protection system has the same function as the corresponding module in the master-end protection system, and is used to generate the slave-end reference point line-mode current. i r1 _ filter _ loc _ remote ( t It is transmitted to the master protection system via its communication module.

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