A flexible low-frequency transmission line inverse time protection method and system

Abstract

The present invention designs a flexible low-frequency transmission line inverse time protection method and system, belonging to the field of power system relay protection. The present invention uses a single-ended electrical quantity to form a criterion. After a fault occurs, a signal acquisition device is used to sample the current and voltage, and the obtained voltage and current are subjected to Karen Bell transformation to obtain the line mode and zero mode components, and the integral value of the fault modulus component is used to construct the transient current energy, and the transient current energy is used to construct a new inverse time characteristic curve equation, and a transition resistance compensation coefficient is introduced to improve the performance of the inverse time protection. The present invention aims to speed up the action time of the protection when a fault occurs in the flexible low-frequency transmission line, and uses the fault component to construct the transient current energy value. The frequency offset does not affect the numerical calculation. The criterion is constructed only by detecting the size of the transient current energy at different fault locations and different transition resistance faults. It is easy to implement, improves the speed of protection, and has high universality and accuracy.

Description

Technical Field

[0001] The invention relates to a flexible low-frequency transmission line inverse time protection method and system, belonging to the field of power system relay protection. Background Art

[0002] In recent years, my country's offshore wind power has received extensive attention and development, and its offshore installed capacity accounts for one-third of the world's total. There are three ways to connect offshore wind power to the grid: high-voltage direct current, high-voltage alternating current and low-frequency transmission. Low-frequency transmission has a much higher transmission capacity than high-voltage alternating current transmission over medium and long distances, and its economic benefits are greater than those of high-voltage direct current transmission. It has great development potential in the field of offshore wind power. With the application of permanent magnet wind turbines in low-frequency offshore wind farms, affected by the control strategy, the fault characteristics of wind farms when the transmission line fails are significantly different from those of traditional synchronous power supplies, which will lead to refusal to operate and false operation problems in traditional power frequency quantity protection methods. Traditional stage current protection is widely used in power systems of various voltage levels. Its advantages are its simple setting and wiring methods and reliable action characteristics. This type of protection uses a combination of instantaneous current quick-break and time-limited current quick-break to protect the entire length of the line, and ensures selectivity by gradually increasing the protection action time limit. However, when a fault occurs at the end of the line, the backup protection action time at the power supply end is often longer. This is because the staged current protection needs to gradually extend the protection action time to ensure selectivity, resulting in a slower response time for the backup protection at the power supply end. Since the capacity of the grid-connected wind farm, the location and type of the fault, and the uncertainty of wind speed will cause changes in the output power of the wind farm, when the wind farm is connected to the grid through the tie line, the transmitted power and the voltage at the end of the protection will fluctuate due to the above situation, which causes the conventional line protection to sometimes fail to meet the protection requirements.

[0003] The inverse time protection action time can be automatically adjusted according to the size of the action. Different system operation modes only affect the action time. In addition, the inverse time protection adopts a unified action curve, which naturally meets the requirements of protection sensitivity. Summary of the invention

[0004] The purpose of the present invention is to provide a flexible low-frequency transmission line inverse time protection method and system, aiming to solve the technical problems of false operation and refusal to operate of protection when the fault distance is far and the high-resistance grounding fault occurs, and the protection speed is insufficient under the high-resistance grounding fault.

[0005] To achieve the above-mentioned purpose, the technical solution of the present invention is to provide a flexible low-frequency transmission line inverse time protection method and system. This method uses the line mode current component and the zero mode current component after the fault to construct the transient current energy value, and uses it as the variable of the inverse time characteristic equation to construct the inverse time action equation, which can achieve the protection speed and selectivity for both phase-to-phase faults and ground faults. In traditional inverse time protection, when the transition resistance is high, it will cause the protection action time to be prolonged. In the case of a high-resistance grounding fault, the present invention will calculate the action delay according to the inverse time protection II stage, and introduce the transition resistance compensation coefficient at the same time, which effectively solves the problem of insufficient protection speed under high-resistance grounding faults. The specific steps are:

[0006] Step 1: Sample the current and voltage using a signal acquisition device, and perform Karenberg transformation on the current and voltage to extract line mode current and zero mode current;

[0007] Step 2: Determine the fault type based on the zero-mode current;

[0008] Step 3: Calculate transient current energy based on line mode current and zero mode current;

[0009] Step 4: When the transient current energy is greater than the starting threshold of section I, the inverse time characteristic equation of section I is constructed; when the transient current energy is between section II and the starting threshold of section I, the inverse time characteristic equation of section II is constructed; when the transient current energy is less than the starting threshold of section II, the protection will not start.

[0010] The Step 1 is specifically as follows:

[0011] Step 1.1: Install a voltage and current signal acquisition device on the field side of the transmission line, and use the signal acquisition device to sample the voltage and current after a fault occurs on the transmission line;

[0012] Step 1.2: Define the sampling time window length, perform Karenberg transformation on the collected fault voltage and current within the time window length, and extract the line mode current With zero mode current .

[0013] The specific step 2 is: when there is a ground fault, there will be a zero-mode current, but when there is a phase-to-phase fault, there will be no zero-mode current. As the zero-mode current setting value, the fault type is determined by:

[0014]

[0015] in, Take it as 5A. If the above formula is established, it is judged as a ground fault, and the transient current energy is calculated using the zero modulus; otherwise, it is a phase-to-phase fault, and the transient current energy is calculated using the line modulus.

[0016] The Step 3 is specifically as follows:

[0017] Step 3.1: Line mode current With zero mode current To perform integral processing:

[0018]

[0019]

[0020] In the formula, is the transient current energy, =0, 1; is the sampling time; is the actual sampling time; is the sampling window length; is the modulus current fault component; , are the modulus electrical quantities after and before the fault respectively; T is the power frequency cycle;

[0021] Step 3.2: To facilitate the calculation of discrete sampling data, the formula is discretized to obtain:

[0022] In the formula, for The number of internal sampling points, To protect the sampling point after startup.

[0023] The Step 4 is specifically as follows:

[0024] Step 4.1: Introduce transition resistance compensation coefficient ;

[0025] Specifically, under the condition that the fault position remains unchanged, the short-circuit current value decreases with the increase of transition resistance, and the protection delay will become longer. Therefore, the transition resistance compensation coefficient is introduced to improve the protection performance. It is now defined as is the starting transition resistance compensation coefficient threshold, then ; In the formula is the error coefficient, and the value range of the present invention is 1.0~1.3;

[0026] Step 4.2: Define the start interval of the inverse time protection stage I as , using transient current energy to construct the inverse time characteristic equation: ;

[0027] The starting interval of the inverse time protection stage II is defined as , using transient current energy to construct the inverse time characteristic equation: ;

[0028] In the formula, is the action time, , is the shape coefficient of the inverse time curve, which is 0~2 in the present invention; , is the time setting coefficient, which is set according to the transient current energy of section I and section II. and are the transition resistance compensation coefficients of section Ⅰ and section Ⅱ respectively, and They are the activation quantities of the protection devices for stage I and stage II respectively.

[0029] Protection device activation amount It should be greater than the transient current energy value measured by the protection when a high-resistance grounding fault occurs at the end of the flexible low-frequency transmission line; It should be set to a smaller value to ensure that the protection can be activated when a high-resistance ground fault occurs;

[0030] If the transient current energy is less than the stage II starting threshold, the protection will not start.

[0031] To achieve the above purpose, the present application also proposes a flexible low-frequency transmission line inverse time protection system, comprising:

[0032] Signal acquisition module, used to obtain voltage and current data at the field-side protection installation after a fault occurs;

[0033] The integral numerical calculation module is used to perform Karenberg transformation on the acquired fault current and voltage data to extract the line mode current and Zero mode current , and perform integral processing;

[0034] The fault judgment module is used to construct the zero-mode current setting judgment criterion and output the judgment result.

[0035] The signal acquisition module specifically includes:

[0036] Data acquisition unit: used to collect voltage and current analog quantities at the field-side protection installation location;

[0037] Analog-to-digital conversion unit: used to convert the analog quantities of voltage and current into digital quantities.

[0038] The integral numerical calculation module specifically includes:

[0039] Transient current energy extraction unit: used to perform Karenberg transformation on the acquired fault current data to extract line mode current and Zero mode current ;

[0040] Integral unit: used to integrate line mode current and Zero mode current Perform integration processing.

[0041] The fault determination module specifically includes:

[0042] Zero mode current setting unit: used to set zero mode current;

[0043] Fault judgment unit: used to construct zero-mode current setting criterion. If zero-mode current exists, it is a ground fault, otherwise it is a phase-to-phase fault.

[0044] The beneficial effects of the present invention are:

[0045] (1) The present invention performs Karenbel transformation on the extracted voltage and current data through the acquisition unit, performs integration processing, calculates the transient current energy value, and sets a two-stage inverse time protection equation. For metal grounding faults or low-resistance faults, the action delay is calculated according to protection stage I, which effectively ensures the protection speed and can simultaneously target phase-to-phase faults and grounding faults;

[0046] (2) The present invention calculates the action delay according to the protection stage II under high-resistance grounding fault, and introduces the transition resistance compensation coefficient, which effectively solves the problem of insufficient protection speed under high-resistance grounding fault. It can effectively prevent the single-phase grounding fault from developing into a phase-to-phase fault when the protection does not operate for a long time, thereby avoiding endangering the system safety.

[0047] (3) The present invention does not require active signal injection, does not require additional equipment, and has a low cost;

[0048] (4) The present invention has been verified through a large number of simulations and is less affected by frequency offset and does not affect the calculation of transient current energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a simulation model topology diagram of the present invention;

[0050] Figure 2 It is the inverse time protection flow chart of the present invention;

[0051] Figure 3 is a block diagram of the inverse time protection system of the present invention;

[0052] Figure 4 It is the two-stage inverse time protection action curve diagram of the present invention;

[0053] Figure 5 1 is a zero-mode current waveform diagram of different transition resistances according to Example 1 of the present invention;

[0054] Figure 6 is the action time of the protection before and after the transition resistance compensation in Embodiment 1 of the present invention;

[0055] Figure 71 is a zero-mode current waveform diagram of different fault distances according to Embodiment 2 of the present invention;

[0056] Figure 8 It is the action time of the protection before and after the transition resistance compensation in Example 2 of the present invention. DETAILED DESCRIPTION

[0057] The present invention is further described below in conjunction with the accompanying drawings and specific implementation examples.

[0058] Embodiment 1: Flexible low frequency transmission line simulation model system Figure 1 As shown in the figure, the offshore wind farm outputs low-frequency power, which is transmitted to the low-frequency side of the M3C converter via the 16.67Hz transmission line, and the M3C power frequency side is connected to the 50Hz grid. The total length of the line is 100km, and the voltage level is 330kV. The fault is set to occur 50km away from the wind farm, and the transition resistances are 0.01Ω, 50Ω, and 100Ω respectively. The fault type is A-phase grounding fault, and the sampling rate is 10kHz.

[0059] A flexible low-frequency transmission line inverse time protection method, the process is as follows Figure 2 As shown, the specific steps are:

[0060] Step 1: Sample the current and voltage using a signal acquisition device, and perform Karenberg transformation on the current and voltage to extract line mode current and zero mode current;

[0061] Step 1.1: Install a voltage and current signal acquisition device on the field side of the transmission line, and use the signal acquisition device to sample the voltage and current after a fault occurs on the transmission line;

[0062] Step 1.2: Define the sampling time window length as 2ms, perform Karenberg transformation on the collected fault voltage and current within the time window length, and extract the line mode current With zero mode current ;

[0063] Step 2: Determine the fault type based on the zero-mode current;

[0064] Since zero-mode current will appear in case of ground fault, but no zero-mode current will appear in case of phase-to-phase fault, As the zero-mode current setting value, the fault type is determined by:

[0065]

[0066] in Take 5A, in this embodiment It is established, and the fault type is judged to be a ground fault. The transient current energy is calculated using zero modulus.

[0067] Step 3: Calculate transient current energy ;

[0068] Step 3.1: Obtain the line mode current With zero mode current To perform integral processing:

[0069]

[0070]

[0071] In the formula, is the transient current energy, =0, 1; is the sampling time; is the actual sampling time; is the sampling window length; is the modulus current fault component; , They are the modulus electrical quantities before and after the fault respectively; T is the power frequency period, which is 20ms.

[0072] Step 3.2: To facilitate the calculation of discrete sampling data, discretize the formula:

[0073] Where: for The number of internal sampling points is 20 in this embodiment. To protect the sampling point after startup. A phase A ground fault occurs 50 km away from the wind farm, and the transition resistance is 0.01Ω, 50Ω, and 100Ω respectively. Finally, the transient current energy in this embodiment is W i1 =0.5330kA, W i2 =0.4034kA, W i3 =0.3221kA.

[0074] like Figure 5 As shown, when a fault occurs at the same location and only the transition resistance is changed, as the fault duration increases, the difference between the fault current components under different transition resistances gradually increases, and the smaller the transition resistance, the larger the value of the fault current component.

[0075] Step 4: After the fault occurs, when the transient current energy is greater than the starting threshold of section I, the inverse time characteristic equation of section I is constructed; when the transient current energy is between section II and the starting threshold of section I, the inverse time characteristic equation of section II is constructed; when the transient current energy is less than the starting threshold of section II, the protection will not start.

[0076] Step 4.1: Introduce transition resistance compensation coefficient :Under the condition that the fault position remains unchanged, the short-circuit current value decreases with the increase of transition resistance, and the protection delay will become longer. Therefore, the transition resistance compensation coefficient is introduced to improve the protection performance. Now define is the starting transition resistance compensation coefficient threshold, then ; In the formula is the error coefficient, which is taken as 0.9. Then the transition resistance compensation coefficient is . Figure 6 It is the protection action time before and after the transition resistance compensation coefficient is introduced in this embodiment.

[0077] Step 4.2: Define the start interval of the inverse time protection stage I as , using transient current energy to construct the inverse time characteristic equation: ; Define the start interval of the inverse time protection II as , using transient current energy to construct the inverse time characteristic equation: If the transient current energy is less than the stage II starting threshold, the protection will not start.

[0078]

[0079] In the formula, is the action time, , is the shape coefficient of the inverse time curve, which is 0~2 in the present invention; , is the time setting coefficient, which is set according to the transient current energy of section I and section II respectively. and are the transition resistance compensation coefficients of section Ⅰ and section Ⅱ respectively, and The protection device activation amount is respectively the protection device activation amount of stage Ⅰ and stage Ⅱ; It should be greater than the transient current energy value measured by the protection when a high-resistance grounding fault occurs at the end of the flexible low-frequency transmission line; It should be set to a smaller value to ensure that the protection can be activated when high-resistance grounding occurs.

[0080] Figure 4 This is the two-stage inverse time protection action curve after the transition resistance compensation coefficient is introduced.

[0081] In this embodiment kA, kA, , , , , then a phase A grounding fault occurs 50km away from the wind farm, the transition resistances are 0.01Ω, 50Ω, and 100Ω, and the action times are t1=0.1788s, t2=0.1849s, and t3=0.1918s, respectively.

[0082] Figure 3 A functional block diagram of a flexible low-frequency transmission line inverse time protection system provided by the present invention includes:

[0083] Signal acquisition module, used to obtain voltage and current data at the field-side protection installation after a fault occurs;

[0084] The integral calculation module is used to perform Karenberg transformation on the acquired fault current data to extract the line mode current and Zero mode current , and integrate it to get the transient current energy;

[0085] The fault judgment module is used to construct the zero-mode current setting judgment criterion and output the judgment result.

[0086] The flexible low-frequency transmission line inverse time protection system is characterized in that the signal acquisition module specifically includes:

[0087] Data acquisition unit: used to collect voltage and current analog quantities at the field-side protection installation location; in this embodiment, the sampling frequency is 10kHz;

[0088] Analog-to-digital conversion unit: used to convert the analog quantities of voltage and current into digital quantities.

[0089] The flexible low-frequency transmission line inverse time protection system is characterized in that the integral value calculation module specifically includes:

[0090] Transient current energy extraction unit: used to perform Karenberg transformation on the acquired fault current data to extract line mode current and Zero mode current ;

[0091] Integral unit: used to integrate line mode current and Zero mode current The transient current energy is obtained by integrating the current.

[0092] The flexible low-frequency transmission line inverse time protection system is characterized in that the fault determination module specifically includes:

[0093] Zero mode current setting unit: used to set zero mode current; =5A.

[0094] Fault judgment unit: used to construct zero-mode current setting criterion. If zero-mode current exists, it is a ground fault, otherwise it is a phase-to-phase fault.

[0095] In this embodiment, when a phase A grounding fault occurs 50 km away from the wind farm and the transition resistance is 0.01Ω, 50Ω, and 100Ω respectively, the inverse time protection starts to operate 0.1788s, 0.1849s, and 0.1918s after the fault occurs.

[0096] Example 2: The offshore wind farm outputs low-frequency power, which is transmitted to the low-frequency side of the M3C converter via a 16.67Hz transmission line, and the M3C power frequency side is connected to the 50Hz grid. The line is 100km long and the voltage level is 330kV. The faults are set to occur at 10km, 50km, and 90km away from the wind farm, respectively, the transition resistance is 50Ω, the fault type is A-phase grounding fault, and the sampling rate is 10kHz.

[0097] A flexible low-frequency transmission line inverse time protection method, the specific steps are:

[0098] Step 1: Sample the current and voltage using a signal acquisition device, and perform Karenberg transformation on the current and voltage to extract line mode current and zero mode current;

[0099] Step 1.1: Install a voltage and current signal acquisition device on the field side of the transmission line, and use the signal acquisition device to sample the voltage and current after a fault occurs on the transmission line;

[0100] Step 1.2: Define the sampling time window length as 2ms, perform Karenberg transformation on the collected fault voltage and current within the time window length, and extract the line mode current With zero mode current ;

[0101] Step 2: Determine the fault type based on the zero-mode current;

[0102] Since zero-mode current will appear in case of ground fault, but no zero-mode current will appear in case of phase-to-phase fault, As the zero-mode current setting value, the fault type is determined by:

[0103]

[0104] in Take 5A, in this embodiment It is established, and the fault type is judged to be a ground fault. The transient current energy is calculated using zero modulus.

[0105] Step 3: Calculate transient current energy ;

[0106] Step 3.1: Obtain the line mode current With zero mode current To perform integral processing:

[0107]

[0108]

[0109] In the formula, is the transient current energy, =0, 1; is the sampling time; is the actual sampling time; is the sampling window length; is the modulus current fault component; , They are the modulus electrical quantities before and after the fault respectively; T is the power frequency period, which is 20ms.

[0110] Step 3.2: To facilitate the calculation of discrete sampling data, discretize the formula:

[0111] Where: for The number of internal sampling points is 20 in this embodiment. is the sampling point after zero-mode current setting. The fault is set to occur at 10km, 50km, and 90km away from the wind farm, the transition resistance is 50Ω, and the fault type is A-phase grounding fault. Finally, the transient current energy in this embodiment is obtained as W. i1 =0.5566kA, W i2 =0.4034kA, W i3 =0.3311kA.

[0112] like Figure 7 As shown, only the fault location is changed. As the fault duration increases, the fault current component continues to rise, the difference in fault current components between different fault locations gradually increases, and the smaller the fault distance, the larger the fault current component.

[0113] Step 4: After the fault occurs, when the transient current energy is greater than the starting threshold of section I, the inverse time characteristic equation of section I is constructed; when the transient current energy is between section II and the starting threshold of section I, the inverse time characteristic equation of section II is constructed; when the transient current energy is less than the starting threshold of section II, the protection will not start.

[0114] Step 4.1: Introduce transition resistance compensation coefficient :Under the condition that the fault position remains unchanged, the short-circuit current value decreases with the increase of transition resistance, and the protection delay will become longer. Therefore, the transition resistance compensation coefficient is introduced to improve the protection performance. Now define is the starting transition resistance compensation coefficient threshold, then ; In the formula is the error coefficient, which is taken as 0.9. Then the transition resistance compensation coefficient is . Figure 8 It is the protection action time before and after the transition resistance compensation coefficient is introduced in this embodiment.

[0115] Step 4.2: Define the start interval of the inverse time protection stage I as , using transient current energy to construct the inverse time characteristic equation: ; Define the start interval of the inverse time protection II as , using transient current energy to construct the inverse time characteristic equation: If the transient current energy is less than the stage II starting threshold, the protection will not start.

[0116]

[0117] In the formula, is the action time, , is the shape coefficient of the inverse time curve, which is 0~2 in the present invention; , is the time setting coefficient, which is set according to the transient current energy of section I and section II. and are the transition resistance compensation coefficients of section Ⅰ and section Ⅱ respectively, and The protection device activation amount is respectively the protection device activation amount of stage Ⅰ and stage Ⅱ; It should be greater than the transient current energy value measured by the protection when a high-resistance grounding fault occurs at the end of the flexible low-frequency transmission line; It should be set to a smaller value to ensure that the protection can be activated when high-resistance grounding occurs.

[0118] In this embodiment kA, kA, , , , The faults are set to occur at 10km, 50km, and 90km away from the wind farm, the transition resistance is 50Ω, the fault type is a phase A grounding fault, and the action times are t1=0.1780s, t2=0.1849s, and t3=0.1908s respectively.

[0119] The present embodiment provides a functional block diagram of a flexible low-frequency transmission line inverse time protection system, including:

[0120] Signal acquisition module, used to obtain voltage and current data at the field-side protection installation after a fault occurs;

[0121] The integral calculation module is used to perform Karenberg transformation on the acquired fault current data to extract the line mode current and Zero mode current , and integrate it to get the transient current energy;

[0122] The fault judgment module is used to construct the zero-mode current setting judgment criterion and output the judgment result.

[0123] The flexible low-frequency transmission line inverse time protection system is characterized in that the signal acquisition module specifically includes:

[0124] Data acquisition unit: used to collect voltage and current analog quantities at the field-side protection installation location; in this embodiment, the sampling frequency is 10kHz;

[0125] Analog-to-digital conversion unit: used to convert the analog quantities of voltage and current into digital quantities.

[0126] The flexible low-frequency transmission line inverse time protection system is characterized in that the integral value calculation module specifically includes:

[0127] Transient current energy extraction unit: used to perform Karenberg transformation on the acquired fault current data to extract line mode current and Zero mode current ;

[0128] Integral unit: used to integrate line mode current and Zero mode current The transient current energy is obtained by integrating the current.

[0129] The flexible low-frequency transmission line inverse time protection system is characterized in that the fault determination module specifically includes:

[0130] Zero mode current setting unit: used to set zero mode current; =5A.

[0131] Fault judgment unit: used to construct zero-mode current setting criterion. If zero-mode current exists, it is a ground fault, otherwise it is a phase-to-phase fault.

[0132] In this embodiment, the fault is set to occur at 10 km, 50 km, and 90 km away from the wind farm, the transition resistance is 50Ω, the fault type is a phase A grounding fault, and the inverse time protection starts to operate 0.1780s, 0.1849s, and 0.1908s after the fault occurs.

[0133] Compared with the traditional overcurrent inverse time method, the flexible low-frequency transmission line inverse time protection method can solve the problems of false operation and refusal to operate when the fault distance is far and the high-resistance grounding fault occurs, and can better improve the protection speed; when the transition resistance in the traditional inverse time protection is high, it will cause the protection action time to be prolonged. The present invention will calculate the action delay according to the inverse time protection II stage under the high-resistance grounding fault, and introduce the transition resistance compensation coefficient at the same time, which effectively solves the problem of insufficient protection speed under the high-resistance grounding fault.

[0134] The specific implementation modes of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above implementation modes, and various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention.

Claims

1. A flexible low-frequency transmission line inverse time protection method, characterized in that: Step 1: Sample the current and voltage using a signal acquisition device, and perform Karenberg transformation on the current and voltage to extract line mode current and zero mode current; Step 2: Determine the fault type based on the zero-mode current; Step 3: Calculate transient current energy based on line mode current and zero mode current; Step 4: When the transient current energy is greater than the start threshold of stage I, the inverse time characteristic equation of stage I is constructed; when the transient current energy is between stage II and the start threshold of stage I, the inverse time characteristic equation of stage II is constructed; when the transient current energy is less than the start threshold of stage II, the protection does not start; The Step 4 is specifically as follows: Step 4.1: Introduce transition resistance compensation coefficient ; Step 4.2: Define the start interval of the inverse time protection stage I as , using transient current energy to construct the inverse time characteristic equation: ; The starting interval of the inverse time protection stage II is defined as , using transient current energy to construct the inverse time characteristic equation: ; In the formula, is the action time, , is the shape coefficient of the inverse time curve; , is the time setting coefficient, which is set according to the transient current energy of section I and section II. and are the transition resistance compensation coefficients of section Ⅰ and section Ⅱ respectively, and They are the activation quantities of the protection devices of stage I and stage II respectively. is the transient current energy.

2. The flexible low-frequency transmission line inverse time protection method according to claim 1 is characterized in that: The Step 1 is specifically as follows: Step 1.1: Install a voltage and current signal acquisition device on the field side of the transmission line, and use the signal acquisition device to sample the voltage and current after a fault occurs on the transmission line; Step 1.2: Define the sampling time window length, perform Karenberg transformation on the collected fault voltage and current within the time window length, and extract the line mode current With zero mode current .

3. The flexible low-frequency transmission line inverse time protection method according to claim 1 is characterized in that: The specific step 2 is: when there is a ground fault, there will be a zero-mode current, but when there is a phase-to-phase fault, there will be no zero-mode current. As the zero-mode current setting value, the fault type is determined by: ； If the above formula is established, it is judged as a ground fault, and the transient current energy is calculated using zero modulus; Otherwise it is a phase-to-phase fault, and the line modulus is used to calculate the transient current energy.

4. The flexible low-frequency transmission line inverse time protection method according to claim 1 is characterized in that: The Step 3 is specifically as follows: Step 3.1: Line mode current With zero mode current To perform integral processing: ； ； In the formula, is the transient current energy, =0, 1; is the sampling time; is the actual sampling time; is the sampling window length; is the modulus current fault component; , are the modulus electrical quantities after and before the fault respectively; T is the power frequency cycle; Step 3.2: Discretize the formula and get: ; In the formula, for The number of internal sampling points, To protect the sampling point after startup.

5. A system for implementing the flexible low-frequency transmission line inverse time protection method as claimed in claim 1, characterized in that: include: Signal acquisition module, used to obtain voltage and current data at the field-side protection installation after a fault occurs; The integral numerical calculation module is used to perform Karenberg transformation on the acquired fault current and voltage data to extract the line mode current and Zero mode current , and perform integral processing; The fault judgment module is used to construct the zero-mode current setting judgment criterion and output the judgment result.

6. The system according to claim 5, characterized in that The signal acquisition module specifically includes: Data acquisition unit: used to collect voltage and current analog quantities at the field-side protection installation location; Analog-to-digital conversion unit: used to convert the analog quantities of voltage and current into digital quantities.

7. The system according to claim 5, characterized in that The integral numerical calculation module specifically includes: Transient current energy extraction unit: used to perform Karenberg transformation on the acquired fault current data to extract line mode current and Zero mode current ; Integral unit: used to integrate line mode current and Zero mode current Perform integration processing.

8. The system according to claim 5, characterized in that The fault determination module specifically includes: Zero mode current setting unit: used to set zero mode current; Fault judgment unit: used to construct zero-mode current setting criterion. If zero-mode current exists, it is a ground fault, otherwise it is a phase-to-phase fault.

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