Fault direction judgment method for double-fed wind power ac line of associated MMC converter station

Abstract

The application discloses a method for judging the fault direction of a double-fed wind power AC line of a connected MMC converter station, establishes a double-fed wind power AC sending-out line system of the connected MMC converter station, analyzes the fault of the double-fed wind power AC sending-out line system, collects current and voltage values through a current and voltage transformer, calculates a generalized Jaccard coefficient J of a fault protection criterion, and if the generalized Jaccard coefficient J is greater than a setting value J set , it is determined that it is a positive direction fault, a circuit breaker acts, a first-end circuit breaker and a last-end circuit breaker are tripped to disconnect the line, and if the calculated J set , it is a reverse direction fault, it is determined that the fault occurs outside the area, and the protection is locked out. The method for judging the fault direction of the double-fed wind power AC sending-out line of the connected MMC converter station solves the problem that the traditional technology cannot be applied due to the influence of the frequency deviation of a DFIG on the judgment of the fault direction of the double-fed wind power AC sending-out line of the connected MMC converter station.

Description

Technical Field

[0001] This invention belongs to the field of power system relay technology, specifically relating to a method for determining the fault direction of a doubly fed wind power AC line in an interconnected MMC converter station. Background Technology

[0002] According to a report by the Global Wind Energy Council (GWEC), global wind power installations nearly reached 100 GW in 2021, marking the second consecutive year that global installed capacity has approached 100 GW. Onshore wind power accounted for 82.3 GW, or 83%, while offshore wind power reached 16.8 GW, a 161% increase compared to 2020. By the end of 2021, the cumulative global wind power installed capacity had reached 840 GW. Modular Multilevel Converter-High Voltage Direct Current (MMC-HVDC) technology, based on modular multilevel converters, has been widely adopted. Unlike traditional DC transmission, it features independent control of active and reactive power, lower switching losses, stronger fault tolerance, and stronger fault ride-through capability. Compared to previous AC transmission systems, it offers lower cost, lower losses, more flexible operation, lower harmonic content, and suitability for passive power transmission systems, making it the most suitable high-voltage DC converter for my country's national conditions to date. However, flexible DC transmission systems also pose challenges to traditional relay protection. The damage caused by wind farm faults will also increase significantly, posing challenges to the stability and security of the power grid.

[0003] As the first line of defense for the safe and stable operation of the power grid, relay protection plays a crucial role in quickly and reliably identifying and effectively isolating faults when they occur. This is essential for preventing further deterioration of the system's operating condition and ensuring the efficient and stable transmission and utilization of electrical energy. For doubly-fed wind power AC transmission systems connected to MMC converter stations, which contain a large number of power electronic devices, the fault characteristics of doubly-fed wind power transmission lines are complex, and the phase angle of the fault current is often affected by the control strategy. Traditional protection principles based on current phase difference design face performance degradation and even incorrect operation. Therefore, the integration of wind power into MMC-HVDC presents significant challenges to the reliability and sensitivity of traditional relay protection schemes.

[0004] For MMC AC lines, a common approach is to first derive the equivalent model of the MMC converter, then analyze the adaptability of negative-sequence directional elements in flexible DC AC lines. It was found that longitudinal protection based on negative-sequence directional elements may fail to operate or malfunction during asymmetrical faults in the AC grid. Therefore, based on this, and considering the difference in voltage and current relationships at the protection installation point under forward and reverse faults, a fault direction determination method utilizing time-domain information is proposed, based on the protection concept of model identification. The voltage and current at the protection installation point satisfy a positive resistance-inductance model under forward faults and a negative resistance-inductance model under reverse faults. A generalized Jaccard algorithm is introduced to reflect the model differences, and the threshold value setting avoids the influence of low-voltage ride-through in doubly-fed wind power. Finally, simulation tests demonstrate that the proposed fault determination method operates reliably and sensitively under various fault conditions and exhibits good noise immunity. Summary of the Invention

[0005] The purpose of this invention is to provide a fault direction determination method for doubly fed wind power AC transmission lines of interconnected MMC converter stations, which solves the problem that the fault direction determination of doubly fed wind power AC transmission lines of interconnected MMC converter stations is affected by the DFIG frequency offset and therefore cannot be applied in the traditional technology.

[0006] The technical solution adopted in this invention is a method for determining the fault direction of a doubly fed wind power AC line in an MMC converter station, which specifically includes the following steps:

[0007] Step 1: Establish a doubly fed wind power AC transmission line system connected to the MMC converter station;

[0008] Step 2: Based on Step 1, conduct fault analysis on the doubly fed wind power AC transmission line system of the connected MMC converter station.

[0009] Step 3: Collect current and voltage values ​​using current and voltage transformers;

[0010] Step 4: Calculate the fault protection criterion, which is the generalized Jaccard coefficient J of the differential values ​​of the equivalent voltage and current; if the generalized Jaccard coefficient J is greater than the setting value J... set If the fault is identified as a positive fault, the circuit breaker will trip, with both the first and last circuit breakers disconnecting the line. This is as calculated in J. <J set If the fault occurs in the opposite direction, it is determined that the fault occurred outside the protection zone, and the protection will be blocked.

[0011] The invention is further characterized in that,

[0012] The protection system for the doubly fed wind power AC transmission line of the connected MMC converter station established in step 1 has the following specific circuit structure: it includes a doubly fed wind turbine, a W bus and a V bus. The W bus is connected to the V bus through a line. The W bus is connected to the doubly fed wind turbine through a step-down transformer. The V bus is connected to the MMC converter station through a step-up transformer.

[0013] At the outlet of bus W, a first-end circuit breaker and a first-end current and voltage transformer for detecting the first-end circuit breaker are installed. The first-end circuit breaker is connected to the action controller w. The first-end current and voltage transformer and the action controller w are all connected to the programmable processor w. At the inlet of bus V, an end circuit breaker and an end-end current and voltage transformer for detecting the current and voltage values ​​of the end circuit breaker are installed. The end circuit breaker is connected to the action controller v. The end-end current and voltage transformer and the action controller v are all connected to the programmable processor v.

[0014] Step 2 is as follows:

[0015] Forward and reverse fault analyses were performed on the doubly-fed wind power AC transmission line system of the MMC converter station in step 1. Equivalent models for forward and reverse faults were established, and the differences between the equivalent models were studied to obtain the equivalent voltage value u. bb and the differential value of current i bb The generalized Jaccard coefficient is used to determine the direction of the fault.

[0016] Voltage u at the wind turbine side protection installation point during forward fault w Current i w It should meet the following requirements:

[0017]

[0018] In the formula, the injection current at the fault point is i F The transition resistance is R F ;R Lw L Lw These are the line resistance and inductance from the fault point to the W side, respectively.

[0019] Simplifying equation (1) yields the voltage u. w for:

[0020]

[0021] In the formula, let R′ Lw It is the equivalent resistance;

[0022] Using the equivalent voltage value u bb and the differential value of current i bb Equivalent to formula (2), as follows:

[0023] u bb =L Lw i bb (3)

[0024] Where the equivalent voltage u bb =u w -R′ Lw i w Differential value of current When a positive fault occurs in the doubly fed wind power AC transmission line system of the connected MMC converter station, the equivalent voltage value u bb and the differential value of current i bb The waveforms are positively correlated, and the generalized Jaccard coefficient is greater than zero;

[0025] Voltage u at the wind turbine side protection installation point for reverse fault w Current i w It should meet the following requirements:

[0026]

[0027] In the formula, the injection current at the fault point is i F The transition resistance is R F ;R Lw L Lw These are the line resistance and inductance from the fault point to the W side, respectively.

[0028] Simplifying equation (4), we can obtain the voltage u. w for:

[0029]

[0030] In the formula, the equivalent resistance

[0031] Using the equivalent voltage value u bb and the differential value of current i bb Equivalent to formula (5), as follows:

[0032] u bb =-L Lw i bb (6)

[0033] Wherein, the equivalent voltage u bb =u w +R′ Lw i w Differential value of current When a reverse fault occurs in the doubly fed wind power AC transmission line system connected to the MMC converter station, the equivalent voltage value u bb and the differential value of current i bbThe waveform is negatively correlated, and the generalized Jaccard coefficient is less than zero; however, the generalized Jaccard coefficient is greater than zero when a positive fault occurs; therefore, the equivalent voltage value u can be used to determine the relationship between the waveform and the generalized Jaccard coefficient. bb and the differential value of current i bb The generalized Jaccard coefficient is used to determine the direction of the fault.

[0034] Step 3 is implemented as follows:

[0035] The PSCAD simulation experiment simulates different types of faults in the doubly fed wind power AC line system of the connected MMC converter station, and the fault current and voltage data are imported into the MATLAB program.

[0036] Step 4 is implemented as follows:

[0037] In step 4, programmable processors w and v are used to read and process the voltage and current data sampled in step 3. Using the read current value, voltage value, and voltage and current value before one cycle, the equivalent voltage value u is calculated. bb and the differential value of current i bb .

[0038] Step 4 Equivalent voltage value u bb and the differential value of current i bb The generalized Jed coefficient similarity is:

[0039]

[0040] In the formula, u bb For the equivalent voltage, i bb This is the differential value of the current.

[0041] The fault criteria are as follows:

[0042] J(u bb i bb )≥J set (8)

[0043] Among them, J set This is the setpoint value.

[0044] In step 2, the set value J set It is 0.1.

[0045] The beneficial effects of this invention are that the fault direction determination method for doubly-fed wind power AC transmission lines of interconnected MMC converter stations solves the problem in traditional technologies where fault direction determination for doubly-fed wind power AC transmission lines of interconnected MMC converter stations is affected by DFIG frequency offset, rendering it inapplicable. By using generalized Jaccard coefficients to calculate the equivalent voltage and current differential similarity, forward and reverse faults are effectively identified, improving the reliability of the protection. Attached Figure Description

[0046] Figure 1 This is a circuit diagram of the protection system for the doubly fed wind power AC transmission line of the MMC converter station connected to this invention.

[0047] Figure 2 This is the equivalent circuit diagram of the protection system for the doubly fed wind power AC transmission line of the MMC converter station connected to this invention when a positive fault occurs;

[0048] Figure 3 This is the equivalent circuit diagram of the protection system for the doubly fed wind power AC transmission line of the MMC converter station connected to this invention when a reverse fault occurs;

[0049] Figure 4 This is the working principle of traditional directional elements;

[0050] Figure 5 These are the operating values ​​of the power directional element on the power grid system side under traditional directional element conditions;

[0051] Figure 6 This refers to the operating value of the power directional element when there is a frequency offset phenomenon on the fan side under the conventional directional element;

[0052] Figure 7 This is a flowchart of the fault direction determination process for the doubly fed wind power AC transmission line of the MMC converter station connected to the present invention.

[0053] Figure 8 This is a schematic diagram of the voltage waveform at both ends of the line when a two-phase short-circuit fault occurs in the system of the doubly fed wind power AC transmission line method of the MMC converter station of the present invention.

[0054] Figure 9 This is a schematic diagram of the current waveforms at both ends of the line when a two-phase short-circuit fault occurs in the doubly fed wind power AC transmission line method of the MMC converter station connected to the present invention.

[0055] Figure 10 This invention relates to the method for constructing a doubly fed wind power AC transmission line to an MMC converter station, where the equivalent voltage u at both ends of the line is measured when a two-phase short-circuit fault occurs in the system. bb Waveform diagram;

[0056] Figure 11 This invention relates to the method for constructing a doubly fed wind power AC transmission line to an MMC converter station, where the differential current i at both ends of the line is measured when a two-phase short-circuit fault occurs in the system. bb Waveform diagram;

[0057] Figure 12 This is a diagram showing the calculation results of fault criteria under different fault locations in the method for doubly fed wind power AC transmission lines of interconnected MMC converter stations in this invention;

[0058] Figure 1In the diagram, 1. Doubly fed wind turbine, 2. Doubly fed wind power transmission line, 3. Step-down transformer, 4. W bus, 5. Head-end circuit breaker, 6. Head-end current and voltage transformers, 7. Line, 8. Programmable processor w, 9. Action controller w, 10. End circuit breaker b, 11. End current and voltage transformers, 12. Programmable processor v, 13. Action controller v, 14. V bus, 15. Step-up transformer, 16. MMC converter station side line, 17. MMC converter station. Detailed Implementation

[0059] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0060] This invention provides a method for determining the fault direction of a doubly fed wind power AC transmission line connected to an MMC converter station, which is implemented according to the following steps:

[0061] Step 1: Establish a doubly fed wind power AC transmission line system connected to the MMC converter station;

[0062] Step 1, please follow these steps: Figure 1 As shown, a protection system for the doubly fed wind power AC transmission line of the connected MMC converter station is established. Its specific circuit structure is as follows: it includes a doubly fed wind turbine generator 1, W bus 4 and V bus 14. W bus 4 is connected to V bus 14 through line 7. W bus 4 is connected to the doubly fed wind turbine generator 1 through step-down transformer 3. V bus 14 is connected to the MMC converter station 17 through step-up transformer 15.

[0063] At the outlet of busbar 4, a first-end circuit breaker 5 and a first-end current and voltage transformer 6 for detecting the first-end circuit breaker are installed. The first-end circuit breaker 5 is connected to the action controller w9. The first-end current and voltage transformer 6 and the action controller w9 are both connected to the programmable processor w8. At the inlet of busbar 14, an end circuit breaker 10 and an end current and voltage transformer 11 for detecting the current and voltage values ​​of the end circuit breaker are installed. The end circuit breaker 10 is connected to the action controller v13. The end current and voltage transformer 11 and the action controller v13 are both connected to the programmable processor v12.

[0064] Step 2: Based on Step 1, conduct fault analysis on the double-fed wind power AC transmission line system of the connected MMC converter station, and propose a method for determining the fault direction of the double-fed wind power AC transmission line of the connected MMC converter station.

[0065] Step 2, specifically implemented as follows: Figure 2-3The paper presents a fault analysis of the doubly fed wind power AC transmission line system of the interconnected MMC converter station, and obtains the forward fault equivalent model and the reverse fault equivalent model. Based on the model differences, a fault direction judgment method for the doubly fed wind power AC transmission line of the interconnected MMC converter station is established.

[0066] Step 3: Acquire current and voltage values ​​using current and voltage transformers. Current and voltage values ​​at the first-end circuit breaker 5 and the last-end circuit breaker 10 on the line are acquired using the first-end current and voltage transformer 6 and the last-end current and voltage transformer 7, respectively. The first-end current and voltage transformer 6 inputs the measured current and voltage values ​​into the programmable processor w8, and the last-end current and voltage transformer 11 inputs the measured current and voltage values ​​into the programmable processor v12.

[0067] Step 3 is implemented as follows: Simulate different types of faults in the doubly fed wind power AC line system of the connected MMC converter station through PSCAD simulation experiments, and import the fault current and voltage data into the MATLAB program.

[0068] Step 4: Calculate the fault protection criterion, which is the generalized Jaccard coefficient of the differential values ​​of the equivalent voltage and current. The calculated generalized Jaccard coefficient J is greater than the setting value J. set If the fault is identified as a positive fault, the circuit breaker will trip, with the first-end circuit breaker 5 and the last-end circuit breaker 10 tripping to disconnect the line. This is as calculated in J. <J set If the fault occurs in the opposite direction, it is determined that the fault occurred outside the zone, and the protection is blocked.

[0069] The setting value J in step 4 set It is 0.1.

[0070] Step 4 is specifically implemented as follows: Programmable processors w and v read and process the voltage and current data sampled in step 3. Using the read current and voltage values ​​and a voltage and current value before one cycle, the equivalent voltage value u is calculated. bb and the differential value of current i bb Then calculate the generalized Jaccard coefficients of the equivalent voltage and current differentials. If the final calculated generalized Jaccard coefficient J is less than the setting value J... set If the fault is determined to be reverse, sampling continues. If the value is greater than the set value J... set If the fault is determined to be positive, proceed to step 5.

[0071] Step 5: The first-end circuit breaker 5 and the last-end circuit breaker 10 trip to disconnect the line, completing the determination of the fault direction of the doubly fed wind power AC line of the connected MMC converter station using the generalized Jaccard coefficient.

[0072] The construction of the directional element in step 2 is as follows:

[0073] Forward and reverse fault analyses were performed on the doubly-fed wind power AC transmission line system of the MMC converter station in step 1. Equivalent models for forward and reverse faults were established, and the differences between the equivalent models were studied to obtain the equivalent voltage value u. bb and the differential value of current i bb The generalized Jaccard coefficient is used to determine the fault direction criterion.

[0074] Voltage u at the wind turbine side protection installation point during forward fault w Current i w It should meet the following requirements:

[0075]

[0076] In the formula, the injection current at the fault point is i F The transition resistance is R F ;R Lw L Lw These are the line resistance and inductance from the fault point to the W side, respectively.

[0077] Simplifying equation (1) yields the voltage u. w for:

[0078]

[0079] In the formula, let R′ Lw It is the equivalent resistance.

[0080] Using the equivalent voltage value u bb and the differential value of current i bb Equivalent to formula (2), as follows:

[0081] u bb =L Lw i bb (3)

[0082] Where the equivalent voltage u bb =u w -R′ Lw i w Differential value of current When a positive fault occurs in the doubly fed wind power AC transmission line system of the connected MMC converter station, the equivalent voltage value u bb and the differential value of current i bb The waveforms are positively correlated, and the generalized Jaccard coefficient is greater than zero;

[0083] Voltage u at the wind turbine side protection installation point for reverse fault w Current i w It should meet the following requirements:

[0084]

[0085] In the formula, the injection current at the fault point is i F The transition resistance is R F ;R Lw L Lw These are the line resistance and inductance from the fault point to the W side, respectively.

[0086] Simplifying equation (4), we can obtain the voltage u. w for:

[0087]

[0088] In the formula, the equivalent resistance

[0089] Using the equivalent voltage value u bb and the differential value of current i bb Equivalent to formula (5), as follows:

[0090] u bb =-L Lw i bb (6)

[0091] Wherein, the equivalent voltage u bb =u w +R′ Lw i w Differential value of current When a reverse fault occurs in the doubly fed wind power AC transmission line system connected to the MMC converter station, the equivalent voltage value u bb and the differential value of current i bb The waveform is negatively correlated, and the generalized Jaccard coefficient is less than zero; however, the generalized Jaccard coefficient is greater than zero when a positive fault occurs; therefore, the equivalent voltage value u can be used to determine the relationship between the waveform and the generalized Jaccard coefficient. bb and the differential value of current i bb The generalized Jaccard coefficient is used to determine the direction of the fault;

[0092] The generalized Jaccard coefficient amplifies the correlation assessment of vector features in the numerator through multiplication, while reducing the influence of similar components between two vectors in the denominator, resulting in a more sensitive overall assessment of waveform characteristics. Furthermore, the generalized Jaccard coefficient can accurately quantify and identify the correlation between two waveforms in a power system during a fault, regardless of the specific circumstances. Therefore, it is well-suited for determining the correlation between the equivalent voltage and current derivatives, and its mathematical expression is:

[0093]

[0094] In the formula, u bb For the equivalent voltage, i bb This is the differential value of the current.

[0095] The criterion for directional elements is:

[0096] J(u bb i bb )≥0.1 (8)

[0097] If the protection of the doubly fed wind power AC transmission line of the connected MMC converter station meets the criterion, it is considered that a positive direction fault has occurred; otherwise, it is considered a directional fault.

[0098] The advantages of the generalized Jaccard coefficient are as follows: In the numerator, the generalized Jaccard coefficient amplifies the correlation assessment of vector features through multiplication, while reducing the influence of similarity between two vectors in the denominator, resulting in a more sensitive overall assessment of waveform characteristics. Furthermore, the generalized Jaccard coefficient can accurately quantify and identify the similarity between two signals in a power system during a fault under any circumstances, making it highly suitable for fault direction determination in doubly-fed wind power AC lines connected to MMC converter stations.

[0099] I. Applicability Analysis of Traditional Directional Elements

[0100] In my country, the positive direction of directional elements is defined as follows: current flowing from the busbar to the line is positive, and current flowing from the line to the busbar is negative. Since the occurrence of a fault does not affect the voltage phase, traditional directional elements use the busbar voltage direction as a reference direction. However, the current direction changes before and after a fault, resulting in a significant phase difference. Generally, the Fourier transform algorithm is used to calculate the phase difference between the voltage and the fault current, determining whether this phase difference falls within the directional element's operating range, thereby identifying the fault direction.

[0101] Depend on Figure 4 As shown, the phase angle difference between the forward fault current vector and the reverse fault current vector is close to 180 degrees, while the voltage vector remains in phase regardless of whether a fault is in forward or reverse direction. The phase angle difference between the current vector and the voltage vector is significant during forward and reverse faults. Traditional directional elements are constructed using this difference, such as power directional elements. Here, we analyze the applicability of power directional elements.

[0102] Power directional elements only need to determine the direction of a fault based on the phase difference between voltage and current. Their wiring method is convenient for engineering applications. However, if a fault occurs near the protection installation location, measurement errors can easily occur due to voltage drop, resulting in a dead zone. In subsequent research, to reduce the dead zone, a 90° wiring method is generally used, replacing the phase voltage with the line voltage. The calculation formula is as follows:

[0103]

[0104] —Protect the line voltage at the installation location

[0105] —Protect the phase current at the installation location

[0106] In practical applications, the relationship between phase current and line voltage is as follows: i a Corresponding to u bc i b Corresponding to u ac i c Corresponding to u ab .

[0107] Because the short-circuit current supplied by doubly-fed induction generators (DFIGs) has a variable frequency and is no longer the power frequency current, the phase difference between the current vector and voltage vector extracted using the Fourier algorithm is no longer accurate. This affects the operating range of directional elements that rely on the phase difference between the forward and reverse currents and the reference voltage to determine the fault direction. Therefore, 90° connection directional elements are no longer suitable for use in AC transmission lines of DFIGs.

[0108] After setting a short circuit in the doubly-fed induction generator (DFIG) AC transmission system, the operating performance of the conventional power direction element on the turbine side was verified. The pre-fault wind speed was changed so that the frequency of the AC component of the post-fault short-circuit current was 40Hz. The operating values ​​of the power direction element are as follows: Figure 5 and Figure 6 As shown.

[0109] from Figure 5 and Figure 6 It can be seen that when the power directional element is applied to the AC transmission line of the doubly fed wind power, the operating value of the directional element on the wind power side is not fixed due to the frequency offset characteristics of the doubly fed wind power, and the operating performance is affected.

[0110] II. New Fault Direction Determination Method

[0111] 1) Fault criteria

[0112] To avoid the frequency offset characteristics of doubly-fed induction generator (DFIG) wind power, a novel fault direction determination method utilizes time-domain information and introduces a generalized Jaccard coefficient to determine the fault direction. The criterion for the AC transmission line system of the DFIG wind power connected to the MMC converter station is as follows:

[0113] J(u bb i bb )≥0.1 (9)

[0114] If the protection of the doubly fed wind power AC transmission line of the connected MMC converter station meets the criterion, it is considered that a positive direction fault has occurred; otherwise, it is considered a directional fault.

[0115] 2) Simulation verification

[0116] A two-phase fault occurs in the doubly-fed wind power AC transmission line system connected to the MMC converter station. The fault occurs 3 seconds after the fault occurs. The voltage and current waveforms of phase A at the installation points of the forward and reverse fault protection are as follows: Figure 8 and Figure 9 As shown.

[0117] In the method for calculating the equivalent voltage u at both ends of the line when a two-phase short-circuit fault occurs in the system of the doubly fed wind power AC transmission line of the connected MMC converter station, the method is as follows: bb and the differential value of current i bb The waveform diagrams are as follows: Figure 10 and Figure 11 As shown.

[0118] Based on the calculation of the generalized Jaccard coefficient, the fault direction determination method yields a result of 0.7714 for a forward fault and -0.725 for a reverse fault. This demonstrates that the novel fault direction determination method can effectively identify the fault direction in two-phase inter-phase fault scenarios.

[0119] The fault direction determination method proposed in this invention was simulated and tested under various fault scenarios, such as fault location, fault type, sampling noise, wind speed, sampling rate, and data window length, to verify the performance of the proposed fault direction determination method under different conditions.

[0120] The specific process is as follows:

[0121] (1) The impact of sampling rate and data window length

[0122] First, the sampling rate and data window length were tested. Since the current sampling signal is actually an array of discrete data, the number of discrete data will affect the performance of the fault direction determination method proposed in this invention. The data window length and sampling rate together determine the number of discrete data. Therefore, it is necessary to conduct simulation tests on the impact of sampling rate and data window length on the proposed fault direction determination method. 1kHz, 1.2kHz, 4kHz, and 5kHz were selected, and the data window lengths were selected as 10ms and 20ms, respectively. Taking a two-phase short-circuit fault scenario as an example, the performance of the sampling rate and data window length on the proposed fault direction determination method was tested. The test results are shown in Table 1.

[0123] Table 1. Calculation results under different sampling rates and data window lengths.

[0124]

[0125] As shown in Table 1, although the fault direction determination method proposed in this invention can operate reliably under the tested sampling rate and data window length, the operating values ​​differ under different test conditions. It can be seen that the higher the sampling rate and the longer the data window length, the greater the difference in the calculated values ​​for forward and reverse faults. Since the fault direction determination method proposed in this invention is used for backup protection, the requirements for reliability are higher than those for speed and economy. Therefore, a sampling rate of 1kHz and a data window length of 20ms were set to test other influencing factors.

[0126] (2) The impact of fault location

[0127] The location of the fault also affects the fault voltage and current, thus influencing the performance of the fault direction determination method. Therefore, this invention takes a two-phase interphase fault as an example and conducts simulation tests on different locations of the fault in the forward and reverse directions of the protected line to verify the performance of the fault direction determination method. The simulation results are as follows: Figure 12 As shown.

[0128] from Figure 12 It can be seen that under two-phase interphase faults, the change of fault location does not affect the performance of the fault direction determination method proposed in this invention. For forward faults, since the fault voltage and current conform to the positive resistance-inductance model, and the conformity degree of the positive resistance-inductance model is positive and close to 1, the proposed fault direction determination method reliably determines the fault direction as forward. For reverse faults, since the fault voltage and current conform to the negative resistance-inductance model, and the conformity degree of the negative resistance-inductance model is negative and close to -1, the proposed fault direction determination method reliably determines the fault direction as reverse. Therefore, the fault direction determination method proposed in this invention can reliably operate in three-phase fault scenarios regardless of the fault location.

[0129] (3) The impact of fault type

[0130] The above tests all used two-phase-to-phase faults as examples to verify the performance of the fault direction determination method. The fault type also significantly affects the performance of the fault direction determination method. Therefore, this section conducts simulation tests on the fault direction determination method proposed in this invention for three-phase short circuits, two-phase-to-ground short circuits, and single-phase-to-ground short circuits. Simultaneously, multiple fault types are set to occur at different fault locations for comprehensive testing. The simulation results are shown in Table 2.

[0131] Table 2 Calculation results at different fault locations

[0132]

[0133]

[0134] As can be seen from Table 2, the fault direction determination method proposed in this invention can not only reliably determine the fault direction under various fault types, but also reliably determine the fault direction when various types of faults occur at different fault locations. Together with the simulation in the previous section, this demonstrates that the fault direction determination method proposed in this invention can reliably determine the direction when various fault types occur at various fault locations.

[0135] (4) Noise impact

[0136] Sampling noise can affect the calculation results of the direction determination algorithm. Therefore, it is necessary to conduct simulation tests on the performance of the proposed fault direction determination method under different noise conditions. This section presents simulation tests for fault scenarios with different sampling noise levels under different types of faults. The results are shown below:

[0137] Table 3 Calculation results of fault direction determination method under different noise levels

[0138]

[0139]

[0140] As can be seen from Table 3, the calculated values ​​of the fault direction judgment method proposed in this paper are affected to some extent after Gaussian white noise with different signal-to-noise ratios is artificially added to the discrete current sampling signal, but it does not affect the overall fault direction judgment. The test results show that the fault direction judgment method proposed in this paper can still reliably judge the fault direction when the noise is 15dB. In actual engineering, the maximum noise of power lines is generally 25dB, so the fault direction judgment method proposed in this paper has good noise resistance.

[0141] (5) Wind speed influence

[0142] For double-fed induction generators (DFIGs), the output current waveform is affected by different wind speeds. Therefore, this paper also conducted simulation tests on the impact of wind speed on the proposed fault direction determination method, and the results are shown below:

[0143] Table 4 Calculation results of fault direction determination method under different wind speeds

[0144]

[0145] According to Table 4, wind speed will also cause fluctuations in the calculated values ​​of the fault direction determination method, but it will not affect the determination of the fault direction. Therefore, the direction determination scheme proposed in this paper is not affected by the wind speed before the fault, and its reliability and sensitivity both meet the requirements.

[0146] This invention presents a fault direction determination method for doubly-fed induction generator (DFIG) AC lines in interconnected MMC converter stations. Starting from full-time-domain current information and focusing on model identification in DFIG interconnected MMC systems, it utilizes generalized Jaccard coefficients to calculate the correlation between the equivalent voltage and current differentials at both ends, constructing a fault direction determination method. This method can adapt to various fault conditions at lower sampling rates, and its reliability and sensitivity meet requirements under different sampling rates, data window lengths, fault types, and fault locations. It solves the problem that existing fault direction determination methods for doubly-fed induction generator (DFIG) AC lines in interconnected MMC converter stations are incompatible due to the frequency offset characteristics and control strategies of DFIG wind power. This invention is highly practical and beneficial for reliable fault diagnosis and clearance.

Claims

1. A method for determining the fault direction of a doubly-fed wind power AC line in an interconnected MMC converter station, characterized in that, The specific steps are as follows: Step 1: Establish a doubly-fed AC transmission line system for wind power connected to the MMC converter station; specifically, implement it as follows: The protection system for the AC transmission line of the doubly fed wind power of the MMC converter station is established, and its specific circuit structure is as follows: it includes a doubly fed wind turbine (1), a W bus (4) and a V bus (14). The W bus (4) is connected to the V bus (14) through the line (7). The W bus (4) is connected to the doubly fed wind turbine (1) through the step-down transformer (3). The V bus (14) is connected to the MMC converter station (17) through the step-up transformer (15). At the outlet of the W bus (4), a first-end circuit breaker (5) and a first-end current and voltage transformer (6) for detecting the first-end circuit breaker are provided. The first-end circuit breaker (5) is connected to the action controller w (9). The first-end current and voltage transformer (6) and the action controller w (9) are both connected to the programmable processor w (8). At the inlet of the V bus (14), an end circuit breaker (10) and an end current and voltage transformer (11) for detecting the current and voltage values ​​of the end circuit breaker are provided. The end circuit breaker (10) is connected to the action controller v (13). The end current and voltage transformer (11) and the action controller v (13) are both connected to the programmable processor v (12). Step 2: Based on Step 1, conduct a fault analysis of the doubly-fed wind power AC transmission line system of the interconnected MMC converter station; details are as follows: Forward and reverse fault analyses were performed on the doubly-fed wind power AC transmission line system of the MMC converter station in step 1. Equivalent models for forward and reverse faults were established, and the differences between the equivalent models were studied to obtain the equivalent voltage values. u bb and the differential value of current i bb The generalized Jaccard coefficient is used to determine the direction of the fault. Voltage at the wind turbine side protection installation point during forward faults u w 、 Current i w It should meet the following requirements: In the formula, the injection current at the fault point is ， Transition resistance is ; , These are the line resistance and inductance from the fault point to the W side, respectively. Simplifying formula (1) yields the voltage. u w for: In the formula, let , It is the equivalent resistance; Using equivalent voltage value u bb and the differential value of current i bb Equivalent to formula (2), as follows: The equivalent voltage Differential value of current When a positive fault occurs in the doubly fed wind power AC transmission line system connected to the MMC converter station, the equivalent voltage value is... u bb and the differential value of current i bb The waveforms are positively correlated, and the generalized Jaccard coefficient is greater than zero; Voltage at the wind turbine side protection installation point for reverse faults u w 、 Current i w It should meet the following requirements: In the formula, the injection current at the fault point is ， Transition resistance is ; , These are the line resistance and inductance from the fault point to the W side, respectively. Simplifying formula (4), we can obtain the voltage. u w for: In the formula, the equivalent resistance ; Using equivalent voltage value u bb and the differential value of current i bb Equivalent to formula (5), as follows: Among them, equivalent voltage Differential value of current When a reverse fault occurs in the doubly fed wind power AC transmission line system connected to the MMC converter station, the equivalent voltage value is... u bb and the differential value of current i bb The waveform is negatively correlated, and the generalized Jaccard coefficient is less than zero; however, the generalized Jaccard coefficient is greater than zero when a positive fault occurs; therefore, it can be determined based on the equivalent voltage value. u bb and the differential value of current i bb The generalized Jaccard coefficient is used to determine the direction of the fault; Step 3: Collect current and voltage values ​​using current and voltage transformers; Step 4: Calculate the fault protection criterion, which is the generalized Jaccard coefficient of the differential values ​​of the equivalent voltage and current. J If the generalized Jaccard coefficient J Greater than the set value J set ， If the fault is determined to be in the forward direction, the circuit breaker will trip, with the first-end circuit breaker (5) and the last-end circuit breaker (10) tripping to disconnect the line, as calculated. J < J set If the fault occurs in the opposite direction, it is determined that the fault occurred outside the protection zone, and the protection will be blocked.

2. The fault direction determination method for the doubly fed wind power AC transmission line of the interconnected MMC converter station according to claim 1, characterized in that, Step 3 is implemented as follows: The PSCAD simulation experiment simulates different types of faults in the doubly fed wind power AC line system of the connected MMC converter station, and the fault current and voltage data are imported into the MATLAB program.

3. The fault direction determination method for the doubly-fed wind power AC transmission line of the interconnected MMC converter station according to claim 1, characterized in that, In step 4, programmable processors w and v are used to read and process the voltage and current data sampled in step 3. Using the read current value, voltage value, and voltage and current value before one cycle, the equivalent voltage value is calculated. u bb and the differential value of current i bb .

4. The fault direction determination method for the doubly fed wind power AC transmission line of the interconnected MMC converter station according to claim 1, characterized in that, Step 4 Equivalent voltage value u bb and the differential value of current i bb The generalized Jed coefficient similarity is: In the formula, Equivalent voltage This is the differential value of the current; The fault direction determination method includes the following fault criteria: in, J set This is the setpoint value.

5. The method for determining the fault direction of a doubly-fed wind power AC line in an interconnected MMC converter station according to claim 1, characterized in that, The setting value mentioned in step 2 J set It is 0.1.

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