Fault identification method for MTDC power grid based on specific frequency band transient energy ratio

Abstract

The application discloses a MTDC power grid fault discrimination method based on specific frequency band transient energy ratio, researches a fault voltage traveling wave, and utilizes a Bergeron equivalent circuit under different fault conditions to deduce voltage traveling wave characteristics on both sides of a current limiting reactor when an in-zone fault, a positive out-of-zone fault and a reverse out-of-zone fault occur, defines specific frequency band transient energy ratio according to the difference in specific frequency band content of voltage traveling waves on both sides of the current limiting reactor, utilizes the difference in high-frequency energy ratio on both sides of the current limiting reactor when in-zone and out-of-zone faults occur to construct a protection criterion to identify a fault area, and selects a fault pole according to the amplitude difference of positive and negative current limiting reactors when different fault poles occur. A DC power grid model is built by using PSCAD / EMTDC simulation software, and simulation verification shows that the application can accurately identify a fault, has certain anti-transition resistance and anti-noise interference capabilities, can realize full-line rapid action, and has simple fault criterion and high equipment reliability.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC line protection technology, specifically relating to a fault identification method for MTDC power grids based on transient energy ratios in a specific frequency band. Background Technology

[0002] With the rapid depletion of traditional energy sources such as fossil fuels and the increasingly serious environmental problems, the world is facing the issue of energy structure adjustment. In 2020, my country proposed the "dual carbon target". In the future, a large number of distributed energy sources will be connected to the grid. The uncertainty of clean energy has brought huge challenges to the power grid. High voltage direct current (MMC-HVDC) transmission technology using modular multilevel converters (MMC) has broad application prospects due to its advantages such as large transmission capacity, high power quality and strong scalability. In order to further improve operational flexibility and ensure power supply reliability, it is increasingly used to build large-scale multi-terminal flexible DC power grids.

[0003] Because the fault current rises faster in flexible DC lines, higher demands are placed on protection. Rapid and reliable fault identification and improved fault identification accuracy in DC lines have become urgent problems to be solved in flexible DC transmission systems. Protection for multi-terminal flexible DC lines mainly relies on traditional DC protection methods. For DC grid area identification, existing methods can be classified into protection schemes based on dual-terminal quantities and single-terminal quantities. Longitudinal protection typically depends on the reliability of communication equipment and is affected by fault distance and communication delay, making it difficult to meet the speed requirements of the main protection and usually serving as backup protection. Single-terminal quantity protection mainly consists of traveling wave protection and boundary protection. Existing single-terminal quantity-based protection schemes suffer from problems such as high influence from transition resistance and noise, complex implementation processes, high equipment requirements, and limited applicability. Summary of the Invention

[0004] The purpose of this invention is to provide a fault identification method for MTDC power grids based on transient energy ratios in a specific frequency band, which solves the problems of low reliability and significant noise impact in fault current identification of flexible straight lines.

[0005] The technical solution adopted in this invention is an MTDC power grid fault identification method based on transient energy ratio in a specific frequency band, which is implemented according to the following steps:

[0006] Step 1: Collect the line voltage signal in real time, calculate the fault voltage change rate, and start the protection based on the fault voltage change rate;

[0007] Step 2: After the protection is started, the fault voltage traveling wave is decoupled using the Kelvin transform matrix to obtain the 1-mode and 0-mode components. The 1-mode component is used as the linear modulus.

[0008] Step 3: Perform a generalized S-transform on the fault voltage line modulus on both sides of the current-limiting reactor to extract the high-frequency components in the 5-10KHz frequency band, and calculate the high-frequency transient energy ratio based on this frequency band.

[0009] Step 4: Set the fault area judgment conditions based on the high-frequency transient energy on both sides of the current-limiting reactor to determine the fault area; set the protection criteria based on the maximum difference between the positive and negative current-limiting reactor voltage values ​​to determine the fault pole.

[0010] The invention is further characterized by:

[0011] Step 1 is as follows: Calculate the fault voltage change rate using the real-time acquired line voltage signal, set the voltage start criterion threshold to a voltage change rate of 500kV / ms, and start the protection when the measured voltage change rate exceeds the voltage start criterion threshold; otherwise, do not start the protection and repeat the acquisition of the line voltage signal.

[0012] The expression for the Kelvin transformation matrix in step 2 is:

[0013]

[0014] Where F0 is the 0-mode voltage or current, and F1 is the 0-mode and 1-mode voltage or current; F P F represents the positive voltage or current. n This refers to the negative voltage or current.

[0015] Step 3 is as follows:

[0016] The linear modulus, after undergoing the generalized S-transform, becomes a time-frequency matrix. Assuming a row of the matrix represents a specific frequency band and the columns represent time points, the transient energy of the linear modulus at a given frequency band is defined as:

[0017]

[0018] Where K represents an element in a row of the matrix after S-transformation, that is, an element of the signal in a certain frequency band, and N represents the number of sampling points. The transient voltage energy on the line side and the bus side in the K frequency band is obtained as follows:

[0019]

[0020] The high-frequency transient energy ratio on both sides of the current-limiting reactor is then obtained as follows:

[0021]

[0022] The protection R is extracted using the generalized S-transform. AB The transient energy in a specific frequency band on both sides of the side-limiting reactor is calculated using the defined transient energy ratio formula, resulting in the following transient energy ratio within that specific frequency band:

[0023]

[0024] In the formula, K1-K2 are specific frequency bands selected, with K1 = 5KHz and K2 = 10KHz. The high-frequency transient energy ratio is calculated based on this frequency band.

[0025] The fault area determination condition in step 4 is: set the fault threshold G outside the area. set1 Fault threshold G in the area set2 ,

[0026] If the high-frequency transient energy ratio obtained in step 3 is less than G set1 The fault is determined to be outside the reverse zone. If it is greater than G... set1 And less than G set2 The fault is determined to be outside the positive zone and is greater than G. set2 The fault is outside the designated area.

[0027] In step 4, the protection criterion is set based on the maximum difference between the positive and negative limiting current reactor voltage values ​​to determine the fault pole. The specific process is as follows:

[0028] When a unipolar fault occurs, the non-faulty pole can still operate normally during this time, while the current in the faulty pole will rise rapidly in a short period of time. According to the expression, the voltage of the fault-limiting current reactor changes significantly due to the change in current, while the voltage amplitude of the non-fault-limiting current reactor does not change significantly. When a bipolar short-circuit fault occurs, the positive and negative poles are in a series conducting state, and the current changes in the positive and negative poles are basically the same. Therefore:

[0029] △u L =max|u L+ |-max|u L- | (5)

[0030] In the formula u L+ and u L- These are the voltage values ​​of the positive and negative limiting current reactors, respectively. When a fault occurs at the positive terminal, the current rise causes u... L+ The amplitude rises rapidly while u L- The change is very small; conversely, when the negative electrode fails, u... L- The changes are significant; in bipolar faults, both changes are substantial, and because they are connected in series, Δu L Approaching zero, depending on the type of fault, Δu L Based on the different magnitudes of change, the protection criteria are as follows:

[0031]

[0032] In the formula: K rel Represents the reliability coefficient, Δu LsetThis represents the setpoint value.

[0033] The beneficial effects of this invention are:

[0034] This invention presents a fault identification method for MTDC power grids based on specific frequency band transient energy ratios. By studying the traveling wave of fault voltage and utilizing the Berylone equivalent circuit under different fault conditions, the characteristics of the traveling wave on both sides of the current-limiting reactor under faults within the fault zone, faults outside the forward fault zone, and faults outside the reverse fault zone are derived. A specific frequency band transient energy ratio is defined based on the difference in the specific frequency band content of the traveling wave on both sides of the current-limiting reactor. A protection criterion is constructed using the difference in the high-frequency energy ratio on both sides of the current-limiting reactor under faults within and outside the fault zone to identify the fault area. Simultaneously, fault pole selection is performed based on the amplitude difference between the positive and negative limiting current reactors when faults occur at different fault poles. A DC power grid model is built using PSCAD / EMTDC simulation software. Simulation verification shows that this invention can accurately identify faults, has a certain resistance to transition resistance and noise interference, can achieve full-line high-speed operation, and has simple fault criteria and high equipment reliability. Attached Figure Description

[0035] Figure 1 This is a flowchart of the MTDC power grid fault identification method based on the transient energy ratio of a specific frequency band, according to the present invention.

[0036] Figure 2 This is an equivalent model diagram of bipolar Berylone in this invention;

[0037] Figure 3 This is a fault diagram within the equivalent region of Berylone in this invention;

[0038] Figure 4 This is the end-point fault diagram within the equivalent region of Berylone in this invention;

[0039] Figure 5 This is the equivalent positive region outside the fault diagram of Berylone in this invention;

[0040] Figure 6 This is the equivalent reverse region external fault diagram of Berylone in this invention;

[0041] Figure 7 This is the traveling wave transmission diagram in this invention;

[0042] Figure 8 This is a diagram of the four-terminal flexible DC power grid topology in this invention;

[0043] Figure 9 This is a schematic diagram of fault verification within the area in an embodiment of the present invention;

[0044] Figure 10 This is a schematic diagram of the fault voltage change rate in an embodiment of the present invention;

[0045] Figure 11The bipolar short circuit Δu at different fault locations in the embodiments of the present invention L Schematic diagram of the changes;

[0046] Figure 12 This is a schematic diagram of noise interference verification in an embodiment of the present invention. Detailed Implementation

[0047] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0048] This invention relates to an MTDC power grid fault identification method based on transient energy ratios in a specific frequency band, such as... Figure 1 As shown, please follow these steps:

[0049] The line voltage signal is collected in real time, the fault voltage change rate is calculated, and the protection is activated based on the fault voltage change rate. The specific process is as follows: the fault voltage change rate is calculated using the line voltage signal collected in real time, the voltage activation criterion threshold is set to a voltage change rate of 500kV / ms, the protection is activated when the measured voltage change rate exceeds the voltage activation criterion threshold, otherwise the protection is not activated, and the line voltage signal is collected repeatedly.

[0050] After the protection is activated, the Kelvin transform matrix is ​​used to decouple the fault voltage traveling wave to obtain the 1-mode and 0-mode components, and the 1-mode component is used as the linear modulus.

[0051] The expression for the Kelvin transformation matrix is:

[0052]

[0053] Where F0 is the 0-mode voltage or current, and F1 is the 0-mode and 1-mode voltage or current; F P F represents the positive voltage or current. n This refers to the negative voltage or current.

[0054] The generalized S-transform is a further derivation of the S-transform. The S-transform is equivalent to a short-time Fourier transform with a normalized Gaussian window or a wavelet transform with phase correction. Furthermore, by introducing adjustment factors a and b, the original Gaussian window function is transformed from 1 / f to b / f. a This allows for better adjustment of the time and frequency domain windows by adjusting the size of the adjustment factor. The expression is as follows:

[0055]

[0056] In the formula: τ represents the time shift parameter, and h(f,τ-t) represents the Gaussian window function.

[0057] In practical engineering applications, the acquired signals are all discrete signals. Therefore, the discrete form expression of signal y(t) is:

[0058]

[0059] The fault signal is subjected to a generalized S-transform to obtain a time-frequency matrix with N+1 / 2 rows and N columns. Different rows in the matrix represent different frequencies, and different columns represent different times. The first row represents the zero-frequency component. The frequencies of the rows are arranged in an arithmetic progression, which is related to the sampling frequency and the number of sampling points of the signal. The expression for the frequency difference between the rows is as follows:

[0060]

[0061] In the formula: fs represents the sampling frequency, N is the number of sampling points, and T is the sampling interval.

[0062] A generalized S-transform is performed on the fault voltage line modulus on both sides of the current-limiting reactor to extract the high-frequency components in the 5-10kHz frequency band, and the high-frequency transient energy ratio is calculated based on this frequency band; the specific process is as follows:

[0063] The linear modulus, after generalized S-transformation, becomes a time-frequency matrix. Assuming a row of the matrix represents a frequency band and the columns represent time points, this invention uses a sampling frequency of 50 kHz. The transient energy of the linear modulus in a certain frequency band is defined as:

[0064]

[0065] Where K represents an element in a row of the matrix after S-transformation, that is, an element of the signal in a certain frequency band, and N represents the number of sampling points. The transient voltage energy on the line side and the bus side in the K frequency band is obtained as follows:

[0066]

[0067] The high-frequency transient energy ratio on both sides of the current-limiting reactor is then obtained as follows:

[0068]

[0069] The voltage ratio across the current-limiting reactor is less than 1 during a reverse fault. During a forward fault, the ratio expression is the same both inside and outside the forward region, and both are greater than 1. This difference is more pronounced at high frequencies. When studying the initial traveling wave propagation path of faults outside and inside the forward region, the traveling wave propagation diagram is as follows: Figure 7 As shown, it was found that the number of current-limiting reactors passing through the fault outside the positive zone is greater than that passing through the fault inside the positive zone. Due to the high-frequency blocking effect of the current-limiting reactor, the high-frequency signal of the voltage traveling wave is attenuated. Moreover, the frequency domain expressions of the two sides of the current-limiting reactor are the same when the fault occurs inside and outside the positive zone. Therefore, the transient energy ratio of a specific frequency band when the fault occurs inside the positive zone is greater than that when the fault occurs outside the positive zone. Based on this difference, the protection R is extracted through generalized S-transform. ABThe transient energy in a specific frequency band on both sides of the side-limiting reactor is calculated using the defined transient energy ratio formula, resulting in the following transient energy ratio within that specific frequency band:

[0070]

[0071] In the formula, K1-K2 are specific frequency bands selected, with K1 = 5KHz and K2 = 10KHz. The high-frequency transient energy ratio is calculated based on this frequency band.

[0072] The fault area is determined by setting the fault area judgment conditions based on the high-frequency transient energy on both sides of the current-limiting reactor; the fault pole is determined by setting the protection criterion based on the maximum difference between the positive and negative current-limiting reactor voltage values.

[0073] The fault zone determination condition is: a fault threshold G outside the set zone. set1 Fault threshold G in the area set2 ,

[0074] A generalized S-transform is performed on the voltage line mode components, and the transient energies within the 5kHz-10kHz frequency bands on both sides are extracted. If the high-frequency transient energy ratio is less than G... set1 The fault is determined to be outside the reverse zone. If it is greater than G... set1 And less than G set2 The fault is determined to be outside the positive zone and is greater than G. set2 The fault is outside the designated area.

[0075] When the transient energy ratio is less than G set1 When the fault is greater than G, it is determined to be a reverse fault. set1 And less than G set2 When the fault is determined to be outside the positive zone, and the value is greater than G... set2 The fault is determined to be within the designated area, and the setting criteria are as follows:

[0076]

[0077] In the formula G set1 With G set2 All are set values, and the reliability coefficient K is set. rel Set to 1.2, where K1 and K2 represent the selected frequency bands.

[0078] The protection criterion is set based on the difference between the maximum voltage values ​​of the positive and negative limiting current reactors. The specific process for determining the fault pole is as follows:

[0079] When a unipolar fault occurs, the non-faulty pole can still operate normally during this time, while the current in the faulty pole will rise rapidly in a short period of time. According to the expression, the voltage of the fault-limiting current reactor changes significantly due to the change in current, while the voltage amplitude of the non-fault-limiting current reactor does not change significantly. When a bipolar short-circuit fault occurs, the positive and negative poles are in a series conducting state, and the current changes in the positive and negative poles are basically the same. Therefore:

[0080] △u L =max|u L+ |-max|u L- | (14)

[0081] In the formula u L+ and u L- These are the voltage values ​​of the positive and negative limiting current reactors, respectively. When a fault occurs at the positive terminal, the current rise causes u... L+ The amplitude rises rapidly while u L- The change is very small; conversely, when the negative electrode fails, u... L- The changes are significant; in bipolar faults, both changes are substantial, and because they are connected in series, Δu L Approaching zero, depending on the type of fault, Δu L Based on the different magnitudes of change, the protection criteria are as follows:

[0082]

[0083] In the formula: K rel Represents the reliability coefficient, Δu Lset This represents the setpoint value.

[0084] If a line fault occurs, the fault is initiated by extracting 1ms data before and after the fault initiation, and the transient energy ratio G and Δu within the specific frequency band are calculated. L Based on the conditions in steps four and five, fault identification is performed, and the corresponding circuit breaker operates to isolate the fault.

[0085] Example 1

[0086] For protection A, when a fault occurs on line L1, the protection must operate reliably, and it must correctly identify and avoid faults outside the positive and negative directions. To ensure the accuracy and comprehensiveness of the measured values, this invention sets the location of the negative direction fault at the outlet of converter station A, and the location of the positive direction fault outside the line at the outlet of converter station B. The faults within the line are located at different positions on line L1, including the outlet, midpoint, and end of the line, to obtain the transient energy ratio G and the voltage of the current-limiting reactor Δu. L As shown in Table 1.

[0087] Table 1 shows that when a fault occurs within the zone, the transient energy ratio in the 5-10kHz frequency band remains relatively stable with little change. However, the transient energy ratio is significantly lower when a fault occurs in the reverse direction than when a fault occurs in the forward direction. Conversely, when a fault occurs outside the zone in the forward direction (i.e., at the MMCB exit), the G value measured by protection A is lower than that for faults occurring within the zone. This verifies the correctness of the theoretical analysis above in identifying faults inside and outside the zone. Extensive simulations have shown that G... set1 With G set2 It can be set to 1 and 10 respectively. Different fault locations G and Δu L The measured values ​​(single-pole short circuit) are shown in Table 2.

[0088] Table 1

[0089]

[0090] Table 2

[0091]

[0092] Theoretical Analysis:

[0093] 1. Fault transient characteristics analysis

[0094] After a fault occurs, the main components of the first stage of the fault circuit include the lines and the converter. In the faulty circuit, the converter can be equivalent to an RLC series circuit, and its expression in the frequency domain is:

[0095]

[0096] In the formula R eq L represents the sum of the on-resistances of each bridge arm. eq For the reactors in each bridge arm to be of equal value, C eq The equivalent capacitor for the submodule participating in the discharge in the first stage.

[0097] 2. Fault Analysis within the Zone

[0098] 2.1 Internal line fault

[0099] like Figure 2 As shown, ZC1 and ZC0 are the equivalent impedances of the line-mode and zero-mode components, respectively. Since the traveling wave attenuation of the line-mode components is small, and they have advantages such as fast propagation speed and rich fault information, this paper chooses to analyze and solve them in a line-mode network. The equivalent power supply in the line-mode equivalent circuit can be expressed as:

[0100]

[0101] Equivalent circuit of fault in the area, such as Figure 3 As shown, where U f1 B represents the fault equivalent power source. AB1 B fA1The power supply is derived from the fault based on the fault's first traveling wave, and Z in the figure is the equivalent power source. mA U represents the equivalent impedance of converter A. b1 (jω) and U L1 (jω) represent the voltages on the bus side and the line side of the current-limiting reactor, respectively. Figure 3 The equivalent line-mode voltage U of the fault f1 (jω) is:

[0102]

[0103] In the formula U f (jω) The frequency domain expression of the fault equivalent voltage, R f Indicates the transition resistance, due to B fA1 If the value is zero, substituting it into formula 16 yields:

[0104] B AB1 (jω)=2e -γ (jω )l U f1 (jω) (19)

[0105] Based on the equivalent circuit diagram of the fault within the zone, the expressions for the voltage on the line side and the voltage on the bus side of the current-limiting reactor during a unipolar fault can be derived as follows:

[0106]

[0107] In the formula U S,L1 with U S,b1 The voltages on the line side and bus side when a single-pole short circuit occurs, and the voltage when a double-pole short circuit occurs are:

[0108]

[0109] In the formula U P,L1 with U P,b1 These represent the line-side and bus-side voltages when a double-pole short circuit occurs, where Z... mL With Z mb for:

[0110]

[0111] According to the formula above, in the event of a single-pole grounding fault or a double-pole short-circuit fault, the voltages on the line side and bus side of the current-limiting reactor both satisfy the following relationship:

[0112]

[0113] 2.2. Terminal fault within the area

[0114] When a fault occurs at the end of the zone, Figure 8Taking fault F2 as an example, since the fault occurs at the end of the line, no new fault port is generated, and the fault excitation voltage source U... f1 (jω) acts directly on the equivalent circuit of converter station B, and R is also used. AB When considering the object of study, based on the time difference between the first wave and the protection installation location, B can be derived. BA1 (jω)=B AC1 (jω)=B BD1 (jω)=0, so the equivalent circuit is as follows: Figure 4 As shown, the voltage ratio between the line side and the bus side of the current-limiting reactor still satisfies Equation 22.

[0115] 3. External fault

[0116] 3.1. Fault outside the forward zone

[0117] When a positive external fault occurs, the equivalent circuit of the fault line is similar to that of an internal end-of-zone fault, such as... Figure 5 The fault equivalent voltage source shown acts between the current-limiting reactor and the converter when an external fault occurs.

[0118] Through circuit derivation, the expressions for the line side and bus side of the current-limiting reactor when a single-phase ground fault and a bipolar short circuit occur are as follows:

[0119]

[0120]

[0121] Z in the formula W1 With Z W0 The equivalent impedances are shown in Equation 26. Finally, it is derived that the voltage ratio on both sides of the current-limiting reactor on the A side of the converter station still satisfies Equation 22 when there is a fault outside the positive zone.

[0122]

[0123] When a reverse fault occurs outside the zone, the fault equivalent power supply acts between converter station A and the current-limiting reactor. Since the equivalent circuit on the converter station B side is not considered in the analysis, it can be ignored, and B... AB1 The value is zero, and the equivalent circuit diagram is as follows: Figure 6 As shown.

[0124] Similar to the analysis above, the voltage ratio across the current-limiting reactor can be obtained as follows:

[0125]

[0126] For protecting R ABThe voltage ratios on both sides of the current-limiting reactor on side A of the converter station were analyzed under both intra-zone and extra-zone fault conditions. The results were obtained for L under single-pole grounding and double-pole short-circuit faults, respectively. AB The voltage ratios of the line side and bus side, as well as the voltage ratios on both sides, show a significant difference between forward and reverse faults, especially at high frequencies. It can be easily deduced that during a forward fault, the high-frequency transient voltage on the line side is greater than that on the bus side, while during a reverse fault, the high-frequency transient voltage on the line side is less than that on the bus side. The expressions for faults within and outside the forward fault zone are the same. When studying the first traveling wave propagation path of faults within and outside the forward fault zone, the traveling wave propagation diagram is as follows: Figure 7 As shown, it was found that the number of current-limiting reactors passing through the fault outside the positive zone is greater than that passing through the fault inside the positive zone. Due to the high-frequency blocking effect of the current-limiting reactor, the high-frequency signal of the voltage traveling wave is weakened. Moreover, the frequency domain expressions of the two sides of the current-limiting reactor are the same when the fault occurs inside and outside the positive zone. Therefore, the transient energy ratio of a specific frequency band when the fault occurs inside the positive zone is greater than that when the fault occurs outside the positive zone.

[0127] Simulation analysis:

[0128] To analyze the feasibility of the proposed protection scheme, a system was built in PSCAD / EMTDC based on the existing four-terminal Zhangbei flexible DC power grid. Figure 8 The MTDC power grid shown adopts true bipolar operation mode, and the converter station outlet is equipped with a current-limiting reactor. The converter station and line parameters of the flexible DC system are shown in Tables 3 and 4.

[0129] Table 3 MMC Parameters

[0130]

[0131] Table 4 Line Parameters

[0132]

[0133] by Figure 8 Taking a fault within the L1 line fault zone as an example, with a transition resistance of 0.01Ω and a fault time of 1s, the simulation results are as follows: Figure 9 As shown, where Figure 10 This represents the voltage change rate when a double-pole short circuit occurs at the midpoint of the line. The protection system activates when the absolute value of the detected voltage change rate exceeds 500 kV / ms. If the voltage change rate exceeds the set threshold 0.38 ms after the fault occurs, data is collected for 1 ms before and after activation. After line-mode transformation, a generalized S-transform is used to extract the 5-10 kHz high-frequency quantities on the line side and bus side of the current-limiting reactor, and the high-frequency transient energy ratio is calculated. Figure 9The figure shows the transient energy ratios at different fault locations when a single-pole ground fault and a double-pole short circuit occur. It can be seen that the transient energy ratio is not significantly affected by the fault type, and since G is in ratio form, it is also less affected by the transition resistance and the fault location. Figure 9 When a single-pole short circuit and a double-pole short circuit occur at different fault locations within the region, the ratio of the fault transient energy is greater than G. set2 This indicates a fault within the zone, and it can be seen that G is almost unaffected by the fault type. Figure 11 The pole selection results are shown for bipolar short circuits at different fault locations.

[0134] Example 2

[0135] Verification of simulation results for faults with different transition resistances:

[0136] Fault location as follows Figure 1 As shown in Table 5, the transition resistances at F1-F7 are set to 0.01Ω, 100Ω, and 300Ω, respectively. The simulation results for different transition resistances at different locations are shown in Table 5. As can be seen from the theoretical derivation above, the transition resistance does not affect the transient energy ratio. However, from the derivation process, the transition resistance has a certain impact on the voltage line modulus on both sides of the current-limiting reactor. Nevertheless, the results show that the protection method proposed in this paper can correctly identify faults inside and outside the zone for different types of high-resistance faults.

[0137] Table 1

[0138]

[0139]

[0140] Example 3

[0141] noise interference

[0142] Because the protected receiver and transmitter transmit signals in the form of signals, they are more susceptible to noise interference, such as Figure 12 As shown, since the signal-to-noise ratio of current measuring devices is greater than or equal to 30 dB, the method of the invention improves upon this by providing protection to R. AB The protection system's noise immunity was verified by introducing 30dB of noise. The verification results are shown in the figure. The simulation results show that the transient energy ratio is slightly affected by noise interference, but it does not affect the judgment of the results. Therefore, the protection system can still operate correctly under a certain degree of noise interference.

[0143] Through the above methods, this invention provides an MTDC power grid fault identification method based on a specific frequency band transient energy ratio. By studying the traveling wave of fault voltage and utilizing the Berylone equivalent circuit under different fault conditions, the characteristics of the traveling wave on both sides of the current-limiting reactor under faults within the fault zone, faults outside the forward fault zone, and faults outside the reverse fault zone are derived. A specific frequency band transient energy ratio is defined based on the difference in the specific frequency band content of the traveling wave on both sides of the current-limiting reactor. A protection criterion is constructed to identify the fault region using the difference in the high-frequency energy ratio on both sides of the current-limiting reactor when faults occur inside or outside the fault zone. Simultaneously, fault pole selection is performed based on the amplitude difference between the positive and negative limiting current reactors when faults occur at different fault poles. A DC power grid model is built using PSCAD / EMTDC simulation software. Simulation verification shows that this invention can accurately identify faults, has a certain resistance to transition resistance and noise interference, can achieve full-line high-speed operation, and has simple fault criteria and high equipment reliability.

Claims

1. A method for fault identification in MTDC power grids based on transient energy ratios in a specific frequency band, characterized in that, The specific steps are as follows: Step 1: Collect the line voltage signal in real time, calculate the fault voltage change rate, and start the protection based on the fault voltage change rate; Step 2: After the protection is started, the fault voltage traveling wave is decoupled using the Kelvin transform matrix to obtain the 1-mode and 0-mode components. The 1-mode component is used as the linear modulus. Step 3: Perform a generalized S-transform on the fault voltage line modulus on both sides of the current-limiting reactor to extract the high-frequency components in the 5-10KHz frequency band, and calculate the high-frequency transient energy ratio based on this frequency band. Step 4: Set the fault area judgment conditions based on the high-frequency transient energy ratio on both sides of the current-limiting reactor to determine the fault area; set the protection criteria based on the maximum difference between the positive and negative current-limiting reactor voltage values ​​to determine the fault pole.

2. The MTDC power grid fault identification method based on transient energy ratio in a specific frequency band according to claim 1, characterized in that, Step 1 is as follows: Calculate the fault voltage change rate using the real-time acquired line voltage signal, set the voltage start criterion threshold to a voltage change rate of 500kV / ms, and start the protection when the measured voltage change rate exceeds the voltage start criterion threshold; otherwise, do not start the protection and repeat the acquisition of the line voltage signal.

3. The MTDC power grid fault identification method based on transient energy ratio in a specific frequency band according to claim 1, characterized in that, The expression for the Kelvin transformation matrix in step 2 is: Where F0 is the 0-mode voltage or current, and F1 is the 0-mode and 1-mode voltage or current; F P F represents the positive voltage or current. n This refers to the negative voltage or current.

4. The MTDC power grid fault identification method based on transient energy ratio in a specific frequency band according to claim 1, characterized in that, Step 3 is as follows: The linear modulus, after undergoing the generalized S-transform, becomes a time-frequency matrix. Assuming a row of the matrix represents a specific frequency band and the columns represent time points, the transient energy of the linear modulus at a given frequency band is defined as: Where K represents an element in a row of the matrix after S-transformation, that is, an element of the signal in a certain frequency band, and N represents the number of sampling points. The transient voltage energy on the line side and the bus side in the K frequency band is obtained as follows: The high-frequency transient energy ratio on both sides of the current-limiting reactor is then obtained as follows: The protection R is extracted using the generalized S-transform. AB The transient energy in a specific frequency band on both sides of the side-limiting reactor is calculated using the defined transient energy ratio formula, resulting in the following transient energy ratio within that specific frequency band: In the formula, K1-K2 are specific frequency bands selected, with K1 = 5KHz and K2 = 10KHz. The high-frequency transient energy ratio is calculated based on this frequency band.

5. The MTDC power grid fault identification method based on transient energy ratio in a specific frequency band as described in claim 1, characterized in that, The fault area determination condition in step 4 is: setting an external fault threshold G. set1 Fault threshold G in the area set2 , If the high-frequency transient energy ratio obtained in step 3 is less than G set1 The fault is determined to be outside the reverse zone. If it is greater than G... set1 And less than G set2 The fault is determined to be outside the positive zone and is greater than G. set2 The fault is outside the designated area.

6. The MTDC power grid fault identification method based on transient energy ratio in a specific frequency band as described in claim 1, characterized in that, The specific process for determining the fault pole by setting the protection criterion based on the maximum difference between the positive and negative limiting current reactor voltage values ​​in step 4 is as follows: When a unipolar fault occurs, the non-faulty pole can still operate normally during this time, while the current in the faulty pole will rise rapidly in a short period of time. According to the expression, the voltage of the fault-limiting current reactor changes significantly due to the change in current, while the voltage amplitude of the non-fault-limiting current reactor does not change significantly. When a bipolar short-circuit fault occurs, the positive and negative poles are in a series conducting state, and the current changes in the positive and negative poles are basically the same. Therefore: △u L = max|u L+ |-max|u L- (5) In the formula u L+ and u L- These are the voltage values ​​of the positive and negative limiting current reactors, respectively. When a fault occurs at the positive terminal, the current rise causes u... L+ The amplitude rises rapidly while u L- The change is very small; conversely, when the negative electrode fails, u... L- The changes are significant; in bipolar faults, both changes are substantial, and because they are connected in series, Δu L Approaching zero, depending on the type of fault, Δu L Based on the different magnitudes of change, the protection criteria are as follows: Where: K rel Represents the reliability coefficient, Δu Lset This represents the setpoint value.

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