Multi-phase-mode conversion decoupling method for same-tower double-circuit alternating current-direct current (AC-DC) system

Abstract

A multi-phase-mode transformation decoupling method for a same-tower double-circuit alternating current-direct current system comprises the following steps: analyzing coupling characteristics of polar lines in the same-tower double-circuit alternating current-direct current system, and constructing an impedance matrix; constructing a loop decoupling matrix to realize decoupling among loops of the same-tower double-loop AC-to-DC system, and proposing a fault loop discrimination matrix based on the decoupling; constructing an interelectrode decoupling matrix to realize decoupling among polar lines of the same-tower double-circuit alternating current-to-direct current system; and determining the number of fault loops based on the sudden change condition of the modulus obtained by the fault loop discrimination matrix, and adaptively selecting an inter-electrode decoupling matrix of the same scale to realize decoupling. The method lays a theoretical foundation for subsequent fault feature analysis, and has the advantages of being good in decoupling effect, small in calculation amount and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of analysis of coupling characteristics of a power transmission system, and particularly relates to a decoupling method for multiple phase-mode transformations of a same-tower double-circuit AC-to-DC conversion system. Background Art

[0002] Currently, large-scale new energy sources represented by wind power and photovoltaic power are continuously connected to regional power grids. The volatility of the active power output of new energy and the increasing demand for power supply pose challenges to the power transmission and consumption capabilities of regional power grids. Compared with AC systems, DC systems have both a larger transmission capacity and fast and flexible power control. By adding a DC system or converting some AC lines to DC, an embedded DC system with both the sending and receiving ends within the same regional power grid is formed, which is an effective solution to improve the power transmission and consumption capabilities of regional power grids. Currently, the Yangzhou-Zhenjiang AC-to-DC conversion project in Jiangsu is the first embedded DC power transmission project in China. This project plans to convert 6 conductors in the 220 kV Wufengshan crossing line into a 3-circuit bipolar DC power transmission system, forming a same-tower three-circuit DC power transmission system. Compared with traditional single-circuit DC power transmission systems, its coupling characteristics are more complex, the fault types are more diverse, and there is currently no effective decoupling method, which brings great challenges to fault identification and line selection.

[0003] Currently, in AC systems, phase-mode transformations such as Clarke transformation, Karenbauer transformation, and Wedpohl transformation are usually used to achieve phase decoupling. Similarly, there is also a pole-mode transformation in DC systems to achieve pole-line decoupling. For a same-tower multi-circuit DC power transmission system, loop decoupling needs to be performed first, and then phase decoupling. Typical solutions include the six-sequence component method for a same-tower double-circuit power transmission system and the twelve-sequence component method for a same-tower four-circuit power transmission line. However, the above methods cannot achieve complete decoupling of a same-tower three-circuit power transmission system. In addition, there are also methods that form an impedance matrix by measuring the self-impedance and mutual-impedance between each line and achieve decoupling based on the principle of matrix diagonalization. However, such methods have a large amount of calculation and poor generality. Therefore, there is currently a lack of quantitative analysis of the coupling characteristics of a same-tower three-circuit DC power transmission system and corresponding decoupling methods. Summary of the Invention

[0004] To achieve complete decoupling of each line in the same-tower double-circuit alternating current to direct current conversion system, the present invention provides a multi-phase-mode transformation decoupling method for the same-tower double-circuit alternating current to direct current conversion system. This method proposes a new loop decoupling matrix, constructs a fault loop discrimination matrix based on this loop decoupling matrix, and uses this fault loop discrimination matrix to decouple the fault characteristic quantities to obtain the characteristic modulus reflecting the faults of each loop. For pole-to-pole decoupling, a new pole-to-pole decoupling matrix with an adaptable transformation scale is constructed using the Clarke phase-mode transformation matrix, and after a fault, a phase-mode transformation matrix of the same scale is adaptively selected according to the number of fault loops to achieve line decoupling. This method lays a theoretical foundation for subsequent fault characteristic analysis and has the advantages of excellent decoupling effect and small computational complexity.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A multi-phase-mode transformation decoupling method for the same-tower double-circuit alternating current to direct current conversion system, comprising the following steps:

[0007] Step 1: Analyze the coupling characteristics of the pole lines in the same-tower double-circuit alternating current to direct current conversion system and construct an impedance matrix;

[0008] Step 2: Construct a loop decoupling matrix to achieve decoupling between the loops of the same-tower double-circuit alternating current to direct current conversion system, and based on this, propose a fault loop discrimination matrix;

[0009] Step 3: Construct a pole-to-pole decoupling matrix to achieve decoupling between the pole lines of the same-tower double-circuit alternating current to direct current conversion system;

[0010] Step 4: For fault analysis, based on the mutation of the modulus obtained from the fault loop discrimination matrix, determine the number of fault loops, and adaptively select a pole-to-pole decoupling matrix of the same scale to achieve decoupling.

[0011] In the said Step 1, the constructed impedance matrix Z is:

[0012]

[0013] In formula (1): z s1 is the self-impedance of each pole line of Circuit I and Circuit II; z n1 is the mutual impedance between two pole lines in Circuit I or Circuit II; z s2 is the self-impedance of each pole line of Circuit III; z n2 is the mutual impedance between two pole lines in Circuit III; z p2 is the mutual impedance between each pole line between Circuit I and Circuit II; z p1 is the mutual impedance between each pole line between Circuit I and Circuit III and between Circuit II and Circuit III. In the said Step 2, a loop decoupling matrix P1 is proposed, and its decoupling effect is as follows:

[0014]

[0015]

[0016] Introduce elementary matrices for correction to obtain:

[0017]

[0018] In formula (4): E 12 , E 34 , E 56 are elementary matrices. Left multiplying them by the matrix respectively means adding the 1st, 3rd, and 5th rows of the matrix to the 2nd, 4th, and 6th rows respectively;

[0019] The corrected loop decoupling matrix is:

[0020] P2 = (E 12 E 34 E 56 )P1(5);

[0021] After using the loop decoupling matrix P2 to achieve decoupling of each loop, use the Clarke phase-mode transformation matrix P c to further perform decoupling between polar lines, as shown in formula (6):

[0022]

[0023] In formula (6): P c is the Clarke phase-mode transformation matrix; O is the zero matrix.

[0024] Finally, the fault loop discrimination matrix obtained is:

[0025]

[0026] In step 3, to achieve decoupling between poles, introduce the decoupling matrix P3:

[0027]

[0028] Take the inter-pole decoupling matrix P5 = P3 to decouple the impedance matrix Z, as shown in formulas (9) and (10) respectively:

[0029]

[0030] The obtained decoupling matrices are completely decoupled in odd rows (or columns) and even rows (or columns). Therefore, P4 and P5 can be used for decoupling respectively, and the moduli obtained by complete decoupling are extracted to form the modulus matrix. Taking the voltage mutation as an example for analysis, as shown in formulas (14) - (16).

[0031] In step 4, when analyzing the fault characteristics of the double-circuit on the same tower to straight-line conversion system, first use the loop decoupling matrix P2 for decoupling, and determine the number of faulted lines according to the mutation of the characteristic modulus.

[0032] Taking the decoupling of the voltage mutation amount during the fault of three lines as an example, as shown in Equation (11):

[0033]

[0034] In Equation (11): u x (t) is the voltage mutation amount before and after the fault at the beginning or end of the pole line. The subscripts 1P, 1N, 2P, 2N, 3P, and 3N are the positive and negative poles of the first, second, and third lines respectively; u1(t), u3(t), and u5(t) are the moduli reflecting whether there are faults in the first, second, and third lines respectively.

[0035] The number of faulted circuits can be determined by the mutation conditions of △u1(t), △u3(t), and △u5(t); specifically as follows:

[0036] It can be seen from Equation (11) that after the fault occurs, the moduli u1(t), u3(t), and u5(t) obtained through the fault circuit discrimination matrix respectively reflect the faults of the first, second, and third lines. As Figures 4(a) to 4(i) shown, among them, Figures 4(a) and 4(b) represent the fault of the first line. After the fault occurs, u1(t) has a large mutation, and the moduli corresponding to the non-faulted circuits are clamped near zero; Figures 4(c) and 4(d) represent the simultaneous faults of the first and second lines. After the fault occurs, u1(t) and u3(t) have large mutations, and the moduli corresponding to the non-faulted circuits are clamped near zero; Figures 4(e) and 4(f) represent the simultaneous faults of the first, second, and third lines. After the fault occurs, u1(t), u3(t), and u5(t) have large mutations. Therefore, the faulted circuit can be determined by the mutation conditions of △u1(t), △u3(t), and △u5(t), and the following fault startup criterion is proposed:

[0037]

[0038] In the above formula: △u act is the fault startup value.

[0039] In step 4, use the decoupling matrix of the same scale to decouple between the pole lines. For example:

[0040] When only one line is faulty, take:

[0041]

[0042] When two lines are faulty, take:

[0043]

[0044] When a three - circuit line fault occurs, take P5 = P3. Taking the three - circuit line fault as an example, the decoupling through P4 and P5 is as shown in Equations (14) and (15):

[0045]

[0046] In the formula: ux′(t) is the voltage modulus obtained after decoupling through P4, and the subscripts 1, 2, 3, 4, 5, 6 are the modulus labels respectively; ux″(t) is the voltage modulus obtained after decoupling through P5, and the subscripts 1, 2, 3, 4, 5, 6 are the modulus labels respectively.

[0047] Take the completely decoupled moduli in Equations (14) and (15) to form the final modulus matrix as:

[0048] u = [u1″(t) u2′(t) u3″(t) u4′(t) u5″(t) u6′(t)] (16);

[0049] In Equation (16): u1″(t), u3″(t), u5″(t) respectively represent the voltage moduli obtained after decoupling through P5, and the subscripts represent the modulus numbers. The technical effects of the multi - phase - mode transformation decoupling method for the same - tower double - circuit AC - to - DC conversion system of the present invention are as follows:

[0050] 1) The present invention makes a qualitative analysis of the complex coupling characteristics of the same - tower double - circuit AC - to - DC conversion system for the first time and proposes an effective decoupling scheme, overcoming the problem of the reduced adaptability of the traditional decoupling scheme.

[0051] 2) Compared with the decoupling scheme based on the matrix diagonalization principle, the present invention has simple calculation, strong generality and significant application value.

[0052] 3) The novel inter - pole decoupling matrix and loop decoupling matrix proposed by the present invention cooperate with each other. The proposed novel inter - phase decoupling matrix can identify the faulty loop, and then only perform inter - pole decoupling on the faulty loop, significantly reducing the subsequent calculation amount. Moreover, the proposed decoupling scheme can effectively eliminate the coupling effect between poles, laying a theoretical foundation for subsequent research. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The present invention will be further described below in conjunction with the drawings and examples;

[0054] Figure 1 It is a schematic diagram of the pole - line layout method of the same - tower double - circuit AC - to - DC conversion system.

[0055] Figure 2 It is a flow chart of the fault decoupling of the same - tower double - circuit AC - to - DC conversion system.

[0056] Figure 3It is a model diagram of the double-circuit same-tower AC-to-DC conversion system.

[0057] Figure 4(a) shows the fault loop identification result of a single-circuit single-pole fault (1P);

[0058] Figure 4(b) shows the fault loop identification result of a single-circuit two-pole fault (1P-1N);

[0059] Figure 4(c) shows the fault loop identification result of a two-circuit two-pole fault (1P-2N);

[0060] Figure 4(d) shows the fault loop identification result of a two-circuit three-pole fault (1P-1N-2P);

[0061] Figure 4(e) shows the fault loop identification result of a two-circuit four-pole fault (1P-1N-2P-2N);

[0062] Figure 4(f) shows the fault loop identification result of a three-circuit three-pole fault (1P-2N-3P);

[0063] Figure 4(g) shows the fault loop identification result of a three-circuit four-pole fault (1P-1N-2P-3N);

[0064] Figure 4(h) shows the fault loop identification result of a three-circuit five-pole fault (1P-1N-2P-2N-3P);

[0065] Figure 4(i) shows the fault loop identification result of a three-circuit six-pole fault (1P-1N-2P-2N-3P-3N).

[0066] Figure 5(a) shows the decoupling result of the fault line of a single-circuit two-pole fault (1P-1N);

[0067] Figure 5(b) shows the decoupling result of the fault line of a two-circuit four-pole fault (1P-1N-2P-2N);

[0068] Figure 5(c) shows the decoupling result of the fault line of a three-circuit six-pole fault (1P-1N-2P-2N-3P-3N). Specific implementation manner

[0069] A multi-phase mode transformation decoupling method for the double-circuit same-tower AC-to-DC conversion system analyzes the coupling characteristics of the pole lines in the double-circuit same-tower AC-to-DC conversion system and constructs an impedance matrix. A new loop decoupling matrix is constructed to achieve decoupling between the loops of the double-circuit same-tower AC-to-DC conversion system. Based on this, a fault loop discrimination matrix is proposed. A new inter-pole decoupling matrix is constructed to achieve decoupling of each pole in the loop. The loop decoupling matrix and the inter-pole decoupling matrix are fused to construct a decoupling matrix applicable to the double-circuit same-tower AC-to-DC conversion system, and a corresponding decoupling scheme is proposed. Specifically, it includes the following steps:

[0070] Step 1: As Figure 1As shown in the figure, the AC-to-DC conversion system with double circuits on the same tower converts the six AC conductors in the original double-circuit line on the same tower into three bipolar DC systems, and each bipolar system uses two conductors. Since the coupling relationship between the six lines is complex, if the self-impedance and mutual impedance of each line are considered separately, the impedance matrix formed cannot be decoupled. Therefore, each circuit is regarded as a whole. Assuming that the mutual impedance between each circuit remains unchanged and considering the influence of floor height and symmetrical arrangement of pole lines, the impedance matrix is constructed as follows:

[0071]

[0072] In the formula: z s1 is the self-impedance of each pole line of Circuit I and Circuit II; z n1 is the mutual impedance between the two pole lines in Circuit I or Circuit II; z s2 is the self-impedance of each pole line of Circuit III; z n2 is the mutual impedance between the two pole lines in Circuit III; z p2 is the mutual impedance between each pole line between Circuit I and Circuit II; z p1 is the mutual impedance between each pole line between Circuit I and Circuit III and between Circuit II and Circuit III.

[0073] Step 2: Since the existing loop decoupling matrix cannot decouple the AC-to-DC conversion system with double circuits on the same tower, a new loop decoupling matrix P1 is proposed, and its decoupling effect is as follows:

[0074]

[0075] As shown in Equation (3), the impedance matrix corresponding to each circuit in the decoupled matrix is an asymmetric matrix, which makes it difficult to decouple the subsequent pole lines. Considering the characteristics of the decoupled impedance matrix, elementary matrices are introduced for correction, and we can obtain:

[0076]

[0077] In the formula: E 12 , E 34 , E 56 are elementary matrices. Left-multiplying the matrix by them respectively means adding the first, third, and fifth rows of the matrix to the second, fourth, and sixth rows respectively.

[0078] The corrected loop decoupling matrix is:

[0079] P2 = (E 12 E 34 E 56 )P1 (5);

[0080] After using the P2 matrix to achieve the decoupling of each loop, the Clarke phase-mode transformation matrix P c is further used for decoupling between the pole lines, as shown in Equation (6).

[0081]

[0082] Where: P c is the Clarke phase-mode transformation matrix; O is the zero matrix.

[0083] As shown in Equation (6), although the P2 matrix realizes the loop decoupling of the same-tower double-circuit AC-to-DC conversion system, when further decoupling between pole lines using the Clarke phase-mode transformation matrix P c there will be a situation of missing moduli, that is, one modulus in each circuit is constantly 0. However, the P2 matrix can identify the fault loop, which is beneficial to reducing the workload of decoupling. Finally, the fault loop discrimination matrix can be obtained as:

[0084]

[0085] Step 3: To achieve decoupling between poles, introduce the decoupling matrix P3:

[0086]

[0087] Take the inter-pole decoupling matrix P5 = P3 to decouple Z, as shown in Equations (9) and (10) respectively.

[0088]

[0089] The obtained decoupling matrices are completely decoupled in odd rows (or columns) and even rows (or columns) respectively. Therefore, P4 and P5 can be used for decoupling respectively, and the moduli obtained by complete decoupling are extracted to form the modulus matrix.

[0090] Step 4: When analyzing the fault characteristics of the same-tower double-circuit AC-to-DC conversion system, first use the P2 matrix for decoupling, and determine the number of fault lines according to the mutation of the characteristic moduli. Taking the decoupling of the voltage mutation amount in the case of a three-circuit line fault as an example, as shown in Equation (11).

[0091]

[0092] Where: u x (t) is the voltage mutation amount at the beginning or end of the pole line before and after the fault, and the subscripts 1P, 1N, 2P, 2N, 3P, 3N are the positive and negative poles of the first, second, and third circuits respectively; u1(t), u3(t), u5(t) are the moduli reflecting whether there are faults in the first, second, and third circuits respectively.

[0093] The number of fault loops can be determined through the mutation conditions of △u1(t), △u3(t), △u5(t), and then use the decoupling matrix of the same scale for decoupling between pole lines. For example, when only one circuit has a fault, take:

[0094]

[0095] When there are two-circuit line faults, take:

[0096]

[0097] When there are three-circuit line faults, take P5 = P3. Taking the three-circuit line fault as an example, decoupling through P4 and P5 is as shown in Equations (14) and (15).

[0098]

[0099]

[0100] Where: ux′(t) is the voltage modulus obtained after decoupling through P4, and the subscripts 1, 2, 3, 4, 5, and 6 are the modulus labels respectively; ux″(t) is the voltage modulus obtained after decoupling through P5, and the subscripts 1, 2, 3, 4, 5, and 6 are the modulus labels respectively.

[0101] Take the fully decoupled moduli in Equations (14) and (15) to form the final modulus matrix as:

[0102] u = [u1″(t) u2′(t) u3″(t) u4′(t) u5″(t) u6′(t)] (16);

[0103] The specific process of the decoupling method proposed by the present invention is as Figure 2 shown.

[0104] Verification example:

[0105] Carry out the verification of the decoupling scheme for the same-tower double-circuit AC-to-DC conversion system. Build the same-tower double-circuit AC-to-DC conversion system model as Figure 3 shown in the power system simulation software. The voltage levels of the three DC systems are all 200 kV, and the transmission capacity is 1.2 million kilowatts.

[0106] 1). Fault loop identification verification:

[0107] To verify the effect of the fault loop discrimination matrix proposed by the present invention, take single-circuit single-pole fault (1P), single-circuit two-pole fault (1P-1N), two-circuit two-pole fault (1P-2N), two-circuit three-pole fault (1P-1N-2P), two-circuit four-pole fault (1P-1N-2P-2N), three-circuit three-pole fault (1P-2N-3P), three-circuit four-pole fault (1P-1N-2P-3N), three-circuit five-pole fault (1P-1N-2P-2N-3P), three-circuit six-pole fault (1P-1N-2P-2N-3P-3N) as examples for verification, and normalize the obtained data. Take the fault identification threshold u set= 0.1. When it exceeds this value, a circuit fault is determined. This covers all fault types of the double-circuit on the same tower conversion to a straight-line system. Due to the large number of fault types, the fault pole in the brackets is taken as an example for verification, and the results are as Figures 4(a) to 4(i) shown.

[0108] As Figures 4(a) to 4(i) shown, for the faults of one-circuit, two-circuit, and three-circuit lines, the decoupling is carried out using the fault circuit discrimination matrix proposed in the present invention, and the obtained u1(t), u3(t), and u5(t) can reflect the faults of Circuit I, Circuit II, and Circuit III. For example, when a single-circuit single-pole fault (1P) occurs, that is, a fault occurs in Circuit I, the corresponding modulus u1(t) changes suddenly and exceeds the threshold, while u3(t) and u5(t) reflecting the faults of Circuit II and Circuit III always remain near 0 and do not exceed the threshold, so it is determined that there is a fault in Circuit I; when a two-circuit three-pole fault (1P-1N-2P) occurs, that is, faults occur in Circuit I and Circuit II, the corresponding moduli u1(t) and u3(t) change suddenly and exceed the threshold, while u5(t) reflecting the fault of Circuit III always remains near 0 and does not exceed the threshold, so it is determined that there are faults in Circuit I and Circuit II; when a three-circuit three-pole fault (1P-2N-3P) occurs, that is, faults occur in Circuit I, Circuit II, and Circuit III, the corresponding moduli u1(t), u3(t), and u5(t) all change suddenly and exceed the threshold, so it is determined that there are faults in Circuit I, Circuit II, and Circuit III. It can be seen that the fault circuit discrimination matrix proposed in the present invention can effectively identify the circuit where the fault pole is located and reduce the calculation amount of subsequent decoupling.

[0109] 2). Verification of fault pole decoupling:

[0110] To verify the effect of the decoupling method proposed in the present invention, a single-circuit two-pole fault (1P-1N), a two-circuit four-pole fault (1P-1N-2P-2N), and a three-circuit six-pole fault (1P-1N-2P-2N-3P-3N) are taken as examples for verification.

[0111] When a grounding fault occurs on the DC pole line, the voltage mutation amount is u x (t). If it is a positive pole grounding fault, the fault voltage decreases from a positive value, then ux(t) < 0, and it can be further expressed as ux(t) = -u F , where u F represents the additional power supply, and u F > 0; similarly, if it is a negative pole grounding fault, then u x (t) = u F . The voltage mutation amounts on the fault pole lines will all couple out voltage mutation amounts on the remaining non-fault pole lines, and their magnitudes are determined by the coupling coefficients between the pole lines. Referring to the operation of the double-circuit on the same tower line, the coupling coefficients between different pole lines are between 0.16 and 0.26.

[0112] Considering that the difference in coupling coefficients does not affect the analysis of fault characteristics, it is approximately considered that the coupling amounts of the non-fault poles are equal, denoted as ku x (t), 0 < k < 1. Therefore, the voltage mutation amounts corresponding to the three fault types are [-u F u F , [-u F u F -u F u F , [-u F u F -u F u F -u F u F . Here, considering that after the fault loop is identified, decoupling is performed using a decoupling matrix of the same scale and substituting into Eqs. (12) to (15), we can obtain

[00] ,

[0000] , [000000]. There is no coupling relationship between the components, and the corresponding simulation results are as shown in Figures 5(a) to 5(c) Figure. It can be seen that each modulus is close to 0, which is consistent with the theory. Therefore, the method proposed in the present invention can achieve complete decoupling of the same-tower double-circuit crossover-to-straight system.

Claims

1. A decoupling method for multiple phase mode transformation of a same-tower double-circuit conversion from crossover to straight configuration system, characterized in that It includes the following steps: Step 1: Analyze the coupling characteristics of the pole lines in the same-tower double-circuit AC-to-DC conversion system, and construct an impedance matrix; Step 2: Construct a loop decoupling matrix to achieve decoupling between the loops of the same-tower double-circuit AC-to-DC conversion system, and based on this, propose a fault loop discrimination matrix; Step 3: Construct an inter-pole decoupling matrix to achieve decoupling between the pole lines of the same-tower double-circuit AC-to-DC conversion system; Step 4: Based on the mutation of the moduli obtained from the fault loop discrimination matrix, determine the number of fault loops, and adaptively select an inter-pole decoupling matrix of the same scale to achieve decoupling.

2. The method for decoupling multiple phase-mode transformation of a same-tower double-circuit retrofit and straightening system according to claim 1, characterized in that: In the said Step 1, the constructed impedance matrix Z is: In Equation (1): z s1 is the self-impedance of each pole line of the first and second circuits; z n1 is the mutual impedance between two pole lines in the first or second circuit; z s2 is the self-impedance of each pole line of the third circuit; z n2 is the mutual impedance between two pole lines in the third circuit; z p2 is the mutual impedance between each pole line between the first and second circuits; z p1 is the mutual impedance between each pole line between the first and third circuits and between the second and third circuits.

3. The multiple phase-mode transformation decoupling method for the same-tower double-circuit retrofit and straightening system according to claim 2, characterized in that: In the said Step 2, a loop decoupling matrix P1 is proposed, and its decoupling effect is as follows: Introduce an elementary matrix for correction to obtain: In formula (4): E 12 , E 34 , E 56 are elementary matrices. Premultiplying the matrix by them respectively means adding the 1st, 3rd, and 5th rows of the matrix to the 2nd, 4th, and 6th rows respectively; The corrected loop decoupling matrix is: P2 = (E 12 E 34 E 56 )P1(5).

4. A decoupling method for multiple phase-mode conversion of a same-tower double-circuit retrofit and straightening system according to claim 3, characterized in that: After decoupling each loop by using the loop decoupling matrix P2, the Clarke phase-mode transformation matrix P is used c Further decoupling between the polar lines is performed as shown in Equation (6): In Equation (6): P c is the Clarke phase transformation matrix; O is the zero matrix; The finally obtained fault loop discrimination matrix is:

5. A decoupling method for multiple phase-mode transformation of a same-tower double-circuit retrofit to straight system according to claim 4, characterized in that: In the said Step 3, to achieve inter-pole decoupling, an inter-pole decoupling matrix P3 is introduced: Obtain the inter-electrode decoupling matrix P5 = P3 decouples the impedance matrix Z, as shown in Equations (9) and (10) respectively: The obtained decoupling matrices are completely decoupled in the odd rows or columns, and in the even rows or columns respectively. Therefore, P4 and P5 can be used for decoupling respectively.

6. A decoupling method for multiple phase-mode conversion of a same-tower double-circuit retrofit and straightening system according to claim 5, characterized in that: In the said Step 4, when analyzing the fault characteristics of the same-tower double-circuit AC-to-DC conversion system, first use the loop decoupling matrix P2 for decoupling, and determine the number of fault lines according to the mutation of the characteristic moduli.

7. A decoupling method for multiple phase-mode transformation of a same-tower double-circuit retrofit-to-straight system according to claim 6, characterized in that: When a three-circuit line fails, decouple the voltage mutation quantity, specifically as shown in Equation (11): In formula (11): u x (t) is the voltage mutation before and after the fault at the beginning or end of the polar line. The subscripts 1P, 1N, 2P, 2N, and 3N are the positive and negative poles of the first, second, and third circuits respectively; u1(t), u3(t), and u5(t) are the moduli reflecting whether there are faults in the first, second, and third circuits respectively.

8. A decoupling method for multi-phase mode transformation of a same-tower double-circuit conversion from cross-arm to vertical configuration system according to claim 7, characterized in that: The number of fault loops can be determined through the mutation of △u1(t), △u3(t), and △u5(t); specifically as follows: As can be seen from Equation (11), after a fault occurs, the moduli u1(t), u3(t), and u5(t) obtained through the fault loop discrimination matrix respectively reflect the faults of the first, second, and third circuit lines; the fault loop can be determined through the mutation of △u1(t), △u3(t), and △u5(t), and the following fault starting criterion is proposed: In the above formula: △u act is the fault starting value.

9. The decoupling method of multiple phase-mode conversion for the same-tower double-circuit retrofit and straightening system according to claim 8, characterized in that: In Step 4, use a decoupling matrix of the same scale to perform decoupling between the pole lines, including: When only one circuit line fails, take: When two circuit lines fail, take: When a three-circuit line fails, take P5 = P3. Taking the three-circuit line failure as an example, decoupling through P4 and P5 is shown in Equations (14) and (15) as follows: In the formula: ux′(t) is the voltage modulus obtained after decoupling by P4, and the subscripts 1, 2, 3, 4, 5, and 6 are the modulus labels respectively; ux″(t) is the voltage modulus obtained after decoupling by P5, and the subscripts 1, 2, 3, 4, 5, and 6 are the modulus labels respectively.

10. A decoupling method for multiple phase mode transformation of a same-tower double-circuit conversion from crossover to vertical system according to claim 9, characterized in that: The finally formed modulus matrix composed of the completely decoupled moduli in Equation (14) and Equation (15) is: u = [u1″(t) u2′(t) u3″(t) u4′(t) u5″(t) u6′(t)] (16); In Equation (16): u1″(t), u3″(t), and u5″(t) respectively represent the voltage moduli obtained after decoupling by P5, and the subscripts represent the modulus numbers.

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