A method for calculating power frequency side fault voltage based on iterative calculation

Abstract

The application discloses a M3C power frequency side fault voltage calculation method based on iterative calculation, and relates to the establishment of the relationship between the output positive and negative sequence currents and the output positive and negative sequence voltages at the M3C converter power frequency side port in the single-phase grounding fault composite sequence fault type during the asymmetric fault of the flexible low-frequency system; the admittance matrix Y of the flexible low-frequency system is obtained according to the single-phase grounding fault composite sequence fault type and the impedance parameters of the flexible low-frequency system; the node voltage equation under the single-phase grounding fault composite sequence fault type is constructed; the iterative node voltage equation and the correction equation are obtained. Compared with the prior art, the application realizes the calculation method of the fault point voltage and the converter feeding current after the fault, and improves the adaptability of the fault protection.

Description

Technical Field

[0001] This invention relates to the fields of new energy grid connection and flexible low-frequency power transmission, and proposes a method for calculating the fault point voltage and current applicable to asymmetrical faults on the M3C power frequency side. Background Technology

[0002] With the rapid development of power electronic switching and control modulation technology, the Modular Multilevel Matrix Converter (M3C) has emerged. Its advantages, such as low output harmonic content, wide conversion frequency range, and high withstand voltage rating, provide strong technical support for the development of flexible low-frequency power transmission systems. Low-frequency AC power transmission technology effectively reduces circuit transmission losses and charging capacitance by lowering the frequency of electrical energy, and is also suitable for direct connection of low-speed renewable energy wind turbines. In traditional power systems, the system power supply can be represented by a simple Thevenin equivalent circuit. When a grid fault occurs, fault analysis is performed using nodal equations. However, the M3C converter, under asymmetrical fault conditions, is modeled as a voltage-controlled current source. The coupling relationship between its positive and negative sequence currents and the positive and negative sequence voltages at the ports is complex, making it difficult to calculate using traditional nodal voltage methods.

[0003] In research on voltage equalization strategies for M3C converters during asymmetrical faults, the paper "Frequency Division and Hierarchical Control of Modular Multilevel Matrix Converters for Flexible Low-Frequency Transmission" proposes a method of arm voltage hierarchical control. This method ensures that the arm voltages of the M3C converter remain consistent during asymmetrical faults, achieving good metastable state characteristics while preventing excessive voltage in some arms from damaging power electronic devices. However, this scheme introduces negative sequence current to improve power flow between arms, and the positive and negative sequence equivalent loops of the converter are interconnected and coupled, making the calculation of the fault point voltage and converter feed-out current after the fault difficult, and also posing a challenge to the adaptability of existing protection systems. Summary of the Invention

[0004] For flexible low-frequency transmission systems, this invention proposes a method for calculating the fault point voltage on the M3C power frequency side based on iterative calculation. Based on iterative calculation and analysis of faults, the fault point voltage is accurately calculated.

[0005] This invention is achieved using the following technical solution: A method for calculating the power frequency side fault voltage of an M3C based on iterative calculation, comprising: Step 1: During an asymmetrical fault in the flexible low-frequency system, establish the relationship between the output positive and negative sequence currents and output positive and negative sequence voltages at the power frequency side port of the M3C converter for the fault type of single-phase ground fault composite sequence, as shown in the following formula:

[0006] In the formula, U 1 represents the positive sequence voltage amplitude at the power frequency side port of the converter. U 2 represents the amplitude of the negative sequence output voltage at the power frequency side port of the converter. I 1. Output positive sequence current amplitude at the power frequency side port of the converter. I 2. Amplitude of the output negative sequence current at the power frequency side port of the converter. Ψ 1 represents the output positive sequence voltage phase at the power frequency side port of the converter. Ψ 2 represents the output negative sequence voltage phase at the power frequency side port of the converter. φ 1 represents the output positive sequence current phase at the power frequency side port of the converter. φ 2 represents the output negative sequence current phase at the power frequency side port of the converter; Establish the positive sequence voltage amplitude of the M3C converter's power frequency side output. U 1 and phase Positive sequence voltage at the fault point The relationship is shown in the following formula:

[0007]

[0008] In the formula, This is the positive sequence impedance from the fault point to the converter outlet. The positive sequence impedance from the fault point to the converter outlet. The phase; Establish the negative sequence voltage amplitude of the M3C converter's power frequency side output. U 2 and phase With the positive and negative sequence voltages at the fault point , The relationship is shown in the following formula:

[0009]

[0010] In the formula, This is the negative sequence impedance from the fault point to the converter outlet. The phase of the negative sequence impedance from the fault point to the converter outlet; Establish the positive sequence current output of the M3C converter Negative sequence current The expression is shown in the following formula: ; Step 2: Obtain the admittance matrix of the flexible low-frequency system based on the fault type of the single-phase ground fault composite sequence and the impedance parameters of the flexible low-frequency system. As shown in the following formula:

[0011] In the formula, Z s1 The positive-sequence impedance from the fault point to the system. Z s2 Z is the negative sequence impedance from the fault point to the system. s0 Z is the zero-sequence impedance from the fault point to the system. m1 Z is the positive sequence impedance from the fault point to the converter outlet. m2 Z represents the negative sequence impedance from the fault point to the converter outlet. m0 The zero-sequence impedance is from the fault point to the converter outlet. Step 3: Construct the node voltage equations under the fault type of single-phase ground fault composite sequence, as shown below:

[0012] In the formula: , For the voltage of an independent node, For power frequency system voltage, , These are the positive and negative sequence currents output by the M3C converter; Step 4, Set the number of iterations k Based on the node voltage equation in step 3, the first... k The nodal voltage equation for the next iteration is shown below:

[0013] In the formula, k This represents the number of iterations. The solution to the node voltage equation yields the first... k The node voltage value obtained in the next iteration; Step 5, after k The iterative processing from step 1 to step 4 yields the following corrected equation: No. k The relationship between the positive, negative, and zero-sequence voltages at the fault point and the node voltages obtained from the next iteration is shown in the following equation:

[0014] k The positive and negative sequence currents of the M3C converter output obtained from the next iteration are shown in the following formula: ; Step 6: Exit the iteration; the process ends.

[0015] Compared with the prior art, the present invention can achieve the following beneficial technical effects: 1) A method for calculating the fault point voltage and converter feed-out current after a fault has been implemented; 2) Improved the adaptability of fault protection; 3) It lays the theoretical foundation for the development of line fault analysis methods and new protection principles for flexible low-frequency transmission systems. Attached Figure Description

[0016] Figure 1 A schematic diagram of an existing flexible low-frequency system and its faults; Figure 2 A flowchart of a method for calculating the power frequency side fault voltage of an M3C based on iterative calculation; Figure 3 This is a single-phase grounding composite sequence network diagram. Detailed Implementation

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] like Figure 1 As shown, the existing flexible low-frequency system consists of a low-frequency wind farm, low-frequency lines, an M3C converter, power frequency lines, a 35:17 transformer, a 17:35 transformer, and a power frequency system power supply. The 35:17 and 17:35 transformers are step-up and step-down transformers, respectively. 35kV represents the bus voltage level in this circuit.

[0019] Asymmetrical fault types include single-phase ground fault, two-phase short-circuit fault, and two-phase short-circuit ground fault. This invention provides a method for calculating the fault voltage and current on the M3C power frequency side based on iterative calculation, and takes a single-phase ground fault as an example to analyze the calculation steps of the fault point voltage.

[0020] like Figure 2 As shown, the present invention provides a method for calculating the fault voltage on the power frequency side of an M3C based on iterative calculation. The specific process includes the following steps: Step 1: During an asymmetrical fault in the flexible low-frequency system, establish the relationship between the fault point voltage and the converter output current for the fault type of a single-phase ground fault composite sequence. Specifically, based on the inter-module uniform control strategy, a negative sequence current is introduced during an asymmetrical fault in the flexible low-frequency system to ensure that the average capacitor voltage of the three sub-converters (u, v, w) on the power frequency side is equal under asymmetrical fault conditions. Establish the relationship between the output positive and negative sequence currents and output positive and negative sequence voltages at the power frequency side port of the M3C converter under asymmetrical fault conditions, as shown in the following equation: (1) In the formula, U 1 represents the positive sequence voltage amplitude at the power frequency side port of the converter. U 2 represents the amplitude of the negative sequence output voltage at the power frequency side port of the converter. I1. Output positive sequence current amplitude at the power frequency side port of the converter. I 2. Amplitude of the output negative sequence current at the power frequency side port of the converter. Ψ 1 represents the output positive sequence voltage phase at the power frequency side port of the converter. Ψ 2 represents the output negative sequence voltage phase at the power frequency side port of the converter. φ 1 represents the output positive sequence current phase at the power frequency side port of the converter. φ 2 represents the output negative sequence current phase at the power frequency side port of the converter; According to Kirchhoff's laws, the positive sequence voltage amplitude output from the power frequency side of the M3C converter... U 1 and phase Positive sequence voltage at the fault point The relationship is shown in the following formula: (2) (3) In the formula, This is the positive sequence impedance from the fault point to the converter outlet. The positive sequence impedance from the fault point to the converter outlet. The phase; Similarly, the negative sequence voltage amplitude output from the power frequency side of the M3C converter... U 2 and phase With the positive and negative sequence voltages at the fault point , The relationship is shown in the following formula: (4) (5) In the formula, This is the negative sequence impedance from the fault point to the converter outlet. The phase of the negative sequence impedance from the fault point to the converter outlet; In summary, the positive sequence current output by the M3C converter... The negative sequence current is represented by the positive sequence voltage at the fault point. The fault point is represented by positive and negative sequence voltages. For convenience, the positive and negative sequence currents output by the M3C converter are represented by functions as follows: (6) Step 2, calculate the system admittance matrix, as follows: Figure 3 Taking the single-phase ground fault composite sequence network diagram shown as an example, the admittance matrix of the system is calculated based on the fault type of the single-phase ground fault composite sequence and the impedance parameters of the flexible low-frequency system. As shown in the following formula: (7) In the formula,Z s1 The positive-sequence impedance from the fault point to the system. Z s2 Z is the negative sequence impedance from the fault point to the system. s0 Z is the zero-sequence impedance from the fault point to the system. m1 Z is the positive sequence impedance from the fault point to the converter outlet. m2 Z represents the negative sequence impedance from the fault point to the converter outlet. m0 The zero-sequence impedance is from the fault point to the converter outlet. Step 3: Construct the node voltage equations under the fault type of single-phase ground fault composite sequence, so as to... Figure 3 Taking the single-phase ground fault composite sequence network diagram shown below as an example, the node voltage equations are as follows: (8) In the formula: , For the voltage of an independent node, For power frequency system voltage, , These are the positive and negative sequence currents output by the M3C converter; Step 4, Set the number of iterations k Based on the node voltage equation in step 3, the first... k The nodal voltage equation for the next iteration is shown below: (9) In the formula, k This represents the number of iterations. Initial value of positive sequence current iteration at the output of M3C converter Set to the rated value during normal operation, with its phase matching the system voltage, and the initial value for the iteration of the negative sequence current. Set to 0; The solution to the equation yields the first... k The node voltage value obtained in the next iteration; Step 5, after k The iterative processing from step 1 to step 4 yields the corrected equation; No. k The relationship between the positive, negative, and zero-sequence voltages at the fault point and the node voltages in the next iteration is shown in the following equation: (10) The corrected equation is shown in equation (11): (11) In the above corrected equation, the converter output current is calculated using the fault point voltage; Step 6: Determine whether to exit the iteration, and set the... k The node voltages obtained from the iteration are respectively compared with the...k The node voltages obtained from the -1st iteration are subtracted. ,when Medium amplitude If the maximum value is not greater than the given value, the result can be considered to meet the calculation accuracy, and the iteration will be terminated; otherwise, the iteration will return to step (4) and continue. The condition for exiting the iteration is shown in the following equation: (12) In the formula: This represents the maximum permissible error.

Claims

1. A method for calculating the power frequency side fault voltage of an M3C based on iterative calculation, characterized in that, include: Step 1: During an asymmetrical fault in the flexible low-frequency system, establish the relationship between the output positive and negative sequence currents and output positive and negative sequence voltages at the power frequency side port of the M3C converter for the fault type of single-phase ground fault composite sequence, as shown in the following formula: In the formula, U 1 represents the positive sequence voltage amplitude at the power frequency side port of the converter. U 2 represents the amplitude of the negative sequence output voltage at the power frequency side port of the converter. I 1. Output positive sequence current amplitude at the power frequency side port of the converter. I 2. Amplitude of the output negative sequence current at the power frequency side port of the converter. Ψ 1 represents the output positive sequence voltage phase at the power frequency side port of the converter. Ψ 2 represents the output negative sequence voltage phase at the power frequency side port of the converter. φ 1 represents the output positive sequence current phase at the power frequency side port of the converter. φ 2 represents the output negative sequence current phase at the power frequency side port of the converter; Establish the positive sequence voltage amplitude of the M3C converter's power frequency side output. U 1 and phase Positive sequence voltage at the fault point The relationship is shown in the following formula: In the formula, This is the positive sequence impedance from the fault point to the converter outlet. The positive sequence impedance from the fault point to the converter outlet. The phase; Establish the negative sequence voltage amplitude of the M3C converter's power frequency side output. U 2 and phase With the positive and negative sequence voltages at the fault point , The relationship is shown in the following formula: In the formula, This is the negative sequence impedance from the fault point to the converter outlet. The phase of the negative sequence impedance from the fault point to the converter outlet; Establish the first k The positive sequence current output of the M3C converter in the next iteration Negative sequence current The expression is shown below: ； Step 2: Obtain the admittance matrix of the flexible low-frequency system based on the fault type of the single-phase ground fault composite sequence and the impedance parameters of the flexible low-frequency system. As shown in the following formula: In the formula, Z s1 The positive-sequence impedance from the fault point to the system. Z s2 Z is the negative sequence impedance from the fault point to the system. s0 Z is the zero-sequence impedance from the fault point to the system. m1 Z is the positive sequence impedance from the fault point to the converter outlet. m2 Z represents the negative sequence impedance from the fault point to the converter outlet. m0 The zero-sequence impedance is from the fault point to the converter outlet. Step 3: Construct the node voltage equations under the fault type of single-phase ground fault composite sequence, as shown below: In the formula: , For the voltage of an independent node, For power frequency system voltage, , These are the positive and negative sequence currents output by the M3C converter; Step 4, Set the number of iterations k Based on the node voltage equation in step 3, the first... k The nodal voltage equation for the next iteration is shown below: In the formula, k This represents the number of iterations. The solution to the node voltage equation yields the first... k The node voltage value obtained in the next iteration; Step 5, after k The iterative processing from step 1 to step 4 yields the following corrected equation: No. k The relationship between the positive, negative, and zero-sequence voltages at the fault point and the node voltages obtained from the next iteration is shown in the following equation: k The positive and negative sequence currents of the M3C converter output obtained from the next iteration are shown in the following formula: Step 6: Exit the iteration; the process ends. Further steps include: Determine whether to exit the iteration, and set the first iteration to... k The node voltages obtained from the iteration are respectively compared with the... k The node voltages obtained from the -1st iteration are subtracted. ,when Medium amplitude If the maximum value is not greater than the given value, the result can be considered to meet the calculation accuracy, and the iteration will be terminated; otherwise, it will return to step 4 and continue the iteration. The condition for exiting the iteration is shown in the following equation: In the formula, This represents the maximum permissible error.

2. The method for calculating the M3C power frequency side fault voltage based on iterative calculation according to claim 1, characterized in that, Initial value of positive sequence current iteration at the output of M3C converter Set to the rated value during normal operation, and the initial value for the iteration of the negative sequence current. Set it to 0.

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