Fault Analysis Method Based on Time Domain Simplified Model of ACC-HVDC DC Fault Current

Abstract

The present invention belongs to the technical field of high-voltage power transmission and distribution systems, and in particular relates to a fault analysis method based on a simplified time-domain model of an ACC-HVDC direct current fault current, comprising the following steps: S1, establishing a fault equivalent circuit; S2, simplifying the circuit and converting it to the complex frequency domain; S3, performing Thevenin equivalent on the AC portion of the simplified circuit in S2 to obtain a simplified fault model; S4, solving the simplified fault model to obtain a complex-frequency-domain expression circuit for the direct current fault current; S5, ignoring the AC and DC equivalent resistances of the ACC in the complex-frequency-domain expression circuit to obtain a simplified complex-frequency-domain expression for the direct current fault current; and S6, processing the simplified complex-frequency-domain expression for the direct current fault current in conjunction with an inverse Laplace transform to obtain a simplified analytical model of the ACC direct current fault current in a unified time domain across all stages. This method can effectively perform comprehensive transient analysis on a post-fault ACC-HVDC system to achieve quantitative calculation of various fault electrical quantities, thereby improving the reliability and stability of the ACC in the high-voltage power transmission and distribution system.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-voltage power transmission and distribution systems, and in particular relates to a fault analysis method based on a simplified time-domain model of an ACC-HVDC direct current fault current. Background Art

[0002] Actively commutated converters (ACCs), as current source converters with controllable shutdown, were initially used in motor drive applications. With breakthroughs in the technology of the new generation of reverse-resistance integrated gate commutated thyristors (IGCTs), their application in high-voltage power transmission and distribution has gradually gained attention in academia. By utilizing fully controlled devices, ACCs can completely avoid commutation failures and can also power passive networks. Compared to modular multilevel converters, ACCs can control DC voltage polarity reversal and have the ability to clear DC fault currents, eliminating the need for expensive additional current limiting devices, DC circuit breakers, and other equipment.

[0003] After a DC fault occurs in the converter, the transient response of various electrical quantities in the ACC-HVDC system (i.e., a high-voltage direct current transmission system using ACC as its core equipment) differs significantly from the steady-state characteristics during rated operation, making it prone to problems such as equipment overvoltage and overcurrent. Therefore, comprehensive transient analysis of the system after the fault and quantitative calculation of various fault electrical quantities are crucial for optimizing protection schemes and system planning and selection. Although ACC has the ability to control DC voltage polarity reversal and clear DC fault currents, this reduces reliance on traditional, expensive current limiting devices and DC circuit breakers. However, how to quickly and effectively implement these control strategies when a fault occurs, and how these strategies affect system stability, are technical issues that require in-depth research.

[0004] After a fault occurs on the ACC DC side, the transient response of the system's electrical quantities differs significantly from the steady-state characteristics under rated operating conditions, which may lead to problems such as overvoltage and overcurrent in the equipment. Therefore, it is necessary to comprehensively analyze the transient process after the fault and achieve quantitative calculation of each fault electrical quantity in order to optimize the protection scheme and system planning and selection. The presence of commutation capacitors on the ACC AC side causes its transient voltage to produce non-power frequency distortion under the influence of the AC system and bridge arm commutation, resulting in a high order transient model and more complex fault characteristics. Most existing studies have failed to fully consider the bridge arm commutation caused by the control system and the transient characteristics of the filter inductor and commutation capacitor under the action of the AC system, which makes it difficult to accurately describe and mathematically model the ACC DC fault characteristics.

[0005] Therefore, how to effectively conduct a comprehensive transient analysis of the ACC-HVDC system after a fault to achieve quantitative calculation of each fault electrical quantity, thereby improving the reliability and stability of ACC in the high-voltage transmission and distribution system, has become an urgent problem to be solved. Summary of the Invention

[0006] To address the shortcomings of the above-mentioned existing technologies, the present invention provides a fault analysis method based on a simplified time-domain model of ACC-HVDC DC fault current. This method can effectively perform comprehensive transient analysis of the ACC-HVDC system after a fault, thereby achieving quantitative calculation of various fault electrical quantities, thereby improving the reliability and stability of ACC in high-voltage transmission and distribution systems.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The fault analysis method based on the simplified time domain model of the ACC-HVDC DC fault current includes the following steps:

[0009] S1. Establish a fault equivalent circuit for the receiving-end converter of ACC-HVDC;

[0010] S2. Based on the fault equivalent circuit, derive the relationship between the voltage and current of the equivalent circuit, simplify the circuit using the substitution principle and transform it into the complex frequency domain;

[0011] S3. In the complex frequency domain, perform Thevenin equivalent on the AC part of the simplified circuit in S2 to obtain a simplified fault model.

[0012] S4. Solve the simplified fault model after Thevenin equivalent to obtain the complex frequency domain expression circuit of the DC fault current including the commutation characteristic coefficient;

[0013] S5. Ignore the AC and DC equivalent resistance of ACC in the complex frequency domain expression circuit and obtain the simplified complex frequency domain expression of DC fault current;

[0014] S6. The simplified complex frequency domain expression of DC fault current is processed according to the partial fraction method combined with the inverse Laplace transform to obtain a simplified analytical model of the ACC DC fault current in the unified time domain at all stages.

[0015] S7, taking the period from the start of each commutation after the ACC fault occurs to the next commutation as a stage, and obtaining the initial values ​​of the circuit elements in stage n;

[0016] S8. Based on the initial values ​​of the circuit elements in the nth stage, the fault current analytical coefficients and the commutation characteristic coefficients of the simplified analytical model of the unified time domain of the entire stage are updated to obtain the simplified analytical model of the DC fault current in the nth stage in the time domain;

[0017] S9. Use the simplified time-domain analytical model of each stage of the DC fault current to conduct a comprehensive transient analysis of the ACC-HVDC system after the fault, and achieve quantitative calculation of each fault electrical quantity.

[0018] Preferably, in S1, the fault equivalent circuit established includes the branches of the phases corresponding to the upper bridge arm, the lower bridge arm and the non-conducting bridge arm during the commutation period, the capacitor voltage, the inductor current and the equivalent voltage source of the AC system of each branch, the voltage at the DC outlet of the converter, the total equivalent resistance and inductance on the DC side, and the commutation capacitor.

[0019] Preferably, after the Thevenin equivalent is performed in S3, the equivalent impedance Z of the AC part is e and equivalent voltage source U oc The expression of (s) is:

[0020]

[0021] Where, L ac is the AC side inductance, R ac is the equivalent resistance on the AC side, s is a complex variable, ω is the power frequency angular frequency, C f is the commutation capacitor, K1-K5 are the commutation characteristic coefficients of ACC.

[0022] Preferably, the expression of each commutation characteristic coefficient is:

[0023]

[0024] Wherein, the subscripts x and y represent the branches of the phases corresponding to the upper bridge arm and the lower bridge arm respectively during this commutation period; Respectively represent the phase angles of the AC equivalent voltage sources on branches x and y at the initial moment; i x (0),i y (0) represents the inductor current of x and y branches at the initial moment; u x (0),u y (0) represents the capacitor voltage of x and y branches at the initial moment; U eq Indicates three-phase AC voltage.

[0025] Preferably, in S4, the complex frequency domain expression circuit of the DC fault current including the commutation characteristic coefficient is obtained as follows:

[0026]

[0027] Among them, a0-a3, b0-b6 are constant coefficients; L dc is the DC side inductance; R dc is the total equivalent resistance on the DC side.

[0028] Preferably, in S5, the obtained simplified complex frequency domain expression of the DC fault current is:

[0029]

[0030] Preferably, in S6, the obtained simplified analytical model of the unified time domain of the ACC DC fault current in all stages is:

[0031] i dc (t) = I DC +Mcosωt+Nsinωt+M′cosω d t+N′sinω d t;

[0032] Among them, I DC , M, N, M', N', ω d is the fault current analysis coefficient.

[0033] Preferably, the expression of each fault current analysis coefficient is:

[0034]

[0035] Where ω′ represents the resonant angular frequency.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. Accurate simplified time-domain model. By constructing and simplifying the fault equivalent circuit of the receiving-end converter of ACC-HVDC, this method derives a simplified analytical model of the DC fault current in the time domain. This model not only considers the impact of the commutation process but also achieves a unified description of all fault phases through conversion from the complex frequency domain to the time domain, thereby improving the accuracy of transient analysis.

[0038] 2. Efficient fault analysis. Utilizing mathematical tools such as the method of partial fractions and the inverse Laplace transform, this method converts complex frequency-domain expressions into easily manageable time-domain expressions. This enables rapid and accurate analysis of the DC fault current's behavior and the specific values ​​of each fault electrical quantity after a fault occurs, improving the efficiency and accuracy of fault analysis.

[0039] 3. Enhanced system reliability. By performing a comprehensive transient analysis of the ACC-HVDC system after a fault, this method reveals the fault's impact on the system, providing a scientific basis for developing effective fault response strategies. Furthermore, quantitative calculation of each fault electrical quantity helps promptly identify potential safety hazards and implement appropriate preventive measures, thereby enhancing the system's reliability and stability in high-voltage transmission and distribution environments.

[0040] 4. This method considers the transient characteristics of the filter inductor and commutation capacitor under the action of the AC system, analyzes the relevant electrical quantities after the fault, and can accurately describe the ACC DC fault characteristics.

[0041] 5. Broad application prospects. This method is not only applicable to ACC-HVDC systems, but can also provide a reference and reference for fault analysis of other types of HVDC transmission systems. With the continuous development of HVDC transmission technology, the application prospects of this technical solution will be even broader.

[0042] In summary, this method can effectively perform a comprehensive transient analysis of the ACC-HVDC system after a fault, so as to achieve quantitative calculation of various fault electrical quantities, thereby improving the reliability and stability of ACC in the high-voltage transmission and distribution system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to make the purpose, technical solutions and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0044] Figure 1 Flowchart of this method;

[0045] Figure 2 Schematic diagram of a fault equivalent circuit in the embodiment;

[0046] Figure 3 A schematic diagram of a circuit simplified and converted to the complex frequency domain in an embodiment;

[0047] Figure 4 Schematic diagram of a simplified fault model in an embodiment. DETAILED DESCRIPTION

[0048] The following is a further detailed description through specific implementation methods:

[0049] Example:

[0050] like Figure 1 As shown, this embodiment discloses a fault analysis method based on a simplified time domain model of ACC-HVDC DC fault current, comprising the following steps:

[0051] S1. Divide each phase change after an ACC fault occurs and before the next phase change into one phase. Establish a fault equivalent circuit for the ACC-HVDC receiving-end converter.

[0052] The circuit diagram of the fault equivalent circuit is as follows Figure 2 In the figure, the subscripts x, y, and z represent the phases corresponding to the upper bridge arm, the lower bridge arm, and the non-conducting bridge arm in this commutation cycle, respectively. J 、i J 、u eqJ(J=x, y, z) are the capacitor voltage, inductor current and AC system equivalent voltage source of each branch, respectively. dc is the voltage at the DC output of the converter. The diode D is used to reflect the unidirectional current flow characteristics of the reverse resistance type switching device, R dc , L dc is the total equivalent resistance and inductance of the DC side, C f is the commutation capacitor. ac is the AC side inductance, R ac is the equivalent resistance on the AC side.

[0053] Since the steady-state commutation capacitor voltages are three-phase symmetrical, the three-phase commutation capacitor voltages have the following relationship:

[0054] -u z =u x +u y .

[0055] By analyzing the nodes and loops of the above fault equivalent circuit, we can obtain:

[0056]

[0057] Combining the above two formulas, we can get:

[0058]

[0059] It can be seen that under the transient state of a DC short-circuit fault, the voltage drop of the non-conducting phase branch of the ACC is always 0, that is, the neutral point of the commutation capacitor and the neutral point of the AC power supply equivalent to the valve side are always at the same potential.

[0060] S2. Based on the fault equivalent circuit, derive the relationship between the voltage and current of the equivalent circuit, simplify the circuit using the substitution principle and transform it into the complex frequency domain. Figure 3 shown.

[0061] S3. In the complex frequency domain, perform Thevenin equivalent on the AC part of the circuit simplified in S2 to obtain a simplified fault model; Figure 4 As shown, the equivalent impedance Z of the AC part e and equivalent voltage source U oc The expression of (s) is:

[0062]

[0063] Where, L ac is the AC side inductance, R ac is the equivalent resistance on the AC side, s is a complex variable, ω is the power frequency angular frequency, C f is the commutation capacitor.

[0064] K1-K5 are the commutation characteristic coefficients of ACC. The expressions of each commutation characteristic coefficient are:

[0065]

[0066] Wherein, the subscripts x and y represent the branches of the phases corresponding to the upper bridge arm and the lower bridge arm respectively during this commutation period; Respectively represent the phase angles of the AC equivalent voltage sources on branches x and y at the initial moment; i x (0),i y (0) represents the inductor current of x and y branches at the initial moment; u x (0),u y (0) represents the capacitor voltage of x and y branches at the initial moment; U eq Indicates three-phase AC voltage.

[0067] S4. Solve the simplified fault model after Thevenin equivalent to obtain the complex frequency domain expression circuit of the DC fault current including the commutation characteristic coefficient. The complex frequency domain expression circuit is:

[0068]

[0069] Among them, a0-a3, b0-b6 are constant coefficients; L dc is the DC side inductance; R dc is the total equivalent resistance on the DC side.

[0070] S5. Ignore the AC / DC equivalent resistance of the ACC in the complex frequency domain expression circuit and obtain the simplified complex frequency domain expression of the DC fault current. The simplified complex frequency domain expression of the DC fault current is:

[0071]

[0072] S6. The simplified complex frequency domain expression of the DC fault current is processed according to the partial fraction method combined with the inverse Laplace transform, and the simplified analytical model of the ACC DC fault current in the unified time domain at all stages is obtained as follows:

[0073] i dc (t) = I DC +Mcosωt+Nsinωt+M′cosω d t+N′sinω d t;

[0074] Among them, I DC , M, N, M', N', ω d is the fault current resolution coefficient. The expression of each fault current resolution coefficient is:

[0075]

[0076] Where ω′ represents the resonant angular frequency.

[0077] S7, taking the period from the start of each commutation after the ACC fault occurs to the next commutation as a stage, and obtaining the initial values ​​of the circuit elements in stage n;

[0078] S8. Based on the initial values ​​of the circuit elements in the nth stage, the fault current analytical coefficients and the commutation characteristic coefficients of the simplified analytical model of the unified time domain of the entire stage are updated to obtain the simplified analytical model of the DC fault current in the nth stage in the time domain;

[0079] S9. Use the simplified time-domain analytical model of each stage of the DC fault current to conduct a comprehensive transient analysis of the ACC-HVDC system after the fault, and achieve quantitative calculation of each fault electrical quantity.

[0080] By constructing and simplifying the fault equivalent circuit of the receiving-end converter of an ACC-HVDC system, this method derives a simplified analytical model of the DC fault current in the time domain. This model not only accounts for the effects of the commutation process but also, through conversion from the complex frequency domain to the time domain, achieves a unified description of all fault phases, thereby improving the accuracy of transient analysis. Furthermore, by utilizing mathematical tools such as the method of partial fractions and the inverse Laplace transform, this method converts complex complex frequency domain expressions into more manageable time domain expressions. This enables rapid and accurate analysis of the DC fault current's behavior and the specific values ​​of various fault electrical quantities after a fault occurs, improving the efficiency and accuracy of fault analysis. Furthermore, by performing a comprehensive transient analysis of the ACC-HVDC system after a fault, this method reveals the fault's impact on the system, providing a scientific basis for developing effective fault response strategies. Furthermore, the quantitative calculation of various fault electrical quantities facilitates the timely identification of potential safety hazards and the implementation of appropriate preventive measures, thereby enhancing the reliability and stability of the system in high-voltage transmission and distribution environments. This method considers the transient characteristics of the filter inductor and commutation capacitor under the influence of the AC system, analyzes relevant electrical quantities after a fault, and accurately describes the characteristics of ACC DC faults. Furthermore, this method is not only applicable to ACC-HVDC systems but also provides a reference for fault analysis in other types of HVDC transmission systems. With the continuous development of HVDC transmission technology, the application prospects of this technical solution will be even broader.

[0081] This method can effectively perform a comprehensive transient analysis of the ACC-HVDC system after a fault, so as to achieve quantitative calculation of various fault electrical quantities, thereby improving the reliability and stability of ACC in the high-voltage transmission and distribution system.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the technical solutions. Those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present invention that do not depart from the purpose and scope of the technical solutions of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A fault analysis method based on a simplified time-domain model of ACC-HVDC DC fault current, characterized in that: The following steps are involved: S1. Establish a fault equivalent circuit for the receiving-end converter of ACC-HVDC; S2. Based on the fault equivalent circuit, derive the relationship between the voltage and current of the equivalent circuit, simplify the circuit using the substitution principle and transform it into the complex frequency domain; S3. In the complex frequency domain, perform Thevenin equivalent on the AC part of the simplified circuit in S2 to obtain a simplified fault model. S4. Solve the simplified fault model after Thevenin equivalent to obtain the complex frequency domain expression circuit of the DC fault current including the commutation characteristic coefficient; S5. Ignore the AC and DC equivalent resistance of ACC in the complex frequency domain expression circuit and obtain the simplified complex frequency domain expression of DC fault current; S6. The simplified complex frequency domain expression of DC fault current is processed according to the partial fraction method combined with the inverse Laplace transform to obtain a simplified analytical model of the ACC DC fault current in the unified time domain at all stages. S7, taking the period from the start of each commutation after the ACC fault occurs to the next commutation as a stage, and obtaining the initial values ​​of the circuit elements in stage n; S8. Based on the initial values ​​of the circuit elements in the nth stage, the fault current analytical coefficients and the commutation characteristic coefficients of the simplified analytical model of the unified time domain of the entire stage are updated to obtain the simplified analytical model of the DC fault current in the nth stage in the time domain; S9. Use the simplified time-domain analytical model of each stage of the DC fault current to conduct a comprehensive transient analysis of the ACC-HVDC system after the fault, and achieve quantitative calculation of each fault electrical quantity.

2. The fault analysis method based on the simplified time-domain model of ACC-HVDC DC fault current according to claim 1, characterized in that: In S1, the fault equivalent circuit established includes the branches corresponding to the phases of the upper bridge arm, the lower bridge arm, and the non-conducting bridge arm during the commutation period, the capacitor voltage, the inductor current, and the equivalent voltage source of the AC system in each branch, the voltage at the DC outlet of the converter, the total equivalent resistance and inductance on the DC side, and the commutation capacitance.

3. The fault analysis method based on the simplified time-domain model of ACC-HVDC DC fault current according to claim 2, characterized in that: After the Thevenin equivalent is performed in S3, the equivalent impedance Z of the AC part e and equivalent voltage source U oc The expression of (s) is: Where, L ac is the AC side inductance, R ac is the equivalent resistance on the AC side, s is a complex variable, ω is the power frequency angular frequency, C f is the commutation capacitor, K1-K5 are the commutation characteristic coefficients of ACC.

4. The fault analysis method based on the simplified time-domain model of ACC-HVDC DC fault current according to claim 3, characterized in that: The expressions of each commutation characteristic coefficient are: Wherein, the subscripts x and y represent the branches of the phases corresponding to the upper bridge arm and the lower bridge arm respectively during this commutation period; Respectively represent the phase angles of the AC equivalent voltage sources on branches x and y at the initial moment; i x (0),i y (0) represents the inductor current of x and y branches at the initial moment; u x (0),u y (0) represents the capacitor voltage of x and y branches at the initial moment; U eq Indicates three-phase AC voltage.

5. The fault analysis method based on the simplified time-domain model of ACC-HVDC DC fault current according to claim 4, characterized in that: In S4, the complex frequency domain expression circuit of the DC fault current including the commutation characteristic coefficient is obtained as follows: Among them, a0-a3, b0-b6 are constant coefficients; L dc is the DC side inductance; R dc is the equivalent resistance on the DC side.

6. The fault analysis method based on the simplified time-domain model of ACC-HVDC DC fault current according to claim 5, characterized in that: In S5, the simplified complex frequency domain expression of the DC fault current is obtained as follows:

7. The fault analysis method based on the simplified time-domain model of ACC-HVDC DC fault current according to claim 6, characterized in that: In S6, the unified time-domain simplified analytical model of the ACC DC fault current in all stages is obtained as follows: i dc (t)=I DC +Mcosωt+Nsinωt+M′cosω d t+N′sinω d t; Among them, I DC , M, N, M', N', ω d is the fault current analysis coefficient.

8. The fault analysis method based on the simplified time-domain model of ACC-HVDC DC fault current according to claim 7, characterized in that: The expressions of the fault current analysis coefficients are: Where ω′ represents the resonant angular frequency.

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