A Fault Analysis Method Based on CSC-HVDC Valve Group Interaction Model

Abstract

The present invention discloses a fault analysis method based on a CSC-HVDC valve group interaction model. The method includes obtaining operating state electrical parameters of a CSC-HVDC system to be tested, constructing a CSC-HVDC valve group interaction model for performing fault analysis, using the operating state electrical parameters of the CSC-HVDC system to be tested as input to the CSC-HVDC valve group interaction model, and outputting dynamic characteristic parameters of the CSC-HVDC system to be tested. Fault analysis of the CSC-HVDC system to be tested is then performed based on the obtained dynamic characteristic parameters. The model establishes a dynamic analysis equivalent circuit of the CSC-HVDC by qualitatively analyzing the coupling characteristics of the AC measurements of upper and lower CSC-HVDC valve groups under the action of grid impedance. The model can more accurately describe the dynamic characteristics of a CSC-HVDC system connected to a weak grid. When a DC fault occurs in the CSC-HVDC system, the present invention can more accurately describe the complex coupling characteristics and variation patterns of key electrical quantities, thereby helping to improve the accuracy of CSC-HVDC system fault analysis.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage direct current (HVDC) transmission, and in particular to a fault analysis method based on a CSC-HVDC valve group interaction model. Background Art

[0002] Actively commutated current-source converters (CSCs), as current-source converters with controllable shutdown, have gained increasing attention in the field of DC transmission in recent years. Because they utilize fully controlled devices, the CSC's arm commutation process is independent of the AC grid voltage, completely avoiding commutation failures. Compared to LCCs, they are better able to handle AC-side faults, improving their ability to support weak AC systems and providing power to passive networks. Compared to MMCs, CSCs also have inherent advantages in handling DC faults. The inherent smoothing inductance at their DC outputs effectively limits the speed of fault current development. Furthermore, CSCs can control DC voltage polarity reversal and possess the ability to clear DC fault currents, eliminating the need for expensive additional current limiting devices, DC circuit breakers, and other equipment. Therefore, they are well-suited for building new DC transmission systems.

[0003] Traditional three-level CSC units suffer from large AC and DC harmonics and limited power transmission levels. To address this, Li Zixin's team at the Institute of Electrical Engineering has proposed a phase-shifted, 12-pulse CSC-HVDC system (hereinafter referred to as CSC-HVDC) suitable for high-voltage, high-power transmission scenarios. This system consists of two cascaded three-level CSC units. The CSC-HVDC boasts superior voltage and power levels, control flexibility, and harmonic characteristics compared to conventional three-level CSC systems, making it more suitable as a converter in HVDC transmission systems. Current research on CSC-HVDC systems focuses on steady-state modeling, modulation control, and analysis and design of related equipment.

[0004] In recent years, some researchers have established CSC-HVDC transient state-space equations. However, these equations equate the AC side to a symmetrical constant sinusoidal voltage source and assume that the converter is connected to an infinite grid, decoupling the upper and lower CSC-HVDC valve groups at the AC busbar. Therefore, they are essentially still transient models of three-level CSC units and are not suitable for analyzing the transient characteristics of CSC-HVDC when connected to weak AC systems. Currently, there is little research on CSC-HVDC transient modeling and DC fault analysis. In existing technologies, insufficient model accuracy leads to deviations between the predicted dynamic characteristic parameters and the actual situation. Furthermore, existing models have limited analytical capabilities in the presence of transient faults or disturbances, failing to accurately capture the dynamic behavior of the system. This makes it difficult to accurately analyze the system during fault analysis. Furthermore, existing literature does not consider the interaction between the upper and lower CSC-HVDC valve groups. Summary of the Invention

[0005] To address the shortcomings of the above-mentioned prior art, the present invention provides a fault analysis method based on the CSC-HVDC valve group interaction model. By qualitatively analyzing the coupling characteristics of the AC measurements of the upper and lower valve groups of the CSC-HVDC under the action of grid impedance, a dynamic analysis equivalent circuit of the CSC-HVDC is established, thereby deriving the CSC-HVDC valve group interaction model. The dynamic characteristic parameters are obtained using this model, which helps to improve the accuracy of fault analysis.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A fault analysis method based on a CSC-HVDC valve group interaction model includes the following steps:

[0008] S1. Obtaining electrical parameters of the working state of the CSC-HVDC system to be tested;

[0009] S2. Constructing a CSC-HVDC valve group interaction model for fault analysis, using the operating state electrical parameters of the CSC-HVDC system to be tested as input to the CSC-HVDC valve group interaction model, and outputting dynamic characteristic parameters of the CSC-HVDC system to be tested, and then performing fault analysis on the CSC-HVDC system to be tested based on the obtained dynamic characteristic parameters;

[0010] The specific steps for constructing the CSC-HVDC valve group interaction model are as follows:

[0011] S21. Based on the operating state electrical parameters of the CSC-HVDC system, adopt the Thevenin equivalent for the AC system, determine the AC system equivalent impedance from the short-circuit ratio and the impedance angle, and obtain an AC system equivalent impedance model, wherein the AC system equivalent impedance model includes an equivalent ideal voltage source, an equivalent inductor, and an equivalent resistor;

[0012] S22, ignoring the excitation branch and transformer losses, equating the converter transformer to an ideal transformer with series leakage reactance, and obtaining a dynamic analysis equivalent circuit of the CSC-HVDC system according to step S21;

[0013] S23. Based on the dynamic analysis equivalent circuit of the CSC-HVDC system, using Kirchhoff's law and combining the electrical coupling on both sides of the converter transformer of the CSC-HVDC system, establish a filter inductance state space of the CSC-HVDC system;

[0014] S24. Using a switching function model of modulation theory and combining it with a dynamic analysis equivalent circuit of the CSC-HVDC system, a unified state space of the commutation capacitors under the switching state of each bridge arm is established;

[0015] S25. Assume that the external characteristics of the external DC system satisfy the algebraic differential equation Thus, the DC side state space of the CSC-HVDC system is established; where u D Indicates the voltage at the DC outlet, k D and k' D They represent the coefficients of the CSC DC current and its derivative in the DC system port external voltage equation, i dc represents the DC current, represents the first-order differential of DC current, X D Represents the algebraic differential combination of the external system state variables and input excitation variables in the port external voltage equation;

[0016] S26. Based on the filter inductor state space, the unified state space of the commutation capacitor in the switching state of each bridge arm, and the DC side state space, matrix partitioning is performed to obtain a CSC-HVDC valve group interaction model.

[0017] Specifically, in step S21, the calculation formulas for the equivalent impedance, equivalent inductance, and equivalent resistance of the AC system are:

[0018]

[0019] Where Z s Indicates the equivalent impedance of the AC system, U pN Indicates the rated voltage of the AC bus, P dN Indicates the system rated active power, SCR indicates the AC system short circuit ratio, Ls Indicates equivalent inductance, R s Indicates equivalent resistance, Represents the impedance angle of the AC system.

[0020] Specifically, in step S23, the step of establishing the CSC-HVDC system filter inductance state space includes:

[0021] S231, performing node loop analysis on the dynamic analysis equivalent circuit of the CSC-HVDC system to obtain basic electrical equations;

[0022] S232. Based on Hoff's law, analyze both sides of the converter transformer to obtain the voltage relationship between the two sides of the converter transformer, the AC bus voltage equation, and the current relationship between the upper and lower valve group transformers;

[0023] S233. Perform mathematical operations on the relationship equation obtained in step S232 to express the leakage reactance current of the delta-connected converter transformer with the filter inductor current of the corresponding valve group, thereby unifying the state variables of the upper and lower valve groups.

[0024] S234, establishing a voltage drop equation between the neutral point of the converter transformer on the valve side of the lower valve group and the neutral point of the commutation capacitor;

[0025] S235 . Construct the filter inductance state space of the CSC-HVDC system using the relationship equations obtained in steps S231 to S234 .

[0026] Specifically, in step S235, the CSC-HVDC system filter inductor state space is expressed as:

[0027]

[0028] Where u jy Indicates the voltage of the commutation capacitor of the lower valve group, j=a,b,c,L y It represents the sum of the leakage reactance and filter inductance of the converter transformer connected to the lower valve group Y, k y Indicates the transformer ratio of the lower valve group, L s Indicates equivalent inductance, i jy Indicates the filter inductor current of the lower valve group, Indicates the first-order differential of the lower valve group filter inductor current, R s Indicates equivalent resistance, k d Indicates the transformer ratio of the upper valve group, i jd Indicates the upper valve group filter inductor current, Indicates the first-order differential of the upper valve group filter inductor current, u eqj represents an equivalent ideal voltage source, L f Indicates the filter inductance, L σd Indicates the leakage reactance of the converter transformer, ujd Indicates the upper valve group commutation capacitor voltage.

[0029] Specifically, in step S24, the switching function model of the modulation theory is:

[0030]

[0031] Where S ji Represents the three-phase switching function of the upper and lower valve groups.

[0032] Specifically, the unified state space of the commutation capacitor in the switching state of each bridge arm is expressed as:

[0033]

[0034] C f represents the commutation capacitance, Indicates the first-order differential of the upper valve group commutation capacitor voltage, Indicates the first-order differential of the commutation capacitor voltage of the lower valve group, i dc Indicates direct current.

[0035] Specifically, in step S25, the DC side state space of the CSC-HVDC system is expressed as follows based on the DC outlet voltage and the DC current expression:

[0036]

[0037] Where, L dc Indicates the DC side filter inductor, represents the first-order differential of DC current, u dcd 、u dcy 、u dc Respectively represent the upper valve group, lower valve group and total DC side voltage.

[0038] Specifically, in step S26, the CSC-HVDC valve group interaction model is expressed as:

[0039]

[0040] Where x d Represents the upper valve group state variable, x d =[i ad i bd i cd u ad u bd u cd ] T ;x y Represents the state variable of the lower valve group, x y =[i ay i by icy u ay u by u cy ] T ,u eq represents the AC equivalent sinusoidal excitation source variable, u eq =[u eqa u eqb u eqc ] T ; A dD 、A yD 、A Dd 、A Dy represents the AC / DC interaction matrix; B d 、B y Represents the system communication matrix; A d 、A y It respectively represents the interaction between the state variables of the upper and lower valve groups.

[0041] Specifically, the working state parameters include electrical equipment parameters and electrical transient parameters; the electrical equipment parameters include the DC side smoothing reactance parameters of the converter, the valve outlet commutation capacitor parameters, the AC filter inductor parameters, the leakage reactance ratio of the converter transformer, the AC system short-circuit ratio and impedance angle parameters; the electrical transient parameters include DC current and voltage, the commutation capacitor voltage of the two valve groups and the filter inductor current of the two valve groups.

[0042] Specifically, the dynamic characteristic parameters include the time domain differential components of the DC current and voltage, the voltage of the commutation capacitor of the two-valve group, and the current of the filter inductor of the two-valve group.

[0043] Compared with the prior art, the present invention has the following technical effects:

[0044] By constructing an interaction model between CSC-HVDC valve groups, the present invention can more accurately and clearly describe the dynamic characteristics of the CSC-HVDC system under conditions such as transient faults, disturbances, or operating mode switching. This model can serve as the model basis for stability analysis and fault characteristic analysis, enabling comprehensive and accurate transient analysis of the system. The model features consider the impact of the AC system impedance on the dynamic behavior of the CSC-HVDC system, thereby reflecting the strength and characteristics of the AC system. At the same time, the system also considers the interaction between the upper and lower valve groups, and can more accurately describe the dynamic characteristics of the weak grid-connected CSC-HVDC system. When a DC fault occurs in the CSC-HVDC system, the present invention can more accurately describe the complex coupling characteristics and variation patterns of various key electrical quantities, thereby helping to improve the accuracy of CSC-HVDC system fault analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] 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:

[0046] Figure 1 The present invention discloses a flow chart of a fault analysis method based on an interaction model between CSC-HVDC valve groups;

[0047] Figure 2 This is a dynamic analysis equivalent circuit diagram of the CSC-HVDC according to an embodiment of the present invention;

[0048] Figure 3 This is a step response comparison diagram of an embodiment of the present invention;

[0049] Figure 4 Graph showing the fitting results of capacitor voltage and inductor current according to an embodiment of the present invention;

[0050] Figure 5 This is a fitting result diagram of the trigger angle, phase-locked loop synchronization phase, and bridge arm switch state according to an embodiment of the present invention;

[0051] Figure 6 The DC side voltage u of the two valve groups in the embodiment of the present invention is dcd ,u dcy And the total DC output voltage u dc The fitting result graph of ;

[0052] Figure 7 Schematic diagram of DC fault current and its rate of change under different transition resistances using different fault analysis methods according to an embodiment of the present invention. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but only represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0054] The present invention will be described in further detail below with reference to the accompanying drawings.

[0055] In the prior art, line converters (LCCs) and voltage source converters (VSCs) have become widely used converters in high-voltage direct current (HVDC) transmission. However, the above converters still face some technical challenges. When the receiving AC power grid is disturbed or fails, LCC is very prone to commutation failure, resulting in serious power loss. In addition, AC filters and reactive power compensation equipment significantly increase the footprint of the converter station and may cause transient overvoltage problems. On the other hand, although VSC can avoid commutation failure, when applied to high-voltage scenarios, a large number of submodules lead to high costs and large valve hall sizes. VSC-HVDC will also suffer from extremely high fault currents after a DC short circuit fault. In response to the above problems, the present invention proposes a fault analysis method based on a CSC-HVDC valve group interaction model. By considering the interaction between valve groups through the CSC-HVDC valve group interaction model, the dynamic characteristics of the CSC-HVDC with weak grid connection can be more accurately described. The valve group interaction mechanism is analyzed based on the constructed model, and the model of the present invention is further simplified according to the interaction strength, making it applicable to different scenarios.

[0056] The present invention proposes a fault analysis method based on the interaction model between CSC-HVDC valve groups by using the constructed CSC-HVDC valve group interaction model. Figure 1 As shown, the method includes:

[0057] S1. Obtaining electrical parameters of the working state of the CSC-HVDC system to be tested;

[0058] S2. Constructing a CSC-HVDC valve group interaction model for fault analysis, using the operating state electrical parameters of the CSC-HVDC system to be tested as input to the CSC-HVDC valve group interaction model, and outputting dynamic characteristic parameters of the CSC-HVDC system to be tested, and then performing fault analysis on the CSC-HVDC system to be tested based on the obtained dynamic characteristic parameters;

[0059] The specific steps for constructing the CSC-HVDC valve group interaction model are as follows:

[0060] S21. Based on the operating state electrical parameters of the CSC-HVDC system, adopt the Thevenin equivalent for the AC system, determine the AC system equivalent impedance from the short-circuit ratio and the impedance angle, and obtain an AC system equivalent impedance model, wherein the AC system equivalent impedance model includes an equivalent ideal voltage source, an equivalent inductor, and an equivalent resistor;

[0061] S22, ignoring the excitation branch and transformer losses, equating the converter transformer to an ideal transformer with series leakage reactance, and obtaining a dynamic analysis equivalent circuit of the CSC-HVDC system according to step S21;

[0062] S23. Based on the dynamic analysis equivalent circuit of the CSC-HVDC system, using Kirchhoff's law and combining the electrical coupling on both sides of the converter transformer of the CSC-HVDC system, establish a filter inductance state space of the CSC-HVDC system;

[0063] S24. Using a switching function model of modulation theory and combining it with a dynamic analysis equivalent circuit of the CSC-HVDC system, a unified state space of the commutation capacitors under the switching state of each bridge arm is established;

[0064] S25. Assume that the external characteristics of the external DC system satisfy the algebraic differential equation Thus, the DC side state space of the CSC-HVDC system is established; where u D Indicates the voltage at the DC outlet, k D and k' D They represent the coefficients of the CSC DC current and its derivative in the DC system port external voltage equation, i dc represents the DC current, represents the first-order differential of DC current, X D Represents the algebraic differential combination of the external system state variables and input excitation variables in the port external voltage equation;

[0065] S26. Based on the filter inductor state space, the unified state space of the commutation capacitor in the switching state of each bridge arm, and the DC side state space, matrix partitioning is performed to obtain a CSC-HVDC valve group interaction model.

[0066] By constructing an interaction model between CSC-HVDC valve groups, the present invention can more accurately and clearly describe the dynamic characteristics of the CSC-HVDC system under conditions such as transient faults, disturbances, or operating mode switching. This model can serve as the model basis for stability analysis and fault characteristic analysis, enabling comprehensive and accurate transient analysis of the system. The model features consider the impact of the AC system impedance on the dynamic behavior of the CSC-HVDC system, thereby reflecting the strength and characteristics of the AC system. At the same time, the system also considers the interaction between the upper and lower valve groups, and can more accurately describe the dynamic characteristics of the weak grid-connected CSC-HVDC system. When a DC fault occurs in the CSC-HVDC system, the present invention can more accurately describe the complex coupling characteristics and variation patterns of various key electrical quantities, thereby helping to improve the accuracy of CSC-HVDC system fault analysis.

[0067] In this embodiment, the use of dynamic characteristic parameters to perform fault analysis on the CSC-HVDC system is a technology that is widely known and applied in the technical field and is not described in detail in the present invention. Instead, the CSC-HVDC valve group interaction model constructed in the present invention is described in detail.

[0068] In this embodiment, in step S21, the calculation formulas for the equivalent impedance, equivalent inductance, and equivalent resistance of the AC system are:

[0069]

[0070] Where Z s Indicates the equivalent impedance of the AC system, U pN Indicates the rated voltage of the AC bus, P dN Indicates the system rated active power, SCR indicates the AC system short circuit ratio, L s Indicates equivalent inductance, R s Indicates equivalent resistance, Represents the impedance angle of the AC system.

[0071] Figure 2 The dynamic analysis equivalent circuit of the CSC-HVDC system in this embodiment is shown in FIG. 1 , where the leakage reactance L σd , L σy is transferred to the valve side, k d , k y The upper valve group transformer adopts YD connection method, while the lower valve group transformer adopts YY connection method. f and C f are filter inductor and commutation capacitor respectively, u dc and i dc Represents DC voltage and current respectively, i' jd 、i ji 、u ji (j=a, b, c, i=d, y) represent the leakage current, filter inductor current and commutation capacitor voltage of the upper and lower valve groups respectively, u pj and i pj Indicates the voltage at the AC bus and the grid-connected current.

[0072] In this embodiment, in step S23, the step of establishing the CSC-HVDC system filter inductance state space includes:

[0073] S231. Perform node loop analysis on the dynamic analysis equivalent circuit of the CSC-HVDC system to obtain a basic electrical equation; the equation is:

[0074]

[0075] Where i jy Indicates the three-phase filter inductor current, u jd Indicates the upper valve group commutation capacitor voltage, u jy Indicates the commutation capacitor voltage, u eqj represents an equivalent ideal voltage source.

[0076] S232. Based on Hoff's law, analyze both sides of the converter transformer to obtain the voltage relationship between the two sides of the converter transformer, the AC bus voltage equation, and the current relationship between the upper and lower valve group transformers;

[0077] Voltage relationship on both sides of the converter transformer:

[0078] AC bus voltage equation:

[0079] The current relationship between the upper and lower valve group transformers: i pj =(k d i j ' d +k y i jy )(j=a,b,c).

[0080] S233. Through mathematical operations on the relationship equation obtained in step S232, the leakage reactance current of the delta-connected converter transformer is expressed by the filter inductor current of the corresponding valve group to achieve the unification of the state variables of the upper and lower valve groups. Specifically,

[0081] By summing both sides of the sub-equations of the current relationship between the upper and lower valve group transformers and combining them with the basic electrical equations, we can obtain:

[0082]

[0083] The voltage relationship on both sides of the converter transformer is summed up on both sides and substituted into the AC bus voltage equation to obtain:

[0084]

[0085] Combining the basic electrical equation with the equation obtained by summing both sides of the current relationship between the upper and lower valve group transformers and combining it with the basic electrical equation, we can get

[0086] The above formula shows that the CSC-HVDC grid-connected current satisfies the first-order differential relationship. Further combined with the steady-state initial state, we can obtain:

[0087]

[0088] The above formula shows that although the CSC-HVDC grid-connected current will be distorted under transient conditions, its sum is always 0. That is, although the CSC-HVDC system grid-connected current will be distorted under transient conditions such as DC side faults and operating conditions changes, and is not a three-phase symmetrical sine wave, its three-phase sum is always 0. Combined with the formula It can be seen that the three-phase leakage current of the delta-connected transformer also satisfies the sum of 0:

[0089] Then the leakage current of the upper valve group commutation transformer and the filter inductor current satisfy:

[0090] Take the first two equations above as an example to make a difference, and combine them with We can get: i ad -i bd =2i a ' d -i c ' d -i b ' d =3i a ' d ;

[0091] Therefore, by taking the difference between each sub-equation, the three-phase leakage current of the upper valve group can be expressed as the filter inductor current:

[0092]

[0093] Based on this, the upper valve group state variable i' jd Replaced by the filter inductor current i jd , to achieve unification with the state variables of the lower valve group.

[0094] S234, establishing a voltage drop equation between the neutral point of the converter transformer on the valve side of the lower valve group and the neutral point of the commutation capacitor;

[0095] Write the voltage drop equation between the neutral point N of the transformer on the valve side of the lower valve group and the neutral point N' of the capacitor, and sum them up to get:

[0096]

[0097] Adding the three-phase components together, we get:

[0098] So we can get u NN' =0, which means that the neutral point of the Y-connected transformer on the lower valve group side and the neutral point of the commutation capacitor still maintain the same potential during transient conditions such as DC short circuit and operating condition changes, thus:

[0099] S235 . Construct the filter inductance state space of the CSC-HVDC system using the relationship equations obtained in steps S231 to S234 .

[0100] The CSC-HVDC system filter inductance state space is expressed as:

[0101]

[0102] Where u jy Indicates the voltage of the commutation capacitor of the lower valve group, j=a,b,c,L yIt represents the sum of the leakage reactance and filter inductance of the converter transformer connected to the lower valve group Y, k y Indicates the transformer ratio of the lower valve group, L s Indicates equivalent inductance, i jy Indicates the filter inductor current of the lower valve group, Indicates the first-order differential of the lower valve group filter inductor current, R s Indicates equivalent resistance, k d Indicates the transformer ratio of the upper valve group, i jd Indicates the upper valve group filter inductor current, Indicates the first-order differential of the upper valve group filter inductor current, u eqj represents an equivalent ideal voltage source, L f Indicates the filter inductance, L σd Indicates the leakage reactance of the converter transformer, u jd Indicates the upper valve group commutation capacitor voltage.

[0103] In this embodiment, in step S24, the switching function model of the modulation theory is:

[0104]

[0105] Where S ji Represents the three-phase switching function of the upper and lower valve groups;

[0106] Combined with the dynamic analysis of the equivalent circuit, the unified state space of the commutation capacitor in the switching state of each bridge arm is expressed as:

[0107]

[0108] C f represents the commutation capacitance, Indicates the first-order differential of the upper valve group commutation capacitor voltage, Indicates the first-order differential of the commutation capacitor voltage of the lower valve group, i dc Indicates direct current.

[0109] In this embodiment, in step S25, the DC side state space of the CSC-HVDC system is expressed as follows based on the DC outlet voltage and the DC current expression:

[0110]

[0111] Where, L dc Indicates the DC side filter inductor, represents the first-order differential of DC current, u dcd 、u dcy 、u dc Respectively represent the upper valve group, lower valve group and total DC side voltage.

[0112] In this embodiment, in step S26, the CSC-HVDC valve group interaction model is expressed as:

[0113]

[0114] Where x d Represents the upper valve group state variable, x d =[i ad i bd i cd u ad u bd u cd ] T ;x y Represents the state variable of the lower valve group, x y =[i ay i by i cy u ay u by u cy ] T ,u eq represents the AC equivalent sinusoidal excitation source variable, u eq =[u eqa u eqb u eqc ] T ; A dD 、A yD 、A Dd 、A Dy represents the AC / DC interaction matrix, A Dd =S d T / (-C f ), A Dy =S y T / (-C f ), A dD =S d / (L dc +k' D ), A yD =S y / (L dc +k' D ), where S d =[000S ad S bd S bd ], S y =[000S ay S by S by ]; B d 、B y represents the system AC action matrix; the specific expressions of the remaining coefficient sub-matrices are as follows:

[0115]

[0116]

[0117] In this embodiment, the working state parameters include electrical equipment parameters and electrical transient parameters. The electrical equipment parameters include the DC side smoothing reactance parameters of the converter, the valve outlet commutation capacitor parameters, the AC filter inductor parameters, the leakage reactance ratio of the converter transformer, the AC system short-circuit ratio and impedance angle parameters. The electrical transient parameters include DC current and voltage, the commutation capacitor voltage of the two valve groups, and the filter inductor current of the two valve groups; the dynamic characteristic parameters include DC current and voltage, the commutation capacitor voltage of the two valve groups, and the time domain differential of the filter inductor current of the two valve groups.

[0118] Example:

[0119] In order to verify the accuracy of the established CSC-HVDC valve group interaction model, its step response is compared with the electromagnetic transient simulation value of the detailed model in PSCAD. The main system parameters are shown in Table 1.

[0120] Table 1 System simulation parameters

[0121]

[0122] The current reference value of the constant current controller is set to step from 3kA to 3.5kA at 2s. The step response comparison of the two models is as follows: Figure 3 As shown, Figure 3 In the figure, blue represents the model of this embodiment, and red represents the electromagnetic transient simulation model. It can be seen that the step response of the CSC-HVDC valve group interaction model is basically consistent with the PSCAD detailed model response, verifying the accuracy of the constructed model.

[0123] Coefficient submatrix A d 、A y Respectively reflects the interaction between the state variables of the upper and lower valve groups, which is called the valve group self-matrix. Note that each sub-matrix of the valve group self-matrix is ​​a diagonal matrix, which indicates that the three-phase branches on the AC side of a single valve group of CSC-HVDC are independent of each other. dD 、A yD 、A Dd 、A Dy It reflects the mutual influence between the state variables of the upper and lower valve groups and the DC current, which is called the AC / DC interaction matrix. d 、B y It reflects the effect of the AC system on the state variables of the upper and lower valve groups, and is called the AC system effect matrix.

[0124] Focus on the coefficient submatrix Ayd and A dy , which respectively reflect the effect of the state variables of the lower valve group on the upper valve group and the effect of the state variables of the upper valve group on the lower valve group, so it is defined as the valve group interaction matrix. Observe the element K in the valve group interaction matrix. 6 / 8 From the expressions of K7, it can be seen that the coupling relationship between CSC-HVDC valve groups mainly depends on the equivalent impedance of the AC system, the leakage reactance of the commutation transformer, the value of the filter inductance and the transformer ratio.

[0125] In order to verify the correctness of the fault analysis method based on the CSC-HVDC valve group interaction model embodied in this embodiment, a CSC-HVDC DC fault simulation model was built based on the PSCAD / EMTDC electromagnetic transient simulation platform. A metallic short circuit fault was set at the rectifier station outlet. The algorithm in this paper was implemented through Matlab platform programming to perform analytical calculations and compared with the simulation values. In this verification example, the solid lines in each figure represent the simulation values, and the dotted lines / shaded areas represent the numerical analytical values. Figures 4 to 6 It can be seen that the coupled system model proposed in this embodiment can more accurately describe the complex coupling characteristics and variation patterns of key electrical quantities after a DC fault occurs in the CSC-HVDC, meeting the needs of comprehensive and accurate transient analysis of the system. The DC fault current is also a key focus after a DC side fault occurs in the converter. It is of great significance for the selection of related equipment, protection design, and system planning. In addition, for current source topologies, the current flowing through the switching devices in the converter's conducting bridge arm is the DC fault current, and its rate of change is also of reference value for the selection of switching devices. Figure 7 The calculated and simulated values ​​of the DC fault current and its rate of change under different transition resistances using the analysis method of the embodiment are given, which reflects the advantage of the analysis method of this embodiment in terms of accuracy.

[0126] The DC fault current calculation errors of the DC side discharge algorithm only, the DC voltage source discharge algorithm, and the analysis method of this embodiment under different transition resistance conditions are shown in Table 2, and the algorithm numbers are 1, 2, and 3 respectively.

[0127] Table 2 DC fault current calculation error

[0128]

[0129] It can be seen that the calculation error of the pure DC discharge algorithm will increase significantly over time, and it can only describe the general development trend of the fault current. In addition, its error tends to decrease with the increase of transition resistance, but the overall accuracy is still poor. The DC voltage source discharge model takes into account the discharge loop composed of AC side capacitors and inductors. It is relatively accurate in the early stage of metallic short-circuit faults at the converter outlet. However, since this method is based on the AC decoupling model, it cannot accurately calculate the DC fault current on a longer time scale when the AC system is weak. At the same time, the analytical formula ignores the DC side resistance. When the short-circuit transition resistance is large, its calculation accuracy will be greatly reduced. The calculation results of the coupling model used in the embodiment can accurately fit the electromagnetic transient simulation values ​​under different DC side transition resistances. The mean absolute percentage error (MAPE) within 10ms is 0.14%, 0.16% and 0.15%, respectively, verifying the accuracy of the proposed CSC-HVDC fault analysis method.

[0130] In summary, by constructing an interaction model between CSC-HVDC valve groups, the present invention can more accurately and clearly describe the dynamic characteristics of the CSC-HVDC system in the event of transient faults, disturbances or operating mode switching. It can serve as a model basis for stability analysis and fault characteristic analysis, and realize comprehensive and accurate transient analysis of the system. The model characteristics take into account the influence of the AC system impedance on the dynamic behavior of the CSC-HVDC system, thereby reflecting the strength and characteristics of the AC system. At the same time, the system also takes into account the interaction between the upper and lower valve groups, and can more accurately describe the dynamic characteristics of the weak grid-connected CSC-HVDC system. When a DC fault occurs in the CSC-HVDC system, the present invention can more accurately describe the complex coupling characteristics and change laws of various key electrical quantities, thereby providing more accurate basic data for CSC-HVDC system fault analysis, and helping to improve the accuracy of CSC-HVDC system fault analysis.

[0131] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described with reference to the preferred embodiments of the present invention, it should be understood by those skilled in the art that various changes can be made in form and details without departing from the spirit and scope of the present invention as defined in the appended claims.

Claims

1. A fault analysis method based on the CSC-HVDC valve group interaction model, characterized in that: The steps include: S1. Obtaining electrical parameters of the working state of the CSC-HVDC system to be tested; S2. Constructing a CSC-HVDC valve group interaction model for fault analysis, using the operating state electrical parameters of the CSC-HVDC system to be tested as input to the CSC-HVDC valve group interaction model, and outputting dynamic characteristic parameters of the CSC-HVDC system to be tested, and then performing fault analysis on the CSC-HVDC system to be tested based on the obtained dynamic characteristic parameters; The specific steps for constructing the CSC-HVDC valve group interaction model are as follows: S21. Based on the operating state electrical parameters of the CSC-HVDC system, adopt the Thevenin equivalent for the AC system, determine the AC system equivalent impedance from the short-circuit ratio and the impedance angle, and obtain an AC system equivalent impedance model, wherein the AC system equivalent impedance model includes an equivalent ideal voltage source, an equivalent inductor, and an equivalent resistor; S22, ignoring the excitation branch and transformer losses, equating the converter transformer to an ideal transformer with series leakage reactance, and obtaining a dynamic analysis equivalent circuit of the CSC-HVDC system according to step S21; S23. Based on the dynamic analysis equivalent circuit of the CSC-HVDC system, using Kirchhoff's law and combining the electrical coupling on both sides of the converter transformer of the CSC-HVDC system, establish a filter inductance state space of the CSC-HVDC system; S24. Using a switching function model of modulation theory and combining it with a dynamic analysis equivalent circuit of the CSC-HVDC system, a unified state space of the commutation capacitors under the switching state of each bridge arm is established; S25. Assume that the external characteristics of the external DC system satisfy the algebraic differential equation Thus, the DC side state space of the CSC-HVDC system is established; where u D Indicates the voltage at the DC outlet, k D and k' D They represent the coefficients of the CSC DC current and its derivative in the DC system port external voltage equation, i dc represents the DC current, represents the first-order differential of DC current, X D Represents the algebraic differential combination of the external system state variables and input excitation variables in the port external voltage equation; S26. Based on the filter inductor state space, the unified state space of the commutation capacitor in the switching state of each bridge arm, and the DC side state space, matrix partitioning is performed to obtain a CSC-HVDC valve group interaction model.

2. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 1 is characterized in that: In step S21, the calculation formulas for the equivalent impedance, equivalent inductance, and equivalent resistance of the AC system are: Where Z s Indicates the equivalent impedance of the AC system, U pN Indicates the rated voltage of the AC bus, P dN Indicates the system rated active power, SCR indicates the AC system short circuit ratio, L s Indicates equivalent inductance, R s Indicates equivalent resistance, Represents the impedance angle of the AC system.

3. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 1 is characterized in that: In step S23, the step of establishing the CSC-HVDC system filter inductance state space includes: S231, performing node loop analysis on the dynamic analysis equivalent circuit of the CSC-HVDC system to obtain basic electrical equations; S232. Based on Hoff's law, analyze both sides of the converter transformer to obtain the voltage relationship between the two sides of the converter transformer, the AC bus voltage equation, and the current relationship between the upper and lower valve group transformers; S233. Perform mathematical operations on the relationship equation obtained in step S232 to express the leakage reactance current of the delta-connected converter transformer with the filter inductor current of the corresponding valve group, thereby unifying the state variables of the upper and lower valve groups. S234, establishing a voltage drop equation between the neutral point of the converter transformer on the valve side of the lower valve group and the neutral point of the commutation capacitor; S235 . Construct the filter inductance state space of the CSC-HVDC system using the relationship equations obtained in steps S231 to S234 .

4. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 3 is characterized in that: In step S235, the CSC-HVDC system filter inductor state space is expressed as: Where u jy Indicates the voltage of the commutation capacitor of the lower valve group, j=a,b,c,L y It represents the sum of the leakage reactance and filter inductance of the converter transformer connected to the lower valve group Y, k y Indicates the transformer ratio of the lower valve group, L s Indicates equivalent inductance, i jy Indicates the filter inductor current of the lower valve group, Indicates the first-order differential of the lower valve group filter inductor current, R s Indicates equivalent resistance, k d Indicates the transformer ratio of the upper valve group, i jd Indicates the upper valve group filter inductor current, Indicates the first-order differential of the upper valve group filter inductor current, u eqj represents an equivalent ideal voltage source, L f Indicates the filter inductance, L σd Indicates the leakage reactance of the converter transformer, u jd Indicates the upper valve group commutation capacitor voltage.

5. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 1 is characterized in that: In step S24, the switching function model of the modulation theory is: Where S ji Represents the three-phase switching function of the upper and lower valve groups.

6. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 5 is characterized in that: The unified state space of the commutation capacitor in the switching state of each bridge arm is expressed as: C f represents the commutation capacitance, Indicates the first-order differential of the upper valve group commutation capacitor voltage, Indicates the first-order differential of the commutation capacitor voltage of the lower valve group, i dc Indicates direct current.

7. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 1 is characterized in that: In step S25, the DC side state space of the CSC-HVDC system is expressed as follows based on the DC outlet voltage and the DC current expression: Where, L dc Indicates the DC side filter inductor, represents the first-order differential of DC current, u dcd 、u dcy 、u dc Respectively represent the upper valve group, lower valve group and total DC side voltage.

8. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 1 is characterized in that: In step S26, the CSC-HVDC valve group interaction model is expressed as: Where x d Represents the upper valve group state variable, x d =[i ad i bd i cd u ad u bd u cd ] T ;x y Represents the state variable of the lower valve group, x y =[i ay i by i cy u ay u by u cy ] T ,u eq represents the AC equivalent sinusoidal excitation source variable, u eq =[u eqa u eqb u eqc ] T ; A dD 、A yD 、A Dd 、A Dy represents the AC / DC interaction matrix; B d 、B y Represents the system communication matrix; A d 、A y It respectively represents the interaction between the state variables of the upper and lower valve groups.

9. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 1 is characterized in that: The working state electrical parameters include electrical equipment parameters and electrical transient parameters; the electrical equipment parameters include the DC side smoothing reactance parameters of the converter, the valve outlet commutation capacitor parameters, the AC filter inductor parameters, the leakage reactance ratio of the converter transformer, the AC system short-circuit ratio and impedance angle parameters; the electrical transient parameters include DC current and voltage, the commutation capacitor voltage of the two valve groups and the filter inductor current of the two valve groups.

10. The fault analysis method based on the CSC-HVDC valve group interaction model according to claim 1, characterized in that: The dynamic characteristic parameters include the direct current and voltage, the voltage of the commutation capacitor of the two valve groups, and the time domain differential components of the filter inductor current of the two valve groups.

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