Fault analysis method based on CSC-HVDC valve group interaction model

Abstract

The invention discloses a fault analysis method based on an interaction model between CSC-HVDC valve banks, and the method comprises the steps: obtaining the working state electrical parameters of a to-be-detected CSC-HVDC system, and constructing the interaction model between the CSC-HVDC valve banks for fault analysis, and taking the working state electrical parameters of the CSC-HVDC system to be detected as input of the CSC-HVDC valve group interaction model, outputting to obtain dynamic characteristic parameters of the CSC-HVDC system to be detected, and performing fault analysis on the CSC-HVDC system to be detected according to the obtained dynamic characteristic parameters. According to the model, a dynamic analysis equivalent circuit of the CSC-HVDC is established by qualitatively analyzing coupling characteristics of alternating current sides of an upper valve group and a lower valve group of the CSC-HVDC under the action of power grid impedance, the dynamic characteristics of the weak power grid connection CSC-HVDC system can be described more accurately, and when the CSC-HVDC system has a direct current fault, the dynamic characteristics of the weak power grid connection CSC-HVDC system can be analyzed more accurately. The CSC-HVDC system fault analysis method can more accurately describe the complex coupling characteristics and change rules of the key electrical quantities, thereby helping to improve the CSC-HVDC system fault analysis precision.

Description

Technical Field

[0001] The present invention relates to the technical field of high voltage direct current 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 converter (CSC) is a controllable shutdown current source converter. Its application in the field of DC transmission has gradually attracted attention from the academic community in recent years. Due to the use of fully controlled devices, the bridge arm commutation process of CSC does not depend on the voltage on the AC grid side, so the commutation failure problem can be completely avoided. Compared with LCC, it can better cope with AC side faults, which is conducive to improving the support capacity for weak AC systems and can also supply power to passive networks. Compared with MMC, CSC also has natural advantages in DC fault handling. The inherent smoothing inductance at its DC outlet can effectively limit the development speed of fault current, and CSC can control the polarity reversal of DC voltage and has the ability to clear DC fault current. There is no need to equip additional expensive current limiting devices, DC circuit breakers and other equipment, so it is very suitable for building a new DC transmission system.

[0003] Traditional three-level CSC units have problems such as large AC and DC harmonics and limited power transmission levels. For this reason, Li Zixin's team at the Institute of Electrical Engineering proposed a phase-shifted 12-pulse CSC-HVDC system (hereinafter referred to as CSC-HVDC) suitable for high-voltage and high-power transmission scenarios. It is composed of two cascaded three-level CSC units. The voltage power level, control flexibility, and harmonic characteristics of CSC-HVDC are better than those of ordinary three-level CSC systems, so it is more suitable as a converter in high-voltage DC transmission systems. At present, research on CSC-HVDC systems is mostly focused on modeling, modulation control, and analysis and design of related equipment in steady state.

[0004] In recent years, some scholars have established the transient state space equation of CSC-HVDC, but they regard the AC side as equivalent to a symmetrical constant sinusoidal voltage source, and believe that the converter is connected to an infinite power grid, thereby decoupling the upper and lower valve groups of CSC-HVDC at the AC bus. Therefore, it is essentially still a transient model of a three-level CSC unit, and is not suitable for analyzing the transient characteristics of CSC-HVDC when connected to a weak AC system. At present, there are few studies on transient modeling and DC fault analysis of CSC-HVDC. In the existing technology, due to the lack of model accuracy, the dynamic characteristic parameters predicted by the model deviate from the actual situation. At the same time, the existing model has limited analysis capabilities in the case of transient faults or disturbances, and cannot accurately capture the dynamic behavior of the system, making it impossible to accurately analyze the system during fault analysis. In addition, the existing literature does not consider the interaction between the upper and lower valve groups of CSC-HVDC. Summary of the invention

[0005] In view of the deficiencies 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 the grid impedance, a dynamic analysis equivalent circuit of the CSC-HVDC is established, thereby deriving the CSC-HVDC valve group interaction model, and using the model to obtain dynamic characteristic parameters, 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 comprises the following steps:

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

[0009] S2, constructing a CSC-HVDC valve group interaction model for fault analysis, taking the working state electrical parameters of the CSC-HVDC system to be detected as input of the CSC-HVDC valve group interaction model, outputting dynamic characteristic parameters of the CSC-HVDC system to be detected, and then performing fault analysis on the CSC-HVDC system to be detected according to the obtained dynamic characteristic parameters;

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

[0011] S21, based on the working state electrical parameters of the CSC-HVDC system, adopting the Thevenin equivalent of the AC system, determining the equivalent impedance of the AC system by the short-circuit ratio and the impedance angle, and obtaining an equivalent impedance model of the AC system, wherein the equivalent impedance model of the AC system includes an equivalent ideal voltage source, an equivalent inductance, and an equivalent resistance;

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

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

[0014] S24, using the switching function model of modulation theory and combining the dynamic analysis equivalent circuit of the CSC-HVDC system to establish a unified state space of the commutation capacitor under the switching state of each bridge arm;

[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 direct current, represents the first-order differential of DC current, X D The algebraic differential combination of the external system state variables and the 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 switch 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] In the formula, Z s Indicates the equivalent impedance of the AC system, U pN Indicates the rated voltage of the AC bus, P dN Indicates the rated active power of the system, SCR indicates the short-circuit ratio of the AC system, Ls Represents equivalent inductance, R s Represents equal value 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, performing mathematical operations on the relational equation obtained in step S232, and expressing the leakage reactance current of the delta-connected converter transformer with the filter inductor current of the corresponding valve group, so as to unify 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 through the relationship equations obtained in steps S231 to S234 .

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

[0027]

[0028] In the formula, u jy Indicates the voltage of the lower valve group commutation capacitor, 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 represents the equivalent inductance, i jy Indicates the filter inductor current of the lower valve group, Represents 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 filter inductor current of the upper valve group, Indicates the first-order differential of the upper valve group filter inductor current, u eqj represents an equivalent ideal voltage source, L f Represents the filter inductance, L σd represents 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] In the formula, 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 lower valve group commutation capacitor voltage, i dc Indicates direct current.

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

[0036]

[0037] Where, L dc represents 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] In the formula, 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 A represents the system communication matrix; d , A y It respectively represents the interaction relationship between the state variables of the upper and lower valve groups.

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

[0042] Specifically, 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.

[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 in the event of transient faults, disturbances or operating mode switching, and can serve as a model basis for stability analysis and fault characteristic analysis, thereby achieving 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 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 change laws of each key electrical quantity, thereby helping to improve the accuracy of the CSC-HVDC system fault analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to make the purpose, technical solution 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 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 is a diagram showing the fitting results of capacitor voltage and inductor current according to an embodiment of the present invention;

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

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

[0052] Figure 7 A schematic diagram of DC fault current and its change rate under different transition resistances using different fault analysis methods in 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 claimed invention, 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 belong to the scope of protection of the present invention.

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

[0055] In the prior art, line converters (LCC) and voltage source converters (VSC) 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 and 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 cost and large valve hall size. VSC-HVDC will also suffer from extremely high fault current 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 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, so that it is suitable for different scenarios.

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

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

[0058] S2, constructing a CSC-HVDC valve group interaction model for fault analysis, taking the working state electrical parameters of the CSC-HVDC system to be detected as input of the CSC-HVDC valve group interaction model, outputting dynamic characteristic parameters of the CSC-HVDC system to be detected, and then performing fault analysis on the CSC-HVDC system to be detected according to the obtained dynamic characteristic parameters;

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

[0060] S21, based on the working state electrical parameters of the CSC-HVDC system, adopting the Thevenin equivalent of the AC system, determining the equivalent impedance of the AC system by the short-circuit ratio and the impedance angle, and obtaining an equivalent impedance model of the AC system, wherein the equivalent impedance model of the AC system includes an equivalent ideal voltage source, an equivalent inductance, and an equivalent resistance;

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

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

[0063] S24, using the switching function model of modulation theory and combining the dynamic analysis equivalent circuit of the CSC-HVDC system to establish a unified state space of the commutation capacitor under the switching state of each bridge arm;

[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 direct current, represents the first-order differential of DC current, X D The algebraic differential combination of the external system state variables and the 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 switch 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 in the event of transient faults, disturbances or operating mode switching, and can serve as a model basis for stability analysis and fault characteristic analysis, thereby achieving 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 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 change laws of each key electrical quantity, thereby helping to improve the accuracy of the CSC-HVDC system fault analysis.

[0067] In this embodiment, using dynamic characteristic parameters to perform fault analysis on the CSC-HVDC system is a technology that is widely recognized and applied in the technical field and is not described in detail in the present invention. Now, the CSC-HVDC valve group interaction model constructed in the present invention is introduced 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] In the formula, Z s Indicates the equivalent impedance of the AC system, U pN Indicates the rated voltage of the AC bus, P dN Indicates the rated active power of the system, SCR indicates the short-circuit ratio of the AC system, L s Represents equivalent inductance, R s Represents equal value resistance, Represents the impedance angle of the AC system.

[0071] Figure 2 is the dynamic analysis equivalent circuit of the CSC-HVDC system in this embodiment, where the leakage reactance L σd , L σy is transferred to the valve side, k d , k y They are the transformation ratios of the upper and lower valve group transformers, the upper valve group transformer adopts the YD connection method, and the lower valve group transformer adopts the 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, performing 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] In the formula, i jy Represents the three-phase filter inductor current, u jd Indicates the upper valve group commutation capacitor voltage, u jy represents 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 operation of the relational equation obtained in step S232, the leakage current of the delta-connected converter transformer is represented by the filter inductor current of the corresponding valve group, so as to unify the state variables of the upper and lower valve groups, specifically:

[0081] The sum of both sides of the sub-equations of the current relationship between the upper and lower valve group transformers and combined with the basic electrical equations yields:

[0082]

[0083] The voltage relationship on both sides of the converter transformer is summed up 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, and further combined with the steady-state initial state, it can be obtained:

[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 is distorted under transient conditions such as DC side faults and operating conditions changes, it is not a three-phase symmetrical sine wave, but its three-phase sum is always 0. It can be seen that the three-phase leakage current of the delta-connected transformer also satisfies that its sum is 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 subtracting each sub-equation, the three-phase leakage current of the upper valve group can be expressed by the filter inductor current:

[0092]

[0093] Based on this, the i' in the upper valve group state variable can be 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 indicates 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 processes such as DC short circuit and operating condition changes, thus:

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

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

[0101]

[0102] In the formula, u jy Indicates the voltage of the lower valve group commutation capacitor, 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 represents the equivalent inductance, i jy Indicates the filter inductor current of the lower valve group, Represents 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 filter inductor current of the upper valve group, Indicates the first-order differential of the upper valve group filter inductor current, u eqj represents an equivalent ideal voltage source, L f Represents the filter inductance, L σd represents 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] In the formula, 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 lower valve group commutation capacitor voltage, 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 according to the DC outlet voltage using the expression of DC current:

[0110]

[0111] Where, L dc represents 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] In the formula, 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 quantity parameters. The electrical equipment parameters include converter DC side smoothing reactance parameters, valve outlet commutation capacitor parameters, AC filter inductance parameters, converter transformer leakage reactance ratio, AC system short circuit ratio and impedance angle parameters. The electrical transient quantity parameters include DC current and voltage, two-valve group commutation capacitor voltage, and two-valve group filter inductor current; the dynamic characteristic parameters include DC current and voltage, two-valve group commutation capacitor voltage, and time domain differential of two-valve group filter inductor current.

[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, which verifies the accuracy of the constructed model.

[0123] Coefficient sub-matrix A d , A y It reflects the relationship between the state variables of the upper and lower valve groups, which is called the valve group self-matrix. It is noted 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. Coefficient submatrix B 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 , K 7 From the expression, 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 implementation, a CSC-HVDC DC fault simulation model was built based on the PSCAD / EMTDC electromagnetic transient simulation platform, and a metallic short circuit fault was set at the outlet of the rectifier station. The algorithm in this paper was implemented through Matlab platform programming for analytical calculation and compared with the simulation value. In this verification example, the solid lines in each figure represent the simulation value, and the dotted lines / shaded areas represent the numerical analytical values. Figures 4 to 6 It can be seen that the coupling system model proposed in this embodiment can more accurately describe the complex coupling characteristics and change laws of various key electrical quantities after a DC fault occurs in CSC-HVDC, and can meet the needs of comprehensive and accurate transient analysis of the system. The DC fault current is also the focus of attention 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 the current source topology, the current flowing through the switch device of the converter's conducting bridge arm is the DC fault current, and its change rate is also of reference value for the selection of the switch device. Figure 7 The calculated values ​​and simulated values ​​of the DC fault current and its change rate 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 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 circuit composed of AC side capacitors and inductors. It is more accurate in the early stage of metallic short circuit fault at the converter outlet. However, since this method is based on the AC decoupling model, when the AC system is weak, it is also impossible to accurately calculate the DC fault current on a longer time scale. 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, and the mean absolute percentage error (MAPE) within 10ms is 0.14%, 0.16% and 0.15%, respectively, which verifies the accuracy of the proposed CSC-HVDC fault analysis method.

[0130] In summary, the present invention can accurately and clearly describe the dynamic characteristics of the CSC-HVDC system in the event of transient faults, disturbances or operating mode switching by constructing an interaction model between CSC-HVDC valve groups, and can serve as a model basis for stability analysis and fault characteristic analysis, thereby realizing a 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 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 changing laws of each key electrical quantity, 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 rather than to limit it. 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 may 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 the working state electrical parameters of the CSC-HVDC system to be tested; S2, constructing a CSC-HVDC valve group interaction model for fault analysis, taking the working state electrical parameters of the CSC-HVDC system to be detected as input of the CSC-HVDC valve group interaction model, outputting dynamic characteristic parameters of the CSC-HVDC system to be detected, and then performing fault analysis on the CSC-HVDC system to be detected according to the obtained dynamic characteristic parameters; The specific steps of constructing the CSC-HVDC valve group interaction model are as follows: S21, based on the working state electrical parameters of the CSC-HVDC system, adopting the Thevenin equivalent of the AC system, determining the equivalent impedance of the AC system by the short-circuit ratio and the impedance angle, and obtaining an equivalent impedance model of the AC system, wherein the equivalent impedance model of the AC system includes an equivalent ideal voltage source, an equivalent inductance, and an equivalent resistance; S22, ignoring the excitation branch and transformer losses, the converter transformer is equivalent to an ideal transformer with series leakage reactance, and according to step S21, a dynamic analysis equivalent circuit of the CSC-HVDC system is obtained; S23, based on the dynamic analysis equivalent circuit of the CSC-HVDC system, using Kirchhoff's law, combined with the electrical coupling on both sides of the converter transformer of the CSC-HVDC system, establish the filter inductance state space of the CSC-HVDC system; S24, using the switching function model of modulation theory and combining the dynamic analysis equivalent circuit of the CSC-HVDC system to establish a unified state space of the commutation capacitor under the switching state of each bridge arm; 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 direct current, represents the first-order differential of DC current, X D The algebraic differential combination of the external system state variables and the 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 switch 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: In the formula, Z s Indicates the equivalent impedance of the AC system, U pN Indicates the rated voltage of the AC bus, P dN Indicates the rated active power of the system, SCR indicates the short-circuit ratio of the AC system, L s Represents equivalent inductance, R s Represents equal value 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, performing mathematical operations on the relational equation obtained in step S232, and expressing the leakage reactance current of the delta-connected converter transformer with the filter inductor current of the corresponding valve group, so as to unify 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 through 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 inductance state space is expressed as: In the formula, u jy Indicates the voltage of the lower valve group commutation capacitor, 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 represents the equivalent inductance, i jy Indicates the filter inductor current of the lower valve group, Represents 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 filter inductor current of the upper valve group, Indicates the first-order differential of the upper valve group filter inductor current, u eqj represents an equivalent ideal voltage source, L f Represents the filter inductance, L σd represents 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: In the formula, 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 each bridge arm switching state 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 lower valve group commutation capacitor voltage, 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, according to the DC outlet voltage and the expression of DC current, the DC side state space of the CSC-HVDC system is expressed as: Where, L dc represents 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: In the formula, 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 A represents the system communication matrix; d , A y It respectively represents the interaction relationship 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 parameters include electrical equipment parameters and electrical transient quantity parameters; the electrical equipment parameters include converter DC side smoothing reactance parameters, valve outlet commutation capacitor parameters, AC filter inductance parameters, converter transformer leakage reactance ratio, AC system short-circuit ratio and impedance angle parameters; the electrical transient quantity parameters include DC current and voltage, two-valve group commutation capacitor voltage and two-valve group filter inductance current.

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 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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