LCC-HVDC sending end system impedance modeling method considering frequency coupling

Abstract

The invention provides an LCC-HVDC sending end system impedance modeling method considering frequency coupling, relates to the field of extra-high voltage direct current transmission, and solves the problem that converter small signal modeling cannot be directly linearized due to time-varying characteristics of rectification and an inverter. The method comprises the following steps: carrying out impedance admittance modeling on a rectification circuit structure and a control structure of the LCC-HVDC sending end system to obtain mathematical expressions of a thyristor converter, a direct current bus, a current loop and a phase-locked loop, and then establishing an impedance model of the LCC-HVDC sending end system by adopting an impedance analysis method according to the obtained mathematical expressions; and establishing system equivalent impedance of the LCC-HVDC sending end system by applying a frequency coupling phenomenon through a mathematical expression of self-impedance and accompanying impedance in an impedance model of the LCC-HVDC sending end system. According to the method, the safe and stable influence factors of the LCC-HVDC sending end system can be analyzed, and then the safe and stable operation interval of the system is determined.

Description

Technical Field

[0001] The present invention relates to the field of UHVDC transmission, and is applied to the sending end system of LCC-HVDC. Specifically, it relates to an impedance modeling method for the sending end system of LCC-HVDC considering frequency coupling. Background Art

[0002] With the continuous growth of the global demand for clean energy, the power industry is undergoing unprecedented restructuring. Since there are rich hydropower resources in the southwest, thermal power and wind power resources in the northwest of China, how to efficiently and stably transmit these energy sources to economically developed but energy-scarce regions such as North China, Central China, East China and South China has become one of the key challenges in the development of the current power system. In this context, the line-commutated converter high-voltage direct current (LCC-HVDC) transmission technology has received extensive attention due to its significant advantages in long-distance and large-capacity power transmission.

[0003] However, although the LCC-HVDC transmission technology does not have the angle stability problem in traditional AC power grids, the interaction between it and a large number of power electronic equipment widely used in wind farms and other new energy power stations has brought new challenges to the safe and stable operation of the power system. Specifically, these power electronic devices may lead to frequent resonance accidents, seriously threatening the stability and security of the power system and possibly causing huge economic losses. Therefore, in order to solve these problems and ensure the safe and stable operation of the power system, it is urgent to develop a new system impedance modeling method.

[0004] The existing analysis methods for LCC-HVDC systems do not fully consider the interaction with large-scale power electronic equipment, resulting in an incomplete understanding of the actual operating conditions of the system. In the face of a complex and changing power system environment, the current technology is difficult to accurately predict and evaluate the stability performance of the system under different operating conditions, thus limiting the development of optimal operation strategies. In addition, due to the lack of effective means to determine the safe and stable operation range of the sending end system of LCC-HVDC, it not only affects the effective utilization of power resources, but also increases the risk of power system failures.

[0005] Therefore, in order to more effectively utilize China's rich clean energy resources, realize energy complementarity and optimal allocation in different regions, ensure the safe and stable operation of the power system, and promote the sustainable and healthy development of the economy, it is urgent to develop a new impedance analysis-based impedance modeling method for the sending end system of LCC-HVDC. This method should not only help analyze the factors affecting the safety and stability of the sending end system of LCC-HVDC, but also determine the safe and stable operation range of the system, which can be used as the basis for putting forward effective operation suggestions to cope with the challenges faced by the current power system. Summary of the Invention

[0006] Based on the current situation in the background technology, the object of the present invention is to solve the problem that still exists in the small-signal modeling of converters in DC transmission systems, that is, the problem of being unable to be directly linearized due to the time-varying characteristics of rectifiers and inverters; therefore, the present invention proposes an impedance modeling method for the sending-end system of LCC-HVDC considering frequency coupling. This method can analyze the factors affecting the safety and stability of the sending-end system of LCC-HVDC, and then determine the safe and stable operation range of the system; through the application of this method, effective operation suggestions for ensuring the safe and stable operation of the system can be put forward.

[0007] The present invention adopts the following technical solutions to achieve the object:

[0008] An impedance modeling method for the sending-end system of LCC-HVDC considering frequency coupling, the method includes the following steps:

[0009] S1. According to the rectifier circuit structure of the sending-end system of LCC-HVDC, impedance admittance modeling is carried out to obtain the mathematical expressions of the thyristor converter and the DC bus in the rectifier circuit structure;

[0010] S2. According to the control structure of the rectifier circuit in the sending-end system of LCC-HVDC, impedance admittance modeling is carried out to obtain the mathematical expressions of the current loop and the phase-locked loop in the control structure;

[0011] S3. Based on the respective mathematical expressions of the thyristor converter, the DC bus, the current loop and the phase-locked loop, an impedance model of the sending-end system of LCC-HVDC is established by using the impedance analysis method;

[0012] S4. Through the mathematical expressions of the self-impedance and the adjoint impedance in the impedance model of the sending-end system of LCC-HVDC, the system equivalent impedance of the sending-end system of LCC-HVDC is established by applying the frequency coupling phenomenon.

[0013] Further, in step S1, for the mathematical expression of the thyristor converter, the self-admittance Y of the sending-end system of LCC-HVDC is defined SA and the adjoint admittance Y AA After the single-frequency input and double-frequency output effects in the sending-end system of LCC-HVDC, the modeling of the thyristor converter is completed and its mathematical expression is determined.

[0014] Further, in step S1, for the mathematical expression of the DC bus, when modeling, the average model is adopted for modeling, and the transformer impedance of the sending-end system of LCC-HVDC is referred to the primary side to obtain the corresponding relationships of DC current, DC voltage, AC current and AC voltage.

[0015] Preferably, the relationships of direct current, direct current voltage, alternating current, and alternating current voltage are transformed into the frequency domain and linearized for small signals to obtain the mapping relationship between small signals and frequency between AC and DC, thereby completing the modeling of the DC bus and determining its mathematical expression.

[0016] Preferably, in step S2, after considering the constraint conditions of the asymmetry of the dq axes of the converters in the control structure, the mathematical expressions of the current loop and the phase-locked loop in the control structure are obtained.

[0017] Specifically, in step S3, when establishing the impedance model, the content of the impedance model includes the important components in the LCC-HVDC sending-end system, namely the rectifier circuit, the phase-locked loop, the control structure, and the AC power grid.

[0018] Furthermore, in step S4, based on the mathematical expressions of the self-impedance and the adjoint impedance in the impedance model, corresponding to the self-admittance Y SA and the adjoint admittance Y AA of the LCC-HVDC sending-end system, when applying the frequency coupling phenomenon, the coupling path is unified into the additional admittance Y C_eq to obtain the system equivalent impedance Z ac_eq considering the frequency coupling effect.

[0019] The present invention also provides a computer system, including a memory, a processor, and a computer program stored on the memory, and the processor executes the computer program to implement the steps of the foregoing impedance modeling method for the LCC-HVDC sending-end system considering frequency coupling.

[0020] In summary, due to the adoption of the present technical solution, the beneficial effects of the present invention are as follows:

[0021] The core of the present invention lies in being able to accurately characterize the impedance characteristics of the LCC-HVDC sending-end system. By comprehensively considering the influence of the grid impedance and frequency coupling, the present invention constructs an AC equivalent impedance model that is closer to the actual operating conditions. This model not only enhances the understanding of the internal dynamic characteristics of the LCC-HVDC sending-end system but also provides a solid theoretical basis for in-depth analysis of the stability when the UHVDC system is interconnected with the new energy system.

[0022] The AC equivalent impedance model established by using the method of the present invention provides a new perspective and tool for studying the factors affecting the safety and stability of the LCC-HVDC sending-end system. It enables researchers to more accurately determine the safe and stable operating range of the system, thereby providing a scientific basis for the optimal operation of the power system. In addition, the operation suggestions proposed based on this model can effectively guide the system operation and maintenance in practice, significantly improving the stability and safety of the power system.

[0023] The present invention also demonstrates great potential in enhancing the compatibility between UHV DC transmission systems and new energy grid connection. By solving the problems of insufficient accuracy and difficulty in comprehensively reflecting the interactive effects between systems in traditional technologies, the present invention opens up a new way for the safe and efficient transmission of clean energy. This not only helps to ensure the power supply safety in energy-scarce but economically developed regions of our country, but also promotes energy complementarity and optimal allocation among regions, further driving the sustainable development of the economy. Therefore, while promoting the technological innovation of the power industry, the present invention also makes important contributions to social and economic development. Brief Description of the Drawings

[0024] Figure 1 Schematic diagram for a brief description of the overall process of the method of the present invention;

[0025] Figure 2 Schematic diagram of the structure of the LCC rectifier circuit in the method of the present invention;

[0026] Figure 3 Schematic diagram of the control structure of the LCC rectifier circuit in the method of the present invention;

[0027] Figure 4 Schematic diagram of the single-phase equivalent circuit of the LCC-HVDC rectifier terminal in the method of the present invention;

[0028] Figure 5 Time-domain relationship diagram between the output of the LCC controller and the trigger pulse in the method of the present invention;

[0029] Figure 6 Schematic diagram of the small-signal path of system frequency coupling in the method of the present invention. Detailed Embodiments

[0030] To make the objectives, 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 accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0031] 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 present invention claimed, but merely represents the selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0032] Embodiment

[0033] An impedance modeling method for the LCC-HVDC sending-end system considering frequency coupling, Figure 1A brief process of the method is shown, which can be referred to synchronously; the general steps of the method are as follows:

[0034] S1. According to the rectifier circuit structure of the LCC-HVDC sending-end system, impedance admittance modeling is carried out to obtain the mathematical expressions of the thyristor converter and the DC bus in the rectifier circuit structure;

[0035] S2. According to the control structure of the rectifier circuit in the LCC-HVDC sending-end system, impedance admittance modeling is carried out to obtain the mathematical expressions of the current loop and the phase-locked loop in the control structure;

[0036] S3. Based on the respective mathematical expressions of the thyristor converter, the DC bus, the current loop and the phase-locked loop, an impedance model of the LCC-HVDC sending-end system is established by using the impedance analysis method;

[0037] S4. Through the mathematical expressions of the self-impedance and the adjoint impedance in the impedance model of the LCC-HVDC sending-end system, and applying the frequency coupling phenomenon, a system equivalent impedance of the LCC-HVDC sending-end system is established.

[0038] In this embodiment, the preferred detailed content of each step will be introduced in detail according to the above step sequence.

[0039] Step S1 can be referred to Figure 2 For the schematic diagram of its application structure, that is, the system structure of the LCC rectifier terminal. In this step, for the mathematical expression of the thyristor converter, the self-admittance Y SA and the adjoint admittance Y AA of the single-frequency input and double-frequency output effects in the LCC-HVDC sending-end system are as follows:

[0040]

[0041] In the formula, Y SA (ω p ) represents the self-admittance of the LCC-HVDC sending-end system at the frequency of ω p , and Y AA (ω p ) represents the adjoint admittance of the LCC-HVDC sending-end system at the frequency of ω p ; V p (ω p ) and I p (ω p ) are respectively the voltage disturbance input and the current disturbance output at the frequency of ω p ; I p (ω p -2ω1) is the current disturbance output at the frequency of ω p -2ω1, and ω pω represents the disturbance frequency on the AC side, and ω1 represents the fundamental frequency; thus, the modeling of the thyristor converter is completed and the above mathematical expressions are determined.

[0042] At the same time, for the mathematical expressions of the DC bus, when modeling, the averaging model is adopted. The transformer impedance of the LCC-HVDC sending-end system is referred to the primary side to obtain the DC current, DC voltage, AC current, and AC voltage. The corresponding relationships are as follows:

[0043]

[0044] In the formula, v dcr is the DC voltage, i dc is the DC current, v x is the corresponding value of the AC voltage after conversion by the transformer turns ratio in the calculation of the operating point; i p is the AC current, m v is the mapping relationship between the three-phase AC voltage and the DC voltage, and m i is the mapping function from the DC current to the AC current.

[0045] As an optimization of this embodiment, the relationships of the DC current, DC voltage, AC current, and AC voltage are subjected to frequency-domain transformation and small-signal linearization to obtain the mapping relationship between the small signals and frequencies between the AC and DC sides, as shown in the following formula:

[0046]

[0047] In the formula, ω p - ω1 represents the disturbance frequency ω on the AC side p After the transformation between the AC and DC sides regarding the fundamental frequency ω1, it appears as a disturbance with a frequency of ω p - ω1 on the DC side; represents the existence of two disturbance frequencies, namely ω p frequency and ω p - 2ω1 frequency; M v and M i respectively represent the steady-state values of the voltage and current mapping relationships, and the two are used to describe the voltage, current, and their components m v (ω p ) and m i (ω p ) in the modeling of the control structure of the rectifier circuit; V x is the steady-state value of the AC voltage, and I dcr is the steady-state value of the DC current; thus, the modeling of the DC bus is completed and the above mathematical expressions are determined.

[0048] In step S2, after considering the constraint condition of the asymmetry between the d-axis and q-axis of the converter in the control structure, the mathematical expressions of the current loop and the phase-locked loop in the control structure are obtained. The preferred method is as follows:

[0049] Figure 3 The structure diagram of the LCC rectifier controller with examples is given, and the scheme of this step is applied to it. When modeling the control structure in this step, for the small-signal disturbance path, ω p is defined as the disturbance output of the controller in the control structure at the frequency, and A(ω p 0, then:

[0050] A(ω p -ω1) = α(ω p -ω1) + θ(ω p -ω1)

[0051] In the formula, A(ω p -ω1) is the defined disturbance output of the controller, and α(ω p -ω1) and θ(ω p -ω1) are obtained from the circuit of the control structure itself, as shown in the following formula:

[0052]

[0053] In the formula, α(ω p -ω1) is the trigger angle output of the current loop, θ(ω p -ω1) is the phase-locked loop angle output, k d is used to describe the conversion relationship between the angle and radian of the trigger angle; ω p represents the disturbance frequency on the AC side, ω1 represents the fundamental frequency; G i (ω p -ω1) represents the sampling delay of the control loop in the control structure, H i (ω p -ω1) represents the PI controller of the current loop control; v p represents the voltage at the PCC point, i dc represents the DC current; T rp (ω p -ω1) represents the closed-loop transfer function of the phase-locked loop at the rectifier end; thus, the modeling of the current loop and the phase-locked loop is completed and the above mathematical expressions are determined.

[0054] In step S3, using the impedance analysis method, based on the disturbance relationship between the AC voltage and the current, the following formula is determined:

[0055]

[0056] In the formula, v x is the corresponding value of the AC voltage after being converted by the transformer turns ratio in the working point calculation, vp represents the PCC point voltage, ω p represents the disturbance frequency on the AC side, ω1 represents the fundamental frequency, i p is the alternating current, Z t is the transformer equivalent impedance.

[0057] In this embodiment, combined with Figure 4 The single-phase equivalent circuit of the LCC-HVDC rectifier end is displayed. The disturbance relationship between the AC voltage and current is combined with the mathematical expressions obtained in steps S1 and S2 to obtain the self-admittance Y of the LCC-HVDC sending end system. SA and the adjoint admittance Y AA The expression is as follows:

[0058]

[0059] In the formula, I dcr is the steady-state value of DC current; M v1 、M v2 、M i1 and M i2 Represent the steady-state value of each mapping function; G c , G p , A1, A2, B1, B2 are represented as follows:

[0060]

[0061] A1=Z gdc (ω p -ω1)-6V x G c (ω p -ω1)-3M v2 Z t (ω p -2ω1)[M i2 +I dcr G c (ω p -ω1)]

[0062] A2=z gdc (ω p -ω1)-6V x G c (ω p -ω1)-3M v1 Z t (ω p -2ω1)[M i1 +I dcr G c (ω p -ω1)]

[0063] B1=3Mv1 +6V x G p (ω p -ω1)+3M v2 Z t (ω p -2ω1)I dcr G p (ω p -ω1)

[0064] B2 = 3M v1 +6V x G p (ω p -ω1)+3M v1 Z t (ω p )I dcr G p (ω p -ω1)

[0065] In the above formulas, k d is used to describe the conversion relationship between the angle and radian of the triggering angle; G i (ω p ) represents the sampling delay of the control loop in the control structure, H i (ω p ) represents the PI controller of the current loop control; T rp (ω p ) represents the closed-loop transfer function of the phase-locked loop at the rectifier end; V x is the steady-state value of the AC voltage, I dcr is the steady-state value of the DC current, Z gdc is the equivalent impedance of the DC power grid. Figure 5 Figure shows the time-domain relationship diagram between the output of the LCC controller and the trigger pulse, which can be compared and referred to during the actual application and execution of the method.

[0066] The content of step S4 can be referred to Figure 6 for illustration. In this embodiment, based on the mathematical expressions of the self-impedance and the adjoint impedance in the impedance model, corresponding to the self-admittance Y Sa and the adjoint admittance Y AA of the LCC-HVDC sending-end system, when the frequency coupling phenomenon is applied, the coupling path is unified into the additional admittance Y C_eq , so as to obtain the system equivalent impedance Z ac_eq considering the frequency coupling effect, as follows:

[0067]

[0068] In the formula, ω p represents the AC-side disturbance frequency, v ac , iac represent the disturbance voltage and disturbance current respectively; Y SA_eq is the self-admittance considering the frequency coupling effect, Y C_eq is as follows:

[0069]

[0070] In the formula, ω1 represents the fundamental frequency, * represents the conjugate, Y AA_eq is the adjoint admittance considering the frequency coupling effect, Z g is the grid impedance.

[0071] The above method of this embodiment can be effectively applied to a computer system, which includes a memory, a processor, and a computer program stored on the memory. When the processor executes the computer program, the steps of the impedance modeling method of the LCC-HVDC sending-end system considering frequency coupling can be realized.

Claims

1. A method for modeling impedance of LCC-HVDC sending-end system considering frequency coupling, characterized in that: The method comprises the following steps: S1. Based on the rectifier circuit structure of the LCC-HVDC sending-end system, impedance-admittance modeling is performed to obtain the mathematical expressions of the thyristor converter and the DC bus in the rectifier circuit structure; S2. Based on the control structure of the rectifier circuit in the LCC-HVDC sending end system, impedance-admittance modeling is performed to obtain the mathematical expressions of the current loop and the phase-locked loop in the control structure; S3. Based on the mathematical expressions corresponding to the thyristor converter, DC bus, current loop and phase-locked loop, an impedance model of the LCC-HVDC sending-end system is established using impedance analysis method; S4. By using the mathematical expressions of self-impedance and accompanying impedance in the impedance model of the LCC-HVDC sending-end system and applying the frequency coupling phenomenon, the system equivalent impedance of the LCC-HVDC sending-end system is established.

2. The LCC-HVDC sending-end system impedance modeling method according to claim 1, characterized in that: In step S1, for the mathematical expression of the thyristor converter, the self-admittance Y of the LCC-HVDC sending end system is defined as SA and the adjoint admittance Y AA The single-frequency input and dual-frequency output effects in the LCC-HVDC sending-end system are as follows: Where Y SA (ω p ) represents ω p The self-admittance of the LCC-HVDC sending end system at the frequency, Y AA (ω p ) represents ω p The accompanying admittance of the LCC-HVDC sending end system at the frequency; V p (ω p ) and I p (ω p ) are ω p Voltage disturbance input and current disturbance output at frequency; I p (ω p -2ω1(ω p Current disturbance output at -2ω1 frequency, ω p Represents the disturbance frequency on the AC side, and ω1 represents the fundamental frequency; thereby, the modeling of the thyristor converter is completed and the above mathematical expression is determined.

3. The LCC-HVDC sending-end system impedance modeling method according to claim 1, characterized in that: In step S1, for the mathematical expression of the DC bus, an average model is used to model the transformer impedance of the LCC-HVDC sending-end system to the primary side, and the DC current, DC voltage, AC current and AC voltage are obtained. The corresponding relationship is as follows: In the formula, v dcr is the DC voltage, i dc is the DC current, v x is the corresponding value of the AC voltage after the transformer ratio conversion in the working point calculation; i p is the alternating current, m v is the mapping relationship between the three-phase AC voltage and DC voltage, m i is the mapping function from DC current to AC current.

4. The LCC-HVDC sending-end system impedance modeling method according to claim 3, characterized in that: The relationship between DC current, DC voltage, AC current and AC voltage is transformed into the frequency domain and processed into small signal linearly to obtain the mapping relationship between small signal and frequency between AC and DC, as shown in the following formula: In the formula, ω p -ω1 represents the disturbance frequency ω on the AC side p After the conversion of the fundamental frequency ω1 between AC and DC, it becomes ω on the DC side. p -ω1 frequency disturbance; Represents the existence of two disturbance frequencies, namely ω p Frequency and ω p -2ω1 frequency; M v With M i They represent the steady-state values ​​of the voltage and current mapping relationship, which are used to model the voltage, current and their small signal disturbance frequency components m in the control structure of the rectifier circuit. v (ω p ) and m i (ω p ) for description; V x is the steady-state value of the AC voltage, I dcr is the steady-state value of the DC current; thus, the DC bus modeling is completed and the above mathematical expression is determined.

5. The LCC-HVDC sending-end system impedance modeling method according to claim 1, characterized in that: In step S2, after considering the constraint condition of the converter dq axis asymmetry in the control structure, the mathematical expressions of the current loop and the phase-locked loop in the control structure are obtained.

6. The LCC-HVDC sending-end system impedance modeling method according to claim 5, characterized in that: In step S2, when modeling the control structure, for the small signal disturbance path, define ω p The disturbance output of the controller in the control structure at the frequency is A(ω p ),but: A(ω p -ω1)=α(ω p -ω1)+θ(ω p -ω1) In the formula, A(ω p -ω1) is the controller disturbance output, α(ω p -ω1) and θ(ω p -ω1) is obtained by the control structure's own circuit, as follows: In the formula, α(ω p -ω1) is the current loop trigger angle output, θ(ω p -ω1) is the phase-locked loop angle output, k d Used to describe the conversion relationship between the trigger angle and radians; ω p represents the disturbance frequency on the AC side, ω1 represents the fundamental frequency; G i (ω p -ω1) represents the sampling delay of the control loop in the control structure, H i (ω p -ω1) represents the PI controller of current loop control; v p represents the PCC point voltage, i dc represents direct current; T rp (ω p -ω1) represents the closed-loop transfer function of the phase-locked loop at the rectifier end; thereby completing the modeling of the current loop and the phase-locked loop and determining the above mathematical expression.

7. The LCC-HVDC sending-end system impedance modeling method according to claim 1, characterized in that: In step S3, when establishing the impedance model, the impedance model content includes important components of the LCC-HVDC sending end system, namely, the rectifier circuit, the phase-locked loop, the control structure and the AC power grid.

8. The LCC-HVDC sending-end system impedance modeling method according to claim 1, characterized in that: In step S3, the impedance analysis method is used to determine the following equation based on the disturbance relationship between the AC voltage and the current: In the formula, v x is the corresponding value of the AC voltage after the transformer ratio conversion in the working point calculation, v p represents the PCC point voltage, ω p represents the disturbance frequency on the AC side, ω1 represents the fundamental frequency, i p is the alternating current, Z t is the transformer equivalent impedance; Combining the above formula with the mathematical expressions obtained in steps S1 and S2, we can obtain the self-admittance Y of the LCC-HVDC sending end system: SA and the adjoint admittance Y AA The expression is as follows: In the formula, I dcr is the steady-state value of DC current; M v1 、M v2 、M i1 and M i2 Represent the steady-state value of each mapping function; G c , G p , A1, A2, B1, B2 are represented as follows: A1=Z gdc (oh p -ω1)-6V x G c (oh p -ω1)-3M v2 Z t (oh p -2ω1)[M i2 +I dcr G c (oh p -ω1)] A2=z gdc (oh p -ω1)-6V x G c (oh p -ω1)-3M v1 Z t (oh p -2ω1)[M i1 +I dcr G c (oh p -ω1)] B1=3M v1 +6V x G p (oh p -ω1)+3M v2 Z t (oh p -2ω1)I dcr G p (oh p -ω1) B2=3M v1 +6V x G p (oh p -ω1)+3M v1 Z t (oh p )I dcr G p (oh p -ω1) In the above formulas, k d Used to describe the conversion relationship between the trigger angle and radians; G i (ω p ) represents the sampling delay of the control loop in the control structure, H i (ω p ) represents the PI controller of the current loop control; T rp (ω p ) represents the closed-loop transfer function of the phase-locked loop at the rectifier end; V x is the steady-state value of the AC voltage, I dcr is the steady-state value of DC current, Z gdc is the equivalent impedance of the DC grid.

9. The LCC-HVDC sending-end system impedance modeling method according to claim 1, characterized in that: In step S4, based on the mathematical expressions of the self-impedance and the accompanying impedance in the impedance model, the self-admittance Y corresponding to the LCC-HVDC sending end system is calculated. SA and the adjoint admittance Y AA , when the frequency coupling phenomenon is applied, the coupling path is unified with the additional admittance Y C_eqc , thus obtaining the system equivalent impedance Z considering the frequency coupling effect ac_eq , as follows: In the formula, ω p represents the disturbance frequency on the AC side, v ac 、i ac Represent the disturbance voltage and disturbance current respectively; Y SA_eq Y is the self-admittance considering the frequency coupling effect. C_eq Then the formula is as follows: In the formula, ω1 represents the fundamental frequency, * represents conjugation, and Y AA_eq To consider the accompanying admittance of the frequency coupling effect, Z g is the grid impedance.

10. A computer system comprising a memory, a processor and a computer program stored in the memory, characterized in that: The processor executes the computer program to implement the steps of the LCC-HVDC sending-end system impedance modeling method considering frequency coupling according to any one of claims 1 to 9.

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