MMC direct current side impedance modeling method considering direct current line

Abstract

The invention relates to an MMC direct-current side impedance modeling method considering a direct-current line, and belongs to the technical field of stability analysis of a flexible direct-current power transmission system. Firstly, an MMC main circuit small signal frequency domain linearization model is established based on a multi-harmonic linearization method, then a small signal frequency domain linearization model controlled by a current inner loop, a ring current, a power outer loop and a direct current voltage outer loop in an MMC control link is established, an MMC direct current side impedance model is established by associating a main circuit model with a control link model, and further, the MMC direct current side impedance model is established. An MMC inter-station direct current line is considered, and an MMC direct current side impedance model considering the direct current line is established; compared with a conventional method, the method has the advantages that the model precision is improved, the stability problem of the direct-current power grid can be analyzed more accurately, and the method is of great significance to engineering construction of a flexible direct-current power transmission system.

Description

Technical Field

[0001] The present invention relates to the technical field of stability analysis of flexible direct current transmission systems, and in particular to a harmonic transmission mechanism of an MMC under direct current side excitation, and in particular to a DC side impedance modeling method of an MMC taking into account a DC line, which is suitable for stability analysis between stations in a flexible direct current transmission system. Background Art

[0002] The high-voltage direct current transmission (MMC-HVDC) system based on modular multilevel converter (MMC) has been widely used due to its advantages such as flexible control, low switching loss and modular design. However, due to the interaction between MMC stations, resonant instability may occur. At present, the MMC DC side impedance modeling method proposed in the prior art mainly focuses on the impedance characteristics of the MMC itself, and rarely considers the internal harmonic transmission mechanism of the MMC. In addition, the influence of the DC line cannot be ignored. When the DC line is long, the distributed parameters such as line resistance, inductance and capacitance will increase with the increase of line length, thereby changing the resonant frequency point of the system. In some cases, a longer line may cause the resonant frequency to fall into a wide frequency range, causing wide-band oscillation. Therefore, it is necessary to analyze the resonance problem of the MMC-HVDC system to avoid causing large-scale instability problems. To analyze the resonance problem, it is first necessary to establish a suitable MMC model. Summary of the invention

[0003] The purpose of the present invention is to provide a method for modeling the DC side impedance of MMC taking into account the DC line, which solves the problem of inaccurate DC side impedance modeling of MMC due to failure to consider the influence of the DC line in the prior art. The present invention first establishes a small signal frequency domain linearization model of the MMC main circuit based on the multi-harmonic linearization method, and then establishes a small signal frequency domain linearization model of the current inner loop, circulating current, power outer loop and DC voltage outer loop control in the MMC control link, and establishes the MMC DC side impedance model by associating the main circuit model and the control link model. The modeling method of the present invention can effectively judge the stability of the DC power grid, which is of great significance to the engineering construction of the flexible DC transmission system.

[0004] The above-mentioned object of the present invention is achieved by the following technical solutions:

[0005] The DC side impedance modeling method of MMC considering the DC line presents each variable in the main circuit equation in the form of a 2n+1 harmonic component matrix, thereby highlighting the frequency coupling characteristics of each variable inside the MMC in the main circuit equation. In the control system, the harmonic component characteristics circulating inside each link are identified according to the harmonic frequency to characterize the coupling relationship between different variables in each link, while considering the influence of the DC line, and finally using frequency domain modeling to study the coupling characteristics of the nth harmonic in the MMC converter, including the following steps:

[0006] Step (1), constructing a small signal frequency domain linearization model of the main circuit of the MMC: In order to take into account the internal harmonic coupling characteristics of the MMC, the phase sequence relationship or the differential mode and common mode components between the three phases are described by the modulus function, and each variable is represented as a vector form considering different harmonic orders to establish a main circuit model;

[0007] Step (2), constructing a small signal frequency domain linearization model of the control link of the MMC: establishing the corresponding relationship between the small signal harmonic disturbance quantity of the input and output of each control link, that is, the relationship between the input voltage or current and the output voltage or current;

[0008] Step (3), constructing the MMC DC side impedance model: adopting two control modes, constant power control and constant DC voltage control, for the outer loop respectively, establishing the corresponding relationship between the main circuit equation and the switching coefficient equation, and constructing the MMC DC disturbance matrix;

[0009] Step (4), constructing the MMC DC side impedance model taking into account the DC line: the MMC DC side impedance model established in step (3) is combined with the DC line equivalent model to construct the MMC DC side impedance model taking into account the DC line.

[0010] The following is a description of each step:

[0011] The main circuit frequency domain linearization model of the MMC described in step (1) is constructed by:

[0012] The variables x of different harmonic orders (including MMC upper and lower bridge arm voltage, current, modulation function, DC voltage, AC voltage, bridge arm equivalent inductance and capacitance) are expressed in vector form x = [x -k ,...,x -1 ,x0,x1,...,x k ] T , where k is the harmonic order, the mathematical model of MMC in the frequency domain is expressed as:

[0013]

[0014] In the formula, v u and v l Indicates the voltage of the upper and lower bridge arms of MMC, i u and i l Indicates the upper and lower bridge arm current of MMC, m u and m l Represents the modulation function of the upper and lower bridge arms of MMC, v g is the AC voltage, Y L and Z C is the bridge arm admittance and capacitance impedance matrix after the disturbance is injected.

[0015] Furthermore, auxiliary functions S1(k) and S2(k) are established to represent the harmonic phase sequence and output properties of MMC, which are expressed as:

[0016]

[0017] Then the frequency domain small signal model of the MMC upper bridge arm is expressed as:

[0018]

[0019] In the formula, the superscript ^ represents the disturbance; Y L and Z C is the bridge arm admittance and capacitance impedance matrix after the disturbance is injected, M u 、V u ,I u The frequency domain matrices representing the upper bridge arm modulation function, the upper bridge arm voltage and the upper bridge arm current, respectively, and Respectively represent the DC side voltage and AC port phase voltage disturbance, is the upper arm current disturbance, is the modulation coefficient disturbance of the upper bridge arm, is the capacitance voltage disturbance of the upper bridge arm equivalent submodule;

[0020] The bridge arm admittance Y after the disturbance is injected L and the capacitive impedance Z C The expression is:

[0021]

[0022] Where diag[] is the diagonal matrix to find the sign.

[0023] The frequency domain linearization model of the control part of the MMC described in step (2) is constructed by:

[0024] In order to analyze the interaction stability between DC grids, it is assumed that the equivalent impedance of each AC system is small, so the system model can be simplified, ignoring the influence of AC grid strength and phase-locked loop, and the small signal model of the upper bridge arm modulation function is Expand as follows:

[0025]

[0026] Where D i , D c , D p , D q , D dc They represent the disturbance modulation functions of current inner loop control, circulating current control, active power control, reactive power control, and DC voltage control respectively. impact.

[0027] The MMC DC side impedance model described in step (3) is constructed by:

[0028] Bringing the control part modeling into the main circuit equation, the disturbance of the modulation function of each control mode can be uniformly described as and The relevant form is obtained, and the expression of the MMC DC side impedance is:

[0029]

[0030] In the formula, B i and B v Indicates the bridge arm disturbance current and DC voltage disturbances DC side admittance Y dc impact.

[0031] The construction of the MMC DC side impedance model taking into account the DC line in step (4) is specifically:

[0032] The equivalent model of the DC line can be represented by a pi-type equivalent circuit, which equates the transmission line and the constant power station to a separate impedance Z ∑ , expressed as:

[0033]

[0034] Where Z PQ , Z Vdc Z represents the DC impedance of constant active and reactive power control and constant voltage control. RL Represents the equivalent impedance of resistance and inductance, Z C Represents the equivalent impedance of a capacitor.

[0035] The beneficial effects of the present invention are as follows: the MMC DC side impedance modeling method taking into account the DC line proposed in the present invention establishes an MMC DC side impedance model taking into account the DC line based on the multi-harmonic linearization method, which can more comprehensively consider the characteristics of the DC line at different harmonic frequencies compared to the conventional method, so that the established impedance model can more accurately reflect the operation of the actual system. In the design stage of the DC power grid, the modeling method can provide a scientific basis for the selection of line parameters, the layout of the converter station and the formulation of the control strategy. By analyzing the DC side impedance under different design schemes, the optimal design scheme can be selected to improve the performance and reliability of the system and reduce the construction cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The drawings described herein are used to provide further understanding of the present invention and constitute a part of this application. The illustrative examples of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0037] Figure 1 It is a control system diagram of the MMC of the present invention;

[0038] Figure 2 It is a schematic diagram of the equivalent impedance of the DC side of the power transmission line of the present invention;

[0039] Figure 3 The impedance sweep and simulation verification of the MMC constant power station of the present invention;

[0040] Figure 4 The impedance sweep and simulation verification of the MMC constant DC voltage station of the present invention;

[0041] Figure 5 The impedance sweep and simulation verification of the MMC constant power station taking into account the DC line of the present invention;

[0042] Figure 6 The influence of the change of the current inner loop parameters of the present invention on the MMC impedance characteristics;

[0043] Figure 7 The influence of the power outer loop parameter change on the MMC impedance characteristics of the present invention;

[0044] Figure 8 The influence of the change of the parameters of the DC voltage outer loop of the present invention on the impedance characteristics of the MMC;

[0045] Fig. 9 It is the Bode analysis diagram of the example of the present invention;

[0046] Fig.10 It is the active power variation diagram of the calculation example of the present invention;

[0047] Fig.11 This is the FFT analysis diagram of active power of the example of the present invention. DETAILED DESCRIPTION

[0048] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0049] See also Figures 1 to 11As shown, the MMC DC side impedance modeling method taking into account the DC line of the present invention first establishes a small signal frequency domain linearization model of the MMC main circuit based on the multi-harmonic linearization method, and then establishes a small signal frequency domain linearization model of the current inner loop, circulating current, power outer loop and DC voltage outer loop control in the MMC control link, and establishes the MMC DC side impedance model by associating the main circuit model and the control link model. Further, considering the MMC inter-station DC line, the MMC DC side impedance model taking into account the DC line is established. The present invention can effectively judge the stability of the DC power grid through a designed MMC DC side impedance modeling method taking into account the DC line, which is of great significance to the engineering construction of the flexible DC transmission system.

[0050] See also Figure 1 to Figure 2 As shown, the MMC DC side impedance modeling method taking into account the DC line of the present invention comprises the following steps:

[0051] Step (1), constructing a small signal frequency domain linearization model of the main circuit of the MMC: In order to take into account the internal harmonic coupling characteristics of the MMC, the phase sequence relationship or the differential mode and common mode components between the three phases are described by the modulus function, and each variable is represented as a vector form considering different harmonic orders to establish a main circuit model; the variables x of different harmonic orders (including the upper and lower bridge arm voltages, currents, modulation functions, DC voltages, AC voltages, bridge arm equivalent inductances, and capacitances of the MMC) are represented as vector forms x = [x -k ,...,x -1 ,x0,x1,...,x k ] T , where k is the harmonic order, the mathematical model of MMC in the frequency domain is expressed as:

[0052]

[0053] In the formula, v u and v l Indicates the voltage of the upper and lower bridge arms of MMC, i u and i l Indicates the upper and lower bridge arm current of MMC, m u and m l Represents the modulation function of the upper and lower bridge arms of MMC, v g is the AC voltage, Y L and Z C is the bridge arm admittance and capacitance impedance matrix after the disturbance is injected.

[0054] Furthermore, auxiliary functions S1(k) and S2(k) are established to represent the harmonic phase sequence and output properties of MMC, which are expressed as:

[0055]

[0056] Then the frequency domain small signal model of the MMC upper bridge arm is expressed as:

[0057]

[0058] In the formula, the superscript ^ represents the disturbance; Y L and Z C is the bridge arm admittance and capacitance impedance matrix after the disturbance is injected, M u 、V u ,I u The frequency domain matrices representing the upper bridge arm modulation function, the upper bridge arm voltage and the upper bridge arm current, respectively, and Respectively represent the DC side voltage and AC port phase voltage disturbance, is the upper arm current disturbance, is the modulation coefficient disturbance of the upper bridge arm, is the capacitance voltage disturbance of the upper bridge arm equivalent submodule;

[0059] The bridge arm admittance Y after the disturbance is injected L and the capacitive impedance Z C The expression is:

[0060]

[0061] Where diag[] is the diagonal matrix to find the sign.

[0062] Step (2), constructing a small signal frequency domain linearization model of the control link of the MMC: establishing the corresponding relationship between the small signal harmonic disturbance quantity of the input and output of each control link, that is, the relationship between the input voltage or current and the output voltage or current; specifically:

[0063] In order to analyze the interaction stability between DC grids, it is assumed that the equivalent impedance of each AC system is small, so the system model can be simplified, ignoring the influence of AC grid strength and phase-locked loop, and the small signal model of the upper bridge arm modulation function is Expand as follows

[0064]

[0065] Where D i , D c , D p , D q , D dc They represent the disturbance modulation functions of current inner loop control, circulating current control, active power control, reactive power control, and DC voltage control respectively. impact.

[0066] The following is the modeling of each control link.

[0067] (1) Current inner loop control

[0068] When the DC side of the MMC is disturbed, the AC side will also be disturbed. The current inner loop control acts on the output current of the MMC AC side. Since the output current only includes the positive or negative sequence differential mode component of the bridge arm current, that is, "|S1(k)||S2(k)|=1", the current inner loop control has no effect on the output current. The impact of D i It is expressed as:

[0069]

[0070] In the formula, G i Indicates the effect of the inner current loop control on the AC current disturbance The role of is expressed as:

[0071]

[0072] In the formula, H i,k is the PI control link of the current inner loop, k pi , k ii Its proportional and integral parameters.

[0073] (2) Circulation Control

[0074] The circulating current control acts on the double frequency circulating current component in the three-phase bridge arm current. Since the double frequency circulating current component only includes the positive or negative sequence common mode component of the bridge arm current, that is, it satisfies "|S1(k)|(1-|S2(k))=1", the circulating current control has no significant effect on the current. The impact of D c It is expressed as:

[0075]

[0076] In the formula, G c Indicates the effect of circulating current control on AC current disturbance The role is expressed as:

[0077]

[0078] In the formula, K c =2ω0L is the decoupling coefficient k of the circulating control link pc and k ic are the proportional and integral parameters of the circulating flow control PI controller.

[0079] (3) Outer loop control

[0080] 1) Fixed active / reactive power control

[0081] The disturbance values ​​of active power and reactive power are expressed as:

[0082]

[0083] Where V d 、V q ,I d ,I q is the dq axis steady-state component of the voltage and current at the PCC point. Assuming that the phase-locked loop has ideal working performance, when the system is in steady-state operation, V d ≈V1,V q ≈0 can be ignored, then the frequency domain form of equation (18) is expressed as:

[0084]

[0085] Since the AC side voltage and current only contain non-zero sequence differential mode components, after Park transformation, V p 、V q ,I p ,I q The non-zero elements of correspond to the zero-sequence circulating current components, that is, they satisfy "(1-|S1(k))(1-|S2(k))=1", and according to the Park transform, the frequency is f p The small signal component of f p -f1 and f p +f1 small signal components, so distributed around the diagonal elements, the above matrix is ​​expressed as:

[0086]

[0087] Since the active power and reactive power only act on the reference values ​​of the positive sequence current inner loop control d-axis and q-axis, The impact of D p , D q It is expressed as:

[0088]

[0089] In the formula, G ir Indicates the effect of the current control inner loop on the inner loop reference value, G P , G Q represents the effect of the active / reactive control outer loop on the active / reactive power. Its non-zero elements correspond to the zero-sequence circulating current component, that is, it satisfies "(1-|S1(k))(1-|S2(k))=1". According to Park transformation, the non-zero elements are distributed on the upper and lower sides of the diagonal elements. Then G p , G q Respectively expressed as:

[0090]

[0091] Among them, H p,k and H q,k It is expressed as:

[0092]

[0093] In the formula, k pp , k pi , k qp , k qi are the proportional and integral parameters of the active / reactive PI controller.

[0094] 2) Constant DC voltage control

[0095] Similar to the constant active power control, the constant DC voltage control acts alone on the reference value of the positive sequence current inner loop control d axis. The impact of D dc It is expressed as:

[0096]

[0097] In the formula, G dc represents the effect of the fixed DC voltage outer loop on the DC voltage. The non-zero elements correspond to the zero-sequence circulating current component, that is, "(1-|S1(k))(1-|S2(k))=1". According to Park transformation, the non-zero elements are distributed on the upper and lower sides of the diagonal elements. Then the DC voltage control matrix G under AC disturbance injection is dc It is expressed as:

[0098] G dc (:,4)=0.5[00 H dc,-1 0H dc,-1 00] T (18)

[0099] Among them, H dc,k It is expressed as:

[0100]

[0101] In the formula, k dcp , k dci are the proportional and integral parameters of the constant DC voltage PI controller.

[0102] Step (3), constructing the MMC DC side impedance model: adopting two control modes, constant power control and constant DC voltage control, for the outer loop respectively, establishing the corresponding relationship between the main circuit equation and the switching coefficient equation, and constructing the MMC DC disturbance matrix; specifically:

[0103] Bringing the control part modeling into the main circuit equation, the disturbance of the modulation function of each control mode can be uniformly described as and The relevant form is obtained, and the expression of the MMC DC side impedance is:

[0104]

[0105] In the formula, B i and B v Indicates the bridge arm disturbance current and DC voltage disturbances DC side admittance Y dc The influence of is expressed as:

[0106]

[0107] Step (4), constructing the MMC DC side impedance model taking into account the DC line: The MMC DC side impedance model established in step (3) is combined with the DC line equivalent model to construct the MMC DC side impedance model taking into account the DC line, specifically:

[0108] The equivalent model of the DC line can be represented by a pi-type equivalent circuit, which equates the transmission line and the constant power station to a separate impedance Z ∑ , expressed as:

[0109]

[0110] Where Z PQ , Z Vdc Z represents the DC impedance of constant active and reactive power control and constant voltage control. RL Represents the equivalent impedance of resistance and inductance, Z C Represents the equivalent impedance of a capacitor.

[0111] Example:

[0112] (1) Impedance model verification

[0113] In order to verify the accuracy of the established MMC converter station impedance model, a simulation software was built including Figure 2 The MMC converter station simulation model of the control system is shown in Table 1. The main system parameters are shown in Table 1. The comparison between the theoretical impedance curve of the MMC converter station and the simulation measurement results is shown in Table 1. Figures 3 to 5 As shown, the results show that the analytical model is in good agreement with the scanning results, verifying the accuracy of the analytical model.

[0114] Table 1 Main parameters of the system

[0115]

[0116] according to Figure 5 When the DC line is taken into account, the impedance characteristics of the constant power station side in most frequency bands above 100Hz are capacitive, while the impedance characteristics of the constant DC voltage station in this area are inductive. When the parameters of the two are not designed reasonably, resonance points are likely to exist and oscillations are caused. Therefore, it is necessary to further analyze the influence of the MMC control system parameters on the impedance characteristics of the converter station.

[0117] (2) Impedance characteristics analysis

[0118] The impedance model is closely related to the control link of MMC. The characteristics of the control link are also included in the established DC side impedance model. In order to clarify the influence of the control link on the MMC impedance characteristics, the MMC impedance characteristics are analyzed by changing the parameters of the current inner loop, power outer loop, and DC voltage outer loop.

[0119] In order to analyze the influence of the change of the inner current loop parameters on the MMC impedance characteristics, the outer loop control is firstly cancelled. The analysis results are as follows: Figure 6 As shown, as the current inner loop k pi The increase in parameters mainly affects the amplitude and phase near the fundamental frequency. For the frequency band below 20Hz, the MMC impedance amplitude increases and the phase decreases, while there is almost no effect for the frequency band above 100Hz. ppq The parameter changes are basically the same as those in the inner loop, such as Figure 7 shown.

[0120] like Figure 8 As shown in the figure, the influence of the change of the DC voltage outer loop parameters on the impedance characteristics, as k pdc As the parameters increase, the MMC impedance amplitude increases, and changes greatly below 20Hz and near the fundamental frequency. The phase changes with k in the 50-200Hz frequency range. pdc The increase in the damping moves towards the negative damping direction, which can easily lead to the risk of resonance.

[0121] (3) Simulation Verification

[0122] Based on the established impedance model, the oscillation problem is analyzed, and the simulation results are as follows: Figures 9 to 11 shown. Fig. 9 is the Bode analysis graph, when k pdc =0.2, there is a resonance point at 135Hz at the two end converter stations, and the phase angle difference is 179.7°. At this time, the system is stable. From the above analysis, it can be seen that increasing k pdc The parameter tends to introduce more negative damping at 100-200Hz for constant DC voltage stations, so adjusting k pdc =5, at this time the phase angle difference is 187° and the system becomes unstable. Fig.10 This is the active power simulation diagram. At 6s, k pdc Adjusted from 0.2 to 5, the system becomes unstable, and the FFT analysis results are as follows Fig.11 As shown in Figure 2, the system is unstable at this time, and the simulation results are consistent with the analysis results.

[0123] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made to the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for modeling the DC side impedance of an MMC taking into account the DC line, characterized in that: The variables in the main circuit equation are presented in the form of a 2n+1 harmonic component matrix to highlight the frequency coupling characteristics of the variables inside the MMC in the main circuit equation; and in the control system, the harmonic component characteristics circulating inside each link are identified according to the harmonic frequency to characterize the coupling relationship between different variables in each link, while considering the influence of the DC line, and finally the frequency domain modeling is used to reflect the coupling characteristics of the nth harmonic in the MMC converter, which includes the following steps: Step (1), constructing a small signal frequency domain linearization model of the main circuit of the MMC: In order to take into account the internal harmonic coupling characteristics of the MMC, the phase sequence relationship or the differential mode and common mode components between the three phases are described by the modulus function, and each variable is represented as a vector form considering different harmonic orders to establish a main circuit model; Step (2), constructing a small signal frequency domain linearization model of the control link of the MMC: establishing the corresponding relationship between the small signal harmonic disturbance quantity of the input and output of each control link, that is, the relationship between the input voltage or current and the output voltage or current; Step (3), constructing the MMC DC side impedance model: adopting two control modes, constant power control and constant DC voltage control, for the outer loop respectively, establishing the corresponding relationship between the main circuit equation and the switching coefficient equation, and constructing the MMC DC disturbance matrix; Step (4), constructing the MMC DC side impedance model taking into account the DC line: the MMC DC side impedance model established in step (3) is combined with the DC line equivalent model to construct the MMC DC side impedance model taking into account the DC line.

2. The MMC DC side impedance modeling method taking into account the DC line according to claim 1, characterized in that: The construction of the small signal frequency domain linearization model of the main circuit of the MMC described in step (1) is specifically: The variable x of different harmonic orders is expressed in vector form x=[x -k ,...,x -1 ,x0,x1,...,x k ] T , where k is the harmonic order, the mathematical model of MMC in the frequency domain is expressed as: In the formula, v u and v l Indicates the voltage of the upper and lower bridge arms of MMC, i u and i l Indicates the upper and lower bridge arm current of MMC, m u and m l Represents the modulation function of the upper and lower bridge arms of MMC, v g is the AC voltage, Y L and Z C is the bridge arm admittance and capacitance impedance matrix after the disturbance is injected; Furthermore, auxiliary functions S1(k) and S2(k) are established to represent the harmonic phase sequence and output properties of MMC, which are expressed as: Then the frequency domain small signal model of the MMC upper bridge arm is expressed as: In the formula, the superscript ^ represents the disturbance; Y L and Z C is the bridge arm admittance and capacitance impedance matrix after the disturbance is injected, M u 、V u ,I u The frequency domain matrices representing the upper bridge arm modulation function, the upper bridge arm voltage and the upper bridge arm current, respectively, and Respectively represent the DC side voltage and AC port phase voltage disturbance, is the upper arm current disturbance, is the modulation coefficient disturbance of the upper bridge arm, is the capacitance voltage disturbance of the upper bridge arm equivalent submodule; The bridge arm admittance Y after the disturbance is injected L and the capacitive impedance Z C The expression is: Where diag[] is the diagonal matrix to find the sign.

3. The MMC DC side impedance modeling method taking into account the DC line according to claim 1, characterized in that: The construction of the small signal frequency domain linearization model of the control link of the MMC described in step (2) is specifically: Ignoring the influence of AC grid strength and phase-locked loop, the small signal model of the upper bridge arm modulation function Expand as follows: Among them, D i , D c , D p , D q , D dc They represent the disturbance modulation functions of current inner loop control, circulating current control, active power control, reactive power control, and DC voltage control respectively. impact.

4. The MMC DC side impedance modeling method taking into account the DC line according to claim 1, characterized in that: The MMC DC side impedance model described in step (3) is constructed by: The control part model is brought into the main circuit equation, and the disturbance of the modulation function of each control mode is uniformly described as and The relevant form, the MMC DC side impedance expression is obtained as: In the formula, B i and B v Indicates the bridge arm disturbance current and DC voltage disturbances DC side admittance Y dc impact.

5. The MMC DC side impedance modeling method taking into account the DC line according to claim 1, characterized in that: The construction of the MMC DC side impedance model taking into account the DC line in step (4) is specifically: The DC line equivalent model is represented by a pi-type equivalent circuit, which equates the transmission line and the constant power station to a separate impedance Z ∑ , expressed as: Among them, Z PQ , Z Vdc Z represents the DC impedance of constant active and reactive power control and constant voltage control. RL Represents the equivalent impedance of resistance and inductance, Z C Represents the equivalent impedance of a capacitor.

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