Impedance modeling and grid-connected stability analysis method based on flexible excitation synchronous machine

Abstract

The invention discloses an impedance modeling and grid-connected stability analysis method based on a flexible excitation synchronous machine, and the method comprises the steps: dividing the flexible excitation synchronous machine according to a topological structure and a control block diagram of the flexible excitation synchronous machine; according to topological structure division of the flexible excitation synchronous machine, modeling is carried out on the synchronous machine, an excitation system machine side and an excitation system network side; a unified impedance model considering a phase-locked loop, a current loop, a DC voltage loop, a reactive power loop, an excitation voltage loop and a dual-channel proportional link is established, and the accuracy of the impedance model is verified through simulation frequency sweep. And converting the 2 * 2 admittance matrix into an equivalent SISO model to analyze the stability of the system so as to position the negative resistance frequency band existing in the system. And finally, verifying the accuracy of grid-connected stability analysis through the simulation model. Therefore, the method is more suitable for simpler working conditions and has very high applicability.

Description

Technical Field

[0001] The present invention relates to the field of grid-connected stability analysis of power systems, and in particular to an impedance modeling and grid-connected stability analysis method based on a flexible excitation synchronous machine. Background Art

[0002] In current technology, synchronous units represented by thermal power account for 57% of the total installed capacity, and still occupy a large proportion of the overall power system, which has an important impact on system stability. Excitation control of synchronous units is the most important means of stability control. At present, the self-parallel excitation method based on thyristors is the most widely used excitation method. However, the thyristor static self-parallel excitation method faces complex oscillation problems, and the damping control method of conventional excitation is single. Analysis shows that the damping control of conventional excitation can play a good damping effect near 1Hz-2Hz, but in the sub- / super-synchronous frequency band above 5Hz, the oscillation suppression effect is relatively limited.

[0003] As a compensation voltage source on the line, series compensation can improve the transmission capacity of the line and improve reactive power balance. However, its series compensation capacitor will change the system impedance, causing a negative resistance frequency band in the system, thereby inducing complex oscillation problems such as low-frequency, sub / super synchronous oscillation, etc., which seriously threatens the stability of the power system.

[0004] Conventional excitation synchronous machines may cause complex oscillations after being connected to the grid through series compensation. DN Walker et al. recorded in the document titled Results of subsynchronous resonance test at Mohave (IEEE Transactions on Power Apparatus and Systems, 1975, 94(5): 1878-1889) that subsynchronous oscillations may occur when a synchronous machine is connected to the grid through series compensation, resulting in damage to the main shaft of the generator set; LI Changyu et al. proved in the document titled Research on sub-synchronous oscillation suppression of flexible HVDC with thermal power units based on external loop damping control (Journal of Electric Power Science and Technology, 2024, 39(2): 20-27) that when the synchronous unit is close to the flexible DC transmission, subsynchronous oscillation of the synchronous machine shaft system may occur.

[0005] In recent years, a flexible excitation synchronous machine topology has been proposed. Based on the existing conventional excitation synchronous machine, the excitation device is improved and a damping channel is added, which can effectively suppress the risk of complex oscillation. In the paper titled Design and Implementation of Voltage Source Converter Excitation System to Improve Power System Stability (IEEE Transactions on Industry Applications, 2016, 52(4): 2778-2788), ZHU Chen analyzed the action mechanism of the dual damping channel in the low frequency band by the complex torque coefficient method. However, the damping effect in the wide frequency band is not intuitive enough and the mechanism is not clear. At present, the impedance analysis method is widely used to analyze the system stability.

[0006] The impedance analysis method can analyze broadband oscillation problems more clearly. At present, the new energy grid-connected equipment represented by double-fed wind turbines and photovoltaic power generation has analyzed the system stability problems of single and multiple machines through impedance modeling. For flexible excitation synchronous machines, there is no relevant impedance modeling work, and the relationship between its excitation / reactive channel and the speed and active power of the synchronous machine is not clear, and the grid stability law is not clear.

[0007] In summary, there is an urgent need to establish a unified impedance model that takes into account the phase-locked loop, current loop, DC voltage loop, reactive power loop, excitation voltage loop and dual-channel proportional link based on the analysis of the topological structure of the flexible excitation synchronous machine to verify the accuracy of the grid-connected stability analysis. Summary of the invention

[0008] In view of the above, the present invention proposes an impedance modeling and grid-connected stability analysis method based on a flexible excitation synchronous machine. The corresponding flexible excitation synchronous machine analytical model includes complex links such as the synchronous machine, the excitation system and its control links, and the dual damping channel. The corresponding stability analysis judgment criterion is the equivalent SISO impedance stability analysis method.

[0009] The first aspect of the present invention is an impedance modeling and grid-connected stability analysis method based on a flexible excitation synchronous machine, comprising the following steps:

[0010] S1: According to the topological structure and control block diagram of the flexible excitation synchronous machine, the flexible excitation synchronous machine is divided into three main parts: synchronous machine, excitation system machine side and excitation system grid side;

[0011] S2: According to the topological structure of the flexible excitation synchronous machine, the three main parts are modeled respectively. In the modeling process, the control links including the excitation voltage loop, current loop, phase-locked loop, DC voltage loop and reactive power loop, the proportional link of the dual damping channel, the lead-lag link, the filter, the notch filter and other links are considered;

[0012] S3: Considering frequency coupling, the grid impedance matrix is ​​calculated as follows:

[0013]

[0014] Where: Z g is the grid impedance matrix, Z 11 (s) is the positive sequence impedance of the power grid in the complex frequency domain, Z 22 (s) is the negative sequence impedance of the power grid in the complex frequency domain, and s is the Laplace operator;

[0015] S4: According to the grid impedance matrix Z g and the analytical model of the admittance matrix of the flexible excitation synchronous machine Y fes , based on the equivalent SISO impedance stability criterion, the stability of the flexible excitation synchronous machine grid-connected system through series compensation is judged.

[0016] Furthermore, the first step of the modeling step in step S2 is to establish an analytical model of the synchronous machine admittance matrix without considering the excitation system:

[0017]

[0018] Where ΔI=[Δi d Δi f Δi D Δi q Δi g Δi Q ] T , A2=[010000] T ; where the subscripts d, q, f, D, Q and g represent the components of the corresponding variables, dq axis, excitation winding f, d axis damping winding D and q axis damping winding Q and g respectively; ΔI is the small signal of the current vector of each winding, Δu dq , Δi dq is the small signal of the voltage and current vector at the machine end, Δu f is the small signal of excitation voltage.

[0019] Furthermore, the second step of the modeling step in step S2 is to establish an analytical model of the admittance matrix on the machine side of the excitation system, which includes an automatic voltage regulator (Automatic Voltage Regulator, AVR) and a power system stabilizer (Power System Stabilizer, PSS).

[0020] Furthermore, the third step of the modeling step in step S2 is to establish an analytical model of the admittance matrix on the grid side of the excitation system. This part mainly includes AC / DC converter, DC / DC chopper and reactive power compensation controller (RPDC). Since the AC / DC converter is affected by control actions such as DC voltage control and phase-locked control, it is a dq asymmetric system with frequency coupling characteristics. This part uses the harmonic linearization method for impedance modeling. Considering that the DC capacitance of the DC / DC chopper is large, the influence of DC voltage disturbance on the excitation voltage output is ignored.

[0021] Furthermore, the specific implementation of judging the stability of the series-compensated grid-connected system of the flexible excitation synchronous machine in step S4 is as follows: using the equivalent SISO impedance stability analysis method, the off-diagonal elements of the admittance matrix of the flexible excitation synchronous machine system and the grid impedance are converted to the main diagonal, and the equivalent SISO impedance ZSISO and the grid positive sequence impedance Z11(s) are simultaneously drawn in the Bode diagram, and then it is judged whether the phase difference at the intersection of the amplitudes of the two impedance curves exceeds 180°: if so, the system is judged to be unstable; if not, the system is judged to be stable.

[0022] The second aspect of the present invention is an impedance modeling and grid-connected stability analysis device based on a flexible excitation synchronous machine, comprising the following modules:

[0023] Division module: According to the topological structure and control block diagram of the flexible excitation synchronous machine, the flexible excitation synchronous machine is divided into the synchronous machine, the machine side of the excitation system and the grid side of the excitation system;

[0024] Modeling module: According to the topological structure of the flexible excitation synchronous machine, the synchronous machine, the machine side of the excitation system and the grid side of the excitation system are modeled respectively;

[0025] Calculation module: Under frequency coupling, the grid impedance matrix Z is calculated g ;

[0026] Analysis and judgment module: according to the grid impedance matrix Z g and the analytical model of the admittance matrix of the flexible excitation synchronous machine Y fes , based on the equivalent SISO impedance stability criterion, the stability of the flexible excitation synchronous machine grid-connected system through series compensation is judged.

[0027] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0028] 1. The present invention establishes a unified impedance model of a flexible excitation synchronous machine that takes into account the phase-locked loop, current loop, DC voltage loop, reactive power loop, excitation voltage loop and dual-channel proportional link, and verifies the accuracy of the impedance model through simulated frequency sweep. As an essential component of the impedance analysis method, the impedance model is the basis for subsequent grid-connected stability analysis and improvement. The model fully considers the situation in which the grid-side converter and the synchronous machine form a coupling relationship through the damping channel, and has the characteristics of easy analysis, simplicity and clarity, and high accuracy. The grid-connected stability is analyzed by the equivalent SISO method, which can accurately analyze the stability of the flexible excitation synchronous machine connected to the grid through series compensation under complex conditions.

[0029] 2. The impedance modeling and grid-connected stability analysis method based on the flexible excitation synchronous machine of the present invention is not only applicable to complex situations that require the complete consideration of the analytical model of the flexible excitation synchronous machine, but can also be applied to simpler working conditions by simplifying the flexible excitation synchronous machine model accordingly. Therefore, the present invention has strong applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0031] Figure 1 It is a schematic diagram of the steps of the impedance modeling and grid-connected stability analysis method based on the flexible excitation synchronous machine of the present invention;

[0032] Figure 2 This is a single machine infinite system diagram using flexible excitation in the present invention;

[0033] Figure 3 AVR and PSS structure diagram of the present invention;

[0034] Figure 4 This is a verification diagram of the analytical model of the flexible excitation synchronous machine of the present invention;

[0035] Figure 5 The equivalent SISO model of the present invention and the Bode diagram of the power grid impedance;

[0036] Figure 6 This is a diagram showing the FFT analysis results of the grid-connected point current of the present invention. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0038] The present invention discloses an impedance modeling and grid-connected stability analysis method based on a flexible excitation synchronous machine. The method is targeted at the flexible excitation synchronous machine, and takes into account the impedance models of the phase-locked loop, current loop, DC voltage loop, reactive power loop, excitation voltage loop and dual-channel proportional link, and comprehensively considers the model of the grid-side converter. Its internal control link is not only affected by its own small voltage and current disturbances, but also responds when the voltage and current at the machine end of the synchronous machine oscillate. This coupling relationship is analyzed in detail during the modeling process. The three parts of the synchronous machine, the machine side of the excitation system, and the grid side of the excitation system are analyzed separately, and finally the whole machine admittance analytical model of the flexible excitation synchronous machine is obtained by aggregation at the PCC point. The accuracy of the flexible excitation synchronous machine analytical model derived by the present invention is proved by frequency sweep verification on the Matlab / Simulink electromagnetic simulation platform; the equivalent SISO impedance stability analysis method is adopted to convert the non-diagonal elements of the admittance matrix of the flexible excitation synchronous machine and the grid impedance to the main diagonal, and the equivalent SISO impedance and the grid positive sequence impedance are judged. The phase difference at the intersection of the resistance curve amplitude is used to judge the stability of the flexible excitation synchronous machine connected to the grid through series compensation. It can be used for system stability analysis under complex conditions of flexible excitation application in actual engineering. Compared with the existing flexible excitation synchronous machine model established by the complex torque coefficient method, it is more intuitive, clear, accurate and complete, thus revealing the damping support effect and mechanism of the dual damping channel of the flexible excitation system on the overall equipment, and can more accurately and quantitatively analyze the damping support effect of the dual damping channels on the overall equipment under the separate and joint action, providing guidance for the adjustment and optimization of relevant parameters of the dual damping channels in actual engineering.

[0039] like Figure 1 As shown, the impedance modeling and grid-connected stability analysis method based on the flexible excitation synchronous machine of the present invention includes the following steps:

[0040] (1) The excitation system of the flexible excitation synchronous machine can realize the functions of the existing excitation system and provide an additional damping channel by adjusting the reactive power exchanged with the grid side. Therefore, it is called a flexible excitation system. Figure 2 It is a single-machine infinite system using flexible excitation.

[0041] (2) The flexible excitation synchronous machine is mainly composed of a synchronous machine and a flexible excitation system. First, the impedance model of the former is obtained by the small signal method, which serves as the basis for subsequent overall impedance modeling.

[0042] The construction of the synchronous machine impedance matrix is ​​actually to analyze the small disturbance Δu of the machine terminal voltage. dq The current response to a small disturbance Δi dq This paper adopts the sixth-order model of the synchronous machine. According to the electromagnetic equation of the synchronous machine, the s operator is used to replace the derivative term d / dt, and the expression of small signal voltage disturbance and current response is obtained.

[0043] Δu=(R+sX)Δi;

[0044] Where Δu=[Δu d Δu f 0 Δu q 0 0] T , Δi=[-Δi d Δi f Δi D -Δi q Δi g Δi Q ] T , Wherein, the subscripts d, q, f, D, Q and g represent the components of the corresponding variables, i.e., the dq axis, the excitation winding f, the d-axis damping winding D and the q-axis damping winding Q and g, respectively; ω0 is the rated angular velocity of the rotor.

[0045] From the above formula, we can get the current vector of each winding ΔI and the voltage and current vector of the machine end Δu dq , Δi dq and excitation voltage Δu f The small signal frequency domain relationship between

[0046]

[0047] In the formula, ΔI=[Δi d Δi f Δi D Δi q Δi g Δi Q ] T ;

[0048] (3) The machine side of the flexible excitation system mainly includes the automatic voltage regulator (AVR) and the power system stabilizer (PSS). The construction of the excitation system impedance matrix is ​​actually to analyze the small disturbance Δu of the excitation voltage. f The small disturbance Δu of the synchronous machine terminal voltage dq , current response to small disturbance Δi dqAccording to the topological structure of AVR and PSS, the expression of small signal voltage and current disturbance and excitation voltage response can be derived.

[0049] Δu f =-H AVR H r Δu t +H AVR Δu PSS ;

[0050] In the formula, K a With T a is the gain and time constant of the power amplifier link, T c With T c1 is the leading phase compensation time constant, T b With T b1 is the time constant of the delayed phase compensation; T r is the rectifier time constant; Δu f , Δu t and Δu PSS They are the excitation voltage, machine terminal voltage and PSS output voltage small signal respectively.

[0051] Since the power unit of the excitation system shares the terminal voltage signal with the excitation regulator, There is nonlinearity in u 2 However, the frequency domain relationship between the terminal voltage disturbance and the excitation response can be obtained by local linearization. t Differentiating it gives:

[0052]

[0053] In the formula, u t 、u d0 and u q0 They are the steady-state value of the machine-end voltage and the dq-axis components respectively.

[0054] There is a relationship between the PSS output voltage small signal and the PSS input small signal:

[0055] Δu PSS =H PSS ΔP a ;

[0056] In the formula, H PSS is the transfer function of the PSS link, and the input variable is the acceleration power P a =P m -P e , P m and P eare the mechanical power and electromagnetic power of the synchronous machine respectively. The former remains constant, that is:

[0057]

[0058] Transfer function H PSS The expression is:

[0059]

[0060] In the formula, i d0 and i q0 are the dq axis components of the machine-end current respectively; T hp and T lp are the time constants of high-pass and low-pass filters respectively; K PSS is the proportionality coefficient of PSS; T e With T e1 is the leading phase compensation time constant, T d With T d1 is the time constant of the delayed phase compensation.

[0061] According to the above formula, the frequency domain relationship of the synchronous machine small signal can be derived as follows:

[0062]

[0063] Based on the above formula, the machine-side partial admittance model of the synchronous machine and the excitation system can be obtained as follows:

[0064]

[0065]

[0066] Where I is the 2*2 unit matrix.

[0067] The above formula obtains the machine-side admittance expression of the flexible excitation synchronous machine in the dq coordinate system, but the broadband impedance modeling is established in the abc coordinate system. The impedance in the dq coordinate system is converted to the broadband impedance in the abc coordinate system using the linear transformation matrix. The conversion relationship is:

[0068]

[0069] In the formula, Y m It is the positive and negative sequence admittance on the machine side of the flexible excitation synchronous machine.

[0070] The grid side of the flexible excitation system mainly includes AC / DC converter, DC / DC chopper and reactive power compensation controller (RPDC). Since the AC / DC converter is affected by control effects such as DC voltage control and phase-locked control, it is a dq asymmetric system with frequency coupling characteristics. This part uses the harmonic linearization method for impedance modeling. Considering the large DC capacitance of the DC / DC chopper, the influence of DC voltage disturbance on the excitation voltage output is ignored.

[0071] According to the converter topology, its average model is:

[0072]

[0073] Where, L is the AC filter inductance of the converter; u abc and i abc is the three-phase voltage and current of the AC grid-connected port of the converter; u iabc is the voltage at the three-phase output port of the converter.

[0074] According to the control link of the converter, it mainly includes phase-locked loop, current loop, DC voltage loop and reactive power loop, and the corresponding transfer function is:

[0075]

[0076]

[0077]

[0078]

[0079] According to the model of each control link of the converter and the relationship between the voltage and current components at the grid connection point, the relationship between the voltage disturbance and the current response can be obtained as shown below:

[0080]

[0081] Since the complete modeling expression is relatively complex, the following sets of expressions are introduced to simplify it:

[0082] The following are two sets of expressions related to reactive power control, where the former represents the influence of positive and negative sequence voltage and current disturbance components on reactive power disturbance components, and the latter represents the influence of reactive power disturbance components on positive and negative sequence current disturbance components.

[0083]

[0084]

[0085] In the formula, HRPDC is the transfer function of the reactive power compensation controller. Its topology and transfer function are similar to those of PSS and are not listed here. i is the ratio of the current at both ends of the excitation transformer; K m is the modulation coefficient; V dc0 is the steady-state value of DC voltage; u0 and i0 are the fundamental frequency voltage and current vectors of the converter; M0 is the fundamental frequency component of the modulation signal; M p With M p2 To simplify the expression, the steady-state current at the synchronous machine end is expressed as:

[0086]

[0087] In the formula, u t0 with i t0 is the fundamental frequency voltage and current vector of the synchronous machine end; Y m11 , Y m12 , Y m21 With Y m22 It is the positive sequence, positive sequence coupling, negative sequence coupling and negative sequence admittance of the flexible excitation synchronous machine.

[0088] From the above formula, the 2*2 admittance matrix model of the grid-side frequency coupling characteristics of the flexible excitation synchronous machine can be obtained as follows:

[0089]

[0090] (4) Based on the above modeling, the admittance of the flexible excitation synchronous machine is the machine side part and the grid side part connected in parallel at the PCC point through the excitation transformer:

[0091]

[0092] The accuracy of the analytical model of the admittance matrix of the flexible excitation synchronous machine can be verified by Matlab / Simulink simulation. In the simulation, the synchronous machine system parameters and related control parameters are shown in the following table. Figure 4 The comparison chart of the analytical model and simulation results of the flexible excitation synchronous machine is shown in Table 1 below.

[0093] Table 1

[0094] parameter symbol Numeric Nominal Votage / V <![CDATA[V n ]]> 13800 Nominal Power / MW <![CDATA[P n ]]> 75 Stator Resistance / pu <![CDATA[R s ]]> <![CDATA[2.85*10 -3 ]]> Stator Inductance / pu <![CDATA[L d ,L q ]]> 0.114,0.114 Excitation Resistance / pu <![CDATA[R f ]]> <![CDATA[5.79*10 -4 ]]> Excitation Inductance / pu <![CDATA[L f ]]> 0.0114 DC Bus Voltage / V <![CDATA[V dc0 ]]> 750 DC Bus Capacitor / F C 1.5 AC Filter Inductor / mH L 0.25

[0095] from Figure 3 It can be found that the admittance matrix analytical model of the flexible excitation synchronous machine derived by the present invention is consistent with the simulation measurement results, indicating that the admittance matrix analytical model derived by the present invention can accurately describe the admittance of the flexible excitation synchronous machine itself.

[0096] (5) In order to study the stability of the flexible excitation synchronous machine connected to the grid through series compensation, a simulation model was established in Matlab / Simulink. Figure 2 The electromagnetic transient simulation model of the flexible excitation synchronous machine connected to the infinite power grid through series compensation shown in the topology.

[0097] Use the equivalent SISO impedance transformation method to analyze grid stability: draw the equivalent SISO impedance Z in the Bode diagram at the same time SISO and the grid positive sequence impedance Z gp , by observing whether the phase difference of the phase-frequency characteristic curve at the intersection of the two amplitude-frequency characteristic curves exceeds 180°, the stability of the interconnected system can be judged: if the phase difference at a certain intersection exceeds 180°, the interconnected system faces the risk of oscillation instability; if the phase difference at all intersections is less than 180°, the interconnected system can operate stably. Considering series compensation, the 2*2 admittance matrix of the flexible excitation synchronous machine is converted into a single-input single-output model, and compared with the grid impedance, the Bode diagram is obtained as follows Figure 5 shown.

[0098]

[0099] In the formula, Z SISO is the equivalent SISO impedance; Z gn is the negative sequence impedance of the power grid, expressed as Z gn =(s-j2ω1)L g .

[0100] By analyzing its stability through the Bode diagram, it can be seen that the phase difference corresponding to the intersection of the equivalent SISO model amplitude curve of the flexible excitation synchronous machine and the grid impedance amplitude curve is 185°, which is greater than 180°. There is a negative resistance frequency band at the frequency point corresponding to the intersection, that is, 20Hz and 80Hz where the frequency is coupled, and obvious harmonic resonance will occur during operation.

[0101] Through the established electromagnetic transient simulation model, we can get the scenario where the grid-connected point current gradually diverges after the system is connected to the series compensation, indicating that the system changes from stable to unstable. The FFT analysis of the grid-connected point current is as follows: Figure 6 It can be found that the grid-connected current has obvious 20Hz and 80Hz harmonic resonance, which is consistent with Figure 5 This is consistent with the prediction results, proving the reliability of the grid-connected stability analysis using the equivalent SISO model in the present invention.

[0102] The present invention also discloses an impedance modeling and grid-connected stability analysis device based on a flexible excitation synchronous machine, comprising the following modules:

[0103] Division module: According to the topological structure and control block diagram of the flexible excitation synchronous machine, the flexible excitation synchronous machine is divided into the synchronous machine, the machine side of the excitation system and the grid side of the excitation system;

[0104] Modeling module: According to the topological structure of the flexible excitation synchronous machine, the synchronous machine, the machine side of the excitation system and the grid side of the excitation system are modeled respectively;

[0105] Calculation module: Under frequency coupling, the grid impedance matrix Z is calculated g ;

[0106] Analysis and judgment module: according to the grid impedance matrix Z g and the analytical model of the admittance matrix of the flexible excitation synchronous machine Y fes , based on the equivalent SISO impedance stability criterion, the stability of the flexible excitation synchronous machine grid-connected system through series compensation is judged.

[0107] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An impedance modeling and grid-connected stability analysis method based on flexible excitation synchronous machine, characterized in that: The following steps are involved: S1: According to the topological structure and control block diagram of the flexible excitation synchronous machine, the flexible excitation synchronous machine is divided into the synchronous machine, the machine side of the excitation system and the grid side of the excitation system; S2: According to the topological structure of the flexible excitation synchronous machine, the synchronous machine, the machine side of the excitation system and the grid side of the excitation system are modeled respectively; S3: Under frequency coupling, the grid impedance matrix Z is calculated g ; S4: According to the grid impedance matrix Z g and the analytical model of the admittance matrix of the flexible excitation synchronous machine Y fes , based on the equivalent SISO impedance stability criterion, the stability of the flexible excitation synchronous machine grid-connected system through series compensation is judged.

2. The impedance modeling and grid-connected stability analysis method of the flexible excitation synchronous machine according to claim 1 is characterized in that: In step S2, the synchronous machine, the excitation system machine side and the excitation system grid side are modeled respectively. The first step of the modeling is to establish an analytical model Y of the synchronous machine admittance matrix without considering the excitation system. fes , the expression is as follows: Where ΔI=[Δi d Δi f Δi D Δi q Δi g Δi Q ] T , A2=[010000] T ; where the subscripts d, q, f, D, Q and g represent the components of the corresponding variables, dq axis, excitation winding f, d axis damping winding D and q axis damping winding Q and g respectively; ΔI is the small signal of the current vector of each winding, Δu dq , Δi dq is the small signal of the voltage and current vector at the machine end, Δu f is the small signal of excitation voltage.

3. The impedance modeling and grid-connected stability analysis method of the flexible excitation synchronous machine according to claim 1 is characterized in that: The second step of the modeling step in step S2 is to establish an analytical model of the admittance matrix on the grid side of the excitation system. This part mainly includes an AC / DC converter, a DC / DC chopper and a reactive power compensation controller RPDC. Since the AC / DC converter is affected by DC voltage control and phase-locked control and is a dq asymmetric system with frequency coupling characteristics, this part uses a harmonic linearization method for impedance modeling. The DC / DC chopper ignores the influence of DC voltage disturbances on the excitation voltage output.

4. The impedance modeling and grid-connected stability analysis method of a flexible excitation synchronous machine according to claim 1, characterized in that: The third step of the modeling step in step S2 is to establish an analytical model of the admittance matrix on the machine side of the excitation system, which includes the automatic voltage regulator AVR and the power system stabilizer PSS.

5. The impedance modeling and grid-connected stability analysis method of a flexible excitation synchronous machine according to claim 1, characterized in that: The grid impedance matrix calculated in step S3 is as follows: Among them, Z g is the grid impedance matrix, Z 11 (s) is the positive sequence impedance of the power grid in the complex frequency domain, Z 22 (s) is the negative sequence impedance of the power grid in the complex frequency domain, and s is the Laplace operator.

6. The impedance modeling and grid-connected stability analysis method of a flexible excitation synchronous machine according to claim 1, characterized in that: The stability of the flexible excitation synchronous machine system connected to the grid through series compensation in step S4 is determined by adopting the equivalent SISO impedance stability analysis method, converting the non-diagonal elements of the flexible excitation synchronous machine system admittance matrix and the grid impedance to the main diagonal, and drawing the equivalent SISO impedance Z in the Bode diagram at the same time. SISO and the grid positive sequence impedance Z 11 (s), and then determine whether the phase difference at the intersection of the two impedance curve amplitudes exceeds 180°: if so, the system is determined to be unstable; if not, the system is determined to be stable.

7. An impedance modeling and grid-connected stability analysis device based on flexible excitation synchronous machine, characterized in that: Includes the following modules: Division module: According to the topological structure and control block diagram of the flexible excitation synchronous machine, the flexible excitation synchronous machine is divided into the synchronous machine, the machine side of the excitation system and the grid side of the excitation system; Modeling module: According to the topological structure of the flexible excitation synchronous machine, the synchronous machine, the machine side of the excitation system and the grid side of the excitation system are modeled respectively; Calculation module: Under frequency coupling, the grid impedance matrix Z is calculated g ; Analysis and judgment module: according to the grid impedance matrix Z g and the analytical model of the admittance matrix of the flexible excitation synchronous machine Y fes , based on the equivalent SISO impedance stability criterion, the stability of the flexible excitation synchronous machine grid-connected system through series compensation is judged.

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