A symmetrical phase-locked loop circuit, a grid-connected converter system and a stability analysis method

Abstract

The present invention provides a symmetrical phase-locked loop circuit, a grid-connected converter system and a stability analysis method, wherein the symmetrical phase-locked loop comprises: a voltage conversion unit converts the observer grid-connected point voltage output by the back electromotive force observer into a q-axis component value and a d-axis component value; a first phase-locked unit obtains the grid-connected point voltage q-axis amplitude according to the d-axis component value output by the voltage conversion unit and the AC grid voltage, and inputs the grid-connected point voltage q-axis amplitude into the voltage conversion unit; a second phase-locked unit obtains the grid-connected point voltage d-axis phase according to the q-axis component value output by the voltage conversion unit and the grid-connected point angular frequency, and inputs the grid-connected point voltage d-axis phase into the voltage conversion unit; a PI control unit generates a grid-connected point phase according to the grid-connected point voltage q-axis amplitude output by the first phase-locked unit and the grid-connected point voltage d-axis phase output by the second phase-locked unit. The present invention can effectively improve the weak grid synchronization stability, and can improve the converter system stability analysis efficiency, and has strong robustness.

Description

Technical Field

[0001] The present invention relates to the technical field of phase-locked loops, and in particular to a symmetrical phase-locked loop circuit based on a back electromotive force observer, a grid-connected converter system and a stability analysis method. Background Art

[0002] In recent years, with the continuous expansion of the scale of new energy applications, voltage source converter (VSC) has received widespread attention as the grid interface in the new energy grid-connected system, and the synchronous reference frame phase-locked loop (SRF-PLL) plays an important role in the stability of grid synchronization, providing important support for providing reliable and high-quality electric energy.

[0003] However, the access of a large number of grid-connected converter devices to the grid will not only change the traditional grid structure, but also bring new challenges to the stable operation of the grid. Especially when the AC grid is relatively weak, the dynamics of the grid-connected converter itself and its interaction with the AC grid are more obvious. When the grid voltage disturbance is introduced from the point of common coupling (PCC), the SRF-PLL dynamics will affect the converter output impedance, and the higher grid impedance will aggravate this effect, resulting in the SRF-PLL being unable to achieve the expected synchronization performance with the AC grid and unable to achieve accurate phase locking under weak grid conditions, which seriously affects the stability of the grid-connected equipment and further affects the safety and stability of the grid system. In addition, in the small signal stability analysis of the grid-connected converter system using the traditional SRF-PLL, not only is it necessary to simultaneously calculate the characteristic trajectories of the two eigenvalues ​​in the impedance matrix, but also the asymmetry of the second-order impedance matrix leads to a large amount of stability analysis calculation and a complex analysis process. Summary of the invention

[0004] The purpose of the present invention is to provide a symmetrical phase-locked loop circuit, a grid-connected converter system and a stability analysis method based on a back-electromotive force observer, which can effectively solve the above-mentioned technical defects, improve the weak network synchronization stability, simplify the small signal stability analysis process, and improve the efficiency and robustness of the converter system stability analysis.

[0005] In order to achieve the above objectives, it is necessary to provide a symmetrical phase-locked loop circuit, a grid-connected converter system and a stability analysis method in response to the above technical problems.

[0006] In a first aspect, an embodiment of the present invention provides a symmetrical phase-locked loop circuit, comprising a symmetrical phase-locked loop, and a back electromotive force observer connected to an input end of the symmetrical phase-locked loop; the symmetrical phase-locked loop comprises:

[0007] A voltage conversion unit, used for converting the observer grid-connected point voltage output by the back electromotive force observer into a q-axis component value and a d-axis component value;

[0008] A first phase locking unit, configured to obtain a q-axis amplitude of a grid-connected point voltage according to a d-axis component value output by the voltage conversion unit and the AC grid voltage, and input the q-axis amplitude of the grid-connected point voltage into the voltage conversion unit;

[0009] A second phase locking unit, configured to obtain a d-axis phase of the grid-connected point voltage according to the q-axis component value output by the voltage conversion unit and the grid-connected point angular frequency, and input the d-axis phase of the grid-connected point voltage into the voltage conversion unit;

[0010] The PI control unit is used to generate a grid connection point phase according to the q-axis amplitude of the grid connection point voltage output by the first phase locking unit and the d-axis phase of the grid connection point voltage output by the second phase locking unit.

[0011] Furthermore, the first phase-locked unit includes a first calculation unit and an integration unit connected in sequence; and the second phase-locked unit includes an integration unit and a second calculation unit connected in sequence.

[0012] Furthermore, the calculation model of the symmetrical phase-locked loop circuit is:

[0013]

[0014] Among them, u αβ and i gαβ Respectively represent the voltage and current at the grid connection point; e αβ and Respectively represent the equivalent internal potential and grid current; K pMPLL and K iMPLL represents the PI control parameter of the back-EMF observer; L g and R g represents equivalent inductance and equivalent resistance; s represents frequency domain.

[0015] In a second aspect, an embodiment of the present invention provides a grid-connected converter system, which includes a main circuit unit and a VSC control unit; the VSC control unit includes a current inner loop control unit and the above-mentioned symmetrical phase-locked loop circuit.

[0016] Furthermore, the complex impedance model of the main circuit unit is:

[0017]

[0018] In the formula,

[0019]

[0020] Among them, U dq , U dq 、V dq and E dqRespectively represent the grid connection point voltage, parallel branch capacitor voltage, converter output voltage and AC grid voltage; I dq and I gdq Respectively represent the grid connection point current and AC grid current; L g and R g Represents equivalent inductance and equivalent resistance; Z out and ω s They represent the equivalent output impedance of the grid-connected converter and the grid-connected point angular frequency respectively; Δ represents the small signal processing of the variable; j represents the imaginary unit in the complex plane; R c and C f They represent the equivalent resistance and equivalent filter capacitance of the parallel branch respectively;

[0021] The complex impedance model of the symmetrical phase-locked loop circuit is:

[0022]

[0023] In the formula,

[0024] H pll =-jG pll / s

[0025]

[0026] Among them, θ dq and e dq Respectively represent the phase and back electromotive force of the grid-connected point; G pll Represents the complex impedance of the PI control unit in the symmetrical phase-locked loop circuit; ω t Represents the back electromotive force parameter.

[0027] In a third aspect, an embodiment of the present invention further provides a stability analysis method for a grid-connected converter system, the method comprising the following steps:

[0028] Obtain the voltage and current at the grid connection point, and calculate the equivalent internal potential through the back-EMF observer;

[0029] Calculating the strong grid phase of the equivalent internal potential according to a symmetrical phase-locked loop circuit;

[0030] Establishing a complex impedance small signal model of the grid-connected converter system according to the coefficient complex impedance model of the strong grid phase;

[0031] According to the complex impedance small signal model, stability analysis is performed on the grid-connected converter system.

[0032] Furthermore, the calculation model for calculating the equivalent internal potential by the back electromotive force observer is:

[0033]

[0034] Among them, u αβ and i gαβ Respectively represent the voltage and current of the grid connection point; e αβ and Respectively represent the equivalent internal potential and grid current; K pMPLL and K iMPLL represents the PI control parameter of the back-EMF observer; L g and R g represents equivalent inductance and equivalent resistance; s represents frequency domain.

[0035] Furthermore, the complex impedance small signal model is expressed as:

[0036]

[0037] In the formula,

[0038]

[0039] in, Represents the equivalent impedance of the VSC converter; Z out Represents the equivalent output impedance of the grid-connected converter; G ci and G dei Indicates digital-to-analog conversion; G del Indicates VSC system delay; U dq ,I dq ,V dq They represent the steady-state value of the converter outlet voltage, the steady-state value of the grid-connected point voltage and the steady-state value of the grid-connected point current in the dq coordinate system respectively; Δ represents the small signal processing of the variable.

[0040] The step of performing stability analysis on the grid-connected converter system according to the complex impedance small signal model comprises:

[0041] Furthermore, based on the complex impedance small signal model, an improved generalized Nyquist criterion is used to perform stability analysis on the grid-connected converter system.

[0042] Furthermore, the improved generalized Nyquist criterion is expressed as:

[0043]

[0044] Among them, Z s and They represent the grid-connected point equivalent impedance and the VSC converter equivalent impedance respectively; Z sd and Z sq They represent the equivalent impedance Z of the grid connection point respectively. s The d component and q component in the dq coordinate system; and Respectively represent the equivalent impedance of VSC converter The d component and q component in the dq coordinate system.

[0045] Based on the symmetrical phase-locked loop circuit provided by the present application, the voltage conversion unit converts the observer grid-connected point voltage output by the back electromotive force observer into a q-axis component value and a d-axis component value; the first phase-locked unit obtains the q-axis amplitude of the grid-connected point voltage according to the d-axis component value output by the voltage conversion unit and the AC power grid voltage, and inputs the q-axis amplitude of the grid-connected point voltage into the voltage conversion unit; the second phase-locked unit obtains the d-axis phase of the grid-connected point voltage according to the q-axis component value output by the voltage conversion unit and the grid-connected point angular frequency, and inputs the d-axis phase of the grid-connected point voltage into the voltage conversion unit; the PI control unit generates a grid-connected point phase according to the q-axis amplitude of the grid-connected point voltage output by the first phase-locked unit and the d-axis phase of the grid-connected point voltage output by the second phase-locked unit; compared with the prior art, the present invention can effectively improve the weak grid synchronization stability, and can simplify the process of converter system stability analysis, improve the efficiency of small signal model stability analysis, and has strong robustness. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is a structural block diagram of a symmetrical phase-locked loop circuit provided in an embodiment of the present invention;

[0047] Figure 2 is a schematic diagram of the structure of a symmetrical phase-locked loop circuit provided in an embodiment of the present invention;

[0048] Figure 3 is a structural schematic diagram of a grid-connected converter system provided in an embodiment of the present invention;

[0049] Figure 4 is a schematic diagram of a complex impedance small signal model provided in an embodiment of the present invention;

[0050] Figure 5 is a flow chart of a stability analysis method for a grid-connected converter system provided in an embodiment of the present invention;

[0051] Figure 6 is a schematic diagram comparing stability analysis between the improved generalized Nyquist criterion provided in an embodiment of the present invention and the traditional generalized Nyquist criterion;

[0052] Figure 7 It is a comparison diagram of synchronization performance between a traditional phase-locked loop circuit provided in an embodiment of the present invention and a back-electromotive force symmetrical phase-locked loop circuit proposed in the present invention under weak network conditions. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical scheme and beneficial effects of the present application clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. Obviously, the embodiments described below are part of the embodiments of the present invention and are only used to illustrate the present invention, but are not used to limit the scope of the present invention. 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.

[0054] like Figure 1 As shown, the symmetrical phase-locked loop circuit provided by the embodiment of the present invention includes a symmetrical phase-locked loop 1 (ModifiedSymmetrical PLL), and a back electromotive force observer 2 (BEMF-obsever) connected to the input end of the symmetrical phase-locked loop 1; the symmetrical phase-locked loop 1 includes:

[0055] A voltage conversion unit 11, used for converting the observer grid-connected point voltage output by the back electromotive force observer 2 into a q-axis component value and a d-axis component value;

[0056] Specifically, Figure 2 As shown, the voltage conversion unit converts the observer grid-connected voltage e output by the back electromotive force observer 2 into αβ Depend on αβ The coordinate system is converted to the dq coordinate system to obtain the corresponding d-axis component value e d and the q-axis component value e q The following corresponding first phase locking unit 12 and second phase locking unit 13 are input respectively for phase locking processing;

[0057] A first phase locking unit 12 is used to obtain a q-axis amplitude of the grid connection point voltage according to the d-axis component value output by the voltage conversion unit 11 and the AC grid voltage, and input the q-axis amplitude of the grid connection point voltage into the voltage conversion unit 11;

[0058] Specifically, Figure 2 As shown, the first phase-locking unit includes a first calculation unit and an integration unit connected in sequence, and the d-axis component value output by the voltage conversion unit 11 and the AC grid voltage are processed by the first calculation unit to obtain E m -e d Input integration unit for processing K pMPLL and K iMPLL represents the PI control parameter of the back EMF observer), the q-axis amplitude θ of the grid-connected point voltage is obtained q Input voltage conversion unit 11;

[0059] A second phase locking unit 13 is used to obtain a d-axis phase of the grid-connected point voltage according to the q-axis component value output by the voltage conversion unit 11 and the grid-connected point angular frequency, and input the d-axis phase of the grid-connected point voltage into the voltage conversion unit 11;

[0060] Specifically, Figure 2 As shown, the second phase-locked unit includes an integral unit and a second calculation unit connected in sequence, and the q-axis component value e output by the voltage conversion unit 11 is q Input integration unit for processing K pMPLL and K iMPLL represents the PI control parameter of the back electromotive force observer) and then input into the second calculation unit for processing and the grid-connected point angular frequency ω s The combined voltage d-axis phase θ of the grid connection point is obtained d Input voltage conversion unit 11;

[0061] The PI control unit 14 is used to generate a grid connection point phase according to the grid connection point voltage q-axis amplitude output by the first phase locking unit 12 and the grid connection point voltage d-axis phase output by the second phase locking unit 23;

[0062] Specifically, according to the obtained grid connection point voltage q-axis amplitude θ q The d-axis phase of the grid-connected point voltage is θ d , using the following matrix calculation method, we can get the grid connection point phase θ dq , to achieve phase θ d and amplitude θ q To perform symmetrical phase locking:

[0063]

[0064] Where e represents the base of natural logarithms and j represents the imaginary unit in the complex plane.

[0065] In summary, according to the above symmetrical phase-locked loop circuit, writing the KVL equation can be obtained:

[0066]

[0067] Since the design of the differential link in the controller is relatively difficult, this embodiment preferably uses an integral link instead of a differential link, that is:

[0068]

[0069] Combining equations (1) and (2), the calculation model for designing a symmetrical phase-locked loop circuit based on back-EMF observer is:

[0070]

[0071] Among them, uαβ and i gαβ Respectively represent the voltage and current at the grid connection point; e αβ and Respectively represent the equivalent internal potential and grid current; K pMPLL and K iMPLL represents the PI control parameter of the back-EMF observer; L g and R g represents equivalent inductance and equivalent resistance; s represents frequency domain.

[0072] like Figure 3 As shown, the grid-connected converter system of the symmetrical phase-locked loop circuit based on the above-mentioned back electromotive force observer provided in an embodiment of the present invention includes a main circuit unit (Main Circuit) and a VSC control unit (Control System); the VSC control unit (Control System) includes a current inner loop control unit (PI of Inner Current Loop) and Figure 2 The above symmetrical phase-locked loop circuit (BEFS-PLL) shown

[0073] Specifically, the complex impedance model of the main circuit unit is:

[0074]

[0075] Where, the coefficient of complex impedance It is expressed as:

[0076]

[0077] Among them, U dq , U dq 、V dq and E dq Respectively represent the grid connection point voltage, parallel branch capacitor voltage, converter output voltage and AC grid voltage; I dq and I gdq Respectively represent the grid connection point current and AC grid current; L g and R g Represents equivalent inductance and equivalent resistance; Z out and ω s They represent the equivalent output impedance of the grid-connected converter and the grid-connected point angular frequency respectively; Δ represents the small signal processing of the variable; j represents the imaginary unit in the complex plane; R c and C f They represent the equivalent resistance and equivalent filter capacitance of the parallel branch respectively;

[0078] based on Figure 3-4 The structure diagram of the grid-connected converter system shown in the figure controls the back electromotive force in the control system. and the back electromotive force Δe in the main circuitdq The relationship between them is:

[0079]

[0080] In summary, the complex impedance model of the symmetrical phase-locked loop circuit is:

[0081]

[0082] In the formula,

[0083] H pll =-jG pll / s

[0084]

[0085] Among them, θ dq and e dq Respectively represent the phase and back electromotive force of the grid-connected point; G pll Represents the complex impedance of the PI control unit in the symmetrical phase-locked loop circuit; ω t Represents the back electromotive force parameter.

[0086] The complex impedance model of the inner current loop control unit is:

[0087]

[0088] Then the complex impedance expression of the current inner loop of the grid-connected converter system can be obtained as:

[0089]

[0090] Among them, G ci and G dei Represents the relevant digital-to-analog conversion; They represent the converter outlet voltage, grid connection point voltage and grid connection point current in the dq coordinate system respectively;

[0091] like Figure 5 As shown, an embodiment of the present invention provides a stability analysis method for a grid-connected converter system of a symmetrical phase-locked loop circuit based on a back electromotive force observer, comprising the following steps:

[0092] S11, obtaining the voltage and current of the grid connection point, and calculating the equivalent internal potential through the back electromotive force observer;

[0093] Specifically, based on formula (3), the calculation model for calculating the equivalent internal potential through the back electromotive force observer is:

[0094]

[0095] Among them, uαβ and i gαβ Respectively represent the voltage and current of the grid connection point; e αβ and Respectively represent the equivalent internal potential and grid current; K pMPLL and K iMPLL represents the PI control parameter of the back-EMF observer; L g and R g represents equivalent inductance and equivalent resistance; s represents frequency domain;

[0096] In this embodiment, preferably, the PI control unit parameters are designed by introducing quantitative ω t , we can simplify (4) as follows:

[0097]

[0098] Among them, the PI control parameters are defined as:

[0099] K pMPLL =ω t L g , K iMPLL =ω t R g

[0100] Based on formula (5), the back electromotive force can be calculated by the voltage and current at the PCC point, and the phase of the back electromotive force can be calculated by inputting it into the phase-locked loop, which serves as the key to the converter control.

[0101] S12, calculating the strong network phase of the equivalent internal potential according to the symmetrical phase-locked loop circuit;

[0102] Specifically, the mathematical model for calculating the strong grid phase of the equivalent internal potential through the symmetrical phase-locked loop circuit based on the back electromotive force of the grid-connected converter is:

[0103]

[0104] in, represents the back electromotive force in the dq coordinate system of the control system;

[0105] The coefficient complex impedance model corresponding to the above formula (6) is:

[0106]

[0107] Among them, θ dq and They represent the phase of the grid connection point and the back electromotive force of the PI control unit respectively; G pll Represents the complex impedance of the PI control unit in the symmetrical phase-locked loop circuit; ω t Represents the back electromotive force parameter and is an inherent constant.

[0108] S13. Establishing a complex impedance small signal model of the grid-connected converter system according to the coefficient complex impedance model of the strong grid phase; wherein the complex impedance small signal model can be understood as a small signal model constructed based on the modeling idea of ​​taking the ratio of the grid connection point voltage and current as the complex impedance in the frequency domain of the VSC output;

[0109] Specifically, Figure 4 The complex impedance small signal model shown is expressed as:

[0110]

[0111] In the formula,

[0112]

[0113] in, Represents the equivalent impedance of the VSC converter; Z out Represents the equivalent output impedance of the grid-connected converter; G ci and G dei Indicates digital-to-analog conversion; G del Indicates VSC system delay; U dq ,I dq ,V dq They represent the steady-state value of the converter outlet voltage, the steady-state value of the grid-connected point voltage and the steady-state value of the grid-connected point current in the dq coordinate system respectively; Δ represents the small signal processing of the variable;

[0114] S14. Performing stability analysis on the grid-connected converter system according to the complex impedance small signal model; wherein the step of performing stability analysis on the grid-connected converter system according to the complex impedance small signal model comprises:

[0115] According to the complex impedance small signal model, an improved generalized Nyquist criterion is used to perform stability analysis on the grid-connected converter system;

[0116] Specifically, the improved generalized Nyquist criterion is expressed as:

[0117]

[0118] Among them, Z s and They represent the grid-connected point equivalent impedance and the VSC converter equivalent impedance respectively; Z sd and Z sq They represent the equivalent impedance Z of the grid connection point respectively. s The d component and q component in the dq coordinate system; and Respectively represent the equivalent impedance of VSC converter The d component and q component in the dq coordinate system;

[0119] When stability analysis is performed based on the improved generalized Nyquist criterion, the analyzed grid-connected converter system can be judged to be stable when and only when the characteristic trajectory of one of the eigenvalues ​​of the complex impedance L does not surround the critical point (-1, 0), while the traditional generalized Nyquist stability criterion is that the system under study is stable when and only when the characteristic trajectory of the eigenvalue of the system impedance matrix does not surround the critical point (-1, 0); that is, the traditional generalized Nyquist criterion needs to calculate the characteristic trajectories of two eigenvalues ​​in the impedance matrix at the same time, and due to the asymmetry of the second-order matrix during calculation, the amount of calculation is large and the analysis process is complicated; Figure 6 As shown, compared with the traditional generalized Nyquist criterion for stability analysis, the improved Nyquist criterion proposed in the present invention only needs to calculate a part of the two conjugate eigenvalue trajectories in the complex impedance, and the overall characteristic curve can be drawn through conjugate symmetry. Only half of the eigenvalue trajectory needs to be calculated, which reduces the amount of calculation and realizes the effective simplification of the eigenvalue analysis of the complex impedance matrix, thereby simplifying the process of stability analysis and judgment;

[0120] In addition, if Figure 7 As shown in FIG. 1 , under the extremely weak network condition with a short circuit ratio of 0.96, a switching comparison is made between the existing S-PLL (a common phase-locked loop without a back electromotive force observer) and the BEFS-PLL (an improved symmetric phase-locked loop based on a back electromotive force observer) proposed in the present invention. Under the error condition O d = 1.5 to switch the short-circuit ratio. It is found that under weak network conditions, the improved symmetrical phase-locked loop can maintain good stability, while the traditional phase-locked loop oscillates and d Under the three different estimation error conditions of =0.5, 1, and 1.5, the stability margin of the grid-connected converter system is greatly improved, with strong robustness and great advantages in practical applications.

[0121] The grid-connected converter system based on the symmetrical phase-locked loop circuit provided by the present invention effectively improves the synchronization performance of the grid-connected converter under weak grid conditions by introducing a symmetrical phase-locked loop circuit in the VSC control unit for phase-locked control, and provides reliable guarantee for the stable operation of the grid-connected converter system; in addition, based on the stability analysis method provided by the improved grid-connected converter system, by obtaining the voltage and current of the grid connection point, and calculating the equivalent internal potential through the back electromotive force observer, the strong grid phase of the equivalent internal potential is calculated according to the symmetrical phase-locked loop circuit, and then according to the coefficient complex impedance model of the strong grid phase, a complex impedance small signal model of the grid-connected converter system is established, and according to the complex impedance small signal model, the grid-connected converter system is subjected to stability analysis using the improved generalized Nyquist criterion, which can effectively simplify the process of stability analysis of the converter system, improve the efficiency of stability analysis of the small signal model, and has strong robustness, and has great advantages in practical application.

[0122] Each embodiment in this specification is described in a progressive manner, and the same or similar parts of each embodiment can be directly referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. It should be noted that the technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of the technical features in the above-mentioned embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0123] The above-mentioned embodiments only express several preferred implementation modes of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in the technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be regarded as the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be based on the protection scope of the claims.

Claims

1. A symmetrical phase-locked loop circuit, characterized in that: The invention comprises a symmetrical phase-locked loop and a back electromotive force observer connected to an input end of the symmetrical phase-locked loop; the symmetrical phase-locked loop comprises: A voltage conversion unit, used for converting the observer grid-connected point voltage output by the back electromotive force observer into a q-axis component value and a d-axis component value; A first phase locking unit, configured to obtain a q-axis phase of a grid connection point voltage according to a d-axis component value output by the voltage conversion unit and the AC grid voltage, and input the q-axis phase of the grid connection point voltage into the voltage conversion unit; A second phase locking unit, configured to obtain a d-axis phase of the grid-connected point voltage according to the q-axis component value output by the voltage conversion unit and the grid-connected point angular frequency, and input the d-axis phase of the grid-connected point voltage into the voltage conversion unit; A PI control unit, configured to generate a grid connection point phase according to a grid connection point voltage q-axis phase output by the first phase locking unit and a grid connection point voltage d-axis phase output by the second phase locking unit; The first phase-locked unit includes a first calculation unit and an integration unit connected in sequence; the second phase-locked unit includes an integration unit and a second calculation unit connected in sequence; The calculation model of the symmetrical phase-locked loop circuit is: Among them, u αβ and i gαβ Respectively represent the voltage and current of the grid connection point; e αβ and Respectively represent the equivalent internal potential and grid current; K pMPLL and K iMPLL represents the PI control parameter of the back-EMF observer; L g and R g represents equivalent inductance and equivalent resistance; s represents frequency domain.

2. A grid-connected converter system, characterized in that: The grid-connected converter system comprises a main circuit unit and a VSC control unit; the VSC control unit comprises a current inner loop control unit and the symmetrical phase-locked loop circuit according to claim 1.

3. The grid-connected converter system according to claim 2, characterized in that: The complex impedance model of the main circuit unit is: In the formula, Among them, U dq , U dq 、V dq and E dq Respectively represent the grid connection point voltage, parallel branch capacitor voltage, converter output voltage and AC grid voltage; I dq and I gdq Respectively represent the grid connection point current and AC grid current; L g and R g Represents equivalent inductance and equivalent resistance; Z out and ω s They represent the equivalent output impedance of the grid-connected converter and the grid-connected point angular frequency respectively; Δ represents the small signal processing of the variable; j represents the imaginary unit in the complex plane; R c and C f They represent the equivalent resistance and equivalent filter capacitance of the parallel branch respectively; The complex impedance model of the symmetrical phase-locked loop circuit is: In the formula, H pll =-jG pll / s Among them, θ dq and e dq Respectively represent the phase and back electromotive force of the grid-connected point; G pll Represents the complex impedance of the PI control unit in the symmetrical phase-locked loop circuit; ω t Represents the back electromotive force parameter.

4. A stability analysis method for a grid-connected converter system as claimed in claim 3, characterized in that: The method comprises the following steps: Obtain the voltage and current at the grid connection point, and calculate the equivalent internal potential through the back-EMF observer; Calculating the strong grid phase of the equivalent internal potential according to a symmetrical phase-locked loop circuit; Establishing a complex impedance small signal model of the grid-connected converter system according to the coefficient complex impedance model of the strong grid phase; According to the complex impedance small signal model, stability analysis is performed on the grid-connected converter system.

5. The stability analysis method of the grid-connected converter system according to claim 4, characterized in that: The calculation model for calculating the equivalent internal potential by the back electromotive force observer is: Among them, u αβ and i gαβ Respectively represent the voltage and current of the grid connection point; e αβ and Respectively represent the equivalent internal potential and grid current; K pMPLL and K iMPLL represents the PI control parameter of the back-EMF observer; L g and R g represents equivalent inductance and equivalent resistance; s represents frequency domain.

6. The stability analysis method of the grid-connected converter system according to claim 4, characterized in that: The complex impedance small signal model is expressed as: In the formula, in, Represents the equivalent impedance of the VSC converter; Z out Represents the equivalent output impedance of the grid-connected converter; G ci and G dei Indicates digital-to-analog conversion; G del Indicates VSC system delay; U dq ,I dq ,V dq They represent the steady-state value of the converter outlet voltage, the steady-state value of the grid-connected point voltage and the steady-state value of the grid-connected point current in the dq coordinate system respectively; Δ represents the small signal processing of the variable.

7. The stability analysis method of the grid-connected converter system according to claim 4, characterized in that: The step of performing stability analysis on the grid-connected converter system according to the complex impedance small signal model comprises: According to the complex impedance small signal model, an improved generalized Nyquist criterion is used to perform stability analysis on the grid-connected converter system.

8. The stability analysis method of the grid-connected converter system according to claim 7, characterized in that: The improved generalized Nyquist criterion is expressed as: Among them, Z s and They represent the grid-connected point equivalent impedance and the VSC converter equivalent impedance respectively; Z sd and Z sq They represent the equivalent impedance Z of the grid connection point respectively. s The d component and q component in the dq coordinate system; and Respectively represent the equivalent impedance of VSC converter The d component and q component in the dq coordinate system.

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