A high-order phase-locked loop design method for renewable energy grid-connected inverters

Abstract

The present invention discloses a high-order phase-locked loop design method suitable for a new energy grid-connected inverter. First, the operating scenario of the new energy grid-connected inverter is analyzed to determine the variation range of the complex impedance of the power grid; then the allowable variation range of the impedance of the new energy grid-connected inverter is calculated, so that the new energy grid-connected system remains stable in all operating scenarios; on this basis, the impedance transfer function of the new energy grid-connected inverter is obtained through an intelligent optimization algorithm; then, according to the impedance analytical model of the grid-connected inverter, the expected phase-locked loop transfer function is obtained; finally, according to the expected transfer function, the structure and parameters of the phase-locked loop controller are designed. The present invention reversely determines the structure and parameters of the phase-locked loop controller from the perspective of the stability of the new energy grid-connected system, thereby ensuring the stability of the new energy grid-connected system in a wide range of operating conditions; better dynamic response performance can also be obtained during frequency detection and phase detection.

Description

Technical Field

[0001] The present invention belongs to the field of renewable energy grid-connected power generation, and in particular relates to a high-order phase-locked loop design method suitable for renewable energy grid-connected inverters. Background Art

[0002] In recent years, new renewable energy generation technologies such as photovoltaics and wind power have developed rapidly due to their advantages of being clean, environmentally friendly, and inexhaustible. As an interface device between new energy generation and the power grid, the role of three-phase grid-connected inverters in modern power systems has become increasingly prominent. An important component of its control system is the grid synchronization technology, which usually uses a phase-locked loop to obtain the phase angle of the grid voltage to achieve synchronous operation between the grid-connected inverter and the grid. Existing studies have shown that the performance of the phase-locked loop will directly affect the stability of the grid-connected inverter under weak grid conditions. Therefore, it is of great significance to design a phase-locked loop with excellent performance.

[0003] The current standard phase-locked loop is implemented in a synchronous reference coordinate system, but it is easily affected by input voltage imbalance and distortion. Therefore, a dual second-order generalized integrator phase-locked loop (DSOGI-PLL) is generally used to effectively eliminate harmonic interference by adding a low-pass filter in the loop, while also having frequency adaptation capabilities. However, the existing DSOGI-PLL design has the following problems:

[0004] 1. When the phase-locked loop controller adopts typical PI control, the DSOGI-PLL will have large overshoot or oscillation during frequency detection and phase detection, and the dynamic performance is poor;

[0005] 2. The existing DSOGI-PLL design method is difficult to achieve high-order transfer function design, so the filtering effect is not good and the anti-interference ability to input high-frequency harmonics is poor;

[0006] 3. The current phase-locked loop structure design only considers its own performance, but does not consider the impact on system stability.

[0007] In response to the above problems, a high-order phase-locked loop design method suitable for new energy grid-connected inverters is proposed. Summary of the invention

[0008] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a high-order phase-locked loop design method suitable for new energy grid-connected inverters. From the perspective of the stability of the new energy grid-connected system, the structure and parameters of the phase-locked loop controller are reversely determined, which can not only obtain better dynamic response performance and higher filtering capability, but also ensure the stability of the new energy grid-connected system within a wide range of operating conditions.

[0009] The purpose of the present invention can be achieved through the following technical solutions:

[0010] A high-order phase-locked loop design method suitable for a new energy grid-connected inverter comprises the following steps:

[0011] S1. Analyze the operation scenarios of the new energy grid-connected inverter and determine the variation range of the grid complex impedance;

[0012] S2. Calculate the allowable variation range of the impedance of the new energy grid-connected inverter according to the variation range of the complex impedance of the power grid obtained in S1, so that the new energy grid-connected system remains stable in all operating scenarios;

[0013] S3. Design the impedance transfer function of the new energy grid-connected inverter according to the allowable variation range of the impedance of the new energy grid-connected inverter in S2;

[0014] S4, using the impedance transfer function of the new energy grid-connected inverter designed in S3, to obtain the desired phase-locked loop transfer function;

[0015] S5. Design the structure and parameters of the phase-locked loop controller according to the desired phase-locked loop transfer function obtained in S4.

[0016] Furthermore, the operation scenarios of the new energy grid-connected inverter in S1 include changes in the grid structure or the operation mode of the grid;

[0017] The complex impedance of the power grid in S1 is obtained by analytical derivation or impedance measurement;

[0018] The method for determining the variation range of the complex impedance of the power grid in S1 is as follows: under different operating scenarios of the new energy grid-connected inverter, the complex impedance of the power grid in a specific frequency band is obtained respectively, and the corresponding impedance amplitude-phase variation curve is drawn. The boundary of the impedance amplitude-phase curve set under each operating scenario is the variation range of the complex impedance of the power grid.

[0019] Furthermore, the allowable variation range of the impedance of the new energy grid-connected inverter in S2 is: Assume that the phase-frequency curve of the impedance of the new energy grid-connected inverter and the curve after the phase-frequency curve of the grid impedance in each operating scenario is shifted upward by 180° and intersects at the frequency point f c1 、f c2 ,…,f ck , k represents the kth operating scenario, and the grid impedance amplitudes corresponding to the above frequency points in the amplitude-frequency curve are |Z sc1 |、|Z sc2 |, ..., |Z sck |, the corresponding impedance amplitudes of the new energy grid-connected inverter are |Z ic1 |、|Z ic2 |, ..., |Z ick |, then when |Z is satisfied ic1 |>|Z sc1 |∩|Z ic2 |>|Z sc2|∩…∩|Z ick |>|Z sck |, the new energy grid-connected system can remain stable in all operating scenarios, and all |Z ic1 |>|Z sc1 |∩|Z ic2 |>|Z sc2 |∩…∩|Z ick |>|Z sck The impedance curve set is the allowable variation range of the impedance of the new energy grid-connected inverter.

[0020] Furthermore, the allowable variation range of the impedance of the new energy grid-connected inverter in S2 is determined by the following method:

[0021] S2.1. Based on the complex impedance amplitude-frequency variation curves of the power grid under different operating scenarios in S1, the maximum value of the complex impedance amplitude of the power grid corresponding to each frequency point within the measurement frequency band is taken out. smax1 |、|Z smax2 |, …, |Z smaxm |, m represents the mth frequency point within the measurement frequency band;

[0022] S2.2, connect |Z in S2.1 on the amplitude-frequency curve smax1 |、|Z smax2 |, ..., |Z smaxm |, and obtain the maximum positive change boundary of the complex impedance amplitude-frequency of the power grid under different operation scenarios;

[0023] S2.3. All areas within the measurement frequency band that exceed the maximum positive change boundary of the grid complex impedance amplitude-frequency in S2.2 are the allowable change range of the impedance of the new energy grid-connected inverter on the amplitude-frequency diagram.

[0024] Furthermore, the method for determining the allowable variation range of the impedance of the new energy grid-connected inverter meets the following premise: the phase-frequency curve of the impedance of the new energy grid-connected inverter has at least one intersection with the curve of the grid impedance phase-frequency curve after shifting upward by 180° within a given frequency band.

[0025] Furthermore, the design method of the impedance transfer function of the new energy grid-connected inverter in S3 is as follows:

[0026] The measurement frequency range of the grid impedance in S3.1 and S1 is [ω 0 ,ω m ], the transfer functions of the grid complex impedance and the impedance of the new energy grid-connected inverter in this frequency band are Z s (s), Z i (s), let s = jω, then the amplitude and phase response curves of the impedance of the grid and the new energy grid-connected inverter are obtained by the following formulas, where ω∈[ω 0 ,ωm ]:

[0027]

[0028]

[0029] S3.2. According to the allowable variation range of the impedance of the new energy grid-connected inverter in S2, the constraints of the impedance transfer function of the grid-connected inverter are determined as follows:

[0030] (1) In the frequency band [ω 0 ,ω m ], there is at least one ω c ∈[ω 0 ,ω m ], so that the following equation holds:

[0031] ∠Z i (jw c )=∠Z s (jw c )+180°

[0032] (2) For any ω∈[ω 0 ,ω m ], both satisfy:

[0033] |Z i (jw)|>|Z s (jw)|;

[0034] S3.3. Based on the constraints in S3.2, the maximum mean value of the damping ratio of the renewable energy grid-connected system is taken as the optimization goal, and the inverter impedance transfer function that can make the renewable energy grid-connected system most stable is solved.

[0035]

[0036] Furthermore, the damping ratio mean value calculation process of the new energy grid-connected system is as follows:

[0037] S3.3.1. Order

[0038]

[0039] In the above formula: Z s (s) is the impedance transfer function of the power grid, Z i (s) is the impedance transfer function of the new energy grid-connected inverter;

[0040] S3.3.2: Calculate all poles P of the transfer function G(s) in S3.3.1 t =c t +jd t , t=1,2,…,T;

[0041] S3.3.3: Obtain the tth damping ratio ζ of the renewable energy grid-connected system t and the mean damping ratio ζ av :

[0042]

[0043]

[0044] Furthermore, the desired phase-locked loop transfer function calculation process in S4 is as follows: According to the impedance analytical model of the new energy grid-connected inverter:

[0045]

[0046] Furthermore, the least squares method is used to solve the desired phase-locked loop transfer function H(s), and the process is as follows:

[0047] S4.1. Extract the impedance model expression related to the phase-locked loop transfer function in the grid-connected inverter impedance analytical model;

[0048] S4.2, let the relevant impedance model expression in S4.1 be the same as the one optimized in S3.3 Corresponding elements are equal:

[0049]

[0050] At the same time Subscripts w1 to wg represent dd or dq or qd or qq; h 1 The impedance analytical model Z of the grid-connected inverter is iw1 The relationship between the phase-locked loop transfer function H(s); g The impedance analytical model Z of the grid-connected inverter is iwg The relationship between the phase-locked loop transfer function H(s);

[0051] S4.3. Find the first-order derivative f'(H(s)) of the function f(H(s)) in S4.2 with respect to H(s);

[0052] S4.4. Let the derivative in S4.3 be 0, that is, f'(H(s)) = 0, so as to obtain the desired phase-locked loop transfer function:

[0053]

[0054] p is the order of the numerator, q is the order of the denominator; b i 、a j is the corresponding coefficient;

[0055] The phase-locked loop transfer function H(s) satisfies:

[0056] (1) pq ≥ 2;

[0057] (2)b 0 =a 0 .

[0058] Furthermore, the phase-locked loop controller in S5 adopts a dual second-order generalized integrator phase-locked loop, and the design process of the phase-locked loop controller is as follows:

[0059] S5.1. Calculate the transfer function G of the standard three-phase synchronous phase-locked loop according to the desired phase-locked loop transfer function H(s) obtained in S4. p (s):

[0060]

[0061] In the above formula: G opu (s) 1,1 is the matrix G opu (s) The first row and first column element, G opw (s) 1 is the matrix G opw The first element of (s), the matrix G opu (s) and G opw The expression of (s) is as follows:

[0062]

[0063] k s is the damping coefficient of DSOGI, ω n is the rated frequency of the power grid of DSOGI;

[0064] S5.2. Using the transfer function G calculated in S5.1 p (s), determine the phase-locked loop controller G c Structure and parameters of (s):

[0065]

[0066] Beneficial effects of the present invention:

[0067] 1. The high-order phase-locked loop design method for the new energy grid-connected inverter proposed in the present invention reversely determines the structure and parameters of the phase-locked loop controller from the perspective of the stability of the new energy grid-connected system, thereby ensuring the stability of the new energy grid-connected system in a wide range of operating conditions;

[0068] 2. The high-order phase-locked loop design method for new energy grid-connected inverter proposed in the present invention, wherein the DSOGI-PLL controller has high-order characteristics, so it has strong filtering capability and strong anti-interference ability to input high-frequency harmonics;

[0069] 3. The high-order phase-locked loop design method suitable for new energy grid-connected inverter proposed in the present invention can also obtain better dynamic response performance during frequency detection and phase detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0071] Figure 1 It is a schematic diagram of the overall process of the present invention;

[0072] Figure 2 is a block diagram of a grid complex impedance measurement according to an embodiment of the present invention;

[0073] Figure 3 Schematic diagram of the allowable variation range of impedance of the new energy grid-connected inverter according to an embodiment of the present invention;

[0074] Figure 4 is a schematic diagram of frequency response comparison results of an embodiment of the present invention;

[0075] Figure 5 is a schematic diagram of step response comparison results of an embodiment of the present invention;

[0076] Figure 6 4 is a schematic diagram of system stability comparison results of an embodiment of the present invention. DETAILED DESCRIPTION

[0077] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not 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.

[0078] This embodiment is based on a direct-drive wind turbine grid-connected model simulation system. The variation range of the complex impedance of the power grid is determined by measurement. From the perspective of the stability of the new energy grid-connected system, the allowable variation range of the impedance of the direct-drive wind turbine grid-connected inverter is calculated. Then, an intelligent optimization algorithm is used to design the impedance transfer function of the grid-connected inverter, and the expected phase-locked loop transfer function is obtained. Finally, the structure and parameters of the DSOGI-PLL controller are designed.

[0079] like Figure 1 As shown, a high-order phase-locked loop design method suitable for a new energy grid-connected inverter specifically includes the following steps:

[0080] S1. Analyze different operation scenarios of direct-drive wind turbine grid-connected inverters, including changes in grid structure and grid operation mode, and use the impedance measurement method based on the dq coordinate system to obtain the grid complex impedance in each operation scenario. The measurement block diagram of the grid complex impedance is as follows: Figure 2 As shown;

[0081] The S1 specifically includes the following steps:

[0082] S1.1, fix the system phase angle, set the q-axis current disturbance to 0, apply a sinusoidal disturbance signal of a specific frequency to the d-axis current, and use the inverse Park transform to convert it into a three-phase current disturbance;

[0083] S1.2, inject the three-phase current disturbance in S1.1 into the system from the PCC point (common connection point) through a controlled current source, measure the system response, i.e., the PCC point voltage and grid-side current, and transform them into the dq coordinate system using Park transformation;

[0084] S1.3. For the PCC point voltage and grid-side current in the dq coordinate system in S1.2, the corresponding voltage at the disturbance frequency is separated using the parameter identification algorithm: u d1 and u q1 And the current component: i d1 and i q1 ;

[0085] S1.4, set the d-axis current disturbance to 0, apply the same sinusoidal disturbance signal as in S1.1 to the q-axis current, and convert it into a three-phase current disturbance;

[0086] S1.5, inject the three-phase current disturbance in S1.4 into the system from the PCC point through a controlled current source, measure the PCC point voltage and grid-side current, and convert them into the dq coordinate system;

[0087] S1.6. For the PCC point voltage and grid-side current in the dq coordinate system in S1.5, the corresponding voltage at the disturbance frequency is separated using the parameter identification algorithm: u d2 and u q2 And the current component: i d2 and i q2 ;

[0088] S1.7. Using the voltage and current values ​​obtained in S1.3 and S1.6, calculate the dq coordinate system impedance matrix Z of the power grid at this frequency. s :

[0089]

[0090] In different operating scenarios of the direct-drive wind turbine grid-connected inverter, the grid complex impedance in the frequency band of 2π to 2000πrad / s is measured, and the corresponding impedance amplitude-phase change curves are drawn. The boundary of the impedance amplitude-phase curve set in each operating scenario is the change range of the grid complex impedance, such as Figure 3 shown.

[0091] S2. Calculate the allowable variation range of the impedance of the direct-drive wind turbine grid-connected inverter based on the variation range of the grid complex impedance obtained in S1, so that the wind turbine grid-connected system remains stable in all operating scenarios;

[0092] The allowable variation range of the impedance of the grid-connected inverter of the direct-drive wind turbine is: Assume that the phase-frequency curve of the grid-connected inverter impedance and the curve after the grid impedance phase-frequency curve under each operating scenario is shifted upward by 180° and intersects at the frequency point f c1 、f c2 ,…,f ck , where k represents the kth operating scenario. The grid impedance amplitudes corresponding to the above frequency points in the amplitude-frequency curve are |Z sc1 |、|Z sc2 |, …, |Z sck |, the corresponding impedance amplitudes of the new energy grid-connected inverter are |Z ic1 |、|Z ic2 |, …, |Z ick |, then when |Z is satisfied ic1 |>|Z sc1 |∩|Z ic2 |>|Z sc2 |∩…∩|Z ick |>|Z sck |, the new energy grid-connected system can remain stable in all operating scenarios, and all |Z ic1 |>|Z sc1 |∩|Z ic2 |>|Z sc2 |∩…∩|Z ick |>|Z sck The impedance curve set of | is the allowable variation range of the grid-connected inverter impedance;

[0093] The S2 specifically includes the following steps:

[0094] S2.1. Based on the complex impedance amplitude-frequency variation curves of the power grid under different operating scenarios in S1, the maximum value of the complex impedance amplitude of the power grid corresponding to each frequency point within the measurement frequency band is taken out. smax1 |、|Z smax2 |, …, |Z smaxm |, where m represents the mth frequency point within the measurement frequency band;

[0095] S2.2, connect |Z in S2.1 on the amplitude-frequency curve smax1 |、|Z smax2 |, ..., |Z smaxm |, and obtain the maximum positive change boundary of the complex impedance amplitude-frequency of the power grid under different operation scenarios;

[0096] S2.3. All areas within the measurement frequency range that exceed the maximum positive change boundary of the grid complex impedance amplitude-frequency in S2.2 are the allowable change range of the wind turbine grid-connected inverter impedance on the amplitude-frequency diagram, such as Figure 3 The shaded area in .

[0097] Among them, the phase-frequency curve of the grid-connected inverter impedance has at least one intersection point within a given frequency band with the curve of the grid impedance phase-frequency curve shifted upward by 180°.

[0098] S3, according to the allowable variation range of the impedance of the wind turbine grid-connected inverter in step S2, using a genetic algorithm to design the impedance transfer function of the grid-connected inverter;

[0099] The S3 specifically includes the following steps:

[0100] The measurement frequency range of the grid impedance in S3.1 and S1 is 2π~2000πrad / s. The transfer functions of the grid complex impedance and the wind turbine grid-connected inverter impedance in this frequency range are Z s (s), Z i (s), let s = jω, then the amplitude and phase response curves of the impedance of the grid and the new energy grid-connected inverter can be obtained by the following formulas, where ω∈[2π,2000π]:

[0101]

[0102]

[0103] S3.2. According to the allowable variation range of the impedance of the direct-drive wind turbine grid-connected inverter in S2, the constraints of the grid-connected inverter impedance transfer function are determined as follows:

[0104] (3) In the frequency band [2π, 2000π], there is at least one ω c ∈[2π, 2000π], so that the following holds:

[0105] ∠Z i (jw c )=∠Z s (jw c )+180°

[0106] (4) For any ω∈[2π, 2000π], it satisfies:

[0107] |Zi (jw)|>|Z s (jw)|

[0108] S3.3. Based on the constraints in S3.2, taking the maximum mean value of the wind turbine grid-connected system damping ratio as the optimization goal, a genetic algorithm is used to solve the inverter impedance transfer function that can make the direct-drive wind turbine grid-connected system most stable.

[0109]

[0110] In the above formula:

[0111]

[0112] The calculation process of the damping ratio mean value of the direct-drive wind turbine grid-connected system is as follows:

[0113] S3.3.1. Order

[0114]

[0115] In the above formula: Z s (s) is the impedance transfer function of the power grid, Z i (s) is the impedance transfer function of the direct-drive wind turbine grid-connected inverter;

[0116] S3.3.2: Calculate all poles P of the transfer function G(s) in S3.3.1 t =c t +jd t , t=1,2,…,T;

[0117] S3.3.3: Obtain the tth damping ratio ζ of the wind turbine grid-connected system t and the mean damping ratio ζ av :

[0118]

[0119]

[0120] S4. Using the impedance transfer function of the direct-drive wind turbine grid-connected inverter designed in S3 According to its impedance analysis model:

[0121]

[0122] in:

[0123]

[0124] In the above formula: U dc is the steady-state value of the DC bus voltage; C dc is the DC capacitance; Rf is the equivalent resistance of the grid-connected filter; L f is the equivalent inductance of the grid-connected filter; G gi (s) is the transfer function of the current inner loop PI link; I gd and I gq is the steady-state component of the grid-side current in the dq coordinate system of the AC system; G dc (s) is the transfer function of the voltage outer loop PI controller; U pccd and U pccq is the steady-state component of the voltage at PCC in the dq coordinate system of the AC system; H(s) is the transfer function of the phase-locked loop control system.

[0125] The least squares method is used to solve the desired phase-locked loop transfer function H(s), the process is as follows:

[0126] S4.1. Take out the impedance model expression related to the phase-locked loop transfer function in the grid-connected inverter impedance analytical model:

[0127]

[0128] S4.2, let the relevant impedance model expression in S4.1 be the same as the one optimized in S3.3 Corresponding elements are equal:

[0129]

[0130] Right now:

[0131]

[0132] At the same time Wherein the subscripts w1~wg represent dd or dq or qd or qq; h g The impedance analytical model Z of the grid-connected inverter is iwg The relationship between the phase-locked loop transfer function H(s); 1 The impedance analytical model Z of the grid-connected inverter is idq The relationship between the phase-locked loop transfer function H(s); 2 The impedance analytical model Z of the grid-connected inverter is iqq The relationship between the phase-locked loop transfer function H(s);

[0133] S4.3. Find the first-order derivative f'(H(s)) of the function f(H(s)) in S4.2 with respect to H(s);

[0134] S4.4. Let the derivative in S4.3 be 0, that is, f'(H(s)) = 0, so as to obtain the desired phase-locked loop transfer function:

[0135]

[0136] The phase-locked loop transfer function H(s) satisfies:

[0137] (1) pq ≥ 2;

[0138] (2)b 0 =a 0 .

[0139] p is the order of the numerator, q is the order of the denominator; b i 、a j is the corresponding coefficient, that is:

[0140]

[0141] In the above formula: T ob =0.003, q=4.

[0142] S5. According to the desired phase-locked loop transfer function obtained in S4, the structure and parameters of a dual second-order generalized integrator phase-locked loop (DSOGI-PLL) controller are designed;

[0143] The S5 comprises the following steps:

[0144] Step S5.1: Calculate the transfer function G of the standard three-phase synchronous phase-locked loop according to the desired phase-locked loop transfer function H(s) obtained in S4. p (s):

[0145]

[0146] In the above formula: G opu (s) 1,1 is the matrix G opu (s) The first row and first column element, G opw (s) 1 is the matrix G opw The first element of (s), the matrix G opu (s) and G opw The expression of (s) is as follows:

[0147]

[0148] k s is the damping coefficient of DSOGI, ω n is the rated frequency of the power grid of DSOGI;

[0149] In the above formula: k s =1.056,ω n =2π×50rad / s;

[0150] S5.2. Using the transfer function G calculated in S5.1 p(s), determine the phase-locked loop controller G c Structure and parameters of (s):

[0151]

[0152] The present method is further described below in conjunction with the results of a specific embodiment, and is analyzed by comparing with a traditional phase-locked loop design method, wherein the design parameters of the traditional method are: k s =1.056, k p =138.23, k i =7961.48.

[0153] (1) Comparison between frequency response and step response

[0154] Figure 4 and Figure 5 The comparison results of the frequency response and step response of the traditional method and the present method are given. It can be seen that compared with the traditional method, the H(s) designed by the present invention has no resonance peak in the frequency response and no overshoot in the step response. In addition, due to q=4, the designed H(s) has better filtering performance, which proves that it has strong anti-interference ability to input high-frequency harmonics.

[0155] (2) Frequency change comparison

[0156] When the input frequency changes from 50Hz to 51Hz, the results show that the overshoot produced by the traditional method is as high as 42%, and the stabilization time is as high as 46.24ms, while the stabilization time of this method can be reduced by 27.40ms.

[0157] (3) Phase change comparison

[0158] When the input phase changes by 0.1 rad (5.73°), the phase detection of this method has the same dynamic performance as the frequency detection, which again shows that the proposed method can achieve fast response without overshoot.

[0159] (4) Comparison of amplitude changes

[0160] When the input amplitude changes by 0.1pu, this method does not affect the steady-state performance of the system. At the same time, the transient changes produced by the proposed method are smaller, which proves its superior filtering performance.

[0161] (5) Voltage imbalance comparison

[0162] When the input voltage becomes unbalanced, both methods can eliminate the unbalanced components in the steady state, which shows that the proposed method can achieve the designed high-order H(s) without affecting the basic function of the DSOGI-PLL.

[0163] (6) Comparison of harmonic distortion

[0164] When the input voltage is distorted by 0.1pu5th and 0.1pu7th harmonics, the method proposed in the present invention can generate lower ripples on the detected frequency and phase compared with the traditional method because it has a fourth-order closed-loop system transfer function.

[0165] (7) System stability comparison

[0166] Figure 6 The system stability comparison results based on the traditional phase-locked loop design method and the phase-locked loop design method proposed by the present invention are given. The curve in the figure is the output active power of the direct-drive wind turbine in the simulation model. It can be seen that the direct-drive wind turbine grid-connected system based on the traditional phase-locked loop design method is difficult to maintain stability, while the phase-locked loop designed by this method will not affect the system stability.

[0167] The results of the above embodiments show that from the perspective of the stability of the new energy grid-connected system, the proposed method can optimize the solution of H(s) and reversely design the structure and parameters of the phase-locked loop without affecting the stability of the system. It can further improve the performance of the phase-locked loop itself and has the advantage of improving both the dynamic performance and the steady-state performance of the system.

[0168] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0169] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A high-order phase-locked loop design method suitable for new energy grid-connected inverters. It is characterized in that The steps include: S1. Analyze the operation scenarios of the new energy grid-connected inverter and determine the variation range of the grid complex impedance; S2. Calculate the allowable variation range of the impedance of the new energy grid-connected inverter according to the variation range of the complex impedance of the power grid obtained in S1, so that the new energy grid-connected system remains stable in all operating scenarios; S3. Design the impedance transfer function of the new energy grid-connected inverter according to the allowable variation range of the impedance of the new energy grid-connected inverter in S2; S4, using the impedance transfer function of the new energy grid-connected inverter designed in S3, to obtain the desired phase-locked loop transfer function; S5. Designing a phase-locked loop controller structure and parameters according to the desired phase-locked loop transfer function obtained in S4; The design method of the impedance transfer function of the new energy grid-connected inverter in S3 is as follows: The measurement frequency range of the grid impedance in S3.1 and S1 is [ω 0 ,ω m ], the transfer functions of the grid complex impedance and the impedance of the new energy grid-connected inverter in this frequency band are Z s (s), Z i (s), let s = jω, then the amplitude and phase response curves of the impedance of the grid and the new energy grid-connected inverter are obtained by the following formulas, where ω∈[ω 0 ,ω m ]: S3.

2. According to the allowable variation range of the impedance of the new energy grid-connected inverter in S2, the constraints of the impedance transfer function of the grid-connected inverter are determined as follows: (1) In the frequency band [ω 0 ,ω m ], there is at least one ω c ∈[ω 0 ,ω m ], so that the following equation holds: ∠Z i (jw c )=∠Z s (jw c )+180° (2) For any ω∈[ω 0 ,ω m ], both satisfy: |From i (jw)|>|From s (as above)|; S3.

3. Based on the constraints in S3.2, the maximum mean value of the damping ratio of the new energy grid-connected system is taken as the optimization target, and the intelligent optimization algorithm is used to solve the inverter impedance transfer function that can make the new energy grid-connected system most stable. The calculation process of the damping ratio mean value of the new energy grid-connected system is as follows: S3.3.

1. Order In the above formula: Z s (s) is the impedance transfer function of the power grid, Z i (s) is the impedance transfer function of the new energy grid-connected inverter; S3.3.2: Calculate all poles P of the transfer function G(s) in S3.3.1 t =c t +jd t , t=1,2,…,T; S3.3.3: Obtain the tth damping ratio ζ of the renewable energy grid-connected system t and the mean damping ratio ζ av : The calculation process of the desired phase-locked loop transfer function in S4 is as follows: According to the impedance analytical model of the new energy grid-connected inverter: The least squares method is used to solve the desired phase-locked loop transfer function H(s), the process is as follows: S4.

1. Extract the impedance model expression related to the phase-locked loop transfer function in the grid-connected inverter impedance analytical model; S4.2, let the relevant impedance model expression in S4.1 be the same as the one optimized in S3.3 Corresponding elements are equal: At the same time Subscripts w1 to wg represent dd or dq or qd or qq; h 1 The impedance analytical model Z of the grid-connected inverter is iw1 The relationship between the phase-locked loop transfer function H(s); g The impedance analytical model Z of the grid-connected inverter is iwg The relationship between the phase-locked loop transfer function H(s); S4.

3. Find the first-order derivative f'(H(s)) of the function f(H(s)) in S4.2 with respect to H(s); S4.

4. Let the derivative in S4.3 be 0, that is, f'(H(s)) = 0, so as to obtain the desired phase-locked loop transfer function: p is the order of the numerator, q is the order of the denominator; b i 、a j is the corresponding coefficient; The phase-locked loop transfer function H(s) satisfies: (1) pq ≥ 2; (2)b 0 =a 0 。 2. A high-order phase-locked loop design method suitable for a new energy grid-connected inverter according to claim 1, It is characterized in that The operation scenarios of the new energy grid-connected inverter in S1 include changes in the grid structure or the operation mode of the grid; The complex impedance of the power grid in S1 is obtained by analytical derivation or impedance measurement; The method for determining the variation range of the complex impedance of the power grid in S1 is as follows: under different operating scenarios of the new energy grid-connected inverter, the complex impedance of the power grid in a specific frequency band is obtained respectively, and the corresponding impedance amplitude-phase variation curve is drawn. The boundary of the impedance amplitude-phase curve set under each operating scenario is the variation range of the complex impedance of the power grid.

3. A high-order phase-locked loop design method suitable for a new energy grid-connected inverter according to claim 1, It is characterized in that The allowable variation range of the impedance of the new energy grid-connected inverter in S2 is: Assume that the phase-frequency curve of the impedance of the new energy grid-connected inverter and the curve after the phase-frequency curve of the grid impedance in each operating scenario is shifted upward by 180° and intersects at the frequency point f c1 、f c2 ,…,f ck , k represents the kth operating scenario, and the grid impedance amplitudes corresponding to the above frequency points in the amplitude-frequency curve are |Z sc1 |、|Z sc2 |, …, |Z sck |, the corresponding impedance amplitudes of the new energy grid-connected inverter are |Z ic1 |、|Z ic2 |, …, |Z ick |, then when |Z is satisfied ic1 |>|Z sc1 |∩|Z ic2 |>|Z sc2 |∩…∩|Z ick |>|Z sck |, the new energy grid-connected system can remain stable in all operating scenarios, and all |Z ic1 |>|Z sc1 |∩|Z ic2 |>|Z sc2 |∩…∩|Z ick |>|Z sck The impedance curve set is the allowable variation range of the impedance of the new energy grid-connected inverter.

4. A high-order phase-locked loop design method suitable for a new energy grid-connected inverter according to claim 3, It is characterized in that The allowable variation range of the impedance of the new energy grid-connected inverter in S2 is determined as follows: S2.

1. Based on the complex impedance amplitude-frequency variation curves of the power grid under different operating scenarios in S1, the maximum value of the complex impedance amplitude of the power grid corresponding to each frequency point within the measurement frequency band is taken out. smax1 |、|Z smax2 |, …, |Z smaxm |, m represents the mth frequency point within the measurement frequency band; S2.2, connect |Z in S2.1 on the amplitude-frequency curve smax1 |、|Z smax2 |, ..., |Z smaxm |, and obtain the maximum positive change boundary of the complex impedance amplitude-frequency of the power grid under different operation scenarios; S2.

3. All areas within the measurement frequency band that exceed the maximum positive change boundary of the grid complex impedance amplitude-frequency in S2.2 are the allowable change range of the impedance of the new energy grid-connected inverter on the amplitude-frequency diagram.

5. A high-order phase-locked loop design method suitable for a new energy grid-connected inverter according to claim 4, It is characterized in that The method for determining the allowable variation range of the impedance of the new energy grid-connected inverter meets the following premise: the phase-frequency curve of the impedance of the new energy grid-connected inverter has at least one intersection with the curve of the grid impedance phase-frequency curve after shifting upward by 180° within a given frequency band.

6. A high-order phase-locked loop design method suitable for a new energy grid-connected inverter according to claim 1, It is characterized in that The phase-locked loop controller in S5 adopts a dual second-order generalized integrator phase-locked loop. The design process of the phase-locked loop controller is as follows: S5.

1. Calculate the transfer function G of the standard three-phase synchronous phase-locked loop according to the desired phase-locked loop transfer function H(s) obtained in S4. p (s): In the above formula: G opu (s) 1,1 is the matrix G opu (s) The first row and first column element, G opw (s) 1 is the matrix G opw The first element of (s), the matrix G opu (s) and G opw The expression of (s) is as follows: k s is the damping coefficient of DSOGI, ω n is the rated frequency of the power grid of DSOGI; S5.

2. Using the transfer function G calculated in S5.1 p (s), determine the phase-locked loop controller G c Structure and parameters of (s):

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