Multi-inverter grid-connected system broadband oscillation suppression method based on impedance collaborative reconstruction

Abstract

The invention provides a multi-inverter grid-connected system broadband oscillation suppression method based on impedance collaborative reconstruction, and the method comprises the steps: firstly carrying out the frequency domain small signal modeling, and constructing a station-level system frequency domain impedance model on the basis of a single machine; therefore, the design of an additional control branch is expanded, a power signal is superposed to the control branch through an additional damping control branch so as to realize the improvement of the impedance amplitude of the inverter, and the phase improvement of a capacitive frequency band is realized by adopting an active damping strategy based on a filter; secondly, constructing an impedance collaborative reconstruction method under a multi-inverter system, establishing a two-stage optimization structure, comprehensively solving the position and parameters of a control branch, and achieving the suppression of system oscillation. According to the invention, through collaborative optimization, conflicts of different control strategies are reduced, and the influence of a coupling relationship is minimized; furthermore, through cooperation of the multiple inverters, dual optimization of damping in the space and the frequency domain is achieved, and efficient suppression of broadband oscillation of the multi-inverter grid-connected system is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electric power, in particular to a wide-frequency oscillation suppression method for a multi-inverter grid-connected system based on impedance coordination reconstruction. BACKGROUND

[0002] With the acceleration of global energy transformation process, large-scale development and utilization of renewable energy such as solar energy and wind energy have become an important direction of the development of power system. As a key link of new energy access to power grid, the stability problem of multi-inverter grid-connected system is increasingly prominent. When a large number of inverters are connected in parallel to the power grid, the interaction of system impedance characteristics will cause wide-frequency oscillation phenomenon, which seriously threatens the safe and reliable operation of the power grid. This oscillation phenomenon covers a wide frequency range from subsynchronous to super-synchronous, and presents complex characteristics of nonlinearity and multi-band coupling, so the traditional single-machine stability control strategy has been difficult to effectively cope with.

[0003] In the prior art, the research on oscillation suppression of inverter grid-connected system mainly focuses on active damping control of single inverter. For example, by introducing virtual impedance or current feedback link in the control loop, the amplitude-frequency characteristic of single machine is improved to suppress the oscillation in a specific frequency band. However, this kind of method only considers single machine independent control, ignoring the impedance coupling effect when multiple inverters are connected in parallel. When multiple inverters are connected in parallel, the impedance characteristics of each unit will produce nonlinear interaction through the grid impedance, resulting in the failure of single-machine optimization strategy at the system level. Studies have shown that the total impedance of multiple inverters connected in parallel is not a simple superposition of the impedance of each single machine, and the amplitude and phase will produce a "neutralizing effect" due to the phase difference between units, so that the improvement of single-machine amplitude-frequency characteristic may not reach the optimal improvement of system-level impedance characteristics due to the phase difference exceeding the critical value.

[0004] In addition, the existing technology lacks systematic design for wide-frequency oscillation suppression of multi-inverter grid-connected system. Most schemes only optimize a single frequency band, such as high frequency or subsynchronous frequency band, and cannot achieve full coverage of wide frequency domain. For example, the phase correction method based on high-pass filter can improve the stability in low frequency band, but may introduce new phase lag in medium and high frequency bands, resulting in a decrease in system damping. At the same time, the existing methods generally use fixed parameter control strategy, which is difficult to adapt to the dynamic changes of power grid operating conditions, and the suppression effect is significantly reduced in weak power grid or harmonic background.

[0005] In the engineering implementation level, the existing technology also faces the contradiction between cost and efficiency of multi-machine coordinated control. The traditional full-machine set reconstruction scheme needs to deploy complex control strategies for each inverter, which not only increases the hardware cost, but also may cause new stability problems due to inconsistent parameter setting.

[0006] In summary, the existing technology has the following deficiencies in wide-frequency oscillation suppression of multi-inverter grid-connected system:

[0007] (1) There is a lack of in-depth analysis of the multi-machine impedance coupling mechanism, and the nonlinear laws of amplitude coupling and phase angle neutralization have not been revealed;

[0008] (2) A single branch control strategy cannot take into account the coordinated optimization of amplitude-frequency and phase-frequency characteristics;

[0009] (3) Insufficient wide-band coverage capability, making it difficult to cope with multi-band coupled oscillations;

[0010] (4) The project has poor adaptability and cannot meet the constraints of improving control effects and controlling costs in actual stations.

[0011] Therefore, it is urgent to propose an oscillation suppression method that takes into account the amplitude-phase coupling characteristics of multiple machines and has both broadband optimization capabilities and engineering feasibility, so as to solve the problems of low broadband oscillation suppression efficiency and poor adaptability of multi-inverter grid-connected systems in the existing technology. Summary of the Invention

[0012] In response to the problems existing in the prior art, the present invention provides a method based on impedance collaborative reconstruction, which realizes a method for efficiently suppressing broadband oscillations in a multi-inverter grid-connected system through hierarchical collaborative control and double-layer objective function optimization.

[0013] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0014] The present invention provides a method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance cooperative reconstruction, which comprises the following steps:

[0015] The frequency domain small signal impedance model of a single grid-connected inverter is obtained based on the small disturbance linearization method;

[0016] Constructing a station-level frequency-domain impedance model including multiple inverters based on the frequency-domain small-signal impedance model of the single grid-connected inverter;

[0017] Based on the frequency domain impedance model of a single inverter, an additional damping control branch and an active damping compensation branch are established;

[0018] Based on the additional damping control branch and the active damping compensation branch, a station-level objective function and a unit-level objective function are constructed, and an impedance collaborative reconstruction process is formulated through the station-level objective function and the unit-level objective function to collaboratively calculate the access strategy and parameter configuration of the additional damping control branch and the active damping compensation branch;

[0019] According to the access strategy and parameter configuration, the additional damping control branch and the active damping compensation branch are connected to the corresponding inverter, and the system frequency domain impedance is reconstructed to suppress the broadband oscillation of the multi-inverter grid-connected system.

[0020] Optionally, the frequency domain small signal impedance model obtaining method of the single grid-connected inverter comprises the following steps:

[0021] Constructing a topology structure of the single grid-connected inverter;

[0022] Establishing a frequency domain steady-state model of a nonlinear system based on the topology structure;

[0023] Superimposing a disturbance signal on the frequency domain steady-state model in a steady state, and performing linearization processing on a nonlinear link in the frequency domain steady-state model to obtain a frequency domain small signal impedance model of the single grid-connected inverter.

[0024] Optionally, in the construction of the field station level frequency domain impedance model, the impedance of the multiple inverters is recorded in a complex polar coordinate form;

[0025] The field station total impedance is a ratio of the product of the impedances of the inverters and the sum of the combined impedances;

[0026] The field station phase angle is a sum of the phase angles of the units minus a neutralized phase angle generated by the superposition of the impedance vectors.

[0027] Optionally, based on the amplitude-frequency characteristic of the field station level frequency domain impedance model, an additional damping control branch is established with the terminal power as an input signal;

[0028] The additional damping control branch comprises a filter link, a gain link, a phase compensation link and a limiting link;

[0029] The gain link outputs a compensation signal.

[0030] Optionally, based on the phase-frequency characteristic of the field station level frequency domain impedance model, an active damping compensation branch is established with a current feedback as an input signal;

[0031] The active damping compensation branch comprises a low-pass filter and a notch filter connected in series.

[0032] Optionally, the field station level target function takes the optimization of the spatial damping distribution as a target, and comprises a sum of a normalized ratio of impedance amplitudes before and after reconstruction at a characteristic frequency and a phase angle boundary distance;

[0033] The phase angle boundary distance is a ratio of a minimum value of a difference between a characteristic impedance phase angle and 90 degrees and 90 degrees.

[0034] Optionally, the unit level target function takes the optimization of the wide frequency band impedance as a target, and comprises a weighted sum of an absolute value of a difference between a target phase angle and an actual phase angle and a weighted sum of impedance amplitudes.

[0035] Optionally, when the access strategy and the parameter configuration are cooperatively calculated, the following steps are included:

[0036] The optimal access position and minimum access number of the additional damping control branch and the active damping compensation branch are calculated by the station-level target function;

[0037] The optimized filter parameters and gain coefficients are calculated by the unit-level target function, and the priority of amplitude and phase angle optimization is dynamically adjusted by a weight coefficient;

[0038] The comparison between the control parameters obtained by the unit-level target optimization and the initial parameters determines whether to perform the double-layer optimization in a loop according to the impedance reconstruction process.

[0039] Optionally, when the additional damping control branch and the active damping compensation branch are accessed to the corresponding inverters, the line impedance is calculated by series-parallel connection equivalence.

[0040] Optionally, the method further comprises the following steps:

[0041] After the system frequency-domain impedance is reconstructed, a station power grid simulation model is established by a simulation method, and an abnormal working condition is set to verify whether the time-domain power oscillation amplitude and / or the harmonic distortion rate meet the threshold value.

[0042] Compared with the prior art, the method has the following beneficial effects:

[0043] The method based on impedance collaborative reconstruction is proposed, the impedance characteristics are reconstructed by hierarchical collaborative control and double-layer target function optimization, the conflict of different control strategies is reduced, and the influence of the coupling relationship is minimized; further, the damping is optimized in space and frequency domain through the collaborative cooperation of multiple inverters, the resonance suppression capability of the weak link of the system is improved at the station level, the impedance characteristics in a wide frequency band are improved at the unit level, and the best collaborative effect of the additional control branch is ensured to realize efficient suppression of wide-frequency oscillation of the multi-inverter grid-connected system; furthermore, the method has general applicability and strong adaptability, and for similar engineering conditions, the position and number of the additional control branch can be designed, and the control parameters can be adjusted to achieve the purpose of oscillation suppression. BRIEF DESCRIPTION OF DRAWINGS

[0044] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings described below only show some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0045] Figure 1 is a method flowchart of the embodiment of the present application;

[0046] Figure 2 is an inverter control structure control block diagram of the embodiment of the present application;

[0047] Figure 3a is the two-machine coupled impedance magnitude plot of an embodiment of the application;

[0048] Figure 3b is the two-machine coupled impedance phase angle plot of an embodiment of the application;

[0049] Figure 4a is the two-machine impedance vector composition plot (frequency domain impedance is resistive-inductive) of an embodiment of the application;

[0050] Figure 4b is the two-machine impedance vector composition plot (frequency domain impedance is resistive-capacitive) of an embodiment of the application;

[0051] Figure 5 is the additional control block diagram of an embodiment of the application;

[0052] Figure 6a is the inverter impedance Bode plot magnitude plot after additional damping control of an embodiment of the application;

[0053] Figure 6b is the inverter impedance Bode plot phase angle plot after additional damping control of an embodiment of the application;

[0054] Figure 7a is the inverter impedance Bode plot magnitude plot of an embodiment of the application with active damping control branch added;

[0055] Figure 7b is the inverter impedance Bode plot phase angle plot of an embodiment of the application with active damping control branch added;

[0056] Figure 8 is the simulation system diagram of an embodiment of the application;

[0057] Figure 9a is the time domain waveform plot of combined strategy 1-3 in scenario one of an embodiment of the application;

[0058] Figure 9b is the time domain waveform plot of combined strategy 4-6 in scenario one of an embodiment of the application;

[0059] Figure 10a is the resonant mode change plot before suppression in the simulation system of an embodiment of the application;

[0060] Figure 10b is the bus current time domain waveform plot before suppression in the simulation system of an embodiment of the application;

[0061] Figure 11a is the resonant mode change plot after suppression in the simulation system of an embodiment of the application;

[0062] Figure 11bis the time-domain current waveform graph of the suppression of the post-confluence point in the simulation system of the embodiment of the present application;

[0063] Figure 12 is the harmonic distortion rate and harmonic component amplitude graph of the confluence point of the embodiment of the present application.

[0064] In the figure: 1, first curve, 2, second curve, 3, third curve, 4, first filter link, 5, first gain link, 6, first phase compensation link, 7, first limiting link, 8, second filter link, 9, second gain link, 10, second phase compensation link, 11, second limiting link, 12, solid line, 13, first dotted line, 14, first dotted line, 15, first dashed line, 16, second dotted line, 17, second dotted line, 18, second dashed line. DETAILED DESCRIPTION

[0065] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0066] It should be noted that: similar labels and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0067] It is worth noting that: the methods used in the present application are conventional methods unless otherwise specified; the raw materials and devices used are conventional commercially available products unless otherwise specified, and their sources are not specifically limited.

[0068] It should also be noted that, in order to facilitate understanding, the steps of the method are described in a certain order in the specific embodiments of the present application, but those skilled in the art can change the order of the steps according to actual needs, so it cannot be used as a limiting condition; further, in the description of the following specific embodiments, the upper and lower indices of each parameter should be understood as the difference between similar marks unless otherwise specified, representing the parameter of the index-related or corresponding device, and cannot be understood as a specific model or special mark.

[0069] Please refer to Figure 1 As shown in the figure, a multi-inverter grid-connected system wide-frequency oscillation suppression method based on impedance collaborative reconstruction includes the following steps:

[0070] Step 1: modeling of the frequency domain impedance of the multi-inverter grid-connected system.

[0071] The frequency domain small signal impedance model of the single grid-connected inverter is obtained based on a small interference linearization method; the method for obtaining the frequency domain small signal impedance model of the single grid-connected inverter comprises the following steps:

[0072] The control structure of the single grid-connected inverter is established; as shown in the following figure: Figure 2 The figure shows the control structure of the grid-side converter of the inverter. dref and i qref are the dq-axis current reference values of the inverter, respectively. dref and u qref are the dq-axis voltage reference values of the inverter, respectively. dpu and u qpu are the dq-axis voltage unit values of the inverter, respectively. dpu and i qpu are the dq-axis current unit values of the inverter, respectively, L is the outlet inductance, and ω g is the angular frequency under the power frequency. abc and i abc are the measured voltage and current values at the outlet, respectively. in is the current inner loop transfer function. dq is the measured dq-axis voltage value, θ is the phase angle output by the phase-locked loop (PLL), and V dc is the capacitor voltage value. The finally output u dref , u qref enter the PWM power electronic switch after Park inverse transformation.

[0073] The frequency domain steady-state model of the nonlinear system is established based on the control structure; in view of the existence of the nonlinear links such as power electronic equipment and Park transformation, it is necessary to firstly perform small interference linearization modeling.

[0074] The disturbance signal is superimposed on the system at the steady state, and the nonlinear links in the frequency domain steady-state model are linearized to obtain the frequency domain small signal impedance model of the single grid-connected inverter. In the steady-state working state of the system, the small disturbance signal is superimposed, the nonlinear links involved are linearized by using Taylor series expansion, Fourier transform and other methods, the input-output linearization model of the grid-connected converter is established, the voltage and current small signal response relationship at the outlet of the fan is obtained, and the response-to-disturbance ratio is the impedance of the system at the disturbance frequency. The small signal impedance model of the grid-connected inverter can be established by traversing the disturbance frequency. The expression of the inverter frequency domain impedance obtained by derivation and calculation is as follows:

[0075]

[0076] In the formula, L is the inductance value at the outlet of the fan, G U,tf,u is the disturbance transfer function of the frequency domain small signal disturbance from the AC side voltage to the valve side voltage at the corresponding frequency disturbance component, and GPLL,tf,u G is the disturbance transfer function of the phase locked loop output phase to the disturbance of the valve side voltage I,tf,u G is the disturbance transfer function of the frequency domain small signal disturbance quantity from the AC side current to the corresponding frequency disturbance component in the valve side voltage PLL H is the disturbance transfer function of the AC side voltage to the phase locked loop output phase PLL G is the phase locked loop transfer function lpf G is the power filter link transfer function F G is the dq axis voltage filter link transfer function i , G v are the transfer functions of the voltage and current value normalization respectively, and K is a constant m K is a constant m0 , I m0 are the AC voltage and current values in steady state operation dref0 , U qref0 are the dq axis reference values in steady state operation respectively; s is a frequency domain complex variable; s0 is a frequency domain complex variable corresponding to the power frequency, j is an imaginary unit, and ω p is an angular frequency variable K is the voltage and current phase angle difference in steady state operation i = ω g L.

[0077] Step 2: Based on the derived frequency domain small signal impedance model of the grid-connected inverter, a field station level frequency domain impedance model containing multiple inverters is constructed, and the coupling influence law of amplitude characteristics and phase angle characteristics is explored.

[0078] Specifically, it is assumed that a new energy field station can be aggregated into an n-inverter connected system, and the impedance of multiple inverters is recorded in the form of complex polar coordinates, that is, the impedance of each inverter is Z1∠θ1, Z2∠θ2…Z n ∠θ n ; the total impedance of the field station is the ratio of the product of the impedance of each inverter to the sum of each combined impedance, so the overall impedance Z eq of the field station is expressed as:

[0079]

[0080] In the formula, Z i is the impedance of the i-th inverter to the grid connection point, which includes the impedance of the i-th inverter and the equivalent impedance of the line.

[0081] As can be seen from formula (3), the overall impedance of the field station is not a linear superposition of the impedance of each inverter in the system, and the phase and amplitude are the result of the nonlinear coupling of the impedance of each inverter.

[0082] Based on the impedance model, the amplitude and phase expressions of the station impedance are analyzed respectively, and the influence of multi-machine amplitude and phase coupling on the overall damping level of the station is explored.

[0083] In terms of amplitude-frequency characteristics, according to formula (3), the amplitude expression of the total impedance of the station is:

[0084]

[0085] As can be seen from formula (4), the amplitude characteristics of the station admittance are composed of two kinds of machine coupling components: one is the square sum of the modulus of each inverter admittance, that is, Under the action of this component, improving the amplitude characteristics of any inverter can improve the amplitude-frequency characteristics of the station; the other is the product sum of the modulus of any two machine admittances and the cosine value of the phase angle difference, that is, This component is affected by the amplitude of each machine and the phase difference between machines. When the phase angles of the machines in the station are equal, the sensitivity of the influence of single-machine amplitude-frequency characteristics on the amplitude-frequency characteristics of the station reaches the maximum. When the phase differences between the machines in the station are not equal due to the influence of factors such as access distance and control parameters, the impedance improvement and phase compensation of a single machine may not optimally improve the overall impedance characteristics at the station level.

[0086] In this embodiment, taking a two-machine system as an example, when the frequency domain impedance of machine 1 is determined, the variation law of the total impedance of the station with the amplitude Z2 and the phase angle θ2 of machine 2 is as shown in Figure 3a and Figure 3b The single-machine frequency domain impedance is set to be resistive and inductive. By comparing the third curve 3 and the first curve 1, when the value increases, the total impedance amplitude increases accordingly, but when the phase angle θ2 of machine 2 is greater than θ 2th , the total impedance amplitude of the first curve 1 is greater than the lowest point of the amplitude of the third curve 3, which indicates that increasing the impedance amplitude of a single machine may not be able to improve the overall damping level of the station. By comparing the first curve 1 and the second curve 2, the total impedance amplitude of the station does not change monotonously with θ2. When θ2=θ1, the total impedance amplitude of the station will have a minimum point, which indicates that the mutual coupling between the impedance angles of multiple machines may affect the contribution effect of the optimization of single-machine amplitude-frequency characteristics on the overall damping of the station.

[0087] In terms of phase-frequency characteristics, the phase angle of the station is the sum of the phase angles of each machine minus the neutral phase angle generated by the superposition of impedance vectors; according to formula (3), the phase angle expression of the total impedance of the station is:

[0088] θ=θ1+θ2...+θ n -(θ add-1 +θ add-2 ...θ add-n ) (5)

[0089] θ add-i =arg(Z1∠θ1...Zi-1 ∠θ i-1 +Z i-1 ∠θ i-1 ...+Z n ∠θ n ) (6)

[0090] where θ add-i is the phase angle of the impedance vector after the superposition of (n-1) inverters. θ add-i is between the phase angles of all the participating impedances, which is defined as the neutral phase angle in this paper. According to equation (5), the total phase angle of the station impedance is composed of the linear superposition of the phase angles of each unit and the neutral phase angle caused by the nonlinear coupling. The neutral phase angle is affected by both the amplitude and the phase angle of each unit. If only the phase angle of a single unit is improved without considering the amplitude, the impedance vector dynamic coupling may not reach the optimal impedance characteristics, as shown in Figure 4a and Figure 4b .

[0091] Therefore, in the process of improving the impedance characteristics of the station, the coupling effect of the amplitude and phase angle of multiple inverters must be considered, and the impedances of multiple inverters must be adjusted coordinately.

[0092] The above steps use the frequency domain small signal injection method to derive the transfer function expression of the nonlinear devices of the grid-connected inverter, and on this basis, the frequency domain impedance model of the multi-inverter grid-connected system is constructed, and the coupling relationship between the impedance amplitude and phase frequency characteristics of the station and the amplitude and phase frequency characteristics of a single machine is derived and analyzed. The coupling relationship between the system frequency domain impedance information and the amplitude and phase frequency characteristics is progressively obtained, which establishes the foundation for impedance reconstruction.

[0093] Step 3: Design an additional damping control branch based on the amplitude frequency characteristic improvement, and an active damping branch based on the phase frequency characteristic optimization.

[0094] Specifically, the input signal that causes the system to be unstable in the control system is superimposed to the control loop to form a feedback loop to improve the amplitude frequency response of the inverter. Considering that the selection of the damping controller input signal should accurately reflect the characteristics or sources of system oscillation, and be easy to measure and control, the terminal power is selected as the input signal of the additional control. The control channel is as shown in Figure 5The additional damping control branch mainly includes a filter link, a gain link, a phase compensation link and a limiting link. The filter link is mainly used for screening the oscillation component signal, the phase compensation link is used for correcting and compensating the phase difference, and the gain link is used for forming a compensation signal. Thus, the active power P and the reactive power Q in the terminal power are processed in two ways. The active power P sequentially passes through the first filter link 4, the first gain link 5, the first phase compensation link 6 and the first limiting link 7. Correspondingly, the reactive power Q sequentially passes through the second filter link 8, the second gain link 9, the second phase compensation link 10 and the second limiting link 11.

[0095] In combination with formula (1), the impedance expression of the inverter after the additional damping control is derived as follows:

[0096]

[0097] In the formula, G I,tf.u (s) is increased to G' I,tf,u (s), and G extra (s) is the transfer function of the control channel; T1 is the time constant of the phase advance correction link, used for improving the phase advance of a specific frequency band; T2 is the time constant of the phase lag correction link, used for suppressing high-frequency noise, and T1 and T2 jointly adjust the phase characteristics of the compensation signal; z is the damping ratio of the notch filter, used for controlling the attenuation degree and bandwidth of the notch filter to a specific frequency component. The smaller z is, the greater the attenuation depth of the notch filter is and the narrower the bandwidth is. The frequency domain impedance Bode diagram after the additional damping control is added is shown in Figure 6a and Figure 6b . In the diagram, the solid line 12 is the frequency domain impedance curve Z VSC , the first dash line 13 corresponds to the parameters: Z SD , F f = 200, the second dash line 14 corresponds to the parameters: Z SD , F f = 250, and the first dotted line 15 corresponds to the parameters: Z SD , F f = 300.

[0098] The phase-frequency characteristics of the inverter determine the oscillation interval of the inverter. As shown in Figure 6a and Figure 6b , the impedance of the inverter is capacitive in the super-synchronous and high-frequency bands. If the impedance is close to the inductive impedance, the resonance risk with the inductive power grid can be reduced, and the phase margin in the sub-synchronous frequency band is more concerned. According to the active damping control principle based on the notch filter, the impedance characteristics of the inverter as a whole are changed by means of the series filter in the current feedforward channel, the phase response of the original control system is changed by means of the notch filter, and the purpose of improving the damping of the system is achieved. The structure of the active damping compensation branch is shown in Figure 5The middle dotted line area shown, which includes a low-pass filter and a trap in series. Where, G se (s) is the transfer function of the desired filter. For filter selection, considering that a single trap or filter is more difficult to achieve the phase correction in the 0-300Hz longer frequency band, therefore, the series combination of low-pass filter and trap is selected to achieve the above purpose.

[0099] After adding the filter-based active damping control, the inner loop transfer function changes as shown in equation (9).

[0100] H′ in (s) = H in (s)G se (s) (9)

[0101] After adding the filter-based active damping control, the frequency domain impedance Bode diagram is shown as Figure 7a and Figure 7b The second dotted line 16 corresponds to the parameters: Z AD , K = 0.02; the second dotted line 17 corresponds to the parameters: Z AD , K = 0.04; the second dotted line 18 corresponds to the parameters: Z AD , K = 0.06. Further, Z SD is the additional damping control, F f is the machine terminal power input signal; Z AD is the active damping control, and K is the current feedback quantity.

[0102] The above steps start from the amplitude-frequency characteristic and the phase-frequency characteristic, respectively, to build the inverter impedance design method facing the amplitude-frequency and phase-frequency characteristic optimization. The additional damping control branch improves the amplitude-frequency characteristic, and the active damping control strategy improves the phase characteristic. The target is divided to realize impedance characteristic reconstruction, reduce the conflict of different control strategies, and minimize the influence of coupling relationship.

[0103] Step 4: Based on the additional damping control branch and the active damping compensation branch, the station-level target function and the unit-level target function are constructed.

[0104] Specifically, a hierarchical impedance collaborative reconstruction method at the station level and unit level is constructed under a multi-inverter system. Through the collaborative cooperation of multiple inverters, the optimal reconstruction of damping in both spatial and frequency domains is achieved. Through the station-level objective function and the unit-level objective function, the access strategy and parameter configuration of the additional damping control branch and the active damping compensation branch are collaboratively calculated. That is, the station-level impedance collaborative compensation aims to achieve the optimal spatial distribution of system damping, and the access location of the amplitude and phase compensation branches is optimized. At the same time, considering that the introduction of the compensation branch may affect the power output characteristics of the unit, when selecting the access location of the compensation branch, the number of connected units should be reduced as much as possible to achieve the optimal distribution of damping space at the minimum control cost, ensuring that the remaining units can still have normal external transmission capabilities.

[0105] The station-level objective function aims to optimize the spatial damping distribution and includes the normalized ratio of the impedance amplitudes before and after reconstruction at the characteristic frequency, as well as the sum of the phase angle boundary distances; the phase angle boundary distance is the ratio of the minimum difference between the characteristic impedance phase angle and positive and negative 90 degrees to 90 degrees. Specifically, if the system currently detects the existence of m characteristic frequencies, the upper-level station-level impedance collaborative compensation optimization objective is defined as:

[0106]

[0107] N xi≠0 ≤N max (13)

[0108] Where Z=[Z1,Z2,...Z n ] T The impedance decision vector is defined as follows:

[0109]

[0110] Among them, Z eq (jω res ) is the impedance of the grid-connected system at the characteristic frequency after the compensation position is selected, Z eq0 (jω res ) represents the impedance of the grid-connected system at the characteristic frequency when impedance cooperative reconstruction is not performed, Z max (jω res ) represents the maximum value among all decision vectors. i The three optional expressions for represent the equivalent impedance from the i-th inverter to the grid connection point, including the i-th inverter impedance and the equivalent line impedance. When connecting the additional damping control branch and the active damping compensation branch to the corresponding inverter, if some inverters share a section of collector line that is then combined at the junction point, the line impedance can be calculated using series-parallel equivalents.

[0111] The target function MOR(Z) of formula (11) represents the normalized ratio of the impedance amplitude of the multi-inverter grid-connected system at the characteristic frequency under the selected compensation position, quantifying the impedance strength of the system at the key frequency; the target function PBD(Z) of formula (12) is defined as the phase angle boundary distance, measuring the distance of the characteristic impedance phase angle from the ideal reactance boundary (±90°) under different compensation positions; considering that the amplitude and the phase angle have different dimensions, the normalized ratio is used. At the same time, the maximum number N of inverters that can be put into the control strategy is set max . N xi≠0 , that is, the number of values in the decision vector that are not equal to Z VSC .

[0112] Through the above method, the optimal position of the amplitude and phase optimization branch access can be finally obtained, realizing the double-objective balance of maximum damping performance and minimum control cost.

[0113] After completing the compensation branch position and control strategy selection, further optimization and setting of the compensation control parameters are needed, considering the coupling effect between the amplitude and the phase angle, fine-tuning the impedance characteristics of the compensation branch in the full attention frequency band, and ensuring its best synergistic effect.

[0114] Taking the station-level synergistic optimization result as input, p inverters are put into the additional damping control strategy, and q inverters are put into the active damping control strategy, and the convergence point is defined as the impedance solving point, and the formula is:

[0115]

[0116] The unit-level target function takes wide-band impedance optimization as the goal, including the weighted sum of the absolute value of the difference between the target phase angle and the actual phase angle, and the weighted sum of the impedance amplitude; specifically, the unit-level impedance synergistic compensation optimization target is set as:

[0117]

[0118] T=[f low ,k,f trap ,ζ f ] (16)

[0119]

[0120] In the formula, Z S,SD and Z S,AD are the total impedance of the inverters implementing additional damping control optimization and active damping strategy optimization, including the impedance of the inverters and the equivalent impedance of the line, and Z S represents the coupling impedance of the inverters to be modified. The optimized filter parameters and gain coefficients are calculated through the unit-level target function, wherein the parameters to be set are the low-pass filter characteristic frequency f lowactive damping gain coefficient k, notch filter characteristic frequency f trap and damping ratio ζ f N SD and N AD respectively are the number of inverters using additional damping control and active damping strategy, m is an integer; θ ref is the target phase angle, the target is to make it resistive in sub / super synchronous and medium-high frequency band, and the phase angle is far away from the zero degree axis as much as possible, so as to reduce the risk of control unit resonance; At the same time, the amplitude-frequency characteristic needs to be considered, and the target phase angle should not be too large.

[0121] Considering the comprehensive influence of amplitude-frequency characteristic, the priority of amplitude and phase angle optimization is dynamically adjusted through weight coefficient, that is, setting weight coefficients α1, α2, to balance the weight relationship between amplitude and phase angle.

[0122] The above steps design the impedance reconstruction method at the station level and the unit level, and through the cooperation of multiple inverters, the double optimization of damping in space and frequency domain is realized. The station level improves the resonance suppression ability of the weak link of the system, and the unit level improves the impedance characteristic in wide frequency band, so as to ensure that the additional control branch can play its best synergistic effect.

[0123] Step 5: According to the station level impedance space coordination compensation and unit level compensation branch control parameter optimization in step 4, the impedance coordination reconstruction process of multiple inverter grid-connected system is formulated according to the following steps. As shown in the attached Figure 1 .

[0124] Step 1: Establish the frequency domain impedance model of the inverter in the multiple inverter grid-connected system, construct the frequency domain impedance model of different additional controls, and obtain the station topology information and grid-side impedance model.

[0125] Step 2: Based on the system topology, form the s-domain node admittance matrix, carry out modal analysis, and identify the system characteristic frequency.

[0126] Step 3: Based on the characteristic frequency obtained in step 2, according to the station level impedance coordination compensation target function proposed in section 3.1, traverse the candidate position, and determine the best coordination compensation branch position.

[0127] Step 4: Based on the selected optimal branch position, according to the unit level compensation branch control parameter optimization method described in section 3.2, traverse the control parameter range space, if the optimal parameter result is inconsistent with the initial control parameter, update the control parameter and return to step 2, if consistent, then step 5.

[0128] Step 5: Output the final compensation position decision variable and optimal control parameter, and determine the impedance coordination reconstruction scheme.

[0129] Further, after reconstructing the system frequency domain impedance, a field station power grid simulation model is established by simulation method, and abnormal working conditions are set to verify whether the time domain power oscillation amplitude and / or harmonic distortion rate meets the threshold value.

[0130] Specifically, taking this embodiment as an example, a simulation system as shown in Figure 8 is set, and simulation verification is carried out, and the solving result is shown in Table 1 parameter setting and solving result according to the foregoing solving compensation branch access position and control parameter.

[0131] Table 1 Parameter setting and solving result

[0132]

[0133] In order to verify the effectiveness of the system impedance coordination reconstruction method proposed in this paper, two simulation scenarios of super-synchronous oscillation and high-frequency resonance are set.

[0134] Scenario one: change the impedance parameters of the power grid side to simulate the small disturbance scenario of the power grid, and excite 80Hz super-synchronous oscillation. In this scenario, the oscillation curves of the system under different control combination strategies are simulated and tested respectively. The control strategy combination is shown in Table 2 strategy combination.

[0135] Table 2 Strategy combination

[0136]

[0137] When nodes 10 and 11 select different control strategies, the time domain waveform and data analysis result at the bus 3 of the station collection point are shown in Figure 9a and Figure 9b . Specifically, it can be known that when only single-machine control optimization is performed (strategies 1-3), the oscillation cannot be quickly attenuated to time domain waveform stability. In comparison, the active waveform oscillation range of the active damping strategy is smaller than that of the additional damping control. When two-machine joint control optimization is implemented (strategies 4-6), the oscillation is effectively suppressed. However, the strategy 6 using the impedance coordination reconstruction method has a smaller active waveform impact range and faster oscillation convergence speed at the initial disturbance moment, and shows more superior damping capacity. Further, the system oscillation component and current waveform before and after the introduction of the strategy 6 are shown in Figure 10a and Figure 10b and Figure 11a , Figure 11b According to the time domain and frequency domain analysis results, it can be known that when the coordination reconstruction strategy is not used, the system resonance mode amplitude is significantly larger than that of the system after the impedance coordination reconstruction, and the super-synchronous oscillation component is reduced to 10% of the original, verifying the effectiveness of the impedance coordination reconstruction in suppressing the system super-synchronous oscillation. It should be noted that the change of the inverter impedance causes the change of the system node admittance matrix, thereby causing the 1Hz small shift of the resonance frequency point. Within a reasonable change range, this offset can be ignored.

[0138] Scenario 2: Changing the grid side impedance parameters, when a harmonic source appears on the grid side, it induces the fifth harmonic resonance of the system. After using different control strategies, the fifth harmonic distortion rate of the system is as follows: Figure 12 As shown. Figure 12 It can be seen that compared with no control strategy, the collaborative control strategy proposed in this paper can effectively reduce the harmonic distortion rate from 3.755% to 2.046%. Compared with the combination of strategies 2-4, the impedance reconstruction strategy proposed in this paper has the best suppression effect. However, compared with strategies 5 and 6, it can be seen that the suppression effect of the collaborative control strategy proposed in this paper is slightly weaker than that of the double-added damping control. The reason is that in order to ensure the oscillation suppression capability of subsynchronous / supersynchronous oscillations, the suppression capability of some high-frequency resonances is weakened during the optimization process. Overall, it shows excellent oscillation suppression performance within the full bandwidth of 0-300Hz.

[0139] Combining all the steps of this embodiment, we first derived a frequency-domain impedance model for a single-unit grid-connected inverter, and then obtained a frequency-domain impedance model for a multi-inverter grid-connected system. We explored the amplitude coupling and phase-angle neutralization characteristics of the multi-inverter system, and revealed the impact of optimizing the amplitude-frequency or phase-frequency characteristics of a single unit on the overall impedance characteristics of the station. Secondly, we designed an additional damping control branch and an active damping compensation branch for optimizing the amplitude-frequency and phase-frequency characteristics, respectively. The former aims to improve the inverter's amplitude-frequency characteristics, while the latter improves the inverter's phase-frequency characteristics. Furthermore, with the goals of optimizing spatial damping distribution and wide-band oscillation suppression, we constructed a coordinated reconstruction technology for the multi-inverter grid-connected system at the station and unit levels, and formulated an impedance coordinated reconstruction process. This approach achieved the goals of optimizing the insertion locations of the amplitude and phase compensation branches and optimizing the wide-band impedance characteristics. Finally, we established a multi-inverter grid-connected simulation model within a simulation software / environment, verifying the effectiveness of the proposed impedance coordinated reconstruction design method.

[0140] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance cooperative reconstruction, characterized by: The steps include: The frequency domain small signal impedance model of a single grid-connected inverter is obtained based on the small disturbance linearization method; Constructing a station-level frequency-domain impedance model including multiple inverters based on the frequency-domain small-signal impedance model of the single grid-connected inverter; Based on the frequency domain impedance model of a single inverter, an additional damping control branch and an active damping compensation branch are established; Based on the additional damping control branch and the active damping compensation branch, a station-level objective function and a unit-level objective function are constructed, and access strategies and parameter configurations of the additional damping control branch and the active damping compensation branch are collaboratively calculated through the station-level objective function and the unit-level objective function; According to the access strategy and parameter configuration, the additional damping control branch and the active damping compensation branch are connected to the corresponding inverter, and the system frequency domain impedance is reconstructed to suppress the broadband oscillation of the multi-inverter grid-connected system.

2. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: The method for obtaining the frequency domain small signal impedance model of a single grid-connected inverter comprises the following steps: Construct the topology of a single grid-connected inverter; Establishing a frequency domain steady-state model of the nonlinear system based on the topological structure; The frequency domain steady-state model of the nonlinear system is superimposed with a disturbance signal in a steady state, and a linearization process is performed on the nonlinear links in the frequency domain steady-state model to obtain a frequency domain small signal impedance model of a single grid-connected inverter.

3. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: In constructing the station-level frequency-domain impedance model, the impedances of multiple inverters are recorded in complex polar coordinate form; The total impedance of the station is the ratio of the product of the impedances of each inverter to the sum of the impedances of each combination; The station phase angle is the sum of the phase angles of each unit minus the neutral phase angle generated by the superposition of impedance vectors.

4. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: Based on the amplitude-frequency characteristics of the station-level frequency-domain impedance model and with the machine-end power as the input signal, the additional damping control branch is established; The additional damping control branch includes a filtering link, a gain link, a phase compensation link and a limiting link; The gain link outputs a compensation signal.

5. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: Based on the phase-frequency characteristics of the station-level frequency-domain impedance model, the active damping compensation branch is established with the current feedforward channel as the control access point; The active damping compensation branch includes a low-pass filter and a notch filter connected in series.

6. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: The station-level objective function aims to optimize the spatial damping distribution, including the sum of the normalized ratios of the impedance amplitudes before and after reconstruction at the characteristic frequency, and the sum of the phase angle boundary distances; The phase angle boundary distance is the ratio of the minimum value of the difference between the characteristic impedance phase angle and positive and negative 90 degrees to 90 degrees.

7. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: The unit-level objective function aims at wide-band impedance optimization, and includes a weighted sum of the absolute value of the difference between the target phase angle and the actual phase angle, and a weighted sum of the impedance amplitude.

8. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: The collaborative calculation of the access strategy and parameter configuration includes the following steps: Calculating the optimal connection position and minimum number of the additional damping control branch and the active damping compensation branch through the station-level objective function; Calculating optimized filter parameters and gain coefficients through the unit-level objective function, and dynamically adjusting the priority of amplitude and phase angle optimization through weight coefficients; By comparing the control parameters obtained by unit-level target optimization with the initial parameters, it is determined whether to perform double-layer optimization in a loop according to the impedance reconstruction process.

9. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: When the additional damping control branch and the active damping compensation branch are connected to the corresponding inverter, the line impedance is calculated by series-parallel equivalent.

10. The method for suppressing broadband oscillations in a multi-inverter grid-connected system based on impedance coordinated reconstruction according to claim 1, characterized in that: The following steps are also included: After reconstructing the system frequency domain impedance, a station power grid simulation model is established through simulation method, and abnormal operating conditions are set to verify whether the time domain power oscillation amplitude and / or harmonic distortion rate meet the threshold.

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