Design method and system for suppressing VSC frequency coupling oscillation based on feedforward compensation

Abstract

The present invention belongs to the technical field related to power grids, and provides a design method and system for suppressing VSC frequency coupling oscillation based on feedforward compensation. The technical solution is as follows: establish a frequency coupling admittance model, and based on this admittance model, use the generalized Nyquist stability criterion for stability analysis; give an index for quantitatively describing the degree of frequency coupling of the system, and conduct sensitivity analysis on the frequency coupling admittance model and the index for three factors affecting the frequency coupling of the system to find out the control links with greater influence on frequency coupling. For these control links, adopt a feedforward compensation suppression strategy, calculate the influence of the disturbance signal of this control link on the subsequent control signal, and feed opposite signals into the corresponding input and output signals to cancel the influence brought by this disturbance and improve the stability of the system. This suppression method is simple and easy to implement, without complex tuning calculations.

Description

Technical Field

[0001] The present invention belongs to the technical field related to power grids, and particularly relates to a design method and system for suppressing VSC frequency coupling oscillation based on feedforward compensation. Background Art

[0002] The statements in this part only provide background technical information related to the present invention, and do not necessarily constitute prior art.

[0003] With the continuous increase in the proportion of new energy power generation in the power grid, power electronic devices, as power change interfaces, are widely used in new energy grid connection. Due to the complex coupling relationship between grid-connected inverters and the AC power grid, a series of oscillation problems may be caused during the process of new energy grid connection through inverters, and such oscillation problems have received extensive attention in recent years. Research and engineering practice have both shown that such oscillation problems can exhibit the characteristics of coexistence of oscillation frequency coupling, that is, when a disturbance of a certain frequency is applied, while generating a response component of the same frequency, another frequency component will also be generated. This phenomenon is called the frequency coupling phenomenon of grid-connected inverters.

[0004] When analyzing frequency coupling oscillation, although some scholars have considered the frequency coupling effect and established a frequency coupling admittance model for voltage source grid-connected inverters and modular multilevel converters, they only conduct a qualitative sensitivity analysis on the influencing factors of the frequency coupling admittance model and the factors affecting the degree of frequency coupling, and do not give an index that can quantitatively describe the degree of system frequency coupling.

[0005] On the other hand, in order to suppress the oscillation problem of grid-connected inverters and improve the stability of the system, some scholars have proposed corresponding control strategies:

[0006] The first one is additional damping control. Additional damping control is to change the control structure of the converter or add an auxiliary control link to achieve the purpose of adjusting the damping characteristics and ensuring the stability of the system. The design of the additional damping control strategy requires the state space model of the grid-connected inverter. When a large number of new energy power generation devices and grid-connected inverters are put into the system, the order of the system state space model will increase sharply, and the implementation difficulty of the control algorithm is greater and the calculation is more complex;

[0007] The second one is to change the impedance characteristics of the grid-connected inverter by adding additional oscillation suppression devices or changing the hardware circuit, which is also an oscillation suppression control strategy. However, the additional control devices added by this method increase the system cost and also increase the loss;

[0008] The third is the impedance reshaping strategy. By means of certain control strategies, the output impedance characteristics of the inverter are changed so that the Nyquist curve of the ratio of the grid impedance to the inverter impedance no longer encloses the point (-1, j0). The control strategy for impedance reshaping has relatively strict requirements for parameter tuning, and it is necessary to take into account both the oscillation suppression effect and the external characteristics of the inverter, with relatively high complexity. Summary of the Invention

[0009] In order to solve at least one of the technical problems existing in the above-mentioned background technology, the present invention provides a design method and system for a VSC frequency coupling oscillation suppression strategy based on feedforward compensation, proposes an index that can quantitatively express the degree of frequency coupling of grid-connected inverters, and conducts a quantitative sensitivity analysis on the relationship between this index and system control parameters to find out the factors with greater influence. Furthermore, for the control link corresponding to this influencing factor, a frequency coupling oscillation suppression strategy based on feedforward compensation is proposed to offset the influence brought by the disturbance signal and achieve the purpose of improving stability.

[0010] In order to achieve the above object, the present invention adopts the following technical solutions:

[0011] The first aspect of the present invention provides a design method for a VSC frequency coupling oscillation suppression strategy based on feedforward compensation, including the following steps:

[0012] Comprehensively consider various factors affecting frequency coupling, and establish a frequency coupling admittance model for the VSC grid-connected system;

[0013] Introduce a quantization index describing the degree of frequency coupling of the system, analyze the sensitivity of the quantization index to different control parameters to determine the key factors affecting the frequency coupling admittance model;

[0014] Based on the sensitivity analysis results, for the control link corresponding to the key factor, design a frequency coupling oscillation suppression strategy based on feedforward compensation, calculate the influence of the disturbance signal on the subsequent control signal, and feed in a quantity opposite to the influence in the input and output signals of this control link to suppress frequency coupling oscillation.

[0015] Further, the process of comprehensively considering various factors affecting frequency coupling and establishing a frequency coupling admittance model for the VSC grid-connected system includes:

[0016] Construct an average value model of the VSC grid-connected inverter;

[0017] Based on the harmonic linearization method, inject a disturbance voltage signal at the grid connection point of the three-phase grid-connected inverter, consider the influence of the phase-locked loop, current loop, and DC voltage loop on frequency coupling, and derive the small-signal models of the control links of the phase-locked loop, current loop, and DC voltage loop in the frequency domain;

[0018] Among them, the considerations of the influence of the phase-locked loop, current loop, and DC voltage loop on frequency coupling include: the dq control asymmetry introduced by the phase-locked loop, current loop, and DC voltage loop, and the transfer function corresponding to each loop determines the degree of dq control asymmetry;

[0019] Based on the small-signal model and combined with the constructed average-value model of the VSC grid-connected inverter, the frequency-coupling admittance model of the VSC grid-connected system is obtained.

[0020] Furthermore, the process of comprehensively considering various frequency-coupling influencing factors and establishing the frequency-coupling admittance model of the VSC grid-connected system includes:

[0021] Construct the average-value model of the VSC grid-connected inverter;

[0022] Based on the harmonic linearization method, inject a disturbance voltage signal at the grid connection point of the three-phase grid-connected inverter. Considering the influence of the phase-locked loop, current loop, and DC voltage loop on frequency coupling, deduce the small-signal models of the control links of the phase-locked loop, current loop, and DC voltage loop in the frequency domain;

[0023] Among them, the considerations of the influence of the phase-locked loop, current loop, and DC voltage loop on frequency coupling include: the dq control asymmetry introduced by the phase-locked loop, current loop, and DC voltage loop, and the transfer function corresponding to each loop determines the degree of dq control asymmetry;

[0024] Based on the small-signal model and combined with the constructed average-value model of the VSC grid-connected inverter, the frequency-coupling admittance model of the VSC grid-connected system is obtained.

[0025] Furthermore, the method also includes, after establishing the frequency-coupling admittance model of the VSC grid-connected system, using the generalized Nyquist stability criterion to conduct a stability analysis of the established VSC grid-connected system, including:

[0026] According to the frequency-coupling admittance model matrix and the grid impedance matrix of the VSC grid-connected system, calculate the loop ratio matrix of the system, and define the product of the system admittance model matrix and the grid impedance matrix as the loop ratio matrix in this scenario;

[0027] Calculate the eigenvalues of the loop ratio matrix of the system. Only when neither of the two eigenvalue trajectories encloses the point (-1, j0), the system is stable.

[0028] Furthermore, the quantization index for describing the degree of system frequency coupling is:

[0029]

[0030] In the formula, |Y 21 |, |Y 12 |, |Y 11 | are respectively the off-diagonal elements Y in the frequency-coupling admittance matrix21 , Y 12 and the admittance magnitude of the main diagonal element Y 11 ; the greater the degree of frequency coupling, the greater the DOFC value.

[0031] Furthermore, the sensitivity of the analysis quantization index to different control parameters includes:

[0032] Only retain the influence of one factor on the admittance model, ignore the remaining factors, and perform sensitivity analysis on the control parameters in this factor, including:

[0033] Take the maximum value corresponding to the quantization index amplitude-frequency curve under each control parameter. The maximum value and this control parameter form a corresponding point. Continuously change the control parameter, and establish a function relationship curve between different control parameters and the maximum value of the quantization index through fitting. Use the average value of the slopes of all points on the curve as the sensitivity of the control parameter to the quantization index.

[0034] Furthermore, the calculation formula for the sensitivity of the control parameter to the quantization index is:

[0035]

[0036]

[0037] In the formula, DOFC max1 , DOFC max2 are the maximum DOFC values corresponding to when a certain parameter changes. X1 and X2 are the values of this parameter before and after the change, including the PLL bandwidth, current control dq asymmetry, or DC capacitor. K i is the slope of any point on the curve, sen is the sensitivity of DOFC to this parameter, and n is the number of points taken on the relationship curve.

[0038] Furthermore, for the control link corresponding to the key factor, design a frequency coupling oscillation suppression strategy based on feedforward compensation, including:

[0039] Feedforward compensation suppression strategy for the influence of the PLL:

[0040] Regard the offset between the output angle of the PLL and the angle of the signal being tracked during the PLL transient as a disturbance, quantitatively analyze the influence of the output angle disturbance of the PLL on the current signal and the modulation signal, and feed into the inner current control loop a quantity opposite to the influence of the analyzed output angle disturbance on the current signal and the modulation signal;

[0041] Among them, the influence of the output angle disturbance of the PLL on the current signal is:

[0042] The influence of the output angle disturbance of the PLL on the modulation signal is:

[0043] in, Steady-state current dq component, F p (s) is the transfer function of the phase-locked loop, is the disturbance component of the q-axis voltage, is the steady-state modulation signal, Δθ is the output angle disturbance;

[0044] Feedforward compensation suppression strategy for the DC voltage loop influence:

[0045] According to the average model of the VSC grid-connected inverter, the average model is combined and the DC voltage signal is retained to obtain the frequency domain expression of the DC voltage disturbance. This disturbance is injected into the input end of the DC voltage loop control link and subtracted from the actual DC voltage to offset the disturbance in the DC voltage.

[0046] A second aspect of the present invention provides a VSC frequency coupling oscillation suppression strategy design system based on feedforward compensation, comprising:

[0047] Frequency coupling admittance building module, which is used to comprehensively consider various frequency coupling influencing factors and establish the frequency coupling admittance model of the VSC grid-connected system;

[0048] Sensitivity analysis module, which is used to introduce quantitative indicators to describe the degree of system frequency coupling, analyze the sensitivity of the quantitative indicators to different control parameters, and determine the key factors affecting the frequency coupling admittance model;

[0049] The oscillation suppression module is used to design a frequency-coupled oscillation suppression strategy based on feedforward compensation for the control link corresponding to the key factors based on the sensitivity analysis results, calculate the influence of the disturbance signal on the subsequent control signal, and suppress the frequency-coupled oscillation by feeding the opposite amount to the influence into the input and output signals of the control link.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1. The present invention focuses on solving the problem of lack of quantitative sensitivity calculation in the analysis of influencing factors of frequency coupling oscillation in VSC grid-connected systems, as well as the problem that existing oscillation suppression strategies do not consider frequency coupling, are difficult to implement, and require complex setting calculations. This can provide a basis for further research on the problem of frequency coupling oscillation.

[0052] 2. The index DOFC for quantitatively describing the frequency coupling degree in the VSC grid-connected system proposed in the present invention has a simple form, is easy to calculate, and can intuitively reflect the frequency coupling degree of the system. The amplitude-frequency characteristics of this index can be calculated, and it can be directly seen that within different frequency bands, the relationship of the frequency coupling degree of the system. At the same time, horizontal analysis and comparison can be carried out to obtain the change of this index within the entire frequency band when the control parameters of the system change.

[0053] 3. For the three influencing factors causing frequency coupling in the present invention, when analyzing the change of the control parameters corresponding to a certain factor, how the frequency coupling admittance model and the frequency coupling degree of the system change. Through this analysis method, the control link with the greatest influence on the frequency coupling admittance model and the coupling degree of the system can be found, which can be used as the entry point for improving the system stability in engineering applications, and the sensitivity analysis is no longer just qualitative analysis.

[0054] 4. The frequency coupling oscillation suppression strategy based on feedforward compensation and aimed at the influence of the phase-locked loop and the DC voltage loop proposed in the present invention does not require complex tuning calculations, does not require additional control devices, does not change the steady-state external characteristics of the inverter, and is easy to implement. And it can effectively improve the stability of the system and suppress the frequency coupling oscillation of the system.

[0055] Advantages of additional aspects of the present invention will be partially given in the following description, partially will become obvious from the following description, or will be understood through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The specification drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0057] Figure 1 It is a flowchart of the design method of the VSC frequency coupling oscillation suppression strategy based on feedforward compensation in the first embodiment of the present invention;

[0058] Figure 2 It is a control block diagram of the VSC grid-connected system in the first embodiment of the present invention;

[0059] Figure 3 It is an analysis diagram of the influence of the current loop asymmetry degree on the admittance model in the first embodiment of the present invention;

[0060] Figure 4 It is an analysis diagram of the influence of the current loop asymmetry degree on the index DOFC in the first embodiment of the present invention;

[0061] Figure 5 It is a function relationship curve diagram of the maximum value of DOFC and the current loop asymmetry degree in the first embodiment of the present invention;

[0062] Figure 6 This is the analysis diagram of the influence of the PLL bandwidth on the admittance model in the first embodiment of the present invention;

[0063] Figure 7 This is the analysis diagram of the influence of the PLL bandwidth on the index DOFC in the first embodiment of the present invention;

[0064] Figure 8 This is the curve graph of the functional relationship between the maximum value of DOFC and the PLL bandwidth in the first embodiment of the present invention;

[0065] Figure 9 This is the analysis diagram of the influence of the DC capacitor on the admittance model in the first embodiment of the present invention;

[0066] Figure 10 This is the analysis diagram of the influence of the DC capacitor on the index DOFC in the first embodiment of the present invention;

[0067] Figure 11 This is the curve graph of the functional relationship between the maximum value of DOFC and the DC capacitor in the first embodiment of the present invention;

[0068] Figure 12 This is the block diagram of the feedforward compensation suppression strategy for the influence of the PLL in the first embodiment of the present invention;

[0069] Figure 13 This is the grid-connected current after adding the suppression strategy for the influence of the PLL in the first embodiment of the present invention;

[0070] Figure 14 This is the Fourier decomposition of the grid-connected current before adding the suppression strategy in the first embodiment of the present invention;

[0071] Figure 15 This is the Fourier decomposition of the grid-connected current after adding the suppression strategy in the first embodiment of the present invention;

[0072] Figure 16 This is the block diagram of the feedforward compensation suppression strategy for the influence of the DC voltage loop in the first embodiment of the present invention. Detailed implementation manners

[0073] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0074] It should be noted that the following detailed descriptions are all illustrative and are intended to provide further explanations of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0075] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should also be understood that when the terms "comprising" and / or "including" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0076] Regarding the existing research mentioned in the background technology, there is a lack of an index that can quantitatively describe the degree of frequency coupling of grid-connected inverters, and there is no quantitative sensitivity analysis of the relationship between this index and system control parameters. In order to find out the factors that have a greater impact on the system frequency coupling degree, it is necessary to conduct a quantitative sensitivity analysis of the influencing factors of frequency coupling. The existing oscillation suppression measures either do not consider the system frequency coupling or adopt the methods of virtual impedance and additional damping control. These two methods have relatively strict requirements for parameter tuning and need to consider both the oscillation suppression effect and the external characteristics of the inverter at the same time, with a relatively high complexity.

[0077] The present invention proposes a design method and system for suppressing VSC frequency coupling oscillation based on feedforward compensation. First, a frequency coupling admittance model is established. Based on this admittance model, the generalized Nyquist stability criterion is used for stability analysis. An index that can quantitatively describe the system frequency coupling degree is given, and sensitivity analysis is carried out on the frequency coupling admittance model and the index for three factors affecting system frequency coupling. The control links that have a greater impact on frequency coupling are found. For these control links, a feedforward compensation suppression strategy is adopted, and the influence of the disturbance signal of this control link on the subsequent control signal is calculated. By feeding opposite signals into the corresponding input and output signals, the influence brought by this disturbance is offset, thereby improving the stability of the system. This suppression method is simple and easy to implement, without complex tuning calculations.

[0078] Embodiment 1

[0079] As Figure 1 shown, this embodiment provides a design method for suppressing VSC frequency coupling oscillation based on feedforward compensation, including the following steps:

[0080] Step 1: Comprehensively consider various factors affecting frequency coupling and establish a frequency coupling admittance model for the VSC grid-connected system;

[0081] Step 2: Introduce a quantization index describing the system frequency coupling degree, analyze the sensitivity of the quantization index to different control parameters, and determine the key factors affecting the frequency coupling admittance model;

[0082] Step 3: Based on the sensitivity analysis results, for the control links corresponding to the key factors, design a frequency coupling oscillation suppression strategy based on feedforward compensation, calculate the influence of the disturbance signal on the subsequent control signal, and suppress the frequency coupling oscillation by feeding a quantity opposite to the influence into the input and output signals of this control link.

[0083] To more clearly illustrate the technical solution of this application, it will be further described by way of examples below.

[0084] Among them, in step 1, the comprehensive consideration of various frequency coupling influencing factors and the establishment of a frequency coupling admittance model for the VSC grid-connected system include:

[0085] Step 101: Construct the average value model of the VSC grid-connected inverter:

[0086]

[0087]

[0088] In the formula, L is the AC side filter inductor; i a , i b , i c , u a , u b , u c are the three-phase grid-connected point currents and voltages respectively; u ia , u ib , u ic are the three-phase voltages at the inverter output port respectively; I dc , C dc , u dc are the DC side current, capacitor and voltage respectively.

[0089] Step 102: Combine the average value model of the inverter, apply a disturbance voltage at the grid-connected point, and obtain a frequency coupling admittance model considering various influencing factors by modeling each control link in the frequency domain; it specifically includes the following steps:

[0090] Step 201: Based on the method of harmonic linearization, inject a positive sequence disturbance voltage signal with a certain frequency of f p at the grid-connected point of the three-phase grid-connected inverter:

[0091] Taking phase a as an example, the time-domain expressions of the grid-connected point voltage and current at this time:

[0092]

[0093]

[0094] In the formula, u o , u p , un are the power frequency voltage amplitude, the disturbance voltage amplitude, and the coupled voltage amplitude, respectively; are the initial phases of the disturbance voltage, the coupled voltage, the power frequency current, the disturbance current, and the coupled current, respectively; i o 、i p 、i n are the power frequency current amplitude, the disturbance current amplitude, and the coupled current amplitude, respectively; f n is the coupled frequency, f o is the power frequency.

[0095] It should be noted that the time-domain expressions of the grid-connected point voltage and current of the other two phases are the same as the derivation process of phase a, and will not be elaborated here.

[0096] Step 202: Considering the influence of the phase-locked loop, current loop, and DC voltage loop on frequency coupling, the small-signal models of the above three control links are derived in the frequency domain;

[0097] The fundamental reason for frequency coupling is the asymmetry of dq-axis control, which may exist in the phase-locked loop, current control loop, DC voltage control loop and other links. Specifically, it includes:

[0098] The dq control asymmetry introduced by the phase-locked loop:

[0099] θ pll =0·u d +F p (s)·u q (5)

[0100] The dq control asymmetry introduced by the current control loop:

[0101]

[0102] The dq control asymmetry introduced by the DC voltage control loop:

[0103] I dr =U dc ·F u (s), I qr =0 (7)

[0104] Among them, θ pll is the output angle of the phase-locked loop, u d 、u q are the voltage dq-axis components respectively, S d 、S q are the modulation signal dq components respectively, I d 、I q are the current dq components respectively, F d (s)、F q (s) are the current control dq-axis transfer functions respectively, Kd is the decoupling coefficient, I dr and I qr are the reference current values of the d and q axes respectively, U dc is the DC voltage, F u (s) is the open-loop transfer function of the DC voltage control.

[0105] It can be seen that the transfer functions F p (s), F d (s), F q (s), and F u (s) corresponding to the phase-locked loop, the circuit control loop, and the DC voltage control loop determine the degree of dq control asymmetry.

[0106] Taking the current loop as an example, according to Figure 2 the control structure of the current loop in, the frequency-domain expressions of the d-axis and q-axis modulation signals can be obtained as follows:

[0107]

[0108]

[0109]

[0110] In the formula: F d (s) is the transfer function of the d-axis current control, F q (s) is the transfer function of the q-axis current control, K d is the decoupling coefficient, S d [±(f p -f o )], S q [±(f p -f o )] are the frequency-domain forms of the modulation signals of the d-axis and q-axis respectively, I dr [±(f p -f o )], I qr [±(f p -f o )] are the frequency-domain forms of the reference currents of the d-axis and q-axis respectively, I d [±(f p -f o )], I q [±(f p -f o )] are the frequency-domain forms of the d-axis and q-axis currents respectively, f p is the frequency of the disturbance signal, k dp , k di are the proportional and integral coefficients of the d-axis current control respectively, k qp , k qiThey are the proportional and integral coefficients for q-axis current control respectively.

[0111] It should be noted that the derivation processes of other links are the same as that of the current loop. When deriving the small-signal models of the three control links, namely the phase-locked loop, the current loop, and the DC voltage loop, in the frequency domain, the factors causing the dq-axis asymmetry of the VSC grid-connected control that results in frequency coupling are considered, including three factors: the phase-locked loop control of the system only orients the q-axis voltage, the output of the DC voltage control is only the d-axis current reference value, and the PI parameters of the current loop control for the dq-axis are not necessarily equal.

[0112] Step 203: By modeling each control link, the frequency-domain expression of the inverter output port voltage is finally obtained and substituted into the corresponding frequency-domain models of Equations (1) and (2), and then the frequency-coupling admittance model considering various influencing factors can be obtained:

[0113]

[0114] Among them, the factor representing the influence of the phase-locked loop disturbance on the admittance model:

[0115]

[0116] Among them, s is the Laplace operator, is the factor representing the degree of current loop asymmetry, K m is the modulation coefficient, u dco is the DC side voltage given value, H p (s) is the transfer function related to the phase-locked loop, K d is the feed-forward coefficient in the inverter control, I o 、S o are the power frequency components of the grid-connected current and the modulation signal respectively, are the conjugate components of the power frequency components of the grid-connected current and the modulation signal respectively, where:

[0117]

[0118]

[0119]

[0120]

[0121]

[0122] Among them, k pp and k pi are the proportional coefficient and integral coefficient of the phase-locked loop PI control respectively, F d (s) is the transfer function of the d-axis current control; Fq (s) is the transfer function for q-axis current control;

[0123] The factor used to represent the influence of the current loop on the admittance model:

[0124]

[0125] The factor used to represent the influence of the DC voltage loop on the admittance model:

[0126]

[0127]

[0128] where, U o is the power frequency component in the frequency domain form of the grid-connected voltage, and F u (s) is the transfer function for DC voltage control.

[0129] Among them, in step 2, a quantization index describing the degree of system frequency coupling is introduced, and the sensitivity of the quantization index to different control parameters is analyzed to determine the key factors affecting the frequency-coupled admittance model;

[0130] In this embodiment, an index for quantitatively expressing the degree of frequency coupling (Degree of frequency coupling, DOFC) is introduced and calculated according to the following formula:

[0131]

[0132] In the formula, |Y 21 |, |Y 12 |, |Y 11 | are the admittance magnitudes of the off-diagonal elements Y 21 , Y 12 and the main diagonal element Y 11 in the frequency-coupled admittance matrix, respectively. It is easy to know that the above-defined DOFC index can well quantitatively describe the degree of system frequency coupling. The greater the degree of frequency coupling, the greater the DOFC, and vice versa;

[0133] When considering only one of the three factors and considering the change of the control parameter of one of the factors, at this time, since the other two factors causing system frequency coupling are ignored, the frequency-coupled admittance model at this time will be simplified. In the process of admittance modeling, the finally obtained admittance model will contain factors corresponding to three parts of factors. The simplification means that the factors corresponding to this part are 0, specifically:

[0134] When analyzing the influence of the asymmetry of dq control of the current loop on frequency coupling, it is considered that the DC capacitor is infinite and the PLL bandwidth is very small, and the same applies to the rest.

[0135] Analyze how the amplitude of the frequency coupling admittance model of the analysis system changes and how the DOFC changes, so as to obtain the influence of this control parameter on the frequency coupling admittance model and the degree of frequency coupling.

[0136] To quantitatively describe the degree of asymmetry of the dq control of the current loop, define V:

[0137]

[0138] In the formula, k dp and k qp are the proportionality coefficients of the PI control of the d-axis and q-axis of the current loop respectively.

[0139] To analyze the influence of the three main factors causing frequency coupling on the degree of frequency coupling of the system and the magnitude of the admittance, below we will only consider one factor at a time, while ignoring other factors, and conduct a sensitivity analysis on the control parameters in this factor.

[0140] Analyze the influence of the three factors on the DOFC index, so as to conduct a quantitative sensitivity analysis. Specifically: take the maximum value corresponding to the DOFC amplitude-frequency curve under each control parameter. The maximum value and this control parameter form a corresponding point. Continuously change the control parameter, and establish a function relationship curve between different control parameters and the maximum value of the quantization index through fitting. Through this curve, the sensitivity of DOFC to this control parameter can be seen more intuitively. Take the average value of the slopes of all points on the curve as the sensitivity of the control parameter to the quantization index.

[0141] The calculation formula for the sensitivity of the influence of the control parameter on the quantization index is:

[0142]

[0143]

[0144] In the formula, DOFC max1 and DOFC max2 are the maximum values of DOFC corresponding to when a certain parameter changes respectively. X1 and X2 are the values before and after the change of this parameter, including the PLL bandwidth, the dq asymmetry of the current control or the DC capacitor., K i is the slope of any point on the curve., sen is the sensitivity, and n is the number of points on the curve of the relationship between the selected index and the control parameter.

[0145] The first one is to only consider the influence of the current loop:

[0146] Assume that the DC bus capacitor in the model is very large at this time, and the bandwidth of the phase-locked loop is small. It is approximately considered that the voltage disturbance has no influence on the angle output by the phase-locked loop at this time, and the DC voltage is a constant constant. Therefore, H p(s) is always 0, and all four elements in the column vector of Equation (12) are 0. Since the voltage disturbance has no effect on the DC voltage, at this time, only the asymmetry of the current loop is the only factor causing the system frequency coupling. Therefore, all four elements in Equation (19) are also 0. Then, by taking the derivative of the admittance matrix according to Equation (11), the admittance model considering only the influence of the current loop can be obtained as follows:

[0147]

[0148] Taking the inverse of Equation (25), it can be found that -D 12 represents Y 12 , -D 21 represents Y 21 . According to Equations (17) and (18), it can be seen that both D 12 and D 21 contain factors representing the asymmetry degree of the dq axes of the current loop and the amplitude expression will contain the difference information of the dq-axis PI parameters. Thus, it can be seen that when the asymmetry degree of the dq axes of the current loop is greater, the amplitudes of Y 12 , Y 21 become smaller and smaller, the amplitudes of the non-coupling terms Y 11 , Y 22 remain almost unchanged, and the degree of frequency coupling is lower. Define a parameter V that can quantitatively describe the asymmetry degree of the dq control of the current loop. Figure 3 shows the change of the frequency-coupling admittance model when V decreases from 10 to 2.5. Figure 4 shows the corresponding change of the DOFC. Figure 5 is the function relationship curve between the maximum value of the DOFC and the parameter V. Furthermore, the sensitivity of the influence of the current asymmetry degree on the DOFC can be obtained as 1.2374.

[0149] Second, only considering the influence of the phase-locked loop:

[0150] Assume that in the current model, the dq axes of the current loop are completely symmetric, and the DC capacitor is large, and the DC voltage is constant. At this time, all four elements in Equation (19) are also 0, and is 0. The frequency coupling of the grid-connected inverter is completely caused by the characteristics of the phase-locked loop. According to Equation (8), the admittance model matrix at this time can be obtained as follows:

[0151]

[0152] It can be seen from the expressions of Y 12 and Y 21 that the factor H p(s) is on the molecule. Therefore, when increasing the PI parameters controlled by the phase-locked loop, that is, increasing the phase-locked loop bandwidth, the coupling terms Y 12 and Y 21 in the frequency coupling admittance model will also become larger and larger, the amplitudes of the non-coupling terms Y 11 and Y 22 are almost unchanged, and the degree of frequency coupling is getting higher and higher. Figure 6 shows the change of the admittance model during the process of the phase-locked loop bandwidth changing from 100 Hz to 20 Hz. Figure 7 shows the corresponding change of the DOFC. Similarly, Figure 8 shows the relationship curve between the maximum value of the DOFC and the phase-locked loop bandwidth. Through calculation, the sensitivity of the phase-locked loop bandwidth to the DOFC can be obtained as 3.513.

[0153] Thirdly, only consider the influence of the DC voltage loop:

[0154] If the phase-locked loop bandwidth is very small and the dq control of the current loop is symmetric, the influences of the phase-locked loop and the current loop are ignored. At this time, only the DC voltage loop affects the frequency coupling characteristics of the system. H p (s) is always 0, and is also 0. The admittance model at this time can be simplified as:

[0155]

[0156] Keeping other parameters unchanged and increasing the size of the DC bus capacitor is equivalent to reducing the bandwidth of the DC voltage controller. The coupling terms Y 12 and Y 21 in the frequency coupling admittance model will also become smaller and smaller, the amplitudes of the non-coupling terms Y 11 and Y 22 are almost unchanged, and the degree of frequency coupling is lower. Figure 9 shows the change of the frequency coupling admittance model when the capacitor changes from 100 mF to 20 mF. Figure 10 is the corresponding change of the DOFC. Similarly, Figure 11 is the relationship curve between the maximum value of the DOFC and the capacitor, and it can be obtained that the sensitivity of the DOFC to the DC capacitor is 1.698.

[0157] To sum up, whether directly according to the change of the DOFC or according to the calculation result of the sensitivity, it can be seen that the sensitivities of different control parameters to the frequency coupling are different, and it can be seen that the influence of the DC capacitor on the degree of frequency coupling is greater than that of the current loop asymmetry but less than the phase-locked loop bandwidth. Therefore, the phase-locked loop and the DC voltage loop are the two control links with greater influence.

[0158] In Step 3, based on the sensitivity analysis results, for the control links corresponding to the key factors, a frequency coupling oscillation suppression strategy based on feedforward compensation is designed to calculate the influence of the disturbance signal on the subsequent control signal, and by feeding a quantity opposite to the said influence into the input and output signals of this control link, the frequency coupling oscillation is suppressed.

[0159] Feedforward compensation suppression strategy for the influence of the phase-locked loop:

[0160] When the system oscillates, the phase-locked angle output by the phase-locked loop is no longer the steady-state value. Instead, it contains a disturbance. According to the structure of the phase-locked loop, this disturbance comes from the disturbance component of the q-axis voltage. Moreover, the influence of this disturbance will bring disturbances to the current signal and modulation signal of the grid-connected inverter, and the above two signals run through the entire inverter control loop, thereby affecting the final output of the grid-connected inverter.

[0161] Regarding the deviation between the phase-locked loop output angle and the angle of the signal to be tracked in the phase-locked transient as a disturbance, quantitatively analyze the influence of the output angle disturbance of the phase-locked loop on the current signal and modulation signal, and feed a quantity opposite to the influence of the analyzed output angle disturbance on the current signal and modulation signal into the inner current control loop;

[0162] Among them, the quantitative analysis of the influence of the phase-locked loop output angle disturbance on the current and modulation signals includes:

[0163] Considering the phase-locked loop output angle after disturbance:

[0164] θ = θ s +Δθ (28)

[0165] According to the phase-locked loop structure, the disturbance angle:

[0166]

[0167] In the formula: is the disturbance component of the q-axis voltage; F p (s) is the transfer function of the phase-locked loop; θ is the phase-locked loop output angle considering the disturbance; θ s is the phase-locked loop output angle at steady state; Δθ is the output angle disturbance.

[0168] The grid-connected current considering the phase-locked loop output angle disturbance in the synchronous rotating coordinate system is:

[0169]

[0170] In the formula, I dq is the dq current considering the phase-locked loop output angle disturbance, I αβ is the current in the stationary coordinate system considering the disturbance, ΔI αβis the self-disturbance of the stationary coordinate current, and Δθ is the output angle disturbance of the phase-locked loop. is the dq component of the steady-state current. is the steady-state current in the stationary coordinate system. is the self-disturbance of the dq current.

[0171] Expand and linearize according to Euler's formula:

[0172] e -jΔθ = cosΔθ - jsinΔθ ≈ 1 - jΔθ (31)

[0173] Combining equations (30) and (31), we can obtain:

[0174]

[0175] Here, only the influence of the output angle disturbance of the phase-locked loop on the current is considered. It can be seen that only is the influence of the output angle disturbance of the phase-locked loop on the current.

[0176] Substitute the output angle disturbance of the phase-locked loop in equation (29) into I dq Δθ:

[0177]

[0178] Figure 12 is the control block diagram of the suppression strategy for the influence of the phase-locked loop. The above derivation has obtained Figure 12 The parts within the two dashed boxes above show the influence of the output angle disturbance of the phase-locked loop on the current.

[0179] Similarly, the modulation signal after considering the influence of the phase-locked loop in the synchronous rotating coordinate system is:

[0180]

[0181] In the formula: S dq is the modulation signal considering the output angle disturbance of the phase-locked loop; S αβ is the modulation signal of the stationary coordinate system considering the disturbance; is the steady-state modulation signal; ΔS dq is the self-disturbance of the modulation disturbance signal.

[0182] Similarly, expand and linearize according to Euler's formula, and substitute equation (31) into equation (34) to obtain:

[0183]

[0184] Similarly, only considering the influence brought by the output angle disturbance of the phase-locked loop, only is the influence of the output angle disturbance of the phase-locked loop on the modulation signal.

[0185] Based on the relationship between the steady-state operating point of the modulation signal and the steady-state operating point of voltage and current, it can be obtained that:

[0186]

[0187] Substitute the steady-state expression of the above modulation signal into and also substitute Δθ with Equation (29), obtaining

[0188] So far, the following part within the dotted line box shown in Figure 12 has been obtained, which is the influence of the phase-locked loop output angle disturbance on the modulation signal.

[0189] To verify the suppression effect, in MATLAB / Simulink, a VSC grid-connected system is built. By increasing the grid inductance, the system becomes unstable at this time and the grid-connected current oscillates. When the system runs to 0.65 s, the above suppression strategy is added to the control loop. Figure 13 is the time-domain waveform of the grid-connected current of phase a. It can be seen from Figure 13 that after adding the suppression strategy, the grid-connected current no longer oscillates and the system tends to be stable. The grid-connected current before and after adding the suppression strategy is respectively subjected to Fourier decomposition, and the results are respectively as shown in Figure 14 and Figure 15 It can be seen that after adding the suppression strategy, the total harmonic distortion rate has been reduced, and the ratio of the coupling component to the oscillation component has also decreased, and the frequency coupling of the system has been suppressed.

[0190] Feedforward compensation suppression strategy for the influence of the DC voltage loop:

[0191] Similarly, there is also a compensation strategy for the DC voltage loop. Due to the power balance between the AC and DC sides, when there is a voltage disturbance on the AC side, not only the disturbance component of the same frequency appears on the DC side, but also the coupling frequency component appears. And the output result of the DC voltage loop is the reference value of the current loop control. Therefore, these components can be compensated and offset, so that the current reference value is the same as that in the steady state.

[0192] According to Equations (1) and (2), when there are disturbances in the AC side voltage and current, the DC side voltage will also generate disturbances of the same frequency. Substitute Equation (1) into Equation (2) to eliminate the inverter output port voltages u ia 、u ib 、u ic , and retain the DC voltage u dc . And transform the substituted formula into a frequency-domain expression, and the frequency-domain expression of the DC voltage under disturbance can be obtained:

[0193]

[0194] where U p and U n are the frequency-domain forms of the disturbance voltage and the coupling voltage in the grid-connected voltage, respectively.

[0195] According to Figure 16 the control block diagram and transfer function of the inverter DC voltage loop shown, the frequency-domain expression of the d-axis current reference value can be obtained:

[0196] I dr [±(f p -f o )] = U dc [±(f p -f o )]F u (s) (38)

[0197] It can be seen that at this time, the d-axis current reference value will contain disturbances with the same frequency as the DC voltage, and the current reference value, as the input of the current loop, will further affect the output of the current loop. Therefore, according to the frequency-domain expression of the DC voltage disturbance derived from Equation (37), this disturbance quantity is injected at the input end of the DC voltage loop control link, and subtracted from the actual DC voltage, so as to cancel the disturbance quantity in the DC voltage. To achieve the purpose of suppressing oscillation, the specific control block diagram is as Figure 16 shown, and the DC voltage disturbance is within the dashed box.

[0198] Embodiment 2

[0199] This embodiment provides a system for designing a VSC frequency coupling oscillation suppression strategy based on feedforward compensation, including:

[0200] A frequency coupling admittance construction module, which is used to comprehensively consider various frequency coupling influencing factors and establish a frequency coupling admittance model for the VSC grid-connected system;

[0201] A sensitivity analysis module, which is used to introduce a quantization index describing the degree of system frequency coupling, analyze the sensitivity of the quantization index to different control parameters, and determine the key factors affecting the frequency coupling admittance model;

[0202] An oscillation suppression module, which is used to design a frequency coupling oscillation suppression strategy based on feedforward compensation for the control link corresponding to the key factors based on the sensitivity analysis results, calculate the influence of the disturbance signal on the subsequent control signal, and feed in a quantity opposite to the influence in the input and output signals of this control link to suppress frequency coupling oscillation.

[0203] Embodiment 3

[0204] This embodiment provides a computer-readable storage medium with a computer program stored thereon. When the program is executed by a processor, it implements the steps in the method for designing a VSC frequency coupling oscillation suppression strategy based on feedforward compensation as described above.

[0205] Embodiment 4

[0206] This embodiment provides a computer device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the program, it implements the steps in the method for designing a VSC frequency coupling oscillation suppression strategy based on feedforward compensation as described above.

[0207] Embodiment 5

[0208] This embodiment provides a program product, which is a computer program product including a computer program. When the computer program is executed by a processor, it implements the steps of the method for designing a VSC frequency coupling oscillation suppression strategy based on feedforward compensation as described above.

[0209] Those skilled in the art should understand that the embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of hardware embodiments, software embodiments, or embodiments combining software and hardware aspects. Moreover, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memories and optical memories, etc.) containing computer-usable program code.

[0210] The present invention is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present invention. It should be understood that each process and / or block in the flowcharts and / or block diagrams, as well as the combination of processes and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate a device for implementing the specified functions in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0211] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured article including an instruction device, and the instruction device implements the specified functions in Figure 1 one process or multiple processes and / or blocks Figure 1The functions specified in one or more boxes.

[0212] These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable device provide for implementing the steps of the functions specified in one or more processes and / or boxes Figure 1 One process or more processes and / or boxes Figure 1 The steps of the functions specified in one or more boxes.

[0213] Those of ordinary skill in the art can understand that all or part of the processes of implementing the methods in the above embodiments can be completed by instructing relevant hardware through a computer program. The said program can be stored in a computer-readable storage medium. When the program is executed, it can include the processes of the embodiments of the above methods. Among them, the said storage medium can be a magnetic disk, an optical disk, a read-only memory (ROM), or a random access memory (RAM), etc.

[0214] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A design method for VSC frequency coupling oscillation suppression strategy based on feedforward compensation, characterized in that: The steps include: Taking into account various frequency coupling influencing factors, a frequency coupling admittance model of the VSC grid-connected system is established; A quantitative index describing the degree of system frequency coupling is introduced, and the sensitivity of the quantitative index to different control parameters is analyzed to determine the key factors affecting the frequency coupling admittance model. Based on the sensitivity analysis results, a frequency coupling oscillation suppression strategy based on feedforward compensation is designed for the control link corresponding to the key factors, the influence of the disturbance signal on the subsequent control signal is calculated, and the frequency coupling oscillation is suppressed by feeding the opposite amount to the input and output signals of the control link; The quantitative index describing the degree of system frequency coupling is: ， In the formula, , , are the sub-diagonal elements in the frequency-coupled admittance matrix , and the main diagonal elements The greater the frequency coupling, DOFC The larger the value.

2. The method for designing a VSC frequency coupling oscillation suppression strategy based on feedforward compensation according to claim 1, characterized in that: The process of establishing the frequency coupling admittance model of the VSC grid-connected system by comprehensively considering various frequency coupling influencing factors includes: Construct the average value model of VSC grid-connected inverter; Based on the harmonic linearization method, a disturbance voltage signal is injected into the grid connection point of the three-phase grid-connected inverter. Considering the influence of the phase-locked loop, current loop and DC voltage loop on frequency coupling, the small signal models of the phase-locked loop, current loop and DC voltage loop control links are derived in the frequency domain. Among them, the effects of the phase-locked loop, current loop, and DC voltage loop on frequency coupling are considered, including: the dq control asymmetry introduced by the phase-locked loop, current loop, and DC voltage loop, and the transfer function corresponding to each loop determines the degree of dq control asymmetry; Based on the small signal model and the constructed average value model of the VSC grid-connected inverter, the frequency-coupled admittance model of the VSC grid-connected system is obtained.

3. The VSC frequency coupling oscillation suppression strategy design method based on feedforward compensation according to claim 1, characterized in that: The method further includes, after establishing the frequency-coupled admittance model of the VSC grid-connected system, using the generalized Nyquist stability criterion to perform stability analysis on the constructed VSC grid-connected system, including: According to the frequency-coupled admittance model matrix of the VSC grid-connected system and the grid impedance matrix, the loop ratio matrix of the system is calculated, and the product of the system admittance model matrix and the grid impedance matrix is ​​defined as the loop ratio matrix in this scenario; Calculate the characteristic roots of the loop ratio matrix of the system. The system is stable only when the two characteristic root trajectories do not surround the point (-1, j0).

4. The VSC frequency coupling oscillation suppression strategy design method based on feedforward compensation according to claim 1, characterized in that: The analysis quantifies the sensitivity of the indicators to different control parameters, including: Only one factor is retained to determine its influence on the admittance model, and the rest of the factors are ignored. A sensitivity analysis is then performed on the control parameters of this factor, including: Take the corresponding maximum value on the amplitude-frequency curve of the quantitative index under each control parameter. The maximum value forms a corresponding point with the control parameter. The control parameter is continuously changed. A functional relationship curve between different control parameters and the maximum value of the quantitative index is established by fitting. The average value of the slope of all points on the curve is taken as the sensitivity of the control parameter to the quantitative index.

5. The VSC frequency coupling oscillation suppression strategy design method based on feedforward compensation according to claim 1, characterized in that: The calculation formula of the sensitivity of the control parameter to the quantitative index is: ， ， In the formula, , They are respectively the corresponding Maximum value, , are the values ​​before and after the change of this parameter, including the phase-locked loop bandwidth, current control dq asymmetry or DC capacitance, is the slope of any point on the curve, for For the sensitivity of this parameter, n is the number of points on the relationship curve.

6. The VSC frequency coupling oscillation suppression strategy design method based on feedforward compensation according to claim 1, characterized in that: The control link corresponding to the key factors is designed with a frequency coupling oscillation suppression strategy based on feedforward compensation, including: Feedforward compensation suppression strategy for phase-locked loop effects: The deviation between the phase-locked loop output angle and the angle of the tracked signal in the phase-locked transient state is regarded as a disturbance, and the influence of the phase-locked loop output angle disturbance on the current signal and the modulation signal is quantitatively analyzed. The opposite amount of the influence of the output angle disturbance on the current signal and the modulation signal obtained by the analysis is fed into the current control inner loop; Among them, the influence of the output angle disturbance of the phase-locked loop on the current signal is: , The effect of the output angle disturbance of the phase-locked loop on the modulation signal is: ; in, Steady-state current dq component, is the transfer function of the phase-locked loop, is the disturbance component of the q-axis voltage, is the steady-state modulation signal, is the output angle disturbance; Feedforward compensation suppression strategy for the DC voltage loop influence: According to the average model of the VSC grid-connected inverter, the average model is combined and the DC voltage signal is retained to obtain the frequency domain expression of the DC voltage disturbance. This disturbance is injected into the input end of the DC voltage loop control link and subtracted from the actual DC voltage to offset the disturbance in the DC voltage.

7. A VSC frequency coupling oscillation suppression strategy design system based on feedforward compensation, characterized in that: include: Frequency coupling admittance building module, which is used to comprehensively consider various frequency coupling influencing factors and establish the frequency coupling admittance model of the VSC grid-connected system; Sensitivity analysis module, which is used to introduce quantitative indicators to describe the degree of system frequency coupling, analyze the sensitivity of the quantitative indicators to different control parameters, and determine the key factors affecting the frequency coupling admittance model; An oscillation suppression module is used to design a frequency coupling oscillation suppression strategy based on feedforward compensation for the control link corresponding to the key factor based on the sensitivity analysis result, calculate the influence of the disturbance signal on the subsequent control signal, and suppress the frequency coupling oscillation by feeding the opposite amount to the influence into the input and output signals of the control link; The quantitative index describing the degree of system frequency coupling is: ， In the formula, , , are the sub-diagonal elements in the frequency-coupled admittance matrix , and the main diagonal elements The greater the frequency coupling, DOFC The larger the value.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps in the method for designing a VSC frequency coupling oscillation suppression strategy based on feedforward compensation as described in any one of claims 1 to 6 are implemented.

9. A computer device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the steps in the method for designing a VSC frequency coupling oscillation suppression strategy based on feedforward compensation are implemented as described in any one of claims 1 to 6.

Images