CN110729729B - Voltage feed-forward link action analysis method for voltage source type converter under weak grid connection - Google Patents
Voltage feed-forward link action analysis method for voltage source type converter under weak grid connection Download PDFInfo
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Abstract
The invention discloses a voltage feedforward link action analysis method of a voltage source type converter under weak grid connection, which is characterized in that a mathematical model is established according to the typical topology of the voltage source type converter under the weak grid connection; linearizing the mathematical model to obtain a small-signal mathematical model of the voltage source type converter under the condition of weak power grid connection; simplifying a small-signal mathematical model to obtain a single-input single-output analytic transfer function capable of representing the influence between current loop current disturbance and output voltage disturbance thereof, namely directly expressing the influence of a voltage feedforward link on the control stability of the voltage source type converter as an independent open-loop transfer function; and analyzing the stability influence of the voltage feedforward link of the power grid on the voltage source type converter under weak grid connection according to the obtained open-loop transfer function, and guiding the control design of the grid-connected converter. The method is simple and practical, and has great significance for ensuring accurate analysis and evaluation of the influence of the voltage feedforward link on the grid-connected stability of the voltage source type converter.
Description
Technical Field
The invention belongs to the field of analysis of stability of grid-connected voltage source converters, and particularly relates to an effect analysis method for a voltage feedforward link of a voltage source converter under weak grid connection.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
The flexible direct-current transmission technology based on the voltage source type converter has the advantages of being free of commutation failure, capable of conducting black start of a power grid and the like, and is considered to be an effective means capable of replacing the conventional direct-current transmission technology based on the power grid converter. However, as the power transfer capacity of the dc link increases, the receive grid strength drops rapidly. Taking the east China area as an example, the power grid strength of a part of direct current centralized feed-in nodes in the N-2 operation mode is reduced to be below 2, and a corresponding area becomes an extremely weak power grid in the operation mode. The existing research shows that when the strength of a power grid is reduced to an extremely weak power grid, a flexible direct-current system based on a voltage source type converter cannot stably operate, so that key links and leading factors influencing the stable operation of the flexible direct-current system need to be effectively analyzed.
Due to the characteristic that the voltage feedforward control can improve the dynamic characteristic of current output under disturbance, the voltage feedforward control is widely applied to the control of the grid-connected voltage source type current converter. However, existing research shows that the grid voltage feedforward link under the weak grid connection condition will deteriorate the stability of the grid-connected system.
The inventor finds that the existing analysis method for the influence of the voltage feedforward link on the stability of the converter is mainly divided into a small signal analysis method and an impedance modeling analysis method. However, the existing analysis method based on small signals depends on the establishment of a state space matrix, the order of a model can be up to tens of orders, and the modeling process is complex, so that the analysis process is difficult.
In another impedance-based analysis method, the impedances at different frequencies are coupled under the action of the phase-locked loop, the modeling difficulty is further complicated after the outer loop control is considered, the original single-input-output system is converted into a multi-input-output system, and the complexity and difficulty of the overall analysis are increased. The two methods cannot directly explain the influence of the voltage feedforward link on the stability analysis of the grid-connected converter system under the weak power grid condition, the analysis only gives a result through numerical calculation, and the direct influence of the introduction of the voltage feedforward link on the stability of the grid-connected converter system cannot be directly explained.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a voltage feedforward link action analysis method of a voltage source type converter under weak power grid connection, the method simplifies a high-order multi-input multi-output small signal model into a single-input single-output analytic transfer function based on a small signal model, directly expresses the influence of the voltage feedforward link on the control stability of the voltage source type converter into an independent open-loop transfer function, and more accurately and directly analyzes the influence of the voltage feedforward link on a grid-connected converter system.
In order to achieve the above object, one or more embodiments of the present invention provide the following technical solutions:
the voltage feedforward link action analysis method of the voltage source type converter under weak grid connection comprises the following steps:
establishing a mathematical model according to the typical topology of the voltage source type current converter under the weak grid connection;
linearizing the mathematical model to obtain a small-signal mathematical model of the voltage source type converter under the condition of weak power grid connection;
simplifying a small-signal mathematical model to obtain a single-input single-output analytic transfer function capable of representing the influence between current loop current disturbance and output voltage disturbance thereof, namely directly expressing the influence of a voltage feedforward link on the control stability of the voltage source type converter as an independent open-loop transfer function;
and analyzing the influence of the voltage feedforward link of the power grid on the stability of the voltage source type converter under the weak grid connection according to the obtained transfer function.
According to a further technical scheme, the following assumptions are required before establishing a mathematical model according to a typical topology of a voltage source type converter under weak grid connection, specifically:
the time delay of the control link can be ignored relative to the time constant of the current loop, and the transfer function G can be approximately consideredd≈1；
The outer loop control is slow, the disturbance response of the outer loop control to the inner loop control is considered to be slow, the dynamic influence of the outer loop control is ignored, and delta udc≈0。
According to the further technical scheme, a main circuit model of the voltage source type converter under a dq coordinate system is established according to the typical topology of the voltage source type converter, and the expression mode of the main circuit model under the dq coordinate system is as follows:
wherein R isp,LpRepresenting the filter resistance and the inductance of the converter; rg,LgRepresenting the equivalent resistance and inductance of the power grid; rac＝Rg+Rp,Lac＝Lg+LpRepresenting the total equivalent resistance and inductance on the ac side. u. ofcd,ucqRepresenting the component of the outlet voltage of the AC side of the converter under a dq coordinate system; u. ofd,uqRepresenting the component of the PCC node voltage in the dq coordinate system; u. ofgd,ugqRepresenting the component of the equivalent potential of the alternating current power grid in a dq coordinate system; i.e. id,iqRepresenting the components of the current in the dq coordinate system. Wherein the grid strength is characterized by a Short Circuit Ratio SCR (SCR).
Establishing a mathematical model of the control system under a dq coordinate system;
the mathematical model of the current inner loop control system is divided into a phase-locked loop mathematical model and a current loop mathematical model. The invention adopts a PCC node voltage orientation method, so that theta exists under the steady state conditionpll0＝θabc0And (5) 0, namely, the phase-locked loop tracking phase angle is the same as the voltage phase angle of the PCC node. In the transient process of load disturbance and the like, a tracking phase angle error, namely theta, exists in the tracking phase angle of the phase-locked loop in the transition processpll≠θsThe influence of this phase angle error needs to be taken into account in the analysis of the small signal. Therefore, the present invention divides the coordinate system into a control coordinate system and a main circuit coordinate system. The relationship between the control coordinate system and the main circuit coordinate system is:
where f represents an electrical component, such as a corresponding voltage, current, etc. The subscripts d, q represent the representation of the corresponding components in the dq coordinate system; superscript cf represents a representation of the corresponding component in the control coordinate system; thetapllRepresenting the tracking phase angle of the phase locked loop.
The open-loop transfer function and the closed-loop transfer function of the phase-locked loop control system can be expressed as:
wherein G ispll(s) and Tpll(s) represents the open and closed loop transfer function, k, of the phase locked loopppllRepresenting the proportionality coefficient, k, of the phase-locked loop PIipllAnd the integral coefficient represents the PI link of the phase-locked loop. The proportional coefficient and the integral coefficient can be calculated by the following formula.
Where ξ represents the damping ratio coefficient, typically taken as 0.707; subscript 0 represents the steady state value of its corresponding component; u shaped0I.e. representing the d-axis voltage steady-state component; omegapllRepresenting the phase-locked loop bandwidth.
The mathematical model of current loop control considering voltage feedforward is:
the current loop control without voltage feed forward is:
the current loop PI element is represented as:
where the superscript "-ref" represents the reference value under the corresponding component. k is a radical ofpgscRepresenting the proportionality coefficient of the current loop PI; k is a radical ofigscRepresents the integral coefficient of the current loop PI. The calculation method of the current loop proportionality coefficient and the integral coefficient comprises the following steps:
wherein, ω isCLRepresenting the design bandwidth of the current loop
The expression of the modulation link is as follows:
wherein theta ispllRepresenting the tracking phase angle, u, of the phase-locked loopdcRepresenting the dc side voltage of the inverter. And obtaining a main circuit parameter mathematical model and a control system mathematical model of the grid-connected converter in conclusion.
According to the further technical scheme, a main circuit parameter mathematical model and a control system mathematical model of the grid-connected converter are linearized, and the components are converted into f by applying the assumption1＝f0Substituting the form of + delta f and eliminating the steady-state component to obtain the component delta i of the current dq axisd、ΔiqAnd its reference value
The relationship between;
further simplification of the above equation yields dq-axis current perturbations Δ id、ΔiqExpression (c):
wherein:
according to the further technical scheme, the method is obtained according to the relation between each component and the reference value of the current loop dq coordinate system:
the relationship between the dq-axis current component and its reference value translates into a typical unity negative feedback system, and the stability of the current loop depends only on the open loop gain G0I.e. to convert a high-dimensional state space equation into a simple one-dimensional transfer function model.
In a further technical scheme, a unit power factor control method U is adopted by considering a grid-connected voltage source type current converterq0＝0,Iq0When the value is 0, the open-loop gain under the condition of not adopting the feedforward control of the power grid can be further simplified;
G0＝G1Gcouple；
wherein:
according to the open-loop transfer function G0And directly analyzing the control stability of the grid-connected voltage source type converter without a voltage feedforward link by using a stability analysis method in a control theory according to a Berde diagram or a Nyquist criterion. According to the open-loop transfer function T0The influence of the voltage feedforward link on the stability of the current loop can be directly analyzed.
Further, if the power grid strength is infinite, L may be considered to beg≈0,RgAbout 0, then the current disturbance Δ id、ΔiqBecomes:
i.e. the strength of the grid is infinite,the stability of the current loop is only related to the control of the current loop and the main circuit parameters of the VSC, and is not related to the parameters of the phase-locked loop and the strength of the power grid. Accordingly, at this time GcoupleAnd the voltage feedforward link does not influence the stability of the current loop when the power grid strength is infinite, namely the power grid strength is 0. And as the grid strength decreases, GcoupleThe influence of (2) is gradually increased, and the stability of the current loop is enhanced. Due to cancellation of G when using voltage feed-forwardcoupleThe voltage feed-forward link under the condition of weak power grid deteriorates the stability of the current loop.
The invention discloses a voltage feedforward link action analysis system of a voltage source type converter under weak grid connection, which comprises the following steps:
the model building module is used for building a mathematical model according to the typical topology of the voltage source type current converter under the weak grid connection;
the linearization processing module linearizes the mathematical model to obtain a small-signal mathematical model of the voltage source type converter under the weak power grid connection condition;
the transfer function establishing module is used for simplifying a small-signal mathematical model to obtain a single-input single-output analytic transfer function which can represent the influence between current loop current disturbance and output voltage disturbance thereof, namely, the influence of a voltage feedforward link on the control stability of the voltage source type converter is directly expressed as an independent open-loop transfer function;
and the stability analysis module is used for analyzing the stability influence of the voltage feedforward link of the power grid on the voltage source type converter under the weak grid connection according to the obtained transfer function.
The above one or more technical solutions have the following beneficial effects:
aiming at a voltage source type converter under weak network connection, a high-order multi-input multi-output small signal model is simplified into a single-input single-output analytic transfer function.
The method can directly analyze the influence of the voltage feedforward link of the power grid on the voltage source type grid-connected converter under the weak grid connection condition, directly express the influence of the voltage feedforward link on the control stability of the voltage source type converter as an independent open-loop transfer function, and more accurately and directly analyze the influence of the voltage feedforward link on the grid-connected converter system. The method has the advantages of being good in extension characteristic, accurate, rapid, simple and practical, and significant in guaranteeing accurate analysis and evaluation of the influence of a voltage feedforward link on the grid-connected stability of the voltage source type converter.
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The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention.
Fig. 1 is a schematic diagram illustrating a topology and control of a typical grid-connected voltage source converter according to an embodiment of the present disclosure;
FIG. 2 is a schematic diagram of a relative position in coordinate space of an embodiment of the present disclosure;
FIG. 3(a) is a small signal model control block diagram of a d-axis current loop of an embodiment of the present disclosure;
FIG. 3(b) is a small signal model control block diagram of a q-axis current loop of an embodiment of the present disclosure;
fig. 4(a) is a relationship between d-axis current and reference value thereof in case of weak grid condition (SCR ═ 1.2) and without grid voltage feedforward link according to an embodiment of the present disclosure;
fig. 4(b) is a bode diagram of an open-loop transfer function of a network voltage feedforward link and a network voltage feedforward link not adopted after calculation of an analytic function obtained by the present invention under a weak network condition (SCR ═ 1.2) according to an embodiment of the present disclosure;
fig. 4(c) is a Nyquist plot of the open-loop transfer function of the network voltage feedforward link and the network voltage feedforward link not adopted after the analytic function calculation obtained by the present invention is adopted under the weak network condition (SCR ═ 1.2) according to the embodiment of the present disclosure;
fig. 5 illustrates the effect of grid strength on coupling terms introduced without a grid feed-forward link in an embodiment of the present disclosure.
Detailed Description
It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The embodiments and features of the embodiments of the present invention may be combined with each other without conflict.
The general idea provided by the invention is as follows:
aiming at the voltage source type converter under weak network connection, a high-order multi-input multi-output small signal model is simplified into a single-input single-output analytic transfer function, the influence of a voltage feedforward link on the control stability of the voltage source type converter is directly expressed into an independent open-loop transfer function, and the influence of the voltage feedforward link on a grid-connected converter system is more accurately and directly analyzed.
Example one
The embodiment discloses a voltage feedforward link action analysis method of a voltage source type converter under weak grid connection, which comprises the following steps:
(1) establishing a mathematical model and a basic assumption according to the typical topology of the voltage source type converter under weak network connection;
(2) linearizing the mathematical model in the step (1) to obtain a small-signal mathematical model of the grid-connected converter under the weak grid connection condition;
(3) simplifying the small signal model in the step (2) to obtain a transfer function capable of representing the influence between the current disturbance of the current loop and the current reference value disturbance of the current loop;
(4) extracting a transfer function G only including the influence of a voltage feedforward link according to the transfer function obtained in the step (3)couple；
(5) According to the transfer function obtained in the step (4), comparing the stability of the grid-connected converter system under the weak grid condition when the voltage feedforward control is used with the stability when the voltage feedforward control is not used according to the amplitude margin, the phase margin and the Nyquist criterion;
(6) analyzing a voltage feedforward link pair coupling term G under different power grid strengths according to the transfer function obtained in the step (5) and according to the amplitude margin, the phase margin and the Nyquist criterioncoupleThe mechanism of influence of (a) guides the design of the control strategy.
In the specific embodiment, a reasonable assumption is made according to the control of a typical voltage source type grid-connected inverter:
as shown in fig. 1, in order to consider the influence of the grid voltage feedforward link on the stability of the voltage source type grid-connected converter, a reasonable assumption is adopted to simplify the analysis process.
The time delay of the control link can be ignored relative to the time constant of the current loop, and the transfer function G can be approximately consideredd≈1。
The outer loop control is slow, the disturbance response of the outer loop control to the inner loop control is considered to be slow, the dynamic influence of the outer loop control is ignored, and the delta u is considered to bedc≈0。
In a specific embodiment, a main circuit model of the voltage source type grid-connected converter is established.
Wherein R isp,LpRepresenting the filter resistance and the inductance of the converter; rg,LgRepresenting the equivalent resistance and inductance of the power grid; rac＝Rg+Rp,Lac＝Lg+LpRepresenting the total equivalent resistance and inductance on the ac side. u. ofcd,ucqRepresenting the component of the outlet voltage of the AC side of the converter under a dq coordinate system; u. ofd,uqRepresenting the component of the PCC node voltage in the dq coordinate system; u. ofgd,ugqRepresenting the component of the equivalent potential of the alternating current power grid in a dq coordinate system; i.e. id,iqRepresenting the components of the current in the dq coordinate system. Wherein the grid strength is characterized by a Short Circuit Ratio SCR (SCR).
In a specific embodiment, a mathematical model of the control system in the dq coordinate system is established. The invention adopts a PCC node voltage orientation method, so that theta exists under the steady state conditionpll0＝θabc0And (5) 0, namely, the phase-locked loop tracking phase angle is the same as the voltage phase angle of the PCC node. In the transient process of load disturbance and the like, a tracking phase angle error, namely theta, exists in the tracking phase angle of the phase-locked loop in the transition processpll≠θabcThe influence of this phase angle error needs to be taken into account in the analysis of the small signal. Therefore, the present invention divides the coordinate system into a control coordinate system and a main circuit coordinate system. The relationship between the control coordinate system and the main circuit coordinate system is shown in fig. 2, and the conversion relationship between the coordinate systems is:
where f represents an electrical component, such as a corresponding voltage, current, etc. The subscripts d, q represent the representation of the corresponding components in the dq coordinate system; superscript cf represents a representation of the corresponding component in the control coordinate system; thetapllRepresenting the tracking phase angle of the phase locked loop.
The open-loop transfer function and the closed-loop transfer function of the phase-locked loop control system can be expressed as:
wherein G ispll(s) and Tpll(s) represents the open and closed loop transfer function, k, of the phase locked loopppllRepresenting the proportionality coefficient, k, of the phase-locked loop PIipllAnd the integral coefficient represents the PI link of the phase-locked loop. The proportional coefficient and the integral coefficient can be calculated by the following formula.
Where ξ represents the damping ratio coefficient, typically taken as 0.707; subscript 0 represents the steady state value of its corresponding component; u shaped0I.e. representing the d-axis voltage steady-state component; omegapllRepresenting the phase-locked loop bandwidth.
The mathematical model of current loop control considering voltage feedforward is:
the current loop control without voltage feed forward is:
the current loop PI element is represented as:
where the superscript "-ref" represents the reference value under the corresponding component. k is a radical ofpgscRepresenting the proportionality coefficient of the current loop PI; k is a radical ofigscRepresents the integral coefficient of the current loop PI. The current loop proportionality coefficient and integral coefficient are calculated as:
wherein,ωCLrepresenting the design bandwidth of the current loop
The expression of the modulation link is as follows:
in a specific embodiment, a current loop small signal model of the voltage source type grid-connected converter is established:
linearizing the main circuit parameter mathematical model and the control system mathematical model of the grid-connected converter, applying the above assumptions, and dividing the above components by f1＝f0Substituting the form of + delta f and eliminating the steady-state component to obtain the component delta i of the current dq axisd、ΔiqAnd its reference value
The relationship between:
further simplification of the above equation yields dq-axis current perturbations Δ id、ΔiqExpression (c):
wherein:
where the subscript 0 represents the steady state value of the respective variable at the operating point. Δ represents a small signal component; the superscript-ref represents the reference value of the corresponding component; hpllRepresenting a phase-locked loop closed-loop transfer function; l isacRepresenting the total inductance value on the AC side; l isgRepresenting an equivalent grid strength inductance value; omega represents the power frequency angular velocity; gCLRepresents a current loop PI;
the above-mentioned currentThe relationship between the small signal component and its reference value can be obtained as the block diagram shown in fig. 3, and based on the relationship between the current small signal component and its reference value, it can be seen that, after the small signal model in the present invention is adopted, the current loop can be abstracted to a typical unit feedback system, and its stability depends only on the open loop gain G0I.e. to convert a high-dimensional state space equation into a simple one-dimensional transfer function model. Therefore, only G needs to be considered in the analysis process0The stability of (1) is sufficient. Considering that grid-connected voltage source type current converter generally adopts a unit power factor control method, U is providedq0＝0,Iq0When the value is 0, the open-loop gain under the condition of not adopting the feed-forward control of the power grid is as follows:
G0＝G1Gcouple (16)
wherein:
in the above formula, GcoupleI.e. represents the coupling term resulting from the absence of the grid voltage feed forward link.
According to the open-loop transfer function G0And GcoupleThe stability of the control of the grid-connected voltage source type converter without the voltage feedforward link and the influence of the voltage feedforward link on the control can be directly analyzed by using a stability analysis method in a control theory according to a Berde plot or a Nyquist criterion.
And abstracting an expression of the influence of the voltage feedforward link. The method specifically comprises the following steps:
it can be seen from the above that, when the power grid strength is weak, the influence of the voltage feedforward link on the stability of the current loop is generated by the combined action of the phase-locked loop, the current loop, the power grid equivalent impedance and the like. By means of a mathematical tool, computational analysis can find that GcoupleThe stability of the current loop is mainly reflected in the cut-off frequency of the middle frequency bandThe nearby part has the compensation effect of phase and amplitude, and mainly has the compensation effect of phase. Using a voltage feed-forward loop, GcoupleThe compensation of (a) is counteracted and a larger phase lag is generated around the cut-off frequency compared with the case of not using a voltage feedforward loop and using a voltage loop.
In a specific embodiment, if the grid strength is infinite, L may be considered to beg≈0,RgWhen 0 is applied, equation (15) becomes:
namely, when the power grid strength is infinite, the stability of the current loop is only related to the control of the current loop and the main circuit parameters of the VSC, and is not related to the phase-locked loop parameters and the power grid strength. Accordingly, at this time GcoupleAnd the voltage feedforward link does not influence the stability of the current loop when the power grid strength is infinite, namely the power grid strength is 0. And as the grid strength decreases, GcoupleThe influence of (2) is gradually increased, and the stability of the current loop is enhanced. Due to cancellation of G when using voltage feed-forwardcoupleThe voltage feed-forward link under the condition of weak power grid deteriorates the stability of the current loop.
In order to verify the calculation method provided by the invention, the stability of the grid-connected inverter system is analyzed by using the open-loop transfer function model established in the invention and utilizing the classical control theory. The control parameters are set to be that SCR is 1.2, the bandwidth of a current loop is 150Hz, and the bandwidth of a phase-locked loop is 40Hz and 50 Hz. The results of the experiment are shown in FIGS. 3(a) to 3 (b). The experimental result shows that when the bandwidth of the phase-locked loop is 40Hz, the phase-locked loop can be kept stable without adopting a voltage feedforward loop and a voltage loop, the amplitude margins are respectively 1.6dB and 1.39dB, and the Neisseria diagram does not surround (-1j, 0); when the phase-locked loop is 50Hz, the phase-locked loop can be stable without adopting a voltage feedforward loop, and the system is unstable after adopting a voltage link, namely, the conclusion that the voltage feedforward link deteriorates the system stability under the condition of weak power grid connection is verified. The amplitude margins of the two are respectively 1.34dB and-0.225 dB. When voltage feed-forward is adopted, the Nquist curve of the Nyquist plot passes through the point (-1j,0) twice after the bandwidth of the phase-locked loop is changed to 50Hz, which also corresponds to the situation that the converter is unstable under the operation condition. The consistency of the above results indicates the effectiveness of the assay proposed herein.
Fig. 4 shows an analysis of the effect of grid strength on coupling terms introduced without grid voltage feed forward. It can be seen from the figure that the coupling term action region is basically in the middle and low frequency band, and the attenuation is 0 in the high frequency band. And along with the reduction of the power grid strength, the amplitude attenuation effect and the phase compensation effect of the coupling terms generated near the shearing frequency corresponding to the voltage feedforward link are enhanced, so that the stability of the grid-connected converter system under weak grid connection is directly improved.
Example two
The present embodiment aims to provide a computing device, including a memory, a processor, and a computer program stored on the memory and executable on the processor, wherein the processor executes the program to implement the following steps, including:
(1) establishing a mathematical model and a basic assumption according to the typical topology of the voltage source type converter under weak network connection;
(2) linearizing the mathematical model in the step (1) to obtain a small-signal mathematical model of the grid-connected converter under the weak grid connection condition;
(3) simplifying the small signal model in the step (2) to obtain a transfer function capable of representing the influence between the current disturbance of the current loop and the current reference value disturbance of the current loop;
(4) extracting a transfer function G only including the influence of a voltage feedforward link according to the transfer function obtained in the step (3)couple
(5) And (4) analyzing the stability of the grid-connected converter system under the weak grid condition according to the transfer function obtained in the step (4) and according to the amplitude margin, the phase margin and the Nyquist criterion, and comparing the stability of the grid-connected converter system under the condition of using voltage feedforward control with the stability of the grid-connected converter system under the condition of not using voltage feedforward control.
(6) Analyzing a voltage feedforward link pair coupling term G under different power grid strengths according to the transfer function obtained in the step (5) and according to the amplitude margin, the phase margin and the Nyquist criterioncoupleThe influence of (c).
EXAMPLE III
An object of the present embodiment is to provide a computer-readable storage medium.
A computer-readable storage medium, on which a computer program is stored which, when executed by a processor, performs the steps of:
(1) establishing a mathematical model and a basic assumption according to the typical topology of the voltage source type converter under weak network connection;
(2) linearizing the mathematical model in the step (1) to obtain a small-signal mathematical model of the grid-connected converter under the weak grid connection condition;
(3) simplifying the small signal model in the step (2) to obtain a transfer function capable of representing the influence between the current disturbance of the current loop and the current reference value disturbance of the current loop;
(4) extracting a transfer function G only including the influence of a voltage feedforward link according to the transfer function obtained in the step (3)couple
(5) And (4) analyzing the stability of the grid-connected converter system under the weak grid condition according to the transfer function obtained in the step (4) and according to the amplitude margin, the phase margin and the Nyquist criterion, and comparing the stability of the grid-connected converter system under the condition of using voltage feedforward control with the stability of the grid-connected converter system under the condition of not using voltage feedforward control.
(6) Analyzing a voltage feedforward link pair coupling term G under different power grid strengths according to the transfer function obtained in the step (5) and according to the amplitude margin, the phase margin and the Nyquist criterioncoupleThe influence of (c).
Example four
The invention discloses a voltage feedforward link action analysis system of a voltage source type converter under weak grid connection, which comprises the following steps:
the model building module is used for building a mathematical model according to the typical topology of the voltage source type current converter under the weak grid connection;
the linearization processing module linearizes the mathematical model to obtain a small-signal mathematical model of the voltage source type converter under the weak power grid connection condition;
the transfer function establishing module is used for simplifying a small-signal mathematical model to obtain a single-input single-output analytic transfer function which can represent the influence between current loop current disturbance and output voltage disturbance thereof, namely, the influence of a voltage feedforward link on the control stability of the voltage source type converter is directly expressed as an independent open-loop transfer function;
and the stability analysis module is used for analyzing the stability influence of the voltage feedforward link of the power grid on the voltage source type converter under the weak grid connection according to the obtained transfer function.
The steps involved in the apparatuses of the above second, third and fourth embodiments correspond to the first embodiment of the method, and the detailed description thereof can be found in the relevant description of the first embodiment. The term "computer-readable storage medium" should be taken to include a single medium or multiple media containing one or more sets of instructions; it should also be understood to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by a processor and that cause the processor to perform any of the methods of the present invention.
Those skilled in the art will appreciate that the modules or steps of the present invention described above can be implemented using general purpose computer means, or alternatively, they can be implemented using program code that is executable by computing means, such that they are stored in memory means for execution by the computing means, or they are separately fabricated into individual integrated circuit modules, or multiple modules or steps of them are fabricated into a single integrated circuit module. The present invention is not limited to any specific combination of hardware and software.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the present invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive efforts by those skilled in the art based on the technical solution of the present invention.
Claims (8)
1. The voltage feedforward link action analysis method of the voltage source type converter under weak grid connection is characterized by comprising the following steps:
establishing a mathematical model according to the typical topology of the voltage source type current converter under the weak grid connection;
according to the typical topology of the voltage source type converter, a main circuit model of the voltage source type converter under a dq coordinate system is established;
the following is obtained according to the relationship between each component and its reference value in the current loop dq coordinate system:
the relationship between the dq-axis current component and its reference value translates into a typical unity feedback system whose stability depends only on the open-loop gain G0Converting a state space equation with high dimensionality into a simple one-dimensional transfer function model;
G0＝Gωq0-Gsd0-Gωq0Gsd0+Gωd0Gsq0；
considering that the grid-connected voltage source type converter adopts a unit power factor control method, an open-loop transfer function under the condition of not adopting the feedforward control of a power grid can be further simplified;
G0＝G1Gcouple；
wherein:
according to the open-loop transfer function G0The stability of the control of the grid-connected voltage source type converter without a voltage feedforward link can be directly analyzed by using a stability analysis method in a control theory according to a Berde plot or a Nyquist criterion, and the stability of the control of the grid-connected voltage source type converter without the voltage feedforward link is determined according to an open-loop transfer function G0The influence of the voltage feedforward link on the stability of the current loop can be directly analyzed;
linearizing the mathematical model to obtain a small-signal mathematical model of the voltage source type converter under the condition of weak power grid connection;
simplifying a small-signal mathematical model to obtain a single-input single-output analytic transfer function capable of representing the influence between current loop current disturbance and output voltage disturbance thereof, namely directly expressing the influence of a voltage feedforward link on the control stability of the voltage source type converter as an independent open-loop transfer function;
and analyzing the influence of the voltage feedforward link of the power grid on the stability of the voltage source type converter under the weak grid connection according to the obtained transfer function.
2. The method for analyzing the voltage feedforward link action of the voltage source type converter under the weak grid connection as claimed in claim 1, wherein the following assumptions are required before establishing the mathematical model according to the typical topology of the voltage source type converter under the weak grid connection, specifically:
the time delay of the control link can be ignored relative to the time constant of the current loop, and the transfer function G can be approximately consideredd≈1；
The outer loop control is slow, the disturbance response of the outer loop control to the inner loop control is considered to be slow, the dynamic influence of the outer loop control is ignored, and delta udc≈0。
3. The method for analyzing the voltage feedforward loop action of the voltage source type converter under the weak grid connection as claimed in claim 2, wherein a control model of the voltage source type converter under the dq coordinate system and a conversion relation model between the control coordinate system and the main circuit coordinate system are established.
4. The method for analyzing the voltage feedforward loop action of the voltage source converter under the weak grid connection of claim 1, wherein a voltage source converter grid-connected system modulation model is established.
5. The method as claimed in claim 3, wherein the mathematical model of the main circuit parameters of the voltage source converter and the mathematical model of the control system are linearized and the current dq axis component Δ i is obtained by applying the above assumptionsd、ΔiqAnd its reference value
The relationship between them.
6. Voltage source type transverter voltage feedforward link effect analytic system under weak electric wire netting connection, characterized by includes:
the model building module is used for building a mathematical model according to the typical topology of the voltage source type current converter under the weak grid connection;
according to the typical topology of the voltage source type converter, a main circuit model of the voltage source type converter under a dq coordinate system is established;
the following is obtained according to the relationship between each component and its reference value in the current loop dq coordinate system:
the relationship between the dq-axis current component and its reference value translates into a typical unity feedback system whose stability depends only on the open-loop gain G0Converting a state space equation with high dimensionality into a simple one-dimensional transfer function model;
G0＝Gωq0-Gsd0-Gωq0Gsd0+Gωd0Gsq0；
considering that the grid-connected voltage source type converter adopts a unit power factor control method, an open-loop transfer function under the condition of not adopting the feedforward control of a power grid can be further simplified;
G0＝G1Gcouple；
wherein:
according to the open-loop transfer function G0The stability of the control of the grid-connected voltage source type converter without a voltage feedforward link can be directly analyzed by using a stability analysis method in a control theory according to a Berde plot or a Nyquist criterion, and the stability of the control of the grid-connected voltage source type converter without the voltage feedforward link is determined according to an open-loop transfer function G0The influence of the voltage feedforward link on the stability of the current loop can be directly analyzed;
the linearization processing module linearizes the mathematical model to obtain a small-signal mathematical model of the voltage source type converter under the weak power grid connection condition;
the transfer function establishing module is used for simplifying a small-signal mathematical model to obtain a single-input single-output analytic transfer function which can represent the influence between current loop current disturbance and output voltage disturbance thereof, namely, the influence of a voltage feedforward link on the control stability of the voltage source type converter is directly expressed as an independent open-loop transfer function;
and the stability analysis module is used for analyzing the stability influence of the voltage feedforward link of the power grid on the voltage source type converter under the weak grid connection according to the obtained transfer function.
7. A computing device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, wherein the processor executes the program to perform steps comprising:
establishing a mathematical model according to the typical topology of the voltage source type current converter under the weak grid connection;
according to the typical topology of the voltage source type converter, a main circuit model of the voltage source type converter under a dq coordinate system is established;
the following is obtained according to the relationship between each component and its reference value in the current loop dq coordinate system:
the relationship between the dq-axis current component and its reference value translates into a typical unity feedback system whose stability depends only on the open-loop gain G0Converting a state space equation with high dimensionality into a simple one-dimensional transfer function model;
G0＝Gωq0-Gsd0-Gωq0Gsd0+Gωd0Gsq0；
considering that the grid-connected voltage source type converter adopts a unit power factor control method, an open-loop transfer function under the condition of not adopting the feedforward control of a power grid can be further simplified;
G0＝G1Gcouple；
wherein:
according to the open-loop transfer function G0The stability of the control of the grid-connected voltage source type converter without a voltage feedforward link can be directly analyzed by using a stability analysis method in a control theory according to a Berde plot or a Nyquist criterion, and the stability of the control of the grid-connected voltage source type converter without the voltage feedforward link is determined according to an open-loop transfer function G0The influence of the voltage feedforward link on the stability of the current loop can be directly analyzed;
linearizing the mathematical model to obtain a small-signal mathematical model of the voltage source type converter under the condition of weak power grid connection;
simplifying a small-signal mathematical model to obtain a single-input single-output analytic transfer function capable of representing the influence between current loop current disturbance and output voltage disturbance thereof, namely directly expressing the influence of a voltage feedforward link on the control stability of the voltage source type converter as an independent open-loop transfer function;
and analyzing the influence of the voltage feedforward link of the power grid on the stability of the voltage source type converter under the weak grid connection according to the obtained transfer function.
8. A computer-readable storage medium, on which a computer program is stored which, when executed by a processor, performs the steps of:
establishing a mathematical model according to the typical topology of the voltage source type current converter under the weak grid connection;
according to the typical topology of the voltage source type converter, a main circuit model of the voltage source type converter under a dq coordinate system is established;
the following is obtained according to the relationship between each component and its reference value in the current loop dq coordinate system:
the relationship between the dq-axis current component and its reference value translates into a typical unity feedback system whose stability depends only on the open-loop gain G0Converting a state space equation with high dimensionality into a simple one-dimensional transfer function model;
G0＝Gωq0-Gsd0-Gωq0Gsd0+Gωd0Gsq0；
considering that the grid-connected voltage source type converter adopts a unit power factor control method, an open-loop transfer function under the condition of not adopting the feedforward control of a power grid can be further simplified;
G0＝G1Gcouple；
wherein:
according to the open-loop transfer function G0The stability of the control of the grid-connected voltage source type converter without a voltage feedforward link can be directly analyzed by using a stability analysis method in a control theory according to a Berde plot or a Nyquist criterion, and the stability of the control of the grid-connected voltage source type converter without the voltage feedforward link is determined according to an open-loop transfer function G0The influence of the voltage feedforward link on the stability of the current loop can be directly analyzed;
linearizing the mathematical model to obtain a small-signal mathematical model of the voltage source type converter under the condition of weak power grid connection;
simplifying a small-signal mathematical model to obtain a single-input single-output analytic transfer function capable of representing the influence between current loop current disturbance and output voltage disturbance thereof, namely directly expressing the influence of a voltage feedforward link on the control stability of the voltage source type converter as an independent open-loop transfer function;
and analyzing the influence of the voltage feedforward link of the power grid on the stability of the voltage source type converter under the weak grid connection according to the obtained transfer function.
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