CN110334935B

CN110334935B - Method and device for evaluating transient stability of grid-connected converter and storage medium

Info

Publication number: CN110334935B
Application number: CN201910573107.8A
Authority: CN
Inventors: 袁豪; 洪潮; 张帆; 张野; 杨健; 李俊杰; 孙鹏伟
Original assignee: CSG Electric Power Research Institute; China Southern Power Grid Co Ltd
Current assignee: CSG Electric Power Research Institute; China Southern Power Grid Co Ltd
Priority date: 2019-06-27
Filing date: 2019-06-27
Publication date: 2021-11-19
Anticipated expiration: 2039-06-27
Also published as: CN110334935A

Abstract

The invention discloses a method, a device and a storage medium for evaluating transient stability of a grid-connected converter, wherein the method comprises the following steps: acquiring a first curve of the change of the current of the VSC along with time; judging whether the first curve meets a non-return point or not; if so, drawing a second curve between the unbalanced power and the current at two ends of the direct current capacitor of the VSC, and calculating to obtain the instability value of the VSC according to the second curve; if not, drawing a third curve of the direct-current voltage error of the VSC along with the change of time, obtaining a fitting curve according to the third curve, and calculating the attenuation coefficient of the fitting curve; when the attenuation coefficient is larger than 0, obtaining the instability value of the VSC; and when the attenuation coefficient is not more than 0, drawing a fourth curve between the unbalanced power and the current at two ends of the direct current capacitor of the VSC, constructing a virtual non-return point, and calculating to obtain the stability margin of the VSC. The method can quantitatively evaluate the stability margin of the VSC direct-current voltage, and makes up the technical defect that the transient stability of the VSC direct-current voltage can only be analyzed through numerical simulation at present.

Description

Method and device for evaluating transient stability of grid-connected converter and storage medium

Technical Field

The invention relates to the technical field of transient stability analysis of electrical equipment, in particular to a method and a device for evaluating transient stability of a grid-connected converter and a storage medium.

Background

The rapid development of concentrated and distributed renewable energy power generation (such as wind power and photovoltaic power generation) and the like enables Voltage Source Converters (VSCs) to be widely distributed in modern power systems. The direct-current voltage (the state quantity of the direct-current bus capacitor) of the VSC has the instability problems of monotonous change, continuous oscillation and the like due to energy accumulation and exchange under the system transient condition.

However, the prior art transient stability analysis for inverters is generally directed to inverters that employ a Virtual Synchronous Generator (VSG) control strategy or a droop control strategy. At present, however, vector control strategies based on terminal voltage orientation are still commonly adopted by VSC grid-connection equipment in the power system. Moreover, the equipment characteristics of the VSC and the conventional synchronous generator are greatly different, and an EEAC method, which is a transient stability quantitative evaluation method for the synchronous generator, cannot be directly applied to quantitative evaluation of the transient stability of the VSC direct-current voltage; the VSC based on VSG and droop control strategy and the VSC based on vector control strategy have great difference in dynamic characteristics. Therefore, the method for analyzing and judging the transient stability of the VSC based on the VSG and the droop control strategy cannot be applied to the VSC based on the vector control strategy, so that the VSC under the vector control strategy based on terminal voltage orientation is caused, a large blank exists in a quantitative evaluation technology of the transient stability margin of the direct current voltage, the method mainly depends on numerical simulation at present, but the single numerical simulation cannot provide quantitative information of the stability.

Disclosure of Invention

The embodiment of the invention aims to provide a method, a device and a storage medium for evaluating the transient stability of a grid-connected converter, which can provide a quantitative evaluation index for measuring the transient stability margin of a VSC direct-current voltage.

In order to achieve the above object, an embodiment of the present invention provides a method for evaluating transient stability of a grid-connected converter, including the following steps:

acquiring a first curve of the current of the grid-connected converter along with the time change;

judging whether the first curve meets a non-return point or not;

if so, drawing a second curve between unbalanced power at two ends of a direct current capacitor of the grid-connected converter and the current, and calculating to obtain a monotonically diverging instability value of the grid-connected converter according to the second curve;

if not, drawing a third curve of the direct-current voltage error of the grid-connected converter changing along with time to obtain a plurality of first inflection points of the third curve, fitting according to the first inflection points to obtain a fitting curve, and calculating an attenuation coefficient of the fitting curve;

when the attenuation coefficient is larger than 0, obtaining a instability value of the grid-connected converter oscillation divergence as the attenuation coefficient;

and when the attenuation coefficient is not more than 0, drawing a fourth curve between the unbalanced power and the current at two ends of the direct current capacitor of the corresponding grid-connected converter under the condition, constructing a virtual non-return point on the fourth curve to obtain a virtual fifth curve, and calculating to obtain the stability margin with the non-monotonous divergence of the stable track of the grid-connected converter according to the fifth curve.

Preferably, the obtaining of the first curve of the current of the grid-connected converter changing with time specifically includes:

establishing a simulation model, taking the direct-current voltage error and the active current of the grid-connected converter as an observation object, and obtaining a first curve of the current of the grid-connected converter along with the time change through numerical integration, wherein the numerical integration is

Wherein i_diThe active current of the grid-connected converter is the active current of the grid-connected converter; Δ u_dcThe error is the direct-current voltage error of the grid-connected converter; k is a radical of_idcThe integral parameter is the integral parameter of a direct current voltage PI controller of the grid-connected converter; u shape_dcIs the combination ofThe dc voltage of the grid converter;

and the reference value is the direct-current voltage reference value of the grid-connected converter.

Preferably, the determining whether the first curve encounters a non-return point specifically includes:

identifying a curved segment with the slope larger than 0 in the first curve, wherein the first curve comprises n curved segments with the slope larger than 0, and n is larger than or equal to 1;

judging whether the first curve meets a non-return point in a curve segment with the slope larger than 0; the non-return point refers to a first node when the area of the grid-connected converter, from which the direct-current capacitance electric field can be reduced, reaches a maximum value, and the area of the grid-connected converter, from which the direct-current capacitance electric field can be reduced, at the first node is still smaller than the area of the grid-connected converter, from which the direct-current capacitance electric field can be increased.

Preferably, if so, drawing a second curve between the unbalanced power at the two ends of the dc capacitor of the grid-connected converter and the current, and calculating a monotonically diverging instability value of the grid-connected converter according to the second curve, specifically including:

if the first curve meets a non-return point in a curve segment with the slope larger than 0 in the previous i segments, drawing a second curve between unbalanced power and current at two ends of a direct current capacitor of the grid-connected converter, and calculating to obtain a monotonically diverging instability value of the grid-connected converter according to the second curve; wherein i is more than or equal to 1 and less than or equal to n, and n is more than or equal to 3;

when i is equal to 1, the instability value of the grid-connected converter which is monotonously dispersed is

When i is 2, the instability value of the grid-connected converter which is monotonously dispersed is

When i is 3, the loss of the grid-connected converter is monotonously dispersedA stable value of

Wherein A is_decIs the area of the grid-connected converter in which the electric field energy of the DC capacitor is reduced, A_incThe area of the electric field energy of the direct current capacitor of the grid-connected converter can be increased.

Preferably, if not, drawing a third curve of the direct-current voltage error of the grid-connected converter changing along with time to obtain a plurality of first inflection points of the third curve, fitting according to the first inflection points to obtain a fitting curve, and calculating an attenuation coefficient of the fitting curve, specifically including:

if the first curve does not meet a non-return point in the curve segment with the slope larger than 0, drawing a third curve of the direct-current voltage error of the grid-connected converter changing along with time to obtain a plurality of first inflection points of the third curve;

the first inflection point comprises a second inflection point with a direct-current voltage error value smaller than 0 and a third inflection point with a direct-current voltage error value larger than 0, and the second inflection point is mirror-symmetrical to one side of the third inflection point along a straight line with the direct-current voltage error value of 0 on the third curve to obtain a plurality of fourth inflection points with the direct-current voltage error values larger than 0;

connecting the third inflection point and the fourth inflection point to obtain a fitting curve;

and calculating the attenuation coefficient of the fitting curve according to the fitting curve.

Preferably, when the attenuation coefficient is not greater than 0, drawing a fourth curve between the unbalanced power and the current at the two ends of the dc capacitor of the grid-connected converter corresponding to the situation, constructing a virtual non-return point on the fourth curve to obtain a virtual fifth curve, and calculating a stability margin with a non-monotonously divergent stability trajectory of the grid-connected converter according to the fifth curve, specifically including:

when the attenuation coefficient is not more than 0, drawing a fourth curve between unbalanced power at two ends of the direct current capacitor of the corresponding grid-connected converter and the current under the condition;

finding a second node with a direct-current voltage error equal to 0 in a curve corresponding to the change of the direct-current voltage error along with time in the fourth curve, and continuously increasing the electric field energy at the second node at any moment to enable unbalanced power at two ends of a direct-current capacitor of the grid-connected converter to be continuously converted from a negative value to a positive value along with the increase of the current, so that a virtual fifth curve is obtained;

obtaining a virtual non-return point according to a third node with unbalanced power equal to 0 at two ends of a direct current capacitor of the grid-connected converter, and meanwhile obtaining the area which can be reduced by a virtual electric field of the grid-connected converter;

and calculating to obtain the stability margin of the grid-connected converter with the stable track not monotonously diverged according to the fifth curve.

Preferably, the calculation formula of the stability margin is

A_dec.potIs the area of the grid-connected converter that can be reduced by the virtual electric field.

The embodiment of the invention also provides a device for evaluating the transient stability of the grid-connected converter, which comprises the following steps:

the curve acquisition module is used for acquiring a first curve of the current of the grid-connected converter along with the change of time;

the judging module is used for judging whether the first curve meets a non-return point or not;

the monotone divergence evaluation module is used for drawing a second curve between unbalanced power and current at two ends of a direct current capacitor of the grid-connected converter if the monotone divergence evaluation module is used for obtaining a monotone divergence instability value of the grid-connected converter according to the second curve;

the fitting module is used for drawing a third curve of the direct-current voltage error of the grid-connected converter changing along with time if the direct-current voltage error of the grid-connected converter does not change, obtaining a plurality of first inflection points of the third curve, fitting according to the first inflection points to obtain a fitting curve, and calculating an attenuation coefficient of the fitting curve;

the evaluation oscillation divergence module is used for obtaining the instability value of the oscillation divergence of the grid-connected converter as the attenuation coefficient when the attenuation coefficient is larger than 0;

and the stability evaluation and non-divergence module is used for drawing a fourth curve between unbalanced power and current at two ends of the direct current capacitor of the grid-connected converter corresponding to the condition when the attenuation coefficient is not greater than 0, constructing a virtual non-return point on the fourth curve to obtain a virtual fifth curve, and calculating a stability margin with a non-monotonous divergence of the stability locus of the grid-connected converter according to the fifth curve.

The invention correspondingly provides an apparatus for evaluating transient stability of a grid-connected converter, which includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, and when the processor executes the computer program, the processor implements any one of the above-mentioned methods for evaluating transient stability of a grid-connected converter.

The embodiment of the invention also provides a computer-readable storage medium, which includes a stored computer program, wherein when the computer program runs, the apparatus where the computer-readable storage medium is located is controlled to execute any one of the above methods for evaluating transient stability of a grid-connected converter.

Compared with the prior art, the method, the device and the storage medium for evaluating the transient stability of the grid-connected converter provided by the embodiment of the invention have the advantages that the disturbed track of the grid-connected converter is specifically analyzed, the disturbed track is divided into three conditions of monotonous divergence, oscillatory divergence and stable non-divergence, and the disturbed track of each specific condition is evaluated by adopting an adaptive parameter index. The set of calculated indexes provided by the invention can be used as a scale to quantitatively evaluate the transient stability margin of the VSC direct-current voltage, and the technical defect that the transient stability of the VSC direct-current voltage can be analyzed only by depending on a numerical simulation method in the prior art is greatly overcome. The quantitative evaluation index provided by the technical scheme has good monotonicity, and is very convenient for analyzing the influence rule of different factors on the transient stability of the VSC grid-connected system, so that the optimal design of the VSC controller can be effectively guided.

Drawings

Fig. 1 is a schematic flowchart of a method for evaluating transient stability of a grid-connected converter according to embodiment 1 of the present invention.

Fig. 2 is a schematic diagram of a typical vector control strategy and a circuit structure of the grid-connected VSC provided by the invention.

FIG. 3 shows i of one time interference of grid-connected VSC provided by the invention_di-t plot.

FIG. 4 shows the sum of the interference of the grid-connected VSC provided by the invention at a certain time_diΔ u for t plot_dc-t plot.

FIG. 5 shows P of one time interference of grid-connected VSC provided by the invention_u-i_diGraph is shown.

FIG. 6 shows monotone divergence i of a certain disturbance of grid-connected VSC provided by the invention_di-t plot and Δ u_dcSchematic of the plot of-t.

FIG. 7 shows the i of oscillation divergence of a grid-connected VSC subjected to interference at a certain time, provided by the invention_di-t plot and Δ u_dcSchematic of the plot of-t.

Fig. 8 is a schematic diagram of oscillation convergence when the grid-connected VSC provided by the present invention is disturbed at a certain time.

Fig. 9 is a schematic diagram of oscillation divergence of the grid-connected VSC provided by the present invention when the grid-connected VSC is disturbed at a certain time.

Fig. 10 is a schematic diagram of a method for acquiring a virtual non-return point (VERP) of a grid-connected VSC stable trajectory provided by the present invention.

FIG. 11 shows a virtual P that is interfered by a VSC in a grid-connected manner according to the present invention_u-i_diGraph is shown.

Fig. 12 is a schematic structural diagram of an apparatus for evaluating transient stability of a grid-connected converter according to embodiment 2 of the present invention.

Fig. 13 is a schematic diagram of an apparatus for evaluating transient stability of a grid-connected converter according to embodiment 3 of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Referring to fig. 1, which is a schematic flowchart of a method for evaluating transient stability of a grid-connected converter according to embodiment 1 of the present invention, the method includes steps S1 to S6:

s1, acquiring a first curve of the current of the grid-connected converter along with the change of time;

s2, judging whether the first curve meets a non-return point;

s3, if yes, drawing a second curve between unbalanced power at two ends of a direct current capacitor of the grid-connected converter and the current, and calculating to obtain a monotonically diverging instability value of the grid-connected converter according to the second curve;

s4, if not, drawing a third curve of the direct-current voltage error of the grid-connected converter changing along with time to obtain a plurality of first inflection points of the third curve, fitting according to the first inflection points to obtain a fitting curve, and calculating an attenuation coefficient of the fitting curve;

s5, when the attenuation coefficient is larger than 0, obtaining a instability value of the grid-connected converter oscillation divergence as the attenuation coefficient;

and S6, when the attenuation coefficient is not greater than 0, drawing a fourth curve between the unbalanced power and the current at two ends of the direct current capacitor of the corresponding grid-connected converter under the condition, constructing a virtual non-return point on the fourth curve to obtain a virtual fifth curve, and calculating to obtain a stability margin with the non-monotonous divergence of the stable track of the grid-connected converter according to the fifth curve.

The grid-connected Converter in the present invention is referred to as a Voltage Source Converter (VSC). Fig. 2 is a schematic diagram of a circuit topology and a control structure of a VSC grid-connected system based on a typical vector control strategy. Wherein the variables and symbols in fig. 2 have the meanings shown in table 1. The VSC vector control strategy can realize active and reactive power decoupling control. Active power control includes cascaded dc voltage control and d-axis current (active current component) control. The output of the direct voltage control is a d-axis current reference value of the current control. The reactive power control includes cascaded reactive control and q-axis current (reactive current component) control. The output of the reactive control is a q-axis current reference value of the current control. There are various control strategies for reactive power control, such as ac voltage control, constant reactive power control, and constant reactive current control. The VSC tracks the phase position of the grid-connected terminal voltage through a phase-locked loop. The output angle of the phase-locked loop is used for mutual transformation between the dq coordinate system and the abc coordinate system.

Table 1 table of indices of variables and symbols in fig. 2

The method for evaluating the transient stability of the grid-connected converter provided by the embodiment 1 of the invention comprises the following specific steps:

after the power system receives the disturbance, the curve of the VSC variable changing along with the time is called the disturbed track of the VSC. Referring to fig. 3, fig. 3 shows a motion trajectory of the VSC disturbed at point o. Using i as the VSC current_diShowing that time is represented by t, and a first curve of the current of the grid-connected converter along with the time is obtained, namely i of VSC is obtained_di-t plot. As can be seen from FIG. 3, i after the o-point perturbation_diPositive increase until the direction of movement changes after point a, i_diStarting to decrease negatively, point a is a rocking inflection point. Similarly, the points b, c, d, and e are also the swing inflection points. According to i_diWhether the moving direction of (2) is changed or not, obtaining i_di-t several swinging inflection points of the curve, denoted FEP.

Judging whether the first curve meets a non-return point, namely judging i_diWhether the t curve encounters a no-return point, if i_diThe t curve meets the non-return point and is intuitiveIs reflected in i_diIs further increased, and i_diAfter further increase, unbalanced power at two ends of the direct-current capacitor of the grid-connected converter is not less than 0, but is more than 0 and continuously increases, so that vicious circle is caused, and active current i is caused_diAnd the direct-current voltage is monotonously dispersed in the swing process, namely the grid-connected converter is monotonously dispersed after being interfered; if i_diAnd the-t curve does not meet the non-return point, and represents that the grid-connected converter can oscillate and diverge or is stable after being interfered. For convenience of the following description, the non-return point will be denoted as NRP.

If so, drawing a second curve between the unbalanced power at the two ends of the direct-current capacitor of the grid-connected converter and the current, and calculating to obtain a monotonically diverging instability value of the grid-connected converter according to the second curve. P is used for unbalanced power at two ends of direct current capacitor of VSC_uThe second curve may be represented by P_u-i_diAnd (4) curve representation. In this case, it is shown that the grid-connected converter is monotonously divergent after being disturbed. For the VSC monotonously divergent instability locus, the transient instability degree of the DC voltage can be reflected by the difference value of the increasing area of the DC capacitance electric field and the decreasing area of the electric field at the NRP, namely the larger the negative absolute value of the difference value is, the more the instability degree is.

If not, drawing a third curve of the direct-current voltage error of the grid-connected converter changing along with time to obtain a plurality of first inflection points of the third curve, fitting according to the first inflection points to obtain a fitting curve, and calculating the attenuation coefficient of the fitting curve. The DC voltage error of VSC is calculated by delta u_dcThe third curve may be expressed as Deltau_dcT curve representation, fitting curve representation l_zThe attenuation coefficient is denoted as τ. According to plotted Δ u_dcThe-t curve can preliminarily judge whether the VSC disturbed track oscillates and converges or oscillates and diverges, the oscillation and convergence is realized when the range of the up-and-down fluctuation is smaller, and the oscillation and divergence is realized when the range of the up-and-down fluctuation is larger. However, the instability degree of VSC is quantitatively evaluated by calculating a fitting curve l_zIs obtained from the attenuation coefficient tau. Wherein a curve l is fitted_zIs obtained by fitting a first inflection point, and the attenuation coefficient tau is obtained by fitting a curve l_zIs obtained by the calculation of (1).

And when the attenuation coefficient tau is larger than 0, obtaining the instability value of the grid-connected converter oscillation divergence as the attenuation coefficient. When the attenuation coefficient tau is larger than 0, the range representing the up-and-down fluctuation of the VSC disturbed track is larger and larger, namely oscillation divergence. The larger the value of the damping coefficient tau, the faster the physically disturbed VSC trajectory will oscillate in subsequent swings.

When the attenuation coefficient is not more than 0 and represents that the fluctuation range of the VSC disturbed track is smaller and smaller, namely oscillation convergence, a fourth curve between the unbalanced power at the two ends of the direct-current capacitor of the corresponding grid-connected converter and the current under the condition is drawn, namely the P of the VSC_u-i_diCurve line. And constructing a virtual non-return point on the fourth curve, marking the point as VNRP for convenient description, obtaining a virtual fifth curve according to the VNRP, and calculating to obtain a stability margin with the stable track of the grid-connected converter not monotonously diverged according to the fifth curve.

In the embodiment 1 of the invention, the disturbed trajectory of the grid-connected converter is specifically analyzed, the disturbed trajectory is divided into three conditions of monotonous divergence, oscillatory divergence and stability non-divergence, the disturbed trajectory of each specific condition adopts an adaptive parameter index to evaluate the instability degree of the disturbed trajectory, a certain index is provided as a scale to quantitatively evaluate the transient stability margin of the VSC direct-current voltage, and the technical defect that the transient stability of the VSC direct-current voltage can only be analyzed by depending on a numerical simulation method in the prior art is greatly overcome. The technical scheme solves the calculation problem that people cannot judge the stability margin of the stable case and cannot know the instability degree of the unstable case, and is greatly convenient for people to evaluate the influence of a certain parameter or a certain control on the transient stability of the VSC direct-current voltage.

As an improvement of the above scheme, the obtaining of the first curve of the current of the grid-connected converter changing with time specifically includes:

Wherein i_diThe active current of the grid-connected converter is the active current of the grid-connected converter; Δ u_dcThe error is the direct-current voltage error of the grid-connected converter; k is a radical of_idcThe integral parameter is the integral parameter of a direct current voltage PI controller of the grid-connected converter; u shape_dcThe direct current voltage of the grid-connected converter is obtained;

Specifically, a simulation model is established, a direct-current voltage error and active current of the VSC are used as observation objects, a first curve of the current of the grid-connected converter changing along with time is obtained through numerical integration, and the numerical integration is

Wherein i_diIs the active current of the grid-connected converter; Δ u_dcThe error is the direct-current voltage error of the grid-connected converter; k is a radical of_idcThe integral parameter is the integral parameter of a direct current voltage PI controller of the grid-connected converter; u shape_dcThe direct current voltage of the grid-connected converter;

the direct-current voltage reference value of the grid-connected converter.

DC voltage error delta u of grid-connected converter_dcAnd an active current i_diFor an observation object, acquiring i of the grid-connected converter after the interference at the point o through the numerical integration_diGraph of-t as shown in FIG. 3 and Δ u_dcThe plot of-t is shown in FIG. 4. As can be seen from FIGS. 3 and 4, after the o-point perturbation, Δ u_dc>0，i_diPositive increase until point a (Δ u)_dc0) the direction of the back movement is changed, and Δ u_dc<0，i_diA negative decrease is initiated. For ease of understanding, the oa swing phase is referred to as the active current i_diThe 1 st pendulum of the motion, the ab segment is its 2 nd pendulum, and so on, and the bc, cd and de segments are called their 3 rd, 4 th and 5 th pendulums, respectively. At i_diIn the plot of-t, in terms of Δ u _dc0 is gotTo i_diT several swing inflection points of the curve, i.e. points a, b, c, d, e are referred to as swing inflection points (FEP) of the corresponding swing, each time Δ u_dcWhen the value is 0 (points a, b, c, d, and e in fig. 3), i_diThe direction of motion will change. In other words, if FEP (Δ u) is encountered during a certain oscillation_dc0) indicating that the wobble is finished and that the next wobble is about to start. In addition,. DELTA.u_dc>The swing at 0 is called the nth positive swing, that is, in fig. 3, the oa segment corresponds to N-1, the bc segment corresponds to N-2, and the de segment corresponds to N-3, where N is greater than or equal to 1.

As an improvement of the above scheme, the determining whether the first curve encounters a non-return point specifically includes:

It should be noted that, neglecting the VSC resistive losses, the relationship between the dc voltage dynamics and the input and output power of the VSC can be expressed as a primary equation

Wherein, C_dcA DC capacitor of the VSC;

is a direct current voltage reference value of the VSC; p_inActive power input for the VSC; p is active power output to a power grid by the VSC; p_uIs the unbalanced power at the two ends of the VSC direct current capacitor.

The first sub-equal sign is simultaneously paired with i_diThe integral can be obtained as the second expression

Wherein i_di1And Δ u_dc1Respectively represent i at the start of integration_diAnd Δ u_dcValue i_di2And Δ u_dc2Representing the value at the end of integration.

The expression to the left of the second equation equal sign represents the change in electric field energy across the VSC dc capacitance. When P is present_uAnd di_diWhen the signs of the two capacitors are the same, the electric field on the direct current capacitor can be increased; otherwise, the electric field can be reduced. According to P_uAnd di_diWhether the symbols of (a) are the same or not, the right formula of the second formula is divided into two parts, specifically

And order

Then there is

Wherein A is_incAnd A_decEach represents P_u-i_diThe area of the curve where the dc capacitance of the VSC increases the electric field energy and the area where the electric field energy decreases is schematically shown in fig. 5. Assuming the beginning of the wobble P_u>0, then Δ u_dc>The DC capacitance electric field energy of the 0 VSC, A_incIncrease when P is_uWhen changing from positive to negative, point A in FIG. 5 is shown_incReaches a maximum value after A_decInitially increased, Δ u_dcBegins to decrease when P_uWhen the value is changed from negative to positive, the point B, A in FIG. 5 is corresponded_decA maximum value is reached.

For the VSC monotone divergence instability track, when the disturbed track reaches the point B, A_decIs still less than A_incI.e. deltau_dcIs still greater than zero, thereby promoting i_diGo toStep size is increased, i_diAfter further enlargement, P_uBut is instead greater than zero and continuously increased, so vicious circle results in active current i_diAnd the direct voltage is monotonically diverging during the swing. We call point B the non-return point (denoted as NRP) of the VSC, as shown in fig. 5. Therefore, for VSC monotonic divergent instability trajectory, it can be used at A at NRP_decAnd A_incThe difference value of the DC voltage reflects the transient instability degree of the DC voltage, namely A_dec－A_incThe more negative the absolute value of (a), the more destabilizing it is.

Specifically, the first curve includes n curved segments with slopes greater than 0; wherein n is more than or equal to 1. By definition, a curve with a slope greater than 0 is a positive pendulum. And identifying a curve section with the slope larger than 0 in the first curve, judging whether the first curve meets a non-return point in the curve section with the slope larger than 0, judging whether the interference received by the VSC is monotonously dispersed according to a judgment result, if the non-return point is met, the interference received by the VSC is monotonously dispersed, otherwise, the interference is not monotonously dispersed. Preferably, n is 3, although monotonic divergence may occur in any minor positive pendulum of the disturbed trajectory, the embodiment only focuses on the monotonic instability situation of the VSC direct-current voltage in the first three positive pendulums, and for the later pendulums of the disturbed trajectory, the embodiment considers that the disturbed trajectory belongs to the category of oscillation divergence instability.

When VSC direct voltage transient state unstability, its unstability orbit mainly has two kinds: one is a monotonous divergent instability locus corresponding to i_di-t plot and Δ u_dcThe graph of-t is shown in FIG. 6 (point o is the disturbance starting point), and the other is the oscillation divergence instability track corresponding to i_di-t plot and Δ u_dcThe plot of-t is shown in FIG. 7. The physical mechanisms of the two types of instability tracks are different, the former is mainly due to insufficient restoring force of the VSC in the transient process, and the latter is mainly due to insufficient damping force of the VSC. In addition, it is worth explaining that the monotonic divergence of the VSC disturbed trajectory may occur in any forward swing, and fig. 6 and 7 are only schematic diagrams of the disturbed forward swing.

As an improvement of the above scheme, if so, drawing a second curve between unbalanced power at two ends of a dc capacitor of the grid-connected converter and the current, and calculating a monotonically diverging instability value of the grid-connected converter according to the second curve, specifically including:

When i is 3, the instability value of the grid-connected converter which is monotonously dispersed is

Specifically, if the first curve is i of VSC_di-t curve meets a non-return point in the first i section of the curve where the slope is greater than 0, then a second curve between unbalanced power and current across the VSC dc capacitor, P, is plotted_u-i_diAnd calculating a instability value of the VSC monotone divergence according to the second curve.

The instability value of the VSC monotonously divergence is expressed by eta, when i is more than or equal to 1 and less than or equal to n, and n is more than or equal to 3, the eta value can be calculated by adopting different formulas according to specific conditions, specifically; when i is equal to 1, the instability value of the grid-connected converter which is monotonously dispersed is

Wherein A is_decArea reduced by the electric field energy of the DC capacitor of the grid-connected converter, A_incThe area of the electric field energy of the direct current capacitor of the grid-connected converter is increased.

Since the non-return point refers to the area A where the VSC DC capacitance electric field energy is reduced_decFirst node when maximum value is reached, and A_decIs less than A_incTherefore, the eta value calculated according to the above formula is smaller than 0, so that whether the interference suffered by the VSC is monotonously dispersed can be judged according to whether the calculated eta value is smaller than 0, if so, the interference suffered by the VSC is monotonously dispersed, otherwise, the interference is not monotonously dispersed. The eta value has good monotonicity and can be used as a monotonically diverging instability value of the grid-connected converter, and the larger the absolute value of the eta value is, the more the monotonically diverging instability degree of the grid-connected converter is.

As an improvement of the above scheme, if not, drawing a third curve of the direct-current voltage error of the grid-connected converter changing with time to obtain a plurality of first inflection points of the third curve, fitting according to the first inflection points to obtain a fitted curve, and calculating an attenuation coefficient of the fitted curve, specifically including:

Specifically, if the first curve is i of VSC_diThe curve-t does not meet a non-return point in the curve segment with the slope greater than 0 in the previous i segments, which means that the interference suffered by the VSC is not monotonously dispersed, the estimation of the instability degree of the interference suffered by the VSC cannot be obtained according to the calculation eta value, other parameters are required to be utilized, and therefore a third curve, namely delta u, of the change of the direct-current voltage error of the grid-connected converter along with time needs to be drawn_dc-t curves, resulting in several first inflection points of the third curve. Observation of Δ u_dcTrend over time t, to quantitatively evaluate this trend, Δ u_dcAll first inflection points in the t-curve are uniformly adjusted to the same direction (i.e. Δ u)_dc>0 or Δ u_dc< 0). Wherein the first inflection point includes a DC voltage error value Δ u_dcSecond inflection point less than 0 and DC voltage error value Deltau_dcA third inflection point that is greater than 0,

in this embodiment, Δ u_dcAll first inflection points in the t-curve are uniformly adjusted to Δ u_dc>In the direction of 0, specifically: on the third curve, a second inflection point is along a straight line delta u which is a straight line with the DC voltage error value of 0_dcObtaining a plurality of DC voltage error values delta u on one side of the third inflection point in mirror symmetry of 0_dcA fourth inflection point greater than 0.

Will delta u_dcAfter the second inflection point needing to be adjusted in the t curve is adjusted, connecting the third inflection point and the fourth inflection point to obtain a fitting curve l_z. As shown in FIGS. 8 and 9, the VSC DC voltage error Δ u_dcSchematic diagram of motion trajectory when no monotonic divergence occurred in the first 5 swings. In FIGS. 8 and 9, points b 'and d' are Δ u_dc<Point b, d relative straight line delta u at 0_dc0 pairThis is called the dot. Fitting an exponential curve lz by a curve fitting method according to the five points a, b ', c, d' and e, and assuming the function A_me^τtWhere τ is the attenuation coefficient of the fitted curve lz.

Calculating the attenuation coefficient tau of the fitting curve lz according to the fitting curve lz, wherein the embodiment is based on the physical understanding of damping, and delta u in the first 5 swings (including the first 3 forward swings) of the VSC disturbed trajectory_dcAnd estimating the positive and negative damping of the VSC grid-connected system according to the swing amplitude of the t curve relative to the balance point. If τ is negative (as shown in fig. 8), it indicates that the system damping is positive, the disturbed track oscillation converges, and the smaller τ value, the faster the subsequent oscillation of the physically disturbed track will converge; conversely, if τ is positive (as shown in FIG. 9), it indicates that the system damping is negative, the disturbed trajectory will oscillate away, and the greater the value of τ, the faster the physically disturbed trajectory will oscillate away in subsequent oscillations.

When the attenuation coefficient tau is larger than 0, the fact that the grid-connected converter diverges the oscillation is indicated, and the larger the attenuation coefficient tau is, the faster the grid-connected converter diverges the oscillation is, and therefore the instability value of the oscillation divergence of the grid-connected converter can be obtained through the attenuation coefficient tau.

As an improvement of the above scheme, when the attenuation coefficient is not greater than 0, a fourth curve between the unbalanced power and the current at the two ends of the dc capacitor of the grid-connected converter corresponding to the situation is drawn, a virtual no-return point is constructed on the fourth curve to obtain a virtual fifth curve, and a stability margin in which a stable trajectory of the grid-connected converter is not monotonically diverged is calculated according to the fifth curve, specifically including:

Specifically, when the attenuation coefficient τ is not greater than 0, it indicates that the trajectory of the VSC is neither monotonously divergent nor oscillatory divergent, and is in a stable state, and at this time, the stability degree of the stable trajectory of the VSC needs to be measured, and since the stable disturbed trajectory only has FEP and no NRP in each swing, a virtual no-return point needs to be constructed, and the virtual no-return point is denoted as VNRP.

The VNRP may be obtained by switching operations as follows, and a schematic diagram is shown in fig. 10, specifically:

disconnecting k in DC Voltage control Circuit at FEP_pdcBranch circuit, and carry out numerical integration switching, the switching numerical integration is i_di＝k_idc∫Δu_dcdt，Δu_dcIs ═ epsilon, where k_idcThe integral parameter is the integral parameter of the direct current voltage PI controller of the grid-connected converter; epsilon is a given grid-connected converter direct-current voltage small error.

The physical explanation is that it is assumed at FEP of the VSC stable trajectory (at this time, Δ u_dc0) continues to inject a certain electric field energy into the dc capacitor, i_diWill not swing back but continue to increase until P_uStart from negative to positive, i.e. to VNRP (at this point P_u0). The trace from FEP to VNRP is fictitious, passing through fictitious P_u-i_diCurve to calculate potential electric field energy reduction area A_dec.pot(equivalent to the extra injected electric field energy) that physically reflects the transient stability of the dc voltage, which can be used as a stability margin with a stable trajectory that does not diverge monotonically.

Specifically, when the attenuation coefficient τ is not greater than 0, a fourth curve P between the unbalanced power at the two ends of the dc capacitor of the corresponding grid-connected converter and the current under the condition is drawn_u-i_diFinding out a second node (at the moment, delta u) corresponding to the change of the direct-current voltage error along with the time in the fourth curve, wherein the second node is the FEP position of the VSC stable track, and the second node is a certain direct-current voltage error in the curve corresponding to the change of the direct-current voltage error along with the time is equal to 0_dc0), the electric field energy continues to increase at the second node at a time, causing an unbalanced power P across the dc capacitance of the VSC_uContinues to follow the current i_diThe increase is shifted from a negative value to a positive value, thereby obtaining a virtual fifth curve, which is shown in fig. 11.

According to unbalanced power P at two ends of direct current capacitor of grid-connected converter_uA third node equal to 0 obtains a virtual non-return point VNRP and simultaneously obtains the area which can be reduced by the virtual electric field of the grid-connected converter;

As an improvement of the scheme, the calculation formula of the stability margin is

Specifically, the stability margin of the stable locus of the VSC, which is not monotonously dispersed, is represented by gamma, and the calculation formula of gamma is as follows by referring to the calculation process of the instability value eta value of the monotonously dispersed locus

A_dec.potThe area of the virtual electric field energy of the grid-connected converter can be reduced.

Because the gamma value is obtained under the condition that the attenuation coefficient tau is not more than 0, for the VSC stable disturbed track, the quantitative index of the stability margin can be measured by the tau value and the gamma value together, and tau is less than or equal to 0 and gamma is greater than or equal to 0. The stability margin that the stable track of the grid-connected converter is not monotonously dispersed can be obtained through the attenuation coefficient tau and the gamma value, the smaller the attenuation coefficient tau is, the better the stability degree that the grid-connected converter is stable and does not oscillate and disperse, and the larger the gamma value is, the better the stability degree that the grid-connected converter is stable and does not monotonously disperse is.

In conclusion, the invention provides quantitative indexes eta, tau and gamma for measuring transient stability margin of VSC direct-current voltage based on VSC disturbed track, and provides a calculation method thereof in detail. When tau is less than or equal to 0 and gamma is greater than or equal to 0, the transient state of the VSC direct-current voltage is stable, when tau is greater than 0, the VSC direct-current voltage oscillates, diverges and is unstable, when eta is less than 0, the VSC direct-current voltage monotonically diverges and is unstable, and the index has good monotonicity. In summary, the smaller the τ value, the better the stability of the VSC dc voltage that does not oscillate and diverge in the transient process, and the larger the γ value, the better the stability of the VSC dc voltage that does not monotonically diverge in the transient process.

It is worth noting that the present invention is equally applicable to VSCs that employ other control strategies. More broadly speaking, the invention is applicable to any grid-connected converter with a direct current bus capacitor branch circuit on the circuit structure.

Referring to fig. 12, a schematic structural diagram of an apparatus for evaluating transient stability of a grid-connected converter according to embodiment 2 of the present invention is shown, where the apparatus includes:

the curve acquisition module 11 is used for acquiring a first curve of the current of the grid-connected converter changing along with time;

a judging module 12, configured to judge whether the first curve encounters a non-return point;

the monotonous divergence evaluation module 13 is configured to draw a second curve between unbalanced power at two ends of a direct current capacitor of the grid-connected converter and the current if the monotonous divergence evaluation module is used, and calculate a monotonous divergence instability value of the grid-connected converter according to the second curve;

the fitting module 14 is configured to draw a third curve of the direct-current voltage error of the grid-connected converter changing with time if the direct-current voltage error of the grid-connected converter does not change with time, obtain a plurality of first inflection points of the third curve, fit according to the first inflection points to obtain a fitting curve, and calculate an attenuation coefficient of the fitting curve;

the oscillation divergence evaluation module 15 is used for obtaining the instability value of the oscillation divergence of the grid-connected converter as the attenuation coefficient when the attenuation coefficient is larger than 0;

and the stability evaluation and non-divergence module 16 is configured to draw a fourth curve between the unbalanced power and the current at the two ends of the dc capacitor of the grid-connected converter corresponding to the situation when the attenuation coefficient is not greater than 0, construct a virtual non-return point on the fourth curve to obtain a virtual fifth curve, and calculate a stability margin with a non-monotonous divergence of the stability trajectory of the grid-connected converter according to the fifth curve.

Preferably, the curve obtaining module 11 is specifically configured to:

(ii) a Wherein i_diThe active current of the grid-connected converter is the active current of the grid-connected converter; Δ u_dcThe error is the direct-current voltage error of the grid-connected converter; k is a radical of_idcThe integral parameter is the integral parameter of a direct current voltage PI controller of the grid-connected converter; u shape_dcThe direct current voltage of the grid-connected converter is obtained;

Preferably, the judging module 12 specifically includes:

the identification unit is used for identifying a curve segment with a slope larger than 0 in the first curve, wherein the first curve comprises n curve segments with slopes larger than 0, and n is larger than or equal to 1;

the judging unit is used for judging whether the first curve meets a non-return point in a curve segment with the slope larger than 0; the non-return point refers to a first node when the area of the grid-connected converter, from which the direct-current capacitance electric field can be reduced, reaches a maximum value, and the area of the grid-connected converter, from which the direct-current capacitance electric field can be reduced, at the first node is still smaller than the area of the grid-connected converter, from which the direct-current capacitance electric field can be increased.

Preferably, the evaluation monotonic divergence module 13 is specifically configured to:

Preferably, the fitting module 14 specifically includes:

the second drawing unit is used for drawing a third curve of the direct-current voltage error of the grid-connected converter changing along with time if the first curve does not meet a non-return point in a curve segment with the slope larger than 0, so as to obtain a plurality of first inflection points of the third curve;

the auxiliary unit is used for enabling the first inflection point to comprise a second inflection point with a direct-current voltage error value smaller than 0 and a third inflection point with a direct-current voltage error value larger than 0, and enabling the second inflection point to be in mirror symmetry to one side of the third inflection point along a straight line with the direct-current voltage error value of 0 on the third inflection point to obtain a plurality of fourth inflection points with the direct-current voltage error values larger than 0;

the third drawing unit is used for connecting the third inflection point with the fourth inflection point to obtain a fitting curve;

and the first calculation unit is used for calculating the attenuation coefficient of the fitted curve according to the fitted curve.

Preferably, the evaluation stable non-divergence module 16 specifically comprises:

the fourth drawing unit is used for drawing a fourth curve between the unbalanced power at the two ends of the direct-current capacitor of the corresponding grid-connected converter and the current under the condition when the attenuation coefficient is not greater than 0;

a fifth drawing unit, configured to find a second node where a dc voltage error in a curve corresponding to a change of the dc voltage error with time is equal to 0 in the fourth curve, and increase electric field energy at the second node at any time, so that unbalanced power at two ends of a dc capacitor of the grid-connected converter continues to be converted from a negative value to a positive value as the current increases, thereby obtaining a virtual fifth curve;

the virtual construction unit is used for obtaining a virtual non-return point according to a third node with unbalanced power equal to 0 at two ends of a direct current capacitor of the grid-connected converter and obtaining the area of the grid-connected converter with reduced virtual electric field energy;

and the second calculating unit is used for calculating and obtaining the stability margin of the grid-connected converter with the stable track not monotonously diverged according to the fifth curve.

Preferably, the calculation formula of the stability margin is

Referring to fig. 13, the device for evaluating transient stability of a grid-connected converter according to embodiment 3 of the present invention includes a processor 10, a memory 20, and a computer program stored in the memory 20 and configured to be executed by the processor 10, and when the computer program is executed by the processor 10, the method for evaluating transient stability of a grid-connected converter according to any of the above embodiments is implemented.

Illustratively, the computer program may be divided into one or more modules/units, which are stored in the memory 20 and executed by the processor 10 to implement the present invention. One or more of the modules/units may be a series of computer program instruction segments capable of performing specific functions, which are used to describe the execution of a computer program in a process for evaluating transient stability of a grid-connected converter. For example, the computer program may be divided into a curve acquisition module, a judgment module, an evaluation monotonic dispersion module, a fitting module, an evaluation oscillatory dispersion module, and an evaluation stable non-dispersion module, and each module has the following specific functions:

The device for evaluating the transient stability of the grid-connected converter can be a desktop computer, a notebook computer, a palm computer, a cloud server and other computing equipment. The device for evaluating the transient stability of the grid-connected converter can comprise, but is not limited to, a processor and a memory. It will be understood by those skilled in the art that the schematic diagram 13 is merely an example of an apparatus for evaluating transient stability of a grid-connected converter, and does not constitute a limitation of the apparatus for evaluating transient stability of a grid-connected converter, and may include more or less components than those shown in the drawings, or may combine some components, or different components, for example, the apparatus for evaluating transient stability of a grid-connected converter may further include an input/output device, a network access device, a bus, and the like.

The Processor 10 may be a Central Processing Unit (CPU), other general purpose Processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), an off-the-shelf Programmable Gate Array (FPGA) or other Programmable logic device, discrete Gate or transistor logic, discrete hardware components, etc. The general purpose processor may be a microprocessor or the processor 10 may be any conventional processor or the like, the processor 10 being the control center of the apparatus, and various interfaces and lines connecting the various parts of the overall apparatus for evaluating transient stability of a grid-connected converter.

The memory 20 can be used for storing the computer program and/or the module, and the processor 10 implements various functions of the apparatus for evaluating transient stability of the grid-connected converter by running or executing the computer program and/or the module stored in the memory 20 and calling data stored in the memory 20. The memory 20 may mainly include a storage program area and a storage data area, wherein the storage program area may store an operating system, an application program required by at least one function (such as a sound playing function, an image playing function, etc.), and the like; the storage data area may store data (such as audio data, a phonebook, etc.) created according to the use of the cellular phone, and the like. In addition, the memory 20 may include high speed random access memory, and may also include non-volatile memory, such as a hard disk, a memory, a plug-in hard disk, a Smart Media Card (SMC), a Secure Digital (SD) Card, a Flash memory Card (Flash Card), at least one magnetic disk storage device, a Flash memory device, or other volatile solid state storage device.

Wherein, the device integrated module for evaluating the transient stability of the grid-connected converter can be stored in a computer readable storage medium if the module is realized in the form of a software functional unit and sold or used as an independent product. Based on such understanding, all or part of the flow of the method according to the embodiments of the present invention may also be implemented by a computer program, which may be stored in a computer-readable storage medium and can implement the steps of the embodiments of the method when the computer program is executed by a processor. The computer program includes computer program code, and the computer program code may be in a source code form, an object code form, an executable file or some intermediate form. The computer readable medium may include: any entity or device capable of carrying computer program code, recording medium, U.S. disk, removable hard disk, magnetic disk, optical disk, computer Memory, Read-Only Memory (ROM), Random Access Memory (RAM), electrical carrier wave signals, telecommunications signals, software distribution media, and the like. It should be noted that the computer readable medium may contain other components which may be suitably increased or decreased as required by legislation and patent practice in jurisdictions, for example, in some jurisdictions, in accordance with legislation and patent practice, the computer readable medium does not include electrical carrier signals and telecommunications signals.

The embodiment of the present invention further provides a computer-readable storage medium, where the computer-readable storage medium includes a stored computer program, and when the computer program runs, the apparatus where the computer-readable storage medium is located is controlled to execute the method for evaluating transient stability of a grid-connected converter according to any one of the above embodiments.

To sum up, the method, the device and the storage medium for evaluating the transient stability of the grid-connected converter provided by the embodiment of the invention specifically analyze the disturbed track of the grid-connected converter, divide the disturbed track into three conditions of monotonous divergence, oscillation divergence and stability non-divergence, and evaluate the instability degree of the disturbed track of each specific condition by adopting an adaptive parameter index, and provide quantitative indexes eta, tau and gamma for measuring the transient stability margin of the VSC direct current voltage, and provide a calculation method thereof in detail, when tau is less than or equal to 0 and gamma is greater than or equal to 0, the VSC direct current voltage is transient stable, when tau is greater than 0, the VSC direct current voltage oscillates and is unstable, and when eta is less than 0, the VSC direct current voltage is monotonously diverged and unstable. The set of calculation indexes provided by the invention can be used as a scale to quantitatively evaluate the transient stability margin of the VSC direct-current voltage, and the technical defect that the transient stability of the VSC direct-current voltage can be analyzed only by depending on a numerical simulation method in the prior art is greatly overcome. The technical scheme solves the calculation problem that people cannot judge the stability margin of the stable case and cannot know the instability degree of the unstable case, greatly facilitates the evaluation of the influence of certain parameters or certain control on the transient stability of the VSC direct-current voltage by people, provides quantitative evaluation indexes with good monotonicity, is very convenient for the analysis of the influence rule of different factors on the transient stability of the VSC grid-connected system, and can effectively guide the optimization design of the VSC controller.

While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention.

Claims

1. A method for evaluating transient stability of a grid-connected converter is characterized by comprising the following steps:

judging whether the first curve meets a non-return point or not;

2. The method for evaluating transient stability of a grid-connected converter according to claim 1, wherein the obtaining of the first curve of the current of the grid-connected converter over time specifically comprises:

3. The method for evaluating transient stability of a grid-connected converter according to claim 1, wherein the determining whether the first curve encounters a non-return point specifically comprises:

4. The method according to claim 3, wherein if the transient stability of the grid-connected converter is determined, a second curve between the unbalanced power at the two ends of the dc capacitor of the grid-connected converter and the current is plotted, and a monotonically diverging instability value of the grid-connected converter is calculated according to the second curve, specifically comprising:

5. The method for evaluating the transient stability of the grid-connected converter according to claim 3, wherein if not, drawing a third curve of the direct-current voltage error of the grid-connected converter changing with time to obtain a plurality of first inflection points of the third curve, obtaining a fitted curve according to the fitting of the first inflection points, and calculating an attenuation coefficient of the fitted curve specifically comprises:

6. The method according to claim 1, wherein when the attenuation coefficient is not greater than 0, a fourth curve between the unbalanced power and the current at two ends of the dc capacitor of the grid-connected converter corresponding to the situation is drawn, a virtual non-return point is constructed on the fourth curve to obtain a virtual fifth curve, and a stability margin with a stability trajectory not monotonically diverging is calculated according to the fifth curve, and specifically the method includes:

7. The method for evaluating transient stability of grid-connected converter according to claim 6, wherein the calculation formula of the stability margin is

8. An apparatus for evaluating transient stability of a grid-connected converter, comprising:

9. An apparatus for evaluating transient stability of a grid-connected converter, comprising a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein the processor, when executing the computer program, implements the method for evaluating transient stability of a grid-connected converter according to any one of claims 1 to 7.

10. A computer-readable storage medium, comprising a stored computer program, wherein the computer program, when executed, controls an apparatus in which the computer-readable storage medium is located to perform the method for evaluating transient stability of a grid-connected converter according to any one of claims 1 to 7.