Method for constructing safe operation domain of grid-connected converter

Abstract

Provided in the present invention is a method for constructing a safe operation domain of a grid-connected converter. The method comprises: firstly, setting an analysis range and analysis precision; determining the maximum quiescent current transmission limit of a grid-connected converter system; after obtaining a region in which a quiescent current injected into a power grid by a converter satisfies the constraint of an expression, determining whether an operating point is within a domain of definition, determining the stability of the system at the current operating point, and determining whether the operating point exceeds an analysis upper limit; and finally, determining an interval consisting of all the operating points with stable small signals, that is, a safe operating domain. The present invention takes the maximum transmission limit and dynamic characteristics of the system into comprehensive consideration, and proposes the use of a safe operation domain to describe the stability of a full operating interval of a converter. By using a safe operating domain, the transmission limit and dynamic characteristics of a converter can be comprehensively reflected, and the stability of a grid-connected converter in the full operating interval can be visually evaluated, thereby avoiding the limitation of traditional methods only being able to display the system stability at specific operating points.

Description

A method for constructing a safe operation domain of a grid-connected converter Technical Field

[0001] The present invention belongs to the technical field of inverters, and in particular relates to a method for constructing a safe operation domain of a grid-connected converter. Background Art

[0002] As energy demand for daily life and industrial production continues to increase, renewable energy sources such as wind power and photovoltaics are increasingly integrated into power grids. Grid-connected converters, the interface between renewable energy and the grid, have attracted widespread attention for their operational characteristics. Grid-connected converters are typical time-varying systems, with operating points typically shifting constantly. These operating point changes can alter system characteristics, so investigating the dynamic characteristics of all operating points within the full operating range is crucial for guiding converter operation and regulation.

[0003] Existing grid-connected converter stability analysis methods typically linearize the time-varying system into a linear time-invariant model at a specific operating point, then analyze the system's dynamic characteristics near that operating point. State-space methods and impedance / admittance methods are two commonly used approaches. However, these traditional methods can only analyze the system's dynamic characteristics near the selected operating point. When the actual operating point deviates from the selected operating point, significant analysis errors can occur.

[0004] Compared to traditional methods based on a single operating point, methods based on multiple discrete operating points can more accurately describe the dynamic characteristics of time-varying systems. However, the accuracy of this method is often directly related to the number of selected operating points. When there are many operating points, the analysis process must be repeated multiple times, which greatly reduces the efficiency of the analysis. For time-varying systems, each operating moment corresponds to an operating point. As the operating moments extend infinitely, the number of system operating points continues to increase. Therefore, this method cannot traverse all operating moments and, therefore, cannot fully analyze the dynamic characteristics of the system throughout its entire operating cycle.

[0005] To overcome these issues, some studies have incorporated the operating point as a variable in the analysis model, enabling direct analysis of the impact of the time-varying operating point. Representative methods include the probabilistic small-signal method and the three-dimensional Bode plot method. The probabilistic small-signal method studies the probability density function of the system's critical eigenvalues ​​to determine the probabilities of system stability and instability. The probabilistic small-signal method can investigate the long-term stability of a system while accounting for the random characteristics of the operating point and provide guidance for system operation. However, probabilistic methods struggle to provide definitive conclusions about grid-connected system stability. The three-dimensional Bode plot method introduces the operating point variable into the traditional admittance model to establish a binary model related to the operating point. Based on this binary model, a three-dimensional Bode plot is used to characterize the dynamic characteristics of the time-varying system, and amplitude and phase contours are then plotted to determine the system's stability range. Analysis methods based on the three-dimensional Bode plot and amplitude and phase contours can clearly determine the influence of the time-varying operating point on stability and, consequently, identify the system's stability range. However, this method is based on the classical Bode criterion, which is a sufficient condition for system stability, but not a necessary and sufficient condition, which may lead to inaccurate analysis conclusions; in addition, this method is only applicable to the analysis of a single operating point variable. When multiple operating point variables are involved, due to dimensionality limitations, the three-dimensional Bode diagram is difficult to fully characterize the dynamic characteristics of the system, and the complexity of the stability analysis will be greatly increased.

[0006] The safe operating region can intuitively characterize the changes in multiple variables simultaneously, providing a graphical tool for describing the operating limits of a system. Based on the relative position of a particular operating point to the boundary of the feasible region, its stability can be easily determined. Furthermore, a wealth of other information can be simultaneously obtained, such as stability margins along different directions and appropriate adjustment directions for improving stability. However, existing research has focused on studying the feasible region of a system when adjustable parameters (control parameters, circuit parameters, etc.) vary, with limited research on the feasible region of time-varying operating points. The feasible region of adjustable parameters can guide system design and tuning. During system operation, adjustable parameters often remain unchanged; however, the operating point is affected by external factors such as renewable energy output and load switching, and exhibits random time-varying characteristics. Therefore, an analytical method is urgently needed that can comprehensively describe the dynamic characteristics of a system under time-varying operating points during the system's invisible process.

[0007] Summary of the Invention

[0008] To solve the above problems, the purpose of the embodiments of the present invention is to provide a method for constructing a safe operating domain of a grid-connected converter, so as to solve the problem that the existing technology is easily affected by external factors such as new energy output and load switching, and is difficult to analyze due to the random time-varying characteristics of the system operation, and to achieve an intuitive representation of the system stability in a multi-dimensional working range.

[0009] To solve the above technical problems, the technical solution adopted by the present invention is a method for constructing a safe operation domain of a grid-connected converter, comprising the following steps:

[0010] S1. Determine the method for representing the working range of the grid-connected converter;

[0011] S2. Determine the domain of the grid-connected converter;

[0012] S3. Set the analysis range and analysis accuracy;

[0013] S4, determine whether the working point is within the definition domain;

[0014] S5. Determine the stability of the grid-connected converter system at the current operating point;

[0015] S6. Determine whether the upper limit of analysis is exceeded;

[0016] S7. Determine the safe operating region of the grid-connected converter.

[0017] Furthermore, the S1 is specifically as follows: determining the grid-connected converter system operating point:

[0018] Λ is the grid-connected converter system operating point, λ l is the lth operating point variable, Represents l-dimensional space; the l-dimensional infinite interval composed of all working points is the working interval Ω of the grid-connected converter Λ .

[0019] Furthermore, the S2 is specifically as follows:

[0020] In steady state, due to the directional effect of the phase-locked loop, the q-axis component of the PCC voltage U q0 is 0, that is: U0=U d0 +jU q0 =U d0 , U q0 =0 (2)

[0021] Among them, U0 is the steady-state operating point of the output voltage vector, U d0 is the active component of U0, U q0 is the reactive component of U0, j is the imaginary unit;

[0022] The grid voltage amplitude is constant:

[0023] Among them, U g0 is the steady-state operating point of the grid voltage vector, U gd0 For U g0 Active component, U gq0 For U g0The reactive component of |·| represents the modulus of the corresponding physical quantity, U g is the steady-state grid voltage;

[0024] For the filter inductance and line impedance, we have:

[0025] U g0 is the steady-state operating point of the grid voltage vector, I0 is the steady-state operating point of the output current vector, and Y f (0) is the filter admittance at 0 Hz, E0 is the steady-state operating point of the potential vector in the converter, Z g (0) is the grid impedance at 0 Hz;

[0026] The q-axis component of the PCC voltage in steady state is shown in equation (5): q0 =U gq0 +I q0 Z gd (0)+I d0 Z gq (0) (5)

[0027] Among them, I d0 is the active component of I0, I q0 is the reactive component of I0, Z gd (0) is the d-axis component of the grid impedance at 0 Hz, Z gq (0) is the q-axis component of the grid impedance at 0 Hz; U g0 The reactive component U gq0 Satisfaction|U gq0 |≤U g For extremely weak grids or serious grid fault conditions, the PLL may not be able to control the q-axis voltage to zero, so the system has no steady-state operating point. The maximum static current transfer limit of the grid-connected converter is shown in formula (6): |I q0 Z gd (0)+I d0 Z gq (0)|≤U g (6)

[0028] For a given line impedance and grid voltage amplitude, the converter injects a limited current into the grid. For full active current injection, the maximum static current injected by the converter into the grid is the ratio of the grid voltage amplitude to the line reactance. During severe grid faults where full reactive current is required, the maximum static current is limited to the ratio of the grid voltage amplitude to the line resistance. This ultimately determines the domain of the grid-connected converter, Ω. def The region where the static current injected by the converter into the grid satisfies the constraint of formula (6).

[0029] Furthermore, the S3 is specifically as follows: select the initial working point Λ1, analysis accuracy ΔΛ and analysis upper limit Λ in the definition domain m , where: Λ1={λ 1(1) ,λ 2(1) ,...,λ l(1)} ΔΛ={Δλ1,Δλ2,...,Δλ l} Λ m ={λ 1(m1) ,λ 2(m1) ,...,λ l(m1)} (7)

[0030] Among them, λ l(1) represents the lth initial operating point variable, Δλ l represents the lth analysis precision variable, λ l(m1) represents the lth analysis upper limit variable;

[0031] Determine the total number of working points M included in the analysis range:

[0032] Where i is the count value.

[0033] Furthermore, the S4 is specifically: when the working point Λ=(I d0 ,I q0 ) satisfies |I q0 Z gd (0)+I d0 Z gq (0)|≤U g When , it is determined that the working point is within the definition domain; U g is the steady-state grid voltage; I d0 is the active component of I0, I q0 is the reactive component of I0, Z gd (0) is the d-axis component of the grid impedance at 0 Hz, Z gq (0) is the q-axis component of the grid impedance at 0 Hz.

[0034] Furthermore, the S5 is specifically as follows: when the current working point of the grid-connected converter is at Λ k When , the converter linearization is:

[0035] Where m is the transfer function T(s,Λ k ) molecular order; n is the transfer function T(s,Λ k ) denominator order; s is the differential operator; Y(·) is the frequency domain form of the system output at the corresponding operating point; R(·) is the frequency domain form of the system input at the corresponding operating point; K(·) is the transfer function T(s,Λ k )’s gain;i (·) is the transfer function T(s,Λ k )’s i-th zero point; p i () is the transfer function T(s,Λ k )’s i-th extreme point;

[0036] T(s,Λ k The set of all extreme points of ) is P, If any p i ∈P has a negative real part, then the system is at the operating point Λ k The small signal is stable.

[0037] Furthermore, the S6 is specifically as follows:

[0038] Jump to the next working point Λ k+1 , to determine whether the working point exceeds the analysis upper limit Λ m If k≥M, continue to determine the safe operating region of the grid-connected converter; if k<M, return to S3.

[0039] Furthermore, in the converter domain Ω def The interval consisting of all small signal stable operating points is called the safe operating region Ω ope :

[0040] Furthermore, the grid-connected converter system includes a current controller, an AC voltage controller, a DC voltage controller, an active power controller, a reactive power controller, and a phase-locked loop.

[0041] Furthermore, the current controller adopts vector current control in the dq coordinate system; the active and reactive currents are jointly controlled by the AC voltage controller, DC voltage controller, active power controller, and reactive power controller; and the phase-locked loop is used to maintain synchronization between the converter and the grid.

[0042] The present invention provides the following beneficial effects: It comprehensively considers the system's maximum transmission limits and dynamic characteristics, and proposes the use of a safe operating region to describe the converter's stability over its entire operating range. This safe operating region comprehensively reflects the converter's transmission limits and dynamic characteristics, allowing for intuitive assessment of the converter's stability over its entire operating range. This avoids the limitation of traditional methods, which only display system stability at specific operating points. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0044] Figure 1 is a block diagram of a three-phase grid-connected converter system, (a) is a structural diagram of the main circuit and control system, and (b) is a structural diagram of the phase-locked loop;

[0045] FIG2 is a diagram showing the maximum static current transfer limit of a grid-connected converter;

[0046] Figure 3 is a schematic diagram of a safe operation domain;

[0047] Figure 4 is a general flow chart of the construction of a secure operation domain;

[0048] FIG5 is a diagram of the safe operating domain of a grid-connected converter, where (a) is the safe operating domain of the present invention and (b) is the selection of observation points;

[0049] Figure 6 shows the simulation results when the operating point changes from A→C→E. (a) is the output current waveform, (b) is the current trajectory when the operating point changes from A→C, and (c) is the current trajectory when the operating point changes from C→E.

[0050] Figure 7 shows the simulation results when the operating point changes from B→C→D. (a) is the output current waveform, (b) is the current trajectory when the operating point jumps from B→C, and (c) is the current trajectory when the operating point jumps from C→D. DETAILED DESCRIPTION

[0051] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0052] This paper comprehensively considers the maximum transmission limit and dynamic characteristics of the converter and proposes a method for constructing a safe operating domain of the grid-connected converter to achieve an intuitive representation of the system stability in the multi-dimensional working range. The actual phase angle of the PCC voltage is θ r During dynamic regulation, since the PLL cannot track the input voltage in real time, θ≠θ r In order to distinguish it from θ r The corresponding actual dq coordinate system is denoted as dq c . Accordingly, dq is marked with a superscript c. c Similarly, the physical quantities in the two-phase stationary coordinate system αβ are marked with the superscript s. For example, the αβ coordinate system, the dq coordinate system and the dq c The PCC voltage vectors in the coordinate system are denoted as u s , u and u c , and there are

[0053] u α 、u β u s The α-axis component and β-axis component of u d 、u q are the d-axis component and q-axis component of u respectively; u c The dc-axis component and qc-axis component of ; j is the imaginary unit.

[0054] Figure 1 is a simplified single-line diagram of a three-phase grid-connected converter. The DC link capacitor is large enough to maintain the DC voltage U dc constant; the filter inductor L is used on the AC side to filter out the switching ripple. The current controller (CC) adopts vector current control in the dq coordinate system. According to different control objectives, the active and reactive current references can be provided by the DC voltage controller (DVC), AC voltage controller (AVC), active power controller (APC) and reactive power controller (RPC), as shown in Figure 1(a); the phase-locked loop (PLL) is used to keep the converter synchronized with the grid, and its schematic diagram is shown in Figure 1(b). In the figure, θ is the phase angle of the PLL orientation; ω1 is the rated angular velocity; F p (s) is the PI regulator, F p (s) = K pp +K ip The CC, PLL, and power circuit constitute the minimum system that enables the converter to operate in a controllable manner. The present invention uses the system shown in FIG1 as an example for subsequent analysis.

[0055] Example 1 Construction of Safe Operating Region of Grid-Connected Converter

[0056] S1. Determine the working range representation method

[0057] The system operating point can be determined by the l-dimensional real vector Λ

[0058] λ l is the lth operating point variable, Represents l-dimensional space.

[0059] The l-dimensional infinite interval composed of all working points is called the working interval Ω of the grid-connected converter Λ For example, the present invention uses a two-dimensional vector Λ=[I d0,I q0 ]Describe the working range of the grid-connected converter Ω Λ .

[0060] S2. Determine the domain of the grid-connected converter

[0061] A prerequisite for small-signal stability analysis is the existence of a steady-state operating point. When the converter's output quiescent current exceeds a certain limit, the system no longer has a steady-state operating point. Regardless of the control strategy employed or the control parameters adjusted, the converter cannot be stabilized. Therefore, when conducting stability analysis on a grid-connected converter, the maximum quiescent current transfer limit should be determined.

[0062] In steady state, due to the directional effect of the phase-locked loop, the q-axis component of the PCC voltage U q0 is 0, that is: U0=U d0 +jU q0 =U d0 ,U q0 =0

[0063] U0 is the steady-state operating point of the output voltage vector, U d0 is the active component of U0, U q0 is the reactive component of U0, and j is an imaginary unit.

[0064] At the same time, it is assumed that the grid voltage amplitude is constant, that is:

[0065] U g0 is the steady-state operating point of the grid voltage vector, U gd0 For U g0 Active component, U gq0 For U g0 The reactive component of |·| represents the modulus of the corresponding physical quantity.

[0066] Among them, U g is the steady-state grid voltage,

[0067] In addition, for the filter inductance and line impedance, there is

[0068] U g0 is the steady-state operating point of the grid voltage vector, I0 is the steady-state operating point of the output current vector, and Y f (0) is the filter admittance at 0 Hz, and E0 is the steady-state operating point of the potential vector in the converter.

[0069] The q-axis component of the PCC voltage in steady state can be expressed as U q0 =U gq0 +I q0 Z gd (0)+I d0 Zgq (0)

[0070] I q0 is the active component of I0, I q0 is the reactive component of I0, Z gd (0) is the d-axis component of the grid impedance at 0 Hz, Z gq (0) is the q-axis component of the grid impedance at 0 Hz.

[0071] Steady-state grid voltage q-axis component U gq0 Satisfaction|U gq0 |≤U g

[0072] From the above formula, we can get that for extremely weak power grid (Z g (0) extremely) or in a serious fault in the power grid (U g0 Under extremely small working conditions, the PLL may not be able to control the q-axis voltage to zero, so the system has no steady-state operating point; the maximum static current transfer limit of the grid-connected converter can be expressed as |I q0 Z gd (0)+I d0 Z gq (0)|≤U g

[0073] From the above equation, it can be seen that for a given line impedance and grid voltage amplitude, the converter can only inject a limited current into the grid. For full active current injection, the maximum static current injected by the converter into the grid is the ratio of the grid voltage amplitude to the line reactance; during severe grid faults where full reactive current is required, the maximum static current is limited to the ratio of the grid voltage amplitude to the line resistance. As shown in Figure 2, the shaded area (Ω def ) shows the area where the static current injected by the converter into the grid satisfies the constraint, which is the definition domain of the grid-connected converter.

[0074] S3. Set the analysis range and analysis accuracy

[0075] Select the initial working point Λ1, analysis accuracy ΔΛ and analysis upper limit Λ within the definition domain m . Where Λ1={λ 1(1) ,λ 2(1) ,...,λ l(1)} ΔΛ={Δλ1,Δλ2,...,Δλ l} Λ m ={λ 1(m1) ,λ 2(m1) ,...,λ l(m1)}

[0076] From this, we can determine the total number of working points M included in the analysis range is

[0077] i is the count value.

[0078] S4. Determine whether the working point is within the definition domain

[0079] When the working point (I d0 ,I q0 ) satisfies |I q0 Z gd (0)+I d0 Z gq (0)|≤U g , it can be determined that the operating point is within the domain of definition; otherwise, the operating point is not within the domain of definition.

[0080] U g is the steady-state grid voltage; I d0 is the active component of I0, I q0 is the reactive component of I0, Z gd (0) is the d-axis component of the grid impedance at 0 Hz, Z gq (0) is the q-axis component of the grid impedance at 0 Hz.

[0081] S5. Determine the stability of the system at the current working point

[0082] When the current operating point of the grid-connected converter is at Λ k When , the converter can be linearized as:

[0083] m is the transfer function T(s,Λ k ) molecular order; n is the transfer function T(s,Λ k ) denominator order; s is the differential operator; Y(·) is the frequency domain form of the system output at the corresponding operating point; R(·) is the frequency domain form of the system input at the corresponding operating point; K(·) is the transfer function T(s,Λ k )’s gain; i (·) is the transfer function T(s,Λ k )’s i-th zero point; p i () is the transfer function T(s,Λ k )’s ith extreme point.

[0084] T(s,Λ k The set of all extreme points of ) is P, If any p i ∈P has a negative real part, then the system is at the operating point Λ k The small signal is stable.

[0085] S6. Determine whether the upper limit of analysis is exceeded

[0086] Jump to the next working point Λ k+1, to determine whether the working point exceeds the analysis upper limit Λ m If k ≥ M, continue to determine the safe operating region of the grid-connected converter; if k < M, return to step 3 and reanalyze.

[0087] S7. Determine the safe operating area of ​​the grid-connected converter

[0088] In the converter domain Ω def The interval consisting of all small signal stable operating points is called the safe operating region Ω ope : Working range of grid-connected converter Ω Λ , domain Ω def and safe operating region Ω ope The relationship is shown in Figure 3. The flow chart of Example 1 of the present invention is shown in Figure 4.

[0089] Example 2 Effectiveness Analysis

[0090] To better demonstrate the effectiveness of the stability analysis method and stability region characterization described in this invention, a simulation example is used below to verify the effectiveness. According to the topology shown in Figure 1, a simulation is built in MATLAB / Simulink, and the simulation parameters are shown in Table 1.

[0091] Table 1 Main circuit parameters

[0092] The safe working area of ​​the three-phase grid-connected converter system of this application is drawn as shown in Figure 5(a). In order to observe the dynamic characteristics of the system and the influence of system parameters on the safe area when the working point changes, as shown in Figure 5(b), in the working range {I d0 ,I q0}, five points A to E are selected as observation points; the coordinate values ​​of the five points are: A={155,90} B={160,75} C={160,90} D={160,105} E={165,90}

[0093] Figure 6 shows the simulation results when the operating point changes from A to C to E. Figure 6(a) shows the output current waveform, and Figures 6(b) and (c) show the output current trajectory within the operating range. As can be seen from the figures, when the operating point changes from A to C, the current trajectory converges to point C, and the system is stable. However, when the operating point changes from C to E, the current trajectory diverges, and the system becomes unstable.

[0094] Figure 7 shows the simulation results when the operating point changes from B to C to D. Figure 7(a) shows the output current waveform, and Figures 7(b) and (c) show the output current trajectory within the operating range. As can be seen from the figures, when the operating point changes from B to C, the current trajectory converges to point C, and the system is stable. However, when the operating point changes from C to D, the current trajectory diverges, and the system becomes unstable.

[0095] This invention comprehensively considers the system's maximum transmission limits and dynamic characteristics, proposing the use of a safe operating region to describe the converter's stability over its entire operating range. This safe operating region comprehensively reflects the converter's transmission limits and dynamic characteristics, allowing for a direct assessment of the converter's stability over its entire operating range. This avoids the limitation of traditional methods, which only display system stability at specific operating points.

[0096] Each embodiment in this specification is described in a related manner. Similar parts between the various embodiments can be referred to in conjunction with each other. Each embodiment focuses on the differences between the other embodiments. In particular, the system embodiment is generally similar to the method embodiment, so the description is relatively simple. For related parts, refer to the description of the method embodiment.

[0097] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are included in the scope of protection of the present invention.

Claims

1. A method for constructing a safe operation domain of a grid-connected converter, characterized in that: The following steps are involved: S1. Determine the method for representing the working range of the grid-connected converter; S2. Determine the domain of the grid-connected converter; S3. Set the analysis range and analysis accuracy; S4, determine whether the working point is within the definition domain; S5, judging the stability of the grid-connected converter system at the current working point; S6, judging whether the upper limit of analysis is exceeded; S7. Determine the safe operating region of the grid-connected converter.

2. The method for constructing a safe operation domain of a grid-connected converter according to claim 1, characterized in that: The S1 is specifically as follows: Determine the grid-connected converter system operating point: Λ is the grid-connected converter system operating point, λ l is the lth working point variable, Represents l-dimensional space; the l-dimensional infinite interval composed of all working points is the working interval Ω of the grid-connected converter Λ .

3. The method for constructing a safe operation domain of a grid-connected converter according to claim 1, characterized in that: The S2 is specifically as follows: In steady state, due to the directional action of the phase-locked loop, the q-axis component of the PCC voltage U q0 is 0, that is: U0=U d0 +jU q0 =U d0 , U q0 =0 (2) Among them, U0 is the steady-state operating point of the output voltage vector, U d0 is the active component of U0, U q0 is the reactive component of U0, j is an imaginary unit; The grid voltage amplitude is constant: Among them, U g0 is the steady-state operating point of the grid voltage vector, U gd0 For U g0 The active component, U gq0 For U g0 The reactive component of |·| represents the modulus of the corresponding physical quantity, U g is the steady-state grid voltage; For the filter inductor and line impedance, we have: U g0 is the steady-state operating point of the grid voltage vector, I0 is the steady-state operating point of the output current vector, Y f (0) is 0Hz The filter admittance is E0, the steady-state operating point of the potential vector in the converter, and Z g (0) is the grid impedance at 0 Hz; The q-axis component of the PCC voltage in steady state is shown in equation (5): U q0 =U gq0 +I q0 Z gd (0)+I d0 Z gq (0) (5) Among them, I d0 is the active component of I0, I q0 is the reactive component of I0, Z gd (0) is the d-axis component of the grid impedance at 0 Hz, Z gq (0) is the q-axis component of the grid impedance at 0 Hz; U g0 The reactive component U gq0 Satisfaction | gq0 |≤U g ; For extremely weak grids or in severe grid fault conditions, the PLL may not be able to control the q-axis voltage to zero, so that the system has no steady-state operating point; The maximum static current transmission limit of the grid-connected converter is shown in formula (6): |I q0 Z gd (0)+I d0 Z gq (0)|≤U g (6) For a given line impedance and grid voltage amplitude, the converter injects a limited current into the grid. For full active current injection, the maximum static current injected by the converter into the grid is the ratio of the grid voltage amplitude to the line reactance. During severe grid faults where full reactive current is required, the maximum static current is limited to the ratio of the grid voltage amplitude to the line resistance. The domain of the grid-connected converter is finally determined as Ω. def The static current injected by the converter into the grid satisfies the constraint of equation (6).

4. The method for constructing a safe operation domain of a grid-connected converter according to claim 1, characterized in that: The S3 is as follows: select the initial working point Λ1, the analysis accuracy ΔΛ and the analysis upper limit Λ in the definition domain m , where: Λ1={λ 1(1) ,λ 2(1) ,...,λ l(1) } ΔΛ={Δλ1,Δλ2,...,Δλ l } m ={λ 1(m1) ,λ 2(m1) ,...,λ l(m1) } (7) Among them, λ l(1) represents the lth initial operating point variable, Δλ l represents the lth analytical precision variable, λ l(m1) represents the lth analysis upper limit variable; Determine the total number of working points M included in the analysis range: Wherein, i is the count value.

5. The method for constructing a safe operation domain of a grid-connected converter according to claim 1, characterized in that: The S4 is specifically: when the working point Λ=(I d0 ,I q0 ) satisfies |I q0 Z gd (0)+I d0 Z gq (0)|≤U g When , it is determined that the working point is within the definition domain; U g is the steady-state grid voltage; I d0 is the active component of I0, I q0 is the reactive component of I0, Z gd (0) is the d-axis component of the grid impedance at 0 Hz, Z gq (0) is the q-axis component of the grid impedance at 0 Hz.

6. The method for constructing a safe operation domain of a grid-connected converter according to claim 1, characterized in that: The S5 is specifically as follows: when the current working point of the grid-connected converter is at Λ k When , the converter linearization is: Where m is the transfer function T(s,Λ k ) molecular order; n is the transfer function T(s,Λ k ) is the denominator order; s is the differential operator; Y(·) is the frequency domain form of the system output at the corresponding operating point; R(·) is the frequency domain form of the system input at the corresponding operating point; K(·) is the transfer function T(s,Λ k )’s gain; i (·) is the transfer function T(s,Λ k )’s i-th zero point; p i () is the transfer function T(s,Λ k )’s ith extreme point; T(s,Λ k The set of all the poles of ) is P, If any p i ∈P has a negative real part, then the system is at the operating point Λ k The small signal is stable.

7. The method for constructing a safe operation domain of a grid-connected converter according to claim 1, characterized in that: The S6 is specifically as follows: Jump to next working point Λ k+1 , determine whether the working point exceeds the analysis upper limit Λ m , if k ≥ M, continue to determine the safe working region of the grid-connected converter; If k<M, return to S3.

8. The method for constructing a safe operation domain of a grid-connected converter according to claim 1 or 6, characterized in that: In the converter domain Ω def The interval consisting of all small signal stable working points is called the safe working region Ω ope :

9. The method for constructing a safe operation domain of a grid-connected converter according to claims 1 to 8, characterized in that: The grid-connected converter system comprises a current controller, an AC voltage controller, a DC voltage controller, an active power controller, a reactive power controller and a phase-locked loop.

10. The method for constructing a safe operation domain of a grid-connected converter according to claim 9, characterized in that: The current controller adopts vector current control in the dq coordinate system; the active and reactive currents are jointly controlled by the AC voltage controller, the DC voltage controller, the active power controller, and the reactive power controller; the phase-locked loop is used to keep the converter synchronized with the power grid.

Images