Method, system and computer readable storage medium for accurate quantification of multi-inverter transient stability

Abstract

The application discloses a kind of based on quasi-steady assumption's follow network-grid type inverter parallel grid system transient stability accurate quantification method, system and computer readable storage medium, comprising the following steps, reveal the relative power angle between follow network inverter and grid inverter in transient state is in a quasi-steady mode;Propose a kind of parallel grid system decoupling method based on the foregoing quasi-steady discovery, the four-order coupling model of original follow network-grid inverter parallel grid system is decomposed into two two-order sub-models;For the above decoupling model, an iterative algorithm is proposed to fully consider the influence of dynamic interaction and damping term between inverters, to realize the accurate quantification of the transient stability boundary of follow network-grid inverter parallel grid system. The method reveals the quasi-steady mode of the relative power angle between follow network inverter and grid inverter, and constructs the approximate mapping relationship between the operating power angle of heterogeneous grid-connected inverter in transient state, thereby realizing the decoupling and order reduction of the model.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics, and particularly relates to a method and system for accurately quantifying transient stability of a grid-following-grid-forming inverter parallel grid-connected system based on a quasi-steady-state assumption, and a computer readable storage medium. BACKGROUND

[0002] With the continuous increase of renewable energy generation grid-connected installed capacity, a single type of grid-connected inverter cannot simultaneously meet the requirements of transient support and fast response. Therefore, a microgrid composed of grid-following inverter and grid-forming inverter in parallel is an important part of a double high power system. How to accurately obtain the transient stability boundary of the grid-following-grid-forming inverter parallel grid-connected system under the complex dynamic interaction of the heterogeneous grid-connected inverters is still an academic blank. SUMMARY

[0003] In order to overcome the above-mentioned defects of the prior art, the application provides a method and system for accurately quantifying transient stability of a grid-following-grid-forming inverter parallel grid-connected system based on a quasi-steady-state assumption, and a computer readable storage medium.

[0004] The technical scheme of the application is as follows:

[0005] The application provides a method for accurately quantifying transient stability of a grid-following-grid-forming inverter parallel grid-connected system based on a quasi-steady-state assumption, which comprises the following steps:

[0006] It is disclosed that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode under transient state, and it is assumed that the relative dω 21 / dt is zero, and ω 21 is the difference between the frequency of the grid-forming inverter and the frequency of the grid-following inverter.

[0007] Under this assumption, an iterative algorithm is constructed to decompose the original four-order coupled model of the grid-following-grid-forming inverter parallel grid-connected system into two two-order sub-models.

[0008] The dynamic interaction and damping term influence between the inverters are fully considered to realize accurate quantification of the transient stability boundary of the grid-following-grid-forming inverter parallel grid-connected system.

[0009] Further, it is disclosed that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode under transient state, and the steps comprise: the four-order state space equation of the grid-following-grid-forming inverter parallel grid-connected system is:

[0010]

[0011] where state variables δ1, δ2, ω1, ω2 represent the phase and frequency of the grid- following inverter output current and the phase and frequency of the grid-forming inverter output voltage, respectively; the difference between δ2 and δ1 is defined as the system relative power angle δ 21 , and the difference between ω2 and ω1 is defined as the system relative frequency ω 21 ; P m1 , P E1 , P int1 , D1, D int1 and J1 represent the equivalent mechanical power, the equivalent maximum electromagnetic power, the equivalent interaction power, the equivalent self-damping coefficient, the equivalent interaction damping coefficient and the equivalent inertia of the grid-following inverter, respectively, and their expressions are shown in equation (2); P m2 , P E2 , P int2 , D2 and J2 represent the equivalent mechanical power, the equivalent maximum electromagnetic power, the equivalent interaction power, the equivalent self-damping coefficient and the equivalent inertia of the grid-forming inverter, respectively, and their expressions are shown in equation (2); constants θ1-θ5 are coefficients related to the grid impedance parameters, and their expressions are shown in equation (4); is the power factor angle of the grid-following inverter;

[0012]

[0013] where I ref , K i and K p are the reference current amplitude, the phase-locked loop integral coefficient and the phase-locked loop proportional coefficient of the grid-following inverter, respectively; where P ref , D p , E and J are the reference active power, the virtual damping, the output voltage amplitude and the virtual inertia of the grid-forming inverter, respectively; where Vg is the grid phase voltage amplitude; where ω n is the system nominal frequency, constants a1-a5 are coefficients related to the grid impedance, and constant C q is a coefficient related to the line impedance and the grid-following inverter current reference value I ref and the power factor angle , and its expression is shown in equation (3);

[0014]

[0015] where Y GFM and Y GFL are the line admittance from the grid-connection point to the common coupling point for the grid-forming inverter and the grid-following inverter, respectively, and Y g is the line admittance from the common coupling point to the infinite grid;

[0016] Numerical simulation calculation of equation (1) can find that the derivative of the system relative frequency with respect to time dω 21 / dt converges rapidly to near zero;

[0017] The values ​​of δ1 and δ2 reach their farthest points around 0.18s-0.25s, while dω 21 The value of / dt converges to a relatively small range within 0.1s. This indicates that the grid-connected inverter and the grid-linked inverter capture each other and move together with the grid in quasi-steady-state (QSS) mode, interacting with the grid. Because the timescale of the grid-connected inverter is slower, the dynamics of the grid-linked inverter are stretched by the interaction with the grid-connected inverter. Under the QSS assumption, assuming relative dω... 21 / dt is zero:

[0018]

[0019] Furthermore, an iterative algorithm is constructed to decompose the original fourth-order coupling model of the grid-connected inverter-grid-connected inverter parallel system into two second-order sub-models. The steps include:

[0020] By solving equation (4), we can derive the mapping function that determines the relationship between δ1 and δ2: δ 2QSS (δ1)andδ 1QSS (δ2), they are inverse functions of each other:

[0021]

[0022]

[0023] The coefficients A1-A4 and B1-B4 are given by equations (5) and (6), respectively:

[0024]

[0025] like Figure 6 As shown, by observing the QSS error δ 2QSS -δ2 and δ 1QSS The time-domain variation of -δ1 shows that the error converges rapidly to zero within 0.05s, verifying the high accuracy of the QSS hypothesis. Therefore, the quantitative analysis results based on the QSS hypothesis are reliable.

[0026] Using the QSS assumptions in (3)-(4), the original coupled fourth-order system is decomposed into two decoupled second-order systems:

[0027]

[0028] Taking full account of the dynamic interaction and damping effects between inverters, the precise quantification of the transient stability boundary of the grid-connected inverter-grid-connected inverter parallel system is achieved through the following steps:

[0029] (1) Determine whether QSS decoupling will occur

[0030] For and QSS equations (5) and (6) may not always have real solutions, where δ 1SEP ,δ 2SEP ,δ 1UEP ,δ 2UEP are stable and unstable equilibrium points of the original fourth-order state-space equation (11), respectively. This leads to the possibility that the QSS relationship may be broken when the operating point enters the non-real solution region, which in turn leads to LOS. The conditions for equations (5) and (6) to have real solutions are:

[0031]

[0032] As long as formula (10) is always satisfied, the system will not have QSS decoupling phenomenon

[0033] (2) If QSS decoupling does not occur, calculate the transient stability power angle-frequency boundary of the grid-connected inverter-grid-connected inverter parallel grid-connected system with the unstable equilibrium point as the upper boundary of the transient stability power angle:

[0034] Only when the system does not have QSS decoupling phenomenon, the stability of the grid-connected inverter and the grid-connected inverter is twin entangled. Therefore, the present invention only derives the boundary of the grid-connected inverter.

[0035] The definite integral of δ2 in (8) from x 2b to x 2a can be obtained as:

[0036]

[0037] Where ω 2b and ω 2a represent the frequency corresponding to x 2b and x 2a . Equation (10) is the law of conservation of energy of the grid-connected inverter under the QSS assumption. Considering that the critical condition for transient stability of the grid-connected inverter is that the frequency ω2 just slows down to zero at δ 2UEP , substituting x 2b = δ 2UEP , ω 2b = 0 into equation (12), the stable boundary of the grid-connected inverter can be derived as:

[0038]

[0039] Formula (13) is a frequency-power angle mapping function under critical stability condition.The positive sign in formula (13) represents a right swing process, and the negative sign represents a left swing process.Formula (13) is an implicit function equation about ω2, and an iterative algorithm shown in formula (13) can be used for solving:

[0040]

[0041] In the formula is a frequency distribution function of the jth iteration

[0042] (3) If QSS decoupling occurs, the QSS decoupling point is taken as the upper boundary of the transient stable power angle to iteratively calculate the transient stable power angle-frequency boundary of the parallel grid-connected system of the grid-following inverter and the grid-forming inverter.

[0043] The minimum power angle values that do not satisfy formula (11) but are in the interval [δ 1SEP ,δ 1UEP ] and [δ 2SEP ,δ 2UEP ] are defined as the QSS decoupling angles δ 1QSSB and δ 2QSSB , respectively.

[0044]

[0045] Therefore, when δ1∈[δ 1QSSB ,δ 1UEP ], the QSS state is no longer maintained, and the transient stability boundary needs to be reconsidered. The upper power angle boundaries of the grid-following and grid-forming grid-connected inverters should be redefined as δ 1QSSB and δ 2QSSB . Therefore, the stability boundary of the grid-forming grid-connected inverter is modified as:

[0046]

[0047] Similar to formula (13), formula (16) can also be iteratively solved:

[0048]

[0049] Since formula (17) excludes the QSS decoupling region, the stability of the grid-following and grid-forming grid-connected inverters is still twin entangled, and the stability boundary of the grid-following grid-connected inverter does not need to be derived again.

[0050] The second aspect of the present application proposes an accurate quantification system for transient stability of a grid-following-grid-forming type inverter parallel grid-connected system based on quasi-steady state assumption, comprising:

[0051] A mode revealing module is configured to reveal that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady state mode under transient state.

[0052] Model decomposition module: for constructing an iterative algorithm, the original four-order coupled model of grid-connected inverter-grid forming inverter parallel grid system is decomposed into two two-order sub-models;

[0053] Precise quantification module: fully considering the dynamic interaction and damping term influence between inverters, the transient stability boundary of grid-connected inverter-grid forming inverter parallel grid system is accurately quantified.

[0054] The third aspect of the application provides a computer readable storage medium, the computer readable storage medium stores a computer program, and the computer program is executed by a processor to realize the steps of the method according to any one of claims 1 to 4.

[0055] Technical effects and advantages of the application: by revealing the quasi-steady state mode of the relative power angle between the grid-connected inverter and the grid forming inverter, an approximate mapping relationship between the operating power angle of the heterogeneous grid-connected inverter in the transient state is constructed, thereby realizing the decoupling and reduction of the model. The method breaks through the shortcomings of the existing method that is not applicable to high-order systems, is conservative and cannot accurately estimate the transient boundary, and provides a theoretical basis for online transient stability monitoring of the grid-connected inverter-grid forming inverter parallel grid system. The method has good development potential and popularization space. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 It is a structure diagram of grid-connected inverter-grid forming inverter parallel system;

[0057] Figure 2 It is a time domain simulation test schematic diagram for a four-order model;

[0058] Figure 3 It is a time domain diagram of QSS assumption error;

[0059] Figure 4 It is a system decoupling schematic diagram based on QSS;

[0060] Figure 5 It is a flow chart of a transient stability accurate quantification method for grid-connected-grid forming inverter parallel grid system based on quasi-steady state assumption;

[0061] Figure 6 It is a time domain simulation diagram. DETAILED DESCRIPTION

[0062] The technical solutions in the embodiments of the application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the application, not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0063] Embodiment one

[0064] The purpose of the present application is to realize the accurate quantification of the transient stability boundary of the grid-following inverter-grid-forming inverter parallel grid-connected system. By revealing the quasi-steady state mode of the relative power angle between the grid-following inverter and the grid-forming inverter, an approximate mapping relationship between the operating power angle of the heterogeneous grid-connected inverter in transient state is constructed, thereby realizing the decoupling and reduction of the model, and realizing the accurate quantification of the transient stability boundary of the grid-following inverter-grid-forming inverter parallel grid-connected system. In order to achieve the above purpose, the technical scheme adopted by the present application is: a grid-following-grid-forming inverter parallel grid-connected system transient stability accurate quantification method based on quasi-steady state assumption, comprising the following steps:

[0065] Step 1, by revealing that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady state mode in transient state.

[0066] Step 2, a parallel grid-connected system decoupling method based on the aforementioned quasi-steady state discovery is proposed, which decomposes the original four-order coupled model of the grid-following inverter-grid-forming inverter parallel grid-connected system into two two-order sub-models.

[0067] Step 3, for the above decoupled model, an iterative algorithm is proposed to fully consider the influence of dynamic interaction and damping term between inverters, and realize the accurate quantification of the transient stability boundary of the grid-following inverter-grid-forming inverter parallel grid-connected system.

[0068] Preferably, in the above grid-following inverter-grid-forming inverter parallel grid-connected system transient stability accurate quantification method based on quasi-steady state assumption, the implementation of step 1 comprises:

[0069] The analyzed grid-following inverter-grid-forming inverter parallel grid-connected system structure and controller schematic diagram is as shown in Figure 1 , wherein: the grid-following inverter and the grid-forming inverter are connected in parallel through impedance lines Y GFL and Y GFM at S, and are connected to the grid through impedance lines Y g . V g and θ g are the amplitude and phase of the grid voltage respectively. L f and C f are the inductance and capacitance of the output filter.

[0070] The control of the grid-following inverter includes a phase-locked loop and a current loop. K p and K i are the proportional coefficient and integral coefficient of the phase-locked loop. θ PLL and ω PLL are the output phase and frequency of the phase-locked loop. ω nis the system nominal frequency. The current loop is much faster than the phase-locked loop, which can be neglected when analyzing the transient stability. Therefore, the output current I GFL is assumed to be equal to the current reference value I ref , i.e. where I ref is the amplitude of the current reference value of the grid-connected inverter, is the power factor angle, I refd and I refq represent the d-axis and q-axis components of the current reference value, respectively.

[0071] The control of the grid-forming inverter includes the active power controller and the reactive power controller of the virtual synchronous generator, as well as the voltage and current inner loops. The dynamics of the inner loops are much faster than the virtual synchronous generator control, which is also neglected, i.e. the output voltage of the grid-forming inverter is assumed to be E ∠ θ VSG , where E and θ VSG are the output voltage amplitude and phase of the active power controller and the reactive power controller, respectively. J and D P are the virtual inertia and virtual damping of the active power controller, respectively. P ref and Q ref are the active and reactive references of the grid-forming inverter. k q and V n are the proportional coefficient and nominal voltage of the reactive power controller, respectively. P GFM and Q GFM are the active and reactive output powers of the grid-forming inverter. Neglecting the dynamic influence of the voltage and current inner loops with a faster time scale, the four-order state space equations of the grid-connected grid-forming inverter parallel grid system under the power angle scale are shown in Fig. 2. Figure 1

[0072]

[0073] where the state variables δ1, δ2, ω1, ω2 represent the output current phase of the grid-connected inverter, the frequency of the grid-connected inverter and the output voltage phase and frequency of the grid-forming inverter, respectively. The expressions of the equivalent coefficients are shown in [1]. The difference between δ2 and δ1 is defined as the relative power angle δ 21 of the system, and the difference between ω2 and ω1 is defined as the relative frequency ω 21 of the system. P m1 , P E1 , P int1 , D1, D int1 and J1 represent the equivalent mechanical power, the equivalent maximum electromagnetic power, the equivalent mutual power, the equivalent self-damping coefficient, the equivalent mutual damping coefficient and the equivalent inertia of the grid-connected inverter, respectively, and their expressions are shown in equation (2); P m2 , P E2 , P int2 ​D2 and J2 represent the equivalent mechanical power, the maximum equivalent electromagnetic power, the equivalent interactive power, the equivalent self-damping coefficient and the equivalent inertia of the grid inverter, respectively, and their expressions are shown in Equation (2); the constants θ1-θ5 are coefficients related to the grid impedance parameters, and their expressions are shown in Equation (4). For the power factor angle of the grid-connected inverter;

[0074]

[0075] Among them, I ref K i and K p These are the reference current amplitude, phase-locked loop integral coefficient, and phase-locked loop proportional coefficient of the grid-connected inverter, respectively; where P... ref D p E and J represent the reference active power, virtual damping, output voltage amplitude, and virtual inertia of the grid-connected inverter, respectively; where Vg is the grid phase voltage amplitude; and constants a1-a5 are coefficients related to grid impedance, and constant C... q To match the line impedance and grid inverter current reference value I ref and power factor angle The relevant coefficients are expressed as shown in (3);

[0076]

[0077] Among them, Y GFM and Y GFL Y represents the line admittance between the grid connection point and the common coupling point of the grid-connected inverter and the grid-connected inverter, respectively. g Let dω be the line admittance between the point of common coupling and the infinite power grid. Numerical simulation of equation (1) reveals that the derivative of the system's relative frequency with respect to time, dω... 21 / dt converges rapidly to near zero. For example... Figure 2 As shown, the values ​​of δ1 and δ2 reach their farthest points around 0.18s-0.25s, while dω 21 The value of / dt converges to a relatively small range within 0.1s. This indicates that the grid-connected inverter and the grid-linked inverter capture each other and move together with the grid in quasi-steady-state (QSS) mode, interacting with the grid. Because the timescale of the grid-connected inverter is slower, the dynamics of the grid-linked inverter are stretched by the interaction with the grid-connected inverter. Under the QSS assumption, assuming relative dω... 21 / dt is zero:

[0078]

[0079] In the above-mentioned accurate quantification method of transient stability of grid-connected system with grid-following inverter and grid-forming inverter in parallel based on quasi-steady-state assumption, the implementation of step 2 comprises:

[0080] By solving equation (4), it can be deduced that δ1 and δ2 are mapping functions of each other: δ 2QSS (δ1) and δ 1QSS (δ2) are inverse functions of each other:

[0081]

[0082] Where coefficients A1-A4 and B1-B4 are given by equation (7) and equation (8) respectively:

[0083]

[0084] Figure 3 The time-domain variation values of QSS errors δ 2QSS -δ2 and δ 1QSS -δ1 are shown, and it can be seen that the errors converge to zero rapidly within 0.05s, verifying the high accuracy of the above-mentioned QSS assumption. Therefore, the quantification analysis result based on the QSS assumption is reliable. As shown in equation (3)-(4), the original coupled fourth-order system is decomposed into two decoupled second-order systems using the QSS assumption: Figure 4

[0085]

[0086]

[0087] Preferably, in the above-mentioned accurate quantification method of transient stability of grid-connected system with grid-following inverter and grid-forming inverter in parallel based on quasi-steady-state assumption, the implementation of step 3 comprises:

[0088] Step 3.1 judges whether QSS decoupling will occur

[0089] For and QSS equations (5) and (6) may not always have real solutions, where δ 1SEP , δ 2SEP , δ 1UEP , and δ 2UEP are stable and unstable equilibrium points of the original fourth-order state space equation (1). This leads to the possibility that the QSS relationship will be broken when the operating point enters the non-real solution region, thereby causing LOS. The condition for equation (5) and equation (6) to have real solutions is:

[0090]

[0091] ​As long as formula (11) is satisfied, the system will not appear QSS decoupling phenomenon

[0092] Step 3.2 If QSS decoupling does not appear, the transient stability power angle upper boundary of the unstable equilibrium point is taken as the transient stability power angle-frequency boundary of the grid-connected inverter-grid-connected inverter parallel grid-connected system, and iteration calculation is carried out:

[0093] Only when the system does not appear QSS decoupling phenomenon, the stability of the grid-connected inverter and the grid-connected inverter is twin entangled. Therefore, the patent only derives the boundary of the grid-connected inverter.

[0094] The integral of δ2 in (8) is from x 2b to x 2a , and the definite integral can be obtained as:

[0095]

[0096] Where ω 2b and ω 2a represent the frequency corresponding to x 2b and x 2a . Formula (12) is the energy conservation law of the grid-connected inverter under the QSS assumption. Considering that the critical condition of transient stability of the grid-connected inverter is that the frequency ω2 is just reduced to zero at δ 2UEP , x 2b = δ 2UEP , ω 2b = 0 is substituted into formula (12), the stable boundary of the grid-connected inverter can be derived as:

[0097]

[0098] Formula (13) is the frequency-power angle mapping function under the critical stability condition. The positive sign in (11) represents the right swing process, and the negative sign represents the left swing process. Formula (13) is an implicit function equation about ω2, which can be solved by using the iteration algorithm shown in formula (13):

[0099]

[0100] In the formula, f is the frequency distribution function of the jth iteration

[0101] Step 3.3 If QSS decoupling appears, the QSS decoupling point is taken as the transient stability power angle upper boundary, and the transient stability power angle-frequency boundary of the grid-connected inverter-grid-connected inverter parallel grid-connected system is iteratively calculated.

[0102] Define that (11) is not satisfied, but in [δ 1SEP , δ 1UEP ]&[δ 2SEP , δ 2UEPThe minimum power angle values in the interval are respectively taken as the QSS decoupling angles δ 1QSSB and δ 2QSSB :

[0103]

[0104] Therefore, when δ1∈[δ 1QSSB , δ 1UEP ], the QSS state is no longer maintained, and the transient stability boundary needs to be reconsidered. The upper power angle boundary of the grid-forming and grid-following inverters should be redefined as δ 1QSSB and δ 2QSSB . Therefore, the stability boundary of the grid-forming inverter is modified as:

[0105]

[0106] Similar to (13), formula (16) can also be solved iteratively:

[0107]

[0108] Since (17) excludes the QSS decoupling region, the stability of the grid-forming and grid-following inverters is still twin entangled, and there is no need to derive the stability boundary of the grid-following inverter.

[0109] Embodiment Two

[0110] The embodiment is implemented by adopting the following technical scheme, and the accurate quantification method for transient stability of a grid-following inverter-grid-forming inverter parallel grid-connected system based on a quasi-steady state assumption includes the following steps,

[0111] Step 1, the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady state mode under transient state;

[0112] Step 2, a parallel grid-connected system decoupling method based on the aforementioned quasi-steady state discovery is proposed, and the original four-order coupled model of the grid-following inverter-grid-forming inverter parallel grid-connected system is decomposed into two two-order sub-models;

[0113] Step 3, for the above decoupled model, an iterative algorithm is proposed to fully consider the influence of dynamic interaction and damping terms between inverters, and to realize accurate quantification of the transient stability boundary of the grid-following inverter-grid-forming inverter parallel grid-connected system.

[0114] Further, the implementation of step 3 includes:

[0115] Step 3.1, judge whether QSS decoupling will occur

[0116] Step 3.2 If QSS decoupling does not occur, take the unstable equilibrium point as the upper boundary of the transient stable power angle to iteratively calculate the transient stable power angle-frequency boundary of the grid-following inverter-grid-forming inverter parallel grid-connected system

[0117] Step 3.3 If QSS decoupling occurs, take the QSS decoupling point as the upper boundary of the transient stable power angle to iteratively calculate the transient stable power angle-frequency boundary of the grid-following inverter-grid-forming inverter parallel grid-connected system.

[0118] In specific implementation, as shown in the following table, the method for accurate quantification of transient stability of a grid-following inverter-grid-forming inverter parallel grid-connected system based on quasi-steady-state assumption is provided. Figure 5 It is disclosed that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode in the transient state; a parallel grid-connected system decoupling method based on the foregoing quasi-steady-state discovery is proposed, which decomposes the original four-order coupled model of the grid-following inverter-grid-forming inverter parallel grid-connected system into two two-order sub-models; for the above decoupled model, an iterative algorithm is proposed to fully consider the influence of dynamic interaction and damping terms between inverters, so as to realize accurate quantification of the transient stability boundary of the grid-following inverter-grid-forming inverter parallel grid-connected system; and a theoretical basis is provided for engineering implementation of online transient stability monitoring of the grid-following inverter-grid-forming inverter parallel grid-connected system.

[0119] The above only describes the preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for accurately quantifying the transient stability of a grid-connected inverter parallel system based on the quasi-steady-state assumption, characterized in that, Includes the following steps: This study reveals that the relative power angle between the grid-connected inverter and the grid-connected inverter is in a quasi-steady-state mode under transient conditions, and assumes that the relative dω is in this state. 21 / dt is zero, ω 21 This is the difference between the frequency of the grid-connected inverter and the frequency of the grid-connected inverter. Under this assumption, an iterative algorithm is constructed to decompose the original fourth-order coupling model of the grid-connected inverter-grid-connected inverter parallel grid system into two second-order sub-models. By fully considering the dynamic interaction and damping effect between inverters, the transient stability boundary of the grid-connected inverter-grid-connected system is accurately quantified.

2. The method for accurately quantifying the transient stability of a grid-connected inverter parallel grid-connected system based on quasi-steady-state assumptions, as described in claim 1, is characterized in that: The process reveals that the relative power angle between the grid-connected inverter and the grid-connected inverter is in a quasi-steady-state mode under transient conditions. The steps include: the fourth-order state-space equation of the grid-connected inverter-grid-connected inverter parallel system is: The state variables δ1, ω1, δ2, and ω2 represent the phase and frequency of the grid-connected inverter's output current and the phase and frequency of the grid-connected inverter's output voltage, respectively; the difference between δ2 and δ1 is defined as the system's relative power angle δ. 21 The difference between ω2 and ω1 is defined as the system's relative frequency ω. 21 ;P m1 P E1 P int1 D1, D int1 J1 and J2 represent the equivalent mechanical power, maximum equivalent electromagnetic power, equivalent interactive power, equivalent self-damping coefficient, equivalent interactive damping coefficient, and equivalent inertia of the grid-connected inverter, respectively, and their expressions are shown in equation (2); P m2 P E2 P int2 D2 and J2 represent the equivalent mechanical power, the maximum equivalent electromagnetic power, the equivalent interactive power, the equivalent self-damping coefficient and the equivalent inertia of the grid inverter, respectively, and their expressions are shown in Equation (2); the constants θ1-θ5 are coefficients related to the grid impedance parameters, and their expressions are shown in Equation (4). For the power factor angle of the grid-connected inverter; Among them, I ref K i and K p These are the reference current amplitude, phase-locked loop integral coefficient, and phase-locked loop proportional coefficient of the grid-connected inverter, respectively; where P... ref D p E and J represent the reference active power, virtual damping, output voltage amplitude, and virtual inertia of the grid-connected inverter, respectively; where V g The phase voltage amplitude of the power grid; where constants a1-a5 are coefficients related to the power grid impedance, and constant C q To match the line impedance and grid inverter current reference value I ref and power factor angle The relevant coefficients are expressed as shown in (3); Among them, Y GFM and Y GFL Y represents the line admittance between the grid connection point and the common coupling point of the grid-connected inverter and the grid-connected inverter, respectively. g The line admittance between the common coupling point and the infinite power grid; Numerical simulation of formula (1) reveals that the derivative of the system's relative frequency with respect to time, dω 21 / dt converges rapidly to near zero; The values ​​of δ1 and δ2 reach their farthest points around 0.18s-0.25s, while dω 21 The value of / dt converges to a relatively small range within 0.1s; this shows that the grid-connected inverter and the grid-connected inverter capture each other and move together with the grid in the quasi-steady-state QSS mode; due to the slower time scale of the grid-connected inverter, the dynamics of the grid-connected inverter are stretched by the interaction with the grid-connected inverter; under the QSS assumption, assuming relative dω 21 / dt is zero:

3. The method for accurately quantifying the transient stability of a grid-connected inverter parallel system based on quasi-steady-state assumptions, as described in claim 2, is characterized in that: An iterative algorithm is constructed to decompose the original fourth-order coupling model of the grid-connected inverter-grid-connected inverter parallel system into two second-order sub-models. The steps include: By solving equation (4), we can derive the mapping function that determines the relationship between δ1 and δ2: δ 2QSS (δ1) and δ 1QSS (δ2), they are inverse functions of each other: The coefficients A1-A4 and B1-B4 are given by equations (7) and (8), respectively: By observing the QSS error δ 2QSS -δ2 and δ 1QSS The time-domain variation of -δ1 shows that the error converges rapidly to zero within 0.05s, verifying the high precision of the QSS hypothesis. Therefore, the quantitative analysis results based on the QSS hypothesis are reliable. Using the QSS assumptions in (3)-(4), the original coupled fourth-order system is decomposed into two decoupled second-order systems:

4. The method for accurately quantifying the transient stability of a grid-connected inverter parallel system based on the quasi-steady-state assumption, as described in claim 3, is characterized in that: Taking full account of the dynamic interaction and damping effects between inverters, the precise quantification of the transient stability boundary of the grid-connected inverter-grid-connected inverter parallel system is achieved through the following steps: (1) Determine whether QSS decoupling will occur. for QSS equations (5) and (6) may not always have real solutions, where δ 1SEP ,δ 2SEP ,δ 1UEP ,δ 2UEP These are the stable and unstable equilibrium points of the original fourth-order state-space equation (1), respectively. This leads to the possibility that the QSS relationship may be broken when the operation point enters the non-real solution region, resulting in LOS. The conditions for equations (5) and (6) to have real solutions are: As long as formula (11) is always satisfied, the system will not exhibit QSS decoupling. (2) If QSS decoupling does not occur, the transient stable power angle-frequency boundary of the grid-connected inverter-grid-connected system is iteratively calculated using the unstable equilibrium point as the upper boundary of the transient stable power angle: Only when the system does not exhibit QSS decoupling, the stability of the grid-connected inverter and the grid-connected inverter are inextricably linked; therefore, only the boundary of the grid-connected inverter is derived. For δ2 in (10) from x 2b To x 2a By taking the definite integral, we can obtain: Where ω 2b and ω 2a This indicates that the frequency corresponds to x. 2b and x 2a Equation (12) is the energy conservation law of the grid inverter under the QSS assumption; considering that the critical condition for the transient stability of the grid inverter is that the frequency ω2 is exactly at δ 2UEP The deceleration is zero at point x 2b =δ 2UEP ,ω 2b Substituting 0 into equation (12), the stability boundary of the grid inverter can be derived: Equation (13) is the frequency-power angle mapping function under critical stability conditions; the positive sign in (13) indicates the right swing process, and the negative sign indicates the left swing process; Equation (13) is an implicit function equation about ω2, which can be solved using the iterative algorithm shown in Equation (13): In the formula ω2 j The frequency distribution function of the j-th iteration (3) If QSS decoupling occurs, the transient stability power angle-frequency boundary of the grid-connected system with the QSS decoupling point as the upper boundary of the transient stability power angle is iteratively calculated; The definition does not satisfy (11) but in [δ 1SEP ,δ 1UEP ]&[δ 2SEP ,δ 2UEP The minimum power angle value within the interval is respectively used as the QSS decoupling angle δ 1QSSB and δ 2QSSB : Therefore, when δ1∈[δ 1QSSB ,δ 1UEP When this occurs, the QSS state is no longer maintained, and the transient stability boundary needs to be reconsidered; the power angle boundary for grid-connected and grid-connected inverters should be redefined as δ. 1QSSB and δ 2QSSB Therefore, the stability boundary of the grid inverter is modified as follows: Similar to (13), formula (16) can also be solved iteratively: Since (17) excludes the QSS decoupling region, the stability of the grid-connected inverter and the grid-connected inverter is still twin entangled, and there is no need to derive the stability boundary of the grid-connected inverter.

5. A system for accurately quantifying the transient stability of a grid-connected inverter parallel system based on quasi-steady-state assumptions, comprising: Mode revealing module: used to reveal the relative power angle between the grid-connected inverter and the grid-connected inverter in a quasi-steady state mode under transient conditions; And assume that at this time, relative to dω 21 / dt is zero, ω 21 This is the difference between the frequency of the grid-connected inverter and the frequency of the grid-connected inverter. Model decomposition module: Under this assumption, it is used to construct an iterative algorithm to decompose the original fourth-order coupled model of the grid-connected inverter-grid-connected inverter parallel grid system into two second-order sub-models. Precise Quantization Module: Fully considers the dynamic interaction and damping effect between inverters to achieve precise quantization of the transient stability boundary of the grid-connected inverter-grid-connected system.

6. The system for accurate quantification of transient stability of a grid-connected inverter parallel grid-connected system based on quasi-steady-state assumptions as described in claim 5, characterized in that: In the mode revealing module, it is revealed that the relative power angle between the grid-connected inverter and the grid-connected inverter is in a quasi-steady-state mode under transient conditions. The steps include: The fourth-order state-space equation of the grid-connected inverter-grid-connected inverter parallel grid-connected system is: The state variables δ1, δ2, ω1, and ω2 represent the phase and frequency of the grid-connected inverter's output current and the phase and frequency of the grid-connected inverter's output voltage, respectively; the difference between δ2 and δ1 is defined as the system's relative power angle δ. 21 The difference between ω2 and ω1 is defined as the system's relative frequency ω. 21 ;P m1 P E1 P int1 D1, D int1 J1 and J2 represent the equivalent mechanical power, maximum equivalent electromagnetic power, equivalent interactive power, equivalent self-damping coefficient, equivalent interactive damping coefficient, and equivalent inertia of the grid-connected inverter, respectively, and their expressions are shown in equation (2); P m2 P E2 P int2 D2 and J2 represent the equivalent mechanical power, the maximum equivalent electromagnetic power, the equivalent interactive power, the equivalent self-damping coefficient and the equivalent inertia of the grid inverter, respectively, and their expressions are shown in Equation (2); the constants θ1-θ5 are coefficients related to the grid impedance parameters, and their expressions are shown in Equation (4). For the power factor angle of the grid-connected inverter; Among them, I ref K i and K p These are the reference current amplitude, phase-locked loop integral coefficient, and phase-locked loop proportional coefficient of the grid-connected inverter, respectively; where P... ref D p E and J represent the reference active power, virtual damping, output voltage amplitude, and virtual inertia of the grid-connected inverter, respectively; where Vg is the grid phase voltage amplitude; and constants a1-a5 are coefficients related to grid impedance, and constant C... q To match the line impedance and grid inverter current reference value I ref and power factor angle The relevant coefficients are expressed as shown in (3); Among them, Y GFM and Y GFL Y represents the line admittance between the grid connection point and the common coupling point of the grid-connected inverter and the grid-connected inverter, respectively. g The line admittance between the common coupling point and the infinite power grid; Numerical simulation of formula (1) reveals that the derivative of the system's relative frequency with respect to time, dω 21 / dt converges rapidly to near zero; The values ​​of δ1 and δ2 reach their farthest points around 0.18s-0.25s, while dω 21 The value of / dt converges to a relatively small range within 0.1s; this shows that the grid-connected inverter and the grid-connected inverter capture each other and move together with the grid in the quasi-steady-state QSS mode; due to the slower time scale of the grid-connected inverter, the dynamics of the grid-connected inverter are stretched by the interaction with the grid-connected inverter; under the QSS assumption, assuming relative dω 21 / dt is zero:

7. The system for accurate quantification of transient stability of a grid-connected inverter parallel grid-connected system based on quasi-steady-state assumption as described in claim 6, characterized in that: In the model decomposition module, an iterative algorithm is constructed to decompose the original fourth-order coupled model of the grid-connected inverter-grid-connected inverter parallel grid system into two second-order sub-models. The steps include: By solving equation (4), we can derive the mapping function that determines the relationship between δ1 and δ2: δ 2QSS (δ1)andδ 1QSS (δ2), they are inverse functions of each other: The coefficients A1-A4 and B1-B4 are given by equations (5) and (6), respectively: By observing the QSS error δ 2QSS -δ2 and δ 1QSS The time-domain variation of -δ1 shows that the error converges rapidly to zero within 0.05s, verifying the high precision of the QSS hypothesis. Therefore, the quantitative analysis results based on the QSS hypothesis are reliable. Using the QSS assumptions in (3)-(4), the original coupled fourth-order system is decomposed into two decoupled second-order systems:

8. The system for accurate quantification of transient stability of a grid-connected inverter parallel grid-connected system based on quasi-steady-state assumption as described in claim 6, characterized in that: The precise quantization module fully considers the dynamic interaction and damping term effects between inverters to achieve precise quantization of the transient stability boundary of the grid-connected inverter-grid-connected system. The steps include: (1) Determine whether QSS decoupling will occur. for QSS equations (3) and (4) may not always have real solutions, where δ 1SEP ,δ 2SEP ,δ 1UEP ,δ 2UEP These are the stable and unstable equilibrium points of the original fourth-order state-space equation (1), respectively. This leads to the possibility that the QSS relationship may be broken when the operation point enters the non-real solution region, resulting in LOS. The conditions for equations (3) and (4) to have real solutions are: As long as formula (11) is always satisfied, the system will not exhibit QSS decoupling. (2) If QSS decoupling does not occur, the transient stable power angle-frequency boundary of the grid-connected inverter-grid-connected system is iteratively calculated using the unstable equilibrium point as the upper boundary of the transient stable power angle: Only when the system does not exhibit QSS decoupling, the stability of the grid-connected inverter and the grid-connected inverter are inextricably linked; therefore, only the boundary of the grid-connected inverter is derived. For δ2 in (10) from x 2b To x 2a By taking the definite integral, we can obtain: Where ω 2b and ω 2a This indicates that the frequency corresponds to x. 2b and x 2a Equation (10) is the energy conservation law of the grid inverter under the QSS assumption; considering that the critical condition for the transient stability of the grid inverter is that the frequency ω2 is exactly at δ 2UEP The deceleration is zero at point x 2b =δ 2UEP ,ω 2b Substituting 0 into equation (12), the stability boundary of the grid inverter can be derived: Equation (11) is the frequency-power angle mapping function under critical stability conditions; the positive sign in (11) indicates the right swing process, and the negative sign indicates the left swing process; Equation (13) is the implicit function equation with respect to ω2, which can be solved using the iterative algorithm shown in Equation (13): In the formula ω2 j The frequency distribution function of the j-th iteration (3) If QSS decoupling occurs, the transient stability power angle-frequency boundary of the grid-connected system with the QSS decoupling point as the upper boundary of the transient stability power angle is iteratively calculated; The definition does not satisfy (11) but in [δ 1SEP ,δ 1UEP ]&[δ 2SEP ,δ 2UEP The minimum power angle value within the interval is respectively used as the QSS decoupling angle δ 1QSSB and δ 2QSSB : Therefore, when δ1∈[δ 1QSSB ,δ 1UEP When this occurs, the QSS state is no longer maintained, and the transient stability boundary needs to be reconsidered; the power angle boundary for grid-connected and grid-connected inverters should be redefined as δ. 1QSSB and δ 2QSSB Therefore, the stability boundary of the grid inverter is modified as follows: Similar to (13), formula (16) can also be solved iteratively: Since (17) excludes the QSS decoupling region, the stability of the grid-connected inverter and the grid-connected inverter is still twin entangled, and there is no need to derive the stability boundary of the grid-connected inverter.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, implements the steps of the method as claimed in any one of claims 1 to 4.

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