Quasi-steady-state hypothesis-based transient stability accurate quantification method and system for network following-constructing type inverter parallel grid-connected system, and computer readable storage medium

Abstract

The invention discloses a quasi-steady-state hypothesis-based transient stability accurate quantification method and system for a network following-construction type inverter parallel grid-connected system, and a computer readable storage medium, and the method comprises the following steps: revealing that a relative power angle between a network following inverter and a network construction inverter is in a quasi-steady-state mode in a transient state; a parallel grid-connected system decoupling method based on quasi-steady state discovery is provided, and an original four-order coupling model of a following grid-constructing grid-connected inverter parallel grid-connected system is decomposed into two second-order sub-models; for the above decoupling model, an iterative algorithm is provided to fully consider the dynamic interaction and damping item influence between inverters, and the accurate quantification of the transient stability boundary of the following-grid-construction grid-connected inverter parallel grid-connected system is realized. According to the method, by revealing a quasi-steady-state mode of a relative power angle between a grid-following inverter and a grid-constructing inverter, an approximate mapping relation between operation power angles of heterogeneous grid-connected inverters in a transient state is constructed, so that decoupling and order reduction of the model are realized.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics technology, and in particular relates to a method, system and computer-readable storage medium for accurately quantifying transient stability of a grid-following-grid-building inverter parallel-connected grid-connected system based on a quasi-steady-state assumption. Background Art

[0002] With the continuous increase in the installed capacity of renewable energy power generation connected to the grid, a single type of grid-connected inverter can no longer meet the requirements of transient support and rapid response at the same time. Therefore, the microgrid composed of grid-connected inverters and grid-following inverters in parallel is an important part of the dual-high power system. How to accurately obtain the transient stability boundary of the grid-connected-grid-connected-grid-connected inverter parallel grid-connected system under the influence of the complex dynamic interaction of heterogeneous grid-connected inverters is still an academic blank. Summary of the invention

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

[0004] The technical solution of the present invention is as follows:

[0005] The first aspect of the present invention proposes a method for accurately quantifying transient stability of a grid-connected inverter parallel-connected grid-connected system based on a quasi-steady-state assumption, comprising the following steps:

[0006] It is revealed that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode under transient conditions;

[0007] An iterative algorithm is constructed to decompose the original fourth-order coupling model of the grid-connected inverter parallel-connected system into two second-order sub-models;

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

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

[0010]

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

[0012]

[0013] Among them, I ref , K i and K p are the reference current amplitude, phase-locked loop integral coefficient and phase-locked loop proportional coefficient of the grid-following converter respectively; ref , D p , E and J are the reference active power, virtual damping, output voltage amplitude and virtual inertia of the grid-connected converter respectively; Vg is the grid phase voltage amplitude; constants a1-a5 are coefficients related to grid impedance, constant C q is the line impedance and the grid-following converter current reference value I ref and power factor angle The relevant coefficient is expressed as shown in (3);

[0014]

[0015] Among them, Y GFM and Y GFL are the line admittances from the grid-connected point to the common coupling point of the grid-forming converter and the grid-following converter, Y g is the line admittance from the common coupling point to the infinite power grid;

[0016] By numerically simulating formula (1), it can be found that the derivative of the relative frequency of the system with respect to time dω is 21 / dt converges rapidly to near zero;

[0017] The values ​​of δ1 and δ2 reach their farthest points at around 0.18s-0.25s, while dω 21The value of / dt has converged to a relatively small range within 0.1s. It can be seen that the grid-connected inverter and the grid-following inverter capture each other and move together over time in the quasi-steady state (QSS) mode and interact with the grid. Due to the slower time scale of the grid-connected inverter, the dynamics of the grid-following inverter are prolonged by the interaction with the grid-connected inverter. Under the QSS assumption, assuming that the 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 parallel grid-connected system into two second-order sub-models, and the steps include:

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

[0021]

[0022] The coefficients A1-A4 and B1-B4 are given by equations (5) and (6) respectively:

[0023]

[0024] like Figure 6 As shown, by observing the QSS error δ 2QSS -δ2 and δ 1QSS -δ1 time domain variation value, it can be seen that the error quickly converges to zero within 0.05s, verifying the high accuracy of the above QSS assumption. Therefore, the quantitative analysis results based on the QSS assumption are reliable;

[0025] Using the QSS assumption in (3)-(4), the original coupled fourth-order system is decomposed into two decoupled second-order systems:

[0026]

[0027] Fully consider the dynamic interaction and damping effects between inverters to achieve accurate quantification of the transient stability boundary of the grid-connected inverter parallel grid-connected system. The steps include:

[0028] (1) Determine whether QSS decoupling will occur

[0029] for QSS equations (5) and (6) may not always have real solutions, where δ 1SEP ,δ 2SEP ,δ 1UEP,δ 2UEP are the stable equilibrium point and unstable equilibrium point of the original fourth-order state space equation (11), respectively. This means that when the operating point enters the non-real solution region, the QSS relationship may be broken, leading to LOS. The conditions for equations (5) and (6) to have real solutions are:

[0030]

[0031] As long as formula (10) is always satisfied, the system will not experience QSS decoupling phenomenon.

[0032] (2) If QSS decoupling does not occur, the unstable equilibrium point is used as the upper boundary of the transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel-connected system:

[0033] Only when the system does not have the QSS decoupling phenomenon, the stability of the grid and the grid-connected inverter is twin entangled. Therefore, the patent of this invention only derives the boundary of the grid-connected inverter.

[0034] For δ2 in (8) from x 2b to x 2a By taking the definite integral we can get:

[0035]

[0036] where ω 2b and ω 2a Indicates the frequency corresponding to x 2b and x 2a Formula (10) is the energy conservation law 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 is just within δ 2UEP The speed is reduced to zero, and x 2b =δ 2UEP ,ω 2b =0 is substituted into formula (12), and the stability boundary of the grid-connected inverter can be derived:

[0037]

[0038] Formula (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. Formula (13) is an implicit function equation about ω2, which can be solved by the iterative algorithm shown in Formula (13):

[0039]

[0040] Where ω2 j is the frequency distribution function of the jth iteration

[0041] (3) If QSS decoupling occurs, the QSS decoupling point is used as the upper boundary of the transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel-connected system.

[0042] The definition does not satisfy (11) but in [δ 1SEP ,δ 1UEP ]&[δ 2SEP ,δ 2UEP ] are taken as the QSS decoupling angle δ 1QSSB and δ 2QSSB :

[0043]

[0044] Therefore, when δ1∈[δ 1QSSB ,δ 1UEP ], the QSS state is no longer maintained, and the transient stability boundary needs to be reconsidered. The power angle boundary of the grid-connected and grid-following inverters should be redefined as δ 1QSSB and δ 2QSSB Therefore, the stability boundary of the grid-connected inverter is modified as follows:

[0045]

[0046] Similar to (13), formula (16) can also be solved iteratively:

[0047]

[0048] 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.

[0049] The second aspect of the present invention proposes a transient stability accurate quantification system of a grid-following-grid-building inverter parallel-grid-connected system based on a quasi-steady-state assumption, comprising:

[0050] Mode revealing module: used to reveal that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode in a transient state;

[0051] Model decomposition module: used to construct an iterative algorithm to decompose the original fourth-order coupling model of the grid-connected inverter parallel grid-connected system into two second-order sub-models;

[0052] Precise quantification module: fully considers the dynamic interaction between inverters and the influence of damping terms to achieve precise quantification of the transient stability boundary of the grid-connected inverter parallel-connected system.

[0053] A third aspect of the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of any one of the methods of claims 1 to 4 are implemented.

[0054] The technical effects and advantages of the present invention are as follows: 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 angles of heterogeneous grid-connected inverters under transient conditions is constructed, thereby achieving decoupling and order reduction of the model. This method overcomes the shortcomings of existing methods that are not applicable to high-order systems and are too conservative to accurately estimate transient boundaries, and provides a theoretical basis for the engineering implementation of online transient stability monitoring of grid-following and grid-forming grid-connected inverter parallel grid-connected systems. This method has good development potential and promotion space. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is the structural diagram of the parallel system of grid-type converter and grid-type converter;

[0056] Figure 2 This is a schematic diagram of the time domain simulation test of the fourth-order model;

[0057] Figure 3 It is the time domain diagram of QSS hypothesis error;

[0058] Figure 4 It is a schematic diagram of system decoupling based on QSS;

[0059] Figure 5 This is a flow chart of a method for accurately quantifying transient stability of a grid-following-grid-building inverter parallel-grid-connected system based on a quasi-steady-state assumption of the present invention;

[0060] Figure 6 This is a time domain simulation diagram. DETAILED DESCRIPTION

[0061] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0062] Embodiment 1

[0063] The purpose of the present invention is to achieve accurate quantification of the transient stability boundary of the grid-connected and grid-forming inverter parallel grid-connected system. 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 angles of heterogeneous grid-connected inverters under transient conditions is constructed, thereby achieving decoupling and order reduction of the model, and achieving accurate quantification of the transient stability boundary of the grid-connected and grid-forming inverter parallel grid-connected system. To achieve the above purpose, the technical solution adopted by the present invention is: a method for accurate quantification of the transient stability of the grid-connected and grid-forming inverter parallel grid-connected system based on the quasi-steady-state assumption, comprising the following steps:

[0064] 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 under transient conditions.

[0065] Step 2: A parallel grid-connected system decoupling method based on the aforementioned quasi-steady-state discovery is proposed, and the original fourth-order coupling model of the grid-connected inverter parallel grid-connected system is decomposed into two second-order sub-models.

[0066] Step 3: For the above decoupling model, an iterative algorithm is proposed to fully consider the dynamic interaction between inverters and the influence of damping terms, so as to achieve accurate quantification of the transient stability boundary of the grid-connected inverter parallel grid-connected system.

[0067] Preferably, in the above-mentioned method for accurately quantifying transient stability of a grid-connected inverter parallel-connected system based on a quasi-steady-state assumption, the implementation of step 1 includes:

[0068] The structure and controller diagram of the grid-connected inverter parallel grid-connected system analyzed are as follows: Figure 1 As shown, the grid-following converter and the grid-building converter are connected through the impedance line Y GFL and Y GFM Connect in parallel at S and through the resistive line Y g Grid-connected. g and θ g are the amplitude and phase of the grid voltage respectively. f and C f are the inductance and capacitance of the output filter.

[0069] The control of the grid-following converter includes a phase-locked loop and a current loop. p and K i θ is the proportional coefficient and integral coefficient of the phase-locked loop. PLL and ω PLL is the output phase and frequency of the phase-locked loop. n is the nominal frequency of the system. The dynamics of the current loop are much faster than those of the phase-locked loop and can be ignored when analyzing transient stability. Therefore, it can be assumed that the output current I GFL Equal to the current reference value Iref ,Right now Among them I ref is the current reference amplitude of the grid-following converter, is the power factor angle, I refd and I refq They represent the d-axis and q-axis components of the current reference value respectively.

[0070] The control of the grid-connected converter includes the active power controller and reactive power controller of the virtual synchronous generator, as well as the voltage and current inner loop. The dynamics of the inner loop are much faster than those of the virtual synchronous generator control.

[0071] Therefore, it is also ignored, that is, assuming that the output voltage of the grid-type converter is E∠θ VSG , where E and θ VSG J and D are the output voltage amplitude and phase of the active power controller and reactive power controller respectively. P are the virtual inertia and virtual damping of the active power controller respectively. ref and Q ref It is the active and reactive reference of the grid-connected converter. q and V n are the proportional coefficient and nominal voltage of the reactive power controller respectively. GFM and Q GFM are the active power and reactive power output of the grid-connected converter respectively. Ignoring the dynamic influence of the voltage and current inner loop with a faster time scale, Figure 1 The fourth-order state space equation of the grid-connected inverter parallel grid-connected system shown is:

[0072]

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

[0074]

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

[0076]

[0077] Among them, Y GFM and Y GFL are the line admittances from the grid-connected point to the common coupling point of the grid-forming converter and the grid-following converter, Y g is the line admittance from the common coupling point to the infinite power grid. By numerically simulating formula (1), it can be found that the derivative of the relative frequency of the system with respect to time dω 21 / dt quickly converges to near zero. Figure 2 As shown, the values ​​of δ1 and δ2 reach their farthest points at around 0.18s-0.25s, while dω 21 The value of / dt has converged to a relatively small range within 0.1s. It can be seen that the grid-connected inverter and the grid-following inverter capture each other and move together over time in the quasi-steady state (QSS) mode and interact with the grid. Due to the slower time scale of the grid-connected inverter, the dynamics of the grid-following inverter are prolonged by the interaction with the grid-connected inverter. Under the QSS assumption, assuming that the relative dω 21 / dt is zero:

[0078]

[0079] In the above-mentioned method for accurately quantifying transient stability of grid-connected inverter parallel-connected grid-connected system based on quasi-steady-state assumption, the implementation of step 2 includes:

[0080] By solving equation (4), we can derive the mapping function of δ1 and δ2 as the relationship between them: 2QSS (δ1)andδ 1QSS (δ2), they are inverse functions of each other:

[0081]

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

[0083]

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

[0085]

[0086] Preferably, in the above-mentioned method for accurately quantifying transient stability of a grid-connected inverter parallel-connected system based on a quasi-steady-state assumption, the implementation of step 3 includes:

[0087] Step 3.1 Determine whether QSS decoupling will occur

[0088] for QSS equations (5) and (6) may not always have real solutions, where δ 1SEP ,δ 2SEP ,δ 1UEP ,δ 2UEP are the stable equilibrium point and unstable equilibrium point of the original fourth-order state space equation (1), respectively. This means that when the operating point enters the non-real solution region, the QSS relationship may be broken, leading to LOS. The conditions for equations (5) and (6) to have real solutions are:

[0089]

[0090] As long as formula (11) is always satisfied, the system will not experience QSS decoupling phenomenon.

[0091] Step 3.2 If QSS decoupling does not occur, the unstable equilibrium point is used as the upper boundary of transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel grid-connected system:

[0092] Only when the system does not have the QSS decoupling phenomenon, the stability of the grid and the grid-connected inverter is twin entangled. Therefore, the patent of this invention only derives the boundary of the grid-connected inverter.

[0093] For δ2 in (8) from x 2b to x 2a By taking the definite integral we can get:

[0094]

[0095] where ω 2b and ω 2a Indicates 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 for transient stability of the grid-connected inverter is that the frequency ω2 is just within δ 2UEP The speed is reduced to zero, and x 2b =δ 2UEP ,ω 2b =0 is substituted into formula (12), and the stability boundary of the grid-connected inverter can be derived:

[0096]

[0097] Formula (13) 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. Formula (13) is an implicit function equation about ω2, which can be solved by the iterative algorithm shown in Formula (13):

[0098]

[0099] Where ω2 j is the frequency distribution function of the jth iteration

[0100] Step 3.3 If QSS decoupling occurs, the QSS decoupling point is used as the upper boundary of the transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel-connected system.

[0101] The definition does not satisfy (11) but in [δ 1SEP ,δ 1UEP ]&[δ 2SEP ,δ 2UEP ] are taken as the QSS decoupling angle δ 1QSSB and δ 2QSSB :

[0102]

[0103] Therefore, when δ1∈[δ 1QSSB ,δ 1UEP ], the QSS state is no longer maintained, and the transient stability boundary needs to be reconsidered. The power angle boundary of the grid-connected and grid-following inverters should be redefined as δ 1QSSB and δ 2QSSB Therefore, the stability boundary of the grid-connected inverter is modified as follows:

[0104]

[0105] Similar to (13), formula (16) can also be solved iteratively:

[0106]

[0107] 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.

[0108] Embodiment 2

[0109] This embodiment is implemented by the following technical solution, and is aimed at a method for accurately quantifying transient stability of a grid-connected inverter parallel-connected system based on a quasi-steady-state assumption, including the following steps:

[0110] Step 1: reveal that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode in a transient state;

[0111] Step 2: propose a decoupling method for parallel grid-connected systems based on the aforementioned quasi-steady-state discovery, and decompose the original fourth-order coupling model of the parallel grid-connected inverter system with the grid-connected ...

[0112] Step 3: For the above decoupling model, an iterative algorithm is proposed to fully consider the dynamic interaction between inverters and the influence of damping terms, so as to achieve accurate quantification of the transient stability boundary of the grid-connected inverter parallel grid-connected system.

[0113] Further, the implementation of step 3 includes:

[0114] Step 3.1 Determine whether QSS decoupling will occur

[0115] Step 3.2 If QSS decoupling does not occur, the unstable equilibrium point is used as the upper boundary of transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel grid-connected system.

[0116] Step 3.3 If QSS decoupling occurs, the QSS decoupling point is used as the upper boundary of the transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel-connected system.

[0117] When implementing it, Figure 5 As shown in the figure, a precise quantification method for transient stability of grid-connected inverter parallel grid-connected system based on quasi-steady-state assumption is proposed. 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; a decoupling method for parallel grid-connected system based on the aforementioned quasi-steady-state discovery is proposed, and the original fourth-order coupling model of the grid-connected inverter parallel grid-connected system is decomposed into two second-order sub-models; for the above decoupling model, an iterative algorithm is proposed to fully consider the dynamic interaction and damping terms between inverters, and realize the precise quantification of the transient stability boundary of the grid-connected inverter parallel grid-connected system; it provides a theoretical basis for the engineering implementation of online transient stability monitoring of the grid-connected inverter parallel grid-connected system.

[0118] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for accurately quantifying transient stability of a grid-connected inverter parallel-connected system based on a quasi-steady-state assumption, characterized in that: The following steps are involved: It is revealed that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode under transient conditions; An iterative algorithm is constructed to decompose the original fourth-order coupling model of the grid-connected inverter parallel-connected system into two second-order sub-models; The dynamic interaction between inverters and the influence of damping terms are fully considered to achieve accurate quantification of the transient stability boundary of the grid-connected inverter parallel-connected system.

2. According to claim 1, a method for accurately quantifying transient stability of a grid-connected inverter parallel-connected system based on a quasi-steady-state assumption, characterized in that: It is revealed that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode in a transient state. The steps include: The fourth-order state space equation of the grid-following-grid-forming grid-connected inverter parallel grid-connected system is: The state variables δ1, δ2, ω1, ω2 represent the output current phase and frequency of the grid-connected inverter and the output voltage phase and frequency of the grid-connected inverter. The difference between δ2 and δ1 is defined as the relative power angle δ 21 , define the difference between ω2 and ω1 as the system relative frequency δ 21 ;P m1 , P E1 , P int1 , D1, D int1 and J1 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-following converter, respectively. Their expressions are shown in formula (2); m2 , P E2 , P int2 , D2 and J2 represent the equivalent mechanical power, the maximum equivalent electromagnetic power, the equivalent interaction power, the equivalent self-damping coefficient and the equivalent inertia of the grid-connected converter, respectively, and their expressions are shown in formula (2); constants θ1-θ5 are coefficients related to the grid impedance parameters, and their expressions are shown in formula (4); is the power factor angle of the grid-following converter; Among them, I ref , K i and K p are the reference current amplitude, phase-locked loop integral coefficient and phase-locked loop proportional coefficient of the grid-following converter respectively; ref , D p , E and J are the reference active power, virtual damping, output voltage amplitude and virtual inertia of the grid-connected converter respectively; Vg is the grid phase voltage amplitude; constants a1-a5 are coefficients related to grid impedance, constant C q is the line impedance and the grid-following converter current reference value I ref and power factor angle The relevant coefficient is expressed as shown in (3); Among them, Y GFM and Y GFL are the line admittances from the grid-connected point to the common coupling point of the grid-forming converter and the grid-following converter, Y g is the line admittance from the common coupling point to the infinite power grid; By numerically simulating formula (1), it can be found that the derivative of the relative frequency of the system with respect to time dω is 21 / dt converges rapidly to near zero; The values ​​of δ1 and δ2 reach their farthest points at around 0.18s-0.25s, while dω 21 The value of / dt has converged to a relatively small range within 0.1s; it can be seen that the grid-connected inverter and the grid-following inverter capture each other and move together over time in the quasi-steady state (QSS) mode and interact with the grid; due to the slower time scale of the grid-connected inverter, the dynamics of the grid-following inverter are prolonged by the interaction with the grid-connected inverter; under the QSS assumption, assuming that the relative dω 21 / dt is zero:

3. According to claim 2, a method for accurately quantifying transient stability of a grid-connected inverter parallel-connected system based on a quasi-steady-state assumption, characterized in that: An iterative algorithm is constructed to decompose the original fourth-order coupling model of the grid-connected inverter parallel grid-connected system into two second-order sub-models. The steps include: By solving equation (4), we can derive the mapping function of δ1 and δ2 as the relationship between them: 2QSS (δ1)andδ 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 -δ1 time domain variation value, it can be seen that the error quickly converges to zero within 0.05s, verifying the high accuracy of the above QSS assumption; therefore, the quantitative analysis results based on the QSS assumption are reliable; Using the QSS assumption in (3)-(4), the original coupled fourth-order system is decomposed into two decoupled second-order systems:

4. According to claim 3, a method for accurately quantifying transient stability of a grid-connected inverter parallel-connected system based on a quasi-steady-state assumption, characterized in that: Fully consider the dynamic interaction and damping effects between inverters to achieve accurate quantification of the transient stability boundary of the grid-connected inverter parallel grid-connected system. The steps include: (1) Determine whether QSS decoupling will occur for and QSS equations (5) and (6) may not always have real solutions, where δ 1SEP ,δ 2SEP ,δ 1UEP ,δ 2UEP are the stable equilibrium point and unstable equilibrium point of the original fourth-order state space equation (1), respectively. This means that when the operating point enters the non-real solution region, the QSS relationship may be broken, leading to 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 experience QSS decoupling phenomenon. (2) If QSS decoupling does not occur, the unstable equilibrium point is used as the upper boundary of the transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel-connected system: Only when the system does not have the QSS decoupling phenomenon, the stability of the grid-connected inverter and the grid-connected inverter is twin entangled; therefore, the patent of this invention only derives the boundary of the grid-connected inverter; For δ2 in (10), we can 2b to x 2a By taking the definite integral we can get: where ω 2b and ω 2a Indicates the frequency corresponding to x 2b and x 2a ; Equation (12) is the energy conservation law 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 is just within δ 2UEP The speed is reduced to zero, and x 2b =δ 2UEP ,ω 2b =0 is substituted into formula (12), and the stability boundary of the grid-connected inverter can be derived: Formula (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; Formula (13) is an implicit function equation about ω2, which can be solved by the iterative algorithm shown in Formula (13): In the formula is the frequency distribution function of the jth iteration (3) If QSS decoupling occurs, the QSS decoupling point is used as the upper boundary of the transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel-connected system; The definition does not satisfy (11) but in [δ 1SEP ,δ 1UEP ]&[δ 2SEP ,δ 2UEP ] are taken as the QSS decoupling angle δ 1QSSB and δ 2QSSB : Therefore, when δ1∈[δ 1QSSB ,δ 1UEP ], the QSS state is no longer maintained, and the transient stability boundary needs to be reconsidered; the power angle boundary of the grid-connected and grid-following inverters should be redefined as δ 1QSSB and δ 2QSSB Therefore, the stability boundary of the grid-connected 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 transient stability accurate quantification system for grid-following and grid-building inverter parallel grid-connected system based on quasi-steady-state assumption, comprising: Mode revealing module: used to reveal that the relative power angle between the grid-following inverter and the grid-forming inverter is in a quasi-steady-state mode in a transient state; Model decomposition module: used to construct an iterative algorithm to decompose the original fourth-order coupling model of the grid-connected inverter parallel grid-connected system into two second-order sub-models; Precise quantification module: fully considers the dynamic interaction between inverters and the influence of damping terms to achieve precise quantification of the transient stability boundary of the grid-connected inverter parallel-connected system.

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

7. The transient stability accurate quantification system of the grid-following-grid-building inverter parallel-grid-connected system based on the quasi-steady-state assumption according to claim 6 is characterized by: In the model decomposition module, an iterative algorithm is constructed to decompose the original fourth-order coupling model of the grid-connected inverter parallel grid-connected system into two second-order sub-models. The steps include: By solving equation (4), we can derive the mapping function of δ1 and δ2 as the relationship between them: 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 -δ1 time domain variation value, it can be seen that the error quickly converges to zero within 0.05s, verifying the high accuracy of the above QSS assumption; therefore, the quantitative analysis results based on the QSS assumption are reliable; Using the QSS assumption in (3)-(4), the original coupled fourth-order system is decomposed into two decoupled second-order systems:

8. According to claim 6, a transient stability accurate quantification system of a grid-following-grid-building inverter parallel-connected grid-connected system based on a quasi-steady-state assumption, characterized in that: In the precise quantification module, the dynamic interaction and damping effects between inverters are fully considered to achieve precise quantification of the transient stability boundary of the grid-connected inverter parallel grid-connected system. The steps include: (1) Determine whether QSS decoupling will occur for and QSS equations (3) and (4) may not always have real solutions, where δ 1SEP ,δ 2SEP ,δ 1UEP ,δ 2UEP are the stable equilibrium point and unstable equilibrium point of the original fourth-order state space equation (1), respectively. This means that when the operating point enters the non-real solution region, the QSS relationship may be broken, leading to 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 experience QSS decoupling phenomenon. (2) If QSS decoupling does not occur, the unstable equilibrium point is used as the upper boundary of the transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel-connected system: Only when the system does not have the QSS decoupling phenomenon, the stability of the grid-connected inverter and the grid-connected inverter is twin entangled; therefore, the patent of this invention only derives the boundary of the grid-connected inverter; For δ2 in (10), we can 2b to x 2a By taking the definite integral we can get: where ω 2b and ω 2a Indicates the frequency corresponding to x 2b and x 2a ; Formula (10) is the energy conservation law 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 is just within δ 2UEP The speed is reduced to zero, and x 2b =δ 2UEP ,ω 2b =0 is substituted into formula (12), and the stability boundary of the grid-connected inverter can be derived: Formula (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; Formula (13) is the implicit function equation about ω2, which can be solved by the iterative algorithm shown in Formula (13): Where ω2 j is the frequency distribution function of the jth iteration (3) If QSS decoupling occurs, the QSS decoupling point is used as the upper boundary of the transient stability power angle to iteratively calculate the transient stability power angle-frequency boundary of the grid-connected inverter parallel-connected system; The definition does not satisfy (11) but in [δ 1SEP ,δ 1UEP ]&[δ 2SEP ,δ 2UEP ] are taken as the QSS decoupling angle δ 1QSSB and δ 2QSSB : Therefore, when δ1∈[δ 1QSSB ,δ 1UEP ], the QSS state is no longer maintained, and the transient stability boundary needs to be reconsidered; the power angle boundary of the grid-connected and grid-following inverters should be redefined as δ 1QSSB and δ 2QSSB Therefore, the stability boundary of the grid-connected 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, and when the computer program is executed by a processor, the steps of any one of the methods of claims 1 to 4 are implemented.

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