On-line method for transient stability of grid-forming converter considering load fluctuation
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SOUTHWEST JIAOTONG UNIV
- Filing Date
- 2026-04-28
- Publication Date
- 2026-08-07
AI Technical Summary
当电网发生故障导致等值电网电压幅值骤降时，构网型变流器的P–f下垂与Q–V下垂将通过PCC功率平衡产生耦合响应，使功角δ与输出电压幅值V出现显著暂态偏移；当该偏移越过系统可恢复域边界时，可能引发失步或电压塌陷等暂态失稳问题
[0060]1）以诺顿聚合等效显式引入PCC并联电力电子负荷的影响，功率表达式同时包含并网通道项、导纳耗散/吸收项与电流源交换项，能够反映多种负荷并行及工况变化引起的稳定边界漂移。
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Abstract
Description
Technical Field
[0001] This invention relates to stability analysis of traction power supply systems and power electronic grid-connected operation monitoring technology, and particularly to an online method for determining the transient stability of grid-connected converters considering load fluctuations. Background Technology
[0002] Against the backdrop of increasing renewable energy share and deepening power electronics integration in traction power supply systems, grid-connected converters are gradually shifting from traditional "grid-following" operation to fulfilling functions such as voltage / frequency support, weak grid stability, and fault ride-through. Simultaneously, the loads connected in parallel to the traction grid's PCC side are highly nonlinear, rapidly controlled power electronic loads. Their equivalent admittance and equivalent current injection change rapidly with the number of loads, traction / braking conditions, current limiting strategies, and external voltage disturbances. When a grid fault causes a sudden drop in the equivalent grid voltage amplitude, the P-f droop and Q-V droop of the grid-connected converter will generate a coupled response through PCC power balance, causing significant transient shifts in the power angle δ and output voltage amplitude V. When this shift exceeds the system's recoverable domain boundary, it may trigger transient instability problems such as loss of synchronism or voltage collapse. Existing analyses often neglect or weaken the impact of parallel power electronic load branches in the PCC, and offline simulations relying on complete control details struggle to adapt to the parameter uncertainties brought about by parallel loads and rapid switching of operating conditions, making it difficult to form online-updable and interpretable stability criteria. Therefore, it is necessary to propose an online discrimination method that can explicitly incorporate the influence of equivalent branches of PCC parallel loads and quickly calculate the stability boundary based on measurement information. Summary of the Invention
[0003] To address the aforementioned technical problems, this invention proposes an online transient stability assessment method for grid-type converters that considers load fluctuations. By establishing a power analytical expression that includes online dynamic updates of Norton equivalent load branches and a dimensionality-reduced coupling model of droop control, and solving for the recoverable domain boundary of the phase plane, this method can eliminate dependence on internal load parameters and accurately reflect changes in operating conditions. This results in an online transient stability criterion with clear physical meaning and easy engineering deployment.
[0004] The online transient stability assessment method for grid-type converters considering load fluctuations includes the following steps:
[0005] Step S1: Establish an online transient stability discrimination model for grid-connected converters that considers parallel power electronic loads at PCC points, and obtain the analytical relationship of power-power angle-voltage coupling and phase plane criterion before and after the fault from the model;
[0006] Step S2: Real-time acquisition of PCC three-phase voltage, current and grid voltage amplitude. Extract the fundamental phasor and calculate the active power. reactive power , and angle With converter output voltage ;
[0007] Step S3: Based on the power electronic load information or online identification results, update the load equivalent parameters to form a load group equivalent model consistent with the current operating state;
[0008] Step S4: Under the updated parameter conditions, solve the quadratic equation obtained by rearranging the reactive power droop equation. Take a physically feasible positive real root or feasible solution, and calculate... The phase plane dynamics relationship is obtained;
[0009] Step S5: Determine the stable boundary of the phase plane based on the droop control law, and calculate the limit resection angle. Critical value of grid voltage after fault ;
[0010] Step S6: Transient stability margin is determined by work angle margin. With voltage margin Characterization, using the sign of the transient stability margin as a criterion for system stability; when the power angle margin Greater than zero and voltage margin If the margin is greater than zero, the system is considered to be in a stable state; if any margin is less than or equal to 0 or negative, the system is considered to be critical or unstable, indicating insufficient stability margin and triggering suppression measures and warning messages.
[0011] Furthermore, the online transient stability discrimination model for grid-connected converters considering parallel power electronic loads at the PCC point in step S1 includes:
[0012] The Norton model equates the Kth power electronic load on the PCC side as a current source with parallel admittance, i.e. ,in
[0013]
[0014] In the formula, For equivalent admittance, Let be the real part of the equivalent admittance of the load. The imaginary part of the equivalent admittance of the load
[0015] When N loads are connected in parallel to the PCC, the equivalent load admittance and equivalent current source can be obtained by aggregation:
[0016]
[0017]
[0018] in, These are the real and imaginary parts of the equivalent admittance after N loads are connected in parallel; The equivalent current source phasor is used; in online implementation, the fundamental phasor sequence within a short time window is used to update the load equivalent parameters.
[0019] Furthermore, in step S1, in the power-power angle-voltage coupling analytical relationship, the active power output by the grid-connected converter... With reactive power The parsing expression is:
[0020]
[0021]
[0022] in, For grid-connected reactors, and These correspond to the real and imaginary parts of the aggregated equivalent admittance, respectively. and The corresponding equivalent current source amplitude and phase angle.
[0023] Furthermore, the large-signal transient relationship of the transient stability online discrimination model in step S1 is as follows:
[0024]
[0025]
[0026] in, Power command given rated voltage amplitude The active and reactive power reference values given for network-type control. These are the active power-frequency ratio and the reactive power-voltage droop coefficient or equivalent gain, respectively.
[0027] Furthermore, the acquisition of electrical parameters of the PCC port in step S2 includes:
[0028] Gongjiao Defined as the difference in phase angle between the output voltage of the grid-connected converter and the grid side:
[0029]
[0030] In the formula: This is the voltage output by the inverter. This refers to the grid-side voltage.
[0031] The voltage amplitude of the grid before and after the fault is obtained by online estimation of the fundamental voltage amplitude of the PCC and the equivalent model on the grid-connected side.
[0032] Furthermore, in step S4, the quadratic equation obtained by rearranging the reactive power droop equation is solved. The method is as follows:
[0033] The quadratic equation for V is obtained by rearranging the reactive power droop equation.
[0034]
[0035] in
[0036]
[0037] And select those that satisfy The feasible solution as , to transform the system Coupling transformed into only containing Equivalent one-dimensional dynamic .
[0038] Furthermore, This yields the standard form of the active power balance condition:
[0039]
[0040] in
[0041]
[0042]
[0043] If and only if satisfying The system at that time has an equilibrium point on the phase plane;
[0044] Small-angle equilibrium point of the system in the phase plane Balance point with large angle They are respectively:
[0045]
[0046]
[0047] Small corner equilibrium point For a stable equilibrium point, the large-angle equilibrium point It is an unstable equilibrium point.
[0048] Furthermore, in step S5, the large-angle unstable equilibrium point is... Defined as the limiting cut-off angle of the phase plane stable boundary Critical grid voltage after a fault at the phase-plane stability boundary for:
[0049]
[0050] in, The expression is:
[0051]
[0052] for The positive root is obtained from the quadratic equation in V.
[0053] Furthermore, in step S6, a transient stability margin index is defined, which is used to determine the remaining capacity of the system to return to the recoverable domain from a fault state under fault disturbance; wherein, the power angle margin is defined as...
[0054]
[0055] Voltage margin is defined as
[0056]
[0057] In the formula, The limit cut-off angle represents the upper bound of the work angle corresponding to the boundary of the recoverable domain. The power angle corresponding to the moment the fault is cleared. This represents the voltage amplitude of the power grid after the fault. To ensure the system still possesses the critical grid voltage value within the recoverable domain; the power angle margin reflects the remaining margin from the current power angle to the limit cutoff angle, and the voltage margin reflects the remaining margin from the current grid voltage to the critical voltage; when and The system is determined to be stable; if any margin is less than or equal to 0 or negative, it is determined to be critical or unstable, indicating insufficient stability margin and triggering alarms or control strategies.
[0058] Furthermore, the specific application methods of the system stability determination include: when and When the fault is cleared, the system has a positive margin, and the power angle trajectory can return to the recoverable domain, indicating transient stability; when or At this time, the system is in a critical stable state and is sensitive to fault duration and voltage amplitude disturbances; when or When the system loses its recoverable domain, there is a risk of transient instability, which should trigger support or suppression measures and output alarm information.
[0059] Compared with the prior art, the present invention can achieve the following beneficial effects:
[0060] 1) The influence of parallel power electronic loads of PCC is explicitly introduced by Norton polymer equivalent. The power expression simultaneously includes grid connection channel terms, admittance dissipation / absorption terms and current source exchange terms, which can reflect the stability boundary drift caused by multiple loads running in parallel and changes in operating conditions.
[0061] 2) The recoverable domain boundary is constructed with the phase plane stable / unstable equilibrium point as the core. The criterion has a clear physical meaning and can directly form the limit cut-off angle and voltage critical value index, which is convenient for engineering application and interpretation.
[0062] 3) Online implementation mainly relies on information such as PCC port voltage, current and grid voltage amplitude, and allows updating of load equivalent parameters through operating data, avoiding strong dependence on internal load control parameters, and is suitable for complex traction network scenarios.
[0063] 4) Output stability margin can provide a direct basis for fault isolation tuning, online correction of droop parameters and triggering of stability enhancement strategies. Attached Figure Description
[0064] Figure 1 This is a flowchart of the online transient stability assessment of grid-type converters considering load fluctuations, as proposed in this invention.
[0065] Figure 2 This is a schematic diagram of the grid-connected topology of the PCC parallel power electronic load of the present invention.
[0066] Figure 3 This is a schematic diagram of the active-frequency and reactive-voltage droop control structure of the present invention.
[0067] Figure 4 This is the invention Phase diagram and schematic diagram of equilibrium point and stability boundary.
[0068] Figure 5 This is a grid-connected converter model for PCC point parallel power electronic loads provided in the embodiments of the present invention.
[0069] Figure 6 The active power P, reactive power Q, and power angle at the PCC point under conditions of relatively mild voltage drop, as provided in this embodiment of the invention, are... Simulation results of phase a grid current Iga.
[0070] Figure 7 The active power P, reactive power Q, and power angle at the PCC point under conditions of increased voltage drop, as provided in this embodiment of the invention, are... Simulation results of phase a grid current Iga.
[0071] Figure 8 The fault time angle provided in the embodiment of the present invention Less than the limit resection angle Active power P, reactive power Q, and power angle at point PCC Simulation results of phase a grid current Iga.
[0072] Figure 9The fault time angle provided in the embodiment of the present invention Greater than the limit resection angle Active power P, reactive power Q, and power angle at point PCC Simulation results of phase a grid current Iga. Detailed Implementation
[0073] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. The technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0074] An online transient stability assessment method for grid-type converters considering load fluctuations, the specific assessment process of which is as follows: Figure 1 As shown, the specific steps include the following:
[0075] Step S1: Establish an online transient stability discrimination model for a grid-connected converter considering parallel power electronic loads at the PCC point. Its physical structure is as follows: Figure 2 As shown, the active-frequency and reactive-voltage droop control structures are as follows: Figure 3 As shown, the analytical relationship between power-power angle-voltage coupling before and after the fault and the phase plane criterion are obtained from this model;
[0076] Step S2: Real-time acquisition of PCC three-phase voltage, current and grid voltage amplitude. Extract the fundamental phasor and calculate the active power. reactive power , and angle With converter output voltage ;
[0077] Step S3: Based on the power electronic load information or online identification results, update the load equivalent parameters to form a load group equivalent model consistent with the current operating state;
[0078] Step S4: Under the updated parameter conditions, solve the quadratic equation obtained by rearranging the reactive power droop equation. Take a physically feasible positive real root or feasible solution, and calculate... The phase plane dynamics relationship is obtained;
[0079] Step S5: Determine the stable boundary of the phase plane based on the droop control law, and calculate the limit resection angle. Critical value of grid voltage after fault ;
[0080] Step S6: Transient stability margin is determined by work angle margin. With voltage margin Characterization, using the sign of the transient stability margin as a criterion for system stability; when the power angle margin Greater than zero and voltage margin If the margin is greater than zero, the system is considered to be in a stable state; if any margin is less than or equal to 0 or negative, the system is considered to be critical or unstable, indicating insufficient stability margin and triggering suppression measures and warning messages.
[0081] Furthermore, the online transient stability discrimination model for grid-connected converters considering parallel power electronic loads at the PCC point in step S1 includes:
[0082] The Norton model equates the Kth power electronic load on the PCC side as a current source with parallel admittance, i.e. ,in
[0083]
[0084] In the formula, For equivalent admittance, Let be the real part of the equivalent admittance of the load. The imaginary part of the equivalent admittance of the load
[0085] When N loads are connected in parallel to the PCC, the equivalent load admittance and equivalent current source can be obtained by aggregation:
[0086]
[0087]
[0088] in, These are the real and imaginary parts of the equivalent admittance after N loads are connected in parallel; The equivalent current source phasor is used; in online implementation, the fundamental phasor sequence within a short time window is used to update the load equivalent parameters.
[0089] Furthermore, in step S1, in the power-power angle-voltage coupling analytical relationship, the active power output by the grid-connected converter... With reactive power The parsing expression is:
[0090]
[0091]
[0092] in, For grid-connected reactors, and These correspond to the real and imaginary parts of the aggregated equivalent admittance, respectively. and The corresponding equivalent current source amplitude and phase angle.
[0093] Furthermore, the large-signal transient relationship of the transient stability online discrimination model in step S1 is as follows:
[0094]
[0095]
[0096] in, Power command given rated voltage amplitude The active and reactive power reference values given for network-type control. These are the active power-frequency ratio and the reactive power-voltage droop coefficient or equivalent gain, respectively.
[0097] Furthermore, the acquisition of electrical parameters of the PCC port in step S2 includes:
[0098] Gongjiao Defined as the difference in phase angle between the output voltage of the grid-connected converter and the grid side:
[0099]
[0100] In the formula: This is the voltage output by the inverter. This refers to the grid-side voltage.
[0101] The voltage amplitude of the grid before and after the fault is obtained by online estimation of the fundamental voltage amplitude of the PCC and the equivalent model on the grid-connected side.
[0102] Furthermore, in step S4, the quadratic equation obtained by rearranging the reactive power droop equation is solved. The method is as follows:
[0103] The quadratic equation for V is obtained by rearranging the reactive power droop equation.
[0104]
[0105] in
[0106]
[0107] And select those that satisfy The feasible solution as , to transform the system Coupling transformed into only containing Equivalent one-dimensional dynamic .
[0108] Furthermore, This yields the standard form of the active power balance condition:
[0109]
[0110] in
[0111]
[0112]
[0113] If and only if satisfying The system described above has an equilibrium point on the phase plane, such as... Figure 4 As shown;
[0114] Small-angle equilibrium point of the system in the phase plane Balance point with large angle They are respectively:
[0115]
[0116]
[0117] Small corner equilibrium point For a stable equilibrium point, the large-angle equilibrium point It is an unstable equilibrium point.
[0118] Furthermore, in step S5, the large-angle unstable equilibrium point is... Defined as the limiting cut-off angle of the phase plane stable boundary Critical grid voltage after a fault at the phase-plane stability boundary for:
[0119]
[0120] in, The expression is:
[0121]
[0122] for The positive root is obtained from the quadratic equation in V.
[0123] Furthermore, in step S6, a transient stability margin index is defined, which is used to determine the remaining capacity of the system to return to the recoverable domain from a fault state under fault disturbance; wherein, the power angle margin is defined as...
[0124]
[0125] Voltage margin is defined as
[0126]
[0127] In the formula, The limit cut-off angle represents the upper bound of the work angle corresponding to the boundary of the recoverable domain. The power angle corresponding to the moment the fault is cleared. This represents the voltage amplitude of the power grid after the fault. To ensure the system still possesses the critical grid voltage value within the recoverable domain; the power angle margin reflects the remaining margin from the current power angle to the limit cutoff angle, and the voltage margin reflects the remaining margin from the current grid voltage to the critical voltage; when and The system is determined to be stable; if any margin is less than or equal to 0 or negative, it is determined to be critical or unstable, indicating insufficient stability margin and triggering alarms or control strategies.
[0128] Furthermore, the specific application methods of the system stability determination include: when and When the fault is cleared, the system has a positive margin, and the power angle trajectory can return to the recoverable domain, indicating transient stability; when or At this time, the system is in a critical stable state and is sensitive to fault duration and voltage amplitude disturbances; when or When the system loses its recoverable domain, there is a risk of transient instability, which should trigger support or suppression measures and output alarm information.
[0129] To further illustrate the technical solution of the present invention, in conjunction with... Figure 4 The transient evolution physical process and stability boundary determination of the embodiments of the present invention are described in detail below:
[0130] like Figure 4 The diagram shown illustrates the large-signal phase plane and equilibrium point boundary of a grid-connected converter. The implementation and judgment process is as follows:
[0131] 1. Before the fault, the system was operating stably at the initial power angle. This refers to the intersection of the blue line and the horizontal axis in the diagram. When a fault occurs, influenced by the equivalent parameters of the parallel load and the sudden drop in grid voltage, the system state transitions to the post-fault phase plane trajectory, which is represented by the red and dashed lines in the diagram. The power angle... A transient shift begins due to power imbalance.
[0132] 2. If the power balance condition is met after the fault, the system has two intersection points on the phase plane: the smaller angle balance point on the left is the stable balance point. The large angle equilibrium point on the right is an unstable equilibrium point. This invention defines the unstable equilibrium point as the absolute boundary of the recoverable domain, i.e., the limiting resection angle. .
[0133] 3. The system calculates the power angle at the time of fault clearing in real time. The work angle margin is obtained. .when and When the fault occurs, it indicates that the power angle did not cross the boundary during the fault period, and the system trajectory can still return to the stable equilibrium point, which is judged as transient stability; or At this time, the system is in a critical stable state and is sensitive to fault duration and voltage amplitude disturbances; when or When this happens, the system trajectory will cross the recoverable domain and enter a state of continuous oscillation, which is determined to be transient instability.
[0134] like Figure 5 The figure shows a grid-connected converter model with parallel power electronic loads at PCC points. The transient analysis theory mentioned above will be verified by simulation. Different cases will be analyzed below.
[0135] like Figure 6 As shown, under conditions of minor voltage drop, the active power P at point PCC drops momentarily during the fault and then gradually recovers to its commanded value; the reactive power Q and power angle... The current Iga of phase a of the power grid gradually increases after the fault and eventually reaches a new steady-state value, and the system returns to a stable state.
[0136] Figure 7 As shown, when the voltage drop intensifies, the observed P, Q, Severe low-frequency oscillations were observed in both the waveforms of Iga and IG, indicating that the grid-connected inverter will lose synchronization with the grid during this transient process, and the grid-connected system will no longer be stable.
[0137] like Figure 8 As shown, the power angle at the time of the fault Less than the limit resection angle At this time, the grid-connected inverter can return to stability after a brief adjustment process.
[0138] like Figure 9 As shown, the power angle at the time of the fault Greater than the limit resection angle At that time, the grid-connected inverter experienced a brief period of low-frequency oscillation, which gradually reached a new steady state in the new cycle.
[0139] The above descriptions are merely preferred embodiments of this application, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.
Claims
1. An online method for determining the transient stability of a grid-type converter considering load fluctuations, characterized in that, Includes the following steps: Step S1: Establish an online transient stability discrimination model for grid-connected converters that considers parallel power electronic loads at PCC points, and obtain the analytical relationship of power-power angle-voltage coupling and phase plane criterion before and after the fault from the model; Step S2: Real-time acquisition of PCC three-phase voltage, current and grid voltage amplitude. Extract the fundamental phasor and calculate the active power. reactive power , and angle With converter output voltage ; Step S3: Based on the power electronic load information or online identification results, update the load equivalent parameters to form a load group equivalent model consistent with the current operating state; Step S4: Under the updated parameter conditions, solve the quadratic equation obtained by rearranging the reactive power droop equation. Take a physically feasible positive real root or feasible solution, and calculate... The phase plane dynamics relationship is obtained; Step S5: Determine the stable boundary of the phase plane based on the droop control law, and calculate the limit resection angle. Critical value of grid voltage after fault ; Step S6: Transient stability margin is determined by work angle margin. With voltage margin Characterization, using the sign of the transient stability margin as a criterion for system stability; When the angle margin Greater than zero and voltage margin If the margin is greater than zero, the system is considered to be in a stable state; if any margin is less than or equal to 0 or negative, the system is considered to be critical or unstable, indicating insufficient stability margin and triggering suppression measures and warning messages.
2. The online transient stability determination method for grid-type converters considering load fluctuations according to claim 1, characterized in that, The online transient stability discrimination model for grid-connected converters considering parallel power electronic loads at PCC points in step S1 includes: The Norton model equates the Kth power electronic load on the PCC side as a current source with parallel admittance, i.e. ,in In the formula, For equivalent admittance, Let be the real part of the equivalent admittance of the load. The imaginary part of the equivalent admittance of the load When N loads are connected in parallel to the PCC, the equivalent load admittance and equivalent current source can be obtained by aggregation: in, These are the real and imaginary parts of the equivalent admittance after N loads are connected in parallel; The equivalent current source phasor is used; in online implementation, the fundamental phasor sequence within a short time window is used to update the load equivalent parameters.
3. The online transient stability determination method for grid-type converters considering load fluctuations according to claim 1, characterized in that, In step S1, the active power output of the grid-connected converter in the power-power angle-voltage coupling analytical relationship is... With reactive power The parsing expression is: in, For grid-connected reactors, and These correspond to the real and imaginary parts of the aggregated equivalent admittance, respectively. and The corresponding equivalent current source amplitude and phase angle.
4. The online transient stability determination method for grid-connected converters considering load fluctuations according to claim 1, characterized in that, The large-signal transient relationship of the transient stable online discriminant model in step S1 is as follows: in, Power command given rated voltage amplitude The active and reactive power reference values given for network-type control. These are the active power-frequency ratio and the reactive power-voltage droop coefficient or equivalent gain, respectively.
5. The online transient stability determination method for grid-type converters considering load fluctuations according to claim 1, characterized in that, The acquisition of electrical parameters of the PCC port in step S2 includes: Gongjiao Defined as the difference in phase angle between the output voltage of the grid-connected converter and the grid side: In the formula: This is the voltage output by the inverter. This refers to the grid-side voltage. The voltage amplitude of the grid before and after the fault is obtained by online estimation of the fundamental voltage amplitude of the PCC and the equivalent model on the grid-connected side.
6. The online transient stability determination method for grid-type converters considering load fluctuations according to claim 1, characterized in that, In step S4, the quadratic equation obtained by rearranging the reactive power droop equation is solved. The method is as follows: The quadratic equation for V is obtained by rearranging the reactive power droop equation. in And select those that satisfy The feasible solution as , to transform the system Coupling transformed into only containing Equivalent one-dimensional dynamic .
7. The online transient stability determination method for grid-connected converters considering load fluctuations according to claim 6, characterized in that, make This yields the standard form of the active power balance condition: in If and only if satisfying The system at that time has an equilibrium point on the phase plane; Small-angle equilibrium point of the system in the phase plane Balance point with large angle They are respectively: Small corner equilibrium point For a stable equilibrium point, the large-angle equilibrium point It is an unstable equilibrium point.
8. The online transient stability determination method for grid-type converters considering load fluctuations according to claim 1, characterized in that, In step S5, the large-angle unstable equilibrium point is... Defined as the limiting cut-off angle of the phase plane stable boundary Critical grid voltage after a fault at the phase-plane stability boundary for: in, The expression is: for The positive root is obtained from the quadratic equation in V.
9. The online transient stability determination method for grid-type converters considering load fluctuations according to claim 1, characterized in that, In step S6, a transient stability margin index is defined, which represents the remaining capacity of the system to return to the recoverable domain from a fault state under fault disturbance; wherein, the power angle margin is defined as... Voltage margin is defined as In the formula, The limit cut-off angle represents the upper bound of the work angle corresponding to the boundary of the recoverable domain. The power angle corresponding to the moment the fault is cleared. This represents the voltage amplitude of the power grid after the fault. To ensure the system still possesses the critical grid voltage value within the recoverable domain; the power angle margin reflects the remaining margin from the current power angle to the limit cutoff angle, and the voltage margin reflects the remaining margin from the current grid voltage to the critical voltage; when and The system is determined to be stable; if any margin is less than or equal to 0 or negative, it is determined to be critical or unstable, indicating insufficient stability margin and triggering alarms or control strategies.
10. The online transient stability determination method for a grid-connected converter considering load fluctuations according to claim 9, characterized in that, The specific application methods of the system stability determination include: when and When the fault is cleared, the system has a positive margin, and the power angle trajectory can return to the recoverable domain, indicating transient stability; when or At this time, the system is in a critical stable state and is sensitive to fault duration and voltage amplitude disturbances; when or When the system loses its recoverable domain, there is a risk of transient instability, which should trigger support or suppression measures and output alarm information.