Reverse injection-based network following and network construction cooperative low-frequency oscillation suppression method

Abstract

The invention, which relates to the technical field of the electric power system, discloses a reverse-injection-based network following and network construction cooperative low-frequency oscillation suppression method comprising the following steps: extracting low-frequency oscillation components in a GFM converter; setting a reverse injection response threshold, and comparing the extracted low-frequency oscillation component with the reverse injection response threshold; for a single GFL converter, when a low-frequency oscillation component exceeds a reverse injection response threshold, controlling the GFL converter to inject low-frequency oscillation components with equal amplitude and opposite directions; and for a plurality of GFL converters, when a low-frequency oscillation component exceeds a reverse injection response threshold value, a self-adaptive reverse injection distribution strategy based on a dynamic power margin is adopted, and a total reverse injection current component is distributed to each GFL converter. According to the invention, the problem that low-frequency oscillation caused by the GFM converter is propagated along the power grid in the GFL-GFM hybrid system and further affects the normal operation of other equipment and the stability of the system is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method for suppressing low-frequency oscillations in a coordinated manner based on reverse injection and grid following. Background Art

[0002] With the widespread adoption of grid-following (GFL) and grid-forming (GFM) converters, hybrid operation of the two has become the norm in power systems with a high proportion of renewable energy. GFM, with its voltage source characteristics, can simulate the inertia and damping characteristics of synchronous generators, providing frequency and voltage support for the system. GFL, on the other hand, offers strong current control and fast power response capabilities, making the two naturally complementary.

[0003] However, the virtual inertia control introduced by GFM also introduces new challenges: its active power control component exhibits second-order dynamic characteristics, making it prone to low-frequency power oscillations under grid disturbances. These low-frequency oscillations can propagate along the grid, inducing overload or even damage to other equipment. Current research on GFM-GFL hybrid systems focuses on small-signal stability modeling and system ratio optimization. For example, methods such as impedance modeling and state-space modeling are used to analyze the impact of converter parameters on system stability, while also exploring the optimal capacity ratio boundaries for both the grid-forming and grid-following converters based on indicators such as the grid short-circuit ratio.

[0004] While research has recognized the low-frequency oscillation problem of GFMs, existing solutions primarily focus on improving the GFM's inherent damping capacity. These include introducing an additional damping controller, superimposing an angular frequency deviation compensation mechanism, or enhancing its frequency response by drawing on the structure of a power system stabilizer (PSS). While these approaches have suppressed low-frequency oscillations to a certain extent, they often suffer from complex parameter adjustment, mutual constraints on dynamic response, or impacts on inertial support accuracy. Consequently, they fail to fully tap the complementary potential of GFM-GFL hybrid systems. Summary of the Invention

[0005] In response to the above-mentioned deficiencies in the prior art, the present invention provides a method for suppressing low-frequency oscillations in a grid-following and grid-building collaborative manner based on reverse injection, which solves the problem that low-frequency oscillations caused by GFM converters propagate along the grid in the GFL-GFM hybrid system, thereby affecting the normal operation of other equipment and system stability.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a method for suppressing low-frequency oscillations in a coordinated manner by following and building a network based on reverse injection, comprising the following steps: S1: Extract low-frequency oscillation components in GFM converter; S2: setting a reverse injection response threshold and comparing the extracted low-frequency oscillation component with the reverse injection response threshold; S3: When the amplitude of the low-frequency oscillation component is greater than or equal to the reverse injection response threshold, output the intermediate threshold coefficient , otherwise output the intermediate threshold coefficient , and start the delay mechanism to make the output threshold coefficient Maintain the state of 1 for 0.2 seconds and then switch to 0; S4: For a single GFL converter, when the low-frequency oscillation component exceeds the reverse injection response threshold, the GFL converter is controlled to inject low-frequency oscillation components of equal amplitude and opposite direction to achieve positive and negative component cancellation at the PCC point, thereby suppressing the propagation of low-frequency oscillation; S5: For multiple GFL converters, when the low-frequency oscillation component exceeds the reverse injection response threshold, an adaptive reverse injection distribution strategy based on dynamic power margin is adopted to distribute the total reverse injection current component to each GFL converter to suppress the propagation of low-frequency oscillation.

[0007] Furthermore, the S1 includes the following sub-steps: S11: Obtain the output current component of the GFM converter based on the phase angle of the GFL converter and ; S12: According to the current component and Extract the steady-state DC component and , and then the current component and Subtract the steady-state DC component and , obtain the low-frequency oscillation component and , the formula is:

[0008] in, is the dq axis current component, is the steady-state DC component of the dq axis, is the dq axis low frequency oscillation component, is the time constant of the low-pass filter, is the Laplace operator.

[0009] Furthermore, the delay mechanism is implemented as follows: When the threshold coefficient is output at the previous moment When When , clear the timer; when ,or If the timer recording time does not exceed 0.2 seconds, keep .

[0010] Furthermore, the step S4 of controlling the GFL converter to inject low-frequency oscillation components with equal amplitudes and opposite directions comprises the following sub-steps: S41: The low frequency oscillation component and Multiply by the output threshold coefficient , obtain the correction amount; S42: Set the initial current reference value of the GFL converter and Subtract the correction amount to get the new current reference value and ; S43: Set the new current reference value and The current is input into the inner loop of the GFL converter, and the inner loop outputs a low-frequency oscillation component with the same amplitude and opposite direction as the GFM converter in real time, achieving the cancellation of positive and negative components at the PCC point and suppressing the propagation of low-frequency oscillation.

[0011] Furthermore, the adaptive reverse injection allocation strategy based on dynamic power margin in S5 includes the following sub-steps: S51: Calculate the adjustable margin of each GFL converter in real time, where the adjustable margin is the difference between the current rated capacity of the GFL converter and the current output power; S52: Calculate the proportion of each GFL converter in the total margin based on the adjustable margin, and use it as the weighting coefficient of the current GFL converter. The formula is:

[0012] in, is the weighting coefficient, is the adjustable margin, is the number of GFL converters, is the rated capacity of the GFL converter; S53: Allocate the total reverse injection current component to each GFL converter according to the weighted coefficient, and set the upper and lower limits of the injection amount to .

[0013] The beneficial effects of the present invention are as follows: by introducing the GFM current low-frequency component detection and GFL reverse injection control strategy, the present invention effectively suppresses the propagation of low-frequency oscillation in the parallel GFM and GFL system, which has the following advantages: Suppressing the Propagation of Low-Frequency Oscillations: In traditional parallel systems, low-frequency power oscillations generated by GFMs under disturbances can easily propagate through the grid, causing system resonance. This invention extracts the low-frequency oscillation component from the GFM output current and injects a compensating current in the opposite direction into the GFL. This achieves partial cancellation of the oscillation component, mitigating its impact on other system units at the source and significantly improving the dynamic stability of the grid-connected system.

[0014] Adaptive judgment mechanism enhances control continuity: To address the problem of repeated start and stop of inhibitory control caused by frequent disturbances during system operation, the present invention introduces a delayed judgment mechanism to ensure that control can be maintained for a period of time when the disturbance just falls below the threshold, effectively avoiding false triggering and frequent switching, and improving the robustness and practicality of the control strategy.

[0015] Adaptive reverse injection allocation based on dynamic power margin: To address the common problem of multiple GFLs connected in parallel in new energy stations, and the inconsistent operating conditions of different GFLs, this invention further proposes an injection allocation mechanism based on dynamic power margin. This ensures that each GFL participates in control according to its capacity, avoiding overload or insufficient response of a single device. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a flow chart of a method for suppressing low-frequency oscillations in coordination with network following and network building based on reverse injection.

[0017] Figure 2 Schematic diagram of the parallel system of grid-following and grid-forming converters.

[0018] Figure 3 Schematic diagram of the converter control structure.

[0019] Figure 4 This is a control diagram of the low-frequency oscillation suppression strategy.

[0020] Figure 5 Schematic diagram of the GFM oscillation component extraction module.

[0021] Figure 6 Schematic diagram of the threshold judgment link.

[0022] Figure 7 Schematic diagram of the effect of the delay judgment link.

[0023] Figure 8 Schematic diagram of the modified structure of the GFL current reference value.

[0024] Figure 9 Schematic diagram of the multi-GFL collaborative reverse injection control structure based on power margin.

[0025] Figure 10 This is the power response curve of GFM and GFL under GFM disturbance.

[0026] Figure 11 The comparison diagram of the active power output of the PCC point when the suppression strategy is adopted is shown below.

[0027] Figure 12 The comparison diagram of the active power output of the PCC point when the suppression strategy is adopted is shown below. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0029] like Figure 1 As shown, a method for suppressing low-frequency oscillations in coordination with network following and network building based on reverse injection includes the following steps: S1: Extract low-frequency oscillation components in GFM converter; S2: setting a reverse injection response threshold and comparing the extracted low-frequency oscillation component with the reverse injection response threshold; S3: When the amplitude of the low-frequency oscillation component is greater than or equal to the reverse injection response threshold, output the intermediate threshold coefficient , otherwise output the intermediate threshold coefficient , and start the delay mechanism to make the output threshold coefficient Maintain the state of 1 for 0.2 seconds and then switch to 0; S4: For a single GFL converter, when the low-frequency oscillation component exceeds the reverse injection response threshold, the GFL converter is controlled to inject low-frequency oscillation components of equal amplitude and opposite direction to achieve positive and negative component cancellation at the PCC point, thereby suppressing the propagation of low-frequency oscillation; S5: For multiple GFL converters, when the low-frequency oscillation component exceeds the reverse injection response threshold, an adaptive reverse injection distribution strategy based on dynamic power margin is adopted to distribute the total reverse injection current component to each GFL converter to suppress the propagation of low-frequency oscillation.

[0030] In one embodiment of the present invention, Figure 2 As shown, Figure 2 The following is a schematic diagram of the parallel system of grid-following and grid-forming converters. E 、 I L is the output voltage and current of the converter; V 、 I is the filter output voltage and current; L f 、 C f 、 R f They are filter inductance, capacitance and parasitic resistance respectively; L l 、 R lis the inductance and resistance of the inverter side line; L g 、 R g is the grid-side line inductance and resistance; V pcc 、 V g are the voltage at the point of common connection (PCC) and the grid-side voltage, respectively. The subscript “1” represents the parameters on the GFL converter side, and “2” represents the parameters on the GFM converter side.

[0031] The control structures of the two are as follows Figure 3 As shown in Figure 2, all control variables and subsequent analysis parameters are expressed in per-unit values. The present invention adopts the following symbol conventions: subscript " dq " represents the component of the variable in the dq coordinate system, and the superscript "*" represents the reference value in the control link.

[0032] Figure 3 Figure (a) shows the control structure of the GFL converter, which uses the simplest constant current control. This control is synchronized with the grid through a phase-locked loop (PLL), and the current inner loop is used to control the current to track the reference value. V 1d 、 V 1q for V dq component of 1; ω 1. θ 1 is the frequency and phase angle generated by the phase-locked loop, ω B is the reference value of angular frequency, s stands for Laplace operator; I 1d * 、 I 1q * is the current reference value.

[0033] Figure 3 Figure (b) shows the control structure of the GFM converter, which uses the most typical virtual synchronous generator (VSG) control. This control uses the active loop to simulate the synchronous generator's rotor motion equations to generate frequency and phase angle, and the reactive loop to control voltage. J and D are the virtual inertia and damping coefficient; P 2. P 2 * is the active power and its reference value; ω 2. θ 2 is the frequency and phase angle generated by the active loop; kq is the droop coefficient of the reactive loop; Q 2. Q 2 * is reactive power and its reference value; V 2d * 、 V 2q * Voltage reference value generated for the reactive loop.

[0034] When the grid-connected converter in the system is disturbed or fails, it may cause low-frequency oscillation. This type of oscillation has the characteristics of wide propagation range and long duration, and is easy to spread to the entire power system through the common connection point (PCC), thereby affecting the stable operation of other equipment in the system. Figure 2 The present invention proposes a GFL / GFM collaborative low-frequency oscillation suppression strategy based on reverse injection, which detects the low-frequency oscillation component in the output current of the GFM converter and injects a compensating current with opposite phase into the GFL converter to achieve dynamic cancellation of the oscillation power, thereby weakening its propagation path and improving system damping. Furthermore, considering that different units in a multi-GFL system have different reverse injection capabilities due to differences in operating status, resource conditions (such as light, wind speed), etc., the present invention introduces an adaptive injection allocation mechanism based on dynamic power margin to ensure that each GFL unit reasonably shares the suppression task according to its current tolerance, avoiding overload or insufficient participation of a single unit. The specific design process is as follows: like Figure 4 The figure shows the control diagram of the low frequency oscillation suppression strategy proposed by the present invention. When the GFM converter has low frequency oscillation, its output current contains the fundamental component. I 20 and low-frequency oscillation components I 2h This strategy extracts the oscillating component of the current I 2h , and set the threshold I th .when I 2h When the threshold is exceeded, the GFL converter is controlled to inject an oscillation component with equal amplitude and opposite direction. I 2h , thereby achieving the offset of positive and negative components at the PCC point and suppressing the propagation of low-frequency oscillations into the power system.

[0035] In step S1, the low frequency oscillation component in the GFM converter is extracted. The present invention designs the following Figure 5The oscillation component extraction module shown in the figure. First, the phase angle of the GFL converter is used θ 1 pair of GFM output current I 2. Perform dq coordinate transformation. Since subsequent control operations are all implemented in GFL, the phase angle of GFL needs to be used. θ 1 rather than the GFM phase angle itself. The current components obtained by transformation I 2d 、 I 2q The steady-state DC component is extracted through low-pass filters respectively I 2d0 、 I 2q0 , then subtract the DC component from the original signal to obtain the low-frequency oscillation component I 2dh 、 I 2qh .

[0036] The S1 includes the following steps: S11: Use the phase angle of the GFL converter to perform dq coordinate transformation on the output current of the GFM converter to obtain the current components and ; S12: The current component and The steady-state DC components are extracted by low-pass filters. and , and then the current component and Subtract the steady-state DC component and , obtain the low-frequency oscillation component and , the formula is:

[0037] in, is the dq axis current component, is the steady-state DC component of the dq axis, is the dq axis low frequency oscillation component, is the time constant of the low-pass filter, is the Laplace operator.

[0038] In step S2, since the actual power system is often affected by factors such as environmental changes during operation, it is easy to generate small perturbations. If the GFL responds to these small perturbations in steady state, it may lead to unnecessary control actions, thereby affecting the stable operation of the system. Therefore, it is necessary to design a threshold judgment link to judge the extracted oscillation components to avoid triggering suppression control due to perturbations. The threshold judgment mechanism is as follows: Figure 6 shown.

[0039] In order to prevent the suppression control from being interrupted immediately when the oscillation component just drops below the threshold, a delay mechanism is introduced into the judgment logic: K 1 switches from 1 to 0, the output threshold coefficient K T Maintaining 1 delays for 0.2 seconds. The effect of the delay judgment logic is as follows Figure 7 shown.

[0040] The delay mechanism is implemented as follows: When the threshold coefficient is output at the previous moment When When , clear the timer; when ,or If the timer recording time does not exceed 0.2 seconds, keep .

[0041] In step S4, the threshold coefficient is obtained K T After that, the current reference value of GFL can be adjusted to make it output the reverse low-frequency oscillation component - I 2h ,like Figure 8 shown.

[0042] The step S4 of controlling the GFL converter to inject low-frequency oscillation components with equal amplitude and opposite direction includes the following steps: S41: The low frequency oscillation component and Multiply by the output threshold coefficient , obtain the correction amount; S42: Set the initial current reference value of the GFL converter and Subtract the correction amount to get the new current reference value and ; S43: Set the new current reference value and The current is input into the inner loop of the GFL converter, and the inner loop outputs a low-frequency oscillation component with the same amplitude and opposite direction as the GFM converter in real time, achieving the cancellation of positive and negative components at the PCC point and suppressing the propagation of low-frequency oscillation.

[0043] Considering that GFLs are usually deployed in renewable energy stations, and that multiple devices of the same model are often operated in parallel within the stations, the operating conditions of different devices vary due to distributed factors such as wind speed and sunlight, resulting in different actual output powers. The output power of some GFL units is close to the rated value, and the adjustment margin available for reverse injection is small; while other units have lower power output and greater reverse injection capabilities. In order to improve the overall oscillation suppression capability of the system, it is necessary to adaptively improve the aforementioned reverse injection control strategy in a multi-GFL scenario. The specific improvement methods are as follows: The adaptive reverse injection allocation strategy based on dynamic power margin in S5 includes the following steps: S51: Calculate the adjustable margin of each GFL converter in real time, where the adjustable margin is the difference between the current rated capacity of the GFL converter and the current output power; S52: Calculate the proportion of each GFL converter in the total margin based on the adjustable margin, and use it as the weighting coefficient of the current GFL converter. The formula is:

[0044] in, is the weighting coefficient, is the adjustable margin, is the number of GFL converters, is the rated capacity of the GFL converter; S53: Allocate the total reverse injection current component to each GFL converter according to the weighted coefficient to ensure that the injection burden matches the equipment capacity. The injection volume of each GFL is , and set the upper and lower limits of injection volume to .

[0045] To prevent individual devices from being overloaded due to excessive injection, the upper and lower limits of injection volume are set as This strategy distributes reverse injection tasks according to capabilities, forming a multi-point collaborative and resource-balanced oscillation suppression mechanism, significantly enhancing the overall stability of the system. Figure 9 As shown, it should be noted that when the system contains multiple GFM devices, the extracted current signal It represents the sum of all GFM output currents, i.e. the total current of multiple GFMs connected in parallel.

[0046] In this embodiment, in order to verify the effectiveness of the proposed control strategy, a MATLAB / Simulink platform was built as follows: Figure 2 The main simulation parameters of the system are shown in Table 1.

[0047] Table 1 Simulation parameters

[0048] In order to verify whether the proposed strategy can drive the GFL to emit a power component opposite to the low-frequency oscillation of the GFM, a 0.2MW step disturbance is applied to the active power instruction of the GFM at 4s in the simulation. The power response curves of the GFM and GFL are shown in the figure below. Figure 10 shown.

[0049] from Figure 10 It can be observed that when the GFM power exhibits low-frequency oscillations, the GFL power exhibits an opposite oscillation trend, with clear symmetry between the two. This indicates that the GFL successfully injects an opposite oscillating power component. Furthermore, after the oscillating component is suppressed, the GFL power returns to its original steady-state value, demonstrating that the proposed strategy successfully suppresses oscillations without affecting the GFL's original steady-state operation.

[0050] To further validate the effectiveness of the proposed "adaptive reverse injection allocation strategy based on dynamic power margin," a system scenario was constructed, consisting of three GFL units and two GFM units. The active power output of the three GFL units was set to 0.7 pu, 0.8 pu, and 0.9 pu, respectively, to simulate power margin variations under different operating conditions. During the simulation, active power command step disturbances of -0.1 MW and -0.2 MW were injected into the two GFM units at 4 seconds, respectively, to examine the response of each GFL unit to the reverse injection. Figure 11 The active output responses of three GFL units are demonstrated to verify the allocation rationality and vibration suppression effect of the proposed strategy under multi-machine coordination.

[0051] It can be observed that the three GFL units each bear different magnitudes of reverse injection components based on their own dynamic power margins. GFL1 has the largest margin and bears the majority of the reverse injection task; GFL3 has the smallest margin and only injects a smaller compensation component. This result demonstrates that the proposed allocation mechanism can achieve reasonable coordination among the units, effectively avoid overloading individual units, and improve the overall system's responsiveness and operational safety.

[0052] In order to further verify the control effect of the proposed suppression strategy on the propagation of low-frequency oscillation, the dynamic response of the PCC point output active power when the suppression strategy is not adopted and when the suppression strategy is enabled is compared and analyzed. The results are as follows: Figure 12 shown.

[0053] Depend on Figure 12 It can be seen that when the suppression strategy is not adopted, the active power output of the PCC point shows obvious low-frequency oscillation, which has a tendency to propagate to the power system, which may have an adverse impact on other equipment; after adopting the suppression strategy, the output power of the PCC point is stable and has no fluctuations, verifying the effectiveness of the proposed method.

[0054] The simulation results demonstrate that the proposed reverse injection suppression strategy can effectively guide the GFL converter to actively inject compensating power in the opposite direction of the GFM low-frequency oscillation component, dynamically offsetting the low-frequency oscillation and thereby blocking its propagation path within the system, thereby improving the overall stability of the power system. Furthermore, the reverse injection allocation mechanism based on dynamic power margin can dynamically allocate injection tasks based on the current operating status of each GFL unit, fully leveraging the control potential of each unit, avoiding single-unit overload, and improving the system's adaptability and robustness under different operating conditions, further validating the practical value of this allocation strategy. Furthermore, the results also confirm the effectiveness of the designed delay judgment mechanism in maintaining control continuity, further demonstrating the strategy's excellent dynamic adaptability and engineering feasibility.

[0055] This paper proposes a reverse-injection-based method for suppressing low-frequency oscillations in both grid-following and grid-building collaboration. This strategy innovatively leverages the superior current control capabilities of the GFL to detect low-frequency oscillations in the GFM output current in real time. It then injects the opposite low-frequency component into the GFL's current command. The GFL actively outputs a power signal opposite to the GFM oscillation component, effectively canceling the GFM's low-frequency oscillations and blocking the grid-side propagation path. This method, which requires no changes to the GFM's parameters and boasts excellent system compatibility and scalability, provides a new approach for improving the operational stability of GFM-GFL hybrid systems.

[0056] To avoid frequent activation and deactivation of the control strategy in conditions bordering on disturbances, this paper introduces an adaptive judgment mechanism based on time delay. This mechanism, by setting a hold time, ensures that even if the oscillation indicator momentarily falls below the activation threshold, control can still be maintained for a period of time, preventing false triggering and control oscillation. This enhances the control continuity and robustness of the strategy, making it particularly suitable for multi-disturbance scenarios in real engineering applications.

[0057] To address the issue of uneven power output caused by parallel operation of multiple GFLs in practical renewable energy stations, this paper proposes a dynamic power margin allocation mechanism. When reverse injection control is initiated, the current available margin of each GFL device is calculated in real time and weighted to allocate the required reverse power injection. This ensures that each device participates in control according to its own capabilities, improving the strategy's applicability and engineering scalability in multi-GFL scenarios.

[0058] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and it should be understood that the scope of protection of the present invention is not limited to such specific descriptions and embodiments. Those skilled in the art can make various other specific variations and combinations based on the technical teachings disclosed in the present invention without departing from the essence of the present invention, and such variations and combinations are still within the scope of protection of the invention.

Claims

1. A method for suppressing low-frequency oscillations in coordination with network building and network following based on reverse injection, characterized in that: The following steps are involved: S1: Extract low-frequency oscillation components in GFM converter; S2: setting a reverse injection response threshold and comparing the extracted low-frequency oscillation component with the reverse injection response threshold; S3: When the amplitude of the low-frequency oscillation component is greater than or equal to the reverse injection response threshold, output the intermediate threshold coefficient , otherwise output the intermediate threshold coefficient , and start the delay mechanism to make the output threshold coefficient Maintain the state of 1 for 0.2 seconds and then switch to 0; S4: For a single GFL converter, when the low-frequency oscillation component exceeds the reverse injection response threshold, the GFL converter is controlled to inject low-frequency oscillation components of equal amplitude and opposite direction to achieve positive and negative component cancellation at the PCC point, thereby suppressing the propagation of low-frequency oscillation; S5: For multiple GFL converters, when the low-frequency oscillation component exceeds the reverse injection response threshold, an adaptive reverse injection distribution strategy based on dynamic power margin is adopted to distribute the total reverse injection current component to each GFL converter to suppress the propagation of low-frequency oscillation.

2. The method for suppressing low-frequency oscillations in coordination with network building based on reverse injection according to claim 1 is characterized in that: The S1 includes the following steps: S11: Obtain the output current component of the GFM converter based on the phase angle of the GFL converter and ; S12: According to the current component and Extract the steady-state DC component and , and then the current component and Subtract the steady-state DC component and , obtain the low-frequency oscillation component and , the formula is: in, is the dq axis current component, is the steady-state DC component of the dq axis, is the dq axis low frequency oscillation component, is the time constant of the low-pass filter, is the Laplace operator.

3. The method for suppressing low-frequency oscillations in coordination with network building based on reverse injection according to claim 1, characterized in that: The delay mechanism is implemented as follows: When the threshold coefficient is output at the previous moment When When , clear the timer; when ,or If the timer recording time does not exceed 0.2 seconds, keep .

4. The method for suppressing low-frequency oscillations in coordination with network building based on reverse injection according to claim 3 is characterized in that: In S4, the GFL converter is controlled to inject low-frequency oscillation components with equal amplitude and opposite direction. The following steps are included: S41: The low frequency oscillation component and Multiply by the output threshold coefficient , obtain the correction amount; S42: Set the initial current reference value of the GFL converter and Subtract the correction amount to get the new current reference value and ; S43: Set the new current reference value and The current is input into the inner loop of the GFL converter, and the inner loop outputs a low-frequency oscillation component with the same amplitude and opposite direction as the GFM converter in real time, achieving the cancellation of positive and negative components at the PCC point and suppressing the propagation of low-frequency oscillation.

5. The method for suppressing low-frequency oscillation in coordination with network building based on reverse injection according to claim 1 is characterized in that: The adaptive reverse injection allocation strategy based on dynamic power margin in S5, The following steps are included: S51: Calculate the adjustable margin of each GFL converter in real time, where the adjustable margin is the difference between the current rated capacity of the GFL converter and the current output power; S52: Calculate the proportion of each GFL converter in the total margin based on its adjustable margin, and use it as the weighting coefficient of the current GFL converter. The formula is: in, is the weighting coefficient, is the adjustable margin, is the number of GFL converters, is the rated capacity of the GFL converter; S53: Allocate the total reverse injection current component to each GFL converter according to the weighted coefficient, and set the upper and lower limits of the injection amount to .

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