A grid frequency dip based gfm inverter current limiting method

Abstract

The application provides a GFM inverter current limiting method based on grid frequency drop, and relates to the technical field of power electronic control. The method comprises the following steps: collecting the output current of the GFM inverter and performing overcurrent detection, if overcurrent, the current limiting algorithm is put into operation; when the grid frequency recovers to the rated frequency, the current limiting algorithm is cut off; the current limiting algorithm comprises the following steps: based on the output voltage and the output current of the GFM inverter, the active power is calculated, and the angular frequency of the inverter is further calculated; based on the angular frequency of the inverter, the power angle increment is calculated, and the active power increment is further calculated; the difference between the active power and the active power increment is taken as the substitute active power, the droop control is performed, and the current limiting is realized. According to the current limiting algorithm, the grid angular frequency can be quickly tracked, the GFM inverter can quickly reach a new working point after the grid fault, and the dynamic performance of the GFM inverter can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of power electronic control technology, and in particular to a current limiting method for GFM inverters based on grid frequency drops. Background Technology

[0002] In recent years, new energy sources, represented by wind power and photovoltaic power generation, have experienced rapid growth, with inverters, primarily powered by power electronics, serving as the interface for grid connection. As the proportion of traditional synchronous generators decreases, energy storage, represented by pumped hydro storage and electrochemical energy storage, is gradually increasing. Full-power pumped hydro storage power stations and electrochemical energy storage power stations also use inverters as the grid connection interface. However, current grid-connected inverters typically employ grid-following (GFL) control to regulate the grid-connected current. Because GFL inverters generally lack spinning reserve capacity and rotational inertia, they cannot provide an inertial response similar to that of traditional synchronous generators. Conversely, grid-forming (GFM) inverters employ a power synchronization strategy similar to synchronous generators, possessing virtual inertia and damped response, giving them a natural advantage in grids with weak system strength and low physical inertia.

[0003] However, during grid frequency dips, the frequency changes of GFM inverters, which possess virtual inertia, lag behind the grid frequency, resulting in frequency deviation. This leads to an increase in the power angle between the GFM inverter and the grid, potentially causing overcurrent. Therefore, addressing overcurrent during grid frequency dips becomes a critical issue, and GFM inverters also need to possess a certain degree of fault recovery capability. Furthermore, commonly used current limiting strategies require corresponding control methods for activation and deactivation. How to quickly activate and deactivate overcurrent protection and current limiting strategies, avoiding repeated switching of current limiting strategies during overcurrent periods, and resetting the control enable signal after fault recovery are also key issues in switching strategies.

[0004] To address the overcurrent issue in GFM inverters during grid faults, current limiting strategies can be added to the control circuit. Currently, current limiting strategies can be categorized into three types: direct current limiting, indirect current limiting, and hybrid current limiting. Direct current limiting primarily uses current limiters to restrict the current reference value. However, direct current limiting causes the inverter to exhibit current source characteristics, potentially leading to voltage outer loop saturation. Furthermore, the voltage control loop is difficult to restore after the fault is cleared, compromising the stable operation of the GFM inverter and even potentially causing transient instability. Indirect current limiting can be categorized into: modifying the power reference value, virtual impedance, and voltage limiters. Modifying the power reference value limits the output current by limiting the output power of the GFM inverter. However, this method suffers from poor dynamic response characteristics of the output current due to the low bandwidth of external control. The virtual impedance-based current limiting method reduces the output current of the GFM inverter by effectively increasing its total output impedance. However, the required virtual impedance varies depending on the severity of the fault, making precise design of the virtual impedance challenging. Adding a voltage limiter limits the output current by adjusting the amplitude and phase of the reference voltage generated by the power outer loop. To ensure the fault recovery capability of this method, appropriate anti-saturation design is required for the outer loop controller.

[0005] Furthermore, current reports on grid current limiting mostly focus on overcurrent caused by grid voltage dips. There is a lack of mechanistic analysis and corresponding current limiting methods for overcurrent issues caused by grid frequency drops. Summary of the Invention

[0006] To address the aforementioned technical problems in the prior art, this invention aims to provide a current limiting method suitable for solving overcurrent problems caused by power grid frequency drops.

[0007] Therefore, this invention provides a current limiting method for GFM inverters based on grid frequency drops, the specific technical solution of which includes:

[0008] The output current of the GFM inverter is collected and overcurrent detection is performed. If the output current is overcurrent, the current limiting algorithm is activated; when the grid frequency returns to the rated frequency, the current limiting algorithm is deactivated.

[0009] The rate limiting algorithm includes:

[0010] The active power is calculated based on the output voltage and output current of the GFM inverter after dq transformation.

[0011] The angular frequency of the GFM inverter is calculated based on the active power through droop control.

[0012] Calculate the power angle increment based on the angular frequency of the GFM inverter;

[0013] Calculate the active power increment based on the power angle increment;

[0014] The difference between the active power and the active power increment is used to replace the active power, and droop control is performed to achieve current limiting.

[0015] The power angle increment is calculated based on the angular frequency of the GFM inverter, specifically including:

[0016] ;

[0017] In the formula, For the increment of the work angle, This represents the phase of the output voltage of the GFM inverter. The phase of the grid voltage. This represents the start time of the frequency drop. This is the end time of the frequency drop in the GFM inverter. The angular frequency of the GFM inverter. This is the angular frequency of the power grid.

[0018] Preferably, the output current of the GFM inverter is collected and overcurrent detection is performed. If the output current is overcurrent, the current limiting algorithm is activated; when the grid frequency returns to the rated frequency, the current limiting algorithm is deactivated. Specifically, the following steps are included:

[0019] Step S11: Acquire the output current of the GFM inverter And perform overcurrent detection;

[0020] If the current If there is an overcurrent, a high-level signal is generated and input to the S terminal of the first SR latch, causing the first SR latch to output a high level and control the activation of the current limiting algorithm.

[0021] Step S12: After the current limiting algorithm is activated, the high-level signal at the S terminal of the first SR latch is converted to a low-level signal, and the low-level signal output by the second SR latch is input to the R terminal of the first SR latch, so that the current limiting algorithm is continuously activated.

[0022] Step S13: After implementing the current limiting algorithm, continuously monitor the grid frequency. If the power grid frequency When the frequency is restored to the rated frequency of the power grid, a high-level signal is generated and input to the S terminal of the second SR latch, causing the second SR latch to output a high level and input to the R terminal of the first SR latch, causing the first SR latch to output a low-level signal, thereby controlling the current limiting algorithm to be cut off.

[0023] Furthermore, after the current limiting algorithm is removed, the R terminals of both the first SR latch and the second SR latch are set to 1.

[0024] Preferably, the active power is calculated based on the output voltage and output current of the GFM inverter after dq transformation, specifically including:

[0025] ;

[0026] In the formula, This refers to the active power output of the GFM inverter. Let be the voltage of the GFM inverter in the dq rotating coordinate system. Let be the current conjugate value of the GFM inverter in the dq rotating coordinate system. This indicates taking the real part.

[0027] Preferably, the angular frequency of the GFM inverter is calculated based on the active power through droop control, specifically including:

[0028] The angular frequency of the GFM inverter is calculated using droop control with a low-pass filter. ,

[0029] The formula is as follows:

[0030] ;

[0031] In the formula, This is the active power droop control coefficient. The time constant of the low-pass filter. For the Lagrangian operator, This is a reference value for the active power output of the GFM inverter. This refers to the active power output of the GFM inverter. This is the rated angular frequency of the GFM inverter.

[0032] Preferably, the active power increment is calculated based on the power angle increment, specifically including:

[0033] ;

[0034] In the formula, For active power increment, This represents the effective value of the output voltage of the GFM inverter. This is the effective value of the grid voltage. For equivalent line reactance, This refers to the rated power angle of the GFM inverter. For the increment of the work angle, This refers to the rated value of the active power output of the GFM inverter.

[0035] Furthermore, it also includes:

[0036] When implementing the current limiting algorithm, a virtual impedance is simultaneously applied. ;

[0037] Among them, virtual impedance satisfy

[0038] ;

[0039] In the formula, The rated voltage of the power grid. This represents the maximum value of the grid voltage phase. This represents the maximum output current of the GFM inverter. This is the equivalent line impedance of the power grid.

[0040] As can be seen, the technical solution provided by this invention can quickly track the grid angular frequency according to the current limiting algorithm, so that the GFM inverter can quickly reach a new operating point after the grid frequency drops and the resulting overcurrent, which can effectively improve the dynamic performance of the GFM inverter. Attached Figure Description

[0041] Figure 1 This is a structural diagram of the droop inverter topology and its control system used in this invention.

[0042] Figure 2 This is a schematic diagram of the switching strategy based on the SR latch in this invention.

[0043] Figure 3 This is a schematic diagram of the power angle change corresponding to the power grid frequency drop in this invention.

[0044] Figure 4 This is a schematic diagram of adaptive frequency tracking based on frequency drop in this invention.

[0045] Figure 5 This is a voltage relationship diagram under the maximum active power support in this invention.

[0046] Figure 6 This is a block diagram of a voltage and current dual-loop control system with added variable virtual impedance in this invention.

[0047] Figure 7 The waveforms showing the frequency and power angle increments of the current limiting method in this invention are shown.

[0048] Figure 8 The output waveform of the GFM inverter with the current limiting method added in this invention is shown. Detailed Implementation

[0049] The technical solution provided by the present invention will be further described in detail below with reference to the accompanying drawings.

[0050] This invention provides a current limiting method for GFM inverters based on grid frequency dips. This method analyzes the overcurrent generation mechanism based on power angle changes and proposes an adaptive frequency tracking strategy based on frequency dips. This strategy adaptively reduces the power angle increment between the GFM inverter and the grid during grid frequency dips, thereby achieving current limiting. Furthermore, since a residual power angle exists from the start of a fault until the current limiting strategy is implemented, leading to an increase in output current, this invention also proposes a variable virtual impedance calculation method based on the residual power angle to mitigate overcurrent caused by the residual power angle.

[0051] like Figure 1 As shown, this invention employs a typical droop-controlled three-phase grid-connected inverter topology and control system. Among them, The DC side voltage of the GFM inverter is typically supplied by photovoltaic or wind turbines; the AC side of the GFM inverter uses an LC filter to remove high-frequency harmonics, and finally passes through the grid-side equivalent inductance via the point of common coupling (PCC). Connected to the power grid, the grid voltage is The control system of a GFM inverter includes power calculation, power control, and a voltage PI controller. v PI controller G C and modulation stage.

[0052] Among them, the voltage at point PCC and current The voltage in the dq rotating coordinate system is obtained after dq transformation. and current Thus, the active power can be calculated. and reactive power .

[0053] in,

[0054] ;

[0055] In the formula, This refers to the active power output of the GFM inverter. Let be the current conjugate value of the GFM inverter in the dq rotating coordinate system. This indicates taking the real part.

[0056] Phase obtained by active droop control Used for dq transformation, while the voltage amplitude obtained from reactive power droop control is used as the d-axis voltage reference value. Set the voltage reference value. Compared with actual value The difference is controlled by the voltage PI controller G. v Obtain the current reference value Then the current reference value Compared with the actual current value The difference is controlled by the current PI controller G. C The modulated voltage is obtained, and finally, a pulse signal to drive the switching transistor is generated through sinusoidal pulse width modulation (SPWM). The actual current value i Ldq By sampling the output current of the GFM inverter It is obtained through dq transformation.

[0057] The switching logic of the current limiting strategy is as follows: During normal operation, the current limiting algorithm is not activated. When the output current exceeds the set threshold, the current limiting algorithm is switched to suppress the fault current. When the grid frequency recovers, it switches back to normal operation mode. Furthermore, the output current should be limited to the allowable range, i.e., the amplitude of the output current should be less than 1.3 pu. Therefore, the current change caused by a drop in grid frequency should be less than 0.3 pu. To retain a certain safety margin, the overcurrent threshold is set to 1.2 pu.

[0058] This invention proposes an event-triggered SR latch to implement the switching logic of the rate limiting strategy. The corresponding switching control block diagram is as follows: Figure 2 As shown. First, the three-phase current is measured. The current is then compared with a current threshold. A high-level signal is generated when an overcurrent is detected. During a grid frequency dip, the current limiting algorithm must remain active until the frequency recovers once an overcurrent is detected. Secondly, the current limiting algorithm can be deactivated when the grid frequency is detected to have returned to the rated frequency, typically 50Hz.

[0059] The main steps of the current limiting algorithm include: calculating the active power based on the output voltage and output current of the GFM inverter after dq transformation; calculating the angular frequency of the GFM inverter based on the active power through droop control; calculating the power angle increment based on the angular frequency of the GFM inverter; calculating the active power increment based on the power angle increment; replacing the active power with the difference between the active power and the active power increment, and performing droop control to achieve current limiting.

[0060] It's important to note that overcurrent caused by other factors does not result in frequency drops or power angle increases, therefore the aforementioned current limiting algorithm will not function. Thus, it's not necessary to pre-detect for frequency drops before implementing the current limiting algorithm. This allows for the implementation of the current limiting algorithm with minimal delay when facing overcurrent caused by frequency drops.

[0061] The truth table of the latches is shown in Table 1. When an overcurrent is detected, S = 1 and R = 0 for SR latch 1. At this time, SR latch 1 outputs a high level, and the control mode switches to the current limiting algorithm. After the current limiting algorithm is activated, SR latch 1 should remain at a high level to avoid repeated switching of the current limiting algorithm.

[0062] Table 1 Truth Table of SR Latch

[0063]

[0064] During a power grid fault (frequency drop causing overcurrent), SR latch 2 outputs a low-level signal. When the power grid frequency recovers to its rated frequency, the power grid frequency recovery detection outputs a high-level signal, and the Q of SR latch 2 equals the R of SR latch 1. Therefore, when the power grid frequency recovers, the R of SR latch 1 (enabling / disabling the current limiting algorithm) will be at a high level, i.e., the current limiting algorithm is switched off. After the current limiting algorithm is switched off, the R of all SR latches is set to 1 to clear the current state for subsequent overcurrent applications.

[0065] In practice, overcurrent detection uses high-speed sampling. Upon detecting an overcurrent, the protection logic is triggered directly, without following a specific timing sequence. That is, both the control logic and protection logic trigger event interruptions.

[0066] Power angle increment during grid frequency sag like Figure 3 As shown. Among them, The rated angular frequency, This is the minimum angular frequency, and also the threshold for under-frequency load shedding (UFLS). t0 is the start time of the frequency drop, t... grid It is the end time of the power grid frequency drop, t inv It is the end time of the frequency drop in the GFM inverter.

[0067] When the grid frequency decreases at a certain rate of change, due to the virtual inertia inside the GFM inverter, the rate of change of the GFM inverter's angular frequency lags behind the grid's angular frequency.

[0068] Gongjiao This means that the phase difference between the output voltage of the GFM inverter and the grid voltage increases, thus increasing the power angle increment. This can be deduced as:

[0069] ;

[0070] In the formula, This represents the phase of the output voltage of the GFM inverter. The phase of the grid voltage. This represents the start time of the frequency drop. This is the end time of the frequency drop in the GFM inverter. This is the angular frequency of the power grid.

[0071] The above The following formula for droop control with a low-pass filter is used:

[0072] ;

[0073] In the formula, This is the active power droop control coefficient. The time constant of the low-pass filter. For the Lagrangian operator, This is a reference value for the active power output of the GFM inverter. This is the rated angular frequency of the GFM inverter.

[0074] Therefore, the power angle difference between the GFM inverter and the grid increases slowly over time, and the active power... This will therefore increase. In extreme cases, when the virtual inertia approaches infinity, the active power... This will continue to increase as the grid frequency decreases, thus triggering overcurrent.

[0075] In summary, the increased power angle is the main cause of overcurrent in GFM inverters during frequency dips. Therefore, reducing the power angle difference between the GFM inverter and the grid during grid frequency dips is a key measure for current limiting.

[0076] Rated power angle It can be represented as:

[0077] ;

[0078] In the formula, This refers to the rated active power output of the GFM inverter. This is the reference voltage for the GFM inverter. The rated voltage of the power grid. This is the equivalent line impedance of the power grid.

[0079] Therefore, the increase in active power due to the power angle difference It can be represented as:

[0080] ;

[0081] In the formula, This represents the effective value of the output voltage of the GFM inverter. This is the effective value of the grid voltage. For equivalent line reactance, This is the rated power angle of the GFM inverter.

[0082] Adaptive frequency tracking strategies based on frequency drop, such as Figure 4 As shown, this strategy calculates the corresponding power angle increment by detecting the angular frequency deviation between the GFM inverter and the grid. The power increment resulting from the power angle increment is then calculated. During normal operation, the power increment of the GFM inverter is set to 0; when an output current overcurrent is detected, the control mode switches to adaptive frequency tracking control; when the grid frequency returns to normal, the operating mode switches back to normal operation. The proposed strategy not only calculates adaptive coefficients based on the angular frequency deviation to track the grid angular frequency more quickly, but also enables the GFM inverter to quickly reach a new operating point after a grid fault, effectively improving the dynamic performance of the GFM inverter.

[0083] Considering that the output current is at its maximum when the reactive current on the grid side is zero under maximum active power support, we can obtain:

[0084] ;

[0085] In the formula, This represents the maximum output current of the GFM inverter. This represents the maximum active power of the GFM inverter, which is the maximum output current.

[0086] Under maximum active power support, the voltage relationship when the reactive power on the grid side is zero is as follows: Figure 5 As shown. The relationship between grid voltage, grid impedance voltage, and GFM inverter output voltage can be expressed as:

[0087] ;

[0088] In the formula, This refers to the output voltage of the GFM inverter under maximum active power support. and These are the equivalent line impedance and virtual impedance of the power grid, respectively. Overcurrents occurring between the occurrence of a fault and the activation of the current-limiting strategy require the application of a variable virtual impedance. .

[0089] Depend on Figure 5 It can be seen that when the power angle reaches its maximum value, the voltage drop between the line impedance and the virtual impedance also reaches its maximum value. To limit the fundamental current after the grid frequency drop, the line impedance and the applied variable virtual impedance should satisfy the following conditions:

[0090] ;

[0091] Conservatively, the variable virtual impedance should satisfy

[0092] ;

[0093] In the formula, This represents the maximum value of the grid voltage phase. During the implementation of variable virtual impedance and current limiting algorithms, these are applied synchronously.

[0094] The aforementioned variable virtual impedance is adaptively calculated through residual power angle increment, which can adaptively respond to overcurrent events caused by the time difference of the current limiting strategy without the need for complicated current limiting design.

[0095] In the event of a grid frequency drop, the reference voltage of the droop control reactive voltage link... It can be calculated as:

[0096] .

[0097] Furthermore, the voltage and current dual-loop control block diagram after adding the variable virtual impedance element is as follows: Figure 6 As shown. Among them, and The voltage drop is the result of decomposing the variable virtual impedance in the dq rotating coordinate system.

[0098] ;

[0099] in, , for By adding a variable virtual impedance element in the dq rotating coordinate system, we can not only utilize the decoupled output current of the GFM inverter in the dq rotating coordinate system, but also reduce the impact of higher harmonics of the output current on the variable virtual impedance element, which is easy to implement in engineering.

[0100] To verify the correctness and effectiveness of the proposed strategy, this invention first builds a simulation model in Matlab / Simulink to simulate the system time constant. The virtual inertia is set to 5 seconds. The grid frequency is set to drop to 49 Hz at a rate of 2 Hz / s starting at 0.6 s, and then rise back to 50 Hz at a rate of 2 Hz / s starting at 1.8 s. The main simulation parameters are shown in Table 2.

[0101] Table 2 Main Simulation Parameters

[0102]

[0103] Figure 7 and Figure 8 The waveforms showing the frequency and power angle increments and the output waveform of the GFM inverter are shown for switching between the proposed adaptive frequency tracking strategy and the variable virtual impedance current limiting strategy. It can be observed that when the grid frequency... As the frequency decreases at 2Hz / s, the output current of the GFM inverter gradually increases. When the current reaches the threshold, the protection logic of the SR latch is triggered, and then the current-limiting strategy is implemented to achieve rapid overcurrent protection. Although there is still a residual power angle increment between the GFM inverter and the grid, the variable virtual impedance calculated from the power angle increment can effectively suppress overcurrent. Furthermore, due to the presence of the variable virtual impedance, the active power decreases slightly, but recovers rapidly, thus providing active power support to the grid. The output current remains within a feasible range, with no overcurrent phenomenon. When the grid frequency recovers, the current-limiting strategy is disconnected, achieving rapid fault recovery. This is the output frequency of the GFM inverter.

[0104] In summary, the technical solution provided by this invention can quickly track the grid angular frequency based on the current limiting algorithm, enabling the GFM inverter to quickly reach a new operating point after an overcurrent caused by a frequency drop in the grid, thus effectively improving the dynamic performance of the GFM inverter.

[0105] Furthermore, the application of the SR latch can achieve rapid switching of the current limiting algorithm through a simple topology. After the current limiting algorithm is cut off, setting the R terminal of the SR latch to 1 can ensure a rapid response to overcurrent caused by the next frequency drop. The method for switching virtual impedance provided by this invention can design virtual impedance more accurately by decoupling the output current of the GFM grid-type inverter in the dq rotating coordinate system, and weaken the influence of high-order harmonics of the output current on the variable virtual impedance link. It can enhance the synchronous stability of the power grid while suppressing overcurrent during frequency drops, and can promote the fault recovery of the power grid.

Claims

1. A current limiting method for a GFM inverter based on grid frequency dips, characterized in that, include: The output current of the GFM inverter is collected and overcurrent detection is performed. If the output current is overcurrent, the current limiting algorithm is activated. When the grid frequency returns to the rated frequency, the current limiting algorithm is disconnected. The rate limiting algorithm includes: The active power is calculated based on the output voltage and output current of the GFM inverter after dq transformation. The angular frequency of the GFM inverter is calculated based on the active power through droop control. Calculate the power angle increment based on the angular frequency of the GFM inverter; Calculate the active power increment based on the power angle increment; The difference between the active power and the active power increment is used to replace the active power, and droop control is performed to achieve current limiting. The calculation of the power angle increment based on the angular frequency of the GFM inverter specifically includes: ； In the formula, For the increment of the work angle, This represents the phase of the output voltage of the GFM inverter. The phase of the grid voltage. This represents the start time of the frequency drop. This is the end time of the frequency drop in the GFM inverter. The angular frequency of the GFM inverter. This is the angular frequency of the power grid.

2. The current limiting method for a GFM inverter based on grid frequency dips as described in claim 1, characterized in that, The process involves collecting the output current of the GFM inverter and performing overcurrent detection. If the output current is overcurrent, a current limiting algorithm is activated. When the grid frequency returns to the rated frequency, the current limiting algorithm is deactivated. This process includes the following steps: Step S11: Acquire the output current of the GFM inverter And perform overcurrent detection; If the current If there is an overcurrent, a high-level signal is generated and input to the S terminal of the first SR latch, causing the first SR latch to output a high level and control the activation of the current limiting algorithm. Step S12: After the current limiting algorithm is activated, the high-level signal at the S terminal of the first SR latch is converted to a low-level signal, and the low-level signal output by the second SR latch is input to the R terminal of the first SR latch, so that the current limiting algorithm is continuously activated. Step S13: After implementing the current limiting algorithm, continuously monitor the grid frequency. If the power grid frequency When the frequency is restored to the grid's rated frequency, a high-level signal is generated and input to the S terminal of the second SR latch, causing the second SR latch to output a high level and input to the R terminal of the first SR latch, causing the first SR latch to output a low-level signal, thus controlling the current limiting algorithm to be cut off.

3. The current limiting method for a GFM inverter based on grid frequency dips as described in claim 2, characterized in that, After the current limiting algorithm is removed, the R terminals of both the first SR latch and the second SR latch are set to 1.

4. The current limiting method for a GFM inverter based on grid frequency dips as described in claim 1, characterized in that, The output voltage and output current of the GFM inverter are converted to dq values, and the active power is calculated accordingly, specifically including: ； In the formula, This refers to the active power output of the GFM inverter. Let be the voltage of the GFM inverter in the dq rotating coordinate system. Let be the current conjugate value of the GFM inverter in the dq rotating coordinate system. This indicates taking the real part.

5. The current limiting method for a GFM inverter based on grid frequency dips as described in claim 1, characterized in that, The calculation of the angular frequency of the GFM inverter based on active power and through droop control specifically includes: The angular frequency of the GFM inverter is calculated using droop control with a low-pass filter. The formula is as follows: ； In the formula, This is the active power droop control coefficient. The time constant of the low-pass filter. For the Lagrangian operator, This is a reference value for the active power output of the GFM inverter. This refers to the active power output of the GFM inverter. This is the rated angular frequency of the GFM inverter.

6. The current limiting method for a GFM inverter based on grid frequency dips as described in claim 1, characterized in that, The calculation of active power increment based on power angle increment specifically includes: ； In the formula, For active power increment, This represents the effective value of the output voltage of the GFM inverter. This is the effective value of the grid voltage. For equivalent line reactance, This refers to the rated power angle of the GFM inverter. For the increment of the work angle, This refers to the rated value of the active power output of the GFM inverter.

7. The current limiting method for a GFM inverter based on grid frequency dips as described in claim 1, characterized in that, Also includes: When implementing the current limiting algorithm, simultaneously implement the variable virtual impedance. ; Among them, variable virtual impedance satisfy: ； In the formula, This is the rated voltage of the power grid. This represents the maximum value of the grid voltage phase. This represents the maximum output current of the GFM inverter. This is the equivalent line impedance of the power grid.

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