GFM inverter frequency drop current limiting method based on power angle increment

Abstract

The invention provides a GFM inverter frequency drop current limiting method based on power angle increment, and relates to the technical field of power electronic control. The method comprises the steps of collecting an output current of a GFM inverter and performing overcurrent detection, and if overcurrent occurs, inputting a current limiting algorithm; when the power grid frequency recovers to the rated frequency, cutting off; the current limiting algorithm comprises the following steps: calculating active power based on the output voltage and the output current of the GFM inverter, and further calculating the angular frequency of the inverter; calculating a power angle increment based on the angular frequency of the inverter, and further calculating an active power increment; and taking a difference value obtained by subtracting the active power increment from the active power as alternative active power, and performing droop control to realize current limiting. According to the method, the angular frequency of the power grid can be quickly tracked according to the current limiting algorithm, so that the GFM inverter can quickly reach a new working point after the power grid breaks down, and the dynamic performance of the GFM inverter can be effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of power electronic control, and in particular to a frequency drop current limiting method for a GFM inverter based on power angle increment. Background Art

[0002] In recent years, new energy sources such as wind power and photovoltaic power generation, which are connected to the grid with inverters based on power electronics as interfaces, have grown rapidly. As the proportion of traditional synchronous generators decreases, energy storage represented by pumped storage and electrochemical energy storage has gradually increased. Full-power pumped storage power stations and electrochemical energy storage power stations also use inverters as grid-connected interfaces. However, grid-following (GFL) control is usually used to adjust the grid-connected current in current-connected inverters. Since GFL inverters generally lack rotating reserve capacity and rotational inertia, they cannot provide inertial responses similar to traditional synchronous generators. On the contrary, grid-forming (GFM) inverters adopt a power synchronization strategy similar to synchronous generators, with virtual inertia and damping response, and have natural advantages in power grids with weak system strength and low physical inertia.

[0003] However, during the period when the grid frequency drops, the frequency change of the GFM inverter with virtual inertia will lag behind the grid frequency, and there will be a frequency deviation, which will cause the power angle between the GFM inverter and the grid to increase, and even cause overcurrent. Therefore, how to deal with overcurrent when the grid frequency drops becomes a key issue, and the GFM inverter also needs to have a certain fault recovery capability. In addition, the commonly used current limiting strategies also require corresponding control methods for input and removal. How to quickly realize the input and removal of overcurrent protection and current limiting strategies, avoid repeated switching of current limiting strategies during overcurrent, and reset the control enable signal after fault recovery is also a key issue of the switching strategy.

[0004] In order to solve the overcurrent problem of GFM inverter during grid fault, a current limiting strategy can be added to the control link. At present, there are three types of current limiting strategies: direct current limiting, indirect current limiting, and mixed current limiting. Direct current limiting is mainly a current limiter, which limits the current reference value. However, the direct current limiting method causes the inverter to present a current source characteristic to the outside, and may also cause the voltage outer loop to saturate, and the voltage control loop is difficult to recover after the fault is cleared. The stable operation of the GFM inverter cannot be guaranteed, and even transient instability problems may occur. Indirect current limiting can be divided into: modifying the power reference value, virtual impedance, and voltage limiter. The current limiting method of modifying the power reference value limits the output current by limiting the output power of the GFM inverter. However, this method will cause the dynamic response characteristics of the output current to deteriorate due to the low bandwidth of external control. The current limiting method using virtual impedance is to reduce the output current of the GFM inverter by equivalently increasing the total output impedance of the GFM inverter. However, when the degree of fault is different, the corresponding virtual impedance is also different, and the precise design of the virtual impedance is more difficult. The method of adding a voltage limiter limits the output current by adjusting the reference voltage amplitude and phase generated by the power outer loop. To ensure the fault recovery capability of this method, it is necessary to perform appropriate anti-saturation design on the outer loop controller.

[0005] In addition, most of the current reports on grid current limiting remain at the level of overcurrent caused by grid voltage sag. There is currently a lack of corresponding mechanism analysis and current limiting methods for overcurrent problems caused by grid frequency drops. Summary of the invention

[0006] In order to solve the above technical problems existing in the prior art, the present invention aims to provide a circuit method suitable for solving the overcurrent problem caused by power grid frequency drop.

[0007] To this end, the present invention provides a GFM inverter frequency drop current limiting method based on power angle increment, and the technical solution specifically includes:

[0008] Collect the output current of the GFM inverter and perform overcurrent detection. 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 removed;

[0009] Wherein, the current limiting algorithm includes:

[0010] Based on the output voltage and output current of the GFM inverter, the active power is calculated after dq transformation;

[0011] Based on the active power, the angular frequency of the GFM inverter is calculated through droop control;

[0012] Based on the angular frequency of the GFM inverter, the power angle increment is calculated;

[0013] Based on the power angle increment, calculate the active power 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] Preferably, the output current of the GFM inverter is collected and overcurrent detection is performed. If the output current is overcurrent, a current limiting algorithm is put into operation; when the grid frequency is restored to the rated frequency, the current limiting algorithm is cut off, which specifically includes the following steps:

[0016] Step S11: Collect the output current i of the GFM inverter abc And perform over-current detection;

[0017] If the current i abc If the current is too high, a high-level signal is generated and input to the S terminal of the first SR latch, so that the first SR latch outputs a high level and controls the current limiting algorithm.

[0018] Step S12: After the current limiting algorithm is put into operation, the high level signal input to the S terminal of the first SR latch is converted into a low level signal, and the second SR latch outputs a low level signal input to the R terminal of the first SR latch, so that the current limiting algorithm is continuously put into operation;

[0019] Step S13: After the current limiting algorithm is put into operation, the grid frequency f is continuously detected. If the grid frequency f is restored to the rated frequency of the grid, a high-level signal is generated and input to the S end of the second SR latch, so that the second SR latch outputs a high level and inputs the R end of the first SR latch, so that the first SR latch outputs a low-level signal to control the removal of the current limiting algorithm.

[0020] Further, after the current limiting algorithm is cut off, the R terminals of the first SR latch and the second SR latch are both set to 1.

[0021] Preferably, the active power is calculated based on the output voltage and output current of the GFM inverter after dq transformation, specifically including:

[0022]

[0023] Where P is the active power output by the GFM inverter, v odq is the voltage of the GFM inverter in the dq rotating coordinate system, is the current conjugate value of the GFM inverter in the dq rotating coordinate system, and Re represents the real part.

[0024] Preferably, based on the active power, the angular frequency of the GFM inverter is calculated by droop control, specifically including:

[0025] Calculation of the angular frequency ω of the GFM inverter by droop control with a low-pass filter inv ,

[0026] The formula is as follows:

[0027]

[0028] Where D f is the active power droop control coefficient, T f is the time constant of the low-pass filter, s is the Laplace operator, P* is the reference value of the active power output by the GFM inverter, P is the active power output by the GFM inverter, ω ref is the rated angular frequency of the GFM inverter.

[0029] Preferably, the power angle increment is calculated based on the angular frequency of the GFM inverter, specifically including:

[0030]

[0031] Where Δδ is the power angle increment, δ inv is the phase of the GFM inverter output voltage, δ grid is the grid voltage phase, t0 is the start time of frequency drop, t inv is the end time of GFM inverter frequency drop, ω inv is the angular frequency of the GFM inverter, ω grid is the grid angular frequency.

[0032] Preferably, calculating the active power increment based on the power angle increment specifically includes:

[0033]

[0034] Where ΔP is the active power increment, V is the effective value of the GFM inverter output voltage, E is the effective value of the grid voltage, X is the equivalent line reactance, δ rate is the rated power angle of the GFM inverter, Δδ is the power angle increment, P rate It is the rated value of the active power output of the GFM inverter.

[0035] Furthermore, it also includes:

[0036] When the current limiting algorithm is activated, the virtual impedance X is also activated. vir ;

[0037] Among them, the virtual impedance X vir satisfy

[0038]

[0039] In the formula, E rate is the rated voltage of the power grid, δ max is the maximum value of the grid voltage phase, I maxis the maximum value of the GFM inverter output current, X g is the equivalent line impedance of the power grid.

[0040] It can be seen that the technical solution provided by the present invention can quickly track the grid angular frequency according to the current limiting algorithm, so that the GFM inverter can quickly reach a new working point after an overcurrent caused by a frequency drop in the grid, which can effectively improve the dynamic performance of the GFM inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The structure diagram of the droop inverter topology and its control system used in the present invention.

[0042] Figure 2 Schematic diagram of the switching strategy based on the SR latch in the present invention.

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

[0044] Figure 4 It is a schematic diagram of adaptive frequency tracking based on frequency drop in the present invention.

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

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

[0047] Figure 7 It is the frequency and power angle increment waveform of the current limiting method used in the present invention.

[0048] Figure 8 The output waveform of the GFM inverter with the current limiting method added in the present invention. DETAILED DESCRIPTION

[0049] Hereinafter, the technical solution provided by the present invention will be further elaborated in detail with reference to the accompanying drawings.

[0050] The present invention provides a GFM inverter frequency drop current limiting method based on power angle increment. The method analyzes the mechanism of overcurrent generation based on the power angle change, and proposes an adaptive frequency tracking strategy based on frequency drop. The strategy can adaptively reduce the power angle increment between the GFM inverter and the power grid during the power grid frequency drop, thereby achieving the purpose of current limiting. In addition, since there will be a residual power angle from the beginning of the fault to the input of the current limiting strategy, the output current increases. Therefore, the present invention also proposes a variable virtual impedance calculation method based on the residual power angle to alleviate the overcurrent caused by the residual power angle.

[0051] like Figure 1As shown, the present invention adopts the topological structure and control system of a typical droop control type three-phase grid-connected inverter. dc The DC side voltage of the GFM inverter is generally provided by photovoltaic or wind turbines. The AC side of the GFM inverter uses an LC filter to filter out high-frequency harmonics, and finally passes through the grid-connected point (point of common coupling, PCC) through the grid-side equivalent inductance L g Connected to the grid, the grid voltage is V g The control system of GFM inverter includes power calculation, power control, voltage PI controller G v , Current PI controller G C and modulation stages.

[0052] Among them, the voltage u at the PCC point oαβ and current i oαβ After dq transformation, the voltage v in the dq rotating coordinate system is obtained odq and current i odq , thereby calculating the active power P and reactive power Q.

[0053] in,

[0054]

[0055] Where P is the active power output by the GFM inverter, is the current conjugate value of the GFM inverter in the dq rotating coordinate system, and Re represents the real part.

[0056] The phase θ obtained by active droop control is used for dq conversion, and the voltage amplitude obtained by reactive droop control is used as the d-axis voltage reference value. The voltage reference value With the actual value u odq The difference is passed through the voltage PI controller G v Get the current reference value Then the current reference value The actual current value i Ldq The difference is passed through the current PI controller G C The modulated wave voltage is obtained, and finally a pulse signal for driving the switch tube is generated through sinusoidal pulse width modulation (SPWM). Ldq By sampling the GFM inverter output current i Lαβ Obtained by dq transformation.

[0057] The switching logic of the current limiting strategy is as follows: During normal operation, the current limiting algorithm is not used. 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 the normal working mode. In addition, the output current should be limited to the allowable range, that is, the amplitude of the output current should be less than 1.3pu. Therefore, the current change caused by the grid frequency drop should be less than 0.3pu. In order to retain a certain safety margin, the invention sets the overcurrent threshold to 1.2pu.

[0058] The present invention proposes an event-triggered SR latch to implement the current limiting strategy switching logic. The corresponding switching control block diagram is as follows: Figure 2 First, detect the three-phase current i abc And compare it with the current threshold. When overcurrent is detected, a high level signal is generated. During the period of grid frequency drop, once overcurrent is detected, the current limiting algorithm must remain in the state until the frequency recovers. Secondly, when it is detected that the grid frequency returns to the rated frequency, generally 50Hz, the current limiting algorithm can be cut off.

[0059] The main steps of the current limiting algorithm include: calculating the active power after dq transformation based on the output voltage and output current of the GFM inverter; calculating the angular frequency of the GFM inverter through droop control based on the active power; 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, performing droop control, and realizing current limiting.

[0060] It should be noted that the overcurrent caused by other factors will not cause frequency drop or power angle increment, so the above current limiting algorithm cannot run. Therefore, it is not necessary to pre-detect whether the frequency drop occurs before the current limiting algorithm is put into use. In this way, when facing overcurrent caused by frequency drop, it is convenient to put into use the current limiting algorithm with a smaller delay.

[0061] Among them, the latch truth table is shown in Table 1. When overcurrent is detected, S=1 and R=0 of 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 put into use, SR latch 1 should maintain a high level to avoid repeated switching of the current limiting algorithm.

[0062] Table 1 SR latch truth table

[0063] S R state 1 0 1 0 1 0 0 0 Keep

[0064] During a grid fault (frequency drop causing overcurrent), SR latch 2 outputs a low level signal. When the grid frequency returns to the rated frequency, the grid frequency recovery detection outputs a high level signal, and the Q of SR latch 2 is equal to the R of SR latch 1. Therefore, when the grid frequency is restored, the R of SR latch 1 (enable / disable current limiting algorithm) will be at a high level, that is, the current limiting algorithm is cut out. When the current limiting algorithm is cut off, the R of all SR latches is set to 1 to clear the current state for subsequent overcurrent use.

[0065] In practice, overcurrent detection uses high-speed sampling, and the protection logic is directly triggered after overcurrent is detected, not triggered in sequence. That is, the control logic triggers the event interrupt, and the protection logic triggers the event interrupt.

[0066] The power angle increment Δδ during the grid frequency drop is as follows: Figure 3 As shown. Among them, ω0 is the rated angular frequency, ω grid_min is the minimum angular frequency and also the threshold of under-frequency load shedding (UFLS). t0 is the start time of frequency drop, t grid is the end time of the grid frequency drop, t inv It is the end time of GFM inverter frequency drop.

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

[0068] The power angle δ, that is, the phase difference between the output voltage of the GFM inverter and the grid voltage, increases, and the power angle increment Δδ can be derived as follows:

[0069]

[0070] In the formula, δ inv is the phase of the GFM inverter output voltage, δ grid is the grid voltage phase, t0 is the start time of frequency drop, t inv is the end time of GFM inverter frequency drop, ω grid is the grid angular frequency.

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

[0072]

[0073] Where D f is the active power droop control coefficient, T f is the time constant of the low-pass filter, s is the Laplace operator, P* is the reference value of the active power output by the GFM inverter, ωref 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 P will increase accordingly. In extreme cases, when the virtual inertia tends to infinity, the active power P will continue to increase as the grid frequency decreases, causing overcurrent.

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

[0076] Rated power angle δ rate It can be expressed as:

[0077]

[0078] Where P rate is the rated value of the GFM inverter output active power, V rate is the reference voltage of the GFM inverter, E rate is the rated voltage of the power grid, X g is the equivalent line impedance of the power grid.

[0079] Therefore, the active power ΔP increased due to the power angle difference can be expressed as:

[0080]

[0081] Where V is the effective value of the GFM inverter output voltage, E is the effective value of the grid voltage, X is the equivalent line reactance, E is the effective value of the grid voltage, and δ rate is the rated power angle of the GFM inverter.

[0082] Adaptive frequency tracking strategy based on frequency drop Figure 4 As shown. This strategy calculates the corresponding power angle increment Δδ by detecting the angular frequency deviation between the GFM inverter and the power grid, and then calculates the power increment generated by the power angle increment. When the GFM inverter is working normally, the power increment is set to 0; when the output current is detected to be overcurrent, the control mode is switched to adaptive frequency tracking control; when the grid frequency is detected to be restored to normal, the working mode is switched to normal working state. The proposed strategy can not only calculate the adaptive coefficient according to the angular frequency deviation to track the grid angular frequency faster, but also enable the GFM inverter to quickly reach a new working point after a grid fault occurs, effectively improving the dynamic performance of the GFM inverter.

[0083] Considering that the reactive current on the grid side is zero under the maximum active power support, the output current is maximum, and we can get:

[0084]

[0085] In the formula, I max is the maximum output current of the GFM inverter, P max The maximum active power of the GFM inverter output current maximum value.

[0086] Under the 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 the grid voltage, grid impedance voltage and GFM inverter output voltage can be expressed as:

[0087]

[0088] Where V Pmax is the output voltage of the GFM inverter under maximum active power support, X g and X vir are the equivalent line impedance and virtual impedance of the power grid respectively. The overcurrent caused from the occurrence of the fault to the input of the current limiting strategy requires the input of a variable virtual impedance X vir .

[0089] Depend on Figure 5 It can be seen that when the power angle reaches the maximum value, the voltage drop between the line impedance and the virtual impedance also reaches the maximum value. In order to limit the fundamental current after the grid frequency drops, the line impedance and the variable virtual impedance should meet

[0090]

[0091] Conservatively, the variable virtual impedance should satisfy

[0092]

[0093] In the formula, δ max It is the maximum value of the grid voltage phase. Among them, the variable virtual impedance and current limiting algorithm are put into operation synchronously.

[0094] The variable virtual impedance is obtained by adaptively calculating the residual power angle increment, and can adaptively respond to overcurrent events caused by the time difference of putting the current limiting strategy into operation without complicated current limiting design.

[0095] In the case of grid frequency drop, the reference voltage U of the reactive voltage link is controlled by droop ref It can be calculated as:

[0096]

[0097] In addition, the voltage and current dual-loop control block diagram after adding the variable virtual impedance link is as follows: Figure 6 As shown. Among them, U cvd with U cvqis the voltage drop after the variable virtual impedance is decomposed in the dq rotating coordinate system,

[0098]

[0099] Among them, R vir =X vir / σ, σ is the ratio of X / R. Adding a variable virtual impedance link in the dq rotating coordinate system can not only utilize the output current of the GFM inverter after decoupling in the dq rotating coordinate system, but also weaken the influence of the output current high-order harmonics on the variable virtual impedance link, which is easy to implement in engineering.

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

[0101] Table 2 Main simulation parameters

[0102] parameter Numeric parameter Numeric <![CDATA[DC bus voltage U dc > 800V <![CDATA[Grid-side equivalent inductance L g > 5mH <![CDATA[Switching frequency f s > 10kHz <![CDATA[Grid-side equivalent resistance R g > 0.9Ω <![CDATA[Filter inductor L f > 10mH <![CDATA[Reactive droop coefficient D q > <![CDATA[3.333*10 -4 ]]> <![CDATA[Filter capacitor C f > 25μF <![CDATA[Active droop coefficient D p > <![CDATA[3*10 -4 ]]> <![CDATA[Current loop proportionality coefficient k ip > 10 <![CDATA[Active power reference value P ref > 20kW <![CDATA[Adaptive frequency tracking k p > 3000 <![CDATA[Voltage loop proportionality coefficient k up > 0.5 <![CDATA[Adaptive frequency tracking k i > 1 <![CDATA[Voltage loop integral coefficient k ui > 100 <![CDATA[Cutoff frequency ω c > 600rad / s

[0103] Figure 7 and Figure 8 The frequency and power angle increment waveforms and the output waveform of the GFM inverter are shown in Figure 2. grid When decreasing at a rate of 2Hz / s, the output current of the GFM inverter begins to gradually increase. When the current reaches the threshold, the protection logic of the SR latch is triggered, and then the current limiting strategy is put into use to achieve fast overcurrent protection. Although there is still a residual power angle increment between the GFM inverter and the grid, the variable virtual impedance calculated by the power angle increment can effectively suppress overcurrent. In addition, due to the existence of variable virtual impedance, the active power will decrease slightly, but the active power will recover quickly, thereby providing active power support for the grid. The output current is also within the feasible range, and there is no overcurrent phenomenon. When the grid frequency recovers, the current limiting strategy is cut off and faster fault recovery is achieved. Among them, f inv is the output frequency of the GFM inverter.

[0104] In summary, the technical solution provided by the present 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 an overcurrent caused by a frequency drop in the grid, which can effectively improve the dynamic performance of the GFM inverter.

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

Claims

1. A GFM inverter frequency drop current limiting method based on power angle increment, characterized in that: The following steps are involved: Step S1: Collect the output current of the GFM inverter and perform overcurrent detection. 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 cut off; Wherein, the current limiting algorithm includes: Based on the output voltage and output current of the GFM inverter, the active power is calculated after dq transformation; Based on the active power, the angular frequency of the GFM inverter is calculated through droop control; Based on the angular frequency of the GFM inverter, the power angle increment is calculated; Based on the power angle increment, calculate the active power 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.

2. A GFM inverter frequency drop current limiting method based on power angle increment as claimed in claim 1, characterized in that: Step S1 specifically includes: Step S11: Collect the output current i of the GFM inverter abc And perform over-current detection; If the current i abc If the current is too high, a high-level signal is generated and input to the S terminal of the first SR latch, so that the first SR latch outputs a high level and controls the current limiting algorithm. Step S12: After the current limiting algorithm is put into operation, the high level signal input to the S terminal of the first SR latch is converted into a low level signal, and the second SR latch outputs a low level signal input to the R terminal of the first SR latch, so that the current limiting algorithm is continuously put into operation; Step S13: After the current limiting algorithm is put into operation, the grid frequency f is continuously detected. If the grid frequency f is restored to the rated frequency of the grid, a high-level signal is generated and input to the S end of the second SR latch, so that the second SR latch outputs a high level and inputs the R end of the first SR latch, so that the first SR latch outputs a low-level signal to control the removal of the current limiting algorithm.

3. A GFM inverter frequency drop current limiting method based on power angle increment as claimed in claim 2, characterized in that: After the current limiting algorithm is cut off, the R terminals of the first SR latch and the second SR latch are both set to 1.

4. A GFM inverter frequency drop current limiting method based on power angle increment as claimed in claim 1, characterized in that: The active power is calculated based on the output voltage and output current of the GFM inverter after dq transformation, specifically including: Where P is the active power output by the GFM inverter, v odq is the voltage of the GFM inverter in the dq rotating coordinate system, is the current conjugate value of the GFM inverter in the dq rotating coordinate system, and Re represents the real part.

5. A GFM inverter frequency drop current limiting method based on power angle increment as claimed in claim 1, characterized in that: The method of calculating the angular frequency of the GFM inverter through droop control based on the active power specifically includes: Calculation of the angular frequency ω of the GFM inverter by droop control with a low-pass filter inv , The formula is as follows: Where D f is the active power droop control coefficient, T f is the time constant of the low-pass filter, s is the Laplace operator, P* is the reference value of the active power output by the GFM inverter, P is the active power output by the GFM inverter, ω ref is the rated angular frequency of the GFM inverter.

6. A GFM inverter frequency drop current limiting method based on power angle increment as claimed in claim 1, characterized in that: The calculation of the power angle increment based on the angular frequency of the GFM inverter specifically includes: Where Δδ is the power angle increment, δ inv is the phase of the GFM inverter output voltage, δ grid is the grid voltage phase, t0 is the start time of frequency drop, t inv is the end time of GFM inverter frequency drop, ω inv is the angular frequency of the GFM inverter, ω grid is the grid angular frequency.

7. A GFM inverter frequency drop current limiting method based on power angle increment as claimed in claim 1, characterized in that: The step of calculating the active power increment based on the power angle increment specifically includes: Where ΔP is the active power increment, V is the effective value of the GFM inverter output voltage, E is the effective value of the grid voltage, X is the equivalent line reactance, δ rate is the rated power angle of the GFM inverter, Δδ is the power angle increment, P rate It is the rated value of the active power output of the GFM inverter.

8. A method for limiting current when frequency drops of a GFM inverter based on power angle increment according to claim 1, characterized in that: Also includes: When the current limiting algorithm is activated, the variable virtual impedance X is also activated simultaneously. vir ; Among them, the variable virtual impedance X vir satisfy In the formula, E rate is the rated voltage of the power grid, δ max is the maximum value of the grid voltage phase, I max is the maximum value of the GFM inverter output current, X g is the equivalent line impedance of the power grid.

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