Method and system for judging stability of grid-connected / off-grid inverter based on overall impedance

Abstract

The invention discloses a grid-connected / off-grid inverter stability judgment method and system based on overall impedance, and the method comprises the steps: voltage stability judgment and current stability judgment, in the voltage stability judgment, the voltage stability of a node to be subjected to stability judgment is judged according to the maximum value of an amplitude-frequency curve of a parallel overall impedance Bode diagram of an inverter system; in the current stability judgment process, the current stability of the branch to be subjected to stability judgment is judged according to the minimum value of an amplitude-frequency curve of a series overall impedance Bode diagram of the inverter system; and determining the stability of the inverter system during grid connection or grid disconnection by combining the judgment results of the voltage stability judgment and the current stability judgment, and determining the dominant electrical quantity of oscillation if the oscillation is generated. According to the method, the stability of the inverter system can be directly and accurately judged, the dominant electrical quantity of oscillation is confirmed, and the method can be suitable for stability analysis of grid-connected and off-grid inverter systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of inverter stability analysis and detection, and mainly relates to a method and system for judging the stability of grid-connected / off-grid inverters based on overall impedance. Background Art

[0002] With the rapid development of new energy power generation, inverters, as key power electronic converters for new energy power generation, in order to improve the penetration rate and utilization rate of new energy, it is required that in addition to having the ability to operate in parallel with the grid, some inverters also need to have the ability to operate independently to form a grid. Therefore, its control mode has changed from a single current source mode to a hybrid mode of current source and voltage source, which means that the inverter faces more complex stability problems and there is a risk of broadband oscillation.

[0003] Existing research shows that the impedance method is an effective method for analyzing the small-signal stability of inverters. At present, the method based on impedance ratio has been widely used in the interactive stability problem between grid-connected inverters and the power grid, and it judges the stability of the system according to the Nyquist stability criterion. However, on the one hand, the method based on impedance ratio needs to first judge whether the inverter is a current source type or a voltage source type, which weakens to a certain extent the advantage that the impedance method is applicable to the "black box" model; on the other hand, the impedance ratio method assumes that the inverter is stable when operating without load. In actual applications, a voltage source type inverter may face stability problems when operating without load, so it does not meet the premise of using the impedance ratio method.

[0004] In addition, there is also some research on the method based on impedance sum, which judges the stability of the system by the equivalent RLC method. However, it needs to first judge the characteristic frequency, otherwise there will be a situation of misjudging stability. At the same time, due to the existence of loads and lines, when oscillations occur, they often show voltage and current oscillations with the same frequency.

[0005] Therefore, neither the method based on impedance ratio nor the method based on impedance sum has found out whether the dominant electrical quantity of oscillation is voltage or current. At present, there is a lack of a judgment method that is direct and accurate, can find out the dominant electrical quantity of oscillation, and is applicable to the stability judgment of grid-connected / off-grid inverters. Summary of the Invention

[0006] In view of the problems existing in the prior art, the present invention proposes a method and system for judging the stability of grid-connected / off-grid inverters based on overall impedance, including voltage stability judgment and current stability judgment. In the voltage stability judgment, according to the maximum value of the amplitude-frequency curve of the Bode diagram of the parallel overall impedance of the inverter system, the voltage stability of the node to be judged for stability is judged; in the current stability judgment, according to the minimum value of the amplitude-frequency curve of the Bode diagram of the series overall impedance of the inverter system, the current stability of the branch to be judged for stability is judged; combining the judgment results of the voltage stability judgment and the current stability judgment, the stability of the inverter system during grid connection or off-grid is determined. If oscillation occurs, the dominant electrical quantity of the oscillation can be determined. The method of the present invention can directly and accurately judge the stability of the inverter system and confirm the dominant electrical quantity of the oscillation, and can be applied to the stability analysis of grid-connected and off-grid inverter systems.

[0007] To achieve the above object, the technical solution adopted by the present invention is: a method for judging the stability of grid-connected / off-grid inverters based on overall impedance, including voltage stability judgment and current stability judgment,

[0008] In the voltage stability judgment, according to the Bode diagram of the parallel overall impedance of the inverter system, the voltage stability of the node to be judged for stability is judged;

[0009] In the current stability judgment, according to the Bode diagram of the series overall impedance of the inverter system, the current stability of the branch to be judged for stability is judged;

[0010] Combining the judgment results of the voltage stability judgment and the current stability judgment, the stability of the inverter system during grid connection or off-grid is determined. If oscillation occurs, the dominant electrical quantity of the oscillation can be determined.

[0011] As an improvement of the present invention, the voltage stability judgment specifically includes the following steps:

[0012] S1-1: Obtain the system parallel overall impedance corresponding to the node to be judged for stability;

[0013] S1-2: Judge the system characteristic frequency according to the maximum value of the amplitude-frequency curve of the Bode diagram of the parallel overall impedance;

[0014] S1-3: Judge the system voltage stability according to the characteristic frequency obtained in step S1-2 and the phase-frequency curve of the Bode diagram of the parallel overall impedance.

[0015] As an improvement of the present invention, the current stability judgment specifically includes the following steps:

[0016] S2-1. Obtain the system series overall impedance corresponding to the branch to be judged for stability;

[0017] S2-2. Determine the system characteristic frequency based on the minimum value of the magnitude-frequency curve of the series overall impedance Bode diagram;

[0018] S2-3. Determine the system current stability based on the characteristic frequency and the phase-frequency curve of the series overall impedance Bode diagram.

[0019] As another improvement of the present invention, in the step S1-2, the calculation method of the maximum value of the magnitude-frequency curve of the parallel overall impedance Bode diagram is: Denote the characteristic frequency as f vm_n , and its characteristic is that when the frequency f vm = f vm_n , the parallel overall impedance obtains a maximum value in the neighborhood of (f vm_n - δ, f vm_n + δ), where δ = 0.1f vm_n .

[0020] As another improvement of the present invention, the method for determining the system voltage stability according to the characteristic frequency and the phase-frequency curve of the parallel overall impedance Bode diagram in the step S1-3 is specifically: Determine whether the phase-frequency curve of the parallel overall impedance Bode diagram has a positive slope crossing 180°. When it does not have a positive slope crossing 180°, the voltage of the node to be judged for stability generates a decaying oscillation; when it has a positive slope crossing 180°, record the crossing frequency as f vp_n , if f vp_n ≠ f vm_n , then the voltage of the node to be judged for stability generates a divergent oscillation; if f vp_n = f vm_n , then the voltage of the node to be judged for stability generates an equal-amplitude oscillation.

[0021] As yet another improvement of the present invention, the calculation method of the minimum value of the magnitude-frequency curve of the series overall impedance Bode diagram in the step S2-2 is specifically: Denote the characteristic frequency as f cm_n , and its characteristic is that when the frequency f cm = f cm_n , the series overall impedance obtains a minimum value in the neighborhood of (f cm_n - δ, f cm_n + δ), where δ = 0.1f cm_n .

[0022] As yet another improvement of the present invention, the method for determining the system current stability according to the characteristic frequency and the phase-frequency curve of the series overall impedance Bode diagram in the step S2-3 is specifically: Determine whether the phase-frequency curve of the series overall impedance Bode diagram has a negative slope crossing 180°. When it does not have a negative slope crossing 180°, the current of the branch to be judged for stability generates a decaying oscillation; when it has a negative slope crossing 180°, record the crossing frequency as f cp_n , if f cp_n ≠ fcm_n , the current of the branch to be judged for stability will generate divergent oscillation; if f cp_n = f cm_n , the current of the branch to be judged for stability will generate equal-amplitude oscillation.

[0023] To achieve the above object, the technical solution adopted by the present invention is also: a stability judgment system for grid-connected / off-grid inverters based on overall impedance, including a computer program, and when the computer program is executed by a processor, the steps of any of the above methods are implemented.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The stability judgment method proposed by the present invention is applicable to grid-connected and off-grid inverter systems, and is applicable to inverter systems composed of inverters in current source mode and voltage source mode, and has a unified stability criterion.

[0026] (2) The stability judgment method proposed by the present invention can directly and accurately judge the stability of the inverter system, and can confirm the dominant electrical quantity of oscillation, and is applicable to the stability analysis of grid-connected and off-grid inverter systems.

[0027] (3) The method of the present invention has the advantages of being not limited by the inverter control and operation mode and accurate stability judgment, which is beneficial to guiding the design of inverter parameters and enhancing the system stability. Description of the Drawings

[0028] Figure 1 is the circuit topology diagram of the three-phase inverter system in Embodiment 1 and Embodiment 2 of the present invention;

[0029] Figure 2 is the schematic diagram of the branch to be judged for stability and the node to be judged for stability in Embodiment 1 of the present invention;

[0030] Figure 3 is the flow chart of the steps of the method for judging the stability of grid-connected / off-grid inverters based on overall impedance of the present invention;

[0031] Figure 4 is the schematic diagram of the parallel overall impedance when the inverter operates independently with no load, 10% load and 20% load in Embodiment 2 of the present invention;

[0032] Figure 5 is the voltage and current change diagram when the load of the inverter operating independently suddenly drops from 20% to 0 in Embodiment 2 of the present invention;

[0033] Figure 6 is the voltage and current change diagram when the load of the inverter operating independently suddenly drops from 20% to 10% in Embodiment 2 of the present invention;

[0034] Figure 7It is a Fourier analysis result diagram of the output voltage of the inverter in Embodiment 2 of the present invention when it is no-load.

[0035] Figure 8 For the grid-side line inductance L of the inverter when it is operating in parallel in Embodiment 2 of the present invention g = 0uH, 7uH and 350uH series overall impedance schematic diagram;

[0036] Figure 9 For the grid-side line inductance L of the inverter when it is operating in parallel in Embodiment 2 of the present invention g = When 350uH is switched to Lg = 7uH and Lg = 0uH, the output voltage and current waveform diagram;

[0037] Figure 10 For the inverter in Embodiment 2 of the present invention when it is operating in parallel and the grid-side line inductance L g = Fourier analysis result diagram of the output current of the inverter when 7uH. Specific embodiments

[0038] The present invention will be further clarified below in conjunction with the drawings and specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0039] Embodiment 1

[0040] A method and system for judging the stability of a parallel / stand-alone inverter based on overall impedance. First, the overall impedance of the inverter system is obtained through existing impedance sweep equipment or theoretical modeling methods, and then, according to the Bode diagram of the parallel overall impedance of the inverter system, the voltage stability of the node to be judged for stability is judged. Then, according to the Bode diagram of the series overall impedance of the inverter system, the current stability of the branch to be judged for stability is judged. Finally, the stability of the inverter system during grid connection or stand-alone operation is determined by comprehensively considering the judgment results of voltage stability judgment and current stability judgment.

[0041] As Figure 1 shown in the circuit topology diagram of the parallel / stand-alone three-phase inverter system, it is composed of an inverter, a transmission line, a power grid and a load. Figure 1 Among them, L f is the inverter filter inductor, R f is the total resistance of the inductor branch, C f is the inverter filter capacitor, R d is the damping resistance of the capacitor branch, V dc is the DC bus voltage, Z g is the sum of the grid impedance and the line impedance, u g is the grid voltage, Z load is the load impedance.

[0042] Figure 2The impedance schematic diagram of the grid-connected / off-grid inverter system is shown, and the branches and nodes to be judged for stability are marked in the figure. According to the method proposed by the present invention, to judge the stability of branch current, the series overall impedance of the inverter system needs to be obtained. Corresponding to Figure 2 In, the series overall impedance Z s The expression of is Z s =Z inv +Z g / / Z load , where Z inv Is the impedance of the inverter. For the branch to be judged for stability, Z g And Z load Are in parallel and then in series with Z inv . According to the method proposed by the present invention, to judge the stability of node voltage, the parallel overall impedance of the inverter system needs to be obtained. Corresponding to Figure 2 In, the parallel overall impedance Z p The expression of is Z p =Z inv / / Z g / / Z load , where Z inv Is the impedance of the inverter. For the node to be judged for stability, Z inv , Z g And Z load Are in parallel with each other.

[0043] The grid-connected / off-grid inverter stability judgment method based on overall impedance according to the present invention, as Figure 3 Shown, includes voltage stability judgment and current stability judgment. In judging voltage stability, the following steps are specifically included:

[0044] S1-1, Obtain the system parallel overall impedance Z p Corresponding to the node to be judged for stability; The parallel overall impedance Z p Is obtained by means of an existing impedance sweep device and impedance theory modeling.

[0045] S1-2, Judge the system characteristic frequency according to the maximum value of the amplitude-frequency curve of the Bode diagram of the parallel overall impedance Z p ; Specifically, denote the characteristic frequency as f vm_n , and its characteristic is that when the frequency f vm =f vm_n , the parallel overall impedance Z p Obtains the maximum value in the neighborhood of (f vm_n -δ, f vm_n +δ). Generally, n≥1, δ = 0.1f vm_n .

[0046] S1-3, According to the characteristic frequency and the parallel overall impedance Z pJudge the system voltage stability by the phase-frequency curve of the Bode diagram.

[0047] First, judge the parallel overall impedance Z p Whether the phase-frequency curve of the Bode diagram has a positive slope crossing 180°. When it does not have a positive slope crossing 180°, the voltage of the node to be judged for stability generates a decaying oscillation; when it has a positive slope crossing 180°, record the crossing frequency as f vp_n , if f vp_n ≠f vm_n , then the voltage of the node to be judged for stability generates a divergent oscillation; if f vp_n =f vm_n , then the voltage of the node to be judged for stability generates an equal-amplitude oscillation.

[0048] In judging the current stability of the system, it specifically includes the following steps:

[0049] S2-1. Obtain the system series overall impedance Z corresponding to the branch to be judged for stability s ; The series overall impedance Z s Is obtained by means of an existing impedance sweep device and impedance theory modeling.

[0050] S2-2. Judge the system characteristic frequency according to the amplitude-frequency curve of the Bode diagram of the series overall impedance Z s ; Specifically, record the characteristic frequency as f cm_n , and its characteristic is that when the frequency f cm =f cm_n , the series overall impedance Z s Obtains a minimum value in the neighborhood of (f cm_n -δ, f cm_n +δ). Generally, n≥1 and δ=0.1f cm_n .

[0051] S2-3. Judge the system current stability according to the characteristic frequency and the phase-frequency curve of the Bode diagram of the series overall impedance Z s ;

[0052] First, judge whether the phase-frequency curve of the Bode diagram of the series overall impedance Z s Has a negative slope crossing 180°. When it does not have a negative slope crossing 180°, the current of the branch to be judged for stability generates a decaying oscillation; when it has a negative slope crossing 180°, record the crossing frequency as f cp_n , if f cp_n ≠f cm_n , then the current of the branch to be judged for stability generates a divergent oscillation; if f cp_n =f cm_n , then the current of the branch to be judged for stability generates an equal-amplitude oscillation.

[0053] Judge the stability of the grid-connected / off-grid inverter system by comprehensively considering voltage stability and current stability. Specifically, based on the judgment results of steps S1 and S2, determine whether the inverter system will oscillate during grid connection and off-grid operation and determine the dominant electrical quantity of the oscillation.

[0054] Embodiment 2

[0055] In this embodiment, a typical grid-connected / off-grid inverter system is adopted. The main circuit of the inverter system is as Figure 1 shown. Among them, the DC side of the main circuit part can be regarded as a DC source with a constant voltage. The inverter part is realized by a three-phase full-bridge inverter circuit composed of 6 IGBTs. The current output by the bridge arm is filtered by LC and then the voltage and current are output, which can operate in grid-connected mode and off-grid with a load.

[0056] In this embodiment, the effective value of the line voltage output by the inverter is 315V, the rated capacity of the inverter is 500kVA, and the inverter operates independently with a load off-grid.

[0057] A method for judging the stability of a grid-connected / off-grid inverter based on the overall impedance includes voltage stability judgment and current stability judgment. In the voltage stability judgment, the following steps are included:

[0058] S1-1: Obtain the system parallel overall impedance corresponding to the node to be judged for stability, as Figure 4 shown. Figure 4 It is a schematic diagram of the parallel overall impedance of the inverter operating independently with no load, 10% load, and 20% load; combining circuit principles, when the inverter operates off-grid with no load, the load impedance is infinite, and the parallel overall impedance of the inverter system is the impedance of the inverter. As the load increases, the load impedance decreases, and the parallel overall impedance of the inverter system is the parallel combination of the inverter impedance and the load impedance.

[0059] S1-2: Judge the system characteristic frequency according to the maximum value of the amplitude-frequency curve of the Bode diagram of the parallel overall impedance; from Figure 4 it can be seen that when there is a maximum value in the parallel overall impedance of the inverter system, the corresponding frequencies are respectively denoted as f vm(0％) = 495Hz, f vm(10％) = 468Hz, f vm(20％) = 449Hz.

[0060] S1-3: Judge the system voltage stability according to the characteristic frequency obtained in step S1-2 and the phase-frequency curve of the Bode diagram of the parallel overall impedance.

[0061] From Figure 4It can be seen that when the inverter operates with a 20% load, the phase-frequency curve of the parallel impedance of the inverter system does not have a positive crossing of 180°, and at this time, the voltage of the inverter system is stable. When the inverter operates with a 10% load, the phase-frequency curve of the parallel impedance of the inverter system has a positive crossing of 180°, and the crossing frequency is equal to the characteristic frequency, that is, f vp(10％) = f vm(10％) = 468 Hz. When the inverter operates under no-load conditions, the phase-frequency curve of the parallel impedance of the inverter system has a positive crossing of 180°, and the crossing frequency f vp(0％) = 467 Hz, while the characteristic frequency is 495 Hz, f vp(0％) ≠ f vm(0％) . Therefore, when the inverter operates independently under no-load conditions, the voltage is unstable and divergent oscillations will occur; when the inverter operates independently with a 10% load, the voltage is critically stable and equal-amplitude oscillations will occur; when the inverter operates independently with a 20% load, the voltage is stable and damped oscillations will occur.

[0062] Figure 5 FIG. is the voltage and current variation diagram when the load of the inverter operating independently in this embodiment suddenly drops from 20% to 0; it can be seen from the figure that at 4.8 s - 5 s, the inverter operates with a 20% load and the output voltage is stable. At 5 s, the load of the inverter suddenly drops to no-load, the output current of the inverter is 0, and the output voltage is unstable, resulting in divergent oscillations. Figure 6 FIG. is the voltage and current variation diagram when the load of the inverter operating independently in this embodiment suddenly drops from 20% to 10%; it can be seen from the figure that at 4.8 s - 5 s, the inverter operates with a 20% load and the output voltage is stable. At 5 s, the load of the inverter suddenly drops to 10%, and the output voltage of the inverter is critically stable, resulting in equal-amplitude oscillations. This is consistent with the stability judgment result proposed by the present invention. In addition, through Fourier analysis of the output voltage of the inverter, it is obtained that Figure 7 , it can be seen that the oscillation frequencies are 447.5 Hz and the coupling frequency is 547.5 Hz, which are close to the corresponding characteristic frequency of 449 Hz, and the error is -0.33%.

[0063] In this embodiment, the effective value of the output line voltage of the inverter is 315 V, the rated capacity of the inverter is 50 kVA, and the inverter operates in parallel with the grid. In the current stability judgment, the following steps are included:

[0064] S2-1. Obtain the overall series impedance of the system corresponding to the branch to be judged for stability, as shown in Figure 8 . Combining the circuit principle, when the inverter operates in parallel with the grid and the grid is an ideal grid, the grid impedance is 0, and the overall series impedance of the inverter system is the impedance of the inverter. As the grid impedance increases, the overall series impedance of the inverter system is the sum of the inverter impedance and the grid impedance.

[0065] S2-2. Determine the system characteristic frequency based on the minimum value of the amplitude-frequency curve of the Bode diagram of the series overall impedance; from Figure 8 it can be seen that when L g = 350 uH, 7 uH, and 0 uH (L g is the sum of the line inductance and the grid inductance), there are minimum values in the series overall impedance of the inverter system, and the corresponding frequencies are recorded as characteristic frequencies, where f cm(350uH) = 2.7 Hz, f cm(7uH) = 4.4 Hz, f cm(0uH) = 4.8 Hz.

[0066] S2-3. Judge the system current stability according to the characteristic frequency and the phase-frequency curve of the Bode diagram of the series overall impedance. From Figure 8 it can be seen that when L g = 350 uH, there is no negative crossing of 180° in the phase-frequency curve of the series impedance of the inverter system, and at this time the current of the inverter system is stable. When L g = 7 uH, there is a negative crossing of 180° in the phase-frequency curve of the series impedance of the inverter system, and the crossing frequency f cp(7uH) = f cm(7uH) = 4.4 Hz. When L g = 0 uH, there is a negative crossing of 180° in the phase-frequency curve of the series impedance of the inverter system, and the crossing frequency f cp(0uH) = 5 Hz, f cp(0uH) ≠ f cm(0uH) . Therefore, when L g = 350 uH, the output current of the inverter decays and oscillates. When L g = 7 uH, the output current of the inverter oscillates with equal amplitude. When L g = 0 uH, the output current of the inverter diverges and oscillates.

[0067] Figure 9 This is the output voltage and current waveform diagram of the inverter in Embodiment 2 of the present invention when it is connected to the grid and the line inductance L g on the grid side is switched from 350 uH to Lg = 7 uH and Lg = 0 uH; Figure 9 It can be seen from that the first sub-diagram is the voltage at the point of common coupling, the second sub-diagram is the output current waveform of the inverter when L g = 350 uH suddenly changes to L g = 7 uH at 2 s, and the third sub-diagram is the output current waveform of the inverter when L g = 350 uH suddenly changes to L g = 0 uH at 2 s. From Figure 9 it can be seen that when L g suddenly changes from 350 uH to 7 uH, the output current of the inverter oscillates with equal amplitude and is in a critically stable state; when L gWhen it mutates from 350 μH to 0 μH, the output current of the inverter diverges and oscillates, being in an unstable state, which is consistent with the result obtained by the stability analysis method proposed by the method of the present invention. In addition, through the Fourier analysis of the output current of the inverter, it is obtained that Figure 10 , and it can be seen that the oscillation frequency is 4 Hz, which is basically consistent with the corresponding characteristic frequency.

[0068] To sum up, the present invention provides a method for judging the stability of parallel / grid-connected inverters based on the overall impedance. First, the overall impedance of the inverter system is obtained through existing impedance sweep equipment or theoretical modeling methods. According to the Bode diagram of the parallel overall impedance of the inverter system, the voltage stability of the node to be judged for stability is judged. According to the Bode diagram of the series overall impedance of the inverter system, the current stability of the branch to be judged for stability is judged. Finally, the stability of the inverter system is detected and judged by synthesizing the results of voltage stability and current stability. The method of the present invention is not limited by the control and operation mode of the inverter and has the advantages of accurate stability judgment. It can directly and accurately judge the stability of the inverter system and can confirm the dominant electrical quantity of the oscillation, which is beneficial to guiding the parameter design of the inverter and enhancing the system stability.

[0069] It should be noted that the above content only illustrates the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. For those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and retouches can be made, and these improvements and retouches all fall within the protection scope of the claims of the present invention.

Claims

1. A method for judging the stability of grid-connected / stand-alone inverters based on overall impedance, characterized in that: It includes voltage stability judgment and current stability judgment. In the voltage stability judgment, according to the Bode diagram of the parallel overall impedance of the inverter system, the voltage stability of the node to be judged for stability is judged. In the current stability judgment, according to the Bode diagram of the series overall impedance of the inverter system, the current stability of the branch to be judged for stability is judged. Combining the judgment results of the voltage stability judgment and the current stability judgment, the stability of the inverter system during grid connection or off-grid is determined. If oscillation occurs, the dominant electrical quantity of the oscillation can be determined.

2. The method for judging the stability of the grid-connected / off-grid inverter based on the overall impedance according to claim 1, wherein: The voltage stability judgment specifically includes the following steps: S1-1: Obtain the parallel overall impedance of the system corresponding to the node to be judged for stability. S1-2: Judge the system characteristic frequency according to the maximum value of the amplitude-frequency curve of the Bode diagram of the parallel overall impedance. S1-3: Judge the system voltage stability according to the characteristic frequency obtained in step S1-2 and the phase-frequency curve of the Bode diagram of the parallel overall impedance.

3. The method for judging the stability of an on-grid / off-grid inverter based on overall impedance according to claim 2, characterized in that: The current stability judgment specifically includes the following steps: S2-1. Obtain the series overall impedance of the system corresponding to the branch to be judged for stability. S2-2. Judge the system characteristic frequency according to the minimum value of the amplitude-frequency curve of the Bode diagram of the series overall impedance. S2-3. Judge the system current stability according to the characteristic frequency and the phase-frequency curve of the Bode diagram of the series overall impedance.

4. The method for judging the stability of the grid-connected / off-grid inverter based on the overall impedance according to claim 2, characterized in that: In the said step S1-2, the calculation method for the maximum value of the amplitude-frequency curve of the parallel overall impedance Bode diagram is as follows: Denote the characteristic frequency as f vm_n , and its characteristic is that when the frequency f vm = f vm_n , the parallel overall impedance obtains the maximum value within the neighborhood of (f vm_n - δ, f vm_n + δ), where δ = 0.1f vm_n .

5. The method for judging the stability of the grid-connected / off-grid inverter based on the overall impedance according to claim 2, wherein: The method for judging the system voltage stability according to the characteristic frequency and the phase-frequency curve of the parallel overall impedance Bode diagram in the steps S1-3 is specifically as follows: Judge whether the phase-frequency curve of the parallel overall impedance Bode diagram has a positive slope crossing 180°. When it does not have a positive slope crossing 180°, the voltage of the node to be judged for stability generates a decaying oscillation; when it has a positive slope crossing 180°, record the crossing frequency as f vp_n , if f vp_n ≠f vm_n , then the voltage of the node to be judged for stability generates a divergent oscillation; if f vp_n =f vm_n , then the voltage of the node to be judged for stability generates an equal-amplitude oscillation.

6. The method for judging the stability of the grid-connected / off-grid inverter based on the overall impedance according to claim 3, wherein: The calculation method of the minimum value of the amplitude-frequency curve of the Bode diagram of the overall series impedance in step S2-2 is specifically as follows: Denote the characteristic frequency as f cm_n , and its characteristic is that when the frequency f cm = f cm_n , the overall series impedance obtains the minimum value in the neighborhood of (f cm_n - δ, f cm_n + δ), where δ = 0.1f cm_n .

7. The method for judging the stability of the grid-connected / off-grid inverter based on the overall impedance according to claim 3, wherein: The method of judging the system current stability according to the characteristic frequency and the phase-frequency curve of the Bode diagram of the series overall impedance in step S2-3 is specifically as follows: Judge whether the phase-frequency curve of the Bode diagram of the series overall impedance has a negative slope crossing 180°. When it does not have a negative slope crossing 180°, the current of the branch to be judged for stability generates damped oscillation. When its slope is negatively crossing 180°, record the crossing frequency as f cp_n , if f cp_n ≠f cm_n , then the current of the branch to be judged stable will generate divergent oscillation; if f cp_n =f cm_n , then the current of the branch to be judged stable will generate equal-amplitude oscillation.

8. An on-grid / off-grid inverter stability judgment system based on overall impedance, including a computer program, characterized in that: When the computer program is executed by a processor, it implements the steps of any of the above methods.

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