A method for non-sensing switching between on-grid and off-grid of photovoltaic storage inverter system

Abstract

The present invention belongs to the technical field of energy storage inverters, and particularly relates to a method for seamless grid-connected and off-grid switching of a photovoltaic energy storage inverter system. This method mainly addresses the issue of whether the system can achieve seamless switching from grid-connected to off-grid when the grid is lost. First, discrete Fourier transform is used to quickly identify abnormal grid points from the amplitude-frequency characteristics. Then, based on the constructed expected value of the grid voltage, it is determined whether a grid loss fault or a low voltage ride-through occurs at the abnormal grid point, and the judgment time is controlled within 1 / 4 power frequency cycle. Therefore, on the one hand, the present invention can control the grid-connected and off-grid switching time within about 7 ms, overcoming the problem of slow grid fault detection in the existing inverter grid-connected and off-grid mode conversion method, and can achieve seamless grid-connected and off-grid switching, meeting the requirement that important loads on the user side can still operate stably during large grid faults. On the other hand, the present invention can also be compatible with low voltage ride-through conditions, ensuring that it will not mistakenly switch to the off-grid mode under low voltage ride-through conditions and improving the accuracy of grid-connected and off-grid switching.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy storage inverters, and particularly relates to a method for seamless grid-connected and off-grid switching of a photovoltaic energy storage inverter system. Background Art

[0002] Currently, the environment and energy remain the focus issues of global concern. Based on this, new energy technologies have been vigorously developed in recent years. While the technology is developing, more challenges and technical requirements are faced, including photovoltaic energy storage technology. With the continuous expansion of photovoltaic power generation, users' demands for photovoltaic power generation are no longer limited to grid connection. More attention is turned to energy storage. Therefore, the requirements for grid-connected and off-grid switching of distributed photovoltaic power generation systems have been improved, and it is hoped that millisecond-level grid-connected and off-grid switching can be achieved. Only in this way can it be ensured that critical loads will not stop due to unplanned faults of the large power grid or other faults that may cause disconnection from the power grid.

[0003] Regarding the grid-connected and off-grid switching schemes of photovoltaic energy storage inverter systems, more are based on the detection of grid voltage amplitude faults, grid voltage frequency faults, or the loss of the zero crossing point of the grid voltage to determine whether to switch to the off-grid mode. This means that it takes at least one power frequency cycle from the actual loss of the power grid to the detection of this fault. Taking a 50Hz power frequency as an example, that is, at least 20ms is required. In this process, the disconnection action time of the grid-side relay and the time of off-grid power generation wave are not included. Thus, it seems that such grid-connected and off-grid switching schemes require at least more than 20ms to complete, and the time may not meet the requirements of seamless grid-connected and off-grid switching, and the user experience is lacking.

[0004] Some of the existing grid-connected and off-grid switching schemes of photovoltaic energy storage inverter systems have a long grid fault detection period and cannot achieve millisecond-level grid-connected and off-grid switching. This may cause the important loads on the user side to lose power and stop, affecting the user's product experience. Another part can achieve ms-level switching, but more is to judge whether a grid fault occurs according to the change of the grid voltage amplitude. This method is prone to mis-switching to the off-grid mode under low voltage ride-through conditions, and the accuracy of grid loss detection is relatively low.

[0005] The low voltage ride-through condition refers to the low voltage ride-through condition. Under this condition, when the terminal voltage of the photovoltaic power generation system drops to a certain value, it is required not to disconnect from the power grid but to continue to operate, and even provide a certain amount of reactive power for the system to help the system restore voltage. As shown in the appendix Figure 1 As shown, at the moment of entering the low voltage ride-through condition, the grid voltage is also distorted to a certain extent. At this time, the amplitude-frequency characteristics of the grid voltage are somewhat similar to grid-connected and off-grid switching to a certain extent. Therefore, it is necessary to distinguish the low voltage ride-through condition here. Summary of the Invention

[0006] The present invention proposes a method for seamless grid-connected and off-grid switching of a photovoltaic energy storage inverter system, which overcomes the problem of slow grid fault detection in the existing inverter grid-connected and off-grid mode conversion method, controls the grid-connected and off-grid switching time within 10 ms, so as to achieve seamless grid-connected and off-grid switching. In addition, it can reduce the possibility of erroneously switching to the off-grid mode under low voltage ride-through conditions and improve the accuracy of grid-connected and off-grid switching, thus solving the above problems.

[0007] The technical solution of the present invention is realized as follows:

[0008] A method for seamless grid-connected and off-grid switching of a photovoltaic energy storage inverter system includes the following steps:

[0009] S1. In the grid-connected mode, the grid voltage is sampled in real time to obtain the real-time sampled value of the grid voltage and save it;

[0010] S2. Calculate the effective value of the grid voltage and save and update it in real time;

[0011] S3. Perform discrete Fourier transform on the real-time sampled value of the grid voltage and observe whether the amplitudes at each frequency point are normal;

[0012] S4. When an abnormal amplitude at a certain frequency point of the grid voltage is observed, enter the grid loss detection logic and save the effective value of the previous power frequency cycle before determining the abnormal grid voltage;

[0013] S5. Construct an expected grid voltage value according to the effective value of the grid voltage in the previous power frequency cycle before determining the abnormal grid voltage and the current phase-locked angle of the phase-locked loop;

[0014] S6. Compare the current real-time sampled value of the grid voltage with the expected grid voltage value constructed in step S5. If there is no fixed proportional relationship between the two and the duration reaches the set threshold, it is determined that the grid is lost and the system switches to the off-grid mode; otherwise, it continues to operate in the grid-connected mode.

[0015] Through the above technical solution, the standard for seamless grid-connected and off-grid switching is that the time from grid loss to the system switching to the off-grid mode is less than 10 ms; through the solutions of steps S1 - S6 of the present invention, the time of this grid-connected and off-grid switching solution is less than 10 ms.

[0016] Optionally, in step S2, the effective value of the grid voltage is calculated according to the real-time sampled value of the grid voltage and the zero-crossing interruption of the grid voltage. The update period of the effective value of the grid voltage corresponds to the power frequency cycle. Among them, the calculation formula of the effective value of the grid voltage is as follows:

[0017] ;

[0018] V grid_RMS : the effective value of the grid voltage;

[0019] v grid_n : The real-time sampled value of the grid voltage at the n th sampling point;

[0020] M : The number of sampling points within one grid cycle.

[0021] Optionally, in step S3, the discrete Fourier transform is performed on the grid voltage in the form of a sliding window, and the time step of the sliding window is 1 ms. Here, the sliding window is for the input signal of the discrete Fourier transform, that is, the real-time sampled value of the grid voltage.

[0022] Through the above technical solution, the purpose of performing the discrete Fourier transform is to obtain the amplitude-frequency characteristic diagram of the grid voltage.

[0023] Optionally, in step S3, the resolution calculation formula of the Fourier transform is as follows:

[0024] Δ f = f sample / N ;

[0025] Where, Δ f represents the resolution, f sample represents the sampling frequency, N represents the window signal length;

[0026] For an AC signal with a power frequency period, the signal length N 1 that can be sampled within one power frequency period depends on the sampling frequency and the power frequency;

[0027] N 1 = f sample / f 0;

[0028] Where, f 0 represents the power frequency;

[0029] For the discrete Fourier transform of an AC signal, the time length of the window is taken as an integer multiple of the AC period. Therefore, the relationship between the window signal length N and the signal length N 1 that can be sampled within one power frequency period is:

[0030] N = k*N 1;

[0031] Where, k is a positive integer;

[0032] Δ f= f 0 / k ;

[0033] In the present invention k Take 5, resolution Δ f = 10 Hz.

[0034] Through the above technical solution, the purpose of calculating the resolution of the Fourier transform is to obtain the frequency points of the grid voltage amplitude-frequency diagram, so as to select an appropriate window signal length, that is: the sliding window length of the previous step.

[0035] Optionally, in step S3, the calculation formula of the discrete Fourier transform is as follows:

[0036] ;

[0037] : Sampling value of the n th sampling point;

[0038] : Amplitude of the m th frequency point; (0 ≤ m ≤ N - 1)

[0039] N : Number of data for one discrete Fourier transform, that is, the above window signal length;

[0040] j : Imaginary part representation;

[0041] : Natural number.

[0042] Optionally, in step S4, when it is observed that the change of the grid voltage corresponding frequency point does not meet the set threshold, it is preliminarily determined that the grid voltage is abnormal and the grid loss detection logic is entered; among them, the set threshold in step S4 includes the DC component threshold and the sum threshold of the amplitudes of the frequency points other than 50 Hz. The DC component threshold is set to 0.2, and the sum threshold of the amplitudes of the frequency points other than 50 Hz is set to 0.1.

[0043] Optionally, in step S5, the calculation formula of the grid voltage expected value is as follows:

[0044] ;

[0045] V grid_RMS : Effective value of grid voltage;

[0046] θ : Current phase-locked loop phase-locked angle;

[0047] f: Industrial frequency period, 50 Hz;

[0048] t : Time.

[0049] Optionally, in step S6, the set threshold is 5 ms. For an AC signal with a 50 Hz industrial frequency period, this is 1 / 4 of an industrial frequency period.

[0050] After adopting the above technical solution, the beneficial effects of the present invention are:

[0051] A method for seamless grid-connected and off-grid switching of a photovoltaic energy storage inverter system proposed by the present invention mainly aims at the problem of whether the system can achieve seamless switching from grid-connected to off-grid when the power grid is lost. First, discrete Fourier transform is used to quickly identify abnormal grid points from the amplitude-frequency characteristics, and then according to the constructed expected value of the grid voltage, it is judged whether a grid loss fault or a low voltage ride-through occurs at the abnormal grid point. The judgment time is controlled within 1 / 4 of an industrial frequency period. Therefore, on the one hand, the present invention can control the grid-connected and off-grid switching time within about 7 ms, overcoming the problem of slow grid fault detection in the existing inverter grid-connected and off-grid mode conversion methods, and can achieve seamless grid-connected and off-grid switching, meeting the requirement that important loads on the user side can still operate stably during large power grid faults; on the other hand, the present invention can also be compatible with low voltage ride-through conditions, and can avoid mis-switching to the off-grid mode under low voltage ride-through conditions, improving the accuracy of grid-connected and off-grid switching.

[0052] The present invention adopts a sliding window discrete Fourier transform to quickly and effectively discover abnormal grid voltage points from the amplitude-frequency characteristics; utilizes the inertia of the phase-locked loop, that is, when a grid loss occurs, due to the inertia of the phase-locked loop circuit, the angle output by the phase-locked loop can still maintain the normal value within a few ms. Combining the effective value of the grid voltage saved before the grid anomaly, a reasonable expected value of the grid voltage can be constructed; according to the proportional relationship between the real-time sampled value of the grid voltage and the expected value of the grid voltage, it is judged whether the grid is lost. If there is a fixed proportional relationship between the two, it means that it is not a grid loss but a voltage ride-through, and the amplitude of the grid voltage has changed, otherwise it means that the grid voltage is lost and the sampled value of the grid voltage at the grid connection port is distorted; from the 1 ms sliding window for discovering abnormal grid voltage to the stable threshold of 5 ms for judging grid loss, including switching to the off-grid mode and stably generating waves, the whole process only takes about 7 ms, and seamless grid-connected and off-grid switching can be achieved. Description of the Drawings

[0053] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0054] Figure 1 is a flowchart of a method for seamless switching between grid-connected and off-grid modes of a photovoltaic energy storage inverter system in an embodiment;

[0055] Figure 2 is the grid voltage amplitude-frequency diagram under low voltage ride-through conditions in the background art;

[0056] Figure 3 is the grid voltage waveform and phase-locked angle waveform at the grid connection port when the large power grid is lost in an embodiment;

[0057] Figure 4 is the grid voltage waveform and phase-locked angle waveform at the grid connection port during low voltage ride-through in an embodiment;

[0058] Figure 5 is the grid voltage amplitude-frequency diagram during normal operation in an embodiment;

[0059] Figure 6 is the grid voltage amplitude-frequency diagram during grid-connected and off-grid switching in an embodiment. Detailed implementation manners

[0060] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0061] The embodiments of the present application disclose a method for seamless switching between grid-connected and off-grid modes of a photovoltaic energy storage inverter system. Embodiment

[0062] According to Figures 1 to 6 shown, a method for seamless switching between grid-connected and off-grid modes of a photovoltaic energy storage inverter system includes the following steps:

[0063] S1: In the grid-connected mode, the grid voltage is sampled in real time and saved;

[0064] S2: Calculate the effective value of the grid voltage according to the real-time sampling value of the grid voltage and the zero-crossing interruption of the grid voltage, and update it in real time; according to the characteristics of the effective value of the AC signal, it can be known that the update period of the effective value of the grid voltage corresponds to the power frequency period; among them, the calculation formula of the effective value of the grid voltage is as follows:

[0065] ;

[0066] V grid_RMS : Effective value of grid voltage;

[0067] v grid_n : Then The real-time sampled value of the grid voltage at a sampling point;

[0068] M : The number of sampling points within one grid cycle.

[0069] S3: Perform discrete Fourier transform on the real-time sampled values of the grid voltage within a continuous period of time, and observe the amplitude change at the corresponding frequency point; here, to ensure rapid and effective detection of grid voltage anomalies, discrete Fourier transform is performed on the grid voltage in the form of a sliding window. The time step of window sliding can be determined according to actual requirements. For example, if the purpose of the present invention is to accelerate the grid connection and disconnection switching time, then the time step of window sliding should be less than the grid connection and disconnection switching time expected by the present invention. In the present invention, the time step of window sliding is selected as 1 ms;

[0070] Among them, the sliding window here is for the input signal of discrete Fourier transform, that is, the real-time sampled value of the grid voltage. The algorithm calculation of the sliding window belongs to the prior art and will not be elaborated here.

[0071] S4: When it is observed that the change of the grid voltage at the corresponding frequency point does not meet the set threshold, initially determine that the grid voltage is abnormal, enter the grid loss detection logic, and save the effective value of the grid voltage in the previous power frequency cycle;

[0072] Such as Figure 5 and Figure 6 shown, when grid connection and disconnection switching occurs, the amplitude-frequency diagram obtained by discrete Fourier transform of the grid voltage is significantly different from that during normal operation. There are two obvious differences: one is that the DC component increases significantly, and the other is that the amplitudes of other frequency points except 50 Hz (power frequency cycle) increase significantly;

[0073] It can be seen from this that there are two set thresholds in step S4, including the DC component threshold and the sum threshold of the amplitudes of frequency points except 50 Hz: the DC component threshold is used to determine whether the DC component is abnormal. If the DC component is greater than the DC component threshold, one determination condition is met. In the present invention, the DC component threshold is set to 0.2; the sum threshold of the amplitudes of frequency points except 50 Hz is used to determine whether the sum of the amplitudes of frequency points except 50 Hz is abnormal. If the sum of the amplitudes of frequency points except 50 Hz is greater than the sum threshold of the amplitudes of frequency points except 50 Hz, the second determination condition is met. In the present invention, the sum threshold of the amplitudes of frequency points except 50 Hz is set to 0.1.

[0074] S5: Construct the expected value of the grid voltage based on the effective value of the grid voltage in the previous power frequency cycle saved and the current phase-locked angle of the phase-locked loop;

[0075] Among them, the calculation formula of the expected value of the grid voltage is as follows:

[0076] ;

[0077] V grid_RMS : The effective value of the grid voltage;

[0078] θ : The phase-locked angle of the current phase-locked loop;

[0079] f : The power frequency period, 50 Hz;

[0080] t : Time.

[0081] S6. Compare the real-time sampled value of the current grid voltage with the constructed expected value of the grid voltage. If there is no fixed proportional relationship between the two and the duration reaches the set threshold, it is determined that the grid voltage is lost, and at the same time, switch to the off-grid mode; otherwise, continue to work in the grid-connected mode.

[0082] For the sliding window of the discrete Fourier transform of the grid voltage in S3, in addition to paying attention to the sliding time step, the size of the window itself and the sampling frequency of the grid voltage directly determine the resolution of the Fourier transform; Δ f = f sample / N ;

[0083] Among them, Δ f is the resolution, f sample is the sampling frequency, N is the window signal length;

[0084] For the AC signal of the power frequency period, the signal length N 1 that can be sampled within one power frequency period N 1 = f sample / f 0;

[0085] Among them, f 0 is the power frequency;

[0086] For the discrete Fourier transform of the AC signal, it is only necessary to take the time length of the window as an integer multiple of the AC period. Therefore, the relationship between the window signal length N and the signal length N 1 that can be sampled within one power frequency period is: N = k* N 1;

[0087] Among them, kis a positive integer and can take values of 1, 2, 3......

[0088] Therefore, the resolution can also be expressed as: Δ f = f 0 / k；

[0089] It can be found that the resolution size has nothing to do with the sampling frequency at this time, and is only related to the power frequency size and the window time length. The larger the window time length, that is k the larger, the smaller Δ f the smaller, that is, the higher the resolution, and the denser the frequency distribution that can be observed; Therefore, in the present invention, the window signal length is taken as five power frequency cycles, that is k = 5, then for the grid voltage with a 50Hz power frequency cycle, the resolution Δ f = 10Hz.

[0090] The calculation of the resolution is related to the density of the frequency after Fourier transform. Taking a resolution of 10Hz as an example, the abscissa of the amplitude-frequency diagram obtained after Fourier transform is distributed at 0Hz, 10Hz, 20Hz, etc., which are multiples of 10Hz. The smaller the resolution, the more beneficial it is for analyzing data, because after the resolution becomes smaller, the number of points on the amplitude-frequency diagram that can be obtained is more, so that more difference points in the amplitude-frequency characteristics under the two conditions of grid connection and off-grid can be obtained. However, due to the limitations of sampling accuracy and operation speed, overall, the present invention selects 10Hz as the resolution. The amplitude-frequency diagram obtained at this resolution can already intuitively distinguish the changes in the grid voltage before and after grid connection turning to off-grid. Therefore, the calculation of the resolution is to obtain the amplitude-frequency diagram under a suitable frequency distribution, and the appropriate discrimination criterion is mainly whether as much useful information as possible can be obtained at this resolution, that is, whether normal grid connection and abnormal grid connection can be clearly distinguished.

[0091] The present invention explains how to select a suitable window signal length (i.e., the sliding window length in the previous step) when the sampling frequency is determined. There are two criteria for determination: one is that the window signal length can cover at least the number of real-time sampling values of the grid voltage in one AC cycle, because the discrete Fourier transform processes discrete and periodic time series; the other is that the resolution calculated based on the window signal length and the sampling frequency is as small as possible, so that a spectrum signal as dense as possible can be obtained;

[0092] The calculation formula of the discrete Fourier transform is as follows:

[0093] ;

[0094] : The sampling value of the n th sampling point;

[0095] : Them The amplitude of a frequency point; (0 ≤ m ≤ N -1)

[0096] N : The number of data points for a single discrete Fourier transform, i.e., the above-mentioned window signal length;

[0097] j : Representation of the imaginary part;

[0098] : Natural number;

[0099] According to the above, the selected resolution is 10 Hz. Then m = 1 means that the frequency point is 10 Hz, and X[1] represents the amplitude of the 10 Hz frequency point. By analogy, X m represents m * 10 Hz frequency point amplitude. Among them, the special case is m = 0, X[0] represents the DC component. For a normal grid voltage, its DC component is almost zero.

[0100] For the basis of the preliminary determination of grid abnormality in S4, it is that the change of the corresponding frequency point does not conform to the threshold. Mainly, after the grid is disconnected without a plan, although the grid voltage value collected at the grid connection port will not immediately drop to 0 V, there will be a certain distortion, so there will be an obvious change in the amplitude-frequency characteristic. Therefore, it can be used to preliminarily judge whether the grid voltage is abnormal based on this point.

[0101] For the construction of the grid voltage expectation value in S5, it depends to a large extent on the inertia of the phase-locked loop. As shown in the appendix Figure 2 When the grid is lost, because the phase-locked loop has a certain inertia, the output angle of the phase-locked loop is still the normal angle within several ms. Based on this, the grid voltage expectation value can be constructed.

[0102] For S6, the reason for taking the comparison value of the grid voltage expectation value and the real-time sampled value of the grid voltage as the judgment object instead of their difference is that although the grid voltage will gradually decrease to 0 V when the grid is lost, due to the effect of the inverter output filter capacitor, the change speed of the grid voltage value collected at the grid connection port is relatively slow and will not directly become 0 V. Therefore, under the premise of requiring a short switching time, comparing the difference does not have much significance. In addition, the low-ride-through working condition needs to be considered. Therefore, comprehensively, the ratio of the two is selected as the judgment object.

[0103] The grid loss detection logic refers to the content of steps S5 and S6.

[0104] For the above-mentioned consideration of the low-ride-through condition, the difference between the expected grid voltage value and the real-time sampled grid voltage value cannot be used as the judgment object. The reason is that during the period of low-ride-through switching, the grid voltage waveform will also have a certain distortion. Therefore, first, the low-ride-through condition will reach the condition for initially judging grid abnormality, that is, S4 passes. Then, to prevent mis-switching to the off-grid mode under the low-ride-through condition, it is considered that the grid voltage waveform during the low-ride-through process is a sine waveform with a reduced amplitude, which has a fixed proportional relationship with the constructed expected grid voltage value. In this way, the low-ride-through condition can be distinguished from the grid-disconnection condition.

[0105] For S6, the duration threshold for judging that there is no fixed proportional relationship between the expected grid voltage value and the real-time sampled grid voltage value needs to be carefully measured. If this value is too small, it may lead to false alarms; if it is too large, it may lead to too long switching time between grid connection and disconnection. Therefore, in the present invention, this time threshold is set to 5 ms. For an AC signal with a 50 Hz power frequency cycle, it is 1 / 4 of a power frequency cycle. According to the requirements of GB / T 14715-2017, the switching time of the UPS (uninterruptible power supply) should be less than 10 ms. In this way, it can be called seamless switching. Therefore, the time from the abnormal disconnection of the grid to the detection of the grid disconnection must be less than 10 ms. Therefore, 5 ms is selected as the judgment threshold here, taking into account that a part of the action time is reserved after the system determines the grid connection and disconnection.

[0106] In S5, after judging the loss of the grid, the off-grid mode can be switched. mainly, the control mode of the inverter is changed from vector control to constant voltage-frequency ratio (VF) control. The whole process is at the time level of the switching period, that is, at the microsecond level;

[0107] Combined with setting the window sliding step of the discrete Fourier transform to 1 ms in S3, the time threshold for judging the loss of the grid to be 5 ms in S6, and the time for switching from the grid-connected mode to the off-grid mode being at the microsecond level, when the grid is disconnected, the total time for the photovoltaic energy storage inverter system involved in the present invention to switch from grid connection to off-grid can be controlled within about 7 ms. 7 ms ≈ sliding step 1 ms (selected according to actual experimental debugging and can be adjusted as appropriate according to specific conditions) + time threshold for judging the loss of the grid 5 ms + operation time of this algorithm inside the software (this algorithm is at the interrupt level and the interrupt period is at the microsecond level).

[0108] A method for seamless switching between grid-connected and off-grid modes in a photovoltaic energy storage inverter system according to the present invention. When operating in normal grid-connected mode, the grid voltage is sampled in real time and discrete Fourier analysis is performed to observe whether the amplitude at the corresponding frequency point is abnormal. At the same time, the effective value of the grid voltage is saved in real time. When the amplitude at the corresponding frequency point is observed to be abnormal, it is preliminarily determined that the grid voltage is abnormal, and the grid loss detection logic is entered. When the grid loss detection logic is entered, the expected value of the grid voltage is constructed based on the effective value of the grid voltage obtained in the previous power frequency cycle before the grid voltage is determined to be abnormal and the current phase-locked loop latch angle, and the real-time value of the current grid voltage is compared with it. If the relationship between the two does not meet the set threshold in the grid-connected mode and lasts for a continuous period of time, it is determined that the grid voltage is lost and the system switches to the off-grid mode; otherwise, it continues to operate in the grid-connected mode.

[0109] A method for seamless switching between grid-connected and off-grid modes in a photovoltaic energy storage inverter system according to the present invention mainly aims at a solution that enables the system to quickly switch from grid-connected to off-grid mode when there is an unplanned large grid loss. It strives to achieve a switching time of the order of milliseconds to meet the user's requirement for seamless switching between grid-connected and off-grid modes, and can ensure that important loads can continue to operate during an unplanned large grid fault. For the scheme of switching from off-grid to grid-connected, the commonly used pre-synchronization control scheme on the market is continued. When the grid recovery is detected, the output of the inverter is first adjusted to track the voltage and phase of the large grid. After meeting the grid-connected conditions, the mode is switched, and the control mode of the inverter is changed from constant voltage frequency ratio (VF) control to vector control. During this process, there is only a microsecond-level switching in the inverter output when the control mode changes, and the time can be almost ignored, which is also a seamless switching.

[0110] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for seamless switching between grid-connected and off-grid modes in a photovoltaic energy storage inverter system, characterized in that, It includes the following steps: S1. Under the grid-connected mode, the grid voltage is sampled in real time, and the real-time sampling value of the grid voltage is obtained and saved; S2. Calculate the effective value of the grid voltage and save and update it in real time; S3. Perform discrete Fourier transform on the real-time sampling value of the grid voltage, and observe whether the amplitudes at each frequency point are normal; S4. When the amplitude at a certain frequency point of the grid voltage is observed to be abnormal, enter the grid loss detection logic, and save the effective value of the previous power frequency cycle before determining that the grid voltage is abnormal; S5. Construct the expected value of the grid voltage according to the effective value of the grid voltage in the previous power frequency cycle before determining that the grid voltage is abnormal and the current phase-locked angle of the phase-locked loop; S6. Compare the current real-time sampling value of the grid voltage with the expected value of the grid voltage constructed in step S5. If there is no fixed proportional relationship between the two and the duration reaches the set threshold, it is determined that the grid is lost, and the system switches to the off-grid mode. Otherwise, it continues to work in the grid-connected mode; In step S3, the resolution calculation formula of the Fourier transform is as follows: Δ f = f sample / N ; Among them, Δ f represents the resolution, f sample represents the sampling frequency, N represents the window signal length; For an AC signal with a power frequency period, the signal length that can be sampled within one power frequency period N 1 depends on the sampling frequency and the power frequency; N 1= f sample / f 0； Among them, f 0 represents the power frequency; For the discrete Fourier transform of an AC signal, the time length of the window is taken as an integer multiple of the AC period. Therefore, the window signal length here N and the signal length that can be sampled within one power frequency period N The relationship between them is as follows: N = k*N 1； wherein, k positive integer; Δ f = f 0 / k ; Among them, k Take 5, resolution Δ f = 10 Hz; In step S3, the calculation formula of the discrete Fourier transform is as follows: ； : Sampling value of the n th sampling point; : The amplitude of the m th frequency point; (0 ≤ m ≤ N - 1) N : The number of data points for a single discrete Fourier transform, i.e., the length of the above window signal; j : Imaginary part representation; : Natural numbers; In step S5, the calculation formula of the expected value of the grid voltage is as follows: ； V grid_RMS : RMS value of grid voltage; θ : Current phase-locked angle of the phase-locked loop; f : Power frequency period, 50 Hz; t : Time.

2. A method for seamless switching between grid-connected and off-grid operation of a photovoltaic energy storage inverter system according to claim 1, characterized in that, In step S2, according to the real-time sampling value of the grid voltage and the zero-crossing interruption of the grid voltage, calculate the effective value of the grid voltage. The update period of the effective value of the grid voltage corresponds to the power frequency cycle. Among them, the calculation formula of the effective value of the grid voltage is as follows: ； V grid_RMS : RMS value of grid voltage; v grid_n : The n real-time sampled value of the grid voltage at the M : The number of sampling points within one power grid cycle.

3. A method for seamless switching between grid-connected and off-grid operation of a photovoltaic energy storage inverter system according to claim 1, characterized in that In step S3, it is selected to perform discrete Fourier transform on the grid voltage in the form of a sliding window. The time step of the sliding window is 1 ms. Among them, the sliding window is for the input signal for discrete Fourier transform, that is, the real-time sampling value of the grid voltage.

4. A method for seamless switching between grid-connected and off-grid modes of a photovoltaic energy storage inverter system according to claim 1, characterized in that, In step S4, when it is observed that the change of the grid voltage corresponding to the frequency point does not meet the set threshold, it is preliminarily determined that the grid voltage is abnormal, and the grid loss detection logic is entered; among them, the set threshold in step S4 includes the DC component threshold and the sum threshold of the amplitudes of the frequency points other than 50 Hz. The DC component threshold is set to 0.2, and the sum threshold of the amplitudes of the frequency points other than 50 Hz is set to 0.

1.

5. A method for seamless switching between grid-connected and off-grid operation of a photovoltaic energy storage inverter system according to claim 1, characterized in that, In step S6, the set threshold is 5 ms. For the AC signal of the 50 Hz power frequency cycle, it is 1 / 4 of a power frequency cycle.

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