Novel grid-connected and off-grid non-inductive switching method for optical storage inverter system

Abstract

The invention belongs to the technical field of energy storage inverters, and particularly relates to a novel grid-connected and off-grid non-inductive switching method for an optical storage inverter system. The method mainly aims at the problem that when a power grid is lost, whether a system can be switched from a grid-connected state to an off-grid state or not, firstly, discrete Fourier transform is adopted, a power grid abnormal point is quickly recognized from amplitude-frequency characteristics, then whether a power grid loss fault or a low-voltage fault occurs at the power grid abnormal point is judged according to a constructed power grid voltage expectation value, and then the power grid loss fault or the low-voltage fault occurs at the power grid abnormal point is judged. And the judgment time is controlled to be 1 / 4 of the power frequency period, so that on one hand, the grid-connected and off-grid switching time can be controlled to be about 7ms, the problem of slow grid fault detection of an existing inverter grid-connected and off-grid mode switching method is solved, grid-connected and off-grid non-inductive switching can be realized, and the grid-connected and off-grid switching efficiency is improved. The requirement that the important load on the user side can still operate stably when the large power grid breaks down is met; and on the other hand, the method can be compatible with a low-pass working condition, the off-grid mode cannot be switched by mistake under the low-pass working condition, and the accuracy of grid-connected and off-grid switching is improved.

Description

Technical Field

[0005] The present invention belongs to the technical field of energy storage inverters, and in particular relates to a novel grid-connected and off-grid senseless switching method for a photovoltaic energy storage inverter system. Background Art

[0006] At present, the environment and energy are still the focus of global attention. Based on this, new energy technologies have been vigorously developed in recent years. At the same time, they face more challenges and technical requirements, including photovoltaic energy storage technology. With the continuous expansion of photovoltaic power generation, users' demand for photovoltaic power generation is no longer limited to grid connection, but more attention is turned to energy storage. Therefore, the requirements for grid connection and off-grid connection of distributed photovoltaic power generation systems have been improved. It is hoped that millisecond-level grid connection and off-grid switching can be achieved to ensure that critical loads will not shut down due to unplanned failures of the large power grid or other failures that may cause disconnection from the grid.

[0007] The on-grid and off-grid switching schemes of photovoltaic storage inverter systems are mostly based on the detection of grid voltage amplitude faults, grid voltage frequency faults or grid voltage zero-crossing loss faults to determine whether to switch to off-grid mode. This means that at least one power frequency cycle is required from the actual loss of the grid to the detection of the fault. Taking 50Hz power frequency as an example, it takes at least 20ms. This process does not include the grid-side relay disconnection action time and the off-grid wave generation time. In this way, this type of on-grid and off-grid switching scheme requires at least 20ms to complete, which may not meet the requirements of on-grid and off-grid seamless switching, and the user experience is lacking.

[0008] Some of the existing on-grid and off-grid switching solutions for photovoltaic storage inverter systems cannot achieve millisecond-level on-grid and off-grid switching due to the long grid fault detection cycle. This may cause important loads on the user side to lose power and shut down, affecting the user's product experience. Although the other part can achieve ms-level switching, it is more based on the change in grid voltage amplitude to determine whether a grid fault has occurred. This type of method is prone to mistakenly switching to off-grid mode under low-throughput conditions, and the accuracy of grid loss detection is low.

[0009] Low voltage ride-through condition refers to low voltage ride-through condition, under which the photovoltaic power generation system is required to maintain operation without disconnecting from the grid when the terminal voltage drops to a certain value, and even provide a certain amount of reactive power to help the system restore voltage. Figure 1 As shown in the figure, when entering the low-through condition, the grid voltage is also distorted to a certain extent. At this time, the amplitude-frequency characteristics of the grid voltage are similar to the grid-connected and off-grid switching to a certain extent, so it is necessary to distinguish the low-through condition here. Summary of the invention

[0010] The present invention proposes a novel method for seamless on-grid and off-grid switching of a photovoltaic storage inverter system, which overcomes the problem of slow grid fault detection in the existing inverter on-grid and off-grid mode conversion method and controls the on-grid and off-grid switching time to within 10ms, thereby achieving seamless on-grid and off-grid switching. In addition, it can reduce the possibility of erroneous switching of the off-grid mode under low-throughput conditions, improve the accuracy of on-grid and off-grid switching, and solve the above-mentioned problems.

[0011] The technical solution of the present invention is achieved in this way:

[0012] A novel method for non-grid switching of a photovoltaic energy storage inverter system, comprising the following steps:

[0013] S1. In the grid-connected mode, the grid voltage is sampled in real time to obtain and save the real-time sampling value of the grid voltage;

[0014] S2, calculate the effective value of the grid voltage and save and update it in real time;

[0015] S3, perform discrete Fourier transform on the real-time sampling value of the grid voltage, and observe whether the amplitude of each frequency point is normal;

[0016] S4. When the grid voltage amplitude at a certain frequency point is observed to be abnormal, the grid loss detection logic is entered, and the effective value of the power frequency cycle before the grid voltage is determined to be abnormal is saved;

[0017] S5, constructing an expected value of the grid voltage according to the effective value of the grid voltage in the power frequency cycle before determining that the grid voltage is abnormal and the current phase-locked loop phase-locked angle;

[0018] 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 switched to the off-grid mode. Otherwise, continue to work in the grid-connected mode.

[0019] Through the above technical solution, the standard for seamless switching between on-grid and off-grid mode is that the time from grid loss to system switching to off-grid mode is less than 10ms; the present invention realizes this on-grid and off-grid switching solution in less than 10ms through the solution of steps S1-S6.

[0020] Optionally, in step S2, the effective value of the grid voltage is calculated according to the real-time sampling value of the grid voltage and the zero-crossing interruption of the grid voltage, and the update cycle of the effective value of the grid voltage corresponds to the power frequency cycle, wherein the calculation formula of the effective value of the grid voltage is as follows:

[0021]

[0022] V grid_RMS : Effective value of grid voltage;

[0023] vgrid_n : The real-time sampling value of the grid voltage at the nth sampling point;

[0024] N: The number of sampling points in one power grid cycle.

[0025] Optionally, in step S3, a 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, wherein the sliding window is for an input signal for discrete Fourier transform, that is, a real-time sampling value of the grid voltage.

[0026] Through the above technical solution, the purpose of performing discrete Fourier transform is to obtain the amplitude-frequency characteristic diagram of the grid voltage.

[0027] Optionally, in step S3, the resolution calculation formula of Fourier transform is as follows:

[0028] Δf=f sample / N;

[0029] Where Δf represents the resolution, f sample represents the sampling frequency, and N represents the window signal length;

[0030] For AC signals with power frequency cycles, the signal length N1 that can be sampled within one power frequency cycle depends on the sampling frequency and the power frequency;

[0031] N1=f sample / f0;

[0032] Among them, f0 represents the power frequency;

[0033] For the discrete Fourier transform of AC signals, the time length of the window is taken as an integer multiple of the AC cycle, so the relationship between the window signal length N and the signal length N1 that can be sampled within one power frequency cycle is:

[0034] N = k * N1;

[0035] Where k is a positive integer;

[0036] Δf=f0 / k;

[0037] In the present invention, k is set to 5, and the resolution Δf=10 Hz.

[0038] Through the above technical solution, the purpose of calculating the resolution of Fourier transform is to obtain the frequency points of the grid voltage amplitude-frequency diagram, so as to select the appropriate window signal length, that is, the sliding window length of the previous step.

[0039] Optionally, in step S3, the calculation formula of discrete Fourier transform is as follows:

[0040]

[0041] x[n]: the sampling value of the nth sampling point;

[0042] X[m]: the amplitude of the mth frequency point; (0≤m≤N-1)

[0043] N: The number of data in a discrete Fourier transform, that is, the length of the window signal mentioned above;

[0044] j: imaginary part representation;

[0045] e: natural number.

[0046] Optionally, in step S4, when it is observed that the grid voltage corresponding to the frequency point change does not meet the set threshold, the grid voltage is preliminarily determined to be abnormal, and the grid loss detection logic is entered; wherein, the set threshold in step S4 includes a DC component threshold and a threshold for the sum of the amplitudes of frequency points other than 50 Hz, the DC component threshold is set to 0.2, and the threshold for the sum of the amplitudes of frequency points other than 50 Hz is set to 0.1.

[0047] Optionally, in step S5, the calculation formula of the expected value of the grid voltage is as follows:

[0048] V grid_expected =V grid_RMS sin(2πft+θ);

[0049] V grid_RMS : Effective value of grid voltage;

[0050] θ: current phase-locked loop phase-locked angle;

[0051] f: power frequency cycle, 50Hz;

[0052] t: time.

[0053] Optionally, in step S6, the threshold is set to 5 ms, which is 1 / 4 of the power frequency period for an AC signal with a power frequency period of 50 Hz.

[0054] After adopting the above technical solution, the beneficial effects of the present invention are:

[0055] The present invention proposes a novel grid-connected and off-grid senseless switching method for a photovoltaic storage inverter system, which mainly aims at the problem of whether the system can achieve grid-connected to off-grid senseless switching when the power grid is lost. Firstly, discrete Fourier transform is adopted to quickly identify grid abnormal points from the amplitude-frequency characteristics, and then it is judged whether the grid abnormal point is a grid loss fault or a low-voltage breakdown according to the constructed grid voltage expected value. The judgment time is controlled to 1 / 4 of the power frequency cycle. Therefore, on the one hand, the present invention can control the grid-connected and off-grid switching time to about 7ms, overcoming the problem of slow grid fault detection in the existing inverter grid-connected and off-grid mode conversion method, and can achieve grid-connected and off-grid senseless switching, meeting the requirement that important loads on the user side can still operate stably when a large power grid fails; on the other hand, the present invention is also compatible with low-voltage breakdown conditions, and can avoid mistaken switching to the off-grid mode under low-voltage breakdown conditions, thereby improving the accuracy of grid-connected and off-grid switching.

[0056] The present invention adopts a discrete Fourier transform of a sliding window to quickly and effectively find abnormal points of grid voltage from the amplitude-frequency characteristics; utilizes the inertia of a phase-locked loop, that is, when a grid is lost, due to the inertia of the phase-locked loop, the angle of the phase-locked output can still maintain a normal value within several ms, and a reasonable expected grid voltage value can be constructed in combination with the effective value of the grid voltage saved before the grid abnormality; determines whether the grid is lost based on the proportional relationship between the real-time sampling value of the grid voltage and the expected value of the grid voltage, if the two have a fixed proportional relationship, it indicates that the grid is not lost, but voltage crossing occurs, and the grid voltage amplitude changes, otherwise it indicates that the grid voltage is lost, and the grid voltage sampling value at the grid-connected port is distorted; from the discovery of the grid voltage abnormality, which requires a sliding window of 1 ms, to the determination of the grid loss stable threshold of 5 ms, including switching to the off-grid mode for stable wave generation, the entire process only takes about 7 ms, and can achieve a senseless switching between the grid and the off-grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0058] Figure 1 This is a flow chart of a novel grid-connected and off-grid senseless switching method for a photovoltaic storage inverter system in an embodiment;

[0059] Figure 2 It is the voltage amplitude-frequency diagram of the power grid under the low-through condition in the background technology;

[0060] Figure 3 The grid voltage waveform and phase-locking angle waveform of the grid port when the large grid is lost in the embodiment;

[0061] Figure 4 The voltage waveform of the grid at the grid-connected port and the phase-locked angle waveform during low-throughput in the embodiment;

[0062] Figure 5 is a voltage amplitude-frequency diagram of the power grid during normal operation in the embodiment;

[0063] Figure 6 It is a diagram of the voltage amplitude-frequency of the power grid when switching between on-grid and off-grid in the embodiment. DETAILED DESCRIPTION

[0064] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0065] The embodiment of the present application discloses a novel method for seamless on-grid and off-grid switching of a photovoltaic energy storage inverter system.

[0066] Example

[0067] according to Figures 1 to 6 As shown, a novel method for non-grid switching of a photovoltaic energy storage inverter system includes the following steps:

[0068] S1: In the grid-connected mode, the grid voltage is sampled in real time and saved;

[0069] S2: Calculate the effective value of the grid voltage based on 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 cycle of the effective value of the grid voltage corresponds to the power frequency cycle; the calculation formula of the effective value of the grid voltage is as follows:

[0070]

[0071] V grid_RMS : Effective value of grid voltage;

[0072] v grid_n : The real-time sampling value of the grid voltage at the nth sampling point;

[0073] N: The number of sampling points in one power grid cycle.

[0074] S3: Perform discrete Fourier transform according to the real-time sampling value of the grid voltage within a continuous period of time, and observe the amplitude change of the corresponding frequency point; here, in order to ensure that the abnormal grid voltage can be detected quickly and effectively, the grid voltage is selected to be discrete Fourier transform in the form of a sliding window, and the time step of the window sliding can be determined according to actual needs. For example, the purpose of the present invention is to speed up the time of switching between on-grid and off-grid, so the time step of the window sliding should be less than the expected on-grid and off-grid switching time of the present invention. The present invention selects the window sliding time step as 1ms;

[0075] The sliding window here is for the input signal for discrete Fourier transform, that is, the real-time sampling value of the grid voltage. The algorithm calculation of the sliding window belongs to the existing technology and will not be described here.

[0076] S4: When it is observed that the grid voltage corresponding to the frequency point change does not meet the set threshold, the grid voltage is initially determined to be abnormal, and the grid loss detection logic is entered, and the effective value of the grid voltage in the previous power frequency cycle is saved;

[0077] like Figure 5 and Figure 6 As shown in the figure, when the grid-on and off-grid switching occurs, the amplitude-frequency diagram obtained by discrete Fourier transform of the grid voltage is obviously different from the amplitude-frequency diagram during normal operation. There are two obvious differences: first, the DC component is significantly increased, and second, the amplitude of other frequency points except 50Hz (power frequency cycle) is significantly increased;

[0078] It can be seen that there are two set thresholds in step S4, including a DC component threshold and a sum of amplitude thresholds of frequency points other than 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 of the determination conditions is met, and the DC component threshold is set to 0.2 in the present invention; the sum of amplitude thresholds of frequency points other than 50 Hz is used to determine whether the sum of amplitudes of frequency points other than 50 Hz is abnormal. If the sum of amplitudes of frequency points other than 50 Hz is greater than the sum of amplitude thresholds of frequency points other than 50 Hz, the second determination condition is met, and the sum of amplitude thresholds of frequency points other than 50 Hz is set to 0.1 in the present invention.

[0079] S5: constructing an expected value of the grid voltage according to the saved effective value of the grid voltage in the previous power frequency cycle and the current phase-locked loop phase-locked angle;

[0080] The calculation formula of the expected value of the grid voltage is as follows:

[0081] V grid_expected =V grid_RMS sin(2πft+θ);

[0082] V grid_RMS : Effective value of grid voltage;

[0083] θ: current phase-locked loop phase-locked angle;

[0084] f: power frequency cycle, 50Hz;

[0085] t: time.

[0086] S6. Compare the current real-time sampling value of the grid voltage with the expected value of the constructed 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 switched to the off-grid mode. Otherwise, it continues to work in the grid-connected mode.

[0087] 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 grid voltage sampling frequency directly determine the resolution of the Fourier transform; Δf = f sample / N;

[0088] Where Δf is the resolution, f sample is the sampling frequency, N is the window signal length;

[0089] For AC signals with power frequency cycles, the signal length N1 that can be sampled within one power frequency cycle depends on the sampling frequency and the power frequency; N1 = f sample / f0;

[0090] Among them, f0 is the power frequency;

[0091] For the discrete Fourier transform of AC signals, the time length of the window can be taken as an integer multiple of the AC cycle. Therefore, the relationship between the window signal length N and the signal length N1 that can be sampled within one power frequency cycle is: N = k*N1;

[0092] Among them, k is a positive integer, which can be 1, 2, 3, etc.

[0093] Therefore, the resolution can also be expressed as: Δf = f0 / k;

[0094] It can be found that the resolution has nothing to do with the sampling frequency at this time, but is only related to the power frequency size and the window time length. The larger the window time length, that is, the larger k is, the smaller Δf is, that is, the higher the resolution is, and the denser the frequency distribution that can be observed is; so in the present invention, the window signal length is five power frequency cycles, that is, k=5, then for the grid voltage with a 50Hz power frequency cycle, the resolution Δf=10Hz.

[0095] The calculation of resolution is related to the density of frequency after Fourier transform. Taking 10Hz resolution as an example, the horizontal coordinates of the amplitude-frequency diagram obtained after Fourier transform are distributed at 0Hz, 10Hz, 20Hz, etc., which are multiples of 10Hz. The smaller the resolution, the more beneficial it is to data analysis, because when the resolution is small, more points of the amplitude-frequency diagram can be obtained, so that more difference points of the amplitude-frequency characteristics under the two working conditions of grid-connected and off-grid can be obtained. However, due to the limitations of sampling accuracy and computing speed, the present invention chooses 10Hz as the resolution. The amplitude-frequency diagram obtained at this resolution can intuitively distinguish the change of grid voltage before and after the grid-connected to off-grid, so the resolution is calculated to obtain the amplitude-frequency diagram under the appropriate frequency distribution. The appropriate judgment criterion is mainly whether as much useful information as possible can be obtained at this resolution, that is, whether normal grid-connected and abnormal grid-connected can be clearly distinguished.

[0096] The present invention explains how to select a suitable window signal length (i.e., the sliding window length of the previous step) when the sampling frequency is determined. There are two criteria for determination: first, the window signal length can cover the number of real-time sampling values ​​of the grid voltage of at least one AC cycle, because the discrete Fourier transform processes discrete, periodic time series; second, the resolution calculated based on the window signal length and the sampling frequency is as small as possible, so that the densest possible spectrum signal can be obtained;

[0097] The calculation formula for discrete Fourier transform is as follows:

[0098]

[0099] x[n]: the sampling value of the nth sampling point;

[0100] X[m]: the amplitude of the mth frequency point; (0≤m≤N-1)

[0101] N: The number of data in a discrete Fourier transform, that is, the length of the window signal mentioned above;

[0102] j: imaginary part representation;

[0103] e: natural number;

[0104] According to the above, the resolution has been selected as 10Hz. When m=1, it means that the frequency point is 10Hz, and X[1] represents the amplitude of the 10Hz frequency point. Similarly, X[m] represents the amplitude of the m*10Hz frequency point. The special thing is that when m=0, X[0] represents the DC component. For normal grid voltage, its DC component is almost zero.

[0105] The basis for the preliminary judgment of grid abnormality in S4 is that the change of the corresponding frequency point does not meet the threshold value. It mainly refers to that after the unplanned disconnection of the grid, the grid voltage value collected by the grid-connected port will not drop to 0V immediately, but there will be a certain distortion, which will result in obvious changes in the amplitude-frequency characteristics. Therefore, it is possible to preliminarily judge whether the grid voltage is abnormal based on this point.

[0106] The construction of the expected value of the grid voltage in S5 depends largely on the inertia of the phase-locked loop, as shown in the attached Figure 2 As shown, when the power grid is lost, because the phase-locked loop has a certain inertia, the phase-locked loop output angle is still a normal angle within a few milliseconds. Based on this, the expected value of the power grid voltage can be constructed.

[0107] For S6, the comparison value between the expected grid voltage value and the real-time grid voltage sampling value is used as the judgment object instead of the difference between the two. The reason is that, although the grid voltage will slowly decrease to 0V when the grid is lost, due to the effect of the inverter output filter capacitor, the grid voltage value collected by the network port changes relatively slowly and will not directly become 0V. Therefore, under the premise of requiring a short switching time, the comparison of the difference is not very meaningful. In addition, the low-throw working condition must be considered. Therefore, the ratio of the two is selected as the judgment object on the whole.

[0108] The grid loss detection logic refers to the contents of step S5 and step S6.

[0109] For the above-mentioned low penetration condition, the difference between the expected value of the grid voltage and the real-time sampling value of the grid voltage cannot be used as the judgment object. The reason is that during the period of low penetration switching, the grid voltage waveform will also be distorted to a certain extent. Therefore, the low penetration condition will first meet the conditions for preliminary judgment of grid abnormality, that is, S4 is passed. Then, in order to prevent the grid-off mode from being mistakenly switched under the low penetration condition, the grid voltage waveform during the low penetration process is considered to be a sinusoidal waveform with reduced amplitude, which has a fixed proportional relationship with the constructed expected value of the grid voltage. In this way, the low penetration condition can be distinguished from the grid disconnection condition.

[0110] For S6, the duration threshold for determining that the expected value of the grid voltage and the real-time sampling value of the grid voltage have no fixed proportional relationship needs to be carefully measured. If the value is too small, it may lead to false alarms, and if it is too large, it may lead to too long switching time. Therefore, the present invention sets the time threshold to 5ms, which is 1 / 4 of the power frequency cycle for AC signals with a 50Hz power frequency cycle. According to GB / T14715-2017, the switching time of UPS (uninterruptible power supply) should be less than 10ms, which can be called inductive switching. Therefore, the time from abnormal disconnection of the grid to the detection of grid disconnection must be less than 10ms. Therefore, 5ms is selected as the judgment threshold here, which takes into account the reservation of a part of the action time after the system determines that the grid disconnection occurs.

[0111] After determining that the grid is lost in S5, the off-grid mode can be switched, which mainly involves changing the inverter control mode from vector control to constant voltage-frequency ratio (VF) control. The entire process takes a switching cycle level of time, i.e., microseconds.

[0112] Combined with the discrete Fourier transform window sliding step size set to 1ms in S3, the time threshold for determining grid loss in S6 is 5ms, and the time for switching from grid-connected mode to off-grid mode is in the microsecond level, when the grid is disconnected, the total time for the photovoltaic storage inverter system involved in the present invention to switch from grid-connected to off-grid can be controlled at about 7ms. 7ms≈sliding step size 1ms (selected according to actual experimental debugging, can be selected as appropriate depending on the specific situation) + grid loss time threshold 5ms + calculation time of the algorithm inside the software (the algorithm is at the interrupt level, and the interrupt cycle is in the microsecond level).

[0113] A novel grid-connected and off-grid senseless switching method for a photovoltaic storage inverter system in the present invention samples the grid voltage in real time and performs discrete Fourier analysis when working in normal grid-connected mode, observes whether the amplitude of the corresponding frequency point is abnormal, and saves the effective value of the grid voltage in real time; when the amplitude of the corresponding frequency point is observed to be abnormal, it is preliminarily determined that the grid voltage is abnormal, and enters the grid loss detection logic; after entering the grid loss detection logic, the grid voltage expected value is constructed according to 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 locking angle, and the current grid voltage real-time value is compared with it. If the relationship between the two does not meet the set threshold value in the grid-connected mode and continues for a period of time, it is determined that the grid voltage is lost and switched to the off-grid mode, otherwise it continues to operate in the grid-connected mode.

[0114] A novel grid-connected and off-grid senseless switching method for a photovoltaic storage inverter system in the present invention is mainly aimed at a solution in which the system can quickly switch from grid-connected to off-grid when an unplanned large power grid is lost, and strives to achieve ms-level switching, meet users' requirements for grid-connected and off-grid senseless switching, and ensure that important loads can continue to operate in the event of an unplanned large power grid failure; the solution for switching from off-grid to grid-connected continues the pre-synchronization control solution currently commonly used on the market. When the grid is detected to be restored, the output of the inverter is first adjusted to track the voltage and phase of the large power grid. After the grid-connected conditions are met, the mode is switched, and the inverter control mode is changed from constant voltage-frequency ratio (VF) control to vector control. In this process, the inverter output only switches at the microsecond level when the control mode is changed, and the time is almost negligible, which is also a senseless switching.

[0115] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A novel method for non-grid switching between photovoltaic and energy storage inverter systems, characterized in that: The following steps are involved: S1. In the grid-connected mode, the grid voltage is sampled in real time to obtain and save the real-time sampling value of the grid voltage; 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 amplitude of each frequency point is normal; S4. When an abnormal amplitude value at a certain frequency point of the grid voltage is observed, the grid loss detection logic is entered, and the effective value of the power frequency cycle before the grid voltage abnormality is determined is saved; S5, constructing an expected value of the grid voltage according to the effective value of the grid voltage in the power frequency cycle before determining that the grid voltage is abnormal and the current phase-locked loop phase-locked angle; 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 switched to the off-grid mode. Otherwise, continue to work in the grid-connected mode.

2. According to the novel photovoltaic storage inverter system on-grid and off-grid senseless switching method of claim 1, it is characterized in that: In step S2, the effective value of the grid voltage is calculated according to the real-time sampling value of the grid voltage and the zero-crossing interruption of the grid voltage. The update cycle of the effective value of the grid voltage corresponds to the power frequency cycle. The calculation formula of the effective value of the grid voltage is as follows: V grid_RMS : Effective value of grid voltage; v grid_n : Real-time sampling value of the grid voltage at the nth sampling point; N: The number of sampling points in one power grid cycle.

3. According to the novel photovoltaic energy storage inverter system on-grid and off-grid senseless switching method of claim 1, it is characterized in that: In step S3, a 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, wherein the sliding window is for an input signal for discrete Fourier transform, that is, a real-time sampling value of the grid voltage.

4. According to the novel photovoltaic storage inverter system on-grid and off-grid senseless switching method of claim 1, it is characterized in that: In step S3, the resolution calculation formula of Fourier transform is as follows: Δf=f sample / N; Where Δf represents the resolution, f sample represents the sampling frequency, and N represents the window signal length; For AC signals with power frequency cycles, the signal length N1 that can be sampled within one power frequency cycle depends on the sampling frequency and the power frequency; N1=f sample / f0; Among them, f0 represents the power frequency; For the discrete Fourier transform of AC signals, the time length of the window is taken as an integer multiple of the AC cycle, so the relationship between the window signal length N and the signal length N1 that can be sampled within one power frequency cycle is: N = k * N1; Where k is a positive integer; Δf=f0 / k; Wherein, k is 5 and the resolution Δf=10 Hz.

5. According to the novel photovoltaic energy storage inverter system on-grid and off-grid senseless switching method of claim 1, it is characterized in that: In step S3, the calculation formula of discrete Fourier transform is as follows: x[n]: the sampling value of the nth sampling point; X[m]: the amplitude of the mth frequency point; (0≤m≤N-1) N: The number of data in a discrete Fourier transform, that is, the length of the window signal mentioned above; j: imaginary part representation; e: natural number.

6. According to the novel photovoltaic energy storage inverter system on-grid and off-grid senseless switching method of claim 1, it is characterized in that: In step S4, when it is observed that the grid voltage corresponding to the frequency point change does not meet the set threshold, the grid voltage is preliminarily determined to be abnormal, and the grid loss detection logic is entered; wherein, the set threshold in step S4 includes a DC component threshold and a threshold for the sum of the amplitudes of the frequency points other than 50 Hz, the DC component threshold is set to 0.2, and the threshold for the sum of the amplitudes of the frequency points other than 50 Hz is set to 0.

1.

7. According to claim 1, a novel method for switching between on-grid and off-grid without sensing of a photovoltaic energy storage inverter system is characterized in that: In step S5, the calculation formula of the expected value of the grid voltage is as follows: V grid_expected =V grid_RMS sin(2πft+θ); V grid_RMS : Effective value of grid voltage; θ: current phase-locked loop phase-locked angle; f: power frequency cycle, 50Hz; t: time.

8. According to claim 1, a novel method for switching between on-grid and off-grid without sensing of a photovoltaic energy storage inverter system is characterized in that: In step S6, the threshold is set to 5 ms, which is 1 / 4 of the power frequency period for an AC signal with a power frequency period of 50 Hz.

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