Grid-connected system and zero-sequence resonance protection method and device thereof

Abstract

The present application discloses a grid-connected system and a zero-sequence resonance protection method thereof. The method includes: when the grid-connected system is in a grid-connected state, resonant information extraction is performed on the zero-sequence electrical signal output by the photovoltaic storage converter in real time to obtain a zero-sequence resonance signal within a preset frequency range; a sliding frequency search is performed on the zero-sequence resonance signal, and the effective value of the zero-sequence resonance amplitude of each frequency sweep point is calculated according to a sliding window algorithm; it is determined whether the effective value of the zero-sequence resonance amplitude of each frequency sweep point is greater than the zero-sequence resonance protection threshold; if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is greater than the zero-sequence resonance protection threshold, the local frequency search of the zero-sequence resonance signal is controlled to determine the zero-sequence resonance frequency; within the preset protection time, if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is continuously greater than the zero-sequence resonance protection threshold, the zero-sequence resonance protection is triggered to shut down the photovoltaic storage converter. The present application can perform zero-sequence resonance protection and lock the zero-sequence resonance frequency of the photovoltaic storage converter under grid-connected conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic storage converters, and in particular to a grid-connected system and a zero-sequence resonance protection method and device thereof. Background Art

[0002] Household photovoltaic power storage inverter is a power electronic device that integrates photovoltaic power generation and energy storage system, mainly used in homes or small commercial places. These systems can effectively use solar energy to generate electricity and store excess electricity for use at any time.

[0003] The main working modes of household photovoltaic storage inverters include: (1) Self-generation and self-use mode. In this case, the energy storage system gives priority to self-generated energy and only obtains electricity from the grid when power generation is insufficient. When the sun is sufficient, the excess electricity is stored in the battery for subsequent use. (2) Peak shaving and valley filling mode. In this case, the energy is charged from the grid during the low electricity price period and the electricity in the energy storage battery is used during the peak electricity price period to optimize the electricity cost. (3) Grid-connected mode. In this case, the energy storage system runs in parallel with the grid. When power generation exceeds consumption, the excess power is transmitted to the grid to obtain electricity subsidies; when power is insufficient, electricity can be purchased from the grid. (4) Off-grid mode. In this case, the energy storage system is not connected to the grid and operates entirely on solar power generation and reserve power. It is suitable for remote areas or places where the grid is unstable.

[0004] The above functions all rely on the stable operation of household photovoltaic storage inverters. However, in actual operating conditions, due to the influence of factors such as sudden changes in grid system parameters, transient changes in grid structure and operating status, and excessive line impedance, household photovoltaic storage inverters are prone to zero-sequence resonance when connected to the grid, which can cause grid failure and damage equipment in severe cases. Summary of the invention

[0005] In view of this, the present invention aims to solve one of the problems in the related art at least to a certain extent. To this end, the object of the present invention is to provide a grid-connected system and a zero-sequence resonance protection method and device thereof.

[0006] The present application provides a zero-sequence resonance protection method, which is applied to a grid-connected system. The grid-connected system includes a photovoltaic storage converter, a relay and a power grid, wherein the relay is arranged between the photovoltaic storage converter and the power grid, and when the relay is closed, the grid-connected system is in a grid-connected state, and the zero-sequence resonance protection method includes: when the grid-connected system is in the grid-connected state, the resonance information of the zero-sequence electrical signal output by the photovoltaic storage converter is extracted in real time to obtain a zero-sequence resonance signal within a preset frequency range; the zero-sequence resonance signal is searched for a sliding frequency point, and the effective value of the zero-sequence resonance amplitude of each frequency sweep point is calculated according to a sliding window algorithm; it is determined whether the effective value of the zero-sequence resonance amplitude of each frequency sweep point is greater than the zero-sequence resonance protection threshold; if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is greater than the zero-sequence resonance protection threshold, the local frequency search of the zero-sequence resonance signal is controlled to determine the zero-sequence resonance frequency point; within the preset protection time, if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is continuously greater than the zero-sequence resonance protection threshold, the zero-sequence resonance protection is triggered to shut down the photovoltaic storage converter.

[0007] In some embodiments, the zero-sequence resonance protection method includes: if the effective value of the zero-sequence resonance amplitude at the current frequency scanning point is less than or equal to the zero-sequence resonance protection threshold, the zero-sequence resonance protection is not triggered, and the zero-sequence resonance signal sliding frequency point search continues.

[0008] In certain embodiments, when the grid-connected system is in the grid-connected state, the resonance information of the electrical signal output by the photovoltaic storage inverter is extracted in real time to obtain a zero-sequence resonance signal within a preset frequency range, including: in the grid-connected state, collecting the zero-sequence voltage signal output by the photovoltaic storage inverter; determining the preset frequency range according to the equivalent inductance and capacitance parameters of the photovoltaic storage inverter and the equivalent inductance parameters of the power grid; and using a bandpass filter to filter the zero-sequence voltage signal according to the preset frequency range to extract the zero-sequence resonance signal within the preset frequency range.

[0009] In some embodiments, the sliding frequency search of the zero-sequence resonance signal and the calculation of the effective value of the zero-sequence resonance amplitude according to the sliding window algorithm include: determining the sliding window depth of the sliding frequency search within the preset frequency range according to the resonance frequency of the zero-sequence resonance signal and the bandpass frequency of the bandpass filter; determining the number of sweeping frequency points according to the preset frequency step, and determining the zero-sequence voltage corresponding to each sweeping frequency point according to the zero-sequence resonance signal; and calculating the effective value of the zero-sequence resonance amplitude of the sweeping frequency point through a preset function formula according to the sliding window depth and the zero-sequence voltage corresponding to the sweeping frequency point.

[0010] In some embodiments, the preset function is:

[0011]

[0012] Where Vres is the effective value of the zero-sequence resonance amplitude, Vi is the zero-sequence voltage corresponding to the i-th sweep frequency point, and n is the sliding window depth.

[0013] In some embodiments, the calculating of the effective value of the zero-sequence resonance amplitude of the scanning point by a preset function formula according to the sliding window depth and the zero-sequence voltage corresponding to the scanning frequency point includes: calculating a plurality of zero-sequence resonance amplitude values ​​of the current scanning frequency point in a plurality of resonance cycles by the preset function formula according to the sliding window depth and the zero-sequence voltage corresponding to the current scanning frequency point; and calculating the effective value of the zero-sequence resonance amplitude corresponding to the current scanning frequency point by taking the average value of the plurality of zero-sequence resonance amplitude values.

[0014] In some embodiments, if the effective value of the zero-sequence resonance amplitude at the current frequency point is greater than the zero-sequence resonance protection threshold, controlling the local frequency search of the zero-sequence resonance signal, and determining the zero-sequence resonance frequency point includes: obtaining the current frequency point and the forward frequency point and the backward frequency point with a preset optimization step length separated from the current frequency point; respectively calculating multiple zero-sequence resonance amplitude values ​​of the forward frequency point, the current frequency point, and the backward frequency point; comparing the sizes of the multiple zero-sequence resonance amplitude values ​​according to a preset arbitration principle to determine the maximum value of the zero-sequence resonance amplitude; and taking the frequency point corresponding to the maximum value of the zero-sequence resonance amplitude as the zero-sequence resonance frequency point.

[0015] In some embodiments, the zero-sequence resonance protection method includes: within a preset protection time, if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is continuously less than or equal to the zero-sequence resonance protection threshold, controlling to exit the local frequency search process and switch to the sliding frequency search process.

[0016] The present application also provides a zero-sequence resonance protection device, which is applied to a grid-connected system. The grid-connected system includes a photovoltaic storage converter, a relay and a power grid. The relay is arranged between the photovoltaic storage converter and the power grid, and when the relay is closed, the grid-connected system is in a grid-connected state. The zero-sequence resonance protection device includes: a signal extraction module, a sliding search module, a judgment module, a local search module and a control module. The signal extraction module is used to extract resonance information of the zero-sequence electrical signal output by the photovoltaic storage inverter in real time when the grid-connected system is in the grid-connected state, so as to obtain a zero-sequence resonance signal within a preset frequency range; the sliding search module is used to perform a sliding frequency point search on the zero-sequence resonance signal, and calculate the effective value of the zero-sequence resonance amplitude of each frequency sweep point according to the sliding window algorithm; the judgment module is used to judge whether the effective value of the zero-sequence resonance amplitude of each frequency sweep point is greater than the zero-sequence resonance protection threshold; the local search module is used to control the local frequency point search of the zero-sequence resonance signal to determine the zero-sequence resonance frequency point if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is greater than the zero-sequence resonance protection threshold; the control module is used to trigger the zero-sequence resonance protection to shut down the photovoltaic storage inverter within a preset protection time if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point continues to be greater than the zero-sequence resonance protection threshold.

[0017] The present application also provides a grid-connected system. The grid-connected system includes the zero-sequence resonance protection device described in the above-mentioned embodiments. The grid-connected system also includes a photovoltaic storage converter, a relay and a power grid, wherein the relay is arranged between the photovoltaic storage converter and the power grid, and when the relay is closed, the grid-connected system is in a grid-connected state. The grid-connected system is used to implement the zero-sequence resonance protection method described in any one of the above-mentioned embodiments.

[0018] In this way, the zero-sequence resonance protection method of the present application can perform zero-sequence resonance protection on the photovoltaic storage converter under grid-connected operating conditions, and there is no need to actively introduce additional signals into the grid-connected system or change the normal operating state of the system to trigger or detect resonance, thereby minimizing the impact on the quality of the grid-connected current. In addition, the zero-sequence resonance protection method of the present application can not only provide resonance-related protection in a timely manner, but also lock the zero-sequence resonance frequency of the grid-connected system before breaking free from the fault, providing a basis for optimizing the control strategy for engineering development technicians.

[0019] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0021] Figure 1 is a schematic diagram of the structure of a grid-connected system of certain embodiments of the present application;

[0022] Figure 2 It is a flowchart of a zero-sequence resonance protection method for a grid-connected system according to certain embodiments of the present application;

[0023] Figure 3 is a schematic structural diagram of a zero-sequence resonance protection device for a grid-connected system according to certain embodiments of the present application;

[0024] Figure 4 It is a schematic diagram of a scenario of a sliding frequency point search in a zero-sequence resonance protection method for a grid-connected system in certain implementation modes of the present application;

[0025] Figure 5 It is a flowchart of a zero-sequence resonance protection method for a grid-connected system according to certain embodiments of the present application;

[0026] Figure 6 is an equivalent structural schematic diagram of a grid-connected system of certain embodiments of the present application;

[0027] Figure 7 It is a flowchart of a zero-sequence resonance protection method for a grid-connected system according to certain embodiments of the present application;

[0028] Figure 8 It is a flowchart of a zero-sequence resonance protection method for a grid-connected system according to certain embodiments of the present application;

[0029] Fig. 9 It is a flowchart of a zero-sequence resonance protection method for a grid-connected system according to certain embodiments of the present application;

[0030] Fig.10 It is a schematic diagram of a scenario when searching for a local frequency point in a zero-sequence resonance protection method for a grid-connected system according to certain embodiments of the present application;

[0031] Fig.11 It is a schematic diagram of electrical signals of a grid-connected system in the related art that does not use a zero-sequence resonance protection method;

[0032] Fig.12 It is a schematic diagram of electrical signals of a grid-connected system using a zero-sequence resonance protection method in certain embodiments of the present application. DETAILED DESCRIPTION

[0033] The embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0034] In the description of the present invention, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance, or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0035] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation" and "connection" should be understood in a broad sense, and may refer to fixed connection, detachable connection, or integral connection; mechanical connection, electrical connection, or mutual communication; direct connection, indirect connection through an intermediate medium, internal connection between two elements, or interaction between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0036] The disclosure below provides many different embodiments or examples to implement different structures of the present invention. In order to simplify the disclosure of the present invention, the parts and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present invention. In addition, the present invention can repeat reference numbers and / or reference letters in different examples, and this repetition is for the purpose of simplification and clarity, which itself does not indicate the relationship between the various embodiments and / or settings discussed.

[0037] The embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0038] See also Figure 1 and Figure 2 The present application provides a zero-sequence resonance protection method, which is applied to a grid-connected system 100. The grid-connected system 100 includes a photovoltaic storage converter 20, a relay 30 and a power grid 40. The relay 30 is arranged between the photovoltaic storage converter 20 and the power grid 40. When the relay 30 is closed, the grid-connected system 100 is in a grid-connected state. The zero-sequence resonance protection method includes:

[0039] 01: When the grid-connected system is in the grid-connected state, the resonance information of the zero-sequence electrical signal output by the photovoltaic storage converter is extracted in real time to obtain the zero-sequence resonance signal within the preset frequency range;

[0040] 02: Perform sliding frequency search on the zero-sequence resonance signal, and calculate the effective value of the zero-sequence resonance amplitude of each sweep frequency point according to the sliding window algorithm;

[0041] 03: Determine whether the effective value of the zero-sequence resonance amplitude of each frequency sweep point is greater than the zero-sequence resonance protection threshold;

[0042] 04: If the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is greater than the zero-sequence resonance protection threshold, the control performs a local frequency search on the zero-sequence resonance signal to determine the zero-sequence resonance frequency point;

[0043] 05: Within the preset protection time, if the effective value of the zero-sequence resonance amplitude of the current sweep frequency point continues to be greater than the zero-sequence resonance protection threshold, the zero-sequence resonance protection is triggered to shut down the solar storage converter.

[0044] See also Figure 3 The present application also provides a zero-sequence resonance protection device 10. The zero-sequence resonance protection device 10 includes: a signal extraction module 11, a sliding search module 12, a judgment module 13, a local search module 14 and a control module 15.

[0045] Step 01 can be implemented by the signal extraction module 11 , step 02 can be implemented by the sliding search module 12 , step 03 can be implemented by the judgment module 13 , step 04 can be implemented by the local search module 14 , and step 05 can be implemented by the control module 15 . That is, the signal extraction module 11 is used to extract the resonance information of the zero-sequence electrical signal output by the photovoltaic storage inverter in real time when the grid-connected system is in the grid-connected state, so as to obtain the zero-sequence resonance signal within the preset frequency range; the sliding search module 12 is used to perform a sliding frequency search on the zero-sequence resonance signal, and calculate the effective value of the zero-sequence resonance amplitude of each scanning point according to the sliding window algorithm; the judgment module 13 is used to judge whether the effective value of the zero-sequence resonance amplitude of each scanning point is greater than the zero-sequence resonance protection threshold; the local search module 14 is used to control the zero-sequence resonance signal and lock the zero-sequence resonance frequency if the effective value of the zero-sequence resonance amplitude of the current scanning point is greater than the zero-sequence resonance protection threshold; the control module 15 is used to trigger the zero-sequence resonance protection to shut down the photovoltaic storage inverter within the preset protection time if the effective value of the zero-sequence resonance amplitude of the current scanning point is continuously greater than the zero-sequence resonance protection threshold.

[0046] Specifically, when the grid-connected system 100 is in the grid-connected state, the resonance information of the zero-sequence electrical signal output by the photovoltaic storage inverter 20 is extracted in real time to obtain a zero-sequence resonance signal within a preset frequency range. Among them, the zero-sequence electrical signal output by the photovoltaic storage inverter can be a zero-sequence voltage signal or a zero-sequence current signal, which is not limited here. That is, the zero-sequence resonance protection method of the present application can extract the resonance information by real-time acquisition of the zero-sequence voltage signal or the zero-sequence current signal output by the photovoltaic storage inverter, thereby obtaining a zero-sequence resonance voltage signal within a preset frequency range, or a zero-sequence resonance current signal within a preset frequency range.

[0047] It can be understood that when the photovoltaic storage inverter 20 is a household photovoltaic storage inverter, the resonance detection process of the household photovoltaic storage inverter will be interfered by high-frequency and low-frequency bands. The high-frequency band interference is mainly caused by the interference of the switching frequency signal of the switching device in the inverter, and the low-frequency band interference is mainly caused by the power frequency signal of the power grid and the interference of odd-numbered power frequency signals. Therefore, only the resonance information of the zero-sequence electrical signal in the preset frequency range is extracted to obtain the zero-sequence resonance signal in the preset frequency range, so as to realize the accurate extraction of the resonance information of the zero-sequence electrical signal and reduce the interference of high frequency and low frequency at the same time. For example, the preset frequency range can be 1K~2K, or other range values, which are not limited here.

[0048] Then, the zero-sequence resonance signal is searched for sliding frequency points, and the effective value of the zero-sequence resonance amplitude at each frequency sweep point is calculated using the sliding window algorithm. Figure 4 As shown in the figure, the sliding frequency search is to change the sweep frequency point of the sliding frequency search by gradually changing the passband frequency of the bandpass filter within the preset frequency range. Each adjustment corresponds to a new sweep frequency point, so that the entire frequency range can be fully scanned. This method ensures that no possible resonant frequency point is missed, thereby improving the integrity of the search.

[0049] In the sliding frequency search for the zero-sequence resonance signal, multiple frequency sweep points can be determined first, and the multiple frequency sweep points can be scanned in chronological order, and then the effective value of the zero-sequence resonance amplitude of each frequency sweep point can be calculated according to the sliding window algorithm.

[0050] In addition, since the sliding frequency search does not rely on specific frequency information, it is applicable to various types of grid-connected systems and circuits. Regardless of how the resonant frequency of the grid-connected system changes, the sliding frequency search can effectively search. In addition, the present application can quickly filter out the frequency points with small zero-sequence resonance amplitudes through the sliding frequency search, thereby narrowing the scope of subsequent local frequency search and improving search efficiency.

[0051] For example, for a zero-sequence resonant signal with a preset frequency range of 1K to 2K, multiple frequency sweep points can be determined according to a certain frequency step length, namely [v1, v2, v3, ...vk ], then, the corresponding zero-sequence resonance amplitude effective value at each frequency sweep point can be calculated according to the sliding window algorithm. It can be understood that the sliding window algorithm is to maintain a window (or "sliding window"), slide the window on the sequence data, and perform some operation or judgment on the data in the window. As the window slides, new elements will be added to the window, and the elements in the window will be updated or moved to the edge of the window.

[0052] Afterwards, it is determined whether the effective value of the zero-sequence resonance amplitude is greater than the zero-sequence resonance protection threshold. If the effective value of the zero-sequence resonance amplitude is greater than the zero-sequence resonance protection threshold, a local frequency search is performed on the zero-sequence resonance signal to determine the zero-sequence resonance frequency.

[0053] That is, the zero-sequence resonance protection threshold is the protection threshold of the sliding frequency search. Assume that the amplitude value of the protection threshold of the sliding frequency search is V limit0 If the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is greater than the amplitude value V of the protection threshold of the sliding frequency search limit0 , the zero-sequence resonance signal is switched from a sliding frequency search to a local frequency search, and a more detailed search is performed to ensure that the true zero-sequence resonance frequency is found. The zero-sequence resonance protection method of the present application can use a method combining a sliding frequency search with a local frequency search to perform a frequency search on the zero-sequence resonance signal, thereby accurately determining the true zero-sequence resonance frequency.

[0054] Afterwards, within the preset protection time, if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point continues to be greater than the zero-sequence resonance protection threshold, the zero-sequence resonance protection is triggered to shut down the photovoltaic storage inverter. It can be understood that after locking the zero-sequence resonance frequency, the system needs a certain amount of time to determine whether this zero-sequence resonance phenomenon continues to exist, or whether it has reached a level that may damage the equipment or the power grid. This judgment time is the preset protection time. The preset protection time is set to ensure that the system has enough time to confirm the authenticity and severity of the resonance before taking any protection action, so as to avoid unnecessary protection actions caused by short-term, unstable or misjudged resonance phenomena.

[0055] The preset protection time may be, for example, 0.14S, 0.15S, 0.16S, 0.17S, 0.18S, 0.19S, 0.2S, 0.21S, 0.22S or 0.21S, and is not limited here.

[0056] Triggering the zero-sequence resonance protection to shut down the photovoltaic storage inverter means that the photovoltaic storage inverter can be disconnected from the power grid in time, thereby protecting the electrical equipment.

[0057] In addition, it is understandable that in the analysis of grid-connected systems, in order to detect or analyze certain phenomena (such as resonance), it may be necessary to artificially inject specific signals (such as disturbance signals) into the grid-connected system. However, this method may interfere with the normal operation of the grid-connected system, affect the quality of the grid-connected current, and even have adverse effects on the power grid and other equipment.

[0058] In this application, by adopting a zero-sequence resonance protection method combining sliding frequency search and local frequency search, it is possible to accurately detect the zero-sequence resonance phenomenon in the system and take corresponding protection measures in time without actively introducing additional signals into the grid-connected system or changing the normal operating state of the system to trigger or detect resonance, that is, without injecting disturbances. This not only minimizes the impact on the quality of grid-connected current, but also enables zero-sequence resonance protection to be performed without affecting the normal operation of the system, providing a strong guarantee for the safe and stable operation of the grid-connected system, and improving the stability and reliability of the grid-connected system.

[0059] In this way, the zero-sequence resonance protection method of the present application can perform zero-sequence resonance protection on the photovoltaic storage converter under grid-connected operating conditions, and there is no need to actively introduce additional signals into the grid-connected system or change the normal operating state of the system to trigger or detect resonance, thereby minimizing the impact on the quality of the grid-connected current. In addition, the zero-sequence resonance protection method of the present application can not only provide resonance-related protection in a timely manner, but also lock the zero-sequence resonance frequency of the grid-connected system before breaking free from the fault, providing a basis for optimizing the control strategy for engineering development technicians.

[0060] In some embodiments, the zero-sequence resonance protection method includes:

[0061] 06: If the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is less than or equal to the zero-sequence resonance protection threshold, the zero-sequence resonance protection is not triggered, and the zero-sequence resonance signal sliding frequency search continues.

[0062] Please combine Figure 3 , step 06 can be implemented by the control module 15. That is, the control module 15 is also used to not trigger the zero-sequence resonance protection if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is less than or equal to the zero-sequence resonance protection threshold, and continue to search the sliding frequency point of the zero-sequence resonance signal.

[0063] That is, when the zero-sequence resonance signal is scanned and searched through the sliding frequency point search, when the effective value of the zero-sequence resonance amplitude at the current scanning frequency point is less than or equal to the zero-sequence resonance protection threshold, it can be judged that the grid-connected system 100 has no resonance trend at this time, and it can be determined that the current scanning frequency point is not a frequency point near the zero-sequence resonance frequency point.

[0064] Therefore, there is no need to trigger the zero-sequence resonance protection at this time, and the sliding frequency search of the zero-sequence resonance signal continues, thereby continuing to search the frequency region where the zero-sequence resonance frequency point is located.

[0065] See also Figure 5 In some embodiments, step 01 includes:

[0066] 011: In the grid-connected state, collect the zero-sequence voltage signal output by the photovoltaic storage converter;

[0067] 012: Determine the preset frequency range according to the equivalent inductance and capacitance parameters of the photovoltaic storage converter and the equivalent inductance parameters of the power grid;

[0068] 013: A bandpass filter is used to filter the zero-sequence voltage signal according to a preset frequency range, and a zero-sequence resonance signal within the preset frequency range is extracted.

[0069] Please combine Figure 3 , step 011, step 012 and step 013 can be implemented by the signal extraction module 11. That is, the signal extraction module 11 is also used to collect the zero-sequence voltage signal output by the photovoltaic storage converter in the grid-connected state; determine the preset frequency range according to the equivalent inductance and capacitance parameters of the photovoltaic storage converter and the equivalent inductance parameters of the power grid; use a bandpass filter to filter the zero-sequence voltage signal according to the preset frequency range, and extract the zero-sequence resonance signal within the preset frequency range.

[0070] Specifically, see Figure 1 and Figure 6 , Figure 1 The circuit diagram of the grid-connected system is shown in Figure 2. Figure 6 for Figure 1 Circuit equivalent diagram of the grid-connected system.

[0071] like Figure 1 As shown, in the grid-connected state, the zero-sequence voltage signal output by the photovoltaic storage converter is collected, and the zero-sequence voltage value in the zero-sequence voltage signal can be v0. v0 can be calculated by the following formula 1:

[0072] v0=v a +v b +v c …………………………Formula 1

[0073] Where, v0 represents the zero-sequence voltage value; v a 、v b 、v c Represent the three-phase capacitor voltages respectively.

[0074] Afterwards, the preset frequency range is determined according to the equivalent inductance and capacitance parameters of the photovoltaic storage converter and the equivalent inductance parameters of the power grid, that is, the preset frequency range of the frequency sweep point in the zero-sequence voltage signal can be determined.

[0075] Specifically, the preset frequency range can be calculated by the following formula 2:

[0076]

[0077] Where L1 is the inverter equivalent inductance of the photovoltaic storage converter, L2 is the grid-connected equivalent inductance, and L g is the equivalent inductance of the power grid, C f It is the equivalent filter capacitor on the grid side.

[0078] It can be understood from the above formula 2 that the system resonant frequency will shift with the change of the equivalent inductance parameters of the power grid. In order to effectively extract the resonance information without affecting normal operation, detection and protection are required for specific frequency bands. The resonance detection of household energy storage converters will be interfered by high-frequency and low-frequency bands. The high-frequency interference mainly revolves around the switching frequency, and the low-frequency interference is mainly the power frequency and odd multiples of the power frequency. Therefore, the resonance extraction algorithm needs to reduce the interference of high and low frequencies at the same time, and only extract the resonant frequency band.

[0079] According to the characteristics of the resonant frequency distribution and the interference source, the present application can use a bandpass filter to filter the zero-sequence voltage signal to extract a zero-sequence resonant signal within a preset frequency range.

[0080] In this way, the present application can first filter the zero-sequence voltage signal including the high-voltage frequency band, the low-voltage frequency band and the resonant frequency band through a bandpass filter to obtain a zero-sequence resonant signal in a preset frequency range, so as to more accurately determine whether it is necessary to trigger the zero-sequence resonance protection and more accurately determine the zero-sequence resonance frequency.

[0081] See also Figure 7 In some embodiments, step 02 includes:

[0082] 021: Determine a sliding window depth for sliding frequency point search within a preset frequency range according to the resonant frequency of the zero-sequence resonant signal and the passband frequency of the bandpass filter;

[0083] 022: Determine the number of frequency sweep points according to the preset frequency step, and determine the zero-sequence voltage corresponding to each frequency sweep point according to the zero-sequence resonance signal;

[0084] 023: According to the sliding window depth and the zero-sequence voltage corresponding to the sweep frequency point, the effective value of the zero-sequence resonance amplitude of the sweep frequency point is calculated by a preset function.

[0085] Please combine Figure 3Step 021, step 022 and step 023 can be implemented by a sliding search module 12, which is used to determine the sliding window depth of the sliding frequency point search within a preset frequency range according to the resonant frequency of the zero-sequence resonant signal and the bandpass frequency of the bandpass filter; determine the number of sweeping points according to the preset frequency step, and determine the zero-sequence voltage corresponding to each sweeping point according to the zero-sequence resonant signal; and calculate the effective value of the zero-sequence resonance amplitude of the sweeping point through a preset function formula according to the sliding window depth and the zero-sequence voltage corresponding to the sweeping point.

[0086] Specifically, the sliding window depth for searching the sliding frequency points within the preset frequency range is determined according to the zero-sequence resonant signal. It can be understood that the resonant frequency f1 can be obtained from the zero-sequence resonant signal. Since the preset frequency range is the frequency range obtained by filtering through a bandpass filter, the sliding window depth n for searching the sliding frequency points within the preset frequency range can be determined based on the multiple relationship between the bandpass frequency f2 of the bandpass filter and the resonant frequency f1 of the zero-sequence resonant signal. For example, if the resonant frequency f1 of the zero-sequence resonant signal is 1KHz, and the bandpass frequency f2 of the bandpass filter is 20KHz, then the sliding window depth n=20KHz / 1KHz=20 can be determined. In this way, the present application can fully reflect the periodic information of the zero-sequence resonant signal with the minimum sliding window depth.

[0087] Afterwards, the number of frequency sweep points is determined according to the preset frequency step. Among them, the zero-sequence resonant signal includes the corresponding relationship between the frequency of the frequency sweep point and the zero-sequence voltage. The preset frequency step can be a pre-set value or a self-set value, which is not limited here. For example, the preset frequency range is 1KHz to 2KHz, and the preset frequency step is 0.2KHz. Then, according to the preset frequency step, the frequency sweep points in the preset frequency range of 1KHz to 2KHz are determined to be 1.2KHz, 1.4KHz, 1.6KHz, 1.8KHz and 2KHz respectively, that is, the number of frequency sweep points at this time is 5.

[0088] It can be understood that, since the zero-sequence resonance signal includes the correspondence between the frequency of the frequency sweep point and the zero-sequence voltage, the zero-sequence voltage corresponding to each frequency sweep point can be determined according to the zero-sequence resonance signal.

[0089] After determining the sliding window depth and the zero-sequence voltage corresponding to the frequency sweep point, the effective value of the zero-sequence resonance amplitude at the frequency sweep point can be calculated by a preset function. It can be understood that the effective value of the zero-sequence resonance amplitude provides a quantitative indicator of the resonance strength. The effective value of the zero-sequence resonance amplitude takes into account the entire waveform of the zero-sequence resonance signal and can more accurately reflect the severity of the zero-sequence resonance phenomenon.

[0090] In one embodiment, the preset function is:

[0091]

[0092] Where Vres is the effective value of the zero-sequence resonance amplitude, Vi is the zero-sequence voltage corresponding to the i-th sweep frequency point, and n is the sliding window depth.

[0093] In this way, the present application can accurately calculate the effective value of the zero-sequence resonance amplitude corresponding to each frequency sweep point through the above preset function.

[0094] See also Figure 8 In some embodiments, step 023 includes:

[0095] 0231: Calculate multiple zero-sequence resonance amplitude values ​​of the current sweep frequency point under multiple resonance cycles through a preset function according to the sliding window depth and the zero-sequence voltage corresponding to the current sweep frequency point;

[0096] 0232: Take the average value of multiple zero-sequence resonance amplitude values ​​to calculate the effective value of the zero-sequence resonance amplitude corresponding to the current frequency sweep point.

[0097] Please combine Figure 3 , step 0231 and step 0232 can be implemented by the sliding search module 12. The sliding search module 12 is used to calculate multiple zero-sequence resonance amplitude values ​​of the current scanning frequency point under multiple resonance cycles according to the sliding window depth and the zero-sequence voltage corresponding to the current scanning frequency point through a preset function; and calculate the effective value of the zero-sequence resonance amplitude corresponding to the current scanning frequency point by taking the average value of the multiple zero-sequence resonance amplitude values.

[0098] Specifically, the resonant period refers to the time required for the voltage, current or other physical quantities in the grid-connected system to complete a complete periodic change at a specific frequency. In the resonant state, these physical quantities of the grid-connected system will oscillate periodically at a fixed frequency, which is the resonant frequency, and the time required to complete a complete oscillation is the resonant period.

[0099] That is to say, the zero-sequence resonance protection method of the present application can average out instantaneous fluctuations caused by noise, interference or measurement errors by continuously detecting multiple resonance cycles, thereby improving the accuracy of the calculation result of the effective value of the zero-sequence resonance amplitude.

[0100] In this way, the zero-sequence resonance protection method of the present application can be closer to the actual resonance amplitude by taking the average value of multiple zero-sequence resonance amplitude values ​​of multiple resonance cycles as the effective value of the zero-sequence resonance amplitude corresponding to the current sweep frequency point.

[0101] See also Fig. 9 In some embodiments, step 04 includes:

[0102] 041: Get the current frequency and the forward and backward frequencies with a preset step length from the current frequency;

[0103] 042: Calculate multiple zero-sequence resonance amplitude values ​​of the forward frequency point, the current frequency point and the backward frequency point respectively;

[0104] 043: Compare the magnitudes of multiple zero-sequence resonance amplitudes according to a preset arbitration principle to determine the maximum zero-sequence resonance amplitude;

[0105] 044: The frequency corresponding to the maximum value of the zero-sequence resonance amplitude is taken as the zero-sequence resonance frequency.

[0106] Please combine Figure 3 , step 041, step 042, step 043 and step 044 can be implemented by the local search module 14. That is, the local search module 14 is used to obtain the current frequency point and the forward frequency point and the backward frequency point with a preset step length from the current frequency point; calculate multiple zero-sequence resonance amplitude values ​​of the forward frequency point, the current frequency point and the backward frequency point respectively; compare the magnitudes of multiple zero-sequence resonance amplitude values ​​according to the preset arbitration principle to determine the maximum zero-sequence resonance amplitude; and use the frequency point corresponding to the maximum zero-sequence resonance amplitude as the zero-sequence resonance frequency point.

[0107] Understandably, sliding scanning is a preliminary frequency search method, which usually scans the entire preset frequency range at a certain preset frequency step to find possible resonance points. During the sliding frequency search process, the zero-sequence resonance amplitude of each frequency point is detected, but the preset frequency step is relatively large, and only the frequency range where the zero-sequence resonance point is located can be roughly determined.

[0108] After the frequency range of the resonance point is initially determined through the sliding frequency search process, in order to improve the search accuracy, the local frequency search process will reduce the search step size, that is, reduce the optimization step size. This is done to perform a more detailed search in a smaller frequency range, so as to find the zero-sequence resonance frequency point more accurately.

[0109] Therefore, the local frequency search is based on the principle of maximizing the zero-sequence resonance amplitude output value. First, the preset optimization step length is reduced based on the preset frequency step length of the sliding scan, and then forward optimization and backward optimization are performed around the current frequency point. Each step of frequency calculation includes three points: the current frequency point, the forward frequency point and the backward frequency point, where the forward frequency point is the current frequency point plus the preset optimization step length, and the backward frequency point is the current frequency point minus the preset optimization step length. The zero-sequence resonance amplitude values ​​of the forward frequency point, the current frequency point and the backward frequency point are calculated respectively, and the optimization direction of the next resonance cycle is arbitrated based on the three extraction results.

[0110] Assume that the zero-sequence resonance amplitude value of the forward frequency is A1, the zero-sequence resonance amplitude value of the current frequency is B1, and the zero-sequence resonance amplitude value of the backward frequency is C1. Fig.10 , then the pre-set arbitration principle can be:

[0111] (1) A1>B1>C1: The extracted frequency needs to be increased in the next resonance cycle.

[0112] (2) C1>B1>A1: The extracted frequency needs to be lowered in the next resonance cycle.

[0113] (3) B1>A1>C1: In the next resonance cycle, the preset optimization step length is reduced, and the optimization is continued between the current frequency point and the forward frequency point, thereby determining the maximum value of the zero-sequence resonance amplitude.

[0114] (4) B1>C1>A1: In the next resonance cycle, the preset optimization step length is reduced to optimize between the current frequency point and the forward frequency point, thereby determining the maximum value of the zero-sequence resonance amplitude.

[0115] If other amplitude value comparison results appear, it is invalid extraction, and the process of performing local frequency point search at the current frequency point is repeated again.

[0116] The conditions for exiting the local frequency search can be set in the preset arbitration principle as follows:

[0117]

[0118] Where v0 is the zero-sequence resonance amplitude value at the current frequency; v 0_L is the zero-sequence resonance amplitude value of the backward frequency; v 0_H is the zero-sequence resonance amplitude value of the forward frequency point. η error ratio, η set The error ratio exit threshold is set.

[0119] That is to say, if the result difference of the zero-sequence resonance amplitude value of the current frequency, forward frequency and backward frequency is within a certain range, it is considered that the local frequency search is completed, and the current frequency is locked as the zero-sequence resonance frequency of the grid-connected system, that is, the frequency corresponding to the maximum value of the zero-sequence resonance amplitude can be determined as the zero-sequence resonance frequency.

[0120] In this way, the zero-sequence resonance protection method of the present application can lock the zero-sequence resonance frequency through a local frequency search process before escaping from a fault, and can provide a basis for optimizing control strategies for engineering development technicians.

[0121] In some embodiments, the zero-sequence resonance protection method includes

[0122] 07: Within the preset protection time, if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is continuously less than or equal to the zero-sequence resonance protection threshold, the control exits the local frequency search process and switches to the sliding frequency search process.

[0123] Please combine Figure 3, step 06 can be implemented by the control module 15. That is, the control module 15 is also used to control the exit of the local frequency search process and the transition to the sliding frequency search process if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is continuously less than or equal to the zero-sequence resonance protection threshold within the preset protection time.

[0124] That is, if within the preset protection time, the effective value of the zero-sequence resonance amplitude of the current scanning frequency point continues to be less than the zero-sequence resonance protection threshold, this may mean that there is no obvious resonance phenomenon at the current frequency point in the grid-connected system, or the original resonance phenomenon has been significantly weakened.

[0125] In this case, continuing to perform detailed local optimization near the current frequency point cannot obtain more effective results. Therefore, the local frequency point search process can be controlled to exit and switch back to the sliding frequency point search process to redetermine the zero-sequence resonant frequency point.

[0126] In this way, the zero-sequence resonance protection method of the present application can also be converted from a local frequency search to a sliding frequency search process, and can continue to re-search the zero-sequence resonance frequency without increasing the risk of damage to the grid-connected system, thereby more accurately determining the zero-sequence resonance frequency to accurately trigger the zero-sequence resonance protection.

[0127] See also Figure 1 The present application also provides a grid-connected system 100. The grid-connected system 100 includes the zero-sequence resonance protection device 10 in the above-mentioned embodiment. The grid-connected system 100 also includes a photovoltaic storage converter 20, a relay 30 and a power grid 40. The relay 30 is arranged between the photovoltaic storage converter 20 and the power grid 40. When the relay 30 is closed, the grid-connected system 100 is in a grid-connected state. The grid-connected system 100 is used to implement the zero-sequence resonance protection method described in any one of the above-mentioned embodiments.

[0128] See also Fig.11 If the grid-connected system 100 does not use the zero-sequence resonance protection method of the present application, the grid-connected system will run for a long time without being shut down, which will reduce the life of the power electronic equipment. Fig.12 When the grid-connected system 100 uses the zero-sequence resonance protection method of the present application, once the zero-sequence resonance is triggered, the grid-connected system 100 can shut down the equipment in time within the preset protection time, thereby protecting the electrical equipment and improving the life of the electrical equipment.

[0129] In this way, the grid-connected system 100 of the present application can apply the above-mentioned zero-sequence resonance protection method to the photovoltaic storage converter 20 under the grid-connected operating condition, and there is no need to actively introduce additional signals into the grid-connected system or change the normal operating state of the system to trigger or detect resonance, thereby minimizing the impact on the grid-connected current quality. In addition, the zero-sequence resonance protection method of the present application can not only provide resonance-related protection in a timely manner, but also lock the zero-sequence resonance frequency of the grid-connected system 100 before breaking free from the fault, providing a basis for optimizing the control strategy for engineering development technicians.

[0130] The above embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the attached claims.

Claims

1. A zero-sequence resonance protection method, applied to a grid-connected system, characterized in that: The grid-connected system comprises a photovoltaic storage converter, a relay and a power grid, wherein the relay is arranged between the photovoltaic storage converter and the power grid, and when the relay is closed, the grid-connected system is in a grid-connected state, and the zero-sequence resonance protection method comprises: When the grid-connected system is in the grid-connected state, resonant information is extracted from the zero-sequence electrical signal output by the photovoltaic storage converter in real time to obtain a zero-sequence resonant signal within a preset frequency range; Performing a sliding frequency search on the zero-sequence resonance signal, and calculating the effective value of the zero-sequence resonance amplitude at each frequency sweep point according to a sliding window algorithm; Determine whether the zero-sequence resonance amplitude effective value of each frequency sweep point is greater than the zero-sequence resonance protection threshold; If the effective value of the zero-sequence resonance amplitude at the current frequency scanning point is greater than the zero-sequence resonance protection threshold, controlling a local frequency search of the zero-sequence resonance signal to determine the zero-sequence resonance frequency point; Within the preset protection time, if the effective value of the zero-sequence resonance amplitude at the current frequency sweep point is continuously greater than the zero-sequence resonance protection threshold, the zero-sequence resonance protection is triggered to shut down the photovoltaic storage converter.

2. The zero-sequence resonance protection method according to claim 1, characterized in that: The zero-sequence resonance protection method comprises: If the effective value of the zero-sequence resonance amplitude at the current frequency scanning point is less than or equal to the zero-sequence resonance protection threshold, the zero-sequence resonance protection is not triggered, and the zero-sequence resonance signal sliding frequency point search continues.

3. The zero-sequence resonance protection method according to claim 1, characterized in that: When the grid-connected system is in the grid-connected state, extracting resonance information from the electrical signal output by the photovoltaic storage converter in real time to obtain a zero-sequence resonance signal within a preset frequency range includes: In the grid-connected state, collecting a zero-sequence voltage signal output by the photovoltaic storage converter; Determining the preset frequency range according to equivalent inductance and capacitance parameters of the photovoltaic storage converter and equivalent inductance parameters of the power grid; A bandpass filter is used to filter the zero-sequence voltage signal according to the preset frequency range, and the zero-sequence resonance signal within the preset frequency range is extracted.

4. The zero-sequence resonance protection method according to claim 3, characterized in that: The performing sliding frequency search on the zero-sequence resonance signal and calculating the effective value of the zero-sequence resonance amplitude according to a sliding window algorithm comprises: Determining a sliding window depth for sliding frequency point search within the preset frequency range according to the resonant frequency of the zero-sequence resonant signal and the passband frequency of the bandpass filter; Determine the number of frequency sweep points according to a preset frequency step, and determine the zero-sequence voltage corresponding to each frequency sweep point according to the zero-sequence resonance signal; According to the sliding window depth and the zero-sequence voltage corresponding to the frequency sweeping point, the effective value of the zero-sequence resonance amplitude of the frequency sweeping point is calculated by a preset function.

5. The zero-sequence resonance protection method according to claim 4, characterized in that: The preset function is: Where Vres is the effective value of the zero-sequence resonance amplitude, Vi is the zero-sequence voltage corresponding to the i-th sweep frequency point, and n is the sliding window depth.

6. The zero-sequence resonance protection method according to claim 4, characterized in that: The calculating, according to the sliding window depth and the zero-sequence voltage corresponding to the frequency sweep point, of the effective value of the zero-sequence resonance amplitude of the frequency sweep point by a preset function formula comprises: According to the sliding window depth and the zero-sequence voltage corresponding to the current frequency sweep point, a plurality of zero-sequence resonance amplitude values ​​of the current frequency sweep point under a plurality of resonance cycles are calculated by the preset function formula; The effective value of the zero-sequence resonance amplitude corresponding to the current frequency sweep point is obtained by taking an average value of the multiple zero-sequence resonance amplitude values.

7. The zero-sequence resonance protection method according to claim 1, characterized in that: If the effective value of the zero-sequence resonance amplitude at the current frequency point is greater than the zero-sequence resonance protection threshold, controlling the local frequency point search of the zero-sequence resonance signal to determine the zero-sequence resonance frequency point includes: Obtain a current frequency point and a forward frequency point and a backward frequency point separated from the current frequency point by a preset optimization step length; Calculating a plurality of zero-sequence resonance amplitude values ​​of the forward frequency point, the current frequency point and the backward frequency point respectively; Compare the magnitudes of the plurality of zero-sequence resonance amplitude values ​​according to a preset arbitration principle to determine the maximum zero-sequence resonance amplitude; The frequency point corresponding to the maximum value of the zero-sequence resonance amplitude is used as the zero-sequence resonance frequency point.

8. The zero-sequence resonance protection method according to claim 1, characterized in that: The zero-sequence resonance protection method comprises: Within the preset protection time, if the effective value of the zero-sequence resonance amplitude of the current frequency sweep point is continuously less than or equal to the zero-sequence resonance protection threshold, the control exits the local frequency search process and switches to the sliding frequency search process.

9. A zero-sequence resonance protection device, characterized in that: Applied to a grid-connected system, characterized in that the grid-connected system comprises a photovoltaic storage converter, a relay and a power grid, the relay is arranged between the photovoltaic storage converter and the power grid, when the relay is closed, the grid-connected system is in a grid-connected state, and the zero-sequence resonance protection device comprises: A signal extraction module, used for extracting resonance information of the zero-sequence electrical signal output by the photovoltaic storage converter in real time when the grid-connected system is in the grid-connected state, so as to obtain a zero-sequence resonance signal within a preset frequency range; A sliding search module, used to perform sliding frequency point search on the zero-sequence resonance signal, and calculate the effective value of the zero-sequence resonance amplitude of each sweep frequency point according to a sliding window algorithm; A judging module, used to judge whether the effective value of the zero-sequence resonance amplitude of each frequency sweep point is greater than a zero-sequence resonance protection threshold; A local search module, configured to control a local frequency search of the zero-sequence resonance signal to determine a zero-sequence resonance frequency point if the effective value of the zero-sequence resonance amplitude at the current frequency scanning point is greater than the zero-sequence resonance protection threshold; The control module is used to trigger the zero-sequence resonance protection to shut down the photovoltaic storage converter if the effective value of the zero-sequence resonance amplitude at the current frequency sweep point is continuously greater than the zero-sequence resonance protection threshold within a preset protection time.

10. A grid-connected system, characterized in that: The invention comprises the zero-sequence resonance protection device as described in claim 9, wherein the grid-connected system further comprises a photovoltaic storage inverter, a relay and a power grid, wherein the relay is arranged between the photovoltaic storage inverter and the power grid, and when the relay is closed, the grid-connected system is in a grid-connected state, and the grid-connected system is used to implement the zero-sequence resonance protection method as described in any one of claims 1 to 8.

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