An islanding detection method and device

Abstract

Embodiments of the present application relate to the technical field of power systems, and provide an islanding detection method and device, which include performing periodic bilateral reactive power disturbances on a energy storage converter in a distributed generation grid-connected system; when the frequency change at the point of common coupling of the power grid during the periodic bilateral reactive power disturbances meets a first preset condition, performing unilateral reactive power disturbances on the energy storage converter, and determining the frequency change amount at the point of common coupling of the power grid after the unilateral reactive power disturbances to obtain a unilateral disturbance frequency difference; when the unilateral disturbance frequency difference is greater than a first preset frequency change amount threshold, determining that the distributed generation grid-connected system is in an islanding state. The present application realizes effective detection of the islanding phenomenon when the output power of the energy storage converter is highly matched with the load power.

Description

Technical Field

[0001] This application relates to the technical field of power systems, and in particular, to an islanding detection method and device. Background Art

[0002] At present, with the rapid development of renewable energy, especially the continuous maturity of photovoltaic and wind power generation technologies, distributed generation systems have been widely applied globally. These systems usually operate in parallel with the power grid, which can improve energy utilization efficiency and reduce the impact on the environment. However, when the power grid interrupts power supply due to electrical faults, misoperations, or natural factors during the grid-connected operation of distributed generation systems, the power generation devices are disconnected from the power grid and continue to supply power to local loads, forming an autonomous power supply island that cannot be controlled by the public power grid. This situation not only poses a threat to the safe and stable operation of the power system but also may endanger the safety of maintenance personnel and equipment.

[0003] In related technologies, the frequency change of the point of common coupling (PCC) of the power grid in a distributed generation grid-connected system is monitored to determine whether an islanding phenomenon occurs. However, when the output power of the power generation device (i.e., the output power of the energy storage converter) is highly matched with the load power, the frequency change of the PCC after island formation may become extremely small, resulting in the inability to effectively detect the islanding phenomenon.

[0004] Therefore, the inventors recognize that it is impossible to effectively detect the islanding phenomenon when the output power of the energy storage converter is highly matched with the load power. Summary of the Invention

[0005] One or more embodiments of this application provide an islanding detection method and device to solve or at least partially alleviate the problem in related technologies that it is impossible to effectively detect the islanding phenomenon when the output power of the energy storage converter is highly matched with the load power.

[0006] In the first aspect of this application, an islanding detection method is provided, adopting the following technical solution:

[0007] An islanding detection method includes: performing periodic bilateral reactive power disturbances on an energy storage converter in a distributed generation grid-connected system, where the disturbance directions of the i-th bilateral disturbance period and the (i + 1)-th bilateral disturbance period in the periodic bilateral reactive power disturbances are opposite, and i is an integer greater than or equal to 1; when the frequency change of the point of common coupling of the power grid during the periodic bilateral reactive power disturbances satisfies a first preset condition, performing unilateral reactive power disturbance on the energy storage converter, and determining the frequency change amount of the point of common coupling of the power grid after the unilateral reactive power disturbance to obtain a unilateral disturbance frequency difference; when the unilateral disturbance frequency difference is greater than a first preset frequency change amount threshold, determining that the distributed generation grid-connected system is in an islanding state.

[0008] In one embodiment, the first preset condition includes: the frequency change of the point of common coupling of the power grid corresponding to each of the continuous M bilateral disturbance periods satisfies a second preset condition, and / or the frequency of the point of common coupling of the power grid within a first time period is greater than a reference frequency or the frequency of the point of common coupling of the power grid within the first time period is less than the reference frequency.

[0009] In one embodiment, the islanding detection method further includes: obtaining a first frequency change trend and a first frequency change amount of the point of common coupling of the power grid corresponding to the (i + 1)-th bilateral disturbance period during the periodic bilateral reactive power disturbances, and a second frequency change trend of the point of common coupling of the power grid corresponding to the i-th bilateral disturbance period; when the first frequency change trend is opposite to the second frequency change trend, and the first frequency change amount is greater than a second preset frequency change amount threshold, determining that the frequency change of the point of common coupling of the power grid satisfies the second preset condition, where the second preset frequency change amount threshold is less than the first preset frequency change amount threshold.

[0010] In one embodiment, the performing periodic bilateral reactive power disturbances on an energy storage converter in a distributed generation grid-connected system includes: respectively determining that the reactive power disturbance amount in the positive direction is a first disturbance amount and the reactive power disturbance amount in the negative direction is a second disturbance amount, where the first disturbance amount is determined by the following formula:

[0011]

[0012] The second disturbance amount is determined by the following formula:

[0013]

[0014] where Q dis1 is the first disturbance amount, Q dis2is the second disturbance quantity, P is the active power output by the energy storage converter, and Q f is the quality factor of the load, and Δf dis is the preset frequency offset, and f g is the grid rated frequency; the energy storage converter is subjected to the periodic bilateral reactive power disturbance according to the first disturbance quantity and the second disturbance quantity.

[0015] In one embodiment, when the frequency change of the grid connection point during the periodic bilateral reactive power disturbance satisfies a first preset condition, performing unilateral reactive power disturbance on the energy storage converter includes: determining a target disturbance duration of the unilateral reactive power disturbance; determining a target disturbance quantity of the unilateral reactive power disturbance; determining a target disturbance direction according to a first frequency of the grid connection point and the grid rated frequency, where the first frequency is the frequency of the grid connection point at a first moment; and performing the unilateral reactive power disturbance on the energy storage converter according to the target disturbance duration, the target disturbance quantity, and the target disturbance direction.

[0016] In one embodiment, determining the target disturbance duration of the unilateral reactive power disturbance includes: when the frequency of the grid connection point is greater than a reference frequency within a first duration, or when the frequency of the grid connection point is less than the reference frequency within the first duration, determining the target disturbance duration according to the reference frequency, the grid rated frequency, and a second duration; when the frequency change of the grid connection point corresponding to each of the continuous M bilateral disturbance periods during the periodic bilateral reactive power disturbance satisfies a second preset condition, determining the target disturbance duration as the second duration, where the second duration is a reference time constant.

[0017] In one embodiment, determining the target disturbance direction according to the first frequency of the grid connection point and the grid rated frequency includes: when the first frequency is less than the grid rated frequency, determining the target disturbance direction as the positive direction; when the first frequency is greater than or equal to the grid rated frequency, determining the target disturbance direction as the negative direction.

[0018] In one embodiment, the islanding detection method further includes: when the frequency change of the grid connection point during the periodic bilateral reactive power disturbance satisfies the first preset condition, updating the reference frequency to the first frequency of the grid connection point, where the first frequency is the frequency of the grid connection point at a first moment, and the first moment is the moment when it is determined that the first preset condition is satisfied.

[0019] In one embodiment, before it is determined that the frequency change of the point of common coupling of the power grid during the periodic bilateral reactive power perturbation satisfies the first preset condition, the islanding detection method further includes: obtaining the frequency of the point of common coupling of the power grid collected in real time to obtain a second frequency, where the second frequency represents the frequency of the point of common coupling of the power grid at a second moment, and the second moment is the current moment; in the case where the second frequency is greater than a third frequency and the third frequency is greater than or equal to a fourth frequency, obtaining the count value of the timer at the second moment to obtain the current offset time; in the case where the current offset time is greater than or equal to the first duration, determining that the frequency of the point of common coupling of the power grid within the first duration is greater than the reference frequency; in the case where the second frequency is greater than the third frequency and the third frequency is less than the fourth frequency, controlling the timer to be cleared; where the third frequency represents the frequency of the point of common coupling of the power grid at a third moment, the fourth frequency represents the frequency of the point of common coupling of the power grid at a fourth moment, the third moment is the previous acquisition moment of the second moment; and the fourth moment is the previous acquisition moment of the third moment.

[0020] In one embodiment, the islanding detection method further includes: in the case where the second frequency is less than the third frequency and the third frequency is less than or equal to the fourth frequency, obtaining the count value of the timer at the current moment to obtain the current offset time; in the case where the current offset time is greater than or equal to the first duration, determining that the frequency of the point of common coupling of the power grid within the first duration is less than the reference frequency; in the case where the second frequency is less than the third frequency and the third frequency is greater than the fourth frequency, controlling the timer to be cleared; and in the case where the second frequency is equal to the third frequency, controlling the timer to be cleared.

[0021] In a second aspect of the present application, an islanding detection device is provided, adopting the following technical solution:

[0022] An islanding detection device includes a first perturbation module for performing periodic bilateral reactive power perturbation on a energy storage converter in a distributed generation grid-connected system, where the perturbation directions of the i-th bilateral perturbation period and the (i + 1)-th bilateral perturbation period in the periodic bilateral reactive power perturbation are opposite, and i is an integer greater than or equal to 1; a second perturbation module for performing unilateral reactive power perturbation on the energy storage converter in the case where the frequency change of the point of common coupling of the power grid during the periodic bilateral reactive power perturbation satisfies a first preset condition, and determining the frequency change amount of the point of common coupling of the power grid after the unilateral reactive power perturbation to obtain a unilateral perturbation frequency difference; and a determination module for determining that the distributed generation grid-connected system is in an island state in the case where the unilateral perturbation frequency difference is greater than a first preset frequency change amount threshold.

[0023] Compared with the related art, one or more embodiments of the present application include at least one of the following beneficial technical effects:

[0024] Apply small-amplitude reactive power disturbances in two directions to the energy storage converter alternately, that is, periodic bilateral reactive power disturbances, and detect the frequency change of the grid connection point in real time, which helps to detect potential frequency anomalies in a timely manner and improve the accuracy of island state detection. Since when the output power of the energy storage converter matches the load power, giving a certain reactive power disturbance to the energy storage converter will cause a frequency change at the grid connection point, and the frequency change is proportional to the reactive power disturbance amount. Therefore, when the frequency change at the grid connection point satisfies the first preset condition, that is, when it is initially confirmed that the distributed generation grid-connected system is in the island state when the output power of the energy storage converter matches the load power, the periodic bilateral reactive power disturbance to the energy storage converter is converted into a unilateral reactive power disturbance, which changes the frequency of the grid connection point more significantly, thereby breaking the power balance state, further detecting the island state, and improving the sensitivity and accuracy of island state detection. After applying the unilateral reactive power disturbance to the energy storage converter, if the unilateral disturbance frequency difference is greater than the first preset frequency change amount threshold, it is determined that the distributed generation grid-connected system is in the island state at this time, which helps to reduce the probability of misjudgment of the island state and improve the accuracy of island state detection. The present invention can effectively detect the island state of the distributed generation grid-connected system when the output power of the energy storage converter matches the load power by setting the first preset condition and combining periodic bilateral reactive power disturbance with unilateral reactive power disturbance. Description of the Drawings

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings of the embodiments will be briefly introduced below. Obviously, the drawings described below only relate to some embodiments of the present application and do not limit the present application.

[0026] Figure 1 It is an equivalent circuit diagram of the distributed generation grid-connected system according to the present application.

[0027] Figure 2 It is a control block diagram of the energy storage converter according to the present application.

[0028] Figure 3 It is a schematic flowchart of the island detection method according to some embodiments of the present application.

[0029] Figure 4 It is a specific flowchart of the island detection method according to some embodiments of the present application.

[0030] Figure 5 It is a system architecture diagram of the island detection device according to some embodiments of the present application.

[0031] Figure 6 It is a system architecture diagram of an electronic device according to some embodiments of the present application. Specific embodiments

[0032] To make the objectives, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings showing multiple embodiments of the present application. It should be understood that the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments described in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present application.

[0033] Unless otherwise defined, all technical and scientific terms used in the present application have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs; the terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "including", "comprising", "having", "possessing", "containing", "including" and the like in the specification and claims of the present application and the above drawings are open-ended terms. Therefore, a method or device "including", "comprising", "having" one or more steps or elements has one or more steps or elements, but is not limited to only having these one or more elements. The terms "first", "second" and the like in the specification and claims of the present application or the above drawings are used to distinguish different objects and are not used to describe a specific order or primary-secondary relationship. In addition, the terms "first", "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, features defined with "first", "second" may explicitly or implicitly include one or more of such features. In the description of the present application, unless otherwise specified, the meaning of "a plurality" is two or more.

[0034] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "center", "lateral", "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "axial", "radial", "circumferential" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the indicated device or element must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation to the present application.

[0035] In the description of the present application, it should be noted that, unless otherwise clearly specified and defined, the terms "installed", "connected", "linked", and "attached" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific circumstances.

[0036] The mention of "embodiment" in the present application means that the specific features, structures, or characteristics described in connection with the embodiment can be included in at least one embodiment of the present application. The phrase appearing in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in the present application can be combined with other embodiments.

[0037] As mentioned above, it should be emphasized that when the term "comprising / including" is used in this specification, it is used to clearly indicate the existence of the described features, integers, steps, or components, but does not exclude the existence or addition of one or more other features, integers, steps, components, or groups of features, integers, steps, components. As used in the present application, the singular forms "a", "an", and "the" also include the plural forms unless the context clearly indicates otherwise.

[0038] The terms "a" and "an" in this specification can mean one, but can also be consistent with the meaning of "at least one" or "one or more". The term "about" generally means the mentioned value plus or minus 10%, or more specifically plus or minus 5%. The term "or" used in the claims means "and / or" unless clearly indicated otherwise to refer only to alternative options.

[0039] The term "and / or" in the present application is merely a description of the association relationship of the associated objects, indicating that three relationships can exist. For example, A and / or B can represent three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in the present application generally represents an "or" relationship between the associated objects before and after.

[0040] Figure 1 is the equivalent circuit diagram of the distributed generation grid-connected system. Refer to Figure 1, the distributed generation grid-connected system includes a power generation device, a local load, and a power grid. Among them, the power generation device includes a direct current (DC) power source and a power conditioning system (PCS). The PCS is a power regulator or inverter responsible for converting direct current into alternating current and injecting it into the power grid. The direct current (DC) power source usually comes from solar photovoltaic panels or other renewable energy sources. The point of common coupling (PCC) is the place where the power generation device is connected to the power grid. The transformer steps up the alternating current output by the PCS to a voltage level suitable for transmission. The local load adopts the form of a parallel resistor-inductor-capacitor (RLC). The circuit breaker controls the connection between the PCS and the power grid. In the normal operation state of the distributed generation grid-connected system, the circuit breaker is closed, and the power generation device and the power grid are connected to jointly supply power to the local load. In the island state of the distributed generation grid-connected system, the circuit breaker is opened, and the power generation device independently supplies power to the local load.

[0041] Figure 2 is the control block diagram of the PCS. Using the control block diagram shown in Figure 2 to control the PCS, the PCS operates in a constant power mode, and the output power of the PCS remains unchanged before and after the power grid is disconnected (i.e., before and after the island is formed). The PCS control mainly includes a phase-locked loop (PLL), a power loop, and a current inner loop. The double closed-loop control enables the output power of the PCS to change with the change of the power reference. The specific implementation method of the control of the PCS belongs to the prior art and will not be elaborated here.

[0042] In the island state of the distributed generation grid-connected system, if the output power of the PCS does not match the load power, the PCC frequency will change. Specifically, when the output power of the power generation device is greater than the load power, the PCC frequency will rise; conversely, when the output power of the power generation device is less than the load power, the PCC frequency will fall. Therefore, by monitoring the PCC frequency, it is possible to detect whether the distributed generation grid-connected system is in the island state.

[0043] The output power of the power generation device matches the load power, that is, the electric energy generated by the power generation device exactly meets the load demand. At this time, the distributed generation grid-connected system enters the island state (corresponding to the relay in Figure 1 being disconnected), and the PCC frequency remains stable, that is, the change may be very small, even close to zero. Therefore, it is difficult to effectively detect that the distributed generation grid-connected system is in the island state by monitoring the frequency change of the PCC.

[0044] When the distributed generation grid-connected system is operating normally, according to the power flow before islanding formation and circuit system analysis, the active power and reactive power consumed by the local load satisfy the following formula:

[0045]

[0046]

[0047] Among them, P load is the active power consumed by the local load before islanding formation, Q load is the reactive power consumed by the local load before islanding formation, P inv is the active power output by the PCS before islanding formation, Q inv is the reactive power output by the PCS before islanding formation, ΔP is the active power output from the power grid side to the PCC, ΔQ is the reactive power output from the power grid side to the PCC, V 0 is the PCC voltage before islanding formation, f 0 is the PCC frequency before islanding formation, R is the equivalent resistance of the local load, L is the equivalent inductance of the local load, and C is the equivalent capacitance of the local load.

[0048] After islanding formation, the circuit breaker opens, and the active power and reactive power consumed by the local load are provided solely by the PCS. The active power consumed by the local load after islanding formation and the reactive power consumed by the local load after islanding formation satisfy the following formula:

[0049]

[0050]

[0051] Among them, P * load is the active power consumed by the local load after islanding formation, Q * load is the reactive power consumed by the local load after islanding formation, P inv is the active power output by the PCS before islanding formation, Q inv is the reactive power output by the PCS before islanding formation, V i is the steady-state voltage of the PCC after islanding formation, f i is the steady-state frequency of the PCC after islanding formation, R is the equivalent resistance of the local load, L is the equivalent inductance of the local load, and C is the equivalent capacitance of the local load.

[0052] Based on the local load characteristics, the resonance frequency of the local load and the quality factor of the local load can be obtained. The resonance frequency of the local load and the quality factor of the local load satisfy the following formula:

[0053]

[0054]

[0055] Among them, f LC is the resonant frequency of the local load, Q f is the quality factor of the local load, R is the equivalent resistance of the local load, L is the equivalent inductance of the local load, and C is the equivalent capacitance of the local load.

[0056] According to the active power consumed by the local load and the quality factor of the local load, the reactive power output by the PCS after islanding can be obtained. The reactive power output by the PCS after islanding satisfies the following formula:

[0057]

[0058] Among them, Q * inv is the reactive power output by the energy storage converter after islanding, P is the active power output by the energy storage converter, Q f is the quality factor of the local load, f LC is the resonant frequency of the local load, f i is the steady-state frequency of the PCC after islanding.

[0059] According to the active power consumed by the local load after islanding and the reactive power consumed by the local load after islanding, the reactive power output by the PCS can be linearly differentiated, as shown in the following formula:

[0060]

[0061] Among them, ΔP inv is the change in the active power output by the PCS after islanding, ΔQ inv is the change in the reactive power output by the PCS after islanding, Δf i is the change in the PCC frequency after islanding, f i is the steady-state frequency of the PCC after islanding, P inv is the active power output by the PCS before islanding, R is the equivalent resistance of the local load, L is the equivalent inductance of the local load, and C is the equivalent capacitance of the local load.

[0062] According to the quality factor of the local load and the linearly differentiated reactive power output by the PCS, the change in the PCC frequency after islanding can be obtained. The change in the PCC frequency after islanding satisfies the following formula:

[0063]

[0064] Among them, Δf iis the change in PCC frequency after islanding formation, ΔP inv is the change in active power output by PCS after islanding formation, Q f is the quality factor of the local load, f LC is the resonant frequency of the local load, f i is the steady-state frequency of PCC after islanding formation, f is the first frequency of PCC, ΔQ inv is the change in reactive power output by PCS after islanding formation, P inv is the active power output by PCS before islanding formation.

[0065] When the output power of PCS matches the power consumed by the local load, the frequency of PCC remains unchanged before and after islanding formation. The steady-state frequency of PCC after islanding formation is equal to the resonant frequency of the local load. The formula for the change in PCC frequency after islanding formation can be simplified as:

[0066]

[0067] where, Δf i is the change in PCC frequency after islanding formation, ΔQ inv is the change in reactive power output by PCS after islanding formation, P inv is the active power output by PCS before islanding formation, Q f is the quality factor of the local load, f i is the steady-state frequency of PCC after islanding formation.

[0068] As can be seen from the above formula, the change in PCC frequency after islanding formation has a linear relationship with the change in reactive power output by PCS after islanding formation. By disturbing the reactive power output by PCS, the frequency of PCC under islanding state can be shifted, so as to detect the islanding state.

[0069] Figure 3 is a schematic flow chart of an islanding detection method according to some embodiments of the present application.

[0070] One or more embodiments of the present application disclose an islanding detection method. Referring to Figure 3 , the islanding detection method includes:

[0071] S100: Perform periodic bilateral reactive power disturbance on the energy storage converter in the distributed generation grid-connected system, where the disturbance direction of the i-th bilateral disturbance period in the periodic bilateral reactive power disturbance is opposite to that of the (i + 1)-th bilateral disturbance period, and i is an integer greater than or equal to 1.

[0072] The periodic bilateral reactive power disturbance is carried out according to a certain period, and a reactive power disturbance is applied once within each period. The direction of the reactive power disturbance in the periodic bilateral reactive power disturbance alternates between positive and negative directions, that is, a positive reactive power disturbance is applied within one period, and a negative reactive power disturbance is applied within the next period.

[0073] In some embodiments, the time interval of the bilateral disturbance period of the periodic bilateral reactive power disturbance (such as 50 ms) is set, as well as the disturbance amount of the positive-direction disturbance and the disturbance amount of the negative-direction disturbance. For example, within the i-th bilateral disturbance period, if a positive reactive power disturbance is applied to the PCS, the frequency change of the PCC within the i-th bilateral disturbance period is monitored. Then, within the (i + 1)-th bilateral disturbance period, a negative reactive power disturbance is applied to the PCS, and the frequency change of the PCC within the (i + 1)-th bilateral disturbance period is continuously monitored. Conversely, within the i-th bilateral disturbance period, if a negative reactive power disturbance is applied to the PCS, the frequency change of the PCC within the i-th bilateral disturbance period is monitored. Then, within the (i + 1)-th bilateral disturbance period, a positive reactive power disturbance is applied to the PCS, and the frequency change of the PCC within the (i + 1)-th bilateral disturbance period is continuously monitored. The positive and negative reactive power disturbances are alternately applied. After each bilateral disturbance period ends, the frequency change of the PCC in each bilateral disturbance period (the absolute value of the difference between the frequency at the end of the period and the frequency at the start of the period) is recorded. Here, i represents the serial number of the bilateral disturbance period, starting from the first period and increasing sequentially.

[0074] S200: When the frequency change of the point of common coupling of the power grid during the periodic bilateral reactive power disturbance satisfies the first preset condition, perform a unilateral reactive power disturbance on the energy storage converter, and determine the frequency change amount of the point of common coupling of the power grid after the unilateral reactive power disturbance to obtain the unilateral disturbance frequency difference;

[0075] When the output power of the PCS matches the load power, the PCS operates at a unity power factor, and the active power output from the power grid side to the PCC is balanced with the reactive power output from the power grid side to the PCC, satisfying the following formula:

[0076]

[0077] where f g is the rated frequency of the power grid, f 0 is the PCC frequency before islanding formation, f i is the steady-state PCC frequency after islanding formation, f LC is the resonant frequency of the local load.

[0078] Under this operating condition, if no disturbance is applied to the PCS, the frequency of the PCC remains unchanged before and after the formation of the island. The steady-state frequency of the PCC after the formation of the island is equal to the resonance frequency of the local load. It is impossible to determine whether an island is formed by monitoring the frequency change of the PCC. In the embodiment of the present application, bilateral reactive power disturbance is applied to the PCS, and it is possible to determine whether an island is formed by monitoring the frequency change of the PCC. After the formation of the island, there is a relationship between the frequency change amount of the PCC and the reactive power change amount output by the PCS as shown in Equation 1:

[0079]

[0080] where Δf i is the frequency change amount of the PCC after the formation of the island, ΔQ inv is the reactive power change amount output by the PCS after the formation of the island, P inv is the active power output by the PCS before the formation of the island, Q f is the quality factor of the local load, and f i is the steady-state frequency of the PCC after the formation of the island.

[0081] Therefore, when the output power of the PCS is highly matched with the load power, the frequency of the PCC remains unchanged before and after the formation of the island. However, if a certain reactive power disturbance is applied to the PCS, this disturbance will cause a frequency change of the PCC, and the frequency change is proportional to the reactive power disturbance amount. Therefore, in this embodiment, periodic bilateral reactive power disturbance is applied to the PCS, and the frequency change of the PCC is monitored. The frequency change of the PCC satisfies the first preset condition to preliminarily confirm that the distributed generation grid-connected system is in the island state when the output power of the PCS is matched with the load power. When the frequency change of the PCC satisfies the first preset condition, the periodic bilateral reactive power disturbance of the PCS is converted into unilateral reactive power disturbance, that is, a large reactive power disturbance in one direction (positive large reactive power disturbance or negative large reactive power disturbance) is applied to the PCS to change the frequency of the PCC more greatly, thereby breaking the power balance state and further detecting the island state, improving the sensitivity and accuracy of the island state detection. At the same time, the frequency change of the PCC before and after the unilateral reactive power disturbance is monitored, that is, the unilateral disturbance frequency difference. When the frequency change of the PCC does not satisfy the first preset condition, the periodic bilateral reactive power disturbance of the PCS is continued to determine whether the first preset condition is satisfied and to monitor whether the formation of an island exists.

[0082] It should be noted that the reactive power disturbance amount applied by the unilateral reactive power disturbance is greater than the reactive power disturbance amount applied by the periodic bilateral reactive power disturbance.

[0083] S300: When the unilateral disturbance frequency difference is greater than the first preset frequency change threshold, it is determined that the distributed generation grid-connected system is in an island state.

[0084] Specifically, compare the calculated unilateral disturbance frequency difference with the first preset frequency change threshold. If the unilateral disturbance frequency difference is greater than the first preset frequency change threshold, it is determined that the distributed generation grid-connected system is in an island state, and corresponding protection measures are immediately taken. For example, trigger the over / under voltage or over / under frequency protection function of the PCS itself to achieve island protection and prevent potential safety hazards. If the unilateral disturbance frequency difference is less than or equal to the first preset frequency change threshold, stop the unilateral reactive power disturbance of the PCS and switch to the periodic bilateral reactive power disturbance of the PCS until the first preset condition is met and then perform the unilateral reactive power disturbance again.

[0085] Among them, the unilateral disturbance frequency difference is determined by the following formula:

[0086]

[0087] Among them, Δf uni-dir is the unilateral disturbance frequency difference, f is the first frequency of the PCC, and f base is the reference frequency. Generally, the PCS can operate normally within the frequency range of 48.5Hz to 50.5Hz. If the rated frequency is selected as the reference frequency, the PCS frequency is long-term in (48.5, 50) ∪ (50, 50.5) Hz, and the frequency offset will be repeatedly triggered. Therefore, in this application, the reference frequency is defaulted to the grid rated frequency.

[0088] The first preset frequency change threshold is determined by the following formula:

[0089]

[0090] Among them, Δf uni-dir,set is the first preset frequency change threshold, T uni-dir is the target disturbance duration of the unilateral reactive power disturbance, K is the disturbance coefficient of the unilateral reactive power disturbance, f g is the grid rated frequency, Q f is the quality factor of the load, and P is the active power output by the energy storage converter.

[0091] Apply small - amplitude reactive - power disturbances in two directions alternately to the PCS, that is, periodic bilateral reactive - power disturbances, and detect the frequency change of the PCC in real - time, which helps to detect potential frequency anomalies in a timely manner and improve the accuracy of islanding - state detection. Since when the output power of the PCS matches the load power, giving a certain reactive - power disturbance to the PCS will cause the frequency change of the PCC, and the frequency change is proportional to the reactive - power disturbance amount. Therefore, when the frequency change of the PCC meets the first preset condition, that is, when it is preliminarily confirmed that the distributed - generation grid - connection system is in the islanding state under the condition that the output power of the PCS matches the load power, the periodic bilateral reactive - power disturbance of the PCS is converted into a unilateral reactive - power disturbance to change the frequency of the PCC more significantly, thereby breaking the power - balance state, further detecting the islanding state, and improving the sensitivity and accuracy of islanding - state detection. After applying the unilateral reactive - power disturbance to the PCS, if the unilateral - disturbance frequency difference is greater than the first preset frequency - change amount threshold, it is determined that the distributed - generation grid - connection system is in the islanding state at this time, which helps to reduce the probability of misjudgment of the islanding state and improve the accuracy of islanding - state detection. By setting the first preset condition and combining periodic bilateral reactive - power disturbances with unilateral reactive - power disturbances, the present invention can effectively detect the islanding state of the distributed - generation grid - connection system when the output power of the PCS matches the load power.

[0092] In some embodiments, the first preset condition includes: the frequency change of the point of common coupling of the grid corresponding to each of the bilateral - disturbance periods in M consecutive bilateral - disturbance periods all meets the second preset condition, and / or, the frequency of the point of common coupling of the grid within the first time period is greater than the reference frequency or the frequency of the point of common coupling of the grid within the first time period is less than the reference frequency.

[0093] In at least one embodiment, when the distributed - generation grid - connection system goes into islanding, whether the output power of the PCS matches the load power is divided into two working conditions:

[0094] Condition 1: When the output power of the PCS does not match the load power, if the distributed generation grid-connected system is in an island state, the PCC frequency will change within the bilateral disturbance period. Since the change in the PCC frequency within a single bilateral disturbance period may be caused by various factors, such as transient events or noise. Monitoring only the change in the PCC frequency within one period to meet the second preset condition is prone to misjudgment. Therefore, the change in the PCC frequency within consecutive M bilateral disturbance periods meeting the second preset condition is used as the trigger condition for unilateral reactive power disturbance, that is, when the change in the PCC frequency within consecutive M bilateral disturbance periods meets the second preset condition, it is preliminarily judged that the distributed generation grid-connected system may be in an island state, and unilateral reactive power disturbance is performed on the PCS. Then, further judge whether the distributed generation grid-connected system is in an island state to reduce misjudgment caused by accidental events and improve the robustness and reliability of island state detection.

[0095] Condition 2: When the output power of the PCS does not match the load power, the frequency of the PCC will change before and after the formation of the island. After the formation of the island, the steady-state frequency of the PCC is not equal to the resonant frequency of the local load, and the relationship between the change in the PCC frequency after the formation of the island and the change in the reactive power output by the PCS after the formation of the island does not satisfy Equation 1. In this condition, the frequency change caused by bilateral reactive power disturbance is covered, and it is impossible to preliminarily judge whether the distributed generation grid-connected system may be in an island state by the change in the PCC frequency within consecutive M bilateral disturbance periods meeting the second preset condition.

[0096] When the PCS operates in the constant power mode, the output power of the PCS remains approximately the same before and after the formation of the island, and the power consumed by the local load remains unchanged before and after the grid is disconnected. According to the reactive power consumed by the local load before the formation of the island and the reactive power consumed by the local load after the formation of the island, the relationship between the reactive power output from the grid side to the PCC and the deviation of the PCC frequency before and after the formation of the island is:

[0097]

[0098] When Δf is much smaller than f 0 , after arrangement, it can be obtained:

[0099]

[0100] Among them, ΔQ is the reactive power output from the grid side to the PCC, f 0 is the PCC frequency before the formation of the island, Δf is the deviation of the PCC frequency before and after the formation of the island, V 0 is the PCC voltage before the formation of the island, L is the equivalent inductance of the local load, and C is the equivalent capacitance of the local load.

[0101] As can be seen from the above formula, there is a linear relationship between the PCC frequency deviation before and after islanding and the reactive power output from the grid side to the PCC. When the output power of the PCS does not match the power consumed by the load, the PCC frequency after islanding always shifts in one direction. That is, if the grid side sends reactive power to the PCC before islanding, the PCC frequency after islanding will increase, and at this time, the PCC frequency within a short time after islanding is greater than the reference frequency; if the grid side absorbs reactive power from the PCC before islanding, the PCC frequency after islanding will decrease, and at this time, the PCC frequency within a short time after islanding is less than the reference frequency, and the PCC frequency after islanding always shifts in one direction. It is known that after islanding occurs, the circuit state of the PCC generally changes continuously for a long time and does not return to normal within a short time. Therefore, when the condition that the PCC frequency is greater than the reference frequency within the first time period is set as the first preset condition, or when the condition that the PCC frequency is less than the reference frequency within the first time period is set as the first preset condition, that is, when the PCC frequency continuously shifts in the same direction as the reference frequency for a time reaching the threshold (the first time period), the distributed generation grid-connected system may be in the islanding state, and the bilateral reactive power disturbance to the PCS is converted into a unilateral reactive power disturbance. That is, when the PCC frequency is greater than the reference frequency within the first time period, or when the PCC frequency is less than the reference frequency within the first time period, the first preset condition is met, and a unilateral reactive power disturbance is applied to the PCS.

[0102] Apply small - amplitude reactive - power disturbances in two directions alternately to the PCS, that is, continuously perform periodic bilateral reactive - power disturbances. The periodic bilateral reactive - power disturbances will not cause significant impact on the power quality in the distributed - generation grid - connected system. When the frequency change of the PCC in each bilateral disturbance period within M consecutive bilateral disturbance periods satisfies the second preset condition, it can be preliminarily detected that the distributed - generation grid - connected system is in the islanding state under the condition that the output power of the PCS matches the load power, and then switch to unilateral reactive - power disturbance for further islanding detection. At the same time, by monitoring that the deviation time of the PCC frequency in the same direction reaches the first duration, that is, the frequency of the PCC is greater than the reference frequency within the first duration, or the frequency of the PCC is less than the reference frequency within the first duration, it can be preliminarily detected that the distributed - generation grid - connected system is in the islanding state under the condition that the output power of the PCS does not match the load power, and then switch to unilateral reactive - power disturbance for further islanding detection, that is, when the first preset condition is satisfied, the periodic bilateral reactive - power disturbance of the PCS is converted to unilateral reactive - power disturbance. The setting of the first preset condition covers the preliminary judgment of islanding formation in two cases of whether the output power of the PCS matches the load power. Through the setting of double - trigger conditions, islanding formed under both conditions can be effectively detected, eliminating the blind area in detecting islanding formation, thus realizing blind - area - free detection of the islanding state. In addition, through whether the first preset condition is satisfied for preliminary detection of islanding formation, when the first preset condition is satisfied, perform unilateral reactive - power disturbance on the PCS. By whether the unilateral disturbance frequency difference after the unilateral reactive - power disturbance is greater than the first preset frequency - change amount threshold, further judge whether the distributed - generation grid - connected system is in the islanding state, which helps to reduce the probability of misjudgment of the islanding state and improve the accuracy of islanding - state detection.

[0103] In some embodiments, the islanding detection method further includes: obtaining the first frequency - change trend and the first frequency - change amount corresponding to the (i + 1)-th bilateral disturbance period of the point of common coupling of the power grid during the periodic bilateral reactive - power disturbance process, and the second frequency - change trend corresponding to the i - th bilateral disturbance period of the point of common coupling of the power grid; when the first frequency - change trend is opposite to the second frequency - change trend and the first frequency - change amount is greater than the second preset frequency - change amount threshold, determining that the frequency change of the point of common coupling of the power grid satisfies the second preset condition, where the second preset frequency - change amount threshold is less than the first preset frequency - change amount threshold.

[0104] In at least one embodiment, the PCC frequency before and after applying periodic bilateral reactive power disturbances is recorded, the frequency change trend (rising or falling) of the i-th bilateral disturbance period and the frequency change trend (rising or falling) of the (i + 1)-th bilateral disturbance period are determined, and the frequency change amount of the (i + 1)-th bilateral disturbance period is calculated. When the frequency change trend of the (i + 1)-th bilateral disturbance period is opposite to that of the i-th bilateral disturbance period, that is, the frequency of the i-th bilateral disturbance period rises while the frequency of the (i + 1)-th bilateral disturbance period falls, or the frequency of the i-th bilateral disturbance period falls while the frequency of the (i + 1)-th bilateral disturbance period rises, and when the frequency change amount of the (i + 1)-th bilateral disturbance period is greater than the second preset frequency change amount threshold, this (i + 1)-th bilateral disturbance period meets the second preset condition and is recorded as an effective disturbance. The effective disturbances are counted. After detecting an effective disturbance, the number of effective disturbances is incremented by 1, and the bilateral reactive power disturbance for the next period continues. If a non-effective disturbance (not meeting the second preset condition) occurs after multiple consecutive effective disturbances, the number of effective disturbances is cleared and re-counted, and the bilateral reactive power disturbance for the next period continues. If the effective disturbances occur continuously for M times, the periodic bilateral reactive power disturbance to the PCS is converted to a unilateral reactive power disturbance to prevent false triggering of the unilateral reactive power disturbance due to normal fluctuations in the grid frequency.

[0105] Among them, the set second preset frequency change amount threshold is smaller than the first preset frequency change amount threshold. In the case where the PCS output power and the load consumption power are highly matched, the frequency change of the PCC may be very small. By setting a lower second preset frequency change amount threshold, these small frequency changes can be captured, so as to detect potential islanding states earlier. Subsequently, the distributed generation grid-connected system is finally confirmed whether it is in the islanding state through a higher first preset frequency change amount threshold. The setting of this two-level threshold realizes multi-level verification, improves the sensitivity of islanding state detection, reduces the probability of misjudgment of the islanding state, and is conducive to eliminating the detection blind area of the islanding state.

[0106] Assume that the frequency change amount (the first frequency change amount) of the (i + 1)-th bilateral disturbance period is known as Δf bi-dir(i+1) , the frequency change amount of the i-th bilateral disturbance period is Δf bi-dir(i) , the second preset frequency change amount threshold is Δf bi-dir,set , and the (i + 1)-th bilateral disturbance period is an effective disturbance, then the following formula is satisfied:

[0107]

[0108] When the output power of the PCS is highly matched with the power consumed by the load, by determining whether the first frequency change trend corresponding to the obtained PCC in the (i + 1)-th bilateral disturbance period is opposite to the second frequency change trend corresponding to the i-th bilateral disturbance period, and at the same time comparing the magnitude of the first frequency change amount with the second preset frequency change amount threshold, it is possible to effectively capture minute frequency changes and distinguish whether the distributed generation grid-connected system is operating normally or may be in an islanding state, thereby detecting a potential islanding state earlier and detecting the islanding state more accurately, which is beneficial to eliminating the detection blind area of the islanding state.

[0109] In some embodiments, referring to Figure 4 , the periodic bilateral reactive power disturbance to the energy storage converter in the distributed generation grid-connected system includes: respectively determining that the reactive power disturbance amount in the positive direction is the first disturbance amount and the reactive power disturbance amount in the negative direction is the second disturbance amount, where the first disturbance amount is determined by the following formula:

[0110]

[0111] The second disturbance amount is determined by the following formula:

[0112]

[0113] where Q dis1 is the first disturbance amount, Q dis2 is the second disturbance amount, P is the active power output by the energy storage converter, Q f is the quality factor of the load, Δf dis is the preset frequency offset, f g is the grid rated frequency; the periodic bilateral reactive power disturbance is performed on the energy storage converter according to the first disturbance amount and the second disturbance amount.

[0114] In at least one embodiment, when the distributed generation grid-connected system is operating normally, due to the clamping effect of the grid, the PCC frequency is consistent with the grid frequency. When there is a large active power or reactive power output from the grid side to the PCC before and after the formation of the island, the islanded grid will naturally become unstable, resulting in significant changes in frequency or voltage amplitude. These changes make it easier for the frequency or amplitude protection device to detect the islanding state, so as to take protection measures in a timely manner to ensure the safe and stable operation of the system. Considering the most severe working conditions, the PCS operates at unity power factor and satisfies the following formula:

[0115]

[0116] Among them, ΔP is the active power output by the power grid side to the PCC before and after islanding, ΔQ is the reactive power output by the power grid side to the PCC before and after islanding, and j is the imaginary unit. Then, the PCS and the load form a power balance, and the following formula is satisfied at this time:

[0117]

[0118] Among them, f g is the rated frequency of the power grid, f 0 is the PCC frequency before islanding, f i is the steady-state PCC frequency after islanding, f LC is the resonant frequency of the load.

[0119] According to the above formula and the power output by the PCS when the distributed generation grid-connected system is in the island state, it can be known that when the distributed generation grid-connected system is in the island state, the following formula is satisfied between the frequency offset generated by the PCC due to the disturbance and the reactive power disturbance amount output by the PCS:

[0120]

[0121] Among them, Q dis is the reactive power disturbance amount output by the PCS after islanding, P inv is the active power output by the PCS before islanding, Q f is the quality factor of the load, f g is the rated frequency of the power grid, Δf dis is the preset frequency offset.

[0122] First, determine the first disturbance amount in the positive direction and the second disturbance amount in the negative direction. Then, perform periodic bilateral reactive power disturbance on the PCS according to these two disturbance amounts to generate a measurable frequency change at the PCC, which is beneficial to timely detecting the islanding phenomenon and preventing unnecessary safety risks and equipment damage.

[0123] In some embodiments, referring to Figure 4 , when the frequency change of the point of common coupling of the power grid during the periodic bilateral reactive power disturbance satisfies the first preset condition, performing unilateral reactive power disturbance on the energy storage converter includes: determining the target disturbance duration of the unilateral reactive power disturbance; determining the target disturbance amount of the unilateral reactive power disturbance; determining the target disturbance direction according to the first frequency of the point of common coupling of the power grid and the rated frequency of the power grid, where the first frequency is the frequency of the point of common coupling of the power grid at the first moment; performing the unilateral reactive power disturbance on the energy storage converter according to the target disturbance duration, the target disturbance amount, and the target disturbance direction.

[0124] In at least one embodiment, according to the requirements and test standards of the distributed generation grid-connected system being in the islanding state, a reasonable target disturbance duration for unilateral reactive power disturbance is set to ensure that the unilateral reactive power disturbance can generate a significant frequency change within a sufficient time, facilitating islanding state detection.

[0125] Determine the target disturbance amount according to the following formula to ensure that the unilateral reactive power disturbance can effectively affect the frequency of the PCC:

[0126]

[0127] Where Q uni-ref is the target disturbance amount of the unilateral reactive power disturbance at the disturbance moment t, P ref is the active power reference value, and K is the preset desired frequency change rate during unilateral reactive power disturbance.

[0128] Determine the target disturbance direction according to the first frequency of the PCC and the grid rated frequency, and perform unilateral reactive power disturbance on the PCS according to the target disturbance duration, target disturbance amount, and target disturbance direction. Specifically, within the target disturbance duration, apply the target disturbance amount in the target disturbance direction to the PCS.

[0129] Among them, the set target disturbance duration of the unilateral reactive power disturbance should at least meet the requirement that the PCS should have the ability to quickly detect islanding and immediately disconnect from the grid, and the anti-islanding protection action time should not be greater than 2 s, that is, meet the following formula:

[0130]

[0131] Where T uni-dir is the target disturbance duration of the unilateral reactive power disturbance, T bi-dir is the time interval of the bilateral disturbance period of the periodic bilateral reactive power disturbance, Δt is the first duration, and M is the number of times of the periodic bilateral reactive power disturbance.

[0132] By reasonably setting the disturbance parameters of the unilateral reactive power disturbance, that is, the target disturbance duration, target disturbance amount, and target disturbance direction, and performing unilateral reactive power disturbance on the PCS, not only can the islanding state be detected without affecting the normal operation of the distributed generation grid-connected system, avoiding system instability or equipment damage caused by excessive disturbance, but also a significant frequency change can be generated at the PCC in a short time, improving the sensitivity and accuracy of islanding state detection.

[0133] In some embodiments, determining the target disturbance duration of the unilateral reactive power disturbance includes: when the frequency of the point of common coupling (PCC) of the power grid is greater than the reference frequency within the first duration, or when the frequency of the PCC of the power grid is less than the reference frequency within the first duration, determining the target disturbance duration according to the reference frequency, the rated frequency of the power grid, and the second duration; when the frequency variations of the PCC corresponding to each of the continuous M bilateral disturbance periods satisfy the second preset condition, determining the target disturbance duration as the second duration, where the second duration is the reference time constant.

[0134] It should be noted that when the output power of the power generation device does not match the load power, the distributed generation grid-connected system enters the island state at this time, and the PCC frequency will change. The island state can be detected by monitoring the frequency change of the PCC. However, it takes a certain amount of time for the frequency change to reach a set threshold, and this time cannot be controlled. That is, the time of island detection cannot be accurately controlled only by monitoring the frequency change of the PCC.

[0135] In at least one embodiment, when the frequency of the PCC is less than the reference frequency within the first duration or the frequency of the PCC is greater than the reference frequency within the first duration, the target disturbance duration of the unilateral reactive power disturbance is determined by the following formula:

[0136]

[0137] where T uni-dir is the target disturbance duration of the unilateral reactive power disturbance, T is the second duration (reference time constant), f g is the rated frequency of the power grid, and f base is the reference frequency.

[0138] It can be seen from the above formula that when the degree of deviation of the PCC frequency from the reference frequency is greater, the target disturbance duration of the unilateral reactive power disturbance will be shorter; when the degree of deviation of the PCC frequency from the reference frequency is smaller, the target disturbance duration of the unilateral reactive power disturbance will be longer, which can ensure that the next step of confirming whether the distributed generation grid-connected system is indeed in the island state can be carried out faster in the case of significant deviation of the PCC frequency.

[0139] The second preset condition is that during the periodic bilateral reactive power disturbance process, the first frequency change trend corresponding to the (i + 1)-th bilateral disturbance period of the point of common coupling (PCC) of the power grid is opposite to the second frequency change trend corresponding to the i-th bilateral disturbance period, and the first frequency change amount corresponding to the (i + 1)-th bilateral disturbance period of the PCC of the power grid is greater than the second preset frequency change amount threshold. When the frequency change of the PCC corresponding to each bilateral disturbance period in M consecutive bilateral disturbance periods satisfies the second preset condition, the target disturbance duration of the unilateral reactive power disturbance is determined by the following formula:

[0140]

[0141] where T uni-dir is the target disturbance duration of the unilateral reactive power disturbance, and T is the second duration (benchmark time constant).

[0142] As can be seen from the above formula, the target disturbance duration of the unilateral reactive power disturbance always remains at a fixed value, that is, the second duration.

[0143] By corresponding to two working conditions of whether the output power of the PCS matches the power consumed by the load, the target disturbance duration of the unilateral reactive power disturbance is accurately controlled, making the island detection more accurate. When the PCC frequency changes greatly, the target disturbance duration is reduced to respond quickly; while in the case of relatively stable PCC frequency changes, the standard target disturbance duration (second duration) is maintained to avoid unnecessary interference, effectively balancing the relationship between the detection speed and accuracy, thereby achieving more reliable island detection.

[0144] When the frequency of the PCC of the power grid is greater than the reference frequency during the first duration, or the frequency of the PCC of the power grid is less than the reference frequency during the first duration, and when the frequency change of the PCC corresponding to each bilateral disturbance period in M consecutive bilateral disturbance periods satisfies the second preset condition, the target disturbance duration is determined according to the reference frequency, the rated frequency of the power grid, and the second duration.

[0145] By monitoring the comparison between the frequency of the PCC and the reference frequency within the first time period, the deviation of the first frequency of the PCC can be quickly determined. If the frequency of the PCC is always higher than the reference frequency, or the frequency of the PCC is always lower than the reference frequency, it indicates that when the output power of the PCS does not match the load power before and after the occurrence of the islanding, the distributed generation grid-connected system may already be in an islanding state or there may be potential frequency abnormalities. At this time, the target disturbance duration is determined as the second time period (reference time constant) to ensure that the unilateral reactive power disturbance generates a significant frequency change within a reasonable time, improving the sensitivity and accuracy of the detection. When the PCC frequency change within the bilateral disturbance periods of M consecutive periodic bilateral reactive power disturbances meets the second preset condition, it indicates that when the output power of the PCS matches the load power before and after the occurrence of the islanding, the distributed generation grid-connected system may already be in an islanding state or there may be potential frequency abnormalities. At this time, the target disturbance duration is determined according to the reference frequency, the grid rated frequency, and the second time period to ensure that the unilateral reactive power disturbance generates a significant frequency change within a reasonable time, improving the sensitivity and accuracy of the detection.

[0146] In some embodiments, determining the target disturbance direction according to the first frequency of the point of common coupling of the grid and the grid rated frequency includes: when the first frequency is less than the grid rated frequency, determining the target disturbance direction as the positive direction; when the first frequency is greater than or equal to the grid rated frequency, determining the target disturbance direction as the negative direction.

[0147] In at least one embodiment, when the first frequency of the PCC is less than the grid rated frequency, the target disturbance direction of the unilateral reactive power disturbance is determined as the positive direction. By applying a positive-direction unilateral reactive power disturbance to the PCS, the frequency of the PCC can be increased to approach the grid rated frequency, which helps to generate a significant frequency change in a short time and facilitates the detection of the islanding state. When the first frequency of the PCC is greater than or equal to the grid rated frequency, the target disturbance direction of the unilateral reactive power disturbance is determined as the negative direction. By applying a negative-direction unilateral reactive power disturbance to the PCS, the frequency of the PCC can be reduced to move away from the grid rated frequency, which also helps to generate a significant frequency change in a short time and facilitates the detection of the islanding state.

[0148] Determining the positive and negative of the target disturbance direction of the unilateral reactive power disturbance by comparing the magnitude of the first frequency of the PCC with the grid rated frequency helps to generate a significant frequency change in a short time, facilitates the detection of whether the distributed generation grid-connected system is in an islanding state, and at the same time avoids the instability of the distributed generation grid-connected system or equipment damage caused by excessive disturbance, ensuring the continuous and stable operation of the distributed generation grid-connected system.

[0149] In some embodiments, referring to Figure 4, the islanding detection method further includes: when the frequency change of the point of common coupling (PCC) of the power grid during the periodic bilateral reactive power perturbation satisfies the first preset condition, updating the reference frequency to the first frequency of the PCC of the power grid, where the first frequency is the frequency of the PCC of the power grid at a first moment, and the first moment is the moment when it is determined that the first preset condition is satisfied.

[0150] In at least one embodiment, when the frequency of the PCC is less than the reference frequency during the first time period, or when the frequency of the PCC is greater than the reference frequency during the first time period, or when the frequency change of the PCC corresponding to each bilateral perturbation period in M consecutive bilateral perturbation periods satisfies the second preset condition, the periodic bilateral reactive power perturbation of the PCS is converted into unilateral reactive power perturbation, and the reference frequency is updated. The updated reference frequency is the first frequency of the PCC, which is used to determine the unilateral perturbation frequency difference and is also used to detect whether the distributed generation grid-connected system is in the islanding state next time.

[0151] When the first preset condition is satisfied, the reference frequency is updated to the first frequency of the PCC, which can calibrate the parameters of the islanding state detection in real time and ensure the reliability of the detection result. Especially when the power grid frequency fluctuates greatly, updating the reference frequency in time can reduce the false alarm rate, improve the accuracy of the islanding state detection, and thus enhance the safety and reliability of the distributed generation grid-connected system.

[0152] In some embodiments, before it is determined that the frequency change of the PCC of the power grid during the periodic bilateral reactive power perturbation satisfies the first preset condition, the islanding detection method further includes obtaining the frequency of the PCC of the power grid collected in real time to obtain a second frequency, where the second frequency represents the frequency of the PCC of the power grid at a second moment, and the second moment is the current moment; when the second frequency is greater than a third frequency and the third frequency is greater than or equal to a fourth frequency, obtaining the count value of the timer at the second moment to obtain the current offset time; when the current offset time is greater than or equal to the first time period, determining that the frequency of the PCC of the power grid is greater than the reference frequency during the first time period; when the second frequency is greater than the third frequency and the third frequency is less than the fourth frequency, controlling the timer to be cleared; where the third frequency represents the frequency of the PCC of the power grid at a third moment, the fourth frequency represents the frequency of the PCC of the power grid at a fourth moment, the third moment is the previous acquisition moment of the second moment; the fourth moment is the previous acquisition moment of the third moment.

[0153] It should be noted that the timer starts timing when the system starts running.

[0154] In at least one embodiment, before determining that the first preset condition is met and converting the bilateral reactive power disturbance of the PCS into a unilateral reactive power disturbance, the PCC frequency collected in real time is obtained. The PCC frequency data collected in real time is stored in a local cache or a database for subsequent frequency comparison and calculation. If the PCC frequency (second frequency) collected at the second moment is greater than the PCC frequency (third frequency) collected at the third moment, and the PCC frequency (third frequency) collected at the third moment is greater than the PCC frequency (fourth frequency) collected at the fourth moment, then the timer value at the second moment is obtained, and the offset time (current offset time) at the second moment is compared with the first duration. When the current offset time is greater than or equal to the first duration, it is considered that the frequency of the PCC within the first duration in the first preset condition is greater than the reference frequency; when the current offset time is less than the first duration, the timer value is cleared, and the PCC frequency collected in real time is continuously obtained until the current offset time is greater than or equal to the first duration. If the PCC frequency (second frequency) collected at the second moment is greater than the PCC frequency (third frequency) collected at the third moment, and the PCC frequency (third frequency) collected at the third moment is less than the PCC frequency (fourth frequency) collected at the fourth moment, the timer value is cleared, and the PCC frequency collected in real time is continuously obtained until the second frequency is less than the third frequency and less than the fourth frequency, or the second frequency is greater than the third frequency and greater than the fourth frequency.

[0155] Among them, the second moment is the current moment, the third moment is the previous acquisition moment of the second moment; the fourth moment is the previous acquisition moment of the third moment.

[0156] By collecting the PCC frequency data in real time, it is ensured that the detection process is based on the latest and most accurate data. Through multi-step frequency relationship judgment, that is, the judgment of the magnitudes of the first frequency, the second frequency, and the third frequency, it is ensured that the PCC frequency changes are verified at multiple time points, and the change trend of the PCC frequency can be identified more accurately. If the PCC frequency increases with time, that is, the first frequency is greater than the second frequency, and the second frequency is greater than the third frequency, then the continuously increasing time, that is, the current offset time, is obtained. By judging the magnitude relationship between the current offset time and the first duration to determine whether the second moment meets the condition that the frequency of the PCC within the first duration in the first preset condition is greater than the reference frequency, it is ensured that the initial confirmation of the islanding state is only carried out when the frequency changes continuously in one direction and stably, reducing misjudgment caused by short-term fluctuations.

[0157] In some embodiments, the islanding detection method further includes: when the second frequency is less than the third frequency and the third frequency is less than or equal to the fourth frequency, obtaining the count value of the timer at the current moment to obtain the current offset time; when the current offset time is greater than or equal to the first duration, determining that the frequencies of the point of common coupling (PCC) of the power grid within the first duration are all less than the reference frequency; when the second frequency is less than the third frequency and the third frequency is greater than the fourth frequency, controlling the timer to be cleared; and when the second frequency is equal to the third frequency, controlling the timer to be cleared.

[0158] In at least one embodiment, before determining that the bilateral reactive power disturbance of the PCS is converted into a unilateral reactive power disturbance when the first preset condition is satisfied, the PCC frequency collected in real time is obtained, and the PCC frequency data collected in real time is stored in a local cache or a database for subsequent frequency comparison and calculation. If the PCC frequency (second frequency) collected at the second moment is less than the PCC frequency (third frequency) collected at the third moment, and the PCC frequency (third frequency) collected at the third moment is less than the PCC frequency (fourth frequency) collected at the fourth moment, then the timer value at the second moment is obtained, and the offset time (current offset time) at the second moment is compared with the first duration. When the current offset time is greater than or equal to the first duration, it is considered that the frequencies of the PCC within the first duration in the first preset condition are all less than the reference frequency; when the current offset time is less than the first duration, the timer value is cleared, and the PCC frequency collected in real time is continuously obtained until the current offset time is greater than or equal to the first duration. If the PCC frequency (second frequency) collected at the second moment is less than the PCC frequency (third frequency) collected at the third moment, and the PCC frequency (third frequency) collected at the third moment is greater than the PCC frequency (fourth frequency) collected at the fourth moment, the timer value is cleared, and the PCC frequency collected in real time is continuously obtained until the second frequency is less than the third frequency is less than the fourth frequency, or the second frequency is greater than the third frequency is greater than the fourth frequency.

[0159] Wherein, the second moment is the current moment, the third moment is the previous acquisition moment of the second moment; the fourth moment is the previous acquisition moment of the third moment.

[0160] By collecting the frequency data of the PCC in real time, it is ensured that the detection process is based on the latest and most accurate data. Through multi-step frequency relationship judgment, that is, the judgment of the magnitudes of the first frequency, the second frequency, and the third frequency, it is ensured that the change of the PCC frequency is verified at multiple time points, and the change trend of the PCC frequency can be identified more accurately. If the PCC frequency decreases with time, that is, the first frequency is less than the second frequency, and the second frequency is less than the third frequency, then obtain the continuously increasing time, that is, the current offset time. By judging the magnitude relationship between the current offset time and the first duration, it is determined whether the second moment satisfies the condition that the frequency of the PCC is less than the reference frequency within the first duration in the first preset condition, which ensures that the preliminary confirmation of the islanding state is only carried out when the frequency changes continuously in one direction and stably, reducing misjudgment caused by short-term fluctuations.

[0161] One or more embodiments of the present application also disclose an islanding detection device 500. Referring to Figure 5 , the islanding detection device 500 includes:

[0162] A first perturbation module 510, configured to perform periodic bilateral reactive power perturbation on the energy storage converter in the distributed generation grid-connected system, wherein the perturbation directions of the i-th bilateral perturbation period and the (i + 1)-th bilateral perturbation period in the periodic bilateral reactive power perturbation are opposite, and i is an integer greater than or equal to 1;

[0163] A second perturbation module 520, configured to perform unilateral reactive power perturbation on the energy storage converter when the frequency change of the grid common coupling point during the periodic bilateral reactive power perturbation satisfies a first preset condition, and determine the frequency change amount of the grid common coupling point after the unilateral reactive power perturbation to obtain a unilateral perturbation frequency difference;

[0164] A first determination module 530, configured to determine that the distributed generation grid-connected system is in an islanding state when the unilateral perturbation frequency difference is greater than a first preset frequency change amount threshold.

[0165] In some embodiments, the first preset condition of the second perturbation module 520 includes: the frequency change of the grid common coupling point corresponding to each of the continuous M bilateral perturbation periods satisfies a second preset condition, and / or, the frequency of the grid common coupling point within a first duration is greater than a reference frequency or the frequency of the grid common coupling point within a first duration is less than a reference frequency.

[0166] In some embodiments, the islanding detection device 500 further includes a first acquisition module and a second determination module: the first acquisition module is configured to acquire a first frequency change trend and a first frequency change amount corresponding to the i + 1-th bilateral disturbance period of the point of common coupling of the power grid during the periodic bilateral reactive power disturbance, and a second frequency change trend corresponding to the i-th bilateral disturbance period of the point of common coupling of the power grid; the second determination module is configured to determine that the frequency change of the point of common coupling of the power grid satisfies the second preset condition when the first frequency change trend is opposite to the second frequency change trend and the first frequency change amount is greater than a second preset frequency change amount threshold, where the second preset frequency change amount threshold is less than the first preset frequency change amount threshold.

[0167] In some embodiments, the first disturbance module 510 is specifically configured to separately determine that the reactive power disturbance amount in the positive direction is a first disturbance amount and the reactive power disturbance amount in the negative direction is a second disturbance amount, where the first disturbance amount is determined by the following formula:

[0168]

[0169] The second disturbance amount is determined by the following formula:

[0170]

[0171] where Q dis1 is the first disturbance amount, Q dis2 is the second disturbance amount, P is the active power output by the energy storage converter, Q f is the quality factor of the load, Δf dis is the preset frequency offset, f g is the rated frequency of the power grid; the energy storage converter is subjected to the periodic bilateral reactive power disturbance according to the first disturbance amount and the second disturbance amount.

[0172] In some embodiments, the second disturbance module 520 is specifically configured to determine a target disturbance duration of the unilateral reactive power disturbance; determine a target disturbance amount of the unilateral reactive power disturbance; determine a target disturbance direction according to a first frequency of the point of common coupling of the power grid and the rated frequency of the power grid; and perform the unilateral reactive power disturbance on the energy storage converter according to the target disturbance duration, the target disturbance amount, and the target disturbance direction.

[0173] In some embodiments, the second disturbance module 520 includes a third determination module: the third determination module is configured to determine the target disturbance duration according to the reference frequency, the rated grid frequency, and the second duration when the frequency of the point of common coupling of the power grid is greater than the reference frequency within the first duration, or when the frequency of the point of common coupling of the power grid is less than the reference frequency within the first duration; when the frequency variations of the point of common coupling of the power grid corresponding to each of the continuous M bilateral disturbance periods satisfy a second preset condition, determine the target disturbance duration as the second duration, where the second duration is a reference time constant.

[0174] In some embodiments, the second disturbance module 520 further includes a fourth determination module: the fourth determination module is configured to determine that the target disturbance direction is the positive direction when the first frequency is less than the rated grid frequency; and determine that the target disturbance direction is the negative direction when the first frequency is greater than or equal to the rated grid frequency.

[0175] In some embodiments, the islanding detection device 500 further includes an update module: the update module is configured to update the reference frequency to the first frequency of the point of common coupling of the power grid when the frequency variation of the point of common coupling of the power grid during the periodic bilateral reactive power disturbance process satisfies the first preset condition, where the first frequency is the frequency of the point of common coupling of the power grid at the first moment, and the first moment is the moment when the first preset condition is determined to be satisfied.

[0176] Before determining that the frequency variation of the point of common coupling of the power grid during the periodic bilateral reactive power disturbance process satisfies the first preset condition, in some embodiments, the islanding detection device 500 further includes a second acquisition module and a fifth determination module: the second acquisition module is configured to acquire the frequency of the point of common coupling of the power grid collected in real time to obtain a second frequency, where the second frequency represents the frequency of the point of common coupling of the power grid at the second moment, and the second moment is the current moment; the fifth determination module is configured to, when the second frequency is greater than a third frequency and the third frequency is greater than or equal to a fourth frequency, acquire the count value of the timer at the second moment to obtain the current offset time; and when the current offset time is greater than or equal to the first duration, determine that the frequency of the point of common coupling of the power grid within the first duration is greater than the reference frequency; when the second frequency is greater than the third frequency and the third frequency is less than the fourth frequency, control the timer to be cleared; where the third frequency represents the frequency of the point of common coupling of the power grid at the third moment, the fourth frequency represents the frequency of the point of common coupling of the power grid at the fourth moment, the third moment is the previous acquisition moment of the second moment; and the fourth moment is the previous acquisition moment of the third moment.

[0177] In some embodiments, the islanding detection device 500 further includes a sixth determination module: the sixth determination module is configured to, when the second frequency is less than the third frequency and the third frequency is less than or equal to the fourth frequency, obtain the count value of the timer at the current moment to obtain the current offset time; when the current offset time is greater than or equal to the first duration, determine that the frequencies of the point of common coupling of the power grid within the first duration are all less than the reference frequency; when the second frequency is less than the third frequency and the third frequency is greater than the fourth frequency, control the timer to be cleared; and when the second frequency is equal to the third frequency, control the timer to be cleared.

[0178] One or more embodiments of the present application also disclose an electronic device 600. Referring to Figure 6 , the electronic device 600 includes a memory 610 and a processor 620; the memory 610 is configured to store a computer program; the processor 620 is configured to, when executing the computer program, implement the islanding detection method as described above.

[0179] Or, an electronic device 600 includes a memory 610 and a processor 620 coupled to the memory 610; the memory 610 is configured to store a computer program; the processor 620 is configured to, when executing the computer program, perform the following operations: perform periodic bilateral reactive power disturbances on the energy storage converter in the distributed generation grid-connected system, where the disturbance directions of the i-th bilateral disturbance period and the (i + 1)-th bilateral disturbance period in the periodic bilateral reactive power disturbances are opposite, and i is an integer greater than or equal to 1; when the frequency change of the point of common coupling of the power grid during the periodic bilateral reactive power disturbances satisfies a first preset condition, perform a unilateral reactive power disturbance on the energy storage converter, and determine the frequency change amount of the point of common coupling of the power grid after the unilateral reactive power disturbance to obtain a unilateral disturbance frequency difference; when the unilateral disturbance frequency difference is greater than a first preset frequency change amount threshold, determine that the distributed generation grid-connected system is in an island state.

[0180] One or more embodiments of the present application also disclose a computer-readable storage medium. A computer program is stored on the computer-readable storage medium, and when the computer program is executed by a processor, the islanding detection method as described above is implemented.

[0181] Alternatively, a non-volatile computer-readable storage medium stores a computer program thereon. When the computer program is executed by a processor, the processor performs the following operations: performing periodic bilateral reactive power disturbances on a energy storage converter in a distributed generation grid-connected system, wherein the disturbance directions of the i-th bilateral disturbance period and the (i + 1)-th bilateral disturbance period in the periodic bilateral reactive power disturbances are opposite, and i is an integer greater than or equal to 1; when the frequency change of a point of common coupling of the power grid during the periodic bilateral reactive power disturbances meets a first preset condition, performing unilateral reactive power disturbances on the energy storage converter, determining a frequency change amount of the point of common coupling of the power grid after the unilateral reactive power disturbances, and obtaining a unilateral disturbance frequency difference; when the unilateral disturbance frequency difference is greater than a first preset frequency change amount threshold, determining that the distributed generation grid-connected system is in an island state.

[0182] The foregoing has described the basic principles, main features and advantages of the present application. Those skilled in the art of this industry should understand that the present application is not limited by the above embodiments. What is described in the above embodiments and the specification is only the principle of the present application. Without departing from the spirit and scope of the present application, the present application will have various changes and improvements, and these changes and improvements all fall within the scope of the present application claimed. The scope of protection claimed by the present application is defined by the appended claims and their equivalents.

Claims

1. An island detection method, characterized in that: include: Performing periodic bilateral reactive power disturbance on the energy storage converter in the distributed power generation grid-connected system, wherein the disturbance direction of the i-th bilateral disturbance period in the periodic bilateral reactive power disturbance is opposite to the disturbance direction of the i+1-th bilateral disturbance period, and i is an integer greater than or equal to 1; In the case where the frequency change of the common coupling point of the power grid during the periodic bilateral reactive power disturbance satisfies the first preset condition, a unilateral reactive power disturbance is performed on the energy storage converter according to a target disturbance duration, and the frequency change amount of the common coupling point of the power grid after the unilateral reactive power disturbance is determined to obtain a unilateral disturbance frequency difference, wherein the target disturbance duration of the unilateral reactive power disturbance is confirmed by two working conditions of the first preset condition, including: When the frequency of the grid common coupling point is greater than the reference frequency during the first duration, or when the frequency of the grid common coupling point is less than the reference frequency during the first duration, determining the target disturbance duration according to the reference frequency, the grid rated frequency and the second duration; When the frequency change of the power grid common coupling point corresponding to each of the bilateral disturbance periods in the consecutive M bilateral disturbance periods satisfies the second preset condition, the target disturbance duration is determined as the second duration, wherein the second duration is a reference time constant; When the unilateral disturbance frequency difference is greater than a first preset frequency change threshold, it is determined that the distributed generation grid-connected system is in an island state.

2. The island detection method according to claim 1, characterized in that: The first preset condition includes: the frequency change of the power grid common coupling point corresponding to each of the bilateral disturbance periods in the M consecutive bilateral disturbance periods satisfies the second preset condition, and / or the frequency of the power grid common coupling point within the first time length is greater than the reference frequency or the frequency of the power grid common coupling point within the first time length is less than the reference frequency.

3. The island detection method according to claim 2, characterized in that: The island detection method further comprises: Acquire a first frequency change trend and a first frequency change amount of the common coupling point of the power grid corresponding to the (i+1)th bilateral disturbance period during the periodic bilateral reactive power disturbance, and a second frequency change trend of the common coupling point of the power grid corresponding to the (i)th bilateral disturbance period; When the first frequency change trend is opposite to the second frequency change trend and the first frequency change amount is greater than a second preset frequency change amount threshold, it is determined that the frequency change of the power grid common coupling point meets the second preset condition, The second preset frequency change threshold is smaller than the first preset frequency change threshold.

4. The island detection method according to claim 1, characterized in that: The periodic bilateral reactive power disturbance of the energy storage converter in the distributed generation grid-connected system comprises: The reactive power disturbance amount in the positive direction is determined as the first disturbance amount and the reactive power disturbance amount in the negative direction is determined as the second disturbance amount, wherein the first disturbance amount is determined by the following formula: The second disturbance amount is determined by the following formula: Among them, Q dis1 is the first disturbance quantity, Q dis2 is the second disturbance, P is the active power output by the energy storage converter, Q f is the quality factor of the load, Δf dis is the preset frequency offset, f g is the grid rated frequency; The periodic bilateral reactive power disturbance is performed on the energy storage converter according to the first disturbance amount and the second disturbance amount.

5. The island detection method according to claim 1, characterized in that: When the frequency change of the common coupling point of the power grid during the periodic bilateral reactive power disturbance meets the first preset condition, performing unilateral reactive power disturbance on the energy storage converter according to the target disturbance duration includes: Determining the target disturbance duration of the unilateral reactive power disturbance; Determining a target disturbance amount of the unilateral reactive power disturbance; Determining a target disturbance direction according to a first frequency of the grid common coupling point and a grid rated frequency, wherein the first frequency is the frequency of the grid common coupling point at a first moment; The unilateral reactive power disturbance is performed on the energy storage converter according to the target disturbance duration, the target disturbance amount, and the target disturbance direction.

6. The island detection method according to claim 5, characterized in that: The determining of the target disturbance direction according to the first frequency of the grid common coupling point and the grid rated frequency comprises: When the first frequency is less than the rated frequency of the power grid, determining the target disturbance direction to be a positive direction; When the first frequency is greater than or equal to the rated frequency of the power grid, the target disturbance direction is determined to be a negative direction.

7. The island detection method according to claim 1, characterized in that: The island detection method further includes: when a frequency change of a grid common coupling point during the periodic bilateral reactive power disturbance satisfies the first preset condition, updating the reference frequency to a first frequency of the grid common coupling point, wherein the first frequency is the frequency of the grid common coupling point at a first moment, and the first moment is a moment when it is determined that the first preset condition is satisfied.

8. The island detection method according to claim 1, characterized in that: Before determining that the frequency change of the grid common coupling point during the periodic bilateral reactive power disturbance process satisfies the first preset condition, the island detection method further includes: Acquire the frequency of the common coupling point of the power grid collected in real time to obtain a second frequency, wherein the second frequency represents the frequency of the common coupling point of the power grid at a second moment; When the second frequency is greater than the third frequency, and the third frequency is greater than or equal to the fourth frequency, obtaining the count value of the timer at the second moment to obtain the current offset time; In a case where the current offset time is greater than or equal to a first duration, determining that the frequency of the grid common coupling point within the first duration is greater than a reference frequency; When the second frequency is greater than the third frequency and the third frequency is less than the fourth frequency, controlling the timer to be reset; The third frequency represents the frequency of the grid common coupling point at a third moment, the fourth frequency represents the frequency of the grid common coupling point at a fourth moment, the third moment is a collection moment before the second moment, and the fourth moment is a collection moment before the third moment.

9. The island detection method according to claim 8, characterized in that: The island detection method further comprises: When the second frequency is less than the third frequency, and the third frequency is less than or equal to the fourth frequency, obtaining the count value of the timer at the current moment to obtain the current offset time; when the current offset time is greater than or equal to the first duration, determining that the frequency of the common coupling point of the power grid within the first duration is less than the reference frequency; When the second frequency is less than the third frequency and the third frequency is greater than the fourth frequency, controlling the timer to be reset; When the second frequency is equal to the third frequency, the timer is controlled to be cleared.

10. An island detection device, characterized in that: include: A first disturbance module is used to perform periodic bilateral reactive power disturbance on the energy storage converter in the distributed power generation grid-connected system, wherein the disturbance direction of the i-th bilateral disturbance period in the periodic bilateral reactive power disturbance is opposite to the disturbance direction of the i+1-th bilateral disturbance period, and i is an integer greater than or equal to 1; a second disturbance module, for performing unilateral reactive power disturbance on the energy storage converter according to a target disturbance duration, when the frequency change of the common coupling point of the power grid during the periodic bilateral reactive power disturbance satisfies a first preset condition, and determining the frequency change of the common coupling point of the power grid after the unilateral reactive power disturbance to obtain a unilateral disturbance frequency difference, wherein the target disturbance duration of the unilateral reactive power disturbance is confirmed by two working conditions of the first preset condition, including: when the frequency of the common coupling point of the power grid is greater than a reference frequency within a first duration, or when the frequency of the common coupling point of the power grid is less than the reference frequency within the first duration, determining the target disturbance duration according to the reference frequency, the rated frequency of the power grid and a second duration; when the frequency change of the common coupling point of the power grid corresponding to each of the bilateral disturbance periods in M ​​consecutive bilateral disturbance periods satisfies the second preset condition, determining the target disturbance duration as the second duration, wherein the second duration is a reference time constant; The determination module is used to determine that the distributed generation grid-connected system is in an island state when the unilateral disturbance frequency difference is greater than a first preset frequency change threshold.

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