Island detection method and device

Abstract

The embodiment of the invention relates to the technical field of power systems, and provides an island detection method and device, and the method comprises the steps: carrying out the periodic bilateral reactive power disturbance of an energy storage converter in a distributed power generation grid-connected system; under the condition that the frequency change of the power grid common coupling point in the periodic bilateral reactive power disturbance process meets a first preset condition, performing unilateral reactive power disturbance on the energy storage converter, and determining the frequency variation of the power grid common coupling point after the unilateral reactive power disturbance to obtain a unilateral disturbance frequency difference; and when the unilateral disturbance frequency difference is greater than a first preset frequency variation threshold, determining that the distributed power generation grid-connected system is in an island state. Under the condition that the output power of the energy storage converter is highly matched with the load power, the island phenomenon is effectively detected.

Description

Technical Field

[0001] The present application relates to the technical field of power systems, and in particular to an island 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 power generation systems have been widely used around the world. These systems usually operate in parallel with the power grid, which can improve energy efficiency while reducing the impact on the environment. However, when the distributed power generation system is connected to the grid, when the power grid is interrupted due to electrical failure, misoperation or natural factors, the power generation device will be disconnected from the grid and continue to supply power to the local load, forming an autonomous power supply island that cannot be controlled by the public grid. This situation not only poses a threat to the safe and stable operation of the power system, but may also endanger the safety of maintenance personnel and equipment.

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

[0004] Therefore, the inventors realize that when the output power of the energy storage converter is highly matched with the load power, the islanding phenomenon cannot be effectively detected. Summary of the invention

[0005] One or more embodiments of the present application provide an island detection method and device to solve or at least partially alleviate the problem in the related art that the island detection cannot be effectively detected when the output power of the energy storage converter is highly matched with the load power.

[0006] In a first aspect of the present application, a method for detecting an island is provided, which adopts the following technical solution: A method for detecting an island comprises: performing a periodic bilateral reactive power disturbance on an energy storage converter in a 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; performing a unilateral reactive power disturbance on the energy storage converter when the frequency change of a common coupling point of a power grid during the periodic bilateral reactive power disturbance satisfies a first preset condition, and determining the frequency change amount of the common coupling point of the power grid after the unilateral reactive power disturbance to obtain a unilateral disturbance frequency difference; and determining that the distributed power generation grid-connected system is in an island state when the unilateral disturbance frequency difference is greater than a first preset frequency change amount threshold.

[0007] In one of the embodiments, 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 period is greater than the reference frequency or the frequency of the power grid common coupling point within the first time period is less than the reference frequency.

[0008] In one of the embodiments, the island detection method further includes: obtaining a first frequency change trend and a first frequency change amount of the common coupling point of the power grid corresponding to the i+1th 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, determining that the frequency change of the common coupling point of the power grid meets the second preset condition, wherein the second preset frequency change amount threshold is less than the first preset frequency change amount threshold.

[0009] In one embodiment, the periodic bilateral reactive power disturbance of the energy storage converter in the distributed power generation grid-connected system includes: determining the reactive power disturbance amount in the positive direction as the first disturbance amount and the reactive power disturbance amount in the negative direction as the second disturbance amount, respectively, wherein the first disturbance amount is determined by the following formula:

[0010] The second disturbance amount is determined by the following formula:

[0011] 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 rated frequency of the power grid; and performing the periodic bilateral reactive power disturbance on the energy storage converter according to the first disturbance amount and the second disturbance amount.

[0012] In one of the embodiments, when the frequency change of the common coupling point of the power grid during the periodic bilateral reactive power disturbance meets a first preset condition, a unilateral reactive power disturbance is performed on the energy storage converter, including: determining a 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 common coupling point of the power grid and a rated frequency of the power grid, wherein the first frequency is the frequency of the common coupling point of the power grid 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 amount, and the target disturbance direction.

[0013] In one of the embodiments, determining the target disturbance duration of the unilateral reactive power disturbance includes: when the frequency of the common coupling point of the power grid is greater than the 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 cycles in M ​​consecutive bilateral disturbance cycles satisfies a second preset condition, determining the target disturbance duration as the second duration, wherein the second duration is a reference time constant.

[0014] In one of the embodiments, determining the target disturbance direction according to the first frequency of the common coupling point of the power grid and the rated frequency of the power grid includes: when the first frequency is less than the rated frequency of the power grid, determining the target disturbance direction as a positive direction; when the first frequency is greater than or equal to the rated frequency of the power grid, determining the target disturbance direction as a negative direction.

[0015] In one of the embodiments, the island detection method further includes: when the frequency change of the common coupling point of the power grid during the periodic bilateral reactive power disturbance satisfies the first preset condition, updating the reference frequency to the first frequency of the common coupling point of the power grid, wherein the first frequency is the frequency of the common coupling point 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.

[0016] In one embodiment, before determining that the frequency change of the grid common coupling point during the periodic bilateral reactive power disturbance satisfies the first preset condition, the island detection method further includes: acquiring the frequency of the grid common coupling point acquired in real time to obtain a second frequency, wherein the second frequency represents the frequency of the grid common coupling point at a second moment, and the second moment is the current moment; when the second frequency is greater than the third frequency, and the third frequency is greater than or equal to the fourth frequency, acquiring 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 duration, determining that the frequency of the grid common coupling point 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, controlling the timer to be cleared; wherein 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, and the third moment is a previous acquisition moment of the second moment; and the fourth moment is a previous acquisition moment of the third moment.

[0017] In one embodiment, the island 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 time duration, determining that the frequency of the common coupling point of the power grid within the first time 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, controlling the timer to be reset.

[0018] In a second aspect of the present application, an island detection device is provided, which adopts the following technical solution: An island detection device includes a first disturbance module, which is used to perform periodic bilateral reactive power disturbance on an energy storage converter in a 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 that of the i+1-th bilateral disturbance period, and i is an integer greater than or equal to 1; a second disturbance module, which is used to perform unilateral reactive power disturbance on the energy storage converter when the frequency change of a grid common coupling point during the periodic bilateral reactive power disturbance meets a first preset condition, and determine the frequency change of the grid common coupling point after the unilateral reactive power disturbance to obtain a unilateral disturbance frequency difference; and a determination module, which is used to determine that the distributed power generation grid-connected system is in an island state when the unilateral disturbance frequency difference is greater than a first preset frequency change threshold.

[0019] Compared with the related art, one or more embodiments of the present application include at least one of the following beneficial technical effects: Small-amplitude reactive power disturbances in two directions are applied alternately to the energy storage converter, i.e., periodic bilateral reactive power disturbances, and the frequency changes at the common coupling point of the power grid are detected in real time, which helps to timely discover potential frequency anomalies and improve the accuracy of island state detection. Since, when the output power of the energy storage converter matches the load power, a certain reactive power disturbance given to the energy storage converter will cause the frequency change at the common coupling point of the power grid, and the frequency change is proportional to the reactive power disturbance. Therefore, when the frequency change at the common coupling point of the power grid meets the first preset condition, i.e., when the distributed generation grid-connected system is preliminarily confirmed to be in an island state when the output power of the energy storage converter matches the load power, the periodic bilateral reactive power disturbance of the energy storage converter is converted into a unilateral reactive power disturbance, and the frequency of the common coupling point of the power grid is changed to a greater extent, thereby breaking the power balance state, further detecting the island state, and improving the sensitivity and accuracy of the island state detection. After the energy storage converter is subjected to unilateral reactive power disturbance, 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 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 the periodic bilateral reactive power disturbance with the unilateral reactive power disturbance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] 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 in the following description only relate to some embodiments of the present application, rather than limiting the present application.

[0021] Figure 1 It is an equivalent circuit diagram of the distributed power generation grid-connected system according to the present application.

[0022] Figure 2 A control block diagram of an energy storage converter according to the present application.

[0023] Figure 3 It is a flowchart of an island detection method according to some embodiments of the present application.

[0024] Figure 4 It is a schematic diagram of a specific process of an island detection method according to some implementation modes of the present application.

[0025] Figure 5 4 is a system architecture diagram of an island detection device according to some embodiments of the present application.

[0026] Figure 6 A system architecture diagram of an electronic device according to some embodiments of the present application. DETAILED DESCRIPTION

[0027] In order to make the purpose, 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 in conjunction with the accompanying drawings showing multiple embodiments of the present application. It should be understood that the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments described in the present application, all other embodiments obtained by ordinary technicians in this field without spending creative work will fall within the scope of protection of the present application.

[0028] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as those generally understood by technicians in the technical field of this application; the terms used in the specification of the application are only for the purpose of describing specific implementation methods, and are not intended to limit this application; the terms "including", "comprising", "having", "having", "containing", "including", etc. in the specification and claims of this application and the above-mentioned figure description are open-ended words. Therefore, "including", "comprising", "having" is a method or device with one or more steps or elements, for example, which has one or more steps or elements, but is not limited to having only these one or more elements. The terms "first", "second", etc. in the specification and claims of this application or the above-mentioned figures are used to distinguish different objects, rather than to describe a specific order or a primary and secondary relationship. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, the meaning of "multiple" is two or more.

[0029] In the description of the present application, it should be understood that the terms "center", "lateral", "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0030] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", 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 a direct connection, or an indirect connection through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0031] Reference to "embodiment" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described in this application may be combined with other embodiments.

[0032] As mentioned above, it should be emphasized that when the term "includes / comprising" is used in this specification, it is used to clearly indicate the presence of the features, integers, steps or components, but does not exclude the presence or addition of one or more other features, integers, steps, components or groups of features, integers, steps, components. As used in this application, the singular forms "a", "an" and "the" also include plural forms, unless the context clearly indicates otherwise.

[0033] The words "a" and "an" in this specification may mean one, but may also be consistent with the meaning of "at least one" or "one or more". The term "about" generally means plus or minus 10% of the mentioned value, or more specifically plus or minus 5%. The term "or" used in the claims means "and / or" unless it is clearly stated that it refers to only alternative solutions.

[0034] The term "and / or" in this application is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this application generally indicates that the associated objects before and after are in an "or" relationship.

[0035] Figure 1 For the equivalent circuit diagram of the distributed generation grid-connected system, refer to Figure 1The distributed generation grid-connected system includes a generating device, a local load, and a power grid. The generating device includes a direct current (DC) power supply and a power conditioning system (PCS). The PCS is a power conditioner or inverter responsible for converting direct current into alternating current and injecting it into the power grid. The direct current (DC) power supply usually comes from solar photovoltaic panels or other renewable energy sources. The point of common coupling (PCC) of the power grid is where the generating device is connected to the power grid. The transformer boosts the alternating current output by the PCS to a voltage level suitable for transmission. The local load adopts a parallel resistor-inductor-capacitor (RLC) form. The circuit breaker controls the connection between the PCS and the power grid. When the distributed generation grid-connected system is in normal operation, the circuit breaker is closed, and the generating device is connected to the power grid to supply power to the local load together. When the distributed generation grid-connected system is in an island state, the circuit breaker is opened, and the generating device independently supplies power to the local load.

[0036] Figure 2 For the control block diagram of the PCS, use Figure 2 The control block diagram shown controls the PCS so that the PCS is in constant power working mode. The PCS output power 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 makes the PCS output power change with the power setting. The specific implementation method of the PCS control belongs to the prior art and will not be repeated here.

[0037] When the distributed generation grid-connected system is in an island state, 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 decrease. Therefore, by monitoring the PCC frequency, it is possible to detect whether the distributed generation grid-connected system is in an island state.

[0038] The output power of the generator matches the load power, that is, the power generated by the generator just meets the load demand. At this time, the distributed generation grid-connected system enters the island state (corresponding to Figure 1 The relay in the PCC is disconnected), the PCC frequency remains stable, that is, the change may be very small or even close to zero. Therefore, it is difficult to effectively detect that the distributed generation grid-connected system is in an island state by monitoring the frequency change of the PCC.

[0039] When the distributed generation grid-connected system operates normally, according to the power flow and circuit system analysis before the island is formed, it can be known that the active power and reactive power consumed by the local load satisfy the following formula:

[0040]

[0041] Among them, P load is the active power consumed by the local load before the island is formed, Q load is the reactive power consumed by the local load before the island is formed, P inv is the active power output by the PCS before islanding, Q inv is the reactive power output by the PCS before islanding, ΔP is the active power output by the grid side to the PCC, ΔQ is the reactive power output by the grid side to the PCC, V0 is the PCC voltage before islanding, f0 is the PCC frequency 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.

[0042] When an island is formed, the circuit breaker is opened, and the active power consumed by the local load and the reactive power consumed by the local load are provided by the PCS alone. The active power consumed by the local load and the reactive power consumed by the local load after the island is formed satisfy the following formula:

[0043]

[0044] Among them, P * load is the active power consumed by the local load after the island is formed, Q * load is the reactive power consumed by the local load after the island is formed, P inv is the active power output by the PCS before islanding, Q inv is the reactive power output by the PCS before island formation, V i is the steady-state voltage of the PCC after island formation, f i is the steady-state frequency of the PCC after island 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.

[0045] Based on the local load characteristics, the resonant frequency of the local load and the quality factor of the local load can be obtained. The resonant frequency of the local load and the quality factor of the local load satisfy the following formula:

[0046]

[0047] 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.

[0048] 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 the island is formed can be obtained. The reactive power output by the PCS after the island is formed satisfies the following formula:

[0049] Among them, Q * inv is the reactive power output by the energy storage converter after the island is formed, 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 PCC after islanding.

[0050] According to the active power consumed by the local load after the island is formed and the reactive power consumed by the local load after the island is formed, the PCS output reactive power differential can be linearized, as shown in the following formula:

[0051] Where ΔP inv is the change in active power output by the PCS after islanding, ΔQ inv is the reactive power change of PCS output after islanding, Δf i is the PCC frequency change after island formation, f i is the steady-state frequency of PCC after island formation, 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.

[0052] According to the quality factor of the local load and the differential linearized PCS output reactive power, the PCC frequency change after the island is formed can be obtained. The PCC frequency change after the island is formed satisfies the following formula:

[0053] Where Δf i is the PCC frequency change after island formation, ΔP inv is the change in active power output by the PCS after the island is formed, Q f is the quality factor of the local load, f LC is the resonant frequency of the local load, f iis the steady-state frequency of PCC after island formation, f is the first frequency of PCC, ΔQ inv is the reactive power change of PCS output after island formation, P inv It is the active power output by the PCS before islanding occurs.

[0054] When the PCS output power matches the local load power consumption, the PCC frequency remains unchanged before and after the island is formed. The steady-state frequency of the PCC after the island is formed is equal to the resonant frequency of the local load. The formula for the PCC frequency change after the island is formed can be simplified as:

[0055] Where Δf i is the PCC frequency change after island formation, ΔQ inv is the reactive power change of PCS output after island formation, P inv is the active power output by the PCS before islanding, Q f is the quality factor of the local load, f i is the steady-state frequency of PCC after islanding.

[0056] It can be seen from the above formula that the change in PCC frequency after the island is formed is linearly related to the change in reactive power output by the PCS after the island is formed. The PCC frequency can be shifted in the island state by disturbing the reactive power output by the PCS, thereby detecting the island state.

[0057] Figure 3 It is a flowchart of an island detection method according to some embodiments of the present application.

[0058] One or more embodiments of the present application disclose a method for detecting an island. Figure 3 , the island detection methods include: S100: 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.

[0059] The periodic bilateral reactive power disturbance is carried out according to a certain period, and a reactive power disturbance is applied once in each period. The reactive power disturbance direction of the periodic bilateral reactive power disturbance is alternately carried out in the positive and negative directions, that is, a positive reactive power disturbance is applied in one period, and a negative reactive power disturbance is applied in the next period.

[0060] In some embodiments, the time interval of the bilateral disturbance period of the periodic bilateral reactive power disturbance (such as 50 ms) and the disturbance amount of the positive direction disturbance and the disturbance amount of the negative direction disturbance are set. For example, in the i-th bilateral disturbance period, if a positive reactive power disturbance is applied to the PCS and the frequency change of the PCC in the i-th bilateral disturbance period is monitored, then in the i+1-th bilateral disturbance period, a negative reactive power disturbance is applied to the PCS and the frequency change of the PCC in the i+1-th bilateral disturbance period is continued to be monitored. Conversely, in the i-th bilateral disturbance period, if a negative reactive power disturbance is applied to the PCS and the frequency change of the PCC in the i-th bilateral disturbance period is monitored, then in the i+1-th bilateral disturbance period, a positive reactive power disturbance is applied to the PCS and the frequency change of the PCC in the i-th bilateral disturbance period is monitored. Positive and negative reactive power disturbances are applied alternately. After each bilateral disturbance cycle, the frequency change of each bilateral disturbance cycle of PCC (the absolute value of the difference between the frequency at the end of the cycle and the frequency at the beginning of the cycle) is recorded, where i represents the sequence number of the bilateral disturbance cycle, starting from the first cycle and increasing sequentially.

[0061] S200: 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 a unilateral reactive power disturbance on the energy storage converter, 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; When the output power of the PCS matches the load power, the PCS operates at unity power factor, and the active power output from the grid side to the PCC is balanced with the reactive power output from the grid side to the PCC, satisfying the following formula:

[0062] Among them, f g is the grid rated frequency, f0 is the PCC frequency before islanding, and f i is the PCC steady-state frequency after island formation, f LC is the resonant frequency of the local load.

[0063] Under this condition, if no disturbance is made to the PCS, the frequency of the PCC remains unchanged before and after the island is formed, and the steady-state frequency of the PCC after the island is formed is equal to the resonant 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, a bilateral reactive power disturbance is made to the PCS, and it is possible to determine whether an island is formed by monitoring the frequency change of the PCC. The change in the frequency of the PCC after the island is formed and the change in the reactive power output of the PCS after the island is formed satisfy the relationship 1:

[0064] Where Δf i is the PCC frequency change after island formation, ΔQ inv is the reactive power change of PCS output after island formation, P inv is the active power output by the PCS before islanding, Q f is the quality factor of the local load, f i is the steady-state frequency of PCC after islanding.

[0065] Thus, when the output power of PCS is highly matched with the load power, the frequency of PCC remains unchanged before and after the island is formed. However, if a certain reactive power disturbance is given to PCS, the disturbance will cause the frequency change of PCC, and the frequency change is proportional to the reactive power disturbance. Therefore, in this embodiment, the PCS is subjected to periodic bilateral reactive power disturbance and the frequency change of PCC is monitored. The frequency change of PCC satisfies the first preset condition for preliminarily confirming that the distributed generation grid-connected system is in an island state when the output power of PCS matches the load power. When the frequency change of PCC satisfies the first preset condition, the periodic bilateral reactive power disturbance of PCS is converted into a unilateral reactive power disturbance, that is, a larger reactive power disturbance in one direction (a larger reactive power disturbance in the positive direction or a larger reactive power disturbance in the negative direction) is applied to PCS, and the frequency of PCC is changed more significantly, thereby breaking the power balance state, further detecting the island state, and improving the sensitivity and accuracy of the island state detection. At the same time, the frequency change of 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 is continued to be performed on the PCS to determine whether the first preset condition is satisfied and to monitor whether an island is formed.

[0066] It should be noted that the reactive power disturbance amount imposed by the unilateral reactive power disturbance is greater than the reactive power disturbance amount imposed by the periodic bilateral reactive power disturbance.

[0067] S300: When the unilateral disturbance frequency difference is greater than a first preset frequency change threshold, determining that the distributed generation grid-connected system is in an island state.

[0068] Specifically, the calculated unilateral disturbance frequency difference is compared 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 taken immediately, for example, triggering the PCS's own over / undervoltage or over / underfrequency protection function 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 performing unilateral reactive power disturbance on the PCS, and switch to periodic bilateral reactive power disturbance on the PCS until the first preset condition is met and unilateral reactive power disturbance is performed again.

[0069] Among them, the unilateral disturbance frequency difference is determined by the following formula:

[0070] Where Δf uni-dir is the unilateral perturbation frequency difference, f is the first frequency of PCC, and f base It is the reference frequency. Generally, PCS can maintain normal operation within the frequency range of 48.5Hz~50.5Hz. If the rated frequency is selected as the reference frequency, the PCS frequency will be at (48.5,50)∪(50,50.5)Hz for a long time, and the frequency deviation will be triggered repeatedly. Therefore, this application sets the reference frequency as the grid rated frequency by default.

[0071] The first preset frequency change threshold is determined by the following formula:

[0072] Where 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.

[0073] Small-amplitude reactive power disturbances in two directions are applied alternately to the PCS, i.e., periodic bilateral reactive power disturbances, and the frequency changes of the PCC are detected in real time, which is helpful to timely discover potential frequency anomalies and improve the accuracy of island state detection. Since a certain reactive power disturbance is given to the PCS when the PCS output power matches the load power, the frequency change of the PCC will be caused by the frequency change of the PCC, and the frequency change is proportional to the reactive power disturbance. Therefore, when the frequency change of the PCC meets the first preset condition, that is, when the distributed generation grid-connected system is preliminarily confirmed to be in an island state when the PCS output power matches the load power, the periodic bilateral reactive power disturbance of the PCS is converted into a unilateral reactive power disturbance, and the frequency of the PCC is changed to a greater extent, thereby breaking the power balance state, further detecting the island state, and improving the sensitivity and accuracy of the island state detection. After the unilateral reactive power disturbance is applied to the PCS, 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 at this time, which helps to reduce the probability of misjudgment of the island state and improve the accuracy of the island state detection. The present invention can effectively detect the islanding state of the distributed generation grid-connected system when the PCS output power matches the load power by setting the first preset condition and combining the periodic bilateral reactive power disturbance with the unilateral reactive power disturbance.

[0074] In some embodiments, 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.

[0075] In at least one embodiment, when an island occurs in the distributed generation grid-connected system, whether the output power of the PCS matches the load power is divided into two working conditions: Working condition 1: When the output power of 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 cycle. Since the PCC frequency change within a single bilateral disturbance cycle may be caused by a variety of factors, such as transient events or noise. It is easy to cause misjudgment if only the PCC frequency change within one cycle is monitored to meet the second preset condition. Therefore, the PCC frequency change within M consecutive bilateral disturbance cycles that meets the second preset condition is used as the trigger condition for unilateral reactive power disturbance. That is, when the PCC frequency change within M consecutive bilateral disturbance cycles meets the second preset condition, it is preliminarily judged that the distributed generation grid-connected system may be in an island state, and a unilateral reactive power disturbance is performed on the PCS. Then, it is further judged whether the distributed generation grid-connected system is in an island state, so as to reduce misjudgments caused by accidental events and improve the robustness and reliability of island state detection.

[0076] Working condition 2: When the output power of PCS does not match the load power, the frequency of PCC will change before and after the island is formed. The steady-state frequency of PCC after the island is formed is not equal to the resonant frequency of the local load. The change in the frequency of PCC after the island is formed and the change in the reactive power output of PCS after the island is formed do not satisfy the relationship 1. Under this working condition, the frequency change caused by the 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 satisfying the second preset condition through the frequency change of PCC within M consecutive bilateral disturbance cycles.

[0077] When the PCS operates in constant power mode, the PCS output power remains approximately the same before and after the island is formed, 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 island is formed and the reactive power consumed by the local load after the island is formed, 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 island is formed can be obtained as follows:

[0078] At that time, Δf was much smaller than f0, and we can get:

[0079] Among them, ΔQ is the reactive power output from the grid side to the PCC, f0 is the PCC frequency before islanding, Δf is the deviation of the PCC frequency before and after islanding, V0 is the PCC voltage before islanding, L is the equivalent inductance of the local load, and C is the equivalent capacitance of the local load.

[0080] It can be seen from the above formula that there is a linear relationship between the PCC frequency deviation before and after the island is formed and the reactive power output from the grid side to the PCC. When the PCS output power does not match the load consumption power, the PCC frequency after the island is formed always shifts in one direction, that is, if the grid side sends reactive power to the PCC before the island is formed, the PCC frequency after the island is formed will increase, and at this time, the PCC frequency is greater than the reference frequency in a short time after the island is formed; if the grid side absorbs reactive power from the PCC before the island is formed, the PCC frequency after the island is formed will decrease, and at this time, the PCC frequency is less than the reference frequency in a short time after the island is formed, and the PCC frequency always shifts in one direction after the island is formed. It is known that after the island occurs, the circuit state of the PCC will generally continue to change for a long time and will not return to normal in a short time. Therefore, when the frequency of PCC in the first time period is greater than the reference frequency and is set as the first preset condition, or when the frequency of PCC in the first time period is less than the reference frequency and is set as the first preset condition, that is, when the PCC frequency continuously deviates in the same direction from the reference frequency for a time period that reaches a threshold value (first time period), the distributed generation grid-connected system may be in an island state, and the bilateral reactive power disturbance to PCS will be converted into a unilateral reactive power disturbance, that is, when the frequency of PCC in the first time period is greater than the reference frequency, or when the frequency of PCC in the first time period is less than the reference frequency, the first preset condition is met and a unilateral reactive power disturbance will be performed on PCS.

[0081] Small-amplitude reactive power disturbances are applied alternately in two directions to the PCS, that is, periodic bilateral reactive power disturbances are continuously performed, and the periodic bilateral reactive power disturbances will not have a significant impact on the power quality in the distributed power generation grid-connected system. When the frequency change of the PCC in each bilateral disturbance cycle in the M consecutive bilateral disturbance cycles meets the second preset condition, it can be preliminarily detected that the distributed power generation grid-connected system is in an island state under the working condition that the output power of the PCS matches the load power, and then turn to unilateral reactive power disturbance for further island detection; at the same time, by monitoring the deviation time of the PCC frequency in the same direction to reach the first duration, that is, the frequency of the PCC in the first duration is greater than the reference frequency, or the frequency of the PCC in the first duration is less than the reference frequency, it can be preliminarily detected that the distributed power generation grid-connected system is in an island state under the working condition that the output power of the PCS does not match the load power, and then turn to unilateral reactive power disturbance for further island detection, that is, when the first preset condition is met, the periodic bilateral reactive power disturbance of the PCS is converted into a unilateral reactive power disturbance. The setting of the first preset condition covers the preliminary judgment of the formation of an island under the two working conditions of whether the PCS output power matches the load power. The island formed under the two working conditions can be effectively detected by setting the dual trigger conditions, which can eliminate the blind spot when detecting the formation of the island, thereby realizing the blind spot-free detection of the island state. In addition, the preliminary detection of the formation of the island is performed by whether the first preset condition is met. When the first preset condition is met, the PCS is subjected to a unilateral reactive power disturbance. Whether the unilateral disturbance frequency difference after the unilateral reactive power disturbance is greater than the first preset frequency change threshold is further judged to determine whether the distributed power generation grid-connected system is in an island state, which helps to reduce the probability of misjudgment of the island state and improves the accuracy of the island state detection.

[0082] In some embodiments, the island detection method also includes: obtaining a first frequency change trend and a first frequency change amount of the common coupling point of the power grid corresponding to the i+1th 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, determining that the frequency change of the common coupling point of the power grid meets the second preset condition, wherein the second preset frequency change amount threshold is less than the first preset frequency change amount threshold.

[0083] In at least one embodiment, the PCC frequency before and after the periodic bilateral reactive power disturbance is applied is recorded, the frequency change trend (increase or decrease) of the i-th bilateral disturbance cycle and the frequency change trend (increase or decrease) of the i+1-th bilateral disturbance cycle are determined, and the frequency change amount of the i+1-th bilateral disturbance cycle is calculated. When the frequency change trend of the i+1-th bilateral disturbance cycle is opposite to the frequency change trend of the i-th bilateral disturbance cycle, that is, the frequency of the i-th bilateral disturbance cycle increases and the frequency of the i+1-th bilateral disturbance cycle decreases, or the frequency of the i-th bilateral disturbance cycle increases and the frequency of the i+1-th bilateral disturbance cycle decreases, and when the frequency change amount of the i+1-th bilateral disturbance cycle is greater than the second preset frequency change amount threshold, this i+1-th bilateral disturbance cycle meets the second preset condition and is recorded as a valid disturbance. The valid disturbance is counted and counted, and after the valid disturbance is detected, the number of valid disturbances is increased by 1, and the next cycle of bilateral reactive power disturbance is continued. If an ineffective disturbance occurs after multiple consecutive effective disturbances (not meeting the second preset condition), the number of effective disturbances is reset and recounted, and the next cycle of bilateral reactive power disturbances is continued. If the effective disturbance occurs M times in a row, the periodic bilateral reactive power disturbance to the PCS is converted into a unilateral reactive power disturbance to prevent the false triggering of unilateral reactive power disturbances due to normal fluctuations in the grid frequency.

[0084] Among them, the second preset frequency change threshold is set to be smaller than the first preset frequency change threshold. When the PCS output power is highly matched with the load consumption power, the frequency change of the PCC may be very small. By setting a lower second preset frequency change threshold, these tiny frequency changes can be captured, thereby discovering potential islanding states earlier. Subsequently, the higher first preset frequency change threshold is used to finally confirm whether the distributed generation grid-connected system is in an islanding state. This two-level threshold setting realizes multi-level verification, improves the sensitivity of islanding state detection, reduces the probability of misjudgment of islanding state, and is conducive to eliminating the blind spot of islanding state detection.

[0085] Assume that the frequency change of the i+1th bilateral perturbation cycle (the first frequency change) is known to be Δf bi-dir(i+1) , the frequency change of the i-th bilateral perturbation period is Δf bi-dir(i) , the second preset frequency change threshold is Δf bi-dir,set , the i+1th bilateral perturbation period is a valid perturbation, then the following formula is satisfied:

[0086] In the case where the PCS output power is highly matched with the load consumption power, by determining whether the first frequency change trend corresponding to the acquired PCC in the i+1th bilateral disturbance period is opposite to the second frequency change trend corresponding to the i-th bilateral disturbance period, and comparing the first frequency change amount with the second preset frequency change amount threshold, it is judged whether the frequency change of the PCC meets the second preset condition, which can effectively capture slight frequency changes and distinguish whether the distributed power generation grid-connected system is in normal operation or is likely to be in an island state, thereby discovering potential island states earlier and detecting island states more accurately, which is conducive to eliminating the detection blind spot of the island state.

[0087] In some embodiments, reference Figure 4 The periodic bilateral reactive power disturbance of the energy storage converter in the distributed power generation grid-connected system includes: determining the reactive power disturbance amount in the positive direction as the first disturbance amount and the reactive power disturbance amount in the negative direction as the second disturbance amount, wherein the first disturbance amount is determined by the following formula:

[0088] The second disturbance amount is determined by the following formula:

[0089] 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 rated frequency of the power grid; and performing the periodic bilateral reactive power disturbance on the energy storage converter according to the first disturbance amount and the second disturbance amount.

[0090] In at least one embodiment, when the distributed generation grid-connected system is operating normally, the PCC frequency is consistent with the grid frequency due to the clamping effect of the grid. When the active power or reactive power output from the grid side to the PCC is large before and after the island is formed, the island 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 island state, so that protective measures can be taken in time to ensure the safe and stable operation of the system. Considering the worst working conditions, the PCS operates at a unity power factor and satisfies the following formula:

[0091] Among them, ΔP is the active power output from the grid side to the PCC before and after the island is formed, ΔQ is the reactive power output from the grid side to the PCC before and after the island is formed, and j is a complex unit. Then the PCS and the load form a power balance, and the following formula is satisfied at this time:

[0092] Among them, f g is the grid rated frequency, f0 is the PCC frequency before islanding, and f i is the PCC steady-state frequency after island formation, f LC is the resonant frequency of the load.

[0093] According to the above formula and the power output of PCS when the distributed generation grid-connected system is in an island state, it can be known that the frequency offset caused by the disturbance of PCC and the reactive power disturbance output of PCS when the distributed generation grid-connected system is in an island state satisfy the following formula:

[0094] Among them, Q dis is the reactive power disturbance of PCS output after island formation, 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 grid rated frequency, Δf dis is the preset frequency offset.

[0095] First, the first disturbance in the positive direction and the second disturbance in the negative direction are determined. Then, the PCS is subjected to periodic bilateral reactive power disturbances based on these two disturbances to produce measurable frequency changes in the PCC, which is conducive to timely detection of islanding phenomena and prevention of unnecessary safety risks and equipment damage.

[0096] In some embodiments, reference Figure 4 , 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, the unilateral reactive power disturbance of 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 common coupling point of the power grid and the rated frequency of the power grid, wherein the first frequency is the frequency of the common coupling point 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.

[0097] In at least one embodiment, based on the requirements and test standards of the distributed power generation grid-connected system in an island state, a reasonable target disturbance duration of the unilateral reactive power disturbance is set to ensure that the unilateral reactive power disturbance can produce significant frequency changes in a sufficient period of time to facilitate island state detection.

[0098] The target disturbance amount is determined according to the following formula to ensure that the unilateral reactive power disturbance can effectively affect the frequency of the PCC:

[0099] Among them, Q uni-ref is the target disturbance amount of the unilateral reactive power disturbance at the disturbance time t, P ref is the active power reference value, and K is the expected frequency change rate when there is a preset unilateral reactive power disturbance.

[0100] The target disturbance direction is determined according to the first frequency of the PCC and the rated frequency of the power grid, and a unilateral reactive power disturbance is performed on the PCS according to the target disturbance duration, the target disturbance amount and the target disturbance direction. Specifically, within the target disturbance duration, the target disturbance amount in the target disturbance direction is applied to the PCS.

[0101] Among them, the target disturbance duration of the unilateral reactive power disturbance is set to at least meet the requirement that the PCS should have the ability to quickly detect the island and immediately disconnect from the grid, and the anti-islanding protection action time is no more than 2s, that is, it meets the following formula:

[0102] Among them, 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 time length, and M is the number of periodic bilateral reactive power disturbances.

[0103] By reasonably setting the disturbance parameters of unilateral reactive power disturbance, namely the target disturbance duration, target disturbance amount and target disturbance direction, unilateral reactive power disturbance is performed on PCS. This can not only detect the islanding state without affecting the normal operation of the distributed power generation grid-connected system, thus avoiding system instability or equipment damage caused by excessive disturbance, but also produce significant frequency changes in PCC in a short time, thus improving the sensitivity and accuracy of islanding state detection.

[0104] In some embodiments, determining the target disturbance duration of the unilateral reactive power disturbance includes: 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 cycles in M ​​consecutive bilateral disturbance cycles satisfies a second preset condition, determining the target disturbance duration as the second duration, wherein the second duration is a reference time constant.

[0105] It should be noted that the output power of the power generation device does not match the load power. At this time, the distributed power generation grid-connected system enters an island state, 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 by only monitoring the frequency change of the PCC.

[0106] In at least one embodiment, when the frequency of the PCC in the first duration is less than the reference frequency or the frequency of the PCC in the first duration is greater than the reference frequency, the target disturbance duration of the unilateral reactive power disturbance is determined by the following formula:

[0107] Among them, 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 grid, f base is the reference frequency.

[0108] It can be seen from the above formula that the greater the degree to which the PCC frequency deviates from the reference frequency, the shorter the target disturbance duration of the unilateral reactive power disturbance will be; the smaller the degree to which the PCC frequency deviates from the reference frequency, the longer the target disturbance duration of the unilateral reactive power disturbance will be, which can ensure that when the PCC frequency deviates significantly, the next step can be carried out more quickly to confirm whether the distributed generation grid-connected system is indeed in an island state.

[0109] The second preset condition is that during the periodic bilateral reactive power disturbance, the first frequency change trend of the grid common coupling point corresponding to the i+1 bilateral disturbance period is opposite to the second frequency change trend corresponding to the i bilateral disturbance period, and the first frequency change amount corresponding to the grid common coupling point in the i+1 bilateral disturbance period is greater than the second preset frequency change amount threshold. When the frequency change of the PCC corresponding to each bilateral disturbance period in the consecutive M bilateral disturbance periods meets the second preset condition, the target disturbance duration of the unilateral reactive power disturbance is determined by the following formula:

[0110] Among them, T uni-dir is the target disturbance duration of the unilateral reactive power disturbance, and T is the second duration (reference time constant).

[0111] It can be seen from the above formula that the target disturbance duration of the unilateral reactive power disturbance is always maintained at a fixed value, that is, the second duration.

[0112] The target disturbance duration of unilateral reactive power disturbance is precisely controlled by corresponding to the two working conditions of whether the PCS output power matches the load power consumption, making island detection more accurate. When the PCC frequency changes greatly, the target disturbance duration is reduced to enable rapid response; while when the PCC frequency changes relatively smoothly, the standard target disturbance duration (second duration) is maintained to avoid unnecessary interference, effectively balancing the relationship between detection speed and accuracy, thereby achieving more reliable island detection.

[0113] When the frequencies of the grid common coupling points within the first time period are all greater than the reference frequency, or the frequencies of the grid common coupling points within the first time period are all less than the reference frequency, and when the frequency changes of the grid common coupling points corresponding to each bilateral disturbance period in M ​​consecutive bilateral disturbance periods meet the second preset condition, the target disturbance duration is determined according to the reference frequency, the grid rated frequency and the second time period.

[0114] By monitoring the comparison between the frequency of PCC and the reference frequency within the first time period, the deviation of the first frequency of PCC can be quickly determined. If the frequency of PCC is always higher than the reference frequency, or the frequency of PCC is always lower than the reference frequency, it means that when the PCS output power does not match the load power before and after the island occurs, the distributed generation grid-connected system may have been in an island state or there is a potential frequency anomaly. At this time, the target disturbance duration is determined as the second duration (reference time constant) to ensure that the unilateral reactive power disturbance produces a significant frequency change within a reasonable time, thereby improving the sensitivity and accuracy of detection. When the PCC frequency change within the bilateral disturbance period of M consecutive periodic bilateral reactive power disturbances meets the second preset condition, it means that when the PCS output power matches the load power before and after the island occurs, the distributed generation grid-connected system may have been in an island state or there is a potential frequency anomaly. At this time, the target disturbance duration is determined according to the reference frequency, the rated frequency of the power grid and the second duration to ensure that the unilateral reactive power disturbance produces a significant frequency change within a reasonable time, thereby improving the sensitivity and accuracy of detection.

[0115] In some embodiments, determining the target disturbance direction according to the first frequency of the common coupling point of the power grid and the rated frequency of the power grid includes: when the first frequency is less than the rated frequency of the power grid, determining the target disturbance direction as a positive direction; when the first frequency is greater than or equal to the rated frequency of the power grid, determining the target disturbance direction as a negative direction.

[0116] In at least one embodiment, when the first frequency of the PCC is less than the rated frequency of the power grid, the target disturbance direction of the unilateral reactive power disturbance is determined to be in the positive direction. By applying a unilateral reactive power disturbance in the positive direction to the PCS, the frequency of the PCC can be increased to be closer to the rated frequency of the power grid, which helps to produce significant frequency changes in a short time and facilitates the detection of the island state. When the first frequency of the PCC is greater than or equal to the rated frequency of the power grid, the target disturbance direction of the unilateral reactive power disturbance is determined to be in the negative direction. By applying a unilateral reactive power disturbance in the negative direction to the PCS, the frequency of the PCC can be reduced to be away from the rated frequency of the power grid, which also helps to produce significant frequency changes in a short time and facilitates the detection of the island state.

[0117] By comparing the first frequency of the PCC with the rated frequency of the power grid, the positive or negative direction of the target disturbance of the unilateral reactive power disturbance is determined, which helps to produce significant frequency changes in a short time, facilitates the detection of whether the distributed generation grid-connected system is in an island state, and 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.

[0118] In some embodiments, reference Figure 4The island detection method further includes: when the frequency change of the common coupling point of the power grid during the periodic bilateral reactive power disturbance satisfies the first preset condition, updating the reference frequency to the first frequency of the common coupling point of the power grid, wherein the first frequency is the frequency of the common coupling point 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.

[0119] In at least one embodiment, when the frequency of the PCC is less than the reference frequency within the first time period, or when the frequency of the PCC is greater than the reference frequency within the first time period, or when the frequency change of the PCC corresponding to each bilateral disturbance period in the M consecutive bilateral disturbance periods meets the second preset condition, the periodic bilateral reactive power disturbance of the PCS is converted into a unilateral reactive power disturbance, and the reference frequency is updated. The updated reference frequency is the first frequency of the PCC, which is used to determine the unilateral disturbance frequency difference, and is also used to detect whether the distributed generation grid-connected system is in an island state next time.

[0120] When the first preset condition is met, the reference frequency is updated to the first frequency of the PCC, which can calibrate the parameters of the island state detection in real time to ensure the reliability of the detection results. Especially when the grid frequency fluctuates greatly, timely updating of the reference frequency can reduce the false alarm rate and improve the accuracy of island state detection, thereby improving the safety and reliability of the distributed generation grid-connected system.

[0121] In some embodiments, before determining that the frequency change of the grid common coupling point during the periodic bilateral reactive power disturbance satisfies the first preset condition, the island detection method further includes acquiring the frequency of the grid common coupling point collected in real time to obtain a second frequency, wherein the second frequency represents the frequency of the grid common coupling point at a second moment, and the second moment is the current moment; when the second frequency is greater than the third frequency, and the third frequency is greater than or equal to the fourth frequency, acquiring 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 duration, determining that the frequency of the grid common coupling point 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, controlling the timer to be cleared; wherein 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, and the third moment is a previous acquisition moment of the second moment; and the fourth moment is a previous acquisition moment of the third moment.

[0122] It should be noted that the timer starts timing when the system starts running.

[0123] In at least one embodiment, before determining that the first preset condition is met and the bilateral reactive power disturbance of the PCS is converted into a unilateral reactive power disturbance, the real-time collected PCC frequency is obtained, and the real-time collected PCC frequency data is stored in a local cache or 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 obtained and 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 real-time collected PCC frequency continues to be 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 continues to be acquired 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.

[0124] Among them, the second moment is the current moment, the third moment is the collection moment before the second moment; and the fourth moment is the collection moment before the third moment.

[0125] By collecting the frequency data of 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 size of the first frequency, the second frequency and the third frequency, it is ensured that the PCC frequency change is verified at multiple time points, and the change trend of the PCC frequency can be more accurately identified. If the PCC frequency increases with the growth of time, that is, the first frequency is greater than the second frequency, and the second frequency is greater than the third frequency, then the time of continuous growth is obtained, that is, the current offset time. By judging the size of the current offset time and the first time length, it is determined whether the second moment satisfies the first preset condition that the frequency of the PCC is greater than the reference frequency within the first time length, ensuring that the preliminary confirmation of the island state is only performed when the frequency continues to change in one direction and the change is stable, reducing misjudgment caused by short-term fluctuations.

[0126] In some embodiments, the island detection method also includes obtaining the count value of the timer at the current moment to obtain the current offset time when the second frequency is less than the third frequency and the third frequency is less than or equal to the fourth frequency; determining that the frequency of the common coupling point of the power grid within the first time period is less than the reference frequency when the current offset time is greater than or equal to the first time period; controlling the timer to be cleared when the second frequency is less than the third frequency and the third frequency is greater than the fourth frequency; and controlling the timer to be cleared when the second frequency is equal to the third frequency.

[0127] In at least one embodiment, before determining that the first preset condition is met and the bilateral reactive power disturbance of the PCS is converted into a unilateral reactive power disturbance, the real-time collected PCC frequency is obtained, and the real-time collected PCC frequency data is stored in a local cache or 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 obtained and 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 less than the reference frequency; when the current offset time is less than the first duration, the timer value is cleared, and the real-time collected PCC frequency continues to be 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 continues to be acquired 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.

[0128] Among them, the second moment is the current moment, the third moment is the collection moment before the second moment; and the fourth moment is the collection moment before the third moment.

[0129] By collecting the frequency data of 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 size of the first frequency, the second frequency and the third frequency, it is ensured that the PCC frequency change is verified at multiple time points, and the change trend of the PCC frequency can be more accurately identified. If the PCC frequency decreases with the increase of time, that is, the first frequency is less than the second frequency, and the second frequency is less than the third frequency, then the time of continuous growth is obtained, that is, the current offset time. By judging the size of the current offset time and the first time length, it is determined whether the second moment satisfies the first preset condition that the frequency of the PCC is less than the reference frequency within the first time length, ensuring that the preliminary confirmation of the island state is only performed when the frequency continues to change in one direction and the change is stable, reducing misjudgment caused by short-term fluctuations.

[0130] One or more embodiments of the present application also disclose an island detection device 500. Figure 5 , the island detection device 500 comprises: The first disturbance module 510 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; The second disturbance module 520 is used to perform unilateral reactive power disturbance on the energy storage converter 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, and determine the frequency change amount of the common coupling point of the power grid after the unilateral reactive power disturbance to obtain the unilateral disturbance frequency difference; The first determination module 530 is configured 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 variation threshold.

[0131] In some embodiments, the first preset condition of the second disturbance module 520 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.

[0132] In some embodiments, the island detection device 500 also includes a first acquisition module and a second determination module: the first acquisition module is used to obtain the first frequency change trend and the first frequency change amount of the common coupling point of the power grid corresponding to the i+1th bilateral disturbance period during the periodic bilateral reactive power disturbance, and the second frequency change trend of the common coupling point of the power grid corresponding to the i-th bilateral disturbance period; the second determination module is used to determine that the frequency change of the common coupling point of the power grid meets 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, wherein the second preset frequency change amount threshold is less than the first preset frequency change amount threshold.

[0133] In some embodiments, the first disturbance module 510 is specifically used to determine the reactive power disturbance amount in the positive direction as the first disturbance amount and the reactive power disturbance amount in the negative direction as the second disturbance amount, respectively, wherein the first disturbance amount is determined by the following formula:

[0134] The second disturbance amount is determined by the following formula:

[0135] 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 rated frequency of the power grid; and performing the periodic bilateral reactive power disturbance on the energy storage converter according to the first disturbance amount and the second disturbance amount.

[0136] In some embodiments, the second disturbance module 520 is specifically used to determine the target disturbance duration of the unilateral reactive power disturbance; determine the target disturbance amount of the unilateral reactive power disturbance; determine the target disturbance direction according to the first frequency of the common coupling point 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.

[0137] In some embodiments, the second disturbance module 520 includes a third determination module: the third determination module is used to determine the target disturbance duration according to the reference frequency, the rated frequency of the power grid and the second duration when the frequency of the power grid common coupling point is greater than the reference frequency within the first duration, or when the frequency of the power grid common coupling point is less than the reference frequency within the first duration; when the frequency change of the power grid common coupling point corresponding to each of the bilateral disturbance cycles in M ​​consecutive bilateral disturbance cycles meets the second preset condition, the target disturbance duration is determined as the second duration, wherein the second duration is a reference time constant.

[0138] In some embodiments, the second disturbance module 520 also includes a fourth determination module: the fourth determination module is used to determine that the target disturbance direction is a positive direction when the first frequency is less than the rated frequency of the power grid; and to determine that the target disturbance direction is a negative direction when the first frequency is greater than or equal to the rated frequency of the power grid.

[0139] In some embodiments, the island detection device 500 also includes an update module: the update module is used to update the reference frequency to the first frequency of the grid common coupling point when the frequency change of the grid common coupling point during the periodic bilateral reactive power disturbance meets the first preset condition, wherein the first frequency is the frequency of the grid common coupling point at a first moment, and the first moment is the moment when it is determined that the first preset condition is met.

[0140] In some embodiments, before determining that the frequency change of the grid common coupling point during the periodic bilateral reactive power disturbance satisfies the first preset condition, the island detection device 500 further includes a second acquisition module and a fifth determination module: the second acquisition module is used to acquire the frequency of the grid common coupling point acquired in real time to obtain a second frequency, wherein the second frequency represents the frequency of the grid common coupling point at a second moment, and the second moment is the current moment; the fifth determination module is used to, when the second frequency is greater than the third frequency and the third frequency is greater than or equal to the fourth frequency, acquire 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 duration, determine that the frequency of the grid common coupling point 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; wherein 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, and 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.

[0141] In some embodiments, the island detection device 500 also includes a sixth determination module: the sixth determination module is used to obtain the count value of the timer at the current moment to obtain the current offset time when the second frequency is less than the third frequency and the third frequency is less than or equal to the fourth frequency; when the current offset time is greater than or equal to the first duration, determine 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, control the timer to be cleared; when the second frequency is equal to the third frequency, control the timer to be cleared.

[0142] One or more embodiments of the present application also disclose an electronic device 600. Figure 6 The electronic device 600 includes a memory 610 and a processor 620; the memory 610 is used to store a computer program; the processor 620 is used to implement the above-mentioned island detection method when executing the computer program.

[0143] In other words, 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 perform the following operations when executing the computer program: performing periodic bilateral reactive power disturbance on an energy storage converter in a distributed power generation grid-connected system, wherein the i-th bilateral disturbance cycle in the periodic bilateral reactive power disturbance is opposite to the disturbance direction of the i+1-th bilateral disturbance cycle, and i is an integer greater than or equal to 1; performing unilateral reactive power disturbance on the energy storage converter when the frequency change of a grid common coupling point during the periodic bilateral reactive power disturbance satisfies a first preset condition, and determining the frequency change of the grid common coupling point after the unilateral reactive power disturbance to obtain a unilateral disturbance frequency difference; determining that the distributed power generation grid-connected system is in an island state when the unilateral disturbance frequency difference is greater than a first preset frequency change threshold.

[0144] One or more embodiments of the present application further disclose a computer-readable storage medium. The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the island detection method described above is implemented.

[0145] In other words, a non-volatile computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the processor performs the following operations: performing periodic bilateral reactive power disturbance on an energy storage converter in a 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; performing unilateral reactive power disturbance on the energy storage converter when the frequency change of a common coupling point of a power grid during the periodic bilateral reactive power disturbance satisfies a first preset condition, and determining the frequency change amount of the common coupling point of the power grid after the unilateral reactive power disturbance to obtain a unilateral disturbance frequency difference; and determining that the distributed power generation grid-connected system is in an island state when the unilateral disturbance frequency difference is greater than a first preset frequency change amount threshold.

[0146] The above describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited by the above embodiments, and the above embodiments and the specification only describe the principles of the present application. The present application may have various changes and improvements without departing from the spirit and scope of the present application, and these changes and improvements fall within the scope of the present application for which protection is sought. The scope of protection claimed by the present application is defined by the attached 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; When the frequency change of the common coupling point of the power grid during the periodic bilateral reactive power disturbance satisfies the first preset condition, performing a unilateral reactive power disturbance on the energy storage converter, 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; 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 includes: Determining a 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: Determining the target disturbance duration of the unilateral reactive power disturbance includes: 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 common coupling point of the power grid corresponding to each of the bilateral disturbance cycles in M ​​consecutive bilateral disturbance cycles meets the second preset condition, the target disturbance duration is determined as the second duration, wherein the second duration is a reference time constant.

7. 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.

8. 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.

9. 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.

10. The island detection method according to claim 9, 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.

11. 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; The second disturbance module is used to perform unilateral reactive power disturbance on the energy storage converter 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, and determine the frequency change amount of the common coupling point of the power grid after the unilateral reactive power disturbance to obtain the unilateral disturbance frequency difference. 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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