Methods, devices, and electronic equipment for identifying island phenomena

Abstract

This application provides a method, apparatus, and electronic device for determining islanding phenomena. The method includes: acquiring the frequency of the power grid to obtain an initial frequency; determining a reference frequency if the initial frequency is greater than a first threshold and less than a second threshold; calculating the difference between the initial frequency and the reference frequency to obtain a frequency offset; adjusting the direction of the reactive power in the power grid to the disturbance direction if the absolute value of the frequency offset is greater than or equal to a preset threshold; determining the disturbance pulse width based on the frequency offset and adjusting the pulse width of the reactive power in the power grid to the disturbance pulse width; determining the disturbance amplitude based on the frequency offset and adjusting the amplitude of the reactive power in the power grid to the disturbance amplitude; after adjusting to the disturbance amplitude, acquiring the current frequency of the power grid; and determining that islanding phenomena have occurred in the power grid if the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold. This application solves the problem of poor performance in existing islanding detection methods.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of detection of grid island phenomenon, in particular, to a method and device for determining island phenomenon, computer readable storage medium and electronic equipment. BACKGROUND

[0002] The distributed power generation system represented by photovoltaic and wind power generation has the characteristics of clean and environmental protection, flexible power generation. The island effect of grid-connected distributed power system is one of the major technical problems in its development and application. The island effect refers to the independent system formed by the distributed power system with local load running when the distributed power system is disconnected from the grid due to faults or maintenance, etc. Unpredictable island of distributed power generation system may cause electrical damage to user equipment, affect power quality, and even endanger human safety and equipment safety. Taking island detection is a favorable measure for island protection.

[0003] Passive island detection usually monitors system parameters of grid-connected inverters, such as voltage, frequency, phase and harmonic distortion. When one or more of these parameters deviate from the allowed range, it is considered that an island phenomenon occurs. The passive island detection method has a relatively large undetectable area. Active detection methods can reduce the undetectable area by introducing appropriate disturbances to observe whether the parameters exceed the normal range. The method based on reactive power disturbance can be conveniently applied to the current mainstream d, q-axis current decoupling control system. Some of these methods have a long disturbance interval, which has the defect of not being able to respond in time to unpredictable island phenomena. The single long disturbance time also affects the steady-state power quality. Some methods have blind spots when the island resonance frequency is close to the rated frequency of the grid. Some methods are prone to misjudging normal fluctuations in the grid as an island. In addition, existing methods generally cannot directly support the low voltage ride-through of inverters and support the reactive power of the grid.

[0004] Therefore, in order to meet the development needs of the distributed power system, it is an important problem to be solved to establish a reliable and efficient island detection method with high steady-state power quality. SUMMARY

[0005] The main purpose of the present application is to provide a method and device for determining island phenomenon, computer readable storage medium and electronic equipment, to at least solve the problem of poor effect of the island detection method in the prior art.

[0006] To achieve the above objectives, according to one aspect of this application, a method for determining islanding phenomena is provided, comprising: applying a steady-state positive and negative bidirectional reactive power disturbance to a power grid; acquiring the frequency of the power grid to obtain an initial frequency; determining a reference frequency when the initial frequency is greater than a first threshold and less than a second threshold, wherein the first threshold is an underfrequency protection frequency and the second threshold is an overfrequency protection frequency; the steady-state positive and negative bidirectional reactive power disturbance is used to detect islanding phenomena in the power grid; calculating the difference between the initial frequency and the reference frequency to obtain a frequency offset; and maintaining the power grid frequency at a set frequency when the absolute value of the frequency offset is less than a preset threshold. The reactive power of the grid is the steady-state positive and negative bidirectional reactive power disturbance. When the absolute value of the frequency offset is greater than or equal to the preset threshold, the direction of the reactive power of the grid is adjusted to the disturbance direction. The pulse width of the disturbance is determined according to the frequency offset, and the pulse width of the reactive power of the grid is adjusted to the disturbance pulse width. The amplitude of the disturbance is determined according to the frequency offset, and the amplitude of the reactive power of the grid is adjusted to the disturbance amplitude. After adjusting to the disturbance amplitude, the current frequency of the grid is obtained. When the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold, it is determined that the grid has experienced the islanding phenomenon.

[0007] Optionally, if the absolute value of the frequency offset is greater than or equal to a preset threshold, adjusting the direction of the reactive power of the power grid to the direction of disturbance includes: if the frequency offset is greater than the preset threshold, adjusting the positive disturbance in the steady-state positive and negative bidirectional reactive power disturbance to a negative disturbance; if the frequency offset is less than the negative preset threshold, adjusting the negative disturbance in the steady-state positive and negative bidirectional reactive power disturbance to the positive disturbance.

[0008] Optionally, after adjusting to the disturbance amplitude, obtaining the current frequency of the power grid includes: after adjusting to the disturbance amplitude, receiving an islanding detection reactive power command and a reactive power dispatch command, wherein the islanding detection reactive power command is a command indicating islanding detection of the power grid, and the reactive power dispatch command is a command indicating dispatching the reactive power of the power grid; obtaining the AC voltage of the power grid, and if the AC voltage is within a preset voltage range, receiving a reactive voltage ride-through command, wherein the reactive voltage ride-through command is a command indicating voltage ride-through of the power grid; and obtaining the current frequency of the power grid upon receiving the islanding detection reactive power command, the reactive power dispatch command, and the reactive voltage ride-through command.

[0009] Optionally, the power grid includes an inverter, and the method further includes: after determining that the power grid has experienced the islanding phenomenon, disconnecting the inverter; if the initial frequency is less than or equal to the first threshold, determining that the power grid is underfrequency protection and disconnecting the inverter; if the initial frequency is greater than or equal to the second threshold, determining that the power grid is overfrequency protection and disconnecting the inverter.

[0010] Optionally, determining the reference frequency includes: an acquisition step: acquiring the frequency of the power grid for a predetermined number of cycles within a past time period to obtain multiple historical frequencies, wherein the past time period is the time period before the time corresponding to the initial frequency is acquired and adjacent to the time corresponding to the initial frequency; a calculation step: calculating the average value of the multiple historical frequencies to obtain the reference frequency.

[0011] Optionally, the method further includes: if the current frequency is greater than the first threshold and less than the second threshold, determining the usage time of the reference frequency; if the usage time is greater than a preset time, repeating the acquisition step and the calculation step to update the reference frequency.

[0012] Optionally, determining the disturbance pulse width based on the frequency offset includes: obtaining a first mapping relationship between the frequency offset and the disturbance pulse width, and determining the disturbance pulse width corresponding to the frequency offset based on the first mapping relationship; determining the disturbance amplitude based on the frequency offset includes: obtaining a second mapping relationship between the frequency offset and the disturbance amplitude, and determining the disturbance amplitude corresponding to the frequency offset based on the second mapping relationship.

[0013] According to another aspect of this application, an islanding detection device is provided, comprising: a first determining unit, configured to apply a steady-state positive and negative bidirectional reactive power disturbance to a power grid, acquire the frequency of the power grid, obtain an initial frequency, and determine a reference frequency if the initial frequency is greater than a first threshold and less than a second threshold, wherein the first threshold is an underfrequency protection frequency and the second threshold is an overfrequency protection frequency, and the steady-state positive and negative bidirectional reactive power disturbance is used to detect islanding in the power grid; and a first adjusting unit, configured to calculate the difference between the initial frequency and the reference frequency to obtain a frequency offset, and maintain the reactive power of the power grid if the absolute value of the frequency offset is less than a preset threshold. The rate is the steady-state positive and negative bidirectional reactive power disturbance. If the absolute value of the frequency offset is greater than or equal to the preset threshold, the direction of the reactive power of the power grid is adjusted to the disturbance direction. The second adjustment unit is used to determine the disturbance pulse width according to the frequency offset and adjust the pulse width of the reactive power of the power grid to the disturbance pulse width. The second determination unit is used to obtain the current frequency of the power grid after adjusting to the disturbance amplitude. If the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold, the power grid is determined to have the islanding phenomenon.

[0014] According to another aspect of this application, a computer-readable storage medium is provided, the computer-readable storage medium including a stored program, wherein, when the program is executed, it controls the device on which the computer-readable storage medium is located to perform any of the determination methods described above.

[0015] According to another aspect of this application, an electronic device is provided, comprising: one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, the one or more programs including methods for performing any of the aforementioned determining methods.

[0016] By applying the technical solution of this application, the frequency of the power grid is obtained to obtain an initial frequency. If the initial frequency is greater than a first threshold and less than a second threshold, a reference frequency is determined. The difference between the initial frequency and the reference frequency is calculated to obtain a frequency offset. If the absolute value of the frequency offset is greater than or equal to a preset threshold, the direction of the reactive power of the power grid is adjusted to the disturbance direction. The disturbance pulse width is determined based on the frequency offset, and the pulse width of the reactive power of the power grid is adjusted to the disturbance pulse width. The disturbance amplitude is determined based on the frequency offset, and the amplitude of the reactive power of the power grid is adjusted to the disturbance amplitude. After adjusting to the disturbance amplitude, the current frequency of the power grid is obtained. If the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold, islanding is determined to have occurred in the power grid. Compared with existing islanding detection methods, which may affect normal power quality and are inaccurate in judging islanding phenomena, this application can efficiently complete islanding detection for the most severe operating conditions, reduce the impact on steady-state power quality, easily accommodate bidirectional reactive power dispatch and low-voltage ride-through, and accurately detect islanding phenomena in the power grid. Attached Figure Description

[0017] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0018] Figure 1 A hardware structure block diagram of a mobile terminal providing a method for determining the islanding phenomenon according to an embodiment of this application is shown.

[0019] Figure 2 A flowchart illustrating a method for determining an islanding phenomenon provided by an embodiment of this application is shown.

[0020] Figure 3 This illustration shows a schematic diagram of the first mapping relationship between the disturbance pulse width and the frequency offset in a method for determining the islanding phenomenon provided by an embodiment of this application;

[0021] Figure 4 This illustration shows a second mapping relationship between disturbance amplitude and frequency offset in a method for determining islanding phenomena provided by an embodiment of this application;

[0022] Figure 5 The illustration shows a schematic diagram of the overall three-dimensional frequency feedback and reactive power disturbance when the frequency drops in a method for determining islanding phenomena provided by an embodiment of this application.

[0023] Figure 6 The illustration shows a schematic diagram of the overall three-dimensional frequency feedback and reactive power disturbance when the frequency drops in a method for determining islanding phenomena provided by an embodiment of this application.

[0024] Figure 7 The diagram illustrates the overall control implementation of the island detection method in an embodiment of this application for determining island phenomena.

[0025] Figure 8 A schematic diagram illustrating a specific method for determining islanding phenomena provided by an embodiment of this application is shown;

[0026] Figure 9 The diagram shows a simulation verification system of the island detection method in a specific method for determining island phenomena provided in an embodiment of this application.

[0027] Figure 10 The diagram illustrates a system frequency when the islanding resonant frequency is slightly lower than the power frequency, as provided in a specific method for determining islanding phenomena according to an embodiment of this application.

[0028] Figure 11 The illustration shows a schematic diagram of reactive power when the islanding resonant frequency is slightly lower than the power frequency in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0029] Figure 12 The diagram illustrates the steady-state grid connection point voltage harmonic distribution in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0030] Figure 13 The diagram illustrates a system frequency when a steady-state normal frequency change is misjudged in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0031] Figure 14 The illustration shows a schematic diagram of reactive power when a steady-state normal frequency change is misjudged in a specific method for determining islanding phenomena provided by an embodiment of this application;

[0032] Figure 15 The diagram illustrates the system frequency during reactive power scheduling in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0033] Figure 16 The diagram illustrates reactive power during reactive power dispatch in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0034] Figure 17 The diagram illustrates the effective value of the grid connection point voltage in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0035] Figure 18 The diagram illustrates the system frequency during low-voltage ride-through in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0036] Figure 19The diagram illustrates a reactive power command during low voltage ride-through in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0037] Figure 20 The diagram illustrates the reactive power during low-voltage ride-through in a specific method for determining islanding phenomena provided by an embodiment of this application.

[0038] Figure 21 A structural block diagram of an islanding phenomenon determination device provided by an embodiment of this application is shown.

[0039] The above figures include the following reference numerals:

[0040] 102. Processor; 104. Memory; 106. Transmission device; 108. Input / output device. Detailed Implementation

[0041] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0042] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0043] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0044] For ease of description, the following explains some of the nouns or terms used in the embodiments of this application:

[0045] Island phenomenon: also known as island effect, refers to the independent system formed when a distributed power system goes offline due to faults or maintenance, and the local load is driven by the distributed power system.

[0046] As described in the background section, existing island detection methods are ineffective. To address this issue, embodiments of this application provide a method, apparatus, computer-readable storage medium, and electronic device for determining island phenomena.

[0047] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0048] The methods and embodiments provided in this application can be executed on a mobile terminal, computer terminal, or similar computing device. Taking running on a mobile terminal as an example, Figure 1 This is a hardware structure block diagram of a mobile terminal for a method of determining the islanding phenomenon according to an embodiment of the present invention. For example... Figure 1 As shown, a mobile terminal may include one or more ( Figure 1 Only one is shown in the diagram. A processor 102 (which may include, but is not limited to, a microprocessor MCU or a programmable logic device FPGA, etc.) and a memory 104 for storing data are also shown. The mobile terminal may further include a transmission device 106 for communication functions and an input / output device 108. Those skilled in the art will understand that... Figure 1 The structure shown is for illustrative purposes only and does not limit the structure of the mobile terminal described above. For example, the mobile terminal may also include components that are more... Figure 1 The more or fewer components shown, or having the same Figure 1 The different configurations shown.

[0049] The memory 104 can be used to store computer programs, such as application software programs and modules, like the computer program corresponding to the method for determining the islanding phenomenon in this embodiment of the invention. The processor 102 executes various functional applications and data processing by running the computer program stored in the memory 104, thereby implementing the above-described method. The memory 104 may include high-speed random access memory and may also include non-volatile memory, such as one or more magnetic storage devices, flash memory, or other non-volatile solid-state memory. In some instances, the memory 104 may further include memory remotely located relative to the processor 102, and these remote memories can be connected to the mobile terminal via a network. Examples of the aforementioned networks include, but are not limited to, the Internet, corporate intranets, local area networks, mobile communication networks, and combinations thereof. The transmission device 106 is used to receive or send data via a network. Specific examples of the aforementioned networks may include wireless networks provided by the mobile terminal's communication provider. In one example, the transmission device 106 includes a network interface controller (NIC), which can be connected to other network devices via a base station to communicate with the Internet. In one example, the transmission device 106 may be a radio frequency (RF) module, which is used to communicate with the Internet wirelessly.

[0050] This embodiment provides a method for determining the islanding phenomenon running on a mobile terminal, computer terminal or similar computing device. It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions. Also, although the logical order is shown in the flowchart, in some cases the steps shown or described can be executed in a different order than that shown here.

[0051] Figure 2 This is a flowchart of a method for determining the islanding phenomenon according to an embodiment of this application. For example... Figure 2 As shown, the method includes the following steps:

[0052] Step S201: Apply steady-state positive and negative bidirectional reactive power disturbance to the power grid, obtain the frequency of the power grid, obtain the initial frequency, and determine the reference frequency when the initial frequency is greater than the first threshold and less than the second threshold. The first threshold is the underfrequency protection frequency and the second threshold is the overfrequency protection frequency. The steady-state positive and negative bidirectional reactive power disturbance is used to detect the islanding phenomenon of the power grid.

[0053] Specifically, this application provides an islanding detection method for inverter-type distributed power sources based on three-dimensional frequency feedback. First, the reactive power command corresponding to the inverter's participation in bidirectional reactive power dispatch of the power grid is denoted as Q. ref1 The reactive power disturbance command corresponding to islanding detection is denoted as Q.ref2 The reactive power command of the outer loop control loop during inverter low-voltage ride-through is denoted as Q. ref3 These three factors together constitute the inverter's total reactive power command. The islanding detection reactive power disturbance command, i.e., the steady-state positive and negative bidirectional reactive power disturbance, is denoted as Q. ref2 In steady state, it is set to a bidirectional, periodic square wave with zero intervals. The first half-cycle is 20ms long (one full frequency cycle), containing a positive square wave with a pulse width of 7ms and an amplitude of 0.015pu. The second half-cycle is 20ms long (one full frequency cycle), containing a negative square wave with a pulse width of 7ms and an amplitude of -0.015pu. The total period is 40ms. Then, the over / under frequency protection frequency is set, with the over frequency protection frequency denoted as f. HI The underfrequency protection frequency is denoted as f. LW According to relevant documents, f HI 50.2Hz can be selected, f LW A frequency of 47.5Hz is acceptable, but this threshold can also be modified according to other standards. The real-time frequency of the detection system is the initial frequency f. If f... LW <f<f HI This indicates that the system's real-time frequency f has not exceeded the over / under frequency protection threshold. The reference frequency is then determined and denoted as f. base And it remains unchanged within the longest allowed island detection time of 2 seconds.

[0054] Step S202: Calculate the difference between the initial frequency and the reference frequency to obtain the frequency offset. If the absolute value of the frequency offset is less than a preset threshold, maintain the reactive power of the power grid as a steady-state positive and negative bidirectional reactive power disturbance. If the absolute value of the frequency offset is greater than or equal to the preset threshold, adjust the direction of the reactive power of the power grid to the disturbance direction.

[0055] Specifically, the real-time frequency f minus the reference frequency f base The frequency offset Δf is calculated, with a preset threshold of 0.15Hz. When its absolute value |Δf| ≥ 0.15Hz, the system is triggered to enter the three-dimensional frequency feedback-reactive power disturbance stage. The duration limit of each round of three-dimensional frequency feedback-reactive power disturbance is the maximum allowable islanding detection time. Frequency feedback is performed in three dimensions: disturbance direction, disturbance width, and disturbance amplitude. The first step is frequency feedback based on the disturbance direction of reactive power, which requires adjusting the direction of reactive power in the grid to the disturbance direction. The specific adjustment method will be explained below.

[0056] Step S203: Determine the pulse width of the disturbance based on the frequency offset, and adjust the pulse width of the reactive power of the power grid to the pulse width of the disturbance; determine the amplitude of the disturbance based on the frequency offset, and adjust the amplitude of the reactive power of the power grid to the amplitude of the disturbance.

[0057] Specifically, after frequency feedback based on the disturbance direction according to reactive power, frequency feedback is performed based on the disturbance pulse width and disturbance amplitude. Q ref2 The width of a single perturbation artery is denoted as Q. pw2 The steady-state pulse width is 7ms. After entering the frequency feedback stage, the pulse width gradually increases with the frequency offset. The maximum pulse width corresponds to half a disturbance period, 20ms. The disturbance pulse width-frequency offset curve is shown below. Figure 3 As shown. Disturbance artery width Q pw2 When the upper limit is reached, the corresponding frequency offset limit is denoted as |Δf|. lim It is the reference frequency and the lower frequency threshold f. LW The difference, multiplied by a certain margin, is expressed as: |Δf| lim =0.9(f base -f LW Here, f is adopted. LW Instead of f HI The frequency offset limit is determined because the difference between the underfrequency protection threshold stipulated by the Energy Bureau and the power frequency is significantly greater than that of the overfrequency threshold, making islanding detection more difficult. Furthermore, when the disturbance pulse width reaches 20ms, there will be no interval between the disturbance square waves. Disturbance pulse width T pw2 The expression is as follows: Frequency feedback of disturbance amplitude: Generally, inverters are allowed to operate for short periods at 1.1 times the apparent power, and the inverter's reactive power limit Q max It can be expressed as: The value is typically a fixed 0.46, where S is the inverter's rated apparent power and P is the inverter's real-time active power output. When |Δf| just reaches 0.15Hz, the reactive power limit obtained from the above formula is maintained at the maximum islanding detection time of 2s specified by the National Energy Administration, denoted as Q. max2 This is used to calculate the specific disturbance amplitude-frequency offset curve during the frequency feedback reactive power disturbance. However, the final total reactive power command of the inverter still needs to be limited by the real-time reactive power limit. When |Δf| exceeds 0.15Hz, the disturbance amplitude-frequency offset curve is as follows: Figure 4 As shown. Figure 4 In the middle, Q mag2 For reactive power disturbance command Q ref2 The absolute value of the amplitude, its upper limit Q mag2lim The constant value is 0.46: Q mag2lim =Q max2 -(Q ref1 +Q ref3 ), Q mag2lim The reactive power support command Q was taken into consideration. ref1 With voltage ride-through command Q ref3 The utilization of the inverter's reactive power output capacity. Therefore, Q mag2 The expression is:

[0058]

[0059] The overall three-dimensional frequency feedback—reactive power disturbance implementation diagram is as follows: Figure 5 and Figure 6 As shown. Figure 5 The corresponding frequency decreases. Figure 6 This corresponds to an increase in frequency.

[0060] Step S204: After adjusting to the disturbance amplitude, obtain the current frequency of the power grid. If the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold, determine that the power grid has an islanding phenomenon.

[0061] Specifically, during a round of frequency feedback-reactive disturbance, if the system frequency reaches the over / under frequency protection threshold, it is considered that the initial frequency deviation exceeding the limit is caused by islanding, and the inverter's output circuit breaker is then disconnected to complete the anti-islanding protection.

[0062] This embodiment allows for the acquisition of the power grid frequency to obtain an initial frequency. If the initial frequency is greater than a first threshold and less than a second threshold, a reference frequency is determined. The difference between the initial frequency and the reference frequency is calculated to obtain a frequency offset. If the absolute value of the frequency offset is greater than or equal to a preset threshold, the direction of the power grid's reactive power is adjusted to the disturbance direction. The disturbance pulse width is determined based on the frequency offset, and the pulse width of the power grid's reactive power is adjusted to the disturbance pulse width. The disturbance amplitude is determined based on the frequency offset, and the amplitude of the power grid's reactive power is adjusted to the disturbance amplitude. After adjustment to the disturbance amplitude, the current frequency of the power grid is acquired. If the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold, islanding is determined to have occurred in the power grid. Compared to existing islanding detection methods, which may affect normal power quality and have inaccurate islanding detection, this application can efficiently complete islanding detection under the most severe operating conditions, reduce the impact on steady-state power quality, easily accommodate bidirectional reactive power dispatch and low-voltage ride-through, and accurately detect islanding in the power grid.

[0063] In specific implementation, step S202, when the absolute value of the frequency offset is greater than or equal to a preset threshold, adjusts the direction of the reactive power of the power grid to the direction of disturbance. This can be achieved through the following steps: when the frequency offset is greater than the preset threshold, the positive disturbance in the steady-state bidirectional reactive power disturbance is adjusted to a negative disturbance; when the frequency offset is less than the negative preset threshold, the negative disturbance in the steady-state bidirectional reactive power disturbance is adjusted to a positive disturbance. This method divides the absolute value of the frequency offset into two cases for disturbance, thus applying different disturbances in the two cases to accurately detect islanding phenomena.

[0064] Specifically, as mentioned above, the preset threshold can be 0.15Hz, and the frequency feedback method for the disturbance direction is as follows: when Δf < -0.15Hz, the negative disturbance in the original steady-state bidirectional disturbance is flipped into a positive disturbance; when Δf > 0.15Hz, the positive disturbance in the original steady-state bidirectional disturbance is flipped into a negative disturbance.

[0065] To accurately obtain the current frequency of the power grid when the grid meets the detection conditions, step S204, after adjusting to the disturbance amplitude, can be achieved through the following steps: After adjusting to the disturbance amplitude, receive the islanding detection reactive power command and the reactive power dispatch command, where the islanding detection reactive power command indicates islanding detection of the power grid, and the reactive power dispatch command indicates dispatching the reactive power of the power grid; obtain the AC voltage of the power grid, and if the AC voltage is within a preset voltage range, receive the reactive voltage ride-through command, where the reactive voltage ride-through command indicates voltage ride-through in the power grid; upon receiving the islanding detection reactive power command, the reactive power dispatch command, and the reactive voltage ride-through command, obtain the current frequency of the power grid. This method obtains the current frequency only after the power grid receives the command, i.e., when the detection conditions are met, thus obtaining an accurate current frequency and accurately detecting whether islanding has occurred.

[0066] In the specific implementation process, as mentioned above, the reactive power command corresponding to the inverter's participation in the bidirectional reactive power dispatch of the power grid is first denoted as Q. ref1 The reactive power disturbance command corresponding to islanding detection is denoted as Q. ref2 The reactive power command of the outer loop control loop during inverter low-voltage ride-through is denoted as Q. ref3 These three factors together constitute the inverter's total reactive power command. Upon receiving the total reactive power command, the current frequency is then obtained.

[0067] In some alternative implementations, the power grid includes an inverter, and the method further includes the following steps: after determining that islanding has occurred in the power grid, disconnecting the inverter; if the initial frequency is less than or equal to a first threshold, determining that the power grid is under-frequency protection and disconnecting the inverter; if the initial frequency is greater than or equal to a second threshold, determining that the power grid is over-frequency protection and disconnecting the inverter. This method disconnects the inverter upon determining that islanding has occurred in the power grid, thus preventing power grid failures.

[0068] In the specific implementation process, after detecting the system's real-time frequency f, if it is higher than f... HI or below f LW If this occurs, the over / under frequency protection will be triggered, directly disconnecting the inverter output circuit breaker within 0.2 seconds. Furthermore, the circuit breaker will also be disconnected after it is determined that islanding has occurred in the power grid. This prevents power grid failures caused by islanding from affecting the normal operation of the power grid.

[0069] To accurately determine the reference frequency, step S201 can be achieved through the following steps: Acquisition step: Acquire the frequency of the power grid for a predetermined number of cycles within a past time period to obtain multiple historical frequencies. The past time period refers to the time period preceding and adjacent to the moment corresponding to the initial frequency. Calculation step: Calculate the average of the multiple historical frequencies to obtain the reference frequency. This method obtains the reference frequency by calculating the average of multiple historical frequencies within a past time period, thus enabling objective and accurate determination of the reference frequency.

[0070] In practice, provided that the system's real-time frequency f does not exceed the over / under frequency protection threshold, the predetermined number is generally 6 cycles. The average system frequency over the past 6 power frequency cycles is taken as the dynamic reference frequency and denoted as f. base And it remains unchanged within the maximum allowed island detection time of 2 seconds. The number of pre-determined islands can also be other numbers, and this application does not impose a specific limit on the number of pre-determined islands.

[0071] In some optional implementations, the method further includes the following steps: if the current frequency is greater than a first threshold and less than a second threshold, determine the usage time of the reference frequency; if the usage time is greater than a preset time, repeat the acquisition and calculation steps to update the reference frequency. This method updates the reference frequency in a timely manner, thus maintaining the real-time nature of island monitoring and enabling more accurate detection of islanding phenomena.

[0072] Specifically, if the reference frequency is used for more than a preset time, such as 2 seconds, the reference frequency will be updated. In practical applications, if the system frequency does not reach the set over / under frequency protection threshold during a round of frequency feedback—reactive power disturbance, it is considered that no islanding phenomenon has occurred, and the previous frequency deviation exceeding the limit is within the normal range of frequency fluctuation. In this case, the reference frequency f also needs to be updated. base The update brings the frequency offset to within ±0.15Hz, and the perturbation mode is restored to the steady-state bidirectional, narrow pulse width, low amplitude perturbation.

[0073] To accurately determine the disturbance pulse width and disturbance amplitude, step S203, determining the disturbance pulse width based on the frequency offset, can be achieved through the following steps: obtaining a first mapping relationship between the frequency offset and the disturbance pulse width, and determining the disturbance pulse width corresponding to the frequency offset based on the first mapping relationship; determining the disturbance amplitude based on the frequency offset can be achieved through the following steps: obtaining a second mapping relationship between the frequency offset and the disturbance amplitude, and determining the disturbance amplitude corresponding to the frequency offset based on the second mapping relationship. This method determines the disturbance pulse width and disturbance amplitude based on the mapping relationship of the frequency offset, thus accurately determining the disturbance pulse width and disturbance amplitude.

[0074] In the specific implementation process, the perturbation artery width-frequency offset curve is as follows: Figure 3 As shown. Disturbance artery width Q pw2 When the upper limit is reached, the corresponding frequency offset limit is denoted as |Δf|. lim It is the reference frequency and the lower frequency threshold f. LW The difference, multiplied by a certain margin, is expressed as: |Δf| lim =0.9(f base -f LW Here, f is adopted. LW Instead of f HI The frequency offset limit is determined because the difference between the underfrequency protection threshold stipulated by the Energy Bureau and the power frequency is significantly greater than that of the overfrequency threshold, making islanding detection more difficult. Furthermore, when the disturbance amplitude reaches 20ms, there will be no interval between the disturbance square waves. When |Δf| exceeds 0.15Hz, the disturbance amplitude-frequency offset curve is as follows... Figure 4 As shown. Figure 4 In the middle, Q mag2 For reactive power disturbance command Q ref2 The absolute value of the amplitude, its upper limit Q mag2lim The constant value is 0.46: Q mag2lim =Q max2 -(Q ref1 +Q ref3 ), Q mag2lim The reactive power support command Q was taken into consideration. ref1 With voltage ride-through command Q ref3 The utilization of the inverter's reactive power output capacity.

[0075] Figure 7 The diagram shown illustrates the overall control implementation of this islanding detection method. Reactive power disturbance command Q. ref2The derivation process is as follows: 1) The pulse generator provides a reference pulse, which is then widened by a monostable multivibrator 1 into a positive initial trigger square wave Pp0 with a pulse width of 7 ms and a value of 1; Pp0 is delayed by one power frequency cycle and then logically inverted to become a negative initial trigger square wave Pn0 with a pulse width of 7 ms and a value of -1. 2) The frequency offset flag F1 is set to 1 when the frequency offset Δf > 0.15 Hz; the frequency offset flag F2 is set to 1 when Δf < -0.15 Hz; F1 and F2 pass through an exclusive - OR circuit and the "interval time - limit link (T1 < t < T2)" to obtain the frequency offset flag F30. When |Δf| exceeds 0.15 Hz, F30 is set to 1. In the interval time - limit link, T1 is taken as half of the disturbance period length, i.e., 20 ms, so as to avoid the subsequent flip signal from being inverted within a single disturbance square wave period, and T2 is taken as the maximum allowable island detection duration of 2 s to stop the frequency feedback - reactive power disturbance within the time limit. 3) F30 and F1 pass through AND gate 1 and a single - input comparator 1 to obtain a positive disturbance flip signal TurnP. When TurnP is -1, it means that Δf > 0.15 Hz and the positive disturbance needs to be flipped to a negative disturbance. After multiplying TurnP by Pp0, a trigger square wave Pp1 with values of -1 and 0 is obtained, and Pp1 is then multiplied by the disturbance amplitude Q mag22 to get Q ref2 . 4) Similarly, after multiplying the negative disturbance flip signal TurnN by Pn0, a trigger square wave Pn1 with values of 1 and 0 is obtained, and after multiplying Pn1 by the disturbance amplitude Q mag22 it also gets Q ref2 . The derivation process of the reactive power disturbance amplitude Q mag22 is as follows: 1) F30 is widened by a monostable multivibrator 2 into a frequency offset flag F31 with a fixed duration equal to the maximum allowable island detection time of 2 s. F31 serves as the hold signal of sample - and - hold 1, limiting the real - time Q max obtained from equation (3) to the Q max2 in step (3) for calculating the disturbance amplitude Q mag2 of the real - time frequency feedback in equation (5). Thus, in each round of disturbance, the slope k Q of the disturbance amplitude - frequency offset curve can remain unchanged. 2) Q mag2 passes through limiter 1 and is then multiplied by the sum of the absolute values of Pp1 and Pn1, i.e., InPN1. Through sample - and - hold 2, Q mag21 is obtained. The multiplication by InPN1 is to keep the disturbance amplitude at 0 when the trigger square wave is 0. The sum of the absolute values of the trigger square wave, InPN1, also serves as the hold signal of sample - and - hold 2, making the amplitude of each disturbance square wave remain constant within its pulse width, rather than the time - varying value calculated by equation (3). 3) Finally, F3 serves as the control signal of dual - input selector 1. When |Δf| exceeds 0.15 Hz, the calculated value Q mag21 is selected as the reactive power disturbance amplitude Q mag22 , rather than the fixed value in the steady state. The reactive power disturbance pulse width Tpw22 The process is as follows: 1) Calculate the real-time perturbation pulse width Tpw2 that varies with the frequency offset using equation (2), and then pass it through a limiting stage with a maximum of 20ms and a minimum of 7ms. 2) Use the sum of the absolute values ​​of the trigger square waves InPN1 as the holding signal for sample / hold 3, so that each perturbation square wave has a fixed pulse width for each perturbation, instead of the time-varying value calculated by equation (2). 3) Then, obtain the previous step length value T through the "take the previous step length value" stage. pw21 Otherwise, the pulse width signal would have a causal contradiction. 4) Finally, through the dual-input selection stage, T pw22 Choose 7ms for steady state and T for frequency feedback-reactive power disturbance phase. pw21 .also, Figure 7 The invention also demonstrates a voltage ride-through control loop compatible with islanding detection. The voltage ride-through strategy used in this invention involves using the difference ΔU between the AC voltage reference value and the actual value, passed through a proportional-integral (PI) stage, to derive the corresponding outer-loop reactive power command Q when the AC voltage reaches the voltage ride-through threshold. ref3 To prevent this command from increasing too rapidly during voltage dips, the original q-axis current command i in the inner loop needs to be adjusted. qref1 Above, the i generated by superimposing ΔU through a gain circuit containing a voltage dead zone qref3 It can play a certain role in limiting the growth of reactive current.

[0076] To enable those skilled in the art to better understand the technical solution of this application, the implementation process of the method for determining the islanding phenomenon of this application will be described in detail below with reference to specific embodiments.

[0077] This embodiment relates to a specific method for determining the islanding phenomenon, such as... Figure 8 As shown, distributed photovoltaic power generation systems have a wide range of applications and are an important representative of distributed power systems. We will take them as an example to illustrate this. Figure 8 The simulation verification system diagram for the island detection method built on the PSCAD / EMTDC platform is shown in Table 1. The main parameters of the photovoltaic simulation system are shown in Table 1, and the following steps are included:

[0078] Step S1: The effectiveness of the proposed islanding detection strategy is verified by using the case of a load quality factor of 2.5, which is the most difficult to detect according to IEEE std.929. Figure 9 The parallel load parameters are 500Ω / / 643mH / / 15.91549μF. The photovoltaic system operates at rated output in steady state, with the inverter outputting 200kW (1p.u.) active power and 0kVar (0p.u.) reactive power. In steady state, the load and inverter power are matched. Combined with the grid-connected transformer, the total load resonant frequency is 49.95Hz, and the total load quality factor is 2.495. At 3.0s... Figure 9Circuit breaker 1 in the system disconnects to form an islanded system.

[0079] Table 1 Main parameters of the photovoltaic simulation system

[0080]

[0081] Step 2: Using the islanding detection method of this invention, the steady-state disturbance is bidirectional (positive and negative). At 3.0396s, the frequency offset exceeds -0.15Hz, and the system automatically enters the frequency feedback-reactive power disturbance stage. The negative disturbance flips to positive, and the reactive power disturbance amplitude and pulse width are calculated in real time based on the frequency offset. Both the disturbance amplitude and pulse width increase with the frequency offset. When the frequency reaches the underfrequency protection threshold, islanding detection is completed. Due to system inertia, the increase in inverter output reactive power lags behind the change in reactive power command. If 49.5Hz, adopted in most literature, is used as the standard for successful islanding detection, this method completes islanding detection in 0.2s; if 47.5Hz, as stipulated by the National Energy Administration, is used, this method takes 0.44s, and the circuit breaker 3 on the inverter side is triggered to open at 3.44s, completing the anti-islanding protection. During this process, the grid connection point frequency, the islanding detection reactive power command, and the inverter reactive power output are as follows: Figure 10 and Figure 11 As shown.

[0082] Step S3: Figure 12 The figure shows the amplitudes of the 2nd to 7th harmonics of the phase voltage at the grid connection point in steady state. This invention benefits from the ultra-low amplitude and ultra-narrow pulse width of the disturbance in steady state, resulting in a total harmonic distortion (THD) of only 0.323%. The inverter's output reactive power in steady state is only within ±0.0008 pu. This invention has almost no impact on the AC voltage quality and steady-state reactive power during grid connection.

[0083] Step S4: Unaffected by misjudgment of steady-state normal frequency changes: The grid frequency changes from 50Hz to 49.84Hz in 3 seconds, due to f base The frequency remained at 50 Hz. At 3.03974 s, a |Δf| exceeding 0.15 Hz was detected, triggering frequency feedback—a reactive power disturbance. The frequency and reactive power of this process are as follows: Figure 13 and Figure 14 As shown. After approximately 2 seconds of frequency feedback—reactive power disturbance, f base The frequency was updated to 49.84 Hz, representing the bidirectional disturbance when the disturbance returns to steady state. During the feedback disturbance, due to the clamping effect of the main power grid, the system frequency and reactive power did not change significantly. This invention has the ability to address misjudgments of normal steady-state frequency changes.

[0084] Step S5: Supports inverter participation in bidirectional reactive power dispatch of the grid: The inverter issues 0.8 pu of active power in steady state. Operating condition 1 is set as follows: Reactive power support command Q is issued at 3.0s.ref1 The reactive power gradually increases from 0 to 0.4 pu over 1 second; Condition 2 is: at 3.0 seconds, the reactive power support command Q... ref1 It gradually decreased from 0 to -0.4pu over 1 second. Figure 15 and Figure 16 The diagram shows the system frequency and reactive power during this process. In this invention, the islanding detection strategy does not affect the inverter's participation in bidirectional reactive power dispatching of the grid when it is not operating at full capacity. It can provide reactive power support to both the load and the grid, and also absorb excess reactive power from the grid. The system frequency remains consistently at 50Hz, and the reactive power output follows reactive power commands well.

[0085] Step S6: Enables inverter low-voltage ride-through: Turn on at 3.0s. Figure 6 The voltage drop is simulated using impedance to achieve a duration of 2 seconds and an amplitude of approximately 0.6U. N The voltage dropped. Figure 17 The figure shows the grid connection point voltage when the low voltage ride-through module is engaged and locked, respectively. The islanding detection strategy is engaged throughout both operating conditions. Figure 18 , Figure 19 and Figure 20 The figure shows the frequency and islanding reactive power command Q when the low voltage ride-through control and islanding detection strategies are in operation. ref2 and the reactive power Q output by the inverter. PV .Depend on Figure 17 It can be seen that after implementing the low-voltage ride-through control method used in this invention, the AC voltage increased from 0.574 pu to 0.595 pu, an increase of approximately 0.021 pu. In summary... Figure 17 , Figure 18 , Figure 19 and Figure 20 The inverter can simultaneously engage both islanding detection and low voltage ride-through strategies, and maintain grid-connected operation during voltage dips to achieve low voltage ride-through.

[0086] This application also provides an apparatus for determining islanding phenomena. It should be noted that this apparatus can be used to execute the method for determining islanding phenomena provided in this application. This apparatus is used to implement the embodiments and preferred embodiments; details already described will not be repeated. As used below, the term "module" can refer to a combination of software and / or hardware that performs a predetermined function. Although the apparatus described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0087] The following describes the device for determining the islanding phenomenon provided in the embodiments of this application.

[0088] Figure 21This is a schematic diagram of a device for determining the islanding phenomenon according to an embodiment of this application. Figure 21 As shown, the device includes:

[0089] The first determining unit 10 is used to apply steady-state positive and negative bidirectional reactive power disturbance to the power grid, obtain the frequency of the power grid, obtain the initial frequency, and determine the reference frequency when the initial frequency is greater than a first threshold and less than a second threshold. The first threshold is the underfrequency protection frequency and the second threshold is the overfrequency protection frequency. The steady-state positive and negative bidirectional reactive power disturbance is used to detect the islanding phenomenon of the power grid.

[0090] Specifically, this application provides an islanding detection method for inverter-type distributed power sources based on three-dimensional frequency feedback. First, the reactive power command corresponding to the inverter's participation in bidirectional reactive power dispatch of the power grid is denoted as Q. ref1 The reactive power disturbance command corresponding to islanding detection is denoted as Q. ref2 The reactive power command of the outer loop control loop during inverter low-voltage ride-through is denoted as Q. ref3 These three factors together constitute the inverter's total reactive power command. The islanding detection reactive power disturbance command, i.e., the steady-state positive and negative bidirectional reactive power disturbance, is denoted as Q. ref2 In steady state, it is set to a bidirectional, periodic square wave with zero intervals. The first half-cycle is 20ms long (one full frequency cycle), containing a positive square wave with a pulse width of 7ms and an amplitude of 0.015pu. The second half-cycle is 20ms long (one full frequency cycle), containing a negative square wave with a pulse width of 7ms and an amplitude of -0.015pu. The total period is 40ms. Then, the over / under frequency protection frequency is set, with the over frequency protection frequency denoted as f. HI The underfrequency protection frequency is denoted as f. LW According to relevant documents, f HI 50.2Hz can be selected, f LW A frequency of 47.5Hz is acceptable, but this threshold can also be modified according to other standards. The real-time frequency of the detection system is the initial frequency f. If f... LW <f<f HI This indicates that the system's real-time frequency f has not exceeded the over / under frequency protection threshold. The reference frequency is then determined and denoted as f. base And it remains unchanged within the longest allowed island detection time of 2 seconds.

[0091] The first adjustment unit 20 is used to calculate the difference between the initial frequency and the reference frequency to obtain the frequency offset. When the absolute value of the frequency offset is less than a preset threshold, the reactive power of the power grid is kept as a steady-state positive and negative bidirectional reactive power disturbance. When the absolute value of the frequency offset is greater than or equal to the preset threshold, the direction of the reactive power of the power grid is adjusted to the disturbance direction.

[0092] Specifically, the real-time frequency f minus the reference frequency f baseThe frequency offset Δf is calculated, with a preset threshold of 0.15Hz. When its absolute value |Δf| ≥ 0.15Hz, the system is triggered to enter the three-dimensional frequency feedback-reactive power disturbance stage. The duration limit for each round of three-dimensional frequency feedback-reactive power disturbance is the maximum allowable islanding detection time. Frequency feedback is performed in three dimensions: the disturbance direction, disturbance width, and disturbance amplitude. The first step is frequency feedback based on the disturbance direction of reactive power, which requires adjusting the direction of reactive power in the grid to the disturbance direction. The specific adjustment device will be described below.

[0093] The second adjustment unit 30 is used to determine the pulse width of the disturbance based on the frequency offset and adjust the pulse width of the reactive power of the power grid to the pulse width of the disturbance, and to determine the amplitude of the disturbance based on the frequency offset and adjust the amplitude of the reactive power of the power grid to the amplitude of the disturbance.

[0094] Specifically, after frequency feedback based on the disturbance direction according to reactive power, frequency feedback is performed based on the disturbance pulse width and disturbance amplitude. Q ref2 The width of a single perturbation artery is denoted as Q. pw2 The steady-state pulse width is 7ms. After entering the frequency feedback stage, the pulse width gradually increases with the frequency offset. The maximum pulse width corresponds to half a disturbance period, 20ms. The disturbance pulse width-frequency offset curve is shown below. Figure 3 As shown. Disturbance artery width Q pw2 When the upper limit is reached, the corresponding frequency offset limit is denoted as |Δf|. lim It is the reference frequency and the lower frequency threshold f. LW The difference, multiplied by a certain margin, is expressed as: |Δf| lim =0.9(f base -f LW Here, f is adopted. LW Instead of f HI The frequency offset limit is determined because the difference between the underfrequency protection threshold stipulated by the Energy Bureau and the power frequency is significantly greater than that of the overfrequency threshold, making islanding detection more difficult. Furthermore, when the disturbance pulse width reaches 20ms, there will be no interval between the disturbance square waves. Disturbance pulse width T pw2 The expression is as follows: Frequency feedback of disturbance amplitude: Generally, inverters are allowed to operate for short periods at 1.1 times the apparent power, and the inverter's reactive power limit Q max It can be expressed as: The value is typically a fixed 0.46, where S is the inverter's rated apparent power and P is the inverter's real-time active power output. When |Δf| just reaches 0.15Hz, the reactive power limit obtained from the above formula is maintained at the maximum islanding detection time of 2s specified by the National Energy Administration, denoted as Q. max2This is used to calculate the specific disturbance amplitude-frequency offset curve during the frequency feedback reactive power disturbance. However, the final total reactive power command of the inverter still needs to be limited by the real-time reactive power limit. When |Δf| exceeds 0.15Hz, the disturbance amplitude-frequency offset curve is as follows: Figure 4 As shown. Figure 4 In the middle, Q mag2 For reactive power disturbance command Q ref2 The absolute value of the amplitude, its upper limit Q mag2lim The constant value is 0.46: Q mag2lim =Q max2 -(Q ref1 +Q ref3 ), Q mag2lim The reactive power support command Q was taken into consideration. ref1 With voltage ride-through command Q ref3 The utilization of the inverter's reactive power output capacity. Therefore, Q mag2 The expression is:

[0095] The overall three-dimensional frequency feedback—reactive power disturbance implementation diagram is as follows: Figure 5 and Figure 6 As shown. Figure 5 The corresponding frequency decreases. Figure 6 This corresponds to an increase in frequency.

[0096] The second determining unit 40 is used to obtain the current frequency of the power grid after adjusting to the disturbance amplitude, and to determine that the power grid has an islanding phenomenon when the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold.

[0097] Specifically, during a round of frequency feedback-reactive disturbance, if the system frequency reaches the over / under frequency protection threshold, it is considered that the initial frequency deviation exceeding the limit is caused by islanding, and the inverter's output circuit breaker is then disconnected to complete the anti-islanding protection.

[0098] This embodiment allows for the acquisition of the power grid frequency to obtain an initial frequency. If the initial frequency is greater than a first threshold and less than a second threshold, a reference frequency is determined. The difference between the initial frequency and the reference frequency is calculated to obtain a frequency offset. If the absolute value of the frequency offset is greater than or equal to a preset threshold, the direction of the power grid's reactive power is adjusted to the disturbance direction. The disturbance pulse width is determined based on the frequency offset, and the pulse width of the power grid's reactive power is adjusted to the disturbance pulse width. The disturbance amplitude is determined based on the frequency offset, and the amplitude of the power grid's reactive power is adjusted to the disturbance amplitude. After adjustment to the disturbance amplitude, the current frequency of the power grid is acquired. If the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold, islanding is determined to have occurred in the power grid. Compared to existing technologies where islanding detection devices may affect normal power quality or have inaccurate islanding detection, this application can efficiently perform islanding detection under the most severe operating conditions, reduce the impact on steady-state power quality, easily accommodate bidirectional reactive power dispatch and low-voltage ride-through, and accurately detect islanding in the power grid.

[0099] In its specific implementation, the first adjustment unit includes a first adjustment module and a second adjustment module. The first adjustment module is used to adjust the positive disturbance in the steady-state bidirectional reactive power disturbance to a negative disturbance when the frequency offset is greater than a preset threshold. The second adjustment module is used to adjust the negative disturbance in the steady-state bidirectional reactive power disturbance to a positive disturbance when the frequency offset is less than a preset negative threshold. This device applies disturbances based on the absolute value of the frequency offset in two different cases, allowing for the application of different disturbances in each case to accurately detect islanding phenomena.

[0100] Specifically, as mentioned above, the preset threshold can be 0.15Hz, and the frequency feedback method for the disturbance direction is as follows: when Δf < -0.15Hz, the negative disturbance in the original steady-state bidirectional disturbance is flipped into a positive disturbance; when Δf > 0.15Hz, the positive disturbance in the original steady-state bidirectional disturbance is flipped into a negative disturbance.

[0101] To accurately obtain the current frequency of the power grid when the grid meets the detection conditions, the second determining unit includes a first receiving module, a second receiving module, and a first acquiring module. The first receiving module, after adjusting to the disturbance amplitude, receives islanding detection reactive power commands and reactive power dispatch commands. The islanding detection reactive power command indicates islanding detection of the power grid, and the reactive power dispatch command indicates reactive power dispatch of the power grid. The second receiving module acquires the AC voltage of the power grid. If the AC voltage is within a preset voltage range, it receives a reactive voltage ride-through command, indicating voltage ride-through in the power grid. The first acquiring module acquires the current frequency of the power grid upon receiving the islanding detection reactive power command, reactive power dispatch command, and reactive voltage ride-through command. This device acquires the current frequency only after the power grid receives the command, i.e., when the detection conditions are met, thus obtaining an accurate current frequency and accurately detecting whether islanding has occurred.

[0102] In the specific implementation process, as mentioned above, the reactive power command corresponding to the inverter's participation in the bidirectional reactive power dispatch of the power grid is first denoted as Q. ref1 The reactive power disturbance command corresponding to islanding detection is denoted as Q. ref2 The reactive power command of the outer loop control loop during inverter low-voltage ride-through is denoted as Q. ref3 These three factors together constitute the inverter's total reactive power command. Upon receiving the total reactive power command, the current frequency is then obtained.

[0103] In some optional implementations, the power grid includes an inverter, and the device further includes a first disconnection unit, a second disconnection unit, and a third disconnection unit. The first disconnection unit disconnects the inverter after determining that islanding has occurred in the power grid. The second disconnection unit disconnects the inverter if the initial frequency is less than or equal to a first threshold value, indicating that the power grid is under-frequency protection, and disconnects the inverter accordingly. The third disconnection unit disconnects the inverter if the initial frequency is greater than or equal to a second threshold value, indicating that the power grid is over-frequency protection, and disconnects the inverter accordingly. This device disconnects the inverter upon determining that an islanding phenomenon has occurred in the power grid, thus preventing power grid failures.

[0104] In the specific implementation process, after detecting the system's real-time frequency f, if it is higher than f... HI or below f LW If this occurs, the over / under frequency protection will be triggered, directly disconnecting the inverter output circuit breaker within 0.2 seconds. Furthermore, the circuit breaker will also be disconnected after it is determined that islanding has occurred in the power grid. This prevents power grid failures caused by islanding from affecting the normal operation of the power grid.

[0105] To accurately determine the reference frequency, the first determining unit includes a second acquisition module and a calculation module. The second acquisition module acquires the frequency of the power grid for a predetermined number of cycles over a past time period, obtaining multiple historical frequencies. The past time period is the time period preceding and adjacent to the moment corresponding to the initial frequency. The calculation module calculates the average of the multiple historical frequencies to obtain the reference frequency. This device obtains the reference frequency by calculating the average of multiple historical frequencies over a past time period, thus enabling objective and accurate determination of the reference frequency.

[0106] In practice, provided that the system's real-time frequency f does not exceed the over / under frequency protection threshold, the predetermined number is generally 6 cycles. The average system frequency over the past 6 power frequency cycles is taken as the dynamic reference frequency and denoted as f. base And it remains unchanged within the maximum allowed island detection time of 2 seconds. The number of pre-determined islands can also be other numbers, and this application does not impose a specific limit on the number of pre-determined islands.

[0107] In some optional embodiments, the device further includes a third determining unit and a repeating unit. The third determining unit is used to determine the usage time of the reference frequency when the current frequency is greater than a first threshold and less than a second threshold. The repeating unit is used to repeat the acquisition and calculation steps to update the reference frequency when the usage time is greater than a preset time. This device updates the reference frequency in a timely manner, thus maintaining the real-time performance of islanding monitoring and enabling more accurate detection of islanding phenomena.

[0108] Specifically, if the reference frequency is used for more than a preset time, such as 2 seconds, the reference frequency will be updated. In practical applications, if the system frequency does not reach the set over / under frequency protection threshold during a round of frequency feedback—reactive power disturbance, it is considered that no islanding phenomenon has occurred, and the previous frequency deviation exceeding the limit is within the normal range of frequency fluctuation. In this case, the reference frequency f also needs to be updated. base The update brings the frequency offset to within ±0.15Hz, and the perturbation mode is restored to the steady-state bidirectional, narrow pulse width, low amplitude perturbation.

[0109] To accurately determine the disturbance pulse width and disturbance amplitude, the second adjustment unit includes a first determining module and a second determining module. The first determining module is used to acquire a first mapping relationship between the frequency offset and the disturbance pulse width, and determine the disturbance pulse width corresponding to the frequency offset based on the first mapping relationship. The second determining module is used to determine the disturbance amplitude based on the frequency offset through the following steps: acquiring a second mapping relationship between the frequency offset and the disturbance amplitude, and determining the disturbance amplitude corresponding to the frequency offset based on the second mapping relationship. This device determines the disturbance pulse width and disturbance amplitude based on the mapping relationship of the frequency offset, thus accurately determining the disturbance pulse width and disturbance amplitude.

[0110] In the specific implementation process, the perturbation artery width-frequency offset curve is as follows: Figure 3 As shown. Disturbance artery width Q pw2 When the upper limit is reached, the corresponding frequency offset limit is denoted as |Δf|. lim It is the reference frequency and the lower frequency threshold f. LW The difference, multiplied by a certain margin, is expressed as: |Δf| lim =0.9(f base -f LW Here, f is adopted. LW Instead of f HI The frequency offset limit is determined because the difference between the underfrequency protection threshold stipulated by the Energy Bureau and the power frequency is significantly greater than that of the overfrequency threshold, making islanding detection more difficult. Furthermore, when the disturbance amplitude reaches 20ms, there will be no interval between the disturbance square waves. When |Δf| exceeds 0.15Hz, the disturbance amplitude-frequency offset curve is as follows... Figure 4 As shown. Figure 4 In the middle, Q mag2 For reactive power disturbance command Q ref2 The absolute value of the amplitude, its upper limit Q mag2lim The constant value is 0.46: Q mag2lim =Q max2 -(Q ref1 +Q ref3 ), Q mag2lim The reactive power support command Q was taken into consideration. ref1 With voltage ride-through command Q ref3 The utilization of the inverter's reactive power output capacity.

[0111] Figure 7 The diagram shown illustrates the overall control implementation of this islanding detection method. Reactive power disturbance command Q. ref2The derivation process is as follows: 1) The pulse generator provides a reference pulse, which is then widened by a monostable multivibrator 1 into a positive initial trigger square wave Pp0 with a pulse width of 7 ms and a value of 1; Pp0 is delayed by one power frequency cycle and then logically inverted to become a negative initial trigger square wave Pn0 with a pulse width of 7 ms and a value of -1. 2) The frequency offset flag F1 is set to 1 when the frequency offset Δf > 0.15 Hz; the frequency offset flag F2 is set to 1 when Δf < -0.15 Hz; F1 and F2 pass through an exclusive OR circuit and the "interval time limit circuit (T1 < t < T2)" to obtain the frequency offset flag F30. When |Δf| exceeds 0.15 Hz, F30 is set to 1. In the interval time limit circuit, T1 is set to half of the disturbance period length, 20 ms, to avoid the subsequent flip signal from being inverted within a single disturbance square wave, and T2 is set to the maximum allowable island detection duration, 2 s, to stop the frequency feedback - reactive power disturbance time limit. 3) F30 and F1 pass through AND gate 1 and a single-input comparator 1 to obtain a positive disturbance flip signal TurnP. When TurnP is -1, it indicates that Δf > 0.15 Hz and the positive disturbance needs to be flipped to a negative disturbance. After multiplying TurnP by Pp0, a trigger square wave Pp1 with values of -1 and 0 is obtained. Pp1 is then multiplied by the disturbance amplitude Q mag22 to obtain Q ref2 . 4) Similarly, after multiplying the negative disturbance flip signal TurnN by Pn0, a trigger square wave Pn1 with values of 1 and 0 is obtained. Pn1 is multiplied by the disturbance amplitude Q mag22 and Q ref2 is also obtained. The reactive power disturbance amplitude Q mag22 is derived as follows: 1) F30 is widened by a monostable multivibrator 2 into a frequency offset flag F31 with a fixed duration equal to the maximum allowable island detection time, 2 s. F31 serves as the hold signal for sample / hold 1, limiting the real-time Q max obtained from equation (3) to the Q max2 in step (3) for calculating the disturbance amplitude Q mag2 of the real-time frequency feedback in equation (5). Thus, in each round of disturbance, the slope k Q of the disturbance amplitude - frequency offset curve can remain constant. 2) Q mag2 passes through limiter 1 and is then multiplied by the sum of the absolute values of Pp1 and Pn1, InPN1. Through sample / hold 2, Q mag21 is obtained. Multiplying by InPN1 is to keep the disturbance amplitude at 0 when the trigger square wave is 0. The sum of the absolute values of the trigger square waves, InPN1, also serves as the hold signal for sample / hold 2, keeping the amplitude constant within the pulse width of each disturbance square wave, rather than the time-varying value calculated by equation (3). 3) Finally, F3 serves as the control signal for dual-input selector 1, selecting the calculated value Q mag21 as the reactive power disturbance amplitude Q mag22 when |Δf| exceeds 0.15 Hz, rather than the fixed value in the steady state. The reactive power disturbance pulse width Tpw22 The process is as follows: 1) Calculate the real-time perturbation pulse width Tpw2 that varies with the frequency offset using equation (2), and then pass it through a limiting stage with a maximum of 20ms and a minimum of 7ms. 2) Use the sum of the absolute values ​​of the trigger square waves InPN1 as the holding signal for sample / hold 3, so that each perturbation square wave has a fixed pulse width for each perturbation, instead of the time-varying value calculated by equation (2). 3) Then, obtain the previous step length value T through the "take the previous step length value" stage. pw21 Otherwise, the pulse width signal would have a causal contradiction. 4) Finally, through the dual-input selection stage, T pw22 Choose 7ms for steady state and T for frequency feedback-reactive power disturbance phase. pw21 .also, Figure 7 The invention also demonstrates a voltage ride-through control loop compatible with islanding detection. The voltage ride-through strategy used in this invention involves using the difference ΔU between the AC voltage reference value and the actual value, passed through a proportional-integral (PI) stage, to derive the corresponding outer-loop reactive power command Q when the AC voltage reaches the voltage ride-through threshold. ref3 To prevent this command from increasing too rapidly during voltage dips, the original q-axis current command i in the inner loop needs to be adjusted. qref1 Above, the i generated by superimposing ΔU through a gain circuit containing a voltage dead zone qref3 It can play a certain role in limiting the growth of reactive current.

[0112] The device for determining the islanding phenomenon includes a processor and a memory. The first determining unit, the first adjusting unit, the second adjusting unit, and the second determining unit are all stored as program units in the memory. The processor executes these program units to achieve their respective functions. Modules can all reside in the same processor; alternatively, the modules can be located in different processors in any combination.

[0113] The processor contains a kernel, which retrieves the corresponding program units from memory. One or more kernels can be configured, and adjusting kernel parameters can accurately detect islanding phenomena.

[0114] The memory may include non-permanent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM, and the memory includes at least one memory chip.

[0115] It is obvious to those skilled in the art that the modules or steps of the present invention described above can be implemented using general-purpose computing devices. They can be centralized on a single computing device or distributed across a network of multiple computing devices. They can be implemented using computer-executable program code, and thus can be stored in a storage device for execution by a computing device. In some cases, the steps shown or described can be performed in a different order than those described herein, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. Thus, the present invention is not limited to any particular combination of hardware and software.

[0116] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0117] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0118] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0119] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process.Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0120] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0121] Memory may include non-persistent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0122] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0123] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0124] As can be seen from the above description, the embodiments of this application achieve the following technical effects:

[0125] The method for determining islanding in this application involves: acquiring the grid frequency to obtain an initial frequency; determining a reference frequency if the initial frequency is greater than a first threshold and less than a second threshold; calculating the difference between the initial frequency and the reference frequency to obtain a frequency offset; adjusting the direction of the grid's reactive power to the disturbance direction if the absolute value of the frequency offset is greater than or equal to a preset threshold; determining the disturbance pulse width based on the frequency offset and adjusting the grid's reactive power pulse width to the disturbance pulse width; determining the disturbance amplitude based on the frequency offset and adjusting the grid's reactive power amplitude to the disturbance amplitude; and after adjusting to the disturbance amplitude, acquiring the grid's current frequency. If the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold, islanding in the grid is determined. Compared to existing islanding detection methods that may affect normal power quality and are inaccurate in determining islanding, this application can efficiently complete islanding detection under the most severe operating conditions, reduce the impact on steady-state power quality, easily accommodate bidirectional reactive power dispatch and low-voltage ride-through, and accurately detect islanding in the grid.

[0126] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method of determining islanding, characterized by, The method comprises the following steps: applying a steady-state bidirectional reactive power disturbance to a power grid, obtaining a frequency of the power grid, and obtaining an initial frequency; determining a reference frequency when the initial frequency is greater than a first threshold value and less than a second threshold value, wherein the first threshold value is an under-frequency protection frequency, and the second threshold value is an over-frequency protection frequency; the steady-state bidirectional reactive power disturbance is used for islanding detection of the power grid; calculating a difference between the initial frequency and the reference frequency to obtain a frequency offset; when the absolute value of the frequency offset is less than a preset threshold value, maintaining the reactive power of the power grid as the steady-state bidirectional reactive power disturbance; when the absolute value of the frequency offset is greater than or equal to the preset threshold value, adjusting the direction of the reactive power of the power grid to a disturbance direction; determining a disturbance pulse width according to the frequency offset, and adjusting the pulse width of the reactive power of the power grid to the disturbance pulse width; determining a disturbance amplitude according to the frequency offset, and adjusting the amplitude of the reactive power of the power grid to the disturbance amplitude; after adjusting to the disturbance amplitude, obtaining a current frequency of the power grid; when the current frequency is less than or equal to the first threshold value, or greater than or equal to the second threshold value, determining that the power grid has the islanding phenomenon; determining a disturbance pulse width according to the frequency offset, comprising: obtaining a first mapping relationship between the frequency offset and the disturbance pulse width, and determining the disturbance pulse width corresponding to the frequency offset according to the first mapping relationship; determining a disturbance amplitude according to the frequency offset, comprising: obtaining a second mapping relationship between the frequency offset and the disturbance amplitude, and determining the disturbance amplitude corresponding to the frequency offset according to the second mapping relationship.

2. The determination method according to claim 1, characterized in that, when the absolute value of the frequency offset is greater than or equal to the preset threshold value, adjusting the direction of the reactive power of the power grid to the disturbance direction, comprising: when the frequency offset is greater than the preset threshold value, adjusting the positive disturbance in the steady-state bidirectional reactive power disturbance to a negative disturbance; when the frequency offset is less than the negative preset threshold value, adjusting the negative disturbance in the steady-state bidirectional reactive power disturbance to the positive disturbance.

3. The determination method according to claim 1, characterized in that, after adjusting to the disturbance amplitude, obtaining a current frequency of the power grid, comprising: after adjusting to the disturbance amplitude, receiving an island detection reactive instruction and a reactive scheduling instruction, wherein the island detection reactive instruction is an instruction for island detection of the power grid, and the reactive scheduling instruction is an instruction for scheduling the reactive power of the power grid; obtaining an alternating current voltage of the power grid; when the alternating current voltage is within a preset voltage range, receiving a reactive voltage ride-through instruction, wherein the reactive voltage ride-through instruction is an instruction for voltage ride-through of the power grid; when the island detection reactive instruction, the reactive scheduling instruction and the reactive voltage ride-through instruction are received, obtaining the current frequency of the power grid.

4. The determination method according to claim 1, characterized in that, The power grid comprises an inverter, and the method further comprises: after determining that the power grid has the islanding phenomenon, disconnecting the inverter. In a case where the initial frequency is less than or equal to the first threshold value, it is determined that the power grid is under-frequency protection, and the inverter is disconnected; In a case where the initial frequency is greater than or equal to the second threshold value, it is determined that the power grid is over-frequency protection, and the inverter is disconnected.

5. The determination method according to claim 1, characterized in that, The method further comprises: an acquisition step of acquiring frequencies of a predetermined number of cycles of the power grid in a past time period to obtain a plurality of historical frequencies, wherein the past time period is a time period before and adjacent to a time point corresponding to the initial frequency; a calculation step of calculating an average value of the plurality of historical frequencies to obtain the reference frequency.

6. The determination method according to claim 5, characterized in that, The method further comprises: In a case where the current frequency is greater than the first threshold value and less than the second threshold value, a use time of the reference frequency is determined; In a case where the use time is greater than a preset time, the acquisition step and the calculation step are repeated to update the reference frequency.

7. An islanding determination apparatus characterized by comprising: The method further comprises: a first determination unit configured to apply a steady-state bidirectional reactive power disturbance to the power grid, acquire a frequency of the power grid to obtain an initial frequency, and determine a reference frequency in a case where the initial frequency is greater than a first threshold value and less than a second threshold value, wherein the first threshold value is an under-frequency protection frequency, the second threshold value is an over-frequency protection frequency, and the steady-state bidirectional reactive power disturbance is used for islanding detection of the power grid; a first adjustment unit configured to calculate a difference between the initial frequency and the reference frequency to obtain a frequency offset, maintain a reactive power of the power grid as the steady-state bidirectional reactive power disturbance in a case where an absolute value of the frequency offset is less than a preset threshold value, and adjust a direction of the reactive power of the power grid to a disturbance direction in a case where the absolute value of the frequency offset is greater than or equal to the preset threshold value; a second adjustment unit configured to determine a disturbance pulse width according to the frequency offset, adjust a pulse width of the reactive power of the power grid to the disturbance pulse width, determine a disturbance amplitude according to the frequency offset, and adjust an amplitude of the reactive power of the power grid to the disturbance amplitude; a second determination unit configured to acquire a current frequency of the power grid after adjustment to the disturbance amplitude, and determine that the power grid has the islanding phenomenon in a case where the current frequency is less than or equal to the first threshold value or greater than or equal to the second threshold value. The second adjustment unit comprises a first determination module and a second determination module, The first determination module is configured to acquire a first mapping relationship between the frequency offset and the disturbance pulse width, and determine a disturbance pulse width corresponding to the frequency offset according to the first mapping relationship. The second determination module is configured to acquire a second mapping relationship between the frequency offset and the disturbance amplitude, and determine a disturbance amplitude corresponding to the frequency offset according to the second mapping relationship.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium comprises a stored program, wherein the program controls a device in which the computer-readable storage medium is located to perform the determination method of any one of claims 1 to 6 when the program is executed.

9. An electronic device, comprising: The method further comprises: One or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, the one or more programs including programs for performing the determining method of any one of claims 1 to 6.

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