Island phenomenon determination method and apparatus, and electronic device

Abstract

The invention provides an island phenomenon determination method and apparatus, and an electronic device. Comprising the following steps: acquiring the frequency of a power grid to obtain an initial frequency, and determining a reference frequency when the initial frequency is greater than a first threshold and less than a second threshold; calculating a difference value between the initial frequency and the reference frequency to obtain a frequency offset, and adjusting the direction of reactive power of the power grid to be a disturbance direction when the absolute value of the frequency offset is greater than or equal to a preset threshold value; determining a disturbance pulse width according to the frequency offset, adjusting the pulse width of the reactive power of the power grid to be 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 be the disturbance amplitude; and after the disturbance amplitude is adjusted, acquiring the current frequency of the power grid, and when the current frequency is less than or equal to a first threshold value and greater than or equal to a second threshold value, determining that the island phenomenon occurs in the power grid. Through the island detection method and device, the problem that an island detection method in the prior art is poor in effect is solved.

Description

Technical Field

[0001] The present application relates to the field of detection of power grid islanding phenomenon, and in particular, to a method, device, computer-readable storage medium and electronic device for determining islanding phenomenon. Background Art

[0002] Distributed power generation systems represented by photovoltaic and wind power generation are clean, environmentally friendly and flexible in power generation. The islanding effect of grid-connected distributed power systems is one of the major technical difficulties in their development and application. The islanding effect refers to the formation of an independent system in which the distributed power system operates with local loads when the distributed power system is disconnected from the grid due to a fault or maintenance. Unpredictable islanding of distributed power generation systems may cause electrical damage to user equipment, affect power quality, and even endanger human safety and equipment safety. Taking islanding detection is a favorable measure for anti-islanding protection.

[0003] Passive island detection usually monitors the system parameters of the grid-connected inverter, such as voltage, frequency, phase and harmonic distortion. When one or more of these parameters deviate from the allowed range, an island phenomenon is considered to have occurred. Passive island detection methods have a relatively large undetectable zone. Active detection methods can reduce the undetectable zone by introducing appropriate interference to observe whether the parameters are beyond the normal range. Methods based on reactive power disturbances can be easily applied to today's mainstream d and q axis current decoupling control systems. Some of these methods have a long disturbance interval and have the defect of not being able to respond to unpredictable island phenomena in a timely manner. Their single long disturbance time will also affect the steady-state power quality. Some methods have blind spots for situations where the island resonance frequency is close to the rated frequency of the grid. There are also methods that easily misjudge normal grid fluctuations as islands. In addition, existing methods are generally not directly compatible with the low voltage ride-through of the inverter and the reactive power of the supporting grid.

[0004] Therefore, in order to adapt to the development needs of distributed power supply systems, it is an important issue that needs to be solved urgently to establish a reliable, efficient, and high-steady-state power quality island detection method. Summary of the invention

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

[0006] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a method for determining an island phenomenon 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, obtaining an initial frequency, and 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, the second threshold is an overfrequency protection frequency, and the steady-state positive and negative bidirectional reactive power disturbance is used to detect an island phenomenon 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 when the absolute value of the frequency offset is less than a preset threshold. The reactive power of the power 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 power grid is adjusted to the disturbance direction; the disturbance pulse width is determined according to 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 according to 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, and 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 island phenomenon occurs in the power grid.

[0007] Optionally, when 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, including: when the frequency offset is greater than the preset threshold, the positive disturbance in the steady-state positive and negative 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 positive and negative bidirectional reactive power disturbance is adjusted to the positive disturbance.

[0008] Optionally, after adjusting to the disturbance amplitude, obtaining the current frequency of the power grid, including: 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 representing an island detection for the power grid, and the reactive scheduling instruction is an instruction representing scheduling of the reactive power of the power grid; obtaining the AC voltage of the power grid, and when the AC voltage is within a preset voltage range, receiving a reactive voltage crossing instruction, wherein the reactive voltage crossing instruction is an instruction representing voltage crossing in the power grid; upon receiving the island detection reactive instruction, the reactive scheduling instruction and the reactive voltage crossing instruction, obtaining the current frequency of the power grid.

[0009] Optionally, the power grid includes an inverter, and the method further includes: after determining that the island phenomenon occurs in the power grid, disconnecting the inverter; when the initial frequency is less than or equal to the first threshold, determining that the power grid is under-frequency protection and disconnecting the inverter; when the initial frequency is greater than or equal to the second threshold, determining that the power grid is over-frequency protection and disconnecting the inverter.

[0010] Optionally, determining the reference frequency includes: an acquisition step of acquiring the frequency 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 the moment corresponding to the initial frequency is acquired and adjacent to the moment corresponding to the initial frequency; a calculation step of calculating an average value of the plurality of historical frequencies to obtain the reference frequency.

[0011] Optionally, the method also includes: when the current frequency is greater than the first threshold and less than the second threshold, determining the usage time of the reference frequency; when 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 according to 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 according to the first mapping relationship; determining the disturbance amplitude according to 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 according to the second mapping relationship.

[0013] According to another aspect of the present application, a device for determining an islanding phenomenon is provided, comprising: a first determining unit, configured to apply a steady-state positive and negative bidirectional reactive power disturbance to a power grid, obtain the frequency of the power grid, obtain an initial frequency, and determine 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, the second threshold is an overfrequency protection frequency, and the steady-state positive and negative bidirectional reactive power disturbance is used to detect an islanding phenomenon in the power grid; a first adjusting unit, configured to calculate a difference between the initial frequency and the reference frequency to obtain a frequency offset, and when the absolute value of the frequency offset is less than a preset threshold, maintain the reactive power of the power grid at a predetermined value. The rate is the steady-state positive and negative bidirectional reactive power disturbance, and 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; a second adjusting 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, determine the disturbance amplitude according to the frequency offset, and adjust the amplitude of the reactive power of the power grid to the disturbance amplitude; a second determining unit is used to obtain the current frequency of the power grid after adjusting to the disturbance amplitude, and determine that the island phenomenon occurs in the power grid when the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold.

[0014] According to another aspect of the present application, a computer-readable storage medium is provided, wherein the computer-readable storage medium includes a stored program, wherein when the program is executed, the device where the computer-readable storage medium is located is controlled to execute any one of the determination methods.

[0015] According to another aspect of the present 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 are configured to be executed by the one or more processors, and the one or more programs include means for executing any one of the determination methods.

[0016] Applying the technical solution of the present application, the frequency of the power grid is obtained, the initial frequency is obtained, and when the initial frequency is greater than the first threshold and less than the second threshold, the reference frequency is determined; the difference between the initial frequency and the reference frequency is calculated to obtain the frequency offset, and 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; the disturbance pulse width is determined according to 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 according to 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, and 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 power grid has an islanding phenomenon. Compared with the prior art, the islanding detection method may affect the normal power supply quality and the islanding phenomenon judgment is inaccurate, the present application can efficiently complete the islanding detection of the most serious working conditions, can reduce the impact on the steady-state power quality, can easily be compatible with the system for bidirectional reactive scheduling and low voltage ride-through, and can accurately detect the islanding phenomenon in the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings constituting part of the present application are used to provide a further understanding of the present application. The exemplary embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0018] Figure 1 A hardware structure block diagram of a mobile terminal for executing an islanding phenomenon determination method provided by an embodiment of the present application is shown;

[0019] Figure 2 A schematic flow chart of a method for determining an island phenomenon provided by an embodiment of the present application is shown;

[0020] Figure 3 A schematic diagram showing a first mapping relationship between a disturbance pulse width and a frequency offset in a method for determining an island phenomenon provided in an embodiment of the present application is shown;

[0021] Figure 4 A schematic diagram showing a second mapping relationship between a disturbance amplitude and a frequency offset in a method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0022] Figure 5 A schematic diagram of overall three-dimensional frequency feedback and reactive disturbance when the frequency drops in a method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0023] Figure 6 A schematic diagram of overall three-dimensional frequency feedback and reactive disturbance when the frequency drops in a method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0024] Figure 7 A schematic diagram of overall control implementation of an island detection method in a method for determining an island phenomenon provided by an embodiment of the present application is shown;

[0025] Figure 8 A schematic diagram of a specific method for determining an island phenomenon provided by an embodiment of the present application is shown;

[0026] Fig. 9 A schematic diagram of a simulation verification system for an island detection method in a specific method for determining an island phenomenon provided in an embodiment of the present application is shown;

[0027] Fig.10 A schematic diagram of system frequency when the island resonance frequency is slightly lower than the power frequency in a specific method for determining the island phenomenon provided by an embodiment of the present application is shown;

[0028] Fig.11 A schematic diagram of reactive power when the island resonance frequency is slightly lower than the power frequency in a specific method for determining the island phenomenon provided by an embodiment of the present application is shown;

[0029] Fig.12 A schematic diagram of the voltage harmonic distribution of a steady-state grid-connected point in a specific method for determining an islanding phenomenon provided in an embodiment of the present application is shown;

[0030] Fig.13 A schematic diagram of system frequency when a steady-state normal frequency change is misjudged in a specific method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0031] Fig.14 A schematic diagram of reactive power when a steady-state normal frequency change is misjudged in a specific method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0032] Fig.15 A schematic diagram of system frequency during reactive power dispatch in a specific method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0033] Fig.16 A schematic diagram of reactive power during reactive scheduling in a specific method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0034] Fig.17 A schematic diagram of the effective value of the grid connection point voltage in a specific method for determining the islanding phenomenon provided in an embodiment of the present application is shown;

[0035] Fig.18 A schematic diagram of system frequency during low voltage ride-through in a specific method for determining an islanding phenomenon provided in an embodiment of the present application is shown;

[0036] Fig.19A schematic diagram of reactive power instruction during low voltage ride-through in a specific method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0037] Fig. 20 A schematic diagram of reactive power during low voltage ride-through in a specific method for determining an islanding phenomenon provided by an embodiment of the present application is shown;

[0038] Fig.21 A structural block diagram of a device for determining an island phenomenon provided in an embodiment of the present application is shown.

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

[0040] 102, processor; 104, memory; 106, transmission device; 108, input and output devices. DETAILED DESCRIPTION

[0041] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0042] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work 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 and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequential order. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present application described here. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0044] For the convenience of description, some nouns or terms involved in the embodiments of the present application are explained below:

[0045] Island phenomenon: also known as the island effect, refers to the independent system formed when the distributed power system is disconnected from the grid due to failure or maintenance, in which the local load is driven by the distributed power system.

[0046] As introduced in the background technology, the effect of the island detection method in the prior art is relatively poor. To solve the problem of the poor effect of the island detection method, the embodiments of the present application provide a method, device, computer-readable storage medium and electronic device for determining the island phenomenon.

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

[0048] The method embodiments provided in the embodiments of the present application can be executed in a mobile terminal, a computer terminal or a similar computing device. Taking running on a mobile terminal as an example, Figure 1 FIG. 1 is a hardware structure block diagram of a mobile terminal of a method for determining an island phenomenon according to an embodiment of the present invention. Figure 1 As shown, the mobile terminal may include one or more ( Figure 1 Only one is shown in the figure) a processor 102 (the processor 102 may include but is not limited to a processing device such as a microprocessor MCU or a programmable logic device FPGA) and a memory 104 for storing data, wherein the mobile terminal may also include a transmission device 106 and an input / output device 108 for communication functions. It can be understood by those skilled in the art that Figure 1 The structure shown is only for illustration and does not limit the structure of the mobile terminal. Figure 1 More or fewer components as shown, or with Figure 1 Different configurations are shown.

[0049] The memory 104 can be used to store computer programs, for example, software programs and modules of application software, such as the computer program corresponding to the method for determining the island phenomenon in the embodiment of the present invention. The processor 102 executes various functional applications and data processing by running the computer program stored in the memory 104, that is, the above method is implemented. The memory 104 may include a high-speed random access memory, and may also include a non-volatile memory, such as one or more magnetic storage devices, flash memory, or other non-volatile solid-state memory. In some examples, the memory 104 may further include a memory remotely arranged relative to the processor 102, and these remote memories can be connected to the mobile terminal via a network. Examples of the above-mentioned network include but are not limited to the Internet, an intranet, a local area network, a mobile communication network, and a combination thereof. The transmission device 106 is used to receive or send data via a network. The specific example of the above-mentioned network may include a wireless network provided by a communication provider of the mobile terminal. In one example, the transmission device 106 includes a network adapter (Network Interface Controller, referred to as NIC), which can be connected to other network devices through a base station so as 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] In this embodiment, a method for determining an island phenomenon running on a mobile terminal, a computer terminal or a similar computing device is provided. It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0051] Figure 2 is a flow chart of a method for determining an island phenomenon according to an embodiment of the present application. Figure 2 As shown, the method comprises the following steps:

[0052] Step S201, applying a steady-state positive and negative bidirectional reactive power disturbance to the power grid, acquiring the frequency of the power grid, obtaining an initial frequency, and 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, the second threshold is an overfrequency protection frequency, and the steady-state positive and negative bidirectional reactive power disturbance is used to detect an islanding phenomenon in the power grid;

[0053] Specifically, the present application provides an inverter-type distributed power supply island detection method based on three-dimensional frequency feedback. First, the reactive power instruction corresponding to the inverter participating in the bidirectional reactive power dispatch of the power grid is recorded as Q ref1 , the reactive power disturbance command corresponding to the island detection is recorded as Qref2 The reactive power command of the outer loop control link of the inverter low voltage ride-through is recorded as Q ref3 The three together constitute the total reactive power command of the inverter. The island detection reactive disturbance command, that is, the steady-state positive and negative bidirectional reactive power disturbance, is recorded as Q ref2 , in steady state, it is set to a positive and negative bidirectional periodic square wave with zero interval, the first half cycle is 1 power frequency cycle 20ms long, including a positive square wave with a pulse width of 7ms and an amplitude of 0.015pu, and the second half cycle is 1 power frequency cycle 20ms long, including a negative square wave with a pulse width of 7ms and an amplitude of -0.015pu, and the total cycle is 40ms. Then set the over / under frequency protection frequency, the over frequency protection frequency is recorded as f HI , the under-frequency protection frequency is recorded as f LW According to relevant documents, f HI 50.2Hz is acceptable, f LW The threshold can be 47.5Hz, and it 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 , indicating that the system real-time frequency f does not exceed the over / under frequency protection threshold, and the reference frequency is determined and recorded as f base , and keep it unchanged within the longest allowed island detection time of 2s.

[0054] Step S202, calculating 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 maintained 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.

[0055] Specifically, the real-time frequency f minus the reference frequency f base The frequency offset Δf is obtained, and the preset threshold can be 0.15Hz. When its absolute value |Δf| ≥ 0.15Hz, the system is triggered to enter the three-dimensional frequency feedback-reactive disturbance stage. The duration limit of each round of three-dimensional frequency feedback-reactive disturbance is the maximum allowable island detection time. The three-dimensional frequency feedback of the disturbance direction, disturbance pulse width, and disturbance amplitude of reactive power is carried out. The first is the frequency feedback based on the disturbance direction of reactive power. The direction of the reactive power of the power grid needs to be adjusted to the disturbance direction. The specific adjustment method will be described below.

[0056] Step S203, determining the 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 the disturbance amplitude according to the frequency offset, and adjusting the amplitude of the reactive power of the power grid to the disturbance amplitude;

[0057] Specifically, after the frequency feedback based on the disturbance direction of reactive power, the frequency feedback is performed based on the disturbance pulse width and the disturbance amplitude. ref2 The single perturbation artery width is recorded as Q pw2 , the pulse width is 7ms in steady state. After entering the frequency feedback stage, the pulse width gradually increases with the frequency offset. The maximum pulse width corresponds to half a disturbance cycle of 20ms. The disturbance pulse width-frequency offset curve is shown in Figure 3 As shown. Disturbance artery width Q pw2 When the upper limit is reached, the corresponding frequency offset limit is recorded as |Δf| lim , which is the reference frequency and the frequency lower limit threshold f LW The difference is multiplied by a certain margin, and the expression is: |Δf| lim =0.9(f base -f LW ), where f LW Rather than f HI The frequency offset limit is obtained because the difference between the under-frequency protection threshold and the power frequency specified by the Energy Bureau is significantly greater than the over-frequency, making it more difficult to detect islands. In addition, 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, the inverter is allowed to work at 1.1 times the apparent power for a short time. The reactive power limit of the inverter is Q max It can be expressed as: Generally, it is set to 0.46, where S is the rated apparent power of the inverter and P is the real-time active power output by the inverter. When |Δf| just reaches 0.15Hz, the reactive power limit obtained by the above formula is kept for the maximum island detection time of 2s specified by the Energy Bureau, recorded as Q max2 , which is used to calculate the specific disturbance amplitude-frequency offset curve during the frequency feedback reactive disturbance. However, the total reactive power command of the inverter is still subject to real-time reactive power limit. When |Δf| exceeds 0.15Hz, the disturbance amplitude-frequency offset curve is as follows: Figure 4 shown. Figure 4 In, Q mag2 is the reactive disturbance command Q ref2 The absolute value of the amplitude, its upper limit Q mag2lim The fixed value is 0.46: Q mag2lim =Q max2 -(Q ref1 +Q ref3 ), Q mag2lim Considering the reactive support command Q ref1 With voltage ride-through instruction Q ref3 The occupation of the inverter's reactive output capacity. Therefore, Q mag2 The expression is:

[0058]

[0059] The overall three-dimensional frequency feedback-reactive disturbance implementation diagram is as follows: Figure 5 and Figure 6 shown. Figure 5 The corresponding frequency decreases, Figure 6 Corresponding to the case of frequency increase.

[0060] Step S204, after adjusting to the disturbance amplitude, obtaining the current frequency of the power grid, and determining that an islanding phenomenon occurs in the power grid when the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold.

[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 offset exceeds the limit due to the islanding phenomenon, and the inverter's output circuit breaker is immediately disconnected to complete the anti-islanding protection.

[0062] Through this embodiment, the frequency of the power grid can be obtained, and the initial frequency can be obtained. When the initial frequency is greater than the first threshold and less than the second threshold, the reference frequency is determined; the difference between the initial frequency and the reference frequency is calculated to obtain the frequency offset, and 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; the disturbance pulse width is determined according to 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 according to 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, and 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 power grid has an island phenomenon. Compared with the prior art, the island detection method may affect the normal power supply quality and the island phenomenon judgment is inaccurate, the present application can efficiently complete the island detection of the most serious working condition, can reduce the impact on the steady-state power quality, can easily be compatible with the system for bidirectional reactive scheduling and low voltage ride-through, and can accurately detect the island phenomenon in the power grid.

[0063] In the specific implementation process, step S202 adjusts the direction of the reactive power of the power grid to the disturbance direction when the absolute value of the frequency offset is greater than or equal to the preset threshold, which can be achieved by the following steps: when the frequency offset is greater than the preset threshold, the positive disturbance in the steady-state positive and negative 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 positive and negative 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, so that different disturbances can be applied in the two cases to accurately detect the islanding phenomenon.

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

[0065] In order to accurately obtain the current frequency of the power grid when the power grid meets the detection conditions, step S204 obtains the current frequency of the power grid after adjusting to the disturbance amplitude, which can be achieved by the following steps: after adjusting to the disturbance amplitude, receiving the island detection reactive instruction and the reactive dispatch instruction, wherein the island detection reactive instruction is an instruction for characterizing the island detection of the power grid, and the reactive dispatch instruction is an instruction for characterizing the dispatch of the reactive power of the power grid; obtaining the AC voltage of the power grid, and receiving the reactive voltage ride-through instruction when the AC voltage is within the preset voltage range, wherein the reactive voltage ride-through instruction is an instruction for characterizing the voltage ride-through of the power grid; obtaining the current frequency of the power grid when the island detection reactive instruction, the reactive dispatch instruction and the reactive voltage ride-through instruction are received. The method obtains the current frequency when the power grid receives the instruction, that is, when it meets the detection conditions, so that the accurate current frequency can be obtained and whether the island phenomenon occurs can be accurately detected.

[0066] In the specific implementation process, as mentioned above, the reactive power instruction corresponding to the inverter participating in the bidirectional reactive power dispatch of the power grid is first recorded as Q ref1 , the reactive power disturbance command corresponding to the island detection is recorded as Q ref2 The reactive power command of the outer loop control link of the inverter low voltage ride-through is recorded as Q ref3 , the three together constitute the total reactive power command of the inverter. When the total reactive power command is received, the current frequency is obtained.

[0067] In some optional embodiments, the power grid includes an inverter, and the method further includes the following steps: after determining that the power grid has an islanding phenomenon, disconnecting the inverter; when 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; when 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. The method disconnects the inverter when determining that the power grid has an islanding phenomenon, thereby avoiding power grid failure.

[0068] In the specific implementation process, after the system real-time frequency f is detected, if it is higher than f HI or below f LW , the over / under frequency protection is triggered, and the inverter output circuit breaker is directly disconnected within 0.2s. And after determining that the grid has an islanding phenomenon, the circuit breaker is also disconnected, which can prevent the grid from failing due to the islanding phenomenon and affecting the normal operation of the grid.

[0069] In order to accurately determine the reference frequency, the reference frequency in step S201 can be determined by the following steps: an acquisition step: acquiring the frequency of a predetermined number of cycles of the power grid in the past time period to obtain multiple historical frequencies, wherein the past time period is a time period before the moment corresponding to the acquisition of the initial frequency and adjacent to the moment corresponding to the initial frequency; a calculation step: calculating the average value of the multiple historical frequencies to obtain the reference frequency. This method obtains the reference frequency by calculating the average value of multiple historical frequencies in the past time period, so that the reference frequency can be determined objectively and accurately.

[0070] In the specific implementation process, under the premise that the system real-time frequency f does not exceed the over / under frequency protection threshold, the predetermined number is generally 6 cycles, and the average value of the system frequency in the past 6 power frequency cycles is taken as the dynamic reference frequency and recorded as f base , and keep it unchanged within the longest allowed island detection time of 2s. The predetermined number can also be other numbers, and this application does not impose specific restrictions on the predetermined number.

[0071] In some optional implementations, the method further includes the following steps: when the current frequency is greater than the first threshold and less than the second threshold, determining the usage time of the reference frequency; when the usage time is greater than the preset time, repeating the acquisition step and the calculation step to update the reference frequency. The method updates the reference frequency in a timely manner, so that the real-time nature of island monitoring can be maintained to more accurately detect the island phenomenon.

[0072] Specifically, when the reference frequency usage time exceeds the preset time, for example, 2s, the reference frequency is updated. In actual application, if the system frequency does not reach the set over / under frequency protection threshold during a round of frequency feedback-reactive disturbance, it is considered that no islanding phenomenon has occurred, and the previous frequency offset limit is a frequency fluctuation within the normal range. At this time, the reference frequency f base Update, so that the frequency offset becomes within ±0.15Hz, and the disturbance mode is restored to the bidirectional, narrow pulse width, low amplitude disturbance in the steady state.

[0073] In order to accurately determine the disturbance pulse width and the disturbance amplitude, step S203 determines the disturbance pulse width according to the frequency offset by 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 according to the first mapping relationship; determining the disturbance amplitude according to the frequency offset can be achieved by 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 according to the second mapping relationship. The method determines the disturbance pulse width and the disturbance amplitude according to the mapping relationship of the frequency offset, so that the disturbance pulse width and the disturbance amplitude can be accurately determined.

[0074] In the specific implementation process, the perturbation pulse 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 recorded as |Δf| lim , which is the reference frequency and the frequency lower limit threshold f LW The difference is multiplied by a certain margin, and the expression is: |Δf| lim =0.9(f base -f LW ), where f LW Rather than f HI The frequency offset limit is obtained because the difference between the under-frequency protection threshold and the power frequency specified by the Energy Bureau is significantly greater than the over-frequency, making it more difficult to detect islands. In addition, when the disturbance pulse width 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 shown. Figure 4 In, Q mag2 is the reactive disturbance command Q ref2 The absolute value of the amplitude, its upper limit Q mag2lim The fixed value is 0.46: Q mag2lim =Q max2 -(Q ref1 +Q ref3 ), Q mag2lim Considering the reactive support command Q ref1 With voltage ride-through instruction Q ref3 The occupation of the inverter's reactive output capacity.

[0075] Figure 7 The figure shows the overall control implementation diagram of the island detection method. ref2The derivation process is as follows: 1) The pulse generator provides a reference pulse, which is then widened by the 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, 20 ms, to avoid the subsequent flip signal from being inverted within a single disturbance square wave, and T2 is taken as the maximum allowable island detection duration, 2 s, to stop the frequency feedback - reactive power disturbance within the time limit. 3) F30 and F1 pass through AND gate 1 and single-input comparator 1 to obtain the 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. 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 also obtains Q ref2 . The derivation process of the reactive power disturbance amplitude Q mag22 is as follows: 1) F30 is widened by the 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 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 unchanged. 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 pulse width T of the reactive power disturbancepw22 The process of obtaining is as follows: 1) Calculate the real-time disturbance pulse width Tpw2 that changes with the frequency offset through formula (2), and pass through the limiting link with a maximum of 20ms and a minimum of 7ms. 2) The sum of the absolute values ​​of the trigger square wave InPN1 is used as the holding signal of the sampling / holding 3, so that each disturbance square wave has a fixed pulse width at each disturbance, rather than the time-varying value calculated by formula (2). 3) Then, through the link of "taking the previous step length value", the previous step length value T of the calculated pulse width is obtained. pw21 , otherwise the pulse width signal will have a causal contradiction. 4) Finally, through the dual input selection link, T pw22 Select 7ms in steady state and T in frequency feedback-reactive disturbance stage. pw21 .also, Figure 7 The voltage ride-through control link compatible with island detection is also demonstrated. The voltage ride-through strategy used in the present invention is that when the AC voltage reaches the voltage ride-through threshold, the difference ΔU between the AC voltage reference value and the actual value is used to pass through the proportional integral link (PI link) to obtain the corresponding outer loop reactive power command Q ref3 To prevent the command from increasing too fast during voltage drop, the original q-axis current command i qref1 The superposition of ΔU generates i through the gain link containing the voltage dead zone qref3 , which can play a certain role in limiting the growth of reactive current.

[0076] In order to enable those skilled in the art to more clearly understand the technical solution of the present application, the implementation process of the method for determining the island phenomenon of the present application will be described in detail below in conjunction with specific embodiments.

[0077] This embodiment relates to a specific method for determining an island phenomenon, such as Figure 8 As shown, distributed photovoltaic power generation system has a wide range of applications and is an important representative of distributed power supply system. It is taken as an example for explanation. Figure 8 The simulation verification system diagram of the island detection method built on the PSCAD / EMTDC platform is shown in Table 1. The main parameters of the photovoltaic simulation system include the following steps:

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

[0079] Table 1 Main parameters of photovoltaic simulation system

[0080]

[0081] Step 2: Adopt the island detection method of the present invention, which is a positive and negative bidirectional disturbance in steady state. At 3.0396s, the frequency offset exceeds -0.15Hz, and the system automatically enters the frequency feedback-reactive disturbance stage. The negative disturbance is flipped to positive, and the reactive disturbance amplitude and reactive disturbance pulse width are calculated in real time according to the frequency offset. The disturbance amplitude and pulse width both increase with the frequency deviation. When the frequency touches the under-frequency protection threshold, the island detection is completed. Due to system inertia, the increase in reactive power output by the inverter has a certain lag compared to the change in reactive command. If 49.5Hz adopted by most literature is used as the standard for successful island detection, this method takes 0.2s to complete the island detection; if 47.5Hz stipulated by the Energy Bureau is used as the standard, this method takes 0.44s, and the circuit breaker 3 on the inverter side can be triggered to disconnect at 3.44s to complete the anti-island protection. In this process, the grid connection point frequency, the island detection reactive command and the inverter reactive output are as follows: Fig.10 and Fig.11 shown.

[0082] Step S3: Fig.12 The figure shows the 2-7 harmonic amplitudes of the phase voltage at the grid connection point in steady state. The present invention benefits from the ultra-low amplitude and ultra-narrow pulse width of the disturbance in steady state, and the total harmonic distortion rate THD is only 0.323%. The inverter output reactive power in steady state is only between ±0.0008pu. The present invention hardly affects the AC voltage quality and steady-state reactive power when connected to the grid.

[0083] Step S4: Not affected by the misjudgment of steady-state normal frequency change: The grid frequency changes from 50Hz to 49.84Hz at 3s. base Still at 50Hz, at the following 3.03974s, it is detected that |Δf| exceeds 0.15Hz, thus triggering frequency feedback-reactive disturbance. The frequency and reactive power of this process are as follows: Fig.13 and Fig.14 As shown. After about 2s of frequency feedback-reactive disturbance, f base Updated to 49.84Hz, the disturbance is restored to a two-way disturbance in steady state. During the feedback disturbance, due to the clamping effect of the large power grid, the system frequency and reactive power do not change much. The present invention has the ability to deal with the misjudgment of steady-state normal frequency changes.

[0084] Step S5: The inverter can be supported to participate in the bidirectional reactive power dispatch of the power grid: the inverter generates an active power of 0.8 pu in steady state, and the working condition 1 is set as follows: at 3.0 s, the reactive power support instruction Qref1 From 0 to 0.4pu after 1s; Working condition 2: At 3.0s, the reactive support instruction Q ref1 It gradually decreases from 0 to -0.4pu in 1s. Fig.15 and Fig.16 The system frequency and reactive power of the process are shown. The island detection strategy in the present invention does not affect the inverter's participation in the bidirectional reactive dispatch of the power grid when it is not fully powered. It can provide reactive support to the load and the power grid, and absorb excess reactive power from the power grid. The system frequency is always maintained at 50Hz, and the reactive output follows the reactive command well.

[0085] Step S6: Supporting the inverter to perform low voltage ride-through: Connecting at 3.0s Figure 6 The voltage drop simulation impedance in the circuit is used to achieve a duration of 2s and an amplitude of about 0.6U N voltage drop. Fig.17 The figure shows the grid connection point voltage when the low voltage ride-through module is activated and blocked respectively. The island detection strategy is fully activated in both working conditions. Fig.18 , Fig.19 and Fig. 20 The frequency and island reactive power command Q when low voltage ride-through control and island detection strategy are put into use ref2 , and the reactive power Q output by the inverter PV .Depend on Fig.17 It can be seen that after the low voltage ride-through control used in the present invention is put into use, the AC voltage increases from 0.574pu to 0.595pu, with an increase of about 0.021pu. Fig.17 , Fig.18 , Fig.19 and Fig. 20 , the inverter can simultaneously invest in the island detection strategy and the low voltage ride-through strategy, and maintain grid-connected operation during the voltage drop to achieve low voltage ride-through.

[0086] The present application embodiment also provides a device for determining an island phenomenon. It should be noted that the device for determining an island phenomenon in the embodiment of the present application can be used to execute the method for determining an island phenomenon provided in the embodiment of the present application. The device is used to implement the embodiments and preferred implementation modes, and the descriptions that have been made will not be repeated. As used below, the term "module" can implement a combination of software and / or hardware for a predetermined function. Although the device described in the following embodiments is preferably implemented in software, the implementation of hardware, or a combination of software and hardware, is also possible and conceived.

[0087] The following is an introduction to the islanding phenomenon determination device provided in the embodiment of the present application.

[0088] Fig.21Schematic diagram of an islanding phenomenon determination device according to an embodiment of the present application. Fig.21 As shown, the device comprises:

[0089] A first determination unit 10 is used to apply a steady-state positive and negative bidirectional reactive power disturbance to the power grid, obtain the frequency of the power grid, obtain an initial frequency, and determine 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, the second threshold is an overfrequency protection frequency, and the steady-state positive and negative bidirectional reactive power disturbance is used to detect an islanding phenomenon in the power grid;

[0090] Specifically, the present application provides an inverter-type distributed power supply island detection method based on three-dimensional frequency feedback. First, the reactive power instruction corresponding to the inverter participating in the bidirectional reactive power dispatch of the power grid is recorded as Q ref1 , the reactive power disturbance command corresponding to the island detection is recorded as Q ref2 The reactive power command of the outer loop control link of the inverter low voltage ride-through is recorded as Q ref3 The three together constitute the total reactive power command of the inverter. The island detection reactive disturbance command, that is, the steady-state positive and negative bidirectional reactive power disturbance, is recorded as Q ref2 , in steady state, it is set to a positive and negative bidirectional periodic square wave with zero interval, the first half cycle is 1 power frequency cycle 20ms long, including a positive square wave with a pulse width of 7ms and an amplitude of 0.015pu, and the second half cycle is 1 power frequency cycle 20ms long, including a negative square wave with a pulse width of 7ms and an amplitude of -0.015pu, and the total cycle is 40ms. Then set the over / under frequency protection frequency, the over frequency protection frequency is recorded as f HI , the under-frequency protection frequency is recorded as f LW According to relevant documents, f HI 50.2Hz can be taken, f LW The threshold can be 47.5Hz, and it 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 , indicating that the system real-time frequency f does not exceed the over / under frequency protection threshold, and the reference frequency is determined and recorded as f base , and keep it unchanged within the longest allowed island detection time of 2s.

[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 maintained 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 obtained, and the preset threshold can be 0.15Hz. When its absolute value |Δf| ≥ 0.15Hz, the system is triggered to enter the three-dimensional frequency feedback-reactive disturbance stage. The duration limit of each round of three-dimensional frequency feedback-reactive disturbance is the maximum allowable island detection time. The three-dimensional frequency feedback of the disturbance direction, disturbance pulse width, and disturbance amplitude of reactive power is carried out. The first is the frequency feedback based on the disturbance direction of reactive power. The direction of the reactive power of the power grid needs to be adjusted to the disturbance direction. The specific adjustment device will be described below.

[0093] The second adjustment unit 30 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, determine the disturbance amplitude according to the frequency offset, and adjust the amplitude of the reactive power of the power grid to the disturbance amplitude;

[0094] Specifically, after the frequency feedback based on the disturbance direction of reactive power, the frequency feedback is performed based on the disturbance pulse width and the disturbance amplitude. ref2 The single perturbation artery width is recorded as Q pw2 , the pulse width is 7ms in steady state. After entering the frequency feedback stage, the pulse width gradually increases with the frequency offset. The maximum pulse width corresponds to half a disturbance cycle of 20ms. The disturbance pulse width-frequency offset curve is shown in Figure 3 As shown. Disturbance artery width Q pw2 When the upper limit is reached, the corresponding frequency offset limit is recorded as |Δf| lim , which is the reference frequency and the frequency lower limit threshold f LW The difference is multiplied by a certain margin, and the expression is: |Δf| lim =0.9(f base -f LW ), where f LW Rather than f HI The frequency offset limit is obtained because the difference between the under-frequency protection threshold and the power frequency specified by the Energy Bureau is significantly greater than the over-frequency, making it more difficult to detect islands. In addition, 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, the inverter is allowed to work at 1.1 times the apparent power for a short time. The reactive power limit of the inverter is Q max It can be expressed as: Generally, it is set to 0.46, where S is the rated apparent power of the inverter and P is the real-time active power output by the inverter. When |Δf| just reaches 0.15Hz, the reactive power limit obtained by the above formula is kept for the maximum island detection time of 2s specified by the Energy Bureau, recorded as Q max2, which is used to calculate the specific disturbance amplitude-frequency offset curve during the frequency feedback reactive disturbance. However, the total reactive power command of the inverter is still subject to real-time reactive power limit. When |Δf| exceeds 0.15Hz, the disturbance amplitude-frequency offset curve is as follows: Figure 4 shown. Figure 4 In, Q mag2 is the reactive disturbance command Q ref2 The absolute value of the amplitude, its upper limit Q mag2lim The fixed value is 0.46: Q mag2lim =Q max2 -(Q ref1 +Q ref3 ), Q mag2lim Considering the reactive support command Q ref1 With voltage ride-through instruction Q ref3 The occupation of the inverter's reactive output capacity. Therefore, Q mag2 The expression is:

[0095] The overall three-dimensional frequency feedback-reactive disturbance implementation diagram is as follows: Figure 5 and Figure 6 shown. Figure 5 The corresponding frequency decreases, Figure 6 Corresponding to the case of frequency increase.

[0096] The second determination unit 40 is used to obtain the current frequency of the power grid after adjusting to the disturbance amplitude, and 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 offset exceeds the limit due to the islanding phenomenon, and the inverter's output circuit breaker is immediately disconnected to complete the anti-islanding protection.

[0098] Through this embodiment, the frequency of the power grid can be obtained, and the initial frequency can be obtained. When the initial frequency is greater than the first threshold and less than the second threshold, the reference frequency is determined; the difference between the initial frequency and the reference frequency is calculated to obtain the frequency offset, and 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; the disturbance pulse width is determined according to 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 according to 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, and 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 power grid has an islanding phenomenon. Compared with the prior art, the islanding detection device may affect the normal power supply quality and the device with inaccurate judgment of the islanding phenomenon, the present application can efficiently complete the islanding detection of the most serious working conditions, can reduce the impact on the steady-state power quality, can easily be compatible with the system for bidirectional reactive scheduling and low voltage ride-through, and can accurately detect the islanding phenomenon in the power grid.

[0099] In the specific implementation process, the first adjustment unit includes a first adjustment module and a second adjustment module, wherein the first adjustment module is used to adjust the positive disturbance in the steady-state positive and negative 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 positive and negative bidirectional reactive power disturbance to a positive disturbance when the frequency offset is less than a negative preset threshold. The device divides the absolute value of the frequency offset into two cases for disturbance, so that different disturbances can be applied in the two cases to accurately detect the islanding phenomenon.

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

[0101] In order to accurately obtain the current frequency of the power grid when the power grid meets the detection conditions, the second determination unit includes a first receiving module, a second receiving module and a first acquisition module, wherein the first receiving module is used to receive the island detection reactive instruction and the reactive dispatch instruction after adjusting to the disturbance amplitude, wherein the island detection reactive instruction is an instruction to characterize the island detection of the power grid, and the reactive dispatch instruction is an instruction to characterize the dispatch of the reactive power of the power grid; the second receiving module is used to obtain the AC voltage of the power grid, and when the AC voltage is within the preset voltage range, the reactive voltage ride-through instruction is received, wherein the reactive voltage ride-through instruction is an instruction to characterize the voltage ride-through of the power grid; the first acquisition module is used to obtain the current frequency of the power grid when the island detection reactive instruction, the reactive dispatch instruction and the reactive voltage ride-through instruction are received. When the power grid receives the instruction, that is, when the detection conditions are met, the device obtains the current frequency, so that the accurate current frequency can be obtained and whether the island phenomenon occurs can be accurately detected.

[0102] In the specific implementation process, as mentioned above, the reactive power instruction corresponding to the inverter participating in the bidirectional reactive power dispatch of the power grid is first recorded as Q ref1 , the reactive power disturbance command corresponding to the island detection is recorded as Q ref2 The reactive power command of the outer loop control link of the inverter low voltage ride-through is recorded as Q ref3 , the three together constitute the total reactive power command of the inverter. When the total reactive power command is received, the current frequency is obtained.

[0103] In some optional embodiments, the power grid includes an inverter, and the device further includes a first disconnection unit, a second disconnection unit, and a third disconnection unit, wherein the first disconnection unit is used to disconnect the inverter after determining that the power grid has an islanding phenomenon; the second disconnection unit is used to determine that the power grid is under-frequency protection and disconnect the inverter when the initial frequency is less than or equal to the first threshold; the third disconnection unit is used to determine that the power grid is over-frequency protection and disconnect the inverter when the initial frequency is greater than or equal to the second threshold. When the device determines that the power grid has a phenomenon, it disconnects the inverter, which can avoid power grid failure.

[0104] In the specific implementation process, after the system real-time frequency f is detected, if it is higher than f HI or below f LW , the over / under frequency protection is triggered, and the inverter output circuit breaker is directly disconnected within 0.2s. And after determining that the grid has an islanding phenomenon, the circuit breaker is also disconnected, which can prevent the grid from failing due to the islanding phenomenon and affecting the normal operation of the grid.

[0105] In order to accurately determine the reference frequency, the first determination unit includes a second acquisition module and a calculation module, wherein the second acquisition module is used to obtain the frequency of a predetermined number of cycles of the power grid in the past time period to obtain multiple historical frequencies, wherein the past time period is a time period before the moment corresponding to the acquisition of the initial frequency and adjacent to the moment corresponding to the initial frequency; the calculation module is used to calculate the average value of the multiple historical frequencies to obtain the reference frequency. The device obtains the reference frequency by calculating the average value of the multiple historical frequencies in the past time period, so that the reference frequency can be determined objectively and accurately.

[0106] In the specific implementation process, under the premise that the system real-time frequency f does not exceed the over / under frequency protection threshold, the predetermined number is generally 6 cycles, and the average value of the system frequency in the past 6 power frequency cycles is taken as the dynamic reference frequency and recorded as f base , and keep it unchanged within the longest allowed island detection time of 2s. The predetermined number can also be other numbers, and this application does not impose specific restrictions on the predetermined number.

[0107] In some optional embodiments, the device further includes a third determination unit and a repeating unit, wherein the third determination unit is used to determine the usage time of the reference frequency when the current frequency is greater than the first threshold and less than the second threshold; and the repeating unit is used to repeat the acquisition step and the calculation step to update the reference frequency when the usage time is greater than the preset time. The device updates the reference frequency in a timely manner, so that the real-time nature of island monitoring can be maintained to more accurately detect the island phenomenon.

[0108] Specifically, when the reference frequency usage time exceeds the preset time, for example, 2s, the reference frequency is updated. In actual application, if the system frequency does not reach the set over / under frequency protection threshold during a round of frequency feedback-reactive disturbance, it is considered that no islanding phenomenon has occurred, and the previous frequency offset limit is a frequency fluctuation within the normal range. At this time, the reference frequency f base Update, so that the frequency offset becomes within ±0.15Hz, and the disturbance mode is restored to the bidirectional, narrow pulse width, low amplitude disturbance in the steady state.

[0109] In order to accurately determine the disturbance pulse width and the disturbance amplitude, the second adjustment unit includes a first determination module and a second determination module, the first determination module is used to obtain a first mapping relationship between the frequency offset and the disturbance pulse width, and determine the disturbance pulse width corresponding to the frequency offset according to the first mapping relationship; the second determination module is used to determine the disturbance amplitude according to the frequency offset, which can be achieved by 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 according to the second mapping relationship. The device determines the disturbance pulse width and the disturbance amplitude according to the mapping relationship of the frequency offset, so that the disturbance pulse width and the disturbance amplitude can be accurately determined.

[0110] In the specific implementation process, the perturbation pulse 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 recorded as |Δf| lim , which is the reference frequency and the frequency lower limit threshold f LW The difference is multiplied by a certain margin, and the expression is: |Δf| lim =0.9(f base -f LW ), where f LW Rather than f HI The frequency offset limit is obtained because the difference between the under-frequency protection threshold and the power frequency specified by the Energy Bureau is significantly greater than the over-frequency, making it more difficult to detect islands. In addition, when the disturbance pulse width 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 shown. Figure 4 In, Q mag2 is the reactive disturbance command Q ref2 The absolute value of the amplitude, its upper limit Q mag2lim The fixed value is 0.46: Q mag2lim =Q max2 -(Q ref1 +Q ref3 ), Q mag2lim Considering the reactive support command Q ref1 With voltage ride-through instruction Q ref3 The occupation of the inverter's reactive output capacity.

[0111] Figure 7 The figure shows the overall control implementation diagram of the island detection method. 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. After delaying Pp0 by one power frequency cycle and then taking the logical inverse, it becomes 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 taken as half of the disturbance period length, i.e., 20 ms, 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 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. After multiplying Pn1 by the disturbance amplitude Q mag22 it also obtains Q ref2 . 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 of 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, i.e., 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 of obtaining is as follows: 1) Calculate the real-time disturbance pulse width Tpw2 that changes with the frequency offset through formula (2), and pass through the limiting link with a maximum of 20ms and a minimum of 7ms. 2) The sum of the absolute values ​​of the trigger square wave InPN1 is used as the holding signal of the sampling / holding 3, so that each disturbance square wave has a fixed pulse width at each disturbance, rather than the time-varying value calculated by formula (2). 3) Then, through the link of "taking the previous step length value", the previous step length value T of the calculated pulse width is obtained. pw21 , otherwise the pulse width signal will have a causal contradiction. 4) Finally, through the dual input selection link, T pw22 Select 7ms in steady state and T in frequency feedback-reactive disturbance stage. pw21 .also, Figure 7 The voltage ride-through control link compatible with island detection is also demonstrated. The voltage ride-through strategy used in the present invention is that when the AC voltage reaches the voltage ride-through threshold, the difference ΔU between the AC voltage reference value and the actual value is used to pass through the proportional integral link (PI link) to obtain the corresponding outer loop reactive power command Q ref3 To prevent the command from increasing too fast during voltage drop, the original q-axis current command i qref1 The superposition of ΔU generates i through the gain link containing the voltage dead zone qref3 , which can play a certain role in limiting the growth of reactive current.

[0112] The islanding phenomenon determination device includes a processor and a memory, wherein the first determination unit, the first adjustment unit, the second adjustment unit, and the second determination unit are all stored in the memory as program units, and the processor executes the program units stored in the memory to implement corresponding functions. The modules are all located in the same processor; or, the modules are located in different processors in any combination.

[0113] The processor includes a kernel, which calls the corresponding program unit from the memory. One or more kernels can be set, and the island phenomenon can be accurately detected by adjusting the kernel parameters.

[0114] The memory may include non-permanent memory in a computer-readable medium, 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] Obviously, those skilled in the art should understand that the above modules or steps of the present invention can be implemented by a general computing device, they can be concentrated on a single computing device, or distributed on a network composed of multiple computing devices, they can be implemented by a program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, and in some cases, the steps shown or described can be executed in a different order than here, or they can be made into individual integrated circuit modules, or multiple modules or steps therein can be made into a single integrated circuit module for implementation. Thus, the present invention is not limited to any specific combination of hardware and software.

[0116] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that include computer-usable program code.

[0117] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0118] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0119] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 A step that specifies a function 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] The memory may include non-permanent memory in a computer-readable medium, random access memory (RAM) and / or non-volatile memory in the form of read-only memory (ROM) or flash RAM. The memory is an example of a computer-readable medium.

[0122] Computer readable media include permanent and non-permanent, removable and non-removable media that can be implemented by any method or technology to store information. Information can be computer readable instructions, data structures, program modules 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 technology, compact disk read-only memory (CD-ROM), digital versatile disk (DVD) or other optical storage, magnetic cassettes, magnetic disk storage or other magnetic storage devices or any other non-transmission media that can be used to store information that can be accessed by a computing device. As defined herein, computer readable media does not include temporary computer readable media (transitory media), such as modulated data signals and carrier waves.

[0123] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

[0124] From the above description, it can be seen that the above embodiments of the present application achieve the following technical effects:

[0125] In the method for determining the island phenomenon of the present application, the frequency of the power grid is obtained to obtain the initial frequency, and when the initial frequency is greater than the first threshold and less than the second threshold, the reference frequency is determined; the difference between the initial frequency and the reference frequency is calculated to obtain the frequency offset, and 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; the disturbance pulse width is determined according to 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 according to 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, and 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 power grid has an island phenomenon. Compared with the prior art, the island detection method may affect the normal power supply quality and the island phenomenon judgment is inaccurate, the present application can efficiently complete the island detection of the most serious working conditions, can reduce the impact on the steady-state power quality, can easily be compatible with the system for bidirectional reactive scheduling and low voltage ride-through, and can accurately detect the island phenomenon in the power grid.

[0126] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for determining an island phenomenon, characterized in that: include: Applying a steady-state positive and negative bidirectional reactive power disturbance to a power grid, acquiring the frequency of the power grid, obtaining an initial frequency, and 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, the second threshold is an overfrequency protection frequency, and the steady-state positive and negative bidirectional reactive power disturbance is used to detect an islanding phenomenon in the power grid; Calculate the difference between the initial frequency and the reference frequency to obtain a frequency offset, and when the absolute value of the frequency offset is less than a preset threshold, maintain the reactive power of the power grid as the steady-state positive and negative bidirectional reactive power disturbance, and when 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; 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, determine the disturbance amplitude according to the frequency offset, and adjust the amplitude of the reactive power of the power grid to the disturbance amplitude; After adjusting to the disturbance amplitude, a current frequency of the power grid is acquired, and 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 islanding phenomenon occurs in the power grid.

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 a preset threshold, adjusting the direction of the reactive power of the power grid to the disturbance direction includes: When 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; When the frequency offset is less than the negative preset threshold, the negative disturbance in the steady-state positive and negative bidirectional reactive power disturbance is adjusted to the positive disturbance.

3. The determination method according to claim 1, characterized in that: After adjusting to the disturbance amplitude, obtaining the current frequency of the power grid includes: After adjusting to the disturbance amplitude, receiving an island detection reactive power instruction and a reactive power dispatch instruction, wherein the island detection reactive power instruction is an instruction for performing island detection on the power grid, and the reactive power dispatch instruction is an instruction for dispatching reactive power of the power grid; Acquiring an AC voltage of the power grid, and receiving a reactive voltage ride-through instruction when the AC voltage is within a preset voltage range, wherein the reactive voltage ride-through instruction is an instruction indicating that voltage ride-through occurs in the power grid; When the island detection reactive power instruction, the reactive power dispatch instruction and the reactive voltage ride-through instruction are received, the current frequency of the power grid is acquired.

4. The determination method according to claim 1, characterized in that: The power grid includes an inverter, and the method further includes: After determining that the islanding phenomenon occurs in the power grid, disconnecting the inverter; When the initial frequency is less than or equal to the first threshold, determining that the power grid is in under-frequency protection, and disconnecting the inverter; When the initial frequency is greater than or equal to the second threshold, it is determined that the power grid is in overfrequency protection, and the inverter is disconnected.

5. The determination method according to claim 1, characterized in that: Determine the base frequency, including: Acquisition step: acquiring the frequency 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 the moment corresponding to the initial frequency is acquired and adjacent to the moment corresponding to the initial frequency; Calculation step: Calculate the average value of multiple historical frequencies to obtain the reference frequency.

6. The determination method according to claim 5, characterized in that: The method further comprises: When the current frequency is greater than the first threshold and less than the second threshold, determining the usage time of the reference frequency; When the usage time is greater than a preset time, the obtaining step and the calculating step are repeated to update the reference frequency.

7. The determination method according to claim 1, characterized in that: Determining the disturbance pulse width according to the frequency offset includes: 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 according to the first mapping relationship; Determining a disturbance amplitude according to the frequency offset includes: A second mapping relationship between the frequency offset and the disturbance amplitude is acquired, and the disturbance amplitude corresponding to the frequency offset is determined according to the second mapping relationship.

8. A device for determining an island phenomenon, characterized in that: include: a first determination unit, configured to apply a steady-state positive and negative bidirectional reactive power disturbance to a power grid, obtain a frequency of the power grid, obtain an initial frequency, and determine 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, the second threshold is an overfrequency protection frequency, and the steady-state positive and negative bidirectional reactive power disturbance is used to detect an islanding phenomenon in 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, and when the absolute value of the frequency offset is less than a preset threshold, maintain the reactive power of the power grid to be the steady-state positive and negative bidirectional reactive power disturbance, and when 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 be the disturbance direction; a second adjustment unit, configured to determine a 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, determine a disturbance amplitude according to the frequency offset, and adjust the amplitude of the reactive power of the power grid to the disturbance amplitude; The second determining unit is used to obtain the current frequency of the power grid after adjusting to the disturbance amplitude, and determine that the islanding phenomenon occurs in the power grid when the current frequency is less than or equal to the first threshold and greater than or equal to the second threshold.

9. A computer-readable storage medium, characterized in that: The computer-readable storage medium includes a stored program, wherein when the program is executed, the device where the computer-readable storage medium is located is controlled to execute the determination method according to any one of claims 1 to 7.

10. An electronic device, characterized in that: include: One or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and are configured to be executed by the one or more processors, and the one or more programs include a method for executing the determination method described in any one of claims 1 to 7.

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