A three-phase inverter and its island detection method

Abstract

The present application provides a three-phase inverter and an island detection method thereof. The island detection method obtains the frequency of each phase on the AC side of the three-phase inverter and uses any phase frequency among the phase frequencies as the disturbance frequency. The disturbance frequency is then used to perform reactive power disturbance. The method can then determine whether an islanding effect occurs based on the current frequency of each phase on the AC side of the three-phase inverter after the disturbance. That is, the present application calculates the frequency of each phase on the AC side of the three-phase inverter separately to ensure that the frequency change of the islanding instant, including single-phase islanding, can be captured. After the reactive disturbance is performed, the protection can be triggered, thereby realizing the detection of the islanding effect.

Description

Technical Field

[0001] The present application relates to the technical field of inverter islanding detection, and in particular to a three-phase inverter and an islanding detection method thereof. Background Art

[0002] In such Figure 1 In the photovoltaic grid-connected power generation system shown, if a power outage occurs on the grid side and the inverter load matches its output, the inverter will operate normally. This phenomenon is known as the islanding effect. The islanding effect in a photovoltaic grid-connected power generation system poses a threat to sensitive loads and the safety of maintenance personnel. Therefore, it is necessary to detect the presence of islanding effects in the photovoltaic grid-connected power generation system during inverter operation.

[0003] Common islanding effect detection methods include negative-sequence current perturbation and reactive power perturbation. The commonly used reactive power perturbation method, based on frequency offset, injects reactive power to increase the frequency offset. The more reactive power injected, the greater the frequency offset. This positive feedback triggers overfrequency or underfrequency protection, thereby detecting the islanding effect. However, because the injected reactive power is derived from a three-phase voltage phase-locked loop (PLL), when a single-phase islanding effect occurs, the three-phase inverter will not detect the frequency change at the moment of islanding. Therefore, it will not inject reactive power to cause the frequency change and provide protection, resulting in detection failure. In other words, the problem of detecting single-phase islanding effects in three-phase inverters needs to be urgently addressed. Summary of the Invention

[0004] In view of this, the present application provides a three-phase inverter and an islanding detection method thereof, so as to realize single-phase islanding effect detection for the three-phase inverter.

[0005] To achieve the above objectives, this application provides the following technical solutions:

[0006] A first aspect of the present application provides an islanding detection method for a three-phase inverter, comprising:

[0007] Obtain the frequency of each phase on the AC side of the three-phase inverter;

[0008] Using any phase frequency among the phase frequencies as the disturbance frequency;

[0009] Performing reactive power disturbance at the disturbance frequency;

[0010] Determine whether the islanding effect occurs.

[0011] Optionally, determine whether an islanding effect occurs, including:

[0012] determining whether overfrequency protection or underfrequency protection is triggered according to the current frequency of each phase on the AC side of the three-phase inverter after the disturbance;

[0013] If the overfrequency protection or the underfrequency protection is triggered, it is determined that an islanding effect occurs.

[0014] Optionally, judging whether to trigger overfrequency protection or underfrequency protection according to the current frequency of each phase on the AC side of the three-phase inverter after the disturbance includes:

[0015] Determining whether the maximum value among the current frequencies of the phases triggers the overfrequency protection;

[0016] as well as,

[0017] It is determined whether a minimum value among the current frequencies of the phases triggers the under-frequency protection.

[0018] Optionally, determine whether an islanding effect occurs, including:

[0019] Determine whether a difference between the current frequencies of the phases on the AC side of the three-phase inverter after the disturbance exceeds a preset threshold;

[0020] If any difference exceeds the preset threshold, it is determined that an islanding effect occurs.

[0021] Optionally, the preset threshold is 0.2 Hz.

[0022] Optionally, determine whether an islanding effect occurs, including:

[0023] respectively determining whether the current frequency of each phase on the AC side of the three-phase inverter changes continuously in the same direction after the disturbance;

[0024] If any relative frequency changes continuously in the same direction, it is determined that an islanding effect occurs.

[0025] Optionally, determining whether an islanding effect occurs includes at least two of the following:

[0026] determining whether overfrequency protection or underfrequency protection is triggered according to the current frequency of each phase on the AC side of the three-phase inverter after the disturbance; and determining that an islanding effect occurs if the overfrequency protection or the underfrequency protection is triggered;

[0027] Determine whether the difference between the current frequencies of the phases on the AC side of the three-phase inverter after the disturbance exceeds a preset threshold; if any difference exceeds the preset threshold, determine that an islanding effect occurs;

[0028] as well as,

[0029] It is determined whether the current frequency of each phase on the AC side of the three-phase inverter changes continuously in the same direction after the disturbance; if the current frequency of any phase changes continuously in the same direction, it is determined that an islanding effect occurs.

[0030] Optionally, using any phase frequency among the phase frequencies as the disturbance frequency includes:

[0031] The frequency of a phase with the largest deviation from the rated frequency among the phase frequencies is determined as the disturbance frequency.

[0032] Optionally, obtain the frequency of each phase on the AC side of the three-phase inverter, including:

[0033] Passing a phase-locked loop or a frequency-locked loop to each phase on the AC side of the three-phase inverter to obtain the frequency of the corresponding phase;

[0034] or,

[0035] Frequency capture is performed based on hardware to obtain the frequencies of each phase.

[0036] A second aspect of the present application provides a three-phase inverter, comprising: a three-phase inverter circuit and a controller;

[0037] The three-phase inverter circuit is controlled by the controller;

[0038] The controller is used to execute the islanding detection method for the three-phase inverter as described in any one of the first aspects above.

[0039] The present application provides a method for detecting an islanding effect in a three-phase inverter. The method obtains the frequency of each phase on the AC side of the three-phase inverter and uses any of the phase frequencies as the disturbance frequency. The method then performs a reactive power disturbance at the disturbance frequency, and can then determine whether an islanding effect has occurred based on the current frequency of each phase on the AC side of the three-phase inverter after the disturbance. That is, the present application calculates the frequency of each phase on the AC side of the three-phase inverter separately to ensure that the frequency change of the islanding effect, including single-phase islanding, can be captured. After the reactive power disturbance is performed, the protection can be triggered, thereby realizing the detection of the islanding effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings to be used in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0041] Figure 1 A schematic diagram of the structure of a photovoltaic grid-connected power generation system provided by existing technology;

[0042] Figure 2 Flowchart of the islanding detection method for a three-phase inverter provided in an embodiment of the present application;

[0043] Figure 3 A specific flow chart of the islanding detection method for a three-phase inverter provided in an embodiment of the present application;

[0044] Figure 4 Another specific flow chart of the islanding detection method for a three-phase inverter provided in an embodiment of the present application;

[0045] Figure 5 A schematic diagram of the structure of a three-phase inverter provided in an embodiment of the present application;

[0046] Figure 6 A schematic diagram of another application scenario of the three-phase inverter provided in an embodiment of the present application. DETAILED DESCRIPTION

[0047] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0048] In this application, the terms "comprises," "comprising," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not preclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0049] The present application provides an islanding detection method for a three-phase inverter, so as to realize single-phase islanding effect detection for the three-phase inverter.

[0050] like Figure 2 As shown, the island detection method of the three-phase inverter includes:

[0051] S101. Obtain the frequency of each phase on the AC side of a three-phase inverter.

[0052] The step S101 may specifically be Figure 3 The S201 or S202 shown in the figure can be selected and used according to the specific application environment, and both are within the scope of protection of this application.

[0053] S201 , obtaining the frequency of each phase on the AC side of the three-phase inverter through a software algorithm.

[0054] The software algorithm can specifically be a single-phase phase-locked loop or a single-phase frequency-locked loop, as long as it can obtain the frequency of the phase, and both are within the protection scope of this application.

[0055] S202: Perform frequency capture based on hardware to obtain the frequency of each phase.

[0056] The hardware may specifically refer to an eCAP port of a DSP (Digital Signal Processing), which is only an example and is not limited thereto.

[0057] That is, in practical applications, the step S101 may specifically be: using software algorithms or hardware to obtain the frequencies of the three phases U, V, and W on the AC side of the three-phase inverter, and record them as f u 、f v 、f w ; Then execute step S102.

[0058] S102. Use any phase frequency among the phase frequencies as the disturbance frequency.

[0059] Different from the frequency obtained based on the three-phase phase-locked loop in the prior art, this embodiment uses the three-phase frequency f obtained in step S101 u 、f v 、f w Any phase frequency in the inverter is used as the disturbance frequency. If a three-phase or single-phase islanding effect occurs, the frequency of each phase on the AC side of the three-phase inverter will change accordingly. At this time, using any phase frequency as the disturbance frequency can make the frequency offset of each phase larger through reactive power disturbance, thereby forming positive feedback and triggering overfrequency or underfrequency protection.

[0060] S103: Perform reactive power disturbance at the disturbance frequency.

[0061] The specific execution process of the reactive power disturbance can be referred to in the prior art and will not be described in detail here.

[0062] S104: Determine whether an islanding effect occurs.

[0063] If a three-phase or single-phase islanding effect occurs, the reactive power disturbance process in step S103 will cause a larger offset in the frequency of each phase on the AC side of the three-phase inverter. After positive feedback is formed, if the over-frequency or under-frequency protection of the three-phase inverter is triggered, it can be determined that an islanding effect has occurred, thereby enabling detection of the islanding effect.

[0064] In actual applications, the over-frequency and under-frequency protection can be the same as the over-frequency and under-frequency protection in the prior art, that is, the frequency obtained by a phase-locked loop or a frequency-locked loop based on the three-phase voltage is used for over-frequency and under-frequency protection; or, the criteria provided in the following embodiments can be used, which are not limited here and are all within the scope of protection of this application.

[0065] The islanding detection method for the three-phase inverter provided in this embodiment performs frequency calculation on each phase on the AC side of the three-phase inverter separately, ensuring that the instantaneous frequency changes of islanding, including single-phase islanding, can be captured. After a reactive disturbance is performed, protection can be triggered, thereby realizing the detection of the islanding effect, avoiding the problem of failure of single-phase islanding detection in three-phase inverters in the prior art.

[0066] On the basis of the above embodiment, preferably, the island detection method of the three-phase inverter, step S102, can be specifically Figure 3 As shown in: Determine the frequency of one phase with the largest deviation from the rated frequency among all phase frequencies as the disturbance frequency.

[0067] If the rated frequency of each phase is f o , then step S102 is to calculate |f i -f o |, i∈(u, v, w), obtain the three-phase frequency f in step S101 u 、f v 、f w Deviation from rated frequency f o The maximum single-phase frequency is used as the disturbance frequency f x Then, step S103 is performed with the disturbance frequency f x Performing reactive power disturbance will form faster positive feedback, triggering over-frequency and under-frequency protection more quickly.

[0068] In addition, in the islanding detection method for the three-phase inverter, step S104 can adopt a more preferred implementation than that in the prior art, such as the following three implementations:

[0069] (1) Step S104 may include Figure 4 As shown in:

[0070] S301 : Determine whether overfrequency protection or underfrequency protection is triggered according to the current frequency of each phase on the AC side of the three-phase inverter after the disturbance.

[0071] The step S301 may specifically be: determining whether the maximum value of each current frequency triggers over-frequency protection, and whether the minimum value of each current frequency triggers under-frequency protection.

[0072] That is, the current frequency (still in terms of f) calculated by the three single-phase phase-locked loops or single-phase frequency-locked loopsu 、f v 、f w Indicates), the maximum value max(f u 、f v 、f w ) for over-frequency protection, and the minimum value min(f u 、f v 、f w ) for under-frequency protection; compared with the prior art scheme of using a phase-locked loop or frequency-locked loop based on three-phase voltage to obtain the frequency for over- and under-frequency protection, this scheme can trigger over- and under-frequency protection faster, thereby realizing island detection faster.

[0073] If the overfrequency protection or underfrequency protection is triggered, it is determined that an islanding effect occurs.

[0074] (2) Step S104 may also be implemented in another form, such as Figure 4 As shown in:

[0075] S302 , determining whether the difference between the current frequencies of the phases on the AC side of the three-phase inverter after the disturbance exceeds a preset threshold.

[0076] The difference between the current frequencies of the phases, i.e., the interphase frequency difference |fm-fn| at the same moment, where m, n∈(u, v, w) and m≠n, is determined to determine whether it exceeds a preset threshold ferr. The preset threshold ferr can be, but is not limited to, 0.2 Hz. The smaller the preset threshold ferr, the faster the protection. In practical applications, the specific protection time can be determined based on national standards, and an appropriate preset threshold ferr value can be selected. If any difference exceeds the preset threshold ferr, an islanding effect is determined.

[0077] Under normal circumstances, the frequencies of the three-phase voltages on the AC side of a three-phase inverter are the same, but may be different when a single-phase is islanded. Therefore, step S302 utilizes this feature to achieve both islanding protection and faster protection, meeting standard requirements.

[0078] (3) This step S104 can also be performed by Figure 4 As shown in:

[0079] S303 , respectively determining whether the current frequency of each phase on the AC side of the three-phase inverter changes continuously in the same direction after the disturbance.

[0080] After each execution of step S103, the three-phase inverter can recalculate the current frequency of each phase on the AC side through the corresponding single-phase phase-locked loop or single-phase frequency-locked loop; and based on the multiple calculation results, determine whether the current frequency of each phase has been changing continuously in one direction; if the current frequency of any phase changes continuously in the same direction, it can be determined in time that an islanding effect has occurred without having to wait until the over-frequency or under-frequency protection is triggered or the frequency difference between the phases exceeds the preset threshold.

[0081] It is worth noting that the step S104 may include any one of the steps S301, S302 and S303; in practical applications, the step S104 may include at least two of the steps S301, S302 and S303. Figure 4 The example of including all three at the same time is used for demonstration. At this time, when any one of them occurs, it can be determined that there is an island effect, and then island protection is performed; it depends on the specific application environment and is within the protection scope of this application.

[0082] The island detection method of the three-phase inverter provided in this embodiment can calculate the frequency of each phase U, V, and W of the AC side of the three-phase inverter in sequence through a software algorithm or based on the frequency capture of the hardware eCAP port; and further obtain the deviation from the rated frequency f o Maximum phase frequency; deviation from rated power f o The maximum phase frequency is the disturbance frequency f x , and perform reactive power disturbance; finally, the reactive power disturbance result is combined with over-frequency and under-frequency protection, phase frequency difference and frequency change direction to judge whether the single-phase islanding effect occurs, which solves the problem of failure of single-phase islanding detection in three-phase inverters using existing conventional reactive power disturbance method.

[0083] Another embodiment of the present application provides another three-phase inverter, which is as follows: Figure 5 As shown, it includes: a three-phase inverter circuit 101 and a controller 102; wherein:

[0084] The three-phase inverter circuit 101 is controlled by a controller 102. In practical applications, the controller 102 specifically obtains the voltage and current on the DC side and AC side of the three-phase inverter circuit 101 through corresponding sensors, and realizes on-off control of each switch tube in the three-phase inverter circuit 101 through a suitable driving circuit. The specific structure and principle can be referred to the existing technology and will not be repeated here.

[0085] Different from the prior art, the controller 102 is used to execute the islanding detection method for the three-phase inverter as described in any of the above embodiments. The execution process and principle of the islanding detection method can be found in the above embodiments and will not be repeated here.

[0086] The three-phase inverter calculates the frequency of each phase on the AC side of the three-phase inverter separately, ensuring that the instantaneous frequency change of islanding, including single-phase islanding, can be captured. After a reactive disturbance is performed, protection can be triggered, thereby realizing the detection of the islanding effect, avoiding the problem of failure of three-phase inverters to detect single-phase islanding in the prior art.

[0087] Moreover, in order to detect the islanding effect more quickly, the frequency of one phase with the largest deviation from the rated frequency as described in the above embodiment can be used as the disturbance frequency, the maximum value of the current frequency after the disturbance can be used for overfrequency protection, and the minimum value can be used for underfrequency protection, the frequency difference between phases exceeding the preset threshold can be used as one of the islanding effect detection criteria, and the current frequency continuously changing in the same direction can be used as one of the islanding effect detection criteria.

[0088] It is worth noting that the three-phase inverter can be Figure 1 The inverter after the PV array shown in Figure 6 The DC side shown in the figure is also connected to a three-phase photovoltaic inverter of the energy storage system, all of which are within the protection scope of this application; its AC side is connected to the power grid through a transformer, and its AC side can operate with load, and then through the above-mentioned island detection method, island detection including single-phase island detection can be achieved.

[0089] The same or similar parts between the various embodiments in this specification can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system or system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. The system and system embodiments described above are merely schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment. Ordinary technicians in this field can understand and implement it without making any creative efforts.

[0090] Professionals may further appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the above description has generally described the components and steps of each example according to their functions. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians may use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of the present invention.

[0091] With respect to the above description of the disclosed embodiments, the features described in the various embodiments in this specification may be interchanged or combined to enable those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for detecting an islanding of a three-phase inverter, characterized in that: include: Obtain the frequency of each phase on the AC side of the three-phase inverter; Determine the frequency of the phase with the largest deviation from the rated frequency among the phase frequencies as the disturbance frequency; Performing reactive power disturbance at the disturbance frequency; Determine whether the island effect occurs.

2. The islanding detection method for a three-phase inverter according to claim 1, wherein: Determine whether the island effect occurs, including: determining whether overfrequency protection or underfrequency protection is triggered according to the current frequency of each phase on the AC side of the three-phase inverter after the disturbance; If the overfrequency protection or the underfrequency protection is triggered, it is determined that an islanding effect occurs.

3. The islanding detection method for a three-phase inverter according to claim 2, wherein: According to the current frequency of each phase on the AC side of the three-phase inverter after the disturbance, it is determined whether overfrequency protection or underfrequency protection is triggered, including: Determining whether the maximum value among the current frequencies of the phases triggers the overfrequency protection; as well as, It is determined whether a minimum value among the current frequencies of the phases triggers the under-frequency protection.

4. The islanding detection method for a three-phase inverter according to claim 1, wherein: Determine whether the island effect occurs, including: Determine whether a difference between the current frequencies of the phases on the AC side of the three-phase inverter after the disturbance exceeds a preset threshold; If any difference exceeds the preset threshold, it is determined that an islanding effect occurs.

5. The islanding detection method for a three-phase inverter according to claim 4, wherein: The preset threshold is 0.2 Hz.

6. The islanding detection method for a three-phase inverter according to claim 1, wherein: Determine whether the island effect occurs, including: respectively determining whether the current frequency of each phase on the AC side of the three-phase inverter changes continuously in the same direction after the disturbance; If any relative frequency changes continuously in the same direction, it is determined that an islanding effect occurs.

7. The islanding detection method for a three-phase inverter according to claim 1, wherein: Determine whether the island effect occurs, including at least two of the following: determining whether overfrequency protection or underfrequency protection is triggered according to the current frequency of each phase on the AC side of the three-phase inverter after the disturbance; and determining that an islanding effect occurs if the overfrequency protection or the underfrequency protection is triggered; Determine whether a difference between the current frequencies of the phases on the AC side of the three-phase inverter after the disturbance exceeds a preset threshold; If any difference exceeds the preset threshold, it is determined that an islanding effect occurs; as well as, It is determined whether the current frequency of each phase on the AC side of the three-phase inverter changes continuously in the same direction after the disturbance; if the current frequency of any phase changes continuously in the same direction, it is determined that an islanding effect occurs.

8. The islanding detection method for a three-phase inverter according to any one of claims 1 to 7, characterized in that: Obtain the AC side frequency of each phase of the three-phase inverter, including: Passing a phase-locked loop or a frequency-locked loop to each phase on the AC side of the three-phase inverter to obtain the frequency of the corresponding phase; or Frequency capture is performed based on hardware to obtain the frequencies of each phase.

9. A three-phase inverter, characterized in that: include: Three-phase inverter circuit and controller; The three-phase inverter circuit is controlled by the controller; The controller is configured to execute the islanding detection method for a three-phase inverter according to any one of claims 1 to 8.

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