Active island detection method for photovoltaic inverter without external frequency detection circuit
Through software phase lock loop calculation of the real-time phase of the photovoltaic inverter and superimposed microstep length, active island detection of photovoltaic inverters without external frequency detection circuit is realized, solving the problems of false alarms and high costs in the existing technology, and improving the accuracy and stability of detection.
Patent Information
- Application Number
- CN202411981690.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-12-31
- Publication Date
- 2025-04-11
AI Technical Summary
The active island detection technology of existing photovoltaic inverters is prone to false alarms due to fluctuations and oscillations in the power grid frequency, and requires external frequency detection circuits, which increases cost and complexity.
Using a method without an external frequency detection circuit, the grid voltage is collected through a software phase lock loop, iteratively calculates the real-time phase of the power grid, and calculates the timer interrupt time based on the real-time phase, superimposes the micro-step length to the PWM output, and determines whether the photovoltaic inverter is in an isolated island state.
It realizes that the island state of the photovoltaic inverter is more convenient and accurate without adding active frequency shift and frequency detection circuits, reducing costs and improving detection stability.
Smart Images
Figure CN119805056A_ABST
Abstract
Description
Technical Field
[0001] The present application relates to the field of photovoltaic detection technology, and in particular to a photovoltaic inverter active island detection method without an external frequency detection circuit. Background Art
[0002] Grid-connected photovoltaic inverters have been widely used in recent years, and household micro inverters have also been widely used in Europe and the United States in recent years. In grid-connected photovoltaic inverters, the island detection function is essential to determine the effectiveness of power transmission between the inverter and the grid.
[0003] The existing island detection technologies include active and passive ones. For occasions with higher requirements, it is generally required that the equipment must have active island detection. Active island detection is mostly implemented by active frequency shifting: when an island occurs, the inverter output frequency is changed to exceed the preset value, and then the inverter output frequency abnormality is detected through the frequency detection circuit to implement island detection. However, for power grids in different situations, their characteristics will be different, and the power grid frequency detection circuit mostly detects the moment when the power grid crosses zero. When the power grid crosses zero slightly, it is easy to cause frequency detection abnormality and generate false alarms. Summary of the invention
[0004] In view of the shortcomings of the prior art, this application provides a photovoltaic inverter active island detection method without an external frequency detection circuit. This application provides a solution for active island detection without an external frequency detection circuit through a software phase-locked loop. This application specifically adopts the following technical solutions.
[0005] Firstly, to achieve the above-mentioned purpose, a photovoltaic inverter active island detection method without an external frequency detection circuit is proposed, and the steps include: collecting the instantaneous value of the grid voltage, and then iteratively calculating the real-time phase of the grid according to the instantaneous value of the grid voltage; calculating the interruption time of the next grid cycle of the timer according to the real-time phase, wherein the interruption time is superimposed with a micro-step length on the grid cycle; superimposing the micro-step length on the PWM output end of the photovoltaic inverter; and judging that the photovoltaic inverter is in an island state when the cumulative interruption time of the timer reaches a preset value.
[0006] Optionally, in any of the above-described active island detection methods for photovoltaic inverters without an external frequency detection circuit, the interruption time is the grid phase zero-crossing moment calculated by software phase locking plus a micro-step length.
[0007] Optionally, in any of the above methods for active islanding detection of photovoltaic inverters without an external frequency detection circuit, the micro-step size is a preset fixed value.
[0008] Optionally, in an active island detection method for a photovoltaic inverter without an external frequency detection circuit as described above, the step of superimposing a microstep length on the PWM output end of the photovoltaic inverter includes: triggering a timer to output an interrupt signal after superimposing the microstep length on the zero-crossing moment of the grid phase calculated by software phase locking; triggering a trigonometric function generation unit to output phase information based on the interrupt signal; and triggering a PWM unit to output a drive signal corresponding to the phase to a drive circuit based on the phase information, thereby triggering the photovoltaic inverter to feed electrical energy into the grid.
[0009] Optionally, an active island detection method for a photovoltaic inverter without an external frequency detection circuit as described above, wherein, in a non-island state, the timer interrupt signal is synchronized with the grid phase zero-crossing moment calculated by software phase-locked phase-locking; in an island state, the period of the timer interrupt signal is the cumulative value of the photovoltaic inverter electric energy signal phase zero-crossing moment and a number of micro-steps; when the period of the timer interrupt signal exceeds a preset value, it is determined that the photovoltaic inverter is in an island state.
[0010] Optionally, in any of the above-mentioned methods for active island detection of photovoltaic inverters without an external frequency detection circuit, the interrupt signal period T of the timer is ,in, is the grid frequency corresponding to the grid phase zero crossing point, is a microstep length, is the switching frequency of the PV inverter.
[0011] At the same time, in order to achieve the above-mentioned purpose, the present application also provides a photovoltaic inverter active island detection system without an external frequency detection circuit, which includes: a grid voltage sampling module, which is connected to the grid and is used to collect the instantaneous value of the grid voltage; a phase-locked loop module, which is connected to the output end of the grid voltage sampling module and is used to iteratively calculate the real-time phase and frequency of the grid according to the instantaneous value of the grid voltage; a timer, which outputs an interrupt signal based on the real-time phase and frequency of the grid and superimposes the micro-step length according to the grid cycle; a trigonometric function generation unit, which is triggered by the interrupt signal to output phase information, so as to trigger the PWM unit to output a drive signal corresponding to the phase to the drive circuit based on the phase information, thereby triggering the photovoltaic inverter to feed electrical energy to the grid; the system determines that the photovoltaic inverter is in an island state when the period of the timer interrupt signal exceeds a preset value.
[0012] Optionally, in the photovoltaic inverter active island detection system without an external frequency detection circuit as described above, the output time of the interrupt signal of the timer is the time corresponding to the zero-crossing time of the grid phase output by the phase-locked loop module superimposed on the micro-step length.
[0013] Optionally, in any of the above-described active islanding detection systems for photovoltaic inverters without an external frequency detection circuit, the micro-step size is a preset fixed value.
[0014] Optionally, in the photovoltaic inverter active island detection system without an external frequency detection circuit as described above, the switching period of the timer is inversely proportional to the grid period and inversely proportional to the switching frequency of the photovoltaic inverter.
[0015] Beneficial Effects
[0016] The active island detection method for photovoltaic inverters without external frequency detection circuits provided in the present application first iteratively calculates the real-time phase of the grid according to the instantaneous value of the grid voltage, then calculates the interruption time of the next grid cycle of the timer according to the real-time phase, and superimposes the corresponding micro-step length in the interruption time, and superimposes the micro-step length to the PWM output end of the photovoltaic inverter, so that when the cumulative interruption time of the timer reaches a preset value, it is judged that the photovoltaic inverter is in an island state. The present application sets the output frequency of the inverter to be related to the timer interrupt, and sets the timer interrupt time to an active change value, which is slightly increased (or slightly decreased) each time, so that the island state of the inverter is identified based on the accumulation of the timer step length. The present application uses a software phase-locked loop to collect the grid voltage, which is also used for software phase locking while realizing over-voltage and under-voltage protection, and detects the real-time phase of the grid. It can detect the island state of the inverter more conveniently and accurately at a lower cost without adding active frequency shifting and frequency detection circuits.
[0017] Other features and advantages of the present application will be set forth in the following description, and in part will become apparent from the description, or may be understood by practicing the present application. BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The accompanying drawings are used to provide a further understanding of the present application and constitute a part of the specification. Together with the embodiments of the present application, they are used to explain the present application and do not constitute a limitation of the present application. In the accompanying drawings:
[0019] Figure 1 It is a principle block diagram of the active island detection solution of the present application;
[0020] Figure 2 It is a flow chart of the active island detection method without external frequency detection circuit of the present application;
[0021] Figure 3 It is the signal waveform of the active island detection scheme without external frequency detection circuit of the present application;
[0022] Figure 4 It is a circuit schematic diagram of the active island detection solution without external frequency detection circuit of the present application. DETAILED DESCRIPTION
[0023] In order to make the purpose and technical solution of the embodiment of the present application clearer, the technical solution of the embodiment of the present application will be clearly and completely described in conjunction with the drawings of the embodiment of the present application. Obviously, the described embodiment is a part of the embodiment of the present application, not all of the embodiments. Based on the described embodiment of the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.
[0024] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art to which this application belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless defined as herein.
[0025] The term “connection” as used in this application may mean a direct connection between components or an indirect connection between components via other components.
[0026] Figure 1 According to the present application, a photovoltaic inverter active island detection method without an external frequency detection circuit is provided, and the detection steps include:
[0027] First, the instantaneous value of the grid voltage is collected through the grid voltage sampling module.
[0028] Then the phase-locked loop module is used to iteratively calculate the real-time phase of the power grid according to the instantaneous value of the power grid voltage;
[0029] Then, calculating the interruption time of the next power grid cycle of the timer according to the real-time phase, wherein the interruption time is superimposed with a micro-step length on the aforementioned power grid cycle;
[0030] Finally, the micro-step length is superimposed on the PWM output of the photovoltaic inverter;
[0031] Therefore, when the cumulative interruption time of the timer reaches a preset value, it is determined that the photovoltaic inverter is in an island state.
[0032] In the above process, the grid phase zero-crossing moment calculated by the software phase-locking is superimposed on the micro-step length and set as the interrupt signal output moment of the timer, and the micro-step length preset as a fixed value can be superimposed on the PWM output end of the photovoltaic inverter. Thereby adjusting the output frequency of the inverter so that the frequency is directly related to the timer interrupt. Therefore, when the inverter is disconnected from the grid, since each interruption time of the timer includes an active change value, a slight increase (or slight decrease) can be achieved each time through the micro-step length, thereby affecting the voltage zero-crossing point detected by the phase-locked loop module. Therefore, the present application uses a software phase-locked loop to collect the grid voltage for over-voltage and under-voltage protection, and also uses it for software phase-locking. The real-time phase of the grid is detected by the micro-step length accumulated on the interrupt signal triggered by the phase-locking, and the island state of the inverter is determined in time when the phase deviation exceeds the preset value.
[0033] Specific reference Figure 4 The present application provides a photovoltaic inverter active island detection system without an external frequency detection circuit, which includes:
[0034] The grid voltage sampling module is connected to the grid and is used to collect the instantaneous value of the grid voltage, which is also the output voltage of the inverter. The software not only calculates the effective value for grid over-voltage and under-voltage protection, but its instantaneous value of the grid voltage is also the input of the phase-locked loop;
[0035] A phase-locked loop module is connected to the output end of the grid voltage sampling module. The module uses multiple iterations of the instantaneous value of the grid voltage to calculate the real-time phase of the grid through digital signal processing;
[0036] The timer is based on the real-time phase and frequency of the power grid, and outputs an interrupt signal according to the power grid cycle and superimposed micro-step length. This module uses the phase of the software phase-locked loop as input to calculate the required interrupt time, which directly reflects the output frequency of the inverter. This interrupt time is slightly increased (or slightly reduced) on the basis of the phase calculation time as an active disturbance of the power grid to evaluate the islanding state.
[0037] a trigonometric function generating unit, which is triggered by the interrupt signal to output phase information, so as to trigger the PWM unit to output a driving signal corresponding to the phase to the driving circuit based on the phase information, thereby triggering the photovoltaic inverter to feed electric energy into the power grid;
[0038] The system slightly increases the PWM update module based on the timer interrupt time, so that the module is updated every time the phase of the phase-locked loop passes zero. As a result, the module, as an action module, can reflect the multi-superimposed micro-step length in the timer interrupt signal on the output frequency of the inverter, so as to judge that the photovoltaic inverter is in an island state when the period of the terminal signal exceeds the preset value.
[0039] Reference Figure 2 as well as Figure 3, the specific principle is:
[0040] Based on the instantaneous value of the grid voltage collected by the grid voltage sampling module, a fixed micro-step is superimposed on the grid phase zero-crossing moment calculated by the software phase lock, and the micro-step can correspond to subtracting a number of milliseconds from the zero-crossing moment, or can correspond to adding a number of milliseconds after the zero-crossing moment, thereby outputting a periodic interrupt signal. The interrupt signal triggers the trigonometric function generation unit to output phase information, thereby triggering the PWM unit to output a drive signal corresponding to the phase to the drive circuit based on the phase information, triggering the photovoltaic inverter to feed electric energy into the grid. In this process, due to the traction of the grid signal, the interrupt signal output by the timer is submerged by the grid signal in the non-island state, and each cycle is kept in step with the grid phase zero-crossing moment calculated by the software phase lock, and the two are always only different by a fixed micro-step, so there will be no change in the interrupt cycle; while in the island state, since the grid itself cannot provide a signal to the sampling module, the grid phase zero-crossing moment obtained by the sampling module after sampling will completely correspond to the signal cycle output by the inverter, that is, after the inverter experiences several interruptions in the island state, the phase zero-crossing moment of its output signal will gradually shift due to the accumulation of micro-steps introduced by each interruption. At this time, the period of the timer interrupt signal is the cumulative value of the phase zero-crossing moment of the photovoltaic inverter power signal and several micro-steps. Therefore, when the period of the timer interrupt signal exceeds the preset value, it can be determined that the photovoltaic inverter is in an island state.
[0041] In order to ensure the timeliness of island detection in the above process, the switching period T of the timer is generally set to ;in, The grid frequency corresponding to the grid phase zero crossing point; It is a preset fixed value, indicating a micro-step length; is the switching frequency of the photovoltaic inverter, which can generally be set to 50000.
[0042] refer to Figure 2 The above system can be operated specifically in the following manner.
[0043] The first step is that the inverter performs grid parameter detection in standby mode. It can be connected to the grid only when the grid has no over-voltage or under-voltage, and no over-frequency or under-frequency.
[0044] Then the second step is to start the inverter and connect it to the grid. The timer interruption time is the initial value, which is linearly related to the normal grid frequency (50HZ or 60HZ).
[0045] In the third step, the grid voltage sampling module collects the instantaneous value of the grid voltage. The software phase-locked loop module iterates multiple times according to the instantaneous value of the grid voltage, performs digital signal processing and calculates the real-time phase of the grid.
[0046] The fourth step is to calculate the interruption time of the next power grid cycle of the timer based on the real-time phase information. For example, the output time of the interruption signal is set to the zero-crossing time of the power grid phase output by the phase-locked loop module plus a micro-step. That is, slightly increase or decrease some steps on the calculated time, and the slightly increased or decreased steps are the disturbance marks of the power grid frequency.
[0047] Therefore, in the absence of islands, the grid frequency is around 50HZ (or 60HZ). Due to the control of the grid, the phase information of each cycle of the software phase-locked loop is not much different from that of the previous cycle, so the update of the timer interrupt time is also not much different each time;
[0048] However, when an island occurs, since the timer interrupt time has a micro-increment (or micro-decrement) each time, which is the fixed output of the inverter itself each time, when the island occurs, the grid has been disconnected, and the phase of the phase-locked loop is no longer constrained by the grid, and this micro-increment (or micro-decrement) of the timer will affect the phase of the phase-locked loop. Therefore, in the island state, the phase information of each cycle of the software phase-locked loop will maintain a slight difference from the previous cycle due to the micro-step length superimposed on the timer interrupt signal. This micro-increment (or micro-decrement) will continue to accumulate with the duration of the island state, and continue to increase (or decrease) until it reaches the preset value, at which time the island is detected.
[0049] In the present application, it is preferred to set the update of the timer interrupt time at the time when the software phase lock calculates the zero-crossing moment of the power grid phase, so as to reduce the impact of the disturbance on the power grid.
[0050] In summary, this application has the following advantages:
[0051] 1. No hardware frequency detection circuit, reducing costs;
[0052] Second, compared with hardware, the zero-crossing moment may be disturbed or oscillated and unstable, resulting in inaccurate frequency detection; using software phase lock, the zero-crossing moment is unique and stable, the timer interrupt time is calculated accurately, and by slightly increasing or decreasing the timer value, when an island appears, it will gradually increase to (or decrease to) the preset value.
[0053] The above is only an implementation method of the present application, and its description is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present application. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present application, which all belong to the protection scope of the present application.
Claims
1. A method for active island detection of photovoltaic inverter without external frequency detection circuit, characterized in that the steps include: Collect the instantaneous value of the grid voltage, and then iteratively calculate the real-time phase of the grid based on the instantaneous value of the grid voltage; calculating an interruption time of a next grid cycle of the timer according to the real-time phase, wherein the interruption time is superimposed with a micro-step length on the grid cycle; Superimpose the microstep length to the PWM output of the photovoltaic inverter; When the cumulative interruption time of the timer reaches a preset value, it is determined that the photovoltaic inverter is in an island state.
2. The photovoltaic inverter active island detection method without external frequency detection circuit as claimed in claim 1, characterized in that: The interruption time is the zero-crossing time of the grid phase calculated by the software phase-locking plus the micro-step length.
3. The active island detection method for photovoltaic inverter without external frequency detection circuit as claimed in claim 1, characterized in that: The micro-step length is a preset fixed value.
4. The active island detection method for photovoltaic inverter without external frequency detection circuit as claimed in claim 1, characterized in that: The steps of adding microstep length to the PWM output of the photovoltaic inverter include: The micro-step length is superimposed on the zero-crossing moment of the grid phase calculated by the software phase lock, and then the timer outputs an interrupt signal; Triggering a trigonometric function generation unit to output phase information based on the interrupt signal; The PWM unit is triggered based on the phase information to output a driving signal corresponding to the phase to the driving circuit, thereby triggering the photovoltaic inverter to feed electric energy into the power grid.
5. The active island detection method for photovoltaic inverter without external frequency detection circuit as claimed in claim 1, characterized in that: In the non-islanding state, the interrupt signal of the timer keeps pace with the grid phase zero-crossing moment calculated by the software phase lock; In the island state, the period of the timer interrupt signal is the cumulative value of the zero-crossing moment of the photovoltaic inverter power signal phase and several micro-steps; When the period of the timer interrupt signal exceeds a preset value, it is determined that the photovoltaic inverter is in an island state.
6. The active island detection method for photovoltaic inverter without external frequency detection circuit as described in claims 1-5, characterized in that: The interrupt signal period T of the timer is ,in, is the grid frequency corresponding to the grid phase zero crossing point, is a microstep length, is the switching frequency of the PV inverter.
7. A photovoltaic inverter active island detection system without external frequency detection circuit, characterized in that: include: A grid voltage sampling module, which is connected to the grid and is used to collect the instantaneous value of the grid voltage; A phase-locked loop module is connected to the output end of the grid voltage sampling module and is used to iteratively calculate the real-time phase and frequency of the grid according to the instantaneous value of the grid voltage; A timer, which is based on the real-time phase and frequency of the power grid and outputs an interrupt signal according to the power grid cycle superimposed with a micro-step length; a trigonometric function generating unit, which is triggered by the interrupt signal to output phase information, so as to trigger the PWM unit to output a driving signal corresponding to the phase to the driving circuit based on the phase information, thereby triggering the photovoltaic inverter to feed electric energy into the power grid; The system determines that the photovoltaic inverter is in an island state when the period of the timer interrupt signal exceeds a preset value.
8. The photovoltaic inverter active island detection system without external frequency detection circuit as claimed in claim 6, characterized in that: The output time of the interrupt signal of the timer is the time corresponding to the superposition of the micro-step length at the zero-crossing time of the power grid phase output by the phase-locked loop module.
9. The photovoltaic inverter active island detection system without external frequency detection circuit as claimed in claim 6, characterized in that: The micro-step length is a preset fixed value.
10. The photovoltaic inverter active island detection system without external frequency detection circuit as described in claims 6-8, characterized in that: The switching period of the timer is inversely proportional to the grid period and inversely proportional to the switching frequency of the photovoltaic inverter.