Island detection method and device, grid-connected inverter and computer readable storage medium

Abstract

The application provides an island detection method and device, a grid-connected inverter and a computer readable storage medium. The island detection method comprises: continuously detecting a voltage frequency at a point of common coupling. A frequency deviation of the voltage frequency between two adjacent detections is calculated according to the voltage frequency. When the frequency deviation is less than a preset frequency threshold, a voltage phase is obtained through a preset closed-loop phase-locked manner. When the frequency deviation is greater than or equal to the preset frequency threshold, the voltage phase is obtained through a preset open-loop phase-locked manner. A reference current of frequency disturbance is determined according to the frequency deviation, the voltage frequency and the voltage phase. The grid-connected inverter is controlled to output a current to the point of common coupling according to the reference current. When the voltage frequency is out of a preset range, it is confirmed that the island effect occurs. Thus, the time delay of detecting the island effect is reduced, and the detection speed of detecting the island effect is improved.

Description

Technical Field

[0001] This application relates to the field of grid-connected power supply system technology, specifically to islanding detection methods, devices, grid-connected inverters, and computer-readable storage media. Background Technology

[0002] As a microgrid system, a grid-connected power supply system connects to the traditional centralized power grid via a point of common coupling (PCC). A grid-connected power supply system typically includes power supply equipment and a grid-connected inverter. The inverter converts the direct current (DC) output from the power supply equipment into alternating current (AC), which is then output to the grid and loads. During grid connection, the AC voltage output by the inverter needs to be synchronized with the grid voltage in terms of frequency and phase. When the grid loses power due to a fault, if the fault cannot be detected in time, some lines that are supposed to be disconnected from the grid may still be energized, resulting in an "islanding effect." Islanding can lead to power supply safety issues. Related technologies typically inject disturbances in relevant electrical parameters (such as phase disturbances, frequency disturbances, and current disturbances) into the PCC to detect islanding; however, slow islanding detection can still affect power supply safety. Summary of the Invention

[0003] In view of this, this application provides an islanding detection method, apparatus, grid-connected inverter, and computer-readable storage medium, aiming to solve the problem of how to quickly perform islanding detection in a grid-connected power supply system.

[0004] The first aspect of this application provides an islanding detection method applied to the controller of a grid-connected inverter. The method includes: continuously detecting the voltage frequency at the point of common coupling (PCC); calculating the frequency deviation of the voltage frequency between two adjacent detections based on the voltage frequency; obtaining the voltage phase using a preset closed-loop phase-locked loop (PLL) method when the frequency deviation is less than a preset frequency threshold; obtaining the voltage phase using a preset open-loop PLL method when the frequency deviation is greater than or equal to the preset frequency threshold; determining a reference current for injecting frequency disturbance based on the frequency deviation, voltage frequency, and voltage phase; controlling the grid-connected inverter to output current to the PCC based on the reference current; and confirming the occurrence of islanding when the voltage frequency exceeds a preset range.

[0005] The islanding detection method in this embodiment first calculates the frequency deviation between two adjacent detections based on the voltage frequency at the detected common coupling point. When the frequency deviation is less than a preset frequency threshold, the voltage phase is detected using a preset closed-loop phase-locked loop (PLL) method, and a reference current with frequency perturbation is injected into the common coupling point. When the frequency deviation is greater than or equal to the preset frequency threshold, it indicates that an islanding effect may occur. Since the frequency perturbation injected for islanding detection is a perturbation with positive feedback, if a closed-loop PLL based on negative feedback is used at this time, the negative feedback caused by the closed-loop PLL will have a certain offsetting effect on the perturbation with positive feedback injected for islanding detection, which may offset part of the frequency shift effect, resulting in a longer delay in detecting the islanding effect or failure to detect the islanding effect. Therefore, when the frequency deviation is greater than or equal to the preset frequency threshold, an open-loop PLL method is used to detect the voltage phase, and a reference current with frequency perturbation is determined based on this to continue injecting frequency perturbation into the common coupling point. When the voltage frequency at the common coupling point exceeds the preset range, an islanding effect is confirmed. Since open-loop phase-locked loops do not have negative feedback, they do not cancel out the positive feedback disturbances injected into islanding detection. Therefore, when a power grid fault occurs and power is lost, the frequency disturbances will accumulate rapidly, causing the voltage frequency at the common coupling point to quickly exceed the preset range. This reduces the time delay in detecting islanding effects and improves the detection speed of islanding effects.

[0006] In one embodiment, the islanding detection method further includes: confirming that no islanding effect has occurred when the voltage frequency does not exceed a preset range.

[0007] Since the voltage frequency at the common coupling point does not exceed the preset range, it indicates that the power grid is in the connected state and no power outage has occurred. The voltage frequency can be clamped near the power grid frequency, that is, no islanding effect has occurred.

[0008] In another embodiment, determining the reference current for injecting frequency disturbance based on frequency deviation, voltage frequency, and voltage phase includes: determining the frequency disturbance amount based on a preset gain coefficient and frequency deviation; calculating the reference phase of the reference current based on the voltage phase, voltage frequency, and frequency disturbance amount; and determining the reference current based on a preset current amplitude and reference phase.

[0009] Since the reference phase of the reference current changes with the frequency disturbance, as the frequency disturbance increases, the reference phase of the reference current also increases. The increase in the reference phase leads to a further increase in the frequency disturbance, which can quickly accumulate frequency disturbances and improve the detection speed of islanding effects.

[0010] In another embodiment, the island detection method further includes: when switching from a preset closed-loop phase-locked loop mode to a preset open-loop phase-locked loop mode, determining the first phase most recently detected by the preset closed-loop phase-locked loop mode as the initial phase value of the preset open-loop phase-locked loop mode.

[0011] This helps maintain phase continuity when switching phase-locked loop (PLL) modes, avoids re-tracking phases after switching modes, and ensures a smooth transition when switching PLL modes, thus avoiding affecting the operation of the grid-connected inverter.

[0012] In another embodiment, the island detection method further includes: when switching from a preset open-loop phase-locked loop mode to a preset closed-loop phase-locked loop mode, determining the second phase most recently detected by the preset open-loop phase-locked loop mode as the initial phase value of the preset closed-loop phase-locked loop mode.

[0013] In another embodiment, controlling the grid-connected inverter to output current to the point of common coupling based on a reference current includes: acquiring the current output current of the grid-connected inverter; determining a drive signal based on the reference current, the current output current, and a preset inverter control algorithm; and controlling the grid-connected inverter to output current to the point of common coupling using the drive signal. Therefore, the output current of the grid-connected inverter will have frequency disturbances. By detecting whether the voltage frequency at the point of common coupling is within a preset range, it can be determined whether islanding has occurred.

[0014] In another embodiment, the above-mentioned islanding detection method further includes: disconnecting the grid-connected inverter from the power grid when an islanding effect is confirmed to have occurred.

[0015] When an islanding effect is confirmed, it indicates a fault in the power grid. In this case, disconnecting the grid-connected inverter from the grid in a timely manner can prevent the connection point from becoming energized and causing a safety accident.

[0016] A second aspect of this application provides an islanding detection device, comprising: a voltage frequency detection module for continuously detecting the voltage frequency at a common coupling point; a frequency deviation calculation module for calculating the frequency deviation of the voltage frequency between two adjacent detections based on the voltage frequency; a voltage phase acquisition module for detecting the voltage phase using a preset closed-loop phase-locked loop (PLL) method when the frequency deviation is less than a preset frequency threshold, and for detecting the voltage phase using a preset open-loop PLL method when the frequency deviation is greater than or equal to the preset frequency threshold; a reference current acquisition module for determining a reference current for injecting frequency disturbances based on the frequency deviation, voltage frequency, and voltage phase; and an islanding effect confirmation module for controlling the grid-connected inverter to output current to the common coupling point based on the reference current, and for confirming the occurrence of an islanding effect when the voltage frequency exceeds a preset range.

[0017] A third aspect of this application provides a grid-connected inverter, which includes a controller and an inverter circuit. The controller is used to execute the steps of the islanding detection method provided in this application.

[0018] A fourth aspect of this application provides a computer-readable storage medium for storing a computer program or code, which, when executed by a processor, implements the steps of the island detection method provided in this application. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a grid-connected power supply system provided as an example.

[0020] Figure 2 This is a schematic diagram illustrating the frequency shift of the current provided as an example.

[0021] Figure 3 This is a flowchart of an island detection method provided in one embodiment of this application.

[0022] Figure 4 This is a schematic diagram of the structure of a three-phase SRF-PLL provided as an example.

[0023] Figure 5 This is a flowchart of an island detection method provided in another embodiment of this application.

[0024] Figure 6 This is a schematic diagram of the island detection device provided in one embodiment of this application.

[0025] Figure 7 This is a schematic diagram of the structure of a grid-connected inverter provided in one embodiment of this application. Detailed Implementation

[0026] It should be noted that the terms "first" and "second" in the specification, claims and drawings of this application are used to distinguish similar objects, rather than to describe a specific order or sequence.

[0027] It should also be noted that the methods disclosed in the embodiments of this application or the methods shown in the flowcharts include one or more steps for implementing the method. Without departing from the scope of the claims, the execution order of multiple steps can be interchanged, and some steps can also be deleted.

[0028] Some embodiments will now be described with reference to the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0029] Figure 1 This is a schematic diagram of a grid-connected power supply system provided as an example.

[0030] like Figure 1 As shown, the grid-connected power supply system 10 includes a grid-connected inverter 110 and a power supply device 130. When the grid-connected power supply system is connected to the power grid 120, it can draw power from the power grid 120 to supply power to important loads connected to the grid-connected power supply system 10. Figure 1 (not shown in the image) supplies power, or feeds 120V from the grid to power household loads (such as...). Figure 1 The load 140 in the circuit is powered by the power supply device 130. The power supply device 130 is electrically connected to the grid-connected inverter 110 and is used to input DC power to the grid-connected inverter 110. The grid-connected inverter 110 is powered via an AC output interface (such as...). Figure 1 The live wire (L terminal) and neutral wire (N terminal) are electrically connected to the power grid 120 and the load 140. The grid-connected inverter 110 is used to convert the DC power output by the power supply equipment 130 into AC power, and then input AC power to the power grid 120 and the load 140.

[0031] During grid connection, the grid-connected inverter 110 is connected to the grid 120 via a common coupling point (PCC). The first switch (K1) between the grid-connected inverter 110 and the grid 120 is closed, and / or the second switch (K2) between the grid-connected inverter 110 and the load 140 is closed. The AC output of the grid-connected inverter 110 generally needs to be synchronized with the frequency and phase of the voltage of the grid 120. When the grid 120 stops supplying power due to a fault, K1 and K2 close. If the fault cannot be detected in time, parts of the grid 120 that are not supplying power may still be energized (e.g., the grid-connected inverter 110 continues to supply power to the load 140), resulting in an "islanding effect." The islanding effect can lead to power supply safety issues. For example, when maintenance personnel perform repairs, because K1, K2, and the load 140 are still energized, electric shock may occur.

[0032] In related technologies, islanding detection is typically achieved by injecting disturbances in relevant electrical parameters (such as phase disturbances, frequency disturbances, and current disturbances) into the point of common coupling (PCC). For example, a grid-connected inverter injects a reference current with frequency disturbances into the PCC. Figure 2As shown, assuming the current injected into the grid by the inverter is synchronized with the grid voltage, the standard waveform of this current is curve S1, and the frequency of current S1 is the grid voltage frequency. The waveform of the reference current is curve S2. The frequency of the reference current deviates from the grid voltage frequency, being slightly higher than the grid voltage frequency. When the grid is supplying power normally, the voltage frequency at PCC is clamped to the grid voltage frequency by the AC power supplied by the grid, and the frequency disturbance of the reference current will not affect the voltage frequency at PCC. Conversely, when the grid loses power due to a fault, the voltage frequency at PCC changes with the waveform of the reference current. After a certain period of accumulation, the frequency deviation exceeds the preset range, thus detecting the islanding effect. However, slow islanding detection can still affect power supply security. For example, if the voltage frequency at PCC reaches a new equilibrium and matches the grid voltage frequency before islanding is detected, the islanding effect will not be detected.

[0033] Based on this, this application provides an islanding detection method, apparatus, grid-connected inverter, and computer-readable storage medium. First, the frequency deviation is calculated based on the detected voltage frequency at the common coupling point (CCP). When the frequency deviation is less than a preset frequency threshold, the voltage phase is detected using a preset closed-loop phase-locked loop (PLL) method, and a reference current with frequency disturbance is injected into the CCP. When the frequency deviation is greater than or equal to the preset frequency threshold, it indicates that islanding may have occurred. Since the frequency disturbance injected for islanding detection is a positive feedback disturbance, if a negative feedback-based closed-loop PLL is used, the negative feedback caused by the closed-loop PLL will have a certain offsetting effect on the positive feedback disturbance injected for islanding detection, potentially canceling out part of the frequency shift effect, resulting in a longer delay in detecting the islanding effect or failure to detect the islanding effect. Therefore, when the frequency deviation is greater than or equal to the preset frequency threshold, an open-loop PLL method is used to detect the voltage phase, and a reference current with frequency disturbance is determined based on this to continue injecting frequency disturbance into the CCP. When the voltage frequency at the CCP exceeds a preset range, islanding is confirmed to have occurred. Since open-loop phase-locked loops do not have negative feedback, they do not cancel out the positive feedback disturbances injected into islanding detection. Therefore, when a power grid fault occurs and power is lost, the frequency disturbances will accumulate rapidly, causing the voltage frequency at the common coupling point to quickly exceed the preset range. This reduces the time delay in detecting islanding effects and improves the detection speed of islanding effects.

[0034] Figure 3 This is a flowchart of an islanding detection method provided in one embodiment of this application. The islanding detection method is applied to a grid-connected inverter.

[0035] like Figure 3 As shown, the island detection method may include the following steps:

[0036] S301 continuously monitors the voltage frequency at the common coupling point.

[0037] In this embodiment, the controller of the grid-connected inverter (hereinafter referred to as the controller) can use zero-crossing interpolation to detect the voltage frequency at the common coupling point. Specifically, the controller calculates the voltage frequency by detecting the duration of the voltage signal at the common coupling point between two zero-crossing points.

[0038] It is understood that the controller can continuously detect the voltage frequency at the common coupling point according to a preset control cycle. The control cycle can be set as needed.

[0039] S302, calculate the frequency deviation of the voltage frequency between two adjacent detections based on the voltage frequency.

[0040] As can be understood, frequency deviation refers to the frequency difference between two consecutive voltage detections. After the controller detects f1 and f2 twice consecutively, it calculates the difference between f1 and f2 to obtain the frequency deviation Δf. For example, if the voltage frequency detected last time was 52Hz and the voltage frequency detected this time is 53Hz, then the frequency deviation is 1Hz. As another example, if the voltage frequency detected last time was 51Hz and the voltage frequency detected this time is 49Hz, then the frequency deviation is -2Hz.

[0041] S303 obtains the voltage phase through a preset closed-loop phase-locked loop method when the frequency deviation is less than a preset frequency threshold.

[0042] The preset frequency threshold can be set as needed. For example, the preset frequency threshold can be set to 3Hz, 5Hz, etc.

[0043] It is understandable that, here, "frequency deviation less than preset frequency threshold" means that the absolute value of the frequency deviation is less than the preset frequency threshold.

[0044] In this embodiment, the closed-loop phase-locked loop (PLL) method is a negative feedback-based PLL. The closed-loop PLL method can adjust the frequency deviation according to the negative feedback amount, so that the voltage phase and frequency output by the PLL are synchronized with the power grid, achieving voltage phase and frequency tracking and locking.

[0045] Take the Synchronous Reference Frame Phase-Locked Loop (SRF-PLL) as an example. Figure 4As shown, the three-phase SRF-PLL 400 includes a phase detector (PD) 410, a loop filter (LF) 420, and a voltage-controlled oscillator (VCO) 430. The working process of the three-phase SRF-PLL 400 is as follows: the output signal is fed back to the input terminal. After loop adjustment, the angular frequency of the output signal is made consistent with the input signal, thus maintaining a constant phase difference between the input and output signals. This means the loop reaches a "locked-in" state, achieving phase-locking of the input signal. When applied to a grid-connected inverter, the input signal is the voltage signal at the common coupling point. When the grid is supplying power normally, this voltage signal is the grid voltage.

[0046] Continue to refer to Figure 4 PD 410 is used to detect the phase difference between the input and output signals and convert the detected phase difference into a voltage signal. LF 420 is used to filter out high-frequency components in the voltage signal output by PD 410. VCO 430 is used to adjust the frequency and phase of the output signal based on the voltage signal output by PD 410, and the phase of the adjusted output signal is fed back to PD 410, thus completing the closed-loop control based on negative feedback.

[0047] For example, PD 410 includes a Clark transformation module 411 and a Park transformation module 412. The Clark transformation module 411 transforms the three-phase input signals Va, Vb, and Vc to a stationary αβ coordinate system, obtaining two-phase output signals Vα and Vβ. The Park transformation module 412 transforms the Vα and Vβ output from the Clark transformation module 411 to a rotating dq coordinate system based on the detected phase difference, obtaining output signals Vd and Vq. Vd is the voltage amplitude. When the phase-locked loop achieves phase locking, i.e., the phase difference between the output signal and the input signal is 0, then Vq is 0, meaning the given value of Vq is 0.

[0048] The LF 420 includes a proportional-integral controller (PI) 421. The PI 421 is used to perform proportional and integral control on the output Vq of the Park converter module 412 based on a given value of 0.

[0049] VCO 430 includes an integrator 431. The integrator 431 integrates the calculation result output by the PI controller 421 to obtain the phase θ. The phase θ output by the integrator 431 is used as negative feedback and fed back to the Park converter module 412, where it is subtracted from the phase of the input signal, thereby achieving closed-loop control. When Vq is 0, it indicates that the rotation speed and phase of the dq coordinate system are synchronized with the power grid; therefore, the corresponding phase θ is the voltage phase of the power grid.

[0050] In one embodiment, the closed-loop phase-locked loop (PLL) method is Kalman phase-locked loop (PLL). The specific implementation process of Kalman PLL can be found in related technologies, and will not be elaborated upon here.

[0051] It is understandable that when the frequency deviation is small, it means that there is no possibility of islanding effect. At this time, locking the voltage phase through closed-loop phase-locked loop can quickly and accurately track the voltage phase of the power grid compared with open-loop phase-locked loop, thus improving the phase-locking accuracy.

[0052] S304, when the frequency deviation is greater than or equal to the preset frequency threshold, obtains the voltage phase through a preset open-loop phase-locked loop.

[0053] In this embodiment, the open-loop phase-locked loop (PLL) is a PLL without feedback control. When the frequency deviation is greater than or equal to a preset frequency threshold, it indicates that islanding may occur. During islanding detection, since the open-loop PLL lacks negative feedback, it does not cancel out the positive feedback disturbance injected during islanding detection. Therefore, the frequency disturbance accumulates rapidly, which helps reduce the time delay in detecting islanding and improves the detection speed.

[0054] In one embodiment, the open-loop phase-locked loop (PLL) method is zero-crossing PLL. The specific implementation process of zero-crossing PLL can be found in related technologies, and will not be elaborated upon here.

[0055] S305 determines the reference current for injecting frequency disturbance based on frequency deviation, voltage frequency, and voltage phase.

[0056] In this embodiment, after the controller obtains the voltage phase through closed-loop phase-locked loop or open-loop phase-locked loop, it can calculate the reference phase of the reference current based on the voltage phase, frequency deviation and voltage frequency, and then determine the reference current according to the preset current amplitude and reference phase.

[0057] In one embodiment, the controller can calculate the reference current according to formula (1):

[0058] I ref =I*sinθ k (1)

[0059] Among them, I ref The reference current is represented by I, the preset current amplitude is represented by θ. k The reference phase θ represents the injected frequency perturbation. k Controlled between 0 and 2π. When θ k When θ is greater than 2π, k The difference between 2π and θ is assigned to θ. k θk It is determined by voltage phase, frequency deviation, and voltage frequency. Among them, frequency disturbance is positively correlated with frequency deviation.

[0060] In another embodiment, the controller can calculate the reference current according to formula (2):

[0061] I ref =I*cosθ k (2)

[0062] S306 controls the grid-connected inverter to output current to the common coupling point based on the reference current.

[0063] In this embodiment, after determining the reference current, the controller can disturb the output current frequency of the grid-connected inverter by the frequency disturbance of the reference current, thereby determining whether an islanding effect has occurred by detecting whether the voltage frequency at the common coupling point is within a preset range.

[0064] It is understandable that if the power grid fails and supply is not provided, the voltage frequency at the point of common coupling (PCC) will lose its clamping effect. When the islanding effect is detected using frequency perturbation, taking a negative frequency deviation as an example, it indicates that the voltage frequency at PCC is decreasing. Due to the presence of the phase-locked loop (PLL), the output current frequency of the grid-connected inverter will decrease. Based on Ohm's law, the voltage frequency at PCC will further decrease as the output current frequency decreases, achieving positive feedback perturbation until the voltage frequency exceeds the preset range.

[0065] S307: When the voltage frequency exceeds the preset range, an islanding effect is confirmed.

[0066] It is understandable that when the voltage frequency at the point of common coupling (PCC) is within the preset range, it indicates that the voltage frequency disturbance is normal, thus confirming that the power grid is in a normal power supply state. The voltage frequency at PCC can be clamped, and frequency disturbances will not affect the voltage at PCC. When the voltage frequency exceeds the preset range, it indicates that the voltage frequency disturbance is abnormal, thus confirming the occurrence of an islanding effect.

[0067] The preset range can be set as needed. For example, assuming the power grid voltage frequency is 50Hz, the preset range can be set to be greater than or equal to 48Hz and less than or equal to 52Hz.

[0068] In the above embodiments, when the frequency deviation is less than a preset frequency threshold, the voltage phase is detected using a preset closed-loop phase-locked loop (PLL) method, and a reference current with frequency perturbation is injected into the common coupling point. When the frequency deviation is greater than or equal to the preset frequency threshold, it indicates that an islanding effect may occur. Since the frequency perturbation injected for islanding detection is a perturbation with positive feedback, if a closed-loop PLL based on negative feedback is used at this time, the negative feedback caused by the closed-loop PLL will have a certain offsetting effect on the perturbation with positive feedback injected for islanding detection, which may offset part of the frequency shift effect, resulting in a longer delay in detecting the islanding effect or failure to detect the islanding effect. Therefore, when the frequency deviation is greater than or equal to the preset frequency threshold, an open-loop PLL method is used to detect the voltage phase, and a reference current with frequency perturbation is determined based on this to continue injecting frequency perturbation into the common coupling point. When the voltage frequency at the common coupling point exceeds the preset range, an islanding effect is confirmed to have occurred. Since open-loop phase-locked loops do not have negative feedback, they do not cancel out the positive feedback disturbances injected into islanding detection. Therefore, when a power grid fault occurs and power is lost, the frequency disturbances will accumulate rapidly, causing the voltage frequency at the common coupling point to quickly exceed the preset range. This reduces the time delay in detecting islanding effects and improves the detection speed of islanding effects.

[0069] In one embodiment, the islanding detection method further includes: confirming that no islanding effect has occurred when the voltage frequency does not exceed a preset range.

[0070] It is understandable that since the voltage frequency at the common coupling point does not exceed the preset range, it indicates that the power grid is in the connected state and no power outage has occurred, that is, no islanding effect has occurred.

[0071] In another embodiment, determining the reference current for injecting frequency disturbance based on frequency deviation, voltage frequency, and voltage phase includes: determining the frequency disturbance amount based on a preset gain coefficient and frequency deviation; calculating the reference phase of the reference current based on the voltage phase, voltage frequency, and frequency disturbance amount; and determining the reference current based on a preset current amplitude and reference phase.

[0072] For example, the controller can calculate the reference phase according to formula (3):

[0073] θ k =θ+2π*(f+kp*Δf)*t (3)

[0074] Where, θ k Let θ represent the reference phase, θ represent the voltage phase obtained via closed-loop or open-loop phase-locked loop (PLL) mode, f represent the voltage frequency at the common coupling point, Δf represent the frequency deviation, kp represent the gain coefficient, and t represent time. (kp*Δf) is the injected frequency disturbance.

[0075] As can be understood, the gain factor is used to amplify frequency deviations, thereby accelerating the accumulation of frequency disturbances. The gain factor can be set as needed. For example, the gain factor can be set to 2, 5, or 10, etc.

[0076] In the above embodiments, since the reference phase of the reference current changes with the frequency disturbance, as the frequency disturbance increases, the reference phase of the reference current also increases, which leads to an increase in the real-time phase of the output current, which in turn leads to a further increase in the frequency disturbance, thus achieving positive feedback disturbance. In this way, when the islanding effect occurs, the frequency disturbance can be accumulated quickly, which helps to reduce the time delay in detecting the islanding effect and improve the detection speed of the islanding effect.

[0077] In another embodiment, the islanding detection method further includes disconnecting the grid-connected inverter from the power grid when an islanding effect is confirmed to have occurred.

[0078] Understandably, when an islanding effect is confirmed, it indicates that the power grid is unable to supply power normally. At this point, the connection between the grid-connected inverter and the grid should be disconnected to de-energize that part of the circuit and ensure safety. Figure 1 As shown in the example, at this time, K1 can be controlled to disconnect, so that the connection between the grid-connected inverter and the grid is disconnected. At this time, the part of the circuit connected to the grid-connected inverter in the grid is no longer energized and can be repaired normally.

[0079] In another embodiment, the island detection method further includes: when switching from a preset closed-loop phase-locked loop mode to a preset open-loop phase-locked loop mode, determining the first phase most recently detected by the preset closed-loop phase-locked loop mode as the initial phase value of the preset open-loop phase-locked loop mode.

[0080] Understandably, due to the accumulation of frequency disturbances, the frequency deviation gradually increases. When the frequency deviation is greater than or equal to a preset frequency threshold, the controller switches the phase-locked loop (PLL) mode from closed-loop to open-loop. To ensure a smooth switch, the controller determines the first phase detected most recently via closed-loop PLL as the initial phase value for open-loop PLL. This helps maintain phase continuity during the switch and avoids the need for re-tracking after the switch, thus preventing disruption to the stability of the grid-connected inverter's output current.

[0081] In another embodiment, the island detection method further includes: when switching from a preset open-loop phase-locked loop mode to a preset closed-loop phase-locked loop mode, determining the second phase most recently detected by the preset open-loop phase-locked loop mode as the initial phase value of the preset closed-loop phase-locked loop mode.

[0082] It is understandable that when the power grid experiences a brief fault and is quickly restored, the frequency deviation gradually increases with the accumulation of frequency disturbances during the fault period. When the frequency deviation is greater than or equal to a preset frequency threshold, the controller switches the phase-locked loop (PLL) mode from closed-loop to open-loop. After the power grid is restored, the frequency deviation gradually decreases with the reduction of frequency disturbances. When the frequency deviation is less than the preset frequency threshold, the controller switches the PLL mode from open-loop to closed-loop. To ensure a smooth switching of PLL modes, the controller determines the second phase most recently detected through open-loop PLL as the initial phase value for closed-loop PLL. This helps maintain phase continuity during switching PLL modes and avoids the need for re-tracking phases after switching modes.

[0083] In another embodiment, controlling the grid-connected inverter to output current to the point of common coupling based on a reference current includes: acquiring the current output current of the grid-connected inverter; determining a drive signal based on the reference current, the current output current, and a preset inverter control algorithm; and controlling the grid-connected inverter to output current to the point of common coupling using the drive signal.

[0084] The inverter control algorithm can be a sinusoidal pulse width modulation (SPWM) method. For example, the inverter control algorithm can be a current loop-based SPWM closed-loop control method. The implementation of the SPWM closed-loop control method can refer to relevant technologies, such as a PI controller, which will not be elaborated on in this application.

[0085] In the above embodiment, after acquiring the reference current, the controller uses the frequency disturbance of the reference current as the control target of the grid-connected inverter's output current based on the inverter control algorithm, thus obtaining a drive signal with frequency disturbance information. The drive signal is then used to control the grid-connected inverter to output current to the point of common coupling. Therefore, the output current of the grid-connected inverter will carry frequency disturbance. When a grid fault prevents power supply, this frequency disturbance will accumulate. At this time, by detecting whether the voltage frequency at the point of common coupling is within a preset range, it can be determined whether an islanding effect has occurred.

[0086] Figure 5 This is a flowchart of another embodiment of the islanding detection method provided in this application. The islanding detection method is applied to grid-connected inverters.

[0087] like Figure 5 As shown, the island detection method may include the following steps:

[0088] S501 continuously monitors the voltage frequency at the common coupling point.

[0089] S502, calculates the frequency deviation of the voltage frequency between two adjacent detections based on the voltage frequency.

[0090] S503, determine whether the frequency deviation is greater than or equal to the preset frequency threshold.

[0091] If the frequency deviation is greater than or equal to the preset frequency threshold, then steps S504 and S506 to S508 are executed; otherwise, steps S505 and S506 to S508 are executed.

[0092] S504 obtains the voltage phase through a preset open-loop phase-locked loop.

[0093] S505 obtains the voltage phase through a preset closed-loop phase-locked loop.

[0094] S506 determines the reference current for injecting frequency disturbance based on frequency deviation, voltage frequency, and voltage phase.

[0095] S507 controls the grid-connected inverter to output current to the common coupling point based on the reference current.

[0096] S508 determines whether the voltage frequency exceeds the preset range.

[0097] If the voltage frequency exceeds the preset range, proceed to step S509; otherwise, proceed to step S510.

[0098] S509, the islanding effect has been confirmed.

[0099] S510, confirming that no islanding effect has occurred.

[0100] In the above embodiments, when the frequency deviation is less than a preset frequency threshold, the voltage phase is detected using a preset closed-loop phase-locked loop (PLL) method, and a reference current with frequency perturbation is injected into the common coupling point. When the frequency deviation is greater than or equal to the preset frequency threshold, it indicates that an islanding effect may occur. Since the frequency perturbation injected for islanding detection is a perturbation with positive feedback, if a closed-loop PLL based on negative feedback is used at this time, the negative feedback caused by the closed-loop PLL will have a certain offsetting effect on the perturbation with positive feedback injected for islanding detection, which may offset part of the frequency shift effect, resulting in a longer delay in detecting the islanding effect or failure to detect the islanding effect. Therefore, when the frequency deviation is greater than or equal to the preset frequency threshold, an open-loop PLL method is used to detect the voltage phase, and a reference current with frequency perturbation is determined based on this to continue injecting frequency perturbation into the common coupling point. When the voltage frequency at the common coupling point exceeds the preset range, an islanding effect is confirmed to have occurred. Since open-loop phase-locked loops do not have negative feedback, they do not cancel out the positive feedback disturbances injected into islanding detection. Therefore, when a power grid fault occurs and power is lost, the frequency disturbances will accumulate rapidly, causing the voltage frequency at the common coupling point to quickly exceed the preset range. This reduces the time delay in detecting islanding effects and improves the detection speed of islanding effects.

[0101] Figure 6 This is a schematic diagram of an islanding detection device provided in one embodiment of this application. The islanding detection device can be a device in a grid-connected inverter, or a device electrically connected to the grid-connected inverter.

[0102] like Figure 6 As shown, the islanding detection device 600 includes a voltage frequency detection module 610, a frequency deviation calculation module 620, a voltage phase acquisition module 630, a reference current acquisition module 640, and an islanding effect confirmation module 650.

[0103] The voltage frequency detection module 610 is used to continuously detect the voltage frequency at the common coupling point.

[0104] The frequency deviation calculation module 620 is used to calculate the frequency deviation of the voltage frequency between two adjacent detections based on the voltage frequency.

[0105] The voltage phase acquisition module 630 is used to detect the voltage phase using a preset closed-loop phase-locked loop (PLL) method when the frequency deviation is less than a preset frequency threshold; and to detect the voltage phase using a preset open-loop PLL method when the frequency deviation is greater than or equal to the preset frequency threshold. Specifically, the voltage phase acquisition module 630 includes a closed-loop PLL module 631 and an open-loop PLL module 632. The closed-loop PLL module 631 is used to detect the voltage phase using a preset closed-loop PLL method when the frequency deviation is less than the preset frequency threshold. The open-loop PLL module 632 is used to detect the voltage phase using a preset open-loop PLL method when the frequency deviation is greater than or equal to the preset frequency threshold.

[0106] The reference current acquisition module 640 is used to determine the reference current for the injected frequency disturbance based on the frequency deviation, voltage frequency, and voltage phase.

[0107] The islanding effect confirmation module 650 is used to control the grid-connected inverter to output current to the common coupling point according to the reference current; and to confirm the occurrence of islanding effect when the voltage frequency exceeds the preset range.

[0108] In one embodiment, the islanding effect confirmation module 650 is further configured to disconnect the grid-connected inverter from the power grid when an islanding effect is confirmed to have occurred.

[0109] In another embodiment, the islanding effect confirmation module 650 is also used to confirm that no islanding effect has occurred when the voltage frequency does not exceed a preset range.

[0110] In another embodiment, the reference current acquisition module 640 determines the reference current for the injected frequency disturbance based on the frequency deviation, voltage frequency, and voltage phase, including: determining the frequency disturbance amount based on a preset gain coefficient and frequency deviation; calculating the reference phase of the reference current based on the voltage phase, voltage frequency, and frequency disturbance amount; and determining the reference current based on a preset current amplitude and reference phase.

[0111] In another embodiment, the islanding detection device 600 further includes a phase-locked loop (PLL) mode switching module (not shown). The PLL mode switching module is used to, when switching from a preset closed-loop PLL mode to a preset open-loop PLL mode, determine the first phase most recently detected by the preset closed-loop PLL mode as the initial phase value of the preset open-loop PLL mode; and, when switching from the preset open-loop PLL mode to the preset closed-loop PLL mode, determine the second phase most recently detected by the preset open-loop PLL mode as the initial phase value of the preset closed-loop PLL mode.

[0112] In another embodiment, the islanding effect verification module 650 controls the grid-connected inverter to output current to the point of common coupling based on a reference current, including: acquiring the current output current of the grid-connected inverter; determining a drive signal based on the reference current, the current output current, and a preset inverter control algorithm; and controlling the grid-connected inverter to output current to the point of common coupling via the drive signal.

[0113] It is understood that the module division described above is a logical functional division, and there may be other division methods in actual implementation. Furthermore, the functional modules in the various embodiments of this application can be integrated into the same processing unit, or each module can exist physically separately, or two or more modules can be integrated into the same unit. The integrated modules described above can be implemented in hardware or in a combination of hardware and software functional modules.

[0114] Figure 7 This is a schematic diagram of the structure of a grid-connected inverter provided in one embodiment of this application.

[0115] like Figure 7 As shown, the grid-connected inverter 110 includes a controller 111 and an inverter circuit 112. The controller 111 is used to execute the steps of the islanding detection method provided in this embodiment. The inverter circuit 112 is used to output current to the point of common coupling according to the signal or command output by the controller 111.

[0116] The controller 111 can be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or any conventional processor.

[0117] It is understood that the structure illustrated in this embodiment does not constitute a specific limitation on the grid-connected inverter. In other embodiments, the grid-connected inverter may include more or fewer components than illustrated, or combine some components, or split some components, or have different component arrangements.

[0118] This application also provides a computer-readable storage medium for storing computer programs or code, which, when executed by a processor, implement the steps of the island detection method provided in this application.

[0119] Computer-readable storage media include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storing information (such as computer-readable instructions, data structures, program modules or other data). Computer-readable storage media include, but are not limited to, random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory, compact disc read-only memory (CD-ROM), digital versatile disc (DVD) or other optical disc storage, magnetic cartridges, magnetic tapes, disk storage or other magnetic storage devices, or any other medium that can be used to store desired information and is accessible to a computer.

[0120] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the spirit and scope of the technical solutions of this application.

Claims

1. An islanding detection method, applied to a grid-connected inverter, characterized in that, The method includes: Continuously monitor the voltage frequency at the common coupling point; Calculate the frequency deviation of the voltage frequency between two adjacent detections based on the voltage frequency; When the frequency deviation is less than a preset frequency threshold, the voltage phase is detected by a preset closed-loop phase-locked loop method. When the frequency deviation is greater than or equal to the preset frequency threshold, the voltage phase is detected by a preset open-loop phase-locked loop method; The reference current for injecting frequency disturbance is determined based on the frequency deviation, the voltage frequency, and the voltage phase. The grid-connected inverter is controlled to output current to the common coupling point according to the reference current. When the voltage frequency exceeds the preset range, an islanding effect is confirmed to have occurred.

2. The island detection method as described in claim 1, characterized in that, The method further includes: When the voltage frequency does not exceed the preset range, it is confirmed that the islanding effect has not occurred.

3. The island detection method as described in claim 1, characterized in that, The step of determining the reference current for the injected frequency disturbance based on the frequency deviation, the voltage frequency, and the voltage phase includes: The frequency disturbance is determined based on the preset gain coefficient and the frequency deviation; The reference phase of the reference current is calculated based on the voltage phase, the voltage frequency, and the frequency disturbance. The reference current is determined based on the preset current amplitude and the reference phase.

4. The island detection method as described in claim 1, characterized in that, The method further includes: When switching from the preset closed-loop phase-locked loop mode to the preset open-loop phase-locked loop mode, the first phase most recently detected by the preset closed-loop phase-locked loop mode is determined as the initial phase value of the preset open-loop phase-locked loop mode.

5. The island detection method as described in claim 1, characterized in that, The method further includes: When switching from the preset open-loop phase-locked loop mode to the preset closed-loop phase-locked loop mode, the second phase most recently detected by the preset open-loop phase-locked loop mode is determined as the initial phase value of the preset closed-loop phase-locked loop mode.

6. The island detection method as described in claim 1, characterized in that, The step of controlling the grid-connected inverter to output current to the point of common coupling based on the reference current includes: Obtain the current output current of the grid-connected inverter; The drive signal is determined based on the reference current, the current output current, and the preset inverter control algorithm. The drive signal controls the grid-connected inverter to output current to the common coupling point.

7. The island detection method as described in claim 1, characterized in that, The method further includes: When an islanding effect is confirmed, disconnect the grid-connected inverter from the power grid.

8. An islanding detection device, applied to a grid-connected inverter, characterized in that, The island detection device includes: The voltage frequency detection module is used to continuously detect the voltage frequency at the common coupling point; The frequency deviation calculation module is used to calculate the frequency deviation of the voltage frequency between two adjacent detections based on the voltage frequency. A voltage phase acquisition module is used to detect the voltage phase using a preset closed-loop phase-locked loop method when the frequency deviation is less than a preset frequency threshold; and to detect the voltage phase using a preset open-loop phase-locked loop method when the frequency deviation is greater than or equal to the preset frequency threshold. A reference current acquisition module is used to determine a reference current for the injected frequency disturbance based on the frequency deviation, the voltage frequency, and the voltage phase. The islanding effect confirmation module is used to control the grid-connected inverter to output current to the common coupling point according to the reference current; and to confirm the occurrence of islanding effect when the voltage frequency exceeds the preset range.

9. A grid-connected inverter, characterized in that, The grid-connected inverter includes a controller and an inverter circuit, wherein the controller is used to perform the islanding detection method as described in any one of claims 1 to 7.

10. A computer-readable storage medium for storing computer programs or code, characterized in that, When the computer program or code is executed by a processor, the island detection method as described in any one of claims 1 to 7 is implemented.

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