Active islanding detection method, device and system for variable frequency energy storage power supply

Abstract

The application discloses a kind of active island detection method, device and system of variable frequency energy storage power supply, method includes: determining variable frequency energy storage power supply grid-connected mode operation, actively superimposes low-frequency disturbance signal in control current;Collect the electrical sampling value of point of common coupling, extract multiple impedance characteristics under low-frequency disturbance signal;Based on the preset judgment rule corresponding to each of multiple impedance characteristics, and the preset passive auxiliary judgment rule, whether island state occurs is comprehensively judged;Low-frequency disturbance signal is attenuated, and the actual operating point before power grid is disconnected is used as initial value to initialize the state of virtual synchronous generator function, and switch to off-grid network mode.Detection blind area elimination: based on impedance response, grid-connected mode / island state impedance ratio reaches 100~10000 times, even in extreme power matching condition Still reliable detection can be made, eliminate detection blind area.

Description

Technical Field

[0001] This application relates to the field of variable frequency energy storage power supplies, specifically to an active islanding detection method, device, and system for variable frequency energy storage power supplies. Background Technology

[0002] The islanding effect refers to the situation where, when the power grid is disconnected due to a fault or maintenance, distributed power sources (such as power conversion systems (PCS), also known as variable frequency energy storage power sources) fail to detect the grid disconnection in time and continue to supply power to local loads, forming a self-sufficient "island".

[0003] The hazards of islanded operation include: 1. Personal safety risks: maintenance personnel may mistakenly believe that the line is de-energized, resulting in a high risk of electric shock; 2. Equipment damage risk: islanded operation may generate a huge inrush current when reconnected to the grid; 3. Deterioration of power quality: voltage / frequency may deviate significantly from the rated value when there is no grid support; 4. Protection malfunction: the protection device may malfunction due to phase inconsistency during reclosing.

[0004] Traditional island detection methods are mainly divided into two categories: The first type is passive island detection: Islanding is identified by monitoring parameters such as voltage amplitude, frequency, phase jump, and harmonic content at the point of common coupling (PCC) to determine if they exceed limits. Typical methods include: voltage / frequency over-limit protection (Over Voltage Protection (OVP) / Under Voltage Protection (UVP) / Over Frequency Protection (OF) / Under Frequency Protection (UFP)), voltage phase jump detection (PJD), rate of change of frequency (ROCOF) detection, and harmonic / unbalance detection.

[0005] Passive islanding detection has inherent limitations: when the local load matches the PCS output power (i.e., the quality factor (Qf) is high), the voltage / frequency change after islanding is minimal, making it difficult to detect islanding within the specified time, resulting in a detection blind zone. According to relevant standards, the islanding detection time should be ≤2s, which passive methods struggle to meet under high quality factor loads.

[0006] The second type is active island detection: By injecting small disturbances into the power grid, the system parameters after islanding are continuously shifted until they exceed limits, thereby disrupting power balance and eliminating detection blind spots. Typical methods include: 1. Active Frequency Drift (AFD): Injecting a frequency shift related to the voltage zero-crossing point into the current reference; 2. Sandia Frequency Shift (SFS) / Active Frequency Drift with Positive Feedback (AFDPF): Adding positive feedback to AFD to accelerate frequency shift; 3. Slip-Mode Frequency Shift (SMS): Introducing phase shift disturbances into the phase-locked loop (PLL) phase-locked stage; 4. Active Impedance Method: Injecting periodic impedance disturbances; 5. Active Voltage Shift: Superimposing DC or low-frequency components into the voltage reference.

[0007] Active detection also has its inherent drawbacks: 1. Conflict with Virtual Synchronous Generator (VSG) / grid control: Active frequency shifting requires the inverter to operate in grid-connected mode (current source), while the energy storage emergency power supply needs to switch to grid-connected mode (voltage source) after islanding. The two modes have a fundamental conflict in terms of frequency / voltage control. 2. Affecting power quality: Disturbance injection will introduce harmonics and interharmonics, reducing the power quality during grid-connected operation. 3. Multi-machine parallel interference: When multiple PCS are operating in parallel, the disturbances of each machine may cancel each other out or be superimposed, leading to detection failure or misjudgment. 4. Conflict with inertial response / primary frequency regulation: When the energy storage PCS is connected to the grid, it needs to provide inertial response and primary frequency regulation. These functions themselves require frequency offset operation, which may contradict the disturbance direction of the active frequency shifting method.

[0008] Meanwhile, variable frequency energy storage power supplies have special requirements. They have both grid-connected operation and off-grid emergency power supply functions, and their islanding detection faces special challenges: 1. Dual-mode operation: When connected to the grid, it operates in grid-connected mode (current source), and when off-grid, it needs to quickly switch to grid-connected mode (voltage source) to provide emergency power supply; 2. Seamless switching requirements: Emergency power supply requires rapid (<100ms) switching to grid-connected mode after islanding detection to avoid load power outages; 3. VSG grid-connected capability: Energy storage PCS usually has VSG (Virtual Synchronous Generator) function, and operates in grid-connected mode after islanding to provide voltage / frequency support for local loads; 4. Primary frequency regulation / inertia response: When connected to the grid, it needs to participate in grid frequency regulation, and these functions are coupled with active frequency shift disturbances; 5. Low Voltage Ride Through (LVRT) / High Voltage Ride Through (HVRT) ride-through: When the grid voltage drops / rises, it needs to provide reactive power support, and voltage / frequency anomalies during the ride-through are unlikely to trigger islanding protection.

[0009] In summary, traditional energy storage PCS solutions generally employ passive islanding detection (voltage / frequency over-limit protection), which has the following problems: 1. Large detection blind zone: Under high-quality loads with Qf>1.0, the voltage / frequency deviation after islanding is extremely small, making reliable detection difficult within 2 seconds; 2. Long detection time: Passive methods rely on over-limit judgment, triggering protection only when parameters exceed the threshold, resulting in a delayed response; 3. Conflict with LVRT: During low-voltage ride-through, the voltage drops significantly, easily confused with islanding undervoltage, requiring complex judgment and shielding logic; 4. Unreliable emergency switching: Passive detection may be delayed by several seconds, during which the load may shut down due to voltage / frequency deterioration, negating the purpose of emergency power supply.

[0010] However, the active islanding detection method does not take into account the special characteristics of energy storage emergency power supplies. Direct application will lead to: conflicts with VSG network control, failure to establish voltage / frequency normally after islanding; disturbance injection affecting the accuracy of primary frequency regulation and inertial response; and decreased detection reliability in multi-machine parallel scenarios. Summary of the Invention

[0011] To address the aforementioned issues, this application proposes an active islanding detection method for variable frequency energy storage power supplies, comprising: The variable frequency energy storage power supply is determined to operate in grid-connected mode, and a low-frequency disturbance signal is actively superimposed onto the control current; Electrical sampling values ​​at the common coupling point are collected, and multiple impedance characteristics under the low-frequency disturbance signal are extracted using signal processing techniques. Based on the preset judgment rules corresponding to each of the multiple impedance characteristics, and the preset passive auxiliary judgment rules, a comprehensive judgment is made on whether an islanding state has occurred through redundant judgment logic. If this occurs, the low-frequency disturbance signal will be attenuated, and the virtual synchronous generator function will be initialized based on the actual operating point before the grid disconnection as the initial value, and then switched to the off-grid network mode.

[0012] In one example, the variable frequency energy storage power supply is determined to be operating in grid-connected mode. A low-frequency disturbance signal is actively superimposed onto the control current, specifically including: To determine the grid-connected operation mode of the variable frequency energy storage power supply, a low-frequency sinusoidal disturbance component is superimposed on the q-axis component of the current reference. The amplitude range of the low-frequency sinusoidal disturbance component is 0.5% to 2% of the rated current, and the frequency range is 0.5 Hz to 5 Hz.

[0013] In one example, electrical sampling values ​​at the common coupling point are acquired, and multiple impedance characteristics under the low-frequency disturbance signal are extracted using signal processing techniques, specifically including: Collect electrical sampling values ​​at the common coupling point; the electrical sampling values ​​include: three-phase voltage signals and three-phase current signals; The electrical sample values ​​are converted to a two-phase stationary coordinate system using Clarke transformation; For the reference signal corresponding to the two-phase stationary coordinate system, orthogonal demodulation is performed with the sine reference signal and cosine reference signal of the disturbance frequency of the low-frequency disturbance signal, respectively; The baseband complex components of the low-frequency disturbance signal are extracted by low-pass filtering. Based on the baseband complex components, a number of corresponding impedance characteristics are determined; the impedance characteristics include: impedance amplitude, impedance phase, and impedance change rate.

[0014] In one example, based on the preset judgment rules corresponding to each of the multiple impedance characteristics, and the preset passive auxiliary judgment rules, redundant judgment logic is used to comprehensively determine whether an islanding state has occurred, specifically including: For each impedance feature among impedance amplitude, impedance phase, and impedance change rate, it is compared with the preset threshold in the preset judgment rule corresponding to the impedance feature to determine whether the impedance feature meets the criterion condition and obtain the corresponding feature judgment result. Based on the preset passive auxiliary judgment rules, the corresponding auxiliary judgment results are obtained according to whether the voltage or frequency exceeds the limit. If, through redundant judgment logic, it is determined that there is a combination of judgment results that is true among the preset multiple combinations of judgment results, then an islanding state is determined to have occurred; the combination of judgment results is selected from the feature judgment results and the auxiliary judgment result combinations.

[0015] In one example, the combination of judgment results includes: the characteristic judgment result corresponding to the impedance amplitude and the characteristic judgment result corresponding to the impedance phase, the characteristic judgment result corresponding to the impedance amplitude and the characteristic judgment result corresponding to the impedance change rate, and the characteristic judgment result corresponding to the impedance change rate and the auxiliary judgment result corresponding to the passive auxiliary judgment rule.

[0016] In one example, the low-frequency disturbance signal is attenuated, and the virtual synchronous generator function is initialized based on the actual operating point before grid disconnection as the initial value, and then switched to off-grid / grid-connected mode, specifically including: For the low-frequency disturbance signal, the disturbance is reduced to zero through linear attenuation; The virtual synchronous generator function is initialized by determining the initial value of the angular frequency based on the last phase-locked frequency before the grid is disconnected, the initial value of the voltage based on the last voltage value of the common coupling point, and the initial value of the phase angle based on the last phase-locked angle of the phase-locked loop. Activate the sub-module corresponding to the virtual synchronous generator function to close the network mode control loop; Send a command to disconnect the common coupling point switch, and wait for the switch to confirm the disconnection before entering the off-grid network construction mode.

[0017] In one example, the method further includes: For multiple variable frequency energy storage power supplies operating in parallel, a low-frequency disturbance signal with a different disturbance frequency is allocated to each variable frequency energy storage power supply through orthogonal allocation, and the injection of the low-frequency disturbance signal is periodically started and stopped by encoding the time pattern. The detection results of the islanding status of each frequency conversion energy storage power supply are shared through the communication bus. If a first frequency conversion energy storage power source exists and its corresponding detection result indicates that it is in an islanded state, then the other second frequency conversion energy storage power sources are confirmed, and based on the confirmation result, it is determined whether to switch to the off-grid network mode.

[0018] In one example, the method further includes: During low-voltage ride-through or high-voltage ride-through, the injection of the low-frequency disturbance signal is suspended, and the preset judgment rule corresponding to the impedance characteristic is suspended, while the preset passive auxiliary judgment rule is retained. After a low-voltage ride-through or a high-voltage ride-through has ended, the grid-connected impedance baseline should be re-acquired.

[0019] On the other hand, this application also proposes an active islanding detection device for variable frequency energy storage power supplies, comprising: The low-frequency disturbance injection module determines the grid-connected operation mode of the variable frequency energy storage power supply and actively superimposes a low-frequency disturbance signal into the control current. The common coupling point impedance response feature extraction module collects electrical sampling values ​​of the common coupling point and extracts multiple impedance features under the low-frequency disturbance signal through signal processing technology. The islanding comprehensive determination module, based on the preset judgment rules corresponding to each of the multiple impedance characteristics, and the preset passive auxiliary judgment rules, comprehensively determines whether an islanding state has occurred through redundant judgment logic; If a disturbance attenuation and smooth switching module occurs, it initializes the virtual synchronous generator function with the actual operating point before grid disconnection as the initial value and switches to off-grid mode.

[0020] On the other hand, this application also proposes an active islanding detection system for variable frequency energy storage power supply, including an active islanding detection device as described in the above example, as well as a control platform consisting of a virtual synchronous generator grid controller, a common coupling point switch, and a dual-core digital signal processor, wherein the dual-core digital signal processor includes a first processor responsible for system management and a second processor responsible for real-time control loop.

[0021] The active islanding detection method for variable frequency energy storage power supplies proposed in this application can bring the following benefits: 1. Elimination of Detection Dead Zone: Traditional passive methods have a dead zone when Qf>1.0, and AFD / SFS still have residual dead zones at high Qf. This application is based on impedance response, with an impedance ratio of 100~10000 times in grid-connected mode / islanded mode, and can still reliably detect even under extreme power matching conditions, thus eliminating the detection dead zone.

[0022] 2. Improved detection speed: The impedance change rate criterion can capture impedance steps within <50ms after the power grid is disconnected, which is 10 to 40 times faster than the passive method (0.5~2s) and 10 to 40 times faster than the traditional active method (0.5~2s waiting for frequency offset accumulation).

[0023] 3. Improved power quality: The increase in total harmonic distortion (THD) of low-frequency disturbances is less than 0.1%, which is significantly lower than that of AFD / SFS (1~5%), meeting the stringent requirements of harmonic limits in relevant standards.

[0024] 4. Improved emergency power supply reliability: The total switching time from island detection to VSG network operation is <100ms (20~50ms of disturbance attenuation + 10~30ms of VSG initialization + 10~20ms of switching action), ensuring uninterrupted power supply to the load and realizing true emergency power supply function.

[0025] 5. Multi-machine scalability: The detection reliability does not decrease with the increase of the number of parallel units, breaking through the performance bottleneck of traditional active methods when multiple machines are connected in parallel. Attached Figure Description

[0026] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a flowchart illustrating the active islanding detection method for variable frequency energy storage power supply in the embodiments of this application. Figure 2 This is a schematic diagram illustrating the injection principle of a low-frequency disturbance signal in one scenario of an embodiment of this application. Figure 3 This is a schematic diagram illustrating the impedance characteristic extraction of a common coupling point under one scenario in an embodiment of this application. Figure 4 This is a schematic diagram illustrating the comprehensive determination of the island state under one scenario in an embodiment of this application. Figure 5 This is a schematic diagram comparing impedance amplitudes in grid-connected mode and islanded mode under one scenario in an embodiment of this application; Figure 6 This is a comparison curve of the detection time of various methods under different quality factor loads in one scenario of the embodiments of this application; Figure 7 This is a spectrum diagram showing the orthogonal allocation of disturbance frequencies of multiple frequency conversion energy storage power supplies in parallel under one scenario in this application embodiment; Figure 8 This is a timing diagram of LVRT / HVRT collaborative processing in one scenario of this application embodiment; Figure 9 This is a schematic diagram of the module structure of the active islanding detection device for the frequency conversion energy storage power supply in the embodiments of this application; Figure 10 This is a schematic diagram of the active islanding detection system for the variable frequency energy storage power supply in this embodiment of the application. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] The technical solutions provided by the various embodiments of this application are described in detail below with reference to the accompanying drawings.

[0029] like Figure 1 As shown in the figure, this application provides an active islanding detection method for variable frequency energy storage power supplies, including: S101: Determines the grid-connected operation mode of the variable frequency energy storage power supply and actively superimposes a low-frequency disturbance signal into the control current.

[0030] Specifically, the operation mode of the variable frequency energy storage power supply in grid-connected mode is determined, and a low-frequency sinusoidal disturbance component is superimposed on the q-axis component of the current reference; wherein the amplitude range of the low-frequency sinusoidal disturbance component is 0.5% to 2% of the rated current, and the frequency range is 0.5Hz to 5Hz.

[0031] Grid-connected mode refers to the variable frequency energy storage power supply being connected to the power grid through a common coupling point, employing a grid-following control strategy, and using the grid voltage as a reference for synchronization, thus achieving the working state of transmitting or absorbing electrical energy from the grid. In this mode, the variable frequency energy storage power supply is equivalent to a controlled current source, and its output voltage and frequency follow the grid.

[0032] In vector control of motors or inverters, the current in the three-phase stationary coordinate system is transformed into a dq coordinate system that rotates synchronously with the grid voltage via Park transformation. In this rotating dq coordinate system, the current is decoupled into two independent control components: the q-axis component (reactive power / frequency control) and the d-axis component (active power / voltage control). Here, the q-axis component referenced by the current refers to the current command value used to control the inverter's reactive power output or frequency / phase regulation.

[0033] like Figure 2 As shown, the low-frequency disturbance signal refers to the low-frequency sinusoidal signal superimposed on the q-axis component of the current reference in grid-connected mode. The low-frequency disturbance signal can be represented as shown in Formula 1: Formula 1; in, This is a low-frequency disturbance signal. The disturbance amplitude is set to 0.5% to 2% of the rated current, and can be dynamically adjusted. The disturbance frequency is 0.5Hz to 5Hz, far from the fundamental frequency and characteristic harmonic frequency; t is time; the disturbance is only activated in grid-connected mode and automatically turned off in grid-connected / off-grid mode.

[0034] The use of low-frequency disturbances instead of the fundamental frequency shift of traditional AFD avoids conflicts with VSG inertia control and primary frequency regulation. The power fluctuations generated by the low-frequency disturbances are absorbed by the low impedance on the grid side, and have little impact on the voltage / frequency at the PCC point; however, after islanding occurs, the power fluctuations of these low-frequency disturbances produce a detectable voltage response on the high impedance of the local load.

[0035] S102: Collect electrical sampling values ​​at the common coupling point and extract multiple impedance characteristics under the low-frequency disturbance signal through signal processing technology.

[0036] Specifically, such as Figure 3 As shown, it is necessary to extract the common coupling point at the disturbance frequency. The impedance characteristic at that point can be expressed as shown in Formula 2: Formula 2; in, Let be the complex impedance at time t. , For three-phase voltage signals and three-phase current signals in The complex components corresponding to the frequencies.

[0037] Based on this, electrical sampling values ​​are collected at the common coupling point; these electrical sampling values ​​include three-phase voltage signals and three-phase current signals. The common coupling point is the electrical connection point between distributed power sources (such as variable frequency energy storage power sources) and the power grid, and is a key physical node for grid connection, islanding detection, and power quality monitoring. The electrical sampling values ​​are collected in real time by voltage / current sensors and quantized by an analog-to-digital converter into instantaneous digital signals of three-phase voltage and three-phase current, which serve as the basic input data for subsequent coordinate transformation and impedance calculation.

[0038] The Clarke transform converts the electrical sampled values ​​to a two-phase stationary coordinate system. The Clarke transform synthesizes the electrical sampled values ​​(including three-phase voltage / current signals) in the three-phase stationary coordinate system (A, B, C) onto the α and β axes according to spatial vector projection relationships, forming a two-phase orthogonal time-varying signal set. This step transforms three mutually coupled AC signals into two independent AC signals, creating conditions for subsequent orthogonal demodulation in the stationary coordinate system and extraction of complex impedance components at specific frequencies.

[0039] For the reference signal corresponding to the two-phase stationary coordinate system, and the sinusoidal reference signal with the disturbance frequency of the low-frequency disturbance signal respectively ( ), cosine reference signal ( Quadrature demodulation is performed. Quadrature demodulation is a signal processing technique that multiplies the input signal by two reference signals of the same frequency with a 90° phase difference (sine and cosine), and then performs low-pass filtering to extract the amplitude and phase information of the component in the input signal that is in phase with the reference frequency. In this embodiment, the electrical sample value is multiplied by the reference signal (αβ signal) corresponding to the two-phase stationary coordinate system, and then by the sine reference signal of the disturbance frequency to extract the impedance component in phase with the reference signal; at the same time, it is multiplied by the cosine reference signal of the disturbance frequency (with a 90° phase lag) to extract the quadrature component.

[0040] Low-pass filtering is used to extract the baseband complex components of low-frequency disturbance signals. The cutoff frequency of the low-pass filter can be set to... At this point, the three-phase voltage signal and three-phase current signal at point PCC are obtained. Complex components at each frequency , .

[0041] Based on the baseband complex components, several corresponding impedance characteristics are determined; these characteristics include impedance amplitude, impedance phase, and impedance rate of change. In actual calculations, the corresponding impedance characteristics are calculated according to Formula 3. Formula 3; in, For complex impedance, For the α-axis voltage baseband complex component. For the baseband complex components of the β-axis voltage, For the α-axis current baseband complex component. For the baseband complex component of the β-axis current.

[0042] After obtaining the complex impedance, the impedance amplitude is obtained by taking the modulus of the complex impedance, which characterizes the overall magnitude of the system impedance at the disturbance frequency; the impedance phase is obtained by taking the phase angle of the complex impedance, which characterizes the phase difference between voltage and current at the disturbance frequency; the impedance rate of change is obtained by differentiating the impedance amplitude with respect to time, which is used to capture the step change in impedance amplitude at the moment the power grid is disconnected.

[0043] Islanding is detected by impedance response rather than traditional voltage / frequency over-limit detection. It takes advantage of the order-of-magnitude difference in impedance between grid-connected and islanded states (|Z|≈0.01~0.1Ω in grid-connected mode, |Z|≈1~100Ω in islanded mode). Even when power matching (Qf is high), the impedance amplitude is still significantly different, which fundamentally eliminates the detection blind zone.

[0044] S103: Based on the preset judgment rules corresponding to each of the multiple impedance characteristics, and the preset passive auxiliary judgment rules, a comprehensive judgment is made on whether an islanding state has occurred through redundant judgment logic.

[0045] Specifically, such as Figure 4 As shown, for each impedance characteristic among impedance amplitude, impedance phase, and impedance change rate, it is compared with a preset threshold in the preset judgment rule corresponding to that impedance characteristic to determine whether the impedance characteristic meets the criterion condition and obtain the corresponding feature judgment result. The preset judgment rule refers to the judgment criterion of setting preset thresholds for impedance amplitude, impedance phase, and impedance change rate for comparison, which is used to determine whether each impedance characteristic meets the characteristic condition of islanding state.

[0046] Preset judgment rules may include: The impedance magnitude criterion is shown in Formula 4: Formula 4; in, The characteristic judgment result corresponding to the impedance amplitude (the characteristic judgment result takes the value 1 or 0, which corresponds to true or false, that is, to the existence of an island state or the absence of an island state, respectively. The characteristic judgment result will not be described in detail below). The impedance amplitude, This is a preset threshold value corresponding to the impedance amplitude, which is set according to the grid short-circuit capacity and local load range, with a typical value of 0.5~2.0Ω. For example... Figure 5 As shown, in the islanded state, the impedance amplitude is large, exceeding the preset threshold.

[0047] The impedance phase criterion is shown in Formula 5: Formula 5; in, The result is the judgment result corresponding to the impedance phase. The impedance phase angle, The reference impedance phase angle (also known as the load impedance angle). This is a preset threshold corresponding to the impedance phase. In islanded mode, the impedance phase angle should be close to the reference impedance phase angle, and the absolute value of the difference between the impedance phase angle and the reference impedance phase angle should be less than this preset threshold. In grid-connected mode, the impedance phase angle is close to 0°, and the absolute value of the difference between the two should be greater than this preset threshold.

[0048] The impedance change rate criterion is shown in Formula 6: Formula Six; in, This is the judgment result corresponding to the rate of change of impedance. The rate of change of impedance, This is a preset threshold corresponding to the impedance change rate. The impedance change rate is used to capture the impedance surge at the moment of grid disconnection, enabling rapid detection. In islanded conditions, the impedance change rate will suddenly increase, exceeding this preset threshold. By utilizing the step characteristic of impedance from low to high at the moment of grid disconnection using the impedance change rate criterion, rapid detection in less than 50ms can be achieved, which is faster than traditional passive methods.

[0049] Based on preset passive auxiliary judgment rules, corresponding auxiliary judgment results are obtained according to whether the voltage and frequency exceed the limits. The passive auxiliary judgment rules are based on traditional criteria such as whether the voltage amplitude and frequency at the common coupling point exceed the limits, serving as a backup supplementary channel for active impedance detection.

[0050] Passive auxiliary criteria, as shown in Formula 7: Formula 7; in, This is the auxiliary judgment result corresponding to the passive auxiliary judgment. It is similar to the feature judgment result, taking the value 1 or 0, which corresponds to true or false, that is, to the existence of an island state or the absence of an island state, respectively. , These refer to voltage exceeding limits and frequency exceeding limits, respectively. ∪ represents an OR operation, indicating that if at least one of the two conditions is true, the result is true. When an islanded state occurs, the voltage / frequency continues to decrease, eventually exceeding the limits, while the traditional criteria are retained as backup protection.

[0051] Through redundant judgment logic, if it is determined that there is a combination of judgment results that is true among the preset multiple combinations of judgment results, then it is determined that an island state has occurred; the combination of judgment results is selected from the combination of feature judgment results and auxiliary judgment results.

[0052] Redundant decision-making logic is a mechanism that improves decision reliability by introducing multiple independent criteria or channels and conducting a comprehensive vote. Even if some criteria fail, the system can still make the correct decision based on the other criteria. In this application, the redundant decision-making logic combines multiple judgment results and conducts a comprehensive decision according to a majority voting mechanism. That is, when at least N of the multiple judgment results are true, the final decision will be in an islanded state to avoid misjudgment or omission of a single criterion. Here, N is set based on actual needs.

[0053] Furthermore, the combination of judgment results includes: the characteristic judgment result corresponding to the impedance amplitude and the characteristic judgment result corresponding to the impedance phase. ∧ The characteristic judgment results corresponding to the impedance amplitude and the characteristic judgment results corresponding to the impedance change rate () ∧ ), the characteristic judgment result corresponding to the impedance change rate and the auxiliary judgment result corresponding to the passive auxiliary judgment rule ( ∧ ), where ∧ represents the AND operation, indicating that both conditions must be true for the result to be true.

[0054] For each combination of judgment results, there is either a combination of two feature judgment results, or a combination of one feature judgment result and one auxiliary judgment result. If both features are true within the combination, the combination is considered true. For multiple combinations of judgment results, if at least N (e.g., 1) of them are true, an island state is considered to exist, as shown in Formula 8. Formula 8; in, The result of the island state detection is 1 or 0, which corresponds to true or false, that is, whether the island state exists or not.

[0055] S104: If this occurs, the low-frequency disturbance signal will be attenuated, and the virtual synchronous generator function will be initialized based on the actual operating point before the grid disconnection as the initial value, and then switched to the off-grid network mode.

[0056] Specifically, for low-frequency disturbance signals, linear attenuation is used to reduce the disturbance to zero. This can be represented by Formula Nine: Formula Nine; in, Let t be the amplitude of the disturbance at the current time. The initial disturbance amplitude, This is the preset total decay time, typically 20-50ms. Based on this, the disturbance amplitude is... The disturbance is linearly attenuated to zero. This linear attenuation allows the disturbance to exit smoothly, avoiding voltage / frequency surges to the system caused by sudden changes. At the same time, it creates a controllable handover window for VSG takeover, ensuring a smooth transition between grid connection and grid construction modes.

[0057] Perform state initialization of the virtual synchronous generator function to determine the initial value of the angular frequency based on the last phase-locked frequency before disconnecting the grid. The initial voltage value is determined based on the final voltage value at the common coupling point. And based on the final phase-locked loop phase angle defect initial value, Choosing this value as the initial value, instead of starting from the rated value, ensures that the electrical quantities (frequency, voltage, phase) of the VSG at the moment of takeover are completely consistent with those before the grid was disconnected, avoiding shocks caused by jumps and achieving true seamless switching.

[0058] Activate the sub-modules corresponding to the virtual synchronous generator function to close the grid-connected control loop. These sub-modules include inertia control, excitation regulation, and virtual impedance. The core functional modules of the VSG—inertia control (simulating the rotor inertia of a synchronous generator to suppress frequency abrupt changes), excitation regulation (controlling the output voltage amplitude), and virtual impedance (suppressing parallel circulating current)—are activated sequentially, and the entire grid-connected control loop is then connected at the software level.

[0059] Send a command to disconnect the common coupling point switch, and wait for confirmation of disconnection before entering the off-grid connection mode. Send a command to trip the physical switch at the common coupling point, cutting off the last electrical connection between the PCS and the power grid. After the switch actually disconnects, the system officially enters a completely independent off-grid emergency power supply state.

[0060] 1. Elimination of Detection Dead Zone: Traditional passive methods have a dead zone when Qf>1.0, and AFD / SFS still have residual dead zones at high Qf. This application is based on impedance response, with an impedance ratio of 100~10000 times in grid-connected mode / islanded mode, and can still reliably detect even under extreme power matching conditions, thus eliminating the detection dead zone.

[0061] 2. Improved detection speed: The impedance change rate criterion can capture impedance steps within <50ms after the power grid is disconnected, which is 10 to 40 times faster than the passive method (0.5~2s) and 10 to 40 times faster than the traditional active method (0.5~2s waiting for frequency offset accumulation).

[0062] 3. Improved power quality: The increase in total harmonic distortion (THD) of low-frequency disturbances is less than 0.1%, which is significantly lower than that of AFD / SFS (1~5%), meeting the stringent requirements of harmonic limits in relevant standards.

[0063] 4. Improved emergency power supply reliability: The total switching time from island detection to VSG network operation is <100ms (20~50ms of disturbance attenuation + 10~30ms of VSG initialization + 10~20ms of switching action), ensuring uninterrupted power supply to the load and realizing true emergency power supply function.

[0064] 5. Multi-machine scalability: The detection reliability does not decrease with the increase of the number of parallel units, breaking through the performance bottleneck of traditional active methods when multiple machines are connected in parallel.

[0065] Tables 1 and 2 below show a comparison between this application and passive island detection, and with traditional active detection methods (AFD / SFS / SMS). Meanwhile, as... Figure 6 As shown, the detection time comparison curves of various methods under different quality factor loads are displayed. Compared with the passive and active detection methods in traditional schemes, the detection time of this application is shorter.

[0066] Table 1. Comparison of this application with passive islanding detection Comparison items Passive detection This application Detection blind spots There is a blind zone when Qf>1.0 No dead zone, impedance difference always exists Detection time 0.5s~2s (>2s during matching) <50ms~200ms Reliability during power matching Low High (impedance difference does not depend on power matching) LVRT compatibility Requires complex shielding logic, prone to misjudgment. The logic of pausing and blocking is clear. Emergency switching speed Slow (waiting for the limit to be exceeded to trigger) Fast (<100ms switching) Table 2. Comparison of this application with traditional active detection methods (AFD / SFS / SMS) Comparison items AFD / SFS / SMS This application Disturbance type Fundamental frequency shift low-frequency current disturbance VSG compatibility Incompatible Compatible (frequency band separation) Compatibility with primary frequency modulation conflict Collaboration (without interference) Impact of grid-connected power quality Significant (THD increase of 1-5%) Extremely small (THD increase <0.1%) Multi-machine parallel reliability Low (perturbation superposition / cancellation) High (frequency domain orthogonality + master-slave collaboration) After the island mode switch difficulty Smoothing (perturbation decay + continuous VSG initialization) Detection mechanism Frequency / phase persistent shift Sudden change in impedance response Detection blind zone (high Qf) SFS can be reduced but not eliminated. No blind spots In one embodiment, such as Figure 7 As shown, when multiple variable frequency energy storage power supplies are operating in parallel, the following coordination strategy is adopted: For multiple variable frequency energy storage power sources operating in parallel, a low-frequency disturbance signal with a different disturbance frequency is assigned to each variable frequency energy storage power source through orthogonal allocation, as shown in Formula 10: Formula 10; in, Let be the disturbance frequency of the nth variable frequency energy storage power supply. The perturbation frequency is based on Δf, which is the frequency interval step size. Typically, Δf ≥ 5Hz needs to be set to ensure frequency domain orthogonality.

[0067] The system encodes time patterns to periodically activate and deactivate the injection of low-frequency disturbance signals. The receiver then decodes these patterns to distinguish between local disturbance responses and external interference.

[0068] The islanding status detection results of each variable frequency energy storage power source are shared via a communication bus. If a first variable frequency energy storage power source is found to be in an islanded state, the other second variable frequency energy storage power sources confirm this and determine whether to switch to off-grid mode based on the confirmation results. Each variable frequency energy storage power source shares impedance detection results via CAN / RS485 communication. When any variable frequency energy storage power source detects an islanded state, it broadcasts a flag. Other variable frequency energy storage power sources respond after confirmation. For example, they can respond to the islanded state and switch modes, or broadcast their own detection results to other variable frequency energy storage power sources. Mode switching will only occur when at least a certain number of variable frequency energy storage power sources are confirmed to be in an islanded state.

[0069] The orthogonal distribution of disturbance frequencies makes the impedance characteristic extraction of each variable frequency energy storage power source completely independent in the frequency domain, fundamentally solving the problem of disturbance superposition / cancellation when multiple units are connected in parallel, and the detection reliability does not decrease with the increase of the number of units connected in parallel.

[0070] In one embodiment, the design criteria for key parameters are explained as shown in Table 3: Table 3 Key Parameter Design Criteria

[0071] In one embodiment, it needs to be processed in conjunction with LVRT / HVRT.

[0072] like Figure 8 As shown, during low-voltage ride-through or high-voltage ride-through, the injection of low-frequency disturbance signals is suspended, as is the preset judgment rule corresponding to the impedance characteristics, while the preset passive auxiliary judgment rule is retained. During LVRT / HVRT ride-through, active islanding detection requires special handling: Disturbance injection suspension: Disturbance injection is suspended during LVRT / HVRT (…). =0), recovers after crossing; impedance criterion shielding: during crossing, the impedance amplitude criterion and impedance change rate criterion are shielded, and only the passive auxiliary criterion is retained.

[0073] After a low-voltage ride-through (LVRT) or high-voltage ride-through (HVRT) is completed, the grid-connected impedance baseline is re-acquired. Post-ride-through impedance baseline update: After the LVRT / HVRT is completed, the grid-connected impedance baseline is re-acquired to avoid impedance estimation offset caused by the ride-through.

[0074] Meanwhile, the embodiments of this application are compatible with 5-segment LVRT (0.1pu~0.9pu, 0.15s~3s) and 3-segment HVRT (1.1pu~1.2pu, 0.5s~2s), and are also compatible with multi-segment crossing criteria.

[0075] like Figure 9 As shown in the illustration, this application also provides an active islanding detection device for a variable frequency energy storage power supply, comprising: The low-frequency disturbance injection module 901 determines the grid-connected operation mode of the variable frequency energy storage power supply and actively superimposes a low-frequency disturbance signal into the control current. The common coupling point impedance response feature extraction module 902 collects electrical sampling values ​​of the common coupling point and extracts multiple impedance features under the low-frequency disturbance signal through signal processing technology. The islanding comprehensive determination module 903, based on the preset judgment rules corresponding to the multiple impedance characteristics and the preset passive auxiliary judgment rules, comprehensively determines whether an islanding state has occurred through redundant judgment logic. If a disturbance attenuation and smooth switching module 904 occurs, it initializes the virtual synchronous generator function with the actual operating point before the grid disconnection as the initial value and switches to the off-grid connection mode.

[0076] like Figure 10 As shown in the embodiment of this application, an active islanding detection system for variable frequency energy storage power supply is also provided, including the active islanding detection device as described above, as well as a control platform consisting of a virtual synchronous generator grid controller, a common coupling point switch, and a dual-core digital signal processor. The dual-core digital signal processor includes a first processor CPU1 responsible for system management and a second processor CPU2 responsible for real-time control loop.

[0077] The various embodiments in this application are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the device and system embodiments are basically similar to the method embodiments, so the descriptions are relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0078] The apparatus and system provided in this application are one-to-one with the method. Therefore, the apparatus and system also have similar beneficial technical effects as their corresponding methods. Since the beneficial technical effects of the method have been described in detail above, the beneficial technical effects of the apparatus and system will not be repeated here.

[0079] The above description is merely an embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. An active islanding detection method for a variable frequency energy storage power supply, characterized in that, include: The variable frequency energy storage power supply is determined to operate in grid-connected mode, and a low-frequency disturbance signal is actively superimposed on the control current; Electrical sampling values ​​at the common coupling point are collected, and multiple impedance characteristics under the low-frequency disturbance signal are extracted using signal processing techniques. Based on the preset judgment rules corresponding to each of the multiple impedance characteristics, and the preset passive auxiliary judgment rules, a comprehensive judgment is made on whether an islanding state has occurred through redundant judgment logic. If this occurs, the low-frequency disturbance signal will be attenuated, and the virtual synchronous generator function will be initialized based on the actual operating point before the grid disconnection as the initial value, and then switched to the off-grid network mode.

2. The active islanding detection method for variable frequency energy storage power supply according to claim 1, characterized in that, To ensure the inverter energy storage power supply operates in grid-connected mode, a low-frequency disturbance signal is actively superimposed onto the control current, specifically including: To determine the grid-connected operation mode of the variable frequency energy storage power supply, a low-frequency sinusoidal disturbance component is superimposed on the q-axis component of the current reference. The amplitude range of the low-frequency sinusoidal disturbance component is 0.5% to 2% of the rated current, and the frequency range is 0.5 Hz to 5 Hz.

3. The active islanding detection method for variable frequency energy storage power supply according to claim 1, characterized in that, Electrical sampling values ​​at the common coupling point are collected, and multiple impedance characteristics under the low-frequency disturbance signal are extracted using signal processing techniques, specifically including: Collect electrical sampling values ​​at the common coupling point; the electrical sampling values ​​include: three-phase voltage signals and three-phase current signals; The electrical sample values ​​are converted to a two-phase stationary coordinate system using Clarke transformation; For the reference signal corresponding to the two-phase stationary coordinate system, orthogonal demodulation is performed with the sine reference signal and cosine reference signal of the disturbance frequency of the low-frequency disturbance signal, respectively; The baseband complex components of the low-frequency disturbance signal are extracted by low-pass filtering. Based on the baseband complex components, a number of corresponding impedance characteristics are determined; the impedance characteristics include: impedance amplitude, impedance phase, and impedance change rate.

4. The active islanding detection method for variable frequency energy storage power supply according to claim 3, characterized in that, Based on the preset judgment rules corresponding to each of the multiple impedance characteristics, and the preset passive auxiliary judgment rules, a comprehensive judgment is made on whether an islanding state has occurred through redundant judgment logic, specifically including: For each impedance feature among impedance amplitude, impedance phase, and impedance change rate, it is compared with the preset threshold in the preset judgment rule corresponding to the impedance feature to determine whether the impedance feature meets the criterion condition and obtain the corresponding feature judgment result. Based on the preset passive auxiliary judgment rules, the corresponding auxiliary judgment results are obtained according to whether the voltage or frequency exceeds the limit. If, through redundant judgment logic, it is determined that there is a combination of judgment results that is true among the preset multiple combinations of judgment results, then an islanding state is determined to have occurred; the combination of judgment results is selected from the feature judgment results and the auxiliary judgment result combinations.

5. The active islanding detection method for variable frequency energy storage power supply according to claim 4, characterized in that, The combination of judgment results includes: the characteristic judgment result corresponding to the impedance amplitude and the characteristic judgment result corresponding to the impedance phase, the characteristic judgment result corresponding to the impedance amplitude and the characteristic judgment result corresponding to the impedance change rate, and the characteristic judgment result corresponding to the impedance change rate and the auxiliary judgment result corresponding to the passive auxiliary judgment rule.

6. The active islanding detection method for variable frequency energy storage power supply according to claim 1, characterized in that, The low-frequency disturbance signal is attenuated, and the virtual synchronous generator function is initialized based on the actual operating point before grid disconnection as the initial value. Then, the system is switched to off-grid / grid connection mode, specifically including: For the low-frequency disturbance signal, the disturbance is reduced to zero through linear attenuation; The virtual synchronous generator function is initialized by determining the initial value of the angular frequency based on the last phase-locked frequency before the grid is disconnected, the initial value of the voltage based on the last voltage value of the common coupling point, and the initial value of the phase angle based on the last phase-locked angle of the phase-locked loop. Activate the sub-module corresponding to the virtual synchronous generator function to close the network mode control loop; Send a command to disconnect the common coupling point switch, and wait for the switch to confirm the disconnection before entering the off-grid network construction mode.

7. The active islanding detection method for variable frequency energy storage power supply according to claim 1, characterized in that, The method further includes: For multiple variable frequency energy storage power supplies operating in parallel, a low-frequency disturbance signal with a different disturbance frequency is allocated to each variable frequency energy storage power supply through orthogonal allocation, and the injection of the low-frequency disturbance signal is periodically started and stopped by encoding the time pattern. The detection results of the islanding status of each frequency conversion energy storage power supply are shared through the communication bus. If a first frequency conversion energy storage power source exists and its corresponding detection result indicates that it is in an islanded state, then the other second frequency conversion energy storage power sources are confirmed, and based on the confirmation result, it is determined whether to switch to the off-grid network mode.

8. The active islanding detection method for variable frequency energy storage power supply according to claim 1, characterized in that, The method further includes: During low-voltage ride-through or high-voltage ride-through, the injection of the low-frequency disturbance signal is suspended, and the preset judgment rule corresponding to the impedance characteristic is suspended, while the preset passive auxiliary judgment rule is retained. After a low-voltage ride-through or a high-voltage ride-through has ended, the grid-connected impedance baseline should be re-acquired.

9. An active islanding detection device for a variable frequency energy storage power supply, characterized in that, include: The low-frequency disturbance injection module determines the grid-connected operation mode of the variable frequency energy storage power supply and actively superimposes a low-frequency disturbance signal into the control current. The common coupling point impedance response feature extraction module collects electrical sampling values ​​of the common coupling point and extracts multiple impedance features under the low-frequency disturbance signal through signal processing technology. The islanding comprehensive determination module, based on the preset judgment rules corresponding to each of the multiple impedance characteristics, and the preset passive auxiliary judgment rules, comprehensively determines whether an islanding state has occurred through redundant judgment logic; If a disturbance attenuation and smooth switching module occurs, it initializes the virtual synchronous generator function with the actual operating point before grid disconnection as the initial value and switches to off-grid mode.

10. An active islanding detection system for a variable frequency energy storage power supply, characterized in that, The system includes the active islanding detection device as described in claim 9, as well as a control platform comprising a virtual synchronous generator network controller, a common coupling point switch, and a dual-core digital signal processor, wherein the dual-core digital signal processor includes a first processor responsible for system management and a second processor responsible for real-time control loops.

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