Differential protection method and device for micro-grid and inverter type island micro-grid system

Abstract

This invention discloses a differential protection method, device, and inverter-islanded microgrid system for microgrids. The method includes: acquiring three-phase current signals at a first target end and a second target end, respectively, to obtain a first current signal and a second current signal; then, based on a rotation angle synchronized with the fundamental frequency of the microgrid, performing synchronous rotational coordinate transformation on the first and second current signals to extract the corresponding first and second direct-axis current components; next, calculating the similarity index between the first and second direct-axis current components; if all the continuously calculated similarity indices fall within a preset tripping threshold range, and the number of consecutive falls reaches a preset sampling number threshold, then it is determined that an internal fault has occurred in the target protected line, and tripping commands are output to the circuit breakers at the first and second target ends, respectively. This invention achieves a balance between low cost, high reliability, and strong anti-interference capability.

Description

Technical Field

[0001] This invention relates to the field of power system protection technology, and in particular to differential protection methods, devices, and inverter-islanded microgrid systems for microgrids. Background Technology

[0002] With the global energy structure transformation and the rapid development of renewable energy technologies, the penetration rate of inverter-based distributed generation (IBDG), represented by photovoltaic and wind power, in microgrids is increasing. Especially in the inverter-based islanded microgrid (IBIMG) mode, the system is disconnected from the main grid, and the internal fault current is almost entirely supplied by the IBDG. However, due to the inherent limitations of the overcurrent capability of power electronic devices and control strategies (such as grid-following or grid-forming), the short-circuit current output capability of IBDG is extremely limited, typically only 1.2 to 2.0 times the rated current, and the fault current characteristics (amplitude, phase, and decay rate) are controlled to exhibit low amplitude, weak feedback, and nonlinear characteristics. This is fundamentally different from the sufficient short-circuit current (usually 6 to 10 times the rated current) provided by traditional synchronous generators. As a result, traditional overcurrent protection and distance protection based on amplitude discrimination or impedance measurement are unable to distinguish between weak fault current and normal load fluctuations, and are prone to false tripping or failure to trip. Especially in high-resistance ground fault scenarios (fault resistance can reach hundreds of ohms), the protection sensitivity is seriously insufficient and there is a protection dead zone.

[0003] Currently, protection technologies for microgrids can be mainly divided into the following three categories:

[0004] The first category is traditional protection schemes based on power frequency electrical quantities. These schemes (such as overcurrent protection and distance protection) are simple in principle, low in implementation cost, and have low requirements for hardware sampling rate and communication bandwidth. However, their reliability drops significantly in microgrids: because the short-circuit current amplitude provided by the IBDG is limited and difficult to distinguish from the load current, overcurrent protection cannot reliably identify faults; at the same time, the bidirectional power flow characteristic in microgrids renders the impedance criterion of distance protection ineffective, failing to meet the basic reliability requirements for safe operation of microgrids.

[0005] The second category is novel protection schemes based on transient components. These schemes (such as traveling wave protection and transient energy protection) utilize the high-frequency components at the initial moment of a fault, theoretically offering extremely fast response and being unaffected by weak feed currents, resulting in extremely high reliability. However, they are costly: these technologies rely on acquisition equipment with extremely high sampling rates (reaching hundreds of kHz or even MHz) and dedicated communication links with extremely low latency, leading to huge hardware investments and making them difficult to promote and popularize in small and medium-sized microgrid projects.

[0006] The third type is the longitudinal differential protection scheme based on steady-state quantities. This scheme utilizes the amplitude and phase relationship of the currents at both ends of the line, naturally possessing the ability to handle bidirectional power flow and weak feeder current, with good reliability and moderate cost. However, its anti-interference capability (robustness) is poor: on the one hand, the capacitive current generated by the distributed capacitance in the microgrid will produce a huge unbalanced current when there is an external fault in the line, leading to maloperation of the protection; on the other hand, this type of scheme is extremely sensitive to high-resistance ground faults, and factors such as current transformer (CT) saturation and measurement noise can easily destroy the protection criteria, resulting in insufficient anti-interference capability under complex operating conditions.

[0007] In summary, existing microgrid protection technologies are caught in a dilemma, with structural defects that make it difficult to balance cost, reliability, and anti-interference capabilities. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to address the above-mentioned shortcomings of the prior art by proposing a differential protection method, device and inverter islanded microgrid system for microgrids. This method can take into account low cost, high reliability and strong anti-interference capability.

[0009] In a first aspect, the present invention provides a differential protection method for a microgrid, the method comprising:

[0010] Three-phase current signals are acquired at the first target end to obtain the first current signal; three-phase current signals are acquired at the second target end to obtain the second current signal; wherein, the first target end and the second target end are the opposite ends of the same target protected line in the microgrid;

[0011] Based on a rotation angle synchronized with the fundamental frequency of the microgrid, a synchronous rotation coordinate transformation is performed on the first current signal to extract the corresponding first direct-axis current component; and based on the rotation angle, a synchronous rotation coordinate transformation is performed on the second current signal to extract the corresponding second direct-axis current component.

[0012] Calculate the similarity index between the first direct-axis current component and the second direct-axis current component;

[0013] The similarity index is used to determine whether an internal fault has occurred in the target protected line. When an internal fault is determined to have occurred, tripping commands are output to the circuit breakers at the first target end and the second target end, respectively, so as to realize differential protection for the microgrid.

[0014] Furthermore, based on a rotation angle synchronized with the fundamental frequency of the microgrid, a synchronous rotational coordinate transformation is performed on the first current signal to extract the corresponding first direct-axis current component; and based on the rotation angle, a synchronous rotational coordinate transformation is performed on the second current signal to extract the corresponding second direct-axis current component, specifically including:

[0015] A reference coordinate system is constructed that rotates synchronously with the fundamental frequency of the microgrid. The rotation angle of the reference coordinate system is generated by the phase-locked loop tracking the fundamental voltage phase or fundamental current phase of the microgrid in real time.

[0016] Using the rotation angle, a synchronous rotating coordinate transformation is performed on the first current signal to convert the first current signal from the stationary three-phase coordinate system to the synchronous rotating coordinate system, obtaining the first direct-axis current component, the first quadrature-axis current component, and the first zero-sequence current component, and extracting the first direct-axis current component from them; and using the rotation angle, a synchronous rotating coordinate transformation is performed on the second current signal to convert the second current signal from the stationary three-phase coordinate system to the synchronous rotating coordinate system, obtaining the second direct-axis current component, the second quadrature-axis current component, and the second zero-sequence current component, and extracting the second direct-axis current component from them.

[0017] Furthermore, the synchronous rotating coordinate transformation employs the Park transformation.

[0018] Furthermore, determining whether an internal fault has occurred in the target protected line based on the similarity index includes:

[0019] If all the similarity indices calculated consecutively fall within the preset tripping threshold range, and the number of consecutive falls reaches the preset sampling threshold, then the protected line is determined to have an internal fault.

[0020] The preset sampling threshold is set based on the magnitude of the capacitance effect of the target protected line, specifically including:

[0021] When the capacitance effect of the target protected line is lower than the first preset capacitance standard, the preset sampling number threshold is equal to the first preset value.

[0022] When the capacitance effect of the target protected line is higher than the second preset capacitance standard, the preset sampling number threshold is equal to the second preset value, and the second preset value is greater than the first preset value.

[0023] Furthermore, determining whether an internal fault has occurred in the target protected line based on the similarity index also includes:

[0024] If the calculated similarity index does not fall within the preset tripping threshold range, or if the number of consecutive falls within the range does not reach the preset sampling threshold, it is determined that the target protected line has not experienced an internal fault, and no tripping command is output.

[0025] Furthermore, the similarity index between the first direct-axis current component and the second direct-axis current component is calculated, specifically including:

[0026] Within the sliding time window, based on the sampling sequence of the first direct-axis current component and the sampling sequence of the second direct-axis current component, the waveform consistency index between the two is calculated using a correlation measurement algorithm to obtain the similarity index.

[0027] The sliding time window moves continuously along the time axis.

[0028] Furthermore, a correlation measurement algorithm is used to calculate the waveform consistency index between the two, resulting in a similarity index, which specifically includes:

[0029] Using the Pearson correlation coefficient algorithm, within a sliding time window, the first mean of the sampling sequence of the first direct-axis current component and the second mean of the sampling sequence of the second direct-axis current component are calculated respectively. Based on the sampling sequences of the first direct-axis current component, the second direct-axis current component, the first mean, and the second mean, the covariance between the sampling sequences of the first and second direct-axis current components, the first standard deviation of the sampling sequence of the first direct-axis current component, and the second standard deviation of the sampling sequence of the second direct-axis current component are calculated.

[0030] The ratio of the covariance to the product of the first standard deviation and the second standard deviation is used as the similarity index.

[0031] Furthermore, the length of the sliding time window is half the period of the microgrid's fundamental frequency.

[0032] In a second aspect, the present invention provides a differential protection device for a microgrid, the device comprising:

[0033] The acquisition unit is used to acquire three-phase current signals at the first target end to obtain a first current signal; and to acquire three-phase current signals at the second target end to obtain a second current signal; wherein the first target end and the second target end are the opposite ends of the same target protected line in the microgrid;

[0034] The transformation unit, connected to the acquisition unit, is used to perform synchronous rotational coordinate transformation on the first current signal based on a rotation angle synchronized with the fundamental frequency of the microgrid, so as to extract the corresponding first direct-axis current component; and to perform synchronous rotational coordinate transformation on the second current signal based on the rotation angle, so as to extract the corresponding second direct-axis current component.

[0035] The calculation unit, connected to the transformation unit, is used to calculate the similarity index between the first direct-axis current component and the second direct-axis current component;

[0036] The judgment unit, connected to the calculation unit, is used to determine whether the target protected line has an internal fault based on the similarity index.

[0037] The execution unit, connected to the determination unit, is used to output trip control signals to the circuit breaker at the first target end and the circuit breaker at the second target end respectively when an internal fault occurs in the protected line of the determined target.

[0038] Thirdly, the present invention provides an inverter-type islanded microgrid system, the system comprising:

[0039] The target protected line is located within an inverter-type islanded microgrid;

[0040] The first circuit breaker is installed at the first target end of the target protected line and is used to disconnect the electrical connection between the first target end and the target protected line when a trip control command is received.

[0041] The second circuit breaker is installed at the second target end of the target protected line and is used to disconnect the electrical connection between the second target end and the target protected line when a trip control command is received.

[0042] Among them, the second target end and the first target end are the opposite ends of the target protected line, and the first target end and the second target end are connected through a communication link;

[0043] And a differential protection device for the microgrid as described in the second aspect, wherein the differential protection device is configured at a first target end or a second target end and is communicatively connected to a first circuit breaker and a second circuit breaker, respectively, to output a trip control command when it is determined that an internal fault has occurred in the target protected line.

[0044] This invention acquires three-phase current signals at both ends of the protected line, extracts the direct-axis current component using a rotating coordinate transformation synchronized with the microgrid's fundamental frequency, calculates a similarity index based on the direct-axis current component, and identifies internal faults by combining this with an adaptively set threshold for the number of consecutive samplings. This method does not rely on the absolute amplitude of the fault current, but rather reflects the consistency of the current direction at both ends of the line through waveform similarity. Therefore, it achieves high sensitivity, high reliability, and strong robustness protection for microgrid line faults under conditions of low sampling rate, low communication bandwidth, and low computational complexity. Specific beneficial effects are as follows:

[0045] 1. Significantly reduces hardware costs and communication burden:

[0046] This invention only needs to transmit the direct-axis current component obtained after synchronous rotating coordinate transformation, without transmitting the original three-phase instantaneous sampled values, effectively compressing the communication data dimension from three phases to a single channel. Simultaneously, since the criterion is based on the consistency of the overall waveform shape rather than high-frequency details, the system can meet protection performance requirements at a lower sampling rate, significantly reducing the demand on the dynamic range of the analog-to-digital converter chip, communication link bandwidth, and local storage resources. The similarity index calculation logic is simple, requiring no high-performance processor or an artificial intelligence model trained on a large amount of historical data, effectively reducing the device's computational overhead, hardware cost, and operating energy consumption.

[0047] 2. Significantly improves fault detection sensitivity and reliability:

[0048] The fault criterion of this invention is based on the waveform similarity change caused by the difference in current direction, rather than relying on the magnitude of the current amplitude. This fundamentally overcomes the problem of insufficient sensitivity in traditional protection schemes caused by the limitation of short-circuit current in power electronic interface power supplies, and can reliably detect high-resistance grounding faults. Furthermore, the direct-axis current component naturally integrates positive-sequence and negative-sequence dynamic information in a synchronous rotating coordinate system, eliminating the need for additional symmetrical component decomposition, and enabling unified identification of various complex fault types such as symmetrical faults, asymmetrical faults, transitional faults, and closing faults. Further, by adaptively setting the threshold for the number of consecutive samplings based on the strength of the capacitance effect of the protected line—using a smaller threshold when the line capacitance effect is weak to speed up the action, and a larger threshold when the capacitance effect is significant to extend the confirmation time—false judgments caused by transient processes are effectively suppressed, balancing the speed and reliability of the protection.

[0049] 3. Possesses strong anti-interference capabilities and adaptability to various operating conditions:

[0050] Calculating the similarity index within a sliding time window possesses inherent statistical smoothing properties, effectively suppressing the impact of measurement noise and transient disturbances. Furthermore, the internal fault criterion essentially relies on the significant decrease in similarity caused by the opposite current directions at both ends, exhibiting strong robustness against typical interference factors such as waveform distortion caused by current transformer saturation, minor time synchronization deviations, and charging current generated by line distributed capacitance. Under various normal operating conditions (such as load surges, capacitor switching, and distributed power source start-up and shutdown), the direct-axis currents at both ends of the line always maintain the same direction, and the similarity index remains stably within the positive range, far from the preset tripping threshold range, thus effectively avoiding protection maloperation and demonstrating excellent engineering applicability and operational stability.

[0051] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of the present invention, nor is it intended to limit the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0052] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. The above and other features and advantages will become more apparent to those skilled in the art from the detailed description of exemplary embodiments with reference to the accompanying drawings, in which:

[0053] Figure 1 This is a schematic diagram of a differential protection method for a microgrid provided in an embodiment of the present invention;

[0054] Figure 2 This is a schematic diagram of a differential protection device for a microgrid provided in an embodiment of the present invention.

[0055] Reference numerals: 10, acquisition unit; 20, transformation unit; 30, calculation unit; 40, judgment unit; 50, execution unit. Detailed Implementation

[0056] It is understood that the specific embodiments and accompanying drawings described herein are merely for explaining the invention and are not intended to limit the invention.

[0057] It is understood that, without conflict, the various embodiments and features in the embodiments of the present invention can be combined with each other.

[0058] It is understood that, for ease of description, only the parts related to the present invention are shown in the accompanying drawings, while the parts unrelated to the present invention are not shown in the drawings.

[0059] It is understood that each unit or module involved in the embodiments of the present invention may correspond to only one entity structure, or may be composed of multiple entity structures, or multiple units or modules may be integrated into one entity structure.

[0060] It is understood that, without conflict, the functions and steps marked in the flowcharts and block diagrams of this invention may occur in a different order than that marked in the accompanying drawings.

[0061] It is understood that the flowcharts and block diagrams of this invention illustrate the possible architecture, functions, and operations of systems, apparatuses, devices, and methods according to various embodiments of this invention. Each block in the flowchart or block diagram may represent a unit, module, program segment, or code, containing executable instructions for implementing the specified function. Furthermore, each block or combination of blocks in the block diagram and flowchart can be implemented using a hardware-based system to achieve the specified function, or using a combination of hardware and computer instructions.

[0062] It is understood that the units and modules involved in the embodiments of the present invention can be implemented by software or by hardware. For example, the units and modules can be located in a processor.

[0063] Example 1:

[0064] This embodiment provides a differential protection method for microgrids, applicable to microgrid systems including inverter-based distributed generation (IBG), and particularly demonstrating outstanding applicability in inverter-based islanded microgrids (IBIMG). In IBIMG operation mode, the system is disconnected from the main grid and relies entirely on the IBDG for voltage and frequency support. Operating conditions are complex and variable, while fault current characteristics are often weak and lack clear identifiability. Due to limitations imposed by the overcurrent capacity of power electronic devices and control strategies (such as grid-connected or grid-connected types) on IBDGs, their short-circuit current output typically only reaches 1.2 to 2 times the rated current, far lower than the 6 to 10 times rated current provided by traditional synchronous generators. This makes it difficult for traditional overcurrent protection and distance protection based on current amplitude or impedance characteristics to effectively distinguish between internal faults and normal disturbances. In scenarios such as high-resistance grounding faults (transition resistance can reach hundreds of ohms), they are prone to failure to operate, exhibiting a significant protection dead zone. Furthermore, they are prone to maloperation during transient processes such as load changes and capacitor switching, resulting in severely insufficient reliability. To address these challenges, the method employed in this embodiment synchronously acquires three-phase current signals at both ends of the protected line, extracts the direct-axis current components of each component using a rotating coordinate transformation strictly synchronized with the microgrid's fundamental frequency, and calculates the similarity index between the direct-axis current components at both ends. This index reflects the consistency of current direction—during internal faults, the current directions at both ends are opposite, and the similarity index decreases significantly; during normal or external faults, the currents are in the same direction, and the similarity index remains high. This mechanism does not rely on the absolute amplitude of the fault current, thus it can still sensitively and reliably identify various fault types, such as high-resistance grounding faults, asymmetrical faults, and symmetrical faults, even under conditions of limited short-circuit current and extremely weak fault characteristics. Simultaneously, thanks to the inherent robustness of the similarity criterion and the use of the direct-axis component, this method has strong suppression capabilities against typical interference factors such as measurement noise, waveform distortion caused by current transformer (CT) saturation, and charging current generated by line distributed capacitance. It maintains stable discrimination performance even under complex operating conditions such as random start-up and shutdown of distributed power sources and rapid load switching, effectively avoiding false tripping. Therefore, the differential protection scheme implemented in this embodiment is not only suitable for high-penetration IBDG microgrids operating in islanded mode, but can also be seamlessly applied to grid-connected modes or other microgrid architectures with power electronic interface power supplies. Through unified criterion logic, it achieves line protection with high sensitivity, high reliability, and strong anti-interference capabilities, providing strong support for the safe and stable operation of microgrids.

[0065] like Figure 1 As shown, this embodiment provides a differential protection method for a microgrid, specifically including steps S1 to S4.

[0066] Step S1: Collect three-phase current signals at the first target end to obtain the first current signal; collect three-phase current signals at the second target end to obtain the second current signal; wherein, the first target end and the second target end are the opposite ends of the same target protected line in the microgrid.

[0067] Taking a microgrid differential protection scenario as an example, a transmission or distribution line to be protected is selected as the target protected line in the system. This line has two separate endpoints in its physical structure, defined as the first target end (denoted as M end) and the second target end (denoted as N end), which are the opposite ends of the line. High-precision current transformers (CTs) are installed at both the M end and the N end to collect the three-phase current signals Ia, Ib, and Ic flowing through their respective endpoints in real time. The transformation ratio of the CT is reasonably configured according to the rated operating current of the microgrid, for example, using a standard transformation ratio of 600 / 5 A to balance measurement accuracy and dynamic response capability under fault current. The collected three-phase analog current signals are then synchronously digitized through a high-precision analog-to-digital converter circuit, with the sampling rate set to 2.4kHz to ensure accurate capture of the fundamental and main harmonic components while avoiding excessive data throughput. Through this process, a complete three-phase digital current sequence is generated at the M end as the first current signal; simultaneously, another corresponding three-phase digital current sequence is generated at the N end as the second current signal. These two sets of signals together constitute the basic input data for subsequent synchronous rotating coordinate transformation and differential criterion calculation, providing key information support for achieving high-sensitivity and high-reliability line protection.

[0068] Step S2: Based on the rotation angle synchronized with the fundamental frequency of the microgrid, perform a synchronous rotation coordinate transformation on the first current signal to extract the corresponding first direct-axis current component; and, based on the rotation angle, perform a synchronous rotation coordinate transformation on the second current signal to extract the corresponding second direct-axis current component.

[0069] As a specific implementation method, this step specifically includes: constructing a reference coordinate system that rotates synchronously with the fundamental frequency of the microgrid. The rotation angle of the reference coordinate system is generated by a phase-locked loop tracking the fundamental voltage phase or fundamental current phase of the microgrid in real time. Then, using the rotation angle, a synchronous rotating coordinate transformation is performed on the first current signal, converting it from a stationary three-phase coordinate system to a synchronous rotating coordinate system to obtain a first direct-axis current component, a first quadrature-axis current component, and a first zero-sequence current component, from which the first direct-axis current component is extracted; and, using the rotation angle, a synchronous rotating coordinate transformation is performed on the second current signal, converting it from a stationary three-phase coordinate system to a synchronous rotating coordinate system to obtain a second direct-axis current component, a second quadrature-axis current component, and a second zero-sequence current component, from which the second direct-axis current component is extracted.

[0070] As a more specific implementation method, the synchronous rotating coordinate transformation employs the Park transformation.

[0071] For example, in this embodiment, based on a rotation angle strictly synchronized with the fundamental frequency of the microgrid, the three-phase current signals acquired at the first target end and the second target end are subjected to synchronous rotating coordinate transformation to extract key feature quantities for fault diagnosis—namely, the first and second direct-axis current components. Specifically, a reference coordinate system that rotates synchronously with the fundamental frequency of the microgrid (e.g., 50 Hz or 60 Hz) is first constructed. The real-time rotation angle of this coordinate system is generated by a high-precision digital phase-locked loop (PLL) module. The PLL ensures that the constructed reference coordinate system is always dynamically synchronized with the system fundamental frequency by tracking the fundamental voltage phase at the common coupling point of the microgrid in real time, or by directly tracking the fundamental phase of the line current, thereby effectively eliminating the influence of frequency offset and phase drift on the subsequent transformation results. Subsequently, using this synchronous rotation angle, a synchronous rotating coordinate transformation is performed on the three-phase current signal acquired at the first target end. This transformation employs the classic Parker transformation method, accurately converting the current vector in the stationary three-phase coordinate system to the aforementioned synchronous rotating coordinate system, yielding three components: the first direct-axis current component, the first quadrature-axis current component, and the first zero-sequence current component. The first direct-axis current component is extracted separately as the core signal for subsequent fault analysis. Similarly, using the same rotation angle, a completely identical Parker transformation is performed on the three-phase current signal acquired at the second target end, mapping it from the stationary three-phase coordinate system to the same synchronous rotating coordinate system, similarly obtaining the second direct-axis current component, the second quadrature-axis current component, and the second zero-sequence current component. The second direct-axis current component is extracted for calculating the differential protection criterion. Because the direct-axis current component in the synchronous rotating coordinate system can centrally reflect the dynamic characteristics of positive-sequence and negative-sequence currents, and has a natural ability to suppress zero-sequence interference components and high-frequency noise, it is particularly suitable for microgrid scenarios with limited short-circuit current and weak fault characteristics, effectively supporting high-sensitivity fault identification based on current direction consistency.

[0072] Step S3: Calculate the similarity index between the first direct-axis current component and the second direct-axis current component.

[0073] As a specific implementation method, the similarity index between the first direct-axis current component and the second direct-axis current component is calculated, specifically including:

[0074] Within the sliding time window, based on the sampling sequence of the first direct-axis current component and the sampling sequence of the second direct-axis current component, a correlation measurement algorithm is used to calculate the waveform consistency index between the two to obtain the similarity index; wherein, the sliding time window moves continuously along the time axis.

[0075] As a specific implementation method, a correlation measurement algorithm is used to calculate the waveform consistency index between the two, resulting in a similarity index, which specifically includes:

[0076] First, the Pearson correlation coefficient algorithm is used to calculate the first mean and the second mean of the sampling sequences of the first and second direct-axis current components within a sliding time window. Based on the sampling sequences of the first and second direct-axis current components, the first mean, and the second mean, the covariance, the first standard deviation, and the second standard deviation of the sampling sequences of the first and second direct-axis current components are calculated. Then, the ratio of the covariance to the product of the first and second standard deviations is calculated as the similarity index.

[0077] As one specific implementation method, the length of the sliding time window is half the fundamental period of the microgrid.

[0078] For example, in this embodiment, the process of calculating the similarity index between the first direct-axis current component and the second direct-axis current component is as follows:

[0079] First, real-time interaction of the direct-axis current components at both ends of the protected line is achieved through a communication link. This communication link adopts the IEC 61850 standard: the sampled values ​​of the direct-axis current components are transmitted through the Sampled Value (SV) protocol, while trip commands are issued through the GOOSE protocol; under the condition of a sampling rate of 2.4 kHz, the communication bandwidth required for single-channel data transmission is approximately 316.8 kilobits per second, which can meet the system's requirements for real-time performance and reliability.

[0080] Subsequently, a continuously sliding time window along the time axis is set within the protection device, with its length set to half the fundamental period of the microgrid. Taking a system frequency of 60 Hz as an example, the fundamental period is approximately 16.67 milliseconds, and half the period is approximately 8.33 milliseconds; at a sampling rate of 2.4 kHz, this window contains exactly 20 consecutive sampling points. This sliding window is used to synchronously capture the latest sampling sequence of the first and second direct-axis current components at the current moment.

[0081] Within this sliding time window, the Pearson correlation coefficient algorithm is used to calculate the waveform consistency index between the two direct-axis current component sampling sequences. This index is the similarity index (denoted as DACCSIR). The formula for calculating the direct-axis current component similarity index (DACCSIR) is as follows:

[0082]

[0083] Wherein, RM and RN are the relays at both ends of the protection line, , , i and j are the mean values ​​of the direct-axis current components at both ends, respectively, and i and j are the starting and ending sampling point numbers of the moving half-cycle window, respectively.

[0084] The calculation process specifically includes the following steps:

[0085] The first step is to calculate the arithmetic mean of the first direct-axis current component within the current 20 sampling points, which is taken as its first mean; at the same time, the arithmetic mean of the second direct-axis current component within the same window is calculated, which is taken as its second mean.

[0086] The second step is to calculate the covariance between the two sets of sampling sequences: For each sampling point in the window, first subtract the first mean from the first direct-axis current component value of that point to obtain the first deviation; then subtract the second mean from the second direct-axis current component value at the same time to obtain the second deviation; multiply the two deviations together, and then average the product of all twenty sampling points. The resulting value is the covariance.

[0087] The third step is to calculate the standard deviation of the two sampling sequences respectively: for the first direct-axis current component, the average of the squared deviations of each sample value from the first mean is calculated, and then the square root of the average value is taken to obtain the first standard deviation; the same operation is performed on the second direct-axis current component to obtain the second standard deviation.

[0088] The fourth step is to divide the covariance by the product of the first standard deviation and the second standard deviation. The resulting ratio is the similarity index (DACCSIR) for the current window.

[0089] This similarity index has a clear fault detection significance: when the system is operating normally or an external fault occurs, the direct-axis current components at both ends of the line are in phase and have highly similar waveforms, and the similarity index approaches positive one; however, when an internal fault occurs in the line, the current directions at both ends are opposite, and the direct-axis current components show a significant negative correlation, and the similarity index will fall into the preset trip margin range, i.e., between -0.5 and -1. By determining whether the index falls into this range consecutively multiple times, internal faults can be reliably identified, and differential protection can be triggered.

[0090] It should be noted that:

[0091] (1) In this embodiment, the Pearson correlation coefficient algorithm is used to calculate the similarity index between the first direct-axis current component and the second direct-axis current component. This is only one specific implementation method. In practical applications, other algorithms that can quantify the consistency or correlation between two signal waveforms can also be used to achieve the same purpose. For example, Spearman's rank correlation coefficient, Kendall's correlation coefficient, cosine similarity, peak value of cross-correlation function, normalized form of dynamic time warping (DTW) distance, or similarity indexes constructed based on Euclidean distance, Mahalanobis distance, etc. As long as the selected algorithm can effectively reflect the consistency or opposite characteristics of the two direct-axis current components in phase and waveform morphology within the sliding time window, and can distinguish between internal faults and normal / external fault conditions, it can be used as a reasonable alternative to the "correlation measurement algorithm" in this scheme, and all of them are within the protection scope of this invention.

[0092] (2) In this embodiment, the length of the sliding time window is set to half the period of the fundamental frequency of the microgrid (for example, about 8.33 milliseconds in a 60 Hz system, corresponding to 20 sampling points at a sampling rate of 2.4 kHz), which is only a preferred implementation. In practical applications, the length of the sliding time window can be flexibly adjusted according to the system frequency characteristics, protection speed requirements, and anti-interference performance requirements. For example, a complete fundamental frequency period, a quarter period, or an integer multiple of multiple periods can also be used as the window length; in microgrids with large frequency fluctuations, an adaptive window with a fixed time length (such as 10 milliseconds or 20 milliseconds) or a variable-length sliding window strategy that dynamically adjusts the window size based on the transient characteristics of the fault can also be used. As long as the selected window length can effectively capture the steady-state or transient waveform characteristics of the direct-axis current component and support reliable discrimination of the consistency of the current direction at both ends, it can be used as a reasonable alternative to the sliding time window setting in this scheme, and all fall within the protection scope of this invention.

[0093] Step S4: Determine whether an internal fault has occurred in the target protected line based on the similarity index, and when an internal fault is determined to have occurred, output trip commands to the circuit breakers at the first target end and the second target end respectively to realize differential protection of the microgrid.

[0094] As one specific implementation method,

[0095] Determining whether the target protected line has an internal fault based on the similarity index includes: if all the continuously calculated similarity indices fall within the preset tripping threshold range, and the number of consecutive falls reaches the preset sampling threshold, then the target protected line is determined to have an internal fault.

[0096] The preset sampling threshold is set according to the capacitance effect of the target protected line. Specifically, when the capacitance effect of the target protected line is lower than the first preset capacitance standard, the preset sampling threshold is equal to the first preset value; when the capacitance effect of the target protected line is higher than the second preset capacitance standard, the preset sampling threshold is equal to the second preset value, and the second preset value is greater than the first preset value.

[0097] As a specific implementation method, determining whether an internal fault has occurred in the target protected line based on the similarity index also includes:

[0098] If the calculated similarity index does not fall within the preset tripping threshold range, or if the number of consecutive falls within the range does not reach the preset sampling threshold, it is determined that the target protected line has not experienced an internal fault, and no tripping command is output.

[0099] This embodiment provides a differential protection method suitable for microgrids containing inverter-type distributed generation (IBDG), with significant advantages, especially in islanded operation mode. The method first synchronously acquires three-phase current signals at both ends of the protected line, requiring the synchronization error to be strictly controlled within ±26 microseconds (60Hz system) or ±31 microseconds (50Hz system), which can be achieved through IEEE 1588 PTP or hardware pulse time synchronization. Next, using a rotating coordinate transformation (such as the Parker transformation) strictly synchronized with the system fundamental frequency, the direct-axis current component (Id_R) reflecting the positive and negative sequence dynamic characteristics at both ends is extracted. Subsequently, within a sliding time window (preferably a half-cycle window, such as approximately 8.33ms at 60Hz, corresponding to 20 sampling points at a 2.4kHz sampling rate, but can also be flexibly replaced with a full-cycle or fixed-duration window), the similarity index (DACCSIR) of the direct-axis current components at both ends is calculated based on a correlation measurement algorithm (such as Pearson correlation coefficient, or can be replaced by cosine similarity or cross-correlation function, etc.) to quantify waveform consistency: during normal operation or external faults, the index tends to +1, while during internal faults, due to the opposite current direction, the index will significantly decrease to the tripping threshold range of -0.5 to -1. The criterion logic does not depend on the absolute amplitude of the fault current, but rather monitors whether the DACCSIR continuously falls within the tripping range and reaches the preset continuous sampling number threshold (N_TM). This threshold has adaptive characteristics: it is set according to the magnitude of the capacitance effect of the target protected line; a smaller value (such as 5 times) is used for short lines with low capacitance effect to take into account speed, while a larger value (such as 20 times) is used for long lines with significant capacitance effect to enhance anti-interference. Once the criteria are met, the device sends a trip command to the circuit breakers at both ends of the line via IEC 61850 standard communication—the SV protocol transmits data and the GOOSE protocol transmits commands. This method can complete detection in just 2.5 to 10.8 milliseconds at a low sampling rate (2.4 kHz), effectively identifying various complex faults such as high-resistance grounding (transition resistance reaching hundreds of ohms), symmetrical / asymmetrical faults, transition faults, and faults caused by closing. It exhibits strong robustness against short-circuit current constraints, CT saturation, capacitive current interference, and load disturbances, achieving a balance between high sensitivity and high reliability.

[0100] Example 2:

[0101] like Figure 2 As shown, this embodiment provides a differential protection device for a microgrid. The device includes a data acquisition unit 10, a conversion unit 20, a calculation unit 30, a judgment unit 40, and an execution unit 50 connected in sequence. Each unit works together to achieve the differential protection function for the microgrid lines.

[0102] Specifically, the acquisition unit 10 is used to acquire three-phase current signals at two opposite ends of the same target protected line in the microgrid—namely, the first target end and the second target end—to obtain the first current signal and the second current signal.

[0103] The transformation unit 20 is connected to the acquisition unit 10 and is configured to perform synchronous rotational coordinate transformation on the first current signal based on a rotation angle synchronized with the fundamental frequency of the microgrid, and extract the corresponding first direct-axis current component from it; at the same time, based on the same rotation angle, the transformation is performed on the second current signal to extract the corresponding second direct-axis current component from it.

[0104] The calculation unit 30 is connected to the transformation unit 20 and is used to calculate the similarity index between the extracted first direct-axis current component and the second direct-axis current component to characterize the consistency of the current waveforms at both ends of the line.

[0105] The determination unit 40 is connected to the calculation unit 30 and is used to determine whether the similarity index obtained by multiple consecutive calculations falls into the preset tripping threshold range, and whether the number of times it falls into the range reaches the preset sampling number threshold; if the above conditions are met at the same time, it is determined that the target protected line has an internal fault.

[0106] The execution unit 50 is connected to the judgment unit 40 and is used to output trip control signals to the circuit breakers set at the first target end and the second target end respectively when the judgment result is an internal fault, so as to realize the rapid isolation of the faulty line and thus complete the differential protection of the microgrid.

[0107] The apparatus in this embodiment is capable of performing the method in Embodiment 1.

[0108] Example 3:

[0109] This embodiment provides an inverter-type islanded microgrid system. The system employs a longitudinal differential protection architecture and is designed for weakly fed grid environments containing high-penetration inverter-type distributed power sources. The system includes:

[0110] The target protected line, as the critical path for power transmission, is located inside the inverter-type islanded microgrid and is the functional coverage object of differential protection.

[0111] The first circuit breaker and the second circuit breaker are mechanically installed at the first target end and the second target end of the target protected line, respectively. Together, they constitute the physical boundary of the protected area and are used to perform fault isolation operations. The first target end and the second target end are the opposite ends of the line, forming the electrical boundary of the protection range.

[0112] A communication link connects the first target end and the second target end, and is used to realize the real-time transmission and sharing of data between the two ends, ensuring the synchronous interaction of information between the two ends;

[0113] The differential protection device, as described in Embodiment 2, adopts a single-end centralized or double-end distributed configuration, is set at the first target end or the second target end, and establishes a communication connection with the first circuit breaker and the second circuit breaker respectively; the device collects and compares the current characteristics at both ends of the line, and when it determines that an internal fault has occurred in the target protected line, it synchronously outputs trip control commands to the first circuit breaker and the second circuit breaker to achieve rapid isolation and disconnection of the faulty line.

[0114] Based on functional modules, the system specifically includes a current acquisition module, a signal processing module, an exponent calculation module, a fault diagnosis module, a communication module, and an execution module. The hardware implementation schemes for each module are as follows:

[0115] Current acquisition module: It adopts a current transformer (CT) with an accuracy class of 0.2 and a sampling rate of 2.4kHz to ensure high-precision and high-fidelity acquisition of three-phase current signals, which meets the microgrid protection requirements for accurate capture of steady-state and transient current characteristics.

[0116] Signal processing module: Based on a dedicated digital signal processor (DSP) chip, it realizes synchronous rotating coordinate transformation (i.e., Park transformation), with a computing performance of no less than 100 MIPS. It can complete the transformation from a three-phase stationary coordinate system to a synchronous rotating coordinate system in real time and stably output the direct-axis current component.

[0117] Index Calculation Module: Integrates a high-performance floating-point arithmetic unit (FPU) for fast calculation of similarity index (such as Pearson correlation coefficient) between the direct-axis current components at both ends. The delay of a single calculation is no more than 1 millisecond, ensuring the timeliness of the criterion.

[0118] Communication module: Configured with fiber optic communication interface, supporting GOOSE (General Object-Oriented Substation Events) and SV (Sample Value) protocols under the IEC 61850 standard, while also being compatible with IEEE 1588 PTP (Precision Time Protocol) to achieve high-precision time synchronization. The end-to-end communication delay is controlled within 20 milliseconds to ensure the synchronization of data between the two ends and the reliability of command transmission.

[0119] Fault Judgment Module: The tripping logic judgment is implemented using a Field Programmable Gate Array (FPGA) chip. Its parallel processing capability is used to monitor the similarity index for multiple consecutive cycles in real time and compare the thresholds to ensure that a tripping decision is generated quickly when the judgment conditions are met.

[0120] Execution module: Equipped with a highly reliable relay output interface, it is used to receive trip commands and drive the circuit breakers at both ends of the line to perform tripping operations, thereby achieving rapid isolation of the fault area.

[0121] The present invention will be further described in detail below with reference to exemplary embodiments. Taking the IEEE 14-node microgrid system as an example, this system includes six inverter-type distributed generation (IBDG) units with a rated capacity of 6 MVA, operating at a voltage level of 20 kV and a frequency of 60 Hz. First, the differential protection device described in this embodiment is deployed at both ends of each protected line. The current transformer (CT) ratio is uniformly set to 600 / 5 A, and a two-way communication link is constructed through single-mode optical fiber to ensure high bandwidth and low latency data transmission. Then, the parameters are configured: the sampling rate is set to 2.4 kHz, the sliding time window length corresponds to the fundamental half-cycle, i.e., 20 sampling points; the tripping threshold range is set to −1 to −0.5, and the continuous sampling number threshold N_TM is adaptively set according to the line capacitance effect—for lines with a length of less than 10 km and negligible capacitance effect, N_TM is set to 5. During normal operation or when an external fault occurs, the direct-axis current components at both ends of the line are in phase, and the calculated similarity index (DACCSIR) remains stably around +1, and the device does not trigger an action. When a high-resistance single-phase ground fault occurs on the line (transition resistance reaches 500 Ω), the direct-axis currents at both ends show a significant negative correlation due to their opposite directions, and the DACCSIR rapidly drops to -0.5371, falling into the preset tripping threshold range. After this state is satisfied for 5 consecutive samplings, the fault judgment module determines it as an internal fault and synchronously sends a tripping command to the circuit breakers at both ends of the line via the IEC 61850 GOOSE protocol. The entire fault detection process takes only 4.2 ms, and the total tripping time from the occurrence of the fault to the circuit breaker completing the trip does not exceed 25.3 ms. For long lines with significant capacitive effects (such as 20 km), the system automatically adjusts N_TM to 20 to enhance the ability to suppress transient charging currents. At this time, the fault detection time is still controlled within 10.8 ms, fully meeting the technical requirements of microgrids for fast and reliable protection.

[0122] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A method of differential protection for a microgrid, characterized by, The method includes: Three-phase current signals are acquired at the first target end to obtain the first current signal; three-phase current signals are acquired at the second target end to obtain the second current signal; wherein, the first target end and the second target end are the opposite ends of the same target protected line in the microgrid; Based on a rotation angle synchronized with the fundamental frequency of the microgrid, the first current signal is subjected to a synchronous rotation coordinate transformation to extract the corresponding first direct-axis current component; and based on the rotation angle, the second current signal is subjected to a synchronous rotation coordinate transformation to extract the corresponding second direct-axis current component. Calculate the similarity index between the first direct-axis current component and the second direct-axis current component; Based on the similarity index, it is determined whether an internal fault has occurred in the target protected line. When an internal fault is determined to have occurred, tripping commands are output to the circuit breakers at the first target end and the second target end, respectively, so as to realize differential protection for the microgrid.

2. The differential protection method for a microgrid according to claim 1, characterized in that, The first current signal is subjected to synchronous rotation coordinate transformation based on a rotation angle synchronized with the fundamental frequency of the microgrid, so as to extract the corresponding first straight-axis current component. Furthermore, based on the rotation angle, a synchronous rotation coordinate transformation is performed on the second current signal to extract the corresponding second direct-axis current component, specifically including: A reference coordinate system is constructed that rotates synchronously with the fundamental frequency of the microgrid. The rotation angle of the reference coordinate system is generated by a phase-locked loop tracking the fundamental voltage phase or fundamental current phase of the microgrid in real time. Using the rotation angle, a synchronous rotating coordinate transformation is performed on the first current signal to convert the first current signal from the stationary three-phase coordinate system to the synchronous rotating coordinate system, obtaining a first direct-axis current component, a first quadrature-axis current component, and a first zero-sequence current component, and extracting the first direct-axis current component from them; and using the rotation angle, a synchronous rotating coordinate transformation is performed on the second current signal to convert the second current signal from the stationary three-phase coordinate system to the synchronous rotating coordinate system, obtaining a second direct-axis current component, a second quadrature-axis current component, and a second zero-sequence current component, and extracting the second direct-axis current component from them.

3. The differential protection method for microgrids according to claim 2, characterized in that, The synchronous rotational coordinate transformation employs the Parker transformation.

4. The differential protection method for a microgrid according to claim 1, characterized in that, The step of determining whether the target protected line has an internal fault based on the similarity index includes: If all the similarity indices calculated consecutively fall within the preset tripping threshold range, and the number of consecutive falls reaches the preset sampling threshold, then it is determined that the target protected line has an internal fault. The preset sampling threshold is set based on the magnitude of the capacitance effect of the target protected line, specifically including: When the capacitance effect of the target protected line is lower than the first preset capacitance standard, the preset sampling number threshold is equal to the first preset value. When the capacitance effect of the target protected line is higher than the second preset capacitance standard, the preset sampling number threshold is equal to the second preset value, and the second preset value is greater than the first preset value.

5. The differential protection method for a microgrid according to claim 4, characterized in that, The step of determining whether the target protected line has an internal fault based on the similarity index further includes: If the similarity index calculated consecutively does not fall within the preset tripping threshold range, or the number of consecutive falls within the range does not reach the preset sampling threshold, then it is determined that the target protected line has not experienced an internal fault, and no tripping command is output.

6. The differential protection method for a microgrid according to any one of claims 1 to 5, characterized in that, The calculation of the similarity index between the first direct-axis current component and the second direct-axis current component specifically includes: Within the sliding time window, based on the sampling sequence of the first direct-axis current component and the sampling sequence of the second direct-axis current component, a correlation measurement algorithm is used to calculate the waveform consistency index between the two to obtain the similarity index. The sliding time window moves continuously along the time axis.

7. The differential protection method for a microgrid according to claim 6, characterized in that, The process of calculating the waveform consistency index between the two using a correlation measurement algorithm to obtain the similarity index specifically includes: Using the Pearson correlation coefficient algorithm, within the sliding time window, the first mean of the sampling sequence of the first direct-axis current component and the second mean of the sampling sequence of the second direct-axis current component are calculated respectively; based on the sampling sequence of the first direct-axis current component, the sampling sequence of the second direct-axis current component, the first mean, and the second mean, the covariance between the sampling sequence of the first direct-axis current component and the sampling sequence of the second direct-axis current component, the first standard deviation of the sampling sequence of the first direct-axis current component, and the second standard deviation of the sampling sequence of the second direct-axis current component are calculated; The similarity index is the ratio of the covariance to the product of the first standard deviation and the second standard deviation.

8. The differential protection method for a microgrid according to claim 6, characterized in that, The length of the sliding time window is half the fundamental period of the microgrid.

9. A differential protection device for a microgrid, characterized in that, The device includes: The acquisition unit is used to acquire three-phase current signals at the first target end to obtain a first current signal; and to acquire three-phase current signals at the second target end to obtain a second current signal; wherein the first target end and the second target end are the opposite ends of the same target protected line in the microgrid; A transformation unit, connected to the acquisition unit, is used to perform a synchronous rotation coordinate transformation on the first current signal based on a rotation angle synchronized with the fundamental frequency of the microgrid, so as to extract the corresponding first direct-axis current component; and to perform a synchronous rotation coordinate transformation on the second current signal based on the rotation angle, so as to extract the corresponding second direct-axis current component. A calculation unit, connected to the transformation unit, is used to calculate the similarity index between the first direct-axis current component and the second direct-axis current component; The determination unit, connected to the calculation unit, is used to determine whether the target protected line has an internal fault based on the similarity index. An execution unit, connected to the determination unit, is used to output trip control signals to the circuit breaker at the first target end and the circuit breaker at the second target end respectively when it is determined that an internal fault has occurred in the target protected line.

10. An inverter-type islanded microgrid system, characterized in that, The system includes: The target protected line is located within an inverter-type islanded microgrid; A first circuit breaker is installed at the first target end of the target protected line and is used to disconnect the electrical connection between the first target end and the target protected line when a trip control command is received. The second circuit breaker is installed at the second target end of the target protected line and is used to disconnect the electrical connection between the second target end and the target protected line when a trip control command is received. Wherein, the second target end and the first target end are the opposite ends of the target protected line, and the first target end and the second target end are connected through a communication link; And the differential protection device for the microgrid as described in claim 9, wherein the differential protection device is configured at the first target end or the second target end, and is communicatively connected to the first circuit breaker and the second circuit breaker respectively, so as to output a trip control command when it is determined that an internal fault has occurred in the target protected line.

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