Power distribution system fault direction identification method based on rotating coordinate system current characteristics

Abstract

The application provides a power distribution system fault direction identification method based on rotating coordinate system current characteristics. The method comprises the following steps: periodically collecting current data of a local end of a power distribution system line by using a current sensor; if the three-phase current increments of the power distribution system line are approximately the same, it is judged that the power distribution system line has a three-phase fault; otherwise, it is judged that the power distribution system line has an asymmetric short-circuit fault; calculating the d-axis current and the q-axis current at the directional element in the rotating coordinate system by using the Park transformation, and extracting the direct current component, the fundamental frequency component and the second harmonic component in the d-axis current by using the continuous variable mode decomposition method; comparing the d-axis current increment with a set threshold value, and judging whether the power distribution system line has a forward fault or a reverse fault. The direction element provided by the method has an identification time of less than 5 ms under a 200Ω high resistance fault, is not affected by the type of an inverter power supply, and can effectively identify the fault direction of the power distribution system.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, and in particular to a method for identifying the fault direction of a power distribution system based on the current characteristics of a rotating coordinate system. Background Technology

[0002] The large-scale integration of distributed generation has transformed traditional distribution networks from a unidirectional radial structure into a complex network with multiple sources and bidirectional interaction. This transformation renders the assumptions of current amplitude-time characteristics and the unidirectionality of fault current, upon which the original protection systems relied, invalid. Especially under fault scenarios, the superposition of fault feed current from distributed generation and short-circuit current on the system side may lead to misjudgment or deterioration of the sensitivity of protection devices. In this case, current direction elements become the key criteria for accurately identifying the source and receiver of the fault. Their direction discrimination capability directly affects the selectivity and speed of protection, thereby impacting the accuracy of fault isolation and the reliability of system power supply.

[0003] With the increasing penetration of distributed power sources, the dynamic operating conditions of distribution networks exhibit strong uncertainty and spatiotemporal coupling characteristics. Traditional overcurrent protection is ill-suited to the complex variations in fault current amplitude, phase, and distribution characteristics under bidirectional power flow scenarios. Current directional elements, by capturing the phase information of the current vector, can effectively distinguish the fault contribution from the system side and the distributed power source side, providing a core criterion for resolving issues such as protection coordination mismatch and islanding detection failure. Especially in weak power grids with a high proportion of inverter-type power sources, their low inertia and weak short-circuit capacity characteristics further exacerbate the difficulty of direction discrimination. There is an urgent need for current directional elements based on novel measurement principles or adaptive algorithms to cope with the multi-scale coupling characteristics of system transient processes and ensure the reliability of protection system operation under complex operating boundaries.

[0004] In high-proportion distributed power distribution systems, fault current flows bidirectionally. Overcurrent protection lacks directional identification capabilities, making it prone to false tripping or failure to trip. Furthermore, existing directional elements require voltage information acquisition, increasing operating costs. To address this issue, a fault directional element that relies solely on current amplitude-frequency information in a rotating coordinate system is proposed. Summary of the Invention

[0005] The embodiments of the present invention provide a method for identifying the fault direction of a power distribution system based on the current characteristics of a rotating coordinate system, so as to effectively identify the fault direction of the power distribution system.

[0006] To achieve the above objectives, the present invention adopts the following technical solution.

[0007] A method for fault direction identification in a power distribution system based on the current characteristics of a rotating coordinate system includes:

[0008] Current data at the local end of the power distribution system line is periodically collected using a current sensor;

[0009] The phase current increment is calculated based on the collected current data. If the phase current increment is greater than the set phase current increment threshold, it is determined that a fault has occurred in the power distribution system line, and the fault time is marked.

[0010] If the three-phase current increments of the power distribution system lines are approximately the same, it is determined that a three-phase fault has occurred in the power distribution system lines; otherwise, it is determined that an asymmetrical short-circuit fault has occurred in the power distribution system lines.

[0011] The d-axis and q-axis currents at the directional element in the rotating coordinate system are calculated using the Park transform, and the DC component, fundamental frequency, and second harmonic component in the d-axis current are extracted using the continuous variable mode decomposition method.

[0012] Under three-phase faults, if the d-axis current increment is greater than the set current increment threshold and there is no fundamental frequency component, then a forward fault is determined to have occurred downstream of the directional element of the power distribution system line; otherwise, a reverse fault is determined to have occurred upstream of the directional element of the power distribution system line. Under asymmetrical faults, if the d-axis current has a second harmonic component and the second harmonic current energy is greater than the set current energy threshold, then a forward fault is determined to have occurred downstream of the directional element of the power distribution system line; otherwise, a reverse fault is determined to have occurred upstream of the directional element of the power distribution system line.

[0013] Preferably, the method of periodically collecting current data at the local end of the power distribution system line using a current sensor includes: setting up multiple sampling points at the local end of the power distribution system line, and periodically collecting the current value of each sampling point using a current sensor.

[0014] Preferably, the step of calculating the phase current increment based on the collected current data, and determining that a fault has occurred in the power distribution system line if the phase current increment is greater than a set phase current increment threshold, and marking the fault time, includes:

[0015] The phase current increment is calculated based on the collected current data. The calculation method for the phase current increment is as follows:

[0016]

[0017] Where K is the current increment, K set Let i be the maximum increment value of the previous sampling period. j (k) i j (k-1) Let N represent the current value at the j-th sampling point in the k-th and (k-1)-th sampling periods, respectively, where N is the number of calculation points, which increases with time.

[0018] If the phase current increment is greater than the preset phase current increment threshold, a fault is determined to have occurred in the power distribution system line, and the fault time is marked; otherwise, if the phase current increment is not greater than the preset phase current increment threshold, the process returns to normal and continues to periodically collect the current value of each sampling point using the current sensor.

[0019] Preferably, the statement that if the three-phase current increments of the power distribution system lines are approximately the same, then a three-phase fault is determined to have occurred in the power distribution system lines; otherwise, determining that an asymmetrical short-circuit fault has occurred in the power distribution system lines includes:

[0020] When a fault is determined to have occurred in a power distribution system line, the method for calculating the correlation of current increments is as follows:

[0021]

[0022]

[0023] Where p(·) represents the correlation, p set The threshold for the correlation coefficient is given. Subscripts A, B, and C indicate the current phase of the distribution system lines, and COV represents the covariance. K represents the standard deviation. A K B K C These represent the current increments of phases A, B, and C, respectively; p(K A K B K C ) represents the correlation coefficient of any two phases among the current increments of the three phases A, B, and C; p AB That is, the correlation coefficient between the current increments of phases A and B; if K A K B K C Correlation characterization between Greater than the set correlation coefficient threshold If the condition is met, a three-phase fault is considered to have occurred; otherwise, an asymmetrical short-circuit fault is considered to have occurred.

[0024] Preferably, the step of calculating the d-axis and q-axis currents at the protection point in a rotating coordinate system using Park transform, and extracting the DC component, fundamental frequency, and second harmonic component from the d-axis current using continuous variable mode decomposition, includes:

[0025] The Parker transformation is used to calculate the d-axis and q-axis currents at a directional element in a rotating coordinate system. The formula for the Parker transformation is as follows:

[0026]

[0027] in, This represents the rotational angular velocity of the current in phase A, where a, b, and c represent phases A, B, and C, respectively, and i d iq i and i0 represent the d-axis, q-axis, and zero-sequence current, respectively. a i b i c These represent the three-phase currents A, B, and C, respectively.

[0028] The DC, fundamental frequency, and second harmonic components of the d-axis current are extracted using the continuous variable mode decomposition method. The continuous variable mode decomposition method is as follows:

[0029]

[0030] in, For the input d-axis current signal i d The frequency domain representation of (t), where α is the weighting factor and L is the index of the currently extracted sub-mode. Let L be the center frequency of the Lth submode. and Let be the frequency domain representations of the Lth submode at the nth and (n+1)th iterations, respectively. Let ω be the frequency domain representation of the Lagrange multipliers in the nth iteration. i The center frequency of the extracted sub-mode;

[0031] The initial frequency signal is decomposed, and modal components of different frequencies are obtained through successive decompositions. This includes time-frequency components of various frequencies, based on the center frequency of the modal components. The modal components with a center frequency close to 0Hz are taken as DC components, the modal components with a center frequency close to 50Hz are taken as fundamental frequency components, and the modal components with a center frequency close to 100Hz are taken as second harmonic components. The allowable frequency error range is ±5Hz. If the center frequency differs from the required frequency by more than this range, it is considered that the frequency component does not exist. For example, if a 43Hz component is obtained by decomposition, it is not considered that the second harmonic component exists.

[0032] Preferably, under a three-phase fault, if the d-axis current increment is greater than a set current increment threshold and there is no fundamental frequency component, then a forward fault is determined to have occurred downstream of the directional element of the power distribution system line; otherwise, a reverse fault is determined to have occurred upstream of the directional element of the power distribution system line. Under an asymmetrical fault, if the d-axis current has a second harmonic component and the second harmonic current energy is greater than a set current energy threshold, then a forward fault is determined to have occurred downstream of the directional element of the power distribution system line; otherwise, a reverse fault is determined to have occurred upstream of the directional element of the power distribution system line. This includes:

[0033] Select the d-axis current DC and harmonic increments at 0Hz, 50Hz, and 100Hz;

[0034] Under three-phase faults, the fault is identified by using the d-axis current increment being greater than a threshold and the fundamental frequency component. The calculation method is as follows:

[0035]

[0036]

[0037] (1)

[0038] Among them, △I dc and △I 50 I represents the d-axis DC increment and the 50Hz component increment. dcf and I dcp I represents the DC flow rate before and after the fault. 50f and I 50p This represents the fundamental frequency components before and after the fault, with the subscript "set" indicating the threshold.

[0039] If the d-axis current increment is greater than the threshold and there is no fundamental frequency component, i.e., the above equation (1) holds, then it is determined that a positive fault has occurred downstream of the directional element of the power distribution system line; otherwise, it is determined that a reverse fault has occurred upstream of the directional element of the power distribution system line.

[0040] Under asymmetrical faults, the fault direction is determined by using the d-axis 100Hz current energy exceeding a threshold. The calculation method is as follows:

[0041] (2)

[0042] (3)

[0043] Among them, E 100 This represents the current energy at 100Hz along the d-axis, where N represents the total number of calculation points within the time window, j represents the j-th point, and i 100 E represents the 100Hz current component. set Indicates the current energy threshold;

[0044] If the d-axis current has a second harmonic component and the energy of the second harmonic current is greater than the threshold, i.e., the above equations (2) and (3) are true, then it is determined that a positive fault has occurred downstream of the directional element of the power distribution system line; otherwise, it is determined that a reverse fault has occurred upstream of the directional element of the power distribution system line.

[0045] As can be seen from the technical solutions provided by the embodiments of the present invention above, the directional element proposed by the method of the present invention has an identification time of less than 5ms under a 200Ω high resistance fault and is not affected by the inverter power supply access type, thus effectively identifying the fault direction of the power distribution system.

[0046] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and will become apparent from the description or may be learned by practice of the invention. Attached Figure Description

[0047] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 A flowchart illustrating a method for identifying the fault direction of a power distribution system based on the amplitude-frequency characteristics of current in a rotating coordinate system, provided in an embodiment of the present invention.

[0049] Figure 2 A simplified topology diagram of an inverter power distribution system is provided for an embodiment of the present invention.

[0050] Figure 3 This invention provides a diagram of a forward and reverse three-phase short-circuit fault. Figure 3 (a) is a waveform diagram of a positive three-phase fault decomposition. Figure 3 (b) is a waveform diagram of reverse three-phase fault decomposition;

[0051] Figure 4 This invention provides a forward and reverse asymmetric short-circuit fault diagram. Figure 4 (a) is a waveform diagram of positive asymmetric fault decomposition. Figure 4 (b) is a waveform diagram of reverse asymmetric fault decomposition. Detailed Implementation

[0052] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0053] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. It should be understood that when we say an element is “connected” or “coupled” to another element, it can be directly connected or coupled to the other element, or there may be intermediate elements. Furthermore, “connected” or “coupled” as used herein can include wireless connections or couplings. The term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0054] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0055] To facilitate understanding of the embodiments of the present invention, the following will provide further explanation and description with reference to the accompanying drawings and several specific embodiments. These embodiments do not constitute a limitation on the embodiments of the present invention.

[0056] The processing flow of a power distribution system fault direction identification method based on the current amplitude-frequency characteristics in a rotating coordinate system provided by this invention is as follows: Figure 1 As shown, the processing steps include the following:

[0057] Step S10: Periodically collect current data at this end of the power distribution system line using a current sensor;

[0058] Step S20: Calculate the phase current increment. If the increment is greater than the threshold, a fault is determined to have occurred, and the fault time is marked. Otherwise, return.

[0059] Step S30: If the three-phase current increments are approximately the same, a three-phase fault is considered to have occurred; otherwise, an asymmetrical short-circuit fault is considered to have occurred.

[0060] Step S40: Calculate the d-axis current and q-axis current at the direction element in the rotating coordinate system using Park transform, and extract the DC component, fundamental frequency and second harmonic component from the d-axis current using continuous variable mode decomposition method.

[0061] The requirement is to attach a summary Figure 2The arrow at point A indicates the positive direction of power flow. Under a three-phase fault, if the d-axis current increment is greater than the set current increment threshold and there is no fundamental frequency component, then the fault is determined to have occurred downstream of the directional element, i.e., the power distribution system line is considered to have a positive fault; otherwise, the power distribution system line is determined to have a reverse fault.

[0062] Under asymmetrical fault conditions, if the d-axis current has a second harmonic component and the energy of this second harmonic current exceeds a set current energy threshold, then a forward fault is determined to have occurred in the power distribution system line; otherwise, a reverse fault is determined to have occurred. (See attached abstract.) Figure 2 The arrow at point A indicates the positive direction of the power flow. A fault in the opposite direction indicates a fault upstream of the directional element.

[0063] Specifically, step S10 includes setting multiple sampling points at the local end of the power distribution system line and periodically collecting the current value of each sampling point using a current sensor. Typically, the sampling points of the directional element are set at the beginning of the line. To prevent reverse fault current from the distributed power source from causing maloperation of the current protection, the directional element is installed at the circuit breaker and switch between the distributed power source and the substation, and the sampling points are also located there.

[0064] Specifically, step S20 above includes the following: the method for calculating the phase current increment is as follows:

[0065]

[0066] Where K is the current increment, K set Let i be the maximum increment value of the previous sampling period. j (k) i j (k-1) Let N represent the current value at the j-th sampling point in the k-th and (k-1)-th sampling periods, respectively, and let N be the number of calculation points, which increases with time.

[0067] The protection is initiated by using phase current increment. If the phase current increment is greater than the preset phase current increment threshold, a fault is determined to have occurred in the power distribution system line, and the fault time is marked. Otherwise, if the phase current increment is not greater than the preset phase current increment threshold, the process returns to normal and continues to periodically collect the current value of each sampling point using the current sensor.

[0068] Specifically, step S30 includes: when it is determined that a fault has occurred in the power distribution system line, the current increment correlation method is used to distinguish between three-phase faults and asymmetrical faults. The current flowing through the line is defined as three phases A, B, and C. If all three phases are faulty, it is a three-phase fault. If two or one of the phases is faulty, it is an asymmetrical fault.

[0069] The correlation of current increments is expressed as:

[0070]

[0071]

[0072] Where p(·) represents the correlation, p set The threshold for the correlation coefficient is given. Subscripts A, B, and C indicate the current phase of the distribution system lines, and COV represents the covariance. K represents the standard deviation. A K B K C These represent the current increments of phases A, B, and C, respectively; p(K A K B K C ) represents the correlation coefficient of any two phases among the current increments of the three phases A, B, and C; p AB This is the correlation coefficient between the current increments of phases A and B.

[0073] If the increments of the three-phase currents A, B, and C are approximately the same, that is, the correlation characteristic p among the three-phase currents A, B, and C is greater than the set correlation coefficient threshold, then a three-phase fault is considered to have occurred; otherwise, an asymmetrical short-circuit fault is considered to have occurred.

[0074] Specifically, step S40 includes: calculating the d-axis current and q-axis current at the directional element in the rotating coordinate system using the Park transformation. The Park transformation is used to convert the three-phase currents (A, B, C) from the three-phase coordinate system to the rotating coordinate system, and is a commonly used transformation matrix in power systems. The rotating coordinate system is a coordinate system in which the d-axis and q-axis are perpendicular to each other, which can convert the time-varying current into a DC current that rotates with the coordinate system, thereby greatly simplifying the control. The directional element is the one proposed in this invention, which is installed at the circuit breaker or switch of the line. The d-axis current and q-axis current are obtained from the three-phase current through the Park transformation. The three-phase current is transformed to generate the d-axis current, q-axis current, and zero-sequence current.

[0075] The Parker transform is represented as:

[0076]

[0077] in, This represents the rotational angular velocity of the current in phase A, where a, b, and c represent phases A, B, and C, respectively, and i d i q i and i0 represent the d-axis, q-axis, and zero-sequence current, respectively. a i b i c These represent the three-phase currents A, B, and C, respectively.

[0078] The DC, fundamental frequency, and second harmonic components of the d-axis current are extracted using the continuous variable mode decomposition method. The continuous variable mode decomposition method is as follows:

[0079]

[0080] in, For the input d-axis current signal i d The frequency domain representation of (t) is given, where α is a weighting factor used to balance the importance of different optimization criteria. Let L be the center frequency of the Lth submode. Let L be the frequency domain representation of the Lth submode in the (n+1)th iteration. Let ω be the frequency domain representation of the Lagrange multipliers in the nth iteration. i The center frequency of the extracted sub-mode.

[0081] The initial frequency signal is decomposed, and modal components of different frequencies are obtained through successive decompositions. This includes time-frequency components of each frequency. Based on the center frequency of the modal components... The modal components with a center frequency close to 0Hz are considered as DC components, those with a center frequency close to 50Hz are considered as fundamental frequency components, and those with a center frequency close to 100Hz are considered as second harmonic components. The allowable frequency error range is ±5Hz. If the center frequency differs from the desired frequency by more than this range, the frequency component is considered to be non-existent. For example, if a 43Hz component is obtained after decomposition, the second harmonic component is not considered to exist.

[0082] By detecting the center frequency of the submodal components, the d-axis DC increment and harmonic DC increment of 0Hz, 50Hz and 100Hz are selected.

[0083] Under three-phase faults, fault identification is based on the d-axis current increment being greater than a threshold and the fundamental frequency component. The calculation method is as follows:

[0084]

[0085]

[0086] (1)

[0087] Among them, △I dc and △I 50 I represents the d-axis DC increment and the 50Hz component increment. dcf and I dcp I represents the DC flow rate before and after the fault. 50f and I 50p This represents the fundamental frequency components before and after the fault, and the subscript "set" indicates the threshold.

[0088] Under three-phase faults, if the d-axis current increment is greater than the threshold and there is no fundamental frequency component, i.e., the above equation (1) holds, then it is considered a forward fault; otherwise, it is a reverse fault.

[0089] Under asymmetrical faults, the fault direction is determined by utilizing the 100Hz d-axis current energy exceeding a threshold. The calculation method is as follows:

[0090] (2)

[0091] (3)

[0092] Among them, E 100 The current energy represents 100Hz, N represents the total number of calculation points within the time window, j represents the j-th point, and i 100 E represents the 100Hz current component. set This indicates the current energy threshold.

[0093] Under asymmetrical faults, if the d-axis current has a second harmonic component and the energy of the second harmonic current is greater than the threshold, i.e., the above equations (2) and (3) hold, then it is considered a positive fault; otherwise, it is a reverse fault.

[0094] Example 1

[0095] This invention provides a method for fault direction element in a power distribution system based on the amplitude-frequency characteristics of current in a rotating coordinate system, specifically including:

[0096] The current data at both ends of the power distribution system line is collected using a current sensor, with a sampling frequency of 4kHz;

[0097] Calculate the phase current increment. If the increment is greater than the threshold, a fault is determined to have occurred, and the fault time is marked; otherwise, return. The phase current calculation formula is:

[0098]

[0099] Where K is the current increment, K set Let i be the maximum increment value of the previous sampling period. j (k) i j (k-1) Let N represent the current value at the j-th sampling point in the k-th and (k-1)-th sampling periods, respectively, and let N be the number of calculation points, which increases with time.

[0100] If the correlation coefficient of the three-phase current increment is greater than the threshold of 0.7, a three-phase fault is considered to have occurred; otherwise, it is considered an asymmetrical short-circuit fault.

[0101]

[0102]

[0103] Where p(·) is the correlation coefficient, p set The threshold values ​​are for the correlation coefficient. Subscripts A, B, and C indicate phase, and COV represents the covariance. It represents the standard deviation.

[0104] Calculate the d-axis and q-axis currents at the directional element in a rotating coordinate system using the Park transformation;

[0105]

[0106] in, This represents the rotational angular velocity of the current in phase A, where a, b, and c represent phases A, B, and C, respectively, and i d i q i and i0 represent the d-axis, q-axis, and zero-sequence current, respectively.

[0107] The DC component, fundamental frequency, and second harmonic component of the d-axis current are extracted using the continuous variable mode decomposition method.

[0108]

[0109]

[0110] in, Let α be the frequency domain representation of the input d-axis current signal i(t), where α is a weighting factor used to balance the importance of different optimization criteria. Let L be the center frequency of the Lth submode. Let L be the frequency domain representation of the Lth submode in the (n+1)th iteration. Let ω be the frequency domain representation of the Lagrange multipliers in the nth iteration. i The center frequency of the extracted sub-mode.

[0111] Under a three-phase fault, if the d-axis current increment is greater than the threshold of 0.25 pu (relative to the d-axis current before the fault) and the fundamental frequency component is less than 0.01 pu, it is considered a forward fault; otherwise, it is a reverse fault.

[0112]

[0113]

[0114]

[0115] Among them, △I dc and △I 50 I represents the d-axis DC increment and the 50Hz component increment. dcf and I dcp I represents the DC flow rate before and after the fault. 50f and I50p This represents the fundamental frequency components before and after the fault, and the subscript "set" indicates the threshold.

[0116] Under asymmetrical fault conditions, if the d-axis current has a second harmonic component and the energy of the second harmonic current is greater than the threshold of 0.01, it is considered a forward fault; otherwise, it is a reverse fault. The calculation method is as follows:

[0117]

[0118]

[0119] Among them, E 100 The current energy represents 100Hz, N represents the total number of calculation points within the time window, j represents the j-th point, and i 100 E represents the 100Hz current component. set This indicates the current energy threshold.

[0120] Example 2

[0121] A simplified topology diagram of an inverter power distribution system provided in this embodiment of the invention is shown below. Figure 2 As shown.

[0122] (1) When a three-phase positive fault occurs on the line, only the DC component of the d-axis is detected at protection A, and the V increment of the DC component is much greater than 0.25pu. When a reverse fault occurs, the increment is about 0.2pu, and the fundamental frequency component is greater than 0.05pu. Therefore, there is a significant difference in the criteria for positive and reverse faults.

[0123] (2) When a forward asymmetrical fault occurs on the line, the DC component and the second harmonic component of the d-axis are detected at protection point A. When a reverse fault occurs, the increment is approximately 0.2 pu, and only the DC component and the fundamental frequency component are detected. Therefore, there is a significant difference in the criteria for forward and reverse faults.

[0124] The above characteristics indicate that the d-axis current components are significantly different during positive and reverse faults, which can be used to construct a direction criterion.

[0125] To verify the application performance of the proposed protection method, a simulation platform was built on the PSCAD platform. Figure 2 A simplified topology diagram of the inverter power distribution system. The directional element is installed at terminal A, and the sampling frequency is 4kHz.

[0126] The proposed protection process steps are as follows:

[0127] 1) Collect current signal data at both ends of the power distribution system line, with a sampling frequency of 4kHz;

[0128] 2) Calculate the phase current increment K. If the increment is greater than the threshold K... setIf so, a fault is determined to have occurred, and the fault time is marked;

[0129] 3) Calculate the correlation coefficient of the three-phase current increment. If the coefficient is greater than the threshold of 0.7, it is a three-phase fault; otherwise, it is an asymmetrical fault.

[0130] 4) Extract the DC component, fundamental frequency, and second harmonic component from the d-axis current using the continuous variable mode decomposition method;

[0131] 5) Under three-phase faults, if the DC increment of the d-axis current is greater than the threshold of 0.25pu (relative to the d-axis current before the fault) and the fundamental frequency component is less than 0.01pu, it is considered a forward fault; otherwise, it is a reverse fault.

[0132] 6) Under asymmetrical faults, if the d-axis current has a second harmonic component and the energy of the second harmonic current is greater than the threshold, it is considered a positive fault; otherwise, it is a reverse fault.

[0133] Simulation tests were conducted using electromagnetic transient simulation software (Power Systems Computer Aided Design / Electromagnetic Transients including DC EMTDC, PSCAD / EMTDC) to verify the relay protection performance at different fault locations.

[0134] Figure 3 The results of d-axis current decomposition for forward and reverse three-phase faults are shown. In the forward fault, the increment is 5 p.u., no fundamental frequency component is detected, which meets the criterion requirements and the fault direction is correctly identified. In the reverse fault, the DC component increment of the d-axis current is less than 0.2 pu, and the fundamental frequency is 0.04 pu at 5 ms, which meets the reverse criterion and the direction element can be correctly identified.

[0135] Figure 4 The results of d-axis current decomposition for asymmetrical faults in both positive and negative directions are shown. In the case of a positive fault, the second harmonic energy is 0.4861, which is greater than the threshold, and is therefore identified as a positive fault. In the case of a negative fault, no second harmonic is detected, and it is therefore identified as a negative fault. Therefore, the directional element can be correctly identified.

[0136] All of the above results were correctly identified.

[0137] In summary, this invention analyzes and concludes that the synchronous power supply output fault current amplitude increment is significant in the rotating coordinate system, and that asymmetrical faults contain a distinct second harmonic component. In contrast, inverter power supplies contain both DC and attenuated fundamental frequency components. Based on this difference, the phase angle information of the second harmonic component is converted into waveform comparison, improving the protection action speed. Under three-phase faults, a dual criterion of d-axis DC increment and fundamental frequency component amplitude change is constructed; under asymmetrical faults, a dual criterion of second harmonic waveform energy is proposed. The proposed directional element has an identification time of less than 5ms under a 200Ω high-resistance fault and is unaffected by the inverter power supply connection type.

[0138] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of one embodiment, and the modules or processes shown in the drawings are not necessarily essential for implementing the present invention.

[0139] As can be seen from the above description of the embodiments, those skilled in the art can clearly understand that the present invention can be implemented by means of software plus necessary general-purpose hardware platforms. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in various embodiments or some parts of the embodiments of the present invention.

[0140] The various embodiments in this specification 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, for apparatus or system embodiments, since they are basically similar to method embodiments, the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. The apparatus and system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0141] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for identifying the fault direction of a power distribution system based on the current characteristics of a rotating coordinate system, characterized in that, include: Current data at the local end of the power distribution system line is periodically collected using a current sensor; The phase current increment is calculated based on the collected current data. If the phase current increment is greater than the set phase current increment threshold, it is determined that a fault has occurred in the power distribution system line, and the fault time is marked. If the three-phase current increments of the power distribution system lines are roughly the same, then it is determined that a three-phase fault has occurred in the power distribution system lines. Otherwise, it is determined that an asymmetrical short circuit fault has occurred in the power distribution system. The d-axis and q-axis currents at the directional element in the rotating coordinate system are calculated using the Park transform, and the DC component, fundamental frequency, and second harmonic component in the d-axis current are extracted using the continuous variable mode decomposition method. Under a three-phase fault, if the d-axis current increment is greater than the set current increment threshold and there is no fundamental frequency component, it is determined that a forward fault has occurred downstream of the directional element of the power distribution system line; otherwise, it is determined that a reverse fault has occurred upstream of the directional element of the power distribution system line. Under asymmetrical fault conditions, if the d-axis current has a second harmonic component and the energy of the second harmonic current is greater than the set current energy threshold, then a forward fault is determined to have occurred downstream of the directional element of the power distribution system line; otherwise, a reverse fault is determined to have occurred upstream of the directional element of the power distribution system line. If the three-phase current increments of the power distribution system lines are approximately the same, then it is determined that a three-phase fault has occurred in the power distribution system lines. Otherwise, to determine if an asymmetrical short-circuit fault has occurred in the power distribution system, the following criteria should be considered: When a fault is determined to have occurred in a power distribution system line, the method for calculating the correlation of current increments is as follows: Where p(·) represents the correlation, p set The threshold for the correlation coefficient is given. Subscripts A, B, and C indicate the current phase of the distribution system lines, and COV represents the covariance. K represents the standard deviation. A K B K C These represent the current increments of phases A, B, and C, respectively; p(K A K B K C ) represents the correlation coefficient of any two phases among the current increments of the three phases A, B, and C; p AB That is, the correlation coefficient between the current increments of phases A and B; if K A K B K C Correlation characterization between Greater than the set correlation coefficient threshold If the condition is met, a three-phase fault is considered to have occurred; otherwise, an asymmetrical short-circuit fault is considered to have occurred.

2. The method according to claim 1, characterized in that, The method of periodically collecting current data at the local end of the power distribution system line using a current sensor includes: setting up multiple sampling points at the local end of the power distribution system line, and periodically collecting the current value at each sampling point using a current sensor.

3. The method according to claim 1 or 2, characterized in that, The process of calculating the phase current increment based on the collected current data, and determining that a fault has occurred in the power distribution system line if the phase current increment exceeds a set phase current increment threshold, and marking the fault time, includes: The phase current increment is calculated based on the collected current data. The calculation method for the phase current increment is as follows: Where K is the current increment, K set Let i be the maximum increment value of the previous sampling period. j (k) i j (k-1) Let N represent the current value at the j-th sampling point in the k-th and (k-1)-th sampling periods, respectively, where N is the number of calculation points, which increases with time. If the phase current increment is greater than the preset phase current increment threshold, a fault is determined to have occurred in the power distribution system line, and the fault time is marked; otherwise, if the phase current increment is not greater than the preset phase current increment threshold, the process returns to normal and continues to periodically collect the current value of each sampling point using the current sensor.

4. The method according to claim 3, characterized in that, The method of calculating the d-axis and q-axis currents at the protection point in a rotating coordinate system using Park transform, and extracting the DC component, fundamental frequency, and second harmonic component from the d-axis current using continuous variable mode decomposition, includes: The Parker transformation is used to calculate the d-axis and q-axis currents at a directional element in a rotating coordinate system. The formula for the Parker transformation is as follows: in, This represents the rotational angular velocity of the current in phase A, where a, b, and c represent phases A, B, and C, respectively, and i d i q i and i0 represent the d-axis, q-axis, and zero-sequence current, respectively. a i b i c These represent the three-phase currents A, B, and C, respectively. The DC, fundamental frequency, and second harmonic components of the d-axis current are extracted using the continuous variable mode decomposition method. The continuous variable mode decomposition method is as follows: in, For input d-axis current signal i d The frequency domain representation of (t), where α is the weighting factor and L is the index of the currently extracted sub-mode. Let L be the center frequency of the Lth submode. and Let be the frequency domain representations of the Lth submode at the nth and (n+1)th iterations, respectively. Let ω be the frequency domain representation of the Lagrange multipliers in the nth iteration. i The center frequency of the extracted sub-mode; The initial frequency signal is decomposed, and modal components of different frequencies are obtained through successive decompositions. This includes time-frequency components of various frequencies, based on the center frequency of the modal components. The modal component with a center frequency close to 0Hz is taken as the DC component, the modal component with a center frequency close to 50Hz is taken as the fundamental frequency component, and the modal component with a center frequency close to 100Hz is taken as the second harmonic component. The allowable frequency error range is ±5Hz. If the center frequency differs from the required frequency by more than this range, it is considered that the frequency component does not exist.

5. The method according to claim 4, characterized in that, Under a three-phase fault, if the d-axis current increment is greater than the set current increment threshold and there is no fundamental frequency component, then it is determined that a positive fault has occurred downstream of the directional element of the power distribution system line; otherwise, it is determined that a reverse fault has occurred upstream of the directional element of the power distribution system line. Under asymmetrical fault conditions, if the d-axis current has a second harmonic component and the energy of the second harmonic current is greater than the set current energy threshold, then it is determined that a positive fault has occurred downstream of the directional element of the power distribution system line. Conversely, if the fault is not detected, it is determined that a reverse fault has occurred upstream of the directional element in the power distribution system line, including: Select the d-axis current DC and harmonic increments at 0Hz, 50Hz, and 100Hz; Under three-phase faults, the fault is identified by using the d-axis current increment being greater than a threshold and the fundamental frequency component. The calculation method is as follows: （1） Among them, △I dc and △I 50 I represents the d-axis DC increment and the 50Hz component increment. dcf and I dcp I represents the DC flow rate before and after the fault. 50f and I 50p This represents the fundamental frequency components before and after the fault, with the subscript "set" indicating the threshold. If the d-axis current increment is greater than the threshold and there is no fundamental frequency component, i.e., the above equation (1) holds, then it is determined that a positive fault has occurred downstream of the directional element of the power distribution system line; otherwise, it is determined that a reverse fault has occurred upstream of the directional element of the power distribution system line. Under asymmetrical faults, the fault direction is determined by using the d-axis 100Hz current energy exceeding a threshold. The calculation method is as follows: （2） （3） Among them, E 100 The current energy at 100Hz along the d-axis represents the total number of calculation points within the time window, and j represents the j-th point. 100 E represents the 100Hz current component. set Indicates the current energy threshold; If the d-axis current has a second harmonic component and the energy of the second harmonic current is greater than the threshold, i.e., the above equations (2) and (3) are true, then it is determined that a positive fault has occurred downstream of the directional element of the power distribution system line; otherwise, it is determined that a reverse fault has occurred upstream of the directional element of the power distribution system line.

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