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

Abstract

The invention provides a power distribution system fault direction identification method based on current characteristics of a rotating coordinate system. The method comprises the following steps: periodically acquiring current data of a home terminal of a power distribution system line by using a current sensor, if three-phase current increments of the power distribution system line are roughly the same, judging that the power distribution system line has a three-phase fault, otherwise, judging that the power distribution system line has a three-phase fault; judging that the power distribution system line has an asymmetric short-circuit fault; park transform is used to calculate d-axis current and q-axis current at a direction element under a rotating coordinate system, and a continuous variable mode decomposition method is used to extract a direct current component, a fundamental frequency and a second harmonic component in the d-axis current. And 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. According to the method, the identification time of the directional element under the 200-ohm high-resistance fault is less than 5 ms, the directional element is not influenced by the access type of the inverter power supply, and the fault direction of the power distribution system can be effectively identified.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power system relay protection technology, and in particular to a power distribution system fault direction identification method based on current characteristics in a rotating coordinate system. BACKGROUND

[0002] The large-scale access of distributed power sources has gradually changed the traditional power distribution network from a one-way radial structure to a complex network form of multi-source two-way interaction. This change has caused the assumptions of the original protection system, which relies on the current amplitude-time characteristics and the single nature of the fault current direction, to fail. In particular, under fault scenarios, the fault feeder current of the distributed power source and the system-side short-circuit current superimposed may cause protection misjudgment or sensitivity degradation. At this time, the current direction element becomes the key criterion for accurately identifying the fault source and the receiving end, and its direction discrimination ability is directly related to the realization of protection selectivity and speed, and thus affects the fault isolation accuracy and the power supply reliability of the system.

[0003] With the increase of the penetration rate of distributed power sources, the dynamic operating conditions of the power distribution network present strong uncertainty and space-time coupling characteristics, and the traditional overcurrent protection cannot adapt to the complex changes in the fault current amplitude, phase, and distribution characteristics under the two-way flow scenario. The current direction element can effectively distinguish the fault contribution of the system side and the distributed power source side by capturing the phase information of the current vector, providing a core criterion for solving the protection mismatch, island detection failure, and other problems. In particular, in a weak power grid with a high proportion of inverter-type power sources, its low inertia and weak short-circuit capacity characteristics further aggravate the difficulty of direction discrimination, and there is an urgent need for a current direction element based on new measurement principles or adaptive algorithms to cope with the multi-scale coupling characteristics of the system transient process and ensure the action reliability of the protection system under complex operating boundaries.

[0004] The fault current of the high-proportion distributed power source power distribution system flows in both directions, and the overcurrent protection lacks direction discrimination ability and is prone to misoperation / refusal, and the existing direction element needs to collect voltage information, increasing the operating cost. To solve this problem, a fault direction element is proposed, which only relies on the current amplitude-frequency information in the rotating coordinate system. SUMMARY

[0005] Embodiments of the present application provide a power distribution system fault direction identification method based on current characteristics in a rotating coordinate system to effectively identify the fault direction of the power distribution system.

[0006] To achieve the above purpose, the present application adopts the following technical solutions.

[0007] A power distribution system fault direction identification method based on current characteristics in a rotating coordinate system, comprising:

[0008] Periodically collecting current data of the local end of the power distribution system line by using a current sensor;

[0009] According to the collected current data, phase current increment is calculated, if the phase current increment is greater than a set phase current increment threshold, it is determined that the power distribution system line has a fault, and the fault time is marked;

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

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

[0012] Under a three-phase fault, if the d-axis current increment is greater than a set current increment threshold and no fundamental frequency component exists, it is determined that the downstream of the directional element of the power distribution system line has a forward fault, otherwise, it is determined that the upstream of the directional element of the power distribution system line has a reverse fault; under an asymmetric fault, if the d-axis current has a double-frequency component and the double-frequency current energy is greater than a set current energy threshold, it is determined that the downstream of the directional element of the power distribution system line has a forward fault, otherwise, it is determined that the upstream of the directional element of the power distribution system line has a reverse fault.

[0013] Preferably, the periodic collection of current data at the local end of the power distribution system line by using the current sensor comprises: setting multiple sampling points at the local end of the power distribution system line, and periodically collecting the current values of each sampling point by using the current sensor.

[0014] Preferably, the calculation of the phase current increment according to the collected current data, if the phase current increment is greater than a set phase current increment threshold, it is determined that the power distribution system line has a fault, and the fault time is marked, comprises:

[0015] The phase current increment is calculated according to the collected current data, and the calculation method of the phase current increment is:

[0016]

[0017] Wherein, K is the current increment, K set The maximum increment value of the last sampling period is set as Kmax, The current values of the jth sampling point in the kth sampling period and the k-1th sampling period are respectively represented as Ij(k) and Ij(k-1), N is the number of calculation points, which increases with time;

[0018] If the phase current increment is greater than a pre-set phase current increment threshold, it is determined that the power distribution system line has a fault, and the fault time is marked, otherwise, if the phase current increment is not greater than the pre-set phase current increment threshold, it is returned, and the current values of each sampling point are periodically collected by using the current sensor.

[0019] Preferably, if the increments of three-phase currents of the power distribution system line are approximately the same, it is determined that a three-phase fault occurs in the power distribution system line, otherwise, it is determined that an asymmetric short-circuit fault occurs in the power distribution system line, comprising:

[0020] When it is determined that a fault occurs in the power distribution system line, the calculation method of the current increment correlation is as follows:

[0021] p(K A , K B , K C ) p set

[0022]

[0023] Wherein, p(·) is a correlation representation, p set is a correlation coefficient threshold, subscripts A, B, and C represent the phase of the current of the power distribution system line, COV represents the covariance, and σ represents the standard deviation; K A , K B , and K C are the increments of the A, B, and C three-phase currents respectively; p(K A , K B , K C ) represents the correlation coefficient of any two of the A, B, and C three-phase current increments; p AB is the correlation coefficient of the A and B phase current increments; if the correlation representation p(K A , K B , K C ) between K A , K B , and K C is greater than the set correlation coefficient threshold p set , it is considered that a three-phase fault occurs, otherwise, it is considered that an asymmetric short-circuit fault occurs.

[0024] Preferably, the d-axis current and the q-axis current at the protection in the rotating coordinate system are calculated by using the Park transformation, and the direct current component, the fundamental frequency component, and the second harmonic component in the d-axis current are extracted by using the continuous variable mode decomposition method, comprising:

[0025] The d-axis current and the q-axis current at the directional element in the rotating coordinate system are calculated by using the Park transformation, and the calculation formula of the Park transformation is as follows:

[0026]

[0027] Wherein, θ represents the rotational angular velocity of the A phase current, a, b, and c represent the A, B, and C phases respectively, i d , i q , and i0 represent the d-axis, q-axis, and zero sequence currents respectively, i a , ib , i c A, B, C represent three-phase currents respectively;

[0028] The continuous variable mode decomposition method is used to extract the DC, fundamental frequency and second harmonic components in the d-axis current, and the continuous variable mode decomposition method is:

[0029]

[0030] Wherein, is the frequency domain representation of the input d-axis current signal i d (t), α is the weight factor, ω k is the center frequency of the kth submode, is the frequency domain representation of the kth submode in the n+1th iteration, is the frequency domain representation of the Lagrange multiplier in the nth iteration, ω i is the center frequency of the extracted submode;

[0031] is the frequency signal of the initial decomposition, and the modal components of different frequencies are obtained by k times of decomposition , wherein each frequency time-frequency component is included, according to the center frequency ω k of the modal component, the modal component with a center frequency close to 0Hz is regarded as a DC component, the modal component with a center frequency close to 50Hz is regarded as a fundamental frequency component, and the modal component with a center frequency close to 100Hz is regarded as a second harmonic component, the allowed frequency error interval is ±5Hz, and when the center frequency and the required frequency differ by more than the interval, it is considered that there is no frequency component, for example, if a 43Hz component is obtained, it is not considered that there is a second harmonic component.

[0032] Preferably, under 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 judged that the downstream of the directional element of the distribution system line occurs forward fault; otherwise, it is judged that the upstream of the directional element of the distribution system line occurs reverse fault; under asymmetric fault, if the d-axis current exists a double frequency component, and the double frequency current energy is greater than the set current energy threshold, it is judged that the downstream of the directional element of the distribution system line occurs forward fault; otherwise, it is judged that the upstream of the directional element of the distribution system line occurs reverse fault, comprising:

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

[0034] Under three-phase fault, the d-axis current increment greater than the threshold and the fundamental frequency component are used to distinguish the fault, and the calculation method is:

[0035] ΔI dc = I dcf -Idcp

[0036] ΔI 50 =I 50f -I 50p

[0037] ΔI 50 <ΔI 50set &ΔI dc >ΔI dcset (1)

[0038] wherein, △I dc and △I 50 are d-axis direct current increment and 50Hz component increment, I dcf and I dcp represent direct current before and after fault, I 50f and I 50p represent fundamental frequency component before and after fault, subscript set represents threshold value;

[0039] If d-axis current increment is greater than threshold value, and no fundamental frequency component exists, that is, formula (1) above is established, then it is judged that positive fault occurs downstream of directional element of distribution system line; otherwise, it is judged that reverse fault occurs upstream of directional element of distribution system line;

[0040] Under asymmetric fault, d-axis 100Hz current energy greater than threshold value is used to distinguish fault direction, and its calculation method is

[0041]

[0042] E 100 >E set (3)

[0043] wherein, E 100 represents d-axis 100Hz current energy, N represents total point number in time window, j represents jth point, i 100 represents 100Hz current component, E set represents current energy threshold value;

[0044] If d-axis current exists double frequency component, and double frequency current energy is greater than threshold value, that is, formula (2) and formula (3) above are established, then it is judged that positive fault occurs downstream of directional element of distribution system line; otherwise, it is judged that reverse fault occurs upstream of directional element of distribution system line.

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

[0046] Additional aspects and advantages of the application will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the attendant drawings or can be learned by practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0047] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiment description. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative labor.

[0048] Figure 1 A processing flow chart of a power distribution system fault direction identification method based on a rotating coordinate system current amplitude frequency characteristic provided for the embodiment of the present application;

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

[0050] Figure 3 A forward and reverse three-phase short-circuit fault diagram provided for the embodiment of the present application. Figure 3 (a) is a forward three-phase fault decomposition waveform diagram, Figure 3 (b) is a reverse three-phase fault decomposition waveform diagram.

[0051] Figure 4 A forward and reverse asymmetric short-circuit fault diagram provided for the embodiment of the present application. Figure 4 (a) is a forward asymmetric fault decomposition waveform diagram, Figure 4 (b) is a reverse asymmetric fault decomposition waveform diagram. DETAILED DESCRIPTION

[0052] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the drawings, wherein the same or similar notations represent the same or similar elements or elements with the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are only used to explain the present application, and cannot be interpreted as a limitation of the present application.

[0053] It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. It is further understood that the terms "comprise" (or comprise), "comprises" (or comprises) and "comprising" (or comprises) when used in this specification, specify the presence of stated features, integers, steps, operations, elements, or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or groups thereof. It is further understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements can be present. In addition, the use of "connection" or "coupling" herein also includes wireless connection or coupling. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0054] It is to be understood that the terms so used are intended to encompass like

[0055] For the purpose of promoting an understanding of the principles of the application, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will, nevertheless, be understood that no limitation of the scope of the application is thereby intended, such alterations and further modifications in the illustrated device being contemplated as falling within the scope of the application.

[0056] The processing flow of the power distribution system fault direction identification method based on the current amplitude-frequency characteristics of the rotating coordinate system provided by the embodiments of the application is shown in Figure 1 The processing steps include the following steps:

[0057] In step S10, the current data of the local end of the power distribution system line is periodically collected by using a current sensor.

[0058] In step S20, the phase current increment is calculated, and if the increment is greater than a threshold value, it is determined that a fault occurs, and the fault time is marked, otherwise, it returns.

[0059] In step S30, if the three-phase current increments are substantially the same, it is considered that a three-phase fault occurs, otherwise, it is considered that an asymmetric short-circuit fault occurs.

[0060] In step S40, the d-axis current and the q-axis current at the directional element in the rotating coordinate system are calculated by using the Park transformation, and the direct current component, the fundamental frequency component and the second harmonic component in the d-axis current are extracted by using the continuous variable mode decomposition method.

[0061] The regulation is as follows: Figure 2The arrow at the middle A indicates the positive direction of the tidal current. In the case of three-phase fault, if the d-axis current increment is greater than the set current increment threshold value and no fundamental component exists, it is judged that the fault occurs downstream of the directional element, that is, the distribution system line is considered to have a positive fault; otherwise, it is judged that the distribution system line has a reverse fault.

[0062] In the case of asymmetric fault, if the d-axis current has a double-frequency component and the double-frequency current energy is greater than the set current energy threshold value, it is judged that the distribution system line has a positive fault; otherwise, it is judged that the distribution system line has a reverse fault. Still taking Figure 2 The arrow at the middle A indicates the positive direction of the tidal current. The reverse fault is the fault occurring upstream of the directional element.

[0063] Specifically, the above step S10 specifically includes: setting multiple sampling points at the local end of the distribution system line, and periodically collecting the current values of each sampling point by using a current sensor. The sampling point of the directional element is usually set at the head end of the line. In order to prevent the reverse fault current fed by the distributed power supply from causing the current protection to malfunction, the directional element is installed at the circuit breaker and switch between the distributed power supply and the substation, and the sampling point is also located at this position.

[0064] Specifically, the above step S20 specifically includes: the phase current increment calculation method is:

[0065]

[0066] Wherein, K is the current increment, K set Let be the maximum increment value of the last sampling period, i j (k) , i j (k-1) respectively represent the current value of the jth sampling point in the kth sampling period and the k-1th sampling period, and N is the calculation point number, which increases with time.

[0067] The phase current increment is used to start the protection. If the phase current increment is greater than the pre-set phase current increment threshold value, it is determined that the distribution system line has a fault, and the fault time is marked. Otherwise, the current values of each sampling point are periodically collected by using the current sensor.

[0068] Specifically, the above step S30 specifically includes: when it is determined that the distribution system line has a fault, the current increment correlation method is used to distinguish three-phase fault and asymmetric fault. The current flowing in the line is defined as A, B and C three phases. If all three phases have a fault, it is a three-phase fault. If two or one phase has a fault, it is an asymmetric fault.

[0069] The current increment correlation is represented as:

[0070] p(K A ,K B ,K C )>p set

[0071]

[0072] wherein p(·) is a correlation representation, p set is a correlation coefficient threshold, subscripts A, B, C represent current phases of the power distribution system line, COV represents covariance, and σ represents standard deviation. K A , K B , K C are current increments of phases A, B, and C, respectively; p(K A , K B , K C ) represents a correlation coefficient of any two of the current increments of phases A, B, and C; and p AB is a correlation coefficient of the current increments of phases A and B.

[0073] If the current increments of phases A, B, and C are substantially the same, i.e., the correlation representation p between the current increments of phases A, B, and C is greater than the set correlation coefficient threshold, i.e., p(A, B, C) > p

[0074] Specifically, the above step S40 specifically comprises: calculating d-axis current and q-axis current at a directional element in a rotating coordinate system by using a Park transformation, the Park transformation being used for converting ABC three-phase current from a three-phase coordinate system to a rotating coordinate system, the rotating coordinate system being a commonly used conversion matrix for power systems, the rotating coordinate system being a coordinate system in which a d-axis and a q-axis are perpendicular to each other, and being capable of converting time-varying current into a direct current rotating with the coordinate system, thereby greatly simplifying control; the directional element being the method proposed in the application, and being installed at a circuit breaker or a switch of the line; and the d-axis current and the q-axis current being obtained by converting three-phase current by using the Park transformation, and the three-phase current generating d-axis current, q-axis current, and zero sequence current after conversion.

[0075] The Park transformation is represented as:

[0076]

[0077] wherein θ represents an A-phase current rotation angular velocity, a, b, and c represent phases A, B, and C, respectively, i d , i q , and i0 represent d-axis current, q-axis current, and zero sequence current, respectively, and i a , i b , and i c represent three-phase currents A, B, and C, respectively.

[0078] The continuous morphological decomposition method is used to extract the DC, fundamental and second harmonic components in the d-axis current, and the continuous morphological decomposition method is as follows:

[0079]

[0080] wherein, is the frequency domain representation of the input d-axis current signal i d (t), and α is a weight factor used to balance the importance of different optimization criteria, and ω k is the center frequency of the kth submode, is the frequency domain representation of the kth submode in the n+1th iteration, is the frequency domain representation of the Lagrange multiplier in the nth iteration, and ω i is the center frequency of the extracted submode.

[0081] is the frequency signal of the initial decomposition, and the modal components of different frequencies are obtained by k times of decomposition wherein the time-frequency components of each frequency are included. According to the center frequency ω k of the modal component, the modal component with a center frequency close to 0Hz is regarded as a DC component, the modal component with a center frequency close to 50Hz is regarded as a fundamental component, and the modal component with a center frequency close to 100Hz is regarded as a second harmonic component. The allowed frequency error interval is ±5Hz, and when the center frequency deviates from the required frequency by more than the interval, it is considered that there is no frequency component, for example, if a 43Hz component is obtained by decomposition, it is not considered that there is a second harmonic component.

[0082] By detecting the center frequency of the submode component, the d-axis DC increment and the harmonic current increment of 0Hz, 50Hz and 100Hz are selected;

[0083] Under three-phase fault, the fault is judged by using the d-axis current increment greater than the threshold value and the fundamental component, and the calculation method is as follows:

[0084] ΔI dc = I dcf -I dcp

[0085] ΔI 50 = I 50f -I 50p

[0086] ΔI 50 <ΔI 50set &ΔI dc >ΔI dcset (1)

[0087] wherein, ΔI dc and ΔI 50For d-axis DC increment and 50Hz component increment, I dcf And I dcp Indicates the direct current before and after the fault, I 50f And I 50p Indicates the fundamental frequency component before and after the fault, and the subscript set indicates the threshold value.

[0088] Under three-phase fault, if the d-axis current increment is greater than the threshold value, and there is no fundamental frequency component, that is, formula (1) above is established, it is considered to be a forward fault, and vice versa.

[0089] Under asymmetric fault, the d-axis 100Hz current energy is greater than the threshold value to distinguish the fault direction, and the calculation method is

[0090]

[0091] E 100 >E set (3)

[0092] Wherein, E 100 Indicates the 100Hz current energy, N indicates the total number of calculation points in the time window, j indicates the jth point, i 100 Indicates the 100Hz current component, E set Indicates the current energy threshold value.

[0093] Under asymmetric fault, if the d-axis current exists a double frequency component, and the double frequency current energy is greater than the threshold value, that is, formulas (2) and (3) above are established, it is considered to be a forward fault, and vice versa.

[0094] Example one

[0095] The power distribution system fault direction element method based on the current amplitude-frequency characteristics of the rotating coordinate system provided by the embodiment of the application, specifically comprises:

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

[0097] The phase current increment is calculated, if the increment is greater than the threshold value, it is determined that a fault occurs, the fault time is marked, and vice versa, it returns; the phase current calculation formula is:

[0098]

[0099] Wherein, K is the current increment, K set Let the maximum increment value of the last sampling period be i j (k) , i j (k-1)I(k) and I(k-1) represent the current value at the jth sampling point in the kth and (k-1)th sampling period, respectively, and N is 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 value 0.7, it is considered that a three-phase fault occurs, otherwise it is an asymmetric short-circuit fault.

[0101] p(K A ,K B ,K C )>p set

[0102]

[0103] where p(·) is the correlation coefficient, p set is the correlation coefficient threshold value, the subscripts A, B and C represent the phase, COV represents the covariance, and σ represents the standard deviation.

[0104] The d-axis current and the q-axis current at the directional element in the rotating coordinate system are calculated by using the Park transformation.

[0105]

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

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

[0108]

[0109] where is the frequency domain representation of the input d-axis current signal i(t), α is a weight factor for balancing the importance of different optimization criteria, ω k is the center frequency of the kth submode, is the frequency domain representation of the kth submode in the n+1th iteration, is the frequency domain representation of the Lagrange multiplier in the nth iteration, ω i is the center frequency of the extracted submode.

[0110] Under a three-phase fault, if the d-axis current increment is greater than the threshold value 0.25 p.u. (relative to the d-axis current before the fault) and the fundamental frequency component is less than 0.01 p.u., it is considered to be a forward fault, otherwise it is a reverse fault.

[0111] ΔI dc =I dcf -I dcp

[0112] ΔI 50 =I 50f -I 50p

[0113] ΔI 50 <ΔI 50set &ΔI dc >ΔI dcset

[0114] where, △I dc and △I 50 are d-axis DC increment and 50Hz component increment, I dcf and I dcp represent DC component before and after fault, I 50f and I 50p represent fundamental component before and after fault, subscript set represents threshold value.

[0115] Under asymmetric fault, if d-axis current exists double frequency component, and double frequency current energy is greater than threshold value 0.01, it is considered as forward fault, otherwise it is reverse fault, and its calculation method is

[0116]

[0117] E 100 >E set

[0118] where, E 100 represents 100Hz current energy, N represents total calculation point number in time window, j represents jth point, i 100 represents 100Hz current component, E set represents current energy threshold value.

[0119] Example two

[0120] The simplified topology diagram of the inverter power distribution system provided by the embodiment of the application is as shown in Figure 3 .

[0121] (1) when three-phase forward fault occurs in the line, the protection A only detects d-axis DC component, and the V increment of the DC component is far greater than 0.25p.u., when reverse fault occurs, the increment is about equal to 0.2p.u., and the fundamental component is greater than 0.05p.u., therefore, the criterion has obvious difference when forward and reverse faults occur.

[0122] (2) when forward asymmetric fault occurs in the line, the protection A detects d-axis DC component and double frequency component, when reverse fault occurs, the increment is about equal to 0.2p.u., and only detects DC component and fundamental component, therefore, the criterion has obvious difference when forward and reverse faults occur.

[0123] The above features can indicate that the d-axis current components are obviously different in forward and reverse faults, and a direction criterion can be formed according to the same.

[0124] To verify the application performance of the protection method, a simplified topology of an inverter power distribution system is built on a PSCAD simulation platform. Figure 1 A directional element is installed at the A end, and the sampling frequency is 4 kHz.

[0125] The steps of the protection procedure are as follows:

[0126] 1) The current signal data at both ends of the power distribution system line are collected, and the sampling frequency is 4 kHz;

[0127] 2) The phase current increment K is calculated, and if the increment is greater than the threshold value K set , it is determined that a fault occurs, and the fault time is marked;

[0128] 3) The correlation coefficient of the three-phase current increment is calculated, and if the coefficient is greater than the threshold value 0.7, it is a three-phase fault, otherwise it is an asymmetric fault;

[0129] 4) The continuous variable mode decomposition method is used to extract the DC component, fundamental frequency component and second harmonic component in the d-axis current;

[0130] 5) Under three-phase fault, if the DC increment of the d-axis current is greater than the threshold value 0.25 p.u. (relative to the d-axis current before the fault), and the fundamental frequency component is less than 0.01 p.u., it is considered to be a forward fault; otherwise, it is a reverse fault;

[0131] 6) Under asymmetric fault, if the d-axis current has a double-frequency component, and the double-frequency current energy is greater than the threshold value, it is considered to be a forward fault; otherwise, it is a reverse fault.

[0132] Simulation tests are performed using electromagnetic transient simulation software (Power Systems Computer Aided Design / Electromagnetic Transients including DC, PSCAD / EMTDC), and the protection performance is verified for different fault locations.

[0133] Figure 3 The d-axis current decomposition results of forward and reverse three-phase faults are shown. The increment is 5 p.u. under forward fault, and no fundamental frequency component is detected, meeting the criterion requirement, and the fault direction is correctly identified. Under reverse fault, the DC component increment of the d-axis current is less than 0.2 p.u., and the fundamental frequency is 0.04 p.u. at 5 ms, meeting the reverse criterion, and the directional element can correctly identify the direction.

[0134] Figure 4 a and Figure 4b shows the positive and negative asymmetrical fault d-axis current decomposition results. When the positive fault, the second harmonic energy is 0.4861, which is greater than the threshold, and the positive fault is determined. When the negative fault, the second harmonic is not detected, and the negative fault is determined. Therefore, the direction element can correctly identify.

[0135] The above results are all correct.

[0136] In summary, the analysis of the present application shows that the synchronous machine power output fault current amplitude increment is significant and asymmetric, and contains obvious second harmonic components. The inverter power contains DC components and attenuated fundamental components. Based on this difference, the phase angle information of the second harmonic component is converted into waveform comparison to improve the protection action speed. Under three-phase fault, the dual criterion of d-axis direct current increment and fundamental component amplitude change is constructed. Under asymmetric fault, the dual criterion of second harmonic waveform energy is proposed. The identification time of the proposed direction element under 200Ω high resistance fault is less than 5ms, and is not affected by the type of inverter power.

[0137] Those skilled in the art can understand that the drawings are only schematic of an embodiment, and the modules or flows in the drawings are not necessarily essential for implementing the present application.

[0138] From the above description of the embodiments, those skilled in the art can clearly understand that the present application can be implemented by means of software and the necessary general hardware platform. Based on such understanding, the technical solutions of the present application can be embodied in the form of a software product, which can be stored in a storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes a number of instructions to make a computer device (which can be a personal computer, server, or network device, etc.) execute the methods described in various embodiments or some parts of the embodiments.

[0139] Each embodiment in the specification is described in a progressive manner, and the same or similar parts between each embodiment can be referred to each other, and each embodiment mainly describes the difference from other embodiments. Especially, for the device or system embodiment, since it is basically similar to the method embodiment, it is described more simply, and the related parts can be referred to the part of the method embodiment. The above described device and system embodiments are only schematic, and the units described as separate components can be or can not be physically separated, and the components displayed as units can be or can not be physical units, that is, they can be located in one place, or can be distributed on multiple network units. According to the actual needs, part or all of the modules can be selected to achieve the purpose of the embodiment. Those skilled in the art can understand and implement without creative labor.

[0140] The above description is only preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A power distribution system fault direction identification method based on rotating coordinate system current characteristics, characterized in that, The utility model relates to a method for detecting the fault of power distribution system, comprising: Periodically collecting the current data of the local end of the power distribution system line by using the current sensor; Calculating the phase current increment according to the collected current data, and determining that the power distribution system line has a fault if the phase current increment is greater than the set phase current increment threshold value, and marking the fault time; If the three-phase current increments of the power distribution system line are roughly the same, determining that the power distribution system line has a three-phase fault, otherwise; Determining that the power distribution system line has an asymmetric short-circuit fault; Calculating the d-axis current and q-axis current at the directional element in the rotating coordinate system by using the Park transformation, and extracting the direct current component, fundamental frequency component and second harmonic component in the d-axis current by using the continuous variable mode decomposition method; Under the three-phase fault, if the d-axis current increment is greater than the set current increment threshold value and no fundamental frequency component exists, determining that the downstream of the directional element of the power distribution system line has a forward fault, otherwise, determining that the upstream of the directional element of the power distribution system line has a reverse fault; Under the asymmetric fault, if the d-axis current has a double-frequency component and the double-frequency current energy is greater than the set current energy threshold value, determining that the downstream of the directional element of the power distribution system line has a forward fault, otherwise, determining that the upstream of the directional element of the power distribution system line has a reverse fault.

2. The method of claim 1, wherein, The periodically collecting the current data of the local end of the power distribution system line by using the current sensor comprises: setting a plurality of sampling points at the local end of the power distribution system line, and periodically collecting the current value of each sampling point by using the current sensor.

3. The method according to claim 1 or 2, characterized in that, The calculating the phase current increment according to the collected current data, and determining that the power distribution system line has a fault if the phase current increment is greater than the set phase current increment threshold value, and marking the fault time comprises: The calculating the phase current increment according to the collected current data comprises: where K is the current increment, K set is the maximum increment value of the last sampling period, i j (k) , i j (k-1) respectively represent the current value of the jth sampling point in the kth sampling period and the k-1th sampling period, and N is the calculation point number, which increases with time. If the phase current increment is greater than the pre-set phase current increment threshold value, determining that the power distribution system line has a fault, marking the fault time, otherwise, returning and continuing to periodically collect the current value of each sampling point by using the current sensor.

4. The method of claim 3, wherein, If the three-phase current increments of the power distribution system line are roughly the same, determining that the power distribution system line has a three-phase fault, otherwise; Determining that the power distribution system line has an asymmetric short-circuit fault comprises: When determining that the power distribution system line has a fault, the current increment correlation calculation method comprises: p(K A , K B , K C )- set where p(·) is a correlation representation, p set is a correlation coefficient threshold, subscripts A, B, C represent current phase of the power distribution system line, COV represents covariance, and σ represents standard deviation; K A , K B , and K C are three-phase current increments of A, B, and C, respectively; p(K A , K B , and K C ) represents a correlation coefficient of any two of the three-phase current increments of A, B, and C; p AB is a correlation coefficient of the current increments of A and B; if the correlation representation p(K A , K B , and K C ) between K A , K B , and K C is greater than a set correlation coefficient threshold p set , it is considered that a three-phase fault occurs, otherwise, it is considered that an asymmetric short-circuit fault occurs.

5. The method of claim 4, wherein, The calculating the d-axis current and q-axis current at the protection in the rotating coordinate system by using the Park transformation, and extracting the direct current component, fundamental frequency component and second harmonic component in the d-axis current by using the continuous variable mode decomposition method comprises: The calculating the d-axis current and q-axis current at the directional element in the rotating coordinate system by using the Park transformation, and the calculation formula of the Park transformation comprises: wherein θ represents the A-phase current rotational angular velocity, a, b, c respectively represent A, B, C phases, i d , i0 respectively represent d-axis, q-axis and zero-sequence currents, i q , i0 respectively represent d-axis, q-axis and zero-sequence currents, i a , i0 respectively represent d-axis, q-axis and zero-sequence currents, i b , i0 respectively represent d-axis, q-axis and zero-sequence currents, i c respectively represent A, B, C three-phase currents; The extracting the direct current component, fundamental frequency component and second harmonic component in the d-axis current by using the continuous variable mode decomposition method comprises: wherein is an input d-axis current signal i d is a frequency domain representation of (t), a is a weighting factor, and ω k is a center frequency of the kth submode, is a frequency domain representation of the kth submode at the n+1th iteration, is a frequency domain representation of the Lagrange multiplier at the n th iteration, and ω i is a center frequency of the extracted submode; For the initially decomposed frequency signal, modal components of different frequencies are obtained by k times of decomposition The time-frequency components containing each frequency are obtained according to the center frequency ω of the modal component k The modal component with the center frequency close to 0 Hz is regarded as a direct current component, the modal component with the center frequency close to 50 Hz is regarded as a fundamental frequency component, and the modal component with the center frequency close to 100 Hz is regarded as a second harmonic component, the frequency error interval allowed is ± 5 Hz, when the center frequency is different from the required frequency by more than the interval, it is considered that the frequency component does not exist, for example, if a component of 43 Hz is obtained, it is not considered that the second harmonic component exists.

6. The method of claim 5, wherein, Under the three-phase fault, if the d-axis current increment is greater than the set current increment threshold value and no fundamental frequency component exists, determining that the downstream of the directional element of the power distribution system line has a forward fault, otherwise, determining that the upstream of the directional element of the power distribution system line has a reverse fault; Under asymmetric fault, if the d-axis current has a double frequency component and the double frequency current energy is greater than a set current energy threshold, it is determined that a forward fault occurs downstream of the directional element of the distribution system line; On the contrary, it is determined that a reverse fault occurs upstream of the directional element of the distribution system line, comprising: Selecting the d-axis current direct current and harmonic increments of 0 Hz, 50 Hz and 100 Hz; Under three-phase fault, the fault is determined by using the d-axis current increment greater than the threshold value and the fundamental frequency component, and the calculation method is: ΔI dc = I dcf - I dcp ΔI 50 = I 50f - I 50p ΔI 50 < ΔI 50set & ΔI dc > ΔI dcset (1) where ΔI dc and ΔI 50 are the d-axis direct current increment and 50 Hz component increment, I dcf and I dcp denote the direct current before and after the fault, I 50f and I 50p denote the fundamental frequency component before and after the fault, and the subscript set denotes the threshold value; If the d-axis current increment is greater than the threshold value and no fundamental frequency component exists, i.e., the above formula (1) is established, it is determined that a forward fault occurs downstream of the directional element of the distribution system line; on the contrary, it is determined that a reverse fault occurs upstream of the directional element of the distribution system line; Under asymmetric fault, the fault direction is determined by using the d-axis 100 Hz current energy greater than the threshold value, and the calculation method is E 100 > E set (3) wherein E 100 represents the d-axis 100 Hz current energy, N represents the total number of points calculated within the time window, j represents the jth point, i 100 represents the 100 Hz current component, E set represents the current energy threshold value; If the d-axis current has a double frequency component and the double frequency current energy is greater than the threshold value, i.e., the above formula (2) and formula (3) are established, it is determined that a forward fault occurs downstream of the directional element of the distribution system line; on the contrary, it is determined that a reverse fault occurs upstream of the directional element of the distribution system line.

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