Fault discrimination phase transient state quantity selection method suitable for phase-to-phase fault direction identification

Abstract

The invention belongs to the technical field of power distribution network phase-to-phase fault direction identification, and particularly relates to a fault discrimination phase transient state quantity selection method suitable for phase-to-phase fault direction identification. The method comprises the following steps: S1, measuring the voltage and current at a protection position, and if the voltage and current meet duset / dtgt; [delta] 1 or diset / dtgt; delta 2, protection starting; s2, three-phase fault voltage and current at a protection position are collected, and a three-phase current detail coefficient is calculated; and S3, calculating the modulus maximum of the detail coefficient of each phase of current, obtaining the modulus maximum of the detail coefficient of the three-phase current in the data window, determining a fault discrimination phase, and identifying a fault by using the sampling data of the fault discrimination phase. The method provided by the invention is easy to implement, can reliably judge the fault direction under the condition of interphase short circuit, does not depend on power supply characteristics, only needs single-end electrical quantity, is not limited by communication channels, and has good generalization performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of phase-to-phase fault direction identification in distribution networks, and in particular relates to a method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification. Background Art

[0002] Driven by the "dual carbon" goal, the new distribution network will gradually shift from being dominated by synchronous power sources to being dominated by new energy sources, and the high proportion of distributed photovoltaic access is bound to become one of the main characteristics of the distribution network. However, unlike synchronous power sources, the fault response characteristics of power electronic equipment such as distributed photovoltaics are highly nonlinear, especially with the continuous increase in access proportion and capacity, which makes the nonlinear characteristics of distribution network failures more significant. In addition, the high proportion of distributed access of photovoltaic power sources makes the distribution network present new characteristics of multiple points and wide areas. Therefore, the high proportion of distributed photovoltaic access brings challenges to the safe operation and reliable power supply of the distribution network. The traditional protection principle cannot be fully applied to the new distribution network with a high proportion of new energy access, and it is very easy to have the risk of reduced sensitivity or even false operation.

[0003] As the capacity and proportion of distributed photovoltaic access to the distribution network continues to increase, the ability of downstream photovoltaics to supply fault current continues to increase when protecting against reverse faults, resulting in reduced sensitivity of line current protection or even false operation. The introduction of direction criteria can effectively solve the problem of reverse false operation of distribution line current protection after a high proportion of distributed photovoltaic access. The polarity characteristic is defined as the sign characteristic corresponding to the voltage / current extreme value after the fault. When the first extreme value of the extracted voltage / current is positive, it means that the electrical quantity has a positive polarity characteristic; when the first extreme value of the extracted voltage / current is negative, it means that the electrical quantity has a negative polarity characteristic. Analysis shows that when the frequency is greater than the critical frequency f shown in the following formula c When the photovoltaic impedance is inductive, when the protection is in the positive direction, the polarities of the voltage and current first extremes at the protection position are opposite; when the protection is in the reverse direction, the polarities of the voltage and current first extremes are the same. Therefore, the polarity characteristics of the first extremes of the voltage and current high-frequency transient quantities can be used to reliably identify the fault direction of the AC distribution network connected to distributed photovoltaics. It should be noted that, as shown in the following formula, the critical frequency f c It is related to the capacitance and inductance parameters of the photovoltaic inverter. Therefore, in the scenario where multiple distributed photovoltaic inverters are connected, the critical frequency f of different photovoltaics can be calculated based on various photovoltaic parameters. c1 、f c2 , ..., f cn , ensuring that the frequency of the extracted voltage and current transient quantities is greater than the maximum critical frequency of each photovoltaic, can ensure that the proposed directional protection criterion is not affected by factors such as photovoltaic capacity.

[0004] Where, L1 is the inductance of the filter on the inverter side; L2 is the inductance of the filter on the grid side; C f is the filter capacitor; C d is the DC side capacitor.

[0005] However, when a phase-to-phase short circuit occurs in the distribution network, the maximum and first extreme values ​​of the transient voltage and current of the fault phase are different, and the accuracy of the protection measurement device is greatly limited. The selection of the fault discrimination phase will affect the reliability and sensitivity of the protection. Therefore, in view of this situation, it is necessary to study the selection method of the transient quantity of the fault discrimination phase suitable for phase-to-phase fault direction identification. Summary of the invention

[0006] In view of this, the present invention provides a method for selecting a fault discrimination phase transient quantity suitable for identifying the direction of a phase-to-phase fault.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: A method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification comprises the following steps: S1. When a three-phase short circuit occurs in the distribution network, measure the voltage and current at the protection point of the relay protection device. When |d u_set / dt|>Δ1or|d i_set / dt|>Δ2, the protection of the relay protection device is activated, Δ1 represents the protection activation threshold value based on the voltage change rate modulus, Δ2 represents the protection activation threshold value based on the current change rate modulus, and t represents time; S2. After the protection is started, the three-phase fault voltage u=[u a u b u c ] and three-phase fault current i=[i a i b i c ], calculate the three-phase current detail coefficient d 1_A (k), d 1_B (k), d 1_C (k); S3. Use the ergodic method to calculate the maximum modulus of the three-phase current detail coefficients. The maximum modulus of the three-phase current detail coefficients in the data window are D 1_A , D 1_B , D 1_C , determine the fault discrimination phase, and use the fault discrimination phase to identify the fault.

[0008] Furthermore, in step S1, a voltage detail coefficient threshold value d is set. u_set , current detail coefficient threshold d i_set; The point where the voltage transient amplitude is greater than the voltage detail coefficient threshold value is determined as the starting point for obtaining the first extreme value of the voltage, and the point where the current transient amplitude is greater than the current detail coefficient threshold value is determined as the starting point for obtaining the first extreme value of the current, so as to ensure that the first extreme value of the fault transient voltage and current is extracted to avoid the influence of harmonics during normal operation.

[0009] Furthermore, in step S2, the three-phase current detail coefficient is calculated based on the wavelet algorithm, and the formula is: , Where, d 1_A (k), d 1_B (k), d 1_C (k) are the kth detail coefficients of the three-phase currents A, B, and C respectively; I A (n), I B (n), I C (n) are the measured sampling values ​​of the three-phase currents A, B, and C respectively; n is the number of sampling points in the protection data window.

[0010] Furthermore, in step S3, the modulus maximum value of the three-phase current detail coefficient is calculated as follows: .

[0011] Furthermore, in step S3, the method for determining the fault discrimination phase is: When D 1_A ≥D 1_B And D 1_A ≥D 1_C , then within the protection time window, the modulus maximum value of the transient current of phase A is the largest, and phase A is taken as the fault judgment phase; When D 1_B ≥D 1_A And D 1_B ≥D 1_C , then within the protection time window, the modulus maximum value of the transient current of phase B is the largest, and phase B is used as the fault judgment phase; When D 1_C ≥D 1_A And D 1_C ≥D 1_B , then within the protection time window, the modulus maximum value of the transient current of phase C is the largest, and phase C is used as the fault judgment phase.

[0012] Furthermore, in step S3, the specific steps of using the fault identification phase to identify the fault include: S31, extracting the voltage transient quantity and current transient quantity of the fault judgment phase, and obtaining the transient current first extreme value I of the fault judgment phase required for the transient quantity direction judgment Tran_IE and the first extreme value of transient voltage U Tran_IE ; S32. Fault identification is performed using a directional protection criterion based on transient voltage and current polarity comparison.

[0013] Further, in step S32, according to the transient current first extreme value I Tran_IE and the first extreme value of transient voltage U Tran_IE , and the criterion for fault identification based on the directional protection criterion based on the comparison of transient voltage and transient current polarity is: .

[0014] For AC distribution networks with a high proportion of new energy and energy storage connected, the transient voltage and current are reverse polarity during positive direction faults, while the transient voltage and current are the same polarity during reverse direction faults, so that reliable identification of phase-to-phase short-circuit faults can be achieved. However, when a three-phase short-circuit fault occurs in the distribution network, the modulus maximum and first extreme values ​​of the transient voltage and current of the three phases are different, and the accuracy of the protection measurement device is greatly limited. The selection of the fault determination phase will affect the reliability and sensitivity of the protection. Therefore, this patent proposes a fault determination phase selection method suitable for the identification of the direction of transient phase-to-phase faults. The transient voltage and current polarity characteristics of the fault determination phase are used to identify faults, which can improve the reliability and sensitivity of the protection action.

[0015] The beneficial effects of the present invention are: As the scale of new energy power sources and energy storage connected to the distribution network gradually increases, the structure of the traditional distribution network has changed from a simple radial shape to a multi-terminal network with changeable current flow direction. Therefore, the traditional three-stage current protection is no longer applicable. Therefore, the present invention proposes a directional protection criterion based on the transient polarity of voltage and current. The proposed method has a simple principle and is easy to implement. It can reliably determine the fault direction in the case of phase-to-phase short circuit. The proposed method does not depend on the power supply characteristics, only requires single-ended electrical quantities, and is not limited by communication channels, and has good generalizability.

[0016] In addition, the transient quantity selection method described in the present application measures the voltage and current at the protection point and sets a threshold value to determine whether the fault initiation conditions are met. It can detect faults in time and start the protection mechanism, improve the sensitivity and accuracy of fault detection, and avoid the occurrence of false operations and missed operations; by collecting the three-phase fault voltage and current at the protection point and calculating the three-phase current detail coefficient, the fault judgment phase can be determined, and the location and severity of the fault can be further determined, which is very important for achieving fault location and isolation, and can help quickly restore power supply to non-fault areas and reduce power outage time and losses; by calculating the maximum value of the modulus of the detail coefficient of each phase current and determining the fault judgment phase, the nature and type of the fault can be further analyzed, which can help select appropriate protection strategies and repair methods, and improve the efficiency and reliability of fault handling; by using the transient voltage and current polarity characteristics of the fault judgment phase to identify faults, fault diagnosis and early warning functions are provided, helping to take preventive measures in time and reduce fault risks and losses. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of AC distribution network connected to photovoltaic power generation.

[0018] Figure 2 This is an example diagram of the simulation verification of the impact of fault phase selection. DETAILED DESCRIPTION

[0019] In order to better understand the present invention, the content of the present invention is further clearly described below in conjunction with the embodiments and the accompanying drawings, but the protection content of the present invention is not limited to the following embodiments. In the following description, a large number of specific details are provided to provide a more thorough understanding of the present invention. However, it is obvious to those skilled in the art that the present invention can be implemented without one or more of these details.

[0020] In the present application, the first extreme value of the fault transient voltage and current refers to the first peak value of the voltage and current waveform captured in the transient process after a fault occurs in the distribution network, that is, the first extreme value.

[0021] The purpose of this embodiment is to provide a method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification, including the following steps S1-S5.

[0022] S1. When a three-phase short circuit occurs in the distribution network, measure the voltage and current at the protection point of the relay protection device. If |d u_set / dt|>Δ1or|d i_set / dt|>Δ2 , then the protection of the relay protection device is started, Δ1 represents the protection starting threshold value based on the voltage change rate modulus, Δ2 represents the protection starting threshold value based on the current change rate modulus, and t represents time.

[0023] S2. After the protection is started, the three-phase fault voltage u=[u a u b u c ] and three-phase fault current i=[i a i b i c ], calculate the three-phase current detail coefficient d 1_A (k), d 1_B(k)、d1_C (k).

[0024] S3. Use the traversal method to calculate the maximum modulus of the current detail coefficient of each phase, and the maximum modulus of the three-phase current detail coefficient in the data window are D 1_A , D 1_B、D1_C , determine the fault discrimination phase, and use the fault discrimination phase to identify the fault.

[0025] It should be noted that in step S1, when extracting the fault transient voltage and current first extreme value, the voltage and current detail coefficient threshold value d is set. u_set d i_set， The point where the voltage transient amplitude is greater than the voltage detail coefficient threshold value is determined as the starting point for obtaining the first extreme value of the voltage, and the point where the current transient amplitude is greater than the current detail coefficient threshold value is determined as the starting point for obtaining the first extreme value of the current, so as to ensure that the first extreme value of the fault transient voltage and current is extracted without the influence of harmonics during normal operation. In step S2, the formula for calculating the three-phase current detail coefficient based on the wavelet algorithm is: , Where, d 1_A (k), d 1_B (k), d 1_C (k) are the kth detail coefficients of the three-phase currents A, B, and C respectively; I A (n), I B (n), I C (n) are the measured sampling values ​​of the three-phase currents A, B, and C respectively; n is the number of sampling points in the protection data window.

[0026] In step S3, the maximum value of the three-phase current detail coefficient is calculated as follows: .

[0027] In order to improve the reliability of the proposed directional protection criterion, the present application selects the phase with the largest transient current modulus maximum value as the fault determination phase to identify the fault direction. Specifically, in step S3, the method for determining the fault determination phase is: If D 1_A ≥D 1_B且D1_A ≥D 1_C， Then, within the protection time window, the modulus maximum value of the transient current of phase A is the largest, and phase A is used as the fault judgment phase; If D 1_B ≥D 1_A且D1_B ≥D 1_C， Then, within the protection time window, the modulus maximum value of the transient current of phase B is the largest, and phase B is used as the fault judgment phase; If D 1_C ≥D 1_A且D1_C ≥D 1_B， Then, within the protection time window, the modulus maximum value of the transient current of phase C is the largest, and phase C is used as the fault judgment phase. In step S3, the specific steps of using the fault identification phase to identify the fault include: S31, extracting the voltage transient quantity and current transient quantity of the fault judgment phase, and obtaining the transient current first extreme value I of the fault judgment phase required for the transient quantity direction judgment Tran_IE and the first extreme value of transient voltage U Tran_IE ; S32. Fault identification is performed using a directional protection criterion based on comparison of transient voltage and transient current polarities.

[0028] In step S5, according to the transient current first extreme value I Tran_IE and the first extreme value of transient voltage U Tran_IE , and the fault identification criterion based on the comparison of transient voltage and transient current polarity is .

[0029] This embodiment uses Figure 1 Taking the protection point R1 as an example, when a two-phase short circuit fault occurs in the opposite direction at the protection point, its voltage and current are as follows Figure 2 As shown in (a)-(b).

[0030] The wavelet algorithm is used to extract the high-frequency components of the three-phase voltage and three-phase current, and the transient voltage and current are obtained as follows: Figure 2 As shown in (c)-(e). From the simulation results, it can be seen that the first extreme values ​​of the transient current of phases A, B, and C measured at the protection are 0.017kA, -0.015kA, and -0.002kA, respectively, and the first extreme values ​​of the transient voltage are 0.585kV, -0.464kV, and -0.107kV, respectively. The three-phase transient voltage and current are quite different, and the minimum amplitude of the transient current is smaller than the minimum amplitude of the transient voltage. Among them, the first extreme value of the transient current of phase C is only -0.002kA. When this value does not meet the measurement accuracy requirements of the current transformer, the protection will mistakenly extract the second extreme point 0.010kA as the first extreme value, which will affect the correct identification of the directional protection criterion, such as Figure 2 (e) as shown.

[0031] In order to improve the reliability of the proposed directional protection criterion, this embodiment selects the phase with the largest transient current modulus maximum value as the fault judgment phase to identify the fault direction. Figure 2 (f) It can be obtained that the maximum values ​​of the transient current modulus of the three phases are 0.038kA, 0.027kA and 0.017kA respectively, and the maximum value of the transient current modulus of phase A is the largest within the protection time window. Therefore, for protection R1, the proposed directional protection scheme selects the voltage and current sampling values ​​of phase A to identify the fault direction.

[0032] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in the field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for selecting a transient state quantity for fault discrimination suitable for phase-to-phase fault direction identification, characterized in that: The following steps are involved: S1. When a three-phase short circuit occurs in the distribution network, measure the voltage and current at the protection point of the relay protection device. When |d u_set / dt|>Δ1or|d i_set / dt|>Δ2, the protection of the relay protection device is activated, Δ1 represents the protection activation threshold value based on the voltage change rate modulus, Δ2 represents the protection activation threshold value based on the current change rate modulus, and t represents time; S2. After the protection is started, the three-phase fault voltage u=[u a u b u c ] and three-phase fault current i=[i a i b i c ], calculate the three-phase current detail coefficient d 1_A (k), d 1_B (k), d 1_C (k); S3. Use the ergodic method to calculate the maximum modulus of the three-phase current detail coefficients. The maximum modulus of the three-phase current detail coefficients in the data window are D 1_A , D 1_B , D 1_C , determine the fault discrimination phase, and use the fault discrimination phase to identify the fault.

2. A method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification according to claim 1, characterized in that: In step S1, the voltage detail coefficient threshold value d is set u_set , current detail coefficient threshold d i_set ; The point where the voltage transient amplitude is greater than the voltage detail coefficient threshold value is determined as the starting point for obtaining the first extreme value of the voltage, and the point where the current transient amplitude is greater than the current detail coefficient threshold value is determined as the starting point for obtaining the first extreme value of the current, so as to ensure that the first extreme value of the fault transient voltage and current is extracted to avoid the influence of harmonics during normal operation.

3. A method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification according to claim 2, characterized in that: In step S2, the three-phase current detail coefficient is calculated based on the wavelet algorithm, and the formula is: ， Where, d 1_A (k), d 1_B (k), d 1_C (k) are the kth detail coefficients of the three-phase currents A, B, and C respectively; I A (n), I B (n), I C (n) are the measured sampling values ​​of the three-phase currents A, B, and C respectively; n is the number of sampling points in the protection data window.

4. A method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification according to claim 3, characterized in that: In step S3, the method for calculating the modulus maximum value of the three-phase current detail coefficient is: 。 5. A method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification according to claim 4, characterized in that: In step S3, the method for determining the fault discrimination phase is: When D 1_A ≥D 1_B And D 1_A ≥D 1_C , then within the protection time window, the modulus maximum value of the transient current of phase A is the largest, and phase A is taken as the fault judgment phase; When D 1_B ≥D 1_A And D 1_B ≥D 1_C , then within the protection time window, the modulus maximum value of the transient current of phase B is the largest, and phase B is used as the fault judgment phase; When D 1_C ≥D 1_A And D 1_C ≥D 1_B , then within the protection time window, the modulus maximum value of the transient current of phase C is the largest, and phase C is used as the fault judgment phase.

6. A method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification according to claim 5, characterized in that: In step S3, the specific steps of using the fault identification phase to identify the fault include: S31, extracting the voltage transient quantity and current transient quantity of the fault judgment phase, and obtaining the transient current first extreme value I of the fault judgment phase required for the transient quantity direction judgment Tran_IE and the first extreme value of transient voltage U Tran_IE ; S32. Fault identification is performed using a directional protection criterion based on transient voltage and current polarity comparison.

7. A method for selecting a fault discrimination phase transient quantity suitable for phase-to-phase fault direction identification according to claim 6, characterized in that: In step S32, according to the transient current first extreme value I Tran_IE and the first extreme value of transient voltage U Tran_IE , and the criterion for fault identification based on the directional protection criterion based on the comparison of transient voltage and transient current polarity is: 。

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