Power distribution network fault direction discrimination method based on current distortion rate

Abstract

The invention belongs to the technical field of fault discrimination of a power distribution network containing distributed energy, and particularly relates to a power distribution network fault direction discrimination method based on a current distortion rate, and the method comprises the steps: collecting a three-phase current at a protection part of an upstream line of a distributed power supply; derivation is carried out on the sampled three-phase current to amplify waveform distortion characteristics, and the total harmonic distortion rate of the current after derivation is calculated; and judging the direction of the fault according to the current harmonic total distortion rate. The fault direction can be accurately identified, and protection maloperation caused by reverse current is avoided.

Description

Technical Field

[0001] This application belongs to the field of fault identification technology for distribution networks containing distributed energy resources, and particularly relates to a method for fault direction identification in distribution networks based on current distortion rate. Background Technology

[0002] With the large-scale integration of distributed power sources such as photovoltaics and wind power, the traditional single-source radial structure that distribution networks have long relied on has been completely disrupted. The original unidirectional power flow distribution has transformed into a complex network supported by multiple power sources. This structural change directly leads to significant variations in the magnitude, direction, and duration of fault currents, causing frequent maloperation and failure to operate in traditional overcurrent protection systems, seriously threatening the safe and stable operation of the distribution network. The integration of distributed power sources significantly alters the fault characteristics of the distribution network; therefore, the integration of distributed power sources will have a certain impact on the directional elements of protection in the distribution network. In traditional relay protection of distribution networks, the direction of short-circuit current is determined by the current distortion rate, based on the characteristic changes of the current waveform during a fault and the output differences of different power sources. Under normal operation, the current waveform of the distribution network is close to a sine wave with a regular shape; however, after a fault occurs, the current waveform will be distorted, and the distortion characteristics are closely related to the fault direction. Summary of the Invention

[0003] The technical problem this application aims to solve is to provide a method for determining the direction of faults in distribution networks based on current distortion rate. This method distinguishes the fault direction by extracting the total harmonic distortion rate of the fault current: the fault current waveform on the system power supply side is relatively smooth with a low distortion rate; while on the distributed power supply side, due to the nonlinear control of the converter, the fault current has a high harmonic content and a significantly higher distortion rate. By setting a dynamic distortion rate threshold and combining it with overcurrent protection criteria to form a dual-blocking logic of "direction determination + overcurrent detection," the fault direction can be accurately identified, avoiding maloperation of protection caused by reverse current.

[0004] This application is implemented as follows: A method for determining the direction of faults in a distribution network based on current distortion rate includes: Three-phase current is collected at the upstream line protection point of the distributed power source; The distortion characteristics of the amplified waveform after sampling of the three-phase current are derived, and the total harmonic distortion rate of the current after the derivative is calculated. The direction of the fault is determined by the total harmonic distortion rate of the current.

[0005] Furthermore, the three-phase current is the three-phase fault current collected after a fault is detected by the current protection element at the line protection point.

[0006] Furthermore, the distortion characteristics of the amplified waveform after differentiating the sampled three-phase current include: Calculate the first derivative of the current in each phase; The amplitudes of the fundamental wave and each harmonic are calculated using the Fast Fourier Transform method. Calculate the total harmonic distortion rate of the current based on the amplitude of each harmonic.

[0007] Furthermore, the direction of the fault is determined based on the total harmonic distortion of the current, including: When the total harmonic distortion rate of one phase current exceeds the current harmonic distortion rate threshold, the fault is determined to be located in the opposite direction of the protection. When the total harmonic distortion rate of any phase current after differentiation of the three-phase fault current is detected to be below the current harmonic distortion rate threshold, the fault is located in the positive direction of the protection.

[0008] Furthermore, the current harmonic distortion rate threshold is the current waveform distortion rate threshold measured at the line protection point when a three-phase short-circuit fault is applied at the end of the line.

[0009] Furthermore, the formula for calculating the total harmonic distortion rate of the current is: , The total harmonic distortion of the current; The amplitude of the nth current harmonic is obtained by fast Fourier transform (FFT). This represents the amplitude of the fundamental current.

[0010] Furthermore, when the fault is in the forward direction and the fault current exceeds the overcurrent protection threshold, the AND gate output of the overcurrent protection element at the protection point is 1, and the circuit enters the time-delay tripping stage; if the fault is in the reverse direction, or the fault current does not exceed the overcurrent protection threshold, the AND gate output is 0, and the protection is blocked.

[0011] Compared with existing technologies, this application offers the following advantages: the short-circuit current of the system power supply contains multiple sequence components, including positive and negative sequences. By performing mathematical calculations on the fault current, the differences between waveforms can be further amplified, thereby calculating the current distortion rate. This parameter reflects the degree to which the current waveform deviates from a sine wave. During a forward fault, the current is mainly dominated by one side of the power supply, and its distortion rate will exhibit a specific pattern and may exceed a preset threshold. During a reverse fault, the current source and path change, and the distortion rate characteristics will also change accordingly. By detecting the current distortion rate in real time and comparing it with a preset threshold, the relay protection device can determine the direction of the short-circuit current, thus providing a crucial basis for whether the protection device should operate. Attached Figure Description

[0012] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying 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. Wherein: Figure 1 This is a flowchart of a method for determining the direction of a distribution network fault based on current distortion rate, provided in an embodiment of this application. Figure 2 This is an equivalent model diagram of power distribution including distributed power sources provided in the embodiments of this application; Figure 3 This is a schematic diagram illustrating the coordination principle between the direction criterion and current protection provided in the embodiments of this application; Figure 4 This is a diagram of the f1 fault simulation model provided in the embodiments of this application; Figure 5 This is a simulation model diagram of the f2 fault provided in the embodiments of this application. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0014] like Figure 1 As shown. A method for determining the direction of a distribution network fault based on current distortion rate, comprising: The upstream line protection point of the distributed power source collects three-phase current; the three-phase current is the three-phase fault current collected after a fault occurs, detected by the current protection element at the line protection point.

[0015] The distortion characteristics of the amplified waveform after sampling of the three-phase current are derived, and the total harmonic distortion rate of the current after the derivative is calculated. The direction of the fault is determined by the total harmonic distortion rate of the current.

[0016] This application is referred to Figure 2 As shown, assuming a three-phase short-circuit fault occurs on the upstream line of the photovoltaic grid connection point, taking phase A as an example, the instantaneous expression of the fault current on the system side in the stationary coordinate system can be expressed as: (1), In the formula: Fundamental component amplitude; for Initial phase of phase current; and This is the DC component; Let be the decay time constant. The fault current expression is denoted as: (2), In the formula: , , These represent the amplitude of the fundamental component, the initial value of the DC component, and the amplitude of the harmonic component of the fault current of the distributed power source, respectively. For the first Second harmonic frequency; , The decay time constant; , The initial phase angles of each wave component are given.

[0017] Before calculating the total harmonic distortion rate of the current, a spectrum analysis is performed using Fourier transform. The aim is to minimize the impact of the attenuated DC component without reducing the distortion characteristics of the fault current.

[0018] Taking the first derivative of the fault current, we obtain the following formulas, focusing mainly on the relative magnitudes of the fundamental component and other components.

[0019] (3), (4), The system-side fault current waveform is smooth with low distortion. As shown in the equation, after differentiation, the amplitude of the fundamental component increases. The DC component amplitude increases by a factor of two. Times. Decay time constant This is related to the system impedance parameters and typically lasts for tens of milliseconds. Therefore Much smaller than in general The content of the attenuated DC component is significantly reduced compared to the fundamental frequency component, meaning the component affecting the Fourier transform is weakened. Therefore, the main component of the fault current on the system side after differentiation is still a standard sine wave, and the waveform distortion rate remains at a low level.

[0020] The fault current waveform on the distributed power source side exhibits significant distortion during the transient process lasting tens of milliseconds after the fault occurs. After differentiation, besides the increase in the amplitude of the fundamental component... In addition to the multiple increase, the amplitudes of the second, third, and other higher harmonics also increase accordingly. Times. Due to After differentiating the fault current, the nonlinear characteristics become more prominent, and the harmonic content increases significantly.

[0021] Based on the existing three-stage overcurrent protection configuration, the circuit breaker is installed at the upper end of the line. On this basis, a direction discrimination element is added at the upstream protection of the distributed power source to implement the method of this application. When a fault is detected, the three-phase fault current is collected. The first derivative of each phase current is calculated according to equation (5). .

[0022] , In the formula: These are discrete sampling times; The sampling interval; for Current sampling value at any given time.

[0023] After differentiation, the Fast Fourier Transform method is used to calculate... The amplitude of each harmonic is determined, and the total harmonic distortion (THD) of the current is calculated. The formula for calculating the total harmonic distortion of the current is: , The total harmonic distortion of the current; The amplitude of the nth current harmonic is obtained from the Fast Fourier Transform calculation. This represents the amplitude of the fundamental current.

[0024] In the formula: This is the calculated value of the waveform distortion rate; The amplitude of the nth current harmonic is calculated using the Fast Fourier Transform (FFT). To fully consider the nonlinear fault characteristics of photovoltaics and improve the sensitivity of the criterion, harmonics below 1 kHz are included in the calculation range of waveform distortion rate.

[0025] When the protection detects that the waveform distortion rate of the fault current derivative of one phase exceeds the overcurrent protection threshold... hour, The fault is determined to be located in the opposite direction of the protection; when the waveform distortion rate of the derivative of any one phase current in the three-phase fault current detected by the protection is below the threshold... If the fault is located in the positive direction of the protection, then the fault is located in the positive direction of the protection.

[0026] The current harmonic distortion rate threshold is selected by avoiding the maximum fault current waveform distortion rate. A three-phase short-circuit fault is applied at the end of the line, and the protection current distortion rate threshold is measured at the line protection point.

[0027] Combine the distortion rate criterion with the original current protection criterion, such as Figure 3As shown, when the fault is in the positive direction of the protection and the fault current exceeds the overcurrent protection threshold, the AND gate output of the overcurrent protection element at the protection point is 1, and the circuit enters the time-delay tripping stage; if the fault is in the opposite direction of the protection, or the fault current does not exceed the overcurrent protection threshold, the AND gate output of the overcurrent protection element at the protection point is 0, and the protection is locked.

[0028] To further verify the effectiveness of this application, a specific example is used to verify the effect: (1) Power supply parameters: The base voltage of the distribution network system is 10.5kV, the base capacity is 40MVA, and the system-side impedance is j0.45Ω.

[0029] (2) Line parameters, see Figure 2 As shown, The distribution network lines AB, BC, CD, DE, AF, FG, and GH are all overhead lines with lengths of 10km, 12km, 14km, 8km, 6km, 8km, and 7km respectively. Their unit resistance and impedance are r=0.069Ω / km and x=0.099Ω / km. The loads connected to the ends of busbars B, C, F, G, and H are 3MW, 4MW, 3MW, 3MW, and 2MW respectively, with a power factor of 1 for all of them. DG is connected to busbar C and has a capacity of 3MW.

[0030] (3) Threshold selection: Three-phase short circuits were applied at the ends of lines AB, BC, CD, and DE respectively. The current distortion rate threshold data measured at protection points 1, 2, 3, and 4 are recorded in Table 1.

[0031] Table 1. Distortion rate thresholds for each protection current: Protected location Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Protection 1 5.54 8.76 7.39 Protection 2 6.25 7.33 9.46 Protection 3 9.11 8.49 7.66 Protection 4 6.79 5.51 9.88 1) A fault occurred at f1: Faults ABC and AB are applied at point f1 respectively. The fault simulation model diagram is shown below. Figure 4 As shown, the data for protection 1, 2, 3, and 4 are recorded as shown in Tables 2 and 3.

[0032] Table 2. Protection direction discrimination under different fault types: Fault type Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Direction judgment Overcurrent Criterion Protective actions ABC 5.11 5.44 6.28 just √ √ AB 4.19 3.28 0.94 just √ √ Table 3. Protection direction discrimination under different fault types: Fault type Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Direction judgment Overcurrent Criterion Protective actions ABC 70.82 81.26 79.35 opposite √ × AB 66.18 57.30 20.14 opposite √ × Table 4. Protection 3-direction discrimination under different fault types: Fault type Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Direction judgment Overcurrent Criterion Protective actions ABC 1.74 1.75 3.25 just × × AB 1.74 1.76 3.23 just × × Table 5. Protection 4-direction discrimination under different fault types: Fault type Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Direction judgment Overcurrent Criterion Protective actions ABC 1.74 1.76 3.24 just × × AB 1.74 1.73 3.24 just × × As can be seen from the data in Tables 1-5, when a short circuit fault occurs at point f1, the current distortion rate of protection 1 and protection 2 changes significantly. When the power flow reverses in the protection 2 line, the direction of the short circuit current can be accurately determined according to the direction criterion proposed in this application, thus blocking the protection and avoiding maloperation of the protection, ensuring the safe and stable operation of the power grid.

[0033] 2) A fault occurred at f2: Faults ABC and AB are applied at point f2 respectively. The fault simulation model diagram is shown below. Figure 5 As shown, the data for protection 1, 2, 3, and 4 are recorded as shown in Tables 6 to 9.

[0034] Table 6. Protection 1 Direction Judgment under Different Fault Types: Fault type Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Direction judgment Overcurrent Criterion Protective actions ABC 4.15 3.12 6.37 just × × AB 1.23 1.97 2.09 just × × Table 7. Protection direction discrimination under different fault types: Fault type Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Direction judgment Overcurrent Criterion Protective actions ABC 50.98 49.09 48.88 opposite √ × AB 38.77 39.09 10.14 opposite √ × Table 8. Protection 3-direction discrimination under different fault types: Fault type Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Direction judgment Overcurrent Criterion Protective actions ABC 5.21 8.02 4.19 just √ √ AB 4.68 5.24 6.11 just √ √ Table 9. Protection 4-direction discrimination under different fault types: Fault type Phase A distortion rate / % Phase B distortion rate / % C-phase distortion rate / % Direction judgment Overcurrent Criterion Protective actions ABC 1.28 0.73 0.28 just × × AB 0.88 0.76 0.17 just × × As can be seen from the data in Tables 6 to 9, when a short circuit fault occurs at point f2, and the power flow of protection line 2 is reversed, if the direction of the fault current cannot be accurately determined, protection line 2 will malfunction. According to the direction criterion proposed in this application, the direction of the short circuit current can be accurately determined, and the protection can be blocked, thus ensuring the safe and stable operation of the power grid.

[0035] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for determining the direction of faults in a distribution network based on current distortion rate, characterized in that, include: Three-phase current is collected at the upstream line protection point of the distributed power source; The distortion characteristics of the amplified waveform after sampling of the three-phase current are derived, and the total harmonic distortion rate of the derived current is calculated. The direction of the fault is determined by the total harmonic distortion of the current.

2. The method for determining the direction of faults in a distribution network based on current distortion rate according to claim 1, characterized in that, The three-phase current is the three-phase fault current collected after a fault is detected by the current protection element at the line protection point.

3. The method for determining the direction of faults in a distribution network based on current distortion rate according to claim 1, characterized in that, The distortion characteristics of the amplified waveform after differentiating the sampled three-phase current include: Calculate the first derivative of the current in each phase; The amplitudes of the fundamental wave and each harmonic are calculated using the Fast Fourier Transform method. Calculate the total harmonic distortion rate of the current based on the amplitude of each harmonic.

4. The method for determining the direction of a distribution network fault based on current distortion rate according to claim 1, characterized in that, The direction of the fault is determined based on the total harmonic distortion of the current, including: When the total harmonic distortion rate of one phase current exceeds the current harmonic distortion rate threshold, the fault is determined to be located in the opposite direction of the protection. When the total harmonic distortion rate of any phase current after differentiation of the three-phase fault current is detected to be below the current harmonic distortion rate threshold, the fault is located in the positive direction of the protection.

5. The method for determining the direction of faults in a distribution network based on current distortion rate according to claim 4, characterized in that, The current harmonic distortion rate threshold is the current waveform distortion rate threshold measured at the line protection point when a three-phase short-circuit fault is applied at the end of the line.

6. The method for determining the direction of faults in a distribution network based on current distortion rate according to claim 1, characterized in that, The formula for calculating the total harmonic distortion rate of current is: , The total harmonic distortion of the current; The amplitude of the nth current harmonic is obtained from the Fast Fourier Transform calculation. This represents the amplitude of the fundamental current.

7. The method for determining the direction of a distribution network fault based on current distortion rate according to claim 4, characterized in that, When the fault is in the positive direction and the fault current exceeds the overcurrent protection threshold, the AND gate output of the overcurrent protection element at the protection point is 1, and the circuit enters the delayed tripping stage. If the fault is in the opposite direction, or the fault current does not exceed the overcurrent protection threshold, the AND gate output is 0, and the protection is locked.

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