A Method for Selecting the Line with Single-Phase High-Resistance Grounding Fault in a Flexible Grounding Distribution Network

Abstract

The invention discloses a single-phase high-resistance grounding fault line selection method for a flexible grounding distribution network. First, after a fault occurs in the distribution network and the shunt small resistor is put into operation, high-precision current transformers are used to collect the three-phase bus voltages and the zero-sequence current signals at the heads of all lines. Secondly, after determining the fault phase according to the amplitude difference of the three-phase voltages, the line-to-line voltages of the non-fault phases are calculated. Then, the FFT is used to extract the power frequency components and calculate the phase differences between the zero-sequence currents at the heads of all lines and the line-to-line voltages of the non-fault phases. Finally, by comparing the magnitudes of the phase differences and a set threshold value, the fault line is determined. The method of the invention is simple and easy to implement, has a high line selection accuracy rate, the required three-phase bus voltages and zero-sequence current signals at the heads of lines are easy to obtain, and the zero-sequence voltage does not need to be measured; the sampling frequency is low and the number of measurement points arranged is small, and the method has high economy.

Description

[0001] The present invention belongs to the technical field of power grid grounding fault line selection, and specifically relates to a method for selecting the single-phase high-resistance grounding fault line in a flexible grounding distribution network. Background Art

[0002] With the development of the power system, the increase in the cable rate has led to an increasingly large system capacitive current. The compensation capacity of the arc suppression coil cannot fully compensate the fault current. Therefore, some resonant grounding substations and newly built substations are gradually being transformed or constructed into substations with a neutral point grounded through a small resistor. Since the small resistor grounding method cannot distinguish between instantaneous grounding faults and permanent grounding fault types, it trips the circuit breakers of all lines for all single-phase grounding types, increasing the number of trips and reducing the power supply reliability. The flexible grounding method (the method of connecting a small resistor in parallel with the arc suppression coil) only trips for permanent grounding faults. For instantaneous grounding faults, due to the compensation effect of the arc suppression coil, the residual grounding current is reduced to a very small value, making it difficult to form a re-ignition, which can effectively ensure that the power supply to users will not be interrupted when an instantaneous grounding fault occurs. It has been widely applied in many places in China.

[0003] High-resistance grounding faults such as wire drops on the grass occur frequently in the flexible grounding system. The transition resistance during a high-resistance grounding fault is generally above 100 Ω, and the zero-sequence current may be less than the setting value, resulting in the refusal of protection to operate. If the faulty line cannot be removed for a long time, it may lead to an expansion of the fault range and affect the power supply reliability; at the same time, the high voltage near the fault point is likely to cause step voltage or touch voltage, threatening personal safety. Therefore, the problem of the refusal of protection to operate for high-resistance grounding faults in the flexible grounding system needs to be solved. Summary of the Invention

[0004] To solve the above problems mentioned in the background art, the present invention proposes a method for selecting the single-phase high-resistance grounding fault line in a flexible grounding distribution network.

[0005] To achieve the above technical objectives and reach the above technical effects, the present invention is realized through the following technical solutions: A method for selecting the single-phase high-resistance grounding fault line in a flexible grounding distribution network, which includes the following steps:

[0006] (1) After the parallel small resistor is put into operation when a fault occurs in the distribution network, use a high-precision current transformer to collect the three-phase bus voltages and the zero-sequence current signals at the heads of all lines.

[0007] (2) After determining the faulty phase according to the amplitude difference of the three-phase voltages, calculate the line voltages of the non-faulty phases. The specific steps are as follows:

[0008] (a) Calculate the amplitude differences between every two of the three-phase bus voltages:

[0009]

[0010]

[0011]

[0012] Wherein, is the phase voltage of phase A of the busbar, is the phase voltage of phase B of the busbar, is the phase voltage of phase C of the busbar; ΔU AB is the amplitude difference between phases A and B, ΔU BC is the amplitude difference between phases B and C, ΔU CA is the amplitude difference between phases C and A.

[0013] (b) Compare the amplitude differences to determine the faulty phase:

[0014]

[0015] (c) Calculate the line voltages of the non-faulty phases:

[0016]

[0017] Wherein, is the line voltage between phases A and B of the busbar, is the line voltage between phases B and C of the busbar, is the line voltage between phases C and A of the busbar;

[0018] (3) Use FFT to extract the power frequency components and calculate the phase differences between the zero-sequence currents at the heads of each line and the line voltages of the non-faulty phases. Among them, the phase differences are:

[0019]

[0020] Wherein, is the phase of the zero-sequence current at the head of the kth line, is the phase of the line voltage between phases B and C of the busbar, is the phase of the line voltage between phases C and A of the busbar, is the phase of the line voltage between phases A and B of the busbar, k is the line number, and n is the total number of distribution network lines.

[0021] (4) Compare the phase differences with the set threshold value to determine the faulty line. Among them, the principle for setting the threshold value θ is: considering the measurement errors of ±10% of the current transformer and the influence of the line impedance angle at the same time, the present invention sets θ to 20°; compare the phase differences and the set threshold value θ, and the determination principle for determining the faulty line is:

[0022] (a) If then the kth line is a normal line;

[0023] (b) If Then the k-th line is the faulty line.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] A method for selecting a high-resistance grounding fault line in a flexible grounding distribution network disclosed by the present invention determines the faulty line by utilizing the difference that when a high-resistance grounding fault occurs on a line and a shunt small resistor is put into operation, the line voltage of the non-faulty phase of the bus is approximately orthogonal to the phase of the zero-sequence current at the head of the faulty line, while being approximately in the same phase as the phase of the zero-sequence current at the head of the sound line. The present invention is simple and easy to implement, has a high line selection accuracy rate, the required three-phase voltage of the bus and the zero-sequence current signal at the head of the line are easy to obtain, and there is no need to measure the zero-sequence voltage; the sampling frequency is low and the number of measurement points is small, having high economy. Description of the Drawings

[0026] Figure 1 It is a flow chart of the fault line selection method of the present invention;

[0027] Figure 2 It is a simulation model diagram of a 10 kV neutral point flexible grounding distribution network in an embodiment of the present invention. Detailed Embodiments

[0028] The following further describes the present invention with reference to the drawings.

[0029] The present invention proposes a method for selecting a single-phase high-resistance grounding fault line in a flexible grounding distribution network, and its overall process is as Figure 1 shown, including the following steps:

[0030] (1) After a fault occurs in the distribution network and a shunt small resistor is put into operation, use a high-precision current transformer to collect the three-phase voltage of the bus and the zero-sequence current signals at the heads of all lines.

[0031] (2) After determining the faulty phase according to the three-phase voltage amplitude difference, calculate the line voltage of the non-faulty phase. The specific steps are as follows:

[0032] (a) Calculate the amplitude differences between every two of the three-phase voltages of the bus:

[0033]

[0034]

[0035]

[0036] In the formula, is the voltage of phase A of the bus, is the voltage of phase B of the bus, is the voltage of phase C of the bus; ΔU AB is the amplitude difference between phases A and B, ΔU BC is the amplitude difference between phases B and C, ΔUCA is the amplitude difference between phases C and A.

[0037] (b) Compare the amplitude difference to determine the faulty phase:

[0038]

[0039] (c) Calculate the line voltage of the non-faulty phase:

[0040]

[0041] In the formula, is the line voltage between phases A and B of the bus, is the line voltage between phases B and C of the bus, is the line voltage between phases C and A of the bus;

[0042] (3) Use FFT to extract the power frequency component and calculate the phase difference between the zero-sequence current at the head of each line and the line voltage of the non-faulty phase. Among them, the phase difference between the zero-sequence current at the head of each line and the line voltage of the non-faulty phase is:

[0043]

[0044] In the formula, is the phase of the zero-sequence current at the head of the k-th line, is the phase of the line voltage between phases B and C of the bus, is the phase of the line voltage between phases C and A of the bus, is the phase of the line voltage between phases A and B of the bus, k is the line number, and n is the total number of distribution network lines.

[0045] (4) Compare the phase difference with the set threshold to determine the faulty line. Among them, the principle for setting the threshold θ is: Considering the measurement error of ±10% of the current transformer and the influence of the line impedance angle at the same time, the present invention sets θ to 20°; Compare the phase difference and the set threshold θ, and the determination principle for determining the faulty line is:

[0046] (a) If then the k-th line is a normal line;

[0047] (b) If then the k-th line is a faulty line.

[0048] Simulation verification

[0049] To verify the reliability and effectiveness of the present invention, the present invention constructs a simulation model topology diagram of a 10 kV neutral point flexibly grounded distribution network as shown in Figure 2 The simulation system includes 5 outgoing lines l 1 ~l5 , E is the main network voltage source, regarded as an infinite power source, with its impedance approximately 0, R N is a small resistor connected in parallel to the neutral point, S is the switch for inserting the small resistor after the fault, L is the arc suppression coil for neutral grounding, f is the fault point, R f is the fault transition resistance. Set a phase A ground fault on the l 1 line, and simulate different fault transition high resistances, different fault distances, and different fault initial phase angles. The simulation sampling frequency is set to 2 kHz, and the fault line selection results are shown in Table 1 below. The fault distance represents the distance between the fault point and the line head end.

[0050] Table 1 Fault line selection results under different fault transition high resistances, fault distances, and fault initial phase angles

[0051]

[0052]

[0053] It is found that the fault line selection results under different fault transition high resistances, fault distances, and fault initial phase angles are all correct, and it has good practicability in on-site application. Therefore, the fault line selection method proposed by the present invention is basically not affected by the fault transition high resistance, fault distance, and fault initial phase angle.

[0054] The above are only the preferred embodiments of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for selecting the faulty line of single-phase high-resistance grounding fault in a flexible grounding distribution network, which is characterized by the following steps: Step 1) After the parallel small resistor is put into operation when a fault occurs in the distribution network, use a high-precision mutual inductor to collect the three-phase bus voltage and the zero-sequence current signals at the head of all lines; Step 2) After determining the faulty phase according to the amplitude difference of the three-phase voltages, calculate the line voltage of the non-faulty phase; Step 3) Use FFT to extract the power frequency component and calculate the phase difference between the zero-sequence current at the head of each line and the line voltage of the non-faulty phase; Step 4) Compare the size of the phase difference and the set threshold to determine the faulty line; In step 3), the phase difference φ between the zero-sequence current at the head of each line and the voltage of the non-faulty phase line k is calculated as follows: Wherein, is the phase of the zero-sequence current at the head of the k-th line, is the phase of the line voltages of phases B and C of the busbar, is the phase of the line voltages of phases C and A of the busbar, is the phase of the line voltages of phases A and B of the busbar, k is the line number, and n is the total number of distribution network lines; in the step 4), the principle for setting the threshold θ is: considering both the ±10% measurement error of the mutual inductor and the influence of the line impedance angle, the present invention sets θ to 20°.

2. The method for selecting the faulty line of single-phase high-resistance grounding fault in a flexible grounding distribution network according to claim 1, characterized in that: In the said Step 2), after determining the faulty phase according to the amplitude difference of the three-phase voltages, calculate the line voltage of the non-faulty phase, and its specific steps are as follows: (a) Calculate the amplitude differences between any two of the three-phase voltages of the busbar: Wherein, is the voltage of phase A of the busbar, is the voltage of phase B of the busbar, is the voltage of phase C of the busbar; is the amplitude difference between phases A and B, is the amplitude difference between phases B and C, is the amplitude difference between phases C and A; (b) Compare the amplitude differences to determine the faulty phase: ; (c) Calculate the line voltages of non-faulty phases: In the formula, is the line voltage between phases A and B of the busbar, is the line voltage between phases B and C of the busbar, is the line voltage between phases C and A of the busbar.

3. The method for selecting the faulty line of single-phase high-resistance grounding fault in a flexible grounding distribution network according to claim 1, characterized in that: In the said step 4), compare the phase difference φ k with the set threshold θ, and the determination principle for the faulty line is as follows: (a) If φ k ≤ θ, then the k-th line is a normal line; (b) If φ k > θ, then the k-th line is the faulty line.

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