A mine selective leakage protection method based on a standard current change trend

Abstract

The application discloses a new mine selective leakage protection method based on a standard current change trend and belongs to the field of coal mine low-voltage power grid fault diagnosis. In view of the problems that the commonly used selective leakage protection device has the problems of malfunction, misoperation, low-voltage switch leakage misoperation or step jump, and low line selection accuracy, the application proposes a mine selective leakage protection method based on a standard current change trend by connecting a switchable middle resistor in parallel with a zero sequence reactor of an additional direct current power supply device. Firstly, the zero sequence current flowing through the zero sequence reactor and each line before and after the input of the parallel middle resistor is obtained, the standard current of the zero sequence current of each line is obtained by taking the amplitude of the zero sequence current of the zero sequence reactor as a reference value, the change trend of the standard current of each line before and after the input of the middle resistor is compared, the standard current of the fault line shows an upward trend after the input of the middle resistor, while the standard current of the healthy line is unchanged, and finally the fault line selection is realized. The method has strong transition resistance capacity, only needs to measure the amplitude of the zero sequence current of each line, does not need to calculate the phase, is not affected by the phase measurement error and non-synchronous sampling, and is not affected by the transition resistance in principle.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of coal mine low-voltage power grid fault diagnosis, in particular to a new method of mine selective leakage protection based on the change trend of per-unit current. BACKGROUND

[0002] Among the electrical fault categories of coal mine underground low-voltage power grid, single-phase leakage fault has the highest occurrence probability, accounting for about 80% of the total. The air humidity is high in the underground environment, and when an electric shock accident occurs to the operating personnel, it is easy to cause more serious harm to the human body. The insulation damage of cable line caused by single-phase grounding may further develop into inter-phase short circuit, leading to explosion of coal dust and gas.

[0003] The low-voltage power grid in the underground often adopts the leakage protection method of additional DC power supply method in cooperation with other selective leakage protection. The additional DC power supply method requires the installation of three-phase reactors and zero-sequence reactors at the total feeder switch. After the low-voltage power grid compensated by the zero-sequence reactor occurs single-phase grounding fault, the fault characteristic quantity is not significant, resulting in frequent occurrence of protection refusal, misoperation and other problems of the commonly used selective leakage protection device in actual application, low selection accuracy, and leakage misoperation or over-grade tripping of low-voltage switch, which has buried huge safety hazards for mine production. SUMMARY

[0004] In order to solve the problems mentioned in the background, the present application proposes a new method of mine selective leakage protection based on the change trend of per-unit current.

[0005] In order to achieve the above technical purpose and achieve the above technical effect, the present application realizes the following technical scheme: a new method of mine selective leakage protection based on the change trend of per-unit current, comprising the following steps:

[0006] Step 1: detecting the low-voltage power grid by the additional DC power supply method, if a fault is found, collecting and calculating the low-frequency steady-state current of the zero-sequence reactor and the low-frequency steady-state current flowing through the zero-sequence current transformer at the head of each line;

[0007] Step 2: the zero-sequence reactor is connected in parallel with a switchable middle resistor, collecting the zero-sequence current flowing through the middle resistor after being put into operation, if it is less than the threshold value, it is judged that the system has three-phase symmetric leakage; otherwise, the low-frequency steady-state current of the zero-sequence reactor and the low-frequency steady-state current flowing through the zero-sequence current transformer at the head of each line are collected and calculated again;

[0008] Step 3: calculating the per-unit current I * of each line before and after the middle resistor is put into operation, and comparing the per-unit coefficient β with the theoretical minimum value of the per-unit coefficient of the fault line β g . β > β gThe line with β < 1.15 is a healthy line, and if the unit coefficients β of all lines are less than 1.15, it is determined that the bus is faulty.

[0009] Further, the zero sequence current signal acquisition time is 0.02s and 0.07s after the fault is found by the additional DC power supply method.

[0010] Further, the resistance R in parallel is 500Ω. N

[0011] Further, the resistance R in parallel is 0.04s after the fault is found by the additional DC power supply method.

[0012] Further, the unit current I of the kth line before the middle resistance in step 3 is put into operation k.1 * The calculation formula is:

[0013]

[0014] Among them, I C0k.1 is the zero sequence current amplitude of the kth line before the middle resistance is put into operation, is the zero sequence current amplitude of the zero sequence reactor before the middle resistance is put into operation.

[0015] Further, the unit current I of the kth line after the middle resistance in step 3 is put into operation k.2 * The calculation formula is:

[0016]

[0017] Among them, I C0k.2 is the zero sequence current amplitude of the kth line after the middle resistance is put into operation, is the zero sequence current amplitude of the zero sequence reactor after the middle resistance is put into operation.

[0018] Further, the unit coefficient β calculation formula in step 3 is:

[0019]

[0020] Further, the minimum value β of the unit coefficient of the fault line in step 3 is g The calculation formula is:

[0021]

[0022] Among them, ω is the power frequency angular frequency, 3C 0Σ is the total distributed capacitance of the system, d max is the ratio of the maximum distributed capacitance of the system to the total distributed capacitance, v is the system detuning degree, R N is the resistance in parallel.​

[0023] Further, the threshold value set in step 2 is 10mA.

[0024] Further, the additional DC power supply method detects again after 0.04s after the fault occurs, and if the fault disappears, it is considered as a transient fault.

[0025] Further, the system detuning degree v in step 3 is-35%.

[0026] Further, the line with a per unit coefficient β less than 1.15 is considered as a healthy line.

[0027] Further, the line with a per unit coefficient β greater than β g less than 1.15 is considered as a fault line.

[0028] Further, if the per unit coefficient β of each line is less than 1.15, it is considered as a bus fault.

[0029] The disclosed new method for selective leakage protection for mine based on per unit current change trend, by collecting the low-frequency steady-state current of the zero sequence reactor before and after the parallel resistor is put into operation and the low-frequency steady-state current flowing through the zero sequence current transformer at the first end of each line, to realize the identification of the fault line, the principle is clear, easy to realize, and has strong practicability, only the amplitude of the zero sequence current of each line needs to be measured, without calculating the phase, not affected by the phase measurement error and non-synchronous sampling, strong transition resistance capacity, and the method is not affected by the transition resistance in principle, has high engineering application value. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 It is a new method for selective leakage protection for mine based on per unit current change trend

[0031] Figure 2 It is a low-frequency zero sequence equivalent model simulation topology of the zero sequence reactor parallel resistor grounding system

[0032] Figure 3 It is a zero sequence current waveform diagram of different lines and zero sequence reactors when line 3 occurs 1000Ω single-phase high resistance grounding fault DETAILED DESCRIPTION

[0033] The application will be described in more detail below in combination with the drawings and simulation cases.

[0034] A new method for selective leakage protection for mine based on per unit current change trend, as shown in Figure 1 , includes the following operation steps:

[0035] Step 1: Detect low-voltage power grid by additional DC power method, if fault is found, collect and calculate low-frequency steady-state current of zero sequence reactor and low-frequency steady-state current flowing through each line head zero sequence current transformer;

[0036] Step 2: Connect the shiftable middle resistance in parallel with the zero sequence reactor, collect the zero sequence current flowing through the middle resistance after being connected, if it is less than the threshold value, it is judged that the system has three-phase symmetry leakage; otherwise, collect and calculate the low-frequency steady-state current of the zero sequence reactor and the low-frequency steady-state current flowing through each line head zero sequence current transformer again;

[0037] Step 3: Calculate the per-unit current I * of each line before and after the middle resistance is connected, and compare the per-unit coefficient β with the theoretical minimum value β g of the per-unit coefficient of the fault line. g The line with β>1.15 is the fault line, the line with β<1.15 is the healthy line, and if the per-unit coefficient β of each line is less than 1.15, it is judged that the bus fault occurs.

[0038] Further, the additional DC power method detects the fault.

[0039] Further, the zero sequence current signal collection time is 0.02s and 0.07s after the additional DC power method finds the fault.

[0040] Further, the parallel middle resistance R N is 500Ω.

[0041] Further, the parallel middle resistance time is 0.04s after the additional DC power method finds the fault.

[0042] Further, the per-unit current I k.1 of the kth line before the middle resistance is connected in step 3 is calculated. * The calculation formula is:

[0043]

[0044] Wherein, I C0k.1 is the zero sequence current amplitude of the kth line before the middle resistance is connected, is the zero sequence current amplitude of the zero sequence reactor before the middle resistance is connected.

[0045] Further, the per-unit current I k.2 of the kth line after the middle resistance is connected in step 3 is calculated. * The calculation formula is:

[0046]

[0047] Wherein, I C0k.2 is the zero sequence current amplitude of the kth line after the middle resistance is connected, The zero sequence current amplitude of the zero sequence reactor after the middle resistance is input.

[0048] Further, the per-unit coefficient β calculation formula in step 3 is:

[0049]

[0050] Further, the theoretical minimum value β of the per-unit coefficient of the fault line in step 3 is: g The calculation formula is:

[0051]

[0052] Where ω is the power frequency angular frequency, 3C 0Σ is the total distributed capacitance of the system, d max is the ratio of the maximum distributed capacitance of a single line to the total distributed capacitance of the system, and v is the system off-tuning degree, R N is the parallel middle resistance;

[0053] Further, the threshold current flowing through the parallel middle resistance in step 2 is set to 10 mA.

[0054] Further, the additional DC power supply method detects again at 0.04 s after the fault occurs, and if the fault disappears, it is considered to be a transient fault.

[0055] Further, the system off-tuning degree v in step 3 is-35%.

[0056] Further, the line with a per-unit coefficient β less than 1.15 is considered to be a healthy line.

[0057] Further, the line with a per-unit coefficient β greater than β g is considered to be a fault line to be cut off.

[0058] Further, if the per-unit coefficients β of all lines are less than 1.15, it is considered to be a busbar fault.

[0059] A simulation model is built by using PSCAD / EMTDC to verify the practical value of the new mine selective leakage protection method based on the per-unit current change trend proposed by the application, the line length of the simulation model is marked in Figure 2 , and the simulation model includes 5 lines of different types, and the specific line types are as follows: line 1 is MYP-3×25, lines 2-3 are MYP-3×35, and lines 4-5 are MYP-3×95; L1 and L2 are three-phase reactors and zero sequence reactors, respectively, the zero sequence reactor L2 is 1.741H, R N is a parallel middle resistance of 500Ω, S is a controllable switch, U Z , R Z , and C ZDC power supply, current limiting resistor, DC blocking capacitor in the additional DC power supply device, U Z 36V, current limiting resistor R Z 50Ω, at this time the ratio d of the maximum distributed capacitance of a single line and the total distributed capacitance of the system can be calculated max 29.79%, the theoretical minimum value of the fault line unit coefficient β g 2.08.

[0060] Table 1 Unit coefficients of each line when line 3 is single-phase grounded through different transition resistances

[0061]

[0062] Table 3 Unit coefficients of each line when the bus is single-phase grounded through different transition resistances

[0063]

[0064] Table 4 Unit coefficients of each line when line 2 is single-phase grounded under different degrees of de-tuning

[0065]

[0066]

[0067] Table 5 Unit coefficients of each line when line 4 is single-phase grounded through different fault grounds

[0068]

[0069] From the data in Tables 1-5 above, it can be found that the method disclosed in the present application can accurately identify line faults of various types using zero sequence current signals, and can also accurately identify in complex working conditions, meeting the needs of real working conditions.

[0070] Figure 3 (a)-(d) are respectively zero sequence current waveform diagrams of different lines and zero sequence reactors when line 3 occurs 1000Ω single-phase high resistance grounding fault, it can be observed that Figure 3 (a) and (c) can be found that Figure 3 (d) all appear the phenomenon of decreasing zero sequence current amplitude, and have the same change trend, and Figure 3 the zero sequence current amplitude of the fault line in (b) increases after the middle resistor is put into operation, which is opposite to the change trend of the zero sequence currents of the remaining lines, therefore, the change trend is reflected by calculating the unit coefficient, the line with unit coefficient β less than 1.15 is regarded as a healthy line, and the line with unit coefficient β greater than 2.08 is regarded as a fault line to be cut off.

[0071] The above and embodiments are merely used to illustrate the technical solutions of the present application, but not to limit the present application; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those skilled in the art that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A novel method for selective leakage current protection in mines based on the per-unit current variation trend, characterized in that, Includes the following steps: Step 1: Use the additional DC power supply method to detect the low-voltage power grid. If a fault is found, collect and calculate the low-frequency steady-state current of the zero-sequence reactor and the low-frequency steady-state current flowing through the zero-sequence current transformer at the beginning of each line. Step 2: Connect the zero-sequence reactor in parallel with the switchable resistor, and collect the zero-sequence current flowing through the switchable resistor after it is connected. If it is less than the threshold value, it is determined that the system has a three-phase symmetrical leakage current; otherwise, collect and calculate the low-frequency steady-state current of the zero-sequence reactor and the low-frequency steady-state current flowing through the zero-sequence current transformer at the beginning of each line again. Step 3: Calculate the per-unit current I of each line before and after the resistor is connected. * And compare the per-unit coefficient β with the theoretical minimum per-unit coefficient β of the faulty line. g ;β>β g The line with β < 1.15 is a faulty line, and the line with β < 1.15 is a healthy line. If the per-unit coefficient β of each line is less than 1.15, it is judged as a bus fault. The per-unit current I of the k-th line before the resistor is applied in step 3. k.1 * The calculation formula is: Among them, I C0k.1 Let be the zero-sequence current amplitude of the k-th line before the medium resistance is applied. The zero-sequence current amplitude of the zero-sequence reactor before the medium resistance is applied; After the medium resistor is applied, the per-unit current I of the k-th line is... k.2 * The calculation formula is: Among them, I C0k.2 Let be the zero-sequence current amplitude of the k-th line after the medium resistance is applied. The zero-sequence current amplitude of the zero-sequence reactor after the medium resistance is applied; The formula for calculating the per-unit coefficient β in step 3 is as follows: Among them, I k.1 * Let I be the per-unit current of the k-th line before the medium resistance is applied. k.2 * Let be the per-unit current of the k-th line after the medium resistor is applied.

2. A novel method for selective leakage current protection in mines based on per-unit current variation trends as described in claim 1, characterized in that, The theoretical minimum value β of the per-unit coefficient for the faulty line mentioned in step 3 g The calculation formula is: Where ω is the power frequency angular frequency, 3C 0Σ Let d be the total distributed capacitance of the system. max R is the ratio of the maximum distributed capacitance of a single line to the total distributed capacitance of the system, where ν is the system detuning degree. N For parallel resistors; Lines with a per-unit coefficient β less than 1.15 are considered faulty lines and disconnected.

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