CN110568312A - Phase current distortion-based single-phase earth fault line selection method for neutral point ungrounded system - Google Patents

Abstract

The invention discloses a phase current distortion-based single-phase earth fault line selection method for a neutral point ungrounded system, which is characterized by comprising the following steps of: 1. extracting each phase voltage before and after the single-phase earth fault of the system respectively, and judging the suddenly-reduced phase as a fault phase; 2. extracting effective current values before and after each line fault phase fault3. Calculating the current distortion rate epsilon of each line fault phase_i1、ε_i2…ε_in(ii) a 4. Selecting a fault phase current distortion rate ε_ii>Line i of 0 is a fault line, and the phase current distortion rate epsilon of the rest faults_in(n≠i)<The line of 0 is a normal line. Compared with other single-phase earth fault line selection methods of a system with no grounding of a neutral point, the method has the advantages of low cost, simple algorithm and simple realization; the simulation test shows that the material is,The line selection result is not influenced by the factors such as the size of grounding resistance, the distance between a fault point and a bus, the length and the number of lines and the like. The method is beneficial to fast troubleshooting and improves the power supply reliability of the power distribution network.

Description

Phase current distortion-based single-phase earth fault line selection method for neutral point ungrounded system

Technical Field

the invention relates to the technical field of power distribution network ground fault line selection, in particular to a phase current distortion-based single-phase ground fault line selection method for a neutral point ungrounded system.

Background

Operational experience has shown that single phase ground faults in power distribution systems account for over 80% of all types of ground faults. The neutral point ungrounded system is widely applied to medium and low voltage power distribution networks at home and abroad due to the characteristics of small fault current, symmetrical line voltage, continuous operation of load for 1-2 h and the like when single-phase ground fault occurs. However, since the faulty phase-to-ground voltage is 0 (metallic ground), the healthy phase-to-ground voltage rises to the original valuein addition, if the line selection and fault elimination are not performed in a short time, the inter-phase breakdown may be caused, and further serious three-phase short-circuit fault may be caused. In particular, the cable-powered enterprises in metallurgy, chemistry and the like have long troubleshooting time on single-phase grounding, and often cause the phenomena of equipment insulation aging, even cable blasting and the like.

In the existing line selection technology, the detection signals can be divided into two types according to the difference:

(1) The method is characterized in that a signal current injection device is connected to the secondary side of a voltage transformer, when a single phase is grounded, the device induces signal current to a fault phase (flows out through a grounding point), and a detector is used for detecting the signal current to realize line selection, but the accuracy of the line selection method is not high for intermittent arc grounding or instantaneous grounding faults;

(2) The method utilizes the characteristics that the amplitude of zero-sequence current of a fault line in an SFB frequency band is maximum and the polarity is opposite to that of the zero-sequence current of a sound line to select the line, and in practical application, the definition of the SFB frequency band depends on specific systems and lines, and for a high-resistance grounding fault, the content of the zero-sequence current of the line can be too low to be detected, thereby causing the failure of line selection.

In recent years, wavelet analysis has good localization and multiresolution characteristics in a time-frequency domain, and is particularly sensitive to sudden change, so that the application of the wavelet analysis in single-phase earth faults of a power distribution network becomes a hot research.

Disclosure of Invention

the invention provides a phase current distortion-based single-phase earth fault line selection method for a neutral point ungrounded system, aiming at overcoming the defects of the existing single-phase earth fault line selection technology of a power distribution network, simplifying a line selection algorithm, and improving the accurate line selection efficiency and the power supply reliability of the power distribution network.

The invention is realized by the following technical scheme:

phase current distortion based single-phase earth fault line selection method for a neutral point ungrounded system, characterized in that the method comprises the following steps:

Step 1: extracting each phase voltage before and after the single-phase earth fault of the system respectively, and judging the suddenly-reduced phase as a fault phase;

Step 2: extracting effective current values before and after each line fault phase fault

and step 3: calculating the current distortion rate epsilon of each line fault phase_i1、ε_i2…ε_in；

And 4, step 4: selecting a fault phase current distortion rate ε_ii>line i of 0 is a fault line, and the phase current distortion rate epsilon of the rest faults_in(n≠i)<The line of 0 is a normal line.

Step 3, the fault phase current distortion rate epsilon_inIs composed of

Compared with the prior art, the invention has the following advantages and beneficial effects:

1. The line selection algorithm is simple. Aiming at the single-phase earth fault of a system with a non-grounded neutral point, after the voltage collapse phase is judged to be a fault phase, the line can be correctly selected only by extracting the current effective values of the fault phases of all lines before and after the fault and carrying out simple operation.

2. Compared with a signal injection line selection method represented by an S injection method, the method can be used in cooperation with a voltage and current monitoring device, an additional signal injection device and a detection device are not needed, and the cost is low; and for intermittent earth fault, the effective value of phase current is increased after line fault, and the distortion rate of phase current is epsilon_iAlso positive, does not affect the accuracy of the method for selecting lines.

3. Compared with a typical zero sequence amplitude-to-amplitude ratio phase method, aiming at the problem that the zero sequence current content is too low to be detected due to high-resistance grounding, the method uses a phase current effective value, and does not influence the current monitoring device on correct extraction and calculation of the current monitoring device.

4. because the line selection method is based on the steady-state fault characteristics (phase current effective values) of respective lines, the accuracy of the line selection method is not influenced by factors such as the transformer acquisition quantity as the steady-state quantity, the transient characteristics (fault initial phase angle, transient high frequency, attenuation factor and the like), the number of system lines and the length of the system lines.

5. The line selection result of the method is not influenced by the size of the grounding resistance value.

drawings

fig. 1 is a simplified model of a single-phase earth fault of a typical distribution network neutral ungrounded system.

FIG. 2 is a composite sequence network based on Karrenbauer phase-mode transformation and boundary conditions.

Fig. 3 is a transient equivalent circuit of a single-phase earth fault of a neutral point ungrounded system.

Fig. 4 is a fault line selection flowchart.

Fig. 5 shows the line fault phase currents for a 10 Ω single-phase ground fault.

Fig. 6 shows the fault phase current of the fault line for different ground resistances.

FIG. 7 shows robust Line (Line1) fault phase current for different ground resistance conditions

Detailed Description

The invention will now be further described with reference to the following theoretical examples and drawings:

Taking a typical domestic neutral point ungrounded system as an example, a simplified single-phase ground fault model is shown in fig. 1, and it is assumed that three-phase system voltage and line parameters are symmetrical and distributed capacitance between wires is ignored. The system has n overhead outgoing lines, whereinThe equivalent impedance of the nth wire and the end load is determined by the line length l, the transformer and the load:

Wherein the content of the first and second substances,The equivalent lumped impedance of the nth wire (l < 100 km);And respectively calculating the leakage impedance and the load impedance of the transformer from the nth wire to the high-voltage side.

When line₂When single-phase earth fault occurs at the point f, simultaneous boundary conditions and Karrenbauer phase-mode transformation can be obtained:

wherein U is_0、1、2，I_0、1、2Voltage, current, R of 0, 1, 2 mode network respectively_fIs a ground resistor.

From equation (3), the equivalent composite network model shown in FIG. 2 can be obtained for the fault current I_faThe model is strictly equivalent to the system shown in fig. 1, wherein the virtual power supply is

U_f'＝U₀'+U₁'+U₂'＝U_A (4)

in the formula of U₀'，U₁'，U₂' power supply in 0, 1, 2 mode networks, respectively.

To facilitate the model derivation, the equivalent composite network model can be simplified into a second-order equivalent circuit as shown in fig. 3, and the accuracy thereof can satisfy various engineering calculations and simulation verifications, wherein L, R, C is the simplified and combined equivalent values:

In the formula R₀、L₀、C₀、C'₀Respectively a resistance, an inductance and two-side capacitances of an n-type equivalent model in a 0-mode impedance network; r₁、L₁Respectively, the equivalent resistance and inductance in a 1-mode (or 2-mode) impedance network.

establishing a second order differential equation of the model:

Combining the initial condition of system fault (setting the voltage at two ends of capacitor C as U when the fault occurs)₀) To solve the fault current of

In the formula:

The above mathematical model of fault current shows that when the system is in line₂After the f point of the fault is generated and the single-phase earth fault enters the steady state (I)_faexponential partial decay to 0), I) whether over-damped or under-damped_faAll being sinusoidal in magnitude, i.e.

For the convenience of analysis, the fault current is setWherein I_favIs I_faThe steady-state effective value of (a),Is a fault initial phase angle; let A phase power supply voltage beWherein

For a robust line in the system shown in FIG. 1_n(n ≠ 2), phase A current before and after faultAre respectively as

In the formulaIs the fault point to ground voltage.

Then line of the robust line_n(n ≠ 2) has an A-phase current distortion rate of

in the formula

Line for fault line₂Before and after failure of A-phase currentAre respectively as

Then the line of the faulty line₂has an A-phase current distortion rate of

In the formula Z_φFor a virtual power supply U_f' equivalent impedance at both ends, Z_eqFor equivalent impedance Z on any bus before system failure_eq1//Z_eq2//…//Z_eqn。

The above analysis shows that: when single-phase earth fault occurs in the non-earthed system of the neutral point, the current distortion rate epsilon of the fault phase of the sound line_{I (health)}Less than 0; current distortion rate epsilon of fault phase of fault line_{i (failure)}＞0。

As can be seen from the equations (12) and (14), the phase current distortion rate is derived from the effective current values of the line itself before and after the fault, and is not affected by other lines and fault transient factors (fault initial phase angle, attenuation factor, etc.), and the transition resistance R_fThe smaller the difference between the phase current distortion rate of the fault line and the healthy line is, the better the line selection effect is. However, it should be noted that, in order to reduce the error, the acquisition calculation should be performed after the grounding transient process is completed, and the line selection step is shown in fig. 4, considering that the transient process is finished after (3-5) τ (circuit time constant) in engineering.

Second, simulation result verification

the system f point was set to single phase ground through a 10 Ω resistor at 0.3S, with each line fault (a) phase current as shown in fig. 5. According to the parameters of the model, the fault transient process (3-5 tau) is approximately 0.9-1.5 ms, namely the system enters a steady state 1.5ms after the fault, it is easy to see that only the current (effective value) of the fault phase of the fault line is increased after the fault, the current (effective value) of the fault phase of the other healthy lines is reduced, and the distortion rate of each phase current is shown in table 1.

TABLE 110 Ω phase-to-phase grounding fault phase current distortion rate for each line

simulation results show that:

Only the fault phase current amplitude of the fault line is increased after the fault, and the distortion rate is positive; the phase current amplitude of the healthy line fault is reduced after the fault, and the distortion rate is negative.

the fault phase currents of the fault lines and the sound lines (taking Line1 as an example) for different grounding resistance values are shown in fig. 6 and 7; the phase current distortion rate of each line fault and the line selection result are shown in table 2.

TABLE 2F points of the fault phase current distortion rate and line selection results for each line under different grounding resistance values

Simulation results show that:

The smaller the grounding resistance is, the farther the values of the equations (10) and (12) are from 0, the larger the difference of the fault phase current distortion rate between the fault line and the healthy line is, and the better the effect of the line selection method is; for high-resistance grounding (the actual resistance value is generally lower than 2.7K omega), the fault line can be correctly selected by the line selection method as long as the acquisition precision of the mutual inductor is high enough.

the phase current distortion rate of each line fault and the line selection results for different grounding positions are shown in Table 4, wherein f in Table 3₁～f₅Are respectively Line₁～Line₅Grounding positions at 2km, 4km, 8km, 12km and 19km above the bus.

TABLE 3 phase current distortion rate of fault and line selection results for each line under different grounding positions

Simulation results show that:

For different grounding positions, the line selection method can correctly select the fault line; and the closer the grounding position is to the bus, the larger the distortion rate of the fault phase current of the fault line is, and the better the effect of the line selection method is.

Because the line selection method is based on the steady-state fault characteristics (phase current effective values) of respective lines, the accuracy of the line selection method is not influenced by factors such as the transformer acquisition quantity as the steady-state quantity, the transient characteristics (fault initial phase angle, transient high frequency, attenuation factor and the like), the number of system lines and the length of the system lines.

Claims (2)

1. Phase current distortion based single-phase earth fault line selection method for a neutral point ungrounded system, characterized in that the method comprises the following steps:

Step 1: extracting each phase voltage before and after the single-phase earth fault of the system respectively, and judging the suddenly-reduced phase as a fault phase;

Step 2: extracting effective current values before and after each line fault phase fault

And step 3: calculating the current distortion rate epsilon of each line fault phase_i1、ε_i2…ε_in；

And 4, step 4: selecting a fault phase current distortion rate ε_ii>Line i of 0 is a fault line, and the phase current distortion rate epsilon of the rest faults_in(n≠i)<The line of 0 is a normal line.

2. method for fault line selection according to claim 1, characterized in that epsilon in step 3_inSetting by the following formula:

In the formula of_inThe fault phase current distortion rate for the nth line.