RU2738469C1 - Method for determination of feeder with single-phase earth fault in three-phase electric networks with inefficiently earthed neutral - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: use: in electric power engineering. According to the method, transient phase voltages on the buses of the power centre are recorded and by means of their processing the fact of arc short circuit to ground is established, as well as the damaged phase by the highest phase voltage derivative at phase breakdown moment of breakdown; in the voltage derivative over the damaged phase du _d/dt find its maximum at the moment of breakdown of the insulation and calculating pre-breakdown time relative to the previous breakdown Δt _pre = t_m1 - t_m0, which should exceed 1 ms, if this condition is not satisfied, then such closure (phase failure breakdown) processing is not performed; when condition Δt _pre > 1 ms are used to determine polarity of mean pre-breakdown voltage on the damaged phase and average currents of zero sequence in all feeders extending from busbars; for this from time of maximum of derivative t _m1 are backed to beginning of oscillogram of voltage on damaged phase for time Δt _U ≅(0.025...0.1)∙T₀ (T₀ is the industrial frequency period), during which average mean pre-breakdown voltage U _p.av. is calculated; from moment of maximum derivative voltage on damaged phase t _m1 for time equal to one third of period of free charge exchange oscillations of electric network Δt _I , determining zero-sequence average currents in all outgoing feeders, leaving two with maximum modulus values; polarities of these two zero sequence sign(I _0av.) are compared with polarity of average pre-breakdown voltage on the damaged phase sign(U _p.av.), one of two allocated feeders, in which polarity of average current of zero sequence coincides with polarity of average pre-breakdown voltage on damaged phase, that is performed condition sign(U _p.av.)=sign(I _0av.), is the feeder with single-phase ground fault.

EFFECT: technical result is increase in reliability and veracity of determination of feeder with single-phase unstable arc closure to earth in electric networks with inefficient earthed neutral.

1 cl, 2 dwg, 1 tbl

Description

The proposed invention relates to the electric power industry and can be used to determine a feeder (supply line) with a single-phase arc fault (ODS) to earth in three-phase electrical networks (ES) with an ineffectively grounded neutral: with an isolated neutral grounded through an arc suppression coil or a high-resistance resistor.

A known method for determining a feeder with a single-phase arc earth fault in an ES with an ineffectively grounded neutral, based on the analysis of the steady-state values of the higher harmonic components of the zero sequence current (I _0V.G. ) in the feeders departing from the power supply unit [Gelfand Y.S. Relay protection of distribution networks. M., 1982]. In this method, the selection of a damaged feeder with a single-phase earth fault is carried out by comparing (comparing) the values of high-frequency components in zero sequence currents (NP): in the damaged connection, the relative content of higher harmonic components I _0v.g. has a maximum value.

Due to the fact that in this method a steady-state current value is used as an input measured value, the method turns out to be unsuitable for the selection of feeders with short-term self-clearing faults, while this information is important for assessing the state of insulation and conducting preventive tests on lines with weakened insulation. as well as for locating the places of single-phase faults.

In addition, there is a known method for determining a feeder with a single-phase arc earth fault in three-phase ES [L.Ye. Dudarev, V.V. Zubkov, V.I. Stasenko. Comprehensive earth fault protection. - Power plants, No. 7, 1981], adopted as a prototype and applicable for short-term self-correcting earth faults, according to which, at the moment of phase insulation breakdown, the zero sequence transient voltage is recorded on the buses of the power supply center (main power point (GPP) or distribution substation ( RP)) and the first half-waves of high-frequency zero-sequence currents in outgoing feeders. The half-waves of transient voltages and currents determine the initial sign of the power direction for all feeders. A feeder with a power direction sign opposite to the others is considered damaged.

In this method, depending on the initial sign of the power direction, the threshold devices are triggered, leading to the triggering of the return device, which, in the absence of sustained damage, after some time t _bw, returns the threshold devices to their initial state. This time t _bg is selected based on the condition of detuning from various kinds of interference (for example, caused by switching in the network or the appearance of higher harmonic components in the voltage) and ranges from 7 to 10 ms, i.e. t _zd close to the industrial frequency half period Τ _0/2. The specified response delay of the reset device is determined by the duration of the existence of the zero sequence voltage 3U ₀ with interference. If the interference is not eliminated within the specified time or reappears at the end of the interval t _bg , leading to the appearance of voltage 3U ₀ , then a false operation (alarm) occurs.

In this method, non-selective determination of the feeder is possible due to the distortion of the phase relationships between current and voltage, which occurs at the time of a single-phase earth fault in a wave reflected from the end of the damaged line [Borukhman V.A. On the operation of selective earth fault protection in 6-10 kV networks and measures to improve them. - Power engineer, No. 1, 2000]. Distortions in the voltage wave are also introduced by the measuring electromagnetic voltage transformer due to the rather high frequency spectrum of the wave, reaching tens - hundreds of kilohertz. The frequency characteristics of electromagnetic measuring voltage transformers are such that they are capable of transforming periodic components only up to several kilohertz: at higher frequencies, the interturn capacitance of the primary winding affects [Gelfand Ya.S. Relay protection of distribution networks. - M., 1982, p. 294]. In this case, the power sign is different. By limiting the band of recorded frequencies to increase the selectivity of the selection of the feeder with a ground fault, the field of application of this method is significantly reduced, especially for cable electrical networks, in which wave processes are high-frequency due to the small lengths of the lines.

The analysis of the above prior art indicates that the objective (technical result) of the invention is to increase the reliability and reliability of the method for determining a feeder with a single-phase unstable arc earth fault in an ES with an ineffectively grounded neutral.

This is achieved by the fact that, in the known method for isolating a damaged feeder, transient NP currents in the outgoing feeders are recorded. Zero-sequence current transformers (TTNP) are phased uniformly - so that with a positive pre-breakdown phase voltage at the moment of a phase-to-ground short circuit, a positive high-frequency voltage surge is formed at the terminals of the TTNP with load resistors connected to them. By analyzing the transient phase voltages, the fact of an earth fault and a damaged phase is established [RF Patent No. 2232456 (from 11.10.2002). A method for recognizing a single-phase arc earth fault and a damaged phase in distribution networks with a resonant-grounded neutral // Kachesov VE / BI No. 19, 2004]. The damaged phase is isolated according to the maximum modulus of the phase voltage derivative. For the damaged phase, on the digitally recorded oscillogram of the transient voltage u _p (t), by calculating the finite differences, the derivative of the measured voltage d u _p / dt is determined and the time moment t _{m1 of the} maximum modulus of the voltage derivative is recorded. The time interval is measured until the previous moment of phase insulation breakdown, i.e. to the previous maximum of the derivative modulo Δt _pre = t _m1 -tm0. If a primary insulation breakdown is considered, then deliberately take Δt _before > 1 ms; if Δt _pre <1 ms, then the current maximum of the derivative (current breakdown of phase insulation) is excluded from processing. If the time interval Δt _before exceeds 1 ms, i.e. Δt _pre > 1 ms, then from the moment of time t _m1 are shifted to the beginning of the oscillogram u _p (t) for the averaging (integration) time equal to Δt _U ≅ (0.025 ... 0.1) T ₀ , i.e. by the time t ₀ = t _m1 -t _U. Further, from this point in time to time t _m1 , the average value of the pre-breakdown phase voltage is found

where U _{p, k} - instantaneous phase voltages on the damaged phase;

- количество отсчетов фазного напряжения на поврежденной фазе;

- the number of phase voltage readings on the damaged phase;

h - sampling step in time on the oscillogram.

Further, its sign is determined - sign (U _p.w. ). From the moment of time t _m1 for all oscillograms of the registered currents of the NP i ₀ in the outgoing feeders, their

average values on a limited time interval Δt _I :

- количество отсчетов тока НП;Where

- number of readings of the NP current;

- треть периода свободных перезарядных колебаний в ЭС;

- a third of the period of free recharge oscillations in the ES;

L _S - leakage inductance (short circuit) of the step-down transformer in the center of the power supply;

S _f and S _ff - phase and phase-to-phase capacities of the electrical network.

Of all the average currents, the NI leaves the two largest in absolute value and determines their signs. A feeder in which the sign of the average value of the current NI sign (I _0.av ) coincides with the sign of the average pre-breakdown voltage on the damaged phase, i.e. sign (I _0.cp ) = sign (U _p.cp ), is considered damaged.

Figure 1 shows a fragment of a three-phase ES, in which the proposed method is implemented; Fig. 2 shows the calculated curves for single-phase ground faults: NP currents in undamaged i _0np and damaged i _0p feeders, phase voltage on the damaged phase u _p and its derivative d u _p / dt.

In the ES shown in Fig. 1, the energy from the GPP - step-down substation 1 is transmitted through lines 2 to the RP, transformer substations (TP) or load. At the GPP on the outgoing feeders there are installed ТТНП (3) connected to load resistors 4 and a device for determining the feeder with a ground fault.

The method is carried out as follows.

In the process of a single-phase earth fault on any of the outgoing lines 2 (figure 1) on the buses of the power supply center 1 phase voltages are recorded in digital form, for which, for example, a monitoring system for transient electromagnetic processes can be used [Kachesov V.E., Larionov V.N., Ovsyannikov A.G. On the results of monitoring overvoltage at single-phase arc earth faults in distribution cable networks. - Power stations, No. 8, 2002], and recognize the type of damage - a single-phase earth fault and a damaged phase. Voltage measurement is performed on the GPP buses using electromagnetic measuring transformers. To recognize a single-phase arc earth fault, various methods are used, for example, for an electrical network, the neutral of which is grounded through an arc suppression coil, a method is used based on the property of a significantly lower effective voltage on the damaged phase in relation to the voltages on the undamaged phases, measured over a limited time interval (~ 3 ms) after breakdown of phase isolation and self-extinguishing of the grounding arc [RF Patent No. 2232456 (dated 11.10.2002). A method for recognizing a single-phase arc fault to ground and a damaged phase in distribution networks with a resonant-grounded neutral // Kachesov V.E. / BI No. 19, 2004]. For electrical networks with an isolated neutral, a method is used, which is based on the property of constant voltage at the neutral of the ES after self-extinguishing of the grounding arc [RF Patent No. 2356062. A method for recognizing a single-phase arc fault to earth and a damaged phase in distribution networks with isolated neutral / L.V. Bogdasheva, V.E. Kachesov; BI No. 14, 2009].

Recognition of the damaged phase is performed by calculating the derivatives of all phase voltages and comparing them at the moments of phase insulation breakdown - on the damaged phase, the derivative of the voltage d u _p / dt is maximum. From the entire array of the derivative d u _p / dt select the maximum derivative - for example t _m1 , and calculate the pre-breakdown time with respect to the previous breakdown Δt _pre = t _m1 -t _m0 , which should exceed 1 ms [Fig. 2]. If this condition is not met, then the processing of such a short circuit (phase insulation breakdown) is not performed. When the condition Δt _pre > 1 ms is satisfied, the polarities of the pre-breakdown average voltage on the damaged phase and the average zero sequence currents in all feeders outgoing from the buses of the power supply center are determined. From the time of the derivative maximum t _m1 retreat to the beginning of the oscillogram of the voltage on the damaged phase for the time Δt _U ≅ (0.025 ... 0.1) T ₀ , during which the average value of the voltage U _{p.w. is calculated.} ... From the moment of time of the maximum of the derivative of the voltage on the damaged phase t _m1 during one third of the period of free recharge oscillations Δt _I , the average value of the zero sequence current is determined. Of all the mean currents of the NP, two are left with the maximum values in absolute value. The polarities of the average values of these two zero-sequence currents sign (I _0cp. ) _Are compared with the polarity of the voltage on the damaged phase sign (U _p.w. ): a feeder whose polarity of the average zero-sequence current coincides with the polarity of the average pre-breakdown voltage on the damaged phase, t .e. the condition sign (U _p.w. ) = sign (I _0av. ) is _satisfied , is a feeder with a single-phase earth fault.

The selected range of averaging (integration) time is due to the fact that with frequent breakdowns of phase insulation, when there are small time intervals between them, and the transient process of breakdowns itself is stochastic, the determination of the average value of the pre-breakdown voltage outside the specified range ΔtU≅ (0.025 ... 0.1) T ₀ may lead to an inaccurate result, since the calculation is performed during a transient alternating process. Inaccurate voltage calculation can even lead to incorrect determination of its polarity. Since the identification of the damaged feeder is performed according to the polarities of the mean current and voltage, then the damaged feeder can be erroneously identified, i.e. the reliability of the determination will be low.

For computer curves of NP currents and voltage on the damaged phase, obtained using the program for calculating transient electromagnetic processes EMTP [N.W. Dommel, "Digital computer solution of electromagnetic transients in single and multiphase networks," IEEE Trans., Vol. PAS-88, pp. 382-399, April 1969] and presented in Fig. 2, calculations of the average pre-breakdown voltages and average currents of the NP for two breakdowns of phase insulation, which occur at voltages that differ approximately by 2 times: ~ 8000 and 4000 V. The calculated value of Δt _I for the considered ES was 570 μs. The table shows the average values of pre-breakdown voltages at each breakdown of phase insulation and average NP currents in undamaged and damaged feeders. The signs of the average NP currents in the damaged feeder coincide with the signs of the average pre-breakdown voltages in the damaged phase during both breakdowns, which makes it possible to single out the damaged feeder from all those connected to the buses.

Thus, the determination of a feeder with a single-phase ground fault in three-phase electrical networks with any method of ineffective neutral grounding is performed by means of a reliably established fact of a ground fault based on the analysis of the registered phase voltages of the ES, determination of the damaged phase by the maximum modulus of the phase voltage derivative, and coincidence of the mean the values of the pre-breakdown voltage on the damaged phase and the average value of the zero sequence current in the outgoing damaged feeder, measured during one third of the period of free recharge oscillations in the ES. A positive effect is the reliability of the selection of feeders with a ground fault even in ES with short feeders, when high-frequency fluctuations in the electrical network are transformed to the low-voltage side of voltage measuring transformers with significant amplitude and phase distortions.

Claims (1)

A method for determining a feeder with a single-phase arc fault to earth in three-phase electrical networks with an ineffectively grounded neutral, based on the registration of the zero sequence transient voltage on the buses of the power supply center, characterized in that the transient phase voltages on the buses of the power supply center are recorded and, through their processing, the fact of an arc fault is established to the ground, as well as the damaged phase by the largest derivative of the phase voltage at the moment of phase insulation breakdown; in the derivative of the voltage on the damaged phase d u _p / d t, find its maximum at the moment of insulation breakdown and calculate the pre-breakdown time with respect to the previous breakdown Δ t _pre = t _m1 - t _m0 , which should exceed 1 ms, if this condition is not met, then the processing of such a short circuit (breakdown of phase insulation) is not performed; when the condition Δt _pre > 1ms is fulfilled, the polarities of the average pre-breakdown voltage on the damaged phase and the average zero sequence currents in all feeders leaving the buses are determined; for this, from the time of the derivative maximum t _{m1, they} retreat to the beginning of the oscillogram of the voltage on the damaged phase for the time Δ t _U ≅ (0.025 ... 0.1) T ₀ ( T ₀ is the power frequency period), during which the average pre-breakdown voltage U _{p.av. is calculated.} ; from the moment of time of the maximum of the derivative of the voltage on the damaged phase t _m1 for a time equal to a third of the period of free recharging oscillations of the electrical network Δ t _I , the average zero-sequence currents in all outgoing feeders are determined, two of them are left with the maximum modulo values; the polarities of these two zero-sequence currents sign ( I _0av. ) are compared with the polarity of the average pre-breakdown voltage on the damaged phase sign ( U _p.av. ), one of the two selected feeders, in which the polarity of the mean zero-sequence current coincides with the polarity of the average pre-breakdown voltage on damaged phase, i.e. the condition sign ( U _p.w. ) = sign ( I _0av. ) is _satisfied , is a feeder with a single-phase earth fault.

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