DonNTU   Masters' portal

Abstract

Contents

General characteristics of

Relevance of the topic. In an ever-deteriorating technical condition of networks fore the problem of maintaining the required level at a sufficiently reliable electricity supply to consumers by extending the service life of electrical equipment with worn insulation. According to the operating experience of the most common type of injury in these networks are single-phase earth fault, constituting up to 90% of the total number of violations of the normal operation of the network. Studies show that in this situation the most effective solution to this problem lies in the optimization and management of the neutral network.

Objective: To develop a mathematical model and the study of transients in the distribution networks of the 6-35kV, working with the neutral grounded through the coil arcing, the arc phase to earth faults, and taking measures to improve the reliability of electrical equipment in these networks.

Scientific novelty: the transients investigated and proposed new circuit design to limit the surge in the networks of six-phase circuit at 35 kV on the ground.

practical value: the practical implementation of the proposed solutions and recommendations based on analysis of the results will significantly improve the working conditions of electrical equipment and distribution networks.

Methods: in the accepted method of mathematical modeling of electrical networks with resonance-compensated neutral.

Testing operation: results of the studies were presented at the Student Conference Chair ES "Science Days 2012" and reported at the Ukrainian student research conference in the city of Sevastopol.

Introduction

The most common types of damage to the electrical networks of all classes are single-phase voltage circuit to the ground. According to the distribution networks of the 6-10kV, they account for at least 80-90% of the total number of cases of a malfunction of the network. In most cases, these closures are accompanied by partial combustion of the arcs in the place of breakdown or ceiling insulation and complex transient overvoltage with high multiplicity. From the analysis of operating experience different purpose electrical networks (mining, metallurgy, chemistry and coke chemistry, as well as the network's own needs TPP) that if ever deteriorating technical condition them to within 60-80% of these closures are currently developing a multi-seat breakdown of damaged insulation on the phase line to line, or go into korotkiezamykaniya with the consequences. And, in good coincidence of data from different sources about 35% of these cases is explained by the thermal effect of the grounding of the arc and about 65% - the effects of the resulting arc voltage surges. Hence it is obvious that the main direction of improving the reliability of the considered networks, is combating the effects of single-phase earth fault. On the basis of generalization of operating experience and special studies in this work task was to evaluate the effectiveness of currently used means of combating the influence of ground arcs and emerging at the same voltage.

most common at present means of protection of distribution networks from the effects of single-phase ground fault is an arcing coil (DHA). However, in a steady deterioration of networks and the lack of commercially available tools to configure the coil, its effectiveness is very low, and in many cases it is even degrade the performance of networks due to the fact that creating a great imbalance in phase voltages drastically shortens the life of isolation, and thus increases accident in nets.

Therefore, this study was to develop a measure to improve the reliability of electrical networks with resonance-compensated neutral. For this purpose, the study conducted transients in these networks with arc ground faults. The results of these studies suggested measures to improve the reliability of the network and made recommendations for their implementation, which will improve safety and reliability of service distribution networks.

1. The current state of the problem and how to solve it

1.1 Work 6-35kV networks in modern conditions.

widely used at present arc suppression coils (DHA) as a means of protecting electrical equipment from the effects of single-phase circuits because of the asymmetry of the phases not only improve network performance, but rather create a more unfavorable conditions for the isolation of electrical equipment. Voltage distortions in phase dramatically shorten the life of electrical insulation throughout the network-related and do not provide the required performance in the mode of arc fault phase to earth as the multiplicity of the surge on the elements of the network increases in proportion to the degree of voltage unbalance, reaching 3Uf and more. Because of these reasons, the accident rate in modern networks reach 120-140 damage per year for every 100 km of transmission lines, up to 80% of which are developing in line to line short-circuit or multi-seat breakdown of damaged insulation on the phase.

In the case of high-level operation of the network with the DHA used properly compensated capacitive currents has the following advantages:

• reduces the earth fault current to a minimum value (in the limit to the active component and higher harmonics). That it provides safe dugogashenie and safety during the spreading of current;

• facilitates the requirements for the grounding devices;

• limits the overvoltage arising from the arc ground faults, to values ??of 2.5-2.6 (with the degree of mismatch compensation) for the safe isolation of electrical equipment;

• significantly reduces the rate of recovery voltage on the affected phase, which contributes to the restoration of the dielectric properties of the fault in the network after each extinction intermittent arc ground;

• prevents threw reactive power to the power sources for arc-circuit to earth than the quality of stored electric power for consumers;

• prevents the development of the network ferroresonance processes (unauthorized neutral bias);

• provides long-term operation of cable lines or network shutting phase;

• provides a high percentage of samopogasaniya grounding of electrical arcs.

1.2 Current state of the arc surges in distribution networks stress the 6-35kV, working with the neutral grounded through the coil arcing

Among the numerous investigations of switching overvoltages arising from any kind of opening and closing electrical circuits, the greatest amount of research was devoted to a very common voltage spikes with arc ground faults in high voltage networks, working with isolated neutral.

founder of the research was a surge Petersen, who in 1916 developed a theory to explain the physical nature of the emergence of the maximum surge.

In 1923, Peters and Slepian offered another theory, fundamentally different from the theory of Petersen. Later, these theories

supplemented by various authors on the basis of theoretical and laboratory studies on the maximum surge levels and forms of development. In 1957

N.N.Belyakovym was published in the theory of surge arc earth fault in networks with isolated neutral.

process of the maximum surge in line with the theory of Petersen has the following characteristics:

a) Re-grounding plugs arc presented in the form of metal closures. In this regard, does not take into account the presence of the arc voltage-current dependence, which is in effect for high frequency currents is explicitly dynamic nature, ie the arc voltage has no clearly defined peak extinction and ignition, as is usually assumed for the static characteristics. Deionization process is delayed for a change of current in the ignition of Petersen duge.Povtornye occur regularly after each half-cycle at a maximum voltage of the damaged phase, when the supply voltage is equal to the maximum value. The maximum voltage can reach a value of 7.5 Uf.

b) the duration of the arc re-ignition of each half-period equal to the free oscillations, despite the fact that the current value and its rate of change with each half-life increases, and increases its thermal and ionizing effects.

c) Each extinguishing the arc in the network there is increasing DC voltage bias Usm.

d) Restoration of the damaged phase voltage after arc extinction has an oscillatory character with a high peak, exceeding the value of phase voltage. However, it is assumed that the dielectric strength of the fault increases faster than the recovery voltage.

d) For each half-cycle voltage changes its sign. Characteristic features of

surge on the theory of Slepian and Peters are:

a) Repetitive firing is also represented in the form of a metallic circuit to the ground. They occur regularly in each period at the maximum stress on the affected phase (the first and all subsequent ignition, respectively, ± and ± 2 Uf Uf).

b) the duration of the arc re-ignition of each half-period equal to the power frequency.

c) Since the arc extinction occurs whenever the current passes through the zero-frequency value, the extinction peaks are absent. Restoration of the damaged phase voltage after arc extinction occurs smoothly with the industrial frequency.

d) The same (except the first) each time the ignition voltage of the arc formed by the unchanging initial and final stress on the damaged phases, respectively, and ± ± 0.5Uf 1.5Uf.

d) Overvoltage not change sign.

The origin of the maximum surge Belyakov must match the two main conditions in one cycle, namely:

a) The first arc ignition should occur ranbshe maximum EMF faulted phase to the time of extinction (the maximum stress on the damaged phase), the first peak recovery voltage reached the value 0.4Uf.

b) The second arc ignition, at which the trailing phase having the highest voltage 3.2Uf must occur exactly at the time when the faulted phase voltage is approximately equal to 2.2Uf, ie more than the first ignition.

1.3 Review of existing methods of surge suppression in the 6-35kV networks.

Network of the same nominal voltage at different ways of neutral grounding are some differences in the technical and economic indicators. Method of neutral grounding in the first place affect the magnitude of fault current to earth. Therefore, all electrical RB network, depending on the current divides into a network with small and large networks with a ground fault current. According to the accepted norms in Ukraine, 6-10 kV networks are networks with a low current circuit to the ground.

2. Development of a mathematical model for the study of transients in the network's own needs TPP

For the analysis of transients in the distribution network with a 6-35kV arc ground faults is taken as the basis of the scheme shown in Fig. A.

Figure 1 - Schematic diagram of the distribution network with a resonant earthed

In contrast to the well-known mathematical models of power supply systems of this type will be considered:

1) ground fault in the stator windings of induction motors and consideration of their impact on the character of the processes depending on the distance from the point of closure of the findings of stator;

2) displacement of the neutral network in the pre-damage mode due to unbalanced load or different phases of the active-phase conduction and capacitive isolation between phases;

3) the presence of a special connecting the transformer for partial neutral grounding through a resistance or a current limiting reactor;

4) the presence of nonlinear surge arresters connected to 6kV busbars;

5) different conditions of the arc - the arc extinction in passing through the zero-frequency current or current high-frequency oscillations;

6) different values ??of the breakdown arc gap with repeated intermittent ignition of the arc.

In preparing the plan take into account the substitution of relatively small length of cable connections for the conditions of distribution networks (up to 0.5km) can be taken for all elements of the study focused network settings. We also consider the network under study as a linear, ie saturation of the individual elements is neglected. Based on the foregoing, in Fig. 2 shows the equivalent circuit of the studied networks, adopted on the basis of a mathematical model.

Figure 2 - Equivalent circuit of the site of the distribution network, working with the resonance-compensated neutral

This equivalent circuit is represented phase AC power supply emf, the leakage inductance L and the resistance of R. In the equivalent circuit network is taken into account capacitances (Ca, Cb, Cc) and resistances (Rua, Rub, Ruc) phase to earth insulation, inductive-capacitive (M, D) between phases bonds, which has a capacity of leakage resistance RT. In the neutral of the transformer can be connected to current limiting resistor RD or arcing reactor LD. High-voltage induction motor is included in the equivalent circuit subtransient phase AC leakage inductance L1 and resistances R1. In one phase of the electric motor can change the location of a single-phase ground fault along the winding through the introduction of variable resistors R11, R12, and the leakage inductance L11, L12. The chain phase to earth fault in motor winding simulates its capacity Cz and the resistance of the arc Rz. Zinc-oxide surge suppressors (ARF), mounted on busbars and the findings of engines are taken into account the nonlinear dependence of the resistance of the current or voltage.

mathematical model is described by the following system of differential equations:

3. Results of the study of transients in the distribution network with a resonance-compensated neutral and development activities to improve the working conditions of electrical equipment in these networks

As a result, a large amount of studies that were carried out using a mathematical model for different parameters and mode of neutral grounding distribution network is established that the main factor that determines the nature and magnitude of transient overvoltage in PTG in a network with a capacity of DHA phase to ground and line to line capacitance, inductance and transformers, power supply, the nature of the load resistance in place of the phase to earth fault, etc. To limit the multiplicities of overvoltages in the network with the specified parameters have a decisive importance: the value of the instantaneous value of voltage on the injured at the time of the initial phase of the arc ignition, the moment of arc extinction and voltage during the second and subsequent ignition of the arc.

below shows the calculated waveforms of transient processes in the studied networks with different settings and modes of adjustment DHA. The first and subsequent breakdowns occurred at the maximum voltage the faulted phase, and the extinction of the arc at the time of passage of the current of industrial frequency (Fig. 3) and the total fault current (Fig. 4) through zero.

The most common method at present to reduce the effects of the emergency-phase arcing fault is the reactor (DGR), which retains the advantages of networks with isolated neutral. The effectiveness of the giant dipole resonance is largely determined by the degree of its setting on the value of the capacitive earth fault current. In this connection the reactor contributes to the neutral network: a significant reduction of heat loss in the arc gap by reducing the earth fault current to the level of the active component of the higher harmonics and unbalance currents, and as a consequence, willful extinction of the arc, reducing the number of re-ignition of the arc due to a significant reduction speed (up to hundreds of periods of industrial frequency) on the arc voltage recovery interval after the extinction of the arc, with the exception of repeated breakdowns in the arc gap at voltages greater than phase, which prevents the escalation of overvoltages in the network with resonant neutral grounding and allows you to keep the multiplicity of the arc voltage surge at the first insulation fault , ie, in the range (2.4-2.6) Uf. However, as shown by our extensive research to achieve the expected results only in a strictly symmetrical networks with a coefficient of adjustment of the GDR in the range (0.98-1.02), which is well illustrated by the calculated oscillograms of the voltage and current in the network of the GDR in setting it up close to the resonance (Fig. 3).

Rastroyka compensation leads to a significant decrease in the efficiency of the use of the GDR. For example, rastroyka compensation for more than 5% leads to a sharp increase in the multiplicity of the surge, and when you set up the GDR in the overcompensation mode (recommended PTE) on the multiplicity of the 25% surge in the network of the GDR can already achieve 3Uf and more. In the event of a network voltage unbalance in phases of unbounded multiplicity surge could increase dramatically, because here, as well as in networks with isolated neutral overvoltage increases in proportion to the value of the neutral displacement. The growth surge helped by the fact that the incorrect setting of the GDR process is equalizing the stress phase, after the extinction of the arc has the character of the beats (Fig. 4), the amplitude and frequency are determined by the degree of compensation and rastroyki Q oscillatory circuit. When fine-tuning of the GDR or in a small overcompensation, the occurrence of beats in OZNZ possible if you disable connection with a large capacitive current recharge. The danger lies in the beat that for paper-Shrove cable insulation, which samovostanavlivaetsya, re-closure of the damaged phase may occur at voltages close to 2Uf, resulting in maximum overvoltages on healthy phases.

Figure 3 - Process for the closure of phase with the ground in the network of the GDR in setting it up close to the resonance (phase fault current to earth - 30 A)

Figure 4 - Process for the closure of phase with the ground in the network of the GDR (the current phase of the circuit to earth - 30 A, overcompensation 10%)

plans to improve the working conditions of electrical networks with a large capacitive current circuit is considered to be a promising transition to the combined method of grounding through the shunt reactors to existing high-resistance resistors. The right value of the shunt resistor of the GDR to: eliminate the possibility of a significant shift of the resonance even at the neutral setting of the reactor and the presence of voltage unbalance in the network in phases, effectively limit the multiplicity of the arc voltage surge to a level that is determined by the first phase of the closure on the ground, ie to (2.3-2.5) Uf; eliminates the possibility of beating the voltage on phases after the extinction of the arc, even with considerable rastroyke reactor and as a consequence, to avoid the possible appearance of multiple re-ignition of the arc on the damaged phase at voltages above the breakdown phase, excluding the possibility of resonant processes in network with GHD and ferroresonance processes in transformer voltage to reduce the level of harmonics in the network are generated unbalanced load and the GDR, to improve the selectivity and reliability of protection against earth faults in compensated neutral networks.

Conclusions

  1. Under these circumstances, a permanent reduction of the insulating ability of the electrical insulation of distribution networks and a lack of funds for the replacement or repair of the above quality of electrical equipment and the lack of reliable means of protection against surges effective solution to the problem of reliability of power supply systems to be found in the optimization of the neutral network.
  2. most common at present means of protection of distribution networks from the effects of single-phase arcing hamykany a coil connected to the neutral point in the network. However, in modern conditions of poor insulation of electrical networks and the availability of voltage unbalance in phases on the effectiveness of DHA is very low, and sometimes creating large Pecos in phases, it significantly reduces the service life of electrical insulation. A large amount of research
  3. established modes of networking with DHA, made a physical model of cable networks has shown that a change in a wide range of active conduction through the insulation is less than displaces the neutral power supply than the change in capacitance of the conduction phase with respect to ground.
  4. Settlement and experimental studies of transient processes in a network with DHA in the single-phase ground faults, made with the use of mathematical and physical models of the network have shown that under conditions of mismatch and the presence of DHA in the network even allowed to PTE (up to 15%) of the bias voltage neutral isklyuchaetvozmozhnost the expected results from the use of DHA.
  5. significant increase of the efficiency of DHA can be achieved by connecting the coil parallel to the necessary size of the active resistor to mitigate the surge to a safe value for the electrical equipment, balancing phase voltages and the possibility of successful application of protective residual phase fault to earth.
  6. We give an estimate of the cost-effectiveness studies, the issues of safety and industrial hygiene, and protection of the environment from harmful emissions from thermal power plants.

References

  1. Sivokobylenko VF Dergilev MP Modes neutral distribution networks 6-10 kV. - Sat scientific papers DonSTU. Series: Electrical and Power Engineering, Vol. 67: - Donetsk: Donetsk National Technical University, 2003. - S. 49 - 58.
  2. Chernikov AA Compensation emkostnvh currents in networks with ungrounded neutral. M., "Energy" in 1974. 96C
  3. Report on the scientific - research work "Study of surge and development of measures to restrict their networks sobsvennyh needs TPP block, ¹ 76085390, Donetsk. In 1976. 156s.
  4. Pereh³dn³ processes in systems elektropostachannya Vlasna needs elektrostants³y: Navch. pos³bnik / VF Sivokobilenko, VK Lebedºv - Donetsk: Donetsk National Technical University PBA, 2002. - 136 p.
  5. Dolginov A. High Voltage Engineering in electrical engineering. - Moscow: "Energy", 1968. - 464.
  6. Sivokobylenko VF Lebedev, VK, Mahinda Silva. Analysis of the processes arc earth fault in networks of their own needs and the CHP plant. - Sat scientific papers DonSTU. Series: Electrical and Power Engineering, Vol. 17: - Donetsk: Donetsk State Technical University, 2000. - S. 129 - 133.
  7. Likhachev, FA Surge in the networks of their own needs / / Electric Station. - 1983. - ¹ 10. - C. 37 - 41.
  8. Silberman VA, Epstein IM et al Influence of neutral grounding of auxiliary unit of 500 MW at the overvoltage protection relay and work. / / Electricity. - 1987. - ¹ 12. - S. 52 - 56.
  9. Dergilev MP, VK Obabkov Irreducible multiplicity surge in the 6-35 kV network with resistive grounded neutral. / / Science, Engineering and Business in the energy sector. - Ekaterinburg. - 2002. - ¹ 5. - S. 10 - 14.
  10. Gindulin FA, Goldstein, VG, Dulzon AA, FA Khalilov Overvoltage in networks 6-35 kV. - Moscow: Energoatomizdat, 1989. - 234 p.
  11. Sirota IM, Kislenko SN Mikhailov, AM Neutral modes of electrical networks. - Kiev.: Science. Dumka, 1985. - 190 p.
  12. Likhachev FW Earth fault in networks with isolated neutral, and with compensation of capacitive currents. - Moscow: Energiya, 1971. - 254 p.