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Abstract

Сontent

Introduction

The experience of the operation of electrical networks 6-10 kV shows that they are characterized by a very high accident rate, and that a large part (90%) of violations of normal operation of these networks is caused by damage to the insulation relative to the earth that lead to the emergence of single-phase short circuits on the earth (SPSCE).

Capacitive current is the main component of the fault current to earth, the magnitude of which depends on the total network capacity. Capacity, and therefore fault currents to earth are small in networks of small extent. Capacitive currents reach tens or hundreds of amps with the development of networks and increase their total length. Single-phase ground fault lead to the occurrence of the intermittent arc and to the development of electromagnetic oscillations at each ignition and extinction of the arc in such networks. Oscillations, superimposed on each other can cause significant over-voltage, commensurate with the level of network isolation. This is the reason that in most cases a single-phase fault to develop in interfacial, which then lead to the breakdown of insulation in many places, damage to the equipment and the poor reliability of power supply [2-5].

Efforts aimed at solving problems related to improving the reliability of networks of 6-10 kV, maintaining the required performance levels of insulation, preventing SPSCE and related negative consequences, are concentrated on developing, improving and enhancing the effectiveness of the methods and means restrictions capacitive currents and overvoltages in the network.

The main objectives of the solutions to these problems are:

The relevance and urgency of the solution of these tasks is determined by the fact that:

1. The relevance of the topic

Mode the neutral ground largely determines the nature of electromagnetic transients when insulation breakdown phase of the network on the ground and extinguishing of the grounding arc, the likelihood and degree of danger of surges during intermittent arc damage.

In electrical networks 6-10 kV fundamentally there are various methods of neutral grounding (isolated, earthed through an arc suppression reactor, low-ohmic or high-ohmic resistors) [10]. For each of the methods of grounding proposed and are used various means to limit the capacitive currents and overvoltage (arc suppression reactor, automation settings, resistors of different designs and with different characteristics, surge suppressor).

Most of the single-phase ground faults in networks with such modes of grounding , especially in the initial stage of damage, have unstable intermittent arc [3].

In these conditions require decisions and actively discuss the questions: what is the best method of grounding and under what conditions to give preference, what to means of current SPSCE limitation and overvoltage to use, how to choose their parameters.

Currently, when a constantly deteriorated insulation of electrical equipment of power supply systems of own needs of thermal power plants due to lack of funds for replacement and efficient recovery of worn-out electrical equipment the urgency of this problem increases even more. So as a reliable means of protection against arc-over voltages are missing, a successful solution can only be found in the optimization of neutral networks own needs in combination with various circuit solutions.

2. The purpose and objectives of the study

The purpose and objectives of the research: development of mathematical models and the study of transition processes in arc-circuit phase to earth in distribution networks 6-10 kV, working with different modes of the neutral ground; the analysis of the influence of neutral grounding on the parameters of the arc voltage.

The object of study: electromagnetic transients in an arc earth fault in networks 6-10 kV with various methods of neutral grounding.

Subject of research: parameters of the arc overvoltages during ground faults in networks 6-10 kV with various methods of neutral grounding.

Research methods: when performing work used the theory of the electromagnetic field, the theory of electrical circuits. To assess the effect of neutral grounding on the parameters arc voltage surge applied mathematical simulation of electromagnetic transients. Model of electromagnetic transient processes in electrical networks obtained using the method of phase coordinates. To develop a network model applied the method of structural modeling.

3. Mathematical model for the study of transients in the auxiliary network TPP

For the analysis of transients in the auxiliary network TPP at arc ground faults will take as a basis the scheme of power supply of the network of own needs of TPP is shown in fig.1.

pic1

Figure 1 – Diagram of the network of own needs TPP

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

1) ground fault in the stator windings of induction motors and the consideration of their impact on the character of the processes depending on the degree of remoteness of a point of the circuit from the findings of the stator;

2) the displacement of the neutral network in mode before the accident due to the unbalanced load phases or different active-capacitive-conductivity of phasic and interfacial insulation;

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

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

5) different conditions burning of the arc - extinguishing of the arc at the zero crossing of the current of industrial frequency or current of high frequencies;

6) various size breakdown of the arc gap when intermittent arc re-ignites.

When drawing up the circuit of substitution, given the relatively small length of the cable connections for the conditions of own needs of power plants (up to 0,5 km), can be taken for all elements studied network lumped parameters. We will also consider the the studied network linear, i.e. the saturation of the individual elements is neglected. In fig.2 shows an equivalent circuit of the studied network, adopted on the basis of the mathematical model.

pic1

Figure 2 – Equivalent circuit for the own needs of 6-10 kV
(animation: 5 frames, 9 cycles, 68 kB)

In this equivalent circuit: the power source presents EMF of the phase, the leakage inductance L and resistance R. Network taken into account capacitances (Ca, Cb, Cc) and resistances (Rua, Rub, Ruc) isolation of the phases to earth, inductive–capacitive (M, Cm) interfacial connections, the capacity of which has an active leakage resistance Rm. Special of connecting transformer introduced into the scheme of the phase values of the leakage inductance LT and active resistance RT. In the neutral of the transformer can be connected current limiting resistor RD or arc suppression reactor LD. High voltage induction motor is included in the equivalent circuit of the phase subtransition inductances scattering L1 and resistance R1. In one of the phases of the electric motor can change the place of occurrence of single-phase earth-fault along the winding by the introduction of variable resistances R11, R12 and inductances scattering L11, L12. Circuit circuit phase to earth in the motor winding is simulated by its capacitance Cz and active arc resistance Rz. Zinc oxide surge arresters installed on the busbar or the findings of the engines takes into account non-linear relationships of their active resistance of current or voltage [8].

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

pic1

where p - operator differentiation.

4. The results of the study of transients in the network of own needs of power plants at arc ground faults

As a result a large amount of research carried out using mathematical models, for different settings and mode neutral grounding of the network of own needs TPP is established that the main factor t that determines the nature of transients and the magnitude of the overvoltage when single-phase ground faults in networks with isolated neutral is the capacity of the phase to earth and interfacial capacity, inductance of power supply and transformers, the nature of the load resistance in place of circuit phase to earth, etc. For the occurrence of limit multiplicities of the overvoltage in the network with the specified parameters have a crucial: the magnitude of the instantaneous value of the voltage on the damaged phase at the time of the initial ignition of the arc, the time of extinction of the arc and the voltage at the second and subsequent ignition of the arc. Below shows the calculated waveforms of transients in the network of own needs TPP at arc ground faults

Below shows the calculated waveforms of transients in the network of own needs TPP at arc ground faults. First and future breakouts occurred when the maximum voltage of the faulted phase, and the extinguishing of the arc at the time of passage of current of industrial frequency (fig.3) and a total current of the circuit (fig.4) through zero.

pic1

Figure 3 – The process of arc short circuit of a phase to earth in a network with isolated neutral (current circuit phase to earth – 30 A, the extinguishing of the arc at the zero crossing of the current after the decay of its high frequency component).

pic1

Figure 4 – The process of arc short circuit of a phase to earth in a network with isolated neutral (current circuit phase to earth – 30 A, the extinguishing of the arc at the moment of passing the full current of the circuit to zero

Maximum voltage anticipatory phase after insulation breakdown reaches (2,4–2,5)Uf, and the subsequent breakouts of the magnitude of overvoltages on healthy phases rises. For of the network of own needs TPP with their characteristic parameters, the magnitude of the overvoltage can be (3,2–3,5)Uf. With the appearance in the network of voltage unbalance on phase overvoltage can grow significantly, as studies have found that the multiplicity of the arc overvoltage grows approximately in proportion to the amount of displacement of a neutral.

The most common currently way to reduce accidental consequences from a single-phase fault is an arc suppression reactor, which retains the advantages of networks with isolated neutral. The efficacy of suppression reactor is largely determined by the degree of configuration on the value of the capacitive current earth fault. In this case, the connection of the reactor to the neutral contributes to exception repeated breakdowns on the arc gap when the voltage is greater than the phase, making it impossible escalating surges in network with resonant neutral grounding and allows you to keep the multiplicity of the arc voltage level of the first insulation breakdown, i.e. within (2,4–2,6)Uf. However, to achieve the expected results only in a strictly symmetrical networks when the ratio settings of this resonance within (0,98–1,02)Uf, that is well illustrated by the waveforms obtained by the calculation of the change of voltage and current in the network of this resonance in terms of setting it close to resonance (fig.5).

pic1

Figure 5 – The process of the closure phase to earth in networks with resonance in the set-up conditions close to resonance (fault current phase to earth – 30 A)

Detuning compensation leads to a significant decrease in efficiency from the use of this resonance. Detuning compensation more than 5% leads to a sharp increase in the number of surge, when adjusting resonance mode overcompensation (recommended PTE) 25% ratio of overvoltage in the network with this resonance can reach 3Uf and more [9]. The growth overvoltages is contribute by the fact that when inaccurate setting of the DGR process of equalizing voltages of the phases after extinction of the arc is in the nature of the beats (fig.6), the amplitude and the frequency of which is determined by the degree of setting compensation and quality factor of the oscillating circuit. The growth surge is facilitated by the fact that when inaccurate setting of the arc suppression reactor process of equalizing voltages of the phases after extinction of the arc is in the nature of the beats (fig.6), the amplitude and the frequency of which is determined by the degree setting compensation and quality factor of the oscillating circuit.

pic1

Figure 6 – The process of the closure phase to earth in a network with arc suppression reactor (current circuit phase to earth – 30 A, overcompensation 10%)

In networks with a large capacitive current circuit perspective is the transition to a combined method of grounding, by parallel connection to existing reactors high-resistance resistors. The correct value of shunt resonance resistor allows to exclude the possibility of significant displacement of a neutral, even when the resonant tuning of the reactor and the presence in the network of voltage unbalance on phases; effectively restrict the multiplicity of the arc overvoltages to a level that is determined by the first circuit phase to earth, i.e., before (2,3–2,5)Uf, to exclude completely the possibility of beating the voltage on phases after extinction of the arc when setting reactor and as a consequence, to avoid the possibility of multiple re-ignitions of the arc on the damaged phase at breakdown voltages above phase. The calculated waveform of the transition process in the network thermal power plants with a resistance-grounded neutral is presented in fig.7.

pic1

Figure 7 –The process of the closure phase to earth in a network with a resistance-grounded neutral

Conclusions

In the result of the work was the analysis of processes that take place in networks of TPP auxiliary 6-10 kV at arc ground faults. It should be noted that the research was carried out taking into account the current state of the network based on real operational data.

The main results are as follows:

1. In the current environment, a persistent reduction in the insulation properties of the electrical insulation of distribution networks and the lack of funds for replacement or repair of electrical equipment, the lack of reliable means of protection against overvoltages effective solution to the problem of reliability of power supply systems should be sought in the optimization mode of the neutral network.

2. Main reason for the high failure rate of electrical equipment in the networks of the middle class are arc voltage surge that occurs when intermittent the nature of the arc in the place of the breakdown phase insulation to earth.

3. The problem of improving the reliability of distribution networks 6-10 kV is a composite of many tasks, effective solutions can be found for each network individually based on its characteristic features on the basis of combined use of relay protection, enhance the grounding, the use of limiters series surge arresters with different thresholds, limitations and system quick and auto-bypass faulted phase.

4. An effective solution to the problem of improving the reliability of distribution networks 6-10 kV can be found through a large amount of scientific and experimental research.

*When writing this abstract master's work is not yet complete. The full text of work and materials on the topic can be obtained from the author or his supervisor in January 2016.

Список источников

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