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Abstract

The content of the abstract

Introduction

Electrical networks can be grounded or isolated neutral. Network 6, 10 and 35 kV working with insulated neutral transformers. Network 660, 380 and 220 can be both isolated and grounded neutral. Even with good insulation of current carrying parts of the network from the ground, the conductors have a connection with the land.

The breakdown phase of the ground in networks with isolated neutral is dangerous, so that the capacitive currents can become quite large and dangerous. For example the breakdown of insulation from one phase to the ground on the ungrounded body of unit may be a voltage equal to the voltage phase, and when you touch the current through the damaged insulation passes through the earth, the human and closed on phase, that is, there is a closed circuit in which large currents flow, which are fatal.

1. Theme urgency

Protect devices against the breakdown phase of the ground in networks with isolated neutral including a high-resistance load (human) are important because they can prevent an emergency situation and to save the life of a person as well as reduce the risk of equipment damage. There are many of different devices and techniques to protect against the breakdown of the land but they do not work perfectly and have their own advantages and disadvantages, that is, the development of new means of protection is important.

2. Goal and tasks of the research

The main goal of master's work is to develop a system of determining the faulted phase in industrial networks with isolated neutral, which will be designed for detection of ground faults in mine three-phase networks with voltage up to1 kV.

Main tasks of the research:

  1. Analysis of the characteristics of three-phase networks, as an object of study.
  2. Analysis of the processes taking place during a ground fault through a high-resistance load and low-resistance load.
  3. Modelling of processes occurring during an earth fault.
  4. Analysis of the available methods to determine the faulted phase with the constructed model.
  5. Develop workable algorithm of definition the faulted phase.

Research object: three-phase circuit with isolated neutral.

Research subject: control parameters of three-phase circuit with isolated neutral.

3. The choice of control method

Timely and accurate determination of the mining power network phase with ground leakage, is a major factor in the protection of human, which connected to a phase voltage. Fast way to solution of this problem described in [1]. Its essence is based on the hypothesis of sinusoidal variation of the neutral potential u N (t) lies in the definition of its argument (initial phase) of two adjacent points of the instantaneous value of u N (t 1) and u N (t an + Δt). If the difference in phase between these points Δωt fixed and varies from 10 to 20 degrees, the problem reduces to the solution of the system:

формула (1);                                                    (1)

формула (2);                                                    (2)

where U Nmax, ψ, - respectively, the amplitude and initial phase (argument) of neutral relative to earth;

Eliminating from (1 and 2) the amplitude U Nmax, we obtain the function y (ψ) and its derivative y '(ψ):

формула (3);                                            (3)

формула (4);                                                      (4)

From the analysis of (3) and (4). First, during the two functions ψ of π and, consequently, on the interval 0 ... 2π dependence (3), in contrast to the system (1, 2) has two roots, one of which "extra". Its easy to set up a test in (1) or (2).

Secondly, the derivative y '(ψ) is always positive. Consequently, the function of y (ψ) is always increasing and within a period (0 ... π) has only one root, which can easily be distinguished by using the property function (3) that it has discontinuities of the second kind for the values of the argument ψ, meet the condition: ω∙t1 + Δωt + ψ = 0, π, 2π etc.

Lack of first option is a low accuracy is determined by the argument of ψ because of the influence, first of all, dynamic components of the transitional regime, which is accompanied by a process of diversion, the presence of higher harmonics, the transverse asymmetry of the network, and a number of other factors. Its main advantages are: relatively easy separation of roots depending (4), their identification and verification.

The second version of implementing digital DPES considers the availability exponential part of the transition process in the event of leakage and is based on the assumption that the instantaneous value of neutral respect to the law:

формула (5);       (5)

where the α – the time (angle) of leakage; τ – time constant of exponential component.

Unlike the first version of the solution (5) requires four values of the neutral potential, taken at regular intervals Δωt. As a result, we arrive to a system of four equations (the number four unknowns ψ, U Nmax, τ, α.):

формула (6),         (6)

where t1 – time of first reference; k = 0 ... 3 – point defining the next three frames.

animation of the transition process in the network, appearing for the closure phase to the ground

Рис.1 Animation of the transition process in the network, appearing for the closure phase to the ground.
The animation consists of 7 shots with a delay of 100 milliseconds between shots; delay before the repeat is 200 ms, the number of cycles of reproduction is limited to the 6th. The volume of the animation is 139 kb.

Conclusion

  1. Justified the urgency of the problem.
  2. Completed the analysis of existing methods and means of determining the breakdown phase of the ground in networks with insulated neutral.
  3. Selected and validated method of measurement.

This master's work is not completed yet. Final completion: December 2012. The full text of the work and materials on the topic can be obtained from the author or his head after this date.

References

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