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Abstract

Contents

Introduction

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Nowadays life without electricity is practically impossible. It is used in all areas of human activity, and its necessity for the further development of human society is obvious. Since the birth of power engineering the question of unceasing electricity supply of good quality was an essential part of the general concept of electrification. It has not lost its relevance today. It acquires special relevance in view of the current situation for today in Ukraine's power engineering. At permanently worsening technical condition of main power equipment, much of which outlived its lifespan, the probability and frequency of accidents increases dramatically, which will certainly lead to a decrease in the quality and reliability of power supply.

Electricity can be transferred over long distances, which can cater to a wide range of consumers, including the regions that are not secured enough resources of fossil fuels [1].

1. Theme urgency

Electrical networks of 6-35kW are widely used and working with isolated neutral. The most typical consumers are the motor load [2].

Single-phase earth faults are the most widespread type damage to the electrical medium voltage networks (within 90% of the total number of violations of the normal operation of networks in terms of their purpose and embodiment) [3].

The fight against them is a strategic direction of work to improve the reliability of electricity supply [4].

2. Goal and tasks of the research

The aim is to develop a program for the calculation of transient processes and to assess the impact of the engine load parameters arc overvoltage during earth faults in distribution networks with isolated neutral with Microsoft Visual Studio C++.

The main tasks of the research:

  1. Development of a mathematical model of the system "the power supply – cable – electric engine."
  2. Writing a program for calculating the system in the programming language Visual C++.
  3. Analysis of transient processes during earth faults in distribution networks with isolated neutral.
  4. Assessing the impact of the engine load on parameters of arc overvoltage.

Object of research : typical scheme of power supply of electric energy.

Subject of research : the impact of the engine load on parameters of arc overvoltage during earth faults in distribution networks with isolated neutral.

As part of the master's work expected to generate actual scientific results in the following directions:

  1. Development of a program in Microsoft Visual Studio C++ for calculating the typical scheme of three-phase network.
  2. The study of overvoltage and impact of engine load on the parameters of transient processes during earth faults.

Mathematical analysis, computer calculations and experimental researches on the model of the network and in real-world networks have shown that one of the reasons of simultaneous damage to a few items of electrical equipment can be resonant overvoltage, which occurs in the discharge circuit of shutting phase. Most of the multiplicity and the probability of their occurrence increases dramatically with increasing network parameters and power of electro receivers. Localized at the connections of large inductive resistance, they can occur simultaneously in several connections, which can cause damage to electrical equipment group [5].

3. An approach to the development of a mathematical model

There is accepted typical scheme of power supply of consumers to conduct research of transient processes in the distribution networks of 6-35kV with isolated neutral and assessing the impact of the engine load.

Each section of the busbar system in the considered networks is powered by a quite powerful (40 MVA or more) of three-phase oil transformers. The higher and lower windings of a power transformer connected in a star to simplify and to make greater universality of the model. This makes it possible to model network with isolated neutral and with the neutral, partly grounded through the various sizes small active resistances R, and is also with the compensation capacitive currents on the ground through the regulated arc suppression reactors.

The complete replacement scheme network

Picture 1 – The complete replacement scheme network

It takes into account the following important factors in the accepted replacement scheme:

  1. All elements, including engine load, in the replacement scheme are distributed parameters, which will provide the most comprehensive understanding of the physics of processes in the arc faults of phase to ground in networks with compensation of capacitive currents.
  2. It takes into account the active phase conduction of leaks through the insulation, which according to measurements in a constantly deteriorating technical condition of the electrical network becomes the real value and, in some cases, become comparable to the capacitive conductivity.
  3. Engine load is taken into account, and thus for the first time given the task to assess the impact a lot of contour of each connection on the quality and quantity of transient processes at single-phase ground faults on both the health and the phases of the damaged network.
  4. The equivalent circuit allows to investigate the influence of voltage imbalance in phases, created the capacitance and the active conductances of leaks through the insulation on the multiplicity of the arc overvoltage in networks with isolated neutral and compensation of capacitive current to ground.

I form the graph of the studied network for definition independent circuits. In this case, take the branch phase to phase capacitances and active resistances of leakage through the isolation to the branches of a tree (thick lines), denote by a single line on the graph. Also for the branch of a tree take the branch with an active resistance to the transformer neutral. Remaining branches of the graph is the chords that form a closed circuit. Arrows indicate the direction of branches.

The graph of researched network

Picture 2 – The graph of researched network

Having a graph of the network will select 25 independent circuits, with the positive direction of the circuits given by direction of chords in circuit.

For these circuits make up the differential equations, for example:

Differential equations for determining the voltage across the capacitor are determined from the first Kirchhoff's law and have the form:

EMF of three-phase power can be represented as a sinusoidal function of time:

To solve the system of differential equations on computers I have taken the implicit Euler's method, which has good numerical stability, accuracy, relative simplicity and high speed calculation.

The formula of Euler's method with independent variable x in a matrix form is:

To solve the differential equation implicit by the Euler method it needs to transform the original system to the form:

which then can be solved as a matrix equation.

The resulting matrix equation is solved at each step of the integration by the computer program. I’ll make a program in the programming language Visual C++ to conduct research.

Conclusion

It examines the actual scientific question in the master's work. All researchers have argued that the kind of load will have no effect on the parameters of transient processes during earth faults. However, there is a task to check it out on the example of the scheme with the engine load in the distribution network with isolated neutral in this master's work.

The main stages:

  1. Development of a mathematical model of the system.
  2. Writing a program for calculating the system.
  3. Carried out experimental researches and calculations on a computer.
  4. Analysis of the results and assess the impact of engine loads.

In writing this essay master's work is not complete.

References

  1. Гительман Л.Д. Эффективная энергокомпания. Электронная библиотека. 2002. – 545 с.
    [electronic resource]. - Access mode: http://www.ekon.oglib.ru.
  2. Сивокобыленко В.Ф., Лебедев В.К. Выбор величины резистора для заземления нейтрали в сетях 6-35 кВ. Вісник приазовського державного технічного університету. Вип.№18. 2008 р.
    [electronic resource]. - Access mode: http://archive.nbuv.gov.ua.
  3. Сивокобыленко В.Ф., Лебедев В.К., Коваленко Е.В., Маслова А.И. Создание математической модели для сети 6-10 кВ. Публикация: Донецкий национальный технический университет.
    [electronic resource]. - Access mode: http://et–school.com.ua.
  4. Хижняк Алексей Николаевич. Современные способы защиты от однофазных замыканий в сетях с изолированной нейтралью. 2006.
    [electronic resource]. - Access mode: http://www.masters.donntu.ru.
  5. Агутин А.Н., Дергилёв М.П.. Переходные процессы в сетях с двигательной нагрузкой при дуговых замыканиях фазы на землю. 2011.
    [electronic resource]. - Access mode: http://masters.donntu.ru.