Evtushenko Sergey
Electrotechnical faculty
Chair electric systems
Speciality "Electric systems and networks"
Working out of the electronic methodical grant "Electromagnetic transients in electric systems"
Leader of work: Larin Arkadiy
ABSTRACT
The maintenance
1. INTRODUCTORY PART
URGENCY
THE PURPOSE AND WORK PROBLEMS
2. THE BASIC PART
THE THEORETICAL ANALYSIS
3. FINAL PART
CONCLUSIONS
THE LITERATURE
URGENCY.
The course "Transients in electric systems" is one of main for electropower specialities and specialisations.
Transients arise in electric systems as at normal operation (inclusion and switching-off of loadings, the power supplies, separate chains and others), and emergencies (breakage a chain or its separate phase, short circuit, loss of the car from synchronism etc.). Their studying cannot be end in itself. It is necessary all for clear representation of the reasons of occurrence and physical essence of these processes, and also for working out of practical criteria and their methods estimations so that it was possible to expect and in advance to prevent dangerous consequences of such processes. To put it briefly, it is important to understand transients, but it is even more important to be able to operate them meaningly.
At any transient there is to some extent a change of an electromagnetic condition of elements of system and balance infringement between the moment on a shaft of each rotating car and the electromagnetic moment.
As a result of this infringement speeds of rotation of cars accordingly change, i.e. some cars test braking, while others – acceleration. Such position exists until regulating devices will not restore a normal condition if it in general is realizable under the changed conditions.
From the told follows, that transient is characterised by set of electromagnetic and electromechanical changes in system.
By means of this degree work it is possible to show efficiency of use of computer technologies at studying of properties of transients, and also to be convinced,
That use of computers and software package MathCAD, resolves
To reduce expenses of time for calculations and to raise their accuracy.
THE PURPOSE AND WORK PROBLEMS.
As calculation of electromagnetic transient usually understand calculation of currents and pressure in the considered scheme under the set conditions. Depending on appointment of such calculation find the specified sizes for the set moment of time or find its change during all transient. Thus the decision is usually spent for one or several branches and scheme points.
To number of problems for which practical decision make such calculations, concern:
- Comparison, an estimation and a choice of the scheme of electric connections as separate installations (stations, substations), and systems as a whole;
- Revealing of working conditions of consumers at emergency operation;
- A choice of devices and conductors and their check on working conditions at short circuits;
- Designing and adjustment of devices of relay protection and automation;
- Definition of conditions of nonsynchronous inclusion of synchronous cars and inclusion by their way of self-synchronisation;
- Constructive the decision of elements of switching centres and, in particular, wires on the big working currents;
- Definition of number of the earthed neutral and their placing in system;
- A choice of number and capacity of the compensating devices extinguishing an arch;
- Definition of influence of transmission lines on communication and alarm system wires;
- Designing and check of protective grounding;
- Selection of characteristics of rated sportsmen for protection against repressure (including protection of condensers, installations of longitudinal indemnification);
- Carrying out of various tests;
- The analysis of failures which have occurred.
The automatic control of excitation is now generally applied to all power plants (generators), attached to electrical systems. We recalculate the main problem (that relates to static properties of the system and that are considered by the method of small oscillations) that can be solved by adjusting the excitation:
- Rise of the boundaries of the transmitting power by controlling the EMF of the generator and eliminating the factors that could cause the system self-oscillation without the boundaries of tolerance;
- Improvement of the quality of system regime at the expense of maintaining the voltage in the initial transmission and provide possible quick cut-off of small fluctuations which occurs in normal operation and adversely affect the quality of the system;
- Improvement of the system parameters specifically, changing the natural frequency of vibration and eliminate the possibility of resonance oscillations.
In order for the excitation regulators to satisfactorily solve the above problems, you need some ways to choose the parameters of the system excitation, including the parameters of most regulators. The solution of each task superimpose on the choice of the parameters of their regulatory requirements, which changes with the change of the scheme and parameters of the regulated system, as well as changing its regime.
Task analysis – The verification of the stability of the system and determination of the quality of the transition process with the given parameters and control system;
The synthesis task – when, on the basis of certain requirements for sustainability and quality of the transition process regulated system is determined by the type of controller (block diagram of control system) and its parameters.
During the analysis of the regulated system, exactly as in the synthesis, it is necessary to mathematically describe the processes that occurs in it, so that appropriate solutions to differential equations to find all the necessary coefficients, which is characterized by the consistent work for a given system or to the given parameters to check whether or not the system is stable.
Drawing up such an equation and analysis of its properties for the excitation of a strong regulatory action would be the main aim and objective of the thesis. To achieve the objectives of the work were raised and the following tasks:
- The software development for the study of theoretical principles of analysis of static stability of power systems with AVR strong action;
- Development of a software to perform calculations as regards to the condition determinations of the static stability of the method of small vibrations;
- The study of an electrical system with AVR strong action;
- Development of guidelines for the establishment of the laboratory work to examine the conditions of static stability of a simple electrical system with ARV strong action.
THE THEORETICAL ANALYSIS.
For this we will consider the initial moment of sudden disfunction of the regime. All the values in the initial moment of sudden disfunction of the regime can be obtained from their particular solution for t = 0. Moreover, since the inductance circuits eliminate the sudden current change, then the value of the latter in the initial moment of the transition process is well known: it is preserved such as in the end of the previous regime. Since the task set is limited to considering only the initial moment, the rotation of the rotor and the resulting change in inductance of the machine, it apparently doesn’t play any role. In other words, in this instance, the machine can be viewed as a transformer. We will pay attention to the balance of the magnetic flux in the longitudinal axis of the synchronous machine rotor under steady symmetrical mode of operation with lags in phase current (Figure 1).
Figure 1 – The balance of magnetic flux
in the longitudinal axis of thea – in the previous mode, b – at the time of sudden
change in regime.
In accordance with the Lenz law, the increment of the flux will cause a backlash of the law of excitation, and the flux linkage increments must compensate each other, i.e.
or
In loaded machine the flux makes up some constant fraction of the flux, which is characterized by the dispersion of the winding excitation coefficient.
Thus, the dispersion of the excitation winding does not make it possible to characterize the machine at the beginning moment of the transient process reactivity dispersion of the stator emf flux, as the latter preterepvaet change from the current increment, whose value is yet to be determined. If the result is regarded as flux linkage in a no-load mode, then its part that is connected to the stator, will be:
Let us give a more vivid expression of the form:
This emf corresponds to the flux linkage:
which is called the transverse transient EMF. Reactivity
Figure 2 – The vector diagramme of the synchronous car working with
Lagging behind a current
Let's present the vector diagramme which have developed in MathCAD
Figure 3 – Vector diagramme developed in program MathCAD
CONCLUSIONS
1. The regulation of strong action (without the sensitivity zone) extends the stable operation and allows to operate at angles greater than 90. Limiting angle is much less than the smaller constant time exciter.
2. The smaller the time constant of the exciter, the smaller the maximum possible amplification coefficient. This means that choosing such a big amplification coefficient, in which the voltage maintained constant, we can not ensure it to work with large marginal angles. If we abandon the rigid support voltage, allowing its drop with and increase in load, then the stable operation can be achieved at angles of 90 or more.
THE LITERATURE
1. Gorev A.A., Transients of the synchronous car,The state power publishing house, 1950.
2. Shchedrin N.N., Currents of short circuit of high-voltage systems, ONTI, 1935.
3. Shchedrin N.N., Uljanov S.A., Problem by calculation of short circuits, The state power publishing house, 1955.
4. Uljanov S.A., Electromagnetic transients in electric systems, publishing house "Energy", 1964.
5. Atabekov G. I, Theoretical bases of relay protection of high-voltage networks, The state power publishing house, 1957.
6. Rjudenberg R., Transients in electric systems, publishing house of the foreign literature, 1955.
7. Methods of calculations limiting on static stability of modes of power supply systems [the Electronic resource]. Mode of access: http://referats.allbest.ru/programming/103021.html
8. Chair ESIS working out of the automated control system electric systems [the Electronic resource]. Mode of access: http://etf.donntu.ru
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