Mikhail Dmitriev
Еlectrotechnical faculty (ET)
Department electrical power plants (EPP)
Speciality "Electrical power plants" (EPP)
"Improving the working conditions of measuring voltage transformers in electric networks with a small capacitive current of ground fault "
Scientific adviser: Mikhail Dergilyov
Abstract on the final work
Contents
Work general characteristic
Theme urgency
The work purpose
Scientific novelty
Practical value
Research methods
1 Modern conditions of the problem and ways of its decision
1.1 Work of networks 6-10 kV in modern conditions
1.2 Basic theories used for the analysis of ways of restriction of overvoltage at single-phase short circuit on the earth
1.3 Review of existing ways of restriction of overvoltage in networks 6-10 kV
2 Theoretical and experimental researches of transients in networks with VT at phase short circuit to the earth
2.1 Equivalent circuit of a network
2.2 Basic results of work
2.3 Analysis of results of research
The list of publications
Work general characteristic
Theme urgency. Systems of electrosupply of Ukraine are intended for satisfaction of requirements of consumers in electric energy according to the set level of reliability, safety and quality, with the minimum losses of the electric power, stimulating besides economy of power consumption. In the conditions of constantly worsening technical condition of networks on the foreground the maintenance problem at necessary enough level reliability of electrosupply of consumers at the expense of prolongation of service life of an electric equipment with the worn out isolation is put forward. According to operating experience by the most widespread kind of damage to these networks single-phase short circuits on the earth, components to 90 % from the general number of infringements of normal work of a network are. Operating experience of considered networks shows, that one of the most damaged elements of networks are measuring transformers of pressure. In the literature there are many inconsistent data about the reasons of damages of transformers. The positive decision on the given question can be found only on the basis of in-depth study of processes in networks with pressure transformers at arc short circuits on the earth in different operating modes of these networks.
The work purpose: the analysis by means of physical and mathematical modelling of transients at deaf and arc short circuits on the earth in networks with the isolated neutral. On the basis of the received results to establish the reasons of damage of transformers of pressure and to develop ways of the decision on perfection of working conditions of measuring transformers in considered networks.
According to it the primary goals of work are: 1) carrying out theoretical and experimental researches by definition of qualitative and quantitative parametres of transients in networks with the isolated neutral at arc short circuits on the earth; 2) the process analysis ferroresonance in the pressure transformer; 3) carrying out of experimental researches according to possibility of occurrence of the resonant phenomena in electric networks different in parametres with transformers of volotage at arc short circuits on the earth; 4) working out of actions and circuit decisions on perfection of working conditions of measuring transformers in considered networks.
Scientific novelty: new ideas about the reasons of damage of measuring transformers of pressure in electric networks with the isolated neutral are offered and circuit decisions on perfection of transformers in the specified operating modes are offered.
Practical value: practical realisation of decisions offered in work will allow to improve essentially working conditions of measuring transformers and to lower their damageability in networks of a considered class of pressure.
Research methods:theoretical and experimental researches of transients with use of physical and mathematical modelling of electric networks with the isolated neutral.
THE WORK MAINTENANCE
1 MODERN CONDITIONS OF THE PROBLEM AND WAYS OF ITS DECISION.
1.1 Work of networks 6-10 kV in modern conditions.
Widely applied now arcing coils as a protection frame of an electric equipment from consequences of single-phase short circuits because of asymmetry presence on phases not only do not improve network work, and on the contrary, create more adverse conditions for work of isolation of an electric equipment. Warps of pressure on phases sharply reduce isolation service life of all электрически the connected network and do not provide demanded efficiency of protection of an electric equipment of networks in a mode of arc short circuits of a phase on the earth including measuring transformers of voltage since growth of frequency rate of overvoltage on network elements is proportional to degree of asymmetry of voltage on phases, and can reach sizes (3-4) Uf and more. Because of the specified reasons breakdown susceptibility in modern networks reaches 120-140 damages in a year on each 100 km of lines, to 80 % from which develop in междуфазные short circuits or many-placed breakdowns of isolation on the damaged phase.
1.2 Basic theories used for the analysis of ways of restriction of overvoltage at single-phase short circuit on the earth [1].
Among researches of the numerous switching overvoltage arising owing to any short circuits and disconnections of electric chains, the greatest quantity of researches has been devoted rather widespread overvoltage at arc short circuits on the earth in the high-voltage networks working with the isolated neutral.
The founder of researches of these overvoltage was Petersen which in 1916 has developed the theory explaining physical essence of process of occurrence of the maximum overvoltage.
In 1923 Peters and Slepian have offered other theory which is essentially distinct from theory Petersen.
Later these theories were supplemented with various authors on the basis of theoretical and laboratory researches concerning levels of the maximum overvoltage and the form of their development.
In 1957 N.N.Beljakovym has been published the theory of occurrence of repressure at arc short circuits on the earth also in networks with the isolated neutral.
Process of occurrence of the maximum overvoltage according to theory Petersen has following prominent features:
Repeated ignitions of an earthing arch are represented in the form of metal short circuits. In this connection presence at an arch volt-ampernoj of dependence which actually for currents of high frequency has obviously dynamic character is not considered, i.e. pressure on an arch has no accurately expressed peaks of clearing and ignition as it is usually considered to be for the static characteristic. Process deionization is late concerning change of a current in an arch.
Repeated ignitions on Petersen occur regularly through each half-cycle at the maximum voltage on the damaged phase, when pressure of the power supply to equally maximum value. Maximum an overstrain can reach sizes 7.5 Uf.
Duration of burning of an arch at each repeated ignition is equal to a half-cycle of free fluctuations in spite of the fact that the size of a current and speed of its change with each half-cycle increases, and also its thermal and ionising action increases.
After each clearing of an arch in a network there is an accruing constant voltage of displacement Uсм.
Voltage Restoration on the damaged phase after arch clearing has oscillatory character with the high-frequency peak exceeding size of phase voltage. However, it is assumed, that dielectric durability of a place of damage accrues faster, rather than restored voltage.
At each half-cycle of an overstrain change the sign.
Characteristic features of occurrence of overvoltage under Peters's theory and Slepian are:
Repeated ignitions are represented also in the form of metal short circuit on the earth. They occur regularly through each period at the maximum value of voltage on the damaged phase (at the first and all subsequent ignitions accordingly ± Uf and ±2 Uf).
Duration of burning of an arch at each repeated ignition is equal to a half-cycle of industrial frequency.
As arch clearing occur at each passage of a current of industrial frequency through zero value clearing peaks are absent. Voltage restoration on the damaged phase after arch clearing occurs smoothly to industrial frequency.
Identical (except for the first) an overstrain at each ignition of an arch are formed as a result of not changing initial and final pressure on the damaged phases, accordingly ±0.5Uf and ±1.5Uf.
Overstrain of a sign do not change.
For occurrence of the maximum overvoltage on Belyakov coincidence of two basic conditions in one cycle is necessary, namely:
The First ignition of an arch should occur before a maximum э.д.с. The damaged phase, that by the clearing moment (a pressure maximum on the damaged phase) the first peak of restored pressure reached sizes 0.4Uf.
The Second ignition of an arch at which on a lagging behind phase there are the greatest of an overstrain 3.2Uf, should occur during the moment at the voltage of the damaged phase approximately equal 2.2Uf, i.e. more than the first ignition [1, 2].
1.3 Review of existing ways of restriction of overvoltage in networks 6-10 kV.
Networks of the same rated voltage at different ways of grounding of a neutral have a number of distinctions in technical and economic indicators. The way of grounding of a neutral first of all influences size of a current of short circuit on the earth. Therefore PUE all electric networks, depending on current size, subdivides into networks with small and networks with the big current of short circuit on the earth. According to the norms of a network accepted in Ukraine 6-10 kV concern networks with a small current of short circuit on the earth [2].
2 THEORETICAL AND EXPERIMENTAL RESEARCHES OF TRANSIENTS IN NETWORKS WITH VT AT PHASE SHORT CIRCUIT TO THE EARTH
2.1 Equivalent circuit of a network
For theoretical researches for a basis the network represented on fig. 1 in which the power supply is presented by the EMF and equivalent circuit parametres is taken, the network is presented phase to phase by capacity and capacity of phases in relation to the ground, the measuring transformer is represented in the form of inductance of dispersion of windings and their active resistance, the active resistor is included in neutral of the measuring transformer. The mode of short circuit of a phase on the earth was simulated by short circuit and disconnection of knife switch S [3].
Drawing 1 — the Basic equivalent circuit of an electric network with VT, for research of transients in a mode of arc short circuits of a phase on the ground
2.2 Basic results of work:
1) As a result of the spent researches it is established, that one of the possible reasons of damage VT in networks with the isolated neutral at arc short circuits on the earth is ferroresonance the processes arising in a network after clearing of an arch in a place of short circuit of a phase on the earth that is illustrated by the oscillogram presented on drawing 2.
2) By results of research it is offered a number of the actions which practical realisation will allow to improve essentially working conditions VT, in considered networks with the isolated neutral and to lower their damageability.
2.3 Analysis of results of research
On the basis of great volume of the researches executed with use of mathematical and physical models in a network with VT and separate experiences in real networks, we find out essentially new phenomena, about the reasons of damageability of measuring transformers in electric networks with the isolated neutral at arc and alternating short circuits of a phase on the earth, demanding essentially new approaches to a choice of means and circuit decisions for perfection of working conditions VT in considered networks.
At a choice of protection frames for each concrete network its specific features, in particular should be considered: its parametres, an isolation condition, a category of consumers, presence of protection frames from short circuits on the earth, the requirement to an electrosecurity etc.
Drawing 2 — the Settlement oscillogram of transients in a network with VT at arc short circuits of a phase on the ground.
(Animation: volume — 76 KB; size — 76 KB; number of frames — 4; delay between frames — 50 ms; number of cycles of recurrence — 7.)
THE LIST OF PUBLICATIONS
- Лихачев Ф.В. Замыкания на землю в сетях с изолированной нейтралью и с компенсацией емкостных токов. — Москва: Энергия, 1971. — 254 с.
- Евдокунин Г. А., Гудилин С. В., Корепанов А. А. Выбор способа заземления нейтрали в сетях 6-10 кВ. Электричество, 1998, N 12
- Брянцев А.М., Базылев Б.И., Бики М.А., Уколов С.В., Долгополов А.Г.. Лурье А.И., Евдокунин Г.А., Славин Г.А. Управляемые подмагничиванием шунтирующие реакторы - новое электротехническое оборудование. Электротехника, N 7, 1999.
!!! When writing this abstract the master’s qualification work is notcompleted. Date of final completion of work: December, 2010. Full text of the work and materials on a work theme can be received from the author or his scientific supervisor after that date.
