In the conditions of constantly deteriorating technical condition of networks on the foreground there is the problem of maintaining a necessary level of reliability of electrosupply of consumers due to the extension of service life of electrical equipment with the deteriorated insulation. According to the operating experience of the most widespread kind of damage in these networks are single-phase short circuit on the ground, constituting up to 90% of the total number of violations of normal operation of the network. As show researches in the current situation the most effective solution of this problem lies in optimization and management of the neutral network.
Development of the physical model of a network with motor load and analysis, with its help, processes for the deaf and the arc short circuits in networks with insulated neutral. On the basis of the obtained results to develop new circuit decisions to limit the surge in распредсетях 6 — 10kv fault phase on the ground. In accordance with that the main tasks are:
* Improvement and development of the physical model of the network created in the laboratory of ТЭВН chair of the CM.
* the Development of methods of carrying out experiments and processing of results of research.
* Research on a physical model of transient processes and the analysis of the obtained results.
* the Development of ways and new circuit solutions for overvoltage restrictions.
There are also new circuit decisions on limiting overvoltage in the network of 6-10 Kv fault phase on the ground.
The practical implementation of the proposed solutions and recommendations on the basis of the analysis of the received results will allow significantly improve the operating conditions of electrical equipment and distribution networks.
In the adopted method physical modeling of electric networks with motor load, developed and implemented a new methodology of processing of results of research.
Study results were presented at the student scientific conference of the CIS "Day of science 2011".
1.1 the Work 6 — 10 Kv grids in modern conditions
Widely used in the present transition coil (DHA) as the means of protection of electrical equipment against the effects of single-phase circuits because of the asymmetry of the phases is not only not improve the work of the network, and on the contrary, create more unfavourable conditions for the insulation of electrical equipment. Bias voltage phases dramatically reduce the life of the insulation of all electrically connected network and do not provide the required performance in the mode of the arc — phase on the ground because the growth multiplicity overvoltage in the network elements is proportional to the degree of asymmetry voltage reaches a value of 3Uф and more. Because of these reasons the accident rate in modern networks reach 120 — 140 damage per year for every 100 km of lines, up to 80% of which are in the interphase short circuit or multi breakdowns isolation of the faulty phase [1].
1.2 Basic theory, used for analysis of means to limit the voltage at single-phase short circuit to earthIN the number of studies numerous switching overvoltages due to any kind of fault and размыканий electrical circuits, the largest number of research was devoted to the very common overvoltages at arc short circuits on the ground in the high — voltage networks, working with insulated neutral.
1.3 Review of existing methods of limiting overvoltage in the network of 6 — 10 Kvthe Network of one and the same nominal voltage at different ways of neutral earthing have a number of differences in technical and economic indicators. Method of neutral grounding in the first turn, affects the size of the earth fault current. Therefore INSTALLATIONS all electrical network, depending on the value of the current, falls into the network with small and network with a large current circuit on the ground. According to accepted in Ukraine with the network of 6-10 Kv belong to networks with a small current circuit on the ground.
MENTATION of THE PHYSICAL MODEL of the DISTRIBUTION NETWORK WITH MOTOR LOAD AND the CHOICE of TECHNIQUE OF EXPERIMENTSStudies were carried out on specially developed models распредсети with motor load and created in the laboratory of the TVD Donntu, in accordance with the calculation scheme of fig. 2.1. At the same time, according to the operating experience of the nature of the damage to electrical equipment for overvoltage in the electric arc short circuits phase on the ground in the networks of their own needs, there were held two series of experiments. In the first — closure phase on the ground was carried out in the supply cable network, and the voltage measured at different points of the network, including the sealing off the stator windings of the electric motors. In the second — isolated winding motor on the body at different distances from the beginning, and the voltage measured at the same point of the network, as in the first series. To assess the influence of the parameters of mains supply on the surge in the experiments were modelled by an equivalent capacity of a network of own needs of blocks of 200, 300 and 800МВт by connecting a different number of cable cells. Simulation of the phase ground short circuit in different points of the cable network was carried out with the help of the cut, connected to the model cells cable network and tap the stator windings of the electric motors [3].
Direct oscillography of transient processes for research overvoltage with such a large number of factors, influencing them, would require significant expenses of time for carrying out of experiment and subsequent processing of the information received. The matter is that the transition processes in the arc short circuits phase on the ground is, as is known, the short-term and repeated periodically, so their view is possible only after the disturbances and photographic processing film. It is more convenient such short-term phenomenon receive in the form of photographs, that enables you to view a large number of oscillograms of the studied processes and to select for the subsequent record on a film the most characteristic in terms of the stated tasks, and information. In this work it is reached by the periodization of the transition process, i.e. repeat with the imposition of overlapping images on each other with the help of a managed device (fig.2.1), showing the process of the phase ground short circuit in each half — cycle of industrial frequency. Photography phenomenon with the screen электронопроминевого oscillograph С1 — 81 was made with the help of the camera. The resulting minimum number of waveform has significantly reduced the amount of time used on their treatment.
Figure 2.1 – The concept of the physical model of the distribution network with motor load
For the convenience of measurements waveform their increase at a print on photo was selected in such a way that one cell scale of the oscilloscope was the size of 10х10мм. At the same time, in the case of absence of special provisions, for large-scale unit voltage is taken amplitude phase voltage, equivalent to 0.5 cell, and the time scale of the taken out of the calculation of 100мкс one cell oscilloscope screen.
For an example in fig. 2.2а, b are given waveform transient processes in healthy phases, shot according to the developed technique, nature and value overvoltage are in good agreement with the waveforms obtained in real networks.
Figure 2.2 a,b — intact phase
the Frequency of oscillations is determined by the settings network and simulated networks, it ranged from hundreds of Hz up to several кГц.Максимальная multiplicity overvoltage on healthy phases obtained in the experiments, was 3.2 U f. at the same time on the other healthy phase overvoltage amounted to 2.4 U f. voltage in the neutral has also a kind of high-frequency oscillations superimposed on the bias voltage neutral, but in contrast to the voltage on healthy phases, the frequency of oscillations is noticeably higher. Therefore traditionally been used for studies of these processes шлейфным oscilloscopes with mechanical scan these fluctuations are not fixed. At the same time, the resulting surge reach two and more times the.
To assess the influence of the parameters of the network and establishment of maximum multiplicity of overvoltage in the network elements, removed on the same distance from the power source studies have been carried out for networks with characteristic for various operating modes of the network parameters for the current circuit. Experiments have been carried out at currents in the range of 2,2A to 20A.
the Study is performed for different parameters networks, have shown, that the multiplicity of overvoltage, depends on the parameters of the network, in all electrically connected network in different places: on the buses, the terminals of electric motors of different power, the length of the cable differ from each other and in magnitude and frequency parameters. The most high overvoltage take place at the terminals of electric motors of the most remote from the power supply. With engines smaller capacity the voltage is less than the engines greater power. For the scheme they amounted to 3.6 U F against 3.2 U F on tyre section of the network to a motor нагрузкой.На tire and cable length value should be approximately the same [1].
3. the analysis findingsOn the basis of a large amount of research carried out with the use of mathematical and physical models of network and separate experiments in real networks, we have found a fundamentally new phenomenon, requiring a new approach to the selection and arrangement of the means of protection from electric arc voltage.
When you select neutral for each specific network should take into account the specific features of, in particular: its parameters, the state of isolation, category of consumers, the means of protection against short circuits on the ground, requirements for electrical safety, etc.
1) Developed a physical model of the distribution network with the motor load.
2) Were Studied on a physical model of transient processes in systems with motor load at arc short circuits phase on the ground and the analysis of the obtained results.
3) Has Developed methods of restrictions and a number circuit solutions to reduce the frequency and duration of arc overvoltage Made the circuit of the developed methods.
4) The efficiency of developed methods in the physical model and the existing substation.
When writing this master of the abstract work is not completed yet. Date of final completion: December, 2011 Full text of the work and materials on the subject can be obtained from the author or her supervisor after this date.