Khudchenko Oleg
Faculty: ElectrotechnicalSpeciality: Power stations
Theme of master's work: Isolation defects frequency method development of definition in thermoelectric power station auxiliaries networks
Scientific adviser: Grebchenko Nickolay
In our time humanity practically can not do without electric power, it found the application in all areas of human activity, and its necessity for further development of human society is obvious. From times of origin of energy a question of trouble-free serve of electric power of necessary quality was inalienable part of general conception of electrification. It did not lose the actuality and to date. It acquires the special actuality taking into account a situation folded on a today's moment in the power engineering specialists of Ukraine. In the conditions of permanent the getting worse technical state of basic power equipment considerable part of which produced the resource, probability and frequency of origin of emergency situations increases sharply, that certainly results in the decline of quality and reliability of power supply
At the same time electric isolation of networks of own needs of thermal power-stations and electric machines, electric installation by tension 6 – 10 kV is an important element, which provides reliability, accident-freeness, safety. Due to an exposure and, the most important, to the done early exposure of senescence or wear of isolation of elements of electric network, it is possible to avoid violations in a power supply and failures, related to the loss of electric installations of the dielectric properties an isolation. Due to it is possible considerably to promote reliability, safety and economy of work of the electric stations. In these terms a question is actual about determination of defects of isolation due to the examined frequency method.
In basis of work the task of improvement of method of imposition of spectrum of frequencies is put on the protected electric network, thus, these frequencies must differ from industrial. A scientific novelty consists of that is laid on none or two signals, namely a spectrum of frequencies is in a certain range ( for example from 60 to 50000 Hz).
Basis of master's degree work is development of method which would allow practically to promote safety and accident-freeness of networks of own needs of the thermal electric stations. The supposed practical value of method gotes herein.
Variety of kinds and character of damages, and also some universal method of determination of defects of isolation does not allow to develop a structure and terms of work of networks of own needs of TES. It is yet more difficult to create some universal apparatus. There are such methods of determination of defects of isolation: 1. method of analysis of recurrent tension, 2. location method, 3. wave method, 4. loop method.
1. The method of analysis of recurrent tension is based on measuring and analysis of dependences on time of charging current in the process of charging of capacity of the diagnosed cable by permanent tension of small size, not having influence, and the restored (recurrent) tension on an isolation cable in the isolation of cable after his brief discharging. Procedure of diagnostics of cables with the paper-impregnated with isolation by tension to 35 kV it is necessary to conduct on the following chart: 1 phase. Charging of capacity of cable by permanent tension 1 kV from the source of permanent tension during sufficient great (no less than 30 mines) while for the even charging of all elements of capacity of cable with measuring of size of current of charging, characterizing the degree of moisture of isolation. 2 phase. Brief discharging of capacity of cable (during a few seconds) through bit resistance. There is a digit of basic capacity of cable in this phase, and charge, contained in a «polarization capacity»,to the characterizing the state isolation of cable, saved from very large permanent time of digit. 3 phase. Measuring of temporal descriptions of the restored (recurrent) tension in the process of recharging of capacity of cable after completion of the brief discharging of cable. Upon termination of process of measuring of recurrent tension a cable runs down through bit resistance to the complete flowing down of remaining charge. After it procedure of diagnostics repeats oneself on the described chart at charging of cable by permanent tension of 2 kV. The got curves of recurrent tension, characterizing state and degree of senescence of isolation of the diagnosed cable, are in future analysed, and the followings descriptions are determined:
- maximal size of recurrent tension;
- time of achievement of maximal size of recurrent tension;
- speed of growth of recurrent tension.
The coefficients of linearity are determined also on correlation of maximal sizes and speeds of growth of recurrent tension at two sizes of charge tension (2 and 1 kV). What anymore the coefficients of linearity (diagnostic factor) differ from sizes, characteristic for a «new» isolation, the higher degree of senescence of isolation of the diagnosed cable.
2. The location method is based on measuring of time between the moments of parcel in the line of sounding electric impulse and arrival to beginning of line of impulse, reflected from the site of damage. The location industrial metrologies are subdivided into automatic and manual. The first are used for VL, plugged in a being under working tension electric network. At the wearing-out of relay defence an automatic location seeker, which fixes the sought after distance for time, less one tenth of second, is started. If as a result of successful AUTORECLOSING a line remains in-process, on the basis of the got measuring it is possible to execute prophylactic repair.
Measuring in the period of burning of arc of KZ is an important condition on VL. After going out of arc on damaged VL to get the necessary reflected impulse at manual location not succeeded in most cases. On KL manual location is very effective.
3. A wave method is based on measuring of time between the moments of achievement of ends of line by fronts of electromagnetic waves, arising up in the site of damage.
In case of occurring of hasp of isolation on earth in some point, tension in this point becomes equal to the zero. Hereupon in the place of electrical failure are spreading in both sides electromagnetic waves by tension – U, aspiring at a speed of v to spread a zero potential on all line.
If a damage happened in the middle of line, fronts of waves arrive at both ends simultaneously. Taking into account that length of line of L is known, it is possible to define distance to the site of damage: As exactness of measuring of interval Dt makes units of microseconds, with the same exactness it is necessary to conduct the synchronous account of time on both ends of line At the modern level of technique a parcel is possible from one (anchorman) end of line on other (slave) clock pulses, providing attachment of moments of counting out.
4. A loopback method is based on measuring of resistance the direct current of cuttings-off of tendons of cable. In those cases, when lived, reserved in the site of damage on a shell, does not have a precipice and, in addition, there is one «healthy» tendon in a cable, distance-finding to the site of damage it is possible to carry out a loopback method, based on the use of bridge of direct-current.
For realization of chart of the loopback measuring it is necessary on one end of cable bus to connect the damaged and undamaged tendons a bridge (by a section no less, what at the tendons of cable), providing a contact with small resistance. Usually a bridge is made from a flexible multicore copper rope with reliable clamp a composition-metal. On other end of cable with the same tendons a measuring bridge joins in with a pointer galvanometer and battery.
Tension of battery for the feed of bridge depends on transitional resistance in the site of damage of cable and can be chosen on the basis of the followings reference correlations:
| Transitional resistance is in the site of damage, Ohm | 100 | 1000 | 10000 |
| Tension of battery,V | 4-6 | 20-30 | 100-250 |
A method is a method of determination of defects of isolation without the removal of working tension. Essence of it consists of that on an electric network the spectrum of frequencies is laid on different from industrial frequency of f = 50 Hz. It is carried out by the generator of frequency. Since on a network a signal is imposed, it is necessary to take off and process it. Then by appearance findings it is possible to conduct an estimation about the state of isolation.
More evidently essence of the examined method can be presented by the following animated flow diagram of method:
A chart, presented on a picture 2, can the most suitable chart serve for realization of method.
the frequency method of determination of defects of isolation is developed, the use of which will be instrumental in the increase of safety and accident-freeness of networks of own needs of the thermal electric stations. Presently (May, 2009) master's degree work is in the stage of development. A fair copy will be ready in December, 2010. With questions it is possible to speak directly to me e-mail or to the leader of master's work.
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