In English


Göteborg : Chalmers tekniska högskola, 2011. 69 s.
[Examensarbete på avancerad nivå]

Electricity is a necessity of the modern life due to its everywhere usage. Its proper continuity depends on the reliability of the equipments used in power system. The transformer is one of the basic components which are commonly used in the power system, generation, transmission or distribution. Its high cost and difficult maintenance highlights its importance in power system. Proper protection is necessary to protect this costly equipment from any damage. Therefore, surge arresters are installed near to the transformer to protect from high voltages. However, during lightning and switching of circuit breakers, re-strikes and pre-strikes during opening and closing of a circuit breaker may lead to a broad band frequency spectrum of overvoltages. Such overvoltages may have an oscillating character and lower amplitude at the transformer terminals. In addition they may arrive at the transformer input without any changes in amplitude and waveform so the surge arrestor will not able to detect any overvoltages near the terminals as well as at the input sides. . If the oscillating frequency component of the external overvoltage is equal to the natural frequency of the windings, then the magnitude of the internal resonance overvoltage has its maximum value. Therefore, overvoltages generated in transients and switching stages in electrical power equipment may be dangerous for the insulating system despite the applied overvoltage protection. The resonance frequencies for transformer winding have been experimentally determined. The frequency band taken into account was between 10Hz and 10MHz, in addition the band has been divided into lower frequency band (10 Hz-200 KHz) and higher frequency band (200 KHz-10 MHz). Then applying sinusoidal waves with the resonance frequencies, voltage stresses on different nodes along the winding have been located. From these voltages stressed nodes it was concluded that the stressed nodes are shifting from centre of the winding towards the upper part of the winding as from lower to higher resonance frequencies. After that the voltage distribution at different resonance frequencies was analyzed. At lower frequency band, the voltage stress between turn to ground in terms of standing wave was observed whereas at higher frequency band inter layer stress was the concluded result. Voltage stress on different nodes under wider frequency was also investigated where the same behavior was seen. Step responses were also applied at the input to investigate the resonance frequencies and their voltage stressed along the length of the windings. Finally the results for sinusoidal wave and step response were compared.

Nyckelord: Reactor, transformer, winding, very-fast transients, transient voltage stress

Publikationen registrerades 2013-01-18. Den ändrades senast 2013-06-03

CPL ID: 171671

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