Analyzing switching transients of parallel connected transformers

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Prikler László
Department of Electric Power Engineering

The transient processes caused by transformer energizing are in focus since the invention of the transformers. Their physical description is mature, their problems are well known and there are many solutions to these problems.

Sympathetic interaction became the subject of thorough studies in the last two decades. This phenomenon appears always together with the transformer inrush, if to the busbar, to which the incoming transformer is connecting another transformer is already connected. In such a case the also the operating transformer will receive a current-transient, and the transient of the connecting transformer will be prolonged.

This interaction was observed in Kelenföld between the transformers in the 120/10 kV/kV substation and the transformers in the power station near to the substation. An EMTP-model was prepared to simulate the incident.

The result of the simulation was that by such network composition and transformers the sympathetic transient doesn’t diminish considerably the life span of the transformers in the power station. The magnitude of the current in the power plant transformers grows not higher than one and a half of the nominal value.

I’ve analyzed the effect of the sympathetic currents on the differential protection too. Fault trip can be generated both by the parallel transformers in the power station and that in the substation, if the synchronous generators are switched off. The sympathetic magnetizing current is rich in 2nd and 5th harmonic as well as the inrush current according to the simulation results. Therefore the harmonic restraint is useful also in that situation.

We don’t have to take into consideration the problem of the fault trip, if the inrush current of the incoming transformer is minimized by controlled switching. This solution may also increase the life span of the transformers. This method can be used successfully by the transformers in the substation according to the results of the simulation. However the results by the transformers in the power station are not so favourable. These have a cable connection to their circuit breaker which is longer than 600 meters. The capacitance of the cable causes a large oscillation in the flux in the iron core of the transformer by switching off, and this can mislead the controller regarding the optimal switch on time.

The credibility of the simulation is proved by comparison with another simulation, that used a similar model and had also measurements on the modelled network. An ultimate proof would be a measurement on the network which was modelled in this work. That’s why I made a measurement program which can be found at the end of this work.


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