Lightning-induced overvoltages in medium voltage distribution systems and customer experienced voltage spikes

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Aalto-yliopiston teknillinen korkeakoulu | Doctoral thesis (monograph)
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Verkkokirja (1950 KB, 178 s.)
TKK dissertations, 209
In Finland, distribution transformers are frequently subjected to lightning strokes for which they are continuously protected by spark-gaps. So, the breakdown probability of medium voltage (MV) spark-gaps is modeled using the Gaussian distribution function under an impulse voltage test in accordance with the IEC 60060-1 standard. The model is presented in the form of the well-known Gaussian tail probability. Accordingly, a modified probabilistic model is proposed to study the effect of impulse voltage superimposed on the ac voltage on the breakdown probability of MV spark-gaps. The modified model is verified using experimental data, where the experimental setup is arranged to generate a range of impulse voltages superimposed on the ac voltages. The experimental verification shows evidence of the efficacy of the proposed probabilistic model. Furthermore, the proposed model is used to evaluate single-phase, two-phase and three-phase spark-gap breakdown probabilities in the case of lightning induced overvoltages. These breakdown probabilities are used along with the simplified Rusck expression to evaluate the performance of MV overhead lines above a perfectly conducting ground under lightning-induced overvoltages using a statistical approach. In order to study the overvoltages propagating through the transformer to its low voltage side, the high frequency model of the transformer is investigated. First, the investigation is carried out using model introduced by Piantini at no-load condition. This model is modified to take more than one resonance frequency into consideration. Therefore, the frequency response of the simulated transient voltage is improved. A verification of the modified model is carried out through the comparison between the experimental and simulation results, in which the time domain simulation is carried out using ATP/EMTP while MATLAB is used to identify the model parameters. As this model is found suitable only for unloaded transformer, an accurate and simplified model is proposed concerning unloaded and loaded conditions as well. The proposed high frequency transformer model is experimentally verified under different balanced load conditions considering two different practical distribution transformers. Then the impact of low voltage (LV) network feeder numbers, lengths, types and loads on the lightning-induced overvoltage reached at the service entrance point is investigated with and without MV spark-gap operation. The high frequency model representation of the distribution transformer and low voltage network are combined in a single arrangement in the environment of ATP/EMTP. A simplified low voltage surge arrester model is represented and verified. Finally, a study is carried out to mitigate the overvoltages by allocating the surge arrester at secondary side of the distribution transformer with concerning MV spark-gap operation.
Supervising professor
Lehtonen, Matti, Prof.
MV spark-gap, distribution function, lightning-induced overvoltage, statistical approach, high frequency transformer model, LV surge arrester
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