Nehmdoh A. Sabiha

Probabilistic model for MV spark-gap characteristics with lightning induced overvoltage superimposed on AC voltage

In this paper, the breakdown probability of 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 introduced 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 results show 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. Finally, 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 voltages using a statistical approach.

Earth Fault Distance Estimation Using Active Traveling Waves in Energized-Compensated MV Networks

In Nordic countries, distribution networks are traditionally unearthed and increasingly compensated. For such networks, switching their neutral point to the earth is practically applied through a resistor for better selectivity functions of earth faults. In this paper, the neutral switching and, consequently, an arrival time of the aerial mode traveling wave reflected from the fault point are utilized to accurately determine the earth fault distance. A concept to create traveling waves is implemented by earthing the neutral via a controlled thyristor that provides a short period of high fault current and produces traveling waves to estimate the fault distance. Much higher transient signals are generated by earthing through an opposite charged capacitor. A capacitor-resistor divider is utilized to measure the reflected surge over a heterogeneous distribution feeder. An adaptive setting is proposed for stamping the arrival surge. The results provide evidence of the efficacy of the proposed fault distance estimation.

Experimental verification of distribution transformer model under lightning strokes

In this paper, the distribution transformer model under lightning impulses introduced by Piantini at no load conditions is investigated. This model is modified where this modification takes more than one resonance frequency into consideration during the model parameters calculation. Therefore, the frequency response of the simulated transient voltage is improved comparing with Piantini model. The 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. The comparison showed a good agreement between the simulation and experimental results.

Probabilistic Risk Assessment of MV Insulator Flashover Under Combined AC and Lightning-Induced Overvoltages

The effect of lightning-induced overvoltages is more profound in overhead distribution lines due to their limited height and low insulation level. Consequently, there is a high risk of line insulation flashover when exposed to lightning-induced overvoltages. This paper presents a probabilistic method to assess the risk of insulator flashover in medium-voltage overhead lines due to lightning-induced overvoltages. In order to accomplish this, a modified Gaussian cumulative distribution function has been used to predict the probability of single-phase, two-phase, and three-phase flashover of insulators under combined ac- and lightning-induced overvoltages. The validity of the modified probabilistic model is confirmed through experiments carried out in the high-voltage laboratory. Next, Monte Carlo simulations were performed on the simplified Ruscks model to generate the distribution of peak lightning-induced overvoltages. Finally, the risk of insulator flashover is calculated based on the distributions of lightning-induced overvoltages and insulator flashover voltages. The proposed procedure could be considered beneficial to select the optimum insulation level required against lightning-induced overvoltages by distinguishing between single-phase and multiphase flashover faults.