Iranian Journal of Electrical and Electronic Engineering

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Insulator strings with several material and profiles are very common in overhead transmission lines. However, the electric field and voltage distribution of insulator string is uneven which may easily lead to corona, insulators’ surface deterioration and even flashover. So the calculation of the electric field and voltage distribution along them is a very important factor in the operation time. Besides, no remarkable endeavor regarding insulator material and profile and their impacts upon the electric field and voltage distribution has been made so far. In this paper several 230-kV insulator strings with different porcelain and glass units were simulated using 3-D FEM based software, and their electric fields and voltage distributions were calculated and compared together, to investigate the effect of insulator types on these quantities. Tower and conductors were included in all simulations and also the effect of corona ring on voltage and electric field distribution over insulator strings with different insulator types was investigated. Reported results show the dependency of voltage distribution to insulator material and profile.

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In this article, a novel concept is introduced to improve the radio frequency (RF) linearity of partially-depleted (PD) silicon-on-insulator (SOI) MOSFET circuits. The transition due to the non-zero body resistance (RBody) in output conductance of PD SOI devices leads to linearity degradation. A relation for RBody is defined to eliminate the transition and a method to obtain transition-free circuit is shown. 3-D Simulations of various body-contacted devices are carried out to extract the transition-free body resistances. To identify the output conductance transition-free concept and its application to RF circuits, a 2.4 GHz low noise amplifier (LNA) is analyzed. Mixed mode device-circuit analysis is carried out to simultaneously solve device transport equations and circuit spice models. FFT calculations are performed on the output signal to compute harmonic distortion figures. Comparing the conventional body-contacted and transition-free SOI LNAs, third harmonic distortion (HD3) and total harmonic distortion (THD) are improved by 16% and 24%, respectively. Two-tone test is used to analyze third order intermodulation distortions. OIP3 is improved in transition-free SOI LNA by 17% comparing with the conventional body-contacted SOI LNA. These results show the possibility of application of transition-free design concept to improve linearity of RF SOI MOSFET circuits.

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Overvoltage at the insulator terminal caused by a lightning strike can occur in two ways, i.e., a direct lightning strike on the phase line and ground wire. The insulator can be exposed to the phenomenon of back flashover (BFO) if the terminal voltage of the insulator is higher than its insulator critical voltage The lightning current characteristics are distinguished by the maximum current and the steepness. Differences in the characteristics in this study are identified as International Electrical Commission (IEC) and Conseil International des Grands Reseaux Electriques (CIGRE) impulse waveform standards. The footing-tower grounding system comes in different configurations, such as horizontal, vertical, and grid. Alternative transient program (ATP) software was used for simulating lightning strikes on ground wire and phase lines. The results exhibit that the highest critical voltage of the insulators on the footing tower through grid grounding when the surge current strikes ground wire (3308kV – 3395 kV), with the magnitude of the lightning current ranging from (48 kA – 3395 kA). For lightning direct stroke on the phase line, the critical voltage on vertical grounding is highest on (2938 kV -3021 kV).  The surge current flow footing-tower is highest on the grid. The currents magnitude flow in footing tower were influenced by impedance of grounding.