Transient electric field computation for composite cross-arm in 750 kV AC transmission line under lightning impulse voltage

The transient electric field distribution under lightning impulse voltage (LIV) is vital to the reliability of 750 kV composite cross-arm. However, the transient electric field of electrical equipment under impulse voltage has usually been substituted by the equivalent sinusoidal voltage, without considering the variation of permittivity with frequency. Using the frequency decomposition computation method (FDCM) based on finite element method (FEM), this paper presents the study of transient electric field under LIV for a 750 kV composite cross-arm. The LIV waveform has been decomposed via Fourier transformation. The complex dielectric functions of the insulation materials used in composite cross-arm have been investigated, and the computation parameters for the dc and each harmonic component of LIV have been obtained. Then, the transient electric field distribution can be computed through the superposition of responses from dc and each harmonic component. Moreover, the computation results have been compared with that using traditional method. The comparison indicates that the potential distribution of composite cross-arm is similar. The maximum percentages of electric field strength difference on fittings and silicone rubber sheath are 2.9 %, 14 %, respectively.