Research on the Characteristics of Transient Sheath Overvoltage in Underground Cable Lines

Increasingly more incidents of insulation breakdown in underground cable network have been found in recent years. Considering the unique structure of single core cable compare with overhead line (OHL), this presentation focused on the characteristics of transient overvoltage in metallic sheath of cables. Electromagnetic transient model of typical 110kV cable network in Shaanxi province is established with the use of EMT-type program. Firstly, it is found that sheath voltage in system frequency 50Hz is much lower than the conduction voltage of sheath voltage limiter (SVL). After that, difference of closing timings of circuit breaker is considered to study the influence of residual charge on voltage. Calculation results indicate that sheath overvoltage of single-end bonding points becomes more severe than cross-bonding points, due to the fact that the close loop for residual charge exists while employing cross-bonding configuration. Moreover, profile of frequency characteristics is much rougher while considering the effect of residual charge indicating more severe oscillation of sheath voltage. Hence, more severe electric stresses are imposed on cable insulation.