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Underground cables will be applied more often within new connections of the extra-high-voltage (EHV) transmission network. Since EHV underground cables are a relatively new technology, not much is known about their behavior in large transmission networks. Underground cable connections (consisting of cables, joints, and terminations) are, in general, less reliable than traditional overhead lines, mainly because of their much longer repair time. This can negatively influence the reliability of the transmission network as well. It is therefore of interest to study the reliability of underground cable connections in detail. This paper concentrates on the reliability of EHV underground cable and overhead line connections. Based on actual failure statistics of individual components, the reliability of a connection is analyzed, and which factors are of influence and what measures can be taken to improve the reliability are studied. An optimized cable repair process and a configuration with additional disconnectors are two solutions.

Integration of long EHV XLPE underground cables in traditionally OHL-based transmission systems can result in abnormal transients caused by large cable shunt capacitance. Transients associated with switching of long cables may result in severe temporary or transient overvoltages. This paper investigates energization transients related to the installation of long 380 kV cables in transmission systems. Voltages and currents at multiple locations were analyzed in the time-domain after energization of different cable lengths in a 80 km mixed OHL-cable connection in the future Dutch 380 kV grid. Moreover, zero-missing phenomenon, as one of the likely events during energization transients of shunt compensated cables, and its countermeasures are discussed. The study comprises time-domain simulations in PSCAD/EMTDC environment, where an accurate distributed frequency-dependent parameter model of the complete Dutch 380 kV grid is developed.