Steady-State And Transient Ehv Ac Cable Shunt Reactive Compensation Assessment

In recent years attention towards underground cables (UGCs) has been stimulated by the development and performance enhancement of XLPE-insulated cables at HV and EHV level. The liberalisation of the energy market with the related need to connect new power plants and strengthen the grid, in conjunction with public opposition to the construction of new overhead lines, are increasingly prompting the installation of underground HV and EHV a.c transmission lines. The possibility of integrating power transmission and other services (i.e. railway and highway transport, bridges, galleries) in the same corridor or in the same "structure" is another significant technical challenge favouring underground technologies. This field has been advocated by Cigré which has investigated the topic by means of several working groups [1-4]. The paper analyzes some issues related to the electrical behaviour of long EHV a.c. cable lines both in steady-state and in transient conditions. In particular, the paper deals with the choice of the shunt reactive compensation degree from a twofold standpoint, that is, steady-state operation and network transients. The capability charts are a useful tool for highlighting the steady-state operating characteristics of a cable link, giving an immediate outlook of admissible active and reactive power flows along the link. The same charts also show regimes associated to undesirable voltages, and can be used to estimate the needed shunt compensation. The size of shunt compensation, lumped and equally shared at cable line terminals, can be evaluated in closed-form, based on steady-state line no-load energization/load rejection constraints. The paper points out that the rated line- and cable-charging breaking current of standard EHV circuit breakers should be checked as it may set significant minimum shunt compensation requirements, even in powerful meshed networks. It is also of interest to perform transient analyses of the cable line, aimed at verifying shunt reactor preliminary rating and determine other required characteristics of shunt reactors. With the increasing use of cables in the EHV network the likelihood of low-order harmonic resonance, owing to the large cable capacitance, is becoming of more concern. The low order harmonic resonance can give rise to temporary overvoltages, dangerous to line apparatuses due to their sustained nature. Notably, the energy absorption withstand capability of commonly adopted surge arresters can be exceeded. The paper shows that the feasibility of a long UGC line depends, besides the steady-state operation, on its interaction with the network in various operating conditions.