Very long EHV cables and mixed overhead-cable lines. Steady-state operation

The paper deals with very long EHV cables (say, over 30 km) and their application in EHV mixed lines, i.e. those formed by a cable line between two sections of overhead lines. Such very special lines could be built for instance at the northern borders of Italy, laying the cable in the service gallery of the extra-long high-speed railway tunnels planned for construction under the Alps. EHV cable lines over 25-30 km long require the compensation of the reactive power generated by the cable, by means of fixed or variable shunt devices. After presenting expressions for the calculation of the maximum permissible length of EHV cable lines, the paper shows that 420 kV-50 Hz cable lines long up to 90-100 km, transmitting an active power up to 90% of the cable thermal limit, could be operated by using only two shunt reactors at the cable terminals. Fixed shunt reactors are adequate if the mixed line is practically symmetrical, i.e. the two stretches of the overhead line are of same length. On the other hand, if one of the overhead line sections is much longer than the other, or the terminal voltages of the mixed line are rather different, variable compensation allows to fully exploit the carrying capacity of the cable. The paper presents an algorithm to find the optimal taps of the variable shunt reactor. Results of the steady-state simulation of two hypothetical 420 kV-50 Hz mixed lines are finally reported, also showing that shunt reactors contain efficiently temporary overvoltages due to load rejection and no load energization.