Steady-state operating conditions of very long EHVAC cable lines

In the present paper the authors investigate steady-state capabilities of very long EHVAC underground cable lines, without intermediate compensating stations, installed in meshed networks. Analytical formulas of cable length-loading relationship are presented to this purpose. The study shows that, in optimal operating conditions, most of the ampacity of such long underground cable lines can be exploited for active power transmission. Non-optimal operating envelopes are studied, showing the effect of terminal voltages and line losses on cable utilization and evidencing limited derating under realistic hypotheses. A parametric analysis of loading limits and possible voltage violations along the cable is shown; a simple criterion for optimal utilization of lossy cable lines is also proposed. Line-end shunt compensation requirements for integration of long EHVAC cable lines in transmission networks are then specified. Studies performed for both ideal and real cables and shunt reactors show that active power transmission of 100 km long, 400 kV-50 Hz underground cable lines can attain 90% of their thermal limit, without intermediate compensating stations. Excess reactive power and temporary overvoltages are effectively controlled by line-end compensation around 90%.