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%.     

Operating capability of ac EHV mixed lines with overhead and cables links

A previous paper has been devoted to the operating capability and constraints involved in power transmission of long ac cable links. In this paper the approach has been extended in order to show the transmission performance of some typical mixed configurations with overhead lines (OHL) and cable lines (UGC). These new capability charts are enhanced with other informative parameters, which allow having a wider knowledge of the regimes of a given mixed line. The necessity of limiting the switching overvoltages during composite line energization, by means of shunt reactive compensation, is investigated and criteria for choosing the compensation degree are also presented.