IMPROVEMENT OF TRANSMISSION-LINE GROUND CIRCUIT CURRENT-CARRYING LIMIT AFTER SYSTEM INTERCONNECTIONS

When one or more transmission lines are added to an existing transmission system, the intensity Of the fault currents flowing along the ground wires near the substations may exceed the ground wire thermal limit. In such cases, it is generally preferred to improve the existing ground wire conductivity by replacing, say a steel ground wire with an ACSR ground wire along the first spans near the substation. Generally, this procedure requires that the transmission line be de-energized, what may sometimes be considered unacceptable. This paper deals with optional procedures which can be implemented in the first line kilometers near the substations keeping the existing lines energized. Such procedures include sectionalizing of ground wires, use of continuous counterpoise and use of additional overhead ground wires installed below the conductors. This paper introduces also a technique called by us ''tower foot resistance adjustment'', which complements the corrective effects of the mentioned procedures. The extension of the ''Decoupled Method'' [5,6,4,3,2,7,1] to represent the above suggested procedures is detailed in this paper which also includes a practical example occurred when the 220 kv Paraguayan system was connected to the Itaipu Collector System. The existing 220 kV lines have steel ground wires and leave the Acaray 250 MW Hydroelectric Power Plant. These transmission lines have been designed some 20 years ago to feed the main 220 kV Paraguayan network, i. e., a long time before the itaipu Power Plant location studies had been started. The connection between the Itaipu System and the Paraguayan System is presently done by two 5 km long 220 kV transmission lines, (L-1) and (L-2) having ACSR ground wires, as shown in Figure 1.