Whatever the length of the line C is about 8 to 9.5 nF/km (may go up to about 12 nF/km with bundled conductor lines) . For a short line the total Capacitance ( C x length) is small so that the value of Vs, Is, Regulation and Efficiency are almost the same with or without taking into account the total capacitance- error is within 2 to 3%. Hence it is OK to neglect the effect of capacitance.
Capacitance effect is due to the tie line and ground and it is in every sort of lines may it be SHORT ,MEDIUM OR LONG TRANSMISSION LINE
Capacitance effect is present in small tie lines also but it so small that it can be neglected.