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This paper presents optimum required rating of series and shunt flexible ac transmission systems controllers for EHVAC long transmission lines. This is achieved by computing `optimum compensation requirement' (OCR) for different loading conditions of EHVAC transmission systems. The OCR enables improvements to be made to the system loadability, total compensation requirement, line voltage profile and efficiency of power transmission. Two schemes of locating FACTS devices in long distance EHV transmission lines is discussed. In the first scheme, an UPFC of equal rating series and shunt compensator is connected on both ends of the line. In the second scheme SVC of equal rating is connected on both ends with a TCSC at the middle of the same line. For each scheme, requirement of series compensation Mvar, shunt compensation Mvar, Voltages (Vmax, Vmin), total Mvar requirement, Efficiency for surge impedance load is calculated and presented. The results are reported for line operation at surge impedance loading and a line angle of 30deg, chosen from the point of view of stability for voltage levels of 500 kv, 760 kv and 1100 kv under different operating conditions.