POLYPHASE CIRCUITS 



121 



to the high-tension line at 60,000 volts what must be the gen- 

 erator's excitation? What is the combined efficiency of the gen- 

 erator and the transformers at this load? If this load were 

 removed what would be the high-tension line voltage if the 

 excitation of the generator were unchanged? Use (a) the syn- 

 chronous impedance method and (b) the magnetomotive-force 

 method when calculating the field current of the generator. 



68. A lOOO-kv.-a., 2400-volt alternator delivers energy to a 

 high-tension transmission line through step-up transformers both 

 the low- and high-ten>ion windings of which are connected in Y. 

 The armature windings of the generator which are connected in 

 delta have an effective resistance of 0.20 ohm per phase. The 

 resistance of the field circuit is 0.427 ohm. 



GENERATOR CHARACTERISTICS 



The friction and windage loss is 8.2 kw. 



TRANSFORMER CHARACTERISTICS 



The transformers have a ratio of transformation of 1390:12,700 

 vnlts. With the low-tension winding short -circuited and with 

 332 volts impressed on the high-tension winding the current is 

 26.2 amperes and the power is 3120 watts. The core loss at the 

 rated voltage is 2220 watts. 



The heaviest load that the high-tension line requires is 1000 kw. 

 at 0.88 power factor and t In- necessary line voltage is 24,500 volts. 

 Whatmustbethotrnnin.il voltage of tin -exciter for this load? 

 What is the combined Hi* iency of the generator and the trans- 

 I'onmTs at this l<>:id'.' 



69. A 1500-kv.-a., 5500-volt alternator delivers energy to a 

 high-ten-ion tran-miioii line through three step-up transformers, 

 the low-ten-ion windings of which are connected in delta and the 

 high-tension windings in V. Tin- neutrals of the generator and 

 of the high-ten-i.,!! transformer \\indinL:- in grounded The 

 transformers have a ratio of transformation of 3.2 to 1.0. The 

 calculated resistance and reactance of the high-tension line are 



