1919.] 
Departmental Reports. 
139 
condensers, but it is usually found necessary to supplement these devices 
by synchronous condensers located at the substation end of the various 
transmission-lines, and where employed it is best to adjust the condenser 
for a 95- or 96-per-cent, power factor. This value keeps the size of 
condenser within reasonable limits; and it will be found, owing to 
the variation of current along the transmission-line, that power can be 
transmitted more economically, generally speaking, at 94 to 97 per cent, 
power factor than at unity power factor. The latter requires a much 
larger machine compared with the size required for, say, a 97-per-cent, 
power factor. 
The conditions above mentioned entail the normal use of synchronous 
condensers to adjust or control the load factor, but cases arise in which 
they are put to a different use—viz., to hold and to maintain a constant 
potential difference between the two ends of the transmission-line, or 
between bus-bars at power-house and substation ; in which case they are 
designed of such dimensions as to enable a leading power factor to be 
obtained under certain conditions of load. 
At a difference of potential of 100,000 volts between phases there is 
no difficulty in transmitting power up to 100 miles or thereabouts with 
due regard to the economical use of the conductor and with due regard to 
limits imposed by good regulation. Between 100 miles and 150 miles 
good regulation is possible with less economy in transmission, but beyond 
150 miles the difficulties begin to increase, so that it is necessary to 
employ synchronous condensers in the special manner described. 
In Table 10 is set out the calculations of the disruptive critical volt' 
age, the visual critical voltage, and the loss due to ionization for a 7/14 
conductor with 6 ft. spacing on a 66,000-volt three-phase transmission 
system, from which it will be seen that this size of conductor is in the 
margin of possible use. 
Diagrammatic Representation. 
It is always advisable to represent the characteristics of a trans¬ 
mission-line graphically, and to take account of the impedence of the 
transformers at each end and of the impedance-coils if used. If the 
characteristics of the transformers to be used are not known the following 
values may be taken as representative : transformer reactance voltage, 5 
to 6 per cent.; transformer resistance voltage, 0-7 to 0*8 per cent. Each 
set of impedance-coils will have about the same ratio of reactance voltage 
to the line voltage as the transformers, but the resistance voltage is 
usually less and between 0T5 and 0-2 per cent. D 74 is a representation 
of Table 9 combined with transformer characteristics at each end having 
6 per cent, reactance and 0*8 per cent, resistance, together with the 
characteristic of an impedance-coil at the power-station or sending end, 
having 6 per cent, reactance and 0-2 per cent, resistance in term of the 
line voltage. 
The total inclusive voltage drop is 11*3 per cent., of which 4-8 per 
cent, is in transformers and impedance-coils, and the remainder, or 6 5 per 
cent., in the line. And inasmuch as the regulation of a generator is 
limited to 10 or 12 per cent, it will be realized that the reactance of 
the transformer and the choke-coils places a serious limitation upon the 
range of transmission and upon the amount of power to be transmitted 
economically, because, generally speaking, only about half, or say 5 or 6 
per cent., is available for transmission purposes. 
