OVER-VOLTAGE PROTECTION 595 



turn cause an additional voltage rise at no load, which is not 

 permissible. 



The voltage rise caused by the charging current in a long line 

 may cause a breakdown of the air nearest the line conductor and 

 cause corona which may seriously increase the transmission losses. 

 They may also unduly strain other insulations on the system and 

 affect the operation of the lightning arresters, the normal voltage 

 range of which should be kept within reasonable limits for satis- 

 factory operation. On the other hand it is well known how the 

 operation of motors is affected by voltage variations and that 

 the life of lamps is seriously reduced if the voltage is too high, 

 not to speak of the unpleasantness of a variation in the intensity 

 of the illumination, which of course accompanies a fluctuation in 

 the voltage. 



From the above it is imperative that the regulation of a modern 

 system be kept within certain permissible limits, and with high- 

 voltage systems this is most readily accomplished by installing 

 synchronous condensers with automatic voltage regulators in the 

 substation. As previously stated, the large-capacity currents 

 of long-distance lines cause a rise of voltage from the generator to 

 receiver at light load, while at full load the lagging current taken 

 by the load will cause a drop of voltage from generator to receiver. 

 It is, therefore, evident that the voltage may be kept constant or 

 within certain limits, at the receiving end, if a synchronous con- 

 denser is installed there, and its field adjusted so as to make it 

 take a lagging current at no load and a leading current at full load ; 

 in the first case to offset the effect of the line capacity and in the 

 second to offset the surplus lagging load current. 



Resonance must also be guarded against, as it can give rise 

 to large currents which may open the circuit protecting devices 

 and interrupt the service, or the potential may be raised to a value 

 at which the installation of the system is broken down. In an 

 electric circuit the inductive reactance and the capacity reactance 

 oppose each other. If of equal value they neutralize each other, 

 in which case the resistance of the circuit limits the value of the 

 current. This may, therefore, reach very high values and when 

 passing through the inductance and capacity the voltage at these 

 would in turn be very high. 



To illustrate this further; assume a circuit having a resistance 

 of say 50 ohms and a capacity reactance of 1000 ohms, then th^ 



